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Moisture-Cured Polyurethane Coating Systems
Productivity and Peformance Advantages
Moisture-cured urethanes are capturing a growing share of new coating projects due to their long-term corrosion protection, resistance to chemicals and graffiti, and superior color appearance and gloss retention. With the wide variety of moisture-cured urethanes available today, engineers can specify complete moisture-curing systems including primers, intermediate, and topcoats. Moisture-cured urethanes offer cost and time savings because of their surface tolerance, quick drying, applicator friendliness, and application under adverse weather conditions.
One of the major advantages of moisture-cured polyurethanes is that they can be applied to steel substrates with minimal surface preparation. Most often, hand/power-tool cleaning is the only requirement for surface preparation. However, abrasive blasting to white metal is the best method. It is also the most expensive. In comparison to abrasive blasting, the amount of waste generated by hand/power-tool cleaning is dramatically reduced, as is the expense of shrouding the structure and cost of waste disposal.
Moisture-Curing Chemistry
The key to all polyurethane coatings is the isocyanate group, which is composed of nitrogen, carbon and oxygen. These groups react with any compound containing an active hydrogen, e.g. alcohols, amines or other polyurethane and ureas. For moisture-curing polyurethane, the active hydrogen containing compound is from atmospheric moisture. The reaction in the coating involves a two-stage process, with the water and the isocyanate groups first producing an amine and carbon dioxide. The amine then reacts with other isocyane groups to form an urea until all available isocyanates are consumed. Carbon dioxide generated during the process leaves the film through evaporation. Because surface moisture actually completes the chemical reaction in moisture-cured polymers, these coatings adhere well to visibly damp surfaces. Moisture-cured urethanes penetratre into pores and tight crevices where moisture is usually present, to form strong chemical bonds. Since the moisture is consumed in the process the risk of blisters caused by the water trappend under the coating film is greatly reduced.
The unique chemistry of moisture-cured urethanes does have its limitations. If there is too much moisture on the surface of the substrate, curing will occur so quickly that carbon dioxide can become trapped, resulting in pinholes or voids in the coating film. Conversely, in cold temperatures, below 15°F, moisture levels in the air drop significantly, so the reaction rate of moisture-cured urethanes is slowed. However, the safe application window for these coatings is still quite large.
Moisture-curing polyurethanes can be designed with a wide range of physical properties, from soft and flexible to hard and rigid. In abrasion resistance tests (ASTM D 4060), the coating average only about 10 to 30 milligrams film loss. Impact resistance for the moisture-cured urethanes is also very impressive. There are several of these coatings on the market capable of withstanding 160 in. lbs. Direct impact and elongation exceeding 100 percent.
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