High-strength polyvinyl alcohol fiber and process for its production

Abstract: Provided is a process for producing high-strength polyvinyl alcohol fiber which comprises extruding a polyvinyl alcohol dope solution through a spinneret (1) into a coagulating bath (4), the spinneret being located such that substantially only its dope-extruding surface contacts with the coagulating bath. This process can produce stably and at a low cost polyvinyl alcohol fibers with excellent strength. Also provided is a polyvinyl alcohol fiber having a primary roughened surface structure comprising a plurality of comparatively flat projections continuously extending along the fiber axis and having a large width and a comparatively small height and a plurality of recessions continuously extending along the fiber axis and having a comparatively small depth, said projections and recessions being arranged alternately, and a secondary roughened surface structure comprising super-fine projections and recessions that are present on the primary roughened surface. This fiber has excellent reinforcement effect for cement and like materials.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a process for producing at a low cost polyvinyl alcohol (hereinafter referred to as PVA) fiber having high strength and to a novel fiber obtained by said process. Description of the Prior Art PVA fiber has higher strength and elastic modulus than polyamide, polyester and polyacrylonitrile fibers and has been principally used in industrial fields. In recent years it has also been used as a replacement for asbestos in reinforcing cement and for reinforcing rubber, plastics, and like materials. Several processes are known for obtaining PVA fibers having still higher strength. See, for example US-A-4,440,711 which discloses a process which employs the concept of gel-spinning and super-drawing, which has first been established with diluted solution of high-molecular-weight polyethylene, and US-A- 4,698,194 (corresponding to EP-A-0 146 084) which comprises using an organic solvent for dope solution and conducting dry-jet-wet spinning of the dope. The dry-jet-wet spinning employed in the above inventions comprises extruding PVA solution through a nozzle and, via an air layer, introducing the extruded solution into a coagulating bath. Although the process utilizes a diluted solution, the solution cannot be a very diluted one since the extruded solution must pass through an air layer while maintaining the form of continuous streams. With a very diluted PVA solution, the polymer solution tends to stick to spinneret surface and stable spinning is difficult to achieve. Where a spinneret having holes with small pitch is used, the polymer streams just after the extrusion tend to contact with and stick to each other in the air and stable spinning is not possible. If the spinning is ever conducted, the solidified filaments obtained become what are called stuck filaments, which will undergo filament breakage upon heat drawing in a high ratio, whereby high-strength fiber is difficult to obtain. To solve this problem, the spinneret used must have a hole allocation with a large pitch, which means the spinneret cannot have very many holes, thus creating another problem of high production cost in commercial production. A large-diameter spinneret may be employed for increasing the number of holes per spinneret, but it will have disadvantages of difficult handling and, in particular, readily causing extrusion to be uneven. Accordingly, the dry-jet-wet spinning is associated with several problems from the viewpoint of commercial production. Wet spinning of PVA fiber is also known. See, for example Japanese Patent Publication No. 16675/1968. In the wet spinning, although it creates no such problems as associated with dry-jet-wet spinning, there has not been obtained any fiber having high strength and high modulus. The present inventors have pursued the reason why conventional wet spinning cannot give a high-strength fiber and found the following particular point. In the wet spinning, the spinneret used and the dope piping connected thereto are immersed in coagulating bath, and hence the coagulating bath temperature and the dope temperature influence each other. With a large difference between the two temperatures, there occur temperature unevennesses between the dope piping and the center and peripheral part of the spinneret used, resulting in a viscosity unevenness of the dope, whereby regular and uniform extrusion is impossible. This fact is one of the causes obstructing production of high-strength PVA fiber, and is very much so in particular with a low coagulating bath temperature of 20°C or below. It is further known from document FR-A-1 369 805 a process for producing fibres by wet-spinning vinyl chloride solutions having a high syndiotacticity index, wherein the contact between the spinning solution and the coagulating bath is limited to the exterior surface of the spinneret. However said document, although referring in passing to vinyl polymer solutions tout court, does not mention at all PVA, and the temperatures of the coagulating bath are such high as 30° to 80° C, whilst there is no hint that the fibres thus fabricated have a high value of the tenacity. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide, while employing the wet spinning system enabling employment of spinneret with very many holes, a process for producing high-strength PVA fiber stably and at low cost which assures regular and uniform extrusion. Another object of the present invention is to provide a novel high-strength PVA fiber obtained by this process. Thus, the present invention provides a process for producing high-strength PVA fiber as claimed in independent claim 1. The present invention also provides a high-strength PVA fiber as claimed in independent claim 7. BRIEF DESCRIPTIONS OF THE DRAWINGS A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: FIGURES 1 through 3 are schematic views illustrating the spinning process of the present invention; FIGURE 4 is a schematic view illustrating a preferred embodiment of the spinning process of the present invention; FIGURES 5 and 6 are schematic views illustrating conventional dry-jet-wet spinning process and wet spinning process, respectively; FIGURE 7 shows an electron photomicrograph with a magnification of 10,000 of an example of the surface structure of the fiber obtained by the process of the present invention; and FIGURE 8 shows an electron photomicrograph with a magnification of 10,000 of an example of the surface structure of the fiber obtained by known dry-jet-wet spinning process; where 1, 2, 3, 4 and 5 represents spinneret, spinning dope, coagulating liquid, coagulating bath and heat-insulating or heating means, respectively; L means the immersion length as referred to in the present invention, FIGURES 1 and 4 show flow-down wet spinning, the latter showing that the surface of the coagulating bath just touches the dope-extruding surface of spinneret by action surface tension, and FIGURES 2 and 3 show flow-up wet spinning. DETAILED DESCRIPTION OF THE INVENTION The PVA used in the present invention has an average polymerization degree obtained by viscosity measurement on its aqueous solution at 30°C of at least 1,500. With polymerization degrees lower than 1,500, high-strength PVA fibers cannot be obtained. A viscosity average polymerization degree of at least 3,000, preferably at least 4,000 will more readily give a high-strength PVA fiber. A viscosity average polymerization degree of even at least 7,000 still more readily gives a high-strength PVA fiber because of smaller number of molecular ends causing microstructural faults to generate. There is no particular limitation to the saponification degree of the PVA used, but it is preferably at least 98.5 mol%, and more preferably at least 99.9 mol% with which, in particular, hot water resistance is excellent. The PVA may be a copolymer PVA copolymerized with not more than 10 mol%, preferably not more than 2 mol% of another monomer having vinyl group, such as ethylene, itaconic acid or vinylpyrrolidone. Any solvent that dissolves PVA can be used in the present invention and its examples are dimethyl sulfoxide (hereinafter referred to as "DMSO"), dimethylformamide, dimethylimidazolidinone, water, glycerine, ethylene glycol, mixtures of 2 or more of the foregoing, such as DMSO/water, dimethylformamide/ethylene glycol, ethylene glycol or glycerine/water, n-propanol or isopropanol/water, and aqueous solutions of thiocyanates and the like. It is, however, preferred that from among the above solvents those giving a gelation temperature of the dope of not higher than 50°C be selected since in the present invention wet spinning into a coagulating bath having comparatively low temperature is effective in obtaining a fiber having high strength and a novel surface structure. In view of the above, glycerine alone or ethylene glycol alone is not preferred because they give comparatively higher gelation temperature. DMSO dissolves PVA at 80°C or below, which minimizes decrease in the polymerization degree of PVA and is hence particularly preferred. The PVA concentration in the spinning dope used is selected depending on the polymerization degree of the PVA and the type of solvent, but generally is 2 to 30% by weight, preferably 3 to 20% by weight. Since the present invention aimes at a high-strength fiber, the PVA concentration preferably is as low as possible, which, with only a small entanglement of molecules, enables high-draft drawing, within a limit not to create spinning troubles such as frequent filament breakage, generation of uneven filaments and sticking between filaments. The spinning dope may incorporate, depending on the intended use of the obtained fiber, various additives, e.g. a color such as pigment, antioxidant, ultraviolet absorber, surfactant, pH adjusting agent such as acid and gelation accelerating agent such as boric acid, in required amounts. It is often preferred, for solvents having a comparatively high freezing point, such as DMSO, to be added with a substance having coagulating function, such as methanol, in an amount not to cause PVA to coagulate, since such addition protects the spinning dope from freezing even when the coagulating bath temperature is set below the freezing temperature of the solvent used. The coagulating bath used in the invention comprises an organic solvent having coagulating function for PVA, such as methanol, ethanol and acetone, as a principal component. These solvents may also be used in combination with each other or with the solvent for spinning dope. With conventional water-based coagulating baths employed in known wet spinning process, it is generally difficult to obtain a novel fiber having a special surface structure as referred to in the present invention, since they coagulate the extruded dope too rapidly. The coagulating bath temperature employed is 20°C or below. If the temperature exceeds 20°C, the coagulated fiber will have many voids, become opaque and nonuniform and be not of high strength. The coagulating bath temperature is more preferably 15°C or below, and most preferably 10°C or below. Too low a coagulating bath temperature, however, causes the spinning dope extruded through spinneret to freeze, whereby extrusion is impossible. The process of the present invention is characterized in that the spinneret (the term "spinneret" herein means an integral spinneret including spinneret case to fix the spinneret, filter, flow straightening plate and the like) used is, instead of being completely immersed in the coagulating bath as seen in the usual wet spinning process (cf. FIGURE 6), located such that substantially only its dope-extruding surface contacts with the coagulating bath having a bath temperature of 20°C or below, the term "substantially only its dope-extruding surface contacts with the coagulating bath" herein means that, with reference to FIGURES 1 and 2, the length, L, of the spinneret part, with its end surface extruding the dope, being immersed in and directly touching the coagulating bath and directly receiving the influence of the coagulating bath temperature, is not more than 20 mm, preferably not more than 10 mm. Further, with reference to FIGURE 3, where the side of a spinneret is partly or even fully covered with a heat-insulating or heating means, the immersion length, L, means the length of the part of the spinneret immersed in and directly touching the coagulating bath, i. e. the length between the dope-extruding surface and the end of such heat-insulating or heating medium covering the spinneret side. It is the most preferred that, as shown in FIGURE 4, only the dope-extruding surface of the spinneret touch the coagulating bath by action of surface tension, i.e. L = 0. It is also very much preferred that, in flow-up wet spinning process as in FIGURE 3, the side of the spinneret be fully covered with a heat-insulating or heating medium, so that only its dope-extruding surface directly touches the coagulating bath, i.e. L = 0. Dry-jet-wet spinning process has been used for producing high-strength PVA, its principal advantage being considered to be that spinning dope and coagulating bath do, not contact with each other and are independent with respect to thermal conductivity so that their temperatures can be set largely apart from each other. On the other hand, conventional wet spinning process for obtaining high-strength PVA enables the spinning dope to be of lower PVA concentration than dry-jet-wet spinning and thus can use a spinning dope with smaller molecular-chain entanglement, whereby the process has a possibility of obtaining fiber having higher strength. Nevertheless, the wet spinning process has not been capable of providing high-strength PVA fiber; because, as described hereinbefore, when in this process a large temperature difference between spinning dope and coagulating bath is employed, extrusions through spinneret holes become nonuniform, creating uneven coagulation and large variation in the finenesses of obtained filaments. This tendency is more marked when an organic solvent is used. The present invention is based on the finding that if wet spinning is conducted through a spinneret located such that substantially only its dope-extruding surface contacts with the coagulating bath used the immersion length, L, of said spinneret in said coagulating bath being not more than 20 mm, the degree of the above unevenness of coagulation and and the fineness variation described above are, unexpectedly, much smaller even under a large temperature difference between the spinning dope and coagulating bath used than those with conventional wet spinning process. With an immersion length exceeding 20 mm, amount of heat exchange between the coagulating bath and the spinning dope inside the spinneret will become large to increase local temperature nonuniformities in the dope or coagulating bath, thereby increasing coagulation unevenness and fineness variation. The immersion length is preferably not more than 10 mm. In the most preferred embodiment of the present invention, only the dope-extruding surface of the spinneret used contacts with the coagulating bath. This state can readily be realized in flow-down system as shown in FIGURE 4 by action of surface tension between the spinneret surface and the coagulating bath. In the case of FIGURE 4, where only the dope-extruding surface contacts with the coagulating bath, a slight rippling on the coagulating bath surface might cause the surface to detach from the dope-extruding surface of the spinneret. In practice, unexpectedly, it has been confirmed experimentally that the coagulating bath cannot be separated from the dope-extruding surface by a slight or a relatively large rippling and that this state is maintained for a long period of time. This is considered to be due to contribution from surface tension. The present invention not only is applicable to flow-down system as shown in FIGURES 1 and 4, but produces the same effect with flow-up system as shown in FIGURE 2 as long as substantially only the dope-extruding surface of the spinneret contacts with the coagulating bath, that is, the immersion length is within the above-described range. It may often work, as stated before, to mount a cover made of a plastic or, preferably, plastic foam having high heat-insulating property around the spinneret used to suppress the temperature decrease at its dope-extruding surface and to prevent the coagulating bath temperature from being influenced by the dope temperature. It is further preferred to mount a heating means to heat the dope-extruding part of the spinneret, either singly or in combination with a heat-insulating means, around the spinneret, thereby preventing the move of heat between the dope-extruding part and the coagulating bath. The extruded dope is taken up, while being coagulated, on a 1st roller located in or above the coagulating bath at a bath draft (ratio of the linear speed of the dope passing the spinneret holes to the circumferential speed of the 1st roller) of preferably 0.1 to 0.5. The coagulated gel is formed into fiber in the following manner. The solvent and other extractables in the gel are washed and extracted off with an extracting liquid such as methanol or water, and the residual fiber-shaped gel is dried. Before the drying, it is preferred that the fiber be wet drawn in a ratio of at least 2 by 1-stage or, preferably, multi-stage drawing, which prevents sticking during drying. The wet drawing ratio is more preferably 2.5 to 5.5. A wet drawing ratio of 6 or more should be avoided because it causes filament breakage to occur frequently or the fiber cross-section to deform. The drying temperature is preferably 30 to 150°C from the viewpoint of drying efficiency and the properties of finished fiber. The thus dried fiber is then heat drawn at high temperature and in a high ratio to orient and crystallize its molecules, thereby becoming high-strength fiber. The heat drawing is conducted at a temperature of preferably at least 210°C, more preferably 220 to 250°C to a total drawing ratio of preferably at least 16, more preferably at least 18, most preferably at least 20. The process of the present invention provides a uniform as-spun fiber with little uneven coagulation and only a small fineness variation, thereby enabling high-draft drawing to obtain high-strength fiber. The heat drawing can be conducted by dry heating, in a heating medium such as silicone or by wet heating such as in high-temperature steam, and by one-stage or 2- or more stage drawing. The thus heat drawn fiber may then be, as required, heat treated or heat shrunk. As stated heretofore, the gist of the present invention lies in shortening of the immersion length of spinneret and lowering of coagulation bath temperature down below 20°C to increase the gelation or solidification speed and slowly extract the solvent used. This process gives as-spun fiber of little uneven coagulation and fineness variation. Heat drawing the thus obtained uniform as-spun fiber in a high drawing ratio of at least 16 can give, stably and at a low cost, high-strength and high-modulus PVA fiber. It has been found that the fiber obtained by the above-described process of the present invention has not only high strength and modulus, but high resistance to transversal abrasion, as well as higher reinforcement function for matrices such as cement than that of known high-strength PVA fiber. The mechanism of this high reinforcement function is attributable to a special surface structure of the fiber. Thus, the fiber of the present invention has a high tensile strength of at least 15 g/d and, at the same time, carries on its surface a plurality of longitudinally extending and comparatively flat projections with small height and comparatively large width, and recesses, the projections and recesses being arranged parallel and alternately. In other words, the fiber of the present invention has on its surface stripes of low, wide "ridges" continuously and longitudinally and extending parallel along the fiber axis. This surface structure is not like the known structure of medium-strength PVA fiber shown for example in Japanese Patent Publication No. 32144/1987, that comprises large and rough "pleats" having a sharp-angled top, or like one of the high-strength PVA fiber obtained by dry-jet-wet spinning shown for example in US-A-4,698,711, that is comparatively small and principally comprises superfinely roughened surface, but is characterized by a so to speak arithmetic mean of these two. For the purpose of defining the fiber of the present invention, it is necessary to precisely describe the fine surface structure by accurate observation method. The usual scanning electronmicroscopy requires metal-coating the surface of the specimen to be observed to a thickness of at least 0.01µm, which then renders it impossible to accurately reproduce the finely roughened surface structure characteristic of this fiber. In the present invention, a replica method described hereinlater was therefore employed and surface observation was conducted by electron microscopy. FIGURE 7 is a photograph obtained by reverse-printing to a total magnification of 10,000 an electron photomicrographic negative at a magnification of 5,000 of a replica of the surface of the fiber of the present invention. FIGURE 8 is a reverse-printed photograph at a total magnification of 10,000 of a surface replica of the fiber obtained in Comparative Example 1. The surface of the fiber of the present invention is characterized by a "two-fold" roughened surface structure comprising a plurality of comparatively large primary projections and recesses arranged alternately and extending continuously along the fiber axis and a plurality of secondary fine projections and recesses that are present on and definitely finer than the former. The primary roughened surface has a plurality of longitudinally extending projections and recesses having a width of 0.1 to 2µ/m, preferably 0.2 to 1µm, and a height, or depth, of 0.05 to 0.4µm, more preferably 0.07 to 0.2µm. If the height of the projections exceeds 0.4µm, the fiber will tend to be of low strength or low resistance to abrasion in a direction perpendicular to the fiber axis. Another, preferred feature of the fiber of the present invention is that its primary projections and recesses have, while their width is as fine as not more than 2µ m as large a length as at least 10µm. With a length of primary projections or recesses of less than 10µm, the fiber will not, like that with too narrow projections, produce satisfactory reinforcement effect. It is more preferred that the primary projections and recesses have a length of at least 10 mm. Then, where this fiber is cut to lengths of 10 mm and served for reinforcing purpose, the primary projections and recessions continuously extend all through the lengths, which is considered to enhance the reinforcement effect. The secondary projections and recesses are super-fine and have a width and height of both 0.01 to 0.05µm. To summarize, the high-strength PVA fiber of the present invention has a tensile strength of at least 15 g/d and has the following surface structure as observed on the reverse print of a transmission-type electron photomicrograph of a replica film of the surface: (1) Width of the primary projections or recesses : 0.1 to 2µm (2) Depth of the primary projections or recesses : 0.05 to 0.4µm (3) Width and height of the secondary projections or recesses: 0.01 to 0.05µm. It is more preferred that the above primary roughened structure of the PVA fiber of the present invention comprise a plurality of flat and wider projections and narrower recesses more concretely, the fiber surface comprises a plurality of longitudinally extending comparatively flat and wider primary projections and a plurality of comparatively low and narrower primary recesses each of which lies between two neighboring ones of the primary projections, the ratios of the width of the primary projections w 1 to the width w 2 and depth d of the primary recesses w 1 /w 2 and w 1 /d being both at least 1. It is not clear why the afore-described process of the present invention provides a fiber having high strength and the two-fold finely roughened surface structure. The formation of the primary roughened structure is considered to be deeply related with wet spinning into a low-temperature coagulating bath comprising principally an organic solvent. Where wet spinning is conducted with a water-based coagulating bath, as shown in the afore-mentioned Japanese Patent Publication No. 32144/1987, the obtained fiber has a more coarsely roughened and more readily crushable surface structure with projections and recesses having a width of 0.5 to 2µm and a depth of 0.5 to 1µ m than that of the fiber of the present invention. Solidification in an organic solvent coagulating bath has a different mechanism from that in a water-based coagulating bath, the former leading to more uniform solidification, which provides, estimatedly, a more finely roughened surface structure. However, even where an organic solvent coagulating bath is used, in dry-jet-wet spinning by which extruded polymer streams first contact with an inert gas, the rougher primary projections and recesses do not form, while finer projections and recesses corresponding to the secondary roughened surface of the fiber of the present invention do form, as shown in FIGURE 8. This is considered to be due to the following. In wet spinning process, coagulation of extruded dope starts just after the extrusion, particularly on its surface. Then the extruded dope coagulates while the tension on the coagulated fiber directly applies to the spinneret hole, whereby the viscoelastic state of the dope just before extrusion is memorized on the surface of the coagulated fiber and then subjected to relaxation. On the other hand, in dry-jet-wet spinning, dope is extruded into an atmosphere of an inert gas and hence solidifies slowly, whereby the original viscoelastic state undergoes relaxation before the solidification and the surface of the dope stream also solidifies after the relaxation. Thus, the large difference in surface structure of fibers obtained by wet spinning and dry-jet-wet spinning is that the solidification speed of the surface of extruded polymer stream just after extrusion is much larger with wet spinning, where relaxation occurs after surface coagulation, than with dry-jet-wet spinning, where solidification occurs after relaxation. From the fact that the secondary roughened surface structure with wet spinning has about the same fineness as that with dry-jet-wet spinning, this micro-finely roughened secondary structure is considered to have developed during the period of removal of solvent from solidified fiber and actualized through processes of wet drawing and dry heat drawing. The methods for determining the polymerization degree of PVA and the tensile strength of fiber are herewith described. Measurements of the width, depth and length of the primary projections and recessions and the width and depth of the secondary projections and recessions of fiber by electron photomicroscopy are also descibed. (i) Polymerization degree of PVA JIS K6726 is applied to determine the intrinsic viscosity [η ] of an aqueous solution at 30°C and the polymerization degree P A is calculated by: log P A , = 1.63 log ([η] x 10 4 /8.29) (ii) Fiber strength The tensile strength of a previously conditioned fiber specimen is determined in accordance with JIS L1013 under conditions of gauge length of 4 cm, rate of extension of 100%/min and an initial load of 0.25 g/d. An average of n = 10 is reported. The fineness in deniers is determined by weight measurement. (iii) Width, depth and length of primary roughened surface structure and size of secondary roughened surface structure, of fiber, A 1st-stage molding replica of a specimen fiber is prepared by pressing the specimen onto a polyethyl methacrylate film at 120°C/0.8 kg/cm 2 . The replica is shadowed by vacuum depositing a platinum/palladium alloy in a direction perpendicular to the fiber axis and at an angle of  against the film surface, where tan = 0.7. The shadowed replica is reinforced by vacuumdepositing carbon thereon in the direction perpendicular to the fiber axis and the film surface and then the polyethyl methacrylate film is dissolved off. The 2-stage replica thus prepared is held on a sheet mesh and photographed with a transmission-type electron photomicrographer at a magnification of 5,000. The width and length of fine projections and recesses are measured on a reverse-printed photograph enlarged to a magnification of 30,000 and the depth is calculated from the above shadowing angle. The photograph used in this measurement is taken on a specimen replica obtained by shadowing a 1st-stage replica with an alloy by vacuum deposition at a specified angle. The photograph should therefore be observed while the angle at which the shadowing was done is taken into consideration. The projections and recesses in the present specification appear on the above photograph by bright parts and dark parts respectively. Thus, the continuous projections appear as bright stripes and the continuous recesses as dark stripes. The width of a projection and a recess therefore is determined from the photograph as the width of the bright stripe and dark stripe, respectively, along a straight line drawn in a direction perpendicular to the fiber axis. The width is measured on a multiplicity of bright stripes and dark stripes on the photograph of a specimen filament, and this measurement is conducted on at least 5 specimens to determine the maximum and minimum values. The depth, or height, of a projection or a recess is, strictly speaking, to be measured on a number of photographs of cross-sections of a specimen fiber. The present inventors, however, have found that the depth thus determined is nearly equal to that calculated from the width obtained from the above-mentioned electron photomicrograph and shadowing angle. In the present specification, the calculated value is therefore employed for convenience' sake instead of time-consuming measurements on cross-sectional photographs. In the above measurements on photomicrographs, stereo-graphical view of a specimen fiber is projected on a plane. To minimize errors occurring at the edge parts, only the central part within about 60% of fiber diameter is observed while 20% each corresponding to edge parts is excluded. Other features of the invention will become apparent in the course of the following descriptions of exemplary embodiments. EXAMPLES Example 1 A PVA solution was prepared by dissolving a PVA having a polymerization degree of 3,900 and a saponification degree of 99.9 mol% in dimethyl sulfoxide at 30°C under an atmosphere of nitrogen to a concentration of 9% by weight. The spinning dope thus obtained was, at 60°C, extruded through a spinneret with 300 holes with a pitch of 2.1 mm and having a hole diameter of 0.16 mm down into a coagulating bath comprising methanol/dimethyl sulfoxide = 65/35 at 2°C, to effect wet spinning. At this time the spinneret was located such that only its dope-extruding surface contacted with the coagulating bath by action of surface tension, as shown in FIGURE 4. The bath draft was 0.21. The solidified fiber obtained was passed through a methanol bath to be extracted off of dimethyl sulfoxide and be drawn in 2 stages to a total wet drawing ratio of 4.2 and then dried by hot air flow at 70°C. The as-spun fiber thus obtained was heat drawn through a heating oven having a temperature gradient of 175-195-233°C to a total drawing ratio of 20.5. The fiber thus obtained showed a tensile strength of 20.1 g/d. The replica of the surface of the fiber was prepared in the afore-described manner and photographed with an electron microscope at a magnification of 5,000. The negative was enlarged to a magnification of 10,000 and printed. FIGURE 7 shows the photograph. The primary and secondary projections and recessions were measured for their sizes on a 30,000-enlarged reverse-printed photograph. The widths of the primary projections and recesses ranged from 0.2 to 1.0µm and 0.1 to 0.8µm respectively and the depths ranged from 0.1 to 0.2µm. The lengths of the projections and recesses were confirmed to be at least 50µm, from the fact that the same roughened pattern was observed on a spot shifted 50µm longitudinally. The sizes of the secondary projections and recesses ranged from 0.02 to 0.03µm. Accordingly, the strength and all the sizes of the primary and secondary roughened surface structures were within the scope defined by the present invention. The fiber was dispersed in a matrix of Portland cement and the mixture was wet-formed and cured. The thus obtained cement board had excellent flexural strength, proving high reinforcement effect of the fiber. The fiber also gave an excellent result in a test for resistance to abrasion in a direction perpendicular to the fiber axis. Comparative Example 1 An attempt was made to conduct dry-jet-wet spinning of the same spinning dope as used in Example 1, by providing, as shown in FIGURE 5, an air layer of about 5 mm thick between the spinneret surface and the coagulating bath. It was found impossible to continue spinning because of too low a dope viscosity. Then, the PVA concentration was increased to 12% by weight and the 5-mm thick air layer provided as above, another dry-jet-wet spinning was attempted under the same conditions of coagulating bath composition, wet drawing, extraction and drying as those in Example 1. The dried fiber however could not but be drawn only to a total drawing ratio of 17. The obtained fiber showed a tensile strength of 17.1 g/d, lower than that of Example 1 but higher than conventional PVA fibers. FIGURE 8 shows a 10,000-enlarged electron photomicrograph of the fiber surface. Observation of a further 60,000-enlarged photomicrograph revealed that the surface structure had projections and recesses having a size of 0.01 to 0.02µm corresponding to the secondary projections and recesses of the fiber of the present invention, but that the structure had no larger ones corresponding to primary projections and recesses thus being definitely different from that of Example 1. The obtained fiber was formed into a cement board in the same manner as in Example 1. The cement board obtained showed a lower flexural strength than that in Example 1. Example 2 A PVA solution was prepared by dissolving a PVA having a polymerization degree of 7,600 and a saponification degree of 99.9 mol% in dimethyl sulfoxide to a concentration of 6% by weight. The spinning dope thus obtained was, at 55°C, extruded through a spinneret with 300 holes with a pitch of 2.1 mm and having a hole diameter of 0.16 mm down into a coagulating bath comprising methanol/dimethyl sulfoxide = 65/35 at 5°C, to effect wet spinning in the same manner as in Example 1 and as shown in FIGURE 4. The fiber was heat drawn at 243°C and upto a total draft of 19.2 was possible. The fiber thus obtained showed a high tensile strength of 21.8 g/d. Observation according to the method of the present invention on the surface of the fiber revealed that the widths of the primary projections and recesses were 0.3 to 0.8µm and 0.1 to 0.5µm respectively and the depths were 0.1 to 0.2µm. The lengths of the projections and recesses were 120µm. The sizes of the secondary projections and recesses were 0.02 to 0.03µm. Example 3 A PVA solution was prepared by dissolving under an atmosphere of nitrogen a PVA having a viscosity average polymerization degree of 4,100 and a saponification degree of 99.8 mol% in dimethyl sulfoxide at 80°C to a concentration of 9% by weight. The spinning dope thus obtained was, at 65°C, extruded through a spinneret with 40 holes with a pitch of 2 mm and having a hole diameter of 0.12 mm down into a coagulating bath comprising methanol/dimethyl sulfoxide = 7/3 at 0°C, to effect wet spinning, while the spinneret was so located as to touch the coagulating bath only with its dope-extruding surface by action of the surface tension of the coagulating bath as shown in FIGURE 4. The solidified fiber was passed through a methanol bath where dimethyl sulfoxide was extracted off and the fiber was wet drawn in 2 stages to a wet drawing ratio of 4, and then dried by hot air flow at 90°C. The fiber was then heat drawn through a hot air oven having a temperature gradient of 170-190-235°C to a total drawing ratio of 22. Operation all through these processes was stable without any trouble such as wrapping around a roll, and the obtained fiber had a round cross-section. The fiber showed a tensile strength of 20.8 g/d, elastic modulus of 475 g/d and a variation of single filament fineness of 8%. Comparative Example 2 An attempt was made to conduct dry-jet-wet spinning as shown in FIGURE 5 using the same spinning dope and spinneret as used in Example 3. With the hole pitch of 2 mm, the extruded dope streams stuck to each other and stable spinning was impossible. The spinneret was changed to one with 40 holes with a pitch of 5 mm and having a hole diameter of 0.12 mm, and dry-jet-wet spinning was conducted through an air layer of 5 mm thick and into the coagulating bath. The PVA dope, same as that used in Example 3, was here of too low a viscosity due to low concentration, and hence filament breakages occurred partly to cause roller wrapping, whereby stable operation could not be continued. Comparative Example 3 Example 3 was repeated except for immersing the spinneret as shown in FIGURE 1 in the coagulating bath, by a length of 35 mm, to effect wet spinning. No stable extrusion through the spinneret was possible, with the extruded dope streams showing a large size variation. The variation of fineness of the as-spun filaments was as large as 19%. The as-spun fiber was drawn in the same manner as in Example 3, where frequent filament breakages occurred and the total drawing ratio had to be decreased to 17. The fiber thus obtained showed a low yarn strength of 16.5 g/d. Comparative Example 4 The same spinning dope as used in Example 3 was extruded through a spinneret with 300 holes with a pitch of 0.8 mm and having a hole diameter of 0.11 mm up into a coagulating bath comprising methanol/dimethyl sulfoxide = 85/15 at 5°C, while the spinneret was immersed in the bath by an immersion length of 80 mm, as shown in FIGURE 6, to effect flow-up wet spinning. Microscopy on the cross-section of the obtained fiber revealed a large unevenness and the fineness variation of single filaments was found to be as large as 34%. Example 4 A PVA solution was prepared by dissolving a PVA having a viscosity average polymerization degree of 4,100 and a saponification degree of 99.8 mol% in dimethyl sulfoxide at 80°C under an atmosphere of nitrogen to a concentration of 8% by weight. The spinning dope thus obtained was, at 60°C, extruded through a spinneret with 300 holes with a pitch of 1.8 mm and having a hole diameter of 0.11 mm down into a coagulating bath comprising methanol/dimethyl sulfoxide = 6/4 at 2°C, to effect flow-down type wet spinning as shown in FIGURE 1, while the immersion length of the spinneret was set to 4 mm. The solidified fiber was passed through a methanol bath where dimethyl sulfoxide was extracted off and the fiber was wet drawn in 3 stages to a total wet drawing ratio of 3.5, and then dried by hot air flow at 90°C. The as-spun fiber thus obtained was then heat drawn through a hot air oven having a temperature gradient of 180-200-240°C to a total drawing ratio of 21. Operation all through these processes was stable without any trouble such as wrapping, and the obtained fiber had a small variation of single filament fineness of 5% and an excellent tensile strength of 21.5 g/d. Comparative Example 5 Example 4 was repeated except that the coagulation bath temperature was set to 25°C. The solidified fiber was whitened, and the heat drawn fiber showed a yarn strength of only 18.7 g/d perhaps because of the presence of many voids. Example 5 A PVA solution was prepared by dissolving a PVA having a viscosity average polymerization degree of 8,000 and a saponification degree of 99.5 mol% in a mixed solvent of dimethyl sulfoxide/water = 8/2 to a concentration of 6% by weight. The spinning dope thus obtained was, at 90°C, extruded through a spinneret with 1,000 holes with a pitch of 1.8 mm and having a hole diameter of 0.18 mm down into a coagulating bath, with the spinneret touching the coagulating bath only with, its dope-extruding surface as shown in FIGURE 4, to effect flow-down type wet spinning. The coagulating bath comprised ethanol/water = 95/5 and was at a temperature of -2°C. The solidified fiber was drawn through an ethanol bath in 2 stages to a total wet drawing ratio of 3, and then dried by hot air flow at 100°C to be an as-spun fiber with almost no water or ethanol. The as-spun fiber was then heat drawn through a radiation type hollow tube heater having a temperature gradient of 170-250°C to a total drawing ratio of 20.4. The obtained fiber had a high yarn strength of 21.8 g/d. P.S.: To convert g/d to cN/tex, multiply by 8.826.

Claims: A process for producing high-strength polyvinyl alcohol fiber which comprises dissolving in a solvent a polyvinyl alcohol having a viscosity average polymerization degree of at least 1,500 and extruding the obtained dope solution through a spinneret into a coagulating bath, comprising an organic solvent and having a bath temperatue of 20°C or below said spinneret being located such that substantially only its dope-extruding surface contacts with the coagulating bath, the immersion length, L, of said spinneret in said coagulating bath being not more than 20 mm. A process for producing high-strength polyvinyl alcohol fiber according to Claim 1, wherein said polyvinyl alcohol has a viscosity average polymerization degree of at least 3,000. A process for producing high-strength polyvinyl alcohol fiber according to Claim 1 or 2, wherein said coagulating bath has a bath temperature of 15°C or below. A process for producing high-strength polyvinyl alcohol fiber according to any of the Claims 1 to 3, wherein said spinneret is covered, on all of its side that is immersed in the coagulating bath, with a heat-insulating means or heating means so that only the dope-extruding surface of said spinneret touches the coagulating bath and flow-up wet spinning is conducted. A process for producing high-strength polyvinyl alcohol fiber according to any of the Claims 1 to 4, wherein the extruded fiber before drying is wet drawn in one or more stages to a total wet drawing ratio of 2 to 6. A process for producing high-strength polyvinyl alcohol fiber according to any of the Claims 1 to 5, wherein the fiber after drying is dry heat drawn at not lower than 210°C in one or more stages to a total drawing ratio including wet drawing ratio of at least 16. A high-strength polyvinyl alcohol fiber made of a polyvinyl alcohol having a viscosity average polymerization degree of at least 1,500 and having a tensile strength of at least 132.39 cN/tex (15 g/d), said fiber having a primary roughened surface structure comprising a plurality of longitudinally extending comparatively flat projections and having a large width and a comparatively small height and a plurality of longitudinally extending recesses having a comparatively small depth, said projections and recesses being arranged alternately, and a secondary roughened surface structure comprising superfine projections and recesses that are present on the primary roughened surface, said projections and recesses having the following sizes: (1) width of the primary projections or recesses: 0.1 to 2µm; (2) depth of the primary projections or recesses :0.05 to 0.4µm; and (3) width and depth of the secondary projections or recesses: 0.01 to 0.05µm. A high-strength polyvinyl alcohol fiber according to Claim 7, wherein said primary and secondary projections and recesses have a length of at least 10 µm. A high-strength polyvinyl alcohol fiber according to Claim 7 or 8, wherein the ratios of the width of said primary projections to the width and depth of said primary recesses are both at least 1.

Inventors: KUBOTSU AKIRA, NARAMURA SYUNPEI, OHMORY AKIO, SANO HIROFUMI, SANO TOMOYUKI, YOSHIMOCHI HAYAMI, KUBOTSU, AKIRA, NARAMURA, SYUNPEI, OHMORY, AKIO, SANO, HIROFUMI, SANO, TOMOYUKI, YOSHIMOCHI, HAYAMI
Applicants: KURARAY CO, KURARAY CO., LTD.
Application year: 1990
Main classification: D01F6
IPC codes: D01F6, D01D5