The fate of organotin biocides in marine antifouling elastomers.
The technique of tin-119m Mossbauer spectroscopy has been used to study the chemical and structural changes undergone by a series of triorganotin biocides when dispersed in neoprene-based marine antifouling elastomers. Chemical speciation using tin-119 nuclear magnetic resonance spectroscopy and gas chromatography, indicate that the majority of the tri-n-butyltin (TBT) biocides are converted to TBT-chloride, TBT-stearate and di-n-butyltin distearate during the polymer processing and curing conditions.Similar studies of triphenyltin (TPT)-containing elastomers demonstrate the extensive chemical degradation suffered by these compounds upon incorporation into the elastomer matrix.Spectroscopic and chromatographic analyses of a marine-exposed elastomer, originally containing bis(TBT)oxide (TBTO), revealed the presence of monobutyl- and dibutyltin compounds as degradation products. A "backscatter"Mossbauer technique showed that the ultimate degradation product, distributed in the near surface regions of the sample, was stannic oxide.The possible binding of organotin biocides with components of the coating formulation was investigated by variable temperature Mossbauer measurements on TBT-chloride systems. At temperatures below 60K, the organotin was shown to exist as a polymeric compound in which the tin atom is penta-coordinate. Application of the Debye model of solids yielded a value of the recoilless fraction at 80K, f(80K), of 0.30 and the vibrational freedom of the tin atom was isotropic. This associated structure was observed to break down upon dispersion into uncured neoprene. The Mossbauer parameters and f(80K) = 0.17 were consistent with the presence of discrete, teracoordinate molecules.In the authentic coating matrix, the Mossbauer parameters indicated a tetracoordinate geometry for the biocide. However, the large f(80K) = 0.34 is more typical of the pentacoordinate structure exhibited by pure TBT-chloride.The presence of TBT and TPT additives on the curing behaviour of the basic coating formulation was investigated and it was found that TBTO effectively delayed the onset of curing in this system. This effect was attributed to interfering reactions between the organotin and the accelerated zinc oxide cure system, and to the existence of a competing crosslinking reaction directly involving TBTO. The TPT biocides were seen to promote the onset of curing and this was attributed to the formation of tin-based Lewis acid catalysts during the chemical breakdown of the original biocide.
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Sheffield Hallam University (United Kingdom).