Crystallinity effects in polylactic acid-based foams

The use of carbon dioxide for the production of polylactic acid foams has been increasingly studied recently. Much of the reported work has used supercritical carbon dioxide as a processing or foaming medium. However, a new foaming process which uses sub-critical liquid carbon dioxide conditions has been developed by the Biopolymer Network Limited. It is generally recognised that processing crystalline thermoplastics, including crystalline polylactic acid polymers, into foam products using carbon dioxide is difficult due to lower solubility of liquid carbon dioxide in polymers and the increased polymer rigidity hindering the expansion of the polymer matrix. However, fundamental studies, and the associated knowledge of the foaming mechanisms at play, have allowed some new approaches to be developed within sub-critical carbon dioxide conditions which overcome many of these difficulties in polylactic acid foams. In this study, several commercially available grades of polylactic acid, ranging from crystalline to amorphous, were processed using carbon dioxide via the Biopolymer Network Limited process (sub-critical carbon dioxide). By adequately adjusting the process parameters, different crystalline grades were successfully foamed to low density. The resulting foams exhibited low to high levels of crystallinity, and consequently displayed varying thermal or physical properties. Cellular structures were observed by scanning electron microscopy. Crystallinity and thermal behaviour of the foamed samples were characterised using differential scanning calorimetry. Mechanical properties and dimensional stability were also investigated. It was shown that the selection of polylactic acid feedstock, and its associated processing conditions, had a significant impact on the final quality and properties of the foams.