JP6356463B2 - Totally aromatic polyamide fiber - Google Patents

Description

  The present invention relates to wholly aromatic polyamide fibers. More specifically, the present invention relates to a wholly aromatic polyamide fiber excellent in mechanical properties such as breaking strength and elongation.

Conventionally, fibers made of wholly aromatic polyamides produced from aromatic diamines and aromatic dicarboxylic acid dihalides are very useful as high-strength fibers and have excellent heat resistance and flame retardancy. Are known.
In addition, fully aromatic polyamide fibers have higher dimensional stability, higher elastic modulus, and better fatigue resistance than general-purpose fibers. For example, rubber such as tire cords, power transmission belts, and conveyor belts. It is suitably used for reinforcing materials, seat belts, fishing nets, safety nets and the like.
And in the above-mentioned various products, there is a strong demand for further workability and fatigue resistance, and in order to satisfy it, it was necessary to improve the elongation while ensuring the mechanical strength of the wholly aromatic polyamide fiber. .

However, since the wholly aromatic polyamide fiber forms a fiber structure having a high degree of crystallinity and a highly oriented polymer chain, the polymer structure is extremely stiff and it is difficult to improve the elongation. Therefore, development of high-strength organic fibers with high elongation has been made.
For example, Patent Documents 1 to 4 propose various polyhexamethylene adipamide fibers having high dimensional stability, high elastic modulus, excellent heat resistance and fatigue resistance, and methods for obtaining the fibers. Yes. However, although the fibers described in Patent Documents 1 to 4 have sufficient elongation at break, it is difficult to say that they have sufficient strength compared to wholly aromatic polyamide fibers.
In recent years, a method for obtaining a wholly aromatic polyamide having a high elongation at break has been proposed, but it is difficult to say that the fiber described in Patent Document 5 has sufficient breaking strength.

JP 58-208413 A JP 59-026517 A JP-A-1-168913 JP-A-3-199421 JP 2012-207325 A

  The present invention has been made to solve the problems of the prior art as described above, and its object is to provide a wholly aromatic polyamide fiber having high elongation and at the same time high strength. It is in.

  The present inventor has intensively studied to solve the above problems. As a result, the structural repeating unit (I) represented by the chemical formula (I), the structural repeating unit (II) represented by the chemical formula (II), the structural repeating unit (III) represented by the chemical formula (III), Has been found to be excellent in mechanical strength while having high elongation, and the present invention has been completed.

  That is, the present invention relates to a structural repeating unit (I) represented by the chemical formula (I), a structural repeating unit (II) represented by the chemical formula (II), and a structural repeating unit (III) represented by the chemical formula (III). A wholly aromatic polyamide fiber comprising a wholly aromatic polyamide, wherein the structural repeating unit (II) is a sum of the structural repeating unit (I), the structural repeating unit (II), and the structural repeating unit (III). 10 to 30 mol%, and the structural repeating unit (III) is 20 to 40 mol% based on the total of the structural repeating unit (I), the structural repeating unit (II), and the structural repeating unit (III). It is a wholly aromatic polyamide fiber.

  The wholly aromatic polyamide fiber of the present invention is a fiber having a high strength and dramatically improved elongation at break. For this reason, according to the fiber of this invention, the various fiber products in which high elongation is requested | required can be provided.

Hereinafter, embodiments of the present invention will be described in detail.
<Aromatic polyamide>
[Structure of wholly aromatic polyamide]
The wholly aromatic polyamide constituting the fiber of the present invention comprises a structural repeating unit (I) represented by the chemical formula (I), a structural repeating unit (II) represented by the chemical formula (II), and a chemical formula (III). It is a wholly aromatic polyamide containing the structural repeating unit (III) represented in a specific ratio.

[Structural repeat unit ratio]
In the wholly aromatic polyamide constituting the fiber of the present invention, the ratio of the structural repeating unit (II) is based on the sum of the structural repeating unit (I), the structural repeating unit (II), and the structural repeating unit (III). It is 20-40 mol%, Preferably it is 25-35 mol%. If it is less than 20 mol%, it will be difficult to develop desired strength and elongation. On the other hand, when it exceeds 40 mol%, the strength of the fiber is lowered, which is not preferable.

At the same time, the ratio of the structural repeating unit (III) is 10 to 30 mol%, preferably 15 to 25%, based on the total of the structural repeating unit (I), the structural repeating unit (II), and the structural repeating unit (III). Mol%. If the amount is less than 10 mol%, the predetermined elongation is not exhibited. On the other hand, if it exceeds 30 mol%, the strength of the fiber decreases, which is not preferable.
In the present invention, by setting the ratio of the structural repeating unit (II) and the structural repeating unit (III) within the above composition range, mechanical properties having a tensile strength of 24 cN / dtex or more and simultaneously an elongation of 5.0% or more can be obtained. Can be expressed.

<Method for producing wholly aromatic polyamide>
The wholly aromatic polyamide used in the present invention can be produced according to a conventionally known method. For example, it can be obtained by subjecting a dicarboxylic acid dichloride component and an aromatic diamine component to low-temperature solution polymerization or interfacial polymerization in an amide polar solvent.

[Raw materials for wholly aromatic polyamides]
(Aromatic diamine component)
Examples of the aromatic diamine component used in the present invention include p-phenylenediamine, 2-chloro p-phenylenediamine, 2,5-dichloro p-phenylenediamine, 2,6-dichloro p-phenylenediamine, and m-phenylenediamine. 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, 3,4'-diaminodiphenyl ether However, it is not limited to these.

  In the present invention, at least three types of aromatic diamine components constituting the structural repeating unit (I), the structural repeating unit (II), and the structural repeating unit (III) may be used according to the ratio. is important. For example, in the present invention, it is preferable to use p-phenylenediamine, 3,4'-diaminodiphenyl ether, and 4,4'-diaminodiphenyl ether as the diamine component.

(Aromatic dicarboxylic acid dichloride component)
Examples of the aromatic dicarboxylic acid dichloride component used in the present invention include isophthalic acid chloride, terephthalic acid chloride, 2-chloroterephthalic acid chloride, 2,5-dichloroterephthalic acid chloride, and 2,6-dichloroterephthalic acid chloride. 2,6-naphthalenedicarboxylic acid chloride and the like, but are not limited thereto. Of these, terephthalic acid dichloride is preferably used.

[Polymerization of wholly aromatic polyamide]
(Polymerization solvent)
Examples of the solvent for polymerizing a wholly aromatic polyamide include organic polar amide solvents such as N, N-dimethylformamide, N, N-dimethylacetamide, N-methyl-2-pyrrolidone and N-methylcaprolactam, tetrahydrofuran And water-soluble ether compounds such as dioxane, water-soluble alcohol compounds such as methanol, ethanol and ethylene glycol, water-soluble ketone compounds such as acetone and methyl ethyl ketone, and water-soluble nitrile compounds such as acetonitrile and propionitrile. These solvents can be used alone or as a mixed solvent of two or more. The solvent is preferably dehydrated.

[Inorganic salt]
In order to increase the solubility, an appropriate amount of a known inorganic salt may be added before, during or after the polymerization. Examples of such inorganic salts include lithium chloride and calcium chloride.

[Raw material composition ratio]
The ratio of the aromatic diamine component to the aromatic dicarboxylic acid dichloride component is preferably 0.90 to 1.10, more preferably 0.95 to 1 as the molar ratio of the aromatic dicarboxylic acid dichloride component to the aromatic diamine component. .05. When the molar ratio of the aromatic dicarboxylic acid chloride component is less than 0.90 or exceeds 1.10, the reaction with the aromatic diamine component does not proceed sufficiently and a high degree of polymerization cannot be obtained, which is not preferable.

[Reaction conditions]
The reaction conditions for the aromatic dicarboxylic acid chloride component and the aromatic diamine component are not particularly limited. The reaction between acid chloride and diamine is generally rapid, and the reaction temperature is preferably, for example, in the range of -25 ° C to 100 ° C, more preferably in the range of -10 ° C to 80 ° C.

[Other polymerization conditions]
The terminal of the wholly aromatic polyamide may be sealed. In the case of sealing with an end-capping agent, for example, phthalic acid chloride and its substituted product, aniline and its substituted product can be used.
In addition, an aliphatic or aromatic amine or a quaternary ammonium salt can be used in combination to capture an acid such as hydrogen chloride to be generated.
After completion of the reaction, a basic inorganic compound such as sodium hydroxide, potassium hydroxide, calcium hydroxide, calcium oxide or the like may be added as necessary to carry out a neutralization reaction.

[Post-polymerization, etc.]
The wholly aromatic polyamide obtained as described above can be put into a poor solvent such as alcohol and water, precipitated, and taken out in the form of pulp. The wholly aromatic polyamide taken out can be dissolved again in another solvent and used for molding. However, the solution obtained by the polymerization reaction can be used as it is as a molding solution. The solvent used for once taking out and then dissolving again is not particularly limited as long as it dissolves the wholly aromatic polyamide, but the solvent used for the polymerization of the wholly aromatic polyamide is preferable.

[Other additives]
In addition, in this invention, a filler can be used together in the range which does not impair a physical property. Examples of the filler used include non-fibrous fillers such as fibrous or plate-like, scale-like, granular, indeterminate, and crushed products. Specific examples include glass fibers, PAN-based and pitch-based carbon fibers. , Stainless steel fibers, metal fibers such as aluminum fibers and brass fibers, organic fibers other than wholly aromatic polyamide fibers, gypsum fibers, ceramic fibers, zirconia fibers, alumina fibers, silica fibers, titanium dioxide fibers, silicon carbide fibers, rock wool, Potassium titanate whisker, barium titanate whisker, aluminum borate whisker, silicon nitride whisker, mica, layered clay mineral, talc, kaolin, silica, calcium carbonate, glass beads, glass flakes, glass microballoon, clay, molybdenum disulfide, straw Stenite, titanium dioxide, oxidation Lead, calcium polyphosphate, graphite, metal powders, metal flakes, metal ribbons, metal oxides, other than boron nitride particles of the present invention, carbon powder, graphite, carbon flake, such flake carbon. Moreover, said filler can also be used in combination of 2 or more types.

  In addition to the polymer used in the present invention and the resulting fiber, in addition to these, various additives such as antioxidants, ultraviolet absorbers, anti-degradation agents such as light stabilizers, lubricants, antistatic agents, and release agents. In addition, colorants such as plasticizers and pigments may be used in combination. The amount of the additive used can be appropriately selected according to the type of additive as long as the original physical properties of the resulting fiber are not impaired.

<Method for producing wholly aromatic polyamide fiber>
In producing the wholly aromatic polyamide fiber of the present invention, the solvent is removed after wet spinning or dry spinning of the wholly aromatic polyamide solution (which may be the polymer dope produced during the production of the wholly aromatic polyamide).

[Preparation of spinning solution (polymer dope)]
In the production of fibers, an isotropic spinning solution (dope) in which wholly aromatic polyamide is dissolved in an organic solvent is obtained. The method for adjusting the spinning solution (polymer dope) is not particularly limited. The organic solvent dope after solution polymerization may be used as it is, or the obtained wholly aromatic polyamide may be dissolved again in an organic solvent. Here, as the solvent used when the wholly aromatic polyamide is dissolved again, the solvent used for the polymerization of the wholly aromatic polyamide described above can be used.

[Spinning / Coagulation]
Using the isotropic dope (spinning solution) adjusted as described above, fibers are formed by a wet spinning method or a semi-dry semi-wet spinning method provided with an air gap. That is, first, the spinning solution (dope) obtained as described above is discharged from a nozzle and then brought into contact with a coagulating liquid in a coagulating bath through or without an air gap to form a coagulated yarn.
As the coagulation bath, a poor solvent of wholly aromatic polyamide is used, but usually a good solvent is added to solidify so that the solvent of the wholly aromatic polyamide dope quickly escapes and the defect of the wholly aromatic polyamide fiber does not occur. Adjust the speed. In general, it is preferable to use water as the poor solvent and a solvent for the wholly aromatic polyamide dope as the good solvent. The weight ratio of good solvent / poor solvent is generally preferably in the range of 15/85 to 40/60, although it depends on the solubility and coagulability of the wholly aromatic polyamide.

[Other processes]
After the solidified yarn is pulled up from the coagulating liquid, a final type wholly aromatic polyamide fiber can be obtained by a known method. For example, the solvent is removed from the undrawn yarn formed by carrying out the water washing step, the drawing is carried out as necessary, and the final fiber is oriented by drawing as necessary after passing through the drying step and the like. Can be obtained.

[Stretching process]
The fibers of the present invention are preferably stretch-oriented. The drawing method is not particularly limited, and may be any of not only washing drawing in a coagulated yarn state and boiling water drawing but also heating drawing in a dry yarn state. Moreover, there is no restriction | limiting in particular about a draw ratio, However, It is preferable that it is 5 times or more, and it is further more preferable that it is 8 times or more. By controlling the draw ratio, the elongation and strength of the resulting aromatic polyamide fiber can be controlled.

The fibers of the present invention are preferably highly oriented and crystallized such that the degree of crystal orientation determined by wide-angle X-ray diffraction is 85% or more and the degree of crystallinity is 70% or more. When either one or both of the crystal orientation degree and the crystallinity degree are low, the mechanical properties of the resulting fiber are likely to be insufficient even when heat (drawing) treatment is performed.
In the case of carrying out hot stretching, the temperature is preferably 300 to 550 ° C., more preferably 350 to 500 ° C., although it depends on the polymer skeleton of the wholly aromatic polyamide, and the draw ratio is preferably 4 times. As described above, more preferably 4 to 10 times.

<Aromatic polyamide fiber>
(Single yarn fineness)
The single yarn fineness of the wholly aromatic polyamide fiber of the present invention is preferably 0.5 to 50 dtex, more preferably 1.0 to 10 dtex. If it is less than 0.5 dtex, the spinning property may become unstable. Moreover, since the specific surface area of a fiber becomes large, it is easy to receive light resistance deterioration. On the other hand, if it exceeds 50 dtex, the specific surface area of the fiber becomes small, and it is difficult to undergo light resistance deterioration, but coagulation tends to be incomplete in the spinning process. The physical properties also tend to decrease.

(Tensile strength)
The higher the tensile strength, the better. If it is less than 24 cN / dtex, the characteristics as a high-strength fiber are insufficient. Therefore, it is preferably 24 cN / dtex or more.

(Elongation at break)
The breaking elongation is preferably 5.0% or more. If it is less than 5.0%, the durability required for applications such as rubber reinforcement cannot be satisfied.

(Use)
The wholly aromatic polyamide fiber of the present invention can constitute fiber structures such as braids, ropes, twisted cords, yarns, and cotton in addition to fabrics such as woven fabrics, knitted fabrics, and nonwoven fabrics.

  Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples. However, these Examples and Comparative Examples are for helping understanding of the present invention, and do not limit the scope of the present invention.

<Measurement and evaluation method>
In Examples and Comparative Examples, the following items were measured and evaluated by the following methods.

[Fineness]
It measured according to JIS-L-1015.

[Tensile strength of fiber, elongation at break, elastic modulus]
Using a tensile tester (manufactured by INSTRON Co., Ltd., trade name: INSTRON, model: 5565 type), measurement was performed under the following conditions based on the procedure of ASTM D885.
(Measurement condition)
Measurement sample length: 500 mm
Chuck pulling speed: 250mm / min
Initial load: 0.2 cN / dtex

<Example 1>
[Polymerization of type wholly aromatic polyamide]
N-methyl-2-pyrrolidone (NMP) 2.139 L, paraphenylenediamine 28.5 g (25 mol%), 3,4'-diaminodiphenyl ether 21.1 g (10 mol%), 4,4'-diaminodiphenyl ether 31. 7 g (15 mol%) was weighed and added, and dissolved at room temperature. To the obtained diamine solution, 106.9 g (50 mol%) of terephthalic acid chloride was added and reacted to polymerize the polymer. Subsequently, 172.2 g of NMP dispersion containing 22.5% by mass of calcium hydroxide was added for neutralization to obtain a polymer solution.

[Preparation of spinning solution (polymer dope)]
A solution of the obtained copolymer ( inherent viscosity (IV) measured at 30 ° C. with respect to a solution having a polymer concentration of 0.5 g / dl in 98% concentrated sulfuric acid at 4.12) was adjusted to a concentration of 6% by mass. A solution for spinning (polymer dope) was prepared by dissolving in NMP.

[Spinning / Coagulation]
Using the obtained dope, it was discharged from a spinneret with 25 holes, spun into an aqueous solution having an NMP concentration of 30% by mass through an air gap of about 10 mm, and solidified (semi-dry semi-wet spinning method). Washing with water, drying, and subsequent stretching at 9.4 times at a temperature of 500 ° C. followed by winding to obtain a wholly aromatic polyamide fiber. Table 1 shows the physical properties and the like of the obtained fibers.

<Example 2>
[Polymerization of wholly aromatic polyamide]
N-methyl-2-pyrrolidone (NMP) 2.139 L, paraphenylenediamine 28.5 g (25 mol%), 3,4′-diaminodiphenyl ether 31.7 g (15 mol%), 4,4′-diaminodiphenyl ether 21. 1 g (10 mol%) was weighed and added, and dissolved at room temperature. To the obtained diamine solution, 110.5 g (50 mol%) of terephthalic acid chloride was added and reacted to polymerize the polymer. Subsequently, 178.0 g of NMP dispersion containing 22.5% by mass of calcium hydroxide was added for neutralization to obtain a polymer solution.

[Preparation of spinning solution (polymer dope)]
A solution of the obtained copolymer ( inherent viscosity (IV) measured at 30 ° C. for a solution with a polymer concentration of 0.5 g / dl in 98% concentrated sulfuric acid at 4.10) was adjusted to a concentration of 6% by mass with NMP. To prepare a spinning solution (polymer dope).

[Spinning / Coagulation]
Using the obtained dope, it was discharged from a spinneret with 25 holes, spun into an aqueous solution having an NMP concentration of 30% by weight through an air gap of about 10 mm (semi-dry semi-wet spinning method), washed with water, It was dried and then stretched 8.6 times at a temperature of 500 ° C. and then wound up to obtain a wholly aromatic polyamide fiber. Table 1 shows the physical properties and the like of the obtained fiber.

<Example 3>
[Polymerization of wholly aromatic polyamide]
N-methyl-2-pyrrolidone (NMP) 2.139 L, paraphenylenediamine 42.2 g (25 mol%), 3,4'-diaminodiphenyl ether 31.7 g (20 mol%), 4,4′-diaminodiphenyl ether 10. 6 g (5 mol%) was weighed and added, and dissolved at room temperature. To the obtained diamine solution, 110.5 g (50 mol%) of terephthalic acid chloride was added and reacted to polymerize the polymer. Subsequently, 178.0 g of NMP dispersion containing 22.5% by mass of calcium hydroxide was added for neutralization to obtain a polymer solution.

[Preparation of spinning solution (polymer dope)]
A solution of the obtained copolymer ( inner viscosity (IV) 3.95 measured at 30 ° C. for a solution having a polymer concentration of 0.5 g / dl in 98% concentrated sulfuric acid) was adjusted to a concentration of 6% by mass. To prepare a spinning solution (polymer dope).

[Spinning / Coagulation]
Using the obtained dope, it was discharged from a spinneret with 25 holes, spun into an aqueous solution having an NMP concentration of 30% by weight through an air gap of about 10 mm (semi-dry semi-wet spinning method), washed with water, It was dried and then stretched at a temperature of 520 ° C. by 10.3 times, and then wound up to obtain a wholly aromatic polyamide fiber. Table 1 shows the physical properties and the like of the obtained fiber.

<Comparative Example 1>
[Polymerization of wholly aromatic polyamide]
To 2.139 L of N-methyl-2-pyrrolidone (NMP), 42.2 g (25 mol%) of paraphenylenediamine and 52.8 g (25 mol%) of 3,4′-diaminodiphenyl ether were weighed and charged at room temperature. Dissolved. To the obtained diamine solution, 110.5 g (50 mol%) of terephthalic acid chloride was added and reacted to polymerize the polymer. Subsequently, 178.0 g of NMP dispersion containing 22.5% by mass of calcium hydroxide was added for neutralization to obtain a polymer solution.

[Preparation of spinning solution (polymer dope)]
A solution of the obtained copolymer ( inherent viscosity (IV) measured at 30 ° C. for a solution having a polymer concentration of 0.5 g / dl in 98% concentrated sulfuric acid at 3.38) was adjusted to a concentration of 6% by mass with NMP. To prepare a spinning solution (polymer dope).

[Spinning / Coagulation]
Using the obtained dope, it was discharged from a spinneret with 25 holes, spun into an aqueous solution having an NMP concentration of 30% by weight through an air gap of about 10 mm (semi-dry semi-wet spinning method), washed with water, It was dried and then stretched 11.0 times at a temperature of 530 ° C. and then wound up to obtain a wholly aromatic polyamide fiber. Table 1 shows the physical properties and the like of the obtained fiber.

<Comparative example 2>
[Polymerization of wholly aromatic polyamide]
To 2.139 L of N-methyl-2-pyrrolidone (NMP), 42.2 g (25 mol%) of paraphenylenediamine and 52.8 g (25 mol%) of 4,4′-diaminodiphenyl ether were weighed and added at room temperature. Dissolved. To the obtained diamine solution, 110.5 g (50 mol%) of terephthalic acid chloride was added and reacted to polymerize the polymer. Subsequently, 178.0 g of NMP dispersion containing 22.5% by mass of calcium hydroxide was added for neutralization to obtain a polymer solution.

[Preparation of spinning solution (polymer dope)]
A solution of the obtained copolymer ( inherent viscosity (IV) measured at 30 ° C. for a solution having a polymer concentration of 0.5 g / dl in 98% concentrated sulfuric acid at 4.23) was adjusted to a concentration of 6% by mass with NMP. To prepare a spinning solution (polymer dope).

[Spinning / Coagulation]
Using the obtained dope, it was discharged from a spinneret with 25 holes, spun into an aqueous solution having an NMP concentration of 30% by weight through an air gap of about 10 mm (semi-dry semi-wet spinning method), washed with water, All the aromatic polyamide fibers were obtained by drying and then stretching after being 5.0 times stretched at a temperature of 460 ° C. Table 1 shows the physical properties and the like of the obtained fiber.

<Comparative Example 3>
[Polymerization of wholly aromatic polyamide]
N-methyl-2-pyrrolidone (NMP) 2.139 L, paraphenylenediamine 28.5 g (25 mol%), 3,4'-diaminodiphenyl ether 10.6 g (5 mol%), 4,4′-diaminodiphenyl ether 31. 7 g (20 mol%) was weighed and added, and dissolved at room temperature. To the obtained diamine solution, 106.9 g (50 mol%) of terephthalic acid chloride was added and reacted to polymerize the polymer. Subsequently, 172.2 g of NMP dispersion containing 22.5% by mass of calcium hydroxide was added for neutralization to obtain a polymer solution.

[Preparation of spinning solution (polymer dope)]
A solution of the obtained copolymer ( inner viscosity (IV) measured at 30 ° C. for a solution having a polymer concentration of 0.5 g / dl in 98% concentrated sulfuric acid at 4 ° C. of 4.12) was adjusted to a concentration of 6% by mass with NMP. To prepare a spinning solution (polymer dope).

[Spinning / Coagulation]
Using the obtained dope, it was discharged from a spinneret with 25 holes, spun into an aqueous solution with an NMP concentration of 30% by mass through an air gap of about 10 mm (semi-dry semi-wet spinning method), washed with water, It was dried and then stretched 6.1 times at a temperature of 460 ° C., and then wound up to obtain a wholly aromatic polyamide fiber. Table 1 shows the physical properties and the like of the obtained fiber.

  The wholly aromatic polyamide fiber of the present invention has high breaking strength and elongation, and is very useful for industrial applications such as rubber cords such as tire cords, power transmission belts, conveyor belts, seat belts, fishing nets, safety nets, etc. It is.

Claims (2)

A wholly aromatic group comprising a structural repeating unit (I) represented by chemical formula (I), a structural repeating unit (II) represented by chemical formula (II), and a structural repeating unit (III) represented by chemical formula (III) A wholly aromatic polyamide fiber made of polyamide,
The structural repeating unit (II) is 20 to 35 mol% based on the total of the structural repeating unit (I), the structural repeating unit (II), and the structural repeating unit (III),
A wholly aromatic polyamide fiber in which the structural repeating unit (III) is 15 to 30 mol% based on the total of the structural repeating unit (I), the structural repeating unit (II), and the structural repeating unit (III).

  The wholly aromatic polyamide fiber according to claim 1, which has a breaking elongation of 5.0% or more and a tensile strength of 24 cN / dtex or more.