The present invention relates to tin or bismuth catalysts useful in the cure of polyol/polyisocyanate reaction mixtures and more particularly to improving the long-term storage stability of such catalysts in the form of a complex with an excess of mercapto compound and to the use of such catalyst systems with mercapto resin/acrylic resin blends for cure with, for example, vaporous tertiary amines.
In the polyurethane coatings field, for example, catalyzed aliphatic urethane coating systems having both extended open times and fast "on demand" or "trigger" cures have been realized by the utilization of mercapto complexing agents to block the activity of a tin or bismuth catalyst. As described by Dammann, et al. in U.S. Pat. No. 4,788,083, the disclosure of which is expressly incorporated herein by reference, such complexed catalysts remain essentially inactive until unblocked in the presence of an amine catalyst or heat. Though the addition of organic solvents to disperse the catalyst complex was described by Dammann, et al., only methyl amyl ketone is reported in the examples.
In commercial practice the Dammann, et al. complex catalyst is quite stable for storage times of ranging up, say, about 8 months, depending upon formulation ingredients, storage conditions, and like factors. However, manufacture of larger batches for shipment of the catalyst complex overseas necessitated longer storage times. Catalyst complex stability emerged as a roadblock to such long term storage. Degradation of the catalyst complex was linked to the formation of hemimercaptals and mercaptals as determined by NMR analysis.
Thus, it was postulated that catalyst complex instability was due to an unintended side reaction of excess mercapto compound with the solvent, notably ketone and acetate solvents. As excess mercapto compound was consumed by the solvent over a period of time, the catalyst complex became unstable, i.e., reactive, due to the excess mercapto presence being depleted. Accordingly, it was apparent that a formulation change was in order so that long term storage of the catalyst complex could be implemented.
Aliphatic urethane coating systems curable with vaporous amine catalyst have been described in U.S. Pat. No. 4,517,222 which is an improvement on so-called chamber cure systems such as described in, for example, U.S. Pat. No. 4,753,825. Subsequently, polymercapto resins were developed for cure with such vaporous amine catalyst method, as described in U.S. Pat. No. 4,753,825. The foregoing catalyst complexes also can be used to cure the polymercapto resins of the '825 patent. Use of polymercapto resins enables the use of all-aliphatic isocyanate curing agents and results in coatings with unusually high gloss. However, such resins actually have been found to be too reactive for commercial use. Further, it now has been determined that ketonic and other carbonyl solvents in the formulation also react unintendedly with the polymercapto resins. Thus, it is apparent that there is a need to modify polymercapto resin systems to minimize unintended side reaction with solvents used and to enable their use in commercial settings. The foregoing patents are expressly incorporated herein by reference.