Electrodeposition of Coatings III — Cathodic ED

摘要

Cathodic (cationic) electrodeposition coatings deposit positively charged polymer micelles along with pigment particles and additives onto negatively charged objects. Cathodics arrived in the marketplace roughly 10 years after anodic (anionic) coatings. It had been long believed that cathodic polymers would give better corrosion resistance due to their alkaline nature, high saponification resistance, and lack of substrate attack. Two problems stood in the way of development—cure of the highly basic film and the cor-rosivity of the acidic bath solutions. The former was solved by using blocked isocyanate crosslinkers that are stable at bath temperatures, but unblock when baked at moderately high temperatures (330-360°F). The latter can be worked around by using corrosion resistant tanks, pipes, and fittings, but epoxide resins reacted with tertiary amine salts or amines to form quaternary ammonium salts or amine salts provide higher bath pH and lower corrosivity. Common types of polymers used are amine or sulfonium functional epoxies and acrylics, which are neutralized with organic acids. Epoxies crosslinked with aromatic urethanes are preferred for highly corrosion-resistant primers, such as those used for virtually all automobiles and light trucks. Acrylics crosslinked with aliphatic urethanes provide bright, glossy one-coats that have good outdoor durability and sufficient corrosion resistance for many applications such as agricultural implements, lawn and garden equipment, appliances, and electrical equipment.