3106 Chapter 25 



Table 25-8. — Variation in bursting and tensile strengths, due to fines content, of hand- 

 sheets made from beaten white oak kraft pulp (Horn 1978) 



Proportion of fines' Burst Tensile 



(percent) factor strength 



Psi 

 0^ 65 9,500 



18. 8^ 56 8,950 



'Fines defined as that fraction of furnish passing 200-mesh screen; this fraction was comprised 

 primarily of parenchyma cells plus a small amount of short fiber segments and vessel element 

 fragments. 



%ines removed and fiber fraction beaten to 400 ml Canadian Standard Freeness. 



^Fractionated fiber furnish beaten to 400 ml Canadian Standard Freeness, and fines added. 



Modulus of elasticity. — Horn (1978) found that the best single factor for 

 predicting modulus of elasticity of unbeaten pulps was unextracted specific 

 gravity of wood (r = -0.730). The second-best was L/T (r = 0.663). Multiple 

 regression analysis suggested that, of the fiber parameters, fibril angle and L/T 

 could account for 89 percent of the variation in modulus of elasticity of unbeaten 

 pulps. Thus, for unbeaten pulps, stiffness was promoted by use of low-density 

 woods having long slender fibers with small fibril angles. 



For beaten pulps, the best indicator was L/T ratio (r = 0.772), and the second 

 best was unextracted specific gravity (r = -0.680). Multiple regression analy- 

 ses indicated that 95 percent of the variation in modulus of elasticity could be 

 accounted for by fibril angle and L/T ratio. 



Horn (1978) concluded that attainment of good stiffness properties in paper 

 made from hardwoods is greatly dependent upon fiber characteristics that pro- 

 mote fiber bonding, i.e., flexibility, collapsibility, and conformability. Also, 

 Horn found that sheet density accounted for 92 percent of the variation in 

 modulus of elasticity when using unbeaten pulps and 82 percent when using 

 beaten hardwood kraft pulps from this study. 



Press drying of hardwood paper and paperboard (see section 25-4) may 

 significantly alter relationships observed in handsheets formed by the traditional 

 standard methods. 



PULPING 



There are three principal methods for producing pulp fibers from wood: 



• by application of mechanical force 



• by reacting wood with chemicals at elevated temperatures 



• by combination of the two foregoing methods 



Selection of pulping process depends on wood species, product requirements, 

 and a host of economic and environmental factors. Primarily, however, process 

 selection is influenced by the completeness with which lignin must be removed 



