ASTM D7490 determines the surface tension of solid coatings, substrates, and pigments from contact angle measurements. Two liquids with known properties are placed on the surface; their contact angles are measured optically, and the values are used to solve for the solid’s dispersive and polar components. MatX Lab runs the method on coated panels, films, treated polymers, and pigment compacts, aligned with ASTM D7490.
What Is ASTM D7490?
ASTM D7490 is the standard test method to measure the surface tension of solid coatings, substrates, and pigments based on contact angle determination. A goniometer deposits a drop of liquid in a controlled manner onto the solid surface and captures its image, which the software uses to calculate the point at which liquid, solid, and air intersect.
Two liquids are used. The water has a large polar component, while diiodomethane is completely non-polar. By analyzing both, two equations are generated with two unknowns using the Owens-Wendt-Kaelble equation.
Adding the dispersive and polar components gives total solid surface tension, which for the materials this method covers generally falls between 20 and 60 dynes/cm. A low value means a liquid will bead rather than spread, which is the practical meaning of a wetting or adhesion problem.
ASTM D7490 Scope and Applications
The procedure works well for surfaces of smooth, isotropic, rigid solids which are not altered by either test liquid. Surfaces that are rough, porous, swelling, and soluble cannot be used for this procedure because the drop will not lie on a surface of homogeneous chemistry.
Applications:
- Adhesion prediction. Establishing whether a substrate has enough surface energy for a coating, ink, or adhesive to wet it properly before bonding.
- Surface treatment verification. Quantifying the effect of corona, plasma, flame, or chemical treatment on a polymer surface, and checking how quickly that effect decays.
- Contamination detection. Finding release agent, oil, or silicone transfer, all of which drop surface tension sharply.
- Formulation work. Matching a coating’s own surface tension against the substrate it must wet.
When a quick yes-or-no answer for hydrophobic contaminants is needed instead of a value, ASTM F21 by atomizer fog method will be faster and will require no instrument time at all. The medical device work using this method includes materials surface-level analysis. Biocompatibility testing or device certification is not performed at MatX Lab.
ASTM D7490 Test Procedure
Contact angle is a surface measurement in the strictest sense. What sits in the top few molecular layers controls the result, and handling puts things there.
| Step | What happens |
| Specimen handling | Handle surfaces at the edges with clean gloves or tweezers. A fingerprint changes the answer more than most process variables do. |
| Surface inspection | The area to be measured is examined for texture, gloss variation, and visible contamination, all of which are recorded before testing. |
| Conditioning | Specimens equilibrate in the test environment. Relative humidity is held above 40% when water is used, since a drying drop reads a changing angle. |
| Liquid preparation | Deionized water and diiodomethane are dispensed through a clean syringe fitted with a fine stainless needle, typically 27 gauge. |
| Drop placement | A drop of consistent volume is deposited without the needle touching the surface, and imaging starts immediately. |
| Angle measurement | The static contact angle is read from the fitted drop profile within seconds of placement, before evaporation or spreading changes it. |
| Replication | Measurements are repeated at several separate locations for each liquid, and the drop positions do not overlap. |
| Calculation | The angle pairs are solved through the Owens-Wendt-Kaelble equations for the dispersive component, the polar component, and their total. |
Limitations: Surface roughness undermines the model, and a matte surface yields results based on geographic rather than chemical properties. The absorbent, porous surface absorbs the water droplet, and the contact angle continues to decrease. Corona-treated polymers will heal over time, meaning that a corona-treated polymer film that is tested one week later will have a different contact angle than when it was first measured. Water vaporizes quickly under low-humidity conditions, causing the contact angle to change while you are watching it; hence the need to set a minimum humidity threshold for the practice. Diiodomethane is toxic and light-sensitive, requiring special handling. The contact angle predicts wetting, not adhesion; thus, a surface energy test does not imply bond strength.
ASTM D7490 Sample Requirements and Test Conditions
| Parameter | Typical requirement |
| Sample size | 25 mm x 25 mm minimum, 50 mm square preferred for multiple drops |
| Surface | Smooth, non-porous, chemically uniform across the measured area |
| Thickness | Any, provided the specimen lies flat on the stage |
| Replicates | Five drop locations per liquid per specimen |
| Test liquids | Deionized water and diiodomethane |
| Relative humidity | Above 40% when water is used |
| Handling | Edge handling only, shipped interleaved and unwrapped from adhesive films |
Ship specimens the way you would ship a bonding surface. Adhesive tape, foam packaging, and plastic bags all transfer material onto a surface, and that transfer is exactly what this test detects.
Tell us whether you care about the as-received condition or the cleaned condition. We do not clean specimens without instruction, and in contamination investigations, the as-received surface is the whole point.
ASTM D7490 Test Results and Reporting
Your report gives total solid surface tension in dynes/cm with its dispersive and polar components stated separately.
Individual contact angles for both liquids appear per drop location, along with the mean and spread for each. We record the test conditions: temperature, relative humidity, drop volume, and time between placement and measurement. Representative drop images accompany the numbers.
If the surface was not uniform across the sample, we mention this in our report and state where the high and low measurements were taken. A surface that showed a range of 20 degrees across five spots says something about process uniformity.
ASTM D7490 FAQs
What surface tension do I need for good adhesion?
As a general rule, the substrate must be higher than the coating/adhesive, with a 10-dynes/cm gap being the norm. This is only a general rule to help with the screening process but does not replace an adhesion test.
How soon after treatment should I test?
As soon as practical, and record the time difference. Corona and plasma surface treatments begin to degrade after just a few hours for some polymers. Thus, include the time difference in the results, not the background.