wood-based composite board - usda forest service · 2 wood-based composite board figure 1. common...

WOOD-BASED COMPOSITE BOARD

ZHIYONG CAIUSDA Forest Service, Madison,

WI

INTRODUCTION

The term wood-based composite is used to describe anywood material bonded together with adhesives. The basicwood elements in the production of wood-based compos-ites can be in a great variety of sizes and geometries:fibers, sawdust, shavings, larger particles composed ofmany fibers, flakes, strands (Fig. 1), and veneers. Theseelements can be used alone or in combination. The choice isalmost unlimited. Wood-based composite boards are madefrom these wood elements in a panel form.

Maloney [1] proposed a logical basis for classifyingwood composites in a bigger family of the wood composite(Table 1). For purposes of this article, these classifi-cations have been slightly modified from those in theoriginal version to reflect the latest product develop-ments. This article covers only the subgroup listed under‘‘composite materials’’ and processes used to manufacturewood-based composite materials. It describes conventionalwood-based composite panels and structural compositematerials intended for general construction and/or interioruse. This article also describes wood–nonwood composites.

Conventional wood-based composite products are man-ufactured primarily from wood with only a few percentresin and other additives. Product types can be subcatego-rized on the basis of the physical configuration of the woodelements used to make these products: veneer, particle,strand, or fiber (Fig. 2). Morphology of the wood elementsinfluences the properties of composite materials, and canbe controlled by selection of the wood raw material andby the processing techniques used to generate the woodelements. Composite properties can also be controlled bysegregation and stratification of wood elements havingdifferent morphologies in different layers of the compositematerial. In conventional wood-based composites, prop-erties can also be controlled by the use of adhesiveswith different curing rate in different layers. Varying thephysical configuration of the wood element, adjusting thedensity profile of the composite, adjusting adhesive resin,or adding chemical additives is just a few of the manyways to influence the properties. Wood-based compositesare used for a number of structural and nonstructuralapplications including panels for exterior, interior, and fur-niture uses. Performance standards are in place for manyconventional wood-based composite products (Table 2).

Generally, wood-based composites provide uniformand predictable in-service performance, largely as aconsequence of standards used to monitor and controltheir manufacturing process. The mechanical properties ofwood composites depend on a variety of factors, includingwood species, forest management regimes (naturally

Wiley Encyclopedia of Composites, Second Edition. Edited by Luigi Nicolais and Assunta Borzacchiello.© 2012 John Wiley & Sons, Inc. Published 2012 by John Wiley & Sons, Inc.

regenerated, intensively managed), the type of adhesive,geometry of the wood elements (fibers, flakes, strands,particles, veneer, lumber), and density of the final product[9]. Many of the mechanical properties of wood-based com-posites tabulated in this article were originally reportedin technical and scientific literature. Consequently, theyshould not be used for direct computation of design values.They do, however, provide excellent baseline informationon the properties of wood-based composites.

There are a wide range of engineering properties thatare used to characterize the performance of wood-basedcomposites. Mechanical properties are frequently usedto evaluate wood-based composites for structural andnonstructural applications. Static elastic and strengthproperties are the primary criteria to select materials or toestablish design/product specifications. Elastic propertiesinclude modulus of elasticity (MOE) in bending, tension,and compression. Strength properties usually reportedinclude modulus of rupture (MOR) in bending, compres-sion strength parallel to surface, tension strength paral-lel to surface, tension strength perpendicular to surface(internal bond strength), shear strength, fastener-holdingcapacity, and hardness.

Many of the questions that arise with wood-basedcomposites have to do with their mechanical proper-ties; especially how the properties of one type of materialcompared to clear wood and other wood products. Whilean extensive review that compares all the properties ofwood-based materials and products is beyond the scopeof this article, Table 3 provides some insight into howthe static bending properties of these materials vary, andhow their properties compare with clear wood. Althoughmost wood composites might not have as high mechanicalproperties as solid wood, they provide very consistent anduniform performance [10].

WOOD ELEMENTS

Conventional wood-based composites are composed pri-marily of wood elements (often 90% or more by mass)bound together with a resin and other additives. Figure 1shows the relative size of the common wood elements usedin wood-based composites from top, left clockwise: shav-ings, sawdust, fiber, large particles, flakes, and strands.Figure 2 shows the various composite products.

ADHESIVES

Commonly used resin or binder systems in wood-basedcomposites include phenol-formaldehyde (PF), urea-formaldehyde (UF), melamine-formaldehyde, and iso-cyanate. The selection of the resin system is dependent onthe process, cost, product standards, and applications.

Phenol-formaldehyde

PF resins, commonly referred to as phenolic resins, aretypically used in the manufacture of construction ply-

1

In: Wiley Encyclopedia of Composites, Second Edition. Edited by Luigi Nicolais and Assunta Borzacchiello.© 2012 John Wiley & Sons, Inc. Published 2012 by John Wiley & Sons, Inc. 2012

2 WOOD-BASED COMPOSITE BOARD

Figure 1. Common wood elements used in wood-based com-posites from top, left clockwise: shavings, sawdust, fiber, largeparticles, flakes, and strands.

Table 1. Classification of Wood-Based Compositesa

Veneer-based materialPlywoodLaminated veneer lumber (LVL)Parallel-strand lumber (PSL)

LaminatesGlue-laminated timbersOverlayed materialsLaminated wood–nonwood compositesb

Multi wood composites (COM-PLYc)

Composite boardsFiberboard (low-, medium- or high-density)HardboardParticleboardWaferboardFlakeboardOriented strand board (OSB)Laminated strand lumber (LSL)Oriented strand lumber (OSL)

Edge-adhesive-bonded materialEdge-glued and ripped panels for lumber

SystemsI-beamsT-beam panelsStress-skin panelsTrusses-metal tooth connectedBox-beamsStructural insulated panels (SIPS)

Wood–nonwood compositesWood fiber–polymer compositesInorganic-bonded composites

aSource: Adapted from Ref. 1.bPanels or shaped materials combined with nonwood materials such asmetal, plastic, and fiberglass.cRegistered trademark of APA—The Engineered Wood Association.

wood and oriented strand board (OSB) where exposureto weather during construction is a concern. Phenolicresins are relatively slow-curing compared with otherthermosetting resins. In hot-pressed wood-based compos-ites, use of phenolic resin necessitates longer press timesand higher press temperatures. Hot-stacking of pressed

Figure 2. Examples of various composite products. From clock-wise from top left: laminated veneer lumber, parallel strandlumber, laminated strand lumber, plywood, oriented strand board,particleboard, and fiberboard.

material shortly after emergence from the press is a fairlycommon industrial practice, used to attain adequate resincure without greatly extending press time. Significantheat exposure associated with pressing of phenolic-bondedcomposites commonly results in a noticeable reduction intheir hygroscopicity. Cured phenolic resins remain chem-ically stable at elevated temperatures, even under wetconditions. The PF resin bonds are sometimes referred toas being boil-proof because of their ability to maintainthe structural integrity and adequate bonding after boil-ing water test. The inherently darker color of PF resincompared with other resins may make them aestheti-cally unsuitable for product applications such as interiorpaneling and furniture.

Urea-formaldehyde

UF resins are typically used in the manufacture of prod-ucts used in interior applications, for example, particle-board and medium-density fiberboard (MDF). They cureat lower temperatures than PF resins. Excessive heatexposure will result in chemical break-down of cured UFresins. Therefore UF-bonded panels are typically cooledafter emergence from the press. UF resins are the lowestcost thermosetting adhesive resins. They offer light color,which often is a requirement in the manufacture of dec-orative products. However, the release of formaldehydefrom products bonded with UF is a growing health andenvironmental concern.

Melamine-formaldehyde

Melamine-formaldehyde (MF) resins are used primarilyfor decorative laminates, paper treating, and paper coat-ing. They are typically more expensive than PF resins.MF resins may, despite their high cost, be used in bondingconventional wood-based composites. When used in thisapplication, they typically are blended with UF resins.Melamine-UF resins are used where an inconspicuous(light color) adhesive is needed, and greater water resis-tance that can be attained with UF resin is required.

WOOD-BASED COMPOSITE BOARD 3

Table 2. Commercial Product or Performance Standards for Wood-Based Composites

ProductCategory

Applicable Standard Name of Standard Source

Oriented strandboard (OSB)

PS 2–04 Voluntary product standard PS 2–04performance standard forwood-based structural-use panels

[2]

Particleboard ANSI A208.1–1999 Particleboard standard [3]Fiberboard ANSI A208.2–2002 MDF standard [4]

ANSI A135.4–2004 Basic hardboard [5]ANSI A135.5–2004 Prefinished hardboard paneling [6]ANSI A135.6–2006 Hardboard siding [7]ANSI A194.1 Cellulosic fiberboard [8]

Table 3. Static Bending Properties of Different Wood and Wood-Based Composites

Material Specific Gravity Static Bending Properties

Modulus of Elasticity Modulus of Rupture

GPa (×106 psi) MPa (psi)

Clear woodWhite oak 0.68 12.27 (1.78) 104.80 (15,200)Red maple 0.54 11.31 (1.64) 92.39 (13,400)Douglas-fir (coastal) 0.48 13.44 (1.95) 85.49 (12,400)Western white pine 0.38 10.07 (1.46) 66.88 (9,700)Longleaf pine 0.59 13.65 (1.98) 99.97 (14,500)Panel productsHardboard 0.9–1.0 3.10–5.52 (0.45–0.80) 31.02–56.54 (4,500–8,200)Medium-density fiberboard 0.7–0.9 3.59 (0.52) 35.85 (5,200)Particleboard 0.6–0.8 2.76–4.14 (0.40–0.60) 15.17–24.13 (2,200–3,500)Oriented strand board 0.5–0.8 4.41–6.28 (0.64–0.91) 21.80–34.70 (3,161–5,027)Wood–nonwood compositesWood plastic 0.8–1.1 1.53–4.23 (0.22–0.61) 25.41–52.32 (3,684–7,585)

Isocyanates

Isocyanate as diphenylmethane di-isocyanate (MDI) resinis commonly used as an alternative to PF resin, primarilyin composite products fabricated from strands. Polymericdiphenylmethane di-isocyanate (pMDI) resin, which isclosely related to MDI resin, is also commonly used inthis application. Isocyanate resins are typically morecostly than PF resins, but have more rapid cure rates,and will tolerate higher moisture contents in the woodsource. Isocyanate resin is sometimes used in core layersof strand-based composites, with slower-curing PF resinused in surface layers. Facilities that use MDI are requiredto take special precautionary protective measures, asthe uncured resin can result in chemical sensitizationof persons exposed to it. Cured isocyante resin poses norecognized health concerns.

Bio-Based Adhesives

Bio-based adhesives, primarily protein glues, were widelyused prior to the early 1970s in construction plywood. Inthe mid 1970s, they were supplanted by PF adhesives onthe basis of the superior bond durability provided by phe-nolics. Several soy-protein-based resin systems, with bonddurabilities similar to those provided by PF resins, have

recently been developed and commercialized. Durableadhesive systems may also be derived from tannins orfrom lignin. Tannins are natural phenol compounds thatare present in the bark of a number of tree species. Thetannins can be extracted from bark, modified, and reactedwith formaldehyde to produce an intermediate polymerthat is a satisfactory thermosetting adhesive. Lignin-basedresins have also been developed from spent pulping liquor,which is generated when wood is pulped for paper orchemical feedstocks. Significant research on thermoset-ting resins derived from tannin and pulping liquors wasundertaken in the late 1970s and early 1980s; the impetusfor the research was that the technology was potentiallyviable but implementation depended on the cost of alter-native petrochemicals. The technology that resulted fromthe research did not, however, become, or at least didnot remain, commercially successful. The reason was thatpetroleum prices decreased in the late 1980s, makingpetroleum-derived phenol inexpensive, and thus alterna-tives to it economically unattractive. In the manufactureof wet-process fiberboard, lignin, which is an inherentcomponent of lignocellulosic material, is frequently usedas binder [11], although ‘‘natural’’ lignin bonding may beaugmented with small amounts of PF resin.


ADDITIVES

A number of additives are used in the production of con-ventional composite products. One of the most notableadditives is wax, which is used to provide finished prod-ucts with some resistance to liquid water absorption. Inparticle- and fiberboard products, wax emulsions providelimited-term water resistance and dimensional stabilitywhen the board is wetted. Even small amounts (0.5%–1%)act to retard the rate of liquid water pickup for limitedtime periods. These improved water penetration proper-ties are important for ensuring the success of subsequentsecondary gluing operations and for providing protectionon accidental wetting of the product during and afterconstruction. The water repellency provided by wax haspractically no effect on dimensional changes or wateradsorption of composites exposed to vaporous moistureequilibrium conditions. Other additives used for specialtyproducts include preservatives, mildewcides, fire retar-dants, and impregnating resins such as waxes and oils toimpart some water resistance. They are more thoroughlydiscussed in the section titled ‘‘Specialty Composites’’.

ORIENTED STRAND BOARD

OSB is an engineered structural-use panel manufacturedfrom thin wood strands bonded together with waterproofresin, typically PF or MDI. Since its debut in 1978,OSB has been rapidly accepted in new residential con-struction in many areas of North America. It is usedextensively for roof, wall, and floor sheathing in residentialand commercial construction. The wood strands typicallyhave an aspect ratio (strand length divided by width) ofat least 3. OSB panels are usually made of three layers

of strands, the outer faces having longer strands alignedin the long-direction of the panel and a core layer that iscounter-aligned or laid randomly using the smaller strandsor fines. The orientation of different layers of alignedstrands gives OSB its unique characteristics, includinggreater bending strength and stiffness in the oriented oraligned direction. Control of strand size, orientation, andlayered construction allows OSB to be engineered to suitdifferent uses.

OSB technology and the raw material used originallyevolved from waferboard technology for which aspen wasthe predominant wood species used. As the industrylearned to control strand size, placement, and orientation,the performance and utility of OSB products improved tothe point that they could perform similar to structuralplywood. As a result, product acceptance and the industryexpanded as OSB began to replace softwood plywood inconstruction applications.

Raw Materials

In North America, aspen is the predominant wood used forOSB. Other species than aspen, such as Southern Pine,spruce, birch, yellow-poplar, sweetgum, sassafrass, andbeech are also suitable raw materials for OSB production.High-density species such as beech and birch are oftenmixed with low-density species such as aspen to maintainpanel properties [12].

Manufacturing Process

To manufacture OSB, debarked logs are sliced into long,thin wood elements called strands. The strands are dried,blended with resin and wax, and formed into thick,loosely consolidated mats that are pressed under heatand pressure into large panels. Figure 3 shows an OSB

Figure 3. Schematic of OSB manufacturing process. Source: Courtesy of TECO, Sun Prairie, Wisconsin. Used with permission.


manufacturing process. A more detailed description ofeach individual manufacturing step is as follows.

During stranding, logs are debarked and then sent to asoaking pond or directly to the stranding process. Long logdisk or ring stranders are commonly used to produce woodstrands typically measuring 114–152 mm (4.5–6 in.) long,12.7 mm (0.5 in.) wide, and 0.6–0.7 mm (0.023–0.027 in.)thick. Green strands are stored in wet bins and dried ina traditional triple-pass dryer, a single-pass dryer, a com-bination triple-pass/single-pass dryer, or a three-sectionconveyor dryer. A recent development is a continuouschain dryer, in which the strands are laid on a chain matthat is mated with an upper chain mat and the strandsare held in place as they move through the dryer. Theintroduction of new drying techniques allows the use oflonger strands, reduces surface inactivation of strands,and lowers dryer outfeed temperatures. Dried strands arescreened and sent to dry bins.

Dried strands are blended with adhesive and waxin a highly controlled operation, with separate rotatingblenders used for face and core strands. Typically, differ-ent resin formulations are used for face and core layers.Face resins may be liquid or powdered phenolics, whereascore resins may be phenolics or isocyanates. Several dif-ferent resin application systems are used; spinning diskresin applicators are frequently used.

The strands with adhesive applied are sent to matformers. Mat formers take on a number of configura-tions, ranging from electrostatic equipment to mechanicaldevices containing spinning disks to align strands alongthe panel’s length and star-type cross-orienters to positionstrands across the panel’s width. All formers use the longand narrow characteristic of the strand to place it betweenthe spinning disks or troughs before it is ejected onto amoving screen or conveyor belt below the forming heads.Oriented layers of strands within the mat are droppedsequentially onto a moving conveyor. The conveyor carriesthe mat into the press.

Once the mat is formed, it is hot-pressed. In hot-pressing, the loose layered mat of oriented strands iscompressed under heat and pressure to cure the resin.As many as 16 3.7- by 7.3-m (12- by 24-ft) panels maybe formed simultaneously in a multiple-opening press.A more recent development is the continuous press forOSB. The press compacts and consolidates the orientedand layered mat of strands and heats it to 177–204◦C(350–400◦F) to cure the resin in 3–5 min.

OSB Grade Marks and Product Certification

OSB that has been grade marked is produced to complywith voluntary industry product performance standards.These inspection or certification programs also generallyrequire that the quality control system of a productionplant meets specified criteria. OSB panels conforming tothese product performance standards are marked withgrade stamps.

PARTICLEBOARD

The particleboard industry initially used cut flakes as araw material. However, economic concerns prompted the

development of the ability to use sawdust, planer shav-ings, and to a lesser extent, the use of mill residues andother relatively homogeneous waste materials producedby other wood industries. Particleboard is produced bymechanically reducing the wood raw material into smallparticles, applying adhesive to the particles, and consoli-dating a loose mat of the particles with heat and pressureinto a panel product.

Particleboard is typically made of three layers. Butunlike OSB, the faces of particleboard usually consist offine wood particles while the core is made of coarser par-ticles. The result is a smoother surface for laminating,overlaying, painting, or veneering. Particleboard is read-ily made from virtually any wood material and from avariety of agricultural residues. Low-density insulating orsound-absorbing particleboard can be made from kenafcore or jute stick. Low-, medium-, and high-density panelscan be produced with cereal straw, which has begun tobe used in North America. Rice husks are commerciallymanufactured into medium- and high-density products inthe Middle East.

All other things being equal, reducing lignocellulosicmaterials to particles requires less energy than reducingthe same material into fibers. However, particleboard isgenerally not as strong as fiberboard because the fibrousnature of lignocellulosics, that is, their high aspect ratio,is not exploited as well. Particleboard is widely used infurniture, where it is typically overlaid with other materi-als for decorative purposes. It is the predominant materialused in ready-to-assemble furniture. Particleboard canalso be used in flooring systems, in manufactured houses,for stair treads, and as underlayment. Thin panels canalso be used as a paneling substrate. Since most applica-tions are interior, particleboard is usually bonded with aUF resin, although PF and MF resins are sometimes usedfor applications requiring more moisture resistance.

Manufacturing Process

All particleboards are currently made using a dry process,where air or mechanical formers are used to distributethe particles prior to pressing. The various stepsinvolved in particleboard manufacturing include particlepreparation, particle classification and drying, adhesiveapplication, mat formation, pressing, and finishing.

Standard particleboard plants based on particulatematerial use combinations of hogs, chippers, hammer-mills, ring flakers, ring mills, and attrition mills. To obtainparticleboards with good strength, smooth surfaces, andequal swelling, manufacturers ideally use a homogeneousraw material.

Particles are classified and separated to minimize neg-ative effect on the finished product. Very small particles(fines) increase particle surface area and thus increaseresin requirements. Oversized particles can adverselyaffect the quality of the final product because of internalflaws in the particles. While some particles are classifiedthrough the use of air streams, screen classification meth-ods are the most common. In screen classification, theparticles are fed over a vibrating flat screen or a seriesof screens. The screens may be wire cloth, plates with


holes or slots, or plates set on edge. Particles are conveyedby mechanical means or by air. The choice of conveyingmethod depends on the size of the particles. In air convey-ing, care should be taken that the material does not passthrough many fans, which reduces the size of the particles.In some types of flakes, damp conditions are maintainedto reduce break-up of particles during conveying.

Desirable particles have a high degree of slenderness(long, thin particles), no oversize particles, no splinters,and no dust. Depending on the manufacturing process,the specifications for the ideal particle size are different.For a graduated board, wider tolerances are acceptable.For a three-layer board, the core particles are longer andsurface particles shorter, thinner, and smaller. For a five-or multilayer board, the furnish for the intermediate layerbetween the surface and core has long and thin particles forbuilding a good carrier for the fine surface and to give theboards high bending strength and stiffness. Particleboardto be used for quality furniture uses much smaller coreparticles. The tighter core gives a better quality edgewhich allows particleboard to compete more favorablywith median density fiberboard.

The raw materials (or furnish) for these products donot usually arrive at the plant at a low enough moisturecontent for immediate use. Furnish that arrives at theplant can range from 10% to 200% dry basis moisture con-tent. For use with liquid resins, for example, the furnishmust be reduced to about 2%–7% moisture content. Themoisture content of particles is critical during hot-pressingoperations and depends on whether resin is to be addeddry or in the form of a solution or emulsion. The moisturecontent of materials leaving the dryers is usually in therange of 4%–8%. The main methods used to dry parti-cles are rotary, disk, and suspension drying. A triple-passrotary dryer consists of a large horizontal rotating drumthat is heated by either steam or direct heat. Operat-ing temperatures depend on the moisture content of theincoming furnish. The drum is set at a slight angle, andmaterial is fed into the high end and discharged at the lowend. A series of flights forces the furnish to flow from oneend to the other three times before being discharged. Therotary movement of the drum moves the material frominput to output.

Frequently used resins for particleboard include UFand, to a much lesser extent, PF, melamine-formaldehyde,and isocyanates. The type and amount of resin used forparticleboard depend on the type of product desired. On thebasis of the weight of dry resin solids and ovendry weightof the particles, the resin content can range between 4%and 10%, but usually ranges between 6% and 9% forUF resins. The resin content of the outer face layers isusually slightly higher than that of the core layer. UFresin is usually introduced in water solutions containingabout 50%–65% solids. Besides resin, wax is added toimprove short-term moisture resistance. The amount ofwax ranges from 0.3% to 1% based on the ovendry weightof the particles.

After the particles have been prepared, they are laidinto an even and consistent mat to be pressed into a panel.This is accomplished in batch mode or usually by contin-uous formation. The batch system traditionally employs a

caul or tray on which a deckle frame is placed. The matis formed by the back-and-forth movement of a tray orhopper feeder. The mat is usually cold pressed to reducemat thickness prior to hot pressing. The production ofthree-layer boards requires three or more forming sta-tions. The two outer layers consist of particles that differin geometry from those in the core. The resin content ofthe outer layers is usually higher (about 8%–15%) thanthat of the core (about 4%–8%).

In continuous mat-forming systems, the particles aredistributed in one or several layers on traveling caulsor on a moving belt. Mat thickness is controlled volu-metrically. The two outer face layers usually consist ofparticles that differ in geometry from those in the core.Continuous-formed mats are often pre-pressed, with eithera single-opening platen or a continuous press. Pre-pressingreduces mat height and helps to consolidate the mat forpressing.

After pre-pressing, the mats are hot-pressed into pan-els. Presses can be divided into platen and continuoustypes. Further development in the industry has madepossible the construction of presses for producing increas-ingly larger panel sizes in both single- and multi-openingpresses. Both of these types of presses can be as wide as3.7 m (12 ft). Multi-opening presses can be as long as 10 m(33 ft) and single-opening presses up to 30.5 m (100 ft) long.

Alternatively, a few particleboards are made by theextrusion process. In this system, formation and pressingoccur in one operation. The particles are forced into along, heated die (made of two sets of platens) by means ofreciprocating pistons. The board is extruded between theplatens. The particles are oriented in a plane perpendicu-lar to the plane of the board, resulting in properties thatdiffer from those obtained with flat pressing.

After pressing, panels are trimmed to obtain the desiredlength and width and to square the edges. Trim lossesusually amount to 0.5%–8%, depending on the size of thepanel, the process employed, and the control exercised.Trimmers usually consist of saws with tungsten carbidetips. After trimming, the panels are sanded or planedprior to packaging and shipping. Particleboards may alsobe veneered or overlaid with other materials to provide adecorative surface, or they may be finished with lacqueror paint. Treatments with fire-resistant chemicals are alsoavailable.

Particleboard Grade Marks and Product Certification

Particleboard that has been grade marked ensures that theproduct has been periodically tested for compliance withvoluntary industry product performance standards. Theseinspection or certification programs also generally requirethat the quality control system of a production plant meetsstrict criteria. Particleboard panels conforming to theseproduct performance standards (i.e., ANSI A208.1–1999)are marked with grade stamps.

FIBERBOARD

The term fiberboard includes hardboard, MDF, and insu-lation board. Several things differentiate fiberboard from


particleboard, most notably the physical configuration ofthe wood element. Because wood is fibrous by nature, fiber-board exploits the inherent strength of wood to a greaterextent than does particleboard.

To make fibers for composites, bonds between the woodfibers must be broken. Attrition milling, or refining, is theeasiest way to accomplish this. During refining process,material is fed between two disks with radial grooves. Asthe material is forced through the preset gap between thedisks, it is sheared, cut, and abraded into fibers and fiberbundles. Grain has been ground in this way for centuries.Refiners are available with single- or double-rotatingdisks, as well as steam-pressurized and un-pressurizedconfigurations.

Refining can be augmented by steaming or chemi-cal treatments. Steaming the lignocellulosic weakens thelignin bonds between the cellulosic fibers. As a result,fibers are more readily separated and usually are lessdamaged than fibers processed by dry processing meth-ods. Chemical treatments, usually alkali, are also usedto weaken the lignin bonds. All of these treatments helpincrease fiber quality and reduce energy requirements, butthey may reduce fiber yield and modify the fiber chemistryas well. For MDF, steam-pressurized refining is typical.

Fiberboard is normally classified by density and canbe made by either dry or wet processes. Dry processesare applicable to boards with high- (hardboard) andmedium-density fiberboard. Wet processes are applicableto both high-density hardboard and low-density insula-tion board. The following sections briefly describe themanufacturing of high- and medium-density dry-processfiberboard, wet-process hardboard, and wet-processlow-density insulation board. Suchsland and Woodson[11] and Maloney [13] provide more detailed information.

Dry-Process Fiberboard

Dry-process fiberboard is made in a manner similar toparticleboard. Resin (UF or melamine-UF) and otheradditives may be applied to the fibers by sprayingin short-retention blenders or introduced, as the wetfibers are fed from the refiner into a blow-line dryer.Alternatively, some fiberboard plants add the resin in therefiner. The adhesive-coated fibers are then air-laid intoa mat for subsequent pressing, much the same as matformation for particleboard.

Pressing procedures for dry-process fiberboard differsomewhat from particleboard procedures. After the fibermat is formed, it is typically pre-pressed in a band press.The densified mat is then trimmed by disk cutters andtransferred to caul plates for the hardboard pressing oper-ation; for MDF, the trimmed mat is transferred directlyto the press. Many dry-formed boards are pressed inmulti-opening presses. Continuous pressing using large,high-pressure band presses is also gaining in popularity.Panel density is a basic property and an indicator of panelquality. Since density is greatly influenced by moisturecontent, this is constantly monitored by moisture sensorsusing infrared light.

ANSI A208.2 classifies MDF by physical and mechan-ical properties, and identifies dimensional tolerances andformaldehyde emission limits [4].

Wet-Process Hardboard

Wet-process hardboards differ from dry-process fiber-boards in several significant ways. First, water is usedas the distribution medium for forming the fibers intoa mat. The technology is really an extension of papermanufacturing technology. Secondly, some wet-processboards are made without additional binders. If thelignocellulosic contains sufficient lignin and if lignin isretained during the refining operation, lignin can serve asthe binder. Under heat and pressure, lignin will flow andact as a thermosetting adhesive, enhancing the naturallyoccurring hydrogen bonds.

Refining is an important step for developing strength inwet-process hardboards. The refining operation must alsoyield a fiber of high ‘‘freeness;’’ that is, it must be easy toremove water from the fibrous mat. The mat is typicallyformed on a Fourdrinier wire, like papermaking, or oncylinder formers. The wet process employs a continuouslytraveling mesh screen, onto which the soupy pulp flowsrapidly and smoothly. Water is drawn off through thescreen and then through a series of press rolls, which usea wringing action to remove additional water.

Wet-process hardboards are pressed in multi-openingpresses heated by steam. The press cycle consists of threephases and lasts 6 to 15 min. The first phase is conductedat high pressure, and it removes most of the water whilebringing the board to the desired thickness. The primarypurpose of the second phase is to remove water vapor.The final phase is relatively short and results in the finalcure. A maximum pressure of about 5 MPa (725 lb/in2) isused in all three phases. Heat is essential during pressingto induce fiber-to-fiber bond. A high temperature of up to210◦C (410◦F) is used to increase production by causingfaster evaporation of the water. Lack of sufficient moistureremoval during pressing adversely affects strength andmay result in ‘‘springback’’ or blistering.

Wet-formed composite technology has lost marketshare compared to dry-formed technology over the lastfew decades because of processing speed and perceivedenvironmental issues related to process water. However,wet-formed technology does offer unique opportunities forforming geometric shapes that yield enhanced structuralperformance and decrease weight, elimination of fiberdrying prior to forming, and reduced need for adhesiveresins. It also greatly increases the ability to use recycledpaper and some other woody fibers. Recent advancesin process wastewater recycling and remediation alsobode well for wet-formed technologies. Wet-formedcomposites may soon experience a renaissance and againbecome a significant technology because of its reducedenergy-demands, increased composite structural perfor-mance and decreased weight, and the virtual eliminationof (or drastic reduction in) process water concerns.

Posttreatment of Wet- and Dry-Process Hardboard

Several treatments are used to increase the dimensionalstability and mechanical performance of hardboard. Heattreatment, tempering, and humidification may be donesingularly or in conjunction with one another.


Heat treatment—exposure of pressed fiberboard todry heat—improves dimensional stability and mechani-cal properties, reduces water adsorption, and improvesinterfiber bonding.

Tempering is the heat treatment of pressed boards,preceded by the addition of oil. Tempering improves boardsurface hardness and is sometimes done on various typesof wet-formed hardboards. It also improves resistance toabrasion, scratching, scarring, and water. The most com-mon oils used include linseed oil, tung oil, and tall oil.

Humidification is the addition of moisture to bring theboard moisture content to levels roughly equivalent tothose anticipated in its end-use environment. Initially, apressed board has almost no moisture content. When theboard is exposed to air, it expands linearly by taking on3% to 7% moisture. Continuous or progressive humidifiersare commonly used for this purpose. Air of high humidityis forced through the stacks where it provides water vaporto the boards. Another method involves spraying water onthe back side of the board.

Several techniques are used to finish fiberboard: trim-ming, sanding, surface treatment, punching, and emboss-ing. Trimming consists of reducing products into standardsizes and shapes. Generally, double-saw trimmers are usedto saw the panels. Trimmers consist of overhead-mountedsaws or multiple saw drives. Trimmed panels are stackedin piles for future processing. If thickness tolerance is criti-cal, hardboard is sanded prior to finishing. S1S (smooth onone side) panels require this process. Sanding reducesthickness variation and improves surface paintability.Single-head, wide-belt sanders are used with 24- to 36-gritabrasive. Surface treatments improve the appearance andperformance of boards. Panels are cleaned by sprayingwith water and then dried at about 240◦C (464◦F) for30 seconds. Panel surfaces are then modified with paperoverlay, paint, or stain or are printed directly on thepanel. Punching changes panels into the perforated sheetsused as peg board. Embossing consists of pressing theunconsolidated mat of fibers with a textured form. Thisprocess results in a slightly contoured panel surface thatcan enhance the resemblance of the panel to that of sawnor weathered wood, brick, and other materials.

Cellulosic Board

Cellulosic boards are low-density, wet-laid panel productsused for insulation, sound deadening, carpet underlay-ment, and similar applications. In the manufacture ofcellulosic board, the need for refining and screening isa function of the raw material available, the equipmentused, and the desired end-product. Cellulosic boards typi-cally do not use a binder, and they rely on hydrogen bondsto hold the board components together. Sizing agents areusually added to the furnish (about 1%) to provide thefinished board with a modest degree of water resistanceand dimensional stability.

Like the manufacture of wet-process hardboard, cellu-losic board manufacture is a modification of papermaking.A thick fibrous sheet is made from a low-consistency pulpsuspension in a process known as wet felting. Felting canbe accomplished through use of a deckle box, Fourdrinier

screen, or cylinder screen. A deckle box is a bottomlessframe that is placed over a screen. A measured amountof stock is put in the box to form one sheet; vacuum isthen applied to remove most of the water. The use ofFourdrinier screen for felting is similar to that for paper-making, except that line speeds are reduced to 8–18 m/min(25–60 ft/min).

Cellulosic board formed in a deckle box is usuallycold-pressed to remove most of the free water after themat is formed. Compression rollers on the Fourdriniermachines squeeze out some of the free water. The wetmats are then dried to the final moisture content. Dryersmay be a continuous tunnel or a multi-deck arrangement.The board is generally dried in stages at temperaturesranging from 120 to 190◦C (248–374◦F). Typically, about2 to 4 h are required to reduce moisture content to about1%–3%.

After drying, some boards are treated for various appli-cations. Boards may be given tongue-and-groove or shiplapedges or can be grooved to produce a plank effect. Otherboards are laminated by means of asphalt to produce roofinsulation.

Cellulosic fiberboard products include sound-deadeningboard, roof insulation boards, structural and nonstructuralsheathings, backer board, and roof decking in variousthicknesses. A grade mark stamp will be given for thesecellulosic fiberboard products conforming to ASTM C208[14].

SPECIALTY COMPOSITES

Special-purpose composite materials are produced toobtain enhanced performance properties like waterresistance, mechanical strength, acidity control, and fire,decay and insect resistance. Overlays and veneers canalso be added to enhance both structural properties andappearance.

Moisture-Resistant Composites

Sizing agents can be used to make composites resistant tomoisture. The common size agents include rosin, wax, andasphalt. Sizing agents cover the surface of fibers, reducesurface energy, and render the fibers relatively hydropho-bic. Sizing agents can be applied in two ways. In the firstmethod, water is used as a medium to ensure thoroughmixing of sizing and fiber. The sizing is precipitated fromthe water and is fixed to the fiber surface. In the secondmethod, the sizing is applied directly to the fibers.

Flame-Retardant Composites

Two general application methods are available for improv-ing the fire performance of composites with fire-retardantchemicals. One method consists of pressure impregnat-ing the wood with waterborne or organic solvent-bornefire-retardant chemicals [15]. The second method consistsof applying fire-retardant chemical coatings to the woodsurface. The pressure impregnation method is usuallymore effective and longer lasting; however, this tech-nique is standardized only for plywood. It is not gener-ally used with structural flake/particle/fiber composites


as it can cause swelling that permanently damages thewood–adhesive bonds in the flake/particle/fiber compos-ite and results in the degradation of some physical andmechanical properties of the composite. For wood in exist-ing constructions, surface application of fire-retardantpaints or other finishes offers a possible method to reduceflame spread.

Preservative-Treated Composites

Composites can be protected from the attack of decayfungi and harmful insects by applying selected chemicalsas wood preservatives. The degree of protection obtaineddepends on the kind of preservative used and the ability toachieve proper penetration and retention of the chemicals.Wood preservatives can be applied using pressure ornon-pressure processes [16]. As in the application offire-retardant chemicals, the pressurized application ofwood preservatives is generally performed after manufac-ture and is standardized for plywood. Post-manufacturepressure treatments are not standardized for all types offlake/particle/fiber composite as it can sometimes causedamage to wood–adhesive bonds that in turn reducesphysical and mechanical properties of the composite.Preservatives can be added in the composite manufac-turing process, but the preservative must be resistant tovaporization during hot pressing. Proprietary flakeboardand fiberboard products with incorporated non-volativepreservatives have been commercialized. Common preser-vative treatments include ammoniacal copper quat (ACQ),copper azol (CA), and boron compounds.

WOOD-NON-WOOD COMPOSITES

Wood may be combined with inorganic materials andwith plastics to produce composite products with uniqueproperties. Wood-non-wood composites typically containcomminuted wood elements suspended in a matrix mate-rial (for example in fiber-reinforced gypsum board, or inthermoplastic material), in which the proportion of woodelements may account for appreciably less than 50% ofproduct mass.

Composites made from wood and other materials createenormous opportunities to match product performance toend-use requirements. The following discussion includesthe most common type of wood-non-wood composites: inor-ganic bonded and wood-thermoplastic composites.

Inorganic–Bonded Composite Materials

Inorganic-bonded wood composites have a long and variedhistory that started with commercial production in Austriain 1914. They are now used in many countries in the world,mostly in panel form. A plethora of building materials canbe made using inorganic binders and lignocellulosics, andthey run the normal gamut of panel products, siding,roofing tiles, and precast building members.

Inorganic-bonded wood composites are molded productsor boards that contain between 10% and 70% by weightwood particles or fibers and conversely 90% to 30% inor-ganic binder. Acceptable properties of an inorganic-bonded

wood composite can be obtained only when the woodparticles are fully encased within the binder to make acoherent material. This differs considerably from the tech-nique used to manufacture thermosetting-resin-bondedboards where flakes or particles are ‘‘spot welded’’ by abinder applied as a finely distributed spray or powder.Because of this difference and because hardened inorganicbinders have a higher density than that of most thermoset-ting resins, the required amount of inorganic binder perunit volume of composite material is much higher thanthat of resin-bonded wood composites. The properties ofinorganic-bonded wood composites are significantly influ-enced by the amount and nature of the inorganic binderand the woody material as well as the density of thecomposites.

Inorganic-bonded composites are made by blending pro-portionate amounts of lignocellulosic fiber (or delignifiedfiber derived from wood) with inorganic materials in thepresence of water and allowing the inorganic material tocure or ‘‘set up’’ to make a rigid composite. A unique featureof inorganic-bonded composites is that their manufactureis adaptable to either end of the cost and technology spec-trum. This is facilitated by the fact that no heat is requiredto cure the inorganic material. This versatility of manu-facture makes inorganic-bonded composites ideally suitedto a variety of lignocellulosic materials.

Inorganic binders fall into two main categories:gypsum-bonded and cement-bonded. Magnesia andPortland cement are the most common cement binders.Gypsum and magnesia cement are sensitive to moisture,and their use is generally restricted to interior applica-tions. Composites bonded with Portland cement are moredurable than those bonded with gypsum or magnesiacement and are used in interior and exterior applications.Some inorganic-bonded composites are very resistantto deterioration by decay fungi and insects. Most haveappreciable fire resistance.

Gypsum-Bonded Composites. Paper-faced gypsumboards have been widely used since the 1950’s for theinterior lining of walls and ceilings, where they havegenerically been called drywall because they commonlyreplace wet plaster systems. These panels are critical forgood fire ratings in walls and ceilings. Paper-faced gypsumboards (and glass fiber-faced gypsum panels), also finduse as exterior wall sheathing. Gypsum sheathing panelsare primarily used in commercial construction, usuallyover steel studding and are distinguished from regulargypsum wallboard by their water repellent additives inthe paper facings and gypsum core. The facings of drywalland of gypsum sheathing panels are adhered to thegypsum core, providing the panels with impact resistance,and bending strength and stiffness. The paper facings ofgypsum panels are derived from recycled paper fiber.

An alternative to use of adhered facings is to incorpo-rate lignocellulosic fiber (typically recycled paper fiber) inthe gypsum core to make what are termed fiber-reinforcedgypsum panels. In the production process, a paste of gyp-sum and water is mixed with the recycled paper fiberand extruded into a panel (formed on a belt), withoutfacings. Shortly after formation, the panel is dried in an


oven. Bonding occurs between the gypsum and the fiber ashydrate crystals form.

Fiber-reinforced gypsum panels are typically strongerand more resistant to abrasion and indentation thanpaper-faced drywall panels, and also have a moderatefastener-holding capability. They are marketed for useas interior finish panels (drywall). Additives can pro-vide a moderate degree of water resistance, for use assheathing panels, floor underlayment, roof underlaymentor tile-backer board.

Cement-Bonded Composites. The properties of cement-bonded composites are influenced by wood element char-acteristics (species, size, geometry, chemical composition),cement type, wood-water-cement ratio, environmentaltemperature, and cure time [17]. They are heavier thanconventional wood-based composites, but lighter than con-crete. Therefore they can replace concrete in construction,specifically in applications that are not subjected to loads.Wood-cement composites provide an option for using woodresides, or even agricultural residues. However speciesselection can be important as many species contain sugarsand extractives that retard the cure of cement [12].

Fewer boards bonded with magnesia cement have beenproduced than cement- or gypsum-bonded panels, mainlybecause of price. One successful application of magnesiacement is a low-density panel made for interior ceilingand wall applications. In the production of this panelproduct, wood wool (excelsior) is laid out in a low-densitymat. The mat is then sprayed with an aqueous solu-tion of magnesia cement, pressed, and cut into panels.Other processes have been suggested for manufactur-ing magnesia-cement-bonded composites. One applicationmay be to spray a slurry of magnesia cement, water, andlignocellulosic fiber onto existing structures as fireproof-ing. Extrusion into a pipe-type profile or other profiles isalso possible.

The most widely used inorganic-bonded composites arethose bonded with Portland cement. Portland cement,when combined with water, immediately reacts in aprocess called hydration to eventually solidify into asolid stone-like mass. Successfully marketed Portland-cement-bonded composites consist of both low-densityproducts made with excelsior and high-density productsmade with particles and fibers.

Low-density products may be used as interior ceilingand wall panels in commercial buildings. In addition to theadvantages described for low-density magnesia-bondedcomposites, low-density composites bonded with Portlandcement offer sound control and can be quite decorative.In some parts of the world, these panels function ascomplete wall and roof decking systems. The exteriorof the panels is coated with stucco, and the interior isplastered. High-density panels can be used as flooring,roof sheathing, fire doors, load-bearing walls, and cementforms. Fairly complex molded shapes can be molded orextruded, such as decorative roofing tiles or non-pressurepipes.

The largest volume of cement-bonded wood-basedcomposite materials manufactured in North America is

fiber-cement siding. Fiber-cement siding incorporatesdelignified wood fiber into the portland cement matrix.

Ceramic-Bonded Composites. In the recent years a newclass of inorganic binders, non-sintered ceramic inorganicbinders, has been developed. These non-sintered ceramicbinders are formed by acid–base aqueous reaction betweena divalent or trivalent oxide and an acid phosphate orphosphoric acid. The reaction slurry hardens rapidly, butthe rate of setting can be controlled. With suitable selectionof oxides and acid-phosphates, a range of binders may beproduced. Recent research suggests that phosphates maybe used as adhesives, cements, or surface augmentationmaterials to manufacture wood-based composites [18].

Wood–Thermoplastic Composite Materials

Wood-thermoplastic composites have become a widely rec-ognized commercial product in construction, automotive,furniture, and other consumer applications in the lastdecade [19]. Commercialization has been primarily due topenetration into the construction industry, first as deck-ing and window profiles, followed by railing, siding, androofing. Interior molding applications are also receivingattention. The automotive industry has been a leader inusing wood-thermoplastic composites for interior panelparts, and is leading the way in furniture applications.

The class of materials can include lignocellulosicsderived from wood or other natural sources and differentthermoplastics including virgin or recycled polypropylene,polystyrene, vinyls, and polyethylenes. Other materialscan be added to affect processing and product performanceof wood–thermoplastic composites. These additivescan improve bonding between the thermoplastic andwood component (for example, coupling agents), productperformance (impact modifiers, UV stabilizers, flameretardants), and processability (lubricants).

The manufacture of thermoplastic composites is usu-ally a two-step process. The raw materials are first mixedtogether, and the composite blend is then formed into aproduct. The combination of these steps is called in-lineprocessing, and the result is a single processing step thatconverts raw materials to end products. In-line processingcan be very difficult because of control demands and pro-cessing trade-offs. As a result, it is often easier and moreeconomical to separate the processing steps [20].

There are two main types of wood-thermoplastic com-posites. In the first, the lignocellulosic component servesas a reinforcing agent or filler in a continuous thermo-plastic matrix. In the second, the thermoplastic serves asa binder to the majority lignocellulosic component. Thepresence or absence of a continuous thermoplastic matrixmay also determine the processability of the compositematerial.

In composites with high thermoplastic content, thethermoplastic component is in a continuous matrix andthe lignocellulosic component serves as a reinforcementor filler. The lignocellulosic content is typically less than60% by weight. In the great majority of reinforced ther-moplastic composites available commercially, inorganicmaterials (for example, glass, clays, and minerals) are


used as reinforcements or fillers. Lignocellulosic materi-als offer some advantages over inorganic materials; theyare lighter, much less abrasive, and renewable. Lignocel-lulosics serve to reinforce the thermoplastic by stiffeningand strengthening, and can improve thermal stability ofthe product compared with that of unfilled material.

In composites with low thermoplastic content, the ther-moplastic component is not continuous, acting more as abinder for the fiber much the same way as a thermoset-ting resin rather than a matrix material. Thermoplasticcontent is typically less than 30% by weight. In theirsimplest form, lignocellulosic particles or fibers can bedry-blended with thermoplastic granules, flakes, or fibersand pressed into panel products. An alternative is to usethe thermoplastic in the form of a textile fiber. The thermo-plastic textile fiber enables a variety of lignocellulosics tobe incorporated into a low-density, non-woven, textile-likemat. The mat may be a product in itself, or it may beconsolidated into a high-density product.

REFERENCES

1. Maloney TM. Terminology and products definitions—A sug-gested approach to uniformity worldwide. In: Proceedings,18th international union of forest research organization worldcongress; 1986 September; Ljubljana, Yugoslavia. Yugoslavia(LJU): IUFRO World Congress Organizing Committee; 1986.

2. NIST. Voluntary product standard PS 2–04. Performancestandard for wood-based structural-use panels. NationalInstitute of Standards and Technology. Gaithersburg (MD):United States Department of Commerce; 2004.

3. CPA. Particleboard, ANSI A208.1–1999. Gaithersburg (MD):Composite Panel Association; 1999.

4. CPA. Medium density fiberboard (MDF), ANSI A208.2–2002.Gaithersburg (MD): Composite Panel Association; 2002.

5. CPA. Basic hardboard, ANSI A135.4–2004. Gaithersburg(MD): Composite Panel Association; 2004a.

6. CPA. Prefinished hardboard paneling, ANSI A135.5–2004.Gaithersburg (MD): Composite Panel Association; 2004b.

7. CPA. Hardboard siding, ANSI A135.6–2006. Gaithersburg(MD): Composite Panel Association; 2006.

8. AHA. Cellulosic fiberboard, ANSI/AHA A194.1–1985. Pala-tine (IL): American Hardboard Association; 1985.

9. Cai Z. Selected properties of MDF and flakeboard overlaidwith fiberglass mats. Forest Prod J 2006;56(11,12): 142–146.

10. Forest Product Laboratory. Wood handbook—Wood as anengineering material. Chapters 11, 12. General TechnicalReport FPL-GTR-190. Madison (WI): USDA Forest Service,Forest Products Laboratory; 2010.

11. Suchsland O, Woodson GE. Fiberboard manufacturing prac-tices in the United States, Agriculture Handbook 640. Wash-ington (DC): U. S. Department of Agriculture; 1986.

12. Bowyer JL, Shmulsky R, Haygreen JG. Forest productsand wood science. 5th ed. Blackwell Publishing Professional:Ames (IA); 2007.p. 558.

13. Maloney TM. Modern particleboard and dry-process fiber-board manufacturing. San Francisco (CA): Miller FreemanPublications; 1993.

14. ASTM 2001. ASTM C208–95(2001). Specification for cellu-losic fiber insulating board.

15. AWPA. Commodity specification H: Fire retardants. AWPAStandard U-1: USS: User standard for treated wood. Birm-ingham (AL): AWPA Book of Standards; 2007a.

16. AWPA. Commodity specification F: Wood composites. AWPAStandard U-1: USS: User standard for treated wood. Birm-ingham (AL): AWPA Book of Standards; 2007b.

17. Jorge FC, Pereira C, Ferreira JMF. Wood-cement composites:a review. Holz Roh Werkst 2004;62,: 370–377.

18. Jeong S Wagh A. Cementing the gap between ceramics,cements and polymers. Mater Technol 2003;18(3): 162–168.

19. Oksman Niska K, Sain M, editors. Wood-polymer Composites.Cambridge England: Woodhead Publishing Limited; 2008.

20. Clemons CM. Wood-Plastic composites in the United States:The interfacing of two industries. Forest Prod J 2002;52(6):10–18.

wood-based composite board - usda forest service · 2 wood-based composite board figure 1. common...

Documents