Preparation Of Bagasse Nanocellulose And Its Influence Mechanism Of Thermal Stability

Bagasse is the main residual product after the sugar production from sugarcane.The rational use of bagasse can improve the profit and market competitiveness of sugar enterprises,and is of great significance for maintaining the safety of sugar supply for our country.Bagasse can be prepared into cellulose nanofibrils(CNFs).The CNF is a second generation renewable resource.By scientific means,which opens up a new direction for expanding the use of bagasse.Because of the high mechanical properties,high specific surface area,and reproducibility,CNFs can be used as a reinforcing filler for polymers to produce composite materials with improved properties.CNFs is unstable at high temperatures,while most polymers typically have a processing temperature above 200°C.The thermal stability of CNFs directly determines whether it can be applied to the preparation of related composite materials.The CNFs with higher thermal stability prepared by different methods can also improve the overall thermal stability of the composite materials,and can further improve its characteristics,which can expand the application field of the composite materials.This work systematically studied the preparation of bagasse CNFs and the influence mechanism of thermal stability.First,different hemicellulose content of bagasse CNFs was prepared by mixing bleached bagasse pulp and purified bagasse pulp in different proportions.The effect of hemicellulose on the thermal stability of bagasse CNFs was studied.Secondly,bagasse CNFs with different lignin contents was prepared by mixing bagasse raw materials and bleached bagasse pulp according to different proportions.The effect of lignin on the thermal stability of bagasse CNFs was studied.Thirdly,unbleached bagasse pulp was pretreated by using of cellulase,xylanase,cold alkali and combining with mechanical method to preparing bagasse CNFs.The effect of crystal structure on the thermal stability of bagasse CNFs was studied.Fourthly,unbleached bagasse pulp was pretreated by using of cellulase,cold alkali,glycerol and combining with mechanical method to preparing bagasse CNFs.The effect of crystallinity on the thermal stability of bagasse CNFs was studied.The research results provide a theoretical basis for the pyrolysis mechanism and high value utilization of CNFs.Cellulose nanofibrils were produced from bagasse pulps with differing hemicellulose contents by ultrafine grinding and high-pressure homogenization.The contribution of hemicellulose to the thermal and physical performance of CNFs films was studied.The results show that a decrease in hemicellulose content can enhance the crystallinity of CNFs and the ordered crystallization zone can improve the thermal stability.The activation energy of bagasse nanocellulose was calculated by Flynn-Wall-Ozawa(FWO)method.When the content of hemicellulose in the CNF is decreased,the crystallinity is increased,the activation energy is increased,and the thermal stability of the CNF is improved.When the hemicellulose content in CNF is 21.7 wt.%,T_d is 341°C,and when the hemicellulose content in CNF is 9.7 wt.%,T_d is 349°C.The average activation energies of NO-HCNF,L-HCNF,ML-HCNF,MH-HCNand H-HCNF were713.03,518.93,462.62,421.78 and 211.11 kJ/mol,respectively,during the conversion rate of 30%to 90%.The effects of lignin on the thermal stability of bagasse CNFs were investigated.The CNFs were prepared with different lignin content bagasse pulp using ultrafine grinding combined with high-pressure homogenization.Scanning electron microscopy,Fourier transform infrared spectroscopy,X-ray diffraction and thermogravimetric analysis were used to study the influence mechanisms of lignin content on the thermal stability of the CNFs.The thermal stability of cellulose was tested by thermogravimetric analyzer at different heating rates,and the activation energy of bagasse cellulose nanofibrils was calculated by Flynn-Wall-Ozawa method.The results showed that the average width of CNFs prepared by the mechanical method was approximately 20 nm.The higher the lignin contents in the CNFs,the lower the crystallinity and the better the thermal stability.The thermal decomposition activation energy of CNF fluctuates with the change of conversion rate.Under the same conversion rate,the higher the lignin contents in the CNFs,the larger the activation energy value.The average activation energies of NO-LCNF,L-LCNF,ML-LCNF,MH-LCNF,and H-LCNF were 208.14,254.49,412.95,530.54 and 652.10 kJ/mol,respectively,during the conversion rate of 20%to 90%.Xylanase,cellulase,low-concentration cold alkali,xylanase combined with cold alkaline,and cellulase combined with cold alkali were used to pretreat unbleached bagasse pulp from which CNFs,about 30 nm in diameter,were successfully prepared through ultrafine grinding and high-pressure homogenization.The effect of different pretreatment methods on the thermal stability of CNFs was studied.X-ray diffraction analysis showed that enzyme pretreatment increased the crystallinity of CNFs.After low-concentration cold alkali pretreatment,the crystallinity of CNFs significantly reduced and the crystal structure of the cellulose changed from type I to type II.Thermogravimetric analysis showed that enzyme pretreatment could improve the thermal stability of CNFs,while cellulase pretreatment had a more significant effect than xylanase pretreatment on the thermal stability of CNFs.Low-concentration cold alkali pretreatment changed the crystal structure of cellulose and significantly improved the thermal stability of CNFs.CNFs prepared by pretreatment with xylanase combined with cold alkali exhibited the best thermal stability.The values of Tonset and Td increased from 303?to325?and from 330?to 355?,respectively.CNFs prepared by cellulase combined with cold alkali treatment produced more non-type II regenerated cellulose and had lower thermal stability.The use of enzymes and low-concentration cold alkali pretreatment combined with ultrafine grinding and high-pressure homogenization is an environment-friendly method for preparing CNFs with high thermal stability.Enzymatic pretreatment causes the cellulose to swell,thereby facilitating mechanical grinding and reducing energy consumption.The use of low-concentration cold alkali reduces the consumption of alkali and clean water.Cellulose nanofibrils was prepared successfully by unbleached bagasse pulp by means of enzymatic pretreatment combined with ultrafine grinding and high-pressure homogenization.The effect of pretreatment with cellulase and a glycerol/low-concentration cold alkali mixture on the thermal stability of prepared CNFs was studied.This was the first time that glycerol was shown to prevent low-concentration cold alkali from converting cellulose structure from type I to type II.The thermogravimetric analysis results shown that the thermal stability gradually increased as the CNFs crystallinity rised,and there was a significant improvement in the maximum thermal degradation temperature.This result indicated that cellulase or glycerol/low-concentration cold alkali mixture pretreatment combined with ultrafine grinding and high pressure homogenization is an environmentally friendly method to prepare high-thermal-stability CNFs,which will both conserve energy and eliminate chemical use.