Pulp and Paper 3115 



While variations as prominent as those shown in figure 25-17 (bottom) are not 

 uncommon, current practice when drying newsprint to 8-percent moisture con- 

 tent might give moisture differences across the sheet of ± y4-percent. Metcalfe 

 (1968) states that with proper attention to drying conditions, moisture differ- 

 ences across the sheet can be reduced to 0.3 percent. 



Dryers on modem paper machines are fast, large, and expensive. Newsprint 

 running at 2,500 lineal fpm might pass over 50 rolls each 5 feet in diameter and 

 25 feet long. Temperature of the first rolls is usually about 180°F, and of the 

 main group of rolls about 240°F. Passage through such a dryer would reduce the 

 moisture content of newsprint from about 62 percent to about 9 percent. 



The reader desiring a more fundamental discussion of fiber and mat drying 

 phenomena is referred to Wrist (1966) and Han and Matters (1966). Other 

 papers, that should be helpful to readers desiring additional information include: 

 Coulson (1981); Gardner (1970); Han (1970); Hankin et al. (1970); Herdman 

 (1970); lannazzi and Strauss (1970); Janett (1970); Kennedy (1970); Khandel- 

 wal (1970); Lee and Hinds (1981); Race (1970); Rantala (1970); Sledge and 

 Knox (1981); Wieselman (1970); Rhorer (1971); and Wahlstrom (1970, 1981). 



25-4 PRESS DRYING 



The shortness and thick walls of hardwood fibers limit their use in paper- 

 boards and papers requiring high strength. Extensive beating of such fibers to 

 improve their bonding capability develops excessive fines that cause drainage 

 problems or low wet- web strengths that prevent their use on fast paper machines 

 (Klungness and Sanyer 1981). 



As Whitney (1980) has noted, the corrugated container is the principal prod- 

 uct of the paperboard industry. Unbleached kraft board, which mostly goes into 

 the linerboard of corrugated boxes, dominates paperboard production (fig. 25- 

 8). Thus, if use of hardwood pulps is to be greatly expanded in southern mills, 

 their utility in corrugated containers must be enhanced by new technology. 



V. C. Setterholm and his associates at the U.S. Forest Products Laboratory in 

 Madison, Wisconsin have made significant progress in developing such technol- 

 ogy (Setterholm et al. 1975; Setterholm and Benson 1977; Byrd 1979ab; Setter- 

 holm 1979; Horn 1979; Setterholm and Ince 1980). They found that press 

 drying of hardwood paper, particularly heavier weights such as linerboard and 

 corrugating medium for containers (fig. 25-18), offers outstanding potential. In 

 press drying (fig. 25-19) stiff pulp fibers are dried under a compressive force that 

 induces greater conformability and interfiber bonding, as well as more restraint 

 during drying, than attained by conventional methods for drying paper. The 

 result is paper having the highest specific strength and stiffness available for any 

 given pulp furnish. These improvements over conventionally dried paper in- 

 crease with increased fiber stiffness; press drying improves paper from high- 

 yield pulp fibers more than that from low-yield well-beaten fibers. Setterholm 

 (1979) found that high-yield hardwood pulp fibers could be press dried to make 

 linerboard (fig. 25-18 top) having higher compression strength and stability than 



