Metal Finishing Guide Book


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(a) (b) (c) 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. solvent evaporation. If solvents evaporate too quickly from the coating surface, once again, an impermeable dry ���skin��� will form. This thin layer will prevent further solvent evaporation; solvent entrapment caused by this mechanism will result in the same detrimental voids that jeopardize the coating strength. In addition, delamination due to solvent entrapment can be exacerbated by environmental factors affecting the product after application. External effects, including contaminants such as air, moisture, or other environmental chemicals, can gradually diffuse through the product that has been damaged by solvent entrapment (regardless of the cause) and collect in the resulting voids. A relative pressure 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 applications where large temperature or pressure fluctuations are common, making it easier for the bubbles to ���pop.��� The two following case histories illustrate the effects of solvent entrapment. CASE #1: Early in his career, Ron Joseph was employed by a paint manufacturing company. He was asked to attend the inspection of a railcar that had been painted six years earlier with a new, untried coating 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 automotive 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 577

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