EP0136345A1

EP0136345A1 - Poly(vinylidene fluoride) film, uses thereof, and method of manufacture

Info

Publication number: EP0136345A1
Application number: EP84901251A
Authority: EP
Inventors: Michael Alan Marcus
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1983-02-24
Filing date: 1984-02-21
Publication date: 1985-04-10
Also published as: WO1984003250A1; EP0136345A4

Abstract

Nouveau film de poly(fluorure de vinylidène) et processus de fabrication. Ce film possède des constantes diélectriques supérieures qui n'étaient disponibles avant la présente invention que dans un film de poly(fluorure de vinylidène) de phase béta. Un tel film est particulièrement utile pour servir de composant diélectrique dans des condensateurs de blessures (Fig. 4). Le procédé préférentiel de la présente invention (M, 70, Y, w) comprend l'allongement du film de poly(fluorure de vinylidène), pendant qu'il est en condition fondue, à un degré permettant de réduire l'épaisseur du film à une valeur ne dépassant pas environ le 1/50 de l'épaisseur originale.New poly (vinylidene fluoride) film and manufacturing process. This film has higher dielectric constants which were only available before the present invention in a polyvinylidene fluoride beta phase film. Such a film is particularly useful for serving as a dielectric component in wound capacitors (Fig. 4). The preferred process of the present invention (M, 70, Y, w) comprises stretching the polyvinylidene fluoride film, while it is in molten condition, to a degree allowing the thickness of the film to be reduced at a value not exceeding approximately 1/50 of the original thickness.

Description

POLYVINYLIDENE FLUORIDE FILM, USES THEREOF, AND METHOD OF MANUFACTURE The inventions disclosed herein relate to polyvinylidene fluoride (PVF₂) films, their uses, and methods of manufacture. More particularly, one invention relates to a PVF« film having a unique combination of piezoelectric and electrical properties. Another related invention pertains to a method for producing such a film. A further related invention pertains to the use of such a film as the dielectric component in electrical capacitors.

Many electronic circuits, such as photoflash circuits of cameras, use small capacitors to store and deliver large amounts of charge, for example, sufficient to fire a flashlamp. To overcome certain disadvantages associated with the conventional electrolytic capacitors commonly used in such circuits, attempts have been made to produce "dry" capacitors using polymeric films, particularly PVF₂ films. To date, such efforts have ⁿot proven entirely successful.

Heretofore, only one of the several polycrystalline phases of PVF2 films has been considered useful as the dielectric component in capacitors. Specifically, only the beta-phase material has been considered useful because of its relatively high dielectric constant compared to the other polycrystalline phases. The beta-phase material is produced by first cooling the PVF^ film as it is extruded from a melt, and thereafter stretching it at temperatures significantly below its melt point, e.g. at temperatures between 60° and 1Q0°C. Such stretching of the film at a cooled temperature serves to convert the alpha-phase crystals (which is the phase of the extruded and cooled film) to beta-phase crystals. In using beta-phase PVF« films in capacitors, difficulties have been encountered in maintaining

yU E ,

OMPI 5 ipo structural integrity during use. Such difficulties are attributed to the fact that beta-phase PVF« film, unlike alpha-phase PVFn film, has a relatively high piezoelectric constant (e.g., greater than 10 meters/volt) upon being "poled"; i.e. subjected to a high electric field for a prolonged period of time (e.g. one hour) at a certain temperature, for example, room temperature. When beta-phase films are used as capacitors, such poling unavoidably takes place as they store charge. After poling occurs, the PVF„ film within the capacitor undergoes dimensional changes each time the capacitor is called upon to βtore charge. Eventually, such dimensional changes cause stress fractures which have an adverse affect on the capacitor's capability to store charge. While alpha-phase PVF₂ films have far less piezoelectric activity and, hence, would not be as subject to such structural defects, such films have not been considered to be useful in capacitors because of their relatively low dielectric constant (typically less than a value of 10).

According to one of the inventions herein described, there is provided a predominantly alpha-phase PVF₂ film which, while retaining its desirably low piezoelectric activity, exhibits a relatively high dielectric constant. According to a related invention, such a film is produced by stretching an extruded PVF₂ film, while still molten, to reduce the film thickness to less than about l/50th of its extruded thickness. The resulting predominantly alpha-phase film has a significantly increased dielectric constant (having a value of at least 12) and a low piezoelectric activity

—12 (less than 4 x 10 meters/volt) which renders it useful as the dielectric component in a wound capacitor. Such use constitutes a further related invention herein. The different inventions herein will be better understood from the ensuing detailed description, reference being made to the accompanying drawings in which:

Fig. 1 is a plot of an infrared absorbance spectrograph of a PVF₂ film made in accordance with the invention;

Fig. 2 is a schematic illustration, partially in section, of apparatus used to manufacture the P F₂ film; Fig. 3 is a graph illustrating the relation¬ ship, in one example, of the surface speed of the chill wheel to the dielectric constant of the PVF₂ film;

Fig. 4 is a fragmentary sectional view of one- half of a capacitor in which the PVF₂ film is used, such view being taken along a radius of the capacitor; and

Fig. 5 is a schematic view of a camera utiliz¬ ing the invention.

We have found that a predominantly alpha-phase PVF« film can be prepared having the desirable dielectric constants of beta-phase PVF₂, and the piezoelectric properties, and their attendant advantages, of alpha-phase PVF₂. Capacitors made from such dielectric film have superior properties, including stable dimensional properties. Such capacitors are useful in a wide range of circuits.

More specifically, stretching PVF₂ film suf¬ ficiently while it is still in the molten state so as to prevent formation of the beta-phase, causes the film to have dielectric constants ranging from 12 to about 16 and higher. Proper stretching causes the film to have final thicknesses no greater than 5 microns, and as thin as 1 micron, and still be free of pinholes and voids. When the crystalline structure of the finished film is examined, it is found to be predominantly alpha-phase. As used herein, "predominantly alpha-phase" means at

OMPI least 75% (by weight) of the crystalline portion of the film is alpha-phase in crystalline structure.

In the embodiments which follow, the film of the invention is described as useful to form a dry capacitor wherein no dielectric liquid is included to remove air pockets. In addition, useful forms of the capacitor of this invention include those wherein the film of this invention is combined with such a dielec¬ tric liquid to form a wet capacitor. Furthermore, the film is useful beyond its use in a capacitor, for exam¬ ple as tubing insulation, diaphragms for instruments or pumps, and protective surfaces for materials exposed to weather or corrosion.

To determine the percent of alpha-phase present in the film's crystalline structure, infrared absorption spectroscopy is used, as is described in U.S. Patent No. 4,298,719, col. 5, lines 23-42. Specifically, the absorption spectroscopy curve is examined for the curve values at 510 and 530 cm^"1, where 510 is character- istic of beta-phase and 530 of alpha-phase. The amount of absorbancy D is measured as the area under the curve for the peak in question. Thus, the proportion of the alph -phase crystals present, by crystalline weight, compared to the total crystalline weight (alpha-phase plus beta-phase) , is determined by the equation

% - — ^D5—30 X 100

^D510 ^{+ D}530

(The third crystalline phase, gamma, is so small in quantity that it can be ignored.) Referring to the curve of Fig. 1, D53_Q and ~ Q are readily measured from the points of the curve identified, and the per¬ centage determined. For the particular curve shown as Fig. 1, the percent that is alpha-phase is greater than 90%. The apparatus and method apparent from Fig. 2 are particularly useful in manufacturing sheets of PVF₂ of this invention. A conventional extruder 10 is supplied with particulate PVF₂ polymer via a hopper 20 so that the screw 30 of the hopper is driven at a desired RPM by motor 40. Any weight average molecular weight (Mw) of the PVF is useful, as long as it insures the PVF„ is in particulate form. For example, a Mw of 10⁵ is useful. Heaters, not shown, preferably supply auxiliary heat to extruder 10. Molten polymer is delivered from the extruder to a conventional die 50 having a rectang¬ ular opening 60 with a fixed length and a variable width "w." The hot polymer melt M flows out of die 50 across a distance "Y" to a conventional, rapidly rotating sur¬ face such as chill wheel or roller 70 operated at RPM's and temperatures hereinafter described. After solidi¬ fying on the chill wheel 70, the film is carried off to edge slitters 80 and take-up roller 90 that operates at RPM's sufficient to maintain tension on the film and avoid wrinkling. Optionally, an air jet 100 or a vacuum holddown (not shown) is added to temporarily "pin" the polymer film to chill wheel 70. Temperature control means, such as a coolant, can be added to wheel 70, to maintain the temperature of the surface of the wheel below the melt temperature (160-185°C) of the PVF₂. However, to insure that the crystalline structure is predominantly alpha-phase, the temperature must be no cooler than that which will ensure the stretching occurs while the film is molten, as is well known. The exact critical temperature will depend upon the size of wheel 70, and the speed of its operation. Most preferably, such surface temperature is maintained at a value between about 40°C and about 120°C. The features of the apparatus of Fig. 2 that are considered important to the invention are the relative speeds of the extruded melt M and the surface speed of wheel 70, flow distance Y, and die opening width w. As to the relative speeds, the desired properties of the final film, including thickness and dielectric constant, are achieved only if the surface speed of wheel 70 is selected to be a high multiple of the lineal speed of extruded melt M. The necessary RPM for the chill wheel depends upon the particular apparatus selected, and is readily determined for a given apparatus by experimentation. Fig. 3 is a plot of the RPM's needed for a 20 cm diameter wheel 70 to produce dielectric constants K of at least 12 when the melt M is extruded at a lineal speed of about 34.5 cm/min from a die opening width w ■ 254 y. This particular apparatus should be operated with wheel 70 rotated at a minimum of about 57 RPM, a value which, when converted to 43 cm/sec peripheral speed, is at least about 105 times that of the lineal speed of melt M. In another form of the apparatus (Examples 1-4 hereinafter) , it was found that the ratio of surface speed of the chill wheel and the lineal speed of extru¬ ded melt should be at least about 45 for best results.

Faster relative speeds of rotation of wheel 70 will provide even larger dielectric constants and thin- ner films, and greater uniaxial orientation within the film.

Although an exact understanding of the mechan¬ ism has not been achieved and is not needed to practice this invention, it is believed that the high dielectric constants, and the relative lack of piezoelectric activity when poled, of the film of the invention are achieved by stretching the film, or equivalently, reducing its thickness, by a particular amount. Specifically, the film is stretched or reduced in thick- ness by a stretch ratio of at least about 50, during or before the chilling of the film below its molten point. For example, if the film as extruded from the die has an initial thickness of 254 microns, it should have a final thickness after stretching that is no greater than about 5 microns (1/51 reduction) to insure that the high dielectric constants and low piezoelectric constants are achieved. A final thickness greater than 5 microns also provides such constants, if the initial extruded thick¬ ness is also larger than the final thickness by a factor of 50. E.g., an initial thickness of 500 microns, when reduced to 10 microns by the procedure of this inven¬ tion, can be expected to have a dielectric constant of at least 12 and a piezoelectric constant no greater than about 4 X 10^{" 1 Z} meters/volt when poled as described above. A variety of flow distances Y is useful within the invention. The most critical aspect of distance Y is that it not be so large as to allow the melt M to solidify before reaching wheel 70, or so as to prevent adherence of the film to wheel 70. Useful values of Y range from about 0.1 to about 5 cm. Most preferably, distance Y does not exceed about 2.5 cm.

Preferably, die opening width w is selected to minimize the thickness of melt M that is extruded, thereby reducing the final thickness of the film that is achieved. Useful values of width w range from 25 to about 1000 μm, with 250 μm being preferred. Thus, although final film thicknesses greater than 5 μm are also useful, if the film is to be used in a photoflash capacitor as in the preferred embodiment, the final thickness should be <5 μm, using a stretch ratio >.50.

The individual components of the afore- described apparatus are conventional. Useful examples include the Brabender Model 2523 Deluxe Vented Extruder, and chrome-plated stainless steel chill wheels operated

OMPI - IPO at from about 30 to about 80 revolutions per minute, depending upon the diameter of the wheel.

Alternatively, the film is formed with the aforesaid properties by extruding melt M onto a plastic support, such as poly(ethylene terephthalate), not shown. This support with the PVF₂ still molten there¬ on is partially wrapped around wheel 70 so that both the support and the PVF₂ are stretched by the rapid rota¬ tion of the wheel. Yet another alternative manufacturing technique comprises the coextrusion of such a plastic support along with the PVF₂, so that both are driven (not shown) by wheel 70, while still molten, and thereby stretched. It is readily apparent from the preceding description that the manufacturing process is improved in that only uniaxial stretching is required. Thus, the additional equipment that would be needed to obtain biaxial stretching is not necessary. The PVF₂ film described above has the follow¬ ing superior properties, in addition to the afore¬ mentioned high dielectric constant and low piezoelectric constant: reduced dissipation factors and high voltage breakdown strengths. Therefore, the film is useful in a variety of applications, particularly those requiring high dielectric constants. One such use is as the di¬ electric for a capacitor. A capacitor 200, Fig. 4, is prepared from sheets of the afore-described PVF₂ film, by applying conductive, metallic electrode layers 212 and 212' on two such PVF₂ sheets 214 and 214' so that the edges 216 and 216', respectively, of the two sheets are left uncoated with metal. Any conventional pro¬ cedure can be used to apply the metallic layers. The insulative thickness of the sheets, that is, the thickness measured without including the metallic layers, is preferably no greater than about 5 microns.

O PI_ The metallic layers have any suitable resistivity, for example, 1 to 4 ohms/square, with thicknesses preferably from 500 to 2000A. The thus-coated sheets (identified as composites A and B) are then wrapped in interleaved relation around a core 220 of any desired shape, one composite stacked on the other, so that edges 216 and 216' are at opposite ends of the core. Soft conductive metal pieces 221, 221', such as flame sprayed metal, are applied at the edges 222 and 224 of the wrappings so as to separately electrically interconnect all of the layers 212 at one end, and all the layers 212' at the other. The metal pieces 221 and 221' are wired to the capacitor's lead wires, not shown, and encasing plastic ends 230 and a cover 240 are applied. The capacitor constructed as described above is useful in any electrical circuit. Its shape is that of the core 220. It is particularly useful in flash apparatus for cameras. The increased C/V values permit the capacitor to have reduced dimensions, a property especially needed in new lines of pocket cameras being introduced by camera makers. As depicted in Fig. 5, such a camera 300 comprises flash apparatus that includes an electronic flash tube 318 which is wired to a high voltage power supply 326 via a control circuit 324. Power supply 326 also supplies power to the lens motor drive circuit 330 that is controlled by an optional automatic focus detector 328. The drive circuit in turn operates the positioning of lens 342 via motor 332 so that the image "I" is properly focused on film 344. All these components are generally described in U.S. Patent No. 4,291,958, issued September 29, 1981, by Lee Frank et al.

The firing means for the flash apparatus includes the flash control circuit 324 and of course power supply 326. Control circuit 324 in turn includes two capacitors--one which is a triggering capacitor (not

- shown) controlled by the circuit 324, and the other of which is the firing capacitor that supplies the energy to actually fire tube 318. The capacitor of this inven¬ tion is particularly useful as the firing capacitor. The capacitor is fired and the tube flashed when the camera shutter release button (not shown) is actuated, if the camera needs additional light for the exposure in question. Examples

The following examples further illustrate the invention. Examples 1-8

Two different forms of the apparatus shown in Fig. 2 and described above, were used to prepare PVF₂ film. The following features of the apparatus were selected:

Table I - Apparatus Parameters

Examples 1-4 Examples 5-8 Screw Diameter 1.9 cm 2.54 cm

Length/Diameter Ratio i 25/1 24/1

Compression Ratio 3/1 3/1

No. of Feed Flights 15 12

No. of Taper Flights 5 6

No. of Metering Flightss 5 6

No. of Heating Zones in Barreell 3 3 No. of Heating Zones in Die 1 1 Temperature of Heating (See Table II) (See Table

III)

Width of Die 10 cm 15 cm

Die Opening 254 μ 305 μ

Chill Wheel Diameter 7.6 cm 20.3 cm

Chill Wheel RPM (See Table II) (See Table III)

Extruder Motor Horse¬ power 3.0 3.0 Extruder RPM 10 15

Wind-up Drive No Yes

Air pinning No Yes

Die opening geometry horizontal vertical Chill wheel temperature 50°C 40°C-70°C

Table Tl - Extrusion Parameters for Examples 1-4

Chill Wheel

Lineal Speed Chill Speed * Temperature of Zone (°C) of Extruded Wheel Melt Example 1 2 3 Die Melt(cm/sec) RPM Lineal Speed

1 220 220 230 220 0.229 27 46.9

2 220 220 230 220 II 40 69.6

3 220 220 230 220 II 50 87.0

4 220 220 230 220 II 62 107.8

Table III - Extrusion Parameters for Examples 5-8

Lineal Chill Wheel Melt Speed of Chill Speed *

Temperature of Zone Temper-Extruded Wheel Melt

Example 1 2 3 Die ature Melt RPM Lineal Speed

5 177 199 232 232 209 0.575 cm/sec 68 126

6 177 199 232 232 212 " 88 163

7 177 204 238 241 218 94 173

8 182 210 246 246 237 " 102

After each film was stretched and rolled onto roller 90, it was measured for its crystalline phase. All examples were found by infrared absorption spectro¬ scopy to have predominantly alpha-phase crystalline structure. This was confirmed by X-ray analysis wherein the crystalline structure was found to be at least 95% by weight alpha-phase.

Each example was also measured for thickness, birefringence (Δn) , dielectric constant at 1000 Hz, charge density C/V, and dissipation factors, reported in Table IV hereinafter. Thickness measurements were made by three different techniques as follows:

In the first method, the films were placed between a flat gauge block and the head of a miniature linear variable differential transformer (Daytronic

Model DC20A LVDT). The LVDT developed an output voltage proportional to distance from a reference position. The transducer output was amplified with a Daytronic Model 300D transducer amplifier indicator followed by a C3140 operational amplifier with a gain of 20. Voltage through the LVDT was measured with the film samples both in place and out of position. The difference between readings yielded a voltage proportional to the sample thickness. A calibration curve was made using conven- tional 6, 9 and 16 μ thick biaxially stretched PVF₂ film obtained from Kureha Chemical Industry Co., Ltd., Japan, and 12.5 μ and 25.4 μ polyethylene tere- phthalate shim stock. All measurements were made at least 4 times and the average value determined. In the second method of thickness determination an IR interference technique was used. Constructive interference between the direct ray and the ray which is internally reflected once off each film surface occurs in transmission when 1) mλ ■ 2nt

O ?. r , vrι?c where t is the film thickness, n is the index of refraction of the film, λ is the incident wavelength and m is an integer. Two different wavelengths were selected to obtain constructive interference. The num- ber of fringes between interference maxima (Δm) is given by

2) Δm - 2nt (1/λι - l/λ₂), or

Δm

Fourier Transform Infrared Spectra (FTIR) were obtained on each example. The film thicknesses were determined from the interference fringes observed in the 4000-1800 cm^{" **} range. The third method of film thickness measurement utilized a precision micrometer gauge (Federal Gauge Model E3BS-2) with 2.54 μm scale divisions. The exam¬ ple films were folded 4, 8 and 16 times with special care to avoid wrinkling. A static neutralizer gun obtained from Quantum Instruments was used to eliminate static charges during folding.

The average thicknesses measured by each of these techniques were then further averaged to obtain the results set forth in Table IV. Birefringence was determined using a polarizing microscope equipped with a Berek compensator. The bire¬ fringence is given by the equation:

4) Δn - R/t where R is the retardation of the film measured by rota- ting a calcite crystal to the two positions of maximum extinction, and t is the thickness already determined. Dielectric constants were calculated from the equation

Ct 5) K -

A X 8.85 X 10 12 where C (capacitance) was measured on a dielectric bridge at 1000 Hz after a measured area A of the film was electroded.

Voltage breakdown strengths were determined by ramping a high voltage power supply through the examples deposited with 800A thick aluminum electrodes, while monitoring the current flow. Breakdown was defined to be the voltage at which the current surged from less than 1 μ amp to greater than 10 μ amps. The values listed in Table IV are average values for 10 samples. Charge density C/V was of course calculated from the equation

6) C/V - K X 8.85 X 10^"X2/ t². The dissipation factors were obtained as phase measurements made directly on the dielectric bridge noted above for the dielectric constants, as is conven¬ tional.

Table IV

Voltage

Average Breakdown Dissipation"

Thickness Strength Factor

Example (microns) Δn* ^1000 (v/j^j) C/V (farads/m³) (+ .0005)

1 4.7+0.4 .0087+.0004 12.2+1.0 230 4.9 .0120

2 2.4+0.2 .0154+.0014 11.8+1.0 220 18.1 .0105

3 1.9+0.1 .0205+.0020 12.6+0.7 237 30.9 .0105

4 1.55+0.05 .0232+.0013 13.9+0.5 232 51.2 .0100

5 3.8+0.3 .0123+.0013 11.9+1.0 218 7.3 .0110

6 1.85+0.05 .0224+.0012 14.1+0.5 227 36.5 .0100

7 1.55+0.05 .0287+.0014 14.6+0.5 245 53.8 .0100

8 1.2+0.1 .0338+.0035 15.8+1.4 258 97.1 .0095

* Δn values greater than 0.004 are characteristic of uniaxial stretching, since they cannot be produced using biaxial stretching.

_

In addition to the properties listed in Table IV, piezoelectric constants were also measured for Examples 1, 3, 5, 6 and 8 by stressing the films along their length and measuring the induced charge, after poling at 1 megavolt/cm at room temperature (25°C) for 1 hour. Example 1 was found to have a piezoelectric con¬ stant of 1.2 X 10^{" 12} meters/volt, Ex. 3 was 1.9 X 10^~12, Ex. 5 was 2.1 X 10^'12, Ex. 6 was 2.6 X 10^{' 12} and Ex. 8 was 3 X 10^{" 12} meters/volt. Examples 2, 4 and 7 not tested are presumed to have a value less than that of Ex. 8, inasmuch as Ex. 8 has the highest birefringence and dielectric constant, which, as is well known, produce the highest piezoelectric constant when poled. Thus, the examples of the invention produced a piezoelectric constant which in many cases is an order of magnitude lower than that occurring in beta-phase PVF₂ having comparable dielectric constants.

As Comparative Examples, Example 1 and Example 5 were each repeated, except that the RPM of the chill wheel was reduced to only 17 and 21.3, respectively. This produced an average final thickness of the PVF₂ film that was 8.8 microns and 7.3 microns, respectively, a reduction in thickness of only 1/28.4 and 1/41.8, respectively. This was found to produce dielectric con¬ stants of only 10.3 and 9.4, respectively, demonstrating that the stretch ratio needs to be at least about 50 to obtain Applicant's results. Example 9 - Use of a Plastic Support The procedure of Example 5 was repeated, except that the die opening was 1016 μm and a layer of unoriented polyester support having a thickness of about 152 μm and traveling at a speed of about 30 m/min was brought into contact by a nip roller with the PVF₂ film right after the PVF₂ film contacted the chill wheel. Thus the PVF₂ film was sandwiched between the chill wheel and the polyester support. The surface of the chill wheel also had a lineal speed of about 30 m/min, and the temperature of the chill wheel was 60°C. Additionally, but of lesser importance, the extruder had a screw length/diameter ratio of about 35/1, the width of the die was about 46 cm and the diameter of the chill wheel was about 35 cm.

The resulting stretched PVF₂ film had a thickness of 17.8 μm, which was about l/57th that of the original thickness of 1016 μm. This film was found to have greater than 90% by weight alpha-phase crystalline structure, a dielectric constant of 12.8 at 1000 hz, a dissipation factor of 0.011 and a breakdown strength of in excess of 2.0 MV/cm. As a Comparative Example No. 3, the process of

Example 9 was repeated, except that the chill wheel was refrigerated with chilled water so as to have a surface temperature of 10°C. The resulting PVF₂ film had the same properties as that of Example 9, except that greater than 75% of the crystalline structure (by weight) was beta-phase and the dissipation factor was increased to 0.018 (at 1000 hz). This demonstrated the importance of stretching the PVF₂ while still molten, since the use of a chill wheel at 10°C cooled the film, before stretching, sufficiently below the melt condition as to produce predominantly beta-phase, an undesired result.

Claims

WHAT IS CLAIMED IS :

1. A predominantly alpha-phase poly(vinylidene fluoride) film characterized by a dielectric constant of at least about 12 and a piezoelectric constant no greater than about 4 X 10^{" i2} meters/volt when poled at 1 MV/cm at room temperature for 1 hour.

2. A film as defined in claim 1 wherein its thickness is no greater than 5 microns.

3. A film as defined in claim 1 wherein said film has a birefringence number characteristic of uniaxial stretching.

4. A film as defined in claim 3, wherein said number is greater than 0.004.

5. A wound capacitor comprising a poly(vinylidene fluoride) film positioned between a pair of electrodes, characterized in that said film comprise predominantly alpha-phase poly(vinylidene fluoride) having a dielectric constant of at least 12 and a piezoelectric constant no greater than 4 X 10^{" 12} meters/volt when poled at 1 MV/cm at room temperature for 1 hour.

6. A capacitor as defined in claim 5 wherein said film has a breakdown voltage of at least 200 v/μ. .

7. A method for making predominantly alpha- phase poly(vinylidene fluoride) film having a dielectric constant greater than 12 and a piezoelectric constant no greater than 4 x 10 -12 meters/volt when poled at 1

Megavolt/cm at room temperature for 1 hour, said method including the steps of a) extruding molten poly(vinylidene fluoride) generally in the shape of a film, and b) stretching said film while still molten; characterized in that said film is stretched during said step b) by an amount effective to reduce the film thickness to a value no greater than about l/50th the original thickness, whereby the dielectric constant is increased to at least about 12.

8. The method of claim 7 wherein said film is subjected to a cooling treatment upon being stretched to lower the film temperature to between 40°C and 70 C.