Free Radical Mediated Cellulose Degradation during High Consistency Ozonation

Abstract

A study was performed on the ozonation of cotton linters at high consistency, 40%, and pH3, in order to shed some light on the mechanism of degradation of cellulose during pulp bleaching. Ferryl ions (FeO2+), produced in the reaction of O3 with Fe(II), was used to initiate free radical chain reactions to investigate the relation between chain reactions and degradation. The degradation was evaluated by viscometry. The degradation of cellulose responded more strongly to the propagation than to the initiation of the chain reactions. Also, the results suggest that the reactions responsible for initiation do not in themselves contribute to the observed degradation. Complementary experiments were made on cellulose beads (<5% crystalline). In contrast to the results from the cotton linters, the degradation of cellulose beads appears to correspond directly to ferryl ion formation. In non‐structured cellulose (cellulose beads), cellulose is highly accessible. The degradation is thus directly linked to the total amount of radical attack. In naturally structured cellulose (cotton linters) only cellulose at fibril surfaces is accessible and will react. A chemical attack of fast‐reacting agents on surfaces in structured cellulose should thus not be highly efficient in lowering the average chain length. What distinguishes radical chain reaction degradation in this context is that the spreading chain reaction is governed by the time of each step in the reaction cycle. The explanation that best fits our results is that of a free radical chain reaction, where the propagation step is rapid, which also implies that degradation is concentrated to zones along fibrils or fibers.