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Partition coefficient

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Description of key information

The log Pow of bis(3,5,5-trimethylhexanoyl) peroxide was estimated at 7.0288. However, in view of the rapid hydrolysis the key value for the CSA is the log Pow of the breakdown product 3,5,5-trimethylhexanoic acid, estimated by QSAR at 3.34.

Key value for chemical safety assessment

Log Kow (Log Pow):

Additional information

As the test item bis(3,5,5-trimethylhexanoyl) peroxide is a substance difficult to test (instantaneous hydrolytical degradation) the log Kow was estimated using EPISuite (KOWWIN), and corresponding QMRF and QPRF documents were prepared. The predicted value is log Pow = 7.0288. The test item is considered to be in the applicability domain of the model.

However, the parent substance bis(3,5,5-trimethylhexanoyl) peroxide itself is not relevant since it will not be present in the environment.

The rapid hydrolytical breakdown of the peroxide requires consideration of the degradation products – as far as known – in the environmental fate assessment. In the context of the hydrolysis study (see IUCLID section 5.1.2) two breakdown products could be identified:

·        3,5,5-trimethylhexanoic acid (CAS no. 3302-10-1), and

·        2,4,4-trimethylpentan-1-ol (CAS no. 16325-63-6)

The partitioning coefficients of these two degradation products were also estimated using the QSAR model KOWWIN, with the following results:

·        3,5,5-trimethylhexanoic acid: log Pow = 3.34

·        2,4,4-trimethylpentan-1-ol: log Pow = 2.62

An additional five degradation products were detected in the hydrolysis study, but proved to be non-identifiable. Nevertheless, considering the molecular structure of the parent substance it may be assumed that also the unidentified breakdown products are smaller molecules than the parent, and more hydrophilic. Therefore, the log Pow of 3,5,5-trimethylhexanoic acid of 3.34 may be taken as a representative worst case figure for the entire spectrum of degradation products.

In conclusion, although the log Pow of bis(3,5,5-trimethylhexanoyl) peroxide can be estimated by QSAR at 7.03, the rapid hydrolysis of the peroxide requires using data on the hydrolysis products for the chemical safety assessment. The representative worst case figure is the log Pow for the breakdown product 3,5,5-trimethylhexanoic acid, which has been estimated by QSAR at 3.34. This figure is therefore adopted as the key value for the CSA.

Approach for regulatory interpretation of the model result: The regulatory cut-off value for the log Pow ranges between 3 (potential for bioaccumulation and adsorption) and 4 (classification and labelling/PBT assessment).

Outcome and conclusion: The estimated log Pow for the environmentally relevant hydrolysis product is moderate (3.34). According to the guidance on data requirements and chemical safety assessment, adsorption and bioaccumulation should therefore be addressed in more detail. With respect to the PBT assessment, however, it can be assumed that the affinity of bis(3,5,5-trimethylhexanoyl) peroxide and its hydrolysis products to the lipids of an organism is insufficient to exceed the B criterion.