Determination of long-term radiation ageing in polymers - Part 2: Procedures for predicting ageing at low dose rates

IEC TS 61244-2:2014, which is a technical specification, applies to procedures for predicting ageing of polymeric materials at low dose rates. The object is to present three methods which can be used to extrapolate data obtained from high dose rate experiments to the low dose rates typical of service conditions. The techniques described are methods which have been found to be useful for a range of elastomeric, thermoplastic and thermoset materials. This edition includes the following significant technical changes with respect to the previous edition:
a) examples and background information moved to annexes;
b) examples updated with more recent references.

Détermination du vieillissement à long terme sous rayonnement dans les polymères - Partie 2: Méthodes pour prédire le vieillissement à faible débit de dose

L'IEC TS 61244-2:2014, qui est une spécification technique, s'applique aux procédures de prévision du vieillissement des matériaux polymères à faible débit de dose. Il s'agit de présenter trois méthodes qui peuvent être utilisées pour extrapoler, à de faibles débits de dose typiques des conditions de service, des données obtenues à partir d'expériences à débit de dose élevé. Les techniques décrites dans les articles suivants sont des méthodes qui se sont révélées utiles pour une gamme de matériaux élastomères et thermoplastiques. Cette édition inclut les modifications techniques majeures suivantes par rapport à l'édition precedente:
a) exemples et informations de fond déplacés dans les annexes;
b) exemples mis à jour avec les dernières références.

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Status
Published
Publication Date
21-Aug-2014
Current Stage
PPUB - Publication issued
Start Date
22-Aug-2014
Completion Date
22-Aug-2014
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IEC TS 61244-2
Edition 2.0 2014-08
TECHNICAL
SPECIFICATION
SPECIFICATION
TECHNIQUE
Determination of long-term radiation ageing in polymers –
Part 2: Procedures for predicting ageing at low dose rates
Détermination du vieillissement à long terme sous rayonnement dans les
polymères –
Partie 2: Méthodes pour prédire le vieillissement à faible débit de dose
IEC TS 61244-2:2014-08(en-fr)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC TS 61244-2
Edition 2.0 2014-08
TECHNICAL
SPECIFICATION
SPECIFICATION
TECHNIQUE
Determination of long-term radiation ageing in polymers –
Part 2: Procedures for predicting ageing at low dose rates
Détermination du vieillissement à long terme sous rayonnement dans les
polymères –
Partie 2: Méthodes pour prédire le vieillissement à faible débit de dose
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX
ICS 17.240; 29.035.01 ISBN 978-2-8322-1828-0

Warning! Make sure that you obtained this publication from an authorized distributor.

Attention! Veuillez vous assurer que vous avez obtenu cette publication via un distributeur agréé.

® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale
---------------------- Page: 3 ----------------------
− 2 − IEC TS 61244-2:2014 © IEC 2014
CONTENTS

FOREWORD......................................................................................................................... 4

1 Scope ............................................................................................................................ 6

2 Normative references .................................................................................................... 6

3 General ......................................................................................................................... 6

4 Power law extrapolation method ..................................................................................... 7

4.1 Description ........................................................................................................... 7

4.2 Test procedure ...................................................................................................... 7

4.3 Determination of model parameters ....................................................................... 7

4.4 Limitations ............................................................................................................ 8

5 Superposition of time dependent data ............................................................................ 9

5.1 Description ........................................................................................................... 9

5.2 Test procedure ...................................................................................................... 9

5.3 Determination of model parameters ..................................................................... 10

5.4 Limitations .......................................................................................................... 13

6 Superposition of DED data ........................................................................................... 14

6.1 Description ......................................................................................................... 14

6.2 Test procedure .................................................................................................... 14

6.3 Evaluation ........................................................................................................... 14

6.4 Limitations .......................................................................................................... 15

Annex A (informative) Behaviour of polymeric materials in radiation environments ............. 17

Annex B (informative) Examples of use of the power law method ........................................ 19

B.1 General ............................................................................................................... 19

B.2 Polypropylene filaments ...................................................................................... 19

B.3 Crosslinked polyethylene (XLPE) ......................................................................... 19

Annex C (informative) Use of the superposition principle .................................................... 21

Annex D (informative) Examples of use of the superposition of time dependent data ........... 23

D.1 Ethylene propylene (EPDM) elastomer ................................................................. 23

D.2 Nitrile elastomer .................................................................................................. 23

D.3 Ethylene vinyl acetate (EVA) polymer .................................................................. 23

Annex E (informative) Examples of use of the superposition of dose to equivalent

damage (DED) data ............................................................................................................ 26

E.1 General ............................................................................................................... 26

E.2 Neoprene cable jacket ......................................................................................... 26

E.3 Chlorosulphonated polyethylene (CSPE) cable jacket .......................................... 26

E.4 Crosslinked polyolefin (XLPO) cable insulation .................................................... 26

E.5 Poly vinyl chloride (PVC) cable jacket .................................................................. 26

Bibliography ....................................................................................................................... 30

Figure 1 – Interpolation of the end-point dose (schematic), showing a plot of relative

elongation at break plotted vs dose with interpolation of DED values at 0,75 and 0,5 .............. 8

Figure 2 – Extrapolation of end-point dose to lower dose rates (schematic) C showing

the plot of DED values vs dose rate....................................................................................... 8

Figure 3 – Limitations – Extrapolation of DED near thermal ageing limit (schematic) ............. 9

Figure 4 – Determining shift factors a (T,0) for thermal ageing ............................................. 10

Figure 5 – Superposition of data to yield master curve ......................................................... 11

---------------------- Page: 4 ----------------------
IEC TS 61244-2:2014 © IEC 2014 – 3 –

Figure 6 – Determination of activation energy E ................................................................... 11

Figure 7 – Determination of shift factors a (T, ) for combined thermal-radiation

ageing, relative to the master curve in Figure 4 ................................................................... 12

Figure 8 – Fitting experimental values of a (T, ) to the empirical model Equation (2) ......... 13

Figure 9 – Calculated DED using Equation (5) ..................................................................... 13

Figure 10 – DED values under combined thermal-radiation conditions (schematic) ............... 15

Figure 11– Superposition of DED data (schematic) .............................................................. 15

Figure A.1 – Schematic illustrating the types of dose rate effects which can occur in

radiation aged polymeric materials ...................................................................................... 18

Figure B.1 – Elongation at break of polypropylene irradiated in air (from [10]) ...................... 19

Figure B.2 – Extrapolation of end-point dose from data in Figure B.1 ................................... 20

Figure B.3 – Dose required to reach 100 % elongation at 20 °C for an XLPE cable

insulation material [11] ........................................................................................................ 20

Figure C.1 – Schematic – Superposition principle for thermal ageing ................................... 21

Figure C.2 – Schematic – Superposition principle for combined thermal-radiation

ageing ................................................................................................................................ 22

Figure D.1 – Experimental data for EPDM elastomer fitted to the superposition model .......... 24

Figure D.2 – Calculated DED for 50 % compression set at 20 °C ......................................... 24

Figure D.3 – Calculated DED for 50 % compression set at 40 °C ......................................... 25

Figure D.4 – Calculated DED for e/e = 0,5 ......................................................................... 25

Figure E.1 – Superposition of DED data at 50 °C for a neoprene cable jacket

material [7] ......................................................................................................................... 27

Figure E.2 – Superposition of DED data for several different CSPE cable jacket

materials ............................................................................................................................ 28

Figure E.3 – Superposition of DED data for a XLPO cable insulation material [7] ................. 29

Figure E.4 – Superposition of DED data for PVC showing complex dose rate

dependence – Homogeneous oxidation data only ................................................................ 29

---------------------- Page: 5 ----------------------
− 4 − IEC TS 61244-2:2014 © IEC 2014
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
DETERMINATION OF LONG-TERM RADIATION AGEING IN POLYMERS –
Part 2: Procedures for predicting ageing at low dose rates
FOREWORD

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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is

indispensable for the correct application of this publication.

9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of

patent rights. IEC shall not be held responsible for identifying any or all such patent rights.

The main task of IEC technical committees is to prepare International Standards. In

exceptional circumstances, a technical committee may propose the publication of a technical

specification when

• the required support cannot be obtained for the publication of an International Standard,

despite repeated efforts, or

• the subject is still under technical development or where, for any other reason, there is the

future but no immediate possibility of an agreement on an International Standard.

Technical specifications are subject to review within three years of publication to decide

whether they can be transformed into International Standards.

IEC TS 61244-2, which is a technical specification, has been prepared by IEC technical

committee 112: Evaluation and qualification of electrical insulating materials and systems.

This second edition cancels and replaces the first edition published in 1996 and constitutes a

technical revision.
---------------------- Page: 6 ----------------------
IEC TS 61244-2:2014 © IEC 2014 – 5 –

This edition includes the following significant technical changes with respect to the previous

edition:
a) examples and background information moved to annexes;
b) examples updated with more recent references.
The text of this technical specification is based on the following documents:
Enquiry draft Report on voting
112/288/DTS 112/305/RVC

Full information on the voting for the approval of this technical specification can be found in

the report on voting indicated in the above table.

This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

A list of all parts in the IEC 61244 series, published under the general title Determination of

long-term ageing in polymers, can be found on the IEC website.

The committee has decided that the contents of this publication will remain unchanged until

the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data

related to the specific publication. At this date, the publication will be
• transformed into an International standard,
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
---------------------- Page: 7 ----------------------
− 6 − IEC TS 61244-2:2014 © IEC 2014
DETERMINATION OF LONG-TERM RADIATION AGEING IN POLYMERS –
Part 2: Procedures for predicting ageing at low dose rates
1 Scope

This part of IEC TS 61244, which is a technical specification, applies to procedures for

predicting ageing of polymeric materials at low dose rates.

The object is to present three methods which can be used to extrapolate data obtained from

high dose rate experiments to the low dose rates typical of service conditions. These methods

assume that homogeneous oxidation has been achieved under the test conditions. The

techniques described in the following clauses are methods which have been found to be

useful for a range of elastomeric, thermoplastic and thermoset materials. The procedures

require a considerable number of test data to enable predictions to be made under low dose

rate conditions.
2 Normative references

The following documents, in whole or in part, are normatively referenced in this document and

are indispensable for its application. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any
amendments) applies.

IEC 60544-2, Guide for determining the effects of ionizing radiation on insulating materials –

Part 2: Procedures for irradiation and test

IEC 61244-1, Determination of long-term radiation ageing in polymers – Part 1: Techniques

for monitoring diffusion-limited oxidation
3 General

The general guidelines of IEC 60544-2 shall be used in the selection of specimen types,

radiation source, dosimetry and temperature control. All irradiations shall be carried out in air

or at constant oxygen overpressure, although as noted in IEC 61244-1, oxygen overpressure

techniques entail some risk of over-ageing the samples. The homogeneity of oxidation

through the specimen thickness can be checked using profiling techniques such as those

described in IEC 61244-1. The test report shall include details of the irradiation source, dose

rate, atmosphere, temperature, sample type and thickness.

All of the procedures described require extensive data obtained over considerable periods of

time. Each method has been found to be of practical use within its limitations, but none of the

methods can be used where there is more than one mechanism operating with different

apparent activation energies.

The power-law extrapolation method (Clause 4) is the simplest of the predictive techniques

and requires the least amount of experimental data. This procedure cannot be used at dose

rates low enough for thermal ageing to dominate, but appears to be valid for extrapolation of

data obtained at near ambient temperatures (20 °C to 30 °C) for polymers such as polyolefins.

Because of the limited data involved, caution should be used in extrapolating by more than a

factor of 10 in dose rate.
---------------------- Page: 8 ----------------------
IEC TS 61244-2:2014 © IEC 2014 – 7 –
Both of the superposition methods can make use of data obtained under combined

thermal/radiation ageing and are able to predict behaviour in the dose rate regime where

thermal degradation is important, but require considerably more experimental data than the

power-law extrapolation method. The superposition of time-dependent data (Clause 5) is not

applicable to all materials; for instance, it cannot be used with materials which exhibit

complex dose rate effects. Where it is applicable, the procedure does lend itself to calculation

of the effects of quite complex temperature-dose rate conditions. The superposition of dose to

equivalent damage (DED) data (Clause 6) can be used for most materials but, like all of the

procedures, it cannot be used to extrapolate through thermal transitions of the polymer.

The general behaviour of polymeric materials aged in radiation environments is described in

Annex A.
4 Power law extrapolation method
4.1 Description

This method is based on the extrapolation of radiation ageing data obtained under isothermal

conditions in air or in oxygen overpressure over a range of dose rates. The upper limit to the

dose rate is such that homogeneous oxidation conditions are achieved. The test data

obtained at the different dose rates are used to determine endpoint criteria which are

extrapolated graphically to the service dose rate.
4.2 Test procedure

The maximum dose rate at which homogeneous oxidation will occur in the test material shall

be assessed. Information in the literature can be used to support an estimation of the

maximum dose rate, or to calculate or measure the oxidation layer thickness (IEC 61244-1).

Once the maximum dose rate has been established, at least two (preferably three) other dose

rates shall be selected, such that the dose rate range covers at least one order of magnitude.

For each of the dose rates selected, samples of the polymer shall be exposed to radiation for

at least four ageing times and a property measured that is sensitive to the degradation of the

material.

NOTE For cable insulation materials, the measured property would usually be elongation at break; for seal

materials, compression set would be appropriate. Suggested properties for other types of component are given in

IEC 60544-2.
4.3 Determination of model parameters

The measured damage parameter is plotted against absorbed dose to establish the endpoint

at each dose rate. A number of endpoint criteria can be interpolated from the graph (Figure 1);

typical endpoint criteria can be the reduction of elongation at break e to 100 % or 50 %

absolute. A sufficient number of absorbed doses shall be used to enable the endpoint criterion

to be established without extrapolation.

The dose at which the end point criterion is reached, i.e. dose to equivalent damage (DED), is

plotted against the dose rate in a log-log plot (Figure 2). For most polymers in the radiation-

dominated region, this plot is found to be linear, with a slope of n, enabling extrapolation to

lower dose rates [1] . The endpoint dose is then given by
= K⋅ D
DED = (1)
where
___________
Numbers in square brackets refer to the Bibliography.
---------------------- Page: 9 ----------------------
− 8 − IEC TS 61244-2:2014 © IEC 2014
D is the dose rate;

K and n are empirical constants specific to the material tested. The value of the parameter

n = <1 and is usually in the range 0 to 0,4.
Experimental data at
constant dose rate
0,75
0,5
0,25
DED 0,75 DED 0,5
Dose
IEC

Figure 1 – Interpolation of the end-point dose (schematic), showing a plot of relative

elongation at break plotted vs dose with interpolation of DED values at 0,75 and 0,5

e/e = 0,5
e/e = 0,75
log dose rate
IEC
NOTE The slope of the plot for each end-point criterion is the parameter n.

Figure 2 – Extrapolation of end-point dose to lower dose rates (schematic) showing the

plot of DED values vs dose rate
4.4 Limitations

This procedure can be a useful method for estimating the behaviour of some polymers at low

dose rates but reference to Figure A.1 immediately shows its potential limitations. For all

materials they have to break down at dose rates low enough for thermal ageing to become

dominant (Figure 3). On a log-log plot of DED versus dose rate used for extrapolation, the

e/e
log DED
---------------------- Page: 10 ----------------------
IEC TS 61244-2:2014 © IEC 2014 – 9 –

thermal ageing limit is represented by a line of slope = 1, i.e. constant time conditions,

whereas the slope of the extrapolated data is generally < 1. Extrapolation to dose rates within

the thermally dominated region would give unrealistically high values for the predicted DED.

This problem can be partially accounted for if separate thermal ageing data are available;

these would allow determination of the appropriate thermal only result. If the additional data

indicate that thermal effects will dominate, the thermal results can be used for predictions.

The power law extrapolation method also cannot be used for materials which exhibit complex

dose rate effects such that the log-log plot of DED versus dose rate is non-linear.

Thermal ageing
limit (slope =1)
Extrapolated
DED value
Actual DED value
log dose rate
IEC

NOTE Extrapolation using the parameter n will give significantly higher estimates of DED if extrapolations are

made near to the thermal ageing limit.

Figure 3 – Limitations – Extrapolation of DED near thermal ageing limit (schematic)

Although the linear extrapolation method assumes that homogeneous oxidation conditions

have been obtained in all of the experiments, it appears to be useful in some materials at

dose rates where heterogeneous oxidation would be expected to occur. This may arise

because cracks generated in the thin oxidized surface layer can then propagate through the

bulk unoxidized material, so that the observed macroscopic properties are determined by

degradation in that sur
...

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