Cellulose reinforced high density polyethylene composites — Morphology, mechanical and thermal expansion properties

A comparative study was made to find out how two cellulose based micron-sized fillers affect the morphological, thermal, and mechanical properties of high density polyethylene (HDPE). The fillers used in this study were commercial microcrystalline cellulose (MCC) and viscose fibers (filler content of 5, 10, 20 and 40 wt.%). The composites were prepared by melt mixing using maleic anhydride grafted polyethylene (PEgMA) as a polymeric compatibilizer in order to improve the chemical compatibility of the fillers and the matrix. SEM images of the composites revealed that the PEgMA improved adhesion between the cellulose fillers and the HDPE matrix. Cellulose was found to have a clear effect on the thermal dimensional stability of the HDPE. The viscose fibers and the MCC decreased the coefficients of the linear thermal expansion (CLTE) of the HDPE matrix in the flow direction, CLTEs (at 25 °C) as low as 37 ppm/°C (40 wt.% of viscose fiber) and 79 ppm/°C (40 wt.% of MCC) were achieved for the composites compared to 209 ppm/°C for the HDPE. The Young’s modulus and the tensile strength of the HDPE significantly increased and elongation at break decreased with a filler content. Enhanced adhesion in the presence of the PEgMA increased the mechanical properties of the composites, especially tensile strength. The highest values for Young’s modulus and tensile strength, and the lowest values for CLTE in the flow direction were achieved with the viscose fibers. The measured Young’s moduli for the viscose fiber composites were compared to those calculated by composite theories. The Halpin–Tsai composite theory successfully predicted the elastic moduli for the viscose fiber composites.