The application of blocked polyfunctional isocyanate as a cross-linking agent in biodegradable extruded poly(lactic acid) foam

Abstract

A polyfunctional isocyanate was prepared and was blocked by methanol to limit its premature reactivity with water or other nucleophiles. The methanol-blocked polyfunctional isocyanate was used as a cross-linking agent to improve the melt strength and foamability of poly(lactic acid) (PLA). The effect of the blocked polyfunctional isocyanate (BPI) content on the melting behavior, crystallization, degree of cross-linking, and melt strength of PLA was investigated, and the cellular morphologies of the PLA foams obtained by chemical foaming extrusion were studied, as well. The cold crystallization temperature increased with increasing BPI proportion and the melting peak changed from a single to multiple peaks upon the addition of BPI to PLA. The ∆Hc, ∆Hm, and Xc values initially increased and then decreased with increasing BPI content. It can be attributed to the effect of cross-linking on crystallization behavior of PLA. The degree of cross-linking increased with the BPI content of the PLA mixtures. The melt strength of the PLA mixture increased with increasing proportions of BPI, whose incorporation led to a decrease in the void fraction, cell size, and open cell content of the PLA foams but an increase in the cell density. When BPI was added to the PLA, the cell morphologies of the PLA mixtures were obviously enhanced.