IEC TS 62073:2016
(Main)Guidance on the measurement of hydrophobicity of insulator surfaces
Guidance on the measurement of hydrophobicity of insulator surfaces
IEC TS 62073:2016(E) specifies the measurement of the hydrophobicity of the shed and housing material of composite insulators for overhead lines, substations and equipment or ceramic insulators covered or not covered by a coating. The obtained value represents the hydrophobicity at the time of the measurement. The object of this technical specification is to describe three methods that can be used to determine the hydrophobicity of insulators. The determination of the ability of water to wet the surface of insulators may be useful to evaluate the condition of the surface of insulators in service, or as part of the insulator testing in the laboratory. This second edition cancels and replaces the first edition published in 2003. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition:
a) Changed wettability to hydrophobicity throughout the document
b) Redefined the criteria for the determination of hydrophobicity class in paragraph 3.4.
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IEC TS 62073 ®
Edition 2.0 2016-02
TECHNICAL
SPECIFICATION
Guidance on the measurement of hydrophobicity of insulator surfaces
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IEC TS 62073 ®
Edition 2.0 2016-02
TECHNICAL
SPECIFICATION
Guidance on the measurement of hydrophobicity of insulator surfaces
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.080.10 ISBN 978-2-8322-3169-2
– 2 – IEC TS 62073:2016 © IEC 2016
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Terms and definitions . 7
3 Methods for measurement of hydrophobic properties . 9
3.1 General . 9
3.2 Method A – Contact angle method . 9
3.2.1 General . 9
3.2.2 Equipment . 9
3.2.3 Measurement procedure . 9
3.2.4 Static contact angle measurements . 10
3.2.5 Dynamic contact angle measurements . 10
3.2.6 Evaluation . 10
3.3 Method B – Surface tension method . 11
3.3.1 General . 11
3.3.2 Safety precautions . 11
3.3.3 Equipment and reagents . 11
3.3.4 Measurement procedure . 12
3.3.5 Evaluation . 12
3.4 Method C – The spray method . 12
3.4.1 General . 12
3.4.2 Equipment . 12
3.4.3 Measurement procedure . 13
3.4.4 Evaluation . 13
3.5 Documentation . 14
Annex A (normative) Guidelines regarding the applicability and comments on the
limitations of the different methods described in this technical specification . 15
A.1 General . 15
A.2 Typical results obtained with the three methods . 15
Annex B (normative) Method A – Contact angle method . 17
Annex C (normative) Method B – Surface tension method . 18
Annex D (normative) Method C – Spray method . 20
Figure 1 – Definition of the static contact angle . 8
Figure 2 – Definition of the advancing angle (θ ) and the receding angle (θ ) inside a
a r
liquid drop resting on an inclined solid surface . 8
Figure 3 – Measurements of the advancing angle (θ ) and the receding angle (θ ) by
a r
adding or withdrawing water from a droplet. 10
Figure B.1 – Measurement of the advancing angle (θ ) and the receding angle (θ ) by
a r
using the captive bubble technique. 17
Figure D.1 – Examples of surfaces with hydrophobicity class (HC) from 1 to 6 . 20
Table 1 – Criteria for the determination of hydrophobicity class (HC) . 14
Table C.1 – Concentrations of ethylene-glycol-monoethyl-ether (cellosolve),
formamide mixtures used in measuring surface tension of insulator surfaces in the
range 30 mN/m to 56 mN/m (T = 20 °C) . 18
Table C.2 – Concentrations of distilled water and formamide mixture used in
measuring surface tension of insulator surfaces in the range 58 mN/m to 73 mN/m
(T = 20 °C) . 19
Table C.3 – Concentrations of distilled water and sodium chloride in mixtures used in
measuring surface tension of insulator surfaces in the range 73 mN/m to 82 mN/m
(T = 20 °C) . 19
– 4 – IEC TS 62073:2016 © IEC 2016
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
GUIDANCE ON THE MEASUREMENT OF HYDROPHOBICITY
OF INSULATOR SURFACES
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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The main task of IEC technical committees is to prepare International Standards. In
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specification when
• the required support cannot be obtained for the publication of an International Standard,
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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 62073, which is a technical specification, has been prepared by IEC technical committee
36: Insulators.
This second edition cancels and replaces the first edition published in 2003. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) Changed wettability to hydrophobicity throughout the document
b) Redefined the criteria for the determination of hydrophobicity class in paragraph 3.4;
The text of this technical specification is based on the following documents:
Enquiry draft Report on voting
36/363/DTS 36/367/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.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC website 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.
A bilingual version of this publication may be issued at a later date.
– 6 – IEC TS 62073:2016 © IEC 2016
INTRODUCTION
The wetting properties of a surface by water are commonly described by the terms
hydrophobic (or hydrophobicity) and hydrophilic (or hydrophilicity). A hydrophobic surface is
water-repellent, while a surface that is easily wetted by water is hydrophilic.
The wetting phenomenon of a surface is complex and many different parameters can
influence its hydrophobic properties. Some important parameters include: type of insulator
material, surface roughness, heterogeneities of the surface, chemical composition (e.g. due to
ageing) and presence of pollution. For insulator materials in common use, the hydrophobic
properties can change over time, due to the influence of the ambient conditions. This change
can be either reversible or irreversible. Thus, the result of the measurement of the
hydrophobicity may be influenced by the ambient conditions and the HV corona or dry-band
arcing to which the insulator has been previously exposed. This dynamic behaviour of the
hydrophobicity is more or less specific to different insulator materials. These types of
materials, which have an ability to retain and transfer hydrophobicity, are commonly called
Hydrophobicity Transfer Materials (HTM).
The dynamic behaviour of the hydrophobicity exhibited by insulator materials is due to their
chemical composition. Different processes such as oxidation, hydrolysis, migration of low
molecular weight compounds, formation of complex compounds between e.g. siloxanes and
water, rotation of flexible polymer chains, inter- and intra-molecular rearrangements,
microbiological growth, deposition of contaminants, adhesion and encapsulation of
contaminant particles, may take place at different rates, depending on material and ambient
conditions. Thus hydrophobicity along and around an insulator can vary, due to differences in
the exposure to solar radiation, rain, corona discharges, deposited pollution, etc. Therefore,
hydrophobicity of insulators is usually measured on several separate areas of the insulator.
Measurement of the hydrophobicity of a surface is readily performed in the laboratory on well
defined, homogeneous, smooth and planar surfaces of prepared specimens. In the case of
insulators, for which non-destructive measurements are usually required (and where cut-out of
material samples is usually not desired), these conditions do not exist and measurement with
high precision is a difficult task. This is especially true when the measurement has to be
performed on an insulator installed in an overhead line, substation or even in a high voltage
test set-up in the laboratory.
Previously wettability class (WC) was used as equivalent technology.
GUIDANCE ON THE MEASUREMENT OF HYDROPHOBICITY
OF INSULATOR SURFACES
1 Scope
The methods described in this technical specification can be used for the measurement of the
hydrophobicity of the shed and housing material of composite insulators for overhead lines,
substations and equipment or ceramic insulators covered or not covered by a coating. The
obtained value represents the hydrophobicity at the time of the measurement.
The object of this technical specification is to describe three methods that can be used to
determine the hydrophobicity of insulators. The determination of the ability of water to wet the
surface of insulators may be useful to evaluate the condition of the surface of insulators in
service, or as part of the insulator testing in the laboratory.
2 Terms and definitions
For the purposes of this document, the following definitions apply.
2.1
hydrophobicity
state of a surface with a low surface tension and thus is water-repellent
2.2
hydrophilicity
state of a surface with a high surface tension and thus is wetted by water (in the form of a
film)
2.3
surface tension
region of finite thickness (usually less than 0,1 µm) in which the composition and energy vary
continuously from one bulk phase to the other
Note 1 to entry: The pressure (force field) in the interfacial zone has a gradient perpendicular to the interfacial
boundary. A net energy is required to create an interface (surface) by transporting the matter from the bulk phase
to the interfacial (surface) zone. The reversible work required to create a unit interfacial (surface) area is the
surface tension and is defined thermodynamically as follows:
∂G
γ =
∂A
T,P,n
where
γ is the surface (interfacial) tension or surface energy;
G is the Gibbs free energy of the total system;
A is the surface (interfacial) area;
T is the temperature;
P is the pressure;
n is the total number of moles of matter in the system.
The surface tension (γ) is usually expressed in mN/m (1 mN/m = 1 dyn/cm).
– 8 – IEC TS 62073:2016 © IEC 2016
2.4
static contact angle
angle of a drop of liquid resting on the surface of a solid, and a gas is in contact with both, the
forces acting at the interfaces are in balance
Note 1 to entry: These forces are due to surface tensions acting in the direction of the respective surfaces. From
Figure 1 it follows that:
γ cosθ =γ −γ
GL s GS SL
where
θ is the static contact angle of the edge of the drop with the solid surface,
s
γ is the surface tension of the gas-liquid interface,
GL
γ is the surface tension of the gas-solid interface, and
GS
γ is the surface tension of the solid-liquid interface.
SL
Note 2 to entry: The above equation (Young´s equation) is only valid for ideal and smooth surfaces.
Air
γ
Liquid GL
θ
S
γ γ
SL GS
Solid
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Figure 1 – Definition of the static contact angle
The right side of the above equation (the difference between the surface tensions of the gas-solid and the solid-
liquid interfaces) is defined as the surface tension of the solid surface. It is not a fundamental property of the
surface but depends on the interaction between the solid and a particular environment.
When the gas is air saturated with vapour of the liquid, γ will be the surface tension of the liquid. If the contact
GL
angle is 0°, the liquid is said to just wet the surface of the solid, and in this particular case (since cos θ = 1), the
s
surface tension of the solid will be equal to the surface tension of the liquid.
2.5
advancing and receding contact angle (dynamic contact angles)
angles of a droplet on an inclined solid surface that exhibits two different angles
Note 1 to entry: The advancing contact angle (θ ) is the angle inside the water droplet between the solid surface
a
and the droplet surface at the lower part of the droplet on the inclined surface (see Figure 2). The receding contact
angle (θ ) of a droplet on an inclined surface is the angle inside the droplet between the solid surface and the
r
droplet surface at the droplet rear (highest part on the inclined surface). If the receding contact angle is zero, a
completely wetted trace of water is formed as the drop moves along the solid inclined surface (see Figure 2). The
general physical relation between the advancing and receding contact angle and the static contact angle defined in
2.4 is: θ ≤ θ ≤ θ .
r s a
Air
θ
r
Liquid
θ
a
Solid
IEC
Figure 2 – Definition of the advancing angle (θ ) and the receding angle (θ )
a r
inside a liquid drop resting on an inclined solid surface
2.6
hydrophobicity class
HC
specific level of the scale used in the spray method (Method C)
Note 1 to entry: Seven classes, HC/1 to HC/7, have been defined. HC/1 corresponds to the most hydrophobic
surface and HC/7 to the most hydrophilic surface.
3 Methods for measurement of hydrophobic properties
3.1 General
Three methods for measurement of the hydrophobicity, differing in accuracy, simplicity, size
of measured surface area and applicability, are described in this technical specification and
are as follows:
a) the contact angle method;
b) the surface tension method;
c) the spray method.
Guidance relative to the specific use of the three methods is found in Annex A.
3.2 Method A – Contact angle method
3.2.1 General
The contact angle method is a measurement that involves the evaluation of the contact angle
formed between the edge of a single droplet of water and the surface of a solid material. If
done on a horizontal surface, the advancing and receding contact angles can be measured by
adding water to or withdrawing water from the droplet.
The contact angles depend strongly on the surface roughness and contact angles measured
on polluted surfaces may differ significantly from contact angles measured on smooth, clean
and planar surfaces.
3.2.2
...
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