JP5307776B2 - Heat-resistant non-woven fabric and molded product obtained by heating the same - Google Patents

Description

  The present invention comprises a flame retardant nonwoven fabric having both excellent flame retardancy and tensile strength, and a molded product obtained by heating the same, comprising a polyetherimide (hereinafter sometimes abbreviated as PEI) fiber as a main constituent. It is related to the body and is exposed to a particularly high temperature environment in the fields of general industrial materials, electrical and electronic materials, medical materials, agricultural materials, optical materials, aircraft / automobile / ship materials, apparel, etc. It can be used very effectively for applications with many occasions.

  Conventionally, development of a flame retardant nonwoven fabric has been demanded in many fields. For example, non-woven fabrics are widely used in the fields of electrical / electronic materials, filters, heat insulation materials, medical materials, and industrial materials. In some of these fields, there is a particularly high demand for the development of flame retardant non-woven fabrics. ing.

  As the non-woven fabric having flame retardancy, a non-woven fabric composed of PEI fibers alone or a composite non-woven fabric of PEI fibers and inorganic fibers is disclosed (for example, see Patent Document 1). However, in the nonwoven fabric of Patent Document 1, it is said that impregnation with a chlorinated aliphatic hydrocarbon compound such as methylene chloride or trichloromethane is necessary for bonding PEI fibers to each other, but by using a solvent, PEI is used. May affect fiber properties. In view of the possibility that these solvents may affect the human body and the environment in recent years, the above invention is a manufacturing method with high cost burden in terms of environmental burden and solvent recovery, and development of alternative technologies has been desired. .

  Further, a heat-resistant flame retardant paper made of PEI fibers is disclosed (for example, see Patent Document 2). However, when the manufacturing method of patent document 2 is seen, the PEI fiber is made and the fibers are glued together using a surfactant, and the fibers are not entangled three-dimensionally. For this reason, since it does not have the characteristics such as breathability, stretchability, bulkiness, texture, and followability specific to nonwoven fabrics obtained by three-dimensional entanglement, it has been restricted in the field of use.

  Also, a polyethylene naphthalate long fiber having a melting point of 270 ° C. or higher and a birefringence (Δn) of 0.22 or higher and a low-oriented polyethylene naphthalate long fiber having a Δn of 0.01 or lower are mixed. A heat-resistant nonwoven fabric characterized by being thermocompression bonded is disclosed (for example, see Patent Document 3). However, the nonwoven fabric of the present invention does not have flame retardancy, and the glass transition temperature of the polyethylene naphthalate fiber used here is 121 ° C., and the mechanical properties such as elastic modulus are the glass transition temperature of the polymer. In view of the large drop in the actual use, there was a limit in actual use.

  Further, a heat-resistant nonwoven fabric obtained by blending heat-resistant fibers and unstretched polyphenylene sulfide fibers to form a web, and plasticizing and fusing the unstretched fibers under pressure is disclosed (for example, (See Patent Document 4). However, the degree of glass transition of the polyphenylene sulfide fiber used here is 85 ° C., and as described above, it is accompanied by a large change in physical properties at this temperature, and is restricted in practical use.

Japanese Patent Laid-Open No. 3-180588 JP-A-1-132898 Japanese Patent No. 3605231 Japanese Patent Publication No. 3-25537

  Therefore, the object of the present invention is not only excellent in flame retardancy but also provides a nonwoven fabric having both low water absorption and tensile strength and a molded body obtained by heating the nonwoven fabric without requiring a special process at low cost. That is.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have made PEI fibers having a specific structure as a main constituent, and a three-dimensional entangled nonwoven fabric has excellent flame retardancy and low water absorption. Furthermore, the present invention was completed by finding that it has tensile strength.

That is, the present invention is a non-woven fabric containing amorphous PEI fibers whose main repeating structural unit is represented by the following formula, and is a flame retardant non-woven fabric that satisfies all of the following (1) to (4).
(1) Part or all of the PEI fiber is three-dimensionally entangled,
(2) The tensile strength in a certain direction is at least 10 kg / 5 cm,
(3) The carbonization length measured according to JIS A1322 test method is 5 cm or less,
(4) The content rate of the said PEI fiber is 50 to 100 weight%.

  In the present invention, the amorphous PEI fiber preferably has a fiber strength at room temperature of 2.0 cN / dtex or more, a dry heat shrinkage at 200 ° C. of 5% or less, and an equilibrium moisture content of 3.0% or less. More preferably, the above-mentioned flame-retardant nonwoven fabric, which is a fiber that has not been stretched in the fiber production process in melt spinning, and more preferably the above-mentioned flame retardant nonwoven fabric, It is a flame retardant nonwoven fabric.

  Furthermore, this invention relates to the molded object characterized by heat-sealing a part or all of an amorphous PEI fiber by heating the said flame-retardant nonwoven fabric.

Hereinafter, the present invention will be described in detail. The flame-retardant nonwoven fabric of the present invention contains amorphous PEI fibers at least having a main repeating structural unit represented by the structural formula described later, and a part or all of the amorphous PEI fibers are three-dimensionally entangled. Need to be. Since the glass transition temperature of the PEI fiber having such a structure is 200 ° C. or higher, the heat resistance of the non-woven fabric formed therefrom is also high. Moreover, the three-dimensional entanglement causes the fibers to be entangled, and the strength and texture that can withstand actual use can be expressed.
There is no limitation in the manufacturing method of the nonwoven fabric of this invention, A spunlace nonwoven fabric, a needle punch nonwoven fabric, a steam jet nonwoven fabric, a spun bond nonwoven fabric, a melt blown nonwoven fabric is mentioned. Among these, a spunlace nonwoven fabric, a needle punched nonwoven fabric, and a steam jet nonwoven fabric are preferable from the viewpoint of production efficiency and high functionality. Moreover, you may add a hot press process to the obtained nonwoven fabric according to the objective suitably.

And the nonwoven fabric of this invention is characterized by the tensile strength of a certain fixed direction being 10 kg / 5cm or more. Here, the certain direction referred to in the present invention may be any of the longitudinal direction, the lateral direction, and the oblique direction of the fiber web.
When nonwoven fabrics are used in applications such as electrical / electronic materials, filters, heat insulation materials, medical materials, industrial materials, etc., many do not have a certain strength, resulting in poor handling and yield. Or, there is a risk of causing problems in product safety such as durability. For example, if the paper used for the separator for electrolytic capacitors is weak in mechanical strength, the paper may be cut when the capacitor element is manufactured, or cracks may cause electrical shorts. Since it becomes an impediment to performance improvement, at least the tensile strength in a certain direction needs to be 10 kg / 5 cm or more. From the viewpoint of process passability and durability, 11 kg / 5 cm or more is preferable, and 12 kg / 5 cm or more is more preferable. In addition, the tensile strength of a nonwoven fabric is measured by the method mentioned later.

  Furthermore, the nonwoven fabric of the present invention is characterized in that the carbonization length measured in accordance with the JIS A1322 test method is 5 cm or less. This is because if it is larger than 5 cm, it cannot be said that it has sufficient flame retardancy and may cause a problem in product safety. In order to enhance safety, it is preferably 4 cm or less, and more preferably 3 cm or less.

  Moreover, the flame-retardant nonwoven fabric of this invention requires that the content rate of PEI fiber is 50 to 100 weight%. When the PEI fiber is less than 50% by weight, sufficient flame retardancy is not exhibited, which is not preferable. It is preferably 55 to 100% by weight, and more preferably 60 to 100% by weight.

  Next, the PEI polymer constituting the nonwoven fabric of the present invention will be described. The PEI polymer used in the present invention is a polymer containing an aliphatic, alicyclic or aromatic ether unit and a cyclic imide as a repeating unit, and any one having amorphous and melt moldability. There is no particular limitation. Moreover, as long as the effect of the present invention is not impaired, the main chain of the PEI polymer contains a cyclic imide, a structural unit other than an ether bond, such as an aliphatic, alicyclic or aromatic ester unit, or an oxycarbonyl unit. May be.

  As a specific PEI polymer, a polymer represented by the following general formula is preferably used. Where R1 is a divalent aromatic residue having 6 to 30 carbon atoms, R2 is a divalent aromatic residue having 6 to 30 carbon atoms, and 2 to 20 carbons. A divalent selected from the group consisting of an alkylene group having an atom, a cycloalkylene group having 2 to 20 carbon atoms, and a polydiorganosiloxane group chain-terminated with an alkylene group having 2 to 8 carbon atoms. Organic group.

  As said R1, R2, what has an aromatic residue shown by the following formula group, for example is used preferably.

  The PEI polymer constituting the flame-retardant nonwoven fabric of the present invention preferably has a glass transition temperature of 200 ° C. or higher. A glass transition temperature of less than 200 ° C. is not preferable because the resulting nonwoven fabric may have poor heat resistance. In addition, the higher the glass transition temperature of the PEI polymer, the more preferable it is because a nonwoven fabric excellent in heat resistance can be obtained. However, when fusing, the fusing temperature also becomes high, and the polymer is decomposed at that time. It is not preferable because it may cause The glass transition temperature of the PEI polymer is preferably 200 to 230 ° C, and more preferably 205 to 220 ° C.

  In the present invention, 2,2-bis [4- (2,3-dicarboxyphenoxy) phenyl] propane dianhydride mainly having a structural unit represented by the following formula from the viewpoint of amorphousness, melt moldability, and cost. A condensate of the product with m-phenylenediamine or p-phenylenediamine is preferably used. This polyetherimide is commercially available from Servic Innovative Plastics under the trademark “Ultem”.

  The molecular weight of the PEI polymer constituting the flame-retardant nonwoven fabric of the present invention is not particularly limited. However, when considering the mechanical properties, dimensional stability, and process passability of the obtained fiber and nonwoven fabric, the weight average molecular weight ( Mw) is preferably 1000 to 80000. The use of a polymer having a high molecular weight is preferable because it is excellent in terms of fiber strength, heat resistance and the like, but Mw is preferably 2000 to 50000 from the viewpoint of resin production cost, fiberization cost, and the like, and is 3000 to 40000. More preferred.

  The amorphous PEI fiber constituting the flame-retardant nonwoven fabric of the present invention preferably has a dry heat shrinkage at 200 ° C. of 5% or less. More specifically, in all temperature ranges from 100 to 200 ° C., the dry heat shrinkage rate is preferably −1 to 5%. When the dry heat shrinkage rate exceeds 5%, the dimensional change of the product at the time of processing or use becomes large, and it cannot be said that it has heat resistance. Moreover, even if it is less than -1%, it is not preferable for the same reason. More preferably, the dry heat shrinkage is −1 to 4.5%, and more preferably 0 to 4%. The dry heat shrinkage referred to here is a value measured by the method described later.

  The amorphous PEI fibers constituting the flame retardant nonwoven fabric of the present invention preferably have a fiber strength at room temperature of 2.0 cN / dtex or more. When the fiber strength is less than 2.0 cN / dtex, the card passing property of the web is deteriorated or the usage is restricted, which is not preferable. More preferably, it is 2.3 to 4.0 cN / dtex, and further preferably 2.5 to 4.0 cN / dtex.

  The flame-retardant nonwoven fabric of the present invention is also characterized by a low water content, and can provide a nonwoven fabric with less deformation due to moisture absorption and less strength deterioration. The equilibrium moisture content may be, for example, 3.0% or less, preferably 2.0% or less, and more preferably 1.0% or less. In addition, the equilibrium moisture content here is a value measured by the method described in the Example mentioned later.

  The amorphous PEI fibers constituting the flame retardant nonwoven fabric of the present invention are not limited to the effects of the present invention, but are antioxidants, antistatic agents, radical inhibitors, matting agents, ultraviolet absorbers. , Flame retardants, inorganic substances, and the like may be included. Specific examples of such inorganic substances include carbon nanotubes, fullerene, talc, wollastonite, zeolite, sericite, mica, kaolin, clay, pyrophyllite, silica, bentonite, alumina silicate and other silicates, silicon oxide, magnesium oxide, Metal oxides such as alumina, zirconium oxide, titanium oxide and iron oxide, carbonates such as calcium carbonate, magnesium carbonate and dolomite, sulfates such as calcium sulfate and barium sulfate, calcium hydroxide, magnesium hydroxide and aluminum hydroxide Hydroxides, glass beads, glass flakes, glass powders, ceramic beads, boron nitride, silicon carbide, carbon black and silica, graphite and the like are used. Furthermore, for the purpose of improving the hydrolysis resistance of the fiber, an end group blocking agent such as a mono- or diepoxy compound, a mono- or polycarbodiimide compound, a mono- or dioxazoline compound, or a mono- or diazirine compound may be included.

  Next, the manufacturing method of the amorphous PEI fiber which comprises the flame-retardant nonwoven fabric of this invention is demonstrated. A known melt spinning apparatus can be used in the production of the amorphous PEI fibers constituting the flame retardant nonwoven fabric of the present invention. That is, it is obtained by first melt-kneading PEI polymer pellets with a melt extruder, introducing the molten polymer into a spinning cylinder, measuring it with a gear pump, and winding the yarn discharged from the spinning nozzle. The take-up speed at that time is not particularly limited, but it is not preferred if molecular orientation occurs on the spinning line, and it is preferably taken in the range of 500 m / min to 4000 m / min. If it is less than 500 m / min, it is not preferable from the viewpoint of productivity. On the other hand, if it exceeds 4000 m / min, not only the molecular orientation is sufficient to cause shrinkage at high temperatures, but also fiber breakage tends to occur. Therefore, it is not preferable.

In the production of the amorphous PEI fibers constituting the flame-retardant nonwoven fabric of the present invention, it is preferable not to perform a stretching step in order to ensure the heat resistance of the fibers. This is because the PEI fibers of the present invention are completely amorphous, and thus entropy shrinkage due to increased molecular motion occurs at high temperatures when drawn as in the conventional fiber manufacturing process. Therefore, it is accompanied by large shrinkage, and it is impossible to achieve heat resistance that can withstand actual use, such as moldability and dimensional stability of the obtained molded body.
Therefore, in the present invention, a PEI fiber having high heat resistance and a flame-retardant nonwoven fabric comprising the same can be produced by not drawing, while considering the feature of “amorphous”.

  The fineness of the amorphous PEI fibers constituting the flame-retardant nonwoven fabric of the present invention is not particularly limited as long as it can be used in the nonwoven fabric production process. However, if it is less than 0.8 dtex, the card has a card passability. For example, the fineness may be about 0.8 to 15.0 dtex, more preferably 0.8 to 10.0 dtex, and still more preferably 1. It may be 0.0 to 8.0 dtex.

  Although fiber length is not specifically limited, About 10 mm-102 mm are preferable from the point of card | curd passage property, Furthermore, 20 mm-64 mm are more preferable in order to raise the entanglement of fibers and to give intensity | strength.

There is no particular limitation on the basis weight of the nonwoven fabric, and an appropriate basis weight is selected depending on the intended use. However, when the basis weight is less than 10 g / m ² , the formation unevenness is increased, which is not preferable. From the point of productivity, 10-700 g / m < ² > is preferable, More preferably, it is 20-700 g / m < ² >.

Furthermore, it is also possible to contain organic or inorganic heat resistant fibers as long as the effects of the present invention are not impaired. By combining the amorphous PEI fiber of the present invention with heat-resistant fibers, a flame-retardant nonwoven fabric having both flame resistance, heat resistance, and mechanical properties can be produced.
For example, examples of inorganic fibers include glass fibers, carbon fibers, silicon carbide fibers, alumina fibers, ceramic fibers, and various metal fibers (for example, gold, silver, copper, iron, nickel, titanium, stainless steel, etc.). .
Examples of organic fibers include fibers made of heat-resistant polymers such as wholly aromatic polyester fibers, polyphenylene sulfide fibers, para-aramid fibers, polysulfonamide fibers, phenol resin fibers, polyimide fibers, and fluorine fibers. Can do. Among these heat resistant fibers, from the viewpoint of improving the flame retardancy of the obtained paper, wholly aromatic polyester fibers, glass fibers, polyphenylene sulfide fibers, para aramid fibers, polysulfonamide fibers, and the like are preferable.

Furthermore, the flame-retardant nonwoven fabric of this invention is heated, and the molded object excellent in the flame retardance can be obtained by carrying out the melt | fusion process of one part or all part of PEI type fiber.
Examples of the method for producing the molded body include calendar roll processing, hot press molding, embossing, and the like. Examples of the obtained molded body include flat plates, deep-drawn molded products, flange molded products, and the like.

  The flame-retardant nonwoven fabric and molded body of the present invention not only have flame retardancy but also have low water absorption and tensile strength, and can be manufactured at low cost without requiring any special process. It is extremely useful in the fields of filters, heat insulating materials, medical materials and industrial materials.

EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited at all by this Example. In the following examples, the physical properties of polymers and nonwoven fabrics were measured by the following methods.

[Fiber strength cN / dtex]
According to the JIS L1013 test method, the pre-humidified yarn was measured for breaking strength at a test length of 20 cm, an initial load of 0.25 cN / dtex and a tensile strength of 50% / min, and an average of n = 10 Value was adopted. The fiber strength (dtex) was determined by a mass method.

[Dry heat shrinkage%]
The fiber cut out to 10 cm was calculated from the fiber length (Xcm) after being held for 10 minutes in an air thermostat kept at 200 ° C. with the end not fixed, using the following formula.
Dry heat shrinkage (%) = <X / 10> × 100

[Weight per unit g / m ² ]
It measured according to the JIS L1913 test method, and adopted the average value of n = 3.

[Thickness mm]
The thickness was measured according to the JIS L1913 test method, and an average value of n = 3 was adopted. However, in Comparative Example 4 only, when a constant load was applied during measurement, the thickness was crushed and an accurate numerical value could not be measured. Therefore, the apparent thickness was measured, and an average value of n = 3 was adopted.

[Tensile strength kg / 5cm]
In accordance with the JIS L1913 test method, a test piece having a width of 5 cm and a length of 15 cm is grasped at a holding interval of 10 cm, stretched at a tensile speed of 20 cm / min using a constant speed extension type tensile tester, and the load value at the time of cutting is pulled. The average value of n = 3 was adopted.

[Carbonization length cm]
In accordance with the JIS A1322 test method, the carbonization length when heated for 10 seconds with a Meckel burner 50 mm away from the lower end of the sample was measured for the lower end of the sample placed at 45 ° C., and the average value of n = 3 was calculated. Adopted.

[Equilibrium moisture content]
Consistent with JIS L1013 test method, after the sample was completely dried in an atmosphere of 120 ° C, it was adjusted at a temperature of 20 ° C and a relative humidity of 65% RH for 72 hours, and contained in the sample relative to the mass of the sample in the absolutely dry state. The moisture content was calculated and expressed as a percentage (%).

<Reference Example: Production of Amorphous PEI Fiber>
(1) “ULTEM9001” manufactured by Servic Innovative Plastics was vacuum-dried at 150 ° C. for 12 hours.
(2) The polymer of (1) above was discharged from a round hole nozzle under the conditions of a spinning head temperature of 390 ° C., a spinning speed of 2000 m / min, and a discharge rate of 50 g / min, single fiber fineness: 2.2 dtex, strength: 2.6 cN / Dtex, dry heat shrinkage at 200 ° C .: 3.5% multifilament was obtained.
The obtained multifilament was crimped and then cut to produce a short fiber having a fiber length of 51 mm.

[Example 1]
The amorphous PEI short fibers obtained in the reference example are put on a card to produce a fiber web having a basis weight of 40 g / m ² . Next, the web is placed on the support net of the hydroentanglement machine, and while feeding at a speed of about 50 m / min, a water junction nozzle arranged in a row with a nozzle diameter of 100 μm and a hole pitch of 0.6 mm is used in the laminate. Then, water having a water pressure of 20 to 100 kgf / cm ² was jetted on both sides, the staples were entangled and integrated, and then a dry heat treatment was performed at a temperature of 110 to 160 ° C. to obtain a nonwoven fabric. Table 1 shows the physical properties of the obtained nonwoven fabric.

[Examples 2 to 4]
Nonwoven fabrics of Examples 2 to 4 were obtained in the same manner as in Example 1 except that the basis weight of the fiber web was changed to the weight described in Table 1. Table 1 shows the physical properties of the obtained nonwoven fabric.

[Examples 5 to 6]
Amorphous PEI short fibers used in Example 1 and polyarylate fibers (“Vectran” manufactured by Kuraray Co., Ltd.), short fiber fineness: 2.8 dtex, fiber length: 51 mm, melting point: 370 ° C., tensile strength: 20 cN / dtex) was used at a rate shown in Table 1 to prepare a nonwoven fabric with a basis weight of 40 g / m ^{2 in} the same manner as in Example 1. Table 1 shows the physical properties of the obtained nonwoven fabric.

[Example 7]
The nonwoven fabric produced in Example 1 was hot compression molded at a temperature of 240 ° C. and a pressure of 20 MPa for 1 minute to produce a board-shaped molded body. Table 1 shows the physical properties of the obtained molded body.

[Comparative Example 1]
40 parts by weight of amorphous PEI short fibers used in Example 1, polyester fiber (“T-471” manufactured by Toray Industries, Inc.), short fiber fineness: 1.6 dtex, fiber length: 51 mm, melting point: 260 ° C. Tensile strength: 5.9 cN / dtex) A non-woven fabric having a basis weight of 40 g / m ² was produced in the same manner as in Example 1 using 60 parts by weight. Table 1 shows the physical properties of the obtained nonwoven fabric.

[Comparative Example 2]
30 parts by weight of amorphous PEI short fibers used in Example 1 and rayon fibers (“Corona” manufactured by Daiwa Spinning Co., Ltd., short fiber fineness: 1.7 dtex, fiber length: 40 mm, melting point: 260 ° C., tensile (Strength: 2.7 cN / dtex) A non-woven fabric having a basis weight of 40 g / m ² was produced in the same manner as in Example 1 using 70 parts by weight. Table 1 shows the physical properties of the obtained nonwoven fabric.

[Comparative Example 3]
A non-woven fabric having a basis weight of 40 g / m ² was produced in the same manner as in Example 1 using 100 parts by weight of rayon fiber used in Comparative Example 2. Table 1 shows the physical properties of the obtained nonwoven fabric.

[Comparative Example 4]
In Example 1, the nonwoven fabric was produced by the same method as Example 1 except not performing a hydroentanglement process. Table 1 shows the physical properties of the obtained nonwoven fabric.

[Comparative Example 5]
The nonwoven fabric produced in Comparative Example 1 was subjected to hot compression molding in the same manner as in Example 7 to produce a board-like molded body. Table 1 shows the physical properties of the obtained molded body.

As is clear from Table 1, the nonwoven fabrics obtained in Examples 1 to 6 are composed of specific PEI fibers and heat-resistant fibers, so that not only has a short charcoal fire length but also flame retardancy, The equilibrium moisture content of the nonwoven fabric was also kept at a low level. Further, the molded body obtained in Example 7 has a board shape in which fibers are sufficiently bonded by thermocompression bonding, and has high flame retardancy.
Moreover, even if the comparative example 4 is using PEI fiber, since the three-dimensional entanglement is not implemented, it does not have the required intensity | strength which fibers are sufficiently entangled.
Moreover, since the content of PEI fiber does not satisfy the constituent requirements of the present invention, Comparative Example 5 is not only insufficient in adhesion between fibers by thermocompression bonding and does not have good moldability, but also difficult. Flammability could not be expressed.

  The flame-retardant nonwoven fabric and molded article of the present invention not only have flame retardancy, low water absorption, and tensile strength, but also can be manufactured at low cost without requiring a special process. Used extremely effectively for applications that are frequently exposed to high temperature environments in the fields of materials, medical materials, agricultural materials, optical materials, aircraft / automobile / marine materials, apparel, etc. be able to.

Claims (5)

A non-woven fabric containing amorphous polyetherimide fiber having at least a main repeating structural unit represented by the following formula and satisfying all of the following (1) to (4).
(1) Part or all of the polyetherimide fiber is three-dimensionally entangled,
(2) The tensile strength in a certain direction is at least 10 kg / 5 cm,
(3) The carbonization length measured according to JIS A1322 test method is 5 cm or less,
(4) The content of the polyetherimide fiber is 50 to 100% by weight.

  The amorphous polyetherimide fiber has a fiber strength at room temperature of 2.0 cN / dtex or more, a dry heat shrinkage at 200 ° C. of 5% or less, and an equilibrium moisture content of 3.0% or less. The flame-retardant nonwoven fabric according to claim 1.   The flame-retardant nonwoven fabric according to claim 1 or 2, wherein the amorphous polyetherimide fiber is a fiber that has not been stretched in a fiber production process in melt spinning.   Furthermore, the flame-retardant nonwoven fabric as described in any one of Claims 1-3 formed by containing a heat resistant fiber.   A molded article, wherein the nonwoven fabric according to any one of claims 1 to 4 is heated and part or all of the amorphous polyetherimide fiber is thermally fused.