Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 91 of 331

Fig. 3. Unblocking/curing reaction of blocked polyisocyanates. rate), and pigment and flow aids. Isophorone diisocyanate is reacted with trimethylolpropane (IPDI/TMP)and then blocked with -caprolactam (Fig. 4).This polyisocyanate, which is the lead- ing choice for powder coatings,is coreacted with polyesters. The crosslinker must be a solid with aTg of 55°C. The polyester must also be a solid with aTg between 50 and 60°C. The coreactants are melt blended,extruded, cooled, and pulver- ized to produce the final powder coating. Powder coatings can be applied by fluidized bed or electrostatic spray. A coated metal substrate requires heat to melt and flow the powder and to unblock the isocyanate to complete the cure. Standard curing conditions are 10 minutes at 360°F for catalyzed coatings and 30 minutes at 360°F for uncatalyzed coatings. Table IV lists the physical prop- erties of the isophorone diisocyanate blocked polyisocyanate and polyester resin used in powder coatings. Urethane powder coatings for metals are a rapidly growing segment of the coat- ings industry. Finished coatings have good chemical resistance, corrosion resis- tance, and mechanical performance. In addition, they have very good exterior col- or and gloss retention. Because finished products are available in all colors, clear coats and textured coatings, they are in demand for appliance, automotive wheel covers and trim, playground equipment, and garden tractors. POLYURETHANE COATINGS FOR PLASTIC SUBSTRATES Since the invention of phenolic resin 80 years ago, plastics have come a long way, replacing traditional materials such as metal, wood, and glass in many appli- cations. In 1992, more than 60 billion pounds of plastics were used in the United States. Compared with traditional materials, plastics are cost effective, inherently corrosion resistant, and can be easily processed into complicated shapes. With these advantages, plastics are used today in almost all major in- dustries, ranging from construction to health care. Unlike metal or wood, the colored surface finishes of plastic products for most applications usually are achieved not by painting of the products. In- stead, they are usually accomplished by making the products from colored resins that are manufactured by compounding or mixing solid pigment or liq- uid colorant into the resins during the resin manufacturing stage. One of the 90

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