Retention of thermal antioxidants in polyethylene by silane coupling agents. III. Copper deactivators

1992

This paper is the first in a series of related papers describing the application of a diffusion/reaction model to the loss of antioxidants from polyolefins in hot‐water applications. The model, which is derived in detail, describes the time evolution of antioxidant concentration profiles in the exposed material in terms of adjustable parameters. The parameters describe the rates of diffusion, evaporation, extraction, and chemical reaction of antioxidant. Parameter values are determined by least‐squares fitting of the calculated concentration profiles to experimental profiles. The model is applied to a commercial medium density polyethylene pipe material, where antioxidant concentration data from thermal analysis is available for water/air (internal/external) exposure at three temperatures. A comparison of parameter values with literature data is undertaken. The temperature dependence of the parameters is considered and activation energies are compared with literature values. The relative importances of the loss mechanisms are discussed. The effect of boundary conditions on parameter values is considered by application of the model to a limited amount of available data for air/air and water/water exposures. The results indicate that for water/air exposure, extraction by the water phase is the dominating loss mechanism.

Section: Parameters: Diffusion Coefficientsupporting

confidence: 51%

Section: Parameters: Water Extraction and Surface Evaporationmentioning

confidence: 70%

Modeling of antioxidant loss from polyolefins in hot‐water applications. I: Model and application to medium density polyethylene pipes

Smith

Karlsson

1992

Antioxidant efficiency loss by precipitation and diffusion to surrounding media in polyethylene hot‐water pipes

Viebke

Hedenqvist

1996

The extensive initial loss of efficiency of antioxidant [4,4′‐thiobis(3‐methyl‐6‐tertbutylphenol)], during hydrostatic pressure testing of medium density polyethylene pipes at 95 and 105°C has been studied. Samples from the pressure‐tested pipes were subsequently investigated by scanning electron microscopy and energy dispersive spectroscopy. Antioxidant‐containing particles were found in the pipe wall. The particles had evolved during the early stages of exposure and consisted of antioxidant that had phase separated and precipitated from the polymer matrix because the initial antioxidant concentration was greater than the solubility limit at the pressure testing temperatures. Relative antioxidant concentration profiles over the pipe wall, as measured by calorimetric oxidation induction time analysis, showed losses of >80% of the initial effective antioxidant content during the first 1000 h of pressure testing. A model assuming diffusion of antioxidant into precipitates was developed. Values for the diffusivity of the antioxidant in different parts of the pipe wall were obtained by fitting a previously developed model to long‐term antioxidant concentration profiles. Using these values, and by using the distance between two adjacent precipitates as an adjustable parameter, it was possible to fit the newly developed model to the experimental short‐term antioxidant concentration profiles.

Long‐term properties of hot‐water polyolefin pipes—a review

Viebke

Leijström

et al. 1994

184

This is a review on the long‐term behavior of polyolefin pipes used in hot‐water environments. Included in the review is work done on pipes of crosslinked polyethylene, isotactic poly(butene‐1), isotactic polypropylene and medium‐density polyethylene, and in particular the extensive work performed at Studsvik, Sweden. A collective view of the changes in antioxidant concentration profiles and molecular/physical structure accompanying hot‐water exposure is presented. Failure at high stress levels is preceded by gross deformation (Stage I failure), whereas at lower stresses fracture is brittle either without any signs of thermal oxidation (Stage II) or induced by a strong and spatially concentrated thermal oxidation (Stage III). It is shown that the Stage III lifetime can be divided into three phases, denoted Regimes A, B, and C. They involve internal precipitation of antioxidant from a supersaturated solution (Regime A), diffusion‐controlled migration of antioxidant to the surrounding media (Regime B), and degradation of the polymer by thermal oxidation (Regime C). For hitherto reported cases the Regime B life constitutes 80% to 90% of the entire life. A presentation is also made of current lifetime extrapolation methods.