Effect of membrane structure of waterborne coatings on the transport process of corrosive medium

Abstract

In this study, gravimetric method and electrochemical impedance spectroscopy combined with atomic force microscopy (AFM) and contact angle measurement were used to study the transport of corrosive medium in waterborne thermosetting and thermoplastic coatings with different structures. Three typical kinds of resin membrane of waterborne coatings (waterborne acrylic, waterborne fluorocarbon, and waterborne epoxy coatings) were analyzed. Results show that the transport of corrosive medium in waterborne thermoplastic coatings and waterborne thermosetting coatings is consistent. The transport in these two kinds of coatings can be divided into three stages. In the first stage, transport is primarily affected by the water concentration in the coating and the medium content is linear with t1/2, which satisfies the Fick diffusion law. Then, medium transport deviates from Fick diffusion and the medium content reaches saturation over time. During condensation of polymer particles in thermoplastic waterborne acrylic coating and waterborne fluorocarbon coating, the intermolecular diffusion of molecular segments on the particle interface is limited. The presence of microstructural defects leads to limited densification of coating and easier formation of medium channels. The waterborne fluorocarbon coating contains symmetric fluorocarbon bonds, resulting in low surface energy, and the hydrophobic properties result in a slower medium transport rate. During coagulation of the latex particles in the waterborne epoxy coating, the crosslinking and curing reaction of the particle interface greatly reduces the microstructural defects, thereby increasing the compactness and resulting in the slowest medium transmission process.