Continuous processing of low-density, microcellular poly(lactic acid) foams with controlled cell morphology and crystallinity

Poly(lactic acid) (PLA) represents perhaps the most viable environmentally-sustainable alternative to petrochemical-based plastics. This paper reports the continuous processing of PLA foams with a microcellular structure, a high expansion ratio, and varied microcell morphology and crystallinity. The extrusion process, which can be easily scaled-up, takes advantage of the tailored physical properties of PLA and the plasticizing effect of the supercritical blowing agent. Three grades of PLA with different molecular weight and branching topology are used. The processing parameters are optimized based on the well-characterized thermal and rheological properties of PLAs and diffusion properties of PLA/CO2 mixture. In general, melt strength governs cell morphology, with cell density, closed-cell content, and expansion ratio increasing as a function of both molecular weight and branching density. Influences of shearing and dissolved-CO2 on crystallization of PLA are characterized and they are believed to induce crystallinity in the foams. In the case of branched PLA, crystallization allows high-expansion-ratio microcellular foams to be stably produced over a wide temperature window. By controlling crystallinity, foams with similar cell morphology but varied mechanical properties and surface gloss are also produced. X-ray diffraction of the foams confirms that crystallization is governed by shearing in the die, and the crystallites are mainly of α-form.

► An extrusion process for making low-density, microcellular PLA foams was developed.
► The mechanical strength and cell morphology of the foams can be well-controlled.
► The influence of branching on PLA rheology and foaming behavior was studied.
► The influences of shearing and branching on PLA crystallization were characterized.