JPH02144132A - Porous polyolefin film - Google Patents

Abstract

PURPOSE:To enhance reliability with respect to safety and hygiene by using a specified blended polymer and forming a porous structure in which spaces defined with lamellae and many fibrils connecting the lamellae communicate with each other all over the structure. CONSTITUTION:A blended polymer consisting of 95-40wt.% polyolefin and 5-60wt.% hydrophilic polyolefin is melted and extruded and the resulting unstretched film is stretched to obtain a porous polyolefin film. Since this film has a structure obtd. by splitting folded molecules between lamellae of the unstretched film into fibrils by stretching, spaces are left around many fibrils connecting the lamellae arranged in the longitudinal direction of the film and the resulting structure ranges from one side of the film to the other side. Since a solvent and additives are not used in production, the porous polyolefin film has high reliability with respect to safety and hygiene and also has superior strength characteristics.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a hydrophilic porous film suitable for medical and industrial filtration, separation, etc.

(Prior Art) Separation using polymer membranes has been widely used in the past, and various materials have been developed for polymer membranes.

Among them, a crystalline thermoplastic polymer is melt-shaped into a film, stretched at a relatively low temperature to generate crazes in the amorphous regions between crystalline lamellae, and preferably further hot-stretched to form a film. Products that have a porous structure throughout do not require additives or solvents to form micropores, making them suitable for filtration and applications where impurities or compounds should not be leached into the system.
It is attracting attention as a porous film for separation.

Such films are described in USP 3,679,538,
It is disclosed in Japanese Patent Publication No. 55-32531.

Such films are made only of polyolefins such as polypropylene and polyethylene, and as such materials are inherently hydrophobic, they require extremely large pressures to filter aqueous liquids such as aqueous solutions, making them impractical. . Therefore, when such a film is used for filtration of aqueous liquids, it is first made hydrophilic with alcohol or a surfactant before use.

Further, as a method of making a hydrophobic film hydrophilic, the hydrophobic film is coated with a hydrophilic organic hydrocarbon monomer such as acrylic acid, methacrylic acid, or vinyl acetate, and then irradiated with ionizing radiation of about 1 to 10 megarads. A method of chemical fixation was disclosed in Japanese Patent Application Laid-open No. 5
6-3+! It is disclosed in Japanese Patent No. 1333.

On the other hand, as an application of blending technology, two different polymers are blended and melt-spun, and then stretched to cleave the interface between the different polymers to create continuous micropores that communicate from the outer wall surface to the inner wall surface of the hollow fiber peripheral wall. Hydrolysis of side chain groups present in the constituent polymers to form a microporous hollow fiber with a heterogeneous micropore wall surface in which at least a portion of the wall surface is composed of different polymers with different properties. , a method for producing hydrophilic porous hollow fibers whose pore surfaces are made hydrophilic by post-treatment such as sulfonation is disclosed in JP-A-55-13.
It is disclosed in Japanese Patent No. 7208.

(Problems to be solved by the invention) Porous membranes can be used for medical purposes such as plasma separation, infusion filtration, plasma protein separation, and the production of sterile water, and for industrial purposes such as the production of IC cleaning water, food processing water, etc. It is used to purify water for other processes, and in recent years, it has been widely used to purify water for domestic use, restaurants, etc. All of these applications are water-based processes, and the elution of foreign substances from the porous membrane material is undesirable because it can lead to safety issues and deterioration of the quality of purified water or aqueous solutions. It is requested.

On the other hand, films made by melt-forming polyolefin polymers and making them porous by stretching them are excellent because they do not use extraction additives or solvents, so there is no need to worry about elution when using the membrane. Treatment with alcohol or surfactant is temporary hydrophilization, and if it is used for filtration etc. after hydrophilization treatment, the alcohol and surfactant will migrate to the purified water side and cause this. Before filtration, it is necessary to thoroughly wash and remove these hydrophilizing agents as they may contaminate the membrane.Moreover, if the membrane surface is dried under such conditions, the membrane surface will return to hydrophobicity, so after hydrophilizing, it must be replaced with water. The problem is that it must be kept in contact with water.

Furthermore, the method described in JP-A No. 56-313333 achieves permanent hydrophilicity because the group expressing hydrophilicity is chemically fixed, but it requires irradiation with ionizing radiation. Therefore, large-scale equipment is required, and the process stability is not sufficient (there is also the risk of damaging the membrane material, making it difficult to operate and manage the treatment process).

In addition, the fiber described in JP-A-55-137208, which is made porous by melt-spinning and drawing a blend of different polymers, generates crazes in the amorphous part between the lamellae and fibrillates it. Rather, it cleaves the interface between different polymers, and therefore the pore inner surface area is smaller (and the pore diameter This method is affected by the blending state of different polymers and uses the lack of affinity between different polymers to create pores, so it is relatively difficult to create fine blends and has the problem of large variations in pore size. In addition, post-treatments such as hydrolysis and sulfonation are required to make them hydrophilic, which also poses the problem of complicating the process.

(Means for solving the problem) As a result of intensive study in view of the above situation, the present inventors found that
By using polyolefin, we generate crazes in the amorphous parts between the lamellae using a melt-forming and stretching process, and then fibrillate the resulting porous film, making it porous. As a result of extensive research into a porous film that has permanent hydrophilicity suitable for processing and can be produced by an industrially advantageous method, the present invention was arrived at.

That is, the gist of the present invention is that 95 to 40% by weight of polyolefin
A porous polyolefin film made of a blend polymer of and 5 to 60% by weight of a hydrophilic polyolefin, the film being surrounded entirely from one surface to the other by lamellae and a large number of fibrils connecting the lamellae. The porous polyolefin film is characterized by having a porous structure formed by communicating spaces.

Polyolefins used in the present invention include polyethylene, polypropylene, poly3-methylbutene-1
, poly-4-methylpentene-1, and the like. or,
In the present invention, the hydrophilic polyolefin blended with the polyolefin is preferably a polyolefin modified so that the contact angle with water is 80° or less, and preferably 70° or less, when measured by making a film of the hydrophilic polyolefin. More preferably, it is a polyolefin modified in the following manner. Examples of such modified polymers include those in which hydroxyl groups, carboxyl groups, amino groups, sulfonic acid groups, polyoxyethylene groups, etc. are bonded to the molecular chains of various polyolefins, including ethylene and vinyl alcohol. Examples include copolymers of ethylene and vinyl acetate, copolymers of ethylene and maleic anhydride, copolymers of ethylene and polyoxyethylene chemically bonded, and metal ion crosslinked polyolefins.

Here, the reason why we limited the polymers to be blended with polyolefins to hydrophilic polyolefins is that when the above-mentioned polyolefins are blended with polyolefins, good affinity is obtained between the two. It does not significantly inhibit the formation of a highly oriented, highly crystalline lamellar structure in the unstretched film obtained by shaping, and further,
Since peeling at the blend polymer interface is difficult to occur, it is possible to obtain an excellent porous membrane structure similar to that obtained when using a polyolefin single polymer.Moreover, it has a hydrophilic group, so permanent hydrophilicity is obtained, and water It has the excellent feature that it can easily pass through moisture and moisture, and since the pores are the spaces around the fibrils, even if one spot becomes clogged, it can be easily bypassed, so there is virtually no clogging. . The mixing ratio of this polyolefin and the hydrophilic polyolefin needs to be 95 to 40% by weight of the polyolefin and 5 to 60% by weight of the hydrophilic polyolefin. This is because when a highly hydrophilic polyolefin is used, it can exhibit hydrophilicity with a relatively small amount of blending, and conversely, when such a hydrophilic polyolefin is blended in a large amount, it can cause the formation of lamellar crystals. This makes it difficult to obtain a good porous structure, and if the hydrophilicity is relatively low and has more of the characteristics of polyolefins, on the other hand, it is necessary to It is necessary to blend them in a targeted amount, and even if they are blended in a large amount, the formation of lamellar crystals will not be inhibited. The mixing ratio of polyolefin and hydrophilic polyolefin is preferably 90 to 50% by weight of polyolefin and 10 to 50% by weight of hydrophilic polyolefin. If the ratio of polyolefin is less than the above-mentioned lower limit, it is difficult to obtain a sufficiently uniform porous structure, which is undesirable, and if it exceeds the above-mentioned upper limit, hydrophilicity becomes insufficient, which is not preferred.

The porous structure of the porous film of the present invention is obtained by stretching an unstretched film obtained by melt-extruding a crystalline polymer, stretching the folded molecules between the lamellae of the unstretched film, and cleaving it into fibrils. Because it has a structure obtained by using lamellae, there is a space around the many fibrils arranged in the longitudinal direction of the film that connect the lamellae, and this structure is connected from one surface of the film to the other. be.

Below, the method for manufacturing a porous film of the present invention will be explained.

First, the above-mentioned polyolefin and hydrophilic polyolefin are blended, but this blend must be sufficiently uniform, and the above-mentioned polymers may be blended in advance using a blender such as a V-type blender, or melt-blended using a melt extruder. It is preferable that the pelletized material is then passed through an extruder for film production.

Next, this blended polymer is molded into an unstretched film using a conventional film extruder. As the film extruder, either a T-type die or a double tube die can be used. When a double-tube die is used, a cylindrical film is obtained, which can be subjected to the subsequent stretching process in its cylindrical form, and there is no need to cut it into a flat film during the manufacturing process. Furthermore, by adjusting the amount of internal air blown during film molding, it is possible to mold a film with a thickness and width that suit the purpose.

The extrusion temperature suitable for stably obtaining an unstretched film for obtaining the porous film of the present invention is determined by the balance between the type of polymer used, melt index, discharge rate adopted, cooling conditions, winding speed, etc. The thickness and width of the desired film can be set appropriately within a stable range (usually, the melting point of the polymer with the higher melting point (hereinafter referred to as m) among the polymers to be blended is set as appropriate. ) from 2
Molding may be carried out at a temperature that is 0° C. or higher and does not exceed a temperature that is 100° C. higher than the melting point (m,). If molding is performed at a temperature lower than the lower limit of this temperature range, the resulting unstretched film will be highly oriented, but the maximum stretching ratio will be low when making the film porous by stretching in the later stretching step, and the maximum stretching ratio will be low. This is not preferable because it becomes difficult to obtain sufficient porosity. Conversely, spinning at a temperature exceeding the upper limit of the above temperature range is also not preferred, since it is difficult to obtain a high porosity.

Polymer extruded at appropriate extrusion temperature is 1-5000
It is preferable to take it in at a draft of 10 to 2000, more preferably a draft of 10 to 2000. It is preferable to rapidly cool the film immediately after the die so that the film can be stably drawn off by the first roller it comes into contact with after exiting the die, and it is preferable to use an air knife or other cooling device.

The unstretched film obtained in this way may be stretched as is, but in order to enhance oriented crystallization, the unstretched film may be stretched at a constant length or at a temperature below mph and within a range that does not substantially damage the structure of the unstretched yarn. The film may be annealed in a relaxed state and then stretched.

Stretching is m, -80°C or lower, and m knee is 220°C or higher,
It is preferable to carry out a combination of cold stretching at mpH-110°C to mpH-90°C and then hot stretching at mpo-60°C to mp14-5°C. The hot stretching may be a multi-stage stretching of two or more stages. That is, in a highly oriented crystalline unstretched film, exfoliation of the crystal interface occurs first through cold stretching, and then develops into a micro-boarded laminated structure through thermoplastic stretching in the subsequent hot stretching step.
How uniformly microcracking can occur during the initial cold stretching is a major technical point in ensuring product homogeneity and process stability.

From this point of view, it is preferable that the stretching section be 50 mm or less. If the hot stretching temperature is higher than the above upper limit, the film becomes transparent and the desired porous structure cannot be obtained.If the hot stretching temperature is lower than the above lower limit, the lower the temperature, the lower the porosity. Therefore, it is not desirable.

The magnification of cold stretching and hot stretching depends on the porosity of the porous film, etc.
Although it may be set appropriately according to the desired quality performance, the stretching ratio in cold stretching is preferably 5 to 100%, and the stretching ratio in hot stretching is such that the total stretching amount of cold stretching and hot stretching is 150%. It is preferable to set it to 700%. If the total amount of stretching exceeds 700%, the film will be frequently cut during stretching, which is not preferable. The porous polyolefin film obtained in this way is almost stable in shape by hot stretching, but if necessary, the porous polyolefin film can be
Heat setting may be performed at a temperature of 1-60° C. to 5° C. under tension or under relaxed conditions.

According to the studies of the present inventors, the desired quality performance of the porous film, such as porosity and rejection rate in filtration, can be appropriately achieved by changing the temperature and magnification of the cold stretching and hot stretching. Example) The present invention will be further explained using examples below. In the examples, the crystallinity of the blend polymer was calculated by integrating the diffraction intensity in all directions using a wide-angle X-ray diffraction device and using the following formula. Ta.

Crystallinity χ. = (integral value of total diffraction intensity - integral value of diffraction intensity of amorphous portion)/integral value of total diffraction intensity Also, the degree of crystal alignment can be determined using a wide-angle X-ray diffractometer using the film MD of the diffraction intensity of the (110) plane. The half value of the distribution in the direction was determined using the following formula.

Crystal order degree = (H(2,.I/(180-Hll,.I
)X100(%) However, Hu + u: Half value of (110) plane. Also, the contact angle is determined by making a non-porous film of hydrophilic polyolefin alone and using a Kyowa contact angle meter (Kyowa Scientific ■).
The measurements were made using a known method.

Example 1 Density 0, 964 g/cm' Melt index 0
．． 35 high-density polyethylene (HIZEX 5202B manufactured by Mitsui Petrochemical ■) and a copolymer of polyethylene and acrylic acid crosslinked with zinc ions (Himilan-1702 manufactured by Mitsui Polychemical ■), the contact angle measured as a film was 69° ) in a V-type blender at a ratio of 1:1, dried, and heated to 180°C using a double tube structure die with an outlet diameter of 60mm and an annular slit width of 1mm.
Melt extrusion was performed at °C. Air is blown into the molded tubular film so that its diameter is equal to the diameter of the die, and cooling air at a temperature of 25°C is blown over the entire outer wall of the tubular film at a position 10 cm above the die to cool it.
It was taken off at a speed of 27 m/min by nip rolls at a position of m. The draft at this time was 213. This unstretched film was annealed by passing it in a fixed length over rollers heated to 113° C. in its tubular form.

The degree of crystallinity of this unstretched film was 65%, and the degree of crystal orientation was 77%. Furthermore, this annealed film was cold-stretched by 35% between nip rollers maintained at 25°C. The roller interval at this time was 35 mm. Following this cold stretching, hot stretching was performed on a heated surface heated to 110°C until the total stretching amount reached 400%, and further,
The porous film was continuously produced by performing long-leg heat setting for 20 seconds on a heating surface heated to 115°C.

The resulting porous film had a wall thickness of 23 μm and a porosity of 67%.
The water permeability pressure (water pressure when water permeates from one surface of the film to the other) was 0.9 kg/cm''.

Example 2 High-density polyethylene with a density of 0, 964 g/c+n'' and a melt index of 035 and a crosslinked product of a copolymer of polyethylene and acrylic acid using zinc ions were blended in a V-type blender at a ratio of 1=1 and dried. Instead of a material, we used high-density polyethylene with a density of 0, 964 g/cm" and a melt index of 0.3 (HIZEX 5200S, manufactured by Mitsui Petrochemicals) and an ethylene-vinyl alcohol copolymer (Soarnol A, manufactured by Nippon Gosei Chemicals, made into a film. The procedure was the same as in Example 1, except that the samples (with a measured contact angle of 56°) were blended in an extruder at a ratio of 7:3 and dried. The crystallinity of the unstretched film after heat treatment is 7.
0%, and the degree of crystal alignment was 80%.

The porous film obtained after stretching and heat setting has a wall thickness of 21
μm, porosity 64%, and water permeability pressure 1.3 kg/cm”.

Comparative Example 1 Example 1 except that the same high-density polyethylene was used alone instead of the blend of high-density polyethylene with a density of 0.964 g/cm'' and a crosslinked product of a copolymer of polyethylene and acrylic acid with zinc ions. A porous film was obtained in the same manner as above.The obtained porous film had a wall thickness of 23 μm,
The porosity was 70% and the water permeability pressure was 5.3 kg/c+n''.

(Effects of the Invention) According to the method of the present invention, no extracting additives or solvents are used in producing the porous film, so it is highly reliable in terms of safety and hygiene.Moreover, the porous structure of the film It has excellent strength characteristics because it is a continuous space surrounded by lamellae and a large number of fibrils connecting the lamellae from one surface to the other surface, and even if a part becomes clogged, it can be bypassed. Because it has a structure in which an infinite number of baths can be formed by passing from one surface to the other, it is characterized by less clogging, and is durable even without any other special hydrophilic treatment using only melt spinning and stretching. Due to its hydrophilic properties, it is an excellent porous film that can be advantageously used in a variety of applications including medical and food industries.

Patent applicant: Mitsubishi Rayon Co., Ltd.

Claims (1)

[Claims] 1) A porous polyolefin film made of a blend polymer of 95 to 40% by weight of polyolefin and 5 to 60% by weight of hydrophilic polyolefin, in which lamella-to-lamellae are formed throughout from one surface to the other surface of the film. A porous polyolefin film characterized by having a porous structure in which spaces surrounded by a large number of connecting fibrils are connected to each other.