Melt versus solvent coating: Structure and properties of block-copolymer-based pressure-sensitive adhesives

Pressure-sensitive adhesives (PSAs) composed of a styrene-isoprene-styrene triblock copolymer and a midblock-associating resin were prepared via solvent and hot-melt coating. The formulations and thermal histories up to the point of coating were identical, yet significant differences in the properties were observed as a function of the coating method. The solvent-coated PSA showed superior shear holding power, and the hot-melt-coated PSA performed better in tack and peel tests. Two factors resulting from the processing conditions were responsible for these property differences. The quick cooling process occurring after hot-melt coating led to a poorly defined microstructure and, therefore, less physical crosslinking. Rheological data for melt-pressed and solvent-cast PSA films confirmed these microstructural differences. The increased solubility of the tackifier in the solvent additionally created a composition gradient in the solvent coating. Annealing improved the long-range order of both hot-melt and solvent coatings, producing a body-centered cubic microstructure identified by small-angle X-ray scattering. This microstructure improved the shear strength of both types of adhesive coatings, whereas the peel and tack properties of the solvent coatings remained inferior to those of the hot-melt coatings because of differences in the surface compositions. (C) 2002 Wiley Periodicals, Inc.