The lack of lignocellulosic materials has been a key factor that hinders the development of the pulp and paper making industry.The high yield pulp from bamboo is attracting interests due to the advantages of high yield and environmentally-friendly.However,the poor bleachability and brightness stability has been a key point that hinders the development of bamboo chemo-mechanical pulp(CMP).In this study,we improved the liquid absorption of the bamboo by using the twin screw extreder(TSE)instead of model screw device(MSD)which as a result improved the bleaching ability and physical property of the bamboo pulp.Three chemical pretreatment technolygies are investigated to evaluate their effects on the pulp brightness.An optimized physico-chemical pretreatment technology was proposed which significantly improved the bleachability of the bamboo chemimechanical pulp.Furthermore,a combined ethanol-water media was established to enchance the bleachability and stability of the pulp.The mechanism of the aforementioned system on improving the bleach ability of the bamboo chemimechanical pulp was studied and the results can give a base of chemimechanical pulp theory and technique.The bamboo chips were extruded with MSD and TSE,respectively,and then,the compressed stocks were pretreated with neutral sodium sulfite and were refined for pulp bleaching.The bleachability and physical properties of the pulp were compared from different extruding methods.The liquid absorbency of the MSD extruded materials was 1.32g/g,while TSE extruded materials was 4.50g/g,improved 240%,which is beneficial to the subsequent impregnation and bleach.When bleached with 12%H2O2,the TSE-CMP achieved the brightness of 57.6%ISO,while the MSD-TMP only reached 49.6%ISO,improved 8.0%ISO.the pulp brightness increased from 49.6%ISO to 57.6%ISO.At the same time,a slight increment of tensile index was obtained while the tear index and the burst index were slightly decreased.The chemical pretreatments were performed with neutral sodium sulfite,sodium bisulfate and alkaline sodium sulfite,respectively.In consideration of the energy consumption,pollution load,bleachability and physical properties of the pulp,the pretreated method conducted with neutral sulfite is superior to the others.The optimized conditions for the neutral sodium sulfite pretreatment is:Na2SO3:2%,DTPA:0.4%,tempure:130 oC,time:10 min.Compared with alkali sodium sulfite combined with MSD,neutral sodium sulfite combined with TSE could improve the brightness of 12.4%ISO.To improve the efficiency of the H2O2bleaching,the water solvent was substituted with ethanol,propanol,and ethanediol,respectively.The results indicated that all of the three solvents can improve the pulp brightness,among which the ethanol and propanol are similar and better than the ethanediol.After being bleached with 9%H2O2,the brightness was increased by 4.9%ISO,5.0%ISO and 2.7%ISO,respectively.Moreover,the brightness was increased by 7.3%ISO,7.2%ISO and 4.3%ISO when the bleaching was accomplished with12%H2O2.Based on the above results,the ethanol was chosen as the solvent in the following experiments.The bleaching conditions using the ethanol-water solvent were 2%Na2Si O3,0.5%DTPA,time:90 min,temperature:95 oC,consistency:10%,ratio of ethanol:50%(w/w).In the optimized condition of 12%H2O2and 9%Na OH,the pulp brightness reached 74.2%ISO,7.2%ISO higher than the traditional bleaching technology with water solvent.The bleaching properties using the ethanol-water solvent were investigated.The results showed that such process can enhance the brightness stability,in addition to increasing its brightness and strength properties of bleached pulp.Furthermore,the brightness reached77.4%ISO after bleached with 15%H2O2.However,the bleaching efficiency decreased at the H2O2dose higher than 15%.Specifically,the pulp brightness was only 82%ISO at the H2O2dose of 25%,which reached“brightness ceiling”of ethanol-water solvent bleaching system.Brightness increments of ethanol-water solvent bleaching system were positively correlated with the amount of hydrogen peroxide,as increasing the H2O2dose from 9%to 25%,the brightness increments of 4.8%ISO and 10.3%ISO was observed.At the same brightness,bleached pulps from ethanol-water solvent bleaching system have higher stability than traditional methods.When bleached with 15%H2O2,ethanol-water solvent bleaching system could improve tensile index 15.1%,burst index 26.4%,tear index 7.0%of bleached pulp than traditional water solvent methodsThe lignin structure of the bamboo chips,CMP pulps and bleached pulps were investigated.The results show that the lignin structure of the bamboo species belong to the G-S-H type,the G-type which is difficult to be degraded has higher content.The contents of o-quinone,o-phenol in the bamboo material were higher,which makes it difficult to get a higher brightness.When using the ethanol-water media,the cracked lignin fragments could dissolve into the solvent more easily which improve the accessibility of hydrogen peroxide and ligin,and reduce the formation of secondary chromophoric groups.At the such ethanol-water solvent,the chromophoric group of conjugate carbonyl,aldehyde groups,o-quinone were destroyed more selectivly by H2O2,reduced H2O2dose when bleached to the same brightness.At the same target brightness,the selectivity of carbonyl and aldehyde groups with the characteristics of photoyellowing is higher and the removal rate is higher,which is one of the reasons for making the bleached pulp brightness more stability.When the amount of hydrogen peroxide is less than 15%,the damage of hydrogen peroxide to quinone structure can be strengthened,which is the reason why the“brightness ceiling”was broken in ethanol-water solvent.The bleaching pulp(77.4%ISO,bleached by 15%H2O2)contains quinone structure formed by the breaking ofβ-O-4 structure and condensation structure together,and quinone structure transformed from o-phenol.It is difficult to be decolorized by hydrogen peroxide,which might be the reason of the“brightness ceiling”in the ethanol-water solvent bleaching system. |