US3900678A

US3900678A - Composite filaments and process for the production thereof

Info

Publication number: US3900678A
Application number: US064020A
Authority: US
Inventors: Itsuho Aishima; Noboru Fukuma; Hisaya Sakura; Hiroshi Chayamiti; Toshio Okamoto; Yuzuru Doi; Hiroshi Henmi
Original assignee: Asahi Chemical Industry Co Ltd
Current assignee: Asahi Kasei Corp; Asahi Chemical Industry Co Ltd
Priority date: 1965-10-23
Filing date: 1970-07-20
Publication date: 1975-08-19
Anticipated expiration: 1992-08-19

Abstract

Composite filaments comprised of two components disposed in side-by-side or eccentric sheath-core relationship. A first component is a crystalline polypropylene homopolymer. The second component is a random or block copolymer of propylene and another olefin. The composite filaments have a crimp-developing force of more than 15%.

Description

United States Patent Aishima et a1.

[ Aug. 19, 1975 COMPOSITE FILAMENTS AND PROCESS FOR THE PRODUCTION THEREOF Inventors: .Itsuho Aishima; Noboru Fukuma;

Hisaya Sakura; Hiroshi Chayamiti; Toshio Okamoto; Yuzuru Doi; Hiroshi Henmi, all of Nobeoka, Japan Assignee: Asahi Kasei Kogyo Kabushiki Kaisha, Osaka. Japan Filed: July 20, 1970 Appl. No.: 64,020

Related U.S. Application Data Continuation of Ser No. 588,332. Oct. 21. 1966, abandoned.

Foreign Application Priority Data Oct. 23. 1965 Japan 40-64794 Febv 25. 1966 Japan 41-11104 U.S. Cl. 428/374; 264/171; 264/D1G. 26

Int. Cl D02g 3/02 Primary Examiner-Daniel J. Fritsch Attorney, Agent, or FirmWaters, Schwartz & Nissen 5 7 ABSTRACT Composite filaments comprised of two components disposed in side-by-side or eccentric sheath-core relationship. A first component is a crystalline polypropylene homopolymer. The second component is a random or block copolymer of propylene and another olefin. The composite filaments have a crimpdeveloping force of more than 15%.

3 Claims, N0 Drawings COMPOSITE FILAMENTS AND PROCESS FOR THE PRODUCTION THEREOF This application is a continuation of application Ser. No. 588,332, filed Oct. 21, 1966, now abandoned.

This invention relates to composite filaments and a process for the production thereof. More particularly, the invention is concerned with composite filaments comprising crystalline polypropylene and random or block copolymers of crystalline propylene.

It is well known that crimpable filaments can be formed by the conjugate-spinning of two or more polymers in combination. However, reports of studies on composite filaments of polypropylene are relatively few in number. That is, as the known literature disclosing methods for the preparation of composite filaments of polypropylene, there are for example, only Japanese Pat. Publication No. 9,533/65, British Pat. No. 979,083 and Belgian Pat. No. 657,850. Substantially all of the composite filaments obtained according to the above methods are crimpable filaments of the type which immediately develop crimps by relaxing after drawing (this type will be referred to as sensitive type hereinafter). However, the composite filaments of the inventions mentioned above do not develop substantially effective crimps by mere relaxing after drawing. On the other hand, there are crimpable filaments of the type which develop no crimps unless they aresubjected to heat treatment under nontensioned state after drawing this type will be referred to as latent type hereinafter). Few crimpable filaments of the latent type are known.

Kasen Geppo (Monthly Report of Chemical Fibers), Vol. 202, May 1965 discloses at page 3 that generally, latent type composite filaments have a low stress in developing crimps and, under a tensioned state, they develop crimps with difficulty even when subjected to a crimp-developing treatment, such as heat or the like. According to the tests of the present inventors, it has been found that every composite polypropylene filament obtained in accordance with the above-cited methods is substantially deprived of crimps by heat treatment under a slight stress and is a composite filament which is inferior in this respect.

Further, the sensitive type composite filaments according to the above methods have such a tendency that the crimps are straightened by application of a relatively small force, and this property is a disadvantage in certain of their uses.

On the other hand, the journal Zairyo (Materials), Vol. 14, No. 136 describes at page 29 thatfilaments obtained by subjecting polypropylene and polyethylene to conjugate spinning suffer from such a drawback that the two components readily separate from each other and hence are not desirable as composite filaments.

An object of the, present invention is to provide crimpable polypropylene composite filaments having superior crimpability and a favorable hand in which the two components employed are excellent in mutual adhesion, and a process for producing the same.

Another object of the invention is to provide latent type polypropylene composite filaments and a process for preparing the same. I

A further object of the invention is to provide polypropylene composite filaments of high crimp developing force and a process for the production thereof.

In accordance with the present invention, the first composite filament is formed by subjecting to conjugate spinning a random copolymer of propylene and 0.5 to 20 mole of another olefin, and crystalline poly-. propylene, while the second composite filament is formed by subjecting to conjugate spinning a block copolymer of propylene and 1.0 to 40 mole of another olefin and crystalline polypropylene.

The spinning material for the latent type composite filament of the first composite filament comprises as one component a crystalline copolymer of propylene and 0.5 to 20 mole of another olefin and as the other component crystalline polypropylene. The above composite filament is prepared by subjecting the two components to melt-spinning to form a side-by-side type or eccentric sheath-core type filament and. then drawing the filament to such an extent that substantially no crimps are developed when the filament has been relaxed after drawing. The thus obtained drawn filament is subjected, either as such or after knitting or weaving, to a shrinking heat treatment in a non-tensioned state.- to develop crimps. V I

The spinning material for the latent type crimpable composite filament of the second composite filament comprises as one component a crystalline block copolymer of propylene and 1.0 to 40 mole of other olefin and as the other component crystalline polypropylene having an MFI similar to, and differing by not more than 5.0 from that of said block copolymer. The above composite filament is prepared by forming the two components into a side-by-side type or eccentric sheath-core type composite filament and drawing the filament, followed by relaxing. The thus obtained drawn filament is subjected, either as such or after knitting or weaving, to heat treatment in a non-tensioned state to develop crimps.

A characteristic of the present invention is that crystalline polypropylene is used as one component and such a crystalline propylene random polymer or block copolymer as described is used as the other component.

In the present invention, the polypropylene and copolymers employed as the starting materials are those having intrinsic viscosities of from O.6 to 3.0, preferably from 1.0 to 2.5.

The intrinsic viscosity [1 referred to herein is defined by the following equation:

7 (.l/lo) 1 wherein t and t are, respectively, times (seconds) required for tetralin and a tetralin solution of the polymer or copolymer to drop at C. using an improved Ostwalds viscometer; and C is the concentration (g/lOO cc.) of the solution.

- The term MFI employed in the present invention is defined as follows:

A polypropylene polymer or a propylene copolymer.

ular weight modifier such as hydrogen or by suitably selecting the polymerization conditions. Alternatively, the polymer having the desired intrinsic viscosity can be obtained by thermally depolymerizing at the finishing step a high intrinsic viscosity polymer once obtained at the polymerization step, or by thermally depolymerizing said polymer in the presence of a small amount of an organo-tin compound. The' extent of depolymerization of the one component crystalline polypropylene during the period of from immediately after polymerization to a time when it is formed into a spinning material, more accurately into a composite filament, is one of the factors dominating the properties of the resulting composite filament.

The copolymer, which is the other component of the starting material employed in the present invention, is obtained by copolymerizing, in the presence of a catalyst, propylene with another olefin such as ethylene, but ene-l, pentenel, hexene-l heptene-l, octene-l, 4-methylpentene-l, 3-methylbutene-l or styrene. As thecopolymerization catalyst there is used a known stereospecific polymerization catalyst capable of polymerizing propylene to crystalline polypropylene. Typical of such catalyst is, for example, one comprising an organic compound of a metal of Groups I to III of the Periodic Table and a halide of a transition element of Groups IV to VIII of the same Table.

The preparation of the random copolymer is effected according to a process for the polymerization of propylene. In this case, the adoption of a continuous polymerization process is desirable in order to control the copolymerization ratio of the resulting random copolymer. The proportion of the other olefin to be random copolymerized with propylene is from 0.5 to 20 mole based on the copolymer. In case the proportion is less than 0.5 mole the desired effect of the present invention is difficulty attainable. Further, in case the proportion is more than 20 mole the resulting copolymer is excessively lowered in melting point and the adhesion of the two components is deteriorated, with the result that a composite filament obtained by use of such copolymer is greatly lowered in utility. The content of the other olefin in the random copolymer is preferably from 3 to mole The extent of depolymerization of the random copolymer during the period of from immediately after copolymerization to a time when it is formed into a spinning material, more accurately into a composite filament, is also one of the factors dominating the properties of the resulting composite filament, like the content of the other olefin in the copolymer.

The preparation of the block copolymer is carried out according to a known process, such as for example, the process disclosed by E. G. Kontos in Journal of Polymer Science, Vol. 6 l page 62 1962) or the process revealed by G. Pier in Macromolecular Chemie, Vol. 44, page 347 (1961 That is, one block-copolymerization process is effected by periodically and alternately contacting with a polymerization catalyst a mixture of propylene and one other olefin or a mixture of two or more olefins including or not including propylene. Another copolymerization process is carried out by periodically and alternately contacting with a polymerization catalyst an olefin mixture composed mainly of propylene and one other olefin, or a mixture of two or more olefins including or not including propylene. The resulting copolymer is a non-statistic copolymer having in alternating order in the molecular chain a monomer composed mainly of propylene and a different monomer composed mainly of other olefin.

The proportion of other olefin to be blockcopolymerized with propylene is preferably in the range of from 1.0 to mole Where the proportion is less than 1.0 mole the resulting composite filament shows no sufficient development of crimps as a composite filament. Further, where the proportion is more than 40 mole the resulting composite filament is deteriorated in adhesion to one component crystalline polypropylene and is greatly lowered in utility as a composite filament.

In order to periodically and alternately contact said olefins with a stereospecific polymerization catalyst, subsequent olefin monomers should be introduced either after applying the operation of deterging the cata lyst surface with an inert gas such as nitrogen or helium to exclude preceding olefin monomers or after reducing the pressure of the reaction system to remove preceding olefins from the catalyst surface. It is also possible to attain the desired effect by the application of said two operations in combination. More simply, the block-copolymerization can be effected by merely feeding olefins periodically and alternately to the catalyst surface without conducting the above removing operations. Olefin monomers may be alternated not only one time but also any desired times. The polymerization process may be effected in any of continuous or batch-wise manner, but a continuous polymerization process is desirable in order to control the copolymer ization ratio of the resulting copolymer.

In order to obtain the random or block copolymer having a desired intrinsic viscosity, a desired amount of a molecular weight modifier such as hydrogen is fed to the polymerization system, whereby the molecular weight of the copolymer can be controlled.fAlternatively, the molecular weight can be controlled by elevating the reaction temperature to frequently cause a chain transfer reaction. Another procedure to control the molecular weight is that the molecular weight control is not effected in the polymerization step but the resulting high intrinsic viscosity copolymer is thermally depolymerized, before entering the spinning step, in the presence of a slight amount of an organo-tin compound,

Either one or both of the polymer and the random or block copolymer, which are the two components constituting the composite filament of the present invention may contain, if necessary, up to 10% by weight of additives. Such additives are various heat stabilizers, light stabilizers, gas-fading stabilizers, titanium dioxide and other pigments, brightening agents and dyeing additives. The dyeing additives include compounds of metals such as Ni, Al and Zn; nitrogen-containing compounds such as polyaminotriazole, polyamides, polyalkyleneimide and low molecular weight amines; polyesters; polycarbonates and other various compounds known to the art. Ordinarily, these additives are added in an amount of less than 10% by weight of the polymer or copolymer, whereby the object of addition is achieved and the composite filament of the present invention is obtained.

For the production of the composite filaments of the present invention, any spinning and drawing apparatuses known to the art are usable ln spinning a composite filament of polypropylene according to the conventional method, there frequently occurs, due to the difference in viscoelastic behavior of melts of the two components, such as phenomenon that molten filaments extruded from a spinneret bend immediately below the orifices. Accordingly, there are many cases where uniform filaments are difficulty spun in a stable state. In accordance with the present invention, however, the above drawbacks encountered in the spinning according to the conventional method are entirely overcome by suitable selection of the depolymer ization degree, intrinsic viscosity and percentage of nheptane extraction residue of crystalline polypropylene; the olefin component, mole of olefin content, depolymerization degree, intrinsic viscosity and percentage of n-heptane extraction residue of the copolymer to be combined with the crystalline polypropylene; and spinning conditions, whereby an excellent latent type composite filament is obtained. This is one of the great characteristics of the present invention.

According to the present invention, the spinning is effected at a temperature of from 200 to 350C., preferably from 240 to 300C. As the spinneret, one forming the two components into the side-by-side/ type or one forming them into the eccentric sheath-core type may be used. However, the use of side-by-side type spinneret is preferred in view of thecrimping charac teristics of the resulting filaments. The weight ratio of the two components in the filament is variable within the range of from :90 to 90: 10, but favorable results can be attained when the weight ratio is between 30:70

and 70:30. 1

1n the present invention, the drawing ,is effected in the same manner as in the conventional method. That is, any of cold drawing and hot-drawing may be adopted. The drawing ratio varies depending on the take up velocity but'is ordinarily from 2 to 9 times.

The preparation of block copolymer having different MFl value may be effected, as mentioned before, by selection and combination ,of the depolymerization degree, intrinsic viscosity and percentage. of n-heptane extraction residue of crystalline polypropylene; the olefin component, mole of olefin content, depolymerization degree, intrinsic viscosity and percentage of nheptane extraction residue; and spinning conditions such as spinning temperature, cooling conditions, take up velocity and denier of single filament.

The heat treatment to fix the developed crimps and the heat treatment to develop crimps from latent type composite filaments are effected at temperatures in the range of from 60C. to a temperature 10C. lower than the melting point of the copolymer component employed. The heat treatment is preferably performed by use of steam at 90 120C. or hot water at 90 95C. For the heat treatment, a treating time of from 5 to 60 minutes is sufficient.

It will be clear from the above illustration that in accordance with the present invention, not only composite filament yarns but also composite staple fibers can be produced. It is a great characteristic in quality of the composite filaments obtained according to the present invention that the composite filaments of the latent type, are excellent in crimp-developing force.

The crimpdeveloping force referred to is defined, for convenience, as follows:

A piece of the drawn filament obtained is treated with boiling water under a load of 4.0. mg/denier S ubsequently, the filament is taken out of the boiling water is subjected to a load of 50 mg/denier in place of the load of 4.0 mg/denier, and the length L of the filament is measured. Thereafter, said load is changed to a load of l mg/denier and the length S of the filament in this state is measured. The crimp-developing force is computed according to the following equation:

Crimp-developing force L S/S X 100.

Composite polypropylene filaments according to the conventional methods have a crimp-developing force of less than about 5% and scarcely develop crimps. In contrast thereto, the composite filaments in accordance with the present invention show a crimp developing force of more than 15% in the case of 15 denier monofilament yarn and develop excellent crimps.

The latent type composite filaments of the present invention are as easy as in the case of the ordinary filament yarns in handling at the secondary processing stage such as knitting or weaving, and are far more excellent than the sensitive type crimpable filaments. It is therefore a great effect of the presentinvention that such latent type composite filaments, which are marked in crimp-developing force and are high in industrial value, can be obtained with ease.

The crimpsof composite polypropylene filaments according to the conventional method have suffered from such a drawback that the crimps of the filaments are easily straightened by application of a relatively small force, whereas the composite filaments of the present invention have such a great advantage that they can withstand against a relatively large force. I

It is an important feature of the composite filaments of the present invention that the two components employed are markedly excellent in adhesion and that the crimped yarns obtained therefrom are favorable in hand.

The present invention will be illustrated further in detail with reference to the following examples:

EXAMPLE 1 Propylene was continuously polymerized, with addition of a small amount of hydrogenjin n-hexane in the presence of a catalyst comprising TiCl and Al( Et) Cl, and the resulting polymer was purified. The thus obtained crystalline polypropylene was in the form of a powder and had an intrinsic viscosity of 2.50 and a nheptane extraction residue of 96.1% This crystalline polypropylene was incorporated with 0.1% of lonol as a stabilizer and was subjected to a pelletizer to form pellets having an intrinsic viscosity of 1 .80. This pellet was used as one component of spinning material.

On the other hand, propylene containing 7 mole of ethylene was continuously polymerized in n-hexane in the presence of the aforesaid catalyst, and the resulting polymer was purified. The thus obtained crystalline ethylene-propylene landom copolymer was in the form of a powder and had an intrinsic viscosity of 12, a nheptane extraction residue of and a melting point of C. crystalline ethylene-propylene copolymer was incorporated with 0.1% of dibutyltin maleate and was subjected to a pelletizer to obtain pellets. The thus obtained pellet was further incorporated with 0.1% of lonol and was again subjected to a pelletizer to A form a pellet having an intrinsic viscosity of 1.50. This pellet was used as the other component of spinning material.

By means of a conjugate-spinning apparatus provided with a side-by-side type conjugate-spinning spinneret having one orifice of 0.5 mm in diameter, the above two starting pellets were extruded in equal volume at a spinneret temperature of 280C, and the resulting filament was taken up at a velocity of 200 m/min. to obtain an undrawn composite filament of 105 denier/ 1 filament. Using an ordinary drawtwister, the undrawn composite filament was hot-drawn under the conditions of a hot plate temperature of 90C., a drawing ratio of 5.5 times and a drawing velocity of 1 14 m/min. The drawn filament developed no crimps at all even when it had been released from the bobbin and relaxed. The drawn filament was treated in boiling water for 30 minutes under no tension to obtain a beautiful crimped yarn having 370 crimps per 25 mm and a crimp elongation of 180%. The crimp-developing force of the drawn filament was 40%.

The crimp elongation was measured in the following manners:

The crimped filament obtained was subjected to an initial load of 1 mg/denier and the length S of the yarn was measured. Subsequently, the filament was subjected to a load of 50 mg/denier and the length L of the filament was measured. The crimp elongation was calculated as follows:

Crimp elongation L S/S X 100 EXAMPLE 2 Propylene containing 5 mole of butene-l was copolymerized according to the same procedure as adopted for the preparation of the copolymer of Example 1. The resulting powdery crystalline butenepropylene copolymerhad an intrinsic viscosity of 13 and a n-heptane extraction residue of 83%. This butenepropylene copolymer was treated in the same manners as in Example 1 to form pellets having an intrinsic viscosity of 1.60.

The above pellet and the polypropylene pellet having an intrinsic viscosity of 1.80, which had been used in Example 1, was incorporated, respectively, with 1.5% of nickel stearate to prepare two spinning materials.

The two spinning materials were spun and drawn in the same manners as in Example 1, except that the spinneret temperature adopted was 270C, the drawing ratio 4.5 times and the hot plate temperature 60C., to obtain a drawn filament of 15 denier/1 filament. This drawn filament did not develop any substantial crimps, even when relaxed. The drawn filament was treated under no tension in boiling water for 30 minutes to obtain a beautiful crimped filament having a crimp elongation of 120%. The above drawn filament had a crimp-developing force of 32%.

EXAMPLE 3 The crystalline polypropylene pellet obtained in Example 1 was used as one spinning material. The pellet had an MFI of 13.5.

On the other hand, propylene incorporated with a small amount of hydrogen was polymerized for 10 minutes in n-hexane in the presence of the aforesaid catalyst. After purging the system with nitrogen for 2 minutes, ethylene was fed for 2 minutes. Subsequently, after purging the system with nitrogen for 1 minute, propylene incorporated with a small amount of hydrogen was again polymerized for 10 minutes. The above operations were repeated to effect continuous polymerization. The resulting polymer was purified with hydrochloric acid-containing methanol. The thus obtained powdery crystalline ethylene-propylene block copolymer had an intrinsic viscosity of 2.00 and a nheptane extraction residue of According to infrared analysis, the copolymer had an ethylene content of 10.2 mole This copolymer was incorporated with 0.1% of lonol and was subjected to a pelletizer to obtain pellets having an intrinsic viscosity of 1.60 and an MP1 of 16. 1. This pellet was used as the other spinning material.

Using a conjugate-spinning apparatus provided with a side-by-side conjugate-spinning spinneret having one orifice of 0.5 mm in diameter, the above two spinning materials were extruded in equal volume at a spinneret temperature of 280C, and the resulting filament was taken up at a velocity of 200 m/min. to obtain an undrawn composite filament of denier/1 filament. By means of an ordinary drawtwister, the composite filament was hotdrawn under the conditions of a hot plate temperature of 90C., a drawing ratio of 5.5 times and a drawing velocity of 200 m/min. The drawn filament developed no crimps at all even when released from the bobbin and relaxed. The drawn filament was treated under no tension for 30 minutes in boiling water to obtain a crimped filament having 22 crimps per 25 mm and a crimp elongation of 13.8%. The drawn filament had a crimp-developing force of 38%.

EXAMPLE 4 The polypropylene prepared in Example 1 was used as one component of composite filament.

On the other hand, propylene was polymerized for 15 minutes in n-hexane in the presence of the same catalyst as in Example 1. Thereafter, the feeding of propylene was discontinued and a mixture of 60 mole of propylene and 40 mole of ethylene was immediately fed for 2 minutes. Subsequently, propylene was again fed for 15 minutes. The above operations were repeated to effect continuous polymerization, and the resulting polymer was purified. The thus obtained powdery crystalline ethylenepropylene block copolymer had an intrinsic viscosity of 14.0 and a n-heptane extraction residue of 83%. According to infrared analysis, the copolymer had an ethylene content of 8.0 mole This crystalline ethylenepropylene block copolymer was incorporated with 0.1% of dibutyltin maleate and was subjected to a pelletizer to form pellets. The pellet was further incorporated with 0.1% of lonol as a heat stabilizer and was again subjected to a pelletizer to obtain pellets having an intrinsic viscosity of 1.75 and an MP1 of 5.3. This pellet was used as the other component of composite filament.

Using a conjugate-spinning apparatus provided with a side-by-side type conjugate-spinning spinneret having one orifice of 0.5 mm in diameter, the above two components were extruded in equal amount at a spinneret temperature of 295C, and the resulting filaments were taken up at a velocity of 300 m/min. The thus obtained undrawn composite filament of 300 denier/ 1 filament was subjected to a drawtwister and was drawn on a hot plate at C. under the conditions of a drawing ratio of 5.0 times and a drawing velocity of m/min. The drawn filament was released from the bobbin but developed no crimps. This drawn filament was treated under non-tensioned state for 10 minutes in boiling water to obtain a crimped filament having 1 1 crimps per 25 mm.

fin of the copolymer being selected from the group consisting of ethylene, butene-l, hexene-l, octene-l, 4-methyl'pentenel B-rnethylbutene-l and styrene, said composite filament having a crimp-developing force of more than 15 2. The composite filament of claim 1 in which the second component is a random copolymer containing about 3 to 10 mole ethylene.

3. The composite filament of claim 1 wherein the crimp-developing force of the filament is 30 to 40 F ORM PO-1050 (10-69) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 900 78 Dated August 19 1975 Inventor(s) Itsuho AiSh-lma et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

The third inventor should read Hisaya Sakurai Signed and Scaled this Arrest:

RUTH C. MASON C- MARSHALL DAN" Arresting Officer I (n m m issionrr of lime)"; and Tmdenurks USCOMM-DC OO37G-P us. GOVERNMENT PRINTING OFFICE 869 Q30

Claims

1. A COMPOSITE FILAMENT COMPRISED OF TWO COMPONENTS DISPOSED IN SIDE-BY-SIDE RELATIONSHIP OR ECCENTRIC SHEATH-CORE RELATIONSHIP, A FIRST COMPONENT BEING CRYSTALLINE POLYPROPYLENE HOMOPOLYMER AND A SECOND COMPONENT BEING SELECTED FROM THE GROUP CONSISTING OF A RANDOM COPOLYMER OF PROPYLENE AND 0.5 TO 20 MOLE % OF ANOTHER OLEFIN AND A BLOCK COPOLYMER OF PROPYLENE AND 1.0 TO 40 MOLE % OF ANOTHER OLEFIN, SAID OTHER OLEFIN OF THE COPOLYMER BEING SELECTED FROM THE GROUP CONSISTING OF ETHYLENE, BUTENE-1, HEXENE-1, OCTENE1, 4-METHYLPENTENE-1, 3-METHYLBUTENE-1 AND STYRENE, SAID COMPOSITE FILAMENT HAVING A CRIMP-DEVELOPING FORCE OF MORE THAN 15%.

2. The composite filament of claim 1 in which the second component is a random copolymer containing about 3 to 10 mole % ethylene.

3. The composite filament of claim 1 wherein the crimp-developing force of the filament is 30 to 40 %.