JPH053084B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a polypropylene film which is used as an electrical insulating layer while being immersed in electrical insulating oil. Electrical insulation paper has traditionally been used as the oil-immersed insulation layer (or dielectric layer) of oil-immersed electrical equipment (capacitors, transformers, cables, motors, generators, etc.), but recently polypropylene film has been used. The case came out. This film not only has a much higher withstand voltage than electrically insulating paper, but also has several advantages, such as a small dielectric loss tangent and a dielectric constant close to that of insulating oil. However, conventional polypropylene films have the disadvantage of extremely large swelling due to oil, and therefore, when used for oil-immersed insulation applications,
There were various restrictions. For example, when a capacitor element is made by winding a polypropylene film and immersed in insulating oil, the film swells with the oil, the element becomes tightly wound and hard, and the flow of insulating oil between the layers becomes poor. This causes the problem of putting it away. As an emergency measure to avoid this, there is a method of winding the element loosely when first winding it, but if you wind it too loosely, it is likely that the winding will be misaligned, and
Wrinkles are also likely to occur. An object of the present invention is to provide a polypropylene film that has improved the above-mentioned drawback, namely, the tendency to swell. In order to achieve the above object, the present invention has the following configuration, that is, after melt extruding polypropylene, 80 ~
A method for producing an oil-immersed polypropylene film for electrical insulation, which comprises winding it around a cooling drum kept at 120°C, cooling and solidifying it, and then rolling it so that the crystal melting point after oil immersion is 166°C or higher. It is something. Polypropylene (hereinafter abbreviated as PP) herein refers to polypropylene having an isotactic degree of 90% or more, preferably 95% or more, more preferably 97% or more, and a melt index of 0.1 to 100 g/10
minutes, preferably in the range of 1 to 40 g/10 minutes, as swelling by the insulating oil is reduced. Further, when the melt index is within the above range, the amount eluted into the insulating oil is reduced and the viscosity of the insulating oil does not increase, which is preferable. Among the above-mentioned PPs, those having a melt crystallization temperature (T _nc ) in the range of 95 to 130°C, more preferably
The temperature is in the range of 105 to 124°C. If T _nc is within the above range, oil swelling due to insulating oil will be small and film forming properties will be good, making it possible to make a homogeneous film, and as a result, insulation defects will be reduced. preferable. The PP film obtained by the production method of the present invention can be used as an oil-immersed electrical insulator either as a PP film alone or as an integrated PP film laminated with electrical insulating paper. Electrical insulating paper (hereinafter referred to as paper), which is laminated and integrated on one or both sides of this PP film, is a natural paper whose main component is cellulose (JIS
C 2301, 2302, 2303, 2304, 2306, 2307 and
2308) and synthetic paper. Particularly preferred paper has a density of 0.6 to 0.9 g/cm ³ ,
The thickness is in the range of 10 to 50 μm. The crystalline melting point of the polypropylene film of the present invention after oil immersion must be 166°C or higher, preferably 168°C or higher. If the crystal melting point (T _n ) of the film after oil immersion is less than 166°C, preferably less than 168°C, the polypropylene film will undergo large swelling due to oil during oil immersion, so When used as an oil-immersed gas insulation layer for oil-immersed electrical equipment, the swelling of the polypropylene film impairs the flow of insulating oil between the insulation layers, causing dielectric breakdown. Of course, it is necessary that the T _n before oil immersion is 166°C or higher, but the degree of decrease in T _n due to oil immersion varies greatly depending on the microstructure such as the orientation state and crystalline state of the polypropylene film. , when using dodecylbenzene oil, Table 1
It becomes like this.

[Table] As is clear from Table 1, even if the T _n is high before oil immersion, the T _n decreases significantly after oil immersion, and moreover, the T _n after oil immersion is 166°C. In the case of biaxially oriented polypropylene with less than 10%, it undergoes large oil swelling in oil and cannot be used as an oil-immersed insulating film. In this way, it can be seen that T _n after oil immersion, rather than T _n before oil immersion, has a close correlation with oil swelling. Note that after oil immersion, the polypropylene film is 80%
Refers to items that have been immersed in insulating oil at ℃ for 24 hours or more. In addition, the insulating oil is measured using dodecylbenzene oil or other insulating oil as desired. Furthermore, the density of the film of the present invention after oil immersion is 0.885.
~0.915g/ ^cm3 , particularly preferably 0.895~0.912g/cm3
Preferably it is in the cm ³ range. When the density is within the above range, swelling due to insulating oil is reduced and the mechanical strength of the insulating layer can be maintained. In addition, the birefringence of the film of the present invention after oil immersion is
It is preferably in the range of 0.020 to 0.035, preferably 0.025 to 0.032. If the birefringence is within this range, the swelling caused by the insulating oil will be small, and the film will be less likely to crack, making dielectric breakdown less likely. Next, when a polypropylene film is used alone, it is preferable that the surface roughness (Rmax) of one or both sides of the film of the present invention is in the range of 1 to 50 μm, preferably 2 to 40 μm. If it is within this range, the interlayer flowability of the insulating oil will be improved, dielectric breakdown will be less likely to occur, and the pressure resistance characteristics will be improved. Of course, it is clear that even when the polypropylene film and the electrically insulating paper are laminated and integrated, the surface roughness described above may be applied as necessary. Further, in the case of the film of the present invention, it is preferable that the higher-order structure composed of crystals and amorphous after oil immersion has a checkered pattern arrangement. Such an arrangement is a very rigid structure and is effective in preventing swelling of the film after oil immersion. The existence of this checkered arrangement can be confirmed from four-point interference of small-angle X-ray scattering. In the case of the film of the present invention, the longitudinal heat shrinkage rate before oil immersion is 0.1 to 5%, preferably 0.5 to 3%.
By setting the value within this range, the film can be made more suitable for oil-immersed electrical insulation. If the heat shrinkage rate is within this range, the insulating layer will be tightly wound and wrinkles will not occur, and since it will not stretch in the longitudinal direction in the insulating oil, the wound insulating layer will not loosen. . An example of a method for keeping the heat shrinkage rate in the longitudinal direction within this range is to apply it during the film forming process, but it is also possible to apply it during the film forming process, but it is also possible to keep the heat shrinkage rate in the longitudinal direction while keeping the polypropylene film wound in a roll for a long time while relaxing it in the longitudinal direction. (1 to 50 hours) A method of aging treatment at 80 to 140°C is preferred. Next, an example of the method for producing the polypropylene (PP) film of the present invention will be described. Melt and extrude PP resin and extrude it into a sheet from a die, which is
Wrapped around a cooling drum kept at a high temperature of 120℃,
After the polymer is kept at a temperature close to the melt crystallization temperature T _nc of the polymer, it is cooled and solidified. In this way, polymer
By cooling and solidifying at a high temperature near T _nc , many crystals are generated in the PP sheet, and the degree of crystallinity of the PP sheet becomes large, and the density of the PP sheet becomes large. this
Insert the PP sheet between a set of rolling rolls,
Rolling is performed so that the rolling ratio (the value obtained by dividing the sheet thickness before rolling by the sheet thickness after rolling) is 5 to 12 times, preferably 7 to 10 times. The rolling process imparts molecular orientation to the PP sheet, and some of the amorphous parts are oriented and crystallized. At this time, the crystal melting point after oil immersion is 166℃
It is necessary to roll it so that it becomes the above. Note that the PP sheet, which is cooled and solidified at the above-mentioned high temperatures and has a high degree of crystallinity, has many crystalline parts even after rolling, and the higher-order structure consisting of crystals and amorphous after oil immersion has a checkerboard pattern. Easier to arrange. The rolling pressure is 10 to 3000Kg/cm, more preferably
A suitable range is 100 to 1000 Kg/cm, and the temperature of the rolling roll is preferably 60 to 160°C, preferably 80 to 150°C. When rolling, wetting the surface of the PP sheet with liquid (water, aqueous surfactant solution, alkylene glycol, polyalkylene glycol, glycerin, electrical insulation oil, etc.) before rolling facilitates uniform high-magnification rolling. The film obtained by rolling (usually has a thickness in the range of 10 to 300 μm),
Reheat to 100-150℃ and reheat to 0.5 to the original size in the longitudinal direction.
Heat treat for 1 to 20 seconds while giving 10% relaxation. If necessary, apply 600 to 6000 J/
Corona discharge treatment may be performed with an amount of electrical energy of about m ² . If this treatment is performed in an atmosphere such as air, nitrogen, or carbon dioxide, the surface tension will be improved and adhesive properties will be excellent. Then heat this film to 90-140℃ as needed.
The film is passed through heated embossing rolls to roughen one or both sides of the film so that the surface roughness (Rmax) is in the range of 1 to 50 μm, preferably 2 to 40 μm. As described below, the film of the present invention is produced by melt extruding polypropylene, winding it around a cooling drum kept at a high temperature of 80 to 120°C, cooling and solidifying it, and then rolling it. be. At this time, rolling may be combined with stretching using closely spaced rolls or the like. It is clear that instead of embossing, the surface may be roughened by sandblasting, etching, or the like. The method of laminating and integrating PP film and paper is not particularly limited, but when using an adhesive, lamination using a thermosetting adhesive is preferred. By using a thermosetting adhesive, it is possible to suppress the swelling of the laminated film due to oil, particularly the drawback that the laminated film stretches in the longitudinal direction and width direction due to swelling with oil. As thermosetting adhesives, amino-based, phenolic-based, resorcinol/formaldehyde-based, xylene-based, furan-based, epoxy-based, polyisocyanate-based, unsaturated polyester-based, and thermosetting acrylic resins can be used. Among these, particularly preferred are polyisocyanate adhesives and epoxy adhesives, and most preferred are polyester to polyisocyanate adhesives. Both of them are particularly excellent in that the degree to which they dissolve into the oil and deteriorate the electrical properties of the oil is extremely small. The amount of adhesive used for laminating and integrating is 0.5~
A range of 5 g/m ² (dry weight of adhesive) is good, particularly preferred is a range of 1 to 4 g/m ² . Within this range, the electrical properties (especially dielectric loss tangent) of the laminated and integrated film will not deteriorate, and the film and paper will not separate in oil. Describing a typical method of laminating and integrating, the surface of corona discharge treated PP film is
Apply an organic solvent solution of thermosetting adhesive, and
It is pre-dried with hot air at 50 to 100°C, and paper is placed on this surface and pressed with a heated press roll to form a laminated integrated film. This is aged for 10 to 50 hours at 50℃, or for 1 to 7 days at room temperature. In the present invention, the crystalline melting point of the polypropylene film after oil immersion is set within a certain range, so that the following excellent effects are produced. (1) Less swelling due to insulating oil. (2) The insulating layer is less likely to tighten or loosen. (3) It has a small dielectric loss tangent between 0 and 100°C, and its dielectric constant is usually larger than that of polypropylene. (4) Excellent dielectric breakdown characteristics at low and high voltages. (5) Excellent electrical insulation properties at high temperatures of 80 to 120℃. (6) The polypropylene film of the present invention is easily impregnated with insulating oil, resulting in a highly accurate and reliable electrical insulator. Therefore, the film of the present invention can be used in various oil-immersed electrical equipment such as capacitors, transformers, cables,
It is suitable as an oil-immersed insulation layer for motors, generators, etc. Note that the terms and measurement methods used in the present invention will be summarized and explained below. (1) Degree of isotacticity Extract PP with boiling n-heptane, divide the weight of the extracted residue by the original weight, multiply by 100, and display as a percentage. (2) Melt index: ASTM D-1238-73
Measured under condition L. (3) Melt crystallization temperature (T _nc ): 5 mg of the sample was placed in a PerkinElmer DSC-type, and the atmosphere was replaced with nitrogen. Next, at a heating rate of 20℃/min,
Raise the temperature to ℃ and hold at this 200℃ for 5 minutes. Next, the temperature is lowered at a rate of 20°C/min to draw an exothermic peak associated with crystallization of the sample. Let the temperature at the top of this peak be T _nc . (4) Crystal melting point (T _n ): PerkinElmer DSC
- Put 5 mg of the sample into a mold and raise the temperature at 10° C./min under a nitrogen stream, and let T _n be the temperature at the top of the endothermic peak associated with crystal melting of polypropylene. If the peak is hidden behind another peak and becomes a shoulder, divide the area and create the peak separately. (5) Density: According to ASTM D 1505. (6) Birefringence: Using Atsube's refractometer, measure the refractive index in the longitudinal direction (Ny) and the refractive index in the width direction (Nx) of the film, and calculate the birefringence by subtracting Nx from Ny. Note that a sodium D line is used as a light source during measurement, and methyl salicylate is used as a mounting liquid. (7) Surface roughness (Rmax): Rmax is measured by the method described in JIS B 0601-1976. The cutoff value is 0.8mm. (8) Heat shrinkage rate: from film, length 200mm, width 10
Cut a mm sample (the direction in which the heat shrinkage rate is measured is the length direction). The sample is kept in a hot air circulation oven at 120°C for 15 minutes, then taken out to room temperature and its length is measured. Its length is L
(mm), the thermal contraction rate can be calculated using the following formula. Heat shrinkage rate (%) = 100 x (200-L) / 200 (9) Degree of swelling due to insulating oil: 100mm from the film
Cut out a sample of ×100mm, and calculate the weight of this
(mg). This sample is immersed in alkylbenzene oil at 80°C and left for 24 hours. Next, take out this sample, wipe off the oil on the surface with filter paper, wash it by immersing it in acetone, and completely remove the oil on the surface with gauze soaked in acetone. Measure the weight of this sample and let it be W' (mg), then the degree of swelling (%) can be calculated using the following formula. Swelling degree (%) = 100 x (W'-W)/W (10) Flowability of insulating oil: A capacitor element is made by alternately winding a film and aluminum foil. This is immersed in insulating oil and vacuum impregnated with oil. Thereafter, the device is disassembled and visually determined whether the insulating oil has spread between all layers of the device. Rank A: Evenly distributed over the entire surface Rank B: There are slight oil-free spots Rank C: Oil-free areas exist in a planar manner As an oil-immersed insulating material, it must be rank A. However, in low pressure applications, even rank B may be used. Rank C is unsuitable as an oil-immersed insulating material. (11) Electrical insulating oil: A general term for various known electrical insulating oils, such as mineral oil, castor oil, cottonseed oil, alkylbenzene (eg, dodecylbenzene), diallylalkane, and polybutene oil. In addition, JIS C
Electrical insulating oils having the properties specified in 2320 are preferred. Next, embodiments of the present invention will be described based on Examples. Example 1 PP resin pellets with an isotactic structure content of 97.6%, a melt index of 6 g/10 min, and a T _nc of 110.5°C were fed into an extruder, melted and extruded at 260°C, and discharged into a sheet from a T-shaped nozzle. I forced it. This molten sheet was wound around a cooling drum at 98°C, held on a roll kept at 115°C for about 3 seconds, and then cooled to produce a sheet with a thickness of about 2 mm. This sheet is rolled through a set of rolling rolls (roll diameter
250mm) and rolled 10 times. Rolling pressure is 500Kg/cm, rolling roll temperature 146℃
The sheet surface was wetted with polyethylene glycol and rolled. The resulting 200 μm thick film was placed in an atmosphere at 130°C and rotated once in the longitudinal direction.
% relaxation for 10 seconds. Next, this film is passed between embossing rolls heated to 130℃, and approximately
A 100-mesh sandblasted pattern has been transferred. Next, this film was kept under tension in an atmosphere at 120° C. for 10 hours for aging heat treatment, and then slowly cooled to room temperature. The properties of the thus obtained film are summarized in Table 2.The film of the present invention, which has a high crystal melting point _Tn of 169°C after oil immersion, has a small swelling due to the insulating oil and has a good flowability of the insulating oil. It can be seen that it is extremely useful as an oil-immersed electrical insulating film because of its excellent properties.

[Table] Dodecylbenzene (DDB) is used as the insulating oil.
was used. Comparative Example 1 The PP resin pellets used in Example 1 were fed into an extruder, melted and extruded at 260°C, and discharged into a sheet from a T-shaped nozzle. This molten sheet is wrapped around a cooling drum at 30℃, cooled and solidified, and the thickness is 2 mm.
I made a sheet. This sheet was rolled and heat treated in the same manner as in Example 1, and then subjected to embossing and aging treatment. The thus obtained film of Comparative Example 1 had a crystalline melting point of 168°C before oil immersion, and a crystalline melting point of 165°C after oil immersion.
The degree of swelling due to insulating oil was 3.0%.
As described above, the PP film of Comparative Example 1, which was obtained by rapidly cooling and solidifying the melt-extruded PP sheet, had a mild crystal structure inside the PP, and the degree of expansion of the insulating oil was greater than that of the PP film of Example 1.

Claims (1)

[Claims] 1. Oil-immersed electricity, characterized in that polypropylene is melt-extruded, wound around a cooling drum maintained at 80 to 120°C, cooled and solidified, and then rolled so that the crystal melting point after oil immersion is 166°C or higher. A method for manufacturing polypropylene film for insulation.