Dual effect of dynamic vulcanization of biobased unsaturated polyester: Simultaneously enhance the toughness and fire safety of Poly(lactic acid)

Abstract

To overcome the inherent flammability and brittleness of biobased and biodegradable poly(lactic acid) (PLA) so as to broaden its applications, a new strategy, introducing a dynamic vulcanization of biobased unsaturated polyester (BUP) in PLA substrates, has been employed to improve the fire safety and toughness of PLA simultaneously for the first time. For toughened PLA by BUP (TPLA), only 20 wt% ammonium polyphosphate (APP) needs to be added for achieving UL 94 V-0 rating and a limiting oxygen index (LOI) value of 27%. In contrast, for PLA without BUP, 20 wt% of APP (PLA/APP20) cannot endow it with a fire safety, and only UL 94 V-2 rating and a LOI value of 24% are obtained. Interestingly, the elongation at break and the notched Izod impact strength of TPLA/APP20 are 107.1% and 15.9 kJ/m2, respectively, being 13.7 and 8.8 times than that of neat PLA; the complex viscosities of TPLA/APP are much higher than that of PLA/APP even near the burning temperature, which can reduce the fluidity and avoid further diffusion of flame with the flow of melt. Further, an open cone calorimeter test, XPS, FT-IR and SEM have been used to find out how the toughening treatment affect the fire safety of PLA/APP. All the results demonstrate that for PLA with low melt viscosity and heavy dripping during burning, the toughening treatment via dynamic vulcanization can quickly promote continuous dense char formation benefiting improving the flame retardant efficiency of APP.