# ASTM D7490-13

Designation: D7490 − 13Standard Test Method forMeasurement of the Surface Tension of Solid Coatings,Substrates and Pigments using Contact AngleMeasurements1This standard is issued under the fixed designation D7490; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (´) indicates an editorial change since the last revision or reapproval.1. Scope*1.1 This test method describes a procedure for the measure-ment of contact angles of two liquids, one polar and the othernonpolar, of known surface tension on a substrate, pigment (inthe form of a disk), or cured or air dried coating in order tocalculate the surface properties (surface tension and its disper-sion and polar components) of the solid.1.2 The total solid surface tension range that can be deter-mined using this method is approximately 20 to 60 dyn/cm.1.3 The values stated in CGS units (dyn/cm) are to beregarded as standard. No other units of measurement areincluded in this standard.1.4 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D1193 Specification for Reagent WaterD5725 Test Method for Surface Wettability and Absorbencyof Sheeted Materials Using an Automated Contact AngleTester (Withdrawn 2010)3D7334 Practice for Surface Wettability of Coatings, Sub-strates and Pigments by Advancing Contact Angle Mea-surement3. Terminology3.1 Definitions:3.1.1 contact angle, n—the interior angle that a drop makesbetween the substrate and a tangent drawn at the intersectionbetween the drop and the substrate as shown in Fig. 1; this isthe angle formed by a liquid at the three phase boundary wherea liquid, gas (air) and solid intersect.3.1.2 dispersion component, n—the component of solidsurface tension that is related to intermolecular attractioncaused by nonpolar dispersion forces.3.1.3 polar component, n—the component of solid surfacetension that is related to polar forces, such as hydrogenbonding and ion-dipole forces.3.1.4 solid surface tension, n—the surface tension of a solidsurface; this parameter cannot be measured directly, but mustbe determined by extrapolation of polymer melt or solutiondata to 100 % solids or by contact angles with liquids of knownsurface tension.3.1.5 surface energy, n—excess free energy of surfacemolecules compared to those of the bulk material; arises fromunbalanced molecular cohesive forces at a surface that causethe surface to contract and behave like a film or membrane(units are energy/unit areas such as joules/cm2).3.1.6 surface tension, n—the force necessary to break thesurface of a film of a given length (units are force/length, suchas dyn/cm or newtons/m); the same numerically as surfaceenergy, but different units.4. Summary of Test Method4.1 Contact angles of drops of distilled water and di-iodomethane (methylene iodide) are measured on the surfaceof interest. The two values are then substituted into twoseparate expressions of the Owens-Wendt-Kaelble equation(one for each liquid). This results in two equations in twounknowns, which are then solved for the dispersion and polarcomponents of surface tension. The sum of the components isthe surface tension of the solid.1This test method is under the jurisdiction of ASTM Committee D01 on Paintand Related Coatings, Materials, and Applications and is the direct responsibility ofSubcommittee D01.23 on Physical Properties of Applied Paint Films.Current edition approved July 1, 2013. Published August 2013. Originallyapproved in 2008. Last previous edition approved in 2008 as D7394 – 08. DOI:10.1520/D7490-13.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at service@astm.org. For Annual Book of ASTMStandards volume information, refer to the standard’s Document Summary page onthe ASTM website.3The last approved version of this historical standard is referenced onwww.astm.org.*A Summary of Changes section appears at the end of this standardCopyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States15. Significance and Use5.1 The method described in this standard is based on theconcept that the total free energy at a surface is the sum ofcontributions from different intermolecular forces, such asdispersion, polar and hydrogen bonding. There are othertechniques that employ three components (dispersion, polarand hydrogen bonding). These methods are further compli-cated by needing three to five test liquids and are not practicalfor routine testing. This method uses contact angles of twoliquids to provide data for the calculation of two components,dispersion, γsd, and polar, γsp.5.2 Dispersion and polar component data, along with thetotal solid surface tension, are useful for explaining or predict-ing wetting or adhesion, or both, of coatings on pretreatments,substrates and other coatings. Low solid surface tension valuesoften are a sign of contamination and portend potential wettingproblems. High polar components may signal polar contami-nation. There is evidence in the literature that matching of polarcomponents of topcoats and primers gives better adhesion.45.3 Solid surface tensions of pigments, particularly the polarcomponents, may be useful in understanding dispersion prob-lems or to provide signals for the composition of dispersantsand mill bases. However, comparison of pigments may bedifficult if there are differences in the roughness or porosity, orboth, of the disks prepared from them.5.4 Although this technique is very useful in characterizingsurfaces, evaluating surface active additives and explainingproblems, it is not designed to be a quality control orspecification test.6. Interferences6.1 The following factors may interfere with results:6.1.1 Dirt, fingerprints or other contamination on the surfacebeing tested. Contact angles are very sensitive to surfacecontamination.6.1.2 A rough or porous test surface such that drops sink inrapidly. Such surfaces are most likely found with pigment disksor sanded coatings.6.1.3 A curved test surface such that angles are difficult orimpossible to measure.6.1.4 Low humidity (40 % RH) when water is the testliquid such that the contact angle changes rapidly.7. Apparatus7.1 Goniometer—An instrument consisting of a controlledlight source, a stage to hold the test specimen, and a micro-scope or camera for viewing of the drop on the specimen isrequired. An automated instrument for measuring angle ofcontact and a method for using it can be found in Test MethodD5725.7.2 Hypodermic Syringe—A syringe, such as a 1-mLhypodermic, equipped with a No. 27 stainless steel needle,capable of providing 100 to 200 drops from 1 mL, is suitablefor use with water-like liquids. More viscous liquids mayrequire a needle of different size.8. Reagents and Materials8.1 Water—Type II reagent water (distilled) in accordancewith Specification D1193.8.2 Diiodomethane—reagent grade or better.9. Preparation of Specimens9.1 If the part or panel of interest is too large to fit on thestage, then test specimens should be cut to a size appropriatefor the instrument being used. They shall be cut in such a wayas to be thoroughly representative of the sample.9.2 The areas tested shall not contain visible blemishes ordefects and shall not be touched with the fingers or contami-nated in any other way.9.3 If contamination or improper handling is suspected, thespecimen may be rinsed with water or washed with laboratorydetergent and water. However, cleaning may affect the resultsand must be noted on the report.9.4 Pigment specimens shall be in the form of disks pre-pared in a press such as those used to prepare KBr disks forinfrared analysis.10. Procedure10.1 Test the specimens at a standard temperature of 23 62°C and at a relative humidity ≥50 %, unless otherwise agreedupon.10.2 Set up the goniometer and level the stage according tothe manufacturer’s instructions.10.3 Measure contact angles of water and diiodomethane onthe specimen of interest as described in Practice D7334 or themanufacturer’s literature for the instrument being used.10.4 Make two angle measurements (one on each dropedge) of each of three drops on the specimen. If the contactangles on two edges are significantly different, the valuesshould be eliminated and the test repeated. The contact anglefor the specimen shall be the average of the six anglesmeasured.10.5 Water contact angles must be measured rapidly (within30 s of depositing the drop) to avoid changes in angle as thewater evaporates. A humidity chamber may be used to reducethe rate of evaporation.4Imai, T, Organic Coatings Science and Technology, G. D. Parfitt and A. V.Patsis, eds, Vol. 6, Marcel Dekker, New York, 1984, p. 301.A = contact angleD=dropofliquidP = specimenT = tangent at specimen surfaceFIG. 1 Measuring Angle of ContactD7490 − 13211. Calculation11.1 This method is based on the Owens-Wendt-Kaelbleequation:5,6γ1~11 cos θ!25 @~γldγsd!1/21~γlpγsp!1/2# (1)where:θ = the average contact angle for the test liquid onthe test specimen,γ1= the surface tension of the test liquid in dyn/cm,andγdand γp= the dispersion and polar components of theliquid and the solid, also in dyn/cm.However, since this method specifies two liquids of knownsurface tension (distilled water = 72.8, diiodomethane = 50.8)and known dispersion and polar components (distilled water γd= 21.8, γp= 51.0; diiodomethane γd= 49.5, γp= 1.3), all indyn/cm), we can use these numbers to simplify the O-W-Kequation and turn it into two expressions, one for water and theother for diiodomethane (denoted as i):72.8 ~11 cos θw!/2 5 @~21.8 γsd!1/21~51.0 γsp!1/2# (2)50.8 ~11 cos θi!/2 5 @~49.5 γsd!1/21~1.3 γsp!1/2# (3)Next, let us further simplify the equations by calculating thesquare roots of the values for the dispersion and polarcomponents of the two test liquids and placing them outside theparentheses. This gives the working equations when water anddiiodomethane are used as the test liquids.~36.4!~11 cos θw! 5 @4.67 ~γsd!1/217.14 ~γsp!1/2# (4)~25.4!~11 cos θi! 5 @7.03 ~γsd!1/211.14 ~γsp!1/2# (5)11.2 Determine the cos θ value for each liquid from thecontact angles.11.3 Write two expressions, one for water, and the other fordiiodomethane based on working Eq 4 and Eq 5 in 11.1. Thiswill result in two equations in two unknowns (the unknownsbeing the dispersion and polar components of the solid surface,γsdand γsp, respectively).11.4 Solve for the two unknowns (see Appendix X1 for asample calculation).11.5 Add the dispersion and polar component values to-gether to give the total surface tension.12. Report12.1 Report the following information:12.1.1 Identification of the specimen,12.1.2 Average contact angles for each of the two liquids onthe specimen,12.1.3 Dispersion and polar components of solid surfacetension and the total solid surface tension, and12.1.4 If the specimen was cleaned in any way, this must benoted.13. Precision and Bias13.1 Precision—The precision of this method has not beendetermined by interlaboratory testing, but estimates of preci-sion of measurements on substrates and coatings can be madebased on experience with the method. Variability of measure-ments on pigments is expected to be greater due to differencesin roughness and porosity.13.1.1 Repeatability (substrates and coatings):13.1.1.1 Contact Angle—Two contact angle results (eachthe overall average of two measurements made on each of threedrops) obtained by the same operator should be consideredsuspect (or as an indicator of the presence of localizedcontamination) if they differ by more than 2°.13.1.1.2 Dispersion and Polar Components—Two values,each based on three drops of each test liquid, obtained by thesame operator should be considered suspect (or as an indicatorof the presence of localized contamination) if they differ bymore than 5 dyn/cm.13.1.2 Reproducibility—These measurements rarely if everare carried out on the same specimen in two different labora-tories. therefore, reproducibility is unknown and probably willbe impossible to establish with any certainty.13.2 Bias—It is not possible to determine bias for thismethod because of the lack of availability of a standardsubstrate whose surface can be expected to remain constantlong enough for comparisons to be made with it.14. Keywords14.1 adhesion; contact angles; dispersion and polar compo-nents of solid surface tension; solid surface tension; wetting5Schoff, C. K., “Wettability Phenomena and Coatings” in Modern Approaches toWettability: Theory and Applications, M. E. Schrader and G. Loeb, eds. PlenumPress, New York, 1992, p. 375.6Pierce, P. E. and Schoff, C. K., Coating Film Defects,2ndEdition, Federationof Societies for Coatings Technology, Blue Bell, PA, 1994.D7490 − 133APPENDIX(Nonmandatory Information)X1. SAMPLE CALCULATION FOR DISPERSION AND POLAR COMPONENTS OF SOLID SURFACE TENSION AND TOTALSOLID SURFACE TENSIONX1.1 The following data were collected during root causeanalysis work on a problem involving poor wetting/dewettingof a topcoat over a primer, particularly when the primedsurface was aged.5Contact angles with distilled water anddiiodomethane were measured on an aged specimen, the sameafter washing with a detergent solution, and on a control, a verysimilar primer know to give adequate wetting. Results areshown in Table X1.1.X1.1.1 Let us consider O-W-K working equations Eq 4 andEq 5 for water and diiodomethane that are shown in 11.1:~36.4!~11 cos θw! 5 @4.67 ~γsd!1/217.14 ~γsp!1/2#~25.4!~11 cos θi! 5 @7.03 ~γsd!1/211.14 ~γsp!1/2#Now, we need to change two equations in two unknowns toa single equation with one unknown. To accomplish this, wemultiply both sides of the first equation by 7.03/4.67 andsubtract the second equation above from this new equation:~54.8!~11 cos θw! 5 @7.03 ~γsd!1/2110.75 ~γsp!1/2#2$~25.4!~11 cos θi! 5 @7.03 ~γsd!1/211.14 ~γsp!1/2#%~54.8!~11 cos θw! 2 ~25.4!~11 cos θi! 5 9.61 ~γsp!1/2For the aged problem primer, the water contact angle was63°, so cos θw= 0.454, and the diiodomethane contact anglewas 46°, cos θi= 0.695. Therefore, the expression resultingfrom the subtraction becomes~54.8!~1.454! 2 ~25.4!~1.695! 5 79.7 2 43.1 5 36.6 5 9.61 ~γsp!1/2Therefore, γsp= (36.7/9.61)2= 14.5 dyn/cm. This is thepolar component of the primer.We then can substitute this value into either of the twoequations and calculate γsd, the dispersion component of theprimer. For example, using the working equation for water:γsd5 @~52.9 2 27.1!/4.67#25 30.5 dyn⁄cm~36.4!~1.454! 5 @4.67 ~γsd!1/21~7.14!~14.5!1/2#52.9 5 4.67 ~γsd!1/2127.1The sum of these two values gives the total solid surfacetension, which in this case is 45.0 dyn/