| Chemical impregnation is the main feature of chemical machine pulp,and also the key process for producing chemical machine pulp.Most of the raw materials for pulping are commercial wood chips,which take a long time to transport.The chemical composition and microstructure of wood chips are easy to change due to the air-drying,and the blockage of the pores makes the permeability of the chemical solution worse,which seriously affects the softening effect of impregnation.In the pulping process,the impregnation effect has an important influence on the energy consumption of the refining,the fiber bundle content and the quality of the pulp.Therefore,good prepreg softening of wood chips is the key to improving pulping performance.This article uses commercial poplar as a raw material to study and compare several physical pretreatment methods such as extrusion pretreatment(MSD and TSPI),microwave-assisted pretreatment,and biological enzymatic pretreatment methods to improve the impregnation performance of wood chips and its impact on pulping performance,And preliminary discussed the mechanism of wood chip pretreatment.At the same time,the possibility of using biological enzymes in the pretreatment section of wood chips was discussed,the technological conditions of enzyme treatment were optimized,and the relationship between enzyme pretreatment and pulp performance was analyzed.The effects of pre-treatment methods such as unextruded(control sample),single-screw extrusion(MSD),and double-screw extrusion(TSPI)on the energy consumption of refining,pulp yield,and pulp performance were compared and analyzed.The results of the study show that the liquid absorption of the material obtained by TSPI extrusion(5.20 g/g)is 3.8 times that of the material obtained by MSD extrusion.The results of pulping by different extrusion methods found that the burst index and tensile index of the pulp obtained by twin-screw extrusion were higher than those of the single-screw extrusion and control samples,while the tear index was slightly reduced.The tensile strength,tear strength and bursting strength of the pulp obtained by single spiral extrusion are slightly higher than those obtained by unextruded wood chips.When the Canadian standard freeness is 250m L,the whiteness of the pulp obtained by twin-screw extrusion is 65.48%ISO,which is 9.14%ISO higher than that of the single screw,and 12.94%ISO higher than that of the pulp obtained by unextruded wood chips.At the same time,the mechanism of extrusion to improve impregnation and pulp performance was explored.The higher the degree of extrusion,the lower the fiber bundle content in the pulp.The content of fiber bundles obtained by twin-screw extrusion was about 76%lower than that of MSD.In addition,compared with MSD extrusion and control samples,the bulk density of the material obtained after TSPI extrusion is smaller,and the porosity and pore diameter are larger,which is conducive to improving the penetration rate of the chemical solution and the uniformity of impregnation.The effect of microwave pretreatment on the permeability of wood shows that under the same immersion conditions,the liquid absorption of microwave-impregnated wood chips is significantly higher than that of water bath immersion.Treating the wood veneer with microwave for 5 minutes can reach the water absorption of 45 minutes in the water bath,which significantly improves the impregnation efficiency.Different impregnation methods have significant differences in the whiteness and strength properties of the pulp.The bleaching brightness of the pulp obtained by microwave-assisted impregnation can reach up to 68.4%ISO after two-stage hydrogen peroxide bleaching,which is 3 to 6 percent higher than the steam pretreatment method.The average tensile index of pulp increases by 3 N·m/g,the tear index increases by about 0.2 m N·m~2/g,and the burst resistance index increases by about 0.15k Pa·m~2/g.The microstructure of the wood chips after microwave treatment also has obvious changes,the fissure pore film on the fiber is greatly cracked,and the fibrous pore film on the fiber and the conduit is also broken,so that the water diffusion channel is increased,which is beneficial to the increase of the penetration rate of the chemical solution.It is a very ecological and environmentally friendly way to use biotechnology to modify pulp and improve the pulp performance through enzymatic hydrolysis.The effects of cellulase and xylanase treatment on fiber quality,beating energy consumption and water retention value were studied.On the basis of the optimal p H and temperature for enzyme treatment,the enzyme dosage and treatment time were optimized.By comparing the strength of the pulp,the optimal treatment conditions of cellulase and xylanase were obtained.The optimal enzyme dosage for cellulase pretreatment is 60 IU/g,and the treatment time is 90 min.At this time,the paper tear index,burst index,and tensile index reach 3.32 m N·m~2/g,1.73 k Pa·m~2/g,34.3 N·m/g,which is 14.5%,16.9%and 12.1%higher than the control sample,respectively.The optimal enzyme dosage for xylanase pretreatment pulp is 20 IU/g,and the processing time is 90min.At this time,the paper tear index,burst index,and tensile index reach 3.1m N·m~2/g and 1.66 k Pa·m~2/g,33.5 N·m/g.Compared with the control samples,they were increased by 7.0%,12.2%and 9.5%respectively.Pretreatment with cellulase or xylanase reduced the content of fine fibers in the pulp compared to the control,the average length and width of the fiber were increased,the curl index of the fiber was also improved,and the surface was exposed more Hydroxyl group,which improves the binding force between fibers,is beneficial to the strength properties of pulp.And the fiber is softened during the enzyme treatment,which can reduce the energy consumption of the refining pulp,and the water retention value of the pulp has also been increased to a certain extent. |
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