A study of triorganotin biocides in antifouling coatings.

The technique of tin-119m Mossbauer and NMR spectroscopy has been used to study the chemical and structural changes undergone by a series of triorganotin biocides when dispersed in an antifouling paint and a polyurethane system. In the antifouling paint, based on the Hypalon polymer, tri-n-butyltin chloride, tri-n-butyltin acetate and to-(triphenyltin) oxide are essentially unmodified. Bis-(tri-n-butyltin) oxide, in contrast, was entirely converted into two new species, one of which was identified as tri-n-butyltin chloride. The second component was thought to be a sulphonate ester. However, synthesis of such compounds and subsequent spectroscopic analysis did not support this theory. Triphenyltin acetate experienced some dephenylation when incorporated into the paint, with a diphenyltin species detected by Mossbauer spectroscopy and GC analysis. Triphenyltin chloride suffered more extensive dephenylation, the dried paint containing the mono- and diphenyltin species. The organotin release-rate of eight Hypalon paint samples has been determined using a flow system and analysis of the effluent by graphite furnace atomic absorption spectrometry. All eight samples were shown to have release-rates significantly less than that required for an effective antifouling coating.Incorporation of triorganotin biocides into the polyurethane resulted in physical changes to the polymer and in certain cases, chemical transformations in the organotin. to-(tri-n-butyltin) oxide is entirely converted into new species in the polyurethane. From work carried out on model systems, the likely products are N-stannylcarbamate derivatives, which are thought to be catalysts in polyurethane formation. It is also suggested that the extensive and undesirable foaming observed in such polyurethanes, is due to the formation of isocyanate oligomers which act as branch points for polymer cross-linking.Tri-n-butyltin chloride, to-(triphenyltin) oxide and triphenyltin chloride all catalyze the polyurethane reaction, the latter increasing the setting time rather than reducing it. Mossbauer and infrared evidence from model studies suggests that dative bonding between triorganotins and isocyanates is occuring, via the oxygen of the isocyanate group. It is these complexes which are proposed to be the catalytic species. The relative merits of the triorganotins are discussed with respect to their potential as effective antifouling agents in the polyurethane system.