Determining Critical Surface Tension of Solid Substrates

SpecialChem - Jan 24, 2007


The ease in which an adhesive or sealant wets (makes intimate contact) a substrate surface and the work necessary to separate the adhesive from the substrate can be related to the surface energies of the adhesive, substrate, and subsequent interface. In an ideal situation for spreading or wetting, the surface of the substrate should always have a higher surface energy than that of the liquid adhesive1. Surface energy, (gamma), is used interchangeably with the terms "surface free energy" and "surface tension".

The surface energies of liquids are readily determined by measuring the surface tension with a duNouy ring. A clean platinum ring is placed under the surface of the test liquid, and the liquid is slowly moved downward until the ring breaks through the liquid surface. The force is recorded, and by means of appropriate conversion factors, the surface tension of the liquid is calculated. There are several other common methods described in the literature for measuring the surface tension of liquids.

However, the measuring of surface tension is not as straight forward when it comes to solid surfaces. Direct surface tension measurements on solids are mostly made near the melting point; however, it is the lower temperature properties that mainly concern adhesive studies. Therefore, surface energies of solids are generally indirectly estimated through contact angle measurement methods.

In a contact angle measurement, a drop of liquid is placed upon the surface of a solid. It is assumed that the liquid does not react with the solid and that the solid surface is perfectly smooth and rigid. The drop is allowed to flow and equilibrate with the surface. The measurement of the contact angle, (theta), is usually made with a goniometer that is simply a protractor mounted inside a telescope. The angle that the drop makes with the surface is measured carefully. A diagram of the contact angle measurement is shown in Figure 1.

Schematic diagram of the contact 
        angle and its surface free energy (tension) components
Figure 1: Schematic diagram of the contact angle and its surface free energy (tension) components

A rather simple method of estimating the surface energy of solids was developed by Zisman2. Zisman proposed that a critical surface tension, c, can be estimated by measuring the contact angle of a series of liquids with known surface tensions on the surface of interest. These contact angles are plotted as a function of the LV of the test liquid. The critical surface tension is defined as the intercept of the horizontal line, cos = 1, with the extrapolated straight-line plot of cos against LV as shown in Figure 2. This intersection is the point where the contact angle is 0 degrees. A hypothetical test liquid having this LV would just spread over the substrate.

Zisman 
              plot for low energy polyethylene surface with a liquid series commonly 
              used by Zisman
Figure 2: Zisman plot for low energy polyethylene surface with a liquid series commonly used by Zisman 3

The surface tension value for most inorganic solids is on the order of hundreds or thousands of mJ/m2, and for polymers it is at least an order of magnitude lower. Solid substrates are often considered to be either "high energy surfaces" (metals, glass, ceramics) or "low energy surfaces" (polymers). Organic liquids have surface tensions that are in a similar range as solid polymers. In fact, an epoxy adhesive composition will have approximately the same surface tension in the cured state as it does in the uncured or liquid state. Values of critical surface tensions for common solids and surface tensions of common liquids are shown in Table 1.

Liquids
Surface Tension, dynes/cm
Epoxy resin (DGEBA type)
47
Petroleum lubricating oil
29
19
Glycerol
64
Hexane
18
Acetone
23
n-Propanol
24
Toluene
28
Trichloroethane
25
Methyl ethyl ketone
25
Ethylene glycol
48
Mineral spirits
24
Water
73

Substrates (High Energy)
Critical Surface Tension, dynes/cm
Aluminum
~500
Copper
1360
Nickel
1770
Iron oxide
1357
Beryllium oxide
1360
Lead
442
Silver
890
Glass
~1000

Substrates (Low Energy)
Critical Surface Tension, dynes/cm
Acetal
47
Acrylonitrile butadiene styrene - ABS
35
41
Epoxy - typical amine cure
46
Nylon 6/6
41
Polycarbonate
46
Polyethylene terephthalate - PET
43
Polyimide
40
Polystyrene
33
Polysulfone
41
Polytetrafluoroethylene - PTFE
18
Polyvinyl chloride - PVC
39
Polyethylene
31
Polypropylene
33
Phenolic
52
24
Styrene butadiene rubber
29
Table 1: Room Temperature Surface Tension of Several Liquids (Top) and Critical Surface Tension of Various High Energy (Middle) and Low Energy Substrates (Bottom)

References

  1. Petrie, E.M., "Chapter 2: Theories of Adhesion", Handbook of Adhesives and Sealants, 2nd ed., McGraw-Hill, New York, 2006.
  2. Fox, H. W. and Zisman, W. A., J. Colloid. Science, 5 (1950), p. 514; also Zisman, W. A., "Relation of Equilibrium Contact Angle to Liquid and Solid Constitution", Chapter 1 in Contact Angle, Wettability, and Adhesion, R. F. Gould, ed., American Chemical Society, Washington DC., 1964.
  3. Kinloch, A.J., Adhesion and Adhesives, Chapman and Hall, New York, 1987, p. 25.

Should you have any comments or feedback, please contact me.

Edward M. Petrie

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SpecialChem4Adhesives Members Reactions

Surface Tension Reference Values - Jan 23, 2014
posted by Gregory Gentile, Production / Manufacturing
This is one of the few articles that lists reference values of metals and plastics and explains in an easy to understand format.

Great article - May 30, 2013
posted by Esau Del Rio, R&D - Applied/ Formulation/ Product development at UCLM nanochemistry group
Very instructive, thnks

USING OTHER TOOLING THAN Goniometer - May 06, 2011
posted by Jose Castellanos, Analytical laboratory / Testing at Converteam
I am going to study a new braze material. I want to use other tooling than a Goniometer to make easier and more accurate contact angle measurement. I let you know later if my hypothesis was the right one. Note: I have measure superficial and interfacial tension on liquids since 1990.

fascinating and very useful - Apr 29, 2011
posted by Steve Strunk, Production / Manufacturing at Teledyne Dalsa
Great article. Instructive and very practical method to measure surface energy/tension.

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