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Craters and cratering.

Of all the many defects that can spoil the appearance of coatings, the most frustrating and difficult to combat are the small bowl-shaped depressions that we call craters or fisheyes. Craters form one branch of a family of defects that are caused by flow that is driven by surface tension. When cratering occurs, some kind of low surface tension contaminant is on the substrate, in the paint, or has fallen on the paint. Craters usually occur immediately after the paint is applied, but some baked coatings wait until the oven to crater. Craters form surprisingly quickly. Laboratory observations have shown that, although a crater may grow for several seconds, it will be noticeable in a very short time (0.1-0.5 sec) after a contaminant contacts the paint. Unfortunately, there usually are plenty of contaminants around when paint is applied--oils, lubricants, dirt, fibers, fingerprints, personal care products, overspray, etc.

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There are ways to make paints more crater resistant, but they are not always acceptable to the paint user. For example, reducing flow by adding thickeners or more pigment or using faster solvents often can stop craters. The resultant orange peel (often severe) might be fine for appliances, but definitely is not acceptable for cars. Another preventative measure is to add a surfactant or other surface active material to make the surface more homogeneous and lower in surface tension. The coating becomes better able to wet contaminants and is less apt to form defects on its own. The drawback is that the coating may have such a low solid surface tension on drying or curing that it cannot be topcoated, recoated, or repaired. The trick is to balance flow properties and additives to give some resistance without overly damaging appearance and recoatability. This usually can be done with pigmented coatings, but is very difficult to accomplish with clears.

How do you know if your paint is crater resistant or not? That is the big question and there is no good answer. It is very difficult to measure or even estimate crater robustness, especially with so many different contaminants in the plant or in the field. One type of test is to "insult" the wet coating with a mist or droplets of oil, solvent, or overspray and see whether craters form or not. Another test is to determine whether paint will apply over drops, beads or buttons of low surface tension material without dewetting.

"Coatings Clinic" is intended to provide a better understanding of the many defects and failures that affect the appearance and performance of coatings. We invite you to send your questions, comments, experiences and/or photos of coatings defects of Cliff Schoff, c/o "Coatings Clinic" CoatingsTech, 492 Norristown Rd., Blue Bell, PA 19422; or email publications@coatingstech.org.
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Title Annotation:Coatings Clinic
Author:Schoff, Clifford K.
Publication:JCT CoatingsTech
Date:Apr 1, 2004
Words:466
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