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Nylon 6.6 accelerated aging studies: thermal–oxidative degradation and its interaction with hydrolysis
Polymer Degradation and Stability 88 (2005) 480e488 www.elsevier.com/locate/polydegstab
Nylon 6.6 accelerated aging studies: thermaleoxidative degradation and its interaction with hydrolysis Robert Bernstein*, Dora K. Derzon, Kenneth T. Gillen Sandia National Laboratories, Organic Materials Department, P. O. Box 5800, MS-0888, Albuquerque, NM 87185-0888, United States Received 28 June 2004; received in revised form 21 October 2004; accepted 27 November 2004 Available online 2 February 2005
Abstract Accelerated aging of Nylon 6.6 fibers used in parachutes has been conducted by following the tensile strength loss under both thermaleoxidative and 100% relative humidity conditions. Thermaleoxidative 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 thermaleoxidative 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. Keywords: Nylon; Degradation; Aging; Humidity; Hydrolysis; Oxygen consumption
1. Introduction Certain systems use parachutes as a method of aerodynamic deceleration during deployment. Since individual parachutes may be stored untested at various locations for decades before being called upon to operate properly, understanding the aging effects on these parachutes is critical to gaining confidence in their long-term viability. An extensive aging study was therefore initiated on parachutes. Two important goals of this work are first, predicting parachute lifetimes under various environmental storage scenarios, and second, developing condition-monitoring techniques capable of assessing the state of field-aged materials.
* Corresponding author. Tel.: C1 505 284 3690; fax: C1 505 844 9624. E-mail address: [email protected] (R. Bernstein). 0141-3910/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.polymdegradstab.2004.11.020
Our initial objective is to determine the mechanisms and factors that influence aging of the parachute materials, a critical first step to making valid and reliable predictions. Since parachutes are stored in air environments ranging from hot and humid to cold and dry, we were particularly interested in determining the relative importance of hydrolysis and oxidation on the parachute materials. Nylon and Kevlar are the two organic materials primarily used in the construction of these parachutes. This paper will discuss results to date only from Nylon aging experiments. Ongoing experiments involving Kevlar will be published separately. Nylon is a common polymeric material used for a myriad of things, ranging from stockings, clothing, carpets, automotive components, hot air balloons, and parachutes. It has been extensively studied since it was first synthesized by Carothers at DuPont. Nylon degradation and aging studies throughout the years have led to a number of papers and reviews [1e21].
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In the literature there exists controversy and debate about not only how Nylon degrades, but also concerning the degradation products [6,7]. Numerous studies have been published that investigated high temperature thermal aging of Nylon [1,7,9,11]. Due to the long time required to perform such tests at lower temperatures, extremely high temperatures were typically utilized to bring the aging time scale into a practical regime. A major objective of the current study was to utilize lower temperatures extending over a greater time period to avoid the pitfalls associated with much longer high temperature Arrhenius extrapolations to room temperature. A danger associated with high temperature aging is the possibility of a change in mechanism from that occurring at lower temperatures. In such instances a simple Arrhenius extrapolation of the high temperature data could lead to incorrect predicted lifetimes at lower temperatures. The Nylon of specific interest for this study is Nylon 6.6. The simplified synthetic scheme involves the condensation polymerization of adipic acid and 1,6 hexanediamine (Scheme 1). Since the reverse of the formation reaction would involve simple hydrolysis of the amide bond, one of the goals of this study was to examine how water influenced the degradation of Nylon. Although many Nylon degradation studies focus on the thermal or photochemical aging, few discuss the role of humidity (water) in the degradation. Some elevated temperature studies that did examine humidity did so for samples submerged in water or in sealed vials where the humidity was determined at room temperature. These latter studies did not take into account the change in relative humidity at the elevated aging temperature [5,9]. Other studies examined the hydrolysis of polyamides in the presence of acids [22e28]. The primary focus of this paper involves the tensile strength loss of the Nylon as a function of time under different aging conditions. Although other physical parameters are important for proper parachute deployment and function, such as permeability and elasticity, tensile strength is arguably the most important and the easiest to monitor. The crux of lifetime predictions involves performing accelerated aging studies. Accelerated aging in this case means aging at elevated temperatures to provide data in H2N
a reasonable time frame. The classic approach to extrapolation of high temperature behavior to lower temperatures involves the Arrhenius equation most often applied to iso-damage results (log of time to a specified amount of damage plotted versus inverse absolute temperature). For a number of reasons described in the literature timeetemperature superposition is a better methodology [29]. In brief, timeetemperature superposition is predicted to occur for the data from different aging temperatures if each increase in temperature increases the overall degradation rate by a constant multiplicative factor (the assumed principle underlying accelerated aging). On a logarithmic time graph the various data sets at each temperature should therefore have the same shape. If the assumption is valid, the data from each elevated temperature can be superposed at a reference temperature by multiplying the aging time at the elevated temperature by a constant, referred to as the multiplicative shift factor aT (aT Z 1 by definition at the reference temperature). If the empirically derived shift factors lead to superposition of the data from all accelerated temperatures (verifying the constant acceleration assumption) they can then be plotted on an Arrhenius plot (log aT versus 1/T ) to see if a linear plot (Arrhenius behavior) results [29]. 2. Experimental The materials under investigation were 4448 N (1000 Pound) 5 cm (2 inch) wide Nylon straps, P2666 Mil-TSS333416 Type SV Class D supplied to Sandia National Laboratories from Bally Ribbon Mills, received in the late 1980’s. 2.1. Tensile tests Since measurements on Nylon straps were impractical due to the large size and high tensile strength values, all tensile studies were performed on yarns. Straps were aged under the various environments (thermaleoxidative, 100% RH with either argon or oxygen headspace gas) and periodically yarns in the warp direction of the straps were carefully removed to be tensile tested. The tensile tests were done on an Instron model 1000 using 2.54 cm
NH2
(1,6 -Hexanediamine)
H N
+ O
O OH
HO
O N H
H N O n
H2O
Nylon 6.6
O Adipic Acid Scheme 1.
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(1 inch) diameter capstans wrapped with two loops and the ends held by set screws. This avoided pinching of the sample by the Instron grips. Force at break was measured at a strain rate of 5.1 cm/min (2 inches/min) with an initial gauge length of 7.6 cm (3 inches). Each data point represents the average of approximately 10 yarn samples. Every time tensile measurements were made on aged samples, 10 control (virgin) yarns were also tested in order to generate a large data set for an average virgin tensile strength. The average Nylon yarn virgin sample force at break was 15.3 N (3.44 lbs) with an approximate average standard deviation of 0.44 N (0.1 lbs). With an average warp cross sectional area of 0.022 mm2 this leads to a tensile strength of w687 MPa.
Nylon samples were cut and weighed and placed loosely in the baskets. The baskets were placed inside the cans after deionised water was added to the cans. The cans were sealed by bolting on valve assemblies similar to those used in the oxygen consumption experiments, and then were placed in a freezer overnight. After freezing, the cans were quickly placed on a vacuum manifold, evacuated, and backfilled with the specified pressure of either argon or oxygen. The assemblies were left to thaw overnight and then placed in elevated temperature aging ovens.
3. Results/discussion 3.1. Thermaleoxidative
2.2. Oxygen consumption measurements Oxygen consumption samples were prepared by accurately weighing Nylon and then placing the samples in small (w5e20 ml) stainless steel vials. The vials, with welded mini-conflat flanges, were hermetically sealed by bolting onto mini-conflat flange/Swagelock 4BG stainless steel valve assembles using gold-coated copper gaskets. Samples were evacuated and then backfilled with the specified pressure of oxygen. Quantification of the amount of oxygen consumed was achieved by a commercial gas chromatograph connected to a gas manifold. Further details of the oxygen consumption procedure can be found in the literature [29,30]. 2.3. Humidity experiments Humidity experiments involved much larger stainless steel cans (ca. 600 ml). Excess water was placed in the bottom of the can to guarantee 100% RH at the aging temperature. An internal stainless steel sample basket supported by metal legs was used in order to keep the Nylon in the 100% RH atmosphere above the water.
It should be noted that all samples that were used in these studies were retrieved after being stored in a benign environment (indoors and in windowless cabinets) in Albuquerque for approximately two decades. Thermale oxidative accelerated aging studies at temperatures ranging from 37 C to 138 C constituted the initial phase of this program. The samples were aged in air circulating ovens in uncovered glass jars to minimize excessive airflow issues, but still allowing some degree of air circulation. The aging time ranged from weeks at high temperatures to years at the lowest temperatures. Periodically samples were removed from the ovens and the tensile strength measured. The thermaleoxidative data obtained thus far (138 C, 124 C, 109 C, 99 C, 95 C, 80 C, 64 C, 48 C, 37 C) are plotted in Fig. 1. By focusing on the portion of the curve critical to the application of the material (tensile strength loss up to w50%), we are able to get reasonable timeetemperature superposition (Fig. 2). An Arrhenius plot (log of shift factor, aT for the Nylon tensile strength versus inverse absolute temperature) reveals the possibility of curvature (Fig. 3).
Fig. 1. Percent tensile strength remaining versus days at temperature for thermaleoxidative aging of Nylon at various temperatures. (A) Linear time scale and (B) Log time scale. Legend is the same for both graphs.
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Fig. 2. Timeetemperature superposed data for Nylon, percent tensile strength remaining versus shifted time. Data are shifted to 109 C (reference temperature).
The line in the graph (a linear fit ignoring the 80 C and 64 C data) yields an activation energy of w77.3 kJ/mol (18.5 kcal/mol). Unfortunately, the results hinting at curvature come mainly from the 64 C data. Although the amount of degradation occurring after approximately three years at 64 C was fairly small (w10% drop in tensile strength) the drop was faster than originally anticipated based on the initial high temperature linear extrapolation. This possible deviation from linearity could be indicative of a curvature and a change in the mechanism at lower temperatures. Caution should be used in any interpretation of the shift factor for these incomplete data, and over the period of the next few years the validity of these data points will be verified. It should be noted that the data obtained at the two lowest temperatures (48 C and 37 C) were not included in the timeetemperature superposition analysis because there
Fig. 3. Arrhenius plot of the shift factors for Nylon tensile data. Log of the shift factors obtained in Fig. 2 plotted versus inverse absolute temperature.
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is an insufficient amount of degradation to generate a plausible shift factor. Interestingly in the shift factor graph (Fig. 3) the 95 C and 99 C data points appear to be reversed. This could stem from slight differences in airflow patterns between the two ovens where the temperature difference is not very large. Although the samples at these two temperatures were not different in colour, heterogeneity in aging was noticed in other sample sets due to distinct colour differences. Clearly this is a topic of great importance with regard to accelerated aging studies and warrants further investigation. One problem with this study is that aging temperatures are well above any that would be seen by a parachute in the field. Unfortunately, the changes in the tensile strength of the Nylon at lower aging temperatures are so small that it is well beyond the detection limit of our tensile technique. Therefore, there is no tensile information in the temperature range near room temperature and extrapolation of data with evidence of non-Arrhenius behavior would be problematic. Due to the extremely long time required to obtain tensile strength data at the lower temperatures, our oxygen consumption technique was used to examine the chemistry of thermaleoxidative aging both at high temperatures overlapping the tensile measurements and in the lower temperature (extrapolation) regime. Oxygen consumption is a technique that measures how much oxygen has chemically reacted with the polymer. This stems from the fact that thermaleoxidative degradation is driven by the reaction of materials with oxygen in the air. Oxygen consumption measurements have been a powerful tool allowing an understanding of the degradation in a time frame/ temperature region that is inaccessible to physical property measurements [29e35]. Oxygen consumption can first be checked to confirm its correlation with some physical property change in a region where they both can be measured. Then, if this correlation is confirmed, the sensitivity of oxygen consumption allows it to be used at much lower temperatures in regions where physical property changes cannot be measured. The technique, while quite detailed in practice, is quite simple in theory. It entails quantifying the amount of oxygen consumed by a polymer as it ages. The experiments involve placing a known amount of polymer into a sealed vial of known volume and filling it with a certain pressure of oxygen. The amount of oxygen remaining after aging is quantified via gas chromatography. In order to eliminate any influence of oxygen pressure on the rate of degradation, the partial pressure of oxygen in the vial is set such that the average partial pressure for the experiment at the aging temperature is approximately 17.3 kPa (130 torr) which is the approximate partial pressure of oxygen in our laboratory (Albuquerque, NM). This assures that during the oven
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exposure the sealed sample has an average oxygen pressure close to that of the accelerated aged tensile samples in the air circulating ovens. Knowledge of all the parameters from the start (fill pressure, volume, mass of polymer, etc.), coupled with the measured amount of oxygen after aging lead to the rate of oxygen consumption, which can then be processed to give the moles of oxygen consumed by the polymer versus aging time [29,30]. When calculating the rate of consumption the data point used for the time axis is the midpoint of the exposure; for the total moles consumed it is the total time of exposure. Results show that the oxygen consumption rate decreased as the aging temperature is decreased, and in most cases the rate of oxygen consumption decreased with time (Fig. 4). This is typical for many commercial polymers [30,32e35]. It should be noted that at the two highest temperatures (138 C and 124 C) the upswing of the oxygen consumption coincides with w50% tensile strength remaining. If 50% loss of tensile strength is arbitrarily chosen as a failure criterion, this implies that the rate of oxidation begins to increase near failure, similar to so called ‘‘induction time’’ behavior. The number of moles of oxygen consumed versus time can be calculated by integrating the consumption results leading to Fig. 5. The total moles of oxygen consumed can then be timeetemperature superposed (Fig. 6) to the same reference temperature used for the tensile strength (109 C) leading to the oxygen consumption shift factors shown in the figure. The resulting superposition is excellent and an Arrhenius plot of the oxygen consumption shift factors shows linearity throughout the temperature range examined, from 138 C to 37 C (Fig. 7). These observations provide evidence that the overall oxidation mechanism remains the same throughout the tempera-
Fig. 4. Rate of Nylon oxygen consumption versus time at the indicated temperatures.
Fig. 5. Total moles of oxygen consumed by Nylon versus aging time at the indicated aging temperatures.
ture range examined. The activation energy for the thermaleoxidation based on the oxygen consumption is ca. 117 kJ/mol (27.9 kcal/mol). Comparison of the Nylon shift factors for tensile strength and oxygen consumption (both normalized to 109 C) is quite revealing (Fig. 8). It appears that the shift factors have two different slopes. This means that the tensile strength loss must involve a mechanism that does not correlate directly with the total consumption of oxygen. 3.2. 100% RH argon Although thermaleoxidative damage is an expected degradation pathway, examining the chemistry of Nylon indicates that another pathway is also likely. As discussed earlier, Nylon is a polyamide with an amide bond as the main functional unit. Amide bonds are known to be susceptible to hydrolysis because it is the reverse of the formation reaction (Scheme 1). To assess the importance of the pure hydrolysis degradation
Fig. 6. Timeetemperature superposed data for moles oxygen consumed by Nylon versus shifted time. Data are shifted to 109 C (reference temperature).
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Fig. 7. Arrhenius plot of Nylon shift factors obtained from oxygen consumption data shifted to 109 C (reference temperature).
pathway, investigations were first conducted under a high relative humidity in the absence of oxygen. The initial experiments reported here were performed at 100% relative humidity (RH) due to ease of experimental setup; future work will involve lower relative humidities. Since a high temperature range was desired (O100 C), a sealed container was necessary to inhibit the water from evaporating. The sealed canister assured a constant 100% RH even at the elevated temperatures. Thus far at all temperatures, the time dependence of the drop in tensile strength appears to be relatively linear (Fig. 9A). When compared to the results for thermale oxidative oven aging at the same temperature (Fig. 1) it is clear that the Nylon samples exposed under purely hydrolysis conditions exhibited a greatly accelerated degradation rate. The 95 C data set could appear different in shape due to scatter, or could represent the beginning of a mechanism change at the lower temperatures. With the
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Fig. 8. Arrhenius plot of shift factors for oxygen consumption (triangles) and tensile strength (circles) both normalized to 109 C.
current data set the answer is unclear. Again emphasizing the results down to w50% loss of tensile strength, timeetemperature superposition of the data from 95 C to 138 C is shown in Fig. 10. The 80 C data are left out of the superposition graph due to the limited amount of data and the insignificant change in tensile strength which makes any shift factor questionable. Even though the scatter is fairly large at 95 C, an Arrhenius plot of the empirically derived shift factors suggests reasonable linearity (Fig. 11). Experiments at lower temperatures are underway, but due to the lengthy time scale required, they will not be completed for quite some time. The data as presented lead to an activation energy of w102 kJ/mol (24.3 kcal/mol) for 100% RH under an argon atmosphere. 3.3. 100% RH oxygen When high humidity is present during aging of the Nylon material in the field, air will also be available,
Fig. 9. Percent tensile strength remaining as a function of days aged at various temperatures for 100% RH plus argon atmospheres. (A) Linear time scale (lines are shown to guide the eye) and (B) Log time scale.
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Fig. 10. TimeeTemperature superposed tensile strength data for Nylon aged at 100% RH under an argon atmosphere. Data are shifted to 109 C (reference temperature).
implying that oxidation as well as hydrolysis degradation pathways will be operative. Carrying out accelerated aging experiments under the combined action of oxidation and hydrolysis is complex for several reasons. The main problem is assuring that the oxygen partial pressure is maintained near that of oxygen in air throughout the exposure. This can be difficult under high temperature and high humidity conditions due to the high vapour pressures of water operative at high temperatures. For instance, in flowing systems, oxygen and water vapour would have to be properly mixed and maintained throughout aging, not an easy situation. Our approach is to use a sealed system containing a reservoir of liquid water and to fill the container with oxygen such that the average pressure during the experiment will be close to ambient oxygen pressure. This is similar to the oxygen consumption experiments and those results are
Fig. 11. Arrhenius plot of Nylon tensile strength shift factors obtained from Fig. 10 for 100% RH under an argon atmosphere.
used in designing these experiments. However, since oxygen consumption may increase when humidity is present, we have initiated investigations attempting to quantify the oxygen consumed during humidity aging. At all the temperatures examined, the degradation in tensile strength in the 100% RH environment containing oxygen was approximately linear with respect to time (Fig. 12A) and therefore the curves at each temperature had similar shapes when plotted versus the logarithm of time (Fig. 12B). In this case there are sufficient data to get a good timeetemperature superposition (Fig. 13). An Arrhenius plot of the empirically derived shift factors gives reasonably linear results as seen in Fig. 14. An activation energy value of w94 kJ/mol (22.5 kcal/ mol) is calculated for the degradation of Nylon under 100% RH and w130 torr oxygen atmosphere. It should be noted that this activation energy is between the activation energy found for 100% RH under an argon atmosphere (102 kJ/mol) and the activation energy
Fig. 12. Percent tensile remaining as a function of days aged at various temperatures under 100% RH and an oxygen partial pressure averaging w130 torr. (A) Linear time scale (lines are shown to guide the eye) and (B) Log time scale. Legend is the same for both graphs.
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Fig. 13. Timeetemperature superposed tensile strength data for Nylon, at 100% RH under an oxygen atmosphere. Data are shifted to 109 C (reference temperature).
calculated for the thermaleoxidative aging (77.3 kJ/ mol). This could provide a clue to the studies underway attempting to better understand the mechanism(s) at work during the degradation. 3.4. Enhanced rate Upon analysis of the data it becomes quite apparent that the rate of degradation via a hydrolysis pathway proceeds at a much higher rate than thermaleoxidative. Inspection of the 124 C tensile data for all the aging conditions highlights this observation (Fig. 15). Examination of the rates of tensile strength change as a function of time under different conditions reveals that a dramatic increase in the ‘‘rate’’ (slope) of degradation occurs when both O2 and H2O are present. The slope of the argon and 100% RH result is w7 times higher than that of the thermaleoxidative slope,
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Fig. 14. Arrhenius plot of the shift factors for Nylon tensile strength data at 100% RH under an oxygen atmosphere of w130 torr. Log of the shift factors obtained in Fig. 13 plotted versus inverse absolute temperature.
whereas the slope of the oxygen and 100% RH result is w23 times higher. The data set for the 109 C, and 95 C experiments demonstrate similar patterns of behavior. This rate enhancement when both oxygen and water are present was not expected nor can be easily explained but is critical for predictions. The chemical reasons for this enhancement are currently under investigation.
4. Conclusions The rate of Nylon tensile strength loss is shown to be dramatically increased in 100% RH environments when compared to thermaleoxidative conditions. This is not surprising because hydrolysis is the known ‘textbook’ pathway for humidity degradation. It is also the reverse reaction for the simplified synthesis of Nylon.
Fig. 15. Nylon percent tensile strength remaining as a function of days exposed to 124 C: Air (circles), argon and 100% RH (squares), and ca. 130 torr oxygen and 100% RH (triangles). (A) Linear time scale and (B) Log time scale. Legend is the same for both graphs.
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The studies also reveal a significant interactive effect between oxidation and hydrolysis. This observation underscores the importance of assuring that approximately atmospheric amounts of oxygen are present during accelerated hydrolysis exposures. Ongoing studies at lower temperatures should yield more information and better predictive capabilities. Future work will involve lower relative humidities and attempt to better understand the chemistry and mechanistic relationship between oxygen and water that causes the rate enhancement.
5. Acknowledgements The authors thank Michelle M. Shedd, Alex R. Griego, and Mike Malone for their contributions in obtaining portions of the data. Also, Larry Whinery for helpful discussions on the topic of parachutes. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy, National Nuclear Security Administration under contract DE-AC0494AL85000.
References [1] Holland BJ, Hay JN. Thermal degradation of nylon polymers. Polym Int 2000;49:943e8. [2] Thanki PN, Singh RP. Progress in the area of degradation and stabilization of nylon 66. J Macromol Sci Rev Macromol 1998;C38:595e614. [3] Gupta MC, Jais AT. Effect of metal oxides on the kinetics of thermal oxidative degradation of nylon-6. Thermochim Acta 1993;230:155e65. [4] Lann PL, Derevjanik TS, Snyder JW, Ward WC. Nylon degradation with automatic transmission fluid. Thermochim Acta 2000;357:225e30. [5] Ishak ZAM, Berry JP. Hygrothermal aging studies of short carbon fiber reinforced nylon-6.6. J Appl Polym Sci 1994;51: 2145e55. [6] Levchik SV, Weil ED, Lewin M. Thermal decomposition of aliphatic nylons. Polym Int 1999;48:532e57. [7] Kamerbeek B, Kroes GH, Grolle W. Thermal degradation of some polyamides. Soc Chem Ind 1961;13:357. [8] Singh RP, Desai SM, Pathak G. Thermal decomposition kinetics of photo-oxidized nylon 66. J Appl Polym Sci 2003;87:2146e50. [9] Dorst HG, Pflueger R. Long term properties of reinforced nylon 6 and nylon 6.6. Technical papers (Society of Plastics Engineers) 1969;15:299e305. [10] Steppan DD, Doherty MF, Malone MF. A simplified degradation model for nylon 6,6 polymerization. J Appl Polym Sci 1991;42: 1009e21. [11] Peebles Jr LH, Huffman MW. Thermal degradation of nylon 66. J Polym Sci Polym Chem Ed 1971;9:1807e22. [12] Shamey R, Sinha K. A review of degradation of nylon 6.6 as a result of exposure to environmental conditions. Rev Prog Color Relate Top 2003;33:93e107.
[13] Wu C, Mantell SC, Davidson J. Polymers for solar domestic hot water: long-term performance of Pb and nylon 6,6 tubing in hot water. J Solar Energy Eng 2004;126:581e6. [14] Cerruti P, Carfagna C, Rychly J, Matisova-Rychla L. Chemiluminescence from oxidation of polyamide 6,6. I. The oxidation of pure polyamide. Polym Degrad Stab 2003;82:477e85. [15] Negoro S. Biodegradation of nylon and other synthetic polyamides. Biopolymers 2003;9:395e415. [16] Goodman I. The thermal degradation of 66 nylon. J Polym Sci 1954;13:175e8. [17] Herrera M, Wilhelm M, Matuschek G, Kettrup A. Thermoanalytical and pyrolysis studies of nitrogen containing polymers. J Anal Appl Pyrol 2001;58e59:173e88. [18] Sharkey WH, Mochel WE. Mechanism of the photo-oxidation of amides. J Am Chem Soc 1959;81:3000e5. [19] Boasson EH, Kamerbeek B, Algera A, Kroes GH. The photooxidation of nylon 6 and 66. Recl Trav Chim Pays-Bas 1962;81:624e34. [20] Kroes GH. Photooxidation of nylon 6 and 66. Recl Trav Chim Pays-Bas 1963;82:979e87. [21] Alam TM, Winters S, Bernstein R, Derzon D. Solution 17O NMR investigation of oxidation and hydrolysis in nylon. Polym Prepr (Am Chem Soc Div Polym Chem) 2003;44:247e8. [22] Cribbs D, Ogale AA. Hydrolytic degradation of nylon 66 pile carpet fibers. Text Res J 2003;73:98e104. [23] Serpe G, Chaupart N, Verdu J. Aging of polyamide 11 in acid solutions. Polymer 1997;38:1911e7. [24] Heikens D, Hermans PH, Veldhoven HA. Kinetics of the acid hydrolysis of cyclic oligomers from nylon 6 and nylon, 6,6. Makromol Chem 1959;30:154e68. [25] Chaupart N, Serpe G, Verdu J. Molecular weight distribution and mass changes during polyamide hydrolysis. Polymer 1998;39: 1375e80. [26] Hoshino K, Watanabe M. Synthetic polyamides. XVIII. Hydrolysis of polycapramide. J Am Chem Soc 1951;73:4816e8. [27] Ogale AA. Depolymerization of nylon 6: some kinetic modeling aspects. J Appl Polym Sci 1984;29:3947e54. [28] Heikens D. Equal reactivity of amide groups in acid hydrolysis of polyamides from epsilon-aminocaproic acid. J Polym Sci 1959;35:277e80. [29] Gillen KT, Celina M, Clough RL, Wise J. Extrapolation of accelerated aging data e Arrhenius or erroneous? Trends Polym Sci 1997;5:250e7. [30] Wise J, Gillen KT, Clough RL. An ultrasensitive technique for testing the Arrhenius extrapolation assumption for thermally aged elastomers. Polym Degrad Stab 1995;49:403e18. [31] Gillen KT, Celina M, Bernstein R. Validation of improved methods for predicting long-term elastomeric seal lifetimes from compression stresserelaxation and oxygen consumption techniques. Polym Degrad Stab 2003;82:25e35. [32] Celina M, Wise J, Ottesen DK, Gillen KT, Clough RL. Correlation of chemical and mechanical property changes during oxidative degradation of neoprene. Polym Degrad Stab 2000;68:171e84. [33] Celina M, Wise J, Ottesen DK, Gillen KT, Clough RL. Oxidation profiles of thermally aged nitrile rubber. Polym Degrad Stab 1998;60:493e504. [34] Celina M, Graham AC, Gillen KT, Assink RA, Minier LM. Thermal degradation studies of a polyurethane propellant binder. Rubber Chem Technol 2000;73:678e93. [35] Gillen KT, Bernstein R, Derzon DK. Evidence of non-Arrhenius behaviour from laboratory aging and 24-year field aging of polychloroprene rubber materials. Polym Degrad Stab 2005;87:57e67.
Nylon 6.6 accelerated aging studies: thermaleoxidative degradation and its interaction with hydrolysis Robert Bernstein*, Dora K. Derzon, Kenneth T. Gillen Sandia National Laboratories, Organic Materials Department, P. O. Box 5800, MS-0888, Albuquerque, NM 87185-0888, United States Received 28 June 2004; received in revised form 21 October 2004; accepted 27 November 2004 Available online 2 February 2005
Abstract Accelerated aging of Nylon 6.6 fibers used in parachutes has been conducted by following the tensile strength loss under both thermaleoxidative and 100% relative humidity conditions. Thermaleoxidative 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 thermaleoxidative 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. Keywords: Nylon; Degradation; Aging; Humidity; Hydrolysis; Oxygen consumption
1. Introduction Certain systems use parachutes as a method of aerodynamic deceleration during deployment. Since individual parachutes may be stored untested at various locations for decades before being called upon to operate properly, understanding the aging effects on these parachutes is critical to gaining confidence in their long-term viability. An extensive aging study was therefore initiated on parachutes. Two important goals of this work are first, predicting parachute lifetimes under various environmental storage scenarios, and second, developing condition-monitoring techniques capable of assessing the state of field-aged materials.
* Corresponding author. Tel.: C1 505 284 3690; fax: C1 505 844 9624. E-mail address: [email protected] (R. Bernstein). 0141-3910/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.polymdegradstab.2004.11.020
Our initial objective is to determine the mechanisms and factors that influence aging of the parachute materials, a critical first step to making valid and reliable predictions. Since parachutes are stored in air environments ranging from hot and humid to cold and dry, we were particularly interested in determining the relative importance of hydrolysis and oxidation on the parachute materials. Nylon and Kevlar are the two organic materials primarily used in the construction of these parachutes. This paper will discuss results to date only from Nylon aging experiments. Ongoing experiments involving Kevlar will be published separately. Nylon is a common polymeric material used for a myriad of things, ranging from stockings, clothing, carpets, automotive components, hot air balloons, and parachutes. It has been extensively studied since it was first synthesized by Carothers at DuPont. Nylon degradation and aging studies throughout the years have led to a number of papers and reviews [1e21].
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In the literature there exists controversy and debate about not only how Nylon degrades, but also concerning the degradation products [6,7]. Numerous studies have been published that investigated high temperature thermal aging of Nylon [1,7,9,11]. Due to the long time required to perform such tests at lower temperatures, extremely high temperatures were typically utilized to bring the aging time scale into a practical regime. A major objective of the current study was to utilize lower temperatures extending over a greater time period to avoid the pitfalls associated with much longer high temperature Arrhenius extrapolations to room temperature. A danger associated with high temperature aging is the possibility of a change in mechanism from that occurring at lower temperatures. In such instances a simple Arrhenius extrapolation of the high temperature data could lead to incorrect predicted lifetimes at lower temperatures. The Nylon of specific interest for this study is Nylon 6.6. The simplified synthetic scheme involves the condensation polymerization of adipic acid and 1,6 hexanediamine (Scheme 1). Since the reverse of the formation reaction would involve simple hydrolysis of the amide bond, one of the goals of this study was to examine how water influenced the degradation of Nylon. Although many Nylon degradation studies focus on the thermal or photochemical aging, few discuss the role of humidity (water) in the degradation. Some elevated temperature studies that did examine humidity did so for samples submerged in water or in sealed vials where the humidity was determined at room temperature. These latter studies did not take into account the change in relative humidity at the elevated aging temperature [5,9]. Other studies examined the hydrolysis of polyamides in the presence of acids [22e28]. The primary focus of this paper involves the tensile strength loss of the Nylon as a function of time under different aging conditions. Although other physical parameters are important for proper parachute deployment and function, such as permeability and elasticity, tensile strength is arguably the most important and the easiest to monitor. The crux of lifetime predictions involves performing accelerated aging studies. Accelerated aging in this case means aging at elevated temperatures to provide data in H2N
a reasonable time frame. The classic approach to extrapolation of high temperature behavior to lower temperatures involves the Arrhenius equation most often applied to iso-damage results (log of time to a specified amount of damage plotted versus inverse absolute temperature). For a number of reasons described in the literature timeetemperature superposition is a better methodology [29]. In brief, timeetemperature superposition is predicted to occur for the data from different aging temperatures if each increase in temperature increases the overall degradation rate by a constant multiplicative factor (the assumed principle underlying accelerated aging). On a logarithmic time graph the various data sets at each temperature should therefore have the same shape. If the assumption is valid, the data from each elevated temperature can be superposed at a reference temperature by multiplying the aging time at the elevated temperature by a constant, referred to as the multiplicative shift factor aT (aT Z 1 by definition at the reference temperature). If the empirically derived shift factors lead to superposition of the data from all accelerated temperatures (verifying the constant acceleration assumption) they can then be plotted on an Arrhenius plot (log aT versus 1/T ) to see if a linear plot (Arrhenius behavior) results [29]. 2. Experimental The materials under investigation were 4448 N (1000 Pound) 5 cm (2 inch) wide Nylon straps, P2666 Mil-TSS333416 Type SV Class D supplied to Sandia National Laboratories from Bally Ribbon Mills, received in the late 1980’s. 2.1. Tensile tests Since measurements on Nylon straps were impractical due to the large size and high tensile strength values, all tensile studies were performed on yarns. Straps were aged under the various environments (thermaleoxidative, 100% RH with either argon or oxygen headspace gas) and periodically yarns in the warp direction of the straps were carefully removed to be tensile tested. The tensile tests were done on an Instron model 1000 using 2.54 cm
NH2
(1,6 -Hexanediamine)
H N
+ O
O OH
HO
O N H
H N O n
H2O
Nylon 6.6
O Adipic Acid Scheme 1.
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(1 inch) diameter capstans wrapped with two loops and the ends held by set screws. This avoided pinching of the sample by the Instron grips. Force at break was measured at a strain rate of 5.1 cm/min (2 inches/min) with an initial gauge length of 7.6 cm (3 inches). Each data point represents the average of approximately 10 yarn samples. Every time tensile measurements were made on aged samples, 10 control (virgin) yarns were also tested in order to generate a large data set for an average virgin tensile strength. The average Nylon yarn virgin sample force at break was 15.3 N (3.44 lbs) with an approximate average standard deviation of 0.44 N (0.1 lbs). With an average warp cross sectional area of 0.022 mm2 this leads to a tensile strength of w687 MPa.
Nylon samples were cut and weighed and placed loosely in the baskets. The baskets were placed inside the cans after deionised water was added to the cans. The cans were sealed by bolting on valve assemblies similar to those used in the oxygen consumption experiments, and then were placed in a freezer overnight. After freezing, the cans were quickly placed on a vacuum manifold, evacuated, and backfilled with the specified pressure of either argon or oxygen. The assemblies were left to thaw overnight and then placed in elevated temperature aging ovens.
3. Results/discussion 3.1. Thermaleoxidative
2.2. Oxygen consumption measurements Oxygen consumption samples were prepared by accurately weighing Nylon and then placing the samples in small (w5e20 ml) stainless steel vials. The vials, with welded mini-conflat flanges, were hermetically sealed by bolting onto mini-conflat flange/Swagelock 4BG stainless steel valve assembles using gold-coated copper gaskets. Samples were evacuated and then backfilled with the specified pressure of oxygen. Quantification of the amount of oxygen consumed was achieved by a commercial gas chromatograph connected to a gas manifold. Further details of the oxygen consumption procedure can be found in the literature [29,30]. 2.3. Humidity experiments Humidity experiments involved much larger stainless steel cans (ca. 600 ml). Excess water was placed in the bottom of the can to guarantee 100% RH at the aging temperature. An internal stainless steel sample basket supported by metal legs was used in order to keep the Nylon in the 100% RH atmosphere above the water.
It should be noted that all samples that were used in these studies were retrieved after being stored in a benign environment (indoors and in windowless cabinets) in Albuquerque for approximately two decades. Thermale oxidative accelerated aging studies at temperatures ranging from 37 C to 138 C constituted the initial phase of this program. The samples were aged in air circulating ovens in uncovered glass jars to minimize excessive airflow issues, but still allowing some degree of air circulation. The aging time ranged from weeks at high temperatures to years at the lowest temperatures. Periodically samples were removed from the ovens and the tensile strength measured. The thermaleoxidative data obtained thus far (138 C, 124 C, 109 C, 99 C, 95 C, 80 C, 64 C, 48 C, 37 C) are plotted in Fig. 1. By focusing on the portion of the curve critical to the application of the material (tensile strength loss up to w50%), we are able to get reasonable timeetemperature superposition (Fig. 2). An Arrhenius plot (log of shift factor, aT for the Nylon tensile strength versus inverse absolute temperature) reveals the possibility of curvature (Fig. 3).
Fig. 1. Percent tensile strength remaining versus days at temperature for thermaleoxidative aging of Nylon at various temperatures. (A) Linear time scale and (B) Log time scale. Legend is the same for both graphs.
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Fig. 2. Timeetemperature superposed data for Nylon, percent tensile strength remaining versus shifted time. Data are shifted to 109 C (reference temperature).
The line in the graph (a linear fit ignoring the 80 C and 64 C data) yields an activation energy of w77.3 kJ/mol (18.5 kcal/mol). Unfortunately, the results hinting at curvature come mainly from the 64 C data. Although the amount of degradation occurring after approximately three years at 64 C was fairly small (w10% drop in tensile strength) the drop was faster than originally anticipated based on the initial high temperature linear extrapolation. This possible deviation from linearity could be indicative of a curvature and a change in the mechanism at lower temperatures. Caution should be used in any interpretation of the shift factor for these incomplete data, and over the period of the next few years the validity of these data points will be verified. It should be noted that the data obtained at the two lowest temperatures (48 C and 37 C) were not included in the timeetemperature superposition analysis because there
Fig. 3. Arrhenius plot of the shift factors for Nylon tensile data. Log of the shift factors obtained in Fig. 2 plotted versus inverse absolute temperature.
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is an insufficient amount of degradation to generate a plausible shift factor. Interestingly in the shift factor graph (Fig. 3) the 95 C and 99 C data points appear to be reversed. This could stem from slight differences in airflow patterns between the two ovens where the temperature difference is not very large. Although the samples at these two temperatures were not different in colour, heterogeneity in aging was noticed in other sample sets due to distinct colour differences. Clearly this is a topic of great importance with regard to accelerated aging studies and warrants further investigation. One problem with this study is that aging temperatures are well above any that would be seen by a parachute in the field. Unfortunately, the changes in the tensile strength of the Nylon at lower aging temperatures are so small that it is well beyond the detection limit of our tensile technique. Therefore, there is no tensile information in the temperature range near room temperature and extrapolation of data with evidence of non-Arrhenius behavior would be problematic. Due to the extremely long time required to obtain tensile strength data at the lower temperatures, our oxygen consumption technique was used to examine the chemistry of thermaleoxidative aging both at high temperatures overlapping the tensile measurements and in the lower temperature (extrapolation) regime. Oxygen consumption is a technique that measures how much oxygen has chemically reacted with the polymer. This stems from the fact that thermaleoxidative degradation is driven by the reaction of materials with oxygen in the air. Oxygen consumption measurements have been a powerful tool allowing an understanding of the degradation in a time frame/ temperature region that is inaccessible to physical property measurements [29e35]. Oxygen consumption can first be checked to confirm its correlation with some physical property change in a region where they both can be measured. Then, if this correlation is confirmed, the sensitivity of oxygen consumption allows it to be used at much lower temperatures in regions where physical property changes cannot be measured. The technique, while quite detailed in practice, is quite simple in theory. It entails quantifying the amount of oxygen consumed by a polymer as it ages. The experiments involve placing a known amount of polymer into a sealed vial of known volume and filling it with a certain pressure of oxygen. The amount of oxygen remaining after aging is quantified via gas chromatography. In order to eliminate any influence of oxygen pressure on the rate of degradation, the partial pressure of oxygen in the vial is set such that the average partial pressure for the experiment at the aging temperature is approximately 17.3 kPa (130 torr) which is the approximate partial pressure of oxygen in our laboratory (Albuquerque, NM). This assures that during the oven
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exposure the sealed sample has an average oxygen pressure close to that of the accelerated aged tensile samples in the air circulating ovens. Knowledge of all the parameters from the start (fill pressure, volume, mass of polymer, etc.), coupled with the measured amount of oxygen after aging lead to the rate of oxygen consumption, which can then be processed to give the moles of oxygen consumed by the polymer versus aging time [29,30]. When calculating the rate of consumption the data point used for the time axis is the midpoint of the exposure; for the total moles consumed it is the total time of exposure. Results show that the oxygen consumption rate decreased as the aging temperature is decreased, and in most cases the rate of oxygen consumption decreased with time (Fig. 4). This is typical for many commercial polymers [30,32e35]. It should be noted that at the two highest temperatures (138 C and 124 C) the upswing of the oxygen consumption coincides with w50% tensile strength remaining. If 50% loss of tensile strength is arbitrarily chosen as a failure criterion, this implies that the rate of oxidation begins to increase near failure, similar to so called ‘‘induction time’’ behavior. The number of moles of oxygen consumed versus time can be calculated by integrating the consumption results leading to Fig. 5. The total moles of oxygen consumed can then be timeetemperature superposed (Fig. 6) to the same reference temperature used for the tensile strength (109 C) leading to the oxygen consumption shift factors shown in the figure. The resulting superposition is excellent and an Arrhenius plot of the oxygen consumption shift factors shows linearity throughout the temperature range examined, from 138 C to 37 C (Fig. 7). These observations provide evidence that the overall oxidation mechanism remains the same throughout the tempera-
Fig. 4. Rate of Nylon oxygen consumption versus time at the indicated temperatures.
Fig. 5. Total moles of oxygen consumed by Nylon versus aging time at the indicated aging temperatures.
ture range examined. The activation energy for the thermaleoxidation based on the oxygen consumption is ca. 117 kJ/mol (27.9 kcal/mol). Comparison of the Nylon shift factors for tensile strength and oxygen consumption (both normalized to 109 C) is quite revealing (Fig. 8). It appears that the shift factors have two different slopes. This means that the tensile strength loss must involve a mechanism that does not correlate directly with the total consumption of oxygen. 3.2. 100% RH argon Although thermaleoxidative damage is an expected degradation pathway, examining the chemistry of Nylon indicates that another pathway is also likely. As discussed earlier, Nylon is a polyamide with an amide bond as the main functional unit. Amide bonds are known to be susceptible to hydrolysis because it is the reverse of the formation reaction (Scheme 1). To assess the importance of the pure hydrolysis degradation
Fig. 6. Timeetemperature superposed data for moles oxygen consumed by Nylon versus shifted time. Data are shifted to 109 C (reference temperature).
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Fig. 7. Arrhenius plot of Nylon shift factors obtained from oxygen consumption data shifted to 109 C (reference temperature).
pathway, investigations were first conducted under a high relative humidity in the absence of oxygen. The initial experiments reported here were performed at 100% relative humidity (RH) due to ease of experimental setup; future work will involve lower relative humidities. Since a high temperature range was desired (O100 C), a sealed container was necessary to inhibit the water from evaporating. The sealed canister assured a constant 100% RH even at the elevated temperatures. Thus far at all temperatures, the time dependence of the drop in tensile strength appears to be relatively linear (Fig. 9A). When compared to the results for thermale oxidative oven aging at the same temperature (Fig. 1) it is clear that the Nylon samples exposed under purely hydrolysis conditions exhibited a greatly accelerated degradation rate. The 95 C data set could appear different in shape due to scatter, or could represent the beginning of a mechanism change at the lower temperatures. With the
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Fig. 8. Arrhenius plot of shift factors for oxygen consumption (triangles) and tensile strength (circles) both normalized to 109 C.
current data set the answer is unclear. Again emphasizing the results down to w50% loss of tensile strength, timeetemperature superposition of the data from 95 C to 138 C is shown in Fig. 10. The 80 C data are left out of the superposition graph due to the limited amount of data and the insignificant change in tensile strength which makes any shift factor questionable. Even though the scatter is fairly large at 95 C, an Arrhenius plot of the empirically derived shift factors suggests reasonable linearity (Fig. 11). Experiments at lower temperatures are underway, but due to the lengthy time scale required, they will not be completed for quite some time. The data as presented lead to an activation energy of w102 kJ/mol (24.3 kcal/mol) for 100% RH under an argon atmosphere. 3.3. 100% RH oxygen When high humidity is present during aging of the Nylon material in the field, air will also be available,
Fig. 9. Percent tensile strength remaining as a function of days aged at various temperatures for 100% RH plus argon atmospheres. (A) Linear time scale (lines are shown to guide the eye) and (B) Log time scale.
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Fig. 10. TimeeTemperature superposed tensile strength data for Nylon aged at 100% RH under an argon atmosphere. Data are shifted to 109 C (reference temperature).
implying that oxidation as well as hydrolysis degradation pathways will be operative. Carrying out accelerated aging experiments under the combined action of oxidation and hydrolysis is complex for several reasons. The main problem is assuring that the oxygen partial pressure is maintained near that of oxygen in air throughout the exposure. This can be difficult under high temperature and high humidity conditions due to the high vapour pressures of water operative at high temperatures. For instance, in flowing systems, oxygen and water vapour would have to be properly mixed and maintained throughout aging, not an easy situation. Our approach is to use a sealed system containing a reservoir of liquid water and to fill the container with oxygen such that the average pressure during the experiment will be close to ambient oxygen pressure. This is similar to the oxygen consumption experiments and those results are
Fig. 11. Arrhenius plot of Nylon tensile strength shift factors obtained from Fig. 10 for 100% RH under an argon atmosphere.
used in designing these experiments. However, since oxygen consumption may increase when humidity is present, we have initiated investigations attempting to quantify the oxygen consumed during humidity aging. At all the temperatures examined, the degradation in tensile strength in the 100% RH environment containing oxygen was approximately linear with respect to time (Fig. 12A) and therefore the curves at each temperature had similar shapes when plotted versus the logarithm of time (Fig. 12B). In this case there are sufficient data to get a good timeetemperature superposition (Fig. 13). An Arrhenius plot of the empirically derived shift factors gives reasonably linear results as seen in Fig. 14. An activation energy value of w94 kJ/mol (22.5 kcal/ mol) is calculated for the degradation of Nylon under 100% RH and w130 torr oxygen atmosphere. It should be noted that this activation energy is between the activation energy found for 100% RH under an argon atmosphere (102 kJ/mol) and the activation energy
Fig. 12. Percent tensile remaining as a function of days aged at various temperatures under 100% RH and an oxygen partial pressure averaging w130 torr. (A) Linear time scale (lines are shown to guide the eye) and (B) Log time scale. Legend is the same for both graphs.
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Fig. 13. Timeetemperature superposed tensile strength data for Nylon, at 100% RH under an oxygen atmosphere. Data are shifted to 109 C (reference temperature).
calculated for the thermaleoxidative aging (77.3 kJ/ mol). This could provide a clue to the studies underway attempting to better understand the mechanism(s) at work during the degradation. 3.4. Enhanced rate Upon analysis of the data it becomes quite apparent that the rate of degradation via a hydrolysis pathway proceeds at a much higher rate than thermaleoxidative. Inspection of the 124 C tensile data for all the aging conditions highlights this observation (Fig. 15). Examination of the rates of tensile strength change as a function of time under different conditions reveals that a dramatic increase in the ‘‘rate’’ (slope) of degradation occurs when both O2 and H2O are present. The slope of the argon and 100% RH result is w7 times higher than that of the thermaleoxidative slope,
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Fig. 14. Arrhenius plot of the shift factors for Nylon tensile strength data at 100% RH under an oxygen atmosphere of w130 torr. Log of the shift factors obtained in Fig. 13 plotted versus inverse absolute temperature.
whereas the slope of the oxygen and 100% RH result is w23 times higher. The data set for the 109 C, and 95 C experiments demonstrate similar patterns of behavior. This rate enhancement when both oxygen and water are present was not expected nor can be easily explained but is critical for predictions. The chemical reasons for this enhancement are currently under investigation.
4. Conclusions The rate of Nylon tensile strength loss is shown to be dramatically increased in 100% RH environments when compared to thermaleoxidative conditions. This is not surprising because hydrolysis is the known ‘textbook’ pathway for humidity degradation. It is also the reverse reaction for the simplified synthesis of Nylon.
Fig. 15. Nylon percent tensile strength remaining as a function of days exposed to 124 C: Air (circles), argon and 100% RH (squares), and ca. 130 torr oxygen and 100% RH (triangles). (A) Linear time scale and (B) Log time scale. Legend is the same for both graphs.
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The studies also reveal a significant interactive effect between oxidation and hydrolysis. This observation underscores the importance of assuring that approximately atmospheric amounts of oxygen are present during accelerated hydrolysis exposures. Ongoing studies at lower temperatures should yield more information and better predictive capabilities. Future work will involve lower relative humidities and attempt to better understand the chemistry and mechanistic relationship between oxygen and water that causes the rate enhancement.
5. Acknowledgements The authors thank Michelle M. Shedd, Alex R. Griego, and Mike Malone for their contributions in obtaining portions of the data. Also, Larry Whinery for helpful discussions on the topic of parachutes. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy, National Nuclear Security Administration under contract DE-AC0494AL85000.
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