Properties and foaming behaviour of thermoplastic olefin blends based on linear and branched polypropylene

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McCallum, Tara J.
Polypropylene , Thermoplastic , Foaming
The recent commercial availability of branched polypropylenes (PPs) combined with the advent of single-site metallocene catalysts has ignited interest in thermoplastic polyolefin blends (TPOs) with controlled melt strength. These blends have potential applications in a variety of industries including foam processing and extrusion foaming. The main objective of the thesis is to provide a detailed investigation on the rheological, morphological, thermal, mechanical and foaming properties of isotactic polypropylene / high melt strength branched polypropylene homopolymer blends, and of thermoplastic olefin blends using these polypropylenes as matrices. Initial research on the polypropylene blends consisted of a linear high melt flow rate PP and two branched PPs with different melt flow rates. Blends containing branched PPs display evidence of miscibility in the melt state and exhibit high melt elasticity together with significant strain hardening in extensional deformation while retaining good flow properties. Of the two blend systems examined, the blends containing linear and branched PPs with similar melt flow rates have better mechanical properties, higher crystallization temperatures, and higher crystallinities. An investigation into the mechanical, thermal, rheological, morphological, and microcellular foaming behaviour of TPO blends consisting of a blended matrix of linear and branched PP with a dispersed phase of an ethylene-octene copolymer was performed. Blends containing branched PP showed improved stiffness and flexural properties. Given that the morphology and interfacial tension of the blends remain virtually unaffected, these improvements are attributed to the increased crystallinity in the presence of a branched component with higher molecular weight. Varying the amount of branched PP into linear PP during foaming experiments in a batch foaming simulation apparatus caused slower cell growth rates and decreased cell densities, while TPO foams showed polydispersity in the cell sizes, possibly due to the different foaming characteristics of the immiscible components. The addition of talc to TPO blends aims at improving the stiffness and dimensional stability of the material, while lowering material costs. Blends of linear and branched PP with an ethylene-octene copolymer dispersed phase and uncalcinated talc showed similar trends, as well as an expected drop in the elongation at break. There was an increase in the viscosity and crystallinity of the blends, and optimum gains were seen in blends containing 20 wt% branched PP. Increasing the levels of branched PP did not significantly affect the bubble growth rate, or the final cell density during foaming experiments.
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