Influence of structure and chemical composition on oxygen permeability of crosslinked epoxy–amine coatings

The oxygen barrier properties of a series of coatings based on diglycidyl ethers of bisphenol and butanediol reacted with a wide range of amines in different stoichiometric ratios have been investigated. The oxygen permeability was analysed with respect to the molecular structure in general and the concentration of polar functional groups in particular. The results showed that formulations based on aliphatic amines gave good barrier performance, that coatings prepared from aromatic and cyclo-aliphatic amines yielded intermediate barrier properties, and that polyether amine formulations produced very poor barriers. It was also observed that pendant methyl groups had a strong detrimental effect on barrier properties and that using an excess of amine monomer significantly improved the barrier performance. Attempts at predicting the oxygen permeability from physical and chemical characteristics of the coatings showed that the glass transition temperature could not be correlated with the permeability. However, it was found that a polarity index, calculated as the sum of the concentrations of hydroxyl and amine moieties weighted by their individual cohesive energy density, was a good predictor of oxygen permeability for all types of formulations.