Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 89 of 331

trimer blends are used in automotive refinish and automotive OEM to speed up dry time and reduce damage to the clear coats caused by acid rain. POLYOLS Numerous acrylic and polyester polyols are available to use as coreactants with polyisocyanates. The hydroxyl (OH) functionality on the acrylic or polyester structure reacts with the isocyanate (NCO) functionality on the polyisocyanate. Polyol selection plays an important role in achieving performance goals. Lead- ing polyols for automotive refinish and OEM, aircraft, and industrial maintenance are summarized in Table III, along with their physical properties. Acrylic polyols are generally preferred in automotive and industrial maintenance applications. Urethane acrylic coatings have excellent appearance, very good UV stability, and meet the overall performance requirements for these end-use mar- kets. Polyester polyols are preferred for use on aircraft and, to a lesser extent, in in- dustrial maintenance. Urethane polyester coatings have excellent mechanical per- formance (flexibility and toughness) as compared to acrylic polyols, but are lacking in UV resistance. CATALYSTS AND SOLVENTS Coatings are cured (hardened) by the reaction of the polyisocyanate (NCO func- tionality) with the polyol (OH functionality). Coatings can cure at room tem- perature or may be accelerated by heat. The addition of catalysts will speed up the final cure. The catalysts of choice among formulators are dibutyltin dilaurate (DBTDL), tertiary amines, alkali metal octoates, acetyl acetonates, titanates, and blends of these catalysts. Newer catalysts, such as dimethyltin dilaurate and dibutyltin diacetate, are gaining popularity. In addition, formulators blend dif- ferent combinations of catalysts to decrease the overall time required to complete the curing process. Most common organic solvent are suitable (esters, ketones, aromatic hydrocarbons, and so on) as thinners for polyurethanes, provided the solvents do not contain groups that react with isocyanates or contain impurities (alcohols, thiols, amines, carboxylic acids, or water). TWO-COMPONENT AND ONE-COMPONENT COATINGS Two-component polyurethane coatings are formulated from the list of chemi- 88

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