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Air entrapment.

Air bubbles trapped in paint during manufacture or application can result in bubbles, pinholes and crater-like defects in the cured film. Usually these defects are difficult to distinguish from solvent pops or craters, so detective work is needed to identify the root cause. Air entrapment rarely is suspected until after solvent popping and substrate gassing have been ruled out as causes. Figures 1 and 2 provide examples of the effects of the trapping of air in coatings. Figure 2 shows a particularly severe case of air entrapment, such that the film is like a foam.

Bubbles are a possibility wherever there is a stirring or shearing action that can lead to vortexing, turbulence or cavitation. This can occur in manufacturing processes such as stirring, dispersing, pumping and metering and during application processes such as rolling, brushing, curtain coating, and spraying. Spraying provides many opportunities for bubble formation. Air may dissolve under high pressure (as in airless spraying) in the circulation system and be released in the spray droplets or in the film. Atomization may cause bubbles, particularly with worn or damaged gun tips or chipped bells, or produce doughnut or cup-shaped droplets that trap air when they hit the substrate. Even normal spherical spray particles can trap air as they splash onto the substrate. Bubbles also may form because of a tendency of the paint to foam or because air is trapped in a porous substrate.

Most tests for air entrapment involve rapid stirring or shearing (usually with a Waring blender) of the paint in question as well as a control that does not show the defect and comparing the results. The evaluation can be by eye, weight per gallon measurements or after spray-outs, but the idea is to see which paint picks up more air. One difference that I have noticed between solvent popping and air entrapment is that spraying thinner and thinner coatings will eventually get rid of popping, but often makes air entrapment more noticeable.

The best way to avoid air entrapment is to prevent the formation of bubbles in the first place. Proper dispersion and mixing practice can reduce air entrapment during manufacture. Correct matching of the batch size with the size of the manufacturing equipment is essential. Proper choice, maintenance, operation and adjustment of paint application equipment can prevent the trapping of air during application. Airless spray has caused air entrapment problems with a number of waterborne coatings. If bubbles cannot be prevented completely, then keeping the coating surface open longer via slower solvents may allow air to be released before the film sets up.

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"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 to 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 Research
Date:Apr 1, 2005
Words:494
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