| Light weight and environmental protection are the main trends of automotive development.At present,composite materials are increasingly used in the automotive industry,in order to reduce the weight of the car itself and promote the energy saving and emission reduction in the automotive field.The fiber reinforced composites have good air permeability and heat insulation performance,low cost,light weight,and no sharp fragments after collision.People gradually apply it to automotive interior parts.However,the interface compatibility between fiber and resin is poor,and the VOC released by fiber reinforced composites in the narrow space of cars damages the physical and mental health of drivers.Therefore,in order to expand the application scope of fiber composites in the field of automotive interiors,it is urgent to reduce the VOC emission of fiber reinforced composites and improve the mechanical properties of the composites.In this project,the degumming of hemp fiber and the interfacial modification of composites were used to reduce the VOC emission of hemp fiber reinforced polypropylene composites and improve their mechanical properties.In this paper,the method of cryogenic-mechanical treatment was proposed.When hemp fiber is used as a composite reinforcement,the removal of colloid can improve the thermal stability of hemp fiber,reduce the VOC emission of the material,and improve the comprehensive mechanical properties of the composite.The effects of the molding on mechanical properties of hemp fiber reinforced composites were investigated to optimize the molding process parameters.The coupling agent and urea were used to modify the interface of hemp fiber reinforced polypropylene composites.The main conclusions can be summarized as follows:(1)After cryogenic-mechanical treatment,the lignin and hemicellulose content of hemp fiber decreased to 7.16% and 2.82% respectively,and the cellulose content increased to78.93%.The diameter becomes smaller and more uniform,and the tensile properties are lowered.The hemp fiber has a rough surface and many small longitudinal gullies,and the dispersion between the fibers becomes better.The corresponding temperature of maximum decomposition rate increased 24 °C,and the treated hemp fiber has higher thermal stability.The contents of formaldehyde,acetaldehyde and acrolein were higher in the released VOC.In addition,the total VOC emission of hemp fibers decreased after cryogenic-mechanical treatment.(2)Taking the middle hemp fiber treated by cryogenics at 77 K and mechanical treatment as raw material,hemp fiber composite sheets were prepared by single factor test,and the optimum molding process parameters were determined by testing mechanical properties.In the molding temperature of 180 °C,the molding pressure of 9 MPa,the heating time of 6 min,hemp fiber mass fraction of 45%,mechanical properties of composites were better.The tensile strength,flexural strength and impact strength were 26.93 MPa,39.26 MPa and 19.13 k J/m~2,respectively.Compared with the untreated hemp fiber,mechanical properties of composites were improved and VOC emission was decreased.(3)Taking the treated middle hemp fiber modified by urea and coupling agent as the raw material,mechanical properties of the modified composite materials were improved compared with those without modification in different degrees,and the VOC emission was reduced.Compared with tensile strength,bending strength and impact strength of the unmodified composites,mechanical properties of the composites modified by urea were improved by 16.45%,17.32% and 13.23%,respectively,while mechanical properties of the composites modified by coupling agent were improved by 19.53%,20.40% and 11.19%,respectively.The results show that the coupling agent KH-550 can improve the properties of the composites. |
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