DE1704762B2 - Use of a vinylidene fluoride polymer film as a dielectric - Google Patents

Description

In products such as capacitors, many different substances have been used as dielectrics in a sheet-like form together with paper as the base material. With the growing need to manufacture smaller capacitors and components, synthetic resin foils of various types have spread in recent years. The volume ν per capacitance unit, which is a measure of the spatial size of a capacitor, can be expressed by the following equation:

(d + t) d

[mnr /, ix

35

It is

d is the thickness of the foil between the electrodes in millimeters,

K equals 3.6 ■ .τ 10 \
f 'is the dielectric constant of the foil,
t is the thickness of each electrode in millimeters.

It is known that polyvinylidene fluoride films, called PVDF films for short, in comparison with others Synthetic resin foils have a high dielectric constant. It is also known that polyvinyl fluoride films also have a high dielectric constant, which is essentially the same as the dielectric constant of PVDF foils.

Stretched films made from vinylidene fluoride homopolymers are already known from US Pat. No. 3,197,538. With them, the stretching is used, the mechanical Properties to improve. It is also from the reference Kolloid-Zeitschrift 153, 5 to 27 (1957), known that in films made of high polymer plastics, the dielectric loss factor due to stretching can be changed.

The object of the invention is to create an even better dielectric in the form of polyvinylidene films, In addition to an increased dielectric constant, this also means a reduced loss factor having.

This object is achieved by the invention specified in claim 1.

It has been found that when stretching an unoriented film made of a PVDF polymer in at least one direction the dielectric constant of the stretched film is greater. The stretched one The film therefore has a higher dielectric constant than the known PVDF films.

It has also been found that the dielectric loss factor or dielectric Loss angle of a film oriented by stretching is considerably improved and therefore essential is lower than that of an unoriented film. The loss factor is further reduced by heat treatment the stretched film improved. During the heat treatment of the film, it is under an effective state of tension in its direction of orientation to keep. Since it is possible to make the foils thin, they play a major role in reducing the volume of capacitors and similar components.

The PVDF films used according to the invention meet the requirement for small components to a much greater extent than the known PVDF films or polyvinyl fluoride films as well as other synthetic resin films because, in addition to their low thickness, they have a much higher dielectric constant to have.

Since the loss factor continues in the erfindungsgeinäß PVDF films used is considerably less than in the case of the known films, that according to the invention used. Foil is of great practical importance for the manufacture of capacitors measured. The PVDF film used according to the invention can, however, like other dielectric films Substances can also be used for other purposes, for example as a luminescent substance.

It has been found that the dielectric constant of PVDF films used according to the invention is 10 or more and that the loss factor is almost 1% or less.

For the production of the polyvinylidene fluorides suitable for the films used according to the invention a resin is used which is obtained by a known method by polymerizing vinylidene fluoride is made with itself or with a mixture of monomers, 80 weight percent or more vinylidene fluoride contains and is able to form copolymers. A polyvinylidene fluoride that by polymerizing vinylidene fluoride at a temperature below the critical temperature is, is produced, is particularly suitable for the purposes of the invention, since it is a molecular May assume arrangement of greater regularity.

Any known method can be used to produce the unoriented PVDF film, such as the casting process, melt spraying process or calendering process. This is the polyvinylidene fluoride in the presence or absence of solvent, it is formed in a film-like or tubular shape.

The films produced in this way generally have an unoriented structure in which one Can recognize spherulites with the help of a microscope. However, at least they are conceptually infinite small spherulites present as seen under an electron microscope. In all cases became an arbitrary molecular orientation regardless of whether spherulites could be seen in the structure or not detected in unoriented foils by x-ray reflection.

When making these unoriented PVDF films a partial orientation is caused in some cases by some processes at the Manufacturing process, for example by stretching when lifting the film after melt casting and before complete hardening or by applying pressure by means of rollers or rollers hardening. However, these foils do not have a specific directional property, as can be seen by X-ray reflection can prove, and can still be viewed as unoriented slides.

The films used according to the invention are thereby made from essentially unoriented films made that these films are stretched 150% or more in at least one direction. For the In the event that further stretching or shrinking takes place after the first stretching process by heating, should the finally stretched film also be at least 150% in relation to the length of the original, non-oriented slide may have been extended. With a film stretched in this way, it is possible to use a certain arrangement of the molecular chains in at least one direction by X-ray reflection to prove.

The measurement results of the dielectric constant and the dissipation factor made at room temperature of various unoriented PVDF films and of various, oriented by stretching, PVDF films used according to the invention are compiled in Table 1. These measurements were made with a transformer bridge apparatus (manufactured by Ando Denki Co., Japan). Aluminum electrodes vapor-deposited under vacuum were used as measuring electrodes.

As a PVDF resin were used two kinds of resins, namely, resin A, which was prepared by emulsion polymerization at high temperature and at the resolution of 0.4 g of resin in 100 cm ³ of dimethylformamide at 30 ⁰ C has an intrinsic viscosity '/, " , of 1.7, as well as resin B, which was prepared by suspension polymerization at a temperature which was below the critical temperature and has an intrinsic viscosity i _{/ inh} of 1.00.

The unoriented film was prepared by flowing a 20% solution of PVDF in dimethylformamide over a glass plate, gradually increasing the temperature of the solution by exposure to an infrared lamp to evaporate the solvent, the remaining PVDF to a final temperature of 190 to 200 ^G C to melt the PVDF completely, and then quenched the melted PVDF with water to a temperature of 20 ⁰ C or the PVDF in an atmosphere gradually or gradually cooled to room temperature.

The films produced by gradual or gradual cooling were, were disoriented with a structure in which the spherulites were visible to the naked eye or with a hoof optical microscope were visible. The foils made by quenching were also disoriented, but no spherulites could be detected with an optical microscope.

The oriented PVDF films were produced by first placing PVDF powder between two plates with chrome-plated surfaces. The powder enclosed between the plates was then ^{pressed at a temperature of 230 °} C. and a pressure of 100 kg / cm ^{2 for} 5 minutes, and the pressed material was quenched in water at 5 ° C. or gradually cooled to room temperature in air. The film thus obtained was then stretched in one direction to four times its length.

Table 1

Comparison between the dielectric constant (/ ') and the loss factors (tan Λ) of unoriented and oriented PVDF resins

characteristic

Frequency (Hz)
30th

ι 'tan fi

60

120 1 (XH)

10 0 (X)

Foil thickness

tan Λ

Unoriented slide
A, chilled quickly

A, gradually chilled

B, chilled quickly

B, gradually cooled

Commercially available

PVDF film *)

106
10.5
12.2
11.1

7.9
7.7
7.1
6.9

10.3
10.2
11.8
11.1

6.0 6.2

5.3 5.1

9.9 10.0 11.6 11.0 tan Λ

5.0 4.8 4.3

3.7

9.5

9.6

11.2

10.7

tan Λ

3.3
2.2
2.0
1.6

9.1

9.4

11.0

10.6

tan A

2.7 2.0 1.8 1.5

10.0 6.6 9.6 5.5 9.4 4.2 9.2 2.0 8.9 1.9

Folic made from unoriented preproduct, stretched in one direction

A, fast chilled 13.4 2.3 - - 13.3 1.2 12.9 1.4 12.6 2.5

A, gradually chilled 13.0 2.3 - - 12.8 1.8 12.6 1.6 12.1 2.5

B, quick chilled 14.7 1.3 14.6 1.3 14.5 1.3 14.3 1.7 14.1 2.2 B, gradually chilled 14.3 2.4 14.1 1.9 13.9 1.6 13.6 1.4 13.3 2.7

*) PVDF film (/ ,, ",, = 1.7). manufactured by Pensalt Chemical Co .. V.Sl.A. It is believed that the film is produced by emulsion polymerization was produced at a temperature of 100 C or more.

(μηι)

30 30 30 30

48

30 30 30 30

The measurement results shown in Table I show very clearly that stretched films have a higher dielectric constant than unoriented films, namely in all comparative measurements. It is also clear from the table that a significant reduction of the loss factor occurs during stretching at the same time with the increase of the dielectric constant, namely be <a frequency which is equal to or less than 1 kHz.

In all cases, the PVDF resins polymerized at a low temperature have a larger one Dielectric constant than the resins polymerized at a higher temperature. One can do this due to the greater regularity in the crystalline structure of those polymerized at a lower temperature Recycle PVDF resins.

While the stretched oriented PVDF films have a much higher dielectric constant and a lower loss factor in the low frequency range, it has also been found that the loss factor of these stretched oriented PVDF films can be further improved by means of a post-stretching heat treatment. It has been found through numerous tests that it is possible, by means of a precisely monitored heat treatment, to reduce the loss factor tan A to about 0.5% without the high dielectric constant of these PVDF films decreasing.

The films oriented by stretching are put under tension again in their direction of orientation and heat-treated at a temperature between 125 and 175.degree. C., preferably 135 and 170.degree. The loss factor tan 6 of these foils improves from the range 1.0 to 1.5% to a range between 0.5 and 1.0%. A film treated in this way is preferably quenched immediately after the heat treatment, for example with water, as a result of which the improvement in the loss factor tan Λ is even more pronounced.

Under normal conditions, the heat capacity of the film during heat treatment is very low, because the film is very thin. Therefore, when the film is taken out of the heated room, it cools the cool ambient air was very snowy. Under certain Conditions, for example in a manufacturing process, are the surrounding atmosphere not always cool air. The cooling effect after the heat treatment can therefore not be achieved in all cases be ensured, and it is then necessary to use air, water or by rapid cooling make cool surfaces on which the film rolls off.

The oriented structure of the molecular PVDF chains helps to improve the loss factor tan Λ at. This is done by heat treating the film under effective tension performed. The electrical properties attributable to the oriented structure of the film the faster the cooling is carried out after the heat treatment, the more it can be improved will.

ίο To improve the loss factor are the conditions in the heat treatment that takes place after stretching, i.e. the temperature and the effective tension, extremely important. The expression "effective tension" is not necessarily intended to mean be understood that the film is already under tension at the start of the heat treatment. A shrinkage of the film which is within the shrinkage percentage at the heat treatment temperature of the film, is prevented by a stress during the heat treatment, namely by a low one Holding voltage.

This tension can also stretch the film a little. There is one through this tension

elicited elongation by more than 10% with no significant effect. Preferably the tension chosen so that the shrinkage or stretching of the film is less than 5%. If the film in the heat treatment with respect to its original length after stretching by less than 5% contracted or lengthened, then the loss factor tan Λ decreased to a range between 0.5 and 1.0%. On the other hand, if the elongation or shrinkage was more than 10%, then it was none

Improvement of the loss factor noted.

Further, it has been found that a heat treatment temperature within a temperature range in which considerable recrystallization of the PVDF takes place is extremely effective in improving the loss factor. The recrystallization temperature of oriented PVDF films is approximately between 125 and 175 "C. At a temperature of 120 C or below, the improvement in the loss factor tan δ due to the oriented structure of the PVDF molecule chains was very small. On the other hand, the oriented structure was lost a temperature of more than 175 ^° C., as a result of which the dielectric properties, which had been improved by stretching, were also essentially lost.

The substances produced by the process described are illustrated below with the aid of examples to be discribed.

example 1

A polyvinylidene fluoride with "/," ,, voi. 1.00 that through Suspension polymerization had been prepared at a polymerization temperature of 25 ° C Injected in film form by means of an injection mold at a temperature between 230 and 240 ° C. While the PVDF was still in the molten state, it was in contact with rollers heated to 80 ° C brought and cooled very quickly. The unoriented film obtained in this way was at 150''C in its longitudinal direction to 3.5 times (250%) stretched to their original length, resulting in a monoaxially or one-dimensional stretched film.

The ones measured at different frequencies

6j values for the dielectric constant and the loss factor of the unoriented PVDF film and the Monoaxially stretched, oriented PVDF films are shown in Table 2 below.

Table 2

characteristic frequency (Hz) 60 lan Λ M) '' ("/» I lan Λ 4.8 .%, 12.0 Unoriented slide 12.3 6.1 Oriented by stretching

120

KK) O

3.7

tan Λ

K) (HK)

film thickness

WmI

11.5 1.7 11.2 1.8 74

13.9 1.7 13.7 1.7 13.7 1.6 13.5 1.3 13.2 2.4 25

Example 2

Unoriented PVDF film formed in the same biaxially or two-dimensionally oriented film

Way as described in Example 1 was prepared.

was, at 15O ⁰ C in its longitudinal direction around the In Table 3 the measured values for the dielectric

3.5 times (250%) its original length stretched, 20 constant of energy and loss factor at different

a monoaxially stretched film was produced. Frequencies from the unoriented to the monoaxial

This monoaxially stretched film was then used for oriented and biaxially oriented PVDF films

60 'C shown at right angles to the first stretching direction by 3.4 times, their original dimensions stretched so that one

Table 3

characteristic l-frequency
30th (Hz) 60 tan Λ 120 tan Λ KKX) lan / 1 10 (XX) tan Λ Foils
strength tan Λ '' 4.5
1.3
1.1 '' 3.8
1.5
1.3 I. 1.9
1.7
1.5 / * 1.9
2.3
3.2 (<< m) Unoriented slide
Monoaxially oriented
foil
Biaxially oriented film 11.8
13.3
13.2 5.8
1.5
0.9 11.7
13.2
13.1 11.5
13.1
13.0 11.3
12.8
12.8 10.0
12.7
12.7 12th
3

Example 3

A PVDF resin with a _linh of 1.00. which had been produced by suspension polymerization was cast by the melt-casting method and then cooled with water, whereby an unoriented film was formed, which was then ^{stretched at 150 °} C. in its longitudinal direction by four times its original length. Several such films have been made with different stretch ratios and at different temperatures.

Table 4

The loss factor tan 6 (%) was dependent on the relevant manufacturing conditions. In all cases, however, the loss factor was in a range between 1.0 and 1.5%.

The dielectric constant and dissipation factor of these oriented films are none after stretching Have been subjected to heat treatment have been measured at different frequencies. The measurement results are shown in Table 4.

Own
shaft tan Λ. (' 120Hz 1 kHz Dielectric
· constant ' 120Hz 1 kHz 1.27 1.31 13.0 118 frequency 1.01 1.13 60Hz 115 114 Stretched
Foils
sample
I. M) Hz 1.47 1.31 13.1 113 111 II 1.31 1.07 1.45 116 14.0 13.7 IH 1.08 1.03 1.04 114 14.2 14.1 IV 1.36 14.0 V 1.00 14.4 1.11

The sample labeled I in Table 4 was ^{heat-treated in an oven at 140 °} C., the film being exposed to an effective tensile stress which allowed a shrinkage of 5% in the direction of orientation. The film was cooled with water after the heat treatment.

Table 5

10

Other foils were made in a similar manner, but with different heat treatment times. The loss factor tan δ and the dielectric constant / of the foils produced in this way are listed in Table 5.

characteristic

Treatment line (min. |

I 3

lan Λ / 'lan Λ

IO

tan Λ

15th
lan λ

frequency 0.60 13.8 0.55 12.9 0.67 13.1 0.50 12.7 60Hz 0.72 13.7 0.80 12.8 0.61 13.1 0.71 12.6 120Hz 1.02 13.6 0.96 12.7 0.96 13.0 0.97 12.5 1 kHz

Example 4

The stretched film designated II in Table 4 was ^{heat-treated in an oven at 160 °} C. and was exposed to an effective tension by which the film was stretched by 5% in its direction of orientation during the treatment. After the heat treatment, the film was cooled with water.

Other foils were made in a similar manner, but with different heat treatment times. Dei Dissipation factor and dielectric constant of these films were then measured and are in Table 6 shown.

Table 6 Treatment time (Mini 3 12.9 10 , · 15th 12.4 characteristic ] tan Λ 12.7 tan A tan Λ 12.3 lan Λ 1%) 12.6 (%) (% l 12.2 1%) 12.8 0.76 0.56 12.6 0.84 frequency 0.69 13.5 1.06 0.79 12.5 1.16 60Hz 0.82 13.4 1.15 1.02 1.16 120Hz 1.04 13.3 1 kHz

Example 5

The stretched film designated III in Table 4 was heat-treated in an oven at 150 ° C. and exposed to an effective tension which causes the film to shrink by 2% in its orientation allowed. After the heat treatment, the film was cooled with water. Similar films were also used produced in an analogous manner, but with different heat treatment times. The loss factor and the Dielectric constants of these films are shown in Table 7

Table 7

characteristic Treatment time (min I. 12.1 3 13.2 7th 12.2 IO 12.0 1 12.0 tan Λ 13.1 tan Λ 12.1 tan ο 11.9 tan A 11.9 12.9 12.0 11.8 frequency 1.09 Example 6 0.86 0.6? 6OHz 0.65 1.34 1.03 1.02 120Hz 1.15 1.25 1.08 1.06 1 kHz 1.11

PVDF films. manufactured in the same manner ₆₅ lung were suspended, to the 4 ihrei 8 times as those mentioned rnonoaxial oriented original length were stretched in its transverse direction

whereby biaxially oriented films were created. This <

Samples and those four times their original

Were stretched for a long time, but no heat-treated foils were then at various heat levels

exposure times to a temperature of 170 ° C and thereby lengthened by 5% in their longitudinal direction and shrink by 5% in their transverse direction calmly.

Table 8

After the heat treatment, the films were cooled with water. The dissipation factor and the dielectric constant these films were measured and are shown in Table 8.

Samples: Not treated samples, tan Λ biaxial Heat treated samples 12.6 with treatment times of: r 5 minutes 12.4 oriented slide (%) 12.5 tf-m Λ 12.3 1 minute 12.3 3 minutes (%) 12.1 characteristic 1.18 lan A tan h 12.5 1.27 (%) 1%) 12.4 0.87 frequency 1.53 12.2 1.22 60Hz 12.3 0.79 0.86 1.34 120Hz 12.2 0.93 0.90 1 kHz 12.0 1.29 1.40

As seen from the above examples, it is possible by heat treatment of an oriented PVDF film at a temperature of 125-175 ° C, preferably 135-170 ⁰ C, and at the same time stretch or shrinkage of 5% or less, the loss factor tan 6 of the film in the nominal frequency range to 0.5 to 1.0%.

On the other hand, if one allows an elongation or shrinkage of 10%, then one cannot see any improvement in the loss factor. Likewise, no improvement in the loss factor was demonstrated at a heat treatment ^{temperature of 120 ° C.} At a heat treatment ^{temperature of 180 °} C., the quality of the film was impaired.

The following comparative examples serve to show the extreme conditions.

Comparative example 1

Stretched films identical to those indicated by V in Table 4 were subjected to a heat treatment in an oven at 160 ° C. for 5 minutes and at the same time subjected to a tension causing an elongation of 12% or kept in a completely relaxed state so that the film could shrink unaffected. The films heat-treated in this way were then cooled with water. The loss factor tan d and the dielectric constant e 'of these films were measured and are shown in Table 9. The percent shrinkage when the shrinkage was unaffected was about 8%.

Table 9

characteristic

foil

stretched by 12%,
heat treated foil

tan Λ

relaxed,

heat treated

foil

tan fi ι '

frequency
60Hz

1.12

14.0

1.10

Comparative example 2

Stretched films which were identical to the films designated IV in Table 4 of Example 1, were heat-treated in an oven at 120 and 180 ° C for 5 minutes and a tension at the same time exposed, which allowed a shrinkage of 5%. The foils were after the heat treatment with Water cooled.

The loss factor tan f> and the dielectric constant; ■ 'were measured and are shown in Table IC.

Table 10

characteristic

foil

film heat-treated with 120 ° C

tan Λ Γ

heat-treated at 180 ° C foil

tan Λ Ζ

frequency
60Hz

1.10

In the case of heat treatment at 180'C the film became slightly opaque and began to melt in several places on the surface. The original The appearance of the film was severely affected. No measurements were therefore made took.

Claims (2)

Godfather tanmriiche: 1. Use of a film made from a vinylidene fluoride homopolymer or copolymer with more than 80 percent by weight vinylidene fluoride, which ^{is heat-treated by stretching at least 150% in at least one direction above room temperature and in an oriented shape between 125 and 175 °} C. while maintaining such a state of tension, that it could not lengthen or shorten more than 10% in any of the stretching directions as a dielectric. 2. Use according to claim 1, characterized in that that the oriented polyvinylidene fluoride film has a thickness of 30 micrometers or owns less.