Follow-up to ecological risk assessment of organotin substances on domestic substances list: chapter 2
Approximately 70% of the total annual world production of non-pesticidal organotin compounds is used as PVC stabilizers (Lytle et al., 2003). Mono- and dialkyltins (i.e., methyl-, butyl- and octyltins) are used mainly as PVC stabilizers. Sulphur-containing organotins impart heat stability to PVC, whereas non-sulphur-containing organotins (e.g., carboxylates) provide resistance to light and weathering. Some mono- and dialkyltins (e.g., butyltin trichloride and dimethyltin dichloride) are also used in depositing clear, durable tin oxide coatings on reusable glass bottles. Certain dialkyltin compounds are used as catalysts in producing various polymers and esters. Internationally, dialkyltins are also used as stabilizers for lubricating oils, hydrogen peroxide and polyolefins. Some dialkyltins are used as anthelmintics for controlling parasitic worms in poultry (Gitlitz and Moran, 1983). Dioctyltin compounds are generally used as additives for PVC food packaging products.
Tetraalkyltin compounds are used primarily as intermediates in the synthesis of other organotin substances (Gitlitz and Moran, 1983).
It is believed that almost all of the intentional use of tributyltins in Canada is related to its pesticidal properties, which are regulated under the Pest Control Products Act (PCPA). Action has already been taken on tributyltin pesticides with the greatest exposure to the environment. The use of tributyltins in antifouling paint for use on ship hulls has been prohibited in Canada since January 1, 2003 under the PCPA. As of March 1, 2009, two tributyltin active ingredients were registered under the PCPA and are found in six end-use products. The Pest Management Regulatory Agency of Health Canada intends to re-evaluate the remaining uses of tributyltin by 2009-2010, to determine if their use continues to be acceptable under today’s standards for health and environmental protection.
It is recognized that trialkyltins occur as contaminants in other organotin commercial products. For example, Environment Canada (2006) reported that tributyltin can be an impurity at concentrations up to about 20% in tetrabutyltins imported for use in the synthesis of organotin stabilizers. Tributyltin is also present at lower concentrations (up to about 0.5%) in commercial dibutyltin products.
As discussed in section 3.1.2, methyltin compounds may be present in the environment both as a result of natural methylation of inorganic tin (Chau et al., 1980, 1997; Weber and Alberts, 1990) and from industrial uses such as PVC stabilization (Chau et al., 1997).
Butyltin species are not produced biologically from inorganic tin (Maguire, 1992). Mono- and dibutyltin compounds are usually present in the environment as a result of the degradation of tributyltin, as well as from non-pesticidal industrial uses such as PVC stabilization (Chau et al., 1997). In the past, tributyltin compounds have entered the environment mostly from their pesticidal uses. However, tributyltin compounds may also enter the environment because of their presence in other butyltin products and from the environmental breakdown of tetrabutyltins.
Phenyltin compounds are probably present in harbours as a result of antifouling uses (Chau et al., 1997).
In Canada, the largest releases of organotins to the environment from non-pesticidal uses probably occur as a result of manufacturing, formulation and blending processes, principally resulting from release during equipment cleaning operations and during cleaning of liquid residues remaining in shipping containers. These point source releases could result in significant concentrations of organotins in local receiving waters and sediments. A survey of the handling practices of users of organotin stabilizers, conducted by the Vinyl Council of Canada and the Tin Stabilizers Association indicates that mainly totes (semi-bulk), tanker cars and drums are used to ship the substances, although the stabilizers can also be shipped in pails and kegs. Users either manually or automatically transfer the organotin stabilizers from shipping and storage containers to compound mixers. Transfer lines are cleaned on a monthly to infrequent basis, with rinsate being either recycled or directed to wastewater treatment facilities prior to being discharged to receiving waters.
Environment Canada (2006) estimates that, in the absence of stewardship practices, up to 0.4 kg of organotin stabilizers per day could be released into the environment from a facility receiving stabilizers in drums, whereas up to 0.13 kg/day could be released from a facility receiving stabilizers by bulk or tote shipments. It was assumed that there would be no environmental releases from dry blend manufacture of powder coatings.
As noted by Environment Canada (2006), facilities using organotin stabilizers have adopted product stewardship practices that have led to a decrease in the quantity of organotins that could potentially be released to the environment. The Vinyl Council of Canada and the Tin Stabilizers Association developed a guideline for the environmental management of stabilizers in Canada (Vinyl Council of Canada and the Tin Stabilizers Association, 2004). The guideline is applicable to companies that process PVC with a tin-based stabilizer. It is estimated that implementation of the practices in the guideline substantially has substantially decreased the potential release of organotins to the environment. Environment Canada (2006) estimated that with stewardship practices in place, up to 0.0016 kg of organotin stabilizers per day could be released into the environment from a facility. An environmental performance agreement was signed by representatives from Environment Canada, the Vinyl Council of Canada and the Tin Stabilizers Association on March 10, 2008, to ensure industry wide compliance with the guideline.
As stated above, certain alkyltins are used as catalysts and in glass coatings. It is believed that the quantities used for these applications are much less than the quantity used as PVC stabilizers, so potential environmental releases would also be much lower.
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