Tailoring a variety of self-stratifying patterns in a light-curable coating on the substrates with different surface free energies

Abstract

Self-stratification is an ecological method that leads to the stratified structures by one-layer application to reduce solvent emissions and energy consumption. Despite the great importance of the effect of surface properties on the self-stratification phenomena, the influence of the substrate type has not been examined in self-stratifying light-curing systems. This study aimed to design dissimilar self-stratifying patterns by using several substrates namely glass, steel, Al, polyethylene, polyethylene terephthalate, polyvinyl chloride, polypropylene, polystyrene, with different surface free energies (γ) (high γ: 59–63, medium: 41–43 and low: 31–34 mN.m⁻¹) based on incompatible bio-renewable light-curable materials including methacrylated gelatin (GelMA) and acrylated epoxidized soybean oil diluted by the acrylated glycidyl neodecanoate (AESO-GN). To this, the impact of the essential substrate characteristics including γ and roughness on the self-stratifying behavior and other properties of the achieved coatings were studied by using SEM-EDX, contact angle (CA), FTIR-ATR, and the other conventional coating analyses. The results disclosed that the Funke theory had the potential to predict the self-stratifying behavior of the abovementioned light-curing coatings on a variety of substrates. Moreover, it was understood that self-stratification dealt with the γ of the substrate, so that the application and light-curing of coatings on the substrates with high and medium γ resulted in a bi-layered and a gradient concentration self-stratified structures, respectively. Furthermore, it was revealed that applying this coating on substrates with low γ led to an incoherent structure. Consequently, the know-how for tailoring several self-stratified coatings with different gloss values (54–88 at 60°), pendulum hardness (43–76), MEK double rub resistance (25–45)), glass transition temperature (44–72 °C), swelling degree (20–29 %), gel fraction (74–89 %), water CA (61–86°), and cross-cut adhesion (0B4B) was realized. Besides, the findings demonstrated that increasing the surface roughness of steel substrate intensified the self-stratification. Accordingly, the coating-substrate interface is of great importance for self-stratification as its characteristics will identify the stratification pattern.

In addition, it was observed that appropriate bi-layered coatings could not be attained by the consecutive application and curing of GelMA and AESO-GN layers. Consequently, this attempt provided beneficial insight into the design of green coatings via an ecological fast-curing methodology.