Kenneth T. Gillen

An ultrasensitive technique for testing the Arrhenius extrapolation assumption for thermally aged elastomers

Abstract We present a general approach for more confidently correlating accelerated aging results with aging under service conditions using the Arrhenius methodology. We first show that, as a result of complex diffusion-limited oxidation effects, time/temperature correlations may occur for some properties but not for others. To rigorously extrapolate high temperature results to low temperatures, we sought an ultrasensitive technique correlated to macroscopic degradation and capable of measurements at or near service temperatures. We achieved this objective by monitoring oxygen consumption rates at high (accelerated) temperatures, to establish the necessary correlation, and at low temperatures (down to 23 °C), to determine their temperature-dependence in the extrapolation region. Because easily measurable oxygen consumption rates of 10 −13 mol/g s correspond to decades of predicted lifetime for most elastomers, this approach increases confidence in long-term predictions and therefore provides a means of testing Arrhenius extrapolations.

Quantitative model for the time development of diffusion-limited oxidation profiles

We combine diffusion and kinetic expressions with chemical/mechanical property relationships to develop a complete, quantitative model with no arbitrarily adjustable parameters for the time development of diffusion-limited oxidation profiles. Excellent agreement between experimental and theoretical modulus profiles for two materials (a nitrile and a neoprene) confirms our understanding of diffusion-limited oxidation and its time and temperature dependencies. The simple assumption of no time dependencies in any of the model parameters is adequate for predictions relevant to elastomer service lifetimes.

Correlation of chemical and mechanical property changes during oxidative degradation of neoprene

The thermal degradation of a commercial, stabilized, unfilled neoprene (chloroprene) rubber was investigated at temperatures up to 140 C. The degradation of this material is dominated by oxidation rather than dehydrochlorination. Important heterogeneous oxidation effects were observed at the various temperatures investigated using infrared micro-spectroscopy and modulus profiling. Intensive degradation-related spectral changes in the IR occurred in the conjugated carbonyl and hydroxyl regions. Quantitative analysis revealed some differences in the development of the IR oxidation profiles, particularly towards the sample surface. These chemical degradation profiles were compared with modulus profiles (mechanical properties). It is concluded that the profile development is fundamentally described by a diffusion-limited autoxidation mechanism. Oxygen consumption measurements showed that the oxidation rates display non-Arrhenius behavior (curvature) at low temperatures. The current results, when compared to those of a previously studied, clay-filled commercial neoprene formulation, indicate that the clay filler acts as an antioxidant, but only at low temperatures.

Time-temperature-dose rate superposition: A methodology for extrapolating accelerated radiation aging data to low dose rate conditions

Abstract Time-temperature superposition is an empirical approach which has been used in polymers for more than 30 years to make thermal aging predictions at experimentally inaccessible times. Given its historical success, we have expanded this approach to combined radiation-thermal environments, yielding an empirical time-temperature-dose rate shifting procedure. The procedure derives an isothermal curve for a given amount of material damage versus dose rate at a selected reference temperature by finding the Arrhenius activation energy which causes higher-temperature, dose-rate data to superpose when shifted to the reference temperature. The resulting superposed curve extends to much lower dose rates which, in effect, are experimentally inaccessible due to the long time periods which would be required. This procedure therefore allows meaningful predictions to be made for long-term, low dose rate, radiation aging conditions. We have successfully applied the time-temperature-dose rate superposition approach to four different materials. For two of these materials, extrapolated predictions based on the superposed data were found to be in excellent agreement with 12 year, low dose rate aging results. Additional confidence in the approach results from the observation that the empirically-derived activation energies for all four materials can be quantitatively rationalized.

General solution for the basic autoxidation scheme

Abstract The basic autoxidation scheme (BAS), first used by Bolland and Bateman, has been widely applied to oxidation studies for almost 50 years. Steady-state kinetic analysis of this reaction scheme has always depended upon making the simplifying assumptions of long kinetic chain length and/or a specific ratio of termination rate constants. We present and discuss a generalized steady-state kinetic solution to the BAS that is derived without any simplifying assumptions. We also combine this generalized solution with diffusion expressions to derive theoretical oxidation profiles appropriate to diffusion-limited oxidation situations. The modeling predicts conditions yielding unusual dependencies of the oxidation rate on oxygen concentration and on initiation rate, as well as conditions yielding some unusual profile shapes.

Oxygen diffusion effects in thermally aged elastomers

Abstract We have obtained detailed, time-dependent modulus profiles on 2·2-mm thick samples of Neoprene and styrene-butadiene rubber (SBR) that were subjected to aging at temperatures of 80–150°C in air. The materials undergo strongly heterogeneous oxidation throughout this temperature range, with degradation much higher near surface regions than in the sample interior. The samples exhibit a latent onset of heterogeneous effects, particularly at the lower temperatures. As aging proceeds, the modulus in the exterior regions rises rapidly, whereas modulus changes in the interior region may slow down or stop. Numerous experiments with Neoprene formulations employing hindered phenol stabilizers of very different molecular weights demonstrate that heterogeneous oxidation is not the result of selective depletion of antioxidant from the surface regions of the sample. The primary cause of the incipient heterogeneous oxidation effects is a dramatic decrease in oxygen permeability coefficient as aging proceeds.

Rigorous experimental confirmation of a theoretical model for diffusion-limited oxidation

Abstract A rigorous test of theoretical treatments for diffusion-limited oxidation was completed by conducting an extensive series of radiation-initiated oxidation experiments on a commercial EPDM material. Oxidation profiles were monitored from density changes; profiles were obtained versus sample thickness, radiation dose rate and surrounding oxygen partial pressure. The resulting profile shapes and magnitudes could be quantitatively fit with a two-parameter theoretical treatment based on oxidation kinetics containing unimolecular termination reactions. The theoretical parameters derived from fitting allowed quantitative confirmation of a governing theoretical expression relating these parameters to independently measured values for the oxygen consumption and permeation rates.

The wear-out approach for predicting the remaining lifetime of materials

Failure models based on the Palmgren-Miner concept that material damage is cumulative have been derived and used mainly for fatigue life predictions for metals and composite materials. The authors review the principles underlying such models and suggest ways in which they may be best applied to polymeric materials in temperature environments. They first outline expectations when polymer degradation data can be rigorously time-temperature superposed over a given temperature range. For a step change in temperature after damage has occurred at an initial temperature in this range, the authors show that the remaining lifetime at the second temperature should be linearly related to the aging time prior to the step. This predicted linearity implies that it should be possible to estimate the remaining and therefore the service lifetime of polymers by completing the aging at an accelerated temperature. They refer to this generic temperature-step method as the Wear-out approach. They next outline the expectations for Wear-out experiments when time-temperature superposition is invalid. Experimental Wear-out results are then analyzed for one material where time-temperature superposition is valid and for another where evidence suggests it is invalid. In analyzing the data, they introduce a procedure that they refer to as time-degradation superposition. This procedure not only utilizes all of the experimental data instead of a single point from each data set, but also allows them to determine the importance of any interaction effects.

Modulus profiling of polymers

Abstract This paper reviews a new analytical apparatus which yields extremely useful information on mechanical property heterogeneities in polymers. The apparatus, which is based on extensive, but relatively simple, modifications of a commercial thermomechanical analyzer, is capable of obtaining greater than 20 quantitative tensile compliance measurements per millimetre of sample cross-section in less than an hour. Since the inverse of the tensile compliance is closely related to the more commonly measured tensile modulus, we refer to the technique as modulus profiling. A major application of the modulus profiling apparatus involves oxidation studies of polymers and a number of representative examples of its utility for such studies are described.

Oxidation profiles of thermally aged nitrile rubber

The thermal degradation of a commercial, stabilized, unfilled nitrile (Buna-N) rubber material was investigated at temperatures in the range 85–140 °C. The resulting heterogeneous oxidation, due to diffusion limitations in oxygen availability, was studied using infrared microscopy and modulus profiling. Degradation-related spectral changes were observed primarily in the hydroxyl, carbonyl and ester regions; quantitative analysis revealed identical oxidation profiles for these chromophores. These chemical oxidation profiles (carbonyl formation) were correlated with mechanical modulus (hardness) profiles. Degradation of the sample proceeds via a linear increase in the carbonyl concentration, but an exponential increase in the modulus with time. It is concluded that the profile development and aging behavior can be described by a diffusion-limited autoxidation mechanism which can be modeled computationally. The results are compared to those of a previously studied carbon-black-filled material.