Relationship between rheology, morphology and foamability of thermoplastic olefin (TPO) blends

Due to the unique properties of thermoplastic olefins (TPOs), this class of polymer blends has gained a great interest in the polymer processing industry. Although several researches have been devoted to investigating the microstructure and various properties of these materials, the foaming behavior of the TPOs has not been fully understood yet. The aim of the present study is to control the foaming behavior of TPOs via the understanding of the role of rheological and morphological properties in the formation of cellular structures. A particular attention is paid to the phase coalescence phenomenon, which plays a major role in the morphology development of polymer blends. In this regard the extent of flow induced coalescence in TPOs was firstly investigated by means of a rheological approach. A transient rheological model was employed to describe the morphological evolution of the blends during shearing in a homogeneous shear flow field. Investigating TPOs with different concentrations showed that phenomenological transient models can be used to simulate the coalescence behavior and to account for the variation of collision frequency with volume fraction. It was also shown that the coalescence is favored by a decrease of the viscosity of the inclusions. This was attributed to the increased interfacial mobility, which can reduce the drainage time and facilitate the collision of two approaching drops.;To control the extent of phase coalescence, in the second part of this study, a reactive compatibilization was employed. An aminated polypropylene and a maleic anhydride grafted elastomer were used to form copolymers at the interface during a reactive melt blending process. Dynamic rheological measurements showed that the compatibilized blends behave as materials in the sol gel transition featuring infinite zero shear viscosity and power-law behavior of the elastic moduli at low frequencies. The gel-like behavior of the compatibilized blends at low frequency could be described using a generalized Zener model with 3 fractional elements. The compatibilized TPOs also showed significant enhancements in transient shear viscosities as well as a retarded stress relaxation behavior. Such a rheological behavior was attributed to the presence of an interconnected network structure in the compatibilized TPOs as confirmed by atomic force microscopy.;In the last phase of this research, the foaming behavior of the TPOs was investigated by means of a bath setup pressurized with carbon dioxide as blowing agent. The morphology of the TPOs was varied via reactive compatibilization, static annealing and shear induced phase coalescence. The cellular structure of the TPO foams were characterized using a microscopic technique capable of positioning the respective locations of bubbles, dispersed elastomeric phase and the polypropylene matrix. The results showed that the elastomeric phase acts as the nucleating site and the final cellular morphology of the TPOs can be directly correlated to the initial blend microstructure. It was also shown that the melt elongational properties of the TPOs play a major role in controlling the extent of cell coalescence. Reactive compatibilization was found to be capable of modifying the melt foamability via increasing the nucleation rate and controlling the growth stage of the nucleated bubbles.