Effect of Hemicelluloses on Papermaking Properties of Bisulfite and Bisulfite-soda Pulps

Abstract

Bleached bisulfite and bisulfite-soda pulps were made from fir or birch wood (Table 1 and 2), and their paperniaking properties were compared with those of conventional acid sulfite and sulfate pulps.
1. Bisulfite pulp lies generally between conventional acid sulfite and sulfate pulps in papermaking properties. This is probably due to the difference between the properties of their hemicelluloses. For instance, lower beating rate and lower swellability of bisulfite pulp than those of acid sulfite pulp (Fig. 1) are mainly due to the lower amount of hydrophilic hemicellulose (Table 1).
On the other hand, hemicellulose in bisulfite pulp is higher than that in acid sulfite pulp in degree of polymerization (Fig. 3) and viscosity of the aqueous solution, and consequently strength of a fiber. to-fiber bond (Fig. 2 B) and tearing strength of bisulfite pulp are higher than those of acid sulfite pulp. These properties of the hemicellulose in bisulfite pulp are, however, so poorer than those of sulfate pulp hemicellulose, that the tearing strength of bisulfite pulp is inferior to that of sulfate pulp.
2. Bisulfite-soda cooking consists of a sodium bisulfite first stage followed by an alkaline stage. In the alkaline stage of this cooking, a part of hemicellulose in bisulfite pulp, especially in the case of hardwood pulp, is removed (Table 3), and the pulp obtained decreases more than bisulfite pulp in beat ing rate and swellability (Fig. 4). The bisulfite-soda pulp is generally higher in tearing strength than bisulfite pulp (Fig. 5). This is probably due to that the hemicellulose of lower degree of polymeriza tion, existing in the outer layer of fiber, is removed in the second alkaline cooking, and degree of polymerization of residual hemicellulose increases, especially at the points of fiber-to-fiber bond.
In the bisulfite-soda cooking also, degree of polymerization of hemicellulose has already been decreased in the first bisulfite cooking, so that the pulp is lower in tearing strength compared with sulfate pulp.
3. The reason why the pulps produced by new processes such as the Sivola process (acid sulfite first stage followed by an alkaline stage) or the Stora process (bisulfite first stage followed by an acid sulfite stage) are also lower in tearing strength than sulfate pulp, is probably due to the difference of the properties of their hemicelluloses, such as degree of polymerization etc.