Studies on the Manufacture of Low DP Pulps by Irradiation (Part 4)

Abstract

The free radicals formed in the irradiated pulps and the after effect of irradiation (that is, post-irradiation depolymerization) have been investigated.The degree of polymerization (DP) of gamma-irradiated pulps progressively reduces up to a few weeks in air at 20°C, but with heat treatment this period is shortened, for example, with only 15 hours of heat treatment at 60°C, DP of pulps settles down to a definite level.Also radicals formed by irradiation, detected and determined by electron spin resonance (ESR), decay very slowly at 20°C, but heat treatment increases this decaying speed. Although the decrement of radicals and the reduction of DP change in the similar tendency at relatively low temperature, the latter exceeds the former at above 100°C. Therefore, it may be said that, at high temperature, main chain scissions which are independent of the decay of radiation-induced radicals take place in pulps. However, this phenomenon occurs as well in original pulps.Heat treatment has negligible effect on the analytical properties of low DF pulps except that the higher temperature, the more copper number. On the other hand, “true” carbonyl groups estimated by cheletometric titrationmethod after oxidizing to carboxyl arc unchanged. This fact shows that alkali seensitivity of pulps which has been given by irradzation becomes much greater by heat treatrnent.In relation to the measurement of ESR absorption for the determination of radicals in pulps, it is observed that microwave power give a large effect on the pattern of absorption spectra. Namely, although at relatively low microwave power, ESR spectrum of irradiated cellulose shows five peaks, as the power gets higher, especially in cellulose I, the spectrum becomes “difuse”, and finally it shows only three peaks. This is important fact as to the discussion on the positions of radicals in irradiated cellulose, and moreover, considering as well that the patterns of the spectra decayed by heat treatment vary in accordance with temperature, it is appropriate to speculate that several species fadicals exist in irradiated cellulose molecule.