| The absorption of liquids into the structure of paper is a key factor for the end-useperformance of the paper products (e.g. paper cups, paper bags, packaging boxes orliquid containers) and also for the runnability of papermaking processes such as sizepress treatments and printing. The internal sizing or surface sizing of pulp is thecommon method using hydrophobic substances. In addition, the mechanical propertiesof paper product are often lost due to the hydrophilic cellulose. It is fatal shortcomingfor the application of paper/paperboard requiring high degree of dimensional stabilityand low hygroscopicity or expansion. Enzymes, which are proteins catalyzingbiochemical reactions provide some advantages over conventional chemicals. They arebiodegradable, effective under mild conditions and highly specific for certain substratesand reactions. These advantages have promoted an increasing industrial use and marketvalue. Laccase in particular has aroused great interest among researchers due to itsfargoing substrates, oxygen as reactant and only water as byproduct.In this paper, the effect of laccase combined with lauryl gallate (LG) treatement onthe hydrophobicity of unbleached kraft pulp (KP) and old corrugated containerpulp(OCC) were studied. Its influencing factors and optimal process conditions werediscussed. The effect of the laccase/LG treatment on pulp wet strength was also studied.The characteristics of the treated fiber were analyzed. The improvement mechanisms ofpulp hydrophobicity were revealed. The effect of laccase/LG treatment on theantibacterial properties of pulp was shown. The research results in this paper willprovide an environmentally-friendly method to improve the hydrophobicity oflignocellulose. The followings are some main conclusions.The Laccase/LG treatment can improve the hydrophobicity and wet strength of KP.The optimal process conditions of improving the size degree are: the laccase dosage of30U/g dry pulps, LG dosage of2%(w/w), reaction time of2h, temperature of60°C,pH4.5, pulp concentration5%, drying time of30min and dry temperature of105℃.The dry method of handsheet has great influence on the improvement of sizing degree.The highest size degree was obtained when the handsheet was dried under the constant25℃and50%RH conditions. The sizing degree increased by10.5fold under theoptimal conditions. The interlayer bond strength, wet tensile index, the dry/wet tensileindex and wet burst index increased by11.7%,31.1%，61.1%and178.8%, respectively. The larger gram weight of paper help to obtain better improve effect of laccase/LGtreatment on pulp hydrophobicity.The fiber characteristics were improved by laccase/LG treatment. The waterretention value and total carboxyl group of the treated fibers decreased and the kappanumber increased. The initiative contact angle of fiber increased from25°to95°andremains constant in0.4seconds. The capillary rise (Klemm method) and Cobb valuerepresenting water absorption of fibers obviously decreased. The surface charge oftreated fibers decreased by23.6%and the total charge slightly rose. The fibercoarseness increased by26.7%after laccase/LG treatment. The laccase/LG couldobviously enhance the antibacterial properties of KP.In laccase/LG treatment system, the participation of LG enhanced the reactivity offibers to laccase. The product of LG catalyzed by laccase is at291nm wavelength. TheXPS results showed that the C1component increased from20.10%to23.54%; the C2component representing carbohydrates decreased from68.59%to64.57%. The C4(carboxyl group) also decreased. The C1/C2increased from29.3%to36.46%. TheFTIR results of the separated lignin from fiber showed that the LG was grafted on thefibers. A large number of granular substances were observed on the fiber surface oflaccase/LG-treated pulp by AFM.As for the laccase/LG treatment on OCC pulp, the sizing degree of treated pulpincreased from1.1S to32.8S. The interlayer bond strength, wet ring crush index,wet/dry ring crush index, wet tensile index and wet/dry tensile index of treated pulpwere increased by9.1%,27.3%,30.8%,26.5%and37.2%, respectively. The staticcontact angle obviously increased and the Cobb value was decreased by69.1%. Thewater retention value was decreased by17.53%, the coarseness and kappa number ofpulp fiber were increased by33.9%and8%, respectively. The ATR-FTIR resultsshowed that LG was grafted on the fiber surface. The treated fibers become more tightlyand a large number of non-fibrillar substance filled between fibers by ESEMobservation. The AFM results showed that the granular substances covered the fibersurface, which should be the results of the graft or self-polymerization of LG on fibersurface.The comprehensive analysis of above results showed that laccase can provide thenew fiber properties by grafting the compound on fibers. The hydrophobic LG wasgrafted on the fiber by radical coupling reaction, which improved the pulp sizing degree.The increase of covalent bonding contributes to the enhancement of pulp wet strength. |
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