ISO 23706:2020
(Main)Plastics — Determination of apparent activation energies of property changes in standard weathering test methods
Plastics — Determination of apparent activation energies of property changes in standard weathering test methods
This document describes a test method to determine the activation energy (Ea) of photochemical degradation reactions in accelerated weathering tests according to the Arrhenius model in the most comprehensive manner.
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General Information
Standards Content (Sample)
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 2020
© ISO 2020
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ii © ISO 2020 – All rights reserved
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
Foreword
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This document was prepared by Technical Committee ISO/TC 61, Plastics, Subcommittee SC 6, Ageing,
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iv © ISO 2020 – All rights reserved
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.
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vi © ISO 2020 – All rights reserved
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
∫
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.
2 © ISO 2020 – All rights reserved
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.
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 conditio
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