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  W t depending on the local semi-crystalline state is observed. A pronounced maximum in  W t is found about  200 μ m 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  W t ) 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.