The effect of morphology of ternary-phase polypropylene/glass bead/ethylene-propylene rubber composites on the toughness and brittle-ductile transition

Correlation between the morphology and the impact toughness was studied for ternary-phase polypropylene (PP)/glass bead (GB)/ethylene-propylene rubber (EPR) composites containing 5-40 vol % of rigid filler at a fixed volume fraction ratio EPR/GB equal to 0.33. The three following types of phase morphology were obtained: (1) separate dispersion of phases and weak GB-PP adhesion; (2) separate dispersion of phases and high GB-PP adhesion; and (3) encapsulation of GB particles by elastomer shell. Maleated PP or EPR was used for the preparation of second and third types of composites, respectively. Binary composites with either rigid or rubbery phases were prepared as model systems. Young's modulus values of composites containing GB encapsulated by a soft interlayer stay close to neat PP modulus and are located in the bounds corresponding to ternary systems with phase-separate distribution and rubber-modified PP. Notched impact strength was measured by Izod and three-point bending tests. Systems with the phase-separate distribution both with weak and with high adhesion exhibit quasi-brittle fracture. The encapsulation of GB particles by elastomer results in significant improvement of the toughness. An increase in core-shell inclusion fraction leads to intense matrix yielding within a specimen bulk and brittle-ductile transition similar to the rubber-toughened PP. A criterion of brittle-ductile transition was proposed on the basis of load-time curve analysis, which is an accumulation of critical plastic deformation on a crack initiation stage. The stress of start of local failure microprocesses at the inclusion-matrix boundary was found to play the dominating role in energy-dissipating mechanisms. The lowering of the stress at which a local matrix yielding starts at elastomer shell-PP interface compared to the stress of GB debonding is a main source of intensive energy adsorption at the initiation stage in the system with encapsulated GB. The optimal stiffness-toughness balance can be obtained by coating the rigid particles with an elastomer shell. (C) 2002 Wiley Periodicals, Inc.