Polyolefin Formulations for Improved Foaming: Effects of Molecular Structure and Material Properties
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The morphology and mechanical properties of foams made out of ethylene-α-olefin copolymers (EC) having well-characterized rheological properties were investigated. The polymers differed in the amount of comonomer contained, type of comonomer and molecular weight, resulting in variable thermal properties and different rheological responses under shear and extensional flow. All of the octene-based copolymers with comparable rheological properties had similar foam morphology. However grades with low extensional viscosity and low crystallization points resulted in poor foams. Increasing density resulted in a higher secant modulus of the foamed samples. To further investigate the effects of material properties, trimethylolpropane trimethacrylate (TMPTMA) and triallyl trimesate (TAM) coagents were used to generate a series of PP derivatives through radical mediated melt state reactive modification. Coagent modification resulted in pronounced effects in the molecular weights and viscosities of the derivatives. However, evidence of long-chain branching (LCB) was only present in TAM modified PPs. Significant increases in the crystallization temperature, heat of fusion and crystallization rate were attributed to the formation of nanoparticles, which resulted in a heterogeneous nucleation effect, both for crystallization and foaming. Generally lower viscosities, coupled by strain hardening, enhanced nucleation and increased crystallization temperatures induced by the nanoparticles resulted in foams with higher expansion ratios and smaller cells, due to higher rates of cell growth, coupled with suppressed coalescence. Nanocomposites based on isotactic PP and nanosilica (SiO2) were prepared using a co-rotating twin-screw extruder (TSE) in order to investigate foaming on a larger scale. High shear stress, sufficient residence time, and high fill ratio in the melting section of the screw were the most important factors in achieving good nanosilica dispersion. Well-dispersed surface-modified hydrophobic SiO2 particles were effective nucleating agents for foaming, when used at loadings below 1 phr.