Inhibition Of High-yield Pulp And Chemical Pulp By Enzymes Treatment

Pulps, induced by light or heat, tend to lose their brightness and become yellow during a period of deposition. Especially for the lignin-riched high yield pulp, its rapid photoyellowing limits both the applicability and marketability of this material, and decreases the additional value of the pulp products. Up to now, some methods, such as modifying the chemical structure of lignin and adding some UV absorbers to the pulps, etc., have been studied and used for preventing yellowing of pulps. However, high cost and/or low affinity to the cellulose restrict the application of yellowing inhibitors. To solve the problems, some enzymes were applied to treat high yield pulp and chemical pulp, in order to inhibit the pulp yellowing and improve the bleaching efficiency in subsequent chemical bleaching stage. The results showed as follows:Obvious different inhibition effects were obtained by using different kinds of enzymes to pretreat aspen CTMP (chemithermomechanical pulp ) . Crude laccase pretreatment could inhibit the heat- and light-induced yellowing of the aspen CTMP effectively, and increase bleached pulp brightness in subsequent H₂O₂ bleaching. Pretreatment with cellobiose dehydrogenase (CDH) also has showed a positive influence on inhibiting the heat-induced yellowing of the aspen CTMP, while xylanase or cellulase showed little effect on decreasing the PC number of the pulp. Combined with xylanase, CDH or glucose oxidase, laccase pretreatment could be more effective, indicating a potential synergistic effect on inhibition of heat-induced yellowing. During the laccase treatment, the sodium citrate buffer system was better than the sodium acetate, and the primary purified laccase with (NH₄)₂SO₄ deposition was better than crude enzyme for yellowing inhibition of the bleached CTMP.The UV-VIS spectrum showed that the increase of light absorption during the light aging was more obvious than that during heat aging. During light-induced yellowing, the light absorption of pulp changed mainly at wavelength of 420 nm, 345 nm and 327 nm, which corresponding to the quinones, aromatic ketones and aromatic carbonyl group. Compared to the light yellowing, remarkable increase was only found at 420 nm during the heat-induced yellowing, meaning that mechanism of yellowing was different between the light- and heat-induced yellowing. The light-induced yellowing retarded after the laccase treatment, maybe owing to the production of a hydroquinone compound.Crude xylanase has a positive effect on preventing heat-induced yellowing of chemical pulp from aspen and eucalyptus. The brightness of aspen chemical pulp increased by 1～3 % ISO through a subsequent H₂O₂ bleaching. The optimum conditions of xylanase treatment were as follows: xylanase dosage 10 IU/g oven dry pulp, 50℃, initial pH 5.0. The analysis of UV-VIS spectrum showed that the light absorption increase of pulp treated with xylanase was less than that of the control in a wide range band, which means that, many groups had changed during heat-induced yellowing of the chemical pulp.