| Wood panel products are flat, sometimes curved, sheets made from wood-based materials that are bonded together with an adhesive. Panels are typically made of veneer, strands, particles or fibers and bonded with a synthetic or other adhesive cured using heat and pressure. The properties of the panel product are a result of the character of the wood raw material, the adhesive, additives, and the details of the manufacturing process. The most commonly produced wood panel products are plywood, oriented strand board, particleboard, and fiberboard. Other wood-based composite materials may be produced in panel form for specialized applications and markets.
Wood panels may be categorized as structural or non-structural. Structural panels are designed and manufactured to resist forces and play specific roles in structural systems such as houses, buildings, concrete formwork, shipping containers and unit load platforms, etc. Mechanical properties are principal design criteria for these products. Non-structural panels are typically produced for other than structural applications, such as furniture, flooring, doors, etc, where other properties beside strength and stiffness are important.
Plywood is a panel product made by gluing individual plies or thin layers of veneer together. Veneer is a thin sheet of wood that has been mechanically peeled from a log on a lathe in a fashion similar to unrolling toilet paper. Plywood is made from both hardwood and softwood species and may be bonded with many types of adhesive. Sheets of veneer are spread with glue and then laid up into a panel and pressed together with heat. The panel lay-up determines the thickness of the composite sheet and its properties. The grain of each layer is typically oriented 90 degrees to its neighbors to improve properties and dimensional stability.
Synthetic adhesives such as urea-formaldehyde or phenol-formaldehyde resins are often used, but other choices are available including those with lower or no potential for formaldehyde emissions, which can be harmful to humans. The desired level of moisture resistance in the panel is a major factor in adhesive selection.
Softwood plywood is typically used for structural sheathing applications where the panel is nailed to a wooden frame to create walls, roofs and floors. It is also commonly used in shipping containers, furniture frames, truck trailers and many other products. Hardwood plywood is most often manufactured for decorative applications such as paneling, cabinetry, architectural woodwork, etc. The quality and characteristics of the face veneers are of principal importance to the value of hardwood plywood and decorative softwood plywood.
Plywood has replaced solid wood in many applications because of its resistance to splitting, cracking and warping in addition to its cost-effectiveness at covering large areas. Uniformity of strength in the plane of the panel, dimensional stability during moisture change, and ability to fasten close to the edge of the panel are other relative advantages.
The quality or grade of the individual veneers in a plywood panel is very important to the quality of the final product. Veneer quality, in turn, is dependent on the quality and shape of the log from which it is produced, and the quality of the manufacturing process. As a result, not all species and log sizes are desirable for producing veneer for plywood. Smaller diameter logs with larger proportions of knots, and lower density species are less desirable for veneer and plywood manufacture.
Oriented Strand Board (OSB) or Wafer board is comprised of thin wood strands or flakes that are sprayed with resin and wax, formed into a mat that is pressed under heat. Individual flakes and layers of flakes are oriented in different directions in the lay-up process to achieve desired properties—hence the name. OSB panels are used much like plywood as a sheathing material and OSB is seen as a lower-cost substitute in many applications and markets.
The properties of OSB can be modified by using different species, different flake or strand geometries, different adhesives, and by varying manufacturing techniques. Because the principal use of OSB is as a sheathing product, it is manufactured to achieve minimum strength, stiffness, and moisture resistance properties required for that application.
Flakes are typically manufactured directly from logs with special flaking equipment and can be made from a wide variety of species and log sizes. As a result, the manufacture of OSB is somewhat less sensitive to the characteristics of the initial raw material than veneer production. For example, lower density species such as aspen or poplar may be used with OSB. These are not typical raw materials for plywood production. This flexibility translates into a sheathing material with lower cost than plywood, but with comparable properties for some applications.
Particleboard or chipboard is a panel product made from wood particles that are sprayed with wax and adhesive, formed into a mat and then pressed under heat. Particleboard can be made with very smooth surfaces that allow for surface coatings, veneer or overlays to be added that form decorative or working surfaces. Examples are kitchen countertops, cabinets, shelves, audio components, etc. While particleboard is commonly remanufactured into parts and components it is also be used as a flooring material in manufactured homes, an underlayment for flooring surfaces, stair treads, door cores and similar applications.
The raw material furnish for particleboard is generally derived from wood manufacturing residues including wood scrap, shavings, and some sawdust. Residues are mechanically reduced to common sizes and dried. Particleboard panels are typically made in three layers with the surface layer utilizing fines, or smaller particles, to create a smooth surface and a central layer of coarser material. Different types of adhesives are employed depending on the level of moisture resistance necessary. Urea-formaldehyde and melamine-formaldehyde are very common particleboard resins.
Particleboard properties for different applications can be engineered by adjusting the geometry of the particles, resin levels, additives and the board density resulting from pressing.
Medium-density fiberboard (MDF) is formed by mixing wax and resin with wood or other cellulosic fibers and pressing under heat to form panels with specific properties for end uses. The surface of the MDF panel is very flat, smooth, and more uniform than that of particleboard which makes it preferable for finishing with paint, thin laminates or a printed surface. Because it has a more homogeneous density profile than particleboard it is often the choice for products requiring high machinability. MDF is frequently used in cabinets, furniture, molding, millwork, and doors.
The cellulosic furnish for MDF may come from a wide variety of sources including logs, manufacturing residues and urban wood waste. The raw material is chipped to small sizes and then converted to fiber using steam and mechanical action of a disk refiner. Resin and other additives are applied to the fiber which is then formed into a mat which goes though a series of pressing steps to achieve the desired density and other properties.
Hardboard is a variation of fiberboard that is pressed to higher densities than MDF and with other refinements to achieve enhanced properties. Hardboard may be manufactured using a dry process such as described for MDF or a wet process where water is used to form the fibers into a mat rather than air. Some wet process boards do not use synthetic binders but rely on natural adhesion by the chemical constituents of wood. Hardboard is used for exterior siding, wall paneling, furniture and a variety of other products. Masonite is a common name for a type of hardboard in North America.
Wood panels are engineered composite products that are manufactured for specific uses in accordance with accepted commercial product standards and specifications or private user requirements. These standards may prescribe the makeup of a panel product, but most often identify minimum properties such as strength, stiffness, water resistance, dimensional stability, surface quality, machinability, screw-holding ability and other characteristics.
Wood composites in general and panel products specifically are frequently considered as “green” materials because they use relatively little energy to produce, fully utilize log, residue and waste resources as raw materials, and are derived from renewable materials. Panel products may be certified as comprised of sustainably produced raw materials under some certification schemes.
Wood panel products, or their constituent elements, are produced throughout the world to national and international standards and are commonly imported and exported. The demand for panels is forecast to increase well into the next century as quality logs for traditional solid products become increasingly scarce and as designers and consumers gain experience with positive product attributes and new applications. In recent years, wood-based panel products have become increasingly specialized and are used widely, including as a substitute for metals and plastics in some applications.
References
APA-EWS. 2007. Accessed at http://www.apawood.org/ , August 15, 2007.
Baldwin, R.F. 1995. Plywood and Veneer-Based Products, Manufacturing Practices. Miller Freeman Books, San Francisco, CA. 388 p.
Bowyer, J.L., R. Shmulsky and J.G. Haygreen. 2003. Forest Products and Wood Science—An Introduction. 4th Edition. Iowa State Press. Ames, IA 554 p.
Composite Panel Association. 2007. Accessed at http://www.pbmdf.com/AboutCPA/index.asp , August 15, 2007.
CORRIM. 2004. CORRIM Report on Environmental Performance Measures for Renewable Building Materials. Consortium for Research on Renewable Industrial Materials. Accessed at http://www.corrim.org/factsheets/fs_02/index.asp, August 15, 2007
Forest Products Laboratory. 1999 Wood Handbook—Wood as an Engineering Material. Gen. Tech. Report FPL-GTR-113. U. S Department of Agriculture, Forest Service, Madison Wisconsin, 463 p
Thomas E. McLain is Professor and Head, Department of Wood Science & Engineering, Oregon State University, Corvallis, OR 97330, USA
Posted 18 August 2007
Updated 22 August 2007
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