JP2002339156A - Wholly aromatic polyamide fiber comprising para-based homopolymer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain both an aromatic polyamide fiber comprising a para-based homopolymer in which oozing of sodium ion is controlled and a copper-clad laminate using the fiber as a substrate. SOLUTION: This aromatic polyamide fiber comprising a para-based homopolymer in which oozing of sodium ion controlled is controlled is produced by heating and annealing within the temperature range of 290 deg.C to 550 deg.C an aromatic polyamide fiber comprising a para-based homopolymer produced by an ordinary production method. This copper-clad laminate is produced by using the fiber as a substrate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fiber for providing an organic insulating material having a very small decrease in insulation resistance in an electric field under high temperature and high humidity, a prepreg using the insulating material, and a copper clad laminate for a printed wiring board. Regarding the board.

[0002]

2. Description of the Related Art In recent years, copper-clad laminates based on aromatic polyamide fibers having excellent heat-resistant dimensional stability have been developed and are being used for applications requiring high heat-resistant dimensional stability.
The substrate is manufactured by impregnating and drying a varnish such as an epoxy resin into a woven fabric, paper, nonwoven fabric, or the like made of an aromatic polyamide fiber, and laminating and molding a plurality of prepregs.

[0003] For example, "a flexible material characterized by using an electrical insulating layer obtained by impregnating a resin composition comprising an epoxy resin and a rubber-based resin into a nonwoven fabric containing an aromatic polyamide as a main component for a base material and / or a coverlay. "Printed wiring board" (Japanese Patent Application Laid-Open No. 61-100446) or
A flexible substrate formed by impregnating an epoxy resin obtained by adducting an aromatic amine curing agent into an aramid fiber cloth base material of 025 to 0.5 mm, and a wiring layer of a metal foil formed on one or both surfaces of the substrate. Flexible wiring board characterized by comprising "(JP-A-62-283695)
Are disclosed.

[0004] Further, by treating with a coupling agent such as a silane coupling agent, a titanate coupling agent, a zirconate coupling agent, etc., the adhesion between an aramid fiber as a base material and a matrix resin is improved. We also offer copper-clad laminates. However, in the thus obtained substrate, when an electric field is applied between the silver electrodes formed on the substrate, silver migration occurs after a certain period of time, particularly in an electric field under high temperature and high humidity, and as a result, It is often found that the insulation resistance decreases. This phenomenon is caused by the absorption of moisture by the aromatic polyamide fiber, the sodium ions leaching out from the sodium salt in the aromatic polyamide fiber containing a large amount of sodium salt for the reason of the production of the fiber, and the migration of silver is promoted. In particular, in high-density mounting applications such as COB (chip-on-board), a fatal defect occurs because the insulation resistance between electrodes decreases.

As one method for overcoming this problem, Japanese Patent Publication No. 6-71130 (named “prepreg”) discloses an inorganic acid such as concentrated sulfuric acid and a hydroxylation necessary for neutralizing and removing the same during spinning. Aromatic polyamide fiber that can be produced without using sodium, that is, aromatic polyamide fiber obtained by wet spinning a polymerization solution as it is without isolating a polymer obtained by polymerization in an aprotic amide solution It has been proposed to use aromatic polyamide fibers with extremely low sodium content.

That is, since polyparaphenylene terephthalamide fibers are produced by liquid crystal spinning of an optically anisotropic dope of a polymer sulfuric acid solution, although there are ionic impurities such as sodium, sulfate and chlor, copolyparaphenylene. The 3,4′-oxydiphenylene terephthalamide fiber is spun from a homogeneous polymer solution of an organic solvent, and is said to have a small amount of these impurities (Circuit Technology Vol. 5, No. 117, 1990). However, aromatic polyamide fibers that can be produced by this method are limited to those composed of meta- or para-based copolymers, and para-type fibers capable of maximizing the thermal dimensional stability and rigidity characteristic of aromatic polyamide fibers. The property value of a wholly aromatic polyamide fiber made of a homopolymer is almost inferior, and the performance must be sacrificed.

While maintaining the excellent physical properties of the para-homopolymer wholly aromatic polyamide fiber, it is contained by using inorganic hydroxide such as concentrated sulfuric acid derived from the production and by using sodium hydroxide for its removal by neutralization. It is required to prevent the adverse effect of the sodium salt in the fiber, which is to be performed.

[0008]

SUMMARY OF THE INVENTION The present invention uses a para-homopolymer wholly aromatic polyamide fiber as a base material and performs insulation by leaching sodium ions in an electric field under high temperature and high humidity without deteriorating its characteristic values. An object of the present invention is to provide a copper-clad laminate made of an epoxy resin-impregnated substrate with a reduced resistance, which overcomes the above-mentioned conventional disadvantages, and a copper-clad laminate for COB and a prepreg thereof.

[0009]

SUMMARY OF THE INVENTION The present invention relates to (1) a wholly aromatic polyamide fiber comprising a para-type homopolymer containing sodium as a salt in a fiber weight ratio of 2,000 ppm or more, wherein A wholly aromatic polyamide fiber comprising a para-type homopolymer, wherein the amount of sodium extracted by the hot water extraction for 2 hours is 500 ppm or less; (2) a fiber weight ratio of 2000 pp in the fiber;
m or more as a salt, a wholly aromatic polyamide fiber comprising a para-type homopolymer having a temperature range of 290 ° C.
And (3) a prepreg obtained by impregnating a para-homopolymer wholly aromatic polyamide fiber base material with an epoxy resin composition, wherein the fiber is heat-annealed at a temperature of from 550 ° C. to 550 ° C. A prepreg in which the homopolymer wholly aromatic polyamide fiber is the fiber described in (1) or (2), or a laminate of two or more prepregs described in (4) and (3), with a copper foil laminated on the surface and integrated by pressure and heating (5) Temperature range 290
A wholly aromatic polyamide fiber consisting of para homopolymer, characterized in that the fiber is annealed at a temperature of from 550 ° C. to 550 ° C. and the equilibrium moisture content of the fiber is 1.5% by weight or less; A wholly aromatic polyamide fiber comprising para-type homopolymer, wherein the equilibrium moisture content of the fiber is reduced to 1.5% by weight or less by heating and annealing the wholly aromatic polyamide fiber having a temperature of 290 ° C. to 550 ° C. And (7) the method according to (5) or the method according to (6), wherein the heating method is a contact method using a high-temperature metal roll, a non-contact method using a heating furnace or infrared irradiation.

[0010]

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a fiber weight ratio of 200
A wholly aromatic polyamide fiber made of a para-type homopolymer containing sodium of 0 ppm or more as a salt,
Provided is a prepreg obtained by impregnating an epoxy resin composition with a para-homopolymer wholly aromatic polyamide fiber as a base material, wherein the amount of sodium extracted by hot water extraction at 21 ° C. for 2 hours is 500 ppm or less. Further, a copper-clad laminate obtained by laminating two or more prepregs obtained here, laminating a copper foil on the surface thereof, and integrally molding the resultant under pressure and heating, and after etching the copper foil. This also provides a product in which the leaching of sodium ions and sulfate ions from the laminate is suppressed.

The para-homopolymer wholly aromatic polyamide fibers used herein are those comprising the following repeating units (I) and / or (II), and are obtained by polycondensation from paradicarboxylic acid and paradiamine. It is what is being done.

Embedded image (Wherein, Ar1, Ar2, and Ar3 are substituted or unsubstituted aromatic rings, and include paraphenylene, 2,6
-Naphthylene, 4,4'-biphenylene, 2,5-pyridinyl and the like. Examples of the substituent on the aromatic ring of Ar1, Ar2, and Ar3 include an alkyl group having 1 to 3 carbon atoms, an alkoxy group, a halogen atom, and a phenyl group.

In the fiber obtained here, all the aromatic rings constituting the polymer main chain and the aromatic rings are connected at the para position only through the amide bond. A straight-chain or parallel-axis-bonded aromatic residue containing no carbon chain or heteroatom (provided that some of the hydrogen atoms directly bonded to the aromatic ring are substituted with halogen atoms, methyl groups, methoxy groups, etc.) A high modulus fiber having a high molecular orientation and / or a short fiber obtained by crushing and fibrillating the fiber. A representative example of the fiber is polyparaphenylene terephthalamide, which is widely known under the trade name of Kepler.

The fineness of the fiber as a base material thus obtained is 0.1.
Those having 1 to 10 decitex are preferable, and woven fabric, knitted fabric,
Non-woven fabric, in the form of a sheet such as paper or in the form of short fibers that can be dispersed in a resin matrix, when used in the laminated board substrate, other fibers as necessary, for example, a copolymer Wholly aromatic polyamide fibers,
Meta-based wholly aromatic polyamide fiber, polyetherketone fiber, polyetheretherketone fiber, polyetherimide fiber, polyimide fiber, wholly aromatic polyester fiber, polyphenylene sulfide fiber, glass fiber, carbon fiber, ceramic fiber, etc. It can also be used. Its use ratio is at most 40% by weight, preferably at most 30% by weight.

The inventors of the present invention have found that one of the drawbacks of a copper-clad laminate substrate based on para-homopolymer wholly aromatic polyamide fibers is a decrease in insulation resistance under high temperature and high humidity. In view of the fact that it is due to the leaching of sodium ions in the fiber due to the above, as a result of repeated investigations on the method of suppressing this, it was obtained by polymerizing aromatic diamine and aromatic dicarboxylic acid chloride The resulting polymer is isolated and purified from the polymerization solution, then dissolved in concentrated sulfuric acid and subjected to wet spinning called liquid crystal spinning, followed by neutralization, washing, and drying to produce a para-homopolymer wholly aromatic. By heating and annealing the polyamide fiber, it was possible to reduce the equilibrium moisture content and to obtain an unexpected new finding that the leaching of sodium ions can be suppressed.

When the equilibrium moisture content of the fiber exceeds 3.0% by weight, sodium in the substrate is easily ionized in a high-temperature and high-humidity electric field, resulting in poor insulation. Further, the silver electrode formed on the substrate surface is ionized. It is easy to form hydrates, and as a result, it is said that silver oxide precipitates on the positive electrode and silver precipitates on the negative electrode, so-called silver migration occurs, so controlling this point is an important point. .

The present invention has been made based on such findings. In other words, the para-homopolymer wholly aromatic polyamide fibers used as the base material of the copper-clad laminate substrate or the prepreg thereof in the present invention are those subjected to heat annealing. The heating temperature ranges from 290 ° C to 550 ° C, preferably from 400 ° C to 450 ° C,
The heating time is 1 second to 10 seconds, preferably 1 second to 3 seconds. Heat annealing is performed in air or a nitrogen stream. Prior to this, preheating and drying at a lower temperature enables short-time processing.

By performing such a heat annealing treatment, the equilibrium moisture content of the fiber can be reduced to 3.0% by weight or less, further 1.5% by weight or less, and good results can be obtained. That is, even if it is a conventional para-based wholly aromatic polyamide fiber, the fiber in which the leaching due to the ionization of the sodium salt remaining inside the fiber is reduced to a negligible level at a practical level by reducing the equilibrium moisture content. You can do it.

Here, the degree of negligible leaching due to ionization of the sodium salt at a practical level is that sodium leaching from the fiber is obtained by heating about 4 g of para-based wholly aromatic polyamide fiber together with 40 g of pure water in a closed vessel at 121 ° C. After boiling for 2 hours, the absolute value obtained by quantitatively analyzing the sodium dissolved in pure water by ion chromatography is represented by the value obtained by dividing by the weight of the aromatic polyamide fiber before extraction, which exceeds 500 ppm. In some cases, ion leaching occurs as sodium ions into the substrate in a high-temperature and high-humidity electric field, and migration of silver occurs, which means that the concentration is 500 ppm or less. The amount of sodium contained in the extraction water was determined using an ion chromatograph (Model DX320, manufactured by Nippon Dionex Co., Ltd.). Using an atomic absorption spectrometer (Hitachi, Ltd., model Z6100)
This is performed according to JIS K0102 48.2.

The reason why the equilibrium water content can be reduced is that the crystal size of the fiber, particularly the crystal state of the portion constituting the outer skin of the fiber, changes by the heat annealing treatment, resulting in a dense structure and hydrophobicity. We think that it happens because of improvement. As a result, even when the external environment becomes humid, the hydrophobic layer of the fiber skin blocks the penetration of moisture, and thus ionization of sodium is suppressed. The equilibrium moisture content can be measured by measuring and measuring the moisture content of the fiber in an equilibrium state at 20 ° C. and 65% RH based on JIS L1013 chemical fiber filament yarn test method.

The method of heating does not require any particular method, for example, a contact method using a high-temperature metal roll,
A non-contact method such as passing through a heating furnace or irradiating infrared rays can be used. The heat annealing treatment on the fiber can be performed in any step, and is not limited. It may be carried out immediately after the neutralization washing after spinning, or may be a processed product in which the fiber is processed into a woven, knitted, paper-like or non-woven fabric. Furthermore, even in the case of a prepreg obtained by impregnating a workpiece with a resin, the effect of heat annealing can be obtained.

It is needless to say that the effect of the present invention is further enhanced by washing and removing as much as possible the sodium salt which easily falls off the fiber surface layer prior to the heat annealing treatment. In addition, in the para-based wholly aromatic polyamide fiber spun with a concentrated sulfuric acid solvent, the polymer main chain is partially sulfonated, and there is a concern that sodium sulfonate is generated in the neutralization step. Washing and removing ions with diluted sulfuric acid or the like can enhance the effect of the present invention.

In this sense, when the fiber as the base material is used in the form of paper, the fiber surface layer comes into contact with a large amount of water in the paper making step, and a cleaning effect appears. Although it is desirable to heat-anneal the fibers, it is a matter of course that the fibers may be washed and heat-annealed before the paper making step. If the high temperature treatment is not possible after the paper making process due to the heat resistance of the paper making binder, etc., it is a preferable method to perform the heat annealing treatment first.

The present invention relates to a prepreg containing the above fibers as a base material, and is produced by a method known per se. Here, the resin used as the matrix resin is preferably an epoxy resin, For example, diglycidyl ether obtained by reaction of bisphenol A or halogenated bisphenol A with epiacrylhydrin (Epicoat 82 manufactured by Yuka Shell Epoxy Co., Ltd.)
8, 1001, 1002, 1003, 1004, 100
5 etc., or epicoat 5045, 5046, 5048,
5049 etc.)

Alternatively, a polyether-type polyglycidyl ether obtained by reacting a polyhydric alcohol obtained by reacting bisphenol A with an alkylene oxide in the presence of an acid-alkali catalyst and epichlorohydrin (for example, EP-4000 manufactured by Asahi Denka Co., Ltd.) ), A phenol novolak type epoxy (for example, Epicoat 152, 154, etc., manufactured by Yuka Shell Epoxy Co., Ltd.), an orthocresol novolak type epoxy (for example, Epicoat 180S65, manufactured by Yuka Shell Epoxy Co., Ltd.) or a mixture thereof. Can be Where a curing agent is used in combination with the epoxy resin used here, general dicyandiamide, heat-resistant aromatic polyamines, phenol resins and the like can be mentioned. Further, a curing accelerator may be appropriately used in combination, and examples of the curing accelerator include tertiary phosphines such as imidazoles, imidazolines, and triphenylphosphine.

The matrix resin may be used by dissolving it in an appropriate solvent, if necessary, for adjusting the amount of resin adhering thereto. Examples of the appropriate solvent include ketones such as acetone and methyl ethyl ketone, and benzene. , Aromatic hydrocarbons such as toluene and xylene, amides such as N, N'-dimethylformamide and N, N'-dimethylacetamide, and cellosolves such as methyl cellosolve, ethyl cellosolve and butyl cellosolve. The epoxy resin and its curing agent, which are the matrix resin used here, as well as the curing accelerator, when containing ionic impurities, cause a decrease in performance as an insulating substrate. It is needless to say that it is desirable to select and use a small number.

The prepreg thus obtained is formed into an appropriate thickness by a method known per se, and a copper foil is attached to the prepreg, or a copper foil is attached to a plurality of laminated prepregs, followed by pressure molding to form a copper-clad laminate. And Here, pressure molding can be performed by applying a known method. The copper-clad laminate obtained here has a small decrease in insulation resistance under high temperature and high humidity conditions, and is therefore used as a highly reliable component.

[0027]

EXAMPLES The present invention will be described below in more detail with reference to Examples, but the present invention is not limited thereto.

Example 1 PPTA (ηinh = 6.5) obtained by polymerizing 100 mol% of terephthaloyl dichloride and 100 mol% of paraphenylenediamine by a conventional method was dissolved in 99.9% by weight of concentrated sulfuric acid to obtain a polymer. Concentration 19.0% by weight, temperature 80
° C spinning dope, with a pore number of 0.06 mm and 100 pores
After extruding from a die having zero pieces, passing through a slight air gap, coagulating in water at 4 ° C., guiding to a Nelson roller, neutralizing with an 8% by weight aqueous sodium hydroxide solution, and washing with hot water After drying at 110 ° C. for 15 seconds with a hot roller, the product was then led to a hot roller installed for 3 seconds.
After preheating at 50 ° C for 10 seconds, further under tension 500 ° C
And then wound on a bobbin by contacting with a high-temperature hot roller for 1 second.
A wholly aromatic polyamide fiber (A) comprising a decitex para-based homopolymer was obtained.

Example 1 (Fiber (A)) Tensile strength (JIS L1013) (N / tex) 1.59 Tensile modulus (JIS L1013) (N / tex) 99.0 Density (g / cm ³ ) 47 Moisture content (%) 1.5 Content sodium (ppm) 6100 Extracted sodium (ppm) 99 The method of measuring the content sodium and the extracted sodium is as described above.

COMPARATIVE EXAMPLE In the same manner as in Example 1, except that the second hot roller temperature was changed to 250 ° C. instead of 350 ° C. and the high-temperature hot roller treatment at 500 ° C. was omitted. A wholly aromatic polyamide fiber (B) was obtained. Physical properties were measured in the same manner as in Example 1.

Comparative Example (Fiber (B)) Tensile strength (JIS L1013) (N / tex) 2.03 Tensile modulus (JIS L1013) (N / tex) 49.9 Density (g / cm ³ ) 1.44 Moisture percentage (%) 7.0 Sodium content (ppm) 6000 Extracted sodium (ppm) 2400

Example 2 Fiber (A) and fiber obtained in Example 1 and Comparative Example described above
For each of (B), strictly according to the following known production methods
According to the prepreg and its silver electrode insulation resistance
And the degree of migration of silver were measured and the following results were obtained.
Was. Insulation resistance between silver electrodes Fiber (A) Fiber (B) 9.5 × 10^Fifteen 2.8 × 10⁹  Silver migration degree None Large In addition, the process manufactured from fiber (A) as COB parts
No inconvenience observed even after using reprig for one year
Was.

(Preparation of Pre-preg) The above fiber (A) or fiber (B) was cut into a length of 3 mm, dispersed in water, and 55 g / m ² paper was weighed using a tappy type square paper machine. 1 in a hot air drier sandwiched between two metal meshes
Drying was performed at 50 ° C. for 5 minutes. Next, a water-dispersed epoxy resin composition containing 1 ppm of sodium and 150 ppm of chlorine was spray-coated on both sides of the mesh from both sides of the mesh, and further heated at 100 ° C. for 2 minutes in a hot-air dryer for 160 minutes.
Dry at 30 ° C. for 30 minutes. The amount of adhered resin was about 5% by weight. Then, using a pair of metal / elastic roll calender having a metal roll having a surface temperature of 190 ° C., 300 kg / c.
The paper-like material was hot-pressed under the conditions of m and 2 mm / min.
Next, 80 parts by weight of a glycidyl etherified product of a polycondensate of bisphenol A and formaldehyde (epoxy equivalent: 208) and a bisphenol A type epoxy resin (epoxy equivalent:
187) 20 parts by weight and 30 parts by weight of tetrabromobisphenol A were reacted in the presence of 0.03 part by weight of dimethylimidazole to obtain an epoxy resin having an epoxy equivalent of 342 and a bromine content of 23% by weight. Next, a curing agent which is a polycondensate of bisphenol A and formaldehyde was obtained. 56 parts by weight of an epoxy resin, 20 parts by weight of a brominated bisphenol A type epoxy resin (epoxy equivalent: 470, bromine content: 48% by weight), and 24 parts by weight of a curing agent were mixed, and 0.2% of 2-ethyl-4-methylimidazole was added thereto. A mixed solvent of methyl ethyl ketone / ethylene glycol monomethyl ether (mixing ratio by weight: 1/1) was added to an epoxy resin composition consisting of 04 parts by weight to obtain a nonvolatile content of 60% by weight and a bromine content of 2%.
A varnish of 2.5% by weight (based on solid content) was prepared. The varnish was impregnated into the above-mentioned paper-like material and dried at 100 ° C. for 3 minutes to obtain a prepreg having an epoxy resin composition solid content of 70% by weight. Next, copper foil CF made by Fukuda Metal Foil Powder Co., Ltd.
Two sheets of -T9 (1 oz) and four of the prepregs were laminated and pressed with a hot press at 170 ° C. and 40 kg / cm ² for 1 hour to produce a copper-clad laminate.

(Measurement of Insulation Resistance Between Silver Electrodes) A silver paste (No. 4929, manufactured by DuPont) was placed on the substrate after etching the copper foil.
A plurality of electrodes having a gap of 0.5 mm are formed on the surface by screen printing using
Immediately after the humidity control for 96 hours, the insulation resistance was measured by applying a DC voltage of 500 volts. Further, solder reflow was performed at 220 ° C. for 2 minutes. Next, a pressure cooker test was performed for 200 hours under the conditions of 121 ° C. and 2 kg / cm ² while applying a DC voltage of 90 V between the two electrodes.
Immediately after the substrate is conditioned at 20 ° C. and 65% RH for 96 hours, 50
The insulation resistance was measured by applying a DC voltage of 0 volt. (Observation of the degree of migration of silver) Immediately after the completion of the above-mentioned pressure cooker test, the surface state between silver electrodes is observed with an optical microscope.

[0035]

The para-based wholly aromatic polyamide fiber provided by the present invention has a low equilibrium moisture content and an extremely small amount of leached sodium without impairing the specific high molecular orientation and high modulus. A prepreg impregnated with an epoxy resin using a fiber as a base material is extremely suitable for producing a copper-clad laminate. Since the amount of exuded sodium is extremely small, when forming into a copper-clad laminate, the amount of exuded sodium ions from the substrate itself after the etching of the copper foil is extremely small. Silver migration accompanying ionization of water and ionization of sodium hardly occurs, and therefore, a copper-clad laminate with very little decrease in insulation resistance between silver electrodes can be obtained.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H05K 1/03 H05K 1/03 610U F term (Reference) 4F072 AA07 AB06 AB30 AC03 AC05 AD23 AG03 4J002 CL061 DE056 GK01 4L035 BB04 BB05 BB09 BB15 CC03 EE05 EE08 EE20 FF01 MG02

Claims (7)

[Claims] 1. A wholly aromatic polyamide fiber comprising a para-type homopolymer containing sodium as a salt at a fiber weight ratio of 2,000 ppm or more in a fiber, the sodium being extracted by hot water extraction at 121 ° C. for 2 hours. 500
A wholly aromatic polyamide fiber comprising a para-type homopolymer, which has a content of not more than ppm. 2. A wholly aromatic polyamide fiber comprising a para-type homopolymer containing sodium as a salt in a fiber weight ratio of 2,000 ppm or more in a fiber, which is heat-annealed in a temperature range of 290 ° C. to 550 ° C. The fiber of claim 1, wherein 3. A prepreg obtained by impregnating a para-homopolymer wholly aromatic polyamide fiber base material with an epoxy resin composition, wherein the para-homopolymer wholly aromatic polyamide fiber is the fiber according to claim 1 or 2. . 4. A copper-clad laminate obtained by laminating two or more prepregs according to claim 3 on a surface thereof, laminating a copper foil, and integrally forming the same by pressure and heating. 5. A wholly aromatic polyamide fiber comprising a para-type homopolymer, which is heat-annealed in a temperature range of 290 ° C. to 550 ° C. and has an equilibrium moisture content of the fiber of 1.5% by weight or less. 6. A para-aromatic polyamide fiber comprising para-homopolymer, which is heated and annealed at a temperature in the range of 290 ° C. to 550 ° C. to reduce the equilibrium moisture content of the fiber to 1.5% by weight or less. A method for producing a wholly aromatic polyamide fiber comprising a homopolymer. 7. The fiber according to claim 5, wherein the heating method is a contact method using a high-temperature metal roll, a non-contact method using a heating furnace or infrared irradiation.