Applications of instrumented indentation testing for the characterization of polypropylene materials

Abstract

In this study, instrumented indentation testing, or so-called depth-sensing indentation testing, was used to characterize polypropylene (PP) materials. The influence of molecular weight and cooling rate on the hardness was investigated showing that low cooling rates and relatively low molecular weight lead to higher hardness values. In random copolymers of propene with butene and hexene, the correlation between hardness and degree of crystallinity is linear and the conversion to the amorphous state could be detected. Blends of PP with ethylene–propylene elastomer show a strong negative deviation of indentation modulus from linear rule of mixture. This behaviour can be described by Kerner’s model in the case of particulate arrangement and by Davies’ model in the phase inversion region. The addition of high-density polyethylene to PP results in a synergistic behaviour of modulus which can be attributed to nucleation effects. Furthermore, the application of indentation testing to detect physical ageing of a surface of an injection-moulded specimen and heterogeneity of weld lines was shown. Orientations in a biaxially oriented film could be detected using Knoop indenter.