JP5286313B2 - Depth filter type microfiltration membrane and manufacturing method thereof - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a filtration membrane used for removing microorganisms, microorganisms, plants, animal-derived debris in the food industry, pharmaceutical industry, cosmetics industry, and the like, and a method for producing the same.

  Currently, in the filtration process in the food industry, pharmaceutical industry, cosmetics industry, etc., soft and highly compressible particles such as bacterial cells, cell debris and polymer aggregates are removed, and water-soluble polymers such as proteins, amino acids, etc. It is necessary to collect a low molecular weight, and filtration using a filter aid, filtration using a ceramic filtration membrane, and microfiltration using a synthetic polymer membrane are used.

  However, the filtration using a filter aid is problematic in that a large amount of filter aid such as diatomaceous earth is used, and a large amount of hardly decomposable filter residues are generated. In addition, ceramic filtration membranes are expensive, and a problem is that regeneration requires a drug such as alkali. And since the conventional synthetic polymer filtration membrane has a large calorific value at the time of incineration and damages the incinerator, the disposal method after clogging of the filtration membrane has been a problem.

  The inventor has already developed a biodegradable polyester filter membrane such as polybutylene succinate, polylactic acid, polycaprolactone, and a polymer blend thereof (Patent Document 1, Non-Patent Documents 1 to 3). If such a biodegradable polyester filter membrane is used, it becomes possible to decompose the used filter membrane using a composting apparatus, which is a problem when a conventional synthetic polymer membrane is used. Problems related to the disposal method of the filtration membrane can be solved.

  On the other hand, with the aim of creating an environmentally conscious society, polymer materials that use biomass, which is a recyclable resource, are attracting attention. Polylactic acid is an industrially practical product that uses carbon derived from biomass. It is a biomass plastic being produced (Non-patent Document 4). Descriptions of the polylactic acid separation membrane can be found in Patent Documents 2 to 4 and Non-Patent Documents 1 to 2 and 5. However, the polylactic acid membranes described in Patent Documents 2 to 4 can remove particles having a particle size of about 1 μm, but studies aimed at microfiltration membranes that can permeate water-soluble polymers such as proteins have been conducted. Not. The polylactic acid films described in Non-Patent Documents 1 and 2 have been studied using as an index the ability to remove yeast having a particle size of 5 μm, but Non-Patent Document 1 shows that it penetrates Escherichia coli having a particle size of about 1 μm. Has been. On the other hand, the polylactic acid membrane described in Non-Patent Document 5 shows the properties of an ultrafiltration membrane that blocks water-soluble polymers.

  In order to obtain a high filtration rate in microfiltration, the internal structure of the filtration membrane is important. For example, in Patent Documents 5 to 7 and Non-Patent Document 6, when a filtration membrane having an asymmetric structure in which the pore diameter on the surface in contact with the particle suspension is large and the pore diameter is small in the direction of the filtrate is used, the filtration resistance during use is reduced. It is described that an increase in the flow rate is suppressed and a high filtration rate can be obtained. Such a filtration membrane that retains particles in the filtration membrane is called a depth filter. On the other hand, a filtration membrane that blocks particles on the membrane surface is called a screen filter.

  However, these asymmetric filtration membranes are filtration membranes having no biodegradability such as polysulfone membranes.

JP 2008-132415 A JP 2002-20530 A JP 2008-296123 A JP 2009-226256 A Japanese Patent Publication No. 1-443619 Japanese Examined Patent Publication No. 4-68966 JP-A-7-222917

T. Tanaka, et al., J. Membr. Sci., 238, 65-73 (2004). T. Tanaka, et al., J. Chem. Eng. Japan, 39, 144-153 (2006). T. Tanaka, et al., Desalination, 193, 367-374 (2006). Edited by Japan Bioplastics Association, "All about Bioplastic Materials", pp. 37-41, Nikkan Kogyo Shimbun, 2008. A. Moriya, et al., J. Membr. Sci., 342, 307-312 (2009). Tanaka Takaaki et al., Chemical Engineering Papers, 35, 81-86 (2009).

  Therefore, in view of the above problems, the present invention has an asymmetric structure that is biodegradable, blocks particles having a size of about 1 μm, but allows water-soluble polymers to pass therethrough, and can be used as a depth filter. It is an object of the present invention to provide a novel polylactic acid microfiltration membrane and a method for producing the same.

  As a result of various studies to achieve the above problems, a thin film obtained by applying polylactic acid as a material and applying a solution prepared at a high temperature to a mold that allows only heat to pass through is immersed in a low temperature non-solvent. It has been found that a microfiltration membrane having an asymmetric structure that is degradable, blocks particles having a size of about 1 μm, but can permeate water-soluble polymers and can be used as a depth filter, The present invention has been completed.

  That is, the depth filter type microfiltration membrane of the present invention has a polylactic acid solution obtained by dissolving polylactic acid in a solvent to form a thin film, and is maintained at a lower temperature than the solution together with the mold. It was obtained by soaking in a non-solvent.

  The solvent is dimethyl sulfoxide, and the non-solvent is water.

  The method for producing a depth filter type microfiltration membrane of the present invention comprises a polylactic acid solution obtained by dissolving polylactic acid in a solvent to form a thin film, and a polycrystal maintained at a lower temperature than the solution together with the mold. It is immersed in a non-solvent of lactic acid.

  The solvent is dimethyl sulfoxide, and the non-solvent is water.

  According to the present invention, a novel biodegradable, blockable particle having a size of about 1 μm but capable of transmitting a water-soluble polymer and having an asymmetric structure that can be used as a depth filter. A microfiltration membrane made of polylactic acid and a method for producing the same can be provided.

It is an electron micrograph which shows the cross section and surface of a film | membrane produced from the polylactic acid solution which dissolved the polylactic acid in Example 1 of this invention in dimethylsulfoxide. It is an electron micrograph which shows the cross section and surface of a film | membrane produced from the polylactic acid solution which melt | dissolved the polylactic acid in Example 1 of this invention in dimethylsulfoxide, and refine | purified water. It is a graph which shows the time-dependent change of the filtrate amount in the filtration experiment of the lactic acid bacteria suspension in Example 1 of this invention.

  The depth filter type microfiltration membrane of the present invention is a polylactic acid solution obtained by dissolving a polylactic acid in a solvent into a mold to form a thin film, and the non-polylactic acid that is maintained at a lower temperature than the solution together with the mold. It was obtained by immersing in a solvent. Hereinafter, the manufacturing method of the filtration membrane of this invention is demonstrated.

  First, polylactic acid is dissolved in a solvent to obtain a polylactic acid solution. The solvent is not particularly limited as long as it can dissolve polylactic acid, but dimethyl sulfoxide is particularly preferably used in the present invention because a porous film having uniform fine pores can be obtained. . The polylactic acid concentration in the solution is preferably in the range of 5 to 15%. If it is less than 5%, it is difficult to obtain a filtration membrane having sufficient thickness and strength, and if it exceeds 15%, it takes a long time to dissolve polylactic acid. When dimethyl sulfoxide is used as a solvent when dissolving, it is preferable to keep the temperature of the solution at 120 to 140 ° C. in consideration of the boiling point of the solvent and the dissolution rate of polylactic acid. After dissolution, for example, by maintaining the temperature at 70 to 90 ° C., it is possible to ensure safety in operation while maintaining fluidity when pouring into a mold in the next step.

  Next, a polylactic acid solution is put into a mold to form a thin film. As a mold into which the solution is placed, for example, a flat glass plate can be used, or when using a continuous production apparatus, a curved roll mold may be used. As the material of the mold, a material having high thermal conductivity is suitably used in order to successfully perform film formation in the next step. If the solution is cooled when it is put into the mold, there is a possibility that the film formation in the next step may be hindered. Therefore, the mold into which the solution is put is held in advance at a predetermined temperature, for example, 50 to 70 ° C. It is desirable to keep it.

  Then, it is immersed in a non-solvent of polylactic acid maintained at a lower temperature than the solution together with the mold. The non-solvent of polylactic acid is not limited to a specific one as long as it does not dissolve polylactic acid and has high affinity with the above solvent. When dimethyl sulfoxide is used as the solvent, water is particularly preferably used as the non-solvent.

  The polylactic acid solution made into a thin film by putting it in a mold is immersed in a non-solvent of polylactic acid maintained at a lower temperature than the solution together with the mold, whereby the solution is rapidly cooled. As a result, the non-solvent solution molecules are diffused into the polylactic acid solution from the side in contact with the non-solvent, and phase separation is obtained, and at the same time, the side in contact with the mold is cooled by the polylactic acid solution. By performing phase separation and forming a coarse porous structure, a filtration membrane having an asymmetric porous structure is formed. The temperature of the non-solvent of polylactic acid should be lower than the temperature of the solution to be immersed, but in order to obtain a filtration membrane having an asymmetric porous structure, a certain temperature difference between the non-solvent and the solution is necessary. For example, when the temperature of the solution and the mold is 50 to 70 ° C., a filtration membrane having an asymmetric porous structure can be surely obtained by setting the temperature of the non-solvent to 20 to 30 ° C. of room temperature.

  Thereafter, in order to completely extract and remove the solvent, it is desirable to store the obtained filtration membrane in a non-solvent and exchange the non-solvent one to several times.

  The filtration membrane of the present invention obtained as described above is a sheet-like, self-supporting filtration membrane that does not require a support such as a nonwoven fabric or a woven fabric, and is a flat membrane that can be used for an existing membrane folder or the like It is a type. And it functions as a microfiltration membrane that can block particles having a size of about 1 μm and allow water-soluble polymers such as proteins to permeate. In addition, since it has an asymmetric structure, it is an in-membrane particle trapping type filtration membrane, and functions as a depth filter that suppresses formation of a particle layer accompanying filtration and obtains a high filtration rate. Furthermore, since the material is polylactic acid, which is a biodegradable plastic, it can be decomposed by a composting device after use. Therefore, the filtration membrane and filtration residue of the present invention can be effectively used as compost.

  The present invention is not limited to the above embodiment, and various modifications can be made without departing from the spirit of the present invention.

  A microfiltration membrane that can be used as a polylactic acid depth filter according to the present invention was prepared, and its performance was evaluated.

1 Material Poly-L-lactic acid having a molecular weight of 120,000 was used as polylactic acid.

  Purified water produced by an apparatus using a special grade dimethyl sulfoxide manufactured by Wako Pure Chemical Industries as a solvent for polylactic acid and an ion exchange resin as a non-solvent was used.

2 Production of Filtration Membrane First, 7.5 g of polylactic acid was dissolved in 42.5 g of dimethyl sulfoxide in a 100 mL Erlenmeyer flask so that the final concentration of polylactic acid was 15%. More specifically, polylactic acid, dimethyl sulfoxide, and a rotor were placed in an Erlenmeyer flask, the head space was replaced with nitrogen gas, and the cap was covered with a cork stopper covered with aluminum foil. Further, Teflon (registered trademark) tape was wrapped around the side of the cork stopper for sealing. The Erlenmeyer flask was placed on a hot stirrer set at 130 ° C. and stirred for about 6 hours to dissolve polylactic acid in dimethyl sulfoxide. Further, the polylactic acid solution was kept in a constant temperature water bath at 80 ° C. for 30 minutes or more.

  Next, a filtration membrane was produced. The film was produced in a room where the room temperature was set to 25 ° C. A sheet of Teflon (registered trademark) having a width of 8 mm and a thickness of 1 mm was attached to a peripheral portion of a 76 mm × 70 mm glass plate with a double-sided tape, and a mold having a depth of 1 mm with this sheet as a frame was produced. Similarly, a mold having a depth of 0.5 mm was produced using a sheet having a width of 8 mm and a thickness of 0.5 mm. The produced mold was placed on a hot stirrer set at 60 ° C. and kept warm. Then, a slightly larger amount of the polylactic acid solution kept at 80 ° C. was poured into the mold frame. Excess polylactic acid solution was ground using a spatula with straight edges. Then, when the mold into which the polylactic acid solution was poured was put into a stainless steel vat containing 400 mL of purified water, dimethyl sulfoxide was extracted to form a polylactic acid film. Two hours later, the stainless steel vat water was replaced with fresh 400 mL purified water. After 2 hours, the membrane was transferred to a sealable plastic container (container: polypropylene; lid: polyethylene) containing 500 mL of purified water. After another day, the purified water was changed. The produced membrane was stored in purified water.

3 Performance Evaluation of Filtration Membrane (1) Electron Microscope Observation The produced filtration membrane was moistened with moisture and cleaved in liquid nitrogen. After being placed on the sample stage, gold-palladium-alloy was sputter coated. The cross section and surface of the film were observed with an acceleration voltage of 15 kV using a scanning electron microscope. FIG. 1 shows the cross section and surface of a polylactic acid film prepared from a polylactic acid solution prepared by dissolving 7.5 g of polylactic acid in 42.5 g of dimethyl sulfoxide and a mold having a depth of 0.5 mm. A dense film structure was formed on the surface in contact with water, and a rough film structure was formed on the flat plate side which is the glass plate side of the mold.

  When purified water was added to dimethyl sulfoxide, which dissolves polylactic acid, it was possible to change the particles having a rough membrane structure. FIG. 2 shows the cross section and surface of a polylactic acid film prepared from a polylactic acid solution in which 0.4 g of purified water is dissolved after 7.5 g of polylactic acid is dissolved in 42.1 g of dimethyl sulfoxide, and a mold having a depth of 0.5 mm. . Particles with a rough film structure on the flat plate side during film formation became smaller.

(2) Measurement of membrane filtration resistance A filtration experiment for purified water was conducted using a filtration device for a membrane diameter of 25 mm (effective filtration area A = 4.1 cm ² = 4.1 × 10 ⁻⁴ m ² ). This filtration experiment was performed using the filtration membrane of this example and a commercially available cellulose acetate membrane having a nominal pore diameter of 0.2 μm for comparison. The filtration pressure ΔP [Pa] was set to 10 kPa using a nitrogen gas cylinder. Filtration pressure ΔP [Pa], purified water permeation flux J [m / s] (= filtrate amount [m ³ ] / (time [s] × effective filtration area [m ² ]) and water viscosity μ = 8 The filtration resistance R _m [1 / m] of the filtration membrane was calculated from the formula of 9 × 10 ⁻⁴ [Pa.s] (25 ° C.) using the formula of R _m = ΔP / (μ · J).

  As shown in Table 1, the filtration resistance of the filtration membrane of the present invention was about 1.2 to 6.9 times that of a commercially available filtration membrane.

(3) Filtration Experiment of Lactic Acid Bacteria Suspension A filtration experiment was performed using a lactic acid bacterium (Lactobacillus plantarum NBRC 15891 ^T ) having a diameter of 0.7 μm and a length of 2.5 μm. A culture solution obtained by stationary culture of the lactic acid bacteria in MRS medium was diluted 10-fold with purified water to obtain a lactic acid bacteria suspension (corresponding to 0.2 kg / m ³ ). Filtration was performed at a pressure of 10 kPa. The inhibition rate of lactic acid bacteria was evaluated by measuring the absorbance of the suspension and the filtrate at 660 nm. This filtration experiment was performed using the filtration membrane of this example and a commercially available cellulose acetate membrane having a nominal pore diameter of 0.2 μm for comparison.

  As shown in Table 2, the apparent blocking rate of lactic acid bacteria (φ0.7 × 2.5 μm) determined from the turbidity of the liquid before and after filtration was 98% or more. The turbidity of the filtrate of the lactic acid bacteria suspension was similar to that of a commercially available cellulose acetate membrane having a nominal pore size of 0.2 μm. Therefore, it was considered that this was caused by a colored dissolved component in the culture solution.

  FIG. 3 shows an example of the change over time in the amount of filtrate in the filtration experiment of lactic acid bacteria suspension. It was verified that when filtered from the rough surface side, it exhibits characteristics as a depth filter. As a comparison, the results of a filtration experiment as a screen filter that has been filtered from the dense surface side are also shown. As a filtration membrane, a polylactic acid membrane prepared from a polylactic acid solution in which 7.5 g of polylactic acid was dissolved in 42.5 g of dimethyl sulfoxide and a mold having a depth of 1.0 mm was used. In any case, the cell blocking rate was 99% or more. When used as a depth filter, filtration is possible in a shorter time compared to the case of a screen filter, and the usefulness of the filter membrane produced in this example was shown.

(4) Filtration experiment of protein solution The permeability of protein was confirmed by the filtration experiment of the bovine serum albumin solution. For bovine serum albumin, Fraction V manufactured by Sigma-Aldrich was used. The protein solution was prepared by dissolving bovine serum albumin in 10 mM sodium phosphate buffer at pH 6.8 to a concentration of 100 g / m ³ . The protein concentration before filtration and the protein concentration of the filtrate were measured using a Pierce BCA protein quantification kit, and the transmittance was calculated. As the filtration membrane, a filtration membrane prepared from a polylactic acid solution prepared by dissolving 7.5 g of polylactic acid in 42.5 g of dimethyl sulfoxide and a mold having a depth of 1.0 mm was used.

  Table 3 shows protein permeability. The protein transmittance was 75% or more on both the rough surface side and the dense surface side. However, when the filtration was performed from the rough surface side showing the characteristics as a depth filter, the transmittance was as high as 88.5%. there were. From the results of Tables 2 and 3, it was shown that the filtration membrane produced in this example has characteristics as a microfiltration membrane that blocks particles of about 1 μm and permeates the polymer.

Claims (4)

A polylactic acid solution obtained by dissolving polylactic acid in a solvent is put into a thin film and immersed in a non-solvent of polylactic acid maintained at a lower temperature than the solution together with the mold. A depth filter type microfiltration membrane. The depth filter type microfiltration membrane according to claim 1, wherein the solvent is dimethyl sulfoxide and the non-solvent is water. A depth filter characterized in that a polylactic acid solution obtained by dissolving polylactic acid in a solvent is placed in a mold to form a thin film and immersed in a non-solvent of polylactic acid maintained at a lower temperature than the solution together with the mold Type microfiltration membrane manufacturing method. 4. The method for producing a depth filter type microfiltration membrane according to claim 3, wherein the solvent is dimethyl sulfoxide and the non-solvent is water.