Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 94 of 331

In addition to the continuous demands in performance improvement, current and future environmental regulation and legislation continue to mandate fur- ther VOC reduction for all coatings. For solvent-based polyurethane coatings, ear- lier efforts to formulate low-VOC, high-solid coatings were mainly focused on the development of resins with lower viscosity; however, the reduction of resin vis- cosity is often achieved by the reduction of the molecular weight of resin. Un- fortunately, many of the low molecular weight resins also cause the resulting coatings to have less than satisfactory properties; therefore, to develop high-sol- id and low-VOC coatings that also have high performance is quite difficult by us- ing low-viscosity resins alone. In the past few years, polyisocyanate producers began to commercialize a new gen- eration of low-viscosity polyisocyanates such as those based on HDI dimer (ure- tidione). Without solvent, these low-viscosity polyisocyanates by themselves have vis- cosities in the range of a few hundred centipoise. Compared with the convention- al biurets or trimers that have viscosities in the range of thousands of centipoise, these low-viscosity polyisocyanates provide exciting new potential to prepare low VOC coat- ings. Unlike low viscosity resins, these new low viscosity polyisocyanates are man- ufactured by forming different isocyanate oligomers from the diisocyanates; there- fore, they can achieve significant viscosity reduction without affecting polyurethane reaction and the performance of resulting coatings. Recently, reactive diluents such as those based on oxazolidine and aldimine chemistry are also becoming available. These reactive diluents can be used to replace conventional solvents typically used to reduce the viscosity of resins. Be- cause the reactive diluents also react with the polyisocyanates when the polyurethane coatings are cured, they will not evaporate like a conventional sol- vent; therefore, by replacing some solvent in the coating system with reactive diluent, the overall VOC value of that coating is also reduced. Combining the benefits from all these developments, the new generation of polyurethane coatings formulated using an appropriate mixture of resin and reactive dilu- ent and low-viscosity polyisocyanates were developed that can achieve VOC reduction while maintaining excellent surface properties, weatherability, and flexibility, which are essential for plastic substrates. SAFE APPLICATION OF POLYURETHANE COATINGS Like any other coatings, polyurethane coatings are made from a variety of chem- icals. Due largely to misunderstanding and confusion regarding the terms "iso- cyanates," "diisocyanates," "polyisocyanates," "urethanes," and "polyurethanes,"' there may be some misconception on the safety of handling polyurethane coat- ings. Polyurethane coatings, which sometimes are mistakenly called "urethane coat- ings," are formed by the reaction of polyalcohol and polyisocyanate. In some cas- es, a certain amount of polyurea, from the reaction of amine terminated polymer with polyisocyanate, is also present in the polyurethane coatings. The monomer- ic urethanes are not part of the polymer backbone of polyurethane coatings. Polyurethane coatings can be formulated either as one-component or two-com- ponent coatings. The one-component polyurethane coatings can be either ful- ly reacted polyurethane in water-based suspension or emulsion, or as solvent-based systems that cure after application. The solvent-based one-component polyurethane coatings are further divided into two different categories. The moisture-cured one-component polyurethane coatings contain isocyanate ter- 93

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