JP7183073B2 - Aramid paper manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing an electrically insulating material having excellent heat resistance, and more particularly to a method for producing aramid paper that is thin and has high strength and insulation.

For the electrical insulation field where heat resistance is required, the application of molding materials made from heat-resistant polymers has been studied, and improved strength and/or thermal stability manufactured using this heat-resistant polymer Paper with properties has been developed. For example, aramid paper is a synthetic paper made of aromatic polyamide and is used as an electrical insulating material and a base for aircraft honeycombs due to its excellent heat resistance, flame retardancy, electrical insulation, toughness and flexibility. It's here. Among these materials, paper comprising Nomex® fibers from DuPont (USA) is made by mixing short fibers and fibrids of poly(metaphenylene isophthalamide) in water. , and then the mixed slurry is made into paper and then calendered. This paper is known to have excellent electrical insulation properties with still high strength and toughness even at high temperatures.
In recent years, with the trend toward miniaturization and weight reduction of devices that require insulating materials, such as transformers and motors, materials that are thinner and have higher electrical insulating performance are in demand.

As a method for thinning the paper, it is common to reduce the basis weight of the aramid paper obtained, but the density of the aramid paper obtained also decreases accordingly, and sufficient strength and insulation cannot be obtained. There is a problem with On the other hand, as a method for obtaining thin aramid paper, for example, Patent Document 1 discloses aramid paper formed from a mixture of fibrids and short fibers formed from aromatic polyamide between at least a pair of heating elements. A method for producing aramid paper is disclosed, which includes heat-pressing with a heating element sandwiched between them, and the shrinkage of the aramid paper after heat-pressing by the heating element is 3% or less. In this method, a pair of calender rolls are used as a pair of heating elements, and hot-pressing is performed once at a calender roll surface temperature of 250° C. and a roll pressure of 2500 kg/cm ² or 1250 kg/cm ² . It is shown that a thin electrical insulating sheet material with high heat resistance can be obtained by this method (Example). On the other hand, it is shown that aramid paper having desired thickness cannot be obtained when hot-pressing at 330° C., which is a temperature exceeding the glass transition temperature of aramid paper (Comparative Examples 2 and 3).

JP 2016-223021 A

An object of the present invention is to provide a method for producing aramid paper that can produce aramid paper that is thin, has high density, and has excellent electrical properties, as well as mechanical properties, particularly high tensile strength. to provide.

In view of this situation, the present inventors have made intensive studies and found that a sheet-like material formed from a mixture of aramid short fibers and aramid fibrids at a specific ratio is sandwiched between a pair of heating elements and a high pressure is applied. The inventors have arrived at the present invention based on the finding that the above problems can be solved by performing the hot-pressing treatment step under temperature conditions at least twice.
That is, the present invention provides the following [1] to [10].
[1] Aramid short fibers and aramid fibrids are mixed at a mass ratio of 60/40 to 10/90 to form a sheet, and the obtained sheet is sandwiched between a pair of heating elements and weighed at 500 kg/ A method for producing aramid paper comprising performing at least two hot-pressing steps applying a pressure of ² cm or more, wherein the at least two hot-pressing steps are performed at a temperature above the glass transition temperature of aramid. A method for producing aramid paper, comprising a hot-pressing step (a) and a hot-pressing step (b) performed thereafter at a temperature lower than the glass transition temperature of aramid.
[2] The production method according to [1], wherein the hot-pressing treatment step (a) is performed at a temperature that is 15°C or more higher than the glass transition temperature of aramid and lower than the decomposition temperature of aramid.

[3] The production method according to [1] or [2], wherein the hot-pressing step (b) is performed at a temperature between 10° C. and 180° C. lower than the glass transition temperature of aramid.
[4] Any one of [1] to [3], in which a pressure release step is present between the hot and pressure processing steps (a) and (b) to release pressure on the hot and pressure processed sheet. The manufacturing method described in .
[5] The production method according to any one of [1] to [4], wherein the pair of heating elements is a pair of calender rolls.
[6] The production method according to any one of [1] to [5], wherein the aramid constituting the aramid short fibers and aramid fibrids is polymetaphenylene isophthalamide.
[7] Aramid paper made by mixing aramid short fibers and aramid fibrids in a mass ratio of 60/40 to 10/90, having a thickness of 5 to 35 μm and a density of 0.70 to 1.0 g. /cm ³ and a tensile strength of 45 MPa or more.
[8] The aramid paper according to [7], which has a density of 0.75 to 0.95 g/cm ³ .
[9] The aramid paper according to [7] or [8], which has a basis weight of 5 to 25 g/m ² .
[10] The aramid paper according to any one of [7] to [9] obtained by the production method according to any one of [1] to [6].

According to the production method of the present invention, by appropriately adjusting the amount of aramid short fibers and aramid fibrids used as raw materials for producing aramid paper and the pressure applied in the hot and pressure processing process, a wide range of thicknesses, such as Aramid paper having a thickness of up to 200 μm and excellent mechanical properties, electrical properties and heat resistance can be easily produced. In particular, according to the production method of the present invention, aramid paper having a thickness equivalent to or thinner than that of the aramid paper disclosed in Patent Document 1, and having higher mechanical properties, particularly high tensile strength. can be manufactured.
As described above, according to the manufacturing method of the present invention, it is possible to easily manufacture aramid paper with high heat resistance, thinness, and excellent mechanical and electrical properties, which is compatible with miniaturization and weight reduction of transformers, motors, and the like. It is possible to obtain an industrially excellent effect.
The present invention will be described in detail below.

[Aramid]
In the present invention, aramid means a linear polymer compound in which 60% or more of amide bonds are directly bonded to aromatic rings. Examples of such aramids include polymetaphenylene isophthalamide and its copolymers, polyparaphenylene terephthalamide and its copolymers, and copolyparaphenylene/3,4'-diphenylether terephthalamide. These aramids are industrially produced by, for example, a solution polymerization method, a two-step interfacial polymerization method, etc., by a condensation reaction with an aromatic acid dichloride and an aromatic diamine, and are commercially available. , but not limited to. Among these aramids, polymetaphenylene isophthalamide is preferably used because it has properties such as good molding processability, thermal adhesiveness, flame retardancy, and heat resistance.

[Aramid staple fiber]
In the present invention, the aramid staple fiber is a fiber made from aramid as a raw material cut into a predetermined length. , and DuPont under the trade name Nomex (registered trademark), but are not limited to these.
The length of the short aramid fibers can generally be selected from the range of 1 mm or more and less than 25 mm, preferably 2 mm or more and less than 12 mm. When the length of the short fibers is 1 mm or more, the mechanical properties of the sheet material are good. It is preferable because it can be suppressed.
The fiber diameter of the aramid short fibers can be selected, for example, from the range of 0.1-40 μm, preferably 0.5-25 μm, more preferably 1-20 μm.

[Aramid fibrid]
In the present invention, aramid fibrids are film-like microparticles made of aramid, and are sometimes referred to as aramid pulp. Methods for producing aramid fibrids include, for example, the methods described in JP-B-35-11851 and JP-B-37-5732. Aramid fibrids have papermaking properties like ordinary wood (cellulose) pulp, and can be formed into a sheet by a paper machine after being dispersed in water. In this case, a so-called beating treatment can be applied for the purpose of maintaining the quality suitable for papermaking. This beating process can be carried out by disc refiners, beaters, or other papermaking stock processing equipment that exerts a mechanical cutting action. In this operation, the morphological change of the fibrid can be monitored by the freeness specified in JIS P8121. In the present invention, the freeness of the aramid fibrids after beating is preferably in the range of 10 to 300 cm ³ (Canadian standard freeness). A fibrid having a freeness in this range can suppress a decrease in the strength of a sheet-like material formed therefrom. On the other hand, when the freeness is greater than 10 cm ³ , it is possible to suppress the progress of fibrid miniaturization, so that it is possible to suppress the deterioration of the so-called binder function.

[Method for producing aramid paper]
Aramid paper in the present invention is produced by mixing aramid short fibers and aramid fibrids in a mass ratio of 60/40 to 10/90 to form a sheet, and placing the obtained sheet between a pair of heating elements. A method for producing aramid paper, which comprises sandwiching and applying a pressure of 500 kg/cm ² or more at least twice, wherein the heat and pressure processing step performed at least twice increases the glass transition temperature of aramid. It is produced by a production method characterized by including a hot-pressing treatment step (a) at a temperature exceeding the temperature and a subsequent hot-pressing treatment step (b) at a temperature below the glass transition temperature of aramid.
In this production method, first, aramid short fibers and aramid fibrids are mixed in a mass ratio of 60/40 to 10/90, preferably 55/45 to 15/85, more preferably 50/50 to 20/80. A sheet is formed by mixing at a mass ratio. Specifically, for example, after dry blending the aramid short fibers and aramid fibrids, a method of forming a sheet using airflow, aramid short fibers and aramid fibrids are dispersed and mixed in a liquid medium, and then liquid permeation A method of ejecting onto a flexible support such as a net or belt to form a sheet, removing the liquid and drying can be applied. .

In the wet papermaking method, a single or mixed aqueous slurry containing at least aramid fibrids and aramid short fibers is fed to a paper machine and dispersed, and then dewatered, squeezed and dried to wind up as a sheet. method is common. As a paper machine, a fourdrinier paper machine, a cylinder paper machine, an inclined paper machine, and a combination paper machine combining these are used. In the case of production by a combination paper machine, a composite sheet consisting of a plurality of paper layers can be obtained by sheet-forming and uniting slurries with different mixing ratios. Additives such as a dispersibility improver, an antifoaming agent, and a paper strength enhancer are used as necessary during papermaking.
In addition to the above aramid short fibers, other fibrous components (e.g., polyphenylene sulfide fiber, polyetheretherketone fiber, cellulose fiber, PVA fiber, polyester fiber, polyarylate fiber, liquid crystal polyester fiber, polyimide fiber, etc. inorganic fibers such as organic fibers, glass fibers, rock wool, asbestos, and boron fibers) can be added as long as the object of the present invention is not impaired. In this case, the proportion of aramid short fibers in all constituent fibers is 80% by mass or more, more preferably 90% by mass or more.

[Heat and pressure processing]
The sheet-like material obtained as described above is sandwiched between a pair of heating elements, and a hot-pressing process step of applying a pressure of 500 kg/cm ² or more is performed at least twice. Here, the hot pressing treatment step performed at least twice is the hot pressing treatment step (a) at a temperature above the glass transition temperature of aramid, and the subsequent hot pressing at a temperature below the glass transition temperature of aramid. Including processing step (b). By carrying out such a specific hot-pressing process, the thickness of the resulting aramid paper can be reduced, and the density and mechanical strength can be improved.
In the present invention, it is preferable to use a pair of calender rolls as the pair of heating elements.
The pressure is preferably 500-10000 kg/cm ² , more preferably 1000-5000 kg/cm ² .
Here, the hot pressing treatment step (a) is performed at a temperature higher than the glass transition temperature of aramid by 15°C or more, preferably 20°C or more, and less than the decomposition temperature of aramid while applying a pressure of 500 kg/cm ² or more. , preferably at temperatures up to 380°C.
Generally, the glass transition temperature of aramid is considered to be around 280°C, and the decomposition temperature is considered to be around 400°C.
In addition, in the hot-pressing treatment step (b), while applying a pressure of 500 kg/cm ² or more, the temperature is 10°C lower than the glass transition temperature of aramid, preferably 20°C lower to 180°C lower, preferably 100°C. It is better to work at a temperature between low temperatures. In addition, it is more preferable that the heating temperature difference between the heat-pressing treatment steps (a) and (b) is 50° C. or more.
In the present invention, the heating temperature in the hot-pressing treatment step can be expressed as the surface temperature of the heating element, and when calender rolls are used as the heating element, it can be expressed as the surface temperature of the calender rolls.

In the present invention, the mechanical strength can be further improved by performing hot pressing at a temperature exceeding the glass transition temperature, but due to the high temperature of the heating element, the sheet once compressed in the thickness direction by the heating element , immediately after being released from the heating element, it swells in the thickness direction, and this effect is particularly large the thinner it is. Therefore, it has become possible to produce aramid paper that is thin and has both mechanical and electrical properties by performing heat-pressing multiple times with the above temperature difference.
In the method for producing aramid paper of the present invention, it is preferable that there is a pressure release step in which the pressure applied to the hot-pressed sheet is released between the hot-pressing steps (a) and (b). . In this pressure releasing step, the temperature of the aramid paper subjected to the hot and pressure processing is lowered to the glass transition temperature by being released from pressure by a pair of heating elements, preferably a pair of calender rolls, and coming into contact with outside air, preferably air. It is preferably cooled to: Such a pressure release step can be provided by, for example, placing a pair of calender rolls and a pair of calender rolls following the calender rolls apart from each other.
In the method for producing the aramid paper of the present invention, in addition to the above-described heat-pressing treatment steps (a) and (b), a pressurizing treatment step at room temperature may be combined. Also, the hot-pressing treatment step (b) may include a plurality of hot-pressing treatment steps with different pressures and temperatures. Moreover, before performing the hot-pressing treatment step (b), a plurality of hot-pressing treatment steps (a) may be performed.

[Aramid paper]
In the above manufacturing method, by appropriately adjusting the amount of aramid short fibers and aramid fibrids used as raw materials for aramid paper and the pressure applied in the hot-pressing process, a wide range of thicknesses, for example, up to 200 μm (preferably 5 μm or more and 100 μm or less, density of 0.70 to 1.0 g/cm ³ ), excellent mechanical properties, electrical properties and heat resistance. can. In particular, according to the production method of the present invention, aramid paper having a thickness equivalent to or thinner than that of the aramid paper disclosed in Patent Document 1, and having higher mechanical properties, particularly high tensile strength. can be manufactured.
In the present invention, in particular, aramid paper made by mixing aramid short fibers and aramid fibrids at a mass ratio of 60/40 to 10/90 and formed into a sheet, having a thickness of 5 to 35 μm and a density of 0.70 to 0.70 Aramid paper can be produced with a tensile strength of 1.0 g/cm ³ , preferably 0.75-0.95 g/cm ³ and a tensile strength of 45 MPa or more. Here, the thickness is preferably 10-30 μm, more preferably 15-30 μm.
Also, aramid paper having the above properties and having a basis weight of 5 to 25 g/m ² , preferably 10 to 25 g/m ² can be produced.
In the present invention, the tensile strength represents the tensile strength per unit cross-sectional area, and the average value of the longitudinal direction and the transverse direction is taken as the tensile strength. The tensile strength of the aramid paper of the present invention is preferably 45 MPa or higher, more preferably 47 MPa or higher, and still more preferably 50 MPa or higher. The upper limit of tensile strength is preferably 120 MPa.
In the present invention, the glass transition temperature is determined by heating a test piece from room temperature at a rate of 3 ° C./min, measuring the calorific value with a differential scanning calorimeter, drawing two extension lines on the endothermic curve, It is a value obtained from the intersection point of the 1/2 straight line between the values and the endothermic curve. The glass transition temperature of the aramid paper used in the example was 275°C.
According to the production method of the present invention, it is possible to easily produce aramid paper with high heat resistance, thinness, and excellent mechanical and electrical properties, which is compatible with miniaturization and weight reduction of transformers and motors. Industrially excellent effects can be obtained.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. It should be noted that these examples are mere illustrations and are not intended to limit the content of the present invention in any way.
[Measuring method]
(1) Basis Weight, Thickness and Density of Sheet This was carried out according to JIS C 2300-2, and the density was calculated by (basis weight/thickness).
(2) Tensile strength According to ASTM D-828, it is carried out in the longitudinal direction and the transverse direction, and after calculating the average value of both, (tensile strength per unit width / thickness) Tensile strength per unit cross-sectional area calculated as
(3) Dielectric Breakdown Voltage According to ASTM D149, it was carried out by a direct voltage boosting method using an alternating current with an electrode diameter of 51 mm.

[Raw material preparation]
Polymetaphenylene isophthalamide fibrids were produced by a method using a pulp particle production apparatus (wet settling machine) composed of a combination of a stator and a rotor described in Japanese Patent Publication No. 52-15621. This was treated with a beater to adjust the length-weighted average fiber length to 0.9 mm (freeness of aramid fibrids: 100 cm ³ (Canadian standard freeness)). On the other hand, DuPont meta-aramid fibers (Nomex (registered trademark), single filament fineness 2 denier, fiber diameter 15 μm) were cut into lengths of 6 mm (hereinafter referred to as “aramid staple fibers”) to prepare papermaking materials.
[Examples 1 to 4]
The aramid fibrids and the aramid short fibers prepared as described above were each dispersed in water to prepare a slurry. This slurry was mixed so that the aramid fibrids and the aramid short fibers would have the respective compounding ratios (mass ratios) shown in Table 1, and a sheet-like material was produced with a tappy-type paper making machine (cross-sectional area: 625 cm ² ). Next, the obtained sheet was adjusted with a pair of metal calender rolls so that the outer circumference of the roll and the aramid paper in contact with each other was 1 mm. got the paper Table 1 shows the main characteristic values of the aramid paper thus obtained.

Table 1

From the results in Table 1, the aramid paper of the present invention (Examples 1 to 4) is thin and has both mechanical properties and electrical properties by performing a specific heat and pressure process multiple times. I understand. In addition, the aramid paper thus produced is useful as an insulating material for transformers, motors, etc., due to the high heat resistance of the aramid material.

[Comparative Examples 1 to 4]
The aramid fibrids and the aramid short fibers prepared as described above were each dispersed in water to prepare a slurry. This slurry was mixed so that the aramid fibrids and the aramid short fibers would have the respective compounding ratios (mass ratios) shown in Table 2, and a sheet-like material was produced using a tappy-type paper making machine (cross-sectional area: 625 cm ² ). Next, the obtained sheet was adjusted with a pair of metal calender rolls so that the outer peripheral length of the contact between the roll and the aramid paper was 1 mm, and then heat-pressed under the conditions shown in Table 2 to form the aramid paper. Obtained. Table 2 shows the main characteristic values of the aramid paper thus obtained.

Table 2

As is clear from Table 2 above, the aramid papers of Comparative Examples 1 to 4 have a certain degree of thinness, but do not have high density. Furthermore, the aramid papers of Comparative Examples 1, 2 and 4 also have low tensile strength per unit cross-sectional area. Therefore, in order to obtain aramid paper that is useful as a thin insulating material for miniaturizing and reducing the weight of equipment such as transformers and motors, and that is thin and has both mechanical and electrical properties, the above-described examples are required. It has been found to be effective to use aramid paper.

Claims (9)

Aramid short fibers and aramid fibrids are mixed at a mass ratio of 60/40 to 10/90 to form a sheet-like material, the obtained sheet-like material is sandwiched between a pair of heating elements, and the weight is 500 kg/cm ² or more. A method for producing aramid paper comprising performing at least two hot-pressing steps applying a pressure of at least two times, wherein the hot-pressing step at a temperature exceeding 280 ° C. ( A method for producing aramid paper, comprising a) and a subsequent step of hot pressing treatment (b) at a temperature of less than 280°C . 2. The production method according to claim 1, wherein the hot pressing step (a) is carried out at a temperature higher than 280[deg.] C. by 15[deg.] C. or more and lower than the decomposition temperature of aramid. 3. The method according to claim 1 or 2, wherein the hot pressing step (b) is carried out at a temperature between 10°C lower than 280 °C and 180°C lower than 280 °C. 4. The production according to any one of claims 1 to 3, wherein a pressure release step of releasing the pressure applied to the hot-pressed sheet exists between the hot-pressing steps (a) and (b). Method. The manufacturing method according to any one of claims 1 to 4, wherein the pair of heating elements are a pair of calender rolls. The production method according to any one of claims 1 to 5, wherein the aramid constituting the aramid short fibers and aramid fibrids is polymetaphenylene isophthalamide. An aramid paper made by mixing aramid short fibers and aramid fibrids in a mass ratio of 60/40 to 10/90, and having a thickness of 5 to 35 μm and a density of 0.70 to 1.0 g/cm ^{3 .} and an aramid paper having a tensile strength of 45 MPa or more. Aramid paper according to claim 7, having a density of 0.75 to 0.95 g/cm ³ . The aramid paper according to Claim 7 or 8, which has a basis weight of 5 to 25 g/ ^m2 .