Elsevier

Journal of Biotechnology

Volume 78, Issue 1, 29 February 2000, Pages 39-48
Journal of Biotechnology

Treatment of recycled kraft pulps with Trichoderma reesei hemicellulases and cellulases

https://doi.org/10.1016/S0168-1656(99)00232-1Get rights and content

Abstract

Effects of recycling ECF-bleached softwood kraft pulp on pulp properties were evaluated in the laboratory. The tensile strength, fiber flexibility and WRV lost during drying of the pulp were recovered by refining between the cycles which, however, resulted in deteriorated drainage properties. The recycled pulps were treated with purified Trichoderma reesei cellulases and hemicellulases and the changes in fiber properties due to enzymatic treatments were characterized. The endoglucanases (EG I and EG II) significantly improved pulp drainage already at low dosage levels, and EG II was found to be more effective at a given level of carbohydrate solubilization. Combining hemicellulases with the endoglucanase treatments increased the positive effects of the endoglucanases on pulp drainage. However, as a result of the endoglucanase treatments a slight loss in strength was observed. Combining mannanase with endoglucanase treatments appeared to increase this negative effect, whereas the impact of xylanase was not significant. Although the drainage properties of the pulps could be improved by selected enzymes, the water retention capacity of the dried hornified fibers could not be recovered by any of the enzymes tested.

Introduction

The importance of secondary fibers as a raw material in paper production is increasing, which has led to the introduction of recycled fibers to higher paper grades. This requires higher standards for the quality of the recycled pulp. The effect of recycling on pulp properties depends on the unit operations included in the cycle. Generally, it is agreed that pulp properties deteriorate during recycling. The deterioration of the paper-making properties of recycled fibers is mainly due to the irreversible structural changes in the fiber wall caused by drying (Laivins and Scallan, 1993). In addition to drying fiber morphology is also changed by other mechanical and chemical treatments such as pulping, refining and bleaching. Therefore, specification of the operations included in the cycle is essential when the effects of recycling on pulp properties are investigated.

When the fibers are dried they lose their conformability and swelling capacity, which cannot be recovered by rewetting the fibers. The structural changes in the fiber cell wall leading to the changes in pulp properties, i.e. hornification, are a typical feature of chemical pulps. It has been suggested that hornification is caused by irreversible hydrogen bonding between cellulose microfibrils (Jayme and Hunger, 1961, Laivins and Scallan, 1993), resulting in inferior strength properties and bulkier sheet. In contrast to chemical pulp fibers, mechanical pulp fibers may thus even gain strength properties, because they become more flexible when recycled, which increases the bonded area (Howard and Bichard, 1992, Laivins and Scallan, 1993).

Recycled fibers can be upgraded through refining, chemical additives, fractionation or blending with virgin pulp (Szwarcsztajn and Przybysz, 1976, Bhat et al., 1991, Abubakr et al., 1995). However, none of these treatments completely restore the virgin fiber properties. By refining, the bonding ability and tensile strength are restored but at the same time the drainage properties are impaired. The use of enzymes in upgrading secondary fiber has also been extensively studied in recent years (Pommier et al., 1989, Putz et al., 1990, Bhat et al., 1991, Bhardwaj et al., 1995, Bhardwaj et al., 1997, Sarkar et al., 1995). In most of the studies commercial mixtures of different cellulases and hemicellulases have been used to treat recycled pulps and the treatments have resulted in improved drainage properties. Stork et al. (1995) used isolated cellobiohydrolases and endoglucanases of Penicillium pinophilum to treat recycled pulps and measured the effects on the water retention value. They concluded that endoglucanases are necessary for improving the dewatering of pulp.

In this work recycled kraft pulps were treated with purified cellulases and hemicellulases of the industrially important fungus Trichoderma reesei. Bleached softwood kraft pulp was recycled in the laboratory and the effects of recyling on the pulp properties were evaluated. Pulps were treated with various dosages of the individual purified enzymes and with cellulase–hemicellulase combinations at different stages of recyling, and the effects on drainage and swelling, as well as on strength properties, were investigated.

Section snippets

Recycling procedure

A commercial ECF-bleached softwood kraft pulp was used in the experiments. The pulp was recycled in the laboratory as shown in Fig. 1. Each cycle consisted of drying, reslushing and refining steps. The pulp was recycled in two different lines: one with fines retained and the other with fines removed (Attis-filter, 200 mesh) after each refining stage. The pulps were oven-dried at 105°C and disintegrated according to the SCAN-C 18:65 standard method. Refining was carried out in an Escher-Wyss

Effects of recycling on pulp properties

The effects of recycling on the properties of bleached kraft pulp are shown in Table 1, Table 2. Each drying step caused a dramatic drop in the water retention value, indicating hornification. When the pulp was recycled with the fines retained, the WRV decreased from 179 to 128% during the first cycle while the WRV of the long fiber fraction decreased to 110%. However, refining the pulp between cycles restored the WRV of the pulp recycled with fines completely and that of the long fiber

Conclusions

This work evaluated the effects of recycling on the properties of bleached kraft pulp as well as the response of the recycled pulps to treatment with purified enzymes from T. reesei. Each drying step caused a dramatic drop in the WRV, wet fiber flexibility and tensile strength, with a simultaneous increase in tear strength. Refining the pulp between cycles could restore the WRV and tensile strength of the pulp practically to the initial level. Fiber flexibility was also recovered by refining

Acknowledgements

This work was mainly financed by the Finnish Technology Development Centre (TEKES). Tiina Leppänen is thanked for excellent technical assistance.

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