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Significance and Use
Knowledge of the critical surface tension of substrates, primers and other coatings is useful for explaining or predicting wettability by paints and other coatings applied to those surrfaces. Surfaces with low critical surface tensions usually are prone to suffer defects such as crawling, picture framing, cratering and loss of adhesion when painted. Low or irregular values, or both, often are indicative of contamination that could reduce adhesion. Surfaces with high critical surface tensions are easy to wet and usually provide an excellent platform for painting.
The swab, marking pen and draw-down tests all simulate the application of a film
The swab and marking pen techniques are simple and rapid and are particularly useful for testing in the field or on curved, irregular or porous surfaces where contact angles cannot be measured. The drop test does not work well on such surfaces and the draw-down method requires a flat specimen that is relatively large.
The estimation of critical surface tension has been useful in characterizing surfaces before and after cleaning processes such as power washes and solvent wipes in order to evaluate the efficiency of the cleaning.
One or more of these techniques could be the basis of a go/no-go quality control test where if a certain liquid wets, the surface is acceptable for painting, but if that liquid retracts and crawls, the surface is not acceptable.
Another go/no go test is possible where the test liquid is a paint and the surface is a substrate, primer or basecoat. A form of this test has been used for coatings for plastics.
1.1 This practice covers procedures for estimating values of the critical surface tension of surfaces by observing the wetting and dewetting of a series of liquids (usually organic solvents) applied to the surface in question.
1.2 Another technique, measurement of the contact angles, θ, of a series of test liquids and plotting cos θ versus surface tension (Zisman plots), provides data that allow the determination of more exact values for critical surface tension.
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.4 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.
2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.
D2578 Test Method for Wetting Tension of Polyethylene and Polypropylene Films
Nordtest StandardsNTpoly176 Spreading Surface Tension by the Applied Droplet Method.
ICS Number Code 25.220.01 (Surface treatment and coating in general)
UNSPSC Code 73181020(Surface treatment Services)
ASTM D7541-11, Standard Practice for Estimating Critical Surface Tensions, ASTM International, West Conshohocken, PA, 2011, www.astm.orgBack to Top