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Cratering

Also known as
craters, fisheyes

Cratering is a coating defect in which small, round depressions form in a wet film as it dries, sometimes exposing the surface underneath. It is usually caused by a low-surface-tension contaminant, such as a droplet of oil or silicone, that makes the surrounding coating flow away from it.

What is cratering?

A crater is a small, bowl-shaped depression in a coating, often with a raised rim and sometimes a contaminant particle at its center. When the depression goes all the way down and exposes the surface underneath, the defect is often called a fisheye.

Why craters form

Cratering is driven by differences in surface tension. When a spot of the wet film has a lower surface tension than the surrounding coating, the surrounding liquid, with its higher surface tension, pulls away from that spot, carrying coating with it. This flow driven by surface tension gradients is known as the Marangoni effect.

Common sources of low-surface-tension spots:

  • Contamination on the substrate, such as oil, grease, fingerprints, silicone or mold-release agents.
  • Airborne contaminants, such as oil mist from compressed air lines or silicone overspray.
  • Incompatible ingredients in the coating, such as poorly dispersed defoamers or additives.

How cratering is prevented

  • Surface preparation: cleaning and degreasing the substrate before coating.
  • Clean application equipment and air supply.
  • Formulation: lowering the coating’s surface tension so it wets the substrate and is less sensitive to contamination, often with wetting additives, and using flow and leveling additives that even out surface tension across the film.
  • Careful selection of defoamers, which work by being partly incompatible and can cause craters if overdosed.

Frequently asked questions

Craters come from surface tension differences, often due to contamination. Pinholes are tiny holes typically caused by trapped air or solvent escaping as the film dries.

Usually the area must be sanded, cleaned and recoated, after the source of contamination has been removed.

  • Leveling (coatings)

    Leveling is the ability of a wet coating to flow out after application and form a smooth, even film, eliminating brush marks, roller texture and other surface irregularities before it dries. It is driven by surface tension and opposed by the coating's viscosity.

  • Wetting

    Wetting is the ability of a liquid to spread over and stay in contact with a solid surface. It depends on the balance between the liquid's attraction to the surface and to itself, and it is usually measured by the contact angle a droplet makes with the surface.

  • Surface tension

    Surface tension is the energy per unit area, or force per unit length, at the surface of a liquid where it meets a gas such as air. It arises because molecules at the surface have fewer like neighbors to attract them than molecules inside the liquid.

Sources

  1. Organic Coatings: Science and Technology, 4th edition, F. N. Jones, M. E. Nichols and S. P. Pappas, Wiley, 2017