Veneer, Plywood, and Wood Products
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Veneer, Plywood, and I I I Products Wood has been used by the human race for millennia. And, although this product of the forests was used before civilization began, it continues to be a versatile material with widespread engineering applications. In Egyptian times, and later in the Roman empire, craftspeople began sawing thin slices of wood from logs. The process, an arduous one, involved sawing lengthwise down the logs. Applying the resulting thin slices of wood as ornamentation was developed into a technique known as veneering. In later centuries, as furniture making became a sophisticated art, veneering came into wide use. Furniture makers found that a veneer of beautifully grained walnut, for example, could increase the value of a piece made primarily of less-expensive wood. Veneering also gave craftspeople the freedom to use ornate and attractive "flaws," such as burls or knots, which were unreliable for use as structural parts. Beautiful examples of European veneered furniture date from the 17th and 18th centuries, along with fine examples of Chinese veneer. The technique gained even greater popularity in the 19th century, when industrially processed plywood became common enough to be used in everyday furniture for the middle classes. The development of "veneered wood," or plywood, as it is currently known, also follows the growth of the furniture industry. Plywood is a composite wood product made of thin layers of wood glued together and arranged so that the grain directions of adjoining layers are perpendicular to each other. This technique results in a wood slab with material strength equal in both directions. A balanced construction of plywood has an odd number of layers, consisting of an identical assembly on both sides of the center layer, or core. Such a balanced construction works well to minimize warping because the opposing grain directions tend to cancel changes in successive layers. Plywood is frequently used when large areas must be covered with a light, rigid sheeting. The primary softwoods (defined as the wood from coniferous trees) used in plywood construction MRS BULLETIN/SEPTEMBER 1993
include Douglas-fir, hemlock, southern pine, and ponderosa pine; hardwood plywoods are made of oak, yellowpoplar, red gum, elm, birch, ash, cherry, and walnut. Layer thicknesses range from about 1/28 inch to 1/8 inch (1 to 3 mm). Thin exterior veneers also make expensive decorative wood go farther. The most common finished plywoods range in thickness from three-ply (3/8 inch or about 1 cm) to five-ply (3/4 inch or 1.9 cm), though other thicknesses are used for specific applications. The appropriate layers are assembled and arranged in gluing presses, which use either natural adhesives (animal, soybean, starch, or casein) or synthetic resins (phenol- or urea-formaldehyde) to glue the plywood together. Some resins actually produce bonds stronger than the wood, resulting in resistance to interfacial attack by water, weather, steam, microorganisms, insects, and fungus. In the 1860s, U.S. patents were issued for
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