CA1234884A - Polyvinylidene fluoride film for use in wound capacitors - Google Patents

Abstract

Abstract A novel PVF2 film and a process for making it are disclosed. The film has superior dielectric con-stants that were available prior to this invention only in beta phase PVF2 film. Such film is particularly useful to make high energy density capacitors that have particular utility in flash apparatus for cameras.
The preferred process of the invention features stretching the PVF2 film while in a molten condition, by an amount that is effective to reduce the film thick-ness to a value no greater than about 1/50th the orig-inal thickness.

Description

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FIELD OF THE INVENTION
This invention relates to films of polyvinyli-dene fluoride, abbreviated herein as PVF2, dielectric uses thereof, for example, capacitors~ and methods o making such films.
BACKGROUND OF THE INVENT~ON
Many electronic circuits, ~uch as photofla~h circuits of cameras, require ~ capacitor to store ~nd deliver large amounts of charge, for example, to fire the flash. Conventionally, such capacitors sre electro-lytic in nature, thflt is, they use an electrolyte $n sddition to a dielectric oxide fllm on a layer of metal, the oxide film being disposed between the layers of the met&l. Although they are advantageously sm~ll and inexpensive, such cspacitors suff~r from a number of disadvantages: a) They have to be reformed after sitting idle on the shelf--that is, the oxide film becomes deformed, resulting in decreased capacitsnce.
Reforming acts to restore the oxide film. The reforming voltage can be a significant and undesirable draw on the power source when that power source is one or more batteries, as in a cam~ra. b) Electrolytic c~pacitors do not maintain uniform capacitance over time. Vari-ations of from -20% to +15070 of the nominal value are common. This is due in part to thP fact that capaci-tance in such capscitors is sub~ect to vari~nce with changes in temperature. c) The electrolyte causes h$gh leakage currents, high equivalent series reslstance ~nd high dielectric loss tangents or dissipation factors.
d) Electrolytic capacitors are polarized so as to be mountable in a cireult in only one direction. e) Finally, they must be used within the voltage range for which they were designed.
Attempt~ to replace such capac~tor~ w~th dry polymeric film capacltor~ lacking the electrolyte have

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suffered from the fact that the dielectric films have had low dielectric constants and relatively large thick-nesses. Since capacitance C of a capacitor is de~er-mined by the well-known equation l) C 5 KA X 8.85 X 10-l2/ t farads where K is the relatlve dielectric constant~ A i6 the capacitance area in square meter~ and t i6 the thlck-ness of the fllm in meters, the resulting capacit~nces have been less than desired. The capacLtance den~ity C/V of such a capacitor is al80 undesirably reduced, where C/V is determined by the equation 2) C/V = K X 8.85 X 10-l2/t2 farads/m 3 and K and t are as defined above. As a result of K
being lower and t being too large in capacitors using such films, a larger volume capacitor ls required to obtain the same capacitance as in an electrolytic capacitor. Since the trend in csmeras is to make the cameras smaller rather than larger, there is no room for enlarged capacitors in a camera fla~h circuit.
Improvements in both the value of K and of t, that is, a larger K and a smaller t, h~ve been obtained heretofore in PVF2 films. However, prior to this invention only one of three polycrystalline ph~es of such film was thought to produce significant value6 of K. That was ~he beta phase that is produced by first quenching the film as it is extruded from the melt, and thereafter stretching it at temperstures signiflcantly below its melt point (about 185~C), e.g., between 60 and 100C. ~See, for example, Murayama, J. of Pol2m.
Sci., Polym. Physics Ed., Vol. 13, pp. 929-945 (1975), and especially Films A and E or F of Table I, p. 930.) The alpha phase that occurs when the film i6 quenched and not stretched, or stretched while xtill molten9 has not been con~idered particularly useful as a cap~citor dielectric,as its K values have been no better than about lO.

Nevertheless~ ~uch beta phase PVF2 has not been completely successful in capacitors~ During charg-ing of the capacitor, the d~electric film un~void~bly becomes poled. One reason for the laclc of Buccess i8 bel~eved to be the tendency of the beta phase to hAve high piezoelectric properties after it has been ~o poled. That is, unlike alpha ph~se PVF2, bet~ ph~se PVF2 film has high piezoelectric constant~, for example, at leas~ 1 X 10-1l meter~/volt after pol~ng with a field of 1 megavolt/cm (MV/cm) for 1 hour at room temperature. Such piezoelectric properties are detri-mental to capacitors because they cau~e substantial dimensional changes when high electric fields are applied. This property is reported, for example, as 1'ferroelectricity", in Edwards, "Radiation Response and Electrical Proper-ties of Polymer Energy Storage Capacitors: PVF23 Polysulfone, and Mylar") Nasa Conference Publication No. 2186, p. 1 (1981) and especially p. 2. Such dimensional changes in turn tend to stress the capacitor to the point ~t which fractures and other mechanical fa~lures occur. Still another disadvantage of the beta phase PVF2 is that production of it requires &dditional equipment and 6teps in the manufacturing process. That is, after the ~xtruded film is quenched to form the alpha phase, additional equip-ment is required to stretch the film to convert the film to the beta phase. Even if the alpha phase were known heretofore to produce high dielectric constants, whlch is believed not to be the case, the thicknesses l't'l, in equations 1) and 2) above, obtainable for alpha phase film by merely quenching the f~lm without sub6equen~
~tretching, have been too large to produce 8~8nificant values of C or C/V.
Therefore, there has been a great need to pro-vide PVF2 film with the increased dielectric eonstant~
that are characteristic of beta phase PVF2, but ~ 2 withou~ the piezoelectric constantg thAt h~ve been characteristic of poled bet~ phase film.
SUMMARY OF THE INVENTION
I have discovered that PVF2 film c~n be manu-factured with the dielectric constants that heretoforehave been characteristic of only bet~ ph~se PVF2 film, but without the piezoelectric beha~ior of poled beta phase film. Dry fllm capacitors compri6ing such PVF2 film have high energy denslty values coupled w$th dimensionally sound construction. Such capacitors in turn provide improved camera flash ~pparatus having reduced volume, enabling ~maller cameras to be con~
s~ructed.
More specifically, in accord with one aspect of the invention there is provided a ~heet of film com-prising poly(vinylidene fluoride) characterized by a dielectric constant of at least about 12, and a piezo electric constant no greater than about 4 X 10-12 meters/volt when poled at 1 MV/cm for 1 hour at room temperature. Such piezoelectric constants are less than hslf the constants that occur when using beta phase. In 8 preferred embodiment, ~uch a fllm has a crystalline structure that i~ predom~nantly alpha phese.
In accord with another aspect of the invention, there i~ provided a method for forming such ~ film. In the method which comprises the ~teps of B) extruding molten poly(vinylidene fluoride) generelly ~n the shape of a f~lm, ~nd b) stretching said film while 8~ill molten, the improvement resides ~n stretching the film during step b) by an amount effective to reduce the film thickness to a v~lue no gre~ter than about l/50th the originsl thickness.
In ~till another aspect of the inventlon, there is provided a wound capacitor comprising a core and a pair of interleaved ln~ulative sheets each bearing an electrically conductive layer 9 the ~heet~ a) having an insul~tive thickne~s no greater than about 5 microns, ~L~3~
and b) being wrapped around the core with the conductive layer of each member of the pair being ~oined to a sep-arate respective one of a pair of electrodes. The capacitor is improved in that the in~ulative sheets are each a film comprising poly(vinylidene fluoride) char-acterized by dielectric and piezoelectric constant~ ns noted in the previous paragraph. The preferred ~orm~ of such capacitors feature PVF2 sheets that are pre-dominantly alpha phase in cry~talline itructure, wlth film thicknesses and dielectric constants that produce C/V charge densities of at least about 5 farad~/m3.
In accord with yet another ~spect of the lnven-tion, there is provided a flash spparatus for use w~thin a circuit of a camera, comprising an electronic fl~sh tube, and means for firing the flash. The app~ratu6 i~
improved in that the flash firing means includes the capacitor described above.
Thus, this invention advantageously fea~ures a PVF~ film of high dielectric constant, without the piezoelectric drawbacks of beta phase PVF2.
It is another advantageous feature of the invention that such film6 have h~gh voltage breakdown strengths.
Still another advantageous feature ~8 that such PVF2 film has been found to produce superior, reduced dissipation factors, which in turn lnRures that the equivalent series resistance is reduced for a given capacitance.
Still another advantageous feature is that such films are readily made by the 6impler procedure of uni-axial stretching only.
It is a related advantageous feature ~hat dry polymeric f~lm capacitors made from ~uch PVF~ film hsve high energy dencities without the piezoelectric stre6ses here~ofore characteristic of PVF2 film pro~
viding such energy densitie6.

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It is another related advantageous feature of the ~nvention th~t such a capacitor can be made from such film w~thout the c~pacitor cracking or failing dur-ing use with high electric field6.
Yet another advantageous feature i6 that Quch capaci~ors are manufacturable in sizes, for at least certain voltage ratings, that are smaller than compar-ably-rated electrolytic capacitor~, producing photoflash circuits with reduced volumes.
Other advantageous features will become sppar~
ent upon reference to the follow1ng "Descript~on of the Preferred Embodiments", when rePd in light of the attached drawings.
BRIEF DESCRIPTION OF THE DRAWI~GS
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 illustrstion, partially in section, of apparatus used to manufacture the PVF2 film;
Fig. 3 is a graph illus~rating the relation-ship, in one example, of the surface speed of the chill wheel to the dielectric constant of the PVF2 fllm;
F~g. 4 is a fragmentary sectional view of one-half of a capacitor manufactured in accordance with theinvention, taken along a radius of the capacitor; and Fig. 5 is a schematic view of a camera utiliz-lng the flash circu~t o the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
~0 I have discovered that PVF2 film can be pre pared with the dielectric const~nts of bets pha~e PVF2, while retaining the electr~cal properties, and their attendan~ advantages, of alpha phase PVF2.
Capacitors made from such dielectric film have 6uperior properties, including stable dimensional properties.
Such capacitors are useful in a wide range of circui~s.

~ ~ 3 More specific~lly, stretching PVF2 ~llm 8uf-fic~ently while it is still in the molten Rt~te 60 ~6 to prevent formation of the beta phase, causes the film to h~ve 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 pinhole6 and ~oids.
When the cryst~lline structure o~ the finished ~ilm iB
examined, it is found to be predominantly alpha phase.
As used herein, "predominantly alpha phase" means at least 75% ~by weight) of the crystall~ne portion of the fi~m 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 i~ included to remove ~ir pockets. In addition, useful forms of the capacitor of this invention include those wherein the ~ilm of thls 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 ~r pumps, and protective surfaceæ for materials exposed to weather or corrosion.
To determine the percent of alpha phase present in the film's crystalline structure, infrared ab60rption spec~roscopy i~ used, as is described in U.S. Patent No.
4,298,719, col. 5, lines 23-42. Specifically, the absorption 6pectroscopy curve is exsmined for the curve values at 510 and 530 cm~l, where 510 is character-istic of beta phase and 530 of alpha phase. The amountof absorbsncy D is measured as the area under the curve for the peak in question. Thus, ~he proportion of the alpha phase crystal~ present, by crystalline ~eigh~, compared to the total crystalline weight (alpha phase plus beta phase), is determined by the equat~on -8- 12~4~B4 % = 530 X loo D510 + D530 (The third crystalline phase, gamma, is co ~mall in quantity that it can be ignored 3 Referring to the curve of Fig- 1, Ds30 and D510 are readily measured from the point~ of the curve identified, and the per-centage determined. For the particular curve ahown a~
Fig. 1, the percent that is alpha pha6e i8 greater t~lan 90% .
The apparatu~ and method apparen~ from F~g. 2 are particularly useful in manufacturing ~heets of PVF2 of this invention. A conventional extruder 10 is supplied with particulate PVF2 polymer Yia a hopper 20 so that the screw 30 of the hopper i8 driven at B
desired RPM by motor 40. Any weight average molecular weight (Mrw) of the PVF2 is useful, as long as it insures the PVF2 is in particulate form. For example, a Mw of 105 is useful.
Heaters, not shown, preferably ~upply suxiliary heat to extruder 10. Molten polymer i8 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 acro6s a distance "Y" to a conventional, rapidly rotst~ng 6ur-fsce such as chill wheel or roller 70 operated a~ RPM'sand temperatures hereinsfter described. A~ter ~ol~di-fying on the chill wheel 70, the film i8 carried off to edge slitters 80 and tske-up roller 90 ehst operate~ at RPM's sufficient to maintain tension on the film and avoid wr~nkling. Optionally, an air ~et 100 or a vacuum holddown (not shown) i6 added to temporarily "pin" the polymer film to chill wheel 70. Preferably9 temperature control means, such a~ a coolant, are added to wheel 70, to ma~ntain the temperature of the 6urface of the wheel below the melt temperature (160-185~C) of the PVF~.

~234l~
g Most preferably, ~uch surf~ce temperature i6 m~ntained at a value between about 30C and about 120~C.
The features of the app~ratus of Fig. 2 that are consldered important to the invention are the relative speeds of the extruded mel~ M and the surfsce speed of wheel 70, flow distance Y, and die openlng width w. As to the relative speeds, the desired properties of the final film, including thickness ~nd 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 constant~ K of at least 12 when ehe melt M is ex~ruded at a lineal speed of about 34.5 cm/min from a die opening width w ~ 254 ~. This particular apparstus 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 lesst sbou~ 105 times thst of the lineal speed of melt M.
In another form of the apparatus (Example~ 1-4 hereinafter), it was found that the ratio of ~urface 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 con~tants and thin-ner f~lms, and greater uniaxial orieDtation within the film.
Although an exact understanding of the mechan-i~m has not been achieved and is not needed to practice this invention, it i8 believed that the high diele~ric constantR, and the relative lsck of piezoelectric activity when poled, of the film of the inventi~n are achieved by ~tretching the film, or equivalently, -10- ~ ~ 3 4 reducing its thickness, by a particul~r 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 lts molten point.
For example, if the film as extruded from the die h~s an initial thickness of 254 micron6, it should have a f~nal thickness after stretching that i~ no gre~ter ~han about 5 microns (1/51 reduction) to insure that the high dielectric constants and low piezoelectric constants sre ~chieved. A final thickne~ greater than 5 micron~ al80 provid~s 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 hsve a dielec~ric constant ofat least 12 and a piezoelectric constant no greater than about 4 X 10-l2 meters/volt when poled as described above.
A variety of flow dist~nces 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 601idify before reaching wheel 70, or fiQ as to prevent adherence of the film to wheel 70. Useful value~ of Y
range from about 0.1 to about 5 cm. Mo6t preferably, dlstance Y does not exceed about 2.5 cm.
Preferably, die opening width w i~ selected to minimize the thickness of melt M that is extruded, thereby reducing the final thickness of the film that ls achieved. U6eful value6 of width w range from 25 to about 1000 ~, with 250 ~ being preferred. Thus, although final film thicknesses greater than 5 ~ are also useful, if the film is to be u6ed in a photofla~h capacitor ~s in the preferred embodiment, the fin~l thickne~s should be <5 ~, U6~ ng a 6tretch ratlo >50.

~3~

The indi~idusl components ~f the afore-described appar~tus sre conventional. U~eful example~
include the Brabender Model 2523 Deluxe Vented Extruder, and chrome-pl~ted ~tainless steel chill wheel~ operated at from about 30 to about 80 revolution~ per minute, depending upon the diameter of the wheel.
Alternatlvely, the film i8 formed with the aforesaid propertles by extruding melt M onto a plastic 6upport, such as poly(ethylene tereph~halate), not shown. This support with the PVF2 still molten there-on is partially wrapped around wheel 70 so that both the suppor~ and the PVF2 are stretched by ~he rapld rots-tion of the wheel.
Yet another alternative manufacturing technique comprises the coextrusion of such a plastic ~upport along with the P~F2, so that both are driven (not shown~ by wheel 70, while still molten, and thereby ; stretched.
It is readily app~rent from the preceding description that the manufacturing process is improved in thst only uniaxial stretch~ng 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 us~ful in a variety of applications, partlcularly those requiring high dielectric constants. One such u e is as the d~-electric for a capacitor. A capacitor 200, Fig. 4, ~8 prepared from sheet~ of the afore-described PVF2 film, by applying conductive, metallic electrode layerx 212 and 212' on two such PVF2 sheets 214 and 214' so that the edges 216 and 216', respect~vely, of the two sheet~
are left uncoated with metal. Any conventional pro~

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cedure can be used to apply ~he met~llic l~yers. The insulative thickness of the sheet~, that is, the thickness measured without includlng the metallic layers, is preferably no greater than about 5 m~crons.
The met~llic layers have any suitable resistivity, for exsmple, 1 to 4 ohms/square, with thickne6se6 preferably from 500 to 2000A. The thus-coated sheet6 (identlfied as composites A and B) are then wrapped ~n interleaved relation around a core 220 of any de~ired shape~ one composite stacked on ~he other9 60 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 hll 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 ~s 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 v~lues permit the capacitor to have reduced dimensionsg 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 appara~us that includes an electronic flash tube 318 which is wired to a high voltage power supply 326 via a control c~rcuit 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 V~3 motor 332 so that the image t'I" is properly focused on film 344. All these components are generally described in U.S. Patent No. 4,291,958, issued September 29 3 1981, by Lee Frank et al.

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The firing means for the fl~sh appar~tufi includes the flash control circuit 324 and of course power supply 326. Control circuit 324 in turn includes two c~pacitors -one which is a triggering capacitor (not shown) controlled by the circuit 324, and the other of which is the firing capacltor that supplie~ the energy to actu~lly fire tube 318. The c~pacitor of this inven-tion is particularly useful ~s the firing capacitor.
The cap~citor is fired ~nd the tube flashed when ~he camera shutter release button (not shown) i~ actu~ted, if the camera needs additionsl light for the exposure in question.
Examples The following examples further illustrate the invention.
Examplés 1-8 Two different forms of the app&ratus shown in Fig. 2 and described above, were used to prepare PVF2 film. The following features of the appsratus were selected:

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Table I - Ap~ar~tus P~ra_ eters Examp_es 1-4 Examples 5-8 Screw Diameter 1.9 cm 2.54 cm Length/Diameter Ratio 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 Flights 5 6 10 No. of Heating Zones in Barrel 3 3 No. of Heating Zone~ in Die Temperature of Hea~ing ~See T~ble II) (See T~ble III) Width of Die 10 cm 15 cm Die Opening 254 ~ 305 ~
Chill Wheel ~iameter 7.6 cm 20.3 cm Chill Wheel RPM (See Table II) (See Table Extruder Motor Horse-power 3.0 3~0 Extruder RPM 10 15 Wind-up Drive No Yes Air pinning No Yes Die opening geome~ry horizontal vertical Chill wheel temperature 50~C 40UC-70C

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After each film was ~tre~ched and rolled onto roller 90, it w~s measured for its cry6talline phase.
All examples were found by infrared ~bæorption spectro-scopy to have predominantly slph~ ph~se crystalline structure. This was confirmed by X-ray ~nalysi6 wherein the crystalline structure was ~ound to be at lea~t 95 by weight alpha phase.
Each example was also measured for thickness, birefringence (~n), dielectric constQnt ~t 1000 Hz, 10 charge density C/V~ and dissipation factors, reported in Table IV hereinafter~ Thicknesæ measurement6 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 ~ miniature linear variable differential ~ransformer (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 20 operational amplifier with a gain of 20. Voltage through the LVDT was measured with the film 6ampleæ both in place and out of position. The difference between readings yielded ~ voltage proportion~l to the s~mple thickness. A cslibration curve was made using conven-25 tional 6, 9 and 16 ~ thick biaxially stretched PVF2film obtained from Kureha Chem$cal ~ndustry Co., Ltd., Japan, and 12.5 ~ and 25.4 ~ polyethylene tere-phthalate shim stock. All measurements were made at least 4 times and the average value determined.
In the second method of thickness determ~nation an IR interference technique was uæed. Constructive interference between the direct ray and the ray w~ch is internally reflec~ed once off each f{lm Rurfsce occurs in trsnsmission when 1) m~ ~ 2nt -17- ~23~
where t is the film thickness, n i6 the index o~
refraction of the film, ~ i6 the incident wavelength and m i6 an integer. Two different wavelength6 were selected to obtain constructive interference. The num-ber of fringes between interference maxima (~m) i8given by 2) ~m - 2nt (1/~ 2), or l~m

3) t ' 2n~ ( Fourier Transform Infrared Spectra (FTIR) were obtained on each example. The film thicknesses were determined from the interference fr~nges observed in the 4000-1800 cm~l range.
The third method of film thickness ~easurement utilized a precision micrometer gauge (Federal Gauge Model E3BS-2) with 2.54 ~ scale divisionsO The exam-ple films were folded 4, 8 and 16 times with special care to avoid wrinkllng. A 6tatic neutral~zer 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 aver~ged to obtaiD
the re~ults set forth in Table IV.
Birefringence was determined using a polarizing microscope equipped with a Bere~ compensator. The bire-frlngence i~ given by the equation:

4) ~n ~ 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 ~ is the thickness already determlned.
Dielectric constants were calculated from the equation Ct

5) K -A X 8.85 X 10-l2 -lB-where C (capacitance) W8S measured on a dielectric bridge ~t 1000 Hz after a measured ~res A of the film was electroded.
Voltage breakdown strengths were determined by ramping a high voltage power ~upply through the ex~mples deposited wlth 800A thick aluminum electrodes, while monitoring ~he current flow. Breakdown was defined to be the voltage at which the current surged from le88 than 1 ~ ~mp to greater than 10 ~ amps. The values lis~ed in Table IV are average values for 10 sample6.
Charge density C/V was of course c~lculated from the equat~on

6) C/~ ~ K X 8.85 X 10-l2/ t2.
The dissipation factors were obtained as pha~e measurements made directly on the dielectric bridge noted above for the dielectric constants, as i8 conven-tional.

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In addition to th~ propert~es listed ~n Table IV, piezoelectric const~nts were also measured for Examples 1, 3, 5, 6 and 8 by stressing the film6 along their length and measuring the induced charge, after polin~ a~ 1 megavolt/cm at room temper~tur~ ~25UC) for 1 hour. Example 1 wa~ found to h~ve a piezoelectrlc con-stant of 1.2 X 10-12 meter~/volt, Ex. 3 was 1.9 X
10-l2, Ex. 5 was 2.1 X 10-l2, Ex. 6 W88 2.6 X
lo-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 ha6 the highe~t birefringence and dielectrlc constant, which, as is well known, produce the highest piezoelectric constant when poled.
Thus a 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 const~nts.
As Comparative Examples, Example 1 and Example 5 were e&ch repeated, except that the RPM of the chill wheel was reduced to only 17 and 21.3, respectively.
This produced an avera~e final thickness of the PVF2 film that was 8 8 microns and 7.3 microns, respectively~
a reduction ln thickness of only l/2g.4 and 1/41.8, respectively. This was found to produce dielectric con-stants of only 10.3 and 9.4, respectively, de20nstrating that the stretch r~tio needs to be a~ least about 50 to obtain Applicant's results.
The invention has been described in detail with particular reference to preferred embodiment6 thereof~
but it will be understood that vari~tions and modifica tions can be effected within the spirit ~nd ~cope of the invent~on.

Claims (7)

WHAT IS CLAIMED IS:

1. A sheet of film comprising poly(vinylidene flouride) characterized by a dielectric constant measured at 1 KHz, of at least about 12 and a crystalline structure that is predominantly alpha phase.

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

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

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

5. In a wound capacitor comprising a core and a pair of interleaved insulative sheets each bearing an electrically conductive layer, said sheets a) having insulative thickness no greater than about 5 microns, and b) being wrapped around said core with the conductive layer of each member of said pair being joined to a separate respective one of a pair of electrodes;
the improvement wherein said insulative sheets are each a film comprising poly(vinylidene flouride) characterized by a dielectric constant measured at 1 KHz, of at least about 12 and a crystalline structure that is predominantly alpha phase.

6. In a wound capacitor comprising a core and a pair of interleaved insulative sheets each bearing an electrically conductive layer, said sheets being wrapped around said core with the conductive layer of each member of said pair being joined to a separate respective one of a pair of electrodes;
the improvement wherein said insulative sheets are each a film comprising poly(vinylidene flouride) having a crystalline structure that is predominantly alpha phase, and a thickness and a dielectric constant that are effective to provide said capacitor with a charge density of at least about 5 farads/m3.

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