Failure processes in particulate filled polypropylene

In this paper the common degradation effect of silicon oxide-filler on fracture strain and fracture toughness of isotactic polypropylene is investigated, analysing the failure processes in the composite material with mmicroscopic methods. Experiments demonstrate that, although fracture of the polymer regions absorbs considerable energy by plastic deformation, void formation and cracking of the interface between polymer and filler usually requires very little energy. These weak interfaces do not resist cracking and are the cause of brittleness in particulate filled systems.

The crucial parameters influencing the fracture data of the composite were found to be the cvolume fraction of the filler and the interfacil adhesion between polymer matrix and particles. As the interfacial fracture energy is usally much smaller than ther polymer fracture energy, the composite toughness drops when filler is added. Using a model which describes the individual steps of crack formation and final fracture, an attempt is made to expalin a decrease of crack resistance of teh polymer matrix with increasing filler fraction and to calculate the fracture energy of the composite by introducing partial values of crack resistance of the matrix and the interface, respectively. In Addition, it is discussed how a coarse spherulitic morphology of the matrix, as procduced by isothermal crystallisation dfrom the melt, can modify this behaviour.