JP7187911B2 - Hygroscopic acrylonitrile fiber, method for producing said fiber, and fiber structure containing said fiber - Google Patents

Description

TECHNICAL FIELD The present invention relates to a hygroscopic acrylonitrile fiber, a method for producing the fiber, and a fiber structure containing the fiber.

Due to the recent heightened awareness of comfort, there is a demand for the development of materials having hygroscopic functions. For example, crosslinked acrylate fibers obtained by chemically modifying acrylic fibers are known (Patent Document 1). The fiber contains a crosslinked structure and a carboxyl group, and has excellent hygroscopic performance and hygroscopic heat generation performance. However, crosslinked acrylate fibers have the following problems (i) and (ii).

(i) First, the crosslinked acrylate-based fiber exhibits a light pink to dark pink color due to the hydrazine crosslinked structure of the fiber, and thus has a drawback that the field of application is limited.

Regarding this problem, Patent Documents 2 and 3 disclose that the acid treatment A is performed after the cross-linking treatment with a hydrazine compound, and the acid treatment B is performed after the hydrolysis treatment with an alkali, respectively. can be mitigated.

However, even crosslinked acrylate fibers whose whiteness has been improved by the above-described method may become more reddish with the passage of time, heating, washing, and the like. Moreover, the above-described method requires many manufacturing steps, resulting in high manufacturing costs. For this reason, it is still difficult to develop applications.

The cause of the redness in conventional crosslinked acrylate fibers is the crosslinked structure formed by the reaction between the cyano group and hydrazine. However, since crosslinked acrylate fibers contain a large amount of highly hydrophilic carboxyl groups, it is considered difficult to maintain fiber properties due to swelling and dissolution in water without a crosslinked structure. For this reason, it is not easy to remove the crosslinked structure, which is the root cause of redness, and has hardly been studied so far.

Moreover, in fibers using copolymers such as acrylonitrile and methacrylic acid, it has been difficult to improve hygroscopicity while suppressing the degree of swelling in water.

In addition, (ii) the production of crosslinked acrylate fibers requires a step of introducing a crosslinked structure with hydrazine and a hydrolysis step for introducing carboxyl groups. A step is required to remove the residue of Moreover, each of these steps requires a high temperature and a long time. For this reason, it is difficult to manufacture the fiber by continuous processing, and conventionally, batch processing with low productivity has been carried out. Therefore, conventional crosslinked acrylate fibers have low productivity and their manufacturing costs remain high.

As for the acrylic fiber having a carboxyl group, an acrylic fiber made of an acrylonitrile-based polymer containing a monomer having a carboxyl group such as acrylic acid as a copolymer component is known. However, when a large amount of acrylic acid is copolymerized, spinning becomes difficult, so it has been difficult to develop high hygroscopicity. In addition, it tends to be eluted under alkaline conditions such as alkaline soaping in dyeing, which is a problem when it is used for clothing.

As described above, the cross-linked acrylate fibers imparted with hygroscopicity have many manufacturing steps and are low in productivity, or it is difficult to increase the hygroscopicity.

JP-A-5-132858 JP 2010-216051 A JP 2009-114556 A

The present invention was created in view of the current state of the prior art, and its object is to provide a hygroscopic acrylonitrile fiber that can be continuously produced in a simpler process than conventionally. Another object of the present invention is to provide an acrylonitrile-based fiber with little redness and high hygroscopicity.

As a result of intensive studies in order to achieve the above object, the present inventors have found that a spinning stock solution in which an acrylonitrile polymer is dissolved is spun from a nozzle, followed by coagulation, washing, and stretching. By hydrolyzing dry fibers, even with acrylonitrile-based polymer fibers that do not have a crosslinked structure, it is possible to achieve both hygroscopicity and practical fiber properties, less redness, and continuous production. I found it and arrived at the completion of the present invention.

That is, the present invention is achieved by the following means.
(1) A hygroscopic acrylonitrile-based fiber composed of a polymer that does not substantially have a covalently crosslinked structure, and having a carboxyl group content in the fiber of 0.2 to 4.5 mmol/g. and a hygroscopic acrylonitrile fiber characterized by having a saturated moisture absorption rate of 5% by weight or more at 20° C.×65% RH and a water swelling degree of 10 times or less.
(2) The hygroscopic acrylonitrile fiber according to (1), wherein a* is -4 to 2 in the display method described in JISZ8781-4 "CIE 1976 L*a*b* color space".
(3) The hygroscopic acrylonitrile fiber according to (1) or (2), characterized in that the carboxyl groups are uniformly present throughout the fiber.
(4) The hygroscopic acrylonitrile fiber according to any one of (1) and (2), characterized in that the carboxyl groups are localized on the surface layer of the fiber.
(5) The hygroscopic acrylonitrile fiber according to any one of (1) to (4), characterized in that the degree of neutralization of carboxyl groups is 25% or more.
(6) It is characterized by including hydrolyzing the undried fibers obtained through the steps of coagulation, washing, and drawing after spinning a spinning dope containing an acrylonitrile-based polymer from a nozzle (3) A method for producing a hygroscopic acrylonitrile fiber according to .
(7) After spinning a spinning solution containing an acrylonitrile-based polymer from a nozzle, the undried fiber obtained through the steps of coagulation, washing, and drawing is heat-treated to densify the fiber or after densification. The method for producing a hygroscopic acrylonitrile-based fiber according to (4), which comprises hydrolyzing the relaxation-treated fiber.
(8) The method for producing a hygroscopic acrylonitrile fiber according to (6) or (7), wherein the undried fiber has a moisture content of 20 to 250%.
(9) A fiber structure containing the hygroscopic acrylonitrile fiber according to any one of (1) to (5).

The hygroscopic acrylonitrile fiber of the present invention has substantially no crosslinked structure due to covalent bonds. Therefore, since it does not have a hydrazine crosslinked structure, the hygroscopic acrylonitrile-based fiber of the present invention has almost no redness. In addition, the hygroscopic acrylonitrile-based fiber of the present invention does not require a cross-linking introduction step in manufacturing, so the manufacturing process can be greatly reduced, and as a result, continuous production using normal acrylic fiber manufacturing equipment is possible. , is highly productive. Moreover, the hygroscopic acrylonitrile-based fiber of the present invention is suitable for continuous production because it retains practical fiber physical properties without being subjected to a cross-linking treatment.

The hygroscopic acrylonitrile fiber of the present invention is characterized in that it does not substantially have a crosslinked structure due to covalent bonds, unlike conventional hygroscopic acrylonitrile fibers. This eliminates the need for a step of introducing cross-linking, and as a result, it is possible to greatly reduce the number of manufacturing steps, and to perform production using simpler steps than conventional ones. Therefore, continuous production is possible without being limited to batch processing such as the production of conventional crosslinked acrylate fibers. In addition, it is possible to suppress or prevent the fibers from becoming reddish or becoming more reddish over time. In the present invention, the expression "substantially does not have a covalently crosslinked structure" means that <solubility in an aqueous sodium thiocyanate solution>, which will be described later, is 95% or more.

Regarding the suppression of redness, the hygroscopic acrylonitrile-based fiber of the present invention can have a* indicating redness of 2 or less in the display method of JIS Z8781-4, and can also have a* of 0 or less. It is possible. The lower limit of a* is preferably −4, more preferably −3, because if it is too low, other colors other than reddish color become dark.

In addition, the hygroscopic acrylonitrile fiber of the present invention contains a carboxyl group, and the content thereof is 0.2 to 4.5 mmol/g, preferably 0.2 to 4.5 mmol/g, as determined by the method described later. 0.5 to 4.0 mmol/g, more preferably 0.5 to 3.5 mmol/g. When the hygroscopic acrylonitrile fiber of the present invention has a core-sheath structure, it is preferably 0.2 to 2 mmol/g, more preferably 0.5 to 1.0 mmol/g. If the amount of carboxyl groups is less than the lower limit of the above range, the hygroscopic performance described below may not be obtained. Beyond that, it violently swells or dissolves in water, making it difficult to handle.

The hygroscopic acrylonitrile fiber of the present invention has a saturated moisture absorption rate of 5% by weight or more, preferably 10% by weight or more, more preferably 15% by weight or more, at 20° C. and a relative humidity of 65%. It is desirable to have When the saturated hygroscopicity is less than the above lower limit, it is difficult to impart significant hygroscopic performance even when applied to various fiber structures. The upper limit is preferably 35% by weight or less, more preferably 30% by weight or less, from the viewpoint of maintaining fiber physical properties.

The hygroscopic acrylonitrile fiber of the present invention preferably has a water swelling degree of 10 times or less, preferably 8 times or less, more preferably 5 times or less, as determined by the method described later. The hygroscopic acrylonitrile-based fiber of the present invention does not have a cross-linked structure by covalent bonds as described above, and if the degree of swelling with water exceeds 10 times, the fiber becomes brittle and partly falls off. or, in some cases, dissolve, making handling difficult. The lower limit is not particularly limited, but from the viewpoint that the hygroscopic acrylonitrile fiber of the present invention has a saturated moisture absorption rate of 5% by weight or more in an atmosphere of 20° C. and a relative humidity of 65%, it is usually 0.05 times or more. Seem.

Moreover, in the hygroscopic acrylonitrile-based fiber of the present invention, it is desirable that the carboxyl groups are uniformly present throughout the fiber. Here, "uniformly present throughout the fiber" means that the coefficient of variation CV of the magnesium element content in the fiber cross section measured by the measuring method described later is 50% or less. If the carboxyl group is localized, the part becomes embrittled due to moisture absorption and water absorption. The presence of carboxyl groups over the entire fiber suppresses embrittlement even when it absorbs moisture and water, making it easier to obtain fiber physical properties that can withstand practical use without having a crosslinked structure. From this point of view, the CV value is preferably 30% or less, more preferably 20% or less, and still more preferably 15% or less.

However, the hygroscopic acrylonitrile-based fiber of the present invention can employ a core-sheath structure in which carboxyl groups are substantially uniformly present only on the fiber surface, depending on the required physical properties and applications. In this case, the core-sheath structure is composed of a surface layer portion made of a carboxyl group-containing polymer and a center portion made of an acrylonitrile-based polymer. In this way, by having a core-sheath structure consisting of a central portion and a surface layer surrounding it, while obtaining practical fiber physical properties such as hard elasticity in the central portion, moisture absorption rate is high in the surface layer with a high carboxyl group concentration. can be significantly increased.

The surface layer occupies preferably 20 to 80%, more preferably 30 to 70%, of the cross section of the core-sheath structure fiber. If the area occupied by the surface layer is small, functions such as hygroscopicity may not be exhibited sufficiently. .

As for the state of the carboxyl group, it is preferable that the counter ion is a cation other than H when higher hygroscopic performance is desired. More specifically, the proportion of cations other than H as counter ions, that is, the degree of neutralization is preferably 25% or more, more preferably 35% or more, and even more preferably 50% or more.

Examples of cations include alkali metals such as Li, Na and K; alkaline earth metals such as Be, Ca and Ba; metals such as Cu, Zn, Al, Mn, Ag, Fe, Co and _Ni ; Examples include cations such as amines, and multiple types of cations may be mixed. Among them, Li, Na, K, Mg, Ca, Zn and the like are suitable.

Moreover, in the above case, excellent deodorant performance against acidic gases such as acetic acid and isovaleric acid and aldehydes such as formaldehyde can be exhibited. Further, if Mg or Ca ions are used, flame retardancy is high, and if Ag or Cu ions are used, high antibacterial performance can be obtained.

On the other hand, if H is increased as a counter ion of the carboxyl group, the deodorant performance, antiviral performance, and antiallergen performance against amine gases such as ammonia, triethylamine, and pyridine can be enhanced.

The above-described hygroscopic acrylonitrile-based fiber of the present invention can be obtained by subjecting an undried acrylonitrile-based fiber to a hydrolysis treatment. A representative method for producing the hygroscopic acrylonitrile-based fiber of the present invention will be described in detail below.

First, regarding the acrylonitrile-based fiber as a raw material, the acrylonitrile-based polymer constituting the fiber preferably contains acrylonitrile (hereinafter also referred to as AN) as a polymerization composition of 40% by weight or more, more preferably 50% by weight or more, and more preferably 50% by weight or more. contains 85% by weight or more. The AN-based polymer may be an AN homopolymer or a copolymer of AN and other monomers. Other monomers include vinyl halides and vinylidene halides, (meth)acrylic acid esters, methallylsulfonic acid, sulfonic acid group-containing monomers such as p-styrenesulfonic acid and salts thereof, acrylamide, styrene, and vinyl acetate. etc., but is not particularly limited as long as it is a monomer copolymerizable with AN. In addition, the notation of (meta) represents both those with and without the word meta.

Next, using such an AN polymer, fiberization is carried out by wet spinning, and the case where an inorganic salt such as sodium rhodanate is used as a solvent is explained as follows. First, the above AN polymer is dissolved in a solvent to prepare a spinning dope. After the spinning dope is spun from the nozzle, the undried fiber (hereinafter also referred to as gel-like acrylonitrile-based fiber) after being subjected to each step of coagulation, washing, and drawing is adjusted to a moisture content of 20 to 250% by weight, preferably 25 to 130% by weight, more preferably 30 to 100% by weight.

Here, when undried gelatinous acrylonitrile fibers are used as raw fibers to be hydrolyzed, carboxyl groups can be uniformly present throughout the fibers as described above. On the other hand, when hydrolysis treatment is applied to fibers that are densified by further heat-treating gel-like acrylonitrile fibers in an undried state or fibers that are further relaxed after densification are used as raw materials, the carboxyl groups are It can be a core-sheath structure localized in the surface layer.

When gel-like acrylonitrile-based fibers are used as raw fibers, if the moisture content of the gel-like acrylonitrile-based fibers is less than 20% by weight, the chemical agent does not penetrate into the inside of the fibers in the hydrolysis treatment described later, and the carboxyl groups are removed throughout the fibers. may not be able to spawn over time. If it exceeds 250% by weight, the fiber contains a large amount of water and the strength of the fiber becomes too low, which is not preferable because the spinnability is lowered. When more emphasis is placed on high fiber strength, it is desirable to make it within the range of 25 to 130% by weight. There are many methods for controlling the moisture content of gelled acrylonitrile fibers within the above range. 20 times, preferably about 7 to 15 times.

When the gel-like acrylonitrile-based fiber is further heat-treated, for example, by alternately performing dry heat treatment at 110°C and wet heat treatment at 60°C, voids inside the fiber disappear and densified fibers are formed. can get. Further, by performing an autoclave treatment at 120° C. for 10 minutes thereafter, a fiber having a somewhat relaxed fiber structure can be obtained. When these fibers are used as raw materials and subjected to a hydrolysis treatment, which will be described later, the reaction progresses from the surface layer of the fibers, facilitating the formation of a structure such as a core-sheath structure. As the reaction progresses, the degree of swelling with water tends to increase, and the resulting fiber may become difficult to handle.

Gel-like acrylonitrile-based fibers, or fibers that have undergone further heat treatment, are then subjected to a hydrolysis treatment. As a means for such hydrolysis treatment, there is a means of heat treatment in a state of impregnation or immersion in a basic aqueous solution of alkali metal hydroxide, alkali metal carbonate, ammonia or the like, or an aqueous solution of nitric acid, sulfuric acid, hydrochloric acid or the like. mentioned. As specific treatment conditions, various conditions such as the concentration of the treatment agent, reaction temperature, and reaction time may be appropriately set in consideration of the range of the amount of carboxyl groups described above. Impregnated with a treatment agent of up to 20% by weight, preferably 1.0 to 15% by weight, after squeezing, in a moist and hot atmosphere at a temperature of 100 to 140°C, preferably 110 to 135°C, for 10 to 60 minutes. It is preferable industrially and fiber properties to set within the range. Within the above range, there is a tendency that the higher the treatment temperature, the higher the crimp ratio. The moist and hot atmosphere means an atmosphere filled with saturated steam or superheated steam. The treatment hydrolyzes the nitrile groups in the gel-like acrylonitrile-based fibers or the fibers that have been further subjected to the heat treatment to generate carboxyl groups.

The fibers subjected to the hydrolysis treatment as described above contain cations such as alkali metals and ammonium depending on the type of alkali metal hydroxide, alkali metal carbonate, ammonia, etc. used in the hydrolysis treatment. A salt-type carboxyl group to be used as a counterion is generated, but subsequently, if necessary, a treatment for converting the counterion of the carboxyl group may be performed. By performing an ion exchange treatment with an aqueous solution of a metal salt such as nitrate, sulfate or hydrochloride, a salt-type carboxyl group having a desired metal ion as a counter ion can be obtained. Furthermore, by adjusting the pH of the aqueous solution and the concentration and type of the metal salt, it is possible to mix different types of counterions and adjust their proportions.

The hygroscopic acrylonitrile-based fiber according to the present invention is obtained as described above, and each of the above-described treatments can be continuously performed by using existing continuous acrylic fiber production equipment. In addition, if necessary, additional treatments such as washing with water, drying, and cutting into specific fiber lengths may be performed. Although the case where an inorganic salt such as sodium rhodanate is used as a solvent has been described above, the above conditions are the same when an organic solvent is used. However, since the types of solvents are different, the temperature of the coagulation bath is selected to be suitable for the solvent, and the moisture content of the gelled acrylonitrile fiber is controlled within the above range.

Moreover, in the production of the hygroscopic acrylonitrile-based fiber of the present invention, a functional material may be added to the spinning dope. Examples of such functional materials include titanium oxide, carbon black, pigments, antibacterial agents, deodorants, hygroscopic agents, antistatic agents, and resin beads.

Here, in the hygroscopic acrylonitrile-based fiber of the present invention obtained by the above-described production method, the undried gel-like acrylonitrile-based fiber is hydrolyzed, so the chemical is not hydrolyzed sequentially from the fiber surface. It is thought that it permeates deep inside the fiber and hydrolyzes the entire fiber. Furthermore, when viewed microscopically, acrylonitrile-based fibers generally have a mixture of crystalline portions in which the AN-based polymer is oriented and amorphous portions in which the structure is disordered. Thus, it is believed that the crystalline portion is hydrolyzed from the outside while the amorphous portion is hydrolyzed entirely. As a result, after hydrolysis, microscopically, part of the crystalline portion remains as a portion with a high nitrile group concentration without being hydrolyzed, and the amorphous portion becomes a portion with a high carboxyl group concentration. it is conceivable that.

From the above, it is speculated that the structure of the hygroscopic acrylonitrile-based fiber of the present invention obtained by the above-described production method is a structure in which a portion with a high carboxyl group concentration and a portion with a high nitrile group concentration are uniformly present throughout the fiber. be done. And, because of such a structure, it is thought that deterioration of fiber physical properties during moisture absorption and water absorption is suppressed even without having a crosslinked structure by covalent bonding. Further, the hygroscopic acrylonitrile-based fiber of the present invention is, as described above, a fiber obtained by further heat-treating the gel-like acrylonitrile-based fiber in an undried state to densify it, or a fiber that is further subjected to relaxation treatment after densification as a raw material fiber. By adopting as, even when taking a core-sheath structure, the carboxyl groups are uniformly present in the surface layer, and the core has a hard and elastic structure, so even without obtaining a crosslinked structure by covalent bonds, Similarly, it is thought that there is little decrease in fiber physical properties.

As can be seen from the above, in the production method described above, the gel-like acrylonitrile-based fiber is hydrolyzed to obtain the fiber having the properties as described above. Gel-like acrylonitrile fibers, i.e., undried fibers after drawing, are not used, and when the hydrolysis treatment is applied to the dried acrylonitrile fibers, the chemical does not penetrate deep into the fibers, Since the hydrolysis proceeds sequentially from the surface of the fiber, a structure is induced in which the surface of the fiber has the largest number of carboxyl groups and the innermost part of the fiber has the fewest carboxyl groups. A fiber having such a structure is unusable because the polymer constituting the surface of the fiber is eluted into water.

The hygroscopic acrylonitrile-based fiber of the present invention described above can be used alone or in combination with other materials as a useful fiber structure in many applications. In the fiber structure, the content of the hygroscopic acrylonitrile fiber of the present invention is preferably 5% by weight or more, more preferably 10% by weight or more, and still more preferably 20% by weight or more. It is desirable from the viewpoint of obtaining the effect of the polyacrylonitrile-based fiber. Further, the types of other materials are not particularly limited, and commonly used natural fibers, organic fibers, semi-synthetic fibers, and synthetic fibers can be used, and inorganic fibers, glass fibers, etc. can also be used depending on the application. Specific examples include cotton, linen, silk, wool, nylon, rayon, polyester, and acrylic fibers.

Appearance forms of the fiber structure include threads, non-woven fabrics, paper-like materials, sheet-like materials, laminates, cotton-like bodies (including spherical and lumpy ones), and the like. The fiber of the present invention may be contained in the structure by mixing it with other materials and distributing it substantially uniformly. There are those that exist concentrated in a plurality of layers, and those that are distributed in each layer at a specific ratio.

The type of final product (for example, clothing, filters, curtains and carpets, bedding, cushions, insoles, etc.), and how the hygroscopic acrylonitrile fiber of the present invention contributes to the expression of such functions. is determined as appropriate in consideration of

EXAMPLES Examples are shown below to facilitate understanding of the present invention, but these are merely illustrative and the gist of the present invention is not limited by these. In the examples, parts and percentages are expressed by weight unless otherwise specified. In addition, the measurement of each characteristic was calculated|required by the following method.

<Solubility in sodium thiocyanate aqueous solution>
About 1 g of the dried sample is precisely weighed (W1 [g]), 100 ml of 58% sodium thiocyanate aqueous solution is added, and the sample is immersed at 80° C. for 1 hour, filtered, washed with water, and dried. The sample after drying is accurately weighed (W2 [g]) and the solubility is calculated by the following formula.
Solubility [%] = (1-W2/W1) x 100
When the solubility is 95% or more, it is determined that the material does not substantially have a crosslinked structure due to covalent bonds.

<Measurement of carboxyl group content>
About 1 g of a sample is weighed, immersed in 50 ml of 1 mol/l hydrochloric acid for 30 minutes, washed with water, and immersed in pure water at a bath ratio of 1:500 for 15 minutes. After washing with water until the pH of the bath reaches 4 or more, it is dried at 105° C. for 5 hours with a hot air dryer. Approximately 0.2 g of the dried sample (W3 [g]) is weighed, and 100 ml of water, 15 ml of 0.1 mol/l sodium hydroxide and 0.4 g of sodium chloride are added and stirred. The sample is then filtered using a wire mesh and washed with water. Add 2 to 3 drops of phenolphthalein solution to the obtained filtrate (including the washing solution) and titrate with 0.1 mol/l hydrochloric acid according to a conventional method to determine the amount of hydrochloric acid consumed (V1 [ml]), The total amount of carboxyl groups is calculated by the following formula.
Total amount of carboxyl groups [mmol / g] = (0.1 × 15-0.1 × V1) / W3

<Measurement of saturated moisture absorption>
The sample is dried with a hot air dryer at 105° C. for 16 hours, and the weight is measured (W4 [g]). Next, the sample is placed in a constant temperature and humidity chamber adjusted to the conditions of 20° C. and 65% RH for 24 hours. The weight of the moisture-absorbed sample is measured. (W5 [g]). From the above measurement results, it is calculated by the following formula.
Saturated moisture absorption [%] = (W5-W4) / W4 x 100

<Water swelling degree>
After the sample is immersed in pure water, it is dehydrated for 5 minutes at 1200 rpm with a desktop centrifugal dehydrator. After measuring the weight of the sample after dehydration (W6 [g]), the sample is dried at 115°C for 3 hours, the weight is measured (W7 [g]), and the water swelling degree is calculated by the following equation.
Water swelling degree [times] = W6/W7-1

<Neutralization degree>
About 0.2 g of the sample dried at 105° C. for 5 hours in a hot air dryer was precisely weighed (W8 [g]), and 100 ml of water, 15 ml of 0.1 mol/l sodium hydroxide, and 0.4 g of sodium chloride were added thereto. Add and stir. The sample is then filtered using a wire mesh and washed with water. Add 2 to 3 drops of phenolphthalein solution to the obtained filtrate (including washings) and titrate with 0.1 mol/l hydrochloric acid according to a conventional method to determine the amount of hydrochloric acid consumed (V2 [ml]). The amount of H-type carboxyl groups contained in the sample is calculated by the following formula, and the degree of neutralization is determined from the result and the total amount of carboxyl groups described above.
H-type carboxyl group amount [mmol / g] = (0.1 × 15-0.1 × V2) / W8
Neutralization degree [%] = [(total carboxyl group amount - H-type carboxyl group amount) / total carboxyl group amount] x 100

<Hue a* value>
Using 0.5 g of a sample that was opened and dried at 105° C. for 16 hours with a hot air dryer, the color was measured with a color difference meter CR-300 manufactured by Konica Minolta.

<Distribution of carboxyl groups in fiber structure>
The fiber sample is subjected to ion exchange treatment by immersing it in an aqueous solution containing twice the amount of carboxyl groups contained in the fiber, in which magnesium nitrate is dissolved at 50°C for 1 hour, followed by washing with water and drying to remove carboxyl groups. Let the counterion be magnesium. A magnesium salt-type fiber sample is measured by an energy dispersive X-ray spectrometer (EDS) at 10 measurement points at approximately equal intervals from the outer edge to the center of the fiber cross section, and the content of magnesium element at each measurement point is measured. do. A coefficient of variation CV [%] is calculated from the obtained numerical value of each measurement point by the following equation.
Variation coefficient CV [%] = (standard deviation / average value) × 100

<Proportion of the area occupied by the surface layer in the cross section of the fiber with the core-sheath structure>
The sample fiber is immersed in a dyeing bath containing 2.5% cationic dye (Nichilon Black G 200) and 2% acetic acid based on the fiber weight so that the bath ratio is 1:80, and boiled for 30 minutes. After that, it is washed with water, dehydrated and dried. The obtained dyed fiber is thinly sliced perpendicular to the fiber axis, and the cross section of the fiber is observed with an optical microscope. At this time, the central portion made of the acrylonitrile-based polymer is dyed black, and the surface layer portion having many carboxyl groups is green because the dye is not sufficiently fixed. Measure the fiber diameter (D1) in the fiber cross section and the diameter (D2) of the central part dyed black bordering on the part where the color starts to change from green to black, and calculate the surface layer area ratio by the following formula. do. The average value of the surface layer area ratios of 10 samples is taken as the surface layer area ratio of the sample fiber.
Surface layer area ratio (%) = [{((D1)/2) ² π-((D2)/2) ² π}/((D1)/2) ² π] × 100

<Measurement of moisture content of undried fibers after drawing>
After the stretched undried fiber is immersed in pure water, it is dehydrated for 2 minutes with a centrifugal dehydrator (TYPE H-770A manufactured by Kokusan Centrifugal Co., Ltd.) at a centrifugal acceleration of 1100 G (G indicates gravitational acceleration). . After the weight after dehydration is measured (W8 [g]), the undried fibers are dried at 120°C for 15 minutes, the weight is measured (W9 [g]), and the weight is calculated by the following formula.
Moisture content (%) of undried fiber after stretching = (W8-W9)/W8 x 100

<Example 1>
A spinning dope prepared by dissolving 10 parts of an AN polymer consisting of 90% AN and 10% methyl acrylate in 90 parts of a 44% sodium thiocyanate aqueous solution was spun in a coagulation bath at -2.5°C, coagulated, washed with water, and washed with water. It was double-stretched to obtain a gel-like acrylonitrile-based fiber having a moisture content of 35%. The fibers are immersed in a 2.5% sodium hydroxide aqueous solution and squeezed, then hydrolyzed at 123° C. for 25 minutes in a moist and hot atmosphere, washed with water and dried to obtain the hygroscopic acrylonitrile of Example 1. system fibers were obtained. Table 1 shows the evaluation results of the obtained fibers.

<Examples 2 to 5>
In the formulation of Example 1, except that the concentration of the aqueous sodium hydroxide solution was changed to 7.5% in Example 2, 10% in Example 3, 15% in Example 4, and 20% in Example 5. Hygroscopic acrylonitrile fibers of Examples 2 to 5 were obtained in the same manner. Table 1 shows the evaluation results of the obtained fibers.

<Example 6>
The hygroscopic acrylonitrile fiber of Example 3 was immersed in an aqueous nitric acid solution, adjusted to pH 5.0, and heated at 60° C. for 30 minutes. Then, it was washed with water and dried to obtain a hygroscopic acrylonitrile fiber of Example 6. Table 1 shows the evaluation results of the obtained fibers.

<Example 7>
A hygroscopic acrylonitrile fiber of Example 7 was obtained in the same manner as in Example 6 except that the hygroscopic acrylonitrile fiber of Example 5 was used instead of the hygroscopic acrylonitrile fiber of Example 3. . Table 1 shows the evaluation results of the obtained fibers.

<Example 8>
In Example 2, instead of the gel-like acrylonitrile-based fiber, the fiber was alternately subjected to dry heat treatment at 110 ° C. for 2.5 minutes and wet heat treatment at 60 ° C. for 2.5 minutes. A hygroscopic acrylonitrile fiber of Example 8 having a core-sheath structure was obtained in the same manner, except that the densified fiber was used. Table 1 shows the evaluation results of the obtained fibers.

<Example 9>
It has a core-sheath structure in the same manner as in Example 8, except that instead of the densified fibers, relaxed fibers that were further relaxed by autoclaving at 120 ° C. for 10 minutes were used. A hygroscopic acrylonitrile fiber of Example 9 was obtained. Table 1 shows the evaluation results of the obtained fibers.

<Comparative Example 1>
A spinning dope prepared by dissolving 10 parts of an AN polymer consisting of 90% AN and 10% methyl acrylate in 90 parts of a 44% sodium thiocyanate aqueous solution was spun in a coagulation bath at -2.5°C, coagulated, washed with water, and washed with water. After being double-stretched, the fiber was dried under an atmosphere of dry bulb/wet bulb=120° C./60° C. to obtain a raw material fiber. The raw material fibers were treated in a 35% hydrazine aqueous solution at 100°C for 3 hours under the conditions shown in Table 1, then treated in a 5% sodium hydroxide aqueous solution at 90°C for 2 hours, then dehydrated, washed with water, and dried. A fiber having a crosslinked structure and carboxyl groups was obtained. Table 1 shows the evaluation results of the obtained fibers.

<Comparative Example 2>
A spinning dope prepared by dissolving 10 parts of an AN polymer consisting of 88% AN and 12% methacrylic acid in 90 parts of a 44% sodium thiocyanate aqueous solution is spun, coagulated, washed with water, stretched, and dried to give a carboxyl An acrylic fiber with groups was obtained. Table 1 shows the evaluation results of the obtained fibers.

<Comparative Example 3>
The acrylic fiber of Comparative Example 2 was heat-treated in a 1 g/l soda ash aqueous solution at 90° C. for 30 minutes, then washed with water and dried to obtain an acrylic fiber having neutralized carboxyl groups. Table 1 shows the evaluation results of the obtained fibers.

<Comparative Example 4>
An acrylic fiber having neutralized carboxyl groups was obtained in the same manner as in the formulation of Comparative Example 3, except that the treatment temperature with the 1 g/l soda ash aqueous solution was changed to 100°C. Table 1 shows the evaluation results of the obtained fibers.

As shown in Table 1, the hygroscopic acrylonitrile-based fibers of Examples 1 to 9 had a saturated hygroscopicity of 5 at 20°C x 65% RH, even though they did not have a covalently crosslinked structure. % or more and a water swelling degree of 10 times or less. Furthermore, these fibers had a ^* within the range of -4 to 2, and were suppressed in redness.

On the other hand, the conventional crosslinked acrylate fiber of Comparative Example 1 has a crosslinked structure, so it exhibits good properties in terms of saturated moisture absorption and degree of swelling in water, but it is strongly reddish. The acrylic fiber of Comparative Example 2 had a low saturated moisture absorption rate because the carboxyl groups were not neutralized. The fiber of Comparative Example 3 was obtained by neutralizing the acrylic fiber of Comparative Example 2, but while the improvement in saturated moisture absorption was insufficient, the degree of swelling in water increased greatly. In Comparative Example 4, since the neutralization reaction conditions were strengthened, the saturated moisture absorption rate was improved, but the degree of swelling with water became too high, resulting in gelation of the fibers.

Claims (9)

A hygroscopic acrylonitrile fiber composed of a polymer substantially free of covalently crosslinked structures, wherein the amount of carboxyl groups in the fiber is 0.2 to 4.5 mmol/g, and the fiber A hygroscopic acrylonitrile fiber characterized by having a saturated moisture absorption rate of 5% by weight or more at 20°C x 65% RH and a water swelling degree of 10 times or less. 2. The hygroscopic acrylonitrile fiber according to claim 1, wherein a ^* is -4 to 2 in the display method described in JIS Z8781-4 "CIE 1976 L ^* a ^* b ^* color space". 3. The hygroscopic acrylonitrile fiber according to claim 1 or 2, wherein the carboxyl groups are uniformly present throughout the fiber. 3. The hygroscopic acrylonitrile fiber according to claim 1 or 2, wherein the carboxyl groups are localized on the surface layer of the fiber.
The hygroscopic acrylonitrile fiber according to any one of claims 1 to 4, characterized in that the degree of neutralization of carboxyl groups is 25% or more. 4. The method according to claim 3, which comprises hydrolyzing the undried fibers obtained through the steps of coagulation, washing and drawing after spinning the spinning dope containing the acrylonitrile polymer from a nozzle. A method for producing a hygroscopic acrylonitrile fiber. After spinning a spinning stock solution containing an acrylonitrile-based polymer from a nozzle, the undried fiber obtained through each step of coagulation, washing with water, and drawing is heat-treated to densify the fiber or further relaxed after densification. 5. A method for producing a hygroscopic acrylonitrile-based fiber according to claim 4, comprising hydrolyzing the fiber. The method for producing a hygroscopic acrylonitrile fiber according to claim 6 or 7, wherein the undried fiber has a moisture content of 20 to 250%. A fiber structure containing the hygroscopic acrylonitrile fiber according to any one of claims 1 to 5.