The overall objective of this work was to enhance the mechanical properties of thermoplastics by blending with liquid crystalline polymers (LCPs). Injection molding and sheet extrusion studies of blends of poly(ethylene terephthalate) (PET) with several LCPs were conducted with an emphasis on blends containing 50 wt % or less of the LCP. It was seen that significant enhancements (50-350%) in the tensile and flex moduli of PET were achieved by blending with 0-50 wt % LCPs via injection molding. The level of property enhancements was lower in the case of-sheet extrusion due largely to processing limitations which made it difficult to obtain high draw ratios.
Since thermotropic LCPs typically have high melting temperatures it is difficult to blend several thermoplastics such as polypropylene (PP) with these LCPs in the same extruder or molding unit. Thus a blending method (hereon referred to as the dual-extruder mixing method) was developed to overcome this limitation. In this method, the matrix and LCP polymers were plasticated in two separate extruders, subsequently mixed downstream in a static mixer (Kenics) and the melt blend than passed through an appropriate capillary or sheet die to generate strands or sheets, respectively. Using this method, blends of PET and PP with several LCPs were extruded into strands and sheets. In some cases, for example PP and Vectra A900 (LCP), the difference in their normal processing temperatures was in excess of 100°C.
Strands of PET/Vectra A900 70/30 composition ratio were observed to have higher moduli than a blend of the same composition extruded using a single extruder at all the draw ratios tested. This was determined to be due to the different LCP fibrillar morphology in the two cases. In the case of the dual-extruder mixing method, the LcP fibrils were continuous, running the length of the extrudate, and further devoid of any skin-core structure. In contrast, the single-screw extruder blend had a distinct skin-core fibril-droplet type of structure and the LCP fibrils were not continuous. On the basis of other independent experiments, it was confirmed that the LCP fibrils in the dual-extruder mixing method were generated in the static mixer itself whereas the LCP fibrils in the case of single-screw extrusion were generated in the converging section of the die and/or by drawing at the die exit. This difference in the mode of LCP fibril generation in the two cases was attributed to the distributive mixing mechanism of the static mixer compared to the dispersive mixing in the extruder. Strands of PP/LCP and PET/LCP had significantly enhanced tensile moduli compared to the corresponding matrix tensile modulus. Enhancements of 10-20 times that of the pure matrix were achieved when blends containing about 20-30 wt % of the LCP were extruded from the dual-extruder mixing method. The tensile moduli of sheets of PET/LCP and PP/LCP blends were not much higher than that of the corresponding matrix polymer and this was attributed to the low molecular orientation achieved in the sheets due to low draw ratios. The tensile strengths of the majority of the blends were not enhanced to any appreciable degree and poor wetting and adhesion between the thermoplastic-LCP polymers was believed to be the cause.
Comparison of some of the mechanical properties (tensile modulus, tensile strength, flexural modulus) of the thermoplastic/LCP blends generated in this study with data from the literature on thermoplastic/inorganic filler composites showed that when compared on the basis of equal wt % of the reinforcement in the blend, the LCP composites can yield mechanical properties which are in the same range as those obtained using inorganic fillers. / Ph. D.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/37319 |
| Date | 05 February 2007 |
| Creators | Sukhadia, Ashish Mahendra |
| Contributors | Chemical Engineering, Baird, Donald G., Conger, William L., Davis, Richey M., Telionis, Demetrios P., Wilkes, Garth L. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation, Text |
| Format | xxi, 325 leaves, BTD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | OCLC# 24090136, LD5655.V856_1991.S865.pdf |
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