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Non-Arrhenius behavior for oxidative degradation of chlorosulfonated polyethylene materials
Polymer Degradation and Stability 87 (2005) 335e346 www.elsevier.com/locate/polydegstab
Non-Arrhenius behavior for oxidative degradation of chlorosulfonated polyethylene materials Kenneth T. Gillen*, Robert Bernstein, Mathew Celina Sandia National Laboratories, Organic Materials Department, Box 5800, MS-1411, Albuquerque, NM 87185-1411, United States Received 10 August 2004; accepted 18 September 2004
Abstract We have carried out oven aging studies on eight different commercial chlorosulfonated polyethylene cable jacket materials at temperatures ranging from 80 C to 150 C utilizing ultimate tensile elongation as the degradation parameter. For each material, the elongation results were timeetemperature superposed at the lowest aging temperature. When the resulting empirical shift factors were tested for Arrhenius behavior, it was found that the eight materials were Arrhenius at w100 C and higher with very similar activation energies averaging w107 kJ/mol. Longer-term aging results at temperatures lower than 100 C for three of the materials provided evidence for curvature to lower activation energies. For one of these materials, we conducted oxidation rate measurements at six temperatures ranging from 37 C to 108 C. The results offered further evidence for a small drop in activation energy below 100 C. Chemical evidence supporting this change in activation energy was derived from analysis of the production rates of CO2 during oxidation. As the temperature was lowered, the amount of CO2 produced relative to the O2 consumed dropped substantially, implying that the chemistry leading to CO2 becomes less important at lower temperatures. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Aging; Lifetime prediction; Chlorosulfonated polyethylene rubber; Arrhenius extrapolation; Oxygen consumption
1. Introduction Predicting polymer lifetimes has been a continuing important objective for most industrial applications. The most commonly used prediction method involves the so-called Arrhenius approach that assumes that an overall chemical process controls degradation where the rate of deterioration is proportional to exp(Ea/RT ). In this expression, Ea is the Arrhenius activation energy, R is the gas constant and T is the absolute temperature. Data are analyzed under accelerated aging conditions in order to estimate the value of Ea
* Corresponding author. Tel.: C1 5058447494; fax: C1 505 8449781. E-mail address: [email protected] (K.T. Gillen). 0141-3910/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2004.09.004
appropriate to the accelerated conditions. This value of Ea is then assumed to be unchanged at temperatures lower than the accelerated temperature regime, allowing extrapolated predictions at lower temperatures (longer times). Given the extremely complex set of reactions that underlies the degradation of most polymeric materials and the expectation that each reaction will have a different activation energy, it is perhaps naı¨ ve to expect a single Ea value to be valid over an extended temperature range [1]. Unfortunately most accelerated aging studies only access a fairly narrow temperature regime over which it is difficult to observe a change in Ea relative to the uncertainties in the data. Even so, there have been numerous literature examples of nonArrhenius behavior, typically involving a drop in Ea at lower temperatures. Richters observed curvature
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to lower Ea values (‘‘downward curvature’’) when monitoring the oxygen induction period for polypropylene [2]. Similar curvature to lower Ea values was observed for polyethylene materials by Bell Labs workers [3,4]. Bernstein and Lee presented evidence for downward curvature for oven-aged samples of HDPE insulated cables [5]. More recent studies by Gijsman et al. on unstabilized polypropylene [6] and by Rosik et al. on a stabilized ABS polymer [7] observed downward curvature effects. Gugumus recently found evidence of both downward and upward curvature for polypropylene films [8]. The upward curvature was tentatively attributed to titanium complexation by the phenolic antioxidant leading to possible passivation of the transition metal. Except for anomalous effects caused by diffusion-limited oxidation [9], upward curvature is seldom observed. We have been interested for many years in the development of improved methods for analyzing and extrapolating accelerated aging studies [9e16] and have found numerous examples of downward curvature in the Arrhenius plots of both mechanical property [13,15,16] and oxygen consumption [9,11,13e16] data. One of our current interests involves the aging of nuclear power plant safety cables. Such cables can be exposed to fairly high temperature (and occasionally radiation) environments for 40 years or more. If an accident such as a loss of coolant accident (LOCA) occurred, the aged cable must possess sufficient mechanical properties for proper operation during the accident. Predicting the mechanical lifetimes of such cables typically involves the Arrhenius extrapolation of high temperature accelerated aging results. The purpose of the current studies is to test the Arrhenius assumption on eight different commercial chlorosulfonated polyethylene (CSPE) cable jacketing materials to derive better lifetime extrapolations. CSPE jackets are the most commonly used jackets for nuclear power plant safety related low-voltage cables and the eight materials under current study came from cables that are commonly found in US nuclear power plants.
2. Experimental 2.1. Materials Chlorosulfonated polyethylene (CSPE) is obtained via the simultaneous chlorination and chlorosulfonation of polyethylene [17]. It is a polymer that consists of a modified polyethylene backbone with chloro and sulfonylchloride side groups. Crosslinking can be achieved with different curing methods (i.e. sulfur, peroxides, maleimide, and others) to produce a commercial generic ‘‘HypalonÒ’’ rubber. CSPE materials are generally regarded as having good toughness, weatherability, and resistance to oxidation and oil/solvent exposure. At low-chlorine levels some CSPE grades can retain some polyethylene-like properties such as hardness, stiffness and partial crystallinity [17]. The eight CSPE jacket materials currently studied were removed before aging from commercially produced low-voltage cables that are qualified for nuclear power plant safety applications. Table 1 gives details on the manufacturer, the nominal jacket thickness and initial values for tensile properties, modulus [18] and density. Also shown in the table are the abbreviations used for the materials for the remainder of the paper. CSPE-1 represent the thin inner jacket surrounding EPR insulation on individual conductors of a multi-conductor cable whereas all of the remaining materials are the thicker overall jackets from multi-conductor cables. For the tensile testing samples, strips approximately 6.4 mm wide by w150 mm long were cut from the cable jackets before oven aging.
2.2. Oven aging Oven aging was carried out in air-circulating ovens controlled to approximately G1 C. Reported temperatures are averages from calibrated thermocouples located close to the samples and connected to continuous strip-chart recorders. Samples were protected from direct air-flow.
Table 1 Summary of the CSPE cable jacketing materials Cable description
Abbreviation
~Thickness (mm)
Ultimate tensile elongation (%)
Ultimate tensile strength (MPa)
Modulus (MPa)
Density (g/cc)
Anaconda Flameguard Anaconda Flameguard BIW Brandrex Eaton Dekoron Kerite Rockbestos Firewall III Samuel Moore Dekoron
CSPE-1 CSPE-2 CSPE-3 CSPE-4 CSPE-5 CSPE-6 CSPE-7 CSPE-8
0.4 1.8 1.7 1.6 1.4 1.5 1.3 1.2
393 263 257 380 316 300 364 381
15 13 13 16 20 22 16 20
5.3 4.8 8.7 5.1 18 9 4.1 14
1.64 1.58 1.47 1.37 1.47 1.57 1.60 1.46
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2.3. Ultimate tensile elongation tests Tensile tests were performed on an Instron Table Model testing machine equipped with pneumatic grips and having an extensometer clamped to the sample. Testing was conducted at a strain rate of 12.7 cm/min with an initial jaw separation of 5.1 cm. Three repeat samples were typically tested for each aging condition (time and temperature) with the average value of the ultimate tensile elongation reported. 2.4. Oxygen consumption and CO2 production rate measurements A detailed description of the procedures used for these measurements has been described in earlier publications [10,16]. Briefly, these measurements are made by initially placing a known weight of polymer with a known initial quantity of oxygen in a sealed container. After oven aging for a selected period of time, the gas in the container is analyzed for oxygen, CO2 and CO using gas chromatography techniques. The amount of oxygen initially sealed in the container is chosen such that its initial partial pressure at the oven aging temperature is w150 torr and the aging time chosen so that the expected final oxygen partial pressure (at elevated temperature) is w110 torr. The intent of this approach is to have the average oxygen partial pressure during aging approximate the partial pressure of oxygen in Albuquerque (132 torr) so that the oxygen consumption reflects the oxidation appropriate during air-oven aging. After aging, the gas in the container is quantitatively analyzed for the oxygen consumption rate and for the production rates of CO2 and CO. The sample container is then pumped out again for several days (to remove dissolved gases in the polymer), backfilled again with the same initial partial pressure of oxygen and aged for another time interval. By repeating this procedure several times, the time dependence of the rates is obtained as well as the integrated rate (total oxygen consumption) versus time.
component [17]. The DSC scans on six materials were flat and therefore showed no evidence of any crystallinity. DSC scans for the other two materials (CSPE-5 and CSPE-8) showed evidence of minor amounts of some crystalline component around 55 C (Fig. 1) where the melting endotherm is relatively small in comparison with other semi-crystalline PE-based cable materials [19]. Although this temperature region is below the normal region expected for polyethylene crystalline components, the presence of a small crystalline component is the likely reason for the higher modulus values found for these two materials (Table 1).
3. Results and discussion 3.1. Arrhenius analyzes of tensile elongation results For degradation of cable jacketing materials, changes in mechanical properties are of primary interest with most studies focusing on drops in the tensile elongation. With this in mind we carried out accelerated oven aging experiments on the eight CSPE materials (using three to five different aging temperatures for each material) and monitored changes in tensile elongation. Typical average elongation results versus temperature are shown in Fig. 2 for CSPE-6. For this material, data are shown for five aging temperatures ranging from 91 C to 130 C (the unfilled symbols). Since significant degradation takes more than 2 years at 91 C, monitoring of mechanical properties below this temperature would require aging times much longer than are typically available. The data in Fig. 2 are plotted versus log of the aging time since this allows us to visually determine how the shapes of the curves compare as a function of temperature. When raising the aging temperature in order to accelerate degradation, the fundamental underlying assumption is that all reactions are accelerated equally. This constant acceleration (independent of the 0.3
2.5. DSC runs Endotherm (W/g)
Differential scanning calorimetry was carried out on w5 mg samples of each material using a Perkin Elmer DSC 6 calibrated with indium and zinc standards. Runs were conducted under 20 cc/min argon gas flow from 10 C to 180 C at a scan rate of 5 C/min. Commercial CSPE materials used for cables and wires are usually based on either Hypalon-45 (24% chlorine content, density of 1.08 g/cc) or Hypalon-40 (35% chlorine content, density of 1.18 g/cc). Due to the higher polyethylene content, formulations based on Hypalon-45 are semi-crystalline whereas those based on Hypalon-40 (or Hypalon-40S) show no crystalline
CSPE-5 CSPE-8
0.2
0.1
0.0
-0.1 0
20
40
60
80
100
120 140
160
180
Temperature [°C] Fig. 1. DSC scans for the two indicated CSPE jacketing materials.
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aT*t (shifted aging time), days at 100°C
300 300
10
100
1000
250
200
Elongation, %
Elongation, %
250
200
150 91°C
100
101°C
2.8x
111°C
50
120°C
100
130°C 101°C (superposed)
0 100
150
101
50
102
103
Aging time, days Fig. 2. Elongation versus time at the indicated temperatures for CSPE6 plus superposed 101 C data (filled triangles) showing procedure underlying timeetemperature superposition (the 101 C data has been multiplicatively shifted with aT Z 2.8).
level of degradation) assumption implies that the degradation curves at differing aging temperatures will have the same shape when plotted versus log of the aging time. This appears to be a reasonable assumption for the results shown in Fig. 2. To determine the relationship between degradation and temperature we apply the principal of timeetemperature superposition [9e16] to the data by first selecting a reference temperature Tref which is typically the lowest experimental temperature (91 C in the present case). We then take the results at a second temperature T and find the constant multiplicative shift factor aT such that multiplying the times associated with the second temperature by aT gives the best overlap (superposition) of the data from T and Tref. An example of this procedure is shown in Fig. 2 where the 101 C data (open triangles) are shifted to the right (filled triangles) by a multiplicative shift factor aT Z 2.8 in order to best overlap the 91 C results. After this procedure is completed on the 101 C data, the 111 C data are then shifted such that it has the best overlap with the superposed results from 91 C and 101 C (aT Z 6.1 works best in this case). Data at the remaining temperatures are then shifted in a similar fashion leading to their aT values. By definition aT Z 1 for the 91 C reference temperature since the times at this temperature remain constant (multiplied by unity). The resulting superposition plus the empirical shift factors used to achieve superposition for the CSPE-6 data of Fig. 2 are shown in Fig. 3 (lower x-axis). The superposition is excellent as expected given the similarities in curve shapes observed for the five aging temperatures (Fig. 2).
0 10-2
T, °C
aT
91
1
101
2.8
111
6.1
120
15
130
27.5
10-1
100
aT*t (shifted aging time), years at 91°C
101
Fig. 3. Timeetemperature superposition of the elongation results from Fig. 2 at 91 C (lower x-axis). By interpolation of the shift factor results, aT at 100 C is 2.5 which leads to the time scale at 100 C shown on the upper x-axis.
A typical Arrhenius analysis would involve first choosing a ‘‘failure’’ criterion for the data on Fig. 2 (e.g., the time to reach 50% absolute elongation) then determining the times required at each temperature to reach this value. These times would then be plotted on an Arrhenius plot (log of time versus inverse absolute temperature) to see if straight-line (Arrhenius) behavior resulted. This procedure would thus use only one processed datum point per temperature. The timee temperature superposition approach (Fig. 3) on the other hand utilizes every experimental data point in the analyzes. The empirically derived shift factors can then be plotted on an Arrhenius plot to see if Arrhenius behavior is indicated. Such an Arrhenius plot is shown in Fig. 4, where reasonable Arrhenius behavior with an Arrhenius activation energy Ea of w106 kJ/mol is found. If the same Arrhenius behavior remains unchanged at lower aging temperatures, the observed line can be extrapolated as shown allowing predictions to be made at lower temperatures. For instance at 1000/ T Z 0.0031 (w49 C), aT w 0.01 implying 100 times more life at 49 C compared to the life at 91 C. Therefore if the Arrhenius extrapolation assumptions were valid, the predicted behavior of the elongation versus aging time at 49 C would be available by simply multiplying the times on the lower x-axis time scale of Fig. 3 by 100. Later on in this manuscript we will more closely examine this assumption. In a similar fashion interpolation of the plotted results can be used to make interpolated predictions at any temperature in the range
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102
Ea = 106 kJ/mol
101
300
(25.4 kcal/mol)
Elongation, %
Empirical aT for elongation
350
Data region
100
200
T, °C
aT
100
1
120
6
130
11.5
140
26
150
47
150 100
10-1
10-2 2.4
250
Extrapolation region
50 0
2.5
2.6
2.7
2.8
2.9
3.0
3.1
1000/T, K-1
used for oven aging. For example, for 100 C aging, aT w 2.5 so reducing the 91 C time scale by a factor of 2.5 leads to the 100 C time scale plotted as the upper x-axis in Fig. 3. We show this axis since the results for all of the other CSPE materials will be shown at a reference temperature of 100 C (the lowest aging temperature for many of the materials). Based on the above discussion, it is now apparent that the timeetemperature superposed results shown in Fig. 3 contain all of the information shown in Fig. 2 (as well as the values of aT plotted in Fig. 4). For instance if interested in the results at 111 C, one would only have to look at the squares in Fig. 3 coupled with a reduction in the lower x-axis time scale by a factor of 6.1 (the aT appropriate to the 111 C data). Since the quality of the superposition also allows immediate visual testing of the constant acceleration assumption, we will now show only the timeetemperature superposed results for the remaining CSPE materials. Figs. 5e11 show the results for the remaining seven CSPE materials. For CSPE-1 (Fig. 5), CSPE-2 (Fig. 6), CSPE-4 (Fig. 7) and CSPE-5 (Fig. 8) the lowest aging temperature was 100 C; this temperature was therefore chosen as the reference temperature for the superposed results shown in the figures. Since the lowest aging temperature for CSPE-3 (Fig. 9) was 80 C, this temperature was chosen as the reference temperature (lower x-axis); however, for comparison to the other materials, the upper x-axis shows the 100 C time scale. For materials CSPE-7 (Fig. 10) and CSPE-8 (Fig. 11), the lowest aging temperature was 100 C. However, the same exact materials from the same cable spools were used in a joint USeFrench program [20] that oven-aged these two materials at 70 C for 5 years. Including these data in
100
1000
Fig. 5. Timeetemperature superposition of elongation results for CSPE-1.
Figs. 10 and 11 results in a superposed reference temperature of 70 C (lower x-axis); for comparative purposes, the 100 C (interpolated) time scale is shown as the upper x-axis. Comparison of the superposed results for the eight CSPE samples (Figs. 3 and 5e11) shows that all of the materials except CSPE-4 had similar shapes for the time dependence of their degradation. The results for CSPE-4 (Fig. 7) indicated a somewhat more abrupt drop-off just before reaching our arbitrarily chosen ‘‘failure’’ criterion of 50% absolute elongation. This material also displayed fairly high data scatter as a function of aging 300
250
200
Elongation, %
Fig. 4. Arrhenius plot of the empirically derived shift factors for elongation of CSPE-6.
10
aT*t (shifted aging time), days at 100°C
150 T, °C
aT
100
1
110
2.7
125
9
100
50
0 10
100
1000
aT*t (shifted aging time), days at 100°C Fig. 6. Timeetemperature superposition of elongation results for CSPE-2.
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aT*t (shifted aging time), days at 100°C
450
10
300
400
100
350 250
250 200 150
200
Elongation, %
Elongation, %
300
T, °C aT 100
1
110
4
125 12
100
125 10 125
50
T, °C aT
150
80
100
9
140 40
50
0 101
102
1
100
5
110
12
125
48
103 0 10
aT*t (shifted aging time), days at 100°C Fig. 7. Timeetemperature superposition of elongation results for CSPE-4.
time and for the three repeat samples that were averaged at each aging condition. This observation prompted us to run three series of aging experiments at 125 C. The results showed fairly large differences as evidenced by the range of aT values found for these runs (9, 10 and 12 as indicated in Fig. 7). One possible reason for the higher scatter observed for CSPE-4 may be caused by an interface effect acting in concert with the material degradation. When removing the jacket from the cable before aging, we noticed that the jacket covered a wrapping material that left angular channels of
100
1000
aT*t (shifted aging time), days at 80°C Fig. 9. Timeetemperature superposition of elongation results for CSPE-3 at a reference temperature of 80 C (lower x-axis) and at 100 C (upper x-axis).
varying depth on the inside surface of the jacket. When tensile tested to failure, the failure location often appeared to initiate at the point where one of these channels crossed the cut edge of the tensile sample. Thus tensile elongation failure depended on both the material
aT*t (shifted aging time), days at 100°C 400
10
100
1000
103
104
300 350 300
Elongation, %
Elongation, %
250
200
150 T, °C
aT
100
50
100
1
110
2.5
125
9
250 200 T, °C
aT
150 100 50
70
1
100
9.5
110
24
125
76
0 0 101
102 102
103
aT*t (shifted aging time), days at 100°C Fig. 8. Timeetemperature superposition of elongation results for CSPE-5.
aT*t (shifted aging time), days at 70°C Fig. 10. Timeetemperature superposition of elongation results for CSPE-7 at a reference temperature of 70 C (lower x-axis) and at 100 C (upper x-axis).
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aT*t (shifted aging time), days at 100°C 10
400
100
102 1000
350
~107 kJ/mol
101
Empirical aT
Elongation, %
300 250 200 T, °C
aT
70
1
100
9.1
110
22
50
125
70
0
CSPE-1 CSPE-2 CSPE-3
150 100
100
CSPE-4
10-1
CSPE-5 CSPE-6 CSPE-7 CSPE-8
10-2 2.3
102
103
104
aT*t (shifted aging time), days at 70°C Fig. 11. Timeetemperature superposition of elongation results for CSPE-8 at a reference temperature of 70 C (lower x-axis) and at 100 C (upper x-axis).
degradation and the distribution and depth of channels in the tensile specimen. The times required at 100 C to reach 50% absolute elongation for the eight CSPE materials are summarized in Table 2. The results range from a low of 210 d for CSPE-3 to a high of 760 d for the non-conforming CSPE-4. Another interesting comparison involves the shift factors empirically derived from the timeetemperature superposition analyzes and shown in Figs. 3 and 5e11. After normalizing all the aT values to 100 C, they are plotted in Arrhenius fashion in Fig. 12. These results show that at temperatures of 100 C and above all eight materials have similar Ea values equal to approximately 107 kJ/mol (as before the scatter for CSPE-4 is somewhat higher). The more limited data below 100 C seems to imply a curvature to smaller Ea values (‘‘downward’’ curvature). 3.2. Oxygen consumption measurements In numerous recent publications we have shown that oxygen consumption measurements can be useful for quantitatively testing the Arrhenius extrapolation assumption [9e11,13e16,21]. A connection between oxygen consumption and mechanical degradation is anticipated Table 2 Times required for the elongation to drop to 50% absolute at 100 C CSPE
1
2
3
4
5
6
7
8
Time, d
290
340
210
760
280
270
390
360
2.4
2.5
2.6
2.7
2.8
2.9
3.0
1000/T, K -1 Fig. 12. Arrhenius plot of the empirically derived shift factors for elongation of the eight CSPE materials all normalized to unity at 100 C.
for materials where oxidation processes dominate degradation when oxygen is present during the aging. Oxygen consumption measurements are first carried out at temperatures overlapping the mechanical property measurements to confirm that the timeetemperature superposed oxygen consumption results have Ea values close to those found for the mechanical property decay. This offers evidence for the expected connection between oxidation and mechanical property decay. Because oxygen consumption measurements can be made with high sensitivity, these measurements can then be made at much lower temperatures extending well into the normal Arrhenius extrapolation regime. Analyzes of these results allow us to determine whether the Ea for oxidation changes in this region. Past results indicate that for some materials the Ea values found at high temperature remain fairly constant in the extrapolation region [9,10,21]. For other materials the Ea values are found to decrease (downward curvature) indicating that using the Arrhenius extrapolation assumption based on high-temperature mechanical property accelerated aging results overestimates the material lifetime at low temperatures [11,13e16]. As already noted all of the previous studies were done on materials where the presence of oxygen leads to oxidation processes that totally dominate the mechanical property degradation rate. The situation for CSPE materials is more complex since evidence exists that these materials may not be particularly susceptible to oxidation and that other degradation processes may dominate [17,22,23]. With this in mind we did some limited screening of mechanical property degradation rates in inert versus air-aging environments on samples
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400 350
Elongation, %
300 250 200 150
Argon
100 Air
50 0
0
5
10
15
20
25
30
Aging time, d at 138°C Fig. 13. Elongation versus time at 138 C for CSPE-8 in argon and air environments.
10-9 e~ 50%
Oxygen consumption rate, mol g-1s-1
of the CSPE-1, CSPE-2, CSPE-6 and CSPE-8 materials. Results for the CSPE-8 material at 138 C are shown in Fig. 13 and indicate that oxidation effects are the most important mechanism in the degradation at this temperature but that non-oxidative processes are also relevant. The most likely inert process involves dehydrochlorination of isolated chlorines along the polyethylene backbone. Results for the other three materials at 150 C gave similar results with the time required to reach 50% absolute elongation in air generally occurring around three to four times faster than in inert environments. Therefore oxygen consumption measurements for such CSPE materials are definitely related to mechanical degradation but the Ea values for mechanical degradation and oxygen consumption may or may not be precisely correlated dependent on how the importance of the non-oxidative processes varies with aging time and with changes in temperature. With this in mind, we decided to carry out oxygen consumption studies on the CSPE-8 material to quantitatively test how well correlated the oxidation was to the mechanical properties. Since the mechanical property results for several of the materials including CSPE-8 indicated downward curvature below 100 C, probing the oxygen consumption results will allow us to see whether such results offer similar evidence for a drop in Ea. The rate of oxygen consumption for the CSPE-8 material is plotted in Fig. 14 versus aging time at six temperatures ranging from 108 C to 37 C. The rate at each temperature appears to drop slightly at early times followed by a leveling off to a fairly constant rate. To determine whether the rate begins to increase again in the latter stages of degradation, two samples were
10-10
10-11
T, oC 108 95
10-12
80 64 48 37
10-13
Average aging time, days Fig. 14. Oxygen consumption results for CSPE-8 versus aging time at the indicated temperatures. The filled diamonds represent samples that were preaged at 108 C for 102 d and 140 d before 4-d oxygen consumption experiments.
preaged at 108 C for 102 d and 140 d. These samples were then placed in oxygen consumption cells and the rate of oxidation monitored for four additional days at 108 C with the results shown by the filled diamonds in Fig. 14. Since the arrow on the figure corresponds to the approximate time at 108 C for the elongation to reach 50% absolute, it is clear that the oxygen consumption rate begins to increase near mechanical ‘‘failure’’. The oxygen consumption rate results of Fig. 14 can be integrated to give the total oxygen consumption versus aging time at the six temperatures with the results shown in Fig. 15. These data are then timeetemperature superposed at a reference temperature of 37 C leading to the nicely superposed oxygen consumption results given in Fig. 16. As usual the empirically derived shift factors aT are indicated in the figure. These shift factors and those for elongation of the CSPE-8 material are both normalized to 100 C and plotted in Arrhenius fashion as shown in Fig. 17. The results indicate a very close correlation between oxidation and mechanical properties with both sets of aT values having similar slopes (Ea values) both at high and low temperatures. This suggests that oxidation effects dominate mechanical property degradation at low temperatures, therefore implying that the low temperature oxygen consumption aT values should allow better extrapolated lifetime estimates. For instance the superposed results at 100 C (top x-axis of Fig. 11) for this material indicate a 50% absolute elongation lifetime of w1 year. Extrapolating this result to 50 C using the 88 kJ/mol Ea value found from the oxygen consumption results leads to a 50 C ‘‘lifetime’’ of w80 years. This value is
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10-3
101 102 kJ/mol
T, °C
95
100
80
10-4
64
Empirical aT
Oxygen consumption, mol/g
108
48 37
10-1 88 kJ/mol
10-5 10-2 elongation O2 consumption
10-3 2.4
10-6
Fig. 15. The integrated oxygen consumption results from Fig. 14 potted versus aging time at the indicated temperatures.
approximately half of the result that would be predicted from extrapolating the 102 kJ/mol Ea value found from the mechanical property results at 100 C and above. The ‘‘downward’’ curvature in the Arrhenius plot observed for both mechanical properties (Fig. 12) and oxygen consumption (Fig. 17) indicates that a change in the mix of degradation reactions is occurring with aging temperature for CSPE materials. In general such curvature implies that certain higher activation energy
Oxygen consumption, mol/g
10-3
10-4
T, °C
aT
108
780
95
260
80
88
64
21
48
4
37
1
10-5
10-6 -1 10
100
101
2.5
2.6
2.7
2.8
2.9
3.0
3.1
3.2
3.3
1000/T, K-1
Aging time, days
102
aT t, shifted aging time at 37°C, years Fig. 16. Timeetemperature superposition of the integrated oxygen consumption results from Fig. 15. The empirical shift factors used for superposition are shown in the figure.
Fig. 17. Arrhenius plot of elongation and O2 consumption shift factors for CSPE-8 normalized to 100 C.
processes are becoming less important at lower temperatures. In an earlier publication on chloroprene rubber cable jacketing materials we found evidence that the oxidation reactions leading to CO2 production become less important as the aging temperature is reduced, consistent with the observation of similar ‘‘downward’’ curvature in the Arrhenius plots for this material [16]. To determine whether similar evidence exists for the CSPE-8 material, we examined the CO2 results for this material; these results come from the same GC analyzes that yield the oxygen consumption results. In analyzing the oxygen consumption results we must correct for the small amount of oxygen dissolved in the polymer when the GC analyzes the gas phase oxygen surrounding the sample [10]. This correction depends on the solubility coefficient for oxygen SOx in the polymer under study and the ratio of the weight of polymer W to the free volume Vf in the aging cell. For typical worstcase situations involving polymer volume (VP Z W/r where r is the polymer density) approximately equal to the free volume, this correction is around 4% [10]. Unfortunately the correction is much more important for CO2 measurements since the solubility coefficient S for CO2 is typically an order of magnitude higher than it is for O2[24]. This necessitates obtaining an experimental value of S for CO2 in the CSPE-8 material. We accomplish this using a procedure where we determine the pressure drop at equilibrium for a closed container initially containing a known amount of evacuated polymer and a known amount of CO2 gas instantaneously introduced at time zero. This approach described in detail in an earlier manuscript [16], gives a CO2 solubility S Z 1.18 G 0.2 ccSTP/cc/atm. This solubility coefficient can now be used to obtain the
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corrected CO2 partial pressure pc from the measured CO2 partial pressure pm using SWð295Þ pc Z pm 1Cÿ ð1Þ 273 Vf r CO2 produced, mol/g
This equation leads to correction factors ranging from 8% for aging runs carried out at 108 C to 67% for those carried out at 37 C. The resulting values for the integrated CO2 produced versus aging time at the six aging temperatures are shown in Fig. 18. Comparing these results with the oxygen consumption results shown in Fig. 15 immediately indicates that CO2 production becomes less important relative to the oxygen consumed as the temperature drops (e.g., at high temperatures the ratio of CO2 produced to O2 consumed starts out around 20e25% versus w12% at the intermediate temperatures and w7% at the lowest temperature). Timeetemperature superposition of the integrated CO2 results (Fig. 18) together with the aT values used for shifting are shown in Fig. 19. Fig. 20 shows an Arrhenius plot of these shift values normalized to 100 C and compares these to the results for elongation and O2 consumption. It is clear that the Ea for CO2 production (109 kJ/mol) is greater at low temperatures than that of the other two parameters (88 kJ/mol). This implies as noted earlier that the degradation processes leading to CO2 become significantly less important relative to the oxygen consumed as the temperature is reduced. This indicates a change in the mix of degradation reactions as the temperature is lowered consistent with the non-Arrhenius ‘‘downward’’ curvature for both O2 consumption and elongation results. Obviously, the opposite change is expected when the
10-4
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10-6
aT
T, °C
108 3300
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10-8
10-1
100
101
95
1300
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64
49
48
7
37
1
102
103
aT t, shifted aging time at 37°C, years Fig. 19. Timeetemperature superposition of the integrated CO2 production results from Fig. 18. The empirical shift factors used for superposition are shown in the figure.
temperature is increased, with the CO2 producing reactions becoming more important for aging processes at the highest temperatures. Further evidence for a reduction in the importance of the reactions leading to CO2 at lower temperatures was obtained on samples that were first preaged to moderate degradation levels (w100 d at 108 C). After the preaging, O2 consumption and CO2 production rates
101 102 kJ/mol
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Empirical aT
CO2 produced, mol/g
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10-1 88 kJ/mol
10-2
108°C elongation
95°C 80°C
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2.6
2.7
2.8
2.9
3.0
3.1
3.2
3.3
1000/T, K-1 1
10
100
1000
Aging time, days Fig. 18. The integrated CO2 production results for the CSPE-8 material plotted versus aging time at the indicated temperatures.
Fig. 20. Arrhenius plot of the shift factors appropriate to elongation, oxygen consumption and CO2 production versus the inverse aging temperature. The oxygen consumption and CO2 production shift factors are normalized to 100 C.
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were compared at 108 C versus 50 C. CO2 production at 108 C for the moderately oxidized sample represented w24% of the oxygen consumed compared to w12% for measurements made at 50 C indicating that the differences hold both at earlier and later stages of oxidation. In an earlier publication on chloroprene rubber materials we found very similar behaviors including evidence of a drop in the importance of CO2 production relative to oxygen consumption as the aging temperature was lowered and evidence for non-Arrhenius downward curvature from both elongation and oxygen consumption results [16].
345
the United States Department of Energy’s National Nuclear Security Administration under Contract DEAC04-94AL85000. The current studies were part of the Nuclear Energy Plant Optimization (NEPO) program jointly funded by the DOE and by the Electric Power Research Institute (EPRI). The authors appreciate the competent technical assistance provided by Dora Derzon, Mike Malone, Michelle Shedd and Tim Dargaville on the DSC, elongation and oxygen consumption/ CO2 production measurements.
References 4. Conclusions Mechanical property (tensile elongation) results after oven aging at various temperatures were obtained on eight commercial CSPE cable jacketing materials. Analyzes of results at temperatures where significant degradation occurs in up to a 1e2-year time frame (w100 C and higher) indicates that all eight materials show reasonable Arrhenius behavior with similar Arrhenius activation energies Ea (w107 kJ/mol). However, longer-term elongation results on three of the CSPE materials at lower temperatures indicate a drop in Ea as the temperature is lowered (referred to as ‘‘downward’’ curvature in the Arrhenius plot). Oxygen consumption results that allow us to probe a much larger temperature range (108 Ce37 C) were carried out on one of the CSPE materials. The results showed downward curvature behavior consistent with the elongation results for this material and indicated an Ea of approximately 88 kJ/mol below 100 C. Such behavior implies that a high Ea aging process is becoming less important at low temperature and we report evidence from the CO2 production rate consistent with this expectation. Using the lower temperature value of Ea (88 kJ/mol) to extrapolate results from 100 C to 50 C leads to an approximate 50% reduction in predicted lifetime compared to an extrapolation based on the Ea obtained from traditional high-temperature elongation results (100 C and higher). The results of this paper add to the growing evidence that many materials exhibit ‘‘downward’’ curvature in their Arrhenius plots, results that are seldom observed for the narrow range of temperatures typically probed in accelerated aging experiments. As such it indicates the power of using analytical techniques sensitive enough to probe temperatures in the temperature range usually associated with Arrhenius extrapolations. Acnowledgments Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for
[1] Gugumus F. The use of accelerated tests in the evaluation of antioxidants and light stabilizers. In: Scott G, editor. Developments in polymer stabilization-8. London: Elsevier Applied Science; 1987 [chapter 6]. [2] Richters P. Initiation process in the oxidation of polypropylene. Macromolecules 1970;3:262. [3] Howard JB, Gilroy HM. Some observations on the longterm behavior of stabilized polyethylene. Polym Eng Sci 1975; 15:268. [4] Chan MG, Gilroy HM, Martin WM. Long-term aging properties of polyolefins. Proceedings of polyolefins IV, Society of Plastics Engineers Regular Technical Conference, Houston, Texas; 1984. p. 167. [5] Bernstein BS, Lee PN. Proceedings of 24th international wire and cable symposium; November 1975. p. 202. [6] Gijsman P, Hennekens J, Vincent J. The influence of temperature and catalyst residues on the degradation of unstabilized polypropylene. Polym Degrad Stab 1993;39:271. [7] Rosik L, Kovarova J, Pospisil J. Lifetime prediction of ABS polymers based on thermoanalytical data. J Thermal Anal 1996; 46:465. [8] Gugumus F. Effect of temperature on the lifetime of stabilized and unstabilized PP films. Polym Degrad Stab 1999;63:41. [9] Gillen KT, Celina M, Keenan MR. Methods for predicting more confident lifetimes of seals in air environments. Rubber Chem Technol 2000;73:265. [10] Wise J, Gillen KT, Clough RL. An ultrasensitive technique for testing the Arrhenius extrapolation assumption for thermallyaged elastomers. Polym Degrad Stab 1995;49:403. [11] Gillen KT, Celina M, Clough RL, Wise J. Extrapolation of accelerated aging data e Arrhenius or erroneous? Trends Polym Sci 1997;5:250. [12] Gillen KT, Celina M. The wear-out approach for predicting the remaining lifetime of materials. Polym Degrad Stab 2001;71:15. [13] Gillen KT, Celina M, Bernstein R. Validation of improved methods for predicting long-term elastomeric seal lifetimes from compression stress-relaxation and oxygen consumption techniques. Polym Degrad Stab 2003;82:25. [14] Celina M, Wise J, Ottesen DK, Gillen KT, Clough RL. Oxidation profiles of thermally aged neoprene. Polym Degrad Stab 2000;68: 171. [15] Celina M, Gillen KT, Graham AC, Assink RA, Minier LM. Thermal degradation studies of a polyurethane propellant binder. Rubber Chem Technol 2000;73:678. [16] Gillen KT, Bernstein R, Derzon DK. Evidence of non-Arrhenius behavior from laboratory aging and 24-year field aging of polychloroprene rubber materials. Polym Degrad Stab 2005; 87(1):57e67. [17] Baseden GA. In: Morton M, editor. Rubber technology. New York: Van Nostrand Reinhold Co.; 1987. p. 360.
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[18] Gillen KT, Clough RL, Quintana CA. Modulus profiling of polymers. Polym Degrad Stab 1987;17:31. [19] Celina M, Gillen KT, Clough RL. Inverse temperature and annealing phenomena during degradation of crosslinked polyolefins. Polym Degrad Stab 1998;61:231. [20] Nelson CF, Gauthier G, Carlin F, Attal M, Gaussens G, Le Tutour P, et al. Long-term radiation aging and loss-ofcoolant accident (LOCA) testing of electrical cables, Sandia National Laboratories report SAND94-0485; October, 1996.
[21] Celina M, Wise J, Ottesen DK, Gillen KT, Clough RL. Oxidation of thermally aged nitrile rubber. Polym Degrad Stab 1998;60:493. [22] Brydson JA. Rubbery materials and their compounds. London: Elsevier Applied Science; 1988 [chapter 14]. [23] Verbank JJ. Stabilisation of specialty rubbers. In: Scott G, editor. Developments in polymer stabilization-4. London: Applied Science Publishers Ltd.; 1981. p. 262f. [24] Pauly S. Permeability and diffusion data. In: Brandrup J, Immergut EH, Grulke EA, editors. Polymer handbook. 4th ed. New York: John Wiley and Sons; 1999. p. VI/543.
Non-Arrhenius behavior for oxidative degradation of chlorosulfonated polyethylene materials Kenneth T. Gillen*, Robert Bernstein, Mathew Celina Sandia National Laboratories, Organic Materials Department, Box 5800, MS-1411, Albuquerque, NM 87185-1411, United States Received 10 August 2004; accepted 18 September 2004
Abstract We have carried out oven aging studies on eight different commercial chlorosulfonated polyethylene cable jacket materials at temperatures ranging from 80 C to 150 C utilizing ultimate tensile elongation as the degradation parameter. For each material, the elongation results were timeetemperature superposed at the lowest aging temperature. When the resulting empirical shift factors were tested for Arrhenius behavior, it was found that the eight materials were Arrhenius at w100 C and higher with very similar activation energies averaging w107 kJ/mol. Longer-term aging results at temperatures lower than 100 C for three of the materials provided evidence for curvature to lower activation energies. For one of these materials, we conducted oxidation rate measurements at six temperatures ranging from 37 C to 108 C. The results offered further evidence for a small drop in activation energy below 100 C. Chemical evidence supporting this change in activation energy was derived from analysis of the production rates of CO2 during oxidation. As the temperature was lowered, the amount of CO2 produced relative to the O2 consumed dropped substantially, implying that the chemistry leading to CO2 becomes less important at lower temperatures. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Aging; Lifetime prediction; Chlorosulfonated polyethylene rubber; Arrhenius extrapolation; Oxygen consumption
1. Introduction Predicting polymer lifetimes has been a continuing important objective for most industrial applications. The most commonly used prediction method involves the so-called Arrhenius approach that assumes that an overall chemical process controls degradation where the rate of deterioration is proportional to exp(Ea/RT ). In this expression, Ea is the Arrhenius activation energy, R is the gas constant and T is the absolute temperature. Data are analyzed under accelerated aging conditions in order to estimate the value of Ea
* Corresponding author. Tel.: C1 5058447494; fax: C1 505 8449781. E-mail address: [email protected] (K.T. Gillen). 0141-3910/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2004.09.004
appropriate to the accelerated conditions. This value of Ea is then assumed to be unchanged at temperatures lower than the accelerated temperature regime, allowing extrapolated predictions at lower temperatures (longer times). Given the extremely complex set of reactions that underlies the degradation of most polymeric materials and the expectation that each reaction will have a different activation energy, it is perhaps naı¨ ve to expect a single Ea value to be valid over an extended temperature range [1]. Unfortunately most accelerated aging studies only access a fairly narrow temperature regime over which it is difficult to observe a change in Ea relative to the uncertainties in the data. Even so, there have been numerous literature examples of nonArrhenius behavior, typically involving a drop in Ea at lower temperatures. Richters observed curvature
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to lower Ea values (‘‘downward curvature’’) when monitoring the oxygen induction period for polypropylene [2]. Similar curvature to lower Ea values was observed for polyethylene materials by Bell Labs workers [3,4]. Bernstein and Lee presented evidence for downward curvature for oven-aged samples of HDPE insulated cables [5]. More recent studies by Gijsman et al. on unstabilized polypropylene [6] and by Rosik et al. on a stabilized ABS polymer [7] observed downward curvature effects. Gugumus recently found evidence of both downward and upward curvature for polypropylene films [8]. The upward curvature was tentatively attributed to titanium complexation by the phenolic antioxidant leading to possible passivation of the transition metal. Except for anomalous effects caused by diffusion-limited oxidation [9], upward curvature is seldom observed. We have been interested for many years in the development of improved methods for analyzing and extrapolating accelerated aging studies [9e16] and have found numerous examples of downward curvature in the Arrhenius plots of both mechanical property [13,15,16] and oxygen consumption [9,11,13e16] data. One of our current interests involves the aging of nuclear power plant safety cables. Such cables can be exposed to fairly high temperature (and occasionally radiation) environments for 40 years or more. If an accident such as a loss of coolant accident (LOCA) occurred, the aged cable must possess sufficient mechanical properties for proper operation during the accident. Predicting the mechanical lifetimes of such cables typically involves the Arrhenius extrapolation of high temperature accelerated aging results. The purpose of the current studies is to test the Arrhenius assumption on eight different commercial chlorosulfonated polyethylene (CSPE) cable jacketing materials to derive better lifetime extrapolations. CSPE jackets are the most commonly used jackets for nuclear power plant safety related low-voltage cables and the eight materials under current study came from cables that are commonly found in US nuclear power plants.
2. Experimental 2.1. Materials Chlorosulfonated polyethylene (CSPE) is obtained via the simultaneous chlorination and chlorosulfonation of polyethylene [17]. It is a polymer that consists of a modified polyethylene backbone with chloro and sulfonylchloride side groups. Crosslinking can be achieved with different curing methods (i.e. sulfur, peroxides, maleimide, and others) to produce a commercial generic ‘‘HypalonÒ’’ rubber. CSPE materials are generally regarded as having good toughness, weatherability, and resistance to oxidation and oil/solvent exposure. At low-chlorine levels some CSPE grades can retain some polyethylene-like properties such as hardness, stiffness and partial crystallinity [17]. The eight CSPE jacket materials currently studied were removed before aging from commercially produced low-voltage cables that are qualified for nuclear power plant safety applications. Table 1 gives details on the manufacturer, the nominal jacket thickness and initial values for tensile properties, modulus [18] and density. Also shown in the table are the abbreviations used for the materials for the remainder of the paper. CSPE-1 represent the thin inner jacket surrounding EPR insulation on individual conductors of a multi-conductor cable whereas all of the remaining materials are the thicker overall jackets from multi-conductor cables. For the tensile testing samples, strips approximately 6.4 mm wide by w150 mm long were cut from the cable jackets before oven aging.
2.2. Oven aging Oven aging was carried out in air-circulating ovens controlled to approximately G1 C. Reported temperatures are averages from calibrated thermocouples located close to the samples and connected to continuous strip-chart recorders. Samples were protected from direct air-flow.
Table 1 Summary of the CSPE cable jacketing materials Cable description
Abbreviation
~Thickness (mm)
Ultimate tensile elongation (%)
Ultimate tensile strength (MPa)
Modulus (MPa)
Density (g/cc)
Anaconda Flameguard Anaconda Flameguard BIW Brandrex Eaton Dekoron Kerite Rockbestos Firewall III Samuel Moore Dekoron
CSPE-1 CSPE-2 CSPE-3 CSPE-4 CSPE-5 CSPE-6 CSPE-7 CSPE-8
0.4 1.8 1.7 1.6 1.4 1.5 1.3 1.2
393 263 257 380 316 300 364 381
15 13 13 16 20 22 16 20
5.3 4.8 8.7 5.1 18 9 4.1 14
1.64 1.58 1.47 1.37 1.47 1.57 1.60 1.46
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2.3. Ultimate tensile elongation tests Tensile tests were performed on an Instron Table Model testing machine equipped with pneumatic grips and having an extensometer clamped to the sample. Testing was conducted at a strain rate of 12.7 cm/min with an initial jaw separation of 5.1 cm. Three repeat samples were typically tested for each aging condition (time and temperature) with the average value of the ultimate tensile elongation reported. 2.4. Oxygen consumption and CO2 production rate measurements A detailed description of the procedures used for these measurements has been described in earlier publications [10,16]. Briefly, these measurements are made by initially placing a known weight of polymer with a known initial quantity of oxygen in a sealed container. After oven aging for a selected period of time, the gas in the container is analyzed for oxygen, CO2 and CO using gas chromatography techniques. The amount of oxygen initially sealed in the container is chosen such that its initial partial pressure at the oven aging temperature is w150 torr and the aging time chosen so that the expected final oxygen partial pressure (at elevated temperature) is w110 torr. The intent of this approach is to have the average oxygen partial pressure during aging approximate the partial pressure of oxygen in Albuquerque (132 torr) so that the oxygen consumption reflects the oxidation appropriate during air-oven aging. After aging, the gas in the container is quantitatively analyzed for the oxygen consumption rate and for the production rates of CO2 and CO. The sample container is then pumped out again for several days (to remove dissolved gases in the polymer), backfilled again with the same initial partial pressure of oxygen and aged for another time interval. By repeating this procedure several times, the time dependence of the rates is obtained as well as the integrated rate (total oxygen consumption) versus time.
component [17]. The DSC scans on six materials were flat and therefore showed no evidence of any crystallinity. DSC scans for the other two materials (CSPE-5 and CSPE-8) showed evidence of minor amounts of some crystalline component around 55 C (Fig. 1) where the melting endotherm is relatively small in comparison with other semi-crystalline PE-based cable materials [19]. Although this temperature region is below the normal region expected for polyethylene crystalline components, the presence of a small crystalline component is the likely reason for the higher modulus values found for these two materials (Table 1).
3. Results and discussion 3.1. Arrhenius analyzes of tensile elongation results For degradation of cable jacketing materials, changes in mechanical properties are of primary interest with most studies focusing on drops in the tensile elongation. With this in mind we carried out accelerated oven aging experiments on the eight CSPE materials (using three to five different aging temperatures for each material) and monitored changes in tensile elongation. Typical average elongation results versus temperature are shown in Fig. 2 for CSPE-6. For this material, data are shown for five aging temperatures ranging from 91 C to 130 C (the unfilled symbols). Since significant degradation takes more than 2 years at 91 C, monitoring of mechanical properties below this temperature would require aging times much longer than are typically available. The data in Fig. 2 are plotted versus log of the aging time since this allows us to visually determine how the shapes of the curves compare as a function of temperature. When raising the aging temperature in order to accelerate degradation, the fundamental underlying assumption is that all reactions are accelerated equally. This constant acceleration (independent of the 0.3
2.5. DSC runs Endotherm (W/g)
Differential scanning calorimetry was carried out on w5 mg samples of each material using a Perkin Elmer DSC 6 calibrated with indium and zinc standards. Runs were conducted under 20 cc/min argon gas flow from 10 C to 180 C at a scan rate of 5 C/min. Commercial CSPE materials used for cables and wires are usually based on either Hypalon-45 (24% chlorine content, density of 1.08 g/cc) or Hypalon-40 (35% chlorine content, density of 1.18 g/cc). Due to the higher polyethylene content, formulations based on Hypalon-45 are semi-crystalline whereas those based on Hypalon-40 (or Hypalon-40S) show no crystalline
CSPE-5 CSPE-8
0.2
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Temperature [°C] Fig. 1. DSC scans for the two indicated CSPE jacketing materials.
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aT*t (shifted aging time), days at 100°C
300 300
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Elongation, %
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150 91°C
100
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2.8x
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0 100
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Aging time, days Fig. 2. Elongation versus time at the indicated temperatures for CSPE6 plus superposed 101 C data (filled triangles) showing procedure underlying timeetemperature superposition (the 101 C data has been multiplicatively shifted with aT Z 2.8).
level of degradation) assumption implies that the degradation curves at differing aging temperatures will have the same shape when plotted versus log of the aging time. This appears to be a reasonable assumption for the results shown in Fig. 2. To determine the relationship between degradation and temperature we apply the principal of timeetemperature superposition [9e16] to the data by first selecting a reference temperature Tref which is typically the lowest experimental temperature (91 C in the present case). We then take the results at a second temperature T and find the constant multiplicative shift factor aT such that multiplying the times associated with the second temperature by aT gives the best overlap (superposition) of the data from T and Tref. An example of this procedure is shown in Fig. 2 where the 101 C data (open triangles) are shifted to the right (filled triangles) by a multiplicative shift factor aT Z 2.8 in order to best overlap the 91 C results. After this procedure is completed on the 101 C data, the 111 C data are then shifted such that it has the best overlap with the superposed results from 91 C and 101 C (aT Z 6.1 works best in this case). Data at the remaining temperatures are then shifted in a similar fashion leading to their aT values. By definition aT Z 1 for the 91 C reference temperature since the times at this temperature remain constant (multiplied by unity). The resulting superposition plus the empirical shift factors used to achieve superposition for the CSPE-6 data of Fig. 2 are shown in Fig. 3 (lower x-axis). The superposition is excellent as expected given the similarities in curve shapes observed for the five aging temperatures (Fig. 2).
0 10-2
T, °C
aT
91
1
101
2.8
111
6.1
120
15
130
27.5
10-1
100
aT*t (shifted aging time), years at 91°C
101
Fig. 3. Timeetemperature superposition of the elongation results from Fig. 2 at 91 C (lower x-axis). By interpolation of the shift factor results, aT at 100 C is 2.5 which leads to the time scale at 100 C shown on the upper x-axis.
A typical Arrhenius analysis would involve first choosing a ‘‘failure’’ criterion for the data on Fig. 2 (e.g., the time to reach 50% absolute elongation) then determining the times required at each temperature to reach this value. These times would then be plotted on an Arrhenius plot (log of time versus inverse absolute temperature) to see if straight-line (Arrhenius) behavior resulted. This procedure would thus use only one processed datum point per temperature. The timee temperature superposition approach (Fig. 3) on the other hand utilizes every experimental data point in the analyzes. The empirically derived shift factors can then be plotted on an Arrhenius plot to see if Arrhenius behavior is indicated. Such an Arrhenius plot is shown in Fig. 4, where reasonable Arrhenius behavior with an Arrhenius activation energy Ea of w106 kJ/mol is found. If the same Arrhenius behavior remains unchanged at lower aging temperatures, the observed line can be extrapolated as shown allowing predictions to be made at lower temperatures. For instance at 1000/ T Z 0.0031 (w49 C), aT w 0.01 implying 100 times more life at 49 C compared to the life at 91 C. Therefore if the Arrhenius extrapolation assumptions were valid, the predicted behavior of the elongation versus aging time at 49 C would be available by simply multiplying the times on the lower x-axis time scale of Fig. 3 by 100. Later on in this manuscript we will more closely examine this assumption. In a similar fashion interpolation of the plotted results can be used to make interpolated predictions at any temperature in the range
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102
Ea = 106 kJ/mol
101
300
(25.4 kcal/mol)
Elongation, %
Empirical aT for elongation
350
Data region
100
200
T, °C
aT
100
1
120
6
130
11.5
140
26
150
47
150 100
10-1
10-2 2.4
250
Extrapolation region
50 0
2.5
2.6
2.7
2.8
2.9
3.0
3.1
1000/T, K-1
used for oven aging. For example, for 100 C aging, aT w 2.5 so reducing the 91 C time scale by a factor of 2.5 leads to the 100 C time scale plotted as the upper x-axis in Fig. 3. We show this axis since the results for all of the other CSPE materials will be shown at a reference temperature of 100 C (the lowest aging temperature for many of the materials). Based on the above discussion, it is now apparent that the timeetemperature superposed results shown in Fig. 3 contain all of the information shown in Fig. 2 (as well as the values of aT plotted in Fig. 4). For instance if interested in the results at 111 C, one would only have to look at the squares in Fig. 3 coupled with a reduction in the lower x-axis time scale by a factor of 6.1 (the aT appropriate to the 111 C data). Since the quality of the superposition also allows immediate visual testing of the constant acceleration assumption, we will now show only the timeetemperature superposed results for the remaining CSPE materials. Figs. 5e11 show the results for the remaining seven CSPE materials. For CSPE-1 (Fig. 5), CSPE-2 (Fig. 6), CSPE-4 (Fig. 7) and CSPE-5 (Fig. 8) the lowest aging temperature was 100 C; this temperature was therefore chosen as the reference temperature for the superposed results shown in the figures. Since the lowest aging temperature for CSPE-3 (Fig. 9) was 80 C, this temperature was chosen as the reference temperature (lower x-axis); however, for comparison to the other materials, the upper x-axis shows the 100 C time scale. For materials CSPE-7 (Fig. 10) and CSPE-8 (Fig. 11), the lowest aging temperature was 100 C. However, the same exact materials from the same cable spools were used in a joint USeFrench program [20] that oven-aged these two materials at 70 C for 5 years. Including these data in
100
1000
Fig. 5. Timeetemperature superposition of elongation results for CSPE-1.
Figs. 10 and 11 results in a superposed reference temperature of 70 C (lower x-axis); for comparative purposes, the 100 C (interpolated) time scale is shown as the upper x-axis. Comparison of the superposed results for the eight CSPE samples (Figs. 3 and 5e11) shows that all of the materials except CSPE-4 had similar shapes for the time dependence of their degradation. The results for CSPE-4 (Fig. 7) indicated a somewhat more abrupt drop-off just before reaching our arbitrarily chosen ‘‘failure’’ criterion of 50% absolute elongation. This material also displayed fairly high data scatter as a function of aging 300
250
200
Elongation, %
Fig. 4. Arrhenius plot of the empirically derived shift factors for elongation of CSPE-6.
10
aT*t (shifted aging time), days at 100°C
150 T, °C
aT
100
1
110
2.7
125
9
100
50
0 10
100
1000
aT*t (shifted aging time), days at 100°C Fig. 6. Timeetemperature superposition of elongation results for CSPE-2.
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aT*t (shifted aging time), days at 100°C
450
10
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250 200 150
200
Elongation, %
Elongation, %
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T, °C aT 100
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125 12
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50
T, °C aT
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100
5
110
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103 0 10
aT*t (shifted aging time), days at 100°C Fig. 7. Timeetemperature superposition of elongation results for CSPE-4.
time and for the three repeat samples that were averaged at each aging condition. This observation prompted us to run three series of aging experiments at 125 C. The results showed fairly large differences as evidenced by the range of aT values found for these runs (9, 10 and 12 as indicated in Fig. 7). One possible reason for the higher scatter observed for CSPE-4 may be caused by an interface effect acting in concert with the material degradation. When removing the jacket from the cable before aging, we noticed that the jacket covered a wrapping material that left angular channels of
100
1000
aT*t (shifted aging time), days at 80°C Fig. 9. Timeetemperature superposition of elongation results for CSPE-3 at a reference temperature of 80 C (lower x-axis) and at 100 C (upper x-axis).
varying depth on the inside surface of the jacket. When tensile tested to failure, the failure location often appeared to initiate at the point where one of these channels crossed the cut edge of the tensile sample. Thus tensile elongation failure depended on both the material
aT*t (shifted aging time), days at 100°C 400
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Elongation, %
Elongation, %
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aT
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50
100
1
110
2.5
125
9
250 200 T, °C
aT
150 100 50
70
1
100
9.5
110
24
125
76
0 0 101
102 102
103
aT*t (shifted aging time), days at 100°C Fig. 8. Timeetemperature superposition of elongation results for CSPE-5.
aT*t (shifted aging time), days at 70°C Fig. 10. Timeetemperature superposition of elongation results for CSPE-7 at a reference temperature of 70 C (lower x-axis) and at 100 C (upper x-axis).
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aT*t (shifted aging time), days at 100°C 10
400
100
102 1000
350
~107 kJ/mol
101
Empirical aT
Elongation, %
300 250 200 T, °C
aT
70
1
100
9.1
110
22
50
125
70
0
CSPE-1 CSPE-2 CSPE-3
150 100
100
CSPE-4
10-1
CSPE-5 CSPE-6 CSPE-7 CSPE-8
10-2 2.3
102
103
104
aT*t (shifted aging time), days at 70°C Fig. 11. Timeetemperature superposition of elongation results for CSPE-8 at a reference temperature of 70 C (lower x-axis) and at 100 C (upper x-axis).
degradation and the distribution and depth of channels in the tensile specimen. The times required at 100 C to reach 50% absolute elongation for the eight CSPE materials are summarized in Table 2. The results range from a low of 210 d for CSPE-3 to a high of 760 d for the non-conforming CSPE-4. Another interesting comparison involves the shift factors empirically derived from the timeetemperature superposition analyzes and shown in Figs. 3 and 5e11. After normalizing all the aT values to 100 C, they are plotted in Arrhenius fashion in Fig. 12. These results show that at temperatures of 100 C and above all eight materials have similar Ea values equal to approximately 107 kJ/mol (as before the scatter for CSPE-4 is somewhat higher). The more limited data below 100 C seems to imply a curvature to smaller Ea values (‘‘downward’’ curvature). 3.2. Oxygen consumption measurements In numerous recent publications we have shown that oxygen consumption measurements can be useful for quantitatively testing the Arrhenius extrapolation assumption [9e11,13e16,21]. A connection between oxygen consumption and mechanical degradation is anticipated Table 2 Times required for the elongation to drop to 50% absolute at 100 C CSPE
1
2
3
4
5
6
7
8
Time, d
290
340
210
760
280
270
390
360
2.4
2.5
2.6
2.7
2.8
2.9
3.0
1000/T, K -1 Fig. 12. Arrhenius plot of the empirically derived shift factors for elongation of the eight CSPE materials all normalized to unity at 100 C.
for materials where oxidation processes dominate degradation when oxygen is present during the aging. Oxygen consumption measurements are first carried out at temperatures overlapping the mechanical property measurements to confirm that the timeetemperature superposed oxygen consumption results have Ea values close to those found for the mechanical property decay. This offers evidence for the expected connection between oxidation and mechanical property decay. Because oxygen consumption measurements can be made with high sensitivity, these measurements can then be made at much lower temperatures extending well into the normal Arrhenius extrapolation regime. Analyzes of these results allow us to determine whether the Ea for oxidation changes in this region. Past results indicate that for some materials the Ea values found at high temperature remain fairly constant in the extrapolation region [9,10,21]. For other materials the Ea values are found to decrease (downward curvature) indicating that using the Arrhenius extrapolation assumption based on high-temperature mechanical property accelerated aging results overestimates the material lifetime at low temperatures [11,13e16]. As already noted all of the previous studies were done on materials where the presence of oxygen leads to oxidation processes that totally dominate the mechanical property degradation rate. The situation for CSPE materials is more complex since evidence exists that these materials may not be particularly susceptible to oxidation and that other degradation processes may dominate [17,22,23]. With this in mind we did some limited screening of mechanical property degradation rates in inert versus air-aging environments on samples
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400 350
Elongation, %
300 250 200 150
Argon
100 Air
50 0
0
5
10
15
20
25
30
Aging time, d at 138°C Fig. 13. Elongation versus time at 138 C for CSPE-8 in argon and air environments.
10-9 e~ 50%
Oxygen consumption rate, mol g-1s-1
of the CSPE-1, CSPE-2, CSPE-6 and CSPE-8 materials. Results for the CSPE-8 material at 138 C are shown in Fig. 13 and indicate that oxidation effects are the most important mechanism in the degradation at this temperature but that non-oxidative processes are also relevant. The most likely inert process involves dehydrochlorination of isolated chlorines along the polyethylene backbone. Results for the other three materials at 150 C gave similar results with the time required to reach 50% absolute elongation in air generally occurring around three to four times faster than in inert environments. Therefore oxygen consumption measurements for such CSPE materials are definitely related to mechanical degradation but the Ea values for mechanical degradation and oxygen consumption may or may not be precisely correlated dependent on how the importance of the non-oxidative processes varies with aging time and with changes in temperature. With this in mind, we decided to carry out oxygen consumption studies on the CSPE-8 material to quantitatively test how well correlated the oxidation was to the mechanical properties. Since the mechanical property results for several of the materials including CSPE-8 indicated downward curvature below 100 C, probing the oxygen consumption results will allow us to see whether such results offer similar evidence for a drop in Ea. The rate of oxygen consumption for the CSPE-8 material is plotted in Fig. 14 versus aging time at six temperatures ranging from 108 C to 37 C. The rate at each temperature appears to drop slightly at early times followed by a leveling off to a fairly constant rate. To determine whether the rate begins to increase again in the latter stages of degradation, two samples were
10-10
10-11
T, oC 108 95
10-12
80 64 48 37
10-13
Average aging time, days Fig. 14. Oxygen consumption results for CSPE-8 versus aging time at the indicated temperatures. The filled diamonds represent samples that were preaged at 108 C for 102 d and 140 d before 4-d oxygen consumption experiments.
preaged at 108 C for 102 d and 140 d. These samples were then placed in oxygen consumption cells and the rate of oxidation monitored for four additional days at 108 C with the results shown by the filled diamonds in Fig. 14. Since the arrow on the figure corresponds to the approximate time at 108 C for the elongation to reach 50% absolute, it is clear that the oxygen consumption rate begins to increase near mechanical ‘‘failure’’. The oxygen consumption rate results of Fig. 14 can be integrated to give the total oxygen consumption versus aging time at the six temperatures with the results shown in Fig. 15. These data are then timeetemperature superposed at a reference temperature of 37 C leading to the nicely superposed oxygen consumption results given in Fig. 16. As usual the empirically derived shift factors aT are indicated in the figure. These shift factors and those for elongation of the CSPE-8 material are both normalized to 100 C and plotted in Arrhenius fashion as shown in Fig. 17. The results indicate a very close correlation between oxidation and mechanical properties with both sets of aT values having similar slopes (Ea values) both at high and low temperatures. This suggests that oxidation effects dominate mechanical property degradation at low temperatures, therefore implying that the low temperature oxygen consumption aT values should allow better extrapolated lifetime estimates. For instance the superposed results at 100 C (top x-axis of Fig. 11) for this material indicate a 50% absolute elongation lifetime of w1 year. Extrapolating this result to 50 C using the 88 kJ/mol Ea value found from the oxygen consumption results leads to a 50 C ‘‘lifetime’’ of w80 years. This value is
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10-3
101 102 kJ/mol
T, °C
95
100
80
10-4
64
Empirical aT
Oxygen consumption, mol/g
108
48 37
10-1 88 kJ/mol
10-5 10-2 elongation O2 consumption
10-3 2.4
10-6
Fig. 15. The integrated oxygen consumption results from Fig. 14 potted versus aging time at the indicated temperatures.
approximately half of the result that would be predicted from extrapolating the 102 kJ/mol Ea value found from the mechanical property results at 100 C and above. The ‘‘downward’’ curvature in the Arrhenius plot observed for both mechanical properties (Fig. 12) and oxygen consumption (Fig. 17) indicates that a change in the mix of degradation reactions is occurring with aging temperature for CSPE materials. In general such curvature implies that certain higher activation energy
Oxygen consumption, mol/g
10-3
10-4
T, °C
aT
108
780
95
260
80
88
64
21
48
4
37
1
10-5
10-6 -1 10
100
101
2.5
2.6
2.7
2.8
2.9
3.0
3.1
3.2
3.3
1000/T, K-1
Aging time, days
102
aT t, shifted aging time at 37°C, years Fig. 16. Timeetemperature superposition of the integrated oxygen consumption results from Fig. 15. The empirical shift factors used for superposition are shown in the figure.
Fig. 17. Arrhenius plot of elongation and O2 consumption shift factors for CSPE-8 normalized to 100 C.
processes are becoming less important at lower temperatures. In an earlier publication on chloroprene rubber cable jacketing materials we found evidence that the oxidation reactions leading to CO2 production become less important as the aging temperature is reduced, consistent with the observation of similar ‘‘downward’’ curvature in the Arrhenius plots for this material [16]. To determine whether similar evidence exists for the CSPE-8 material, we examined the CO2 results for this material; these results come from the same GC analyzes that yield the oxygen consumption results. In analyzing the oxygen consumption results we must correct for the small amount of oxygen dissolved in the polymer when the GC analyzes the gas phase oxygen surrounding the sample [10]. This correction depends on the solubility coefficient for oxygen SOx in the polymer under study and the ratio of the weight of polymer W to the free volume Vf in the aging cell. For typical worstcase situations involving polymer volume (VP Z W/r where r is the polymer density) approximately equal to the free volume, this correction is around 4% [10]. Unfortunately the correction is much more important for CO2 measurements since the solubility coefficient S for CO2 is typically an order of magnitude higher than it is for O2[24]. This necessitates obtaining an experimental value of S for CO2 in the CSPE-8 material. We accomplish this using a procedure where we determine the pressure drop at equilibrium for a closed container initially containing a known amount of evacuated polymer and a known amount of CO2 gas instantaneously introduced at time zero. This approach described in detail in an earlier manuscript [16], gives a CO2 solubility S Z 1.18 G 0.2 ccSTP/cc/atm. This solubility coefficient can now be used to obtain the
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corrected CO2 partial pressure pc from the measured CO2 partial pressure pm using SWð295Þ pc Z pm 1Cÿ ð1Þ 273 Vf r CO2 produced, mol/g
This equation leads to correction factors ranging from 8% for aging runs carried out at 108 C to 67% for those carried out at 37 C. The resulting values for the integrated CO2 produced versus aging time at the six aging temperatures are shown in Fig. 18. Comparing these results with the oxygen consumption results shown in Fig. 15 immediately indicates that CO2 production becomes less important relative to the oxygen consumed as the temperature drops (e.g., at high temperatures the ratio of CO2 produced to O2 consumed starts out around 20e25% versus w12% at the intermediate temperatures and w7% at the lowest temperature). Timeetemperature superposition of the integrated CO2 results (Fig. 18) together with the aT values used for shifting are shown in Fig. 19. Fig. 20 shows an Arrhenius plot of these shift values normalized to 100 C and compares these to the results for elongation and O2 consumption. It is clear that the Ea for CO2 production (109 kJ/mol) is greater at low temperatures than that of the other two parameters (88 kJ/mol). This implies as noted earlier that the degradation processes leading to CO2 become significantly less important relative to the oxygen consumed as the temperature is reduced. This indicates a change in the mix of degradation reactions as the temperature is lowered consistent with the non-Arrhenius ‘‘downward’’ curvature for both O2 consumption and elongation results. Obviously, the opposite change is expected when the
10-4
10-5
10-6
aT
T, °C
108 3300
10-7
10-8
10-1
100
101
95
1300
80
180
64
49
48
7
37
1
102
103
aT t, shifted aging time at 37°C, years Fig. 19. Timeetemperature superposition of the integrated CO2 production results from Fig. 18. The empirical shift factors used for superposition are shown in the figure.
temperature is increased, with the CO2 producing reactions becoming more important for aging processes at the highest temperatures. Further evidence for a reduction in the importance of the reactions leading to CO2 at lower temperatures was obtained on samples that were first preaged to moderate degradation levels (w100 d at 108 C). After the preaging, O2 consumption and CO2 production rates
101 102 kJ/mol
10-4
10-5
Empirical aT
CO2 produced, mol/g
100
10-6
10-1 88 kJ/mol
10-2
108°C elongation
95°C 80°C
10-7
CO2 production
64°C 48°C
10-4 2.4
37°C
10-8
109 kJ/mol
O2 consumption
10-3
2.5
2.6
2.7
2.8
2.9
3.0
3.1
3.2
3.3
1000/T, K-1 1
10
100
1000
Aging time, days Fig. 18. The integrated CO2 production results for the CSPE-8 material plotted versus aging time at the indicated temperatures.
Fig. 20. Arrhenius plot of the shift factors appropriate to elongation, oxygen consumption and CO2 production versus the inverse aging temperature. The oxygen consumption and CO2 production shift factors are normalized to 100 C.
K.T. Gillen et al. / Polymer Degradation and Stability 87 (2005) 335e346
were compared at 108 C versus 50 C. CO2 production at 108 C for the moderately oxidized sample represented w24% of the oxygen consumed compared to w12% for measurements made at 50 C indicating that the differences hold both at earlier and later stages of oxidation. In an earlier publication on chloroprene rubber materials we found very similar behaviors including evidence of a drop in the importance of CO2 production relative to oxygen consumption as the aging temperature was lowered and evidence for non-Arrhenius downward curvature from both elongation and oxygen consumption results [16].
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the United States Department of Energy’s National Nuclear Security Administration under Contract DEAC04-94AL85000. The current studies were part of the Nuclear Energy Plant Optimization (NEPO) program jointly funded by the DOE and by the Electric Power Research Institute (EPRI). The authors appreciate the competent technical assistance provided by Dora Derzon, Mike Malone, Michelle Shedd and Tim Dargaville on the DSC, elongation and oxygen consumption/ CO2 production measurements.
References 4. Conclusions Mechanical property (tensile elongation) results after oven aging at various temperatures were obtained on eight commercial CSPE cable jacketing materials. Analyzes of results at temperatures where significant degradation occurs in up to a 1e2-year time frame (w100 C and higher) indicates that all eight materials show reasonable Arrhenius behavior with similar Arrhenius activation energies Ea (w107 kJ/mol). However, longer-term elongation results on three of the CSPE materials at lower temperatures indicate a drop in Ea as the temperature is lowered (referred to as ‘‘downward’’ curvature in the Arrhenius plot). Oxygen consumption results that allow us to probe a much larger temperature range (108 Ce37 C) were carried out on one of the CSPE materials. The results showed downward curvature behavior consistent with the elongation results for this material and indicated an Ea of approximately 88 kJ/mol below 100 C. Such behavior implies that a high Ea aging process is becoming less important at low temperature and we report evidence from the CO2 production rate consistent with this expectation. Using the lower temperature value of Ea (88 kJ/mol) to extrapolate results from 100 C to 50 C leads to an approximate 50% reduction in predicted lifetime compared to an extrapolation based on the Ea obtained from traditional high-temperature elongation results (100 C and higher). The results of this paper add to the growing evidence that many materials exhibit ‘‘downward’’ curvature in their Arrhenius plots, results that are seldom observed for the narrow range of temperatures typically probed in accelerated aging experiments. As such it indicates the power of using analytical techniques sensitive enough to probe temperatures in the temperature range usually associated with Arrhenius extrapolations. Acnowledgments Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for
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