Urethane resins are formed by the condensation reaction of a urethane (carbamate) with an aldehyde. The reaction conditions determine whether a distinct compound or a resin is formed.
In an alkaline environment, carbamates react with formaldehyde to produce hydroxymethyl compounds that revert back to their initial constituents at elevated temperatures. In an acidic environment, carbamates undergo cross-linking, forming methylene bridges and yielding resins essential for molding material production.
Urethane-urea-formaldehyde mixed resins are also known. These resins are formed by the reaction of cyanuric acid, polyhydric alcohols, and formaldehyde. They are used as paper and textile auxiliaries and as raw materials for surface coatings.
Another method for producing urethane resins involves converting urea into the corresponding carbamate by heating it in the presence of an alcohol, followed by ammonia elimination. The resulting solution is then reacted with formaldehyde to form a cross-linked resin. These highly viscous solutions are used as label adhesives and for bonding cardboard boxes.
The properties of urethane resins vary depending on the constituent components and the production method. The properties range from oily to brittle, depending on the extent of condensation. Hard resins are obtained when using lower aliphatic and cycloaliphatic alcohols, while higher alcohols yield wax-like resins.
- Amino Resins; Ullmann’s Encyclopedia of Industrial Chemistry. – https://onlinelibrary.wiley.com/doi/10.1002/14356007.a02_115.pub2