Studies of Dimethyl Meta‐Isopropenylbenzyl Isocyanate (TMI) for Preparation of Ambient Cross‐linkable Latexes—Stability of NCO Groups in Latex Form

Abstract

Single-pack, ambient cross-linkable acrylic latexes containing dimethyl metaisopropenylbenzyl isocyanate (TMI) repeat units were prepared by batch and semicontinuous emulsion polymerization processes at 408C using methyl methacrylate (MMA) and n-butyl acrylate (BA) as the principal comonomers. The semicontinuous processes were used to prepare structured latexes in which the locus of functional groups within the particles was controlled. In some polymerizations, methacrylic acid (MAA) was incorporated at similar levels to TMI (2 and 5 wt%). An FT-IR method was established for monitoring loss of isocyanate (NCO) groups during latex storage and was shown to give reliable values of rate coefficients. The results showed that NCO functionality was lost gradually over a period of 1-2 weeks depending on latex parameters. Inclusion of MAA repeat units increased the rate of loss of NCO groups by an order of magnitude when the TMI and MAA were uniformly incorporated. The rate also increased significantly on changing latex pH from acidic (pH 1 -4) to basic (pH 9) using NaOH. Particle morphology and functional group location also were important. Loss of NCO groups was fast for core-shell latexes in which the NCO groups were incorporated only in the shell (with more direct access to water from the continuous phase), but much slower when a core containing the TMI and MAA repeat units was protected from the continuous phase by an unreactive poly(MMA/BA) shell layer. The observations show that TMI-containing latexes have a relatively short shelf-life and emphasize the importance of casting films soon after latex preparation for optimum cross-linking during film formation.