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Wood Fiber Anatomy and Identification
28 WOOD FIBER ANATOMY AND IDENTIFICATION 28,1 FIBER ANALYSIS
high percentage of juvenile wood, fiber length will be shorter than published values. It takes a large investment of time to become proficient at fiber identification, and many references containing micrographs should be used. This chapter allows one to visualize the major differences in pulps that contribute to the papermaking properties and is useful for simple identifications, such as determining when various places in a mill are affected by the changeover of a digester line from one species to another. In mechanical pulps one can learn about the relative morphology of the contributing species by looking for fiber bundles.
Introduction The references for this chapter are found on pages 537—540 of Chapter 25. It is useful to look at wood in terms of its fiber anatomy and properties, since this is how it is used by the pulp and paper industry. There are many excellent references available for fiber analysis (see the Annotated Bibliography in Chapter 25), and much of the information presented in earlier chapters can be used. This chapter gives an overview of fiber morphology from wood and shows what features to look for in fibers for their identification. Information is also given to help determine pulping and bleaching methods used to prepare the pulp. It is ftindamental that the properties on paper depend on the fiber properties and the method of fiber preparation (pulping, bleaching, and fiber processing). It is difficult to separate wood prior to pulping (latewood from earlywood, fast growth wood from slow growth wood, compression wood from normal wood, juvenile wood from mature wood, etc.), so variability of wood must be overcome by varying the processing of the wood and pulp. The availability of and price of wood are as important as the properties of the wood itself. Fiber anatomy considerations Identification of fibers is difficult since there is much less information available than in the wood; identification is often down to a few members within a genus, rather than to a particular species. Other species, like Douglas—fir with spiral thickening, are unique and can be identified as to the species. The situation is complicated by the fact that many papers can contain two, three, or more types of fibers, especially if the paper contains appreciable amounts of recycled fiber. Panshin and de Zeeuw (1980, pp. 657—666) include a key for fiber identification. It is important to have authentic fiber samples of contributing species on hand for comparison with unknown samples. For example, if one is using a lot of "precommercial" thinnings with a
Weight factors When attempting quantitative analysis of fiber mixtures, the number of a particular type of fiber is multiplied by its weight factors since some types of fibers appear to be present in larger amounts than others, such as those with low surface areas. Weight factors are given in T 401 [based largely on Graff, J.H., Paper Trade J. 110(2):37(1940)]. The weight factors for coastal Douglas—fir and loblolly pine are 1.40, for most hardwoods 0.50, and for bleached straw 0.35. The higher the relative surface area (per given mass of pulp), the lower the weight factor. Parham and Gray (1990) give weight factors on page 35 [largely from D.W. Einspahr and J.D. Hankey, Tappi 61(12): 86-87(1978)]. One could determine one's own weight factors for a particular application. Weight factors are a crude method of taking number averages and converting them to weight averages. Fiber pitting Pitting, in general, is very useful in the determination of isolated fibers. However, one must keep in mind that pulping operations and refining are bound to affect the way a pit looks under the microscope. Intervessel pitting of hardwoods and ray cross—field pitting in softwoods are the principal means of identification. Ray cross—field pitting shows whether ray parenchyma and or tracheids are present; it also indicates the height of the rays.
628
FTOER ANALYSIS
Fiber and wood staining Chemical stains can help with the analysis by identifying the type of pulping method used to generate the fibers. This might be more important in brown papers that might have some recycled newsprint rather than in white printing papers, but these might have some bleached CTMP pulp. Hoadley (1990) references examples of species determination of wood using chemical stains including boxelder and other maples, red gum and black tupelo, elms, pond and sand pines, spruces and pines, and other groups. Separation of red and white oaks and red and sugar maples is discussed in Panshin and de Zeeuw (1980). Eastern spruce and balsam fir are separated by the method of Kutscha et al. in Wood Sci. and Tech. 12:293-308(1978). Separation of heartwood and sapwood is referenced (Kutscha and Sachs, 1962). Iodine is used to detect the presence of starch, which might be present in sapwood but not heartwood or in paper such as in the detection of forged currency. It is used with iodide to form the soluble I3' species in water (0.3% I2 and 1.5% KI in water). Safranin O is a general stain for cellulosic materials (red). It is used as a 0.5—1 % solution in water or 50% ethanol. Pulp stains are described in detail in Graff (1940), Isenberg (1967), and TAPPI Standard T401 om-82. Graff (1940) describes three types of stains. (1) Spot stains or groundwood reagents using chemicals such as aniline sulfate, phloroglucinol & hydrochloric acid, and/?—nitroaniline for detection of mechanical pulps in paper. The first class is said here to be of limited value. (2) The iodine/iodide metallic salt stains including the Herzberg stain, the "A" stain, and the "C" stain; these give qualitative information about fibers. (3) Aniline dyes for the determination of the degree of cooking, bleaching, and purity of pulps. Phloroglucinol is specific for lignin, causing it to turn red. It is used as a 2% solution in 18.5% HCl. Isenberg (1967) suggests mixing 1 g in 50 mL of EtOH to which is added 25 mL of concentrated HCl; this is made up immediately before use. One use is to show mechanical pulps in white papers. Mechanical softwood and hardwoods are differentiated with the use of 2% aniline sulfate made acidic (1 drop concentrated H2SO4 per 50
629
mL) followed by 0.02% methylene blue after removal of the first dye by blotting. Softwoods are yellow and hardwoods are bluish green. The Maule reaction can also be used. Mechanical pulp bleached with dithionite no more than a few months old is determined by detecting traces of SO2 or sulfite as H2S liberated by stannous chloride. Several stains are available to determine the level of bleaching (or cooking) of pulps, such as Bright stain. These can be used to check the uniformity of bleaching (and pulping) in the mill (Isenberg, 1967). Stains even exist for the identification of dirt, although scanning electron microscopy (SEM-EDDAX) page 184) has made many of these methods somewhat obsolete. Other techniques Monosaccharide analysis (although tedious) will give a good indication of the ratio of hardwoods to softwoods since galactose and mannose come from softwoods. Dissolving pulps will have low levels of hemicelluloses, of course. 28.2 SOFTWOOD FIBER Fibers over 2 mm long are most often softwood fibers. Be sure to use Table 26-3 (page 550) as a general guide to softwood fiber analysis, and Table 26-4 (page 552) as a summary of some important softwood fiber properties. Fig. 28-1 shows six types of softwood fibers. Parham and Gray (1990) claim that Douglas—fir, redwood, baldcypress, podocarp, parana—pine, sugar pine, white pine, and hard pines should be distinguishable under most circumstances, although these separations require a high degree of skill. Spruce, larch, and hemlock may not be able to be distinguished from each other as with true—fir and western redcedar. If the source of the pulp is known then regional information can be used (i.e., European or American; eastern or western, etc.). Ray cross—field pitting in softwoods is the principal means of identification. Examination of many ray cross—field pits shows whether ray tracheids are present, whether ray tracheids are marginal or interspersed, and may indicate the average height of the rays. For example. Fig. 28-2 shows some fibers with ray cross—field pitting on the tracheids. The
Douglas—fir
Sitka spruce
Southern pine
Western hemlock
Eastern white pine
Western cedar
Fig. 28-1. Various softwood fibers isolated in the laboratory (60x).
SOFTWOOD FIBER
western white pine shows the fenestriform ray cross—field pitting that is characteristic of the white pines and the small pits are from ray tracheids. Ponderosa pine has pinoid ray cross—field pitting with pits that are variable is size and up to 7 pits per cross—field. The Sitka spruce fiber shows piceoid ray cross—field pitting of uniform size with 3 or more pits per cross—field common. 28.3 HARDWOOD FIBER Be sure to use Table 27-1 (page 590) as a general guide to hardwood pulp analysis. Whole fibers less than 2 mm long usually originate from hardwoods. Hardwood fibers are accompanied by vessels or vessel fragments depending on the pulping process. The fibers shown in this chapter were isolated by a laboratory pulping process that
631
leaves the vessels largely intact. Furthermore, parenchyma cells have not been removed as occurs to various degrees in pulp processing. Fig. 28-3 shows fibers from hardwoods. Hardwood libriform fibers or fiber tracheids are not helpful in species determination unless helical thickening is observed. Vascular or vasicentric tracheids [the shape of vasicentric tracheids is observed in red oak (upper right) in Fig. 28-3], indicate the presence of certain species. However, vessel elements offer much information based on their overall size, shape, intervessel pitting, and ray contact pitting. The intervessel pitting of Populus (observe black Cottonwood in Fig. 28-3) and Salix genera are easy to recognize . Ring—porous woods may be difficult to determine since the large vessels are generally fragmented during processing.
Western white pine Ponderosa pine Sitka spruce Fig. 28-2. Portions of fiber tracheids of three softwoods (600x).
Black Cottonwood
red maple
Swamp tupelo
red oak
Eucalyptus grandis
yellow birch
Fig, 28-3. Various hardwood fibers isolated in the laboratory (60x).
high percentage of juvenile wood, fiber length will be shorter than published values. It takes a large investment of time to become proficient at fiber identification, and many references containing micrographs should be used. This chapter allows one to visualize the major differences in pulps that contribute to the papermaking properties and is useful for simple identifications, such as determining when various places in a mill are affected by the changeover of a digester line from one species to another. In mechanical pulps one can learn about the relative morphology of the contributing species by looking for fiber bundles.
Introduction The references for this chapter are found on pages 537—540 of Chapter 25. It is useful to look at wood in terms of its fiber anatomy and properties, since this is how it is used by the pulp and paper industry. There are many excellent references available for fiber analysis (see the Annotated Bibliography in Chapter 25), and much of the information presented in earlier chapters can be used. This chapter gives an overview of fiber morphology from wood and shows what features to look for in fibers for their identification. Information is also given to help determine pulping and bleaching methods used to prepare the pulp. It is ftindamental that the properties on paper depend on the fiber properties and the method of fiber preparation (pulping, bleaching, and fiber processing). It is difficult to separate wood prior to pulping (latewood from earlywood, fast growth wood from slow growth wood, compression wood from normal wood, juvenile wood from mature wood, etc.), so variability of wood must be overcome by varying the processing of the wood and pulp. The availability of and price of wood are as important as the properties of the wood itself. Fiber anatomy considerations Identification of fibers is difficult since there is much less information available than in the wood; identification is often down to a few members within a genus, rather than to a particular species. Other species, like Douglas—fir with spiral thickening, are unique and can be identified as to the species. The situation is complicated by the fact that many papers can contain two, three, or more types of fibers, especially if the paper contains appreciable amounts of recycled fiber. Panshin and de Zeeuw (1980, pp. 657—666) include a key for fiber identification. It is important to have authentic fiber samples of contributing species on hand for comparison with unknown samples. For example, if one is using a lot of "precommercial" thinnings with a
Weight factors When attempting quantitative analysis of fiber mixtures, the number of a particular type of fiber is multiplied by its weight factors since some types of fibers appear to be present in larger amounts than others, such as those with low surface areas. Weight factors are given in T 401 [based largely on Graff, J.H., Paper Trade J. 110(2):37(1940)]. The weight factors for coastal Douglas—fir and loblolly pine are 1.40, for most hardwoods 0.50, and for bleached straw 0.35. The higher the relative surface area (per given mass of pulp), the lower the weight factor. Parham and Gray (1990) give weight factors on page 35 [largely from D.W. Einspahr and J.D. Hankey, Tappi 61(12): 86-87(1978)]. One could determine one's own weight factors for a particular application. Weight factors are a crude method of taking number averages and converting them to weight averages. Fiber pitting Pitting, in general, is very useful in the determination of isolated fibers. However, one must keep in mind that pulping operations and refining are bound to affect the way a pit looks under the microscope. Intervessel pitting of hardwoods and ray cross—field pitting in softwoods are the principal means of identification. Ray cross—field pitting shows whether ray parenchyma and or tracheids are present; it also indicates the height of the rays.
628
FTOER ANALYSIS
Fiber and wood staining Chemical stains can help with the analysis by identifying the type of pulping method used to generate the fibers. This might be more important in brown papers that might have some recycled newsprint rather than in white printing papers, but these might have some bleached CTMP pulp. Hoadley (1990) references examples of species determination of wood using chemical stains including boxelder and other maples, red gum and black tupelo, elms, pond and sand pines, spruces and pines, and other groups. Separation of red and white oaks and red and sugar maples is discussed in Panshin and de Zeeuw (1980). Eastern spruce and balsam fir are separated by the method of Kutscha et al. in Wood Sci. and Tech. 12:293-308(1978). Separation of heartwood and sapwood is referenced (Kutscha and Sachs, 1962). Iodine is used to detect the presence of starch, which might be present in sapwood but not heartwood or in paper such as in the detection of forged currency. It is used with iodide to form the soluble I3' species in water (0.3% I2 and 1.5% KI in water). Safranin O is a general stain for cellulosic materials (red). It is used as a 0.5—1 % solution in water or 50% ethanol. Pulp stains are described in detail in Graff (1940), Isenberg (1967), and TAPPI Standard T401 om-82. Graff (1940) describes three types of stains. (1) Spot stains or groundwood reagents using chemicals such as aniline sulfate, phloroglucinol & hydrochloric acid, and/?—nitroaniline for detection of mechanical pulps in paper. The first class is said here to be of limited value. (2) The iodine/iodide metallic salt stains including the Herzberg stain, the "A" stain, and the "C" stain; these give qualitative information about fibers. (3) Aniline dyes for the determination of the degree of cooking, bleaching, and purity of pulps. Phloroglucinol is specific for lignin, causing it to turn red. It is used as a 2% solution in 18.5% HCl. Isenberg (1967) suggests mixing 1 g in 50 mL of EtOH to which is added 25 mL of concentrated HCl; this is made up immediately before use. One use is to show mechanical pulps in white papers. Mechanical softwood and hardwoods are differentiated with the use of 2% aniline sulfate made acidic (1 drop concentrated H2SO4 per 50
629
mL) followed by 0.02% methylene blue after removal of the first dye by blotting. Softwoods are yellow and hardwoods are bluish green. The Maule reaction can also be used. Mechanical pulp bleached with dithionite no more than a few months old is determined by detecting traces of SO2 or sulfite as H2S liberated by stannous chloride. Several stains are available to determine the level of bleaching (or cooking) of pulps, such as Bright stain. These can be used to check the uniformity of bleaching (and pulping) in the mill (Isenberg, 1967). Stains even exist for the identification of dirt, although scanning electron microscopy (SEM-EDDAX) page 184) has made many of these methods somewhat obsolete. Other techniques Monosaccharide analysis (although tedious) will give a good indication of the ratio of hardwoods to softwoods since galactose and mannose come from softwoods. Dissolving pulps will have low levels of hemicelluloses, of course. 28.2 SOFTWOOD FIBER Fibers over 2 mm long are most often softwood fibers. Be sure to use Table 26-3 (page 550) as a general guide to softwood fiber analysis, and Table 26-4 (page 552) as a summary of some important softwood fiber properties. Fig. 28-1 shows six types of softwood fibers. Parham and Gray (1990) claim that Douglas—fir, redwood, baldcypress, podocarp, parana—pine, sugar pine, white pine, and hard pines should be distinguishable under most circumstances, although these separations require a high degree of skill. Spruce, larch, and hemlock may not be able to be distinguished from each other as with true—fir and western redcedar. If the source of the pulp is known then regional information can be used (i.e., European or American; eastern or western, etc.). Ray cross—field pitting in softwoods is the principal means of identification. Examination of many ray cross—field pits shows whether ray tracheids are present, whether ray tracheids are marginal or interspersed, and may indicate the average height of the rays. For example. Fig. 28-2 shows some fibers with ray cross—field pitting on the tracheids. The
Douglas—fir
Sitka spruce
Southern pine
Western hemlock
Eastern white pine
Western cedar
Fig. 28-1. Various softwood fibers isolated in the laboratory (60x).
SOFTWOOD FIBER
western white pine shows the fenestriform ray cross—field pitting that is characteristic of the white pines and the small pits are from ray tracheids. Ponderosa pine has pinoid ray cross—field pitting with pits that are variable is size and up to 7 pits per cross—field. The Sitka spruce fiber shows piceoid ray cross—field pitting of uniform size with 3 or more pits per cross—field common. 28.3 HARDWOOD FIBER Be sure to use Table 27-1 (page 590) as a general guide to hardwood pulp analysis. Whole fibers less than 2 mm long usually originate from hardwoods. Hardwood fibers are accompanied by vessels or vessel fragments depending on the pulping process. The fibers shown in this chapter were isolated by a laboratory pulping process that
631
leaves the vessels largely intact. Furthermore, parenchyma cells have not been removed as occurs to various degrees in pulp processing. Fig. 28-3 shows fibers from hardwoods. Hardwood libriform fibers or fiber tracheids are not helpful in species determination unless helical thickening is observed. Vascular or vasicentric tracheids [the shape of vasicentric tracheids is observed in red oak (upper right) in Fig. 28-3], indicate the presence of certain species. However, vessel elements offer much information based on their overall size, shape, intervessel pitting, and ray contact pitting. The intervessel pitting of Populus (observe black Cottonwood in Fig. 28-3) and Salix genera are easy to recognize . Ring—porous woods may be difficult to determine since the large vessels are generally fragmented during processing.
Western white pine Ponderosa pine Sitka spruce Fig. 28-2. Portions of fiber tracheids of three softwoods (600x).
Black Cottonwood
red maple
Swamp tupelo
red oak
Eucalyptus grandis
yellow birch
Fig, 28-3. Various hardwood fibers isolated in the laboratory (60x).