JP2009277653A - Electrical insulating paper - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electric insulating paper which is superior in insulation destruction strength, moisture absorption dimensional stability, thermal dimensional stability, and process passage characteristics at the time of paper making. <P>SOLUTION: In a wet nonwoven cloth used as an electrical insulating paper containing a fibrillated aramid fiber and a polyphenylene sulfide short fiber; a percent of non-extended polyphenylene sulfide short fiber of the polyphenylene sulfide short fiber is 95 mass% or higher; a mixing rate of the non-extended polyphenylene sulfide short fiber in the wet nonwoven cloth is 50-99 mass%; and the insulating destruction strength is 15 kV/mm or higher. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an electrical insulating paper.

  An electrically insulating paper obtained by mixing aramid fiber and polyphenylene sulfide fiber is disclosed (Patent Documents 1 and 2). However, these mixed papers have through-holes that allow air to pass therethrough, and since current is passed through the through-hole portions, the dielectric breakdown strength cannot be improved.

  Patent Document 3 discloses a battery separator made of a wet nonwoven fabric containing pulp-like aramid fibers and unstretched polyphenylene sulfide fibers. However, unlike the electrically insulating paper, this wet nonwoven fabric has many voids in the wet nonwoven fabric in order to make the liquid absorption rate of the electrolyte excellent. Since the dielectric breakdown occurs through partial discharge in the gap portion, sufficient breakdown strength cannot be obtained.

JP 7-189169 A JP-A-2-47389 JP 2001-40597 A

  An object of the present invention is to provide an electrical insulating paper that is excellent in dielectric breakdown strength, excellent in hygroscopic dimensional stability and thermal dimensional stability, and excellent in process passability during papermaking.

  As a result of intensive studies to solve this problem, the present invention mainly has the following configuration. That is, a wet nonwoven fabric comprising fibrillated aramid fibers and polyphenylene sulfide short fibers, wherein the fraction of unstretched polyphenylene sulfide short fibers in the polyphenylene sulfide short fibers is 95% by mass or more, and unstretched polyphenylene in the wet nonwoven fabric An electrically insulating paper comprising a wet nonwoven fabric having a mixed ratio of sulfide short fibers of 50 to 99 mass% and a dielectric breakdown strength of 15 kV / mm or more.

  According to the present invention, it is possible to provide an electrical insulating paper that is excellent in dielectric breakdown strength, excellent in hygroscopic dimensional stability and thermal dimensional stability, and excellent in process passage during papermaking.

  The electrical insulating paper of the present invention comprises a wet nonwoven fabric containing fibrillated aramid fibers and polyphenylene sulfide (hereinafter referred to as PPS) short fibers, and the PPS short fibers include unstretched PPS short fibers.

  The fibrillated aramid fiber can be used to eliminate large voids in the wet nonwoven fabric to form small voids, melt the unstretched PPS short fibers, and eliminate the small voids to improve the electrical breakdown strength.

  The wet nonwoven fabric in the present invention contains aramid fibers. By including an aramid fiber, a wet nonwoven fabric excellent in thermal dimensional stability and flame retardancy can be obtained. In particular, since excellent thermal dimensional stability can be obtained, heat shrinkage and the like when paper is made and dried are suppressed, and continuous paper making can be stably performed.

  The aramid fiber is not particularly limited as long as it is a general aromatic polyamide, and examples thereof include polyparaphenylene terephthalamide, copolyparaphenylene 3,4′oxydiphenylene terephthalamide, and polymetaphenylene terephthalamide. Among them, polyparaphenylene terephthalamide, which is easy to fibrillate and has excellent heat resistance, is preferable.

  The aramid fiber is preferably at least partially fibrillated. Here, fibrillation means that a fiber has a portion that is split into two or more in the vertical direction and is thinner than a single fiber. By forming into fibrils, the entanglement between the fibers is improved, and the paper strength is improved, so that a wet nonwoven fabric excellent in process passability during papermaking can be obtained. Further, a dense wet nonwoven fabric can be obtained by reducing the large gaps between fibers with fibrils.

  The aramid fiber is preferably a fibrillated short fiber. Short fibers are easier to obtain wet nonwoven fabrics with better water dispersibility and smaller unevenness per unit area.

  The wet nonwoven fabric in the present invention contains PPS short fibers. By including PPS short fibers, a wet nonwoven fabric excellent in hygroscopic dimensional stability can be obtained.

  PPS is a polymer containing phenylene sulfide units such as p-phenylene sulfide units and m-phenylene sulfide units as repeating units. PPS may be a homopolymer of any of these units, or a copolymer having both units. Moreover, the copolymer with another aromatic sulfide may be sufficient.

  Moreover, as a weight average molecular weight of PPS, 40000-60000 are preferable. By setting it to 40,000 or more, good mechanical properties as PPS fibers can be obtained. Further, by setting the viscosity to 60000 or less, the viscosity of the melt spinning solution is suppressed, and a special high pressure resistant spinning equipment is not required.

  It is important that the PPS short fibers in the wet nonwoven fabric in the present invention include unstretched PPS short fibers. Since unstretched PPS short fibers are easily deformed when heated and pressurized, the voids are crushed and fused with aramid fibers, PPS short fibers (unstretched PPS short fibers, stretched PPS short fibers), etc., constituting the wet nonwoven fabric, A dense wet nonwoven fabric can be obtained. As described above, since at least a part of the unstretched PPS short fibers is fused, the dielectric breakdown strength can be improved and a dielectric breakdown strength of 15 kV / mm or more can be achieved.

  It is important that the fraction of unstretched PPS short fibers in the PPS short fibers contained in the wet nonwoven fabric is 95% by mass or more. By setting it as 95 mass% or more, a dense wet nonwoven fabric can be obtained and a dielectric breakdown strength can be improved.

  The unstretched PPS fiber refers to a PPS fiber obtained without being generally stretched after melt spinning through a die with an extruder-type spinning machine or the like.

  The mixing ratio of unstretched PPS short fibers in the entire wet nonwoven fabric is preferably 50 to 99% by mass. If it is less than 50% by mass, the ratio of unstretched PPS short fibers is not sufficient, the voids in the wet nonwoven fabric cannot be sufficiently crushed, and the intended dielectric breakdown strength cannot be obtained. On the other hand, when the amount is more than 99% by mass, the ratio of aramid fibers is too small to obtain sufficient dimensional stability. Furthermore, the mixing ratio of the unstretched PPS short fibers is preferably 70 to 99% by mass with respect to the entire wet nonwoven fabric.

As the single fiber fineness of the PPS short fibers in the wet nonwoven fabric in the present invention, both undrawn fibers and drawn fibers are preferably 0.05 dtex or more and 5 dtex or less. If it is thinner than 0.05 dtex, the fibers are easily entangled and it is difficult to uniformly disperse them. If it is thicker than 5 dtex, the fiber becomes thick and hard, and the entanglement force between the fibers becomes weak, so that sufficient paper strength cannot be obtained, resulting in a wet nonwoven fabric that is easily torn.
Moreover, as a fiber length of a PPS short fiber, 0.5-15 mm is preferable in both undrawn fiber and a drawn fiber, More preferably, it is 1-8 mm. By setting it as 0.5 mm or more, the strength of the wet nonwoven fabric can be increased by entanglement of the fibers. Moreover, by setting it as 25 mm or less, it can prevent that an unevenness etc. arise by the entanglement of fibers becoming lumps.

  The fibrillated aramid fiber and the PPS short fiber are mixed and obtained to obtain a dry web by a commonly used paper machine, and heat and pressure treatment is performed to melt the unstretched PPS short fiber, thereby crushing the gap. An electrically insulating paper of mm or more can be obtained. When the dielectric breakdown strength is 15 kV / mm or more, it can be used as electrical insulating paper used in motors, transformers, and the like.

  In addition, the dielectric breakdown strength in this invention says the value measured according to JISK6911: 1995. That is, test specimens of about 10 cm × 10 cm were taken from five different places of the sample, the test specimens were sandwiched between disc-shaped electrodes having a diameter of 25 mm and a mass of 250 g, air was used as the test medium, and the test medium was 0.25 kV / sec. While increasing the voltage, an AC voltage having a frequency of 60 Hz is applied to measure the voltage when dielectric breakdown occurs. The obtained dielectric breakdown voltage is divided by the thickness of the central portion measured in advance, and the calculated value is taken as the dielectric breakdown strength.

  Next, a method for producing the wet nonwoven fabric of the present invention will be described.

  The unstretched PPS fiber can be obtained by melt-spinning a PPS polymer with an extruder-type spinning machine or the like and processing it without stretching. Further, the stretched PPS fiber is obtained by melt spinning the PPS polymer with an extruder type spinning machine or the like in the same manner as the unstretched PPS fiber, and is 3.0 times or more, preferably 5.5 times or less, more preferably 3.5 to It can be obtained by stretching in the range of 5.0 times. This stretching may be performed in one stage, but may be performed in two or more stages. In the case of using two-stage stretching, the first stage of stretching is preferably 70% or more of the total magnification, preferably 75 to 85%, and the rest is preferably performed by the second stage of stretching. The obtained undrawn yarn and drawn yarn may be cut without being crimped, or may be cut with crimp. As for the presence or absence of crimp in the PPS short fibers, there are advantages to those having and not having crimps. PPS short fibers having crimps are suitable for obtaining a wet nonwoven fabric having improved strength by improving the entanglement between fibers. On the other hand, short PPS fibers that do not have crimps are suitable for obtaining a uniform wet nonwoven fabric with little unevenness.

  Next, an aramid fiber will be described by taking a para-aramid fiber as an example. Para-aramid fibers can be obtained by using a high-pressure homogenizer and passing a suspension of para-aramid at a high speed through an orifice having a small diameter and discharging the suspension.

  As a means for fibrillating the para-aramid fiber, for example, a method can be used in which a wall is installed near the outlet of the orifice, and the para-aramid fiber immediately after discharge collides with the wall to give an impact to the fiber. Further, after forming a cut fiber, the fiber may be fibrillated by grinding with a Niagara beater, a homogenizer, a disc refiner, a reiki machine, a mortar and mortar, a water jet punch, or the like.

  An example of a method of blending para-aramid fibers and PPS short fibers as described above into a wet nonwoven fabric will be described. First, para-aramid fibers and short PPS fibers are dispersed in water to form a papermaking dispersion.

  The total amount of para-aramid fiber and PPS fiber with respect to the papermaking dispersion is preferably 0.005 to 5% by mass. By making the total amount 0.005% by mass, water in the paper making process can be used efficiently. Moreover, by making it 5 mass% or less, the dispersion state of a fiber becomes good and a uniform wet nonwoven fabric can be obtained.

  The dispersion may be prepared in advance by separately preparing a dispersion of para-aramid fibers and a dispersion of PPS fibers, and then mixing both with a paper machine, or a dispersion containing both may be made directly. It is preferable to make the dispersion of each fiber separately and then mix them in that the stirring time can be controlled separately according to the shape and characteristics of each fiber. Is preferable in terms of process simplification.

  In order to improve water dispersibility, dispersants for papermaking, such as dispersants and oils composed of cationic, anionic, and nonionic surfactants, and antifoaming agents that suppress foaming are added. May be.

  The paper-making dispersion prepared as described above is used to make paper using a round-mesh type, long-mesh type, inclined net-type paper machine, or hand-made paper machine, and this is dried with a Yankee dryer or a rotary dryer. A wet nonwoven fabric obtained by applying a heating / pressurizing treatment to the web can be used as an electrically insulating paper. In the present invention, simultaneous heating and pressurization is referred to as a heating / pressurizing process, and is distinguished from a process in which no pressure is applied only by heating such as drying. The dry web refers to a non-woven fabric that has been subjected to wet papermaking and that has not been subjected to the heating / pressurizing treatment.

  In order to increase the dielectric breakdown strength of the electrical insulating paper, the crystallization heat amount of the dry web is preferably 10 J / g or more. The amount of crystallization heat is preferably 15 J / g or more. In order to set the crystallization heat amount of the dry web to 10 J / g or more, it is important that the unstretched PPS short fibers are not completely crystallized in the paper making process. Specifically, in order to achieve this heat of crystallization, it is preferable to set the drying temperature in the paper making process to (the crystallization temperature of unstretched PPS short fibers + 10 ° C.) or less, more preferably less than the crystallization temperature. It is preferable. In particular, at the crystallization temperature to the crystallization temperature + 10 ° C., the crystallization of the unstretched PPS short fibers is likely to proceed, so it is preferable to shorten the time for passing through the drying step. Here, the drying temperature in the papermaking process refers to the maximum processing temperature (atmospheric temperature) during drying in the papermaking process.

  The amorphous portion of the unstretched PPS short fiber remains by performing a drying treatment at a temperature equal to or lower than (crystallization temperature of unstretched PPS short fiber + 10 ° C.). Amorphous PPS is soft and easily plastically deformed, so when subjected to heat and pressure treatment, it can be deformed to fill voids, reduce the number of through-holes, etc. The breaking strength can be improved. If the drying temperature is too low, the moisture cannot be evaporated and cannot be dried. Therefore, the drying temperature is preferably 80 ° C. or higher, more preferably 95 ° C. or higher.

  The crystallization temperature refers to the apex temperature of the main exothermic peak measured under the same conditions as the crystallization calorimetry in the section [Measurement / Evaluation Method] (1) in the column of Examples described later.

  In the production of the present invention, it is important to include a step of heating and pressurizing a dry web obtained by mixing fibrillated para-aramid fibers and PPS short fibers including unstretched PPS short fibers. By performing the heating and pressurizing treatment, the unstretched PPS short fibers can be melted and softened as described above, the gaps are crushed and the fibers are fused, and a dielectric breakdown strength of 15 kV / mm or more can be achieved. As a means for heating and pressurizing, any means may be used. For example, a hot press using a flat plate, a calendar, or the like can be employed. Among these, a calendar that can be processed continuously is preferable. As the calendar roll, a metal-metal roll, a metal-paper roll, a metal-rubber roll, or the like can be used.

  In the wet nonwoven fabric in the present invention, the temperature condition of the heating / pressurizing treatment is preferably a temperature not lower than the glass transition temperature of the unstretched PPS short fibers and not higher than the melting point, more preferably 160 to 250 ° C, and further preferably 160 to 220 ° C. It is. If the treatment temperature is lower than the glass transition temperature of the unstretched short PPPS fibers, the fibers are not heat-sealed and a dense wet nonwoven fabric cannot be obtained. On the other hand, when the melting point is exceeded, the unstretched PPS short fibers become too soft and stick to a heat and pressure apparatus such as a calender roll or a hot press plate, and stable mass production cannot be performed. In addition, a wet nonwoven fabric can be produced with a rough surface.

  The pressure when calendering is employed as the heating / pressurizing treatment is preferably 98 to 7000 N / cm. The space | interval between fibers can be crushed by setting it as 98 N / cm or more. On the other hand, by setting it to 7000 N / cm or less, it is possible to prevent the wet non-woven fabric from being broken in the heating / pressurizing treatment step and stably perform the treatment. As process speed, 1-30 m / min is preferable, More preferably, it is 2-20 m / min. Good working efficiency can be obtained by setting it as 1 m / min or more. On the other hand, by setting it to 30 m / min or less, heat can also be conducted to the fibers inside the wet nonwoven fabric, and the effect of heat fusion of the fibers can be obtained.

  The wet nonwoven fabric obtained in this way can be used as insulating paper, punched out, bent into a predetermined shape, inserted into a motor, and used as a wedge, slot liner, or interphase paper. Moreover, in a transformer, it can also be used as insulation paper between coil wires or interlayer insulation paper. Furthermore, an epoxy-based or polyester-based pressure-sensitive adhesive can be applied to the wet nonwoven fabric and used as an insulating tape used for fixing coils and lead wires.

[Measurement and evaluation method]
(1) Heat of crystallization About 2 mg of dry web sample after drying (before heating and pressure treatment) is precisely weighed and heated at a rate of temperature increase of 10 ° C. under nitrogen with a differential scanning calorimeter (Shimadzu, DSC-60). The temperature was raised in minutes, and the calorific value of the observed main exothermic peak was measured.

(2) Basis weight According to JIS L 1906: 2000, three 20 cm × 20 cm test pieces were collected per 1 m width of the sample, each mass (g) in the standard state was measured, and the average value per m ² Expressed in mass (g / m ² ).

(3) Thickness According to JIS L 1906: 2000 applied mutatis mutandis to JIS L 1906: 2000, thickness was measured under a pressure of 2 kPa with a pressurizer having a diameter of 22 mm, using a thickness measuring machine at 10 different locations of the sample. In order to calm the thickness, after waiting for 10 seconds, the thickness was measured, and an average value was calculated.

(4) Dielectric breakdown strength Measured according to JIS K 6911: 1995. Test specimens of about 10 cm × 10 cm are collected from five different specimens, and the specimens are sandwiched between disc-shaped electrodes with a diameter of 25 mm and a mass of 250 g, air is used as the test medium, and a voltage is applied at 0.25 kV / second. While increasing, an AC voltage having a frequency of 60 Hz was applied, and the voltage when dielectric breakdown was measured. The obtained dielectric breakdown voltage was divided by the thickness of the central portion measured in advance, and the dielectric breakdown strength was calculated.

(5) Moisture absorption dimensional change rate Test specimens of vertical 25cm x horizontal 5cm were collected from three different specimens, dried in a desiccator containing silica gel for 24 hours at 25 ° C, and spaced 200.0mm apart in the vertical direction. I put a sign of

Next, after leaving the test piece in a constant temperature / humidity chamber adjusted to 90% relative humidity and 25 ° C. for 6 hours, the test piece is taken out from the constant temperature / humidity chamber, and the interval L of the mark is measured within 10 minutes. The dimensional change rate was calculated by the following formula. The average value was calculated and rounded to one decimal place.
Dimensional change rate (%) = (1−L / 200.0) × 100
Here, L: the distance (mm) between the marks after standing at 90% relative humidity.

(6) Thermal dimensional change rate (dry heat shrinkage rate)
Samples of 100.0 mm x 100.0 mm were collected from three different specimens, heat-treated in a hot air dryer at 180 ° C for 4 hours, allowed to stand at 25 ° C for 2 hours, and then the area of the test piece was measured. Then, the thermal dimensional change rate was calculated by the following formula, the average value was calculated and rounded to one decimal place.
Thermal dimensional change rate (%) = {(10000−A) / 10000} × 100
Here, A: Area (mm ² ) of the test piece after the heat treatment.

[Examples 1-6, Comparative Examples 1-5]
Wet nonwoven fabrics were produced by the following methods so that the fraction / mixing ratio and basis weight of each fiber were as shown in Table 1.

(Para-aramid fiber)
As a para-aramid fiber having fibrils, “Kevlar” pulp manufactured by DuPont, product number 1F303 was used.

(Para-aramid fiber dispersion)
The above para-aramid fiber is roughly divided into the numbers shown in Table 1 rounded up to the first decimal place, and each portion is divided into 1 aliquot and put into a domestic juicer mixer with 1 L of water. Was repeated to obtain a dispersion. The stirring time was 15 seconds.

(Unstretched PPS short fiber)
As the unstretched PPS short fibers, “Torcon” manufactured by Toray with a single fiber fineness of 3.0 dtex and a cut length of 6 mm, product number S111 was used.

(Unstretched PPS short fiber dispersion)
The unstretched PPS short fibers are roughly divided into the numbers rounded up to the first decimal place for each mass listed in Table 1, and each aliquot is taken into a household juicer mixer with 1 L of water and stirred. This was repeated to obtain a dispersion. The stirring time was 10 seconds in order to prevent the fibers from getting tangled.

(Drawn PPS short fiber)
As the drawn PPS short fiber, “Torucon” manufactured by Toray Industries, Inc., product number S301 having a single fiber fineness of 1.0 dtex and a cut length of 6 mm was used.

(Dispersion of drawn PPS short fibers)
Each of the above drawn PPS short fibers is roughly equally divided into the numbers obtained by rounding up the first decimal place of the mass shown in Table 1, and each equal portion is poured into a domestic juicer mixer together with 1 L of water and stirred. Was repeated to obtain a dispersion. The stirring time was 10 seconds in order to prevent the fibers from getting tangled.

(Paper)
The fiber dispersion used in each of the examples and comparative examples is a handmade paper machine having a size of 25 cm × 25 cm and a height of 40 cm in which a handmade paper net having a size of 25 cm × 25 cm is installed at the bottom of 140 mesh (Kumaya Riki Kogyo Co., Ltd.) The total amount of the papermaking dispersion was adjusted to 20 L and sufficiently stirred with a stir bar.
Water from the handsheet paper machine was drained, and the wet paper remaining on the paper web was transferred to filter paper.

(Dry)
The above wet paper is put together with the filter paper into a rotary dryer, and the process of drying at a process passing speed of 0.5 m / min and a process length of 1.25 m (processing time 2.5 min) is repeated 3 times for each of the front and back surfaces, a total of 6 times. It was. The drying temperature is shown in Table 1.

(Heat and pressure treatment)
The wet nonwoven fabric subjected to the drying treatment was peeled off from the filter paper and passed through a calendering machine composed of an iron roll and a paper roll. The calendar pressure was 100 kN / 25 cm (4 kN / cm), and the treatment was performed twice, once for each of the front and back surfaces. The processing temperature and processing speed are shown in Table 1.

  In Examples 1 to 6, electrical breakdown strength was high, and electrical insulating paper excellent in hygroscopic dimensional stability and thermal dimensional stability could be obtained. Moreover, when the surface of the test piece was observed with an electron microscope at a magnification of 300, it was confirmed that there was fusion with unstretched PPS short fibers.

  The electrical insulation paper of the present invention can be used as electrical insulation paper used for motors, capacitors, transformers, cables and the like.

Claims (4)

A wet nonwoven fabric comprising fibrillated aramid fibers and polyphenylene sulfide short fibers, wherein the fraction of unstretched polyphenylene sulfide short fibers in the polyphenylene sulfide short fibers is 95% by mass or more, and the unstretched polyphenylene sulfide short in the wet nonwoven fabrics An electrically insulating paper comprising a wet nonwoven fabric having a fiber mixing ratio of 50 to 99 mass% and a dielectric breakdown strength of 15 kV / mm or more. 2. A dry web having a heat of crystallization of 10 J / g or more is obtained by subjecting a dry web to a heating and pressing treatment at a temperature not lower than the glass transition temperature and not higher than the melting point of the unstretched polyphenylene sulfide short fibers. An electrically insulating paper comprising the wet nonwoven fabric described in 1. The electrically insulating paper according to claim 1, wherein a mixing ratio of the unstretched polyphenylene sulfide short fibers in a wet nonwoven fabric is 70 to 99% by mass. The electrically insulating paper according to claim 1, wherein the aramid fiber is a para-aramid fiber.