Metal Finishing Guide Book


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Figure 6. Two coating aluminum extrusions with non-uniform and uniform coating deposits. An analysis by means of a stereomicroscope confirmed what the consultant anticipated. There was excessive solvent entrapped in the primer layer. This is seen by the large cavities in the gray coating and explains why the primer lifted off the polyurethane foam. However, why did the top coat pull away from the primer? It was suspected the primer's top surface had started to oxidize in the strong ultraviolet (UV) light during the hot summer's day, and the oxidized surface was inert to the top coat layer. Although microscopy provided hints of what might have gone wrong, additional analytical tests were conducted to confirm solvent was in fact present in the primer. The client had a limited budget and did not request the consultant prove the UV oxidation theory. However, a test such as x-ray photoelectron spectroscopy (XPS) could have determined if the epoxy had oxidized before the top coat was applied. 3. Acrylic Latex peels off school walls Soon after occupying a new office building, while attaching drawings and other paper items to the walls, employees noticed the white acrylic latex wall paint peeling off. Blue painter's adhesive tape was used to mount the articles. Several classrooms exhibited the same problem, regardless of whether the wall was alongside a window or on the opposite side of the room. Was this the painter's fault? Was the paint itself defective? A microscopic analysis of some small paint samples that had peeled showed that the paint was well applied and at the manufacturer's recommended DFT. However, paint was not peeling off the wall; rather, a coating layer on the drywall, under the paint was coming off with the paint (Figure 5). Therefore, it was the under-coat (plaster) that had poor cohesion and broke away from itself: neither the painter nor the paint was at fault. 4. Coating of aluminum extrusions Aluminum extrusions for architectural purposes are often coated with a polyvinylidene difluoride (PVdF) system. American Architectural Manufacturer's Association (AAMA) Specification 2605, "Voluntary Specification, Performance Requirement and Test Procedures for Superior Performing Organic Coatings on Aluminum Extrusions," is often specified for extrusions that will be exposed to a marine environment in which high humidity, salt-laden air, and fog are prevalent. A client wanted to ensure the extrusions he ordered would meet the 529

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