Graftable antioxidant for peroxide crosslinked polyethylene:stabilisation and performance

Abstract

The overall aim of this work was to investigate antioxidant systems based on three synthesized reactive (graftable) hindered amine stabilisers (g-HAS) used in combination with either synthesized reactive (g-Ph), or conventional, hindered phenols to prevent antioxidant migration and offer effective long term stabilisation under aggressive solvent and water extractive environments, in peroxide crosslinked high density polyethylene (HDPE) targeted for use in water pipe applications (both potable and hot water). This study also addressed the question of interference of the peroxide initiated crosslinking process with grafted and conventional (non-grafted) hindered phenol antioxidants. Pipes and laboratory thin film samples highly crosslinked by peroxides were prepared using commercial and laboratory production methods. The melt grafting of reactive HAS stabilisers on HDPE was optimized along with the polymer crosslinking using two different laboratory developed methods; a two-step process, where the HAS-grafting was achieved in a first step followed by polymer crosslinking, and a one-step method where both grafting and crosslinking took place in one step. The effect of the chemical composition and processing conditions of the reaction system in the two-step method were investigated using an internal batch mixer in order to optimize the extent of grating of the stabilizers. It was found that lower peroxide concentration and a higher processing temperature gave rise to an increase in the level of HAS-grafting with lower extent of HAS-homopolymer formation. In the case of the pipes which were produced using one of two commercial continuous processes, the Engel process (PEXEng) and a High Speed Extrusion-IR Process (PEXHS), the formulations were not optimised due to lack of time but their choice was based on both the experience (by the sponsor company) with commercial pipe production using conventional (non-graftable) antioxidants (AO), and the laboratory-optimised grafting-crosslinking methods developed in this work. PEXHS pipes showed more homogenous AO distribution compared to the PEXEng pipes and this is almost certainly due to the lack of sheer in the Engel process. PEX pipes (e.g. PEXEng) containing the g-HAS (used with a g-Ph or a conventional/non-graftable hindered phenol, (Irganox 1076) were found to have both high AO-retention and high long term polymer thermal stability especially under exhaustive solvent extraction environment, in contrast, similarly prepared pipes but containing conventional AOs (with similar AO functions), were shown to suffer from high AO-losses, thus, resulting in a much lower long term thermal stability, LTTS. Furthermore, the amount of AOs retained in the polymer after the commercial Pipe production processes (e.g. in PEXEng) revealed that the grafted antioxidants, e.g. the g-Ph, (DBPA) was retained to a much higher extent than the commercial hindered phenol Irganox 1076 (retention of 75% vs 50%, respectively). This suggests that the peroxide crosslinking process does not interfere (or interferes much less) with the g-AOs compared to non-graftable conventional AOs. Similarly, a very high retention of over 90% of the g-Ph was found in the PEXHS pipes (e.g. Pipe X6) compared to similar pipes containing Irganox 1076 (PEXHS pipe X1) with retention of only 46% after sequential solvent extraction using DCM/xylene. However, extraction with boiling water has resulted in hydrolysis of the ester groups of the grafted AOs (the g-Ph) resulting in their partial loss in the water extracts. Qualitative analysis of transformation products of g-Ph and of Irganox 1076 (and Irg 1010) obtained from PEXHS pipes extracts in DCM and in boiling water and their identity were determined using HPLC-MS analysis.