Structure-property characterisation of ternary phase polypropylene composites

Abstract

An investigation to study factors controlling the structure and properties of binary- and ternary-phase polypropylene (PP) composites containing ethylene-propylene rubber (EPR) and glass beads has been carried out. The composite structure was evaluated using various techniques including SEM, DSC, XRD and DMA. While the mechanical tests included tensile and impact measurements at ambient temperature, and a fracture toughness test based on the J-integral method carried out at -20 oC. EPR and glass beads were found to influence the structure and properties of polypropylene in different ways. Incorporation of EPR into polypropylene results in an improvement in impact strength and toughness, accompanied by a decrease in tensile strength and modulus. The opposite was found for composites containing glass beads. Polypropylene composites with balanced mechanical properties were achieved by physical blending of this polymer with both EPR and glass beads. The effect of composite structure, composition and processing variables on the properties of the ternary systems were analysed. A study of their morphology has shown that two kinds of phase structure can be formed, either a separate dispersion of the phases, or encapsulation of the filler by rubber. Factors controlling these structures are believed to be due mainly to the surface characteristics of the components. Modification of EPR by maleic-anhydride grafting results in composites with rubber encapsulation of the filler, with FTIR revealing a reaction between these phases. Composites containing unmodified EPR, on the other hand, show separate dispersion of the components. The former composites, with good adhesion at the rubber and filler interface, have noticeably higher impact strength and fracture toughness at and below ambient temperatures, while the latter variant is characterised by higher tensile strength and modulus, accompanied by a lower impact strength. Improvements in impact strength of the composites was also achieved by promoting adhesion between the polymer and filler interface using surfacecoated glass beads, or by increasing the number of rubber particles adhering to the glass bead surfaces using a two-step mixing technique. Results of the present study have thus shown that mechanical properties of ternary phase polypropylene composites can be adjusted, to a certain extent, by controlling their morphologies through the use of suitable functionalised materials and also by using an appropriate compounding methodology.