JP2024033025A - Base material for filters - Google Patents

Abstract

[Solution] In a filter base material for use as a filter medium by bonding porous membranes together by heat fusion, the filter medium is a wet nonwoven fabric containing non-binder fibers and heat-fusible binder fibers. , characterized in that the blending ratio of heat-fusible binder fibers is 20 to 90% by mass and the blending ratio of non-binder fibers is 10 to 80% by mass with respect to the total fiber components contained in the wet-laid nonwoven fabric. Base material for filters. [Effect] A filter base material is obtained that achieves both high collection efficiency and low pressure loss, and also provides high peel strength between the porous membrane and the filter base material. [Selection diagram] None

Description

The present invention relates to a filter base material.

BACKGROUND ART Porous membranes have conventionally been used as filter media for air filters and liquid filters. Examples of the material for the porous membrane include polytetrafluoroethylene (PTFE); polyolefins such as polyethylene and polypropylene (PP); and the like. In particular, these porous membranes can achieve high collection efficiency and low pressure loss, so they are used as air filter media in clean room filters and cyclone vacuum cleaner filters that require high collection efficiency. . In addition, due to its high chemical resistance, it is used as a filter material for liquid filters, such as water purification filters that are often cleaned with strong alkalis such as sodium hydroxide, industrial wastewater, and industrial water filters. It is also widely used. However, as described in Non-Patent Document 1, porous membranes have low mechanical strength and problems with dimensional stability, so it is difficult to use the porous membrane alone. Therefore, there is a need for a filter medium that is a composite of a filter base material and a porous membrane. Nonwoven fabrics are often used as filter base materials from the viewpoint of flexibility and porosity. Hereinafter, the "base material for filters" may be abbreviated as "base material", and the "filter medium for filters" may be abbreviated as "filter material".

The base material and the porous membrane are combined by bonding by thermal fusion, bonding by adhesive, or the like. Bonding with an adhesive is not preferable because the adhesive closes the pores of the porous membrane, leading to concerns about an increase in pressure loss and a decrease in collection efficiency. Therefore, it is preferable to form a composite by bonding by heat fusion. For bonding by heat fusion, lamination processing is performed by heat and pressure treatment, but the heat and pressure treatment increases the density of the base material, which may increase pressure loss. Furthermore, due to insufficient peel strength due to heat-pressure treatment, a problem may arise in which the porous membrane peels off from the base material during use of the filter. Therefore, the filter base material is required to be difficult to obtain high density when bonded by heat fusion and to have high peel strength.

The base material to be composited with the porous membrane is made of, for example, a polyolefin resin or a polyester resin, has a basis weight of 40 g/m ² to 120 g/m ² , a thickness of 0.3 mm to 0.9 mm, and a rupture material. A nonwoven fabric having a strength of 199 kPa to 600 kPa, a puncture strength of 8 N to 24 N, and a porosity of 65% to 90% is disclosed (see Patent Document 1). However, with the base material disclosed in Patent Document 1, there are concerns that the base material becomes dense when bonded by heat fusion, resulting in an increase in pressure loss, and that the peel strength between the porous membrane and the base material is not sufficient. Therefore, there was a concern that problems such as peeling of the porous membrane from the base material would occur.

Patent Document 2 discloses a spunbond nonwoven fabric made of polyester/polyethylene core-sheath composite fibers as a base material (see Patent Document 2). However, even with the base material of Patent Document 2, there are concerns that the base material becomes dense when bonded by heat fusion, increasing pressure loss, and that the peel strength between the base material and the porous membrane is insufficient. There was a concern that problems such as the porous membrane peeling off from the base material would occur.

Therefore, there is a need for a filter base material that does not easily cause pressure loss to increase when bonded together by heat fusion and can provide high peel strength between the porous membrane and the base material.

Masahiro Kitajima, “Fluororesin film and its high functionality,” Valqua Technology Magazine, Winter, 2015, No. 28, page 6

Japanese Patent Application Publication No. 2014-240047 Japanese Patent Application Publication No. 10-211409

An object of the present invention is to obtain a filter base material that achieves both high collection efficiency and low pressure loss, and that provides high peel strength between the porous membrane and the filter base material.

The present inventors conducted research to solve this problem and discovered the following invention.

<1> In a filter base material for use as a filter medium by bonding a porous membrane by heat fusion, the filter medium is a wet nonwoven fabric containing non-binder fibers and heat-fusible binder fibers, A filter characterized in that the blending ratio of heat-fusible binder fibers is 20 to 90% by mass and the blending ratio of non-binder fibers is 10 to 80% by mass with respect to all fiber components contained in the wet-laid nonwoven fabric. Base material for use.
<2> The filter base material according to <1>, wherein the non-binder fibers contain wood pulp.
<3> The filter base material according to <1> or <2>, wherein the non-binder fibers include synthetic fibers.
<4> The filter base material according to any one of <1> to <3>, wherein the material of the porous membrane is polytetrafluoroethylene.

According to the present invention, it is possible to obtain a filter base material that achieves both high collection efficiency and low pressure loss, and also provides high peel strength between the porous membrane and the filter base material.

This will be explained in detail below. The filter base material of the present invention is a filter base material for use as a filter medium by bonding a porous membrane by heat fusion, and the filter medium is composed of non-binder fibers and heat-fusible binder fibers. It is a wet-laid nonwoven fabric containing 20 to 90% by mass of heat-fusible binder fibers and 10 to 80% by mass of non-binder fibers with respect to the total fiber components contained in the wet-laid nonwoven fabric. characterized by something.

Wet-processed nonwoven fabric is a nonwoven fabric manufactured using a wet-process papermaking method that provides a uniform texture. In the wet papermaking method, each fiber is dispersed in water to prepare a slurry. The slurry is made into paper using a paper machine to obtain a wet-laid nonwoven fabric.

As a chemical compounded during papermaking, a wet strength agent may be used to prevent paper breakage in a wet paper state, and an internal sizing agent may be used to stabilize peeling from a Yankee dryer.

In the wet papermaking method, for example, a fourdrinier method, a cylinder method, an inclined wire method, or the like can be used. The filter base material of the present invention can also be manufactured using a combination paper machine in which two or more paper machines of the same or different types selected from these paper machines are installed online. In order to produce a filter base material with excellent uniformity, it is preferable to use a papermaking method that can gently dewater the slurry on a wire, such as a Fourdrinier method or an inclined wire method.

In the present invention, non-binder fibers are characterized in that the cross-sectional shape of the fibers does not change even at the melting point or softening temperature of the heat-fusible binder fibers, or that the fiber shape is maintained even if the cross-sectional shape changes. It is a fiber that plays the role of the main fiber. Note that, in general, the melting point or softening point of synthetic fibers as non-binder fibers is higher than the melting point or softening point of heat-fusible binder fibers. Moreover, the non-binder fiber in the present invention is preferably an organic fiber.

The blending ratio of the non-binder fibers is 10 to 80% by mass, more preferably 20 to 75% by mass, and even more preferably 30 to 70% by mass, based on the total fiber components contained in the wet-laid nonwoven fabric. preferable. If the blending ratio of non-binder fibers is less than 10% by mass, the thickness will decrease too much during bonding by heat fusion, resulting in a base material with too high density, resulting in an excessively high pressure loss in the filter medium. , not suitable as a filter. On the other hand, when the blending ratio of non-binder fibers is more than 80% by mass, there are few heat-fusible binder fibers, so the fibers tend to fall off from the base material, and when used as a filter, the fallen fibers may flow downstream. The problem arises that it has a negative impact.

In the present invention, it is preferred that the non-binder fibers include wood pulp. The wood pulp is not limited to NBKP, LBKP, NBSP, LBSP, or any other type of pulp. If the fibers of the wood pulp are fine, the pressure loss tends to increase, so although there is no particular limitation, it is preferable that the fiber diameter of the wood pulp is large.

When the non-binder fibers include wood pulp, the blending ratio of the wood pulp is not particularly limited, but is preferably less than 50% by mass, and less than 40% by mass, based on the total fiber components contained in the wet-laid nonwoven fabric. More preferably, it is less than 30% by mass. When the blending ratio of wood pulp is 50% by mass or more, pressure loss may increase because the wood pulp closes the pores of the base material, and the wood pulp inhibits adhesion with the porous membrane. Peel strength may decrease.

In the present invention, the non-binder fibers may include synthetic fibers. Synthetic fibers as non-binder fibers include, for example, polyester fibers such as polyethylene terephthalate (PET) fibers and polybutylene terephthalate fibers; polyolefin fibers such as polypropylene fibers and polyethylene fibers; fibers made of polystyrene or modified polymers and copolymers of these polymers; Acrylic fibers; polyacrylonitrile fibers; polyvinyl alcohol fibers; polyamide fibers; urethane fibers; cellulose fibers such as rayon fibers, regenerated cellulose fibers, and solvent-spun cellulose fibers; fibers spun from solutions of collagen, alginic acid, chitin, etc.; etc. As mentioned above, synthetic fibers as non-binder fibers do not change their cross-sectional shape even at the melting point or softening temperature of the heat-fusible binder fiber, or maintain their fiber shape even if the cross-sectional shape changes. It has the characteristic of Therefore, by blending synthetic fibers, it is possible to obtain the effect that the thickness is less likely to decrease during bonding by heat fusion, and the base material is less likely to have high density.

The fineness of the synthetic fiber as a non-binder fiber is preferably 0.1 to 6.0 dtex, more preferably 0.6 to 3.3 dtex. If the fineness of the synthetic fiber is less than 0.1 decitex, papermaking properties tend to be poor and the texture tends to be poor, leading to an increase in cost. On the other hand, when the fineness of the synthetic fiber is more than 6.0 decitex, the texture tends to be poor. The fiber length of the synthetic fiber is preferably 2 to 20 mm, more preferably 3 to 10 mm. When the fiber length of the synthetic fiber is less than 2 mm, the fiber tends to come off from the wire during wet papermaking, and the yield may decrease. On the other hand, if the fiber length of the synthetic fiber exceeds 20 mm, the fibers may clog the screen in the wet papermaking method, making papermaking difficult.

In the present invention, mechanical strength is improved by incorporating into the manufacturing process a step of raising the temperature of the nonwoven fabric to a temperature higher than the softening temperature of the heat-fusible binder fibers.

The fineness of the heat-fusible binder fiber is preferably 0.1 to 6.0 dtex, more preferably 0.2 to 3.3 dtex. If the fineness of the heat-fusible binder fiber is less than 0.1 dtex, the nonwoven fabric will have a small thickness and a high density, which may result in high pressure loss. On the other hand, when the fineness of the heat-fusible binder fiber is more than 6.0 dtex, the texture of the nonwoven fabric becomes poor and may not be suitable as a filter base material. The fiber length of the heat-fusible binder fiber is preferably 2 to 20 mm, more preferably 3 to 10 mm. When the fiber length of the heat-fusible binder fibers is less than 2 mm, the fibers tend to come off from the wire during wet papermaking, and the yield may decrease. On the other hand, if the fiber length of the heat-fusible binder fiber exceeds 20 mm, the fibers may clog the screen in wet papermaking, making papermaking difficult.

Examples of heat-fusible binder fibers include single fibers as well as composite fibers such as core-sheath fibers (core-shell type) and parallel fibers (side-by-side type). Examples of single fibers include fibers such as polyethylene and unstretched polyester. Composite fibers can improve mechanical strength without forming a film on the surface of the nonwoven fabric. Examples of core/sheath fibers include combinations of polypropylene (core) and polyethylene (sheath), combinations of polypropylene (core) and ethylene vinyl alcohol (sheath), and combinations of high melting point polyester (core) and low melting point polyester (sheath). Can be mentioned. In addition, all-melting type single fibers made only of low-melting point resins such as polyethylene tend to form a film during the drying process, which may result in high pressure loss, but it should be used as long as it does not impair its properties. be able to. In this study, core-sheath fibers are preferable as heat-fusible binder fibers, since they have a core portion and are difficult to reduce in thickness when bonded together by heat-fusion bonding.

The content of the heat-fusible binder fiber is 20 to 90% by mass, more preferably 25 to 80% by mass, and 30 to 70% by mass based on the total fiber components contained in the wet-laid nonwoven fabric. It is even more preferable. When the content of heat-fusible binder fibers is less than 10% by mass, there is a problem that there is not enough of a component to stop non-binder fibers, resulting in a large amount of shedding fibers. On the other hand, if the content of heat-fusible binder fibers is more than 90% by mass, the nonwoven fabric will have a small thickness and high density, so the pressure loss will decrease during bonding by heat-fusion because the thickness will decrease. The problem arises that it is not suitable as a filter.

The melting point of the heat-fusible binder fiber is preferably lower than 180°C, preferably lower than 170°C, and more preferably lower than 160°C. If the melting point of the heat-fusible binder fiber is higher than 180°C, it is necessary to apply a high temperature when bonding by heat-fusion, and as a result, the synthetic fiber as a non-binder fiber becomes easily crushed, resulting in pressure loss. may be higher.

In the present invention, as the binder fiber, in addition to the heat-fusible binder fiber, a moist heat-adhesive binder fiber can also be used. As the wet heat adhesive binder fibers, hot water soluble fibers such as polyvinyl alcohol (PVA) fibers can be used. Wet heat adhesive binder fibers hardly dissolve in water at room temperature and maintain their fiber form, but when heated on the dryer surface of the wet papermaking process, they begin to dissolve easily, and at that moment By pressurizing it with pressurizing equipment and then dehydrating and drying it, it re-solidifies and a binder effect is obtained.

The influence of this wet heat adhesive binder fiber on the adhesive strength can be considered from the underwater softening point. The underwater softening point roughly indicates the temperature at which wet paper receives heat from a dryer in a wet papermaking process, and the wet heat adhesive binder begins to dissolve and exhibits adhesive function. The lower the wet heat adhesive binder with a lower softening point in water is used, the easier it becomes to dissolve the wet heat adhesive binder fibers, which is a prerequisite for adhesion, and the greater the adhesive effect becomes. However, if the underwater softening point becomes too low, a problem arises in that it tends to adhere to the dryer. In order for the wet heat adhesive binder to dissolve, the temperature of the wet paper needs to be higher than its softening point in water. Therefore, the higher the drying temperature, the greater the adhesive effect and the higher the strength. When the temperature of the wet paper is below the softening point in water of the wet heat adhesive binder fibers, the wet heat adhesive binder does not dissolve, and therefore the binder effect is completely lost. In the case of a Yankee dryer, the steam temperature of the dryer is about 130 to 160°C, and the temperature of the wet paper in contact with it is thought to be 60 to 90°C, so in order to obtain a sufficient binder effect, it is necessary to It is preferable that the binder fiber has a softening point in water of 65 to 85°C. However, the moisture-heat adhesive binder fibers form a film that closes the pores of the base material, and even if heat-pressure treatment is performed in the absence of moisture, it does not contribute to adhesion to the porous membrane, so the blending ratio is not particularly limited, but when using wet heat adhesive binder fibers, it is preferably less than 20% by mass based on the total fiber components contained in the wet-laid nonwoven fabric.

Further, the fineness of the wet heat adhesive binder fiber is not particularly limited, but is preferably 0.3 to 5.0 dtex, more preferably 1.0 to 3.0 dtex. If the fineness is less than 0.3 decitex, it may fall off from the papermaking wire. On the other hand, if it exceeds 5.0 decitex, it may be difficult to obtain sufficient strength because the specific surface area is small. Considering the viewpoint of texture and dispersibility, the fiber length is suitably 3 to 6 mm.

In the present invention, cotton pulp, straw pulp, bamboo pulp, esparto pulp, bagasse pulp, and hemp pulp can also be used as the non-binder fiber. In addition, inorganic fibers such as glass fibers and carbon fibers can be used, but they may break during bonding by heat fusion, and the strength of the base material may decrease significantly, so it is preferable not to include them. Although included in non-binder fibers in a broad sense, non-binder fibers in the present invention are fibers excluding inorganic fibers.

By bonding the filter base material of the present invention and the porous membrane together by heat fusion, a filter medium is obtained. The bonding can be performed by heat pressure treatment. The heat and pressure treatment can be performed by laminating sheets using a hot press machine or laminating windings together using a thermal calendar. In order to obtain sufficient peel strength between the wet-laid nonwoven fabric and the porous membrane, the temperature during the heat-pressure treatment is preferably 160° C. or higher, which is higher than the melting point of the heat-fusible binder fiber.

In the present invention, porous membranes such as polyolefins such as polyethylene (PE) and polypropylene (PP); polyethylene terephthalate; polyurethane; polyimide; and polytetrafluoroethylene (PTFE) can be used. If a porous membrane with a low heat resistance temperature is used, the porous membrane will soften during heat-pressure treatment, and the pores may become clogged, making it impossible to obtain sufficient Frazier air permeability for use as a filter. Therefore, it is preferable to use polytetrafluoroethylene, which has excellent heat resistance.

The filter base material of the present invention is not particularly limited, but preferably has a basis weight in the range of 30 to 120 g/m ² . When the basis weight is less than 30 g/m ² , the strength of the base material is low and it may be damaged during use as a filter. When it is larger than 120 g/m ² , pressure loss becomes high and it may not be suitable for use as a filter.

The density of the filter base material of the present invention is not particularly limited, but is preferably 0.1 to 0.5 g/cm ³ . The density can be adjusted as appropriate by adjusting the paper-making conditions during wet paper-making, the type of fiber, the blending ratio of fibers, the basis weight, and the thickness. If the density is less than 0.1 g/cm ³ , fibers may fall off during bonding by heat fusion, causing trouble. If the density exceeds 0.5 g/cm ³ , the pressure loss will be high and it may not be suitable as a filter base material.

In the present invention, in order to obtain low pressure loss, it is preferable to increase the Frazier air permeability of the filter medium. The Frazier air permeability is the air permeability defined by method A (Fragir type method) described in JIS L1096:2010. In the present invention, the Frazier air permeability of the filter medium is preferably 10 cm ³ /cm ² ·s or more, more preferably 20 cm ³ /cm ² ·s or more. If the Frazier air permeability is lower than 10 cm ³ /cm ² ·s, the pressure loss will be high and it may not be suitable as a filter medium.

The present invention will be explained in more detail with reference to Examples.

<Heat-fusible binder fiber>
・PET - low melting point PET core-sheath fiber (core-sheath binder, fineness 2.2 dtex, fiber length 5 mm, melting point of sheath part 120°C)

<Wood pulp: non-binder fiber>
・NBKP Chinook (freeness 680ml CSF)

<Synthetic fiber: non-binder fiber>
・PET fiber (normal PET fiber, fineness 1.7 dtex, fiber length 5 mm)

<Inorganic fiber>
・Glass fiber (fiber diameter 9μm, fiber length 6mm)

[Filter material for filters]
Examples 1 to 10, Comparative Examples 1 to 6
Each fiber was dispersed with a pulper so as to have the fiber composition shown in Table 1, and after paper-making with a cylinder paper machine, it was dried with a cylinder dryer to obtain a filter base material made of wet-laid nonwoven fabric. Examples 1 to 5 and Filter media for filters of Comparative Examples 1 to 3 were produced. In addition, a porous PTFE membrane was laminated on the filter base material by heat-pressure treatment using a hot roll at a linear pressure of 20 kgf/cm and a temperature of 160°C, and laminated by thermal fusion bonding. and Comparative Examples 4 to 6, respectively, were produced.

(Evaluation method)
The filter base materials and filter media produced in Examples and Comparative Examples were measured and evaluated by the following methods.

1) Basis weight The basis weight and thickness of the filter base material were measured according to JIS P 8124:2011 and 8118:2014.

2) Collection efficiency (unit: %)
A mixture of JIS Type 8 powder and JIS Type 11 powder at a ratio of 1:1 and diluted with water to a concentration of 0.05% by mass was used as the test liquid, and the filter medium was mixed with water. After moistening with water, 100 ml of the test liquid was filtered with the porous membrane surface of the filter media set on the upstream side, with a filtration area of 14 cm ² and a differential pressure of ΔP = 320 mmHg. The number of 1.0 μm particles was measured using a liquid particle counter (trade name: KL-01) manufactured by Rion, and the collection efficiency was calculated.

3) Frazier air permeability The air permeability required to obtain low pressure loss when using a filter medium as a filter is determined by measuring the Frazier air permeability using method A (Fragile type method) described in JIS L1096:2010. did.

4) Life test The repair efficiency of the filter medium is repeatedly evaluated using the test liquid in 2) above, and the number of times the evaluation is repeated until the Frazier air permeability measured by the method in 3) falls below 10 cm ³ /cm ² ·s The lifespan was evaluated.

5) Peel strength Peel strength was determined by cutting a filter medium 25 mm in the width direction and 150 mm in the flow direction, using a Tensilon universal testing machine, and holding the wet nonwoven fabric and porous membrane between the jigs and pulling as shown below. Visual judgment was made.

○: The porous membrane did not peel off from the base material at all.
△: It was observed that the porous membrane was peeled off from the base material when a strong force was applied. ×: It was observed that the porous membrane was easily peeled off from the base material when a smaller force than △ was applied.
-: Not evaluable

6) Strength during heat-pressure treatment Evaluation was performed by checking the state of the filter base material during heat-pressure treatment.

○: No damage such as tearing of the filter base material was observed during the heat and pressure treatment. ×: Damage such as tearing was observed on the filter base material during the heat and pressure treatment.

The above evaluation results are shown in Table 2. In Table 2, "-" indicates that evaluation was not possible.

The filter base materials of Examples 1 to 10 are filter base materials for use as filter media by bonding porous membranes together by heat-sealing, and the filter media is heat-sealed to non-binder fibers. It is a wet-laid nonwoven fabric containing binder fibers, and the blending ratio of heat-fusible binder fibers is 20 to 90% by mass, and the blending ratio of non-binder fibers is 10 to 80% by mass, based on the total fiber components contained in the wet-laid nonwoven fabric. It is a base material for a filter in mass %. Filter media in which the porous membrane and the filter base materials of Examples 1 to 10 were bonded together by heat fusion exhibited high peel strength. On the other hand, the porous membrane and the filter base material of Comparative Example 1 or Comparative Example 4 could not be bonded together by heat fusion, although no damage was observed in the filter base material during the heat-pressure treatment. .

Furthermore, it can be seen that higher Frazier air permeability and life are obtained than in the filter medium in which the porous membrane and the filter base material of Comparative Example 2 or Comparative Example 5 are bonded together by heat fusion. Furthermore, in Comparative Example 3 and Comparative Example 6, since glass fiber was used, the base material was damaged during the heat and pressure treatment.

The PP porous membranes used in Examples 1 to 5 have a lower heat resistance temperature than the PTFE porous membranes used in Examples 6 to 10, so they soften during hot pressure treatment and some of the pores are blocked. It was done. In Examples 6 to 10, it was possible to obtain filter media with more preferable Frazier air permeability than in Examples 1 to 5.

The filter base material of the present invention can be suitably used as a filter medium by bonding a porous membrane.

Claims (4)

In a filter base material for use as a filter medium by bonding porous membranes together by heat-sealing, the filter medium is a wet-laid nonwoven fabric containing non-binder fibers and heat-fusible binder fibers; A base material for a filter, characterized in that the blending ratio of heat-fusible binder fibers is 20 to 90% by mass and the blending ratio of non-binder fibers is 10 to 80% by mass, based on the total fiber components contained. . The filter substrate according to claim 1, wherein the non-binder fibers include wood pulp. The filter base material according to claim 1 or 2, wherein the non-binder fibers include synthetic fibers. 4. The filter base material according to claim 1, wherein the material of the porous membrane is polytetrafluoroethylene.