Manipulation of chain transfer agent and cross-linker concentration to modify latex micro-structure for pressure-sensitive adhesives

N-Dodecyl mercaptan (NDM) chain transfer agent and allyl methacrylate (AMA) cross-linker were used to manipulate latex properties in a starved seeded semi-batch emulsion polymerization of butyl acrylate (BA) and methyl methacrylate (MMA) or with a third monomer, acrylic acid (AA). Latexes with higher gel content and lower sol polymer molecular weight (Mw) were produced by adding only AMA. On the other hand, latexes with lower gel content and Mw were produced by adding only NDM. In addition, at a constant AMA concentration (0.2 phm), the addition of NDM (0.2 phm) decreased gel content, increased molecular weight between cross-linking points (Mc), and decreased Mw. Adding more NDM (to a total of 0.4 phm) further decreased the gel content, while decreasing the tested Mc and increasing Mw. It was also found that using higher concentrations of both AMA and NDM could produce latex with similar gel content, but smaller Mc and Mw, compared to the latex produced at lower concentrations of both NDM and AMA. Regarding the influence of AA, gel content was increased and Mw was significantly decreased with an increase in AA concentration and a decrease in MMA concentration. The performance of the latexes was evaluated for application as a pressure-sensitive adhesive (PSA).

At a constant cross-linker concentration, one can manipulate the polymer micro-structure by adding varying amounts of chain transfer agent. Three examples of these micro-structures are depicted below which show (l. to r.) a tight gel network with long-chain sol polymers, a loose gel network with shorter sol polymers, and an imperfect gel structure with highly branched sol polymers. By manipulating the micro-structure, pressure-sensitive adhesive performance can be affected.Figure optionsDownload as PowerPoint slide