制浆造纸专业单词7

11 Wood is the principal source of cellulosic fiber for pulp and paper manufacture. At present, wood provides about 93% of the world's virgin fiber requirement, while non-wood sources, mainly bagasse, cereal straws and bamboo, provide the remainder. Approximately one-third of all paper products are recycled into secondary fiber.

2.1 TREE STRUCTURE A tree can be considered to have three general parts: •the crown composed of leaves and branches •the stem •the root system The leaves or needles are the factories where food material is manufactured through photosynthesis to provide the tree with energy and growth. Photosynthesis is the production of carbohydrates from carbon dioxide and water in the presence of chlorophyll and light. Although the crown is both the source of nutrients and the regulating center for wood production, wood is not produced directly by photosynthesis. Rather, wood results through cell divisions of the vascular cambium using energy derived from the products of photosynthesis. After cambial division, each successive cell undergoes enlargement, wall thickening, and lignification.

Figure 2-1 shows a cutaway sketch of a

tree trunk revealing the general structure. Figure 2-2 shows a transverse cross-section. The cambium consists of a thin layer of tissue between the bark and the inner sapwood. In temperate climes, the rate of cambial growth varies with the seasons giving rise to the deposition of thin-wall fiber cells in the spring and more denser thick-wall fibers in the fall. The cambium is dormant during the cooler months of the year. The yearly growth cycle is reflected in the annual rings, the total number of which represent the tree's age. The inner bark (phloem) is a narrow layer of tissue where the carbohydrate-containing sap moves upward and downward through sieve tubes and rays. The outer bark is a collection of dead cells which originally existed in the inner living bark: it is composed of a variety of extraneous components in addition to cellulose, hemicellulose and lignin. The sapwood portion of the tree provides structural support for the crown, acts as a food storage reservoir, and provides the important function of water conduction up from the roots. It is physiologically active (parenchyma cells only) and in continuous communication with the cambium and

Fiber Supply 12 phloem through sap flow from the crown. FIGURE 2-2. Cross-sectional sketch of a mature stem, showing outer bark, inner bark, sapwood and heartwood. The inner heartwood is a core of dead woodcells in the center of the stem whose physiological activity has ceased. It functions only as mechanical support. Heartwood is usually much darker in color than sapwood due to deposition of resinous organic compounds in the cell walls and cavities. Such deposition makes liquor penetration during chemical pulping more difficult in heartwood than in sapwood. In a few species (most notably spruce) the color difference between heartwood and sapwood is minor. At the center of the tree is a small core of soft tissue called the pith. 2.2 CHARACTERISTICS OF WOOD Botanically, woods are classified into two major groups. The gymnosperms are commonly called softwoods or conifers. The angiosperms are the hardwoods or broad-leafed trees, either deciduous or evergreen. The main structural features of each wood group are illustrated in Figures 2-3 and 2-4. Softwoods The vertical structure of conifers is composed almost entirely of long, tapering cells called tracheids. In some species, vertical resin canals are also present. The horizontal system is composed of narrow rays, only one cell in width but often several cells high. Ray cells are of two specialized types: ray parenchyma occurs in all species, while ray tracheids are present in only certain species. several cells high. Ray cells are of two specialized types: ray parenchyma occurs in all species, while ray tracheids are present in only certain species. Seasonal growth is usually

characterized by a denser band of tracheids at the end of the annual ring. This latewood (or summerwood) tissue has quite different properties from the earlywood (or springwood) tissue whose density may be only one-half or one-third that of the latewood. The cell wall itself has a relative density (specific gravity) of about 1.5 (oven-dry basis). The wall of a typical tracheid or "fiber" is composed of several layers. The middle lamella, very high in lignin content, separates two contiguous tracheids. Each tracheid has a primary wall and a three-layered secondary wall with specific alignments of microfibrils. Microfibrils are bundles of cellulose molecules, and their orientation can influence the characteristics of a pulp fiber. The structure of a tracheid is illustrated and explained in Table 2-1 and Figure 2-5. Note that the symbols S3 and T are used interchangeably. Figure 2-6 shows additional detail with respect to microfibrillar layers (or laminae), alignments and textures. Radial cross-sections of four representative North American softwoods