JPS639323B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a polyolefin film for electrical insulation, and particularly to a method for producing a polyolefin film for low loss electrical insulation suitable for ultra-super high voltage OF cables. More specifically, the present invention relates to a method for producing an electrically insulating polyolefin film made of 100% low dielectric loss plastic. Traditionally, kraft paper has been used exclusively as electrical insulating paper for OF cables, etc. in times when there was no need to increase the power transmission capacity of cables.
However, recently, as the demand for electric power has increased around large cities, it has become necessary to transmit ultra-high voltage and large capacity power through a single power transport route. The issues that need to be considered at this time are:
The first problem is that because there is a limit to the finished outer diameter of the cable due to the conduit, etc., it is not possible to increase the number of insulating layers without limit even if the power transmission voltage increases. For this reason, an insulating material with high withstand voltage per unit thickness is required. At present, environmental conditions make it difficult to construct large-capacity power plants near large cities, and this means that electricity will be transmitted over long distances from remote areas to cities. In order to achieve such long-distance, large-capacity power transmission, it is necessary to suppress the dielectric loss of the insulator and keep the heat generation of the cable low. In order to achieve this, it is an advantageous condition to use a so-called low loss material having a small ε and tan δ value. Furthermore, the insulation layer of the OF cable coexists with insulating oil such as mineral oil, paraffin oil, alkylbenzene oil, silicone oil, etc., so it is necessary to avoid swelling and melting of the insulation as much as possible, not only at room temperature but also at high temperature. Particularly in our country, alkylbenzene oils such as DDB (dodecylbenzene oil) are widely used, so there is a need for oil-resistant, low-loss materials or non-polar materials. Furthermore, another important physical property value for insulating materials is the tensile Young's modulus. This must be considered from the viewpoint that the insulating layers wound in multiple layers do not buckle due to "slip between layers" when the cable is wound around a drum or when bent at the rising part from a tunnel. This is a physical property value that cannot be achieved. This physical quantity is
Plastics are smaller than cellulose paper, so materials with high glass transition temperatures and polar substances containing benzene rings in the main chain have been used exclusively, and solutions have been searched for, but with some exceptions, materials with low loss have been used. I've seen results that are inconsistent with performance. Regarding voltage resistance, compared to kraft paper and plastic materials, kraft paper has extremely stable and excellent values, especially when used after being immersed in oil. However, in the case of plastics, especially when made into laminates, it has often been observed that the breakdown voltage decreases rapidly when compared to a single sheet. As mentioned above, considering the characteristics that electrically insulating paper used for OF cables etc. should have, kraft paper has ε = 3.3,
The substance-specific limit value of tan δ = 0.15% requires that new materials be used in future large-capacity power transmission cables. This is why it is desirable to develop new materials that are based on the low dielectric loss characteristics of plastics and also have high withstand voltage, oil resistance, and high Young's modulus. Many attempts have been made to address this problem. For example, materials that are less compatible with insulating oil and have lower loss properties (e.g., polyethylene terephthalate, polycarbonate, etc.) are often extruded into sheets using conventional processing methods and used as a substitute for kraft paper. It has been done. In this case, even if there is no problem with oil resistance, there is still a lack of low loss properties.
The large amount of heat generated from the cable and the low heat transfer characteristics characteristic of plastics were also disadvantageous, making it unsuitable as a material for ultra-high voltage OF cables.
Additionally, such materials increase the tensile Young's modulus in order to satisfy the bending properties of the cable. For this reason, biaxial stretching is often performed.
Although this method is effective in increasing mechanical strength and tear strength, there is no example in which it has been successfully processed into a material with a high Young's modulus suitable for use in insulating paper. In addition, a polypropylene sheet, which is a low-loss material, was made into a biaxially stretched film in the same manner as described above, and its properties were evaluated, but the target values for oil resistance and tensile Young's modulus could not be achieved. As an approach different from the above-mentioned processing of biaxially stretched film, a non-woven fabric type plastic insulating paper in which molten polymer is discharged from a nozzle at high speed and integrated into a flat surface has been studied.
However, these materials generally have poor withstand voltage performance, so although attempts have been made to achieve high airtightness through strong calendering and improve the voltage withstand properties, overall evaluation of Young's modulus, oil resistance, etc. shows that plastic insulating paper is inferior to plastic insulating paper. It has become clear that this is inappropriate. In addition, there are other ways to process plastics into paper, such as papermaking using a mixture of plastic fibers and pulp, which does not require such non-woven fabric processing methods, and which can be used in the conventional kraft paper manufacturing process, or papermaking methods that combine fibers and plastic fibers. has been studied, but no examples have been put into practical use. The inventors took into account the above-mentioned problems with plastic insulating paper, and as a result of intensive research, they found an extruded sheet made of polyolefin, a typical low-loss material, blended with polytetrafluoroethylene. After applying a special stretching process to
For ultra-high voltage power transmission by applying high pressure treatment
The present invention was established based on the discovery that a composite material suitable for OF cable materials can be obtained. The present inventors have already filed a patent application regarding the fact that an excellent low-loss, oil-resistant material can be obtained by using the process of the invention that does not involve high-pressure treatment. That is, a matrix of crystalline polyolefin with an MI value of 10 or more is used, and 0.5 to 20 parts by weight of unfired polytetrafluoroethylene fibers are uniformly dispersed therein so as to match the orientation direction of the matrix, and the thickness is 80 to 250 μm. A polyolefin film for electrical insulation, which is an oriented film with a tensile modulus of 2×10 ⁴ Kg/cm ² or more and a thickness change rate of 10% or less in dodecylbenzene at 100°C,
The specified invention involves uniformly dispersing powdered unfired polytetrafluoroethylene into a crystalline polyolefin powder that has an extremely high melt flow index that does not have sheet formability, and then extruding it with high shear force. Orientation in which a countless number of polytetrafluoroethylene fibers are formed in an extruded sheet using a molding device, and then the thickness is reduced to within a specific range by rolling or stretching, and numerous polytetrafluoroethylene fibers are arranged parallel to the molecular orientation direction of the polyolefin matrix. The second invention is a method for producing a polyolefin film for electrical insulation, which is characterized in that it is made into a film. However, since the material obtained by the above invention uses green tetrafluoroethylene without firing in the processing process, the elastic modulus of the material, especially the high-temperature Young's modulus, cannot be improved more than that of stretched polypropylene. However, there was a drawback that the voltage resistance performance of the stretched film was not improved. The present invention seeks to densify a phase of green polytetrafluoroethylene dispersed in a polyolefin matrix, thereby significantly improving its electrical and mechanical properties. That is, the uniaxially oriented film obtained in the prior invention is treated, for example, by placing it in an oil medium and applying high pressure.
It is not clear why this pressure treatment significantly improves the voltage resistance and elastic modulus of the material, but the matrix polymer itself is transformed by pressure and at the same time is an unsintered material with a much higher melting point than polyolefin. It is inferred that polytetrafluoroethylene transforms to a higher density as a result of being placed in an isotropic pressure field. The electrical insulation sheet obtained by the present invention is a special functional material that has low dielectric loss performance, oil resistance, voltage resistance, and high Young's modulus at the same time. Although it is said that there is a limit to the use of kraft paper, by applying the material manufactured by the method of the present invention to OF cables, it is possible to surpass the technical limit at once, for example, 750KV or 1000KV class OF It is conceivable that the production of cables can also be realized easily. Furthermore, by using the new material manufactured by the method of the present invention for conventional type OF cables of 500KV or less, the power transmission capacity can be increased due to the reduction in heat generation, or the capacity of cable auxiliary equipment such as reactors can be reduced. The ripple effects of this new material are significant, including reduced equipment costs. To further describe the present invention in detail, 0.5 to 20 parts by weight of powdered unsintered polytetrafluoroethylene is mixed with 100 parts by weight of powdered crystalline polyoffine having an MI value of 10 or more. After dispersing the powder particles without causing substantial agglomeration, the dispersion mixture was formed into a sheet by a forming method using a device capable of generating high shear, such as a twin-screw extruder, and then used. At a temperature range of 20° to 50°C lower than the melting point of polyolefin, rolling or stretching or both are performed simultaneously in the machine direction so that the thickness is 1/2 to 1/10, and then 300 kg/cm The processed sheet is placed in a pressure medium of ² to 3000 kg/cm ² and subjected to pressure treatment. The high-pressure atmosphere may be either liquid or gas, but heat-resistant oils having a high boiling point, such as silicone oil, are particularly preferred. Alternatively, when the synthetic insulating paper obtained by the present invention is used for an OF cable or the like, there is no problem in using cable insulating oil. Further, as the high-pressure container, the purpose can be achieved by using a high-pressure cell using a pressurizing method such as a piston W-ring. Further, the temperature in the high pressure treatment in the present invention is not particularly critical as long as it is below the melting point under high pressure of the polyolefin serving as the matrix. Usually a temperature range between 60°C and 100°C is chosen. The reason for this is that if the temperature is over 100℃,
This is because the matrix may undergo thermal deformation when the pressure is removed. This is because at temperatures below 60°C, the hardness of the matrix may not be sufficient to densify the unsintered polytetrafluoroethylene. The amount of pressure used here is usually
Although a pressure of 300 atmospheres or more is sufficient, increasing the pressure excessively is economically disadvantageous because the design of the pressure vessel is limited by the material strength. Therefore, a range of 300 to 3000 atmospheres is usually used, preferably a static pressure of 500 to 1500 atmospheres. When loading a film into a pressure vessel, there is usually no problem in winding it compactly around a metal winding core. After completing the high-pressure treatment, add hot water,
Or, in some cases, by treating with an organic solvent that is compatible with the pressure medium, it may be convenient for handling in subsequent steps, such as paper rolling.
It can also prevent staining of the sheet. The main purpose of the high-pressure treatment in the present invention is to increase the density of the unfired polytetrafluoroethylene, and conditions should be selected that do not have any effect on the polyolefin that serves as the matrix. When polyolefin is placed under high pressure, from around 3,000 atmospheres onwards, the solid phase/liquid phase transition does not occur adjacently, but rather the solid phase undergoes crystal transformation and then changes to the liquid phase. Advantageously, since the present invention does not utilize such a critical region atmosphere, the matrix is merely a pressure transmission medium. By using the method of the present invention, high-density unsintered polytetrafluoroethylene can be dispersed in a polyolefin matrix in the form of fibers, and a material having particularly high withstand voltage and high Young's modulus can be obtained. The breakdown voltage of plastic is particularly sensitive to the voids contained in the material, so by using the treatment method of the present invention, it can be applied not only to synthetic insulating paper but also to the finishing treatment of plastic molded products used under high voltage. Can be used effectively. The present invention will be further explained below with reference to Examples. Examples 1 to 5 To 100 parts by weight of isotactic polypropylene powder with a melt flow index (MI) of 20,
At room temperature, 3 parts by weight of an unsintered Teflon dispersion with a concentration of 60% unsintered Teflon was added, and the mixture was stirred for 5 minutes at 60 rpm using a multipurpose mixer to thoroughly and uniformly disperse the mixture. This for 48 hours, 10 ^-2 mm
It was placed in a vacuum dryer at a temperature of 96° C. under a Hg vacuum to completely remove moisture. In this state, the blended mixed powder is in a fine powder state without self-agglomeration. In order to form this into a sheet, it is once granulated using a pelletizing device. In other words, a 30mm twin screw extruder with different rotations (extrusion temperature
Strands with a diameter of 2 mm were obtained (210 DEG C., cylinder temperature 190 DEG -230 DEG C.), and this was passed through a granulator to obtain pellets consisting of a polypropylene/polytetrafluoroethylene blend composition. At this time, the polytetrafluoroethylene has already been dispersed in the form of fine fibers in the polypropylene matrix, and remains as unfired polytetrafluoroethylene. This state can be easily observed by heating a thin section of the pellet under a polarizing microscope equipped with a heating device to a temperature range above the melting point of polypropylene. The pellets were formed into a sheet with a thickness of 1.2 mm and a width of 30 mm using a 40 mm full-flight single-screw extruder equipped with a T-die, cooling roll, and take-up roll. Next, this sheet was rolled using a two-stage rolling roll with a diameter of 8 cm (roll surface temperature 135°C).
30cm/ while loading back tension 15Kg/cm
A transparent rolled sheet was obtained at a feed rate of 10 min. The sheet thickness at this time was 0.2 mm. The dimensional change in the width direction of the sheet was almost negligible. Next, the sheet thus obtained was subjected to pressure treatment in dodecylbenzene under the conditions shown in Table 1, and the withstand voltage and tensile Young's modulus were measured, and these values are also shown in Table 1. The pressure vessel has a cylinder diameter of 35
A piston-cylinder type airtight container with a Gridziman seal of 500 mm in length and 500 mm in length was used.

[Table] As is clear from the above examples, according to the method of the present invention, an electrically insulating plastic film having various excellent properties can be obtained, and its industrial value is extremely large.

Claims (1)

[Claims] 1 Mix 0.5 to 20 parts by weight of powdered unsintered polytetrafluoroethylene to 100 parts by weight of powdered crystalline polyolefin having an MI value of 10 or more,
After the powders are dispersed without substantial agglomeration, the dispersion mixture is formed into a sheet by a forming method capable of developing high shear forces, and then the melting point of the polyolefin used is higher than the melting point of the polyolefin used.
At a temperature range of 20° to 50°C lower, rolling or stretching or both are performed simultaneously in the machine direction so that the thickness change is 1/2 to 1/10 of the original sheet thickness, and then A method for producing a polyolefin film for electrical insulation, characterized in that the processed sheet is placed in a pressure medium of 300 Kg/cm ² to 3000 Kg/cm ² and subjected to pressure treatment.