(122d) Developing High Impact Poly(lactic acid) (PLA) Blends Through Reactive Blending

Poly(lactic acid) (PLA) is one of a few commercially available biobased thermoplastic polymers and is considered the most promising one to substitute petroleum-based polymers for broad application because of its high strength, modulus and competitive price. However, the brittleness of PLA has prevented it from wide applications. To improve its toughness, PLA has been blended with various rubbery or flexible polymers. While most of these blends exhibit very high ductility compared with neat PLA, they only yield very limited (a few times higher) improvement on impact strength which is more important for many applications. Given the very low impact strength of PLA (ca.10-30 J/m), a few folds of increase is far from sufficient to satisfy most engineering applications. Because in most blends the polymer pair is immiscible, creation of sufficiently strong interphase interaction is a prerequisite for achieving high mechanical properties, particularly impact strength. Reactive blending may provide a crucial solution to realize the strong interfacial adhesion.

In this work, improving the toughness of PLA materials through blending is discussed in detail. In particular, the discussion is focused on our recent investigation of improving impact properties of PLA blends through reactive blending. Our novel PLA ternary blend system consisted of PLA, an epoxy-containing elastomer and a zinc ion ionomer of the ethylene-methyacrylic acid copolymer. The blending was performed in a twin-screw extruder. Mechanical, thermal dynamic mechanical properties and morphology of the blends were thoroughly studied. Blending temperature, elastomer/ionomer ratio, type of cations and neutralization degree of the ionomer all displayed significant effects on impact toughness of the ternary blends. Super toughness was successfully obtained for some blends compounded at 240°C. The curing of the elastomer and reactive interfacial compatibilization between the cured elastomer phase and the PLA matrix were noted to occur simultaneously during the blending. Zinc ionomer initiated and catalyzed the dynamic vulcanization of epoxy-containing elastomer and also promoted the reactive compatibilization at the interface between PLA and elastomer. TEM observation revealed that the ‘salami’-like phase structure was formed in the ternary systems.