Method for producing continuous cellulose fiber having improved thermal stability

Abstract

To provide a method for producing a continuous cellulose fiber having improved stability in an industrially productive manner. Provided is a method for producing a continuous cellulose fiber having improved thermal stability, wherein: the method includes immersing a continuous cellulose fiber in an aqueous medium including a compound (A) that can react with a reducing terminal or hydroxyl group of cellulose, causing the continuous cellulose fiber to absorb the aqueous medium, and then drying; and the series of above steps are carried out continuously while winding in or drawing in the continuous cellulose fiber.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for producing a continuous cellulose fiber having improved thermal stability.BACKGROUND

[0002] Cellulose fibers are fibers that include cellulose, which is the main constituent component of wood, and are classified into plant fibers, regenerated fibers (regenerated cellulose fibers), semi-synthetic fibers, and cellulose nanofibers, etc. Such cellulose fibers are not limited to uses such as for clothing, etc., and are widely used as industrial raw materials for foods, cosmetics, and functional paper, etc.

[0003] In recent years, owing to an increase in environmental awareness, cellulose fibers, which are a biomass-derived material, are employed as resin modifiers and reinforcers (for example, Patent Document 1, etc.). Common methods for cases in which cellulose fibers are used as a resin reinforcer, include a method in which continuous cellulose fibers are produced, then cut to desired lengths and blended in a resin to obtain a fiber-reinforced resin composition, or in which the continuous cellulose fibers are impregnated with a resin and then cut to desired lengths to obtain a resin material reinforced by cellulose fibers (resin-impregnated long fiber bundle), etc. Meanwhile, cellulose has low thermal stability and suffers performance degradation in which the appearance thereof deforms and becomes discolored due to heating. Due thereto, for a resin composition including cellulose fibers, for example, there are cases of appearance defects occurring wherein the cellulose fibers discolor due to a heat treatment when preparing the resin composition or when obtaining a molded article of the resin composition.

[0004] As a method for improving the thermal stability of cellulose, for example, Patent Document 2 proposes a method of dripping a polyhydric alcohol aqueous solution onto a cellulose filter paper and then drying the aqueous solution in a 105°C oven for 24 hours. Further, Patent Document 3 proposes a method for producing cellulose in which yellowing and blacking at a temperature range of up to 300°C is inhibited, the method involving adding a boric acid solution to a cellulose fine powder, then heat-treating the cellulose powder including the boric acid solution at 105°C for 24 hours or more. Patent Document 1: JP 2018-024967 A Patent Document 2: JP 2010-159364 A Patent Document 3: JP 2008-163053 A SUMMARY OF INVENTION

[0005] The methods disclosed in Patent Documents 2 and 3 are batch-type methods, and thus, it is difficult to apply the methods to a continuous fiber. Further, in the foregoing methods, in order to obtain cellulose having improved thermal stability, it is necessary to perform a drying process at a temperature of 100°C or more for 24 hours, and therefore, a method for producing continuous cellulose fibers having improved thermal stability in an industrially productive manner is sought. Thus, the present invention addresses a problem of providing a method for producing a continuous cellulose fiber having improved thermal stability in an industrially productive manner.

[0006] As a result of diligent research, the present inventors discovered that it is possible to produce a continuous cellulose fiber having improved thermal stability by an industrially productive method by immersing a continuous cellulose fiber in an aqueous medium including a compound capable of improving the thermal stability of cellulose by reacting with a reducing terminal or hydroxyl group of the cellulose, and then drying, and further, implementing a series of steps while winding in or drawing in the continuous fiber. That is, the present invention has the following aspects. [1] A method for producing a continuous cellulose fiber having improved thermal stability, wherein: the method includes immersing a continuous cellulose fiber in an aqueous medium including a compound (A) that can react with a reducing terminal or hydroxyl group of cellulose, causing the continuous cellulose fiber to absorb the aqueous medium, and then drying; and the series of above steps are carried out continuously while winding in or drawing in the continuous cellulose fiber. [2] The method for producing a continuous cellulose fiber described in [1], wherein the compound (A) includes at least one compound selected from: a glycosylation agent (a1) of a reducing terminal of the cellulose; and an esterification agent (a2) of a hydroxyl group of the cellulose. [3] The method for producing a continuous cellulose fiber described in [2], wherein: the glycosylation agent (a1) includes a monohydric or polyhydric alcohol; and the esterification agent (a2) includes boric acid. [4] The method for producing a continuous cellulose fiber described in any of [1] to [3], wherein: an immersed length of the continuous cellulose fiber is 1-1,000 cm; and a winding-in speed or a drawing-in speed of the continuous cellulose fiber is 10-1,000 m / minute. [5] The method for producing a continuous cellulose fiber described in any of [1] to [4], wherein the concentration of the compound (A) in the aqueous medium is 0.1-50 mass%. [6] The method for producing a continuous cellulose fiber described in any of [1] to [5], wherein a drying temperature of the continuous cellulose fiber is 10-300°C. [7] The method for producing a continuous cellulose fiber described in any of [1] to [6], wherein the drying includes drying the continuous cellulose fiber with hot air and then performing further drying while bringing the continuous cellulose fiber into contact with a heated cylinder. [8] The method for producing a continuous cellulose fiber described in any of [1] to [7], wherein the continuous cellulose fiber includes a continuous regenerated cellulose fiber, a spun fiber of short-fiber regenerated cellulose, or a spun fiber of natural cellulose.

[0007] According to the present invention, it is possible to provide a method for producing a continuous cellulose fiber having improved thermal stability in an industrially productive manner.DESCRIPTION OF EMBODIMENTS

[0008] Hereinafter, embodiments of the present disclosure will be explained in detail. However, the scope of the present disclosure is not limited to any one embodiment explained here, and various modifications can be made within a scope not departing from the gist of the present disclosure. The various embodiments described herein may be combined with any other feature described herein. Further, with respect to specific parameters, in cases in which multiple upper limit values and lower limit values are described, a suitable numerical value range can be obtained by combining any of the upper limit values and lower limit values among the upper limit values and lower limit values that are described. Further, the lower limit value and / or the upper limit value of the numerical value ranges described herein may be replaced with numerical values that are within the numerical value ranges described and that are indicated in the examples. The expression "X-Y" indicating a numerical value range means "X or more and Y or less". In cases where a specific explanation provided for one embodiment also applies to another embodiment, the explanation may be omitted for the other embodiment. The various configurations in the examples and combinations thereof, etc., are examples. Additions, omissions, substitutions and other modifications can be made to the configurations, as appropriate, within a scope not departing from the gist of the present disclosure. The present disclosure is not limited to the embodiments described below. The various embodiments described herein may be combined with any other feature described herein.[Method for producing continuous cellulose fiber]

[0009] The present embodiment relates to a method for producing a continuous cellulose fiber having improved thermal stability, wherein: the method includes immersing a continuous cellulose fiber in an aqueous medium including a compound (A) that can react with a reducing terminal or hydroxyl group of cellulose, causing the continuous cellulose fiber to absorb the aqueous medium (step (I)), and then drying (step (II)); and the series of above steps are carried out continuously while winding in or drawing in the continuous cellulose fiber. Note that in the present production method, "carried out continuously while winding in or drawing in the continuous cellulose fiber" means that the continuous cellulose fiber is wound in or drawn in from one direction such that a raw material continuous cellulose fiber is continuously processed in the order of step (I) then step (II). Further, "winding in the continuous cellulose fiber" indicates that a continuous cellulose fiber obtained by the production method according to the present embodiment is recovered by being wound in onto a core or reel as a final product. Further, "drawing in the continuous cellulose fiber" includes winding in, by means of a winding machine, etc., a continuous cellulose fiber obtained by the production method according to the present embodiment, and may include unreeling the continuous cellulose fiber in preparation for a different step.<Step (I)>

[0010] The production method according to the present embodiment includes immersing a continuous cellulose fiber in an aqueous medium including a compound (A) that can react with a reducing terminal or hydroxyl group of cellulose and causing the continuous cellulose fiber to absorb the aqueous medium. The production method according to the present embodiment includes causing the compound (A) to react with a reducing terminal or hydroxyl group of cellulose by causing the continuous cellulose fiber to absorb the aqueous medium including the compound (A). Note that "causing the continuous cellulose fiber to absorb the aqueous medium" includes incorporating (water-absorbing) the aqueous medium in voids (pores) inside the continuous cellulose fiber, and also includes: the continuous cellulose fiber absorbing the aqueous medium and swelling; the aqueous medium attaching to the surface of the continuous cellulose fiber; and the aqueous medium being incorporated between fibers. Further, in the continuous cellulose fiber having improved thermal stability that is finally obtained, the compound (A) may be fixed in a state of having reacted with a reducing terminal or hydroxyl group of the cellulose, and may be fixed (attached) to a fiber surface.(Continuous cellulose fiber)

[0011] Cellulose is a plant-derived polysaccharide and is included, for example, in wood, cotton, ramie, linen, hemp, jute, Manila hemp, and sisal, etc. The raw material continuous cellulose fiber (hereinafter sometimes referred to as the "raw material fiber") used in the production method according to the present embodiment may, for example, be a fiber (continuous natural cellulose fiber (spun fiber)) in which natural cellulose fibers (short fibers) extracted from a plant such as the cotton, ramie, linen, hemp, jute, Manila hemp, and sisal, etc., described above are twisted and joined lengthwise in a thread form, and may be a continuous regenerated cellulose fiber. Further, said raw material continuous cellulose fiber may be a monofilament and may be a multifilament. The continuous natural cellulose fiber may include lignin or hemicellulose, etc. However, the cellulose content is preferably 60% or more and more preferably 60-80%. When the raw material fiber is a continuous regenerated cellulose fiber, the production method thereof is also not particularly limited. For example, the continuous regenerated cellulose fiber may be a continuous fiber obtained from regenerated cellulose such as viscose rayon, Lyocell, cupra, or a continuous fiber obtained from regenerated cellulose using a solvent method with a solvent such as an ionic liquid. Further, the continuous regenerated cellulose fiber may be a spun fiber in which regenerated cellulose short fibers are twisted and joined lengthwise in a thread form.

[0012] From the viewpoint that it is easy for the raw material fiber to absorb an aqueous medium in a short time when the raw material fiber is immersed in an aqueous medium including the compound (A) (hereinafter also sometimes referred to simply as the "aqueous medium"), the average fiber diameter of the raw material fiber may be 5-50 µm and may be 7-30 µm. Note that the average fiber diameter of the raw material fiber can be measured with a microscope. That is, a cross-section of the raw material fiber is observed with a microscope and the length of the major diameter of the cross-section is measured. An average value of the length of the major diameter measured for approximately 200 raw material fibers is used as the average fiber diameter. In the case in which the fiber cross-section has an elliptical diameter, the length of the major axis is used as the major diameter. In the case of an irregular shape, the longest section thereof is used as the major diameter.(Aqueous medium including compound (A))

[0013] Step (I) includes immersing the raw material fiber in an aqueous medium including the compound (A). The aqueous medium may be an aqueous solution of the compound (A) or may be an emulsion (dispersion liquid) in which the compound (A) is dispersed. Further, the water component included in the aqueous medium may be pure water, may be ion-exchanged water, may be tap water or industrial water, or may be a mixture thereof. From the viewpoint of cost, the water component is preferably tap water or industrial water. Further, the aqueous medium may also include an arbitrary component other than the water component described above and the compound (A).(Compound (A) which can react with reducing terminal or hydroxyl group of cellulose)

[0014] The aqueous medium includes a compound (A) which can react with a reducing terminal or a hydroxyl group of cellulose. The compound (A) may be a compound having a structure which can react chemically with a reducing terminal or a hydroxyl group of cellulose, or may be a compound having a structure which is capable of acting physically. Further, the positions of the reducing terminal or hydroxyl group of cellulose which can react with the compound (A) are not particularly limited. Thereamong, there is preferably included at least one compound selected from: a glycosylation agent (a1) of a reducing terminal of the cellulose; and an esterification agent (a2) of a hydroxyl group of the cellulose.· Glycosylation agent (a1)

[0015] In one embodiment, the compound (A) can include a glycosylation agent (a1) of a reducing terminal of cellulose. A "glycosylation agent" indicates all compounds which are capable of glycosylating a reducing terminal of cellulose. As long as the glycosylation agent (a1) is a compound capable of glycosylating a reducing terminal of cellulose, there are no particular limitations thereon. However, from the viewpoint that glycosylation of a reducing terminal is easy, it is preferable to include a monohydric or polyhydric alcohol.

[0016] The monohydric or polyhydric alcohol is preferably an alcohol having, for example, a carbon number of 1-20. Specific examples thereof include: monohydric alcohols such as methanol, ethanol, propanol, isopropanol, butanol, ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, benzyl alcohol, and phenethyl alcohol, etc.; chain-like polyhydric alcohols such as mannitol, glycerin, erithritol, pentaerythritol, xylitol, glucitol, and ethylene glycol, etc.; and phenols such as phenol, paracresol, metacresol, orthocresol, ethylphenol, guaiacol, and catechol, etc. The foregoing may be used alone or as a combination of two or more. Among the foregoing, from the viewpoint that it is easy to glycosylate a reducing terminal more effectively, it is preferable that glycerin, mannitol, pentaerythritol, methanol, or ethylene glycol is included, and it is more preferable that glycerin, mannitol, or pentaerythritol is included.

[0017] When a glycosylation agent (a1) is used as the compound (A), the ratio of the glycosylation agent (a1) (concentration of the glycosylation agent (a1)) with respect to the total mass of the aqueous medium is preferably 0.1-50 mass%, more preferably 0.5-20 mass%, and even more preferably 1-15 mass%. By causing the raw material fiber to absorb an aqueous medium in which the concentration of the glycosylation agent (a1) is within the ranges described above, it is easier to obtain a continuous cellulose fiber having improved thermal stability.· Esterification agent (a2)

[0018] In one embodiment, the compound (A) can include an esterification agent (a2) of a hydroxyl group of cellulose. An "esterification agent" indicates all compounds which are capable of esterifying a hydroxyl group of cellulose. As long as the esterification agent (a2) is a compound capable of esterifying a hydroxyl group of cellulose, there are no particular limitations thereon. However, from the viewpoint of avoiding the internal structure of cellulose being destroyed by the esterification agent (a2), it is preferable to include acetic acid, boric acid, or a boric-acid compound, which are weak acids, it is more preferable to include boric acid or a boric acid compound, and it is even more preferable to include boric acid.

[0019] Examples of a boric acid compound include: boric acid salts such as alkali metal salts, alkaline earth metal salts, and metallic salts, etc.; metaboric acid and salts thereof; and polyboric acid and salts thereof, etc. The foregoing boric acid compounds may be used alone or as a combination of two or more.

[0020] When an esterification agent (a2) is used as the compound (A), the ratio of the esterification agent (a2) (concentration of the esterification agent (a2)) with respect to the total mass of the aqueous medium is preferably 0.1-50 mass%, more preferably 0.5-20 mass%, and particularly preferably 1-15 mass%. By causing the raw material fiber to absorb an aqueous medium in which the concentration of the esterification agent (a2) is within the ranges described above, it is easier to obtain a continuous cellulose fiber having improved thermal stability.

[0021] In one embodiment, from the viewpoint of improved thermal stability, the compound (A) preferably includes the esterification agent (a2) and more preferably includes boric acid.

[0022] The compound (A) may include another compound other than the glycosylation agent (a1) and the esterification agent (a2). Examples of the other compound include metal hydrates such as hydrates of aluminum hydroxide, calcium hydroxide, dolomite, hydrotalcite, calcium hydroxide, barium hydroxide, basic magnesium carbonate, zirconium hydroxide, or tin oxide, etc. The foregoing other compounds may be used alone or as a combination of two or more.

[0023] In step (I), the raw material fiber is immersed in an aqueous medium to cause the raw material fiber to absorb the aqueous medium. Step (I) is preferably carried out in conditions wherein the immersed length of the raw material fiber is 1-20 cm and the winding-in speed or drawing-in speed of the raw material fiber is 10-1,000 m / minute. The immersed length can be arbitrarily adjusted in the range of 1-20 cm, and in the same manner, the winding-in speed or drawing-in speed can also be arbitrarily adjusted in the range of 10-1,000 m / minute. Note that "the immersed length of the raw material fiber" indicates the length of the fiber in a section of the raw material that is immersed in the aqueous solution. In one embodiment, step (I) may be carried out in conditions wherein the immersed length of the raw material fiber is 1-1,000 cm and the winding-in speed or drawing-in speed of the raw material fiber is 10-1,000 m / minute. In that case, the immersed length can be arbitrarily adjusted in the range of 1-1,000 cm, and in the same manner, the winding-in speed or drawing-in speed can also be arbitrarily adjusted in the range of 10-1,000 m / minute.

[0024] The temperature of the aqueous medium can be set in accordance with the solubility of the compound (A) in the water component. For example, in the case of boric acid, a saturated aqueous solution concentration at 20°C is less than 5 mass%. As such, in order to make, for example, a 9 mass% boric acid aqueous solution, the temperature of the aqueous medium is preferably set so as to be 50°C or more.

[0025] In a preferred embodiment, step (I) may include attaching the compound (A) to the raw material fiber by immersing the raw material fiber in the aqueous medium to cause the raw material fiber to absorb the aqueous medium. At that time, the concentration of the compound (A) in the aqueous medium, the immersed length described above, and / or the winding-in speed or the drawing-in speed described above may be adjusted so that the attached amount of the compound (A) is 0.1-20 mass% and more preferably 0.3-15 mass% with respect to the total mass of the raw material fiber. By attaching the compound (A) to the raw material, preferably by attaching the compound (A) to at least a portion of the raw material fiber surface, the thermal stability improvement effect is likely to be uniform. In a preferred embodiment, the attached amount of the compound (A) may be 0.01-20 parts by mass, may be 0.01-10 parts by mass, and may be 0.03-5 parts by mass with respect to 100 parts by mass of the raw material fiber. The foregoing embodiment is preferably implemented with the compound (A) including boric acid or including a glycosylation agent (a1) (preferably including glycerin, pentaerythritol, or mannitol). In that case, it is easy to obtain a cellulose fiber having a lower heating weight loss rate.<Step (I')>

[0026] The production method according to the present embodiment preferably includes, after step (I), adjusting the amount of the aqueous medium absorbed by the raw material fiber (step I'). Specifically, the "ratio of the aqueous medium absorbed by the raw material fiber with respect to the total mass of the raw material fiber before absorbing the aqueous medium" (hereinafter referred to as the "aqueous medium ratio") is more preferably adjusted so as to be 200 mass% or less. Here, the "aqueous medium ratio" expresses a total value of the amount of aqueous medium absorbed by the raw material fiber and the amount of aqueous medium in contact with and present around the raw material fiber. Further, the aqueous medium ratio can be calculated as "aqueous medium ratio (%) = (W2-W1) / W1 × 100" wherein W1 is the weight of the raw material fiber before absorbing the aqueous medium and W2 is the weight of the raw material fiber after absorbing the aqueous medium. When, in step (I), an excessive amount of the aqueous medium is absorbed by the raw material fiber or when the amount of the aqueous fiber present around the raw material fiber is too great, the drying time in step (II) described later may become too long causing productivity to deteriorate or causing the thermal stability improvement effect in the obtained continuous fiber to be non-uniform. Due thereto, after step (I) and before implementing step (II), it is preferable to implement step (I'). Note that also from the viewpoint of controlling the amount of the compound (A) that is attached to the fiber performing step (I') is particularly preferable.

[0027] A preferred method for adjusting the aqueous medium ratio is a method in which the raw material fiber is compressed to remove aqueous medium in the raw material fiber and / or present around the raw material fiber and render the aqueous medium ratio so as to be in the range of 200 mass% or less. In one embodiment, the aqueous medium ratio may be 150 mass% or less and may be 100 mass% or less. From the viewpoint of achieving a balance between the thermal stability improvement effect and drying efficiency in step (II), the aqueous medium ratio is preferably 10 mass% or more.

[0028] A preferred method for compressing the raw material fiber is a method in which the raw material fiber is compressed by being pressed by a roller, etc. Specifically, the aqueous medium ratio is more preferably adjusted by removing aqueous medium in the raw material fiber by winding in or drawing in, toward a pressure roller, etc., the raw material fiber that has absorbed the aqueous medium and bringing the raw material fiber into contact with the pressure roller such that a constant pressure is applied thereto. In the case of the foregoing method, there may be one pressure roller or two or more thereof may be set. Note that as pressure roller conditions, it is preferable to determine, by a preliminary test prior to implementing step (I), the aqueous medium ratio when the immersed length and winding-in speed or drawing-in speed are set to specific conditions, and set the pressure of the roller on the basis of the determined value.<Step (II)>

[0029] The production method according to the present embodiment further includes drying the raw material fiber after step (I) (or after steps (I) and (I')). The drying temperature for the raw material fiber is preferably 10-300°C and more preferably 100-280°C. From the viewpoint of being a method that enables industrial production and also in that productivity is likely to improve, a preferred method for drying the raw material fiber is a method in which non-contact drying and contact drying are combined. In one embodiment, a drying method in which hot-air drying and cylinder drying are combined is preferable, and it is particularly preferable for the cylinder drying to be carried out after the hot-air drying. Cylinder drying is a method in which drying is performed while bringing the raw material fiber into contact with a heated cylinder. That is, it is particularly preferable for step (II) to include drying the raw material fiber with hot air, and thereafter, further drying while bringing the raw material fiber into contact with a heated cylinder.

[0030] From the viewpoint of drying efficiency, the temperature of the hot air when drying the raw material fiber is preferably 50-260°C, more preferably 80-200°C, and even more preferably 100-200°C. Further, as an air flow, air is preferable and nitrogen is more preferable. The time during which the hot-air drying is implemented (non-contact drying time) can be adjusted arbitrarily depending on winding-in speed, etc. From the viewpoints of drying efficiency and reducing thermal history on the fiber, the non-contact drying time is preferably 0.01-10 minutes and more preferably 0.1-5 minutes.

[0031] After the hot-air drying (non-contact drying), it is more preferable to further carry out cylinder drying (contact drying). From the viewpoints of drying efficiency and reducing thermal history on the fiber, the cylinder temperature is preferably 50-260°C, more preferably 80-200°C, and even more preferably 100-200°C. Further, from the viewpoint of thermal efficiency, the number of cylinders that contact the raw material fiber is preferably 3-50 and more preferably 5-30.

[0032] The production method according to the present embodiment which includes steps (I) and (II) (preferably steps (I), (I'), and (II)) described above is preferably implemented by using a device that includes: a storage part for storing a raw material fiber; an immersion part including a container for immersing the raw material fiber; a drying part for drying the raw material fiber; and a recovery part for recovering while winding in a continuous cellulose fiber having improved thermal stability or a drawing-in part for drawing in a continuous cellulose fiber having improved thermal stability and unreeling the continuous cellulose fiber in preparation for another production step. By winding in the continuous cellulose fiber with the recovery part or drawing in the continuous cellulose fiber with the drawing-in part, the raw material fiber can be unreeled from the storage part toward the immersion part and the drying part. Hereinafter, there follows an explanation of one example of the production method according to the present embodiment for a case in which a device including a storage part, an immersion part, a drying part, and a recovery part is used.

[0033] The raw material fiber is stored in the storage part in a state of being wound around a bobbin, etc. The storage part includes a width that is wide enough to enable the bobbin to be stored and an unreeling mechanism for unreeling the raw material fiber. Further, there may also be provided a tensioning device for applying a constant tension to the raw material fiber.

[0034] The raw material fiber is first unreeled from the storage part toward the immersion part. The immersion part includes a container that accommodates an aqueous medium. A pressure roller is disposed at an upper section of the container on an outlet (drying part) side of immersion device. The raw material enters from an inlet (storage part) side of the immersion device and is then immersed in the aqueous medium inside the container and wound in at a constant winding-in speed while being caused to absorb the aqueous medium inside the container. Thereafter, the raw material is wound in towards a pressure roller disposed on the outlet side of the immersion part and the aqueous medium ratio is adjusted.

[0035] The raw material that leaves the immersion part is further wound in towards the drying part. The drying part includes a non-contact drying device and a contact drying device. The raw material first enters the non-contact drying device and then enters the contact drying device. The raw material that has been heat-treated by the drying part is further wound in towards the recovery part.

[0036] As described above, the production method according to the present embodiment makes it possible for a continuous cellulose fiber having improved thermal stability to be produced in an industrially productive manner. Note that when a continuous cellulose fiber having improved thermal stability is produced by a device that includes a storage part, an immersion part, a drying part, and a drawing-in part, a regenerated cellulose fiber drawn in by the drawing-in part may further be unreeled in preparation for another production step, for example, toward an impregnation die for producing a fiber-reinforced resin.

[0037] Patent Documents 2 and 3 disclose a thermal stability improvement in cellulose due to a compound such as boric acid or glycerin, etc. In conventional methods, cellulose is added to a dispersion liquid or an aqueous solution including a compound such as boric acid or glycerin, etc., and the entire aqueous solution or dispersion liquid undergoes a drying process at a high temperature to cause a reaction between the boric acid or glycerin and a reducing terminal or hydroxyl group of the cellulose and improve thermal stability. However, such methods are suitable for batch-type production but are ill-suited for continuous fiber production. Further, due to the large amounts of thermal energy that are consumed thereby in industrial production, such methods also have a drawback in terms of cost. The present inventors discovered that it is possible to produce a continuous cellulose fiber having improved thermal stability by immersing a continuous cellulose fiber in an aqueous medium including a compound that can react with a reducing terminal or hydroxyl group of cellulose, such as boric acid or glycerin, etc., causing the continuous cellulose fiber to absorb the aqueous medium, and then recovering the continuous cellulose fiber from the aqueous medium and drying. It was discovered that if the foregoing method is used, not only is a long drying process unnecessary, but surprisingly, the thermal stability of the cellulose was improved to the same extent as cases in which a drying process was performed for a large amount of an aqueous medium.

[0038] In the continuous cellulose fiber having improved thermal stability obtained by the production method according to the present embodiment, discoloration or performance degradation due to heat treatment is unlikely to occur. Due thereto, the continuous cellulose fibers having improved thermal stability obtained by the production method according to the present embodiment can be preferably used in fields such as resin modifiers and reinforcers, etc., which are likely to have a problem wherein discoloration and performance degradation due to heating.EXAMPLES

[0039] Examples are provided below to explain the present disclosure in further detail. However, interpretation of the present disclosure is not limited by the examples provided.

[0040] The following materials were used as the raw material fiber and the compound (A).<Raw material fiber>

[0041] · Regenerated cellulose fibers: regenerated cellulose fibers (Lyocell) obtained by a solvent method. Average fiber diameter: 11 µm, 552 tex, 2700 filament <Compound (A)>

[0042] · Glycosylation agent (a1): glycerin · Esterification agent (a2): boric acid

[0043] [Examples 1 and 2] A device (minisizer, manufactured by Kaji Group, product name: "DCC001P") provided with a storage part, an immersion part, a drying part, and recovery part was used to produce continuous cellulose fibers having improved thermal stability under the conditions shown in Table 1. The thermal stability of the continuous cellulose fibers obtained by the production method of each example was evaluated.(Evaluation of thermal stability)

[0044] The continuous cellulose fiber of Example 1 obtained via a glycerin process was heated for 10 minutes in a 260°C oven. Further, the continuous cellulose fiber of Example 2 obtained via a boric acid process was heated for 10 minutes in a 240°C oven. The discoloration level of the continuous cellulose fiber after being heated was evaluated for each example in accordance with the following criteria. Discoloration levels were compared by deeming the color of a blank 1 (raw material fiber for which a thermal stability improvement process and a heating test were not carried out) to be level 1 and the color of Reference Example 1 (raw material fiber which underwent a heating test without implementing a thermal stability improvement process) to be level 4.(Discoloration level)

[0045] Level 1: approximately the same color as blank 1. Level 2: slightly more discolored than blank 1. Level 3: slightly more discolored than level 2. Level 4: entire fiber approximately the same color as Reference Example 1. TABLE 1 EXAMPLE 1EXAMPLE 2REF EX 1STEP (I)COMPOUND (A) TYPE(-)GLYCERINBORIC ACIDWATER COMPONENT TYPE(-)TAP WATERTAP WATERCOMPOUND (A) CONCENTRATION(MASS%)14.33.8AQUEOUS MEDIUM TEMPERATURE(°C)2323ATTACHED AMOUNT OF COMPOUND (A)(MASS%)5.11.4STEP (I')AQUEOUS MEDIUM RATIO(MASS%)3636STEP (II)NON-CONTACT DRYING (HOT-AIR DRYING)(°C)180180DRYING TEMPERATURECONTACT DRYING (CYLINDER DRYING)(°C)180180DRYING TEMPERATUREWINDING-IN SPEED(m / min)2020IMMERSED LENGTH(cm)3 ~ 103 ~ 10EVALUATION RESULTDISCOLORATION LEVEL AFTER HEATING TEST AT 260°C FOR 10 MIN(-)3-4DISCOLORATION LEVEL AFTER HEATING TEST AT 240°C FOR 10 MIN(-)-34

[0046] As shown in Table 1, it was discovered that by the production method according to the present embodiment, it is possible to produce a continuous cellulose fiber having improved thermal stability. By using, for example, a device used in the examples, the production method according to the present embodiment also makes it possible to industrially produce a continuous cellulose fiber having improved thermal stability.INDUSTRIAL APPLICABILITY

[0047] The production method according to the present embodiment makes it possible for a continuous cellulose fiber having improved thermal stability to be produced in an industrially productive manner. In the continuous cellulose fiber produced by the present embodiment, discoloration and performance degradation due to heating are unlikely to occur, and therefore, the continuous cellulose fiber can be preferably utilized in fields such as resin modifiers and reinforcers, etc.

Claims

1. A method for producing a continuous cellulose fiber having improved thermal stability, wherein: the method comprises immersing a continuous cellulose fiber in an aqueous medium comprising a compound (A) that can react with a reducing terminal or hydroxyl group of cellulose, causing the continuous cellulose fiber to absorb the aqueous medium, and then drying; and the series of above steps are carried out continuously while winding in or drawing in the continuous cellulose fiber.

2. The method for producing a continuous cellulose fiber according to claim 1, wherein the compound (A) comprises at least one compound selected from: a glycosylation agent (a1) of a reducing terminal of the cellulose; and an esterification agent (a2) of a hydroxyl group of the cellulose.

3. The method for producing a continuous cellulose fiber according to claim 2, wherein: the glycosylation agent (a1) comprises a monohydric or polyhydric alcohol; and the esterification agent (a2) comprises boric acid.

4. The method for producing a continuous cellulose fiber according to any one of claims 1 to 3, wherein: an immersed length of the continuous cellulose fiber is 1-1,000 cm; and a winding-in speed or a drawing-in speed of the continuous cellulose fiber is 10-1,000 m / minute.

5. The method for producing a continuous cellulose fiber according to any one of claims 1 to 3, wherein the concentration of the compound (A) in the aqueous medium is 0.1-50 mass%.

6. The method for producing a continuous cellulose fiber according to any one of claims 1 to 3, wherein a drying temperature of the continuous cellulose fiber is 10-300°C.

7. The method for producing a continuous cellulose fiber according to any one of claims 1 to 3, wherein the drying comprises drying the continuous cellulose fiber with hot air and then performing further drying while bringing the continuous cellulose fiber into contact with a heated cylinder.

8. The method for producing a continuous cellulose fiber according to any one of claims 1 to 3, wherein the continuous cellulose fiber comprises a continuous regenerated cellulose fiber, a spun fiber of short-fiber regenerated cellulose, or a spun fiber of natural cellulose.

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