Note: Descriptions are shown in the official language in which they were submitted.
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Process for the production of polyether polyols
The invention relates to an improved process for the production of polyether
polyols
by means of double metal cyanide (DMC) catalysis by polyaddition of alkylene
oxides onto starter compounds having active hydrogen atoms.
Double metal cyanide (DMC) catalysts for the polyaddition of alkylene oxides
onto
starter compounds having active hydrogen atoms have long been known (c.~ for
example US-A 3 404109, US-A 3 829 505, US-A 3 941 849 and US-A 5 158 922).
Using these DMC catalysts for the production of polyether polyols brings
about, in
particular, a reduction in the proportion of monofunctional polyethers with
terminal
double bonds, so-called monvols, in comparison with the conventional
production of
polyether polyols by means of metal hydroxide catalysts. The resulting
polyether
polyols may be further processed to yield high-grade polyurethanes (for
example
elastomers, foams, coatings).
A general feature of DMC-catalysed production of polyether polyols is the
occurrence of an induction period, i.e. the DMC catalyst has initially to be
activated
by a certain minimum quantity of alkylene oxide, before continuous alkylene
oxide
addition can.proceed to polyether chain synthesis (c.f. for example J.F.
Schuchardt,
S.D. Harper, 32nd Annual Polyurethane Technical Marketing Conference, 1989, p.
360 ff.). Induction times are typically between a few minutes and several
hours and
are dependent on reaction parameters such as catalyst concentration, molar
mass of
the starter compound used, reaction temperature and alkylene oxide quantity
during
activation.
DE-OS 15 95 759 describes a process for the production of polyether polyols by
means of DMC catalysis, in which a polyether polyol is produced by
polyaddition of
an epoxide in the presence of a DMC catalyst, a telogen capable of reacting
with the
epoxide and having an average molecular weight of at least 300 g/mol and a
telogen
capable of reacting with the epoxide and having an average molecular weight of
at a
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least 31 glmol. After activation of the mixture, i.e. after the end of the
induction
period, addition of the epoxide continues until the desired polyether OH
number is
reached. A disadvantage of this process is that the entire quantity of starter
compound is initially introduced in full and a high catalyst concentration is
therefore
required, since catalyst activity is markedly reduced by the short-chain
starter
compound initially introduced. One consequence of this is high material costs,
resulting from the large quantities of catalyst which have to be used. In
addition, the
catalyst has to be separated off after alkoxylation, making the process yet
more
complex and costly. Moreover, the catalyst has initially to be activated prior
to
alkoxylation proper. However, the occurrence of such an induction phase
results, on
the one hand, in a deterioration in the space-time yield and, on the other
hand, in an
increased potential risk from the quantity of free alkylene oxide used in the
reactor
for the purpose of activation.
DDR-WP 203 734 discloses a process for DMC-catalysed polyether polyol
production, in which the DMC catalyst is initially introduced alone into the
reactor
and activated with pure propylene oxide. The reaction temperature increases
during
activation from room temperature to 80°C, owing to the exothermic
nature of the
reaction. Once activation is complete (end of the induction period), epoxide
and
starter compound are added simultaneously. The reaction temperature is in the
range
from 20°C to 140°C. In this manner, living prepolymers may be
produced, which,
according to DDR-WP 203 735, may then be further extended with propylene oxide
in a continuous process at a temperature in the range from 40°C -
120°C. The
process described in DDR-WP 203 734 exhibits several disadvantages, however.
On
the one hand, activation of the catalyst is performed using large amounts of
pure
propylene oxide. Pure propylene oxide is potentially extremely hazardous,
since, in
the event of cooling failure, the heat liberated by the exothermic
propoxylation
reaction cannot be absorbed and dissipated by an additional component, e.g. a
starter
compound. The considerable localised heating caused thereby may lead to
explosive
thermal decomposition of the polyethers formed. However, even when the
heatinglcooling circuit is fully functional, localised heating may occur. Due
to the
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small quantity of contents present in the reactor, only a very small heat
exchange
surface is available, such that effective control of the reaction temperature
is very
difficult or wholly impossible, as a result of which thermally induced ageing
and
deactivation of the catalyst cannot be ruled out. Furthermore, in this process
very
high-molecular weight chains are formed by the large quantities of propylene
oxide
used for catalyst activation, resulting in wide molar-mass distribution and a
marked
increase in the viscosity of the polyether polyol, which restricts
considerably the use
of such products for polyurethane applications.
EP-A 879 259 discloses a process in which the alkylene oxide and the starter
compound are again added simultaneously. The DMC catalyst is introduced
previously into the reactor. In addition, a polyether polyol may optionally be
introduced into the reactor. This process also exhibits the disadvantage of
the
occurrence of an induction phase. This is caused by the simultaneous addition
of
epoxide and starter compound during activation or by the starter compound
initially
present, since the starter compound, even in a very low concentration, is
presumed
initially to effect temporary deactivation of the catalyst. This phenomenon is
also
described in WO 98/52689, for example, in which an increase in the activity of
the
DMC catalyst is achieved by the removal of water residues from pre-
propoxylated
starter compounds. Water may also be regarded as a low-molecular weight
starter
compound, such that even slight traces of a low-molecular weight starter
compound
lead to a marked reduction in the activity of the DMC catalyst and thus to a
lengthening of the induction phase. Only after complete activation of the
catalyst,
which is characterised by an accelerated pressure drop in the reactor, may
addition of
the epoxide/starter mixture be continued. Since addition of the
epoxide/starter
mixture proceeds as a function of the concentration ratio in the desired
polyether
polyol, free propylene oxide is present in the reactor during the induction
phase
generally in a very high concentration, such that this process entails a
relatively high
safety risk.
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German patent application 19937114.8 discloses a process, in which a DMC
catalyst
is initially present in a starter compound and the alkylene oxide
concentration
necessary for activation of the catalyst is kept constant during the induction
phase. In
this process too, the catalyst has initially to be activated, before
propoxylation proper
may proceed.
The object of the present invention was therefore to develop a process for DMC-
catalysed production of polyether polyols, which exhibits a markedly reduced.
induction phase or none at all (thus entailing a markedly reduced potential
risk from
free alkylene oxide) and results in polyether polyols with a narrow molar mass
distribution and low viscosity.
It has now been found that polyether polyols may be produced by DMC catalysis
with a markedly reduced induction period or even without an induction period,
if the
DMC catalyst is initially present in an inert suspending agent and is
activated with
pure alkylene oxide, the polyether polyol then being synthesised by the
addition of a
starter compound/alkylene oxide mixture. Using this process, polyether polyols
with
a narrow molar mass distribution and low viscosity are obtained.
The present invention therefore provides a process for the production of
polyether
polyols by polyaddition of alkylene oxides onto starter compounds having
active
hydrogen atoms by means of DMC catalysis, in which the DMC catalyst is
initially
present in an inert suspending agent, is activated with 1-30 wt.% alkylene
oxide,
relative to the total quantity of suspending agent and alkylene oxide, at
temperatures
of between 20°C and 200°C and the polyether polyol is then
synthesised by the
addition of a starter compound/alkylene oxide mixture.
In the process according to the invention, a suspending agent, which does not
react
with the alkylene oxide, is initially introduced into the reactor. The
quantity of non-
activated DMC catalyst necessary for polyaddition is then introduced into the
reactor. The order in which constituents are added is not crucial. It is also
possible
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for the catalyst to be suspended in the inert suspending agent first and the
suspension
then to be introduced into the reactor. An adequate heat exchange surface is
made
available in the reactor by the suspending agent, such that the liberated heat
of
reaction may be very effectively dissipated. Moreover, the suspending agent
provides thermal capacity in the event of cooling failure, such that the
temperature
may in such an instance be kept below the decomposition temperature of the
reaction
mixture.
Suitable suspending agents are all polar-aprotic, weakly polar-aprotic and non-
polar-
aprotic solvents, which, under the conditions described below, do not react
with the
alkylene oxides used for polyaddition. Mention should be made at this point,
by way
of example, of the following polar-aprotic solvents: acetone, methyl ethyl
ketone,
acetonitrile, nitromethane, dimethyl sulfoxide, sulfolan, dimethylformamide,
dimethylacetamide and N-methylpyrrolidone. Non-polar- and weakly polar-aprotic
solvents are preferably used, however. This group includes, for example,
ethers,
such as for example dioxane, diethyl ether, methyl tert.-butyl ether and
tetrahydrofuran, esters, such as for example acetic acid ethyl ester and
acetic acid
butyl ester, hydrocarbons, such as for example pentane, n-hexane, benzene and
alkylated benzene derivatives (toluene, xylene, ethylbenzene) and chlorinated
hydrocarbons, such as for example chloroform, chlorobenzene, dichlorobenzene
and
carbon tetrachloride. Toluene, xylene, ethylbenzene, chlorobenzene and
dichlorobenzene are particularly preferred. ,
The DMC catalysts suitable for the process according to the invention are
known in
principle and described in detail, for example, in JP-A 4 145 123, EP-A 654
302,
EP-A 700 949, EP-A 743 093, EP-A 761 708, WO 97/40086, WO 98/16310, WO
99/19062, WO 99/19063, WO 99/33562, WO 99/46042 and German patent
applications 198 34 572.0, 198 34 573.9, 198 42 382.9, 198 42 383.7, 199 05
611.0,
199 06 985.9, 199 13 260.7, 199 20 937.5, 199 24 672.6. The DMC catalysts
described in German patent application 199 05 611.0 are a typical example,
containing, in addition to a double metal cyanide compound (e.g. zinc
hexacyano-
i
a
!F
a
I
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cobaltate(IIn) and an organic complex ligand (e.g. tert.-butanol), a bile acid
or the
salts, esters or amides thereof.
The DMC catalyst concentration is generally in the range of from 0.0005 wt.%
to
1 wt.%, preferably in the range of from 0.001 wt.% to 0.1 wt.%, particularly
preferably in the range of from 0.001 to 0.01 wt.%, relative to the quantity
of
polyether polyol to be produced.
The DMC catalyst suspension is brought to the temperature necessary for
activation
after its introduction into the reactor. Activation may be performed in the
range of
from 20°C - 200°C, preferably in the range of from 60°C -
180°C, particularly
preferably in the range of from 120°C - 160°C.
Ethylene oxide, propylene oxide, butylene oxide, styrene oxide and mixtures
thereof
are used as alkylene oxides for activating the DMC catalyst. Activation is
particularly preferably performed with propylene oxide or a mixture of
propylene
oxide and ethylene oxide.
Activation of the DMC catalyst may proceed at a reactor pressure in the range
of
from 0.0001 - 20 bar, preferably in the range of from 0.5 - 10 bar,
particularly
preferably in the range of from 1 - 6 bar.
According to the invention, the quantity of alkylene oxide for activating the
DMC
catalyst is 1-30 wt.%, preferably 2-25 wt.%, particularly preferably 3-15
wt.%, in
each case relative to the total quantity of suspending agent and alkylene
oxide.
By performing catalyst activation in this way, the induction times are
markedly
reduced in comparison to the prior art processes. The reaction conditions are
preferably so selected that no induction period occurs. This is generally the
case if
the temperature during catalyst activation amounts to at least 120°C.
The reactor
pressure drops immediately after addition of the alkylene oxide, since, in
contrast to
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the prior art processes, in the process according to the invention no
induction phase
is observed. Allcylene oxide addition does not have to interrupted at any
point during
the reaction and the concentration of free alkylene oxide in the reactor is
very small
at all times during the reaction, which constitutes a great safety advantage.
The
process according to the invention therefore makes it possible for addition of
the
starter compound for polyether polyol synthesis to be commenced immediately
after
alkylene oxide addition starts. This results in an improved space-time yield
at the
same time as an increase in safety during the reaction.
Synthesis of the polyether polyol by polyaddition is performed according to
the
invention by the addition of a starter compound/alkylene oxide mixture. The
starter
compound/alkylene oxide molar ratio is 1:1 to 1:1000, preferably 1:2 to 1:500,
particularly preferably 1:3 to 1:200.
Alkylene oxides preferably used for polyether polyol synthesis are ethylene
oxide,
propylene oxide, butylene oxide and mixtures thereof. Propylene oxide and
mixtures
of propylene oxide and ethylene oxide are particularly preferred. Synthesis of
polyether chains may be performed, for example, with only one monomeric
epoxide
or also randomly with 2 or 3 different monomeric epoxides. Further details may
be
found in Ullmanns Encyclopadie der industriellen Chemie, volume A21, 1992;
p. 670 ff.
Preferred starter compounds having active hydrogen atoms are compounds with
molecular weights of from 18 to 2000 g/mol, preferably 62 to 1000 g/mol, and 1
to
8, preferably 2 to 6, hydroxyl groups. The following may be mentioned by way
of
example: butanol, ethylene glycol, diethylene glycol, triethylene glycol, 1,2
propylene glycol, 1,4-butanediol, 1,6-hexanediol, bisphenol A,
trimethylolpropane,
glycerol, pentaerythritol, sorbitol, cane sugar, degraded starch, water or so-
called
pre-extended starter compounds, which are obtained by alkoxylation from the
above
mentioned compounds.
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Polyaddition may be performed at a reactor pressure in the range from 0.0001 -
20 bar, preferably in the range from 0.5 - 10 bar, particularly preferably in
the range
from 1 - 6 bar. The reaction temperatures range from 20 - 200°C,
preferably from 60
180°C, particularly preferably from 80 - 160°C.
The mixture of alkylene oxide and starter compound for polyether polyol
synthesis
may be introduced into the reactor via a line, in which static mixing elements
may
optionally be installed in order to achieve more homogeneous mixing of
alkylene
oxide and starter compound, or via separate feed sections. However, it is
important
to ensure good mixing of the educts at all times during the reaction. An
elevated
degree of segregation in the reactor, i.e. in the event of poor mixing of the
educts
with the reactor contents, may cause complete or partial deactivation of the
DMC
catalyst owing to a locally increased starter compound concentration.
The molecular weights of the polyether polyols produced using the process
according to the invention are in the range from 400 to 100 000 glmol,
preferably in
the range from 700 to 50 000 g/mol, particularly preferably in the range from
1000
to 20 000 g/mol.
The synthesis of polyether polyols by polyaddition may be performed wholly by
the
addition of a starter compound/alkylene oxide mixture. It is also possible,
however,
initially to synthesise only some of the polyether polyol to be produced by
the
addition of a starter compound/alkylene oxide mixture and then to extend
further the
intermediate product obtained with pure alkylene oxide, preferably propylene
oxide,
or an alkylene oxide mixture, preferably a mixture of propylene oxide and
ethylene
oxide. It is also possible to form several of these blocks from alkylene oxide
or
alkylene oxide mixture, e.g. synthesis of the final polyether polyol may be
achieved
by the initial addition of a mixture of propylene oxide and ethylene oxide to
an
intermediate product, which has been produced by the addition of a starter
compound/propylene oxide mixture, followed by the addition of pure propylene
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oxide. In this way, polyether polyols may be synthesised which have several
defined
blocks (so-called multiblock copolymers).
Using the process according to the invention, it is possible to produce
polyether
polyols with very low double bond contents (_< 6 mMol/kg) even at reaction
temperatures of 145°C and higher, which is not possible with any of the
DMC-
catalysed polyaddition processes known hitherto. The polyether polyols
produced by
the process according to the invention have narrow molar mass distributions
and low
viscosities.
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Examples
Example 1
59 g of xylene and 0.036 g of a double metal cyanide (DMC) catalyst (produced
in
accordance with German patent application 199 OS 611, Example A) were
initially
introduced into a steel reactor with a volume of 2 litres. Once the reaction
temperature of 150°C was reached, 4.78 g of propylene oxide (7.5 wt.%,
relative to
the total quantity of xylene and propylene oxide) were added. As soon as this
addition was discontinued, the pressure fell from 1.75 bar to 1.4 bar (Fig.
1). 1.2 kg
of a mixture of propylene oxide and propylene glycol in a weight ratio of 96/4
(molar ratio 33/1) was then added to the active catalyst suspension over a
period of
2.5 hours. The suspending agent and readily volatile fractions were then
removed by
distillation at 120°C/10 mbar.
A polyether polyol with an OH number of 56 mg KOH/g, a viscosity
(25°C) of
359 mPas and a double bond content of 5 mMol/kg was obtained. The gel
permeation chromatogram shows a very narrow molecular weight distribution
without a high-molecular weight fraction (Fig. 2). All GPC measurements were
performed at 25°C with THF as the mobile solvent. Polystyrenes with
molecular
masses of 162 g/mol, 580 g/mol, 7002 g/mol, 10856 g/mol and 319894 g/mol were
used as standard.
Example 2
59 g of toluene and 0.036 g of the double metal cyanide (DMC) catalyst from
Example 1 were initially introduced into a steel reactor with a volume of 2
litres.
Once the reaction temperature of 150°C was reached, 4.78 g of
propylene oxide
(7.5 wt.%, relative to the total quantity of toluene and propylene oxide) were
added
over a period of approximately 30 seconds. Without interrupting the addition
of
propylene oxide, the addition of propylene glycol (propylene oxide/propylene
glycol
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weight ratio = 96/4) was commenced at this point. After 2.5 hours, 1.2 kg of
the
mixture of propylene oxide and propylene glycol in a weight ratio of 96/4
(molar
ratio 33/1) had been added in full. The suspending agent and readily volatile
fractions were then removed by distillation at 120°C/10 mbar.
A polyether polyol with an OH number of 56 mg KOH/g, a viscosity
(25°C) of
352 mPas and a double bond content of 5 mMol/kg was obtained. The gel
permeation chromatogram shows a very narrow molecular weight distribution
without a high-molecular weight fraction (Fig. 3).
Example 3 (Comparative Example as EP-A 879 259
59 g of a polyoxypropylene diol with an average molecular weight of 2000 g/mol
(OH number 56 mg KOH/g), produced by DMC catalysis, and 0.036 g of the double
1 S metal cyanide (DMC) catalyst from Example 1 were initially introduced into
a steel
reactor with a volume of 2 litres. To activate the catalyst, 4.8 g of a
mixture of
propylene oxide and propylene glycol in a weight ratio of 96:4 (molar ratio
33/1)
were added at 150°C. After 27 minutes, the pressure drop at the end of
the induction
phase typical of catalyst activation could be observed (Fig. 4). 1.2 kg of a
mixture of
propylene glycol and propylene oxide in the ratio 96:4 were then added over a
period of 2.5 hours. The readily volatile fractions were then removed by
distillation
at 120°C/10 mbar.
The double bond content was 5 mMol/kg at an OH number of 56.4 mg KOH/g. The
viscosity was 381 mPas at 25°C.
Example 4 (Comparative Example) Activation with more than 30 wt.% propylene
oxide, relative to the total quantity of toluene and propylene oxide
42 g of toluene and 0.036 g of the double metal cyanide (DMC) catalyst from
Example 1 were initially introduced into a steel reactor with a volume of 2
litres. To
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activate the catalyst, 36 g of propylene oxide (46 wt.% relative to the total
quantity
of toluene and propylene oxide) were added at 105°C. After
approximately 5
minutes, the pressure drop typical of DMC catalyst activation could be
observed in
the reactor. 1.2 kg of a mixture of propylene oxide and propylene glycol in a
weight
ratio of 96/4 (molar ratio 33/1) were then added over a period of 4 hours. The
suspending agent and readily volatile fractions were then removed by
distillation at
120°C/10 mbar.
A polyether polyol with an OH number of 53.3 mg KOH/g, a viscosity
(25°C) of
852 mPas and a double bond content of 5 mMol/kg was obtained. GPC analysis
shows that high-molecular weight chains having a molecular weight in the range
of
> 10° g/mol have formed, which are responsible for this marked increase
in viscosity
(Fig. 5).
Example 5
59 g of toluene and 0.036 g of the double metal cyanide (DMC) catalyst from
Example 1 were initially introduced into a steel reactor with a volume of 2
litres.
Once the reaction temperature of 150°C was reached, 9.6 g of propylene
oxide
(15 wt.°lo, relative to the total quantity of toluene and propylene
oxide) were added.
As soon as this addition was discontinued, a pressure drop could be observed,
indicating activation of the catalyst. 1.2 kg of a mixture of propylene oxide
and
propylene glycol in a weight ratio of 96/4 (molar ratio 33/1) was then added
to the
active catalyst suspension over a period of 2.5 hours. The suspending agent
and
readily volatile fractions were then removed by distillation at
120°C/10 mbar.
A polyether polyol with an OH number of 56 mg KOH/g, a viscosity
(25°C) of
362 mPas and a double bond content of 5 mMol/kg was obtained. The gel
permeation chromatogram shows a very narrow molecular weight distribution
without a high-molecular weight fraction (Fig. 6).
