Measurement of Impact Fracture Toughness of Elastomer Modified Polypropylene Blends in the Brittle- Ductil Regime

LAURA FASCE; VALERIA PETTARIN; PATRICIA FRONTINI

Les Diablerets, Switzerland

Conferencia; Fracture of Polymers, Composites and Adhesives – 3rd ESIS TC4 Conference on Polymers and Composites; 2002

European Structural Integrity Society

The lack of toughness at high testing rate and room temperature, combined with high notch sensitivity exhibited by polypropylene homopolymer (PP) hampers its high performance applications. The usual way to achieve better impact properties for PP is by adding elastomers. The present investigation is concerned with the impact fracture toughness evaluation of novel toughened blends based on commercial polypropylene homopolymer and a thermoplastic elastomeric polyolefin (POEs). PP and POEs form immiscible phase separated blends with the spherical elastomeric inclusions randomly dispersed in the neat PP matrix. Impact testing was carried out on sharp notched SE(B) samples at room temperature and at 1m/s using a Fractovis Ceast falling weight type machine. Specimens of polypropylene homopolymer exhibited brittle behaviour while elastomer modified polypropylene blends showed semi-brittle behaviour. Elastomer modified PP blends displayed non-linear load deflection records in fracture experiments. This pattern, which was really impressive for 20%wt POEs blend, may be caused by plasticity, sub-critical crack growth or both phenomena. In addition, the large scatter found in fracture energy values remind us the so-called ductile-brittle transition regime in ferritic steels. In the blends a damage zone was developed ahead of the crack tip prior to catastrophic crack propagation revealed as a white halo in the fracture surface. Neither, the naked eye nor, scanning electron microscopy (SEM) inspection allowed us to undoubtedly discerned whether this damage zone corresponds to plastic deformation or stable crack growth. Fracture mechanics methodologies already accepted by the scientific community like ESIS TC-4 protocol are not suitable in these cases. The challenge of determining reliable toughness parameters was first faced by applying different approaches available in literature based on modified linear elastic fracture mechanics concepts, including corrected linear elastic fracture mechanics (GCIc) and equivalent energy concept (KEIc). Elastic-plastic fracture mechanics approach was also considered by the evaluation of J -Integral at the instability load point (Jc) . The results were presented in the way of a Weibull plot according to the statistical weakest link model. The incorporation up to a 20% weight of a second dispersed elastomeric phase overall enhanced the measured fracture energy, further elastomer incorporation was less effective in toughening.