Longterm performance of polyolefins in different environments including chlorinated water: antioxidant consumption and migration and polymer degradation

Abstract: The long-term performance of stabilized polyolefins in different environments was studied with focus on antioxidant consumption and migration. Plaques of linear polyethylene (LPE) and branched polyethylene (BPE) were stabilized with Santonox® R (4,4'-Thiobis(6-tert-butyl-3-methylphenol)), Irganox® 1081 (2,2’-Thiobis(4-methyl-6-tertbutylphenol)), or Lowinox® 22M46 (2,2’-Methylenebis(6-tert-butyl-4-methylphenol)). The samples were aged in water and nitrogen at 75, 90 and 95°C. Antioxidant concentration profiles were obtained by oxidation induction time (OIT) measurements using differential scanning calorimetry (DSC). The very flat antioxidant concentration profiles of the plaques exposed to non-aqueous media indicated that the migration of antioxidant to the surrounding medium was controlled by the low evaporation rate at the sample boundary. The samples of BPE and Santonox R were also exposed to air and water saturated with air. The similarity of the antioxidant concentration profiles of Santonox R obtained after ageing in air and nitrogen suggested that the fraction of antioxidant oxidized is negligible in comparison with the loss of antioxidant by migration to the surrounding media. The loss of Santonox R in samples exposed to water saturated with air was faster than for the samples exposed to oxygen-free water. This was due to increased mass transport of the antioxidant from the polymer phase boundary to the water phase when oxygen was present. An unexpected higher migration rate from LPE than from BPE was proposed to be due to the low boundary loss rate in BPE, caused by the presence of a thin liquid-like (oligomeric) surface layer developed during ageing. A quantitative relationship was found between the boundary loss rate to water and the polarity of antioxidants. The antioxidant diffusivities were approximately equal in LPE and BPE, indicating that the constraining effect of the crystals on the non-crystalline fraction did not affect the antioxidant molecules. Results obtained by liquid chromatography of extracts confirmed that the gradual decrease in OIT with increasing ageing time was due to migration of antioxidant to the surrounding medium. Pipes of high-density polyethylene stabilized with hindered phenols and phosphites were exposed to chlorinated water at elevated temperatures. OIT showed that the stabilizing system was rapidly chemically consumed by the action of chlorinated water. Size exclusion chromatography and DSC showed extensive polymer degradation strictly confined to the immediate surface of the unprotected inner wall material and to the amorphous phase of the semicrystalline polymer. The rate of growth of the layer of highly degraded polymer was constant. Pipes of isotactic polybutene-1 were pressure-tested in chlorinated water at a controlled pH, and the lifetime was assessed as a function of temperature and chlorine content. The lifetime shortening in chlorinated water was significant even at relatively low chlorine contents, 0.5 ppm. A further increase of chlorine content led to only a moderate shortening of the lifetime. The temperature dependence of the lifetime data obeyed the Arrhenius law. The decrease of the antioxidant concentration was independent of the chlorine concentration in the range of 0.5-1.5 ppm. The time to reach depletion of the antioxidant system could be predicted by linear extrapolation.