Polymer dielectric

Abstract: A copolymer of 20 to 33 mol.% vinylidene fluoride, 15 to 45 mol.% of ethylene and 30 to 65 mol.% of tetrafluoroethylene gives a dielectric material having a large dielectric constant of 4.0 or above and a small dielectric dissipation factor (tan δ) of 0.8% or below, which is adapted for a condenser film.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a vinylidene fluoride-based polymer dielectric having a high dielectric constant and a low dielectric dissipation factor combination. Description of the Prior Art Polymer dielectrics are widely used, for example, as materials for electric capacitors or condensers mainly of small sizes because they can be easily formed into films. Polymer dielectric materials well known heretofore include polyethylenes, polyethylene terephthalates (hereinafter abbreviated as "PET"), polycarbonates, and polystyrenes. Among these, PET has both a relatively low dissipation factor (hereinafter referred to as "tan δ") of 0.6 % at 20 °C and 1 kilo-hertz (KHz) and an excellent film-forming property, and is widely used as a useful polymer dielectric material. However, while PET has a low tan δ, it also has a small dielectric constant of 3.2 at 20 °C and 1 KHz. Moreover, PET is relatively hygroscopic and therefore has a drawback that its dielectric properties vary according to change in enviromental conditions. On the other hand, polyvinylidene fluoride (hereinafter abbreviated as "PVDF") has the highest level of dielectric constant among the synthetic polymers, has an excellent shapability or processability and also has a low hygroscopicity which is an advantageous feature as a dielectric material when compared with PET. However, this PVDF has a tan δ of 1.0 % or above which is larger than 0.6 % of PET and is not desirable as a material for an usage requiring small dielectric loss. SUMMARY OF THE INVENTION A principal object of the invention is, in view of the above circumstances, to provide a vinylidene fluoride-based polymer dielectric having a low tan δ comparable with that of PET and also having a good shapability or processability, while retaining a dielectric constant higher than that of PET. As a result of our researches, it has been discovered that the above object can be accomplished by a vinylidene fluoride-ethylene-tetrafluoroethylene copolymer formulated in specific component proportions. The polymer dielectric of the present invention is provided based on the above knowledge and, more specifically, comprises a copolymer of 20 to 33 mol.% of vinylidene fluoride, 15 to 45 mol.% of ethylene and 30 to 65 mol.% of tetrafluoroethylene, and has a dielectric constant of 4.0 or larger and a dielectric dissipation factor (tan δ) of 0.8% or less at 20°C and 1 KHz and which is obtainable by a process of suspension polymerization in water or of bulk polymerization, employing a peroxide polymerization initiator or an azo-type polymerization initiator. In the meantime, a vinylidene fluoride-ethylenetetrafluoroethylene terpolymer per se has been known as a kind of tetrafluoroethylene-ethylene-fluoroethylene copolymer (as disclosed in Japanese Patent Publication No. 25415/1973). The series of copolymers was simply developed as a class of heat-resistant copolymers with good shapability, and the vinylidene fluoride-ethylenetetrafluoroethylene terpolymer as disclosed therein has not been recognized at all as a useful dielectric material. Especially, it has been first discovered by us that the terpolymer satisfying the proportions specified by the present invention is a dielectric material provided with both a high dielectric constant and a low tan δ. Further, when a copolymer is produced through solution polymerization as disclosed in the above publication, the resultant copolymer is caused to have a low molecular weight, has a worse shapability into film and fails to give a lower tan δ as desired (See Comparison Example appearing hereinafter). These and other objects and features of the present invention will be more readily apparent in view of the following detailed description and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Figure 1 and Figure 2 are graphs respectively showing the values of dielectric constant and dielectric dissipation factor (tan δ) plotted versus the varying temperature measured at 30 Hz with respect to a terpolymer film obtained in Example 2 appearing hereinafter. DETAILED DESCRIPTION OF THE INVENTION The copolymer constituting the dielectric of the invention contains three components, vinylidene fluoride, ethylene and tetrafluoroethylene as indispensable components, and any two of them fail to give the desired properties. Thus, vinylidene fluoride and ethylene have poor copolymerizability with each other, and a copolymer with a high dielectric constant and low tan δ has not yet been obtained from the combination of these. While copolymers of vinylidene fluoride and tetrafluoroethylene have been widely known, they are caused to have extremely poor shapability and it is difficult to shape them into films adapted for condenser materials if the content of tetrafluoroethylene is large. In contrast thereto, if the vinylidene fluoride content is large, the resultant vinylidene fluoride-tetrafluoroethylene copolymer will have properties similar to those of PVDF having a high dielectric constant but it is also very difficult to lower the tan δ of the copolylmer. Further, while a copolymer of ethylene and tetrafluoroethylene is also known as a heat-resistant resin with good shapability and has a low tan δ, it has a small dielectric constant. In order to obtain the desired dielectric properties in combination, it is very important to satisfy the compositional proportions among the three components. Thus, only a copolymer having a limited composition of 20 to 33 mol.% of vinylidene fluoride, 15 to 45 mol.% of ethylene and 30 to 65 mol.% of tetrafluoroethylene, is used in the invention. If the content of vinylidene fluoride in the product copolymer is less than 20 mol.%, the copolymer is caused to have a dielectric constant below 4.0. In contrast thereto, if the vinylidene fluoride content exceeds 33 mol.%,.the copolymer, while having a large dielectric constant, is caused to have a tan δ of above 0.8 % and fails to give the required dielectric properties in combination. When the ethylene content is less than 15 mol.%, the contents of vinylidene fluoride and tetrafluoroethylene become relatively larger, whereby the resultant copolymer becomes poor in shapability so that it is difficult to shape it into films, and, moreover the tan δ thereof increases. In contrast, when the ethylene content is above 45 %, the copolymerizability of the composition becomes poor, whereby the production of the copolymer becomes difficult. Next, when the tetrafluoroethylene content is below 30 mol.%, the contents of ethylene and vinylidene fluoride having poor copolymerizability with each other become large, and it is difficult to produce a desired random copolymer. When above 65 mol.%, the product copolymer will have good heat-resistance but have extremely poor shapability, whereby it is difficult to form thin films therefrom. While the polymer dielectric of the invention comprises three components of vinylidene fluoride, ethylene and tetrafluoroethylene as indispensable components thereof as described above, it can also contain a minor amount, e.g. 5 mol.% or less thereof, of a monomer copolymerizable with the above three components, with the proviso that the resultant copolymer satisfies the required dielectric properties, that is, a dielectric constant of 4.0 or above and a tan δ of 0.8 % or below at 20 °C and 1 KHz. Examples of such copolymerizable monomers include propylene, isobutylene, vinyl fluoride, trifluoroethylene, trifluoromonochloroethylene and hexafluoropropylene. The copolymer constituting the polymer dielectric of the invention is obtainable by a suspension polymerization process using water as the dispersion medium or a bulk polymerization process, both using as polymerization initiator a peroxide initiator or an azo-type initiator. On the contrary, by a solution or suspension polymerization using an organic solvent as dispersion medium, it is difficult to produce a copolymer having a sufficiently large degree of polymerization, and the resultant copolymer not only is poor in physical properties but also fails to give desired dielectric properties, especially a low tan δ. The polymer dielectric of the present invention also has a good shapability and is readily shaped into a dielectric in the form of film or sheet through heat-forming processes. As the heat forming processes for the production of films or sheets, known forming processes are generally available, including those using hot presses, extruders or calendering rolls. In order to obtain an especially thin film, it is possible to further subject the thus obtained film or sheet to roll pressing or mono-or multi-axial stretching. Particularly, by the stretching, some improvement in dielectric properties can be attained along with the formation of thin films. As has been described hereinabove, the present invention provides a vinylidene fluoride-ethylenetetrafluoroethylene copolymer of a specific composition, which has a dielectric constant larger than that of PET and a dielectric dissipation factor (tan δ) substantially equal to that of PET and also is excellent in anti-hygroscopicity, heat resistance and shapability. Accordingly, the dielectric is especially useful as a material for electric condenser films for which a high dielectric constant and a low dielectric loss are required in combination. When used as a condenser material, the polymer dielectric of the invention is preferably formed into films each of 1 - 200 microns (micrometres), particularly stretched films each of 1 - 100 microns (micrometers) in thickness. Furthermore, these films may be used as a condenser film laminate composed of a plurality of films stacked in layers. In this case, each film layer should preferably have a thickness of 1 to 50 microns (micrometres). The present invention will be explained more specifically hereinbelow by referring to actual examples of experiments and comparison examples. In the following description, the dielectric constants and tan δ represent values obtained by measurement at 20 °C and 1 KHz unless otherwise noted. Example 1 Into a 1 liter-autoclave provided with a stirrer were charged 580 g of distilled water, 0.2 g of methyl-cellulose (suspension aid) and 4.0 g of n-propylperoxydicarbonate. After replacement with nitrogen, 48.6 g (25 mol.%) of vinylidene fluoride, 29.8 g (35 mol.% of ethylene) and 121.6 g (40 mol.%) of tetrafluoroethylene were further charged, and the mixture was subjected to polymerization under stirring at 30 °C. When the pressure was lowered to 9 kg/cm² (882 kPa), well degassed distilled water was charged into the system under nitrogen pressure so as not to lower the pressure below 10 kg/cm² (980 kPa), and the polymerization was continued to obtain a terpolymer. Thus, a copolymer having a composition substantially the same as the charged composition was obtained at a yield of above 98 %. The thus obtained copolymer was a resin excellent in heat resistance and having a melting point of 254 °C and was readily formed into a sheet having a thickness of 150 microns (micrometres) by hot pressing. The pressed sheet had dielectric properties as shown below: Thus, the terpolymer obtained was found to be a useful dielectric material having a large dielectric constant and a relatively small tan δ. Example 2 The terpolymer sheet obtained in Example 1 was stretched in a ratio of 4 times at 100 °C to obtain a film of 70 microns (micrometres) in thickness. Thus, the terpolymer was found to be rich in shapability and readily stretchable. The stretched film had a dielectric constant of 5.15 and a tan δ of 0.62 % at 20 °C and 1 KHz. The variation with temperature of dielectric constant and tan δ of the stretched film was measured at 30 Hz by a rheolograph (mfd. by Toyo Seiki K.K.). The results of the measurement are shown in Figures 1 and 2, respectively. Thus, the terpolymer of the invention has a small temperature dependence of the dielectric constant and tan δ within a practical temperature range and is found to be a useful dielectric also from this view point. Example 3 In a manner similar to that of Example 1, a terpolymer having mole ratios of vinylidene fluoride/ethylene/tetrafluoroethylene of 30/25/45 was produced. The thus obtained copolymer had a melting point of 258 °C and showed the following dielectric properties when formed into a pressed film of 75 microns (micrometres) in thickness: Dielectric constant (ε′): 5.2 tan δ: 0.72 % Comparison Example 1 A terpolymer having the same composition as that of Example 3 was produced through suspension polymerization in a perhalogenated solvent of 1,1,2-trichloro-1,2,2-trifluoroethane. After being fully washed and dried, the copolymer was formed into a pressed film of 75 microns (micrometres) in thickness, which showed the following dielectric properties: Dielectric constant (ε′): 5.3 tan δ : 1.02 % The thus obtained film not only was fragile and poor in physical properties but also had a large tan δ. Comparison Example 2 In a manner similar to that of Example 1, a terpolymer having mol.% ratios of vinylidene fluoride/ethylene/tetrafluoroethylene of 45/10/45 was produced. The copolymer had a melting point of 271 °C, a fluorine content of 67 mol.% and apparently inferior shapability. The copolymer was formed into a sheet of 200 microns (micrometres) thickness, which showed the following dielectric properties: Dielectric constant (ε′): 5.2 tan δ : 0.94 % Thus, it will be understood that a terpolymer having too small an ethylene content not only is poor in shapability but also fails to give the desired dielectric properties because of a large tan δ.

Claims: A capacitor dielectric film comprising a fluorine-containing copolymer having a composition of 20 to 33 mol.% of vinylidene fluoride, 15 to 45 mol.% of ethylene and 30 to 65 mol.% of tetrafluoroethylene, and having a dielectric constant of 4.0 or above and a dielectric dissipation factor of 0.8% or below at 20°C and 1 KHz, and which is obtainable by a process of suspension polymerization in water or of bulk polymerization, employing a peroxide polymerization initiator or an azo-type polymerization initiator. A capacitor dielectric film as claimed in Claim 1 in which the polymerization initiator used is a peroxide initiator. A capacitor dielectric film, as claimed in Claim 1 in which the polymerization initiator used is an azo-type initiator. A capacitor dielectric film as claimed in any of Claims 1 to 3, which is in the form of a film having a thickness of 1 to 200 microns (micrometres). A capacitor dielectric film as claimed in Claim 4, which is in the form of a stretched film having a thickness of 1 to 100 microns (micrometres). A capacitor dielectric comprising a dielectric film laminate having a plurality of dielectric films stacked in layers, each dielectric film comprising a copolymer of 20 to 33 mol.% of vinylidene fluoride, 15 to 45 mol.% of ethylene and 30 to 65 mol.% of tetrafluoethylene, and having a dielectric constant of 4.0 or above and a dielectric dissipation factor of 0.8% or below at 20°C and 1 KHz, and which is obtainable by a process of suspension polymerization in water or of bulk polymerization, employing a peroxide polymerization initiator or an azo-type polymerization initiator. A capacitor dielectric as claimed in Claim 6, wherein each film has a thickness of 1 to 50 microns (micrometres).

Inventors: ARAKAWA NORIYUKI, KAKUTANI HARUKO, SAKAGAMI TERUO, ARAKAWA, NORIYUKI, KAKUTANI, HARUKO, SAKAGAMI, TERUO
Applicants: KUREHA CHEMICAL IND CO LTD, KUREHA KAGAKU KOGYO KABUSHIKI KAISHA
Application year: 1984
Main classification: H01B3
IPC codes: H01G4, C08F214, C08F14, H01B3