High-strength polyethylene fiber

Abstract

The present invention addresses the problem of providing: an ultrahigh-molecular-weight polyethylene multifilament having a high degree of crystalline orientation and a uniform crystal structure; and a method for manufacturing the ultrahigh-molecular-weight polyethylene multifilament. The present invention provides an ultrahigh-molecular-weight polyethylene multifilament comprising two or more single yarns, wherein: the longitudinal direction of the single yarns is defined as the vertical direction; a direction orthogonal to the vertical direction is defined as the horizontal direction; one side surface of the single yarns is irradiated with an X-ray from the horizontal direction, and then the irradiation location is moved in the horizontal direction to continuously irradiate a portion extending to the side surface on the opposite side with the X-ray, such that diffraction peak strength derived from the (110) plane of an orthorhombic crystal are measured at intervals of 1.0 μm; and degrees of crystalline orientation calculated on the basis of the equation below are 0.990-0.999 at all measurement points.　Degree of crystal orientation = (180-H) / 180. In the equation, H represents the full width at half maximum (°) of the diffraction peak derived from the (110) plane of the orthorhombic crystal.

Description

High-strength polyethylene fiber

[0001] This invention relates to ultra-high molecular weight polyethylene multifilaments and a method for producing the same.

[0002] Ultra-high molecular weight polyethylene (UHMWPE) is widely used as a high-performance fiber, leveraging its properties such as high strength, high modulus of elasticity, and excellent impact resistance, which stem from its extremely high molecular weight. Specifically, it is used in high-performance textiles such as sportswear, bulletproof and protective clothing, and protective gloves; various rope products such as tag ropes, mooring ropes, yacht ropes, and construction ropes; braided products such as fishing lines and blind cables; and net products such as fishing nets and ball-catching nets. Furthermore, its application is progressing in chemical filters, battery separators, tent membrane materials, sports equipment such as skis and helmets, and composite materials such as speaker cones, prepregs, and concrete reinforcements.

[0003] In particular, gel spinning is known as a method for increasing the strength and modulus of ultra-high molecular weight polyethylene fibers. Gel spinning is a manufacturing method in which UHMWPE is dissolved in a solvent to form a solution, this solution is extruded from a nozzle, rapidly cooled to form a gel-like fiber body, and then the solvent is removed from this gel-like fiber body while it is stretched at a high magnification. With this method, stretching at a high magnification is possible with less entanglement of molecular chains, which promotes the orientation and crystallization of molecular chains, and as a result, fibers with high strength and high modulus can be obtained.

[0004] In gel spinning, reducing the polyethylene concentration in the solution further reduces molecular chain entanglement, enabling stretching at extremely high draw ratios. As a result, multifilaments with fewer defects and a highly oriented crystalline structure can be obtained. It is known that fibers obtained by this technology can be made even stronger by increasing the draw ratio (for example, Patent Documents 1, 2, and 3).

[0005] Patent No. 4565324, Patent No. 4524644, Patent No. 5696809

[0006] However, even using the manufacturing methods disclosed in these prior art documents, it is still difficult to obtain ultra-high molecular weight polyethylene multifilaments with sufficient strength, and further improvements are needed. While the use of ultra-high molecular weight polyethylene can relatively reduce the number of defects caused by the molecular chain ends, the entanglement between molecular chains also increases with increasing molecular weight. Therefore, it was necessary to lower the polyethylene concentration and relatively increase the organic solvent concentration in the spinning and drawing processes.

[0007] Furthermore, increasing the concentration of organic solvents in the spinning solution requires a longer solvent removal process before the drawing process. As a result, the fibers are drawn without sufficient solvent removal during the drawing process, making them prone to breakage. This makes it difficult to increase the drawing ratio and obtain high-strength multifilaments. Moreover, excessively high organic solvent concentrations cause fusion between single filaments during the solvent removal process, degrading the quality of the multifilaments. This fusion also leads to uneven cross-sectional shape of the multifilaments, causing not only a decrease in strength but also problems such as poor fiber opening when used in impact-resistant components.

[0008] The present invention has been made in view of the above circumstances, and its object is to provide an ultra-high molecular weight polyethylene multifilament having a high degree of crystal orientation and a uniform crystal structure, and a method for producing the same.

[0009] [1] A polyethylene multifilament composed of two or more single filaments, wherein the longitudinal direction of the single filament is defined as the vertical direction, the direction perpendicular to this vertical direction is defined as the horizontal direction, X-rays are irradiated from the horizontal direction onto one side of the single filament, and then the irradiation position is moved horizontally to continuously irradiate the opposite side with X-rays, and the diffraction peak intensity originating from the (110) plane of the ortholombic crystal is measured at 1.0 μm intervals, and the degree of crystal orientation calculated based on the following formula is 0.990 or more and 0.999 or less at all measurement points. Degree of crystal orientation = (180 - H) / 180 In the formula, H represents the full width at half maximum (°) of the diffraction peak originating from the (110) plane of the ortholombic crystal. [2] The polyethylene multifilament according to [1], wherein among all the measurement points, there is a measurement point where the maximum value of the degree of crystal orientation is 0.992 or more. [3] The polyethylene multifilament according to [1] or [2], wherein the difference between the maximum and minimum values ​​of the degree of crystal orientation among all the measurement points is 0.003 or less. [4] The polyethylene multifilament according to any one of [1] to [3], wherein the ratio of the diffraction peak intensity derived from the (200) plane to the diffraction peak intensity derived from the (110) plane of the ortholombic crystal is 0.50 or less. [5] The polyethylene multifilament according to any one of [1] to [4], wherein the crystal size of the (110) plane of the ortholombic crystal is 200 Å or more. [6] The polyethylene multifilament according to any one of [1] to [5], wherein the fineness of the single filament is 0.4 dtex or more and 40 dtex or less. [7] The polyethylene multifilament according to any one of [1] to [6], wherein the tensile strength is 20 cN / dtex or more and the initial modulus of elasticity is 600 cN / dtex or more.

[0010] [8] A fiber processed member comprising polyethylene multifilament according to any one of [1] to [7]. [9] The fiber processed member according to [8], wherein the fiber processed member is at least one selected from the group consisting of fishing line, rope, net, and glove.

[10] An impact-resistant member comprising polyethylene multifilament according to any one of [1] to [7].

[11] The impact-resistant member according to

[10] , wherein the impact-resistant member is at least one selected from the group consisting of unidirectional reinforcing material (UD material), protective material, and bulletproof sheet.

[12] The impact-resistant member according to

[11] , wherein the unidirectional reinforcing material (UD material) is formed by aligning single threads according to [1] in one direction while embedded in resin.

[13] A suture comprising polyethylene multifilament according to any one of [1] to [7].

[14] An interdental brush comprising polyethylene multifilament according to any one of [1] to [7].

[0011]

[15] A method for producing polyethylene multifilament according to any one of [1] to [7], comprising: a dissolution step of dissolving polyethylene in a solvent to prepare a polyethylene solution, wherein polyethylene has an intrinsic viscosity [η] of 10.0 dL / g or more and 40.0 dL / g or less, the repeating units consist substantially of ethylene units, and constituent units derived from monomers other than ethylene amount to 5.0 mol% or less per total monomer unit; a spinning step of discharging the polyethylene solution from a nozzle set to a temperature above the melting point of the polyethylene, and cooling the discharged yarn with a refrigerant of 0°C or more and 60°C or less to obtain an undrawn yarn; and a drawing step of drawing the undrawn yarn while removing the solvent, wherein the number of draws in the drawing step is 1 or more and 5 or less, and the amount of residual solvent in the yarn after the second draw is 3000 ppm or less.

[0012] According to the present invention, it is possible to provide polyethylene multifilaments having a high degree of crystal orientation and a uniform crystal structure, and a method for producing the same. According to one embodiment of the present invention, it is possible to provide a multifilament having high strength.

[0013] This figure shows a schematic diagram of the method for measuring the degree of crystal orientation of a monofilament according to the present invention.

[0014] The inventors considered that in order to achieve further increases in the strength of polyethylene multifilaments, it is necessary to improve the uniformity of the crystal orientation within each monofilament constituting the multifilament and suppress variations in strength between and within the monofilaments. To investigate the degree of crystal orientation, the inventors utilized the large synchrotron radiation facility SPring-8 and used microbeam X-ray diffraction to analyze in detail the crystal structure on the surface of polyethylene monofilaments with a diameter of approximately 5 to 50 μm in the direction perpendicular to the longitudinal direction (horizontal direction). Specifically, as shown in Figure 1, X-rays were irradiated from the horizontal direction (perpendicular to the vertical direction of the monofilament) onto one side of the monofilament, and then the irradiation position was moved horizontally to continuously irradiate the opposite side with X-rays, and the diffraction peak intensities originating from the (110) plane and (200) plane of the ortholombic crystal at each measurement point (1.0 μm interval) were measured. As a result, we discovered that by gel spinning ultra-high molecular weight polyethylene as a low-concentration solution, and then subjecting the resulting undrawn yarn to high-magnification stretching under specific conditions, the (110) plane crystal structure can be highly oriented, and the uniformity of that orientation can be enhanced. Furthermore, we found that the polyethylene multifilament obtained in this way, which has a highly oriented and uniform crystal structure, exhibits high strength, thus completing the present invention.

[0015] The present invention provides a polyethylene multifilament (hereinafter referred to as "multifilament") composed of a plurality of single filaments, wherein the longitudinal direction of the single filament is defined as the vertical direction, the direction perpendicular to this vertical direction is defined as the horizontal direction, X-rays are irradiated from the horizontal direction onto one side of the single filament, and then the irradiation position is moved horizontally to continuously irradiate the opposite side with X-rays, and the diffraction peak intensity is measured at 1.0 μm intervals, and the crystal orientation of the (110) plane of the ortholhombic crystal (hereinafter sometimes referred to as "(110) plane crystal orientation") calculated based on the following formula: Crystal orientation = (180 - H) / 180 (wherein H represents the full width at half maximum (°) of the diffraction peak originating from the (110) plane of the ortholhombic crystal) is 0.990 or more and 0.999 or less at all measurement points.

[0016] The (110)-plane crystal orientation of each single filament constituting the multifilament of the present invention is 0.990 or higher at all measurement points, indicating a high degree of orientation. Furthermore, since the (110)-plane crystal orientation at all measurement points is within the range of 0.990 to 0.999, the variation in crystal orientation within the single filament is extremely small, and the crystal structure is uniformly highly oriented. The multifilament of the present invention and its manufacturing method will be described in detail below.

[0017] Polyethylene The polymer constituting the multifilament according to the present invention is polyethylene whose repeating units are substantially composed of ethylene. So-called ultra-high molecular weight polyethylene (UHMWPE) is preferred. A homopolymer consisting solely of ethylene is preferred, exhibiting excellent mechanical properties. Alternatively, polyethylene may be a copolymer of ethylene and a small amount of other monomers, as long as the objectives of the present invention can be achieved. Specific examples of other monomers include α-olefins, acrylic acid and its derivatives, methacrylic acid and its derivatives, vinylsilane and its derivatives. Specific examples of polyethylene usable in the present invention include polyethylene composed of an ethylene homopolymer, blends of copolymers of ethylene and other monomers (e.g., α-olefins), blends of homopolyethylene and ethylene-based copolymers, and blends of homopolyethylene and other homopolymers such as α-olefins. These polyethylenes may have a partially crosslinked structure, or they may have a partially branched structure such as methyl branching, ethyl branching, or butyl branching. In particular, polyethylene copolymers with α-olefins such as propylene and 1-butene, which contain short-chain or long-chain branches at a rate of less than 20 per 1000 carbon atoms, can also be suitably used in the production of multifilaments according to the present invention. The introduction of moderate branching contributes to improving production stability, especially in the spinning and drawing processes. However, if the amount of branching exceeds 20 per 1000 carbon atoms, the molecular chains become excessively entangled, which is undesirable as it leads to a decrease in spinnability and drawability. Excessive content of monomers other than ethylene also inhibits crystallization and makes high-magnification drawing difficult. Therefore, the content of monomers other than ethylene is preferably 5.0 mol% or less per total monomer unit, more preferably 1.0 mol% or less, and even more preferably 0.2 mol% or less. Most preferably is a homopolymer of ethylene that does not contain monomers other than ethylene.

[0018] Polyethylene may be a polyethylene composition containing various additives as described below, if necessary.

[0019] Furthermore, the polyethylene may be a blend of polyethylenes with different number-average molecular weights or weight-average molecular weights, or a blend of polyethylenes with different molecular weight distributions (Mw / Mn), as long as the intrinsic viscosity [η] is within the predetermined range described later. These blends are also effective in obtaining the desired physical properties and processability.

[0020] Weight-average molecular weight (Mw) As the polyethylene used in the production of multifilaments according to the present invention (hereinafter sometimes referred to as raw material polyethylene), ultra-high molecular weight polyethylene (UHMWPE) is preferred. The weight-average molecular weight (Mw) of polyethylene is not particularly limited, but is preferably 1,260,000 or more and 8,400,000 or less, more preferably 1,400,000 or more and 8,000,000 or less, and even more preferably 1,500,000 or more and 7,000,000 or less. If the weight-average molecular weight (Mw) is less than 1,260,000, the strength and elastic modulus of the resulting multifilament may not be sufficient even after the drawing process described later. In addition, during the spinning and drawing processes, the polyethylene concentration in the solution decreases relatively, and as a result, yarn breakage is more likely to occur in the water tank or drawing oven, making stable production difficult. This is presumably because a decrease in weight-average molecular weight (Mw) increases the number of molecular chain ends per unit cross-sectional area of ​​the multifilament, and these act as structural defects. On the other hand, if the weight-average molecular weight (Mw) exceeds 8,400,000, the tension in the drawing process increases excessively, leading to frequent yarn breakage and a significant decrease in productivity, which is undesirable.

[0021] Calculation of Weight-Average Molecular Weight (Mw) In this invention, the weight-average molecular weight (Mw) is calculated from the value of the intrinsic viscosity [η] measured by the method described later, using the following formula disclosed in "POLYMER HANDBOOK, Fourth Edition J. Brandrup and E.H. Immer, A JOHN WILEY & SONS, Inc. Publication 1999," instead of using gel permeation chromatography (GPC) measurement. Weight-average molecular weight = 5.365 × 10 ４ × (limiting viscosity) １．３７

[0022] Intrinsic viscosity [η] The intrinsic viscosity [η] of the raw polyethylene is preferably 10.0 dL / g or more, more preferably 15.0 dL / g or more. On the other hand, the upper limit is preferably 40.0 dL / g or less, more preferably 38.0 dL / g or less, and even more preferably 36.0 dL / g or less. That is, the intrinsic viscosity [η] of the raw polyethylene is preferably 10.0 dL / g to 40.0 dL / g, more preferably 15.0 dL / g to 38.0 dL / g, and even more preferably 12.0 dL / g to 36.0 dL / g. If the intrinsic viscosity [η] is less than 10.0 dL / g, the strength of the resulting multifilament may be insufficient. On the other hand, if the intrinsic viscosity [η] exceeds 40.0 dL / g, the melt viscosity in processing steps such as spinning and drawing will increase significantly, which may greatly reduce workability and productivity. Therefore, it is preferable to set the intrinsic viscosity [η] within the above range.

[0023] Intrinsic viscosity [η] of multifilament In one embodiment, the intrinsic viscosity [η] of the multifilament itself is preferably 10.0 dL / g or more and 40.0 dL / g or less. Here, the intrinsic viscosity [η] of the multifilament refers to the intrinsic viscosity [η] of the polyethylene single filaments constituting the multifilament. By setting the intrinsic viscosity [η] of the multifilament to 10.0 dL / g or more, high strength and high modulus of elasticity can be imparted to the multifilament. On the other hand, by setting the intrinsic viscosity [η] of the multifilament to 40.0 dL / g or less, the increase in melt viscosity during processing steps such as spinning and drawing can be suppressed, thereby improving workability and productivity. From these viewpoints, the intrinsic viscosity [η] of the multifilament is more preferably 12.0 dL / g or more and 38.0 dL / g or less, even more preferably 15.0 dL / g or more and 36.0 dL / g or less, and particularly preferably 18.0 dL / g or more and 35.0 dL / g or less.

[0024] Single filament fineness The fineness of the single filament constituting the multifilament according to the present invention is preferably 0.4 dtex or more and 40 dtex or less, more preferably 0.5 dtex or more and 30 dtex or less, and even more preferably 0.6 dtex or more and 20 dtex or less. If the single filament fineness is 0.4 dtex or more, it is possible to stably obtain a high-strength multifilament. On the other hand, if the single filament fineness exceeds 40 dtex, the strength of the obtained multifilament tends to decrease, which is undesirable.

[0025] The total fineness of the multifilament according to the present invention is preferably 5 dtex or more and 7000 dtex or less, more preferably 10 dtex or more and 5000 dtex or less, and even more preferably 40 dtex or more and 3000 dtex or less. If the total fineness is 10 dtex or more, it is possible to stably obtain a high-strength multifilament. On the other hand, if the total fineness exceeds 7000 dtex, the strength of the obtained multifilament tends to decrease, which is undesirable.

[0026] In one embodiment, it is preferable that some fusion exists between the individual filaments constituting the multifilament according to the present invention. In the manufacturing process of the multifilament according to the present invention, some fusion may occur between the individual filaments. However, the inventors' studies suggest that such slight fusion does not substantially adversely affect the overall strength of the multifilament. On the other hand, if excessive fusion occurs, the uniformity of the multifilament may be impaired, which may adversely affect, for example, the ability to open fibers when used as an impact-resistant member, or the strength and modulus of elasticity of the final product. Therefore, it is desirable to appropriately control the degree of fusion by removing solvents or controlling the stretching conditions in the manufacturing process.

[0027] The multifilament according to the present invention is composed of two or more single filaments. The number of single filaments is preferably 10 or more, more preferably 15 or more.

[0028] Crystal Structure of Single Filaments Each single filament constituting the multifilament according to the present invention has a highly oriented, uniform crystal structure, as the (110) plane crystal orientation degree is 0.990 or more and 0.999 or less at all measurement points measured by X-ray. In one embodiment, the maximum value of the (110) plane crystal orientation degree at all measurement points is preferably 0.992 or more, more preferably 0.993 or more, even more preferably 0.994 or more, and preferably 0.997 or less. The higher the crystal orientation degree, the more aligned the orientation of the molecular chains in the direction of the fiber axis, and the stronger the bonds between the molecular chains, thus contributing to improved strength. A higher (110) plane crystal orientation degree is preferable, but as an industrially feasible range, the upper limit may be 0.997 or less. The crystal structure, including the crystal orientation degree, the peak intensity ratio of the (200) plane and the (110) plane, and the crystal size in the present invention shall be carried out according to the conditions and methods described in the examples. In this invention, the measurement method is described assuming that the cross-section of the single filament is circular. However, if the cross-section of the single filament is non-circular, such as elliptical, the length (diameter) of the line segment connecting two points on the outer circumference of the single filament on the measurement surface that are at their maximum distance from each other should be measured using X-rays.

[0029] In one embodiment, it is preferable that the degree of (110) plane crystal orientation at all measurement points is within the above range (0.990 to 0.999) because the variation in the degree of crystal orientation within the single filament is extremely small and the crystal structure is uniformly highly oriented. Specifically, the difference between the maximum and minimum values ​​of the degree of (110) plane crystal orientation at the measurement surface (hereinafter sometimes referred to as the "degree of orientation difference") is preferably 0.003 or less, more preferably 0.002 or less. A smaller degree of orientation difference is preferable, but as a range that can be manufactured industrially, the lower limit may be around 0.001.

[0030] In one embodiment, the diffraction peak intensity (I) originating from the (110) plane of the same ortholombic crystal on the measurement surface. １１０ ) Diffraction peak intensity (I ２００ ) ratio (hereinafter referred to as "peak intensity ratio") ２００ / I １１０The peak intensity ratio (determined by) is preferably 0.5 or less, more preferably 0.46 or less, even more preferably 0.40 or less, particularly preferably 0.33 or less, and most preferably 0.32 or less at all measurement points. If the peak intensity ratio is 0.5 or less, it indicates that the molecular chains are highly oriented in the longitudinal direction of the monofilament and are uniformly oriented, suppressing disorder and non-uniformity of orientation, and resulting in a structure with few defects. Monofilaments that satisfy such a peak intensity ratio contribute to increased strength.

[0031] In one embodiment, the difference between the maximum and minimum values ​​of the peak intensity ratio on the measurement surface (hereinafter referred to as "peak intensity ratio difference") is preferably 0.30 or less, and more preferably 0.20 or less. A smaller peak intensity ratio difference indicates higher uniformity of crystal orientation within the single filament and smaller variation in strength within the single filament, thus contributing to improved strength. While a smaller peak intensity ratio difference is preferable, the lower limit may be around 0.001, as this is within the range of industrially feasible manufacturing.

[0032] In one embodiment, the crystal size of the (110) plane of the ortholombic crystal on the measurement surface is preferably 15 nm (150 Å) or larger, more preferably 20 nm (200 Å) or larger, even more preferably 23 nm (230 Å) or larger, particularly preferably 25 nm (250 Å) or larger, and most preferably 25.6 nm (256 Å) or larger. The larger the crystal size, the higher the degree of crystal orientation, and the relatively fewer defects and grain boundaries there are, thus suppressing stress concentration. On the other hand, if the crystal size becomes excessively large, it may cause embrittlement, so it is preferably 40 nm (400 Å) or smaller, more preferably 35 nm (350 Å) or smaller, and even more preferably 30 nm (300 Å) or smaller. That is, the crystal size of the (110) plane is preferably 15 nm to 40 nm, more preferably 20 nm to 5 nm, even more preferably 23 nm to 30 nm, particularly preferably 25 nm to 30 nm, and most preferably 25.6 nm to 30 nm.

[0033] Tensile Strength The multifilament according to the present invention has a tensile strength of 20 cN / dtex or more, preferably 49.0 cN / dtex or more, more preferably 50.0 cN / dtex or more, even more preferably 52.0 cN / dtex or more, even more preferably 54.0 cN / dtex or more, particularly preferably 54.8 cN / dtex or more, and most preferably 55.2 cN / dtex or more. The higher the tensile strength, the more feasible it becomes to develop applications for high-strength materials that were not previously possible. There is no particular upper limit to the tensile strength, but a higher value is preferable. However, from the viewpoint of manufacturing in industrial production, a realistic upper limit of about 85 cN / dtex may be acceptable. The specific method and conditions for measuring the tensile strength are as described in the examples.

[0034] Initial Modulus of Elasticity The multifilament according to the present invention has an initial modulus of elasticity of 600 cN / dtex or higher. Preferably, the initial modulus of elasticity is 1700 cN / dtex or higher, more preferably 1750 cN / dtex or higher, and even more preferably 1800 cN / dtex or higher. A high initial modulus of elasticity exhibits excellent shock absorption, easily dispersing energy even when subjected to instantaneous impact, making it suitable for protective materials and bulletproof applications. It also contributes to improved large deformability, making it less prone to deformation when external forces are applied and providing excellent dimensional stability. However, from the viewpoint of manufacturing in industrial production, a realistic upper limit may be around 2500 cN / dtex.

[0035] Manufacturing Method The multifilament according to the present invention is preferably manufactured using a gel spinning method. Specifically, the manufacturing method of the multifilament according to the present invention includes the following steps: a dissolution step of preparing a polyethylene solution by dissolving polyethylene having a predetermined intrinsic viscosity [η] in a solvent; a spinning step of discharging the polyethylene solution from a nozzle set to a temperature above the melting point of the polyethylene and cooling the discharged yarn with a refrigerant at 10°C to 60°C; a stretching step of stretching the unstretched yarn obtained in the spinning step while removing the solvent; and a winding step of winding the stretched yarn obtained in the stretching step at a temperature of 50°C or lower and under tension conditions of 5 cN / dtex or lower.

[0036] In the dissolution step, polyethylene having a predetermined intrinsic viscosity [η] is dissolved in a solvent to prepare a polyethylene solution. The intrinsic viscosity [η] of the polyethylene used in this step is preferably 10.0 dL / g or more and 40.0 dL / g or less, more preferably 15.0 dL / g or more and 38.0 dL / g or less, even more preferably 20.0 dL / g or more and 36.0 dL / g or less, particularly preferably 25.0 dL / g or more and 36.0 dL / g or less, and most preferably 30.0 dL / g or more and 36.0 dL / g or less. The intrinsic viscosity [η] is preferably adjusted considering the polyethylene concentration described below. For example, if the polyethylene concentration is low, it is desirable to adjust the intrinsic viscosity to be high to prevent thread breakage. Furthermore, since polyethylene is composed of repeating units that are substantially ethylene units, other monomers other than ethylene may be copolymerized to improve properties such as strength and heat resistance. However, the content of other monomers shall be 5.0 mol% or less, more preferably 1.0 mol% or less, and even more preferably 0.2 mol% or less, in monomer units. Here, "monomer units" refers to structural units derived from each monomer that constitutes the polymer. If the content of monomers other than ethylene is excessive, it will inhibit crystallization and become a factor that makes high-magnification stretching difficult. Suitable solvents to use are volatile organic solvents such as decalin and tetralin, or solvents that are solid at room temperature or non-volatile. The polyethylene concentration in the prepared polyethylene solution is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less. The optimal polyethylene concentration needs to be determined considering the intrinsic viscosity [η] of the polyethylene used, the stretching ratio, and the residual solvent in the intermediate yarn. In particular, when using raw materials with high intrinsic viscosity [η], it is preferable to appropriately adjust the polyethylene concentration and appropriately control the entanglement of molecular chains in the raw material. In one embodiment, it is preferable to appropriately control the intrinsic viscosity [η] of polyethylene and the polyethylene concentration in the polyethylene solution, as this reduces the amount of residual solvent in the intermediate yarn and enables stretching at high magnification.

[0037] Various known methods can be used to prepare the polyethylene solution. For example, methods using a twin-screw extruder or methods in which solid polyethylene is suspended in a solvent and then stirred at a high temperature can be used. Preferred mixing conditions are a temperature of 150°C to 200°C and a time of 1 minute to 80 minutes. The mixing temperature is more preferably 160°C to 190°C. The mixing time is more preferably 5 minutes to 60 minutes, and even more preferably 10 minutes to 50 minutes. If the mixing time is less than 1 minute, the dissolution of polyethylene may be insufficient, and a uniform solution may not be obtained. On the other hand, if the mixing time exceeds 80 minutes, the cleavage and crosslinking reactions of polyethylene molecules may proceed excessively, potentially deviating from the range suitable for spinning. As a result, even if a multifilament containing at least five single filaments with a single filament fineness of 3 dtex or more is produced, it becomes difficult to obtain a high-strength multifilament. In this invention, the "range suitable for spinning" refers, for example, to a spinning speed of 10 m / min or more, and a spinning tension of 0.01 cN or more and 300 cN or less per single filament. Depending on the molecular weight and concentration of the polymer, mixing at a temperature exceeding 200°C may be necessary, but the mixing time in the temperature range exceeding 200°C is preferably 30 minutes or less, more preferably 20 minutes or less, and even more preferably 10 minutes or less. If the mixing time in the temperature range exceeding 200°C exceeds 30 minutes, the cleavage and crosslinking reactions of polyethylene molecules proceed excessively, and as a result, even if a multifilament containing at least five single filaments with a single filament fineness of 3 dtex or more is produced, it becomes difficult to obtain a high-strength multifilament.

[0038] Spinning Process The polyethylene solution prepared in the above dissolution process is extruded using an extruder at a temperature preferably 10°C or more, more preferably 20°C or more, and even more preferably 30°C or more higher than the melting point of the polyethylene. Then, it is supplied to a spinneret (spinning nozzle) using a metering device. The time for the polyethylene solution to pass through the orifice of the spinneret is preferably 1 second or more and 8 minutes or less. If the orifice passage time is less than 1 second, turbulence occurs in the flow of the polyethylene solution, making stable discharge difficult, and as a result, the crystal structure within the cross-section of the resulting single filament may become non-uniform. On the other hand, if the orifice passage time exceeds 8 minutes, the polyethylene molecules are discharged with insufficient orientation within the orifice, making it easier for the spinning tension to deviate from the above suitable range. As a result, the crystal structure of the resulting single filament becomes non-uniform, and it becomes difficult to obtain high-strength multifilaments, which is not preferable.

[0039] The polyethylene solution is discharged through a spinneret in which a plurality of orifices are arranged, and a plurality of filaments are formed. At this time, the temperature of the spinneret is at least the melting point of polyethylene or higher, preferably 140°C or higher, and more preferably 150°C or higher. Note that the melting point of polyethylene depends on the type of solvent used, the concentration of the polyethylene solution, and the molecular weight of the polyethylene. The temperature of the spinneret needs to be set within a range that does not exceed the thermal decomposition temperature of polyethylene.

[0040] The polyethylene solution is preferably discharged from a spinneret having orifices with a diameter of 0.2 mm or more and 3.5 mm or less, more preferably 0.5 mm or more and 2.5 mm or less. The discharge amount is preferably 0.1 g / min or more per orifice. At this time, the temperature of the spinneret is preferably set 10°C or more higher than the melting point of polyethylene and lower than the boiling point of the solvent used. If the temperature of the spinneret is near the melting point of polyethylene, the melt viscosity of the polymer is too high, making it difficult to quickly take up the filaments after spinning. On the other hand, if the temperature of the spinneret is above the boiling point of the solvent, the solvent boils immediately after being discharged from the spinneret, resulting in frequent thread breaks, which is not preferable.

[0041] In one embodiment, it is necessary to provide two or more orifices in the spinneret so that the number of monofilaments constituting the multifilament is two or more. In particular, the number of orifices is preferably eight or more. In one embodiment, from the perspective of controlling the residual solvent amount of the intermediate yarn, it is desirable to adjust the total number of orifices (hole number) of the spinneret and the discharge amount from each orifice. Increasing the number of orifices increases the total surface area of the yarns discharged from the spinneret, making it easier for the solvent to volatilize. On the other hand, if the discharge amount is small even when the number of orifices is increased, the amount of solvent supplied to each monofilament may increase and the residual solvent amount may increase. Also, in one embodiment, for example, even if the number of orifices is increased, by increasing the concentration of the raw material polyethylene or appropriately controlling the draw ratio (especially a low ratio in the first stage), it becomes easier to retain the solvent, and the residual solvent amount of the intermediate yarn may be reduced.

[0042] On the surface of the spinneret (the surface on the discharge side of the polyethylene solution), the same number of pores (the ends of the orifices on the spinneret surface side) for discharging the polyethylene solution are formed as the number of orifices. At this time, it is preferable to make the discharge amount of the polyethylene solution from each pore as uniform as possible. For this purpose, it is important to reduce the temperature difference between the pores. Specifically, the difference between the highest temperature and the lowest temperature among the plurality of pores is preferably 10°C or less, more preferably 8°C or less. The method of suppressing the temperature difference between the pores is not particularly limited, but for example, it is effective to shield the spinneret so that it does not directly contact the outside air. Specifically, a method of shielding the spinneret from the outside air with a highly heat-insulating glass shielding plate can be mentioned. Furthermore, by making the difference between the distance between the shielding plate and the closest pore and the distance between the shielding plate and the farthest pore as small as possible, the difference between the highest temperature and the lowest temperature between the pores can be made smaller. With such a configuration, it is possible to equalize the temperature of each pore and thus achieve a uniform discharge amount.

[0043] The atmosphere in which the yarn is extruded from the pores of the spinneret and cooled by the coolant is not particularly limited, but an inert gas atmosphere is preferred. Examples of inert gases include nitrogen and helium. By using an inert gas atmosphere, oxidative degradation of the extruded yarn can be suppressed and stable manufacturing conditions can be ensured.

[0044] Next, the yarn extruded from the spinneret is preferably drawn up at a speed of 800 m / min or less, and more preferably at a speed of 200 m / min or less, while being cooled with a cooling medium. The temperature of the cooling medium is preferably 0°C to 60°C, and more preferably 12°C to 35°C. If the temperature of the cooling medium falls outside this range, the tensile strength of the resulting multifilament may decrease significantly as the single filament fineness increases, which is undesirable. The reason is as follows: In other words, even when the single filament fineness is large, in order to maintain high strength and high modulus, it is important to homogenize the crystal structure throughout the cross-section perpendicular to the longitudinal direction of the single filament. However, if the temperature of the cooling medium is too low, the cooling rate near the center of the cross-section of the single filament will be slower than the cooling rate near the outer surface of the single filament, and as a result, non-uniformity will occur in the crystal structure throughout the cross-section of the single filament. On the other hand, if the temperature of the cooling medium is too high, although the difference in cooling rate between the vicinity of the center and the vicinity of the outer surface of the single filament's cross-section becomes smaller, the time required for cooling increases, causing structural changes to occur at the undrawn stage. As a result, differences in crystal structure are more likely to occur between the vicinity of the center and the vicinity of the outer surface of the single filament's cross-section, which may lead to a decrease in the strength of the single filament and, consequently, a decrease in the strength of the entire multifilament. As the cooling medium, either a liquid that is miscible with the solvent used in the polyethylene dissolution solution or a liquid that is not miscible with said solvent can be used. For example, in the latter case, water can be used. Regardless of miscibility, by selecting an appropriate cooling medium, it is possible to improve the uniformity of the crystal structure throughout the cross-section of the single filament and manufacture a multifilament with high strength and high modulus of elasticity. In one embodiment, controlling the take-up speed of the filament is also preferable from the viewpoint of reducing residual solvent. Even if the take-up speed is within the above range, if the time the filament passes through the water-cooled bath is short, cooling and solvent removal may be insufficient.

[0045] The time required to remove the solvent from the filament after the cooling process should preferably be short, and it is desirable to remove the solvent as quickly as possible after cooling. Details of the solvent removal method will be described later, but the time required for solvent removal, that is, the time until the amount of residual solvent in the multifilament is 10% by mass or less, is preferably within 10 hours, more preferably within 2 hours, and even more preferably within 30 minutes. If the time required for solvent removal exceeds 10 hours, the difference in crystal structure formed between the vicinity of the center and the vicinity of the outer surface in a cross-section perpendicular to the longitudinal direction of the single filament becomes large, and the uniformity of the crystal structure throughout the cross-section of the single filament is easily impaired. As a result, the strength and elastic modulus of the resulting multifilament will decrease, so it is desirable to remove the solvent as quickly as possible.

[0046] The undrawn yarn obtained in the spinning process is subjected to the drawing process either continuously as is or after being wound up. In one embodiment, the drawing process of the present invention is a multi-stage drawing process that includes multiple stages (e.g., three stages) of drawing, in which the undrawn yarn is heated and drawn to a predetermined magnification at each drawing stage. In the present invention, it is particularly preferable to reduce the amount of residual solvent in the yarn after the second stage of drawing (intermediate yarn) to 3000 ppm or less, and then perform the third stage of drawing at a high magnification. By performing the final third stage of drawing at a high magnification, it is possible to obtain a multifilament with a high degree of crystal orientation and small variation in the degree of crystal orientation (i.e., having a uniform crystal structure). The drawing may be performed only once, or in multiple stages, and it is preferable to perform it in one to five stages. It is also possible to heat-dry the undrawn yarn and then perform one or more stages of drawing.

[0047] The following describes a preferred stretching method involving three stages, but the present invention is not limited to three stages, and the number of stretching stages can be changed as appropriate. For example, stretching may be performed only once, or in any number of stages, as long as the desired degree of crystal orientation can be obtained, for example, one to five times. Furthermore, stretching may be performed after heating and drying the unstretched yarn. In this embodiment, the stretching ratio for the first stage is preferably 1.1 to 5.0 times, more preferably 1.5 to 4.0 times, even more preferably 2.9 to 3.6 times, and particularly preferably 2.9 or 3.6 times. The stretching ratio for the second stage is adjusted as appropriate in relation to the stretching ratio for the first stage, but is preferably 1.1 to 10 times, more preferably 2 to 8 times, and even more preferably 2.2 to 5 times. The stretching ratio for the third stage is preferably high from the viewpoint of the degree of crystal orientation, strength, and productivity of the final multifilament (finished yarn). Specifically, the product of the stretching ratios of the first and second stages is considered and adjusted, preferably between 1.1 and 15 times, more preferably between 2 and 12 times, and even more preferably between 5 and 10 times. The product of the stretching ratios of the first and second stages is preferably between 2 and 30 times, more preferably between 4 and 20 times, and even more preferably between 7 and 10 times. By controlling within this range, the amount of residual solvent in the intermediate yarn can be effectively reduced. The product of the stretching ratios of the first, second, and third stages is preferably between 30 and 150 times, more preferably between 40 and 130 times, even more preferably between 50 and 110 times, and particularly preferably between 63 and 73 times. In one embodiment, it is preferable to adjust the stretching ratio considering the stretching temperature to prevent yarn breakage and to obtain a desired degree of crystal orientation. For example, if the drawing temperature is low, increasing the drawing ratio may cause the yarn to break. Conversely, if the drawing temperature remains low and the drawing ratio is set to prevent yarn breakage, the desired degree of crystal orientation and strength may not be obtained.

[0048] The drawing process can be carried out using various known methods, such as a method performed in a heated heat transfer atmosphere or a method using heated rollers. Examples of heat transfer materials include air, inert gases such as nitrogen, water vapor, or various liquid media. Appropriate selection of these drawing methods and heat transfer materials also contributes to controlling the crystal structure of the undrawn yarn and producing multifilaments with high strength and high modulus of elasticity.

[0049] In one embodiment, a step to remove the solvent from the undrawn yarn (desolvation step) may be provided prior to the drawing step. The desolvation step may be a method of removing the solvent while drawing the undrawn yarn, i.e., a method of performing desolvation and drawing simultaneously, or the desolvation step and the drawing step may be performed separately. When the polyethylene concentration of the polyethylene solution is 7% by mass or less, the drawing ratio in the desolvation step is preferably 1.1 times or more and 5.0 times or less, more preferably 1.5 times or more and 4.0 times or less, even more preferably 2.9 times or more and 3.6 times or less, and particularly preferably 2.9 times or 3.6 times. If the drawing ratio in the desolvation step is too low, the single filaments tend to fuse together during the desolvation step, which may reduce the quality of the resulting multifilament (finished yarn). Also, yarn breakage is more likely to occur when drawing in a subsequent step. On the other hand, if the drawing ratio is too high, the solvent removal will be insufficient, and as a result the amount of residual solvent in the intermediate yarn will increase, which is undesirable. The stretching ratio in the desolvation process referred to herein may include the first and second stretching stages in the stretching process described above. As for the method of removing the solvent, if the solvent used is volatile, a removal method by heating is suitable, and if the solvent used is non-volatile, an extraction method using an extractant is effective. Examples of extractants include chloroform, benzene, trichlorotrifluoroethane (TCTFE), hexane, heptane, nonane, decane, ethanol, and higher alcohols.

[0050] In this embodiment, the drawing of the undrawn yarn is preferably carried out by a multi-stage drawing process including multiple drawing steps, as described above. In particular, it is preferable to reduce the amount of residual solvent in the yarn (intermediate yarn) after the second drawing step (immediately before the third drawing step) to 3000 ppm or less, more preferably 2800 ppm or less, even more preferably 2000 ppm or less, even more preferably 1800 ppm or less, particularly preferably 600 ppm or less, and most preferably 440 ppm or less. If the amount of residual solvent in the intermediate yarn exceeds 3000 ppm, crystal orientation and oriented crystallization may be inhibited in the subsequent drawing steps (especially the third drawing step), which may result in the crystal orientation of the resulting multifilament being non-uniform or not sufficiently highly oriented. As a result, it may be difficult to obtain a multifilament having the desired strength and uniform crystal structure. For example, if the product of the stretching ratios of the first, second, and third stages (total stretching ratio) is 30 times or more, it is recommended to set the product of the stretching ratios of the first and second stages to 30 times or less, more preferably 27 times or less, and even more preferably 7 times or more and 10 times or less, in order to reduce the amount of residual solvent in the intermediate yarn to the above range. If the stretching ratio is set too high in the first and second stages of stretching, the amount of residual organic solvent tends to increase. When performing stretching in multiple stages, it is preferable to set the stretching temperature higher as the process progresses to the later stages. Specifically, the stretching temperature for the first stage (OV1) is preferably 80°C or more and 140°C or less, more preferably 100°C or more and 130°C or less, and even more preferably 120°C. The stretching temperature for the second stage (OV2) is preferably 100°C or more and 150°C or less, more preferably 120°C or more and 140°C or less, and even more preferably 135°C. The final, third-stage drawing temperature (OV3) is preferably 80°C to 160°C, more preferably 100°C to 160°C, even more preferably 130°C to 158°C, and particularly preferably 150°C to 158°C. For example, if the drawing temperature is too low, thread breakage may occur at high drawing ratios. In one embodiment, it is preferable to appropriately adjust the drawing ratio and drawing temperature in order to obtain a highly oriented multifilament having a uniform crystal structure.In the drawing process, it is preferable to set the conditions of the heating device (for example, the set temperature in the case of a hot air oven) so that the temperature of the yarn falls within the above temperature range. The temperature of the yarn during drawing can be measured and confirmed using, for example, an infrared camera (for example, FLIR SC640 manufactured by FLIR Systems). Performing such temperature control is also effective for forming an appropriate crystal structure in the drawing process and obtaining a high-strength multifilament.

[0051] The amount of residual organic solvent in the intermediate yarn can be calculated from the difference between the mass of the intermediate yarn before solvent extraction and the mass of the intermediate yarn after solvent extraction. Specifically, when the remaining organic solvent is volatile, the organic solvent can be removed by methods such as heating and drying in an oven or extraction using an appropriate chemical. On the other hand, when the remaining organic solvent is non-volatile, a method of extraction using an appropriate chemical that can dissolve the organic solvent is suitable. The type of chemical used for the extraction of the organic solvent is not particularly limited, and examples include methylene chloride.

[0052] In the present invention, the total drawing time in the drawing process is preferably 0.5 minutes or more and 20 minutes or less, more preferably 1 minute or more and 15 minutes or less, and even more preferably 2 minutes or more and 10 minutes or less. The total drawing time here means the sum of the drawing times of each stage when a multi-stage drawing process is included. If the drawing time exceeds 20 minutes, even if other manufacturing conditions are appropriate, relaxation of the molecular chains may occur during the drawing process, and as a result, the strength of the obtained multifilament may decrease, which is not preferable. On the other hand, although the lower limit of the drawing time is not particularly limited, if it is too short, uniform drawing may be difficult, so the above range is preferable. For example, in the case of three-stage drawing, the drawing time in each stage is preferably 0.2 minutes or more and 10 minutes or less, more preferably 0.5 minutes or more and 5 minutes or less.

[0053] The deformation speed of the multifilament in the drawing process is preferably 0.001 s －１ or more and 0.8 s －１ or less, more preferably 0.01 s －１ or more and 0.2 s －１The following conditions apply. If the deformation rate is too fast, the multifilament may break before the molecular chains are sufficiently oriented, which is undesirable. On the other hand, if the deformation rate is too slow, relaxation of the molecular chains occurs during the stretching process, making it difficult to obtain a high-strength multifilament, which is also undesirable. The deformation rate is calculated from the stretching ratio of the multifilament, the stretching rate, and the length of the stretched section using the following formula: Deformation rate (s) －１ ) = (Exit velocity of stretching - Inlet velocity of stretching) / Length of stretching section The above "length of stretching section" refers to the length of the portion that deforms due to stretching when the single yarn to be measured is fixed using a fixing jig so that its longitudinal direction (fiber axis direction) is parallel to the vertical direction and the horizontal cross-section is the measurement surface, for example, it corresponds to the length of the heating furnace or the distance between heating rollers.

[0054] In addition, in the manufacturing process of multifilaments according to the present invention, the following additives may be added as needed for the purpose of imparting various functions: antioxidants, light stabilizers, pH adjusters, surfactants, thickeners, humectants, dye enhancers, preservatives, fungicides, antistatic agents, pigments, inorganic fibers, other organic fibers, metal fibers, and chelating agents.

[0055] The multifilament according to the present invention can be used in a variety of applications, taking advantage of its excellent properties. Specifically, it can be used in protective woven or knitted fabrics that take advantage of its cut resistance, unidirectional reinforcing materials (UD materials), protective materials, bulletproof materials (bulletproof sheets), ropes, nets, tapes, fishing lines, material protective covers, sheets, kite threads, bowstrings, sailcloths, membrane materials, medical sutures, interdental brushes, artificial tendons, artificial muscles, reinforcing materials for fiber-reinforced resins, cement reinforcing materials, reinforcing materials for fiber-reinforced rubber, machine tool parts, battery separators, chemical filters, and other industrial materials. In particular, the multifilament according to the present invention is suitable for impact-resistant members, such as at least one impact-resistant member selected from the group consisting of unidirectional reinforcing materials (UD materials), protective materials, and bulletproof sheets; sutures; and interdental brushes. The unidirectional reinforcing material (UD material) is made by aligning single filaments constituting the multifilament according to the present invention, or multifilaments that have been opened according to the present invention, or yarns made by bundling multiple filaments, in one direction while embedded in a resin. Furthermore, in the case of multifilaments that have been opened according to the present invention, it is preferable that some of them are fused. For example, in the opened state, separation between individual filaments progresses, so if there is appropriate fusion (local bonding), the cooperation and cohesion between fibers will improve, and as a result it will lead to an improvement in mechanical properties (for example, tensile strength and rigidity), which is preferable.

[0056] This application claims the benefit of priority under Japanese Application No. 2025-017446, filed on 5 February 2025. The entire specification of Japanese Application No. 2025-017446 is incorporated herein by reference.

[0057] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples, and can be implemented with appropriate modifications within the scope that is consistent with the spirit of the preceding and following descriptions, and all such modifications are included within the technical scope of the present invention.

[0058] The characteristic values ​​of the multifilament and monofilament in this embodiment were measured using the following method.

[0059] (1) Intrinsic viscosity [η] The intrinsic viscosity [η] was measured using an Ubbelohde viscometer with decalin as the solvent under conditions of 135°C. Specifically, first, the polyethylene sample to be measured was shredded to a length of approximately 5 mm, and 1% by mass of BHT (trade name "Yoshinox® BHT", manufactured by API Corporation) was added to the sample as an antioxidant. Next, this mixture was added to decalin heated to 135°C and dissolved by stirring for 4 hours to prepare several dilute solutions of different concentrations. The specific viscosity of each dilute solution was measured using an Ubbelohde viscometer. The specific viscosity at each concentration was plotted against the concentration, and a straight line was obtained by applying the least squares method to the obtained plot. The intercept obtained by extrapolating this straight line to zero concentration was taken as the intrinsic viscosity [η] (dL / g).

[0060] (2) Crystal orientation of monofilaments To evaluate the crystal structure inside each monofilament, the crystal size and crystal orientation at the measurement surface were measured by microbeam X-ray diffraction on a cross section perpendicular to the longitudinal direction of each monofilament. The BL03 hutch of the large synchrotron radiation facility SPring-8 was used as the X-ray source, and the wavelength of the X-rays was set to λ = 0.12 nm (= 1.2 Å). The size of the X-ray beam used for measurement was adjusted so that the distance between the two furthest points located on the outer circumference of the X-ray cross section was 7 μm or less. As shown in Figure 1, the monofilament (1a), which is the sample to be measured, was fixed using a fixing jig (1b) installed on the XYZ stage (1c) so that its longitudinal direction was perpendicular to the X-rays and perpendicular to the incident X-rays (horizontal direction). Next, the stage was moved slightly horizontally to align it so that the center of the monofilament on the measurement surface, that is, the center of the distance between the two furthest points located on the outer circumference of the cross section, was at the center of the X-ray beam. Because the beam of SPring-8 is extremely high-intensity, the exposure time for X-ray diffraction measurements was limited to 30 seconds or less to prevent damage to the sample. X-rays were irradiated horizontally onto one side of a single filament (1a), and then the irradiation position (1d) was moved horizontally (in the figure, (A) → (B) → (C)) to continuously irradiate the opposite side of the single filament, and the diffraction peak intensity was measured at 1.0 μm intervals. The fiber diameter of the single filament was measured in advance using a microscope, and the part with the highest intensity in the acquired diffraction pattern was considered the center of the single filament, and the radial range on both sides from there was analyzed as the "single filament portion". A charge-integrating SOI detector SOPHIAS, placed 67 mm away from the sample, was used to acquire the X-ray diffraction pattern. From the obtained X-ray diffraction pattern, the diffraction peak originating from the (110) plane of the ortholombic crystal at each measurement point was identified, and the full width at half maximum H (°) ​​was determined from the profile of the diffraction peak, and the degree of crystal orientation was calculated using the following formula. In the formula, the degree of crystal orientation = (180 - H) / 180, where H represents the full width at half maximum (°) of the diffraction peak originating from the (110) plane of the ortholombic crystal. In this invention, the degree of crystal orientation at multiple measurement points in a cross section perpendicular to the longitudinal direction was quantitatively evaluated for each single filament.

[0061] (3) Peak intensity ratio within monofilament Similar to (2) above, the BL03 beamline of the large synchrotron radiation facility SPring-8 was used as the X-ray source, and for each monofilament, the cross section perpendicular to its longitudinal direction was used as the measurement plane, and X-ray diffraction images were obtained at multiple measurement points on the measurement plane using the measurement method and measurement conditions described in (3) above. From the obtained X-ray diffraction images, the diffraction profile in the equatorial direction at each measurement point was analyzed, and the peak intensity I110 of the diffraction peak originating from the (110) plane of the ortholombic crystal and the peak intensity I200 of the diffraction peak originating from the (200) plane of the ortholombic crystal were determined, respectively, and the peak intensity ratio at each measurement point was calculated using the following formula: Peak intensity ratio = I200 / I110

[0062] (4) Crystal size inside monofilaments Similar to (2) above, the BL03 beamline of the large synchrotron radiation facility SPring-8 was used as the X-ray source, and for each monofilament, the cross section perpendicular to its longitudinal direction was used as the measurement plane, and X-ray diffraction images were obtained at multiple measurement points on the measurement plane using the measurement method and measurement conditions described in (3) above. From the obtained X-ray diffraction images, the diffraction peak originating from the (110) plane of the ortholombic crystal was identified for the equatorial diffraction profile at each measurement point, and its full width at half maximum β was determined. The crystal size (ACS) of the (110) plane of the ortholombic crystal was calculated using Scherrer's equation shown below. The identification of diffraction peaks followed Bunn et al. (Trans. Faday Soc., 35, 482 (1939)). In the formula ACS = Kλ / βcosθ, K is Scherrer's constant (K = 0.9 in this specification), λ is the wavelength of the X-rays used (λ = 0.12 nm), θ is half the diffraction angle of the diffraction peak originating from the (110) plane of the ortholhombic crystal (Bragg angle), and β is the full width at half maximum (in radians) of the diffraction peak originating from the (110) plane of the ortholhombic crystal.

[0063] (5) Tensile Strength (Finished Yarn Strength) The tensile strength of the multifilament was measured in accordance with JIS L 1013 8.5.1. Specifically, using the "Tensilon Universal Material Testing Machine RTF-1310" manufactured by Orientec Co., Ltd., a tensile test was performed with a sample chuck length of 200 mm and an elongation rate of 100 mm / min, under ambient temperature of 20°C and relative humidity of 65%, to obtain a strain-stress curve. The tensile strength and elongation at fracture were determined from the stress and elongation at the fracture point in the obtained strain-stress curve, respectively. The initial modulus of elasticity was calculated from the slope of the tangent line that gives the maximum slope near the origin of the strain-stress curve. For the measurement, the value of 1 / 10 of the mass (g) per 10,000 m of the multifilament was calculated, and this value was applied to the sample as the initial load. The tensile strength, elongation at fracture, and initial modulus of elasticity were each measured 10 times, and the average value was taken as the measured value.

[0064] (6) Fineness The fineness of the single filaments constituting the multifilament was determined as follows. First, single filaments 20 cm in length were taken from five randomly selected locations on the multifilament. Next, the mass of each taken single filament was measured using a precision balance. The average of the five obtained mass values ​​was calculated, and this value was used as the basis to calculate the mass (g) per 10,000 m of single filament, which was then defined as the fineness (dtex) of the single filament.

[0065] (7) Residual solvent in intermediate yarn (ppm) 3 to 7 g of the obtained intermediate yarn was taken, and the amount of residual solvent in the intermediate yarn was calculated from the weight of the intermediate yarn before and after solvent removal using the following formula: Amount of residual solvent in intermediate yarn = (Weight before drying - Weight after drying) / Weight before drying If the solvent used for gel spinning was volatile, the intermediate yarn was placed in a vacuum drying oven at 70°C for 15 hours, and then the weight of the intermediate yarn after vacuum drying was measured. The amount of residual solvent in the intermediate yarn was calculated from the weight obtained before and after drying using the following formula. If the solvent used for gel spinning was non-volatile, it may also be calculated by measuring the weight before and after extraction using an extraction solvent.

[0066] (Example 1) A dispersion with a UHMWPE concentration of 4.0% by mass was prepared using ultra-high molecular weight polyethylene (hereinafter sometimes referred to as "UHMWPE") with an intrinsic viscosity [η] of 31.5 dL / g and decalin (solvent). The obtained dispersion was supplied to a twin-screw extruder and dissolved by a residence time of 8 minutes in a temperature range of 200°C or higher. The material was extruded from a spindle set to a spinning temperature of 180°C at an extrusion rate of 4.8 g / min per single hole. The spindle was provided with 384 orifice holes with a diameter of 0.8 mm. The yarn (UHMWPE solution) extruded from the spindle was taken up at a speed of 39 m / min while being cooled in a water-cooled bath (cooling medium) set to 20°C to obtain an undrawn multifilament consisting of 24 single filaments. Next, the undrawn multifilament was first drawn to a ratio of 2.9 in an oven (OV1) with circulating hot air at 120°C, and then further drawn in an oven (OV2) with circulating hot air at 135°C, adjusting so that the product of the first and second drawing ratios was 8 times. After the second drawing, all the threads obtained from the 16 nozzles were combined and wound up as an intermediate thread. The residual decalin content in this intermediate thread was 440 ppm. Next, this intermediate thread was drawn in an oven (OV3) with circulating hot air at 150°C, adjusting so that the product of the first, second, and third drawing ratios was 63 times. It was then immediately wound up in the drawn state, and a multifilament (finished thread) was obtained that was drawn a total of 63 times through three drawing processes. The crystal size of the (110) plane of the single filaments constituting the obtained multifilament (finished yarn) was measured according to the measurement method described above, and the average value was 259 Å. Furthermore, the minimum value of the degree of crystal orientation was 0.992, and the maximum value of the peak intensity ratio was 0.46.

[0067] (Example 2) An intermediate yarn, which had been stretched up to the second stage in the same manner as in Example 1, was stretched in an oven (OV3) with circulating hot air at 150°C so that the product of the stretching ratios of the first, second, and third stages was 73 times, thereby obtaining a multifilament (finished yarn). The crystal size of the (110) plane of the single filaments constituting the obtained multifilament (finished yarn) was measured, and the average value was 258 Å. In addition, the minimum value of the degree of crystal orientation was 0.992, and the maximum value of the peak intensity ratio was 0.32.

[0068] (Example 3) When the first stage of stretching was set to 3.6 times as in Example 1, the amount of residual solvent (residual decalin) in the intermediate yarn increased to 1800 ppm as the stretching ratio increased. Subsequently, as in Example 1, stretching was performed so that the product of the stretching ratios of the first, second, and third stages was 63 times, and a multifilament (finished yarn) was obtained. The crystal size of the (110) plane of the single filaments constituting the obtained multifilament (finished yarn) was measured, and the average value was 268 Å. In addition, the minimum value of the degree of crystal orientation was 0.994, and the maximum value of the peak intensity ratio was 0.15.

[0069] (Example 4) The extrusion rate per single hole from the spinneret was changed to 4.1 g / min, and a multifilament (finished yarn) was obtained in the same manner as in Example 3. In order to equalize the fineness of the single filaments of the obtained multifilament (finished yarn), the product of the draw ratios of the first, second, and third stages was adjusted to 53 times. The crystal size of the (110) plane of the single filaments constituting the obtained multifilament (finished yarn) was measured, and the average value was 270 Å. The minimum value of the degree of crystal orientation was 0.993, and the maximum value of the peak intensity ratio was 0.33.

[0070] (Example 5) Spinning and drawing were performed with a total of 768 Ophiris holes, a UHMWPE concentration of 5.0% by mass, and a single-hole extrusion rate of 3.2 g / min / hole to obtain multifilaments (finished yarn). Increasing the UHMWPE concentration made it easier to retain the solvent, and the residual solvent amount (residual decalin amount) of the intermediate yarn was 2800 ppm. The crystal size of the (110) plane of the single filaments constituting the obtained multifilaments (finished yarn) was measured, and the average value was 258 Å. Furthermore, the minimum value of the degree of crystal orientation was 0.993, and the maximum value of the peak intensity ratio was 0.31.

[0071] (Example 6) In Example 5, the first-stage stretching ratio was set to 1.5 times, and stretching was performed. As a result, an intermediate yarn with a residual solvent content (residual decalin content) of 600 ppm was obtained. Subsequently, stretching was performed in the same manner as in Example 5 to obtain a multifilament (finished yarn). The crystal size of the (110) plane of the single filaments constituting the obtained multifilament (finished yarn) was measured, and the average value was 256 Å. Furthermore, the minimum value of the degree of crystal orientation was 0.993, and the maximum value of the peak intensity ratio was 0.26.

[0072] (Comparative Example 1) The intermediate yarn obtained in Example 1 was used as the multifilament (finished yarn) as is. That is, the third drawing step in Example 1 was not performed, and the intermediate yarn was used as the finished yarn. The crystal size of the (110) plane of the single yarn constituting the obtained multifilament (finished yarn) was measured according to the measurement method described above, and the average value was 197 Å. In addition, the minimum value of the degree of crystal orientation was 0.987, and the maximum value of the peak intensity ratio was 0.30.

[0073] (Comparative Example 2) The intermediate yarn obtained in Example 6 was used as is as the multifilament (finished yarn). That is, the third drawing step in Example 6 was not performed, and the intermediate yarn was used as the finished yarn. The crystal size of the (110) plane of the single filaments constituting the obtained multifilament (finished yarn) was measured according to the measurement method described above, and the average value was 186 Å. In addition, the minimum value of the degree of crystal orientation was 0.987, and the maximum value of the peak intensity ratio was 0.29.

[0074] (Comparative Example 3) Based on the conditions of Example 1, a spinneret with 768 orifice holes was prepared, the extrusion rate per hole was set to 3.3 g / min, and the extruded yarn was passed through a water-cooled bath at a speed of 45 m / min to cool it. The first and second drawing temperatures were carried out under the same conditions as in Example 1, and the product of the drawing ratios of the first and second stages was adjusted to a total of 7 times. As a result, the residual solvent amount (residual decalin amount) of the intermediate yarn was 3550 ppm. Subsequently, a third drawing was performed, and the product of the drawing ratios of the first, second, and third stages was adjusted to a total of 37 times to obtain a multifilament (finished yarn). The crystal size of the (110) plane of the single filaments constituting the obtained multifilament (finished yarn) was measured according to the measurement method described above, and the average value was 234 Å. In addition, the minimum value of the degree of crystal orientation was 0.983, and the maximum value of the peak intensity ratio was 0.57.

[0075] (Comparative Example 4) Based on the conditions of Example 4, the spinneret was changed to one with 768 orifice holes, and a multifilament (finished yarn) was obtained in the same manner as in Example 4. The residual solvent amount (residual decalin amount) of the intermediate yarn was 10,200 ppm. The crystal size of the (110) plane of the single filaments constituting the obtained multifilament (finished yarn) was measured according to the measurement method described above, and the average value was 233 Å. In addition, the minimum value of the degree of crystal orientation was 0.988, and the maximum value of the peak intensity ratio was 0.55.

[0076] (Comparative Example 5) Based on the conditions of Example 4, the spinneret was changed to one with 768 orifice holes, and the yarn was wound from a water-cooled bath at a speed of 45 m / min. The first stage draw ratio was set to 4.0 times, and a multifilament (finished yarn) was obtained in the same manner as in Example 4. The residual solvent amount (residual decalin amount) of the intermediate yarn was 15,000 ppm. The crystal size of the (110) plane of the single filaments constituting the obtained multifilament (finished yarn) was measured according to the measurement method described above, and the average value was 241 Å. The minimum value of the degree of crystal orientation was 0.989, and the maximum value of the peak intensity ratio was 1.00.

[0077] (Comparative Example 6) For the intermediate yarn obtained in Example 1, the third drawing step was set to a drawing temperature of 147°C (temperature of OV3), and a multifilament (finished yarn) was obtained in the same manner as in Example 1. Because the drawing temperature was set low, yarn breakage occurred at the initial three-stage drawing ratio (product of the drawing ratios of the first to third stages, 63 times). Therefore, the product of the drawing ratios of the first, second, and third stages was changed to 52 times to obtain a multifilament (finished yarn). The crystal size of the (110) plane of the single filaments constituting the obtained multifilament (finished yarn) was measured according to the measurement method described above, and the average value was 253 Å. In addition, the minimum value of the degree of crystal orientation was 0.989, and the maximum value of the peak intensity ratio was 0.51.

[0078] (Comparative Example 7) Based on the conditions of Example 1, a dispersion of UHMWPE with an intrinsic viscosity [η] of 20.0 dL / g and decalin was prepared to a UHMWPE concentration of 4.0% by mass, and spinning was attempted. However, because the molecular weight of UHMWPE was small and the entanglement in the gel was insufficient, yarn breakage occurred frequently in the water tank during the spinning process and in the ovens (OV1, OV2, OV3) during the drawing process, and intermediate yarn and multifilament (finished yarn) could not be obtained.

[0079] (Comparative Example 8) Based on the conditions of Example 1, the UHMWPE concentration was changed to 7.0% by mass, and spinning and drawing were attempted in the same manner as in Example 1. However, the high UHMWPE concentration resulted in excessive entanglement in the gel, making it difficult to increase the drawing ratio in the third stage. The residual solvent amount (residual decalin amount) of the intermediate yarn was 4500 ppm. Finally, the product of the drawing ratios of the first, second, and third stages was set to 40 times to obtain a multifilament (finished yarn). The crystal size of the (110) plane of the single filaments constituting the obtained multifilament (finished yarn) was measured according to the measurement method described above, and the average value was 223 Å. In addition, the minimum value of the degree of crystal orientation was 0.987, and the maximum value of the peak intensity ratio was 0.41.

[0080]

[0081]

[0082] The following observations were made from the results of the examples and comparative examples. In Examples 1 to 6, UHMWPE with a high intrinsic viscosity [η] of 31.5 dL / g was used, and the UHMWPE concentration was set relatively low at 4.0% by mass or 5.0% by mass. Under these conditions, by controlling the amount of residual solvent in the intermediate yarn to 3000 ppm or less, it became possible to perform the third stage of drawing at a high magnification. As a result, the degree of crystal orientation was 0.990 or higher at all measurement points, and the difference between the maximum and minimum values ​​of the degree of crystal orientation was 0.003 or less. This is presumed to be because, by using ultra-high molecular weight polyethylene with a high intrinsic viscosity [η], the entanglement of molecular chains was appropriately formed, improving spinning stability under low concentration conditions, and in addition, by controlling the amount of residual solvent, uniform orientation of molecular chains in the drawing process was promoted. In particular, the low maximum peak intensity ratio of 0.46 or less in Examples 1 to 6 supports the finding of a highly uniform crystal structure.

[0083] Next, in Comparative Examples 1 and 2, the intermediate yarns obtained in Examples 1 and 6 were used directly as the finished yarns, and the third stage of stretching was not performed. As a result, the average crystal size was smaller than in the examples (197 Å, 186 Å), and the minimum value of the degree of crystal orientation was less than 0.990 (0.987), failing to achieve the range defined in the present invention. In Comparative Examples 1 and 2, the desired degree of crystal orientation could not be obtained due to insufficient stretching.

[0084] In Comparative Examples 3, 4, and 5, the residual solvent content in the intermediate yarn exceeded 3000 ppm (3550 ppm, 10200 ppm, and 15000 ppm, respectively). As a result, the minimum value of the degree of crystal orientation was less than 0.990 (0.983 to 0.989), and the maximum value of the peak intensity ratio was 0.55 to 1.00, which was significantly higher than in the Examples. In particular, Comparative Example 5 had an extremely high maximum peak intensity ratio of 1.00, indicating a non-uniform crystal structure. In these comparative examples, although the number of orifices was increased, the amount of solvent dispensed from the orifices was small, which increased the amount of solvent supplied to each single yarn and thus increased the amount of residual solvent.

[0085] In Comparative Example 6, the third-stage stretching temperature was set to 147°C, lower than the 150°C used in Example 1. As a result, the stretching ratio in the third stage did not increase sufficiently, and the minimum value of the degree of crystal orientation was 0.989, not reaching 0.990. Furthermore, the maximum value of the peak intensity ratio was also higher at 0.51 compared to the example. In Comparative Example 6, yarn breakage occurred at high stretching ratios due to the low stretching temperature. When the stretching ratio was changed to one that did not cause yarn breakage, a finished yarn was obtained, but the desired crystal orientation was not achieved.

[0086] In Comparative Example 7, spinning was attempted using UHMWPE with an intrinsic viscosity [η] of 20.0 dL / g at a UHMWPE concentration of 4.0% by mass. However, due to the small molecular weight and insufficient entanglement of molecular chains in the gel, yarn breakage occurred frequently during the spinning and drawing processes, making it impossible to obtain intermediate yarns and multifilaments.

[0087] In Comparative Example 8, the same UHMWPE with an intrinsic viscosity [η] of 31.5 dL / g as in Example 1 was used, and the UHMWPE concentration was set high at 7.0% by mass. As a result, excessive entanglement of molecular chains occurred in the gel, and the third-stage stretching ratio could not be sufficiently increased to obtain the finished yarn. In addition, the amount of residual solvent in the intermediate yarn was also high at 4500 ppm. The obtained multifilament had a low minimum crystal orientation of 0.987, failing to achieve the range defined in this disclosure.

[0088] According to the present invention, it is possible to provide a multifilament with high strength using high molecular weight polyethylene. In a preferred embodiment, it is possible to provide a high-strength multifilament in which the high molecular weight polyethylene is highly oriented and the variation in the degree of crystal orientation within the single filament is small. The multifilament of the present invention has a tensile strength of at least 20 cN / dtex or higher. Furthermore, by using the manufacturing method of the present invention, it is possible to stably manufacture a multifilament with such excellent strength. The high-strength polyethylene multifilament according to the present invention can be applied to a wide range of applications, including fiber processed materials such as fishing lines, ropes, nets, and gloves; impact-resistant materials such as UD materials, protective materials, and bulletproof sheets; medical materials such as sutures and interdental brushes; and functional materials such as battery separators.

[0089] 1a Single thread 1b Fixing jig 1c Stage 1d X-ray irradiation position A, B, C X-ray spot H Horizontal V Vertical

Claims

A polyethylene multifilament composed of two or more single threads, The longitudinal direction of the single filament is defined as the vertical direction, and the direction perpendicular to this vertical direction is defined as the horizontal direction. X-rays are irradiated from the horizontal direction onto one side of the single filament, and then the irradiation position is moved horizontally to continuously irradiate the opposite side with X-rays, thereby measuring the diffraction peak intensity at 1.0 μm intervals. A polyethylene multifilament in which the degree of crystal orientation of the (110) plane of the ortholombic crystal, calculated based on the following formula, is between 0.990 and 0.999 at all measurement points. Crystal orientation degree = (180-H) / 180 In the formula, H represents the full width at half maximum (°) of the diffraction peak originating from the (110) plane of the ortholombic crystal. The polyethylene multifilament according to claim 1, wherein among all the aforementioned measurement points, there is a measurement point where the maximum value of the crystal orientation is 0.992 or greater. 　 The polyethylene multifilament according to claim 1, wherein the difference between the maximum and minimum values ​​of the crystal orientation among all the measurement points is 0.003 or less. 　 Furthermore, the polyethylene multifilament according to claim 1, wherein the ratio of the diffraction peak intensity originating from the (200) plane to the diffraction peak intensity originating from the (110) plane of the ortholombic crystal is 0.50 or less. 　 The polyethylene multifilament according to claim 1, wherein the crystal size of the (110) plane of the ortholombic crystal is 200 Å or more. 　 The polyethylene multifilament according to claim 1, wherein the fineness of the single yarn is 0.4 dtex or more and 40 dtex or less. 　 The polyethylene multifilament according to claim 1, wherein the tensile strength is 20 cN / dtex or more and the initial modulus of elasticity is 600 cN / dtex or more. 　 A fiber processing member comprising polyethylene multifilament according to any one of claims 1 to 7. 　 The fiber processing member according to claim 8, wherein the fiber processing member is at least one selected from the group consisting of fishing line, rope, net, and glove. 　 An impact-resistant member comprising polyethylene multifilament according to any one of claims 1 to 7. 　 The impact-resistant member according to claim 10, wherein the impact-resistant member is at least one selected from the group consisting of unidirectional reinforcing material (UD material), protective material, and bulletproof sheet. 　 The aforementioned unidirectional reinforcing material (UD material) is an impact-resistant member according to claim 11, wherein the single filaments described in claim 1 are embedded in a resin and aligned in one direction. 　 A suture comprising polyethylene multifilament according to any one of claims 1 to 7. 　 An interdental brush comprising polyethylene multifilaments according to any one of claims 1 to 7. 　 A method for producing polyethylene multifilaments according to any one of claims 1 to 7, A dissolution step to prepare a polyethylene solution by dissolving polyethylene in a solvent, wherein polyethylene has an intrinsic viscosity [η] of 10.0 dL / g or more and 40.0 dL / g or less, its repeating units consist substantially of ethylene units, and constituent units derived from monomers other than ethylene amount to 5.0 mol% or less per total monomer unit, A spinning process in which the polyethylene solution is discharged from a nozzle set to a temperature above the melting point of the polyethylene, and the discharged yarn is cooled with a refrigerant at 0°C to 60°C to obtain undrawn yarn, The process includes a stretching step in which the solvent is removed from the unstretched yarn while stretching it, A method for producing polyethylene multifilament, wherein the number of stretching steps in the stretching process is one to five, and the amount of residual solvent in the yarn after the second stretching is 3000 ppm or less.

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