The term urethane and polyurethane apply to certain types of binders used for paints and clear coatings. (The binder is the ingredient that holds the pigment together in a tough, continuous film, and provides film integrity and adhesion.) Urethane binders are made be reacting a compound with an isocyanate group with another group, often a hydroxyl group: R1-N=C=O + R2-OH --> R1-NH-COO-R2. The urethane polymer is also called a polyurethane. Aliphatic solvents and polymers are preferred over aromatic types in that they provide greater flexibility and exterior durability, although they are more costly.
There are different types of urethane systems:
TYPE I: one-component, alkyd reacted with a polyisocyanate; this is the usual "polyurethane" varnish used in residential and commercial applications on bare or stained wood (floors, furniture, doors, etc.); offers greater toughness and abrasion resistance than do unmodified alkyds;
TYPE II: one-component, has a polymer with unreacted isocyanate in the backbone; reacts and crosslinks upon application and exposure to moisture from the air; known as "moisture cure urethane";
TYPE III: one-component, heat-cured; the polymer contains isocyanate that is chemically blocked from reacting with hydrogen donors in the product, until the heating drives off the blocking agent; used in factory application;
TYPE IV: two-component, a polymer containing free isocyanate groups is mixed at time of use with a catalyst of monomeric polyol or polyamine; has short pot life;
TYPE V: two-component, a poly-isocyanate polymer is mixed at time of use with a resin containing active hydrogen groups; limited pot life;
TYPE VI: one-component, unreactive urethane polymer in solvent solution; forms the film upon solvent evaporation, but otherwise does not change (crosslink); PUD: "polyurethane dispersion"; dispersion of unreactive urethane polymer in water; used to modify latex coatings, particularly clear coatings, to boost mar resistance.