Thermal stability of poly(ethylene-co-vinyl acetate) based materials

The thermal stability of poly(ethylene-co-vinyl acetate) based materials has been investigated in support of materials qualification and service life prediction programmes. Poly(ethylene-co-vinyl acetate) is used as a binder phase for boron particles in highly filled (greater than 70 wt%) composites. Studies have been carried out to generate improved understanding of the stability of the binder material and the mechanical response of the highly filled composite. Our studies show that the uncured resin (emulsion) readily accumulates acetic acid through hydrolysis of the pendent acetate groups, which alters the acidity (pH) of the material. Thermal desorption studies in combination with gas-chromatography-mass spectrometry show that the resin readily evolves acetic acid when thermally aged to temperatures up to 75 °C. Gel Permeation Chromatography (GPC) suggests that thermal ageing induces a gradual reduction in the molecular weight of the resin and is linked to the susceptibility of the material to chain scission. Heating at elevated temperatures in excess of 300 °C is shown to induce significant changes in the carbon skeleton through deacetylation and dehydration processes, with the production of unsaturated main chain double bonds. The key insight or improved understanding offered from these studies is the complex mechanical responses of these highly filled materials.