Self-stratified bio-based coatings: Formulation and elucidation of critical parameters governing stratification

Self-stratification is an innovative one-step process used to design multi-functional coatings gathering simultaneously in a one-pot formulation the primer, the intermediate layer and the top coat properties. Many selfstratifying coatings contain oil-based epoxy resins but the literature is scarce in the development of greener solutions. In this work, silicone resins and bio-based epoxy resins were dissolved in various solvent blends, applied on a composite substrate and cured under different conditions to obtain stratified coatings. To reach a perfect stratification, the influence of various parameters including (i) the surface tension and the polarity of the resins, (ii) the solvents volatility, (iii) the curing temperature and (iv) the reactivity of the epoxy/amine reaction was studied by a systematic approach. In accordance with the literature, it was demonstrated that a large difference in surface energy and polarity favors resins separation. The volatility of the solvent blend was also shown to be a key factor in the stratification process. However, the predominant parameter, rarely taken into account, is the curing temperature, which impacts the cross-linking reaction of the epoxy resin. The increase in molecular weight (MW) of epoxy resins due to the cross-linking reaction favors the incompatibility between resins by increasing the difference in MW between epoxy and silicone resins. Thus, optimization of process conditions allowed the design of perfectly stratified bio-based epoxy/silicone coatings. The mechanism of film stratification was also elucidated thanks to in-situ analyses.

Automated summary

Self-stratification is employed in designing bio-based epoxy/silicone coatings, achieving perfect stratification through optimization of process conditions and studying the influence of various parameters. Curing temperature plays a critical role in the cross-linking reaction of epoxy resins, impacting the incompatibility between epoxy and silicone resins. The volatility of the solvent blend is also a key factor in determining the stratification process.