Note: Descriptions are shown in the official language in which they were submitted.
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Process for preparing aqueous polyurethane dispersions by means of flash
evaporation
The invention relates to a novel process for preparing solvent-free or low-
solvent aqueous
polyurethane dispersions from solvent-containing, aqueous polyurethane
dispersions or
solutions by means of flash evaporation of the organic solvents or organic
solvent mixtures,
and also to a process for preparing aqueous polyurethane dispersions by
converting
hydrophilically modified NCO prepolymers or polyurethanes in an optionally
aqueous,
organic solvent or solvent mixture to an aqueous dispersion or aqueous
solution and
subsequently removing the organic solvent or solvent mixture by means of flash
evaporation.
Aqueous polyurethane dispersions are used, for example, for single-component,
isocyanate-
free varnishes, coatings, sealing compositions, adhesives and membranes. Their
importance
has been increasing ever more for many years for ecological (environmental
compatibility,
occupational safety) and economic reasons. The viscosity and the flow behavior
are
independent of the molar mass which can be adjusted over a wide range. In
addition to these
advantages, the possible applications of these low-solvent or solvent-free
products already
correspond substantially to those of the products containing solvent.
Processes for preparing aqueous polyurethane dispersions, here including both
aqueous
dispersions or suspensions of pure polyurethanes and of polyurethaneureas, are
known and are
described, for example, in the following references: Houben-Weyl, Methoden der
organischen
Chemie, Volume E 20, Part I; Ullmann's Encyclopedia of Industrial Chemistry,
Release 2003,
7th Edition, Wiley-VCH Verlag; Adv. Urethane Sci. Technol. 10 (1987), 121-187;
DE 198 12 751; DE 199 57 604; WO 96/40811; US 2002/0028877.
Of the processes mentioned, it is the "acetone process" in analogy to the
teaching of
DE 14 95 745 or to that of DE 14 95 847 that is of particular importance for
the present
invention. In this process, an NCO prepolymer is generally prepared initially
and is dissolved
in an inert solvent (optionally, the NCO prepolymer is also prepared directly
in the inert
solvent), which is followed by chain extension in solution to give the higher
molecular weight
polyurethane. The hydrophilic groups required for the dispersion are
preferably incorporated
' - CA 02571711 2006-12-21
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2
either by incorporation of diols bearing ionic, potentially ionic or nonionic
hydrophilic groups
into the prepolymer or by using appropriate amines as chain extenders. The
dispersion is
effected batchwise in stirred tanks having a stirrer and possibly baffles. The
solvent used is
generally distilled out of the stirred tank immediately after the dispersion.
Despite the
outstanding properties of the products obtained by this procedure, the
"acetone process" has
considerable disadvantages. The long distillation times to remove the solvent
reduce the
space-time yield, increase the preparation costs and have a disadvantageous
effect on the state
of the dispersed particles, especially on their degree of swelling. The long
thermal stress on
the dispersion during distillation of the solvent may additionally lead to
problems in the case
of thermally sensitive dispersions.
Processes for preparing aqueous polyurethane dispersions by continuous
dispersion are also
known. DE 22 60 870 describes, for example, the use of special mixing reactors
which are
based on the cellular flows which form. The removal of the solvent used is
achieved here with
the aid of a thin-film evaporator. Although such an evaporator brings
extremely short
residence times and good heat transfer values, it leads in the case of film-
forming dispersions
to caking on the evaporator surface. In order to circumvent this disadvantage,
DE 3603996
describes a process for continuously preparing aqueous polyurethane
dispersions in barbed
mixers in which the distillative removal of the majority of the solvent is
effected continuously
by means of a circulation evaporator. Owing to the evaporator surfaces which
are always
flooded, no film formation is observed even in the case of products which tend
to film
formation. However, the thus obtained dispersions still have an acetone
content of approx.
10% which has to be removed by a subsequent distillation in a conventional
distillation
vessel.
It is therefore an object of the invention to develop a novel process for
removing solvents
from solvent-containing, aqueous polyurethane dispersions/solutions and a
process for
preparing aqueous polyurethane dispersions by the "acetone process" which is
not burdened
by the aforementioned disadvantages. This object is achieved by the provision
of the process
according to the invention described in detail hereinbelow.
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3
The invention provides a process for preparing solvent-free or low-solvent
aqueous
polyurethane dispersions, which comprises removing the organic solvents or the
organic
solvent mixtures from solvent-containing, aqueous polyurethane dispersions or
solutions by
means of flash evaporation. In a preferred embodiment of the process, the
invention further
provides a process for removing organic solvents or organic solvent mixtures
from solvent-
containing, aqueous polyurethane dispersions or polyurethane solutions by
means of flash
evaporation, and also a process for preparing aqueous polyurethane dispersions
by converting
hydrophilically modified NCO prepolymers or polyurethanes in an optionally
aqueous,
organic solvent or solvent mixture to an aqueous dispersion or solution and
subsequently
removing the organic solvent or solvent mixture by means of flash evaporation.
In the inventive solvent removal, the vapors are generated by passing the
preheated, solvent-
containing, aqueous polyurethane dispersions or solutions into decompression
vessels in
which there is a lower pressure. At the same time, the evaporation of the
solvent from the
dispersion or solution results in the temperature in the remaining liquid
phase falling, so that
the thermal stress is reduced. It is not essential to the invention whether
the removal of the
solvent/solvent mixture is effected in one step or in several stages. The type
of flash
evaporation depends on the composition and the properties of the particular
solvent-
containing, aqueous polyurethane dispersion or solution and of the residual
content of the
solvent/solvent mixture to be attained. For instance, in the case of thermally
sensitive
dispersions, it may be advantageous to carry out the solvent removal in
several stages at low
temperatures, while robust dispersions are freed of solvent/solvent mixture in
fewer stages at
higher temperatures. It is not essential to the invention whether the removal
of the
solvent/solvent mixture is effected batchwise in several steps in a single-
stage flash apparatus,
or batchwise or continuously in a flash apparatus composed of several stages.
It is possible in
each step/in each stage to further reduce the pressure and/or the temperature
and utilize the
energy released, which is obtained by condensation of the vapors, for heating.
A third variant
is the removal of the solvent/solvent mixture in a single-stage flash
evaporation apparatus in
which the polyurethane dispersion or solution is passed from the flash vessel
continuously
through a liquid heater and brought to a higher temperature and a higher
pressure and
subsequently passed back into the flash vessel in which the pressure is lower.
This
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4
continuously reduces the solvent concentration in the aqueous polyurethane
dispersion. When
the solvent-containing, aqueous polyurethane dispersion/solution is prepared
beforehand
batchwise in a stirred tank, preference is given to using this stirred tank as
the flash vessel. In
none of the variants is it essential to the invention how the solvent-
containing, aqueous
polyurethane dispersion or solution is introduced into the flash vessel. This
can be effected,
for example, by nozzle-spraying into the flash chamber or other prior art
methods.
The specific embodiments of the process according to the invention described
are intended
only to illustrate the breadth of applicability, but the process according to
the invention is not
restricted thereto.
The boiling points of the organic solvents or solvent mixtures, or their
azeotropes with water,
are below the boiling point of water at the appropriate pressure at which the
flash evaporation
is carried out. In the solvent removal, a portion of the water is in some
cases also removed, but
this is not essential to the invention.
The advantages of the process according to the invention can be seen in that,
compared to the
prior art "acetone process", the removal of the solvent proceeds distinctly
more rapidly and
product more gently, which achieves products in the process according to the
invention which,
with regard to the properties, are equivalent or superior to the products of
the conventional
"acetone process". Moreover, a distinctly better space-time yield is achieved.
In addition, the
use of the flash evaporation in the case of products tending to film formation
does not lead to
film formation on the evaporator surfaces. The residual contents of the
solvent in the
polyurethane dispersion which are achieved by the application of the process
according to the
invention make a further distillative workup superfluous. The process
according to the
invention may be carried out continuously and batchwise.
In a particularly preferred embodiment of the invention, the polyurethane
dispersions
described in detail hereinbelow are prepared by the process according to the
invention. The
preparation of aqueous polyurethane dispersions by the "acetone process"
comprises the steps
of: preparing a hydrophilically modifiable or modified prepolymer (optionally
in an inert,
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organic solvent/solvent mixture); optionally dissolving in an inert, organic
solvent/solvent
mixture; optionally neutralizing an anionically or cationically modifiable
prepolymer/
polyurethane to give an anionically or cationically modified
prepolymer/polyurethane;
extending the chain; dispersion; removing the solvent/solvent mixture.
5
Anionically or cationically modifiable (i.e. unneutralized) or nonionically
modified
polyurethane prepolymers suitable for the process according to the invention
are known from
the literature and are prepared by polyaddition reactions of polyols and
polyisocyanate
components and optionally solvents/solvent mixtures and/or catalysts.
Polymeric and/or
monomeric polyols having two or more hydroxyl groups reactive toward
polyisocyanates are
used, for example polyester polyols, polyether polyols,
polyhydroxypolycarbonates,
polyhydroxypolyacetals, polyhydroxypolyacrylates, polyhydroxypolyesteramides,
poly-
hydroxypolythioethers, polyalkylenepolyols, polyhydroxypolycaprolactones,
vinyl-modified
polyether polyols, macromonomeric polyols, techelene or polyhydroxy epoxy
resins or
mixtures thereof, and/or any low molecular weight polyols, for example 1,2-
ethanediol,
1,2-propanediol, 1,2-propylene glycol, 1,3-propanediol, 1,3-propylene glycol,
1,4-butanediol,
1,4-butylene glycol, 1,6-hexanediol, 2-methyl-1,3-propanediol, 2,2-dimethylol-
1,3-
propanediol, 1,4-bis(hydroxymethyl)cyclohexane, 1,2,3-propanetriol, 2-
hydroxymethyl-2-
methyl-1,3-propanol, 2-ethyl-2-hydroxymethyl-1,3-propanediol, 2,2-
bis(hydroxymethyl}1,3-
propanediol or mixtures thereof. Examples of preferred polyisocyanate
components are
polyisocyanates, polyisocyanate derivatives or polyisocyanate homologues
having two or
more aliphatic, cycloaliphatic or aromatic isocyanate groups. Especially
suitable are the
polyisocyanates well known in polyurethane chemistry or combinations thereof,
for example
1,4-diisocyanatobutane, 1,6-diisocyanatohexane (HDI), 1,12-
diisocyanatododecane, 1,4-diiso-
cyanatocyclohexane, 1-isocyanato-5-isocyanatomethyl-3,3,5-trimethylcyclohexane
(IPDI),
bis(4-isocyanatocyclohexyl)methane (H12MDI), 1,3-bis(1-isocyanato-l-
methyl)benzene
(XDI), 1,3-bis(1-isocyanato-l-methylethyl)benzene (m-TMXDI), 2,4-
diisocyanatotoluene
(TDI), bis(4-isocyanatophenyl)methane (MDI), 1,6-diisocyanato-2,2,4-(2,4,4)-
trimethyl-
hexane (TMDI) and any isomers, higher homologs or technical-grade mixtures of
the
individual polyisocyanates. In addition, mixtures and derivatives of the
abovementioned
diisocyanates can also be used which have allophanate, biuret, carbodiimide,
isocyanurate,
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6
uretdione or urethane groups, and optionally also blocked polyisocyanates, as
described, for
example, in DE 196 26 886.
The compounds used which have an anionically, cationically and/or nonionically
dispersing
action are those which contain, for example, carboxylate, sulfonate,
phosphonate, sulfonium,
ammonium, phosphonium groups or groups which can be converted to the
aforementioned
groups by salt formation (known as anionically or cationically modifiable
groups/compounds),
and/or polyether groups (known as nonionically emulsifying groups), and which
can be
incorporated into the prepolymers by existing isocyanate-reactive groups, and
having two or
more groups reactive toward polyisocyanates, for example compounds having OH
and/or NH2
groups. Representatives of these compounds are, for example, 2-hydroxymethyl-3-
hydroxypropanoic acid, 2-hydroxymethyl-2-methyl-3-hydroxypropanoic acid, 2-
hydroxy-
methyl-2-ethyl-3-hydroxypropanoic acid, 2-hydroxymethyl-2-propyl-3-
hydroxypropanoic
acid, citric acid, tartaric acid, alanine, taurine, 2-
aminoethylaminoethanesulfonic acid,
polyethylene glycols, polypropylene glycols, polybutylene glycols which have
been started on
alcohols, the block copolymers thereof and monomethyl ethers of these
polyglycols, and also
all polymeric polyols having corresponding modification.
Preferred solvents are solvents inert toward isocyanate groups which have a
boiling point
2o below that of water (at the appropriate pressure at which the flash
evaporation is carried out).
These are, for example, benzene, ethyl acetate, acetone, methyl ethyl ketone,
diethyl ether,
tetrahydrofuran, methyl acetate, acetonitrile, chloroform, methylene chloride,
carbon
tetrachloride, 1,2-dichloroethane, 1,1,2-trichloroethane, tetrachloroethylene
or mixtures
thereof. Preference is given to using water-miscible solvents/solvent
mixtures, very preferably
acetone. However, it is also possible in special cases to use those
solvents/solvent mixtures
which are not inert toward isocyanate groups and have a boiling point below
that of water, for
example alcohols such as methanol, ethanol, or isopropanol. The solvents may
under some
circumstances also contain water.
In addition to the solvents/solvent mixtures which are removed after the
dispersion step, still
further auxiliary solvents which have a boiling point above that of water may
be added, for
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7
example diisopropyl ketone, xylene, dimethylformamide, dimethylacetamide,
dimethyl
sulfoxide, methyl glycol acetate, ethyl glycol acetate, butyl acetate or N-
methylpyrrolidone.
These solvents ultimately remain in the low-solvent dispersions.
The neutralization components used for anionically modifiable polyurethane
prepolymers are
bases, for example tertiary amines, e.g. N,N-dimethylethanolamine, N-
methyldiethanolamine,
triethanolamine, N,N-dimethylisopropanolamine, N-methyldiisopropanolamine,
triiso-
propylamine, N-methylmorpholine, N-ethylmorpholine, triethylamine or ammonia,
or alkali
metal hydroxides, e.g. lithium hydroxide, sodium hydroxide or potassium
hydroxide. For
cationically modifiable polyurethane prepolymers, corresponding acids are
used, for example
formic acid, acetic acid, propionic acid, sulfuric acid, dimethyl sulfate or
succinic acid. In the
case of the nonionically modified polyurethane prepolymers, the neutralization
step is
dispensed with.
In the reaction step in which the molar mass increase takes place, the chain
extender
components used are polyamines having two or more amino groups reactive toward
polyisocyanates. Suitable polyamines are, for example, adipic dihydrazide,
ethylenediamine,
diethylenetriamine, triethylenetetramine, tetraethylenepentamine,
pentaethylenehexamine,
dipropylenetriamine, hexamethylenediamine, hydrazine, isophoronediamine, N-(2-
amino-
ethyl)-2-aminoethanol, 1,3- and 1,4-phenylenediamine, 4,4'-
diphenylmethanediamine, amino-
functional polyethylene oxides or polypropylene oxides, adducts of salts of 2-
acrylamido-2-
methylpropane-l-sulfonic acid and ethylenediamine or any combinations of
polyamines.
The individual process steps of the preparation of aqueous polyurethane
dispersions by the
"acetone process" are effected in any known manner, continuously or batchwise,
according to
the prior art. The dispersion or the mixing the majority of the water is
effected with suitable
mixer units. In the batchwise preparation, these are, for example, stirred
tanks which are
equipped with suitable stirrers and possibly baffles. For the continuous
preparation, in
addition to the abovementioned processes, for example, the stirrer aggregates
or rotor-stator
mixing elements described in GB 14 14 930, DE 22 60 870, DE 23 11 635, DE 23
47 299, DE
23 44 135, DE 33 19 921, DE 36 03 996, US 4742095 or in M. Keyvani, Advances
in
CA 02571711 2006-12-21
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8
Polymer Technology, 22 (2003), 218-224, but also static mixers may be used.
However, these
procedures are not essential to the invention.
What is essential to the invention is only the removal of the solvent/solvent
mixture from
solvent-containing, aqueous polyurethane dispersions or solutions by flash
evaporation.
Suitable apparatus and the principle of the process are described in, for
example, Ullmann's
Encyclopedia of Industrial Chemistry, Release 2003, 7th Edition, Wiley-VCH
Verlag;
Henglein, Lexikon der chemischen Technik [Dictionary of chemical technology],
1 st Edition,
VCH Verlagsgesellschaft, 1988; Vauck, Muller, Grundoperationen chemischer
Verfahrenstechnik [Basic operations of chemical process technology], 11th
Edition, Deutscher
Verlag fiir Grundstoffindustrie, 2000.
In a preferred exemplary embodiment of the process according to the invention,
a solvent-
containing, aqueous polyurethane dispersion or solution is passed from a
reservoir
continuously through a liquid heater and a nozzle into what is known as a
vapor chamber in
which the pressure has been reduced to such an extent that a portion of the
solvent evaporates.
The vapors are removed from the vapor chamber and condensed in a heat
exchanger.
Subsequently, the concentrated dispersion is passed through a further liquid
heater into a
further vapor chamber. For example, the vapors of the first vapor chamber may
be utilized as
a heating medium. In the second vapor chamber, flash evaporation is likewise
effected. The
pressure in the second vapor chamber may be reduced further compared to the
first vapor
chamber. The number of flash evaporation stages depends upon the target
concentration of the
solvent to be achieved. At the end, an aqueous polyurethane dispersion is
obtained which,
depending on the objective, may still have a residual concentration of
solvent.
The examples which follow serve to further illustrate the process according to
the invention,
without it being restricted thereto.
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Example 1
Preparation of a solvent-containing, aqueous polyurethane dispersion
A stirred tank was initially charged with 6792 g of a 50% solution of VESTANAT
T1890
(manufacturer: Degussa AG) in acetone, 2068 g of isophorone diisocyanate, 20 g
of dibutyltin
laurate, 624 g of dimethylolpropionic acid and 3492 g of acetone, and the
temperature was
adjusted to 60 C and the stirrer to 180 rpm. Subsequently, 1248 g of
trimethylolpropane and
9583 g of Oxyester T1136 (manufacturer: Degussa AG) were added. On attainment
of an
NCO number of 0.5%, 226 g of methyl ethyl ketoxime and, I h later, 544 g of
diethylaminoethanol and a solution of 7500 g of VESTANAT B 1358/100
(manufacturer:
Degussa AG), 7500 g of acetone, 128 g of Tinuvine 900 (manufacturer: Ciba
Geigy) and
128 g of Tinuvine 292 (manufacturer: Ciba Geigy) were added. The thus
obtained, acetone-
containing resin solution was subsequently dispersed in 59 800 g of water to
give an acetone-
containing, aqueous polyurethane dispersion (acetone content: 14.4%; solids
content: 25.6%).
Comparative Example A
Conventional removal of the solvent by means of distillation
48 600 g of the acetone-containing, aqueous polyurethane dispersion from
Example I were
heated to 60 C in a reactor having a capacity of approx. 50 1 and an Inter-MIG
stirrer and
attached distillation column, and vacuum was applied. Owing to the vigorous
foaming of the
dispersion, the pressure was reduced slowly, from 600 to 60 mbar within 10.5
h. The resulting
dispersion was corrected using water to a solids content of 33.7% and the
characteristic data
were determined (pH: 8.7; viscosity: 111 mPa*s; average particle size
diameter: 115 nm;
acetone content: 0.39%).
Example 2
Removal of the solvent by means of flash evaporation
48 000 g of the acetone-containing, aqueous polyurethane dispersion from
Example 1 were
heated in a reservoir to a temperature of 59 C at a pressure of I bar and
passed continuously
through a pipeline (internal diameter = 6 mm) into a flash vessel (throughput:
20.2 kg/h) in
which there was a pressure of 137 mbar. The resulting vapors were withdrawn
via a side draw
and condensed in a condenser. The product withdrawn from the bottom had an
acetone
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content of 3.8% and was used in the apparatus two further times. The
conditions were
comparable to the first throughput, only the pressure was reduced further (to
90 mbar and
subsequently to 64 mbar) and the throughput increased (to 25.4 kg/h and
subsequently to
30.1 kg/h). This further reduced the acetone content in stages (to 1.6% and
subsequently to
5 0.4%). The entire process for solvent removal took 5.5 h. The resultant
dispersion was
corrected using water to a solids content of 33.3% and the characteristic data
were determined
(pH: 8.9; viscosity: 95 mPa*s; particle size diameter: 110 nm; acetone
content: 0.41%).
