JPS6142879B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a polyvinylidene fluoride resin stretched film, and more specifically, a method for producing a polyvinylidene fluoride resin stretched film that exhibits excellent piezoelectric properties when subjected to polarization treatment, and the polyvinylidene fluoride resin. This invention relates to the use of stretched films in electrets (piezoelectric materials).

Polyvinylidene fluoride (hereinafter abbreviated as PVDF) resin stretched film has excellent orientation polarization and piezoelectric properties due to its special crystal structure, and is useful for electro-acoustic transducers such as electrets (microphones, speakers, pickups, etc.) It is also known that it can be used as an electronic industrial material such as a capacitor.

Conventionally, as a method for manufacturing a stretched PVDF resin film having such properties, for example, a film of PVDF resin is formed at a temperature above the crystal melting point, and immediately quenched to a temperature below the crystal dispersion temperature to form a film with undeveloped crystals as much as possible. A method has been proposed in which the film is made into a stretched film and then stretched, but in this method, rapid cooling causes a rapid temperature change inside the film during film formation, which causes large distortions in the film structure. In the stretching process, there are drawbacks such as difficulty in stably stretching or inability to increase the stretching ratio, so even if the film thus obtained is subjected to electrification (polarization) treatment under a DC electric field, Its piezoelectric coefficient
g ₃₁ was approximately 1×10 ⁻⁷ to 3×10 ⁻⁷ cgsesu, which had the disadvantage that it was still insufficient for use in the above-mentioned applications.

The present inventors have conducted extensive research with the aim of providing a PVDF resin stretched film that does not have such drawbacks and exhibits excellent orientation polarization and piezoelectric properties. When forming into a film by extrusion method, the formed film is slowly cooled without quenching, and the PVDF resin spherulites are sufficiently developed inside the film, but the unstretched film has almost no distortion in its structure. Once manufactured, heat treatment and quenching treatment are performed to obtain a film with excellent stretchability at low temperatures, and when the thus obtained stretched film is subjected to polarization treatment under a direct current electric field, it can be compared to conventional PVDF-based films. It was discovered that the resin electret exhibits an extremely high piezoelectric constant that is completely unexpected.

Conventionally, when producing stretched stretched films from crystalline polymers such as PVDF resins, the crystalline polymer is formed into a film at a temperature above the crystal melting point, and then immediately heated to a temperature below the crystal dispersion temperature. It is necessary to suppress the formation of crystals by rapidly cooling the film to a temperature of
It is common knowledge in the technical field that subsequent processing, especially stretching, is difficult and that it is extremely difficult to obtain a film with sufficient orientation, and that the development of such crystals should be avoided. Therefore, it must be said that the above-mentioned findings in the present invention are completely unexpected.

Thus, according to the invention, having a spherulitic structure
A method for producing a stretched polyvinylidene fluoride resin film is provided, which comprises heating a PVDF resin film to a temperature near the crystal melting point, then rapidly cooling it to a temperature below the crystal dispersion temperature, and then stretching it.

The essential feature of the method of the present invention is that a PVDF resin film having a spherulite structure, which was conventionally considered difficult to process by stretching, is used as a starting material, and the film is subjected to stretching. It is.

The spherulites present in the PVDF resin film used as the starting material can be developed to an average diameter of at least 5 microns, and generally
It can have an average diameter ranging from 10 to 200 microns. The amount of spherulites at this time is not critical, and the method of the present invention can also be applied when the developed spherulites grow only sparsely in the film and are scattered over less than 1/3 of the surface area. However, in such cases, it is difficult to expect sufficient effects from applying the method of the present invention.

Examples of PVDF resins that can be used include PVDF homopolymers; vinylidene fluoride and other monomers that can be copolymerized with vinylidene fluoride, such as acrylic esters, methacrylic esters, vinyl fluoride, trifluorochloroethylene, tetrafluoroethylene, etc. (in which case the vinylidene fluoride units should represent at least 50% by weight of the copolymer, preferably 70% or more); or copolymers of PVDF with other polymers such as polyacrylic esters, polymethacrylic esters, polyfluorinated fluoride units, etc. Examples include blends with vinyl chloride, polytrifluorochloroethylene, polytetrafluoroethylene, etc. (in which case the PVDF accounts for at least 50% by weight of the blend, preferably 70% or more).

Molding of such a PVDF resin into a film can be carried out by a conventional method, such as a solution casting method or a melt extrusion method, with the former method being particularly advantageous.

Forming a film using the solution casting method of PVDF resin is a method known per se, such as Japanese Patent Publication No. 38-4176.
It can be carried out according to the method described in the publication. Examples of solvents that can be used in this case include dimethylacetamide, diethylacetamide, dimethylformamide, diethylformamide, tetramethylurea, tetraethylurea, dimethyl sulfoxide, triethyl phosphate, propylene carbonate, dimethyl succinate, diethyl oxanate, and diethyl succinate. Suitable are ester, dimethyl phthalate, diethyl phthalate, diethyl adipate, acetone, methyl ethyl ketone, diacetone alcohol, cyclohexanone, isophorone, mesityl oxide, 4-methoxy-4-methylpentanone-2, ethyl amyl ketone, etc. Each can be used alone or in combination of two or more. In addition, in this case, after drying the cast film at a temperature of about 160 to 180°C, the rapid cooling step is omitted and the film is slowly cooled.
A spherulite structure can be developed within the film. The same applies to the case of melt extrusion, and by gradually cooling the extruded film without quenching it, a film with a developed spherulite structure can be obtained. The slow cooling time depends on the temperature of the film, but for example, in the case of a temperature of 150 to 160°C, it is sufficient to slow the film at a rate that maintains the film at that temperature for about 30 seconds to 1 minute. In addition, in the case of an amorphous PVDF resin film that has been formed once, it is heated to at least 100°C, preferably at a temperature in the range of 140°C to the softening temperature of the film, and then slowly cooled to form a spherulite structure. It can be changed to a PVDF resin film with Although the thickness of the PVDF resin film can vary over a wide range depending on the application, a range of 5 to 100 microns is suitable for practical use.

The thus formed PVDF resin film having a spherulite structure is difficult to stretch as described above, or even if it can be stretched, the stretching ratio cannot be increased, making it difficult to achieve the desired excellent properties. It is difficult to process it into a film that has According to the present invention, it has a spherulite structure obtained as described above.
This method is completely different from conventional methods in that, prior to the stretching process, the PVDF resin film is heated to a temperature near the crystal melting point, and then rapidly cooled to below the crystal dispersion temperature.

Here, "temperature near the crystal melting point" means a temperature in the range of several tens of degrees around the crystal melting point, and when using a temperature lower than the crystal melting point (crystal melting point minus 20 degrees Celsius). ) or higher is preferable; on the other hand, if a temperature higher than the crystal melting point is used, a temperature of (crystal melting point plus 50° C.) or lower is suitable. The heat treatment at such temperatures may be carried out either under tension or without tension, but it is desirable to apply a slight tension to avoid excessive deformation or shrinkage of the film. The heating time depends on the temperature, film thickness, etc., but is generally at least 30 seconds or longer, and there is no particular upper limit, but heating for too long will not improve the effect accordingly and is rather wasteful. Generally, 1 to 2 minutes is sufficient. Heating can be carried out by various methods known per se, such as roll heating, furnace heating (hot blast furnace, heat medium furnace, infrared furnace, etc.).

The film thus heat-treated is then rapidly cooled to a temperature below the crystal dispersion temperature, usually below 100°C, more preferably from room temperature to about 60°C. Quenching can be accomplished in a variety of ways, including air cooling, water cooling, and contact with chill rolls.

The film after quenching is further stretched. The stretching can be either uniaxial or biaxial stretching depending on the intended use of the obtained film. Stretching can be carried out by a method known per se, but in the case of uniaxial stretching, the temperature is usually below 100°C, preferably between room temperature and 60°C.
It is advantageous to stretch at a temperature of at least 1.5 times, preferably 2 to 5 times. Biaxial stretching can also be carried out in two stages according to a conventional method, but it is appropriate to simultaneously stretch in two directions at a temperature of 120°C or higher by 1.5 times or more, preferably 2 to 5 times. . In this way, an oriented film of PVDF resin mainly composed of β-type crystals is obtained.

The stretched PVDF resin film produced by the method of the present invention is not only flexible and has excellent mechanical strength, but when polarized (electretized) in a DC electric field as described below, it becomes
It was discovered that the piezoelectric constant exhibited an extremely high piezoelectric constant that was completely unexpected from a system electret.

Thus, according to the present invention, there is also provided a PVDF resin electret obtained by polarizing the PVDF resin stretched film obtained as described above under a direct current electric field. The polarization treatment itself can be performed by a known method. For example, aluminum may be vapor-deposited as electrodes on both sides of the film obtained by the method of the present invention, or the film may be sandwiched between suitable electrode plates, and the film may be heated at 300 to 3000 KV at 60 to 150°C.
After applying a DC voltage of cm, it can be made into an electret by allowing it to cool to room temperature.

The thus obtained electret of the present invention is
While all conventional PVDF electrets have a piezoelectric constant g ₃₁ of the order of 10 ^-7 cgsesu, this is 10 times higher.
It has a piezoelectric constant on the order of 10 ^-6 CGSESU, and is used as a high-performance electro-acoustic transducer such as microphones, speakers, pickups, and headphone, or as an ultra-compact and high-performance transducer because it has a very high dielectric constant. It can be widely used as an electronic material such as performance capacitors.

Next, the present invention will be further explained by examples.

Example 1 100g of PVDF powder was added to dimethylacetamide.
The solution was poured onto a chrome plate, dried at 160°C to remove the solvent, and left to cool to room temperature. A stretched film was obtained. This film was heat treated at 180°C for 2 minutes, then rapidly cooled to 30°C by passing it through water at 30°C, and then heated to 50°C.
It was stretched 4 times in the uniaxial direction at °C. The film thus obtained had a thickness of 40 μm and was flexible and strong.

Aluminum was vapor-deposited on both sides of this stretched film by a conventional method, a DC voltage of 1000 KV/cm was applied at 90° C., and then the film was polarized by being left to cool to room temperature. The piezoelectric constant _g31 of this PVDF film was measured and found to be 2×10 ^-6 cgsesu.

Comparative Example 1 The same dimethylacetamide solution of PVDF used in Example 1 was cast onto a chrome plate in the same manner as in Example 1, dried at 160°C to remove the solvent, and then immediately poured into water at 30°C. and rapidly cooled to 30°C.
The obtained film was uniaxially stretched 4 times at 50° C., and then polarized in the same manner as in Example 1 above. The piezoelectric constant _g31 of the obtained film is 2×
It was 10 ^-7 cgsesu.

Example 2 PVDF/polymethyl methacrylate = 70/30
(weight) of the blend was melt-extruded from an extruder at 255°C, slowly cooled to 100°C,
An unstretched film in which spherulites with an average diameter of 20 μm were sufficiently developed was obtained. After this film was heat treated at 220°C for 1 minute, it was immediately cooled to about 10°C by passing it through water at 0°C, and then stretched 3 times in the uniaxial direction at 50°C. The obtained film had a thickness of 10 μm and was flexible and strong.

When this stretched film was polarized in the same manner as described in Example 1, its piezoelectric constant g ₃₁
showed a high value of 1.0×10 ^-6 cgsesu.

Comparative example 2 PVDF powder was heated to 260 mL using an extruder.
The film obtained by melt extrusion at 0°C and then immediately passed through water at 0°C and quenched to about 20°C is
There was almost no development of spherulites, and even if they existed, they were so minute that the presence of spherulites could not be clearly confirmed. This film was stretched 3 times in the uniaxial direction at 50°C and then polarized in the same manner as described in Example 1. The piezoelectric constant g ₃₁ of the obtained electret was 1.5×10 ^-7 It was nothing more than a simple thing.

Example 3 A copolymer of PVDF/tetrafluoroethylene = 80/20 (by weight) was melt-extruded at 280°C, slowly cooled to 130°C, and an unstretched film with fully developed spheres with an average diameter of 40μ was obtained. Obtained. 200 of this film
After heat treatment at ℃ for 2 minutes, it was immediately cooled to about 30℃ by contacting with a cooling roll at 20℃, and then heated to 70℃.
It was uniaxially stretched 2.5 times. The obtained film is 80μ
It was flexible, strong, and transparent.

When this stretched film was polarized in the same manner as described in Example 1, its piezoelectric constant _g31 was 1.5.
It showed a high value of ×10 ^-6 cgsesu.

Claims (1)

[Claims] 1. A polyvinylidene fluoride resin film having spherulites with an average diameter of 10 to 200 microns is heated to a temperature near the crystal melting point, then rapidly cooled to a temperature of 60°C or less, and then stretched. A polyvinylidene fluoride resin electret obtained by polarization treatment under a DC electric field of 300 to 3000 KV/cm at 60 to 150°C.