JP2002138385A - Nonwoven fabric of staple fiber of polyimide, method for producing the same and prepreg using the nonwoven fabric - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a nonwoven fabric of a polyimide excellent in heat resistance, dimensional stability in use at a high temperature and also in moldability and processability, to provide a method for producing the same and to obtain a prepreg using the nonwoven fabric. SOLUTION: This nonwoven fabric is characterized in that the nonwoven fabric is a one comprising a staple fiber composed of a polyimide as a main constitutent component and the average coefficient of linear expansion (a value measured in the manner described in JIS-K 7197) of the nonwoven fabric at the glass transition point of a main polyimide or below is in the range of -20 to 20 μm/m. deg.C in both the width direction and length direction of the nonwoven fabric.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nonwoven polyimide short fiber nonwoven fabric having excellent heat resistance and dimensional stability under high temperature use, a method for producing the same, and a prepreg using the nonwoven fabric.

[0002]

2. Description of the Related Art In recent years, the technical development of organic materials in the electric, electronic, space, and aviation fields has been remarkable. In these fields, polyimide resins have attracted attention due to their excellent heat resistance, mechanical properties, and electrical properties. Has been attempted. Until now, examples of the use of polyimide resin include varnish (polyimide solution or polyimide precursor) for coating polyimide resin directly on the surface of parts or films, or other materials, or as an adhesive or filler. Solution), which are being developed and used in various forms.

[0003] On the other hand, development and utilization of nonwoven fabrics or papers obtained mainly by processing polyimide fibers, and attempts to obtain polyimide molded articles by using them have been actively conducted. For example, Japanese Patent Application Laid-Open No. 9-52308 discloses an invention of a heat-resistant bag filter using a polyimide nonwoven fabric (web).
In Japanese Patent Application Laid-Open Publication No. H10-209, there is found an invention in which a nonwoven fabric obtained by wet-making a polyimide short fiber is used as a substrate for a printed laminate, but the polyimide used in these inventions was substantially thermoplastic and amorphous. Also, JP-A-6-57529
JP-A-10-1894 discloses a polyimide paper obtained by adding a polyimide precursor solution into a poorly stirred high-speed solvent and depositing the solution to obtain a polyimide paper from polyimide precursor fibrids (highly branched fine fibers). However, the fine fibers constituting the polyimide paper obtained by these inventions were substantially made of non-thermoplastic and amorphous polyimide.

[0004]

The polyimide non-woven fabric and the polyimide paper found in the above-mentioned conventional invention examples are as follows.
Although it has excellent heat resistance, it has the problem of large dimensional changes due to temperature, and it requires printed circuit board substrates and honeycomb core substrates for space and aviation that require dimensional stability at high temperatures. It was unsatisfactory for use. This is because the non-woven fabric and the polyimide fibers that make up the paper are amorphous,
This was due to large thermal expansion. Therefore, by examining the chemical structure of polyimide, polyimide fibers with low thermal expansion have been developed.However, due to problems such as heat resistance and moldability, those that are still satisfactory for use as nonwoven fabric or paper are still in development. Not obtained.

In view of such a situation, an object of the present invention is to
An object of the present invention is to provide a polyimide nonwoven fabric which is excellent in heat resistance, dimensional stability under high temperature use, and excellent in moldability, a method for producing the same, and a prepreg using the nonwoven fabric.

[0006]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to solve the above problems, and as a result, obtained short fibers from crystalline polyimide having thermoplasticity, and used them as main components of the nonwoven fabric. It has been found that the use can solve the above-mentioned problems, and furthermore, a polyimide nonwoven fabric having a characteristic of low hygroscopicity can be obtained, thereby completing the present invention.

That is, the gist of the present invention is as follows. Nonwoven fabric having a short fiber composed of polyimide as a main component, the average coefficient of linear expansion of the nonwoven fabric below the glass transition point of the main polyimide (value measured according to JIS-K7197), the width direction of the nonwoven fabric and A nonwoven fabric characterized by being in the range of -20 to 20 µm / m · ° C in any of the length directions. The nonwoven fabric as described above, wherein the nonwoven fabric has a water absorption of 0.5% or less when left in an environment of 25 ° C. and a relative humidity of 60% for 24 hours using the following formula.

[0008]

(Equation 2) (Wherein, W is shows the mass of the nonwoven fabric after moisture absorption, W ₀ denotes the mass of the nonwoven fabric during bone dry.)

[0009] Any of the above nonwoven fabrics, wherein the polyimide is a crystalline polyimide having thermoplasticity. The nonwoven fabric as described above, wherein the polyimide has a glass transition point of 230 ° C or higher and a melting point of 400 ° C or lower. The nonwoven fabric according to the above or the above, wherein the polyimide is a polyimide having a repeating unit represented by the above general formula (1). Any of the above nonwoven fabrics, wherein the average fiber length of the polyimide short fibers is 1 to 15 mm, and the average fiber diameter is 3 to 30 µm. The method for producing any of the above nonwoven fabrics, comprising: a step of preparing a slurry containing short fibers made of polyimide and wet-making the same; and a step of heating and pressing at a temperature equal to or higher than the glass transition point of the polyimide. A prepreg comprising 70 to 20% by mass of any of the above nonwoven fabrics and 30 to 80% by mass of a thermosetting resin.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
The nonwoven fabric of the present invention mainly comprises short fibers made of polyimide. The polyimide is preferably a thermoplastic polyimide from the viewpoint of spinning and thermal processing.
The glass transition point of the thermoplastic polyimide is preferably 230 ° C. or more, and less than 400 ° C., and more preferably 3 ° C. or less.
It is preferably 50 ° C. or lower. Glass transition point is 230
When the temperature is lower than 400 ° C., the heat resistance is poor, which is not preferable. On the other hand, when the temperature is higher than 400 ° C., thermal processing becomes difficult, which is not preferable.

In general, a polyimide having various properties can be obtained by selecting the chemical structures of a tetracarboxylic acid component and a diamine component as raw materials. Although the chemical structure of the polyimide used in the present invention is not particularly limited, polyimide having a chemical structure represented by the following general formula (1) as a repeating unit is thermoplastic and has excellent heat processability and heat resistance. It is particularly preferable because it is an excellent crystalline polyimide. As a specific example of such a polyimide, a polyimide having a chemical structure represented by the following formula (2) as a repeating unit is commercially available, for example, as Aurum (trade name; Mitsui Chemicals, Inc.).

[0012]

Embedded image (In the formula, R represents a tetravalent aromatic residue selected from a monocyclic aromatic, a condensed polycyclic aromatic, and a non-condensed polycyclic aromatic in which aromatic rings are bonded directly or by a bridge member. Also, X
Is a direct bond, a hydrocarbon group, a carbonyl group, an ether group,
Y _{1 to} Y ₄ represent a divalent residue selected from a thio group or a sulfonyl group, and Y _{1 to} Y ₄ represent a monovalent residue selected from a hydrogen, an alkyl group, an alkoxyl group or a halogen group. )

[0013]

Embedded image

The polyimide fibers used in the present invention include:
It is preferably a fiber made of crystalline polyimide.
Fibers made of crystalline polyimide have a smaller thermal expansion than fibers made of amorphous polyimide, and have a small dimensional change upon repeated heating and cooling, so they have excellent thermal dimensional stability when made into a nonwoven fabric, and It is preferable because the water absorption is also small. As a method of obtaining fibers made of crystalline polyimide, a raw yarn is obtained by melt-spinning using a crystalline polyimide having thermoplasticity as described above, and the raw yarn at this time is heated and drawn to obtain a crystal. The degree of conversion can be increased. The crystallinity of the fiber made of crystalline polyimide is 1 as measured by the X-ray diffraction method.
It is preferably at least 5%, more preferably at least 20%, particularly preferably at least 25%. If the crystallinity is less than 15%, thermal expansion and water absorption tend to increase, which is not preferable. In addition, as the melting point of the crystalline polyimide,
400 ° C. or lower is preferred. When the melting point exceeds 400 ° C,
It is not preferable because melt spinning involves difficulty.

When the polyimide is made into a fiber, a polyimide having various chemical structures may be copolymerized or blended for the purpose of improving various properties, and the effects of the present invention are not impaired. , May be blended with other polymers such as polyarylate, polyolefin, polyamide, polyphenylene sulfide, polyetheretherketone, fluororesin, and further blended with inorganic filler such as titanium oxide, alumina, silica, carbon black. Is also good.

In the nonwoven fabric of the present invention, the polyimide fibers are used in the form of short fibers in which the above-mentioned polyimide fibers are cut short. As the fiber length and fiber diameter of the short fibers, considering the uniformity of the formation of the obtained nonwoven fabric,
The average fiber length is preferably from 1 to 15 mm, and the average fiber diameter is preferably from 3 to 30 μm.

The basis weight of the nonwoven fabric of the present invention is not particularly limited, but is usually preferably 15 to 200 g / m ² from the viewpoint of practicality and productivity. The thickness of the nonwoven fabric of the present invention is not particularly limited, but is preferably from 10 to 800 µm from the same viewpoint.

The nonwoven fabric of the present invention has a small dimensional change due to heat and has excellent dimensional stability under high temperature use.
As the dimensional stability, the average coefficient of linear expansion below the glass transition point of the main polyimide forming the polyimide fiber is -20 in both the width and length directions of the nonwoven fabric.
It is necessary to be within the range of ~ 20 µm / m · ° C (value measured according to JIS-K7197), and -10 ~ 10 µm / m · ° C
And more preferably −5 to 5 μm / m · ° C. Above average linear expansion coefficient is -20 ~ 20μm
When the temperature is out of the range of / m · ° C., the dimensional change under high temperature use becomes large, and it is not suitable for use. In a nonwoven fabric formed of a conventional amorphous polyimide fiber, the above average linear expansion coefficient exceeds 20 μm / m · ° C., but in the present invention, a short-circuit made of the above-described crystalline polyimide is used. By using a short-fiber nonwoven fabric in which fibers are randomly dispersed, a nonwoven fabric having an excellent balance between dimensional stability and directionality can be obtained.

The water absorption of the nonwoven fabric is preferably small, and the water absorption of the nonwoven fabric calculated by the following equation when left in an environment of 25 ° C. and a relative humidity of 60% for 24 hours is 0.5. % Is a preferred embodiment of the present invention.

[0020]

(Equation 3) (Wherein, W is shows the mass of the nonwoven fabric after moisture absorption, W ₀ denotes the mass of the nonwoven fabric during bone dry.)

In general, polyimide is known to have relatively high water absorption due to its imide group having a strong polarity. In the case of a molded product, there is a problem that a dimensional change and a change in electrical characteristics at the time of water absorption are problematic. Has become. In nonwoven fabrics composed of conventional amorphous polyimide fibers, similarly, the size of water absorption has been a problem, but in the present invention, by using small crystalline polyimide short fibers of water absorption, It has become possible to obtain a polyimide nonwoven fabric having low water absorption.

The polyimide nonwoven fabric of the present invention mainly comprises short fibers made of the above-mentioned polyimide as a main component. However, in order to facilitate the papermaking of the nonwoven fabric and to improve various properties of the obtained nonwoven fabric, Short fibers or pulp-like substances made of other organic or inorganic substances, for example, short fibers or pulp-like substances made of aramid, polyester, polyetheretherketone, polyphenylene sulfide, carbon, glass, alumina, etc., and various particulate matters (Filler) may be blended. Further, for the purpose of imparting strength to prevent breakage when handling the nonwoven fabric, for example, a polyimide resin or a precursor thereof, an epoxy resin, or the like may be included. In this case, the polyimide resin or its precursor, an epoxy resin or the like is usually applied, sprayed or impregnated in the form of an emulsion or a solution. These components other than the short fibers made of polyimide may be blended singly or several types within a range that does not impair the purpose of the present invention, but the total mass of the dried non-woven fabric relative to the total mass of the nonwoven fabric of the present invention. Is preferably not more than 40% by mass. The content of the short fibers made of polyimide in the nonwoven fabric of the present invention is preferably 60% or more, and more preferably 90% by mass or more.

The nonwoven fabric of the present invention can be produced by a conventionally known dry or wet papermaking method. Of these, the wet papermaking method is particularly preferably employed because it is suitable for obtaining a short-fiber nonwoven fabric having excellent uniformity in thickness and texture. The wet papermaking apparatus used in the wet papermaking method is not particularly limited, and a known paper machine or the like may be used. Specific examples thereof include a circular net wet paper machine and a short net wet paper machine. And a short net inclined wet paper machine, a long net inclined wet paper machine and the like. It is preferable that a hot air type, a contact type or a radiation type drier is provided in these wet papermaking apparatuses.

In the wet papermaking step, the above-mentioned polyimide short fiber and other components blended as required are mixed with a dispersion medium such as water, and the slurry is dispersed uniformly using a pulper or the like to form a slurry. The nonwoven fabric of the present invention can be obtained by adjusting, wet-making using the above-described apparatus, and then drying. In addition, for the purpose of improving the papermaking property, a process of shrinking the polyimide short fiber with a beater or the like may be performed in advance. When the nonwoven fabric is impregnated or sprayed with an emulsion or solution containing a polyimide precursor, an epoxy resin, or the like, an impregnation device, a spray device, or the like may be provided between the papermaking section and the drying section of the wet papermaking apparatus.

The nonwoven fabric wet-processed by the above-mentioned process usually has a high porosity, and may cause a problem when passing through the subsequent process from the viewpoint of mechanical strength characteristics. Therefore, in order to produce a nonwoven fabric having better strength, the production method of the present invention described below is preferably employed. That is, as the production method of the present invention, in addition to the step of preparing a slurry containing short fibers made of polyimide and wet-making the same, a step of heating and pressing the obtained nonwoven fabric is performed.

The heating temperature at the time of heating and pressing must be higher than the glass transition point of the main polyimide which forms the short fibers made of polyimide. If the heating temperature is lower than the glass transition point, the effect of improving the mechanical strength characteristics cannot be obtained. The heating temperature is preferably not higher than the temperature at which the polyimide melts, and when the polyimide is a crystalline polyimide, the temperature is preferably equal to or lower than the melting point of the polyimide, and more preferably 10 ° C. or lower than the melting point. Preferably it is temperature. When the heating temperature exceeds the temperature at which the polyimide melts,
It is not preferable because the short fibers made of polyimide tend to be fused together extremely and the performance as a nonwoven fabric cannot be exhibited. Further, as the pressurizing pressure at the time of heating and pressurizing, is not particularly limited, there is a tendency that the higher the pressurizing pressure, the lower the porosity of the obtained polyimide nonwoven fabric,
In the present invention, a pressure of about 0.05 to 10 MPa may be usually employed.

The apparatus for performing the above-mentioned heating and pressurization is not particularly limited, and a conventionally known heating press or the like may be used. The apparatus can continuously heat and press a long nonwoven fabric. From this point, calendar roll device,
A double belt press device or the like capable of performing a heat press between a pair of opposed belts made of metal or the like is preferably used.

As described above, the nonwoven fabric of the present invention is excellent in heat resistance and thermal dimensional stability, and has a small water absorption. Therefore, the nonwoven fabric is suitable as a base material of a fiber-reinforced composite material.
It is possible to obtain a fiber-reinforced composite material having excellent properties described above. The prepreg of the present invention using the polyimide nonwoven fabric of the present invention suitable for such use will be described below. The prepreg of the present invention is a prepreg for a fiber-reinforced composite material, comprising the nonwoven fabric of the present invention and a thermosetting resin. The thermosetting resin is not particularly limited, and examples thereof include an epoxy resin, a polyimide resin, a bismaleimide resin, a phenol resin, a modified polyphenylene ether resin, a cyanate resin, and an unsaturated polyester resin.

The mass ratio of the nonwoven fabric and the thermosetting resin in the prepreg of the present invention is 30 to 80% by mass of the thermosetting resin with respect to 70 to 20% by mass of the nonwoven fabric. The nonwoven fabric exceeds 70% by mass or the thermosetting resin is 30%
When the amount is less than mass%, the thermosetting resin is not sufficiently distributed in the nonwoven fabric, and many pores are likely to remain inside the fiber reinforced composite material obtained from the prepreg. On the other hand, if the nonwoven fabric is less than 20% by mass or the thermosetting resin exceeds 80% by mass, the effect of fiber reinforcement by the nonwoven fabric cannot be sufficiently obtained.

The prepreg of the present invention can be easily obtained by impregnating or applying a liquid material such as varnish or emulsion containing a thermosetting resin to the nonwoven fabric of the present invention. As the method, for example, after impregnating or coating the nonwoven fabric of the present invention using a thermosetting resin solution using an impregnating device or a coater, it can be performed by desolvating using a hot air or radiation type dryer. it can. At this time, by setting appropriate drying conditions, the thermosetting resin can be semi-cured, that is, a so-called B stage, and a prepreg excellent in handleability can be obtained.

[0031]

EXAMPLES The present invention will be specifically described below with reference to examples. In Examples 1 to 3, as the short fibers made of polyimide (hereinafter sometimes referred to as polyimide short fibers), the ones according to the following production examples were used. [Production Example of Polyimide Short Fiber] As the polyimide, a crystalline polyimide resin having thermoplasticity (“Auram PL450” manufactured by Mitsui Chemicals, Inc.) was used. The glass transition point of this polyimide measured by DSC was 250 ° C., and the melting point was 38 ° C.
7 ° C. The above polyimide was heated and melted at 400 ° C., melt-spun at a spinning speed of 500 m / min, and heated and drawn at a temperature of 300 ° C. and a draw ratio of 2.5 to obtain a polyimide fiber having a fiber diameter of about 15 μm. . When the crystallinity of this polyimide fiber was measured by an X-ray diffraction method, the crystallinity was 30%. Next, the above polyimide fiber was cut into a fiber length of 5 mm to obtain a polyimide short fiber.

In all the examples and comparative examples,
Each physical property of the nonwoven fabric was measured or evaluated by the following methods. 1. Thickness A sample cut into a width of 50 cm and a length of 40 cm is divided into eight equal parts (two equal parts in width and four equal parts in length), and the thickness of each central part is measured by a contact type digimatic thickness gauge. The average value obtained from the measured values was defined as the thickness of the nonwoven fabric. 2. Porosity The apparent density d ₁ is calculated from the basis weight and thickness of the nonwoven fabric,
Separately, the true density d _{0 of the} nonwoven fabric was calculated from the true density and the content of each material constituting the nonwoven fabric, and then the porosity P was calculated by the following equation.

[0033]

(Equation 4)

3. Formation The uniformity was evaluated by visual observation, and a good one was evaluated as O, and a poor one was evaluated as X. 4. Non-woven fabric cut to an average linear expansion coefficient of 20 mm x 5 mm is absolutely dried (180 ° C
X dried for 2 hours) as a sample, according to JIS-K7
Using a TMA measuring device (TA Instruments, “TMA2940”) according to 197 and applying a tensile load to the sample, heating from 25 ° C to a temperature 20 ° C lower than the glass transition point of polyimide. Was measured by
The measurement was performed twice consecutively for the same sample, and the second value was adopted as the measured value (this excludes factors such as the influence of residual stress on the measured value). 5. Break length and elongation Using a universal testing machine (manufactured by Intesco), JIS-P81
13 was measured. 6. Non-woven fabric cut to 15 cm x 15 cm in water absorption
C. x 2 hours) as a sample at a temperature of 25 ° C.
Put in a thermo-hygrostat set at 60 ° C and 60% relative humidity.
It was made to absorb moisture by leaving it to stand for 4 hours, and was calculated from the absolute dry mass W ₀ and the mass W after moisture absorption by the following formula.

(Equation 5) 7. Resin impregnating property After immersing the nonwoven fabric in an epoxy resin solution having a solid content concentration of 70% by mass prepared by dissolving an epoxy resin (Epicoat 1001; trade name, Yuka Shell Epoxy) in methyl ethyl ketone, pulling it up, and then removing excess epoxy resin solution Was squeezed out with a mangle, and dried with hot air at 130 ° C. for 1 hour. The content of the epoxy resin in the thus obtained product (prepreg) was determined, and the content of 30 to 80% by mass was evaluated as ○, and the others were evaluated as ×, to evaluate the resin impregnation property of the nonwoven fabric.

Example 1 90 parts by mass of polyimide short fiber and 10 parts by mass of para-aramid pulp ("Kevlar" manufactured by Toray DuPont) were poured into water, and uniformly dispersed in water using a pulper to obtain a solid content concentration. A slurry of 0.05% by mass was obtained. Next, using this slurry, a polyimide nonwoven fabric having a width of 60 cm was formed by a short net inclined continuous paper machine with a dryer (manufactured by Saito Iron Works). Furthermore, heating was performed while continuously applying pressure using a double belt press (manufactured by Sandvik) (240
(2 minutes at 300 ° C. and 5 minutes at 300 ° C.) to obtain a polyimide nonwoven fabric with increased strength. Next, epoxy resin (Epicoat 1001; trade name, Yuka Shell Epoxy)
Was dissolved in methyl ethyl ketone to obtain a solid concentration of 7.
The prepreg is obtained by dipping the above-mentioned polyimide nonwoven fabric in a 0% by mass epoxy resin solution and pulling it up, and then squeezing out excess epoxy resin solution with a mangle, followed by hot-air drying at 130 ° C. for 1 hour. I got

Example 2 Polyimide staple fibers were cast in water and uniformly dispersed in water using a pulper to obtain a slurry concentration of 0.05.
A% by weight dispersion was obtained. Next, using this dispersion, a 60-cm-wide polyimide non-woven fabric was formed by a short net inclined continuous paper machine with a dryer (manufactured by Saito Iron Works). At this time, an aqueous solution of a polyimide precursor (concentration: 10% by mass) was sprayed onto the wet paper web using a spray gun in order to increase the handling strength of the polyimide nonwoven fabric during the paper making operation.
After spraying at a setting of 0 g / m ² , it was dried to obtain a polyimide nonwoven fabric. Further, in the same manner as in Example 1, a polyimide nonwoven fabric and a prepreg having increased strength by heating under pressure were obtained.

Example 3 In this example, a polyimide short fiber which was shrunk by being treated with a beater was used. The polyimide short fiber treated with the beater was thrown into water and uniformly dispersed in water using a pulper to obtain a dispersion having a slurry concentration of 0.05% by mass. Next, using this dispersion, a 60-cm-wide polyimide non-woven fabric was formed by a short net inclined continuous paper machine with a dryer (manufactured by Saito Iron Works).
Further, in the same manner as in Example 1, a polyimide nonwoven fabric and a prepreg having increased strength by heating under pressure were obtained. In this example, the papermaking property was improved by treating the polyimide short fibers with a beater, and it was recognized that the fibers were well entangled in the papermaking nonwoven fabric.

Comparative Example 1 Thermoplastic amorphous polyimide ("Ultem" manufactured by GE Plastics, glass transition point 21
0 ° C) (fiber length 5mm, fiber diameter 25μ)
A polyimide nonwoven fabric was formed in the same manner as in Example 1 except that m) was used, and a polyimide nonwoven fabric with increased strength was obtained by heating under pressure.

Comparative Example 2 4.00 g of diaminodiphenyl ether was added to THF 59.
Dissolved in a mixed solvent of 6 g and 15.9 g of methanol;
It was kept at 8 ° C. 3. Add pyromellitic dianhydride to this solution.
When 40 g was added at a time and stirring was continued for 1 hour, a polyamic acid solution in which [η] of the polyamic acid was 1.0 was obtained. This polyamic acid solution has a solid content of 7.5%.
And diluted with THF / methanol (weight ratio 4/1). Next, a continuous homomixer (capacity 5) having a stock solution supply port, a coagulation solution supply port, and a fibrid slurry discharge port.
00 ml, turbine rotation speed 9000 r. p. m), the diluted polyamic acid solution was continuously coagulated by supplying the diluted polyamic acid solution from the stock solution supply port at a supply rate of 24 ml / min and water from the coagulation liquid supply port at a supply rate of 960 ml / min. The fibrid slurry was continuously removed from the fibrid slurry outlet. The generated fibrids were separated by filtration, 1 liter of water was added, and the mixture was stirred and filtered three times. 0.8 g of the fibrids obtained as described above was dispersed in 500 ml of water, and paper was formed into a size of 15 cm × 15 cm. After pressing and dewatering this at a pressure of 3 MPa, 8
Dried at 0 ° C. This was heated at 170 ° C. for 1 hour and at 300 ° C. for 1 hour for imidization to obtain a non-thermoplastic and amorphous polyimide nonwoven fabric. Further, this nonwoven fabric was immersed in the above-mentioned polyamic acid solution (not diluted) for 5 minutes, pulled up and dried, and the mass increased by 70%. This sheet is kept at 80 ° C. for 5 hours,
By heating at 00 ° C. for 5 hours for imidization, a polyimide nonwoven fabric with increased strength was obtained.

Table 1 shows the results of measuring or evaluating various properties of the nonwoven fabric of which strength was increased in each of the polyimide nonwoven fabrics obtained in the examples and comparative examples.

[0041]

【table 1】

[0042]

As described above, the nonwoven fabric of the present invention
Excellent dimensional stability under high temperature use and low water absorption,
It is suitable as a base material for composite materials used in various fields. Furthermore, according to the production method of the present invention, such a nonwoven fabric can be efficiently produced, and the quality and handleability of the nonwoven fabric can be improved. The prepreg of the present invention in which the polyimide nonwoven fabric is impregnated with a thermosetting resin is a prepreg from which a composite material having excellent performance can be obtained because the polyimide nonwoven fabric having the above characteristics is used as a base material.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat ゛ (Reference) D21H 27/12 D21H 27/12 // C08L 101: 00 C08L 101: 00 F term (Reference) 4F072 AA02 AA07 AB07 AB29 AD23 AD45 AG03 AH02 AH25 AH31 AJ04 AL02 AL13 4L047 AA26 AB02 AB06 AB07 BA21 CB07 CB10 CC13 4L055 AF34 AF35 AF44 AF83 AG87 AH37 AH49 BE02 BE08 BE10 EA10 EA16 EA19 EA20 EA32 FA18 FA19 GA01 GA02 GA37

Claims (8)

[Claims] 1. A non-woven fabric mainly comprising short fibers made of polyimide, wherein the average linear expansion coefficient (JIS-K7197) of the non-woven fabric below the glass transition point of the main polyimide.
A non-woven fabric characterized by having a value measured according to the formula (1) in the width direction and the length direction of the non-woven fabric in the range of −20 to 20 μm / m · ° C. 2. The non-woven fabric according to claim 1, wherein a water absorption rate calculated by the following formula when left in an environment of 25 ° C. and a relative humidity of 60% for 24 hours is 0.5% or less. (Equation 1) (Wherein, W is shows the mass of the nonwoven fabric after moisture absorption, W ₀ denotes the mass of the nonwoven fabric during bone dry.) 3. The nonwoven fabric according to claim 1, wherein the polyimide is a crystalline polyimide having thermoplasticity. 4. The nonwoven fabric according to claim 3, wherein the polyimide has a glass transition point of 230 ° C. or higher and a melting point of 400 ° C. or lower. 5. The nonwoven fabric according to claim 3, wherein the polyimide has a repeating unit represented by the following general formula (1). [Formula 1] (In the formula, R represents a tetravalent aromatic residue selected from a monocyclic aromatic, a condensed polycyclic aromatic, and a non-condensed polycyclic aromatic in which aromatic rings are bonded directly or by a bridge member. Also, X
Is a direct bond, a hydrocarbon group, a carbonyl group, an ether group,
Y _{1 to} Y ₄ represent a divalent residue selected from a thio group or a sulfonyl group, and Y _{1 to} Y ₄ represent a monovalent residue selected from a hydrogen, an alkyl group, an alkoxyl group or a halogen group. ) 6. The polyimide short fiber has an average fiber length of 1 to 1.
The nonwoven fabric according to any one of claims 1 to 5, wherein the nonwoven fabric has a diameter of 5 mm and an average fiber diameter of 3 to 30 µm. 7. The method according to claim 1, further comprising the steps of: preparing a slurry containing short fibers made of polyimide to perform wet papermaking; and heating and pressing at a temperature equal to or higher than the glass transition point of the polyimide. The method for producing a nonwoven fabric according to any one of the above. 8. A prepreg comprising 70 to 20% by mass of the nonwoven fabric according to any one of claims 1 to 6 and 30 to 80% by mass of a thermosetting resin.