Xylene-diisocyanate

The Showa Denka Company practices this reaction with a PX—MX mixture (24), whereas Mitsubishi Gas Chemical Company uses high purity MX first to form the dicyanide (25). In both processes, hydrogenation to the diamine follows. y -Xylenediamine is reacted with phosgene to give / -xylene diisocyanate, which is used in urethane resins (26—28). 

An offering by Cytec Specialty Chemicals, the meta isomer of tetramethyl xylene diisocyanate (TMXDI) is interesting because it contains an aromatic ring, but the NCO groups themselves are aliphatic isocyanates and have reaction characteristics typical of aliphatic diisocyanates. It reacts even more sluggishly than the more standard aliphatic isocyanates because of steric interactions, making the reactions easier to control. Compounds such as dimethyl tin dilaurate, lead octoate, or tetrabutyl diacetyl distannox-ane have been shown to be effective catalysts for the isocyanate-hydroxyl reaction. The manufacturer claims that it is less toxic than many other isocyanates. 

In addition, American Cyanamid Co. has commercialized tetra-methyl xylene diisocyanate (TMXDI) in meta and para forms (117). The synthetic method is shown by the model reactions (1) to (3) as shown below. The NCO-groups are produced by the thermal dissociation of urethane groups. Recently, another similar method was disclosed by the same company (149). The NCO groups are also produced by the thermal dissociation of urethane groups. 

Other high growth aliphatic isocyanates currently in production include isophorone diisocyanate IPDI (4,1) ex. Hiils, hydrogenated MDI HMDI (4.2) ex. Mobay, tetramethyl xylene diisocyanate TMXDI (4,3) and (4.4) ex. American Cyanamid and 3-isopropenyl dimethylbenzyl isocyanate TMI (4.5) ex. American Cyanamid [63a]. The latter two... 

Catalyst TDI Isocyanate m-xylene diisocyanate Hexamethylene drisocyanate... 

Nonyellowing urethane sealants can be made using aliphatic or cycloaliphatic polyisocyanates or with compounds where the NCO group is removed from the benzene ring by at least one methylene group. These include hexa-methylene diisocyanate (most frequently as its less toxic biuret with water), methylcyclohexyl diisocyanate, dimer acid diisocyanate, and xylene diisocyanate. Since these materials are considerably more expensive than TDI, usage is limited. 

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