Foaming of poly(ethylene‐co‐vinyl acetate) and poly(ethylene‐co‐vinyl acetate‐co‐carbon monoxide) and their blends with carbon dioxide

Poly(ethylene-co-vinyl acetate) (EVA-25) and poly(ethylene-co-vinyl acetate-co-carbon monoxide) (EVACO-2410) and their blends with EVACO:EVA ratios of 80:20, 60:40, 40:60, and 20:80 were foamed using CO2. These foams are of interest for applications ranging from footwear to medical devices. Foaming experiments were carried out using 1 mm thick melt-extruded films in CO2 at a range of pressures (100, 200, and 300 bar) and temperatures (30, 40, 50, and 60 °C). Foamability of the polymers was explored both under isothermal and gradient temperature conditions. Foams of EVACO-2410 displayed high initial expansions followed by postfoaming relaxation and shrinkage while foams generated from EVA-25 showed more dimensional stability. Blending EVACO-2410 with EVA-25 was explored as an approach to reduce postfoaming relaxation and shrinkage. The surfaces of the foamed samples displayed blistering that was linked to CO2 bubble entrapment and coalescence at the surface. Scanning electron micrographs of the foams generated from blends displayed distinct morphologies reflecting whether the sections were representing the machine- or cross-machine direction of extruded films. In going from EVACO-2410 to EVA-25, the cell densities ranged from about 106 to 1010 cells/cm3. Foams with low bulk densities of about 0.11 g/cm3 could be generated. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45841.