Mathias Christopher Celina

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.

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.

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.

Anomalous aging phenomena in a crosslinked polyolefin cable insulation

Abstract The radiation-thermal degradation of a commercial crosslinked polyolefin (XLPO) cable insulation material was investigated as a function of dose rate and temperature in the range of 22–120°C. Degradative changes in the material were monitored by ultimate elongation, density, gel content, O 2 consumption, infrared spectroscopy, and differential scanning calorimetry. Mechanical aging surprisingly occurred most rapidly at the lowest temperatures. This unusual phenomenon was corroborated by chemical measurements (gel content and density). When samples that had been irradiated at ambient temperature were subsequently annealed at elevated temperatures, recovery of mechanical properties and concurrent changes in gel content and density were observed. The involvement of residual radical species and hydroperoxide intermediates as well as the importance of molecular mobility in the semi-crystalline XLPO as contributors to these anomalous behaviors were evaluated and discussed. The observed inverse temperature effect, where polymer degradation occurs more rapidly at lower temperatures, represents an example in which material aging and life time prediction cannot be handled by conventional approaches, such as the commonly applied Arrhenius methodology.

Density measurements as a condition monitoring approach for following the aging of nuclear power plant cable materials

Monitoring changes in material density has been suggested as a potentially useful condition monitoring (CM) method for following the aging of cable jacket and insulation materials in nuclear power plants. In this study, we compare density measurements and ultimate tensile elongation results versus aging time for most of the important generic types of commercial nuclear power plant cable materials. Aging conditions, which include thermal-only, as well as combined radiation plus thermal, were chosen such that potentially anomalous effects caused by diffusion-limited oxidation (DLO) are unimportant. The results show that easily measurable density increases occur in most important cable materials. For some materials and environments, the density change occurs at a fairly constant rate throughout the mechanical property lifetime. For cases involving so-called induction-time behavior, density increases are slow to moderate until after the induction time, at which point they begin to increase dramatically. In other instances, density increases rapidly at first, then slows down. The results offer strong evidence that density measurements, which reflect property changes under both radiation and thermal conditions, could represent a very useful CM approach.

FTIR emission spectroscopy applied to polymer degradation

Abstract The thermal degradation of some common polymers was investigated using infrared emission spectroscopy. The potential of the technique to contribute to polymer degradation studies is demonstrated by measuring the spectroscopic changes that occur during thermal degradation, oxidation or decomposition of polymers under air at temperatures ranging from 150 to 250 °C. Polymer types studied include EPDM and nitrile rubbers (commercial materials containing some inorganic fillers), PMMA, PAN, PA, PVC and PS. The resulting qualitative changes in the polymers were easily detected, and were correlated with the current knowledge on the degradation mechanisms of these materials. These include simple carbonyl formation and related oxidative reactions, weight loss and volatilization, as well as formation of conjugation and specific polymer reactions. Some fundamental aspects and limitations of the technique are discussed to demonstrate the intrinsic difficulties of attempting to determine precise emittances and ultimately quantitative spectroscopic information. FTIR emission appears to be promising for studying in situ polymer degradation.

17O NMR investigation of oxidative degradation in polymers under γ-irradiation

Abstract The γ -irradiated-oxidation of pentacontane (C 50 H 102 ) and the polymer polyisoprene was investigated as a function of oxidation level using 17 O nuclear magnetic resonance (NMR) spectroscopy. It is demonstrated that by using 17 O labeled O 2 gas during the γ -irradiation process, details about the oxidative degradation mechanisms can be directly obtained from the analysis of the 17 O 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 17 O NMR spectroscopy demonstrates the selectivity of this technique over more conventional approaches.

Pulsed e− beam irradiation of polymers—A comparison of dose rate effects and let effects

Linear Energy Transfer (LET) effects in radiation chemistry are ascribed to the high density of active species in the track structure, resulting in overlapping of spurs. We studied the possibility of spur overlapping in electron beam irradiation at extremely high dose rate, both theoretically and experimentally. Considering differences in overlap mode leads to the concept of a threshold dose rate, above which spur overlapping may occur, and an overlapping dose, which is necessary to cause overlapping of spurs even at higher dose rate than the threshold, depending on lifetime and effective volume of reactive intermediates. Using Sandias pulsed power e - beam system, we irradiated cellulose triacetate (CTA), polymethylmethacrylate (PMMA), polycarbonate (PC) and high density polyethylene (HDPE) at a dose rate as high as 4 x 10 10 Gy/s at room temperature in the absence of oxygen. Comparison of the e - beam results with data obtained from gamma irradiation at 0.5 Gy/s showed no dose rate effects based on discoloration sensitivity for CTA, or on scission probabilities of PMMA and PC. For HDPE, the results indicated a slightly lower rate of gel formation under e - beam irradiation, implying that the crosslinking efficiency may be somewhat reduced at the high dose rate. Published by Elsevier Science Ltd

Inverse temperature and annealing phenomena during degradation of crosslinked polyolefins

Abstract The radiation-thermal degradation of several types of commercial cable insulation materials (semi-crystalline crosslinked polyolefins) was investigated as a function of temperature in the range of 22–120 °C. Mechanical property deterioration surprisingly occurred most rapidly at the lowest temperatures. This unusual phenomenon was corroborated by fundamental differences in the degradation mechanism at elevated and ambient temperatures. Annealing at elevated temperatures of samples that had been aged at or near room temperature led in some cases to significant recovery of mechanical properties (elongation at break) and concurrent changes in gel content (additional crosslinking) and density. The importance of molecular mobility in the semi-crystalline materials and the nature of the additional crosslinking reaction as contributors to these anomalous behaviors are evaluated and discussed. The observed inverse temperature/aging dependence is due to a combination of the semi-crystalline morphology of the materials, and a specific crosslinking reaction, which can act as a repair mechanism at elevated temperatures, but is dormant at ambient conditions.

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.