Electret and electret filter

Abstract

This invention provides an electret with improved charge stability. The peak temperature of the electret after being charged in an electric field, obtained by the TSC method, is above 140°C. The electret contains a nitrogen-containing compound, and in 100 parts by mass of resin contained in the electret, it contains 80-99 parts by mass of polypropylene and 1-20 parts by mass of poly-4-methyl-1-pentene.

Description

Technical Field

[0001] This invention relates to electrets and electret filters. Background Technology

[0002] In the past, porous filters have been used in dust masks, various air conditioning components, air purifiers, cabin filters, and various devices for purposes such as dust collection, protection, and ventilation.

[0003] Among porous filters, those containing fibrous materials are widely used due to their advantages of high porosity, long lifespan, and low air resistance. These fibrous filters capture particles onto the fibers through mechanical trapping mechanisms such as interception, diffusion, and inertial impaction. Moreover, it is known that in practical operating environments, the filter's trapping efficiency reaches a minimum when the aerodynamic equivalent diameter of the captured particles is between 0.1 and 1.0 μm.

[0004] To improve the filter capture efficiency at the aforementioned minimum values, methods using electro-attraction are known. Examples include: methods of imparting a charge to the captured particles, methods of imparting a charge to the filter, methods of placing the filter in an electric field, and combinations of these methods. As methods of imparting a charge to the filter, known methods include: placing the filter between electrodes and dielectrically polarizing it during ventilation; and methods of imparting a long-lived static charge to the insulating material. The latter method, in particular, is widely used as an electret filter because it does not require external power.

[0005] Electrets and electret filters are required to suppress degradation over time during handling, storage, and use, and also to suppress degradation during pleating, injection molding, lamination bonding, and drying. Therefore, techniques for improving charge stability are known, for example (Patent Document 1, Patent Document 2).

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Publication No. 2003-522300

[0009] Patent Document 2: US Patent No. 8,415,416 Summary of the Invention

[0010] The problem that the invention aims to solve

[0011] However, higher performance is required, and there is a desire for electrets and electret filters with higher charge stability than ever before.

[0012] Methods for solving problems

[0013] Therefore, through in-depth research, the inventors discovered that the above-mentioned problems could be solved in the following way, thus completing the present invention.

[0014] [1] An electret, characterized in that the peak temperature obtained by the TSC method after being charged in an electric field is above 140°C, the electret contains a nitrogen-containing compound, and in 100 parts by mass of resin contained in the electret, there are 80 to 99 parts by mass of polypropylene and 1 to 20 parts by mass of poly-4-methyl-1-pentene.

[0015] [2] The electret described in [1] above is characterized by further containing a fatty acid metal salt.

[0016] [3] The electret as described in [1] or [2] above is characterized in that it is a fibrous material.

[0017] [4] The electret described in any of [1] to [3] above is characterized in that the fibrous material is meltblown nonwoven fabric.

[0018] [5] An electret filter that uses an electret described in any of [1] to [4] above.

[0019] Invention Effects

[0020] According to the present invention, electrets and electret filters with excellent charge stability can be obtained. Detailed Implementation

[0021] The following are specific examples of the invention, but the most suitable configuration can be selected for each application within the scope of the spirit of the invention.

[0022] <Polyolefins>

[0023] The electret of the present invention contains a polyolefin, which includes polypropylene and poly-4-methyl-1-pentene.

[0024] In this invention, the polypropylene can be a propylene homopolymer or a copolymer containing units mainly derived from propylene. There are no particular limitations as long as the desired properties are obtained, but the molar fraction of propylene-derived units in the polymer is preferably 80 mol% or more, more preferably 85 mol% or more, further preferably 90 mol% or more, and most preferably 95 mol% or more.

[0025] As a copolymer component other than propylene, various olefins can be included in the polymer. Examples include ethylene and α-olefins with 4 to 20 carbon atoms. More specifically, examples include propylene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene, and 1-eicosene. Only one or more of these components may be present in the copolymer. To improve the flame retardancy and rigidity of the electret, halogenated olefins and alicyclic olefins are preferred.

[0026] In the polypropylene used in this invention, the stereoregularity is preferably 85% or more, more preferably 90% or more, and even more preferably 95% or more. In this case, both isotactic and syndiotactic structures are preferred. When using two or more types of polypropylene, it is preferable that the polyolefin contained therein contains one or more types.

[0027] In this invention, poly-4-methyl-1-pentene can be a homopolymer of 4-methyl-1-pentene or a copolymer with units derived from poly-4-methyl-1-pentene as the main component. There are no particular limitations as long as the desired properties are obtained, but the molar fraction of units derived from 4-methyl-1-pentene contained in the poly-4-methyl-1-pentene is preferably 80 mol% or more, more preferably 85 mol% or more, further preferably 90 mol% or more, and most preferably 95 mol% or more. This is to effectively exhibit the properties of poly-4-methyl-1-pentene. Examples of such copolymers include TPX (registered trademark) DX820 manufactured by Mitsui Chemicals Co., Ltd., which has 97 mol% structural units derived from 4-methyl-1-pentene and 3 mol% structural units derived from 1-decene.

[0028] As a copolymerizing component other than 4-methyl-1-pentene contained in the above copolymer, various olefins may be contained, such as ethylene, propylene, and α-olefins with 4 to 20 carbon atoms. More specifically, examples include propylene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene, and 1-eicosene. It is preferable that one or more of these are contained in the copolymer. The number of carbon atoms in the olefin contained as a copolymerizing component is preferably 6 to 19, more preferably 10 to 18. To improve the flame retardancy and rigidity of the electret, halogenated olefins and alicyclic olefins may also be preferred.

[0029] The melt flow rate (MFR) of the poly-4-methyl-1-pentene used in this invention is preferably 20 to 1000 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. It should be noted that the MFR was measured according to JIS K 7210 (1999) at a test temperature and load of 260°C and 5 kg. However, when using commercially available products, values ​​listed in catalogs, etc., may also be used.

[0030] Regarding the electret of the present invention, the polyolefin content in 100 parts by weight of the electret is preferably 80 parts by weight or more, more preferably 85 parts by weight or more, further preferably 90 parts by weight or more, particularly preferably 95 parts by weight or more, and most preferably 97 parts by weight or more. There is no particular upper limit to the polyolefin content; for example, it is 99.5 parts by weight or less, preferably 99 parts by weight or less. It should be noted that when the resin contained in the left and right sides or the core sheath of fibers such as core-sheath type fibers or parallel type fibers is changed, only the portion containing polyolefin is used as the electret of the present invention.

[0031] Regarding the electret of the present invention, 100 parts by weight of resin contained in the electret contain 80 to 99 parts by weight of polypropylene and 1 to 20 parts by weight of poly-4-methyl-1-pentene, comprising a resin formed by mixing poly-4-methyl-1-pentene and polypropylene. In the present invention, it has been found that by mixing the two in a predetermined amount, an effect of improving the charge stability of polypropylene can be achieved. When the amount of poly-4-methyl-1-pentene is greater than the above, processing becomes difficult; when the amount is less than the above, it is difficult to obtain the effect of improving charge stability. In the electret of the present invention, 100 parts by weight of resin contains preferably 85 to 97 parts by weight of polypropylene, more preferably 90 to 95 parts by weight. In the electret of the present invention, 100 parts by weight of resin contains preferably 3 to 15 parts by weight of poly-4-methyl-1-pentene, more preferably 5 to 10 parts by weight. Furthermore, in the electret of the present invention, the total content of polypropylene and poly-4-methyl-1-pentene in 100 parts by weight of resin is preferably 90 parts by weight or more, more preferably 95 parts by weight or more, further preferably 98 parts by weight or more, and particularly preferably 100 parts by weight.

[0032] In 100 parts by weight of the electret of the present invention, polypropylene is preferably 80 to 99 parts by weight, more preferably 85 to 97 parts by weight. Furthermore, in 100 parts by weight of the electret of the present invention, poly-4-methyl-1-pentene is preferably 1 to 20 parts by weight, more preferably 3 to 15 parts by weight.

[0033] In addition to polypropylene and poly-4-methyl-1-pentene, the electret of the present invention also contains a nitrogen-containing compound. Furthermore, the electret of the present invention preferably also contains a fatty acid metal salt. The nitrogen-containing compound and the fatty acid metal salt will be described below.

[0034] <Nitrogen-containing compounds>

[0035] The electret of the present invention contains a nitrogen-containing compound that promotes the charging of polyolefins through liquid contact. In the case of incompatibility, it may be contained in at least one of poly-4-methyl-1-pentene or polypropylene.

[0036] The content of nitrogen-containing compounds 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. It should be noted that when the electret contains two or more types of fibers, or when one fiber contains two or more types of resins, this refers to the proportion of nitrogen-containing compounds in the polyolefin. Even when the electret contains resins other than polyolefins, these resins can be identified because they are soluble in solvents, acids, alkalis, or have different dyeing properties. Quantitative methods such as DSC and NMR can also be used to identify polyolefins. When the proportion of nitrogen-containing compounds is less than 0.1 parts by mass, the charge decreases, thus reducing filtration characteristics; when it is greater than 5 parts by mass, the hydrophilicity increases, thereby losing stability as an electret.

[0037] In the electret of the present invention, the proportion of nitrogen-containing compound in 100 parts by weight is preferably 0.1 to 5 parts by weight, more preferably 0.5 to 3 parts by weight, and even more preferably 0.75 to 1.5 parts by weight. By setting it within the above range, the filtration characteristics can be improved, and the stability as an electret can be improved.

[0038] There are no particular limitations on the nitrogen-containing compounds as long as they can achieve the desired properties, but hindered amine compounds containing at least one of the 2,2,6,6-tetramethylpiperidine structure and the triazine structure are preferred, and compounds containing the 2,2,6,6-tetramethylpiperidine structure and the triazine structure are more preferred.

[0039] As a hindered amine compound, there are no particular limitations; examples include: poly[{6-(1,1,3,3-tetramethylbutyl)amino-1,3,5-triazine-2,4-diyl}{(2,2,6,6-tetramethyl-4-piperidinyl)imino}hexamethylene{2,2,6,6-tetramethyl-4-piperidinyl)imino}] (Chimassorb 944LD, manufactured by BASF JAPAN), and dimethyl succinate-1-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethyl-4-piperidinyl condensate (Tinuvin 622LD, BASF). 2-[[3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl]methyl]-2-butylmalonic acid bis[1,2,2,6,6-pentamethyl-4-piperidinyl] ester (Tinuvin (registered trademark) 144, manufactured by BASF JAPAN), dibutylamine 1,3,5-triazine·N,N-bis(2,2,6,6-tetramethyl-4-piperidinyl)-1,6-hexamethylenediamine·N-(2,2,6,6-tetramethyl-4-piperidinyl)butylamine condensate (Chimassorb (registered trademark) 2020FDL, manufactured by BASF JAPAN), 2-(4,6-diphenyl-1,3,5-triazine-2-yl)-5-(hexyloxy)-phenol (Tinuvin (registered trademark) 1577FF, manufactured by BASF JAPAN), SABO STAB (registered trademark) UV119 (manufactured by SONGWON), etc. Among these, Chimassorb (registered trademark) 944LD or Chimassorb (registered trademark) 2020FDL, containing a 2,2,6,6-tetramethylpiperidine structure and a triazine structure, are preferred. A hindered amine compound can be used alone or in combination with two or more compounds.

[0040] The nitrogen-containing compound that promotes charging used in this invention only needs to be present at least on the surface of the electret, and can be introduced by means of solution coating, powder adhesion, mixing with polyolefin solution, mixing during polymerization, melt mixing, etc.

[0041] Of the above, melt mixing is superior from the viewpoints of homogeneity and processability. It can be directly mixed with the resin during melt molding. It can also be used directly as a pre-prepared resin mixture containing charged reinforcing agents, or after dilution.

[0042] <Fatty acid metal salts>

[0043] In this invention, charge stability can be further improved by adding one or more fatty acid metal salts. A preferred addition amount is 0.01 to 1 part by mass relative to 100 parts by mass of the polyolefin. The fatty acid metal salt is preferably selected from at least one of fatty acid aluminum salts and fatty acid magnesium salts, more preferably a fatty acid magnesium salt. From the viewpoint of improving charge stability, the fatty acid in the fatty acid metal salt preferably contains at least one selected from monocarboxylic acids and dicarboxylic acids, but it may also contain hydrocarbons other than carboxylic acids.

[0044] The fatty acid metal salt is not particularly limited, but it is preferably a fatty acid metal salt having fatty acid groups with 10 to 50 carbon atoms, more preferably a fatty acid metal salt having fatty acid groups with 12 to 30 carbon atoms, and even more preferably a fatty acid metal salt having fatty acid groups with 16 to 22 carbon atoms. In addition, the fatty acid metal salt preferably has straight-chain fatty acid groups.

[0045] As an aluminum salt of fatty acids, it is preferable to be an aluminum salt of fatty acids having a straight-chain fatty acid group. In addition, the fatty acid chain bonded to aluminum can be one, two, or three, but three is preferred. Specific examples of aluminum salts of fatty acids include: aluminum salts of straight-chain saturated fatty acids such as aluminum laurate, aluminum myristate, aluminum palmitate, aluminum stearate, and aluminum behenate; and aluminum salts of straight-chain unsaturated fatty acids such as aluminum oleate. However, from the viewpoint of melting point and reactivity, aluminum salts of straight-chain saturated fatty acids are preferred, and aluminum stearate is more preferred.

[0046] As a magnesium salt of fatty acids, a magnesium salt of fatty acids having a straight-chain fatty acid group is preferred. In addition, the fatty acid chain bonded to magnesium can be one or two, but two are preferred. Specific examples of magnesium salts of fatty acids include: magnesium salts of straight-chain saturated fatty acids such as magnesium laurate, magnesium myristate, magnesium palmitate, magnesium stearate, and magnesium behenate; and magnesium salts of straight-chain unsaturated fatty acids such as magnesium oleate. However, from the viewpoint of melting point and reactivity, magnesium salts of straight-chain saturated fatty acids are preferred, and magnesium stearate is more preferred.

[0047] In this invention, since charge stability can be improved without the use of additives such as nucleating agents and charge stabilizers, the total content of polypropylene, poly-4-methyl-1-pentene, nitrogen-containing compounds and fatty acid metal salts in the electret of this invention is preferably 95% by mass or more, more preferably 98% by mass or more, further preferably 99% by mass or more, particularly preferably 99.5% by mass or more, and most preferably 100% by mass.

[0048] The electret of the present invention can be used in any desired shape, for example, as a fibrous material, membrane, extruded material, porous membrane, powder, surface coating on other materials, etc., to utilize the function of the electret. Among these, when the electret is used for filter applications, fibrous material (fiber sheet) is particularly preferred.

[0049] The aforementioned fibrous material is preferably a fiber aggregate. Examples of fiber aggregates include: nonwoven fabrics, woven fabrics, cotton-like materials formed from long or short fibers, and fibrous materials obtained from stretch films. A fiber aggregate refers to a material whose fibrous morphology is confirmed when the electret is observed on its surface using a scanning electron microscope, optical microscope, or similar device, and in which at least a portion of the fibers constituting the fiber aggregate are integrated through melting or interweaving.

[0050] When electrets are used for filter applications, the fiber assembly is preferably a nonwoven fabric. Methods for obtaining nonwoven fabrics include: methods for forming short fibers such as single-component fibers, core-sheath type fibers, parallel type fibers, and split type fibers into sheets through methods such as carding, air-flow web formation, and wet papermaking; methods for forming sheets using continuous fibers such as spunbonding, meltblowing, electrospinning, and centrifugal spinning; and other conventionally known methods. From the viewpoint of not needing to treat residual solvents or textile oils adhering to the surface, nonwoven fabrics obtained by spunbonding, meltblowing, melt electrospinning, or melt centrifugal spinning are more preferred, and nonwoven fabrics obtained by spunbonding or meltblowing are particularly preferred.

[0051] In the fiberization or nonwoven fabrication based on the above-described melt extrusion method, melt extrusion is preferably performed 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 with excellent charge quantity and filtration characteristics as an electret can be obtained.

[0052] The average diameter of the fibers constituting the fiber assembly is preferably 0.001–100 μm, more preferably 0.05–50 μm, further preferably 0.1–30 μm, particularly preferably 0.3–25 μm, and most preferably 0.5–20 μm. When the average fiber diameter is greater than 100 μm, it is difficult to obtain practical collection efficiency, and the efficiency decreases significantly during charge decay. When the average fiber diameter is less than 0.001 μm, it is difficult to obtain an electret with an imparted charge. The average fiber diameter can be calculated, for example, by measuring the diameter of 50 fibers in the same field of view without fiber repetition using a scanning electron microscope, and then calculating the average diameter using a geometric mean.

[0053] A fiber aggregate can be composed of a single fiber with the same manufacturing method and raw material, or it can be composed of two or more fibers with different manufacturing methods, raw materials, or average diameters.

[0054] Regarding the electret method in this invention, there are no particular limitations as long as the desired characteristics for using the electret can be obtained, but a method of contacting or colliding with the liquid (liquid contact charging method) is preferred, as an electret with high filtration characteristics can be obtained by the liquid contact charging method. More specifically, a method of contacting or colliding the fiber assembly before electretization with the liquid by means of attraction, pressurization, ejection, etc., is preferred.

[0055] Regarding the liquid used in the liquid contact electrification method, there are no particular restrictions as long as the desired characteristics can be obtained, but water is preferred from the perspective of processability and performance. Alternatively, a liquid in which byproducts (components other than water) have been added can be used instead of water, and the conductivity and pH of the liquid can be adjusted by the type and amount of the byproducts added.

[0056] In the liquid contact electrification method, the pH of the liquid in contact or collision is preferably 1 to 11, more preferably 3 to 9, and even more preferably 5 to 7. Furthermore, the conductivity of the liquid in contact or collision is preferably 100 μS / cm or less, more preferably 10 μS / cm or less, and even more preferably 3 μS / cm or less.

[0057] When the electret of the present invention is used in a filter application, the QF value, which is the filter media quality factor, is, for example, 0.1 mmAq. -1 The preferred value is 0.11 mmAq. -1 The above is more preferably 0.12 mmAq. -1 The above is further preferred to be 0.13 mmAq. -1 The above is particularly preferred, with 0.14 mm Aq. -1 The optimal value is 1.0 mm Aq. -1 The above. In particular, electret filters manufactured by meltblowing with an average fiber diameter of 0.5 to 5 μm that exceed the above-mentioned QF value are especially preferred. If the QF is less than 0.1 mmAq... -1 If the particles are not sufficiently captured by the electret, the filter's performance is inadequate. In this specification, the QF value is calculated based on the airflow resistance along the filter's thickness at a wind speed of 10 cm / s and the particle count value in the 0.3–0.5 μm particle size zone of the laser particle counter.

[0058] When the electret of the present invention is used for filter applications, the particle collection efficiency at an air velocity of 10 cm / s can be adjusted in various ways according to the desired characteristics, but it is preferably 50% or more, more preferably 70% or more, further preferably 90% or more, and most preferably 95% or more. The particle collection efficiency in this specification is calculated based on the number of particles counted by the laser particle counters before and after the filter at a particle size partition of 0.3 to 0.5 μm when air is circulated along the thickness direction of the filter at an air velocity of 10 cm / s.

[0059] When the electret of the present invention is used as a filter, the air resistance at an air velocity 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 air resistance is too low, the filter performance is insufficient; if the air resistance is too high, the advantages of being a filter will be lost.

[0060] The quality factor (QF) of the filter media is approximately 0.1 or less when using unelectretized fiber assemblies. For filter applications, it is preferably 0.5 or more, more preferably 1.0 or more, further preferably 1.1 or more, and most preferably 1.2 or more. In liquid contact electrification methods using water, there is sometimes a heating and drying process. In addition, in filter applications, there are sometimes pleating processes and long-term storage. Therefore, it is preferable that the quality factor (QF) of the filter media is maintained not only immediately after electretization, but also after heat treatment and after long-term storage.

[0061] The charge stability evaluation of the electret in this invention is performed using the TSC (thermal stimulating current) method. Specifically, as shown below.

[0062] (1) Charge the sheet-like fiber aggregate in an electric field to electret.

[0063] (2) Make the electrodes contact the two surfaces of the electret in opposite directions.

[0064] (3) Connect a high-impedance picoampere to the electrodes on both surfaces.

[0065] (4) Place it in a heating bath below 30°C, and after temporarily short-circuiting it, raise the temperature from the set condition of 25°C to 180°C at 5°C / minute.

[0066] (5) Use the electrode temperature as the horizontal axis and the current value as the vertical axis.

[0067] Therefore, the charge transfer characteristics at various temperatures can be measured. There are two types of TSC methods: contact and non-contact. From the perspective of sensitivity and the ability to stably obtain evaluation results by performing a short-circuit operation before heating, the contact method is used in this invention. It should be noted that, as is well known, the depolarization temperature of the TSC method is related to the charge stability of the electret and its performance stability as a filter (e.g., Osaka Institute of Technology Notes, 66(1), 1-18, Japanese Patent No. 3199947, etc.). Furthermore, the inventors have discovered a correlation in charge stability between electrets prepared for use with the TSC method based on corona charging and electrets obtained through the liquid contact method.

[0068] Regarding the polarization in (1) above, in order to observe sufficient depolarization current in the TSC method, a corona discharge method is used to ensure that the charge (amount and polarity) is unevenly distributed in the thickness direction. In order not to affect the measurement results in the TSC method, the atmosphere and sample temperature during charging are below 30°C. Since the corona discharge method in this invention is not for the purpose of surface treatment such as oxidation or etching, the equipment and general conditions used in the manufacture and study of electrets are important.

[0069] After the above-mentioned charging, the TSC method is used for measurement within 10 minutes in an environment with an ambient temperature below 30°C. For materials that have undergone transportation, storage, heating, etc. after the electret is manufactured, they need to be re-electretized using the aforementioned method before measurement. The purpose of the evaluation using the TSC method in this invention is to evaluate the inherent thermoelectric stability of the raw material, in order to eliminate the influence of charge changes caused by various processing and time. That is, even for fibrous materials, laminates, etc., that have undergone various experiences (time, temperature, etc.), it is important to separate the electret portion and perform TSC measurement after charging for evaluation before thermal cleaning. Conversely, electrets and electret filters with high QF and the a / b ratio of this invention have excellent filtration characteristics and charge stability.

[0070] In this invention, when the maximum value of the depolarization current at 50–100°C in the TSC method is defined as 'a', and the maximum value of the depolarization current exceeding 100°C is defined as 'b', the ratio of a / b (where ' / ' represents division) is preferably 0.3 or less, more preferably 0.2 or less, and most preferably 0.1 or less. It is important to use a reasonable value with continuity as the data for the maximum current value shown above. The value is processed after removing instantaneous peak values ​​caused by noise and abnormal values ​​caused by short-circuit current, and a reasonable value is judged based on the values ​​before and after. The smaller the value of a / b, the smaller the depolarization ratio below 100°C, resulting in an electret and electret filter with low charge decay value and excellent practicality in the pleating process, transportation, and storage. In compositions with poor charge stability, continuous depolarization is observed at approximately 50°C to 150°C, with a significant decrease in charge. It should be noted that in this invention, the amount of depolarization charge is expressed as an a / b ratio. Naturally, it is preferable that the amount of depolarization charge is larger on the high-temperature side when the corona is charged.

[0071] Regarding depolarization above 100°C, it is preferable that the peak temperature and the temperature at which the maximum value is displayed are both high, with the peak temperature being 135°C or higher, more preferably 140°C or higher, and most preferably 145°C or higher. The temperature at which the maximum value is displayed is preferably 80°C or higher, more preferably 90°C or higher, further preferably 100°C or higher, and most preferably 110°C or higher. The temperature at which the maximum value is displayed is obtained by using the tangent of the measured values ​​before and after the peak value.

[0072] In this invention, by combining polypropylene and poly-4-methyl-1-pentene, low-melting-point oxidation can be prevented, and the peak temperature typically found in polypropylene can be further increased. There is a correlation between peak temperature and activation energy and relaxation time; by setting it to a high temperature, excellent charge stability and charge lifetime are achieved.

[0073] Furthermore, when electrets are used in filter applications, the trapping properties of small particles can be used to evaluate the charge quantity or charge stability of the electret. In particular, it is effective as an evaluation method when the uneven distribution of charge in the thickness direction is insufficient in the liquid contact method, making direct evaluation by the TSC method difficult.

[0074] In this invention, as a method for evaluating charge quantity or charge stability using the trapping characteristics of microparticles, the filter material quality factor (QF) can be used as a parameter related to charge quantity. Furthermore, as a method for evaluating charge stability, the natural logarithmic ratio of transmittance before and after various treatments (denoted as performance retention rate in this invention) can be used. As for the particles, artificially generated particles can be balanced with an electric charge, or atmospheric dust can be used. Regarding the detector, any detector capable of obtaining either concentration or number of particles can be used, as long as the particle size range can be defined. As an evaluation condition, electrostatic attraction is preferred to be dominant. As an example, with a wind speed of 10 cm / s, the pressure difference before and after the sample is compared using a differential pressure gauge, and the particle number concentration before and after the sample in the diameter range of 0.3–0.5 μm is compared using a laser particle counter. The particle transmittance, particle trapping efficiency, filter material quality factor, and performance retention rate are calculated accordingly. The relationships between these factors are shown below.

[0075] Particle transmittance [-] = (Particle concentration on the downstream side of the sample) / (Particle concentration on the upstream side of the sample)

[0076] Particle capture efficiency [%] = (1 - Particle transmittance [-]) × 100

[0077] Filter media quality factor QF[mmAq] -1 = -(ln(particle transmittance [-])) / (ventilation resistance [mmAq])

[0078] Performance retention rate [-] = ln(particle transmittance after treatment [-]) / ln(particle transmittance before treatment [-])

[0079] Regarding the aforementioned filter media quality factor (QF) value, when using unelectretized fibrous materials, it is generally 0.1 or less; for filter applications, it is preferably 0.5 or more, more preferably 1.0 or more, further preferably 1.1 or more, and most preferably 1.2 or more. In electretization methods based on liquid contact with water, a heat drying process is sometimes involved. Furthermore, in filter applications, pleating processes and long-term storage are sometimes performed. Therefore, it is preferable that the aforementioned filter media quality factor (QF) is maintained not only immediately after electretization but also after heat treatment.

[0080] The electret of the present invention can be used in conjunction with other constituent components as needed. When the electret of the present invention is used for filter applications, it is preferably used in combination with, for example, a pre-filter layer, a fiber protective layer, a reinforcing member, or a functional fiber layer. Electret filters using the electret of the present invention are also included within the scope of the present invention.

[0081] Examples of pre-filter layers and fiber protective layers include spunbond nonwovens, thermally bonded nonwovens, and polyurethane foam. Examples of reinforcing components include thermally bonded nonwovens and various webs. In addition, examples of functional fiber layers include antibacterial, antiviral, and color-coded fiber layers for identification and design purposes.

[0082] The electret of the present invention can be used in a wide range of applications. In particular, for purposes of protection, ventilation, dirt prevention, and water resistance, it is suitable for use in dust masks, dust suits, various air conditioning components, air purifiers, cabin filters, and filters for protective purposes in various devices.

[0083] 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 description in Japanese Patent Application No. 2024-028743, filed on February 28, 2024, are incorporated herein by reference.

[0084] Example

[0085] The present invention will be further described in detail through embodiments, but the following embodiments do not limit the present invention, and all modifications and implementations made without departing from the spirit of the present invention are included within the technical scope of the present invention. First, the measurement method will be described.

[0086] (Depolarization current measurement based on TSC method)

[0087] The measurements were performed using a thermally stimulated current measuring device under the following conditions. It should be noted that, in order to exclude peaks present in polymethylpentene above 200°C and to determine the peak temperature originating from polypropylene, the measurement endpoint was set at 180°C.

[0088] • Corona charging: The sample (fiber sheet) was charged and electretted under the following conditions: a flat needle electrode with an alternating arrangement and a spacing of 10 mm; a 0.5 mm thick silicone rubber sheet with a gap of 10 mm was placed on the grounding surface; a voltage of 20 kV was applied; and the charging time was 30 seconds.

[0089] • TSC electrode: An electretted sample (25 mm in diameter) is sandwiched between a pair of circular (20 mm in diameter) electrodes, with the two electrodes facing each other. The surfaces of the two electrodes are in contact with the surface of the sample.

[0090] • Temperature conditions: Increase temperature at a rate of 5℃ / minute from 25℃ to 180℃.

[0091] • Other: After placing the sample between the electrodes, temporarily short-circuit the two electrodes to serve as the zero point of the current value. Plot the obtained data with temperature [°C] on the horizontal axis and current value [-] on the vertical axis. Set the maximum value (peak height) of the depolarization current above 50°C and below 100°C as a, and set the maximum value (peak height) of the depolarization current above 100°C and below 180°C as b. Calculate the a / b value.

[0092] <Example 1>

[0093] Prepare 90 parts by weight of polypropylene homopolymer, 10 parts by weight of poly-4-methyl-1-pentene resin (DX820, 4-methyl-1-pentene / 1-decene copolymer, manufactured by Mitsui Chemicals, Ltd., MFR: 180 g / 10 min (260°C, 5 kg)), and 1 part by weight of nitrogen-containing compound (Chimassorb 944LD, manufactured by BASF), and mix them in a mixer. Spin the mixture using a meltblown spinning device to obtain a fiber with an area weight of 30 g / m². 2 The obtained fiber sheet (also a fiber assembly, and also a nonwoven fabric) was subjected to depolarization current measurement based on the above-described TSC method. The TSC peak temperature was 153℃, and the a / b value was 0.28.

[0094] <Example 2>

[0095] Prepare 95 parts by weight of polypropylene homopolymer, 5 parts by weight of poly-4-methyl-1-pentene resin (DX310, 4-methyl-1-pentene / hexadecene / octadecene copolymer, manufactured by Mitsui Chemicals, Ltd., MFR: 100 g / 10 min (260°C, 5 kg)), and 1 part by weight of nitrogen-containing compound (Chimassorb 944LD, manufactured by BASF), and mix them in a mixer. Spin the mixture using a meltblown spinning device to obtain a fiber with an area weight of 30 g / m². 2 The obtained fiber sheet was subjected to depolarization current measurement based on the above-described TSC method. The TSC peak temperature was 148℃, and the a / b value was 0.38.

[0096] <Example 3>

[0097] Prepare 90 parts by weight of polypropylene homopolymer, 10 parts by weight of poly-4-methyl-1-pentene resin (DX820, manufactured by Mitsui Chemicals Co., Ltd.), 1 part by weight of nitrogen-containing compound (Chimassorb 944LD, a registered trademark, manufactured by BASF), and 0.1 parts by weight of magnesium stearate, and mix them in a mixer. Spin the mixture using a meltblown spinning device to obtain a fiber with a basis weight of 30 g / m². 2The obtained fiber sheet was subjected to depolarization current measurement based on the above-described TSC method. The TSC peak temperature was 154℃, and the a / b value was 0.05.

[0098] <Example 4>

[0099] Prepare 95 parts by weight of polypropylene homopolymer, 5 parts by weight of poly-4-methyl-1-pentene resin (DX310, manufactured by Mitsui Chemicals Co., Ltd.), 1 part by weight of nitrogen-containing compound (Chimassorb 944LD, a registered trademark, manufactured by BASF), and 0.1 parts by weight of magnesium stearate, and mix them in a mixer. Spin the mixture using a meltblown spinning device to obtain a fiber with a basis weight of 30 g / m². 2 The obtained fiber sheet was subjected to depolarization current measurement based on the above-described TSC method. The TSC peak temperature was 148℃, and the a / b value was 0.07.

[0100] <Comparative Example 1>

[0101] Prepare 100 parts by weight of polypropylene homopolymer and 1 part by weight of a nitrogen-containing compound (Chimassorb 944LD, manufactured by BASF), and mix them. Spin the mixture using a melt-blown spinning device to obtain a fiber with a basis weight of 30 g / m². 2 The obtained fiber sheet was subjected to depolarization current measurement based on the above-described TSC method. The TSC peak temperature was 138℃, and the a / b value was 0.87.

[0102] Based on the results of the examples and comparative examples, by including a nitrogen-containing compound and combining polypropylene and poly-4-methyl-1-pentene, the peak TSC temperature accompanying the melting of polypropylene can be increased without adding nucleating agents, charge stabilizers, etc. Furthermore, according to Examples 1 and 3, and Examples 2 and 4, by adding magnesium stearate, it is possible to maintain the peak TSC temperature at a high temperature while reducing depolarization below 100°C and further improving charge stability.

[0103] Industrial availability

[0104] According to the present invention, electrets and filters with excellent charge stability can be obtained, which can make a significant contribution to industry. The electrets of the present invention are excellent in terms of charge quantity and charge density, and are particularly suitable for use as filters in dustproof clothing, dustproof masks, air purifiers, etc.

Claims

1. An electret characterized in that, The peak temperature obtained by the TSC method after charging in an electric field is above 140 °C. The electret contains a nitrogen-containing compound. In 100 parts by mass of the resin contained in the electret, 80 to 99 parts by mass of polypropylene and 1 to 20 parts by mass of poly-4-methyl-1-pentene are contained.

2. The electret according to claim 1, characterized in that, It also contains a fatty acid metal salt.

3. An electret according to claim 1 or 2, characterised in that, It is a fibrous material.

4. An electret according to claim 1 or 2, characterised in that, It is a melt-blown non-woven fabric.

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