Preparation And Performance Of Soybean Oil Derivatives/bamboo-based Biocomposites

Bamboo is a green material with characteristics of abundance,fastgrowth,and recyclability.Soybean oil has the advantages of abundance,low-price,and molecular designability.The development of biocomposites using bamboo and soybean oil as raw materials can replace petroleum-based products and be a suitable approach to improve the utilization efficiency of biomass resources in China.In this study,four bamboo reinforcements,i.e.,microcrystalline cellulose(MCC),bamboo powders(BP),original bamboo fibers(BF),and bamboo shavings(BS),were used to prepare soybean oil-based composites,respectively.Concerning the different characteristics of bamboo reinforcements,two commercially available soybean oil derivatives,i.e.,epoxidized soybean oil(ESO)and acrylated epoxidized soybean oil(AESO),were used as resin matrix,interfacial compatibilizer,and adhesives,respectively,to develop MCC/AESO and BF/AESO composites,BP/poly(lactic acid)composites,and BS/AESO particleboards,respectively.The main bottlenecks of the composites’ fabrication including low crosslinking density of resins,poor processibility of resins,and poor interfacial adhesion of the bamboo/AESO biocomposites were addressed.The research contents and conclusions are as follows.(1)The curing parameters and kinetics of soybean oil-based resins were studied based on extrapolation method,Kissinger,Crane,and Friedman equations,and the influence of initiators,reactive diluents(RDs),and isocyanate modification on the resins curing were explored.The types of initiators significantly affected the curing process and curing kinetics of AESO resins,and the curing temperatures and reaction activation energies of the resins were significantly correlated with the decomposition temperature and activation energies of the initiators.The number of carbon-carbon double bonds and molecular structure of RDs significantly affected the curing characteristics of the resins.The addition of RDs endowed the reaction more complexity,and the activation energy changed with the C=C conversion rapidly.The addition of hexamethylene diisocyanate(HDI)resulted in the polycondensation between the isocyanate and the hydroxyl groups in the resin system,which increases the complexity of the curing process and makes it more difficult to discuss the curing kinetics of the resins in details.(2)Biocomposites were developed from AESO and microcrystalline cellulose(MCC)by a solution casting method.MCC was grafted with C=C bonds through esterification with methacrylic anhydride(MAA)with the assistance of ultrasonic treatment.The unsaturation degree on the MCC surface was changed by the feeding mass ratios of MAA to MCC during the treatment process.The modified MCC was characterized by FTIR,XPS,NMR,and contact angle measurements.The degrees of substitution and unsaturation of the modified MCC were quantitatively determined by NMR and XPS analyses to demonstrate the grafting efficiency of MAA on MCC.The modified MCC was further used as a reactive reinforcing agent to manufacture MCC/AESO biocomposites.The C=C grafting of MCC significantly improved the flexural strength and modulus,water resistance,storage modulus,glass transition temperature,thermal stability,and interfacial adhesion of the composites.(3)Butyl methacrylate(BMA)and hydroxyethyl methacrylate(HEA)were used to replace styrene as RDs for AESO resins,respectively,to prepare styrene-free BF-mats reinforced AESO composites.The effects of chemical structure of RDs(particularly the hydroxyl groups)on the curing behavior,rheological and mechanical properties of resins,the interfacial bonding between fibers and matrix,and properties of the composites were investigated.The presence of hydrogen bonds in the resin system can increase the interaction force between molecules,restrict the movement of molecular chain,and thus increase the viscosity of the system.However,the polymerization rate of the resins was increased due to the formation of multimers induced by the hydrogen bond.The hydrogen bonds could provide physical cross-linking points for the cured network,which significantly increased the tensile strength,tensile modulus,storage modulus,and glass transition temperature of the resin.At the same time,the hydrogen bonds inside the resins can form a good interfacial adhesion with the fibers and hence improve the mechanical properties of the composites.(4)Micro-scale size BP was suitable for preparing BP/PLA composites.ESO was used to in-situ polymerize at the interface between BP and PLA matrix to prepare highly tough and biodegradable BP/PLA composites where a polymerized ESO interfacial layer inside the composites was formed.The poly(ESO)layer was covalently bonded with both BP and PLA matrix,resulting in an enhanced interfacial adhesion of the biocomposites.The flexible poly(ESO)layer contributed to a simultaneous strengthening and toughening effect on the biocomposites,hence significantly increased the tensile strength,tensile modulus,fracture elongation,and storage modulus of the biocomposites.However,the glass transition temperatures,melting temperatures,and cooling crystallization temperatures of the biocomposites were slightly decreased due to the incorporation of ESO.(5)The BS particleboards were prepared from BS and AESO as adhesives.Hexamethylene diisocyanate(HDI)was used to modify AESO because AESO contains double bonds,hydroxyl groups,and epoxy groups.HDI not only works as a crosslinking agent for AESO to improve cohesion of the adhesives,but also forms chemical bonds between BS and adhesives to improve the bonding strength of the adhesives.The chemical structure,viscosity,curing behavior,and bonding strength of the modified AESO adhesives were characterized.It was found that HDI can react with the hydroxyl and epoxy groups of AESO to form carbamate structure.The addition of HDI significantly reduced the viscosity and maximum curing temperature of AESO,and improved the processibility of the adhesives.Compared to the BS particleboards from AESO,the AESO-HDI bonded particleboards had 173%,74%,and 184% increases in flexural strength,flexural modulus,and internal bonding strength,respectively,as well as 63% and 29% decreases in thickness swelling rate of water absorption and abrasive resistance,respectively.