Engineered Composite Sciences

FS-FPL-4706

Wood Handbook
Chapter 10, Wood-Based Composites and Panel Products

This and the web pages listed in the table of contents below are sections out of chapter 10, Wood-Based Composites and Panel Products, of the wood handbook. The entire Chapter 10 is available for downloading and is in Adobe Acrobat Reader portable document format (.pdf). The Adobe Acrobat Reader (preferably version 3.0 or higher) must be installed on your system to view or print a PDF document. The Acrobat Reader may be part of your Web browser, or you may download it from Adobe free of charge.

Introduction

Because wood properties vary among species, between trees of the same species, and between pieces from the same tree, solid wood cannot match reconstituted wood in the range of properties that can be controlled in processing. When processing variables are properly selected, the end result can sometimes surpass nature's best effort. With solid wood, changes in properties are studied at the molecular level. With reconstituted wood materials, changes in properties are studied at the fiber, particle, flake, or veneer level. Properties of such materials can be changed by combining, reorganizing, or stratifying these elements.

The basic element for composite wood products may be the fiber, as it is in paper, but it can also be larger wood particles composed of many fibers and varying in size and geometry. These characteristics, along with control of their variations, provide the chief means by which materials can be fabricated with predetermined properties.

In any discussion of the strength properties of wood-based panels and other glued wood composites, the first consideration is the constituents from which these products are made (O'Halloran and Youngquist 1984; Youngquist 1987, 1988). The basic wood elements that can be used in the production of wood-based panels are shown in Figure 10-1. The elements can be made in a great variety of sizes and shapes and can be used alone or in combination. The choice is almost unlimited.

Currently, the term composite is being used to describe any wood material glued together. This product mix ranges from fiberboard to laminated beams and components. Table 10-1 shows a logical basis for classifying wood composites proposed by Maloney (1986). For the purposes of this chapter, these classifications were slightly modified from those in the original version to reflect the latest product developments. Composites are used for a number of structural and nonstructural applications in product lines ranging from panels for interior covering purposes to panels for exterior uses in furniture and support structures in many different types of buildings.

Figure 10-2 provides a useful way to further classify wood-based composite materials. This chart presents an overview of the most common types of products discussed in this chapter as well as a quick reference to how these composite materials compare to solid wood from the standpoint of density and general processing considerations. The raw material classifications of fibers, particles, and veneers are shown on the left vertical axis (y axis). Specific gravity and density are shown on the top and bottom horizontal axis (x axis). The right y-axis, wet and dry processes, describes in general terms the processing method used to produce a particular product. Note that both roundwood and chips can serve as sources of fiber for wet-process hardboard. Roundwood or wood in the form of a waste product from a lumber or planing operation can be used for dry-processed products. For medium-density fiberboard (MDF), resin is usually applied to the fiber after the fiber is released from the pressurized refiner. The fiber is then dried, formed into a mat, and pressed into the final product. For other dry-processed products, the material is fiberized and dried and then adhesive is added in a separate operation prior to hot pressing into the final composite product. Figure 10-3 shows examples of some composite materials that are represented in schematic form in Figure 10-2.

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Scope of Chapter

Although there is a broad range of wood composites and many applications for such products, for the purposes of this chapter, wood composites will be grouped into three general categories: plywood, particle and fiber composites, and wood-nonwood composites. Books have been written about each of these categories, and the constraints of this chapter necessitate that the discussion be general and brief. References are provided to more detailed information. Information on glued-laminated (glulam, timbers, and structural composite lumber, including laminated veneer lumber) and glued members with lumber and panel products are presented in Chapter 11 of this handbook. Many composite materials, like fiberboard, medium-density fiberboard, and particleboard, can be made from wood alone or in combination with agricultural fibers (Youngquist and others 1993a, 1994; Rowell and others 1997).

The first category, plywood, is covered in some detail because the process for manufacturing this kind of material is quite different from that used for other composite materials and because there are many different classes and grades of plywood in the marketplace. The second category, composite materials, includes oriented strandboard (OSB), particleboard, and fiberboard. These types of composites undergo similar processing steps, which is discussed in general terms for all the products in the Particle and Fiber Composites section. The first and second categories of composite materials are further generally classified as conventional composite materials. The third category, wood-nonwood composites, discusses products made from combining wood fibers with agricultural fibers, with thermoplastics, and with inorganic materials.

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Types of Conventional Composite Materials

Conventional wood composite materials fall into five main categories based on the physical configuration of the wood used to make the products: plywood, oriented strandboard, particleboard, hardboard, and, cellulosic fiberboard. Within limits, the performance of a conventional type of composite can be tailored to the end-use application of the product. Varying the physical configuration of the wood and adjusting the density of the composites are just two ways to accomplish this. Other ways include varying the resin type and amount and incorporating additives to increase water or fire resistance or to resist specific environmental conditions.

Figure 10-1. Basic wood elements, from largest to smallest (Marra 1979).

Figure 10-2.
Classification of wood composite boards by particle size, density, and process type (Suchsland and Woodson 1986).

Figure 10-3. Examples of various composite products
(from left to right: plywood, OSB, particleboard, MDF, and hardboard).

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