Influence of structure gradients in injection moldings of isotactic polypropylene on their mechanical properties

Abstract

The internal structure of injection molded model components made from a commercial non-nucleated isotactic polypropylene grade is investigated by wide angle X-ray diffraction and polarized-light optical microscopy. Changes in the polymorphic state, degree of crystallinity and spherulitic superstructure are quantified depending on the distance from the outer surface d. Interrelations between structure and local cooling rate as obtained from numerical simulations are considered. The influence of structural features on mechanical properties is studied using thin quasi-homogenous films microtomed at different depth from the injection molded component. A significant variation of the storage part of the tensile modulus E′ and the toughness Wt depending on the local semi-crystalline state is observed. A pronounced maximum in Wt is found about 200μm below the surface where a fine spherulitic superstructure occurs. The changes in E′ are moderate (values ranging from 1.4 to 1.8 MPa) and show a maximum at intermediate depth although the degree of crystallinity is continuously increasing from ≈ 27% at the surface to ≈ 45% in the core region. Relations between structural features and mechanical properties measured at small and large deformation (E′ and Wt) are discussed. General conclusion is that a deeper understanding of relations between structural state of semi-crystalline polymers determined by the processing conditions and mechanical parameters is of major importance for predicting the properties of injection molded components.