ISO 23706:2020

INTERNATIONAL ISO
STANDARD 23706
First edition
2020-08
Plastics — Determination of apparent
activation energies of property changes
in standard weathering test methods
Reference number
ISO 23706:2020(E)
©
ISO 2020

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ISO 23706:2020(E)

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ISO 23706:2020(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and abbreviated terms . 1
3.1 Terms and definitions . 1
3.2 Abbreviated terms . 2
4 Principle . 2
5 Apparatus . 3
5.1 General . 3
5.2 Surface temperature measurement . 3
6 Test specimens. 3
7 Test method . 3
7.1 Test conditions . 3
7.2 Test duration . 4
7.3 Increased/decreased temperatures . 4
7.4 Calculation of activation energy, E .
a 4
7.4.1 Exposures at two different temperatures . 4
7.4.2 Exposures at two or more different temperatures . 5
8 Exposure conditions . 5
9 Procedure. 5
9.1 General . 5
9.2 Mounting the test specimens . 5
9.3 Exposure . 5
9.4 Measurement of radiant exposure . 5
9.5 Determination of changes in properties after exposure . 6
10 Test report . 6
Annex A (normative) Arrhenius concept . 7
Annex B (informative) Examples .10
Annex C (informative) Effective temperature calculation .14
Annex D (informative) Potential contributions to the uncertainty budget .15
Bibliography .17
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ISO 23706:2020(E)

Foreword
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ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
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This document was prepared by Technical Committee ISO/TC 61, Plastics, Subcommittee SC 6, Ageing,
chemical and environmental resistance.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
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ISO 23706:2020(E)

Introduction
Temperature is an important influencing factor on correlation and acceleration in lightfastness and
weathering testing. This is acknowledged in international general weathering standards like ISO 4892-1
or ISO 16474-1.
The Arrhenius concept describes the influence of temperature on the reaction rate of chemical reaction.
This model is also used (with limitations) to describe the reaction rate of photochemical/weathering
reactions. Several assumptions and limitations should be taken into account; the Arrhenius concept
might not be sufficient to describe complex degradation behaviours. However, this basic approach
might help to better understand the influence of temperature on weathering degradation of polymeric
materials.
An important material specific property to describe the temperature influence on degradation reactions
is the activation energy E , which describes the required energy barrier for a chemical reaction to
a
occur. E is only valid for a specific material and a specific degradation pathway. In weathering, this
a
degradation pathway is usually attributed to a specific property change.
The Arrhenius principle is applied to determine the temperature dependency of the thermal
degradation of polymers. The thermal activation energy can be calculated based on ISO 11358-2. There
is no such standard to determine the activation barrier for weathering or photo-chemical degradation
of polymers.
Limited information is available on the activation energies of photochemical degradation/weathering
reactions. To determine activation energies, weathering experiments at least at two different
temperatures are required. Due to this high effort, available activation energies are often calculated
based on unsuitable weathering experiments (for example based on outdoor weathering and accelerated
weathering).
There are some basic test requirements which help to increase the significance of the Arrhenius
concept and the determination of the activation energy in accelerated weathering. These requirements
are described in this document.
NOTE 1 Different property changes, for example yellowing and surface cracking of one material can have
different activation energies.
NOTE 2 In this approach, the activation energy is considered as temperature independent for the selected
temperature range.
The determination of activation energies of photo-chemical degradation processes allows including the
temperature into the evaluation and correlation of different weathering experiments as quantitative
factor. Knowing the activation energy improves the reliability and predictive value of artificial
weathering.
NOTE 3 The use of the Arrhenius concept in artificial weathering simplifies complex chemical degradation
processes and is therefore, in this context, a limited model to estimate the temperature dependency in
weathering. However, by following some basic experimental preconditions, the relevance of the estimates can be
increased.
The International Organization for Standardization (ISO) draws attention to the fact that it is claimed
that compliance with this document may involve the use of a patent.
ISO takes no position concerning the evidence, validity and scope of this patent right.
The holder of this patent right has assured ISO that he/she is willing to negotiate licences under
reasonable and non-discriminatory terms and conditions with applicants throughout the world. In
this respect, the statement of the holder of this patent right is registered with ISO. Information may be
obtained from the patent database available at www .iso .org/ patents.
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ISO 23706:2020(E)

Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights other than those in the patent database. ISO shall not be held responsible for identifying
any or all such patent rights.
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INTERNATIONAL STANDARD ISO 23706:2020(E)
Plastics — Determination of apparent activation energies
of property changes in standard weathering test methods
1 Scope
This document describes a test method to determine the activation energy (E ) of photochemical
a
degradation reactions in accelerated weathering tests according to the Arrhenius model in the most
comprehensive manner.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 4582, Plastics — Determination of changes in colour and variations in properties after exposure to
glass-filtered solar radiation, natural weathering or laboratory radiation sources
ISO 4892-1, Plastics — Methods of exposure to laboratory light sources — Part 1: General guidance
ISO 4892-2, Plastics — Methods of exposure to laboratory light sources — Part 2: Xenon-arc lamps
ISO 4892-3, Plastics — Methods of exposure to laboratory light sources — Part 3: Fluorescent UV lamps
ISO 10640, Plastics — Methodology for assessing polymer photoageing by FTIR and UV/visible spectroscopy
3 Terms, definitions and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1.1
activation energy
E
a
energy, above that of the ground state, which is be added to an atomic or a molecular system to allow a
particular process to take place
–1
Note 1 to entry: It is expressed in J∙mol .
Note 2 to entry: The activation energy, E , is derived from the Arrhenius concept (see Annex A).
a
3.1.2
effective irradiance
E
eff
spectral irradiance triggering the relevant photo-degradation processes for the reactions under
consideration
EE=⋅s ()λλd
effxλ
∫
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ISO 23706:2020(E)

where
–2 −1
E is the spectral irradiance in W·m ·nm ;
λ
s (λ) is the action spectrum normalized to its maximum.
x
Note 1 to entry: If different test conditions are compared using radiation sources with the same spectral
irradiance distribution, often the ratio of the total irradiances is used instead of the ratio of the effective
irradiance.
3.2 Abbreviated terms
CHT chamber air temperature (ambient air temperature)
BST black standard temperature (measured with insulated surface temperature sensor)
BPT black panel temperature (measured with un-insulated surface temperature sensor)
WST white standard temperature (measured with insulated surface temperature sensor)
WPT white panel temperature (measured with un-insulated surface temperature sensor)
4 Principle
The Arrhenius formula deals with effects of varying levels of temperature on property change rates
caused by exposure to solar radiation. This concept is used in the thermal degradation of plastics, for
example in ISO 11358-2 and ISO 2578. To determine the activation energy, E , two or more weathering
a
experiments shall be performed at different temperatures, but under otherwise equal conditions, until
the same specified property change is reached.
The only difference between the exposures shall be the specimen surface temperature. All other
parameters (spectral irradiance, relative humidity, cycles, etc.) shall be the same in all exposures. If
the specimen surface temperature cannot be measured directly and/or controlled, a reference surface
temperature (BST/BPT or WST/WPT, as similar to the relevant specimen surface temperature as
possible) shall be the only varying parameter instead.
Different radiation sources can be used.
If radiation sources are used which emit IR radiation (see Table 1, Type A-1, e.g. Xenon arc according to
ISO 4892-2), the specimen will show an inhomogeneous temperature distribution over the thickness,
due to radiation heating. In such case, only thin samples should be investigated, or the considered
property change should be related to the specimen surface, not to the bulk. If radiation heating occurs,
vary the surface temperature by values of about 5 K to 10 K, as the systematic errors during surface
temperature measurement can cancel each other out.
If radiation sources are used which do not emit IR radiation, but the specimen are cooled from the backside
(see Table 1, Type A-2, e.g. UVA according to ISO 4892-3), the specimen will show an inhomogeneous
temperature distribution over the thickness. In that case the same limitations as above apply.
If radiation sources are used which do not emit IR radiation and the specimen are placed inside a climatic
chamber with homogeneous temperature distribution (see Table 1, Type B e.g. UVA-340 according to
ISO 4892-3), the specimen will show a constant temperature over the thickness. Thus, also for thicker
samples, bulk properties can be investigated. In such case, temperature can be varied in a larger range.
Doing so, it should be taken into account that the activation energy, E , can vary over the temperature,
a
due to differently relevant processes.
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ISO 23706:2020(E)

5 Apparatus
5.1 General
The equipment comprises a climate chamber with a chamber air temperature and relative humidity
measurement device. In the climate chamber included is a radiation source. The radiation source may
generate UV, visible, and infrared radiation similar to solar radiation with appropriate filter systems. A
cooling system for the laboratory simulated solar radiation source and a fixture for the specimens are
included in the chamber as well.
For the test equipment (including chamber design, laboratory radiation source, radiometer, temperature
and humidity control equipment) the criteria as described in ISO 4892-1 and ISO 4892-2 (for Xenon-arc
equipment) and ISO 4892-3 (for fluorescent UV devices) apply.
5.2 Surface temperature measurement
Equipment utilizing sample surface temperature sensors according to EN 16795 should be used.
However, if no direct specimen surface temperature measurement is possible, the black standard or
black panel sensors should be used instead as reference for dark test specimen and white standard or
white panel sensors should be used as reference for light coloured test specimen.
NOTE Surface and bulk temperature of the specimens will typically adjust between CHT or WST and BST,
depending on possible radiation heating, mounting position, or thermal properties of the specimens.
6 Test specimens
The test specimens shall be according to ISO 4892-1.
7 Test method
7.1 Test conditions
Select a standard test method or create a test method as appropriate for the required material
(e.g. ISO 4892-2, Cycle 1 for outdoor weathering of plastics).
The test parameters are:
a) relative spectral irradiance (preferred simulated solar radiation according to ISO/TR 17801);
b) irradiance;
c) chamber air temperature (CHT);
d) surface temperature of the specimen (BST or BPT);
e) relative humidity;
f) rain cycle, if specified/necessary.
NOTE 1 Rain or wetting cycles influence the specimen temperature. If rain cycles are used, the effective
specimen temperature is used for evaluation (see Annex C).
NOTE 2 Rain cycles can increase the uncertainty of the method.
Test at conditions described in the selected test method.
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ISO 23706:2020(E)

7.2 Test duration
The test duration is either defined by the test standard used or as agreed between all interested parties
(test duration, or until a specific property change is achieved).
The evaluation criteria are either defined by the test standard used or as agreed between all interested
parties. The evaluation criteria should be determined after the exposure
For determination of Arrhenius factors, the sample surface temperature is the critical temperature.
The sample surface temperature shall be measured or estimated according to 5.2.
7.3 Increased/decreased temperatures
Test at a reference temperature of, for example, −10 K, −5 K, +5 K, +10 K, or other temperature ranges
compared to standard conditions (e.g. according to ISO 4892-2, Cycle 1). For dark materials, BST or
BPT can be a suitable reference temperature. For light coloured materials WST or WPT as reference is
recommended.
All other parameters [irradiance (E), radiant exposure (H), relative humidity (RH), rain cycle] shall be
the same as described under standard conditions.
The test at increased or decreased temperatures shall be performed until the same property change is
achieved as described under standard conditions.
For determination of the activation energy, E , the sample surface temperature is the critical
a
temperature. The surface temperature shall be measured or estimated according to 5.2.
For the determination of the activation energy, E , the test shall be performed at standard conditions
a
and at least at one increased or decreased temperature or at least two different temperatures within a
range where no temperature influence on the degradation pathway is expected (no T transition).
g
Table 1 — Classification of instrument types with and without radiation heating and test
specimen with inhomogeneous (Type A) and homogeneous (Type B) specimen temperature
Instrument Type Suggested ΔT Temperature control by Constant parameters
K
A-1 Radiation heating (e.g. Xenon e.g. ± 5, ± 10 Adjusting convection E, E , RH, rain cycle
λ
arc) (blower speed) and/or
control of IR radiation
A-2 No radiation heating e.g. ± 5, ± 10 Adjusting CHT E, E , RH, rain or
λ
(e.g. UVA), backside cooling condensation cycle
B No radiation heating e.g. ± 20 Adjusting CHT E, E , RH, rain cycle
λ
(e.g. UVA), homogeneous
specimen temperature
7.4 Calculation of activation energy, E
a
7.4.1 Exposures at two different temperatures
Calculate the activation energy, E , on the basis of two weathering tests according to the logarithmic
a
Arrhenius formula shown in Formula (A.3).
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ISO 23706:2020(E)

7.4.2 Exposures at two or more different temperatures
Determine the activation energy, E , on the basis of two or more weathering tests graphically. When the
a
natural logarithm of k is plotted versus the inverse of the temperature (1/T), the slope is a straight line
with a value equal to –E /R [according to Formula (A.4) and Figure A.1].
a
NOTE 1 The determination of E becomes more reliable, the more data points from different temperatures
a
are used.
NOTE 2 The correlation coefficient can be calculated by Pearson correlation according to ISO 2578:1993,
Annex B for data sets with more than three data points.
8 Exposure conditions
Follow the exposure conditions described in the selected test method or standard. Control the test
parameters as defined in the selected test method or standard or according to ISO 4892-1, ISO 4892-2
or ISO 4892-3.
9 Procedure
9.1 General
It is recommended that at least three test specimens of each material evaluated be exposed in each run
to allow statistical evaluation of the results.
9.2 Mounting the test specimens
Attach the specimens to the specimen holders in the equipment in such a way that the specimens are
not subject to any applied stress. Identify each test specimen by suitable indelible marking, avoiding
areas to be used for subsequent testing. As control, a plan of the test-specimen positions may be made.
If desired, in the case of specimens used to determine change in colour and appearance, a portion
of each test specimen may be shielded by an opaque cover throughout the exposure. This gives an
unexposed area adjacent to the exposed area for comparison. This is useful for checking the progress
of the exposure, but the data reported shall always be based on a comparison with control specimens
stored in the dark.
9.3 Exposure
Before placing the specimens in the test chamber, be sure that the apparatus is operating under the
desired conditions. Programme the apparatus with the selected conditions to operate continuously
for the required number of cycles at the selected exposure conditions. Maintain these conditions
throughout the exposure, keeping any interruptions to service the apparatus and to inspect the
specimens to a minimum.
Expose the test specimens, the radiometer and the surface temperature sensor for the specified period.
If it is necessary to remove a test specimen for periodic inspection, take care not to touch the exposed
surface or alter it in any way. After inspection, return the specimen to its holder or to its place in the
test chamber with its exposed surface oriented in the same direction as before.
9.4 Measurement of radiant exposure
Mount and calibrate the radiometer so that the irradiance at the exposed surface of the test specimen
is reported.
–2
Express the exposure stages in terms of radiant exposures, in joules per square metre (J · m ), in the
–2 –1
wavelength band from 300 nm to 400 nm, or in joules per square metre per nanometre (J · m · nm )
at the wavelength selected (e.g. 340 nm).
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ISO 23706:2020(E)

9.5 Determination of changes in properties after exposure
These shall be determined as specified in ISO 4582 or in ISO 10640, if applicable. Other properties may
be used if agreed upon by all interested parties.
NOTE Different material properties of the same material can act differently. The property choice is of basic
relevance for evaluating the weathering acceleration.
The knowledge of the progress of the considered property change over time for a time-constant
exposure is essential for this method. It shall be measured and included into the Test report.
In case of nonlinear behaviour, the evaluation method possibly should be adopted in an appropriate way.
10 Test report
The test report shall be according to ISO 4892-1.
If applicable, the test report may also contain:
— a graph with the curves giving for each temperature, the values of the characteristic evaluation
properties against time;
— a graph of the logarithm of the times it takes to get to a specific property change at different
temperatures plotted against the reverse temperature;
— the calculated activation energy, E , and the correlation coefficient.
a
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ISO 23706:2020(E)

Annex A
(normative)

Arrhenius concept
A.1 General
The temperature dependence of the rate of photochemical reactions can be estimated using a modified
Arrhenius formula [see Formula (A.1)]:
E
a
−
α
RT
kA=⋅E ⋅e (A.1)
eff
where
k is the reaction rate constant;
A is the Arrhenius factor;
–1 –1
R is the universal gas constant (8,314 J·mol ·K );
T is the absolute temperature (in K);
–1
E is the activation energy (in J·mol ) of the considered property change;
a
E is the effective irradiance;
eff
α is the material-specific coefficient.
Due to the lack of a specific traceable chemical parameter or reaction during a weathering test, usually
a specific macroscopic property change (such as gloss loss or colour change) is taken into account for
the Arrhenius formula.
A.2 Limitations
The reaction rate constant k is only valid for a single change in property or performance.
The range of operating temperature should not include a phase transition.
The overall temperature dependent process leading to a change in performance with or without
radiation should follow an Arr
...