Effect of Hemicelluloses on the Difference of Papermaking Properties between Sulfite and Sulfate Pulps.

Abstract

Bleached sulfite, bisulfite and sulfate pulps were made from fir and birch wood chips (Table 1), and their papermaking properties were compared, especially on the tearing strength.Sulfite pulps are easy to swell by beating operation (Fig. 1), and higher in water retention value and accordingly lower in specific scattering coefficient of dried sheet, namely higher in fiber bonded area, compared with sulfate pulp beaten to the equal freeness (Fig. 2). On the other hand, tensile or bursting strengths of sulfate pulp are generally higher than those of sulfite pulps at the equal freeness (Fig. 3 and 4). This is mainly due to that sulfate pulp is higher in strength of a fiber-to-fiber bond than sulfite pulps. The reason is that tensile and bursting strengths of sulfate pulp are considerably higher than those of sulfite pulps at the equal specific scattering coefficient (Fig. 8), in spite of a little difference between their fiber strengths (Fig. 9).Sulfate pulp is considerably higher in tearing strength than sulfite pulps at the equal freeness (Fig. 3 and 4), in spite of a slight difference in fiber strength and in average fiber length (Fig. 12) between these pulps. On the other hand, in comparison between sulfite and sulfate pulps at the equal specific scattering coefficient, tearing strength of sulfate pulp is higher than that of sulfite pulps in the early stage of beating, but these strengths approach together in the later stage of beating (Fig. 10).These facts are probably due to the following reasons ; (1) The higher tearing strength of sulfate pulp than that of sulfite pulp at the beginning of beating is mainly due to the higher strength of fiber bonds in sulfate pulp, because most part of tearing energy is consumed by the deformation of fiber, destruction of fiber-to-fiber bond and the following inter-fiber friction in this stage. (2) The fact that the tearing strengths of sulfite and sulfate pulps at equal specific scattering coefficient approach together at the later stage of beating, is due to the nearly equal fiber strength of the pulps, because tearing energy is mainly consumed by the cutting of fiber in this stage. (3) In comparison between sulfite and sulfate pulps at the equal freeness, the former is considerably lower than the latter in tearing strength even at the later stage of beating. This is explained by higher swellability and greater fiber bonded area of dried sheet of sulfite pulp, and accordingly the tearing work is performed mainly on the cutting of fiber, and the deformation and slip of fiber are lesser compared with sulfate pulp at the equal freeness.In short, it is considered that the difference of papermaking properties between sulfite and sulfate pulps are mainly due to the difference in swellability and strength of a fiber-to-fiber bond between them.Bisulfite pulp is higher in tearing strength than conventional acid sulfite pulp (Fig. 3. 4 and 10). It is probably due to that the former is poor in swellabilty (Fig. 1 and 2) and higher in strength of a bond than the latter.Further consideration was made on the difference in tearing strength between fir and birch pulps.