Comparison of oxidation induction time measurements with values derived from oxidation induction temperature measurements for EPDM and XLPE polymers.

INTRODUCTION

Polymer insulations used for low-voltage electric control cables in nuclear power plants are exposed to radiation and thermal environments, which accelerate the oxidative degradation process. Oxidation induction time (OIT) analysis provides a useful tool for life-assessment of nuclear-use cables. The usual method for determining OIT is to measure it directly with a differential scanning calorimeter (DSC), which heats a polymer sample in pure oxygen at constant temperature until the antioxidant is entirely consumed. This point is marked by a rapid onset of polymer oxidation, which is an exothermic reaction. The time required to consume the antioxidant is the OIT. As the polymer materials are increasingly damaged by radiation or thermal exposure, the amount of antioxidant is decreased and the OIT shortens to a length indicative of the relative degradation.

A second method can be used to determine OIT. Using measured values of the oxidation induction temperature (OITemperature) and the activation energy, the OIT can be calculated for a given polymeric material. Thermodynamic theory coupled to the oxidation degradation kinetics provides the basis for this calculation. Both methods were applied successfully by Gimzewski to petroleum lubricants (1). An advantage of obtaining the OIT from the OITemperature is that the time required to measure OITemperature is much shorter than the time to measure OIT directly.

The objective of this investigation was to determine if the two methods give the same results for polymer cable insulations. The polymers studied were unaged ethylene propylene diene monomer (EPDM) and unaged crosslinked polyethylene (XLPE). This paper reviews Gimzewski's method for calculating OIT from OITemperature, describes the experimental procedures used in the present research, and compares the results for the two methods for EPDM and XLPE.

THEORY

The theoretical model presented here is the one developed by Gemzewski (1). The model combines oxidative degradation kinetics with fundamental thermodynamic principles used in differential scanning calorimetry. The primary thermally induced oxidation reaction that occurs in the DSC is given by

RH + [O.sub.2] [approaches] R [center dot] + H[O.sub.2]. (1)

where RH is an undamaged polymer chain, R. is a polymer free-radical, and H[O.sub.2]. is a hydroperoxide free-radical. Polymer free-radicals can be produced when energy is imparted to the molecule (for example, by ionizing radiation or elevated thermal exposure) that results in the ejection of an electron. In the absence of antioxidants these free radicals propagate the degradation of the polymer. Antioxidants react preferentially with the free radicals to form inert molecules, thus deterring the degradation of the polymer.

Assuming that the reaction in Eq 1 follows an Arrhenius relationship, as was assumed by Gimzewski (1), the corresponding rate equation for reaction 1 is

d/dt[RH] = -[RH][[O.sub.2]]A[e.sup.-[E.sub.a]/kT] (2)

where A is a constant related to the initial polymer concentration, [E.sub.a] is the activation energy, k is Boltzmann's constant, and T is the test temperature.

Direct measurement of the OIT is done in the isothermal mode of the DSC (time-scanning). In this mode, letting T = [T.sub.iso], Eq 2 can be integrated until the antioxidant is exhausted to give

[[integral of] d[RH]/[RH][[O.sub.2]]A = - [integral of] [e.sup.-[E.sub.a]/kT]] between limits OIT and O = (OIT)[e.sup.-[E.sub.a]k[T.sub.iso]] (3)

Although OIT cannot be solved from this equation, this result will be used in the second method for determining OIT described below.

Alternatively, the DSC can be operated in the temperature scanning mode. This provides a measure of the oxidation induction temperature ([T.sub.ind]), which is the temperature at which the antioxidant is entirely consumed. In this mode the temperature of the polymer sample is raised at a constant rate until the antioxidant is exhausted, i.e., until the oxidation induction temperature is reached. The temperature ramp rate, dT/dt, can be introduced into Eq 2 as follows:

d/dt [RH(dT/dt]) = -[RH][[O.sub.2]] A[e.sup.-[E.sub.a]/kT], (4)

Integration of Eq 4 from the initial temperature, [T.sub.o], to [T.sub.ind] and denoting the ramp rate by [Alpha] give

[integral of] d[RH]/[RH][[O.sub.2]]A = - 1/[Alpha] [integral of] [e.sup.[E.sub.a]/kT] dt between limits [T.sub.ind] and [T.sub.o] (5)

The integrals on the LHS of Eqs 3 and 5 are equal because both are integrated until the antioxidant is exhausted. Therefore the RHS of Eqs 3 and 5 can be set equal. This gives for OIT:

OIT = 1/[Alpha] [e.sup.-[E.sub.a]/[kT.sub.iso]] [integral of] [e.sup.-[E.sub.a]/kT] dT between limits [T.sub.ind] and [T.sub.o]. (6)

By measuring [T.sub.ind] and [E.sub.a], the integral in Eq 6 can be evaluated numerically. Equation 6 provides the basis for a comparison between the OIT measured directly and the OIT calculated from the measured thermal parameters [T.sub.ind] and [E.sub.a]. (The value of [T.sub.o] is unimportant since [T.sub.ind] is always so much greater than [T.sub.o].)

EXPERIMENTAL

Polymers used in this research consisted of two EPDMs and one XLPE in the form of low-voltage electric cable insulation. These particular cables are qualified for use in nuclear reactors for safety control and instrumentation. The two EPDM cables were manufactured by Okonite Co., and were single cable strands from a multi-cable system. EPDM6G is the green strand from a four-conductor cable called Okolon FMR[TM] which is jacketed by Hypalon[TM]. EPDM7J is a single conductor from a three-conductor Okonite cable in which each individual conductor is clad with bonded Hypalon and the bundle is jacketed by another outer layer of Hypalon. The XLPE type is manufactured by Rockbestos, Inc., with the tradename Firewall III[TM]. XLPE8B is singularly spooled and not part of a multiconductor cable. All cables were tested in singular form without the outer Hypalon jacket.

Direct OIT Measurement

OIT measurements were performed with a Perkin-Elmer Thermal Analyzing System TAS 7 and a differential scanning calorimeter DSC 7. Samples were prepared by grinding in a Wiley Mill to a particle size of 20 mesh. A standard U.S. sieve series was used to filter the particles (2). Grinding was performed at room temperature. Comparisons of OITs between samples prepared by grinding and samples prepared by slicing with a razor blade to 20 mesh size particles resulted in reasonable agreement between the two methods (4, 5). For three unaged materials, OITs for all samples prepared by slicing were slightly lower than for those prepared by grinding; the average difference was 3%. The sample mass was held constant at 8.0 mg for all tests. Prepared samples were encapsulated in an aluminum pan with a stainless-steel screen lid mechanically crimped in place. The methodology for performing OIT measurements was based on ASTM guidelines (3) and research by the authors (4, 5).

In the direct measurement of OIT using the isothermal mode of the DSC, samples are immersed in nitrogen purge gas while heated to the isothermal temperature. Pure oxygen is introduced when the desired temperature is reached. OIT is determined graphically from a DSC thermogram. A typical Perkin-Elmer TAS 7 thermogram for a cable insulation material is illustrated in Fig. 1. The y-axis is the differential heat flow needed to maintain the reference and sample assemblies at the isothermal temperature. The curve deviates from the horizontal baseline when the antioxidant is exhausted and the reaction becomes exothermic. The OIT is then defined as the time between the introduction of oxygen into the sample and the intersection of the base fine extension and the exotherm slope.

Two sources of uncertainty are identified for the values of OIT reported here. The first is reproducibility, which varied from [+ or -] 2% for OITs above 100 min to about [+ or -] 10% for OITs below 10 min. The second source exists in the determination of slopes for non-ideal thermograms and the establishment of a baseline for shorter OITs. The two errors were assumed random.

Oxidation Induction Temperature

When the DSC is set to the temperature scanning mode, the polymer sample is subjected to a temperature ramp at a constant rate of change while immersed in oxygen. The antioxidant is consumed in the same manner as before until exhaustion which is marked by a rapid oxidation of the polymer. The temperature at which this exothermic spike occurs is termed the oxidation induction temperature, [T.sub.ind].

Table 1. Measured OIT Values at Varying DSC Isothermal Temperatures.

               DSC Isothermal             OIT
Material   Temperature ([Delta]C)      (minutes)

EPDM6G              195             233 [+ or -] 1
                    205             103 [+ or -] 2
                    215              52 [+ or -] 2
                    225              22 [+ or -] 1
                    235              13 [+ or -] 2

EPDM7J              195             104 [+ or -] 4
                    205              59 [+ or -] 4
                    215              23 [+ or -] 3
                    225              11 [+ or -] 3
                    235              immeasurable

XLPE8B              195             581 [+ or -] 5
                    205             270 [+ or -] 3
                    215             112 [+ or -] 2
                    225              52 [+ or -] 1
                    235              17 [+ or -] 2

Figure 2 shows a typical thermogram depicting an oxidation induction temperature measurement for a cable insulation material. The [T.sub.ind] is measured from the beginning of the test (oxygen is introduced at the outset) to the intersection between the exothermic slope and the horizontal baseline, much in the same way as the OIT. However, the [T.sub.ind] characteristically has a very sharp peak that leaves little margin of error in determining the best-fit slope. The error attached to [T.sub.ind] is below [+ or -] 1 [degrees] C.

A temperature ramp of 5 K/min was used based on Perkin-Elmer's recommendation that a 5-10 K/min rate provides uniform heating of the sample. This is consistent with the rate used by Gimzewski on petroleum products (1). It is recommended that further study be made of the sensitivity of temperature ramp rate on the equivalence of the two methods of determining OIT.

Activation Energy

Experimental measurement of the activation energy requires a bifurcated process that utilizes the DSC. OIT is measured at different DSC test temperatures [TABULAR DATA FOR TABLE 2 OMITTED] within a range of 185 [degrees] C to 240 [degrees] C for each material. The limits are dictated by the composition of polymer cables, specifically the initial antioxidant concentration. OITs are then plotted on a semi-log plot as a function of inverse temperature, which reveals straight-line fits for each material in this temperature range. It is important to measure the activation energy for each specific material in the temperature range near the OITemperature.

Results for OIT as a function of DSC test temperature are given in Table 1 and plotted in Fig. 3. These results show that OIT has an Arrhenius correlation with DSC temperature on the form of

ln(OIT) = A + [E.sub.a]/kT, (7)

where A is the y-intercept. This relationship gives the slope as the measured activation energy, [E.sub.a]. The slopes were calculated from computerized regression fits. Table 2 summarizes the activation energies and oxidation induction temperatures for EPDM6G, EPDM7J, and XLPESB.

[TABULAR DATA FOR TABLE 3 OMITTED]

COMPARISON OF CALCULATED AND MEASURED OIT

In order to compare measured values of OIT with values calculated from OITemperature and activation energies, OITs were measured directly (first method) at several different DSC temperatures ([T.sub.iso]). Next OITs were calculated from Eq 6 (second method) and compared with the measured values. Table 3 lists the results of this comparison. The calculated OITs were in close agreement with the directly measured values. The average absolute value of the difference from the measured value was 4%, with a maximum difference at 7.4%.

The results demonstrate the validity of obtaining OIT from measurement of the OITemperature. Thus, the proposition that the LHS of Eqs 3 and 5 are equal is validated, and the use of a constant value of [E.sub.a] in the integral in Eq 6 is satisfactory for the EPDM and XLPE insulations measured. Thus, the methodology that Gimzewski demonstrated for petroleum lubricants works for EPDM and XLPE insulation.

Comparisons of the two methods were made for unaged materials, and an important use of OIT is the assessment of polymer aging. Therefore, it would be useful to extend the present work to a specific demonstration of the validity of obtaining OIT from OITemperature for aged cable insulation materials.

Although the time required to measure OITemperature is shorter than the time to measure OIT, OIT is generally a more sensitive parameter than OITemperature for experimental comparisons. The lack of sensitivity of OITemperature is observed by comparing the OITemperature values for the three materials in Table 2. Little difference exists between these values. However, the OITs for these materials (at any given DSC temperature) differ significantly, whether measured directly or calculated from the OITemperatures and activation energies.

CONCLUSIONS

Gimzewski's method of coupling oxidation degradation kinetics to thermodynamic theory in order to obtain OIT values from measured values of oxidation induction temperature and activation energy is valid for EPDM and XLPE cable insulation. Measured values of OIT for two EPDM and one XLPE insulations were compared with values calculated from measured oxidation induction temperatures and activation energies, and the average difference between the two methods was only 4%.

ACKNOWLEDGMENTS

This research was performed as part of a larger project supported by the Electric Power Research Institute under Contract No. EPRI RP3427-02. The authors are grateful for the assistance of George Sliter and John J. Carey of EPRI and Gary J. Toman, formerly of Odgen Environmental and Energy Services, throughout the project.

REFERENCES

1. E. Gimzewski, Thermochimica Acta, 198, 133-140 (1992).

2. American Society for Testing and Materials, "Standard Specification for Wire Sieves for Testing Purposes," ASTM E 11 (1987).

3. American Society for Testing and Materials, "Standard Methods of Testing Thermoplastic Insulations and Jackets for Wire and Cable," ASTM D2633-82, reapproved 1989.

4. L. R. Mason, PhD Dissertation, University of Virginia, Charlottesville, Va. (1994).

5. L. R. Mason and A. B. Reynolds, "Reduction of Oxidation Induction Time Testing to Practice as a Life Assessment Technique for Cable Insulation," EPRI TR-106370, Electric Power Research Institute, Palo Alto, Calif. (1996).