A thermotropic liquid crystalline polyester (TLCP), based on hydroxybenzoic acid, hydroquinone and sebacic acid, was used as a processing aid in the extrusion of high molecular mass polyethylene (HMMPE). This TLCP is in the nematic phase at 179.6- 182℃. Capillary rheometry experiments were carried out at two processing temperatures: 190°C and 230°C. At 190°C the TLCP is predominantly nematic and at 230°C it is predominantly isotropic. It is an effective processing aid for HMMPE, particularly at 190°C, with viscosity reductions in excess of 95% for the 0.5 and 1.0 wt% TLCPBIMMPE blends....[ Read more ]

A thermotropic liquid crystalline polyester (TLCP), based on hydroxybenzoic acid, hydroquinone and sebacic acid, was used as a processing aid in the extrusion of high molecular mass polyethylene (HMMPE). This TLCP is in the nematic phase at 179.6- 182℃. Capillary rheometry experiments were carried out at two processing temperatures: 190°C and 230°C. At 190°C the TLCP is predominantly nematic and at 230°C it is predominantly isotropic. It is an effective processing aid for HMMPE, particularly at 190°C, with viscosity reductions in excess of 95% for the 0.5 and 1.0 wt% TLCPBIMMPE blends.

The rheological characteristics of the blends have been linked to the optical textures of the TLCP using hot-stage optical microscopy. From the experimental observations speculations are made about the mechanisms for viscosity reduction. Initial viscosity reductions are caused by TLCP structure effects (fibrillation and phase change), giving rise to fibrillation-induced molecular orientation in the neighboring HMMPE phase. This is only effective when nematic structures are present. At 230°C there is evidence for a phase transition from isotropic to nematic induced by shear. At low TLCP concentrations and wall shear stress values the TLCP migration allows the formation of a lubricant layer between the die wall and the extrudate surface. However, the experimental data shows that the wall slip effect is not the dominant viscosity reduction mechanism. Entrance pressure loss measurements show that the entrance pressure accounts for up to 34% of the overall pressure. They also show that the entrance pressure loss correction is necessary in TLCP blend studies.

Even with such a large viscosity reduction brought about, by the addition of small amounts of TLCP, the mechanical performance of the blend extrudates similar to that of homopolymer extrudates.