Electret and electret filter

Abstract

Provided is an electret having improved charge stability. An electret according to the present invention has a peak temperature of 140°C or higher according to a TSC method after being charged in an electric field. The electret contains a nitrogen-containing compound, and in 100 parts by mass of resin contained in the electret, 80-99 parts by mass are polypropylene, and 1-20 parts by mass are poly-4-methyl-1-pentene.

Description

Electrets and electret filters

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

[0002] BACKGROUND ART Porous filters have conventionally been used in dust masks, various air conditioning elements, air purifiers, cabin filters, and various devices for the purposes of dust collection, protection, ventilation, etc.

[0003] Among porous filters, filters made of fibrous materials are widely used due to their advantages of high porosity, long life, and low airflow resistance. These fibrous filters capture particles on the fibers by mechanical collection mechanisms such as interception, diffusion, and inertial impaction. It is known that in practical use environments, filter collection efficiency reaches a minimum when the aerodynamic equivalent diameter of the particles to be captured is approximately 0.1 to 1.0 μm.

[0004] To improve the filter collection efficiency at the minimum value, methods that incorporate electrical attraction are known. Examples include applying an electric charge to the particles to be collected, applying an electric charge to the filter, placing the filter in an electric field, or a combination of these. Known methods for applying an electric charge to the filter include placing the filter between electrodes and causing dielectric polarization when ventilated, and applying a long-lasting electrostatic charge to an insulating material. The latter method, in particular, is widely used as an electret filter because it does not require energy from an external power source.

[0005] Electrets and electret filters are required to be prevented from deteriorating over time during transportation, storage, and use, as well as during pleating, injection molding, lamination, and drying. To address this issue, techniques for improving charge stability have been proposed (see Patent Documents 1 and 2).

[0006] JP 2003-522300 A U.S. Pat. No. 8,415,416

[0007] However, there is a demand for even higher performance, and there is a demand for electrets and electret filters having higher charge stability than ever before.

[0008] Therefore, as a result of intensive research, the present inventors have found that the above problems can be solved by the following, and have arrived at the present invention. [1] An electret having a peak temperature of 140°C or higher as measured by the TSC method after charging in an electric field, the electret containing a nitrogen-containing compound, and characterized in that, per 100 parts by mass of the resin contained in the electret, the electret contains 80 to 99 parts by mass of polypropylene and 1 to 20 parts by mass of poly-4-methyl-1-pentene. [2] The electret according to [1] above, further containing a fatty acid metal salt. [3] The electret according to [1] or [2] above, characterized in that it is a fibrous material. [4] The electret according to any one of [1] to [3] above, characterized in that the fibrous material is a meltblown nonwoven fabric. [5] An electret filter using the electret according to any one of [1] to [4] above.

[0009] According to the present invention, it is possible to obtain an electret and an electret filter having excellent charge stability.

[0010] Specific examples of the present invention will be given below, but the optimum configuration can be selected for each application in accordance with the spirit of the present invention.

[0011] <Polyolefin> The electret of the present invention comprises a polyolefin, and the polyolefin includes polypropylene and poly-4-methyl-1-pentene.

[0012] In the present invention, the polypropylene may be a propylene homopolymer or a copolymer mainly composed of propylene-derived units. There are no particular limitations on the polypropylene as long as it can provide the required properties, but the molar fraction of propylene-derived units contained in the polymer is preferably 80 mol% or more, more preferably 85 mol% or more, even more preferably 90 mol% or more, and most preferably 95 mol% or more.

[0013] Various olefins may be contained as copolymerization components other than propylene contained in the polymer. Examples include ethylene and α-olefins having 4 to 20 carbon atoms. More specific examples include propylene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene, and 1-eicosene. These may be contained in the copolymer alone or in two or more types. Halogenated olefins and alicyclic olefins are also preferably used to improve the flame retardancy and rigidity of the electret.

[0014] The stereoregularity of the polypropylene used in the present invention is preferably 85% or more, more preferably 90% or more, and even more 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 at least one type is contained in the polyolefin contained.

[0015] In the present invention, poly-4-methyl-1-pentene may be a 4-methyl-1-pentene homopolymer or a copolymer primarily composed of units derived from poly-4-methyl-1-pentene. While there are no particular limitations as long as the required properties are obtained, the molar fraction of 4-methyl-1-pentene-derived units contained in poly-4-methyl-1-pentene is preferably 80 mol% or more, more preferably 85 mol% or more, even more preferably 90 mol% or more, and most preferably 95 mol% or more. This is to effectively develop the properties of poly-4-methyl-1-pentene. An example of such a copolymer is TPX (registered trademark) DX820 manufactured by Mitsui Chemicals, Inc., which contains 97 mol% of structural units derived from 4-methyl-1-pentene and 3 mol% of structural units derived from decene-1.

[0016] Copolymerization components other than 4-methyl-1-pentene contained in the copolymer may include various olefins, such as ethylene, propylene, and α-olefins having 4 to 20 carbon atoms. More specific examples include propylene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene, and 1-eicosene. It is also preferable that one or more of these be contained in the copolymer. The number of carbon atoms of the olefin contained as a copolymerization component is preferably 6 to 19, more preferably 10 to 18. Halogenated olefins and alicyclic olefins are also preferably used to improve the flame retardancy and rigidity of the electret.

[0017] The poly-4-methyl-1-pentene used in the present invention preferably has a melt flow rate (MFR) of 20 to 1,000 g / 10 min, more preferably 50 to 500 g / 10 min, even more preferably 70 to 300 g / 10 min, and particularly preferably 100 to 200 g / 10 min. The MFR is measured in accordance with JIS K 7210 (1999) at a test temperature and load of 260°C and 5 kg, but when a commercially available product is used, the value listed in the catalog or the like may be used.

[0018] In the electret of the present invention, the polyolefin content in 100 parts by mass of the electret is preferably 80 parts by mass or more, more preferably 85 parts by mass or more, even more preferably 90 parts by mass or more, particularly preferably 95 parts by mass or more, and most preferably 97 parts by mass or more. The upper limit of the polyolefin content is not particularly limited, and is, for example, 99.5 parts by mass or less, preferably 99 parts by mass or less. In the case of sheath-core fibers or side-by-side fibers, in which the resins contained in the fibers are different between the left and right sides or the core-sheath, only the part containing the polyolefin is considered to be the electret of the present invention.

[0019] The electret of the present invention comprises a resin blend of poly-4-methyl-1-pentene and polypropylene, containing 80 to 99 parts by weight of polypropylene and 1 to 20 parts by weight of poly-4-methyl-1-pentene per 100 parts by weight of the resin contained in the electret. It has been discovered that mixing the two in predetermined amounts improves the charge stability of polypropylene. If the amount of poly-4-methyl-1-pentene is greater than the above range, processing becomes difficult, and if the amount is less, the effect of improving charge stability is difficult to achieve. For 100 parts by weight of the resin contained in the electret of the present invention, the amount of polypropylene is preferably 85 to 97 parts by weight, more preferably 90 to 95 parts by weight. For 100 parts by weight of the resin contained in the electret of the present invention, the amount of poly-4-methyl-1-pentene is preferably 3 to 15 parts by weight, more preferably 5 to 10 parts by weight. In addition, in 100 parts by mass of the resin contained in the electret of the present invention, the total content of polypropylene and poly-4-methyl-1-pentene is preferably 90 parts by mass or more, more preferably 95 parts by mass or more, even more preferably 98 parts by mass or more, and particularly preferably 100 parts by mass.

[0020] In 100 parts by mass of the electret of the present invention, the polypropylene content is preferably 80 to 99 parts by mass, more preferably 85 to 97 parts by mass. In addition, in 100 parts by mass of the electret of the present invention, the poly-4-methyl-1-pentene content is preferably 1 to 20 parts by mass, more preferably 3 to 15 parts by mass.

[0021] The electret of the present invention contains a nitrogen-containing compound in addition to polypropylene and poly-4-methyl-1-pentene. Furthermore, the electret of the present invention preferably further contains a fatty acid metal salt. The nitrogen-containing compound and the fatty acid metal salt will be described below.

[0022] <Nitrogen-containing compound> The electret of the present invention contains a nitrogen-containing compound that promotes charging of polyolefin by liquid contact. If the compound is not compatible with polyolefin, it is sufficient that the compound is contained in at least one of poly-4-methyl-1-pentene and polypropylene.

[0023] The content of the nitrogen-containing compound relative to 100 parts by mass of polyolefin is preferably 0.1 to 5 parts by mass, more preferably 0.5 to 3 parts by mass, and even more preferably 0.75 to 1.5 parts by mass. Note that when an electret contains two or more types of fibers or when a single fiber contains two or more types of resins, this refers to the proportion of the nitrogen-containing compound contained in the polyolefin. Even when the electret contains resins other than polyolefin, these can be distinguished because they dissolve in solvents and acids and bases or have different dyeing properties, and polyolefins can also be distinguished using quantitative methods such as DSC and NMR. If the content of the nitrogen-containing compound is less than 0.1 parts by mass, the charge amount will be low, resulting in reduced filtration properties. If it is more than 5 parts by mass, the electret will lose stability due to increased hydrophilicity.

[0024] The content of the nitrogen-containing compound in 100 parts by mass of the electret of the present invention is preferably 0.1 to 5 parts by mass, more preferably 0.5 to 3 parts by mass, and even more preferably 0.75 to 1.5 parts by mass. By setting it within the above range, it is possible to improve the filtration characteristics and the stability as an electret.

[0025] 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, and more preferably contains a 2,2,6,6-tetramethylpiperidine structure and a triazine structure.

[0026] The hindered amine compound is not particularly limited, but examples thereof 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}] (Chimasorb (registered trademark) 944LD, manufactured by BASF Japan Ltd.), dimethyl succinate-1-(2hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethyl-4-piperidine polycondensate (Tinuvin (registered trademark) 622LD, manufactured by BASF Japan Ltd.), 2-[[3,5-bis(1,1-dimethylethyl)-4-hydroxy- bis[1,2,2,6,6-pentamethyl-4-piperidinyl]-2-butylpropanedioate (Tinuvin (registered trademark) 144, manufactured by BASF Japan Ltd.), polycondensate of 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 (Chimasorb (registered trademark) 2020FDL, manufactured by BASF Japan Ltd.), 2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-(hexyloxy)-phenol (Tinuvin (registered trademark) 1577FF, manufactured by BASF Japan Ltd.), SABO STAB (registered trademark) UV119 (manufactured by SONGWON Co., Ltd.) and the like. Among these, Chimassorb (registered trademark) 944LD or Chimassorb (registered trademark) 2020FD, which contain a 2,2,6,6-tetramethylpiperidine structure and a triazine structure, are preferred. One type of hindered amine compound may be used alone, or two or more types may be used in combination.

[0027] The nitrogen-containing compound used in the present invention for promoting charging only needs to be present on at least the electret surface, and can be introduced by applying a solution to the electret surface, attaching a powder, mixing a solution into a polyolefin, mixing during polymerization, melt mixing, or the like.

[0028] Of these, the melt mixing method is superior in terms of homogeneity and processability. It may be mixed directly with the resin during melt molding. A pre-prepared resin compound containing a charge enhancer may be used as is or diluted.

[0029] <Fatty Acid Metal Salt> In the present invention, the addition of one or more fatty acid metal salts can further enhance charge stability. A preferred amount is 0.01 to 1 part by mass per 100 parts by mass of polyolefin. The fatty acid metal salt is preferably at least one selected from the group consisting of fatty acid aluminum salts and fatty acid magnesium salts, and more preferably fatty acid magnesium salts. From the viewpoint of enhancing charge stability, the fatty acid in the fatty acid metal salt preferably contains at least one selected from the group consisting of monocarboxylic acids and dicarboxylic acids, but may also contain hydrocarbons other than carboxylic acids.

[0030] The fatty acid metal salt is not particularly limited, but is preferably one having a fatty acid group with 10 to 50 carbon atoms, more preferably one having a fatty acid group with 12 to 30 carbon atoms, and even more preferably one having a fatty acid group with 16 to 22 carbon atoms. Furthermore, the fatty acid metal salt is preferably one having a linear fatty acid group.

[0031] The fatty acid aluminum salt preferably has a linear fatty acid group, and the number of fatty acid chains bonded to the aluminum may be one, two, or three, but is preferably three. Specific examples of the fatty acid aluminum salt include aluminum salts of linear saturated fatty acids such as aluminum laurate, aluminum myristate, aluminum palmitate, aluminum stearate, and aluminum behenate; and aluminum salts of linear unsaturated fatty acids such as aluminum oleate; but from the viewpoints of melting point and reactivity, aluminum salts of linear saturated fatty acids are preferred, and aluminum stearate is more preferred.

[0032] The fatty acid magnesium salt preferably has a linear fatty acid group, and the number of fatty acid chains bonded to the magnesium may be one or two, but is preferably two. Specific examples of the fatty acid magnesium salt include magnesium salts of linear saturated fatty acids such as magnesium laurate, magnesium myristate, magnesium palmitate, magnesium stearate, and magnesium behenate; and magnesium salts of linear unsaturated fatty acids such as magnesium oleate; but from the viewpoints of melting point and reactivity, magnesium salts of linear saturated fatty acids are preferred, and magnesium stearate is more preferred.

[0033] In the present invention, charge stability can be improved without using additives such as a crystal nucleating agent or a charge stabilizer. Therefore, the electret of the present invention has a total content of polypropylene, poly-4-methyl-1-pentene, a nitrogen-containing compound, and a fatty acid metal salt of preferably 95% by mass or more, more preferably 98% by mass or more, even more preferably 99% by mass or more, particularly preferably 99.5% by mass or more, and most preferably 100% by mass.

[0034] The electret of the present invention can be used in any required shape, and the electret function can be utilized, for example, as a fibrous material, a film, an extrusion molding material, a porous membrane, a powder, a surface coating layer on other materials, etc. Among these, when the electret is used for a filter, a fibrous material (fiber sheet) is particularly preferably used.

[0035] The fibrous material is preferably a fiber assembly, and examples of the fiber assembly include fibrous materials such as nonwoven fabrics, woven or knitted fabrics, and cotton-like materials made of long or short fibers, and fibrous materials obtained from stretched films. The fiber assembly refers to an electret that is recognized to have a fibrous form when the surface is observed with a device such as a scanning electron microscope or an optical microscope, and in which at least some of the fibers constituting the fiber assembly are integrated by melting or entangling with each other.

[0036] When the electret is used as a filter, the fiber aggregate is preferably a nonwoven fabric. As a method for obtaining a nonwoven fabric, conventionally known methods can be used, such as a method of forming a sheet from short fibers such as single-component fibers, composite fibers such as core-sheath fibers and side-by-side fibers, and split fibers using carding, airlaid, wet papermaking, etc., or a method of forming a sheet from continuous fibers using a spunbond method, meltblown method, electrospinning method, or force spinning method. From the viewpoint of not requiring treatment of residual solvents and treatment of spinning oils adhering to the surface, nonwoven fabrics obtained by the spunbond method, meltblown method, melt electrospinning method, or melt force spinning method are more preferred, and nonwoven fabrics obtained by the spunbond method or meltblown method are particularly preferred.

[0037] In the production of fibers or nonwoven fabrics by the melting method, the melt extrusion is preferably carried out in the range of 150° C. to 350° C., more preferably in the range of 170° C. to 330° C., even more preferably in the range of 200° C. to 310° C., and most preferably in the range of 230° C. to 300° C. By setting the temperature within this range, an electret filter having excellent charge amount and filtration properties as an electret can be obtained.

[0038] The average diameter of the fibers constituting the fiber assembly is preferably 0.001 to 100 μm, more preferably 0.05 to 50 μm, even more preferably 0.1 to 30 μm, particularly preferably 0.3 to 25 μm, and most preferably 0.5 to 20 μm. If the average fiber diameter is thicker than 100 μm, it is difficult to achieve a practical collection efficiency, and the efficiency decreases significantly during charge decay. If the average fiber diameter is thinner than 0.001 μm, it is difficult to obtain a charged electret. The average fiber diameter can be calculated, for example, using a scanning electron microscope to measure the diameters of 50 fibers in the same field of view, ensuring no overlapping fibers, and then taking the geometric mean.

[0039] The fiber aggregate may be composed of a single fiber made by a single manufacturing method or from a single material, or may be composed of two or more types of fibers that are different in manufacturing method, material, or average diameter.

[0040] The electretization method in the present invention is not particularly limited as long as it can provide desired properties when the electret is used, but a method of contacting or impacting a liquid (liquid contact charging method) is preferred, and an electret having high filtration properties can be obtained by the liquid contact charging method. More specifically, a method of contacting or impacting a liquid with a fiber aggregate or the like before electretization by a method such as suction, pressurization, or spraying is preferred.

[0041] In the liquid contact charging method, the liquid to be contacted or collided with is not particularly limited as long as it can provide the desired characteristics, but water is preferred in terms of ease of handling and performance. Instead of water, a liquid in which a secondary component (a component other than water) has been added to water may be used, and the conductivity and pH of the liquid can be adjusted by the type and amount of the secondary component added.

[0042] The liquid to be contacted or collided with in the liquid-contact charging method preferably has a pH of 1 to 11, more preferably a pH of 3 to 9, and even more preferably a pH of 5 to 7. Furthermore, the liquid to be contacted or collided with in the liquid-contact charging method preferably has a conductivity of 100 μS / cm or less, more preferably 10 μS / cm or less, and even more preferably 3 μS / cm or less.

[0043] When the electret of the present invention is used for a filter, the filter medium quality factor QF value is, for example, 0.1 mmAq －１ or more, and 0.11 mmAq －１ It is preferable that the concentration is 0.12 mmAq or more. －１ More preferably, it is 0.13 mmAq or more. －１ More preferably, it is 0.14 mmAq or more. －１ It is particularly preferable that the concentration is 1.0 mmAq or more. －１ It is most preferable that the QF value is greater than 0.1 mmAq. In particular, it is particularly preferable that the QF value is greater than 0.1 mmAq for an electret filter having an average fiber diameter of 0.5 to 5 μm produced by the meltblown method. －１If the QF value is less than 100%, the particles are not sufficiently captured by the electret, resulting in insufficient filter performance. The QF value in this specification is calculated based on the airflow resistance when air is passed through the filter in the thickness direction at a wind speed of 10 cm / s and the number of particles counted in the particle size range of 0.3 to 0.5 μm using a laser particle counter.

[0044] When the electret of the present invention is used as a filter, the particle collection efficiency at a wind speed of 10 cm / s can be adjusted in various ways depending on the required properties, but is preferably 50% or more, more preferably 70% or more, even more preferably 90% or more, and most preferably 95% or more. In this specification, the particle collection efficiency is calculated based on the number of particles in the particle size range 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 cm / s.

[0045] When the electret of the present invention is used as a filter, the airflow resistance at a wind speed of 10 cm / s is preferably in the range of 0.05 to 50 mmAq, more preferably 0.2 to 30 mmAq, and most preferably 0.5 to 20 mmAq. If the airflow resistance is too small, the performance as a filter becomes insufficient, and if the airflow resistance is too large, the advantages as a filter are lost.

[0046] The value of the quality factor (QF) of filter material in the above-mentioned case of using non-electret fiber aggregate is generally below 0.1, and for filter use, it is preferably above 0.5, more preferably above 1.0, even more preferably above 1.1, and most preferably above 1.2.In the liquid contact charging method using water, there is a heating and drying process, and in filter use, there is a pleating process or long-term storage.Therefore, it is preferable that the quality factor (QF) of filter material mentioned above is maintained not only immediately after electretization, but also after heat treatment or super basic storage.

[0047] In the present invention, the charge stability evaluation of the electret is performed by the TSC (thermally stimulated current) method. Specifically, the process is as follows: (1) A sheet-like fiber assembly is charged in an electric field to form an electret. (2) Electrodes are brought into contact with each other on both surfaces of the electret, facing each other. (3) A high-impedance picoammeter is connected to the electrodes on both surfaces. (4) The sample is placed in a heating bath at 30°C or less, short-circuited, and then heated from 25°C to 180°C at a rate of 5°C / min. (5) The electrode temperature is plotted on the horizontal axis and the current value on the vertical axis. This allows the charge transfer characteristics at each temperature to be measured. While the TSC method can be performed in both contact and non-contact modes, the contact method is used in the present invention from the viewpoints of sensitivity and the ability to obtain stable evaluation results by performing a short-circuit operation before heating. It is known that the depolarization temperature in the TSC method is related to the charge stability of the electret and the performance stability as a filter (for example, Osaka Institute of Technology Bulletin, 66(1), 1-18, Patent No. 3199947, etc.). The present inventors have also found that there is a correlation between the charge stability of electrets prepared by the corona charging method for use in the TSC method and electrets obtained by the liquid contact method.

[0048] For polarization in (1) above, a corona discharge method is used to unevenly distribute charge (quantity and polarity) in the thickness direction so that sufficient depolarization current can be observed in the TSC method. The atmosphere and sample temperature during charging are 30°C or lower so as not to affect the measurement results in the TSC method. Since the corona discharge method in this invention is not intended for surface treatment such as oxidation or etching, it is important that the equipment and conditions are those commonly used in electret manufacturing and research.

[0049] After the charging, measurement by the TSC method is initiated within 10 minutes in an environment with an ambient temperature of 30°C or less. Materials that have undergone transportation, storage, heating, or other processes after electret fabrication must be re-electretized using the aforementioned method and then measured. The purpose of the TSC method evaluation in this invention is to evaluate the essential thermal charge stability of the material and to eliminate the effects of changes in charge quantity due to various processes and the passage of time. In other words, even for fibrous materials, laminates, etc. that have undergone various histories (time, temperature, etc.), it is important to separate the electret portion, charge it, and then perform TSC measurement to evaluate it before thermal cleaning. Paradoxically, electrets and electret filters with a high QF and the a / b ratio of this invention have excellent filtration properties and charge stability.

[0050] In the present invention, where the maximum depolarization current at 50 to 100°C in the TSC method is a and the maximum depolarization current above 100°C is b, the ratio a / b ( / denotes division) is preferably 0.3 or less, more preferably 0.2 or less, and most preferably 0.1 or less. It is essential to use reasonable values ​​with continuity for the maximum current values ​​shown above. The values ​​should be determined by excluding momentary spikes due to noise and abnormal values ​​due to short-circuit current, and should be judged as reasonable values ​​in light of the preceding and following values. The smaller the value of a / b, the smaller the depolarization rate at temperatures below 100°C, resulting in smaller charge decay during the pleating process, transportation, and storage, resulting in electrets and electret filters with excellent practicality. In compositions with poor charge stability, continuous depolarization is observed from approximately 50 to 150°C, resulting in a significant decrease in charge amount. In the present invention, the depolarization charge amount is expressed as the a / b ratio, and it goes without saying that it is preferable for the depolarization charge amount during corona charging to be larger at higher temperatures.

[0051] For depolarization at 100° C. or higher, the peak temperature and the rise temperature showing the maximum value are preferably high, with the peak temperature being 135° C. or higher, more preferably 140° C. or higher, and most preferably 145° C. or higher. The rise temperature is preferably 80° C. or higher, more preferably 90° C. or higher, even more preferably 100° C. or higher, and most preferably 110° C. or higher. The rise temperature is determined as the tangent to the measured values ​​before and after the peak.

[0052] In the present invention, the combination of polypropylene and poly-4-methyl-1-pentene provides a low melting point, prevents oxidation, and also increases the peak temperature that polypropylene normally has. The peak temperature corresponds to the activation energy and relaxation time, and a high peak temperature results in excellent charge stability and charge life.

[0053] Furthermore, when an electret is used as a filter, the fine particle collection property can be used to evaluate the charge amount or charge stability of the electret. In particular, in the liquid contact method, the charge distribution in the thickness direction is insufficient, making it difficult to directly evaluate by the TSC method.

[0054] In the present invention, as a method for evaluating charge amount or charge stability using the collection characteristics of fine particles, the filter quality factor (QF) can be used as a parameter related to charge amount.In addition, as a method for evaluating charge stability, the natural logarithm ratio of transmittance before and after various treatments (referred to as performance maintenance rate in the present invention) can be used.As particles, artificially generated particles can be balanced charged, or atmospheric dust can be used.As for the detector, as long as it can limit the range of particle diameter, it can obtain either concentration or number.As an evaluation condition, it is preferable that electrostatic attraction is dominant.For example, the passing wind speed is 10 cm / s, and the pressure difference before and after the sample is compared using a micro-differential pressure meter, and the particle number concentration before and after the sample in the diameter range of 0.3 to 0.5 μm is compared using a laser particle counter, thereby calculating particle transmittance, particle collection efficiency, filter quality factor, and performance maintenance rate.The respective relationships are shown below. Particle transmittance [-] = (particle number concentration downstream of sample) / (particle number concentration upstream of sample) Particle collection efficiency [%] = (1 - particle transmittance [-]) × 100 Filter material quality factor QF [mmAq －１ ] = - (ln (particle permeability [-]) / (airflow resistance [mmAq])) Performance retention rate [-] = ln (particle permeability after treatment [-]) / ln (particle permeability before treatment [-])

[0055] The value of the quality factor (QF) of filter material in the above-mentioned case of using fibrous material that is not electretized is generally below 0.1, and for filter use, it is preferably above 0.5, more preferably above 1.0, even more preferably above 1.1, and most preferably above 1.2.In the case of the electretization method that uses water and liquid contact, it may be passed through heat drying process, and in filter use, it may be carried out pleating process or long-term storage.Therefore, it is preferable that the quality factor (QF) of filter material is maintained not only immediately after electretization but also after heat treatment.

[0056] The electret of the present invention may be used in combination with other components as needed. When the electret of the present invention is used for a filter, it is also preferable to use it in combination with, for example, a prefilter layer, a fiber protection layer, a reinforcing member, or a functional fiber layer. An electret filter using the electret of the present invention is also included in the scope of the present invention.

[0057] Examples of the prefilter layer and the fiber protection layer include spunbond nonwoven fabric, thermal-bond nonwoven fabric, and urethane foam. Examples of the reinforcing member include thermal-bond nonwoven fabric and various nets. Examples of the functional fiber layer include antibacterial, antiviral, and colored fiber layers for identification and design purposes.

[0058] The electret of the present invention can be used in a wide range of applications. In particular, it can be suitably used as a filter for protecting various devices, such as dust masks, dustproof clothing, various air conditioning elements, air purifiers, cabin filters, and the like, for the purposes of protection, ventilation, antifouling, waterproofing, etc.

[0059] This application claims the benefit of priority based on Japanese Patent Application No. 2024-028743, filed on February 28, 2024. The entire contents of the specification of Japanese Patent Application No. 2024-028743, filed on February 28, 2024, are incorporated herein by reference.

[0060] The present invention will be described in more detail with reference to examples, but the following examples do not limit the present invention, and all modifications and variations within the scope of the present invention are included within the technical scope of the present invention. First, the measurement method will be described.

[0061] (Depolarization Current Measurement by TSC Method) Measurements were performed using a thermally stimulated current (TSC) measurement device under the following conditions. The measurement endpoint was set to 180°C to exclude the polymethylpentene peak present at temperatures above 200°C and determine the peak temperature derived from polypropylene. Corona charging: The sample (fiber sheet) was charged and electretized using flat needle electrodes arranged in a staggered pattern with 10 mm spacing. A 0.5 mm thick silicone rubber sheet was placed on the ground surface. The sample was then charged with a 10 mm gap, an applied voltage of 20 kV, and a charging time of 30 seconds. TSC electrodes: A pair of circular electrodes (20 mm in diameter) sandwiched an electretized sample (25 mm in diameter) between them, with the electrodes facing each other. The surfaces of both electrodes were in contact with the sample surface. Temperature conditions: The temperature was increased from 25°C to 180°C at a rate of 5°C / min. Other: After placing the sample between the electrodes, the electrodes were short-circuited to determine the zero point of the current value. The obtained data was plotted with the temperature [°C] on the horizontal axis and the current value [-] on the vertical axis, and the maximum value (peak height) of the depolarized current at 50°C or higher and 100°C or lower was defined as a, and the maximum value (peak height) of the depolarized current at more than 100°C and 180°C or lower was defined as b, and the a / b value was calculated.

[0062] Example 1 90 parts by mass of polypropylene homopolymer, 10 parts by mass of poly-4-methyl-1-pentene resin (DX820, 4-methyl-1-pentene-1 copolymer, manufactured by Mitsui Chemicals, Inc., MFR: 180 g / 10 min (260°C, 5 kg)), and 1 part by mass of nitrogen-containing compound (Chimassorb (registered trademark) 944LD, manufactured by BASF) were prepared and mixed in a blender. This mixture was spun using a meltblown machine to produce a fiber with a basis weight of 30 g / m. ２ A fiber sheet (both a fiber assembly and a nonwoven fabric) was obtained. The depolarization current of the obtained fiber sheet was measured by the TSC method. The TSC peak temperature was 153°C, and the a / b value was 0.28.

[0063] Example 2 95 parts by mass of polypropylene homopolymer, 5 parts by mass of poly-4-methyl-1-pentene resin (DX310, 4-methyl-1-pentene-hexadecene-octadecene copolymer, manufactured by Mitsui Chemicals, Inc., MFR: 100 g / 10 min (260°C, 5 kg)), and 1 part by mass of nitrogen-containing compound (Chimassorb (registered trademark) 944LD, manufactured by BASF) were prepared and mixed in a blender. This mixture was spun using a meltblown machine to produce a fiber with a basis weight of 30 g / m. ２ The fiber sheet was subjected to depolarization current measurement by the TSC method described above. The TSC peak temperature was 148°C, and the a / b value was 0.38.

[0064] Example 3 90 parts by mass of polypropylene homopolymer, 10 parts by mass of poly-4-methyl-1-pentene resin (DX820, manufactured by Mitsui Chemicals, Inc.), 1 part by mass of nitrogen-containing compound (Chimassorb (registered trademark) 944LD, manufactured by BASF), and 0.1 part by mass of magnesium stearate were prepared and mixed in a blender. This mixture was spun using a meltblown machine to produce a fiber with a basis weight of 30 g / m. ２ The fiber sheet was subjected to depolarization current measurement by the TSC method described above. The TSC peak temperature was 154°C, and the a / b value was 0.05.

[0065] Example 4 95 parts by mass of polypropylene homopolymer, 5 parts by mass of poly-4-methyl-1-pentene resin (DX310, manufactured by Mitsui Chemicals, Inc.), 1 part by mass of nitrogen-containing compound (Chimassorb (registered trademark) 944LD, manufactured by BASF), and 0.1 part by mass of magnesium stearate were prepared and mixed in a blender. This mixture was spun using a meltblown machine to produce a fiber with a basis weight of 30 g / m. ２ The fiber sheet was subjected to depolarization current measurement by the TSC method described above. The TSC peak temperature was 148°C, and the a / b value was 0.07.

[0066] Comparative Example 1 100 parts by mass of polypropylene homopolymer and 1 part by mass of a nitrogen-containing compound (Chimassorb (registered trademark) 944LD, manufactured by BASF) were prepared and mixed. This mixture was spun using a meltblown machine to produce a fiber with a basis weight of 30 g / m. ２ The fiber sheet was subjected to depolarization current measurement by the TSC method described above. The TSC peak temperature was 138°C, and the a / b value was 0.87.

[0067] The results of the Examples and Comparative Examples show that by including a nitrogen-containing compound and combining polypropylene with poly-4-methyl-1-pentene, it is possible to increase the TSC peak temperature associated with melting of polypropylene without adding a crystal nucleating agent, charge stabilizer, etc. Furthermore, Examples 1 and 3 and Examples 2 and 4 show that by adding magnesium stearate, it is possible to reduce depolarization below 100°C while maintaining a high TSC peak temperature, thereby further improving charge stability.

[0068] According to the present invention, an electret and a filter having excellent charge stability can be obtained, which can greatly contribute to industry. The electret of the present invention has excellent charge amount and charge density, and can be suitably used in particular for filters in dustproof clothing, dustproof masks, air purifiers, etc.

Claims

1. An electret having a peak temperature of 140°C or higher as measured by the TSC method after charging in an electric field, the electret containing a nitrogen-containing compound, and containing 80 to 99 parts by mass of polypropylene and 1 to 20 parts by mass of poly-4-methyl-1-pentene per 100 parts by mass of resin contained in the electret.

2. The electret according to claim 1, further comprising a fatty acid metal salt.

3. The electret according to claim 1 or 2, which is a fibrous material.

4. The electret according to claim 1 or 2, which is a meltblown nonwoven fabric.

5. An electret filter using the electret according to claim 1 or 2.