Technical Field

The present invention is directed to a method and apparatus for the production of an electret material.

Background Art

Materials known as "electret" are those pre¬ pared by maintaining a film, sheet, or other shaped article of a plastic composed of a polymeric or organic material such as carnauba wax or naphthalene; a polar resin such as polyfluorovinylidene rosin, a polyvinyl chloride rosin, polycarbonate, polyester, and acrylic rosin, etc.; a non-polar rosin such as polyethylene, polypropylene, polystyrene, etc. or copolymers of them; or a mixture of these materials at a suitable temperature for a period of time under the application of a D.C. potential and cooling the article to room temperature while maintaining the D.C. potential, resulting in the polarization of the material. Because such an electret can maintain its polarized state for a long period of time as well as have excellent workability, toughness and flexibility, the electret is widely used in trans¬ formers such as speakers and microphones and also in other specific fields. A method of producing electrets is disclosed in patent document U.S. -A- 3,885,301. According to this method an electret can be produced by placing a pair of electrodes on the opposite surfaces of a polymer film and applying an electric potential to the electrodes while maintaining portions of the film at an elevated temperature. To polarize different areas of the film, either the electrodes are moved along the opposite sur¬ faces of the film, or the polymer film is moved between the pair of electrodes. The method for the production of electrets disclosed in the above reference requires a rather high voltage source for generating an ectric

field which is applied across the shortest dimension of the film to produce the polarized state of the electret, which requirement increases the cost of the electret.. A further drawback of this method is that it is performed at a relatively low speed.

Disclosure of the Invention

It is, therefore, an object of the invention to manufacture electrets at a relatively low cost, using relatively low voltages and at a relatively high rate of speed.

Thus, according to the invention, there is provided a method of polarizing a dielectric material to form an electret which includes the steps of bringing about a relative movement between the dielectric mater- ial and a pair of electrodes positioned in operative relationship therewith to subject successive portions of the dielectric material to an electric field, and apply¬ ing polarization - enabling energy to the portion of the dielectric material subjected to the electric field, characterized in that said pair of electrodes are ar¬ ranged to be positioned adjacent one surface of said dielectric material-

According to another aspect of the invention, there is provided an apparatus for producing an electret including a pair of electrode numbers connected to a voltage source and positioned, in operation, adjacent a dielectric material to apply an electrical field to an area thereof, and means for applying polarization - enabling energy to said area of the dielectric material, enabling the dielectric material to exhibit a residual polarization, characterized in that said pair of elec¬ trode members are positioned, in operation, adjacent one surface of said dielectric material.

In the preferred embodiment of the invention, two electrical conductors are positioned adjacent one surface of a sheet of polymer-type dielectric material,

which has been heated to a temperature higher than the last transition point of the material and lower than the melting point of the material by a lamp member producing infrared energy focused on the polymer material . The electrical conductors move on a movable support member which also supports the lamp member therebetween. A lens member positioned between the lamp member and the dielectric material focuses the rays of the lamp member onto the material between the electrical conductors. Movement of the support member adjacent the surface of the polymer material supplies a polarizing voltage across a long dimension of the material to produce an electret whose polarization is perpendicular to the polarization of the electret material produced by the mentioned prior art method.

Brief Description of the Drawing

One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a side view of an electret of the prior art showing the direction of the electrical field applied during the polarization of the material.

Fig. 2 is an oblique view of the electret material of Fig. 1. ; Fig. 3 is a side view of an electret material of the present invention showing the direction of the electrical field applied during the polarization of the material;

Fig. 4 is an oblique view of the electret material of Fig. 3;

Fig. 5 is a schematic representation of the top view of a carriage assembly in which is mounted electrical conductors, an infrared lamp and a focus¬ ing lens used in producing the electret; • • Fig. 6 is a schematic representation of the side view of the carriage assembly.

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Best Mode for Carrying Out the Invention

Referring now to Figs. 1 and 2, there is shown views of a prior art electret-forming material 20 in which the electrical field is applied during the polar- ization of the material in a direction through the shortest dimension of the material or perpendicular to the longitudinal axis of the material. This arrangement requires the employment of a high voltage electrical field as indicated by the arrows in Figs. 1 and 2. The present invention directs the electrical field in a direction along the longest dimension of the material as indicated by the arrows shown in Figs. 3 and 4. I -has been found that in moving a pair of electrical conduc¬ tors or electrodes along the surface of the material 20 in a direction parallel to the longitudinal axis of the material, polarization of the material will occur using a voltage which is directly proportional to the distance between the conductors and the material but independent of the thickness of the material. This polarization will be. parallel to the surface of the material, and will extend into the material to a depth which can be controlled by the distance between the surface of the material and the electrical conductors .

Referring now to Figs. 5 and 6, there is shown a schematic representation of the top and front view of an apparatus for polarizing a dielectric material to produce an electret. The apparatus comprises a carriage assembly generally indicated by the numeral 24 and which includes a pair of oppositely positioned support members 26 between which is mounted a source of radiant energy such as a Quartz infrared lamp 28, a focusing lens 30 and a pair of electrical conductors 32 connected to a suitable power supply 34. The assembly 24 is driven along the edge of the material 20. The lamp 28 provides a line source of radiant energy which is focused by the lens 30 to a position between the conductors 32 for heating the material 20 as the carriage assembly 24 moves past the surface of such material.

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The power required to polarize the material 20 varies with the distance between the conductors 32 and the type of material to be polarized. In the present example, a voltage of 320K volts per 2.54 centimeters of distance between the conductors and the material is required. The distance between the lamp 28 and the surface of the material 20 is not critical since the focal length of the lens 30 can be selected with respect to the location of the lamp 28 to focus the radiant energy generated by the lamp 28 at the surface of the material 20. In order to avoid arcing between the conductors 32, the conductors may be coated with an insulating material such as Mylar, a trademark of the DuPont Corporation of Wilmington, Delaware. The con- ductors 32 may engage the surface of the material during the movement of the carriage assembly 24 without detrac¬ ting from the polarization of the material . The speed of the carriage assembly 24 is selected to allow the area of the material heated by the radiant energy of the lamp 28 to become cool while still within the electrical polarizing field of the conductors 32. In the present example, if the thickness of the dielectric material is 0.125 millimeters and the temperature ranges between a high of 120°C and a low of 80°C, a maximum speed of 3.8 meters per second can be attained where the conductors 32 are spaced 2.54 millimeters apart and are located 0.254 millimeter from the surface of the material 20. As the carriage assembly 24 moves along the surface of the polymer material 20, the conductors 32 will generate an electrical field in a direction par¬ allel to the longitudinal axis of the sheet of material 20, as shown in Figs. 3 and 4. The lens 30 will focus the radiant energy generated by the lamp 28 along a line on the surface of the material 20 located between the conductors 32 and heat the material to a level enabling the electrical field produced by the conductors 32 to polarize the material. The movement of the carriage

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asserαbly 24 allows the heated material to cool by the time it reaches the downstream conductor 32, thereby enabling the material to maintain its polarized state.

It will be obvious to those skilled in the art that many modifications of structure, arrangements, elements and components can be made to the illustrated embodiment which are particularly adapted for specific environments. Thus, for example, the carriage assembly 24 can be stationary, while the sheet of dielectric material 20 moves past the assembly 24 to have the material polarized, thereby producing an electret.