ASTM D7334-08
(Practice)Standard Practice for Surface Wettability of Coatings, Substrates and Pigments by Advancing Contact Angle Measurement
Standard Practice for Surface Wettability of Coatings, Substrates and Pigments by Advancing Contact Angle Measurement
SIGNIFICANCE AND USE
This standard is useful for characterizing the wettability of surfaces. A surface that is easy to wet is one over which a coating is more likely to give good adhesion and appearance and less likely to suffer surface tension related defects such as crawling, cratering, pinholing and orange peel.
This standard also can be used to test pigment surfaces for wettability, particularly by potential surfactant- or resin-based dispersants or mill bases. Easily wetted pigments are more likely to be easy to disperse and dispersants/mill bases that wet pigments of interest are more likely to disperse those pigments well.
Although the contact angle is governed by the surface tensions of the test liquid and test surface, the angle cannot provide a surface tension value directly.
A low advancing contact angle value ( 45°) is indicative of wetting and angles of 10 to 20º are indicative of excellent wetting.
Water can be used as a test liquid to establish (via the advancing contact angle) whether a surface is hydrophilic (angle 45º), hydrophobic (angle > 90º) or somewhere in-between (angle of 45 to 90º). Water contact angles have been used to estimate surface cleanliness before and after cleaning operations, ease of wettability of surfaces by waterborne coatings and the effectiveness of rinsing processes.
An organic liquid such as a solvent also can be used to characterize a substrate, coating or pigment. The resultant contact angle will depend on the surface tensions of the liquid and the test surface. A low surface tension (energy) test surface will not be wet by a high surface tension liquid.
In addition to water and solvents, a surfactant dispersion or dispersant solution can be used to test a pigment surface. Any test liquid that is a potential dispersant for a test pigment must wet the pigment well or it will not work as a dispersant.
Contact angle measurements can be used to map surfaces in terms of hydrophilicity, presence of low surface tension components or c...
SCOPE
1.1 This practice covers the measurement of the angle of contact when a drop of liquid is applied to a coated surface, substrate, or preformed disk of pigment.
1.2 There are two types of contact angles, advancing and receding. This standard deals only with advancing contact angles.
1.3 This practice is intended to supplement the manufacturer’s instructions for the device being used to make the measurements, but is not intended to replace them.
1.4 A common test liquid is water, but many other liquids such as solvents, surfactant and dispersant solutions and even liquid paints can be used.
1.5 This practice is based on goniometry, which involves the observation of a sessile drop of test liquid on a solid substrate.
1.6 Although contact angles are governed by surface tension, this standard cannot be used to measure surface tension directly.
1.7 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Standards Content (Sample)
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Designation: D7334 − 08
StandardPractice for
Surface Wettability of Coatings, Substrates and Pigments
by Advancing Contact Angle Measurement
This standard is issued under the fixed designation D7334; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope of Sheeted Materials Using an Automated Contact Angle
Tester
1.1 This practice covers the measurement of the angle of
contact when a drop of liquid is applied to a coated surface,
3. Terminology
substrate, or preformed disk of pigment.
3.1 Definitions:
1.2 There are two types of contact angles, advancing and
receding. This standard deals only with advancing contact 3.1.1 advancing contact angle, n—the contact angle that is
angles. measured immediately after the sessile drop is placed on the
surface.
1.3 This practice is intended to supplement the manufactur-
3.1.1.1 Discussion—The drop has the maximum volume
er’s instructions for the device being used to make the
allowable for the liquid-solid interfacial area: any addition will
measurements, but is not intended to replace them.
make the drop expand and increase the liquid-solid interfacial
1.4 A common test liquid is water, but many other liquids
area. This can be thought of as the “wetting angle” because the
such as solvents, surfactant and dispersant solutions and even
drop is ready to wet additional area. This is the contact angle
liquid paints can be used.
used in this method and in most coating measurements.
1.5 Thispracticeisbasedongoniometry,whichinvolvesthe
3.1.2 contact angle, n—the interior angle that a drop makes
observation of a sessile drop of test liquid on a solid substrate.
between the substrate and a tangent drawn at the intersection
1.6 Although contact angles are governed by surface
between the drop and the substrate as shown in Fig. 1.
tension, this standard cannot be used to measure surface
3.1.2.1 Discussion—This is the angle formed by a liquid at
tension directly.
the three phase boundary where a liquid, gas (air) and solid
1.7 The values stated in SI units are to be regarded as the
intersect.
standard. The values given in parentheses are for information
3.1.3 receding contact angle, n—thecontactanglemeasured
only.
when material is removed from the drop so that it contracts.
1.8 This standard does not purport to address all of the
3.1.3.1 Discussion—The liquid-solid interfacial area will
safety concerns, if any, associated with its use. It is the
decrease. This is the “de-wetting angle.”
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
3.1.4 sessile drop, n—drop of liquid on the upper side of a
bility of regulatory limitations prior to use.
horizontal surface (as in Fig. 1).
3.1.5 surface tension, n, and surface energy, n—the terms
2. Referenced Documents
2 surface tension and surface energy are often used interchange-
2.1 ASTM Standards:
ably.
D1193 Specification for Reagent Water
3.1.5.1 Discussion—They are the same numerically, al-
D5725 Test Method for Surface Wettability andAbsorbency
thoughtheyhavedifferentunits.Theyaremeasuresofaneffect
that arises from unbalanced molecular cohesive forces at a
This practice is under the jurisdiction of ASTM Committee D01 on Paint and
surface that cause the surface to contract and behave like a film
Related Coatings, Materials, and Applications and is the direct responsibility of
or membrane. The surface molecules have higher free energy
Subcommittee D01.23 on Physical Properties of Applied Paint Films.
than those in the bulk material. The excess free energy is what
Current edition approved Feb. 15, 2008. Published March 2008. DOI: 10.1520/
D7334-08.
wecallsurfaceenergy,whichhasunitsofenergy/unitareasuch
For referenced ASTM standards, visit the ASTM website, www.astm.org, or 2
as Joules/cm . However, what we measure is surface tension,
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
the force necessary to break a film of a given length, usually in
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. dynes/cm or Newtons/m.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7334 − 08
Any test liquid that is a potential dispersant for a test pigment
must wet the pigment well or it will not work as a dispersant.
5.8 Contact angle measurements can be used to map sur-
faces in terms of hydrophilicity, presence of low surface
tension components or contaminants, or variations in compo-
sition. Other analytical methods such as infrared microscopy
would be needed to identify the chemical moieties that give the
A = contact angle
D=dropofliquid
contact angle differences.
P = specimen
T = tangent at specimen surface 5.9 This test method can be used on nearly all coatings and
FIG. 1 Measuring Angle of Contact
substrates and may be extended to pigments by compressing
the pigment powder into a solid disk.
6. Interferences
4. Summary of Practice
6.1 The following factors may interfere with results:
4.1 Adrop of a specified volume of water or another agreed
6.1.1 Dirt or fingerprints on the surface being tested.
upon test liquid is applied to a test specimen using a syringe.
6.1.2 Arough or porous test surface such that the drop sinks
4.2 Thecontactangleismeasuredbyeitheroftwomethods:
in rapidly.
4.2.1 by viewing the sessile drop through a microscope
6.1.3 A curved test surface such that angles are difficult to
fitted with a goniometer scale for direct measurement of the
measure.
angle,
6.1.4 Low humidity (< 50 % RH) when water is the test
4.2.2 by capturing an image of the drop, then measuring
liquid such that the contact angle changes rapidly.
with a protractor or using appropriate software to process the
image and measure the angle.
7. Apparatus
5. Significance and Use 7.1 Ahorizontal stage onto which a flat surface is mounted,
7.2 A device (generally a syringe) to place a droplet on the
5.1 This standard is useful for characterizing the wettability
of surfaces. A surface that is easy to wet is one over which a surface.
coating is more likely to give good adhesion and appearance
7.3 An enclosure (if desired) to prevent interference of
and less likely to suffer surface tension related defects such as
vapor-borne impurities and to secure saturation of vapor
crawling, cratering, pinholing and orange peel.
(particularly useful in measuring water angles).
5.2 This standard also can be used to test pigment surfaces
7.4 A light source to illuminate the three-phase c
...
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