The diffusion of water and aviation fuel in polysulphide sealants

The diffusion and permeation of aviation fuel (Avtur) and water in commercial and 'model' polysulphide sealants have been studied using simple gravimetric techniques. For Avtur, the mass uptake behaviour is explicable in terms of normal, Fickian diffusion, and the diffusion coefficient determined has been found to be independent of the experimental method and concentration of the liquid. Water, however, exhibits anomalous diffusion behaviour. Mass uptake by the polysulphides is high and equilibrium is not reached after several months. There is no correlation between the results of permeation, absorption and desorption experiments except at low water concentration. These anomalies have been explained in terms of the formation of water droplets within the rubber. This study shows that the amount of water absorbed is dependent upon the amount and nature of the curing agent residues. The results have been analysed in terms of the osmotic and water vapour pressure and two equations have been derived which satisfactorily predict the amount of water absorbed at equilibrium. It has been shown that the effect of the water droplets on permeation is negligible, and hence the diffusion coefficient found is a measure of the true rate of diffusion. In contrast, the effect of the droplets on mass uptake experiments is large and the apparent overall diffusion rate is greatly reduced. The apparent diffusion coefficient found is dependent upon the water concentration and the concentration of the water-soluble impurities. An equation has been derived showing the relationship between these variables. The results of this work show that the elastic strength of the rubber has negligible effect in determining the water uptake at equilibrium and the apparent diffusion coefficient. However, deterioration in adhesive and cohesive strength occurs with water uptake. At high uptake some of this loss is irreversible. The deterioration in properties is mainly due to breakage of weak physical bonds and plasticisation effects