The extractability of phenolic antioxidants into water and organic solvents from polyethylene pipe materials - Part I

Three commercial anti-oxidant systems have been studied regarding migration and chemical reaction in different polyethylene pipe materials during extraction in organic solvents and in hot water, 95-100 °C, under anaerobic and aerobic conditions. Materials made of crosslinked and non-crosslinked high density polyethylene, low density polyethylene and blends thereof were stabilized with different phenolic antioxidants: Irganox 1330, Irganox 1010, and Irganox 1076. The loss in stability with increasing extraction times was followed by oxidation induction time (OIT) measurements and by measuring the residual amounts of anti-oxidant by extraction. A gradual decrease in OIT and amount of anti-oxidant with increasing treatment time in water was observed for all samples. However, the behaviour of the three antioxidants was dissimilar. For materials containing Irganox 1330 three new species, possibly transformation products of the anti-oxidant, were found within the material. Larger decrease in anti-oxidant content in the polymer compared to corresponding loss in OIT indicates that the new species still have anti-oxidant capacity. On the other hand, corresponding transformation species could not be found within materials stabilized with Irganox 1010 or 1076. It is suggested that these antioxidants, which both contain ester groups, are hydrolyzed and that the new species migrate out into the surrounding water. Solvent extractions showed that crosslinking did not affect the extractability of the antioxidants. Actually, even with water as the extracting media, the network did not show any obstructive effect of the migration of the anti-oxidant. For peroxide crosslinked samples part of the anti-oxidant could not be extracted with solvent but we believe that grafting of antioxidants onto the polymer backbone is the most plausible explanation in this case. Additionally, we see a clear decrease in anti-oxidant loss with increasing crystallinity.