The standard “ASTM D7334-08 Standard practice for surface wettability of coatings, substrates, and pigments by developing contact angle measurement” developed by the American Society for Testing and Materials (ASTM) describes a procedure for measuring the contact angle when a liquid droplet is applied to a coated surface, substrate, or preformed pigment disk.
There are two types of contact angles: advancing and receding. This standard deals only with advancing contact angles. This application is intended to supplement, but not replace, the manufacturer's instructions for the instrument used to make the measurements.
A common test liquid is water, but many other liquids can be used, such as solvents, surfactant and dispersant solutions, and even liquid dyes.
This application is based on goniometry, which involves observing a stationary drop of test liquid on a solid substrate. Although contact angles are governed by surface tension, this standard cannot be used to measure surface tension directly.
The ASTM D7334-08 standard is useful for characterizing the wettability of surfaces. A surface that is easy to wet is one on which a coating is more likely to provide good adhesion and appearance and is less likely to suffer from surface stress-related defects such as creep, cratering, pinholes, and orange peel.
This standard can also be used to test the wettability of pigment surfaces, particularly by potential surfactant or resin-based dispersants or mill bases. Pigments that wet easily are more likely to be easier to disperse, and dispersants/mill bases that wet the pigments of interest are more likely to disperse those pigments well.
The contact angle is governed by the surface tensions of the test liquid and the test surface, but the angle does not directly give a surface tension value.
A low progressive contact angle value (up to 45 degrees) indicates wetting and angles of 10 to 20 degrees indicate excellent wetting.
Water can be used as a test fluid to determine (via progressive contact angle) whether a surface is hydrophilic (angle greater than 45 degrees), hydrophobic (angle less than 90 degrees), or somewhere in between (angle between 45 and 90 degrees). Water contact angles have been used to predict surface cleanliness before and after cleaning operations, ease of wetting of surfaces with water-based coatings, and effectiveness of rinsing operations.
An organic liquid, such as a solvent, can also be used to characterize a substrate, coating, or pigment. The resulting contact angle depends on the surface tensions of the liquid and the test surface. A low surface tension test surface will not be wetted by a high surface tension liquid.
In addition to water and solvents, a surfactant dispersion or dispersant solution can be used to test a pigment surface. Any test liquid that is a potential dispersant for a test pigment must thoroughly wet the pigment or it will not work as a dispersant.
Contact angle measurements can be used to map surfaces for hydrophilicity, the presence of low surface tension components or contaminants, or changes in composition. Other analytical methods, such as infrared microscopy, are needed to identify the chemical moieties that contribute to the contact angle differences.
This testing method can be used on almost all coatings and substrates and can be extended to pigments by compressing the pigment powder into a solid disk.
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