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Comparisons of Microcellular PHBV/PBAT Parts Injection Molded with Supercritical Nitrogen and Expandable Thermoplastic Microspheres: Surface Roughness, Tensile Properties, and Morphology

Solid and foamed tensile bars made of poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV)/ poly (butylenes adipate-co-terephthalate) (PBAT) blends with a weight ratio of 45/55 were prepared via conventional and microcellular injection molding processes, respectively. To fabricate the degradable PHBV/PBAT foamed parts, two kinds of blowing agents, namely, supercritical nitrogen (SCF N2) as the physical blowing agent and expandable thermoplastic microspheres (ETM) as the chemical blowing agent, were employed. Various properties were investigated and compared in terms of surface roughness, mechanical tensile properties, and cell morphologies. Through surface roughness comparisons using a 2D surface roughness analyzer and a 3D white-light interferometer surface profiler, microcellular injection molded parts with ETM exhibit a better surface quality. The tensile property results show that PHBV/PBAT–N foamed with SCF N2 have a longer strain-at-break. The microcellular morphologies and sandwich-like multilayer structure on the fractured cross-sectional surfaces were characterized by using a scanning electron microscope (SEM). As shown by the test results, the cell microstructures - such as cell size, cell density, and multi-layered structures with a foamed core sandwiched by skin layers - played an important role in the surface quality and mechanical properties. The rather interesting and unusual cell morphology of PHBV/PBAT–N explains why the Young's modulus and ultimate strength are lower.