Method for producing porous polymeric membranes and corresponding apparatus

Abstract

The present invention concerns a method for producing porous polymeric membranes (M), comprising a step of solubilizing at least one polymer in a solvent to obtain a polymeric solution; a step of applying the polymeric solution in the form of a polymeric film (16, 116) on a substrate (15, 115); and a step of putting the polymeric film (16, 116) in contact with at least one antisolvent. The invention also concerns an apparatus configured to implement this method.

Description

[0001] “METHOD FOR PRODUCING POROUS POLYMERIC MEMBRANES AND CORRESPONDING APPARATUS”

[0002] FIELD OF THE INVENTION

[0003] The present invention concerns a method for producing porous polymeric membranes. In particular, the present invention concerns a method for producing porous polymeric membranes through phase inversion.

[0004] BACKGROUND OF THE INVENTION

[0005] Methods have long been known for producing porous polymeric membranes, which can be used in various fields such as, for example, in the biomedical sector, in the filtering membranes sector or in the sector of separators for storage systems, such as batteries for example.

[0006] Among the various known methods, those providing a so-called phase inversion are particularly advantageous. Phase inversion provides to deposit, on a substrate, a polymeric film constituting the base of the membrane, and to put it in contact with an antisolvent substance, which leads to the solidification of the film and the creation of pores. In known methods, the step of putting the polymeric film in contact with the antisolvent occurs by immersing the film in a coagulation bath, which contains a solution that represents the antisolvent.

[0007] Although these types of methods are advantageous for making porous polymeric membranes, they are not without disadvantages.

[0008] For example, one disadvantage of known methods is that, due to the use of the coagulation bath, they require large amounts of antisolvent.

[0009] Another disadvantage of known methods is that the reagents involved are dissolved in organic solvents, which leads to sustainability problems with both the method and the membrane obtained.

[0010] Yet another disadvantage of known methods is that they are not easily integratable in industrial production plants, for example in roll-to-roll type lines.

[0011] There is therefore the need to perfect a method for producing porous polymeric membranes that can overcome at least one of the disadvantages of the state of the art.

[0012] To do this, it is necessary to solve the technical problem of making both the method as well as the membrane thus obtained eco-sustainable, less costly in terms of solvents or antisolvent, and easy to integrate into an industrial production line.

[0013] In particular, one purpose of the present invention is to perfect a method for producing porous polymeric membranes that require fewer chemical species, in particular antisolvent, than known methods.

[0014] Another purpose of the present invention is to perfect a method for producing porous polymeric membranes easily integratable into an industrial membrane production line.

[0015] The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.

[0016] SUMMARY OF THE INVENTION

[0017] The present invention is set forth and characterized in the independent claim. The dependent claims describe other characteristics of the present invention or variants to the main inventive idea.

[0018] In accordance with the above purposes and to solve the technical problem described above in a new and original way, also achieving considerable advantages compared to the state of the prior art, a method according to the present invention for producing porous polymeric membranes comprises a step of solubilizing at least one polymer in a solvent; a step of applying the polymeric solution thus obtained in the form of a polymeric film on a substrate, and a step of putting in contact with at least one antisolvent.

[0019] In accordance with one aspect of the present invention, the step of putting in contact with the at least one antisolvent occurs by nebulizing, or spraying, the at least one antisolvent on the polymeric film. Moreover, the solvent in which the at least one polymer is solubilized is an aqueous-based solution, and the antisolvent is an aqueous-based solution.

[0020] Doing so achieves at least the advantage of having an environmentally sustainable method, since the organic solvents are replaced with aqueous solvents, or even water. The use of antisolvent spraying allows to significantly reduce the amount of this reagent, in particular compared to solutions that provide a coagulation bath.

[0021] It should also be noted that the method according to the present invention allows to have greater stability for the substrate, which is no longer immersed in a solution. This allows to obtain porous membranes with different and more easily controllable properties, at least in terms of pores.

[0022] Furthermore, such a method is more easily integratable into an industrial production line, in particular of the roll-to-roll type. It can in fact be provided that the substrate is fed along a feed path continuously; that the application of the polymeric solution in the form of a polymeric film occurs in correspondence with a film application station located along the feed path; that the spraying of the at least one antisolvent occurs in correspondence with a spraying station located along the feed path and downstream of the film application station; that the polymeric membrane thus obtained is separated from the substrate in correspondence with a separation station located along the feed path and downstream of the spraying station.

[0023] In accordance with another aspect of the present invention, the spraying is performed through a spraying member, for example a nozzle, and it is provided to adjust the porosity of the membrane by modifying the distance between the spraying member and the substrate. This modification occurs preferably before the spraying step begins, on the basis of already obtained results.

[0024] According to another aspect of the invention, the distance between the spraying member and the substrate is at least 5 cm, preferably at least 10 cm. It has been discovered that, with the same dispensing speed, some of the membrane’s properties can be modified by modifying the distance between the spraying member and the substrate, as will be explained in detail below.

[0025] In accordance with another aspect of the present invention, the at least one polymer is selected from polymers soluble in water or in any case in aqueous solution, at a temperature higher than 40 °C, preferably higher than 50 °C, more preferably between approximately 60 and 80 °C.

[0026] According to another aspect of the invention, the at least one polymer is selected from polysaccharides, collagen, gelatin, albumin, elastin, polyolefins, polyethers, polyamides, polyesters, derivatives and mixtures thereof.

[0027] Advantageously, the at least one polymer comprises a first polymer selected from polysaccharides, collagen, gelatin, albumin, elastin, polyethers and their derivatives, and a second polymer selected from polyolefins, polyamides, polyesters and their derivatives. More advantageously, the second polymer is selected from polyethylene vinyl alcohol EV OH, Nylon (in particular Nylon 6,6), polylactic acid and their derivatives.

[0028] According to another aspect of the present invention, the aqueous-based solution, in which the at least one polymer is provided, comprises a naturally occurring acid, such as for example acetic acid, lactic acid or formic acid.

[0029] According to another aspect of the present invention, the at least one antisolvent in an aqueous solution.

[0030] In accordance with another aspect of the present invention, the polymeric solution, if necessary, also comprises at least one plasticizer.

[0031] In accordance with the above purposes and to solve the technical problem described above in a new and original way, also achieving considerable advantages compared to the state of the prior art, an apparatus according to the present invention for producing porous polymeric membranes comprises a feed path along which to continuously feed a substrate; a film application station placed along the feed path and in which a polymeric film, comprising at least one polymer, is applied on the substrate; a spraying station located along the feed path downstream of the film application station, in which at least one antisolvent is sprayed on the polymeric film to obtain a polymeric membrane; and a separation station, located along the feed path downstream of the spraying station, in which the polymeric membrane obtained is separated from the substrate.

[0032] Preferably, the apparatus is or comprises a roll-to-roll type line.

[0033] DESCRIPTION OF THE DRAWINGS

[0034] These and other aspects, characteristics and advantages of the present invention will become apparent from the following description of an embodiment, given as a non-restrictive example with reference to the attached drawings wherein:

[0035] - fig. 1 is a schematic view of a spraying station for implementing a method for producing porous polymeric membranes according to the present invention;

[0036] - fig. 2 is a schematic view of a line for producing polymeric membranes;

[0037] - figs. 3 and 4 are micrographs obtained using scanning electron micrography (SEM) of a sample obtained through a method of the prior art and of samples obtained through the method according to the present invention, at two different magnifications.

[0038] We must clarify that the phraseology and terminology used in the present description, as well as the figures in the attached drawings also in relation as to how described, have the sole function of better illustrating and explaining the present invention, their purpose being to provide a non-limiting example of the invention itself, since the scope of protection is defined by the claims.

[0039] To facilitate comprehension, the same reference numbers have been used, where possible, to identify identical common elements in the drawings. It is understood that elements and characteristics of one embodiment can be conveniently combined or incorporated into other embodiments without further clarifications.

[0040] DESCRIPTION OF AN EMBODIMENT OF THE PRESENT INVENTION

[0041] Unless otherwise defined, all the technical and scientific terms used here and hereafter have the same meaning as commonly understood by a person with ordinary experience in the field of the art to which the present invention belongs. Even if methods and materials similar or equivalent to those described here can be used in practice and in the trials of the present invention, the methods and materials are described hereafter as an example. In the event of conflict, the present application shall prevail, including its definitions. The materials, methods and examples have a purely illustrative purpose and shall not be understood restrictively.

[0042] All measurements are carried out at 25 °C (ambient temperature) and at atmospheric pressure, unless otherwise indicated. All temperatures are in degrees Celsius, unless otherwise indicated.

[0043] All percentages and ratios indicated shall be understood to refer to the weight of the total composition (w / w), unless otherwise indicated.

[0044] All percentage ranges indicated here are given with the provision that the sum with respect the overall composition is 100%, unless otherwise indicated.

[0045] All the intervals reported here shall be understood to include the extremes, including those that report an interval “between” two values, unless otherwise indicated.

[0046] The present description also includes the intervals that derive from uniting or overlapping two or more intervals described, unless otherwise indicated.

[0047] The present description also includes the intervals that can derive from the combination of two or more values taken at different points, unless otherwise indicated. Where mentioned, water is understood as distilled water, unless otherwise specified.

[0048] With reference to fig. 1, a spraying station 10 for implementing the method according to the present invention comprises a spraying device 11 equipped with a spraying member 12 comprising an outlet aperture 12 A, for example a nozzle. The spraying device 11 is provided to spray an antisolvent solution. For this purpose, it is fluidically connected to a source 13 of antisolvent solution. The station also comprises a support base 14 on which a substrate 15 can be disposed.

[0049] The substrate 15 can be a sheet of plastic material, for example a sheet of polypropylene PP, polyethylene terephthalate PET, or polytetrafluoroethylene PTFE.

[0050] A film 16 or thin layer of polymeric solution can be applied on the substrate 15, obtained by mixing at least one polymer in an acidic aqueous solution, the polymer being selected from polysaccharides, collagen, gelatin, albumin, elastin, polyolefins, polyethers, derivatives and mixtures thereof. The polymer is preferably soluble in aqueous solutions at high temperatures, that is, greater than 40 °C, but not at ambient temperature, that is, 20-25 °C. The most suitable polymers are those that dissolve at a temperature comprised between 60 °C and 80 °C.

[0051] Some examples of polysaccharides particularly suitable for the formation of a porous membrane include pectins, chitin, chitosan, carrageenan, guar gum, agar, xanthan gum, cellulose derivatives, starch, gelatin, P-glucans, glycosaminoglycans, mucopolysaccharides and derivatives thereof.

[0052] Preferably, the cellulose derivatives are selected from hydroxypropyl methyl cellulose HPMC, hydroxypropyl cellulose HPC, hydroxyethyl cellulose HEC, sodium carboxymethyl cellulose Na-CMC. The glycosaminoglycans GAGs or mucopolysaccharides can be selected from chondroitin sulfate, dermatan sulfate, heparin, heparan sulfate and hyaluronic acid HA.

[0053] Among polyethers we include polyethylene dimethyl ether glycol PEGDME.

[0054] Advantageously, the natural polymer is mixed with synthetic polymers such as polyamides (e.g. Nylon 6,6), polyethylene vinyl alcohol EVOH, poly lactic acid. This makes this method usable even for natural polymers whose solubility in water occurs at ambient temperature. In addition, if necessary, at least one plasticizer is added to these polymeric solutions to improve their detachment from the support. The plasticizer is preferably soluble in aqueous solutions at high temperatures, that is, greater than 40 °C, but not at ambient temperature, that is, 20-25 °C. Some examples of usable plasticizers are dicarboxylic acids or long chain alcohols such as azelaic acid, sebacic acid, dodecanedioic acid, enanthic alcohol, caprylic alcohol and suchlike.

[0055] The acidic aqueous solution is water in which an acid has been dissolved at a specific concentration, so as to obtain a predefined pH. An example of an acid to be used is acetic acid; other commonly used acids are however possible, such as formic acid and lactic acid.

[0056] The sizes of the outlet aperture 12A are such that the antisolvent, provided in the form of a liquid solution, is dispensed in the form of droplets G which are intended to come into contact with the polymeric film 16.

[0057] It has been surprisingly discovered that by modifying the distance D between the outlet aperture 12A of the spraying member 12 and the polymeric film 16, or the substrate 15, it is possible to modify the porosity of the porous polymeric membranes. The distance D corresponds to the spraying distance, that is, the distance travelled by the droplets G of antisolvent before coming into contact with the polymeric film 16.

[0058] With reference to fig. 2, an apparatus 100 for making porous polymeric membranes M comprises a spraying station 110 for spraying an antisolvent onto a polymeric film 116.

[0059] The apparatus 100 is of an industrial type, in particular roll-to-roll, and comprises a long feed path A along which a substrate 115 is fed, in a continuous manner, in the form of a continuous film. The feed path A is defined, in a known manner, by a plurality of rollers 101, possibly motorized. The continuous film substrate 115 can be unwound by a first reel 115 A.

[0060] In this context, the spraying station 110 comprises a spraying device 111 equipped with a spraying member 112 which comprises an outlet aperture 112A for the droplets G of antisolvent. The outlet aperture 112A is favorably linear and disposed transversely, in particular perpendicularly, to the feed path A, so as to affect the polymeric film 116 as much as possible. The spraying device 111 is fluidically connected to a source 113 of antisolvent solution. Along the feed path A there are provided a film application station 102, upstream of the spraying station 110, and a separation station 103, downstream of the spraying station 110.

[0061] The application of the polymeric film 116 onto the substrate 115 occurs in the film application station 102. For this purpose, the film application station 102 comprises a film application device 104 equipped with a linear dispensing mouth 104A disposed transversely, in particular perpendicularly, to the feed path A. The film application device 104 is fluidically connected to a source 105 of polymeric solution at the base of the polymeric film 116.

[0062] The separation of the porous polymeric membrane M from the substrate 115 occurs in correspondence with the separation station 103. The separation station 103 consists of a pair of opposing and reciprocally contacting rollers 106, between which the substrate 115 and porous polymeric membrane M assembly passes, and of two winding rollers 107, 108 downstream of the pair of rollers 106, and reciprocally spaced apart from each other and with respect to the extension of the feed path A beyond the pair of rollers 106.

[0063] The substrate 115 is wound on a first winding roller 107 while the membrane M is wound on the second winding roller 108. The winding rollers 107, 108 are preferably motorized.

[0064] The spraying station 10 as per fig. 1 and the apparatus 100 as per fig. 2 are both configured to perform the method according to the present invention, which substantially provides a phase inversion between a polymeric solution and an antisolvent, but wherein the antisolvent is sprayed onto a film created with the polymeric solution.

[0065] More precisely, the method provides a step of solubilizing at least one polymer in a solvent, so as to obtain a polymeric solution; a step of applying the polymeric solution in the form of a polymeric film onto a substrate; and a step of putting the polymeric film in contact with at least one antisolvent, by spraying the at least one antisolvent, which is in the form of a liquid solution.

[0066] The polymers, the polymeric solution and the at least one antisolvent are as per the description above.

[0067] The application step can provide to apply, on the substrate, a layer of polymeric solution having a thickness between 100 and 200 pm. The thickness can vary depending on requirements and the conditions of the dispensing of the polymeric solution.

[0068] During the spraying of the at least one antisolvent, it is possible to adjust the spraying distance D so as to modulate the porosity of the membrane M that is obtained. According to preferred variants, the spraying distance D is at least 5 cm, more preferably at least 10 cm. The distance D can be increased so as to be equal to 15 cm, 20 cm, 25 cm, 30 cm, 35 cm, 40 cm, 45 cm, 50 cm, 55 cm, 60 cm, 65 cm, 70 cm, 75 cm, 80 cm, 85 cm, 90 cm, 95 cm, 100 cm or even more, depending on requirements. Obviously, the spraying distance D can be adjusted to any value comprised between all the values listed above. The spraying of the antisolvent can occur with a flow comprised between 10 and 150 mL min’1, preferably between 30 and 100 mL min1.

[0069] A practical implementation of the method is outlined in the following examples. EXAMPLE 1

[0070] A polymeric solution was prepared by dissolving chitosan and polyethylene vinyl alcohol in a 1 :9 weight ratio, in a solvent consisting of water and acetic acid in a 1 :1 weight ratio.

[0071] Specifically, in a closed container, about 20 mg of chitosan is solubilized in 2 ml of solvent at ambient temperature for 4 hours under magnetic stirring. Upon complete dissolution of the chitosan, 180 mg of EVOH was introduced into the same container and the solution was left stirring at 70 °C for a period of time comprised between 2 and 6 hours, depending on the granulometry of the EVOH.

[0072] The solution was then deposited onto a substrate, also polymeric, using an MC20 mini-coater.

[0073] More precisely, a PP substrate was prepared (cut into sheets of 10x25 cm) and fixed with magnets on the mini-coater, turning on the vacuum to make it adhere to the surface of the instrument. The bar of the mini-coater was set at a height of 8 mils (203 pm). When the EVOH was completely dissolved, the solution was removed from the bath at 70 °C and deposited on the PP support. It is possible to start the deposit by making the bar advance to the end of the device at a deposit rate of 0.3 cm s’1.

[0074] Upon completion of the deposit, the substrate with the solution deposited thereon was positioned under a professional airbrush. The height between the airbrush and the polymeric layer was adjusted, for example to 30 cm. At this point, the inversion process begins, spraying the antisolvent (distilled water) on the deposited solution for 30 seconds with a flow of between 30 and 100 ml min'1. Spraying the antisolvent causes the solidification of the polymeric film and the formation of a membrane. Finally, the wet membrane was oven dried at 80 °C for 4 hours to make the excess solvent and antisolvent mixture evaporate. The membrane obtained was then detached from the substrate.

[0075] In this example, the substrate selected to test the spray-induced phase inversion is a polypropylene sheet, since it was the substrate that allowed the easiest detachment of the membrane after drying. The samples obtained in this way will be indicated later with the abbreviation PP_S. Different samples were obtained by varying the distance between the airbrush’s nozzle and the substrate, to verify the effects of this parameter on membrane porosity. The tested distances are 10, 30 and 90 cm.

[0076] Membranes were also obtained with a traditional phase inversion method (namely, with immersion of the polymeric film in an antisolvent bath), using a glass plate as a substrate (hereafter, these samples will be indicated with GLASS B) to make a comparison of the properties of the final product obtained with the two methods.

[0077] The membranes thus obtained were characterized using scanning electron microscopy (SEM) technique and the images obtained at 250x and 5000x magnification are shown in figs. 3 and 4, respectively. The Up and Down lines refer to the upper and lower surfaces, respectively, of the membrane obtained, which have a different morphology, since the lower surface is in contact with the substrate while the upper one is exposed to air, and therefore to the droplets of antisolvent solution.

[0078] The images allow to verify that the porous membranes obtained through the method of the present invention have a porosity that is much different from the membrane obtained with the method of the state of the art, on both the upper and lower surfaces. In particular, the porosity of the reference sample on the upper surface is lower than the porosity of the membranes obtained through the present method, while on the lower surface it is higher. That is, the lower surfaces of the three samples of the example are smoother than that of the reference sample, and also smoother than their respective upper surfaces.

[0079] Furthermore, the images show that the spraying distance has a visible and sharp effect on the porosity of the membrane obtained, in particular on its upper surface. The number and average size of the pores increase as the spraying distance increases.

[0080] Also note the clear difference in porosity between the upper and lower surfaces of each sample. Such a difference in porosity can be interesting in terms of making membranes for batteries, for example in the event one battery has one electrode more reactive than the other.

[0081] EXAMPLE 2

[0082] A polymeric solution was prepared by dissolving polyethylene dimethyl ether glycol and polyethylene vinyl alcohol in a 2:8 weight ratio, in a solvent consisting of water and isopropanol in a 3:7 weight ratio.

[0083] Specifically, about 60 mg of PEGDME was mixed in 2 ml of solvent (HsOnsopropanol 3:7) at ambient temperature for 15 minutes under magnetic stirring. Once the PEGDME was completely dissolved, 240 mg of EVOH was introduced into the same container. To solubilize the EVOH, the solution was allowed to stir at 70 °C for 2 to 6 hours, depending on the granulometry of the EVOH.

[0084] Deposit on the substrate and formation of the membrane were performed as described for example 1.

[0085] The membrane obtained has improved mechanical properties, while maintaining the advantageous properties linked to spraying of the antisolvent.

[0086] It is clear that modifications and / or additions of parts may be made to the method and to the apparatus 100 as described heretofore, without thereby departing from the field and scope of the present invention, as defined by the claims.

[0087] It is also clear that, although the present invention has been described with reference to some specific examples, a person of skill in the art will be able to achieve other equivalent forms of method for producing porous polymeric membranes and corresponding apparatus, having the characteristics as set forth in the claims and hence all coming within the field of protection defined thereby.

[0088] In the following claims, the sole purpose of the references in brackets is to facilitate their reading and they must not be considered as restrictive factors with regard to the field of protection defined by the claims.

Claims

CLAIMS1. Method for producing porous polymeric membranes (M), comprising a step of solubilizing at least one polymer in a solvent to obtain a polymeric solution; a step of applying said polymeric solution in the form of a polymeric film (16, 116) on a substrate (15, 115); and a step of putting said polymeric film (16, 116) in contact with at least one antisolvent, characterized in that said step of putting in contact with at least one antisolvent occurs by spraying said at least one antisolvent on said polymeric film (16, 116), and in that said solvent is an aqueous-based solution and said at least one antisolvent is an aqueous-based solution.

2. Method as in claim 1, characterized in that said spraying step is performed by means of a spraying member (12), and in that it provides to adjust the porosity of said membrane (M) by modifying the distance (D) between said spraying member (12) and said substrate (15).

3. Method as in claim 1 or 2, characterized in that said spraying step is performed by means of a spraying member (12), and in that the distance (D) between said spraying member (12) and said substrate (15) is at least 5 cm.

4. Method as in any claim hereinbefore, characterized in that said at least one polymer is selected from polysaccharides, collagen, gelatin, albumin, elastin, polyolefins, polyethers, derivatives and mixtures thereof.

5. Method as in claim 4, characterized in that said at least one polymer comprises a first polymer selected from polysaccharides, collagen, gelatin, albumin, elastin, polyethers and their derivatives, and a second polymer selected from polyolefins, polyamides, polyesters and their derivatives.

6. Method as in claim 5, characterized in that said first polymer is selected from chitosan and polyethylene glycol dimethyl ether and their derivatives, and said second polymer is selected from polyethylene vinyl alcohol EVOH, Nylon, polylactic acid and their derivatives.

7. Method as in any claim hereinbefore, characterized in that said aqueous solution of said at least one polymer contains an acid.

8. Method as in any claim hereinbefore, characterized in that said polymeric solution also comprises at least one plasticizer.

9. Apparatus (100) for producing porous polymeric membranes (M), comprising a feed path (A) along which to continuously feed a substrate (115); a film applicationstation (102) placed along said feed path (A) and in which a polymeric film (116), comprising at least one polymer, is applied on said substrate (115); characterized in that it also comprises a spraying station (110), located along said feed path (A) downstream of said film application station (102), and in which at least one antisolvent is sprayed on said polymeric film (116) to obtain a polymeric membrane (M); and a separation station (104), located along said feed path (A) downstream of said spraying station (110), and in which said polymeric membrane (M) obtained is separated from said substrate (115).

10. Apparatus (100) as in claim 9, characterized in that it comprises a roll-to- roll line.

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