| At present,in order to solve the environmental and ecological problems,it is an important measure to improve the ecological environment that petroleum-based plastics are gradually replaced by renewable and degradable green polymer materials.Polylactic acid(PLA)is produced by biological fermentation,which has the advantages of low energy consumption,pollution-free,renewable and degradable.And PLA can be degraded into carbon dioxide and water to return to nature in many ways after being abandoned,forming a green circulation system.However,PLA is brittle,the preparation process conditions are harsh,and it is difficult to obtain high molecular weight polymers,which leads to high cost and cannot be widely popularized and applied.Straw fiber(CSF)is a kind of renewable biomass materials with abundant reserves and low price.Compounding with PLA can not only reduce the cost,but also improve the toughness of PLA.The interface compatibility and internal structure deterioration caused by the polarity difference between resin and biomass materials in composite materials have always been a problem to be solved,which will affect the basic properties and service life of composite materials.In this paper,CSF/PLA composites with strong hydrophobicity,high strength,high toughness and low cost were prepared by melt blending and injection molding technology with PLA as matrix and CSF as reinforcement.The effects of straw fiber composition,particle size,coating treatment and additives treatment on the mechanical properties,functional group changes,microstructure,crystallinity,water absorption,thermal stability and aging and weatherability of CSF/PLA composites were systematically studied,the interfacial compatibility system of composites was constructed,the composite modification mechanism was revealed,and the cost of CSF/PLA composites was calculated,which provided important theoretical data and technical support for the reuse and application of waste biomass resources.The specific research conclusions were as follows:(1)The effects of CSF composition and particle size on the properties of CSF/PLA composites are investigated by different testing methods,and the results show that the blending modification between CSF and PLA is only a physical change,and no new groups are generated during the processing.Due to its dispersibility and wettability,small particle size CSF is easy to be uniformly dispersed into a PLA matrix to form a stable binary structure.Moreover,the CSF/PLA composites made up of CSFC and CSFS mixed systems with smaller particle size have stronger mechanical strength.And PLA melt can flow into the fluffy pores of CSFC to form a strong mechanical interlock.The highest bending strength and the highest tensile strength of the composites are 87.24 MPa and 54.08 MPa respectively,and the elongation at break is 4.60%.CSF with small particle size has a less negative effect on the interface and less damage to the internal structure of the material.In contrast,CSF with large particle size can cause serious defects in the material structure,mainly in the form of reduced mechanical properties and poor resistance to the invasion of water molecules.The contact angle of the composite material composed of 20-40 mesh CSF can be reduced to 34.49 degrees.(2)The PLA melt is changed into blend modified with different additives and mixed with CSF to produce composites for interfacial control and properties investigation.The results show that the polar free hydroxyl groups of CSF are significantly reduced under the action of additives,and the interfacial bonding properties between CSF and PLA are also enhanced,thereby improving the mechanical properties and boosting the thermal stability of the composites to different degrees.Among them,the modification of PCDI has a prominent gain in many properties of the composite including interfacial compatibility.For example,the most significant improvement of hydrophobicity is achieved,and the contact angle of the composite can be increased to 73.78 degrees.This enhancement of interfacial compatibility lays the foundation for the subsequent study of CSF/PLA composites.(3)Corn stalk fiber(CSF)is cross-linked and coated with epoxy resin and isocyanate to form a core-shell structure,which reduces the polarity difference between CSF and PLA and provides uniformly dispersed physical cross-linking points for the PLA molecular chain.In this way,it constructs compatible interfaces,improves the strength of the interfacial layer,and reduces the hydrophilicity of the composite.Among them,isocyanate can form chemical bonding with CSF,so the bonding strength between the core and shell is significantly improved,and the interfacial compatibility of the composite becomes better.The results show that the mechanical properties and hydrophobicity of the composites with the core-shell structure composed of 10%isocyanate are improved the most,and the mechanical strength is improved while the impact strength is increased by 98.8%.The tensile,bending and impact strength are50.67 MPa,113.39 MPa and 16.44 KJ/m~3,and the saturated water absorption and contact angle are 3.25%and 79.69 degrees respectively.The use of natural biomass as a raw material to prepare composites is a good way to save energy and sequester carbon and meanwhile it also gives the material rich functionality,promoting the sustainable development of polymer materials.(4)The aging durability of CSF/PLA is investigated by both natural and artificial aging methods to compare the effects of CSF content and PCDI on the aging durability of PLA matrix composites.During the aging test,the PLA molecular chains are subjected to both ultraviolet photolysis and hydrolysis respectively,resulting in the phase separation between PLA and CSF.With delay of aging time,the aging treated PLA/CSF composites show a gradual decrease in mechanical strength,while the opposite trend is observed for toughness.The functional change model of tensile strength and aging time of composites is established.It is found that there is a first-order exponential attenuation relationship between them,and the attenuation rate of PCDI modified composites is lower than that of unmodified ones.After UV aging treatment,there is a small decrease in thermal stability of all CSF/PLA materials,and only pure PLA shows a substantial decrease after aging for 1500h,indicating that CSF can have a positive effect on the persistence of thermal stability of PLA matrix materials.The mechanical properties,water resistance and fracture morphology of CSF/PLA composites modified by PCDI can show that PCDI hinders the aging and degradation of PLA matrix composites. |
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