Toluene diisocyanate

• Toluene diisocyanate
• 2,4-tolylene diisocyanate
• Tolylene -2 ,4 -diisocyanate
• 2,4-TDI
• TDI
• 2,4 - diisocyanatotoluene
• 4-methyl- m-phenylene

Colorless to yellowish, pungent-smelling liquid

Liquid

1.22 g · cm -3

21 ° C

251 ° C

1.3 Pa (20 ° C)

• Decomposes with water
• Soluble in ethyl acetate

Risk

• 5110 mg · kg -1 ( LD50, Rat (male ) oral)
• 0.48 mg · h 1 ( LC50, rat, inh. )
• > 9400 mg · kg -1 ( LD50, Rabbit, transdermal)

Template: Infobox chemical / molecular formula search available

Toluene -2 ,4 -diisocyanate ( TDI) is a major isocyanates and an important intermediate in the plastics industry. In the late 1930s, the compound was found next to other isocyanates as an ideal starting material for polyaddition reactions and was developed by Otto Bayer and his coworkers in Leverkusen classified as a promising material for the production of foams ( polyurethanes ).

Properties

Toluene -2 ,4 -diisocyanate is a very toxic liquid. TDI is well soluble in organic solvents (eg, benzene, toluene, and others), it reacts with water to relatively slow decomposition ( formation of polymeric ureas and carbon dioxide ). Therefore, organic solvents must be anhydrous if they are to be used for synthesis with TDI.

The market is dominated by three different isomers or products: The vast majority here, the mixture of 80 % toluene -2 ,4 -diisocyanate and 20% toluene -2 ,6 -diisocyanate under trade names such as " Desmodur T 80 " (Bayer AG) or " Lupranat T 80 " (BASF AG). Further, D 65 of 65% toluene -2 ,4 -diisocyanate and 35% toluene -2 ,6 -diisocyanate. T 100, however, is ,4-diisocyanate from almost 100 % toluene -2.

The worldwide annual capacity from approximately 1.5 million tons (as of 2005) and TDI is thus beside the MDI isocyanate most produced. It is currently produced in large plants around 100,000 tons per year or more, for example in Dormagen, Brunsbuttel, Black Heath (D), in Baytown (USA), in Japan and since 2006 in Caojing (China). Smaller plants in Leverkusen, Tarragona (E ), Antwerp (B) and New Martinsville (USA) closed at the end of the last and beginning of this century in the context of global consolidation.

Production

The production of TDI can be done in different ways:

• The route via the nitration of toluene, and subsequent catalytic hydrogenation with hydrogen to give 2,4- diaminotoluene (2,4- toluenediamine, TDA), which is finally phosgenated, is a technologically advanced process with high yields. A disadvantage of this method is that the highly toxic phosgene must be used as a starting material and that most of such chemical manufacturing plants operate with a slight excess pressure. However, this is still the most economical Basenphosgenierung manufacturing process and the most frequently applied. He has since commissioning of the first large-scale plant in 1953 constantly optimized. To reduce the risk of Phosgenausbruchs, or the exposure of this gas into the environment as low as possible, the security of such a chemical plant are very strictly regulated and redundant. It is subject in Germany the StörfallVO. The basic principle is to keep the amount of phosgene located during the process in circulation as low as possible ( no Phosgenlagertanks etc., see Bhopalunglück ). The system must switch off automatically if interference and the escaping phosgene will be destroyed as soon as possible by ammonia vapor. For this purpose there is in the industry specific safety devices.

By Phosgenation in the gas phase, ie without a solvent as a carrier fluid, a chemical plant can depressurized, ie with a slight vacuum. Gas emissions are therefore extremely difficult even purely physical.

• Alternatively, TDI can be produced by the oxidative dehydrogenation of formamides. Can thus be dispensed to the handling of phosgene. However, this is not is a perfected method.

Basenphosgenierung of TDA

The procedure consists of several steps in a continuous chemical plant. These steps are:

Carriers of the starting materials of phosgene and toluene ( slightly alkaline, therefore the base ) and the reaction products, the solvent is ortho -dichlorobenzene (ODB or ODCB ). The starting materials are both mixed prior to reaction with it; the product TDI must be separated by distillation processes it then. The recovered ODB is used in the circulation process again.

For the production of phosgene, the chlorine gas and carbon monoxide (CO) are first mixed, wherein in this case because of the high activation energy (95 kJ / mol ) or is no reaction. By passage of the mixed gas over the catalyst active carbon, it can be reduced to about 30 kJ / mol. Thus, the reaction of already jumps at about 40-50 ° C. Here, 106.7 kJ / mol of energy is released at full conversion. Thus, the reaction is exothermic, the produced heat of the reactor must be removed by a cooling device.

Of these three compounds, the chlorine has the greatest affinity preference for activated carbon and adsorbed. At the same time ensures that formed and adsorbed phosgene is immediately desorbed by newly flowing chlorine. To fully convert the chlorine, a slight excess of carbon monoxide is ensured. The exit temperature of the resulting gas mixture must be well below 100 ° C, thus no dissociation becomes noticeable. Phosgene begins when the temperature rises again in the starting materials to decay (at 200 ° C are dissociated approximately 5% phosgene at 800 ° C are practically only CO and chlorine before ).

The phosgene is then dissolved in cold ODB. The solubility is high, so that the mass fractions of 60% COCl2 and more can be achieved. Previously melted at about 140 ° C, toluene diamine (TDA) was diluted in hot ODB.

The phosgenation reactions are composed of a small number of co- main reactions and a myriad of side reactions. They fall into two sub-steps, the summarized cold and hot phosgenation. It is important prior to, to use a large excess of phosgene. This should be at least 300 %, but are preferably not higher than 400%. Important factors, the proportion of 2,4 -TDA is in the isomer mixture. The larger this is, the higher the excess phosgene must be because the sterically favorable 4- position of the second amino group can be easily reached from Phosgenmolekülen than the laterally shielded by the methyl group of 6 -amino group of 2,6- TDA. A too high phosgene this would lead to an undesirably rapid Phosgenation.

Arise from the first exothermic reactions of the main cold phosgenation simplified five solid intermediates that form a suspension in ODB:

Carbamyl chloride, bis- carbamic acid, amine hydrochloride, bis- amine hydrochloride, and compounds containing two chemical groups

These intermediates need to be implemented by supplying a lot of heat on. From Carbaminsäurechloriden (HCl) is cleaved by the action of heat endothermic hydrogen chloride, there arises TDI. Furthermore, the acquisition phosgene, the amine hydrochlorides with repeated HCl exclusion also to TDI to offer. The solubility of hydrogen chloride in ODB is virtually non-existent, so that it immediately comes to outgassing of the reaction hydrogen chloride.

During the hot phosgenation the emergence of some undesirable and irreversible, residue- forming secondary compounds ( ureas, biuretes, isocyanurate, and more. ) Is inevitable, however, be kept to a minimum. The course for a high yield ( 95% and above ) must be provided by optimal mixing, flow and temperature conditions already in the cold phosgenation. The structure of the Heißphosgenierreaktoren is extremely important. To avoid unwanted Nachchlorierungen by the hydrogen chloride or Überphosgenierungen, the residence times of the molecules are too tight. In the best case, all molecules should have approximately the same retention time. For this, the tower-like, cylindrical reactors are running slim and high return flows are prevented by many perforated plates, or at least the space between two floors limited ( chambering ). The necessary heat for the endothermic reactions would be supplied to a large extent directly on entry into the reactor for reaction as quickly as possible to achieve TDI. The apparatus material this must Heißphosgeniertürme due to the highly corrosive HCl vapor have a high nickel content (eg Inconel 600).

At the outlet of the Heißphosgenierreaktoren the liquid raw material (about 15 % TDI, 80 % ODB, 5% phosgene and residual products from side reactions ) from the hot process gas ( phosgene, HCl ) is bound separately. The process gases leave the phosgene where the phosgene returned on the one hand back to the Phosgenabsorption, on the other hand, the hydrogen chloride is removed. This is absorbed in a separate plant in about 18 % in an azeotropic hydrochloric acid method. The resulting about 30 % hydrochloric acid is very pure and can be supplied to a plurality of uses.

The raw material is dephosgenated parallel by a first distillation step in which a large part of the still dissolved phosgene is separated by heat. Subsequently, the separation by distillation of the ODB, the residue and cleaning the TDI of undesirable side components, such as hydrolyzable chlorine compounds or carbon tetrachloride occurs. This is carried out, at best, in series-connected distillation columns, but can also be carried out in a single, suitably designed column having, for example, bubble cap trays. Due to the high boiling point of TDI These process steps are carried out as a vacuum distillation.

Use

In the chemical industry TDI is an important intermediate for the production of adhesives, foams for mattresses and upholstery, polyurethanes (eg for shoe soles ), elastomers, coatings, and high-quality coatings for use in the automotive, aircraft or railcar coatings.

Safety

Vapors irritate eyes and respiratory system very strong. After enduring inhalation of vapors pulmonary edema is possible, which may be delayed. The MAK value is very low at 0.005 ppm, because long-term effects on lung sensitization of the respiratory system, leading to asthmatic disorders can be the result.