EP4317550A1

EP4317550A1 - Polyamide multifilament

Info

Publication number: EP4317550A1
Application number: EP22780034.9A
Authority: EP
Inventors: Yudai WATANABE; Yuta Watanabe; Mizuki KUJIRAI
Original assignee: Toray Industries Inc
Current assignee: Toray Industries Inc
Priority date: 2021-03-29
Filing date: 2022-03-14
Publication date: 2024-02-07
Also published as: WO2022209813A1; TW202237917A; JPWO2022209813A1; CN116964259A

Abstract

This polyamide multifilament has a total fineness of 4-100 dtex, a single filament fineness of 1.1-5.0 dtex, a strength-elongation product of 4.5-10.0 cN/dtex, a flatness (b/a) expressed by a major axis b and a minor axis a in a single filament cross section of 6.1-15.0, and a flatness CV value of 2.0 or less.Provided is a polyamide multifilament having a flat cross-section and excellent practical durability of a knitted material or textile for clothing, and having a soft texture and an excellent aesthetic appearance.

Description

TECHNICAL FIELD

The present invention relates to a polyamide multifilament having a flat cross section and suitable for a woven or knitted fabric in clothing applications. More specifically, the present invention relates to a polyamide multifilament capable of providing a woven fabric or a knitted fabric excellent in soft texture, practical durability, and aesthetic appearance when the polyamide multifilament having a flat cross section of the present invention is used for clothing.

BACKGROUND ART

Synthetic fibers such as polyamides and polyesters are widely used in clothing applications and industrial applications because they have excellent characteristics in mechanical and chemical properties. In particular, polyamide fibers are widely used in general clothing applications such as stockings, underwear, and sportswear because they have excellent characteristics in their unique softness, high strength, abrasion resistance, color development, hygroscopicity, and the like.
One of required characteristics for clothing products is texture. Many flat yarns have been proposed as techniques for improving the texture. For example, Patent Document 1 proposes an oval or convex lens-shaped polyamide multifilament having a flatness of 1.5 to 5.0 and a fiber cross-sectional shape line-symmetric with respect to a major axis, and a covering elastic yarn using the same, and Patent Document 2 proposes a covering elastic yarn having a single filament fineness of 0.6 to 1.0 dtex and a cross-sectional shape line-symmetric with respect to a major axis, and stocking using the same. Further, although the intended use described is in airbag applications, Patent Document 3 proposes an airbag base fabric in which a synthetic fiber multifilament having a cross-sectional shape of a single yarn having an oblateness of 1.5 to 8.0 expressed by a ratio a/b of a maximum major axis length a and a maximum minor axis length b is used for both or one of warps and wefts.

PRIOR ART DOCUMENT

PATENT DOCUMENTS

Patent Document 1: International Publication No. 2020/105637
Patent Document 2: Japanese Patent Laid-open Publication No. 2009-203563
Patent Document 3: Japanese Patent Laid-open Publication No. 2003-55861

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

The demand for texture is progressing with times, and when softness is pursued, a yarn with higher flatness is required. However, when the flatness is increased, the strength and the abrasion property are impaired, and thus the practical durability is impaired. Therefore, the flatness of the polyamide multifilament having practical durability in clothing applications is about 5.0 at the highest, and it has been desired to further increase the flatness while maintaining the durability.
In the polyamide multifilament having a flat cross section described in Patent Document 1, since the spinneret hole is minimized in order to achieve high strength, when a cross section having a higher flatness is to be formed, variations in flatness, single filament fineness, and physical properties of raw yarns increase, and deterioration in strength and abrasion properties, fuzz, streaks, and uneven gloss occur, resulting in lack of aesthetic appearance. In addition, even when the condition described in Patent Document 1 that the relaxation of polymer orientation is promoted to lower the solidification point by keeping the atmosphere temperature under the spinneret at a high temperature or the like in order to increase the strength-elongation product is applied to Patent Document 2, a high flatness cannot be obtained in formation of a flat cross section due to lowering of the solidification point, and the texture cannot be differentiated. Furthermore, even when the polyamide multifilament having a flat cross section described in Patent Document 3 is made finer to be used in clothing applications, the slow cooling region is not appropriate, and the cooling difference of single yarns is large, so that variations in flatness, single filament fineness, and physical properties of raw yarns increase, and deterioration in strength and abrasion properties, fuzz, streaks, and uneven gloss occur, resulting in lack of aesthetic appearance.
The present invention is intended to solve the above problems and has an object to provide a polyamide multifilament having a flat cross section and excellent practical durability of a woven or knitted fabric for clothing and having a soft texture and an excellent aesthetic appearance.

SOLUTIONS TO THE PROBLEMS

In order to solve the above-mentioned problems, the present invention employs the following constitution.

(1) A polyamide multifilament having a total fineness of 4 to 100 dtex, a single filament fineness of 1.1 to 5.0 dtex, a strength-elongation product of 4.5 to 10.0 cN/dtex, a flatness (b/a) expressed by a major axis b and a minor axis a in a single filament cross section of 6.1 to 15.0, and a flatness CV value of 2.0 or less.
(2) The polyamide multifilament according to (1), having a flat smoothness of 1.5% or less. $Flat smoothness (%) = (aM - am) / a \times 100$
(aM: maximum minor axis, am: minimum minor axis)
(3) A woven fabric including the polyamide multifilament according to (1) or (2).
(4) A knitted fabric including the polyamide multifilament according to (1) or (2).

EFFECTS OF THE INVENTION

The polyamide multifilament of the present invention is a polyamide multifilament having a high flatness, a high strength-elongation product, and a small variation in flatness between single yarns. Furthermore, the polyamide multifilament of the present invention can provide a woven or knitted fabric for clothing having excellent practical durability and having a soft texture and an excellent aesthetic appearance.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows an embodiment of a manufacturing device that can be preferably used in a method for manufacturing a polyamide multifilament of the present invention.
Fig. 2 is a schematic cross-sectional model diagram showing a spinneret and a heating cylinder that can be preferably used in the method for manufacturing the polyamide multifilament of the present invention.
Fig. 3 shows an embodiment of the shape of a discharge hole of the spinneret that can be preferably used in the method for manufacturing the polyamide multifilament of the present invention.
Fig. 4 shows an embodiment of a transverse section of fiber of the polyamide multifilament of the present invention.
Fig. 5 shows a hole arrangement of the spinneret that can be preferably used in the method for manufacturing the polyamide multifilament of the present invention.
Fig. 6 shows the hole arrangement of a spinneret used in Comparative Example 6.

EMBODIMENTS OF THE INVENTION

Hereinafter, the present invention will be described in more detail.
A polyamide constituting a polyamide multifilament of the present invention is a resin made of a high-molecular-weight product in which what is called a hydrocarbon group is linked to the main chain by an amide bond, and such a polyamide is excellent in yarn-making properties and mechanical properties. Mainly polycaproamide (nylon 6) and poly(hexamethylene adipamide) (nylon 66) are preferable, and polycaproamide (nylon 6) is more preferable because it hardly gels and has good yarn-making properties. The term "mainly" in the above description means that the content of ε-caprolactam units constituting polycaproamide in the case of polycaproamide or hexamethylenediammonium adipate units constituting poly(hexamethylene adipamide) in the case of poly(hexamethylene adipamide) is 80 mol% or more, more preferably 90 mol% or more. The other components are not particularly limited, and examples thereof include aminocarboxylic acid units, dicarboxylic acid units, and diamine units, which are monomers constituting polydodecanoamide, poly(hexamethylene adipamide), poly(hexamethylene azelamide), poly(hexamethylene sebacamide), poly(hexamethylene dodecanoamide), poly(m-xylylene adipamide), poly(hexamethylene terephthalamide), poly(hexamethylene isophthalamide), and the like.
In general, titanium oxide is often used as a matting agent for polyamide multifilaments, and the polyamide of the present invention may also contain titanium oxide as a matting agent. The content of titanium oxide may be appropriately set as long as the effect of the present invention is not impaired, and a preferred range thereof is 0 to 2 wt%. In addition, various additives may be contained in addition to titanium oxide described above as long as the effects of the present invention are not impaired. Examples of the additives include a stabilizer such as a manganese compound, a heat resistance agent, and a flame retardant.
The polyamide multifilament of the present invention is mainly used for clothing applications in which soft texture is required to have a total fineness of 4 to 100 dtex and a single filament fineness of 1.1 to 5.0 dtex. When the total fineness is in the range of 4 to 100 dtex, a clothing product excellent in soft texture can be obtained. If the total fineness is less than 4 dtex, the strength of the raw yarn is insufficient, the tear strength of the woven fabric and the bursting strength of the knitted fabric are poor, and the practical durability of the clothing product is impaired. The total fineness is preferably 70 dtex or less. When the single filament fineness is in the range of 1.1 to 5.0 dtex, a clothing product excellent in soft texture can be obtained. If the single filament fineness is less than 1.1 dtex, raw yarn fuzz and pilling of the woven or knitted fabric easily occur, and the practical durability and aesthetic appearance of the clothing product are impaired. The single filament fineness is preferably 4.0 dtex or less.
The polyamide multifilament of the present invention has a strength-elongation product of 4.5 to 10.0 cN/dtex. Within such a range, the tear strength of the woven fabric, the bursting strength of the knitted fabric, and the abrasion resistance are excellent, and the practical durability of the clothing product can be obtained. If the strength-elongation product is less than 4.5, the practical durability of the clothing product is impaired. The strength-elongation product is preferably 6.0 or more.
The polyamide multifilament of the present invention has a flatness (b/a) represented by a major axis b and a minor axis a of a single filament cross section of 6.1 to 15.0. The flatness referred to herein is obtained, using a cross-sectional image of a fiber obtained by cutting the fiber perpendicularly to the fiber axis and photographing the cross section, by measuring a major axis b of a single filament cross section, drawing straight lines perpendicular to the major axis b from five points obtained by dividing a line segment indicating the major axis b into six equal parts, defining a line segment between the intersections of each of the straight lines and the fiber outer shape as a minor axis aN as shown in Fig. 4(2), and calculating flatness = b/a using an average value (minor axis a) obtained by measuring the five minor axes aN (a1, a2, a3, a4, and a5). The flatness was measured for all filaments, and the average value was taken as the flatness in the present invention. The maximum value of the minor axes aN was defined as aM, and the minimum value was defined as am. By setting the flatness within such a range, the bending softness of the fiber is improved, and a clothing product having a softer texture than that of the prior art can be obtained, which enables differentiation. If the flatness is less than 6.1, a clothing product with a soft texture differentiated from the prior art cannot be obtained. On the other hand, if the flatness exceeds 15.0, the strength-elongation product decreases, and the tear strength of the woven fabric, the bursting strength of the knitted fabric, and the abrasion resistance decrease, so that the practical durability of the clothing product is impaired. The flatness is preferably 7.0 to 14.0, more preferably 8.0 to 11.0.
The polyamide multifilament of the present invention has a flatness CV value of 2.0 or less. Since a flatness of 6.1 to 15.0 represents a highly flat shape, strong gloss is provided because of scattering and reflection of light. On the other hand, due to its shape, bending and twisting easily occur in the long axis direction. Therefore, the appearance of the surface of the fabric tends to look streaky and tends to cause uneven gloss. In addition, the shape is so delicate that fuzz is likely to occur due to damage on the twisted and stressed portion. By setting the flatness CV value to 2.0 or less, a clothing product excellent in aesthetic appearance can be obtained. The flatness CV value here indicates the flatness variation of each single yarn and is a value obtained by dividing the standard deviation value of the flatness of all filaments by the average value of the flatness. In addition, the aesthetic appearance is an expression indicating the beauty of the appearance of the surface of the fabric seen comprehensively without fuzz, streaks, uneven gloss, and the like, which are product defects. If the flatness CV value exceeds 2.0, any one of streaks, uneven gloss, and fuzz or a combination thereof occurs, and the aesthetic appearance is poor. In addition, the abrasion resistance is also poor, and the practical durability of the clothing product is impaired. The flatness CV value is more preferably 1.5 or less.
The cross-sectional shape of the polyamide multifilament according to the embodiment of the present invention is not particularly limited as long as it has a flat shape, and the surface form is also not particularly limited. For example, the polyamide multifilament according to the embodiment of the present invention may have a lens type cross section, a bean type cross section, and a modified cross section having the same number of recesses as the three to eight protrusions. A particularly preferable form is a planar flat shape as exemplified in Fig. 4(1).
The polyamide multifilament of the present invention preferably has a flat smoothness of 1.5% or less. The term "flat smoothness" as used herein refers to the uniformity of the flat minor axis of a single yarn, and for the maximum minor axis aM and the minimum minor axis am of the minor axes aN of the single filament cross section, (aM - am)/a × 100 was calculated, and the average value of all filaments was taken as the flat smoothness. A smaller numerical value indicates a planar type flat cross section, and a larger numerical value indicates a flat cross section having irregularities such as a lens type cross section, a bean type cross section, and a modified cross section having the same number of recesses as the three to eight protrusions. By setting the flat smoothness to 1.5% or less, a planar type flat cross section having small irregularities exemplified in Fig. 4 is obtained. By forming the planar type flat cross section, when a woven fabric is formed, single yarns are easily laminated in the same direction, and the higher the flatness is, the thinner the woven fabric becomes, and the soft texture is improved. In addition, the surface of the woven fabric becomes smooth, strong glossiness is obtained, gloss unevenness is suppressed, and aesthetic appearance is improved. The flat smoothness is more preferably 1.0% or less.
Next, an example of a method for manufacturing the polyamide multifilament of the present invention is specifically described. Fig. 1 shows an embodiment of a manufacturing device preferably used in the method for manufacturing the polyamide multifilament of the present invention.
As for the polyamide multifilament of the present invention, a polyamide is melted, a polyamide polymer is metered and transported with a gear pump and finally extruded from a discharge hole provided in a spinneret 1 to form each filament. As illustrated in Fig. 1, a gas supply device 2 that blows vapor to suppress temporal contamination of the spinneret is provided, and a heating cylinder 3 that surrounds the entire periphery is provided to slowly cool each filament discharged from the spinneret 1 as described above, and the yarn is cooled to room temperature and solidified in a cooling device 4. Thereafter, an oil is applied with an oiling device 5, and the filaments are converged to form multifilaments, the multifilaments are converged with a fluid nozzle device 6, drawn with a take-up roller 7 and a drawing roller 8, and wound with a winding device 9.
In the manufacture of the polyamide multifilament of the present invention, the 98% sulfuric acid relative viscosity of polyamide resin chips to be used is preferably in the range of 2.5 to 4.0. The higher the 98% sulfuric acid relative viscosity is, the easier it is to obtain a high flatness, while the higher the flatness is, the lower the strength-elongation product is. Within such a range, the flatness and the strength-elongation product can be obtained. It is more preferable that the 98% sulfuric acid relative viscosity is 3.5 or less from the viewpoint of yarn making properties because the extrusion pressure of the molten polymer at the time of spinning and its increase rate with time can be suppressed, an excessive load on production equipment can be reduced, a replacement cycle of the spinneret can be extended, and productivity can be ensured.
In the manufacture of the polyamide multifilament of the present invention, the melting is preferably performed at a melting temperature in the range of a temperature (Tm + 20°C) higher by 20°C than the melting point (Tm) of the polyamide or more and a temperature (Tm + 95°C) higher by 95°C than Tm of the polyamide or less. Within such a range, a melt viscosity suitable for melt spinning is obtained, so that stable yarn making is possible.
In the manufacture of the polyamide multifilament of the present invention, in order to achieve a desired flatness and a desired flatness CV value, the discharge hole of the spinneret is optimized, and the shear rate is set to an appropriate value. Fig. 3 shows one embodiment of the hole shape of the discharge hole of the spinneret. The discharge hole has a structure in which round hole portions at both ends are connected via a slit portion, and in order to regulate the flatness to 6.1 to 15.0, a discharge hole width H (mm) of the spinneret 1 is minimized. In addition, in order to regulate the flatness CV value to 2.0 or less, the shear rate is reduced in order to reduce the stress applied to the polymer on the discharge hole wall (periphery). That is, the aspect ratio (discharge hole length N/discharge hole width H shown in Fig. 3) of the discharge hole is set to 15 to 30. Within such a range, it is possible to achieve both high and uniform flatness and excellent productivity. The aspect ratio is preferably 18 to 27.
The discharge hole width H is set to 0.060 to 0.080 mm. The discharge hole width H is more preferably 0.065 to 0.075 mm. The flatness can be achieved by minimizing the hole width of the discharge hole within a range in which the discharged polymer is stably discharged. Furthermore, in order to efficiently obtain a fiber having a flat cross section satisfying the single filament fineness, the oblateness, and the flat smoothness in the present invention, a round hole diameter D shown in Fig. 3 preferably satisfies 1.4H < D < 1.6H.
However, an aspect ratio of 15 to 30 of the discharge hole is a high value, and with such a discharge hole shape, a difference is likely to be made in the cooling profile in each discharge hole and between discharge holes. Therefore, since variation between single yarns due to a difference in formation of a fiber cross section and a difference in solidification point occurs (the flatness CV value increases), a hole arrangement in which cooling air strikes perpendicularly to the direction of the discharge hole length is adopted, so that the cooling profile of the discharge holes can be made uniform, and variation between single yarns (flatness CV value) can be suppressed. In the case of an annular cooling device exemplified in Fig. 5, the hole arrangement is such that a line segment connecting the center point of the spinneret and the center point of the discharge hole length (N/2) perpendicularly intersects a line segment of the discharge hole length.
In the manufacture of the polyamide multifilament of the present invention, in order to achieve a desired flatness and a desired strength-elongation product, although depending on the single filament fineness of the multifilament, a slow cooling region for keeping the atmosphere temperature at a high temperature is provided under the spinneret to sufficiently promote the relaxation of the orientation of the polymer, and then the polymer is rapidly solidified in a cooling region to fix the cross-sectional shape of the fiber. Fig. 2 shows a schematic cross-sectional model diagram showing the spinneret and the heating cylinder.
In the manufacture of the polyamide multifilament of the present invention, the heating cylinder 3 is provided above the cooling device 4 so as to surround each filament on the entire circumference. By providing the heating cylinder 3 above the cooling device 4 and setting the atmosphere temperature in the heating cylinder to be in the range of 280 to 310°C, the relaxation of orientation of the polyamide polymer discharged from the spinneret 1 can be improved. A desired strength-elongation product can be achieved by promoting relaxation of orientation in the slow cooling region from the spinneret surface to the lower surface of the heating cylinder. When the heating cylinder is not provided, the slow cooling region is eliminated, and the relaxation of orientation from the spinneret surface to cooling is insufficient, so that it is difficult to achieve a desired strength-elongation product.
A heating cylinder length L is preferably 30 to 80 mm although it depends on the single filament fineness of the multifilament. By setting the heating cylinder length to 30 mm or more, the distance is sufficient to promote relaxation of polymer orientation, and a desired strength-elongation product is achieved. Further, by setting the heating cylinder length to 80 mm or less, a desired flatness is achieved. The heating cylinder length is more preferably 40 to 70 mm.
The heating cylinder is preferably multitiered. In the total fineness range of the polyamide multifilament of the present invention mainly used for clothing applications, when the temperature distribution in the heating cylinder is constant, heat convection tends to be disturbed, which affects the solidification state of each filament and causes deterioration of U%. Therefore, the heat convection from the upper layer to the lower layer is intentionally created by making the heating cylinder multitiered and gradually lowering the temperature setting from the upper layer to the lower layer, and a descending air current in the same direction as the accompanying flow of the yarn is formed, so that the disturbance of the heat convection in the heating cylinder is suppressed, the yarn swinging is reduced, and the multifilament having a small U% can be obtained. The multitiered heating cylinder is more preferably made of two or more tiers, and a single tier length L1 of the multitiered heating cylinder is preferably in the range of 10 to 25 mm.
In the manufacture of the polyamide multifilament of the present invention, it is important that the cooling device 4 uniformly cools each single yarn, and an annular cooling device is used for the cooling. As an example of the method, either an annular cooling device that blows cooling/rectifying air from the outer peripheral side toward the center side or an annular cooling device that blows cooling/rectifying air from the center side toward the outer periphery is used.
In the case of a uniflow cooling device that blows cooling/rectifying air from one direction, variation in cooling is caused between a single yarn on the front side and a single yarn on the back side of the air outlet, so that the flatness CV value increases.
In order to achieve a desired flatness, the solidification point of the polymer is increased. This is because the elastic force acting on the polymer is directed outward and acts in a direction of minimizing the surface area, so that the work time is shortened. That is, the solidification point of the polymer that has exited the lower surface of the heating cylinder and entered the cooling region is brought as close as possible to the upper end of the cooling region. A vertical distance LS (hereinafter referred to as a cooling start distance LS) from the lower surface of the spinneret to the upper end of the cooling air outlet portion of the cooling device 4 is 30 to 100 mm.
Furthermore, in order to achieve a desired flatness CV value while maintaining high flatness, the polymer entering the cooling region is rapidly cooled as uniformly as possible. A horizontal distance LF (hereinafter referred to as a distance LF between the cooling start point and a yarn) from the upper end of the cooling air outlet portion of the cooling device 4 to a yarn group is 7 to 15 mm. Within such a range, cooling can be performed uniformly by rectification, and the effect of air speed variation is small, so that uniform flatness can be achieved. The starting point of the yarn group is a position farthest from the cooling air outlet portion. The length L can be arbitrarily adjusted by changing the heating cylinder length, and the distance LF can be arbitrarily adjusted by changing a spinneret hole arrangement or the like.
In addition, from the viewpoint of the Nusselt heat exchange equation, as an effective method of bringing the solidification point close to the upper end, it is preferable to increase the cooling air speed, and the range thereof is preferably in the range of 4.0 to 6.0 m/min at the lower end surface of the cooling region although it depends on the single filament fineness of the multifilament. When the cooling air speed is 4.0 m/min or more, the heat exchange rate of the polymer increases, and the solidification point approaches the upper end surface of the cooling region, so that a desired flatness is achieved. On the other hand, the cooling air speed is preferably 6.0 m/min or less from the viewpoint of runnability. Similarly to the above, the cooling air temperature in the cooling region is also an important factor in the heat exchange, and the cooling air temperature is preferably 20°C or less. When the cooling air temperature is 20°C or less, the heat exchange rate of the polymer increases, and the solidification point approaches the upper end surface of the cooling region, so that a desired flatness is achieved.
In the manufacture of the polyamide multifilament of the present invention, the position of the oiling device 5, that is, a vertical distance Lg (hereinafter referred to as an oiling position Lg) from the lower surface of the spinneret to an oiling nozzle position of the oiling device 5 in Fig. 1 is preferably 800 to 1,500 mm, more preferably 1,000 to 1,300 mm, although it depends on the single filament fineness and the cooling efficiency of the filament from the cooling device. When the distance is 800 mm or more, the filament temperature decreases to an appropriate degree at the time of applying the oil, and when the distance is 1,500 mm or less, the yarn swinging due to the descending air current is also small, and a multifilament having a low U% is obtained. The distance is preferably 1,500 mm or less from the viewpoint of increasing the strength because the accompanying flow is reduced due to shortening of the distance from the solidification point to the oiling position, the spinning orientation is suppressed due to reduction of the spinning tension, and the stretchability is excellent. When the distance is 800 mm or more, the yarn bending from the spinneret to an oil supply guide becomes appropriate, the effect of the abrasion on the guide is small, and the reduction in strength increase is reduced.

EXAMPLES

Hereinafter, the present invention will be described in more detail with reference to examples.

A. Strength-Elongation Product

A fiber sample is measured according to Breaking Strength and Elongation Percentage in JIS L 1013 (2010), and a tensile strength-elongation curve is drawn. As the test conditions, the type of the tester was a constant-speed extension shape, a grip interval was 50 cm, and a tensile speed was 50 cm/min. When the tensile strength at the time of breaking was smaller than the maximum strength, the maximum tensile strength and the elongation at that time were measured.
The strength and strength-elongation product were determined from the following equations. $Elongation = elongation length at break (%)$
$\begin{array}{l} Strength = tensile strength at break (cN) / fineness \\ (dtex) \end{array}$
$\begin{array}{l} Strength - elongation product = \{strength (cN / dtex)\} \times \\ \{elongation (%) + 100\} / 100 \end{array}$

B. Total Fineness

A fiber sample was set on a sizing reel having a perimeter of 1.125 m and rotated 500 times to make a looped skein. The skein was dried (105 + 2°C × 60 minutes) with a hot air dryer, and the skein mass was measured with a balance. The fineness was calculated from the value obtained by multiplying the skein mass by the official regain. The official regain was set to 4.5%.

C. Sulfuric Acid Relative Viscosity (ηr)

After dissolving 0.25 g of a polyamide chip sample so as to be 1 g with respect to 100 ml of 98-mass% sulfuric acid, a flow time (T1) at 25°C was measured using an Ostwald viscometer. Subsequently, a flow time (T2) of 98-mass% sulfuric acid alone was measured. The ratio of T1 to T2, that is, T1/T2 was defined as the sulfuric acid relative viscosity.

D. Flatness

A thin section was cut out in the transverse sectional direction at an arbitrary position of the fiber, the transverse section of the fiber was photographed for every filament with a transmission microscope, the image was printed out (SCT-P66 manufactured by Mitsubishi Electric Corporation) at a magnification of 1,000 times and captured (black and white photograph, 400 dpi) using a scanner (GT-5500WINS manufactured by Seiko Epson Corporation), and in a state in which the image was magnified 1,500 times on a display, image processing software (WinROOF) was used. The major axis b of a single filament cross section is measured, straight lines perpendicular to the major axis b are drawn from five points obtained by dividing a line segment indicating the major axis b into six equal parts, a line segment between the intersections of each of the straight lines and the fiber outer shape is defined as the minor axis aN, and flatness = b/a is calculated using an average value (minor axis a) obtained by measuring the five minor axes aN. The flatness was measured for all filaments, and the number average value of the obtained values was taken as the flatness.

E. Flatness CV Value (%)

The standard deviation of the flatness of all the filaments measured above was calculated, and the value obtained by dividing the standard deviation by the average value of the flatness was taken as the flatness CV value.

F. Flat Smoothness

As for the measurement of the flatness, for the maximum minor axis aM and the minimum minor axis am of the minor axes aN of the single filament cross section, (aM - am)/a × 100 was calculated, and the average value of all filaments was taken as the flat smoothness.

G. Evaluation of Fabric

(a) Tear Strength

For a woven fabric product produced by the same production method as in Example 1, tear strength was measured at three arbitrary points in accordance with JIS L 1096 (2010) (8.17 Method A), and the average value thereof was measured. Evaluation was performed according to four ranks by the following criteria.

S: 4.5 N or more
A: 4.1 N less than 4.5 N
B: 3.7 N less than 4.1 N
C: Less than 3.7 N.

(b) Abrasion Resistance

For a woven fabric product produced by the same production method as in Example 1, pilling abrasion resistance was measured at three arbitrary points in accordance with JIS L 1076 (2012) (8.1.1 Method A), and the average value thereof was measured.

S: Fifth grade
A: Fourth grade
B: Third grade
C: Second grade or less

A product ranked as S, A, or B for both the tear strength and the abrasion resistance was judged as passed in terms of durability.

(c) Texture

For a woven fabric product produced by the same production method as in Example 1, softness was relatively evaluated by inspectors (five persons) who had abundant experience in texture evaluation, using as a reference a woven fabric produced by the same production method as in Example 1 with a nylon 6 multifilament having a flat cross section of Comparative Example 9 at the level of the prior art. The evaluation point was given by each inspector, and the result was ranked such that the case where the average value of the points by the five inspectors (rounded off to the whole number) was five was rated as S, four as A, three as B, and one or two as C. A product ranked as S, A, or B was judged as passed in terms of texture.

Five points: Excellent
Four points: Good
Three points: Normal
Two points: Slightly poor
One point: poor.

(d) Aesthetic Appearance

For a woven fabric product produced by the same production method as in Example 1, the appearance of the surface of the fabric comprehensively viewed without fuzz, streaks, uneven gloss, and the like, which are product defects, was evaluated by inspectors (five persons) who had abundant experience in appearance inspection by the following criteria. The evaluation point was given by each inspector, and the result was ranked such that the case where the average value of the points by the five inspectors (rounded off to the whole number) was five was rated as S, four as A, three as B, and one or two as C. A product ranked as S, A, or B was judged as passed in terms of aesthetic appearance.

Five points: No Fuzz, streaks, or gloss unevenness
Four points: No fuzz or streaks
Three points: No fuzz
Two points: No streaks
One point: No gloss unevenness.

[Example 1]

As a polyamide, nylon 6 chips having a sulfuric acid relative viscosity (ηr) of 3.3 and a melting point of 225°C and containing no titanium oxide were dried by a conventional method so as to have a moisture content of 0.03 mass% or less. The obtained nylon 6 chips were melted at a spinning temperature (melting temperature) of 298°C and discharged from a spinneret (discharge amount: 39.2 g/min). As the spinneret, a spinneret having 68 holes and 2 yarns/spinneret and having discharge holes (aspect ratio N/H = 19.7, H = 0.07 mm) having round holes at both ends of a slit as shown in Fig. 3 was used.
Spinning was performed using a spinning machine of the form shown in Fig. 1. The heating cylinder had a heating cylinder length L of 50 mm, and the atmosphere temperature of the heating cylinder was set to 290°C. Each filament discharged from the spinneret was passed through an annular cooling device 4 that blew cooling/rectifying air from the outer peripheral side toward the center side at a cooling start distance LS of 60 mm, a distance LF between the cooling start point and the yarn of 10 mm, an air temperature of 18°C, and an air speed of 5.0 m/min to cool and solidify the yarn to room temperature. Thereafter, an oil was applied at an oiling position Lg from the spinneret surface at a position of 1,300 mm, and each filament was converged to form multifilaments, and convergence was imparted with the fluid nozzle device 6. The convergence was imparted by injecting high-pressure air to the traveling yarns in the entangling device 6. The pressure of the injected air was 0.2 MPa (flow rate: 30 L/min). Thereafter, the yarn was drawn so that the draw ratio between the take-up roller 7 and the drawing roller 8 was 1.8 times and wound at 3,500 m/min with a winding machine 9 to obtain a nylon 6 multifilament of 56 dtex and 34 filaments having a flat cross-sectional shape.
The obtained multifilament was used as a warp and a weft, and a plain weave structure was woven at a warp density of 188 yarns/2.54 cm and a weft density of 155 yarns/2.54 cm. The obtained gray fabric was dyed in the following (a) to (e) to obtain a woven fabric having a warp density of 200 yarns/2.54 cm and a weft density of 160 yarns/2.54 cm.

(a) Refining: 5 ml/L of Noigen WS, 5 g/L of sodium hydroxide, bath ratio of 1 : 50, 95°C × 60 min
(b) Intermediate set: 180°C × 1 min
(c) Dyeing: 1.0% owf of acidic dye (Nylosan Blue-GFL 167% (manufactured by Sandoz International GmbH), 98°C × 60 min
(d) Fixing treatment: 3 g/l of synthetic tannin (NYLONFIX 501 manufactured by SENKA Corporation), 80°C × 20 min
(e) Finishing set: 200°C × 1 min

The results of evaluating the obtained flat nylon 6 multifilament and woven fabric are shown in Table 1.

[Examples 2 and 3] [Comparative Examples 1 and 2]

A flat nylon 6 multifilament of 56 dtex and 34 filaments was obtained in the same manner as in Example 1 except that the heating cylinder length L and the cooling start distance LS were changed as shown in Table 1, and a woven fabric was obtained. The results of evaluation are shown in Table 1.

[Examples 4 and 5]

A flat nylon 6 multifilament of 56 dtex and 34 filaments was obtained in the same manner as in Example 1 except that the sulfuric acid relative viscosity (ηr) of the polyamide was changed as shown in Table 1, and a woven fabric was obtained. The results of evaluation are shown in Table 1.

[Examples 6 and 7] [Comparative Examples 3 and 4]

A flat nylon 6 multifilament of 56 dtex and 34 filaments was obtained in the same manner as in Example 1 except that the aspect ratio of the discharge hole of the spinneret shown in Fig. 3 was changed as shown in Table 2, and a woven fabric was obtained. The results of evaluation are shown in Table 1.

[Example 8]

A flat nylon 6 multifilament of 78 dtex and 24 filaments was obtained in the same manner as in Example 1 except that a spinneret having 48 holes and 2 yarns/spinneret was used, the aspect ratio of the discharge hole of the spinneret shown in Fig. 3 was changed as shown in Table 2, and the discharge amount was changed to 54.6 g/min, and a woven fabric was obtained.

[Example 9]

A flat nylon 6 multifilament of 100 dtex and 24 filaments was obtained in the same manner as in Example 1 except that a spinneret having 48 holes and 2 yarns/spinneret was used, the aspect ratio of the discharge hole of the spinneret shown in Fig. 3 was changed as shown in Table 2, and the discharge amount was changed to 70.0 g/min, and a woven fabric was obtained.

[Comparative Example 5]

A flat nylon 6 multifilament of 56 dtex and 34 filaments was obtained in the same manner as in Example 1 except that the cooling device 4 was changed to a uniflow type in which cooling/rectifying air was blown out in one direction, and a woven fabric was obtained. The results of evaluation are shown in Table 2.

[Comparative Example 6]

A nylon 6 multifilament of 56 dtex and 34 filaments was obtained in the same manner as in Example 1 except that the hole arrangement of the spinneret was changed such that cooling air was blown in parallel to the major axis direction of the flat cross section as shown in Fig. 6, and a woven fabric was obtained. The results of evaluation are shown in Table 2.

[Comparative Examples 7 and 8]

A nylon 6 multifilament of 56 dtex and 34 filaments was obtained in the same manner as in Example 1 except that the distance LF between the cooling start point and the yarn was changed as shown in Table 2, and a woven fabric was obtained. The results of evaluation are shown in Table 2.

[Comparative Example 9]

A flat nylon 6 multifilament of 56 dtex and 34 filaments was obtained in the same manner as in Example 1 except that the discharge hole width H of the spinneret shown in Fig. 3 was changed as shown in Table 2, and a woven fabric was obtained. The results of evaluation are shown in Table 2.

[Table 1]

		Example 1	Example 2	Example 3	Comparative Example 1	Comparative Example 2	Example 4	Example 5
Raw material polymer	Polyamide type	N6	N6	N6	N6	N6	N6	N6
Raw material polymer	Relative viscosity	3.3	3.3	3.3	3.3	3.3	2.7	3.7
Spinning conditions	Aspect ratio of discharge hole (N/H)	19.7	19.7	19.7	19.7	19.7	19.7	19.7
	Discharge hole width H (mm)	0.07	0.07	0.07	0.07	0.07	0.07	0.07
	Discharge hole arrangement	Perpendicular to major axis direction	Perpendicular to major axis direction	Perpendicular to major axis direction	Perpendicular to major axis direction	Perpendicular to major axis direction	Perpendicular to major axis direction	Perpendicular to major axis direction
	Heating cylinder length L (mm)	50	80	30	25	100	50	50
	Cooling method	Annular	Annular	Annular	Annular	Annular	Annular	Annular
	Cooling start distance LS (mm)	60	90	40	35	110	60	60
	Distance LF between cooling start point and yarn (mm)	10	10	10	10	10	10	10
Physical properties	Total fineness (dtex)	56	56	56	56	56	56	56
	Single filament fineness (dtex)	1.65	1.65	1.65	1.65	1.65	1.65	1.65
	Strength-elongation product	7.2	8 . 0	6.0	4.3	8 . 9	6.6	5.0
	Flatness	9.0	6.2	11.0	12.0	4 . 0	6.3	12.0
	Flatness CV value	0.8	1.2	0 . 6	0 . 7	1.5	0.8	1.0
	Flat smoothness (x)	0.7	0.9	0 . 6	0 . 6	1.6	0.8	0 . 9
Evaluation of fabric	Tear strength of fabric	S	S	A	C	S	A	B
	Abrasion resistance	S	A	S	C	S	S	B
	Texture	S	A	S	S	C	B	S
	Aesthetic appearance	S	S	S	S	B	S	S

[Table 2-1]

		Example 6	Example 7	Comparative Example 3	Comparative Example 4	Example 8	Example 9	Comparative Example 5
Raw material polymer	Polyamide type	N6	N6	N6	N6	N6	N6	N6
Raw material polymer	Relative viscosity	3.3	3.3	3.3	3.3	3.3	3.3	3.3
Spinning conditions	Aspect ratio of discharge hole (N/H)	15.7	23.6	10	31	23.6	23.6	19.7
	Discharge hole width H (mm)	0.07	0.07	0.07	0.07	0.07	0.07	0.07
	Discharge hole arrangement	Perpendicular to major axis direction	Perpendicular to major axis direction	Perpendicular to major axis direction	Perpendicular to major axis direction	Perpendicular to major axis direction	Perpendicular to major axis direction	Perpendicular to major axis direction
	Heating cylinder length L (mm)	50	50	50	50	50	50	50
	Cooling method	Annular	Annular	Annular	Annular	Annular	Annular	Uniflow
	Cooling start distance LS (mm)	60	60	60	60	60	60	60
	Distance LF between cooling start point and yarn (mm)	10	10	10	10	10	10	-
Physical properties	Total fineness (dtex)	56	56	56	56	78	100	56
	Single filament fineness (dtex)	1.65	1.65	1.65	1.65	3.25	4.17	1.65
	Strength-elongation product	7.2	6.8	8.2	3.5	7.3	5.0	7.0
	Flatness	7.5	11.5	4.5	16.0	12.0	14.0	6.5
	Flatness CV value	0.8	0.9	2 . 9	1.2	0.8	1.8	2.6
	Flat smoothness (x)	0.9	0.7	3.4	1.1	0.8	1.1	2.1
Evaluation of fabric	Tear strength of fabric	S	A	S	C	S	S	S
	Abrasion resistance	S	A	C	C	S	S	A
	Texture	A	S	C	S	A	A	B
	Aesthetic appearance	S	S	C	A	S	A	C

[Table 2-2]

		Comparative Example 6	Comparative Example 7	Comparative Example 8	Comparative Example 9
Raw material polymer	Polyamide type	N6	N6	N6	N6
Raw material polymer	Relative viscosity	3.3	3.3	3.3	3.3
Spinning conditions	Aspect ratio of discharge hole (N/H)	19.7	19.7	19.7	19.7
	Discharge hole width H (mm)	0 . 07	0.07	0.07	0.1
	Discharge hole arrangement	Parallel to major 3xm direction	Perpendicular to major axis direction	Perpendicular to major axis direction	Perpendicular to major axis direction
	Heating cylinder length L (mm)	50	50	50	50
	Cooling method	Annular	Annular	Annular	Annular
	Cooling start distance LS (mm)	60	60	60	60
	Distance LF between cooling start point and yarn (mm)	10	5	20	10
Physical properties	Total fineness (dtex)	56	56	56	56
	Single filament fineness (dtex)	1.65	1.65	1.65	1.65
	Strength-elongation product	6.9	6.8	7.6	7.0
	Flatness	8.0	9.3	7.7	4 . 8
	Flatness CV value	2.2	2.1	2.3	0 . 9
	Flat smoothness (x)	3.0	1.6	1.7	0.8
Evaluation of fabric	Tear strength of fabric	A	A	S	S
	Abrasion resistance	A	S	A	S
	Texture	S	S	A	C
	Aesthetic appearance	C	C	c	s

DESCRIPTION OF REFERENCE SIGNS

1:: Spinneret
2:: Gas supply device
3:: Heating cylinder
4:: Cooling device
5:: Oiling device
6:: Fluid nozzle device
7:: Take-up roller
8:: Drawing roller
9:: Winding device
L:: Heating cylinder length
LS:: Cooling start distance
LF:: Distance between cooling start point and yarn
Lg:: Oiling position
N:: Discharge hole length
H:: Discharge hole width
D:: Round hole diameter
a:: Minor axis of flat cross section
b:: Major axis of flat cross section

Claims

A polyamide multifilament having a total fineness of 4 to 100 dtex, a single filament fineness of 1.1 to 5.0 dtex, a strength-elongation product of 4.5 to 10.0 cN/dtex, a flatness (b/a) expressed by a major axis b and a minor axis a in a single filament cross section of 6.1 to 15.0, and a flatness CV value of 2.0 or less.
The polyamide multifilament according to claim 1, having a flat smoothness of 1.5% or less,
wherein the flat smoothness (%) = (aM - am)/a × 100,

where aM is a maximum minor axis, and

am is a minimum minor axis.
A woven fabric comprising the polyamide multifilament according to claim 1 or 2.
A knitted fabric comprising the polyamide multifilament according to claim 1 or 2.