Exploiting dynamic thiourethane covalent bonds for enhanced adhesion of UV-curable metal coatings

Abstract

UV-cured coatings with environmental-friendly trait are widely used in various industries. However, high internal stress during photopolymerization leads to low adhesion of coating, which limits its application on metal surface. In this work, polythiourethane-urethane acrylates (PTUA) with dynamic thiourethane bonds were designed and prepared via click reaction of 3,6-dioxa-1,8-octanedithiol (EDDT) and isophorone diisocyanate (IPDI), following by the reaction between 2-hydroxyethyl acrylate (HEA) and IPDI. By adjusting the IPDI/EDDT/HEA ratios, a series of PTUA with different molecular weights were synthesized. The structure of PTUA was characterized by Fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance spectroscopy (1H NMR) and gel permeation chromatography (GPC). The incorporation of dynamic thiourethane bonds in UV-cured PTUA coating could release shrinkage stress via dissociative exchange of thiourethane bonds upon heating. After heating, the adhesion strength of the resulting coatings could be significantly improved. The maximum adhesion strength of the PTUA coating could reach 4.90 MPa, which was 2.5-fold increase compared with the coating without heating. This study demonstrated that adaptable thiourethane network was a promising candidate for UV-cured metal coating with enhanced adhesive performance.