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Development of flax fiber-reinforced polyethylene biocomposites by injection molding




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Flax fiber-reinforced plastic composites have attracted increasing interest because of the advantages of flax fibers, such as low density, relatively high toughness, high strength and stiffness, and biodegradability. Thus, oilseed flax fiber derived from flax straw, a renewable resource available in Western Canada, is recognized as a potential replacement for glass fiber in composites. Among plastics, polyethylene is a suitable material for use as a matrix in composites. However, there are not many studies in this area. Therefore, the main goal of this research was to develop flax fiber-polyethylene (PE) biocomposites via injection molding and investigate the effect of material properties and processing parameters on their properties. Alkali, silane, potassium permanganate, sodium chlorite, and acrylic acid treatments were employed to flax fiber to decrease the hydrophilic of fiber and improve the adhesion between the fiber and the matrix. All chemically treated fiber-HDPE biocomposites had higher tensile strength and lower water absorption compared with non-chemically treated ones. Acrylic acid treatment of the fiber resulted in slight increase in its degradation temperature; using this treated fiber resulted in biocomposites with the best performance. Therefore, the morphological, chemical, and thermal properties of acrylic acid treated fiber were also studied. Linear Low Density Polyethylene (LLDPE) and High Density Polyethylene (HDPE) were the main matrices investigated in this research. Showing a high tensile strength and similar water absorption, HDPE was used as the matrix in further research. Flax fiber with 98-99% purity was chosen as reinforcement since the flax shive mixed with the fiber decreased the tensile and flexural properties but increased the water absorption of the biocomposite. Acrylic acid-treated fiber-HDPE biocomposites had been developed through injection molding under different processing conditions. Increasing the fiber content of biocomposite increased its tensile and flexural strengths, especially flexural modulus, but its water absorption capacity also increased. It was possible to improve the mechanical properties of biocomposites and decrease the water absorption by adjusting injection temperature and pressure. Injection temperature had more influence on the quality of the biocomposite than injection pressure. Injection temperature lower than 195°C was recommended to achieve good composite quality. Melts of HDPE and flax fiber-HDPE biocomposites were categorized as power-law fluids. Apparent viscosity, consistency coefficient, and flow behavior index of biocomposites were determined to study their flow behavior. The statistical relationship of these parameters with temperature and fiber content were modeled using the SAS and SPSS softwares. The injection filling time was related to the material rheological properties: biocomposites required longer filling time than pure HDPE. Low injection temperature also resulted in long filling time.The thermal conductivity, thermal diffusivity, and specific heat of biocomposites containing 10, 20, and 30% fiber by mass were determined in the processing temperature range of 170 to 200°C. Fiber content showed a significant influence on the thermal properties of the biocomposites. The predicted minimum cooling time increased with the thickness of the molded material, mold temperature, and injection temperature, but it decreased with the ejection temperature.



Injection molding, Flax fiber, Biocomposites, Polyethlyene



Doctor of Philosophy (Ph.D.)


Agricultural and Bioresource Engineering


Agricultural and Bioresource Engineering


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