ASTM D7334-08 Standard Practice for Surface Wettability of Coatings, Substrates, and Pigments by Improving Contact Angle Measurement

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 an application 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 practice 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.

For the purposes of this standard, the following terms and definitions apply:

• The advancing contact angle is the contact angle measured immediately after the seated drop is placed on the surface. The drop has the maximum volume allowed for the liquid-solid interface area, any addition will cause the drop to expand and increase the liquid-solid interface area. This can be thought of as the wetting angle as the drop is ready to wet the additional area. This is the contact angle used in this method and most coating measurements.
• The contact angle is the angle made by a droplet between the substrate and a tangent drawn at the intersection between the substrate as shown in the figure. This is the angle made at the three phase boundary where a liquid, gas (air), and solid intersect.
• The receding contact angle is the contact angle measured when the material is removed from the drop and shrinks. The liquid-solid interface area will decrease. This is the wetting angle.
• A dormant drop is a drop of liquid on top of a horizontal surface.
• The terms surface tension and surface energy are often used interchangeably. They are numerically the same, but have different units. They are measures of an effect resulting from unbalanced molecular cohesive forces at a surface that cause the surface to contract and behave like a film or membrane. Surface molecules have a higher free energy than molecules in the bulk material. The excess free energy is what is called surface energy. However, what is measured is surface tension, which is usually the force required to break a film of a given length.

When testing, a specific volume of water or another agreed upon drop of test liquid is applied to a test sample using a syringe. The contact angle is measured by one of two methods:

• By viewing the stationary drop through a microscope fitted with a goniometer scale for direct measurement of the angle.
• By capturing an image of the drop, then measuring it with a protractor or using appropriate software to process the image and measure the angle.

This 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 tension-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.

Although the contact angle is governed by the surface tensions of the test liquid and the test surface, the angle cannot provide a direct surface tension value.

A low advancing 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 degrees 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 test surface with low surface tension (energy) will not be wetted by a liquid with high surface tension.

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 differences in composition. Other analytical methods, such as infrared microscopy, are needed to identify the chemical moieties that give rise to 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.

Our organization, which has been trying to support businesses from every sector with its testing, measurement, analysis and evaluation studies carried out in a wide range for years, has a strong staff that closely follows the developments in the world in the field of science and technology and constantly improves itself. In this context, testing services are also provided to businesses in accordance with the standard "ASTM D7334-08 Standard application for surface wettability of coatings, substrates and pigments by developing contact angle measurement".