The present study focuses on short flax fiber, as well as long flax fiber-reinforced polypropylene (flax/PP) composites, manufactured by the injection molding method. Compounding of flax with two different grades of PP (with and without maleic anhydride (MA-PP) grafting) is carried out by four methods: kneading process, Henschel kinetic mixer, extrusion compounding, and production of long fiber thermoplastic (LFT) granules through pultrusion. The effect of the compounding method and injection molding on the fiber length and mechanical properties of the composites is being investigated. Furthermore, the effect of fiber—matrix adhesion on the mechanical response is being discussed. It can be concluded that the reduction in fiber length, associated with injection molding, did not affect the tensile properties significantly for the studied systems due to improvements in fiber orientation along the polymer flow direction and increased fiber efficiency through dimensional changes due to fiber opening. The addition of MA-PP led to improvements in the tensile strength of injection-molded composites. Kneader compounded composites showed maximum tensile strength as well as stiffness when compared with other compounding methods.
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