3108 Chapter 25 



Timell (1967) found cellulose distribution in fibers oi Betula verrucosa, as 

 follows: 



Cellulose portion of total 

 Cell layer polysaccharides in layer 



Percent 



Middle lamella-primary wall 41 



Secondary wall 48-60 



Cellulose concentration was highest and hemicelluloses lowest, in wall regions 

 nearest fiber lumens. 



Wood rays of hardwoods, e.g., white oak, have appreciably greater lignin 

 content and lower cellulose content than total stemwood (Harlow and Wise 

 1928). Timell (1969) reported that tension wood (figs. 5-71 and 5-72), on a 

 gross weight basis, has less lignin and hemicellulose and more cellulose than 

 normal wood; per fiber, however, there is as much lignin in tension wood as in 

 normal wood. 



Distribution of chemical constituents across annual growth increments of 

 scarlet oak is shown in figure 6- 1 . 



Historically, the first approach to pulping was mechanical. Softwoods, but not 

 hardwoods, can be successfully ground — usually in 4- or 5-foot-long lengths — 

 against a revolving cylindrical stone. (See page 2238, and Koch 1972, p. 1423- 

 1426 for descriptions of the process). Short-fibered southern hardwoods yield 

 principally unusable fines when prepared by this method. In another mechanical 

 process, hardwood chips can be reduced to refiner groundwood fiber by passing 

 the chips through at least two stages of single- or double-disk refiners. This 

 process is described in sections 18-27, 23-6, and 25-5. Mechanical pulps have 

 essentially the same composition as the wood from which they were made, 

 except for loss of a small percentage of water-soluble material. These mechani- 

 cal pulps, retaining all lignin, produce poorly bonded paper with low strength 

 and short life, but good printability (Whitney 1980). Mechanical pulps from 

 hardwoods are usually combined with other types of fiber in papermaking. 



Freeing cellulose fibers and leaving them intact requires a chemical treatment 

 to remove non-cellulosic wood components. In practice neither objective is 

 completely realized, but several processes reach satisfactory compromises at 

 pulp yields of 45-55 percent of wood weight (Whitney 1980). For some special- 

 ized pulps, higher-yield semichemical pulping processes, retaining some hemi- 

 celluloses and lignin, may yield 60 to 90 percent. They usually require some 

 mechanical attrition to disintegrate the wood chips after chemical treatment. 

 Whitney (1980) concluded that all commercial chemical and semichemical 

 pulping processes remove more carbohydrate material than lignin from wood, 

 but that this may not be true in the future. 



The chemical reagents commonly used for pulping can be arranged according 

 to pH and grouped according to pulping process (fig. 25-14). Development of 

 these processes, as described by Whitney (1980), has occurred over the past 150 

 years. 



Alkalies have been used for centuries to separate cellulose fibers from the 

 other components of non woody plants but their successful use for the pulping of 



