Calls to investigate tin use in silicone hull coatings

Apr 21 2017

The re-emergence of organotin in marine hull coatings is of increasing concern in some quarters.

Academics and environmentalists have called on the IMO to investigate the use of tin in silicone-based foul release systems and other ships hull coatings.

While use of the organotin tributyltin (TBT) was banned as an active biocide nearly 10 years ago, the IMO is claimed to ‘have left the door open’ for tin as a catalyst, but according to some academics the amount of organotin used suggested it could be acting as the active agent.

Dr Rik Bruer, a former researcher at     Netherlands research institute, TNO and now managing director of Finsulate, a manufacturer of a non-toxic antifouling wrap, said: “I have seen for myself that something strange happened with the formulation of these foul release coatings. Until 2002, I worked at TNO and at that time the chemistry of the silicone foul release coatings seemed to be in order. There was some organotin in there, but this is known to be a catalyst for curing these coatings.

“About a year ago, I studied the materials safety data sheets of recent versions of these foul release coatings and it turns out that the amount of ‘catalyst’ added is more than 10 times higher compared to 2005. For me there is no debate that there is a purpose beside the catalyst activity and that the risk of spreading tin compounds again to kill marine life is eminent,” he stressed.

Shipowners may have believed that the use of organotins in marine hull coatings was completely outlawed in 2008 with the ban on tributyltin (TBT), but they can still be used as a catalyst if organotin content does not exceed the allowable limit of 250 mg per kg of paint. Dibutyltin and dioctyltin are the organotins under most scrutiny.

Prof Daniel Rittschof, a specialist in barnacles and other arthropods at the Duke University in North Carolina, said: “Part of the problem is that at very low concentrations, less than 1/1,000th of the amount in coatings, organotins cause molluscs to change sex and/or become behaviourally castrated, with male following male pheromones and female following female pheromones.”
While some manufacturers’ safety data sheets have detailed the use of organotins in their paint systems, Rittschof said: “The effects of dibutyltin and probably dioctyltin are similar to TBT. Organotins at very low levels alter enzymes that process steroids, which is why molluscs change sex.”

Boud Van Rompay, chairman of Subsea Industries, the manufacturer of the Ecospeed hull coating system, said: “Only a proper and independent investigation will determine whether some paints exceed the permitted levels. But like most people in the industry, we had thought the days of toxic tin in hull coatings was long gone. It is very worrying to hear this may not be so.”
Van Rompay questioned whether silicone-based hull coatings are relying on the presence of organotins as an active ingredient to prevent fouling, rather than a catalyst to deliver ‘non-stick’ properties to the coating.

Referring to a recent report, he said: “Independent research on the tin content of some foul release coatings appears to have identified levels allegedly exceeding the limits set by the IMO’s MEPC. This tends to support earlier research indicating that this type of paint appeared to be having a toxic effect on aquatic fauna in a way that led to the ban on tributyltin (TBT) in 2008.
“However, it is important that independent laboratories assess the biofouling on these hulls, underwater or in drydock, to measure the levels of toxicity. We need to have clear evidence on how these silicone-based coatings are affecting marine organisms, which are vital to the marine eco system,” he said.

Citing a 2013 study that noted dibutyltin showed toxicity toward fish and shrimps even when used as a catalyst, Van Rompay stressed: “There are studies available in the public domain that should have already raised alarm bells.”


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