Craters and cratering revisited.
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Substrate contaminants cause craters because the paint does not wet the contaminated area or immediately dewets from it. The contaminant may be dirt, grease, one of a number of different oils, fingerprints, mold release agents, or plasticizer leaching from plastic or a previous coating. The solution for substrate-based cratering is to make sure that the surface is clean. The cleaning step in many industrial coating processes receives little attention, but is critical to preventing surface defects such as craters and dewetting. In addition, there should be some kind of wash primer, pretreatment, or conversion coating to enhance wetting and provide a uniform surface for better adhesion. If the paint is designed for oily or dirty surfaces, then it can be tested by being applied over drops or beads of low surface tension material.
Contaminants that exist in or on top of the paint cause craters by a different mechanism. There is a surface tension gradient between the low surface tension contaminant and the relatively high surface tension coating around it. This gradient or, rather, changes in it with time drives flow from the low surface tension spot to the higher surface tension area around it. In order for an inpaint contaminant to cause craters, it must have a low surface tension, be sufficiently incompatible so that it comes to the surface, yet must be able to diffuse enough into the surrounding paint to give a surface tension gradient. In-paint contaminants may come from the paint plant, from shipping, or even on the job or in the customer's plant. Examples include dirt, oil (including from compressed air used in spraying), gel particles, and surfactants. In my experience, solid particles do not cause craters unless they have a certain degree of solubility in the paint or they are semi-solids (such as resin gels) which can absorb and slowly release solvents or other low surface tension components. We sometimes suspected that Teflon [R] or other plastic shavings or particles from stirrer bushings were the cause of cratering, but they almost never were. We even added such particles to paint and could not cause craters.
Batches of paint that crater sometimes can be saved by high shear agitation for an hour or so and I have seen pigmented paints that no longer produced craters after being run through the mill a second time. Anything that disperses the contaminant or causes it to adsorb on pigment will prevent or reduce cratering. Testing for sensitivity to in-paint contaminants is very complicated because it is more of a test of the ability of the paint to assimilate contaminants than it is a crater test. A solvent or additive replacement (probably for other reasons) may completely change the resistance of the paint to a given contaminant.
There are many contaminants that may fall on paint as it is being applied or soon after, including dirt, overspray, oil droplets, soot, fibers, personal care products, chain lubricants, and oven aerosols and drips. The craters form as noted above: flow from low surface tension to high surface tension. Prevention of such craters depends on keeping the painting area ultraclean, having painters wear protective clothing, and developing robust paints that resist contaminants.
Location of contaminant sources and identification of the contaminants themselves usually are difficult. Sometimes, possible sources can be found by inspecting the area where painting is being done and looking for things like dirty substrate, a dirty paint area, paint from one part of a job or plant overspraying another, nearby pumps leaking or spraying oil, oily overhead chains, ovens that smoke, and workers leaving fingerprints on ware about to be painted. Identification of the contaminant usually requires analysis of individual craters by sophisticated instruments such as scanning electron microscopy, infrared microscopy, x-ray photoelectron spectroscopy (XPS or ESCA), and secondary ion mass spectrometry (SIMS). If the coating is baked and the contaminant is volatile such as a hydrocarbon or silicone oil, then it is liable to be driven off in the oven, leaving nothing to analyze.
By Clifford K. Schoff, Schaff Associates
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|Title Annotation:||COATINGS CLINIC|
|Author:||Schoff, Clifford K.|
|Date:||Apr 1, 2012|
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