FR2527468A1 - Selective gas-permeable composite membrane prodn. - by applying a tert. carbon or organo:silicon cpd. to a heat resistant porous polymer membrane by plasma polymerisation - Google Patents

Abstract

A selective gas permeable composite membrane having a bridged structure is produced by a process in which a cpd. contg. tert. carbon atom (e.g. 4-methyl-1-pentene, 4-methyl-2-pentene, 2,4,4-trimethyl-1-pentene, t.-butylamine, t.-butylalcohol, t.-butylchloride, etc.) or a tert. organic Si cpd. (e.g. vinyl trimethylsilane, tetramethylsilane, dimethylamino trimethylsilane, etc.) is supplied to a heat-resistant porous polymer membrane (supporting body) under glow discharge atmos. of up to 5 torr so that a thin membrane (thickness up to 0.3 microns) by plasma polymerisation is formed on the supporting body. The supporting body is (pref.) impregnated with a siloxane cpd. and the siloxane cpd. is bridged and cured in plasma atmos. of a non-polymerising gas of up to 5 torr. By combining different materials, various characteristics such as permeability, heat resistance, chemical resistance, mechanical strength etc. are satisfactory achieved.

Description

 The present invention relates to selectively gas-permeable composite membranes and a process for their production.

More particularly, the invention relates to selectively gas permeable composite membranes comprising a thin layer of structure crosslinked on one side of a porous heat-resistant support, the thin layer being prepared by plasma polymerization of tertiary carbon compounds or tertiary organosilicon compounds. The invention also relates to a process for the production of said membranes.

 In recent years, extensive research has been conducted to obtain the separation and purification of fluid mixtures by the use of selectively permeable membranes instead of conventional techniques such as distillation and low temperature treatment, which are accompanied by of phase change and consume a large amount of energy, as described in US Pat. Nos. 4,230,463 and 4,264,338.

 The separation and purification of fluid mixtures using membranes has already been passed into practical use in many fields. For example, the conversion of seawater to freshwater, plant wastewater discharge and feed concentration have all been carried out on an industrial scale using appropriate membranes. These methods, however, are liquid-liquid separation and liquid-solid separation processes.

However, the gas-gas separation on an industrial scale is virtually unknown.

 It is difficult to use gas separation on an industrial scale using a membrane (technique sometimes referred to as "membrane separation") for the following reasons: (1) the selective permeability of conventional membranes is poor ( more particularly, there is no suitable membrane that selectively allows certain gases to pass through, while practically blocking certain others, making it possible to obtain a high-purity gas, and it is therefore necessary to use a process in several stages in which membrane separation is carried out several times, leading to an increase in the size of the apparatus), and (2) the gas permeability is poor, making it difficult to treat a In addition, when the selective permeability of the membrane is increased, the gas permeability tends to decrease.

However, as gas permeability increases, the selective permeability tends to decrease.

 This makes it difficult to perform membrane separation on an industrial scale.

 For industrial membrane separation, various methods for producing improved membranes have been proposed.

Typical examples include a method in which the casting of a polymer solution is used to produce a symmetrical membrane, wherein the thickness of the active skin layer is as thin as possible, and a method in which independently prepares an ultra-thin membrane corresponding to the active skin layer above and is bonded to a porous support to form a composite membrane, as described in the US Pat.
US Pat. Nos. 3,497,451, 4,155,793 and 4,279,855. These methods, however, do not provide satisfactory improved membranes, although these are conventional techniques for improving gas permeability. of this is that there are no commercially available polymers or copolymers which satisfy all the necessary physical properties, for example selective permeability, gas permeability, heat resistance, chemical resistance and mechanical strength.

 From the point of view of heat resistance, and mechanical strength, various materials may be selected from the currently available porous polymeric materials on the market.

Porous polysulfones, porous polyimides, etc., can be used, but cellulose esters, polyvinyl chloride, polypropylene, polycarbonates, polyvinyl alcohol, etc. are not appreciated.

 As regards heat resistance and mechanical strength, a porous polytetrafluoroethylene carrier is preferred. In addition, it has the advantage of satisfactory high chemical resistance.

 With respect to gas permeability, polytetrafluoroethylene is unsuitable. Materials having satisfactory gas permeability include various rubbers, such as silicone rubbers (eg dimethylsiloxane and phenylsiloxane), natural rubber and polybutadiene. These rubbers, however, have the serious disadvantage of poor mechanical strength. It is possible to incorporate a silica filler into these rubbers to improve the strength. The incorporation of these charges, however, is not preferred because it decreases gas permeability.

 As a result of numerous investigations, we have found that polymer compounds containing a tertiary carbon atom in its recurring unit have excellent gas selective permeability and, in addition, compounds containing a tertiary organic silicon atom instead. of said tertiary carbon atom above also have excellent gas selective permeability. However, these polymers have lower heat resistance, mechanical strength and chemical resistance.

 The present invention therefore relates to an improved menbrane which uses not only a single substance which combines all the above physical properties, but which combines different materials each having excellent physical properties.

 The invention also relates to a selectively gas-permeable composite membrane which is useful for concentrating oxygen in the air, or for separating hydrogen from petroleum gas, etc.

 The applicant has discovered according to the invention that when polymers containing a tertiary carbon atom or a tertiary organic silicon atom are polymerized by corona discharge, at a pressure of 13.46 mbar or less, preferably 6, 73 mbar or less, to form a thin membrane on a heat-resistant porous support, a composite membrane can be obtained which has not only improved selective permeability but also improved mechanical strength and heat resistance.

dans lesquelles X est un radical d'hydrocarbure aliphatique saturé, un radical d'hydrocarbure aliphatique insaturé, un radical d'hydrocarbure aromatique, un radical hétérocyclique, un halogène, un groupe hydroxy, un groupe amino, un groupe amino substitue ou un groupe halo génoamino et l'un des restes R1, R2 et R3 est un atome d'hydrogène ou un groupe méthyle et les deux autres sont identiques ou différents et représentent chacun un groupe méthyle ou un groupe éthyle ; et
(2) un procédé pour la production d'une membrane composite sélectivement perméable aux gaz, qui consiste à introduire un composé contenant un atome de carbone tertiaire ou un composé contenant un atome de silicium organique tertiaire dans une atmosphère sous 6,73 mbar ou moins dans une décharge par effluve et à polymériser sous forme d'une membrane mince sur un support poreux résistant la chaleur.The subject of the invention is therefore
(1) a selectively gas permeable composite membrane comprising a heat-resistant porous support and a thin layer of crosslinked structure on one side of the support, said layer being prepared by plasma polymerization of a compound containing a carbon atom tertiary compound of the general formula (I) or a compound containing a tertiary organic silicon atom of the general formula (II) or (III)

wherein X is a saturated aliphatic hydrocarbon radical, an unsaturated aliphatic hydrocarbon radical, an aromatic hydrocarbon radical, a heterocyclic radical, a halogen, a hydroxy group, an amino group, a substituted amino group or a halo group genoamino and one of the radicals R1, R2 and R3 is a hydrogen atom or a methyl group and the other two are identical or different and each represents a methyl group or an ethyl group; and
(2) a method for producing a gas-permselective composite membrane which comprises introducing a tertiary carbon-containing compound or a tertiary organic silicon-containing compound into an atmosphere of 6.73 mbar or less in a corona discharge and polymerize as a thin membrane on a porous heat-resistant support.

 In the above formulas, examples of hydrocarbon radicals represented by X include alkyl groups, for example methyl, ethyl, pentyl, etc., alkoxy groups, for example methoxy, ethoxy, and the like. and analogues.

Examples of unsaturated aliphatic radicals represented by X include alkenyl groups, for example vinyl, allyl, 3-butenyl, 2-butenyl, 4-pentenyl, etc., alkynyl groups, for example ethyl, and the like. and analogues. Examples of aromatic hydrocarbon radicals represented by X include phenyl etc. Examples of the heterocyclic radicals represented by X include imidazolyl and the like.

Examples of the substituted amino group represented by
X include dimethylamino group, trimethylsilylamino group, and the like. Examples of halogens include chlorine, fluorine, etc.

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

 Among the compounds represented by the general formula (I), compounds having simple structures include tertbutylamine, tert-butyl alcohol and tert-butyl chloride.

Typical examples of compounds of general formula (I) wherein X is a hydrocarbon radical include saturated compounds such as isopentane and isooctane and unsaturated compounds such as pentene derivatives, for example 4-methyl-1-pentene , 4-methyl-2-pentene, 2,4,4-trimethyl-1-pentene and 4,4-dimethyl-1-pentene and octene derivatives, for example iso-octene.

 Compounds which can be introduced in gaseous form into the corona discharge atmosphere are limited to compounds of relatively low boiling points (less than 2000C, preferably less than 1500C) having a vapor pressure of about 6.73 to 1000 mbar. Compounds containing less than 20 carbon atoms, preferably up to about 15 carbon atoms, more preferably C4-C10, may be used according to the invention, while compounds containing 20 or more carbon atoms are not preferred. usable in practice.

 Among the compounds described above, compounds that are partially substituted by fluorine are advantageous from the point of view of their ease of polymerization in the corona and increased chemical resistance. In addition, the compounds of the above formulas in which each of R 1, R 2 and R 3 are methyl are higher in boiling point range and ease of production than those containing the ethyl group, although Compounds in which one or two of the radicals R 1, R 2 and R 3 represent an ethyl group can also be used according to the invention.

 Examples of the compounds containing a tertiary organic silicon atom used according to the invention represented by the formulas (II) and (III) include trimethylchlorosilane, trimethylfluorosilane, trimethylmethoxysilane, methyltrimethoxysilane, trimethoxyphenylsilane and, in addition, it is possible to use various aminosilanes. For the polymerization in the plasma, however, compounds containing no halogen atom, such as tetramethylsilane, hexamethyldisilazane, dimethylaminotrimethylsilane and trimethylsilylimidazole are preferred. Compounds containing a functional group such as a vinyl group, an ethynyl group, a allyl group, etc., for example 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 structure prior to their plasma polymerization. In the compounds, the dehydrogenation or polymerization growth of the vinyl radicals causes the growth of the main chain. By growth of the main chain, a polymeric compound is formed in which branches comprising recurrent methyl side chains are connected to the main chain.

 On the other hand, the side chains growing from the main chain which has been dehydrogenated by the plasma form long branches. The crosslinking frequency between part of the branches and the main chain increases with the branching, finally leading to the formation of a three-dimensional crosslinked structure. As the proportion of the three-dimensional crosslinked structure increases, the strength increases and the thermal deformation properties decrease.

This leads to an improvement of the thermal resistance.

 With a thin membrane in which a number of branches corresponding to the methyl group are formed on the main chain or the side chain, the crystallinity decreases and a structure is formed which allows a sufficient detection of small size differences between the gas molecules. . This increases the selective permeability of the thin membrane. The average collision radius of hydrogen at atmospheric pressure is considered to be 2.9 and that of methane is 3.8. In the case of a membrane comprising dimethylsiloxane, for example, its permeability to hydrogen is substantially the same as the permeability to methane. Indeed, methane2 having a collision radius greater than that of hydrogen, passes through the membrane about 1.2 times faster than hydrogen. This is presumed to be due to the presence of methyl groups attached to the main chain.

 The thickness of the thin membrane to be formed on a support by plasma polymerization varies depending on the time during which the tertiary carbon-containing compound or a tertiary organic silicon atom is introduced into the corona discharge, the flow of compound feed, high frequency power, etc. The corona discharge can be carried out under conditions as described in United States Patents 3,775,308 and 3,847,652. The thin membrane preferably has a thickness of -1 micron or less, and more preferably 0.3 micron less, from the point of view of its selective gas permeability.

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

 When the thin membrane is formed by plasma polymerization under conditions where the thickness is adjusted to 1 micron or less, preferably 0.3 micron or less, if the adhesion between the thin membrane and the support is poor, or if the pore diameter of the support is too large, the membrane tends to have defects. Up to now, no suitable techniques have been known to prevent the existence of these defects.

 Various methods have been proposed for this purpose, including a method in which the pore diameter of the porous support is reduced as described in US Pat. Nos. 3,567,810, 3,709,841, 3,855,122 and 4,026,977. There are, however, few processes capable of solving the above problem while combining the necessary physical properties such as heat resistance and mechanical strength.

 According to another characteristic of the invention, by filling the interior of the pores of the porous heat-resistant support with a siloxane illustrated by the silicone rubbers, and then performing the polymerization with the plasma, the adhesion between the thin membrane thus formed and the support is increased and simultaneously the pore diameter of the support decreases, which reduces the appearance of defects in the thin membrane.

 These siloxanes may be crosslinked within the pores of the porous heat-resistant carrier, as is the case with ordinary 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 the actual materials have excellent characteristics, but also the constitution elements controlling the permeability must be as thin as possible.

The characteristics of the material are evidenced by the unit of the gas permeation coefficient 3 2
P = cm cm / cm .s.mbar
This is calculated with a material thickness of 1 cm. On the other hand, with the composite membrane, the evaluation is carried out with the unit
P = cm3 / cm2 .s.mbar which is the speed of permeation to the thickness of the material itself. In other words, although the permeation rate of a membrane having a thickness of 10 microns is 10 times that of a membrane with a thickness of 1 micron, their permeation coefficients are the same. In practice, the value of the permeation rate is necessary.

 As a result of extensive research on a method for curing siloxanes, the Applicant has found that when the surface layer, which contacts the plasma in a plasma atmosphere using a non-polymerizable gas, is reticulated or hardened ( such as air, N2 Ar, Ne, preferably Ne, Ar), the uncured siloxanes can be extracted and removed from the surface layer of the porous heat-resistant carrier, which does not come into contact with the plasma.

 A membrane of asymmetric structure is thus obtained comprising a surface layer which is composed of the siloxane crosslinked by the plasma and the opposite surface layer in which the siloxane is extracted and eliminated, and no siloxane remains. The thickness of the cured portion is not greater than 1 μm.

 To facilitate the plasma crosslinking step and the uncured siloxane extraction and removal step, it is preferable to use intermediate molecular weight polymers referred to as silicone oils instead of uncured raw rubber.

 The viscosity (250C) of the dimethylsiloxane-illustrated silicone oils, which is commercially available, varies from 0.65.10 to 1 m 2 Is. When the viscosity is 20 × 10 6 m 2 or less, the volatility is high. causes the dissipation of the oil in the plasma atmosphere. By cons, when the viscosity is 50.10-3 m Is or more, it becomes difficult to fill the pores of the porous support heat resistant with silicone oil. In addition, an additional problem arises from the fact that the silicone oil does not penetrate only the pores of the support, but that it is fixed too strongly on the surface of the support.

 Silicone oil excessively bound to the surface is crosslinked by the plasma.

 In the extraction and removal of the uncured component, however, it is likely to separate from the porous polymeric membrane. As a result, a product of uniform quality can not be obtained. Similarly, when the pores of the porous heat-resistant carrier are used for uncured raw rubber instead of silicone oil, there is the problem of fixing the excess rubber on the surface of the support as in the case of high viscosity silicone oil. The use of intermediate molecular weight silicone oil reduces the amount of oil that binds to the surface.

 To further reduce the amount of oil attached to the surface, it is preferred to use the thermal expansion and contraction action of the siloxane. The siloxane is heated to 100-1500C to cause volume expansion and viscosity reduction. The siloxane is used in the state where the volume is increased and the viscosity reduced to impregnate the porous heat-resistant carrier.

When the impregnation is complete, the excess of siloxane attached to the surface is expressed. Then, when the carrier is cooled to room temperature, a volume contraction of about 10% occurs and the siloxane remaining on the surface is absorbed into the pores of the porous support. In all cases, a dimethylsiloxane having a viscosity of between 30.10-6 viscosity and 300-3 m-1 is preferred.

 After the siloxane has been placed in a crosslinked structure, a thin membrane formed by plasma polymerization and having a thickness of 1 micron or less, preferably 0.3 micron or less, can be laminated to the surface. For this purpose, the reactor interior is maintained under a reduced pressure of 6.73 mbar or less, preferably 2.692 mbar or less, and a gaseous mixture of the non-polymerizable gas and a polymerizable gas consisting of the compound represented by the general formula (I), (II) or (III) which is the same as that used in the formation of the thin membrane on the support. When the corona discharge is produced in the reactor by the production of high frequency at a predetermined power of 20 to 500 W, for example 50 W, the polymerizable gas is polymerized by the plasma forming a thin membrane. of the thin membrane on the surface layer of a composite material comprising the cross-linked siloxane and the porous heat-resistant polymeric membrane takes place in the same manner as described for the carrier having no siloxane on its surface.

 A composite membrane prepared under very limited conditions as described above has excellent characteristics of selective permeation of gas mixtures and therefore strongly contributes to the industry as a method of energy-saving gas separation.

 The composite membrane according to the invention is particularly interesting for the separation of oxygen from air and hydrogen from the coke oven gas.

 The following examples illustrate the invention without, however, limiting its scope.

Example 1
A porous polytetrafluoroethylene resin membrane produced by the Applicant Company is impregnated under the name "Fluoropore FP045" (average pore diameter: 0.4 micron) with a solution of a dimethylsiloxane oil produced by the company.
Shin-Etsu Silicone Co., Ltd. under the name "Silicone Oil KF-96" (viscosity: 30.10 m 2 / s) diluted twice in methyl ethyl ketone and then the methyl ethyl ketone is evaporated. The membrane is heated to 1500 ° C. and the silicone oil appearing on the surface of the membrane is eliminated by means of a sponge roll. Then the membrane is allowed to cool.

 The membrane is exposed to a plasma atmosphere at 50 W and 13.56 MHZ and in a nitrogen atmosphere at a pressure of 2.692 mbar for 15 min. The uncured silicone oil is then extracted with methyl ethyl ketone. The membrane is again placed in the plasma apparatus into which 4-methyl-pentene gas is then added with the nitrogen and the plasma polymerization is carried out for 20 minutes.

 The gas permeability of the composite membrane thus prepared is measured. The permeation rates of oxygen and of -7 are 8.91, 2.53 × 10 -7 2 nitrogen and 8.91 × 10 -7 and 2.53 × 10 cm cm / cm 2. , respectively, and the selective permeation coefficient a, IN is therefore 3> 5.

Example 2 22
A polytetrafluoroethylene resin membrane containing in its pores a crosslinked siloxane is prepared in the same manner as in Example 1. On the membrane thus formed, a thin membrane of each of the compounds containing a tertiary organic silicon atom shown in Table I below is formed by plasma polymerization and the gas permeability of each membrane is measured. The results obtained are shown in Table II below.

 It should be understood that the invention is not limited to the preferred embodiments described above by way of illustration and that those skilled in the art may make various modifications and changes thereof without departing from the scope of the invention. and the spirit of the invention.

TABLE I

<tb><SEP><SEP> Conditions <SEP> Polymerization <SEP> By
<tb><SEP> the <SEP> Plasma
<tb> Test <SEP> the <SEP> Power <SEP> Plasma
<tb><SEP> ci <SEP> n0 <SEP> to <SEP> High <SEP> Pressure <SEP> Polymerization
<tb><SEP> frequency <SEP> (mbar) <SEP> (min)
<tb><SEP> (w) <SEP>
<tb><SEP> 1 <SEP><SEP> Tetramethylsilane <SEP> 10 <SEP> 5,384 <SEP> 30
<tb><SEP> 2 <SEP> Dimethylaminosilane <SEP> 60 <SEP> 5,384 <SEP> 15
<tb><SEP> 3 <SEP> Vinyltriethoxysilane <SEP> 80 <SEP> 1.346 <SEP> 20
<tb><SEP> 4 <SEP> Vinyltrimethylsilane <SEP> 30 <SEP> 4,038 <SEP> 20
<Tb>
TABLE he

<tb> Test <SEP> Speed <SEP> of <SEP> permeation <SEP> Speed <SEP> of <SEP> permeation <SEP><SEP> Coefficient <SEP> of
<tb><SEP> n0 <SEP> of <SEP> oxygen <SEP> of <SEP> nitrogen <SEP> permeation
<tb><SEP> 0 <SEP> (P <SEP>) <SEP> Selective
<tb><SEP>(<SEP> 2) <SEP> N2 <SEP>(<SEP> O2 / N2) <SEP>
<tb><SEP> 1 <SEP> 52, C1.10-7 <SEP> 21.54.10-7 <SEP> 7 <SEP> 2.4 <SEP>
<tb><SEP> 2 <SEP> 11.88.10-7 <SEP> 42.35.10-7 <SEP> 2.8 <SEP>
<tb><SEP> 3 <SEP> 17, <SEP> 83.10-7 <SEP> 66,12.10-7 <SEP> 2,7
<tb><SEP> 4 <SEP> 16,34.10-7 <SEP> 40,86.10-7 <SEP> 4,0
<Tb>

Claims (12)

 wherein X represents a saturated aliphatic hydrocarbon radical, an unsaturated aliphatic hydrocarbon radical, an aromatic hydrocarbon radical, a heterocyclic radical, a halogen, a hydroxy group, an amino group, a substituted amino group or a haloamino group and one of the radicals R1, R2 and R3 is a hydrogen atom or a methyl group and the other two are the same or different and each represents a methyl group or an ethyl group.

 a thin layer of structure crosslinked on one side of said membrane, characterized in that said thin layer is prepared by plasma polymerization of a compound corresponding to one of the general formulas (I), (II) or (III)  a porous polymeric membrane resistant to heat; and  1. Selectively permeable composite gas membrane comprising 2. Selectively gas-permeable composite membrane according to claim 1, characterized in that R1, R2 and R3 are methyl groups. 3. Composite membrane according to claim 1 or 2, characterized in that the thin layer of crosslinked structure has a thickness of 0.3 micron or less. 4. Composite membrane according to claim 1 or 2, characterized in that the tertiary carbon compound of formula (I) is selected from G-methyl-1-pentene, 4-methyl-2-pentene, 2,4,4 trimethyl-1-pentene, 4-dimethyl-1-pentene, tert-butylamine, tert-butyl alcohol, tert-butyl chloride and their fluorinated derivatives. 5. Composite membrane according to claim 1 or 2, characterized in that the organic compound of tertiary silicon of formula (II) or (III) is selected from vinyltrimethylsilane, tetramethylsilane, hexamethyldisilazane, dimethylaminotri-methylsilane, trimethylsilylimidazolev vinyltrimethoxysilane, vinyltriethoxysilane and vinyltris (# -methoxyethoxy) silane. Composite membrane according to claim 1 or 2, characterized in that a siloxane is crosslinked in the pores of the porous polymeric heat-resistant membrane. Composite membrane according to claim 1 or 2, characterized in that the heat-resistant porous polymeric membrane is made of polytetrafluoroethylene and has a structure comprising fibers and knots. Process for the production of a selectively gas-permeable composite membrane according to claim 1, characterized in that  we start from a porous polymer membrane resistant to heat as a support  introducing a compound of general formula (I), (II) or (III) in an atmosphere of 6.73 mbar or less in a corona discharge to polymerize the compound and form a thin membrane; and  the thin membrane is laminated on the porous heat-resistant support. A process for producing a selectively gas permeable membrane according to claim 7, characterized in that X is a hydrocarbon or haloamine residue and R1, R2 and R3 are all CH3 groups. 10. Process according to claim 8 or 9, characterized in that the crosslinking of the siloxane is carried out in a plasma atmosphere of a non-polymerizable gas under a pressure of 6.73 mbar or less. 11. Process according to claim 8, characterized in that the tertiary carbon compound of formula (I) is chosen from 4-methyl-1-pentene, 4-methyl-2-pentene and 2,4 4-trimethyl-1-pentene, 4-dimethyl-1-pentene, tert-butylamine, tert-butyl alcohol, tert-butyl chloride and their fluorinated derivatives.  12. Process according to claim 8, characterized in that the tertiary silicon organic compound of formula (II) or (III) is chosen from vinyltrimethylsilane, tetramethylsilane, hexamethyldisilazane, dimethylaminotrimethylsilane, trimethylsilylimidazole, vinyltrimethoxysilane, vinyltriethoxysilane and vinyltris (# -methoxyethoxy) silane.