JP5197912B2 - Method for producing porous body - Google Patents

Description

  The present invention relates to a porous body that can be used for water treatment membranes, filters, adsorbents, nonwoven fabric abrasives, substrates of various functional materials, molds, and the like, and a method for producing the same.

  The porous body is a material used for various applications such as a porous membrane, a filtration filter, an adsorbent, and a nonwoven fabric abrasive. Among these, porous membranes are excellent in separation completeness, compactness, and the like, and are widely used as water treatment membranes for water treatment such as industrial wastewater treatment, sewage wastewater treatment, and water purification. In recent years, the capacity of a hard disk (HD) has been increased by improving the recording density. In order to improve the recording density of HD, further smoothing of the HD surface is necessary. For this purpose, a nonwoven fabric abrasive having pores of nano-order (1 nm or more and less than 100 nm) is desired.

As a conventional porous body, for example, a porous film obtained by stretching and forming melt-formed polyethylene is known (see Patent Documents 1 to 3). The porous membrane can remove unnecessary components such as bacteria and foreign matters in the water to be treated to a high degree and is excellent in water permeability. Therefore, the porous membrane is used in many fields such as water treatment and chemical filtration. However, a conventional porous membrane made of polyethylene has a problem in durability, in particular, chemical resistance against sodium hypochlorite used when cleaning the porous membrane after water treatment. As polyethylene having excellent chemical resistance, ultrahigh molecular weight polyethylene is known. However, ultrahigh molecular weight polyethylene is extremely difficult to be made porous by heat stretching.
In addition, the melt-shaped polyethylene porous film described in Patent Documents 1 to 3 peels between crystal lamellae by stretching polyethylene having a laminated lamella structure in which crystalline lamellae and amorphous are alternately laminated, A fibril is formed between the peeled lamellas to form a porous structure (see, for example, Non-Patent Documents 1 and 2). However, the porous structure obtained by this method is a porous structure having a pore size of submicron order (0.1 μm or more and less than 1 μm) to micron order (1 μm or more and less than 1 mm), although the porosity is high.

  As a porous body made of ultra high molecular weight polyethylene, a porous body in which ultra high molecular weight polyethylene particles are fused (see Patent Document 4), a porous body in which ultra high molecular weight polyethylene particles are sintered and molded (Patent Document) 5) has been proposed. However, since these porous bodies are aggregates of granular ultrahigh molecular weight polyethylene, for example, the tensile strength of the porous body described in Patent Document 5 is about 3 MPa, and the mechanical strength is low. Patent Document 6 discloses a porous fiber having a strength of 5 g / d or more (that is, 0.045 N / dtex or more). However, the melt index (MI) value of the material polyethylene is 0.05 to 1.2, and a low molecular weight polyethylene that can be melt-drawn is used, so that the chemical resistance is low.

  Patent Document 3 describes a method for stretching and making high-density polyethylene having a melt index of 1 to 15 in a stretched manner. However, ultrahigh molecular weight polyethylene is extremely difficult to be stretched and porous by heating. Patent Document 7 discloses a polyolefin hollow fiber-like porous material comprising a polyolefin mixture containing 60 to 95% by weight of polyethylene having a weight average molecular weight of 500,000 or more and 5 to 40% by weight of polypropylene having a weight average molecular weight of 10,000 to 1,000,000. A membrane is described. This porous membrane is manufactured by a so-called thermally induced phase separation method, but has a low breaking strength of 6.54 to 9.147 MPa.

In addition, a method for producing a polyethylene fiber that is drawn after ultra high molecular weight polyethylene is discharged from a spinneret and then treated with a supercritical fluid or a similar fluid is proposed (Patent Document 8). This method aims to improve the strength and elastic modulus of the fiber, and relaxes only the orientation of the outer layer of the fiber by stretching during or after treatment with a supercritical fluid or a similar fluid, and the denseness of the outer layer and the inner layer. By reducing the degree or structural difference, the draw ratio is improved, and high strength and high elasticity are achieved. The production method described in Patent Document 8 is a method for producing polyethylene fibers that are drawn when or after the ultrahigh molecular weight polyethylene discharged by so-called gel spinning is treated with a supercritical fluid or a similar fluid. In this case, a gel-like unstretched yarn having a minimum entanglement structure necessary for stretching is prepared from a quasi-dilute solution in which ultrahigh molecular weight polyethylene is dissolved in a suitable solvent and polymer chains begin to overlap each other, and then gel-like The structure is changed from a folded chain to an extended chain by stretching the yarn at a high magnification (see, for example, Non-Patent Document 3). Therefore, since the molecular chain is highly molecularly oriented by stretching, a porous structure is not obtained.
JP-A-57-66114 JP 58-81611 A JP-A-5-49878 Japanese Patent No. 3484331 Japanese Patent Laid-Open No. 9-3236 JP-A-6-123005 Japanese Patent Laid-Open No. 2001-300275 JP 2003-3323 A The Society of Polymer Science, “Polymer Science One Point-4 Polymer Crystals”, Kyoritsu Shuppan, 1993 Toshihiko Kuroda, Akira Takizawa, Mitsuru Nagasawa, “Basic Physical Properties and Applications of Polymers”, CMC Co., Ltd., published in 1984 Textile Society, 2nd edition Textbook, Maruzen Co., Ltd., published in 1994

Accordingly, an object of the present invention is to provide a porous body having high mechanical strength and excellent chemical resistance.
Another object of the present invention is to provide a porous body having a nano-order pore diameter.
Moreover, the objective of this invention is providing the manufacturing method of the porous body which can be made porous by extending | stretching, even if it is a polymeric material with a high molecular weight.
Another object of the present invention is to provide a method for producing a porous body having a nano-order pore diameter.

The method for producing a porous body according to the present invention is a method for producing a porous body in which a crystalline polyolefin having a viscosity average molecular weight of 100 × 10 ⁴ or more is treated with a supercritical fluid or a subcritical fluid and stretched. The crystalline polyolefin is any one of a film shape, a sheet shape, and a hollow shape.
The drawing of the crystalline polyolefin is preferably performed in a supercritical fluid or a subcritical fluid.

Stretching of the crystalline polyolefin is crystalline polyolefin (melting point -30) ° C. or higher, the crystalline polyolefin (melting point + 5) it is desirable to conduct ° C. at a temperature below.
The main component of the supercritical fluid or subcritical fluid is preferably carbon dioxide.

The porous body of the present invention has high mechanical strength and excellent chemical resistance.
According to the method for producing a porous body of the present invention, even a polymer material having a high molecular weight can be made porous by stretching.

The porous body of the present invention has a nano-order pore size.
According to the method for producing a porous body of the present invention, a porous body having a nano-order pore diameter can be produced.

The present invention will be described below.
<Porous body>
The porous body of the present invention is made of a polymer material containing a crystalline polymer.
The “crystalline polymer” in the present invention is a polymer substance having a considerably ordered molecular arrangement and clear crystal X-ray diffraction.

  Examples of the crystalline polymer include polyolefins such as polyethylene, polypropylene, poly-3-methylbutene-1, and poly-4-methylpentene-1, syn-polystyrene, polyvinyl alcohol, polyvinyl chloride, acrylonitrile-based polymers, and the like. Polyvinyl compounds; crystalline polyvinylidene such as polyvinylidene chloride, polyvinylidene fluoride, and polycyanovinylidene; crystalline fluoropolymers such as polytetrafluoroethylene; polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polylactic acid, polyarylate Polyamides such as polyamide-6, polyamide-6,6, aromatic polyamide, etc .; crystalline polyurethanes; crystalline polyimides; crystalline polyketones; crystalline polyether ether Ton class; crystalline polycarbonates; crystalline polysulfones; crystalline polyether sulfones; crystalline polysulfides; cellulose and cellulose derivatives; proteins, nucleic acids, and the like. Of these, polyolefins, particularly polyethylene and polypropylene, are preferred because they are easily made porous by stretching.

(First porous body)
The first porous body of the present invention is made of a polymer material containing a crystalline polymer having an average molecular weight of 100 × 10 ⁴ or more, and has a tensile strength of 10 MPa or more (hereinafter referred to as porous body (I ).
The average molecular weight of the crystalline polymer is 100 × 10 ⁴ or more, and the upper limit is limited only by practicality. The utility here means that it is available. When the average molecular weight of the crystalline polymer is less than 100 × 10 ⁴ , the mechanical strength and chemical resistance of the porous body are insufficient, and the average molecular weight of the crystalline polymer is set to 700 × 10 ⁴ or less. , Shaping becomes easy. The “average molecular weight” in the present invention is an average molecular weight determined by a solution viscosity method (ASTM D4020) (hereinafter also referred to as viscosity average molecular weight).

Of these crystalline polymers, it is preferably 100 × 10 ⁴ or more polyolefins average molecular weight, the average molecular weight of 100 × 10 ⁴ or more polyethylene (hereinafter also referred to as ultra high molecular weight polyethylene) is particularly preferred. Porous materials made of polyolefins with an average molecular weight of 100 × 10 ⁴ or more, especially ultra-high molecular weight polyethylene, are less susceptible to oxidative degradation even after chemical cleaning using sodium hypochlorite, etc., and maintain tensile strength over a long period of time. Is done.

  The ratio of the crystalline polymer in the polymer material is preferably 10% by mass or more in the polymer material (100% by mass). In the case of a porous body produced by stretching a polymer material containing a crystalline polymer, the property of forming voids between the crystals by stretching is utilized (for example, the above-mentioned non-patent document) 1 and 2). Therefore, when the ratio of the crystalline polymer is less than 10% by mass, there is a possibility that the formation of pores by stretching becomes insufficient. The ratio of the crystalline polymer in the polymer material is preferably 50% by mass or more, more preferably 90% by mass or more, and most preferably 100% by mass in the polymer material (100% by mass).

  The tensile strength of the porous body (I) is 10 MPa or more. A porous body having a tensile strength of 10 MPa or more is a porous body having sufficient mechanical strength required for use in applications such as water treatment. The “tensile strength” in the present invention is a stress when a tensile test is performed on a porous body at a pulling speed of a (mm / min) and a fracture is caused when the test length is a (mm). The range of the test length a depends on the porous body, but it is 2 mm to 200 mm, preferably 5 mm to 100 mm. In addition, the test length said here is the distance between chuck | zippers at the time of performing a tension test. Although the upper limit of the tensile strength of the porous body (I) is not particularly provided, it is about 1000 MPa by analogy with the tensile strength of the fiber made of ultrahigh molecular weight polyethylene.

The shape of the porous body (I) is not particularly limited, and examples thereof include a film shape, a sheet shape, and a hollow shape.
The porous body (I) can be used as a water treatment film, a filter, an adsorbent, a nonwoven fabric abrasive, and the like. Examples of the form of the water treatment membrane include a flat membrane and a hollow fiber membrane.

  The porous body (I) described above is excellent in mechanical strength and exhibits good chemical resistance due to the molecular structure of the crystalline polymer as the material. Furthermore, the higher the crystalline polymer or the higher molecular weight crystalline polymer having few functional groups, double bonds, and side chains, the better the chemical resistance. Therefore, such a porous body is very suitable for, for example, a water treatment application in which washing with sodium hypochlorite is performed, and particularly in a wastewater treatment application used in an aerated environment. Is preferred.

(Second porous body)
The second porous body of the present invention is a porous body (hereinafter referred to as porous body (II)) having a structure in which thin plates containing a crystalline polymer and having a thickness of 5 to 50 nm are laminated.
When the thickness D of the thin plate is less than 5 nm, the strength of the porous body may be weakened, and when D exceeds 50 nm, the porosity may be reduced. The range of D is preferably 10 nm or more and 40 nm or less, and more preferably 15 nm or more and 30 nm or less. The thickness D of the thin plate in the present invention is the length of the short axis of the thin plate.

  The porosity of the porous body (II) is preferably 10 to 80%. If the open area ratio is less than 10%, it may be difficult to sufficiently exhibit the use as a porous body described later. If the open area ratio exceeds 80%, the thin plate constituting the porous body occupies the area. Since the ratio decreases, the strength of the porous body may be insufficient.

The thickness D and the hole area ratio in the present invention are determined as follows.
The outer surface of the obtained porous body is observed using an electron microscope. The electron microscope used is preferably a scanning electron microscope (SEM). The observation magnification cannot be generally described, but is 30,000 times or more, preferably about 100,000 times. If the observation magnification is less than 30,000, it is difficult to determine the thickness D and the hole area ratio of the thin plate. Therefore, in order to determine the thickness D and the aperture ratio, it is preferable to use a field emission SEM. Next, the thickness D of the thin plate is obtained from the obtained SEM photograph. The thickness D of the thin plate is the average value of the thicknesses of any 10 thin plates in the SEM photograph. In addition, the open area ratio is obtained by expressing the ratio of the area of the pore area to the area of the porous body as a percentage from the obtained SEM photograph, and is obtained by the following formula (1). The method for obtaining the thickness D and the aperture ratio of the thin plate is not particularly limited, but it is preferable to use image analysis software. The thickness D and the hole area ratio of the thin plate obtained by image analysis vary slightly depending on image quality adjustment for image analysis and image analysis software, but the difference is within the range of normal experimental error.
Open area ratio (%) = (area of pore area / area of porous body) × 100 (1)

  The shape of the porous body (II) is not particularly limited, and examples thereof include a film shape, a sheet shape, and a hollow shape.

Since the porous body (II) described above has a structure in which thin plates having a thickness of 5 to 50 nm are laminated, the pore diameter is on the order of nanometers. The porous body (II) includes a water treatment membrane for filtering a liquid, a filter, a non-woven polishing material having fine irregularities, a base material for impregnating a functional material into voids of the porous body, a mold, and the like Useful as. In the present invention, the nanometer order is from 1 nm to less than 100 nm, the submicron order is from 0.1 μm to less than 1 μm, and the micron order is from 1 μm to less than 1 mm.
In addition, the porous body (II) is excellent in mechanical strength and exhibits good chemical resistance due to the molecular structure of the crystalline polymer as the material. Furthermore, the higher the crystalline polymer or the higher molecular weight crystalline polymer having few functional groups, double bonds, and side chains, the better the chemical resistance.

<Method for producing porous body>
The method for producing a porous body of the present invention is a method in which a polymer material containing a crystalline polymer is treated and stretched by exposure to a supercritical fluid or a subcritical fluid. That is, the stretching of the polymer material may be performed after (i) exposure to a supercritical fluid or a subcritical fluid, or (ii) in a supercritical fluid or a subcritical fluid. Further, the film may be preliminarily stretched before being exposed to a supercritical fluid or a subcritical fluid.

  The “supercritical fluid” in the present invention refers to a high-density fluid that does not condense even when a temperature and pressure above the critical point are applied. This state has the same diffusivity as a gas while having the same density as a liquid. Therefore, the supercritical fluid is a fluid that penetrates into the details of the polymer material and has a large plasticizing effect.

  The “subcritical fluid” in the present invention is a fluid having a temperature or pressure higher than the critical point, and the high-pressure fluid penetrates to the inside of the polymer material as in the supercritical fluid. Has the effect of plasticizing.

  As the supercritical fluid or subcritical fluid, carbon dioxide (critical temperature: 31.0 ° C., critical pressure: 7.38 MPa) and nitrous oxide (critical temperature: 36. 3) at a temperature and / or pressure above the critical point. Various materials such as ethane (critical temperature: 32.2 ° C., critical pressure: 4.88 MPa), ethylene (critical temperature: 9.34 ° C., critical pressure: 5.04 MPa), etc. may be used. it can.

  Carbon dioxide is suitable as the main component of the supercritical fluid or subcritical fluid. Since carbon dioxide has a critical temperature of 31.0 ° C. and a critical pressure of 7.38 MPa, it is relatively easy to handle, nonflammable, inert, non-toxic, inexpensive, and suitable for supercritical conditions. . In the present invention, “the main component of the supercritical fluid or subcritical fluid is carbon dioxide” means that the ratio of carbon dioxide in the supercritical fluid or subcritical fluid (100 vol%) is 80 to 100 vol%. It means that.

  Examples of the crystalline polymer used in the production method of the present invention include the crystalline polymers exemplified as the material for the porous body of the present invention. Of these, polyolefins, particularly polyethylene and polypropylene, are preferred because they are easily made porous by stretching.

  In the production method of the present invention, the ratio of the crystalline polymer in the polymer material is preferably 10% by mass or more in the polymer material (100% by mass). When the ratio of the crystalline polymer is less than 10% by mass, there is a possibility that the formation of pores by stretching becomes insufficient. The ratio of the crystalline polymer in the polymer material is preferably 50% by mass or more, more preferably 90% by mass or more, and most preferably 100% by mass in the polymer material (100% by mass).

The shape of the polymer material (precursor) before performing the stretching operation is not particularly limited, and examples thereof include a film shape, a sheet shape, and a hollow shape. Moreover, as a shaping method of a precursor, well-known methods, such as sintering shaping | molding, melt shaping, and wet shaping, can be used.
In the production method of the present invention, an entrainer (addition solvent) may be used in either the method (i) or the method (ii).

(Method for producing porous body (I))
By using the production method of the present invention, a porous body (I) made of a polymer material containing a crystalline polymer having an average molecular weight of 100 × 10 ⁴ or more can be obtained. From the viewpoint of enhancing the mechanical strength and chemical resistance of the resulting porous body (I), the average molecular weight of the crystalline polymer is preferably 100 × 10 ⁴ or more, and the upper limit is limited only by practicality. The utility mentioned here means that it is available. When the average molecular weight of the crystalline polymer is 700 × 10 ⁴ or less, shaping is facilitated.

  The production method of the present invention is suitable for ultrahigh molecular weight polyethylene among crystalline polymers. According to the production method of the present invention, even an ultrahigh molecular weight polyethylene can be stretched and made porous, and by stretching and stretching, a porous body (I) having a tensile strength of 10 MPa or more can be easily obtained. It is also possible to obtain a porous body having a tensile strength of 20 MPa or more, and further a porous body having a tensile strength of 50 MPa or more.

According to the production method of the present invention, not only a polymer material containing a crystalline polymer having an average molecular weight of 100 × 10 ⁴ or more, but also a polymer material containing a crystalline polymer having a low average molecular weight can be used. A porous body can be obtained. When a crystalline polymer having an average molecular weight of less than 100 × 10 ⁴ is used, for example, as described in Patent Documents 1, 2, 3, and 6 described above, it is possible to obtain a porous body even by heat stretching. However, a new effect by applying the manufacturing method of the present invention, for example, the stretching ratio can be increased, and by using the manufacturing method of the present invention, a large pore diameter that cannot be achieved by heating stretching alone. And a porous body having a high porosity can be obtained.

The temperature at which a polymer material containing a crystalline polymer having an average molecular weight of 100 × 10 ⁴ or more is exposed to a supercritical fluid or a subcritical fluid (hereinafter referred to as a processing temperature) is high in crystallinity. When the melting point of the molecule is Tm, (Tm−120) ° C. or higher and (Tm + 80) ° C. or lower is preferable, and (Tm−60) ° C. or higher and (Tm + 50) ° C. or lower is more preferable. When the treatment temperature is lower than (Tm-120) ° C., the mobility of the molecular chain of the crystalline polymer is insufficient due to the supercritical fluid or subcritical fluid, so that the porosity due to stretching becomes insufficient. There is a fear. When the treatment temperature is higher than (Tm + 80) ° C., the polymer material is melted, and it may be difficult to perform the stretching operation. When the crystalline polymer is an ultra high molecular weight polyethylene having an average molecular weight of 100 × 10 ⁴ or more, the treatment temperature is preferably 20 ° C. or more and 200 ° C. or less, and more preferably 80 ° C. or more and 180 ° C. or less.

When the polymer material containing a crystalline polymer having an average molecular weight of less than 100 × 10 ⁴ is exposed to a supercritical fluid or a subcritical fluid, the processing temperature is defined as Tm being the melting point of the crystalline polymer. (Tm−120) ° C. or more and (Tm + 5) ° C. or less is preferable, and (Tm−60) ° C. or more and (Tm + 5) ° C. or less is more preferable. When the treatment temperature is less than (Tm−120) ° C., the effect obtained when the production method of the present invention is applied to polyethylene having an average molecular weight of less than 100 × 10 ⁴ , that is, enlargement of the pore size by stretching and improvement of the porosity. There is a risk that the effect will be insufficient. When the treatment temperature is higher than (Tm + 5) ° C., the polymer material is melted, and it may be difficult to perform the stretching operation. When the crystalline polymer is polyethylene having an average molecular weight of less than 100 × 10 ⁴ , the treatment temperature is preferably 20 ° C. or higher and 140 ° C. or lower, more preferably 80 ° C. or higher and 140 ° C. or lower.

The pressure (hereinafter referred to as “treatment pressure”) for treating the polymer material containing the crystalline polymer by exposing it to a supercritical fluid or subcritical fluid is preferably 2 MPa or more, and more preferably 3 MPa or more. If the treatment pressure is less than 2 MPa, the supercritical fluid or subcritical fluid may not sufficiently penetrate into the polymer material. The upper limit of pressure is defined only by the pressure resistance condition of the device. When the crystalline polymer is ultra high molecular weight polyethylene having an average molecular weight of 100 × 10 ⁴ or more, the treatment pressure is preferably 5 MPa or more, and more preferably 8 MPa or more. When the crystalline polymer is polyethylene having an average molecular weight of less than 100 × 10 ⁴ , 2 MPa or more is preferable, and 3 MPa or more is more preferable.

The time for treating the polymer material containing the crystalline polymer by exposure to a supercritical fluid or subcritical fluid (hereinafter referred to as treatment time) is preferably 10 seconds to 5 hours, and preferably 1 minute to 1 hour. Is more preferable. If the treatment time is less than 10 seconds, the supercritical fluid or subcritical fluid may not sufficiently penetrate into the polymer material. Moreover, even if it processes for 5 hours or more, there is hardly any change in the permeation state of the supercritical fluid or the subcritical fluid, which tends to be disadvantageous in terms of productivity and cost of the porous body. When the crystalline polymer is ultra high molecular weight polyethylene having an average molecular weight of 100 × 10 ⁴ or more, the treatment time is preferably from 1 minute to 5 hours, and more preferably from 5 minutes to 1 hour. In the case of polyethylene having an average molecular weight of less than 100 × 10 ⁴ , it is preferably 10 seconds or longer and 5 hours or shorter, and more preferably 1 minute or longer and 1 hour or shorter.

When the crystalline polymer is an ultra high molecular weight polyethylene having an average molecular weight of 100 × 10 ⁴ or more, the stretching operation is preferably performed in the above-described method (ii), that is, in a supercritical fluid or a subcritical fluid.

  In the method for producing a porous body of the present invention, a conventional heating is performed in order to stretch a polymer material containing a crystalline polymer in a state treated with a supercritical fluid or a subcritical fluid, or after treatment. Even a polymer material having a high molecular weight that could not be made porous by stretching by the above method can be made porous. Also, in the case of a polymer material having a molecular weight that can be made porous by stretching by conventional heating, a porous body having a large pore diameter, a porous body having a large porosity, etc. that cannot be achieved by heating stretching alone is obtained. Can do.

  In the production method of the present invention, the mechanism of stretched porosity is not clear, but the mobility of the molecular chain of the crystalline polymer is active by dissolving the supercritical fluid or subcritical fluid in the crystalline polymer. The aggregate of tie molecules that connect the lamellar crystals due to a decrease in the density of the molecular chains in the amorphous region due to impregnation of the supercritical fluid or subcritical fluid, and the separation of the lamellar crystals with a slight stress. As a result, it is estimated that fine fibrils are formed, and as a result, a porous structure is formed.

(Method for producing porous body (II))
In the method for producing the porous body (II), a polymer material containing a crystalline polymer is treated in a supercritical fluid or a subcritical fluid, and the temperature is (Tm−30) ° C. or higher and (Tm + 5) ° C. or lower. This is a method of stretching at a temperature. (Tm is the melting point of the crystalline polymer.)
In order to produce the porous body (II), it is preferably stretched at (Tm−30) ° C. or more and (Tm + 5) ° C. or less in a supercritical or subcritical fluid. When the stretching temperature is not higher than (Tm-30) ° C., not the supercritical state or the subcritical state, the plasticizing effect of the molecular chain of the crystalline polymer to be used may be insufficient. Preferably, it is (Tm-25) ° C. or higher. Further, when the stretching temperature is (Tm + 5) ° C. or higher, melting of the stretched crystalline polymer chain may be preferential. Preferably it is Tm or less.

  The pressure when the crystalline polymer is treated by exposing the polymer material to a supercritical fluid or a subcritical fluid (hereinafter referred to as treatment pressure) is preferably 2 MPa or more, and more preferably 3 MPa or more. If the treatment pressure is less than 2 MPa, the supercritical fluid or subcritical fluid may not sufficiently penetrate into the polymer material. The upper limit of pressure is defined only by the pressure resistance condition of the device.

  The treatment time (hereinafter referred to as treatment time) by exposing the polymer material to the supercritical fluid or subcritical fluid is preferably 10 seconds to 5 hours, more preferably 1 minute to 1 hour. If the treatment time is less than 10 seconds, the supercritical fluid or subcritical fluid may not sufficiently penetrate into the polymer material. Moreover, even if it processes for 5 hours or more, there is hardly any change in the permeation state of the supercritical fluid or the subcritical fluid, which tends to be disadvantageous in terms of productivity and cost of the porous body.

According to the production method of the present invention, either a polymer material containing a crystalline polymer having a low average molecular weight or a polymer material containing a crystalline polymer having a high molecular weight such as an average molecular weight of 100 × 10 ⁴ or more. Even if it exists, porous body (II) can be obtained. The average molecular weight of the crystalline polymer is preferably 700 × 10 ⁴ or less, and more preferably 100 × 10 ⁴ or less capable of melt shaping. The lower limit is not particularly limited, but is usually 1 × 10 ⁴ or more. When the average molecular weight of the crystalline polymer exceeds 700 × 10 ⁴ , it may be difficult to form a thin plate. When the average molecular weight of the crystalline polymer is less than 1 × 10 ⁴ , the strength is insufficient. There is a risk.
In the case of a crystalline polymer that can be melt-shaped, there is a melt flow rate (MFR) (JIS K7120) as a measure for changing the molecular weight. In order to obtain the porous body (II) using polyethylene, the MFR is preferably 0.05 or more, and more preferably 0.1 or more. The upper limit is not particularly limited, but is usually 10 or less.

The shape of the polymer material (precursor) before performing the stretching operation is not particularly limited, and examples thereof include a film shape, a sheet shape, and a hollow shape. Moreover, as a shaping method of a precursor, well-known methods, such as sintering shaping | molding, melt shaping, and wet shaping, can be used.
In the production method of the present invention, an entrainer (addition solvent) may be used in either the method (i) or the method (ii).

  Although the porous mechanism of the porous body (II) is not clear, it is preferable to stretch the crystalline polymer in a supercritical fluid or subcritical fluid as described above in order to produce the porous body. . In the methods described in Patent Literatures 1 to 3, the crystalline lamellae are separated by stretching, and the amorphous chains stretched between the separated lamellas form fibrils, so that the porous structure has a structure in which thin plates are laminated. It does not become a body. The structure formation mechanism of porous body (II) with a structure in which thin plates are laminated is not clear, but it was plasticized by supercritical fluid or subcritical fluid when stretched in supercritical fluid or subcritical fluid It is presumed that the molecular chains are oriented in the stretching direction, and a folded crystal grows around the oriented molecular chains. Accordingly, the thin plate is presumed to be a lamellar crystal of a crystalline polymer. Such a structure is similar to the shish kebab structure, but the conventional shish kebab structure differs from the porous body (II) of the present invention in that the interval between the lamellar crystals is very narrow, while the adjacent shish kebab structure is Since they are not connected to each other and are largely separated from each other, they cannot substantially become porous bodies. Also, the conventional method for forming the shish kebab structure is made from a dilute solution of crystalline polymer. For example, in the case of polyethylene, the Shishikabab structure can be obtained by dissolving polyethylene in a hot xylene solution at about 0.1% by mass and cooling it while rotating and stirring it (edited by “Polymer Chemistry”, edited by the Society of Polymer Science, Basic 2nd edition ", published by Tokyo Kagaku Doujin, 1994). However, with this method, only a small amount of polyethylene having a shish kebab structure can be obtained, and it cannot be used industrially. On the other hand, if the production method of the present invention is used, it is possible to form a shishikabab structure in polyethylene shaped into a fiber shape, a hollow shape, or a film shape, which can be sufficiently utilized industrially. .

Hereinafter, the present invention will be described in more detail with reference to examples. The present invention is not limited to these examples.
The evaluation method in the present example is as follows.

<Observation of porous material>
The surface of the porous body or the cross section of the porous body was observed using a field emission scanning electron microscope JSM-7400F manufactured by JEOL Ltd. (SEM observation). The cross section of the porous body was formed by cleaving the porous body in liquid nitrogen.

<X-ray measurement of porous body>
The small-angle X-ray scattering measurement of the porous body was performed by using Rigaku Co., Ltd., ultra18 and imaging plate, with the sample stretching direction fixed vertically, camera length 1180 mm, output 40 kV-200 mA, and exposure time 2 hours. went. Next, as described in Masao Tsunodo, Masami Kasai, “Polymer X-ray Diffraction”, Maruzen Co., Ltd., published in 1968, Bragg's formula was applied based on long-period diffraction, The pore size of the porous structure was calculated.

<Measurement of tensile strength of porous material>
The tensile strength of the porous body was measured using a tensile tester UCT-500 manufactured by Orientec Co., Ltd. at a chuck distance of 20 mm and a pulling speed of 20 mm / min, or a chuck distance of 5 mm and a pulling speed of 5 mm / min. .

<Thickness D of porous thin plate>
The thickness D of the thin plate was obtained by image analysis of SEM photographs using Image-Pro Plus ver4.5.0.24 manufactured by Planetron Co., Ltd. After adjusting so that the scale of the image analysis apparatus matches the scale of the SEM photograph, the thin plate in the porous body is arbitrarily selected from 10 positions in the SEM photograph, the thickness is obtained, and the average value is obtained as the porous body. The thickness D of the thin plate that constitutes

<Porosity of porous body>
The porosity of the porous body was determined by image analysis of SEM photographs using Image-Pro Plus ver4.5.0.24 manufactured by Planetron Co., Ltd. For image analysis, a place where a scale bar or the like is not displayed from an SEM photograph and a good image quality is cut out into a rectangle, then the median is executed once from the spatial filter, and the photograph is slightly blurred and then brightened. By adjusting the contrast, gamma value, and adjusting so that the thin plate area is white and the pore area is black, the black area (ie the area of the pore area) relative to the total area of the photograph (ie the area of the porous body) The area ratio was determined and expressed as a percentage to determine the hole area ratio.

[Reference Example 1]
As the crystalline polymer, Mitsui Chemicals, High Density Polyethylene, Hi-Zex 2200J (melt flow rate = 5.2, JIS K7120: catalog value) (melting point = 134 ° C., ASTM D2117: catalog value) was used. This high-density polyethylene was hot-pressed at 180 ° C. to produce a film having a thickness of 180 μm. A strip sample having a width of 5 mm and a length of 40 mm was cut from the film.
A weight was fixed to one end of the strip-shaped sample so as to have a tensile stress of 1 MPa, and this was placed in a pressure resistant container, and the inside of the container was replaced with carbon dioxide. The strip-like sample was stretched in supercritical carbon dioxide by processing for 1 hour at a temperature of 130 ° C. and a pressure of 10 MPa inside the container. After completion of the treatment, the inside of the container was reduced to atmospheric pressure, and a film stretched 5 times was collected. The surface SEM observation photograph of the obtained film is shown in FIG. From this photograph, it can be seen that a porous body having a pore diameter of at least submicron or more is obtained.

[Example 1]
As a crystalline polymer, manufactured by Mitsui Chemicals, ultra high molecular weight polyethylene, Hi-Zex Million 630M (viscosity average molecular weight = 590 × 10 ⁴ , ASTM D4020: catalog value) (melting point = 136 ° C., ASTM D2117: catalog value) Using. This ultrahigh molecular weight polyethylene was hot-pressed at 220 ° C. to produce a film having a thickness of 180 μm. A strip sample having a width of 5 mm and a length of 40 mm was cut from the film.
A weight was fixed to one end of the strip-shaped sample so as to have a tensile stress of 1 MPa, and this was placed in a pressure resistant container, and the inside of the container was replaced with carbon dioxide. The strip-shaped sample was stretched in supercritical carbon dioxide by treating the inside of the container at 140 ° C. and a pressure of 15 MPa for 1 hour. After completion of the treatment, the inside of the container was reduced to atmospheric pressure, and the film stretched twice was collected. The surface SEM observation photograph of the obtained film is shown in FIG. From this photograph, it can be seen that a nanoporous body having a pore diameter of at least several tens of nm is obtained, which corresponds to the small-angle X-ray scattering measurement result shown in FIG. Further, the tensile strength of the obtained film was measured at a test length of 10 mm. As a result, the breaking strength was 122 MPa, which was a porous film having sufficient strength.

[Example 2]
The same film as in Example 1 was used, and the film was stretched in the same manner as in Example 1 except that the treatment temperature with supercritical carbon dioxide was 155 ° C., and the film stretched 5 times was collected. The surface SEM observation photograph of the obtained film is shown in FIG. From this photograph, it can be seen that a nanoporous material having a pore diameter of at least several tens of nm is obtained, which corresponds to the small-angle X-ray scattering measurement result shown in FIG. Further, the tensile strength of the obtained film was measured at a test length of 10 mm. As a result, the breaking strength was 130 MPa, which was a porous film having sufficient strength.

[Comparative Example 1]
Using the same film as in Example 1, a strip-shaped sample having a width of 5 mm and a length of 40 mm was cut from the film. A weight was fixed to one end of this strip-shaped sample so as to have a tensile stress of 1 MPa, and stretching was attempted by air heating at 140 ° C. and 155 ° C. under atmospheric pressure. However, the film could not be stretched because it was broken.

[Example 3]
An apparatus shown in FIG. 6 was prepared. In this apparatus, a sapphire glass 1 is attached to the upper and lower surfaces, a pressure vessel 4 provided with a carbon dioxide introduction tube 2 and a carbon dioxide discharge tube 3, a heater 5 embedded in the side wall of the pressure vessel 4, and a sample 6 A pair of chucks 7 for fixing the inside of the pressure vessel 4, and a shaft 8 having one of the chucks 7 fixed to the tip and slidable in the axial direction by a motor (not shown) provided outside the pressure vessel 4. It is comprised roughly and is comprised.

  Using the same film as in Example 1, a strip-shaped sample having a width of 4 mm and a length of 20 mm was cut from the film. This strip sample was subjected to supercritical carbon dioxide treatment and stretching in supercritical carbon dioxide using the apparatus shown in FIG. A strip-shaped sample cut out is attached to this apparatus (test length: 5 mm), and supercritical carbon dioxide treatment is performed for 15 minutes under the conditions of a temperature of 155 ° C. and a pressure of 15 MPa. The film was stretched 4 times at 2 mm / min. The surface SEM observation photograph of the obtained film is shown in FIG. From this photograph, it can be seen that a porous body having a pore diameter of at least submicron or more is obtained. Further, the tensile strength of the obtained film was measured at a test length of 20 mm. As a result, the breaking strength was 34.3 MPa, and it was a porous film having sufficient strength.

[Example 4]
A supercritical carbon dioxide treatment and stretching in supercritical carbon dioxide were performed in the same manner as in Example 3 except that the same film as in Example 1 was used and the stretching ratio in supercritical carbon dioxide was set to 2 times. The surface SEM observation photograph of the obtained film is shown in FIG. From this photograph, it can be seen that a porous body having a pore diameter of at least submicron or more is obtained. Further, the tensile strength of the obtained film was measured at a test length of 5 mm. As a result, the breaking strength was 28.9 MPa, and it was a porous film having sufficient strength.

[Example 5]
Using the same film as in Reference Example 1, the supercritical carbon dioxide treatment conditions were temperature: 120 ° C., pressure: 10 MPa, treatment time: 15 minutes, and the stretching conditions in supercritical carbon dioxide were the stretching speed of 30 m / min. The procedure was the same as in Example 3 except that. The surface SEM observation photograph of the obtained film is shown in FIG. From this photograph, it can be seen that a porous body having a pore size of nanometer order is obtained. Further, as a result of image analysis of this photograph, it was found that a porous body having a thickness of the thin plate D of 21 nm and a porosity of 43% was obtained.

  The porous body of the present invention is useful for water treatment membranes, filters, adsorbents, nonwoven fabric abrasives, functionality and the like. Furthermore, the porous body of the present invention is a water treatment membrane for filtering liquid of nanometer order, a filter, a non-woven polishing material having fine irregularities, and a porous material for impregnating a functional material with voids. It is also useful for substrates. In addition, the method for producing a porous body of the present invention can make a porous material, even a polymer material having a high molecular weight, which cannot be made porous by stretching by conventional heating, and can be made into a water treatment membrane, filter, adsorbent. It is possible to easily produce a porous body useful for a nonwoven fabric abrasive or the like.

3 is a surface SEM observation photograph of the porous body obtained in Reference Example 1. 3 is a surface SEM observation photograph of the porous body obtained in Example 1. 2 is a small-angle X-ray scattering image of the porous body obtained in Example 1. 3 is a surface SEM observation photograph of the porous body obtained in Example 2. FIG. 3 is a small-angle X-ray scattering image of the porous body obtained in Example 2. It is the schematic of the apparatus which can be extended | stretched in the supercritical carbon dioxide process and supercritical carbon dioxide used in Examples 3-5. 4 is a surface SEM observation photograph of the porous body obtained in Example 3. 4 is a surface SEM observation photograph of the porous body obtained in Example 4. 6 is a surface SEM observation photograph of a porous body obtained in Example 5.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Sapphire glass 2 Carbon dioxide introduction tube 3 Carbon dioxide discharge tube 4 Pressure-resistant container 5 Heater 6 Sample 7 Chuck 8 Shaft

Claims (4)

A method for producing a porous body, in which a crystalline polyolefin having a viscosity average molecular weight of 100 × 10 ⁴ or more is treated with a supercritical fluid or a subcritical fluid and stretched,
The method for producing a porous body, wherein the crystalline polyolefin has a film shape, a sheet shape, or a hollow shape. The method for producing a porous body according to claim 1, wherein the crystalline polyolefin is stretched in a supercritical fluid or a subcritical fluid. The stretching of the crystalline polyolefin, of a crystalline polyolefin (melting point -30) ° C. or higher, carried out at (melting point +5) ° C. below the temperature of the crystalline polyolefin, the production method of the porous body according to claim 1 or 2.  The method for producing a porous body according to any one of claims 1 to 3, wherein a main component of the supercritical fluid or subcritical fluid is carbon dioxide.

Images