CHEMICAL BONDINGUpdated: April, 2004 - M. G. Kamath, Atul Dahiya, Raghavendra R. Hegde

(Monika Kannadaguli & Ramaiah Kotra)

1. INTRODUCTION

For more than four decades, almost all nonwovens required a chemical binder in order to provide anymeasure of structural integrity. In addition, the binder was called upon to contribute and convey numerousproperties that were necessary for the effective performance of the fabric.

During this extended period, binders were essentially the weak element in developing fully acceptablenonwoven fabrics. The fibers that were available to the nonwoven industry were the same fibers that wereavailable to the textile and other fiber-based industries; hence, the fibers were fully acceptable. Generally, thebinder limited the performance of the nonwoven fabric.

The deficiencies cited against nonwovens generally were deficiencies attributable to an inadequate binder.Common complaints are as follows:

The fabric doesn't have enough strength.

The fabric is too stiff.

The fabric has inadequate absorbency.

The fabric shows poor laundering ability.

The fabric has inadequate dry cleaning ability.

The fabric simply doesn't feel like a textile.

Consequently, a great deal of effort has been put into the development and continuous improvement ofchemical binders. The steady improvements in nonwovens performance that occurred over a period of manyyears were, in no small measure, due to improvements in the performance and utility of the binder.

In the very early stages of nonwovens development, different types of natural resins and glues were used tobond nonwovens. While they conveyed some integrity and strength to these webs, they also had many glaringdeficiencies. Consequently, synthetic binders were developed to meet the structural and performancerequirements of nonwoven fabrics.

Polyvinyl acetate was the first successful synthetic binder used in substantial volume. This material haddistinctly superior adhesive properties, strength, and performance compared to the early natural adhesives.This binder is flexible and it can be applied to fiber webs by many ways including print bonding.

The industry was faced with the inevitable compromise in fabric properties of nonwovens bonded withsynthetic materials. In order to build strength in the fabric, increasing amounts of resin must be applied,which results in more stiffness. If softness is necessary, it can be achieved, but primarily by sacrificingstrength.

A substantial improvement in this trade-off of strength and softness was achieved with the introduction ofacrylic-based latex binders in the 1950s and 1960s. By proper selection of co-monomers, it is possible tobuild improved softness properties with adequate strength. Consequently, these binders became widely used

by most of the nonwovens industry, despite the somewhat higher cost.

As polymer technology for manufacturers of synthetic binder systems improved, a greater variety of chemicalbuilding blocks became available with much greater flexibility in terms of binder strength, durability, andother properties. The introduction of cross-linkable and self-crosslinking binder polymers turned out anentirely new range of fabric properties. This was particularly noteworthy in durable nonwovens where suchdurability features as washability and dry cleanability were important.

2. PROPERTIES DESIRED IN A BINDER

The construction of a nonwoven with suitable binders is to achieve improved characteristics such as strength,softness, adhesion, firmness, durability, stiffness, fire retardence, hydrophilicity, hydrophobicity,anti-microbial properties, organic compatibility, reduced surface tension, improved dimensional stability andsolvent, wash and acid resistance. The following list illustrates some general considerations required for anideal binder. The required properties can be varied depending on the end-uses.

· Strength: The strength of a nonwoven fabric is more closely related to the strength of the appliedbinder.

Adhesion to Fibers: Even though the mechanism of adhesion is not completely understood, theadhesion strength of the binder-to-fiber bond has to be considered.

Flexibility/handle: The some movements of fibers should be allowed, especially when a soft hand isdesired.

Elastic Recovery: To avoid the permanent deformation of fabric, good elastic recovery is requiredunder strain.

Resistance to washing/ Drying cleaning: Some nonwoven products need durability in cleaningprocesses according to their end-uses.

Resistance to aging: The binder should be stable and not be degraded in the fabric during storage anduse.

Good color and color retention: Diverse ranges of colors are required, and the colorfastness andyellowing problems should be considered.

Economical: Minimizing the cost is an ongoing requirement.

Other special requirements: Such as Flame resistance, resistance to chemicals, air, oxygen, light,heat, etc.

3. HOW BINDERS WORK

The process involves three steps:

Binder application to nonwoven web.1.

Removal moisture or solvent.2.

Formation of strong bond between binder and nonwoven web.3.

In general binders contain polymer produced by the reaction of monomers in presence of initiators or

catalysts. Surfactants are used to stabilize these polymer particles in water during emulsification (Fig. 1)

Fig.1: Emulsion Polymerization – Schematic. [1]

After application of the binder to the nonwoven web, during moisture removal, film formation takes place.This phenomenon is shown in Fig. 2.

Fig.2: Schematic of film formation [1]

Binder factors influencing nonwoven performance are:

Backbone Structure1.

Functional Group2.

Surfactant3.

Process4.

4. CLASSIFICATION OF BINDERS

Due to their diversity, binders may be classified into several categories based on polymer (binder) chemicalstructure, functionality and the type of curing reactions.

4.1. CLASSIFICATION BASED ON CHEMICAL STRUCTURE

There are three main kinds of binders: butadiene copolymers, acrylates, and vinyl copolymers. The chemicalcompositions influence Tg, hardness and softness, hydrophobicity and hydrophilicity, elasticity, aging, anddry tensile strength of binders. The higher the Tg, the higher will be the dry tensile strength of binders.

Butadiene Copolymers

The structure of the main butadiene copolymer is shown as follows:

The butadiene polymers are cross-linked by polysulphides, and their properties are modified by differentcopolymers. The butadiene monomers provide elasticity while styrene and acrylonitrile monomers givetensile strength, and oil and solvent resistance, respectively. Their disadvantages are oxidation and

discoloration due to residual double bonds in their polymer chains.

Acrylic acid derivatives

Acrylic binders are the most widely used and versatile binders available with various modifications. Theproperties of acrylic binders differ according to their derivatives and copolymers. The structures of thecommon acrylic polymers are as follows:

Acrylic Acid Derivatives

They are frequently copolymerized with styrene, acrylonitrile, vinyl chloride or vinyl acetate, depending onthe desirable properties. Some of these properties are hardness from styrene, solvent resistance fromacrylonitrile, flame retardancy from vinyl chloride, and cost benefits from vinyl acetate.

Vinyl copolymers

There are two main binders for vinyl copolymers: vinyl chloride and vinyl acetate. Since the vinyl binders arestiff, they are plasticized externally or internally. As internal plasticizers, ethylene and acrylate monomers areused, and external plasticizers consist of vinyl chloride. Due to its low Tg, vinyl acetate is not that stiff, andits advantage is low cost. The chlorides cause yellowing problems. The chemical structures are closelyrelated Tg and stiffness of binders.

Vinyl acetate

4.2. CLASSIFICATION BASED ON FUNCTIONALITY

The functionality of binders is in the functional groups attached to polymer chains, which influences wet andsolvent properties. To modify binder properties, copolymerization with a small amount of monomers withspecial functionality is performed. The main functionalities in binders are carboxyl and amide side chains.

Carboxyl functionality

This functionality is related to binders containing acrylic acid or methacrylic acid by copolymerization. Thebinders are crosslinkable since the functional group, carboxylic acid, provides sites for crosslinking reactions.

Amide functionality

This functionality is related to binders containing acrylamide by copolymerization. The amide functionalityprovides crosslinking sites, and even the binders are self-crosslinkable.

N-metylol amide (NMA) functionality

This functionality is obtained after acrylamide is reacted with formaldehyde. The binders containing thesubstituted acrylamide groups have self-crosslinking properties and the possible reaction as follows:

Acrylamide

4.3. CLASSIFICATION BASED ON TYPE OF CURING REACTIONS

The classification of reactivity refers to crosslinkability of binders, which is related to reaction with curingresins, crosslinking agents. The most common curing resin is melamine formaldehyde condensate resininvolving reaction of n-methylol groups.

Non-crosslinkable polymers

The polymers do not contain any of the functional groups. They cannot crosslink, even with external curingresins.

Crosslinkable polymers

The polymers contain acid or amide functional groups. They can react with added curing resins, but thedegree of crosslinking is limited.

Self-crosslinking polymers

The polymers contain n-methylol functional groups. They can react with themselves, and a high crosslinkdensity can be obtained by adding curing resins.

Recent trends in chemical bonding: Although nonwoven manufacturers are seeking alternativetechnologies such as thermal bonding, chemical bonding still has its advantages and a promising market.Chemical bonding allows more room for fabric designs and fiber selections. Both disposable and durableproducts are supplied to roll goods producers and fiber manufacturers. On the environmental front,increasingly strict regulations and guidelines are driving a trend towards alternative products andtechnologies. Manufacturers and end-product suppliers alike are seeking ultra-low or formaldehyde-freebinders. The growing consideration of the environmental impact of chemical binder and additives hasbecome a focus of debate on the national and international level.

Latex binder chemical types

A latex polymer consists of an aqueous medium with extremely fine liquid or solid polymer particlesdispersed therein. The latex polymer generally is produced via free radical emulsion polymerization in water,whereby a vinyl monomer is combined with a small amount of other monomer (co-monomer) to create a highmolecular weight polymer. The latex dispersion also will carry surfactants, stabilizers, and other additives toconvey realistic properties to the latex itself.

When used as a binder, the latex typically is combined with other components to provide the formulatedbinder ready for application to the fiber web. The formulated binder conveys many characteristics that are notpossessed by the straight binder. Consequently, there is a substantial chemistry involved in combining thelatex with the other components in order to prepare the formulated binder.

Binders are quite dependent upon the glass transition temperature (Tg) of the monomer unit selected to formthe polymer. Differential Scanning Calorimeter (DSC) is used to determine Tg as shown below (fig. 3):

Fig. 3: Typical DSC graph

The lower the (Tg) of the monomer units, the softer is the resulting polymer. A sampling of the most commonmonomers used in the manufacture of latex polymer for nonwoven binders include the following materials:

Monomer Tg(0C)

Ethylene -125

Butadiene -78

Butyl Acrylate -52

Ethyl Acrylate -22

Vinyl Acetate +30

Vinyl Chloride +80

Methyl Methacrylate +105

Styrene +105

Acrylonitrile +130

The monomers selected for forming the polymeric latex also have considerable influence on the hydrophilicor hydrophobic nature of the binder. This can affect the wet strength of the nonwoven fabric as well as a hostof absorbency characteristics.

With the current capabilities of polymerization chemistry, there is considerable versatility for each chemical

type. Despite this range of properties, the commonly employed nonwovens binders generally arecharacterized by a fairly well defined set of properties. These properties can be modified to some degree byincorporation of other agents, but they provide a useful guide in classifying the kind of performance to beexpected from each type of binder.

5. TYPES OF BINDERS

The following comparison of latex binder chemical types provides an indication of the relative performance,as well as the advantages and disadvantages of each type of binder. As indicated, the binder properties can bemodified considerably by the presence of co-monomers.

i) Acrylic: These binders offer the greatest durability, color stability, and dry/wet performance. Acrylicbinders have the widest range of fabric hand properties. They can be formulated to vary from very soft (Tg =- 40°C) to extremely hard (Tg = 105°C). These binders can be used in virtually all nonwovens applications,although they tend to be more costly. These polymers can be made to cross-link, with substantialimprovement in durability.

ii) Styrenated Acrylics: These are tough, hydrophobic binders. The resulting textile hand ranges fromsoft-to-firm (Tg varies from –20°C to +105°C ).These binders can be used in applications where there is aneed for some wet strength without crosslinking. The use of this type of latex binder does involve somesacrifice in UV and solvent resistance.

iii) Vinyl Acetate (VAC): The vinyl acetate binders are firm (Tg = +30°C to +40°C); however, they arerelatively low cost and find extensive use. They offer good dry strength and toughness, but are somewhathydrophilic and have a tendency to yellow when subjected to heat.

iv) Vinyl Acrylics: These binders are more hydrophobic than the straight VAC binders. They provideexcellent toughness, flexibility, and better color stability. They are the compromise between VAC andacrylic, and can compete on a cost/performance basis. The hand range is limited to intermediate softness (Tg= -10°C) to a firm hand (Tg = +30°C).

v) Ethylene Vinyl Acetate (EVA): These latex binders have a (Tg range of –20°C to +115°C, which isequivalent to soft ranging to an intermediate textile hand. They exhibit high wet strength, coupled withexcellent absorbency. In general, they are less costly than acrylics. They do have a tendency to have more ofan odor compared to other binders. They are used primarily in wipes, air-laid pulp fabrics and similarapplications.

vi) Styrene-Butadiene (S/B, SB, or styrene butadiene rubber): These binders have an excellentcombination of flexibility and toughness. They range in hardness from very soft (Tg = -30°C) to very firm(Tg = +80°C). However, the (Tg of an SB binder is not strictly comparable to other classes of nonwovenbinders. The styrene-to-butadiene ratio (S/B ratio) is the most common method for describing the relativehand resulting from the use of these binders. When cross-linked, this class of binder is very hydrophobic anddurable. They are affected somewhat by heat and light because of their tendency to oxidize.

vii) Polyvinyl Chloride (PVC): The homopolymer of polyvinyl chloride is a very hard, rigid polymer (Tg =+80°C). This polymer must be plasticized to provide flexibility and film-forming properties. Normally, the(PVC) binders used in nonwovens are softened internally by co-polymerizing the vinyl chloride or withsofter acrylic monomers. The hand range of most of these polymers is still relatively firm (Tg is greater thanthe +30°C). Because this type of polymer is a thermoplastic, it performs well in heat and dielectric sealingapplications. This can be an advantage in some uses. The chlorine content of the polymer promotes flameretardency. This feature is one of the primary benefits of utilizing this type of binder. However, the chlorine

also conveys the tendency to yellow upon heat aging, due to elimination of hydrogen chloride from thepolymer.

viii) Ethylene/Vinyl Chloride: Binders in this class have a slightly broader hand range (Tg = 0°C to +30°C)without the external plasticization required of (PVC) binders. The presence of the chlorine again conveyssome flame retardancy. These binders exhibit good acid resistance, fair water resistance, and excellentadhesion to synthetic fibers. There is some tendency to yellow upon aging. In essence, this is an internallyplasticized (PVC) binder, considering the ethylene monomer to be the softener.

6. FORMULATION

i) Ingredients

The formulation of binding solution is an art since many ingredients are involved and many differentpossibilities exist for different end-uses. Some of the characteristics, and the types of formulation agentsutilized to obtain them include the following.

· Surfactants : offer improvement in binder adhesion, stability, and ability to be converted into a foam

External cross-linkers: provide cross-links with binder polymer to provide improved performance

Defoamers: utilized to minimize foam in processing

Repellent agents : convey water or oil repellency

Salts: added to impart low flame response properties and to convey antistatic properties

Thickeners: added to control the rheology of the binder liquid

Catalysts: added to facilitate curing and to promote cross-linking

Acids and bases: added to control pH of the latex

Dyes and pigments: provide color to the binder and fabric

Fillers: added to reduce binder tack and to lower cost

Optical brighteners: added to increase whiteness

Sewing aids: added to provide lubrication during fabrication

The purposes of wetting agents, mainly nonionic or anionic surfactants, are to enhance binder penetrationthrough webs, improve the affinity between binder and fibers. The crosslinker, which has multi-functionalgroups, is generally added to increase crosslink density and to improve durability and resistance todeformation.

ii) Order of Formulation

In terms of adding ingredients into a binding bath, the compatibility of ingredients should be confirmedbecause the orders are extremely important. The milky white color of most binders impedes a check on thewhite-color indication of non-compatible ingredients, so most ingredients are first added to the dilutionwater. After the compatibility is assured, binders are added and then thickeners added to adjust viscosity. Forthe stability of the binding solution, catalysts are added just before application. Some water may be added toreach a desirable solid level. The summarized order is as follows:

· Most ingredients

· Latex binder

· Thickener

· Catalyst

· Some water, and the others, such as dyes and pigments, fillers, clays, optical brighteners, sewing aids,etc.

7. BONDING TECHNOLOGY

Web consolidation or nonwoven bonding processes interlock preferentially arranged fiber or film assembliesby mechanical, chemical, solvent, and/or thermal means. The degree of bonding is a primary factor indetermining fabric integrity (strength), porosity flexibility, softness, and density (loft, thickness).

Bonding may be carried out as a separate and distinct operation, but generally is carried out as a sequentialoperation in tandem with web formation. In some fabric constructions, more than one bonding process maybe used to enhance physical or chemical properties.

Mechanical consolidation methods include needlefelting, stitchbonding, and hydroentangling. Chemicalconsolidation methods involve applying adhesive binders to webs by saturating, spraying, printing, orfoaming techniques. Solvent bonding involves softening or partially dissolving fibers with a solvent toprovide self-bonding surfaces. Thermal bonding involves the use of heat and often pressure to fuse or weldfibers together at points of intersection or in patterned bond sites.

Important issues to consider when choosing the web consolidation methods are economy; versatility; andproduct properties, primarily absorbency, strength, softness, loft, and purity. A recurring issue involvesenvironmental requirements of both the process and the product. Many techniques are done for specificproperties of unique fabrics; therefore, it is difficult to measure differences in cost. In some instances, two ormore bonding techniques compete. The system that is most energy-efficient; environmentally sound orprovides the preferred fabric properties generally dominates.

7.1 CHEMICAL BONDING PROCESSES

Chemical or resin bonding is a generic term for interlocking fibers by the application of a chemical binder.The chemical binder most frequently used to consolidate fiber webs today is a water-borne latex. Most latexbinders are made from vinyl materials, such as polyvinylacetate, polyvinylchloride, styrene/butadiene resin,butadiene, and polyacrylic, or their combinations.

Latexes are extensively used as nonwoven binders, because they are economical, versatile, easily applied,and effective adhesives. The versatility of a chemical binder system can be indicated by enumerating a fewfactors that are considered when such a system is formulated.

The chemical composition of the monomer or backbone material determines stiffness/softness properties,strength, water affinity (hydrophilic/hydrophobic balance), elasticity, durability, and aging. The type andnature of functional side groups determines solvent resistance, adhesive characteristics, and cross-linkingnature. The type and quantity of surfactant used influences the polymerization process, polymer stability, andthe application method.

Chemical binders are applied to webs in amounts ranging from about 5% to as much as 60% by weight. Insome instances, when clays or other weighty additives are included, add-on levels can approach or even

exceed the weight of the web. Waterborne binders are applied by spray, saturation, print, and foam methods.A general objective of each method is to apply the binder material in a manner sufficient to interlock thefibers and provide fabric properties required of the intended fabric usage.

The common methods of bonding include saturation, foam, spray, print and powder bonding. They are brieflyintroduced in the following paragraphs:

i) Saturation

Saturation bonding is used in conjunction with processes which require rapid binder addition, such ascard-bond systems, and for fabric applications which require strength, stiffness, and maximum fiberencapsulation, such as carrier fabrics. Fiber encapsulation is achieved by totally immersing the web in abinder bath or by flooding the web as it enters the nip point of a set of pressure rolls. Excess binder isremoved by vacuum or roll pressure.

Three variations of saturation bonding exist: screen, dip/squeeze, and size-press. Screen saturation is used formedium-weight nonwovens, such as interlinings. Dip/squeeze saturation is used for web structures withstrength sufficient to withstand immersion without support, such as spunbonds. Size-press saturation is usedin high speed processes, such as wet-laid nonwovens. Drying and curing may be carried out on steam-heateddrying cans or in thru-air ovens or perforated-drum dryers. Binder addition levels range from 20% to 60%.Two techniques, single screen saturator and applicator roll technique, are illustrated in fig. 4 & 5. Advantages of this method are simplicity, controllable tensile strength and softness by choice and amount ofbinders. The disadvantages are the great influence of binders on softness, and the limitation in loftiness.

Fig. 4: Saturation bonding

Fig. 5: Applicator roll method

ii) Foam bonding

Foam bonding is a means to apply binder at low water and high binder-solids concentration levels. The basicconcept employed involves using air as well as water as the binder diluent and carrier medium. Foam-bondednonwovens require less energy in drying, since less water is used. The foam is generated by introducing airinto the formulated latex while mechanically agitating the binder solution.

Air/latex dilutions or blow ratios in the order of 5:25 are practiced for various products. With the addition ofa stabilizing agent to the binder solution, the foam can resist collapsing during application and curing, and thebonded fabric will exhibit enhanced loft, hand, and resilience. Non-stabilized foams are referred to as froths;froth-bonded fabrics are similar in properties to some saturation-bonded nonwovens. One example of thisbonding is illustrated in fig. 6. The advantages include less energy required to dry the web, less bindermigration and controllable softness by choices and amount of binders. The disadvantages are difficulties incontrolling process and adequate foaming.

Fig 6: Foam bonding process

iii) Spray bonding

In spray bonding, binders are sprayed onto moving webs. Spray bonding is used for fabric applications thatwhich require the maintenance of highloft or bulk, such as fiberfill and air-laid pulp wipes. The binder isatomized by air pressure, hydraulic pressure, or centrifugal force and is applied to the upper surfaces of theweb in fine droplet form through a system of nozzles.

Lower-web-surface binder addition is accomplished by reversing web direction on a second conveyor andpassing the web under a second spray station. After each spraying, the web is passed through a heating zone

to remove water, and the binder is cured (set/cross-linked) in a third heating zone. For uniform binderdistribution, spray nozzles are carefully engineered. Typical spray bonding is illustrated in fig. 7 & 8

Fig. 7: Schematic of spray bonding process

Fig. 8: Industrial spray bonding process

iv) Print bonding

Print bonding applies binder only in predetermined areas. It is used for fabric applications that require a partof the area of the fabric to be binder-free, such as wipes and coverstocks. Many lightweight nonwovens areprint bonded. Printing patterns are designed to enhance strength, fluid transport, softness, hand, absorbency,and drape. Print bonding is most often carded out with gravure rolls. Binder addition levels are dependent onengraved area and depth as well as binder-solids level. Increased pattern versatility can be achieved with theuse of rotary screen rolls. Drying and curing are carried out on heated drums or steam-heated cans.

In print bonding, high viscose binders are applied to limited, patterned areas. A prewet/prebond step isrequired for enough strength of webs, and typical steps in this bonding are in fig. 9.

Fig. 9: Latex printing process

There are two types of printers: rotary screen and rotogravure printers. Binders are applied through a hollowapplicator roll in rotary screen printer, while in rotogravure printer they are applied by an engraved applicatorroll as shown in fig. 10 a & b. The main advantage is that outstanding softness of nonwoven fabrics withadequate strength can be achieved.

Fig. 10a: Print bonding

Fig. 10b: Printing equipment

v) Powder bonding

In powder bonding, the adhesive powder of thermoplastic polymers is applied onto webs by heat andpressure. Polyesters and polyolefins with low Tg's and molecular weight can be used as powder binders. Atypical bonding line is illustrated in fig. 11 a, b & c. The advantages are the bulky structure of densenonwovens and the applicability of polyester or polypropylene webs. The disadvantage lies in difficulties ofsuitable particle sizes and ranges, and their distribution.

Fig. 11a: Powder bonding line

Fig. 11b: Powder adhesive sprinkling

Fig. 11c: Powder bonding with electrostatic assistance

8. APPLICATIONS

Nonwoven products in which binders are utilized:

Wipes and towels

Medical nonwovens

Roofing products

Apparel interlinings

Filter media

Coating substrates

Automotive trim

Carrier fabrics

Bedding products (high loft)

Furniture applications (high loft)

Apparel

Pillows (high loft)

9. SUMMARY

In the latter part of the 1970s and 1980s, thermal bonding technology grew rapidly, providing the industrywith a realistic method to produce strong and soft nonwoven fabrics without the use of a chemical binder.This development provided substantial advances in performance and properties of many types of nonwovens.One quality of this new bonding technique was that these nonwovens contained no formaldehyde and nochemical additives to cause consumer concern. Naturally, this has depressed the interest of chemical binderswithin the industry and has resulted in a decline in binder usage.

Despite this setback, significant improvements and advances have continued to be made by the syntheticpolymer industry, to the benefit of the range of nonwoven products that continue to utilize chemical bondingmethods. These improvements have involved such developments as formaldehyde-free binders, low-curetemperature binders, complex copolymers with unique characteristics, moldable binders, and others. In thefuture, new types of binders may be combined with the present choices, for example, by co polymerization.In addition, new ideas such as reactive binders which can be covalently bonded with fibers will becontinually investigated.

REFERENCES

1. Michele Mlynar, Rohm & Haas Co, “Chemical Binders”, INTC 2003, Sept 15-18.

2. W.E. Devry, "Latex bonding Chemistry and Processes."

3. Derelich, "Nonwoven Textile Fabrics", Kirk-Othmer: Encyclopedia of Chemical Technology, Vol.16, 3rd Ed, p104-124, 1981.

4. B.M. Lichstein, The Nonwovens Handbook, INDA Association of the Nonwoven Fabric Industry,New York, 1988.

5. J. Lunenschloss and W. Albrecht, Non-woven Bonded Fabrics, John Wiley & Sons Inc., New York, 1985.

6. A.E. Meazey, " Binders used in Bonded Fiber Fabric Production", Nonwovens '71, The textile TradePress, England, 1971.

7. M.F. Meyer and W. A.; Haili, " Nonwovens and Laminates Made with Polyester Adhesive Powders,"Eastman Kodak Company.

8. J.M. Oelkers and E.J. Sweeney," Latex Binders and Bonding Techniques of Disposables", 1988.

9. Ellen Lees Wuagneux, " And how would you like your nonwovens?" Nonwoven Industry Oct. 64-72(1997).

10. INDA, Book of Paper, 1997

11. www.nonwovens.com/facts/technology/binders/binders.htm

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DRY-LAID NONWOVENSUpdated: April, 2004 - Atul Dahiya, M. G. Kamath, Raghavendra R. Hegde

(Xiao Gao and Hsu-Yeh Huang)

1. INTRODUCTION

All nonwoven fabrics are based on a fibrous web. The characteristics of the web determine the physicalproperties of the final product. These characteristics depend largely on the web geometry, which isdetermined by the mode of web formation. Web geometry includes the predominant fiber direction, whetheroriented or random, fiber shape (straight, hooked or curled), the extent of inter-fiber engagement orentanglement, crimp and z-direction compaction. Web characteristics are also influenced by the fiberdiameter, fiber length, web weight, chemical and mechanical properties of the polymer.

The choice of methods for forming webs is determined by fiber length. Initially, the methods for the formingof webs from staple-length fibers were based on the textile carding process, whereas web formation fromshort fibers was based on papermaking technologies. Though these technologies are still in use, newermethods have been developed. For example, webs are formed from long, virtually endless filament directlyfrom bulk polymers; both web and fibers are produced simultaneously.

2. DRY-LAID NONWOVENS FROM STAPLE FIBERS

Staple Fibers:

These fibers are long enough to be handled by conventional spinning equipment. The fibers are 1.2 to 20cmor longer, but not continuous.

3. FOUR PHASES OF THE DRY-LAID MANUFACTURING SYSTEM

3.1 FIBER SELECTION

Some of the factors to be considered in the selection of fibers for dry-laid nonwovens are:

Absorbency

Abrasion resistance

Bursting strength

Permeability

Softness and tear resistance in the fabric

3.2 FIBER PREPARATION

Staple fibers are shipped to the manufacturer in the form of bales and fiber preparation consists of mechanicaland pneumatic processes of handling from the bale to the point where the fiber is introduced into theweb-forming machine. The following processes are included in a typical fiber preparation line:

Bale opening

The bales are unstrapped and placed side-by-side in line with the milling head of a bale opener. Thefibers are picked up from the top of the bales by two opening rolls in conjunction with a partial airvacuum. The opening head traverses back and forth across the bale lay down, starting and stopping on

demand from the blending hopper. This ensures maximum efficiency and blending. The objective ofan opening line is to reduce the size of fiber tufts from the bale to the chute feed, which supplies theweb forming machine.

Fig. 1: Optomix® head

Blending

The blending feeders gently open the tufts of fibers by the interaction of an inclined needle latticeapron and an evener roller equipped with needles. Blending of the tufts from different bales also takesplace in the opening and mixing achieved by the inclined apron and the evener roller. The openedtufts are deposited into a weigh pan controlled by load cells which dump the fibers onto a feedconveyor.

Fig.2: Blend hopper

Coarse opening

The blending conveyor feeds fiber into an opening roll, which has a three-lag pin beater (Kirschnerbeater type) where coarse opening of the fiber tufts takes place.

Fine opening

The fiber opened by the opening roll is transported by air to the feed box of the fine opener. The fineopener consists of two opening rolls, one evener roll and a cylinder roll all of which are wound withmetallic clothing. The opener reduces the tuft size by using the principle of carding points betweenrolls A and B and between rolls B and C. The reduced tufts are transferred to the cylinder roll Dwhich delivers the opened fiber into an air stream to the web-former.

Web-former feeding

The feed system to the web-forming machine is selected based on the type of fiber and the type ofweb-former. Chute feeding is normally used to feed fibers up to 60 millimeters in length. For longerfibers, a hopper feed with a shaker-type chute is used.

Fig. 3: Kirshner beater

Fig. 4: Microtuft opener, FS/52, KD condenser

3.3 WEB FORMATION AND LAYERING

The dry-web process for making a nonwoven consists of basically three methods:

3.3.1 WEB FORMATION

3.3.1.1 MECHANICAL WEB FORMATION (Carding or Garnetting)

Carding

The main objective of carding is to separate small tufts into individual fibers, to begin the process ofparallelization and to deliver the fibers in the form of a web. The principle of carding is themechanical action in which the fibers are held by one surface while the other surface combs the fiberscausing individual fiber separation. At its center is a large rotating metallic cylinder covered with card

clothing. The card clothing is comprised of needles, wires, or fine metallic teeth embedded in a heavycloth or in a metallic foundation. The cylinder is partly surrounded by an endless belt of a largenumber of narrow, cast iron flats positioned along the top of the cylinder. The top of the cylinder maybe covered by alternating rollers and stripper rolls in a roller-top card.

Fig. 5: Basic construction of a card and its parts

The fibers are fed by a chute or hopper and condensed into the form of a lap or batting. This isinitially opened into small tufts by a licker-in, which feeds the fibers to the cylinder. The needles ofthe two opposing surfaces of the cylinder and flats or the rollers are inclined in opposite directionsand move at different speeds. The main cylinder moves faster than the flats and, due to the opposingneedles and difference in speeds, the fiber clumps are pulled and teased apart (fig 6 a & b). In theroller-top card the separation occurs between the worker roller and the cylinder. The stripping rollerstrips the larger tufts and deposits them back on the cylinder. The fibers are aligned in the machinedirection and form a coherent web below the surface of the needles of the main cylinder.

Fig. 6 (a): Carding action [3]

Fig. 6 (b): Carding action

The web is doffed from the surface of cylinder by a doffer roller and deposited on a moving belt. Theorientation ratio of the web at the doffer of a conventional card is approximately 5:1.

Productivity of older roller cards is about 30-50 kg/hour at the width of 1.5~2m. Nowadays, the rollercards of performance up to 1000kg/hour in width 2~3.5m are delivered. Flat carding machines areusually 1m wide and process about 5~50kg/hour [3].

Spinning preparation and carding of staple fibers have been and still are the subjects of studies andpublications concerning various aspects. There is even research on the carding of micro-fibers [4].

· Garnett

Garnetts are similar to roller-top cards. R.L. Street has described the garnett as "a group of rollsplaced in an order that allows a given wire configuration, along with certain speed relationships, tolevel, transport, comb and interlock fibers to a degree that a web is formed."[2] Garnetts are mostlyused to process waddings and for making pads for automobile and bedding industries. It delivers amore random web than a card. Most webs from garnetts are layered by crosslapping to build up thedesired finished nonwoven weight.

3.3.1.2 AERODYNAMIC WEB FORMATION (Air-lay)

The orientation created by carding is effectively improved by capturing fibers on a screen from an air-stream.This is done on a Rando-Webber component. Starting with a lap or plied card webs fed by a feed roller, thefibers are separated by a licker-in or spiked roller and introduced into an air-stream.

The total randomization excludes any preferred orientation when the fibers are collected on the condenserscreen. The web is delivered to a conveyor for transporting to the bonding area. Feeding of theRando-Webber by the cards increases the uniformity of the web. The length of fibers used in air-laying varies

from 2 to 6 cm. The shorter lengths allow higher production speeds. Longer fibers require higher air volume,i.e., a lower fiber density to avoid tangling. Problems associated with air-laying are speed, web uniformityand weight limitations. Due to uniformity problems, it has not been practical to make isotropic webs lighterthan 30g/m2. Air-laying is slower than carding and, hence, more expensive.

Fig. 7: Aerodynamic web production

The aerodynamic web forming process has some typical advantages and disadvantages [3]:

Among the advantages are:

· Isotropic structure of the web

· Voluminous webs can be produced

· Wide variety of processable fibers such as natural, synthetic, glass, steel, carbon, etc.

The main disadvantages are as follows:

Low level of opening fiber material by licker-in

Variable structures of web in width of layer due to irregular air flow close to walls of duct

· Possible entanglement of fibers in air stream

i) Centrifugal dynamic web formation (random card)

The centrifugal dynamic random card forms a web by throwing off fibers from the cylinder onto a dofferwith fiber inertia, which is subject to centrifugal force, in proportion to the square of the rotary speed.Orientation in the web is three-dimensional and is random or isotropic. The random card produces a 12 to

50 g/ m2 web with fine fibers of 1.5 den and a web up to 100 g/m2 with coarse fibers. The production of therandom card is generally about 30 to 50% higher than conventional cards. The machine direction versus thecross-direction strength is better than those produced in the conventional card, but not as good as that of theair-laid webs. The number of machines required in the nonwovens line for the production of multi-layeredwebs can be reduced by the use of the random card. The broad scope of adaptability of the random card forproducing a wide range of nonwovens has led to innovations in this method.

3.3.2 LAYERING

Web formations can be made into the desired web structure by the layering of the webs from either the cardor garnett. Layering can be accomplished in several ways to reach the desired weight and web structure.

· Longitudinal Layering: More than once cards are involved in here. Carded webs from all the cards(placed in a sequence one after the other) are laid above one another on a conveyor belt and laterbonded. Properties of the bonded webs are anisotropic in nature because of the unidirectionalarrangements of fibers. This technique can only be used for making light textiles.

Cross layering: It can be done by using two different devices (cross lappers); vertical and horizontalcross lapper. Vertical lapper consists of a pendulum conveyor after the doffer roll on a card (as shownin fig. 8). Pendulum conveyor reciprocates and lays the carded web in to folds on another conveyorbelt.

Fig. 8: Vertical cross lapper [3]

Perpendicular layering: This technique has an advantage over longitudinal and cross layering becauseof the perpendicular and oriented fibers in the fabric. The bonded webs have high resistance tocompression and show better recovery after repeated loading.

3.4 BONDING AND STABILIZATION OF WEBS

The type of bonding and finishing must also be considered when determining the chemical changes in thefiber properties that may develop, which might affect the end product. Equally important is the performanceof the fiber in fiber preparation (opening and blending) and web formation.

3.4.1 MECHANICAL BONDING

Needlepunching

Needlepunching is a process of bonding nonwoven web structures by mechanically interlocking thefibers through the web. Barbed needles, mounted on a board, punch fibers into the web and then arewithdrawn leaving the fibers entangled. The needles are spaced in a non-aligned arrangement and aredesigned to release the fiber as the needle board is withdrawn. Details

Stitch Bonding

Stitch bonding is a method of consolidating fiber webs with knitting elements with or without yarn tointerlock the fibers. There are a number of different yarns that can be used. Kevlar® is used forstrength in the fabric for protective vests. Lycra® is used for stretch in the fabric. Home furnishingsare a big market for these fabrics. Other uses are vacuum bags, geotextiles, filtration and interlinings.In many applications stitch-bonded fabrics are taking the place of woven goods because they arefaster to produce and, hence, the cost of production is considerably less.

3.4.2 THERMAL BONDING

Thermal bonding is the process of using heat to bond or stabilize a web structure that consists of athermoplastic fiber. All part of the fibers act as thermal binders, thus eliminating the use of latex or resinbinders. Thermal bonding is the leading method used by the cover stock industry for baby diapers.Polypropylene has been the most suitable fiber with a low melting point of approximately 165°C. It is alsosoft to touch. The fiber web is passed between heated calender rollers, where the web is bonded. In mostcases point bonding by the use of embossed rolls is the most desired method, adding softness and flexibilityto the fabric. Use of smooth rolls bonds the entire surface of the fabric increasing the strength, but reducesdrape and softness. Details

3.4.3 CHEMICAL BONDING

Bonding a web by means of a chemical is one of the most common methods of bonding. The chemical binderis applied to the web and is cured. The most commonly used binder is latex, because it is economical, easy toapply and very effective. Several methods are used to apply the binder and include saturation bonding, spraybonding, print bonding and foam bonding. Details

3.4.4 HYDROENTANGLEMENT

Hydroentanglement is a process of using fluid forces to lock the fibers together. This is achieved by finewater jets directed through the web, which is supported by a conveyor belt. Entanglement occurs when thewater strikes the web and the fibers are deflected. The vigorous agitation within the web causes the fibers tobecome entangled. Details

Fig. 9: Schematic of hydroentanglement process

REFERENCES

1. Turbak, A. F., Nonwovens: Theory, Process, Performance and Testing.

2. Street, R.L., "Mechanical Web Formations," 1981 Fiber Fill Conference Proceedings, INDA,Charlotte, NC, p.1, 1981.

3. Jirsak, O., and Wadsworth, L. C., Nonwoven Textiles, Carolina Academic Press, 1999.

4. Leifeld, F., Carding Micro-Fibers, Textile Technology, Melliand English, 2/1993 E43.

Back to Table of Contents

FINISHING OF NONWOVEN BONDED FABRICSUpdated: April, 2004 - M. G. Kamath, Atul Dahiya, Raghavendra R. Hegde

(Haoming Rong & Ramaiah Kotra)

1. INTRODUCTION

The production of nonwoven fabrics is carried out either as a continuous process, with fiber or resin as theinput material and a roll of fabric as output, or as a series of batch processes. Correspondingly, fabricfinishing is carried out either in tandem with web formation and consolidation or off-line as a separateoperation. Nonwoven bonded fabrics are, by definition, textiles and they can be finished in exactly the sameway as other textiles such as woven or knitted fabrics. There are many examples of particular methods andtypes of finishing equipment being used for both kinds of fabrics. Nonwovens may be given one or more of avariety of finishing processes as a means of enhancing fabric performance or aesthetic properties.Performance properties include functional characteristics such as moisture regain and transport, absorbency,or repellency; flame retardancy; electrical response; resistance; and frictional behavior. Aesthetic propertiesinclude various attributes such as appearance, surface texture, color, and odor.

Finishing of nonwoven bonded fabrics can be classified from different ways. Some people believe thatnonwoven finishing processes can be categorized as chemical, mechanical, or thermal-mechanical. Chemicalfinishing involves the application of chemical agents as coatings to fabric surfaces or the impregnation offabrics with chemical additives or fillers. Mechanical finishing involves altering the texture of fabric surfacesby physically reorienting or shaping fibers on or near the fabric surface. Thermal-mechanical finishinginvolves altering fabric dimensions or physical properties using of heat and pressure [1]. Generally, finishingof nonwoven bonded fabrics is classified as Dry finishing or Wet finishing.

2. DRY FINISHING

2.1 SHRINKAGE

The compaction that accompanies shrinkage is useful in obtaining greater basis weight or GSM (grams persquare meter) and density, more bulk, higher strength and improved cleavage properties. Shrinkage occurswhen the fibers are wet or dry depending on the type. Shrinkage by exposure to heat is suitable for anonwoven fabric made predominantly of synthetic fibers and is especially effective if fibers are prone toshrinkage. The web is fed through the heating zone on screen driers. They are usually perforated cylinderdriers with a rotating over feed, whereby the web is fed faster onto the roll than it is drawn off.

A second shrinkage is carried out if the web contains significant amounts of natural fibers. The web isimmersed in a tank of hot water to promote shrinkage and is dried without tension. Some special syntheticfibers shrink both when they are wet and when heated. A variation of wet shrinkage, which aids in savingenergy, is shrinkage in steam. Needling together two types of webs where one shrinks and the other isshrink-proof results in the formation of decorative raised patterns when shrunk. This technique is used in theproduction of sculptured wall and floor coverings.

2.2 WRENCHING

The Clupak process, invented by Sanford Cluett, is similar to the sanforising process first used in the paperindustry in 1957. It was later adopted to wet-laid nonwoven bonded fabrics.

The machinery (fig. 1) consists of a continuous rubber belt, about 25 mm thick, with an intermediate wovenlayer lying on a heated, chromium-plated and polished drying cylinder. The web is pressed against thecylinder at the first point of contact by a non-rotating clamping bar. The rubber cloth is compactedlengthwise, which affects the web between it and the cylinder in the same way thus causing compacting andcrimping of the fibers in the web longitudinally. The web is fed moist, through the gap between the belt andthe cylinder. The compacting is fixed by drying.

The outcome of the Clupak method depends on a number of factors. Hydrophilic fibers are more suitablethan hydrophobic ones. Polyolefin fibers are not suitable due to their lower moisture absorption andsensitivity to heat. Webs in which the fibers are oriented lengthwise give a more pronounced effect thancross-laid or random-laid webs. The degree of wrenching is increased if the moisture content is high - about20% - but if the bonding agent is more than 50% such increases are unattainable. Thermoplastic bondingagents assist wrenching but the web tends to adhere to the cylinder. Elastomer bonding agents due to theirelastic nature almost cancel the wrenching effect.

2.3 CREEPING: THE MICREX-MICROCREEPE PROCESS

In the Micrex process, compaction of the web is so strong that the creeping effect is visible and the increasein extension and basis weight can easily be measured. The surface per unit area is larger and the flexibility isimproved even further than by the Clupak method.

The apparatus for the Micrex process (fig. 2) consists of a rotating conveyor roller, the surface of which hasscrew- shaped grooves in it, and two guide plates - one fixed and one elastic -forming a knee lying againstthe cylinder. Between these is fed the web and nearby is a scrapper-like compressing device inclined at anacute angle to the surface of the roller.

The web is compacted in the first gap, then raises itself from the cylinder in the relaxation zone to becompacted by the scrapper again. The process can be adjusted to produce a fine or coarse crepe withoutsignificant impairment of the mechanical properties despite production speeds of 150-200 m/min since theweb is handled dry and at much lower temperatures as compared to the Clupak method. This method issuitable to creeping longitudinally oriented carded webs, wet or dry-laid random structured webs,spun-bonded and spunlaced products.

2.4 CRABBING, CALENDERING AND PRESSING

These methods are used to improve the surface characteristics of the fabrics, the most important featuresbeing smoothing and patterning. The processes used are continuous and usually involve one or several pairsof rollers operating under pressure.

Glazing or Rolling calender: This method is not particularly important for nonwoven fabrics, with occasionalexceptions. The smooth surface can be obtained usually by selecting an appropriate form of bonding and,especially, for drying a wet-bonded web. Calendering has not met with much success since it is oftenaccompanied by undesirable compression. The only time a rolling calender is used is when two steel rollersare paired to break the so-called 'blotches' in spun-bonded fabrics.

Moire or goffering calender: The calenders are common in nonwoven finishing and are used in thecompacting of the webs made of natural and synthetic fibers. This type of calendering can be considered tobe both a bonding and finishing process. Webs composed of longitudinally oriented cotton or viscose fiberswith a GSM of about 10-30 g/m2 can be stiffened and compacted sufficiently by passing them through agoffering calender when slightly damp. Hot embossing of synthetic fiber webs, even when the fibers arelongitudinally oriented, produces a product remarkably strong due to the fibers melting at the embossedareas. The patterns can be of grid, webbed or point type. The temperature of the heated rollers is generally20-30°C above the melting point of the fibers and the nip roll pressure 20-50dN/cm, depending on thevolume of the web and the proportion of synthetic fibers it contains. If the web is cross-laid, point embossingresults in maximum strength. If the fibers are arranged lengthwise, webbed embossing is employed.

The embossing effect is used to obtain special effects such as leather graining, simulated weave, plaster,brush strokes, cord and mock tiling. Another area in which heated calenders are used is in the manufacture oflaminates. Here thermoplastic fibers, layers of thread or film are placed between two layers of non-plasticweb and are fused together by heat and pressure. Such laminates are used as tablecloths, seat and cushioncovers. Calenders are also used in the transfer printing of the bonded webs.

Roller presses: The oldest form of improving the surface of nonwoven bonded fabrics is the pressing of woolfelts, especially felts for collar linings. This gives a smoother surface finish and also improves strength andluster.

2.5. PERFORATING AND SLITTING

The nonwoven bonded fabrics produced are too stiff and are, therefore, unsuitable for clothing. This isbecause the individual fibers are not free to move in relation to one another, as are threads in woven orknitted fabrics. Perforating and slitting are two methods practiced to improve the fall or drape of nonwovenbonded fabrics.

Perforating: The Artos method is a method of perforating in which the web, which has been bonded byusing chemicals, is perforated with hot needles. This process not only punches holes but also reinforces as aresult of cross-linking and condensation of the bonding agent. The Hungarian firm Temaforg uses a similarmethod to perforate webs made of synthetic fibers to produce nonwoven bonded fabrics which are strong andyet supple enough for use as building and insulation materials.

Slitting: Slitting, originally developed to improve the softness and drape of films was used by the Breveteamcompany for interlinings, in particular for adhesive fixable interlinings. The optimum cut length and distancebetween the slits to get maximum softness and fall without serious reduction of strength can be calculated.The effect of slitting allows greatest flexibility at right angles to the direction of the slit.

The slitting is accomplished by a roller with small blades mounted on it, for example, in an off-setarrangement 1.7 mm apart, making slits of a maximum length of 6.5mm. Rotary knives with spreaders can befitted to the roller, thus making an interrupted cutting edge. Polyethylene or polyamide film shaped bysplitting or embossing and stretching by the Xironet and Smith-Nephew methods make good air permeablebonding layers for laminating nonwoven bonded fabrics.

2.6. SPLITTING, GRINDING, VELOURING AND SINGEING

Splitting: When nonwovens are substituted for leather, the thick layer of needled fabric is split similar to thesplitting of leather to make thinner fabrics. The fabrics used are thick, high strength, firmly bonded, closelyneedled and usually shrunk. The product is thin, supple and like leather is used for slip belts, shoeinterlinings, backing material for shoe uppers and leather bags.

Splitting is done by machines in which a continuous rotation hoop knife is guided with great precision in thegap between two conveyor rollers, the distance between them depending on the thickness and type of fabricrequired.

Grinding and Velouring: Splitting is followed by either ironing and friction calendering or moirecalendering and possibly also grinding and polishing to make the surface even, giving the fabric theappearance of velour or suede. The process is known as velouring. First there are several machines orconsecutive passages to coarsely roughen the surface and then polish it increasingly fine. After grinding, thedust is removed by brushing or beating the fabric or by suction. The distinctive features of such products aretheir soft feel, elegant draping qualities and velvet-like surface.

Singeing: It belongs to the category of a dry finishing process. It is essentially the burning off of protrudingfibers from nonwoven fabrics, particularly needled fabrics. The process is exactly the same as traditionalsingeing and is carried out on gassing frames where the fabric is passed over an open gas flame. The surfaceis made smoother, which simplifies the dusting of filter fabrics.

3. WET FINISHING

3.1 WASHING

The purpose of washing is to remove unwanted substances from the fabric. In a wet process a suitablewashing machine, using water as the washing medium and occasionally a detergent, intensifies the effectsrequired.

Some anionic washing agents also have the effect of softening the fabric; nonionic agents have the advantageof being universally compatible but are more efficient at specific temperatures. As in all wet and dryprocesses the fabric should be subjected to as little tension as possible when being washed and, lengthwise,stretching is undesirable.

3.2. DYEING

Nonwoven fabrics are colored either plain or patterned when they are to be used for decorative purposes.Examples are in wallpapers or floor coverings, table or bed linen or as a furnishing fabric. The interliningsfor shirts or blouses are also colored to match the top fabric. Colors can be divided into dyes and pigments.Dyes have substantivity for fibers, meaning they are attracted from their application media by the fibroussubstrate. Pigments are applied from a latex medium. Both dyes and pigments can be applied at variousstages of the nonwoven process, starting from the polymer or pulp of fibers prior to web formation [2].

Dyeing of polymer: In certain polymers such as polyester, dyes and pigments can be added as a concentrateto the polymer immediately prior to extrusion. This process is referred to as producer coloration or meltdyeing. The color concentrates are usually pellets or beads that contain a high concentration of dyes orpigments. Acrylic polymer can be "gel dyed" with cationic dyes which react with the anionic sites in thepolymer while the polymer is in the final stages of being formed prior to drying. In rayon, pigments can beintroduced to the polymer solution prior to spinning. This is also the case in polypropylene, which has muchless affinity to dyes. The advantages of producer coloring are that the web does not have to withstand therigors of dyeing and the dye fastness is generally superior to dyed webs [3].

Staple and mass dyeing: Dyeing and printing are wet processes and are time, energy and cost-intensive.Wherever possible, coloring of the web is combined with the wet processes necessary for the bonding, or thefiber is dyed in staple form. Mass dyeing plays an important role in the case of synthetic fibers.

Dyeing and bonding: When the web has to be bonded chemically the dye is also added to the vat containingthe bonding agent. The bonding agent may coat the fibers of the web equally, which would make possible theuse of finely dispersed pigment dyes. The bonding agent would then adhere to the surface of the fibers andalso would exhibit the excellent non-fading properties pigments are noted for. This also improves the rubbingfastness when wet or dry and dye fastness to perspiration and ironing. In the case of bonding agents notapplied evenly to all fibers, a dye with affinity to the fibers can be added to the medium containing thebonding agent. Thus even dyeing can be expected despite the uneven distribution of the bonding agent.

If great lengths of web composed of a single type of fiber, bonding and dyeing can be carried out in a singleprocess without difficulty. For example cotton and viscose webs can be dyed with direct dyes, polyamidewebs with acidic dyes and polyester webs with disperse dyes resulting in coloration that is as deep and fast asconventional dyeing. The only consideration is that the pH of the bonding agent be acceptable for the dye.

Subsequent dyeing: It is much more difficult to dye and bond simultaneously if the web is composed of amixture of different kinds of fibers. In this case and also in many cases when the fiber is homogeneous,dyeing is carried out in a later stage. The nonwoven fabric is then treated as a woven of knitted fabric and isdyed in the traditional ways.

Heavy and high bulk fabrics are dyed continuously since jiggers or dyeing beams, which work in batches,can cope with only small quantities and is therefore not economically feasible. It is however possible to dyelight nonwoven fabrics perfectly on dyeing beams. Thermoplasticity, especially of the soft acrylate bondingagents, play an important role in jig dyeing. Polyester nonwovens can be jig dyed at a high temperature, but ithas been found that at temperatures above 102°C the bonding agent begins to make the separate layers of theweb adhere to one another. Consequently the rolls do not unwind properly.

Cold pad batch dyeing: This process was patented by Farbwerke Hoechst for the dyeing of bonded websmade from polyamide by the cold pad-batch method. Nonwoven fabrics meant for curtains and table linenproduced by the melt spinning or card/cross-laying method and bonded with acrylic acid esters are dyed withacid or metal coupled dyes to which acids are added to provide hydrogen bonding together with cold wettingagents to encourage migration. The fabric is then padded, batched and left for 24 hours covered withpolyethylene film to be roller burnished. Later it is given a warm rinse followed by soaping and, thereafter, afurther rinse.

Continuous dyeing: The dyeing of heavy nonwoven fabrics is continuous, usually by the conventionalpad-steam process followed by steaming to fix the dye. Steaming is usually followed by rinsing and washing.

3.3. PRINTING

Due to the increasingly popular use of nonwovens in the home furnishing sector there has been a greatexpansion of the color range and printing methods. The most commonly employed methods are screen androtary screen-printing. The nonwoven fabric is placed on the printing backcloth similar to any other fabricand printed with dyestuffs appropriate for the fabric concerned, partially dried, fixed by steaming andwashed.

Pigment printing is very important since the pigment binders bond the fabric even more. The effect isparticularly marked in spunbonded fabrics. A further consequence is that condensation replaces drying andsteaming. If the thickeners have little body, washing may not be necessary.

Printing of light non-woven bonded fabrics: Pigments are suitable for all kinds of light, non-woven bondedfabrics. The concentration of dye is high in light fabric printing.

Printing heavy non-woven bonded fabrics: The printing paste for the rotary screen printing of heavy needledfabrics have very different rheological properties from the paste suitable for light fabrics. The printing speedis much lower than when printing light fabrics.

Transfer printing: In transfer printing, subliming dyestuffs are transferred from a release paper on to thenon-woven bonded fabric with the aid of heat and pressure. Polyester fiber is more suitable for this method.

3.4. CHEMICAL FINISHES

Nonwovens are finished with various chemicals in order to obtain the specific property depending onend-use. Different chemical finishes are discussed below.

i) Antistats

Static electricity tends to build up in nonwovens made of synthetic fibers due to their lack of moisture regainand conductivity. This can cause problems such as clinging and dragging during processing, apparel thatclings and crackles, dangerous discharge of static electricity in explosive atmospheres and tendency to attractairborne dirt and soil in processing and use. The antistats work in three basic ways. They improve theconductivity of the fibers, coat the fiber with a thin layer of material that will attract a thin layer of moisture,and finish the fabric such that it holds a charge opposite to that normally accumulated on the fiber toneutralize the static charge. Antistats can be either durable or non-durable. Examples of durable antistatsinclude vapor deposited metals, conductive carbon or metallic particles applied by binders, polyamines,polyethoxylated amine and ammonium salts and carboxylic salts. Non-durable antistats usually consist ofinorganic or organic salts or hygroscopic organic materials. Examples are quaternary ammonium salts,imidazoles and fatty amides which are cationic. Anionic antistats include phosphates, phosphate esters,sulfonates, sulfates and phosphonates. Examples of nonionic antistats include glycols, ethoxylated fatty acids,

ethoxylated fatty alcohols and sorbitan fatty acid esters.

ii) Antimicrobials

These are used to control populations of bacteria, fungi, algae and viruses on the substrate. The treatmentusually prevents the biological degradation of the product or prevents the growth of undesirable organisms.Broadly classed, the antimicrobials are either fixed or leachable. The fixed treatments are durable, but theleachable treatments may transfer to the surrounding environment through migration, solubility or abrasion.A generic list of the treatments include alcohols such as isopropanol or propylene glycol, halogens such aschlorine, hypochlorite, iodine, N-chloramine and hexachlorophene, metals such as silver nitrate, mercuricchloride and tin chloride, various peroxides, phenols quaternary ammonium compounds, pine oil derivatives,aldehydes and phosphoric acid esters. Care should be taken in the application of these compounds to preventinactivation, loss of durability or masking of the active ingredient with other finishes.

iii) Water repellents

Water repellent finishes are a type of barrier, which function to lower the critical surface tension of the fibersurface. To be most effective it is important that the fibers are treated evenly on all surfaces to give thelowest critical surface tension possible. Water repellency can be achieved with a variety of chemical finishessuch as waxes, wax dispersions, melamine wax extenders, chrome complexes, silicones, and flourochemicals.The finishes require curing to develop the best repellency and are also prone to destabilizing with shear, heator changes of pH or ionic strength.

iv) Lubricants

Lubricants or slip agents are generally applied as processing aids to help in stretching or to improve theprocessability of nonwovens. They are also applied to aid in sewing, quilting, tufting or other processeswhere needles penetrate the fabric. Lubricants impart the same properties as softeners but specifically reducefiber friction. Common chemicals include sulphonated oils, oil emulsions, silicones, esters, polyethylenedispersions and fatty acid soaps. Many surfactants may also be used. Care should be taken to avoid excessivestrength loss.

v) UV absorbers and polymer stabilizers

Ultraviolet light can do great damage to the polymers causing photo-degradation, yellowing, loss in strengthand fading of the colors. The damage is generally due to the formation of destructive free radicals in thepolymer. The finish can protect the fabric by shielding the fiber or absorbing the light or by chemicallyquenching the free radicals. The three main classes of products used are, substituted benzotriazoles,benzophenones which are uv absorbers, and hindered amines which are free radical reactants. They areapplied from a bath or added to the polymer.

vi) Flame retardants

The finishing of fabrics with flame retardants can reduce the tendency to burn or reduce the tendency topropagate the flame. The flame retardants may char the fuel, quench the reaction of combustion, absorb heator emit cooling gases or replace oxygen. Flame retardants are durable or nondurable. Durable retardantsinclude decabromodiphenyl oxide, antimony oxide, phosphates, brominated esters, PVC and otherchlorinated binders. Nondurables include borates, boric acids, zinc borate, sulfamic acid sulfamates,ammonium phosphates, urea, etc. Hydrated alumina and zinc borate act as smoke supressants. Problems in

the application include odor, yellowing, loss of tensile strength, stiffening, skin irritation and color change orloss.

vii) Softeners

Softeners are applied to improve the aesthetic and functional characteristics of a fabric. The hand, drape,abrasion resistance, sewability and tear strength can be improved with the addition of a softener. It works byreducing the coefficient of friction between the fibers. There are different types of softeners such as anionic(sulfates or sulfonates), cationic (amines and quaternary amines) and nonionic (silicones, ethylene oxidederivatives and hydrocarbon waxes.)

viii) Absorbency and rewetters

Chemicals used to impart hydrophilicity to a nonwoven are referred to as rewetters. These treatmentsincrease the critical surface tension of the fiber making it more wettable. This property is desirable inend-uses such as wipes, hygiene, medical absorbent pads and garments. For hydrophobic fibers the treatmentfacilitates the movement and penetration of the liquid in the capillary channels. Many anionic and nonionicsurfactants, antistats, flame retardants and softeners impart hydrophilicity.

ix) Thermoplastic binders, resins and emulsion polymers

Binders and resins are widely used in the finishing of nonwovens to add strength, control stiffness, addmoldability or pleatability, provide durable flame retardants, color, reduce linting and control shrinkage.They soften when exposed to heat and return to their original state when cooled and, hence, can be set.Emulsion polymers are also called latexes. The common binders, resins and polymers include acrylics, PVC,polyacrylic acid, urethanes, starch, vinyl acetate etc.

x) Thermosetting resins and crosslinking agents

These are used to produce wrinkle resistant or permanent-press textiles. They are used to crosslink cellulosefor wrinkle resistance, crosslink binders for wash durability and solvent resistance. The technology is basedon the ability of formaldehyde to react with cellulose and nitrogen containing resins. The important resintypes are melamine-formaldehyde, urea formaldehyde and dimethyloethylene urea. The reaction is usuallycatalyzed by acids, such as Lowry-Bronsted or Lewis acids. Problems encountered include formaldehydegeneration, tensile loss, discoloration and amine odor.

xi) Soil release

The soil release chemicals reduce the problem of soiling in two ways: repel the stains and soil usingrepellants such as flourochemicals or create a surface that aids the removal of soils when cleaning orlaundering using chemicals based on polyacrylic acid.

xii) Optical brighteners

Optical brighteners or fluorescent whitening agents are organic chemicals that are used like dyes or pigmentsto add brightness to fabrics. These chemicals are colorless but can absorb UV light and reemit it to the visiblerange usually as a blue or blue-green. These products produce very white fabrics or brighten colored fabrics.[8]

3.5 COATING

Coating is a basic and exceptionally important form of finishing for non-woven bonded fabrics. The way inwhich the coating is carried out depends on the substrate, the machinery available, the substance that is to be

applied and, also on the effect desired.

Slop padding: It is one of the best known methods of direct coating. The coating is put on with a rotary roller,the surface of which is covered in the substance to be applied. The slop padding roller is fed directly with thelaminating float by being dipped into it or using special feed rollers.

3.6 LAMINATING

Laminating is the permanent jointing of two or more prefabricated fabrics. Unless one or other of the fabricsdevelops adhesive properties in certain conditions, an additional medium is necessary to secure bonding.

Wet laminating: Adhesives used in the wet process are dissolved or dispersed in a suitable solvent. Thesimplest form of wet laminating consists of applying the adhesive to one of the lengths of material that is tobe joined, and to put the second length on it with the required amount of pressure. Then drying, hardening orcondensing the material that has been joined together is carried out. The solvents can be macromolecularnatural or synthetic substances and water.

Dry laminating: All Kinds of thermoplastics are used for dry laminating. These include powders, plastisols,or melt adhesives, and are applied to the substrates that are to be joined together using suitable machinery.Dry laminated non-woven fabrics have a soft feel.

3.7. FLOCKING

Flocking is a process of making a two-dimensional fabric have a third dimension. It is done by mechanicallyor commonly electrostatically. Depending on how the adhesive is applied, the whole surface can be flockedor patterns can be made. The adhesives are just like what are used in laminating and includepolyvinylchloride plastisols, polyurethane bicomponent adhesives and all kinds of aqueous dispersionadhesives.

4. RESEARCH OF TEXTILE FINISHING: SOLID-ON-SOLID

Many textile manufacturing operations such as dyeing, printing, and finishing of fabrics use wet processingtechniques. These techniques involve using an aqueous solution or bath to apply chemicals to a textilesubstrate, fixing the chemicals to the fiber, scouring or washing to remove loose chemicals, and drying toproduce a finished product. Heating and later evaporating water make these wet processes very energyintensive. Industry experts estimate that wet processes use approximately 60% of the energy consumed in thetextile industry. In addition, shrinking water supplies in many parts of the world have prompted textilemanufacturers to develop methods that reduce water and energy consumption.

Working in this direction Georgia Tech’s School of Textile and Fiber Engineering has investigated three SOStechnologies. One of the techniques used electostatic powder spraygun deposition in the finishing of 100%polypropylene nonwoven fabric. In this work, fluoropolymer in a fine powder was used in making fabrichaving an excellent barrier to oil/solvent and isoproponal. The trials demonstrated for the first time that afinishing process, which combines powder deposition and melt cutting can impart superior barrier propertiesto polypropylene nonwoven substrate. So with the further developments in SOS processing, There can resultthe elimination of the need for steam generation, the elimination of effluents, a decrease in dwell times in thecuring oven, the reduction of water consumption and the saving of energy. These changes can increaseproductivity and reduce costs [5].

REFERENCES

1. www.nonwovens.com

2. J. R. Aspland and C. W. Jarvis, Clemson University, Clemson, SC, The Coloration and Finishingof Nonwoven Fabrics

3. J. K. Vandermaas, Hoechst Celenese Corp and J. R. Aspland, Clemson University, Dyeing ofNonwovens.

4. Frank Baldwin, Precision Fabrics Group Inc, Greensboro, NC, The Chemical Finishing of Nonwovens.

5. http://es.epa.gov/techinfo/facts/solid-cs.html

6. Menachim Lewin and Stephen B. Sello, Chemical Processing of Fibers and Fabrics, FunctionalFinishes, Part B, 1983

7. G H J Van der Walt and N J J Van Rensberg, Low-liquor Dyeing and Finishing, Textile Progress,Vol. 14, No. 12, 1984.

8. Howard L. Needles, Textile Fibers, Dyes, finishes and Process, A Concise Guide, University ofCalifornia, Davis, CA, 1986.

9. J. Robert Wagner, TAPPI Journal, Vol. 66, No. 4, pp 41-43,1983

10. Herman B. Goldstein and Herbert W. Smith, aatcc, Vol. 12, No. 3, pp 49-54, 1980.

Back to Table of Contents

MELT BLOWN TECHNOLOGYUpdated, April, 2004- Atul Dahiya, M. G. Kamath, Raghavendra R. Hegde

(Ramaiah Kotra & Haoming Rong)

Melt blowing (MB) is a process for producing fibrous webs or articles directly from polymers or resins usinghigh-velocity air or another appropriate force to attenuate the filaments. The MB process is one of the newerand least developed nonwoven processes. This process is unique because it is used almost exclusively toproduce microfibers rather than fibers the size of normal textile fibers. MB microfibers generally havediameters in the range of 2 to 4 µm, although they may be as small as 0.1 µm and as large as 10 to 15 µm.Differences between MB nonwoven fabrics and other nonwoven fabrics, such as degree of softness, cover oropacity, and porosity can generally be traced to differences in filament size.

1. HISTORY

The basic technology to produce these microfibers was first developed under U.S. government sponsorship inthe early 1950s. The Naval Research Laboratory initiated this work to produce microfilters for the collectionof radioactive particles in the upper atmosphere. The significance of this work was recognized by an Exxonaffiliate and a development program was initiated in the middle 1960s. Five years later, a patented prototypemodel successfully demonstrated the production of microfibers. At present, Exxon has developed most of thelicenses and/or options to produce microfiber nonwoven and MB equipment.

In the past 20 years there has been some activity outside of the Exxon technology and patents obtained bycompanies such as 3M. The company, 3M, has developed processes for making microfibers and blends ofmicrofibers with textile denier fibers that were apparently beyond Exxon patents. Exxon has continued toaggressively support MB R&D through the years. The major portion of the Exxon-supported effort is nowbeing conducted at the University of Tennessee, Knoxville. North American major MB producers includeHollingsworth and Vose, Kimberly-Clark, 3M, Fleetguard Filter, PGI Nonwovens, BBA Nonwovens, FirstQuality Nonwovens and Johns Manville.

2. PROCESSING

The most commonly accepted and current definition for the MB process is: a one-step process in whichhigh-velocity air blows a molten thermoplastic resin from an extruder die tip onto a conveyor or takeupscreen to form a fine fiberous and self-bonding web.

The MB process is similar to the spunbond (SB) process which converts resins to nonwoven fabrics in asingle integrated process. The schematic of the process is shown MB in figure 1. A typical MB processconsists of the following elements: extruder, metering pumps, die assembly, web formation, and winding.

Fig. 1: Schematic of MB process

2.1 EXTRUDER

The extruder is one of the important elements in all polymer processing. It consists of a heated barrel with arotating screw inside as shown in figure 2. Its main function is to melt the polymer pellets or granules andfeed them to the next step/element. The forward movement of the pellets in the extruder is along the hot wallsof the barrel between the flights of the screw. The melting of the pellets in the extruder is due to the heat andfriction of the viscous flow and the mechanical action between the screw and the walls of the barrel. Thereare four different heaters in the extruder, which are set in incremental order. The extruder is divided in tothree different zones [1]:

i) Feed Zone: In the feed zone the polymer pellets are preheated and pushed to the next zone.

ii) Transition Zone: The transition zone has a decreasing depth channel in order to compress andhomogenize the melted polymer.

iii) Metering Zone: This is the last zone in the extruder whose main purpose is to generatemaximum pressure in order to pump the molten polymer in the forward direction. At this pointthe breaker plate controls the pressure generated with a screen pack placed near to the screwdischarge. The breaker plate also filters out any impurities such as dirt, foreign particle metalparticles and melted polymer lumps.

Fig. 2: Schematic of a screw extruder

2.2. METERING PUMP

The metering pump is a positive-displacement and constant-volume device for uniform melt delivery to thedie assembly. It ensures consistent flow of clean polymer mix under process variations in viscosity, pressure,and temperature. The metering pump also provides polymer metering and the required process pressure. Themetering pump typically has two intermeshing and counter-rotating toothed gears. The positive displacementis accomplished by filling each gear tooth with polymer on the suction side of the pump and carrying thepolymer around to the pump discharge, as shown in figure 3. The molten polymer from the gear pump goes tothe feed distribution system to provide uniform flow to the die nosepiece in the die assembly (or fiberforming assembly).

Fig. 3: Schematic of a metering pump

2.3. DIE ASSEMBLY

The die assembly is the most important element of the melt blown process. It has three distinct components:polymer-feed distribution, die nosepiece, and air manifolds.

i) Feed Distribution

The feed distribution in a melt-blown die is more critical than in a film or sheeting die for tworeasons. First, the melt-blown die usually has no mechanical adjustments to compensate forvariations in polymer flow across the die width. Second, the process is often operated in atemperature range where thermal breakdown of polymers proceeds rapidly. The feed distribution isusually designed in such a way that the polymer distribution is less dependent on the shearproperties of the polymer. This feature allows the melt blowing of widely different polymericmaterials with one distribution system. The feed distribution balances both the flow and theresidence time across the width of the die. There are basically two types of feed distribution thathave been employed in the melt-blown die: T-type (tapered and untapered) and coat hanger type.Presently, the coathanger type feed distribution is widely used because it gives both even polymerflow and even residence time across the full width of the die.

ii) Die Nosepiece

From the feed distribution channel the polymer melt goes directly to the die nosepiece. The webuniformity hinges largely on the design and fabrication of the nosepiece. Therefore, the dienosepiece in the melt blowing process requires very tight tolerances, which have made theirfabrication very costly. The die nosepiece is a wide, hollow, and tapered piece of metal having

several hundred orifices or holes across the width. The polymer melt is extruded from these holes toform filament strands which are subsequently attenuated by hot air to form fine fibers. In a die'snosepiece, smaller orifices are usually employed compared to those generally used in either fiberspinning or spunbond processes. A typical die nosepiece has approximately 0.4-mm diameterorifices spaced at 1 to 4 per millimeters (25 to 100 per inch).

There are two types of die nosepiece used: capillary type and drilled hole type. For the capillarytype, the individual orifices are actually slots that are milled into a flat surface and then matchedwith identical slots milled into a mating surface. The two halves are then matched and carefullyaligned to form a row of openings or holes as shown in Figure 4. By using the capillary type, theproblems associated with precise drilling of very small holes are avoided. In addition, the capillarytubes can be precisely aligned so that the holes follow a straight line accurately. The drilled-holetype has very small holes drilled by mechanical drilling or electric discharge matching (EDM) in asingle block of metal, as shown in Figure 4.

Fig. 4: Schematic of a die nosepiece [1]

During processing, the whole die assembly is heated section-wise using external heaters to attaindesired processing temperatures. It is important to monitor the die temperatures closely in order toproduce uniform webs. Typical die temperatures range from 2l5°C to 340°C.

iii) Air Manifolds

The air manifolds supply the high velocity hot air (also called as primary air) through the slots onthe top and bottom sides of the die nosepiece, as shown in Figure 5. The high velocity air isgenerated using an air compressor. The compressed air is passed through a heat exchange unitsuch as an electrical or gas heated furnace, to heat the air to desired processing temperatures. Theexits from the top and bottom sides of the die through narrow air gaps are shown in Figure 5.Typical air temperatures range from 230°C to 360°C at velocities of 0.5 to 0.8 the speed of sound.

Fig. 5: Schematic showing the air flow through the die assembly

2.4. WEB FORMATION

As soon as the molten polymer is extruded from the die holes, high velocity hot air streams (exiting from thetop and bottom sides of the die nosepiece) attenuate the polymer streams to form microfibers. As the hot airstream containing the microfibers progresses toward the collector screen, it draws in a large amount ofsurrounding air (also called secondary air) that cools and solidifies the fibers, as shown in figure 5. Thesolidified fibers subsequently get laid randomly onto the collecting screen, forming a self-bonded nonwovenweb. The fibers are generally laid randomly (and also highly entangled) because of the turbulence in the airstream, but there is a small bias in the machine direction due to some directionality imparted by the movingcollector. The collector speed and the collector distance from the die nosepiece can be varied to produce avariety of melt-blown webs. Usually, a vacuum is applied to the inside of the collector screen to withdraw thehot air and enhance the fiber laying process.

2.5. WINDING

The melt-blown web is usually wound onto a cardboard core and processed further according to the end-use

requirement. The combination of fiber entanglement and fiber-to-fiber bonding generally produce enoughweb cohesion so that the web can be readily used without further bonding. However, additional bonding andfinishing processes may further be applied to these melt-blown webs.

2.6. BONDING

Additional bonding, over the fiber adhesion and fiber entanglement that occurs at lay down, is employed toalter web characteristics. Thermal bonding is the most commonly used technique. The bonding can be eitheroverall (area bonding) or spot (pattern bonding). Bonding is usually used to increase web strength andabrasion resistance. As the bonding level increases, the web becomes stiffer and less fabric-like.

2.7. FINISHING

Although most nonwovens are considered finished when they are rolled up at the end of the production line,many receive additional chemical or physical treatment such as calendering, embossing, and flameretardance. Some of these treatments can be applied during production, while others must be applied inseparate finishing operations.

3. PROCESS VARIABLES

Process variables can be divided into three groups: machine/operational variables, off-line variables andmaterial variables.

3.1 MACHINE VARIABLES

Machine variables, also called operational variables, are related to the machine and can be changed while theequipment is being operated. These variables include air temperature, polymer/die temperature, die tocollector distance (DCD), collector speed, polymer throughput and air throughput. All of these affect the finalproperties of the nonwoven web.

i) Polymer Throughput and Air Flow: both polymer throughput and air flow rate control the finalfiber diameter, fiber entanglement, basis weight and the attenuating zone.

ii) Polymer/Die and Air Temperature: These variables combined with air flow rate affect theuniformity, shot formation (large globules of nonfibrous polymer larger in diameter than fibers inwebs), rope and fly formation, fabric appearance and feel (soft or stiff).

iii) Die to Collector Distance: This affects the openness of the fabric, thermal bonding among the fibersand basis weight.

3.2 OFF-LINE VARIABLES

Off-line variables are fixed during a process run and can only be changed when the machine is not inoperation. These variables are air gap, air angle, die setback, and die hole size.

i) Die Hole Size: Die hole size along with die set back affects the fiber size.

ii) Air Gap: It affects the degree of fiber breakage by controlling the air exit pressure.

iii) Air Angle: It controls the nature of air flow, i.e. as the air angle approaches 90° it results in a highdegree of fiber separation or turbulence that leads to random fiber distribution. At an angle of 30°, roped or parallel fibers deposited as loosely coiled bundles of fibers are generated. This structure is

undesirable. At angles greater than 30°, attenuation as well as breakage of fibers occurs.

3.3 MATERIAL VARIABLES

Material variables include polymer type, molecular weight, molecular weight distribution, melt viscosity,polymer additives, and polymer pellet size (powdered or granular). The MB process is amenable to a widerange of polymers in terms of viscosities and blends.

4. WEB CHARACTERISTICS AND PROPERTIES

i) Uniformity

The uniformity of the web is controlled by two important parameters: uniform distribution of fiber in the airstream and proper adjustment of the vacuum level under the forming wire or belt. Non-uniform distributionof fiber in the air stream can result from poor die design and from non-uniform ambient airflow into the airstream. The vacuum under the forming media should be adjusted to pull the total air stream through themedia and lock the fibers in place. Generally, the closer the die is to the forming drum or belt, the better theweb uniformity.

ii) Product Characteristics

Melt-blown webs usually have a wide range of product characteristics. The main characteristics andproperties of melt-blown webs are as follows:

1. Random fiber orientation.

2. Lower to moderate web strength, deriving strength from mechanical entanglement and frictionalforces.

3. Generally high opacity (having a high cover factor).

4. Fiber diameter ranges from 0.5 to 30 m, but typically from 2-7 m.

5. Basis weight ranges from 8-350 g/m2, but typically 20-200 g/m2.

6. Microfibers provide a high surface area for good insulation and filtration characteristics.

7. Fibers have a smooth surface texture and are circular in cross-section.

8. Most melt-blown webs are layered or shingled in structure, the number of layers increases with basisweight.

The fiber length in a melt-blown web is variable; it can be produced in the range from a few millimeters toseveral hundred centimeters in length and usually exists over a broad range. The fiber cross-section is alsovariable, ranging from circular to a flat configuration and other variations.

iii) Defects

Three of the major defects that occur in melt-blown production are roping, shot, and fly. Roping is caused byuncontrolled turbulence in the air-stream and by movement of fibers during and after laydown. The defect isobserved as a narrow, elongated, thick streak in the web and resembles a slightly twisted "rope". Shot aresmall, spherical particles of polymer formed during the blowing operation. Shot are generally caused byexcessively high temperatures or by too low a polymer molecular weight. Fly is a defect that does not godirectly into the web, but instead contaminates the surrounding environment. Fly is composed of very short

and very fine microfibers not trapped on the drum or belt during laydown. This can be caused by too violentblowing conditions.

5. PROCESS STRUCTURE PROPERTY

The web structure in a melt-blown product is essentially isotropic. This is not surprising since web formationis an air lay process. This means that the fibers have a random distribution in terms of the machine direction(MD) and cross direction (CD). As a result, the physical properties will normally also be isotropic. If desired,the fiber orientation in the web can be skewed by the use of selected processing conditions.

Polymer throughput of PP has been shown to have a noticeable effect on the physical properties of resultantwebs [2]. The mean fiber diameter, tensile strength, initials modulus, stiffness and web density increase withincreasing throughput. However, the decrease in both breaking strain and the energy required to breakindicates the brittle nature of the web produced at higher throughput. Increasing fiber diameter was attributedto die swell and change in polymer-to-air ratio for a given airflow rate. Increase in airflow rate didn't result inany significant change in average fiber diameter. The die orifice size had only minimal effects on the averagefiber diameter.

The average fiber diameter of the MB microfibers can be controlled by the specific resin employed and theprocessing conditions selected. A typical microfiber can be as fine as 2 µm or less in diameter. The numberof fibers and resulting surface area are greatly increased as fiber diameter decreases.

6. POLYMER TYPE

The type of polymer or resin used will define the elasticity, softness, wetability, dyeability, chemicalresistance and other related properties of formed fibers. One of the advantages of melt-blown technology is tohandle many different polymers as well as mixture of polymers. Some polymers, which can be melt-blown,are listed below. However, the list is not complete.

1. Polypropylene is easy to process and makes good web.

2. Polyethylene is more difficult to melt-blow into fine fibrous webs than is polypropylene.Polyethylene is difficult to draw because of its melt elasticity.

3. PBT processes easily and produces very soft, fine-fibered webs.

4. Nylon 6 is easy to process and makes good webs.

5. Nylon 11 melt-blows well into webs that have very unusual leather like feel.

6. Polycarbonate produces very soft-fiber webs.

7. Poly (4-methyl pentene-1) blows well and produces very fluffy soft webs.

8. Polystyrene produces an extremely soft, fluffy material with essentially no shot defects.

The most widely used polymer that has a high MFR is polypropylene. Polypropylene with its low viscosityhas a low melting point and is easy to draw into fibers. It comprises 70-80% of the total North Americanproduction [3].

The feasibility of MB original and recycled PET has also been studied [4]. PET webs have a strong tendencyto shrink, depending on the airflow rate used. PET webs produced at high airflow rate shrink more than thoseproduced at low airflow rate because of their higher level of molecular orientation. Heat-setting of

melt-blown PET webs or, alternatively, the use of PBT (poly-butylene terephalate) was suggested as apossible means of producing thermally stable melt-blown PET nonwovens.

7. MAJOR PROCESS MODIFICATIONS

In the case of melt-blown web, additional process modifications can provide excellent flexibility in terms ofdesigning new and novel products. Composite formation has been used extensively to supplement the limitedphysical strength of normal melt-blown webs and also to provide enhancement of other properties. Thisfeature has been very effectively exploited in SMS structures based on a three-ply system consisting ofSpunbond/Melt-blown/Spunbond plies to give an enhanced composite structure, supplementing the fiber andweb properties of the melt blown with the strength and toughness of the spunbond surface fabrics.Lamination with a variety of other sheets and webs has also extended the range of properties achievable withmelt-blown systems.

During the MB process other fibers or materials such as activated carbon or super absorbents can be blown into the MB web to enhance, resiliency and absorbency. The most notable success of this is the Coform®structures produced by Kimberly-Clark Corporation. These structures involve an intimate blend of meltblown fibers and short wood pulp fibers. By varying the ratio of the two fiber feeds, a broad range ofproducts can be produced. Such Coform® structures are often combined with a Spunbond (SB) fabric on oneor both surfaces to provide additional product versatility. Blends with other fiber types have also beenproduced, such as 3M's Thinsulate® blends of polypropylene (PP) or polyester staple with MB webs.

8. APPLICATIONS

The melt-blown system is unique because the process generates a fine fiber not available to the othernonwoven processes. Micro-denier fiber (less than 0.1 denier per filament) is not really available as anonwoven fibrous raw material. Hence, the melt-blown process, which can produce such a fiber, opens newvistas of products and applications. At the present time, the following market segments are successfullyserved by melt-blown products:

i) Filtration media

This market segment continues to be the largest single application. The best known application is the surgicalface mask filter media. The applications include both liquid filtration and gaseous filtration. Some of themare found in cartridge filters, clean room filters and others.

ii) Medical fabrics

The second largest MB market is in medical/surgical applications. The major segments are disposable gownand drape market and sterilization wrap segment.

Surgical drapes and gown Sterilization wrap

ii) Sanitary products

MB products are used in three types of sanitary protection products - feminine sanitary napkin,Spunbond-MB diaper top sheet, and disposable adult incontinence absorbent products.

iv) Oil adsorbents

MB materials in variety of physical forms are designed to pick up oily materials. The best known applicationis the use of sorbents to pick up oil from the surface of water, such as encountered in an accidental oil spilland for mats in machine shops and in industrial plants.

v) Apparel

The apparel applications of melt-blown products fall into three market segments: thermal insulation,disposable industrial apparel and substrate for synthetic leather. The thermal insulation applications takesadvantage of microvoids in the structure filled with quiescent air, resulting in excellent thermal insulation.

vi) Hot-melt adhesives

The MB process has a special feature: it can handle almost any type of thermoplastic material. Thus, the taskof formulating a hot-melt adhesive to provide specific properties can be greatly simplified by using the meltblown system to form the final uniform adhesive web.

vii) Electronic specialties

Two major applications exist in the electronics specialties market for melt blown webs. One is as the linerfabric in computer floppy disks and the other as battery separators and as insulation in capacitors.

viii) Miscellaneous applications

Interesting applications in this segment are manufacture of tents and elastomeric nonwoven fabrics whichhave the same appearance as continuous filament spunbonded products.

9. COMPLEXITY OF MELT BLOWN TECHNOLOGY

The melt-blown process is a complex one that involves turbulence, which is poorly understood by the

scientist even today. Isolation of experimental factors is difficult because of highly variable interaction. Themulti-filament environments and factors such as humidity of the processing room and quench air temperaturewildly change boundary conditions [5].

The processing window for successfully melt-blowing polymers is very limited. In order to produce webswith acceptable quality, one has to be in the right range of process parameters and the range varies betweenpolymers. Unlike melt-spinning, there is almost virtually no control over the individual filaments inmelt-blowing. It is very difficult to predict structure/property of melt-blown filaments since the isolation ofvariables is very difficult in a multi-filament environment.

Melt-blown products are difficult to compare to other nonwoven products because they are quite different innature and function. Most nonwoven fabrics are designed to function similarly to woven or knit fabrics andgenerally can be replaced with such fabrics, although usually at a performance and /or financial penalty. Withlimited exceptions such as some wipes, melt-blown products are not designed to function as fabrics. They aregenerally manufactured in sheet form but lack the physical strength of conventional woven or nonwovenfabrics.

On the other hand, despite extensive research and development in this area, there is a paucity of publishedresearch studies, mainly due to the secretive and competitive nature of the work. However, there isconsiderable patented literature available.

10. COMPARISON OF MELT BLOWN AND SPUNBOND

The Spunbond (SB) and MB processes are somewhat identical from an equipment and operator's point ofview. The two major differences between a typical MB process and a spunbond process that uses airattenuation are: i) the temperature and volume of the air used to attenuate the filaments and ii) the locationwhere the filament draw or attenuation force is applied.

A MB process uses large amounts of high-temperature air to attenuate the filaments. The air temperature istypically equal to or slightly greater than the melt temperature of the polymer. In contrast, the SB processgenerally uses a smaller volume of air close to ambient temperature to first quench the fibers and then toattenuate the fibers.

In the MB process, the draw or attenuation force is applied at the die tip while the polymer is still in themolten state. Application of the force at this point is ideal for forming microfibers but does not allow forpolymer orientation to build good physical properties. In the spunbond process, this force is applied at somedistance from the die or spinneret, after the polymer has been cooled and solidified. Application of the forceat this point provides the conditions necessary for polymer orientation and the resultant improved physicalproperties, but is not conductive to forming microfibers.

11. PROCESS EQUIPMENT

Although the MB process is conceptually simple, high-quality webs at commercial scale require preciselydesigned and fabricated equipment. In a manner similar to spunbond technology, many melt-blownweb-manufacturing companies such as 3M and Freudenberg have developed proprietary technology. Most ofthe MB processes in the market now are based on the Exxon process. The process equipment layout, whichcan be vertical or horizontal, is simpler and more compact than that of SB. A vertical layout is preferred whenmultiple dies are used, but horizontal layout is preferred when a single die is used. The vertical spacerequirement, usually a minimal of 20 ft., depends on the die-to-collector distance. The horizontal spacerequirements depend on the total width of the die and end product requirement. Usually three times thevertical space is the minimal requirement for a horizontal space [3]. In summary, a schematic follows thatshows components of a complete melt blowing line.

Fig. 6: Schematic of complete MB line

12. ECONOMICS OF MELT BLOWN WEBS

The economics of MB process is influenced by many factors such as energy, capital investment, andproduction speed conversion. With respect to energy, the MB process requires more energy per pound of webthan does the spunbond process. A typical MB process consumes about 7-8 kWh/kg of polymer process,while a typical SB process consumes 2-3 kWh/kg. MB processing is more energy-intensive because ofcompressed hot air is used for fiber attenuation. About 70% of total energy used for hot air. This result in ahigh production cost. Typically, a 2.0-oz PP SB web cost US $0.12 to $0.24/yd2, while a MB equivalent is$0.32-$0.37/ yd2 [3].

Initial capital investment of a melt-blown line is much lower than that of spunbond line. Typically, the later is3-4 times higher than the former. However the production speed of SB is inherently faster than that of MBbecause of a much larger number of spinneret holes can be incorporated in SB dies than in the typical MBdesign.

13. THE POTENTIAL OF MELT BLOWING

The MB technique for making nonwoven products has been forecast in recent years as one of thefastest-growing in the nonwovens industry. With the current expansion and interest, it cannot be questionedthat MB is well on its way to becoming one of the major nonwoven technologies. Technical developmentsare also on the horizon that will increase the scope and utility of this technology. The application of speciality

polymer structures will no doubt offer new nonwoven materials unobtainable by other competitivetechnologies. The considerable work to modify the blowing step to something more akin to spraying is alsogoing to have an impact on this technology and the products derived from it. So a strong and bright future isforecasted for this technology.

REFERENCES

1. Wadsworth, L. C., Malkan, S. R., A Review of Melt Blowing Technology, INB Nonwovens, p. 2,1991.

2. Malkan, S.R. and Wadsworth, L.C., IND JNR, No.2, pp21-23,1991.

3. Malkan, S., Tappi Journal, Vol.78, No.6, pp185-190, 1995.

4. Bhat, G.S., Zhang, Y., and Wadsworth, L.C., Processing of the Tappi Nonwoven Conference,Macro Island, FL, May, pp61-68, 1992.

5. Vasanthakumar, N., Dissertation, Dimensional Stability of Melt-blown Nonwovens. TheUniversity of Tennessee, May,1995

6. Malkan, S. R., PH.D Dissertation, The University of Tennessee, Knoxville, 1990.

7. Vargas, E., Meltblown Technology Today, Miller Freeman Publications Inc., San Francisco, CA,1989.

8. Bresee, R. R., Ko, W-C., Fiber Formation During Melt Blowing, INTC 2002 InternationalNonwovens Technical Conference Proceedings, Atlanta.

Back to Table of Contents

NANOFIBER NONWOVENSUpdated: June 13, 2005

Raghavendra R Hegde, Atul Dahiya, M. G. Kamath

1. INTRODUCTION

The nonwoven industry generally considers nanofibers as having a diameter of less than one micron,although the National Science Foundation (NSF) defines nanofibers as having at least one dimension of 100nanometer (nm) or less. The name derives from the nanometer, a scientific measurement unit representing abillionth of a meter, or three to four atoms wide.

Nanofibers are an exciting new class of material used for several value added applications such as medical,filtration, barrier, wipes, personal care, composite, garments, insulation, and energy storage. Specialproperties of nanofibers make them suitable for a wide range of applications from medical to consumerproducts and industrial to high-tech applications for aerospace, capacitors, transistors, drug delivery systems,battery separators, energy storage, fuel cells, and information technology [1,2].

Generally, polymeric nanofibers are produced by an electrospinning process (Figure 1). Electrospinning is aprocess that spins fibers of diameters ranging from 10nm to several hundred nanometers. This method hasbeen known since 1934 when the first patent on electrospinning was filed. Fiber properties depend on fielduniformity, polymer viscosity, electric field strength and DCD (distance between nozzle and collector).Advancements in microscopy such as scanning electron microscopy has enabled us to better understand thestructure and morphology of nanofibers. At present the production rate of this process is low and measured ingrams per hour.

Another technique for producing nanofibers is spinning bi-component fibers such as Islands-In-The-Seafibers in 1-3 denier filaments with from 240 to possibly as much as 1120 filaments surrounded by dissolvablepolymer. Dissolving the polymer leaves the matrix of nanofibers, which can be further separated bystretching or mechanical agitation.

The most often used fibers in this technique are nylon, polystyrene, polyacrylonitrile, polycarbonate, PEO,PET and water-soluble polymers. The polymer ratio is generally 80% islands and 20% sea. The resultingnanofibers after dissolving the sea polymer component have a diameter of approximately 300 nm. Comparedto electrospinning, nanofibers produced with this technique will have a very narrow diameter range but arecoarser [3].

2. ELECTROSPINNING PROCESS.

A schematic diagram of electrospinning is as shown in Figure 1. The process makes use of electrostatic andmechanical force to spin fibers from the tip of a fine orifice or spinneret. The spinneret is maintained atpositive or negative charge by a DC power supply. When the electrostatic repelling force overcomes thesurface tension force of the polymer solution, the liquid spills out of the spinneret and forms an extremelyfine continuous filament. It has the misleading appearance of forming multiple filaments from one spinneretnozzle, but current theory is that the filaments do not split.

These filaments are collected onto a rotating or stationary collector with an electrode beneath of the oppositecharge to that of the spinneret where they accumulate and bond together to form nanofiber fabric.

Figure 1. Schematic representation of electrospinning process [4].

The distance between the spinneret nozzle and the collector generally varies from 15 –30 cm. The processcan be carried out at room temperature unless heat is required to keep the polymer in liquid state. The finalfiber properties depend on polymer type and operating conditions. Fiber fineness can be generally regulatedfrom ten to a thousand nanometers in diameter [1,4].

2.1 POLYMER-SOLVENTS USED IN ELECTROSPINNING.

The polymer is usually dissolved in suitable solvent and spun from solution. Nanofibers in the range of 10-to2000 nm diameter can be achieved by choosing the appropriate polymer solvent system [5]. Table 1 gives listof some of polymer solvent systems used in electrospinning.

POLYMER SOLVENTSNylon 6 and nylon 66 Formic AcidPolyacrylonitrile Dimethyl formaldehydePET Trifluoroacetic acid/Dimethyl chloridePVA WaterPolystyrene DMF/TolueneNylon-6-co-polyamide Formic acidPolybenzimidazole Dimethyl acetamidePolyramide Sulfuric acidPolyimides Phenol

Table 1. Polymer solvent systems for electrospinning [6]

2.2 NANOFIBERS FROM SPLITTING BICOMPONENT FIBERS

Figure 2. Nanofibers from Bicomponent fibers [3].

As described above, nanofibers are also manufactured by splitting of bicomponent fibers; most oftenbicomponent fibers used in this technique are islands-in-a-sea, and segmented pie structures. Bicomponentfibers are split with the help of the high forces of air or water jets.

Figure 2 shows the bicomponent nanofiber before and after splitting. A pack of 198 filaments in singleislands is divided into individual filaments of 0.9 µm. In this example, Hills Inc has succeeded in producingfibers with up to 1000 islands at normal spinning rates. Furthermore bi-component fibers of 600 islands havebeen divided into individual fibers of 300 nm [1,3].

3. PROPERTIES OF NANOFIBERS

Nanofibers exhibit special properties mainly due to extremely high surface to weight ratio compared toconventional nonwovens.

Low density, large surface area to mass, high pore volume, and tight pore size make the nanofiber nonwovenappropriate for a wide range of filtration applications [9].

Figure 3 shows how much smaller nanofibers are compared to a human hair, which is 50-150 µm and Figure4 shows the size of a pollen particle compared to nanofibers. The elastic modulus of polymeric nanofibers ofless than 350 nm is found to be 1.0±0.2 Gpa.

Figure 3. Comparison between human hair and nanofiber web [4].

Figure 4. Entrapped pollen spore on nanofiber web [4].

4. APPLICATIONS OF NANOFIBERS

4.1. FILTRATION

Nanofibers have significant applications in the area of filtration since their surface area is substantiallygreater and have smaller micropores than melt blown (MB) webs. High porous structure with high surfacearea makes them ideally suited for many filtration applications. Nanofibers are ideally suited for filteringsubmicron particles from air or water.

Electrospun fibers have diameters three or more times smaller than that of MB fibers. This leads to acorresponding increase in surface area and decrease in basis weight. Table 2 shows the fiber surface area permass of nanofiber material compared to MB and SB fibers [8].

Fiber Type Fiber size, in MicrometerFiber surface area per mass of

fiber material m2/g

Nanofibers 0.05 80

Spunbond fiber 20 0.2

Melt blown fiber 2.0 2

Table 2. Fiber surface area per mass of fiber material for different fiber size [8].

Nanofiber combined with other nonwoven products have potential uses in a wide range of filtrationapplications such as aerosol filters, facemasks, and protective clothing. At present, military fabrics underdevelopment designed for chemical and biological protection have been enhanced by laminating a layer ofnanofiber between the body side layer and the carbon fibers [10].

e-Spin Technologies, Inc has produced a prototype of activated carbon nanofiber web. PAN- basednanofibers were electrospun. Then these webs were stabilized, carbonized, and activated. These activatedPAN nanofibers gave excellent results for both aerosol and chemical filtration [11,12].

Electrospun nanofiber webs are used for very specialized filtration applications. Donaldson is making andmarketing filter media that incorporate electrospun nylon fibers for gas turbines, compressor and generators[13].

4.2. MEDICAL APPLICATION

Nanofibers are also used in medical applications, which include, drug and gene delivery, artificial bloodvessels, artificial organs, and medical facemasks. For example, carbon fiber hollow nano tubes, smaller thanblood cells, have potential to carry drugs in to blood cells [14, 15].

Figure 5. Comparison of red blood cell with nanofibers web [4].

Nanofibers and webs are capable of delivering medicines directly to internal tissues. Anti-adhesion materialsmade of cellulose are already available from companies such as Johnson & Johnson and GenzymeCorporation [2]. Researchers have spun a fiber from a compound naturally present in blood. This nanofiber

can be used as varieties of medical applications such as bandages or sutures that ultimately dissolve in tobody. This nano fiber minimizes infection rate, blood lose and is also absorbed by the body [11].

To meet these varied requirements a layered composite structure is used. The bulk of the filter is generallymade of one or multiple MB layers designed from coarse to fine filaments. This is then combined with ananofiber web. The MB layer provides fluid resistance while the outer nanofiber layer improves smoothnessfor health, wear and comfort.

Nanofibers greatly enhance filtration efficiency (FE). Scientists at the U.S. Army Natick Soldier Centerstudied the effectiveness of nanofibers on filter substrates for aerosol filtration. They compared filtration andfilter media deformation with and without a nanofiber coating of elastic MB and found that the coating ofnanofiber on the substrate substantially increases FE [17].

With most of the nanofiber filter media, a substrate fabric such as SB or MB fabric is used to providemechanical strength, stabilization, pleating, while nanofiber web component is used to increase filtrationperformance [2,18].

4.4 NANOFIBER COMPOSITE CONSTRUCTION:

Nanofibers were applied to 0.6 ounces per square yard (osy) nylon SB material and to 1.0 osy nylon SB asshown in Figure 6 [8].

Figure 6. Nanofiber impregnation to spunbond layers [1,8]

Then two such layers were laminated together. Figure 7 shows three different types of nanofibercomposite fibers designed by altering the thickness and weight of base cloth.

Figure 7. Nanofiber composite fiber layer options [1,8]

The performance and the durability of the composite structure depends on the finished fabric architecture.

The final nanofiber fabric architecture is as shown in Figure 8. The two type of constructions are;

1. The nanofiber/SB layer between outer shell layer fabric and chemical filtration layer.

2. Nanofiber /SB layer is impregnated over the shell fabric and free floats against chemical filtrationlayer.

Figure 8. Nanofiber composite fabric Designs [1,8]

Polymeric nanofiber composites can provide enhanced protection against chemical agent micro droplets,biological aerosols, radioactive ducts, etc.

5. CHALLENGES IN NANOFIBERS

The process of making nanofibers is quite expensive compared to conventional fibers due to low productionrate and high cost of technology. In addition the vapors emitting from electrospinning solution while formingthe web need to be recovered or disposed of in an environmental friendly manner. This involves additionalequipment and cost. The fineness of fiber and evaporated vapor also raises much concern over possible healthhazard due to inhalation of fibers. Thus the challenges faced can be summarized as:

· Economics

· Health hazards

· Solvent vapor

· Packaging shipping handling

Because of its exceptional qualities there is an ongoing effort to strike a balance between the advantages andthe cost [19].

REFERENCES

1. Textile World “Nano Technology and Nonwoven”. P52, November 2003.2. Gajanan Bhat and Youneung Lee, “Recent advancements in Electrospun nanofibers.” Proceedings ofthe twelfth international symposium of Processing and Fabrication of Advanced materials, Ed TSSrivatsan & RA Vain, TMS, 2003.3. www.hillsinc.net

4. Electrostatic spinning of Nanofibers spin Technologies, Chattanooga, TN.5. Timothy Grafe, Kristine Graham, “Polymeric nanofibers and nanofiber webs: A New class ofNonwoven”Presented at INTC 2002:International Technical Conference Atlanta, Georgia, September24-26,2002.6. Www.nano21c.com, Nano Techniques Co., Ltd. “Mass production of Electrospun Nanofibers forfiltration”7. www.ecmjournal.org8. Kristine Graham, Heidi Schreuder-Gibson and Mark Gibson “Incorporation of Electrospunnanofibers in to Functional Structures.” US Army Soldiers Systems Center, Natick, Massachusetts,http://www.inda.org/subscrip/inj04_2/p21-27-graham.pdf.9. Peter P Tsai, “Effect of Electro spinning Material and Conditions upon Residual Electrostatic Chargeof Polymer nanofibers”. Proceedings of the 11th Annual international TANDEC Nonwoven conference,Nov.6-8, 2001, Knoxville, TN, USA.10. Heidi Schreuder-Gibson, Phillip Gibson, “Use of Electrospun Nanofibers for aerosol Filtration intextile structures”. US Army Soldier Systems Center AMSSB-RSS-MS (N) Natick, Massachusetts,http://www.asc2004.com/23rdASC/oral_summaries/L/LO-05.PDF.11. J.Doshi, MH Mainz and GS Bhat, Proceedings of the Tenth TANDEC Nonwoven Conference,Knoxville, TN (2000).12. www.nanospin.com13. www.donaldson.com14. www.ecmjournal.org15. www.zapmeta.com16. G.Wnek, M.Marcus, D.Simpson, and G.Bowlin, “NanoLeters”, 3(2), 213-216,2003.17. H. Schreuder-Gibson and P Gibson, 23rd Army science conference, Florida, December 2002.18. T.Grage, and Kgraham, Int.Non.J.51, 2003. Back to table of contents

NEEDLE PUNCHED NONWOVENSUpdated: April, 2004 - M. G. Kamath, Atul Dahiya, Raghavendra R. Hegde

(Praveen Jana & Xinli Liu)

1. INTRODUCTION

Worldwide, the needlepunching industry enjoys one of the greatest successes of any textile related process[1]. The needlepunching industry around the world is a very exciting and diverse trade involving eithernatural or both natural and synthetic fibers.

2. PROCESS

The needlepunch process is illustrated in fig. 1. Needlepunched nonwovens are created by mechanicallyorienting and interlocking the fibers of a spunbonded or carded web. This mechanical interlocking isachieved with thousands of barbed felting needles repeatedly passing into and out of the web.

Fig. 1: Needle punching process

The major components of the needle loom and brief description of each are as follows:

2.1 THE NEEDLE LOOM (Fig-2 a & b)

The needle board : The needle board is the base unit into which the needles are inserted and held. Theneedle board then fits into the needle beam that holds the needle board into place.The feed roll and exit roll. These are typically driven rolls and they facilitate the web motion as itpasses through the needle loom.The bed plate and stripper plate. The web passes through two plates, a bed plate on the bottom and astripper plate on the top. Corresponding holes are located in each plate and it is through these holes theneedles pass in and out. The bed plate is the surface the fabric passes over which the web passesthrough the loom. The needles carry bundles of fiber through the bed plate holes. The stripper platedoes what the name implies, it strips the fibers from the needle so the material can advance through theneedle loom.

Fig. 2a: Needle loom

Fig. 2b: Needle penetration

2.2 THE FELTING NEEDLE

The correct felting needle can make or break the needle punched product. The proper selection of gauge,barb, point type and blade shape (pinch blade, star blade, conical) can often give the needlepuncher the addededge needed in this competitive industry (fig. 3).

Fig. 2: Types of needles

The gauge of the needles is defined as the number of needles that can be fitted in a square inch area. Thusfiner the needles, higher the gauge of the needles. Coarse fibers and crude products use the lower gaugeneedles, and fine fibers and delicate fibers use the higher gauge needls. For example, a sisal fiber productmay use a 12 to 16 gauge needle and fine synthetics may use 25 to 40 gauge needle [3].

The major components of the basic felting needle are as follows :

The crank: The crank is the 90 degree bend on the top of the needle. It seats the needle when insertedinto the needle board.

The shank: The shank is the thickest part of the needle. The shank is that part of the needle that fitsdirectly in the needle board itself.

The intermediate blade: The intermediate blade is put on fine gauge needles to make them moreflexible and somewhat easier to put inside the needle board. This is typically put on 32 gauge needlesand finer.

The blade: The blade is the working part of the needle. The blade is what passes into the web and iswhere the all important barbs are placed.

The barbs: The barbs are the most important part of the needle. It is the barb that carries and interlocksthe fibers The shape and sized of the barbs can dramatically affect the needled product

The point: The point is the very tip of the needle. It is important that the point is of correct proportionand design to ensure minimal needle breakage and maximize surface appearance.

As the needleloom beam moves up and down the blades of the needles penetrate the fiber batting. Barbs onthe blade of the needle pick up fibers on the downward movement and carry these fibers the depth of thepenetration. The draw roll pulls the batt through the needle loom as the needles reorient the fibers from apredominately horizontal to almost a vertical position. The more the needles penetrate the web the moredense and strong the web becomes generally See fig. 4 a & b. Beyond some point, fiber damage results fromexcessive penetration.

Fig. 4a: Needle Action - Schematic

Fig. 4b: Needle action

3. TYPES OF LOOMS

There are three basic types of needle looms in the needlepunching industry. They are:

The Felting Loom1.The Structuring Loom2.The Random Velour Loom3.

The felting looms are the type just described. These needle looms may have one to four needle boards andneedles from the top, bottom or top and bottom. The primary function of this type of loom is to dointerlocking of fibers resulting in a flat, one dimension fabric. The types of products made with this processand needle loom are diverse and multifaceted. They exist in variety of industrial products, geotextileds,automotives, interlinings, home furnishings, etc. [2].

Structuring looms use what are called fork needles. Instead of carrying fibers into bedplate hole, the fork

needles carry fiber tufts into lamella bars that extend from the entry to the exit of the needle loom (fig. 1).These fork needles carry large tufts of fibers into parallel lamella bars. These bars carry the tuft of fiber fromthe entry to the exit side of the loom. Depending on the orientation of the fork needle, a rib or velour surfaceis introduced (fig. 5). The most popular products made with structuring looms include home and commercialcarpets and floor mats, automotive rib and velour products, wall covering and marine products.

Fig. 5: Structured needling

Random velour looms are the newest type of needle looms, having only been available since the mid 1980's.The random velour looms are used to produce velour surfaces. Unlike the structuring looms, the velourproducts produced by this loom are completely isotropic. It is almost impossible to distinguish the crossdirection from the machine direction.

Unique to this type of needle loom is the bristle-brush, bed-plate system. Special crown type needles or forkneedles are used in this loom design. The needles push fibers into a moving brush bed plate. The fibers arecarried in this brush from the entry to the exit of the loom with zero draft. This allows for the completelynon-linear look, perfect for molded products. Random velour type products have been very popular in theEuropean and Japanese automotive industry. While almost all U.S. automotive producers have the randomvelour machine, this type of product has yet to become popular in this country. The most popular productsmade with this type of needle loom are almost all centered around the automotive industry.

3.1 Machine variable:

The most important machine variable is the depth of penetration and puncture density. The fiber travelthrough the web depends on the depth of penetration of the needle. The maximum penetration is fixed by theneedle of the machine and depends on the length of the three sided shank, the distance between the needleplates, the height of stroke, and the angle of penetration. The greater the depth of penetration, greater is theentanglement of fibers within the fabric because more barbs are employed.

The puncture density i.e. number of punches on the surface of the feed in the web is a complex factor anddepends on

· the density of needles in the needle board (Nd)

· the rate of material feed

· the frequency of punching

· the effective width of the needle board (Nb T)

· the number of runs

The puncture density per run Edpass = [n*F] / [V*W]

Where, n= number of needles within a needleboard

F = frequency of punching

V = rate of material feed

W = effective width of the needle board

The puncture density in the needled fabric Ed NV depends on the number of runs Npass; Ed NV = Edpass *Npass

The thickness, basis weight, bulking density and air permeability - which provide information aboutcompactness of fabric are influenced by a number of factors. If the basis weight of the web and puncturedensity and depth are increased, the web density increases and air permeability is reduced (when finerneedles and longer, finer and more tightly crimped fibers are used). Web density does not increase when finerfibers are needled with coarser needles. There is neither an increase nor a decrease in air permeability if thepuncture density is increased.

As far as the strength of a needled nonwoven web, the situation is similar to that for compactness, namelythat finer needles, finer and longer fibers, greater web basis weight and greater puncture depth and density,result in increased strength of the needled web. However, once a certain critical puncture depth or density hasbeen reached, the rise in strength may be reversed. If the depth of the barb is decreased or the distancebetween the barbs is increased, the dimensional stability is improved during needling, and the web densityand maximum tensile strength in relation to basis weight can be raised.

4. APPLICATIONS

Tennis Court Surfaces1.

Space Shuttle Exterior Tiles2.

Marine Hulls, Headliners3.

Shoe Felts4.

Blankets5.

Automotive Carpeting6.

Automotive Insulation7.

Filters8.

Vinyl Substrate9.

10.Insulator10.

Primary Carpet Backing11.

Fiberglass Insulation Felts12.

Fiberglass Mats13.

Wall coverings14.

Composites15.

Blood Filters16.

Tennis Ball Covers17.

Synthetic Leather18.

Carpet Underlay Pads19.

Auto Trunk Liners20.

Interlinings21.

Papermaker Felts22.

Felts23.

Padding24.

Shoulder Padding25.

Ceramic Insulation26.

Kevlar Bullet Proof Vests27.

5. MARKET & PROSPECTS

In the U.S. the needlepunching sector of the nonwovens industry has always been the black sheep of thenonwovens industry. Needlepunching still has the connotation of being a slow, non-technical technology.Internationally, however, it is interesting to note that these negative connotations relating to needlepunchingare not so prevalent. This is especially true in Asian markets. The simple fact of the matter is that many U.S.,companies do not fully understand the needle punching process nor the emerging and developing markets.Additionally, companies cannot see a profitable future in needlepunching.

Needled felts used for filtration forms only about 10% of the total consumption, which accounts toapproximately $400 million. The Pacific Rim countries like Korea, Taiwan, Indonesia, India and otherSoutheast Asian countries are currently investing heavily in pollution control that will naturally increase themarket for the needled felt products [3].

Needlepunched nonwovens industries are rapidly growing in Latin America too. Due to the extensive use ofneedlepunched nonwovens in automobiles and due to the expansion of the automobile industry in thesecountries, there is tremendous potential for growth [4].

REFERENCES

John Foster, Nonwovens industry, October '92, p 44.1.

Klauss G Maitre, Frontiers of Needle punching; Nonwovens an advanced tutorial, p382.

J. Robert Wagner, Bonding Nonwovens, The Technical needs : Nonwovens for medical/surgical andconsumer uses, p76-77.

3.

Nonwovens Industry, April '94, p 64.4.

Nonwovens Industry, Nov. '93, p 50.5.

Back to Table of Contents

SPUNBOND TECHNOLOGYUpdated, April, 2004- Atul Dahiya, M. G. Kamath, Raghavendra R. Hegde

(Hsu-Yeh Huang and Xiao Gao)

1. INTRODUCTION

Spunbond fabrics are produced by depositing extruded, spun filaments onto a collecting belt in a uniformrandom manner followed by bonding the fibers. The fibers are separated during the web laying process by airjets or electrostatic charges. The collecting surface is usually perforated to prevent the air stream fromdeflecting and carrying the fibers in an uncontrolled manner. Bonding imparts strength and integrity to theweb by applying heated rolls or hot needles to partially melt the polymer and fuse the fibers together. Sincemolecular orientation increases the melting point, fibers that are not highly drawn can be used as thermalbinding fibers. Polyethylene or random ethylene-propylene copolymers are used as low melting bondingsites. Spunbond products are employed in carpet backing, geotextiles, and disposable medical/hygieneproducts. Since the fabric production is combined with fiber production, the process is generally moreeconomical than when using staple fiber to make nonwoven fabrics [1].

2. SPUNBONDING PROCESS

Fig. 1: Flowchart of spunbonding process [3]

3. POLYMER

In general, high molecular weight and broad molecular weight distribution polymers such as PP, PET,Polyamide, etc. can be processed by spunbonding to produce uniform webs. Medium melt-viscosity

polymers, commonly used for production of fibers by melt-spinning, are used.

i) Polypropylene

Isotactic polypropylene is the most widely used polymer for spunbond nonwovens production. It provides thehighest yield (fiber per kilogram) and covering power at the lowest cost because of its low density.Considerable advances have been made in the manufacture of polypropylene resins and additives since thefirst spunbond polypropylene fabrics were commercialized in the 1960s. Although unstabilizedpolypropylene is rapidly degraded by UV light, improved stabilizers permit several years of outdoor exposurebefore fiber properties deteriorate. To reduce cost, scrap or polypropylene fibers of inferior quality may berepelletized and then blended in small amounts with fresh polymer to produce first grade spunbond fabrics.This is very advantageous and important in a highly competitive industry.

ii) Polyester

Polyester is used in a number of commercial spunbond products and offers certain advantages overpolypropylene, although it is more expensive. Unlike polypropylene, polyester scrap is not readily recycled inspunbond manufacturing. Tensile strength, modulus, and heat stability of polyester fabrics are superior tothose of polypropylene fabrics. Polyester fabrics are easily dyed and printed with conventional equipment.

iii) Nylon

Spunbond fabrics are made from both nylon-6, and nylon-6,6. Nylon is highly energy intensive and,therefore, more expensive than polyester or polypropylene. Nylon-6,6 spunbond fabrics are produced withweights as low as 10 g/m2 and with excellent cover and strength. Unlike olefins and polyester fabrics, thosemade from nylon readily absorb water through hydrogen bonding between the amide group and watermolecules.

iv) Polyethylene

The properties of polyethylene fibers that are meltspun by traditional methods are inferior to those ofpolypropylene fibers. Advances in polyethylene technology may lead to the commercialization of spunbondstructures with characteristics not yet attainable with polypropylene. A fiber grade polyethylene wasannounced in late 1986.

v) Polyurethane

A new type of structure was announced in Japan with the commercialization of spunbond fabrics based onthermoplastic urethanes. Although spunbond urethane fabrics have been previously described, this representsthe first commercial production of such fabrics. Unique properties are claimed for this product which appearsto be well suited for apparel and other applications requiring stretch and recovery.

vi) Rayons

Many types of rayons have been successfully processed into usable spunbond webs using wet spinningmethods. The main advantage of rayon is that it provides good drape properties and softness to web.

4. POLYMER COMBINATIONS

Some fabrics are composed of several polymers. A lower melting polymer can function as the binder whichmay be a separate fiber interspersed with higher melting fibers, or two polymers may be combined into asingle fiber type. In the latter case the so-called bi-component fibers possess a lower melting component,which acts as a sheath covering over a higher melting core. Bicomponent fibers are also spun by extrusion of

two adjacent polymers. Polyethylene, nylon-6 and polyesters modified by isophthalic acid are used asbicomponent (lower melting) elements.

5. SPINNING AND WEB FORMATION

Spunbonding combines fiber spinning with web formation by placing the bonding device in line withspinning. In some arrangements the web is bonded in a separate step which, at first glance, appears to be lessefficient. However, this arrangement is more flexible if more than one type of bonding is applied to the sameweb.

Fig.2: Schematic of spunbonding process

The spinning process is similar to the production of continuous filament yarns and utilizes similar extruderconditions for a given polymer. Fibers are formed as the molten polymer exits the spinnerets and is quenchedby cool air. The objective of the process is to produce a wide web and, therefore, many spinnerets are placedside by side to generate sufficient fibers across the total width. The grouping of spinnerets is often called ablock or bank. In commercial production two or more blocks are used in tandem in order to increase thecoverage of fibers.

Before deposition on a moving belt or screen, the output of a spinneret usually consists of a hundred or moreindividual filaments which must be attenuated to orient molecular chains within the fibers to increase fiberstrength and decrease extensibility. This is accomplished by rapidly stretching the plastic fibers immediatelyafter exiting the spinneret. In practice the fibers are accelerated either mechanically or pneumatically. In mostprocesses the fibers are pneumatically accelerated in multiple filament bundles; however, other arrangementshave been described where a linearly aligned row or rows of individual filaments is pneumaticallyaccelerated.

In traditional textile spinning some orientation of fibers is achieved by winding the filaments at a rate ofapproximately 3,200 m/min to produce partially oriented yarns (POY). The POYs can be mechanically drawnin a separate step for enhancing strength. In spunbond production filament bundles are partially oriented bypneumatic acceleration speeds of 6,000 m/min or higher. Such high speeds result in partial orientation and

high rates of web formation, particularly for lightweight structures (17 g/m2). The formation of wide webs athigh speeds is a highly productive operation.

For many applications, partial orientation sufficiently increases strength and decreases extensibility to give afunctional fabric (examples: diaper coverstock). However, some applications, such as primary carpet backing,require filaments with very high tensile strength and low degree of extension. For such application, thefilaments are drawn over heated rolls with a typical draw ratio of 3.5:1. The filaments are then pneumaticallyaccelerated onto a moving belt or screen. This process is slower, but gives stronger webs.

The web is formed by the pneumatic deposition of the filament bundles onto the moving belt. A pneumaticgun uses high-pressure air to move the filaments through a constricted area of lower pressure, but highervelocity as in a venturi tube. In order for the web to achieve maximum uniformity and cover, individualfilaments must be separated before reaching the belt. This is accomplished by inducing an electrostatic chargeonto the bundle while under tension and before deposition. The charge may be induced triboelectrically or byapplying a high voltage charge. The former is a result of rubbing the filaments against a grounded, conductivesurface. The electrostatic charge on the filaments must be at least 30,000 esu/ m2.

Fig. 4: Pneumatic jet for spunbonding

The belt is usually made of an electrically grounded conductive wire. Upon deposition, the belt discharges thefilaments. This method is simple and reliable. Webs produced by spinning linearly arranged filamentsthrough a so-called slot die eliminating the need for such bundle separating devices.

Filaments are also separated by mechanical or aerodynamic forces. The figure below illustrates a method thatutilizes a rotating deflector plane to separate the filaments by depositing them in overlapping loops; suctionholds the fiber mass in place.

Fig. 5: Deflector plane for separation of filaments

For some applications, the filaments are laid down randomly with respect to the direction of the lay downbelt. In order to achieve a particular characteristic in the final fabric, the directionality of the splayed filamentis controlled by traversing the filament bundles mechanically or aerodynamically as they move toward thecollecting belt. In the aerodynamic method, alternating pulses of air are supplied on either side of thefilaments as they emerge from the pneumatic jet.

By proper arrangement of the spinneret blocks and the jets, lay down can be achieved predominantly in thedesired direction. The production of a web with predominantly machine direction and cross-machinedirection filament lay down is shown in the figure below. Highly ordered cross-lapped patterns can begenerated by oscillating filament bundles, as shown.

Fig. 5: Web production with machine and cross machine direction

If the lay down belt is moving and filaments are being rapidly traversed across this direction of motion, thefilaments are being deposited in a zig-zag or sine-wave pattern on the surface of the moving belt. The effectof the traverse motion on the coverage and uniformity of the web has been treated mathematically. The result

is that relationships between the collecting belt speed, period of traverse, and the width of filament curtainbeing traversed determine the appearance of the formed web. The following illustration shows the lay-downfor a process where the collecting belt travels a distance equal to the width of the filament curtain x duringone complete period of traverse across a belt width y. If the belt speed is Vb and the traverse speed is Vt, thenumber of layers deposited, z, is calculated by z = [x Vt/y Vb]. If the traverse speed is twice the belt speedand if x and y are equal, a double coverage occurs over all areas of the belt.

Fig. 6: Web laydown pattern

6. BONDING

Many methods can be used to bond the fibers in the spun web. Although most procedures were developed fornonwoven staple fibers, they have been successfully adapted for continuous filaments. These includemechanical needling, thermal bonding, and chemical bonding. The last two may bond large regions (areabonding) or small regions (point bonding) of the web by fusion or adhesion of fibers. Point bonding results inthe fusion of fibers at points, with fibers between the point bonds remaining relatively free. Other methodsused with staple fiber webs, but not routinely with continuous filament webs include stitch bonding,ultrasonic fusing, and hydraulic entanglement. The last method has the potential to produce very differentcontinuous filament structures, but is more complex and expensive. The choice of a particular bondingtechnique is dictated mainly by the ultimate fabric applications; occasionally a combination of two or moretechniques is employed to achieve bonding.

7. SPUNBOND PROCESS SYSTEM

A number of spunbond processes can be fitted into one of these three routes with appropriate modification.The following are three successful spinning, drawing, and deposition systems merit a brief discussion.

7.1 “DOCAN SYSTEM”

This route was first developed by the Lurgi Kohle & Mineral-Oltechnik GmbH of Germany in 1970. Manynonwoven companies have licensed this route from the Lurgi Corporation for commercial production.[3] Thisroute (chart 2 below) is based on the melt spinning technique. The melt is forced by spin pumps throughspecial spinnerets having a large number of holes. By suitable choice of extrusion and spinning conditions,desired filament denier is attained. The blow ducts located below individual spinnerets continuously cool thefilaments with conditioned air. The force required for filament drawing and orientation is produced by aspecial aerodynamic system. Each continuous filament bundle is picked up by a draw-off jet operated on highpressure air and passed through a guide tube to a separator which effects separation and fanning of thefilaments [8]. Finally, the filament fan leaving the separators is deposited as a random web on a moving sievebelt. The suction below the sieve belt enhances the random lay down of the filaments.

Fig. 7: Schematic of Docan spunbonding system

7.2 “REICOFIL” SYSTEM

This route has been developed by Reifenhauser of Germany. Many nonwovens companies have licensed thisroute from the Reifenhauser GmbH for commercial production. This route (Chart 3 below), is based on themelt spinning technique.[3] The melt is forced by spin pumps through special spinnerets having a largenumber of holes. The primary blow ducts, located below the spinneret block, continuously cool the filamentswith conditioned air. The secondary blow ducts, located below the primary blow ducts, continuously supplythe auxiliary room temperature air. Over the line's entire working width, ventilator-generated underpressuresucks filaments and mixed air down from the spinnerets and cooling chambers. The continuous filaments aresucked through a venturi (high velocity, low pressure zone) to a distributing chamber, which affects fanningand entanglement of the filaments. Finally, the entangled filaments are deposited as a random web on amoving sieve belt. The randomness is imparted by the turbulence in the air stream, but there is a small bias inthe machine direction due to some directionality imparted by the moving belt. The suction below the sievebelt enhances the random lay down of the filaments.

Fig. 8: Schematic of Reicofil spunbonding system

7.3 “LUTRAVIL SYSTEM”

This route was first developed by Carl Freudenberg Company of Germany in 1965. This process isproprietary and is not available for commercial licensing. This route (Chart 4), is based on the melt spinningtechnique. The melt is forced by spin pumps through special spinnerets having a large number of holes. Theprimary blow ducts, located below the spinneret block, continuously cool the filaments with conditioned air.The secondary blow ducts, located below the primary blow ducts, continuously supply controlledroom-temperature air. The filaments are passed through a special device, where high pressure tertiary airdraws and orients the filaments. Finally, the filaments are deposited as a random web on a moving sieve belt[4].

Fig. 9: Schematic of Lutravil spunbonding system

8. CHARACTERISTICS AND PROPERTIES

The spunbonded webs represent a new class of man-made product, with a property combination fallingbetween paper and woven fabric. Spunbonded webs offer a wide range of product characteristics rangingfrom very light and flexible structure to heavy and stiff structure. [4]

· Random fibrous structure

· Generally the web is white with high opacity per unit area

· Most spunbond webs are layered or shingled structure, the number of layers increases withincreasing basis weight

· Basis weights range between 5 and 800 g/m2, typically 10-200 g/ m2

· Fiber diameters range between 1 and 50 um, but the preferred range is between 15 and 35 um

· Web thicknesses range between 0. 1 and 4.0 mm, typically 0.2-1.5mm

· High strength-to-weight ratios compared to other nonwoven, woven, and knitted structures

· High tear strength (for area bonded webs only)

· Planar isotropic properties due to random lay-down of the fibers

· Good fray and crease resistance

· High liquid retention capacity due to high void content

· High in-plane shear resistance, and low drapeability.

Spunbond fabrics are characterized by tensile, tear, and burst strengths, elongation-to-break, weight,thickness, porosity and stability to heat and chemicals. These properties reflect fabric composition andstructure. Comparison of generic stress-strain curves of thermally bonded and needlepunched fabrics showsthat the shape of the load-strain curves is a function of the freedom of the filaments to move when the fabricis placed under stress.

Fig. 9: Typical stress-strain curves

Some applications require special tests for sunlight, oxidation, burning resistance, moisture vapor and liquidtransport, coefficient of friction, seam strength and aesthetic properties. Most properties can be determinedwith standardized test procedures (INDA). Typical physical properties are given below:

Tables 1

Product Basis wt. g/m2

Thickness mm

Tensile St.a

NbTear St. Nb Mullen burst

KPacBonding Method B

Accord 69 144MD175CD

36MD41CD

324 Point thermal

Bidim 150 495 279 1550 NeedlepunchCerex 34 0.14 182MD

116CD40MD32CD

240 ChemicallyInduced area

Corovin 75 130 15 Point thermalLutradur 84 0.44 275MD

297CD86MD90CD

600 CopolymerArea thermal

Polyfelt 137 585 225 1450 NeedlepunchReemay 68 0.29 225MD

180CD45MD50CD

331 CopolymerArea thermal

Terram 137 0.7 850 250 1100 Area thermal[sheath/core]

Trevira 155 630MD 270MD 1520 NeedlepunchTypar 137 0.38 650MD

740CD345MD355CD

1210 Undrawnsegments-areathermal

Tyvek 54 0.15 4.6MD5.1CD

4.5MD4.5CD

Area andpoint thermal

aMD=machine direction; CD=Transverse direction.bTo convert N to pound force, divide by 4.448.cTo convert Kpa to psi, multiply by 0.145.

9. APPLICATIONS

i) Automotive

Today spunbonded webs are used throughout the automobile and in many different applications. One of themajor uses of spunbonded webs in automobile is as a backing for tufted automobile floor carpets. Thespunbonded webs are also used for trim parts, trunkliners, interior door panel, and seat covers.

ii) Civil Engineering

The civil engineering market segment remains the largest single market spunbond webs, constituting over25% of the total. Spunbonded civil engineering webs cover a multiple of related uses, such as, erosioncontrol, revestment protection, railroad beds stabilization, canal and reservoir lining protection, highway andairfield black top cracking prevention, roofing, etc.[6]. The particular properties of spunbonded webs - whichare responsible for this revolution - are chemical and physical stability, high strength/cost ratio, and theirunique and highly controllable structure which can be engineered to provide desired properties [6].

iii) Sanitary and medical

The use of spunbond web as a coverstock for diapers and incontinence devices has grown dramatically in thepast decade. This is mainly because of the unique structure of spunbond, which helps the skin of the user stay

dry and comfortable [7]. Additionally, spunbond webs are cost effective over other conventional nonwovens.Spunbond web, as coverstock, is also widely used in sanitary napkins and to a limited extent in tampons.

In medical applications many traditional materials have been replaced by high performance spunbondedwebs. The particular properties of spunbonded webs, which are responsible for medical use, are:breathability; resistance to fluid penetration; lint free structure; sterilizability; and, impermeability to bacteria.Medical applications include: disposable operating room gowns, shoe covers and sterilizable packaging [7].

iv) Packaging

Spunbonded fabrics are widely used as packaging material where paper products and plastic films are notsatisfactory. The examples include: metal-core wrap, medical sterile packaging, floppy disk liners, highperformance envelopes and stationery products.

REFERENCES

1. Encyclopedia of Polymer Science and Engineering

2. Oldrich Jirsak and Larry C. Wadsworth: ‘Nonwoven Textiles’, Carolina Academic Press, ISBN:0-89089-978-8, 1999

3. Sanjiv R. Malkan and Larry C. Wadsworth: ‘A review on spunbond technology, Part I’, INB,Nonwovens vol.3, 4-14, 1992.

4. Sanjiv R. Malkan and Larry C. Wadsworth: ‘A review on spunbond technology, Part II’, INB,Nonwovens vol.4, 24-33, 1992.

5. ‘Spunbonding ‘, Textile Month, March, 16, 1999.

6. Poter K.; ‘Encyclopedia of chemical technology’, 3rd edition, 16, 72-104

7. Smorada, R. L.; ‘Encyclopedia of polymer science and engineering’, New York, 227-253 NRI, 135,9, 7-10, 1982.

8. Ian Butler; ‘Worldwide prospects for spunbond’, nonwovens world, September 59-63, 1999.

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SPUNLACE (HYDROENTANGLEMENT)Updated: April, 2004 - M. G. Kamath, Atul Dahiya, Raghavendra R. Hegde

(Hsu-Yeh Huang & Xiao Gao)

1. INTRODUCTION

The oldest technique for consolidating fibers in a web is mechanical bonding, which entangles the fibers togive strength to the web [1]. Under mechanical bonding, the two most widely used methods areneedlepunching and spunlacing (hydroentanglement). Spunlacing uses high-speed jets of water to strike aweb so that the fibers knot about one another. As a result, nonwoven fabrics made by this method havespecific properties, as soft handle and drapability. Japan is the major producer of hydroentangled nonwovensin the world. The output of spunlaced fabrics containing cotton was 3,700 metric tons and a significantgrowth in production can still be seen [2]. The biggest producers of spunlaced fabrics in the U.S. are DuPont,Chicopee and Kendall corporations.

This technology was officially introduced by DuPont in 1973 (Sontara®) and is a result of considerable workdone by DuPont and Chicopee (DuPont obtained five patents on spunlaced nonwovens within the period1963-1970. Since the 1990's, the technology has been made more efficient and affordable for moremanufacturers. Majorities of hydroentangled fabrics have incorporated dry-laid webs (carded or air-laid websas precursors). This trend has changed very recently with an increase in wet-laid precursor webs. This isbecause of Dexter making use of Unicharm's technology to make spunlaced fabrics using wet-laid fabrics asprecursors [3].

So far, there are many different specific terms for spunlaced nonwoven like jet entangled, water entangled,and hydroentangled or hydraulically needled. The term, spunlace, is used more popularly in the nonwovenindustry. In fact, the spunlace process can be defined as: the spunlace process is a nonwovens manufacturingsystem that employs jets of water to entangle fibers and thereby provide fabric integrity. Softness, drape,conformability, and relatively high strength are the major characteristics that make spunlace nonwovenunique among nonwovens.

2. PROCESS

Spunlacing is a process [3,5] of entangling a web of loose fibers on a porous belt or moving perforated orpatterned screen to form a sheet structure by subjecting the fibers to multiple rows of fine high-pressure jetsof water (Fig. 1). Various steps are of importance in the hydroentangling process.

Fig. 1: Spunlace process [23]

While some of them are typical in a nonwoven process, some of them are unique to the process ofspunlacing. The steps characteristic for producing hydroentangled nonwoven fabric include:

· Precursor web formation

· Web entanglement

· Water circulation

· Web drying

The formed web (usually air-laid or wet-laid, but sometimes spun bond or melt-blown, etc.) is firstcompacted and prewetted to eliminate air pockets and then water-needled. The water pressure generallyincreases from the first to the last injectors. Pressures as high as 2200 psi are used to direct the water jets ontothe web. This pressure is sufficient for most nonwoven fibers, although higher pressures are used inspecialized applications. It has been argued that 10 rows of injectors (five from each side of the fabric)should achieve complete fabric bonding [12]. Injector hole diameters range from 100-120 m m and the holesare arranged in rows with 3-5 mm spacing, with one row containing 30-80 holes per 25 mm [3]. Theimpinging of the water jets on the web causes the entanglement of fibers. The jets exhaust most of the kineticenergy primarily in rearranging fibers within the web and, secondly, in rebounding against the substrates,dissipating energy to the fibers. A vacuum within the roll removes used water from the product, preventingflooding of the product and reduction in the effectiveness of the jets to move the fibers and causeentanglement (fig.2 a, b & c).

Fig. 2a: Spunlace support wire details

Fig. 2b: Spunlace support wire and the product

Fig. 2c: Spunlace support wire and the product enlarged

Usually, hydroentanglement is applied on both sides in a step-wise manner. As described in the literature [6],the first entanglement roll acts on the first side a number of times in order to impart to the web the desiredamount of bonding and strength. The web then passes over a second entanglement roll in a reverse directionin order to treat and, thereby, consolidate the other side of the fabric. The hydroentangled product is thenpassed through a dewatering device where excess water is removed and the fabric is dried.

Hydroentanglement carried out at standard conditions (six manifolds of needles, 1500 psi, web weighing 68g/m2) requires 800 pounds of water per pound of product [14]. For that reason it is necessary to develop anew filtration system able to effectively supply clean water with this high throughput; otherwise, water jetholes become clogged. This system consists of three stages: chemical mixing and flocculation, dissolved airflotation and sand filtration [14]. Spunlaced fabrics have led to a lot of speculation regarding theirmanufacture because most of the manufacturing process details are considered as proprietary [4].

3. MATERIALS USED IN SPUNLACED TECHNOLOGY

As previously mentioned, hydroentanglement could be carried out using dry-laid (carded or air-laid) orwet-laid webs as a precursor. Most commonly, precursors are mixtures of cellulose and man-made fibers(PET, nylon, acrylics, Kevlar (P84, (imide) etc). In addition, Asahi Chemical Industry [3] has used very finefibers produced from splittable composite fibers to produce hydroentangled substrates for synthetic suedeleather products.

In general, cellulosic fibers are preferred for their high strength, pliability, plastic deformation resistance andwater insolubility. Cellulosic fibers are hydrophilic, chemically stable and relatively colorless. Anotheradvantage is that cellulose has an inherent bonding ability caused by a high content of hydroxyl groups,which attract water molecules. As the water evaporates from the fabric, the hydroxyl groups on fiber surfacelink together by hydrogen bonds.

Influence of cotton micronaire on fabric properties has been studied [14]. Generally, low micronaire cotton isnot recommended for hydroentangled nonwovens because of higher number of neps and small bundles ofentangled fibers, resulting in unsightly appearing fabric. In spite of this, fabrics made with lower micronairefiber show higher strength, probably caused by a higher number of fine fibers and greater surface area.

In addition, greige cotton has been used in spunlacing technology. It has been shown that the absorbency rateincreases with increasing hydroentangling energy. This is the result of oil and wax removal from the fibersurface. These nonwovens can be subsequently bleached, which should raise the strength of the fabric [14].

We can summarize all the processes that can be separated into following categories: [15-19]

3.1 THE CHOICE OF FIBERS

The fiber used in spunlaced nonwoven should think about following fiber characteristics.

· Modulus: Fibers with low bending modulus requires less entangling energy than those with highbending modulus.

Fineness: For a given polymer type, larger diameter fibers are more difficult to entangle than smallerdiameter fibers because of their greater bending rigidity. For PET, 1.25 to 1.5 deniers appear to beoptimum.

Cross section: For a given polymer type and fiber denier, a triangular shaped fiber will have 1.4 timesthe bending stiffness of a round fiber. An extremely flat, oval or elliptical shaped fiber could have only0.1 times the bending stiffness of a round fiber.

Length: Shorter fibers are more mobile and produce more entanglement points than longer fibers.Fabric strength, however, is proportional to fiber length; therefore, fiber length must be selected to givethe best balance between the number of entanglement points and fabric strength. For PET, the fiberlength from 1.8 to 2.4 seems to be best.

Crimp: Crimp is required in staple fiber processing systems and contributes to fabric bulk. Too much

crimp can result in lower fabric strength and entanglement.

Fiber wetability: Hydrophilic fibers entangle more easily than hydrophobic fibers because of the higherdrag forces.

3.2 PRECURSOR WEB FORMATION

Theoretically, any nonwoven web forming process can be used in the spunlace process. It depends on whatkind of products you desire. The general properties of web forming from other process are listed asfollowing:

Isotropic precursor webs can be produced by air laying system.

Carding webs can result in final products, which have higher MD strength than CD strength.

Melt blown webs can be produced with good ‘squareness’ of the web. Wet formed webs can especiallybe produced with good machine direction / cross direction characteristics.

The combinations of various types of precursor webs provide numerous options for using in thespunlace process to create various different composites.

3.3 WEB SUPPORT SYSTEM (CONVEYOR WIRE)

The web support system plays an important part in most nonwoven processes. Especially for the spunlaceprocess, it has a critical role in this process because the pattern of the final fabric is a direct function of theconveyor wire. By special design for the wire, we can have following varied products:

Ribbed and terry cloth-like products

Aperture products

Lace patterns or company logo can be entangled into fabrics

Production of composites

3-D fabric formation

There are two general characteristic wires in spunlace system. The comparison of their properties is listed inTable 1.

Table 1: Comparison of metal and plastic wires

Plastic wire Metal wire

Good flex resistance

Light weight

Easy to install

Corrosion resistant

Difficult seams

Prone to shower damage

Difficult to control knuckle height

Moderate temperature

Poor flex resistance

Heavy weight

Difficult to install

Prone to corrosion

Invisible seam

Shower damage resistance

Easier to control knuckle height

High temperature

In fact, the surface characteristics of the forming wire determine what the nonwoven products will look like.A smooth top surface of forming wire is desired for little or no marking. As for the aperture product, there isa high knuckle in the forming wire. A high knuckle in the wire will give a large hole in the fabric since thehigh-pressure water jets are deflected by the high knuckle.

3.4 THE ENTANGLEMENT UNIT

Hydroentanglement is an energy transfer process where the system provides high energy to water jets andthen transfers the energy to the precursor. In other words, the energy is delivered to the web by the waterneedles produced by the injector. Therefore, we can calculate the energy from the combination of the watervelocity (related to the water pressure) and the water flow rate (related to the diameter of the needles).

Flow rate = P½ x D2 x N x 2572 x 10-8 m3/hour/injector/meter

Energy = P3/2 x D2 x N x 7 x 10-10 KWH/injector/meter

P= water pressure (bar)

D=hole diameter ( m m )

N= number of holes (per injector per meter)

In general, the diameter of water needle ranges from 100 to 170 m m. The highest number of needles is 1666needles per meter of injector, corresponding to the smaller diameter. The water pressure ranges from 30 barsto 250 bars and it is increased stepwise from injector to injector.

3.5 WATER SYSTEM

As we know, water is most critical part in spunlace process. Therefore, there are some requirements for thewater as follows:

Large amount of water – about 606 m3/hr/m/injector for 40 bar and 120? m

Nearly neutral pH

Low in metallic ions such as Ca

No bacteria or other organic materials

3.6 FILTERATION SYSTEM

Due to the large amount of water consumed, the spunlace process requires that it be recycled. Therefore, ahigh quality filtration system is necessary for the spunlace process. Some of special filters are listed asfollowing:

Bag filter

Cartridge filter

Sand filter

3.7 WEB DRYING

When the fabric leaves the entanglement zone the web, it is completely saturated with water. There are a fewsteps to remove water from the fabric. The include:

Vacuum dewatering system

Drying system

4. PARAMETER AFFECTING THE PRODUCT PERFORMANCE PROPERTIES

Both the fiber and web properties have primary effects on the performance of the finished product. Theseparameters comprise of the web material and area basis-weight, and the way in which the web wasmanufactured. As mentioned in literature [12], spunlaced technology demands a high quality web, especiallyin its uniformity and isotropic orientation.

The process variables are considered to have secondary effects on the performance of the finished product.The supporting substrate transport is an important variable influencing the fabric. There are two systems ofentanglement substrate systems: flat and rotary. For the most part [6], there is no difference in the mechanismused to achieve entanglement. The rotary concept uses a compact machine design with ease of sheet run thatprovides entanglement of both sides of the web. Entanglement is nearly achieved with as little as four meters(in the machine direction) of the material. Sometimes the fibers are driven through the substrate wire and, inthe flat concept, it is seen that the wire (along with the fibers) is dragged over the suction box causingdifficulty in the removal of the product. In the rotary concept, this problem is not encountered because thefibers are not pulled along the machine direction.

The substrate texture seems to have important influence on the product. The size of perforations is usuallymeasured in "mesh", which is the count of wires per inch of the substrate. It has been shown [6] thatimposing the same energy into two webs with different substrate meshes, the finer substrate yielded astronger product resulting from finer support. The coarser wire support (20 mesh) gave a bulkier product withmore permeability, but with less strength. Water removal from the fabric was shown to be dependent on themesh of the support belt. The lower the mesh, the more energy that was necessary to remove the remainingwater. In addition to that, the surface of the fabric can be aperture (textured on the surface) with a speciallystructured substrate [13].

The amount of energy delivered in the web is a crucial parameter influencing the fabric structure andproperties since it affects fiber entanglement completeness. "Completeness" is a term that is defined [6] as

"the portion of fibers that are tied together". DuPont patent literature has methods for entanglementcompleteness testing. Water pressure is another parameter related to fabric energy intake. There are severalwater pressure levels used (see Table-2)[12].

Now, higher water pressure machines are mostly used since using high pressure, energy can be delivered intoa web with less water needles and less water. This is economically more useful [12].

Another basic process parameter having influence on the fabric is the speed of the line. If a constant amountof energy is being delivered to a fabric, the speed of the fabric determines how much energy is going to beabsorbed per fabric unit area. Logically, the higher the line speeds, the less the energy that is absorbed by thefabric and the lower the fabric strength that is achieved.

5. PROPERTIES OF SPUNLACED FABRICS

Spunlaced fabrics show high drape, softness and comfortable handle because more fiber entanglement leadsto increased strength without an increase in shear modulus. It has also been shown that there is a relationshipbetween absorbency capacity and hydroentangling energy used. An increase of hydroentangling energyresults in a decrease of absorbency capacity and absorbency rate [14]. Shear modulus remains low and isvirtually independent of the degree of entanglement [7]. The softness of the fabric is explained by the factthat the entangled structures are more compressible than bonded ones, as well as having mobility and partialalignment of fibers in the thickness direction. The absence of a binder is seen to result in a fabric withyarn-like fabric intersections composed of "pseudoyarns". The pseudoyarns are "more highly intereconnectedthan yarns of conventional fabrics because individual fibers can migrate from one pseudoyarn to another.This tends to stabilize the intersection". This pseudoyarn structure seems to be the reason for gooddimensional stability, which is also accountable for drape [4], softness, and good strength/weight propertiesof the fabric, pilling and abrasion behavior.

The strength of hydroentangled fabrics is lower than that of woven and higher than that of knitted fabrics,whereas the wash durability is considerably lower than that of woven or knitted fabrics [11].

6. THE INFLUENCE OF PROPERTIES OF FABRIC ON THE SPUNLACE PROCESS [20]

Spunlaced fabrics are unique among nonwoven fabrics because of the balance achieved between strength andshear modulus. General speaking, spunlaced fabrics rely primarily on fiber-to-fiber friction to achievephysical integrity and are characterized by relatively high strength, softness, drape, conformability andaesthetics closely approaching woven and knitted fabrics. The property map of shear modulus and strength islisted in Fig-4 below

Therefore, the operational condition change in the process will affect directly on the properties of fabrics. Forexample Fig-5, show that the spunlaced fabric has the lowest shear modulus among the nonwoven fabrics andis very close to the shear modulus of woven and knitted fabric. Even if one tries to increase the fabricstrength, it doesn’t increase the shear modulus, as is the case normally for other nonwoven fabrics.

Fig. 5: Fabric strength and Modulus

Fig. 6 shows that the tensile strength of fabric increases with water pressure increase. This is due to the highenergy from water imparted to the fiber entanglement.

Fig. 6: Fabric strength verses water pressure

Generally, the water jet is perpendicular to the fabric. If we change the angle a little, the results show thestrength increases as demonstrated in fig. 7. Additionally fig. 8 shows speeding the speed of conveyor willdecrease the strength of fabric.

Fig. 7: Fabric strength verses jet angle

Fig. 8: Fabric strength verses conveyor speed

In the spunlace process, there are three water jet manifolds at least and the water pressure can be adjustedindividually. Therefore fig 9,10, and11 illustrate the change of water pressure at each of the jet manifolds 1, 2&3.

Fig. 9: Fabric strength verses water pressure in 1st manifold

Fig. 10: Fabric strength verses water pressure in 2nd manifold

Fig. 11: Fabric strength verses water pressure in 3rd manifold

The results show the strength always increases with increasing water pressure. However, the lower waterpressure at the first jet manifold and similar water pressure at second and third, the closer tensile strength forboth MD and CD directions. In other words, the fabric is closer to isotropic properties as shown in Table 2.This is a very important factor in deciding what kind of material property will result.

Table 2.

Injector I (bar) II (bar) III (bar) MD: CD

20 70 100 1.27

30 70 100 1.55

40 70 100 1.51

50 70 100 1.50

60 70 100 1.67

70 70 100 1.93

80 70 100 2.19

90 70 100 2.47

100 70 100 2.23

30 50 100 1.71

30 60 100 1.82

30 70 100 1.55

30 80 100 1.43

30 90 100 1.54

30 100 100 1.29

30 110 100 1.33

30 120 100 1.60

30 70 80 1.33

30 70 90 1.39

30 70 100 1.55

30 70 110 1.95

30 70 120 2.25

Fig. 12: Water pressure verses drape

Fig. 12 shows that there is no significant change for the drape with increasing water pressure which showsthat spunlaced fabrics have good drape even with greater entangling pressure.

Fig. 13: Strength increases when entangled on both sides.

Fig13 shows that the tensile strength of water jet entangling on both sides is much better than that on a singleor multiple treatments on one side.

7. APPLICATIONS

Hydroentanglement is a highly versatile process [8] because it can be used to produce nonwovens with abroad range of end-use properties. These differences are achieved because of a wide range of fibers that areavailable and because of the broad range of possible parameter adjustments. The versatility of thehydroentanglement processes is seen as an advantage because this process can be used to combineconventionally formed webs with melt blown, spunbond webs, paper, other textiles and scrims in order to geta combination of properties that cannot be achieved by the use of a single web.

Spunlace fabrics can be further finished, usually dyed and/or printed, treated with binders to allow for washdurability, or fire retardants can be applied to resist burning. The fabric can be treated by antimicrobial agentsto enhance resistance against microorganisms.

The largest US market [9] for spunlaced fabrics spans from surgical packs and gowns, protective clothing aschemical barriers to wipes, towels and sponges for industrial, medical, food service and consumerapplications. The main reason for wide use of these fabrics in medical applications is based on relatively highabsorption abilities. Another important criterion is absence of a binder in the fabric allowing sterilization ofthe fabric at high temperatures.

There are some applications: [22]

1. Bacteria-proof Cloth (Fig. 14)

· Based on 100% Rayon. The extreme absorption with water and oily stuff are good for yourconvenience.

For our unique green earth, we adopt the recyclable material for protecting the environment.

By special water-processed method, the fluffed cotton cannot easily float away

Easy to wash, quick dry, making a becteriaproof environment.

No Starch, No fluorescence substance, and other chemical medicines.

Fig. 14: Bacteria proof cloth

2. Cleaning Cloth (Fig 15)

· Based on 100% Rayon. The extreme absorption with water and oily stuff provides convenience.

For our unique green earth, we adopt the recyclable material for protecting the environment.

By special water-processed method, the fluffed cotton cannot easily float away.

Easy to wash, quick dry, resulting in a bacteria-proof environment.

No Starch, No fluorescence substance, and other chemical medicine.

Fig. 15: Cleaning cloth

3. Magic Towel (Fig 16)

· Processed by the advanced high-pressure-water method, the magic towel has extreme absorption forwater, oil stuff and so on. Definitely, it has no formaldehyde and gluey substances. That's good foryour health and convenience.

Easy to carry out for picnic, travel, and even as promotion gifts. One can print their LOGO on the tagfor advertisement.

Fig. 16: Magic owel

4. Wet Tissue (Fig 17)

· Processed by the advanced high-pressure-water method. The nonwoven spunlace has noFormaldehyde.

No glue-like substance, tender the soft skin.

For the refreshing experience, it is comfortable for body and parents like it.

The wet tissue is used for make-up, make-up removal, and other facial applications. In fact, it isconvenient all the day.

Fig. 17: Wet tissues

5. Make -up Cotton (Fig 18)

Hi-Tech Nonwoven Spunlace which has no any chemical substance, but does have a soft touch and istender to baby skin.

Saving the lotion and make-up cream. Best absorption, No fluffed cotton.

Best use for make up, wiping lips-sticker, fingernail polish, glasses, leather, jewels, and so on.

Fig. 18: Make up cotton

REFERENCES

The Nonwovens Handbook, INDA, Association of the Nonwoven Fabrics Industry, 19881.

Suzuki, M.: New Nonwoven and its Technical Features, INDEX 84 Congress, Session 2- Componentand Process developments, Geneva, 1984

2.

White, C. F.: Hydroentanglement Technology Applied to Wet Formed and Other Precursor Webs,TAPPI Nonwovens Conference, 1990, 177-187

3.

Vaughn, E.: Spunlaced Fabrics, Canadian Textile Journal, October 1978, 31-364.

Drelich, A.: A Simplified Classification of Nonwoven Fabrics, Sixth Annual Nonwovens Conference,University of Tennessee, Knoxville, 1988

5.

Jaussaud, Jean Paul: Rotary hydraulic Entanglement Technology, Nonwovens in Medical andHealthcare Applications Conference, Nov 10th -12th 1987, Brighton, England

6.

Shivers, Joseph C., Popper, Peter, Saffer, Henry W.: The mechanical and Geometric Properties ofSpunlace Fibrous Structures, INDA- TEC 1976, Symposium Papers

7.

Information brochure for Hydroentanglement technology from Valmet Paper Machinery, HoneycombSystems Inc.

8.

White, C. F.: A review of Hydroentanglement Technology - Development of Future Products andMarkets, Eighth Annual Nonwovens conference, University of Tennessee, Knoxville, 1990

9.

1White, C. F.: Future Directions, Nonwovens in Medical and Healthcare Applications Conference,Session 3: Paper 19, Nov 10th - 12th, 1987, Brighton, England

10.

Connolly, T.J., Parent, L.R.: Influence of Specific Energy on the Properties of HydroentangledNonwoven Fabrics, Tappi Journal, v.76, Aug '93, 135-141

11.

Vuillaume, Andre M.: A Global Approach to the Economics and End Product Quality of SpunlaceNonwovens, Tappi Journal, v.74, Aug '91, 149-152

12.

Widen, Christian B.: Forming Fabrics for Spunlace Applications, Tappi Journal, v.74, May '91,149-153

13.

Allen, Charles H., Jr.: New Development for Spunlacing Cotton, Paper presented at Fiber SocietyConference, University of Tennessee, Knoxville 19th-21st Oct. 1997

14.

EDANA’s 1989 UK Nonwoven Symposium15.

"Spunlace technology today" 198916.

Christian B. Widen , " Forming fabrics for spunlace applications " TAPPI Journal, May 199117.

" Spunlace offers utmost versatility" Nonwovens manufacturing ATI, November 199718.

" Spunlaced nonwovens overview" Nonwoven Industry, Feb. 199919.

" Spunlace Nonwoven " Perfojet, December 199120.

" The study on the mechanical properties of spunlaced nonwoven" 16th polymer symposium Vol.9, PP433-436, Jun.1993

21.

www.jenor.com.tw/pe2a.htm22.

Jurgen Heeler, “Hydroentanglement of short fibers” TANDEC Conference November 200123.

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THERMAL BONDING OF NONWOVEN FABRICSUpdated: April, 2004 - M. G. Kamath, Atul Dahiya, Raghavendra R. Hegde

(Xiao Gao & Hsu-Yeh Huang)

1. INTRODUCTION

There are three major bonding types: chemical bonding / thermal bonding/ mechanical bonding. Thedevelopment of the past few years has shown that the share of thermally bonded webs is growing steadily. [1]

The first thermally bonded nonwovens were produced in 1940s. Initial products used rayon as the carrierfiber and plasticized cellulose acetate (PCA) or vinyl chloride (PVC) as the binder fiber [2]. The viability ofthe thermal bonding process is rooted in the price advantage obtained by lower energy costs. However, thethermal bonding process also addresses the demanding quality requirements of the market place. Thedevelopment of new raw materials, better web formation technologies and higher production speeds havemade thermal bonding a viable process for the manufacture of both durable and disposable nonwovens.

2. BINDERS

Many materials that can be used as a binder for thermally bonded nonwovens.

Binding fibers

Binding powder

Binding web

The following are the essential characteristics of the binder polymer:

Efficient melt flow

Good adhesion to the carrier fiber

Lower melting point than the carrier fiber

Desired stiffness or elasticity.

2.1 BINDING FIBERS

Single-component and bi-component fibers, as binder fibers, are most widely used in thermal bonding ofnonwovens [4]. Single-component fibers are the least sophisticated and most economical because the fibersare often already in existence and low in cost. The type bond that is formed is dependent on several factorsincluding fiber chemistry, morphology, linear density, staple length, crimp, and processing conditions. Themajor disadvantage encountered when using 100 percent single-component fibers is the narrow temperaturerange that is necessary when thermal bonding. If the temperature is too low, there is inadequate bondstrength. If the temperature is too high, the web will melt excessively and lose its identity as a web.

When bi-component fibers are used to produce thermal bonded nonwoven, the acceptable temperature rangefor bonding may be as great as 25°C. When thermal bonding, the high melting portion of the fiber maintainsthe integrity of the web, while the low melting point portion melts and will bond with other fibers at the fibercrossover points. The product produced tends to have bulk and exceptional softness.

2.2 BINDING POWDER

Powdered polymers are sometimes used in thermal bonding of nonwovens. The most prevalent use ispowdered polyethylene. The powder can be applied between layers of fibers when cross-laying, air laying, oras an after treatment. A short exposure in an oven is sufficient to melt and fuse the powder. It is often usedwhen a light weight and open structure is required with a soft hand or when a reinforced, molded product isnecessary.

2.3 BINDING WEB

A very open-structured, low-melting-point thermoplastic fabric is placed between the webs and, duringthermal bonding between the calender rolls, the fabric melts completely bonding the webs together. Thenonwoven produced by this technique is soft and bulky. Thermoplastic coatings and hot melt print bondinghave been used to a limited extent in controlled porosity filters, impermeable membranes and other items.However, the use of this method of bonding is not expected to achieve a high level of importance.

3. METHODS OF THERMAL BONDING

Hot calendering

Belt calendering

Through-air thermal bonding

Ultrasonic bonding

Radiant-heat bonding, etc.

3.1 HOT CALENDERING

There are three main types of hot calendering.

· Area bonding· Point bonding· Embossing.

3.1.1 AREA BONDING

This process involves the use of a calender with a hot metal roll opposed by a wool felt, cotton or specialcomposition roll. Two, three or four roll calenders can be used, depending on the weight of the web to bebonded and the degree of bonding desired. The three-roll calender has the heated roll in the middle while thefour-roll configuration has the heated rolls on the top and bottom, with the two composition roll in themiddle. The amorphous or co-polymeric binder fibers used in this process provide bonding at all cross-overpoints between the carrier and binder fibers. The resultant product - commonly used in electrical insulationand coating substrates - is smooth, thin and stiff. The material is always two sided, but this effect is mostapparent in material processed through two and three roll calenders. Four roll calenders minimize this effect.

The application of heat from the outside produces a material whose inner area is less bonded than its outersurface. This becomes more pronounced as the product weight increases beyond 35 g/m2 and can becomedetrimental unless corrective measures are taken. These include increasing heat, slowing speed, or increasingthe binder/carrier fiber ratio. The two-roll calender is used for low-to-medium weight products with light-to-medium bonding. The three-roll calender is used for special bonding and finish effects on a single surface.The four roll calender produces the widest weight range of materials because it provides more flexibility inthe application of heat. .

Area-bond hot calendering is influenced by five factors:

Heat

Bonding occurs at the surface of the metal roll, which obtains its heat by conduction from heated oilcirculated through its center or from restrictive heating. The composition rolls obtain their heat fromcontact with the heated metal roll. Before the start of a production run, the roll stacks are operateduntil the composition rolls achieve dynamic heat equilibrium.

Pressure

Bonding occurs through simultaneous application of heat and pressure. The heat causes the fiberbinder to become thermoplastic. The pressure enhances mechanical bonding by forcing the binderpolymer to flow in and around the carrier fibers.

Speed

The speed at which the nonwoven passes through the calender, combined with heat and pressureconditions, determines the degree of bonding in the nonwoven. It also determines the throughput rateof the entire nonwoven line and is a critical factor in product cost. The faster the rate, the lower is thecost. This is the primary reason for the recent development of lower melting binders.

Roll combination.

The only practical roll combination for area bonding is a metal roll-felt roll. The metal roll applies theheat. The surface resilience of the felt roll enables uniform application of pressure to all the minutesurface thickness variations throughout the product.

Cooling rolls

The product is warm and thermoplastic as it leaves the calender nip. If the product were to be woundwhile it was still hot, the tension applied to eliminate wrinkles would stretch the web and introduceunrelieved stresses. This would lead to shrinkage whenever post-heat treatments were used. A set oftwo cooling rolls placed immediately after the calendering stage eliminates these unwanted sideeffects.

3.1.2 POINT BONDING

Point-bond hot calendering is the main method of thermally bonding in disposables as diaper, sanitaryproducts, and medical products. This method involves the use of a two-roll nip consisting of a heated malepatterned metal roll and a smooth or patterned metal roll (fig 1 a, b, c & d). This second roll may or may notbe heated, depending on the application. In a typical production line, the web is fed by an apron leading to acalender nip and the fiber temperature is raised to the point at which tackiness and melting cause fibersegments caught between the tips of engraved points and the smooth roll to adhere together. The heating time

is typically of the order of milliseconds [3]. The fabric properties are dependent on the process temperatureand pressure and other parameters like the contact time, quench rate and calender pattern. Experimentalresults show that for a given nip line pressure and calendering speed, the breaking strength reaches amaximum at a critical bonding temperature (fig1e); on keeping the nip line pressure constant, the criticaltemperature was found to be a function of the calendering speed. Fig. 1A,B, C and D Showing details ofpoint bonding

Fig. 1a: Engraved rolls for point bonding [7]

Fig. 1b: Advances in bonding with a release pattern [7]

Fig. 1c: SEM of point bonding [8]

Fig. 1d: SEM of point bonding failures [8]

The maximum strength achieved is influenced by the nip line pressure. This influence depends on the meltingbehavior of the fiber. If the maximum occurs in the softening region, higher pressure yields higher strength.On the other hand, if maximum occurs in the early melting region, a low calendering pressure is desirable.The degree of product bonding depends on the pattern of bond points on the roll surface. Bonded areas arecompressed and densely compacted. Unbonded area is very open, breathable and porous. The productsformed range from thin, closed, inelastic, strong, and stiff to open, bulky, weak, flexible and elasticdepending on the number density, the size and the pattern of the bond points

Fig. 1e: Stress-strain curves for various bonding temp (PP) [8]

3.1.3 EMBOSSING

This method is a figured or sculptured area-bond hot calendering. In this case, though, the area bonding isthree-dimensional. A "bulky but thin" product can be made in any pleasing or functional construction,depending on the faces of the embossing rolls. The calender roll combination has a male patterned heatablemetal roll and a matching female patterned felt roll.

3.2 BELT CALENDERING

Belt calendering is a modified form of hot roll calendering. The two main differences are the time in the nipand the degree of pressure applied. In belt calendering, time in the nip is 1-10 seconds. The pressure appliedis about 1/10th of the pressure applied in the hot calendering process. The belt bonder consists of a heatedroll and a rubber blanket. The nonwoven fabric is heat bonded by running it between the roll and the blanket.Pressure is applied by varying:

The tension on the blanket against the heated roll a.The pressure on the exit guide roll inside the rubber blanket.b.

Belt calendered products are much less dense and papery compared to hot roll calendering. The belt bonderfacilitates the use of binders with sharp melting and flow properties. Such binders can present difficulties in ahot roll calendering process.

3.3 THROUGH-AIR BONDING

Through-air thermal bonding involves the application of hot air to the surface of the nonwoven fabric. Thehot air flows through holes in a plenum positioned just above the nonwoven. However, the air is not pushedthrough the nonwoven, as in common hot air ovens. Negative pressure or suction, pulls the air through theopen conveyor apron that supports the nonwoven as it passes thorough the oven. Pulling the air through thenonwoven fabric allows much more rapid and even transmission of heat and minimizes fabric distortion. (SeeFig.2)

Fig. 2: Through-air bonding

Binders used in through-air thermal bonding include crystalline binder fibers, bi-component binder fibers,and powders. When using crystalline binder fibers or powders, the binder melts entirely and forms moltendroplets throughout the nonwoven's cross-section. Bonding occurs at these points upon cooling. In the case ofsheath/core binder fibers, the sheath is the binder and the core is the carrier fiber. Products manufacturedusing through-air ovens tend to be bulky, open, soft, strong, extensible, breathable and absorbent.Through-air bonding followed by immediate cold calendering results in thicknesses between a hot rollcalendered product and one that has been though-air bonded without compression. Even after coldcalendering, this product is softer, more flexible and more extensible than area-bond hot-calendered material.

3.4 ULTRASONIC BONDING

This process involves the application of rapidly alternating compressive forces to localized areas of fibers inthe web. The stress created by these compressive forces is converted to thermal energy, which softens thefibers as they are pressed against each other. Upon removal from the source of ultrasonic vibration, thesoftened fibers cool, solidifying the bond points. This method is frequently used for spot or patterned bondingof mechanically bonded materials.

No binder is necessary when synthetic fibers are used since these are self-bonding. To bond natural fibers,some amount of synthetic fiber must be blended with the natural fiber. Fabrics produced by this technique aresoft, breathable, absorbent, and strong. This bonding method is used to make patterned composites andlaminates, such as quilts and outdoor jackets.

3.5 RADIANT HEAT BONDING

Radiant heat bonding takes place by exposing the web or mat to a source of radiant energy in the infraredrange. The electromagnetic energy radiated from the source is absorbed by the web, increasing itstemperature. The application of radiant heat is controlled so that it melts the binder without affecting thecarrier fiber. Bonding occurs when the binder resolidifies upon removal of the source of radiant heat. Lowerenergy and equipment costs make this a favored method for processing powder-bonded nonwovens.Versatility and lower shipping costs are also factors. Post-calendered rolls can be shipped in thin, compactedform and rebulked by reapplication of heat, without pressure or restraints, to the desired state at the time ofuse. Powder bonded products made in this manner are soft, open, and absorbent with low-to-mediumstrength. They also can be reactivated by heat for use in the manufacture of laminated composites.

4. ADVANTAGES AND ENERGY COMPARISION

Compared to other bonding processes, thermal bonding and the products thus obtained offer a number ofadvantages [1]:

Quality of product soft and textile-like.1.

High economic efficiency as compared to chemical bonding with binder agents because no waterevaporation is required, i.e., considerable energy saving results. In comparison with chemical bonding,thermal bonding only has a heat energy requirement of 1/4 to 1/6 (also in this respect ecologicallybeneficial).

2.

Less expensive machinery. The capital expenditure, maintenance and operating costs are often lowerbecause no binder preparation station and no binder application units are required.

3.

It is possible to bond even thicker webs uniformly and thoroughly to the core that cannot be achievedby spraying. While a regular bonding effect across the web cross- section can be achieved for a webwith a homogeneous distribution of the binding fibers, spraying only produces a bonding effect in theouter layers of the web.

4.

No binder agents are required and no curing process is needed. Hence, there is no exhaust air orwastewater problem. Objections against certain chemicals can be dropped. Thus, thermal bonding isnon-polluting. (Note: New developments of the binder producers in the meantime have put on themarket new dispersions, which also can be considered ecologically harmless).

5.

As pure polymer fibers or blends can be used for thermal bonding processes, recyclability is 100% inpractice.

6.

Fiber properties can be influenced in an ideal manner (e.g. flame-retardence, nonwovens with highbulk and excellent resilience owing to fiber crimping, heat-insulating characteristics due to hollowfiber, etc.).

7.

It has to be mentioned, however, that not all nonwovens can be processed by thermal bonding in such a waythat the product obtains the requested properties. It can be assumed therefore that binder bonding can alsosecure its market share because the binder producers are trying to develop polymer dispersions which arebiodegradable and, in connection with the fiber polymers using the same polymer basis, allow recycling ofthe respective nonwovens.

5. THERMAL BONDED NONWOVENS IN THE MARKET

A variety of nonwovens made of staple fibers with blends of matrix fibers and bicomponent fibers isproduced on Fleissner hot-air flow-through bonding installations. Other products are bicomponent andbifilament spunbondeds. The following is a list of various thermo-bonded nonwovens: [5]

· Colback by Akzo, for the production of carpet carrier webs, roofing felts, geotextiles etc.

Lutradur by Freudenberg Spunweb for automobile construction, carpet fabrication, civil engineeringand building construction, roofing felt production, furniture industry, filtering technology, andelectronic industry, wall paper fabrication, horticulture and agriculture.

Lutrasil by Freudenberg Spunweb for sanitary products, medical products, automobile construction,furniture, textile processing, bedding, filters, protective clothes, agriculture, packing material.

Freudenberg Colmar Spunweb for roofing industry and many other applications

Celbond by Hoechst for personal products, f urniture, quilts, needled webs made of spunbonded forroofing felts, geotextiles etc.

Cambrelle by ICI for insoles.

Terram by ICI, Exxon for spunbonded geotextiles.

Islands-in-the-Sea or Citrus by BASF for filtration and wiping cloths, made of fibers that can easily bebroken up into microfibers.

ES fibers by Chisso or Danaklon for personal products up to paper maker felts.

Danaklon for short staple fibers in the airlaid fabric sector, etc.

Corebond by DuPont for fiberfill webs, etc. Melty and Bellcombi by Unitika, Kanebo for fiberfillwebs, mattress substitutes etc.

Sofil by Kuraray for various applications.

TBS by Teijin as short staple for wet-laid webs.

Estranal by Toyobo for needle felts.

Wellbond by Wellman: clothing (ski outfits, interlinings), bedding (comforters, mattresses),construction industry (insulating webs, roofing), filters (industry, air, liquids, household), floorcoverings (needle felts), furniture (cushions, etc.)

MERAKLON is introducing a new series of thermal bonding fibers, S2000 for high-speed nonwovenslines. These are available in hydrophilic, hydrophobic and durable strike-through types for bothcoverstock and the production of cloth-like backsheets. [6]

The subsequent table lists the major applications for thermobonded webs (carded webs, wet-laid webs,spunbonds) and the respective preferred bonding method.

Table 1: Applications of thermobonded webs

Type of Web formation Weight range Applications Bonding Type gram/sq.meter Card or aerodynamic 18-25 coverstock calender Light weight sanitary webs incl. Bico & blends Disposables through air bondingSpunbonded 10-25 coverstock calender Light weight sanitary webs Disposables Card or aerodynamic 25-150 Interlinings Calender Through air bondingCard or aerodynamic 100-1000 Filter webs Through air bonding Bulky or needled Card or aerodynamic 80-100 Geotextiles Through air bondingAnd Spunbonded Spunbonded 150-200 Carpet backing Through air bonding Card or aerodynamic 80-2000 Technical textiles Through air bonding Insulation, decoration Wall cover, upholstery Card or aerodynamic 80-2000 Wadding Through air bonding Fiberfill webs

Card or aerodynamic 100-250 Wiping cloth Through air bonding Card or aerodynamic 80-3000 Waste fiber web Through air bonding For various uses Card or aerodynamic 300-600 Needle web Through air bonding Food covering Card or aerodynamic 150-350 Bitumen carrier webs Through air bonding With fiber heat setWet forming machine 20-200 Decoration Through air bonding Tea bag paper Dry-laid paper 25-150 Wiping cloth Through air bonding Sanitary webs Technical products

6. CONCLUSION

Thermal bonding is much less energy intensive, kinder to the environment and more economical than latexbonding. A wide range of products can be made with thermal bonding, depending on the options used forprocessing. The bonding method has a significant effect on product properties. Depending on the bondingmethod, product properties can vary from nonporous, thin, nonextensible, and nonabsorbent to open, bulk,extensible and absorbent. All thermal bonding methods provide strong bond points that are resistant to hostileenvironment and to many solvents too.

REFERENCES

1. Alfred Watzi; "Fusion bonding, thermal bonding and heat-setting of nonwovens-theoreticalfundamentals, practical experience, market trends", Melliand, English, 10/1994, E 217

2. Albert G. Hoyle, "Thermal bonding of nonwoven fabrics", Tappi Journal, July '90, p 85-88.

3. "Thermal bonding", Textile progress vol 2 p 3-11.1995

4. J. Robert Wagner, "The bonding nonwovens, The Technical Needs: nonwovens for medical surgicaland consumer uses", p 70-73.

5. Alfred Watzi; Fusion bonding, thermal bonding and heat-setting of nonwovens-theoreticalfundamentals, practical experience, market trends", Melliand, English, 12/1994. P.E 270

6. "Fibers and Fabrics-Thermal Bonding", Textile Month, March 1999, p 20

7. Steve Gunter, “Advances in Thermal Bonding” INTC Sep 2002

8. Gajanan Bhat, “Structure Development in SBMB Process”, PP Fiber Tech Conf. 2002

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