5 - Pulp Bleaching

Abstract

Bleaching is the treatment of wood pulps with chemical agents to increase their brightness. Bleaching of chemical pulps is achieved by lignin removal that leads to greater fiber-fiber bonding strength in paper, but the strong chemical used in bleaching chemical pulps decreases the length of cellulose molecules, resulting in weaker fibers. Bleaching mechanical pulps is achieved by chemically altering the portions of the lignin molecule that absorb light. Mechanical pulps are bleached with chemicals designed to alter many of the chromophores. Chromophores are most often conjugated double bond systems arising in the lignin of pulps. Other chromophores such as sap-stain induced by microorganisms. Brightening mechanical pulps is accomplished with reducing agents, such as dithionite, or oxidants, such as hydrogen peroxide, often in a single stage process. Whereas chemical pulp bleaching is accomplished with various compounds containing chlorine or oxygen and alkali extractions in several stages. Bleaching of chemical pulps involves the use of chemicals which are more specific to lignin removal than to carbohydrate degradation compared with the chemicals used in pulping.