Diffusion and Existing Form of Water Vapour in Organic Coatings

Abstract

Absorption and desorption of water vapour in alkyd and polyurethane varnish were studied by quartz crystal microbalance(QCM). The absorption and desorption kinetics curves were Fickian in nature when the relative humidity was higher than 30%, while the solubility followed Henry's law. The average diffusion coefficient D increases linearly with relative humidity. A residual amount of water(about 10% of the equilibrium absorption amount under the condition of RH 100%) was found to remain in the coatings after extended exposure to dry air. FTIR analysis showed that the molecular structure of the coatings changed before absorption and after desorption of water vapour. The results showed that the residual water reacted with the coating molecule and H-bond was formed between water molecule and the C = O group. A considerable part of water in the coatings may transport in a chain mode by replacement. The rate of forming H-bond is much faster than that of diffusion. When the relative humidity is higher than 30%, the concentration of the water in the coatings is sufficient to support the reaction; diffusion is the controlling step of the absorption process, so the kinetics is Fickian. When the RH is lower than 30%, the concentration of the water in the coatings is not sufficient to support the reaction; the reaction rate is lowered to the degree that it is comparable to the rate of diffusion, then the kinetics of the whole process begin to deviate from Fick's law.