Investigating foamed polylactic acid films through cell structure alteration and processing conditions

Abstract

Polylactic acid films were foamed through a solid-state process using liquid carbon dioxide as a physical blowing agent. The foamed films were stretched to understand the effect of cell strut alignment on elastic modulus and yield stress normalised with density. Through stretching, cell strut alignment was achieved and verified with scanning electron microscopy. Aligning the cell struts improved the elastic modulus by a factor of 4 and the yield stress by a factor of 5 while decreasing the strain percentage at break. Aligned cell struts axially strain during tensile deformation which requires more force than unaligned cell struts which bend at initial deformations. The flexographic printability and surface topography of the foamed polylactic acid films was also analysed. The temperature used to impregnate carbon dioxide into the polylactic acid films was varied to understand its significance on printability and surface topography. Samples impregnated at ambient temperatures had smooth surface topography and a shiny lustre and good printability, whereas, samples impregnated at sub-ambient temperatures had a dull and rough texture and poor printability. Also, the carbon dioxide content had an effect on the surface topography and printability.