ASTM D7490-08

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: D7490 − 08
StandardTest Method for
Measurement of the Surface Tension of Solid Coatings,
Substrates and Pigments using Contact Angle
Measurements
This standard is issued under the fixed designation D7490; 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 3.1.1 contact angle, n—the interior angle that a drop makes
between the substrate and a tangent drawn at the intersection
1.1 This test method describes a procedure for the measure-
between the drop and the substrate as shown in Fig. 1; this is
ment of contact angles of two liquids, one polar and the other
the angle formed by a liquid at the three phase boundary where
nonpolar, of known surface tension on a substrate, pigment (in
a liquid, gas (air) and solid intersect.
the form of a disk), or cured or air dried coating in order to
3.1.2 dispersion component, n—the component of solid
calculate the surface properties (surface tension and its disper-
surface tension that is related to intermolecular attraction
sion and polar components) of the solid.
caused by nonpolar dispersion forces.
1.2 The total solid surface tension range that can be deter-
3.1.3 polar component, n—the component of solid surface
mined using this method is approximately 20 to 60 dynes/cm.
tension that is related to polar forces, such as hydrogen
1.3 The values stated in SI units are to be regarded as
bonding and ion-dipole forces.
standard. No other units of measurement are included in this
3.1.4 solid surface tension, n—the surface tension of a solid
standard.
surface; this parameter cannot be measured directly, but must
1.4 This standard does not purport to address all of the
be determined by extrapolation of polymer melt or solution
safety concerns, if any, associated with its use. It is the
datato100 %solidsorbycontactangleswithliquidsofknown
responsibility of the user of this standard to establish appro-
surface tension.
priate safety and health practices and determine the applica-
3.1.5 surface energy, n—excess free energy of surface
bility of regulatory limitations prior to use.
molecules compared to those of the bulk material; arises from
2. Referenced Documents unbalanced molecular cohesive forces at a surface that cause
the surface to contract and behave like a film or membrane
2.1 ASTM Standards:
(units are energy/unit areas such as joules/cm ).
D1193 Specification for Reagent Water
D5725 Test Method for Surface Wettability andAbsorbency 3.1.6 surface tension, n—the force necessary to break the
surface of a film of a given length (units are force/length, such
of Sheeted Materials Using an Automated Contact Angle
Tester (Withdrawn 2010) as dynes/cm or newtons/m); the same numerically as surface
energy, but different units.
D7334 Practice for Surface Wettability of Coatings, Sub-
strates and Pigments by Advancing Contact Angle Mea-
4. Summary of Test Method
surement
4.1 Contact angles of drops of distilled water and di-
3. Terminology
iodomethane (methylene iodide) are measured on the surface
3.1 Definitions: of interest. The two values are then substituted into two
separate expressions of the Owens-Wendt-Kaelble equation
(one for each liquid). This results in two equations in two
This test method is under the jurisdiction of ASTM Committee D01 on Paint
unknowns, which are then solved for the dispersion and polar
and Related Coatings, Materials, andApplications and is the direct responsibility of
Subcommittee D01.23 on Physical Properties of Applied Paint Films.
components of surface tension. The sum of the components is
Current edition approved Dec. 15, 2008. Published January 2009. DOI: 10.1520/
the surface tension of the solid.
D7490-08.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
5. Significance and Use
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
5.1 The method described in this standard is based on the
the ASTM website.
3 concept that the total free energy at a surface is the sum of
The last approved version of this historical standard is referenced on
www.astm.org. contributions from different intermolecular forces, such as
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7490 − 08
7. Apparatus
7.1 Goniometer—An instrument consisting of a controlled
light source, a stage to hold the test specimen, and a micro-
scope or camera for viewing of the drop on the specimen is
required. An automated instrument for measuring angle of
contact and a method for using it can be found in Test Method
D5725.
A = contact angle
7.2 Hypodermic Syringe—A syringe, such as a 1-mL hypo-
D=dropofliquid
dermic, equipped with a No. 27 stainless steel needle, capable
P = specimen
T = tangent at specimen surface
of providing 100 to 200 drops from 1 mL, is suitable for use
with water-like liquids. More viscous liquids may require a
FIG. 1 Measuring Angle of Contact
needle of different size.
8. Reagents and Materials
8.1 Water—Type II reagent water (distilled) in accordance
dispersion, polar and hydrogen bonding. There are other
techniques that employ three components (dispersion, polar with Specification D1193.
and hydrogen bonding). These methods are further compli-
8.2 Diiodomethane—reagent grade or better.
cated by needing three to five test liquids and are not practical
for routine testing. This method uses contact angles of two
9. Preparation of Specimens
liquids to provide data for the calculation of two components,
d p
9.1 If the part or panel of interest is too large to fit on the
dispersion, γ , and polar, γ .
s s
stage, then test specimens should be cut to a size appropriate
5.2 Dispersion and polar component data, along with the
for the instrument being used. They shall be cut in such a way
total solid surface tension, are useful for explaining or predict-
as to be thoroughly representative of the sample.
ing wetting or adhesion, or both, of coatings on pretreatments,
9.2 The areas tested shall not contain visible blemishes or
substrates and other coatings. Low solid surface tension values
often are a sign of contamination and portend potential wetting defects and shall not be touched with the fingers or contami-
nated in any other way.
problems. High polar components may signal polar contami-
nation.Thereisevidenceintheliteraturethatmatchingofpolar
9.3 If contamination or improper handling is suspected, the
components of topcoats and primers gives better adhesion.
specimen may be rinsed with water or washed with laboratory
5.3 Solidsurfacetensionsofpigments,particularlythepolar
detergent and water. However, cleaning may affect the results
components, may be useful in understanding dispersion prob-
and must be noted on the report.
lems or to provide signals for the composition of dispersants
9.4 Pigment specimens shall be in the form of disks pre-
and mill bases. However, comparison of pigments may be
pared in a press such as those used to prepare KBr disks for
difficult if there are differences in the roughness or porosity, or
infrared analysis.
both, of the disks prepared from them.
5.4 Although this technique is very useful in characterizing
10. Procedure
surfaces, evaluating surface active additives and explaining
10.1 Test the specimens at a standard temperature of 23 6
problems, it is not designed to be a quality control or
2°C and at a relative humidity≥50 %, unless otherwise agreed
specification test.
upon.
6. Interferences
10.2 Set up the goniometer and level the stage according to
the manufacturer’s instructions.
6.1 The following factors may interfere with results:
6.1.1 Dirt,fingerprintsorothercontaminationonthesurface
10.3 Measure contact angles of water and diiodomethane on
being tested. Contact angles are very sensitive to surface
the specimen of interest as described in Practice D7334 or the
contamination.
manufacturer’s literature for the instrument being used.
6.1.2 Arough or porous test surface such that drops sink in
10.4 Make two angle measurements (one on each drop
rapidly.Suchsurfacesaremostlikelyfoundwithpigmentdisks
edge) of each of three drops on the specimen. If the contact
or sanded coating
...