JP7585653B2 - Electret Filter - Google Patents

Description

The present invention relates to an electret filter.

Traditionally, porous filters have 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.

Among porous filters, filters made of fibrous materials have the advantages of high porosity, long life, and low airflow resistance, and are therefore widely used. These fibrous filters capture particles on the fibers using mechanical collection mechanisms such as interception, diffusion, and inertial collision, but it is known that in practical usage environments, the filter collection efficiency is at a minimum when the aerodynamic equivalent diameter of the particles captured is around 0.1 to 1.0 μm.

To improve the filter collection efficiency at the above-mentioned minimum value, methods that also use electrical attraction are known. For example, there are methods that impart an electric charge to the particles to be collected, methods that impart an electric charge to the filter, methods that place the filter in an electric field, and methods that combine these. Filters that impart an electric charge are widely used as electret filters.

Electret filters that contain, in addition to the main component polypropylene resin, a crystal nucleating agent, modified polypropylene, various metal salts, and the like as necessary have been disclosed (Patent Documents 1 to 4).

Patent No. 6346640 JP 2018-202369 A Special table 2018-523761 publication JP 2019-93367 A

Currently, high spinning temperatures are required for filters in order to increase production speed and reduce fiber diameter. According to the inventor's research, it has become clear that conventionally used nucleating agents begin to decompose thermally at temperatures above 250°C, and that at temperatures above 300°C, not only do they lose their nucleating effect, but also problems such as odor generation due to the decomposition products occur.

Therefore, in view of the above problems, the present invention aims to provide an electret filter that is easy to produce at high temperatures and has excellent electret properties.

The present inventors have conducted extensive research to solve the above problems, and as a result have finally completed the present invention.
(1) An electret filter comprising 90% by weight or more of a polyolefin resin, of which 50% by weight or more is a polypropylene resin, 0.1% by weight or more and 5% by weight or less of a nitrogen-containing compound, and 0.1% by weight or more and 9.9% by weight or less of zinc oxide particles, and characterized in that the crystallization temperature derived from the polypropylene resin after heating to 300°C is 115°C or more.
(2) The electret filter according to (1), wherein the nitrogen-containing compound is a hindered amine compound.
(3) The electret filter according to (1) or (2), which contains a metal salt of a fatty acid having 10 to 50 carbon atoms and/or a fatty acid having 10 to 50 carbon atoms.
(4) The electret filter according to any one of (1) to (3), which is a nonwoven fabric obtained by a melt blown method.

The zinc oxide particles contained in the electret filter of the present invention are not thermally decomposed even at high temperatures, do not lose their nucleating effect, and do not produce odors. Therefore, they act favorably as a nucleating agent in the manufacture of filters. Therefore, the present invention makes it possible to obtain an electret filter with excellent productivity.

The following describes an embodiment of the present invention.

The electret of the present invention contains 90% by weight or more of polyolefin resin, of which 50% by weight or more is polypropylene resin, 0.1% by weight or more and 5% by weight or less of nitrogen-containing compound, and 0.1% by weight or more and 9.9% by weight or less of zinc oxide particles, and has a crystallization temperature derived from the polypropylene resin after heating to 300°C of 115°C or more.

The electret filter of the present invention is made by taking into consideration the freedom of shape and the charge stability of the material itself, and contains 90% by weight or more of hydrophobic and highly electrically resistant polyolefin resin as a constituent material, of which 50% by weight or more is polypropylene resin. In this case, it means that the composition of the sheath part of a homogeneous material or a core-sheath structure, or in the case of a mixed fiber of two or more types, at least one of them is polyolefin and has the composition of the present invention. By containing 90% by weight or more of polyolefin resin, the electric resistance is high and the balance of hydrophobicity, moldability, etc. is good, and in the case of an electret filter, it is possible to obtain an excellent collection efficiency (hereinafter sometimes simply referred to as collection efficiency) for particles with a particle diameter of 0.3 to 0.5 μm, i.e., an excellent practicality.

The electret filter of the present invention is made by mixing one or more nitrogen-containing compounds and zinc oxide particles with the polyolefin resin. This allows for a combination of a specific composition and charging method, resulting in a particularly high initial charge amount and charge stability.

The content of the nitrogen-containing compound is preferably 0.1% to 5% by weight of the electret filter, more preferably 0.5% to 3% by weight, and most preferably 0.75% to 1.5% by weight. If the content is too small, the charge amount will be low, and if it is too large, the electret filter will lose stability due to increased hygroscopicity.

There are no particular limitations on the nitrogen-containing compound as long as it provides the desired properties, but compounds containing both or either an amino group and a triazine structure, such as hindered amine additives, are preferably used.

The hindered amine or triazine additive is not particularly limited, but specifically includes poly[(6-(1,1,3,3-tetramethylbutyl)imino-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 944LD, manufactured by BASF), N,N',4,7-tetrakis[4,6-bis[N-butyl-N-(1,2,2,6,6-pentamethyl-4-piperidyl)amino]-1,3,5-triazin-2-yl]-4,7-diazadecane-1,10-diamine (Chimasorb 119, manufactured by BASF), dimethyl hexanoate-1-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethylpiperidine polycondensate (Tinuvin 622LD, manufactured by BASF) BASF Corporation), 2-(3,5-di-t-butyl-4-hydroxybenzyl)-2-n-butylmalonate bis(1,2,2,6,6-pentamethyl-4-piperidyl) (Tinuvin 144, BASF Corporation), 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 2020FDL, BASF Corporation), and 2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-((hexyl)oxy)-phenol (Tinuvin 1577FF, BASF Corporation).

The content of zinc oxide particles is preferably 0.1% by weight or more and 9.9% by weight or less, more preferably 0.5% by weight or more and 7% by weight or less, even more preferably 0.7% by weight or more and 5% by weight or less, and most preferably 1% by weight or more and 3% by weight or less.

The shape of the zinc oxide particles is not particularly limited, but it is possible to preferably use shapes such as spherical, cylindrical, rectangular, needle-like, and plate-like, more preferably spherical, rectangular, and plate-like, and most preferably spherical. They may also be composed of two or more components, in which case it is preferable that the outer surface of the core-sheath structure is zinc oxide.

There are no particular limitations on the particle size (diameter) of the zinc oxide particles, but the secondary particle size is preferably 5 μm to 0.01 μm, more preferably 0.02 to 2 μm, even more preferably 0.03 to 1.5 μm, and most preferably 0.05 to 1 μm. If the particle size is large, the contact area with the resin component will be insufficient, reducing the effect and causing the resin to lose its molding properties.

It is known that zinc oxide particles are coated with various dispersants and anti-adhesive agents in consideration of fluidity, dispersibility, etc. In the electret filter of the present invention, in the case of fibrous materials, those that are volatilized or decomposed at the time of spinning or masterbatching are preferred. That is, in terms of the heat loss characteristics of these components in nitrogen, it is preferred that the heat loss at 250°C to 550°C is 10% by weight or less, more preferably the heat loss is 5% by weight or less, and most preferably the heat loss is 1% by weight or less. In a preferred embodiment, the zinc oxide particles are dispersed in advance in the same polypropylene resin as that used for spinning, so that only zinc oxide particles remain at the time of masterbatching.

The electret filter of the present invention may contain a fatty acid and/or a fatty acid metal salt in combination with the zinc oxide particles. The fatty acid group is not particularly limited, but one having a linear saturated fatty acid group is preferred, and the fatty acid group preferably has 10 to 50 carbon atoms. Specific examples include lauric acid, myristic acid, palmitic acid, and stearic acid. The metal element is preferably zinc, aluminum, or magnesium. By using the fatty acid and/or fatty acid metal salt in combination with the zinc oxide particles, the free fatty acid from the fatty acid metal salt is reduced and the free ions from the zinc oxide are supplemented. The fatty acid and/or fatty acid metal salt may be added during spinning, or may be mixed with the zinc oxide particles in advance.

The amount of fatty acid and/or fatty acid metal salt preferably used is in the range of 0.1% to 5% by weight of the electret filter, more preferably 0.15% to 2% by weight, and most preferably 0.2% to 1% by weight.

The electret filter of the present invention has a crystallization temperature of 115°C or higher derived from the polypropylene resin after heating to 300°C. The measurement of the crystallization temperature will be described later. The crystallization temperature of polypropylene resin alone is around 110°C, but the electret filter of the present invention has a higher crystallization temperature, which allows the filter to have excellent spinnability at high temperatures during production, improve production speed, and reduce fiber diameter. The present invention can provide an electret filter that is excellent in charge stability and bulkiness by improving productivity.

The electret filter of the present invention may have any shape required, but is preferably formed as a fibrous material.

The fibrous materials 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, and can be molded into appropriate shapes and thicknesses depending on the application. Electret filters made of nonwoven fabrics are particularly suitable for use and handling.

Methods for obtaining nonwoven fabrics include conventional methods such as 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, air-laid, wet papermaking, etc., and obtaining nonwoven fabrics from continuous fibers by spunbonding, melt-blown, electrospinning, force spinning, etc. Among these, nonwoven fabrics obtained by the melt-blown, electrospinning, or force spinning methods, which can easily obtain dense and fine fibers, are preferred from the viewpoint of effectively utilizing the mechanical collection mechanism, and nonwoven fabrics obtained by the melt-blown, molten electrospinning, or molten force spinning methods are more preferred from the viewpoint of not requiring treatment of residual solvents.

Since the electret filter of the present invention has excellent spinnability and electret properties, particularly at high temperatures, when made into a nonwoven fabric, it is preferable to manufacture the filter using the meltblown method or the spunbond method, and it is particularly preferable to manufacture the filter using the meltblown method.

When the electret filter of the present invention is a fibrous material, the fiber diameter is preferably 0.001 to 100 μm, more preferably 0.005 to 20 μm, even more preferably 0.01 to 10 μm, particularly preferably 0.02 to 5 μm, and most preferably 0.03 to 3 μm. If the fiber diameter is thicker than 100 μm, it is difficult to obtain a practical collection efficiency as a filter, and there is a large decrease in efficiency when the charge decays. If the fiber diameter is smaller than 0.001 μm, it is difficult to impart a charge as an electret.

When the electret filter of the present invention is a fibrous material, it may be a homogeneous product made of a single manufacturing method and fiber, or it may be a mixture made of two or more types of fibers that differ in manufacturing method, material, and fiber diameter. As described above, what is necessary for the characteristics to be expressed is that at least a part of the part having electret properties is the composition of the present invention.

The electretization (charging) method for the electret filter of the present invention is not particularly limited as long as it can provide the desired characteristics during use, but a method utilizing contact and collision with a liquid can be preferably used. More specifically, an electret filter with a high charge amount and filtration characteristics can be obtained by contacting a liquid with the filter before electretization and charging it by a method such as suction, pressure, or spraying.

The above liquid is not particularly limited as long as it can provide the desired characteristics, but in terms of ease of handling and performance, it is preferable to use water or a liquid whose main component is water. Furthermore, the conductivity and pH of the water and the subcomponents mixed in can be adjusted as necessary. In the manufacture of electret filters, it is preferable to use water with a pH of 1 to 11 and a conductivity of 100 μS or less.

The electret filter of the present invention may be used in combination with other components as necessary. That is, it may be used in combination with a prefilter layer, a fiber protection layer, a reinforcing member, or a functional fiber layer. The electret filter of the present invention may also be used after being processed into a shape, such as a pleated shape.

The electret filter of the present invention can be used in a wide range of applications due to its functions as a filter, such as dust collection, protection, ventilation, antifouling, and waterproofing, and can be particularly suitably used as a filter for protecting dust masks, various air conditioning elements, air purifiers, cabin filters, and various devices.

Hereinafter, an embodiment of the present invention will be described.
[Filter quality factor (QF value)]
The QF value defined below was used as an index of the electret charge amount and filter performance.
QF [mmAq ⁻¹ ] = − (ln (particle permeability) / (airflow resistance))

(Particle Transmission Rate)
A sample punched to 72 mm in diameter was attached to an adapter with an effective ventilation diameter of 50 mm in diameter, and the particle transmittance was calculated under the following conditions using a light scattering particle counter KC-01E manufactured by Rion Co., Ltd.
Particles evaluated: Atmospheric dust particles Wind speed: 10 cm/s
Particle number: Calculation of particles of 0.3 to 0.5 μm using the light scattering counting method Particle transmittance = Number of particles on downstream side of sample / Number of particles on upstream side of sample

(Ventilation resistance)
A sample punched out to 72 mmφ was attached to an adapter with an effective air flow diameter of 50 mmφ, and pipes with an inner diameter of 50 mm connected to a differential pressure gauge were connected above and below. Air was passed through the sample at 10 cm/s, and the air flow resistance (pressure loss) (mmAq) without any restriction was measured.

[Crystallization temperature]
According to JIS K7121, the transition temperature measurement method for plastics, a Tg-DTA device was used, and the temperature was raised from 50°C to 300°C at a rate of 10°C/min in nitrogen, and then cooled to 50°C. The peak temperature during cooling was taken as the crystallization temperature derived from PP.
Example 1
94% by weight of polypropylene homopolymer with MFR 1200, 1% by weight of BASF Chimassorb 944, and 5% by weight of zinc oxide particles with an average particle size of 0.75 μm and a thermal weight loss of 1% or less at 250 to 550° C. were mixed and spun using a meltblown device at a resin temperature of 265° C. and an air temperature of 265° C. to obtain a nonwoven fabric sheet with a basis weight of 20 g/m ^2. The nonwoven fabric sheet obtained was charged by colliding it with water having an electrical conductivity of 0.7 μS and a pH of 6.8, and then left to dry at 100° C. for 30 minutes to obtain an electret filter. The electret filter had a ventilation resistance of 3.47 mmAq, a QF value indicating filter performance of 1.63 mmAq ⁻¹ , and a crystallization temperature of 138° C.

Example 2
An electret filter was obtained by the same operation as in Example 1, except that the resin temperature and the air temperature during spinning were 300° C. The electret filter had an air resistance of 5.60 mmAq, a QF value of 1.33 mmAq ⁻¹ , and a crystallization temperature of 136° C.

Example 3
The same procedure as in Example 1 was carried out, except that the amount of polypropylene homopolymer was 84.6 parts by weight and the amount of Mitsui Chemicals polymethylpentene resin DX820 was 9.4 parts by weight. The flow resistance was 3.35 mmAq, the QF indicating the filter performance was 1.73 mmAq ^-1 , and the crystallization temperature was 137°C.

Example 4
The same procedure as in Example 1 was carried out except that 0.1 parts by weight of magnesium distearate was added. The air resistance was 3.52 mmAq, the QF value was 1.65 mmAq ^-1 , and the crystallization temperature was 137°C.

<Comparative Example 1>
The same procedure as in Example 1 was carried out, except that 0.3% by weight of dibenzylidene sorbitol was used instead of zinc oxide particles. A benzamide odor was observed during spinning and from the nonwoven fabric sheet. The electret filter had an air resistance of 3.68 mmAq, a QF value of 1.26 mmAq ^-1 , and a crystallization temperature of 112°C.

<Comparative Example 2>
The same procedure as in Example 2 was carried out except that 0.01% by weight of 1,3,5-tris(2,2-dimethylpropionylamino)benzene CasRN.745070-61-5 was used instead of the zinc oxide particles. The electret filter had an air resistance of 6.21 mmAq, a QF value of 1.13 mmAq ^-1 , and a crystallization temperature of 114°C.

<Comparative Example 3>
Except for omitting the zinc oxide particles, the same operation as in Example 1 was carried out. The electret filter had an airflow resistance of 3.72 mmAq ^-1 , a QF value of 1.18 mmAq ^-1 and a crystallization temperature of 110°C.

It can be seen that Examples 1 to 4 are electret filters that have superior filtering performance and productivity at high temperatures compared to Comparative Examples 1 to 3.

The electret filter of the present invention has excellent productivity and electret properties at high temperatures, and can make a significant contribution to the field of filters.

Claims (1)

the composition comprises 90% by weight or more of a polyolefin resin, 50% by weight or more of which is a polypropylene resin, 0.1% by weight or more and 5% by weight or less of a hindered amine compound, 0.1% by weight or more and 9.9% by weight or less of zinc oxide particles, the secondary particle diameter of which is 0.01 to 5 μm and the thermal weight loss at 250 to 550° C. is 1% or less, and a metal salt of a fatty acid having 10 to 50 carbon atoms and/or a fatty acid having 10 to 50 carbon atoms;
Obtaining a nonwoven fabric having a crystallization temperature of 136°C or higher derived from a polypropylene resin alone after heating to 300°C by a melt blowing method;
A method for producing an electret filter , comprising the step of converting the nonwoven fabric into an electret .