The present invention relates to a method for the preparation of monomeric or polymeric aromatic isocyanates by thermal decomposition of monomeric or polymeric aromatic carbamates.

EP-A 611.243 discloses the preparation of organic isocyanates by thermal decomposition of corresponding carbamates dissolved in a solvent to isocyanate and an alcohol in multiple and separate steps. In intermediate steps the solvent is treated with an inert stripping agent to remove the alcohol formed during the thermal treatment and finally an isocyanate-rich solution is recovered.

US-A 5.453.536 describes the pyrolysis of polycarbamates in the substantial absence of a solvent under reduced pressure and at a temperature of about 150 to about 2700C to form the corresponding polyisocyanate and a secondary alcohol. Alcohols containing a halogen atom are not mentioned.

In US-A 4.547.322 a method is described for manufacturing MDI from a N-phenylcarbamate which comprises methylenating a N-phenylcarbamate to produce a dinuclear diphenylmethane dicarbamate and subjecting the latter to a thermal decomposition process which involves dissolving the dinuclear diphenylmethane dicarbamate in a solvent having a boiling point between 120 and 3500C, allowing the solution to flow down in a reactor and come into counterflow contact with a carrier introduced into the reactor upwardly thereby producing an organic hydroxyl compound and recovering said hydroxyl compound as vapor and the carrier at the upper end and the isocyanate solution at the lower end of the reactor.

It is mentioned that the method can be used with, amongst others, 2,2,2-trichioroethyl and 2,2,2-trifluoroethyl substituted N-phenylcarbamates, but no such examples are given.

US-A 3.746.689 discloses the use of halogenated alcohols having 1 to 6 carbon atoms as blocking agents for polyisocyanates.

An improved method has now been found for the preparation of monomeric or polymeric isocyanates by thermolysis of corresponding monomeric or polymeric carbamates.

The invention thus concerns a method for the preparation of aromatic monomeric or polymeric isocyanates by decomposing aromatic monomeric or polymeric carbamates of the formula R1(NHCOOR2)x wherein x is at least 1, R1 is an aromatic radical of valency x and R2 is a monovalent organic radical, into monomeric or polymeric isocyanates of the formula R'(NCO)X and alcohols of the formula R2OH, characterised in that the radical R2 is substituted by a group containing at least one halogen atom.

Organic isocyanates can be obtained at low temperatures in high yields in the absence of a solvent or from concentrated solutions.

R1 is a substituted or unsubstituted, saturated or unsaturated, aromatic hydrocarbon radical optionally containing hetero-atoms.

R2 is preferably substituted by at least one chlorine or fluorine atom.

The carbamate composition which is subjected to decomposition may be a mixture of polymeric carbamate compounds of different functionalities which, upon decomposition, result in a mixture of polymeric isocyanates. It will be understood that in such instances the value for x is an average of the functionalities of all species present in the carbamate mixture. The term 'functionality' as used herein is defined as number averaged functionality.

The average value of x is generally between 1 and 15, preferably between 2 and 10 and more preferably from 2 to 3.

The term "polymeric" as used herein refers to any functionality higher than 1.

Preferred as R' are tolylene, methylene diphenylene or polymethylene polyphenylene radicals or mixtures thereof.

Alcohols which may be formed include, for example, 2,2,2-trifluoroethanol, 2,2,2-trichloroethanol, trichloromethanol, 1,1,1 ,3,3,3-hexafluoroisopropanol, nonafluoro tert.butanol, fluorophenois, chlorophenols and polysubstituted halogenated phenols.

Representative monomeric isocyanates which may be formed include phenylisocyanate, 4-chlorophenylisocyanate, 2-fluorophenylisocyanate, 3,4-dichlorophenyl isocyanate, tolylisocyanate and diisopropylphenylisocyanate.

Examples of difunctional isocyanates which can be made according to the present method include diphenylmethane diisocyanates such as 4,4'-diphenylmethane diisocyanate, 2,4'-diphenylmethane diisocyanate, 2,2'-diphenylmethane diisocyanate and mixtures thereof, toluene diisocyanates such as 2,4-toluene diisocyanate, 2,6-toluene diisocyanate and mixtures thereof, m-phenylene diisocyanate and 1, 5-naphthylene diisocyanate.

The method of the present invention can advantageously be used for the preparation of diphenylmethane diisocyanates, toluene diisocyanates, polymethylene polyphenylene

polyisocyanates, or mixtures of any of these.

Trifunctional and higher functional isocyanates which can be made include 2,4,6-toluene triisocyanate and polymethylene polyphenylene polyisocyanates.

As already mentioned above, any mixtures of mono-, di- and polyfunctional isocyanates may be obtained depending on the composition of the starting carbamate mixture.

The reaction may be carried out in an inert solvent, i.e. any solvent not interacting with isocyanates or alcohols under the applied reaction conditions. However, isocyanates formed in the decomposition reaction can serve as a solvent for the reaction as well.

Suitable inert solvents which may be employed include, for example, aromatic hydrocarbons such as benzene, halogenated aromatic hydrocarbons such as monochlorobenzene, ortho-dichlorobenzene, trichlorobenzene or 1 -chloronaphthalene, alkylated aromatic hydrocarbons like toluene, xylene, ethylbenzene, cumene or tetrahydronaphthalene, other functionalised aromatic hydrocarbons such as anisole, diphenylether, ethoxybenzene, benzonitrile, 2-fluoroanisole, 2,3-dimethylanisole or trifluorotoluene or mixtures thereof.

Preferred solvents comprise monochlorobenzene or ortho-dichlorobenzene.

Any of the abovementioned solvents may also be used to generate the carrier gas.

The carrier gas serves to physically remove any alcohol without forming a chemical bond with it.

Mixtures of at least one of the above solvents with a lower boiling inert solvent used as carrier gas may also be used.

Exemplary of such additional lower boiling solvents are alkanes such as n-pentane, n-hexane, n-heptane or higher or branched alkanes, cyclic alkanes like cyclopentane, cyclohexane or derivatives thereof, halogenated alkanes like chloroform, dichloromethane, carbontetrachloride, and alkanes with other functional groups like diethylether, acetonitrile, dioxane and the like.

The method may be carried out at atmospheric pressure, preferably under nitrogen.

However, in the absence of a solvent, the reaction preferably takes place under reduced pressure. in such case, the pressure is preferably reduced to between 104 and 50 mbar, and more preferably to between 0.1 and 10 mbar.

Superatmospheric pressures may sometimes be required, depending on the type of solvents

used.

The reaction time is dependent on the temperature and on the type and quantity of the carbamate compound, but will normally not exceed 5 hours. Reaction timers of less than 3 hours are common, and reaction times of less than 1 hour have been achieved without any problem.

The reaction temperature largely depends on whether a solvent is present or not. Generally, it will be between 50 and 350°C. In a solvent-free method the temperatures will normally be between the melting point of the carbamate and 3500C, whereas in the presence of a solvent the temperature will preferably be between 100 and 2000C and more preferably between 120 and 1900C.

With the method of the present invention yields of isocyanates of more than 90% can readily be obtained. Yields of at least 95% are possible.

The method may be conducted in any suitable apparatus which can be equipped, if required, with agitation means and heating and/or cooling means to keep the temperature within the desired range. A distiilation column is generally attached to said apparatus.

The method of the present invention may be conducted batchwise or as a semi-continuous or continuous process.

The order of addition of the reactants may be varied to suit the particular apparatus and/or reactants employed.

The presence of any other compounds, such as catalysts or co-reactants, in addition to the carbamate compound and optionally the solvent is generally not required.

The isocyanates and alcohols obtained by this method are generally of high purity and no additional treatment is required to further purify said products. Only the solvent, if present, needs to be removed.

If a particularly high grade of purity is required, the reaction products formed may be subjected to known purification methods, such as filtration, extraction, recrystallisation or distillation.

The invention is illustrated by, but not limited to, the following examples.

Examples Example I Into a suitable flask equipped with a condenser, were placed 3 g of diphenylmethane bis (1,11 ,3,3,3-hexafluoroisopropylcarbamate). The carbamate was heated to a temperature of 240"C. After the carbamate was molten, the pressure was reduced to 0.1 mbar.

1,1,1 ,3,3,3-Hexafluoroisopropanol was removed from the system as the reaction proceeded.

After 25 minutes at 240"C diphenylmethane diisocyanate containing 33.6% by weight NCO-groups remained in the pyrolysis flask.

ExamDle 2 Example 1 was repeated, but 4.2 g of poiyphenylene polymethylene poly (1,11 ,3,3,3-hexafluoroisopropylcarbamate) was used instead of diphenylmethane bis (1,1,1 ,3,3,3.hexafiuoroisopropylcarbamate). The carbamate was heated to a temperature of 220"C. After the carbamate was molten, the pressure was reduced to 2 mbar.

1,1,1,3,3,3-Hexafluoroisopropanol was removed from the system as the reaction proceeded.

After 20 minutes at 220"C polyphenylene polymethylene polyisocyanate containing 30% by weight NCO-groups remained in the pyrolysis flask.

Examole 3 (comparative) Example 1 was repeated, but 2 g of diphenylmethane bis (isopropylcarbamate) was used instead of diphenylmethane bis (1,1,1 ,3,3,3-hexafluoroisopropyl carbamate). The carbamate was heated to a temperature of 220"C. After the carbamate was molten, the pressure was reduced to 1-3 mbar.

Isopropanol was removed from the system as the reaction proceeded. After 20 minutes at 220"C diphenylmethane diisocyanate containing 5.7% by weight NCO-groups remained in the pyrolysis flask.

Examole 4 (comoarative) Example 1 was repeated, but 2.1 g of diphenylmethane bis (1-methoxy-2-propyl carbamate) was used instead of diphenylmethane bis (1,1,1 ,3,3,3-hexafluoro isopropylcarbamate). The carbamate was heated to a temperature of 220°C. After the carbamate was molten, the pressure was

reduced to 1-5 mbar.

1 -Methoxy-2-propanol was removed from the system as the reaction proceeded. After 25 minutes at 220"C diphenylmethane diisocyanate containing 23.5% by weight NCO-groups remained in the pyrolysis flask.

Comparative examples 3 and 4 show that a significantly lower yield is obtained in comparison with example I when the alcohol formed does not contain a halogen atom.

Examole 5 Into a suitable flask equipped with a condenser and an addition funnel, a 5% dispersion of diphenylmethane bis (1,1,1 ,3,3.3-hexafluoroisopropyl carbamate) in chlorobenzene (MCB) was placed. The dispersion was heated to about 134"C and the solvent/alcohol mixture was distilled off. The volume was kept constant in the pyrolysis flask by addition of MCB.

After 1 hour at 134"C diphenylmethane diisocyanate containing 30.5% by weight NCO-groups was obtained.

Examole 6 Example 5 was repeated, but a 10% dispersion of polyphenylene polymethylene poly (1,1,1,3,3,3-hexafluoroisopropylcarbamate) in MCB was used. The dispersion was heated to about 134"C and the solvent/alcohol mixture was distilled off. The volume was kept constant in the pyrolysis flask by addition of MCB.

After 1 hour at 134"C polyphenylene polymethylene polyisocyanate containing 31.2% by weight NCO-groups was obtained.

ExamDle 7 Example 5 was repeated, but a 5% dispersion of diphenylmethane bis (2,2,2-trifluoroethylcarbamate) in ortho-dichlorobenzene (ODCB) was used.

The dispersion was heated to about 180"C and the solvent/alcohol mixture was distilled off. The volume was kept constant in the pyrolysis flask by addition of ODCB.

After 1 hour at 180"C diphenylmethane diisocyanate containing 27.4% by weight NCO-groups was obtained.

Example 8

Example 5 was repeated, but a 5% dispersion of polyphenylene polymethylene poly (2,2,2.trifiuoroethylcarbamate) in a MCBIODCB mixture was used. The dispersion was heated to about 180"C. The volume was kept constant in the pyrolysis flask by addition of ODCB.

After 1 hour at 180"C polyphenylene polymethylene poiyisocyanate containing 28.2% by weight NCO-groups was obtained.

Example 9 (comDarative} Example 5 was repeated, but a 5% dispersion of diphenylmethane bis (isopropylcarbamate) in a MCB/ODCB mixture was used. The dispersion was heated to about 1800C. The volume was kept constant in the pyrolysis flask by addition of ODCB.

After 2 hours at 1800C diphenylmethane diisocyanate containing 8.6% by weight NCO-groups was obtained.

Compared with example 5, this comparative example shows that a much lower yield is obtained when an alcohol not having a halogen atom is split off.

Examole 10 (comparative! Example 5 was repeated, but a 10% dispersion of polyphenylene polymethylene poly (1-methoxy-2-propylcarbamate) in ODCB was used. The dispersion was heated to about 1800C.

The volume was kept constant in the pyrolysis flask by addition of ODCB.

After 21/2 hours at 1800C polyphenylene polymethylene polyisocyanate containing 9% by weight NCO-groups was obtained.

Comparative example 10 again iliustrates, by comparison with example 6, that the yield is significantly lower when an alcohol not according to the invention is formed in the decomposition.

Example 11 Into a suitable flask equipped with a condenser and an addition funnel, a 10% dispersion of toluene 2,4 bis(2,2,2-trifluoroethyicarbamate) in ortho-dichlorobenzene (ODCB) was placed. The dispersion was heated to about 1800C and the solvent/alcohol mixture was distilled off. The volume was kept constant in the pyrolysis flask by addition of ODCB. After 90 minutes at about 180"C toluene diisocyanate containing more than 43% by weight NCO-groups, corresponding to more than 89% yield, was obtained.

Examole 12 Into a suitable flask equipped with a condenser and an addition funnel, a 10% dispersion of toluene 2,4 bis(I 1,1 ,3,3,3-hexafluoroisopropylcarbamate) in ortho-dichiorobenzene (ODCB) was placed. The dispersion was heated to about 1800C and the solvent/alcohol mixture was distilled off. The volume was kept constant in the pyrolysis flask by addition of ODCB. After 90 minutes at about 180"C toluene diisocyanate containing more than 47% by weight NCO-groups, corresponding to about 99% yield, was obtained.