Endoglucanase-assisted Mechanical Grinding For Cellulose Nanofibril Production

Cellulose nanofibrils(CNFs)not only have high specific surface area,high strength,high stiffness and lightweight,but also they are biodegradable and renewable.In recent years,extensive research has been carried out about potential applications of CNFs.Extensive research has also been conducted to develop cost-effective and scalable process for producing CNFs from wood or other plant fibers;however,high energy consumption in mechanical grinding remain the key issues,which impede the further development.Enzyme pretreatment has shown promise for energy savings in CNFs production process.However,there is rarely detailed analysis or specific data presented about energy consumption.Moreover,very few has elucidated the fundamentals behind enzyme actions related to the reduction of the exact energy consumption in the production of CNFs.Therefore,firstly the bleached softwood kraft pulp(BSK)was processed for CNFs production using endoglucanase pretreatment with different dosage followed by mechanical grinding.The energy consumption was directly determined and analyzed with respect to total energy consumption,idle and net energy consumption,effective energy consumption and also they were related to the CNFs yield for exploring the energy saving opportunity.Secondly,in order to further explore the energy saving opportunity with endoglucanase,endoglucanase pretreatment with high dosage was applied followed by 2 h mechanical grinding for CNFs production.And BSK fibers were also first grounded for 0.5 h and then endoglucanase treatment with high dosage followed by mechanical grinding for 1.5 h was applied for CNFs production.Thirdly,the effects of endoglucanase treatment on fiber physical structure,crystalline index and chemical structure of cellulose were investigated,which elucidated the mechanism behind the enhancement of the grinding process by endoglucanase treatment.Fourthly,the effects of endoglucanase treatment on the transparent,mechanical strength and thermal stability of CNFs were investigated in order to develope into an economically viable and into an industrial-scale process for CNFs production.Finally,the bleached bagasse kraft pulp(BBK)fibers were processed for CNFs production using endoglucanase pretreatment and mechanical grinding.The effects of raw materials on mechanical grinding and the according fundamentals behind were investigated.The results support the idea that bagasse,a waste or byproduct from sugar industry can be a promising alternative for nanofibers production.The main studies and results are as follows:(1)With endoglucanase pretreatment,the total energy consumption was reduced by 13%,and CNFs yield was increased by about 27%.At the same time,the effective energy consumption(the net energy consumption per kilogram weight of CNFs produced)was also analyzed.Endoglucanase pretreatment effectively reduced energy consumption by approximately 39%.On the other hand,the fibrillated fibrils prepared by only mechanical grinding consist of randomly entangled sub-micron fibrils,and they showed non-uniform CNFs diameter ranging from several to 50 nm.Endoglucanase pretreatment enhanced the level of fibrillation.Moreover,with endoglucanase pretreatment,the diameters and lengths of CNFs were greatly reduced and became more uniform.For example,when endoglucanase pretreatment with the dosage of 60 U/g followed by mechanical grinding for 2 h was applied for CNFs production,The average diameter of resultant CNFs was about 11 nm and the length was about 200-1000 nm.(2)Endoglucanase pretreatment with high dosage(200 U/g)reduced effective energy consumption by 56%,and with mechanical pre-grinding followed by endoglucanase treatment with high dosage,the effective energy consumption was further reduced by approximately 63%.Moreover,the resultant CNFs yield is 73%and 88%,respectively.In addition,endoglucanase treatment with high dosage enhanced the level of fibrillation,and the diameters of CNFs were reduced and became more uniform.Also,mechanical pre-grinding followed by endoglucanase treatment further strengthened this effect.The average diameters of resultant CNFs were approximately 9 nm,and the CNFs with the diameters in the range of 9-12 nm accounted for about 60%of the total produced CNFs.The length of CNFs was about 200-600 nm.(3)Endoglucanase worked mainly in the regions of amorphous cellulose,and the physical integrity of fiber wall is greatly destructed with fractures and slitting all over the fiber length.With the increase of endoglucanase dosage in the pretreatment,?-glycosidic bond cutting was more significant.As a result,the fiber length was reduced from 2.01 mm to 0.38 mm,and fines content was increased from 37%to 55%.Fiber cutting generates new cross sections,and the specific surface area is increased,both of which will promote fiber swelling thus facilitating the following mechanical grinding.Therefore,it is much easier to disintegrate fibers into micro-and nano-sized fibrils,i.e.,CNFs.The results supported the idea that endoglucanase treatment saved the energy consumption of mechanical grinding and increased CNFs yield.(4)After endoglucanase pretreatment,the films are more densely packed,which increased the light transmittance of fibril films.Moreover,the mechanical pre-grinding followed by endoglucanase pretreatment further increased the light transmittance of fibril films,about 71%,which slightly lower than that(80%)of pure CNFs films.On the other hand,the specific tensile strength of the fibrils films was increased by endoglucanase treatment with the dosage of 0.4 U/g,about 165 kNm/kg.However,with the increase of enzymatic dosage,the specific tensile strength was decreased to a minimum value of 102.12 kN m/kg.Moreover,the specific tensile strength was further decreased to 80.98 kN m/kg by mechanical pre-grinding followed by endoglucanase pretreatment.In addition,elongation at break of the fibril films was decreased to a minimum value of 6.7%.However,the specific elastic modulus was increased from 2.1 MN to 6.3 MN m/kg by endoglucanase pretreatment.After endoglucanase pretreatment,the onset decomposition temperature(Tonset)and the maximum decomposition temperature(Tmax)of fibrillated firils were in the range of 304.4-335.2? and 340.9-361.6?,respectively.This indicated that endoglucanase pretreatment did not deteriorate the thermal stability of CNFs.(5)Bagasse fibers have smaller sizes and higher content of hemicellulose,and thus grinding energy consumption of bagasse fibers was significantly lower than softwood,by 7.31%,and endoglucanase pretreatment further improved the energy efficiency,by 59.71%,and the yield of nanofibers,by 30.57%.In addition,the nanofibers produced from bagasse had more uniform-sizes,and the average diameters of resultant CNFs were approximately 23.6 nm,and which led to a tighter structure of film made from them with higher transparency,about 51%.The tensile strength of nanofibers film from bagasse was even similar with that of wood fibers.The results support the idea that bagasse,a waste or byproduct from sugar industry can be a promising alternative for nanofibers production.