Multistep, continuous preparation of organic polyisocyanates

A multistep process for the continuous preparation of organic polyisocyanates, preferably aliphatic or cycloaliphatic diisocyanates, by reacting the corresponding organic polyamines with carbonic acid derivatives and alcohols to give monomeric polyurethanes and pyrolyzing the latter involves separating the resultant polyisocyanates and worthless residues in certain reaction steps and recycling the reusable by-products into earlier steps.

The present invention relates to a multistep process for the continuous 
preparation of organic, distillable polyisocyanates, preferably aliphatic 
or cycloaliphatic diisocyanates, by reacting the corresponding organic 
polyamines with carbonic acid derivatives and alcohols to give 
low-molecular-weight, monomeric polyurethanes, and pyrolyzing the latter, 
in which the resultant polyisocyanates and worthless residues are 
separated in certain reaction steps, and reusable by-products and 
intermediates are recycled into earlier steps. 
The industrial processes for the preparation of organic polyisocyanates, 
e.g. aromatic, aliphatic or cycloaliphatic polyisocyanates, are based on 
phosgenation of the corresponding organic polyamines to give polycarbamic 
acid chlorides, and thermolysis thereof to give the polyisocyanates and 
hydrogen chloride. Apart from serious environmental, disposal and safety 
problems accompanying the use of phosgene, these processes have further 
crucial disadvantages. Thus, the relatively high basicity of the starting 
polyamines means that the preparation of aliphatic or cycloaliphatic 
polyisocyanates only occurs in quite moderate space-time yields. A further 
disadvantage is the formation of undesired by-products, which can, even in 
traces, result in considerable discoloration of the polyisocyanates. In 
the preparation of hexamethylene 1,6-diisocyanate (HDI), for example, a 
number of by-products are formed, of which the most important, 
6-chlorohexyl isocyanate, additionally has the disadvantage of requiring 
considerable distillative effort for separation from HDI. 
Particular problems in this procedure are the high conversion of chlorine 
into hydrogen chloride via phosgene and carbamoyl chloride, the toxicity 
of the phosgene, and the corrosive properties of the reaction mixture, the 
lability of the solvents which are generally employed, and the formation 
of halogen-containing residues. 
There has therefore been no lack of attempts to prepare organic 
isocyanates, preferably aromatic and (cyclo)aliphatic diisocyanates and/or 
higher-functional polyisocyanates, by a phosgene-free process. 
According to EP-A-0 018 588 (U.S. Pat. No. 4,497,963), aliphatic and/or 
cycloaliphatic diurethanes and/or polyurethanes are prepared by reacting 
primary aliphatic and/or cycloaliphatic diamines and/or polyamines with 
O-alkyl carbamates in the presence of alcohols in an amine NH.sub.2 
group:carbamate:alcohol ratio of from 1:0.8 to 10:0.25 to 50 at from 
160.degree. to 300.degree. C. in the presence or absence of catalysts, 
and, if necessary, removing the resultant ammonia. The resultant 
diurethanes and/or polyurethanes can, if desired, be converted into the 
corresponding diisocyanates and/or higher-functional polyisocyanates. 
Detailed reaction conditions for the thermolysis are not disclosed in the 
patent specification. 
According to EP-A-28 338 (U.S. Pat. No. 4,290,970), aromatic diisocyanates 
and/or polyisocyanates are prepared by a two-step process in which, in the 
first step, primary aromatic diamines and/or polyamines are reacted with 
O-alkyl carbamates in the presence or absence of catalysts and in the 
presence or absence of urea and alcohol to give aryldi- and/or 
-polyurethanes, the resultant ammonia is removed if desired, and the 
resultant aryldi- and/or -polyurethanes are converted, in the second 
reaction step, into aromatic diisocyanates and/or polyisocyanates by 
thermolysis. 
Other publications relate to the partial substitution of urea and/or 
diamines by carbonyl-containing compounds, for example N-substituted 
carbamates and/or dialkyl carbonates, or mono- or disubstituted ureas or 
polyureas (EP-B-27 952 (U.S. Pat. No. 4,388,238), EP-B-27 953 (U.S. Pat. 
No. 4,430,505), EP-B-28 331 (U.S. Pat. No. 4,480,110), EP-A-126 299 (U.S. 
Pat. No. 4,596,678), EP-A-126 300 (U.S. Pat. No. 4,596,679)). 
EP-A-0 320 235 describes a process for the preparation of aliphatic 
O-arylurethanes by reacting (cyclo)aliphatic polyamines with urea and 
aromatic hydroxyl compounds. 
Although the thermolysis of (cyclo)aliphatic and in particular aromatic 
monourethanes and diurethanes to give the corresponding isocyanates and 
alcohol has been known for some time and can be carried out either in the 
gas phase at elevated temperature or in the liquid phase at comparatively 
low temperature, it is, in particular, the undesired side reactions and in 
particular the tendency of the reaction mixtures to form deposits, resins 
and blockages in the reactors and work-up equipment that impair the 
economic efficiency of the processes in the long term. 
Numerous patent applications therefore describe, for example, chemical 
methods, e.g. the use of specific catalysts (DE-C-1 022 222 (U.S. Pat. No. 
2,692,275) or DE-B-19 44 719 (U.S. Pat. No. 3,734,941)) or catalysts in 
combination with inert solvents (U.S. Pat. No. 3,919,279 or DE-A-2 635 490 
(U.S. Pat. No. 4,081,472)), for improving the yield in the thermolysis of 
urethane. 
The thermolysis of hexamethylene-1,6-diethylurethane under pressure in the 
presence of dibenzyltoluene as solvent and in the presence of a catalyst 
mixture comprising methyl toluenesulfonate and diphenyltin dichloride for 
the preparation of hexamethylene 1,6-diisocyanate is described, for 
example, in DE-A-3 108 990 (U.S. Pat. No. 4,388,246). No details are given 
on the preparation and isolation of the starting component and the 
purification and any recovery of the solvent and of the catalyst mixture, 
and it is therefore not possible to judge the economic efficiency of the 
process. 
According to EP-B-0 078 005 (U.S. Pat. No. 4,482,499), urethanes can easily 
be cleaved into the isocyanate and alcohol in a carbon-containing 
fluidized bed without using a catalyst. According to DE-A-32 27 748 (U.S. 
Pat. No. 4,613,466), hexamethylenedialkylurethanes can be cleaved to give 
hexamethylene diisocyanate in the gas phase at above 300.degree. C. in the 
presence or absence of gas-permeable packing materials, for example made 
of carbon, steel, brass, copper, zinc, aluminum, titanium, chromium, 
cobalt or quartz. According to DE-A-32 48 018 (U.S. Pat. No. 4,613,466), 
this process is carried out in the presence of hydrogen halides and/or 
hydrogen halide donors. 
However, this process cannot achieve a yield of hexamethylene diisocyanate 
of &gt;90%, since the cleavage products partially recombine. The necessary 
purification of the hexamethylene 1,6-diisocyanate by distillation may 
further increase the yield losses. 
Furthermore, EP-A-54 817 (U.S. Pat. No. 4,386,033) discloses that 
monocarbamates can be cleaved in good yields at relatively low 
temperatures, preferably under reduced pressure, in the presence or 
absence of catalysts and/or stabilizers and without using solvents. The 
cleavage products (monoisocyanate and alcohol) are removed from the 
boiling reaction mixture by distillation and are collected separately by 
fractional condensation. Ways of partially purging the reaction mixture in 
order to remove the by-products formed on thermolysis are described in 
general form. No mention is made of any industrial use for these residues. 
According to EP-A-0 061 013 (U.S. Pat. No. 4,388,246), the thermolysis of 
aliphatic, cycloaliphatic or aromatic polycarbamates is carried out at 
from 150.degree. to 350.degree. C. and at from 0.001 to 20 bar in the 
presence of inert solvents, in the presence or absence of catalysts and 
hydrogen chloride, organic acid chlorides, alkylating substances or 
organotin(IV) chlorides as aids. The by-products formed can be removed 
continuously from the reactor, for example with the reaction solution, and 
a corresponding amount of fresh or recovered solvent simultaneously 
metered in. Disadvantages in this process are, for example, that the use 
of refluxing solvent results in a reduction in the space-time yield of 
polyisocyanates and, in addition, a large amount of energy is necessary, 
including, for example, for recovery of the solvents. Furthermore, the 
aids employed, which are volatile under the reaction conditions, may 
result in contamination of the cleavage products. Also striking is the 
amount of residues, which is high relative to the polyisocyanate formed 
and, like the low operating pressure, casts doubt on the economic 
efficiency and reliability of the procedure in industry. 
A process for the continuous thermolysis of carbamates, e.g. the 
cycloaliphatic diurethane 
5-(ethoxycarbonylamino)-1-(ethoxycarbonylaminomethyl)-1,3,3-trimethylcyclo 
hexane, which is fed along the inside of a tubular reactor in liquid form 
in the presence of a high-boiling solvent, is described in EP-B-92 738 
(U.S. Pat. No. 4,692,550). This process has the disadvantages of low 
yields and low selectivity in the preparation of (cyclo)aliphatic 
diisocyanates. No results are given for a continuous procedure with 
recovery of the recombined or partially cleaved carbamate, and the work-up 
of the solvent containing the by-products and catalyst is not mentioned. 
EP-A-0 355 443 relates to a circulation process for the preparation of 
(cyclo)aliphatic diisocyanates by conversion of the corresponding diamines 
into diurethanes and thermolysis of the latter. This process minimizes the 
reductions in yield by recycling the product from the urethane cleavage 
step after reaction with alcohol into the urethanization step. 
Non-recyclable by-products are removed by distillative separation of the 
urethanization product mixture, the worthless residue being produced as 
the bottom product and all relatively low-boiling components, including 
the diurethane, being removed at the top of the column. 
This procedure, which involves high investment costs, also has the 
disadvantage of using large amounts of energy, since all the diurethane 
must be evaporated in the presence of catalysts, and, in addition, this 
must be done at a temperature level which is in the region of the urethane 
cleavage temperature. The isocyanate groups which form in the useful 
product react with the urethane groups of the residue to form relatively 
high-molecular-weight, yield-reducing by-products. 
It is an object of the present invention to prepare distillable organic 
polyisocyanates, preferably aliphatic and cycloaliphatic diisocyanates, in 
high selectivity and in high space-time yields in an inexpensive and 
simple manner without using expensive and/or hazardous starting materials 
or aids. 
We have found that, surprisingly, this object is achieved by partial 
purging of worthless by-products before the polyurethane cleavage. 
The present invention accordingly provides a multistep process for the 
continuous preparation of organic polyisocyanates, preferably aliphatic or 
cycloaliphatic diisocyanates, by reacting the corresponding organic 
polyamines, preferably aliphatic or cycloaliphatic diamines, with carbonic 
acid derivatives and alcohols to give polyurethanes, preferably 
diurethanes, and thermolysis thereof, which comprises 
a) reacting organic polyamines, preferably aliphatic or cycloaliphatic 
diamines, with urea and alcohols in the absence or preferably in the 
presence of dialkyl carbonates, alkyl carbamates or mixtures of dialkyl 
carbonates and alkyl carbamates, and in the absence or preferably in the 
presence of catalysts to give polyurethanes, preferably diurethanes, and 
simultaneously removing the resultant ammonia, 
b) removing the alcohol, the dialkyl carbonates and/or alkyl carbamates 
from the resultant reaction mixture and preferably recycling them into 
reaction step a), 
c) dividing the polyurethane-containing reaction mixture, preferably the 
aliphatic or cycloaliphatic diurethane-containing reaction mixture, 
separating one part, by distillation, into a useful product, which 
contains the polyurethanes, preferably diurethanes, and the relatively 
low-boiling by-products and is then combined with the other part of the 
reaction mixture, and a worthless residue, which is removed from the 
preparation process, 
d) continuously pyrolyzing some of the combined, polyurethane-containing 
reaction mixture, preferably the aliphatic or cycloaliphatic 
diurethane-containing reaction mixture, in the liquid phase in the absence 
of solvents in the presence of catalysts at from 200.degree. to 
300.degree. C. and at from 0.1 to 200 mbar, and removing the unthermolyzed 
component of the reaction mixture together with the resultant by-products 
and recycling these into reaction step a), 
e) separating the thermolysis products into a crude polyisocyanate, 
preferably a crude aliphatic or cycloaliphatic diisocyanate, and alcohol 
by rectification, and 
f) purifying the crude polyisocyanate, preferably the crude aliphatic or 
cycloaliphatic diisocyanate, by distillation. 
In a preferred embodiment, the top fraction produced on distillative 
purification of the crude polyisocyanate (f) is recycled into reaction 
step (a), the side fraction, which essentially comprises pure 
polyisocyanate, is fed to a storage tank, and the bottom fraction is 
recycled into reaction step (a) or (d) or (a) and (d). 
The process according to the invention allows distillable organic 
polyisocyanates, preferably aliphatic or cycloaliphatic diisocyanates, to 
be prepared easily in very good yields. The multistep process according to 
the invention has the advantages, in particular, of simple separation and 
removal or recycling of the dialkyl carbonates and/or alkyl carbamates 
formed as intermediates and of the alcohol and removal of the worthless, 
high-boiling by-products by partial purging of high-boiling components. 
In purely formal terms, the process according to the invention can thus be 
balanced schematically by means of the following equation: 
EQU R--(NH.sub.2).sub.n +n H.sub.2 CONH.sub.2 +n ROH.fwdarw.R(NCO).sub.n +2n 
NH.sub.3 +n ROH 
a) To prepare the monomeric polyurethanes, preferably (cyclo)aliphatic 
diurethanes, in reaction step (a), the polyamines, preferably diamines, 
are reacted with urea and an alcohol, expediently in an NH.sub.2 
group:urea: alcohol ratio of from 1:0.9 to 1.3:1 to 5, preferably from 
1:1.0 to 1.2:1.5 to 3, in the absence or preferably in the presence of 
dialkyl carbonates or preferably carbamates or mixtures of dialkyl 
carbonates and carbamates, and in the absence or preferably in the 
presence of catalysts at from 160.degree. to 300.degree. C., preferably at 
from 180.degree. to 250.degree. C., in particular at from 185.degree. to 
240.degree. C., and at a pressure from 0.1 to 60 bar, preferably from 1 to 
40 bar, depending on the alcohol used. These reaction conditions give 
reaction times of from 0.5 to 50 hours, preferably from 3 to 15 hours. 
Amines which are suitable for the preparation of the monomeric 
polyurethanes which can be used according to the invention as 
intermediates are those of the formula R(NH.sub.2).sub.n where R is a 
polyvalent, preferably divalent, organic radical, e.g. a substituted or 
unsubstituted, for example alkyl-substituted, aromatic or preferably 
linear or branched aliphatic or substituted or unsubstituted 
cycloaliphatic radical. Specific examples of aromatic polyamines are 2,4- 
and 2,6-tolylenediamine, 4,4'- 2,4'- and 2,2'-diaminodiphenylmethanes and 
the corresponding isomer mixtures. Examples of suitable aliphatic or 
cycloaliphatic polyamines are: 1,4-butanediamine, 
2-ethyl-1,4-butanediamine, 1,8-octanediamine, 1,10-decanediamine, 
1,12-dodecanediamine, 1,4-cyclohexanediamine, 2-methyl- and 
4-methyl-1,3-cyclohexanediamine, 1,3- and 1,4-diaminomethylcyclohexane- 
Preference is given to 2-methyl-1,5-pentanediamine, 2,2,4- and 
2,4,4-trimethyl-1,6-hexanediamine and in particular 1,6-hexanediamine and 
3-aminomethyl-3,5,5-trimethylcyclohexylamine. 
Suitable alcohols are in principle all aliphatic alcohols, but preference 
is given to those whose boiling points are sufficiently far from the 
boiling point of the polyisocyanate, preferably diisocyanate, obtained by 
thermolysis, so that highly quantitative separation of the thermolysis 
products, polyisocyanate, preferably diisocyanate, and alcohol, is 
possible. 
For these reasons, preference is therefore given to alcohols such as 
methanol, ethanol, n-propanol, n-butanol, isobutanol, n-pentanol, 
isopentanol, n-hexanol, isohexanols, cyclohexanol, 2-ethylhexanol, decanol 
or mixtures of said alcohols, but in particular n-butanol and/or 
isobutanol. 
As stated above, the reaction in step (a) is preferably carried out in the 
presence of dialkyl carbonates, expediently in an amount of from 0.1 to 30 
mol %, preferably from 1 to 10 mol %, or preferably alkyl carbamates, 
expediently in an amount of from 1 to 20 mol preferably from 5 to 15 mol 
%, based on the polyamine, preferably diamine. However, particular 
preference is given to mixtures of dialkyl carbonates and alkyl carbamates 
in said mixing ratios. Preferred dialkyl carbonates and/or carbamates are 
those whose alkyl radicals correspond to the alkyl radical of the alcohol 
used. 
In order to increase the reaction rate, the monomeric polyurethanes, 
preferably diurethanes, can be prepared in the presence of catalysts. 
These are expediently used in amounts of from 0.01 to 20% by weight, 
preferably from 0.05 to 10% by weight, in particular from 0.1 to 5% by 
weight, based on the weight of the polyamine, preferably diamine. Suitable 
catalysts are inorganic or organic compounds which contain one or more 
cations, preferably one cation of metals from groups IA, IB, IIA, IIB, 
IIIA, IIIb, IVA, IVB, VA, VB, VIB, VIIB and VIIIB of the Periodic Table, 
as defined in Handbook of Chemistry and Physics, 14th Edition, published 
by Chemical Rubber Publishing Co., 23 Superior Ave. N.E., Cleveland, Ohio, 
for example halides, such as chlorides and bromides, sulfates, phosphates, 
nitrates, borates, alkoxides, phenoxides, sulfonates, oxides, oxide 
hydrates, hydroxides, carboxylates, chelates, carbonates, thiocarbamates 
and dithiocarbamates. Specific examples which may be mentioned are the 
cations of the following metals: lithium, sodium, potassium, magnesium, 
calcium, aluminum, gallium, tin, lead, bismuth, antimony, copper, silver, 
gold, zinc, mercury, cerium, titanium, vanadium, chromium, molybdenum, 
manganese, iron and cobalt. The catalysts can also be used, without 
detectable disadvantages, in the form of their hydrates or ammoniates. 
Specific examples of typical catalysts are: lithium methoxide, lithium 
ethoxide, lithium propoxide, lithium butoxide, sodium methoxide, potassium 
tertbutoxide, magnesium methoxide, calcium methoxide, tin(II) chloride, 
tin(IV) chloride, lead acetate, lead phosphate, antimony(III) chloride, 
antimony(V) chloride, aluminum acetylacetonate, aluminum isobutylate, 
aluminum trichloride, bismuth(III) chloride, copper(II) acetate, 
copper(II) sulfate, copper(II) nitrate, 
bis(triphenylphosphinoxido)copper(II) chloride, copper molybdate, silver 
acetate, gold acetate, zinc oxide, zinc chloride, zinc acetate, zinc 
acetylacetonate, zinc octanoate, zinc oxalate, zinc hexylate, zinc 
benzoate, zinc undecylenate, cerium(IV) oxide, uranyl acetate, titanium 
tetrabutoxide, titanium tetrachloride, titanium tetraphenoxide, titanium 
naphthenate, vanadium(III) chloride, vanadium acetylacetonate, 
chromium(III) chloride, molybdenum(VI) oxide, molybdenum acetylacetonate, 
tungsten(VI) oxide, manganese(II) chloride, manganese(II) acetate, 
manganese(III) acetate, iron(II) acetate, iron(III) acetate, iron 
phosphate, iron oxalate, iron(III) chloride, iron(III) bromide, cobalt 
acetate, cobalt chloride, cobalt sulfate, cobalt naphthenate, nickel 
chloride, nickel acetate and nickel naphthenate, and mixtures thereof. 
It has proven advantageous for the resultant ammonia to be removed 
immediately from the reaction mixture, for example by distillation. The 
apparatus used for this purpose, for example a distillation column, is 
operated at from 60.degree. to 150.degree. C., preferably at from 
65.degree. to 120.degree. C., so that a coating of ammonium carbamate, 
which is formed in small amounts from ammonia and carbon dioxide due to 
decomposition of urea, can be avoided. 
b) The alcohol, the dialkyl carbonates, if formed or present in the 
reaction mixture, or alkyl carbamates, or mixtures of at least two of 
these components, are removed from the reaction mixture (a) obtained by, 
advantageously, continuous reaction, and preferably recycled into reaction 
step (a). In order to remove the components, the reaction mixture is 
advantageously decompressed from the pressure level of step (a) to a 
pressure in the range from 1 to 500 mbar, preferably from 10 to 100 mbar. 
This gives gaseous vapors which contain most of the alcohol and from 0 to 
30% by weight, preferably from 1 to 10% by weight, of dialkyl carbonate 
and/or from 1 to 50% by weight, preferably from 1 to 20% by weight, of 
alkyl carbamate, and a liquid product which essentially comprises the 
monomeric polyurethane, preferably diurethane, and possibly contains 
oligourea-polyurethanes and high-boiling oligomers. 
The vapors obtained are separated in subsequent, expediently distillative, 
purification steps, preferably by rectification, and the useful products 
(alcohol and alkyl carbamate) isolated in this operation are preferably 
recycled, individually or as a mixture, into reaction step (a) for 
formation of the monomeric polyurethanes. 
c) The liquid reaction mixture (c) containing the monomeric polyurethanes, 
preferably diurethanes, and possibly oligourea-polyurethanes and 
high-boiling oligomers which is obtained in reaction step (b) after 
removal of the vapors is divided into two sub-streams in a weight ratio of 
from 5 to 50:95 to 50 parts by weight, preferably from 10 to 30:90 to 70 
parts by weight. One of the two portions of equal size or preferably the 
smaller portion is separated by distillation in a conventional 
distillation unit, preferably a thin-film evaporator, at from 170.degree. 
to 240.degree. C., preferably at from 180.degree. to 230.degree. C., and 
at from 0.01 to 5 mbar, preferably from 0.1 to 2 mbar, into a useful 
product containing the polyurethanes, preferably diurethanes, and the 
relatively low-boiling by-products, and undistillable by-products, which 
are removed from the preparation process and usually discarded as 
worthless residue. The useful product is combined with the other portion, 
of equal size or preferably larger, and the combined reaction mixture 
containing polyurethanes, preferably diurethanes, is fed to thermolysis. 
This measure in step (c) limits the proportion of undistillable by-products 
in the reaction mixture, which form in the successive sub-reactions and 
would constantly accumulate in the reaction cycle due to recycling of 
useful starting materials, to a content of from 3 to 30% by weight, 
preferably from 5 to 20% by weight, and thus ensures that the reaction 
proceeds in high selectivity and without interruptions. 
d) The reaction mixture containing polyurethanes, preferably diurethanes, 
obtained in reaction step (c) is partially continuously thermolyzed in a 
suitable apparatus in the absence of solvent in a liquid phase in the 
presence of catalysts at from 200.degree. to 300.degree. C., preferably 
from 220.degree. to 280.degree. C., and under a reduced pressure of from 
0.1 to 200 mbar, preferably from 5 to 80 mbar. The conversion of 
polyurethane to polyisocyanate, preferably of diurethane to diisocyanate, 
in the thermolysis apparatus can be selected substantially freely 
depending on the polyurethane used and is expediently in the range from 10 
to 95% by weight, preferably from 40 to 85% by weight, of the polyurethane 
feed. The unthermolyzed component of the reaction mixture, which contains 
unreacted polyurethanes, oligourea-polyurethanes, high-boiling oligomers 
and other reusable and worthless by-products, is separated off, purged 
continuously from the thermolysis apparatus and recycled directly or, if 
desired, after reaction with alcohol, into reaction step (a). 
Examples of catalysts used for the chemical cleavage of the polyurethanes 
are the above-mentioned inorganic and organic compounds which catalyze the 
formation of urethanes. 
Compounds which have proven particularly successful and are therefore 
preferred are dibutyltin dilaurate, iron (III) acetylacetonate, cobalt 
(II) acetylacetonate, zinc acetylacetonate and tin(II) dioctanoate. 
Examples of suitable thermolysis apparatuses are cylindrical reactors, e.g. 
tubular furnaces or preferably evaporators, for example thin-film or bulk 
evaporators, e.g. Robert evaporators, Herbert evaporators, caddle-type 
evaporators and preferably heated cartridge evaporators. 
e) The products formed on thermolysis, which are composed principally of 
alcohol, polyisocyanate, preferably diisocyanate, and partially cleaved 
polyurethanes, are then separated into alcohol and a crude polyisocyanate 
mixture having a polyisocyanate content of from 85 to 99% by weight, 
preferably from 95 to 99% by weight, advantageously with the aid of one or 
more distillation columns, preferably by rectification, at from 
100.degree. to 220.degree. C., preferably from 120.degree. to 170.degree. 
C., and at from 1 to 200 mbar, preferably at from 5 to 50 mbar. The 
relatively high-boiling by-products obtained on distillative separation 
and in particular the uncleaved and partially cleaved polyurethanes are 
preferably recycled into the thermolysis apparatus. 
f) The crude polyisocyanate mixture preferably obtained by rectification is 
purified by distillation at from 100.degree. to 180.degree. C. and at from 
1 to 50 mbar, the individual fractions being recycled or isolated as pure 
product. In the case of purification distillation, which is preferred, the 
top fraction, which preferably comprises polyisocyanate, in particular 
diisocyanate, is, as stated above, recycled into reaction step (a), the 
polyurethane formation, if appropriate after reaction of the free 
isocyanate groups with alcohol, the side fraction, which comprises pure 
polyisocyanate, in particular diisocyanate, preferably in a purity of at 
least 98% by weight, in particular greater than 99% by weight, is taken 
off and stored, and the bottom fraction, in which the essential components 
are the partially cleaved polyurethanes and polyisocyanates, is preferably 
recycled into the thermolysis apparatus. In other variants, however, the 
bottom fraction can be recycled into the distillation column (e) for 
removal of crude polyisocyanate and alcohol or into reaction step (a), the 
polyurethane formation. It is also possible to divide the bottom fraction 
into 2 or 3 product streams, which are preferably recycled into the 
polyurethane formation (a) and the thermolysis apparatus (d) and, if 
desired, into the distillation column (e). 
The multistep process according to the invention for the continuous 
preparation of organic polyisocyanates with recycling and purging of 
by-products allows distillable polyisocyanates, preferably diisocyanate, 
to be prepared in high selectivity and very good yield. The process 
according to the invention is particularly suitable for the preparation of 
aliphatic diisocyanates, such as 2-methylpentane 1,5-diisocyanate, 
isomeric aliphatic diisocyanates having 6 carbon atoms in the alkylene 
radical, and mixtures thereof, and preferably hexamethylene 
1,6-diisocyanate, and cycloaliphatic diisocyanates, in particular 
3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate, by an economical 
method. 
The polyisocyanates prepared are eminently suitable for the preparation of 
plastics containing urethane, isocyanurate, amide and/or urea groups by 
the polyisocyanate polyaddition process. They are furthermore used for the 
preparation of polyisocyanate mixtures which have been modified by means 
of urethane, biuret and/or isocyanurate groups. Such polyisocyanate 
mixtures of aliphatic or cycloaliphatic diisocyanates are used in 
particular for the production of light-stable polyurethane paints and 
coatings.

EXAMPLE 1 
0.879 kg of urea, 0.805 kg of hexamethylene-1,6-diamine and 0.089 kg of 
n-butanol, as well as 3.333 kg of a product mixture containing cleavage 
butanol, part of the reaction mixture from the cleavage of urethane, 
containing the resultant by-products, which principally comprised 
relatively high-molecular-weight compounds containing isocyanurate, 
allophanate, urea and polyurethane groups, and the top fraction from the 
purification distillation of hexamethylene diisocyanate, were added per 
hour at 220.degree.-230.degree. C. and 12 bar into the first reactor of a 
three-stage stirred reactor cascade fitted with heated columns and head 
condensers and a pressure-retention means, containing a mixture of 
hexamethylene-1,6-dibutylurethane and n-butanol in addition to 
hexamethylene-oligourea-polybutylurethanes, dibutyl carbonate and butyl 
carbamate. The resultant ammonia escaping from the refluxing reaction 
mixture was removed via the columns and freed virtually quantitatively 
from butanol in the downstream condensers by fractional condensation. 
The product from the 3rd reactor in the cascade was continuously 
decompressed in a tank operated at 50 mbar. The gaseous vapors passed 
directly into a rectification column, likewise operated at 50 mbar, from 
which about 2.8 kg/h of n-butanol were obtained at the top, 0.06 kg/h of a 
dibutyl carbonate-rich azeotrope were obtained at the side and 0.234 kg/h 
of butyl carbamate were obtained in the stripping section. The n-butanol 
and the butyl carbamate were recycled into the reactor cascade. 
The liquid product from the decompression tank was divided in the 
approximate weight ratio 3:1, and the smaller part was fed to a thin-film 
evaporator, which was operated at 220.degree. C. and 1 mbar, giving 0.071 
kg/h of undistillable residue at the bottom (purging of high-boiling 
components). The hexamethylene-1,6-dibutylurethane which condensed at the 
top was combined with the majority of the liquid product from the 
decompression tank, the dibutyltin dilaurate catalyst unavoidably removed 
with the residue was replaced, and the product was fed in the melt-liquid 
state via a metering device to a steam-heated evaporator reactor with a 
reaction capacity of 2.5 l for homogeneously catalyzed thermolysis. The 
thermolysis at a conversion of about 55% with respect to the 3.81 kg/h of 
hexamethylene-1,6-dibutylurethane employed was carried out at 30 mbar with 
vigorous boiling of the reaction mixture. The gaseous vapors were passed, 
for separation, into a rectification column, from the top of which 1.1 
kg/h of liquid cleavage butanol were removed. Approximately 95% by weight 
crude diisocyanate was obtained at the side. Uncleaved diurethane and 
6-isocyanatohexylbutylurethane were recycled into the evaporator reactor. 
The crude hexamethylene 1,6-diisocyanate obtained in this way was subjected 
to purification distillation, 1.115 kg/h of hexamethylene 1,6-diisocyanate 
having a purity of &gt;99% being obtained at the side of a column operated at 
30 mbar. The bottom product from the purification distillation, which was 
predominantly composed of 6-isocyanatohexylbutylurethane and its 
relatively high-molecular-weight oligomers, was recycled directly into the 
evaporator reactor or into the subsequent rectification column. 
The top product from the purification distillation, combined with the 
cleavage butanol and the product from the urethane Cleavage evaporator 
reactor, which contained the high-boiling by-products, was passed directly 
back into reaction step a), the three-stage stirred reactor cascade. The 
overall selectivity for the conversion of hexamethylene-1,6-diamine feed 
into hexamethylene 1,6-diisocyanate was 97%. 
EXAMPLE 2 
The procedure was similar to that of Example 1, but the 
hexamethylenediamine derivatives in the stirred reactor cascade were 
replaced by corresponding 2-methylpentamethylene-1,5-diamine derivatives, 
dibutyl carbonate and butyl carbamate. 0.886 kg of urea, 0.810 kg of 
2-methylpentamethylene-1,5-diamine and 0.097 kg of n-butanol, as well as 
3.543 kg of a product mixture of cleavage butanol, part of the reaction 
mixture from the cleavage of urethane and the top fraction from the 
purification distillation of diisocyanate were added per hour to this 
mixture at from 220.degree. to 230.degree. C. and at 12 bar. 
The product from the 3rd reactor in the cascade was decompressed at 50 
mbar, and the vapors were fed in gas form into a rectification column; 
about 2.8 kg/h of n-butanol were obtained at the top, 0.06 kg/h of a 
dibutyl carbonate-rich azeotrope were obtained at the side and 0.245 kg of 
butyl carbamate were obtained in the stripping section. The n-butanol and 
the butyl carbamate were recycled into the reactor cascade. 
0.087 kg/h of undistillable residue were obtained in the bottom of the 
thin-film evaporator for partial removal of high-boiling components. 
The cleavage of urethane at a conversion of about 55% with respect to the 
3.80 kg/h of 2-methylpentamethylene-1,5-diurethane employed was carried 
out at 30 mbar. 1.1 kg/h of liquid cleavage butanol were obtained at the 
top of the downstream rectification column and about 95% by weight crude 
diisocyanate was obtained at the side. Purification distillation gave 
1.110 kg/h of 2-methylpentamethylene 1,5-diisocyanate. This gave an 
overall selectivity for the conversion of 
2-methylpentamethylene-1,5-diamine feed into 2-methylpentamethylene 
1,5-diisocyanate of 96%. 
EXAMPLE 3 
The procedure was similar to that of Example 1, but the 
hexamethylenediamine derivatives in the stirred reactor cascade were 
replaced by corresponding 3-aminomethyl-3,5,5-trimethylcyclohexylamine 
derivatives, dibutyl carbonate and butyl carbamate. 0.625 kg of urea, 
0.839 kg of 3-aminomethyl-3,5,5-trimethylcyclohexylamine and 0.061 kg of 
n-butanol, as well as 2.297 kg of a product mixture of cleavage butanol, 
part of the reaction mixture from the cleavage of urethane and the top 
fraction from the purification distillation of diisocyanate were added per 
hour to this mixture at from 220.degree. to 230.degree. C. and at 12 bar. 
The product from the 3rd reactor in the cascade was decompressed at 50 
mbar, and the vapors were fed in gas form into a rectification column; 
about 2.1 kg/h of n-butanol were obtained at the top, 0.05 kg/h of a 
dibutyl carbonate-rich azeotrope were obtained at the side and 0.175 kg of 
butyl carbamate were obtained in the stripping section. The n-butanol and 
the butyl carbamate were recycled into the reactor cascade. 
0.044 kg/h of undistillable residue were obtained in the bottom of the 
thin-film evaporator for partial removal of high-boiling components. 
The cleavage of urethane at a conversion of about 60% with respect to the 
2.94 kg/h of 3-urethanomethyl-3,5,5-trimethylcyclohexylurethane employed 
was carried out at 20 mbar. 0.81 kg/h of liquid cleavage butanol were 
obtained at the top of the downstream rectification column and about 95% 
by weight crude diisocyanate was obtained at the side. Purification 
distillation gave 1.060 kg/h of 
3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate. This gave an 
overall selectivity for the conversion of 
3-aminomethyl-3,5,5-trimethylcyclohexylamine feed into 
3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate of 98%. 
EXAMPLE 4 
The procedure was similar to that of Example 1, but the 
hexamethylenediamine derivatives in the stirred reactor cascade were 
replaced by corresponding 2,2,4(2,4,4)-trimethylhexamethylene-1,6-diamine 
derivatives, dibutyl carbonate and butyl carbamate. 0.746 kg of urea, 
0.923 kg of 2,2,4(2,4,4)-trimethylhexamethylene-1,6-diamine and 0.088 kg 
of n-butanol, as well as 3.379 kg of a product mixture of cleavage 
butanol, part of the reaction mixture from the cleavage of urethane and 
the top fraction from the purification distillation of diisocyanate were 
added per hour to this mixture at from 220.degree. to 230.degree. C. and 
at 12 bar. 
The product from the 3rd reactor in the cascade was decompressed at 50 
mbar, and the vapors were fed in gas form into a rectification column; 
about 2.9 kg/h of n-butanol were obtained at the top, 0.06 kg/h of a 
dibutyl carbonate-rich azeotrope were obtained at the side and 0.239 kg of 
butyl carbamate were obtained in the stripping section. The n-butanol and 
the butyl carbamate were recycled into the reactor cascade. 
0.074 kg/h of undistillable residue were obtained in the bottom of the 
thin-film evaporator for partial removal of high-boiling components. 
The cleavage of urethane at a conversion of about 50% with respect to the 
3.99 kg/h of 2,2,4(2,4,4)-trimethylhexamethylene-1,6-diamine employed was 
carried out at 30 mbar. 0.95 kg/h of liquid cleavage butanol were obtained 
at the top of the downstream rectification column and about 95% by weight 
crude diisocyanate was obtained at the side. Purification distillation 
gave 1.170 kg/h of 2,2,4(2,4,4)-trimethylhexamethylene 1,6-diisocyanate. 
This gave an overall selectivity for the conversion of 
2,2,4(2,4,4)-trimethylhexamethylene-1,6-diamine feed into 
2,2,4(2,4,4)-trimethylhexamethylene 1,6-diisocyanate of 97%. 
EXAMPLE 5 
The procedure was similar to that of Example 1, but the urethanization 
reaction was carried out using a reaction system comprising a reactor 
fitted with heated column and head condenser and a downstream reaction 
column containing 10 trays. About 2.8 kg/h of n-butanol vapor were fed 
into the bottom of the reaction column and passed in gas form in 
countercurrent to the liquid product stream. n-Butanol vapors from the top 
of the reaction column passed directly into the vapor space of the 
reactor. 
The starting materials and the recycling streams from the other reaction 
steps were fed into the reactor. 
The liquid reaction product from the reaction column was decompressed 
continuously in a tank operated at 50 mbar. 
EXAMPLE 6 
The procedure was similar to that of Example 1, but the reaction mixture 
obtained from the urethane synthesis was fed into a column operated at 500 
mbar. About 1.7 kg/h of n-butanol were removed from the top of this 
n-butanol column. The n-butanol was not condensed, but fed, after 
superheating, into the bottom of a downstream stripping column operated at 
100 mbar. The bottom product from the n-butanol column was fed to the top 
of the stripping column and freed from residual n-butanol, but in 
particular from dibutyl carbonate and butyl carbamate, by the n-butanol 
vapor rising in countercurrent. 
The liquid product from the stripping column was divided approximately in 
the weight ratio 3:1 and fed both to the thin-film evaporator for partial 
removal of high-boiling components and to the cleavage of urethane. 
The gaseous vapors from the stripping column passed into a rectification 
column operated at 50 mbar for separation into the discharge streams 
n-butanol, dibutyl carbonate and butyl carbamate mentioned in Example 1. 
EXAMPLE 7 
The procedure was similar to that of Example 1, but the gaseous vapors from 
the cleavage of urethane were passed into a first rectification column at 
the top of which gaseous n-butanol and diisocyanate were obtained. This 
gaseous stream passed into the purification distillation column comprising 
a main column, at the top of which 1.1 kg/h of liquid cleavage butanol 
were obtained, and an ancillary column, which gave 1.115 kg/h of 
hexamethylene 1,6-diisocyanate at the side. 
The n-butanol top product from the main column and the top product from the 
ancillary column, combined with the product from the urethane cleavage 
evaporator reactor, which contained the high-boiling by-products, passed 
directly back into reaction step a), the three-stage reactor cascade. 
The bottom product from the purification distillation column served as 
reflux for the rectification column downstream of the urethane cleavage.