fiber bonding

7/29/2019 Fiber Bonding

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FIBER BONDING

Importance of fiber bonding

The bonding is very much important for the development of

Mechanical strength of paper andRun ability of paper machine or printing press.

Paper may be made from glass fibers, ceramic fibers or synthetic textilefibers such as nylon, polyolefin or acrylics do not take fibers in bondingwithout the addition of adhesives bonding agents or solvents

Factors affecting the fiber bonding are the Physical or chemical properties of the fibers are the resultant of the

contribution of- the species of tree or plant-the subsequent pulping and bleaching processes.-beating during stock preparation,-wet pressing andcalendaring on the paper machine.

In weak fibers choose stronger fiber from a different source oreliminating excessive degradation resulting from pulping andbleaching errors.

When a bonding additive was used in paper, the fiber breakage

increased.

When a surfactant was added, the tensile strength was lower

In a study in which fibers were deliberately weakened by overbleaching, the conclusion was drawn that fiber strength was lessimportant for paper strength than is inter fiber bonding.

A mathematical equation, in which the variables are quantitativelyrelated by the choice of suitable units for the indices:

1 = 1 + 1T F B

T = tensile strengthF = fiber strength indexB = the bond strength index


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Paper is a net work of crossing fibers, the fiber net work influences

the formation and density of fibers, fiber orientation, and the relativebonded area and upon the geometrical and strength properties of thefiber themselves.

When bonding increases strength properties such as tensile andbursting strength increase.

Tearing strength however requires a certain minimum fiber bindingafter which increased fiber bonding reduces tearing strength.

Folding endurance increases as paper becomes brittle.

Both folding endurance and tearing strength are increased by longerfiber and very sensitive to fiber length as well to the degree of fiberbonding.

Opacity of paper is decreased as fiber bonding increases.

Increased beating will increase the fiber bonding, and, as result, thedensity.

Wet pressing slightly favors density increase over fiber bondingincrease.

Calendaring increase density with little increase in fiber bonding.

Internal bonding also reduces surface fuzz a condition caused byloose ends of fibers projecting above the surface of the paper.

One of the consequences of poorly bonded fibers in the surface ofpaper intended to be printed by the offset process is that the tackyinks will pull out loose surface fibers. The detached fibers adhere tothe image transfer blanket and make a defect in the printing on eachsubsequent sheet of paper.

Theories of fiber bonding

Contraction of the web on drying is due to Surface tension of the water film between fibers and Fiber shrinkage

The web contracts initially because of surface tension between fibers andthen contracts further as drying proceeds because of intra-fiber shrinkage.


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Hydrogen bonding and water removal

Effect of surface tension on fiber bonding Surface tension is important in bringing the fibers together during

the drainage and drying of the sheets.

As water is removed, surface tension creates a tremendous forcewhich compacts and draws the fiber into more intimate contact.

When drainage advances to the point that air forms an appreciableproportion of the medium in which the fibers are suspended, thewater air interfacial area increases rapidly

The pulp fibers must approach each other within a few tenths of ananometer to form hydrogen bond. The fibers surfaces approachesto each other when dried from liquids of high surface tension.Tremendous forces from capillary action and hydrogen bondingbring the fiber surfaces very close to gather.


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Surfaces tension forces during paper drying

Swanson shows how some of these forces act. The three diagrams are inthe order of decreasing water content and increasing forces:

a. The web is about 8 % solids or consistency on the Fourdrinier wire inthe hivacs.

b. The force with a surface tension of 75 dynes/cm for water and a fiberdiameter of 0.03 mm would be about 5 kPa; with fibrillation thediameter may be effectively 0.003 mm, giving a force of 50 kPa (7psi). In the region of 20-25% solids the water becomes discontinuousand the surface tension forces decrease

c. As film thickness decreases, the pressure may reach one or twohundred atmospheres.

Surface tension is so important in fiber bonding that thepresence, either from poor washing or deliberate addition, ofeven a small amount of agents that reduce the surface tensionwill result in a sheet of low density.


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This lower density is probably the result of the reduction ofcontracting forces due to reduce surface tension; however, itmay also be the result of the prevention of hydrogen bondingresulting from the adsorption of surfactant which masks thepolar grouping on the surface of the fibers.

Lyne and Gallay proved that the strength at very low solidscontent was due almost entirely to the effect of surfacetension by making sheets of glass fibers which are nonbonding.

Strength of pulp webs with increasing Strength of fiber webs withdryness Increasing dryness

Stages for fiber bonding during dewatering of wet webs

Surface tension is the principal factor in sheet consolidation in theearly stages of drying up to a solid content of about 20 to 25%

During this period there is a rapid decrease in the caliper of the wetweb as water is removed, with increasing air intrusion into the webof fiber and water


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It is concluded that up to 20- 25 % solids, the fibers are held together

by surface tension forces. Surface tension forces decrease withincreasing solids but the hydrogen bonding begins and the strengthincreases as the water is removed.

The plateau due to reaching the maximum effect of surface tension isclearly seen. In the range of 25 to 45 % solids depending upon thetype of pulp. In the low-solids area where surface tension suppliesthe pressure holding the fibers to gather, the resistance to tensilepull is likely to be frictional.

Types of bonds in dried paper

The main forces are:

Primary valence bonds

Polar bond attraction or Van der waals forces that is nonpolar, nonionic polar bonds

Paper is not strong as plastic films, which are crossed linked byprimary valence bonds. In quantitative terms primary valence bondsrequire about 30 kcal/mole for disruption compared with about 5kcal/mole for strong hydrogen bonds, and about 2 kcal/mole forother bonds.

Close proximity of the hydroxyl group is required for hydrogenbonding to occur, approaching to each other to within 2.5 to 3.5that is within atomic distances. Hydrogen bonds are formed byfunctional groups in which the hydrogen is attached to oxygen, mostimportantly, but alternatively to N2 to F. The groups OH and NH2are important examples.

Water is governed in its properties by hydrogen bonding. The abilityof water to disintegrate paper is believed to be the results ofpenetration in to the web structure, followed by substitution of waterto cellulose hydrogen bonds for cellulose to cellulose hydrogenbonds. The ability of water to swell cellulosic structures has thesame explanation in hydrogen bonding

Partial solubility theory

A hypothesis is advanced by Urquhart proposed that celluloseCH2OH is soluble in water in certain stages


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Campbell extended the idea to beaten fibres supposing that thecellulose crystallites on the surface of the fibres become partiallydissolved in water and in this state attach themselves to similarcrystallites on adjacent fibres

Clark suggested the surface of well-beaten is a two dimensionalcolloid system in which the fibrillate have two dimensions in thecolloid range, but are anchored to the fiber in the third dimension

Voyutski proposed Diffusion theory of polymer adhesion.According to him diffusion aided by the microfibrillar nature of thefiber wall.

Nature of fiber surface

The ability of cellulose fibers to produce fiber-to-fiber bonding is

dependent upon the hydrophilic nature of the fiber surface, and theconsequent ability to form hydrogen bonds. The presence of hemicellose material is favourable to fiber bonding

in that it improves the contact of adjacent hydroxyl- containingsurfaces.

The adsorption of hydrophilic materials or surface active agents onthe surface of cellulose fibers greatly decreases the amount of fiberto fiber bonding, probably because these agents are adsorbed on thefiber surface in such a way that the large hydrophilic groupprotrudes from the surface of the fiber and reduces the hydrophilic

nature of the fiber surface In order to soften the paper it is necessary to add long chain amines

to the furnishes. The softness is due to decreased fiber to fiberbonding

The sheet forming properties of the pulp are reduced by treatmentwith tannic acid, probably because of the interface with thehydrophilic character of the fiber surface

Tensile strength of filter paper is much reduced when the paper wasdipped in water solution of cetyldimethylbenzyl ammonium chlorideand then dried but the tensile strength was unaffected when thepaper was dipped in a water solution of Congo red and dried

It is concluded that cationic material was absorbed on the out side ofthe fibers, thus interfering with bonding, while the dye enteredinterstices of the cell wall and was adsorbed there, thus leaving theouter bonding surface of the fiber unaffected.


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Measurement of fiber bonding and strength of bonds

Fiber bonding is the area of the fibers under contact, the number ofbonds within the area of contact, and the strength of each bond.Beating is supposed to increase the bonding of fibers but there is no

method for measuring fiber bonding.

Bonded area from light scattering

The reflectance of light from (white) paper is due to light scatteringfrom the unbonded surfaces of the fibers in the paper and that asbonding is increased the light scattering, hence reflectance willdecrease.

Surface area of the paper is determined by adsorption of nitrogenusing the method of Brunauer, Emmet, and Teller (BET)

Swanson and Steber told that a relative bonded area could bedefined, which would be the fiber area bonded divided by the area ofthe totally bonded area

Ingmanson and Thode explored the idea that the area of the totallyunbonded fibers which was actually available for bonding could bedetermined by changing beating time and wet pressing in otherwiseidentical papers and plotting the optical scattering coefficientagainst the tensile strength

The curve could be extrapolated to zero tensile strength, giving avalue of scattering coefficient presumably corresponding to zerofiber bonding. The relative bonded area would be then calculatedfrom the scattering coefficients: RBA = (St-Su) where St is thescattering coefficient at zero strength and Su is the scatteringcoefficient at some measured strength. Bonded area could beconverted to absolute values by using the calibration factor obtainedfrom the nitrogen adsorption measurement i.e. scattering coefficientin units cm2/gmultiplied by 0.044.

Factor affecting fiber bonding

The degree of bonding between the fibers in a sheet depends upon:

Chemical and physical nature of the fiber surface upon the mannerin which the fibers have been formed in to the sheet of the paper

Fibers in actual contact, the number of bonds per unit area and thenumber of fiber contacts and bonds per unit volume of the fibrousstructure


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The morphology of pulp fibers, notably the cell wall thickness,

spring wood and summer fibers

The effect of beating and fiber morphology, chemical composition

of wood in terms of hemicellulose and lignin, degree ofpolymerization of cellulose

The presence of alumina, sizing, fillers and other additives

Moisture in paper, recycling effect, wet pressing, drying andcalendaring

1. Effect of beating on fiber bonding

Unbeaten pulp does not form in to strong sheet. Even small amount

of beating is required to develop the tearing strength However, primary wall is permeable to water but it does not take part

bond formation. Removal of primary wall exposes secondary wallwhich is made up of three components (S1, S2 and S3 layers)

Drainage resistance (water removal resistance) increases

Tensile strength, tensile stiffness, burst strength, internal bondingstrength and fracture toughness increases

Tear strength of softwood fibers slightly improve at first but thendecreases, where as that of hardwood fibers at first significantlyincreases but then decreases after prolonged refining

Air permeability, bulk, absorbency, opacity and light scatteringdecreases

Brightness slightly decreases.


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Scott bond: Brushing will enhance hydrogen bonding in zdirection of the paper. It is evident from the figure the pliabilityproperty higher in pulp having more brushing action and lowerin case of fiber cutting.

Effect of fiber cutting and brushing on scott bond

Tensile and Mullen properties: The burst and tensile strengthboth increase 2-3 times their initial values as refiningproceeds. Tensile energy absorption (TEA) also increasedsignificantly, which reflects the combined increases in tensileland stretch. In contrast to those properties that dependslargely upon fiber-to-fiber bonding.

Effect of fiber cutting or brushing on tensile and mullenproperties


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Tear strength: The tearing resistance falls off to less than halfthe level for the unbeaten pulp. Mill experience supports thiseffect where it is difficult to obtain a good balance betweentear and tensile strength when attempting to maximize bothproperties.

Effect of fiber cutting or brushing on tear strength

Formation: Refining will produce positive effects on sheetformation. A severe cutting action provides a more rapidimprovement in formation; however, many other propertieswill suffer. Cutting simply makes shorter fibers from longfibers, and the effects are similar to adding more hardwood orfiller to furnish. In general controlled uniform refining of stockis the most effective way to improve sheet formation, althoughchemical formation aids may be needed in special situations.

Effect of fiber cutting or brushing on formation


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Vacuum: Higher couch vacuum results from increasedrefining, which means that the drainage resistance is greater.Uniform gentle refining tends to open up the fibers rather thansimply reduce fiber length. The change will be much largerwith brushing type refining which more effectively closes up

the structure and restricts the movement of air through theweb.

Effect of fiber cutting or brushing on vacuum

Steam drying:The steam requirement for drying increases asthe amount of refining is increased and brushing action clearlyproduces the greatest change.

Effect of fiber cutting or brushing on steam drying sheet

Sheet shrinkage: The shrinkage of the fibers and sheetincreases as refining increases. Refining places water on thefiber wall. When this water is evaporated, the fiber and sheetshrink

Effect of cutting and brushing on sheet shrinkage


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Sheet density and bulk:

Effect of refining on density and bulk of sheet

Porosity:

More uniform, brushing treatment has a strong effect on gurleyporosity. In the cutting mode, the fibers are mainly shortened inlength so they do not collapse readily to ribbons

Effect of refining on porosity


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Smoothness: Cutting mode refining permits some improves packing and

distribution of the fibers in the sheet structure to improvesheet smoothness.

Effect of refining on smoothness

Brightness and opacity:

Brightness, opacity, and bulk (caliper) all decreaseuniformly increased refining.

Effect of refining on brightness and opacity


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2. Effect of conformability, plasticity and swelling upon fiberbonding

An increase in paper density will increase fiber bonding and densityis governed by the degree of plasticity developed wet pressing. It

depends in turn on the amount and type of beating.

However, delamination will increase plasticity and flexibility.

This is called collapse. The lumens of sulphate pulps collapsedmuch less then did those of sulphite pulps, which can be explainedin terms of stiffer cell wall of sulphate pulps.

3. Effect of fibrillation on fiber bonding

External fibrillation, the partial removal of the fiber wall, leaving it stillattached to the fiber. The external fibrillation is the peeling off of thefibrils from the fiber surface and thus the formation of fines.

Internal changes in the wall structure, dislocation within the fiber wall,including the change in fibril, and reduction of crystallinity variouslydescribed as delamination, internal fibrillation or swelling.


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Over all beating increases the conformability and plasticity of fibers

4. Effect of fibrillation on fiber bonding

Fines are divided in to three categories:1. Primary fines: broken parenchymatous cells generated duringpulping

2. Secondary fines Broken primary wall and microfibrillar structure3. Broken microfibrillar structure during recycling of secondary fibers.

Fines are the detached portion of the cellulosic material. Fines may consistof lignin which will have preventive effect on hydrogen bonding. Theydecrease the strength of paper and acts as filler

4. Effect of hemi cellulose on fiber bonding

Hemicelluloses are water soluble (having primary and secondaryhydroxyl groups) and are strongly hydrophilic.

High hemicellulose content favors fiber-to-fiber bonding where aslow hemicellulose content favors opacity and tear strength.

It is well established that softwood hemicellulose is much moreeffective than hard wood cellulose due to the presence of mannoseand other hexose content.

-cellulose is the degraded cellulose produced during cooking and isineffective during cooking.

Alkali soluble hemicellulose increases the strength properties ofpaper.

Sulphite pulp wet readily but soda pulp did not when the samespecies used because alkali in soda process removes alkali solublehemicellulose which causes the fibers to be brittle and fragmentizeon beating instead of developing wetness.

However, wheat straw pulp contains high hemicellulose content thanwood pulp still the properties are poor.

The dimensions of fiber, ratio of non-fibrous cells to fibrous cellsand physical properties of fiber. Cotton is essentially 95% cellulose

The non-cellulosic materials, consisting mostly of waxes,pectinaceous substances, and nitrogenous matter, are located to alarge extent in the primary wall, with small amounts in the lumen.


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5. Effect of lignin on fiber bonding


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6. Effect of recycling on fiber bonding

Recycling results in to decrease in fiber bonding, fiber flexibility andconformability

Burst strength, zero span tensile, tensile strength and double folddecreases whereas tear strength increases due to the effect ofdrying on fiber stiffness. Opacity and stiffness of fiber increases

After 4th recycling fines contents increased up to 16-35 % and causeda severe reduction in drainage.


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7. Effect of water on fiber bonding

Water is essential for the plasticizing of cellulose

Cellulose fibers do not spontaneously dissolve

The never dried wet strength of paper is important to papermachine speed

The rewet strength of paper is important to certain end use

It is tested at a relative humidity of 65 % and temperature 25 0C

8. Effect of alumina, sizing, fillers and other additives onfiber bonding

Any hydrophobic substances like rosin or paraffin will result in avery great decrease in strength

However, fillers like clay and alumina are hydrophilic in naturewill result in a decrease in strength

The loss in strength is less for paper made from slightly beaten or

beaten pulp than for papers made from well beaten pulp. Actuallyrosin will increase the strength of paper if they are slightly beaten

Antifoam added to the paper machine may also reduce inter-fiberbonding

Hydrophilic colloids of which starch, proteins, vegetable gumsand water soluble resins particularly polymers of acrylamide


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absorbed on the fiber surface where they hold the sheet togetherthrough a fiber-to-fiber bonding

The pH slightly effect properties of paper and a very great effecton the permanence of paper

9. Effect of water on fiber bonding

Solvent bonding:Solvents or swelling agents added to gelatinize thefibers which are then bonded by pressure

Thermoplastic fibers: Added as a fiber blend, followed by heat tobond these fibers in the web

Thermoplastic powder:Fine particles (0.002 to 0.005 in) are shifted into the web. These penetrate by gravity and are bonded at fiberintersection by heat. About 15-30 % binders are used

Printing: Thickened binder (e.g. plasticized polyvinyl acetate)isapplied cross wise to the thin web

Saturation:A fluid solution of resin is applied to the web. From 15-50% binder is used to provide a very high degree of bonding

Foaming:A foamed mixture of binder, emulsifier, foaming agent, andthickener is applied to the web and squeeze d in to it by squeezerolls


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1. What is the impact of fiber bonding on followings:(i) Opacity(ii) Fuzz

2. What is the impact of surface tension fiber bonding?

3. Describe the various views given byUrquhart, Campbell, Clark and Voyutskiabout partial solubility theory.

4. Tensile strength of filter paper is much reduced when the paper was dipped inwater solution of cetyldimethylbenzyl ammonium chloride and then dried but thetensile strength was unaffected when the paper was dipped in a water solution ofCongo red and dried. Explain why?

5. Describe various factors which affect fiber bonding.

6. What is the impact of air permeability, bulk, absorbency, opacity and light

scattering and brightness of paper?7. Describe the impact of fiber cutting and fibrillation on Scot bond, tensile,

tear and mullen properties, formation, brightness and opacity, porosity,sheet density and bulk, sheet shrinkage, steam drying and vacuum.

8. Explain the followings:(i) The pH slightly effect properties of paper and a very great effect

on the permanence of paper.(ii) Antifoam added to the paper machine may also reduce inter-

fiber bonding.(iii) Clay and alumina are hydrophilic in nature will result in a

decrease in strength.

(iv) Rosin or paraffin will result in a very great decrease in strength.(v) Up to 4th recycling there is a severe reduction in drainage and

increase in tear strength.(vi) Recycling results decrease in fiber bonding, fiber flexibility and

conformability.(vii) Wheat straw pulp contains high hemicellulose content than

wood pulp still the properties are poor.

fiber bonding

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