Influence of compatibilizers and liquid crystal polymers upon polypropylene blends.

Abstract

The present work was undertaken in an attempt to improve several important properties of polypropylene (PP) by utilizing 5 to 30 wt% of superior polymers in polypropylene polyblends. Mechanical, morphological, rheological, and creep properties of blends of polypropylene and two thermotropic liquid crystal polymers (Rodrun 3000 and Vectra A-950) based on wholly aromatic copolyesters have been investigated. Two compatibilizers (PP-g-AA and PP-g-MA) were selected for these incompatible blends of PP with liquid crystal polymers (LCPs) to improve interfacial properties. Blends were prepared using a single-screw extruder with mixing head. These immiscible blends of PP with two LCPs were significantly improved in morphology and mechanical properties through the selected compatibilizers. In the case of the self-reinforcement of LCP polyblends, LCPs easily form fibrillar structures in the PP matrix and the aspect ratio of these fibrillar structures increases with increased draw ratios. The compatibilizers increased the mechanical properties of polyblend strands considerably up to 5 times in tensile modulus and 3 times in tensile strength over virgin polypropylene. This is comparable or even exceeds the corresponding values of glass-reinforced PP. Considerable creep resistance was observed for PP/LCP polyblends and the creep modulus of reinforced strands showed up to 23 fold increase compared with virgin polypropylene. The influence of liquid crystal polymers (LCPs) on the primary crystallization process of polypropylene was investigated both in isothermal and non-isothermal crystallization behavior with a DSC instrument. The time required to complete the primary crystallization appears to be considerably reduced by the presence of liquid crystal polymers. However, the maximum crystallinity of PP is not affected in the presence of liquid crystal polymers (LCPs) in isothermal crystallization. The melt viscosity of these polyblends is found to be lower than virgin PP. Their reduction in viscosity is dramatic at low shear rates. The significantly higher rate of crystallization of PP and lowered melt viscosity of blends have been projected to be beneficial for injection molding. These compatibilized PP/LCP blends overcome some of the disadvantages of glass fiber-reinforced polypropylene (GRP), namely, high melt viscosities of GRP, difficulties in processing and molding, and abrasiveness to compounding equipments and molds. As a result, LCPs may provide a more efficient reinforcement for polypropylene than does glass fibers, considering all of the above factors.