Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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stripping STRIPPING ORGANIC COATINGS FROM PARTS AND PART CARRIERS USING MOLTEN SALT BATH TECHNOLOGY BY JAMES C. MALLOY KOLENE CORP., DETROIT A molten salt bath is one of the most effective methods available to the met- al-finishing industry for removing organic coatings from metal substrates. This technology is an alternative to burn-off ovens, solvent/aqueous cold- stripping systems, and fluidized beds for stripping part carriers, as well as sal- vaging parts for recoating (Table I). Major benefits of molten salt bath paint stripping include efficient removal of all organic coatings including nylon, polyester, epoxies, fluoropolymers, and hybrid finishes; 100% first-pass strip- ping efficiency; elimination of ash or pigment residue on stripped surfaces; elim- ination of secondary cleaning requirements; stripping speeds ranging from a few seconds to a few minutes; and the ability to be integrated with finishing lines for in-line/on-line stripping. HOW THE PROCESS WORKS A molten salt bath uses inorganic oxidizing salts that become liquid when heat- ed from 600 to 900°F. When work is immersed in a molten salt bath, organic de- posits on the workpiece are converted into an inorganic gas or water-soluble by product (carbon dioxide and water) that is easily treated or disposed. Any inor- ganic constituents, such as pigments and opacifiers, that were present in the coating are sloughed off the workpiece as the organic matrix is destroyed. The stripping process begins with contact between the work and the liquid process chemicals. The salt bath functions as both a heat source to rapidly and uniformly heat work to the bath temperature and as a heat sink that quickly withdraws heat generated by the stripping reaction. (This prevents localized overheating common to other thermal stripping methods and allows thin gauge materials and temperature-sensitive components, such as spring clips, to be processed.) The thermochemical reaction creates a gentle turbulence, which ac- celerated stripping. Stripping is complete when the turbulence stops. The time required to strip organic coatings from parts and part carriers ranges from a few seconds to several minutes. For example, the time required to strip racks with a moderate amount of cured paint or powder in a salt bath is approxi- mately 5 minutes. To strip the same rack in a thermal cleaning oven would take from 8 to 12 hours. Another indicator of the speed of molten salt bath pro- cessing is that equipment can be integrated into finishing lines to provide con- tinuous, in-line stripping at normal line speeds. Variables that dictate process- ing time include the geometry, cross-section, and mass of components being stripped, the coating to be removed, and bath chemistry and temperature. SUBSTRATE FLEXIBILITY A molten salt bath can be used to strip virtually any metallic substrate com- patible with the operating temperature of the salt bath. This includes carbon and alloy steel, aluminum, stainless steel, super alloys, titanium, copper, brass, 260

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