ISO/TR 20470-1:2026

Technical
Report
ISO/TR 20470-1
First edition
New weatherable topcoats as part
2026-03
of an associated protective coating
system —
Part 1:
Weathering of FEVE type
fluoropolymer topcoat
Nouvelles couches de finition résistantes aux intempéries faisant
partie d'un système de revêtement protecteur associé —
Partie 1: Vieillissement aux intempéries de la couche de finition
en fluoropolymère de type FEVE
Reference number
© ISO 2026
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Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 General introduction of FEVE type fluoropolymer . 2
4.1 Chemical structure of FEVE type fluoropolymer .2
4.2 Chemical curing system .2
5 Weatherability of fluoropolymer topcoats . 3
5.1 Weatherability by fluorescent UV condensation cycle testing .3
5.2 Fifteen years weatherability test of FEVE type fluoropolymer topcoats .4
5.3 Actual bridges situation .6
6 Durability of fluoropolymer topcoat system . 6
6.1 Results of collaboration study .6
6.2 Evaluation of the durability .9
6.2.1 Weatherability test during 29 years of fluoropolymer topcoat system .9
6.2.2 Test method .9
6.3 Summary . 12
7 Health and safety .12
Annex A (informative) Examples of bridges with FEVE type fluoropolymer topcoat systems .13
Bibliography .16

iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. 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 document 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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 35, Paints and varnishes, Subcommittee SC 14,
Protective paint systems for steel structures.
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.

iv
Introduction
In ISO 12944-5:2019, 6.2.5, fluoroethylene vinyl ether (FEVE) fluoropolymer topcoats are characterized as
a specialized type of polyurethane based on fluoropolymers. This document describes the characteristics
of the fluoropolymer topcoats with the intention of helping users understand both their advantages and
limitations. Joint research on enhancing the weatherability and durability for a variety of coatings was
launched in 1983 managed by the Public Works Research Institute of the Ministry of Construction in Japan
and was continued until the publication of this document. The fluoropolymer topcoat systems were included
in the joint research and results were obtained on the durability and weatherability over 30 years. These
[1]-[9]
fluoropolymer topcoats systems were reported at several international conferences. This document
incorporates valuable findings from the joint research on weathering tests conducted over the past 30 years,
which are beneficial for users. Moreover, users are encouraged to contribute their own results to enhance
the comprehensiveness of this document. FEVE type waterborne fluoropolymer topcoats and their coating
systems have been developed and used in the market since around 2015, but long-term outdoor weathering
data as has been obtained for the solvent borne type have not been achieved at present.
This document contains weathering performance data such as the rate of gloss retention and chalking
resulting in film thickness loss of the topcoat system, along with chemical analysis data focusing on the
coating film deterioration of fluoropolymer topcoat. All data has been obtained from the outdoor exposure
testing. The chemical analysis relating to coating film deterioration was conducted using FTIR spectroscopy,
covering analysis of both the surface and the cross-section (depth direction from the surface).
Users of this document can use the results of the long-term outdoor exposure test to predict the durability
of the coating system.
Finally, this document points to the technological development of coating materials with state-of-the-art
weathering performance in future, such as extremely slow chalking deterioration. This coating can be the
standard reference for observing and evaluating short-, medium- and long- term weatherable coatings using
the outdoor exposure data of these topcoats.
By using the data in this document, the real time durability of the coating film can be efficiently estimated
using a short-term exposure test or an accelerated weathering test, or both.

v
Technical Report ISO/TR 20470-1:2026(en)
New weatherable topcoats as part of an associated protective
coating system —
Part 1:
Weathering of FEVE type fluoropolymer topcoat
1 Scope
This document describes the basic characteristics of fluoroethylene vinyl ether copolymer (FEVE) type
fluoropolymer topcoats and their coating systems. Information on the weathering performance of the
[1],[4]-[12]
fluoropolymer topcoats and associated coating systems for over 30 years is also provided.
This document covers:
— weathering data of FT (FEVE fluoropolymer topcoat);
— chemical analysis of outdoor exposed coated panels focusing on the paint surface and their cross-
sections.
This document does not include FEVE type waterborne fluoropolymer topcoats and their coating systems.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
imaging infrared
imaging IR
method of measuring and imaging the in-plane distribution of functional groups specific to a compound
using a detector composed of multiple elements
Note 1 to entry: Since it is possible to measure the molecular structure over a wide area, imaging IR is effective for
investigating the cross-sectional structure of surface deterioration due to ultraviolet irradiation and the cross-
sectional layer structure of multilayer films.
3.2
fluoroethylene vinyl ether copolymer
FEVE
resin which consist of fluoro-olefin units, various hydrocarbon vinyl ether units and has pendant OH-groups
Note 1 to entry: Along the polymer chain each vinyl ether unit alternates with fluoro-olefine unit. This sequence is
responsible for weather resistance.

4 General introduction of FEVE type fluoropolymer
4.1 Chemical structure of FEVE type fluoropolymer
Fluoroethylene vinyl ether copolymer (FEVE) used for coating systems of steel structures as topcoats is
an amorphous polymer, soluble in organic solvents and curable at ambient temperature. Waterborne type
FEVE fluoropolymer has also been developed. As shown in Figure 1, FEVE consists of fluoro-ethylene units,
various hydro-carbon vinyl ether units and has pendant hydroxy group (OH) groups. Along the polymer
chain, each vinyl ether unit alternates with fluoroethylene units which protects the rather unstable vinyl
ether unit, chemically and sterically, which can be easily attacked by bases at the hydrogen on the tertiary
carbon and by acids at the oxygen in the ether linkage. The hardness of FEVE coatings is due mainly to
the fluoroethylene units, but the other properties are desired, such as solubility in organic solvents,
compatibility with pigments and dyestuffs and curing reagents such as isocyanates. Gloss, flexibility and
adhesion can be optimized by choosing the appropriate amount of vinyl ether unit. The OH and COOH values
of the polymer are set to be like those of the general polymers with free hydroxyl groups such as polyester,
acrylic and polyether resin.
Key
X F, CF , Cl
R , R alkyd group
1 2
R , R alkylene group
2 4
1 weatherability, durability
2 gloss, hardness, solvent, solubility
3 flexibility
4 adhesion, pigment dispersion
Figure 1 — Molecular structure of the FEVE type fluoropolymer
FEVE resins have characteristics of both fluoropolymers and hydrocarbons. The fluoroethylene groups
are the strength of the FEVE resin. These groups are responsible for the polymer’s high resistance to UV
degradation. The C-F bond is very strong (see Table 1). The energy of this bond is higher than the energy of
UV radiation at 290 nm which is ~411 KJ/mol. The alternating pattern, shown in Figure 1, is critical for the
extreme UV resistance properties. The chemically stable and UV resistant fluoroethylene units sterically
and chemically protect the neighbouring vinyl ether units.
Table 1 — Bond energy of fluoro-chemicals and commodity chemicals
Resin C-C Chain KJ/mol C-F. C-H KJ/mol
Fluoro compound CF3-CF3 414 [F]-CF2-CH3 523
Fluoro compound CF3-CH3 424 CF3CH2-[H] 447
Commodity chemical CH3-CH3 379 CH3CH2-[H] 411
4.2 Chemical curing system
Compatibility of FEVE with curing agents is essential for paint formulation to ensure weather resistant
coatings. FEVE is a fluoropolymer that has free hydroxyl groups which react with suitable isocyanate curing
agents. Aromatic or aliphatic poly-isocyanates can be used as the curing agent component. Almost three days

are required for curing at room temperature when an isocyanate is used with a molar ratio of NCO:OH = 1:1,
though the coating is tack-free after only three hours and blocking-free after eight hours.
5 Weatherability of fluoropolymer topcoats
5.1 Weatherability by fluorescent UV condensation cycle testing
Surface degradation of the fluoropolymer coating and the polyurethane coating was measured using the
IR-ATR (attenuated total reflection) method before and after the fluorescent UV-condensation cycle test.
The fluoropolymer resin cures at normal temperature by reacting with an isocyanate similarly to the
polyurethane. So, the degradation behaviour of the fluoropolymer coating was regarded as decomposition of
−1
the urethane bond. The decrease of the inherent IR absorption for a chemical bond of amide(II)(1 530 cm )
was measured to estimate the degradation.
The degradation mechanism of the urethane bond is considered to be as follows:
Hydrolysis of urethane bonding

R’ NHCOOR” + H O  R’ NH + CO + HOR”
2 2 2
The absorbance was measured to compare the degradation before and after the fluorescent UV-condensation
cycle test.
A
A�= (1)
rat
B
where
A is the absorbance ratio;
rat
-1
A is the absorbance of amide(II) (1 530 cm );
-1
B is the absorbance of methylene (1 470 cm ).
Absorption of amide (II) depends on the degree of degradation. Absorption of methylene did not change and
was independent of degradation time. So, the amide (II) retention was obtained from the absorbance ratio
at the same initial value to that after the fluorescent UV-condensation cycle test (QUV). The larger the value,
the smaller was the degree of degradation.
The amide (II) retention ratio is shown in Figure 2.

Key
X exposure time (×10 hours)
Y retention of amide(II) in %
fluoropolymer coating
polyurethane coating
Figure 2 — retention of amide(II)
Judging from the results obtained from the measurement after 1 000 hours of fluorescent UV-condensation
cycle test, surface degradation of the polyurethane coating was progre
...