DE2845255A1 - METHOD OF POLARIZING THERMOPLASTIC RESIN FILMS - Google Patents

Description

KUREHA KAGAKU KOGYO KABUSHIKI KAISHA, No. 1-8, Horidomecho, Nihonbashi, Chuo-ku, Tokyo, Japan

Method of polarizing thermoplastic resin films

The invention relates to a method of polarizing a thermoplastic resin film between a pair of electrodes. The invention therefore relates to a method for polarizing high molecular weight films in which there is a loss of polarization is reduced under high voltage electrical fields.

So far, electrets, piezoelectric and pyroelectric elements are manufactured according to a method at which the high molecular film is placed between two electrode surfaces and after applying a high voltage to the electrode surfaces is polarized by a high voltage electric field. In this procedure, the degree of polar

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sation with the electric field strength or the voltage. The resulting electrets, piezoelectric elements or pyroelectric elements show excellent behavior. Accordingly, it is desirable to be so high to apply electric fields to the film as it is due the dielectric strength of the film is possible at its polarization temperature.

If a high molecular film contains impurities or has places where it is thinner, dielectric strength is low such a thin spot. In this case, the film can be made locally by applying the high voltage even if other areas of the film have sufficient dielectric strength. As a consequence of this The charges stored between the electrodes are released through this damaged part of the film between the electrodes unload. If the high voltage necessary for polarization has been caused by this damaged part for some time is discharged, a large amount of heat is released, and as a result, the film becomes around the damaged area melt and damage the corresponding areas of the electrodes.

It is now assumed that thin metallic film electrodes on both surfaces of the polarized Film can be applied by vacuum evaporation, plating or pressing and a defective part of the to be polarized The film is further damaged locally by the high voltage to be applied during polarization. First will be the electrodes short-circuited through this damaged area. As a result, the charges flow were stored between the electrodes, and the charges from the power source passed through the damaged part of the Films. This creates heat that melts the film and creates a large hole.

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like the evaporation of the corresponding parts of the electrodes causes. Evaporation of the appropriate part of the electrodes will remove the short circuit, i.e. the electrodes are self-healing, and accordingly the polarization can still be carried out, none the less the part of the film that has been destroyed or corresponds to the damaged parts of the electrodes can no longer be used will.

The invention is accordingly based on the object of creating a polarization method for high molecular weight films, in which the extent of damage caused by discharges during polarization is reduced.

According to the invention this object is achieved in that damage of the electrodes applied to the film surface can be avoided. In addition, high molecular weight Films can be polarized more economically than previously possible, while at the same time increasing the production yield of films with minimal surface changes due to discharges.

Even if the film has areas of low dielectric strength, the damage will be caused by the application of the high voltage limited to the occurrence of a pin hole, but no damage to the film or electrodes can result.

In the method according to the invention for polarizing high molecular weight Films are a layer of dielectric or semiconducting material whose electrical conductivity is greater than that of the film to be polarized, between at least one of the two electrodes un, d the one to be polarized Film laid.

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According to the invention, at least one surface of the film is in contact during the polarization of the high molecular weight film with a layer of dielectric or semiconductor that extends between one of the two electrodes and the film to be polarized, and the difference between the potential of this contact surface and the The potential of the other electrode is applied to the film to be polarized. Since the electrical capacity of the Layer of dielectric or semiconducting material is very small and also the electrical conductivity is low is, even if the film to be polarized has a defect with low electrical resistance, the current flowing through the defective part is very small. In addition, it should be pointed out that even then if a pin hole has formed, it does not enlarge significantly, and the electrode on the opposite one Surface of the field rests, therefore is not damaged.

In summary, the invention is a method for polarizing thermoplastic resin films between a pair of electrodes in which a layer of dielectric or semiconductor with a higher electrical conductivity than that of the thermoplastic resin film to be polarized between at least one of the two electrodes and the thermoplastic resin film is laid.

Further features and advantages of the invention emerge from the claims and from the following description in of the exemplary embodiments are explained in detail. It shows:

Fig. 1 is a schematic side view illustrating the polarization method of the present invention.

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As shown in Figure 1, a film 1, the Example made of polyvinylidene fluoride can be polarized. A film electrode 2 is formed by applying of gold, silver, aluminum, nickel or tin on a surface of the film 1 corresponding to that of this Purpose suitable method such as vacuum evaporation, plating or pressing on, formed. A film 3 is made of a material having higher electrical conductivity than a film manufactured, such as polyvinylidene fluoride, which is a minor Part of an ionic component was added. Film becomes on the surface opposite to the electrode 2 of the film 1 applied. (To give you a better idea: Film 3 is separated from Film 1 in Figure 1, none the less is practically the former in close contact with the latter.) Similar to the case of the film electrode 2, becomes an electrode 2 'is formed on the outer surface of the film 2.

If a polarization is caused by applying a voltage V between the electrodes 2 and 2 ', the electrical Fields E. and E ", which are attached to films 1 and 3, can be expressed by the following equations:

E ₁ - ir ⁶² . > ν

¹ O1d2 + O2d1

^E 2 ^{= {} ~ Z < ~ < ^{) V}

^z Oid2 + Ö2d1

01 and (32 are the electrical conductivities of films 1 and 3, _f and d1 and d2 are the thicknesses of films 1 and 3.

If CJ1 is less than (f2 , then E _{1 is} greater than E, that is, the electric field applied to polarize film 1 is greater than the electric field applied to film 3. If the films be chosen so that Cf 2 is greater than σΊ, E ₁ can be made sufficiently small

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become even if E "increases. Therefore, there will be no voltage breakdown of film 3 occur.

If film 1 has a local defect in which the breakdown voltage is lower than that of the surrounding film surface, the electrical conductivity of the defect is greater than that of the rest of the film. This will allow a large current to flow, and the breakdown at this point will be increased. However, since the film 3 is made of a dielectric or insulating material and the electrical conductivity is very small compared to that of the metal electrode, the current that is brought through film 3 to the surface of the film 3 is small enough to cause the breakdown in to prevent this area. Even if a breakdown is caused, the amount of current flowing through film 3 is small compared to where it is in direct contact with the electrode, and the energy level is low. Therefore, the resin around the areas exposed to the voltage breakdown will never melt. Defects larger than "pin holes" will never arise, the electrodes will never evaporate. Accordingly, it is not desirable that the value οΊ be very large. The upper limit of the value Cf 2 should be approximately 10 GIcm), which is relatively close to the electrical conductivity of a dielectric in semiconductors. It is better to use a dielectric with an electrical conductivity lower than 10 dflcm). The lower limit of the electrical conductivity 02 is higher than the electrical conductivity C Ί. In accordance with the local fluctuations in the electrical conductivity of the film, it is recommended that the lower limit of the electrical conductivity Cf2 is more than 1.5, better still three times, greater than the electrical conductivity Cf1.

Let us now assume the case that the voltage V between the

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Electrodes 2 and 2 'is applied and the film to be polarized 1 has a low resistance defect through which large currents flow compared to the currents in other areas of the film. In this case, the charges that flow in the space between the two films above the defect 4 (later referred to as "the gap 4 '") to the electrode 2. That is, the number of charges in the gap 4' decreases Dielectric or a semiconductor, the electrical conductivity of the film 3 is small. Accordingly, the amount of current ^{flowing from the electrode 2 1} through the film 3 into the space 4 * is small. Although charges have been stored in the remaining area of the gap between films 1 and 3, the amount of charge ^{flowing from there to gap 4 1} is extremely small. Therefore, the charge deficit is not filled up immediately when the amount of charge in the gap 4 'has been decreased, and accordingly no significant current flows through the defect 4. Even if the defect 4 has been damaged by discharge into the gap 4 ¹ , and a "pin hole" has been formed, that is

The size of the hole is very small because the amount of charge that is discharged is extremely small. Later it is possible to apply a higher voltage as the film moves through the Discharges will heal themselves. As described above, in the polarization technique of the present invention, hardly "pin holes" are formed, and even if such a hole is formed, its size remains very small. Hence it is not It is necessary to remove areas with "pin holes" from the polarized film, i.e. the polarized film can without further treatment can be used for almost any purpose. The polarization yield, which in the inventive method

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driving is achieved is practically 100%.

This invention is useful for making an electret with predetermined surface conduction polarization or for the production of piezoelectric elements that have a piezoelectric or pyroelectric effect have due to an internal polarization, applicable. It is especially useful for making piezoelectric Elements or pyroelectric elements that require internal polarization of high accuracy.

The film to be polarized in the present invention may be a high molecular one Be a film with high polarization, such as polyvinylidene fluoride or a polyvinyl fluoride in case that one wants to achieve piezoelectric or pyroelectric characteristics. If you have an electret with a high surface charge a high molecular weight film with high electrical resistance such as polyethylene, Polypropylene, polystyrene, polytetrafluoroethylene or polycarbonate be used. The dielectric or semiconducting film between the film to be polarized and the electrode can be a dielectric or a semiconductor that has a melting point higher than the polarization temperature and has an electrical conductivity higher than that of the film to be polarized at the polarization temperature is. In the event that the film to be polarized consists of polyvinylidene fluoride, the between the film to be polarized and the electrode, film 3 made of resins of relatively high electrical conductivity exist such as polyvinyl fluoride polyamide based resins, ionic ethylene based resins, plastic polyvinyl chloride and plastic copolymer of vinylidene chloride and vinyl chloride. Furthermore, a material can be used this by mixing an ionic material such as salt, potassium chloride, potassium bromide and potassium sulfate with a resin

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such as polyvinylidene fluoride, polyethylene or polypropylene comes about whose electrical conductivity is equal to or less than that of the film to be polarized.

In FIG. 1, the polarizing electrodes are assumed to lie on the film to be polarized and the dielectric one Film is placed on top of the film to be polarized, nonetheless the invention does not preclude them Limited assumptions, i.e. the electrode surfaces above and below the film can of course also be used as polarizing Electrodes are used.

example 1

A monoaxially stretched polyvinylidene fluoride film of 25 yu

2
Thickness and 10,000 cm area stretched about 5 times were used. An aluminum layer was formed on one surface of the film by vacuum evaporation. Similarly, a polyvinylidene fluoride

2
film 25 .u thick and 10,000 cm in area was used, and an aluminum layer was formed on one surface of this film by vacuum evaporation. The two films were placed one on top of the other in such a way that the aluminum layers came to lie on the outside of the films placed one on top of the other. For polarization at 115 ° C. for 20 minutes, 2500 volts direct voltage were applied between the two aluminum layers or electrodes in such a way that the electrode lying directly on the vinylidene fluoride film was negatively charged. After cooling to room temperature, the DC voltage was switched off.

The polyvinylidene fluoride film thus polarized was examined. No defects were found in the film. More precisely, none of the aluminum layers was peeled off or pulled down and there were no holes in that

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Film formed. An aluminum layer was formed by vacuum evaporation on the surface of the film which had not yet been subjected to the vacuum evaporation. The piezoelectric constant of the film was determined to be d _{3 1} = 8.18 χ 10 (cgs-esu). The electrical conductivity (Xlcm) of the polyvinylidene fluoride used here was 3.3 1O ~ ¹⁶ at 25 ° C and 2.1 χ 10 ~ ¹² at 100 ° C. The electrical conductivity of the polyvinylfluoride used here was 5.5 χ 10 ~ ¹⁴ (25 ° C) and 1.2 χ 1θ " ¹⁰ (100 ° C).

Example 2

A polyvinylidene fluoride film monoaxially stretched to about 5.3 times the length and having a thickness of 15 µm and an area of 10,000 cm was applied to a film of 25 µm . Thickness and

ο
10,000 cm area, which consisted of polyvinylidene fluoride mixed with 20 ppm potassium iodide, applied. A pair of electrodes were brought into close contact with both surfaces of the thus layered film. Polarization was achieved by applying a direct voltage of 1400 volts at 115 C for five minutes to the electrodes, the electrode lying directly on the film containing potassium iodide being negative. After the film assembly had cooled, the electrodes were removed from the film layers.

The polarized film showed no "pin holes"

which could have been traced back to dielectric breakdown. After aluminum was vapor-deposited on both surfaces of the polyvinylidene fluoride film without potassium iodide, the piezoelectric constant of the film was determined to be d, ₁ = 8.3 χ 10 (cgs-esu).

The electrical conductivity (vQ, cm) of the polyvinylidene

fluoride film without potassium iodide was 3.4 10 ~ ¹⁶ (25 ° C) and 2.1 χ 10 (100 ° C), which corresponds to within the to-

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The tolerance limits of the values found in Example 1. The electrical conductivity of the polyvinylidene fluoride film with potassium iodide was 5.5 10 (25 ° C) and 4.5 χ 10 ¹⁰ (100 ° C).

Comparative example 1

Aluminum layers were applied to both surfaces of a polyvinylidene fluoride film by vacuum evaporation method

2
25µ thick and 10,000 cm in area formed as in Example 1, but no polarization was applied. 1700 volts DC voltage was applied to the aluminum electrodes at 115 ° C. for 20 minutes, then the film was cooled. The polyvinylidene fluoride film thus treated had 25 locations where voltage breakdown had occurred, and its piezoelectric constant was d., . = 6.9 χ 10 (cgs-esu).

Comparative example 2

Both surfaces of a polyvinylidene fluoride film with

A 10,000 cm surface as in Example 2, but without prior polarization, was provided with aluminum layers by means of vacuum vapor deposition. A DC voltage of 1100 volts was applied to the aluminum electrodes at 115 ° C. for five minutes, after which the film was cooled. The film treated in this way exhibited voltage breakdowns at 65 points. The piezoelectric constant was d_ ₁ = 7.2x10 (cgs-esu).

Comparative example 3

The same polyvinylidene fluoride film as in Example 2 with

2
10 000 cm surface, but which does not cause polarization

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was thrown and a polyethylene terephthalate film with 25 / ι

Thickness and 10 000 cm area with an inherent volume resistance of 1.45 χ 10 were used. These at the Firne were, as shown in Figure 1, arranged around a composite film.

Electrodes were brought into very close contact with both surfaces of the expanded film, and a DC voltage of 1400 volts was applied to the electrodes at 115 ° C for five minutes. The film assembly was then cooled. No voltage breakdown could be found in the polyvinylidene fluoride film thus treated. After aluminum layers were formed on both sides of this film by vacuum evaporation, the piezoelectric constant of this film was determined. The measurement result was very low, namely d., . = 0.5 χ 10 (cgs-esu). The changes in the process steps described here can be made without departing from the scope of this invention.

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Claims (6)

BOEHMERT & BOEHMERT KX 899 Expectations A method of polarizing a thermoplastic resin film between a pair of electrodes, characterized in that that a layer of a dielectric or semiconducting material with higher electrical conductivity than that of the resin to be polarized is arranged so as to be sandwiched between at least one of the described Electrodes and the thermoplastic resin film to be polarized, and a high voltage is applied to the electrodes is created. 2. The method according to claim 1, characterized in that the electrical conductivity of the layer of dielectric or semiconducting material, located between one of the electrodes and the thermoplastic resin film, small —4 r ^ - 1
ner than 10 (Sicm) and more than twice as large as that of the film to be polarized. 3. The method according to claim 1 or 2, characterized in that the electrical conductivity of the layer of dielectric or semiconducting material located between one of the electrodes and the resin film to be polarized, - 7 - 1
less than 10 (, T & cm) and more than 3 times as large as the electrical conductivity of the film to be polarized. - 13 - 909817/0826 ORIGINAL INSPECTED BOEHMERT & BOEHMERT I. 284S255 4. The method according to any one of the preceding claims, characterized in that the film to be polarized is made of polyvinylidene fluoride and, after it has been subjected to polarization, piezoelectric and
shows pyroelectric properties. 5. The method according to any one of the preceding claims, characterized characterized in that in an additional step electrodes on both the thermoplastic film and on the dielectric or semiconducting layer. 6. The method according to claim 4 or 5, characterized in that that the dielectric layer comprises polyvinylidene fluoride with an ionic component mixed therewith. 909817/0826