10. Shelf Life Assessment of Poly(ethylene-co-vinyl acetate) and Polyester Polyol Resins Used as Adhesives

The thermal stability and ageing properties of Vinamul 3161 poly(ethylene-co-vinyl acetate) and AS1160 polyester polyol resins have been investigated in support of shelf life assessment and also to identify storage conditions that may extend product life. These resins are typically used in the production of adhesives for specialised applications either as binders for filler particles or to minimise the relative movement of materials in multi-material assemblies. Our studies confirm that both these resins are susceptible to moisture and hydrolysis chemistry which potentially limits shelf life. The EVA resin readily accumulates acetic acid through hydrolysis of the pendent acetate groups which increases both the acidity (pH) and volatile outgassing characteristics of the material. The temperature sensitivity of pH combined with Arrhenius kinetics was used to identify a useful shelf life for EVA in conditions representative of normal storage conditions. In a separate set of experiments, relatively short-term thermally accelerated ageing studies have been carried out on AS1160 polyester polyol to investigate sensitivity to humidity, temperature and open/close ageing conditions. This material is hygroscopic, readily accumulates moisture and is susceptible to chain scission with molecular weight changes linked to the hydrolysis-esterification equilibrium. These changes do not however adversely impact adhesive bond strength allowing the resin to be potentially used significantly beyond the manufacturer recommended shelf life limit.