Historical Note
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More Ways to Recombine i l l Fibers than You Can Shake a Stick At Wood has been categorized as a renewable resource. In the industrial era, however, a dwindling supply of wood spurred the invention of techniques developed over the last 150 years that exploit a greater proportion of available wood. Their general principles involve the use of thin sheets or strips of wood assembled into final form with the use of adhesives. Lumber cut from round logs produces waste. Eliminating this waste is one motivation for plywood since very little waste is produced when a log is rolled against a blade, creating sheets of wood veneer. These veneer layers, or plies, are laminated with adhesive and pressed together. When the direction of the grain of each lamination is alternated, the resulting boards possess a high degree of tensile strength. Adhesives contribute to plywood's strength by providing the bond between each ply. Early adhesives included animal hide glue and cassava-based vegetable glues, which were water soluble. Identifying waterproof adhesives became key to making plywood widely marketable. John K. Mayo obtained a U.S. patent for plywood in 1865, but the technology did not become known at the time and the patent lapsed. Plywood reappeared in 1905 as a local product displayed at the Lewis and Clark Centennial Exposition in Portland, Oregon. This time, a market developed and plywood began to be used for cabinetry, doors, and car running boards. Although milk-based casein glues were introduced around 1900 and were more water-resistant than earlier adhesives, plywood's primary drawback remained its inability to perform in moist conditions because the adhesives would break down and the plies would delaminate. Plywood is used most effectively in the form of wide, flat sheets, but there are other laminated wood products used as structural beams and columns. Traditionally, structural members are wood pieces cut straight from the tree, but the large assemblies required to span distances upward of 30 feet become prohibitively heavy. In 1901, Otto Hetzer of Weimar, Germany obtained a patent for a straight beam created from laminations of wood boards bonded, as was plywood, with casein adhesive under pressure. The laminations in this beam ran parallel to one another, mimicking the fibrous structure
of a tree. Building up beams from laminations of smaller wood components allowed trees of average size and quality to be used, and reduced waste. Using small pieces of wood permitted the moisture content of the completed beam to be better controlled. Hetzer patented a curved beam in 1906. The curve allowed the beam to behave as an arch. Because this configuration capitalized on wood's natural fiber strength and the manufacturing process permitted defects to be removed, curved laminated beams could span longer distances than lumber beams. Wood now could be used to make beams that spanned as far as steel beams, which were more expensive. The world wars and attendant government-sponsored research helped advance the technology and accept
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