Acceleration Factors for the Oxidative Aging of Polymeric Materials

Chapter 3

Acceleration Factors for the Oxidative Aging of Polymeric Materials

Downloaded by FUDAN UNIV on February 23, 2017 | http://pubs.acs.org Publication Date: December 21, 2007 | doi: 10.1021/bk-2007-0978.ch003

Roger A. Assink, Mathew C. Celina, and Julie M . Elliott Department of Organic Materials, Sandia National Laboratories, Albuquerque, NM 87185-1411

Three methods that were used to measure the chemical changes associated with oxidative degradation of polymeric materials are presented. The first method is based on the nuclear activation of in an elastomer that was thermally aged in an 0 atmosphere. Second, the alcohol groups in a thermally aged elastomer were derivatized with trifluoroacetic anhydride and their concentration measured via F NMR spectroscopy. Finally, a respirometer was used to directly measure the oxidative rates of a polyurethane foam as a function of aging temperature. The measurement of the oxidation rates enabled acceleration factors for oxidative degradation of these materials to be calculated. 180

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© 2008 American Chemical Society

Celina and Assink; Polymer Durability and Radiation Effects ACS Symposium Series; American Chemical Society: Washington, DC, 2007.


27 Predicting a polymer's useful life is difficult because its mechanical properties can be a nonlinear function of aging time. Thus extrapolation of short-term behavior may not be an accurate predictor of long-term behavior. Many lifetime prediction methods revolve around the concept of "accelerated aging" in which high-temperature short-term aging is extrapolated to low temperature long-term aging. Uncertainty arises, however, when the rates need to be extrapolated over an extended temperature range [1]. We are investigating the chemical changes occurring during the oxidative aging of polymers. In addition to providing insight into the aging mechanism, it is often possible to measure chemical changes associated with aging more accurately than mechanical changes associated with aging. This increased sensitivity can be used to measure the very low rate of degradation at ambient conditions, so that extrapolation from high temperature behavior to low temperature behavior can be made in a scientific manner. Three methods that are capable of measuring the chemical changes associated with low levels of oxidative degradation are presented. The first method relies on nuclear activation resulting in the 0(γ,ρ) Ν reaction. This method was used to measure the concentration of oxidative products in a hydroxy-terminated polybutadiene (HTPB) elastomer that had been thermally aged in an atmosphere of 0 . The second method relies on a previous study that has shown that the primary oxidative degradation products of an HTPB elastomer are alcohols. The alcohol groups were fimctionalized with trifluoroacetic anhydride (TFAA) and the concentration of the resulting trifluorinated esters measured by F NMR spectroscopy. Finally, a commercial differential fuel cell respirometer is used to measure oxidation rates of a polyurethane foam as a function of temperature. This instrument has been successfully employed for measuring the respiratory cycles of small animals and insects in real time [2,3]. We modified the operation of the instrument in order to measure the extremely low oxidation rates of the foam at ambient temperatures in a relatively short period of time. The acceleration factors for the oxidation rate are compared to those for the foam's compressive strength and used to predict the service life for the compressive strength of the foam aged under ambient condtions. 18

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Nuclear Activation of

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Samples consisting of two grams of HTPB elastomer were placed in vials containing approximately 125 torr of 0 gas and aged at 95°C for times ranging from 6 to 956 hrs. The samples were removed and the amount of oxygen consumed by the polymer determined by the ultrasensitive oxygen consumption method (UOC) [4,5,6]. The net amount of 0 in each sample was calculated by adding the amount of consumed 0 to the 51 ppm background 0 due to the 1 8

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28 0.204% natural abundance of O. The samples were analyzed on a 20 MeV electron LINAC owed and operated by the Idaho Accelerator Center at Idaho State University. The LINAC operates at a peak energy between 19 and 22 MeV with a beam current of approximately 80 mA. The electrons are allowed to pass through a thin tungsten sheet that converts the electron energy to photon energy through bremsstrahlung production. The neutron detector was composed of four proportional counters consisting of BF gas-filled tubes at a pressure of 70 cm Hg. Futher details of the instrumental procedure have been published [7]. Water enriched with 0 was first analyzed in order to verify N detectability. The output was clearly exponentially with a half-life of 0.419 +/0.33 s compared to a published value of 4.17 s. Figure 1 shows a comparison of the neutron counts from the HTPB samples with their expected 0 concentrations. The linearity is excellent and demonstrates sensivity to 0 at concentrations below 100 ppm. The nuclear activation method measures relative concentrations, so the analysis of samples containing unknown amounts of 0 would need to employ well characterized standards such as the HTPB samples prepared for this study. 3

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29 Chemical Derivatization of Alcohol Groups 13

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In a previous study, C and H NMR were utilized to identify and quantify the oxidative degradation products of an HTPB elastomer [8]. A reliable relationship between extent of oxidation and the nature of the oxidation products was established, with the finding that alcohols comprise approximately 60% of the functional groups produced. In this study, we utilize the anhydride-alcohol esterification reaction shown in Scheme I to convert all of the alcohol groups in HTPB to trifluorinated esters. We have shown that this conversion is relatively straightforward and quantitative. The intensity of the F NMR signal from the trifluorinated esters is then used to determine the concentration of alcohol groups that were present in the polymer. The resultant F NMR signal of the - C F group is over 14,000 times as intense as the C NMR signal of the alcohol so detection of extremely low concentrations is possible.


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Derivatized / Trifluoroacetylated Polymer

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Scheme I. The F derivatization reaction for an alcohol group.

I9

Figure 2. shows the F NMR spectra obtained in approximately 1 minute of 10 mg of HTPB elastomer that had been aged at 80°C and then derivatized with TFFA. The details of the derivatization process and the NMR sample tube configuration required in order to obtain a high resolution F NMR spectrum have been described in the literature [9]. Samples of HTPB elastomer that had been aged at 80°C for times ranging from 14 to 221 days were derivatized and their F NMR spectra recorded. The signal intensities of the derivatization products were converted to absolute alcohol concentrations by comparision to the signal from a solution with a known amount of TFAA. Figure 3 shows the alcohol concentration vs. aging time determined by F labeling and by the UOC method combined with knowledge of the relative distribution of oxidative products obtained by C NMR spectroscopy. The slopes of the two methods are comparable, confirming that each measures the same amount of alcohol production as a function of aging l9

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Figure 2. The F NMR spectrum of the derivatization product of HTPB elastomer aged at 80°C. The arrow denotes the trifluoroester. The upfield resonance originates from the trifluoroacetic acid byproduct.

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Time [d] Figure 3. The alcohol concentration in HTPB as a function of aging time at 80°C. The slopes of the F derivatization and UOC methods are comparable. 19


31 time. The nonzero intercept of the F derivation method is attributed to hydroxy group present in the cured but unaged elastomer.


Differential Respirometer The respirometer, an Oxzilla II Dual Absolute and Differential Oxygen Analyzer from Sable Systems International (Las Vegas, NV), is based on sensitive fuel cell detectors. The output of each fuel cell detector depends on the concentration of oxygen in the air stream directed to that fuel cell. The output of the fuel cell connected to the sample chamber is subtracted from the output of the fuel cell connected to the reference flow and the difference is used to calculate the amount of oxygen that was consumed by the polymer sample during aging. A simplified outline of the arrangement is shown in Scheme II. The respirometer and a gas multiplexer (also purchased from Sable Systems International) are controlled by a laboratory PC, which was also used to record the data. Ultrapure air was supplied by Matheson Inc.

Mass Flow Valve Reference Respirometer Multiplexer Air Bypass L Sample -

Computer

Scheme II. Outline of the respirometer, associated plumbing and data interface.

A known amount of polymer was placed in a sample chamber. The vessel was evacuated; cylinder air at 40 cmVmin was passed over the sample for several minutes and the chamber sealed. After an interval of at least one hour, air was again passed over the sample in order to completely eliminate any contribution originating from ambient air dissolved in the sample. The chamber was removed from the manifold and placed in an air-circulating oven. After aging, the sample chamber was reconnected to the manifold and the short sections of the



32 Connecting gas pathways that had been exposed to atmosphere were evacuated and refilled with cylinder air. At time = 0, the multiplexer was instructed to switch the air flow from the bypass loop to the sample chamber. The difference between the oxygen concentration of its air flow and the reference air flow was recorded as a function of time. The difference reached a maximum after 60 seconds, and recovered after approximately 3 minutes. The total amount of oxygen depletion was calculated from the integral of the difference in oxygen concentration. Details of the procedure and calculations have been published [10]. Figure 4 shows the oxygen deficit trace from a sample chamber containing 0.207g of a carbon filled natural rubber that had been aged for 16 hrs at 80°C. The dual detectors were first balanced with identical air streams flowing through both channels. The hatched area represents the total oxygen deficit which was calculated to be 0.0819 cm 0 at STP or 117 μg 0 . This amount of oxygen corresponds to an oxidation rate of 3.06x10" mol 0 STP/g sample/s. This oxidation rate compared favourably to that measured by the conventional UOC method. 3

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Time [s]

Figure 4. The 0 deficit trace of afillednatural rubber that had been agedfor 16hrat 80° C. 2

3

The oxygen deficit trace of a 0.1 g/cm polyurethane foam sample aged at 23°C for 555 hours is shown in Figure 5. The low density of the foam limited


33 the amount of sample that could be easily placed into the chamber to 1.07 g. Additional material would have lead to compression of the foam and obstruction of the free flow of air through the chamber. Aging at room temperature represents the lowest oxidation rate that needs to be measured for normal aging conditions. The oxygen deficit range of the respirometer was set to maximum sensitivity. Although the trace exhibits considerable fluctuation (the response was not time-filtered), the integral represented by the crosshatched area could be measured reliably. The total deficit equals 0.00149 cm 0 STP or 2.1 μg for the 1.07 g sample and predicts an oxidation rate of 3.12x10" mol 0 STP/g sample/s. Figure 6 shows a comparision of the oxidation rates measured by the respirometer and the UOC method for a range of temperatures. Temperatures for the UOC were limited to 37°C and above, because a year of aging was required for sufficient response at that temperature. The respirometer extended oxidation rates measurements to 23°C and even at that temperature, only 23 days of aging were required. In addition to being able to use shorter aging times, the respirometer data exhibited significantly less scatter than the UOC data. Note that some of the respirometer data overlay each other; there are a total of 6, 4, 3 and 6 measurements at 110, 80, 50 and 23°C, respectively. 3

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Figure 5. The 0 deficit trace of a polyurethane foam that has been agedfor 23 days at 23°C 2



34

The oxidation rates measured by the respirometer were used to calculate acceleration factors for the oxidative aging of the polyurethane foam. The oxidation rates at each temperature were averaged and used to calculate acceleration factors that were normalized to 1.0 at 95°C. The results versus inverse temperature are shown in Figure 7. The acceleration factor plot exhibits some curvature in the lower temperature region of the curve. The activation energy of the relatively linear region between 50 and 110°C is 92 kJ/mol. The purpose of measuring acceleration factors for oxidation is to apply these same acceleration factors in the low temperature region to a mechanical property of interest. The foam is used as a packaging material to protect items from shock and vibration. An important property for the performance of this task is the compressive strength of the foam. The force of foam samples at 50% compression were measured as a function of aging time and temperature. Aging the samples for 14 months caused significant changes in the compressive strength of the foam only at temperatures of 95°C and above. The shape of the compressive strength curves for samples aged from 95 to 140°C are reasonably similar and suggest that a time-temperature superposition is possible. The acceleration factors derived from the time-temperature superpostion are also plotted vs. inverse temperature in Figure 7. The acceleration factors for the


35 compressive strength of the foam correlate reasonably well with the acceleration factors for the rate of oxidation of the foam. If one assumes that the compressive strength is closely coupled to the rate of oxidation over the entire temperature range, then the low temperation acceleration factors for oxidation can be used to predict the behavior of the compressive strength when the foam is aged at low temperatures. Similar assumptions coupling oxidation rates to mechanical properties of polymers have proven quite useful for a variety of polymers.


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Figure 7. The accelerator factors for the oxidation rate and compressive force of a polyurethane foam.

Conclusions We have demonstrated three techniques that measure the chemical changes associated with the oxidative degradation of polymers. Each of these techniques has the potential to measure very low concentrations of chemical change. The ability to measure slow oxidation rates enable the degradation process to be monitored at the low temperatures more closely related to the material's service conditions. Thus, more accurate service life predictions can be made without relying on extrapolation of high temperature behavior to low temperatures.


36 Acknowledgements Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC0494AL85000. The technical assistance of Sean T. Winters and Ana B. Trujillo is gratefully acknowledged.


References 1. Celina, M ; Gillen, K. T.; Assink, R. A. Polym. Degrad. Stab. 2005, 90, 395-404. 2. Greenlee, K. J.; Harrison, J. F. J. Exp. Biol. 2004, 207, 497-508. 3. Josephson, R. K.; Malamud, J. G.; Stokes, D. R. J. Exp. Biol. 2001, 204, 4125-4139. 4. Gillen, K. T.; Clough, R. L. In: Radiation Effects on Polymers; Clough, R. L.; Shalaby, S. W., Eds.; ACS Series #475, Washington D.C. 1991. pp 457472. 5. Wise, J.; Gillen, K. T.; Clough, R. L. Polym. Degrad. Stab. 1995, 49, 403418. 6. Wise, J.; Gillen, K. T.; Clough, R. L. Polymer 1997, 38, 1929-1944. 7. Webb, T; Beezhold, W; DeVeaux, L; Harmon, F; Petrisko, J; Spaulding, R; Assink, R; "Photonuclear and Radiation Effects Testing with a Refurbished 20 MEV Medical Electron LINAC", Particle Accelerator Conference, Knoxville, TN 5/16-20/2005. 8. Harris, D. J.; Assink, R. Α.; Celina, M . Macromolecules 2001, 34, 66956700. 9. Skutnik, J. M.; Assink, R. Α.; Celina, M . Polymer 2004, 45, 7463-7469. 10. Assink, R. Α.; Celina, M.; Skutnik, J. M.; Harris, D. J. Polymer 2005, 46, 11648-11654.


Acceleration Factors for the Oxidative Aging of Polymeric Materials

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