Note: Descriptions are shown in the official language in which they were submitted.
;9~C~
Composite membranes, processes for their preparation and
their use
BACKGROUND OF THE INVENTION
1. FIELD OF THE INVENTION
The invention relates ~o new composite membranes,
processes for their preparation and their use for
removing benzenes optionally substituted by lower alkyl
radicals, hydroxyl, chlorine or bromine from their
mixtures with aliphatic and/or cycloaliphatic hydro-
carbons, alcohols, ethers, ketones and/or carboxylicacid esters or from effluent.
Membranes can be used for removal of substance
mixtures by permeation. A procedure can be followed here
in which, for example, a substance mix~ure in the liquid
phase (feed solution) is brought to one side of the
membrane and one substance therefrom, a cer~ain group
of substances therefrom or a mixture enriched in the one
substance or in the certain group of substances is
removed, also in the liquid form, on the other side of
the membrance (permeation in the narrower sense). The
substance which has passed through the membrance and has
been collected again on the other side or the substance
mixture described is called the permeate. However, it
is also possible to follow the procedure in which, for
example, the feed is brought to the one side of the
membrane in liquid or gaseous form, preferably in liquid
form, and the permeate is removed in the form of a
vapour on the other side and is then condensed (perva-
~5 poration).
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Such permeation processes are useful additions to
other processes of substance removal, such as
distillation or absorption. Permeation, specifically
pervaporation, can be of useful service in particular
in the removal of substance mixtures which boil as
azeotropes.
2. DESCRITPTION OF THE STATE OF THE ART
There have previously been many attempts to adapt
membranes of various polymer materials to individual
specific purposes. It is thus known from US 2,95~,520
to enrich benzene in the permeate and in this way
substantially to separate it off from an azeotropic
benzenelmethanol mixture with the aid of a non-porous
plastic membrane of polyethylene. It is furthermore
known from US 3,776,970 to separate the two aromatic
compounds styrene and ethylbenzene with the aid of a
membrane of certain polyurethane elastomers such that
styrene is enriched in the permeate. It is furthermore
known from German Patent Specification 2,627,629 to
remove benzene and alkylbenzenes from aliphatic
hydrocarbons, cycloaliphatic hydrocarbons, alcohols,
e~hers and carboxylic acid esters with the aid of
polyurethane membranes.
SUMMARY OF THE I NVENTION
It has now been found, surprisingly, that the
removal of benzene~ optionally substituted by lower
alkyl radicals, hydroxyl, chlorine or bromine from their
mixtures with aliphatic and/or cycloaliphatic hydro-
carbons, alcohols, ethers~ ketones and/or carboxylicacid esters or from effluent can be substantially
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improved using ~he composite membrane described below
in comparison with the polyurethane membranes described
in German Paten Specification 2,627,629, these improved
removal effects becoming particularly clear in the field
of mixtures of low aromatic content.
The invention thus relates to composite membranes
consisting of
i) a macroporous membrane of at least two incompatible
p~lymers containing a~ least one filler, whereby such
filler or a mixture of several of them amounts to 30 -
85 % ~f the total weight of the filler~s) and the in-
compatible polymers andii~ a pore-free polyurethane (P~) membrane applied to
i ) .
DETAILED DESCRIPTION OF THE INVENTION
The macroporous membrane according to i) consists
of at least two polymers which are incompatible in
solution, that is to say, lead to phase separation in
a common solution. Further details on incompatible
polymer systems which demix are to be found in the
monograph by Paul J. ~lory, Principles of P~lymer
Chemistry, Ithaca, N.Y., (195~). By dispersing of at
least one insoluble filler in~o this unstable mixture,
this mixture is converted into a stable homogeneous
dispersion. This dispersion is then applied to a
substrate as a casting solution. The macroporous
~0 filler(s)-containing membrane according to i) is
produced from this casting solution by precipi~ation
coagulation, which is also called phase inversion. This
technology of phase inversion is known, for example from
H. Strathmann, Trennungen von mole~ularen Mischungen mit
Hilfe synthetischer Membranen (Separations of Molecular
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Mixtures with the aid of synthetic membranes), Stein-
kopf-Verlag, Darmstadt (1979) and D.R. Lloyds, Materials
Science of Synthetic Membranes, ACS Symp. Ser. 269,
Washington ~.C. (1985),
These publications also describe the typical
membrane structures obtained during precipitation
coagulation. These are always asymmetric membrane
structures with a denser polymer skin on ~he membrane
surface and higher porosities inside the membrane. The
pore structure can be finger-like or foamlil~e, depending
on the recipe of the casting solution. By forming the
denser polymer skin on the membrane surface, the pore
diameters of the conventional memhranes are limited and
as a rule do not exceed values of about 8-10 ~m.
Homogeneous polymer casting solutions are used as
the starting substances in the production of precipi-
tation coagulation membranes of the conventional type,
since otherwise unstable membranes are obtained. For
this reason, typical membrane casting solutions are
formed from a polymer and a solvent or solvent mixture
(for example polyamide in dimethylacetamide or cellulose
acetate in acetone/formamide),
There have already been attempts to produce
membranes having increased permeabilities by specific
recipes of the polymer casting solutions, Membranes are
described in Chem. Pro. Res, ~ev, 22 ~1983), 320-326 or
~ in DE-OS (German Published Specification) 3,149~976
which have been produced using polymer casting solutions
containing water-soluble polymers, such as polyvinyl-
pyrrolidone, which are dissolved out during the
coagulation in water and in this way lead to enlarged
Le A 26 927
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o
pores. Membranes of polymer mixtures have also been
des~ribed. However, the recipes of the corresponding
casting sDlutions are built up in such a way that
homogeneous polymer solutions are obtained on the basis
of the solubility parameters. For example, EP 66,408
describes membranes of a mixture of cellulose acetate
and polymethyl methacrylate which have increased
permeabilities in comparison with the conventional
membranes of only one polymer. However, polymer
combinations with similar solubility parameters and
certain very narrow mixing ratios are depended upon
here.
It has now been found, surprisingly, that macro-
porous membranes of polymers which are incompatible and
immiscible per se can be processed in any desired mixing
ratio to give homogeneous casting solutions if certain
insoluble fillers are dispersed in them and which dis-
play the abovementioned better removal effects in
association with pore-free polyurethane (PU) membranes
applied to them.
For example, if a 20 % strength by weight solution
of polyurethane in dimethylformamide (PUIDMF solution)
and a 20 % strength by weight solution of polyacrylo-
nitrile in dimethylformamide (PANIDMF solution) are
mixed, while stirring, phase separation occurs after the
mixture has stood for a short while. Such mixtures are
~ unstable and are unsuitable as casting solutions for
production of membranes. In contracst, if the same
polymerlDMF solutions are combined with simultaneous or
subsequent dispersing in of fillers, for example talc,
homogeneous stable casting solutions which are suitable
Le A 26 927
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for membrane production by the precipitation coagulation
meLhod are obtained
In comparison with the known membranes, the
membranes produced fr~m such cas~ing solutions have
significantly larger pores on the surface and a very
much higher overall porosity.
As electron microscopy photographs of the cross-
section of these polymer membranes show, these are
structures with a felt-like build-up, whereas the known
asymetric structure build-up with a denser polymer skin
on the membranes surface is almost completely
suppressed. Average pore diameters of up to 3D ~m can
be detected on the membrane surface of a membrane of the
above recipe.
The polymer casting solutions required for
production of such membrane matrices must fulfil the
following conditions:
a) The solutions of ~he individual polymer components
should not be miscible with one another. With
miscible systems, analogously to conventional
casting solu~ions, membrane structures of fine
porosity and pronounced asymmetric structure are
obtained.
b) The solvents of the individual polymer components
must be miscible with one ano~her.
c) To convert the immiscible polymer components into
homogeneous casting solutions, suitable insoluble
fillers, for example inorganic fillers, must be
dispersed in them in an amount which constitutes
30-85 % of the total weight of the filler(s) and
the incompatible polymers. In a preferred variant
the filler~s) constitutes 50-75 ~/. of the total
weight.
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The nature of the filler can in some cases be
important for the stability and homogeneity of the
casting solution. Whereas, for example, cast;ng
solutions of PU/PAN mixtures con$aining t;tan;um d;oxide
(Tio2RKB2~, Bayer AG) or barium sulphate (Blanc Fixe
Mikron~, Sachtleben) having specific surface areas of
about 3 m2/g (particle size about 0.5-1.0 ~m) are less
favourable in respect of stability and homogeneity,
solutions of the same polymer mixture containing talc
(Talc AT 1, Norwegian Talc) show a good homogeneity and
dispersion stability.
Similarly good results could also be obtained with
very fine-grained fillers of high specific surface area,
for example with the titan;um dioxide Degussa P25 (about
40 m2/g) or the silicon diox;de Aerosil 200~, Degussa
(2~0 m2lg). Mixtures of talc with bar;um sulphate or
talc with Tio2 RKB2~ or titanium dioxide P25~, Degussa,
w;th barium sulphate lead to suitable casting solutions.
It was also possible to prepare suitable casting
solutions by dispersing in microcrystalline cellulose
tfor example Arbocel B E 600/30~, J. Rettenmaier &
50hne). Other suitable fillers are CaC03, MgC03, ZnO and
ron oxides.
In addition to the fillers already mentioned, there
may also be mentioned zeolites and bentonites, and
furthermore mix~ures of Tio2 with BaS04 or talc with
BaS04, and furthermore mixtures of Tio2 of large and
small specific surface area, such as Tio2 RKB2~
Bayer/TiO2 P 25~ Degussa, Preferred fillers are: $alc,
microcrystalline cellulose, zeolites, bentonitesg BaS04,
Tio2 and SiO2.
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The func~ion and action of the filler is conversion
of the uns~able inhomogeneous polymer solution into
stable and homogeneous casting solutions; the mechanism
of this "solubilization" is unknown. By informing pre-
liminary tes~s sui~able filler/polymer combinations can
be found.
The pore size is con~rolled via ~he choice of
polymers and the par~icular quan~ities. The fillers have
only a minor influence, if any, on ~he pore sizs. The
particle diameters of the fillers are of smaller order
of size~ namely of from 0.007 - 16 ~m, often 0.3 - 5 ~m,
~han the pore diameters of the polymer membrane
(~ 30 ~m). The process of precipita~ion coagulation in
combination with the type of casting solu~ions described
here is responsible for the pore forma~ion of the mem-
branes according to the inven~ion. The range of the
average pore size of ~he macroporous membranes according
to ~he invention is 10 ~o 30 ~m, preferably 15 to 25 ~m.
Such an average pore size does not exclude the occur-
rence of pores in a range below (for example from 1 ~m~
and in a range above (for example up to 50 ~m).
The following polymer classes, for example, can be
used ~o produce the macroporous filler-containing
membrane according to i): cellulose esters, polyvinyl
esters, polyurethanes, polyacrylic derivatives and
acrylic copolymers, polycarbonates and ~heir copolymers,
~ polysulphones, polyamides, polyimides, polyhydantoins,
polystyrene and styrene copolymers, poly(para-dimethyl-
phenylene oxide), polyvinylidine fluoride, polyacrylo-
nitrile and e~hylene/vinyl ace~a~e copolymers containing
at least 50 % by weigh~ of vinyl acetate,
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Preferably, two or three incompatible polymers from
the class of polyurethanes, polyacrylonitrile, polyvinyl
acetate, polystyrene~ polysulphone, polyvinylidene
fluoride~ polyamide, polyhydantoin and ethylene/vinyl
acetate copolymers containing at least 50 % by weight
of vinyl a etate are employed. Examples o~ binary
incompatible polymer systems are:
- cellulose esters/polyvinyl esters (such as the
cellulose acetate Cellidor CP~/the polyvinyl
acetate Mowilith~)
- polyurethane/polyacrylic derivatives (such as
Desmoderm KBH~/the polyacrylonitrile Dralon ~ or
Desmoderm KBH~/amine modified Dralon A~ or
Desmoderm KB ~ /anionically modified Dralon U~, that
is to say provided with sulphate groups)
- polycarbonate copolymers/polyurethane (such as
polyether polycarbonate/Desmoderm KBH~)
- polyvinyl derivatives/polysulphones (such as
polyvinylidine fluoride/the polysulphone Udel P
1700~)
- polyamides or polyimides/polystyrene or styrene
copolymers
- poly(para-dimethyl-phenylene oxide)/polyvinylidene
fluride and
- polyhydantoin/polystyrene.
Other two-component combinations which may be
mentioned are: Dralon U~/Mowilith~ and Cellidor
CP~/Dralon U~; examples of ternary polymer mixtures are
Cellidor CP~/Dralon U~/polystyrene, Mowilith
R~/Desmoderm KB ~ /polyvinyl chloride and Desmoderm
KB ~ /Mowilith R~/Dralon ~ , it also bein~ possible for
Dralon ~ to be replaced by Dralon A~.
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Preferred binary and ternary polymer sys~ems are;
Desmoderm KBH~/Dralon ~ , Desmoderm KB ~IDralon A~J
Desmoderm KBH~/Mowilith~/Dralon ~, it also being
possible for Dralon ~ to be replaced by Dralon A~ or
Dralon U~.
The chemical structures of the polymers preferably
employed are described in the appendix to the embodiment
examples.
Generally, even 4 or more incompatible polymers can
be used but ~his results, at a higher effort, in no
additional advantage.
The ratio of the amounts of the polymers, which is
required for the pore diameters, in the particular
combinations can be determined by appropriate
experiments.
If the polymers, of which ~here are at least two, are
mixed in approximately the same amounts, as a rule
higher values for the average pore sizes are ob~ained;
if the amounts differ relatively widely, lower values
are obtained. The polymer casting solution when
consisting ot 2 polymers should contain at least 10 %
by weight of one polymer based on the total amount of
all the polymers. With more than 2 incompatible
polymers, this minimum amount of one polymer should be
% by weight of all ~he polymers.
The macroporous filler(s~-containing membrane i)
as a part of the composite membranes according to the
invention has a thickness of from 10 - 200 ~m,
preferably 30 - 100 ~m.
Dimethylformamide (DMF) is a particularly suitable
solvent for the preparation of casting solutions of the
Le A 26 927
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preferred polymer combinations. Other suitable solvents
are, depending on the polymers used: N-methylpyrrolidone
(NMP), dimethyl sulphoxide (DMSO), dimethylacetamide,
dioxolane, dioxane, acetone, methyl ethyl ketone or
Cellosolve~.
The amount of solvent is chosen such that a
viscosity of the casting solution which reaches the
range from 500 to 25,000 mPas is achieved~ As a rule,
this correspondends to a polymer content of 10 to 40 %
by weight in the overall filler(s)-containing casting
solution.
The overall process for the preparation of content
i) in the composite membranes according to the invention
can be described with the aid of a preferred example as
follows: The DMF polymer solutions, in each case about
20 % strength by weight, of Desmoderm KBHR, Mowilith~
and Dralon ~ were mixed with the aid of a high-speed
stirrer (dissolver) to give a homogeneous polymer
casting solution, talc being dispersed in. After
degassing in vacuo, this casting solution was applied
in a layer thickness of, for example, 150 ~m with the
aid of a doctor blade to a carrier substrate and was
dipped in the coagulation bath, for example pure water.
After a residence time of about 2 minutes, the micro-
porous filler-containing membrane formed in this way was
removed from the coagulation bath and dried with warm
air.
Surfactants, for example dioctyl sodium sulpho-
succinate or dodecylbenzenesulphonates, can also be used
to prepare the casting solution in an amount of from 2 -
10 % of the total weight of ~he casting solution.
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Water-soluble polymers, such as cellulose ethers,
polyethylene glycols, polyvinyl alcohol or polyvinyl-
pyrrolidone can also be a constituent of the polymer
casting solution. Other possible additives are so-called
coagulation auxiliaries, such as, for example, cationic
polyurethane dispersions (such as Desmoderm Koagulant
KPK~). The water-soluble polymers and the further
additives can constitute 0 - 10 ~/. of the total weight
of the casting solution.
The carrier substrates used for application of the
casting solution can be one which merely serves for the
production of the macroporous filler-containing membrane
according to i) and is therefore peeled off again after
the coagulation operation on i)~ For this purpose, the
carrier substrate must be smooth and is, for example,
glass, a polyethylene terephthalate film or a sili-
conized carrier material. However, if the composite
membrane according to the invention of i) and ii) is to
be provided with a support material for improving the
Mechanical stability, materials which are permeable to
liquid, such as woven polymer fabric or polymer non-
wovens, to which ~he macroporous filler-containing
membrane i) shows good adhesion are used as the carrier
substrate, The co-use of such a support material (woven
fabric or non-woven) is preferred for the composite
membranes according to the invention. Suitable materials
for this are: polypropylene and polyester non-wovens,
multi-fibrous polyester, polyamide, and glass-fiber
woven fabrics.
It is furthermore known, for increasing the surface
area of membranes, also to use these in the form of
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21)~6~0
tubes, hoses or hollow fibres, as well as in the form
of films, produc~ion of which has jus~ been described.
These tubes, hoses or hollow fibres can be arranged and
used in special separat;on units, which are called
modules~ in order to achieve maximum membrane surface
areas with the minimum possible apparatus volumes. Such
tubes, hoses or hollow fibres can be produced, for
example, by forcing the filler-containing and in this
way stabilized casting solution described above through
the outer annular gap of a concentric two-component die,
whilst a coagulating agent, such as water, is forced
through the central die opening and the casting solu~ion
which issues moreover en~ers a coagulation bath, such
as water; coagulation is in this way performed from the
inside and from the outside.
Af~er coagulation and drying, a pore-free poly-
urethane (PU) membrane is applied to the macroporous
filler-containing membrane i) by the casting technique.
The thickness of this pore-free PU membrane is
0,5 - 500 ~m, preferably 5 - 50 ~m.
Polyurethanes for this pore-free PU membrane ii)
and their preparation are known. Polyurethanes are in
general prepared by reaction of higher molecular weight
di- or polyhydroxy compounds and aliphatic, araliphatic
or aromatic di- or polyisocyanates and if appropriate
so-called chain-lengthening agents.
Examples which may be mentioned of starting
materials containing OH end groups are: polyesters of
carbonic acid and aliphatic dicarboxylic acids having
2 - 10 C atoms~ preferably of adipic and sebacic acid,
with aliphatic dialcohols having 2 - 10 C atoms,
Le A 26 927
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preferably those having 2 to 6 C atoms, it also being
possible for the dialcohols to be used as a mix~ure in
order to lower the melting points of the polyesters~
polyester of low molecular weight aliphatic lactones and
~-hydroxycarboxylic acids, preferably of caprolactone
or ~-hydroxycapric acid, the carboxyl groups of which
have been reacted with diols; and furthermore poly-
alkylkene etherdiols, specifically polytetramethyleneetherdiols, polytrimethylene etherdiols, polypropylene
glycol or corresponding copolyethers.
Aromatic diisocyanates, such as toluylene ~iiso-
cyanate ard m-xylylene diisocyanate, araliphatic diiso-
cyanates, such as diphenylmethane 4,4 -diisocyanate, or
aliphatic and cycloaliphatic diisocyanates, such as
hexamethylene diisocyanate and dicyclohexylmethane 4,4`-
di-isocya~ate, as well as isophorone diisocyanate, are
used as the diisocyanates~
If appropriate, these starting materials can also
be reacted with dialcohols which are additionally
employedg to give so-called prepolymers, and these can
then be polymerized again with further di- or polyhy-
droxy compounds and di- or polyisocyanates and if
appropriate further chain-lengthening agents. In
addition to the two-dimensionally crosslinked poly-
urethanes obtainable by using diols and diisocyanates,
three-dimensionally crosslinked polyurethanes can also
be obtained if trihydroxy compounds and/or polyols
and/or tris- and/or polyisocyanates are simultaneously
used as starting materials in the polymerization.
Three-dimensional crosslinking can also be
achieved, however, if two-dimensionally crosslinked
polyurethanes which still contain free hydroxyl and/or
Le A 26 927
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polycyanate groups are subsequently further reacted wiLh
trifunctional alcohols and/or isocyanates, Such three-
dimensionally crosslinked polyursthanes can likewise be
obtained by subsequent reaction of two-dimensionally
crosslinked polyurethanes containing free isocyanate end
groups with small amounts of polymers having end groups
containing reactive hydrogen atoms, such as formaldehyde
resins or melamine resins. Film-forming elastic poly-
urethanes are preferably used for the pore-free P~
membranes ii), these being prepared as so-called one-
component PU wi~h a characteristic numer (equivalent)
NCO or NCO
OH OH + NH2
of about 1.0, for example in the range from 0,95 to 1.1.
Butane-1,4-diol adipic acid polyester, hexamethylene
1,6-glycol adipic acid polyester and hexane-1,6-diol
poly-carbonate, in particular, are employed here as
diols.
Preferred diisocyanates are isophorone diiso-
cyanate, 4,4 -diisocyanato-diphenylmethane and ~oluylene
diisocyanate. Ethylene glycol, butane-1,4-diol, ethanol-
amine and diamino-dicyclohexyl-methane are preferably
used as chain-lengthening agents,
This group àlso includes polyurethanes which are
prepared from a prepolymer having free hydroxyl groups,
a diol and a diisocyanate with a characteristic number
NCO of about 1,
OH
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9~
Another preferred group of such film-forming poly-
urethanes are so-called two-component PUs" of one of
the abovementioned polyurethanes, which have been cross-
linked by subsequent fur~her polymerization with a
polyol, such as trime~hylolpropane, and if appropriate
a chain-lengthener, such as butylene 1,3-glycol, and a
diisocyanate, This group of two-component PUs also
includes those polyurethanes which have subsequently
been further crosslinkinked with formaldehyde resins or
melamine resins.
Other polyurethanes can of course also be used for
the production of the pore-free PU membranes ii) such
as are used in the composite membranes according to the
invention; only those polyurethanes which dissolve in
the aromatic and aliphatic or cycloaliphatic hydrocar-
bons to be separated are unsuitable.
In addition to the abovementioned casting ~echnique
for application of the pore-free PU membrane ii) onto
the microporous filler-containing membrane i), appli-
cation by extrusion, calendering or the injection
moulding technique is in principle also conceivable.
However, application by the casting technique is
preferred.
Within ~he casting technique, a possible embodiment
is to add acrylates to the PU casting solution, ThPse
added acrylates enable the pore-free PU membrane ii) to
~ crosslink within the composite membranes according to
the invention by UV irradiation or Y radiation or
electron beams and in this way to be stabilized
mechanically.
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Possible acrylates are acrylic a~id esters andlor
S methacrylic acid esters of diols having 4 - 12 C aLoms
or of tri- or tetraalcohols, in particular butane-1,4-
diol acrylate, butanediol bis-methacrylate, and in
particular trime~hylolpropane trisacrylate, trimethylol-
propane trimethacrylate, pentaerythritol tetraacrylate
or pentaerythritol tetramethacrylate, or urethane acry-
lates (for example reaction products of trimethylol-
propane, isophorone diisocyanate and hydroxyethyl acry-
late). Their amount is 4 - 24 % by weight, based on the
~otal amount of polyurethane and acrylates. A cross-
linkable acrylate/polyurethane blend is thus obtained
for ii). Trimethylolpropane trisacrylate is partieularly
preferably employed.
If a~ueous PU dispersions (~ngew. Makromolek.
Chemie 9A (1981) 13~-165) are used for the production
of the pore-free PU membrane ii), these can be cross-
linked with carbodiimides, if appropriate, in order toimprove the mechanical strength.
Plasticizers, such as nonylphenol, or fillers, such
as finely divided SiO2 (for example silica gel or
Aerosil grades from Degussa) and zeolites, can further-
more also be used for production of ~he PU membrane
i i ) -
The invention furthermore relates to production of
composite membranes of the abovementioned type, which
~ is characterized in that
a) at least one insoluble filler is dispersed in a
solution containing at least two incompatible polymers
in amounts which lead to phase separation in the
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Le A 26 927
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;~0~6~
solution whereby such filler or a mixture of several of
them amounts to 30 -85 % of the total weight of the
filler(s) and the incompatible polymers, a homogeneous
casting solution being formed,
b) this solution is processed to membranes in the form
of films, tubes, hoses or hollow fibres and precipita-
tion coagulation is carried out andc) a pore-free PU membrane is applied to the macro-
porous filler~containing membrane obtained in this way.
In the production of the membranes in step b) in
the form of films, the solution is applied to a carrier
substrate and, after the precipitation coagulation in
the manner described above before step c) is carried
out, the coagulate is detached from the carrier sub-
strate.
Preferably, however, this process is modified so
that the carrier substrate is a support material of the
type mentioned, which remains on the composite membrane.
The pore-free PU membrane ii) is then applied in the
casting process in the manner described abo~e.
In the case where the composite membranes according
to the invention are produced in the form of tubes,
hoses or hollow fibres, after production of the macro-
porous filler-containing membrane i), for example by
extrusion and coagulation in the manner described above,
a PU casting solution is applied to the inside of such
tubes, hoses or hollow fibres by casting in order to
produce the pore-free PU membrane ii), the system being
subsequently flushed with an inert gas, if appropriate,
for example in order to avoid sticking of the inside in
the case of hollow fibres. This inert gas can at the
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Le A 26 927
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2~16~
same time be prewarmed in order to effect evaporation
of the solvent from the casting solution. Such a method
of application of ii) is suitable for brin~ing the
mixture to be separated, of benzenes optionally sub-
stituted by lower alkyl radicals, hydroxyl, chlorine or
bromine and aliphatic and/or cycloaliphatic hydro-
carbons, alcohols, ethers, ketones and/or carboxylic
acid esters, or the effluent containing such benzenes
inside these tubes, hoses or hollow fibres and for
removing the permeate enriched in optionally substituted
benzene from the outer surface of the tubes, hoses or
hollow fibres. This type of build-up of the composite
membranes according to the invention is particularly
favourable if a pressure gradient from a higher to a
lower pressure is to be applied from the mixture side
to the permeate side.
In addition, the reverse use is in principle also
possible~ that is to say bringing of the starting
mixture onto the outer surface of the tubes, hoses or
hollow fibres and removal of the permeate from the
inside surface. For this embodiment, the P~ casting
solution for the production of ii) must be brought onto
the outer surface of tubes, hoses or hollow fibres of
the macroporous filler-containing membrane i).
The invention furthermore relates to the use of the
composite membranes described above for removing
benzene, which can be mono-, di- or trisubstituted by
chlorine, bromine, C1-C4-alkyl or hydroxyl from
aliphatic and/or cycloaliphatic hydrocarbons, alcohols,
ethers, ketones and/or carboxylic acid esters or from
effluent.
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Optionally substituted benzenes are: benzene,
toluene, xylene, ethylbenzene, propylbenzene, chloro-
benzene, dichlorobenzene, bromobenzene, phenol or
cresol.
Examples of aliphatic or cycloal;cphatic hydro-
carbons from which the optionally substituted benzene
is to be removed are, for example, straight-chain or
branched hydrocarbons having 5 - 14 C atoms, such as
pentane, hexane, heptane, 2-methyl- and 3-methylhexane,
2,2-dimethylpentane, 2,4-dimethylpentane, 2,2,3-tri-
methylbutane, straigh~-chain or branched tetradecane,
i-octane or cycloaliphatic hydrocarbons, in particular
having 5 and 6 ring C atoms, which can also be substi-
tuted by C1-C~-alkyl~ preferably C1-C4-alkyl and
particularly preferably by methyl and ethyl. These
aliphatic or cycloaliphatic hydrocarbons can be present
individually or as a mixture; mixtures of petrochemical
origin, for example for fuels, are preferably suitable.
Preferred cycloaliphatic hydrocarbons in these are
methylcyclopentane, cyclohexane and methylcyclohexane.
It is also possible for more than one optionally
substituted benzene for removal to be present in the
mixture.
Possible further organic solvents from which
optionally substituted benzenes can be removed with the
aid of the membrane according to the invention are
alcohols, such as ethanol; ethers, such as dioxane;
ketonesf such as cyclohexanone, and carboxylic acid
esters, such as ethyl acetate.
Le A 26 927
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6~
The removal is by liquidlliquid permeation,
gaseous/gaseous pervaporation or liquid/gaseous
pervaporation, preferably by liquidlgaseous
pervaporation. The techniques needed for this are ~nown
to the expert. Preferably, a pressure gradient in the
direction of the permeate is used, for which a reduced
pressure ~for example 1 - 500 mbar~ is applied to the
permeate side.
It is surprising that the composite membranes
according to the invention have a significantly improved
separation factor for optionally substituted benzenes.
The separation factor K, which represents a measure
of the selective permeability of the membrane, is
generally stated as a measure of the removal effect; it
is defined by the following equation:
CAp CBg
o~ = x
CBp CAg
in which
CAp and CBp denote the concentrations of substances
A and B in the permeate (p) and CAg and CBg denote
~he corresponding concentrations in the mixture (g~
to be separated,
and wherein
A in each case denotes the component to be removed,
in the present case the optionally substituted
benzene (or several benzenes) and B denotes the
other or remaining components of the mixture.
Le A 26 927
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~O~L~9~i~
A very surprising effect of the composite membranes
according to the inven~ion is their successful use for
removal of optionall~ substituted benzene from
effluent.
ExamDle 1
a) Production of the macroporous filler-containing
polymer blend membrane:
21.6 g of a 17 % strength Dralon ~ /DMF solution,
65.2 g of a 20 % strength KBH~ polyurethane/DMF
solution, 86.6 9 of a 25 % strength Mowilith 50~/DMF
solution, 22.5 g of sodium dioctyl sulphosuccinate,
14.8 g of talc AT 1, 59.4 9 of barium sulphate (Blanc
Fixe Mikron), 17.3 g of KPK~ (Bayer AG, cationic
polyurethane dispersion) and 140.0 g of DMF were
processed to a homogeneous dispersion with the aid of
a high-speed stirrer (dissolver). After degassing in
vacuo, this casting solution was coated in a layer
thickness of 150 ~m with the aid of a doctor blade onto
a polypropylene non-woven 200 ~m thic~ (type F0 2430
from Freudenberg~ and coagulated in water at 45 for
3 minutes. The polymer matrix formed in this way and
resting on the carrier film was dried by means of warm
air.
b~ Application of the pore-free PU membrane (pro-
duction of the composite membrane according t~ the
invention):
the porous membrane matrix obtained according to
a) was coated with the following polyurethane: 100.0 g
of poly-hexanediol adipate (average molecular weight
about 850j, 57,5 g of isophorone diisccyanate and 23.7 g
Le A 26 927
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;9~0
of isophoronediamine were reacted with one another in
a known manner. A 30 % strength solution (weight/volume)
of this polyureLhane in a mixture of toluene and iso-
propanol (1:1) was f;ltered through a pressure fil~er
and ~he filtrate was left to stand until it was free
from bubbles. This polyurethane casting solution was
applied with a wet application of 100 ~m onto the
macroporous carrier membrane described in a). The
solvent was removed with ~he aid of warm air; the
composite membrane No. Z characterized in Figures l and
2 was in this way obtained.
The membrane No. 3 characterized in Figures 1 and
2 (for comparison) was obtained by coating a polyamide
microfiltration (MF) membrane (Pall, 0.2 ~m~ with the
same polymer casting solution according to b) under ~he
same production parameters.
Example 2 ~for comparison)
Production of the carrier-free polyurethane perva-
poration membrane
The polymer solution described in Example lb) was
coa~ed in a layer thic~ness of 100 ~m onto a transparent
polyethylene terephthalate film (PET film). The solvent
was removed by evaporation with warm air; the membrane
film adhering to the PET film was in this way obtained.
Membrane No. 1 charac~erized in Figures 1 and 2 was
obtained by careful peelinq off from the PET film.
Example 3
Produc~ion of a composite membrane with a pore-free
acrylate/polyurethane blend separating layer:
~5
Le A 26 927
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3.75 g of trimethylolpropane triacrylate
(commercial product from Rohm) and 0.18 g of 1-
hydroxycyclohexylphenyl ~etone (Irgacure 184~,
commercial product from Ciba-Geigy), as a photo-
initiator, were added to a polyurethane casting solution
of 25.0 g of polyurethane (chemical structure as in
Example lb), 37.5 g of toluene and ~7.5 g of isopro-
panol.
The mixture was homogenized by stirring and left
to stand for degassing, This casting solution was then
applied in a layer thickness of 150 ~m to the polymer
blend membrane described in Example la) and the solvent
was subsequently evaporated off. The pore-free acry-
late/polyurethane blend layer formed in this way was
crosslinked with the aid of UV light.
Exposure conditions:
20 Exposure apparatus: Hanovia
Radiation source: medium-pressure
mercury vapour lamp
~amp output: 80 W/cm
Distance between sample and lamp: 11 cm
Belt speed: 10 m/minute
The separation effect and flow characteristics of
this membrane during toluene/cyclohexane separation
\ corresponded to those of the membrane described in
Example 1 (Figure 1). However, improved membrane
stabilities could be observed at high temperatures, e,g,
around 90C.
Le A 26 927
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Example 4
Toluene/cyclohexane separation:
The membranes described in Examples 1 and 2 were
tested with the aid of a pervaporator module, such as
is described, for example, in DE-OS (German Published
Specification) 3,441,190, under the same conditions by
allowing feed solutions of various compositions to flow
\ over. The experimental conditions and the experimental
results are shown in Figures 1 and 2.
The increase in selectivity when the macroporous
polymer blend membrane is used according to the
invention as a composite component in comparison with
membrane No,1 is striking. Whereas the composite
membrane according to the invention remained fully
functional for several days at 50C, polyurethane
membrane No, 1 dissolved after a few hours under th~se
conditions,
~0
Le A 26 927
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Explanatory note on Figures 1 and 2:
The composition of the substance mixture to be
separated (feed) as a function of increasing toluene
content is in each case shown on the abscissa. The
permeate concentration with increasing toluene content
is shown on the ordinate in Figure 1 and the correspond-
ing permeate flow is shown on the ordinate in Figure 2.
Composite membrane No. 2 according to the invention
shows an unexpected increase in selectivity (increase
in Lhe separation factor ~), especially in the region
of low toluene concentrations. The macroporous filler-
containing membrane (i) of at least two incompatible
polymers thus contributes towards the selecting effect,
although it places no resistance against the feed
because of the macroporous structure and thus displays
no corresponding separation action in accordance with
the concept of the solubility/diffusion model. The
composite membrane according to the invention is
additionally overall more mechanically and chemically
stable, even at higher temperatures.
ExamDle 5
Removal of chlorobenzene from an effluent:
The feed solution to be purified was an effluent
which contained 10 % of ethanol and 150 ppm of chloro-
benzene. Composite membrane No. 2 from Example 1 was
used. The feed solution was kept static (without flowing
over) on the membrane (temperature = 30C; permeate
pressure p = 11 mbar).
After 4 hours of testing, the content of chloro-
benze in the feed solution had been reduced to
0,02 ppm-
_e A 26 927
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L6~
Example 6
Separation of benzene/cyclohexane:
Composits membrane No. 2 from Example 1 was used.
Composi~ion of ~he feed solution: 55 % of benzene, 45 %
of cyclohexane.
The experiment was carried out as in Example 4, A
flow of 0.6 l/m2 x hour was determined. Only traces
(~ 0,5 % of cyclohexane~ could be found in the per-
meate.
Exam~le 7
a) Production of a macroporous filler-containing
polymer blend membrane:
21,6 g of a 17 % strength Dralon ~DMF solution,
62,5 g of a 20 % strength KBH~ polyurethane/DMF
solution, 86,6 g of a Z5 % strength Mc,wilith~lDMF
solution, 1,5 g sodium dodecyl benzenesulphonate, 74.2 g
Talc AT 1, and 80.0 g of DMF were processed according
to Example 1 to a macroporous membrane.
b) Application of the pore-free P~ membrane (pro-
duction of the composite membrane accord;ng to the
invention):
The porous membrane matrix obtained according to
a) was coated with the following polyurethane: 100.0 g
of poly-butanediol adipate~ 10.0 g butanediol, and
38.7 g of diphenylmethane diisocyanate were reacted with
one another in a known manner. A 30 % by weight solution
of this polyurethane in a mixture of DMF ard butanol
(3:2) was produced in analogy to Example lb) and coated
onto the support membrane described under a).
Le A 26 927
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~o~g~
A fuel mixture which contained, according to gas-
chromatographic analysisJ 55 components with more than
1 %, was employed for the separation by pervaporation.
The analytical determination of the permeate and
the retentate with respect to aromatic compounds gave,
after a one day pervaporation, the following results:
Retentate Permeate
benzene4 % 10 %
toluene7 % 17 %
o-xylene6 % 8 %
15 p/m-xylene18 % 24 %
As is indicated by the results, the pervaporation
leads to a remarkable derichment with respect to benzene
and toluene.
Appendix:
Chemical structures of the polymers preferably used
Polyurethane (KBH~, Bayer AG)
Thermoplastic polyadduct which was obtained by
reaction of 75 parts of a polyester of adipic acid,
ethylene glycol and 1~4-butanediol (molecular weight =
2,000), 25 parts of a polyester of adipic acid and 1,4-
butanediol (mole~ular weight = 2,250), 25 parts of 1,4-
butanediol and 85 parts of diphenylmethane 4,4 -
diisocyanate.
Dralon ~ ~Bayer AG)
~(~CH2~CH~n~ Mn : 75~000
C=N
Le A 26 927
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q~
Dralc,n ~ (Bayer AG)
Cl H 3
- ( CH2-CH )--( CH2-CH )--( CH2-C ) - Mn : 48, 000
CN C=O CIH2
OCH3 S03Na
91 .5 % b.w. 5.0 % b.w. 3.5% b.w.
15 Dralon A(~ (Bayer AG~
~, . . . . . . _ _ _ _ _ .
ICH3
-(CH2-CH)-- (CH2-CH)--(CH2-C)-
1 1 ¦ Mn : 48, 000
CN C=O C=O IH 0
OCH3 ~ CH2 . CH2 -N ( CH3 ) 2 HS04
91 .4 % b.w. 4.9 % b.w. 3.7 % b.w.
Mowilith 50~ (Polyvinyl acPtate, HoPchst AG)
-(CH2~ClH)n Mn = 73~000
O- I -CH3
L~ A 26 927
_ 29 --
;~16~0
Cationic ~olvurethane disDersion (KPK~, Bayer AG)
The polyurethane dispersiDn serves as a coa~ulation
auxiliary and is a cationic emulsifier-free dispersion
of a reaction product of 200 parts of a polyester of
adipic acid, phthalic acid and ethylene glycol (mole-
cular weight = 1,700), 50 parts of toluylene diiso-
cyanate, 20 parts o~ N-methyldiethanolamine and 6 parts
of p-xylylene dichloride.
Le A 26 927
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