IL30273A - Process for the production of microporous films,membranes or similar sheets of high polymers and products obtained thereby - Google Patents

Description

,ιτΐΰπ my»- ικ ο»ο"ηρ ,Ο»Ο?»Β YIX ? "i»?nn οηνχΜΡ o»psiDn c -moi a* ma ι ο»ΊΟ^ΤDD ,im»yr /r aiapa PROCESS FOR THE PRODUCTION OF MICROPOROUS FILMS, MEMBRANES OR SIMILAR SHEETS OF HIGH POLYMERS AND PRODUCTS OBTAINED THEREBY.

The present Invention relates to a process for the production of ioicroporoue films, membranes or similar sheets of high polymers* It also relates to the products obtained according to this process.

Throughout the present description the term "mlcroporous material*1 designates films, membranes and similar sheets which are gas and vapour permeable and of which the thickness depends on the purpose for which they are to be used.

It is known to obtain ion selective membranes of various synthetic polymers by evaporation of the solvent from a shaped film or by coating a fabric with a synthetic resin which is resistant to water, acids and bases, followed by ionization of the membranes* said ionization serving to render the membranes permeable to certain ions and impermeable to others. However, it is often difficult to obtain, by these known methods. microporous material having the desired porosity.

·,' These known gas permeable membranes.: especially permeable to air and water vapour, have recentl acquired great importance, especially for the manufacture of leather substitutes. Nevertheless, the synthetic polymeric membranes hitherto emp!jj§red have been insufficiently gas permeable. It has bee proposed to admix with the polymer solutions pore producing agents* to convert these solutions into membranes by evaporation of the solvents and then to form pores by appropriate means. However* the membranes produced in this way generally comprise visible pores of non-uniform dimension and dis-strlbution.

It has likewise been proposed to form layers of polymers containing urehane groups dissolved in hygroscopic solvents, to expose the layers formed in this way to a humid atmosphere at'ambient'- ■".""*«V temperature and to then eliminate the residual solvent by washing with water or evaporation. However, this process has a very limited application and requires the use of some specific polymers* Furthermore^ it has been proposed to immerse a freshly formed layer oVsolution suoh as a linear polymer of polyur«thane in succe^" ^sive baths constituted by homogeneous mixtures of non-solvent and solvent liquids for the polymer* tho successive baths becoming gradually richer in non-solvent, and later drying at a temperature which can be around 100°C or less or even at ambient temperature. However* this process leads to membranes having too grainy a surface and an annoying tendency to curl up; Furthermore* this process gives good results only with some specific polymers* ;· The present invention provides a process for the production of a microporous film* membrane or similar sheet of a high polymer* which process comprises successively the steps of shaping (e.g. by casting or extruding f'though any other known method of producing a thin film may likewise be used) a thin film from a solution of a polymer which is insoluble i cold water, coagulating the shaped film in a coagulating bath* washin the coagulated film with water until complete elimination of the solvent* freezing without intermediate drying - the water impregnated film by rapid cooling to a temperature equal to or belo -10°C, and drying the resulting material by slow and progressive heating in a vacuum* The invention likewise provides microporous films, membranes and similar sheets obtained according to the process of the invention* A o the polymers which may be used for carrying out the process according to the invention there may be mentioned as particularly interesting polymers having a basis of aorylonltrile, e.b. (l) aorylonltrile homopolymers or (ii) copolymers of polymer mixtures or gra t polymers containin a major portion of aorylonltrile units and a minor portion of units resulting from one or more ethylenic monomers which are copolymerlaable with aorylonltrile (for example a vinyl compound, ifor instance vinyl chloride and vinyl acetate, vinylidene chloride, acrylic or methaorylic acid, an acylic or methaorylic ester or amide, methacrylonitrile, an aliphatic or arylaliphatic ethylenically unsaturated compound with carboxylic acid functional groups, for instance itaconic acid, or with sulphonic acid functional groups,, for instance a vinyl sulphonic add compound* a sulphonated aromatic derivative, such as stwrene sulphonic acid, allyl- or methallyl- eulphonic add, a vinyl-oxyarene-sulphonic acid, a vinyl derivative of the basic type, for instance a vinyl pyridine or an alkylated derivative thereof, o a vinyl dialkylamlne oxide); alternatively there may be used a segmented polyurethane obtained by reacting a polyether or a polyester with terminal hydrox 1 groups with an excess of an organic dilso-cyanate to form a prepolymer with terminal isocyanate groups, this prepolymer being coupled with an agent containing at least two atoms of replacable hydrogen such as diols, and thiols and preferably one containing two terminal amino groups; furthermore there may be used a polymer having a basis of polyvinyl alcohol containing the chain -(CH2-CHOH)-, which polymer is insoluble in cold water. It is likewise possible to use a mixture of two or more of these polymers or a mixture of one or more of these polymers with one or more other polymers, such as the halides of polyvinyl or polyvinylidene, pqlyamides, polyamides-iinides, polyesters, however, these' should be used in relatively smaller quantities* For example a mixture of two polyurethanes or of a polyurethane and pdyvinyl chloride may be used* Furthermore, it is possible to use, if desired., other polymers of natural or synthetic origin,, which are insoluble in cold ( ' ■ I · ■ ■ . ' '. water. .

The first step of the process according to the invention consists in shaping the film by a known process. For this purpose a polymer solution is used which is obtained either by dissolution of the^ said polymer in an appropriate solvent or by direct polymerization in the solvent* In order to produce the solution* in the case of polymers having an acylonitrile basis or segmented polyurethanes, a water mlsoible solvent is used,, for example dimethylformamide or dimethyl-* sulphoxide. When polyvinyl alcohol is used, this polymer is dissolved in hot water, preferably at 106?C.

The film may be obtained by casting the solution on a smooth surface, fo example a glass plate, or by extrusion or casting the said solution on the final support such as the material which is intended to constitute the body of the leather substitute. The resulting film or which the thickness may vary as required, is then plunged into the coagulating bath* if desired while still on its support. On the other hand, it is possible, if this should be desirable,, to extrude the solution directly into the coagulating bath with or . without support.

In general the coagulating bath contains water and a solvent for the polymert for the polymers with an acylonitrile basis or poly-urethanes, it is possible to use* for example, a mixture of water and dimeth l ormamide or water and dimethylsulphoxide. The composition and the temperature of the. coagulating bath may toary within wide limits dependin on the, required microstructure for the final membrane.

For example, the temperature of the coagulating bath may be as high as 80°C or even higher, or it may go down to or below 0°C, for example to -10 C or even less. However, it is generally advantageous for economic reasons to maintain the bath around room temperature, .

The coagulating bath may, however, consist entirely o water, which,may even be maintained at a j&irly high temperature,, but In that case it is somethlmes preferable to precede the coagulatio b a thermal treatment to eliminate a portion of the solvent so as to avoid the formation of large pores during the coagulation.

In the case of polyvinyl alcohol, the hot solution is coagulated in an aqueous saturated salt solution of a water soluble salt of alkali or alkaline earth metals, preferably an aqueous sodium sulphate solution^ If desired it is possible, moreover, to use a coagulating bath containing a mixture of a solvent and a , . , SC 3148 non-aqueous non-solvent for the extruded polymer ■. ■ ' . · providing that the solvent can he displaced wa^fcfe. water at the time at which the film is washed. However,, it is generally preferable for economic reasons to use water as the non-solvent in the coagulating hath.

It is likewise possible to. cause the coagulation by means' of a chemical reaction, such as for example by decomposition of a cellulose xanthogenate.

As mentioned above, the coagulation conditions, especially the temperature and composition of the bath, ' strongly influence the microstructure of the coagulated film, so that it is possible to obtain a film having the desired microstructure by varying these factors. · · The duration of the coagulation is regulated in · such a way that the film, on leaving the coagulating bath, may be easily manipulated, it being possible to effect coagulation either continuously or discontinuously. If a uniform porosity is desired, it is preferable, however, to. allow the film to remain in the ' coagulating bath for a sufficient time to ensure that coagulation is complete throughout the thickness of the film.

The film is then washed with water, which should preferably be demineralized, until the film is substantially constituted only by the polymer swelled with water; the water must be sufficiently cold to avoid dissolving a polymer which is soluble in hot water, e.g. polyvinyl alcohol. The washing may be effected, for example, on rollers or in a vat. It is very important that at the end of this operation the solvent should be . displaced with water either entirely or at least to the SC 3148 together of the walls of the pores and would thus destroy the structure of the material.

It, is' generally advantageous for' economic reasons to effect continuousl . some or even- all the ahove mentioned successive operation's, including recovery of the solvent from the coagulating bath..

... After washing, drying, of the film must be avoided; it; is' thus preferable to keep it in water until the following operation.

The film is congealed by very, rapid cooling to a temperature equal ..to or /below -10°C, preferably comprised within the range of -15°C to -70°C for economic reasons, but if -desired the temperature ma ■ be very much lower.

The' cooling may be effected by passing the film over a cold body, through a cooling. chamber or by means of any other appropriate apparatus. It is of prime importance that the cooling should be rapid, that is to say that the entire material constituting the' film should reach this low temperature almost instantaneously. :.··'_· The water contained in this film is suddenly congealed for practical purposes to form a very large number of microcrystals which,jneither deteriorate nor modify the structure of the material.

On the other hand, if the cooling is operated too slowly, the crystals would grow and explode' the. material thus destroying the microstructure of the coagulated film. · This rapid cooling seems to fix the micro-structure of the film in the state in which it was 'after coagulation and washing which is in contradistinction to the known drying' processes, which evolve the. microstructure.

SG 314-8 in- .that -case also brusque crystallization of the:- water contained in the .film bursts its structure. For this ■ * ', reason, if the films impregnated' with water are plunged into liquid nitrogen in order to cool them, generally films result which it is no longer possible :to manipulate and which . · disintegrate into dust.. " - ' , The cooled film is- then dried by "desiccation in a vacuum while heating it slowly and progressively at the .same time which causes sublimation of the microcrystals of ice ..contained in the film; a water trap may be placed in the vacuum circuit whi:ch allows the obtaining of a high - vacuum, for example of the order of 10 mm of mercury.

The duration of the heating depends primarily on the thickness and the microstructure of the film, that is to / ' say the number of microcrystals to be sublimed, and may'1 .· - . vary within wide limits ;- it may ^easily be determined by a technician using simple test methods. ' - • Fo ''the last two- operations, i.e. the freezing and ', drying,it is possible to' use a cryodesiccation apparatus capable of ensuring successively .· far reaching- congealing . and desiccation in a .vacuum of the treated film in a- single apparatus. It is likewise possible to effect these operations, (sudden cooling and drying- by sublimation) in two separate apparatuses, the first of these two1 : operations being much more' rapid: than the' second one and thus being possibly effected continuously with the ■ - previous·'operations .

. After the vacuum desiccation treatment, there is : obtained a perfectly dry film which is free of solvent,, SC 3148 and a remarkable suppleness. Membranes obtained, in . this way may be stored in the dry state, cut and manipulated without difficulty.

In certain cases it might be advantageous to replace the water of impregnation, during the course * of the coagulation or during subsequent washing, by a liquid having a sufficient vapour tension to allow its sublimation in a vacuum under good conditions'. However-, for economic reasons it is preferable to use water for this purpose. ' ' " The microporous membranes obtained according to. the invention ma be used in filtration, and dialy:sis techniques. They may find applications especially in the medical field and as separation membranes for batteries or accumulators, fuel cell separators, etc; the invention thus includes such batteries, accumulators and fuel cells with' said membranes as separators.

The membranes obtained by. the process accordin to the inv ith advantage as the skin of be applied to the body of cesses well known to the man skilled in the art, or may be cast directly . - ■ ias a unit* on, the body and treatedywith it by cooling and desiccation according to the process described above. Such synthetic leather is also included in the present invention..

SC 3ϊ¾θ £exia-a£~ microcrystals .

The cooling chamber is immediately connected to a vacuum- circuit, the pressure reached being of the order of 10-4 mm of mercury, and the film- is progress- ively reheated. The temperature is increased. in the. followin way: . ■' .. , ' - in 11/ hours from -62° to -50°C • - in 14 hours from' -50° to -5°G ■ :. . - in 7 hours from -5° to +20°C. - The film withdrawn from the apparatus is. ; perfectly dry and free ' of residual solvent..

C Starting from a polyacrylonitrile o"f^apparent density of 1.17,· the resulting film has an apparent .density of 0.25; it has a specific surface of 216 m 2/g,. - measured by the method BET.

■ Example 2 ' The following materials are charged into. a flask provided with a fractionating column and in which •it is possible to work under a vacuum : " . . .· - purified' adipic acid 1460 g (10 moles) - ethylene glycol 6 g ( 8 moles) - 1 ,2-propylene glycol ·.. · 304 g ( 4 moles). · The mixture is heated durin four hours in an atmosphere of nitrogen. . A vacuum is then set up~. progressively in the flask increasing in such a way that the distillatio of water is maintained. After ten hours ripening at a pressure of 10 mm of mercury and a temperature of 200°C,' a colourless and viscous polyester, of molecular weight 1730 is obtained.- r t a flas m characteristics: 2 -weight per m 212 g -thickness 0*34 urn --water vapour permeability 47.8 em2hou .

On a circular knitting machine a spun yam of titre 150 deniers (167 deoitex) cpnotituted by a nlxtro of fibres 40 mm in length produced starting from the following ie knitted to form an even interlock mcsht -7(¾ of polyhoxamethylene adipamide fibres of 1.5 denier (1.7 deoitex) having a shrinkage in boiling wate below * ■ ~30p of chloroflbrea obtainable eoinmerclally under the name of "Fibravyl", Of 2.4 denier (2.7 deoitex) and having a shrinkage in boiling water of about 50$. (The tens "chlorofibres" is used to designate tho synthetic textile threads and fibres which aro formed fros linear oacrooolecules baaed on or having a predominance in weight o ohlorido taonojaGrs).(-3y chloride aonomore it ie undox1- stood, ening of a double bond and containing one or more cfc^tor^ otoa^CiewecsnnrplG^ vinyl or vinylidene chlorides, which are the inost^uaed)· 2 T e resulting raw tissue weighs 220 g/a and has a thickness of about ■ 2 1.3 mm. After treatment with boiling water and drying, it weighs 440 g/m and has an average thiokness of 1.6 mm.

By scraping in a wire card raising machine and then shearin on a shearing machine there io obtained a tight knitted article with straight and short pile 2 weighing 423 g/a and a thickness of 1*9 cm.

This knitted material ie then impregnated with a solution in dimethyl-formamlde of segmented polyurethano produced as in Example 2 but diluted to a 1¾5> polymer content.

The resulting knitted material is then impregnated, without intermediate drying, with a SC 3148 fresh, solution of segmented polyurethane produced as in Example 2 but having, a concentration of 25 .

The resulting composite article is then placed in. a . ventilated oven for 1 minute at 170°C and then plunged into a water bath at 80°C where it is allowed to ; remain for 10 minutes.

• : It is then carefully rinsed for 16 hours in water.

After keeping it in water for 48 hours, the product is submitted to a rapid cooling followed by-slow reheating in a high vacuum under the same conditions as an Example 2. ■appeara ce/ 'A leather substitute product of -good--erspe^^resuit having the following characteristics : . . - .weight per m 780. g - total thickness ' 2.2 mm - thickness of the skin 0,30 mm 2 - permeability to ' water vapour .6 g/m /hour.

Exam le ^ A water impregnated film obtained according to Example 1 is introduced into a cooling chamber kept at -15°C. The water content in this film becomes, transformed into} immediately <®½-^€t^d--½¾r"^½e--^f½»jiff^.fi microcrystals.

The cooling chamber is then connected to a .vacuum line . to obtain a pressure of 2 mm of mercury.

The temperature is maintained for 2 hours at -15°C. and . then for 1 hour at 0°C and finally for 2 hours at +20°C under that pressure.

The resulting dry film is very supple and has .a , '.. ■ ■ . , SC 318 - water vapour permeability of 117 g/m /hour.

Example 6 A film is cast as in Example 1 but startin with a copolymer constituted by 9 $ of acrylonitrile units and 6$ of methyl methacrylate units instead of a acrylonitrile homopolymer.

The well washed film impregnated with water is introduced into a cooling chamber at -40°C in which the water is immediately congealed in the form of micro-crystals* The chamber is then connected to a vacuum line allowing the achievement of a pressure of 25 x 10 mm . of. mercury. The temperature is maintained for 1 hour at - 0°C, for 21/2. hours, at -25°C,- for 1 hour at -10°C and finally for 2 hours at +20°C under that pressure.

The resulting dry film is very supple and has a water vapour permeability of 104 g/m /hour.

Example' 7 A water impregnated film obtained according to Example 1 is rapidly cooled at -79°C and then introduced into a chamber of . which the pressure is regulated x 10 mm of mercury by means of a vacuum .line and the temperature is maintained for 1 hour at -40°C, for 2" hours at -25°C, for l1/2 hours at -10°C and finally for l1/2 hours, at +20°C. ' ', ' A supple film is obtained having a water vapour permeability, of 117 g/m /hour which is capable ÷of being used for ga's filtration, .

Example 8 SC 31 8 "but starting from' a copolymer" constituted by 9 % of acrylonitrile units and > of methyl methacrylate units is rapidly cooled to -114°0. It is then introduced into a chamber of which the pressure is regulated 25 x 10""^ mm of mercury and of which the temperature is regulated to : -40°C for 1 hour -25°C " 21/2 hours -10°C " l1/2 hours +20°C *» 2 hours.

The resultin supple film has a water vapour -permeability of 104 g/m 2/hour.

Example 9 A water impregnated film obtained according to Example.1 is rapidly cooled to ~134°C and is then reheated, to -40°C and dried^in a vacuum under the same conditions as in Example 8. , The thus treated film has a permeability to' water vapour of 101 g/m /hour.

Example 10 :.' Into a "Pyrex" glass flask of 2 litres capacity there are charged 318 g of toluene and 354- g of a polyester of molecular weight 1770 , hydroxyl number 62.4, acid number 0.8 and melting at 32-33°C, obtained by the action of adipic acid on a mixture of 1 , 2-ethanediol and 1 , 2-propanediol, the molecular ratio of the two dialcohols being 80/20 respectively. 150 g of toluene are distilled in order to SC 314-8 - thickness < 0*21 mm water vapour permeability 31.2 g/m /hour. ..

For comparison purposes the same solution is cast also on to a glass' plate to, form a 1 mm thick film.

*'·.' The plate and film thereon are then exposed in . a medium having 62?S relative' humidity at a temperature . of 23°Q. ■ " ·' : ; , , ' ■ · ■ : ' After exposure for 3 hours the solvent has . completely exuded "from the film. The film is then separated- from the glass plate -and rinsed in a water stream at 50°C for 5 hours.

It is finally dried at 70°C and a translucent film is obtained of which the water vapour permeability y is nil under the conditions of measurement described 1 above .

It is particularly interesting to. find that the process according to the invention enables the . production of microporous membranes having a satisfactory water vapour permeability with polymers which lead to. articles having no permeability when treated according to processes normally used for the manufacture of the e.i-Sir'^fl.eather substitutes.-

Claims (1)

1. A process for the production of a mlcroporous film, membrane or .similar sheet, of a high polymer, which process comprises successivel the steps of shaping a thin film from a solution of a high polymer which is insoluhle in cold nater, coagulating the shaped film in a coagulating hath, washing the coagulated film with water until complete elimination of the solvent, freezing - without intermediate drying - the water impregnated lm, by rapid cooling to a temperature equal to or below -10°C, and drying the resulting material by slow and , progressive heating in a high vacuunu ' · · ■· ' ' ' 2· A process according to Claim 1, in which the high poly- I... .. . ; · .·. mer is an aorylonitrile polymer. 3. A process according to Claim 1, In which the high polymer is. a segmented polyurethanei 4. A process according, to Claim 1, in which the high polyme is a polyvinyl alcohol polymer. 5. A process according to any of the preceding Claims, in which a mixture of two ormoreVpolymers is used for maldng the film, membrane or. similar sheet. 6; A process according to claim 2 or 3, in which the solvent for the.polymer solution is dimethylformamide or diethylsulphoxide. ■ SO 314-8 7· A process according to Claim 4, in which the solvent for the polymer solution is hot water. 8· A process according to Claim 6, in which the coagulating hath is a mixture of water-dimethylformamide or water-dimethylsulphoxide. 9· A process according to Claim 2 or 3» in which the coagulating bath consists of water. Id. . A process according to Claim 9» in which the coagulation is preceded by a thermal treatment to .. eliminate a portion of the solvent so as. to avoid the formation of ; large pores during the coagulation. 11.> A process according to Claim 4, in which the coagulating bath is an aqueous .saturated salt solution.. 12. A- process according to Claim 11, in which . the salt, solution is a 'saturated aqueous . sodium sulphate solution. 13. A process according to any one of the preceding •Claims, in which the washing of the coagulated film is effected with demineralized. water. 14. : A process accordin to any one of the preceding. Claims,., in which the freezing of the water impregnated film and the drying are effected in a cryodesiccation apparatus. V '■ < ■· , . . SC 314-8 15,. Modification of the process claimed, in any one of the preceding Claims, in which the. washing water is replaced with another liquid having a sufficiently high vapour tension to. allow its sublimation in a vacuum under good conditions, the freezing and drying being effected with the film impregnated with said other liquid. 16. A process according to Claim 1 substantially as herein described with reference to any one of the Examples; 17. A microporous film, membrane or similar", sheet, whenever produced by the process claimed in any one of the preceding Claims. 18. Synthetic leather having a skin of a microporous film, membrane or similar sheet as claimed in'the preceding Claim. 19. Batteries j accumulators and fuel cells provided with separators made of a microporous film, membrane or similar sheet as claimed in Claim 18.