CA2209139A1

CA2209139A1 - Preparation of solutions of one or more isocyanates which contain no appreciate amounts of components which give a color, and also the isocyanates themselves

Info

Publication number: CA2209139A1
Application number: CA 2209139
Authority: CA
Inventors: Rupert Kraus; Filip Nevejans; Peter Van Den Abeel; Helmut Tesch; Wilfried Seyfert; Bernd Bruchmann
Original assignee: Individual
Current assignee: BASF SE
Priority date: 1996-06-26
Filing date: 1997-06-25
Publication date: 1997-12-26
Also published as: JPH1095763A; CN1172125A; MX9704810A; KR980002016A; EP0816333A1

Abstract

In a process for preparing solutions containing at least one isocyanate, the above-defined solution is subjected to a treatment with hydrogen after the phosgenation and before the complete removal of the solvent and of residual phosgene. A
process for preparing isocyanates or isocyanate mixtures is provided, and the isocyanates thus prepared can be used for preparing polyurethanes, in particular polyurethane foams.

Description

PREPARATION OF SOLUTIONS OF ONE OR MORE ISOC~ANATES WI~ICH
CONTAIN NO APPRECIATE AMOUNTS OF COMPONENTS WHICH GIVE A
COLOR, AND ALSO THE ISOCYANATES THEMSELVES

The present invention relates to a process for preparing solution6 containing at least one isocyanate, where the above-defined solution is, after the phosgenation and before complete lo removal of the solvent and of residual phosgene, subjected to treatment with hydrogen hydrogenation treatment, a process for preparinq isocyanates or isocyanate mixtures, and also the use of the isocyanates thus prepared for preparing polyurethanes, in particular polyurethane foams. It may be noted here that the isocyanates obtained contain no appreciable amounts of components which give a color, have a low content of hydrolyzable chlorine and have a lighter color than the isocyanate6 of the prior art.

Isocyanates and isocyanate mixtures are prepared according to known methods by phosgenation of the corresponding amines. For polyurethane foams, use is made of, for example, difunctional or polyfunctional aromatic isocyanates of the diphenylmethane diisocyanate group (MDI). owing to the preparation process, the phosgenation and the subsequent work-up (removal of the solvent distilling off monomeric MDI) often results in comparatively dark products which in turn give yellowish polyurethane (PUR) foams or other PUR materials. This is undesirable since the coloration allows slight inhomogeneities to occur, e.g. as streaks in the foams obtained. Light-colored isocyanates or isocyanates which contain a reduced amount of components which give a color are therefore preferred as raw materials. For this reason, there have been many attemps to obtain polyisocyanates, in particular those o~ the diphenylmethane diisocyanate group, having as light as possible a color. Numerous methods are known for empirically lightening the color of MDI. The nature of the colorants causing the problem has hitherto not been satisfactorily established.

The previously known processes can be roughly divided into four groups:

1. Processes in which the starting material diaminodi-phenylmethane (MDA) is subjected to treatment and/or purification.

EP-A-0 546 398 describes a process for preparing polymeric MDI
in which the polymethylenepolyphenylpolyamine used as starting material is acidified before the phosgenation.

EP-A-0 446 781 relates to a process for preparing diaminodiphenylmethanes which are first treated with hydrogen and subsequently subjected to phosgenation, with a lighter-colored MDI being obtained.

The abovementioned processes give only a slight improvement in color, since experience has shown that the colorants in MDI are formed not only from certain secondary components in MDA but also result from color precursors which are formed by secondary reactions during the phosgenation.

2. Solutions based on the phosgenation process US 5 364 9S8 relates to a process for preparing polyisocyanates according to which the phosgene is completely removed at low temperature after the phosgenation and the isocyanate is subsequently treated hot with HCl gas. EP-A-0 581 100 relates to a process for preparing polyisocyanates in which, after the phosgenation, a chemical reducing agent i8 added before removal of solvent with, according to this patent, lighter-colored products likewise being obtained.

CA 02209l39 l997-09-l8 Although these proces6e6 attempt to remove the components causing discoloration at the correct point, their high technical complexity or the high costs make them not efficient enough in terms of their color-lightening effect, since the degradation of color precursors occurs to only a small extent as a result of incomplete chemical reactions.

3. Addition of color-lightening additives to the crude isocyanate product obtained after the phosgenation and before the work-up According to Us 4 465 639, water is added to the crude product obtained after the phosgenation to lighten its color. For the same purpose, EP-A-o 538 500, EP-A-0 445 602 and EP-A-0 467 125 describe the addition of carboxylic acids, alkanols or polyetherols after the phosgenation.

Although the above-described methods of lightening the color are efficient, they have the disadvantage that all the additives undergo reactions with the isocyanates formed as product as well as lightening the color.

4. After-treatment of the final product .

EP-A-0 133 538 describes the purification of isocyanate~ by extraction, giving fractions of a light-colored MDI.

EP-A-0 561 225 relates to a process for preparing isocyanates or isocyanate mixtures which, according to this document, contain no components which give a color. In this process, the isocyanates are, after the phosgenation of the corresponding amines, subjected to a hydrogen treatment at from loO to 180~C
and a pressure of from 3 to 150 bar. According to the examples described in that document, final isocyanate products are hydrogenated as such or in the form of solutions in suitable solvents.

These color-improving after-treatments of the final isocyanate products after complete removal of the solvent at elevated temperature likewise have low efficiency since ~table colorants which are difficult to attack chemically have already been formed as a result of the high temperatures which occur during the work-up, in particular when distilling off the solvent and (in the case of the preparation of polymeric (MDI)) when separating off monomeric MDI.

It is an object of the present invention to provide a novel process for preparing solutions which contain isocyanates, contain no appreciable amounts of components which give a color, have a lighter color than the isocyanates known from the prior art and have a relatively low content of hydrolyzable chlorine. This process should also be able to be included as part of the total process for preparing isocyanates from the corresponding amines.

We have found that this object is achieved by a process for preparing a solution containing at least one isocyanate, which comprises subjecting a solution containing at least one isocyanate to a treatment with hydrogen after the phosgenation and before the complete removal of the solvent. Thus, according to the present invention, the isocyanate is, after the phosgenation of the corresponding amine and the substantial removal of residual phosgene from the solution, subjected without further work-up, i.e. without additional temperature stress, directly to a treatment with hydrogen at relatively low temperatures and pressures.
The present invention further provides a process for preparing an isocyanate or a mixture of two or more isocyanates, which comprises carrying out a treatment with hydrogen as defined in the present application after the phosgenation of the corresponding amine and before the customary removal of the solvent and the thermal after-treatment.

If MDI is used, monomeric MDI may, if desired, be distilled off to leave polymeric MDI containing no appreciable components which give a color. The term "isocyanate which contains no appreciable amounts of components which give a color" used for the purpo6es of the present invention means that isocyanates or solutions of isocyanates whose proportion of components which give a color is generally so low that the iodine color number (ICN) measured in accordance with DIN 6162 is at most about 70, preferably at most about 40 and in particular from about 20 to lo about 30, are obtained according to the present invention.

Furthermore, the isocyanates prepared according to the present invention have a content of hydrolyzable chlorine of less than 1500 ppm, preferably less than I000 ppm and in particular less than 800 ppm, measured in accordance with ASTM D 4663-87. The above term "hydrolyzable chlorine" means the chlorine content of the i60cyanates which originates from compounds which liberate chloride ions on hydrolysis, for example phosgene, HCl or primary and secondary carbamoyl chloride.
In the process of the present invention, it is possible, in principle, to use solutions of all isocyanates which are prepared by phosgenation of the corresponding amines. Examples which may be mentioned are: hexamethylene 1,6-diisocyanate, isophorone diisocyanate, cyclohexyl isocyanate, phenyl isocyanate, 4-tolyl isocyanate, naphthylene 1,5-diisocyanate, tolylene 2,4- or 2,6-diisocyanate or mixtures thereof, diphenylmethane 4,4', 2,4' or 2,2'-diisocyanate or mixtures thereof, and also oligomeric or relatively high molecular weight (polymeric) derivatives of the above diisocyanates.
Among these, preference is given to diphenylmethane diisocyanates (MDI) snd, in particular, polymeric MDI.

As solvent, it is possible to use all the inert aromatic, aliphatic or alicyclic hydrocarbons or halogenated hydrocarbons which are known for the phosgenation process, in which the respective isocyanate is soluble and which are not attacked under the reaction conditions of the phosgenation and the treatment with hydrogenation. Examples of such solvents are aromatic compounds such as monochlorobenzene, dichlorobenzene, toluene, xylenes and naphthalene derivatives, alkanes having from 5 to 12 carbon atoms, e.g. hexane, heptane, octane, nonane and decane, cycloalkanes such as cyclohexane, inert esters and inert ethers, e.g. ethyl acetate or butyl acetate, tetrahydrofuran, dioxane or diphenyl ether.

After the phosgenation and the substantial removal of phosgene and HCl, which generally precede the process of the present invention, the solution of at least one isocyanate subjected to the hydrogenative treatment still contains traces of phosgene and HCl which are, however, not critical for the process.
Traces of HCl may be able to increase the activity of the catalyst used for the treatment with hydrogen. The treatment with hydrogen is preferably carried out directly in the solvent used during the phosgenation.
The reaction conditions for the treatment with hydrogen are generally as follows:

The concentration of the isocyanate in the solvent is generally from about 1 to about 90% by weight, preferably from about 5 to about 50% by weight. The temperature during the hydrogenation is generally from about 25 to about 200~C and preferably from about 70 to 150~C.

The pressure used during the hydrogenation is generally from about 2 x 104 to about 3 x 107 Pa, preferably from about 5 x 104 to about 2.8 x 105 Pa, and in particular from about 1 x 105 to about 2.5 x 105 Pa.

The hydrogenation is generally carried out over a period of from one minute to three hours and preferably from five minutes to sixty minutes. Suitable catalysts are all known heterogeneous hydrogenation catalysts which are stable under strongly acid, anhydrous conditions (HCl) and do not undergo any reactions with isocyanates. Suitable catalysts are in particular the transition metals and transition metal compounds of transition groups I, VII and VIII of the Periodic Table known for hydrogenation; preference is given to using the noble metals such as Pd, Pt, Rh, Ru, etc. These are activated by known methods and may also be in the form of supported lo catalysts, with preferred support materials being activated carbons, aluminum oxides and silicates. The content of the active component is here generally from about 0.5 to about 30%
by weight, preferably from about 2 to about 10% by weight, in each case based on the weight of the catalyst. ~ased on the isocyanate used, the amount of catalyst is generally from about 0.01 to about 20~ by weight, preferably from about 0.5 to about 10% by weight.

The hydrogenative treatment can be carried out under virtually no applied pressure or under superatmospheric pressure by the known processes of continuous or batchwise hydrogenation. Thus, the isocyanate solution can be saturated with hydrogen before contact with the catalyst or hydrogen can be injected after the addition of catalyst to the solution of at least one isocyanate. The hydrogenation can be carried out continuously or batchwise in a suspension of the catalyst or continuously in the downflow mode over a catalyst bed. In the suspension procedure, mixing can be carried out, for example, by means of customary stirrers, gas-introduction stirrers or by means of static mixers. In a continuous process, the hydrogenation can be carried out by known methods in a suspension procedure in a circulating system with nozzle feed and discharge of the catalyst-free product. The continuous procedure over a ca-talyst bed can be carried out, for example, with a cocurrent or countercurrent of H2. The suspended catalyst is separated off and recirculated using prior art methods for separating finely divided solids from solutions.

The resulting solution of at least one isocyanate in the inert solvent can subsequently be worked up in the customary manner by gradual distilling off of the solvent over a plurality of columns under decreasing pressure. Finally, brief heating to high temperatures under reduced pressure enables a solvent-free crude isocyanate to be obtained and this can be subjected to a subsequent distillation, for example to separate monomeric MDI
lo from trimeric, tetrameric and relatively high molecular weight polymeric MDI.

The present invention thus also provides a process for preparing i60cyanates or mixtures of two or more isocyanates in which one reaction stage carried out is a treatment of a 601ution of at least one isocyanate with hydrogen, as described in detail in the present application.

The isocyanates or mixtures of two or more isocyanates prepared by the process of the present invention are used for preparing polyurethanes having a comparatively light color, preferably polyurethane foams.

Compared with the processes of the prior art, the process of the present invention displays, inter alia, the following advantages:

The treatment with hydrogen according to the present invention at the process stage mentioned gives solutions of isocyanates which contain no appreciable amounts of components which give a color and these then lead to significantly lighter-colored isocyanates.

The product obtained is free of additives.

The treatment with hydrogen according to the present invention, which is preferably carried out directly in the solvent used in the phosgenation, can in principle be built into a process for preparing isocyanates in such a way that the total process can be carried out continuously.

Furthermore, the process of the present invention is relatively sparing in its use of energy and resources since overall fewer temperature changes are required and it can be carried out under comparatively low pressu~es and at moderate temperatures.
lo In addition, subsequent treatment and puri~ication of the isocyanate to be obtained as final product is not necessary.

The resulting solution of at least one isocyanate or the resulting isocyanates have a comparatively low content of hydrolyzable chlorine compounds, as defined above, which are likewise mostly destroyed by the treatment with hydrogen.

The process has a high efficiency since precursors of substances which give a color are degraded and as a result it is not necessary to destroy thermodynamically stable colorants as such.

The examples below illustrate the invention.

~AMPT.F~

Preparation of the MDI samples (mix A) 150 g of diaminodiphenylmethane (MDA) raw base dissolved in 1.3 1 of monochlorobenzene were reacted at atmospheric pressure and 50-80~C with 300 g of phosgene dissolved in 1.3 1 of monochlorobenzene in a stirred reactor having a capacity of 6 1. The temperature was increased to about 120~C over a period of from 1 to 2 hours, with the reaction to form the isocyanate (about 180 g) taking place. Subsequently, remaining phosgene and from 50 to 95~ of the monochlorobenzene were distilled off under gentle conditions (80~C, 0.2 x 105 Pa). The reaction mixture, which still contained t~aces of phosgene, was made up to from about 1.5 to 2 1 with fresh solvent (monochlorobenzene or another suitable solvent) until an about 10~ strength solution of the isocyanate in the solvent had been obtained.
This mixture was used for the hydrogenative treatment.

Hydrogenative treatment of the diphenylmethane diisocyanate (MDI) samples.

lo Example 1 In a pressure reactor having a capacity of 4 1 and fitted with a stirrer, 10 g of Pd/C (5~ by weight of palladium) were added to a mix A made up to 2 l with monochlorobenzene and the mixture was stirred for 1 hour at 80~C under an H2 pressure of 5 x 105 Pa. After depressurization, the sample was drained and the catalyst was filtered off quantitatively. The solvent was taken off gently under reduced pressure on a rotary evaporator and the sample was subsequently treated twice for 45 minutes each time at 180~C and O.ol x 105 Pa on a rotary evaporator to remove traces of solvent. The results are shown in Table 1.

Example 2 In a pressure reactor having a capacity of 4 1 and fitted with a stirrer, 10 g of Pd/C (5% by weight of palladium) were added to a mix A made up to 2 1 with toluene and having a residual monochlorobenzene content of about 200 ml and the mixture was treated under an H2 pressure of 2 x 105 Pa for 30 minutes at 120~C. After depressurization, the sample was drained and the-catalyst was filtered off quantitatively. The solvent was taken off gently under reduced pressure on a rotary evaporator and the sample was subsequently treated once for 45 minutes at 180~C and o.Ol x 105 Pa. The results are shown in Table 1.

Example 3 In a pressure reactor having a capacity of 4 1 and fitted with a stirrer, 5 g of Pd/C (5% by weight of palladium) were added to a mix A made up to 2 l with monochlorobenzene and the mixture was treated under an H2 pressure of 2 x 105 Pa for 1 hour at 80~C. After depressurization, the sample was drained and the catalyst was filtered off ~uantitatively. The solvent was taken off gently under reduced pressure on a rotary -evaporator and the sample was subsequently treated once for 45 minutes at 180~C and 0.01 x 105 Pa on a rotary evaporator. The result6 are shown in Table 1.

Example 4 In a pressure reactor having a capacity of 4 l and fitted with a stirrer, 10 g of Pd/Al2o3 (5% by weight of palladium) were added to a mix A made up to 2 1 with monochlorobenzene and the mixture was treated under an H2 pressure of 2 x 105 Pa for 30 Zo minutes at 120~C. After depressurization, the sample was drained and the catalyst was filtered off quantitatively. The solvent was taken off gently under reduced pressure on a rotary evaporator and the sample was subsequently treated once for 45 minutes at 180~C and 0.01 x 105 Pa on a rotary evaporator. The results are shown in Table 1.

COMPARATIVE EX ~ PLE (CE) 1 (as described in EP-A-o 561 225) Mix A was freed of solvent (monochlorobenzene) under reduced pressure and was treated for 45 minutes at 180~C and o.Ol x 105 Pa on a ~otary evaporator. About 175-180 g of isocyanate product were obtained. This was again dissolved completely in monochlorobenzene and, in a pressure reactor having a capacity of 4 1 and fitted with a stirrer, 10 g of Pd/C (5% by weight of palladium) were added and the mixture was treated under an H2 pressure of 5 x 105 Pa for 1 hour at 80~C. After depressurization, the sample was drained and the catalyst was filtered off quantitatively. The solvent was taken off gently under reduced pressure on a rotary evaporator and the sample was subsequently treated for a further 45 minutes at 180~C and o.Ol x 105 Pa on a rotary evaporator. The results are shown in Table 1.

(without hydrogen treatment) Mix A was freed of solvent (monochlorobenzene) under reduced pressure and treated once for 45 minutes at 180~C and 0.01 x 105 Pa on a rotary evaporator, 175 g of final isocyanate product were obtained. The results are shown in Table 1.

The characteristic data of the isocyanates obtained as described in Examples 1 to 4 (according to the present invention) and Comparative Examples 1 and 2 were determined.
The iodine color number which is customarily given for MVI was specifically determined. For thic purpose, the ~amples (diluted with monochlorobenzene; ratio of isocyanate; monochloroben-zene = 1:5) were examined using a comparator (from Hellige) by comparison with color disks (corresponding to certain iodine color numbers) and color numbers were also determined using a photometer (from Dr. Lange, Berlin) in the ICN program mode, with the values obtained being corrected using a factor after calibration with iodine standard solution in accordance with DIN 616Z.

Table 1: Characteristic data for the Examples Example/ NCO (~) EHC (*) DHC (**) ICN 1:5 ICN 1:5 Comp. Ex. (ppm) (ppm) comparator Photometer El 31.2 89 525 25 14 E2 30.5 72 471 18 8 E3 31.2 93 497 20 8 E4 31.3 148 577 30 21 CEl 29.5 92 512 70 64 CE2 31.9 413 998 70 29 In addition, a color number determination for different colors was carried out by measurement of the absorption values at different wavelengths (using a method similar to EP-A-0 676 391 Al). For this purpose, 2 g of product were dissolved in loo ml of monochlorobenzene and measured in 1 cm rectangular cells made of quartz glass.

(*) EHC = Easily hydrolyzable chlorine, measured in accordance with ASTM D 4667-87 (**) DHC = Difficulty hydrolyzable chlorine, measured in accordance with ASTM D 4663-87.
NCO = Isocyanate content, measured in accordance with DIN 53185.

Table 2: Measurement of the absorption values Example/Comp. A [430 nm] A [520 nm] A [600 nm]
Example E1 0.046 0.008 0.002 E2 0.045 0.007 0.002 E3 0.159 0.014 0.003 CE1 0.394 0.043 0.013 CE2 0.163 0.031 0.011 The results show a very good lightening of the color of raw MDI
by hydrogenative treatment, with a simultaneous significant reduction in the values for hydrolyzable chlorine. The NC0 content is reduced only insignificantly.

Claims

1. A process for preparing a solution containing at least one isocyanate, which comprises subjecting a solution containing at least one isocyanate to a treatment with hydrogen after the phosgenation and before the complete removal of the solvent.

2. A process as claimed in claim 1, wherein the treatment with hydrogen is carried out at pressures of from 0.5 x 10 5 to 2.8 x 10 5 Pa.

3. A process as claimed in claim 1 or 2, wherein the treatment with hydrogen is carried out at from 70 to 150°C.

4. A process as claimed in claim 1 or 2, wherein the treatment with hydrogen is carried out in the solvent of the phosgenation.

5. A process as claimed in claim 1 or 2, wherein the isocyanate, of which there is at least one, is an isocyanate of the diphenylmethane diisocyanate (MDI) group.

6. A process for preparing an isocyanate or a mixture of two or more isocyanates, which comprises carrying out a treatment with hydrogen as defined in claim 1 or 2.

7. A process as claimed in claim 6, which is carried out continuously.

8. A process as claimed in claim 6, wherein the product obtained is polymeric MDI.

9. Process for preparing a polyurethane by using an isocyanate or a mixture of two or more isocyanates which is prepared by a process as claimed in claim 6.

10. Process as claimed in claim 9, wherein the polyurethane is a polyurethane foam.