JPS5821413B2 - electret film - Google Patents

Description

Detailed Description of the Invention The present invention has a high charge density, excellent charge retention,
Conventionally, non-porous polymer films related to electret films with high mechanical strength were slowly cooled while applying a DC high voltage near their softening temperature, and then the voltage was removed, or they were exposed to corona discharge, β-rays, Electret films are known that are made into electrets by irradiating them with electron beams, applying a static magnetic field in a molten state, or mechanically deforming them.

When these electret films are used in audio equipment such as microphones and headphones, they are required to have stable charge retention characteristics and, at the same time, are required to have high mechanical strength, particularly Young's modulus.

Up until now, fluororesin-based materials have mainly been put into practical use based on the charge retention properties of the former.As for the latter, fluororesin-based materials belong to the small class of polymers and have not been fully satisfied. It is not something that will be done.

Furthermore, there are almost no examples of electrets made of porous polymer films.

This is probably due to the idea that the withstand voltage of air is significantly lower than that of polymers, so if there are voids in the film, charges will be discharged in those areas, or if the surface area is increased by making it porous, the surface This is presumably because it was considered undesirable as an electret film because of the idea that the amount of creeping along the surface increases.

The present invention breaks such conventional common sense5.
The present invention proposes an electret film that is porous but has excellent charge stability and high mechanical strength.In other words, the present invention proposes an electret film that has an in-plane orientation structure and is flat in a direction perpendicular to the film thickness direction. The present invention relates to an electret film obtained by converting a biaxially stretched polymer film having a large number of microscopic voids into an electret.

The in-plane orientation structure of a film is a state in which the polymers constituting the film are oriented parallel to the film surface, and is usually obtained by a stretching operation.

The porous film used in the present invention can be obtained, for example, by the following method.

(1) A polymer sheet with a fine phase-separated structure is deformed at a temperature far lower than its melting temperature to make it cloudy, and then heated to a temperature at which it can be easily stretched under tension. A method of biaxial stretching.

(2) A method of biaxially stretching a finely foamed polymer sheet.

(3) A method in which a polymer sheet having a fine phase-separated structure is biaxially stretched and then only one of the phases is selectively eluted with a solvent.

Film constituent materials include polyethylene, trifluoropylene, polyolefin resin marked with poly-4-methylpentene-1, polystyrene, polycarbonate, polytetrafluoroethylene, a comonomer of tetrafluoroethylene and hexafluoropropylene, and polyethylene. Vinyritine fluoride, chlorinated polyether, etc., whose insulation resistance is I x 1 o
1! :Ω(7) or more can be the main component.

Various resins, inorganic powders, etc. can be added to these main components as necessary.

The film used in the present invention has flat fine voids in a direction perpendicular to its thickness direction, and the number of voids is 100/n (J) in the film surface direction and 5 in the thickness direction.
It is desirable that the porosity is 0 or more, and the porosity is preferably 2 to 80%.

The method for converting the above film into electret is as follows:
Various known methods such as those described above can be applied as they are.

Although the reason is not clear, the obtained electret film has an initial charge density that is at least equivalent to that of an electret film made of a non-porous film with the same composition.
Usually 10-30% higher.

Furthermore, the charge attenuation property is better than that of a non-porous film.

Next, examples of the present invention will be described.

Example 1 A porous film (porosity 35%, thickness 20μ) containing 20% of high-density polypropylene-based polyethylene (porosity 35%, thickness 20μ) obtained by the above manufacturing method (1) was heated at 110°C. I made an electret and heated it to 40°C195
% Curve A in the drawing shows the charge decay characteristics when left in high humidity.

Curve B in the figure is a non-porous film of the same composition (thickness 20 μm).
) was used to conduct a similar test.

Example 2 A porous polypropylene film (porosity: 20 %, thickness 12μ) and made into electret by electron beam irradiation method.
A comparison of stretched polypropylene films (non-porous, 12μ thick) made into electrets under the same conditions revealed that although the initial charge densities were almost the same,
The charge residual rate after being left at 195% humidity for 500 hours at °C is as follows:
While the former (product of the present invention) has a weight of 50, the latter has a weight of 30.
He was in charge.

Example 3 A porous film with a thickness of 20μ and a porosity of 35 obtained from a blend of 70 parts of polyvinyl fluoride IJden and 30 parts of ethylenetetrafluoroethylene copolymer by the method described in (1) above, Example 2 A film (thickness 10μ) obtained by pressing this film between 170°C heat rolls to make it transparent (non-porous)
The electretization and subsequent charge decay characteristics were measured in the same manner as described above.

The charge residual rate after 500 hours was 40 for the former (product of the present invention).
Section, the latter was Section 10.

As is clear from the above, the electret film of the present invention uses a biaxially stretched polymer film having an in-plane oriented structure and a large number of flat fine voids in a direction perpendicular to the thickness direction of the film. Electret properties such as initial charge density and charge retention properties are superior to those obtained from non-porous films, and the above-mentioned biaxially stretched high strength Since the molecular film has a high withstand voltage, it has the advantage of being able to be made into an electret using the same method as ordinary non-porous films, making it ideal as a composition for R5 acoustic devices such as electret microphones, headphones, and speakers.

[Brief explanation of the drawing]

The drawings are diagrams showing the charge retention properties of an electret film and a non-invented electret film.

Claims (1)

[Claims] 1. An electret film obtained by converting a biaxially stretched polymer film into an electret, which has an in-plane oriented structure and a large number of flat fine voids in a direction perpendicular to the film thickness direction.