ISO/TS 14778:2021

TECHNICAL ISO/TS
SPECIFICATION 14778
First edition
2021-01
Paper and board — Measurement of
water contact angle by optical methods
Papier et carton — Mesurage de l'angle de contact de l'eau par des
méthodes optiques
Reference number
©
ISO 2021
© ISO 2021
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ii © ISO 2021 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and references . 1
4 Principle . 2
5 Reagents . 3
6 Apparatus . 3
7 Sampling . 5
8 Conditioning . 5
9 Preparation of test pieces . 5
10 Procedure. 6
10.1 General . 6
10.2 Mounting of the test piece . 6
10.3 Calibration and adjustment of apparatus . 6
10.4 Method of measurement . 7
11 Expression of results . 8
12 Test report . 8
Annex A (informative) Precision . 9
Bibliography .12
Foreword
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This document was prepared by Technical Committee ISO/TC 6, Paper, board and pulps, Subcommittee
SC 2, Test methods and quality specifications for paper and board.
Any feedback or questions on this document should be directed to the user’s national standards body. A
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iv © ISO 2021 – All rights reserved

Introduction
The interaction between the liquid and the solid phases influences the contact angle. Contact angles
above 90° define a situation where the liquid is repelled by the solid; below 90° defines a situation of
attraction where the liquid wets the surface. The magnitude above or below 90° shows the relative
degree of repulsion or attraction between the two phases.
On many surfaces the contact angle varies with duration of contact through a combination of spreading,
penetration (in the case of porous substrates) and evaporation. Both manual and automated apparatus
are available for optical measurement of contact angle, but automated equipment is preferred for
precision and rapid measurement, and because it is often applicable to a wider combination of liquid
and paper or board samples.
Contact angle measurement is used to predict how liquids interact with paper surfaces. This document
describes the most common form of test, using water as probe liquid, and from the data a probable
interaction between the paper surface and another liquid with comparable surface tension and viscosity
characteristics is often inferred. The veracity of this inference should be tested wherever possible.
Notwithstanding the above, contact angle measurement is used widely as a predictive tool in several
industrial settings, for example:
a) for assessing writing, ruling or printing quality with water-based or solvent based inks (e.g. in
inkjet, gravure or flexographic printing);
b) for gluing applications;
c) for wet offset lithographic printing;
d) for hot-foil applications;
e) for barrier or release coatings;
f) for coating applications.
In some cases, measurements of contact angle are used to calculate two or three components of the
surface energy of the paper or board, which requires the use of two or three liquids of known surface
energy, respectively. In such cases, the values calculated for the components of surface energy will be
[1]
related to the liquids chosen for the analysis . This is not covered in this document.
The test method described in this document is sometimes known as ‘static’ or even ‘sessile drop’,
since on many surfaces the droplet remains static and in equilibrium with the paper surface and air.
Yet on paper and board surfaces the droplet often changes its dimensions with time, due to sorption
and wetting phenomena. This has led some instrument manufacturers and researchers to describe the
automated optical technique outlined in this document as measurement of the ‘dynamic’ contact angle.
This document does not use the term ‘dynamic’ because this nomenclature confuses a measurement
changing with respect to time with one that changes due to a plane of shear. For example, the Willhelmy
plate method of contact angle measurement, which measures the force required to push a solid material
[2]
into and then pull it out from a liquid reservoir, is a true ‘dynamic’ method .
Similarly, this document does not cover the situation where a droplet is placed on a horizontal surface
that is subsequently tilted so that gravity causes the droplet to assume an asymmetric shape, then to
commence movement.
TECHNICAL SPECIFICATION ISO/TS 14778:2021(E)
Paper and board — Measurement of water contact angle by
optical methods
1 Scope
This document specifies the method for optical assessment of the contact angle between water and the
surface of paper and board, where the process of droplet formation, application to planar substrates, or
measurement of the droplet shape in contact with the solid is performed by automated equipment.
The limits of measurement are determined by the capabilities of the instrumentation used. The
instrumental capabilities defined by this document use a digital image capturing system operating at
a minimum of 50 frames per second and needs the ability to perform the first measurement after no
more than 20 ms to 40 ms contact between the droplet and substrate.
The test method is applicable to most kinds of paper or board however it cannot be applicable to
structured materials.
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 186, Paper and board — Sampling to determine average quality
ISO 187, Paper, board and pulps — Standard atmosphere for conditioning and testing and procedure for
monitoring the atmosphere and conditioning of samples
ISO 13530, Water quality – Guidance on analytical quality control for chemical and physicochemical water
analysis
3 Terms and references
For the purposes of this document, the following terms and definitions apply:
3.1
baseline
line of contact between the water droplet and paper or board surface, determined by optical means and
taken as the plane from which the contact angle is measured
Note 1 to entry: Precise measurement of contact angle requires precise assessment of the baseline. For the
optical system to achieve the best estimate of the baseline, it may be necessary to tilt the camera so that it views
the droplet slightly from above, at a shallow angle (typically 0-3° relative to the horizontal), rather than directly
from the side (0°). Experience shows this small change in viewing angle does not affect the measured contact
[3]
angle value but can enhance detection of the baseline .
3.2
contact angle
Θ
angle to a baseline (3.1), formed by means of a tangent on the droplet contour through one of the three-
phase points at the specified contact time
Note 1 to entry: The contact angle is expressed in degrees (°).
3.3
contact time
duration of contact(s) between the water droplet and the test piece surface, as measured automatically
from the instant at which the timing mechanism is triggered to the instant of measurement
3.4
droplet height
H
distance from the top of the droplet in contact with the test piece surface as measured from the baseline
(3.1) at the specified contact time (3.3)
Note 1 to entry: The droplet height is expressed in millimetres.
3.5
droplet base diameter
D
maximum width of the droplet base in contact with the test piece surface, as measured along the
baseline (3.1) at the specified contact time (3.3)
Note 1 to entry: The droplet base diameter is expressed in millimetres.
3.6
three-phase point
point of intersection of the liquid/solid, liquid/gaseous and solid/gaseous boundary lines
4 Principle
A water droplet is applied to the horizontal planar surface of paper or board, the angle of contact it
makes with the material is assessed by optical digital imaging.
Figure 1 — Principle of measurement
2 © ISO 2021 – All rig
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