3102 



Chapter 25 



0.5 1.0 1.5 



FIBER LENGTH (mm) 



2.0 



200 



150 



100 



50 



°0.5 



1.0 



1.5 



FIBER LENGTH (mm J 



M 145 674, M 145 675 

 Figure 25-1 1. — Influence of hardwood kraft pulp fiber length on the tearing resistance 

 of pulp sheets made from the unbleached fibers. (Left) Unbeaten. (Right) Beaten to 40 

 ml Canadian Standard Freeness. (Drawings after Horn 1978.) 



Horn (1978) cooked all the pulps to a comparable lignin content, i.e., to a 

 Kappa number from 18 to 22, and made the morphological measurements (table 

 25-6) before beating the fibers. Mechanical properties of the pulps were deter- 

 mined before and after beating to a Canadian Standard Freeness (CSF) of 400 

 ml, and all pulp handsheets were prepared according to TAPPl Standard Proce- 

 dures (T205-m60). Relationships observed are summarized in table 25-7. 



Tear strength. — Horn (1978) found that tearing strength of sheets made from 

 either unbeaten or beaten hardwood fibers is principally dependent on fiber 

 length (fig. 25-1 1) — longest pulp fibers yielded greatest tearing strength, which 

 contrasts with paper made from softwood pulps in which cross-sectional area 

 and cell-wall thickness are the dominant variables affecting tearing strength 

 (Horn 1974). Fibril angle also showed a significant correlation in unbeaten pulps 

 with tearing strength (fibers with largest fibril angles had greatest tearing 

 strength). Multiple regression analysis showed that the interaction of fiber length 

 and fibril angle could account for 78 percent of the variation in tearing strength 

 of unbeaten pulps — indicating that tearing strength is influenced more by fiber 

 extensibility than fiber strength. In hand sheets made of unbeaten pulp, fiber 

 length was the dominant factor related to tearing strength. 



