PROCESS FOR PRODUCING AN ACRYLIC FIBER HAVING HIGH FIBER CHARACTERISTICS

Abstract: A process for producing an acrylic fiber comprising the steps of: (a) dissolving a polymer having a weight-average molecular weight of at least 5 × l0⁵ and containing at least 80 wt% acrylonitrile in a solvent comprising dimethylformamide or dimethylacetamide at a polymer concentration of 5-l5 wt% to provide a spinning solution having a viscosity of 500-l,500 poises at 45°C; (b) extruding the solution into a gaseous medium to form filaments; (c) introducing the filaments into a coagulation bath comprising water and at least one of dimethyl­ formamide and dimethylacetamide at 0°C or below; and (d) stretching the coagulated filaments to a draw ratio of at least 3 to provide an acrylic fiber having at least 90% X-ray orientation and an X-ray crystallinity parameter of up to l degree. The resulting acrylic fiber has high strength and high modulus.

Description: FIELD OF THE INVENTION The present invention relates to a process for producing an acrylic fiber of high orientation that is useful as a fiber in industrial materials such as ropes and tire cords, or as a reinforcing fiber in composite materials or as a precursor for the manufacture of high-performance carbon fibers. BACKGROUND OF THE INVENTION Acrylic fibers based on acrylonitrile have heretofore been used extensively in the apparel market because of their superior properties such as outstanding resistance to sunlight and dyeability. However, acrylic fibers are low in mechanical strength compared with other synthetic fibers such as nylon and polyesters and little use has been made of them in industrial materials. Therefore, many attempts are proposed to produce acrylic fibers that have mechanical characteristics that would make them suitable for use in industrial materials. Acrylic fibers can be oxidized and carbonized to make carbon fibers and because of their high strength and modulus, carbon fibers are receiving considerable attention as reinforcements in composite materials. Since the physical properties of carbon fibers are determined by the characteristics of the starting acrylic fibers, active efforts are also being made to modify them to make suitable precursors for carbon fibers. According to Japanese Patent Application (OPI) No. 61-152811(1986) (the term "OPI" as used herein means a "published unexamined Japanese patent application") or Japanese Patent Application (OPI) No. 61-6160(1986) (corresponding to European Patent Application 165,372A), which describes the result of such an attempt, a spinning solution of an acrylonitrile based polymer having an intrinsic viscosity of 2.5 to 3.6 is spun by the dryjet wet spinning process, and the resulting coagulated filaments are stretched to a draw ratio of 2 to 10 either after or during washing, the filaments being then dried, collapsed and stretched to a draw ratio of 1.2 to 5 at 180 to 240°C under the action of dry heat so as to produce an acrylic fiber having a strength of 11.9 cN/dtex (13.5 g/d) and a modulus of 207 cN/dtex (235 g/d). Another method for producing an acrylic fiber having improved mechanical characteristics has been proposed in Japanese Patent Application (OPI) No. 61-97415(1986) (corresponding to U.S. Patent 4,658,004) in which an acrylonitrile based polymer having a M w/ M n ratio of not more than 7.0 and a weight-average molecular weight ( M w) of at least 4 x 10⁵ is dissolved in a suitable solvent with defoaming under vacuum, and the resulting spinning solution is spun into filaments, which are then coagulated, stretched in multiple stages under progressively increasing temperatures, and dried under tension at temperatures of not higher than 130°C. According to this method, an acrylic fiber having a tensile strength of 16.6 cN/dtex (18.8 g/d) and a sonic modulus of 3.2 x 10⁴ N/mm² (3.2 x 10¹¹ dyn/cm²) is produced. Further, Japanese Patent Application (OPI) No. 59-199809(1984) (corresponding to U.S. Patents 4,535,027 and 4,659,529) describes a method for producing an acrylic fiber having high strength in which an acrylonitrile based polymer is dissolved in an aqueous solution of rhodanate and is spun. However, these methods have their own problems. First, the method described in Japanese Patent Application (OPI) No. 61-152811(1986) or No. 61-6160(1986) is applicable only to acrylic polymers having a very narrow range of molecular weight (an intrinsic viscosity of 2.5 to 3.6 is equivalent to a molecular weight of about 2.8 x 10⁵ to about 3.8 x 10⁵) and in order to achieve consistent spinning from a polymer having an intrinsic viscosity of more than 3.6, the polymer concentration of the spinning solution must be lowered to about 10 to 15 wt%. However, if a spinning solution having such a low polymer concentration is used, a large quantity of solvent must be removed after the formation of coagulated filaments and this increases the chance of the fiber structure becoming porous and opaque. In addition, if such a spinning solution is subjected to the dryjet wet spinning process which is considered to be most appropriate for the purpose of making high-strength acrylic fibers, individual single years are highly likely to fuse together during the stretching or drying step, thereby making it difficult to manufacture desired multifilaments. These problems associated with the increase in the molecular weight of acrylic polymers are specifically mentioned in Japanese Patent Application (OPI) No. 61-152811(1986). The method described in Japanese Patent Application (OPI) No. 61-97415(1986) starts with an acrylic polymer having a weight-average molecular weight of at least 4 x 10⁵. A polymer having such a high degree of polymerization can only be produced by performing aqueous suspension polymerization in the presence of a dispersion stabilizer such as polyvinyl alcohol. Furthermore, a solution of such a polymer is so viscous that considerable difficulty is involved in defoaming it. In addition, since ease of spinning is directly influenced by the viscosity of a polymer solution, the polymer concentration of the spinning solution used in Japanese Patent Application (OPI) No. 61-97415(1986) has to be lowered compared with ordinary spinning solutions, but then, as already mentioned, the decrease in polymer concentration causes various problems and is not considered an industrially feasible method in view of the fiber quality obtainable and the consistency of spinning operations. As a further problem, in order to have desired physical properties developable in the acrylic fiber prepared from such a spinning solution of low polymer concentration, the fiber must be stretched to a very high draw ratio. As shown in Japanese Patent Application (OPI) No. 61-97415(1986), a draw ratio as high as 36 is necessary in order to produce an acrylic fiber having a tensile strength of 15.2 cN/dtex (17.2 g/d). But stretching by such a high draw ratio generally causes unevenness in fiber fineness and only fibers of low quality will result. It is also well known that as acrylic fibers are stretched, fibrillation progresses and the acrylic fibers which are inherently low in wear resistance, will become even less wear-resistant. The acrylic fiber attained by the spinning technique proposed by Japanese Patent Application (OPI) No. 61-97415(1986) may develop comparatively satisfactory strength, but other mechanical properties such as tensile modulus and knot strength cannot be attained in a balanced way. Therefore, this acrylic fiber does not afford fiber characteristics as good as aromatic polyamide fibers, i.e., Aramid fibers which, as typified by Du Pont's "Kevlar" having a tensile strength of up to 17.7 cN/dtex (20 g/d), are highly adaptable for use as reinforcements in composite materials. The method as described in Japanese Patent Application (OPI) No. 59-199809(1984) employs spinning technique using an inorganic salt-containing aqueous solution such as an aqueous solution of rhodanate. In this case, it is necessary to remove inorganic impurities which cause deterioration in strength after spinning and stretching. Therefore, a complicated washing step is required and as a result, it is undesirable from an industrial point of view. Further, when this acrylic fiber is used for precursors to make carbon fibers, it is necessary to remove inorganic impurities completely since they have an adverse effect on the physical properties of carbon fibers. Therefore, this case also requires complicated washing step for produce satisfactory carbon fibers. US-A-3080210 discloses the preparation of a spinning solution containing 25% of a polymer consisting of acrylonitrile and vinylacetate. Fibers are spun from the spinning solution in a coagulation bath having a temperature of less than 0°C. The aforementioned techniques are addressed solely to enhancement of fiber strength and they are not concerned with efforts to improve the overall mechanical characteristics and morphology of acrylic fibers. As is well known, the physical properties of carbon fibers prepared by oxidizing and carbonizing acrylic fibers are closely related to the physical properties of the starting acrylic fiber and in consideration of changes in the chemical structure of high molecular chain during oxidizing and carbonizing, it would be more advantageous for the purpose of improving the physical properties of carbon fibers to improve the morphology of acrylic fibers, rather than their mechanical properties, i.e., strength and modulus. SUMMARY OF THE INVENTION Accordingly, one object of the present invention is a process for producing a high-strength acrylic fiber. Another object of the invention is a process for producing an acrylic fiber having a morphology preferable for use as a precursor in the manufacture of high-performance carbon fibers. As a result, it has now been found that these and other objects can be attained by using an acrylonitrile based polymer with a weight-average molecular weight of at least 5 x 10⁵ and by subjecting a solution of this polymer to a method of spinning that is compatible with polymers of high molecular weights. That is, the present invention relates to a process for producing an acrylic fiber having a strength of at least 8.83 cN/dtex (10 g/d), a modulus of at least 159 cN/dtex (180 g/d), at least 95% X-ray orientation, and an X-ray crystallinity parameter of up to 0.8 degree, comprising the steps of: (a) dissolving a polymer having a weight-average molecular weight of at least 5 x 10⁵ and containing at least 80 wt% acrylonitrile in a solvent comprising dimethylformamide or dimethylacetamide at a polymer concentration of 5-15 wt% to provide a spinning solution having a viscosity of 50-150 Pa·s (500-1,500 poises) at 45°C; (b) subjecting said spinning solution to dryjet wet spinning to spin into filaments, wherein the spinning nozzle has a nozzle hole diameter of at least 0.15 mm, the air gap between the nozzle face and coagulation bath surface is 3-20 mm, the coagulation bath comprises water and 70-85 wt% of at least one of dimethylformamide and dimethylacetamide and has a temperature of 0°C or below, and the spinning draft is in the range of 1.0-10.0; and (c) stretching the coagulated filaments to a draw ratio of at least 10. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph showing the composition of a spinning solution of a polymer/DMF(dimethylformamide)/water system, in which the preferred range is bounded by a triangle ABC; Fig. 2 is a graph showing the composition of a spinning solution of a polymer/DMAc(dimethylacetamide)/water system, in which the preferred range is also bounded by a triangle A'B'C'; DETAILED DESCRIPTION OF THE INVENTION In the present invention, it is essential to employ an acrylonitrile based polymer having a weight-average molecular weight of at least 5 x 10⁵. An acrylonitrile based polymer having a weight-average molecular weight of less than 5 x 10⁵ cannot be processed into an acrylic fiber having the desired morphology. If a high-strength acrylic fiber is to be attained, stretching by a draw ratio of at least 10, preferably at least 15, is necessary, but this is not possible with an acrylonitrile based polymer whose weight-average molecular weight is less than 5 x 10⁵. Furthermore, a spinning solution having a low-molecular weight acrylonitrile-based polymer dissolved in an organic solvent in high concentration has the disadvantage that it gels easily during spinning and in commercial operations, such gelling will cause undesirable complications such as filter clogging and filament breakage. The acrylonitrile based polymer having a weight-average molecular weight of at least 5 x 10⁵ may be prepared by any method that is capable of producing a polymer that satisfies various requirements for fiber formation. An acrylonitrile based polymer with a weight-average molecular weight of at least 5 x 10⁵ is generally difficult to make by solution polymerization. This method is also unsuitable in terms of production rate, since it provides only a slow polymerization rate. An acrylonitrile based polymer having a weight-average molecular weight of at least 5 x 10⁵ can be produced by suspension polymerization, but if a generally known method of suspension polymerization, which uses water as a polymerization medium is employed, a polymer that would be suitable for fiber formation cannot be attained. A preferred method for producing a high-molecular weight polymer that is suitable for fiber formation is described, for example, in Japanese Patent Application (OPI) No. 61-11310(1986); in this method, a mixture of 10-70 wt% acrylonitrile, 15-60 wt% organic solvent and 15-60 wt% water is polymerized in the presence of a radical initiator and further polymerized with water and/or an organic solvent added in an amount of 1-10 parts by weight per part by weight of the monomer. This method is capable of consistent and efficient production of the desired acrylonitrile based polymer. Illustrative organic solvents that can be used in this method include DMF (dimethylformamide), DMAc (dimethylacetamide), γ-butyrolactone, and DMSO (dimethyl sulfoxide). The composition of the acrylonitrile based polymer used as the starting material in the process of the present invention varies widely with the specific use of the acrylic fiber finally produced. If it is to be used as a high-performance carbon fiber precursor, a carboxylic acid based monomer such as methacrylic acid, acrylic acid or itaconic acid is preferably incorporated as a comonomer in an amount of 0.1-5.0 wt% for the purpose of controlling the rate of oxidization. If the amount of comonomer is less than 0.1 wt%, oxidation will not proceed efficiently unless very high temperatures are used, but in that case, monofilaments of acrylic fiber will fuse together to prevent the manufacture of the desired high-performance carbon fiber. If the comonomer content exceeds 5.0 wt%, not only is the chance of tar formation during oxidation increased, but also a low carbon yield results. If carboxylic acid based monomers are copolymerized with other monomers for the specific purpose of increasing the polymer solubility, the contents of such comonomers are also preferably no more than 5 wt% from the viewpoint of chemical carbon yield. Even if the acrylic fiber produced by the present invention is to be used as a high-strength acrylic fiber, the content of comonomers is preferably kept at no more than 20 wt% in order to retain the good properties of the fiber. If more than 20 wt% comonomers are used, such advantages of the acrylic fiber as high resistance to weather and alkalis will be impaired. Illustrative comonomers include unsaturated monomers such as methacrylic acid, methyl (meth)acrylate, ethyl (meth)acrylate, n-, i- or t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, α-chloroacrylonitrile, 2-hydroxyethyl acrylate, hydroxyethyl acrylate, hydroxyalkyl (meth)acrylate, vinyl chloride, vinylidene chloride, vinyl bromide, vinyl acetate, sodium methallylsulfonate, and sodium-p-sulfonyl phenyl methallyl ether. In addition to these unsaturated monomers, any other monomers that are capable of copolymerizing with acrylonitrile may be used, either independently or in combination. Of these monomers copolymerizable with acrylonitrile, methacrylic acid, methyl (meth)acrylate, vinyl chloride, vinyl acetate, sodium methallylsulfonate, and sodium-p-sulfonyl phenyl methallyl ether are preferably used. The acrylonitrile based polymer having a weight-average molecular weight of at least 5 x 10⁵ is then dissolved in DMF or DMAc to prepare a spinning solution. The polymer concentration of this spinning solution is so adjusted that its viscosity is within the range of 50 to 150 Pa·s (500 to 1,500 poises) at 45°C. If the viscosity of the spinning solution exceeds 150 Pa·s (1,500 poises) at 45°C, a very high pressure will be exerted on the spinning nozzle, spinning solution filter and other components of the spinning equipment so as to shorten their life. Filaments could be spun from a spinning solution of high viscosity if the temperature of the solution was raised, but in that case the solvent and the spinning solution are prone to deteriorate as a result of decomposition. On the other hand, a spinning solution having a viscosity of less than 50 Pa·s (500 poises) at 45°C has reduced spinnability and the coagulated filaments spun from this solution would have a deteriorated structure. In order to produce the acrylic fiber of the present invention, the spinning solution must be spun by the dryjet wet spinning process, and only when its viscosity is within the above-specified range can consistent spinning be accomplished without filament breakage or other difficulties, to produce an acrylic fiber of improved morphology. An acrylic fiber with improved morphology can also be attained using a spinning solution that contains water in addition to the acrylonitrile based polymer and an organic solvent. The use of this water-containing spinning solution has the following major advantages: 1. Compared with a spinning solution that is solely composed of the acrylonitrile based polymer and an organic solvent, the water-containing spinning solution is slow to coagulate and thereby facilitates the production of coagulated filaments having a desired cross-sectional shape; and 2. As the molecular weight of the acrylonitrile based polymer increases, the viscosity of the spinning solution increases too, which prevents consistent spinning, but by incorporating water in the spinning solution, its viscosity can be sufficiently reduced to enable consistent spinning even if the high-molecular weight polymer is present in the spinning solution. The spinning solution may contain any amount of water so long as its stability is ensured, and the range of water content that satisfies this condition varies with the ability of organic solvent to dissolve the acrylonitrile based polymer. Specifically, if the organic solvent is DMF, the water content of the spinning solution is preferably within the range approximately bounded by a triangle ABC in Fig. 1, and if DMAc is used as the organic solvent, the range approximately bounded by A′B′C′ in Fig. 2 is preferred. If the water content exceeds the approximate upper limit of each of these ranges, the polymer will precipitate out of the spinning solution, making subsequent spinning impossible. The advantages of adding water mentioned in the preceding paragraph will not be fully attained if less than 1 wt% water is incorporated in the spinning solution. The spinning solution prepared in the present invention has a reduced concentration of a high-molelcular weight acrylonitrile based polymer. Dimethylformamide (DMF) and dimethylacetamide (DMAc) are two organic solvents that are suitable for spinning fibers from such high-molecular weight polymer for the following two reasons: first, they enable the formation of a stable spinning solution; secondly, coagulated filaments having a nearly circular cross section (shape factor ≦ 16) can be easily produced from the dilute spinning solution. If organic solvents for spinning other than DMF and DMAc are used, the coagulation rate is so fast that coagulated filaments having a nearly circular cross section (shape factor ≦ 16) cannot be produced from the spinning solution of low polymer concentration which is used in the present invention. On the other hand, if DMF or DMAc is used as the organic solvent for spinning in the present invention, coagulated filaments having a shape factor of not more than 16 can be attained over a broad range of coagulation bath conditions even if the spinning solution has a low polymer concentration. The range of applicable coagulation bath conditions can be further extended if a spinning solution containing water in the range indicated in Fig. 1 or 2 is subjected to the dryjet wet spinning process. Incorporation of water has the additional advantage of lowering the viscosity of the spinning solution. The use of a high-molecular weight polymer causes an unavoidable increase in the viscosity of the spinning solution and thereby makes subsequent spinning difficult; however, by adding water, a spinning solution having a viscosity that is suitable for spinning can be prepared. The higher the water content of a spinning solution, the smaller the shape factor of the coagulated filaments that are produced. However, if the water content exceeds the approximate upper limit of the range bounded by the triangle ABC in Fig. 1 or A′B′C′ in Fig. 2, the solubility of the acrylonitrile based polymer becomes too low to prepare a desired spinning solution. If the polymer concentration of the spinning solution is not more than 5 wt%, the chance of void formation in filaments is increased and it becomes difficult to attain coagulated filaments that are homogeneous and which have a shape factor not exceeding 16. If the water content of a spinning solution is less than 1 wt%, the coagulation bath conditions are little different from those when a nonaqueous spinning solution is used, and the advantages of employing a three-component spinning solution based on a polymer/water/DMF or DMAc are substantially lost. The spinning solution thus prepared is spun into filaments by the dryjet wet spinning process, with the spinning nozzle positioned 3 to 20 mm, preferably 3 to 10 mm, above the coagulation bath surface. If the distance between the nozzle and the bath surface is less than 3 mm, problems such as contact between the spinnerette face and the bath surface will frequently occur. If the spinnerette-to-liquid distance exceeds 20 mm, the chance of the extruded filaments breaking and fusing together will increase. The coagulation bath used in the present invention must be in the form of an aqueous solution of a solvent for the acrylonitrile based polymer, with the solvent concentration being in the range of 70 to 85wt% and the temperature not higher than 0°C, preferably not higher than -10°C, more preferably not higher than -20°C. In the present invention which uses an acrylonitrile based polymer having a weight-average molecular weight of at least 5 × 10⁵, it is specifically necessary to control the polymer concentration at a low level to achieve suitable viscosity of the spinning solution. If the spinning solution having such a low polymer concentration is extruded into a coagulation bath having a fairly high temperature, e.g., exceeding 0°C, it is impossible to attain coagulated fibers having a desired structure. Only when coagulation takes place slowly at temperatures no higher than 0°C can dense coagulated filaments be produced from the dilute spinning solution. For this reason, a preferred spinning solvent permits the resulting coagulation bath consisting of an aqueous solution of the solvent to be cooled to the lowest possible temperature. Dimethyl sulfoxide (DMSO), which has a melting point of 18°C, is not suitable since a coagulation bath in the form of its aqueous solution cannot be cooled below 0°C, and dense coagulated filaments cannot be produced from the dilute spinning solution that has been extruded into the coagulation bath of a DMSO-water system. Therefore, DMF and DMAc are two preferred spinning solvents when a spinning solution of low polymer concentration is used. The spinning draft which is determined by the amount in which the spinning solution is extruded from the nozzle and by the speed at which the extruded filaments are taken up is typically set in the range of 1.0-10.0, preferably in the range of 1.5 to 10.0. If the spinning draft exceeds 10.0, frequent filament breakage occurs and if the draft is less than 1.0, fusion bonding of filaments and uneven fineness of fibers will occur. If low-draft spinning is effected in a DMSO-based coagulation bath, the high viscosity of the bath can cause very significant uneven fiber fineness. In this respect, too, DMF and DMAc, which provide coagulation baths of low viscosity, are preferred. In the present invention which uses a dilute spinning solution, filaments have to be spun with a draft of 1.0 or more, preferably 1.5 or more, being applied, so it is preferred to employ a spinning nozzle having a spinnerette-hole size of at least 0.15 mm. The resulting coagulated filaments are stretched in warm water at two or more stages provided that temperature is increased by degrees while being washed to remove the organic solvent from the filaments. Thereafter, the filaments are further stretched at a temperature exceeding 100°C. This second stage of stretching may be effected in a dry heat atmosphere, a steam atmosphere, or a wet heat atmosphere using a high-boiling point heat-transfer medium. In order to produce a high-strength acrylic fiber, the filaments must be stretched to a total draw ratio of at least 10, preferably at least 15. However, if stretching in warm water is immediately followed by dry-heat stretching, a draw ratio of 10 or higher is difficult to achieve, and a draw ratio of 15 or above is even more difficult. It is therefore preferred to attain a total draw ratio of at least 10, preferably at least 15, by first stretching in warm water to a draw ratio of 3 to 6; followed by preliminary drying; another stretching to a draw ratio of 3 to 6 either in a steam atmosphere or in a wet heat atmosphere using a high-boiling heat-transfer medium, followed by dry-heat stretching to a draw ratio of 1.05 to 1.2. A suitable high-boiling heat-transfer medium is a water-soluble polyhydric alcohol such as ethylene glycol, diethylene glycol, triethylene glycol or glycerin. The acrylic fiber of the present invention thus produced has a good morphology, high degree of orientation, and large crystalline size. If it is stretched to a high draw ratio of at least 15, a high-strength acrylic fiber can be attained that has a good balance between morphology and mechanical characteristics such as tensile strength. The high-strength acrylic fiber produced by the method of the present invention finds immediate use in many industrial applications (e.g., canvas, asbestos substitutes, sewing threads, hoses and heavy fabrics) and in fiber reinforcement applications where it is used as a reinforcement of composite materials. The carbon fiber produced by oxidizing and carbonizing the precursor obtained by the method described above is useful not only in sporting goods and recreational applications but also in aircraft and aerospace applications where extremely high reliability is required. The present invention is hereunder described in greater detail with reference to working examples, which are not to be construed as limiting the scope of the present invention. In the examples, fiber properties were measured or determined by the following methods. Unless otherwise indicated, all parts, percents and ratios are by weight. (1) Weight-average molecular weight ( M w): Intrinsic viscosity [η] of polymer was measured in the presence of dimethylformamide as a solvent at 25°C and its M w was calculated by the following formula: [η] = 3.35 x 10⁻⁴ [ M w] 0.72 (2) Monofilament fineness: Measured with a denier computer manufactured by Toyo Baldwin Co., Ltd. (3) Monofilament strength, elongation and modulus: Measured in accordance with JIS L1015. (4) Shape factor: The shape factor was described in Japanese Patent Application (OPI) No. 61-163149(1986). The size measurements of a fiber cross section magnified under microscope was determined and its shape factor calculated by the following formula: Shape factor = (circumferential length)²/cross-sectional area. (5) CV (%): The CV represented coefficient of variation, and was calculated by the following formula: (6) The degree of orientation, π and crystallinity parameter, β were determined by the following procedures: (i) Degree of orientation, π An azimuthal diffraction line profile of an acrylonitrile based fiber was obtained and recorded on paper for reflection in the vicinity of an equatorial scattering angle (2ϑ) of 17° and the degree of its orientation was calculated by the following formula: wherein H 1/2 is the half-width (in degrees) of a peak as measured from a baseline drawn on the recording paper. (ii) Crystallinity parameter, β A diffraction line profile for all scattering angles of an acrylonitrile based fiber was obtained and recorded on paper for reflection in the vicinity of an equatorial scattering angle (2ϑ) of 17° and the degree of its orientation was calculated by the following formula: wherein B is the half-width (in degrees) of a peak as measured from a baseline drawn on the recording paper, and b is the half-width for a reference material (silicone powder). The two factors, π and β, were measured with a Model RAD-A of Rigaku Denki Co., Ltd. under the following conditions: tube voltage and current: 40 kV, 200 mA (β) and 40 kV, 150 mA (π) filter : Ni filter. EXAMPLE 1 Polyacrylonitrile samples (100% acrylonitrile) having varying weight-average molecular weights were prepared by suspension polymerization. Using these polyacrylonitrile samples, spinning solutions having different polymer/DMF/water compositions were prepared. Each of the spinning solutions held in a spin tank at 50°C was spun by dryjet wet spinning, in which they were extruded through a nozzle (with 500 holes having a size of 0.20 mm) into air at 20 °C, then introduced into a coagulation bath (-24°C) composed of 83 wt% DMF and 17 wt% water. The distance between nozzle face and coagulation bath surface was set at 5 mm. The coagulated filaments were stretched in warm water (70°C) to a draw ratio of 2, in boiling water to a draw ratio of 2, and in glycerin (185°C) to a draw ratio of 3.5. Thereafter, the stretched filaments were oiled, dried at 140°C and further stretched under the action of dry heat at 200°C to a draw ratio of 1.15, so as to achieve a total draw ratio of 16.1. The physical properties of the thus prepared polyacrylonitrile fibers are summarrized in Table 1. As is seen from Table 1, acrylic fibers according to the present invention are excellent in the fiber properties. EXAMPLE 2 . Using a water-containing spinning solution having the same composition as Run No. 3 used in Example 1, a series of spinning experiments were conducted under the same conditions except for the nozzle-hole diameter. the results of these experiments are summarized in Table 2. Nozzle-hole diameter (mm) Fiber cross section 0.1 highly unevenness 0.12 unevenness 0.15 uniform 0.20 uniform 0.25 uniform 0.35 uniform COMPARATIVE EXAMPLE 1 Using polyacrylonitrile having a weight-average molecular weight of 6.5 x 10⁵ (corresponding to Run No. 2 in Example 1), spinning solutions having the compositions and viscosities shown in Table 3 were prepared. The results of experiments conducted on these spinning solutions are summarized in Table 3. Run No. Polymer/DMF/water (wt%) Viscosity at 45°C (Pa·s) ((poises)) Remarks 1 12/85/3 49 (490) frequent fiber breakage in coagulation bath 2 14/84/2 160 (1,600) viscous solution stuck to nozzle face Consistent spinning could not be achieved with spinning solutions having viscosities outside the range of 50 to 150 Pa·s (500 to 1,500 poises). EXAMPLE 3 Polyacrylonitrile samples (100% AN) having weight-average molecular weights of 4.8, 5.1, 6.5, 7.2 and 8.0 (x 10⁵) were prepared by suspension polymerization. Using these polymers, spinning solutions composed of 10 wt% polymer, 88 wt% DMAc, and 2 wt% water were prepared. Each of these spinning solutions held in a spin tank at 50°C was spun by dryjet wet spinning, in which they were extruded through a nozzle (with 500 holes having a size of 200 µm) into air at 20°C, then introduced into a coagulation bath (80.0 wt% DMAc and 20.0 wt% Water) at -10°C. The distance between nozzle face and coagulation bath surface was set at 5 mm. The resulting coagulated filaments were stretched first in warm water (70°C) to a draw ratio of 2, then in boiling water to a draw ratio of 2, and finally in glycerin (185°C) to a draw ratio of 3.5. Thereafter, the stretched filaments were oiled, dried at 140°C and further stretched under the action of dry heat at 200°C to a draw ratio of 1.15, so as to attain a total draw ratio of 16.1. The physical properties of the thus prepared acrylic fibers are summarized in Table 4. As is seen from Table 4, acrylic fibers according to the present invention are excellent in the fiber properties. EXAMPLE 4 Polyacrylonitrile (100% AN) having a weight-average molecular weight of 7.1 x 10⁵ was prepared by suspension polymerization. Using this polymer, a spinning solution with a viscosity of 81 Pa·s (810 poises) which was composed of 10 wt% polymer and 90 wt% DMAc was prepared. This spinning solution held in a spin tank at 50°C was spun by dryjet wet spinning, in which it was extruded through a nozzle (with 500 holes having a size of 200 µm) into air at 20°C, then introduced into a coagulation bath (80.0 wt% DMAc) at -10°C. The distance between nozzle face and coagulation bath surface was set at 5 mm. The resulting coagulated filaments with a shape factor of 13.0 were stretched first in warm water (70°C) to a draw ratio of 2, then in boiling water to a draw ratio of 2, and finally in glycerin (185°C) to a draw ratio of 3.5. Thereafter, the stretched filaments were oiled, dried at 140°C and further stretched under the action of dry heat at 200°C to a draw ratio of 1.15, so as to attain a total draw ratio of 16.1 The thus prepared fiber had the following physical properties: fineness, 1.17 dtex (1.05 d); strength (knot strength), 15.7 (4.7) cN/dtex (17.8 (5.3) g/d); elongation (knot elongation), 8.1 (4.0) %; modulus (knot modulus), 216 (152) cN/dtex (245 (172) g/d); degree of X-ray orientation π, 95.6%; and crystallinity parameter β, 0.70°. EXAMPLE 5 An acrylonitrile based copolymer having a weight-average molecular weight of 7 x 10⁵ and containing 3 wt% methacrylic acid as a comonomer was prepared by suspension polymerization. Using this polymer, a spinning solution was prepared and spun as in Example 3. The thus prepared fiber had the following physical properties: fineness, 1.19 dtex (1.07 d); strength (knot strength), 15.5 (4.6) cN/dtex (17.6 (5.2) g/d); elongation (knot elongation), 8.1 (4.1)%; modulus (knot modulus), 212 (150) cN/dtex (240 (170) g/d); degree of X-ray orientation π , 95.3%; and crystallinity parameter β, 0.71°.

Claims: A process for producing an acrylic fiber having a strength of at least 8.83 cN/dtex (10 g/d), a modulus of at least 159 cN/dtex (180 g/d), at least 95% X-ray orientation, and an X-ray crystallinity parameter of up to 0.8 degree, comprising the steps of: (a) dissolving a polymer having a weight-average molecular weight of at least 5 x 10⁵ and containing at least 80 wt% acrylonitrile in a solvent comprising dimethylformamide or dimethylacetamide at a polymer concentration of 5-15 wt% to provide a spinning solution having a viscosity of 50-150 Pa·s (500-1,500 poises) at 45°C; (b) subjecting said spinning solution to dryjet wet spinning to spin into filaments, wherein the spinning nozzle has a nozzle hole diameter of at least 0.15 mm, the air gap between the nozzle face and coagulation bath surface is 3-20 mm, the coagulation bath comprises water and 70-85 wt% of at least one of dimethylformamide and dimethylacetamide and has a temperature of 0°C or below, and the spinning draft is in the range of 1.0-10.0; and (c) stretching the coagulated filaments to a draw ratio of at least 10. A process according to claim 1, wherein said polymer comprises 95 to 99.9 wt% acrylonitrile and 0.1 to 5 wt% of a copolymerizable unsaturated carboxylic acid. A process according to claim 2, wherein said polymerizable unsaturated carboxylic acid is acrylic acid, methacrylic acid or itaconic acid. A process according to claim 1, wherein the concentrations of water and the polymer in a spinning solution using dimethylformamide as a spinning solvent are within amounts defined by an area bounded by straight lines connecting points A, B and C which have coordinates of (1, 15), (1, 5) and (10, 5), respectively, on a rectangular coordinate system in which the water concentration in wt% is plotted on the x-axis and the polymer concentration in wt% is plotted on the y-axis. A process according to claim 1, wherein the concentrations of water and the polymer in a spinning solution using dimethylacetamide as a spinning solvent are within amounts defined by an area bounded by straight lines connecting points A', B' and C' which have coordinates of (1, 15), (1, 5) and (5, 5), respectively, on a rectangular coordinate system in which the water concentration in wt% is plotted on the x-axis and the polymer concentration in wt% is plotted on the y-axis. A process according to claim 1, wherein the coagulated filaments are stretched to a draw ratio of at least 15. A process according to claim 1, wherein said acrylic fiber has a strength of at least 13.2 cN/dtex (15 g/d) and a modulus of at least 177 cN/dtex (200 g/d). A process according to claim 1, wherein said acrylic fiber has a knot strength of at least 3.5 cN/dtex (4 g/d).

Inventors: FURUTANI, YOSHIFUMI, NISHIHARA, YOSHIHIRO, TORAMARU, MASAAKI, YASUNAGA, TOSHIYUKI
Applicants: MITSUBISHI RAYON CO., LTD.
Application year: 1987
Main classification: D01F6
IPC codes: D01F6, D01D5