Fiber dispersion during compounding/injection molding of PP/kenaf composites: Flammability and mechanical properties

The primary aim of this paper is to analyze the fiber length retention capacity of extrusion and injection molding processes for short and long kenaf fiber reinforced polypropylene composites. The composites were manufactured by adding maleic anhydride grafted polypropylene (MAPP) and an antioxidant (Irganox) using twin-screw extrusion and injection molding processes. A higher level of fiber attrition rate was observed during the twin-screw compounding compared to that in the injection molding. During high-temperature shear mixing, both processes caused a reduction in fiber length below the calculated critical fiber length along with considerable fiber damage. The mechanical and thermal properties of the resultant composites were subsequently characterized. Cone calorimeter tests demonstrated that the peak heat release rate for the kenaf fiber composites (30. wt.%) was reduced by 37% compared to pure polypropylene (PP). In the UL-94V tests, the presence of lignin in kenaf reduced the dripping time considerably, which further led to the formation of char residue after burning. Although the fiber length obtained was below the critical fiber length under high shear processing (twin-screw compounding and injection molding), effective fiber dispersion, orientation and opening led to significantly improved mechanical performance of the PP/kenaf composites, irrespective of the initial fiber length.