Mechanical changes of electron beam irradiated ethylene–vinyl acetate copolymer (EVA) film (I)

Electron beam irradiation of ethylene‐vinyl acetate copolymer (EVA) film results in a marked depression of sorption of flavour compounds. This effect is presumed to be brought about by radical reactions, following changes in the structure. The crosslinking density and gel fraction increased with irradiation dose, and the maximum percent gel reached 70% for 20 Mrad‐50g kg⁻¹ EVA film, 35% for 20 Mrad‐95g kg⁻¹ EVA film and 50% for 10 Mrad‐150g kg⁻¹ EVA film. In calculating the ratio of scission to crosslinking events, it was found that EVA film is a polymer of the scission type. The degree of crystallinity, X_c decreased significantly with increasing irradiation dose in all irradiated films. For unirradiated films, X_c decreased with increase in vinyl acetate content, which suggests that crystalline regions of EVA film are primarily composed of ethylene units. Therefore, the scission reaction may predominantly take place in the crystalline region, resulting in the formation of methyl or end vinyl groups. This was supported by measuring the film density, peak melting temperature and stress at yield.