Surface enrichment by conventional and polymerizable sulfated nonylphenol ethoxylate emulsifiers in water-based pressure-sensitive adhesive

Comparisons of properties are made for pressure-sensitive adhesives (PSAs) generated via emulsion polymerization using both conventional and reactive emulsifiers. The emulsifiers are ammonium salts of sulfated nonylphenol ethoxylates with similar chemical structures and hydrophilic-lipophilic balances. The polymerizable surfactant possesses a reactive double bond at its phenyl ring allowing for participation in free-radical polymerization reactions. Incorporation of the polymerizable surfactant into the adhesive polymer was confirmed via reverse-phase liquid chromatography. Surfactant distributions in PSA films were characterized using confocal Raman microscopy and contact angle measurements. Increasing conventional surfactant concentrations in emulsion polymerization reactions resulted in proportional increases in concentrations at the surfaces of latex-cast films. No surface enrichment of adhesive films was observed when reactive emulsifiers were employed. Variations in measured depth profiles and contact angles are consistent with observed performance properties, for example, tack, peel strength, and shear times. Increasing concentrations of conventional surfactants decreased adhesive performance, while adhesives generated with polymerizable surfactants showed little or no such decreases in film performance properties. It is concluded that the polymerizable surfactant is bound in the adhesive in such a way that its migration to film surfaces during drying and formation is inhibited, preventing degradation of adhesive performance.