Effect Of Laccase On Paper Properties And Lignin Structures Of High Yield Pulps

Nowadays, shortage of raw materials and energy is becoming more and more serious to pulp and paper industry. There has been an increasing trend towards more efficient utilization of raw materials by fiber modification. A wide range of chemicals have been developed to improve paper properties. In addition, enzymes have received significant attention for their highly specific effects on cellulose, hemicellulose, lignin and/or extractives. Laccase is a multicopper oxidase enzyme catalyzing the oxidation of lignin in the presence of mediater and oxygen.Lignin-riched fiber surface of mechanical pulp can lead to poor flexibility,low strength of fibers and low brightness and return to yellow easily. Therefore, the mechanical pulp can not completely replace the chemical pulps to produce paper with high quality. It is needed to modify the chemical pulps in order to expanding its scope of application, while their own advantages are kept. In this paper, we studied the lignin structure changes in detail of poplar alkaline perxide mechanical pulp, which modified by the laccase and laccase/mediator system .It was found that the optimum conditions of the two pulps of poplar alkaline peroxide mechanical pulp modified by laccase and laccase/mediator were:pH 5,temperature 50℃,pulp consistency4%, reaction time 60min,the optimum charge of laccase were 2u/g , O2 and the charge of mediator were 0.5%. The results showed that the brightness of the modified pulp were decreased compared to the control pulp, while the tensile index and the tear index were increased.Physical properties of paper were studied by FQA and ESEM.The result showed that the laccase had a great influence on the fibers.Especially on the fiber surface of the pulp, the average length and coarseness of modified fibers changed. Some surface lignin was dissoved after enzyme treatment, while the brightness and strength of paper were both increased. Infrared spectra show that the degradation of lignin happened and part of lignin dissolved during the modification process .The sample of isolated lignin from the control pulp and the modified pulp by the laccase and laccase/meditor by the two-stage method of enzyme-mild acidic hydrolysis solution.The molecular weight distributions of lignin were studied through GPC analysis. The results indicated that: the number average molecular weight of the residual lignin from the first stage of APMP both decresed after the modifiction by laccase and LMS, while the mass average molecular weights both increased. The change of the average molecular weight of the residual lignin of the two stage pulp were similar with the first stage pulp. The despersivity of lignin were both increased obviously after the modification by laccase and LMS.The functional groups of all lignin samples from APMP pulps and modified pulps by laccase and LMS were studied using infrared spectroscopy FTIR and phosphorus nuclear magnetic resonance, qualitatively and quantitatively.During the modification of the first-stage APMP with laccase, the content of aliphatic OH (A-OH) increased, while the contents of COOH and total phenolic OH (T-OH) decreased. The contents of the condensed phenolic OH (C-OH),syringyl phenolic (S-OH),guaiacyl and dementhylated OH (G-OH) and p-hydroxyl-phenyl OH (p-OH) decreased in the residual lignin. During the modification of the second-stage APMP with laccase, the contents of aliphatic OH (A-OH),the contents of COOH and total phenolic OH (T-OH) all decreased. The contents of the condensed phenolic OH (C-OH),syringyl phenolic (S-OH),guaiacyl and dementhylated OH (G-OH) and p-hydroxyl-phenyl OH (p-OH) all decreased in the residual lignin.During modification of the first-stage APMP with LMS process, the content of the aliphatic OH (A-OH),the contents of COOH and the total phenolic OH (T-OH) all decreased. The contents of the condensed phenolic OH (C-OH),syringyl phenolic (S-OH),guaiacyl and the dementhylated OH (G-OH) and p-hydroxyl-phenyl OH (p-OH) all decreased in the residual lignin. During he modification of the second-stage APMP with LMS, the content of aliphatic OH (A-OH) increased, the contents of COOH and total phenolic OH (T-OH) both decreased. The contents of the condensed phenolic OH (C-OH),syringyl phenolic (S-OH),guaiacyl and dementhylated OH (G-OH) and p-hydroxyl-phenyl OH (p-OH) all decreased in residual lignin.