Flax and polypropylene (PP) pultrudates of 4.76 mm in diameter were produced using a multi-die pultrusion system. The flax content was 50 vol.%. Pultrusion conditions were varied to produce four porosity content between 4% and 22%. The pultrudates were pelletized into lengths of 6 and 15 mm, and injection-molded with pure PP to reduce the part flax content to 25 wt.%. Results showed that well consolidated pultrudates, having porosities up to 8%, were more resilient to the pelletizing process such that the pellets remained structurally intact after the process. These pellets went smoothly through the hopper into the injection molding machine screw. Pellets with void content higher than 8% lost integrity during pelletizing leading to uniform mixing. Exposed fibers segregated from the polymer in the hopper leading to ununiform mixing. The highest mechanical properties improvement compared to pure PP was using 15 mm pellets that had a prorosity of 8%. The tensile modulus doubled at 4652 ± 113 MPa. The impact strength increased by almost five times at 10.5 ± 0.7 [Formula: see text]. 15 mm pellets provided 1.7 times the impact strength of the 6 mm pellets. These improvement are attributed to low flax fiber thermal degradation and improved fiber dispersion.
During thermoplastic pultrusion, increasing the pulling speed generally leads to a melted polymer pressure rise in the pultrusion dies. Yet, it was not clear if the polymer pressure’s increase would result in a faster impregnation flow that would compensate for the lower residency time inside the heating dies. Thus, the aim of this study was to verify if the speed-induced pressure can counterbalance the decrease in residency time that is required for impregnation quality and to analyze possible pultrudate morphology’s reconfiguration. Three distant pulling speeds were selected: namely 50, 500 and 1000 mm/min. Using a model, the polymer pressure in the pultrusion dies was computed to reach up to 1.0, 10.42, and 20.6 MPa for the respective pultrusion pulling speed. The morphology’s characterization showed a reconfiguration of the pultrudate at a higher pulling speed leading to larger unimpregnated agglomerations and polymer rich areas. The highest tensile strength achieved was 233.4 ± 1.5 MPa at 50 mm/min and dropped by around 20% when the pulling speed was raised to 1000 mm/min. The pultrudate reconfiguration at higher speed, attributed to delayed melting of PP fibers in underfilled dies, is deemed responsible for the loss in impregnation and mechanical properties.
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