The Reaction of Ozone with Pulp

Abstract

Ozone is a strong electrophile and reacts with functional groups in residual lignin. Conjugated aliphatic double bonds and enol ether structures react with ozone via intermediates to form carbonyl groups and peroxides. Most phenolic groups are oxidized by ozone. Ozone may react also with aromatic lignin structures to form acids. Ozone is better than chlorine and chlorine dioxide in solubilizing lignin by these reactions. Ozone removes most of the residual lignin. This is based on the much faster reaction of ozone with lignin than with polysaccharides. But intermediate inorganic by-products formed by direct decomposition, such as hydroxyl (HO･) and perhydroxyl (HOO･) radicals, can be very reactive with carbohydrates. Carbohydrate degradation occurs from the formation of carbonyl and carboxyl structures on the polysaccharide chains, which induce chain cleavage in alkaline environment. Reducing reaction of ozonized pulp with strong reducing agent such as borohydride converts alkali-sensitive functional groups to alkali-stable ones and prevents chain cleavage reaction. The response of pulp to an ozone treatment is characterized by the amount of lignin removal (as measured by Kappa number) and the degree of cellulose degradation (as measured by pulp viscosity or strength). The cellulose viscosity deceases with the increase of ozone charge. But the viscosity loss due to the introduction of zone in the ECF sequence does not translate into a similar loss in strength.