Effects Of Lignin On The Preparation And Properties Of Mechanically Defibrillated Fibrils

The high-value added and comprehensive utilization of lignocellulose resources has gained extensive attention with the urgent requirement of sustainable development and the diminishment of petroleum resources.Cellulose nanofibrils(CNFs)are biodegradable and renewable,and have unique physical structures,which have been extensively studied and applied as a natural nanomaterial in many fields.However,most previous studies have been mainly focused on the CNFs rather than the lignocellulose nanofibrils(LCNFs).Very limited studies reported the preparation and properties of LCNFs.It is envisioned that using the lignocellulose fibers to produce LCNFs could provide some benefits such as high yields,low production cost and less environmental impacts.More importantly,the LCNFs containing lignin can show many differences in the composition,structure and properties compared to CNFs,which can improve the application potential of nanofibrils in many different fields.Therefore,this thesis mainly investigated the effects of residual lignin on the mechanical defibrillation process of cellulosic fiber,and on the composition,structure and properties of defibrillated cellulose fibrils and films.Moreover,a novel green strategy was applied to produce LCNFs and simultaneously extract bio-products from bagasse in an efficient and energy-saving approach.The main methods and results for this study can be seen as follows:Three types of lignocellulose fibers which contained different amounts of lignin(14.9%,2.4%and 0.1%,respectively)but were similar in cellulose to hemicellulose ratio,were mechanically defibrillated through micro-grinding for producing nanofibrils.Effects of residual lignin on the mechanical defibrillation process were investigated by comparing the differernt lignin-containing fibers in terms of the nanofibrils yields,the morphological development of defibrillated fibrils,and the micro-grinding energy consumption.The results showed that the residual lignin delayed the initial defibrillation of cellulosic fibers due to that it covered the cellulosic fibers which posed a physical barrier to the mechanical defibrillation.However,the residual lignin hypothetically counteracted the recombination reactions between the highly reactive cellulose radicals and impeded the aggregation of defibrillated fibrils through reducing the inter-fibrillar hydrogen bonding.As a result,the residual lignin eventually facilitated the defibrillation of cellulosic fibrils and enhanced the maximum nanofibrils yields by 17.9-30.8%,as well as greatly reduced the diameters of the resultant LCNFs from 32.9 ± 7.7 to 9.2 ± 3.3 nm.The presence of lignin can reduce the shear viscosity of cellulose fibril suspension and thus help to reduce the energy consumption in the process of mechanical defibrillation.This study also investigated the effects of residual lignin on composition,structure and properties of cellulose fibrils and cellulose fibril-based films by comparing two groups of cellulose fibrils which contained different amounts of lignin(15%and 2%,respectively)but were similar in cellulose to hemicellulose ratio.The results showed that lignin affected the size and content of nanofibril in cellulose fibrils,and also the surface area and mesopore structures of fibrils networks through initially delaying but ultimately facilitating the mechanical defibrillation.Lignin increased the mesopore diameter and dewatering of cellulose fibrils network due to its effects on interfering with the interfibrillar hydrogen bonding.However,this effect of lignin lowered the mechanical properties of cellulose fibril-based films such as the tensile strength and elongation at break.Lignin can redistribute on the cellulose fibrils surface due to the increase in the surface area of cellulose fibrils and the change in the physical structures of lignin in the defibrillation process,and this consequently led to the decrease of surface lignin content on the cellulose fibrils,which affected the interfacial properties of fibril films.Lignin acting as a cement in cellulose fibril-based film made the film with a more densely packed structure,a higher density and a smoother surface.Lignin has hydrophobic,thermally stable and UV-adsorbing properties,it can conspicuously increase the hydrophobicity,the thermal stability and UV blocking properties of cellulose fibril-based films.However,it decreased the transparencies of films due to its chromophore groups.The work also optimized the production of LCNFs from bagasse through a novel green approach of combining organosolv pretreatment with micro-grinding.Bagasse was first processed through organosolv pretreatment with trifluoroacetic acid as catalyst to provide cellulosic fibers with full control of chemical components for LCNFs production.Mass balance of the pretreated bagasse was determined by chemical component and total organic carbon analysis,and results indicated that organosolv pretreatment with trifluoroacetic acid as catalyst was an efficient and energy-saving strategy for recovering cellulose and hydrolyzing hemicellulose and lignin.Non-cellulosic contents in cellulosic fibers were optimized for obtaining a high resultant LCNFs yield through micro-grinding.Effects of non-cellulosic components on mechanical defibrillation were comprehensively elucidated:non-cellulosic components were found to be advantageous to improve the resultant LCNFs yield and produce individualized nanofibrils with uniform and small diameters;however,excessive amount of non-cellulosic components reduced the defibrillation efficiency.The highest LCNFs yield of 72%could be produced through mechanical defibrillation of the pretreated bagasse fibers containing non-cellulosic content of 29%,which generated nanofibrils with the number-average diameter of 9 nm.In biorefinery principles,nearly 90%of the bagasse could be effectively utilized through the combination of organosolv pretreatment and micro-grinding,in which 57%was converted to cellulosic fibers for LCNFs preparation and 30%was recovered as valuable bio-products including the hemicellulose-derived xylose(19%)and lignin-derived organosolv lignin(11%).Overall,the combination of organosolv pretreatment and micro-grinding was proved to be a green,efficient and recoverable approach for producing LCNFs and simultaneously extracting bio-products from bagasse.