The effect of injection molding conditions on the mechanical properties of an in situ composite

The effect of processing conditions on the mechanical properties of an injection molded polypropylene/liquid crystalline polymer (PP/LCP) blend was studied. The LCP consisted of a copolymer based on poly(ethylene terephthalate) and poly(hydroxybenzoic acid). Two PP matrices were used to vary the viscosity ratio of the blend. It was found that the highest flexural moduli were obtained for blends with the PP matrix that provided the highest viscosity ratios (closest to unity). These optimal properties were obtained only at one processing temperature, 250 degrees C. At this temperature, the storage (G') and loss moduli (G '') of the neat LCP were approximately equivalent. The optimal properties at temperatures where G' approximate to G '' is in agreement with a prior study of another PP/co-polyester LCP blend [15]. Furthermore, the optimal properties were sensitive to the filling time. This was related to heat transfer via the Graetz number. For thinner molds, higher properties are obtained over a broad range of Graetz numbers (temperatures) while for thicker molds higher properties are obtained only at lower Graetz numbers (temperatures). The dependence of the flexural modulus on the Graetz number was related to the ability to deform the LCP at the advancing front and the ability to freeze the morphology possibly even before cessation of flow.