Investigation of Factors Affecting Stratification Phenomenon in Epoxy-acrylic Coatings

The stratification behaviour of different thermoplastic acrylic copolymers with one epoxy resin and its different curing agents were studied. Xylene and methyl isobutyl ketone were used as solvents of the system. Films with different thicknesses (200, 400, and 800 micron) were applied with a film applicator. Also the effect of epoxy gel time with different curing agents was tested. The results showed that coating stratification was thickness dependent, so that in higher thicknesses and the same compositions more stratified films was obtained. In addition, reduction of gel time of epoxy composition showed untraceable effect on stratification, although long duration of curing has important effect on stratification. The reaction rate of curing agent regardless of gel time is also important. According to the obtained data, it can be concluded that stratification mechanism is not diffusion dominated and it maybe convection dominated. The effect of substrates (aluminium and glass) was investigated for the coatings, and it is found that normally glass substrate yield more stratified coatings. The IR-ATR spectra showed that low surface free energy component could not be found in the bottom layer of stratified films and most of it migrated to the top surface. Our findings to some extent support the proposed theory of minimum surface free energy of the system, but the thickness dependency, reported in our paper cannot be interpreted by current theories.