Generally, two-component films provide higher hardness with enhanced resistance to moisture permeation, soluble salts, and chemicals. In the context of the chemistry detailed in this article, a hardener is defined as a separate ingredient that reacts with the first component of a two-component paint. The pigmented functional resin portion is normally in Part A and the hardener in Part B.
Crosslinked films are not soluble in most solvents; however, solvents swell most crosslinked films. As crosslink density (XLD) increases, solvent swelling decreases. A polyurethane is formed from the reaction of a polyol with an isocyanate prepolymer. The number and type of functional groups and structure dramatically impacts the reaction speed and the cured performance
As Table I illustrates, an amine functional resin can also react with an isocyanate to form a polyurea. The reaction of an amine with an isocyanate is extremely fast and requires mixing at the point of application. For ambient cured urethane paints, normally a slight excess of isocyanate functional groups is used as moisture, which also reacts with isocyanate.
In general, aromatic isocyanates react with hydroxyl groups faster than aliphatic isocyanates and primary functional isocyanates react faster than secondary isocyanates. Also, primary hydroxyl groups react faster than secondary hydroxyl groups.