Comprehensive study on the relationships between the processing, the microstructure, and mechanical properties of injection molded polypropylenes

Abstract

The relationship between the morphologies of different injection molded polypropylenes and their tensile properties measured at high strain rate were investigated. A threefold systematic analysis is proposed by establishing the following relationships: (i) processing-morphology; (ii) processing-mechanical properties; and (iii) morphology-mechanical properties. Experimental techniques were used to assess the structural heterogeneity of the moldings, namely: polarized light microscopy (skin thickness), wide-angle X-ray scattering (crystallinity, molecular orientation, and polymorphism of the skin layer), and differential scanning calorimetry (bulk crystallinity). High velocity tensile tests (1 m/s) were recorded by a high speed camera that runs 20k frames per second. The influence of multiple processing variables on the morphology and properties was studied adopting a structured statistical approach by means of two statistical tools: analysis of variance and response surface methodology. The most important molding variables and their interactions were identified. Straightforward relationships between the morphology and the tensile properties were established by fitting the experimental data to polynomial equations, using a least-square minimization procedure. The melt temperature was identified as the most significant variable for the development of the morphologies and the mechanical response of the moldings. The skin thickness, its crystallinity and molecular orientation (flow direction) influence, to a certain degree, the tensile properties of the materials.