Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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
CA 02209139 1997-09-18
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.
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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.
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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.
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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
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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
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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
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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,
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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
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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.
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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
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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.
COMPARATIVE EXAMPLE 2
(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.
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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.
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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.
