| Natural fibers have many excellent properties over synthetic fibers and can be used to reinforce the polylactide(PLA).They can not only improve the properties of PLA and expand its application,but also achieve the sustainable development.However,the greatest challenge in working with natural fibers reinforced PLA composites is their large variation in properties and characteristics.Some methods for pretreating of fibers had been used to improve their compatibility.Though these methods improved the interfacial adhesion of fibers and matrix,they eliminated some components and destroyed the construction of fibers,which led to the decrease of the strength of fibers and the mechanical properties of composites couldn't be enhanced as expected.In this work,sisal fibers(SFs)were alkali treated with different time to prepare ASFs samples for analyzing the influence of chemical pretreatment on the microstructure,tensile strength of SFs and interfacial bonding between SFs and PLA matrix based on the variety of contents of cellulose,hemicellulose and lignin.Then the untreated SFs(USFs)and the alkali-treated SFs(ASFs)were mixed to make hybird SFs(HSFs)and then blended with PLA matrix to prepare HSFs reinforced PLA composites(PLA/HSFs).The hybrid ratio of USFs and ASFs,the mechanical properties,thermal properties and crystallization behavior of the PLA/HSFs composites were discussed.Moreover,a new processing technology,continous fiber winding process(CFWP),was put forward on account of the defects existing in the conventional processes.Based on the CFWP,a set of fiber winding device was designed and manufactured.The main work and research results was as follows.1)Influence of pretreatment on the microscture,mechanical properties of SFs and interfacial strength of PLA/SFs composites based on the analysis of chemical compositions were studied.The influence rule of the chemical composition for the microstructure and properties of SFs and the interface of SFs and polymer matrix were deserved.With the alkali treatment time increasing,the content of cellulose in SFs increased while the content of hemicellulose decreased,meanwhile the content of lignin decreased firstly and then tended to be stable.The crystal form and microfibril angle of SFs did not change within short alkali treatment time,but would change when the alkali treatment time increased to a certain extent.The crystallinity of SFs improved with the increase of alkali treatment time.An approximately linear relationship was found between the crystallinity of SF and the hemicellulose content.The tensile strength of USFs was higher than that of ASFs.However,the optimum alkali treatment time could make SFs reached the best performances.The proper contents of hemicellulose and lignin were good for both the tensile strength of SFs and the interface strength of SFs and PLA matrix.Meanwhile,it was found that lignin was perhaps more favorable for the interfacial bonding of SFs and PLA.2)HSFs reinforced PLA composites(PLA/HSFs)were designed,the mechanical properties of PLA/HSFs composites were studied and the optimal proportion of cellulose,hemicellulose and ligin were deserved.The COX shear-lag model was modified based on the above research.USFs with the highest tensile strength and ASF1 h with the best interfical bonding with PLA were mixed to make HSFs according to different mass ratios and then blended with PLA matrix to prepare HSF reinforced PLA composites(PLA/HSFs).It was found that the mechanical properties of PLA/HSFs composite were better than those of PLA/USFs and PLA/ASFs.The tensile modulus,flexural strength,flexural modulus and impact strength of PLA/HSFs composite were better than those of PLA.PLA/HSFs composite could reach the best mechanical properties when the hybriding ratio of USFs and ASF1 h was 4: 6.The contents of cellulose,hemicellulose and lignin were 71.4%?16.6% and 12.0% respectively.3)The thermal,dynamic mechanical,rhelogical and interfacial properties of PLA/HSFs composites were studied.The strengthen mechanism of SFs and HSFs for PLA were further discussed.USFs were mixed with ASF1 h the the mass raio of 4:6 to prepare HSFs.Then HSFs were blended with PLA to make PLA/HSFs composite.In addition,pure PLA?PLA/USFs?PLA/ASFs composites were also prepared.The thermal and interfacial properties of PLA/HSFs were investigated and compared with those of PLA?PLA/USFs?PLA/ASFs composite by differential scanning calarimetry(DSC)?XRD?thermal gravimetric(TG)and Scanning Electron Microscope(SEM).The results showed that: The thermal properties of PLA/HSFs were higher than those of PLA.The crystallinity of three composites was higher than that of neat PLA and PLA/HSFs had the maximum crystallinity.The addition of SFs could accelerate crystallization and improve the crystallinity of PLA composites.The polarizing microscope(POM)results demonstrated that transcrystallinity appeared in the PLA composites with the addition of SFs.Another crystal type which was ? occurred in PLA/USFs and PLA/HSFs composites and thus improved the impact strength and thermal stability of PLA composites.4)The continuous fiber winding process(CFWP)were developed and the composites prepared by fiber winding process(PLA/ CWSFs)were studied.A new model for predicting long fiber reinforced polymer composites was put forward.CFWP was low cost,low energy and nontoxicity,and it could achieve continuous winding and reinforcement of natural fiber for PLA matrix.PLA/ CWSFs had excellent mechanical properties.What's more,the manufacturing time was greatly shortened.Tensile property of PLA/ CWSFs was less than that of composites prepared by preimpregnation process(PLA/ PSFs),whereas flexural and impact property were better than those of PLA/ PSFs.In conclusion,CFWP was an excellent and practical process.When preparing composites through continuous fiber winding device,the tensile strength and tensile modulus of composites were improved with the increase of the mass fraction of SFs.However,the rising amplitude was limited when the mass fraction of SFs exceeded 40%.For flexural and impact property of the PLA composites,they rose at first and then decreased after the fiber fraction exceeding 40%. |
PDF Full Text Request