| In recent years,due to a greater concern for environmental protection and energy saving around the world,the lightweight of automobile has become the inevitable trend of the sustainable development of automobile industry.Research suggests that the lightweight material is one of the effective ways to realize the automobile lightweight.Therefore,research and development of the lightweight green composite materials that can meet the requirements of lightweight,environmentally friendly,recyclable and reusable is an important direction of the development in the automotive industry.As a green composite,natural fiber composite has many advantages such as low cost,lightweight,high specific strength,high specific stiffness,renewability,biodegradability,environmentally friendly and less hazardous to health.So the natural fiber composite is a kind of ideal material for automobile lightweight,and it has been widely used in automotive industry.In this dissertation,the natural fibers were modified by active silicon alcohol,amino silicone oil and potassium permanganate respectively,and the automotive green composites with excellent performance were prepared.Then in order to provide theoretical guidance for the surface modification of natural fiber,molecular dynamics simulation was used to study the microscopic mechanism of surface modification of natural fiber.The main contents of this dissertation are as follows:(1)In this study,a new method of fluorine free water repellent modification of natural ramie fiber was developed which are low-cost and environmentally friendly.The results showed that the surface of natural ramie fiber formed a rough surface with micro/nano structures,chemical bonding and low surface energy after surface modification.It can effectively improve the water repellent and heat resistance of ramie fiber by the surface modification with the active silicone and amino silicone oil.The contact angle of ramie fiber surface increased from 0° to 147.2° after surface modification.The temperature of 5%weight loss of the fiber increased from 97.3?to 322.6?,and the temperature of 10%weight loss increased from 316.7? to 328.5? after surface modification.(2)Based on the modified with the high solid content amino silicone oil microemulsion,the high performance modified ramie fiber/polypropylene green composites was prepared by extrusion-injection molding process,and the effects of amino silicone oil microemulsion treatments on the properties of ramie fiber/polypropylene composites were investigated by experiments.The comparison study shows that the heat resistance,compatibility between fibers and polymer matrix and mechanical properties of the composites were significantly improved after surface modification.The heat resistance of ramie fiber/polypropylene composites was improved remarkably,and the oxidation of ramie fiber in the process of forming was avoided modification.The tensile strength,flexural strength and impact strength of the modified ramie fiber/polypropylene composites increased maximally by 15.89%,7.04%,and 36.58%,respectively,compared with the unmodified ramie fiber/polypropylene composites.(3)The surface models of natural fibers before and after modification with amino silicone oil were constructed,and the microscopic mechanism of surface modification at the molecular and atomic scale was studied by molecular dynamics simulation.It was found from molecular simulation that N-H---O and O-H---N hydrogen bonds formed at the interface between amino silicone oil and fiber surface.These intermolecular forces can make the molecular chain of amino silicone oil taking a specific orientation,i.e.the hydrophobic alkyl groups project outward and the polar amino groups point to the fiber surface,and result in the surface property of ramie fiber changing from hydrophilic to hydrophobic.In addition,the amino groups content of amino silicone oil affect the microstructure and properties of amino silicone oil.With the increase of the content of amino groups,the cohesive energy density of the amino silicone oil increases,the free volume voids become smaller,the distance between the free volume voids increases,and the free volume fraction decreases.The decrease of the free volume of amino silicone oil can effectively prevent the water diffusion on the fiber surface,which can lead to the decrease of water absorption of natural fiber and finally resulted in the good comprehensive properties of the RF/PP composites.(4)The effects of KMnO4 treatment on the structure and properties of ramie fiber and ramie fiber/polypropylene composites were investigated with experiments and molecular dynamics simulations at the molecular and atomic levels.The results indicated that the mechanical properties of the composites were significantly improved after surface modification.The tensile strength,flexural strength and impact strength of the modified ramie fiber/polypropylene composites increased maximally by 27.80%,15.71%,and 38.64%,respectively,compared with the unmodified ramie fiber/polypropylene composites.According to the analysis of the experimental results and molecular dynamics simulation results,there are three major reasons for the improvement of the performance of composites after surface modification.Firstly,the KMnO4 treatment can make the ramie fiber becoming rougher,which may lead to the increase of the effective surface area available for contact with the polymer matrix and then the mechanical interlocking between the ramie fiber and the polypropylene matrix can be enhanced.Secondly,the free volume voids of cellulose molecular become smaller,the fraction free volume of cellulose molecular decrease by 9.69%after KMnO4 modification,and this lead to the decrease of water absorption of natural fiber.Finally,the weak van der Waals force between the polypropylene molecule and the cellulose molecule is enhanced after KMnO4 modification.The increase in the weak Van der Waals force at interface can improve the adhesion strength at the interface of the composites and further result in the improvement of the mechanical properties of ramie/polypropylene composites.The modification method and modification mechanism of natural fiber was investigated systemically and natural fiber green composites with excellent performance,environmentally friendliness and low production cost were prepared in this dissertation.Therefore,this research can provide a theoretical basis and good technical support for the processing and application of the automotive natural fiber composites in the future. |
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