Bio-based self-stratifying waterborne coatings via silicone-induced phase separation for enhanced hardness and anti-graffiti performance

This study developed a green and bio-based waterborne coating system that self-stratifies via silicone-induced phase separation. The system is built on a rosin acid-modified epoxy ester/acrylate hybrid resin and includes a small amount of vinyl-terminated silicone oil to promote phase separation. Two resin formulations were prepared, namely a silicone oil-free epoxy ester/acrylate hybrid resin (designated as EAR20, rosin acid constitutes 20 % of the total carboxylic acid content of epoxy ester) and a counterpart containing 2.5 wt% vinyl-terminated silicone oil (designated as EAR80–2.5, rosin acid constitutes 80 % of the total carboxylic acid content of epoxy ester). Self-stratifying coatings were fabricated by blending these two resins, with optimal performance observed at a 2:1 (EAR20:EAR80–2.5) mass ratio. During curing, the silicone-containing resin phase preferentially migrated to the surface, forming a distinct silicone-rich upper layer in a one-step process. This stratified architecture yields a hard, hydrophobic surface while preserving robust adhesion and mechanical integrity in the bulk. Compared to the silicone-free coating (with pure EAR20), the self-stratifying coating exhibited a markedly higher surface hardness (pencil hardness increased from B to H) and significantly improved durability. Water resistance in an immersion test extended from 7 days to 12 days without failure, and abrasion resistance increased from 5 ± 1 to 41 ± 2 cycles. The static water contact angle also rose from 75 ± 2° to 89 ± 2°, indicating a more hydrophobic surface. Furthermore, the silicone-rich top layer imparted anti-graffiti functionality, enabling sprayed ink to be easily wiped off without residue. Overall, this silicone oil-induced stratification strategy synergistically enhances both mechanical robustness and surface properties, offering a promising and sustainable route for developing high-performance waterborne coatings.