Hexamethylene diisocyanate (HDI) is the organic compound with the formula (CH2)6(NCO)2. It is classified as an diisocyanate. It is a colorless liquid.

Aliphatic diisocyanates are used in specialty applications, such as enamel coatings which are resistant to abrasion and degradation by ultraviolet light. These properties are particularly desirable in, for instance, the exterior paint applied to aircraft and vessels. HDI is also sold oligomerized as the trimer or biuret. Although more viscous in these forms, it reduces the volatility and toxicity. At least 3 companies sell material in this form commercially. (source)