Control of adhesive properties through structured particle design of water-borne pressure-sensitive adhesives

Abstract

The concept of using structured particles and the mechanism of film formation from latexes to produce pressure-sensitive adhesive (PSA) films with controlled sub-micron and nanometre scale morphology has been investigated with the objective of enhancing adhesive performance. Structured particle acrylate-based latexes were synthesised by seeded semi-continuous emulsion polymerisation procedures. Aliquots removed at intervals during the polymerisations were analysed for conversion and z-average particle diameter; the results confirm that the particles grew without secondary nucleation or coagulation. Three-layer particles were studied first and comprised a core whose composition was the same for all particles, an interlayer between core and shell which was crosslinked during synthesis (using 1,6-hexanediol diacrylate, HDDA) and a shell which contained diacetone acrylamide (DAAM) repeat units that provided for interfacial crosslinking between particles during film formation by reaction with post-added adipic acid dihydrazide (ADH). The three-layer latexes produced adhesive films with high shear resistance when ADH was added, but peel adhesion was generally low and further reduced by increasing either the thickness or HDDA concentration of the interlayer. Thus two-layer latexes with shell layers containing DAAM but no pre-crosslinked interlayer were the main focus of the study. The effects of core:shell ratio and amount of DAAM on adhesive performance were optimum with a core:shell ratio of 80:20 and 2wt% DAAM in the shell copolymer, for which interfacial crosslinking using the stoichiometric amount of ADH led to a marked increase in shear resistance with only a slight reduction in peel adhesion. Increasing the level of n-dodecylmercaptan (DDM) chain transfer agent used in synthesis of the core increases peel adhesion and reduces shear resistance in the absence of ADH, but high shear resistance can be recovered through interfacial crosslinking with ADH. Adhesive performance can be optimised by using a high DDM level in the core and a low DDM level in the shell, a combination that enables both high peel adhesion and shear resistance to be achieved with the addition of ADH. The results from this study define guiding principles for development of commercial water-borne PSAs that can compete with solvent-borne PSAs in high-performance applications.