The effect of chain transfer agent level on adhesive performance and peel master-curves for acrylic pressure sensitive adhesives

Three series of pressure sensitive adhesives were prepared with fixed glass transition temperature, using emulsion polymerisation. Monomers chosen were butyl acrylate, 2-ethylhexyl acrylate, and methyl methacrylate, along with a small amount of acrylic acid. The proportion of acrylic acid monomer was held constant for each polymer preparation while acrylate ester monomer levels were varied. The chain transfer agent level used was varied for each series of polymerisations. Adhesion performance was assessed by measurement of loop tack, static shear resistance, and through construction of peel master-curves. Peel master-curves were generated through peel tests conducted over a range of temperatures and peel rates and through application of the Boltzmann superposition principle. The presence of high levels of polymer gel was found to obscure the influence of monomer composition on adhesion performance. For polymers with low levels of gel, adhesion performance was correlated with changes in viscosity as comonomer composition was varied. In cohesive failure regions of peel master-curves for polymers with high levels of chain transfer agent, the shift factors aT were found to closely fit the Williams-Landel-Ferry (WLF) equation and allowed the calculation of the WLF parameters, C1 and C2. The cohesive failure regions were successfully shifted to provide a super master-curve and the values of aC used to generate the super master-curve were applied to accurately predict relative static shear resistance levels. For low gel content polymers, loop tack was correlated with log(aC) as EHA levels were increased.