JPH04211925A - Manufacture of electret - Google Patents

Abstract

PURPOSE:To obtain an lectret having a stabilized surface electric charge, by a method wherein treatment performing application of an electric charge and heating alternately is applied repeatedly to a matter to be treated at least two time. CONSTITUTION:A graft polymer comprised by modifying a nonpolar polymer or those polymeric compound, especially preferably a nonpolar polymer modified by unsaturated carboxylic acid or unsaturated epoxy monomer or silane monomer is mentioned as a matter 1 to be treated. The matters 1 to be treated are passed continuously through a device constituted by arranging a pair of electrodes 2, 3 to which a direct current voltage is applied, heating roll 4, then electrodes 22, 32 and a heating roll 42 and continuously electrodes 23, 33 and a heating roll 43, through which application of electric charge and heating treatment are performed a fixed number of times repeatedly and an electret is obtained.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing electrets, and more particularly to a method for producing electrets capable of obtaining electrets with excellent surface charge retention properties.

0002

BACKGROUND OF THE INVENTION Electrets such as electret films and electret fiber sheets are used for various purposes. For example, electret film can be used for acoustic elements such as microphones, speakers, cartridges, etc.
It is used as an element in measuring instruments such as vibration meters and strain meters, and in non-contact switches. Applications are also being attempted in memory devices and in the medical field. In addition, electret fiber sheets are used in household air conditioners, air purifiers,
It is used for various purposes such as dust filters for vacuum cleaners and general air conditioning filters.

[0003] These electrets are produced by converting films, fiber sheets, etc. into electrets. Generally, the thermal electretization method, electroelectretization method, radioelectretization method, magnetoelectretization method, photoelectretization method, mechanoelectretization method, etc. are known as methods of electretization, but they are not actually used. From the viewpoint of performance and cost, the thermal electretization method and the electroelectretization method have been adopted as methods.

0004

[Problems to be Solved by the Invention] However, although the thermal electretization method described above can produce electrets with stable surface charges, the manufacturing method requires keeping the material above its melting point or softening point and applying a high DC voltage. This is a batch method in which the product is cooled down to room temperature while applying an electric current, and as it takes a long time to produce, the productivity is poor. On the other hand, the electroelectretization method is made by applying a DC high voltage to the material at room temperature and injecting charges, so it can be made into electrets in a short time and the productivity is good, but the resulting electret There were problems such as poor stability of surface charge.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a method for producing electrets that can produce electrets with excellent surface charge stability with high productivity.

[0006]

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present invention alternately performs a process of applying a charge to a workpiece and a process of heating it two or more times, preferably three or more times in a row. The present invention provides a method for producing an electret that includes repeated steps. Moreover, the present invention, as a first embodiment,
The present invention provides a method for producing an electret film in which a stretched or unstretched film is subjected to the above steps.

[0007] Furthermore, the present invention has a second embodiment,
The present invention provides a method for producing an electret fiber sheet in which a nonwoven fabric or a woven fabric is subjected to the above steps. Furthermore, the present invention provides, as a third embodiment, a method for producing an electret nonwoven fabric using a fibrous material obtained by subjecting a stretched or unstretched film to the above-described process and defibrating it.

The present invention will be explained in detail below.

The method of the present invention, as shown in FIG.
This is a method that is repeated several times.

Conditions for the charge application treatment and the heat treatment are appropriately selected according to the melting point and form of the object to be treated, desired treatment time, treatment temperature, and the like. Generally, the application of charge is a direct current voltage of 5
This is carried out by applying approximately 20 kV to the object to be processed. In addition, the heat treatment is performed at about 60 to 200°C, and in particular,
It is preferable to carry out the treatment at a temperature in the range from 50° C. lower than the melting point of the material to be treated, since a very stable electret can be produced. Further, the conditions for the application treatment and the conditions for the heat treatment are selected in relation to each other so that an electret having a desired surface charge can be obtained with high productivity. That is, if the application treatment is performed at a temperature slightly lower than the melting point or transition point of the electret, the heat treatment may be performed in a short time.

[0011] Taking a film as an example, the method of applying a charge may be carried out by passing the film between a pair of electrodes to which a DC voltage is applied.

[0012] The heat treatment may be carried out by passing the material through a heating roll adjusted to a predetermined temperature.

[0013] In the method of the present invention, the step of heating after applying the electric charge is repeated two or more times, preferably three to five times.

[0014] The object to be treated to which the method of the present invention can be applied is not particularly limited, and may be, for example, in any form of film, sheet, or fiber. In addition, typical materials for the object to be treated include those whose main component is a polymer compound, and examples of the polymer compound include polyolefins such as polypropylene, polyethylene, polybutene-1, and poly-4-methyl-1-pentene. , polyesters such as polyethylene terephthalate, polycarbonate,
Polyamides such as nylon-6 and nylon-12, acrylic resins such as polymethyl acrylate, acrylic-styrene resins (AS resins), acrylic-butadiene-styrene resins (ABS resins), polyvinyl chloride, polyvinylidene chloride , fluororesins such as polytetrafluoroethylene, etc. Particularly preferred are nonpolar polymers typified by the above-mentioned polyolefins, polystyrene, fluororesins such as polytetrafluoroethylene, and these polymers. Molecular compounds, particularly preferably grafted products obtained by modifying the above-mentioned nonpolar polymers with unsaturated carboxylic acids such as maleic anhydride, acrylic acid, methacrylic acid, or unsaturated epoxy monomers and silane monomers. Can be mentioned. In addition, other resins such as polyesters such as polyethylene terephthalate and polytetramethylene terephthalate, polyamides such as nylon 6, nylon 66, and nylon 12, polycarbonate, polymethyl methacrylate, and polyacrylic acid may be added to these polymer compounds as necessary. Acrylic resins such as ethyl, acrylic-styrene resins (AS resins), acrylic-butadiene-styrene resins (ABS resins),
Polar polymers such as polyvinyl chloride, polyvinylidene chloride, polychlorotrifluoroethylene, polyacetal, and polyacrylonitrile may be added. It is also possible to create a three-component polymer composition by adding a non-polar polymer modified with one of the monomers described above as a third component when mixing the above-mentioned non-polar polymer and polar polymer. good.

Among the modified nonpolar polymers, those grafted with modified monomers are particularly preferred. Various known methods can be used to produce a graft copolymer; for example, a method in which a nonpolar polymer is melted and a modified monomer is added to perform graft copolymerization, or a method in which a nonpolar polymer is melted and a modified monomer is added thereto, or a method in which it is dissolved in a solvent and a modified monomer is added thereto. There is a method of copolymerizing. In any case, in order to carry out efficient graft copolymerization, it is preferable to carry out the reaction in the presence of a radical initiator. As a radical initiator, organic peroxide, organic perester,
Examples include azo compounds, but ionizing radiation, ultraviolet rays, etc. can also be used to generate radicals. Further, the grafting rate is about 0.05 to 15 parts by weight, preferably about 0.05 to 10 parts by weight of the modified monomer per 100 parts by weight of the nonpolar polymer.

[0016] The above-mentioned three-component polymer composition (here, 3
The modified polymer, which is one of the components, is one of the three types mentioned above.
(considering one or more species together as one component), non-polar polymer 60-99% by weight, especially 80-95% by weight, polar polymer 0.5-39.5% by weight, especially 1-10% by weight and 0.5 to 20% by weight of the aforementioned modified nonpolar polymer, especially 4
~10% by weight.

In the method of the present invention, the timing of repeating the heating process after applying the electric charge is appropriately selected depending on the object to be processed. For example, the present invention will be explained based on an example of manufacturing an electret by converting a continuous band-shaped processed material such as a film or a fiber sheet such as a woven fabric or a non-woven fabric into an electret according to the method of the present invention. As shown, a workpiece 1 is passed between a pair of electrodes 2 and 3 to which a DC voltage is applied, a heating roll 4, then electrodes 22 and 32 and a heating roll 42, then electrodes 23 and 33 and a heating roll 43. By continuously passing the electret through a device provided with the electret, the application of electric charge and the heat treatment can be repeated a predetermined number of times to obtain an electret. Here, the object to be processed is, for example, a film, a stretched film obtained by stretching a film, and further, after defibrating this stretched film, cutting it into a predetermined size,
Examples include fiber sheets obtained by opening cotton and then forming it into a sheet, and nonwoven sheets obtained by known methods such as spunbonding and melt blowing. Also,
The fiber sheet-like material may be subjected to heat embossing, needle punching, ultrasonic fusion processing, or the like.

[0018]

[Examples] Examples and comparative examples of the present invention are listed below.
The present invention will be specifically explained.

(Example 1) Polypropylene (manufactured by Mitsui Petrochemical Co., Ltd., B-200)
A charge was applied to the film (thickness: 30 μm) using a pair of electrodes to which a DC voltage of 10 kV was applied at room temperature, and then heated in a heating oven at 60° C. with a humidity of 80% RH. H. So 48
Heat treated for hours. Subsequently, the same electric charge application and heat treatment were repeated to produce an electret film. At this time, the surface charge density (σ48) of the film after each heat treatment was measured according to the method below, and the ratio (σ48/σ0× 100 (%)) was determined as the retention rate of the surface charge density of the obtained electret film, and is shown in FIG. 3 with the retention rate taken as the vertical axis and the number of repetitions of the process taken as the horizontal axis.

Measurement of surface charge density Using a surface charge density measuring device consisting of the circuit shown in FIG. 4, a sample is set on a grounded metal flat plate, and an electrode of the device having a certain area is brought into contact with the surface of the sample. Read the potential V at that time. The surface charge density σ is obtained from the following formula. σ=Q/A=CV/A (coulomb/cm2)
C: Capacity A: Electrode area Q: Amount of electric charge (coulombs)

(Example 2) Instead of the polypropylene film, a film made of a polypropylene composition (polypropylene (B-200)/maleic anhydride modified product of polypropylene (modification rate 3 wt%)/polycarbonate = 90/5/5) was used. (thickness 30μ
An electret film was produced in the same manner as in Example 1, except that m) was used, and the retention rate of its surface charge density was measured. Figure 3 shows the change with respect to the number of repetitions of the process.

[0022]

According to the method of the present invention, electrets with excellent surface charge stability can be obtained with high productivity.

[Brief explanation of the drawing]

FIG. 1 is a flowchart illustrating essential steps of the electret manufacturing method of the present invention.

FIG. 2 is a diagram illustrating an example of steps for continuously implementing the method of the present invention.

FIG. 3 is a diagram showing the measurement results of surface charge density retention in Examples 1 and 2.

FIG. 4 is a circuit diagram illustrating an apparatus used for measuring surface charge density.

[Explanation of symbols]

1 Processed object, 2, 3, 22, 32, 23, 33 Electrode, 4, 4
2,43 Heating roll

Claims (4)

[Claims] 1. A method for producing an electret, comprising the steps of alternately repeating a process of applying a charge to a workpiece and a process of heating it two or more times in succession. 2. A method for producing an electret film, which comprises subjecting a stretched or unstretched film to the above process. 3. A method for producing an electret fiber sheet, in which a nonwoven fabric or a woven fabric is subjected to the above process. 4. A method for producing an electret nonwoven fabric using a fibrous material obtained by subjecting a stretched or unstretched film to the above-mentioned process and defibrating it.