Patent Application
20150231575
This invention relates to a method of treating a substrate, comprising providing a treatment space between at least two opposing electrodes, and filling the treatment space with a gas composition.
Ion exchange membranes have wide applications including energy storage and conversion, electro-dialysis, separation of ionic species in environmental protection and clean production, and nano- or ultra-filtration. Most of ion exchange membranes are composed of functional polymers, in which the charged groups are chemically bonded to the polymer chains.
The charged groups are —SO3−, —COO−, —PO32−-, —PO3H−, and —C6H4O−, etc. for cat-ion exchange membranes and —NH3+, —NRH2+, —NR3+, —PR3+, and —SR2+, etc. for anion exchange membranes. Cat-ion exchange membranes contain negatively charged groups attached to the membrane backbone and allow the passage of cations but reject anions, while anion exchange membranes containing positively charged groups allow the passage of anions but reject cations.
In order to yield long lasting mechanical stable membranes it is advantageous to make the membrane having a porous stable polymer support as substrate and filling the pores with ion exchange material.
The use of hydrophobic, inert materials as the porous substrate will contribute to enhanced mechanical and chemical stability as hydrophobic materials have minimal swelling in aqueous media. Unfortunately, an inert hydrophobic base membrane is not typically compatible with ion exchange material, which is hydrophilic. Incompatibility between the materials of the base membrane and the ion exchange materials can cause membrane defects, such as holes in the membrane, because there is poor interfacial interaction between the base membrane and the ion exchange materials.
In the prior art the use of plasma or corona treatments are disclosed, however, the methods described therein have the disadvantage that only the surface of the porous substrate is affected and not throughout the total porous substrate. As a result, there is a need for improved uniformly activated composite membrane materials compatible with the ion exchange material.
The present invention seeks to provide a method of treating a porous substrate in order to allow manufacturing membranes in an efficient and cost-effective manner, and which solves the problems described above.
In an embodiment, a method according to the preamble above is provided, further comprising placing the substrate, which is a porous substrate in the treatment space, generating an atmospheric pressure glow (APG) discharge plasma between the at least two opposing electrodes, subjecting the porous substrate to the atmospheric pressure glow discharge plasma, thereby creating micro-pores uniformly throughout the porous substrate. The effect of the APG treatment is that the micro-pores are uniformly hydrophilic, allowing e.g. an easy impregnation step with (hydrophilic) ion exchange material.
The atmospheric pressure glow discharge plasma in the treatment space has a specific energy of 10 J/cm2 or higher. This specific energy is advantageous because it can allow the particles in the plasma to penetrate sufficiently deep into the porous substrate.
In one embodiment, the porous substrate comprises a polymer support, e.g. of polyolefin, polyamide, polyester, polyether, acrylic or methacrylic polymers, polystyrene, polyurethane or cellulosic polymer or combinations thereof.
The method further comprises impregnating the substrate with ion exchange material. Thus, an ion exchange membrane is provided. The method may further comprise radiation curing of the porous substrate resulting in a ready to use membrane.
Heating the porous substrate to a temperature between 20 and 50° C. is provided for in a further embodiment, resulting in a more efficient treatment of the porous substrate. Heating may be provided in a direct manner, or in an indirect manner, e.g. using heating of the electrodes.
In a further embodiment, the gas composition comprises a reactive gas and an inert or noble gas. For example, preferably the gas composition comprises 0.1 to 21 vol. % oxygen. This provides sufficient reactive particles in the APG plasma to treat the porous substrate entirely without damaging the substrate. In even further embodiments, the APG treatment is further enhanced by using an inert gas in the gas composition, e.g. nitrogen. Preferably the gas composition comprises 79 to 99.9 vol. % nitrogen, especially 95 to 99.9 vol. % nitrogen.
The micro-pores preferably are between 0.1 μm and 10 μm in size (i.e. average diameter) in a further embodiment (after the APG step). The porous substrate may have a porosity between 25 and 95%. The porosity of the substrate is defined as the percentage of the volume of all the pores in the substrate with respect to the whole volume of the substrate, including the pores.
The porosity values may be obtained by performing the following equation:
porosity (%)={1−{BW/(thickness×p)}}×100%
wherein:
In successfully tested exemplary embodiments, substrates were used having a thickness of 60 μm or less, e.g. 25 μm.
In a further aspect, the present invention relates to the use of a membrane comprising a substrate obtained by one of the present invention method embodiments. Such membranes can e.g. be used in energy storage and energy conversion, electro-dialysis, separation of ionic species e.g. in environmental protection and clean production, and nano- or ultra-filtration.
The present invention will be explained in further detail hereinafter based on a number of exemplary embodiments with reference to the drawings, wherein:
a shows a schematic detail view of an electrode structure of the plasma generation apparatus according to an embodiment of the present invention;
b shows a schematic view in more detail of a further embodiment of the electrode structure;
a shows a top view SEM image of an untreated Solupor™ 10P05A substrate;
b shows a top-side SEM image of a plasma treated Solupor™ 10P05A substrate according to an embodiment of the present invention;
c shows a bottom-side SEM image of a plasma treated Solupor™ 10P05A substrate according to an embodiment of the present invention;
a shows a reference example of a cross-cut of a membrane of the present invention;
b shows a cross-cut of a membrane according to an embodiment of the present invention;
The plasma treatment apparatus 10 is arranged to provide a uniform APG plasma, as opposed to other types of plasma, which are non-uniform (e.g. a corona discharge, or other types of filamentary discharges).
In the treatment space 5, a combination of gasses is introduced from a gas supply device 8. The gas supply device 8 may be provided with storage, supply and mixing components as known to the skilled person. As shown in
Further a side tab 12 may be used to close off the treatment space 5 on one side as is shown in
The plasma treatment apparatus 10 is used according to the present invention embodiments, to treat at least one porous substrate 11 in order to make it suited for use in e.g. an ion exchange membrane, e.g. by filling the pores with ion exchange materials.
The composite membrane then includes a porous base substrate or base membrane 11, which has a plurality of pores. In one embodiment, the base substrate 11 has a three-dimensional matrix or lattice type structure with a plurality of nodes interconnected by a plurality of fibrils. The interconnections between the nodes and fibrils define the pores in the base membrane, which are open spaces or voids. The surfaces of the nodes and fibrils define numerous interconnecting pores that extend completely through the membrane.
The pore sizes in the base membrane 11 may be any size. In one embodiment, the average pore size of the pores in the base membrane is micro-porous. In another embodiment, the average pore size is in the range of about 0.01 μm to about 10 μm, and in another embodiment, the average pore size is in the range of about 0.1 μm to about 5.0 μm.
Porosity of the porous substrate 11, may be in the range from 25 to 95%. The porosity of the porous substrate 11, is defined as the percentage of the volume of all the pores in the substrate with respect to the whole volume of the substrate.
The base membrane 11 may be any material or blend of materials that is suitable for forming a base membrane with an open pore structure. In one embodiment, the base membrane material includes, but is not limited to, polyolefin, polyamide, polyester, polyether, acrylic and methacrylic polymers, polystyrene, polyurethane and cellulosic polymer and combinations thereof.
In an embodiment, the porous substrate are hydrophobic porous membranes made from hydrophobic materials like polyethylene (PE), polypropylene (PP), poly(vinylidene-difluoride) (PVDF), polytrifluorochloroethylene (PTFCE), polytetrafluoroethylene (PTFE), and polysulfone (PS).
In an exemplary embodiment, the base substrate is a microporous polyethylene film from Lydall from the Solupor™ series like for example Solupor™ 3P07A, 5P09A,7P03A, 7P07C, 10P05A, 14P01E, 16P05A and 16P10A. Solupor™ materials are produced by extrusion of a solvent together with a solution comprising a mixture of UHMW and HMW polyethylene, followed by drying the resulting tape and biaxial stretching of the tape, resulting in a micro-porous membrane. This process is described in more detail in EP 504,954 which is hereby incorporated by reference. The Solupor™ materials comprises UHMW polyethylene, and a portion of lower molecular weight polyethylene. The material is bi-axially stretched and has a porosity that can be varied between 45-95%. Depending on the needs, the porosity of the Solupor™ material can be adjusted by controlling the preparation parameters, such as the stretching factor to increase porosity and calendaring to reduce porosity. The person skilled in the art knows what techniques are available to control the porosity and he can easily determine which technique is most suitable for the situation. In addition, EP 0,504,954 describes some of those techniques. Solupor™ materials are commercially available in a range of porosities.
In one embodiment, the base substrate 11 has a thickness of 60 μm. In another embodiment, the base substrate or base membrane 11, has a thickness of about 25 μm or more.
The gases used in the plasma treatment apparatus 10 may vary depending on the intended application. The gasses can be selected for example from noble gases like argon and the like, an inert gas like nitrogen, and reactive gases like oxygen, hydrogen, carbon(di)oxide, nitrogen-oxide combinations of these reactive gases and inert or noble gases.
Both electrodes 2, 3 may have the same configuration being flat orientated (as seen in the embodiments of
In the embodiment depicted in
In the embodiment shown, three substrates 6, 7, 11 are provided to the treatment space 5 simultaneously by rotating supply drums 22, 23. Substrates 7 and 11 may be first laminated using lamination roller 25, which allows the top substrate 11 and other substrate 6 to be treated simultaneously in the treatment space 5 and subjected to an atmospheric pressure glow discharge plasma, thereby creating micro-pores throughout the top substrate 11 and substrate 6. After treatment, the substrate 6, and laminated substrate 7, 11 are received on storage drums 24. The embodiment in
In a further embodiment three substrates are provided to the treatment space 5 simultaneously as shown in
a and 2b show two embodiments of a part of the plasma treatment apparatus 10 with different electrode configurations. Both electrodes 2, 3 can be provided with a dielectric barrier layer 2a, 3a as shown in the embodiment of
In the further embodiment shown in
In the embodiment shown in
The gap distance g as shown in
The dimensions of the electrodes 2, 3, dielectric barrier layers 2a, 3a, and gap g between the electrodes 2, 3 and the total dielectric distance d which is the total dielectric thickness of the dielectric barrier are controlled in a further embodiment, such that the product of gap distance and the total dielectric distance is arranged to be less than or equal to 1.0 mm2 or even more preferred less than 0.5 mm2. This is described in more detail in the published WO2009104957 of the applicant, which is herein incorporated by reference.
In the embodiments described above, the power supply 4 can be a power supply providing a wide range of frequencies. For example, it can provide a low frequency (f=10-500 kHz) electrical signal during the on-time. It can also provide a high frequency electrical signal for example f=450 kHz up to 30 MHz. Also other frequencies can be provided like from 450 kHz to 1 MHz or from 1 to 20 MHz and the like. The on-time may vary from very short, e.g. 20 μs, to short, e.g. 500 μs or as long as 2 minutes or even continuous. The on-time effectively results in a pulse train having a series of sine wave periods at the operating frequency, with a total duration of the on-time. In further treatment experiments, even ‘continuous wave’ (CW) applications of AC power by power supply 4 have been used. Good results have been obtained using a duty cycle of 40% up to 100%.
Very good results can now be obtained by using an atmospheric pressure glow discharge (APG) plasma. Until recently these plasma's suffered from bad stability, but using the stabilization circuits as described in e.g. U.S. Pat. No. 6,774,569, EP-A-1383359, EP-A-1547123 and EP-A-1626613 (which are incorporated herein by reference), very stable APG plasmas can be obtained. In general, these plasmas are stabilized by a stabilization circuit 21 (see description of
Although oxygen as a reactive gas has a many advantages in for example surface modification uses or in uses where oxidative processes occur, other reactive gases might be used also, such as hydrogen, carbon dioxide, ammonia, oxides of nitrogen, and the like.
In the present invention, one or more carrier gasses are used chosen from the group comprising argon, nitrogen, or a combination of both. However, also other inert gasses (e.g. helium or argon) or combinations of inert gasses may be used.
In an embodiment the gas composition in the treatment space 5 comprises nitrogen in the range 79 to 99.9% and oxygen in the range of 0.1 to 21% vol. %, e,g, 95 to 99.9% and oxygen in the range of 0.1 to 5 vol. %. Oxygen concentrations higher than 21 vol. % may result in damage to the porous substrate 11.
In a further embodiment, the substrate 11 is heated during the plasma treatment by e.g. heating the electrodes 2, 3. The temperature of the heated electrodes 2, 3 (and thus the substrate 6, 7, 11) is typically between 20 and 150° C. Examples of such heating embodiments may be found in WO2008147184 from the same applicant, which is herein incorporated by reference. Other heating implementations, direct or indirect may be contemplated in further embodiments.
After the plasma treatment according to the present invention, the porous substrate 11 is volumetrically compatibilized (i.e. uniformly throughout the substrate and not only the top surface of the substrate), which is thought to be the result of volumetric hydrophylization of the micro-pores by oxidation and production of OH-groups as well as to uniform enlargement of the pores of the micro-porous substrate 11 by the plasma treatment.
After the plasma treatment step described above the activated porous substrate 11 is immersed in or impregnated with the ion exchange material, in order to obtain the actual ion exchange membrane. The impregnation step is done via techniques known as such in the art. Various options are available for applying the ion exchange solution components to the plasma treated porous substrates 11 which may include e.g. dip, spray, bar coating steps etc.
After the impregnation the resulting membranes were radiation cured using UV-light to obtain the final membrane ready for use.
As a porous substrate 11 two grades of Solupor™ materials were tested.
As substrates 6 and 7 PET foil type Astera X063.004 from Agfa was used with a thickness of 63 μm.
P1: Solupor™ 10P05A: porosity 83%/thickness 60 μm/mean pore size is 0.5 μm.
P2: Solupor™14P01E: porosity 40%/thickness 25 μm/mean pore size is 0.1 μm.
Both porous substrates 11 (P1, P2) were separately treated on top of a supporting carrier PET foil 7 using an atmospheric pressure glow discharge plasma apparatus as shown in
Note: Table 1 lists Examples 1 to 5 and Reference Examples 8 and 9 which were treated in the atmospheric pressure glow discharge plasma as shown in
The two porous substrates P1, P2 (Solupor™ 10P05A and 14P01E) used above were Corona treated in air using a Softal™ corona VTG250 apparatus at a specific energy of 16 J/cm2 using ceramic electrodes coupled to a Softal™ VTG3005 generator.
The same porous substrates as above however untreated (in plasma or corona environment) were used as reference samples 4 and 5.
Before the impregnation step, all five Examples and nine Reference Examples were checked by a Jeol JSM-6330F Field Emission Scanning Electron Microscope (SEM) SE detector/5 kV Accelerating Voltage/15 mm Working Distance/Aperture 2/Pt-coating, 10 nm thickness/enlargement 2000×).
a is showing the reference SEM image of the top-side Reference Example 6 and
All fourteen samples of above (i.e. Examples 1 to 5 and Reference Examples RE1 to RE9) were impregnated with two kinds of recipes (one C1 recipe for making a cationic membrane and one A1 recipe for making an an-ionic membrane) resulting in 28 samples by a dip-coating of all 14 samples in a bath containing the C1 or A1 recipe-solutions.
Recipe solution C1 and A1 were prepared by mixing the ingredients expressed as wt. % as shown in Table 2 and 3.
After impregnation the samples were radiation cured using a Light Hammer LH6 from Fusion UV Systems fitted with a D-bulb working at 100% intensity with a speed of 30 m/min (single pass). The exposure time was 0.47 seconds.
After the impregnation with Recipe A1 and C1, the samples were inspected visually and by SEM images off cross-cuts of the membranes. Table 4 shows the results of these inspections.
a shows a scanning electron microscope image of a cross section of a comparative membrane prepared using a corona plasma treatment (Reference Example RE4) and
From the Examples 1-5 shown in Tables 1 and 4, it can be inferred that the compatabilitization due to uniform and volumetric impregnation is the result of the plasma treatment according to the present invention. In particular, Reference Examples RE1 and RE8 lack oxygen in the gas composition. Reference Example RE9 shows that more than 21 vol. % oxygen in the gas composition damaged the membrane. Furthermore, the specific energy of the atmospheric pressure glow discharge plasma in the treatment space was rather low for Reference Examples RE2-3, i.e. less than 10 J/cm2. Reference Examples RE6 and RE7 indicate that the corona treatment does not allow for proper impregnation of the porous substrate, hence further proving the advantages of the present invention.
The present invention embodiments have been described above with reference to a number of exemplary embodiments as shown in and described with reference to the drawings. Modifications and alternative implementations of some parts or elements are possible, and are included in the scope of protection as defined in the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1215098.3 | Aug 2012 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2013/051991 | 7/25/2013 | WO | 00 |
