Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 86 of 331

Fig. 1. Schematic of coating formulation process. UV stabilizers, reactive diluents, rheology control). The first polyisocyanate commercialized for a solventborne coating was the re- action product of toluene diisocyanate (TDI) and trimethanol propane (TMP). Commercialized in 1955, this product is still used for metallic industrial fin- ishes as a cobinder for magnetic media (audio/video tapes) and for wood coat- ings; however, the TDI/TMP polyisocyanate yellows severely when exposed to di- rect sunlight, as do all aromatic polyisocyanates. This disadvantage led to the de- velopment of light-stable aliphatic polyisocyanates made from hexamethylene di- isocyanate (HDI), isophorone diisocyanate (IPDI), and their adducts (Fig 2). Three commercial HDI-based polyisocyanates are used in polyurethane coat- ings. Finished coatings based on HDI trimer (polyisocyanurate) chemistry have significant advantages, including low viscosity (reduced VOC), excellent color, and low free monomer content. Two commercial polyisocyanates based on biuret chemistry are also available for making polyurethane coatings. Both HDI bi- urets are higher in color and higher in free monomer content than the HDI trimer. The low-viscosity biuret is higher in isocyanate content and lower in equivalent weight than HDI trimer. The high-viscosity HDI biuret is higher in vis- cosity and similar in isocyanate content and equivalent weight to the HDI trimer. 85

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