Miljøevaluering af organotin i plastprodukter

English Summary

Organotins are compounds with a strong bond between a tin atom and up to four carbon atoms. Di- and monoorganotins are used in the polymer industry primarily as stabilisers and catalysts. Here the risk for the environment is assessed for six parent organotins representing 10 specific organotin compounds and the typical organotins used in the polymer industri.

The environmental evaluation of organotins have been based on practices described by the OECD and the European Commission for deriving PECs and PNECs. A full risk assessment of the ten organotin compounds according to the "Technical Guidance Documents in support of Commission Directive 93/67/EEC on Risk Assessment for New Notified Substances and Commission Regulation (EC) No. 1488/94 on Risk Assessment for Existing Substances" (TGD 1996) is not within the scope of this report. The documentation used varies from original studies according to OECD and other test guidelines with GLP, to published and unpublished references of varying quality.

Mono- and diorganotins are used in the industry as stabilisers and catalysts in the production of various polymers, notably PVC, silicone and polyurethane. A minor amount is also used in paint, lacquer and varnish industry. From the industrial production, processing and formulation of organotins, they may be emitted to air and waste water, but no production of organotins takes place in Denmark, and no production of virgin polymers takes place in Denmark. The release of the compounds to the environment is therefore estimated based on the use pattern and life cycle of the products and the compiled information about possible environmental releases during product life.

In Denmark, organotins have been found in water and sludge. In water, the primary compounds have been mono- and dibutyltin observed mainly near or in harbours where they may occur as degradation products of the antifouling agent, tributyltin, rather than emissions from the use of polymer products. No data from Denmark are available regarding their occurrence in potable water, rain water, ground water or other fresh waters. Organotins have been detected in sediment, but mainly the possible antifouling degradation products mono- and dibutyltin. In sewage sludge butyl-, phenyl- and octyltins have been measured. Phenyltins and octyltin are not used in antifouling products, but octyltins are the main stabilisers in Danish PVC products. Phenyltin is allowed for use in wood preservation.

Organotins often have high molecular weights and occurs as the hydrolysed ionic compounds in water. They have low vapour pressures/Henrys Law constant and do not occur in the atmosphere, with the exception of metyl- and dimetyltin. These compounds are used very little in Denmark compared to other organotins. Organotins are slowly dealkylated in water (half life T_½= 2-17 days) and under aerobic conditions in sediment (T_½= 1-2 years). Only a few data are available for anaerobic conditions, but tributyltins have very little degradation under anaerobic conditions (T_½> 2.5 years).

Organotins are also degraded (dealkylated) relatively slowly in degradation tests. Typically, 20-28% is degraded during 28 days which corresponds to an approximate half-life of 43-63 days.

Organotin compounds have a range of soil-water and sediment-water partition coefficients. Due to the ionic character of organotins in the environment, it is not possible to predict the soil-water partition coefficients from the octanol-water partition coefficients. It is estimated that the adsorption to sewage sludge in sewage treatment plants will be the major route into the environment for organotins emitted from PVC and polymers. The major part of organotins are contained in the products for the product life time, and are ultimately incinerated or disposed of into land fills. Organotins have a low to medium adsorption to soil and organic carbon and may be considered mobile in some soils.

Table 3.1
PEC/PNEC ratios (risk quotient) calculated according to OECD and EU guidelines.

Several organotins can be bioaccumulated (BCF > 100), but BCF is generally not high (40-400). For some invertebrates lacking metabolising enzyme systems BCFs may be up to 1000 times higher (the data are for tributyltin).

Based on the collected data a risk for the aquatic environment cannot be excluded with the present use and emission of dibutyltin and dioctyltin. Dibutyltin is used in polymers, but is estimated to occur in the aquatic environment mainly as a result of degradation of tributyltin used in antifouling paints. The compounds methyltin, butyltin and dimethyltin are classified as "not hazardous for the aquatic environment".

No risk for the terrestrial environment is anticipated for butyltin, dibutyl- or dioctyltin. The assessments are based on average partition coefficients for sediment/water which vary considerably. For dioctyltin the PEC/PNEC ratio in the terrestrial environment is relatively close to 1 (one). Dioctyltin is probably used only in plast and polymer materials, and an increase in the consumption may lead to increased emissions (and a transgression of the PEC/PNEC ratio of 1) as a result of substitution of lead stabilisers. The data set for dioctyltin is, however, limited with respect to partition coefficients and acute toxicity data for algae.

Due to lack of data the environmental risk of octyltin cannot be evaluated. For mono- and dimetyltin the risks to the terrestrial environment cannot be assessed, due to lack of partition coefficients and data on occurrence in the environment.

Organotin compounds may in many products/processes replace lead stabilisers used in PVC today. According to the polymer industry the alternative Ca/Zn-compounds will be used in most instances, and in this project these compounds are assumed not to be of environmental concern. The toxicity of lead in the environment has therefore been compared with organotin in two scenarios, where the PVC stabilisers or their degradation products may occur in the environment:

1. Occurring as inorganic lead or as organotin:
   a. emitted by overflow of sewage to aquatic environment
   b. distributed when amending soil with sewage sludge
2. Occurring as inorganic lead or as inorganic tin:
   a. incinerated with solid waste
   b. deposited in landfills with waste

The compounds may occur in the form they are released from PVC to water (as lead or organotin) and be carried with the sewage flow via storm water overflow to the aquatic environment (1a) or with the sewage sludge to the terrestrial environment (1b).

Organotin PVC stabilisers may also occur as the inorganic degradation product (tin), if the PVC matrix are carried with the waste stream to incineration where the organotin compounds will be oxidised to inorganic tin oxide. In the incineration process airborne tin compounds can be produced, but especially fly ash and filtered particulates are collected. These fractions, and the corresponding lead fraction, are deposited in landfills (2a). From PVC waste deposited without incineration the stabilisers may also eventually be released in their inorganic form (2b).

Based on the available information on ecotoxicological effects concentrations for lead and organotin stabilisers it is assessed that there is an environmental advantage for both the terrestrial and the aquatic environment by using the organotin stabilisers in PVC, when the environment is affected from deposited or incinerated PVC material.

The ecotoxicological data suggests that in is not an environmental advantage to use organotin stabilisers instead of lead when the environment is affected directly from organotins, as is the case with storm water overflow of sewage and amending of soil with sewage sludge.