WO2025115627A1 - Air filter filtration material - Google Patents

Abstract

This air filter filtration material comprises a plurality of layers of nonwoven fabric and has a performance index of at least 0.023. At least one of the layers of nonwoven fabric contains at least two kinds of organic fibers A and B having different fiber diameters. The average fiber diameter of the organic fibers A is 0.01-0.60 μm, and the average fiber diameter of the organic fibers B is 1.0-30 μm.　Provided is an air filter filtration material suitable for an air filter having both high collection efficiency and low pressure loss.

Description

Air filter media

The present invention relates to an air filter medium suitable for air filters.

In recent years, there has been an increasing demand for purifying spaces, and air filters that remove fine dust from the air are being used in a wide range of fields, from living environments to industry, to address health problems caused by dust particles with a particle size of 2.5 μm or less, and to eliminate dust in the manufacturing of semiconductors and pharmaceuticals. In particular, in spaces that require extremely clean air, such as clean rooms and semiconductor manufacturing equipment, high-performance air filters such as HEPA filters (High Efficiency Particulate Air filters) and ULPA filters (Ultra Low Penetration Air filters) are used, and the air filter media for HEPA and ULPA filters are made of nonwoven glass fiber fabric or porous membranes made of PTFE (polytetrafluoroethylene).

However, air filters that use air filter media made of glass fiber or PTFE are both disposed of in landfills after use, which poses a large environmental burden. Furthermore, ULPA filters are used in clean rooms and semiconductor manufacturing equipment in semiconductor manufacturing, but since the use of glass fiber filter media results in the emission of boron, which has a negative effect on semiconductor manufacturing, PTFE filter media is generally selected and used. However, PFOA (perfluorooctanoic acid), a by-product of PTFE manufacturing, is an environmentally destructive substance that is difficult to decompose in nature, and is also toxic as it accumulates in living organisms, creating issues such as the need for alternative materials.

As a result of the recent increase in environmental and compliance awareness, various filter media have been proposed that are made of ultrafine polyester fibers, have a smaller environmental impact, and have excellent filtering performance.

For example, Patent Document 1 proposes a wet-laid nonwoven fabric with a multi-layer structure of two or more layers in which there is no interface between two adjacent layers, using short-cut nanofibers made of a fiber-forming thermoplastic polymer and cut so that the single fiber diameter (D) is 100 to 1,000 nm and the ratio (L/D) of the fiber length (L) to the single fiber diameter (D) is within the range of 100 to 2,500. This proposal claims to be able to provide a filter that has high collection efficiency, low pressure loss, and a long filter life.

Patent Document 2 proposes an air filter medium using a nonwoven fabric containing three types of polyester fibers: nanofibers with a fiber diameter of 200 to 800 nm, fibers thicker than the nanofibers, and binder fibers. According to this proposal, it is possible to achieve low pressure loss and high collection performance with excellent pleating properties and resistance to wind pressure deformation.

JP 2013-126626 A JP 2015-140495 A

The method described in Patent Document 1 can obtain air filter media with excellent high collection efficiency and low pressure loss, but does not achieve the collection efficiency required for HEPA filters or ULPA filters, and is insufficient in terms of achieving both low pressure loss and high collection efficiency.

In addition, the method described in Patent Document 2 can obtain a thin air filter medium with excellent pleatability, and can achieve the collection efficiency required for HEPA and ULPA filters, but the pressure loss is high, and the method is insufficient in terms of achieving both low pressure loss and high collection efficiency.

The object of the present invention is to solve the problems of the above-mentioned conventional technology and to provide an air filter medium suitable for air filters, which has both the high collection efficiency and low pressure loss required for HEPA and ULPA filters, and an air filter using the same.

(1) An air filter medium having a performance index of 0.023 or more, in which at least one of the nonwoven fabrics is laminated, contains at least two types of organic fibers A and B having different fiber diameters, and the organic fibers A have an average fiber diameter of 0.01 to 0.60 μm and the organic fibers B have an average fiber diameter of 1.0 to 30 μm.
(2) The air filter medium according to (1), characterized in that it has a particle collection rate of 99.97% or more for particles having a particle diameter of 0.3 μm.
(3) The air filter medium according to (1), characterized in that it has a particle collection rate of 99.9995% or more for particles having a particle diameter of 0.15 μm.
(4) The air filter medium according to (1), wherein at least one of the laminated nonwoven fabrics has a performance index of 0.023 or more.
(5) The air filter medium according to (1), wherein the nonwoven fabric further contains binder fibers.
(6) An air filter comprising the air filter medium according to any one of (1) to (5).
(7) A fan filter unit equipped with the air filter described in (6).
(8) A clean room or semiconductor manufacturing equipment equipped with the air filter according to (6).

According to the present invention, it is possible to obtain an air filter medium suitable for air filters, which combines the high collection efficiency and low pressure loss required for HEPA and ULPA filters.

The air filter medium of the present invention is an air filter medium that is composed of at least two types of organic fibers, and is characterized in that it is made up of multiple layers of nonwoven fabric containing fiber A with a fiber diameter of less than 0.8 μm and an average fiber diameter of 0.01 to 0.60 μm and fiber B with a fiber diameter of 0.8 μm or more and an average fiber diameter of 1.0 to 30 μm, and has a performance index of 0.023 or more.

[Nonwoven fabric]
First, the nonwoven fabric of the present invention will be described in detail.

The fibers A and B used in the nonwoven fabric of the present invention are organic fibers. The organic fibers of the present invention are fibrous materials mainly composed of organic matter. Specific examples of organic fibers include, but are not limited to, cellulose produced from wood pulp, natural fibers such as cotton, hemp, wool, and silk, regenerated fibers such as rayon, semi-synthetic fibers such as acetate, and synthetic fibers such as polyester, nylon, and acrylic. Among them, synthetic fibers made of thermoplastic polymers are preferable from the viewpoint of mechanical properties and dimensional stability. Specific examples of thermoplastic polymers include polyesters such as polyethylene terephthalate (PET), polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and polylactic acid, polyamides such as polyamide 6, polyamide 66, and polyamide 610, polyolefins such as polyethylene, polypropylene, and polymethylpentene, thermoplastic polymers such as polycarbonate, polyacrylate, polyphenylene sulfide, and thermoplastic polyurethane, and copolymers thereof. Among these, polyesters such as polyethylene terephthalate, polytrimethylene terephthalate, and polybutylene terephthalate, polyamides such as polyamide 6 and polyamide 66, and polyphenylene sulfide are preferred because they combine mechanical properties and heat resistance.

The organic fibers in the present invention may be modified in various ways by adding secondary additives, provided that the effects of the present invention are not impaired. Specific examples of secondary additives include, but are not limited to, compatibilizers, plasticizers, antioxidants, ultraviolet absorbers, infrared absorbers, fluorescent whitening agents, release agents, antibacterial agents, nucleating agents, heat stabilizers, flame retardants, antistatic agents, color inhibitors, regulators, matting agents, defoaming agents, preservatives, gelling agents, latex, fillers, inks, colorants, dyes, pigments, and fragrances. These secondary additives may be used alone or in combination.

The fiber A used in the nonwoven fabric of the present invention has a fiber diameter of less than 0.8 μm and an average fiber diameter of 0.01 to 0.60 μm. The finer the fiber diameter, the higher the specific surface area, which is preferable because it will provide high collection performance when made into a nonwoven fabric, but if the average fiber diameter is 0.01 μm or more, in addition to high collection performance, the nonwoven fabric will have good handleability and moldability during processing, and will have excellent durability when used. It is more preferable that the average fiber diameter is 0.05 μm or more, and even more preferable that it is 0.10 μm or more. On the other hand, if the average fiber diameter is 0.60 μm or less, the effect of the high specific surface area resulting from the fine fiber diameter will provide excellent collection performance when made into a nonwoven fabric. It is more preferable that the average fiber diameter is 0.50 μm or less.

The fiber B used in the nonwoven fabric of the present invention has a fiber diameter of 0.8 μm or more and an average fiber diameter of 1.0 to 30.0 μm. If the average fiber diameter is 1.0 μm or more, it serves as an aggregate in the nonwoven fabric to maintain the shape of the nonwoven fabric, and fiber A is not excessively densified, so that an increase in pressure loss can be suppressed, and a nonwoven fabric that combines high collection performance and low pressure loss can be obtained. The average fiber diameter is more preferably 2.0 μm or more, and even more preferably 3.0 μm or more. On the other hand, if the average fiber diameter is 30.0 μm or less, fiber A, which is thinner than fiber B, is suppressed from falling off during processing of the wet nonwoven fabric described below, and in the obtained nonwoven fabric, it serves as a scaffold for fiber A, and can form a three-dimensionally homogeneous fine space. The average fiber diameter of fiber B is more preferably 28.0 μm or less, and even more preferably 25.0 μm or less.

In the nonwoven fabric of the present invention, various binders such as fibrous or adhesives can be used to physically bond the fibers constituting the nonwoven fabric together by thermal adhesion. The binder in this case is not particularly limited, but for example, a core-sheath fiber in which a thermoplastic polymer with a melting point of 150°C or less is arranged in the sheath can be preferably used. When such a core-sheath fiber is used, after the nonwoven fabric is formed, the sheath component on the surface of the binder fiber is melted and bonded to other fibers constituting the nonwoven fabric by going through a drying process such as a Yankee dryer or an air-through dryer, or a heat treatment process such as a calendar, which is preferable because it is possible to increase the rigidity of the nonwoven fabric. Furthermore, it is preferable because the core component of the binder fiber can contribute to ensuring the strength of the nonwoven fabric. In addition, if the melting point of the core component of the binder fiber is higher than the melting point of the sheath component and the difference between the melting points is 20°C or more, the sheath component on the surface of the binder fiber is easily melted sufficiently, and the decrease in the orientation of the core component is suppressed, and sufficient thermal adhesion and high rigidity can be achieved at the same time, which is preferable. In addition, by making fiber B into an undrawn yarn with a crystallinity of 20% or less, it can be used as a binder. By softening and fluidizing it through heat treatment, it is possible to firmly and uniformly bond the fibers together.

In the nonwoven fabric of the present invention, the blending ratio (weight ratio) of fibers A and B can be calculated by dividing the weight of fiber A or B by the total weight of fibers A and B. When binder fibers are used in addition to fibers A and B, the weight of fiber A or B can be calculated by dividing the weight of fiber A or B by the total weight of fibers A, B and the binder. As a guideline for the blending ratio of fibers A and B, the blending ratio of fiber A is preferably 2 to 55% by weight, and the blending ratio of fiber B is preferably 5 to 90% by weight. If the blending ratio of fiber A is 2.5% by weight or more, it is preferable because excellent collection performance is exhibited when the nonwoven fabric is made due to the effect of the high specific surface area derived from the fine fiber diameter of fiber A. The blending ratio of fiber A is more preferably 3% by weight or more, and even more preferably 5% by weight or more. On the other hand, if the blending ratio of fiber A is 55% by weight or less, excessive densification of fiber A in the nonwoven fabric is suppressed, and an increase in pressure loss can be suppressed, and a nonwoven fabric that combines high collection performance and low pressure loss can be obtained, so this is preferable, and 50% by weight or less is even more preferable. Also, if the blending ratio of fiber B is 5% by weight or more, it can function as an aggregate for maintaining the shape of the nonwoven fabric, and fiber A is not excessively densified, so an increase in pressure loss can be suppressed, and fiber A, which is thinner than fiber B, can fall off during processing of the wet nonwoven fabric described below, so this is preferable. The blending ratio of fiber B is more preferably 7% by weight or more, and even more preferably 10% by weight or more.

The basis weight of the nonwoven fabric in the present invention is preferably 3 to 100 g/ ^m2 . The basis weight of the nonwoven fabric in the present invention refers to a value measured by the method described in the Examples section. If the basis weight is 3 g/ ^m2 or more, it is preferable because a uniform nonwoven fabric with little density difference and excellent durability during use can be obtained. The basis weight is more preferably 5 g/m2 or ^more , and even more preferably 7 g/m2 or ^more . On the other hand, if the basis weight is 100 g/ ^m2 or less, it is preferable because the handling property and molding processability during nonwoven fabric processing are good. The basis weight is more preferably 80 g/m2 or ^less , and even more preferably 70 g/m2 or ^less .

[Collection efficiency, pressure loss]
The performance index is an index for evaluating the performance of a filter and indicates the potential performance of the filter. Specifically, it can be calculated using the following formula from the filter collection efficiency and pressure loss, which will be described later.
Performance index=−ln[{1−capture efficiency (%)/100)}/pressure loss (Pa)]
The higher the figure of merit, the higher the performance of the filter is evaluated, and the figure of merit is an important index for comprehensively evaluating the performance of a filter.

The collection efficiency is an index that indicates how many particles a nonwoven fabric or air filter medium can capture. For example, the number of particles supplied to the nonwoven fabric or air filter medium (upstream particle number) and the number of particles that have passed through the nonwoven fabric or air filter medium (downstream particle number) can be measured using a particle counter or the like, and the particle transmittance can be calculated. The collection efficiency can then be calculated from the particle transmittance using the following formula:
Particle transmittance [%] = (number of downstream particles/number of upstream particles) x 100
Collection efficiency [%] = 100 - particle transmission rate [%]
In the JIS standard (JIS Z 8122), HEPA is stipulated to have a collection efficiency of 99.97% or more for 0.3 μm particles, and ULPA is stipulated to have a collection efficiency of 99.9995% or more for 0.15 μm particles, and when evaluating nonwoven fabrics or air filter media, the particle size to be measured is selected according to the filter grade to be aimed for. The collection efficiency can be automatically measured by using a filter collection efficiency test device (TSI Model 3160) or the like, using a particle counter built into the device.

When air passes through the nonwoven fabric or air filter media, the air flow is impeded and resistance is generated. This resistance causes a difference in air pressure (static pressure) before and after passing through the nonwoven fabric or air filter media, and this differential pressure value is the pressure loss (Pa). If the pressure loss is large, the energy required to pass through the air filter (for example, fan power) becomes large, and energy efficiency decreases. In the JIS standard (JIS Z 8122), the pressure loss for both HEPA and ULPA is specified as 245 Pa or less in the air filter state. However, this is the value after the air filter media is processed into a pleated shape, and the pressure loss of the filter media alone before pleating is preferably 350 Pa or less when air flows at a surface velocity of 3.3 m/min. The pressure loss can be automatically measured using a pressure gauge (including a differential pressure gauge) built into the device by using a filter collection efficiency test device (TSI Model 3160) or the like.

[Air filter media]
The air filter medium of the present invention is a laminated nonwoven fabric, and is constructed by laminating a plurality of the above-mentioned nonwoven fabrics. By laminating a plurality of nonwoven fabrics, the particle permeability of the nonwoven fabric is the product of the particle permeability of each nonwoven fabric, so the particle permeability after laminating a plurality of sheets is small, and the collection efficiency is large. On the other hand, the pressure loss is the sum of the pressure loss of each nonwoven fabric, so it is large by laminating a plurality of sheets. To explain with an example of specific figures, when two nonwoven fabrics with a particle permeability of 10% (collection efficiency of 90%) and a pressure loss of 100 Pa are laminated, the particle permeability is 1% (10% [= 0.1] x 10% [= 0.1] = 1% [= 0.01]), and the pressure loss is 200 Pa (100 Pa + 100 Pa = 200 Pa). In addition, the performance index indicating the potential performance as a filter is 0.023 in both cases of particle permeability of 10% (collection efficiency 90%) and pressure loss of 100 Pa before lamination (one nonwoven fabric), and particle permeability of 1% (collection efficiency 99%) and pressure loss of 200 Pa after lamination (two nonwoven fabrics), and the balance between collection efficiency and pressure loss itself does not change. If the basis weight or the blending ratio of fiber A is increased in an attempt to increase the collection efficiency with one nonwoven fabric, the increase in collection efficiency is accompanied by a larger increase in pressure loss, which tends to worsen the balance between collection efficiency and pressure loss, and the performance index decreases. In other words, in order to improve the performance index, it is possible to reduce the difficulty of manufacturing by using a multilayer structure rather than a single nonwoven fabric. For this reason, when manufacturing a filter medium with a collection efficiency of 99.97%, a pressure loss of 350 Pa, and a performance index of 0.023 required for a HEPA filter, it is less difficult to manufacture by laminating two sheets of nonwoven fabric with a collection efficiency of 98.27%, a pressure loss of 175 Pa, and a performance index of 0.023, or by laminating three sheets of nonwoven fabric with a collection efficiency of 93.31%, a pressure loss of 117 Pa, and a performance index of 0.023, than by constructing it with one sheet of nonwoven fabric. The performance of the laminated nonwoven fabrics does not need to be the same. As an example, by laminating a nonwoven fabric with a collection efficiency of 70.00%, a pressure loss of 100 Pa, and a performance index of 0.012 on the first sheet and a nonwoven fabric with a collection efficiency of 99.90%, a pressure loss of 250 Pa, and a performance index of 0.028 on the second sheet, it is possible to achieve a collection efficiency of 99.97%, a pressure loss of 350 Pa, and a performance index of 0.023. In this way, by using a multi-layer structure, it is expected that the difficulty of manufacturing the nonwoven fabric will be reduced, and as a result, the manufacturing cost will be reduced, but on the other hand, if the number of nonwoven fabrics to be laminated is too large, the work involved in the nonwoven fabric manufacturing process and lamination process will increase, and the cost advantage of the multi-layer structure will be lost. For this reason, the number of nonwoven fabrics to be laminated is preferably 5 or less, and more preferably 3 or less.

The air filter medium of the present invention must have a performance index of 0.023 or more after laminating the nonwoven fabric. If the performance index is 0.023 or more, the air filter medium will have a good balance between collection efficiency and pressure loss, making it useful in various industries, such as for HEPA filters and ULPA filters.

The air filter medium of the present invention can also be configured by further laminating a breathable support material other than the nonwoven fabric of the present invention in order to improve the strength of the filter medium. The material and structure of the breathable support material are not particularly limited, but for example, nonwoven fabric, woven fabric, metal mesh, resin net, etc. are used. Among them, dry nonwoven fabric having thermal fusion properties is preferable in terms of strength, collection ability, flexibility, and workability. Furthermore, the dry nonwoven fabric may be a nonwoven fabric in which some or all of the fibers constituting it have a core/sheath structure, or a two-layer nonwoven fabric consisting of two layers of a low melting point material and a high melting point material. The material of the dry nonwoven fabric is not particularly limited, and may be polyesters such as polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and polylactic acid; polyamides such as polyamide 6, polyamide 66, and polyamide 610; polyolefins such as polyethylene, polypropylene, and polymethylpentene; thermoplastic polymers such as polycarbonate, polyacrylate, polyphenylene sulfide, and thermoplastic polyurethane; and copolymers or composites thereof. For dry nonwoven fabrics with a core/sheath structure, the core component preferably has a higher melting point than the sheath component. For example, examples of combinations of the core/sheath materials include polyethylene terephthalate/polyethylene, and high-melting polyethylene terephthalate/low-melting polyethylene terephthalate. The breathable support material can be bonded to the nonwoven fabric of the present invention by partially melting the breathable support material by heating, or by melting a hot melt resin, by utilizing the anchor effect, or by utilizing adhesion such as a reactive adhesive.

The basis weight and thickness of the breathable support material are not particularly limited, and it may be placed between the nonwoven fabrics of the present invention, or the nonwoven fabrics of the present invention may be laminated together in advance, and the breathable support material may be further laminated on the multi-layered nonwoven fabric.

The air filter medium of the present invention may further have a pre-collection layer laminated thereon (usually on the upstream side of the airflow passing through the filter medium) in order to extend the life of the air filter. The pre-collection layer may be, for example, one obtained by the melt-blown method. Examples of materials for the pre-collection layer include polyesters such as polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and polylactic acid; polyamides such as polyamide 6, polyamide 66, and polyamide 610; polyolefins such as polyethylene, polypropylene, and polymethylpentene; thermoplastic polymers such as polycarbonate, polyacrylate, polyphenylene sulfide, and thermoplastic polyurethane, and copolymers thereof. The pre-collection layer can be bonded to the laminated nonwoven fabric by thermal lamination using, for example, a hot melt resin.

The air filter media of the present invention preferably has a particle collection rate of 99.97% or more for particles with a particle size of 0.3 μm. This makes it possible to use the media as a HEPA filter in clean rooms in the pharmaceutical, medical, food, and semiconductor fields. Furthermore, if the particle collection rate is 99.99% or more for particles with a particle size of 0.15 μm, the media is preferable for use in clean rooms, semiconductor manufacturing equipment, etc., as it can purify the air to an extremely high degree.

The air filter medium of the present invention preferably has a particle collection rate of 99.9995% or more for particles with a particle size of 0.15 μm. This makes it possible to use it as a ULPA filter that can also be used in semiconductor manufacturing equipment.

The air filter medium of the present invention preferably has an average pore size of 0.1 to 10 μm. The average pore size in the present invention refers to the average size of the through holes formed in the nonwoven fabric sheet, and refers to the value measured by the method described in the Examples. An average pore size of 0.1 μm or more is preferable because it ensures a stable fluid flow. On the other hand, an average pore size of 10 μm or less is preferable because it allows the fluid to flow uniformly throughout the entire sheet without disturbing the flow of fluid passing through the wet nonwoven fabric sheet.

The thickness of the air filter medium of the present invention is preferably 0.05 to 1.0 mm. The thickness of the air filter medium in the present invention refers to a value measured by the method described in the Examples. If the thickness is 0.05 mm or more, the handling and molding processability during nonwoven fabric processing are good, and an air filter medium with excellent durability during use can be obtained, which is preferable. In addition, it is preferable because it has good molding processability such as pleating when used as an air filter medium. The thickness is more preferably 0.1 mm or more, and even more preferably 0.15 mm or more. On the other hand, if the thickness is 1.0 mm or less, it is preferable because it can suppress high pressure loss due to densification of the nonwoven fabric. In addition, when pleating is performed to make an air filter, the thickness of the filter medium reduces the contact area between adjacent filter materials, so that the filtration area can be secured and the increase in pressure loss can be suppressed, which is preferable. The thickness is more preferably 0.9 mm or less, and even more preferably 0.8 mm or less.

[Method of manufacturing nonwoven fabric]
Next, an example of a method for producing the nonwoven fabric of the present invention will be described below.

First, fiber B, and, if necessary, short fibers of a fibrous binder, are added to an aqueous medium and stirred with a disintegrator to prepare a fiber dispersion that is uniformly dispersed. In this process, the dispersibility of the fibers can be adjusted by the amount of fiber charged, the amount of aqueous medium, stirring time, etc., and it is preferable that each short fiber is dispersed as uniformly as possible in the aqueous medium. A dispersant may be added to improve the dispersibility of the fibers in the aqueous medium, but it is preferable to keep the amount of dispersant added to a minimum so as not to affect the processability when the nonwoven fabric is subjected to post-processing.

Next, a fiber dispersion of fiber A is prepared in which fiber A is uniformly dispersed in an aqueous medium according to the method described below. This fiber dispersion of fiber A is mixed with the fiber dispersion of fiber B (to which a fibrous binder has been added as necessary) to obtain a papermaking stock solution, which is then wet-woven to obtain a nonwoven fabric in which fiber A is uniformly distributed.

The fiber A of the present invention can be produced by using an island-in-sea fiber made of two or more types of polymers that have different dissolution rates in a solvent. The island-in-sea fiber of the present invention is a fiber having a structure in which island components made of a poorly soluble polymer are scattered in a sea component made of a readily soluble polymer.

The preferred method for spinning this sea-island fiber is sea-island composite spinning by melt spinning, which is highly productive and allows for continuous production. Furthermore, a method using a sea-island composite spinneret is preferred, as it allows for excellent control over fiber diameter and cross-sectional shape.

Specific examples of the sparingly soluble polymers used in the island components in the present invention include, but are not limited to, polyesters such as polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and polylactic acid; polyamides such as polyamide 6, polyamide 66, and polyamide 610; polyolefins such as polyethylene, polypropylene, and polymethylpentene; and thermoplastic polymers such as polycarbonate, polyacrylate, polyphenylene sulfide, and thermoplastic polyurethane, and copolymers thereof.

The readily soluble polymer used in the sea component in the present invention is preferably readily soluble in aqueous solvents or hot water, etc., from the viewpoint of simplifying the dissolution process of the sea component. As the readily soluble polymer in the present invention, it is preferable to use copolymerized polyesters, polylactic acid, polyvinyl alcohol, etc., and in particular, it is preferable to use polyesters copolymerized with polyethylene glycol and sodium 5-sulfoisophthalate, either alone or in combination, and polylactic acid, from the viewpoint of ease of handling and ease of dissolving in low-concentration aqueous solvents.

In the present invention, "easily soluble" means that the dissolution rate ratio (easily soluble polymer/slightly soluble polymer) is 100 or more when the poorly soluble polymer is used as a standard in the solvent used in the dissolution treatment. Considering the simplification and time-saving of the dissolution treatment, it is preferable that this dissolution rate ratio is large, and it is more preferable that the dissolution rate ratio is 1000 or more, and even more preferable that it is 10000 or more. If it is within this range, the dissolution treatment is completed in a short time, and fiber A suitable for the present invention can be obtained without unnecessarily deteriorating the poorly soluble polymer, which is preferable.

In addition, from the viewpoint of solubility in aqueous solvents and ease of treatment of waste liquid generated during dissolution, polyesters copolymerized with 3 to 20 mol% polylactic acid, sodium 5-sulfoisophthalate, and polyesters copolymerized with 5 to 15 wt% polyethylene glycol having a weight average molecular weight of 500 to 3000 in addition to the aforementioned sodium 5-sulfoisophthalate are particularly preferred.

In light of the above, examples of suitable polymer combinations for the aforementioned sea-island fiber include, but are not limited to, a sea component made of either polyester copolymerized with 3-20 mol % sodium 5-sulfoisophthalate and 5-15 wt % polyethylene glycol having a weight average molecular weight of 500-3000, or polylactic acid, and an island component made of either polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, or copolymers thereof.

The spinning temperature of the aforementioned sea-island fiber is preferably set to a temperature at which the poorly soluble and easily soluble polymers, mainly those with high melting points and high viscosity, show fluidity, as determined from the above-mentioned viewpoints. The temperature at which fluidity is shown varies depending on the properties and molecular weight of the polymer, but the melting point of the polymer is a guide, and the spinning temperature can be set to the melting point + 60°C or lower. This range is preferable because it suppresses thermal decomposition of the polymer in the spinning head or spinning pack, suppresses molecular weight reduction, and allows for good sea-island fiber production.

As described above, in the present invention, in order to dissolve and remove the sea component from the sea-island fiber to obtain ultrafine fibers, the sea-island composite spinneret exemplified in Japanese Patent No. 5740877 can be suitably used, but is not limited to this.

The molten yarn extruded from the sea-island composite spinneret is cooled and solidified, and converged by adding an oil agent, and then taken up by rollers with a specified peripheral speed. The take-up speed can be determined based on the discharge rate and the desired fiber diameter, and is preferably 100 to 7000 m/min from the viewpoint of stable production of sea-island fiber. The spun sea-island fiber is preferably stretched from the viewpoint of improving mechanical properties and thermal stability. The spun multifilament may be stretched after being wound once, or may be stretched directly after spinning without being wound.

The sea-island fiber is preferably cut into a tow of several tens to several millions of fibers using a cutting machine such as a guillotine cutter, slicer, or cryostat to the desired fiber length. The fiber length L after cutting is preferably cut so that the ratio (L/R1) of the diameter R1 of the island component of the sea-island fiber (corresponding to the average fiber diameter of fiber A) is 1000 to 6000. This range is preferable because it increases the number of contact points between the fibers when made into a nonwoven fabric, promotes the formation of a bridge structure between the fibers, and enhances the reinforcing effect of the nonwoven fabric. It is preferable that L/R1 is 1000 or more because fiber A is prevented from falling out of the nonwoven fabric during nonwoven fabric processing by wet papermaking. It is more preferable that L/R1 is 1500 or more, and even more preferable that L/R1 is 2000 or more. On the other hand, it is preferable that L/R1 is 6000 or less because it prevents fiber A from aggregating in an aqueous medium and allows a nonwoven fabric with high uniformity to be obtained. L/R1 is more preferably 5500 or less, and even more preferably 5000 or less.

Fiber A can be produced by dissolving and removing the sea component from the aforementioned sea-island fiber. That is, the aforementioned sea-island fiber after cutting is immersed in a solvent capable of dissolving the soluble polymer of the sea component to remove the soluble polymer. When the soluble polymer is copolymerized polyethylene terephthalate or polylactic acid in which sodium 5-sulfoisophthalate or polyethylene glycol is copolymerized, an alkaline aqueous solution such as an aqueous sodium hydroxide solution can be used. When using an alkaline aqueous solution, the bath ratio of the sea-island fiber to the alkaline aqueous solution (weight (g) of the sea-island fiber:weight (g) of the alkaline aqueous solution) is preferably 1:5 to 1:10,000, more preferably 1:10 to 1:5,000. This range is preferable because it prevents unnecessary entanglement of fibers A when the soluble polymer of the sea component dissolves.

The alkali concentration of the alkaline aqueous solution is preferably 0.1 to 5% by weight, and more preferably 0.5 to 3% by weight. This range is preferable because dissolution of the easily soluble polymer of the sea component is completed in a short time, and a fiber dispersion in which fiber A is uniformly dispersed can be obtained without unnecessarily deteriorating the sparingly soluble polymer of the island component. The temperature of the alkaline aqueous solution is not particularly limited, but is preferably 50°C or higher, since this can hasten the progress of dissolution of the easily soluble polymer of the sea component.

In the present invention, the aqueous solution of the easily soluble polymer of the sea component from the sea-island fiber may be used as it is as the fiber dispersion of fiber A, or may be used after adjusting the pH by adding an acid or alkali, or after diluting with water. A dispersant may be added to the fiber dispersion to suppress aggregation of fiber A over time. Types of dispersants include cationic compounds, nonionic compounds, and anionic compounds, among which anionic compounds are preferred from the viewpoint of improving dispersibility due to electrical repulsion in an aqueous medium. The amount of dispersant added is preferably 0.001 to 10 times the weight of fiber A, and within this range, dispersibility of fiber A is ensured without impairing processability during nonwoven fabric processing by wet papermaking, which is preferred.

When fiber B in the present invention is a synthetic fiber made of a thermoplastic polymer, it can be produced by melt spinning, stretching if necessary, and then cutting to the desired fiber length as described above. Here, the fiber length of fiber B is preferably 30 mm or less. If the fiber length is 30 mm or less, the formation of fiber agglomerates caused by the fibers being tightly entangled with each other during dispersion in an aqueous medium is suppressed, and a homogeneous nonwoven fabric can be obtained, which is preferable for use as an air filter medium.

The fiber dispersion of fiber A thus prepared is mixed with the fiber dispersion of fiber B (with a fibrous binder added as necessary) and diluted to a certain concentration, then dehydrated on an inclined wire or cylinder to form a nonwoven fabric by wet papermaking. Equipment used for wet papermaking includes, but is not limited to, a cylinder papermaking machine, a Fourdrinier papermaking machine, an inclined short wire papermaking machine, or a combination of these. In the papermaking process, a three-dimensionally homogenous nonwoven fabric can be produced by controlling the accumulation of fibers during drainage by adjusting the papermaking speed and the amount of fiber and aqueous medium, in addition to the dispersibility of the fibers in the papermaking solution.

The nonwoven fabric formed by wet papermaking is passed through a drying process to remove moisture. As a drying method, a method using hot air ventilation (air through) or a method of contacting with a heated rotating roll (heated calendar roll, etc.) can be suitably adopted from the viewpoint that the nonwoven fabric can be dried and the heat-bonded fibers can be heat-bonded simultaneously.

[Method of manufacturing air filter media]
The air filter medium of the present invention can be manufactured by stacking and arranging a plurality of nonwoven fabrics. When stacking, the stacked nonwoven fabrics may be bonded together. When bonding, it is possible to use a thermal lamination or an adhesive.

When bonding with thermal lamination, the rollers of the thermal lamination device are heated to a temperature higher than the glass transition temperature of the organic fibers and hot melt resin that make up the nonwoven fabric, and then the nonwoven fabrics are passed through, causing some of the nonwoven fabrics to melt between them, or the hot melt resin to melt, creating an anchor effect that bonds the nonwoven fabrics together.

When using an adhesive for bonding, the adhesive is applied in dots to the surface of the nonwoven fabric to be bonded using a dispenser or the like, and the adhesive-coated surface is then aligned with the surface that will come into contact with the other nonwoven fabric to be bonded, allowing the two to be bonded together. There are no particular restrictions on the type of adhesive used for bonding, and water-based, solvent-based, resin-based, etc. can be used.

[Application]
The nonwoven fabric of the present invention has a high collection efficiency, so the air filter material using the nonwoven fabric of the present invention can be suitably used as an air filter material for air purifiers, air conditioners, building air conditioners, industrial clean rooms, and car compartments of automobiles, trains, etc. In addition, in spaces that require extremely high levels of clean air, such as clean rooms and semiconductor manufacturing equipment, it can be suitably used as an air filter material for air filters such as air conditioners for taking in outside air into clean rooms, air conditioners for circulating air in clean rooms, and fan filter units installed on the ceilings of clean rooms and semiconductor manufacturing equipment. Clean rooms and semiconductor manufacturing equipment equipped with air filters using these air filter materials of the present invention are useful in various industries.

Next, the present invention will be described in detail based on examples. However, the present invention is not limited to these examples. The characteristic values in the examples were determined by the following methods.

A. Average Fiber Diameter In an image of the cross section of a fiber taken with a scanning electron microscope (electron microscope SU-1510 manufactured by Hitachi High-Technologies Corporation), the circumscribed circle diameter of the cross section of the fiber was measured for any 100 fibers, and the average was rounded off to one decimal place to obtain the average fiber diameter.

B. Fiber length of organic fibers The fiber lengths of 100 random fibers were measured in an image of the side of the fiber taken with a stereomicroscope (Olympus SZ-61 stereomicroscope), and the average was rounded off to one decimal place to obtain the fiber length. The fiber length of fiber A was measured for the sea-island fiber before removing the sea part.

C. Total volume of organic fibers The volume of each fiber was calculated assuming that the cross section was a perfect circle using the average fiber diameter and fiber length calculated in the above items A and B. The volume of all fibers was then added up to calculate the total volume of the organic fibers.

D. Weight per unit area The nonwoven fabric obtained in each of the Examples and Comparative Examples was used as a sample, and the weight of the nonwoven fabric cut into a 250 mm x 250 mm square was weighed, and the weight per unit area (1 ^m2 ) was converted and rounded off to the nearest tenth to calculate the weight per unit area (g/ ^m2 ). Measurements were performed on three randomly selected points per sample, and the average value was rounded off to the nearest tenth to determine the weight per unit area.

E. Thickness The nonwoven fabric used in the measurement in item D above was used as a sample, and the thickness of the nonwoven fabric was measured using a dial thickness gauge (SM-114 manufactured by TECLOCK Corporation, probe shape 10 mmφ, graduation 0.01 mm, measuring force 2.5 N or less). Measurements were taken at five arbitrary points per sample, and the average value was rounded off to two decimal places to calculate the thickness (mm) of the nonwoven fabric.

F. Porosity The basis weight and thickness of the nonwoven fabric calculated in the above items D and E were used to calculate the following formula, and the value was rounded off to one decimal place to obtain the porosity (%) of the nonwoven fabric.
Porosity (%)=100−[weight per unit area (g/m ² )/(thickness (mm)×fiber density (g/cm ³ ))]×0.1
The fiber density may be the density of the fibers constituting the nonwoven fabric, and in the case of PET, it was calculated as 1.38 g/ ^cm3 .

G. Collection efficiency The nonwoven fabric or air filter medium obtained in the examples and comparative examples was used as a sample, and a sample cut to a diameter of 12 cm was set using a filter collection efficiency tester (Model 3160 manufactured by TSI). The collection efficiency was measured using particles with a particle size of 0.15 μm or 0.3 μm. The collection efficiency (%) of a particle size of 0.15 μm was measured by passing air containing 20,000 to 30,000 sodium chloride particles with an average ^particle size of 0.15 to 0.16 μm at a speed of 31.5 L/min. The collection efficiency (%) of a particle size of 0.30 μm was measured by passing air containing 20,000 to 30,000 sodium chloride particles with an average particle size of 0.30 to 0.31 ^μm at a speed of 31.5 L/min.

H. Pressure loss The nonwoven fabric or air filter medium obtained in the examples and comparative examples was used as a sample, and a sample cut to a diameter of 12 cm was set using a filter collection efficiency tester (TSI Model 3160). The evaluation part was opened to a diameter of 11 cm and the sample was exposed, and the air was passed through at a setting of 31.5 L/min to set the surface velocity of the air to 3.3 m/min, and the pressure loss (Pa) was measured.

I. Performance Index Using the collection efficiency and pressure loss of the nonwoven fabric calculated in the above items G and H, the value calculated by the following formula was rounded off to three decimal places to obtain the performance index (1/Pa) of the nonwoven fabric.
Performance index=−ln[{1−capture efficiency (%)/100)}/pressure loss (Pa)]
Example 1
Polyethylene terephthalate (PET) was used as the island component, and copolymerized PET obtained by copolymerizing 8.0 mol % of sodium 5-sulfoisophthalate and 10 wt % of polyethylene glycol having a molecular weight of 1000 was used as the sea component, and each was vacuum dried for 12 hours at 150° C. Then, the island component and sea component were fed in a blending ratio of 50 wt % to an extruder-type composite spinning machine and melted separately, and the resulting mixture was made to flow into a spinning pack incorporating a sea-island composite spinneret (number of island components: 2000, shape of island components: round) at a spinning temperature of 285° C., and the composite polymer flow was discharged from the discharge hole at a discharge rate of 12 g/min to obtain a spun yarn. The spun yarn was cooled with cooling air at 20°C and 20m/min, oiled with an oiling device to converge the yarn, taken up by a first godet roller rotating at 1000m/min, passed through a second godet roller rotating at the same speed as the first godet roller, and wound by a winder to obtain an undrawn yarn. The undrawn yarn was then drawn 3.4 times between rollers heated to 85°C and 130°C using a drawing machine to obtain a sea-island fiber (island component diameter: 0.20µm).

The resulting sea-island fiber was cut to a fiber length of 0.6 mm. After cutting, the sea-island fiber was treated in a 1% by weight aqueous solution of sodium hydroxide in a bath ratio of 1:100 at 90°C for 30 minutes, and then neutralized with acetic acid to pH=7 to obtain a fiber dispersion of fiber A.

Next, PET staple fibers (fiber diameter 3.0 μm, fiber length 3.0 mm) were used as fiber B at a blend ratio of 65% by weight (blending ratio in the papermaking stock solution), and core-sheath PET staple fibers (core component: PET, sheath component: copolymerized polyester with a melting point of 110°C, copolymerized with 60 mol% terephthalic acid and 40 mol% isophthalic acid as dicarboxylic acid components, 85 mol% ethylene glycol and 15 mol% diethylene glycol as diol components, core-sheath ratio (weight ratio) = 50:50, fiber diameter 10.0 μm, fiber length 5.0 mm) were used as binder fiber C at a blend ratio of 30% by weight (blending ratio in the papermaking stock solution), and the mixture was uniformly mixed and dispersed with water using a disintegrator to prepare a fiber dispersion of fiber B and binder fiber C.

The fiber dispersion of fiber B and binder fiber C was mixed homogeneously with the fiber dispersion of fiber A described above so that the blending ratio of fiber A was 5% by weight, to prepare a papermaking stock solution. This papermaking stock solution was made into paper using a square sheet machine (250 mm square) manufactured by Kumagai Riki Kogyo Co., Ltd., and then dried and heat-treated in a rotary dryer with a roller temperature set to 110°C to obtain nonwoven fabric 1. Nonwoven fabric 2 was obtained using a similar method.

Table 1 shows the evaluation results of the obtained nonwoven fabric 1 and nonwoven fabric 2, as well as the evaluation results of a laminated nonwoven fabric (air filter medium) formed by laminating nonwoven fabrics 1 and 2. Nonwoven fabric 1 and nonwoven fabric 2 had collection efficiencies of 98.2314% and 98.3542%, respectively, for particles with a particle diameter of 0.3 μm, and both had performance indices of 0.024. The collection efficiency of both nonwoven fabrics was below the HEPA standard of 99.97%, but by laminating nonwoven fabric 1 and nonwoven fabric 2, the collection efficiency was increased to 99.9715%, exceeding the HEPA standard, and an air filter medium with a performance index of 0.024 was obtained. In this way, it was confirmed that the collection efficiency can be significantly increased by laminating nonwoven fabrics.

Example 2
Nonwoven fabrics 3 and 4 were produced in the same manner as in Example 1, except that the compounding ratio and basis weight of fiber A and fiber B were changed as shown in Table 1.

Table 1 shows the evaluation results of the obtained nonwoven fabrics 3 and 4, as well as the evaluation results of a laminated nonwoven fabric (air filter medium) formed by laminating nonwoven fabrics 3 and 4. Nonwoven fabrics 3 and 4 had collection efficiencies of 99.3284% and 99.4135%, respectively, for particles with a particle diameter of 0.15 μm, and performance indexes of 0.027 and 0.029. By laminating nonwoven fabrics 3 and 4, the collection efficiency was increased to 99.9955%, resulting in an air filter medium with a high collection efficiency and a performance index of 0.028. It was thus confirmed that the collection efficiency can be significantly increased by laminating nonwoven fabrics.

Example 3
Nonwoven fabrics 5 and 6 were produced in the same manner as in Example 1, except that the blending ratio of fiber A and fiber B was changed as shown in Table 1.

Table 1 shows the evaluation results of the obtained nonwoven fabrics 5 and 6, as well as the evaluation results of a laminated nonwoven fabric (air filter medium) formed by laminating nonwoven fabrics 5 and 6. Nonwoven fabrics 5 and 6 had collection efficiencies of 99.9412% and 99.9135%, respectively, for particles with a particle diameter of 0.15 μm, and performance indices of 0.037 and 0.038. Neither of these collection efficiencies met the ULPA standard of 99.9995%, but by laminating nonwoven fabrics 7 and 8, the collection efficiency reached 99.9999%, exceeding the ULPA standard, and the performance index was 0.036, resulting in an air filter medium. It was thus confirmed that the collection efficiency can be significantly increased by laminating nonwoven fabrics.

Example 4
Nonwoven fabrics 7 and 8 were produced in the same manner as in Example 1, except that the blending ratio of fiber A and fiber B was changed as shown in Table 2 and binder fiber C was not used.

Table 2 shows the evaluation results of the obtained nonwoven fabrics 7 and 8, as well as the evaluation results of a laminated nonwoven fabric (air filter medium) formed by laminating nonwoven fabrics 7 and 8. Nonwoven fabrics 7 and 8 had collection efficiencies of 99.7751% and 99.8742%, respectively, for particles with a particle diameter of 0.15 μm, and performance indexes of 0.036 and 0.038. Neither of these collection efficiencies met the ULPA standard of 99.9995%, but by laminating nonwoven fabrics 7 and 8, the collection efficiency reached 99.9996%, exceeding the ULPA standard, and the performance index was 0.036, resulting in an air filter medium. It was thus confirmed that the collection efficiency can be significantly increased by laminating nonwoven fabrics.

Example 5
Nonwoven fabrics 9 and 10 were produced in the same manner as in Example 1, except that the blending ratio of fiber A and fiber B was changed as shown in Table 2.

The evaluation results of the obtained nonwoven fabric 9 and nonwoven fabric 10 were as follows: Further, a spunbond nonwoven fabric ("Elves S0303WDO" manufactured by Unitika Ltd. (average fiber diameter 24 μm, basis weight 30 g/m2) consisting of fibers with a core/sheath structure using PET as the core and PE as the sheath was placed between the nonwoven fabrics 9 and 10 as a breathable support material ^. The evaluation results of the laminated nonwoven fabric (air filter medium) in which nonwoven fabric 9 and nonwoven fabric 10 are arranged and laminated with 0.15 mm thick particles are shown in Table 1. The nonwoven fabric 9 and nonwoven fabric 10 have collection efficiencies of 99.9125% and 99.9056%, respectively, for particles with a particle size of 0.15 μm, and the performance indexes of both are 0.038. Although the collection efficiencies of both were less than the ULPA standard of 99.9995%, the nonwoven fabric 9, nonwoven fabric 10, and the breathable support material were laminated to obtain a collection efficiency of 99.9999%, which is a collection efficiency exceeding the ULPA standard, and the performance index was 0.038, resulting in an air filter medium. It was confirmed that the collection efficiency can be significantly increased by laminating nonwoven fabrics in this way.

Comparative Example 1
Nonwoven fabrics 11 and 12 were produced in the same manner as in Example 1, except that the compounding ratio of fiber A and fiber B was changed as shown in Table 2.

Table 2 shows the evaluation results of the obtained nonwoven fabrics 11 and 12, as well as the evaluation results of a laminated nonwoven fabric (air filter medium) formed by laminating nonwoven fabrics 11 and 12. Nonwoven fabrics 11 and 12 had collection efficiencies of 96.2653% and 97.8426%, respectively, for particles with a particle diameter of 0.3 μm, and performance indexes of 0.020 and 0.023. When nonwoven fabrics 11 and 12 were laminated, the collection efficiency was 99.9241%, which did not exceed the HEPA standard, and the air filter medium had a performance index of 0.022. It was thus confirmed that the performance index itself, which indicates potential performance, does not improve even when nonwoven fabrics with low performance potential are laminated together.

 

 

Claims (8)

An air filter medium having a performance index of 0.023 or more, in which at least one of the nonwoven fabrics is laminated, and at least two types of organic fibers A and B having different fiber diameters are included, with the average fiber diameter of organic fiber A being 0.01 to 0.60 μm and the average fiber diameter of organic fiber B being 1.0 to 30 μm. The air filter medium according to claim 1, characterized in that it has a particle collection rate of 99.97% or more for particles with a particle size of 0.3 μm. The air filter medium according to claim 1, characterized in that it has a particle collection rate of 99.9995% or more for particles with a particle size of 0.15 μm. The air filter medium according to claim 1, characterized in that at least one of the laminated nonwoven fabrics has a performance index of 0.023 or more. The air filter medium of claim 1, wherein the nonwoven fabric further comprises binder fibers. An air filter using the air filter medium according to any one of claims 1 to 5. A fan filter unit equipped with the air filter according to claim 6. A clean room or semiconductor manufacturing equipment equipped with the air filter according to claim 6.