GB2121314A - Selectively gas-permeable composite membranes and process for the production thereof - Google Patents

Abstract

A selectively gas-permeable composite membrane comprises a heat-resistant porous polymeric membrane and a thin layer of a tertiary carbon or tertiary organic silicon-containing compound provided on one side of the membrane, the layer being prepared by polymerizing the compound by means of plasma. The membrane has highly selective permeability as well as superior strength and heat resistance.

Description

SPECIFICATION Selectively gas-permeable composite membranes and process for the production thereof The present invention relates to selectively gas-permeable composite membranes and a process for the production thereof. More particularly, it relates to selectively gas-permeable composite membranes comprising a thin layer of cross-linked structure provided on one side of a heat-resistant porous support, the thin layer being prepared by polymerizing tertiary carbon-containing compounds or tertiary organosilicon compounds by means of plasma. Also, the present invention relates to a process for producing the membranes.

In recent years, an increasing number of extensive studies have been made to achieve separation and purification of fluid mixtures by the use of selectively permeable membranes in place of conventional techniques such as distillation and low temperature processing, which are accompanied by changes in phase and consume a lot of energy as described in U.S. Patents 4,230,463 and 4,264,338.

Separation and purification of fluid mixtures using membranes has already been put to practical use in several fields. For example, the conversion of sea water into fresh water, disposal of waste water from factories, and the concentration of foods have all been carried out on a commercial scale using appropriate membranes. These processes, however, are liquid-liquid separation and liquid-solid separation. However, gas-gas separation on a commercial scale is practically unknown.

It is difficult to commercially perform the separation of gases using a membrane (hereinafter sometimes referred to as "membrane-separation") because; (1) the selective permeability of conventional membranes is poor more specifically, there is no suitable membrane which selectively allows specific gases to pass therethrough while essentially blocking other gases making it possible to obtain high purity gas and therefore, it is necessary to employ a multi-stage process wherein the membrane-separation is performed repeatedly, which leads to increases in the size of the apparatus); and (2) the gas permeability is poor, which makes it difficult to process a large amount of gas.

Furthermore, when the selective permeability of the membrane is increased, the gas permeability tends to be reduced. However, when gas permeability is increased, the selective permeability tends to be decreased. This makes it difficult to perform membrane-separation on a commercial scale.

In order to achieve commercial membrane-separation, various methods of producing improved membranes have been proposed. Typical examples include a method in which casting of a polymer solution is employed to produce an unsymmetrical membrane wherein the thickness of an active skin layer is made as thin as possible, and a method in which a super-thin membrane corresponding to the above active skin layer is prepared independently and stuck together to a porous support to form a composite membrane as described in U.S. Patents 3,497,451,4,155,793 and 4,279,855. These methods, however, fail to provide satisfactorily improved membranes although they are standard procedures to improve gas permeability.The reason for this is that there are no commercially available polymers or copolymers which meet all the required physical properties, e.g., selective permeability, gas permeability, heat-resistance, chemical resistance, and strength.

From the viewpoints of heat resistance and strength various materials can be chosen from porous polymerous materials now commercially available. Porous polysulfone, polyimide, and so forth may be used, but cellulose ester, polyvinyl chloride, polypropylene, polycarbonate, polyvinyl alcohol, etc. are not much preferred. In view of heat resistance and strength, a porous support made from a polytetrafluoroethylene is most preferred. Furthermore, it has the advantage that its chemical resistance is satisfactorily high.

With regard to gas permeability, the polytetrafluoroethylene is not suitable. Materials having satisfactory gas permeability include various rubbers such as silicone rubbers (e.g., dimethyl siloxane and phenyl siloxane), natural rubber, and polybutadiene. These rubbers, however, suffer from the serious defect of poor strength. It is possible to incorporate a silica filler into such rubber materials for the purpose of improving the strength. Incorporation of such fillers, however, is not preferred since it deteriorates gas permeability.

As a result of various investigations, it has been found that polymeric compounds containing a tertiary carbon atom in the recurring unit thereof have excellent gas selective permeability and, furthermore, those compounds containing a tertiary organic silicon in place of the above tertiary carbon atom are also excellent in gas selective permeability. However, these polymers are inferior in heat resistance, strength, and chemical resistance.

Therefore, an object of the present invention is to provide an improved membrane which does not merely employ a single material meeting all the above physical properties, but which combines different materials having their own excellent physical properties.

Another object of the present invention is to provide a composite membrane having a selective gas-permeability which is useful in concentrating oxygen from the air, separating hydrogen from petroleum gas, etc.

It has now been found according to the invention that when monomers containing a tertiary carbon atom or tertiary organic silicon are polymerized by glow-discharging under reduced pressure of 10 Torr or less, preferably 5 Torr or less, to form a thin membrane on a heat resistant porous support, there can be obtained a composite membrane which is greatly improved not only in selective permeability but also in strength and heat resistance.

The present invention, therefore, provides: (1) a selective gas-permeable composite membrane comprising a heat-resistant porous support and a thin layer of cross-linked structure provided on one side of the support, said thin layer being prepared by polymerizing a tertiary carbon containing compound represented by general formula (I) or tertiary organic silicon-containing compound represented by general formula (II) or (III) by means of plasma::

wherein X is a saturated aliphatic hydrocarbon radical an unsaturated aliphatic hydrocarbon radical, an aromatic hydrocarbon radical, a heterocyclic radical, halogen, hydroxy group, an amino group, a substituted amino group or a halogenamino group and one of Ra, R2 and R3 is a hydrogen atom or a methyl group, which may be and each of the remaining two thereof, which may be the same or different, is a methyl group or an ethyl group, and (2) a process for producing a selective gas-permeable composite membrane which comprises feeding a tertiary carbon or tertiary organic silicon-containing compound into an atmosphere of 5 Torr or less under glow discharge and polymerizing it to form a thin membrane on a heat-resistant porous support.

In the above formulae, examples of the saturated aliphtic hydrocarbon radical represented by X include an alkyl group (e.g. methyl, ethyl, pentyl) and an alkoxy group (e.g. methoxy, ethoxy). Examples of the unsaturated aliphatic radical represented by X include an alkenyl group (e.g. vinyl, allyl, 3-butenyl, 2-butenyl,4-pentenyl) and an alkynyl group (e.g. ethynyl). Examples of the aromatic hydrocarbon radical represented by X include a phenyl group. Examples of the heterocyclic radical represented by X include an imidazolyl group. Examples of the substituted amino group represented by X include a dimethylamino group, a trimethylsilylamino group, etc. Examples of the halogen include chlorine, fluorine, etc.

In general formula (I), the tertiary carbon atom corresponds to the central carbon atom of the above formula.

Of the compounds represented by general formula (I), compounds having simple structures include tert-butylamine, tert-butyl alcohol, and tert-butylchloride. Typical examples of the compounds of general formula wherein X is a hydrocarbon radical include saturated compounds, such as isopentane and isooctane, and unsaturated compounds, such as pentene derivatives, e.g., 4-methyl-1-pentene, 4- methyl-2-pentene, 2,4,4-trimethyl-1 -pentene, and 4,4-dimethyl- 1 -pentene, and octene derivatives, e.g.

iso-octene.

Compounds which can be introduced in a gaseous state into the glow discharge atmosphere are limited to relatively low boiling compounds (below 2000 C, preferably below 1500 C) having a vapor pressure of about 5 to 760 Torr. Thus, compounds containing from up to about 15 carbon atoms, preferably 4 to 10 carbon atoms can be used in the present invention whereas compounds containing 20 or more carbon atoms are not employable practically.

Of the above-described compounds, compounds which are partially substituted by fluorine are advantageous from a viewpoint of ease of plasma-polymerization and increased chemical resistance.

Further, compounds of the above formulae in which each of R1, R2 and R3 represents a methyl group are superior in the boiling point range and ease of production to those containing the ethyl group although compounds in which one or two of R,, R2 and R3 represent an ethyl group can also be used in the present invention.

Examples of the tertiary organic silicon-containing compounds as used herein represented by the following formulae (íí) and (Ill) include trimethylchlorosilane, trimethylfluorosilane, trimethylmethoxysilane, methyltrimethoxy silane, trimethoxyphenylsilane, and additionally, various aminosilane compounds can be used. For plasma polymerization, however, compounds not containing a halogen atom, such as tetramethylsilane, hexamethyldisilazane, dimethylaminotrimethylsilane, and trimethylsilylimidazole, are preferred. Compounds containing a function group such as a vinyl group, an ethynyl group, an allyl group, etc., e.g., vinyltrimethylsilane, vinyltrimethoxysilane, and vinyltris(p-methoxyethoxy) silane are advantageous with respect to the rate of polymerization.

The compounds specified in the invention have a tertiary or tertiary type structure prior to the plasma polymerization thereof. In the compounds, dehydrogenation or growth by polymerization of vinyl radicals causes the main chain to grow. Upon the growth of the main chain, there is formed a polymeric compound in which branches comprising recurring methyl side chains are linked to the main chain. On the other hand, side chains growing from the main chain which has been dehydrogenated by plasma form long branches. The frequency of cross-linking between part of the branches and the main chain increases as the branch grows, finally resulting in the formation of three-dimensional net-like structure.

As the proportion of the three-dimensional net-like structure increases, the strength becomes greater and heat deformation properties are reduced. This leads to an improvement in heat resistance.

With a thin membrane in which a number of branches corresponding to the methyl group are formed onto the main chain or side chain, its crystallinity lowers, and there is formed a structure which makes it possible to sufficiently detect fine differences in size between gas molecules. This will increase the selective permeability of the thin membrane. It is said that the mean collision radius of hydrogen at atmospheric pressure is 2.9 A,and that of methane is 3.8 A. In the case of a membrane comprising dimethylsiloxane, for example, its hydrogen permeability is nearly the same as the methane permeability. In fact, methane having a larger collision radius than hydrogen passes through the membrane about 1.2-times faster than the hydrogen. This is supposed to be ascribed to the presence of methyl groups branching from the main chain.

T#he thickness of the thin membrane to be formed on a support by plasma polymerization varies depending on the time for which the tertiary carbon or tertiary organic silicon-containing compound is supplied under glow discharge, the flow rate of the compound supplied, the high frequency output, and so forth. The glow discharge can be carried out under conditions as described e.g., in U.S. Patents 3,775,308 and 3,847,652. The thin membrane preferably has a thickness of 1 micron or less and more preferably a thickness of 0.3 micron or less in view of its selective gas-permeability.

The support which can be used in the present invention is a heat resistant porous polymer membrane composed of polysulfones, polyimides, cellulose esters, polyvinyl chlorides polypropylene, polycarbonates, polyvinyl alcohols, polytetrafluoroethylenes, etc., with polytetrafluoroethylene being preferred. Preferably, the porous support has a porosity of 30 to 80% and a pore diameter of not larger than 0.2 , preferably not larger than 0.1 y.

When the thin membrane is formed by plasma polymerization under such conditions as to adjust the thickness to 1 micron or less, preferably 0.3 micron or less, if the adhesion between the thin membrane and the support is poor, or the pore diameter of the support is too large, there is a tendency for defects to develop therein. No suitable technique to prevent such defects from occurring has heretofore been known.

Various methods have been proposed for that purpose, including a method in which the pore diameter of the porous support is reduced as described in U.S. Patents 3,567,810, 3,709,841, 3,855,122 and 4,026,977. There are, however, only few methods capable of solving the above problem while meeting the required physical properties, such as heat resistance and strength.

Another feature of the invention is that by filling the inside of pores in the heat-resistant porous support with a siloxane compound exemplified by silicone rubber, and thereafter, by performing plasma polymerization, the adhesion between the thin membrane thus formed and the support is increased and at the same time, the pore diameter of the support is decreased reducing the occurrence of defects in the thin membrane.

These siloxane compounds can be cross-linked inside the pores of the heat-resistant porous support, as is with the usual silicone rubber, by the use of organic peroxides, aliphatic acids, azo compounds, sulfur, etc., or by means of radiation.

In the selectively gas-permeable membrane, not only materials per se have excellent characteristics, but also constitutional elements governing the permeability should be as thin as possible.

The characteristics of material is evaluated by the unit of coefficient of gas permeation: P = cm3.cm/cm2.sec.cmHg This is calculated with the thickness of the material as 1 cm. On the other hand, with the composite membrane, the evaluation is performed by the unit: P = cm3/cm2.sec.cmHg which is the permeation speed at the thickness of material per se. That is, although the permeation speed of a membrane having a thickness of 10 microns is 10 times that of a membrane having a thickness of 1 micron, their coefficients of permeation are the same. In practice, the value of the permeation speed is necessary.

As a result of extensive studies on a method of curing siloxane compounds, it has been found that when the surface layer, which comes into contact with plasma in a plasma atmosphere using unpolymerizable gas (such as air, N2, Ar, Ne, preferably Ne, Ar), is cross-linked or cured, uncured siloxane compounds can be extracted and removed from the surface layer of the heat-resistant porous support, which does not come into contact with the plasma.

Thus there is obtained a membrane of unsymmetrical structure comprising one surface layer which is composed of the siloxane compound cross-linked by plasma, and the reverse surface layer in which the siloxane compound is extracted and removed, and no siloxane compound remains. The thickness of the cured portion is not larger than 1 ju.

In order to make easier the step of cross-linking by plasma and the step of extracting and removing the uncured siloxane compound, it is preferable to employ intermediate molecular weight polymers generally called silicone oil rather than using uncured raw rubber.

The viscosity (at 250C) of silicone oil exemplified by dimethylsiloxane, which is commercially available, ranges from 0.65 cs to 1,000,000 cs. When the viscosity is as low as 20 cs or less, the volatility is high, resulting in the dissipation of the oil in the plasma atmosphere. On the other hand, when the viscosity is as high as 50,000 cs or more, it becomes difficult to fill the pores of the heatresistant porous support with the silicone oil. Furthermore, an additional problem develops in that the silicone oil not only enters the pores of the support, but also excessively attaches to the surface of the support.

The silicone oil excessively attaching to the surface undergoes cross-linking by means of plasma.

In extracting and removing the uncured component, however, it is liable to peel apart from the porous polymeric membrane. Accordingly, a product having uniform quality can not be obtained. Also, when uncured raw rubber except for silicone oil is used to fill the pores of the heat-resistant porous support, the problem of the rubber excessively attaching to the surface of the support arises as in the case of high viscosity silicone oil. The use of intermediate molecular weight silicone oil makes it possible to reduce the amount of the oil being attached to the surface.

In order to further reduce the amount of the oil being attached onto the surface, it is preferable to utilize the thermal expansion and contraction action of the siloxane compound. The siloxane compound is heated to 100 to 1500C to cause an expansion in volume and a reduction in viscosity. In the state that the volume is increased and the viscosity is reduced, the siloxane compound is used to impregnate the heat-resistant porous support therewith. After the impregnation is completed, an excess of siloxane compound attaching to the surface of the support is squeezed out therefrom. Thereafter, when the support is cooled to room temperature, a contraction in volume of about 10% occurs, and the siloxane compound remaining on the surface is absorbed into the pores of the support.In any event, dimethylsiloxane having a viscosity ranging from 30 cs to 300,000 cs is preferred.

After the siloxane compound is formed into a cross-linked structure, a thin membrane formed by plasma polymerization and having a thickness of 1 micron or less, preferably 0.3 micron or less, may be laminated on the surface. For this purpose, the inside of the reactor is maintained at a reduced pressure of 5 Torr or less, preferably 2 Torr or less, and a mixed gas of the unpolymerizable gas and polymerizable gas of the compound represented by general formula (I), (II) or (Ill) which is the same as is used in the formation of the thin membrane on the support is introduced thereinto. When glow discharge is developed in the reactor by the generation of high frequency at a predetermined output of from 20 to 500 W, e.g., 50 W, the polymerizable gas undergoes plasma polymerization to form a thin membrane.

Lamination of the thin membrane on the surface layer of a composite material comprising the crosslinked siloxane compound and the heat-resistant porous polymeric membrane proceeds in the same manner as described for the support having no siloxane compound thereon.

A composite membrane prepared under very limited conditions as described above exhibits excellent characteristics in the selective permeation of gas mixtures, and thus, greatly contributes to industry as an energy-saving gas-separation method.

The composite membrane of the present invention is particularly useful in separating oxygen from the air and hydrogen from coke oven gas.

The invention is explained in detail with reference to the following examples.

EXAMPLE 1 FLUOROPORE FP045 (a porous membrane of a polytetrafluoroethylene, produced by Sumitomo Electric Industries, Ltd.; mean pore diameter: 0.45 micron) was impregnated with a two-fold dilution solution of SILICONE OIL KF-96 (dimethylsiloxane, produced by Shin-Etsu Silicone Co., Ltd.: 30,000 cs) with methyl ethyl ketone, and thereafter, the methyl ethyl ketone was evaporated. The membrane was heated to 150 C, and the silicone oil appearing on the surface of the membrane was removed with a sponge roll. Then, the membrane was allowed to cool.

The membrane was exposed to a plasma atmosphere of 50 W high frequency output, 13.56 MHz, and 2 Torr nitrogen gas for 15 minutes. Then, uncured silicone oil was extracted with methyl ethyl ketone. The membrane was again placed in the plasma apparatus, into which 4-methyl-1-pentene vapor was then introduced along with nitrogen gas, and plasma polymerization was performed for 20 minutes.

The gas permeability of the composite membrane thus prepared was measured. The permeation speeds of oxygen and nitrogen were 1.2 x 10-5 cm3/cm2.sec.cmHg and 3.4 x 10-6 cm3/cm2.sec.cmHg, respectively, and thus, the coefficient of selective permeation was 3.5.

EXAMPLE 2 A polytetrafluoroethylene membrane with a siloxane compound cross-linked in the pores was prepared in the same manner as in Example 1. On the membrane thus formed was provided a thin membrane of each of the tertiary organic silicon-containing compounds shown in Table 1 by means of plasma polymerization. The gas permeability of each membrane was measured with the results shown in Table 2.

TABLE 1 Plasma Polymerization Conditions Run High Frequency Polymerization No. Compound Output Pressure Time (Watts) (Torrs) (Minutes) 1 Tetramethylsilane 10 4 30 2 Dimethylaminosilane 60 4 15 3 Vinyltriethoxysilane 80 1 20 4 Vinyltrimethylsilane 30 3 20 TABLE 2 Run Permeation Speed Permeation Speed Coefficient of No. of Oxygen of Nitrogen Selective Permeation (pro2) (PN2) (a: 02/N2) 1 7.0x10-5 2.9 x 10-5 2.4 2 1.6 x 10-5 5.7 x 10-6 2.8 3 2.4 x 10-5 8.9 x 10-6 2.7 4 2.2 x 10-6 5.5 x 10-' 4.0

Claims (15)

1. A selectively gas-permeable composite membrane comprising: a heat-resistant porous polymeric membrane; and a thin layer of cross-linked structure provided on one side of said membrane, said thin layer being prepared by polymerizing a compound having the general formula (I), (II) or (III)

wherein X represents a saturated aliphatic hydrocarbon radical, an unsaturated aliphatic hydrocarbon radical, an aromatic hydrocarbon radical, a heterocyclic radical, halogen, a hydroxy group, an amino group, a substituted amino group, or a halogenamino group, and one of Ra, R2 and R3 is a hydrogen atom, or a methyl group and each of the remaining two thereof, which may be the same or different, is a methyl group or an ethyl group, wherein the polymerization is carried out by means of plasma.

2. A selectively gas-permeable composite membrane, as claimed in Claim 1, each of Ra, R2, and R3 is a methyl group.

3. A composite membrane as claimed in Claim 1 or 2, wherein the thin layer of cross-linked structure has a thickness of 0.3 micron or less.

4. A composite membrane as claimed in Claim 1,2 or 3, wherein the tertiary carbon-containing compound is selected from 4-methyl-1 -pentene, 4-methyl-2-pentene, 2,4,4-trimethyl-1 -pentene, 4,4- dimethyl-1 -pentene, tert-butylamine, tert-butyl alcohol, tert-butylchloride, and their fluorine-contained derivatives.

5. A composite membrane as claimed in Claim 1,2 or 3, wherein the tertiary organic siiiconcontaining compound is selected from vinyl trimethylsilane, tetramethylsilane, hexamethyldisilazane, dimethylaminotrimethylsilane, trimethylsilylimidazole, vinyltrimethoxysilane, vinyltriethoxysilane, and vinyItris(#-methoxyethoxy)siIane.

6. A composite membrane as claimed in Claim 1,2 or 3, wherein a siloxane compound is crosslinked in the pores of the heat-resistant porous polymeric membrane.

7. A composite membrane as claimed in any preceding Claim, wherein the heat-resistant porous polymeric membrane is made of a polytetrafluoroethylene, and has a structure comprising fibers and knots.

8. A process for producing a selectively gas-permeable membrane, comprising the steps of: providing a heat-resistant porous polymeric membrane as a support; feeding a compound having the general formula (I), (II) or (III)

wherein X represents a saturated aliphatic hydrocarbon radical, an unsaturated aliphatic hydrocarbon radical, an aromatic hydrocarbon radical, a heterocyclic radical, halogen, a hydroxy group, an amino group, a substituted amino group, or a halogenamino group, and one of R1, R2 and R3 is a hydrogen atom, or a methyl group and each of the remaining two thereof which may be the same or different, is a methyl group or an ethyl group, said compound being fed into an atmosphere of 5 Torr less under glow discharge in order to polymerize the compound and form a thin membrane; and laminating the thin membrane on the heat-resistant porous polymeric membrane.

9. A process for producing a selectively gas-permeable membrane as claimed in Claim 8, wherein X is a hydrocarbon or a halogenamine and R1, R2, and R3 are all CH3.

10. A process as claimed in Claim 8 or 9, wherein the cross-linking of the siloxane compound is performed in a plasma atmosphere of unpolymerizable gas at a pressure of 5 Torr or less.

11. A process as claimed in Claim 8 or 9, wherein the tertiary carbon-containing compound is selected from 4-methyl-I -pentene, 4-methyl-2-pentene, 2,4,4-trimethyl- 1 -pentene, 4,4-dimethyl- 1 - pentene, tert-butylamine, tert-butyl alcohol, tert-butylchloride, and their fluorine-containing derivatives.

12. A process as claimed in Claim 8 or 9, wherein the tertiary organic silicon-containing compound is selected vinyl trimethylsilane, tetramethylsilane, hexamethyldisilazane, dimethylaminotrimethylsilane, trimethylsilylimidazole, vinyltrimethoxysilane, vinyltriethoxysilane, and vinyitris(p-methoxyethoxy) silane.

13. A process as claimed in Claim 8, substantially as hereinbefore described in Example 1 or 2.

14. A selectively gas-permeable membrane when produced by a process as claimed in any one of Claims 8 to 13.

15. A selectively gas-permeable membrane as claimed in claim 1, substantially as hereinbefore described in Example 1 or 2.