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.
Table I – Common reactions of isocyanate prepolymers
Aromatic isocyanate prepolymers are used primarily in primers as well as for interior applications where light stability is less of an issue. Aliphatic isocyanate prepolymers are used primarily in applications where exterior weathering and light stability is of primary importance.
Another common ambient cure two-component paint chemistry involves the reaction of an epoxy hardener with that of an amine functional resin. Due to their tenacious adhesion to a variety of surfaces including metals and concrete, epoxy two-component compositions are used in a variety of applications including primers for exterior and interior application and interior coatings.
As Table II illustrates, epoxy groups react with primary amines at ambient temperatures to form secondary amines that in turn react to form tertiary amines. Reactivity of amines increases with the strength of the base and decreases with steric hindrance. The general order of amine reactivity is: primary > secondary > tertiary amines. Aliphatic amines are more reactive than aromatic amines as the former are more basic.
Acrylics containing glycidyl methacrylate or cycloaliphatic epoxies react more rapidly than do aromatic epoxies such as those based on bisphenol A. With the correct catalyst, aliphatic epoxy resins can react with carboxyl functionality even at room temperature. Cycloaliphatic epoxy-based systems also provide improved light stability for exterior applications.
Table II – Example of Reactions of epoxy with amine
When formulating a stoichiometric reaction, it is desirable to discuss reactants in terms of equivalents. Molecular weights of common functional groups are listed in Table III.
Table III – Molecular Weight of Common Functional Groups
|Functional Group||Molecular Weight of Functional Group|
|Terminal Epoxy||43 g/mole|
For example, using the equation above, if the equivalent weight of an isocyanate functional hardener is 200, to have a 1:1 stoichiometric reaction for a polyol with an equivalent weight of 400, a weight ratio of 2:1 polyol:isocyanate is required as the polyol has half the functionality on a weight basis. Other common hardeners and their reactions are listed in Table IV.
Table IV – Other Common Hardeners and Reactions
Cross-linker Functional Group
Triethoxy silane and aliphatic epoxy
|Dual or self cure mechanism||Siloxane & epoxy ester|
A search of Prospector’s search engine provides a number of hardeners, and resins for formulating two-component coatings.
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