1996

Suurnäkki, Anna

Enzyme-aided bleaching of kraft pulps is the major industrial application of hemicellulases in pulp processing. In this work, the role of xylanase and mannanase pretreatments in the ECF and TCF bleaching of softwood pulps produced by different sulphate cooking methods was studied. In addition, the action of hemicellulases in kraft fibres was characterised and exploited in the analysis of the surface composition of kraft pulps. The effects of Trichoderma reesei xylanase and mannanase treatment on pulp bleachability were affected by the pulp origin, pulp production method and bleaching sequence. The xylanase was generally more efficient than mannanase in enhancing the bleachability of softwood pulps. However, MCC pulp was equally or even more susceptible to mannanase-aided than xylanase-aided ECF bleaching. Of the three different mannanases tested for their ability to improve the bleachability of softwood pulps, only the T. reesei mannanase resulted in a significant increase in the bleachability of MCC pulp. The distribution of xylanase and mannanase attack in conventional pine and birch kraft fibres was determined by combining enzymatic treatment to mechanical peeling, FTIR and pore size measurements. In the extensive enzymatic treatments, the T. reesei xylanase could penetrate the fibre wall. In the limited enzymatic treatments, xylanase attacked evenly all the accessible surfaces of pine kraft fibres, whereas in the birch kraft pulp the xylan-rich outer surface of fibres was relatively more attacked than the inner fibre wall. The action of T. reesei mannanase was found to be highly concentrated on the outer surface of pine kraft fibres. The composition of xylan and glucomannan in the accessible surfaces of pine kraft fibres was studied by enzymatic peeling. About 50 % of pulp xylan and glucomannan could be removed from the pulp by T. reesei xylanase and mannanase. Xylan removed enzymatically from the pulp contained relatively less arabinose and uronic acids than the overall xylan in the pulp. The most readily hydrolysed glucomannan was rich in galactose. The mechanisms involved in xylanase- and mannanase-aided bleaching were discussed on the basis of the results obtained. Both reprecipitated xylan and LCC xylan were found to have a role in xylanase aided-bleaching. The bleachability of pine kraft pulp was most affected by the action of xylanase on lignin-bound xylan in all the acccessible pulp surfaces, whereas in birch kraft pulp the effective action of xylanase was more concentrated on the xylan-rich outer surface of fibres. However, the overall effect of xylanase on kraft pulp bleachability was suggested to be based on a broad action of the enzyme throughout the fibre matrix. In the case of mannanase-aided bleaching, the role of the composition and structure of the outer surface of pulp fibres was very pronounced. The knowledge obtained concerning the fibre surfaces and the action of enzymes in fibre matrix can be exploited in the development of pulp and paper applications.