Structural Changes Of Bambusa Chungii SCMP Lignin During Bleaching And Improvement Of Bleachability By Enzyme Pretreatment

In China, the production of machenical and chemimechanical pulp has made great progress, due to its high yield, high bulk and stiffness, superior optical properties and good printability. However, the machenical and chemimechanical pulps could not be used to produce high grade paper product because of its poor bleachability and inferior brightness stability. The changes of lignin substructure and the component content of benzene-ethanol extractives of Bambusa chungii SCMP were investigated during the process of its hydrogen peroxide bleahing and yellowing. The changes of the fiber surface properties and the component content of benzene-ethanol extractives of Bambusa chungii SCMP were discussed during pretreatment with lipase, pectinase and laccase. The mechanism of hydrogen peroxide bleaching and yellowing and the effect of the extractives on the bleachability and brightness stability were also studied.The model of hydrogen peroxide bleaching of Bambusa chungii SCMP was set up by MODDE software. It could be found from the model that there exits the correlation among hydrogen peroxide dosage, ratio of hydrogen peroxide dosage to alkali usage, the ratio of NaOH to Na₂SiO₃ usage and pulp consistency. And hydrogen peroxide dosage, the ratio of hydrogen peroxide dosage to alkali usage and consistency are the main factors that influence the the brightness of bleached SCMP. The optimal conditions of hydrogen peroxide bleaching of Bambusa chungii SCMP were H2O2 6%,NaOH 3%,Na₂SiO₃ 3%,MgSO₄ 0.3%,pulp consistency 20%,temperature 80℃,time 180min. The bleaching model was: Brightness(%ISO)=51.25+4.62*(PCH)-3.68*(P/A)-0.35*(nA/Si)+1.50*C+0.78*(P/A)²-0.85*(C)²-0.81*(PCH)*(P/A)+0.56*(P/A)*(Na/Si)-0.47*(P/A)*(C)From the analysis of Py-GC/MS spectra, lignin isolated from Bambusa chungii SCMP constitutes of gluaiacyl (G), syringyl (S) and p-hydrophenyl (H) units, with a G:S:H molar ratio of 44:29:27. And the molar ratios of G, S and H in lignin isolated from bleached SCMP and yellowed bleached SCMP are 40.8:26.2:33 and 34.9:30.2:34.9, respectively. The data of 31P NMR spectra shows that the contents of alphatic hydrogen group in lignin isolated from SCMP, bleached SCMP and yellowed bleached SCMP are 3.83mmol/g, 4.03mmol/g and 3.74mmol/g, the contents of phenolic hydroxyl group 1.27mmol/g, 1.71mmol/g and 1.18mmol/g, the contents of carboxylic hydrogen group 0.24mmol/g, 0.36mmol/g and 0.35mmol/g, respectively. During hydrogen peroxide bleaching, the ether bond between the lignin units could be broken down, and more alphatic and phenolic hydroxyl groups were formed, but during the process of yellowing, the condensation of lignin would occur.From the analysis of 31P NMR, 13C NMR and 2D NMR (HSQC), it could be concluded that the linkage among lignin units was mainly ?-O-4′aryl ether substructure linkage, there was also some condensed substructure linkage, such as dibenzodioxocins substructures(5′-5′′/α-O-4′/β-O-4′′), phenylcoumaran structures(β-5′/α-O-4′), resinol substructures(β-β′/α-O-γ′/α′-O-γ), spirodienone andβ-1′substructure, . And most of H lignin unit was acetated at ?-position ofβ-O-4′aryl ether linkages. Fraction ofβ-O-4′aryl ether linkages in lignin of Bambusa chungii SCMP was broken down during the process of bleaching and yellowing. Part ofα-CO inβ-O-4′aryl ether linkages was changed intoα-OH during hydrogen peroxide bleaching, and then some of -OH would be changed intoβ-CO during the process of yellowing. Some of Alk-O-Alk ether linkage in dibenzodioxocins substructures was also broken down during bleaching and yellowing. But carbonyl group(CO) in spirodienone substructure and Alk-O-Alk and Alk-O-Aryl linkages in penylcoumaran and resinol substructure would be relative stable during bleaching and yellowing.The bleachability and brightness stability of Bambusa chungii SCMP could be improved after pretreatment with lipase, pectinase and laccase. The opitmal pretreatment conditions were as follows: lipase pretreatment: lipase dosage 10IU/g, pH 8,temperature 35℃, time 40min, pulp consistency 10%; pectinase pretreatment: pectinase dosage 0.5IU/g, pH 10, temperature 30℃, time 30min, pulp consistency 10%; laccase pretreatment: laccase dosage 10IU/g,HBT 0.5%, pH 6, temperature 50℃, oxygen pressure 0.2MPa, pulp consistency 10%. Under the optimal conditions, the brightness of bleached SCMP pretreated by lipase, pectinase and laccase increased from 57.1%ISO to 60.3%ISO,61.1%ISO and 60.0%ISO, respectively. The PC number decreased from 2.97 to 2.03,1.97 and 2.06;the beating degree rose up from 70°SR to 74°SR,72.5°S and 73°SR;the breaking length inceased by 4.1%,5.9% and 3.7%;the bursting index also increaded by 8.4%,6.6%å’Œ8.8%;the tearing index rose up by 5%,8.6% and 4.7%. From the analysis of AFM and XPS, it could be found that 57.45% and 7.57% of fiber surface of Bambusa chungii SCMP were covered with lignin and extractives. After pretreatment with lipase and pectinase, the percentages of lignin covered on the fiber surface were 66.40% and 68.41%, percentages of extractives 3.05% and 2.03%, respectively. After pretreatment with laccse, almost all of the surface of fiber were covered with lignin, and pecentage of extractives on the fiber surfacet could not be calculated.The analysis results of GC-MS indicate that the benzene-ethanol extractives of Bambusa chungii SCMP consist of low molecular linear hydrocarbon, low molecular aromatic substances, fatty alkene and alcohol, fatty acid and sterol. And the main extractives that affect the bleachability and brightness stability are low molecular aromatic substances, fatty acid and sterol. The extractives covered on the fiber surface could be partially removed by enzyme pretreatment, which could evidently improve the bleachability and brightness stability of Bambusa chuangii SCMP.