High-Performance EVA Composite Foams Prepared by Supercritical In-Mold Injection Vulcanization

An ethylene-vinyl acetate copolymer (EVA) has been widely applied in various fields due to its excellent foaming properties and modification compatibility. To address the potential health risks posed by chemical foaming agents, supercritical fluid foaming has been applied through autoclave batch processes. However, due to the required vulcanization process, applying supercritical injection foaming to EVA remains challenging. Considering the advantages of a shorter process and high automation of injection molding, we successfully prepared high-performance EVA composite foams using a newly developed supercritical in-mold vulcanization injection molding (SIVIM) process in this work. The evolution during the in-mold vulcanization process was first analyzed, and the optimal vulcanization temperature and time were found to be 180 °C and 300 s, respectively. Subsequently, the effects of gas concentration and gas counterpressure on product performance were investigated. The results showed that the gas concentration is the key factor influencing foam density. When the gas concentration reached 1.8 wt %, the initial product density decreased to 0.195 g/cm3. Additionally, an increase in gas counterpressure improved nucleation efficiency, resulting in a foam structure with a higher cell density and smaller cell size. Compared to the batch foaming process, the SIVIM process reduces the molding cycle time by over 90%, while producing foams with comparable density and superior overall mechanical properties. This work provides an innovative approach, enabling the production of EVA composite foams through supercritical injection molding, which will significantly promote the sustainable development and modernization of the EVA products.