Water-curable compositions based on thermoplastic polymers having hydrolyzable silane moieties are becoming increasingly interesting as environmental, health and safety concerns increase for other curing technologies. Such compositions have excellent properties of weather-, chemical- and water-resistance, since the alkoxysilyl group is connected to the polymer chain by a carbon-silicon bond, rather than a labile carbon-oxygen-silicon linkage; therefore water-, alkali- and acid-resistance are remarkably improved compared to a system with silicates or silanes added by physical mixing. One disadvantage of water-curable silylated polymer compositions, however, is that they tend to crosslink, especially if dispersed in water, under normal conditions of preparation, handling and storage. As a result, the relatively poor shelf life of such compositions has tended to limit their wide commercial acceptance and has kept the use of silylated polymers to those with very low silane concentrations, typically less than 1.0 weight percent, in waterborne polymeric products.
Modification of water-curable compositions to alleviate the problem of premature crosslinking is described in U.S. Pat. No. 4,526,930 which teaches relatively water-stable, melt-processable, thermoplastic polymers with hydrolyzable silane moieties. These silylated polymers are only activated or made readily water-curable by the reaction herewith of an organotitanate having at least one readily hydrolyzable group, which ester exchanges with an ester group of the silane. Although the titanate functions as a silanol condensation catalyst, it is dispersed in the alkylene-acrylate solid matrix, not in water.
Unexamined Japanese Patent Application No. 6025502 teaches a composition comprising a polymer emulsion obtained by adding a tin catalyst (a diorganotin carboxylate) which is insoluble in water, to silylated vinyl polymers after emulsion polymerization. The addition of a water insoluble tin catalyst, however, is not suitable for such films because defects result from the heterogeneous catalysts and the emulsion polymer mixtures, such as formation of craters and granular particles on the surface and uneven crosslinkage in the film structure. Moreover, the silanes taught therein have alkoxy groups of at least eight carbons long and generally of a straight chain.
This Japanese patent application also teaches non-discriminate curing catalysts generally used for silane/ester hydrolysis and condensation reactions. Similar examples of catalysts for silane ester and silanol-containing compositions can be found in the literature, which disclose catalysts dissolved in organic solvent-based systems to ensure a proper cure.
For example, it has long been known that diorganotin dicarboxylates are catalysts for polymerization of organosilicon compounds. However, in spite of their proven utility, the diorganotin dicarboxylates suffer from several disadvantages. One is the relative instability of the compound as shown by loss of activity upon standing, particularly under moist conditions. The phenomenon is even more pronounced when the catalyst is in the form of an aqueous emulsion. Many tin compounds may also undergo hydrolysis during prolonged storage and revert to catalytically inactive forms.
Thus, it is clear that there is the need for one component, water-based dispersed silylated polymeric systems that have good stability during storage in water and which produce films of good quality upon application and drying.