DE3712492A1 - Gas permeation-selective compound membrane, produced by plasma polymerisation coating - Google Patents

Abstract

Known gas permeation-selective membranes having both high selectivity and high gas flowrates are difficult to produce. The membrane according to the invention includes a hollow fibre substrate having a plasma polymerisation coating on the surface, which coating is formed from a lower-alkyl-substituted disiloxane monomer having a molecular weight of no more than 165, the monomer bearing methyl and/or ethyl groups.

Description

The invention relates to gas permeation selective membranes with a gas permeation selectivity for O₂ / N₂ of more than 2.3, made by application a plasma polymer coating on the Surface of a microporous membrane substrate. The coating can be made from a single special selected monomers are formed or by Co-polymerize two or more specifically selected co-monomers. The body of the substrate retains its original features regarding chemical resistance and mechanical Strength, but receives a plasma polymer coating that effectively bridges the substrate pores, what the use of the composite Gas separation membrane with high grade gasper selectivity and high gas flows allowed. The invention further relates to gas separation modules, which the new membranes of the invention use in the form of hollow fibers.

In the production of conventional gas permeations selective composite membranes is often encountered a variety of problems. These include, for example: low gas flow, caused by the thickness of the coating layer; Difficulty getting a thin one Coating layer; insufficient adhesive strength between the coating layer and the substrate and  Difficulty in making the membrane. It is particularly difficult to gas permeation selective to make composite membranes that both high gas selectivity and high Allow gas flows. In the case of asymmetrical Membranes encompass complicated problems Manufacturing operations and limited selection available raw materials.

In the case of composite membranes, the through Plasma polymer coating techniques formed the plasma coatings are in Networks are highly branched and highly cross-linked segments. The bills effect of the plasma polymer coating with the Substrate and the unique mechanism of plasma Polymer formation leads to excellent adhesion a thin layer of coating on the substrate. Accordingly, a plurality of Researchers examined how plasma polymer coatings work techniques in the manufacture of composite Have gas separation membranes applied to both high degree of permeation selectivity as well as show high-grade gas flows.

European patent application 0 134 055 by Van The Scheer reveals a dense compound Membrane for selective gas separation, which one dense ultra-thin gas-selective film of a plasma Polymer, a dense, highly permeable Interlayer of a conventionally formed poly merisats and a microporous substrate, which  the plasma polymer film and the intermediate layer wearing. The conventional polymer, which directly on an organosiloxane can be applied to the substrate or a co-polymer of siloxanes and polycarbonates be. The top ultra thin gas selective film consists of a silicone-free plasma polymer. This prior art requires in particular the presence of the intermediate layer and shows that their presence is necessary for two important purposes, namely 1. to the plasma Wear polymer coating and so the application to allow an ultra-thin top layer and 2. the gas mixture over the microporous Distribute substrate to allow that the entire composite membrane for gas separation is being used. Van Der Scheer therefore gives in particular that a high degree of gas selectivity in Combination with high gas flows are not expected can if a single plasma layer directly is applied to a microporous substrate. in the immediate and surprising contrast to that Teaching of this publication was the inventor of the present invention is able to put together set membrane of high-grade gas permeation selectivity and for high gas flows that a "soft" ultra-thin plasma polymer layer from a specific organosiloxane monomer low molecular weight immediately on a microporous substrate by means of a new plasma polymerization process applied becomes. In addition, the inventor has unexpectedly found that organosiloxanes as a class of substances  a highly permeation-selective plasma co- Polymerization layer for direct application form microporous substrates when a "soft" Organosiloxane plasma polymer with a "hard" Plasma polymer is copolymerized. The "hard" Plasma polymer is made from an organosilane, a Fluorocarbon or a hydrocarbon Monomers obtained.

U.S. Patent 4,410,338 Yamamoto et al. are gas races membranes revealed a microporous substrate is plasma-coated with a polymer which from one of organosilanes, organosiloxane and olefins selected organic monomers is formed. Yamamote et al. have no meaning the molecular weight or the size of the monomer noticed which is used to manufacture the plasma Coating is used. Unexpectedly has however, the inventor found that a special Group of low molecular weight organosiloxanes weight for the production of gas permeations selective membranes with increased gas selectivity can be used in combination with high gas flow rates , about which Yamamoto et al. report nothing. In addition, the revelation of Yamamoto is silent et al. completely regarding the use of copolymeric plasma coatings, which the inventor developed for the production of gas membranes Has. The gas membranes according to Yamamoto et al. will manufactured in that a microporous substrate in a conventional, high frequency powered Bell jar plasma reactor is housed and  that thereupon a plasma polymer coating brought. The intensity of the plasma discharge zone, which is shaped in such a known plasma reactor is inherently weak and the intensity increases with the distance of the substrate from the plasma generating electrode. Therefore, the Polymer coatings in such a reactor are produced, not intensively cross-linked be because of the intensity of the plasma discharge zone is weak, and also the application rate very low and the composition and uniformity the plasma polymer coating varies with the position of the substrate in the reactor. The The inventor now found that compound gas permeations selective membranes with plasma polymer coatings of certain selected monomers and Co-monomers can be made that a highly cross-linked structure as well as a extraordinarily uniform composition and Have thickness. This is achieved by the Plasma polymer application in tubular high-frequency Plasma reactor with capacitively coupled external electrodes, the plasma Discharge zone on the area between the electrodes restricted and controlled there and by greater energy density than with known ones Plasma reactors can be achieved. In previous ones Investigations, published in "Thin Solid Films ", 118 (1984), pages 187 to 195, under the Title "Preparation of Gas Separation Membranes by Plasma Polymerization with Fluorocompounds " Inventor other, less energy intensive plasma  Polymerization reactor system described in which organic monomers to form coatings polymerized for gas separation membranes, which differ from the monomers according to the invention divorce. The inventor has now determined that certain specific monomers and certain specific combinations of monomers and co-mono mers for the production of gas membranes with high grade Selectivity and high flow rates are used can be. The inventor also found that the plasma polymerization to manufacture of the membranes unexpectedly in the high energy plasma discharge zone of a high frequency operated tubular reactor with capacitive coupled outer electrodes are executed can.

When developing the new invention gas permeation selective composite membranes the inventor had various difficulties overcome that when using the previously available Plasma polymerization techniques occurred. If they are for the production of composite Membranes used suffer from all known Plasma polymer coating methods under certain inherent disadvantages regardless of the type of reactor system used (example wise glass bell reactors, low frequency tandem systems, coil induction coupled tubular high frequency reactors). These disadvantages are general due to the fact that such known plasma polymerizations  of the polymer on a substrate which in a range lower or unequal moderate plasma energy density was placed. These Disadvantages can generally be as follows can be summarized:

• 1. Non-uniformity of the application rate of the Plasma polymers and the composition of the Plasma polymer coating, essentially depending on the position of the substrate in the Reactor.
• 2. Low or uncontrollable energy density levels in known plasma reactors can be found, whether from the bell jar or of the tubular, coil induction coupled High frequency type where the polymer application in the "after glow" zone or low frequency type, where the polymer application in the discharge zone takes place. Low order rates in conventional reactors can also Construction of plasma coating on the inner Electrodes are attributed.
• 3. The impossibility of multiple membrane substrates coat evenly and effectively, attributed to competing shadowing versus plasma discharge and also attributable to the fact that the Polymer application rates essentially from that exact position of the substrate in the reactor depend.  
• 4. Non-uniformity of the coating around that Exterior of the membrane substrate around, for example around the circumference of a fiber.
• 5. Problems caused by unwanted training of various chemical species and the impossibility of efficiently remove chemical waste products.
• 6. The difficulty of all this plasma plating to regulate parameters, especially for the over based on commercial production standards.

Trying to overcome these problems the inventor developed a method in which a Plasma polymer on a microporous membrane substrate is applied, which is characterized by the energy intensive discharge zone in the area between the outer electrodes of a capacitively coupled tube like high-frequency reactor moves. The plasma polymer deposition techniques used in this reactor system are highly reliable, suitable for operation at high production rates and result in a desirably uniform Product. It is in this area of technology well known that the plasma coating of such microporous membrane substrates extraordinary is difficult, especially with continuous commercial manufacturing what on their Sensitivity to loads such as Temperature, pressure, voltage and chemical attack is due to handling. However the inventor unexpectedly found that  microporous membrane substrates in the manner mentioned quickly and effectively and with desirable uniform Results can be plasma coated and novel gas permeation selective compositions Membranes result in both high grade Permeation selectivity as well as high flow rates demonstrate.

The invention provides gas permeation selective microporous composite membranes with a Plasma polymer coating on the surface from specifically selected monomers or one Plasma co-polymer coating from specific Combinations of monomers and co-monomers, such as is described below. These gas membranes unexpectedly show gas selectivity at the same time and gas flows in much higher degrees than those with the previously available plasma-coated Reachable gas membranes.

The invention also provides a novel one Plasma polymerization technology for manufacturing of these gas membranes by working in the intense Plasma discharge zone of a tubular, high frequency-operated reactor with capacitively coupled outer electrodes as described below becomes.

Only by using the specific monomers or by using the specific combination of monomers and co-monomers of this invention and by performing the plasma polymerization in reactor system disclosed here may be the unexpected  Combination of high grade gas flow and higher grade Gas selectivity can be obtained which the allowed gas membranes according to the invention.

The microporous membrane substrates that form the basis the gas permeation-selective microporous form composite membranes of the invention, can polyolefins, fluorinated polyolefins and polysulfones and can be in the form of films, hollow fibers and the like. The explicit mention only of hollow fiber membranes in claims 1 to 10 is not to be understood as a limitation rather, there are also films and other membrane shapes Embodiments of this invention.

There are at least two different types that can be used Plasma polymer coatings:

• i) The plasma polymerization application of a "soft" plasma polymers on a microporous Membrane substrate, being the "soft" plasma polymer is obtained from a specific organosiloxane Low molecular weight monomers, as described below;
• ii) The plasma deposition of a co-polymer of a "soft" plasma polymers and a "hard" Plasma polymers, the "soft" plasma polymer is obtained from an organosiloxane monomer and get the "hard" plasma polymer is made from a monomer of an organosilane, a fluorocarbon or one Hydrocarbon.

The "soft" plasma polymer covers the micropores of the porous substrate, while the "hard" plasma  Polymers copolymerized with the "soft" plasma polymer, the plasma coating a higher selectivity for gas separation.

Also according to this invention, the separation of gaseous mixtures can be achieved in that the novel gas permeability-selective microporous membranes in the form of hollow fibers are installed in a separation module, which is suitably of the type shown in FIGS . 1 and 2.

Brief description of the drawings:

Fig. 1 shows a separation module of the type open at both ends;

Fig. 2 shows a single-ended type and separation module

Fig. 3 shows a partial outline section of the plasma polymerization system.

The microporous membranes that act as substrate membranes according to the invention are all known and readily available commercially. Suitable substrate membranes can be in the form of flat film, hollow fibers and the like are used. Hollow fibers are a currently preferred form because a larger area for gas permeation in a certain volume of space can be obtained. Therefore, a separator such as that Modules according to the invention, more compact if hollow  fibers can be used as a separating membrane. These microporous substrate membranes can be made from polyolefins, such as polyethylene and polypropylene fluorinated polyolefins such as fluorinated Ethylene propylene and made from polysulfones will. The use according to the invention suitable substrate membranes have pore sizes in Range from at least about 200 Å to maximum about 6000 Å. The pores are depending on the manufacturing method of various forms and in general oval to circular. If the pores are not are circular, the small axes are the smallest pores at least about 200 Å long and the major axes of the largest pores are at least about 6000 Å long. Suitable membrane substrates, who meet these requirements are without others in the trade of Mitsubishi Rayon Co., Ltd. available under the trade names KPF 190 M, 270 B and 360 A, and EHF 270 W, 270 T, 270 H, 410 C, 390 C and 390 A, as well as from the Celanese Corporation under the trade names CELGART 2400, 2402, 2500, 2502, 4400, 4410, 4500, 4510, K-442, K-443, X-10 and X-20. Other suitable films Polyolefins and fluorinated polyolefins as well usable hollow fibers are also described in U.S. Patents 3,558,764, issued January 26, 1971; 36 79 538, issued January 25, 1972; 38 01 404 on 2/4/1974; 38 01 692, issued on April 2, 1074; 38 39 240, issued October 1, 1974; 38 39 516, issued October 1, 1974; 38 43 761 on October 22, 1974; 39 20 785, issued on November 18, 1975; 40 58 582, issued November 15, 1977; 41 38 549, issued February 6, 1979; 42 55 376.  issued on March 10, 1981; 42 57 997, issued on March 24, 1981; 42 90 987, issued September 11, 1981 and 44 05 688, issued September 20, 1983, all by Reference to the component of the present disclosure be made.

The "soft" plasma polymers in the plasma poly Merisat coating according to i) are obtained that lower alkyl-substituted as a precursor monomer Disiloxanes with a molecular weight of not more than 165 according to the formula

are used, wherein R₁, R₂, R₃ and R₄ each are selected from methyl and ethyl. examples for are lower alkyl-substituted disiloxanes 1,1,3,3-tetramethyldisiloxane, 1,1,3-trimethyl-3- Ethyl disiloxane; 1,1-dimethyl-3,3-diethyldisiloxane; 1,3-dimethyl-1,3-diethyldisiloxane. A present preferred monomer is 1,1,3,3-tetramethyl disiloxane.

It is not known exactly why this is specific selected class of organosiloxanes plasma coated microporous substrates of such a size superior gas separation properties. It will assumed that the Si-C bonds are the Si-O bonds  weaken when all available bindings of silicon atoms of disiloxane to carbon atoms run, d. H.:

Therefore, in the high energy discharge zone the Si-O bonds are easily broken, causing the Oxygen is released, which is then not longer available for polymerization reactors is. When the remaining units polymerize and be deposited on the substrate, their properties are more similar to plasma Polymers of silicones, i.e. H.:

than the plasma polymers of siloxanes. The silane monomers themselves are simply unable to Plasma polymers of sufficiently large permeation Train selectivity. This problem will the less substituted lower alkyl disiloxanes not found according to formula I. This situation only the inventor noticed.  

The plasma co-polymerization coating according to ii) above is made by co-polymerizing a "soft" Plasma polymers and a "hard" polymer obtained. The "soft" plasma polymer is obtained by Use of any organosiloxane as Precursor monomers, which are not based on the above in Formula I particularly illustrated compounds is limited. In the formation of the co-polymerbe any organosiloxane can be used since the organosiloxanes are only "soft" polymers serve to coat the substrate pores and the specific co-monomers the selectivity inflict that when using the "Soft" monomers cannot be achieved. The "Hard" plasma polymers are obtained in that as a precursor monomer an organosilane, a fluorine carbon or hydrocarbon is used. The organosilanes include lower alkyl-substituted ones Silanes such as tetramethylsilane and lower alkoxy substituted silanes such as tetramethoxysilane. The Fluorocarbons include perfluorinated carbons hydrogens such as tetrafluoroethylene, and the Hydrocarbons include hydrocarbons low molecular weight such as propylene.

The gas membranes used according to the invention the lower alkyl-substituted disiloxanes produced according to formula I show a lot higher rates of gas selectivity and gas flow than those with the similar gas membranes from Yamamoto et al. which can be obtained above were specified. The only one from Yamamoto et al.  called disiloxane is a hexamethyldisiloxane (CH₃) ₃-Si-O-Si- (CH₃) ₃, a peralkyl substituted Disiloxane, which is considerably steric hindered and very much in its molecular structure is more space consuming than the siloxanes according to Formula I above

The inventor unexpectedly found that certain low molecular weight monomers, d. H. the lower alkyl-substituted disiloxanes according to formula I above (e.g. 1,1,3,3- Tetramethyldisiloxane) quickly on the microporous Membrane substrate can be applied when in the Plasma polymerizer according to the invention be polymerized, and then gas permeation result in selective composite membranes, which has an inherent superior gas selectivity and an increased gas flow compared to the gas permeation selective membranes according to Yamamoto et al. or other known gas permeation selective Show membranes.

The plasma polymerization according to the invention includes the activation of the required monomer or Co-monomers within the discharge zone to one high-energy dissociated form that with radicals, Is enriched with electrons and ions (i.e. a plasma), and the application as a plasma polymer or copolymers on the surface of the substrate, which moves through the discharge zone. In practice there is an electrical discharge a high frequency generator to the outer electrodes  of a capacitively coupled tubular Plasma reactor created. The selected monomer or co-monomers are introduced into the reactor and excited to plasma. According to the invention the area of the highest plasma energy density in the Reactor to the area between the electrodes limited, that is, to the plasma discharge zone, and are kept there controlled.

Because the plasma discharge zone is the area of highest energy density within the reactor is it is absolutely necessary that the temperature sensitive Substrate only a short dwell time Reactor has, however, a requirement Application of the plasma coating guaranteed must stay. This is achieved in that the Continuous substrate such as a film or hollow fiber is moved to the plasma discharge zone in which the plasma excited Monomers progressively polymerized and up this moving substrate is applied.

The movement of the substrate through the area of Plasma discharge zone is of course through on the one hand the train speed and on the other hand determines the tension of the substrate. Since the plasma Discharge zone the area of highest energy density in the plasma reactor, the pulling speed must be and regulates the tension together be a residence time in the substrate To give discharge zone to the required Application of the plasma polymer is sufficient,  while on the other hand any damage to the substrate due to the energy density in the discharge zone is avoided. At the same time they have to Train speed and tension together like this be regulated that a wall contact of the substrate or avoid touching other substrates when a plurality of substrates are common be coated.

Earlier researchers especially had the coating polymeric substrates in the field of plasma discharge zone avoided as it is an uncontrollable Degradation of the substrate observed, and have therefore switched to the substrate of the Discharge zone adjacent, but outside this too coat. By precise regulation of the specific Variables given here was the inventor unexpectedly able to be precisely controlled and fully reproducible coatings for Extraction of gas permeation-selective microporous perform composite membranes, the regarding the original properties of the substrate mechanical strength and chemical Resilience is maintained.

A suitable system for the continuous production of the permeation-selective membranes according to the invention is illustrated schematically in FIG. 3. The high-frequency plasma reactor is a tubular reactor 36 with a pair of capacitively coupled outer electrodes 31 , which are arranged at one end of the reactor 36 and are connected externally to a high-frequency generator. The highest energy density is maintained in the area between the electrodes, that is to say in the plasma discharge zone, by regulating both the current from the high-frequency generator and the flow rate of the monomer or co-monomer. If the flow rate is too high, the discharge zone overflows into the area outside the electrodes. Two vacuum chambers 35 A , 35 B are with the reactor 36 in continuous vacuum-tight connection and an outlet to a vacuum pump is designated 38 . The reactor 36 can be formed from any material that is sufficiently resistant to the reaction conditions of the plasma polymerization. At present, quartz, PYREX (trademark) and VYCOR (trademark) have proven to be suitable. In operation, a hollow fiber is continuously fed from a supply spool 33 to a take-up spool 34 through the reactor 36 . The desired precursors of monomers of the co-monomers are admitted into the system through a feed inlet 37 . It is important to minimize the residence time in the plasma discharge zone and to keep the substrate as cold as possible (ie as close as possible to room temperature) in order to prevent damage to the temperature sensitive substrate and to ensure a rapid application rate of the plasma polymer to promote the substrate. In order to achieve the desired thickness of the plasma coating and to maximize the plasma polymer application, the substrate can therefore be passed through the discharge zone several times. In the manufacture of gas permeation selective composite membranes, it is important that the pores of the microporous substrate be completely bridged, and to achieve this goal it is important that an adequate plasma polymer or co-polymer coating be applied. To achieve this, a system can be provided which reverses the direction of movement of the substrate, which allows repeated passage of a continuous length of substrate through the discharge zone to achieve the desired coating. The directional flow of the monomer can be either in the direction of or against the direction of substrate movement through the discharge zone.

The system shown in FIG. 1 can be converted in a simple manner to the continuous production of flat film membranes by providing a reactor 36 and unwinding and winding spools 33, 34 of suitable dimensions for receiving the continuous film. Correspondingly, several fibers can be coated at the same time by using a corresponding reactor 36 and unwinding and winding spools 33, 34 .

The in the discharge zone of the invention Plasma reactor system are generated energy densities much higher than that of conventional Plasma reactors available. This requires that the length of time during which the substrate of the plasma Discharge zone is minimized to  To prevent damage to the membrane substrate. For example, a polyolefin substrate with a Rate to be moved through the plasma discharge zone which is usually 2 cm / sec for each run exceeds. The residence time in the plasma discharge zone naturally varies with the respective substrate. With a more temperature-resistant substrate such as polysulfone can take a long time for that Suspension can be chosen to get the desired plasma Get polymer coating.

The method according to the invention allows simple Application of the required plasma polymer coating on flat surfaces such as films rounded or curved surfaces such as hollow fibers or uneven membrane substrates and on one Plurality of substrates such as films or hollow fibers that are simultaneously through the Move the plasma discharge zone. The reason for this is, that in the system according to the invention the plasma discharge zone across the cross-sectional area of the tubular Reactor kept at a uniform density can be. In conventional plasma reactors, which is a plasma deposition in the discharge zone allow (e.g. low frequency tandem systems) the discharge zone can not be more uniform Density should be kept. They also cover themselves internal electrodes in both conventional Low frequency as well as high frequency reactor systems over time with plasma polymers what the energy density of the discharge zone is further reduced. In plasma reactors where application in the  Post-discharge zone occurs (for example Glass bell and coil induction coupled tube like high-frequency reactors) takes the plasma density and thus the density of the polymer applied with the distance from the discharge zone from and Gas flow patterns become unregulated. In addition, in conventional plasma polymerization reactors Substrate for example on the electrode or the Reactor wall can be arranged. In such conventional Systems are known to vary in quality and uniformity of the coating with the Position of the substrate relative to the plasma discharge. Therefore, if there are several substrates at the same time plasma polymer coated, the varies Composition and uniformity of the plasma polymer coating depending on the position of the respective substrate with regard to the plasma Discharge zone. In addition, a substrate must be carefully arranged in exactly the same position in the reactor when a certain plasma polymer coating repeated on a particular substrate shall be. All of these problems dwell on it Operation of the system, which for example in U.S. Patent 4,410,338 Yamamoto et al. described is. In contrast, in the system according to the invention the quality and uniformity of the Product with each pass and between individual runs because of the even Density safe across the entire discharge zone posed.

The tension of the substrate, which is characterized by the  Plasma discharge zone moving must be as low as possible to keep the original shape and to obtain tensile strength of the substrate membrane, while at the same time a distance that meets the requirements maintained between multiple substrates is and is prevented that a substrate Touched reactor wall. Because of the intensity of the plasma Discharge zone in the system according to the invention is Application rate of the plasma polymer very much higher than that of conventional plasma reactor systems is possible. The plasma polymer application is checked between the electrodes and the Application in the after-discharge area becomes minimal held.

The appropriate distance between the electrodes depends depends on the size of the pipe. In the described here System, the electrodes are approximately 10 to 15 cm spaced and the tube has a diameter of 13 mm. For pipes of larger diameter, the connected to the plasma discharge zone Energy density as close as possible to that of smaller tubular reactor remain approximated. It is extremely important besides the energy density also to regulate the monomer density. Even though when changing the pipe size the monomer density will remain essentially the same, will Changing the size and structure of the system Optimal energy density and monomer density change easily.

Because of the extremely high energy density in the plasma  Discharge zone must be the temperature of the substrate while moving through the plasma discharge zone be kept as low as possible (near the Room temperature) to ensure an even application rate ensure the plasma polymer and Deformation and damage to the substrate prevent. Under normal process conditions will some precipitation of plasma polymers usually occur on the reactor wall. This however, affects the process conditions not and is usually to be expected; the precipitation will be removed during routine maintenance. By keeping the substrate as cold as possible is an application of the plasma polymer on the membrane surface opposite the reactor wall promoted. Additional cooling of the substrate below room temperature can, if desired, applied, assuming that the Residence time in the reactor can be increased and that an application of the plasma polymer on the Membrane substrate is further promoted.

A useful yardstick for determining the change in reaction parameters depending on changes in the tube geometry is the composite discharge parameter W / FM , where W is the wattage of the plasma, F is the flow rate of the monomer or monomers M is the molecular weight of the monomer or monomers. As tube geometry and system size vary, W / FM changes for a given application rate of the plasma polymer or co-polymer, but the optimal value of W / FM varies between one-half and twice the original value for W / FM for a given monomer system . Therefore, for each monomer system, the change in the composite plasma parameter with the change in tube geometry can be expressed as:

(1/2) W _a / F _a M _a < W _b / F _b M _b <2 W _a / F _a Mb _a

where W _b / F _a M _{b is} the composite plasma parameter for a first tubular high frequency reactor as described, and W _a / F _a M _{a is} the composite plasma parameter for a tubular high frequency reactor of different sizes as described above.

The advantages of the plasma polymerization according to the invention systems compared to conventional plasma polymerization systems include:

• 1. The ability to coat both individual as well as multiple substrates with continuous Uniformity of thickness and chemical composition of the plasma polymer.
• 2. The ability to combine high energy densities and high application rates with minimal Residence times of the substrate in the energy-intensive Plasma discharge zone.
• 3. The ability to generate a minor Population range of chemical species in the  Plasma discharge zone compared to conventional ones Plasma reactors, which is uniform Properties of the plasma polymer produced coating shows.
• 4. The ability to remove waste products (i.e. unreacted monomers and possibly harmful corrosive gases) from the discharge zone of the tubular reactor in continuous Way what the effect of their presence minimized. This is due to the fact that the Monomer flow rate is greater than the rate of the substrate so that the waste products can be washed away. Since also that Plasma polymers practically exclusively in the Plasma discharge zone is applied is that Substrate already by the applied plasma Protected polymer layer, which any Prevents damage from these waste products if it is in the after - discharge zone of the Reactor moves. In the system, which by Movement of the substrate in the reverse direction repeated implementation of a continuous Length of the substrate through the discharge zone enables, prevents the speed of the monomer gas flow that any waste products in the after-discharge zone any more uncoated part of the substrate to damage.
• 5. Other advantages include the possibility of Manufacture of composite membranes in one  continuous process with for production rates acceptable at the commercial level and the possibility of plasma coating like that sensitive substrates such as polyolefins more conventional under the operating conditions Plasma reactors are not the preferred membrane substrates apply. As mentioned above, Another advantage is the possibility a plasma coating using the Monomers with a specifically low molecular weight important as described here for conventional Plasma polymerization systems because of their low Application rates are not preferred Form reactants.

Fig. 1 shows a schematic outline of a separation module of the two-ended type using the gas permeation-selective microporous composite membrane of the invention in the form of hollow fibers. A plurality of hollow fibers 11 are preferably used in each module unit, which may be made of glass or any other suitable inert material. The ends of the hollow fibers are each fastened in end caps 12 of the module unit, so that a gaseous mixture flowing in through an inlet 14 a of the module unit and the hollow fibers 11 flow around. The permeate gas exits through outlets 13 , the impermeate gas through an outlet 14 b .

Fig. 2 shows a schematic outline of a single-ended type separation module using hollow fibers as a gas permeation selective microporous composite membrane according to the invention. A plurality of hollow fibers 21 are preferably used in each module unit. The hollow fibers are inserted into a module jacket, which can be formed from glass or any other inert material. The ends of the hollow fibers are fastened within end caps 22 of the module unit, so that a gaseous mixture can flow in through an inlet 24 a of the module and can flow around the hollow fibers 21 . The permeate gas exits through an outlet 23 , the impermeate gas through an outlet 24 b . Other suitable separation modules for the purposes of the invention using the gas permeation selective hollow fiber membranes of the invention are more fully described in US Pat. Nos. 3,821,087, issued June 28, 1974; 38 83 393, issued May 13, 1975 and 41 84 922, issued January 22, 1980, all issued to Knazek et al., Each of which is incorporated herein by reference.

As will be immediately apparent to those skilled in the art, the manner in which the gas permeation selective microporous composite membranes of the invention are used in gas separation processes is only a matter of choice and convenience. It is not limited to the specific devices described herein, but rather can be used in any manner that results in the desired gas separation. The flow J (cm³ / cm² sec.) Through the membrane can generally be expressed as

J = P ( Δ p / l)

where P (cm³ cm / cm² sec. cmHg) is the permeability coefficient and Δ p (cmHg) is the pressure difference across the membrane of thickness l (cm). For a composite membrane, the permeability P / l is suitable for expressing the permeability of gases, since the permeability coefficients of the plasma coating layer and of the substrate cannot be determined independently of one another. In the case of a hollow fiber, the membrane area is calculated using the logarithmic mean of the inside and outside diameter.

The permeation selectivity (P (A) / P (B)) * is also an important factor in the assessment of membrane selectivity for binary gaseous systems AB . The permeation selectivity (P (A) / P (B)) * is obtained according to the following equation:

wherein C _A ₁ and C _B ₁ the mole fractions in the feed stream and C _A ₂ and C _B ₂ each in the permeate for the gaseous components A and B and p ₁ and p ₂ are the absolute pressures in the feed stream and in the permeate, respectively. Ideally, there is no interaction between the gases and no pressure drop in the hollow fiber cavity; then the value of the permeation selectivity (P (A) / P (B)) * is the ratio of the permeability coefficients (P (A) / P (B) .

The gas permeation selective membranes of the invention which has a high selectivity for the gaseous System have oxygen-nitrogen are suitable for oxygen applications enrichment and for cleaning and separation of Nitrogen from the air. According to the invention Membranes that have a high selectivity for the gaseous system showing hydrogen carbon monoxide are suitable for the production of synthetic Fuels from water gas. Membranes with high Selectivity for the nitrogen-hydrogen system are suitable for ammonia synthesis. Membranes with high selectivity for the helium-methane system are suitable in practice for the extraction of Helium from natural gas sources. Membranes that a selectivity for the carbon dioxide system Show methane are suitable for methane purification for biogas, especially by removing Carbon dioxide.

In the following specific experiments Measurements of membrane performance at room temperature performed. The calibration pressure applied was 6.5 Atmospheres. The gas that permeated the membrane was withdrawn at atmospheric pressure. For compressed air was used in the feed stream and the concentration ratio was O₂ / N₂ about 21/79. The component concentrations were determined by gas chromatography (detector: TCD, column: molecular sieve 5A from Shimadzu Co., Japan).  

Manufacturing

Microporous hollow fibers made of polypropylene with a Inside diameter of 100 µm, a porosity of 20% and a wall thickness of 24.5 µm were considered Substrate used. The plasma polymer coating was under the following conditions performed:

• i) Monomer: 1,1,3,3-tetramethyldisiloxane
  Discharge energy: 5 watts, high frequency generator (13.56 MHz)
  Train speed: 2.4 cm / second
  Monomer flow rate: different
• ii) Monomer: 1,1,3,3-tetramethyldisiloxane +
  a) Tetramethoxysilane
  b) tetramethylsilane
  c) tetrafluoroethylene
  d) propylene
  Discharge energy: 5 watts, high frequency generator 13.56 MHz
  Train speed: 2.4 cm / second
  the tensile speed determines the residence time of the fiber in the plasma discharge.

With a typical electrode spacing of 15 cm the plasma coating was applied by the substrate three times through the plasma discharge area was led. Therefore, the total was Dwell distance 45 cm and the dwell time in the Plasma discharge 19 seconds.  

In the production of the plasma co-polymer coating according to ii) the membranes were applied after application of the plasma co-polymer by immersion in anhydrous ethanol for one hour acts. This post-treatment serves the cross cross-linked crystalline structure of the plasma co-polymer to compress and improve gas selectivity the composite membrane.

Trial 1

A microporous hollow fiber was created under the conditions according to i) coated by plasma polymerization. The monomer flow rate and system pressures were as follows:

• Membrane 484-11: monomer flow rate 5.12 SCCM, system pressure P (inlet) 145.3 mtorr, P (outlet) 81.4 mtorr;
• Membrane 484-22: monomer flow rate 3.64 SCCM, system pressure P (inlet) 117.8 mtorr, P (outlet) 55.0 mtorr;
• Membrane 484-32: monomer flow rate 6.66 SCCM, system pressure P (inlet) 173.5 mtorr, P (outlet) 100.5 mtorr;
• Membrane 497-11: monomer flow rate 5.8 SCCM, P (inlet) 160.5 mtorr, P (outlet) 83.2 mtorr;
• Membrane 500-12: monomer flow rate 5.8 SCCM, P (inlet) 157.9 mtorr, P (outlet) 97.2 mtorr.

The membrane performances for the above-mentioned membranes are shown in Table I. The oxygen concentration in the permeate fluctuated between 37.7% and 41.2% (permeation selectivity (P (O₂) / P (N₂)) *: 2.6-3.2) depending on the coating conditions, while the permeability P / l fluctuated between 7.48 x 10 ^-5 and 1.41 x 10 ^-5 cm³ / cm² sec cmHg.

Table I

Membrane performance

Table II

Membrane performance

Trial 2

Microporous hollow fibers were made according to the Conditions according to ii) by plasma polymerization coated. The monomer flow rate and system pressures were as follows:

• Membrane 504-212: monomer flow rate 3.76 SCCM (1,1,3,3-tetramethyldisiloxane / tetramethoxysilane = 50/50), system pressure P (inlet) 118.5 mtorr, P (outlet) 71.5 mtorr;
• Membrane 503-212: monomer flow rate 4.10 SCCM (1,1,3,3-tetramethyldisiloxane / tetramethylsilane = 50/50), system pressure P (inlet) 128.0 mtorr, P (outlet) 80.2 mtorr;
• Membrane 513-122: monomer flow rate 4.20 SCCM (1,1,3,3-tetramethyldisiloxane) + 2.45 SCCM (tetrafluoroethylene), system pressure P (inlet) 132.2 mtorr, P (outlet) 77.2 mtorr;
• Membrane 505-212: monomer flow rate 4.60 SCCM (1,1,3,3-tetramethyldisiloxane) + 1.16 SCCM (propylene), system pressure P (inlet) 114.2 mtorr, P (outlet) 65.9 mtorr;

After the plasma coating, the membranes by immersing in anhydrous ethanol during after an hour.

The membrane performances of the above membranes are given in Table II. The oxygen concentration in the permeate varied between 39.5% and 42.1% (permeation selectivity: (P (O₂) / P (N₂)) *: 3.0-3.4), while the permeability (P / l ) varied between 5.78 × 10 ^-5 and 2.02 × 10 ^-5 cm³ / cm² sec. cmHg.

Claims (29)

1. Gas permeation-selective microporous hollow fiber membrane with a gas permeation selectivity for O₂ / N₂ of more than 2.3, comprising: a microporous hollow fiber substrate and a gas permeation-selective plasma polymer coating on the substrate surface, in which the plasma polymer coating from a lower alkyl -substituted disiloxane monomers with a molecular weight of not more than 165 according to the formula is formed, wherein R₁, R₂, R₃ and R₄ are each selected from methyl and ethyl. 2. hollow fiber membrane according to claim 1, in which the microporous substrate is selected from Polyolefins, fluorinated polyolefins or poly sulfones with pore sizes in the range of at least about 10 Å to a maximum of about 6000 Å. 3. hollow fiber membrane according to claim 2, where the microporous substrate is polypropylene, Polyethylene, fluorinated ethylene propylene or Is polysulfone. 4. hollow fiber membrane according to claim 1, in which the lower alkyl substituted disiloxane monomer is selected from 1,1,3,3, tetramethyldisiloxane; 1,1,3-trimethyl-3-ethyldisiloxane; 1,1-dimethyl 3,3-diethyldisiloxane and 1,3-dimethyl-1,3- Diethyldisiloxane. 5. hollow fiber membrane according to claim 3, in which the lower alkyl substituted disiloxane monomer 1,1,3,3-tetramethyldisiloxane. 6. Hollow fiber membrane with a gas permeation  Selectivity for O₂ / N₂ gas of more than 2.3, comprising: a microporous hollow fiber substrate and a gasper meation-selective plasma polymer coating on the substrate surface where the plasma Polymer coating by copolymerization of a Disiloxane monomers and a co-monomer formed which is made of organosilanes, fluorocarbons and hydrocarbons is selected. 7. hollow fiber membrane according to claim 6, in which the disiloxane monomer is selected from 1,1,3,3-tetramethyldisiloxane; 1,1,3-trimethyl-3- Ethyl disiloxane; 1,1-dimethyl-3,3-diethyldisiloxane and 1,3-dimethyl-1,3-diethyldisiloxane, and the co- Monomers is selected from a lower alkyl silane, a lower alkoxysilane, a perfluorinated Low molecular weight hydrocarbon and a low molecular weight hydrocarbon Weight. 8. hollow fiber membrane according to claim 7, in which the disiloxane 1,1,3,3-tetramethyldisiloxane and the co-monomer is selected from Tetramethoxysilane, tetramethylsilane, tetrafluoro ethylene and propylene. 9. hollow fiber membrane according to claim 6, where the microporous substrate is selected from Polyolefins, fluorinated polyolefins and Polysulfones with pore sizes in the range of at least about 10 Å to a maximum of about 6000 Å.   10. hollow fiber membrane according to claim 9, where the microporous substrate is polypropylene, Polyethylene, fluorinated ethylene propylene or Is polysulfone. 11. A method for producing a gas permeation selective microporous composite membrane comprising the steps:
Application of a high-frequency discharge to the capacitively coupled outer electrodes of a tubular plasma reactor,
Introduction of an organosiloxane monomer, optionally copolymerized with a monomer which is an organosilane, a fluorocarbon or a hydrocarbon, into the reactor to activate the monomer or co-monomer into a plasma which is kept under control within the discharge zone between the reactor electrodes ,
and applying a gas permeation-selective plasma polymer coating to the surface of a microporous substrate moving through the discharge zone. 12. The method according to claim 11, in which the microporous substrate is selected from Films and hollow fibers with pore sizes in the range from at least about 10 Å to a maximum of about 6000 Å. 13. The method according to claim 12, in which the microporous substrate is a hollow fiber is.   14. The method according to claim 11, in which the reactor is designed to be the microporous Continuous substrate Film or a continuous hollow fiber too to edit. 15. The method according to claim 11, where the microporous substrate is polypropylene, Polyethylene, fluorinated ethylene propylene or Is polysulfone. 16. The method according to claim 15, in which the plasma polymer coating an organosiloxane is formed. 17. The method according to claim 16, in which the plasma polymer coating 1,1,3,3-tetramethyldisiloxane is formed. 18. The method according to claim 11, in which the plasma polymer coating is formed is from an organosiloxane, copolymerized with an organosilane, a fluorocarbon or a hydrocarbon. 19. The method according to claim 18, in which the plasma polymer is formed from 1,1,3,3-tetramethyldisiloxane, copolymerized with Tetramethoxysilane, tetramethylsilane, tetrafluoro ethylene or propylene. 20. Module for the separation of gaseous mixtures, comprising:

• a) a jacket device with spaced end regions, which defines an elongated chamber between them,
• b) a plurality of individual gas permeation-selective microporous hollow fibers according to claim 1, which extend substantially parallel and spaced apart in the jacket device, the chamber being divided by the walls of the hollow fibers into an intracapillary space inside the hollow fibers and an extracapillary space outside the hollow fibers and the intracapillary space and the extracapillary space communicate with each other only through the walls of the hollow fibers,
• c) devices connected to the extracapillary space for the passage of gaseous mixtures therethrough, and
• d) Devices for withdrawing permeate gas from the intracapillary space.

21. Module according to claim 20, in which the hollow fiber substrate is selected from Polyolefins, fluorinated polyolefins and polysulfones with pore sizes in the range of at least approximately 10 Å to a maximum of about 6000 Å. 22. Module according to claim 20, where the hollow fiber substrate is polypropylene, Polyethylene, fluorinated ethylene propylene or Is polysulfone. 23. Module according to claim 20,  in which the plasma polymer coating on the Surface of the hollow fiber is formed from 1,1,3,3-tetramethyldisiloxane; 1,1,3-trimethyl-3- Ethyl disiloxane; 1,1-dimethyl-3,3-diethyldisiloxane and 1,3-dimethyl-1,3-diethyldisiloxane. 24. Module according to claim 23, in which the monomer 1,1,3,3-tetramethyldisiloxane is. 25. Module for the separation of gaseous mixtures, comprising:

• a) a jacket device with spaced end regions which define an elongated chamber between them;
• b) a plurality of gas permeation-selective microporous hollow fibers according to claim 6, which extend substantially parallel and spaced apart in the jacket device, the chamber being divided by the walls of the hollow fibers into an intracapillary space in the hollow fibers and an extracapillary space, and wherein the extracapillary space and the intracapillary space communicate with each other only through the walls of the hollow fibers,
• c) devices communicating with the extracapillary space for the passage of gaseous mixtures therethrough and
• d) devices for withdrawing gaseous permeates from the intracapillary space.

26. Module according to claim 25, in which the plasma polymer coating on the Surface of the hollow fibers due to the copolymerization of an organosiloxane monomer, copolymerized is formed with a co-monomer which consists of Organosilanes, fluorocarbons and hydrocarbons fabrics is selected. 27. Module according to claim 26, wherein the monomer is 1,1,3,3-tetramethyldisiloxane and that co-monomers tretramethoxysilane, tetramethylsilane, Is tetrafluoroethylene or propylene. 28. A method of separating gases comprising: Attachment of a gas permeation-selective membrane according to claim 1 with a permeation selectivity of at least 2.3 for O₂ / N₂ in a suitable one Gas separator, and contacting the gaseous Mix with the membrane to effect the desired separation. 29. A method of separating gases comprising: Arrangement of a gas permeation-selective membrane according to claim 6, with a gas permeation selectivity of at least about 2.3 for O₂ / N₂ in a suitable gas separator and contact of the gaseous mixture with the membrane Effect the desired separation.