3104 



Chapter 25 



From extensive studies relating hardwood (mostly tropical) fiber morphology 

 and strength to paper sheet properties, Wangaard and Williams (1970) conclud- 

 ed that tear strength tends to increase with increasing fiber length in the lower 

 range of sheet densities, culminates at a level of sheet density which increases 

 with increasing fiber strength and, at a specified level of fiber strength, at high 

 sheet tear strength declines with increasing fiber length. They found that kraft 

 pulps made from hardwood fibers 1 mm long attained maximum tear strength at 

 sheet densities of 0.62 to 0.85 g/cm^ At the very lowest levels of sheet density 

 there was essentially no effect of fiber length on tearing strength. At some sheet 

 density beyond a critical level of bonding, long fibers had an adverse effect on 

 tear resistance. Readers wanting to further study relationships between hard- 

 wood fiber morphology and pulp sheet mechanical properties should read Tamo- 

 lang and Wangaard (1961), Wangaard (1962), and Kellogg and Wangaard 

 (1964). 



Stretch. — Horn (1978) found that fibril angle and fiber length were individ- 

 ually correlated with stretch properties of paper made from hardwood pulps; 

 those made with long fibers or those having a large fibril angle stretched most. 



In unbeaten pulps, fiber length accounted for 78 percent of the variation in 

 stretch. After beating, the effect of fiber length was negligible and fibril angle 

 was the dominant single variable, accounting for 45 percent of the variation in 

 stretch. 



By multiple regression analyses, fiber coarseness was related to stretch, i.e., 

 paper with the longest fibers of least coarseness stretched most. For unbeaten 

 pulps these two factors accounted for 92 percent of the variation; for beaten 

 pulps they accounted for 75 percent of the variation in stretch. 



60 



45 



30 



15 



0.5 



1.0 



1.5 



2.0 



100 



FIBER LENGTH (mm) 



200 300 400 



L/T RATIO 



500 



M 145 676, M 145 677 



Figure 25-12. — Burst strength of sheets made from unbleached hardwood kraft pulps. 



(Left) Unbeaten, related to pulp fiber length. (Right) Beaten to 400 ml Canadian 



Standard Freeness, related to L/T ratio, i.e., the ratio of fiber length to cell wall 



thickness of the pulp fiber. (Drawings after Horn 1978.) 



