“Biocide free” silicone hull paints contain tin, not so green after all
Unpublished research suggests that “Foul Release” silicone hull coatings, rather then being a biocide-free alternative to copper anti-foulings, actually rely on organotin compounds for their effect.
Tin compounds were banned as anti-fouling agents in 2008, but the ban is avoided by manufacturers’ explanation of the presence of tin as a ‘catalyst’.
If this explanation is successfully challenged, manufacturers and ship owners may be in serious breach of the 2008 ban. Part of the research is presented here.
“Antifoul paints achieved considerable notoriety from the 1970s/1980s (e.g. Blaber, 1970; Alzieu et al., 1986; Thain and Waldock, 1986) onwards. Research clearly demonstrated the impacts and effects of the biocide tributyltin (TBT) intentionally leached from antifoul which deleteriously affected non-target species from gastropods to cetaceans (Bray and Langston, 2006). Despite being introduced in the 1960s and the toxic effects being recognised by the 1980s, the compound was not banned worldwide until the inception of the AFS convention in 2008.
“In the latter years of TBT’s general use, research into alternatives began in the realisation of the compound’s toxicity and as impending legislation caused focus. Alternative compounds were largely based on zinc (Zn) and copper (Cu) often with co-biocides such as photosynthesis inhibiting algaecides (e.g. Diuron, Irgarol). These alternatives can have toxic effects on marine communities (Nehring, 2001; Chesworth et al., 2004) and although initially not thought to be as significant as that of TBT paints, the levels now encountered in marinas and harbours is high enough to adversely affect marine communities and species (e.g. see Neira et al., 2014).
“Against the background of Cu and Zn (with algaecide) antifoul options, silicone coatings were developed under the general descriptor of foul-release coatings (FRC). FRC are based on the premise that the slippery surface will facilitate the detachment of organisms as vessels move through the water. Concerns have been raised over their use as they are easily damaged and not suitable for even moderately aggressive underwater cleaning (Almeida et al., 2007), but , if not cleaned later biofouling stages can penetrate well and become “firmly established” (Carson, et al., 2009). Thus it’s been noted that silicone paints have an “extraordinarily high” possibility of non-indigenous species introduction (Nehring, 2001).
“Although described as non-toxic options for larger vessels, this aspect of silicone paints requires careful consideration. Silicone FRCs have received attention regarding their content and production techniques. Some silicone oils (e.g., polydimethylsiloxanes, (PDMS) in FRCs were identified as toxic (e.g. to barnacle cyprids) and potentially have adverse respiratory implications amongst filtering species (e.g. mussels) due to mechanical blocking (Nendza, 2007). Silicone oils may also affect benthic communities through blocking of pore water ingress; silicone oils have a long sediment residence time (see Nendza, 2007 for general discussion).
“Further to the above, concern has been raised on the use of organotins as catalysts in the production of silicone FRCs (see Nehring, 2001; Watermann et al., 2005; Chambers et al., 2006; ). Organotins as catalysts (Rittschof et al., 2011) are allowed, providing they are not having a biocidal effect; ie organotin compound should not be present in FRC paints above 2,500 mg total Sn per kg of dry paint. Against this background it has been considered that organotin use as a catalyser, (as DBT with TBT impurities, with well recognised adverse effects on marine species – see above) should cease (e.g. see Watermann et al., 2005). Watermann et al., (2005) did not find immediate acute effects from silicone FRCs on test organisms, but questioned the need for the use of organotins (see early concerns by Nehring, 2001). As late as 2012 a general comment was made regarding the chemical consistency of FR coatings in that FR “coatings are presumed environmentally friendly and are not subject to government regulations that apply to coatings containing biocides. However, their biological effects are poorly understood” (Feng et al., 2012). Watermann et al. (2005) note that “it is recommended that copper compounds be used as catalysts [in silicone FRCs] instead of organotin compounds. Even though the [organotin] catalysts are locked in the elastomer, they may be released by abrasion” such as underwater cleaning, ice abrasion or grounding (e.g. Great Barrier Reef, Polar latitudes. See (Haynes & Loong, 2002; Negri et al., 2004; Jones, 2007).
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