Temperature-triggered release of a liquid cross-linker micro-encapsulated in a glassy polymer for low temperature curing

In order to prevent a liquid epoxy cross-linker from premature, Arrhenius-law predicted, reaction with an acid-functional polyester resin, the liquid cross-linker has been physically separated from the resin by encapsulation while release is only possible by a temperature-controlled trigger. The glass transition temperature Tg of the polymeric encapsulant was used to trigger the softening of the polymeric capsule.Epoxidized linseed oil (ELO) cross-linker was encapsulated as 0.1 μm diameter droplets in 16 μm diameter microparticles (poly(N-vinyl)pyrrolidone) (PVP) by means of spray-drying. The resulting nano-composite microparticles were used to cross-link the acid functional polyester in a powder coating.It was found that below Tg the encapsulation considerably slows down the (premature) cross-link reaction. Beyond Tg also a slowdown was found but in this case it was caused by the mere presence of PVP rather than by the state of encapsulation of the cross-linker: PVP as a strong hydron bond acceptor slows down the reaction of epoxy with the acid groups of the polyester.In conclusion, the Tg of the encapsulant was successfully used as a temperature trigger for realizing the controlled release of a liquid reactant in a reactive polymer matrix.