CA2016960A1

CA2016960A1 - Composite membranes, processes for their preparation and their use

Info

Publication number: CA2016960A1
Application number: CA002016960A
Authority: CA
Inventors: Karlheinz Hildenbrand; Rolf Dhein; Willi Meister; Dittmar K. Nerger
Original assignee: Bayer AG
Current assignee: Bayer AG
Priority date: 1989-05-18
Filing date: 1990-05-16
Publication date: 1990-11-18
Also published as: EP0398093A2; JPH0312226A; EP0398093A3; DE3916210A1

Abstract

Composite membranes, processes for their preparation and their use A b s t r a c t Composite membranes of a macroporous filler-containing membrane of at least two incompatible polymers and a pore-free polyurethane membrane applied thereto exhibit an improved action in the removal of benzenes optionally substituted by lower alkyl radicals, hydroxyl, chlorine or bromine from their mixtures with aliphatic and/or cycloaliphatic hydrocarbons, alcohols, ethers, ketones and/or carboxylic acid esters or from effluent.

Le A 26 927-US

Description

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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).

Le A 26 927-US

201~

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 ~5 Le A 26 927 ~o~

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 Le A 26 927 q~

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 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 69~

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.

Le A 26 927 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.

Le A 26 927 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, ~5 ~e A 2h 927 Z~

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~.

Le A 26 927 ~ i9~7~

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 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.

Le A 26 927 ~ 3~

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 Le A ?6 ~27 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 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 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

Le A 26 927 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.

Le A 26 927 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 ~5 Le A 26 927 ;~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 ~5 Le A 26 927 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.
~5 Le A ?6 927 2~

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 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 ~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 ;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 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 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 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 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 ~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 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~
~, . . . . . . _ _ _ _ _ .

-(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 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

Claims

1. Composite membranes consisting of i) a macroporous membrane of at least two incompatible polymers con-taining at least one filler, 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 poly-mers, and ii) a pore-free polyurethane (PU) membrane applied to i).

2. Composite membranes according to claim 1, characterized in that two or three incompatible polymers from the class comprising polyurethanes, polyacrylo-nitrile, polyvinyl acetate, polystyrene, polysulphone, polyvinylidene fluoride, polyamide, polyhydantoin and ethylene/vinyl acetate copolymers containing at least 50 % by weight of vinyl acetate are used for i).

3. Composite membranes according to claim 1, characterized in that one or more fillers from the group comprising talc, microcrystalline cellulose, zeolites, bentonites, SiO2, TiO2 and BaSO4 are used.

4. Composite membranes according to claim 1, characterized in that they additionally contain a support material onto which i) and then ii) are applied.

5. Composite membrane according to claim 1, characterized in that the pore-free membrane ii) is a crosslinked acrylate/polyurethane blend.

6. Composite membranes according to claim 5, characterized in that one of more esters of acrylic acid or methacrylic acid with aliphatic, cycloaliphatic or Le A 26 927 araliphatic diols and/or polyols having three of more OH groups, the diols having 4 - 12 C atoms are used as the acrylate.

7. Composite membranes according to claim 6, characterized in that butane-1,4-diol acrylate, butane-diol bis-methacrylate, trimethylolpropane trisacrylate, trimethylolpropane trimethacrylate, pentaerythritol tetraacrylate or pentaerythritol tetramethacrylate, or urethane acrylates are used as the acrylate,

8. Composite membranes according to claim 7, characterized in that trimethylopropane trisacrylate is used as the acrylate.

9. A process for the preparation of composite membranes, 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 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 preci-pitation coagulation is carried out and c) a pore-free PU membrane is applied to the macroporous filler-containing membrane obtained in this way.

10. The process of claim 9, characterized in that a support material which remains on the composite membrane is used as the carrier substrate for casting the solution containing the incompatible polymers and fillers during production of the composite membranes in the form of films.

Le A 26 927

11. A process for removing benzene, which can be mono-, di- or trisubstituted by chlorine, bromine, hydroxyl or C1-C4-alkyl, from aliphatic and/or cycloaliphatic hydrocarbons, alcohols, ethers, ketones and/or car-boxylic acid esters or from effluent, characterized in that composite membranes are used consisiting of i) a macroporous filler-containing membrane of at least two incompatible polymers and ii) a pore-free polyurethane (PU) membrane applied to i) according to claim 1.

Le A 26 927