Reactive antioxidants for peroxide crosslinked polyethylene

Three synthesised reactive (graftable) antioxidants, r-AO, with hindered phenol and hindered amine antioxidant functions, were examined for their grafting efficiency in polyethylene and their retention and stabilising performance in peroxide-crosslinked polyethylene pipe material. Their level of grafting in un-crosslinked high density polyethylene, HDPE, and the extent of their retention in highly peroxide-crosslinked HDPE (PEX_a) material (both laboratory prepared samples and PEX_a pipes produced by commercial processes) were determined after stringent solvent extraction methodologies. The uniformity of distribution of the r-AOs in the PEX_a pipes (across the length and thickness of the pipes) were examined using infrared-microscopy and compared with similarly produced PEX_a pipes containing commercial antioxidants having the same antioxidant functions. The extent of interference of the graftable hindered phenol and hindered amine antioxidants with the peroxide crosslinking process in the PEX_a materials was assessed by comparing the level of crosslinking achieved against analogously prepared samples containing a conventional hindered phenol antioxidant (Irganox 1076).
The results obtained showed that the presence of the graftable (r-AOs) antioxidants did not affect the level of crosslinking of the PEX_a pipes which remained high (>85%) and that they were retained to a very high extent in the PEX_a material after solvent extraction, and are very uniformly distributed across the length and thickness of PEX_a pipes. The long-term stabilising performance of the graftable r-AOs in the PEX_a material (in both laboratory prepared samples and in peroxide crosslinked pipes produced by commercial processes) was also examined using complimentary methods: oxidative induction time (OIT) before and after solvent extraction, physical embrittlement following oven ageing at 125 °C, as well as by a hydrostatic pressure tests conducted (with water inside and air outside) at 115 °C and 2.5 MPa pressure. The stabilising efficacy of the r-AOs was compared with that of conventionally stabilised PEX_a material containing analogous antioxidant functions produced in the same way. The level of retention (in the pipes) of the graftable antioxidants and their long-term stabilising performance was shown to be significantly higher than that of the conventional AOs under all the test conditions used here.