A tracer study of organotin leaching in antifouling paints

Abstract

A radiotracer method of measuring the loss of organotin antifouling agents from marine paints has been developed using tin-113. The suitability of the isotope has been investigated and shown to be acceptable over the normal sea water pH range. Radiotin labelled TBT-chloride, -fluoride, and TBTO have been prepared and their leaching rates determined from a marine paint which comprised a vinyl copolymeric binder, Rosin and zinc oxide, using small panels exposed to sea water, and on a rotor apparatus to give accelerated results. The order of leaching rate from this paint was shown to be TBTO > TBTCl > TBTF. The change in leaching rate with time for TBTF was found to fit the expression log (LRt) = log (LRo,) - kT. The rotor apparatus allowed a determination of the effective life of a paint in half to one third the time taken in sea water exposure tests. Examination of paint film cross-sections by SEM showed that as leaching occurred, the paint film became cavitated. An electron microprobe study showed that the loss of zinc oxide occurred at a faster rate than the loss of organotin. A study of the loss of organotin from deeper within the paint film showed that this occurred from relatively early stages of exposure. These studies indicated that this paint type allows leaching of organotin by first losing the slightly soluble components and building up a series of cavities which allow in ingress of sea water to the whole depth of the paint film. The nature of the leaching species has been investigated, and it was shown that the tri-n-butyl tin group remained intact. The reaction between TBTO and Rosin was studied by IR spectrophotometry, and showed a cleavage of the Sn-0-Sn bridge.