Influence of drier combination on through-drying in waterborne alkyd emulsion coatings observed with magnetic resonance profiling

Achieving fast and uniform crosslinking in alkyd coatings poses a challenge to formulators that demands a fundamental understanding of drier efficiency. In recognition of this, we have examined the physical changes that accompany autooxidative crosslinking in alkyd films (cast from waterborne emulsions) in the presence of various combinations of metal carboxylate driers. A newly developed type of magnetic resonance (MR) profiling was used in conjunction with conventional techniques: Beck-Koller drying tests, pendulum hardness, and mass uptake. MR profiling noninvasively probes the molecular mobility of the alkyd as a function of depth (with a pixel resolution of about 9 mum), over drying times ranging from minutes to weeks. It thereby indicates drier efficiency via its sensitivity to viscosity build-up during drying and to subsequent film hardening. We show unequivocally that more uniform crosslinking is achieved using a combination of a primary (Co) and a secondary (Ca) drier, in support of conventional belief. Furthermore, these results yield new insight into the chemical mechanisms induced by the driers and are thus of clear benefit to coatings researchers and formulators. Notably, the secondary driers improve the efficiency of the hydroperoxide decomposition reactions, but they are only active during an initial period, after which crosslinking nonuniformity develops.