Role of hemicelluloses in the formation of chromophores during heat treatment of bleached chemical pulps

The heat-induced colour formation of TCF- and ECF-bleached chemical pulps has been investigated. Partial hydrolysis of the pulps and analysis of the carboxyl content of the pulps and hydrolysed matter were carried out. it was found that the yellowing tendency of the pulps corresponds to the content of hydrolysable substances. The hydrolysis leads to a considerable removal of the carboxyl groups in the case of hardwood pulps. Differences between softwood and hardwood pulps are discussed in terms of their hemicellulose content. The yellowed pulps were extracted and the extracts were analysed by different NMR and mass spectrometric techniques. Structures of low molecular products were elucidated and the routes of their formation were proposed. The results suggest that the yellowing proceeds in two main steps, i.e. decay of unstable polysaccharide chains such as hemicelluloses to low molecular carbohydrate compounds that subsequently undergo dehydration and condensation reactions to form Coloured products.