JPH0326091B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> The present invention relates to an electret body that can be used in air filters and the like, and particularly relates to an electret body made of a charged porous body that has excellent moisture resistance and heat resistance and has a long life. <Prior art> The electret porous body of polyolefin is
Since it shows high collection efficiency when used in filter applications, it has high industrial utility value and many proposals have been made. For example, JP-A-60-168511, JP-A-61-
No. 102476 describes a method for charging a porous polyolefin material to make it electret. <Problems to be Solved by the Invention> However, although the polyolefin electret porous body according to the prior art has relatively high initial performance, the melting point of the polyolefin is not very high, so when used above room temperature, the charge disappears quickly. The disadvantage is that it has a short lifespan. Therefore, there is a need for an electret porous body that has even higher initial performance and has a longer life even at temperatures higher than room temperature. <Means for Solving the Problems> The present invention solves the problems of the prior art and provides an electret body that has high performance as a filter and has a long life with little deterioration in performance even at temperatures higher than room temperature. The purpose is to That is, the present invention has a polyolefin resin and a general formula (In the formula, R ¹ and R ² are either an alkyl group or an alkoxy group having 1 to 10 carbon atoms, and may be the same or different, and m and n are each independently 0 to 10.
This invention relates to a porous body obtained from a polyolefin composition made of a sorbitol derivative represented by the number 3) and a charged electret body. The polyolefin resin used in the present invention includes polypropylene, ethylene-propylene copolymer, high-density polyethylene, low-density polyethylene, linear low-density polyethylene, ethylene-vinyl acetate copolymer, or a mixture thereof, unsaturated Examples include those modified with carboxylic acid derivatives. Among the polyolefins, polypropylene or ethylene-propylene block copolymer having a high melting point is preferred. Next, the general formula The sorbitol derivative represented by the formula (wherein R ¹ , R ² , m, and n have the meanings described above) will be described.
R ¹ and R ² are either an alkyl group or an alkoxy group having 1 to 10 carbon atoms, and are methyl, ethyl, propyl, butyl, hexyl, octyl,
Examples include decyl group, methoxy, ethoxy, propoxy, butoxy, hextoxy, octoxy, and detoxy groups, and preferably 1 to 4 carbon atoms.
m and n are each independently a number from 0 to 3, m,
n is preferably 0 to 1. Specifically, dibenzylidene sorbitol, di(p-methylbenzylidene) sorbitol, di(m-methylbenzylidene) sorbitol, (benzylidene)(p-methylbenzylidene) sorbitol, (benzylidene)
(p-ethylbenzylidene) sorbitol, (p-
methylbenzylidene) (p-ethylbenzylidene)
Sorbitol, di(p-ethylbenzylidene) sorbitol, bis(dimethylbenzylidene) sorbitol, di(p-methoxybenzylidene) sorbitol, di(p-isopropylbenzylidene)
Sorbitol etc. are used. The polyolefin composition is made by adding a sorbitol derivative to the above-mentioned polyolefin resin, and although this ratio is not particularly critical, if the sorbitol derivative is too small, the effect of the present invention will be small; on the other hand, if it is added in a certain amount or more, the effect will increase. However, problems such as bleeding occur. Therefore, the sorbitol derivative is preferably added in an amount of 0.02 to 5 parts by weight, more preferably 0.05 to 4 parts by weight, per 100 parts by weight of the polyolefin resin. A polyolefin composition can be obtained by adding a sorbitol derivative to polyolefin resin powder or pellets, uniformly dispersing it using a ribbon blender, Henschel mixer, etc., and then melting and kneading it using an extruder, kneader, etc. Although the effects of the present invention are exhibited by using this polyolefin composition, the reason for this has not yet been clarified academically. However, it is generally said that the charge of the electret is trapped on the surface of the porous material and the interface between the crystal and the amorphous, and that the charge trapped on the interface between the crystal and the amorphous is stable and contributes to the lifetime of the electret. ing. Therefore, it is presumed that the effects of the present invention occur because a large number of fine crystals are generated in the polyolefin composition of the porous body due to the nucleation effect of the sorbitol derivative, and charges are stably trapped at this interface. The porous body of the present invention has a large number of communicating voids and has ventilation performance. This space ratio is 40%
The above range is preferably 50 to 99.9%. Specific examples of the porous body include fibrous sheets, porous films, sponge sheets, foam sheets, and the like. Examples of the fibrous sheet include nonwoven fabrics produced by spunbonding, melt blowing, and flash spinning, fabrics obtained by laminating fibrillated films, and knitted fabrics. Further, the fibrous sheet can also be used after being embossed, pleated, etc. As mentioned above, the electret body of the present invention is capable of trapping many charges at the interface between the polyolefin microcrystal and the amorphous. This has improved the lifespan of nonwoven electret fabrics made of ultrafine fibers with a fiber diameter of 20 μm or less using the melt blow method, flash spinning method, etc., and has also made it possible to manufacture high-performance air filters. Various known methods can be used to electret a porous body. For example, thermal electrets are obtained by raising the temperature of a porous body to near its softening temperature and cooling it while applying a high DC voltage, and electroelectrets that are obtained by applying corona discharge or pulsed high voltage to the surface of the porous body. , radioelectrets that emit gamma rays or electron beams can be used. A preferred method is to bring one side of the porous body into contact with a ground plate and perform corona discharge from the opposite side, as it can charge the porous body in a relatively short time. <Examples> Examples of the present invention will be shown below. The collection efficiency in the example was determined at a flow rate of 6.1 using 0.3μ stearic acid particles generated by the steam-cooled condensation method.
Determine the particle concentration before and after the porous body under cm/sec conditions,
Calculated from that ratio. Incidentally, the particle concentration was measured by a light scattering type light amount integration method. Polymer melt index (hereinafter referred to as MI) is determined according to ASTM-D1238 at a temperature of 190℃ and a charge
Measured under the condition of 2.16Kg, the unit is g/10 minutes. The density of the porous body was determined from the following formula. Density (g/cm ³ ) = Weight per unit area (g) / Thickness (c
m) x unit area (cm ² ) Example 1 100 parts by weight of polypropylene homopolymer pellets of MI53 were mixed with 0.3 parts by weight of dibenzylidene sorbitol, and then melt-kneaded and extruded using an extruder. Next, a nonwoven fabric having a fiber diameter of 1.8 μ, a basis weight of 32 g/m ² and a density of 0.11 g/cm ³ was formed using this composition by a melt blowing method. A voltage of +9 Kv/cm was applied to a needle electrode placed 1 cm above the nonwoven fabric, and corona charging was performed for 30 seconds to form an electret body. The collection efficiency (hereinafter referred to as collection efficiency A) of this was 99.7%. The collection efficiency of this electret body after being stored at 95°C for 5 hours (hereinafter referred to as collection efficiency B) is
The collection efficiency was 99.4%, and the collection efficiency after storage for 30 days at a temperature of 50°C and a humidity of 95% was 99.5%. Comparative Example 1 Using the polypropylene homopolymer used in Example 1, a nonwoven fabric was formed by melt blowing and corona discharge was performed under the same conditions as in Example 1, except that dibenzylidene sorbitol was not added. The collection efficiency A of this nonwoven fabric was 98.4%, and the collection efficiency B after storage at 95° C. for 5 hours was 78.7%. Examples 2 to 8, Comparative Example 2 Polyolefin compositions were prepared using 100 parts by weight of MI62 polypropylene homopolymer pellets and the sorbitol derivatives shown in the table. Using this composition, a nonwoven fabric having a fiber diameter of 1.6 μm, a basis weight of 21 g/m ² , and a density of 0.12 g/cm ³ was formed by melt blowing. This nonwoven fabric was subjected to corona discharge for 1 minute at a voltage of -8 Kv/cm to form an electret body. The collection efficiency of this was measured and the results shown in the table were obtained.

【table】

[Table] Measurement after storage
Example 9 0.5 part by weight of dibenzylidene sorbitol was added to 100 parts by weight of MI12, an ethylene-propylene block copolymer having an ethylene content of 4 wt%, and the mixture was melt-kneaded. Next, a woven fabric with a fiber diameter of 39.4 μ, a basis weight of 100 g/m ² and a density of 0.07 was prepared by a spunbond method. Cut out a 20cm x 20cm sample from this non-woven fabric, sandwich it between two 20cm x 20cm 100μ thick PET films (top and bottom), sandwich it between 15cm x 15cm 2mm thick stainless steel electrodes, and heat. 55 in the furnace
The temperature was raised to ℃. One side of the electrode was grounded, and the other electrode was kept at 55°C for 10 minutes while applying a DC voltage of +5 Kv, and then cooled to room temperature while applying a voltage. The collection efficiency A of this was 83.6%, and the collection efficiency B was 83.1%. Comparative Example 3 A nonwoven fabric was produced under the same conditions as in Example 9, except that dibenzylidene sorbitol was not added, and it was made into an electret. The collection efficiency A was 79.9%, and the collection efficiency B was 57.1%. Example 10 MI16, di(p-
0.2 parts by weight of sorbitol (methylbenzylidene) was added and melt-kneaded. Next, a nonwoven fabric having a fiber diameter of 9.4 μ, a basis weight of 80 g/m ² , and a density of 0.06 was prepared by melt blowing, and then embossing was performed using a pinpoint roll. Cut out a 15cm x 15cm sample from this sample, place a 14cm x 14cm 2mm thick vinyl chloride plate on top, apply an accelerating current of 200Kv from above, and apply a dose of 1Mrad.
was irradiated with an electron beam. The collection efficiency A of this is 93.9
%, and the collection efficiency B was 93.1%. Comparative Example 4 A nonwoven fabric was made and electretized under the same conditions as in Example 10, except that di(p-methylbenzylidene) sorbitol was not added. The collection efficiency A of this is
The collection efficiency B was 81.4% and 62.5%. <Effects of the Invention> As is clear from the above, the present invention has discovered an electret porous sheet that has extremely high collection efficiency and high filter performance with a long life even at temperatures higher than room temperature. .

Claims (1)

[Claims] 1. Polyolefin resin and general formula (In the formula, R ¹ and R ² are either an alkyl group or an alkoxy group having 1 to 10 carbon atoms, and may be the same or different, and m and n are each independently 0 to 10.
An electret body which is a porous body obtained from a polyolefin composition comprising a sorbitol derivative represented by the number 3) and which is electrically charged.