Wood-Based Composite Board

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WOOD-BASED COMPOSITE BOARD regenerated, intensively managed), the type of adhesive, geometry of the wood elements (fibers, flakes, strands, ZHIYONG CAI particles, veneer, lumber), and density of the final product USDA Forest Service, Madison, [9]. Many of the mechanical properties of wood-based com- WI posites tabulated in this article were originally reported in technical and scientific literature. Consequently, they should not be used for direct computation of design values. INTRODUCTION They do, however, provide excellent baseline information on the properties of wood-based composites. The term wood-based composite is used to describe any There are a wide range of engineering properties that wood material bonded together with adhesives. The basic are used to characterize the performance of wood-based wood elements in the production of wood-based compos- composites. Mechanical properties are frequently used ites can be in a great variety of sizes and geometries: to evaluate wood-based composites for structural and fibers, sawdust, shavings, larger particles composed of nonstructural applications. Static elastic and strength many fibers, flakes, strands (Fig. 1), and veneers. These properties are the primary criteria to select materials or to elements can be used alone or in combination. The choice is establish design/product specifications. Elastic properties almost unlimited. Wood-based composite boards are made include modulus of elasticity (MOE) in bending, tension, from these wood elements in a panel form. and compression. Strength properties usually reported Maloney [1] proposed a logical basis for classifying include modulus of rupture (MOR) in bending, compres- wood composites in a bigger family of the wood composite sion strength parallel to surface, tension strength paral- (Table 1). For purposes of this article, these classifi- lel to surface, tension strength perpendicular to surface cations have been slightly modified from those in the (internal bond strength), shear strength, fastener-holding original version to reflect the latest product develop- capacity, and hardness. ments. This article covers only the subgroup listed under Many of the questions that arise with wood-based ‘‘composite materials’’ and processes used to manufacture composites have to do with their mechanical proper- wood-based composite materials. It describes conventional ties; especially how the properties of one type of material wood-based composite panels and structural composite compared to clear wood and other wood products. While materials intended for general construction and/or interior an extensive review that compares all the properties of use. This article also describes wood–nonwood composites. wood-based materials and products is beyond the scope Conventional wood-based composite products are man- of this article, Table 3 provides some insight into how ufactured primarily from wood with only a few percent the static bending properties of these materials vary, and resin and other additives. Product types can be subcatego- how their properties compare with clear wood. Although rized on the basis of the physical configuration of the wood most wood composites might not have as high mechanical elements used to make these products: veneer, particle, properties as solid wood, they provide very consistent and strand, or fiber (Fig. 2). Morphology of the wood elements uniform performance [10]. influences the properties of composite materials, and can be controlled by selection of the wood raw material and by the processing techniques used to generate the wood WOOD ELEMENTS elements. Composite properties can also be controlled by segregation and stratification of wood elements having Conventional wood-based composites are composed pri- different morphologies in different layers of the composite marily of wood elements (often 90% or more by mass) material. In conventional wood-based composites, prop- bound together with a resin and other additives. Figure 1 erties can also be controlled by the use of adhesives shows the relative size of the common wood elements used with different curing rate in different layers. Varying the in wood-based composites from top, left clockwise: shav- physical configuration of the wood element, adjusting the ings, sawdust, fiber, large particles, flakes, and strands. density profile of the composite, adjusting adhesive resin, Figure 2 shows the various composite products. or adding chemical additives is just a few of the many ways to influence the properties. Wood-based composites ADHESIVES are used for a number of structural and nonstructural applications including panels for exterior, interior, and fur- Commonly used resin or binder systems in wood-based niture uses. Performance standards are in place for many composites include phenol-formaldehyde (PF), urea- conventional wood-based composite products (Table 2). formaldehyde (UF), melamine-formaldehyde, and iso- Generally, wood-based composites provide uniform cyanate. The selection of the resin system is dependent on and predictable in-service performance, largely as a the process, cost, product standards, and applications. consequence of standards used to monitor and control their manufacturing process. The mechanical properties of Phenol-formaldehyde wood composites depend on a variety of factors, including PF resins, commonly referred to as phenolic resins,are wood species, forest management regimes (naturally typically used in the manufacture of construction ply-

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. 1 2 WOOD-BASED COMPOSITE BOARD

Figure 1. Common wood elements used in wood-based com- Figure 2. Examples of various composite products. From clock- posites from top, left clockwise: shavings, sawdust, fiber, large wise from top left: laminated veneer lumber, parallel strand particles, flakes, and strands. lumber, laminated strand lumber, plywood, oriented strand board, particleboard, and fiberboard. Table 1. Classification of Wood-Based Compositesa

Veneer-based material material shortly after emergence from the press is a fairly Plywood common industrial practice, used to attain adequate resin Laminated veneer lumber (LVL) cure without greatly extending press time. Significant Parallel-strand lumber (PSL) heat exposure associated with pressing of phenolic-bonded Laminates composites commonly results in a noticeable reduction in Glue-laminated timbers their hygroscopicity. Cured phenolic resins remain chem- Overlayed materials ically stable at elevated temperatures, even under wet Laminated wood–nonwood compositesb Multi wood composites (COM-PLYc) conditions. The PF resin bonds are sometimes referred to as being boil-proof because of their ability to maintain Composite boards the structural integrity and adequate bonding after boil- Fiberboard (low-, medium- or high-density) ing water test. The inherently darker color of PF resin Hardboard compared with other resins may make them aestheti- Particleboard Waferboard cally unsuitable for product applications such as interior Flakeboard paneling and furniture. Oriented strand board (OSB) Laminated strand lumber (LSL) Urea-formaldehyde Oriented strand lumber (OSL) UF resins are typically used in the manufacture of prod- Edge-adhesive-bonded material ucts used in interior applications, for example, particle- Edge-glued and ripped panels for lumber board and medium-density fiberboard (MDF). They cure Systems at lower temperatures than PF resins. Excessive heat I-beams exposure will result in chemical break-down of cured UF T-beam panels resins. Therefore UF-bonded panels are typically cooled Stress-skin panels after emergence from the press. UF resins are the lowest Trusses-metal tooth connected cost thermosetting adhesive resins. They offer light color, Box-beams Structural insulated panels (SIPS) which often is a requirement in the manufacture of decorative products. However, the release of formaldehyde Wood–nonwood composites from products bonded with UF is a growing health and Wood fiber–polymer composites environmental concern. Inorganic-bonded composites aSource: Adapted from Ref. 1. Melamine-formaldehyde bPanels or shaped materials combined with nonwood materials such as metal, plastic, and fiberglass. Melamine-formaldehyde (MF) resins are used primarily cRegistered trademark of APA—The Engineered Wood Association. for decorative laminates, paper treating, and paper coat- ing. They are typically more expensive than PF resins. wood and oriented strand board (OSB) where exposure MF resins may, despite their high cost, be used in bonding to weather during construction is a concern. Phenolic conventional wood-based composites. When used in this resins are relatively slow-curing compared with other application, they typically are blended with UF resins. thermosetting resins. In hot-pressed wood-based compos- Melamine-UF resins are used where an inconspicuous ites, use of phenolic resin necessitates longer press times (light color) adhesive is needed, and greater water resis- and higher press temperatures. Hot-stacking of pressed 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 Product Applicable Standard Name of Standard Source Category Oriented strand PS 2–04 Voluntary product standard PS 2–04 [2] board (OSB) performance standard for wood-based structural-use panels 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 wood White 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 products Hardboard 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 composites Wood plastic 0.8–1.1 1.53–4.23 (0.22–0.61) 25.41–52.32 (3,684–7,585)

Isocyanates recently been developed and commercialized. Durable adhesive systems may also be derived from tannins or Isocyanate as diphenylmethane di-isocyanate (MDI) resin is commonly used as an alternative to PF resin, primarily from lignin. Tannins are natural phenol compounds that in composite products fabricated from strands. Polymeric are present in the bark of a number of tree species. The diphenylmethane di-isocyanate (pMDI) resin, which is tannins can be extracted from bark, modified, and reacted closely related to MDI resin, is also commonly used in with formaldehyde to produce an intermediate polymer this application. Isocyanate resins are typically more that is a satisfactory thermosetting adhesive. Lignin-based costly than PF resins, but have more rapid cure rates, resins have also been developed from spent pulping liquor, and will tolerate higher moisture contents in the wood which is generated when wood is pulped for paper or source. Isocyanate resin is sometimes used in core layers chemical feedstocks. Significant research on thermoset- of strand-based composites, with slower-curing PF resin ting resins derived from tannin and pulping liquors was used in surface layers. Facilities that use MDI are required undertaken in the late 1970s and early 1980s; the impetus to take special precautionary protective measures, as for the research was that the technology was potentially the uncured resin can result in chemical sensitization viable but implementation depended on the cost of alter- of persons exposed to it. Cured isocyante resin poses no native petrochemicals. The technology that resulted from recognized health concerns. the research did not, however, become, or at least did not remain, commercially successful. The reason was that Bio-Based Adhesives petroleum prices decreased in the late 1980s, making Bio-based adhesives, primarily protein glues, were widely petroleum-derived phenol inexpensive, and thus alterna- used prior to the early 1970s in construction plywood. In tives to it economically unattractive. In the manufacture the mid 1970s, they were supplanted by PF adhesives on of wet-process fiberboard, lignin, which is an inherent the basis of the superior bond durability provided by phe- component of lignocellulosic material, is frequently used nolics. Several soy-protein-based resin systems, with bond as binder [11], although ‘‘natural’’ lignin bonding may be durabilities similar to those provided by PF resins, have augmented with small amounts of PF resin. 4 WOOD-BASED COMPOSITE BOARD

ADDITIVES of strands, the outer faces having longer strands aligned in the long-direction of the panel and a core layer that is A number of additives are used in the production of con- counter-aligned or laid randomly using the smaller strands ventional composite products. One of the most notable or fines. The orientation of different layers of aligned additives is wax, which is used to provide finished prod- strands gives OSB its unique characteristics, including ucts with some resistance to liquid water absorption. In greater bending strength and stiffness in the oriented or particle- and fiberboard products, wax emulsions provide aligned direction. Control of strand size, orientation, and limited-term water resistance and dimensional stability layered construction allows OSB to be engineered to suit when the board is wetted. Even small amounts (0.5%–1%) different uses. act to retard the rate of liquid water pickup for limited OSB technology and the raw material used originally time periods. These improved water penetration proper- evolved from waferboard technology for which aspen was ties are important for ensuring the success of subsequent the predominant wood species used. As the industry secondary gluing operations and for providing protection learned to control strand size, placement, and orientation, on accidental wetting of the product during and after the performance and utility of OSB products improved to construction. The water repellency provided by wax has the point that they could perform similar to structural practically no effect on dimensional changes or water plywood. As a result, product acceptance and the industry adsorption of composites exposed to vaporous moisture expanded as OSB began to replace softwood plywood in equilibrium conditions. Other additives used for specialty construction applications. products include preservatives, mildewcides, fire retardants, and impregnating resins such as waxes and oils to Raw Materials impart some water resistance. They are more thoroughly In North America, aspen is the predominant wood used for discussed in the section titled ‘‘Specialty Composites’’. OSB. Other species than aspen, such as Southern Pine, spruce, birch, yellow-poplar, sweetgum, sassafrass, and beech are also suitable raw materials for OSB production. ORIENTED STRAND BOARD High-density species such as beech and birch are often OSB is an engineered structural-use panel manufactured mixed with low-density species such as aspen to maintain from thin wood strands bonded together with waterproof panel properties [12]. resin, typically PF or MDI. Since its debut in 1978, Manufacturing Process OSB has been rapidly accepted in new residential construction in many areas of North America. It is used To manufacture OSB, debarked logs are sliced into long, extensively for roof, wall, and floor sheathing in residential thin wood elements called strands. The strands are dried, and commercial construction. The wood strands typically blended with resin and wax, and formed into thick, have an aspect ratio (strand length divided by width) of loosely consolidated mats that are pressed under heat at least 3. OSB panels are usually made of three layers and 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. WOOD-BASED COMPOSITE BOARD 5 manufacturing process. A more detailed description of development of the ability to use sawdust, planer shav- each individual manufacturing step is as follows. ings, and to a lesser extent, the use of mill residues and During stranding, logs are debarked and then sent to a other relatively homogeneous waste materials produced soaking pond or directly to the stranding process. Long log by other wood industries. Particleboard is produced by disk or ring stranders are commonly used to produce wood mechanically reducing the wood raw material into small strands typically measuring 114–152 mm (4.5–6 in.) long, particles, applying adhesive to the particles, and consoli- 12.7 mm (0.5 in.) wide, and 0.6–0.7 mm (0.023–0.027 in.) dating a loose mat of the particles with heat and pressure thick. Green strands are stored in wet bins and dried in into a panel product. a traditional triple-pass dryer, a single-pass dryer, a com- Particleboard is typically made of three layers. But bination triple-pass/single-pass dryer, or a three-section unlike OSB, the faces of particleboard usually consist of conveyor dryer. A recent development is a continuous fine wood particles while the core is made of coarser par- chain dryer, in which the strands are laid on a chain mat ticles. The result is a smoother surface for laminating, that is mated with an upper chain mat and the strands overlaying, painting, or veneering. Particleboard is read- are held in place as they move through the dryer. The ily made from virtually any wood material and from a introduction of new drying techniques allows the use of variety of agricultural residues. Low-density insulating or longer strands, reduces surface inactivation of strands, sound-absorbing particleboard can be made from kenaf and lowers dryer outfeed temperatures. Dried strands are core or jute stick. Low-, medium-, and high-density panels screened and sent to dry bins. can be produced with cereal straw, which has begun to Dried strands are blended with adhesive and wax be used in North America. Rice husks are commercially in a highly controlled operation, with separate rotating manufactured into medium- and high-density products in blenders used for face and core strands. Typically, differ- the Middle East. ent resin formulations are used for face and core layers. All other things being equal, reducing lignocellulosic Face resins may be liquid or powdered phenolics, whereas materials to particles requires less energy than reducing core resins may be phenolics or isocyanates. Several dif- the same material into fibers. However, particleboard is ferent resin application systems are used; spinning disk generally not as strong as fiberboard because the fibrous resin applicators are frequently used. nature of lignocellulosics, that is, their high aspect ratio, The strands with adhesive applied are sent to mat is not exploited as well. Particleboard is widely used in formers. Mat formers take on a number of configura- furniture, where it is typically overlaid with other materi- tions, ranging from electrostatic equipment to mechanical als for decorative purposes. It is the predominant material devices containing spinning disks to align strands along used in ready-to-assemble furniture. Particleboard can the panel’s length and star-type cross-orienters to position also be used in flooring systems, in manufactured houses, strands across the panel’s width. All formers use the long for stair treads, and as underlayment. Thin panels can and narrow characteristic of the strand to place it between also be used as a paneling substrate. Since most applica- the spinning disks or troughs before it is ejected onto a tions are interior, particleboard is usually bonded with a moving screen or conveyor belt below the forming heads. UF resin, although PF and MF resins are sometimes used Oriented layers of strands within the mat are dropped for applications requiring more moisture resistance. sequentially onto a moving conveyor. The conveyor carries the mat into the press. Once the mat is formed, it is hot-pressed. In hot- Manufacturing Process pressing, the loose layered mat of oriented strands is All particleboards are currently made using a dry process, compressed under heat and pressure to cure the resin. where air or mechanical formers are used to distribute As many as 16 3.7- by 7.3-m (12- by 24-ft) panels may the particles prior to pressing. The various steps be formed simultaneously in a multiple-opening press. involved in particleboard manufacturing include particle A more recent development is the continuous press for preparation, particle classification and drying, adhesive OSB. The press compacts and consolidates the oriented application, mat formation, pressing, and finishing. ◦ and layered mat of strands and heats it to 177–204 C Standard particleboard plants based on particulate ◦ (350–400 F) to cure the resin in 3–5 min. material use combinations of hogs, chippers, hammer- mills, ring flakers, ring mills, and attrition mills. To obtain OSB Grade Marks and Product Certification particleboards with good strength, smooth surfaces, and OSB that has been grade marked is produced to comply equal swelling, manufacturers ideally use a homogeneous with voluntary industry product performance standards. raw material. These inspection or certification programs also generally Particles are classified and separated to minimize neg- require that the quality control system of a production ative effect on the finished product. Very small particles plant meets specified criteria. OSB panels conforming to (fines) increase particle surface area and thus increase these product performance standards are marked with resin requirements. Oversized particles can adversely grade stamps. affect the quality of the final product because of internal flaws in the particles. While some particles are classified PARTICLEBOARD through the use of air streams, screen classification methods are the most common. In screen classification, the The particleboard industry initially used cut flakes as a particles are fed over a vibrating flat screen or a series raw material. However, economic concerns prompted the of screens. The screens may be wire cloth, plates with 6 WOOD-BASED COMPOSITE BOARD holes or slots, or plates set on edge. Particles are conveyed caul or tray on which a deckle frame is placed. The mat by mechanical means or by air. The choice of conveying is formed by the back-and-forth movement of a tray or method depends on the size of the particles. In air convey- hopper feeder. The mat is usually cold pressed to reduce ing, care should be taken that the material does not pass mat thickness prior to hot pressing. The production of through many fans, which reduces the size of the particles. three-layer boards requires three or more forming sta- In some types of flakes, damp conditions are maintained tions. The two outer layers consist of particles that differ to reduce break-up of particles during conveying. in geometry from those in the core. The resin content of Desirable particles have a high degree of slenderness the outer layers is usually higher (about 8%–15%) than (long, thin particles), no oversize particles, no splinters, that of the core (about 4%–8%). and no dust. Depending on the manufacturing process, In continuous mat-forming systems, the particles are the specifications for the ideal particle size are different. distributed in one or several layers on traveling cauls For a graduated board, wider tolerances are acceptable. or on a moving belt. Mat thickness is controlled volu- For a three-layer board, the core particles are longer and metrically. The two outer face layers usually consist of surface particles shorter, thinner, and smaller. For a five- particles that differ in geometry from those in the core. or multilayer board, the furnish for the intermediate layer Continuous-formed mats are often pre-pressed, with either between the surface and core has long and thin particles for a single-opening platen or a continuous press. Pre-pressing building a good carrier for the fine surface and to give the reduces mat height and helps to consolidate the mat for boards high bending strength and stiffness. Particleboard pressing. to be used for quality furniture uses much smaller core After pre-pressing, the mats are hot-pressed into pan- particles. The tighter core gives a better quality edge els. Presses can be divided into platen and continuous which allows particleboard to compete more favorably types. Further development in the industry has made with median density fiberboard. possible the construction of presses for producing increas- The raw materials (or furnish) for these products do ingly larger panel sizes in both single- and multi-opening not usually arrive at the plant at a low enough moisture presses. Both of these types of presses can be as wide as content for immediate use. Furnish that arrives at the 3.7 m (12 ft). Multi-opening presses can be as long as 10 m plant can range from 10% to 200% dry basis moisture con- (33 ft) and single-opening presses up to 30.5 m (100 ft) long. tent. For use with liquid resins, for example, the furnish Alternatively, a few particleboards are made by the must be reduced to about 2%–7% moisture content. The extrusion process. In this system, formation and pressing moisture content of particles is critical during hot-pressing occur in one operation. The particles are forced into a operations and depends on whether resin is to be added long, heated die (made of two sets of platens) by means of dry or in the form of a solution or emulsion. The moisture reciprocating pistons. The board is extruded between the content of materials leaving the dryers is usually in the platens. The particles are oriented in a plane perpendicu- range of 4%–8%. The main methods used to dry parti- lar to the plane of the board, resulting in properties that cles are rotary, disk, and suspension drying. A triple-pass differ from those obtained with flat pressing. rotary dryer consists of a large horizontal rotating drum After pressing, panels are trimmed to obtain the desired that is heated by either steam or direct heat. Operat- length and width and to square the edges. Trim losses ing temperatures depend on the moisture content of the usually amount to 0.5%–8%, depending on the size of the incoming furnish. The drum is set at a slight angle, and panel, the process employed, and the control exercised. material is fed into the high end and discharged at the low Trimmers usually consist of saws with tungsten carbide end. A series of flights forces the furnish to flow from one tips. After trimming, the panels are sanded or planed end to the other three times before being discharged. The prior to packaging and shipping. Particleboards may also rotary movement of the drum moves the material from be veneered or overlaid with other materials to provide a input to output. decorative surface, or they may be finished with lacquer Frequently used resins for particleboard include UF or paint. Treatments with fire-resistant chemicals are also and, to a much lesser extent, PF, melamine-formaldehyde, available. and isocyanates. The type and amount of resin used for particleboard depend on the type of product desired. On the Particleboard Grade Marks and Product Certification basis of the weight of dry resin solids and ovendry weight Particleboard that has been grade marked ensures that the of the particles, the resin content can range between 4% product has been periodically tested for compliance with and 10%, but usually ranges between 6% and 9% for voluntary industry product performance standards. These UF resins. The resin content of the outer face layers is inspection or certification programs also generally require usually slightly higher than that of the core layer. UF that the quality control system of a production plant meets resin is usually introduced in water solutions containing strict criteria. Particleboard panels conforming to these about 50%–65% solids. Besides resin, wax is added to product performance standards (i.e., ANSI A208.1–1999) improve short-term moisture resistance. The amount of are marked with grade stamps. wax ranges from 0.3% to 1% based on the ovendry weight of the particles. After the particles have been prepared, they are laid FIBERBOARD into an even and consistent mat to be pressed into a panel. This is accomplished in batch mode or usually by contin- The term fiberboard includes hardboard, MDF, and insu- uous formation. The batch system traditionally employs a lation board. Several things differentiate fiberboard from WOOD-BASED COMPOSITE BOARD 7 particleboard, most notably the physical configuration of Wet-Process Hardboard the wood element. Because wood is fibrous by nature, fiber- Wet-process hardboards differ from dry-process fiberboard exploits the inherent strength of wood to a greater boards in several significant ways. First, water is used extent than does particleboard. as the distribution medium for forming the fibers into To make fibers for composites, bonds between the wood a mat. The technology is really an extension of paper fibers must be broken. Attrition milling, or refining, is the manufacturing technology. Secondly, some wet-process easiest way to accomplish this. During refining process, boards are made without additional binders. If the material is fed between two disks with radial grooves. As lignocellulosic contains sufficient lignin and if lignin is the material is forced through the preset gap between the retained during the refining operation, lignin can serve as disks, it is sheared, cut, and abraded into fibers and fiber the binder. Under heat and pressure, lignin will flow and bundles. Grain has been ground in this way for centuries. act as a thermosetting adhesive, enhancing the naturally Refiners are available with single- or double-rotating disks, as well as steam-pressurized and un-pressurized occurring hydrogen bonds. configurations. Refining is an important step for developing strength in Refining can be augmented by steaming or chemi- wet-process hardboards. The refining operation must also cal treatments. Steaming the lignocellulosic weakens the yield a fiber of high ‘‘freeness;’’ that is, it must be easy to lignin bonds between the cellulosic fibers. As a result, remove water from the fibrous mat. The mat is typically fibers are more readily separated and usually are less formed on a Fourdrinier wire, like papermaking, or on damaged than fibers processed by dry processing meth- cylinder formers. The wet process employs a continuously ods. Chemical treatments, usually alkali, are also used traveling mesh screen, onto which the soupy pulp flows to weaken the lignin bonds. All of these treatments help rapidly and smoothly. Water is drawn off through the increase fiber quality and reduce energy requirements, but screen and then through a series of press rolls, which use they may reduce fiber yield and modify the fiber chemistry a wringing action to remove additional water. as well. For MDF, steam-pressurized refining is typical. Wet-process hardboards are pressed in multi-opening Fiberboard is normally classified by density and can presses heated by steam. The press cycle consists of three be made by either dry or wet processes. Dry processes phases and lasts 6 to 15 min. The first phase is conducted are applicable to boards with high- (hardboard) and at high pressure, and it removes most of the water while medium-density fiberboard. Wet processes are applicable bringing the board to the desired thickness. The primary to both high-density hardboard and low-density insula- purpose of the second phase is to remove water vapor. tion board. The following sections briefly describe the The final phase is relatively short and results in the final manufacturing of high- and medium-density dry-process cure. A maximum pressure of about 5 MPa (725 lb/in2)is fiberboard, wet-process hardboard, and wet-process used in all three phases. Heat is essential during pressing low-density insulation board. Suchsland and Woodson to induce fiber-to-fiber bond. A high temperature of up to ◦ ◦ [11] and Maloney [13] provide more detailed information. 210 C (410 F) is used to increase production by causing faster evaporation of the water. Lack of sufficient moisture Dry-Process Fiberboard removal during pressing adversely affects strength and Dry-process fiberboard is made in a manner similar to may result in ‘‘springback’’ or blistering. particleboard. Resin (UF or melamine-UF) and other Wet-formed composite technology has lost market additives may be applied to the fibers by spraying share compared to dry-formed technology over the last in short-retention blenders or introduced, as the wet few decades because of processing speed and perceived fibers are fed from the refiner into a blow-line dryer. environmental issues related to process water. However, Alternatively, some fiberboard plants add the resin in the wet-formed technology does offer unique opportunities for refiner. The adhesive-coated fibers are then air-laid into forming geometric shapes that yield enhanced structural a mat for subsequent pressing, much the same as mat performance and decrease weight, elimination of fiber formation for particleboard. drying prior to forming, and reduced need for adhesive Pressing procedures for dry-process fiberboard differ resins. It also greatly increases the ability to use recycled somewhat from particleboard procedures. After the fiber paper and some other woody fibers. Recent advances mat is formed, it is typically pre-pressed in a band press. in process wastewater recycling and remediation also The densified mat is then trimmed by disk cutters and bode well for wet-formed technologies. Wet-formed transferred to caul plates for the hardboard pressing oper- composites may soon experience a renaissance and again ation; for MDF, the trimmed mat is transferred directly become a significant technology because of its reduced to the press. Many dry-formed boards are pressed in energy-demands, increased composite structural perfor- multi-opening presses. Continuous pressing using large, mance and decreased weight, and the virtual elimination high-pressure band presses is also gaining in popularity. of (or drastic reduction in) process water concerns. Panel density is a basic property and an indicator of panel quality. Since density is greatly influenced by moisture Posttreatment of Wet- and Dry-Process Hardboard content, this is constantly monitored by moisture sensors using infrared light. Several treatments are used to increase the dimensional ANSI A208.2 classifies MDF by physical and mechan- stability and mechanical performance of hardboard. Heat ical properties, and identifies dimensional tolerances and treatment, tempering, and humidification may be done formaldehyde emission limits [4]. singularly or in conjunction with one another. 8 WOOD-BASED COMPOSITE BOARD

Heat treatment—exposure of pressed fiberboard to screen, or cylinder screen. A deckle box is a bottomless dry heat—improves dimensional stability and mechani- frame that is placed over a screen. A measured amount cal properties, reduces water adsorption, and improves of stock is put in the box to form one sheet; vacuum is interfiber bonding. then applied to remove most of the water. The use of Tempering is the heat treatment of pressed boards, Fourdrinier screen for felting is similar to that for paper- preceded by the addition of oil. Tempering improves board making, except that line speeds are reduced to 8–18 m/min surface hardness and is sometimes done on various types (25–60 ft/min). of wet-formed hardboards. It also improves resistance to Cellulosic board formed in a deckle box is usually abrasion, scratching, scarring, and water. The most com- cold-pressed to remove most of the free water after the mon oils used include linseed oil, tung oil, and tall oil. mat is formed. Compression rollers on the Fourdrinier Humidification is the addition of moisture to bring the machines squeeze out some of the free water. The wet board moisture content to levels roughly equivalent to mats are then dried to the final moisture content. Dryers those anticipated in its end-use environment. Initially, a may be a continuous tunnel or a multi-deck arrangement. The board is generally dried in stages at temperatures pressed board has almost no moisture content. When the ◦ ◦ board is exposed to air, it expands linearly by taking on ranging from 120 to 190 C (248–374 F). Typically, about 3% to 7% moisture. Continuous or progressive humidifiers 2 to 4 h are required to reduce moisture content to about are commonly used for this purpose. Air of high humidity 1%–3%. is forced through the stacks where it provides water vapor After drying, some boards are treated for various appli- to the boards. Another method involves spraying water on cations. Boards may be given tongue-and-groove or shiplap the back side of the board. edges or can be grooved to produce a plank effect. Other Several techniques are used to finish fiberboard: trim- boards are laminated by means of asphalt to produce roof ming, sanding, surface treatment, punching, and emboss- insulation. ing. Trimming consists of reducing products into standard Cellulosic fiberboard products include sound-deadening sizes and shapes. Generally, double-saw trimmers are used board, roof insulation boards, structural and nonstructural to saw the panels. Trimmers consist of overhead-mounted sheathings, backer board, and roof decking in various saws or multiple saw drives. Trimmed panels are stacked thicknesses. A grade mark stamp will be given for these in piles for future processing. If thickness tolerance is criti- cellulosic fiberboard products conforming to ASTM C208 cal, hardboard is sanded prior to finishing. S1S (smooth on [14]. one side) panels require this process. Sanding reduces thickness variation and improves surface paintability. SPECIALTY COMPOSITES Single-head, wide-belt sanders are used with 24- to 36-grit abrasive. Surface treatments improve the appearance and Special-purpose composite materials are produced to performance of boards. Panels are cleaned by spraying ◦ ◦ obtain enhanced performance properties like water with water and then dried at about 240 C (464 F) for resistance, mechanical strength, acidity control, and fire, 30 seconds. Panel surfaces are then modified with paper decay and insect resistance. Overlays and veneers can overlay, paint, or stain or are printed directly on the also be added to enhance both structural properties and panel. Punching changes panels into the perforated sheets appearance. used as peg board. Embossing consists of pressing the unconsolidated mat of fibers with a textured form. This Moisture-Resistant Composites process results in a slightly contoured panel surface that Sizing agents can be used to make composites resistant to can enhance the resemblance of the panel to that of sawn moisture. The common size agents include rosin, wax, and or weathered wood, brick, and other materials. asphalt. Sizing agents cover the surface of fibers, reduce surface energy, and render the fibers relatively hydropho- Cellulosic Board bic. Sizing agents can be applied in two ways. In the first Cellulosic boards are low-density, wet-laid panel products method, water is used as a medium to ensure thorough used for insulation, sound deadening, carpet underlay- mixing of sizing and fiber. The sizing is precipitated from ment, and similar applications. In the manufacture of the water and is fixed to the fiber surface. In the second cellulosic board, the need for refining and screening is method, the sizing is applied directly to the fibers. a function of the raw material available, the equipment used, and the desired end-product. Cellulosic boards typi- Flame-Retardant Composites cally do not use a binder, and they rely on hydrogen bonds Two general application methods are available for improv- to hold the board components together. Sizing agents are ing the fire performance of composites with fire-retardant usually added to the furnish (about 1%) to provide the chemicals. One method consists of pressure impregnat- finished board with a modest degree of water resistance ing the wood with waterborne or organic solvent-borne and dimensional stability. fire-retardant chemicals [15]. The second method consists Like the manufacture of wet-process hardboard, cellu- of applying fire-retardant chemical coatings to the wood losic board manufacture is a modification of papermaking. surface. The pressure impregnation method is usually A thick fibrous sheet is made from a low-consistency pulp more effective and longer lasting; however, this tech- suspension in a process known as wet felting.Feltingcan nique is standardized only for plywood. It is not gener- be accomplished through use of a deckle box, Fourdrinier ally used with structural flake/particle/fiber composites WOOD-BASED COMPOSITE BOARD 9 as it can cause swelling that permanently damages the wood composite can be obtained only when the wood wood–adhesive bonds in the flake/particle/fiber compos- particles are fully encased within the binder to make a ite and results in the degradation of some physical and coherent material. This differs considerably from the tech- mechanical properties of the composite. For wood in exist- nique used to manufacture thermosetting-resin-bonded ing constructions, surface application of fire-retardant boards where flakes or particles are ‘‘spot welded’’ by a paints or other finishes offers a possible method to reduce binder applied as a finely distributed spray or powder. flame spread. Because of this difference and because hardened inorganic binders have a higher density than that of most thermoset- Preservative-Treated Composites ting resins, the required amount of inorganic binder per unit volume of composite material is much higher than Composites can be protected from the attack of decay that of resin-bonded wood composites. The properties of fungi and harmful insects by applying selected chemicals inorganic-bonded wood composites are significantly influ- as wood preservatives. The degree of protection obtained enced by the amount and nature of the inorganic binder depends on the kind of preservative used and the ability to and the woody material as well as the density of the achieve proper penetration and retention of the chemicals. composites. Wood preservatives can be applied using pressure or Inorganic-bonded composites are made by blending pro- non-pressure processes [16]. As in the application of portionate amounts of lignocellulosic fiber (or delignified fire-retardant chemicals, the pressurized application of fiber derived from wood) with inorganic materials in the wood preservatives is generally performed after manufac- presence of water and allowing the inorganic material to ture and is standardized for plywood. Post-manufacture cure or ‘‘set up’’ to make a rigid composite. A unique feature pressure treatments are not standardized for all types of of inorganic-bonded composites is that their manufacture flake/particle/fiber composite as it can sometimes cause is adaptable to either end of the cost and technology spec- damage to wood–adhesive bonds that in turn reduces trum. This is facilitated by the fact that no heat is required physical and mechanical properties of the composite. to cure the inorganic material. This versatility of manu- Preservatives can be added in the composite manufac- facture makes inorganic-bonded composites ideally suited turing process, but the preservative must be resistant to to a variety of lignocellulosic materials. vaporization during hot pressing. Proprietary flakeboard Inorganic binders fall into two main categories: and fiberboard products with incorporated non-volative gypsum-bonded and cement-bonded. Magnesia and preservatives have been commercialized. Common preser- Portland cement are the most common cement binders. vative treatments include ammoniacal copper quat (ACQ), Gypsum and magnesia cement are sensitive to moisture, copper azol (CA), and boron compounds. and their use is generally restricted to interior applications. Composites bonded with Portland cement are more WOOD-NON-WOOD COMPOSITES durable than those bonded with gypsum or magnesia cement and are used in interior and exterior applications. Wood may be combined with inorganic materials and Some inorganic-bonded composites are very resistant with plastics to produce composite products with unique to deterioration by decay fungi and insects. Most have properties. Wood-non-wood composites typically contain appreciable fire resistance. comminuted wood elements suspended in a matrix material (for example in fiber-reinforced gypsum board, or in Gypsum-Bonded Composites. Paper-faced gypsum thermoplastic material), in which the proportion of wood boards have been widely used since the 1950’s for the elements may account for appreciably less than 50% of interior lining of walls and ceilings, where they have product mass. generically been called drywall because they commonly Composites made from wood and other materials create replace wet plaster systems. These panels are critical for enormous opportunities to match product performance to good fire ratings in walls and ceilings. Paper-faced gypsum end-use requirements. The following discussion includes boards (and glass fiber-faced gypsum panels), also find the most common type of wood-non-wood composites: inor- use as exterior wall sheathing. Gypsum sheathing panels ganic bonded and wood-thermoplastic composites. are primarily used in commercial construction, usually over steel studding and are distinguished from regular gypsum wallboard by their water repellent additives in Inorganic–Bonded Composite Materials the paper facings and gypsum core. The facings of drywall Inorganic-bonded wood composites have a long and varied and of gypsum sheathing panels are adhered to the history that started with commercial production in Austria gypsum core, providing the panels with impact resistance, in 1914. They are now used in many countries in the world, and bending strength and stiffness. The paper facings of mostly in panel form. A plethora of building materials can gypsum panels are derived from recycled paper fiber. be made using inorganic binders and lignocellulosics, and An alternative to use of adhered facings is to incorpo- they run the normal gamut of panel products, siding, rate lignocellulosic fiber (typically recycled paper fiber) in roofing tiles, and precast building members. the gypsum core to make what are termed fiber-reinforced Inorganic-bonded wood composites are molded products gypsum panels. In the production process, a paste of gyp- or boards that contain between 10% and 70% by weight sum and water is mixed with the recycled paper fiber wood particles or fibers and conversely 90% to 30% inor- and extruded into a panel (formed on a belt), without ganic binder. Acceptable properties of an inorganic-bonded facings. Shortly after formation, the panel is dried in an 10 WOOD-BASED COMPOSITE BOARD oven. Bonding occurs between the gypsum and the fiber as fiber-cement siding. Fiber-cement siding incorporates hydrate crystals form. delignified wood fiber into the portland cement matrix. Fiber-reinforced gypsum panels are typically stronger and more resistant to abrasion and indentation than Ceramic-Bonded Composites. In the recent years a new paper-faced drywall panels, and also have a moderate class of inorganic binders, non-sintered ceramic inorganic fastener-holding capability. They are marketed for use binders, has been developed. These non-sintered ceramic as interior finish panels (drywall). Additives can pro- binders are formed by acid–base aqueous reaction between vide a moderate degree of water resistance, for use as a divalent or trivalent oxide and an acid phosphate or sheathing panels, floor underlayment, roof underlayment phosphoric acid. The reaction slurry hardens rapidly, but or tile-backer board. the rate of setting can be controlled. With suitable selection of oxides and acid-phosphates, a range of binders may be produced. Recent research suggests that phosphates may Cement-Bonded Composites. The properties of cement- bonded composites are influenced by wood element char- be used as adhesives, cements, or surface augmentation acteristics (species, size, geometry, chemical composition), materials to manufacture wood-based composites [18]. cement type, wood-water-cement ratio, environmental temperature, and cure time [17]. They are heavier than Wood–Thermoplastic Composite Materials conventional wood-based composites, but lighter than con- Wood-thermoplastic composites have become a widely rec- crete. Therefore they can replace concrete in construction, ognized commercial product in construction, automotive, specifically in applications that are not subjected to loads. furniture, and other consumer applications in the last Wood-cement composites provide an option for using wood decade [19]. Commercialization has been primarily due to resides, or even agricultural residues. However species penetration into the construction industry, first as deck- selection can be important as many species contain sugars ing and window profiles, followed by railing, siding, and and extractives that retard the cure of cement [12]. roofing. Interior molding applications are also receiving Fewer boards bonded with magnesia cement have been attention. The automotive industry has been a leader in produced than cement- or gypsum-bonded panels, mainly using wood-thermoplastic composites for interior panel because of price. One successful application of magnesia parts, and is leading the way in furniture applications. cement is a low-density panel made for interior ceiling The class of materials can include lignocellulosics and wall applications. In the production of this panel derived from wood or other natural sources and different product, wood wool (excelsior) is laid out in a low-density thermoplastics including virgin or recycled polypropylene, mat. The mat is then sprayed with an aqueous solu- polystyrene, vinyls, and polyethylenes. Other materials tion of magnesia cement, pressed, and cut into panels. can be added to affect processing and product performance Other processes have been suggested for manufactur- of wood–thermoplastic composites. These additives ing magnesia-cement-bonded composites. One application can improve bonding between the thermoplastic and may be to spray a slurry of magnesia cement, water, and wood component (for example, coupling agents), product lignocellulosic fiber onto existing structures as fireproof- performance (impact modifiers, UV stabilizers, flame ing. Extrusion into a pipe-type profile or other profiles is retardants), and processability (lubricants). also possible. The manufacture of thermoplastic composites is usu- The most widely used inorganic-bonded composites are ally a two-step process. The raw materials are first mixed those bonded with Portland cement. Portland cement, together, and the composite blend is then formed into a when combined with water, immediately reacts in a product. The combination of these steps is called in-line process called hydration to eventually solidify into a processing, and the result is a single processing step that solid stone-like mass. Successfully marketed Portland- converts raw materials to end products. In-line processing cement-bonded composites consist of both low-density can be very difficult because of control demands and pro- products made with excelsior and high-density products cessing trade-offs. As a result, it is often easier and more made with particles and fibers. economical to separate the processing steps [20]. Low-density products may be used as interior ceiling There are two main types of wood-thermoplastic com- and wall panels in commercial buildings. In addition to the posites. In the first, the lignocellulosic component serves advantages described for low-density magnesia-bonded as a reinforcing agent or filler in a continuous thermo- composites, low-density composites bonded with Portland plastic matrix. In the second, the thermoplastic serves as cement offer sound control and can be quite decorative. a binder to the majority lignocellulosic component. The In some parts of the world, these panels function as presence or absence of a continuous thermoplastic matrix complete wall and roof decking systems. The exterior may also determine the processability of the composite of the panels is coated with stucco, and the interior is material. plastered. High-density panels can be used as flooring, In composites with high thermoplastic content, the roof sheathing, fire doors, load-bearing walls, and cement thermoplastic component is in a continuous matrix and forms. Fairly complex molded shapes can be molded or the lignocellulosic component serves as a reinforcement extruded, such as decorative roofing tiles or non-pressure or filler. The lignocellulosic content is typically less than pipes. 60% by weight. In the great majority of reinforced ther- The largest volume of cement-bonded wood-based moplastic composites available commercially, inorganic composite materials manufactured in North America is materials (for example, glass, clays, and minerals) are WOOD-BASED COMPOSITE BOARD 11 used as reinforcements or fillers. Lignocellulosic materi- 6. CPA. Prefinished hardboard paneling, ANSI A135.5–2004. als offer some advantages over inorganic materials; they Gaithersburg (MD): Composite Panel Association; 2004b. are lighter, much less abrasive, and renewable. Lignocel- 7. CPA. Hardboard siding, ANSI A135.6–2006. 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