Surface Energy Measurement: Contact Angle

Surface Energy Measurement: Contact Angle, Dyne Solutions, and the OWK Model

Adhesive failure is usually a result of the substrate, not the adhesive itself. If the substrate has less surface energy than the liquid being applied, wetting will not occur, and bonding will be impossible. Surface energy measurements provide numbers to indicate this before full-scale production, rather than afterward.

What Is Surface Energy Testing?

Surface energy measures the excess energy at a solid’s surface compared with its bulk, reported in millinewtons per meter, which is numerically the same as the older dynes per centimeter. A liquid spreads on a solid when the solid’s surface energy exceeds the liquid’s surface tension, and it beads up when it does not.

Contact angle is the direct measurement. A droplet is applied to the surface, and the angle between the liquid interface and the solid is measured optically. Small angles indicate good wetting. Water on clean glass is around zero. Water on untreated polypropylene is above 90°.

Surface energy has components, so one liquid is not sufficient. ASTM D7490 uses two liquids, polar and nonpolar, and introduces their contact angles into the Owens-Wendt-Kaelble equations. This process separates the dispersive and polar components, whose sum equals the total. The procedure ranges from 20 to 60 dynes/cm.

That split matters practically. Two substrates can share a total of 40 mN/m while one carries most of it as polar and the other as dispersive, and a polar adhesive will bond well to only one.

Types of Surface Energy Testing

Five approaches:

1. Two-liquid contact angle with a component model. Deionized water supplies the polar measurement and diiodomethane the nonpolar one. ASTM D7490 covers this for solid coatings, substrates, and pigments. Use this route when you need the polar and dispersive split rather than a single figure.
2. Single-liquid contact angle. One liquid can provide a wettability number without separating any components. ASTM D5946 covers the method for corona-treated polymer films, while ASTM C813 covers the method of hydrophobic contamination on glass. Quick and efficient, especially when measuring treatment level.
3. Dyne solution wetting tension. A set of calibrated liquids with known surface tensions is applied in sequence, and the wetting tension is the value at which the film of liquid holds for a defined time before beading. ASTM D2578 covers polyethylene and polypropylene film. It is cheap, fast, and used on production lines to verify corona or flame treatment.
4. Advancing and receding angles. Liquid addition gives the advancing angle, and removal gives the receding angle. Their difference is known as contact angle hysteresis, which provides information about surface roughness and chemical heterogeneity, unlike the static angle.
5. Qualitative assessment. ASTM F21 employs atomized water droplets to detect the presence of hydrophobic coating. This test either passes or fails, provides no numerical results, and takes seconds to perform on the entire surface.

Surface Energy Test Methods and Standards

The models that produce these figures rely on several assumptions. In Owens-Wendt-Kaelble’s model, the assumption is that the surface is smooth, rigid, homogeneous, and non-absorbent. A rough or porous surface violates all of these and renders the calculated figure meaningless in terms of the equation.

Also read: ASTM D7490: Contact Angle Measurements

How to Choose the Right Surface Energy Test

Use cases or scenarios:

1. Do you want the split into polar and dispersive contributions, or simply a wetting value? Formulating an adhesive and analyzing a failed bond both require a split; thus, D7490 with two liquids is the right choice. Checking if your corona treater works as intended requires only one wetting value, and D2578 or D5946 does the job quicker.
2. Is the surface being tested the surface that gets bonded? Release agents, slip additives, and plasticizers transfer to the surface within a few days. A film that is measured at 42 mN/m after treatment will show a value of 36 mN/m after 2 weeks. Always test the material as it will be used.
3. How rough is the surface? Any roughness alters what happens on a flat surface, making the wetting surface more hydrophilic and the non-wetting surface more hydrophobic. Above a few micrometers of roughness, the contact angle no longer measures only chemistry. Roughness should be measured for the same samples, or it should be noted if it was not.
4. Is humidity controlled during the measurement? The contact angle of water varies during evaporation, and evaporation is faster at low humidity. Relative humidity should be kept above 40% when water is used in testing, and the measurement should be done within a certain period after dropping the liquid.

Another factor needs to be decided early. One drop on a 100 mm substrate means only a small portion is characterized, because contamination does not occur uniformly. It would be better to use five drop locations on the substrate.

Surface Energy FAQs