The development of a dual extrusion blending process and composites based on thermotropic liquid crystalline polymers and polypropylene

Abstract

The overall objectives of this work were to improve a dual extrusion process (DEP) which is used to blend thermotropic liquid crystalline polymers (TLCPs) with thermoplastics, determine the mechanism by which TLCP morphology is developed in the DEP and to determine the optimal properties possible in composite materials generated from the blends. The DEP consists of two single screw extruders within which the TLCP and matrix material are plasticated separately. The two continuous polymer streams are joined and then mixed in a series of static mixing elements. Composite materials were formed from pelletized pregenerated strands by processing at temperatures below the melting point of the TLCP.

The DEP was improved by the addition of a gear pump to the TLCP stream, a multiple port phase distribution system, static mixing design, minimization of residence time, die design, and introduction of thermal control over the entire strand production process. The TLCP material was introduced into the matrix phase by means of a multiple port phase distribution system which injected 12 individual TLCP streams parallel to the flow direction of the matrix stream. This design resulted in improvements in the axial continuity of the TLCP phase during mixing and improved radial mixing as compared with a simple T-injection system. Both Kenics and Koch static mixer designs were evaluated in this investigation and it was found that the use of either could produce similar mechanical property enhancement in the resulting blends provided that an excessive number of elements were not used. Furthermore, it was found that the most stable strand materials were formed when the die was designed with respect to the flow exiting the static mixer elements. For example, a dual strand die with each capillary having an L/D ratio of 1 produced the most stable strands when used with the Kenics mixing elements. Finally, it was found that drawing the molten blend strand in a vertical drawing chimney provided a favorable thermal environment and resulted in much higher draw ratios and high mechanical properties of the strand.

The other objectives of this work including the development of morphology and composite materials produced from pregenerated strands is presented in two manuscripts formatted for submission to appropriate journals. Detailed abstracts dealing with these two topics are included therein.