Comparing the Effectiveness of Polylactic Acid Foam Formation through Cross-Linking Versus Enhanced Crystallization Enabled By Solid-State Shear Pulverization

Polylactic acid (PLA) is a biodegradable polymer that has recently become more prevalent in commercial materials industries through its common use in biomedical, packaging, and additive manufacturing applications. As an effort to make PLA materials more environmentally friendly and expand upon possible PLA applications, PLA has also been subject to foam production for added benefits like density reduction, impact and vibration resistance, and thermal insulation. PLA historically has difficulty producing foams, as it has a low crystallization tendency that weakens the structural integrity of the cells formed, causing them to rupture easily. Various methods have been attempted to increase crystallinity and thus enable better foaming, and these have had mixed levels of success. A more commonly used manner of increasing polymer foamability has been through the addition of cross-linking agents that enable the polymer chains to better contain the foam cells. However, there have been limited PLA cross-linkers successfully used in literature to for effective PLA foaming. This paper focuses on the effectiveness of crystallization and cross-linking in PLA foams through solid state sheer pulverization (SSSP) processing. SSSP processing is a form of chilled extrusion that imparts homogenous mixing in solid form. SSSP is advantageous in this study as its low temperature nature prevents the cross-linking agents and chemical foaming agents (CFAs) from prematurely activating due to increased temperatures. The SSSP machine will compound PLA foam formulations together in a single step. The SSSP output will be further processed into foam products through either compression molding or extrusion. The final foam products will be subject to mechanical testing, thermal analysis, and density reduction measurements to generate findings on the advantages and disadvantages of PLA foaming with cross-linking versus enhanced crystallization. The ultimate goal is to generate PLA foams that rely on enhanced crystallization via SSSP only, meaning they are free of any chemical additives.