Microcellular foaming of poly(lactic acid) branched with food-grade chain extenders

This study evaluated the effectiveness and efficiency of two food-grade multifunctional epoxies chain extenders (CE) in branching PLA and improving its foamability. Both CE grades were effective in branching PLA causing increased end mixing torque, shear, elongational viscosities, molecular weight but decreased crystallinity of poly(lactic acid) (PLA) with CE content, due to chain entanglements. CE with low epoxy equivalent weight (EEW) was more efficient than the counterpart with high EEW due to its high reactivity. Neat PLA foams showed poor cell morphology with areas without nucleated cells and had a low expansion, owing to its low elongational viscosity. By contrast, there was a considerable change in the morphology of the PLA foam structure caused by its branching. Chain-extended PLA foams had uniform cell morphology with a high void fraction (up to ~85%) and expansion ratio (an eightfold expansion over unfoamed PLA) due to their high elongational viscosities, suggesting that melt properties of branched PLA were appropriate for optimum cell growth and stabilization during foaming. Overall, CE with low EEW was the most effective grade and 0.25% the optimum content that provided appropriate melt viscosity to produce PLA foams with a homogeneous structure, fine cells, high void fraction, high volume expansion ratio, and cell-population density.