Filter medium for air filter and method for manufacturing same

Abstract

The present invention enables carbon nanotubes to be present not only in both surfaces of a non-woven fabric constituting a filter medium for an air filter but also within the thickness thereof, thereby obtaining good conductivity while achieving non-hydrophilicity.　Provided is a method for manufacturing a filter medium for an air filter, wherein a non-woven fabric is immersed in a dispersion liquid containing carbon nanotubes and then dried to manufacture a filter medium made of the non-woven fabric to which the carbon nanotubes have adhered. The method uses a spunbonded non-woven fabric made of synthetic fibers as the non-woven fabric. As the dispersion liquid, a dispersion liquid which is aqueous and contains a penetrant and a water repellent is used.

Description

Filter media for air filters and method for producing the same

[0001] The present invention relates to a filter medium for an air filter used in, for example, a dust collector for purifying exhaust gas.

[0002] The filters of dust collectors can catch fire. This is because the filter media becomes charged with static electricity due to friction between the airflow containing iron powder and the filter media, causing sparks. For this reason, filter media are made conductive to prevent static electricity.

[0003] Carbon black can be used as a conductive material, but because the particles of carbon black are spherical and coarse, they tend to fall off during post-processing of the nonwoven fabric, such as pleating. Furthermore, to achieve the desired conductivity, a large amount of carbon black must be attached to the nonwoven fabric, which reduces the air permeability.

[0004] To avoid such problems, carbon nanotubes, which have good conductivity and are difficult to fall off, have been used as a conductive material (see Patent Documents 1 and 2 below). Patent Document 1 relates to synthetic fibers that can also be used to manufacture antistatic bag filters, and Patent Document 2 relates to air purification filters, and in both cases, carbon nanotubes are attached to a material (synthetic fibers in Patent Document 1, and a laminate of fibrous materials in Patent Document 2). The carbon nanotubes are attached when the material is immersed in a dispersion liquid containing the carbon nanotubes and then the liquid is removed from the material.

[0005] However, when carbon nanotubes are attached to a laminate of fibrous materials, they have difficulty penetrating the interior, so they end up concentrating on the surface of the laminate and do not provide good conductivity in the thickness direction.Even when carbon nanotubes are attached to fibers, they may fall off when the fibers are processed into a nonwoven fabric, and if they do fall off, the desired conductivity will not be achieved.

[0006] To incorporate hydrophobic carbon nanotubes into the thickness of a nonwoven fabric made of synthetic fibers such as hydrophobic polyester, a surfactant can be used. Specifically, a surfactant is mixed into the dispersion liquid. However, to incorporate the carbon nanotubes between the fibers using the surfactant, not only is a large amount of surfactant required, but even if the carbon nanotubes are successfully attached, the filter media becomes hydrophilic. It is preferable for the air filter's properties to be non-hydrophilic in order to maintain low pressure loss when processing gas containing dust and moisture.

[0007] JP 2010-261108 A JP 2017-51384 A

[0008] Therefore, the main object of the present invention is to make it possible to have carbon nanotubes present within the thickness of the nonwoven fabric, thereby obtaining good electrical conductivity, while at the same time making the fabric non-hydrophilic.

[0009] To this end, the present invention provides the following method for producing a filter medium for an air filter.

[0010] In a method for manufacturing a filter media for an air filter, a nonwoven fabric is immersed in a dispersion containing carbon nanotubes and then dried to produce a filter media consisting of a nonwoven fabric with carbon nanotubes attached thereto. The nonwoven fabric is a spunbonded synthetic fiber nonwoven fabric. The dispersion is either an aqueous dispersion containing a penetrating agent and a water repellent, or an aqueous dispersion containing a penetrating agent. The penetrating agent is preferably a material that dissolves well in water, has a strong degreasing effect, and dries quickly, such as isopropyl alcohol. The water repellent is preferably a material with a high deformation temperature and good acid and alkali resistance, such as an isocyanate-based water repellent or a urethane-based water repellent.

[0011] In this configuration, the penetrant in the dispersion quickly penetrates between the fibers of the nonwoven fabric, causing the carbon nanotubes to adhere to the fibers. Upon drying, the penetrant evaporates, and the water-repellent agent imparts water-repellency to the fibers.

[0012] According to this invention, carbon nanotubes are present not only on both sides of the nonwoven fabric but also within its thickness, so that dust can be collected with low pressure loss even when the gas contains moisture. Moreover, the water repellent agent prevents the nonwoven fabric from becoming hydrophilic, so dust collection is also good.

[0013] FIG.

[0014] An embodiment of the present invention will be described below with reference to the drawings.

[0015] Air filter media (hereafter referred to as "filter media") are primarily used in dust collectors that collect dust in exhaust gases from industrial facilities. They are made of nonwoven fabric, and because static electricity can build up during exhaust gas treatment and cause sparks that could lead to fire, they are made electrically conductive to prevent static buildup. The conductivity is achieved by attaching carbon nanotubes to the nonwoven fabric, which are held on both the front and back of the fabric, as well as within the thickness of the wall.

[0016] Carbon nanotubes are attached to the nonwoven fabric by immersing the nonwoven fabric in a dispersion liquid containing carbon nanotubes and then drying it, resulting in a state in which carbon nanotubes are attached to the surface and interior of the nonwoven fabric as described above. The manufacturing process, as outlined in the schematic diagram of FIG. 1 , includes an immersion step S1 in which the nonwoven fabric 11 is immersed in a dispersion liquid 12, a squeezing step S2 in which the nonwoven fabric 11 is dehydrated while being pulled up after immersion, and a drying step S3 in which the dehydrated nonwoven fabric 11 is dried. That is, the nonwoven fabric 11 is formed into a long length and wound up, and as it is unwound, it passes through the dispersion liquid 12 filled in a vat 13 and is compressed in the thickness direction by a pair of pressure rolls 15 provided upstream of a dryer 14. The nonwoven fabric 11 dehydrated by the pressure rolls 15 passes through the dryer 14 and is wound up into a roll.

[0017] The filter media thus produced may be subjected to necessary processing such as cutting or pleating, as required.

[0018] The nonwoven fabric used is one commonly used for air filters. Among these, synthetic spunbond nonwoven fabrics, especially those made of polyester long fibers, are preferable because they do not fray or produce lint. The desired basis weight, thickness, and air permeability of the nonwoven fabric are selected depending on the object to be filtered. Note that polyester fibers, like polyethylene and polypropylene, are hydrophobic and do not easily dissolve in water.

[0019] The dispersion liquid used is water-based and has low viscosity. The carbon nanotubes dispersed in the dispersion liquid are preferably high-purity single-walled carbon nanotubes that have a large specific surface area, are long, and have a high aspect ratio. The dispersion liquid contains a penetrating agent and a water-repellent agent in addition to the carbon nanotubes. That is, the dispersion liquid is composed of at least water, carbon nanotubes, a penetrating agent, and a water-repellent agent.

[0020] The proportion of carbon nanotubes in the dispersion should be as low as possible while still providing the necessary conductivity, in order to ensure sufficient ventilation and reduce manufacturing costs. Furthermore, if the proportion of carbon nanotubes in the dispersion is high, they may be less likely to penetrate into the nonwoven fabric. A suitable preparation example is as follows:

[0021] A commercially available carbon nanotube aqueous dispersion using water as a dispersion medium is diluted with water, and a penetrating agent and a water-repellent agent are added to prepare a dispersion. It is recommended to mix equal or approximately equal amounts of the penetrating agent and the water-repellent agent. Specifically, if the carbon nanotube concentration in the carbon nanotube aqueous dispersion is 0.2 wt%, the carbon nanotube aqueous dispersion is mixed with water, the penetrating agent, and the water-repellent agent in a ratio of 5:91:2:2, respectively. In this case, the carbon nanotube concentration in the dispersion is approximately 1 wt%.

[0022] Isopropyl alcohol was used as the penetrating agent because it dissolves well in water, has a strong degreasing effect, and dries quickly. A urethane-based water repellent was used as the water repellent because it has a high deformation temperature and is resistant to acids and alkalis. An isocyanate-based water repellent may also be used.

[0023] The dispersion 12 prepared in this manner is thoroughly stirred and filled into a tray 13. The nonwoven fabric 11 is transported and submerged in the dispersion 12 by a dipping roll 16, whereupon the nonwoven fabric 11 absorbs the dispersion 12. At this time, the penetrant acts as a degreasing agent, causing the hydrophobic carbon nanotubes to quickly penetrate between the fibers of the nonwoven fabric. The carbon nanotubes adhere to the fibers within the thickness of the nonwoven fabric, including both the front and back surfaces. Meanwhile, the water repellent agent comes into contact with the fiber surfaces of the nonwoven fabric 11.

[0024] After the immersion step S1, the nonwoven fabric 11 is dehydrated by a pair of pressure rolls 15 in a squeezing step S2 to remove excess water. The pair of pressure rolls 15 is composed of a rubber roller 15a and a metal roller 15b. The rubber roller 15a and the metal roller 15b are each positioned so as to pressurize a predetermined surface of the nonwoven fabric 11. That is, the rubber roller 15a is positioned in contact with the upstream side of the nonwoven fabric 11, which is the side through which unfiltered air enters, and the metal roller 15b is positioned in contact with the downstream side, which is the side through which filtered air exits.

[0025] The metal roller 15b applies a strong pressure, so the low-viscosity aqueous dispersion is strongly dehydrated. On the other hand, the rubber roller 15a applies a lower pressure than the metal roller 15b, so the dehydration is not too strong, allowing the carbon nanotubes and resin components to adhere sufficiently. Moreover, unlike when a pair of pressure rollers is made up of a rubber roller and a rubber roller, sufficient dehydration is possible for drying in the next drying step S3.

[0026] After the squeezing step S2, the nonwoven fabric 11 is transported to the drying step S3 where it is dried, the penetrating agent evaporates, and the carbon nanotubes and the resin component of the water repellent agent that were in the dispersion liquid adhere to the fiber surface of the nonwoven fabric 11.

[0027] The filter media constructed in this manner has carbon nanotubes on both the front and back surfaces of the nonwoven fabric 11 and within the thickness thereof, with the upstream side, where unfiltered air enters, holding more carbon nanotubes than the downstream side, where filtered air exits.

[0028] Because carbon nanotubes are present not only on both the front and back surfaces of the nonwoven fabric but also within its thickness, it is possible to obtain a filter medium with excellent surface conductivity as well as volume conductivity. Furthermore, the amount of carbon nanotubes attached is greater on the upstream side, where dust tends to adhere and become charged, than on the opposite side, and this, combined with the fact that carbon nanotubes are attached even to the interior, results in better antistatic performance.

[0029] Furthermore, the water-repellent agent mixed into the impregnated dispersion prevents the nonwoven fabric from becoming hydrophilic and makes it water-repellent, so pressure loss can be kept low and dust can be collected effectively even when the dust is a gas containing moisture. Carbon nanotubes are naturally highly conductive and do not easily fall off once they adhere to the nonwoven fabric fibers, which gives them the excellent antistatic properties mentioned above. However, the water-repellent agent also adheres to the nonwoven fabric fibers, making the carbon nanotubes even more difficult to fall off.

[0030] Furthermore, the dispersion liquid used in the production is a diluted version of a commercially available aqueous dispersion of carbon nanotubes, which reduces production costs. By limiting the amount of carbon nanotubes in the dispersion liquid to about 1%, the amount used is kept to a minimum, which is effective not only in reducing production costs but also in not inhibiting ventilation.

[0031] By adjusting the blending amounts of the water repellent and penetrant in the dispersion to 2%, the required functions can be achieved with smaller amounts used.

[0032] In addition, the difference in the amount of carbon nanotubes attached on the upstream and downstream sides of the filter media as described above can be achieved by changing the type and combination of pressure rolls used in the squeezing process, so there is no need to add any special configuration, making processing easy.

[0033] A comparative example will be described below to demonstrate the suitability of the above-mentioned example for preparing the dispersion. The processed nonwoven fabric was a spunbond nonwoven fabric made of the same polyester long fibers as described above. Its thickness was 0.62 mm, its basis weight was 260 g / m², and its air permeability was 11 cc / cm²·sec. Comparative Example 1: A commercially available carbon nanotube dispersion using methyl ethyl ketone (MEK) as the solvent was used as the undiluted dispersion, and nonwoven fabric was processed using the same process as described above. The carbon nanotube concentration in the carbon nanotube dispersion was 0.3 wt%.

[0034] Although the methyl ethyl ketone penetrated the nonwoven fabric during the immersion process, the carbon nanotubes remained on the surface of the nonwoven fabric and did not penetrate into the interior, so it was determined that processing was impossible. Comparative Example 2: A commercially available carbon nanotube dispersion using isopropyl alcohol (IPA) as the solvent was used as the dispersion liquid, and nonwoven fabric was processed using the same process as above. The carbon nanotube concentration of the carbon nanotube dispersion liquid was 0.3 wt%.

[0035] Although the methyl ethyl ketone penetrated into the nonwoven fabric during the immersion process, the carbon nanotubes remained on the surface of the nonwoven fabric and did not penetrate into the interior, so it was determined that processing was impossible. Comparative Example 3: A water-based aqueous dispersion was used as the dispersion. A commercially available carbon nanotube aqueous dispersion was used as is, and nonwoven fabric was processed using the same process as described above. The carbon nanotube concentration in the carbon nanotube dispersion was 0.2 wt%.

[0036] During the immersion process, it was confirmed that the carbon nanotubes penetrated the nonwoven fabric. The required conductivity was confirmed when the conductivity was measured. However, it was found that the surfactant (dispersant) mixed into the carbon nanotube aqueous dispersion made the nonwoven fabric hydrophilic, making it unusable for air filters. Furthermore, it became clear that using the undiluted solution would increase manufacturing costs. Comparative Example 4: To avoid hydrophilicity, a water repellent was used, and the effect of the water repellent on conductivity was confirmed. Furthermore, the possibility of dilution was confirmed by diluting the undiluted solution to reduce manufacturing costs.

[0037] The dispersions were prepared using a commercially available carbon nanotube aqueous dispersion and dilution water alone, and a commercially available carbon nanotube aqueous dispersion to which dilution water and a water repellent agent were added. The carbon nanotube aqueous dispersion was the same as that described above, with a carbon nanotube concentration of 0.2 wt %. The water repellent agent used was a fluorine-based agent commonly used for water-repellent finishing of spunbond nonwoven fabrics.

[0038] The results of the conductivity of the samples and the nonwoven fabrics processed using them are shown in Table 1 below. The numbers in Table 1 indicate the blending ratio. Good conductivity is marked with "○", and poor conductivity is marked with "×".

[0039] Samples 5 and 6, which contain a water repellent, also achieved the required conductivity, similar to Samples 2 to 4, which do not contain a water repellent. Moreover, the required conductivity was achieved in both Sample 5, which contains more water repellent than the carbon nanotubes, and Sample 6, which contains less water repellent than the carbon nanotubes. Therefore, it is believed that the water repellent does not adversely affect or inhibit conductivity, and it can be said that the amount of water repellent relative to the carbon nanotubes does not have a significant adverse effect as long as it is at the above level.

[0040] Furthermore, sample 1 had poor conductivity, but samples 2 to 6 had good conductivity. Sample 1 had the lowest blending ratio of carbon nanotube aqueous dispersion, so it was found that conductivity could be obtained even after dilution as long as the carbon nanotube concentration was above a certain level, that is, conductivity could be obtained even after dilution.

[0041] Focusing on the carbon nanotube concentration, the results of Samples 1 and 2 show that in the case of an aqueous dispersion with a carbon nanotube concentration of 0.2 wt%, when the carbon nanotube content is 3% of the total dispersion, the conductive performance is poor, but when the content is 5%, the conductive performance is obtained. In other words, when the carbon nanotube concentration in the dispersion is about 0.8 wt%, the conductivity is insufficient, but when it is about 1 wt%, the required conductivity is obtained. For this reason, it is thought that a carbon nanotube concentration of about 1 wt% is necessary in the dispersion.

[0042] Furthermore, the nonwoven fabrics using Samples 5 and 6 also exhibited water repellency. However, the water repellency was not uniform throughout, and scattered areas where the water repellency was not observed were observed. Comparative Example 5: It was found that the blending ratio of the aqueous dispersion containing 0.2 wt% carbon nanotubes and the blending of a water repellent were better. However, in light of PFAS regulations, there is a demand for the use of water repellents other than fluorine-based ones.

[0043] Therefore, we investigated other water repellents and selected an isocyanate-based water repellent and a urethane-based water repellent from the viewpoint of their high deformation temperature property, water repellency, acid resistance, and alkali resistance, and prepared dispersions using each of them as the water repellent.

[0044] Just to be sure, two types of dispersions with a ratio of [carbon nanotube aqueous dispersion:water:water repellent agent] of [3:95:2] and [5:93:2] were prepared, and the same processing was carried out on the same nonwoven fabric as above.

[0045] All four types of nonwoven fabrics exhibited satisfactory breathability and appeared suitable for use in air filters. However, the nonwoven fabric containing 3% aqueous carbon nanotube dispersion did not exhibit sufficient conductivity. In contrast, the nonwoven fabric containing 5% aqueous carbon nanotube dispersion exhibited conductivity. However, this conductivity was only surface conductivity, and volume conductivity was insufficient. Close observation of the processed nonwoven fabric revealed variations in surface shading. From this, it was possible to infer that there were areas where the carbon nanotubes had not penetrated to the interior, although it is unclear whether this was due to compatibility with the water repellent. Comparative Example 6: A penetrant was mixed to ensure uniform impregnation of the dispersion. While common penetrants such as surfactants could be considered, alcohol was selected to avoid hydrophilicity. Isopropyl alcohol was used because it dissolves well in water, has strong degreasing properties, and dries quickly.

[0046] The ratio of carbon nanotube aqueous dispersion: water: water repellent: penetrant in the dispersion was 5:91:2:2. The reason why the blending ratio of the penetrant to the water repellent was set to the same amount was because a small amount is preferable and in consideration of the balance with the action of the water repellent.

[0047] The nonwoven fabric obtained after processing was checked and found to have good breathability, sufficient water repellency, and good surface and volume conductivity.

[0048] The above description is one embodiment for carrying out the present invention, and the present invention is not limited to the above-mentioned configuration, but other configurations can be adopted.

[0049] For example, the nonwoven fabric does not have to have the thickness or breathability described above.

[0050] The dispersion may be prepared by dispersing carbon nanotubes, in addition to using a commercially available aqueous dispersion of carbon nanotubes.

[0051] S1... Immersion step S2... Squeezing step S3... Drying step 11... Nonwoven fabric 12... Dispersion liquid 15... Pressure roll 15a... Rubber roller 15b... Metal roller

Claims

1. A method for producing a filter media for an air filter, in which a nonwoven fabric is immersed in a dispersion containing carbon nanotubes and then dried to produce a filter media consisting of a nonwoven fabric having carbon nanotubes attached thereto, wherein the nonwoven fabric is a spunbond nonwoven fabric made of synthetic fibers, and the dispersion is an aqueous dispersion containing a penetrating agent and a water repellent.

2. A method for producing a filter media for an air filter, in which a nonwoven fabric is immersed in a dispersion containing carbon nanotubes and then dried to produce a filter media consisting of a nonwoven fabric having carbon nanotubes attached thereto, wherein the nonwoven fabric is a spunbond nonwoven fabric made of synthetic fibers, and the dispersion is an aqueous dispersion containing a penetrating agent.

3. The method for manufacturing filter media for air filters according to claim 1, wherein the dispersion is prepared by diluting a carbon nanotube aqueous dispersion in which carbon nanotubes are dispersed in water with water, and mixing equal amounts of the penetrating agent and the water repellent agent.

4. The method for producing a filter medium for an air filter according to claim 1 or 2, wherein the penetrating agent is isopropyl alcohol.

5. The method for producing a filter medium for an air filter according to claim 1, wherein the water repellent is a urethane-based water repellent.

6. The method for producing a filter medium for air filters according to claim 3, wherein the penetrating agent and the water repellent agent are mixed in a ratio of 2% each of the dispersion liquid.

7. A method for producing filter media for air filters according to claim 1 or 2, further comprising a squeezing step of dehydrating the nonwoven fabric after immersion in the dispersion liquid and before drying, wherein the squeezing step is carried out by compressing with a pair of rotating pressure rolls, the pair of pressure rolls consisting of a rubber roller and a metal roller, and the upstream side, through which unfiltered air enters, is compressed with the rubber roller, and the downstream side, through which filtered air exits, is compressed with the metal roller.

8. A filter medium for an air filter made of spunbond nonwoven fabric, in which carbon nanotubes are present on both the front and back sides of the nonwoven fabric and within the thickness of the fabric, and the amount of carbon nanotubes attached is greater on the upstream side, where unfiltered air enters, than on the downstream side, where filtered air exits.

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