Preparation And Performance Study Of Straw Fiber-based Green Composites

The straw fiber,a by-product of agricultural production,and polylactic acid（PLA）are prepared into composite materials by laminated hot pressing,which not only can deal with waste straw,but also improve the added value of straw fibers.Since straw fiber has polar groups such as hydroxyl groups that make its surface polar,and PLA is a weakly polar polymer,the polarity difference between the two is large,which will make the interface compatibility poor,and will also affect the performance of the composite material.In order to solve the polarity difference between straw fiber and PLA,this paper adopts sodium hydroxide（Na OH）treatment,silane coupling agent KBM403,end-hydroxy hyperbranched polymer HPN202 and HPN202-KBM403combined modification of straw fiber,and through scanning electron microscopy,fourier infrared spectroscopy,microcomputer-controlled electronic universal testing machine,differential scanning calorimetry,thermogravimetric analyzer,energy spectrum and other instruments to characterize the modified straw fibers and straw/PLA composites to study the modification principles of straw fibers.The main conclusions are as follows:（1）By treating straw fiber with sodium hydroxide,some hemicellulose and lignin were removed,which reduced the surface polarity of straw fiber,its surface became rough and increased the contact area and mechanical locking of PLA;the silane coupling agent KBM403can carry out coupling reaction with the hydroxyl group on the surface of straw fiber,thus reducing the polarity of the surface of straw fiber,both modification methods can improve the straw fiber and PLA.The mechanical properties of the composites modified by Na OH and silane coupling agent KBM403 showed a first increase and then decrease,when the mass fraction of Na OH was 6%,the tensile and bending properties of the composites were the best,and when the mass fraction of Na OH was4%,the impact properties were the best;when the mass fraction of silane coupling agent KBM403 was when the mass fraction of silane coupling agent KBM403 is 2%,the tensile,bending and impact properties of the composites reach the maximum;with the addition of Na OH and KBM403,the cooling crystallization peak of the composites appears to be reduced,the crystallization rate is accelerated and the crystallinity in the composite system increases;when the mass fraction of Na OH is 6%and the mass fraction of silane coupling agent KBM403 is 2%,at this time the initial thermal decomposition temperature and the maximum.When the mass fraction of Na OH is 6%and the mass fraction of silane coupling agent KBM403 is 2%,the initial thermal decomposition temperature and the maximum thermal decomposition temperature of the composites increase,and the thermal stability is enhanced.（2）The modification of straw fibers by sol-gel method generated nano-Si O₂on the surface of straw fibers,which increased the surface roughness and decreased the surface polarity of straw fibers,thus improving the interfacial compatibility between straw and PLA.As the mass fraction of ethylene silicate（TEOS）increased,the mechanical properties of the composites increased and then decreased,and the water absorption and thickness swelling also showed a trend of decreasing and then increasing.The high mass fraction of TEOS causes the formation of pores inside the composites,resulting in a decrease in mechanical properties,an increase in water absorption and thickness expansion.When the mass fraction of TEOS is 2%,the flexural and impact properties of the composites reach the maximum,the water resistance is the best,the crystallinity of the composites increases,and the nano-Si O₂as a heterogeneous nucleating agent provides sites for the cooling crystallization process of PLA,and the thermal decomposition temperature in the composite system increases at this time.（3）The end-hydroxy hyperbranched polymer（HPN202）was physically adsorbed on the surface of straw fibers.It was found that as the mass fraction of HPN202 increased,the tensile,bending and impact properties of the straw fiber/Poly（lactic acid）composites showed a tendency to increase and then decrease,and the water resistance showed a phenomenon of decreasing and then increasing.When the mass fraction of HPN202 is 6%,its mechanical properties are the best,at this time the bond between straw fiber and PLA is better,there is no separation of the two phases,and the interfacial compatibility between straw fiber and PLA is enhanced.Moreover,the crystalline area in the composite increases and the composite has the best water resistance,but the addition of HPN202will make the thermal decomposition temperature of the composite decrease,which leads to the decrease of the thermal stability of the composite.（4）After the combined modification of HPN202 and KBM403,the intensity of the hydroxyl group of HPN202 on the surface of straw fibers decreased,indicating that the surface polarity became weaker,and the water resistance of the composites was better than that of the single modification due to the chemical reaction between the hydroxyl group to be modified at the end of HPN202 and KBM403.The cooling crystallization peak in the composites was reduced,indicating that the silane coupling agent molecules acted as a heterogeneous nucleation site during the crystallization of PLA,leading to an increase in the crystallinity in the composites,and thus the mechanical properties of the jointly modified straw fiber/PLA composites were all better than those of the single modification.The thermal decomposition temperature of the composites increased after the joint modification,which was due to the fact that KBM403 would increase the heat resistance of the material surface,and after the joint modification,HPN202 reacted with the coupling agent KBM403,but the degree of polymerization of HPN202itself was low,which caused the reduction of thermal stability in the composite system.Therefore,the thermal stability performance after joint modification is lower than that after coupling agent modification and higher than that after hyperbranched polymer modification.