Diffusion-controlled polymer oxidation

Thermal oxidation of hydrocarbon substrates at low-to-moderate temperature, typically T ≤ 150 • C, results from a radical chain process initiated by hydroperoxide decomposition and displays an induction period. A reliable model exists to simulate oxidation kinetics, but an incertitude remains on initial steps because they are out of reach of all available analytical methods. This work is aimed to have a kinetic approach of the problem, by comparing various mechanisms, i.e., (A) bimolecular decomposition of initially present hydroperoxides; (B) combined uni-and bimolecular decomposition of hydroperoxides; (C) the presence of radicals at the beginning of the exposure; and (D) radicals generation at (low) constant rate from irradiation, for instance by ionizing radiation linked to natural radioactivity or from a direct oxygen-substrate reaction. Scheme A is not realistic at low initial hydroperoxide concentrations. All the other mechanisms generate similar behaviors: the induction time tends toward a constant value almost independent of the nature of initial steps, when the concentration of precursors (initially present hydroperoxides or radicals) or the rate of their initial production (from species other than hydroperoxides) tends toward zero. C 2008 Wiley Periodicals, Inc. Int

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“…The system of differential equations is thus, for a thin sample (oxygen concentration gradient negligible [10])…”

Section: Kinetic Modelmentioning

confidence: 99%

Induction period in the low‐temperature thermal oxidation of saturated hydrocarbons: Example of polyethylene

Richaud

Colin

Fayolle

et al. 2008

Int J of Chemical Kinetics

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“…Donohue and Apostolou 21 suggested, by contrast, that the oxidation reactions were so rapid that oxygen was consumed upon arrival within the bulk sample. Others 19,20 have suggested that the diffusion and reaction rates were of comparable magnitude and that a quasi steady-state oxygen concentration profile should exist in the sample. This matter can be addressed by the data reported in this study.…”

Section: Introductionmentioning

confidence: 99%

“…The assumed oxygen profile can certainly be justified and modeled as a normal diffusion/reaction process. 19,20 However, Sun's assumption of the free-radical profile increasing with depth into the sample cannot be supported. Figure 1 30 shows an Electron Spin Resonance (ESR) depth profile immediately after irradiation and illustrates that the initial free-radical concentration was essentially uniform.…”

Section: Introductionmentioning

confidence: 99%

“…Several investigators have reported on the results of modeling the simultaneous oxygen diffusion and oxidation reactions as a function of depth and time. 19,20,28,29 Several investigators have assumed that the oxygen profile reaches a pseudo steady state in a relatively short time and remains constant thereafter, 19,20 thus simplifying the problem. This assumption is probably reasonable, because the time scale of oxygen diffusion is somewhat shorter than that for the reaction sequence (discussed further below).…”

Section: Introductionmentioning

confidence: 99%

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Oxidation of gamma-irradiated ultrahigh molecular weight polyethylene

Coote

Hamilton

McGimpsey

et al. 2000

J. Appl. Polym. Sci.

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“…When the oxidation rate is higher than the oxygen diffusion rate, an oxidation profile over thickness can be expected [65,66]. Depending on the ratio between oxidation and diffusion rate different profiles are likely.…”

Section: Oxidation Profilesmentioning

confidence: 99%