In several applications 1-component, thermosetting coatings are being used exclusively like in coil coatings, can coatings, or at least predominantly like in automotive coatings. Polyisocyanates as such can`t be used for 1-component systems due to the high reactivity towards the polyols (“2-pack systems”). Therefore, polyisocyanates are thermally reversibly “blocked” with H-acidic components. Reaction of such blocked polyisocyanates does not occur in presence of a polyol at ambient temperature and therefore a stable 1-component system is obtained. A proper selection of the “blocking agent” determines the reactivity, i.e. the splitting temperature of the blocked polyisocyanate and therefore the curing conditions of such an aliphatic, 1-component thermosetting polyurethane system.
Actually only a few blocking agents are of commercial and technological interest: more important are 2-Butanone-oxime (methyl ethyl ketoxime,MEKO), ε-caprolactam and to a limited extend 3,5-dimethylpyrazole.
In specific areas, especially in powder coatings, the uretdion/dimer structure is used to deactivate isocyanate groups thermally reversible without using blocking agents.
Since a significant portion of the blocking agent is released to the air during the curing process, the toxicological and environmental aspects of these substances have to be considered, too.
More detailed information can be taken from the suppliers' Material Safety Data Sheets.