Pulp Strength Delivery

Industrially produced softwood kraft pulps typically show only 60–80% of the

strength performance as compared to laboratory reference pulps. The strength

delivery is defined as the percentage of the tear index at a given tensile index from

an industrially cooked pulp in relation to the corresponding tear index of a laboratory

reference pulp from the same chip material. Extensive mill studies revealed

an average strength delivery from softwood batch cooking of 72% and 73%,

respectively [63,64]. The major reason for the low strength delivery of industrially

cooked pulps has been attributed to the hot blow which exerts a destructive effect

on the pulp fibers [22].The development of a more gentle pump discharge

improved the strength delivery to 84% as compared to only 70% using hot blow

discharge [65]. Surprisingly, the deleterious effect of hot blow on strength delivery

diminishes when cooking is performed according to the displacement technology,

as reported for the Superbatch technology. Mill trials at the Joutseno-Pulp Oy’s

digester house confirmed that when using liquor displacement cooking technique

the blow method made very little difference in the strength delivery. Blowing a

kappa-30 displacement batch cook without terminal displacement directly at cooking

temperature still resulted in about 90% strength delivery as compared to

100% in the case of a cold blow (see Tab. 4.29) [66].

256 4 Chemical Pulping Processes

Tab. 4.29 Strength delivery (percentage of tear index at 70Nm· g–1

tensile index to reference laboratory pulps) of industrially cooked

softwood pulps at different kappa number levels, and for both

cold and hot blow (according to [66]).