Preparation Of Lignocellulosic Nanofibers Using Maleic Acid Pretreatment And Their Film And Hydrogel Properties

Nanocellulose has shown great applications in the fields of optoelectronic,packaging,and biomedical materials due to its small diameter,large specific surface and excellent physicochemical properties.Conventional methods,such as strong acid hydrolysis,TEMPO oxidation,mechanical fibrillation,etc.,have problems on low yield,high energy consumption,severe equipment corrosion and environment pollution.Maleic acid,as a kind of solid organic acid,can weaken the interlayer bonding between microfibrils and endow hydrophilic group on fiber surface.The solubility of maleic acid in aqueous solution is increased quickly with the increased temperature,making it easily crystallized at room temperature for low cost acid recovery,which can reduce the cost of nanocellulose preparation.In this thesis,lignocellulosic nanofibers(LN)was produced using maleic acid pretreatment followed by microfluidization.The effects of maleic acid pretreatment conditions on lignocellulose and LN were systematically studied and the dissociation mechanism and process of producing LN were presented.The effects of acid recycling on the preparation of LN was also investigated,as well as the LN-based films and LN-reinforced polyvinyl alcohol hydrogels were prepared.The main conclusions are listed as follows:1.Maleic acid pretreatment had effects on the yield,degree of polymerization(DP),and morphology of lignocellulose.DP and size of lignocellulose were gradually decreased with the increased acid concentration,reaction temperature or time.The yield reached up to 85% when the acid concentration was not less than 60 wt% and the temperature was not lower than 120 oC.The results showed that the degradation of pulp fiber by maleic acid mainly occured in the amorphous region of cellulose,which broke up the glycosidic bond of cellulose macromolecules and reduced the DP and size of the fiber.Fiber with lower moisture content or lignin content had higher acid hydrolysis efficiency.2.Maleic acid pretreatment had effects on the surface chemical properties of LN.Lignocellulosic nanocrystals(LCNC)of 2-6% was obtained from the supernatant by centrifugating the maleic acid pretreated lignocellulose.The precipitated cellulosic solids were then mechanically fibrillated using a microfluidizer to produce lignocellulosic nanofibrils(LCNF)with a diameter of 7 to 25 nm and a length of 300 to 1500 nm.The results showed that the pretreatment time had the greatest influence on the yield of LCNC,and LCNC diameter was decreased when the time was increased.When using maleic acid,the hydrolyzed cellulosic materials were functionalized through esterification with one carboxyl group,endowing microfibril negatively charged(carboxyl group was less than 0.3 mmol/g).Sample with higher content of carboxyl group was more hydrophilic and could be easily fibrillated into nanofibrils.The mean aspect ratio of LCNF was not affected by processing conditions and the mean height was linearly correlated to the DP of LCNF.Increasing acid pretreatment time resulted in better LCNF dispersibility with more uniform size distribution,however,the entanglement between the fibrils was reduced,resulting in a decrease in shear storage modulus and loss modulus.Pretreated sample with higher lignin content was easier to be mechanically fibrillated to obtain LCNF with higher aspect ratio and better thermal stability.3.The hydrolyzate of maleic acid could be crystallized and recovered at room temperature,and then reused for LN preparation.The results showed that the combined hydrolysis factor(CHF)could be used as a measure of the pretreatment conditions of maleic acid.Under different CHF,the theoretical recovery of acid remains basically unchanged,and the acid recovery rate was up to 84.1%.The times of acid cycle almost had no effects on the physicochemical properties of LN,and the chemical properties of the recovered acid and the fresh acid were consistent after four cycles.4.Flexible LCNF films were prepared by ultrafiltration and drying process.The results showed that lignin weakened the hydrogen bonding force between fibrils and impaired the surface morphology of the film.LCNF film with higher lignin content was relatively rough and hydrophobic,however,the mechanical property was very poor.LCNF was also used to produce reinforced PVA hydrogel.The results showed that adding LCNF to PVA hydrogel formed a three-dimensional interpenetrating network,which improved the viscoelasticity and compression properties of the hydrogel.The compression stiffness and shear storage modulus of the composite hydrogel were increased by 9 times and 17 times,respectively.In summary,maleic acid mainly acted on the amorphous region of cellulose,which broke up the glycosidic bond of cellulose macromolecule,effectively reduced the degree of polymerization and size of the fiber,and endowed better hydrophilicity and water swelling property.The times of acid cycle almost had no effects on the physicochemical properties of LN,and the chemical properties of the recovered acid and the fresh acid were consistent.Preparation of nanocellulose using maleic acid pretreatment is a process technology with low corrosivity,low preparation cost and environmental friendliness,which can provide theoretical support and method reference for the preparation,development and application of nanocellulose.