Isocyanate

Isocyanates are known as the esters of isocyanic acid. The salts of the isocyanic could also expect the term isocyanates, but since they are identical with the salts of cyanic acid are both classified traditionally as cyanates.

Preparing isocyanates

Isocyanates can be prepared by reacting the corresponding amines with phosgene. The synthesis is carried out in two steps via the occurring as an intermediate carbamoyl chloride. In each sub-step, a hydrogen chloride molecule is cleaved:

Alternative synthetic routes are, for example, the catalytic carbonylation of nitro - compounds and amines or the reaction of primary amines with di-tert -butyl dicarbonate ( DIBOC ) in the presence of 4 - ( dimethylamino) pyridine (DMAP).

Chemical Properties

Isocyanates are highly chemically reactive compounds which have the structural element R -N = C = O. The most important reactions of the isocyanate group are addition reactions. So they react with alcohols to form urethanes, with amines to give urea derivatives and with water to Carbamidsäuren, but they are chemically unstable and decay directly with the release of carbon dioxide to the amines.

Urethane:

Urea derivative Education:

Reaction with water:

Isocyanates can dimerize in an equilibrium reaction to uretdions. The dimerization can be accelerated by basic catalysts such as pyridine or tertiary phosphines; aromatic isocyanates dimerize but also without a catalyst.

At higher temperatures, the equilibrium is shifted to the thermally more stable towards isocyanates.

A trimerization to the so-called isocyanurates (1,3,5 -triazine-2 ,4,6- triones ) is possible; as catalysts for this purpose, for example, sodium formate, potassium acetate, tertiary amines or phosphines are suitable. Prepared from diisocyanates Isocyanurates be used as a reactant with a very low vapor pressure for polyurethane systems. As the trimerisation can also be used for the preparation of polyisocyanurates.

Application and use

In the case of the application of isocyanates must distinguish between the very volatile mono-isocyanates and the clearly less volatile diisocyanates and polyisocyanates.

Methyl isocyanate (CH3 -N = C = O) is the simplest ester of isocyanic acid. It is a highly reactive substance, which is used, for example in the manufacture of pesticides.

Industrial use

If connections are made ​​with two isocyanate groups ( diisocyanate ) with divalent alcohols (diols ) to the reaction, so run from polyaddition reactions and there are the various industrial uses polyurethanes used. Is also simultaneously water (eg humidity) present, also the reaction of the unstable Carbamidsäuren expires, which decompose to carbon dioxide elimination. Here, the resulting gas is responsible for the formation of foam in polyurethane foams. The same resulting amine can in turn, as described above, react with another isocyanate to form a urea compound. In the curing of polyisocyanates with water so polyureas are formed. It also helps to component products ( coatings, adhesives, foams ) formulate that relate to the required for curing water as water vapor from the ambient air. Thin layers harden without foaming, as far as the base material has sufficient permeability for carbon dioxide.

The main application of the diisocyanates and blocked isocyanates represents the synthesis of the polyurethanes are the major representatives of the diisocyanates are:

• Toluene -2 ,4 -diisocyanate ( TDI)
• Diphenylmethane diisocyanate and methylene diphenyl diisocyanate (MDI)
• Hexamethylene diisocyanate ( HDI, HMDI)
• Polymeric diphenylmethane diisocyanate (PMDI)
• Isophorone diisocyanate ( IPDI)
• 4,4 '- diisocyanatodicyclohexylmethane ( H 12 MDI )

The latter two aliphatic diisocyanates and their derivatives are mainly used in high quality polyurethane paint systems use.

Medical importance

Damage isocyanates. By reaction with NH 2 and OH groups, the cell membranes of human cells When recording through inhalation of isocyanate -containing vapors it comes to irritation of skin and mucous membranes. In the eye, corneal damage can be caused. In particular HMDI can on the skin hives, contact dermatitis or a toxic dermatitis trigger.

Isocyanates MDI and TDI in particular, can cause allergic reactions of the type I. The thereby developing isocyanate asthma presents itself similar to the classic allergic asthma, however, seems to be subject to slightly different molecular mechanisms. Specific antibodies are rarely detected. A diagnosis is often a challenge test.

Respiratory diseases, which are caused by isocyanates can be recognized as an occupational disease ( BK1315 ). Workers who are regularly exposed to isocyanates must participate in occupational medical examinations.

Toxicity

Isocyanic acid, methyl isocyanate and other isocyanates are highly toxic.

On 3 December 1984 escaped from a faulty tank in a pesticide factory of the American chemical company Union Carbide Corporation near the city of Bhopal (India) about 40 tonnes of methyl isocyanate. The gas cloud killed more than 2,800 people and caused at several hundred thousand other people serious injuries such as eye and mucous membrane damage ( Bhopalunglück ).