ANTIOXIDANT DIFFUSION IN POLYETHYLENE HOT-WATER PIPES

This paper presents oxidation induction time (OIT) data for samples ta ken from a range of polyethylene hot-water pipes before and after hydr ostatic 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 Ficki an 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-prof ile data except for in the case of a pipe with a substantial scatter i n the OIT data, indicative of compositional heterogeneity. The choice of initial conditions for the modeling was critical. The use of initia l conditions based on insufficiently exposed pipes led to an overestim ate of the diffusivities caused by the combined action of Regimes A (i nternal precipitation) and B loss mechanisms. The time period associat ed with Regime A constituted up to 25% of the lifetime for the pipes s tudied. Antioxidants with sterically accessible polar groups skewed a higher melting point and greater interaction with dissolved water and carbon black, Obtained radial dependences of the antioxidant diffusivi ties (D) indicate that the water concentration in the polymer influenc ed D primarily through cluster formation involving water and antioxida nt molecules and by competition between water and antioxidant molecule s about adsorption sites on carbon black particles and to a much lesse r extent by plasticization, antioxidant concentration profiles calcula ted for the failure time interval in pressure testing appeared in the same concentration range, and fracture initiation occurs in the region s in the pipe wall first reaching depletion of the antioxidant system.