阅读说明：本技术 包含间位芳族聚酰胺和聚苯硫醚的湿式无纺布及其层叠片材 (Wet nonwoven fabric comprising meta-aramid and polyphenylene sulfide, and laminated sheet thereof ) 是由 原田大 土仓弘至 于 2018-06-05 设计创作，主要内容包括：为了提供绝缘击穿强度优异、吸湿尺寸稳定性、热尺寸稳定性优异、且撕裂强度、磨损耐久性优异的电绝缘纸以及使用了其的电绝缘片材,提供一种无纺布,其为包含间位芳族聚酰胺纤维和聚苯硫醚短纤维的湿式无纺布,其中,至少一部分经熔接的聚苯硫醚短纤维的混合率为40％以下,无纺布的绝缘击穿强度为17kV/mm以上。(In order to provide an electric insulating paper having excellent dielectric breakdown strength, moisture absorption dimensional stability, thermal dimensional stability, and tear strength and abrasion durability, and an electric insulating sheet using the same, a nonwoven fabric is provided which is a wet nonwoven fabric comprising meta-aramid fibers and polyphenylene sulfide short fibers, wherein the mixing ratio of at least a part of the polyphenylene sulfide short fibers which are welded is 40% or less, and the dielectric breakdown strength of the nonwoven fabric is 17kV/mm or more.)

1. A nonwoven fabric which is a wet-laid nonwoven fabric comprising meta-aramid fibers and polyphenylene sulfide short fibers, wherein the mixing ratio of at least a part of the polyphenylene sulfide short fibers which are welded together is 40% or less, and the nonwoven fabric has a dielectric breakdown strength of 17kV/mm or more.

2. The nonwoven fabric of claim 1, comprising 15% or more meta-aramid fiber.

3. The nonwoven fabric of claim 1 or 2, comprising more than 15% fibrillated meta-aramid fibers.

4. The method for producing a nonwoven fabric according to any one of claims 1 to 3, wherein a wet nonwoven fabric comprising meta-aramid fibers and polyphenylene sulfide short fibers is subjected to a papermaking process and then dried to obtain a dry web, and the dry web is subjected to a heat-pressure treatment at a temperature of not lower than the glass transition temperature of the polyphenylene sulfide short fibers and not higher than the melting point of the meta-aramid fibers.

5. A laminated sheet comprising a thermoplastic resin sheet and the nonwoven fabric according to any one of claims 1 to 3 laminated on at least one surface of the thermoplastic resin sheet.

6. The laminated sheet according to claim 5, wherein the thermoplastic resin constituting the thermoplastic resin sheet is selected from the group consisting of polyethylene terephthalate, polyethylene naphthalate, and polyphenylene sulfide.

7. An electrically insulating paper comprising the nonwoven fabric according to any one of claims 1 to 3.

8. An electrically insulating sheet comprising the laminated sheet of claim 5 or 6.

Technical Field

The present invention relates to a wet-laid nonwoven fabric comprising meta-aramid and polyphenylene sulfide and a laminate (laminated sheet) thereof, which are suitable for use as an electrical insulating paper and an electrical insulating sheet.

Background

There is a demand for paper and laminates thereof in a wide range of fields, and the paper and laminates thereof are used in various members such as filters, separators, and electrical insulating members. Depending on the application, the heat resistance and denseness required for paper and its laminate are various.

In recent years, electric vehicles have been developed due to growing concerns about environmental problems, and in particular, improvement in efficiency and high performance of industrial motors including in-vehicle motors are desired. Among these, the performance of paper, insulating paper or insulating sheet including a laminate thereof used in industrial motors is also required to be improved. The insulating member is required to have heat resistance and durability that differ depending on the use environment, but in industrial motors such as vehicle-mounted motors, the use environment temperature may exceed 100 ℃.

As a material of an insulating paper used in a high temperature environment, for example, an electrical insulating paper obtained by mixing aramid fibers and polyphenylene sulfide fibers, which are heat-resistant fibers, is disclosed in patent documents 1 and 2.

In addition, in order to improve the insulation, it is effective to increase the density of the wet nonwoven fabric, and a method of mixing a pulp-like fiber aggregate may be considered. Further, a method of heating and pressurizing a wet nonwoven fabric to make the structure more dense and improve the insulation characteristics is also known. Further, for example, patent documents 3 and 4 disclose wet nonwoven fabrics containing fibrillated pulp-like aramid fibers and undrawn polyphenylene sulfide fibers.

Disclosure of Invention

Problems to be solved by the invention

However, an insulating member such as insulating paper is required to have not only excellent insulating properties but also mechanical properties and workability, and also workability in disposing and inserting the insulating member into an electric component such as a motor.

Patent documents 1 and 2 disclose electrical insulating papers obtained by mixing aramid fibers and polyphenylene sulfide fibers, but these mixed papers have through holes of a breathable degree, and the dielectric breakdown strength cannot be improved because electricity is applied to the through holes. Further, since the strength and elasticity (Japanese: ハ リ コ シ) of the insulating paper are also weak, it is necessary to bond the film.

Patent document 3 discloses a wet nonwoven fabric containing fibrillated pulp-like aramid fibers and undrawn polyphenylene sulfide fibers and subjected to a hot calendering treatment, but this nonwoven fabric is a nonwoven fabric in which gaps between fibers are reduced by fibrillating the aramid fibers and has an excellent liquid absorption rate of an electrolyte when used as a battery separator. Therefore, it is considered that, in fact, a large number of voids are present in the wet nonwoven fabric, and partial discharge is likely to occur in the void portions, and therefore, not only is sufficient dielectric breakdown strength not obtained, but also the insulation properties are considered to be degraded by long-term use.

Patent document 4 discloses an electrical insulating paper obtained by subjecting a wet nonwoven fabric containing fibrillated aramid fibers and polyphenylene sulfide short fibers to a hot calendering treatment, but the insulating paper specifically disclosed in this document uses para-aramid fibers as the aramid fibers and contains a large amount of undrawn polyphenylene sulfide short fibers, so that the undrawn polyphenylene sulfide short fibers are too molten and filmed by the calendering treatment to become too filmed. As a result, the tear strength is reduced, and the wear durability is insufficient, and there is room for improvement from the viewpoint of paper breakage due to burrs at the end portions of the motor slot (motor slot). In addition, the cost of the insulating paper itself becomes high.

The present invention has been made in view of the problems of the insulating members such as the conventional electric insulating paper, and an object of the present invention is to provide a wet nonwoven fabric excellent in dielectric breakdown strength, moisture absorption dimensional stability, thermal dimensional stability, tear strength, and abrasion durability, a laminate thereof, an electric insulating paper, and an electric insulating sheet using the same.

Means for solving the problems

As a result of intensive studies to solve the problems, the present invention mainly has the following configurations.

(1) A nonwoven fabric which is a wet-laid nonwoven fabric comprising meta-aramid fibers and polyphenylene sulfide short fibers, wherein the mixing ratio of at least a part of the polyphenylene sulfide short fibers which are welded together is 40% or less, and the nonwoven fabric has a dielectric breakdown strength of 17kV/mm or more.

(2) The nonwoven fabric of claim 1, comprising 15% or more meta-aramid fiber.

(3) The nonwoven fabric of claim 1 or 2, comprising more than 15% fibrillated meta-aramid fibers.

(4) The method for producing a nonwoven fabric according to any one of claims 1 to 3, wherein a wet nonwoven fabric comprising meta-aramid fibers and polyphenylene sulfide short fibers is subjected to a papermaking process and then dried to obtain a dry web, and the dry web is subjected to a heat-pressure treatment at a temperature of not lower than the glass transition temperature of the polyphenylene sulfide short fibers and not higher than the melting point of the meta-aramid fibers.

(5) A laminated sheet comprising a thermoplastic resin sheet and the nonwoven fabric according to any one of claims 1 to 3 laminated on at least one surface of the thermoplastic resin sheet.

(6) The laminated sheet according to claim 5, wherein the thermoplastic resin constituting the thermoplastic resin sheet is selected from the group consisting of polyethylene terephthalate, polyethylene naphthalate, and polyphenylene sulfide.

(7) An electrically insulating paper comprising the nonwoven fabric according to any one of claims 1 to 3.

(8) An electrically insulating sheet comprising the laminated sheet of claim 5 or 6.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, by providing the above-described structure, a nonwoven fabric excellent in dielectric breakdown strength, moisture absorption dimensional stability, thermal dimensional stability, tear strength, and abrasion durability, a laminated sheet using the same, and an electrical insulating paper and an electrical insulating sheet can be obtained.

Detailed Description

The present invention is a wet nonwoven fabric comprising meta-aramid fibers and polyphenylene sulfide short fibers, wherein at least a part of the polyphenylene sulfide short fibers are fused at a mixing ratio of 40% or less, and the wet nonwoven fabric has a dielectric breakdown strength of 17kV/mm or more.

The meta-aramid fiber is excellent in heat resistance, thermal dimensional stability, and flame retardancy, and is suppressed in the decrease of mechanical properties even in a high-temperature environment. Further, by heating and pressurizing the polyphenylene sulfide short fibers in combination, the polyphenylene sulfide short fibers can be melted and formed into a film between the meta-aramid fibers, and large gaps in the wet nonwoven fabric can be eliminated. As a result, the electrical insulation breakdown strength can be improved.

The mixing ratio of the meta-aramid fiber is preferably 15% or more, and more preferably 30% or more in the nonwoven fabric from the viewpoint of heat resistance and thermal dimensional stability. The upper limit is preferably 80% or less from the viewpoint of dielectric breakdown strength and hydrolysis resistance. Therefore, when the total mass of the fibers used in the formation of the nonwoven fabric is 100% by mass (hereinafter referred to as "nonwoven fabric-constituting fibers"), the meta-aramid fiber is preferably used at 15% by mass or more, and more preferably at 30% by mass or more. The upper limit is preferably 80% by mass or less.

In the present invention, the meta-aramid means an aromatic polyamide, and an amide group is substituted at the meta position of the benzene ring. The meta-aramid is a linear polymer compound in which 60 mol% or more, preferably 70 mol% or more, more preferably 80 mol% or more, and even more preferably 90 mol% or more of amide bonds are directly bonded to an aromatic ring in terms of chemical structure.

Examples of the meta-aramid include polymetaphenylene isophthalamide (m-phenylene isophthalamide) and a copolymer thereof. These aromatic polyamides can be produced industrially by, for example, a general interfacial polymerization method, a solution polymerization method, or the like, and can be obtained as a commercially available product, but are not limited thereto. Among meta-aramids, polyisophthaloyl metaphenylene diamine is preferably used from the viewpoint of having good properties such as moldability, hot adhesiveness, flame retardancy, and heat resistance.

Examples of the method for fiberizing a meta-aramid include the following methods: a meta-aramid polymer industrially produced by a general method is dissolved in a solvent such as dimethylacetamide, extruded from a die, and desolventized. The following methods are classified according to the manner of desolvation: a dry method of removing the solvent in a high-temperature atmosphere, a wet method of removing the solvent in a poor solvent for meta-aramid such as water, or a combination of these two methods. The meta-aramid fibers obtained by these methods have a uniform cross-sectional shape of 1 fiber in the longitudinal direction of the fiber, and the fibers are not split or branched into 2 or more fibers in the longitudinal direction of the fiber. In the present invention, such a meta-aramid fiber is referred to as a normal meta-aramid fiber.

A typical meta-aramid fiber obtained by the dry method has a flat gourd-like shape in cross section, which is called a dog-bone shape. On the other hand, a typical meta-aramid fiber obtained by a wet process has a shape close to a circle. The fiber cross-sectional shape easily affects the stacking (packing) property between meta-aramid fibers after paper making, and thus the dielectric breakdown strength is changed. In addition, when the dog-bone-shaped meta-aramid is used, the surface smoothness of the obtained wet nonwoven fabric is increased, and the abrasion durability between the wet nonwoven fabrics can be further improved, so that it is preferable to use the meta-aramid fiber obtained by the dry method.

Further, the meta-aramid fiber is preferably fibrillated meta-aramid fiber. Here, fibrillation means that fibers are longitudinally split into 2 or more pieces, and each piece is made thinner than a single fiber or has a portion formed into a thin film. By fibrillation, the interweaving property between fibers is improved and the paper strength is improved, so that a wet nonwoven fabric having excellent process passability in papermaking can be obtained. In addition, large voids between fibers can be further reduced by the filaments, and as a result, an extremely dense wet-laid nonwoven fabric can be obtained, and the insulation breakdown strength is further improved.

The fibrillated meta-aramid fiber is not limited, and the following methods may be mentioned: for example, a method of beating a normal meta-aramid fiber by a mechanical action, and a method of re-precipitating a meta-aramid polymer by dissolving the meta-aramid polymer in a solvent such as dimethylacetamide and extruding the solution while stirring the solution in a poor solvent such as water. The fibrillated meta-aramid fibers may also be fibrillated to a greater degree by further beating them.

The fibrillated meta-aramid fiber may be mixed with a normal meta-aramid fiber and used. The proportion of the fibrillated meta-aramid fiber is preferably 15% or more, more preferably 15 to 50%, even more preferably 20 to 50%, and most preferably 30 to 50% in the nonwoven fabric. Therefore, it is also preferable to use a fibrillated meta-aramid fiber in an amount of 15 mass% or more (more preferably 15 to 50 mass%, even more preferably 20 to 50 mass%, and most preferably 30 to 50 mass%) in the nonwoven fabric constituent fiber. By including fibrillated meta-aramid fibers, the interweaving of the fibers with each other is increased and the strength of the paper is increased. As a result, a wet nonwoven fabric having excellent process passability during papermaking can be obtained. Further, a dense wet-laid nonwoven fabric having a small number of large voids between fibers can be obtained by the filaments. Further, by setting the content of the fibrillated meta-aramid fiber to 50% or less, it is possible to easily perform dewatering, prevent increase in dewatering suction and load on a press roll during papermaking, and obtain a wet nonwoven fabric having excellent paper strength without excessive moisture content after the dewatering step.

Here, regarding the proportion of the fibrillated meta-aramid fiber, in the case of the nonwoven fabric before the heat and pressure treatment (the same applies to the proportion of the constituent fibers of the nonwoven fabric), a 5cm square sample was taken and separated into individual fibers, and then the individual fibers were dried in an oven at 100 ℃. In the case of the sample after the heat and pressure treatment, since the fibers are not easily separated, the ratio of the total area of the portion where the fibers are broken into 2 or more fibers and become thinner than single fibers or the portion where the fibers are thinned is calculated as% in the cross-sectional photograph of the sample with respect to the total area of the nonwoven fabric from which the void (void) portion is removed.

In the present invention, the meta-aramid fiber is a general term for a normal meta-aramid fiber and a fibrillated meta-aramid fiber, and hereinafter, the normal meta-aramid fiber and the fibrillated meta-aramid fiber may be collectively referred to as the meta-aramid fiber.

The wet nonwoven fabric of the present invention contains polyphenylene sulfide short fibers. By containing the polyphenylene sulfide short fibers, a wet nonwoven fabric having excellent moisture absorption dimensional stability can be obtained. In the case of using the polyphenylene sulfide short fiber, the polyphenylene sulfide short fiber is melted by heat and pressure treatment to form a film in the gap and the surface of the meta-aramid fiber, thereby improving hydrolysis resistance.

Polyphenylene sulfide is a polymer containing a phenylene sulfide unit such as a p-phenylene sulfide unit or a m-phenylene sulfide unit as a repeating unit. The polyphenylene sulfide may be a homopolymer having any one of the above repeating units, or may be a copolymer having two units. In addition, copolymers with other aromatic thioethers are also possible.

The polyphenylene sulfide preferably has a weight average molecular weight of 40000 to 60000. Since the polyphenylene sulfide fiber has 40000 or more, good mechanical properties as a polyphenylene sulfide fiber can be obtained. Further, the viscosity of the melt spinning solution is suppressed by 60000 or less, and a spinning facility of a special high pressure resistant standard is not required.

The polyphenylene sulfide short fibers may be unstretched polyphenylene sulfide short fibers or stretched polyphenylene sulfide short fibers, but preferably include unstretched polyphenylene sulfide short fibers that are deformed at a lower temperature and welded. Since the polyphenylene sulfide short fibers are thermoplastic, they deform when heated and pressurized, thereby filling the gaps between the nonwoven fabrics, and are fused with meta-aramid fibers, other polyphenylene sulfide short fibers (unstretched polyphenylene sulfide short fibers, stretched polyphenylene sulfide short fibers), and the like constituting the wet nonwoven fabric, thereby obtaining a dense wet nonwoven fabric. By fusing at least a part of the polyphenylene sulfide short fibers, the dielectric breakdown strength can be improved, and the dielectric breakdown strength can be increased to 17kV/mm or more. At least a part of the fusion may be a portion where the fiber form is retained by the fusion, or the entire fiber may be deformed by the fusion. In the polyphenylene sulfide short fibers contained in the nonwoven fabric of the present invention, at least a part of the polyphenylene sulfide short fibers is welded, and as a result, the form of the fibers is not retained.

The wet nonwoven fabric contains at least a part of fused polyphenylene sulfide short fibers in a proportion of 40% or less, preferably 20 to 40%, more preferably 25 to 40%, and still more preferably 30 to 40%. In the nonwoven fabric, by setting the proportion of the polyphenylene sulfide short fibers at least partially welded to the above range, a dense wet nonwoven fabric having excellent tear strength can be obtained, and the dielectric breakdown strength can be improved. When the proportion of the polyphenylene sulfide short fibers at least partially welded is in a preferable mode (20 to 40%), the voids in the wet nonwoven fabric can be sufficiently filled, and excellent dielectric breakdown strength can be exhibited. On the other hand, when the ratio of the polyphenylene sulfide short fibers at least partially welded is too large, the insulation breakdown strength is not problematic, but the wet nonwoven fabric becomes a film shape and the tear strength is reduced. The ratio of the polyphenylene sulfide short fibers at least partially welded to each other was calculated from the ratio of the total area of 2 or more welded polyphenylene sulfide short fiber portions and the total area of the film-formed polyphenylene sulfide portions to the total area of the nonwoven fabric obtained by removing the void portions in the cross-sectional photograph of the sample after the heat and pressure treatment.

The undrawn polyphenylene sulfide short fiber referred to herein is a polyphenylene sulfide short fiber obtained in a state of being substantially undrawn after melt-spun by passing through a die in an extrusion (extruder) type spinning machine or the like.

In the present invention, as the polyphenylene sulfide short fibers, the undrawn polyphenylene sulfide short fibers and the drawn polyphenylene sulfide short fibers may be used in combination, or only the drawn polyphenylene sulfide short fibers may be used. Hereinafter, the undrawn polyphenylene sulfide short fibers and the drawn polyphenylene sulfide short fibers may be collectively referred to as polyphenylene sulfide short fibers.

When the undrawn polyphenylene sulfide short fiber and the drawn polyphenylene sulfide short fiber are used in combination as the polyphenylene sulfide short fiber, the ratio of the drawn polyphenylene sulfide short fiber is preferably 10 to 40% by mass, more preferably 15 to 30% by mass, in the nonwoven fabric-constituting fiber. On the other hand, the unstretched polyphenylene sulfide is preferably 10 to 40% by mass, more preferably 15 to 30% by mass, in the nonwoven fabric constituting fibers. When the drawn polyphenylene sulfide short fiber and the undrawn polyphenylene sulfide short fiber are used in combination in the above-described range, the polyphenylene sulfide short fiber such as the undrawn polyphenylene sulfide short fiber is melted in a range of 40% or less by heat and pressure treatment and is fused to another fiber, and thus a wet nonwoven fabric having improved adhesiveness and excellent tensile strength and tear strength is formed.

On the other hand, when the drawn polyphenylene sulfide short fibers are partially fused by heating and pressing without using the undrawn polyphenylene sulfide short fibers, the drawn polyphenylene sulfide short fibers are 40 mass% or less, preferably 20 to 40 mass%, more preferably 25 to 40 mass%, and still more preferably 30 to 40 mass% in the nonwoven fabric constituent fibers. When the ratio is too large, the insulation breakdown strength is not a problem, but the wet nonwoven fabric becomes a film and the tear strength is lowered.

In the wet nonwoven fabric of the present invention, the single fiber fineness of the polyphenylene sulfide short fibers is preferably 0.05dtex or more and 5dtex or less in both the undrawn polyphenylene sulfide short fibers and the drawn polyphenylene sulfide fibers. By setting the fiber diameter to 0.05dtex or more, the fibers are not too easily entangled with each other and are easily uniformly dispersed. By setting the fiber diameter to 5dtex or less, the fibers are not excessively thick and hard, and the interlacing force between the fibers can be maintained within an excellent range. As a result, a wet nonwoven fabric which has sufficient paper strength and is less likely to be broken can be produced.

In addition, regarding the fiber length of the polyphenylene sulfide short fiber, both the undrawn polyphenylene sulfide short fiber and the drawn polyphenylene sulfide short fiber are preferably 0.5 to 15mm, and more preferably 1 to 8 mm. By setting the thickness to 0.5mm or more, the strength of the wet nonwoven fabric can be improved by the interweaving of the fibers. Further, by setting the diameter to 15mm or less, it is possible to prevent the fibers from being entangled with each other, for example, to form lumps (Japanese: ダ マ), which may cause unevenness.

The meta-aramid fiber and the polyphenylene sulfide short fiber are mixed and made into a dry web by a paper machine generally used, and the dry web is subjected to heat and pressure treatment to fuse the polyphenylene sulfide short fiber, preferably the undrawn polyphenylene sulfide short fiber, within a certain range to fill the voids, thereby obtaining an electrical insulating paper of 17kV/mm or more. Since the dielectric breakdown strength becomes 17kV/mm or more, it can be used as an electrical insulating paper used in motors, transformers, and the like.

The dielectric breakdown strength in the present invention was calculated by sandwiching a test piece between a disc-shaped upper electrode having a diameter of 25mm and a mass of 250g and a disc-shaped lower electrode having a diameter of 75mm in accordance with JIS C2151 (2006)17.1, applying an alternating voltage having a frequency of 60Hz to a test medium while increasing the voltage by 1.0 kV/sec, and dividing the voltage at the time of dielectric breakdown by the thickness of the center portion measured in advance.

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

The undrawn polyphenylene sulfide fiber can be obtained by melt-spinning a polyphenylene sulfide polymer with an extrusion spinning machine or the like and treating the resultant fiber in a substantially undrawn state. The drawn polyphenylene sulfide fiber can be obtained by melt-spinning a polyphenylene sulfide polymer with an extrusion spinning machine or the like in the same manner as the undrawn polyphenylene sulfide fiber, and is usually drawn in a range of 3.0 times or more, preferably 5.5 times or less, and more preferably 3.5 to 5.0 times. The stretching may be performed in one stage, or may be performed in two or more stages. When the two-stage stretching is used, the stretching in the 1 st stage is preferably 70% or more, preferably 75 to 85% of the total magnification, and the rest is performed by the stretching in the 2 nd and subsequent stages. The obtained undrawn yarn and drawn yarn may be cut (cut) in a state where no crimp is imparted thereto, or may be cut after crimp is imparted thereto. The presence or absence of crimp in the polyphenylene sulfide short fibers has advantages of both the polyphenylene sulfide short fibers having crimp and the polyphenylene sulfide short fibers having no crimp. The crimped polyphenylene sulfide short fiber is suitable for obtaining a wet nonwoven fabric having improved interlacement of fibers and excellent strength. On the other hand, the polyphenylene sulfide short fibers having no crimp are suitable for obtaining a uniform wet nonwoven fabric having small unevenness.

Next, a normal meta-aramid fiber will be described. A normal meta-aramid fiber can be obtained by extruding a spinning solution in which meta-aramid is dissolved from a die into a high-temperature air gap, washing with water, drying, and cutting into a predetermined fiber length.

In addition, the fibrillated meta-aramid fiber can be obtained by extruding a spinning solution in which meta-aramid is dissolved from a die into a poor solvent while stirring at a high speed. In this case, the impact plate may be provided near the discharge port. Alternatively, the fibrillation may be performed by imparting mechanical or chemical action to the normal meta-aramid fiber.

Examples of the mechanism for fibrillating meta-aramid fibers include a Niagara beater (Niagara coater), a homogenizer, a disc refiner (disk refiner), an attritor (japanese: ラ イ カ イ), a pestle and mortar, and a water jet punch (water jet punch). These mechanisms are applicable not only to the usual meta-aramid fibers but also to fibrillated meta-aramid fibers, and a combination of mechanisms may be used depending on the degree of fibrillation.

The degree of fibrillation can be confirmed by the freeness of Canadian freeness tester (Canadian freeness tester) according to JIS P8121-2 (2012), and the freeness is preferably 10 to 900cm³More preferably 10 to 600cm³More preferably 10 to 300cm³. If the degree of fibrillation is too small, that is, the drainage degree is too large, the entanglement by the filaments is reduced, and the paper strength of the dry web is reduced. On the other hand, when the degree of fibrillation is too large, that is, the drainage degree is too small, the efficiency of the fibrillation step is lowered, and the load of the dewatering step in the paper making process is increased.

An example of a method of producing a wet nonwoven fabric by mixing the meta-aramid fiber and the polyphenylene sulfide short fiber as described above is shown.

First, meta-aramid fibers and polyphenylene sulfide staple fibers were dispersed in water, respectively. Further, the dispersions are mixed at a predetermined ratio to prepare a dispersion for papermaking.

The total amount of the meta-aramid fiber and the polyphenylene sulfide short fiber is preferably 0.05 to 5% by mass based on the total weight of the dispersion for papermaking. When the total amount is less than 0.05 mass%, the production efficiency is lowered and the load of the dehydration step is increased. Conversely, if the amount exceeds 5 mass%, the dispersion state of the fibers is deteriorated, and it becomes difficult to obtain a uniform wet nonwoven fabric.

As for the dispersion, a dispersion of meta-aramid fiber and a dispersion of polyphenylene sulfide short fiber may be prepared separately in advance and then mixed, or a dispersion containing both may be prepared directly. From the viewpoint of individually controlling the stirring time according to the shape and characteristics of each fiber, a method of preparing a dispersion liquid of each fiber and mixing the two is preferable, and from the viewpoint of process simplification, a method of directly preparing a dispersion liquid including both is preferable.

In the dispersion for papermaking, a dispersant or an oil agent including a surfactant such as a cationic surfactant, an anionic surfactant or a nonionic surfactant may be added for improving water dispersibility, a viscosity agent for increasing the viscosity of the dispersion to prevent agglomeration of the dispersion for papermaking, a defoaming agent for suppressing generation of bubbles, and the like may be added.

The dispersion for papermaking prepared as described above is made into a web using a paper machine such as a cylinder, fourdrinier, or inclined wire machine or a handsheet machine, and dried by a yankee dryer (yankee dryer), a rotary dryer (rotary dryer), or the like to prepare a dry web. Thereafter, the wet nonwoven fabric was subjected to heat and pressure treatment to obtain a wet nonwoven fabric. In the present invention, the simultaneous heating and pressurizing treatment is referred to as heating/pressurizing treatment, and is distinguished from the treatment in which only heating such as drying is used and pressurization is not performed. The dry web refers to a nonwoven fabric which has not been subjected to the heating and pressing treatment among nonwoven fabrics subjected to wet papermaking.

In the present invention, in order to increase the dielectric breakdown strength, it is preferable to weld a part of the polyphenylene sulfide short fibers (preferably unstretched polyphenylene sulfide short fibers) by heat and pressure treatment, but in this case, it is preferable to suppress crystallization in the production process of the dry net as much as possible. Therefore, the drying temperature in the paper-making step is preferably 80 to 140 ℃, more preferably 90 to 130 ℃. In addition, it is preferable to suppress crystallization by shortening the passage time of the drying step. If the drying is insufficient, the paper strength of the dry web decreases, or rapid thermal shrinkage is likely to occur during the heat and pressure treatment in the next step, thereby causing a sheet of paper. Conversely, when the drying is too much, crystallization of the dry net progresses, and the polyphenylene sulfide short fibers are less likely to be plastically deformed by the subsequent heat and pressure treatment, and as a result, gaps between the fibers of the dry net are less likely to be filled, and the dielectric breakdown strength is reduced. Here, the drying temperature in the paper making step means the highest temperature of the treatment temperature (atmosphere temperature) at the time of drying in the paper making step. In order to facilitate plastic deformation of the polyphenylene sulfide short fibers contained in the dry net by the heat and pressure treatment, the heat of crystallization of the dry net before the heat and pressure treatment is preferably 3J/g or more, and more preferably 5J/g or more.

The crystallization temperature is the peak temperature of the main heat generation peak measured under the same conditions as the measurement of the crystallization heat in [ measurement/evaluation method ] (1) in the section of example (described later).

In the production of the nonwoven fabric of the present invention, it is important to include a step of heating and pressurizing a dry web obtained by mixing meta-aramid fibers and polyphenylene sulfide short fibers. By performing the heating and pressing treatment, the polyphenylene sulfide short fibers are melted and softened as described above to fill the voids and weld the fibers to each other, and the dielectric breakdown strength can be set to 17kV/mm or more. The heating and pressing mechanism may be any mechanism, and for example, a hot press or a calender using a flat plate or the like can be used. Among these, a calender capable of continuous processing is preferable. As the roll of the calender, a metal-metal roll, a metal-paper roll, a metal-rubber roll, or the like can be used.

The temperature conditions for the heat and pressure treatment are favorable for the glass transition temperature of the polyphenylene sulfide short fiber (preferably, undrawn polyphenylene sulfide short fiber) or higher and the melting point of the meta-aramid or lower. When the undrawn polyphenylene sulfide short fibers are fused, the temperature is more preferably 160 to 260 ℃, still more preferably 180 to 240 ℃, and when the drawn polyphenylene sulfide short fibers are fused, the temperature is more preferably 230 to 285 ℃, still more preferably 240 to 280 ℃. When the treatment temperature is lower than the glass transition temperature of the polyphenylene sulfide short fibers, the polyphenylene sulfide short fibers are not thermally fused, and a dense wet nonwoven fabric cannot be obtained. On the other hand, if the melting point of the meta-aramid is exceeded, the dry net becomes too soft and adheres to a heating and pressing device such as a roll of a calender or a plate of a hot press, and thus mass production cannot be stably performed. In addition, the surface of the wet type nonwoven fabric is also roughened.

The glass transition temperature and the melting point are values measured under the same conditions as those in [ measurement/evaluation method ] (2) in the column of example described later.

The pressure when the calendering is used as the heating/pressing treatment is preferably 98 to 7000N/cm. The gap between the fibers can be filled by setting the thickness to 98N/cm or more. On the other hand, by setting 7000N/cm or less, it is possible to prevent damage to the wet nonwoven fabric in the heating/pressing step, and the like, and to stably perform the treatment. The process speed is preferably 1 to 30m/min, more preferably 2 to 20 m/min. By setting the flow rate to 1m/min or more, good work efficiency can be obtained. On the other hand, by setting the thickness to 30m/min or less, heat can be conducted to the fibers even in the wet nonwoven fabric, and the effect of thermal fusion of the fibers can be obtained.

The wet nonwoven fabric obtained in this manner can be formed into a predetermined shape by pressing, bending, or the like as an insulating paper, inserted into a motor, and used as a wedge (wedge), a slot liner (slot liner), or a phase paper. In addition, the transformer can also be used as an inter-coil insulation paper or an interlayer insulation paper. Further, the wet nonwoven fabric may be coated with an epoxy-based or polyester-based adhesive to be used as an insulating tape for fixing a coil or a lead wire.

Further, the web before the heat/pressure treatment and the thermoplastic resin sheet may be subjected to the heat/pressure treatment and then the web and the thermoplastic resin sheet may be bonded to each other. The bonded sheet (laminated sheet) obtained in this way can be formed into a predetermined shape by pressing, bending, or the like as an insulating sheet, inserted into a motor, and used as a wedge, a slot liner, or a phase separation paper. Here, in order to obtain good adhesion between the web and the thermoplastic resin sheet, the heat of crystallization of at least one of the web before the heat and pressure treatment and the thermoplastic resin sheet before the heat and pressure treatment is preferably 10J/g or more. In the case of the laminated sheet, since the dry web and the thermoplastic resin sheet are bonded to each other and are not easily separated from each other, only the dry web is subjected to the heat/pressure treatment under the same conditions as those under which the dry web and the thermoplastic resin sheet are subjected to the heat/pressure treatment, and the ratio of the polyphenylene sulfide short fibers to be welded is calculated from a photograph of a cross section of the dry web in this case.

Preferred examples of the thermoplastic resin constituting the thermoplastic resin sheet include polyethylene terephthalate, polyethylene naphthalate, polyphenylene sulfide, and the like, and 1 or more of them can be used.