Modelling migration of substances from polymers into drinking water. Part 2 – Partition coefficient estimations

Abstract

The permeation through and diffusion/migration of a series of additives, their degradation products and other organic substances from high density polyethylene, polybutylene, polypropylene and cross linked polyethylene films into water was investigated experimentally with two different methods in the 20–60 °C temperature range. Vapour absorption experiments were also conducted in the same temperature range to determine the equilibrium concentration of some organic substances in the same polymers. The goal was to determine partition coefficients, KPW's, of the investigated substances at a polyolefine-water boundary. It was found that in general the temperature dependence of the obtained KPW's was well represented by the van't Hoff law. Some of these results also suggested that the magnitude of the KPW's and their apparent activation enthalpies is influenced by the presence of other substances solved in water. The vapour absorption experiments also provided polyolefin-air partition coefficients, KPA's.In the second part of this work the obtained pools of KPW's, for each of the investigated polymers were used to develop an approach to estimate theoretically and without any further experimentation “conservative” partition coefficients, K+PW's, for any organic substance, with a solubility in water at 25 °C, SW(25), ranging from 0.1 mg/L to 1000 g/L and diffusing/migrating from these polymers into water. For this a linear correlation between the KPW's, and SW(25) of these organic substances is used as starting point. A similar concept was applied to develop a method to estimate “conservative” polyolefin-air partition coefficients K+PA's for any organic substance, with a saturation vapour pressure, PV, ranging from about 1 Pa to 105 Pa.The possibility to estimate such K+PW's is very important when it comes to use efficiently migration modelling as an alternative method to test the compliance of polymeric articles with the existing national and/or European standards for drinking water. The use of polymer specific partition coefficients in migration modelling is required in the framework of the “Migration modelling guideline” recommendation of the German Environment Agency.