The impact of ionic polymer cross-linking on the film formation kinetics of waterborne dispersions designed for use as acrylic pressure-sensitive adhesives (PSAs) was studied with Forster resonance energy transfer (FRET). Aluminum acetylacetonate (Al(acac) 3 ) was added as an ionic cross-linker to a latex with un-cross-linked chains. The polymer interdiffusion (studied with FRET) suggests that the degree of polymer cross-linking in the wet dispersion is small, as the interdiffusion kinetics is similar to that of a latex with un-crosslinked chains only. Cross-linking mainly takes place when the film dries, and it is slower than interdiffusion; thus, a homogeneous film can be formed. Ionic cross-linking in the final film through Al 3+ was proven by a large gel content and an increased cohesion, the latter evaluated with tensile and tack tests. Further aspects studied include the influence of serum pH on interdiffusion and cross-linking reaction and how ionic crosslinking before film formation affects interdiffusion.

Polymer interdiffusion is crucial to obtain continuous films from aqueous polymer dispersions. Here, interdiffusion in films from dispersions of acrylates which were ionically cross-linked prior to film formation was studied by Förster resonance energy transfer analysis. Dispersions of copolymers of n-butyl acrylate, methacrylic acid, and zinc dimethacrylate (ZnDMA) were investigated. ZnDMA was used as a co-monomer to ionically cross link the polymers during the synthesis. Ionic cross-linking does not prevent interdiffusion even at high gel contents. Interdiffusion data were compared with results of tensile tests on final films. Films of ionically cross-linked polymers fracture at both higher stress and strain than films of covalently cross-linked polymers. Further interdiffusion studies addressed effects of temperature and humidity. Both increased temperature and humidity accelerate interdiffusion of ionically crosslinked polymers. Thus, using ZnDMA allows for preparing dispersions which can form continuous films. Also, no volatile organic compounds are released during the film formation.