Correlation between cross-linking of the rubbery phase and aesthetic properties in ABS copolymer

This thesis work, carried out at the Rheology laboratory of Eni Versalis S.p.a., was started with the aim of investigating why ABS copolymers appear significantly different in terms of gloss, with only a change in the swelling index. This is an indirect measure of cross-linking degree of the rubber particles inside ABS. Aesthetic defects in injection- molded objects, such as the gloss transition, must be mitigated as they lead to poor surface quality. Furthermore, in certain types of products, gloss is a fundamental property that must be enhanced. It is known that gloss depends on molding conditions, such as injection speed and mold temperature. In literature, a dimensionless parameter has been proposed, called replication factor R, as responsible for the gloss value. It is defined as the ratio between the pressure, to which the melt is subjected during the injection, and the Young’s modulus E of the polymer layer in contact with the mold wall. The pressure generated by the melt acts as a driving force for replication and is related to injection speed and polymer viscosity. The increasing surface stiffness, during the cooling below the glass transition temperature (Tg), acts as a replication resistance factor and is also related to molding conditions, such as injection speed and mold temperature. A high replication factor is therefore the key point to achieve excellent surface quality. However, this parameter alone cannot explain the different gloss values obtained for the samples at study. These differ for the swelling index value but possess the same R value. It will be proposed that the difference in glossiness is due to the behavior exhibited by the polymer when stretched in the molten state, particularly to its elongational viscosity. It is in fact known that during the filling stage, the flow front presents a divergence giving rise to the so-called “fountain flow”, in which the melt is subjected to very strong elongational stresses. These generate a stronger different orientation of the matrix polymer chains and of the rubber particles on the surface, which show a stretched shape, especially for the sample with more deformable particles (higher swelling index). In any case, many aspects, such as surface roughness, will be investigated to understand how differences in the cross-linking of the rubber can significantly affect gloss.