Process and device for preparing (cyclo)aliphatic biuret groups-containing polyisocyanates

In a process and an apparatus for preparing (cyclo)aliphatic polyisocyanates containing biuret groups from (cyclo)aliphatic diisocyanates and steam or a substance capable of splitting off water as reactants which are mixed with one another in a stirred reactor, the reactants are passes in countercurrent through a cascade-type stirred reactor comprising at least two stages. The dispersion of the gaseous reactant in the liquid reactant is reinforced by baffles installed in the stirred reactor. As baffles, use is made of discs provided with central openings and arranged at a distance from one another and/or strip baffles running in the longitudinal direction of the stirred reactor.

The invention relates to a process and an apparatus for preparing 
polyisocyanates containing biuret groups from cyclo(aliphatic 
diisocyanates using steam or a substance capable of splitting off water as 
reactants which are mixed with one another in a stirred reactor. 
(Cyclo)aliphatic polyisocyanates containing biuret groups are used, inter 
alia, in high-quality light and weather-resistant, two-component PUR 
surface coatings. Other applications such as adhesives and dispersions are 
known. An overview of the literature is given in DE 34 03 277. 
(Cyclo)aliphatic polyisocyanates containing biuret groups are prepared by 
reacting the diisocyanates with a certain amount of biuret-forming agent 
(water or a substance capable of splitting off water) at from 100 to 
200.degree. C. Subsequently, the excess monomeric diisocyanate is removed 
from the crude product thus formed by single-stage or multistage 
distillation. When t-BuOH (or another substance capable of splitting off 
water) is used as a biuret-forming agent, the urethane formed is 
catalytically dissociated into isobutene, carbon dioxide and an 
isocyanatoamine intermediate. However, this requires high reaction 
temperatures (&gt;140.degree. C.) and the reaction products take on a yellow 
discoloration. This is unfavorable since colorless products are required 
for application reasons, eg. in the case of a clear surface coating. In 
addition, the use of substances capable of splitting off water (eg. 
OH-containing molecules) as biuret-forming agents result in formation of 
by-products which do not have a biuret structure and impair the storage 
stability of the desired product or cause other process problems. Water is 
therefore preferred as biuret-forming agent. However, insoluble areas are 
generally formed during the reaction and the products obtained have a poor 
storage stability in terms of redissociation into the monomers. As a 
result, the limit value requiring statutory labeling of 0.5% of free 
monomeric diisocyanate is quickly exceeded, in particular on storage above 
room temperature. Proposals have been made for avoiding these 
disadvantages, (see, for example, EP 259 233 and EP 251 952). These 
documents describe the use of catalytic amounts of protic acids for 
avoiding by-product formation in the synthesis of aliphatic and 
cycloaliphatic polyisocyanates containing biuret groups. 
Nevertheless, use of conventional stirred vessels does not succeed in 
completely reacting the water used as biuret-forming agent with the 
diisocyanate. Water vapor and diisocyanate escape together with the carbon 
dioxide formed in the reaction, in accordance with the existing partial 
pressure. This gas mixture condenses at cold places in the reactor and, in 
particular in the downstream off-gas cooler. There, the diisocyanate 
reacts with the water vapor to form polyureas which finally leads to 
blocking of the off-gas lines and the off-gas condensor. Reliable, 
long-term operation is not possible in this way. 
It is an object of the present invention to provide a reaction procedure 
and an apparatus suitable for this purpose in which the escaping off-gas 
is virtually free of water vapor and therefore no measurable polyurea 
formation occurs in the off-gas system. 
We have found that this object is achieved by the reactants being conveyed 
in countercurrent through a cascade-type stirred reactor comprising at 
least two stages. This effects a fine distribution of the steam 
introduced. The ring gas bubbles lose water vapor on their way through the 
isocyanate solution and become enriched in carbon dioxide. 
The multistage nature of the cascade and the baffles installed in the 
stirred reactor, which baffles prevent the reactants from simply flowing 
straight through the stirred reactor, considerably increases the residence 
time of the gas bubbles in the liquid, so that complete absorption of the 
steam and thus complete reaction can be achieved. 
According to a particularly advantageous embodiment of the process of the 
present invention, the distribution of the gaseous material in the liquid 
reactant is improved by baffles installed in the stirred reactor. The 
baffles which can be used are disks provided with central openings and 
arranged at a distance from one another. In addition, strip baffles 
running in the longitudinal direction of the stirred reactor can be used. 
From 10 to 95% by volume of nitrogen and/or carbon dioxide can be mixed 
into the reactant used as biuret-forming agent, preferably steam. The 
reaction is carried out at from 60 to 200.degree. C., preferably from 100 
to 150.degree. C. The off-gas flowing from the top end of the stirred 
reactor is preferably scrubbed with cold (cyclo)aliphatic diisocyanate 
which is subsequently fed to the process. If the off-gas formed in the 
reaction is conveyed via an off-gas condensor which is additionally 
flushed with cold diisocyanate, no polyurea residues can be detected in 
the off-gas system even when the process has been running for a long time. 
For carrying out the process, the invention provides a stirred reactor 
which comprises an upright tubular vessel in which there is fixed, 
parallel to the longitudinal axis and able to rotate, a drive shaft to 
which are fixed at least two disk stirrers at a distance from one another, 
where between these disk stirrers there are arranged disk baffles fixed to 
the inner wall of the vessel and having a central opening. It has been 
found that, depending on the reaction parameters, it is advantageous to 
use from 2 to 6 disk stirrers and from 1 to 5 disk baffles. The ratio of 
the opening of the disk baffles to their total area is derived from the 
stirrer size. 
According to one embodiment of the stirred reactor of the present 
invention, the inner wall of the reactor can be provided with strip 
baffles running parallel to the longitudinal axis of the reactor and 
extending radially inwards. These can preferably be fixed so as to leave a 
gap between the baffle and the inner wall of the reactor. Advantageously, 
at least 4 strip baffles equally spaced around the reactor wall are 
provided. The strip baffles are standardized, their width being 0.1 D, 
where D is the diameter of the vessel. 
The stirred reactor can be surrounded by a heatable jacket. Advantageously, 
the top end of the stirred reactor is connected to a cooling vessel in the 
lower part of which a cooler is installed. Above this cooler there is 
located an injector for the liquid reactant. This unit opens into an 
off-gas line. Advantageously, the ratio of the height of the stirred 
reactor to its diameter is in the range from 2 to 6 and is preferably 
greater than 4.5. 
Reactors of the above-described construction enable the process of the 
present invention to be carried out particularly advantageously.

In the known plant shown in FIG. 1, the reaction for preparing 
(cyclo)aliphatic polyisocyanates containing biuret groups is carried out 
in a simple stirred reactor 1. Hexamethylene diisocyanate (HDI) as one of 
the two reactants is fed from a vessel 2 via a pump 3 through a line 4 to 
the reactor from above. Catalysts, for example, strong inorganic Lewis or 
Brenstedt acids (cf. DE-A-15 43 178) and/or salts of nitrogenous bases and 
inorganic and/or organic acids (cf. DE-A-19 31 055) can be introduced 
through line 5. In the experimental plant, di-2-ethylhexyl phosphate was 
used as catalyst. As second reactant, steam diluted with gaseous nitrogen 
was introduced through line 6 into the lower part of the stirred reactor. 
The off-gases formed in the reaction, in particular CO.sub.2, are 
conducted away from the top of the stirred vessel 1 through the line 7. 
The product formed in the reaction, namely the polyisocyanate containing 
biuretic groups is taken from the lower end of the vessel 1 through line 8 
and conveyed via the pump 9 to a receiver 10. In the stirred vessel 1 
there is arranged a disk stirrer 12 rotatable about a vertical shaft 11. 
The temperature is the stirred vessel 1 is adjusted by means of a heatable 
jacket 13 surrounding the vessel. 
In the plant according to the present invention shown in FIG. 2, the 
stirred vessel, here likewise denoted by 1, has a cascade-type 
construction, as is shown in detail in FIG. 3. In this vessel four disk 
stirrers 12 are arranged at intervals along the rotating shaft 11 running 
parallel to the longitudinal axis of the vessel 1. Between the disk 
stirrers 12 there are located three disk baffles 14 which are fixed to the 
wall of the stirred vessel 1 and have a circular central opening. In the 
embodiment shown in FIG. 3, the stirred vessel 1 has an internal diameter 
of 100 mm. The opening of the disk baffles 14 has a diameter of 52 mm. The 
disk stirrers 12, of which one is shown in FIG. 4, can have different 
dimensions. In general, standard disk stirrers are used. In the present 
case, the disk of the bottom disk stirrer 12 has a diameter of 37.5 mm. 
The external diameter including the vertical stirring surfaces 12A is 50 
mm. The stirring surfaces 12A have a rectangular shape with a height of 10 
mm and a width of 12.5 mm. The other three disk stirrers 12 have an 
internal diameter of 30 mm and an external diameter of 40 mm, with the 
dimensions of the disks being 8.times.10 mm. In the interior of the 
stirred vessel 1 there are arranged four strip baffles 15 parallel to the 
longitudinal axis of the stirred vessel spaced at 90.degree. from one 
another with a spacing from the wall of 1 mm. The rotating shaft 11 
together with the disk stirrers 12 attached thereto rotates at from 500 to 
900 revolutions per minute. 
In the plant according to the present invention shown in FIG. 2, the 
reactant HDI is also fed to the stirred vessel 1 from the vessel by means 
of the pump 3 through the line 4. The line 4 here leads via an injector 18 
into a cooling vessel 16 fitted with a cooler 17. The off-gas flowing from 
the stirred vessel 1 through line 7A is cooled by means of this cooler. 
Deposition of residues is thereby prevented. From the upper part of the 
cooling vessel 16, the off-gas consisting essentially of CO.sub.2 flows 
away through line 7A. The steam/nitrogen mixture flowing in through line 6 
is fed in at the lower end of the stirred vessel 1. In the embodiment 
shown in FIG. 3, the line 6 enters at the upper end of the stirred vessel 
but the steam flows out at the lower end of the stirred vessel 1 at 6A. 
The product is taken from the lower end of the stirred vessel 1 at 1A and 
is conveyed by means of the pump 9 vial the line 8 to the receiver 10. 
Both the known plant shown in FIG. 1 and the plant according to the present 
invention shown in FIG. 2 were operated continuously and semicontinuously. 
In the semicontinuous procedure, HDI and catalyst are initially charged 
and heated. Subsequently, the mixture of steam and nitrogen is introduced 
continuously, the reaction taking a total of about 3-4 hours. In the 
through-flow procedure, HDI and catalyst are continuously metered into the 
reactor 1 and the mixture of steam and nitrogen is introduced in parallel 
thereto. The crude product of HDI-biuret oligomers and excess monomers is 
continuously discharged to the vessel 10. The crude product is 
subsequently worked up by means of distillation. 
Continuous and batchwise experiments were carried out using the 
abovedescribed plants comprising a simple stirred vessel (FIG. 1) and 
cascade-type stirred vessel (FIG. 2). It has been found that the 
conversion of the water used is significantly higher in the cascade=type 
stirred vessel both in the continuous and the batchwise process. This can 
be seen in the difference between the "actual NCO value" and the "ideal 
NCO value" of the crude product, which are shown in Tables 1 and 2 below. 
The "ideal NCO value" can be calculated using the assumption the 1 mol of 
water reacts with exactly 3 mol of NCO groups. If the actual conversion is 
lower than the ideal conversion, ie. the NCO value is higher (pure HDI has 
50% of NCO), then not all the water could have reacted and the reactor 
used does not have optimum efficiency. An NCO value lower than the "ideal 
NCO value" means that additional side reactions have taken place. 
The experiments carried out using the cascaded stirred vessel of the 
present invention and a simple stirred vessel have given the results shown 
in the tables below. 
TABLE 1 
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Semicontinuous process 
Semicontinuous 
Reaction conditions 
cascaded stirred vessel 
simple stirred vessel 
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Amount of water, g 
24.05 9.00 
Amount of HDI, g 2230.00 600.00 
NCO value (actual) 42.70% 40.70% 
NCO value (ideal, 3 43.10% 40.37% 
mol of NCO/mol of about 50 minutes about 50 
minutes 
H.sub.2 O) 
Steam introduction 3 to 4 hours 3 to 4 hours 
Total residence time 0.20% 
0.20% 
Cat. (di-2-ethylhexyl 
phosphate), 
mol% based on HDI 130.00 130.00 
Temperature, .degree. C. atmospheric atmospheric 
Operating Pressure 
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TABLE 2 
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Continuous process 
Continuous 
Reaction conditions 
cascaded stirred vessel 
simple stirred vessel 
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Amount of water, g/h 
10.02 9.48 
Amount of HDI, g/h 1000.00 1000.00 
NCO value (actual) 43.50% 
44.73% 
NCO value (ideal, 3 43.60% 43.97% 
mol of NCO/mol of 3 hours 1 hour 
H.sub.2 O) 
Residence time 
Cat. (di-2-ethylhexyl 0.20% 0.20% 
phosphate), 
mol% based on HDI 130.00 130.00 
Temperature, .degree. C. atmospheric atmospheric 
Operating Pressure 
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Overall, the experimental results show that the process of the present 
invention using the novel apparatus provided for this purpose leads to 
significantly better results than the known processes carried out using a 
simple stirred vessel.