Injection Molding of Microcomposites Based on Polypropylene and Thermotropic Liquid Crystalline Polymers

Abstract

Abstract This work is concerned with the injection molding of pellets of polypropylene (PP) containing pregenerated microfibrils of thermotropic liquid crystal polymers (TLCPs), referred to as microcomposites. The TLCPs used are HX6000 and Vectra A950. The microcomposites are produced by drawing strands of PP and TLCPs generated by means of a novel mixing technique and pelletizing the strands. The work was undertaken in an effort to improve on the properties observed for in situ composites in which the TLCP fibrils are generated during mold filling. In situ composites usually exhibit highly anisotropic mechanical properties and the properties do not reflect the full reinforcing potential of the TLCP fibers. Factors considered include the effect of in situ composite strand properties on the properties of the injection molded composite, the melt temperature used in injection molding, TLCP concentration, and the melt temperature of the TLCP. It is found that when processed at appropriate temperatures, the TLCP microfibrils are maintained through injection molding, and the composites show a good balance between machine and transverse direction properties (with ratios of the machine and transverse direction moduli ranging from 1.1 toto 1.35). The use of HX6000 rather than Vectra A allows for higher processing temperatures to be used during injection molding. The moduli of the injection molded composites increase with both increasing TLCP composition and in situ composite strand moduli. At 10 wt.-% TLCP the moduli of the injection molded composites approach the predictions of composite theory while at 20 and 30 wt.-% TLCP negative deviations from the theoretical moduli are seen. This is believed to be the result of a loss of fiber aspect ratio from fiber breakage and agglomeration. It is believed that with future studies on the effect of shear and elongational stresses during processing on the aspect ratio of the fibers a means of overcoming this problem may be found. The modulus of a 20 wt.-% HX6000 composite is similar to that of a 20 wt.-% glass composite (3.21 GPa versus 3.66 GPa), but the tensile strength of the HX6000 composite is about 28 % lower than that of the glass composite. It is expected that as the modulus and strength of the reinforcing TLCP fibrils are improved the properties of the injection molded microcomposites should exceed those of glass reinforced composites.