IEC TS 61244-1:2014

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.

General Information

• Status: Published
• Publication Date: 21-Aug-2014
• ICS: 17.240 - Radiation measurements
• ICS: 29.035.01 - Insulating materials in general
• Technical Committee: TC 112 - Evaluation and qualification of electrical insulating materials and systems
• Drafting Committee: WG 2 - TC 112/WG 2
• Current Stage: PPUB - Publication issued
• Start Date: 22-Aug-2014
• Completion Date: 30-Nov-2014

Standards Content (Sample)

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

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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

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
X
CODE PRIX
ICS 17.240; 29.035.01 ISBN 978-2-8322-1827-3

– 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

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

– 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 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 61244-1, 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.
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.
– 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.

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
–1
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 complicating
attempts to use FTIR spectrometry for these materials.

– 8 – IEC TS 61244-1:2014 © IEC 2014
An example of an infra-red profile obtained after microtoming slices off an aged material is
shown in Figure 1 [18]. A polyolefin material was aged in air for 6 days at 100 °C and the
relative oxidation (the absorbance of the carbonyl peak) is plotted versus the depth away from
the air-exposed sample surface. The oxidation drops with depth with an approximate
exponential dependence; similarly shaped profiles are often observed for heat-aged materials
[10, 13, 14].
0 125 250 375 500
Depth  (µm)
IEC
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])
A second infra-red approach is to create multilayer samples by packing thin films together
under mechanical pressure. After ageing, the individual films are separated then individually
analysed. Carbonyl profiles obtained in this fashion for gamma-radiation ageing in air of an
unstabilized low-density polyethylene material are shown in Figure 2 [19]. The profiles are
symmetrical, since both surfaces of the multilayer samples were exposed to air. For these
samples, the profiles show a fairly abrupt transition between completely oxidized and
unoxidized regions, quite different behaviour from the "exponential" shape observed in
Figure 1.
Another interesting advance is the use of micro FTIR spectroscopy as a profiling method.
Jouan and co-workers [20, 21] pioneered this approach and have used it in photo-oxidation
studies to profile the carbonyl peak of a PVC material [20], and product profiles for styrene-
butadiene (SBR) and nitrile rubbers [21]. Figure 3 shows product profiles for an SBR film
photo-oxidized for 100 h and surrounded on both sides by air [21]. In this case, the drop-off in
oxidation away from the surface is similar in shape to the result shown in Figure 1.
Relative oxidation
150 150
100 100
50 50
0 0
0 2 3 4 1 5 10 15 18
1 5 10
15 20 25 27 1 5 10 15 20 25 30 35 40 44
IEC
Key
  0,14
∆  0,30
×  0,45
+ 0,60 MGy (from [19])
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

– 10 – IEC TS 61244-1:2014 © IEC 2014
1,3
0,3
1,1
0,9
0,2
0,7
0,1
0,5
1 35 70 105 140 175 210
d  (µm)
IEC
Key
–1 –1
 carbonyl at 1 717 cm + 1,4-trans at 962 cm
–1
–1
 1,2 at 910 cm
× hydroxyl at 3 440 cm
–1
 1,4-cis at 700 cm d represents the width of the slice (from [21])
Figure 3 – Micro-FTIR spectrophotometric determination of photoproduct
and of residual double-bond profiles in a SBR film photooxidized for 100 h
2.3 Modulus profiling
The modulus profiling technique [14, 15] allows one to obtain rapidly and accurately more
than 20 quantitative tensile compliance (D) measurements per millimeter of sample cross-
section (1/D is closely related to the tensile modulus of the material). This technique is
especially useful for elastomers, since the modulus of such materials is very sensitive to
scission and cross-linking events and therefore to processing and ageing.
The instrument, which is based on modifications of a thermomechanical analyser, is shown
schematically in Figure 4. The apparatus measures the indentation of a tiny, paraboloidally-
tipped indentor into the sample. A tiny vice (shown in the figure), is used to hold the cross-
sectioned samples. After the vice assembly is metallographically polished, indentation
measurements under a chosen load are made at selected locations across the cross-
sectioned surface. An optical microscope and an X-Y-Z linear positioner are used to quantify
the measurement locations. For samples of rectangular cross-section, three samples are held
in the vice in a sandwich arrangement with the profiling done across the middle sample. This
avoids edge artefacts caused by the high-modulus aluminum plates used as part of the vice.
The accuracy (within better than ±10 % of conventional modulus measurements),
reproducibility (typically better than ±5 %) and linearity (with load) of the apparatus have been
demonstrated on a variety of elastomeric materials [14].
Figures 5, 6 and 7 show modulus profile results for 1,68 mm thick sheets of a commercial
fluoro elastomer rubber, which were gamma-radiation aged in air at 70 °C at three different
dose rates. The data are plotted on a normalized thickness basis in which the ordinate, P,
represents the percentage of the distance from one air-exposed surface of the sample to the
opposite air-exposed surface. These profiles have shapes that appear to be intermediate
between the "exponential" or "U-shaped" profiles shown in Figures 1 and 3 and the "step-
∆OD
OD
shaped" profiles shown in Figure 2. For unaged commercial fuoro elastomer, the modulus is
independent of cross-sectional position and equal to 5,4 MPa; this result is denoted by the
horizontal line labelled "unaged". At the highest radiation dose rate of 5,49 kGy/h (Figure 5),
spectacular heterogeneity, caused by diffusion-limited oxidation, develops with ageing.
Oxidative scission occurs near the air-exposed sample surfaces, leading to rapid decreases in
modulus. Ageing occurs under essentially anaerobic (inert) conditions in the sample interior,
yielding a cross-link-dominated increase in modulus. Since the heterogeneity can be observed
after less than 0,04 MGy, which corresponds to relatively moderate changes (10 % to 20 %) in
ultimate tensile properties [6, 7, 11], modulus profiling can clearly be sensitive to the earlier
stages of ageing. Figure 6 gives results at a six times lower dose rate of 0,9 kGy/h, where
diffusion-limited oxidation effects are reduced but still evident. Finally, at 0,14 kGy/h
(Figure 7), oxidation has been slowed down sufficiently to assure homogeneous oxidation
throughout the sample. When high dose-rate is adopted for radiation ageing, which is typically
in accelerated ageing conditions, cross-linking is macroscopically dominant; on the other hand,
in the case of low dose-rate and long-term radiation ageing, scission is dominant. Such
results clearly underscore the danger that occurs whenever important diffusion-limited
oxidation effects exist for accelerated environments, if the accelerated results are used to
make predictions under long-term, low-level environments.
Modulus profiling results [10] for 1,9 mm thick chloroprene rubber samples after ageing in
an air-circulating oven at 150 °C and 100 °C are shown in Figure 8. Significant and
complicated diffusion-limited oxidation effects are evident. At the higher temperature of
150 °C (left plot), diffusion effects exist at the earliest stages of ageing. At the lower
temperature of 100 °C (right plot), diffusion effects appear to be less important. In fact, at
the early stages of ageing, the oxidation appears to be essentially homogeneous. In the
later stages of ageing, however, important diffusion effects become apparent. This
phenomenon, of increasingly important diffusion-limited effects with ageing time, is common
for elastomers which are thermally aged in air. It is often caused by substantial decreases in
oxygen permeation rate which occur as the polymer hardens (modulus increases) with
progressive ageing. Other factors contribute [10], for example, the rate of oxygen
consumption may increase with ageing time. Sorting out these complicated diffusion-limited
effects is clearly necessary if results from accelerated temperature exposures are used to
make long-term predictions at much lower temperatures.
M
Weight tray
Silicone oil
Float
suspension
S
Sample holder
LVDT S
Probe P
Equilibrium
Alignment
float position
device
X-Y-Z
Linear positioner
IEC
NOTE The detailed top view of the sample-holder shows three samples labelled with an S held between metal
plates P. The detail to the left shows a side view of the sample-holder held in the alignment device (from [14, 15]).
Figure 4 – Schematic diagram of modulus profiling apparatus

– 12 – IEC TS 61244-1:2014 © IEC 2014
Unaged
0,04 MGy
0,10 MGy
0,23 MGy
0,49 MGy
0,96 MGy
0,3
0 20 40 60 80 100
P  (%)
IEC
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])
Unaged
0,11 MGy
0,23 MGy
0,47 MGy
0,74 MGy
0,3
0 20 40 60 80 100
P  (%)
IEC
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])
-1
-1
D  (MPa)
D  (MPa)
0,11 MGy
0,21 MGy
0,41 MGy
0,75 MGy
Unaged
0,3
0 20 40 60 80 100
P  (%)
IEC
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])
4 4
10 10
Virgin
Virgin 10 days
1 day 17 days
75 days
2 days
3 3
10 10
4 days 123 days
5 days
156 days
2 2
10 10
1 1
10 10
0 25 50 75 100 0 25 50 75 100
P  (%) P  (%)
IEC
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])
-1
D  (MPa)
-1
D  (MPa)
-1
D  (MPa)
– 14 – IEC TS 61244-1:2014 © IEC 2014
2.4 Density profiling
The density profiling technique [13] is based on the use of a density gradient column to obtain
the density of successive thin slices cut across a sample. It depends on the fact that oxidation
reactions often lead to substantial and easily measurable increases in polymer density.
Although a number of methods are available for measuring density, only the density gradient
column can yield accurate results on the extremely small samples necessary for achieving the
required spatial resolution. To use the density profiling approach, one has to find liquids for
the column that are not significantly absorbed by the polymer, since swelling of the material
will complicate the interpretation of the data. Since absorption of water by most polyolefins is
small, density profiling has been successfully applied to these materials using aqueous salt
solutions. Some example density profiling data are plotted in Figure 9 for two thicknesses of a
commercially formulated EPDM rubber material after radiation ageing in air to 0,32 MGy at
6,65 kGy/h and 70 °C [22]. Since the amount of oxidation is approximately linearly related to
the density increase, density changes are plotted as crosses versus cross-sectional position.
The vertical span of each cross represents the estimated experimental uncertainty of the
measurement, whereas the horizontal span denotes the position and thickness of each slice.
The shape of these profiles again lies in the regime that is intermediate between "U-shaped"
and "step-shaped". As expected, diffusion-limited oxidation effects become less important as
the sample thickness is reduced.
Since density profiling is a much more difficult and time-consuming technique than modulus
profiling, the latter technique is generally preferred where applicable, especially for
elastomers. However, under certain ageing conditions, elastomers may degrade with an
approximate balance between scission and cross-linking events, implying little sensitivity of
modulus to ageing. In such instances, density profiling may prove helpful.
60 60
α = 154,5 α = 55
β = 6 β = 6
50 50
40 40
30 30
20 20
10 10
0 0
0 20 40 60 80 100 0 20 40 60 80 100
P  (%) P  (%)
IEC
NOTE The curves give theoretical fits to the experimental results (from [22, 23]).
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
Another developed technique for monitoring diffusion-limited oxidation effects involves the
use of X-ray microanalysis (XMA) [24]. During oxidation, common products which result on
-4
∆ density  (10 g/cc)
-4
∆ density  (10 g/cc)
the polymer chain are carboxyl containing groups and peroxide groups. After ageing, a cross-
sectional slice of the material is exposed and dipped in a 0,1 N KOH- isopropanol solution in
order to convert these groups to potassium-containing species. After conversion, the profile of
potassium will therefore represent the oxidation profile for these two oxidation species. The
potassium distribution is measured using an electron probe X-ray microanalyser [24].
Figure 10 shows some representative results for an EPDM material which was radiation aged
in air at 1 kGy/h and room temperature. The results indicate that the thickness of the oxidized
region is reasonably constant up to doses of at least 330 kGy, implying that the oxygen
consumption rate and the oxygen permeability coefficients are reasonably constant up to this
dose. Figure 11 shows XMA profiles for this same material after various thermal ageing
exposures. It is interesting to note from the 70 °C results, that homogeneous oxidation occurs
for ageing times up to 4 000 h, but that heterogeneous behaviour becomes significant at
longer times. This clearly indicates that the permeability and/or the consumption rate change
significantly with ageing time. It is also interesting to note the very different profile shapes
observed for the two different ageing environments (radiation versus thermal). In fact, the
general shapes observed with all of the profiling techniques for thermal ageing exposures
(see Figures 1, 8 and 11) and for radiation exposures (see Figures 2, 5, 6, 9 and 10) are
similar.
Sheet surface
330 kGy
Oxidized
Unoxidized
100 100
Original
0 1 2
Depth  (mm)
IEC
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])
XMA  (counts/s of potassium)
– 16 – IEC TS 61244-1:2014 © IEC 2014
400 800
Unoxidized
6 300 h
600 h
Oxidized
5 600 h
500 h
4 800 h
300 h
200 200
...