Determination of long-term radiation ageing in polymers - Part 1: Techniques for monitoring diffusion-limited oxidation

IEC TS 61244-1:2014, which is a technical specification, reviews experimental techniques to quantitatively monitor the effects when oxygen is present during ageing of polymers in various environments including temperature, radiation or ultraviolet. This edition includes the following significant technical changes with respect to the previous edition:
a) numerical simulation of DLO is much improved;
b) geometry of samples has been expanded from only the case of the infinite plane to the cylindrical and the spherical cases.

Détermination du vieillissement à long terme sous rayonnement dans les polymères - Partie 1: Techniques pour contrôler l'oxydation limitée par diffusion

L'IEC TS 61244-1:2014, qui est une spécification technique, passe en revue les techniques expérimentales permettant de contrôler les effets en présence d'oxygène pendant le vieillissement des polymères dans divers environnements comprenant la température, le rayonnement et l'ultraviolet. Cette édition inclut les modifications techniques majeures suivantes par rapport à l'édition précédente:
a) la simulation numérique de DLO a été très améliorée;
b) la géométrie des échantillons a été étendue à partir du cas du plan infini uniquement vers les cas cylindriques et sphériques.

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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 61244-1
Edition 2.0 2014-08
TECHNICAL
SPECIFICATION
SPECIFICATION
TECHNIQUE
colour
inside
Determination of long-term radiation ageing in polymers –
Part 1: Techniques for monitoring diffusion-limited oxidation
Détermination du vieillissement à long terme sous rayonnement dans les
polymères –
Partie 1: Techniques pour contrôler l'oxydation limitée par diffusion
IEC TS 61244-1:2014-08(en-fr)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC TS 61244-1
Edition 2.0 2014-08
TECHNICAL
SPECIFICATION
SPECIFICATION
TECHNIQUE
colour
inside
Determination of long-term radiation ageing in polymers –
Part 1: Techniques for monitoring diffusion-limited oxidation
Détermination du vieillissement à long terme sous rayonnement dans les
polymères –
Partie 1: Techniques pour contrôler l'oxydation limitée par diffusion
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX
ICS 17.240; 29.035.01 ISBN 978-2-8322-1827-3

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 61244-1:2014 © IEC 2014
CONTENTS

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

INTRODUCTION ................................................................................................................... 6

1 Scope ............................................................................................................................ 7

2 Profiling techniques to monitor diffusion-limited oxidation ............................................... 7

2.1 General ................................................................................................................. 7

2.2 Infra-red profiling techniques ................................................................................. 7

2.3 Modulus profiling ................................................................................................. 10

2.4 Density profiling .................................................................................................. 14

2.5 Miscellaneous profiling techniques ...................................................................... 16

3 Theoretical treatments of diffusion-limited oxidation ..................................................... 18

4 Permeation measurements .......................................................................................... 21

5 Oxygen consumption measurements ............................................................................ 21

6 Comparison of theory with experimental results ............................................................ 22

7 Oxygen overpressure technique ................................................................................... 23

8 Summary ..................................................................................................................... 25

Annex A (informative) Derivation of theoretical treatment of diffusion-limited oxidation ........ 26

A.1 General ............................................................................................................... 26

A.2 Numerical simulation ........................................................................................... 29

A.3 Cylindrical and spherical geometries and simulation ............................................. 30

A.4 Time dependence of the simulation...................................................................... 35

Bibliography ....................................................................................................................... 37

Figure 1 – Relative oxidation as determined from the carbonyl absorbance versus

depth away from air-exposed surface of polyolefin material after ageing for 6 days at

100 °C (from [18]) ................................................................................................................. 8

Figure 2 – Depth distribution of carbonyl groups in irradiated (0,69 Gy/s) multilayer

samples composed of 4, 18, 27 and 44 films of 22 µm thickness ............................................ 9

Figure 3 – Micro-FTIR spectrophotometric determination of photoproduct and of

residual double-bond profiles in a SBR film photooxidized for 100 h ..................................... 10

Figure 4 – Schematic diagram of modulus profiling apparatus .............................................. 11

Figure 5 – Modulus profiles of 1,68 mm thick commercial fluoro elastomer samples

after air ageing at 5,49 kGy/h and 70 °C to the indicated radiation doses (from [15]) ............ 12

Figure 6 – Modulus profiles of 1,68 mm thick commercial fluoro elastomer samples

after air ageing at 0,90 kGy/h and 70 °C to the indicated radiation doses (from [15]) ............ 12

Figure 7 – Modulus profiles of 1,68 mm thick commercial fluoro elastomer samples

after air ageing at 0,14 kGy/h and 70 °C to the indicated radiation doses (from [15]) ............ 13

Figure 8 – Modulus profiles of 1,9 mm thick chloroprene rubber samples following

elevated temperature exposures in the presence of air at 150 °C, left plot, and 100 °C,

right plot (from [10]) ............................................................................................................ 13

Figure 9 – Experimental density profiles (crosses) for 0,302 cm (left) and 0,18 cm

(right) thick EPDM sheets after ageing at 6,65 kGy/h and 70 °C in airX-ray

microanalysis ...................................................................................................................... 14

Figure 10 – Effect of total radiation dose on XMA profile for 2 mm thick EPDM sheet

irradiated at 1 kGy/h in air (from [24]) .................................................................................. 15

---------------------- Page: 4 ----------------------
IEC TS 61244-1:2014 © IEC 2014 – 3 –

Figure 11 – XMA profiles of 1 mm thick EPDM sheets after thermal ageingin air (from

[24]) 16

Figure 12 – NMR self-diffusion coefficients versus distance away from sample surface

for low-density polyethylene samples after gamma-irradiation in air or vacuum at 0,6

Gy/sec for the indicated total doses (from [26]) .................................................................... 17

Figure 13 – Chemiluminescence profile for a polypropylene material after gamma

irradiation in air to 0,05 MGy at 2 kGy/h (data from [30]) ...................................................... 17

Figure 14 – Theoretical oxidation profiles for various values of α (indicated in the figure)

with β = 0,1 ......................................................................................................................... 19

Figure 15 – Identical to Figure 14, except that β = 10 .......................................................... 20

Figure 16 – Identical to Figure 14, except that β = 1 000 ...................................................... 20

Figure 17 – Plot of α /(β + 1) versus β, where α denotes the value of integrated
c c

oxidation corresponding to 90 % (from [7, 23]) ..................................................................... 21

Figure 18 – Apparatus used for irradiation under pressurized oxygen conditions ................. 24

Figure 19 – Tensile elongation (left) and tensile strength (right) data for an EPR

material aged at the indicated high and low dose-rates in air and at high dose rate in

the pressurized oxygen apparatus of Figure 18.................................................................... 25

Figure A.1 – Simplified kinetic scheme used to represent the oxidation of polymers

(from [44, 45]) ..................................................................................................................... 26

Figure A.2 – Typical example of normalized concentration of oxygen for cylindrical

shape for β=0,01 from [46] .................................................................................................. 31

Figure A.3 – Typical example of relative oxygen consumption for cylindrical shape for

β=0,01 from [46] ................................................................................................................. 31

Figure A.4 – Typical example of normalized concentration of oxygen for cylindrical

shape for β=100 from [46] ................................................................................................... 32

Figure A.5 – Typical example of relative oxygen consumption for cylindrical shape for

β=100 [46] .......................................................................................................................... 32

Figure A.6 – Typical example of normalized concentration of oxygen for spherical

shape for β=0,01 from [46] .................................................................................................. 33

Figure A.7 – Typical example of relative oxygen consumption for spherical shape for

β=0,01 from [46] ................................................................................................................. 33

Figure A.8 – Typical example of normalized concentration of oxygen for spherical

shape for β=100 from [46] ................................................................................................... 34

Figure A.9 – Typical example of relative oxygen consumption for spherical shape for

β=100 [46] .......................................................................................................................... 34

Figure A.10 – Typical example of time-dependent normalized concentration of oxygen

at the centre from for the case of β=1 [46] ........................................................................... 35

Figure A.11 – Typical example of time-dependent normalized concentration of oxygen

at the centre from for the case of α=50 [46] ......................................................................... 36

---------------------- Page: 5 ----------------------
– 4 – IEC TS 61244-1:2014 © IEC 2014
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
DETERMINATION OF LONG-TERM RADIATION AGEING IN POLYMERS –
Part 1: Techniques for monitoring diffusion-limited oxidation
FOREWORD

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

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• 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 61244-1, 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 1993 and constitutes a

technical revision.
---------------------- Page: 6 ----------------------
IEC TS 61244-1:2014 © IEC 2014 – 5 –

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

edition:
a) numerical simulation of DLO is much improved;

b) geometry of samples has been expanded from only the case of the infinite plane to the

cylindrical and the spherical cases.
The text of this specification is based on the following documents:
Enquiry draft Report on voting
112/287/DTS 112/304/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.

IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates

that it contains colours which are considered to be useful for the correct

understanding of its contents. Users should therefore print this document using a

colour printer.
---------------------- Page: 7 ----------------------
– 6 – IEC TS 61244-1:2014 © IEC 2014
INTRODUCTION

It is usually necessary to estimate the anticipated lifetime of a polymeric material in various

usage environments. For extended lifetimes (years), this often requires the application of

accelerated ageing techniques which typically involve the modelling of results obtained at

higher-than-ambient environmental stress levels. For many practical applications, air is

present during environmental exposures – this usually implies that important oxidation effects

underlie the degradation of the material. Unfortunately, exposure of polymers to air during

ageing often results in inhomogeneously oxidized samples, a complication which affects

attempts both to understand the oxidation process and to extrapolate accelerated exposures

to long-term conditions.

The most important inhomogeneous oxidation complication involves diffusion-limited oxidation.

The significance of this complication in various environments, including thermal [1]

radiation [2 to 4] and ultraviolet [5] has been recognized for many years. Diffusion-limited

oxidation can occur whenever the rate of oxygen consumption in a material is greater than the

rate at which oxygen can be resupplied to the interior of the material by diffusion processes

from the surrounding air atmosphere. Such instances result in a smooth drop in the oxygen

concentration from its equilibrium sorption value at the sample surfaces to a diminished or

non-existent value in the sample interior. This will usually lead to a heterogeneity in the

oxidation across the material, with equilibrium oxidation (e.g. corresponding to air-saturated

conditions) occurring at the sample surfaces, and reduced or little oxidation in the interior.

The importance of the effect will clearly depend upon the material geometry, coupled with the

oxygen consumption rate, the oxygen permeability coefficient and the oxygen partial pressure

surrounding the sample [5 to 8]. Since the oxygen consumption rate will typically depend upon

the environmental stress level (e.g. temperature, radiation dose rate) and both the

consumption rate and the permeability coefficient may change as the material degrades [9,

10], the importance of diffusion-limited oxidation will also vary with stress level and

degradation. This often implies that the percentage of the sample which is oxidized under

accelerated (higher-level) environmental conditions is substantially lower than the percentage

oxidized under lower-level application conditions [5 to 7, 10 to 16]. Thus, as has been clear

for many years, in order to confidently extrapolate shorter-term accelerated simulations to

long-term, air-ageing conditions, a critical requirement is the ability to monitor and

quantitatively understand diffusion-limited oxidation effects.

Since a great deal of progress has recently been made in this area, this goal is now realistic.

The purpose of this specification is to review this area. Clause 2 describes experimental

profiling methods which can be used to monitor diffusion-limited oxidation. Theoretical

descriptions of the phenomenon are briefly given in Clause 3. Since the shapes of the

theoretical profiles depend upon the oxygen permeability coefficient and the oxygen

consumption rate, these quantities are measured or estimated in order to quantitatively

validate the theories. Many experimental methods have been developed for measuring

permeability coefficients and a large number of experimental values are available in the

literature. Clause 4 introduces some of the important literature. Experimental methods for

estimating oxygen consumption rates is briefly reviewed in Clause 5. Experimental data

supporting the theoretical treatments is presented in Clause 6. Once confidence in the

theoretical treatments exists, the theories can be used either to choose experimental ageing

conditions so that diffusion effects are unimportant, or to predict the importance of such

effects. If it is impossible to eliminate diffusion effects under air ageing conditions, increasing

the oxygen pressure surrounding the sample during ageing may, in certain instances, be used

to achieve the desired goal, as outlined in Clause 7 on the oxygen overpressure technique.

Part 2 is published as a separate specification and describes procedures for predicting

radiation ageing at low dose rates.
_____________
Figures in square brackets refer to the Bibliography.
---------------------- Page: 8 ----------------------
IEC TS 61244-1:2014 © IEC 2014 – 7 –
DETERMINATION OF LONG-TERM RADIATION AGEING IN POLYMERS –
Part 1: Techniques for monitoring diffusion-limited oxidation
1 Scope

This part of IEC TS 61244, which is a technical specification, reviews experimental techniques

to quantitatively monitor the effects when oxygen is present during ageing of polymers in

various environments including temperature, radiation or ultraviolet.

Inhomogenous ageing effects caused by diffusion-limited oxidation are often encountered and

provide theoretical equations to estimate their importance. These effects make it difficult to

understand the ageing process and to extrapolate accelerated exposure to long-term

conditions.

It is widely known that mechanical properties degrade prior to electrical properties.These

changes are consequences of chemical changes such as oxidation. In this technical

specification, only mechanical or chemical monitoring techniques are of interest.

This technical specification does not deal with electrical monitoring techniques.

2 Profiling techniques to monitor diffusion-limited oxidation
2.1 General

The presence of diffusion-limited oxidation effects implies that various properties related to

the amount of oxidation will depend upon spatial location in the material. Thus, any technique

which can profile (map) these spatial variations will allow diffusion-limited oxidation to be

monitored. Since polymer geometries utilize cross-sections down to a few millimetres or less,

and since diffusion-limited oxidation effects are operative over such small dimensions, a

useful profiling technique has to have a resolution of at least 100 µm. An additional problem

related to sensitivity is the observation that severe polymer degradation typically corresponds

to less than 1 % of the polymer being oxidized. Thus, a useful profiling technique shall have

reasonable resolution, good sensitivity to the small chemical changes which occur, wide

applicability and relative ease of operation and analysis. A number of particularly useful

techniques are briefly described in this clause.
2.2 Infra-red profiling techniques

Because of the ability to provide detailed chemical information on thin film samples, infra-red

spectroscopy has been used to monitor diffusion-limited oxidation effects for more than

25 years [17]. Any oxidation-sensitive infra-red peak that can be monitored, either as a

function of sample thickness, or as a function of sequentially microtomed slices, will yield

information on oxidation heterogeneities. Many of the studies to date have concentrated on

the carbonyl region (approximately 1 720 cm ) of polyolefin materials, such as polyethylene

and polypropylene, since infra-red peaks in this region are characterized by high extinction

coefficients (high sensitivities) and are usually absent from these materials when unaged.

Since the carbonyl region typically represents a superposition of a number of oxidation

products (e.g. ketones, aldehydes, esters, acids) of differing extinction coefficients at slightly

different wavelengths, simplifying assumptions are often needed to extract semi-quantitative

information. In most cases, either the maximum height of the hybrid carbonyl peak or its area

is chosen. It should be noted that additives present in commercially formulated materials (e.g.

antioxidants, fire retardants) often absorb in the carbonyl region, thereby
...

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