Prediction of Elastomer Lifetimes from Accelerated Thermal-Aging

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 27, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch034

34 Prediction of Elastomer Lifetimes from Accelerated Thermal-Aging Experiments Kenneth T. Gillen, Roger L. Clough, and Jonathan Wise Sandia National Laboratories, Albuquerque, NM 87185

We describe a study aimed at validating the Arrhenius tion methodology.

Ultimate tensile measurements

elastomers after elevated temperature

lifetime

predic-

were made on three

exposures. Although tensile elon-

gation could be analyzed using the Arrhenius

approach, tensile strength

could not. Modulus profiles resolved this inconsistency by showing that complex, diffusion-limited

oxidation effects (involving surface

harden-

ing) were present. The surface (equilibrium) modulus values were correlated

with elongation

and indicated

that elongation

because cracks initiated at the hardened surface and then propagated

is

Arrhenius immediately

through the material. Tensile strength, on the other hand,

is non-Arrhenius

because it depends on the entire material cross sec-

tion. We also introduce

a methodology

based on monitoring

consumption rates that allows us, for the first time, to test the extrapolation

oxygen

Arrhenius

assumption.

B E C A U S E ELASTOMERIC MATERIALS are commonly used for long-term ap

plications (years to decades) at room (ambient) or at moderately elevated temperature conditions, it is important to be able to predict their lifetimes. A common approach involves accelerating the chemical reactions underlying the degradation by aging at several elevated temperatures and monitoring the degradation through changes in ultimate tensile properties [elongation and tensile strength (TS)]. These accelerated thermal-aging results are generally extrapolated to use-temperature conditions by using the Arrhenius method ology (I). This method is based on the observation that the temperature de pendence of the rate of an individual chemical reaction is typically

0065-2393/96/0249-0557$12.00/0 © 1996 American Chemical Society In Polymer Durability; Clough, Roger L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

558

POLYMER DURABILITY

proportional to exp(— EJRT), where E is the Arrhenius activation energy, R is the ideal gas constant, and Τ is the absolute temperature. In general, the aging of a polymer can be described by a series of chemical reactions, each assumed to have Arrhenius behavior. Kinetic analysis of these reactions results in a steady-state rate expression with an Arrhenius temper ature dependence, where E now represents the effective activation energy for the mix of reactions underlying the degradation. If this relative mix of reactions remains unchanged throughout the temperature range under anal ysis, a linear relationship will exist between the logarithm of the time to a certain amount of material property change and 1/Γ. The value of E is then obtained from the slope of the line. If, on the other hand, the relative mix of degradation reactions changes with changes in T, the effective E would be expected to change, and this change would lead to curvature in the Arrhenius plot. Although, at first glance, Arrhenius behavior seems to be valid in many (though not all) instances, closer examination leads to some troubling con cerns. For instance, ultimate tensile elongation results are often used to "con firm" Arrhenius behavior, even though the ultimate TS data available from the same mechanical-property testing (typically not reported) are often nonArrhenius. In addition, many workers use only a single point or a few selected points from each temperature curve for their Arrhenius analysis, thereby elim inating much of their data and significantly depreciating the value of any con clusions. When elastomers age in air environments, the chemical reactions domi nating the long-term degradation usually involve the oxygen dissolved in the material (2). When attempts are made to accelerate these reactions by using elevated temperature (e.g., in air-circulating ovens), complications caused by diffusion-fimited oxidation ( D L O ) typically enter (3-7). D L O can occur when ever the rate of oxygen consumption within the material is greater than the rate at which it can be resupplied by diffusion from the surrounding air at mosphere. This effect results in heterogeneously oxidized materials (equilib rium oxidation occurs at the sample surfaces and reduced or nonexistent oxidation occurs in interior regions). Because this physical phenomenon de pends on both temperature and geometry (i.e., sample thickness), understand ing its significance in accelerated simulations is critical to confident predictive extrapolations. A second common problem with the Arrhenius approach is confirming the assumption that the value of E derived under the accelerated conditions remains constant at lower (extrapolated) temperatures. In an attempt to address these concerns, we have been critically exam ining the Arrhenius approach to better understand its capabilities and limi tations. Our first paper (8) on this subject concentrated on the importance and mechanism of D L O effects for thermal aging of neoprene and styrenebutadiene rubber (SBR) elastomers. This chapter describes a detailed study of a nitrile rubber in which we apply the Arrhenius approach to ultimate a


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In Polymer Durability; Clough, Roger L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

34.

GILLEN ET A L .

559

Prediction of Elastomer Lifetimes

tensile-property data from accelerated experiments and show that the complex results can be rationalized by understanding the underlying D L O effects. We also describe some preliminary results in which we used sensitive oxygenconsumption measurements to determine whether the Ε derived from ac celerated conditions remains constant at the low temperatures appropriate to the extrapolation region. A future paper (9) will provide a more detailed dis cussion of this work. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 27, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch034

Ά

Experimental Compression-molded sheets (~2.0-mm thick) of a typical commercial nitrile rubber formulation (100 parts Hycar 1052 resin, 5 parts per hundred [pph] zinc oxide, 1 pph stearic acid, 1.5 pph 2246 [hindered phenol] antioxidant, 65 pph N774 carbon black, 15 pph Hycar 1312, 1.5 pph sulfur, and 1.5 pph 2,2'-dithiobis[benzothiazole]) and a typical commercial neoprene rubber formulation (100 parts Neoprene G N , 4 pph magnesia, 0.5 pph stearic acid, 5 pph zinc oxide, 60 pph hard clay, and 2 pph 2246 antioxidant) were obtained from Burke Rubber Co. The SBR was a proprietary material obtained in sheets (150 X 150 X 2.2 mm) from Parker Seal Corporation. The copolymer had a styrene:butadiene mon omer ratio of 23:77 and number-average molecular weight —425,000. The material was cross-linked by using a sulfur cure and contained 37% by weight carbon black and a hindered phenol stabilizer. Strips approximately 12-mm wide and 150-mm long were cut from the sheets and aged in air-circulating ovens ( ± 1 °C stability). Tensile testing (12.7-cm/min strain rate; 5.1-cm initial jaw separation) was performed using an Instron Model 1130 testing machine equipped with pneumatic grips and having an extensometer clamped to the sample. For each aging time at a given temperature, three samples of the nitrile and neoprene materials typically were tensile tested; for the SBR material, a single sample was tested under eacn aging condition. Modulus profiles were obtained on sample cross sections by using our modulus profiling instrument, which was described in detail elsewhere (10). This instrument allows us to obtain quantitative values of the inverse tensile compliance, D~\ a quantity closely related to the tensile modulus, with a resolution of —50 μηι. Tensile and modulus (D ) values for the unaged materials are given in Table I. Oxygen consumption measurements were performed by sealing known amounts of the material with 16 cm Hg of 0 in glass containers of known volume. To avoid D L O artifacts and therefore to assure that homogeneous oxidation oc curred during the measurements, the material was cut into sufficiendy thin pieces (5-7). The containers were thermally aged for time periods chosen to consume ~40% of the 0 (to make the average partial pressure of 0 during aging ap proximately equal to ambient conditions in Albuquerque, NM). The remaining Ô content was determined using gas chromatography (7). _1

2

2

2

2

Results and Discussion N i t r i l e R u b b e r . Figure 1 shows normalized elongation results (nor malized elongation is e/e , where e is initial elongation value) at the indicated aging temperatures for the nitrile rubber. Each data point typically represents the average result from three identically aged samples. Estimated experimen0

0


560

POLYMER DURABILITY

Table I. Initial Mechanical Properties (MPa)

Material

e (%)

TS (MPa)

D"

Nitrile SBR Neoprene

570 ± 30 260 ± 30 620 ± 50

15.7 ± 0.7 12.8 ± 1 15.2 ± 1

4.3 ± 0.2 4.95 ± 0.2 7.5 ± 0.3

0

0

1


1

0.1

1

10

100

1000

AGING TIME, DAYS Figure 1. Ultimate tensile elongation (e) of the nitnle rubber normalized to its unaged value (e ) versus aging time in air at the indicated temperatures. 0

tal uncertainties range from about ± 0 . 0 5 at high values of e/e to about ± 0.015 at low values. Smooth curves drawn through these data are used to construct conventional Arrhenius plots at 3 levels of damage (e/e = 0.75, 0.5, and 0.25). These plots (Figure 2) are linear, have identical slopes (from which E = 22 ± 2 kcal/mol is calculated), and therefore confirm Arrhenius behav ior. Having determined E from the processed data, we applied the principle of time-temperature superposition (6, 11, 12) to shift all of the unprocessed raw data from Figure 1 to a selected reference temperature, T . This process is accomplished by multiplying the times appropriate to experiments at each temperature, T, by a shift factor a : 0

0

a

a

ref

T

k /1

A'



34.

GlLLEN ET AL.


2.6

2.8

1000/T, K"

561

3

1

Figure 2. Arrhenius plots of elongation results for the nitrile rubber. Using E = 22 kcal/mol, we obtained excellent superposition, as shown for T = 50 °C in Figure 3. However, use of the same shift factor for the normalized TS data did not result in superposition (Figure 4). Indeed, the TS data could never be superposed because the results dropped in the later stages at high temperatures but increased or dropped less at other temperatures. Such behavior indicates fundamental temperature-dependent changes in the degradation mechanism that are contrary to the assumptions of the Arrhenius methodology. Therefore, we are left with a dilemma as to why the Arrhenius approach works well for the elongation but fails for the TS. This dilemma can be re solved through the use of modulus profiling data. Our modulus profiling ap paratus (10) allows us to quantitatively map modulus values across the cross section of degraded samples with a spatial resolution of approximately 50 μπι. Representative modulus profiles for the nitrile rubber at selected aging times and temperatures are shown in Figure 5. At the highest temperature (125 °C), heterogeneity in the modulus is evident at the earliest aging times and becomes quite pronounced later. This effect is caused by D L O in which the rate of oxygen consumption within the material is greater than the rate at which it can be replenished by diffusion from the surrounding air atmosphere (5-8). For aging experiments at lower temperatures (Figure 5), the importance a

ref



562

POLYMER DURABILITY

1.2 1

ο 0 ο ο ο ο Ο

0.8 CO

1-

0.6 Ο

125C 0 110.7C • 94.8C Δ 80.1 C V 64.5C

0.4 0.2 0 0.1

1

100

10

SUPERPOSED DATA A T 50°C, YEARS

Figure 4, Time-temperature superposition of the normalized TS data for th nitrile rubber obtained by using E = 22 kcal/mol. a



100

4

95°C

65°C

100

Figure 5. Modulus profiles for 2.0-mm thick samples of the nitrile rubber after air-oven aging for various times at the i temperatures. The abscissa, P, refers to the percentage of the total distance from one air-exposed sample surface to the exposed surface.

OH

1000

125°C


564

POLYMER DURABILITY

This change occurs because the oxygen consumption rate decreases more rapidly ( £ = 22 kcal/mol) with decreasing temperature than does the oxygen diffusion rate ( E «=* 9 kcal/mol). Later in the degradation, however, hardening (modulus increases) leads to a significant reduction in the oxygen diffusion rate (8) and results i n the delayed appearance of heterogeneity caused again by D L O effects. Given this complexity, it is now easy to rationalize the TS results shown in Figure 4: TS is a property that depends on the force at break integrated over the cross section of the material, and the nitrile rubber sam ples aged at different temperatures clearly experience very different degrees of degradation in their interior regions. The modulus values at the sample surfaces, however, are not affected by D L O anomalies. Figure 6 shows surface modulus values of the nitrile rubber versus time and temperature; these values correspond to the modulus change expected under equilibrium air (i.e., non-DLO) conditions. Following the pro cedure used for the elongation results in Figures 1-3, we shifted these surface modulus values to a 50 °C reference temperature by using an Arrhenius shift factor chosen to achieve the best superposition. The best superposition oc curred with E = 22 ± 2 kcal/mol (Figure 7). Because this value represents the £ appropriate to the underlying oxidative degradation reactions, and be cause it is exactly the same as that found earlier for the elongation, it is clear why Arrhenius behavior occurred for the elongation. When a material is ten sile tested, cracks can be expected to initiate first at the hardened, oxidized surface; if such cracks quickly propagate through the remainder of the material's cross section, the elongation value will be determined by the oxi dative hardening occurring at the surface. A further confirmation that hard ening of the surfaces determines the elongation results can be seen in Figure 8, which shows the experimental correlation for these properties. This cor relation indicates that severe mechanical degradation (e/e 0.1) will occur when the surface modulus value increases by approximately one order of mag nitude. a


a

a

a

0

The correlation between modulus and elongation is anticipated in most theories of rubber elasticity (13, 14). For both Gaussian and non-Gaussian statistical treatments, the extension ratio at break, λ, is predicted to be pro portional to (M )° , where M represents the molecular weight between crossfinks. In addition, the statistical theories predict that the low strain modulus is directly proportional to the (M )~ . This relationship implies that λ, defined as c

5

c

c

1

λ = 1 + OMe

(2)

where e is the elongation i n percent, should be directly proportional to the inverse square root of the modulus. A test of this prediction for the nitrile data is shown in Figure 9. The results quafitatively follow the predicted correlation but quantitatively follow


GILLEN ET A L .


565


34.

Figure 7. Time-temperature superposition of nitnle rubber surface modulu values obtained by using Ε = 22 kcal/mol. β



566

POLYMER DURABILITY

10°

10

10

1

2

1

NORMALIZED SURFACE V A L U E O F D"

Figure 8. Normalized elongation (e/e ) plotted versus the normalized surface modulus value for the nitrile rubber at the indicated temperatures. 0

1/(D- ) - , M P a ^ 1

0

5

5

Figure 9. Extension ratio, λ, plotted versus the inverse square root of the surface modulus for the nitrile rubber at the indicated temperatures.


34.

GILLEN ET A L .


567

only from X 1.5-3. There are numerous reasons for the quantitative failure of the statistical theories. First, the nitrile rubber is a filled material. In filled materials, the mechanical properties will depend on the type and amount of filler; its size, shape, agglomeration behavior; and the chemical nature of its surfaces. These characteristics make comparisons with theory much more dif ficult (14). Second, besides oxidative aging leading to a reduction in M , the incorporation of polar species containing oxygen may also modify the me chanical properties. Oxidation processes could also influence the chemical na ture of the filler particle surfaces and thereby affect the adhesion of filler to elastomer and hence the mechanical properties. Finally, as λ approaches unity, modulus values begin to rise very rapidly and imply that the material is ap proaching its glassy state where rubber elasticity theory is inappropriate.


c

SBR and Neoprene Results. Our earlier study of the SBR and neoprene materials first used modulus profiling results to show that complex time- and temperature-dependent heterogeneous aging effects occurred dur ing accelerated air-oven aging exposures. By incorporating oxygen permeation and consumption measurements, plus antioxidant assay techniques, we then showed that D L O dominated the observed heterogeneities (8). Figures 10 and 11 show modulus profile results for SBR and neoprene, respectively, at three temperatures. Similar to the analysis done in Figures 6 and 7 for the nitrile rubber, we plotted the surface modulus values versus time and tem perature and found the £ that gives the best superposition of the results. For the SBR, this value occurred at E 24 ± 2 kcal/mol; the resulting super position at a reference temperature of 50 °C is shown in Figure 12. For the neoprene, the best superposition occurred for E « 21.5 ± 2 kcal/mol; the superposed results are shown in Figure 13. Figures 10 and 11 show that, similar to the nitrile rubber, dramatic surface hardening occurs for the SBR and neoprene materials under air-oven aging conditions. We would therefore expect cracks to initiate at the surfaces during tensile testing. If these cracks immediately propagate through the material, the ultimate tensile elongation values should time-temperature superpose with the same value of E as the surface modulus values (24 kcal/mol for SBR and 21.5 kcal/mol for the neoprene). This hypothesis is confirmed by the superposed elongation results shown in Figures 14 and 15. By comparing the results from Figures 12 and 14 and from Figures 13 and 15, it is seen that severe mechanical degradation (e/e « 0.1) occurred when the surface mod ulus value increased by factors of about 5 and about 8, respectively, for the SBR and neoprene materials. Although scatter in the TS data for the SBR material is too large to make definitive conclusions, poor superposition occurred when the neoprene TS data was shifted by using a 21.5 kcal/mol E , as seen in Figure 16. This result, of course, was anticipated for reasons similar to those discussed earlier for the nitrile TS results. a

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POLYMER DURABILITY

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Prediction of Elastomer Lifetimes from Accelerated Thermal-Aging

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