Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 95 of 331

minated polymer. Once applied, the polyisocyanates react with water in the at- mosphere to form the final coatings. The one-component heat-cured polyurethane coatings are formulated using blocked polyisocyanates. These kinds of coatings do not contain free isocyanate functional groups. They will not react with water in the atmosphere, and have to be cured by heat. When heated at an appropriate temperature, the blocked isocyanates in these coatings will unblock to generate free isocyanate functional groups that react with active hydrogens to form the cured coatings. The two-component polyurethane coatings consist of a resin component that has active hydrogen, and a polyisocyanate component. These two components are packaged separately and are mixed immediately before application. Both one-component and two-component polyurethane coatings can be formulated as solvent based or waterborne. To cure polyurethane coatings, polyisocyanates need at least two isocyanate functional groups per polymer chain to react with resins that also have mul- tiactive hydrogens; therefore, monoisocyanates such as methyl isocyanate, are never used in any polyurethane coatings. Diisocyanates that are commonly used are either aromatic diisocyanates such as MDI (diphenylmethane diiso- cyanate), TDI, or aliphatic diisocyanates such as HDI, IPDI, and H12 MDI [bis(4- isocyanatocyclohexyl) methane, which is sometime also called HMDI (hydro- genated MDI)]. Some of these diisocyanates such as HDI, IPDI, and TDI have relatively higher vapor pressure; therefore, in order to ensure the safety in coat- ing applications, these diisocyanates typically are either oligomerized into polyisocyanates, such as biuret, dimer, or trimer, or reacted into higher mole- cular weight prepolymers before they are actually used in polyurethane coat- ings. The oligomer form of HDI and IPDI polyisocyanates or TDI prepoly- mers have very low monomer levels, so the possibility of vaporized monomer- ic diisocyanatein polyurethane coatings is greatly reduced. Polyisocyanates contain isocyanate functional groups that have high reactiv- ity toward chemicals with active hydrogens such as alcohol, amine, and water; therefore, polyisocyanate coatings should be stored under an atmosphere free of moisture. Similar to many other chemicals, isocyanates are irritants to eye, skin, and respiratory system. For some people, exposure to isocyanate could also cause skin and/or respiratory sensitization, resulting in asthmatic symptoms. As a generally practiced safety operation rule, personal protective equip- ment, such as butyl rubber gloves, goggles, coveralls, and respirators, should always be used during a painting operation involving polyurethane coatings. When safe painting procedure and waste disposal procedure are followed, and proper painting equipment used, the application of polyurethane coatings should not cause more industrial hygiene concern than other kinds of sol- vent-based coatings. Fully cured polyurethane coatings do not have any special waste management issues that are not encountered by other coating ingredients such as solvents, pigments, and others. The uncured polyurethane coatings contain isocyanate groups. They should be allowed to react with water or moisture to form inert polyurea before they are disposed. Over the years, polyurethane coating man- ufacturers and polyisocyanate suppliers have developed an in-depth under- standing on the safe handling of isocyanates and related industrial hygiene knowledge. Many of them also have a wide range of education programs that can help coating users handle and apply polyurethane coatings safely. 94

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