Standard Test Method for Surface Wettability of Coatings, Substrates, and Pigments by Contact Angle Measurement Using Portable Goniometers

Importancia y uso:

5.1 This test method is useful for characterizing the wettability of surfaces. A surface that is easy to wet is one over which a coating is more likely to give good adhesion and appearance and less likely to suffer surface-tension-related defects such as cratering, pin holing, orange peel, and fish eyes. Contact angles can be used to measure immediate wettability of surfaces as well as changes due to aging or effects of wear, for example changes in a water repellent coating or film with exposure to environmental or wear testing.

5.2 The wettability of a surface is also a useful control parameter for processes that affect the surface properties of materials, such as molding, forming, machining, cleaning, surface treating, storage, packaging, and handling.

5.3 The contact angle is governed by the surface tensions of the test liquids and the surface energy of the solids. Contact angles of more than one fluid are necessary to calculate the surface energy of a solid. Once the surface energy of the solid has been determined, the contact angle of a single fluid can be correlated to the solid surface energy.

5.4 The contact angle of a liquid is determined by the interaction of the liquid with the uppermost few molecular layers of the solid surface. As such, contact angles are very sensitive to slight variations in sample composition, history, and the presence of small amounts of contaminants.

5.5 Water can be used as a test liquid to establish whether a surface is hydrophilic (<~45°) or hydrophobic. Water contact angles have been used to estimate surface cleanliness before and after cleaning operations, ease of wettability of surfaces by waterborne coatings, or level and uniformity of treatment level by, for example, corona, flame, plasma, or laser ablation. Water contact angles are particularly sensitive to the presence of polar functional groups.

5.6 Factors that may affect contact angles may include surface chemical composition, surface topography (such as those that create air pockets under the droplet, that is, the lotus effect or high-energy roughness that creates microcapillaries, that is, a grit-blasted aluminum surface), and substrate absorbency of the liquid.

5.7 An organic liquid can also be used to characterize a substrate. The contact angle established by non-polar organic liquids will be particularly sensitive to the non-polar nature of the substrate surface.

5.8 If contact angles are determined using two or more liquids having different polar and non-polar characteristics (for example, water and diiodomethane), the total surface energy of the solid may be calculated as the sum of polar and non-polar components, as per Test Method D7490.

5.9 Contact angles can be used to map a surface in terms of hydrophilicity and hydrophobicity, presence of contaminants, and uniformity of treatment. Other analytical techniques such as infrared spectroscopy would be required to determine the origin of any non-uniformity.

Subcomité:

D01.23

Volúmen:

06.01

Palabras clave:

contact angle; goniometer; handheld goniometer; surface energy; surface tension; wettability;