Hexamethylene diisocyanate


 * For other uses of the abbreviation HDI, see HDI (disambiguation).

Hexamethylene diisocyanate (HDI) 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 isophorone diisocyanate) only 3.4 % of the global diisocyanate market in the year 2000. Aliphatic diisocyanates are used 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.

Synthesis
There are two steps to the synthesis of pure HDI:
 * Phosgenation: Reaction of hexamethylene diamine with phosgene to form crude HDI
 * Purification: Distillation of crude HDI to extract pure HDI

Chemistry
HDI is a symmetrical molecule and thus has two isocyanate groups of equal reactivity.