Note: Descriptions are shown in the official language in which they were submitted.
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"A Dielectric Film of a Copolymer of
Vinylidene Fluoride and Tetrafluoroethylene
IR-2828
8ACKGROUND OF THE INVENTION
5This invention relates to a film of a copolymer of vinyl-
idene fluoride and tetrafluoroethylene which demonstrates a
high dielectric constant and good dielectric strength. More
particularly, it relates to a molecularly oriented film of a
copolymer of at least about 85 weight % of vinylidene fluoride
and at least about 3 weight % of tetrafluoroethylene. In
addition, this invention relates to the process of preparing
such dielectric and capacitors utilizing it.
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STATEMENT OF THE INVENTION
This invention is a dielectric comprising a molecularly
oriented film of a copolymer of at least about 85 weight %
of vinylidene fluoride and at least abou~ 3 weight % of tetra-
fluoroethylene based on the weight of said copolymer, saidfilm having a dielectric constant at 100 Hz of at least 10
and a dielectric strength of at least about 400 volts per
micron (~m) of film thickness.
The process for preparing the dielectric comprises
forming a nonfibrous sheet of a copolymer of at least about
85 weight % of vinylidene fluoride and at least about 3
weight % of tetrafluoroethylene based on the weight of the
copolymer, molecularly orienting said sheet by elongating
it in at least one direction relative to a major dimension
thereof to thereby produce a film having a thickness of no
greater than about 75 microns and a dielectric constant
at 100 Hz of at least 10.
Detailed Description of Invention
The dielectric of this invention is a molecularly
oriented copolymer film of vinylidene fluoride and tetra-
fluoroethylene. The weight range of vinylidene fluoride in
the copolymer is from about 85 to 97 % while the range of
tetrafluoroethylene in the copolymer is from about 3 to 15%.
It is preferable, from the standpoint of improved dielectric
properties that the vinylidene fluoride content of the
copolymer be from abou~ 88 to 94 % while the tetrafluoro-
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ethylene content should be about 12 to 6 %. While it ispreferred that the copolymer consist of only vinylidene
fluoride and tetrafluoroethylene monomers, small proportions
of other copolymerizable monomers, preferably fluorine-
containing ethylenically unsaturated monomers, may beincluded.
The copolymers are formed by any convenient polymeriza-
tion procedure including emulsion, suspension and solvent
polymerization techniques. Emulsion and suspension poly-
mersion procedures are preferred. The procedure detailedin U.S. Patent No. 4,360,652 is most preferred.
The copolymer film is prepared by first fabricating
preformed sheets by solvent casting, melt extrusion,
injection molding, pressing or calendaring. Such techniques
are well known and need not be specifically described here.
The preformed sheets are of any thickness which will permit
molecular orientation of the sheets by elongation in at
least one major dimension into films of the desired thick-
ness. Typically,the preform sheets have thicknesses ranging
between about 45 and 200 microns.
In accordance with the process of this invention, the
sheets are molecularly oriented, for example, by stretching,
rolling or blowing, to provide film of improved dielectric
properties. The techniques of molecular orientation of
polymer film are well known and need not be specifically
discussed here. An example of the stretch orientation of
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polyvinylidene fluoride film may be found, for example, in
U.S. Patent No. 3,197,538. The teaching of this patent may
be extended to the copolymer films of this invention. The
fabricated copolymer sheets are oriented uniaxially or
biaxially to provide the desired improvement in dielectric
properties. The orientation ratio will range from about
2:1 to about lO:l for both uniaxial orientation and biaxial
orientation but the preferred orientation range is from
about 4:1 to about 5:1 whether orientation is in one direc-
tion or both. That i5, for example, if biaxial orientationis accomplished by stretching, the film is preferably stretched
from 4 to 5 times in the machine direction and then from 4 to
S times in the transverse direction.
After orientation, the film will have a thickness range
extending up to about 75 microns but preferably no greater
than about 13-15 microns.
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E.YAMPLE
A copolymer resin for this invention is typically made
as follows:
A horizontal stainless steel autoclave equipped with a
stirrer is charged with water and the ammonium salt of a
perfluorooctanoate as an emulsifier. Vinylidene fluoride
monomer and tetrafluoroethylene monomer in the required molar
proportions are added to the pressurized autoclave after
purging with nitrogen. The reactor is Xeated to the
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reaction temperature and the reactants are agitated. A chain
transfer agent may be added at any appropriate stage of the
reaction. Monomers, in the required molar proportions are
continuously fed into the reactor along with an initiator and
chain transfer agent, if desired. Each monomer's pressure is
maintained constant throughout the reaction. Upon completion
of the reaction (effective initiator depletion), the reactor
contents are cooled and the latex product drained from the
reactor. The latex is concentrated and the copolymer resin
recovered for use in the fabrication of a resinous sheet.
The resin is formed into a sheet by extrusion and the
sheet subjected to molecular orientation by stretching to
provide a continuous film of a thickness of about 12 microns.
One fabricated sheet of a copolymer of 90 weight % vinylidene
fluoride and 10 weight % tetrafluoroethylene is unixally
oriented at a ratio of about 3.8 to 1 to form the 12 micron
thick film and another fabricated sheet of the same monomer
content having a greater initial thickness is biaxially
oriented at a machine direction stretch ratio of about 3.5
to 1 and then at a transverse stretch ratio of about 3.9 to
1 to also form a 12 micron thick film. The temperature at
which the films were stretched was 100C for machine direc-
tion stretch and 140C for transverse direction stretch.
The films were tested for dielectric properties and
the results are reported in the following table.
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The data of the above table show that the vinylidene
fluoride-tetrafluoroethylene copolymer film of this invention
is unexpectedly more amenable than the film of vinylidene
fluoride homopolymer to the process conditions of molecular
orientation whereby the dielectric constant is improved when
the film is subjected to conditions developed in heat stretch
orientation.
Further, the dielectric fllm of this invention when
compared to the homopolymer fllm has less loss of dlelectric
constant in thermocycling of the film as would ordlnarily
occur when used in capacitor applicatlons.
Still further, the fllm-of thls lnvention has less
elevatlon in dielectrlc loss with increase in temperature
than the homopolymer film.
Whlle both the homopolymer and the copolymer film both
retain physical stability when heat annealed, the copolymer
has better dielectrlc constant stabllity.
The copolymer film of this invention may be heat
annealed to provlde dlmenslonal stability at elevated tem-
peratures. For example, the fllm may be annealed by holdlngit under tenslon on all sldes whlle subjecting it to a
temperature of from about 120 to 175C and then allowlng
the fllm to cool whlle malntaining it under tenslon.
For use in a capacitor, the film is typically coated,
for example by vapor deposltlon, wlth an electroconductive
coating, for example, silver, copper, aluminum, chromium,
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zinc, tin, nickel or other conductive material. Other
methods of applying the coating are, for example, electro-
plating, spray painting, sputtering, laminating and the like.
Electrode foils and dielectric film may also be wound to
S form the capacitor. Method and materials for the manufactu,re
of capacitors are well known and can be used with the film
of this invention without specific instructions to those
skilled in this art. The coated film, having conductive
leads extending from the electroconductive coating or layer
m~y be rolled into a capacitor.
