Polyurethane polymers are widely used in coatings, foams, elastomers, adhesives, sealants, inks, film, fiber and engineering plastics among other things. Specifically in the coatings area, they are commonly used in industrial, construction, textile, leather, floor finishes and other one- or two-component coatings due to the unique properties of polyurethanes and isocyanate-based reactions. Generally speaking, polyurethanes have good toughness that is primarily due to the molecular structure of the main chain of alternating soft and hard segments. The soft segment imparts flexibility to the polymer while the hydrogen bonds in the hard segment not only provide hardness but also form a physical crosslinking point that facilitates bonding. As a result, these polyurethanes are “soft but not sticky” and “hard but not brittle”, and these performance attributes widen the potential applications for polyurethane coatings.
As specified by ASTM D16, Standard Terminology for Paint, Related Coatings, Materials, and Applications, urethane coatings are classified from Type I to Type VI. Type I urethane coatings are oil based, and these types of urethanes are also called urethane-modified alkyds, whereby the isocyanate fully or partially replaces dibasic acid in the binder. A Type I urethane is usually made of an unsaturated vegetable oil or fatty acid in organic solvent; it is used as varnish and has been prevalent in the wood floor coatings market for more than half a century.1 As shown in Figure 1, the film formation for this type of coating is achieved by lacquer-dry followed by oxidative curing in the presence of a metallic drier. There are two categories of Type I urethanes: oil-modified urethanes (OMU), in which the isocyanate fully replaces dibasic acid in alkyd, and uralkyds, in which the isocyanate only partially replaces the dibasic acid. This urethane binder combines the benefits of urethanes such as abrasion resistance, hydrolytic stability and toughness with the benefits of alkyds such as oxidative crosslinking and use of renewable materials.