Effects of sulphonic and phosphonic acrylic monomers on the crosslinking of acrylic latexes with cycloaliphatic epoxide

Abstract

Two model acrylic latexes were synthesized using methyl methacrylate (MMA) and butyl acrylate (BA) with methacrylic acid (MAA) or 2-hydroxyethyl methacrylate (HEMA). The MAA or HEMA were incorporated to provide the latexes with carboxyl and hydroxyl functionality, respectively. A cycloaliphatic diepoxide (3, 4-epoxycyclohexyl methyl-3′, 4′-epoxycyclo-hexane carboxylate) was used as a crosslinker for both the latexes. The crosslinking of the latexes with the diepoxide was catalyzed using a copolymerizable or free sulphonic or phosphonic acids. The copolymerizable acids (2-sulfoethyl methacrylate (SEM) and acid phosphoxyethyl methacrylate (PEM)) were added during the latex synthesis. The free acids ( p-toluene sulphonic acid (TsOH) and phenylphosphonic acid (PPA)) were added into the latex emulsion shortly before crosslinking. The crosslinked coatings were evaluated in terms of water absorption, gel content, and pull-off adhesion. The crosslinking of the hydroxyl functional latex coatings with the cycloaliphatic epoxide required an acid catalyst, whereas the crosslinking of the carboxyl latex coatings did not need an acid catalyst. Sulphonic and phosphonic acrylic acid monomers not only functioned as catalysts for the crosslinking reactions, but also improved the adhesion and freeze–thaw stability of the latex coatings. In addition, neutralization of the acid catalysts led to reduction of the crosslinker hydrolysis, and consequently enhanced the overall coating properties.