Isophorone diisocyanate. Isophorone diisocyanate (IPDI) is an organic compound in the class known as isocyanates. More specifically, it is an aliphatic diisocyanate. It is produced in relatively small quantities, accounting for (with hexamethylene diisocyanate) only 3.4% of the global diisocyanate market in the year 2000. Aliphatic diisocyanates are used, not in the production of polyurethane foam, but in special applications, such as enamel coatings which are resistant to abrasion and degradation from ultraviolet light. These properties are particularly desirable in, for instance, the exterior paint applied to aircraft.

Properties 
Isophorone diisocyanate (IPDI) stands out as a cycloaliphatic diisocyanate distinguished by its two reactive isocyanate groups, exhibiting differences in reactivity between primary and secondary NCO groups. This unique property ensures high selectivity in reacting with hydroxyl-bearing compounds.

This distinctive attribute proves advantageous in processing low-viscosity prepolymers, resulting in a notably reduced residual content of monomeric diisocyanate. Furthermore, the low viscosity of IPDI-based prepolymers facilitates a decrease in solvent usage. The presence of methyl groups linked to the cyclohexane ring broadens IPDI's compatibility with resins and solvents.

The inherent cycloaliphatic ring confers heightened rigidity and a notably elevated glass transition temperature to IPDI-based products. IPDI itself is a transparent, slightly yellowish, low-viscosity liquid with a solidification point at -60 °C and boiling point at 158 °C. Semi-finished products like NCO-terminated prepolymers exhibit a low tendency to crystallize, remaining in a liquid state and facilitating easy processing.

Synthesis
Isophorone diisocyanate is produced by phosgenation of isophorone diamine in five-step reaction:

 Reaction of acetone with catalyst to form isophorone.
 Isophorone react with HCN to form isophorone nitrile.
 Isophorone nitrile react with ammonia and hydrogen under the influence of catalyst. This reaction create mixture of isophorone diamine conformers (25% cis, 75% trans).
 Reaction of isophorone diamine with phosgene to form isophorone diisocyanate.
 Purification of product by distillation.

Chemistry
IPDI exists in two stereoisomers, cis and trans. Their reactivities are similar. Each stereoisomer is an unsymmetrical molecule, and thus has isocyanate groups with different reactivities. The primary isocyanate group is more reactive than the secondary isocyanate group.

Application 
Isophorone diisocyanate is used in special applications:

 Hard foams and coatings
 Polyurethanes resins (PUR)
 Leather and textile
 Adhesives for battery
 Elastomers and TPU
 PUR fibers and laminates
 Adhesives and glues
 Light-stable PUR
 Aqueous dispersible polymers

Safety 
Isophorone diisocyanate is a highly toxic substance if inhaled. It can cause eye irritation and irreversible eye damage, lung and respiratory damage. It is a skin irritant and causes allergic reactions and may cause skin corrosion on prolonged contact. It is highly hazardous to aquatic environment.

H-statements: H315, H317, H319, H331, H334, H335, H411

P-statements: P260, P273, P280, P305+P351+P338, P308+P313

See also
 Hexamethylene diisocyanate
 Methylene diphenyl diisocyanate 
 Tetramethylxylene diisocyanate
 Toluene diisocyanate

References

External links
 NIOSH Safety and Health Topic: Isocyanates, from the website of the National Institute for Occupational Safety and Health (NIOSH)
 Isophorone diisocyanate - NIOSH Pocket Guide to Chemical Hazards

Isocyanates