Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 268 of 331

(a) result in the same detrimen- tal voids that jeopardize the coating strength. (b) (c) In addition, delamination due to solvent entrapment can be exacerbated by environ- mental factors affecting the product after application. Ex- ternal effects, including con- taminants such as air, mois- ture, or other environmental chemicals, can gradually dif- fuse through the product that has been damaged by solvent entrapment (regardless of the cause) and collect in the re- sulting voids. A relative pres- sure can build up inside the voids, which can cause them to grow in size; in an extreme case, leading to rupture and catastrophic failure of the coating. This mechanism, shown schematically in Figure 4, is further accelerated in ap- plications where large tem- perature or pressure fluctua- tions are common, making it easier for the bubbles to "pop." The two following case his- tories illustrate the effects of solvent entrapment. Figure 2. Coatings that are too thick can entrap solvents, leading to voids in the final coating. These voids jeopardize the integrity of the coating. CASE #1: Early in his career, Ron Joseph was employed by a paint manufacturing company. He was asked to attend the in- spection of a railcar that had been painted six years earlier with a new, untried coat- ing system. Upon arriving at the site he was joined by the customer and several older and more experienced parties. The railcar was painted white (an automo- tive finishand) looked superb. Since the customer intended to evaluate this new coating system, he performed the standard ASTM D3359 "Standard Method for Measuring Adhesion by Tape Test – Method A." Using a sharp knife blade, he scribed a large "X" into the paint, ensuring that the cut penetrated down to the substrate. The customer then lifted the apex of the "X" with his thumb and forefinger and to everyone's horror the entire system lifted from the steel substrate. The coat- ing delaminated like a sheet. Young Joseph was horrified—the paint had been sup- plied by his employer! One could clearly see the clean, un-corroded, abrasive blasted substrate shining like new under the coating system. Moreover, one 267

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