制漿造紙專業(yè)單詞7

1、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, providethe remainder. Approximately one-third of all paper products are

2、recycled into secondary fiber.2.1 TREE STRUCTUREA tree can be considered to have three general parts:? the crown composed of leaves and branches?the stem?the root systemThe leaves or needles are the factories where food material is manufactured through photosynthesis to provide the tree with energy

3、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

4、 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 aFiber Supplytree trunk revealing the general structur

5、e. 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-wa

6、ll 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 narrowlayer of tissue where the carbohydrate-containing sap moves upward a

7、nd 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 suppor

8、t for the crown, acts as a food storage reservoir, and provides the important function of water conduction up from the roots. It isphysiologicallyactive(parenchymacells only) and in continuous communication with the cambium and12phloem through sap flow from the crown.FIGURE2-2. Cross-sectional sketc

9、h of a maturestem, showing outer bark, inner bark, sapwoodand 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

10、 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. Ina few species (most notably spruce) the color difference between heartwood and sapwood is minor. At the center of the tree is

11、 a small core of soft tissue called the pith.2.2 CHARACTERISTICS OF WOODBotanically, 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 feat

12、ures of each wood group are illustrated in Figures 2-3 and 2-4.SoftwoodsThe 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 narrowrays, only one cell in

13、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

14、in only certain species.Seasonal growth is usuallycharacterized 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 latew

15、ood. 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 a

16、nd 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 sy

17、mbols S 3 and T are used interchangeably. Figure 2-6 shows additional detail with respect tomicrofibrillar layers (or laminae), alignments and textures.Radial cross-sections of four representative North American softwoods("typical" pine, western red cedar, Douglas fir, hemlock) are shown u

18、nder increasing magnification in Figures 2-7 to 2-10. Note the distinctive features of each specie and the differences in cell wall thickness between earlywood and latewood. A three-dimensional view of spruce wood is shown in Figure 2-11 and under increasing magnification in Figures 2-12 and 2-13.FI

19、GURE 2-5. Diagram of cell wall organization.Table 2 1 Layers of softwood tracheid (20-40 microns diameter)Middle-bond between fibers,mostlyLamellalignin(Ml)Primary Wall- a thin, relatively impermeable(P)covering about 0.05um thickSecondary- makes up bulk ofcellwall;Wall(S) forms three distinct layer

20、s characterized by different fibril alignments;* S1 is the outer layer of the secondary wall (about 0.1 -0.2 um thick)* S2 forms the main body of the fiber and is from 2 to 10 um thick* S3 is the inner layer of thesecondary wall (about 0.1um thick)Tertiary Wall- same as S3(T)Lumen (L)the central can

21、al of fiber (void)HardwoodsTheprincipalverticalstructureofhardwoods is composed of both relativelylong, narrow cells, called libriformfibers,andmuchshorter,widercells,calledvessels. Vessels in a typical hardwoodeye, incross-section as "pores"oronvertical surfaces us a series of long groove

22、s.Hardwoodsalsohaveaverticalparenchyma system and a horizontal or rayparenchyma system. sample are often largeenough in diameter to be seen easily withthe nakedVessel diameter varies from earlywood to latewood within an annual ring. If this difference is extreme and abrupt, the rings become easy to

23、distinguish, and the wood is termed ring-porous (Figure 2-14). In other species where the gradation in vessel diameter is small and gradual, the annual rings are more difficult to distinguish, and the wood is termed diffuse-porous (Figure 2-15). A three-dimensional view of diffuse-porous white birch

24、 is shown in Figure 2-16.Softwoods vs. HardwoodsThe dramatic difference between softwood and hardwood (e.g. spruce vs. birch I with respect to weight and volume percentages of the various types of fiber cells is illustrated in Table 2-2. Another major difference is the length of the fibers: a typica

25、l relationship between biological age of wood and fiber length (Figure 2-17) shows that softwood fibers are more than twice as long as hardwood fibers.The average relative density (oven-dry14weight per green volume) of commonly used coniferous pulping species ranges from 0.31 for western red cedar t

26、o 0.55 in western larch. This difference in density relates to the fact that cedar has relatively little latewood tissue while larch has a large proportion. Hardwood relative density ranges from 0.30 in black cottonwood-to over 0.60 for rock elm, hickory and white oak which all contain thicker-walle

27、d fibers.TABLE 2-2. Types of cells - spruce vs. birch.Fibers (%)VesselsParenchyma (%)bybybybybybywtvolwt.vol.wtvol.Spruce9995-15birch8665925510A relationship exists (within the respective softwood and hardwood groupings) between wood density and a number of pulping parameters. For example, the yield

28、 of pulp per unit volume of wood is usually directly related to density. A high wood density generally indicates a slower beating response for the pulp, lower tensile, burst and fold strengths, greater bulk, and higher tear strength. As shown in Table 2-3. Hardwoods tend to have higher densities tha

29、n softwoods, and the southern pine species are denser than softwoods from other growth regions. Generally hardwoods contain a larger proportion of holocellulose and less lignin as compared to softwoods, but a greater percentage of extractives. The average compositions and the normal range of values

30、are shown in Figure 2-18.TABLE 2-3. Properties of North Americanlengthdiameterdensity(mm)(microns)(lb/cu ft)Southern RegionLongleaf Pine4.935-4541Shortleaf Pine4.635-4536Loblolly Pine3.635-4536Slash Pine4.635-4543Northeast RegionBlack Spruce3.525-3030White Spruce3.325-3026Jack Pine3.528-4030Balsam F

31、ir3.530-4025Northwest RegionDouglas Fir3.935-4534Western Hemlock4.230-4029Redwood6.150-6525Red Cedar3.530-4023HardwoodsAspen1.0410-2727Birch1.8520-3638Beech1.2016-2245Oaks1.4014-2246Red Gum1.7020-4034BarkBark is the outer covering or rind of woody stems and branches. It is distinct and separable fro

32、m wood. The structure is complex (as compared to wood) because bark contains three types of tissue (cortex, periderm and phloem), each of which has several types of cells. While bark generally is considered to be a contaminant in pulping operations, some types (e.g., western red cedar and aspen) con

33、tain significant quantities of fiber and can be tolerated to an extent in an alkaline pulping system.However, certain bark constituents are resistant to typical pulping conditions, principally cork cells, dense sclereids or "stone cells" (see Figure 2-19), and cells impregnated with extrac

34、tives. The extractives consume relatively largeSpeciesFiberFiberWoodamountsofchemical,whilepartiallypulpedpaniclesremainas dininthefinished pulp.FIGURE 2-19. Schlereid fiber bundle (MacMillan Bloedel Research Ltd.)Greater amounts of bark are tending to be introduced into the pulp mill with the chip

35、furnish because of more intense tree utilization (e.g., whole-tree chiping). Techniques are employed in the pulp mill to remove bark from the chips and remove bark specks from the pulp.2.3EFFECTOFFIBERSTRUCTURE(MORPHOLOGY)ON PAPER PROPERTIESThe properties of paper are dependenton the structural char

36、acteristics of the various fibers that compose the sheet.Undoubtedlythe two most important ofthese characteristics are fiber length and cellwall thickness. Aminimum lengthisrequiredfor interfiberbonding,andlengthisvirtuallyproportionaltotearstrength. Acomparison ofsome softwoodandhardwoodcelltypesis

37、 showninFigure 2-20.Ingeneral,softwoodtracheids with"relatively thin cell walls collapse readily into ribbons during sheet formation (e.g., westernred cedar in Figure 2-21). Tracheids withthicker cell walls resist collapse and do not contribute to interfiber bonding to the sameextent.Thethicker

38、-walledfibers(e.g.,Douglas fir in Figure 2-22) tend to producean open, absorbent, bulky sheet with low burst/tensile strength and high tearingresistance. Withinspecies, the thin-walledearlywood tracheids are relatively flexible, while the latewood tracheids, having as much as.60 to 90 % of their vol

39、ume in cell wall material, are less conformable.To help illustrate the basic principle, Figure 2-23 shows two types of idealized fiber structures. In the upper representation (A), the thick-walled fibers are shown as hollow cylinders; the thin-walled fibers below (B) are shown as ribbon-like element

40、s. The numbers of fibers and contact points are the same in both structures; but the area of contact and potential bonding sites are clearly much greater in the sheet composed of thin-walled fibers.The ratio of pulp fiber length to cell wall thickness (L/T) is sometimes used as an index of relative fiber flexibility. However, a more specific indication of a fiber's behavior is provided by its coarseness value. Fiber coar