LOW-DIELECTRIC CONSTANT PRESS BOARD FOR OIL IMPREGNATION INSULATION

Abstract: The present invention is directed to a low-dielectric constant press board for oil impregnation insulation which comprises a poly(4-methylpentene-1) fiber and/or a poly(3-­ methylbutene-1) fiber and a kraft pulp. This press board may be formed by first laminating the mixed wet sheets of the poly(4-methylpentene-1) fiber and/or the poly(3-methyl­ butene-1) fiber and the kraft pulp, and then heating/press­ ing the laminate integrally. In this case, the wet sheets of the kraft pulp (2) alone may be disposed over and under the mixed wet sheets (1), or over, under and between the mixed wet sheets. Further, the press board may additionally contain at least one selected from polyethylene terephthalate fibers and polyphenylene sulfide fibers in an amount of less than 28 wt%.

Description: The present invention relates to a press board for oil impregnation insulation. More specifically, it relates to a low-dielectric constant press board prepared by mixing a poly(4-methylpentene-1) fiber and/or a poly(3-methyl-butene-1) fiber and a kraft pulp. A conventional press board for oil impregnation insulation comprises 100 wt% of a kraft pulp and has a dielectric constant as high as 4.7. Some of the inventors of the present application have already suggested a low-dielectric constant press board for oil impregnation insulation which comprises a laminate of sheets of a kraft pulp alone and mixed sheets composed of the kraft pulp and 30 to 90 wt% of a polymeric fiber (JP-A-156,386/1987). Examples of insulating paper other than the press board include low-dielectric constant insulating paper which can be made by mixing a pulp-like material (a synthetic fibrid) of a synthetic polymer with a polymeric fiber, and then heating/pressing the mixture to melt and shape it (JP-B-13,912/1963); insulating paper made by mixing a pulp with 30 wt% or more of a polymeric fiber having a different molecular structure, and then shaping the mixture (JP-A-124,811/1982); and insulating paper made by laminating the mixed sheets of a polymeric fiber and a pulp, the polymeric fiber being present in the mixed sheets in different proportions, and then shaping the mixture (JP-A-168,418/1982). The insulating paper described in (JP-B-13,912/1963) mentioned above is made by using, as a pulp, a synthetic fibrid which is a synthetic polymer and heating/pressing the fibrid to melt it. Therefore, its oil impregnation which is necessary as the press board for oil-impregnation insulation is poor and insufficient. The synthetic insulating paper disclosed in (JP-A-124,811/1982) mentioned above has a thickness of 0.2 mm or less and is constituted to be wound around conductors. Therefore, this paper does not have such a compressive strength as to support winding wires and is inconveniently bad in oil impregnation. The semisynthetic insulating paper described in (JP-A-168,418/1982 mentioned above is as thin as 0.15 mm or less and therefore is different from the press board having a thickness of 0.5 mm or more. In addition, it should be noted that this insulating paper is made by heating and melting the polymeric fiber at a high temperature, and that the content of the polymeric fiber therein is as much as 40 wt% or more. Further, since constituted to be wound around conductors, this paper does not have such a compressive strength as to support winding wires, and its oil impregnation is also inconveniently poor. Generally, press boards for oil impregnation insulation have the feature that as the dielectric constant of the press board is decreased, a stress of the electric field which is applied to an oil space between the press board and another insulator is alleviated. From the viewpoint of this effect, it is appreciated that the dielectric constant of the press board is preferably 3.8 or less, more preferably 3.5 or less. As in the case of the present invention, the increase in the content of a poly(4-methylpentene-1) fiber or the like in the press board leads to the diminution in the dielectric constant, so that an effect of mitigating the electric field is built up. On the other hand, however, such a manner causes its compressive strength to decrease disadvantageously. The compressive strength of the press board is required to be 72 N/mm² (7.3 kg/mm²) (80°C) or more. In this respect, it is necessary that the press board has different properties than the above-mentioned insulating papers. Further, when used in high-voltage oil-impregnated electric machines, the press board is also required to be excellent in insulating properties at a high voltage. Accordingly, an object of the present invention is to provide a low-dielectric constant press board for oil impregnation insulation having a thickness of 0.5 mm or more in which the compressive strength is high, the oil impregnation is good, the dielectric constant is small, insulating properties at a high voltage are excellent, and an electric field which easily concentrates in the oil space between insulators is effectively alleviated. It has been found, that for the sake of diminishing the dielectric constant of a press board, it is effective to mix a polymeric synthetic fiber with a kraft pulp, whereas when the content of the polymeric fiber is increased, the compressive strength of the formed press board is decreased. Therefore, research has been conducted for synthetic fibers which are capable of effectively lowering the dielectric constant by these fibers in a relatively small amount, and as a result, it has been found that when a poly(4-methylpentene-1) fiber and/or a poly(3-methylbutene-1) fiber is mixed with a kraft pulp, the dielectric constant of the press board can be decreased and the compressive strength thereof can be maintained above a predetermined level. According to one embodiment of the present invention, there is provided a low dielectric constant press board for oil impregnation insulation which is formed by laminating mixed wet sheets of a poly(4-methylpentene-1) fiber and/or a poly(3-methylbutene-1) fiber and a kraft pulp, the fibers being used in an amount of 5,6 to less than 30 wt-%, based on the press board, and then drying the laminate integrally by heating/pressing at a temperature of 110 to 190 °C under a pressure of 98 to 490 N/cm² (10 to 50 kgf/cm²). According to another embodiment of the present invention there is provided a low-dielectric constant press board for oil impregnation insulation which is formed by mixing a poly(4-methylpentene-1) fiber and/or a poly(3-methylbutene-1) fiber in an amount of 5,6 to less than 30 wt-%, based on the press board, with a kraft pulp to make mixed wet sheets, disposing wet sheets of kraft pulp alone over and under said mixed wet sheets, or over, under and between said mixed wet sheets, and then drying the laminate integrally by heating/pressing at a temperature of 110 to 190 °C under a pressure of 98 to 490 N/cm² (10 to 50 kgf/cm²). The content of the poly(4-methylpentene-1) fiber and/or the poly(3-methylbutene-1) fiber in the press board is preferably 10 wt% or more and less than 30 wt%. Further, when mixed with the kraft pulp, the amount of the poly(4-methylpentene-1) fiber and/or the poly(3-methylbutene-1) fiber is 5.6 wt% or more and less than 30 wt% based on the press board, and it is preferred that one or more of polyethylene terephthalate fibers and polyphenylene sulfide fibers is mixed therewith in an amount of less than 28 wt% as a third fiber. When the latter fiber, i.e., the third fiber is additionally mixed, the dielectric constant of the product is a little worse than in the case of the former fiber alone, but its compressive strength is built up perceptibly. Therefore, the employment of the third fiber can provide the additional advantage that the balance between the dielectric constant and the compressive strength is easily regulated. The above-mentioned two kinds of third fibers have no problems in points of swelling resistance and solubility to an insulating oil. Also in the case that the third fiber is mixed additionally with the above fiber, the total amount of the fibers is preferably 30 wt% or less based on the weight of the press board. The dielectric constant of the press board is preferably 3.8 or less, more preferably 3.5 or less (that of a conventional press board is 4.7) from the viewpoint of the effect of mitigating the electric field in the oil space. For this reason, the content of the poly(4-methylpentene-1) fiber and/or poly(3-methylbutene-1) fiber is preferably 10 wt% or more, more preferably 15 wt% or more, based on the total weight of the press board. On the other hand, the compressive strength of the press board is required to be 72 N/mm² (7.3 kg/mm²) (80°C) or more, and therefore the content of the poly(4-methylpentene-1) fiber and/or poly(3-methylbutene-1) fiber is preferably less than 30 wt%. Also in the case that the above-mentioned third fiber is mixed, it is preferred that the total amount of the fibers is less than 30 wt%. The mixture of these fibers permits not only decreasing the dielectric constant of the press board effectively but also heightening heat resistance and oil resistance thereof. Thickness and length of these fibers are preferably 6 denier or less and 2 to 10 mm, respectively. When the mixed wet sheets and the wet kraft pulp sheets are laminated and heated/pressed in order to form the pressboard, it is preferred that at least top and bottom sheets of, or at least top, bottom and intermediate sheets of the laminate comprise sheets of the kraft pulp alone. In the present invention, with regard to the kraft pulp with which the poly(4-methylpentene-1) fiber and/or poly(3-methylbutene-1) fiber are mixed, and with regard to the kraft pulp for the wet sheets of the kraft pulp alone which are laminated together with the mixed wet sheets, it is preferred that the quality of these kraft pulps is comparable to the grade of the press boards in Class 2 (hereinafter referred to as PB-2) which JIS 2311 prescribes. A freeness of the usable kraft pulp is preferably within the range of 200 to 400 ml (Canadian Standard Freeness-CSF). When the freeness of the kraft pulp is in excess of 400 ml: (beating and entanglement of the pulp are poor), then the press board manufactured therefrom is insufficient in tensile strength and compressive strength. Inversely, when the freeness of the kraft pulp is less than 200 ml (CSF), (the beating makes excessive progress and the strength of the pulp is degraded,) so that the press board manufactured by the use of such a kraft pulp is insufficient in strength and is bad in oil impregnation. The press board of the present invention may be manufactured by first making wet sheets in accordance with a wet sheet making process from an aqueous slurry in which the poly(4-methylpentene-1) fiber and/or the poly(3-methylbutene-1) fiber having a thickness of 6 denier or less and a length of 2 to 10 mm are mixed with the kraft pulp, then laminating the thus made wet sheets as many as their optional number, and finally drying the laminate at a temperature of 110 to 190°C under a pressure of 98 to 490 N/cm² (10 to 50 kgf/cm²). In this case, the drying temperature should be less than the melting points of the respective fibers. This point is different from the case of the insulating papers disclosed in the known literature in which a polymeric fiber is molten. The conventional insulating papers made by melting the polymeric fiber do not have such a compressive strength as to support the winding wire and are poor in oil impregnation. The heating process of the present invention does not intend to melt the fiber but to dry it. However, as a result of this way, the formed press board involves serious problems such as linting and dislocation of the mixed fiber. When used in high-voltage oil-impregnated electric machines, the press board must possess excellent insulating properties even at a high voltage. For this purpose, surface properties of the press board are crucial. Forming the top and bottom of the mixed wet sheets with the sheets of the kraft pulp alone is effective to prevent the linting of the synthetic fiber, and interposing the sheets of the kraft pulp alone between the mixed wet sheets builds up the effect of inhibiting the fibers from dislocating. In the high-voltage oil-impregnated electric machines, the linting and dislocation of the fiber decrease the lightning impulse breakdown voltage. For the prevention of the linting on the surface of the press board, for the prevention of the decrease in the lightning impulse breakdown voltage, and for the prevention of the dislocation of the fiber, it is effective to treat thermally both the surfaces of the press board at a temperature of 220°C or more for a short period of time. The range of the suitable temperature is within 230 to 280°C. By the thermal treatment, the lints on each surface are molten. The thermal treatment may be carried out by the use of one or more hot rolls, a special flatiron suitable for a high temperature, an iron, a hair drier or a hot-air oven. A time period necessary for the thermal treatment depends upon a chosen means for the treatment. For example, in the case that the hot rolls are employed, the thermal treatment can be achieved instantaneously, but when the hot-air oven is used, a period of 1 minute or more is taken. If the treatment temperature is 250°C, a period of about 5 minutes is necessary. When, with the intention of lowering the dielectric constant of the press board, the low-dielectric constant synthetic fiber is mixed with the kraft pulp and is then heated and pressed in a usual manner to form the desired press board, a mesh pattern is more liable to appear on the surface of the press board than in the case of a press board only comprising the kraft pulp. For the press board prepared from the fiber and kraft pulp, the surface roughness; (i.e., a 10 point height,) was measured in accordance with the procedure of JIS B0601, and it was found that the value of the 10 point height was as high as 180 µm or more. Various press boards having different 10 point heights were formed by the use of nets having different meshes, and for these press boards, the relation between the lightning impulse properties and the 10 point height was inspected. As a result, it was found that both the factors were relative to each other. That is, a relation was appreciated in which the smaller the surface roughness of the press board became, the greater the lightning impulse breakdown voltage became. In particular, when the 10 point height is 100 µm or less, the lightning impulse breakdown voltage is high, even in the case that the dielectric constant is 3.5. In order to regulate the 10 point height of the low-dielectric constant press board to 100 µm or less, for example, a fine net having e.g., openings of 150 µm or less (100 meshes or more) may be used in shaping the heated and pressed press board. Alternatively, when a coarse net is employed, the surface of the heated and pressed board may be treated with a calender. In this way, the 10 point height can be regulated to a level of 100 µm or less, and therefore a press board can be obtained having a low dielectric constant and a high lightning impulse breakdown voltage. EXAMPLES Now, the present invention will be described in detail in reference to examples and comparative examples, but it should not be limited at all by these examples. The respective properties of press boards were measured as follows: (1) Dielectric Constant: A specimen (diameter 90 mm) was dried by heating in vacuo and then impregnated with an insulating oil, and the specimen was held between a pair of electrodes manufactured by Nissin Electric Co., Ltd. at a pressure of 2.45 N/cm² (0.25 kgf/cm²). Afterward, the specimen was impregnated with the insulating oil again, and the dielectric constant was then measured. (2) Compressive Strength: Specimens (impregnated with oil) having a size of 13 x 13 mm and a thickness of about 1.6 mm were laminated, till their height had reached a level of about 16 to 18 mm. The breaking load was then measured at a compressive velocity of 0.5 mm/minute at a temperature of 80°C by the use of an autographic recording device, and the aimed compressive strength was calculated from the breaking load and a sectional area of the laminate. (3) Impregnation of Oil: Eight press boards having a size of 50 x 200 mm and a thickness of 1.6 mm were laminated, and of 4 sides and 2 surfaces of the laminate, one face in faces of 50 x (1.6 x 8)mm was kept intact and the remaining 5 faces were coated with an epoxy adhesive. After the curing of the epoxy adhesive, vacuum drying was carried out at 110°C at 40 Pa (0.3 Torr) for 24 hours, and the specimen was then impregnated with an oil. After the specimen was allowed to stand at 25°C under atmospheric pressure for 48 hours, the oil impregnation distance in the layer direction of the press board was measured and compared. (4) Linting on Surface of Press Board: Linting was evaluated by visually observing the surface of the press board. (5) Lightning Impulse Breakdown Voltage: As shown in Fig. 3 attached hereto, a press board 7 for which lightning impulse breakdown voltage would be measured was put between two coils where a conductor 3 was coated with oil-impregnated paper 4 and another coil where another conductor 5 was coated with oil-impregnated paper 6, in order to construct a model for the measurement of the lightning impulse breakdown voltage. The thus constructed model was then immersed into an oil, and after the conductors 3 and 5 had been connected respectively to an application side and a ground side, the lightning impulse breakdown voltage was applied to the model, increasing the voltage one shot by one shot from a low voltage level. At this time, a lightning impulse streamer was delivered from a wedge-like oil space 8 defined between the oil-impregnated paper 4 and the press board 7 to the surface of the oil-impregnated paper 6 of the coil on the ground side, and the streamer was finally passed through the oil-impregnated paper 6, so that an insulation breakdown occurred between the coils. At this time, the applied lightning impulse breakdown voltage was measured. A ratio of the lightning impulse breakdown voltage was sought relatively by regarding, as 100%, a lightning impulse breakdown voltage of the conventional press board PB-2 comprising a kraft pulp alone. (6) Surface Roughness: In accordance with JIS B0601, a 10 point height was measured. Examples 1 to 5 and Comparative Examples 1 to 3 In each case of Examples 1 to 5, a poly(4-methylpentene-1) fiber having a thickness of 0.33 tex (3 denier) and a length of 5 mm was mixed with a kraft pulp having a freeness of 365 ml (CSF) in a proportion shown in Table 1 given below, in order to prepare an aqueous slurry, and wet sheets were made from this slurry in accordance with a wet paper making process. The 19 wet sheets were laminated and then heated/pressed at a temperature of 140°C at a pressure of 392 N/cm² (40 kgf/cm²) in order to dry them and to thereby obtain a press board having a thickness of 1.6 to 1.7 mm integrally. For each press board, dielectric constant, compressive strength and oil impregnation distance were measured, and the results are set forth in Table 1. Further, in each of Comparative Examples 1 to 3, a press board was formed in a proportion of materials shown in Table 1, and for each press board, dielectric constant, compressive strength and oil impregnation distance were measured in like manner, and the results are set forth in Table 1. As be definite from the results in Table 1, if the content of the poly(4-methylpentene-1) fiber is adjusted to be less than 30 wt% from the viewpoint of the compressive strength and is also adjusted to 10 wt% or more from the viewpoint of the dielectric constant, the press board can be obtained in which the dielectric constant is 3.8 or less and the compressive strength is 72 N/mm² (7.3 kg/mm²) or more. Furthermore, since the drying treatment under the heating/pressing operation is carried out at a temperature within the range of 110 to 190°C, the poly(4-methylpentene-1) fiber does not fuse thermally, so that the impregnation of the insulating oil in every example is better than Comparative Example 3 regarding a conventional press board (PB-2) and Comparative Example 1 in which the content of the poly(4-methylpentene-1) fiber is in excess of the upper limit of the present invention. Examples 6 to 11 and Comparative Examples 4 to 7 In each case of Examples 6 to 11, a poly(4-methylpentene-1) fiber having a thickness of 3 denier and a length of 5 mm was mixed with a kraft pulp having a freeness of 365 ml (CSF) in a proportion shown in Table 2 given below in order to prepare an aqueous slurry, and wet sheets were then made from this slurry in accordance with a wet paper making process. Then, the 19 wet sheets were laminated and treated in the same manner as in Examples 1 to 5 in order to form press boards having a thickness of 1.6 to 1.7 mm. The press board formed in each example was passed through hot rolls at a temperature shown in Table 2 so as to perform a surface treatment. Further, in Comparative Examples 4 to 7, press boards were formed in proportions of materials set forth in Table 2 in the same manner as in Examples 1 to 5. Then, the press board formed in each comparative example was passed through hot rolls at a temperature in Table 2 to perform a surface treatment. For the surface-treated press boards formed in these examples and comparative examples, dielectric constant, surface hairiness and oil impregnation distance were measured. The results are set forth in Table 2. As be apparent from the results in Table 2, the treatment by means of hot rolls at 250°C and 280°C permits eliminating the fibrous naps from the surfaces of the press boards. Further, even when the surface treatment is carried out, the impregnation distance of the insulating oil scarcely varies. Unexpectedly, the results in Table 2 indicate that the press boards regarding the present invention are more easily impregnated therewith than the conventional press board (PB-2) of Comparative Example 7. Examples 12 to 15 and Comparative Examples 8 to 10 In each case of these examples and comparative examples, a poly(3-methylbutene-1) fiber having a thickness of 0.33 tex (3 denier) and a length of 5 mm was mixed with a kraft pulp having a freeness of 365 ml (CSF) in a proportion shown in Table 3 given below in order to prepare an aqueous slurry, and wet sheets were made from this slurry in accordance with a wet paper making process. Then, the 19 wet sheets were laminated and dried by heating/pressing for 45 minutes at a temperature of 140°C at a pressure of 392 N/cm² (40 kgf/cm²) in order to integrally form a press board having a thickness of 1.6 to 1.7 mm. For each press board, dielectric constant, compressive strength and oil impregnation distance were measured, and the results are set forth in Table 3. Incidentally, conventional press boards (PB-2) comprising the kraft pulp alone are also exhibited therein as comparative examples. As be definite from the results in Table 3, the content of the poly(3-methylbutene-1 fiber is preferably less than 30 wt%. It can be understood that when the dielectric constant of 3.8 or less is desired, the content of the poly(3-methylbutene-1) fiber is required to be 10 wt% or more. The results in Table 3 indicate that when the content of the poly(3-methylbutene-1) fiber is more than 15 wt% or more and is less than 30 wt%, the formed press board has a dielectric constant of 3.8 or less and a compressive strength of 72 N/mm² (7.3 kg/mm²) or more. Further, since the press boards regarding the present invention are dried at 110 to 190°C in the heating/pressing process, the poly(3-methybutene-1) fiber does not fuse thermally, and therefore all of them are more excellent in insulating oil impregnation than the conventional press board (PB-2). Examples 16 to 21 and Comparative Examples 11 to 14 : In each case of these examples and comparative examples, a poly(3-methylbutene-1) fiber having a thickness of 0.33 tex (3 denier) and a length of 5 mm was mixed with a kraft pulp having a freeness of 365 ml (CSF) in a proportion shown in Table 4 given below in order to prepare an aqueous slurry, and wet sheets were made from this slurry in accordance with a wet paper making process. Then, the 19 wet sheets were laminated and treated in the same manner as in Examples 12 to 15 in order to form press boards having a thickness of 1.6 to 1.7 mm. Each press board was passed through hot rolls at 270°C and 300°C so as to fuse or remove fibrous fluffs from its surface. For each press board, dielectric constant, surface linting and oil impregnation distance were measured, and the results are set forth in Table 4. As be apparent from the results in Table 4, the treatment by the use of the hot rolls at 270°C and 300°C permits eliminating the fibrous fluffs from the surfaces of the press boards. Further, it can be understood that even when the hot roll treatment is carried out, unexpectedly the press boards regarding the present invention are more easily impregnated therewith than the conventional press board (PB-2), though an impregnation velocity of the insulating oil changes slightly. Examples 22 to 25 and Comparative Examples 15 to 19 In each case of these examples and comparative examples, a poly(3-methylbutene-1) fiber having a thickness of 0.33 tex (3 denier) and a length of 5 mm was mixed with a kraft pulp having a freeness of 365 ml (CSF) in a weight ratio of the fiber : the kraft being 11.2 : 88.8 in order to prepare a 1% aqueous slurry. Then, 16 wet sheets (water content was about 80%) in which an absolute dry weight would be 80 g/m² were made from this slurry. Further, a 1% aqueous slurry comprising the kraft pulp alone having a freeness of 365 ml (CSF) was utilized to make 17 wet sheets in which an absolute dry weight would be 9 g/m². These two kinds of wet sheets were alternately laminated as many as required, as shown in Fig. 1. In this drawing, reference numeral 1 is a mixed layer and numeral 2 is a pulp single layer. In this case, the top and bottom sheets of the laminate were constituted by the wet sheets of the kraft pulp alone. The composite wet sheets were heated and pressed at 140°C at 392 N/cm² (40 kgf/cm²) for 45 minutes by the use of a hot press in order to form a press board integrally. In like manner, combination type press boards were formed which were each composed of the mixed wet sheets of the poly(3-methylbutene-1) fiber and the kraft pulp and the wet sheets of the kraft pulp alone. In this case, the mixing ratios of the poly(3-methylbutene-1) fiber : the kraft pulp were 16.8 : 83.2, 22.4 : 77.6 and 28.0 : 72.0, and the top, bottom and intermediate sheets of each combination type press board were constituted by the sheets of the kraft pulp alone. In the combination type press boards, the contents of the poly(3-methylbutene-1) fiber were 10, 15, 20 and 25 wt% in the respective examples. Additionally, in comparative examples, combination type press boards were likewise formed which were each composed of the mixed wet sheets of the poly(3-methylbutene-1) fiber and the kraft pulp and the wet sheets of the kraft pulp alone. In this case, the mixing ratios of the poly(3-methylbutene-1) fiber : the kraft pulp were 5.6 : 94.4 and 39.2 : 60.8, and the top, bottom and intermediate sheets of each combination type press board were constituted by the sheets of the kraft pulp alone. In comparative examples in which the contents of the poly(3-methylbutene-1) fiber in the press boards were 5 and 35 wt%, and in comparative examples in which the kraft pulp was absent and the mixed wet sheets were only used and in which the mixing ratios of the poly(3-methylbutene-1) fiber : the kraft pulp were 15 : 85 and 25 : 75, the results shown in Table 5 were obtained. As be definite from the results in Table 5, in the case of the combination type press boards comprising the wet sheets which are each composed of the poly(3-methylbutene-1) fiber in an amount of 11.2 wt% or more and less than 39.2 wt% and the kraft pulp, having a freeness of 365 ml (CSF) within the range of 200 to 400 ml, in an amount of more than 60.8 wt% and 88.8 wt% or less, the content of the poly(3-methylbutene-1) fiber in these press boards is 10 wt% or more and less than 35 wt%. In these cases, dielectric constant is 3.8 or less, compressive strength is 72 N/mm² (7.3 kg/mm²) (80°C) or more, any linting is not present on the surfaces, and oil impregnation distance is also good. On the contrary, in some comparative examples in which the content of the poly(3-methylbutene-1) fiber is zero, the dielectric constant is high, and also in the case that the content of the fiber is as small as about 5 wt%, the value of the dielectric constant is at a similar level. When the content of the fiber is inversely as great as about 40 wt%, the compressive strength deteriorates remarkably. Further, even when the content of the fiber is 10 wt% or more and less than 30 wt%, the linting is present on the surfaces of the press boards, unless the kraft pulp layers are used for the top and bottom sheets. This fact leads to the degradation in the lightning impulse breakdown voltage. Examples 26 and 30 and Comparative Examples 20 to 24 In each example, a poly(4-methylpentene-1) fiber having a thickness of 0.33 tex (3 denier) and a length of 5 mm was mixed with a kraft pulp having a freeness of 365 ml (CSF) in a weight ratio shown in Table 6 in order to prepare a 1 wt% aqueous slurry. Then, 16 wet sheets (water content was about 80 wt%) in which an absolute dry weight would be 80 g/m² were formed from this slurry. Further, a 1 wt% aqueous slurry comprising the kraft pulp alone having a freeness of 365 ml (CSF) was utilized to make 17 wet sheets in which an absolute dry weight would be 9 g/m². These two kinds of wet sheets were alternately laminated as many as required, as shown in Fig. 1. In this case, the top and bottom sheets of the laminate were made up of the wet sheets of comprising the kraft pulp alone. The composite wet sheets were heated and pressed at 140°C at 392 N/cm² (40 kgf/cm²) for 45 minutes by the use of a hot press in order to form a combination type press board integrally in which the top, bottom and intermediate sheets were composed of the layers of the kraft pulp alone. In Comparative Example 20, no poly(4-methylpentene-1) fiber was used, and in Comparative Examples 21 and 22, combination type press boards were prepared from the mixed wet sheets of the poly(4-methylpentene-1) fiber and the kraft pulp and the sheets of the kraft pulp alone. In this case, the mixing ratios of the kraft pulp : the poly(4-methylpentene-1) fiber were 95 : 5 and 65 : 35, and the top, bottom and intermediate sheets of the press board were made up of by the sheets of the kraft pulp alone. Further, in Comparative Examples 23 and 24, wet sheets in which an absolute dry weight would be 80 g/m² were made in the mixing ratios of a kraft pulp : poly(4-methylpentene-1) fiber were 85 : 15 and 75 : 25. The suitable number of the wet sheets was laminated and then heated and pressed at 140°C at 392 N/cm² (40 kgf/cm²) for 45 minutes by the use of a hot press in order to form combination type press boards integrally. For the thus formed press boards, dielectric constant, compressive strength and linting were measured, and the results are set forth in Table 6. As be apparent from the results in Table 6, when the sheets of the kraft pulp alone are used for the top, bottom and intermediate sheets of the combination type press board and when the content of the poly(4-methylpentene-1) fiber is less than 30 wt% from the viewpoint of the compressive strength and is 10 wt% or more from the viewpoint of the dielectric constant, the press boards can be obtained having the aimed properties in points of the dielectric constant and compressive strength. In addition, the thus obtained press boards have no problems of the linting and dislocation in contrast to such press boards as in Comparative Examples 23 and 24 in which the poly(4-methylpentene-1) fiber and the kraft pulp are merely mixed. Examples 31 to 33 In each example, a poly(4-methylpentene-1) fiber having a thickness of 0.33 tex (3 denier) and a length of 5 mm and a polyethylene terephthalate fiber having a thickness of 1.5 denier and a length of 5 mm were mixed with a kraft pulp having a freeness of 365 ml (CSF) in a proportion shown in Table 7 given below in order to prepare a 1 wt% aqueous slurry. From this slurry, 16 wet sheets were made in which an absolute dry weight would be 80 g/m². Further, a 1 wt% aqueous slurry comprising the kraft pulp alone having a freeness of 365 ml (CSF) was used to make 17 wet sheets in which an absolute dry weight would be 9 g/m². These two kinds of wet sheets were laminated alternately, as shown in Fig. 1. In this case, the top and bottom sheets of the laminate were made up of the wet sheets comprising the kraft pulp alone. This composite wet sheets were heated and pressed at 140°C at 392 N/cm² (40 kgf/cm²) for 45 minutes to form a press board integrally. For the thus formed press board, dielectric constant, compressive strength and linting were measured, and the results are set forth in Table 7. As be apparent from the results in Table 7, with regard to the press boards obtained by first mixing the poly(4-methylpentene-1) fiber and the polyethylene terephthalate fiber with the kraft pulp in order to make the wet sheets, then laminating these wet sheets and the other sheets of the kraft pulp alone so that the optional number of the latter sheets may constitute the top and bottom sheets of the laminate, and finally heating and pressing the laminate integrally, these press boards are a little worse in dielectric constant but are higher in compressive strength than the press boad formed in Example 30 in which the poly(4-methylpentene-1) fiber is only used as the fiber, even though the content of the polymeric fiber based on the total weight of the press board is equal. Therefore, it can be appreciated that the press boards obtained in these examples are well balanced between the dielectric constant and compressive strength. The content of the polyethylene terephthalate fiber is preferably less than 25 wt% based on the total weight of the press board (in terms of a fraction in the mixed wet sheets, the content of the terephthalate fiber is preferably less than 28 wt%). If the content of the polyethylene terephthalate fiber is in excess of this level, the content of the poly(4-methylpentene-1) fiber is restricted, so that the feature of the poly(4-methylpentene-1) fiber will be lost. Further, since the kraft pulp sheets are used for the top and bottom sheets of the press board, the linting and dislocation on the surface thereof can be prevented. Examples 34 to 36 In each example, a poly(4-methylpentene-1) fiber having a thickness of 0.33 tex (3 denier) and a length of 5 mm and a polyphenylene sulfide fiber having a thickness of 4.2 denier and a length of 5 mm were mixed with a kraft pulp having a freeness of 365 ml (CSF) in a proportion in Table 8 given below in order to prepare a 1 wt% aqueous slurry. From this slurry, 16 wet sheets were made in which an absolute dry weight would be 80 g/m². Further, a 1 wt% aqueous slurry comprising the kraft pulp alone having a freeness of 365 ml was used to make 17 wet sheets in which an absolute dry weight would be 9 g/m². These two kinds of wet sheets were laminated alternately, as shown in Fig. 1. In this case, the top and bottom sheets of the laminate were constituted by the wet sheets comprising the kraft pulp alone. The composite wet sheets were heated and pressed at 140°C at 392 N/cm² (40 kgf/cm²) for 45 minutes to form a press board integrally. For the thus formed press board, dielectric constant and compressive strength were measured, and the results are set forth in Table 8. In addition, linting on the press board was also measured. As be apparent from the results in Table 8, as in Examples 31 to 33, with regard to the press boards each obtained by first mixing the poly(4-methylpentene-1) fiber and the polyphenylene sulfide fiber with the kraft pulp in order to make mixed wet sheets, then laminating the thus made mixed wet sheets and other wet sheets comprising the kraft pulp alone so that the optional number of the kraft pulp sheets may constitute the top and bottom sheets of the laminate, and finally heating and pressing the laminate integrally, these press boards are a little worse in dielectric constant but are higher in compressive strength than the press boad formed in Example 30 in which the poly(4-methylpentene-1) fiber alone is used as the fiber, even though the content of the polymeric fiber based on the total weight of the press board is equal. The content of the polyphenylene sulfide fiber is preferably less than 25 wt% based on the total weight of the press board, as in Examples 31 to 33. Further, since the kraft pulp sheets are used for the top and bottom sheets of the press board, the linting and dislocation on the surface can be prevented. Moreover, the combination type press boards obtained in Examples 26 to 36 are more excellent in the impregnation of the insulating oil than conventional press boards, because the former press boards undergo the heating/pressing treatment at 110 to 190°C in order to dry them, so that the thermal fusion of the polymeric fibers is prevented. Fig. 2 shows a relation between a ratio of a lightning impulse breakdown voltage and the dielectric constant of the press board. This ratio of the lightning impulse breakdown voltage is a value measured by the use of the model for a lightning impulse breakdown test and is a relative proportion in the case that the lightning impulse breakdown voltage of the conventional press board PB-2 is regarded as 100%. The result of Fig. 2 indicates that lowering the dielectric constant of the press board effectively increases the breakdown voltage in an oil space between the press board and the kraft insulating sheet layer, and mitigates an electric field therein. As be definite from the results of Examples 31 to 33, when the polyethylene terephthalate fiber having a dielectric constant of 3.15 and the poly(4-methylpentene-1) fiber having a dielectric constant of 2.1 are mixed with the kraft pulp, the dielectric constant of the formed press board can be reduced to 3.8 or less. Therefore, when the poly(3-methylbutene-1) fiber having a dielectric constant of 2.1 and the poly(4-methylpentene-1) fiber having a dielectric constant of 2.1 are mixed with the kraft pulp, the dielectric constant of the formed press board can be reduced to 3.8 or less and simultaneously the comressive strength can be maintained at a level of 72 N/mm² (7.3 kg/mm²) or more. Example 37 and Comparative Example 25 First, 29.5 wt% of a poly(4-methylpentene-1) fiber having a thickness of 0.33 tex (3 denier) and a length of 5 mm was mixed with 70.5 wt% of a kraft pulp having a freeness of 365 ml (CSF) to prepare an aqueous slurry, and wet sheets were then made in accordance with a wet paper making process. Afterward, the 19 wet sheets were laminated. As shown in Fig. 4, nets 9 of 150 µm (100 mesh) were disposed over the top and under the bottom of the wet sheets 10. The laminate was then heated and pressed at a temperature of 140°C under a pressure of 392 N/cm² (40 kgf/cm²) for 45 minutes to integrally form a low-dielectric constant press board of 1.6 mm in thickness. In Comparative Example 25, the same procedure as in Example 37 was repeated with the exception that nets of 270 µm (55 mesh) were used. Table 9 sets forth the opening in µm (mesh numbers) of the used nets, dielectric constants and values (Rz) of a 10-point height of the formed press boards. Kind of Net Dielectric Constant at 20°C Surface Roughness (µm) Example 37 150 µm (100 mesh) 2.90 95 Comp. Ex. 25 270 µm (55 mesh) 2.90 210 Examples 38 to 40 and Comparative Examples 26 to 28 A poly(4-methylpentene-1) fiber having a thickness of 0.33 tex (3 denier) and a length of 5 mm was mixed with a kraft pulp having a freeness of 365 ml (CSF) in a proportion of the fiber : the pulp being 11.2 : 88.8 in order to prepare a 1 wt.% slurry. From this slurry, 16 mixed wet sheets (the content of water was about 80%) were made in which an absolute dry weight would be 80 g/m². Further, another 1 wt.% aqueous slurry was prepared comprising the kraft pulp alone the freeness of which was 365 ml (CSF), and from this slurry, 17 wet sheets were made in which an absolute dry weight would be 9 g/m². The mixed wet sheets and the kraft pulp sheets were alternately laminated as many as required so that the kraft pulp sheets might be used over and under the mixed wet sheets so as to sandwich them. The thus made composite wet sheets were heated and pressed at 140°C under 392 N/cm² (40 kgf/cm²) for 45 minutes by the use of a net of 150 µm (100 mesh) and a hot press in order to form a press board integrally. In like manner, mixed sheets were formed from the kraft pulp and the poly(4-methylpentene-1) fiber in a proportion of the pulp : the fiber being 80 : 20 in Example 39 and 70.5 : 29.5 in Example 40, and these mixed wet sheets and the other 100% kraft pulp sheets were used to form combination type press boards having a low dielectric constant. In this case, the top, bottom and intermediate sheets of the press board were composed of the 100% kraft pulp sheets. Incidentally, in Comparative Examples 26 to 28, a net of 270 µm (55 mesh) was used in like manner to form press boards. In Table 10, there are set forth mesh sizes of the used nets as well as dielectric constants, values (Rz) of a 10 point height and ratios of a lightning impulse breakdown voltage of the thus obtained press boards. As be apparent from the above explanation and examples, in the present invention, the poly(4-methylpentene-1) fiber and/or the poly(3-methylbutene-1) fiber is selected as the fiber which is mixed with the kraft pulp, in order to form the press board having a thickness of 0.5 mm or more. Therefore, the content of the fibers can be reduced to a low level of 10 wt% or more and less than 30 wt% based on the total weight of the press board, and the compressive strength of the press board, which is the essential requirement thereof, can be maintained at 72 N/mm² (7.3 kg/mm²) (80°C) or more, with the dielectric constant thereof kept at 3.8 or less. Further, the compressive strength of the press board can be built up, without sacrificing the dielectric constant, by mixing at least one selected from polyethylene terephthalate fibers and polyphenylene sulfide fibers with the above-mentioned fiber. When the wet sheets made from the kraft pulp alone are used for the top, bottom and intermediate sheets of the mixed wet sheet laminate which is composed of the fiber and the kraft pulp, the linting and dislocation of the fiber can be prevented, whereby the lightning impulse breakdown voltage can be inhibited from deteriorating. In addition, since the heating/pressing process for drying the mixed wet sheets is carried out at a temperature below a melting point of the fiber, the impregnation of the oil into the press board remains good. Furthermore, the linting on the surface of the press board can be eliminated by thermally treating this surface alone at a temperature of 220°C or more, and the lightning impulse breakdown voltage can be maintained at a high level by reducing a value of the 10 point height to 100 µm or less.

Claims: A low-dielectric constant press board for oil impregnation insulation which is formed by laminating mixed wet sheets of a poly(4-methylpentene-1) fiber and/or a poly(3-methylbutene-1) fiber and a kraft pulp, the fibers being used in an amount of 5,6 to less than 30 wt-%, based on the press board, and then drying the laminate integrally by heating/pressing at a temperature of 110 to 190 °C under a pressure of 98 to 490 N/cm² (10 to 50 kgf/cm²). A low-dielectric constant press board for oil impregnation insulation which is formed by mixing a poly(4-methylpentene-1) fiber and/or a poly(3-methylbutene-1) fiber in an amount of 5,6 to less than 30 wt-%, based on the press board, with a kraft pulp to make mixed wet sheets, disposing wet sheets of kraft pulp alone over and under said mixed wet sheets, or over, under and between said mixed wet sheets, and then drying the laminate integrally by heating/pressing at a temperature of 110 to 190 °C under a pressure of 98 to 490 N/cm² (10 to 50 kgf/cm²). A low-dielectric press board for oil impregnation insulation according to any one of Claims 1 or 2 wherein the amount of said poly(4-methylpentene-1) fiber and/or said poly(3-methylbutene-1) fiber based on the press board is 10 wt-% or more and less than 30 wt-%. A low-dielectric press board for oil impregnation insulation according to any one of Claims 1 to 3 wherein said kraft pulp has a freeness of 200 to 400 ml (CSF). A low-dielectric press board for oil impregnation insulation according to any one of Claims 1 to 3 wherein said press board further contains poly-ethylene terephthalate fibers and/or polyphenylene sulfide fibers in an amount of less than 28 wt-%. A low-dielectric press board for oil impregnation insulation according to Claim 1 wherein the surface of the dried laminate has been subjected to a thermal treatment at a temperature of 220 °C or more. A low-dielectric press board for oil impregnation insulation according to any one of Claims 1 to 6 wherein the surface of said press board has a 10 point height of 100 µm or less. A process for producing the low-dielectric constant press board for oil impregnation insulation according to claim 1 which comprises laminating mixed wet sheets of a poly(4-methylpentene-1) fiber and/or a poly(3-methylbutene-1) fiber and a kraft pulp, wherein the fibers are used in an amount of 5,6 to less than 30 wt-%, based on the press board, and then drying the laminate integrally by heating/pressing at a temperature of 110 to 190 °C under a pressure of 98 to 490 N/cm² (10 to 50 kgf/cm²). A process for producing the low-dielectric constant press board for oil impregnation insulation according to claim 2 which comprises mixing a poly(4-methylpentene-1) fiber and/or a poly(3-methyl-butene-1) fiber in an amount of 5,6 to less than 30 wt-%, based on the press board, with a kraft pulp to make mixed wet sheets, disposing wet sheets of kraft pulp alone over and under said mixed wet sheets, or over, under and between said mixed wet sheets, and then drying the laminate integrally by heating/pressing at a temperature of 110 to 190 °C under a pressure of 98 to 490 N/cm² (10 to 50 kgf/cm²). The process according to claim 8 or 9, which comprises the use of a net having openings of 150 µm or less (100 meshes or more) in the step of heating/pressing. The process according to claim 8, which comprises thermally treating the dried laminate at a temperature of 220 °C or more.

Inventors: ENDO, KAORU, HYODO, KENJI, ISHIZUKA, HUMIHIKO, KAMATA, YUZURU, NOZAKI, MASAOKI, OOE, ETSUO, TANI, ICHITARO, TSUKIOKA, HIDEO
Applicants: HITACHI, LTD., MITSUBISHI PAPER MILLS, LTD.
Application year: 1987
Main classification: H01B3
IPC codes: D21H13, H01B3, D21H27, D21H11