Polyurethane Coating
Polyurethane coatings used for marine coatings are made from resins that contain the isocyanate group and are highly reactive with compounds that contain hydroxyl groups (e.g., water and alcohols) which are commonly used for curing agents. Coating films are formed by solvent evaporation followed by chemical reactions between the polyurethane resin and the curing agents. Common curing agents for polyurethane coatings include water and polyols.
Polyurethane coatings, which cure by reaction with water, are called moisture-cured polyurethane coatings. They are packaged in a single can and rely on moisture in the air to cure. This type of coating is commonly used as a cosmetic coating or as a clear varnish.
Polyurethane coatings, like epoxy coatings, must be mixed thoroughly with the curing agent and catalyst before use. After mixing, commonly used coatings have a pot life of 2–6 hours. The coating film is typically dry to touch within 12 hours and fully cured after 14 days at 25 °C. The time to cure is highly dependent on the ambient and surface temperature during the curing period as well as the humidity. The curing reaction slows down rapidly at temperatures below 10 °C.
Polyurethane coatings form tough, chemically resistant coatings and make good, high-gloss cosmetic finishes. They have good abrasion and impact resistance and are particularly useful in high wear areas. They have good weather resistance but lose gloss when exposed to intense sunlight. Weathered polyurethane coatings are often difficult to recoat and topcoats will fail to adhere unless care is taken when repainting aged or damaged areas.
