A rheological study of the cavitation mechanism of rubber particles in ABS copolymers

This thesis is the result of a work carried out in the rheology and processability characterization laboratory of the industrial site of Versalis S.p.a in Mantua. Cavitation of rubber particles in acrylonitrile-butadiene-styrene (ABS) copolymers can be both mechanically and thermally induced. In the latter case cavitation is due to a volume strain generated, on cooling below room temperature, by the mismatch of the values of the thermal expansion coefficients of the glassy matrix and the polybutadiene dispersed rubber. The resulting strong hydrostatic negative stress can also decrease the glass transition temperature value of the rubber particles by several degrees. An energy balance model has been proposed to study this phenomenon, considering a homogeneous spherical rubber particle embedded in a shell of thermoplastic matrix, in which the cavity is supposed to grow at the center of the particle. When the volume strain to which rubber particles are subjected is sufficiently high, the energy balance predicts a small maximum, followed by a more pronounced minimum. Data in the literature suggest that rubber particles are more resistant to cavitation than predicted by the energy minimum. It was proposed that the reason for this is the energy barrier maximum which plays the role of an activation energy for void nucleation. In this work a method based on the detection of isothermal cavitation kinetics is proposed to investigate whether an activation step governs the formation of the voids. The activation energy barrier was measured and discussed at different temperatures for three emulsion-like ABS copolymers with different particle size distribution.