KR800000311B1 - Method of making electret - Google Patents

Abstract

Membranes with a single charge were formed from polymer films by a corona discharge followed by heat treatment. A Teflon film was placed on a metal plate and a needle electrode was placed 15mm above the film. A potential of -6KV was applied and corona discharge was maintained for 30 sec. The film was removed from the metal plate and its charge was detd. to be -8 ×10-8C/Cm2 on the side and +7.8 × 1.0-8C/Cm2 on the opposite side. The film was heated at 155≰ for 10 min to give negative-charged polymer ! membrance.

Description

Preparation of negatively charged polymer membrane

1 is a process chart showing an example of the present invention.

2 is a process chart showing another example of the present invention

3 and 4 are respectively a principle diagram of the head zone for explaining the present invention

5 is a characteristic diagram showing the change over time of the surface charge density for explanation of the present invention

The present invention relates to a method for producing a single-signal charge, that is, a negatively charged polymer film, a so-called electret film.

Currently, electrets have been put to practical use in electrostatic transducers such as microphones. Usually, when electrets are sandwiched between two electrodes, positive and negative charges are applied to both surfaces of the dielectric, respectively. In this way, when the electrets having positive and negative charges on both sides are applied to the electrostatic acoustic transducer, a metal electrode layer is disposed on one side of the electret (dielectric), and the charge on the other side is used. I'm doing it. That is, for example, when the electret is used for the headphones, conventionally, two electrets 1 and 2 arranged inwardly inward as shown in the principle diagram of FIG. Is deposited on a fixed electrode having a negative electrode, i.e., the electrodes 4 and 5, and a conductive vibrating membrane 6 is disposed in the center between the electrets 1 and 2. The diaphragm was configured to be driven by the pressure signal from (t). As such a structure, two electrets 1 and 2 are used for one converter, and dielectric films 1 and 2 which are electrets to the double electrodes 4 and 5, respectively, at the time of manufacture. ) And the electret treatment may be difficult depending on the material of the double electrode.

That is, there are problems such as deformation of the cathode due to heating during the electret treatment and deterioration of the adhesive for bonding the dielectric film to the cathode. In addition, even in the case where electret treatment is performed by charging the surface of the dielectric film by corona discharge, if a large number of pores exist in the dielectric film, the surface of the dielectric film may not be evenly charged.

In order to avoid such a problem, for example, first, the dielectric film is electretized and the electretized dielectric film is deposited on the bipolar electrode. However, in this case, a plurality of perforations are formed in the electret film to form the bipolar electrode. The complex work of attaching and impregnating with the pore of the air is required, and there is a fear of contaminating the electret film during the work, which may cause attenuation of surface charge.

On the other hand, it is conceivable to electret the vibrating membrane as a method of eliminating such difficulties. In this case, however, in order to operate as a vibration membrane, the vibration membrane needs to carry a single sign of electric charge. For example, in the case of application to headphones, as shown in FIG. 4, an electret vibrating membrane 7 having only negative charges is disposed at the center thereof, and a plurality of vibrating membranes 7 are respectively opposed to both sides of the vibrating membrane 7. Place the polarizations 4 and 5 with the perforations 3 in place.

However, as a manufacturing method of the conventional electret, both sides thereof are charged to the government, and the electret charged to a single code is not obtained. As a method of obtaining a single coded electret film, the recent half. Degree. A method of using gravure electricity has been proposed. This is half the dielectric film. Degree. If we investigate the electron beam of high energy by graf electric machine. Immediately after the war, both sides of the membrane become negatively charged and become a electret membrane with a single sign as a whole. According to this method, a single electrified electret film is obtained. Degree. Requires a graph machine.

SUMMARY OF THE INVENTION In view of the above, it is an object of the present invention to provide a method for obtaining a stable electret with both sides being negatively charged using the same equipment as that of a conventional electret.

Next, the present invention will be described in detail with reference to the drawings.

In the present invention, for example, as shown in FIG. 1A, the dielectric film to be electret, that is, the polymer film 10, is first placed on the metal plate 11, and immersed on the upper surface 10A of the polymer film 10. The electrode 12 is disposed at a predetermined interval d only, and the metal plate 11 becomes a positive electrode through the high voltage power supply 13 between the precipitation electrode 12 and the metal plate 11, and the precipitation electrode 12 is the negative electrode. In addition, a predetermined high pressure is applied to cause a corona discharge between the precipitation electrode 12 and the polymer film 10, and the upper surface 10A of the polymer film 10 is negatively charged by the corona discharge. Next, as shown in FIG. 1B, the polymer film 10 is once peeled from the metal plate 11. At this time, discharge occurs between the peeling surface 10B of the polymer film 10 and the surface 11A of the metal plate 11, and the peeling surface 10B of the polymer film is positively charged. That is, as shown in FIG. 3, negative and positive charges are charged to both surfaces 10A and 10B of the polymer film 10, respectively.

After such extension, the polymer film 10 is placed on the metal plate 14 with its positively charged surface 10B beneath, heated in the furnace 15 for a predetermined time, and then cooled. At this time, the metal plate 14 may be grounded. (16) represents a heater. (Figure 1D).

In this way (as shown in FIG. 1E, both surfaces 10A and 10B have a negative charge electret film 17).

That is, as a result of measuring the charges on both surfaces 10A and 10B of the polymer film 10 after cooling, the surface 10B, which had a positive charge before heating, also changed to a negative charge by selecting an appropriate heating time, and failed before heating. Although the charge of the surface 10A that had been lowered was somewhat reduced, it was confirmed that the charge remained, and both surfaces 10A and 10B had negative charges. Here, the initial charge amount due to corona discharge is much larger than the value by the thermal electret, and there is a practically sufficient charge even after the decrease by heat treatment.

The electric field E produced by the unstable charge layer is expressed as E = σ / 2σ ₀ when the charge density is σ and the vacuum dielectric constant is σ ₀ . The discharge limit of the equal electric field in the air at room temperature and atmospheric pressure is 30KV / cm, so the maximum charge density that such an electric charge can have is about 5.3 × 10 ^-9 C / cm ² , and both sides of the polymer film bear government charge. The charge, which can be present more stably if present, or with the electrode contacts, is smaller.

In other words, the charge density required to generate the required electric field may be small.

2 is another example of the present invention. First, as shown in FIG. 1, the polymer film 10 to be electret is placed on the metal plate 11, and the precipitation electrode 12 is disposed to face the upper surface 10A of the polymer film 10 so as to face each other. A predetermined high pressure is applied between the 12 and the metal plate 11 so that the metal plate 1l becomes a positive electrode, and the upper surface 10A of the polymer film 10 is negatively charged by corona discharge (FIG. 2A).

Next, the polymer film 10 is not removed from the metal plate 11, but is disposed in the heating furnace 15 in a state where the polymer film 10 is placed on the metal plate 1l, and heated (annealed) to the polymer 10 for a predetermined time. To cool down. This heat treatment is performed over a longer time than in the case of FIG.

After cooling, the electric charges on both sides 10A and 10B of the polymer membrane 10 were measured, and as shown in FIG. 2C, both sides 10A and 10B were negatively charged by selecting an appropriate heating time. It was confirmed that the electret film 18 having a single sign of charge was obtained. An embodiment of the present invention is shown.

Example 1

A Teflon FEP film (ethylene tetrafluoroethylene hexafluoropropylene copolymer film) having an area of 1.5 × 1.5 cm ² and a thickness of 50 μm was placed on the metal plate 1 l as shown in FIG. ), And apply corona discharge for 30 seconds by applying a voltage of -6KV to the needle. Discharge, was a result of the film was peeled off from the metal plate 11. With this over the electrode plate of the charge measuring device measures the amount of charge needle electrode and the counter discharge surface ^-8 × 10 -8 C / cm and then stopped.

Next, the film was inverted and the amount of charge on the side opposite to the discharge surface of the film was measured. As a result, it was + 7.8 × 10 ^-8 / cm ² . After the measurement, the film is placed on a metal plate with the positively charged surface downward and heated to 155 ° C. for 10 minutes. After heating, as a result of measuring the charge amount of the film, the surface that was initially charged by krona discharge became -1.9 × 10 ^-9 C / cm ² , and the surface that was positively charged was -2.3 × 10 ^-9 C / It became cm ² , and both ends were shown to be negatively charged.

Example 2

A Teflon FEP film having a thickness of 12.7 μm is attached to a metal ring having an outer diameter of 32 mm and an inner diameter of 28 mm, and the film surface is placed on the metal plate 11, and the precipitation electrode 12 is disposed at a position of 15 mm on the film surface. Apply Corona discharge for 30 seconds. After the discharge was stopped, the film was once peeled off from the metal plate 11, and then the film was brought into contact with the other metal plate 14 with the discharge surface facing up, and heated at 160 ° C. for 20 minutes. The charge after heating was found to be -4.7 × 10 ^-9 C / cm ^{2 in the} discharge plane, -4.0 × 10 ^-9 C / cm ² in contact with the metal plate, and both sides were negatively charged. The time-dependent change of the negative charge when this film is left at room temperature is shown in FIG.

In Fig. 5, the curve II represents the charge on the discharge surface, and the curve II represents the charge on the opposite surface. As evident from these characteristics, both sides of the film were negatively charged, and it was confirmed that it was stabilized after 10 days, and it can be seen that the negatively stable without change with time.

Example 3

A Teflon FEP film having an area of 1.5 × 1.5 cm 2 and a thickness of 50 μm was placed on a metal plate, and a precipitation electrode 12 was disposed at a position of 15 mm above the film, and a corona discharge was applied for 30 seconds by applying a voltage of -6 KV to the needle. Next, the film is discharged at 150 ° C. for 1 hour and 30 minutes without being peeled off from the metal plate 11 after discharge. The electric charge on the film after heating was -1.3 × 10 ^-9 C / cm ² , and the surface in contact with the metal plate was -2. It became 4 * 10 <^-9> C / cm <^2> , and it turned out that both surfaces are negatively charged.

[Other Experimental Example (a)]

A Teflon FEP film having an area of 1.5 × 1.5 cm ² and a thickness of 50 μm is placed on the metal plate 11, the deposition plate 11 is disposed at a position of 15 mm on the film, and a voltage of -6 KV is applied to this tip to cause corona discharge. . The discharge surface of the film at this time obtains a charge amount of -5.5 × 10 ^-8 C / cm ² . Next, the film is peeled off from the metal plate 11 and placed on the metal plate again. At this time, the peeling surface of the film is positively charged. After that, a gold mesh is disposed between the needle and the metal plate, and a voltage of about -100 V for the metal plate 11 is applied to the gold plate, and a voltage of -6 KV is applied to the gold plate for corona discharge. The charge amount in this case is the discharge surface in the ^{-5 × 10 -9 C / cm 2} ~-7 × 10 -9 in C / cm ^2, but the surface contacting the metallic sheet ^{+ 1 × 10 -9 C / cm} 2 ~0 It became --1.4 * 10 <^-9> C / cm <^2> , and both surfaces may be negatively charged, but the reproducibility was bad.

[Other Experimental Example (b)]

The star sample (Teflon FEP film) charged by corona discharge in the same manner as in Example 1 has a charge amount of -8.6 × 10 ^-8 C / cm ² , and a surface in contact with a metal plate + 8.3 × 10 ^-8 C / cm ² . Next, the film was placed on the metal plate 14 with the front of the bag underneath, and the charge on both sides of the film after heat treatment at 155 ° C. for 10 minutes under the same conditions as in Example 1 was measured. It had a negative charge of 10 ⁻⁹ C / cm ² , and the surface which had been electrostatic had a static charge of + 2.1 × 10 ⁻⁹ C / cm ² , and the sign did not change.

[Other Experiment Example (c)]

In the same manner as in Example 1, a separate sample (Teflon FEP film) charged by corona discharge was peeled off from the metal plate after charging, and then the films charged on both sides positively and negatively, respectively, were heated without contacting the metal plate. After heating, the respective charges on both sides of the film were measured. As a result, one side remained electrostatically charged and the other side was negatively charged, and no single-charged film was obtained.

Also, when a 50 μm-thick Teflon FEP film is placed on a metal plate and the negative charge is charged on the surface of the film, even if it is -5.5 × 10 ^-8 C / cm ² , the stable charge is when the metal plate is peeled off from the metal plate. ^Since it becomes about ^-9 C / cm <^2> , a discharge generate | occur | produces at the peeling surface at the time of peeling, and a positive charge is charged. Therefore, in theory, if the film on the metal plate is initially charged with a charge of about -5 × 10 ^-9 C / cm ² or less, no discharge is generated at the peeling surface when the film is peeled from the metal plate. An electret film having electric charge is obtained. In practice, however, the reproducibility is poor, and in the case where the heat treatment is performed in various experiments, it is preferable in view of the reproducibility and the magnitude of the charge amount. It is better to do it.

In addition, although the Teflon FEP film was used as a polymer film in the said Example, it was confirmed that the electret film which has a single code | charge charge likewise is obtained also in the case of Teflon TFE film, Tefgel film, etc. in addition.

As described above, according to the present invention, an electret that is easily charged with a single sign, that is, negatively charged, is obtained by performing a process of charging a negative charge to the surface of the polymer film as a corona discharge and heating a polymer film after the corona discharge. There is a number. In addition, there is an advantage that the equipment used in the manufacturing method of the conventional electret can be utilized without requiring any special equipment regarding the manufacturing method of the manufacturing method.

Claims (1)

As shown in the drawings and described in the text, a negatively charged polymer membrane comprising a step of charging a negative charge on one surface of a Teflon (FEP, TFE) or a teflon gel film by corona discharge and heating the film on a metal plate. Recipe.

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