Cell structure and mechanical properties of microcellular PLA foams prepared via autoclave constrained foaming

Microcellular polylactic acid (PLA) foams with various cell size and cell morphologies were prepared using supercritical carbon dioxide (sc-CO2) solid-state foaming to investigate the relationship between the cell structure and mechanical properties. Constrained foaming was used and a wide range of cell structures with a constant porosity of ∼75% by tuning saturation pressure (8–24 MPa) was developed. Experiments varying the saturation pressure while holding other variables’ constant show that the mean cell size and the mean cell wall thickness decreased, while the cell density and the open porosity increased with increase of pressure. Tensile modulus of PLA foams decreased with increasing the saturation pressure, but the specific tensile modulus of PLA foams was still 15–80% higher than that of solid PLA. Tensile strength and elongation at break first increased with increasing saturation pressure up to 16 MPa and then decreased with further increasing saturation pressure (20 MPa and 24 MPa) at which opened-cell structure produced. Compressive modulus, compressive strength, and compressive yield stress also followed the same variation trend. The results indicated that not only cell size plays an important role in properties of PLA foams but also cell morphology can influence these properties significantly.

Constrained Solid-State Foaming of Microcellular Panels

This paper presents a novel constrained foaming process to produce microcellular panels. A systematic study of various process variables is conducted using a two-stage, sliding-level design of experiment approach. The resulting microcellular sheets have thicknesses in the 5 – 15 mm range and densities reductions as low as 92%. It is shown that an unfoamed integral skin of desired thickness can be produced with a microcellular core at various densities. These microcellular panels are envisioned for use in advanced panel systems and sandwich structures, such as in housing construction and other load-bearing applications.