Effect of Flax/PP pellets' morphology on the mechanical properties of injection-molded parts

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 kJ/m(2). 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.