The majority of polyurethanes are derived from aromatic diisocyanates such as toluene diisocyanate (TDI), methylene diphenyl diisocyanate (MDI), and polymeric MDI (PMDI), which will not be discussed in this report. These polyurethanes are used primarily for flexible and rigid foams. The aliphatic diisocyanates (ADIs), such as isophorone diisocyanate (IPDI), hexamethylene diisocyanate (HDI), and hydrogenated MDI (H12MDI), are more expensive to produce, and are therefore used in specialty applications such as light-stable polyurethane coatings. The ADIs that will be discussed in this report are shown below. All are at or near commercial production.
ALIPHATIC DIISOCYANATES
| Abbreviation | Phosgene | Non-Phosgene | | Isophorone diisocyanate | IPDI | 3 | 3 | | Hexamethylene diisocyanate | HDI | 3 | --- | | Methylene dicyclohexyl diisocyanate, (hydrogenated MDI) | H12MDI | 3 | --- | | Xylylene diisocyanate | XDI | 3 | --- | | Trans-1,4-Cyclohexane diisocyanate | t-CHDI | 3 | 3 | | Tetramethyl xylylene diisocyanate | TMXDI | --- | 3 | | Isopropenyl dimethylbenzyl isocyanate | TMI | --- | 3 | | Trimethylhexamethylene diisocyanate | TMHDI | 3 | --- | | Norbornane diisocyanate | NBDI | 3 | --- |
Although an aromatic structure may be present in these molecules, they are referred to as aliphatic diisocyanates because the functional group is not attached to an aromatic ring. The first three diisocyanates listed (IPDI, HDI, and H12MDI) and their respective adducts account for the vast majority of commercial applications.
The majority of ADIs are produced by phosgenation of corresponding diamines. Alternative phosgenation routes involve an intermediate diamine salt. In general, a slurry is formed when a diamine solution is mixed with a phosgene solution (both using the same solvent, such as chlorobenzene). The slurry is treated with excess phosgene at temperatures as high as 130°C (266°F). The diisocyanate is generated through dicarbamoyl chloride and diamine dihydrochloride intermediates. An alternative route is to treat the diamine solution with hydrogen chloride to form a slurry of amine salts, which is then treated with phosgene until a clear solution is obtained. Although this method gives higher yields, the reaction time is much longer. Non-phosgene routes have long been the goal of the industry. These routes are attractive because of the continued environmental and toxicological restrictions regarding the use of chlorine and phosgene. Non-phosgene routes most often involve the conversion of amines to bisureas or biscarbamates before conversion to diisocyanates. Of particular interest is the new urea-based route to IPDI that has been recently commercialized by Hüls. The chemistry of this route is as follows: The actual process comprises three steps with various intermediates. First, IPDA is condensed with urea, in the presence of an alcohol solvent, such as butanol, forming the corresponding bisurea and ammonia. In the next step, the bisurea reacts with the alcohol to form the corresponding biscarbamate and ammonia. The bisurea is essentially converted quantitatively into the biscarbamate at higher temperatures, preferably 180-230°C (356-446°F), and at higher pressure. The ammonia is removed as it is formed, in order to drive the reaction equilibrium towards biscarbamate production. The biscarbamate is cleaved in the presence of a catalyst, without using solvents, to form IPDI and alcohol. Process technology and economics for the larger volume ADI's are presented in the report. A brief market overview including producer capacities, regional supply/demand outlook and applications are also presented. | 
