Design of different self-stratifying patterns in a VOC-free light-curable coating containing bio-renewable materials: Study on formulation and processing conditions

Abstract

The self-stratification is an ecologic approach providing the solvent emission reduction and formation of a laminated multi-functional structure by one-coat application. Although light-curing systems as a point of interest have been developed extensively, the self-stratifying phenomena and its governing parameters have not been investigated in such a fast-curable coating. This study aimed to design solvent-free and light-curable coatings with various self-stratifying behavior using methacrylated gelatin (GelMA) and acrylated epoxidized soybean oil (AESO). The influences of several essential factors including flash-off time, irradiation time, light intensity, photoinitiator concentration (PI%), coating thickness, and raw materials weight ratio on the stratification level were evaluated by SEM-EDX, AFM, ATR-FTIR spectroscopy, water contact angle (WCA), and conventional coating analyses. It was learned that about 10 min of flash-off time was required to start the phase separation and create gradient integrated layered structures. Also, the results demonstrated that the self-stratification was not impressed by prolonging the irradiation time after the formation of the lattice structure. Furthermore, increasing the coating thickness to 300 μm or decreasing it to 30 μm led to uncured or slightly gradient stratified coatings, respectively. Besides, the lower GelMA content resulted in the lower mixture viscosity and sharper gradient self-stratified structures. Moreover, the increasing of crosslinking rate by PI% or light intensity increment changed the bilayer stratified structure into the gradient ones; therefore, self-stratifying in this fast-curing system was both thermodynamically and kinetically controlled. Subsequently, the know-how for preparing different self-stratified coatings with various features, namely swelling degree (11–29%), gel fraction (74–92%), glass transition temperature (43–72 °C), WCA (67.1–81.6°), MEK double rub resistance (25–55), pendulum hardness (45–83), cross-cut adhesion (0B4B), gloss (53–87 at 60°), and surface roughness (72.8–235.7 nm) was achieved. Additionally, the relation of the surface GelMA content and the surface roughness, gloss, hardness, and WCA were derived. Accordingly, this attempt gave a helpful insight into the self-stratification phenomena to establish the optimum formulation and processing conditions as a promising prospect to prepare green coatings for advanced bio and food applications.