No abstract

Thermotropic liquid crystalline polymers (TLCPs) exhibit a number of mechanical and physical properties such as excellent chemical resistance, low permeability, low coefficient of thermal expansion, high tensile strength and modulus, and good impact resistance, which make them desirable as a rotationally molded storage vessel. However, there are no reports in the technical literature of the successful rotational molding of TLCPs. In this article, conditions are identified that lead to the successful rotational molding of a TLCP, Vectra B 950. First, a technique was developed to produce particles suitable for rotational molding because TLCPs cannot be ground into a free‐flowing powder. Second, because the viscosity at low shear rates can be detrimental to the sintering process, coalescence experiments with isolated particles were carried out to determine the thermal and environmental conditions at which sintering should occur. These conditions were then applied to static sintering experiments to determine whether coalescence and densification of the bulk powder would occur. Finally, the powders were successfully rotationally molded into tubular structures in a single axis, lab‐scale device. The density of the molded structure was essentially equivalent to the material density and the tensile strength and modulus were approximately 18 MPa and 2 GPa, respectively. POLYM. ENG. SCI., 45:410–423, 2005. © 2005 Society of Plastics Engineers

SynopsisThe coalescence in air of two polymeric drops into a single drop ͑also referred to as sintering͒ was investigated for two thermotropic liquid crystalline polymers. Initial coalescence via elastic contact was ruled out based on the magnitude of the equilibrium compliance values and the process was, therefore, believed to be driven by surface tension and resisted by means of viscous flow.Remarkably the viscous coalescence model developed for Newtonian fluids ͑an extension of the Frenkel and Eshelby approach͒ agreed well under some conditions of temperature with coalescence data ͑i.e., observation of neck growth under a microscope͒. On the other hand, the extension of the Newtonian model to the viscoelastic case by incorporating the upper convected Maxwell model ͑UCM͒ assuming steady state stresses always underpredicted the rate of coalescence. The viscous neck growth model using the UCM constitutive equation was extended to the transient stress case in order to incorporate the slow growth of viscosity at the startup of flow. The unsteady state UCM approach represented a qualitative improvement over the Newtonian and steady state UCM formulations because it predicted accelerated coalescence, relative to the Newtonian model, by increasing the relaxation time. However, the model was unable to quantitatively predict the experimental coalescence rates, as it overpredicted the acceleration of coalescence.