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ELSJSVIER Nuclear Instruments and Methods in Physics Research B I3 1 (1997) 192- 197

Evolution of ethylene propylene copolymers properties during ageing

R. Clavreul EDF, DER / 77250 Moret SW Loing, France

Abstract

Ageing of Ethylene Propylene copolymer used as joints for electrical materiels in nuclear power plants has been studied in EDF (Electricitt de France). The objective was to determine the elastomer service lifetime in normal and accidental

conditions. This material is thermally stable up to 130°C for long term service and it can be used until a radiation dose of at least 60 kGy. If the probability to have no more waterproofness in normal conditions during 30-50 years at 40°C and at 0.01 Gy/h is low, it could be quite different in accidental conditions where the temperature could be higher than 150°C. That is why we first studied the thermal ageing of this material at constant temperatures from 90°C to 150°C. During ethylene propylene copolymer ageing, we examined the evolution of mechanical properties and morphology. Mechanical properties such as elongation at break and tensile strengh allowed us to appreciate the material service lifetime according to IEC 544-2 (Guide for determining the effects of ionizing radiation on insulating materials). Besides, the evolution of morphology including density and oxidation was examined in order to determine whether crosslinking or chain scission was predominant at high temperatures. The lifetime prediction of ethylene propylene in accidental thermal conditions was estimated by applying a kinetic model where the constant depends on the Arrhenius law. In a second step, we proceeded to a radiation ageing on non-aged ethylene propylene copolymer samples according to normal and accidental conditions. The preliminary results obtained under gamma radiation are presented.

1. Introduction

Ageing of ethylene propylene copolymer used as an insulator for cables under nuclear environment was studied for about 20 years. Nevertheless, the main results have been obtained during the last ten years. Besides, ageing models have been proposed. For example, a practical model has been applied to prediction of EPDM-CSPE and PVC cables in nu- clear plants subjected to thermal (40°C) and radiation (0.01 Gy/h) stresses for periods up to 30 years [l]. The main properties of these insulants would stay

correct during the first 50 years if no accident would happen during this period. Nevertheless, the proper- ties of ethylene propylene copolymer used as pieces of electrical equipments placed in the reactor of nuclear plants could change during accidental condi- tions. These equipments must operate under acciden- tal conditions at a high temperature that can increase from 40°C to 160°C or more, and at a high dose rate of l- 10 kGy/h. That is why we first studied the effect of a high temperature applied to ethylene propylene copolymer used as joints in electrical ma- terials.

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R. Claureul/Nucl. Instr. and Meth. in Phys. Res. B 131 (1997) 192-197 193

2. Literature about ageing of ethylene propylene copolymer

2.1. Thermal ageing

2.1.1. Effect of temperature The elastomer used as joints for electrical materi-

als has the following formula: Ethylene propylene

copolymer = 50%; Carbon black = 40%; Other addi-

tives = 10%. This formulated material can be used until 130°C

for a long time service, or until 170°C for a short time service [2]. So, we decided to proceed to a thermal ageing at constant temperatures: 90°C

110°C 12O”C, 130°C and 170°C.

2.1.2. Mechanism of thermal ageing

The joints in electrical materials for nuclear power plants are directly in contact with air. So, the main

effect of temperature on the elastomer is thermo-

oxidation [5]: Initiation:

R-R+R. +R’. .

Propagation:

ii. +O,+ROi.,

ROi - +R’H+RO,H+R’. .

Termination:

ROi. +ROi. ---) inactive products

ii.. +ROi. + inactive products

According to this mechanism, we can expect to have the elastomer degradation during thermal age- ing: chains scissions and radicals oxidation. This last phenomenon will probably depend on oxygen diffu- sion in the material.

2.1.3. Arrhenius model

The most commonly used model for thermal age- ing is the Arrhenius one [61:

t = t,exp ( E,/RT)

with t = time to reach a predetermined end point; A = constant, E, = activation energy &J/mol), T =

temperature (K).

2.2. Radiation ageing

2.2.1. Effect of gamma radiation

The effect of gamma radiation from 10 Gy/h to

2500 Gy/h was studied four years ago on ethylene propylene copolymer without any additive [3]. These experiments were carried out on 2 mm thick samples

for EDF (Electricite de France) applications:

(1) For low dose rates from 10 Gy/h to 100

Gy/h, the polymer degradation was homogeneous.

These results were adapted to predict the material

lifetime. (21 A change of the polymer degradation appeared

between 100 and 500 Gy/h. The material evolution

became heterogeneous because the oxidation was

controlled by oxygen diffusion. (3) A heterogeneous degradation was then sys-

tematically observed from 500 Gy/h to 2500 Gy/h. The gamma radiation systematically led to a de-

crease in mechanical properties. When temperature and gamma radiations were simultaneously applied, the radiation effect was dominant at low dose rates.

Ethylene propylene copolymer was also studied

by CERN at high dose rates (European Organization for Nuclear Research) [4]: the degradation became severe towards lo6 Gy.

So, we have retained the following dose rates for gamma radiation ageing of elastomeric joints:

(i) 100 Gy/h until 250 kGy for normal condi- tions;

(ii) 30000 Gy/h until 600 kGy for accidental conditions.

2.2.2. Mechanism of radiation ageing

Gamma radiation of high energy in ethylene propylene copolymer lead to the formation of radi- cals, positive ions and energized molecules. These interactions are randomly produced 131:

AB+AB++e-,

AB-+A++i)* +e-,

AB+AB*.

In a second step, secondary reactions occur from primary species:

AB++e--+AB*,

AB*-,A.+ii.,

A++e-+A’ -.

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194 R. Claureul/Nucl. Instr. andkleth. in Phys. Res. B I31 (1997) 192-197

Two global mechanisms are often competitive: chains scissions and crosslinking. The predominant

mechanism depends on the environmental ageing conditions [ 1,8].

3. Ageing characterization methods

The experimental procedure was composed of the following steps [7]:

* Ageing; - Mechanical characterization after ageing:

Elongation at break; Tensile strengh.

The thermal ageing was carried out on dumbell-

shaped tensile test pieces (2 mm thick samples) at constant temperatures in ambient air: 90°C 1 lO”C,

12O”C, 130°C and 150°C. Systematic sampling has then been done in order to determine their time

dependent properties.

* Chemical evolution after ageing:

Morphology: density, hardness;

Oxidation: infrared spectroscopy; Crosslinking: swelling in a solvent; Chains scission: extraction by a solvent.

For the present work, thermal ageing preliminary

accelerated tests have shown that the most sensible characterization method was the “elongation at

break”. So, the following results are presented in

the present paper:

4.1.2. Mechanical properties

The evolution of the mechanical properties with thermal ageing is given in Figs. 1 and 2. We can

observe in Fig. 1 that the decreasing of the elonga- tion at break with respect to the time ageing is as much important as the temperature is high. The tensile strengh in Fig. 2 is also decreasing with the time ageing and the temperature. So, if two mecha- nisms may occur simultaneously during thermal age- ing [I], the chains scissions are predominant.

(i) The mechanical properties which have first We have arbitrarily determined, in accordance to

been applied to populations of IO- 12 aged dumbell- IEC 544, that the ethylene propylene copolymer

shaped pieces in accordance to ISO/R 537; endlife was obtained when the elongation at break

(ii) The density (morphology) which was mea- was 50% of the initial value. So, according to the

sured in accordance to ISO/R 1123. experimental results from which we have checked

4. Experimental results and discussion

4.1. Thermal ageing

4.1 .I. Thermal ageing experiments

0

0 50 100 150 200 250 300 350 400

Tim. ,fl.y*,

Fig. 1. Thermal ageing of ethylene propylene copolymer: Effect of the temperature on the elongation at break.

R. Claureul/ Nucl. Instr. and Meth. in Phys. Res. B 131 (1997) 192-197 195

16

2

0

1 0 SO 100 150 200 250 300 350 400

Time (days)

Fig. 2. Thermal ageing of ethylene propylene copolymer: Effect of the temperature on the tensile strength.

the Arrhenius law, the lifetime t in relation to the temperature T is the following:

Ln( t) = - 33.4 + 15400( l/T)

with the following units: t in seconds; T in Kelvin.

4.1.3. Density The evolution of the density in relation to the time

ageing is given in Fig. 3. The density is increasing with the time ageing and it is more important as the temperature is high. The increasing of the density conforms that oxidation of the elastomer also occurs during the thermal ageing. Infrared spectra obtained in the ethylene propylene copolymer extracted phase (in chloroform) showed the presence of carbonyl bands between 1720 cm- ’ and 1770 cm- ’ .

50 100 150 200 250 300 350

Time (days)

Fig. 3. Thermal ageing of ethylene propylene copolymer: Effect of the temperature on density

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196 R. Claoreul/Nucl. Insrr. andhfeth. in Phys. Res. B 131 (1997) 192-197

Fig. 4. Radiation ageing of ethylene propylene copolymer: Effect of the temperature on the elongation at break.

4.1.4. Conclusion The ethylene propylene copolymer lifetime is

about 20 days at 15O”C, 3 months at 130°C. From the Arrhenius law, it is possible to have a prediction

of the elastomer lifetime at 40-50°C: it is longer than 50 years. These results confirm those previously obtained in cables insulators [l]. The chains scissions mechanism is predominant from 120°C to 150°C. Besides, oxidation has also been characterized. We

have now to check that the ageing mechanisms are the same at lower temperatures.

4.2. Gamma radiation ageing

4.2.1. Gamma radiation ageing experiments The gamma radiation ageing was carried out at a

low dose rate of 100 Gy/h and at three different temperatures: ambient temperature, 50°C and 70°C.

20 --

0 0 100 200 300 l oo 500 500

Dose (kGy)

Fig. 5. Radiation ageing of ethylene propylene copolymer: Effect of the dose rate on the elongation at break.

R. Claureul/ Nucl. Instr. and Meth. in Phys. Res. B 131 (19971 192-197 197

Systematic sampling has then been done from 50 kGy to 150 kGy cumulated doses (Fig. 4).

4.2.2. Mechanical properties

The evolution of the mechanical properties with

gamma radiation has been studied. The elongation at break and the tensile strengh are decreasing with the

cumulated dose until 150 kGy. No significant effect of the temperature from 20°C to 70°C has been

observed. According to these results, we can con-

clude that chains scissions are predominant [I].

4.2.3. Effect of the dose rate

Preliminary experiments about the effect of the dose rate have also been done. These experiments have been carried out at a high dose rate of 30000

Gy/h and at 70°C (Fig. 5). No significative differ- ence has been observed between 100 Gy/h and 30000 Gy/h until 150 kGy. More experiments are

necessary to conclude about the effect of the dose

rate.

4.2.4. Conclusion

Ethylene propylene copolymers have been aged under gamma radiation according to normal and accidental conditions. The main mechanism has been determined: chains scissions. Besides, oxidation probably occurs in ambient air.

5. Conclusion

The evolution of ethylene propylene copolymer properties has been studied during thermal and

gamma radiation ageing. First, thermal ageing has

been carried out at high temperatures from 90°C to

150°C. Several mechanisms occur simultaneously:

chains scissions and oxidation. The Arrhenius law

has been checked and we can make the prediction of

ethylene propylene copolymer lifetime at 40-50°C: it is probably longer than 50 years. Besides, it is

about 3 months at 130°C and 20 days at 150°C. In a

second step, gamma radiation ageing has been done according to normal and accidental conditions. The

preliminary results obtained in normal conditions at

100 Gy/h do not show any effect of the temperature

between 20°C and 70°C and the mechanisms are probably the same as those identified during the

thermal ageing. Besides, the comparison between normal and accidental conditions at 30000 Gy/h

until 150 kGy do not show any significant difference when we examine the mechanical properties. These results would merit to be completed by experiments

at intermediate dose rates between 100 Gy/h and

30000 Gy/h and at higher cumulative doses.

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III. PHOTON/ELECTRON/PROTON IRRADIATION