Electret fiber sheets and electret filters

Abstract

To provide an electret fiber sheet with high filtration characteristics. [Solution] The electret fiber sheet of the present invention has constituent fibers containing polyolefin resin and nitrogen-containing compounds, has a crystallization temperature of 125°C or lower, a 2% elongation stress of 0.12 to 0.18 N / s per unit basis weight, a 3% elongation stress of 0.20 to 0.26 N / s per unit basis weight, and a filter media quality factor (QF) value of 1.2 or higher, as shown by the following formula. QF[mmAq -1 ]=-[ln(1-(particle collection efficiency (%) / 100))] / [airflow resistance (mmAq)]

Description

Technical Field

[0001] The present invention relates to an electret fiber sheet and an electret filter.

Background Art

[0002] Conventionally, air filters have been used to remove atmospheric dust and the like, and fiber sheets are widely used as filter media. Filters made of these fibrous materials capture particles on the fibers by mechanical collection mechanisms such as sieving, diffusion, and inertial collision. However, it is known that in a practical use environment, when the aerodynamic equivalent diameter of the particles to be captured is about 0.1 to 1.0 μm, the filter has a minimum collection efficiency. Therefore, an electret filter that utilizes the electrostatic attraction of electrets to compensate for this weakness is used.

[0003] The performance required for air filters is high collection efficiency and low pressure loss, but high collection efficiency and low pressure loss are in an inverse relationship. Electret filters are widely used to solve this problem.

[0004] In order to improve the collection efficiency of electret filters, it is known that it is preferable to impart an electrostatic charge to fibrous materials by a method of bringing a liquid into contact with or colliding with the fibrous materials (liquid contact charging method) to make them electrets. For example, as an electret filter that balances cost and performance, an electrostatic charge is imparted by bringing a liquid such as water into contact with fibrous materials formed from a mixture in which a nitrogen-containing compound such as a hindered amine compound is added to a resin mainly composed of a polyolefin resin.

[0005] In addition, attempts have been made to simultaneously satisfy high collection efficiency and low pressure loss by adding a crystal nucleating agent or a charge enhancing additive to a thermoplastic resin, or by adding a metal salt to a fiber sheet (see Patent Documents 1 to 3). In addition, a proposal has been made to increase the voids between fibers by forming a non-woven fabric in which fine fibers and thick fibers are mixed, thereby suppressing an increase in pressure loss (Patent Document 4).

[0006] [Patent Document 1] Japanese Patent Publication No. 2018-40098 [Patent Document 2] International Public Publication WO2018 / 105546 [Patent Document 3] Special Publication No. 2018-523761 [Patent Document 4] Japanese Patent Publication No. 2006-037295 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, in recent years, there has been a demand for improved filtration performance in air filters such as air purifier filters and cabin filters, including increased airflow and higher efficiency, and conventionally known methods are insufficient. Therefore, the present invention has been made in view of the above problems, and its objective is to provide an electret fiber sheet and an electret filter that have high filtration performance that simultaneously satisfy high collection efficiency and low pressure loss, which could not be achieved with conventionally known electrets. [Means for solving the problem]

[0008] As a result of diligent research by the inventors, it was discovered that an electret fiber sheet and an electret filter with high filtration performance can be obtained by using the following configuration, leading to the present invention. That is, the present invention is as follows.

[0009] 1. An electret sheet comprising a fiber sheet having constituent fibers containing polyolefin resin and nitrogen-containing compounds, characterized in that the crystallization temperature is 125°C or lower, the stress at 2% elongation per unit basis weight is 0.12 to 0.18, the stress at 3% elongation per unit basis weight is 0.20 to 0.26, and the filter material quality factor (QF) value is 1.3 or higher. 2. The electret sheet according to item 1 above, wherein the average fiber diameter of the constituent fibers is 1.5 to 10 μm. 3. An electret filter using the electret sheet described in 1 or 2 above. [Effects of the Invention]

[0010] The present invention provides electret fiber sheets and electret filters that have high filtration performance while achieving both high collection efficiency and low pressure loss. [Modes for carrying out the invention]

[0011] The present invention will be described in detail below, but the present invention is not limited to the following, and it is possible to implement it with appropriate modifications within the scope that is consistent with the spirit of the preceding and following, and all such modifications are included within the technical scope of the present invention.

[0012] The electret fiber sheet of the present invention has constituent fibers comprising a polyolefin resin and a nitrogen-containing compound.

[0013] <Polyolefin resin> The electret fiber sheet of the present invention uses a polyolefin resin that is hydrophobic and has high electrical resistance, from the viewpoint of freedom of shape and charge stability. Examples of polyolefin resins include homopolymers of olefins such as ethylene, propylene, butylene, hexene, octene, butadiene, isoprene, chloroprene, methyl-1-pentene, and cyclic olefins, or copolymers composed of two or more of the above olefins. One type of polyolefin resin may be selected and used alone, or two or more types may be selected and used in combination. The polyolefin resin preferably contains at least one selected from polyethylene, polypropylene, and polymethylpentene, and more preferably contains polypropylene. The content of the polyolefin resin in 100 parts by mass of electret is preferably 80% by mass or more, more preferably 85% by mass or more, even more preferably 90% by mass or more, even more preferably 95% by mass or more, and particularly preferably 97% by mass or more. There is no particular upper limit to the content of the polyolefin resin, for example, it is 99.5% by mass or less, and preferably 99% by mass or less.

[0014] In the polypropylene resin used in the present invention, the degree of stereoregularity is preferably 85% or more, more preferably 90% or more, even more preferably 90% or more, and particularly preferably 95% or more. In this case, either isotactic or syndiotactic polypropylene can be preferably used. When two or more types of polypropylene are used, it is preferable that one or more types are included in the polyolefin contained.

[0015] The melt flow rate (MFR) of the polyolefin resin used in this invention is not particularly limited, but at a temperature of 230°C and a load of 2.16 kg in accordance with ASTM D 1238, It is preferable that the amount is 10g / 10 minutes or more, more preferably 100g / 10 minutes or more, even more preferably 500g / 10 minutes or more, and particularly preferably 1000g / 10 minutes or more.

[0016] The polyolefin resin used in the present invention may be "derived from fossil raw materials", "derived from plants", or a mixture of both, and a composition advantageous from the viewpoints of availability and LCA can be selected.

[0017] <Nitrogen-containing compound> The content ratio of the nitrogen-containing compound with respect to 100% by mass of the electret fiber sheet of the present invention is 0.1 to 5% by mass, preferably 0.5 to 3% by mass, and more preferably 0.75 to 1.5% by mass. When the content ratio of the nitrogen-containing compound is lower than 0.1% by mass, the charge amount becomes low, resulting in a decrease in filtration performance. When it is higher than 5% by mass, the stability as an electret is lost due to an increase in hygroscopicity.

[0018] The nitrogen-containing compound is not particularly limited as long as it can provide the above-described desired properties, but is preferably a hindered amine compound containing at least one of a 2,2,6,6-tetramethylpiperidyl structure and a triazine structure. More preferably, the hindered amine compound contains a 2,2,6,6-tetramethylpiperidyl structure and a triazine structure.

[0019] Hindered amine compounds are not particularly limited, but examples include poly[{6-(1,1,3,3-tetramethylbutyl)amino-1,3,5-triazine-2,4-diyl}{(2,2,6,6-tetramethyl-4-piperidyl)imino}hexamethylene{(2,2,6,6-tetramethyl-4-piperidyl)imino}] (Kimasorb® 944LD, manufactured by BASF Japan), dimethyl-1-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethyl-4-piperidine polycondensate (Tinuvin® 622LD, manufactured by BASF Japan), and 2-[[3,5-bis(1,1-dimethylethyl)-4-hydroxy Examples include phenylmethyl]-2-butylpropanediate bis[1,2,2,6,6-pentamethyl-4-piperidinyl] (Tinuvin® 144, manufactured by BASF Japan), dibutylamine 1,3,5-triazine·N,N-bis(2,2,6,6-tetramethyl-4-piperidyl)-1,6-hexamethylenediamine·N-(2,2,6,6-tetramethyl-4-piperidyl)butylamine polycondensate (Chimasorb® 2020FDL, manufactured by BASF Japan), and 2-(4,6-diphenyl-1,3,5-triazine-2-yl)-5-(hexyloxy)-phenol (Tinuvin® 1577FF, manufactured by BASF Japan). In particular, it is preferable that the compound contains a 2,2,6,6-tetramethylpiperidine structure and a triazine structure, and more preferably Kimasove® 944LD or Kimasove® 2020FD. One of the above compounds may be used alone as the hindered amine compound, or two or more may be used in combination.

[0020] <Crystallization temperature> The electret fiber sheet of the present invention preferably has a crystallization temperature of 125°C or lower. It is known that the crystallization temperature can be increased by adding various crystallization nucleating agents to polyolefin resin, and while this can suppress fiber fusion, it can lead to yarn breakage due to poor stretching and difficulty in achieving fine denier. In the present invention, an electret with appropriate fusion can be obtained without increasing the crystallization temperature.

[0021] <Tensile stress at elongation> The key point of the present invention is that the tensile stress at 2% elongation per unit basis weight of the electret fiber sheet is 0.12 to 0.18 N / g, and the tensile stress at 3% elongation per unit basis weight is 0.20 to 0.26 N / g. That is, by setting the tensile stress at elongation per unit basis weight as described above, the fusion between fibers is made appropriate, and the increase in voids enables both high collection efficiency and low pressure loss. As a specific method for obtaining such an electret fiber sheet, in the conventionally known melt blowing method, it is preferable to adjust the fiber orientation and the degree of fusion, and the conveying speed, conveyor angle, collection distance, discharge amount, nozzle pitch, nozzle L / D, resin temperature, hot air temperature, hot air flow rate, etc. may be adjusted.

[0022] When the tensile stress at 2% elongation and 3% elongation per unit basis weight is lower than these ranges, the fiber sheet breaks and tears during the winding process of the sheet, which is not preferable. Conversely, when the tensile stress at 2% elongation and 3% elongation per unit basis weight is higher than these ranges, the fusion between fibers is too strong, the voids decrease, and the pressure loss increases, which is not preferable.

[0023] As a method for making the fusion appropriate, a method of adding a nucleating agent can be considered, but since the crystallization temperature increases, the ductility decreases, and it becomes difficult to make the fiber sheet finer and denser. That is, the present invention is to obtain an electret fiber sheet having high filtration performance by setting the tensile stress at 2% elongation and 3% elongation per unit basis weight within the above ranges without increasing the crystallization temperature. The method for measuring the elongation stress will be described later.

[0024] <Shape of electret> The electret fiber sheet of the present invention is a fiber aggregate, and examples of fiber aggregates include fibrous materials such as woven or knitted fabrics, nonwoven fabrics, and cotton-like materials made of long or short fibers, as well as fibrous materials obtained from stretched films. A fiber aggregate refers to a state in which a fibrous form is observed when the surface is observed with a device such as a scanning electron microscope or an optical microscope, and at least some of the fibers constituting the fiber aggregate are integrated by melting or inter-fiber entanglement.

[0025] When electret fiber sheets are used for filter applications, the fiber aggregate is preferably a nonwoven fabric. Methods for obtaining a nonwoven fabric include methods of forming sheets from single-component fibers, composite fibers such as core-sheath fibers and side-by-side fibers, and short fibers such as split fibers by carding, airlaid, or wet papermaking; and methods of forming sheets from continuous fibers using spunbond, meltblown, electrospinning, or force spinning methods. Nonwoven fabrics obtained by spunbond, meltblown, molten electrospinning, or molten force spinning methods are more preferable from the viewpoint of not requiring treatment of residual solvents or spinning oils adhering to the surface, and nonwoven fabrics obtained by meltblown are particularly preferable.

[0026] The average fiber diameter of the constituent fibers of the electret fiber sheet of the present invention is preferably 0.001 to 100 μm, more preferably 0.05 to 50 μm, even more preferably 0.1 to 30 μm, particularly preferably 0.5 to 25 μm, and most preferably 1.5 to 10 μm. If the average fiber diameter of the constituent fibers is thicker than 100 μm, it is difficult to obtain a practical collection efficiency. Also, if the average fiber diameter of the constituent fibers is thinner than 0.001 μm, it is difficult to produce an electret with an applied charge. The fineness is calculated by measuring the diameter of 50 fibers in the same field of view using a scanning electron microscope, ensuring that there is no overlap of fibers, and then calculating the geometric mean.

[0027] The basis weight of the electret fiber sheet of the present invention is 1 to 80 g / m². 2Preferably, it is 5-60 g / m 2 It is more preferable that the amount be 10-50 g / m 2 It is even more preferable that the amount be 15-30 g / m 2 It is particularly preferable that this be the case.

[0028] The electret fiber sheet of the present invention may be a homogeneous material made from a single manufacturing method and material, or it may be a mixture made from two or more materials with different manufacturing methods and materials.

[0029] The electret formation method in the present invention is not particularly limited as long as it allows for the acquisition of desired properties when the electret is used. However, for the electret fiber sheet of the present invention, a method of contacting or impacting a liquid with a fibrous material (liquid contact charging method) is preferred, and an electret with high filtration properties can be obtained by the liquid contact charging method. More specifically, a method of contacting or impacting a liquid with a fibrous material by means of suction, pressurization, or ejection is preferred.

[0030] In the liquid contact charging method, the liquid to be contacted or impacted is not particularly limited as long as the desired properties can be obtained, but water is preferred in terms of handling and performance. A liquid obtained by adding a secondary component (a component other than water) to water may be used instead of water, and the conductivity and pH of the liquid can be adjusted by the type and amount of the secondary component added.

[0031] In the liquid contact charging method, the liquid to be contacted or impacted preferably has a pH of 1 to 11, more preferably 3 to 9, and even more preferably 5 to 7. Furthermore, the liquid to be contacted or impacted preferably has an electrical conductivity of 100 μS / cm or less, more preferably 10 μS / cm or less, and even more preferably 3 μS / cm or less.

[0032] Filters using the electret fiber sheet of the present invention are also included in the scope of the present invention. When the electret fiber sheet of the present invention is used as a filter, the filter media quality factor (QF) value described later is 1.2 mmAq-1 The above is preferable, 1.3 mmAq -1 The above is more preferable. In particular, it is especially preferable that the above QF value is obtained with electret filters with an average fiber diameter of 1.5 to 10 μm manufactured by the meltblown method. 1.2 mmAq -1 If the value falls below this level, the electret filter will not adequately capture the particles, resulting in insufficient filter performance. The QF value used herein is calculated based on the airflow resistance when air is passed through the sheet thickness direction at an airflow velocity of 10.4 cm / s and the number of particles counted by a laser particle counter in the particle size category of 0.3 to 0.5 μm.

[0033] When the electret fiber sheet of the present invention is used as a filter, the particle collection efficiency at a wind speed of 10.4 cm / s can be adjusted in various ways according to the required characteristics. The particle collection efficiency is preferably 50% or more, more preferably 70% or more, even more preferably 80% or more, and particularly preferably 90% or more. The particle collection efficiency described herein is calculated based on the number of particles in the particle size category of 0.3 to 0.5 μm measured by a laser particle counter before and after passing through the filter when air is passed through the filter in the thickness direction at a wind speed of 10.4 cm / s.

[0034] When the electret fiber sheet of the present invention is used as a filter, the airflow resistance at a wind speed of 10.4 cm / s is preferably in the range of 0.05 to 50 mmAq, more preferably 0.2 to 30 mmAq, and particularly preferably 0.5 to 20 mmAq. If the airflow resistance is too low, the filter performance will be insufficient, and if the airflow resistance is too high, the advantages of the electret filter will be lost.

[0035] <Other> The electret fiber sheet and electret filter of the present invention can be used in combination with other components as needed. That is, the electret of the present invention can be used in combination with a pre-filter layer, fiber protection layer, reinforcing member, functional fiber layer, etc.

[0036] Examples of pre-filter layers and fiber protection layers include spunbond nonwoven fabrics, thermal bond nonwoven fabrics, and polyurethane foam, while examples of reinforcing members include thermal bond nonwoven fabrics and various types of nets. Examples of functional fiber layers include antibacterial, antiviral, and colored fiber layers for identification or design purposes.

[0037] The electret fiber sheet of the present invention can be used in a wide range of applications. In particular, it can be suitably used as a filter for protection of various devices, such as dust masks, dustproof clothing, various air conditioning elements, air purifiers, cabin filters, and protective equipment, for purposes such as protection, breathability, stain resistance, and waterproofing.

[0038] The electret fiber sheet of the present invention may contain other polymers, colorants, stabilizers, nucleating agents, and other compounding agents as needed, as long as they do not impair the objectives of the present invention. Examples of components that may be optionally added include conventionally known heat-resistant stabilizers, weather-resistant stabilizers and other stabilizers, slip agents, anti-blocking agents, anti-fogging agents, lubricants, dyes, pigments, natural oils, synthetic oils, and the like. [Examples]

[0039] The present invention will be described in more detail below with reference to examples, but the present invention as well as the following examples It is not restricted by this, and appropriate changes may be made to the extent that they are consistent with the spirit of the preceding and following provisions. It is certainly possible to implement these methods as well, and all of them fall within the technical scope of the present invention. .

[0040] First, I will explain the method for measuring physical properties. (1) Crystallization temperature Using a TA Instruments DSC2500, a 1 mg sample of electret fiber sheet was packed into an aluminum pan, sealed, and heated from 20°C to 180°C under a nitrogen gas atmosphere at a heating rate of 5°C / min to obtain the heating DSC curve. Subsequently, the temperature was cooled down to 20°C at a cooling rate of 5°C / min to obtain the cooling DSC curve. The crystallization temperature was determined by rounding the decimal part of the exothermic peak in the cooling DSC curve.

[0041] (2) Weight and tensile stress per unit weight Samples measuring 50 mm wide and 200 mm long were prepared from electret fiber sheets. The weight of each sample was measured, and the obtained values ​​were converted to a unit area. The arithmetic mean of these values ​​was then rounded to the first decimal place. The samples were placed in a testing machine (Tensilon universal material testing machine) in the longitudinal direction, and tensile tests were performed eight times each in the longitudinal (MD direction) and transverse (TD direction) directions under conditions of a gripping distance of 100 mm and a tensile speed of 300 mm / min. The strength at 2% elongation and the strength at 3% elongation of the sample were divided by the basis weight, respectively, to obtain the stress at 2% elongation and 3% elongation per unit basis weight. The elongation stress is the average value of the longitudinal and transverse directions (n=16).

[0042] (3) Air resistance A sample punched out to 72mmφ is attached to a 50mmφ effective airflow adapter, and a slight differential pressure is applied. Two pipes with an inner diameter of 50 mm, each connected to a meter, were linked vertically. Air was passed through the sample in the direction of the sample thickness at a wind speed of 10 cm / s, and the differential pressure between the top and bottom of the sample without any constriction was measured as the airflow resistance (pressure loss).

[0043] (4) Particle collection efficiency and particle transmission rate A sample punched to 72 mm in diameter was mounted in an adapter with an effective airflow diameter of 50 mm in diameter. Air was passed through the sample in the thickness direction at a wind speed of 10 cm / s, and the particle collection efficiency was measured using a light scattering particle counter KC-01E manufactured by Rion Co., Ltd. in the following manner. Particles evaluated: atmospheric dust particles Wind speed: 10cm / s Particle collection efficiency [%] = (1 - (Number concentration of particles with a particle size of 0.3 to 0.5 μm after sample transmission ÷ Number concentration of particles with a particle size of 0.3 to 0.5 μm before sample transmission)) × 100 Particle transmittance = (Number concentration of particles with a particle size of 0.3 to 0.5 μm after sample transmission ÷ Number concentration of particles with a particle size of 0.3 to 0.5 μm before sample transmission)

[0044] (5) Filter media quality factor (QF) value Using the values ​​of the air permeability resistance measured in (1) above and the particle transmittance measured in (2) above, the QF value was calculated using the following formula. QF[mmAq -1 ]=-[ln(1-(particle collection efficiency (%) / 100))] / [airflow resistance (mmAq)]

[0045] (6) Average fiber diameter Five arbitrary points were selected from an electret fiber sheet sample, and the diameter of a single fiber was measured at each point (n=20) using an electron microscope. The average fiber diameter was then calculated using the arithmetic mean.

[0046] <Example 1> A mixture was prepared by adding 98.925% by mass of polypropylene resin with a melt flow rate (MFR) of 1300 g / 10 min as the polyolefin resin, 1% by mass of Chimassorb® 944, a hindered amine-type additive manufactured by BASF, as the nitrogen-containing compound, and 0.075% by mass of magnesium stearate as other additives. The mixture was melt-spun using a melt-blowing apparatus at 223°C, with a single-hole discharge rate of 0.20 g / min, a collection distance of 300 mm, and a spinning nozzle with a diameter of 0.3 mm to obtain a fiber sheet. The obtained fiber sheet was charged by passing water with an electrical conductivity of 0.7 μS / cm and pH 6.8 from the surface to the back, and then air-dried at 25°C to obtain an electret fiber sheet. The obtained electret fiber sheet was measured according to (1) to (7) above.

[0047] <Example 2> An electret fiber sheet of Example 2 was obtained in the same manner as in Example 1, except that melt spinning was performed using a spinning nozzle with a molten resin temperature of 256°C, a single-hole discharge rate of 0.078 g / min, a collection distance of 180 mm, and a diameter of 0.2 mm.

[0048] <Comparative Example 1> Comparative Example 1's electret fiber sheet was obtained in the same manner as in Example 1, except that melt spinning was performed using a spinning nozzle with a molten resin temperature of 226°C, a single-hole discharge rate of 0.30 g / min, a collection distance of 200 mm, and a diameter of 0.3 mm.

[0049] <Comparative Example 2> Comparative Example 2's electret fiber sheet was obtained in the same manner as in Example 1, except that melt spinning was performed using a spinning nozzle with a molten resin temperature of 252°C, a single-hole discharge rate of 0.12 g / min, a collection distance of 140 mm, and a diameter of 0.2 mm.

[0050] Table 1 shows the measured values ​​for Examples 1 and 2 and Comparative Examples 1 and 2.

[0051] [Table 1]

[0052] From the measurement results of Examples 1 and 2 and Comparative Examples 1 and 2 in Table 1, it can be seen that the electret fiber sheets of Examples 1 and 2, which have a stress of 0.12 to 0.18 N / g at 2% elongation per unit basis weight and a stress of 0.20 to 0.26 N / g at 3% elongation per unit basis weight, exhibit moderate fusion and have a high filter media quality factor (QF). [Industrial applicability]

[0053] The electret fiber sheet of the present invention has excellent filtration performance and can be suitably used as a filter in, for example, dustproof clothing, dustproof masks, air purifiers, etc., and can make a significant contribution to industry. However, its range of applications is not limited to those described herein.

Claims

1. An electret fiber sheet having constituent fibers containing polyolefin resin and nitrogen-containing compounds, The crystallization temperature is 125°C or lower. The stress at 2% tensile stress per unit weight is 0.12 to 0.18 N / s, the stress at 3% tensile stress per unit weight is 0.20 to 0.26 N / s, and, An electret fiber sheet having a filter media quality factor (QF) value of 1.2 or higher, as shown by the following formula. QF[mmAq -1 ] = -[ln(1 - (particle collection efficiency (%) / 100))] / [airflow resistance (mmAq)]

2. The electret fiber sheet according to claim 1, wherein the average fiber diameter of the constituent fibers is 1.5 to 10 μm.

3. An electret filter using an electret fiber sheet according to claim 1 or 2.

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