Dielectric films and process for preparing same

The dielectric film contains from 40 to 97% by volume of a thermoplastic resin, from 2 to 50% by volume of a ceramic dielectric and from 1 to 25% by volume of carbon black. The composite film contain a base film layer composed of said dielectric film and the dielectric film of the thermoplastic resin provided on at least one surface thereof.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention reltes to a dielectric film, a laminated film using 
the same, and a process for preparing the same. 
2. Brief Description of the Prior Art 
A conventional dielectric film having a high dielectric constant is one 
formed from a mixture of a ceramic dielectric with a high dielectric 
constant and a polymer. Where an amount of the high dielectric ceramics is 
increased, processability of the dielectric film resulting therefrom is 
impaired. Where the amount thereof is rendered small, the dielectric 
constant of the resulting dielectric film is decreased. 
As a conventional dielectric body having a high dielectric constant has a 
high dielectric dissipation factor, tan .delta., there is known yet no 
dielectric film having a combination of properties such as high dielectric 
constant, low AC loss, good processability and uniform composition. 
A conventional dielectric film employing a polymer as a matrix material 
presents various problems. For example, it is known that a capacitor or a 
capacitive switch which results from a film of a thermoplastic resin such 
as polyolefin, thermoplastic polyester, polyamide, polycarbonate, 
polyvinyl fluoride, polyvinylidene fluoride, polytetrafluoroethylene or 
polyvinyl chloride possesses an electrostatic capacity that is in 
proportion to a dielectric constant of the film and in inverse proportion 
to a thickness of the film. Although it is preferred that the thickness of 
the film is as thin as possible, there is a limitation of rendering the 
film thinner. For example, a thermoplastic polyester film having the 
thickness of 2 microns and a high Young's modulus is commercially 
available; however, it is very difficult to form a film having a thickness 
of less than 3 microns from the other resins on an industrial scale. If a 
film which is formed from a material having a Young's modulus far smaller 
than that of a film of polyester such as polyvinylidene fluoride or 
polypropylene is wound on itself in roll, a stress resulting from winding 
without causing wrinkles may sometimes exceed the critical stress of the 
elastic limit of the film. This will encounter the difficulty with respect 
to the preparation of a capacitor from such film roll. Such conventional 
film will also give rise to the difficulty in increasing an electrostatic 
capacity of the capacitor so that the resulting capacitor cannot be 
rendered thinner. 
Japanese Patent Publication No. 2,044/1980 discloses a composition 
comprising polyvinylidene fluoride and carbon black. This composition 
permits a large variation in dielectric constant with a slight change in 
an amount of carbon black. It has the tendency that carbon black 
decomposes upon application of a high shear during milling, resulting in a 
variation in dielectric constant even when carbon black is mixed in 
identical amounts. With such composition, it is difficult to form films 
having a uniform thickness on an industrial scale. Where the amount of 
carbon black is rendered too large, such composition has the tendency that 
the dissipation factor, tan .delta., becomes extremely low. 
OBJECTS AND SUMMARY OF THE INVENTION 
Therefore, the primary object of the present invention is to provide a 
dielectric film having a high dielectric constant and a good 
processability. 
Another object of the present invention is to provide a dielectric film 
further having a low AC loss and a uniform composition. 
A further object of the present invention is to provide a dielectric film 
resulting from a film material which has a thickness suitable for 
processability during manufacture and which can form a capacitor or the 
like which a high electrostatic capacity per unit surface area. 
The dielectric film in accordance with one aspect of the present invention 
comprises a dielectric film containing a thermoplastic resin in an amount 
ranging from about 40 to 97% by volume, a ceramic dielectric in an amount 
ranging from about 2 to 50% by volume and carbon black in an amount 
ranging from about 1 to 25% by volume. 
The dielectric film in accordance with another aspect of the present 
invention comprises the dielectric film laminated on one or both surfaces 
of a base film.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The dielectric film in accordance with the present invention comprises a 
thermoplastic resin, a ceramic dielectric and carbon black. 
As the thermoplastic resin for the composition to be used for the present 
invention, a known thermoplastic resin may be used. The thermoplastic 
resins may include non-polar or polar thermoplastic resins. 
Representatives of the non-polar thermoplastic resins may include 
polyethylene (hereinafter referred to as PE) and polypropylene 
(hereinafter referred to as PP), and representatives of the polar ones may 
include a polyvinylidene fluoride resin such as, for example, a 
polyvinylidene fluoride homopolymer (hereinafter referred to as PVDF) or a 
polyvinylidene fluoride copolymer obtainable by the copolymerization of 
more than about 50 mol% of the vinylidene fluoride with a copolymerizable 
monomer such as a fluoride-containing compound, e.g., vinyl fluoride, 
vinylidene fluoride, ethylene chlorofluoride, ethylene tetrafluoride or 
propylene hexafluoride. Among these resins, polyvinylidene fluoride resin, 
polyvinyl fluoride and polytrifluoroethylene that have each a high 
dielectric constant are preferably used. In order to impart a particularly 
high dielectric constant to the resulting film, it is preferred to use the 
polar thermoplastic resin having a relative dielectric constant at 1 KHz 
of preferably larger than about 6, more preferably larger than about 8. 
The amount of the thermoplastic resin in the composition to be used in the 
invention may be in the range generally from about 40 to 97% by volume, 
preferably from about 45 to 95% by volume and, more preferably, from about 
50 to 93% by volume. Where the amount of the thermoplastic resin is too 
small, processability may be impaired. Where the amount thereof becomes 
too much, the dielectric constant of a dielectric film resulting therefrom 
may be rendered smaller. 
As carbon black to be dispersed in the thermoplastic resin, it is 
preferable to use a longer chain of carbon black particles (generally 
called "structure") which gather with each other in clusters. Whether the 
structure is long or short is determined by whether an oil absorption of 
carbon black used is large or small. In general, carbon black having a 
larger oil absorption may impart a higher dielectric constant to the film. 
In order to improve a dielectric constant by the addition of a small 
amount of carbon black, it is preferred to use such carbon black as having 
an oil absorption larger than about 100 cc per 100 grams. In order to 
minimize a variation in dielectric constant with a slight change of 
amounts of carbon black, it is rather preferred to use carbon black having 
a small oil absorption. 
The amount of carbon black in the composition to be used in the present 
invention may be in the range generally from about 1 to 25% by volume, 
preferably from about 2 to 20% by volume and, more preferably, from about 
3 to 15% by volume. A larger amount of carbon black will impair molding 
characteristics. A smaller amount thereof will decrease a dielectric 
constant of the film. 
The ceramic dielectric that is dispersed in the composition in accordance 
with the present invention may include, for example, a ferroelectric such 
as ceramics of the BaTiO.sub.3 ceramic type, of the lead titanozirconate 
type or of the niobic acid type or the like or a ceramic paraelectric 
material such as titanium oxide, alumina or the like. It is preferable to 
use a dielectric having a relative dielectric constant of more than about 
10, more preferably, more than about 50 at 1 KHz because such dielectric 
will provide a higher dielectric constant. 
The amount of the ceramic dielectric in the composition may be in the 
amount ranging preferably from about 2 to 50% by volume, more preferably 
from about 5 to 45% by volume and, particularly preferably from about 10 
to 40% by volume. Where the amount of the ceramic dielectric is rendered 
too large, processability becomes impaired. 
In accordance with the present invention, a size of the ceramic dielectric 
is not restricted to a particular one. Where the size of the dielectric is 
rendered smaller, voids are formed to a lesser extent when the film is 
oriented. The use of the ceramic dielectric having a smaller size, 
however, may present the disadvantages that a homogeneous admixture 
becomes difficult upon extrusion processing and flexibility is rendered 
relatively small. Accordingly, the size of the ceramic to be used may be 
appropriately chosen in accordance with an intended use. 
The composition to be used in the present invention may contain another 
constituent such as a dispersion aid. The composition comprising the 
thermoplastic resin, the ceramic dielectric and carbon black can provide a 
dielectric constant indicating the same tan .delta., that is about several 
tens times that of the composition comprising the thermoplastic resin and 
carbon black. 
The dielectric film in accordance with the present invention may be of the 
type that is formed by a single use of the three-component composition as 
hereinabove set forth or by laminating a dielectric film on a base film. 
Where the dielectric film in accordance with the present invention 
comprises a one-layer structure, it contains the thermoplastic resin in 
the amount ranging from about 40 to 97% by volume, preferably from about 
45 to 95% by volume and more preferably from about 50 to 93% by volume; 
the ceramic dielectric in the amount ranging preferably from about 2 to 
50% by volume and more preferably from about 5 to 45% by volume; and 
carbon black in the amount ranging from about 1 to 25% by volume, 
preferably from about 2 to 20% by volume and more preferably from about 3 
to 15% by volume. Such dielectric film may be prepared in conventional 
manner, for example, merely by extruding the composition comprising the 
three components through an extruder to a film having a desired thickness 
and, when necessary, then orienting the film in a uniaxial or biaxial 
direction. 
Where the dielectric film in accordance with the present invention 
comprises a plural-layer structure, the film may be of the type that is 
called "a composite film", that is to say, that is formed by laminating a 
dielectric layer on at least one surface of a base film. 
In the embodiments of the invention as shown in FIGS. 1 and 2, the 
composite films 1 and 11 are seen, respectively, to comprise the base 
films 2 and 12 and the dielectric layers 3a, 3b and 13a, 13b laminated on 
the both respective surfaces of the base films. In FIGS. 1 and 2, the 
reference numerals 4 and 14 means the thermoplastic resin, and the 
reference numerals 5 and 15 mean carbon black. The base films may be 
prepared from the composition as hereinabove mentioned, that is, the 
homogeneous composition comprising the thermoplastic resin in the amount 
ranging from about 40 to 97% by volume, preferably from about 45 to 95% by 
volume and more preferably from about 50 to 93% by volume; the ceramic 
dielectric in the amount ranging generally from about 2 to 50% by volume 
and preferably from about 5 to 45% by volume; and carbon black in the 
amount ranging from about 1 to 25% by volume, preferably from about 2 to 
20% by volume and more preferably from about 3 to 15% by volume. 
Referring to FIG. 2, the composite film in accordance with the present 
invention may be provided in the base film 12 with linear portions 16 
where no carbon black is dispersed. The provision of such portions is 
preferred because the portions serve as portions that can electrically 
isolate the adjacent portions where carbon black and the ceramic 
dielectric are homogeneously dispersed. The width of the linear portions 
16 may be preferably in the range from about 0.1 to 5 mm. 
In accordance with the present invention, the composite films 1 and 11 may 
be prepared, for example, by concurrently extruding the base films 2 and 
12 and the dielectric layer or layers 3a, 3b and 13a, 13b, respectively, 
in a uniaxial or biaxial direction. The materials to be used for the 
dielectric layers may be suitably chosen such that the resulting 
dielectric layer or layers are bonded by themselves to the base film to a 
sufficient extent. Where the materials for either or both of the base film 
and the dielectric layer or layers do not adhere to each other, it is 
possible to use an adhesive; however, the use of an adhesive leads to the 
provision of an additional dielectric layer, resulting in an increase in 
an overall thickness of the composite film and giving rise to a decrease 
in the average dielectric constant of the overall dielectric layer or 
layers. The materials for the dielectric layers may be a thermoplastic 
resin identical to those enumerated hereinabove for the base film and 
preferably of the type that has a high dielectric constant as, for 
example, a relative dielectric constant of higher than about 10. Such 
resins may preferably include, for example, a fluorine-containing resin 
such as polyvinylidene fluoride, polyvinyl fluoride and 
polytrifluoroethylene. The use of the resins having a high dielectric 
constant is also advantageous for the purpose to increase an electrostatic 
capacity of the composite film. The dielectric layer or layers may contain 
a ceramic dielectric, a dispersing aid, a small amount of carbon black and 
any other additives. Among those additives, the ceramic dielectric may be 
preferably employed to increase an electrostatic capacity of the 
dielectric layer or layers, resulting in an increase of the electrostatic 
capacity of the resulting composite film. The ceramic dielectrics to be 
used for this purpose may be the same as enumerated hereinabove for the 
base film. In instances, however, where the dielectric layer or layers are 
rendered thick, the employment of finely divided particles or powders 
inadmixible with the resins, such as ceramics, may well be avoided because 
such powders will cause a decrease in a volume resistivity. Although the 
thickness of the dielectric layer is not restricted to a particular one, 
it may be generally less than about 5 microns and preferably less than 
about 3 microns. The thickness of the dielectric layer may also be 
generally about less than 2 times, preferably equal to and, more 
preferably, from about 1/2 to 1/50 times, the thickness of the base film. 
The overall thickness of the composite film may range generally from about 
2 to 200 microns and preferably from about 4 to 100 microns although it 
may be chosen in arbitrary manner. The dielectric film comprising the base 
film and the dielectric layers disposed on at least one surface thereof 
obtainable by the concurrent drawing technique may be co-drawn up to 1/10 
or less of the thickness of a film obtainable by separating extruding the 
base film and the dielectric layer or layers to form a composite film and 
then drawing the composite film together. The composite film may also be 
prepared by pressing or casting. 
Turning back to FIG. 1, for example, a ratio in thickness of the base film 
2 to the dielectric layers 3a and 3b may be arbitrarily chosen, although a 
too large ratio is not preferred because the effect to be otherwise 
expected by the composition is decreased. Generally, the thicknesses of 
the dielectric layers 3a and 3b may be each in the range preferably lower 
than 5 microns, more preferably lower than 3 microns and further below 2 
times the thickness of the base film 2, preferably below the same 
thickness thereof and, more preferably, in the range from 1/2 to 1/50. 
Although the thickness of the composite film 1 as a whole may be chosen 
within an arbitrary scope, it may be in the range generally from 2 to 200 
microns, preferably from 4 to 100 microns. 
The composite film 1 in accordance with the present invention may be 
provided at its one side or both sides with a thin electrode by means of 
deposition, sputtering or any other suitable techniques and may be used as 
a dielectric material for a film capacitor such as a laminated capacitor 
or a roll capacitor or a capacitive switch. 
Referring now to FIG. 3, a capacitor resulting from the laminated films in 
accordance with the present invention is seen to comprise the composite 
films (generally referred to as 11) in accordance with the present 
invention with the electrode (generally referred to as 17) deposed 
thereon. In FIG. 3, the thicknesses of the film 11 and, in particular, the 
electrode 17 are exaggerated for convenience in drawing the figure. In the 
capacitor as shown in FIG. 3, the Metallikon (registered trade mark) 
joints 18a and 18b are isolated from the electrodes 17a and 17b, 
respectively, so that, even when the joints are brought into contact with 
the respective side portions of the composite films 11a and 11b, there is 
no risk that circuitry between the electrodes 17a and 17b is caused to be 
formed. It is preferred to provide the linear portions 16 having no carbon 
black at portions that are in contact with the Metallikon (registered 
trade mark) joints 18a and 18b. 
Where a capacitor is formed by laminating the composite film 1 with one 
dielectric layer 3a laminated thereon, the electrostatic capacity C.sub.1 
of the base film layer 2 and the electrostatic capacity C.sub.2 of the 
dielectric layer 3a are represented respectively as follows: 
EQU C.sub.1 =.epsilon..sub.1 S/t.sub.1 (1) 
EQU C.sub.2 =.epsilon..sub.2 S/t.sub.2 (2) 
where .epsilon..sub.1 and .epsilon..sub.2 are each a dielectric constant of 
the respective layers; t.sub.1 and t.sub.2 are each a thickness thereof; 
and S is an area of the film. Thus, the whole electrostatic capacity C of 
the composite film 1 is: 
EQU 1/C=1/C.sub.1 +1/C.sub.2 =t.sub.1 /.epsilon..sub.1 S+t.sub.2 
/.epsilon..sub.2 S (3) 
Therefore, if the amount of carbon black 5 in the base film 2 would 
increase and the dielectric constant .epsilon..sub.1 of the base film 
comes up to infinity, the electrostatic capacity C becomes almost equal to 
C.sub.2, that is, C=C.sub.2. 
Where the base film layer 2 is provided at its both surfaces with the 
dielectric layers 3a and 3b, the electrostatic capacity C.sub.2 and 
C.sub.3 of the dielectric layers 3a and 3b, respectively, are likewise 
represented as follows: 
EQU 1/C=1/C.sub.1 +1/C.sub.2 +1/C.sub.3 =1/C.sub.2 +1/C.sub.3 (4) 
EQU C=C.sub.2 C.sub.3 /(C.sub.2 +C.sub.3) (5) 
If the dielectric layers 3a and 3b are formed from the identical resins and 
in the same thicknesses, C.sub.2 becomes equal to C.sub.3. Thus C=C.sub.2 
/2. 
From the foregoing, even when the dielectric constant .epsilon..sub.1 of 
the base film 2 nears to infinity and where t.sub.1 /.epsilon..sub.1 is 
substantially smaller than t.sub.2 /.epsilon..sub.2, that is, 
.epsilon..sub.2 /.epsilon..sub.1 is sufficiently small with respect to 
t.sub.2 /t.sub.1, a contribution of C.sub.1 to C becomes small. Thus, the 
effect of a thinner thickness of the dielectric layer 3a is produced to an 
appreciably great extent. For example, when t.sub.2 /t.sub.1 =1/10 and 
.epsilon..sub.2 /.epsilon..sub.1 =1/100, the first term of the right 
member in equation (3) is 1/100 of the second term of the right member. 
When .epsilon..sub.2 /.epsilon..sub.1 is 1/1,000, the first term of the 
member in equation (3) is 1/100 of the second term of the right member. 
The base film 2 comprises the thermoplastic resin layer containing carbon 
black 5 and can be oriented with the dielectric layers 3a and/or 3b formed 
on the surface or surfaces thereof so that the thicknesses of the 
dielectric layers 3a and/or 3b may be decreased to about one several 
tenths through 1/10 or less by orientation, thicknesses of the layers and 
the orientation ratio may be suitably selected. A mere concurrent 
extrusion without orientation may also give the composite film 1. 
EXAMPLES 1-15 AND COMATIVE EXAMPLES 1-17 
A mixture comprising a thermoplastic resin, a ceramic and carbon black as 
listed in Table 1 below was blended with a mixture. The thermoplastic 
resin included PVDF, PP and PE. The ceramic included BaTiO.sub.3 powders 
(Designation: Kyolix Ceramic Powder ZU-60B; Kyoritsu Yogyo Genryo K.K.; a 
relative dielectric constant at 1 KHz, 10,000; tan .delta., 2.5%) and 
titanium dioxide TiO.sub.2 (Designation: Rf-101; E. I. duPont de Nemours & 
Co.). Carbon black included one sold under designation "Neo Spectra Mark 
II" and manufactured by Columbian Carbon Nippon K.K. If necessary, a 
titanate series coupling agent (Designation: Plainact-TTS; manufactured by 
Ajinomoto K.K.) was used in the amount of 3 parts by weight per 100 parts 
by weight of the aforesaid three constituents. The resulting mixture was 
then mixed for 15 minutes with a mixing roll having a roll space of 0.1 mm 
and a roll surface temperature of 170.degree. C., and the resulting roll 
sheet was then formed into a press sheet having a thickness of 100 microns 
by passing through ferroplates at a temperature of 220.degree. C. under a 
pressure of 150 kg/cm.sup.2. The press sheet was subjected to deposition 
and provided at its both surfaces with an aluminum layer as an electrode. 
The resulting sheet was measured for its relative dielectric constant and 
tan .delta. at 25.degree. C. and 1 KHz by means of an alternating current 
bridge method. The film was then formed in a sheet having the thickness of 
200 microns, the width of 20 mm and the length of 50 mm and which was 
measured for its flexibility by bending the sheet at an angle of 
180.degree.. The results are shown in Table 1. 
EXAMPLE 16 
A biaxially drawn PVDF film having a thickness of 9 microns and a relative 
dielectric constant at 1 KHz of 10.7 was bonded to one surface of the 
press sheet of Example 4 at a press temperature of 175.degree. C. under a 
press pressure of 50 kg/cm.sup.2 in such a manner that no air was present 
between the bonded surfaces. The film was then deposited on its both 
surfaces with A1 films as electrodes by means of the vacuum deposition 
technique. The dielectric film thus prepared was formed to have a relative 
dielectric constant at 1 KHz of 110 at 25.degree. C. when measured in the 
same manner as in Example 4. Its volume resistivity was 
1.2.times.10.sup.14 ohms-cm at 25.degree. C. in one minute after a direct 
current of 100 volts were applied. The insulation breakdown in accordance 
with the Specification JIS 2138 was 110 KV/mm. 
TABLE 1 
__________________________________________________________________________ 
Characteristics 
Relative 
Composition (% by vol.) 
Coupl- 
Dielect- 
Carbon 
Kinds of 
Kinds of 
ing ric tan .delta. 
Flexi- 
Resin 
Ceramics 
Black 
Resins 
Ceramics 
Agent 
Constant 
(%) 
bility 
__________________________________________________________________________ 
Example 1 
71 25 4 PVDF BaTiO.sub.2 
None 
150 4 Good 
Example 2 
70 " 5 " " " 300 6 " 
Example 3 
69 " 6 " " " 850 7 " 
Example 4 
68 " 7 " " " 5,000 
10 " 
Example 5 
66 " 9 " " Used 
15,000 
17 " 
Example 6 
52 36 12 " " " 80,000 
31 " 
Example 7 
54 43 3 " " " 64 2 " 
Example 8 
90 3 7 " " None 
740 18 " 
Example 9 
81 5 12 " " " 40,000 
22 " 
Example 10 
78 10 14 " " " 45,000 
25 " 
Example 11 
68 25 7 PP " Used 
3,000 
15 " 
Example 12 
64 30 6 PVDF TiO.sub.2 
" 250 18 " 
Example 13 
53 46 1 " BaTiO.sub.2 
" 36 2.8 
" 
Example 14 
68 25 7 PE " " 3,000 
15 " 
Example 15 
65 30 5 PVDF " " 350 6 " 
Comparative 
100 0 0 PVDF -- None 
10 2 Good 
Example 1 
Comparative 
40 60 0 " BaTiO.sub.2 
Used 
75 2 None 
Example 2 
Comparative 
97 0 3 " -- None 
17 3 Good 
Example 3 
Comparative 
96 0 4 " -- " 23 4.5 
" 
Example 4 
Comparative 
94 0 6 " -- " 80 10 " 
Example 5 
Comparative 
93 0 7 " -- " 600 28 " 
Example 6 
Comparative 
91 0 9 " -- " 1,800 
40 " 
Example 7 
Comparative 
89 0 11 " -- " 9,600 
65 " 
Example 8 
Comparative 
80 20 0 " BaTiO.sub.2 
" 22 2 " 
Example 9 
Comparative 
75 25 0 " " " 23 2 " 
Example 10 
Comparative 
70 30 0 PVDF BaTiO.sub.2 
None 
26 2 Good 
Example 11 
Comparative 
60 40 0 " " Used 
36 2 " 
Example 12 
Comparative 
50 50 0 " " " 60 2 Poor 
Example 13 
Comparative 
75 25 0 PP " " 3 1.3 
Good 
Example 14 
Comparative 
96 0 4 " -- " 8 5 " 
Example 15 
Comparative 
70 30 0 PVDF TiO.sub.2 
" 20 3.7 
Poor 
Example 16 
Comparative 
55 45 0 " " " 27 4.1 
Poor 
Example 17 
__________________________________________________________________________ 
EXAMPLE 17 
A biaxially drawn PVDF film having a thickness of 9 microns was bonded to 
the both surfaces of the press sheet of Example 5 under the same press 
conditions as used in Example 16 to give a three-layer dielectric body. 
The dielectric body was then measured for its dielectric constant and 
volume resistance by the same methods as used in Example 16 and gave 75 
and 2.2.times.10.sup.14 ohms-cm, respectively. The insulation breakdown 
voltage thereof measured in accordance with the designation JIS 2318 was 
150 KV/mm. 
EXAMPLE 18 
The three-layer dielectric body of Example 17 was oriented in one axial 
direction three times its length at an orientation speed of 10% per minute 
to give a film having a thickness of 30 microns. The relative dielectric 
constant and the volume resistivity of the resulting film were 48 and 
2.5.times.10.sup.14 ohms-cm, respectively. 
EXAMPLE 19 
A blend was obtained by adding carbon black (furnace black) in the amount 
of 8% by volume to PVDF as a resin for the base film and then subjected to 
melt extrusion so as to disperse the carbon black uniformly to give 
pellets. 
The dielectric layer was laminated on the both surfaces of the base film 
prepared hereinabove by coextruding the PVDF pellets at a temperature of 
240.degree. C. The resulting film was then oriented in a lengthwise 
direction under drawing conditions as mentioned in Table 2 below. 
The thickness of the base film and the dielectric layers disposed on the 
both surfaces thereof after concurrent orientation were found as shown in 
Table 2 below provided that the thicknesses of the two dielectric layers 
were identical to each other. 
An aluminum layer was deposited on the both surfaces of the resulting 
sheet. The relative dielectric constant at 1 KHz and tan .delta. of the 
resulting deposited sheet were found as shown in Table 2 below. The volume 
resistivity was also measured by applying a direct current of 100 volts 
thereto. The results are shown in Table 2. In Table 2 below, and 
electrostatic capacity per unit surface area of the capacitor resulting 
therefrom is also shown. 
COMATIVE EXAMPLES 18-19 
The material for either of the base film (Comparative Example 18) and the 
dielectric layer (Comparative Example 19) as used in EXAMPLE 19 was 
extruded at 240.degree. C. and then drawn by 2.5 times the original length 
at 130.degree. C. The film was then treated in the same manner as in 
Example 19 to give to specimen. Its performance is shown in Table 2 below. 
EXAMPLE 20 
The uniaxially oriented film of Example 21-2 was oriented 4.5 times the 
original length in the lengthwise direction at a temperature of 
170.degree. C. to give a biaxially oriented film having a thickness of 
13.5 microns. The relative dielectric constant, tan .delta. and volume 
resistivity after 1 minute when a direct current of 10 volts was applied 
were 55, 0.02 and 13.times.10.sup.14 ohms-cm, respectively. 
TABLE 2 
__________________________________________________________________________ 
Thickness Volume Electrostatic 
Thickness 
of Dielec- 
Relative Resis- Capacity per 
Drawing 
Drawing 
of Base 
tric Layer 
Dielectric 
tivity Unit Surface 
Temp (.degree.C.) 
Rate (times) 
Film (.mu.) 
(.mu.) 
Constant 
tan .delta. 
(ohm-cm) 
Area (.mu.F/sq.m.) 
__________________________________________________________________________ 
Example 21-1 
140 3.0 45 2.5 103 0.021 
5.2 .times. 10.sup.13 
18 
Example 21-2 
145 2.5 48 6.0 58 0.020 
1.3 .times. 10.sup.14 
8.5 
Example 21-3 
150 2.5 42 9.0 43 0.020 
1.8 .times. 10.sup.14 
6.3 
Example 21-4 
130 2.5 30 15.0 25 0.019 
3.3 .times. 10.sup.14 
3.7 
Example 21-5 
145 3.0 17 1.7 72 0.021 
9.5 .times. 10.sup.13 
31.3 
Comparative 
130 2.5 60 -- 20,000 
0.13 
Inmeasurable 
2,950 
Example 18 
Comparative 
130 2.5 -- 60.0 11 0.015 
1.1 .times. 10.sup.15 
1.6 
Example 19 
__________________________________________________________________________ 
EXAMPLE 21 
The uniaxially oriented film was further oriented 4.6 times its original 
length in the widthwise direction at 168.degree. C. to give a biaxially 
oriented film having a thickness of 4.5 microns. 
The relative dielectric constant, tan .delta. and volume resistivity after 
1 minute when a direct current of 10 volts were applied were respectively 
70, 0.021 and 1.0.times.10.sup.14 ohms-cm. 
EXAMPLE 22 
A dielectric layer comprising a uniform dispersion of BaTiO.sub.3 in PVDF 
in the amount of 20% by volume was laminated on the both surfaces of a 
core layer comprising a base film resin used in Example 19 by concurrently 
extruding the resins at 250.degree. C. and then oriented 3.0 times its 
original length uniaxially in the lengthwise direction to give a laminated 
film having a thickness of 60 microns with each of the dielectric layers 
having a thickness of 3.3 microns. The relative dielectric constant, tan 
.delta. and volume resistivity of the resulting film after 1 minute when a 
direct current of 100 volts were applied were respectively 290, 0.027 and 
1.2.times.10.sup.13 ohms-cm. 
EXAMPLE 23 
A blend was obtained by mixing polypropylene resin as a base film resin 
with 9% by volume of carbon black and melt extruded to give pellets in 
which carbon black was dispersed uniformly in the polypropylene resin. 
Using the polypropylene resin as a dielectric layer resin, the resin was 
laminated on the both surfaces of a core layer comprising the aforesaid 
base film resin. The laminated film was concurrently extruded at a 
temperature of 250.degree. C. and then oriented in the lengthwise 
direction under orientation conditions as shown in Table 3 below. The 
thicknesses of the base film and the dielectric layers are shown in Table 
3 below provided that the thicknesses of the both dielectric layers were 
identical to each other. 
COMATIVE EXAMPLES 20-21 
Each of the base film resin and the dielectric layer resin as used in 
Example 23 was extruded at a temperature of 250.degree. C. and oriented 
7.5 times its original length at a temperature of 135.degree. C. 
On the both surfaces of the resulting film was deposited an aluminum layer 
as an electrode, and the resulting film was measured for its specific 
dielectric constant, tan .delta. and volume intrinsic resistivity after 1 
minute when a direct current of 100 volts was applied. The results are 
shown in Table 3 below. 
TABLE 3 
__________________________________________________________________________ 
Thickness Volume Electrostatic 
Thickness 
of Dielec- 
Relative Resis- Capacity per 
Drawing 
Drawing 
of Base 
tric Layer 
Dielectric 
tivity Unit Surface 
Temp (.degree.C.) 
Rate (times) 
Film (.mu.) 
(.mu.) 
Constant 
tan .delta. 
(ohm-cm) 
Area (.mu.F/sq.m.) 
__________________________________________________________________________ 
Example 23-1 
135 7.5 25 2.5 17 0.0007 
2.6 .times. 10.sup.16 
5.0 
Example 23-2 
135 7.5 12 2.5 10.5 0.0007 
3.7 .times. 10.sup.16 
5.5 
Comparative 
135 7.5 17 -- 15,000 
0.18 
Inmeasurable 
7,800 
Example 20 
Comparative 
135 7.5 -- 17 2.3 0.0006 
2.5 .times. 10.sup.17 
1.2 
Example 21 
__________________________________________________________________________ 
EXAMPLE 24 
A uniaxially oriented film was prepared in the same procedures as in 
Example 21-3 with the exception that the base film resin and the 
dielectric layer resin of Example 34-3 were concurrently extruded so as to 
provide 0.5 mm wide portions having no carbon black at a distance of 10 mm 
in the widthwise direction. The relative dielectric constant, tan .delta. 
and volume resistivity of the resulting film were respectively 40, 0.02 
and 1.8.times.10.sup.14 ohms-cm. The resistivity was higher than 10.sup.15 
ohms at a width of 40 mm and a length of 40 mm, while that of the portions 
containing no carbon black was 380 kiloohms. 
As have been known from the foregoing examples, the dielectric film in 
accordance with the present invention can possess flexibility by choosing 
the composition and kind of raw materials appropriately. For example, 
where a large area is required such as an electric wave absorber or where 
an electrostatic capacitive switch is subject to a stress whenever it is 
applied giving rise to a transformation, a flexibility is required for the 
prevention from breakdown. A film which is used by transformation such as 
a roll capacitor requires flexibility; otherwise breakdown is caused upon 
molding. As have been set forth hereinabove, flexibility becomes an 
important property, and the dielectric film according to the present 
invention can be used for these purposes. 
The composite film in accordance with the present invention possesses 
greatly improved volume intrinsic resistivity and tan .delta. when 
compared to one resulting from a thermoplastic resin containing conductive 
fine powders. The dielectric constant and the electrostatic capacity per 
unit area of the composite film according to the present invention are 
improved to a remarkable extent as compared by the corresponding 
dielectric layer alone. Accordingly, the composite film in accordance with 
the present invention may be particularly suitable for film capacitors and 
the like. 
As have been set forth hereinabove, one embodiment in accordance with the 
present invention comprises the base film comprising the thermoplastic 
resin containing the conductive fine particles provided at least one 
surface thereof with the dielectric layer comprising the thermoplastic 
resin. Accordingly, the present invention can greatly improve an 
electrostatic capacity per unit surface area with a thickness which 
provides a favorable processability for manufacture.