EP4141153A1

EP4141153A1 - Polyamide monofilament

Info

Publication number: EP4141153A1
Application number: EP21817409.2A
Authority: EP
Inventors: Mizuki KUJIRAI; Sumio Yamaguchi; Sho MURATA
Original assignee: Toray Industries Inc
Current assignee: Toray Industries Inc
Priority date: 2020-06-02
Filing date: 2021-05-27
Publication date: 2023-03-01
Also published as: JPWO2021246270A1; CN115427617A; WO2021246270A1; CN115427617B

Abstract

This polyamide monofilament is characterized by including at most one knot having a fiber diameter that is at least 135% of the diameter of the fiber and at most one filament having a fiber diameter that is at most 80% of the diameter of the fiber, wherein the knot and filament are present at a position of 200,000 m in the longitudinal direction of the fiber. Provided is a polyamide monofilament which is suitable for obtaining a filter having excellent filtration performance due to the excellent opening uniformity of a gauze fabric, and has excellent fiber diameter uniformity due to a small number of knots and filaments.

Description

TECHNICAL FIELD

The present invention relates to a polyamide monofilament. More specifically, it relates to a monofilament suitable for producing filters with excellent filtration performance that can serve as automotive filters, medical filters, and acoustic filters. In particular, the present invention provides a polyamide monofilament that contains few knots and thinner filaments and has high uniformity in fiber diameter, thus serving to produce plain gauze fabrics with uniform openings and filters with high filtration performance.

BACKGROUND ART

Having good mechanical characteristics, high chemical resistance, and high heat resistance, polyamide fibers have been widely used for producing clothing, industrial materials, and the like. In the field of industrial material production, in particular, it has been used very widely in the form of filters of monofilament fabrics (screen gauze fabrics) for automotive filters, medical filters, and acoustic filters.
In the field of electronics, which has been growing rapidly in recent years, and in the medical field, where extremely high product quality is required, there is an increasing demand for high quality products having enhanced uniformity without clogging or large openings. In particular, demands are growing for products that are free of fineness unevenness in the length direction and knots, i.e., local fineness irregularities, and many techniques have been proposed with the aim of providing monofilaments that are free of these quality defects.
For example, Patent document 1 describes that knots are caused by gelation products of the polymer getting into the thread to prevent full stretching of these portions and proposes a technique that uses a spinning pack containing a sintered filter of metallic short fibers to ensure sufficient dispersion of the gelation products.
Patent document 2 is focused on the production of a finer monofilament with high tenacity, high modulus, and uniform fiber diameter in the length direction and reports a technique to produce a polyester monofilament of a core-sheath structure having fewer and smaller knots including very small ones made tangible.
Patent document 3 reports a spinneret for melt-spinning that has a discharge hole 1 containing an inflow hole 2, a metering hole 3 having a smaller cross-sectional area than the inflow hole 2, and a relaxation hole 4 having a larger cross-sectional area than the metering hole 3 that are connected in this order. The use of this spinneret for melt-spinning can serve to reduce the frequency of cleaning of the spinneret surface. A melt-spinning method that uses this spinneret for melt-spinning is also reported. The combined use of this spinneret for melt-spinning and the melt-spinning method makes it possible to further reduce the accumulation of dirt on the spinneret surface and reduce the variation in discharge. This is expected to realize the production of filaments with decreased unevenness.

PRIOR ART DOCUMENTS

PATENT DOCUMENTS

Patent document 1: Japanese Unexamined Patent Publication (Kokai) No. 2014-231651
Patent document 2: International Publication WO 2019/044449
Patent document 3: Japanese Unexamined Patent Publication (Kokai) No. HEI 9-268417

SUMMARY OF INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

However, although the technique proposed in Patent document 1 is effective for suppressing the formation of knots, there are no descriptions about thinning of the thread, and the technique cannot serve sufficiently for solving the problem of thinning. So far, quality evaluations for abnormal fineness have been made based on the rate of capturing knots by a slub catcher and fineness unevenness in the length direction (averaged) measured by a Ustertester, but this can allow local outflow of thin filaments, possibly leading to a serious complaint. This conventional technique is also disadvantageous due to unstable discharge from the spinneret that tends to cause fineness unevenness in the length direction.
In addition, although the technique proposed in Patent document 2 can serve to produce a monofilament with few knots in the case of a polyester monofilament thread of a core-sheath structure, polyamide easily forms a heat-degradable polymer that can cause knots because polyamide itself is likely to suffer heat degradation and heat decomposition in the process of producing a polyamide monofilament, and furthermore, sublimates of low molecular weight mers (monomers, oligomers, etc.) are easily formed in larger amounts compared with polyester. Therefore, this conventional technique cannot work sufficiently in suppressing knot formation and thinning in a polyamide monofilament. Specifically, there is the problem that knots and thin filament are likely to occur due to contamination of spinneret surface caused by sublimation of low molecular weight substances.
Furthermore, in the case of Patent document 3, the upper limit of the diameter of the relaxation hole is as small as 0.75 mm, and accordingly, the accumulated dirt density per unit circumferential length is large, leading to the problem of easy knot formation.
Thus, the present invention provides a high quality polyamide monofilament useful for production of filters. It has uniform fineness and hardly suffers knot formation. If knots exist, they can cause thread breakage and scum generation during warping and weaving. In addition, if a monofilament having knots is woven, because the knots have larger diameters than the fiber, the resulting woven fabric will suffer clogging in portions adjacent to the knots. As a result, the woven fabric will have largely deteriorated quality. As another problem, as filters with higher quality have been developed, greater attention is now focused on the existence of thinner filaments that have smaller diameters than the normal portions. If thin filaments exist, they will lead to woven fabrics having larger openings that allow foreign objects to pass through and cause inferior filtration. Therefore, it is extremely important to eliminate thin filaments. The present invention provides a polyamide monofilament that contains few knots and thinner filaments and has high uniformity in fiber diameter, thus serving to produce plain gauze fabrics with uniform openings and filters with high filtration performance.

MEANS OF SOLVING THE PROBLEMS

The present invention adopts the following constitution to meet the object described above.

(1) A polyamide monofilament comprising no more than one knot with a fiber diameter of 135% or more of the fiber diameter, and no more than one thin filament with a fiber diameter of 80% or less of the fiber diameter, existing in the fiber longitudinal direction of 200,000 m.
(2) The polyamide monofilament as set forth in (1) having a fiber diameter CV% of 1% or less as measured over a 200,000 m section in the fiber length direction.
(3)
The polyamide monofilament as set forth in either (1) or (2) comprising no more than ten knots with a fiber diameter of 120% or more and less than 135%, and no more than ten thin filaments with a fiber diameter of more than 80% and 90% or less of the diameter of the fiber, existing in the fiber longitudinal direction of 200,000 m.
(4) The polyamide monofilament as set forth in any one of (1) to (3), wherein the smallest of 50 measurements of the strength-elongation product taken continuously is 90% or more and 100% or less of the average.
(5) The polyamide monofilament as set forth in any one of (1) to (4) having a fineness of 6 to 50 dtex.

ADVANTAGEOUS EFFECTS OF THE INVENTION

The present invention can provide a polyamide monofilament that contains few knots and thinner filaments and has high uniformity in fiber diameter, thus serving to produce plain gauze fabrics with uniform openings and filters with high filtration performance.

BRIEF DESCRIPTION OF THE DRAWINGS

[Fig. 1] This is a longitudinal cross section of a typical discharge hole of a spinneret for melt-spinning designed for producing a polyamide monofilament according to the present invention.
[Fig. 2] This is a schematic diagram of a typical process used for the production method for a polyamide monofilament according to the present invention.
[Fig. 3] This is a longitudinal cross section of a typical spinning pack used for melt-spinning of a polyamide monofilament according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is described in more detail below.
For the present invention, a knot is a lump-like portion extended in the fiber length direction and having an abnormal local fiber diameter. Its fiber diameter is 120% or more of the normal fiber diameter. A thin filament is a portion with a small fiber diameter. Specifically, its diameter is 90% or less of the normal fiber diameter. Knots and thin filaments are examined using an optical profile sensor (PSD-200, manufactured by Sensoptic) under the conditions of a traveling speed of 800 m/min, thread length-directional interval of 0.07 mm, and thread length of 200,000 m. The optical profile sensor records numerical values of fiber diameter, standard deviation of fiber diameter measurements, number and size of knots, and number and size of thin filaments.
For the polyamide monofilament according to the present invention, any 200,000 m section in the fiber length direction contains one or less knot with a diameter 135% or more as large as the fiber diameter and one or less thin filament with a diameter 80% or less as large as the fiber diameter. If the numbers of knots and thin filaments are in these ranges, the monofilament has a high uniformity and can serve to produce plain gauze fabrics with uniform openings and filters with high filtration performance. If there is more than one knot with a diameter 135% or more as large as the fiber diameter, the resulting plain gauze fabric will suffer large openings and clogging in regions adjacent to the knots, leading to a decrease in the uniformity of openings. In addition, it will be caught by reeds used in high-order processing, leading to increased thread breakage occurrence and scum generation. If there is more than one thin filament with a diameter 80% or less as large as the fiber diameter, the resulting plain gauze fabric will suffer large openings, leading to a decrease in the uniformity of openings. If the plain gauze fabric has large openings, it will lead to a filter that allows foreign objects to pass through and suffers a decrease in filtration performance.
For the polyamide monofilament according to the present invention, it is preferable that any 200,000 m section in the fiber length direction contains ten or less knots with a diameter 120% or more and less than 135% as large as the fiber diameter and ten or less thin filaments with a diameter more than 80% and 90% or less as large as the fiber diameter. If the number of knots with a diameter 120% or more and less than 135% as large as the fiber diameter is ten or less, it serves to produce a plain gauze fabric that hardly suffers clogging. The plain gauze fabric will have openings with improved uniformity and serve to produce a filter having enhanced filtration performance. Furthermore, if the number of thin filaments with a diameter more than 80% and 90% or less as large as the fiber diameter is ten or less, it serves to produce a plain gauze fabric that hardly suffers openings. Accordingly, the plain gauze fabric will have openings with improved uniformity and serve to produce a filter having enhanced filtration performance. It is more preferable that any 200,000 m section in the fiber length direction contains five or less knots with a diameter 120% or more and less than 135% as large as the fiber diameter and five or less thin filaments with a diameter more than 80% and 90% or less as large as the fiber diameter.
The polyamide monofilament according to the present invention preferably has a fiber diameter CV% of 1.0% or less as measured over a 200,000 m section in the fiber length direction. The fiber diameter CV% is calculated by dividing the standard deviation of fiber diameter measurements by the average of fiber diameter measurements and represented in percentage. It shows the degree of fineness unevenness in the length direction, and a smaller value indicates a higher uniformity in fiber diameter. If the value of CV% is controlled at 1.0% or less, it serves to produce a monofilament that is free of fineness unevenness in the fiber length direction and high in the uniformity in fiber diameter. It also serves to produce a plain gauze fabric with uniform openings and a filter with high filtration performance. The fiber diameter CV% is more preferably 0.8% or less.
For the polyamide monofilament according to the present invention, the smallest of 50 measurements of the strength-elongation product taken continuously is preferably 90% or more and 100% or less of the average. If knots and thin filaments exist, their sites are likely to act as fracture points and weaken the thread easily, leading to a plain gauze fabric suffering decreased local durability. If it is controlled at 90% or more, it serves to produce a plain gauze fabric with increased durability and a filter with increased durability.
When used for the production of high mesh filters, the polyamide monofilament according to the present invention preferably has a fineness of 6 to 50 dtex. It is more preferably 8 to 47 dtex. In particular, when used for the production of ultrafine high mesh filters, it preferably has a fineness of 6 to 13 dtex. As described later, furthermore, if the fineness is controlled at 50 dtex or less in producing the polyamide monofilament according to the present invention, the occurrence of cooling unevenness can be suppressed in the air-cooling equipment and this serves to produce a polyamide monofilament that is free of knots and thin filaments and ensures high filtration performance. On the other hand, if it is controlled at 6 dtex or more, it serves to suppress variations in discharge and produce a polyamide monofilament that is free of knots and thin filaments and ensures high filtration performance.
The polyamide monofilament according to the present invention preferably has a strength of 4.0 cN/dtex or more and an elongation of 30% to 60%. If they are controlled in these ranges, it ensures the production of a plain gauze fabric with high durability.
The production method for the polyamide monofilament according to the present invention is described below.
For the present invention, polyamide is a polymer in which so-called hydrocarbon groups are connected to the backbone chain via amide bonds, and it is preferable for the polyamide to mainly contain polycaproamide or polyhexamethylene adipamide because such a polymer tends to have high dyeability, high washing fastness, and good mechanical properties. Here, the term "mainly" means that, in the case of polycaproamide, ε-caprolactam units that form polycaproamide are mainly contained, and the polyamide mainly contains polyhexamethylenediamine diammonium adipate units. These units preferably account for 80 mol% or more, more preferably 90 mol% or more. There are no specific limitations on other components and useful ones include, for example, such units as aminocarboxylic acid and dicarboxylic acid that serve as monomers forming polydodecanamide, polyhexamethylene azelamide, polyhexamethylene sebacamide, polyhexamethylene dodecanamide, polymetaxylylene adipamide, polyhexamethylene terephthalamide, polyhexamethylene isophthalamide, etc.
The polyamide may have a polymerization degree appropriately set in a range commonly adopted for the production of industrial fibers, but it is preferable for its 98% sulfuric acid relative viscosity to be in the range of 2.0 to 3.3, and more preferably in the range of 2.4 to 3.3. If it is controlled in this range, a polyamide monofilament having a strength required for plain gauze fabrics can be produced with high thread-making performance.
In producing the polyamide monofilament according to the present invention, the moisture content of the polyamide resin chips used for spinning is preferably adjusted by drying etc. to 0.11% to 0.15%. If it is controlled in this range, it serves to suppress the formation of knots and thin filaments in the polyamide monofilament. The moisture content referred to herein is determined by putting a specimen of the polyamide resin chips in a trace moisture measuring device and evaporating the moisture under the conditions of 230°C and 30 minutes while reading the moisture value.
The melt-spinning temperature of the polyamide resin chips is preferably a temperature higher than the melting point of the polyamide resin. Specifically, melt-spinning is performed preferably at a temperature higher by 20°C to 40°C than the melting point. If it is controlled in this range, it serves to suppress the formation of knots and thin filaments in the polyamide monofilament.
In the production method for the polyamide monofilament according to the present invention, the basic production process may be carried out by a generally known technique, and examples thereof include continuous implementation of spinning and stretching steps (direct spinning and stretching process) and winding up of an unstretched thread implemented first, followed by stretching thereof (two-stage process).
A typical process for the production method for the polyamide monofilament according to the present invention is described below with reference to the schematic process diagram given in Fig. 2. First, polyamide resin chips melted in an extruder is fed to a melt-spinning pack 10 and spun threads are discharged from a round-hole spinneret 11 in which two discharge holes are arranged circumferentially. Then, the threads are cooled in an uniflow type chimney 12 designed to blow air in one direction, and the threads are divided into individual ones, supplied with a spinning oil solution from an oil supply guide 13, taken up by the first godet roller 14, stretched between the second godet roller 15 and the third godet roller 16, heat-treated by the third and fourth godet rollers (16, 17), and wound up on a wind-up device 18.
The melt-spinning pack used for producing the polyamide monofilament according to the present invention is a melt-spinning pack as described in Patent document 1. The melt-spinning pack contains at least a spinneret, a pressure plate, a metal wire filter, and a sand filter layer or a straightening vane, with a sintered filter made of metallic short fibers having a roughly polygonal cross section being provided between the metal wire filter and the sand filter layer or straightening vane.
As a major factor in the formation of knots in a polyamide monofilament, it is considered that a molten polyamide polymer suffering a viscosity variation is thermally modified to form a gelled polymer, and the polymer is not melted completely to allow gelled material to get mixed in the threads as the threads are discharged from spinneret holes, resulting in insufficient stretching in this portion. To suppress the formation of knots, it is necessary to disperse the gelled material. In order to promote the dispersion of the gelled material, the filtering accuracy of the metal wire filter, sand filter layer, and straightening vane contained in the melt-spinning pack is improved, and metallic short fibers having a roughly polygonal shape are used so that the metallic short fibers are entangled, thereby enhancing the filtration performance and dispersibility. The polygonal cross section preferably has a shape containing acute angles. The thermally modified gelled polymer is dispersed finely as it hits the acute angle corners of the cross section. Accordingly, as the angles in the cross section become acuter, the thermally modified gelled polymer can be dispersed more finely.
The spinneret used to produce the polyamide monofilament according to the present invention is a spinneret for melt-spinning that contains a polymer inflow hole, a metering hole having a smaller cross-sectional area than the inflow hole, and a relaxation hole having a larger cross-sectional area than the metering hole that are connected in this order.
In the melt-spinning of polyamide, low molecular components (monomers, oligomers, etc.) of the polymer is sublimated as it is discharged from the spinneret, and it is accumulated over time as dirt around the polymer discharge holes. In general, a mold release agent such as silicone is applied on the polymer-discharging surface of the spinneret for melt-spinning in order to facilitate the release of the polymer. However, as described above, dirt accumulation around the polymer discharge holes and deterioration in polymer releasability act to destabilize the discharge of the polymer from the spinneret, resulting in thread unevenness in the thread length direction, knot and thin filament formation, and thread breakage. As a solution to this, Patent document 3 ( Japanese Unexamined Patent Publication (Kokai) No. HEI-9-268417 ) describes a melt-spinning spinneret for high speed spinning that is designed to take up threads at a high speed of 2,000 m/min or more, in which the ratio (D2/D1) between the metering hole diameter (D1) and the relaxation hole diameter (D2) is 1.75 to 2.5, with the relaxation hole diameter (D2) being 0.40 to 0.75 mm, so that discharge is stabilized to decrease the fineness unevenness and suppress thread breakage, thereby realizing a decrease in required spinneret surface cleaning frequency. Although it is described that the equipment can work effectively down to a single-fiber fineness of 8 denier (9 dtex), discharge of the polymer can become unstable over time to cause thread unevenness defects such as knots and thin filaments even when it is applied to a polyamide monofilament production process in which it is taken up under the conditions of a single-fiber fineness of 6 dtex or more and 1,000 m/min or less.
Fig. 1 shows a typical discharge hole provided in a spinneret used for the present invention. In Fig. 1, the discharge hole 1 contains an inflow hole 2, a metering hole 3, and a relaxation hole 4 that are connected continuously in the flow direction of the molten polymer. In the structure of the discharge hole 1, the metering hole 3 is narrowed to a smaller cross section than the inflow hole 2 and works to measure the molten polymer flowing in from the inflow hole 2. The relaxation hole 4 that follows the metering hole 3 has a larger cross section than the metering hole 3 to serve for decreasing the accumulated dirt density per unit circumferential length and relaxing the pressure on the molten polymer, thereby reducing the ballast.
In the spinneret used for the production process according to the present invention, the use of a relaxation hole that has a diameter (D2) larger than the diameter (D1) of the metering hole as described above tends to cause a drop in the back pressure on the discharge hole. As a countermeasure, it is necessary to decrease the metering hole diameter (D1) to supplement the drop in the back pressure. On the other hand, if the metering hole diameter (D1) is decreased, it becomes more difficult for the molten polymer to fill the entire discharge hole uniformly, making discharge more unstable. The metering hole is preferably designed so that the ratio (L1/D1) between the diameter (D1) and the length (L1) of the metering hole is in the range of 2.0 to 3.5. If it is controlled in this range, it will be possible for the molten polymer to be discharged stably while maintaining a required back pressure and metering accuracy, and this serves to produce a polyamide monofilament containing few knots and thin filaments and having high uniformity in fiber diameter. It is more preferably 2.5 to 3.0.
In the spinneret used for the production process according to the present invention, the ratio (D2/D1) between the metering hole diameter (D1) and the relaxation hole diameter (D2) is preferably 2.6 to 4.0. If it is controlled in this range, the cross section of the molten polymer largely increases as it flows into the relaxation hole that has a larger diameter than the metering hole. Accordingly, the pressure on the molten polymer squeezed in the metering hole is relaxed and this prevents the threads from being caught by the dirt accumulated around the discharge hole, serving to reduce the variations in discharge. It also serves to decrease the rate of shear and stress in the molten polymer, thereby suppressing the generation of frictional heat and reducing dirt accumulation. In addition, the increase in the relaxation hole diameter causes an increase in the circumference relative to the discharge rate of the polymer, which serves to decrease the accumulated dirt density per unit circumferential length. As a result, the molten polymer is discharged stably to provide a polyamide monofilament with a highly uniform fiber diameter. It is more preferably 3.0 to 3.7.
For the spinneret used for the production process according to the present invention, the relaxation hole diameter (D2) is in the range of 0.8 to 1.4 mm. If the relaxation hole diameter (D2) is adjusted in this range, the molten polymer can be discharged stably, and this prevents inclination and twitching of the polymer from being caused by dirt accumulated around the discharge hole. On the other hand, as the discharge is stabilized, inferior discharge is suppressed to prevent the formation of knots and thin filaments, thereby ensuring the production of a polyamide monofilament that shows high filtration performance. In the case where the relaxation hole diameter (D2) is less than 0.8 mm, the accumulated dirt density per unit circumferential length will be large and it tends to cause dirt accumulation around the discharge hole that leads to inclination and twitching of the polymer. In addition, the effect of relaxing the pressure on the molten polymer is weakened and the strain rate of the molten polymer will increase. Accordingly, larger frictional heat will be generated to cause the formation of dirt around the discharge hole, leading to deterioration in discharge stability. In the case where the relaxation hole diameter (D2) is 1.4 mm or more, it will be difficult for the polymer to fill the entire discharge hole (relaxation hole in particular) uniformly, and variations in the discharge state will occur easily, leading to not only the formation of knots and thin filaments but also frequent breakage of the spun threads. It is more preferably 1.0 to 1.2 mm.
In addition, as the relaxation hole 4 is enlarged, the speed of discharge (shear rate) of the polymer from the polymer discharge hole will decrease, and therefore, it is desirable to optimize the spinning rate. When the spinning rate is high, a larger difference between the strain rate and the spinning rate will cause more irregular crystal orientations, leading to more frequent formation of knots and thin filaments. Thus, for the production method for a polyamide monofilament according to the present invention, the spinning rate is 300 to 1,000 m/min, preferably 300 to 600 m/min in a two stage process and 300 to 800 m/min in a single stage process. If it is controlled in this range, it will be possible to produce a polyamide monofilament containing few knots and thin filaments and having high uniformity in fiber diameter.

EXAMPLES

This invention is explained below more specifically with reference to examples. All physical property values adopted in the examples were measured using the methods described below.
A. Number of knots and thin filaments

(1) Mount a package on a creel.
(2) Unwind the thread at a speed of 800 m/min and pass it through an optical inspection device (PSD-200, manufactured by Sensoptic).
(3) Measure the thread diameter at intervals of 0.07 m along the thread. A measuring run lasts for 250 minutes.
(4) Knots with a diameter 135% or more as large as the fiber diameter One measurement that is larger by 35% or more than the normal fiber diameter (135% or more of the fiber diameter) is assumed to represent one knot.
(5) Knots with a diameter 120% or more and less than 135% as large as the fiber diameter One measurement that is larger by 20% or more and less than 35% than the normal fiber diameter (120% or more and less than 135% of the fiber diameter) is assumed to represent one knot.
(6) Thin filaments with a diameter 80% or less as large as the fiber diameter One measurement that is smaller by 20% or more than the normal fiber diameter (less than 80% of the fiber diameter) is assumed to represent one thin filament.
(7) Thin filaments with a diameter more than 80% and 90% or less as large as the fiber diameter

One measurement that is smaller by 10% or more and less than 20% than the normal fiber diameter (more than 80% and 90% or less of the fiber diameter) is assumed to represent one thin filament.

B. CV% of fiber diameter

(1) Mount a package on a creel.
(2) Unwind the thread at a speed of 800 m/min and pass it through an optical inspection device (PSD-200, manufactured by Sensoptic).
(3) Measure the thread diameter at intervals of 0.07 m along the thread. A measuring run lasts for 250 minutes.
(4) The standard deviation of the fiber diameter measurements and the average of the fiber diameter measurements were read, and the CV% in fiber diameter was calculated by the formula given below: $CV % in fiber diameter = (standard deviation of fiber diameter measurements) / (\begin{array}{l} average of \\ fiber diameter measurements \end{array}) \times 100$

C. Strength, elongation, and strength-elongation product

A total of 50 measurements were taken from a fiber sample using a Tensilon (registered trademark) tester manufactured by Orientec Co., Ltd. according to JIS L 1013 (2010) under the conditions of elongation at a constant rate, a clamp-to-clamp distance of 50 cm, and a tension speed of 50 cm/min. The tenacity was determined from the maximum tenacity on the tensile strength-elongation curve while the elongation was determined from the elongation at the maximum tenacity point. In regard to the strength, furthermore, the quotient of the maximum tenacity divided by the total fineness was adopted to represent the strength. The strength-elongation product was calculated by the formula given below. A total of 50 measurements were taken, and the smallest one and the average were determined. $Strength-elongation product = strength [cN / dtex] \times (1 + elongation [%] / 100)$

D. Coefficient of variation in opening size

Polyamide monofilaments were warped at 20 threads/mm using a warping machine and woven at 20 threads/mm (so as to form square openings) using a rapier loom. This test-woven roll was observed at a magnification of 1,000 times using a scanning electron microscope (ESEM-2700, manufactured by Nikon Corporation). A total of 20 openings were selected randomly and the distance between the fibers (the largest distance between fibers in each opening) was measured with an accuracy down to units of 0.1 µm. The coefficient of variation in opening size was calculated by the formula given below. $Coefficient of variation in opening size (%) = (\begin{array}{l} standard deviation of fiber-to-fiber distance \\ measurements \end{array}) / (average of fiber-to-fiber distance measurement) \times 100$
Here, a sample was rated as acceptable if its coefficient of variation in opening size was 3% or less, which is a value generally used as evaluation criteria for high precision filters.

[Example 1]

Fig. 3 shows the melt-spinning pack used. It has a spinneret 30, pressure plate 27, metal wire filter 26, sintered filter 25, and sand filter 24. The spinneret 30 has four discharge holes. As illustrated in Fig. 1, each discharge hole has an inflow hole 2, metering hole 3, and relaxation hole 4, wherein the metering hole diameter (D1) is 0.30 mm, the metering hole length (L1) 0.75 mm, the relaxation hole diameter (D2) 1.0 mm, and the metering hole length (L2) 1.0 mm. The sintered filter 25 is a sintered filter (with a thickness of 2 mm and a filtration accuracy of 40 µm) containing varied stainless steel short fibers each having a roughly polygonal cross section, a length of 1.0 to 3.0 mm, a converted diameter of 30 to 60 µm, and an aspect ratio of 10 to 100.
Nylon 66 chips with a 98% sulfuric acid relative viscosity of 2.8 was dried to an adjusted moisture content of 0.13% and melted at a melting temperature of 290°C. The discharge rate was adjusted so that the polymer discharge rate of the entire melt-spinning pack was 6.5 g/min, and the polymer was fed to the melt-spinning pack and discharged through a round-hole spinneret in which two discharge holes were arranged circumferentially. Then, from a heating device provided on the side face downstream of the spinneret, water vapor at 130°C was sent through a heated gas flow channel (not shown in Figures) to the spinneret at a rate of 150 mg/min per square centimeter of the spinneret area, followed by cooling the threads by an uniflow type chimney designed to blow air in one direction. The threads were divided into individual ones, supplied with a spinning oil solution from an oil supply guide at a deposition rate of 0.5%, and wound up at a spinning rate of 500 m/min. An unstretched thread was stretched 4.3 times by a stretching machine to provide a nylon 66 monofilament.
The resulting nylon 66 monofilament was evaluated in terms of the number of knots, fineness unevenness, strength-elongation product, and coefficient of variation in filter's opening size.
Results are shown in Table 1.

[Examples 2 to 6 and Comparative examples 1 and 2]

Except that the metering hole diameter (D1) 7, metering hole length (L1) 6, relaxation hole diameter (D2) 9, and relaxation hole length (L2) 8 illustrated in Fig. 1 were changed as specified in Table 1, spinning and stretching were carried out by the same procedure as in Example 1 to provide a nylon 66 monofilament. Results are given in Table 1.

[Table 1]

item		Example 1	Example 2	Example 3	Example 4	Example 5	Example 6	Comparative example 1	Comparative example 2
		2-stage	2-stage	2-stage	2-stage	2-stage	2-stage	2-stage	2-stage
spinneret discharge hole	L1 [µm]	0.75	0.60	1.00	0.75	0.75	0.75	0.75	0.75
	D1 [µm]	0.30	0.30	0.30	0.30	0.30	0.30	0.30	0.30
	L2 [µm]	1.0	1.0	1.0	1.0	1.0	1.0	1.0	1.0
	D2 [µm]	1.0	1.0	1.0	0.8	1.2	1.3	0.7	1.5
	L1/D1	2.5	2.0	3.3	2.5	2.5	2.5	2.5	2.5
	D2/D1	3.3	3.3	3.3	2.7	4.0	4.3	2.3	5.0
spinning rate [m/min]		500	500	500	500	500	500	500	500
fineness [dtex]		8	8	8	8	8	8	8	8
number of knots [number/200,000 m]	knots with diameter 135% or more as large as fiber diameter	0	0	0	1	0	1	2	10
number of knots [number/200,000 m]	knots with diameter 120% or more and less than 135% as large as fiber diameter	0	6	1	10	8	16	25	49
number of thin filaments [number/200,000 m]	thin filaments with diameter 80% or less as large as fiber diameter	0	1	0	0	1	1	3	13
number of thin filaments [number/200,000 m]	thin filaments with diameter more than 80% and 90% or less as large as fiber diameter	0	5	3	6	7	14	13	20
fineness unevenness in length direction	CV% in fiber diameter	0.50	0.79	0.65	0.89	0.98	1.10	1.20	1.50
strength-elongation product	average	8.6	8.5	8.6	8.6	8.5	8.5	8.7	8.5
	minimum value	8.2	7.9	8.0	7.9	7.8	7.7	7.8	7.6
	0.9 × average	7.7	7.7	7.7	7.7	7.7	7.7	7.8	7.7
coefficient of variation in opening size [%]		1.6	2.0	1.8	2.6	2.7	3.0	3.1	3.5

[Example 7]

Except that the spinning rate was changed as specified in Table 2, spinning and stretching were carried out by the same procedure as in Example 1 to provide a nylon 66 monofilament. Results are given in Table 2.

[Example 8]

Except for using a spinneret having two discharge holes, adjusting the discharge rate so that the polymer discharge rate of the entire melt-spinning pack was 15 g/min, and changing the metering hole diameter (D1) 7, metering hole length (L1) 6, relaxation hole diameter (D2) 9, and relaxation hole length (L2) 8 as specified in Table 2, spinning and stretching were carried out by the same procedure as in Example 1 to provide a nylon 66 monofilament. Results are given in Table 2.

[Example 9]

Except for using a spinning apparatus as illustrated in Fig. 2 wherein the thread was taken up by the first godet roller 14 at a spinning rate of 760 m/min, stretched 4.1 times between the second godet roller 15 and the third godet roller 16, heat-treated at 170°C by the third and fourth godet rollers (16 and 17), and wound up by the wind-up device 18 at 3,000 m/min, the same procedure as in Example 1 was carried out to provide a nylon 66 monofilament. Results are given in Table 2.

[Comparative example 3]

Except that the discharge hole used had only an inflow hole 2 and a metering hole 3 wherein the metering hole diameter (D1) and metering hole length (L1) were as specified in Table 2, spinning and stretching were carried out by the same procedure as in Example 1 to provide a nylon 66 monofilament. Results are given in Table 2.

[Table 2]

item		Example 7	Example 8	Example 9	Comparative example 3
		2-stage	2-stage	1 step	2-stage
spinneret discharge hole	L1 [µm]	0.75	1.25	0.75	9.5
	D1 [µm]	0.30	0.50	0.30	0.55
	L2 [µm]	1.0	1.5	1.0	-
	D2 [µm]	1.0	1.5	1.0	-
	L1/D1	2.5	2.5	2.5	17.3
	D2/D1	3.3	3.0	3.3	-
spinning rate [m/min]		800	360	760	500
fineness [dtex]		8	47	8	8
number of knots [number/200,000 m]	knots with diameter 135% or more as large as fiber diameter	1	0	0	16
number of knots [number/200,000 m]	knots with diameter 120% or more and less than 135% as large as fiber diameter	8	0	1	56
number of thin filaments [number/200,000 m]	thin filaments with diameter 80% or less as large as fiber diameter	1	0	0	16
number of thin filaments [number/200,000 m]	thin filaments with diameter more than 80% and 90% or less as large as fiber diameter	10	0	0	23
fineness unevenness in length direction	CV% in fiber diameter	1.06	0.67	0.44	1.70
strength-elongation product	average	8.6	7.9	8.9	8.5
	smallest value	7.8	7.4	8.4	7.5
	0.9×average	7.7	7.1	8.0	7.7
coefficient of variation in opening size [%]		2.8	1.2	1.4	3.5

EXPLANATION OF NUMERALS

1: discharge hole
2: inflow hole
3: metering hole
4: relaxation hole
5: inflow hole diameter
6: metering hole length (L1)
7: metering hole diameter (D1)
8: relaxation hole length (L2)
9: relaxation hole diameter (D2)
10: melt-spinning pack
11: spinneret for melt-spinning
12: chimney
13: oil supply guide
14: first godet roller
15: second godet roller
16: third godet roller
17: fourth godet roller
18: wind-up device
19: polymer feeding port
20: upper pack block
21: intermediate pack block
22: lower pack block
23: protruding stage
24: sand filter medium
25: sintered filter
26: metal wire filter
27: pressure plate
28: polymer passage hole
29: packing
30: spinneret
31: discharge hole

Claims

A polyamide monofilament comprising no more than one knot with a fiber diameter of 135% or more of the fiber diameter, and no more than one thin filament with a fiber diameter of 80% or less of the fiber diameter, existing in the fiber longitudinal direction of 200,000 m.
The polyamide monofilament as set forth in claim 1 having a fiber diameter CV% of 1% or less as measured over a 200,000 m section in the fiber length direction.
The polyamide monofilament as set forth in either claim 1 or 2 comprising no more than ten knots with a fiber diameter of 120% or more and less than 135%, and no more than ten thin filaments with a fiber diameter of more than 80% and 90% or less of the diameter of the fiber, existing in the fiber longitudinal direction of 200,000 m.
The polyamide monofilament as set forth in any one of claims 1 to 3, wherein the smallest of 50 measurements of the strength-elongation product taken continuously is 90% or more and 100% or less of the average.
The polyamide monofilament as set forth in any one of claims 1 to 4 having a fineness of 6 to 50 dtex.