Studies of Dimethyl Meta‐Isopropenylbenzyl Isocyanate (TMI) for Preparation of Ambient Cross‐linkable Latexes—Film Formation and Cross‐Linking

Single‐pack, ambient cross‐linkable acrylic latexes containing dimethyl meta‐isopropenylbenzyl isocyanate (TMI) repeat units were prepared by batch and semicontinuous emulsion polymerization processes at 40°C using methyl methacrylate (MMA) and n‐butyl acrylate (BA) as the principal comonomers and methacrylic acid (MAA) as a functional comonomer. Degree of cross‐linking and mechanical properties of films from batch latexes increased as the level of TMI increased from 0–5 wt%. Increasing latex pH (from 1–4) to 9 by adding dilute aqueous NaOH brings about an increase in degree of cross‐linking and mechanical properties of films. This effect is further enhanced by incorporation of MAA repeat units, which also give rise to physical cross‐linking through associations between MAA repeat units. Thus, by use of both TMI and MAA, a synergy is achieved in which the TMI repeat units provide for chemical cross‐linking that is accelerated by the effect of MAA repeat units, which also provide for physical cross‐linking, leading to greater improvements in film mechanical properties than when only TMI or MAA is incorporated. Core‐shell particles with an outer TMI/MAA‐containing layer are more susceptible to aliphatic isocyanate (NCO) hydrolysis, which can be detrimental if the rate of hydrolysis is too high. Although the rate of NCO hydrolysis in the TMI/MAA‐containing phase can be reduced by having an outer phase of unreactive poly(MMA/BA), this also prevents development of strong interfaces between particles during film formation. The TMI latexes are colloidally stable during storage for one year, but suffer from extensive NCO hydrolysis and substantial intraparticle cross‐linking, which converts the particles into microgels. The mechanical properties of films from the stored latexes are far inferior to films cast just after latex preparation due to the poor film integrity resulting from the low level of interparticle chemical cross‐linking. Thus, in order to obtain optimum properties in films, TMI latexes should be used soon after preparation and NCO hydrolysis must proceed at a rate that is synchronized with the particle integration stage of film formation.