Micromorphology and Macromorphology of Recycled Fiber and Wood*
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than many other materials such as metals, ceramics, or polymers.4 In addition, manipulating molecular structure in order to
achieve desired properties is generally not an option, although the genetic improvement of wood and applications of biotechnology represent applications of this line of thinking. However, if larger wood elements are considered (e.g., fiber, particles, or flakes), materials science principles do apply for the manufacture of materials with desired properties.5 Consideration of such elements as the fundamental units to be manipulated in a fabrication process centers largely on their geometry. Geometry and morphology significantly affect the final properties of wood-based composites.5^ In most instances, recycled or secondary sources of wood and/or wood fiber will exhibit at least some degree of comminution relative to the "primary" wood source, i.e., the tree. A Taxonomy of Recycled Wood and Fiber A conceptual framework useful for the initial categorization of secondary wood and fiber materials is Marra's "nonperiodic
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Material Figure 1. U.S. consumption of major industrial raw materials, 1987. Data on consumption of plastics, aluminum, cement, and steel compiled from Modern Plastics and U.S. Bureau of Mines statistics by J.L. Bowyer.2 Data on wood consumption from Ulrich.1 For further information on U.S. consumption of timber products and other industrial raw materials, see: J. Peterson, Evergreen, summer 1993 (Evergreen Foundation, Medford, OR).
* This article is a modification of an article published in Materials Research Society Symposium Proceedings Vol. 266, (1992) p. 47. 22
MRS BULLETIN/FEBRUARY 1994
Micromorphology and Macromorphology of Recycled Fiber and Wood
table of wood elements."9 The table arranges 14 elements in order of decreasing size: logs, lumber, thin lumber/thick veneer, veneer, long flakes/short veneer, chips, flakes, excelsior, strands, particles, fiber bundles, paper fiber, wood flour, and cellulose. The intent was to present an array of basic wood elements that may be combined with one another and/or with other materials to create engineered materials. It was further pointed out that of
the 14 elements, 10 could be generated from residues9 or, in the present context, from recycled or secondary sources. In fact, for certain pallet and perhaps demolition recycling/reuse schemes, this number could be larger; that is, elements ranging from lumber size on down may be recovered, representing 13 of the 14 possible "wood elements." Using such a taxonomy, we may recognize the potential for producing wood elements from any given
secondary source. Presumably, some technical capability either exists or could be developed to reduce any given element to any desired smaller element. Given this taxonomy, we might then consider the point at which major classes of secondary wood and fiber would be classified, then observe the other elements which may be derived from them. Of the major classes considered in this article, pallets repre
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