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Physical & Chemical properties

Water solubility

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The European REACH Grease Thickeners Consortium (ERGTC) is registering a number of grease thickeners under the EU REACH regulation. Most grease thickener substances only exist within a grease base as the substances are manufactured in situ in base oil. Unique interactions occur between the thickener and the base oil in formulated greases. It is proposed that this physical interaction, or matrix effect, has a significant impact on the bioaccessibility of thickeners in situ in base oil. As such, the ERGTC have undertaken testing on base grease containing polyurea substances in base oil in order to assess the bioaccessibility of the substances and to evaluate the implications for the matrix effects on the potential for exposure to these substances.

Bioaccessibility (leaching studies) were conducted in a step-wise approach, starting with the substance predicted by QSAR to have the highest solubility, reaction mass of aniline and m-tolylidene diisocyanate (A054), followed by the substance predicted to be next most soluble, reaction product of 1,5-naphthylene diisocyanate and cyclohexylamine (R59) (Shell 2018). Leaching studies were also undertaken previously on polyurea, produced by reacting toluene diisocyanate with octyl amine and tallow alkyl amines (R37) at 17.8% in medicinal white oil and 17.8% in another base oil (identity not provided) (polyurea thickener 1) and polyurea, produced by reacting toluene diisocyanate with ethylenediamine, tallow alkyl amines and stearyl alcohol (R36) at 15.7% in base oil (identity not provided) (polyurea thickener 2) (Shell 2013). For further details, see additional information on environmental fate and behaviour.

Before conducting the bioaccessibility study to determine the leaching of the thickener from base grease to water, water solubility studies on the isolated substances (100%) were conducted following the same general approach. The studies are not definitive OECD 105 water solubility studies and were not conducted in accordance with Good Laboratory Practice (GLP) but provide a realistic estimate of the solubility value which can be compared against the QSAR predicted water solubility.

On the basis of the water solubility studies on the isolated substances, it was evaluated whether leaching studies on the substances in base oil were relevant. If the solubility of the isolated substance indicated that the thickener would not be present in a water accommodated fraction (WAF) at significant concentrations (i.e. above the analytical limit), the substance was considered not bioaccessible. Leaching studies on substances in base oil are not considered to be relevant or feasible where the isolated substance has a solubility below the limit of detection.

The water solubility study on reaction mass of aniline and m-tolylidene diisocyanate was conducted with isolated substance at a loading rate of 5 mg/L (pre-determined from a visual observation study). Based on WAF concentrations, the water solubility of the test item was determined to be 0.0017, 0.0062, 0.0065 and 0.0114 mg/L after 24, 48, 72 and 144 hours. As such, a leaching study was conducted on the substance at 20% in base oil.

The water solubility study with reaction product of 1,5-naphthylene diisocyanate and cyclohexylamine, was conducted with isolated substance at loading rates of 5 and 10 mg/L (pre-determined from a visual observation study). Analysis of the 24, 48, 72 and 96 hour WAF samples (centrifuged and non-centrifuged) did not detect the substance in any samples at concentrations above the limit of quantification (<0.00064 mg/L). The substance was concluded to be not bioaccessible and a leaching study was not conducted as the isolated substance was not soluble at concentrations above the analytical limit. The conclusion for this substance was applied to other polyurea substances with lower predicted water solubilities.