Polyvinyl alcohol fiber, fiber structure, and method for producing the same

Abstract

[Problem] To provide a polyvinyl alcohol-based fiber having a sufficient shrinkage rate and a sufficient shrinkage stress when water at room temperature or higher, particularly around 35 °C, is absorbed. [Solution] Provided are: a polyvinyl alcohol-based fiber which includes a modified polyvinyl alcohol containing at least 1 mol% of a carboxy group and has a birefringence index of at least 0.040; a fiber structure using said polyvinyl alcohol-based fiber; and a method for producing said polyvinyl alcohol-based fiber.

Description

[Technical Field]

[0001] The present invention relates to polyvinyl alcohol-based fibers, a fiber structure containing the fibers, and a method for producing the polyvinyl alcohol-based fibers.

[0002] Health care products such as disposable diapers and incontinence pads, as well as daily necessities such as sanitary napkins, absorb bodily fluids to maintain cleanliness and play an important role in daily life. Absorbent articles intended to absorb moisture such as body fluids generally have a structure in which a moisture absorber is covered with paper or the like, part of which is secured with a hot melt adhesive or the like, and the moisture absorber is placed between a breathable polymer sheet that comes into direct contact with the wearer's skin and an impermeable nonwoven fabric.

[0003] The moisture absorber uses water-absorbing and shrinkable fibers, which absorb moisture and shrink, ensuring a flow path for moisture to the moisture absorber and bringing the water-absorbing agent into close contact with the human body, thereby preventing leakage.

[0004] As an example of such a water-absorbing shrinkable fiber, Patent Document 1 discloses a high-speed shrinkable fiber that is made from a modified polyvinyl alcohol containing 0.5 to 10 mol % of carboxyl groups, has a maximum shrinkage rate of 30% or more in water at 20°C, takes 10 seconds or less to reach 30% shrinkage, has a shrinkage stress of 0.15 g / d or more in the original length, takes 10 seconds or less to reach a shrinkage stress of 0.15 g / d, and has a shrinkage stress of 0.03 g / d or more in water at 20°C when shrunk 30% from the original length, and has a dissolution loss of 45% or less when dispersed in water at 20°C, is poorly soluble in water, and shrinks in the presence of water.

[0005] Patent Document 2 describes a water-shrinkable polyvinyl alcohol fiber having a shrinkage rate in water at 30°C of 20 to 50%, a shrinkage rate in water at 30°C relative to the maximum shrinkage rate of 0.2 to 0.7, a wet modulus of elasticity of 0.1 to 3 cN / dtex, and an ash content of 0.2 mass% or less. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 62-215011 [Patent Document 2] Japanese Patent Application Laid-Open No. 2001-262432 Summary of the Invention [Problem to be solved by the invention]

[0007] In the case of the above-mentioned healthcare and daily necessities applications, Moisture absorber Since the fiber absorbs moisture at around 35°C, which is close to the body temperature of the human body, the shrinkage rate when the fiber absorbs moisture at a temperature at least above room temperature is important. Furthermore, since the entire moisture absorber needs to shrink, the water-absorbing shrinkable fiber needs to have sufficient shrinkage stress when absorbing moisture at a temperature at least equal to or higher than room temperature.

[0008] Furthermore, when the moisture absorber absorbs a large amount of moisture, its own weight causes it to lose its shape and crack, causing it to move between the breathable polymer sheet and the nonwoven fabric inside the absorbent article, which can result in uneven distribution of the moisture absorber inside the absorbent article, preventing the absorbent article from absorbing enough moisture and causing leakage. To prevent such leakage, the water-absorbing shrinkable fiber is required to have a shrinkage stress that allows the moisture absorber to maintain its shape even after absorbing water.

[0009] However, the high-speed shrinkage fiber described in Patent Document 1 loses 45% or less in dissolution weight when dispersed in water at 20°C, and there is a possibility that the shrinkage stress is insufficient when absorbing water at room temperature or higher. Furthermore, the water-shrinkable polyvinyl alcohol fiber described in Patent Document 2 has a shrinkage rate in water at 30° C. of 20 to 50%, and further improvement in the shrinkage rate is considered necessary.

[0010] Therefore, absorbent articles using a combination of water-absorbent shrinkable fibers and rubber threads have been used to improve the shrinkage rate and shrinkage stress, but the rubber threads are in a shrunk state even before the absorbent article is used, which results in the absorbent article being bulky before use.

[0011] Therefore, there is a demand for shrinkable fibers that have sufficient shrinkage and shrinkage stress when they absorb moisture at temperatures above room temperature, particularly around 35°C.

[0012] An object of the present invention is to provide a polyvinyl alcohol-based fiber that exhibits a sufficient shrinkage rate and shrinkage stress when it absorbs moisture at a temperature above room temperature, particularly around 35°C. Another object of the present invention is to provide a fiber structure that at least partially comprises such a polyvinyl alcohol-based fiber. Another object of the present invention is to provide a method for producing a polyvinyl alcohol-based fiber. [Means for solving the problem]

[0013] That is, the present invention includes the following preferred embodiments. [1] Polyvinyl alcohol-based fibers containing modified polyvinyl alcohol containing 1 mol% or more of carboxyl groups and having a birefringence of 0.040 or more. [2] The polyvinyl alcohol fiber according to [1], wherein the carboxyl group is contained in at least one functional group selected from the group consisting of an acrylic acid group, a methacrylic acid group, and an itaconic acid group. [3] Polyvinyl alcohol-based fibers according to [1] or [2] above, having a crystallinity of 30 to 60%. [4] A fiber structure at least partially comprising the polyvinyl alcohol-based fiber according to any one of [1] to [3]. [5] The fiber structure according to [4], wherein the fiber structure is a nonwoven fabric or a spun yarn.

[0014] The present invention also includes the following preferred embodiments. [6] A method for producing polyvinyl alcohol-based fibers, in which a spinning dope containing 5 to 30% by mass of modified polyvinyl alcohol containing 1 mol% or more of carboxyl groups is wet-spun or dry-wet-spun into a solidification bath mainly containing an organic solvent capable of solidifying polyvinyl alcohol, and the total draw ratio in all processes is 7 times or more at 180°C or higher in any of the processes of wet drawing, drying, dry drawing, and heat treatment, and the drawing tension when the total draw ratio in all processes is 7 times or more is 0.40 cN / dtex or higher. Polyvinyl alcohol fiber Manufacturing method. [7] The method for producing polyvinyl alcohol-based fibers according to [6], wherein the stretching temperature in the dry stretching step is 180°C or higher. [Effects of the Invention]

[0015] The present invention provides polyvinyl alcohol-based fibers that have sufficient shrinkage and shrinkage stress when absorbing moisture at room temperature or higher, particularly at a temperature around 35°C, and a fiber structure at least partially comprising the polyvinyl alcohol-based fibers. Also provided is a method for producing polyvinyl alcohol-based fibers. DETAILED DESCRIPTION OF THE INVENTION

[0016] In the present invention, by using a polyvinyl alcohol-based fiber (hereinafter also referred to as "the present polyvinyl alcohol-based fiber") that contains modified polyvinyl alcohol containing 1 mol % or more of carboxyl groups and has a birefringence of 0.040 or more, it is possible to obtain a water-absorbing shrinkable fiber that has a sufficient shrinkage rate and shrinkage stress when absorbing moisture at a temperature above room temperature, particularly around 35°C.

[0017] Polyvinyl alcohol is obtained by saponifying a vinyl ester polymer obtained by polymerizing a vinyl ester monomer, such as vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, or vinyl versatate, with vinyl acetate being preferred.

[0018] The vinyl ester polymer is preferably one obtained by using one or more vinyl ester monomers as a monomer, more preferably one obtained by using one vinyl ester monomer as a monomer, or may be a copolymer of one or more vinyl ester monomers with another monomer copolymerizable therewith.

[0019] Examples of other monomers copolymerizable with vinyl ester monomers include ethylene; olefins having 3 to 30 carbon atoms such as propylene, 1-butene, and isobutene; acrylic acid or its salts; acrylic acid esters such as methyl acrylate, ethyl acrylate, n-propyl acrylate, i-propyl acrylate, n-butyl acrylate, i-butyl acrylate, t-butyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate, and octadecyl acrylate; methacrylic acid or its salts; methyl methacrylate, ethyl methacrylate, and methacrylic acid. Methacrylic acid esters such as n-propyl methacrylate, i-propyl methacrylate, n-butyl methacrylate, i-butyl methacrylate, t-butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, and octadecyl methacrylate; acrylamide, N-methylacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, diacetone acrylamide, acrylamidopropanesulfonic acid or a salt thereof, acrylamidopropyldimethylamine or a salt thereof, and N-methylolacrylamide or a derivative thereof. Examples of suitable vinyl compounds include acrylamide derivatives; methacrylamide derivatives such as methacrylamide, N-methylmethacrylamide, N-ethylmethacrylamide, methacrylamidepropanesulfonic acid or a salt thereof, methacrylamidepropyldimethylamine or a salt thereof, and N-methylolmethacrylamide or a derivative thereof; N-vinylamides such as N-vinylformamide, N-vinylacetamide, and N-vinylpyrrolidone; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether, and stearyl vinyl ether; vinyl cyanides such as acrylonitrile and methacrylonitrile; vinyl halides such as vinyl chloride, vinylidene chloride, vinyl fluoride, and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; maleic acid or a salt, ester, or acid anhydride thereof; itaconic acid or a salt, ester, or acid anhydride thereof; vinylsilyl compounds such as vinyltrimethoxysilane; and isopropenyl acetate.The vinyl ester polymer may have structural units derived from one or more of these other monomers.

[0020] The modified polyvinyl alcohol containing a carboxy group can be prepared by copolymerizing the vinyl ester monomer with a monomer containing a carboxy group when polymerizing the vinyl ester monomer or, if necessary, when copolymerizing the vinyl ester monomer with another monomer copolymerizable with the vinyl ester monomer, and then saponifying the resulting vinyl ester copolymer; or by subsequently introducing these functional groups into previously synthesized polyvinyl alcohol.

[0021] From the viewpoint of stability during copolymerization reaction with vinyl ester and saponification, the carboxyl group is preferably contained in a functional group such as an acrylic acid group, a methacrylic acid group, or an itaconic acid group, and a modified polyvinyl alcohol obtained by copolymerizing a monomer containing such a functional group with the vinyl ester monomer is preferred. The acrylic acid group refers to a residue obtained by removing hydrogen atoms other than the hydroxyl group from acrylic acid, and there is no particular limitation on the hydrogen atoms to be removed, as long as they are not hydroxyl groups. The same applies to methacrylic acid groups and itaconic acid groups.

[0022] Examples of the monomer containing an acrylic acid group in the functional group include acrylic acid or its salts, and acrylic acid esters such as methyl acrylate, ethyl acrylate, n-propyl acrylate, and isopropyl acrylate. Among these, acrylic acid and methyl acrylate are preferred from the viewpoints of copolymerization reaction with vinyl esters and stability during saponification.

[0023] Examples of the monomer containing a methacrylic acid group include methacrylic acid or its salts, and methacrylic acid esters such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, and isopropyl methacrylate. Among these, methacrylic acid and methyl methacrylate are preferred from the viewpoints of copolymerization with vinyl esters and stability during saponification.

[0024] Examples of the monomer containing an itaconic acid group include itaconic acid or a salt thereof, itaconic acid esters such as monomethyl itaconate, dimethyl itaconate, monoethyl itaconate, and diethyl itaconate, itaconic anhydride or a derivative thereof, etc. Among these, itaconic acid, monomethyl itaconate, and dimethyl itaconate are preferred from the viewpoints of copolymerization reaction with vinyl esters and stability during saponification.

[0025] Among the above monomers, preferred are monomers containing at least one functional group selected from the group consisting of an acrylic acid group, a methacrylic acid group, and an itaconic acid group. The above-mentioned monomers having a functional group may be used alone or in combination of two or more, but it is preferable to use one type of monomer.

[0026] When the modified polyvinyl alcohol is produced by copolymerizing a monomer containing the functional group with a vinyl ester monomer, the content of carboxy groups in the resulting modified polyvinyl alcohol can be adjusted by appropriately adjusting the amount of the monomer during copolymerization. When these functional groups are subsequently introduced into polyvinyl alcohol, the desired content of the functional groups can be achieved by adjusting the amount of polyvinyl alcohol and the amount of the compound having the functional groups. The carboxyl groups in the modified polyvinyl alcohol may be condensed with hydroxyl groups within the molecule or with hydroxyl groups of other molecules to form intramolecular or intermolecular ester bonds, or may exist as metal salts with metals.

[0027] In the present invention, the content of carboxy groups in the modified polyvinyl alcohol (hereinafter also referred to as "modification amount") is 1 mol % or more in the modified polyvinyl alcohol from the viewpoint of shrinkage when absorbing moisture at room temperature or higher and water solubility when discarded. The content of carboxy groups is preferably 1.5 mol % or more, more preferably 2 mol % or more.

[0028] From the viewpoint of shrinkage stress when absorbing moisture at room temperature or above, the carboxy group content is preferably 20 mol % or less, more preferably 6 mol % or less, even more preferably 4 mol % or less, and particularly preferably 3 mol % or less. As long as the effects of the present invention are not impaired, the modified polyvinyl alcohol may contain functional groups other than carboxy groups or may be modified with other components, such as allyl sulfonic acid, vinyl pyrrolidone, and ethylene.

[0029] The degree of polymerization (viscosity average degree of polymerization) of the modified polyvinyl alcohol is preferably 2400 or less, more preferably 1800 or less, from the viewpoints of mechanical strength and suppression of insolubilization due to gelation. If the degree of polymerization is too high, the solubility in water decreases, and disposal of the moisture absorber after use may be an environmental burden. Furthermore, from the viewpoints of suppressing a decrease in spinnability and interfiber agglutination and maintaining the mechanical performance and quality of fibers and fiber structures, the degree of polymerization is preferably 500 or more, more preferably 700 or more, and particularly preferably 1000 or more. The degree of polymerization can be determined by measurement in accordance with JIS K 6726 as described below.

[0030] The polyvinyl alcohol-based fiber contains the modified polyvinyl alcohol. The polyvinyl alcohol-based fiber may contain one or more of the modified polyvinyl alcohols, or may contain a modified polyvinyl alcohol and another polyvinyl alcohol. The polyvinyl alcohol-based fiber may further contain a polymer other than polyvinyl alcohol.

[0031] For example, the polyvinyl alcohol fiber may be: (a) Polyvinyl alcohol-based fibers composed of one or more of the above-mentioned modified polyvinyl alcohols; (b) Polyvinyl alcohol-based fibers composed of the modified polyvinyl alcohol and polyvinyl alcohol having no carboxy group (hereinafter, sometimes referred to as vinyl alcohol polymer (A)). etc. When a polymer other than polyvinyl alcohol is contained, for example, (c) Polyvinyl alcohol-based fibers composed of the modified polyvinyl alcohol, vinyl alcohol polymer (A) and other polymers other than polyvinyl alcohol, or polyvinyl alcohol-based fibers composed of the modified polyvinyl alcohol and other polymers other than polyvinyl alcohol (hereinafter also referred to as "other polymers"). etc.

[0032] In the above (a), the multiple types of modified polyvinyl alcohols differ in at least one of the type of functional group including a carboxyl group, the degree of modification, the degree of saponification, and the degree of polymerization. In the above (b), the modified polyvinyl alcohol and the polyvinyl alcohol polymer (A) may have the same or different degrees of saponification and polymerization.

[0033] When the polyvinyl alcohol-based fiber contains a polyvinyl alcohol other than modified polyvinyl alcohol or a polymer other than polyvinyl alcohol, the content of modified polyvinyl alcohol in the polyvinyl alcohol-based fiber is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, based on the total mass of the polyvinyl alcohol-based fiber being 100% by mass. The present polyvinyl alcohol-based fiber preferably contains 100% by mass of the modified polyvinyl alcohol, and more preferably contains 100% by mass of one type of modified polyvinyl alcohol. Polyvinyl alcohol fibers can be produced by spinning these modified polyvinyl alcohols.

[0034] The polyvinyl alcohol fiber has a birefringence of 0.040 or more. Birefringence is affected by the orientation of the amorphous and crystalline parts of the polymer, residual internal stress, etc. Polyvinyl alcohol fibers with a birefringence of 0.040 or more are thought to exhibit high shrinkage rates and shrinkage stress when they absorb moisture due to the influence of these orientations and residual stress. The birefringence of the polyvinyl alcohol fiber is preferably 0.041 or more, more preferably 0.042 or more, and even more preferably 0.045 or more. There is no particular upper limit to the birefringence, but it is usually 0.052 or less.

[0035] The saponification degree of the polyvinyl alcohol fiber is preferably 97 mol% or more, more preferably 98 mol% or more, and even more preferably 99 mol% or more. If the saponification degree is less than the above range, the mechanical strength of the polyvinyl alcohol fiber may be poor, and for example, the shape retention of the moisture absorber may be insufficient. Furthermore, if the saponification degree is less than the above range, the dissolution rate at around 35°C may be high. A higher saponification degree may enable a higher shrinkage stress. The saponification degree is usually 100 mol% or less, and preferably 99.5 mol% or less.

[0036] As described above, the polyvinyl alcohol fiber may contain one or more modified polyvinyl alcohols. When a single modified polyvinyl alcohol is used, the desired polyvinyl alcohol fiber can be obtained by spinning the modified polyvinyl alcohol having a saponification degree within the above range using the method described below.

[0037] When two or more types of modified polyvinyl alcohols are used, the saponification degrees of the respective modified polyvinyl alcohols are additive. Therefore, the saponification degrees of the respective modified polyvinyl alcohols are determined in advance by measurement or the like, and the saponification degree of the entire polyvinyl alcohol-based fiber is calculated using the following formula (1). The amounts of the polyvinyl alcohols used are then adjusted so that the resulting saponification degrees fall within the above range. The degree of saponification of polyvinyl alcohol can usually be determined by the method described in JIS K 6726.

[0038] Degree of saponification of polyvinyl alcohol fiber (mol%) =[Σ(ni×Mi)] / 100 (1) ni: Degree of saponification of each modified polyvinyl alcohol (mol%) Mi: The proportion of each modified polyvinyl alcohol in the polyvinyl alcohol fiber (mass%)

[0039] If the polyvinyl alcohol-based fiber contains a polymer other than modified polyvinyl alcohol, the proportion of the polymer other than modified polyvinyl alcohol is substituted for Mi in the above formula (1). Furthermore, if the polymer other than modified polyvinyl alcohol is polyvinyl alcohol, the degree of saponification of the polymer is substituted for ni, and if the polymer is other than polyvinyl alcohol, 0 (zero) is substituted.

[0040] The polyvinyl alcohol fiber has a crystallinity of preferably 60% or less, more preferably 50% or less, from the viewpoints of shrinkage properties and water solubility, and preferably 30% or more, more preferably 40% or more, from the viewpoints of shrinkage stress, fiberization, and mechanical strength. The degree of crystallinity can be controlled by the degree of polymerization of the modified polyvinyl alcohol constituting the polyvinyl alcohol fiber, the degree of saponification, the type and degree of modification of the functional group containing a carboxy group, and the like.

[0041] In producing polyvinyl alcohol-based fibers, a spinning dope containing the modified polyvinyl alcohol is prepared. The solvent of the spinning dope may be water, but it is preferable to use an organic solvent for the spinning dope because it can produce fibers with high mechanical properties and dimensional stability, a uniform, substantially circular cross section, and a lower underwater fusion temperature than when the solvent of the spinning dope is water, and therefore the solvent is excellent in water solubility when discarded.

[0042] Examples of organic solvents include polar solvents such as dimethyl sulfoxide (hereinafter also referred to as "DMSO"), dimethylacetamide, dimethylformamide, and N-methylpyrrolidone, polyhydric alcohols such as glycerin and ethylene glycol, mixtures of these with swellable metal salts such as rhodanide, lithium chloride, calcium chloride, and zinc chloride, and mixtures of these solvents with each other or with water. DMSO is particularly preferred in terms of low-temperature solubility, low toxicity, and low corrosivity.

[0043] The concentration of polyvinyl alcohol in the spinning dope is in the range of 5 to 30% by mass. Here, the polyvinyl alcohol concentration refers to the concentration of modified polyvinyl alcohol when the polyvinyl alcohol-based fiber contains only modified polyvinyl alcohol, and refers to the total concentration of both when the polyvinyl alcohol-based fiber contains modified polyvinyl alcohol and the vinyl alcohol polymer (A). When the polyvinyl alcohol-based fiber contains the other polymers, the concentration of the other polymers is not included in the concentration of the polyvinyl alcohol.

[0044] When the solvent of the spinning dope is an organic solvent, it is preferable to dissolve the polymer under stirring under reduced pressure after nitrogen substitution in order to prevent oxidation, decomposition, crosslinking reactions, etc. and to suppress foaming. The liquid temperature of the spinning dope when extruded is preferably in the range of 50 to 150°C, which is a range that does not cause gelation, decomposition, or coloration of the dope.

[0045] When the polyvinyl alcohol-based fiber contains only modified polyvinyl alcohol, the solvent is used to prepare a spinning dope for modified polyvinyl alcohol. When the polyvinyl alcohol-based fiber contains modified polyvinyl alcohol and the vinyl alcohol polymer (A), the modified polyvinyl alcohol and the vinyl alcohol polymer (A) may be mixed in advance when preparing the spinning dope, and the solvent may be used to prepare a spinning dope. Alternatively, the solvent may be used to prepare solutions each containing modified polyvinyl alcohol and the vinyl alcohol polymer (A), and the solutions may then be mixed to prepare a spinning dope. When the polyvinyl alcohol-based fiber further contains the other polymer, the other polymer may be mixed with either or both of the modified polyvinyl alcohol and the vinyl alcohol polymer (A), or the other polymer may be dissolved in the solvent in advance before use.

[0046] Polyvinyl alcohol-based fibers can be produced by spinning the spinning dope prepared as described above. The spinning method is not particularly limited, and examples include dry spinning, wet spinning, and dry-wet spinning. Among these, wet spinning or dry-wet spinning is preferred due to its high productivity, and the spinning dope is discharged into a solidification solution capable of solidifying polyvinyl alcohol. In particular, when discharging the spinning dope from multiple holes, wet spinning is preferred over dry-wet spinning to prevent fibers from sticking together during discharging. The wet spinning method refers to a method in which the spinning dope is discharged directly from the spinneret into a solidification bath, while the dry-wet spinning method refers to a method in which the spinning dope is first discharged from the spinneret into air or an inert gas and then introduced into a solidification bath. In the present invention, solidification refers to the transformation of a fluid spinning dope into a non-fluid solid, and includes both gelation, in which the dope solidifies without changing its composition, and coagulation, in which the dope solidifies with a change in composition.

[0047] When the solvent of the spinning dope is water, for example, a saturated aqueous solution of sodium sulfate can be discharged as the solidification liquid. When the solvent of the spinning dope is an organic solvent, the spinning dope can be discharged into a solidification bath mainly composed of an organic solvent, such as alcohols (e.g., methanol, ethanol, propanol, butanol), ketones (e.g., acetone, methyl ethyl ketone, methyl isobutyl ketone), fatty acid esters (e.g., methyl acetate, ethyl acetate), aromatics (e.g., benzene, toluene), or a mixture of two or more of these. To achieve sufficient solidification deep into the fiber, it is preferable to use a solidification bath containing the solvent of the spinning dope. The mass ratio of the solidification bath to the solvent of the spinning dope is preferably 95 / 5 to 40 / 60, more preferably 90 / 10 to 50 / 50, and most preferably 85 / 15 to 55 / 45. Furthermore, by mixing the solvent of the spinning dope with the solidification bath, the solidification ability can be adjusted and the cost of separating and recovering the solvent of the spinning dope from the solidification bath can be reduced. There are no limitations on the temperature of the solidification bath, but when the spinning dope solvent is an organic solvent, solidification is usually carried out at a solidification bath temperature between -15 and 30°C. From the viewpoints of uniform solidification and energy conservation, the solidification bath temperature is preferably -10 to 20°C, more preferably -5 to 15°C, and particularly preferably 0 to 10°C. If the solidification bath temperature is outside this temperature range, the tensile strength of the resulting fiber may decrease. If the spinning dope is heated to a high temperature, it is preferable to cool the solidification bath to keep the temperature low.

[0048] The fiber is then wet-stretched as needed after being removed from the solidification bath. From the viewpoints of the mechanical properties of the fiber and preventing sticking, wet stretching is preferably performed at a wet stretching ratio of 1.5 to 5 times, particularly 2.5 to 4 times. To prevent sticking of the filaments, it is preferable to increase the wet stretching ratio within a range that does not cause fuzz. In order to increase the wet stretching ratio, it is effective to perform wet stretching in two or more stages during the process of extracting and removing the spinning dope described below.

[0049] When the spinning solution contains an organic solvent, it is preferable to extract and remove the spinning solution solvent from the filaments by contacting the filaments with an extraction bath primarily containing an organic solvent capable of solidifying. The wet drawing and extraction may also be performed in the same process. This extraction process can shorten the residence time in the extraction bath by continuously flowing a pure solidifying solution countercurrently to the running direction of the filaments. This extraction process reduces the amount of spinning solution solvent contained in the filaments to 1% or less, particularly 0.1% or less, of the filament mass, making this method preferable. The contact time is preferably 5 seconds or more, particularly 15 seconds or more. To increase the extraction rate and improve extraction, it is preferable to break up the filaments in the extraction bath. Prior to drying, other methods that are effective in preventing sticking include replacing the solvent in the spinning solution with a solvent that has a high solidifying ability for polyvinyl alcohol, such as ketones such as acetone, methyl ethyl ketone, and methyl isobutyl ketone, applying a hydrophobic oil such as a mineral oil, polyethylene oxide, silicone, or fluorine-based oil in solution or emulsion form, or shrinking the material to relieve shrinkage stress during drying.

[0050] Next, in the drying step, the fibers are preferably dried at 180°C or less, and further dry stretching can improve the mechanical properties of the fibers. The dry stretching conditions can be appropriately selected depending on the properties of the polyvinyl alcohol, particularly the melting point and the desired water fusion temperature. The stretching ratio in the dry stretching after the drying step is preferably about 1.1 to 12 times, and the dry stretching temperature is preferably 160 to 220°C. From the viewpoint of processability and the effect of dry stretching, the dry stretching temperature is preferably 160 to 240°C, and particularly from the viewpoint of shrinkage, 180 to 220°C is more preferable. From the viewpoint of suppressing inter-fiber agglutination and performing efficient stretching, it is preferable to perform dry stretching in multiple stages at 6 times or more, and multi-stage stretching with increasing temperature is particularly preferable.

[0051] After the dry stretching, the polyvinyl alcohol-based fiber can be heat-treated in a heat treatment step to adjust the crystallinity of the fiber to a desired range. The heat treatment temperature is preferably 160 to 240° C. The heat treatment step and the dry stretching step may be performed in the same step.

[0052] In the method for producing polyvinyl alcohol-based fibers, the total draw ratio in all steps is set to 7 times or more at 180°C or higher in any of the wet drawing, drying, dry drawing, and heat treatment steps. Drawing may be performed so that the total draw ratio is 7 times or more in any of the wet drawing, drying, dry drawing, and heat treatment steps. The stretching temperature may be 180°C or higher when the total stretching ratio is 7 times or more in any of the wet stretching, drying, dry stretching, and heat treatment steps. When the total stretching ratio is 7 times or more in any of the wet stretching, dry stretching, and heat treatment steps, it is preferable to set the dry stretching temperature to 180°C or higher from the viewpoint of increasing the birefringence of the resulting polyvinyl alcohol-based fiber. It is preferable to set the stretching temperature to 180°C or higher and the total stretching ratio to 7 times or more in any of the dry stretching and heat treatment steps. The total draw ratio is preferably 7 times or more, more preferably 8 times or more, and even more preferably 9 times or more. There is no upper limit to the total draw ratio, but it is preferably within a range that does not cause fluffing of the resulting polyvinyl alcohol-based fiber, and is usually 20 times or less.

[0053] Furthermore, the stretching tension when the total stretching ratio is 7 times or more is preferably 0.40 cN / dtex or more from the viewpoint of making the birefringence of the resulting polyvinyl alcohol-based fiber 0.040 or more, and more preferably 0.60 cN / dtex or more from the viewpoint of further increasing the birefringence. When making the birefringence of the resulting polyvinyl alcohol-based fiber 0.040 or more, the stretching tension is the stretching tension when the total stretching ratio in all steps, including wet stretching, drying, dry stretching, and heat treatment, is 7 times or more. The stretching tension when stretching does not make the total stretching ratio 7 times or more and when stretching after the total stretching ratio has reached 7 times or more does not necessarily have to be the above-mentioned stretching tension. There is no particular upper limit to the drawing tension, but it is preferable to carry out the drawing within a range that does not cause fluffing of the resulting fibers, and it is usually 2.0 cN / dtex or less.

[0054] The polyvinyl alcohol-based fiber can be produced by the method for producing a polyvinyl alcohol-based fiber of the present invention, in which in any one of the wet stretching, drying, dry stretching, and heat treatment steps, the total stretch ratio in all steps is 7 times or more at 180°C or higher, and the stretching tension when the total stretch ratio in all steps is 7 times or more at 180°C or higher is 0.40 cN / dtex or higher. It is more preferable that the dry stretching temperature is 180°C or higher and the stretching tension during dry stretching is 0.40 cN / dtex or higher. That is, as an example of a method for producing the present polyvinyl alcohol-based fiber, An example of a production method is to wet-spin or dry-wet-spin a spinning dope containing 5 to 30% by mass of modified polyvinyl alcohol containing 1 mol% or more of carboxy groups in a solidification bath mainly containing an organic solvent capable of solidifying polyvinyl alcohol, and to achieve a total draw ratio of 7 times or more at 180°C or higher in all steps in any of the steps of wet drawing, drying, dry drawing, and heat treatment, wherein the draw tension when achieving a total draw ratio of 7 times or more at 180°C or higher in all steps is 0.40 cN / dtex or higher.

[0055] The polyvinyl alcohol fiber preferably has a high shrinkage rate and shrinkage stress when absorbing moisture at room temperature or higher. Therefore, the shrinkage rate of the polyvinyl alcohol fiber at 35°C is preferably 55% or higher, more preferably 60% or higher, and even more preferably 65% ​​or higher. The shrinkage rate and shrinkage stress at 35°C are the shrinkage rate and shrinkage stress when the polyvinyl alcohol fiber absorbs water at 35°C, and artificial urine made by adding urea to physiological saline is used as the water. The shrinkage stress at 35°C is preferably 0.15 cN / dtex or more, more preferably 0.2 cN / dtex or more, and even more preferably 0.23 cN / dtex or more.

[0056] The shrinkage rate of the polyvinyl alcohol fiber at 45°C is preferably 55% or more, more preferably 60% or more, and even more preferably 65% ​​or more. The shrinkage stress at 45°C is preferably 0.15 cN / dtex or more, more preferably 0.2 cN / dtex or more, and even more preferably 0.23 cN / dtex or more. The shrinkage rate and shrinkage stress at 45°C are measured in the same manner as in the measurement of the shrinkage rate and shrinkage stress at 35°C, except that the temperature of the water is 45°C.

[0057] The polyvinyl alcohol fiber shrinks quickly when it absorbs moisture above room temperature, particularly around 35°C, and from the viewpoint of ensuring a moisture flow path, the shrinkage rate after immersion in artificial urine at 35°C for 30 seconds is preferably 25% or more, and more preferably 40% or more.

[0058] From the viewpoint of preventing leakage after absorbing water, it is preferable that the shrinkage rate is maintained, and the shrinkage rate after immersion in artificial urine at 35°C for 60 seconds is preferably 30% or more, more preferably 45% or more. The shrinkage rate after immersion in artificial urine at 35°C for 300 seconds is preferably 40% or more, more preferably 55% or more. Furthermore, it is preferable that the shrinkage rate after 24 hours of immersion in artificial urine at 35°C is maintained within a range of 90% to 110% of the shrinkage rate after 300 seconds.

[0059] When the polyvinyl alcohol fiber is used as part of an absorbent, the resulting absorbent preferably has excellent moisture absorption and water retention. Therefore, the absorbency of the polyvinyl alcohol fiber after immersion for 10 minutes is preferably 4 times or more, more preferably 6 times or more. Furthermore, the absorbency after immersion for 1 hour is preferably 5 times or more, more preferably 6 times or more. From the viewpoint of water retention and shrinkage stress, the dissolution rate is preferably 5% or less. 3% or less is more preferable.

[0060] The water absorption capacity was measured by immersing polyvinyl alcohol fibers in physiological saline at 35°C for a certain period of time, air-drying the fibers, and measuring the mass of the fibers. Mass after immersion divided by mass before immersion The absorbency after 1 hour of immersion is the absorbency after 1 hour of immersion. After measuring the absorption capacity, the polyvinyl alcohol fiber is dried at 120°C for 2.5 hours, and then its mass is measured. The change in mass before and after drying is divided by the mass before drying, and the resulting value is expressed as a percentage. Since the mass usually decreases after drying due to evaporation of water, the dissolution rate is the amount of absorbed water lost due to drying. Therefore, the smaller the dissolution rate, the better the water absorbency and shrinkage stress.

[0061] When the polyvinyl alcohol-based fiber is used as at least a part of a fiber structure described later, the polyvinyl alcohol-based fiber is preferably water-soluble from the viewpoint of disposal of the fiber structure after use. Therefore, from the viewpoint of water solubility, the melt-cutting temperature of the polyvinyl alcohol-based fiber in water is preferably 80°C or less, more preferably 60°C or less. There is no particular lower limit to the severance temperature in water, but it may be room temperature or higher.

[0062] The shrinkage rate, shrinkage stress, absorption rate and elution rate of the present polyvinyl alcohol-based fiber can be controlled by the modification amount of the carboxyl group of the modified polyvinyl alcohol, the degree of saponification, the degree of crystallinity, and further by the concentration of the stock solution, the drawing temperature, the heat treatment temperature and the draw ratio in all steps in the production process of the polyvinyl alcohol-based fiber.

[0063] The tensile strength of the polyvinyl alcohol fiber is preferably 3 cN / dtex or more, more preferably 4 cN / dtex or more. There is no upper limit to the tensile strength, but it may be 25 cN / dtex or less. The tensile strength of the polyvinyl alcohol fiber can be adjusted to a desired value by controlling the drawing conditions, such as the drawing temperature and draw ratio, in the method for producing the polyvinyl alcohol fiber.

[0064] Other commonly used additives may be added to the polyvinyl alcohol fiber, preferably in an amount of 0.1 to 10 parts by mass, based on 100 parts by mass of the total mass of the polyvinyl alcohol fiber.

[0065] The single fiber fineness of the present polyvinyl alcohol-based fiber is not particularly limited, but fibers ranging from 0.1 to 1000 dtex, particularly 0.2 to 100 dtex, and even more preferably 0.5 to 10 dtex, are widely used. The fiber length can be set appropriately depending on the application, but for example, when processed into paper or spun yarn, a fiber length of approximately 1 to 100 mm is preferred. Furthermore, the cross-sectional shape of the present polyvinyl alcohol-based fiber is not limited, but simple, substantially circular fibers are preferred over complex shapes in terms of water dispersibility and product homogeneity.

[0066] The polyvinyl alcohol fiber exhibits excellent shrinkage and shrinkage stress upon absorbing moisture at temperatures above room temperature, particularly around 35°C. The fiber can be used for applications such as moisture absorbers, moisture detection sensors, fishing gear tightening lines, plant grafting cords, root wrapping cords, and food tightening cords. Among these, the fiber is particularly suitable for use in fiber structures such as moisture absorbers. When used as a moisture absorber, the fiber can be processed into, for example, filaments, cut fibers, spun yarns, strings, fabrics such as woven or knitted fabrics or nonwoven fabrics, ropes, etc., with nonwoven fabrics and spun yarns being preferred. Furthermore, the cut fibers are used in lengths of, for example, 0.1 to 50 mm.

[0067] When the present polyvinyl alcohol-based fiber is used as a moisture absorber, the content of the polyvinyl alcohol-based fiber of the present invention in the moisture absorber is preferably 30% by mass or more, more preferably 50% by mass or more, and particularly preferably 70 to 100% by mass, with the total mass of the moisture absorber being 100% by mass.

[0068] Furthermore, as a moisture absorber, the present invention is suitable for use in sanitary materials, particularly absorbent sanitary materials such as diapers and sanitary napkins, especially diapers. The fiber may be used in any form, but significant effects are obtained when it is in the form of a string or nonwoven fabric. It may be integrated with other fabrics such as films, woven fabrics, or nonwoven fabrics to form a product. Furthermore, the present polyvinyl alcohol-based fiber or a fiber structure using the present polyvinyl alcohol-based fiber may be used as part of a product. The use of a string in the form of a spun yarn or filament is preferred from the viewpoints of achieving contraction performance, preventing leakage, and enhancing the fit to the body. For example, by placing a string made of the fiber of the present invention at the end of a diaper, the moisture absorption performance of the diaper can be improved and leakage can be efficiently suppressed.

[0069] In addition to the above-mentioned sanitary materials, the polyvinyl alcohol fiber can be used for moisture detection sensors, fishing tackle tying lines, plant grafting tying lines, root wrapping tying cords, food tying cords, etc.

[0070] The method for producing the spun yarn is not particularly limited, and it can be produced by a known method for producing a spun yarn, such as ring spinning, tow spinning, etc. In order to effectively exhibit the water absorption and shrinkage performance of the present polyvinyl alcohol-based fiber, the thickness of the spun yarn is preferably about 1 to 20 British cotton count.

[0071] Furthermore, when the polyvinyl alcohol-based fiber is used as a dry nonwoven fabric, excellent effects can be obtained in terms of texture, shrinkage performance, and suppression of stuffiness. The method for producing the nonwoven fabric is not particularly limited, but examples include a needle-punching method and a method in which a web containing a heat-fusible fiber is heated. To effectively achieve the water-absorbing shrinkage performance of the polyvinyl alcohol-based fiber, a nonwoven fabric produced without using a binder, preferably a needle-punched nonwoven fabric, is preferred. The thickness of the nonwoven fabric is preferably about 0.5 to 3 mm.

[0072] The method for producing such fabrics is not particularly limited, but from the viewpoint of feel, flexibility, etc., it is preferable to treat a fiber web to produce a dry nonwoven fabric. Suitable methods for producing dry nonwoven fabrics include, for example, forming a web by opening polyvinyl alcohol fiber filaments or the like by the repulsive action caused by frictional charging, or by opening crimped and cut staples with a card or the like, and then thermocompressing the web with a hot embossing roller at an area compression rate of 10 to 50%, more preferably 10 to 30%, i.e., 10 to 50%, more preferably 10 to 30%, of the surface area of ​​the nonwoven fabric. By subjecting a portion of the nonwoven fabric to thermocompression treatment, it is possible to improve the mechanical properties and dimensional stability of the nonwoven fabric without impairing the feel, flexibility, and water solubility of the nonwoven fabric. From the viewpoint of feel, water solubility, etc., the area of ​​each thermocompression-bonded portion should be 4 cm. 2 Below, especially 2cm 2 Below, another 1cm 2 It is preferable that the thickness is 1 mm or less, and from the viewpoint of the mechanical properties of the nonwoven fabric, 2 or more. The thermocompression temperature may be, for example, about 120 to 230°C, and the pressure may be about 1 to 6 MPa. Since the polyvinyl alcohol-based fiber exhibits adhesive properties when dry-heat treated, the fibers can be bonded by such embossing treatment, thereby efficiently improving the mechanical properties of the nonwoven fabric, and the thermocompression treatment also makes it easy to form the nonwoven fabric into a desired shape. For example, it may be formed into a desired shape such as a bag or box. Bag-shaped products are suitable for use as packaging materials. For example, bag-shaped products with sides of about 3 to 10 cm may be used.

[0073] Another method for producing a dry nonwoven fabric is to produce a nonwoven fabric by entangling the fibers using needle punching. In this case, a known needle punching machine is used, and conditions such as needle density, needle type, needle depth, and number of punches can be adjusted depending on the properties of the fibers to produce a dry nonwoven fabric with excellent strength and flexibility. If necessary, entanglement can be optimized using multiple needle punching machines. [Example]

[0074] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way. The artificial urine and physiological saline used in the following measurements are as follows: Artificial urine: Physiological saline solution to which urea has been added at a concentration of 2% by mass. Saline: phosphate buffered saline with a concentration of 0.01 mol / L.

[0075] [Degree of polymerization] It was calculated from the measured value of the intrinsic viscosity [η] of an aqueous solution at 30° C. according to JIS K 6726 using the following formula (2), where P is the average degree of polymerization of polyvinyl alcohol. logP = 1.613 log([η] × 10 4 / 8.29) (2)

[0076] [Saponification degree (mol%)] Measurement was carried out in accordance with JIS K 6726.

[0077] [Fiber strength (cN / dtex)] Measurement was carried out in accordance with JIS L 1013.

[0078] [Birefringence of polyvinyl alcohol fiber] Using a commercially available polarizing microscope, a sodium lamp was used as a light source, and the retardation was calculated by the Berek compensator method with the sample immersed in α-bromonaphthalene.

[0079] [Polyvinyl alcohol fiber crystallinity (%)] Using a Mettler differential scanning calorimeter (DSC-20), 10 mg of a fiber sample was heated at a rate of 20°C / min under nitrogen, and the endothermic heat ΔH (J / g) at the endothermic peak was measured. The crystallinity was calculated from the ratio to the heat of fusion of the complete crystal of polyvinyl alcohol, 174.5 J / g, using the following formula (3): Crystallinity (%)=ΔH(J / g) / 174.5(J / g)×100 (3)

[0080] [Fiber cutting temperature in water (℃)] (WTb in water) A 10 cm long fiber bundle was hung in artificial urine at 2°C with a load of 2.2 mg / dtex, and the water temperature was raised at a rate of 2°C / min. The temperature at which the fiber dissolved and the load dropped was taken as the underwater melting temperature.

[0081] [Water absorption rate of fiber at 35℃] The fiber was precisely weighed, immersed in saline at 35°C for 1 hour, left to stand for 10 minutes to drain, and then its mass was measured. The mass of the fiber before immersion in saline was A (g) and the mass after immersion was B (g), and the water absorption capacity was calculated using the following formula (4): Water absorption capacity (times) = (B) / (A) (4)

[0082] [Fiber elution rate (%)] The fiber was precisely weighed, immersed in saline at 35°C for 1 hour, left to stand for 10 minutes to drain, and then dried at 105°C for 4 hours, after which the mass was measured. The dissolution rate was calculated using the following formula (5), where A (g) is the mass of the fiber before immersion in saline, and C (g) is the mass after immersion and drying. Elution rate (%)=[(A)-(C)] / (A)×100 (5)

[0083] [Fiber shrinkage rate at 35°C (%)] A 10 cm long fiber bundle was hung in artificial urine at 2°C with a load of 2.2 mg / dtex applied, and the water temperature was raised at a rate of 2°C / min. Before the fiber dissolved and the load dropped, the fiber length at 35°C was divided by the original length to obtain the 35°C shrinkage rate.

[0084] [Fiber shrinkage stress at 35°C (cN / dtex)] A 10 cm long fiber bundle was fixed at both ends using a hand-tightened chuck for Cell A with an initial load of 1 / 10 g / dtex, immersed in a water bath, and heated at a rate of 0.9°C / min. The stress at 35°C was measured using an Autograph AG-IS / I / IC / EZGraph.

[0085] [Fiber shrinkage stress at 45°C (cN / dtex)] A 10 cm long fiber bundle was fixed at both ends using a hand-tightened chuck for Cell A with an initial load of 1 / 10 g / dtex, immersed in a water bath, and heated at a rate of 0.9°C / min. The stress at 45°C was measured using an Autograph AG-IS / I / IC / EZGraph.

[0086] [Fiber shrinkage rate after 30 seconds (%)] A load of 0.15 mg / dtex was applied to a 10 cm long fiber bundle, which was then suspended in physiological saline at 35°C, and the shrinkage rate was measured after 30 seconds.

[0087] [Fiber shrinkage rate after 60 seconds (%)] A load of 0.15 mg / dtex was applied to a 10 cm long fiber bundle, which was then suspended in physiological saline at 35°C, and the shrinkage rate was measured after 60 seconds.

[0088] [Fiber shrinkage rate after 300 seconds (%)] A load of 0.15 mg / dtex was applied to a 10 cm long fiber bundle, which was then suspended in physiological saline at 35°C, and the shrinkage rate was measured after 300 seconds.

[0089] [Fiber shrinkage rate after 24 hours (%)] A load of 0.15 mg / dtex was applied to a 10 cm long fiber bundle, which was then suspended in physiological saline at 35°C, and the shrinkage rate was measured after 24 hours.

[0090] [Leak-proof property of nonwoven fabric sheet] Polyvinyl alcohol fiber yarn was spun into a spun yarn of 10 count by a British cotton shrinkage method using known methods such as ring spinning and tow spinning. 2 200 mm of spun yarn was sewn onto both ends of the pulp sheet containing the water-absorbing material. 20 g of artificial urine at 35°C was then poured onto the pulp sheet and left to stand for 1 minute, after which the state of urine leakage was evaluated as ○ if there was no leakage and × if there was leakage.

[0091] [Example 1] Modified polyvinyl alcohol (Elvanol 80-18, manufactured by Kuraray) containing 5.2 mol% acrylic groups, a copolymer of acrylic acid groups and carboxyl-containing methyl acrylate, was dissolved in DMSO with stirring at 90°C for 5 hours to obtain a spinning solution with a polyvinyl alcohol concentration of 22% by mass. This spinning solution was wet-spun through a nozzle with 40,000 holes and a 0.08 mm diameter into a 10°C solidification bath containing 80 / 20 methanol / DMSO and then wet-heat stretched 3.0 times in a 20°C methanol bath. After extracting the DMSO from the yarn with methanol, a spinning oil was added and the yarn was dried at 140°C. The resulting dried yarn was hot-heat stretched at 180°C to a 3.3x draw ratio (total draw ratio TD = 10x). The stretching tension was 0.61 cN / dtex. Polyvinyl alcohol fibers were then produced by dry heat shrinkage at 180°C with a dry heat shrinkage rate of 1%. The birefringence, crystallinity, underwater fusion temperature, water absorption rate, elution rate, shrinkage rate, shrinkage stress, and leakproofness of the nonwoven fabric sheet of the obtained fibers were measured, and the results are shown in Tables 1 and 2.

[0092] [Example 2] Modified polyvinyl alcohol (Elvanol T-25, manufactured by Kuraray) containing 2.5 mol% methacrylic groups, a copolymer of methyl methacrylate containing carboxyl groups, was dissolved in DMSO at 90°C with stirring for 5 hours to obtain a spinning solution with a polyvinyl alcohol concentration of 20% by mass. This spinning solution was wet-spun through a nozzle with 35,000 holes and a 0.08 mm diameter into a 10°C solidification bath containing 70 / 30 methanol / DMSO and then subjected to wet heat stretching at 3.0x in a 20°C methanol bath. After extracting the DMSO from the yarn with methanol, a spinning oil was applied and the yarn was dried at 140°C. The resulting dried yarn was hot-stretched at 180°C at a hot stretch ratio of 3.3x (total stretch ratio TD = 10x). The stretching tension was 0.68 cN / dtex. Polyvinyl alcohol fibers were then produced by dry heat shrinkage at 180°C with a dry heat shrinkage rate of 1%. The birefringence, crystallinity, underwater fusion temperature, water absorption rate, elution rate, shrinkage rate, shrinkage stress, and leakproofness of the nonwoven fabric sheet of the obtained fibers were measured, and the results are shown in Tables 1 and 2.

[0093] [Example 3] Modified polyvinyl alcohol (Elvanol T-25, manufactured by Kuraray) containing 2.5 mol% methacrylic groups, a copolymer of methyl methacrylate containing carboxyl groups, was dissolved in DMSO at 90°C with stirring for 5 hours to obtain a spinning solution with a polyvinyl alcohol concentration of 20% by mass. This spinning solution was wet-spun through a nozzle with 35,000 holes and a 0.08 mm diameter into a 10°C solidification bath containing 70 / 30 methanol / DMSO and then subjected to wet heat drawing at 3.0x in a 20°C methanol bath. After extracting the DMSO from the yarn with methanol, a spinning oil was applied and the yarn was dried at 140°C. The resulting dried yarn was hot-drawn at 200°C at a 4x hot draw ratio (total draw ratio TD = 12x). The drawing tension was 1.15 cN / dtex. Polyvinyl alcohol fibers were then produced by dry heat shrinkage at 200°C with a dry heat shrinkage rate of 1%. The birefringence, crystallinity, underwater fusion temperature, water absorption rate, elution rate, shrinkage rate, shrinkage stress, and leakproofness of the nonwoven fabric sheet of the obtained fibers were measured, and the results are shown in Tables 1 and 2.

[0094] [Example 4] Modified polyvinyl alcohol (Elvanol T-66, manufactured by Kuraray) containing 1.8 mol% methacrylic groups, a copolymer of methyl methacrylate containing carboxyl groups, was dissolved in DMSO at 90°C with stirring for 5 hours to obtain a spinning solution with a polyvinyl alcohol concentration of 20% by mass. This spinning solution was wet-spun through a nozzle with 35,000 holes and a 0.08 mm diameter into a 10°C solidification bath containing 70 / 30 methanol / DMSO and then subjected to wet heat drawing at 3.0x in a 20°C methanol bath. After extracting the DMSO from the yarn with methanol, a spinning oil was applied and the yarn was dried at 140°C. The resulting dried yarn was hot-drawn at 190°C at a hot draw ratio of 3.0x (total draw ratio TD = 9x). The drawing tension was 0.64 cN / dtex. Polyvinyl alcohol fibers were then produced by dry heat shrinkage at 190°C with a dry heat shrinkage rate of 1%. The birefringence, crystallinity, underwater fusion temperature, water absorption rate, elution rate, shrinkage rate, shrinkage stress, and leakproofness of the nonwoven fabric sheet of the obtained fibers were measured, and the results are shown in Tables 1 and 2.

[0095] [Example 5] Modified polyvinyl alcohol (KL-118, manufactured by Kuraray) containing 1.5 mol% itaconic acid groups, a copolymer of itaconic acid and carboxyl-containing itaconic acid, was dissolved in DMSO with stirring at 90°C for 5 hours to obtain a spinning solution with a polyvinyl alcohol concentration of 19% by mass. This spinning solution was wet-spun through a nozzle with 80 holes and a 0.12 mm diameter into a 5°C solidification bath containing 80 / 20 methanol / DMSO, followed by wet-heat drawing at 3.0x in a 20°C methanol bath. The DMSO in the yarn was then extracted with methanol, after which a spinning oil was applied and the yarn was dried at 120°C. The resulting dried yarn was hot-heat drawn at 200°C at a draw ratio of 3.33x (total draw ratio TD = 10.0x). The drawing tension was 0.69 cN / dtex. Polyvinyl alcohol fibers were then produced by dry heat shrinkage at 200°C with a dry heat shrinkage rate of 1%. The birefringence, crystallinity, underwater fusion temperature, water absorption rate, elution rate, shrinkage rate, shrinkage stress, and leakproofness of the nonwoven fabric sheet of the obtained fibers were measured, and the results are shown in Tables 1 and 2.

[0096] [Example 6] Modified polyvinyl alcohol (Elvanol T-66, manufactured by Kuraray) containing 1.8 mol% methacrylic groups, a copolymer of methyl methacrylate containing carboxyl groups, was dissolved in DMSO at 90°C with stirring for 5 hours to obtain a spinning solution with a polyvinyl alcohol concentration of 20% by mass. This spinning solution was wet-spun through a nozzle with 35,000 holes and a 0.08 mm diameter into a 10°C solidification bath containing 70 / 30 methanol / DMSO and then wet-heat stretched 2.5 times in a 20°C methanol bath. The DMSO in the yarn was then extracted with methanol, after which a spinning oil was applied and the yarn was dried at 140°C. The resulting dried yarn was hot-heat stretched at 180°C at a 2.8x draw ratio (total draw ratio TD = 7x). The stretching tension was 0.43 cN / dtex. Polyvinyl alcohol fibers were then produced by dry heat shrinkage at 180°C with a dry heat shrinkage rate of 1%. The birefringence, crystallinity, underwater fusion temperature, water absorption rate, elution rate, shrinkage rate, shrinkage stress, and leakproofness of the nonwoven fabric sheet of the obtained fibers were measured, and the results are shown in Tables 1 and 2.

[0097] [Example 7] Modified polyvinyl alcohol (degree of polymerization 1400) containing 3.0 mol% acrylic groups, a copolymer of acrylic acid groups and methyl acrylate containing carboxyl groups, was dissolved in DMSO with stirring at 90°C for 5 hours to obtain a spinning solution with a polyvinyl alcohol concentration of 21% by mass. This spinning solution was dry-wet spun through a nozzle with 80 holes and a 0.12 mm diameter into a 5°C solidification bath containing 80 / 20 methanol / DMSO, and then wet-stretched 3.0 times in a 20°C methanol bath. The DMSO in the yarn was then extracted with methanol, after which a spinning oil was applied and the yarn was dried at 120°C. The resulting dried yarn was dry-stretched at 180°C to a dry-stretch ratio of 3.3 times (total stretch ratio TD = 10 times). The stretching tension was 0.75 cN / dtex. Polyvinyl alcohol fibers were then produced by dry heat shrinkage at 180°C with a dry heat shrinkage rate of 1%. The birefringence, crystallinity, underwater fusion temperature, water absorption rate, elution rate, shrinkage rate, shrinkage stress, and leakproofness of the nonwoven fabric sheet of the obtained fibers were measured, and the results are shown in Tables 1 and 2.

[0098] [Comparative Example 1] Modified polyvinyl alcohol (5.2 mol% acrylic group, a copolymer of acrylic groups and carboxy-containing methyl acrylate) (Kuraray "Elvanol 80-18") was dissolved in DMSO with stirring at 90°C for 5 hours to obtain a spinning solution with a polyvinyl alcohol concentration of 22% by mass. This spinning solution was wet-spun through a nozzle with 40,000 holes and a 0.08 mm diameter into a 10°C solidification bath containing 80 / 20 methanol / DMSO and then wet-heat stretched 2.5 times in a 20°C methanol bath. After extracting the DMSO from the yarn with methanol, a spinning oil was applied and the yarn was dried at 140°C. The resulting dried yarn was hot-heat stretched at 180°C at a 2.0x hot-heat draw ratio (total draw ratio TD = 5x). The stretching tension was 0.31 cN / dtex. Polyvinyl alcohol fibers were then produced by dry heat shrinkage at 180°C with a dry heat shrinkage rate of 1%. The birefringence, crystallinity, underwater fusion temperature, water absorption rate, elution rate, shrinkage rate, shrinkage stress, and leakproofness of the nonwoven fabric sheet of the obtained fibers were measured, and the results are shown in Tables 1 and 2.

[0099] Comparative Example 2 Polyvinyl alcohol (Kuraray's "27-96") containing no carboxyl groups was dissolved in DMSO at 90°C with stirring for 5 hours to obtain a spinning solution with a polyvinyl alcohol concentration of 21% by mass. This spinning solution was wet-spun through a nozzle with 40,000 holes and a hole diameter of 0.08 mm in a 10°C solidification bath containing methanol / DMSO at a ratio of 70 / 30, and then subjected to wet heat drawing at a ratio of 3.0 in a 20°C methanol bath. Next, the DMSO in the yarn was extracted with methanol, and then a spinning oil was applied and the yarn was dried at 165°C. The resulting dried yarn was dry-heat drawn at 203°C at a dry heat draw ratio of 2.7 (total draw ratio TD = 8.0). The drawing tension was 0.40 cN / dtex. Next, dry heat shrinkage was achieved at 203°C with a dry heat shrinkage of 1%. Polyvinyl alcohol fiber The birefringence, crystallinity, underwater fusion temperature, water absorption capacity, elution rate, shrinkage rate, shrinkage stress, and leakproofness of the nonwoven fabric sheet of the obtained fiber were measured, and the results are shown in Tables 1 and 2.

[0100] Comparative Example 3 Modified polyvinyl alcohol (KL-118, manufactured by Kuraray) containing 1.5 mol% itaconic acid groups, a copolymer of itaconic acid and carboxylated itaconic acid, was dissolved in DMSO with stirring at 90°C for 5 hours to obtain a spinning solution with a polyvinyl alcohol concentration of 20% by mass. This spinning solution was wet-spun through a nozzle with 80 holes and a 0.12 mm diameter into a 5°C solidification bath containing 80 / 20 methanol / DMSO, followed by wet-heat drawing at 1.5x in a 20°C methanol bath. The DMSO in the yarn was then extracted with methanol, after which a spinning oil was applied and the yarn was dried at 120°C. The resulting dried yarn was hot-heat drawn at 190°C at a draw ratio of 3.67x (total draw ratio TD = 5.5x). The drawing tension was 0.34 cN / dtex. Polyvinyl alcohol fibers were then produced by dry heat shrinkage at 190°C with a dry heat shrinkage rate of 1%. The birefringence, crystallinity, underwater fusion temperature, water absorption rate, elution rate, shrinkage rate, shrinkage stress, and leakproofness of the nonwoven fabric sheet of the obtained fibers were measured, and the results are shown in Tables 1 and 2.

[0101] Comparative Example 4 Modified polyvinyl alcohol (degree of polymerization 1600) containing 0.5 mol% methacrylic groups, a copolymer of methyl methacrylate containing carboxyl groups, was dissolved in DMSO at 90°C with stirring for 5 hours to obtain a spinning solution with a polyvinyl alcohol concentration of 20% by mass. This spinning solution was dry-wet spun through a nozzle with 80 holes and a 0.1 mm diameter in a 5°C solidification bath containing 70 / 30 methanol / DMSO, and then wet-stretched 3.0 times in a 20°C methanol bath. The DMSO in the yarn was then extracted with methanol, after which a spinning oil was applied and the yarn was dried at 120°C. The resulting dried yarn was dry-stretched at 180°C to a dry-stretch ratio of 3.3 times (total stretch ratio TD = 10 times). The stretching tension was 0.60 cN / dtex. Polyvinyl alcohol fibers were then produced by dry heat shrinkage at 180°C with a dry heat shrinkage rate of 1%. The birefringence, crystallinity, underwater fusion temperature, water absorption rate, elution rate, shrinkage rate, shrinkage stress, and leakproofness of the nonwoven fabric sheet of the obtained fibers were measured, and the results are shown in Tables 1 and 2.

[0102] Comparative Example 5 Modified polyvinyl alcohol (degree of polymerization 1600) containing 7.0 mol% methacrylic groups, a copolymer of methyl methacrylate containing carboxyl groups, was dissolved in DMSO at 90°C with stirring for 5 hours to obtain a spinning solution with a polyvinyl alcohol concentration of 24% by mass. This spinning solution was wet-spun through a nozzle with 80 holes and a 0.12 mm diameter into a 5°C solidification bath containing 80 / 20 methanol / DMSO and then wet-heat stretched 3.0 times in a 20°C methanol bath. After extracting the DMSO from the yarn with methanol, a spinning oil was added and the yarn was dried at 120°C. The resulting dried yarn was hot-heat stretched at 180°C to a 3.3x draw ratio (total draw ratio TD = 10x). The stretching tension was 0.32 cN / dtex. Polyvinyl alcohol fibers were then produced by dry heat shrinkage at 180°C with a dry heat shrinkage rate of 1%. The birefringence, crystallinity, underwater fusion temperature, water absorption rate, elution rate, shrinkage rate, shrinkage stress, and leakproofness of the nonwoven fabric sheet of the obtained fibers were measured, and the results are shown in Tables 1 and 2.

[0103] [Table 1]

[0104] [Table 2]

[0105] As is clear from Tables 1 and 2, the polyvinyl alcohol fiber contains modified polyvinyl alcohol containing 1 mol % or more of carboxyl groups and has a birefringence of 0.040 or more, and is excellent in shrinkage rate and shrinkage stress at 35°C. For example, as shown in Comparative Examples 1, 3, and 5, polyvinyl alcohol fibers with a birefringence of less than 0.040 have low shrinkage and shrinkage stress, and poor leakproofness when made into nonwoven fabrics, even when they contain modified polyvinyl alcohol containing 1 mol% or more of carboxyl groups. Furthermore, the elution rate is high, and the shrinkage stress is poor in practical use. Comparative Example 2, which is a polyvinyl alcohol fiber having a birefringence of less than 0.040 and not containing modified polyvinyl alcohol containing a carboxy group, has a low shrinkage rate and shrinkage stress, and has poor leakproofness when made into a nonwoven fabric. In Comparative Example 4, which is a polyvinyl alcohol-based fiber containing modified polyvinyl alcohol with a carboxy group content of less than 1 mol%, even though the birefringence is 0.040 or more, the shrinkage rate and shrinkage stress are low, and the leakproofness when made into a nonwoven fabric is poor. Therefore, by using this polyvinyl alcohol fiber as a moisture absorber, it is possible to ensure a moisture flow path and prevent leakage when absorbing moisture above room temperature, particularly around 35°C.

Claims

1. A polyvinyl alcohol fiber containing modified polyvinyl alcohol containing 1 mol % or more of a carboxy group and having a birefringence of 0.040 or more.

2. 2. The polyvinyl alcohol fiber according to claim 1, wherein the carboxyl group is contained in at least one functional group selected from the group consisting of an acrylic acid group, a methacrylic acid group, and an itaconic acid group.

3. 3. The polyvinyl alcohol fiber according to claim 1, wherein the degree of crystallinity is 30 to 60%.

4. A fiber structure comprising at least a portion of the polyvinyl alcohol fiber according to claim 1 or 2.

5. The fiber structure according to claim 4, wherein the fiber structure is a nonwoven fabric or a spun yarn.

6. A method for producing a polyvinyl alcohol-based fiber, comprising wet- or dry-wet-spinning a spinning dope containing 5 to 30% by mass of a modified polyvinyl alcohol containing 1 mol % or more of a carboxyl group in a solidification bath mainly containing an organic solvent capable of solidifying polyvinyl alcohol, and achieving a total draw ratio of 7 times or more in all steps at 180°C or higher in any of the steps of wet drawing, drying, dry drawing, and heat treatment, wherein the drawing tension when achieving a total draw ratio of 7 times or more in all steps is 0.40 cN / dtex or higher.

7. 7. The method for producing polyvinyl alcohol-based fibers according to claim 6, wherein the stretching temperature in the dry stretching step is 180° C. or higher.

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