Improvement Of Interfacial Properties For Ramie Fiber/polypropylene Composites

In recent years, more and more attentions have been paid to natural cellulose fiber reinforced thermoplastic composites. However the low affinity between hydrophilic cellulose fibers and hydrophobic matrices such as polyolefin leads to poor interfacial bonding, suppressing the mechanical performance of the natural cellulose fiber reinforced composites. Therefore, improving the interfacial bonding between cellulose fibers and hydrophobic matrices is a major focus in green composite research. Ramie fibers are abundant and adequately priced in China. Thus, ramie reinforced polymer composites is one of the leading green composites with a great potential to be used in various applications. This study illustrates different ways to enhance the interfacial bonding by polymer modification through compounding, plasma induced ramie fiber surface modification and emulsion polymerization modified ramie fiber to improve the mechanical properties of ramie fiber reinforced polypropylene composites. Theoretical models and finite element analysis are developed and validated with actual experimental results to identify the key influencing factors for the interfacial properties. This is a fundamental study for future development of cellulose fiber reinforced polymer composites.Firstly, EVA(ethylene-vinyl acetate copolymer) is used to compound with polypropylene in order to improve the affiliation of polypropylene to cellulose. The experimental results show that EVA acts as a coupling agent which effectively improves the interfacial bonding between ramie and EVA/PP matrix. Meanwhile, EVA as a disperse phase increased the toughness of polypropylene. As increasing of EVA content in the compounds, the tensile strength and modulus are reduced while the flexural modulus and strength are increased. Further analysis on the interfacial shear strength(IFSS) shows that with an increased EVA content, IFSS initially increases and then decreases with a maximum value at around 8% EVA content.Secondly, plasma treatments with propylene and propane are applied to modify the surface hydrophobicity of ramie fibers to improve their interfacial bonding with polypropylene. The results show that the increased surface roughness of the ramie fiber and the amount of addition polymerized polypropylene or alkyl groups are the key factors determining the ramie surface properties. It is also revealed that with the increasing of exposure time of plasma treatment with propylene the IFSS firstly increased and then decreased. A treatment time of 1 min generated the IFSS peak value, 36.4% higher than that for the untreated ramie fiber. Compared with propane plasma treatment, propylene plasma treatment is more effective in increasing IFSS.Thirdly, emulsions made by 5 acrylic esters with different Tgs are used to introduce emulsion polymerization of acrylic ester on ramie fiber surfaces to improve the interfacial bonding properties of ramie fiber reinforced polypropylene composites. The results show that the dynamic contact angle of the ramie fiber increased from 66.76o to 85o after addition surface polymerization in acrylic ester emulsions. With an increased Tg of the emulsion, the IFSS is firstly increased, when the Tg of emulsion is 83℃, the IFSS increases 47.07%. When the Tg of the emulsion further increases, the IFSS starts to decrease. The tensile strength of treated fiber is higher than untreated fibers and with increasing acrylic ester content in the emulsion the tensile strength increases first and then decreases with a peak value at acrylic ester content of 15%.Ramie fiber reinforced polypropylene composites are made using PP and ramie fiber treated with above three different methods. Bending stiffness and tensile strength are tested with thorough analysis on the bending stress-strain curves and tensile stress-strain curves for all composites. The results show that all surface treatments could improve the interfacial bonding of ramie reinforced PP composites. The tensile and flexural strengths of composites made by emulsion polymerization treated ramie fiber show the biggest improvement. The tensile and flexural strengths of composites made by EVA compounded PP show the smallest improvement. The flexural strength of the composite with plasma treated ramie fibers and that with the EVA compounded PP are comparable.Single fiber pull out model and corresponding equation are developed to further analyze the fundamental factors determining interfacial bonding of the composites. IFSS is analyzed by changing the number of bonding points or the embedded length(equivalent to bonding strength of the micro bonds). A finite element model is proposed for the single fiber pull out experiment and the stress distribution in droplet, fiber and their interface is discussed at different separation distance. The deformation and transition behavior of interface during single fiber pull out test are modeled and the interfacial properties with different surface treatments and theoretical mechanism involved in the process are discussed.