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 entrapment, in law, the instigation of a crime in the attempt to obtain cause for a criminal prosecution. Situations in which a government operative merely provides the occasion for the commission of a criminal act (e.g. 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 trapping, most broadly, the use of mechanical or deceptive devices to capture, kill, or injure animals. It may be applied to the practice of using birdlime to capture birds, lobster pots to trap lobsters, and seines to catch fish. 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 turbulence, state of violent or agitated behavior in a fluid. Turbulent behavior is characteristic of systems of large numbers of particles, and its unpredictability and randomness has long thwarted attempts to fully understand it, even with such powerful tools as or cavitation cavitation Formation of vapour bubbles within a liquid at low-pressure regions that occur in places where the liquid has been accelerated to high velocities, as in the operation of centrifugal pumps, water turbines, and marine propellers. . 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 Atomization The process whereby a bulk liquid is transformed into a multiplicity of small drops. This transformation, often called primary atomization, proceeds through the formation of disturbances on the surface of the bulk liquid, followed by their 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 spher·i·cal adj. Having the shape of or approximating a sphere; globular. 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 porous /por·ous/ (por´us) penetrated by pores and open spaces. po·rous adj. 1. Full of or having pores. 2. Admitting the passage of gas or liquid through pores. substrate. Most tests for air entrapment involve rapid stirring or shearing (usually with a Waring Several people have had the name Waring:
The best way to avoid air entrapment is to prevent the formation of bubbles in the first place. Proper dispersion dispersion, in chemistry dispersion, in chemistry, mixture in which fine particles of one substance are scattered throughout another substance. A dispersion is classed as a suspension, colloid, or solution. 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. [FIGURE 1 OMITTED] [FIGURE 2 OMITTED] "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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