Review of Some Important Issues and Resolutions when Lifetime Predictions of Elastomers are Made Using Accelerated Aging Techniques
Abstract not provided.
Publications
Review of Some Important Issues and Resolutions when Lifetime Predictions of Elastomers are Made Using Accelerated Aging Techniques
Abstract not provided.
Nylon 6.6 accelerated aging studies: Thermal-oxidative degradation and its interaction with hydrolysis
Polymer Degradation and Stability
Accelerated aging of Nylon 6.6 fibers used in parachutes has been conducted by following the tensile strength loss under both thermal-oxidative and 100% relative humidity conditions. Thermal-oxidative studies (air circulating ovens) were performed for time periods of weeks to years at temperatures ranging from 37 °C to 138 °C. Accelerated aging humidity experiments (100% RH) were performed under both an argon atmosphere to examine the 'pure' hydrolysis pathway, and under an oxygen atmosphere (oxygen partial pressure close to that occurring in air) to mimic true aging conditions. As expected the results indicated that degradation caused by humidity is much more important than thermal-oxidative degradation. Surprisingly when both oxygen and humidity were present the rate of degradation was dramatically enhanced relative to humidity aging in the absence of oxygen. This significant and previously unknown phenomena underscores the importance of careful accelerated aging that truly mimics real world storage conditions. Published by Elsevier Ltd.
Condition monitoring approaches applied to a polychloroprene cable jacketing material
Polymer Degradation and Stability
In this paper we examine the utility of several promising material condition monitoring (CM) techniques applied to a commercial polychloroprene cable jacketing material used in nuclear power plant applications. These include two relatively unknown approaches, cross-sectional modulus profiling and NMR T2 relaxation time measurements of solvent-swelled samples, as well as three more commonly used approaches, density, gel fraction and solvent uptake. The results from each approach were compared to tensile elongation measurements, the usual standard approach for monitoring degradation of elastomers. Degradation was carried out at three temperatures and at four combined radiation plus thermal environments, all of which were selected (by theoretical modeling and later confirmed by cross-sectional degradation mapping) such that oxidation proceeded uniformly throughout the cross-section of the material. This allowed macroscopic condition monitoring measurements to be made in the absence of anomalous diffusion-limited oxidation effects. Of the techniques examined, modulus profiling, solvent uptake and NMR T2 measurements correlated extremely well with the elongation measurements and therefore showed substantial potential as CM approaches for this material. This is not unexpected since all of these techniques are sensitive to crosslinking of the material and the deterioration of the elongation is itself dominated by material hardening and thus by crosslinking. Published by Elsevier Ltd.
Parachute materials study : Kevlar degradation
Abstract not provided.
Validation of improved methods for predicting long-term elastomeric seal lifetimes from compression stress-relaxation and oxygen consumption techniques
Proposed for publication in Polymer Degradation and Stability.
Conventional high-temperature compression stress-relaxation (CSR) experiments (e.g., using a Shawbury-Wallace relaxometer) measure the force periodically at room temperature. In this paper, we first describe modifications that allow the force measurements to be made isothermally and show that such measurements lead to more accurate estimates of sealing force decay. We then use conventional Arrhenius analysis and linear extrapolation of the high-temperature (80--110 C) CSR results for two commercial butyl o-ring materials (Butyl-A and Butyl-B) to show that Butyl-B is predicted to have approximately three times longer lifetime at room temperature (23 C). To test the linear extrapolation assumed by the Arrhenius approach, we conducted ultrasensitive oxygen consumption measurements from 110 C to room temperature for the two butyl materials. The results indicated that linear extrapolation of the high temperature CSR results for Butyl-A was reasonable whereas a significant curvature to a lower activation energy was observed for Butyl-B below 80 C. Using the oxygen consumption results to extrapolate the CSR results from 80 C to 23 C resulted in the conclusion that Butyl-B would actually degrade much faster than Butyl-A at 23 C, the opposite of the earlier conclusion based solely on extrapolation of the high-temperature CSR results. Since samples of both materials that had aged in the field for {approx}20 years at 23 C were available, it was possible to check the predictions using compression set measurements made on the field materials. The comparisons were in accord with the extrapolated predictions made using the ultrasensitive oxygen consumption measurements, underscoring the power of this extrapolation approach.
17O NMR investigation of oxidative degradation in polymers under γ-irradiation
Radiation Physics and Chemistry
The γ-irradiated-oxidation of pentacontane (C50H102) and the polymer polyisoprene was investigated as a function of oxidation level using 17O nuclear magnetic resonance (NMR) spectroscopy. It is demonstrated that by using 17O labeled O2 gas during the γ-irradiation process, details about the oxidative degradation mechanisms can be directly obtained from the analysis of the 17O NMR spectra. Production of carboxylic acids is the primary oxygen-containing functionality during the oxidation of pentacontane, while ethers and alcohols are the dominant oxidation product observed for polyisoprene. The formation of ester species during the oxidation process is very minor for both materials, with water also being produced in significant amounts during the radiolytic oxidation of polyisoprene. The ability to focus on the oxidative component of the degradation process using 17O NMR spectroscopy demonstrates the selectivity of this technique over more conventional approaches. © 2001 Elsevier Science Ltd.
Monitoring Thermo-Oxidative Degradation in Elastomers by NMR Relaxation Measurments
Abstract not provided.
Analysis of Hydroperoxides in solid Polyethylene by NMR and EPR Spectroscopy
The authors have shown that the hydroperoxide species in {gamma}-irradiated {sup 13}C-polyethylene can be directly observed by {sup 13}C MAS NMR spectroscopy. The experiment was performed without the need for special sample preparation such as chemical derivatization or dissolution. Annealing experiments were employed to study the thermal decomposition of the hydroperoxide species and to measure an activation energy of 98 kJ/mol. EPR spectroscopy suggests that residual polyenyl and alkylperoxy radicals are predominantly trapped in interracial or crystalline regions, while the peroxy radicals observed after UV-photolysis of hydroperoxides are in amorphous regions.
The wear-out approach for predicting the remaining lifetime of materials
Polymer Degradation and Stability
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.
Aging analyses of aircraft wire insulation
Over the past two decades, Sandia has developed a variety of specialized analytical techniques for evaluating the long-term aging and stability of cable insulation and other related materials. These techniques have been applied to cable reliability studies involving numerous insulation types and environmental factors. This work has allowed the monitoring of the occurrence and progression of cable material deterioration in application environments, and has provided insights into material degradation mechanisms. It has also allowed development of more reliable lifetime prediction methodologies. As a part of the FAA program for intrusive inspection of aircraft wiring, they are beginning to apply a battery of techniques to assessing the condition of cable specimens removed from retired aircraft. It is anticipated that in a future part of this program, they may employ these techniques in conjunction with accelerated aging methodologies and models that the authros have developed and employed in the past to predict cable lifetimes. The types of materials to be assessed include 5 different wire types: polyimide, PVC/Glass/Nylon, extruded XL-polyalkene/PVDF, Poly-X, and XL-ETFE. This presentation provides a brief overview of the main techniques that will be employed in assessing the state of health of aircraft wire insulation. The discussion will be illustrated with data from their prior cable aging studies, highlighting the methods used and their important conclusions. A few of the techniques that they employ are widely used in aging studies on polymers, but others are unique to Sandia. All of their techniques are non-proprietary, and maybe of interest for use by others in terms of application to aircraft wiring analysis. At the end of this report is a list showing some leading references to papers that have been published in the open literature which provide more detailed information on the analytical techniques for elastomer aging studies. The first step in the investigation of aircraft wiring is to evaluate the applicability of their various techniques to aircraft cables, after which they expect to identify a limited subset of techniques which are appropriate for each of the major aircraft wiring types. The techniques of initial interest in the studies of aging aircraft wire are as follows: optical microscopy; mandrel bend test; tensile test/elongation at break; density measurements; modulus profiling/(spatially-resolved micro-hardness); oxygen induction time/oxygen induction temperature (by differential scanning calorimetry); solvent-swelling/gel fraction; infrared spectroscopy (with chemical derivatization as warranted); chemiluminescence; thermo-oxidative wear-out assessment; The first two techniques are the simplest and quickest to apply; those further down the list tend to be more information rich and in some cases more sensitive, but also generally more specialized and more time consuming to run. Accordingly, the procedure will be to apply the simplest tests for purposes of preliminary screening of large numbers of samples. For any given material type, it can be expected that only a limited number of the other techniques will prove to be useful, and therefore, the more specialized techniques will be used on a limited number of selected samples. Samples of aircraft wiring have begun to be released to the authors in late April; they include in this report some limited and preliminary data on these materials.
New methods for predicting lifetimes. Part 2 -- The Wear-out approach for predicting the remaining lifetime of materials
The so-called Palmgren-Miner concept that degradation is cumulative, and that failure is therefore considered to be the direct result of the accumulation of damage with time, has been known for decades. Cumulative damage models based on this concept 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. The authors first consider cases where 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, they 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 may be possible to estimate the remaining lifetime of polymeric materials aging under application ambient conditions by completing the aging at an accelerated temperature. They refer to this generic temperature-step method as the Wear-out approach. They then outline the expectations for Wear-out experiments when time-temperature superposition is invalid, specifically describing the two cases where so-called interaction effects are absent and are present. Finally, they present some preliminary results outlining the application of the Wear-out approach to polymers. In analyzing the experimental Wear-out results, they introduce a procedure that they refer to as time-damage 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.
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