3110 Chapter 25 



Trends shown in figure 25-4 continued during the latter half of the 1970's, 

 i.e., production of kraft, semichemical, and mechanical pulp increased, while 

 that of sulfite pulp declined. Waste paper consumption, for recycling, increased 

 from about 8 million tons in 1950 to nearly 15 million tons in 1978. By 1981 

 about 20 percent of the paper and paperboard consumed in the United States was 

 from recycled paper. 



BEATING 



Paper has strength because wet cellulose surfaces bond to each other when 

 brought together and dried — chiefly by forming hydrogen bonds. The strength 

 of these bonds is enhanced if fibers are collapsed to flexible ribbon-like form 

 (rather than tubular), and if fibers are fibrillated (i.e. , fibrils partly loosened) on 

 ends and surfaces. For adequate sheet strength, most stock is prepared in beaters 

 or refiners which mechanically alter fiber form to yield strong bonds. Most 

 strength properties of paper, particularly tension and bursting strength, are 

 enhanced by beating (fig. 25-15). 



The two major effects of beating are cutting and fibrillation. While cutting 

 shortens average fiber length, and impairs strength if carried to extreme, it 

 forestalls undesirable floculation of clumps of long flexible fibers which other- 

 wise appear as dense mottled areas in paper sheets. 



Fibrillation in beaters or refiners alters fiber surfaces by macerating, fraying, 

 brooming, and generally loosening the fibrillar structure of the cell wall — 

 making more surface on each fiber available for bonding. Beaten fibers (fig. 25- 

 16 bottom) are less well defined than unbeaten ones (fig. 25-16 top), but they 

 bond together to produce stronger, smoother, denser (more compact), and gen- 

 erally better paper. 



Differences in beatability between hardwoods and softwoods are attributable 

 primarily to their differing fiber morphology. Most investigators have found that 

 hardwood pulps are beaten faster and with less energy than softwood pulp. Pulps 

 with a small hardwood component may be beaten as a mixture, but if the 

 hardwood component is near 50 percent or greater, better paper properties may 

 be obtained by beating the hardwood separately from the softwood pulp. There 

 is a very large literature on the subject of beating hardwoods. Following are a 

 few references on the subject: Ai et al. (1978); Bobrov and Ershov (1978); 

 Gorbacheva and Ivanov (1968); Hamada and Matsumoto (1977); Higgins et al. 

 (1973); Hunt and Hatton (1976); Kibblewhite and Brookes (1977); Kijima and 

 Yamakawa (1978ab); Levlin (1976, 1980); Lonnberg (1976); and Marton 

 (1976). 



SHEET FORMATION 



Prepared fibers are continuously formed into paper sheets by suspending the 

 pulped fibers in a water slurry and causing the slurry to flow from a headbox onto 



