Impact of toluene sorption on oligomers and antioxydants migration from HDPE pipes

Abstract

The objective of this work was to investigate the ageing behaviour of filled HDPE samples, immersed into toluene at 60 °C, in terms of migration and extraction of stabilizers and oligomers. Firstly, the high level of solvent solubility (mass uptake of∼13.6 wt%), due to the high toluene/HDPE affinity allowed the oligomers (M < 450 g/mol) to migrate and to finally be extracted (3.5 wt% of the mass sample for an immersion time of 96 h). Secondly, the decrease in OIT values, measured at 200 °C on HDPE samples dried after immersion, also revealed the extraction of antioxydants. On the other hand, the impact of the introduction of spherical (ZnO) or lamellar (graphite and organomodified montmorillonite) fillers up to 15 wt% on toluene sorption and migration phenomena was studied. As a result, the introduction of fillers, whatever the filler type, led to modification of the extraction profile of the oligomers. However, ZnO and MMT fillers had little effect on toluene sorption compared to graphite. Indeed, a significant decrease in the amount of toluene sorbed by the matrix and a significant slowdown in the toluene diffusion rate were demonstrated for amounts of graphite greater than 10 wt%. Interestingly, a reduction in the migration of low molar mass oligomers was evidenced and attributed to possible adsorption on the filler surfaces. Among the fillers used, graphite-type was more effective, reducing the amount of extracted species by a factor of 3.5. Regarding the impact of fillers on the thermo-oxidative behavior, once again, it was shown that graphite, at concentration higher than 10 wt%, allowed to prevent the antioxidant migration and consequently the thermal-oxidative degradation. Finally, and as expected, another way to improve the thermo-oxidative resistance after immersion in toluene was to increase the initial amount of antioxidants by adding Irganox B225 which is an equimassic mixture of Irganox 1010, a primary antioxidant and Irgafos 168, a secondary antioxidant.