Unique thermal behavior of acrylic PSAs bearing long alkyl side groups and crosslinked by aluminum chelate

An acrylic pressure-sensitive adhesive (PSA), bearing octadecyl acrylate, methyl acrylate and acrylic acid groups, and crosslinked by aluminum acetylacetonate (AlACA), displayed behavior unique among acrylic PSAs in that its adhesion, which decreases with an increase in temperature, began to increase again from around 150 °C. In order to understand this behavior, the structure and thermal properties of the PSA were investigated in detail, along with another PSA crosslinked by a covalent crosslinking agent (Az). From thermal mechanical analysis, the PSA with ionic crosslinks (AlACA) showed three softening points at 20, 60, and 160 °C. In comparison, the PSA covalently crosslinked by Az only exhibited two softening points (at 20 and 60 °C). The softening point at 160 °C is clearly related to ionic chelate crosslinking. DSC measurements indicated that the softening point at 20 °C resulted from melting of the ordered octadecyl group, and the softening point at 60 °C was due to an increase in the mobility of the main chain. The temperature dependence of viscoelastic measurements revealed that the viscosity of the PSA crosslinked by AlACA increased at around 160 °C. From these results, we considered that the distinctive adhesion of the PSA crosslinked by AlACA could be due to ligand exchange at the aluminum crosslinking points, which are chelated by carboxy groups built in the main chain.