The present invention relates to a method for the preparation of polymeric isocyanates by thermal decomposition of polymeric ureas.

A number of methods for preparing isocyanates by decomposition of ureas are already known.

EP-A 583.637 discloses the decomposition of trisubstituted ureas at elevated temperature (90-400 OC) and in the presence of a solvent into a volatile monoisocyanate and a secondary amine of which the boiling point is higher than that of the isocyanate and higher than the reaction temperature.

Only examples of the preparation of aliphatic monoisocyanates by this method are given.

US-A 3.936.484 describes the decomposition of trisubstituted ureas at elevated temperatures (above 2300C) and in the presence of an inert carrier to form isocyanates and amines. The isocyanate yield is from 60 to 88%.

FR-A 1.473.821 concerns the pyrolysis into isocyanates and amines of substituted ureas in the liquid phase (temperature less than 2000C) in the presence of a particular class of solvents. The reaction times however are long (6-35 hours) and the isocyanate yield only moderate (60-75%).

The preparation of isocyanates by thermally decomposing dialkylureas in an inert solvent in the presence of co-reagents is known from EP-A 391.716, EP-A 402.020 and EP-A 408.277.

The known methods however have various disadvantages. They either can only be used for producing low boiling isocyanates, need to be carried out in dilute solutions or at elevated temperatures, require the use of co-reagents and/or result in the production of considerable amounts of undesirable by-products.

A method has now been found for the preparation of polymeric isocyanates by thermolysis of corresponding polymeric ureas which does not have the disadvantages associated with the known methods.

The invention thus concerns a method for the preparation of polymeric isocyanates by decomposing at moderate temperature polymeric ureas of the formula R1(NHCONR2R3)x wherein x is at least 2, R' is an organic radical of valency x and R2 and R3 are monovalent organic radicals, into polymeric isocyanates of the formula R1(NCO)x and secondary amines of the formula R2NHR3 characterised in that at least one of the radicals R2 and R3 is bound to the nitrogen atom of the amine by a tertiary carbon atom.

Non-volatiie organic isocyanates can be obtained in high yields via a fast reaction in the absence of a solvent or from concentrated solutions.

R' is a substituted or unsubstituted, saturated or unsaturated, aliphatic, cycloaliphatic or aromatic hydrocarbon radical optionally containing hetero-atoms.

The urea composition which is subjected to decomposition may be a mixture of polymeric urea 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 urea mixture. The term 'functionality' as used herein is defined as number averaged functionality.

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

The term "polymeric" as used herein refers to a functionality of 2 or higher.

Preferred as R1 are tolylene, methylene diphenylene or polymethylene polyphenylene radicals or mixtures thereof.

R2 and R3 may be any monovalent organic radicals, provided one of said radicals contains a tertiary carbon atom by which it is attached to the nitrogen atom.

It is noted that the radicals R2 and R3 and the nitrogen atom of the amine may together also form a heterocyclic compound.

The radical containing a tertiary carbon atom is preferably a tertiary alkyl group having from 4 to 12 carbon atoms, more preferably a tertiary butyl or an at least 2,2-disubstituted piperidine group.

Secondary amines which may be formed upon decomposition include tert.butylmethylamine, tert.butyl ethylamine, tert.butylpropylamine, tert.butylisopropylamine,tert.butyl n-butylamine, tert.butyl sec.butylamine, tert.butyl isobutylamine, di(tert.butyl)amine, and higher linear, branched or cyclic alkyl tert.butylamines, tert. butyl phenylamine, 2,2-dimethylpiperidine, 2,2-diethylpiperidine, 2,2-methylethyl piperidine , 2,2,6-trimethylpiperidine, 2,2,6,6-tetramethylpiperidine and higher linear, branched or cyclic 2,2-dialkylpiperidines and any other substituted 2,2-dialkylpiperidine , and other cyclic 2,2-dialkyl variants like pyrrolidine, morpholine or other cyclic structures containing at least one nitrogen or mixtures thereof.

Functional groups not interacting with secondary amines nor ureas under the applied reaction conditions such as a halogen, nitrile, olefine, ether, cumulene or nitro group may be present as well.

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'-diphenyl methane diisocyanate and mixtures thereof, toluene diisocyanates such as 2,4-toluene diisocyanate, 2,6-toluene diisocyanate and mixtures thereof, m-phenylene diisocyanate, 1,4-butylene diisocyanate, 1,6-hexylene diisocyanate, 1,5-naphthylene diisocyanate, 1,4-cyclohexylene diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, I ,4-xylylene diisocyanate and isophorone diisocyanate.

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

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

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.

The reaction may be carried out in an inert solvent, i.e. any solvent not interacting with the ureas, isocyanates or secondary amines 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, al kylated 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 secondary amine without forming a chemical bond with it.

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

Examples of such additional lower boiling inert 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 urea compound, but will normally not exceed 5 hours. Reaction times 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 and on the type of urea compound used. Generally, it will be between 50 and 350or.

In a solvent-free method the temperatures will preferably be between the melting point of the starting urea compound and 3500C.

When a solvent is present, the temperature will preferably be between 50 and 2000C, and more preferably between 150 and 190or.

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 distillation 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 urea compound and optionally the solvent is generally not required.

The isocyanates and amines 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, crystallisation or distillation.

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

Examples Example 1 Into a suitable flask equipped with a condenser were placed 5 g of diphenylmethane bis (N-t-butylmethylurea). The urea was heated to a temperature of 2200C. After the urea was molten the pressure was reduced to 0.3 mbar. N-t-butylmethylamine was removed from the system as the reaction proceeded. After 13 minutes diphenylmethane diisocyanate was distilled. The decomposition was complete after 17 minutes.

The released diphenylmethane diisocyanate containing 30.7% by weight NCO-groups was recovered.

A tarry residue of 2.6% by weight remained in the pyrolysis flask.

Example 2 Example 1 was repeated, but 3.4 g of diphenylmethane bis(N-t-butylmethylurea) was used instead of 5 g, the pressure was reduced to 2.5 mbar instead of being reduced to 0.3 mbar and the diphenylmethane diisocyanate was not distilled.

After 25 minutes at 2250C diphenylmethane diisocyanate containing 30.2% by weight NCO-groups remained in the pyrolysis flask.

Example 3 Into a suitable flask equipped with a condenser and an addition funnel, a 10% dispersion of diphenylmethane bis (N-t-butylmethylurea) in o-dichlorobenzene (ODCB) was placed. The dispersion was heated to about 1800C and the solvent/amine mixture was distilled off. The volume was kept constant in the pyrolysis flask by addition of ODCB.

After 90 minutes at about 1 800C diphenylmethane diisocyanate containing more than 33% by weight NCO-groups, corresponding to more than 98% yield, was obtained.

Examole 4 (comDarative) Example 3 was repeated, but a 10% dispersion of diphenylmethane bis (diisopropylurea) in ODCB was used.

After 120 minutes at about 1800C diphenylmethane diisocyanate containing 21.8% by weight NCO-groups was obtained.

This comparative example shows that when a secondary amine not according to the invention is split off a significantly lower yield of isocyanates is obtained.

Example 5 Example 3 was repeated, but a 10% dispersion of diphenylmethane bis (N-t-butylmethylurea) in cumene was used.

The dispersion was heated to about 15500 and the solvent/amine mixture was distilled off. The volume was kept constant in the pyrolysis flask by addition of cumene.

After 90 minutes at about 15500 diphenylmethane diisocyanate containing 28% by weight NCO-groups was obtained.

Example 6 Example 3 was repeated, but using mixtures of monochlorobenzene(MCB)/ODCB or toluene/ODCB as a solvent.

In both cases, after 90 minutes at about 1550C (reflux temperature of the mixture) diphenylmethane diisocyanate containing 28% by weight NCO-groups was obtained.

ExamDle 7 Example 3 was repeated, but a 10% dispersion of polyphenylene polymethylene poly(N-t-butylmethylurea) in ODCB/methylenechloride was used.

After 90 minutes at about 1 550C (reflux temperature of the mixture) polyphenylene polymethylene polyisocyanate containing 24% by weight NCO-groups was obtained.

Example 8 Example 3 was repeated, but a 1% dispersion of polyphenylene polymethylene poly(N-t-butylmethylurea) in ODCB/methylenechloride was used.

After 90 minutes at about 1 550C (reflux temperature of the mixture) polyphenylene polymethylene polyisocyanate containing 25.1% by weight NCO-groups was obtained.

Example 9 Into a suitable flask equipped with a condenser and an addition funnel, a 10% dispersion of toluene bis(N-t-butylmethylurea) in ortho-dichlorobenzene (ODCB) was placed. The dispersion was heated to about 180°C and the solvent/amine mixture was distilled off. The volume was kept constant in the pyrolysis flask by addition of ODCB. After 90 minutes at about 180or toluene diisocyanate containing more than 46% by weight NCO-groups, corresponding to more than 95% yield, was obtained.

Example 10 Into a suitable flask equipped with a condenser and an addition funnel, a 10% dispersion of hexamethylene bis(N-t-butylmethylurea) in ortho-dichlorobenzene (ODCB) was placed. The dispersion was heated to about 1800C and the solvent/amine mixture was distilled off. The volume was kept constant in the pyrolysis flask by addition of ODCB. After 90 minutes at about 1800C hexamethylene diisocyanate containing more than 45% by weight NCO-groups, corresponding to more than 90% yield, was obtained.