Antioxidant diffusion in polyethylene hot‐water pipes

Abstract

AbstractThis paper presents oxidation induction time (OIT) data for samples taken from a range of polyethylene hot‐water pipes before and after hydrostatic pressure testing with internal stagnant water and external air. Linear relationships between OIT and antioxidant concentration were established for the antioxidants/polymers used. A model assuming Fickian diffusion of the antioxidant to the surrounding media and chemical consumption of the antioxidant (Regime B model) was applied to the OIT‐profile data. The Regime B model was successfully applied to OIT‐profile data except for in the case of a pipe with a substantial scatter in the OIT data, indicative of compositional heterogeneity. The choice of initial conditions for the modeling was critical. The use of initial conditions based on insufficiently exposed pipes led to an overestimate of the diffusivities caused by the combined action of Regimes A (internal precipitation) and B loss mechanisms. The time period associated with Regime A constituted up to 25% of the lifetime for the pipes studied. Antioxidants with sterically accessible polar groups showed a higher melting point and greater interaction with dissolved water and carbon black. Obtained radial dependences of the antioxidant diffusivities (D) indicate that the water concentration in the polymer influenced D primarily through cluster formation involving water and antioxidant molecules and by competition between water and antioxidant molecules about adsorption sites on carbon black particles and to a much lesser extent by plasticization. Antioxidant concentration profiles calculated for the failure time interval in pressure testing appeared in the same concentration range, and fracture initiation occurs in the regions in the pipe wall first reaching depletion of the antioxidant system.