ISO/TS 14778:2021

Paper and board - Measurement of water contact angle by optical methods

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.

Papier et carton — Mesurage de l'angle de contact de l'eau par des méthodes optiques

General Information

• Status: Published
• Publication Date: 10-Jan-2021
• ICS: 85.060 - Paper and board
• Technical Committee: ISO/TC 6/SC 2 - Test methods and quality specifications for paper and board
• Drafting Committee: ISO/TC 6/SC 2/WG 41 - Contact angle
• Current Stage: 9093 - International Standard confirmed
• Start Date: 04-Jul-2024
• Completion Date: 13-Dec-2025

Overview

ISO/TS 14778:2021 - Paper and board - Measurement of water contact angle by optical methods - defines an automated, optical procedure for assessing the water contact angle on planar paper and board surfaces. The method uses a controlled droplet (commonly distilled water) and digital imaging to determine the droplet shape, base diameter, height and the contact angle after a specified contact time. The specification is intended for automated equipment and is not applicable to structured materials (e.g., highly embossed or non‑planar substrates).

Key topics and technical requirements

• Instrumentation: Automated contact angle apparatus with platform/holder, homogeneous lighting, liquid reservoir and dosing system, droplet application system and a digital video camera.
• Imaging: Minimum image capture rate of 50 frames per second and the ability to perform the first measurement within 20 ms to 40 ms of droplet contact.
• Liquid: Distilled water with surface tension ≥ 72 mN/m at 23 °C (ISO 13530 referenced). Other probe liquids are permitted only with caveats (non‑comparable results).
• Droplet control: Dosing tolerance of ±10% for droplet volume; capillary material and application must minimise wicking and kinetic energy on impact.
• Measured parameters: Contact angle (Θ) relative to an optically determined baseline, droplet height (H) and droplet base diameter (D) at specified contact times.
• Procedure elements: Sampling, conditioning (standard atmosphere per ISO 187), test piece mounting, apparatus calibration, automated measurement sequence and reporting of results.
• Limitations: Instrumental capabilities set the measurement limits; the method assumes a planar substrate and stable baseline detection.

Applications and who uses it

ISO/TS 14778:2021 is used to evaluate wettability and predict liquid‑paper interactions in industrial and laboratory settings. Typical users include:

• Paper and board manufacturers - quality control and product specification
• Coating formulators and converters - assessing coating wetting and adhesion
• Printing houses (inkjet, gravure, flexographic, wet offset) - predicting ink behavior
• Adhesive and lamination developers - gluing performance predictions
• R&D and testing laboratories - surface characterization and process development
• Instrument manufacturers - conformance to imaging and dosing requirements

Practical outcomes include improved printability, coating performance, glue bond reliability and screening of surface treatments. Note that surface energy component calculations require additional liquids and are outside the scope of this document.

Related standards

• ISO 186 - Sampling to determine average quality (paper and board)
• ISO 187 - Standard atmosphere for conditioning and testing
• ISO 13530 - Guidance on analytical quality control for water (reagent requirements)

Keywords: ISO/TS 14778:2021, water contact angle, paper and board, optical methods, sessile drop, automated contact angle measurement, wettability, surface characterization.

Standards Content (Sample)

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
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
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CH-1214 Vernier, Geneva
Phone: [phone]
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Website: www.iso.org
Published in Switzerland
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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electrotechnical standardization.
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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).
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iso/ foreword .html.
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
complete listing of these bodies can be found at www .iso .org/ members .html.
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 rights reserved

Key
1 substrate
D droplet base diameter
H droplet height
Θ contact angle
5 Reagents
5.1 Distilled water, with a surface tension of at least 72 mN/m at 23 °C, as specified in ISO 13530.
NOTE 1 This method can be used with other liquids providing they can be delivered in the form of a droplet
by the liquid delivery system (6.1.3) described in this document, and providing they do not cause substantial
swelling of the substrate over the time scale of the measurements such that the baseline can no longer be defined.
Also, for liquids with high vapour pressure, the rate of evaporation may be too fast to allow the technique to be
used, unless performed in a vapour-saturated atmosphere (e.g. a closed glass cuvette containing a few drops of
solvent). Contact angle values produced using other liquids are not comparable with those produced using water.
NOTE 2 Some contact angle measuring apparatus (6.1) have the facility to heat liquids prior to droplet
formation. Heating alters both surface tension and viscosity, both of which can affect contact angle. If the liquid is
heated, this is a deviation from the standard.
NOTE 3 Many contact angles measuring apparatus (6.1) are also capable of measuring liquid surface tension
and can be used to examine water surface tension. It is recommended to periodically check that the water has not
been contaminated as it would have a direct effect on the contact angle readings.
6 Apparatus
6.1 Contact angle measuring apparatus.
Key
1 platform
2 light source
3 droplet
4 pump
5 liquid reservoir
6 droplet application system
7 video camera
8 substrate
Figure 2 — General schematic of automated contact angle apparatus
For the optical determination of contact angles on a horizontal, planar substrate, any type of apparatus
having the general geometry shown in Figure 2 and further conforming to the requirements in the
following sub clauses is used.
NOTE This schematic is not intended to describe all possible instrumental arrangements. Accordingly,
instruments without a platform, with separated liquid container, with different kind of construction or those
using different viewing arrangements, are also permitted.
6.1.1 Platform with a holder mechanism (key item 1), to secure the test piece, capable of maintaining
flatness of the test piece over the duration of the test. The platform shall be horizontal.
NOTE The “platform” can be a table-top and the holder can be the housing of the device.
6.1.2 Light source (key item 2), capable of illuminating the droplet, with suitable heat filters, if
needed to eliminate heating of the test piece or water droplet. The illumination system shall produce
homogeneous lighting over the entire field of view.
6.1.3 Liquid delivery system (key item 5), consisting of a water reservoir and a dosing mechanism,
capable of producing a water droplet of known volume to a tolerance of +/- 10 %. If the droplet volume is
determined by the pumped volume, are shall be taken that no air bubbles are present as they will cause
significant volume error.
NOTE 1 The volume relates to the droplet produced at the end of a capillary, not necessarily to the volume
applied to the test piece surface. After release of the droplet there will always be a small portion of water
remaining on the capillary, the amount of which will relate to the water, capillary diameter, capillary type and
material, and droplet application technique.
NOTE 2 The dispensing volume and reproducibility of dosing system can be checked using an impermeable
reference surface.
NOTE 3 Some contact angle measuring apparatus enable the volume calculation directly from pendant droplet
in which case air bubbles won’t affect the final droplet volume.
6.1.4 Droplet application system (key item 6), allowing a water droplet of specified volume to be
suspended at the end of a capillary before being applied to a paper or board surface under controlled
conditions. The diameter and material of the capillary shall be chosen for application o
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