Effect of filler particle shape on plastic-elastic mechanical behavior of high density poly(ethylene)/mica and poly(ethylene)/wollastonite composites

It was found in this study that both fillers (mica and wollastonite) trigger an increase in Young's modulus of elasticity with increasing filler concentration in a HDPE composites matrix. In the case of HDPE/mica the same improvement was also found for the upper yield point vs. filler concentration dependencies indicating higher stiffness. However, for the HDPE/wollastonite composites the opposite trend was observed, i.e. a decrease of the upper yield point and strain at break. These findings were also confirmed by mechanical vibration damping testing where there was found a more intense shift of the first resonance frequency peak position to higher frequencies with increasing filler concentrations for HDPE/mica in comparison to HDPE/wollastonite composites. Both composites exhibited decreasing strain at break with increasing filler concentration indicating a more brittle mechanical behavior in comparison to the virgin HDPE polymer matrix. However, for HDPE/wollastonite composites at 5 w. % filler concentration a 15% increase in the magnitude of the strain at break was found indicating an increase in ductility at 50 mm/min deformation rate. Fracture toughness measurements show, that both studied fillers function as the stress concentrators in the HDPE polymer matrix, which was reflected in the exponentially decreasing dependencies of the fracture toughness vs. filler concentrations. SEM analysis of the fracture surfaces show typical elongation bands of high plasticity deformation regions characteristic of typical shearing bands, interpenetrated with cavities created around filler particles. Thermal analysis data showed for HDPE/mica a strong increase of the crystallinity with increasing filler concentration, however in the case of HDPE/wollastonite the opposite effect of a higher amorphous polymer phase content was found.