JP2012136798A - Hollow cellulosic fiber and manufacturing method for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide hollow and extra fine cellulosic fiber that is good at lightness, hygroscopic property and feeling and is suitably used for clothing and the like.SOLUTION: The invention provides hollow cellulosic fiber having a single yarn diameter of 10 μm or less and an ester substitution degree less than 0.5, and a method for manufacturing hollow cellulosic fiber in which cellulose fatty acid ester is eluted from core-sheath composite fiber including a core component of cellulose fatty acid ester and a sheath component of cellulose with an ester substitution degree less than 0.5.

Description

  The present invention relates to a cellulosic fiber and a method for producing the cellulosic fiber. More specifically, the present invention relates to a cellulosic fiber that is hollow and extremely fine and has lightness, moisture absorption, and good texture, and a method for producing such cellulosic fiber.

  Cellulose fibers such as rayon and cupra are known to be excellent in hygroscopicity, color development and gloss, and are suitably used as clothing materials. Such cellulose fibers are produced by wet spinning. This is a spinning method in which cellulose fibers are obtained by discharging a spinning stock solution into a coagulation bath. Conventionally, cellulose fibers having a continuous hollow portion in the fibers and having a single yarn diameter of 10 μm or less. It was difficult to get.

  On the other hand, various methods have been proposed for hollowing out or reducing the fineness of cellulose ester fibers. For example, in Patent Document 1, a spinning stock solution mainly composed of cellulose triacetate and a spinning stock solution mainly composed of cellulose diacetate are used in combination, and dry spinning is performed using a special spinneret. A method for obtaining a cellulose acetate composite fiber is proposed. Further, Patent Document 2 proposes a method for obtaining cellulose fatty acid mixed ester fibers having various hollow cross sections by melt spinning a thermoplastic composition mainly composed of a cellulose fatty acid mixed ester using a special spinneret. ing.

  However, the fineness of the fibers obtained by these methods has been difficult due to the limitation of being hollowed out by the spinneret. Moreover, since it is a cellulose ester fiber, the characteristics resulting from the hydroxyl group of a cellulose, such as a hygroscopic property equivalent to a cellulose fiber, and a specific texture, were not acquired.

  Next, in Patent Document 3, by dissolving and removing cellulose diacetate from a fiber having a layered structure of a cellulose triacetate component and a cellulose diacetate component, 60% or more of all the single fibers have a single yarn fineness of 0.11 dtex or more. Methods have been proposed to obtain cellulose triacetate fibers that are less than 0.89 dtex. Furthermore, in Patent Document 4, sea components are removed from sea-island type composite fibers in which a thermoplastic polymer that can be dissolved in alkaline hot water is used as a sea component, and a composition containing a cellulose ester and a plasticizer is used as an island component. A method for obtaining a fiber having a single yarn fineness of 0.01 to 1 dtex has been proposed.

  However, the fibers obtained by these methods have been able to achieve a single yarn diameter of 10 μm or less, but have not been able to achieve hollowing.

  Thus, the hollow and ultrafine fiber was not obtained also in the cellulose ester fiber. Therefore, hollow and ultrafine cellulose fibers could not be obtained by alkali treatment of such fibers.

JP 2002-013028 A JP 2007-119991 A JP 2001-049520 A JP 2004-084139 A

  It is an object to provide a hollow and ultrafine cellulosic fiber that is excellent in lightness, hygroscopicity, and texture, and that can be suitably used in clothing applications.

  The above-mentioned problems can be solved by a hollow cellulose fiber characterized in that it is made of cellulose having a single yarn diameter of 10 μm or less and an ester substitution degree of less than 0.5.

  In order to solve the above-mentioned problems, the present invention is characterized in that cellulose fatty acid ester is eluted from a core-sheath composite fiber having cellulose fatty acid ester as a core component and cellulose having a degree of ester substitution of less than 0.5 as a sheath component. The manufacturing method of cellulosic fiber is adopted.

  In this case, the core-sheath composite fiber is obtained by removing the sea component from the sea-island composite fiber containing the thermoplastic composition made of cellulose fatty acid ester as the island component and deesterifying the surface layer portion of the island component. More preferably, the sea component is a thermoplastic polymer having solubility in an alkaline aqueous solution, and the cellulose fatty acid ester is preferably selected from cellulose acetate propionate and cellulose acetate butyrate. .

  It is possible to provide a hollow and ultrafine cellulosic fiber that is excellent in lightness, hygroscopicity, and texture, and can be suitably used in clothing applications.

FIG. 1 is a conceptual diagram showing an embodiment of a melt spinning apparatus. FIG. 2 is an explanatory diagram of a method for calculating the diameter of a single yarn.

  The present invention is characterized in that the diameter of the single yarn is 10 μm or less. By setting the diameter of the single yarn to 10 μm or less, the texture in the case of a fiber structure such as a woven fabric or a knitted fabric is improved. When it is larger than 10 μm, the texture is in the direction approaching the conventional product. In addition, when the single yarn fineness is extremely thin, the strength of the final product is lowered, or the process passability in high-order processing is lowered. Therefore, when considering the strength of the final product and process passability, the diameter of the single yarn is more preferably 0.5 to 10 μm, and further preferably 1 to 10 μm.

  The present invention is characterized by comprising cellulose having a degree of ester substitution of less than 0.5. The degree of ester substitution in this case is the average value of the number of esterified moieties among the three hydroxyl groups present in the glucose unit of the cellulosic polymer.

  When the ester substitution degree is less than 0.5, the hygroscopicity when the fiber structure is obtained is excellent. In this case, it is preferable that the moisture absorption rate in the standard state of the fiber structure consisting only of the fibers of the present invention at a temperature of 20 ° C. and a relative humidity of 65% is in the range of 6 to 15%. In this range, the higher the hygroscopicity, the more suitable it can be used for clothing. Therefore, it is preferably 7% or more, and more preferably 8% or more.

  Such cellulose is composed of a combination of glucose units represented by the following structural formula.

[Wherein, R ^{1 to} R ³ are each independently hydrogen or an acyl group. ]

  If the degree of ester substitution is less than 0.5, the combination of glucose units and the acyl group in the glucose unit are not particularly limited, but particularly preferred acyl groups include acetyl, propionyl, butyryl and the like. . Such an acyl group is preferable because it does not impair the hygroscopicity of the hollow cellulose fiber. Moreover, since the hygroscopicity is improved as the ester substitution degree is lower, the ester substitution degree is particularly preferably 0.

  In the present invention, cellulose means a cellulose ester having a degree of ester substitution of less than 0.05, or a cellulose having a degree of ester substitution of 0, and a cellulose fatty acid ester described later has a degree of ester substitution of 0.05. The above cellulose ester is meant. Cellulosic fiber means a fiber made of the above cellulose.

  The cellulose ester means a cellulose polymer in which some or all of the three hydroxyl groups present in the glucose unit of the cellulose polymer are substituted with some functional group.

  The cellulosic fiber of the present invention is characterized by being hollow, and the hollow ratio is preferably in the range of 10 to 50%.

In the present invention, the hollow ratio means that a cross section of the cellulosic fiber (cross section perpendicular to the fiber axis of the hollow fiber) is observed with a scanning electron microscope (magnification 2000 times), and a reflected electron image of the fiber cross section After photographing, the total area of the cross section (including the area of the hollow portion) (Sa) and the area of the hollow portion (Sb) are calculated from the photographed drawing, and are values calculated using the following formula. However, in the case of a hollow fiber having a plurality of hollow portions, Sb means the sum of the areas of the respective hollow portions.
Hollow ratio (%) = (Sb / Sa) × 100

  If the hollowness is 10% or more, the lightness that is the effect of the present invention is clearly expressed. Moreover, the texture in the case of a fiber structure is improved. If the hollow ratio is 50% or less, the mechanical properties of the fiber can be maintained, and when the article becomes a garment, the cross section of the fiber during wear does not occur, which is preferable. The hollow ratio is more preferably 10 to 45%, and further preferably 10 to 40%.

  The tensile strength of the hollow cellulose fiber of the present invention is preferably 0.6 cN / dtex or more. If the tensile strength is 0.6 cN / dtex or more, it is preferable because it is easy to pass through higher processing steps such as weaving and knitting, and the strength of the final product is not insufficient. From the viewpoint of good tensile strength characteristics, the higher the tensile strength, the more the practical range is widened and it can be developed for various uses. However, at present, 2.0 cN / dtex is the upper limit. The tensile strength is more preferably 0.8 cN / dtex or more, and further preferably 1.0 cN / dtex or more.

  The elongation of the hollow cellulose fiber of the present invention is preferably 5 to 50%. When the elongation is 5% or more, fluff does not occur in the spinning process, and fluff and yarn breakage do not occur frequently in high-order processing processes such as weaving and knitting. Property is improved. The elongation is preferably 50% or less. When the elongation is 50% or less, the fiber will not be deformed if the stress is low, and a dyeing defect of the final product due to wetting marks during weaving will not occur. The elongation is more preferably 6 to 45%, further preferably 7 to 40%.

  The final product includes not only fiber structures such as woven fabrics and knitted fabrics, but also clothing and interior products that use the hollow cellulosic fibers of the present invention as a part or all of the material.

  The hollow cellulosic fiber of the present invention can be used for apparel use, medical use, bedding use, interior use, and the like. Since it has lightness, hygroscopicity, and good texture, it is particularly preferable to be used for clothing.

  When used for apparel, it is more preferable to use it in the form of woven fabrics such as taffeta, desine, georgette and twill, or knitted fabrics such as tengu, smooth and tricot.

  The hollow cellulose fiber of the present invention is preferably produced using a sea-island composite fiber as a precursor. By using the sea-island composite fiber as a precursor, a hollow cellulose fiber having a single yarn diameter of 10 μm or less can be stably obtained.

  The sea component of the sea-island composite fiber is preferably a thermoplastic polymer that is soluble in an alkaline aqueous solution.

  The solubility in an alkaline aqueous solution means that a weight loss of 50% by weight or more is observed when immersed in an aqueous solution of sodium hydroxide having a concentration of 2% by weight / L heated to 80 ° C. for 60 minutes. That means. By forming a composite fiber with a sea component made of a thermoplastic polymer that can be dissolved in an alkaline aqueous solution, the sea component is dissolved and removed in a general scouring process or alkali weight reduction process in the processing process of woven or knitted fabric. The island components inside can be completely divided into each. Any thermoplastic polymer that can be dissolved in a neutral or acidic aqueous solution can be used as long as it is soluble in an alkaline aqueous solution.

  Examples of the thermoplastic polymer include polyethylene glycol polymers such as polyethylene oxide (polyethylene glycol), polypropylene glycol, and poly (ethylene glycol-propylene glycol) copolymer, 5-sodium sulfoisophthalic acid copolymer polyethylene terephthalate, Copolymer aromatics such as 5-sodium sulfoisophthalic acid copolymerized polybutylene terephthalate, 5-sodium sulfoisophthalic acid copolymerized polypropylene terephthalate, and aromatic polyester copolymerized with a third component such as isophthalic acid or aliphatic dicarboxylic acid Examples include polyester polymers, aliphatic polyester polymers such as polylactic acid, polyglycolic acid, polybutylene succinate, and polyethylene succinate.

  From the standpoints of solubility in alkaline aqueous solution and yarn-making property, 5-sodium sulfoisophthalic acid copolymerized polyethylene terephthalate, 5-sodium sulfoisophthalic acid copolymerized polybutylene terephthalate, 5-sodium sulfoisophthalic acid copolymerized polypropylene terephthalate, etc. 5-sodium sulfoisophthalic acid copolymer polyester and polylactic acid are preferred.

  The island component of the sea-island composite fiber is preferably a thermoplastic composition comprising a cellulose fatty acid ester.

  The thermoplastic composition comprising a cellulose fatty acid ester is a composition composed of only a cellulose fatty acid ester or a mixture of a cellulose fatty acid ester and a plasticizer.

The cellulose fatty acid ester is one in which part or all of the three hydroxyl groups present in the glucose unit of the cellulose polymer are blocked with an acyl group. Specific examples of the acyl group include an acetyl group, a propionyl group, and a butyryl group. Specific examples of such cellulose fatty acid esters include cellulose fatty acid mixed esters such as cellulose acetate propionate in which an acetyl group and a propionyl group are bonded, and cellulose acetate butyrate in which an acetyl group and a butyryl group are bonded. In addition, it is preferable that each ester substitution degree of an acetyl group and another acyl group satisfy | fills the following formula.
2.0 ≦ (Degree of ester substitution of acetyl group + Degree of ester substitution of other acyl groups) ≦ 2.9

  By using such a fatty acid mixed ester, the spinning property of the sea-island composite fiber is improved.

  The weight average molecular weight (Mw) of the cellulose fatty acid ester is preferably 50,000 to 255,000. When Mw is less than 50,000, thermal decomposition at the time of melt spinning becomes remarkable, and the mechanical properties (such as tensile strength) of the sea-island composite fiber are lowered, which is not preferable. When Mw exceeds 255,000, the melt viscosity becomes very high, so that the yarn-making property at the time of melt spinning is deteriorated. From the viewpoints of mechanical properties and spinning properties during melt spinning, Mw is more preferably 60,000 to 20,000, and even more preferably 80,000 to 20,000,000.

  The thermoplastic composition preferably contains a plasticizer in the range of 0 to 30% by weight. By containing a plasticizer, the heat flow property of the thermoplastic composition is improved, and production by a melt spinning method with high production efficiency is facilitated, which is preferable. On the other hand, if it is 30% by weight or less, melt fracture during melt spinning (when the shear stress of the polymer is high when passing through the spinneret hole, streamline turbulence occurs and the shape of the yarn discharged from the spinneret is irregular. This phenomenon is preferable because it does not occur. The plasticizer is preferably contained in the range of 5 to 27% by weight, and more preferably in the range of 10 to 24% by weight.

  Examples of the plasticizer that can be preferably used for such a purpose include polyhydric alcohol compounds. Specific examples include polyalkylene glycols, glycerin compounds, caprolactone compounds, and the like, and these compounds have good compatibility with cellulose fatty acid esters, so that the thermoplastic effect is good. Of these, polyalkylene glycol is preferred. Specific examples of the polyalkylene glycol include polyethylene glycol, polypropylene glycol and polybutylene glycol having a weight average molecular weight of 200 to 4000. Plasticizers can be used alone or in combination.

  The thermoplastic composition preferably contains a phosphorus antioxidant, and particularly preferably a pentaerythritol compound. When the phosphorus-based antioxidant is contained, the effect of preventing thermal decomposition is good even in the range where the spinning temperature is high and in the low discharge region, and deterioration of the mechanical properties and color tone of the sea-island composite fiber is suppressed. It is preferable that the compounding quantity of phosphorus antioxidant is 0.005-0.5 weight% with respect to this thermoplastic composition.

  The thermoplastic composition is within the range in which melt spinning is possible, such as stearamide, ethylenebisstearylamide, dimethyl silicone, methylphenyl silicone and other lubricants, phosphorus compounds, flame retardants such as silicone compounds, titanium oxide, barium sulfate, etc. Inorganic particles, coloring pigments, dyes, and other additives generally used in fiber applications may be contained. When adding inorganic particles, it is possible to shorten the time required for elution by utilizing voids generated in the surface of the cellulosic fiber by the inorganic particles in the elution step described later. May be colored or the mechanical properties may be slightly deteriorated.

  The sea / island component ratio of the sea-island composite fiber (hereinafter referred to as the sea-island ratio) is preferably 5/95 to 70/30 from the viewpoint of the stability of the composite form, the yarn-making property and the productivity. When the sea component is small, there is a possibility that a composite abnormality such as island components are fused together, and when the sea component is large, the manufacturing cost increases, so the composite ratio is 10/90 to 60/40. It is more preferable that it is 15 / 75-50 / 50. The sea-island ratio in the present invention means an area ratio between the area occupied by the sea component and the area occupied by the island component in the fiber cross section.

  When the sea-island composite fiber is made into a fiber by a melt spinning method, the spinning temperature is preferably 220 ° C to 280 ° C. By setting the spinning temperature to 220 ° C. or higher, the elongational viscosity of the yarn discharged from the spinneret is sufficiently reduced, so that short pitch periodic spots due to melt fracture do not appear, and there is no thickness unevenness. Excellent fibers can be obtained. Furthermore, since the thinning deformation process of the yarn discharged from the spinneret becomes gentle, the fiber microstructure is developed and the fiber characteristics are improved. In addition, the spinning property is stable. Moreover, by setting the spinning temperature to 280 ° C. or lower, it is possible to suppress thermal decomposition of the sea component and the island component, and it is possible to suppress deterioration in mechanical properties and color tone of the composite fiber due to molecular weight reduction. Since deterioration and coloring of the mechanical properties of the composite fiber as a precursor deteriorate the mechanical properties and color tone of the hollow cellulosic fiber, it is preferable to suppress them. The spinning temperature is more preferably 230 ° C to 270 ° C, and further preferably 240 ° C to 260 ° C.

  FIG. 1 is a diagram showing an embodiment of an apparatus used in a method for producing sea-island composite fibers. In FIG. 1, 1 is a spin block, 2 is a melt spinning pack, 3 is a spinneret, 4 is a cooling start point, 5 is a cooling device, 6 is a spun yarn, 7 is an oil supply device, and 8 is a first godet roller. , 9 is a second godet roller, and 10 is a package.

  The sea component and island component in the molten state are pushed out from the discharge holes of the spinneret 3 attached to the lower part of the melt spinning pack 2 attached to the spin block 1. The extruded spun yarn 6 is cooled to the room temperature by the cooling device 5 from the cooling start point 4, the finishing agent is applied by the oil supply device 7, and the first godet roller 8 and the second godet rotating at a predetermined speed. It is wound up as a package 10 by a winder through a dead roller 9.

  The position of the cooling start point 4 is preferably a distance from the lower surface of the spinneret 3 of 0 mm to 200 mm. When the position of the cooling start point is within this range, it is possible to obtain a composite fiber excellent in mechanical properties and uniformity without unevenness of the yarn thickness with good yarn production. The position of the cooling start point is more preferably 0 mm to 190 mm, and further preferably 0 to 180 mm. The cooling start point is the uppermost end of the cooling device.

  The cooling air speed of the yarn discharged from the spinneret is preferably 0.01 m / second to 1 m / second. When the cooling air velocity is within this range, the cooling of the single yarn constituting the yarn discharged from the spinneret becomes uniform, and a composite fiber having excellent uniformity with no thickness unevenness is obtained. The cooling air speed is more preferably 0.1 m / second to 0.5 m / second, and further preferably 0.2 m / second to 0.45 m / second.

  If the cooling air temperature of the yarn discharged from the spinneret is 10 ° C. or higher, the spinning property is not deteriorated, which is preferable. For the purpose of cooling, it is preferably lower than the spinning temperature. Of these, a temperature of 15 to 40 ° C. is more preferable.

  The spinning speed is preferably 750 m / min to 3500 m / min. By setting the spinning speed to 750 m / min to 3500 m / min, a spinning stress that sufficiently promotes the molecular orientation of the obtained fiber is applied, and a composite fiber having excellent fiber characteristics can be obtained. The spinning speed is more preferably 1000 m / min to 3000 m / min, and further preferably 1250 m / min to 2500 m / min.

  When a process using sea-island composite fibers is employed as a precursor, a process of removing sea components from the composite fibers (hereinafter, this process is referred to as sea removal) is also employed.

  When a plasticizer is contained in the island component of the composite fiber, it is preferable to employ a step of extracting the plasticizer from the composite fiber before performing sea removal (hereinafter, this step is referred to as a deplasticizer). . By extracting the plasticizer, it is possible to avoid problems such as contamination of the sea removal device and difficulty in controlling sea removal. The deplasticizer may be used by a method of immersing the composite fiber in water or hot water, alcohol such as methanol or ethanol, or an organic solvent, or continuing to bathe the composite fiber.

  An alkaline aqueous solution is used for sea removal. In the present invention, the alkaline aqueous solution is an aqueous solution having a pH of 8 or more. From the viewpoint of sea removal treatment efficiency, the pH is preferably 9 or more, and more preferably 10 or more. The higher the pH is, the higher the efficiency of sea removal. However, if the pH is too high, it becomes difficult to control sea removal, so that the pH is preferably 12 or less.

  Any compound that can obtain an aqueous solution having a pH in the above range can be used for the preparation of an aqueous solution without any particular limitation. Specific examples thereof include hydroxides such as sodium and potassium, carbonates, peroxides. Examples include carbonates and bicarbonates. From the viewpoint of the price of the compound itself and the efficiency of sea removal, sodium hydroxide and sodium carbonate are preferred.

  During sea removal, the aqueous solution is preferably heated in the range of 20 to 90 ° C. If the temperature is 20 ° C. or higher, sea removal proceeds efficiently, and if the temperature is 90 ° C. or lower, the mechanical properties of the resulting fiber do not deteriorate. From the viewpoint of sea removal efficiency and the mechanical properties of the fiber, it is more preferably 30 to 80 ° C, and further preferably 40 to 70 ° C.

  When adopting a method of immersing the sea-island composite fiber in the aqueous solution at the time of sea removal, the treatment time for the immersion is preferably in the range of 5 to 120 minutes. The optimum treatment time varies depending on the compound dissolved in the aqueous solution and the temperature of the aqueous solution. However, if the treatment time is shorter than 5 minutes, the sea removal is not sufficiently performed. If the treatment time is longer than 120 minutes, the mechanical properties of the fiber obtained after the sea removal are not obtained. Tend to be lower. The treatment time is more preferably 10 to 90 minutes considering sufficient sea removal and the mechanical properties of the fiber. More preferably, it is 15 to 60 minutes. In addition, said process time is the process time supposing the case where it performs without interrupting immersion, after immersing a sea-island composite fiber using one or several processing tanks, it squeezes, and also immerses and squeezes. This is not the case when performing sea removal by repeating, or when sea removal is performed by continuously bathing the aqueous solution on the sea-island composite fiber.

  The device used for sea removal is not particularly limited, but a device capable of stirring an aqueous solution at the time of sea removal or a device capable of moving a composite fiber in an aqueous solution is preferable because the efficiency of sea removal is improved. .

  It should be noted that the high efficiency of sea removal means that industrial effects such as removal of sea components in a short time during sea removal and the reduction of the amount of compound required for the preparation of an aqueous solution occur. is there.

  The hollow cellulosic fiber of the present invention comprises a cellulose fatty acid ester as a core component, and a cellulose fatty acid ester is eluted from a core-sheath composite fiber having a cellulose having a degree of ester substitution of less than 0.5 as a sheath component. Can be obtained.

  The cellulose fatty acid ester as the core component is the same as the cellulose fatty acid ester constituting the island component of the sea-island composite fiber. Moreover, the cellulose whose ester substitution degree of a sheath component is less than 0.5 is the same as the cellulose whose ester substitution degree which comprises the hollow cellulose fiber of this invention is less than 0.5.

  The core / sheath component composite ratio of the core-sheath composite fiber (hereinafter referred to as the core-sheath ratio) is preferably in the range of 10/90 to 50/50.

  In addition, the core-sheath ratio in this invention means the area ratio of the area which a core component occupies and the area which a sheath component occupies in a fiber cross section.

  When the cellulose fatty acid ester is eluted from the composite fiber, if the core-sheath ratio is 10/90 or more, the hollow ratio of the resulting hollow cellulose fiber is 10% or more, and if the core-sheath ratio is 50/50 or less, the hollow ratio is hollow. The rate becomes 50% or less, and the hollow rate can be controlled.

  Such a core-sheath conjugate fiber is a method of deesterifying the surface layer of a cellulose fatty acid ester fiber obtained by removing sea components from the sea-island conjugate fiber to obtain cellulose having an ester substitution degree of less than 0.5. It is possible to obtain in

  Deesterification means that the fatty acid group in the fatty acid ester portion present in the glucose unit of cellulose fatty acid ester is eliminated and substituted with a hydroxyl group.

  Deesterification can be performed by a method such as immersing the cellulose fatty acid ester in an alkaline aqueous solution. Therefore, in the present invention, when a production method using sea-island composite fibers as a precursor is employed, a production method in which deesterification is performed simultaneously with sea removal can also be employed. It is preferable to perform sea removal and deesterification at the same time because the time required for producing the hollow cellulosic fibers is shortened. Even when sea removal and deesterification are performed simultaneously, the seawater treatment conditions already described in this specification can be adopted as the treatment conditions. Moreover, even when only deesterification is performed, the conditions for sea removal can be employed.

  When the core-sheath composite fiber is obtained by deesterification, the treatment time may be adjusted to adjust the core-sheath ratio. To reduce the ratio of the core component and increase the ratio of the sheath component with respect to the core-sheath ratio at a certain processing time, it is necessary to increase the processing time, and to increase the ratio of the core component and decrease the sheath component. What is necessary is just to shorten processing time.

  The elution is to dissolve and remove only the cellulose fatty acid ester from the core-sheath composite fiber to obtain a hollow cellulose fiber made of cellulose having an ester substitution degree of less than 0.5. For elution, ester solvents such as methyl formate, methyl acetate, ethyl acetate and ethyl lactate, ether solvents such as tetrahydrofuran, dioxane and dioxolane, ketone solvents such as acetone, methyl ethyl ketone, cyclohexanone and diacetone alcohol, nitromethane, acetonitrile Nitrogen-containing solvents such as N-methylpyrrolidone and dimethylformamide, glycol solvents such as methyl glycol and methyl glycol acetate, solvents such as methylene chloride, chloroform, tetrachloroethane, dimethyl sulfoxide, propylene carbonate, and these solvents and water Or a mixed solution thereof can be used.

  The treatment liquid used for elution may use the above solvent as it is, or may be used by mixing with water or other liquid. In that case, it is preferable that the solvent contains 30% by weight or more. From the viewpoint of shortening the time required for elution, 50% by weight or more is more preferable, and 70% by weight or more is more preferable.

  The elution can be performed by a method of immersing the core-sheath composite fiber in a treatment liquid or continuing to bathe the core-sheath composite fiber in the treatment liquid. From the viewpoint of easily controlling the elution conditions, a method of immersing the core-sheath composite fiber in the treatment liquid is preferable. When elution is performed, the temperature of the treatment liquid is not particularly limited, but a temperature at which the treatment liquid is in a liquid state is preferable. About processing time, from a viewpoint of fully eluting the cellulose fatty acid ester of a core component, 30 minutes or more are preferable and it is more preferable that it is 1 hour or more. Although an upper limit is not specifically limited, From a viewpoint of the manufacturing efficiency of the hollow cellulose fiber of this invention, it is preferable within 24 hours. The elution apparatus is not particularly limited, but it is preferable to use an apparatus capable of stirring the treatment liquid and an apparatus capable of moving the core-sheath composite fiber in the treatment liquid. By using such an apparatus, the time required for elution is shortened.

  In this invention, it is preferable that finishing agents, such as a softening agent and an electrostatic agent, are provided to the hollow cellulosic fiber. Therefore, it is preferable to employ a step of applying a finishing agent to the cellulosic fibers obtained after elution. Cellulose fibers with very fine single yarns may cause aggregation of the single yarns due to uneven charge on the fiber surface layer or hydrogen bonds between hydroxyl groups present in the glucose unit. If aggregation occurs, the texture of the fiber structure tends to be impaired, and the quality of the fiber structure tends to decrease, such being undesirable.

  Specific examples of the finishing agent include anionic surfactants, cationic surfactants such as polyamide derivatives, nonionic surfactants, coating agents such as aminosilicone and paraffin, and lubricants. In view of the high effect of suppressing aggregation, a cationic surfactant such as a polyamide derivative, aminosilicone and the like are particularly preferable.

  In order to give a finishing agent to the cellulosic fiber, the cellulosic fiber may be immersed in a treatment solution that is an aqueous solution or a water suspension containing 0.05 to 10.00% by weight of the finishing agent. The temperature of the treatment liquid during immersion is not particularly limited, but if the temperature is too high, the mechanical properties of the cellulosic fibers are impaired, so the upper limit is 90 ° C.

  The evaluation method of each physical property is as follows.

(1) Degree of ester substitution 0.9 g of a cellulose sample dried at 80 ° C. for 8 hours was weighed, 35 ml of acetone and 15 ml of dimethyl sulfoxide were added and dissolved, and 50 ml of acetone was further added. While stirring, 30 ml of 0.5N sodium hydroxide aqueous solution was added and reacted for 2 hours. After adding 50 ml of hot water and washing the side of the flask, it was titrated with 0.5N sulfuric acid using phenolphthalein as an indicator. Separately, a blank test was performed in the same manner as the sample. The supernatant of the solution after titration was diluted 100 times, and the composition of the organic acid was measured using an ion chromatograph. The degree of ester substitution was calculated from the measurement results and the results of acid composition analysis by ion chromatography.

The following formula shows an example in which the average value of the ester substitution degree of the acetyl group of the cellulose-based sample and the average value of the ester substitution degree of the acyl group other than the acetyl group are calculated.
TA = (B−A) × f / (1000 × W)
DSace = (162.14 × TA) / [{1− (Mwase−16.00−1.01 × 2) × TA} + {1− (Mwacy−16.00−1.01 × 2) × TA} × (Acy / Ace)]
DSacy = DSace × (Acy / Ace)
TA: Total organic acid amount per 1 g of cellulosic sample (mol / g)
A: Cellulose sample titration (ml)
B: Blank test titration (ml)
f: sulfuric acid titer W: cellulosic sample weight (g)
DSace: average value of ester substitution degree of acetyl group DSacy: average value of ester substitution degree of acyl group Mwash: molecular weight of acetic acid Mwacy: molecular weight of other organic acids Acy / Ace: organic acid other than acetic acid (Ace) and acetic acid ( Acy) molar ratio 162.14: molecular weight of glucose unit 16.00: atomic weight of oxygen 1.01: atomic weight of hydrogen

  In the above work, first, a solution in which a cellulosic sample is dissolved is reacted with an excess aqueous sodium hydroxide solution, and all organics derived from acyl groups such as acetyl group, propionyl group and butyryl group are obtained by back titration with sulfuric acid. Calculate the amount of acid (number of moles per gram of sample), second, calculate the ratio of each organic acid in the total organic acid by ion chromatography using high performance liquid chromatography, and third, glucose that constitutes the cellulosic sample The degree of ester substitution is calculated by using the mathematical formula for calculating the molecular weight of the unit and the values calculated in the first and second.

(2) Measurement of weight average molecular weight (Mw) by gel permeation chromatography (GPC) It was completely dissolved in tetrahydrofuran so that the concentration of cellulose fatty acid ester was 0.15% by weight, and used as a sample for GPC measurement. Using this sample, GPC measurement was performed with Waters 2690 (manufactured by Waters) under the following conditions, and the weight average molecular weight (Mw) was determined in terms of polystyrene.
Column: Two TSK gel GMHHR-H manufactured by Tosoh detector: Waters 2410 (manufactured by Waters) Differential refractometer RI
Moving bed solvent: Tetrahydrofuran flow rate: 1.0 ml / min Injection volume: 200 μl.

(3) Tensile strength and elongation Using an Autograph AG-50NISMS type (manufactured by Shimadzu Corp.) under a temperature of 20 ° C and a humidity of 65%, a tensile test was performed under the conditions of a sample length of 20 cm and a tensile speed of 20 cm / min. The value obtained by dividing the stress (cN) at the point indicated by the maximum load by the fineness (dtex) was taken as the tensile strength (cN / dtex). The elongation at that time was defined as elongation (%). The number of measurements was 5 times, and the average values were taken as tensile strength and elongation.

(4) Observation of fiber cross section (scanning electron microscope)
The cross section of the fiber was observed with a scanning electron microscope, and a reflected electron image of the fiber cross section was taken. Image analysis software was used to calculate the length, area, etc. from the photograph.
Cross-section creation: Fiber embedded in epoxy resin, fiber cut along with resin Cutting direction: Transverse (cut perpendicular to fiber axis)
Observation device: Scanning electron microscope (S-4000 manufactured by Hitachi High-Technologies Corporation)
Observation magnification: 2000 times image analysis software: WinRooF5.0 (manufactured by Mitani Corporation).

(4-1) Diameter of single yarn From a photograph obtained by observation of the fiber cross section, a quadrilateral (four sides) that includes a side parallel to the line connecting the ends of the longest diameter portion of the cross section of the single yarn and circumscribes the cross-sectional shape. The long side length X and the short side length Y were calculated, and the average value of X and Y was taken as the diameter of the single yarn (see FIG. 2). This is performed for 20 single yarns, and is an average value calculated. (That is, the length X of the long side is equal to the length of the longest diameter portion of the cross section of the single yarn.)

(4-2) Hollow ratio The total area Sa of the cross section of the single yarn and the area Sb of the hollow portion are calculated from the photographed drawing obtained by observation of the fiber cross section (scanning electron microscope), and the hollow ratio is calculated using the following formula Was calculated. It should be noted that the solid part and the hollow part in the photographed drawing can be distinguished by the difference in brightness of the image. The hollowness was calculated for 20 single yarns, and the average value was defined as the hollowness. Moreover, if the hollow ratio was 10% or more, it was judged that there was lightness.
Hollow ratio (%) = (Sb / Sa) × 100.

(5) Observation of fiber cross section (transmission electron microscope)
The fiber was embedded with an epoxy resin, and the fiber was cut together with the epoxy resin by using a slice preparation device to obtain an ultrathin slice having a thickness of 1 μm. The obtained ultrathin section was observed with a transmission electron microscope, and an image of the fiber cross section was taken. Image analysis software was used to calculate the length, area, etc. from the photograph.
Method: Ultra-thin section cutting direction: Crossing (cut perpendicular to the fiber axis)
Section preparation apparatus: Ultramicrotome (MT6000 type, manufactured by Sorvall)
Observation device: Transmission electron microscope (H800 manufactured by Hitachi High-Technologies)
Observation magnification: 2000 times image analysis software: (WinRooF5.0, manufactured by Mitani Corporation).

(5-1) Sea-island ratio The area Ta occupied by the sea component and the area Tb occupied by the island component in the cross-section of the single yarn are calculated from the photographed figure obtained by observation of the fiber cross-section (transmission electron microscope), and Ta and Tb The ratio was the sea-island ratio. It should be noted that the sea component and the island component can be distinguished from each other based on the difference in brightness of the image. In addition, the said area Ta and Tb shall use the average value of the area calculated from 20 single yarn, respectively (namely, the value of sea-island ratio is the average value of Ta / average value of Tb).
Example) When Ta = 600 (μm) ² and Tb = 2400 (μm) ² The sea-island ratio is 600/2400 = 20/80.

(5-2) Core-sheath ratio The area Ua occupied by the core component and the area Ub occupied by the sheath component in the cross-section of the single yarn are calculated from the photographed figure obtained by observation of the fiber cross-section (transmission electron microscope), and Ua and Ub The ratio was defined as the core-sheath ratio. In the photograph drawing, the core component and the sheath component can be distinguished by the difference in brightness of the image. In addition, the said area Ua and Ub shall use the average value of the area calculated from 20 single yarn, respectively (namely, the value of a core-sheath ratio is the average value of Ua / average value of Ub).
Example) When Ua = 24 (μm) ² and Ub = 36 (μm) ² The core-sheath ratio is 24/36 = 40/60.

(5-1) Evaluation of texture A 5 m long tubular knitted fabric to be evaluated was prepared, and the texture with a reference tubular knitted fabric was compared. Ten test subjects scored and evaluated the average score based on the following criteria. In addition, the thing with an average score of 4 or more was set as the pass.
5: It was clear that the texture was excellent, and I felt that the texture was warm.
4: The texture is excellent.
3: The texture is slightly good.
2: Equivalent texture.
1: The texture is inferior.

(5-2) Preparation of reference tubular knitted fabric Island component A, which will be described later, is melted at a melter temperature of 270 ° C. and introduced into a melt spinning pack having a spinning temperature of 270 ° C., and a pore diameter D = 0.18 mm, L / It was discharged at a discharge rate of 20.0 g / min from a die having D = 3 and the number of holes = 72. The discharged multifilament is cooled by cooling air (25 ° C., wind speed 0.30 m / sec) from a distance of 40 mm below the base surface (cooling start point), and is converged by applying an oil agent, and then 2000 m / min. Taking up at a speed, a multifilament of 100 dtex-72 filaments was obtained.

  Using this multifilament, 5 meters of 27-gauge cylindrical knitted fabric was produced with a circular knitting machine (NCR-BL type manufactured by Eikoh Industry Co., Ltd.) having a hook diameter of 3.5 inches (89 mm). This cylindrical knitted fabric was immersed in water at room temperature for 20 minutes to deplasticizer, and then filled with an alkaline aqueous solution (compound: sodium hydroxide, concentration: 1.5% by weight) heated to 60 ° C. It was immersed in a treatment tank for 30 minutes and deesterified to obtain a tubular knitted fabric comprising a core-sheath composite fiber having cellulose fatty acid ester as a core component and cellulose having a degree of ester substitution of less than 0.5 as a sheath component. Further, the tubular knitted fabric is dipped in a soft finish processing solution containing 3% by weight of “Viclon 88T” (manufactured by Yushi Kogyo Co., Ltd.), which is a softener mainly composed of a cationic surfactant. The flexible finishing treatment was performed for 15 minutes while heating to 50 ° C. After the flexible finishing treatment, the tubular knitted fabric was dried with a hot air dryer set at 60 ° C. for 1 hour.

(6) Moisture absorption rate 3 g of cylindrical knitted fabric to be evaluated was prepared, and the weight (W ₀ ) when vacuum-dried at 105 ° C. for 10 hours to obtain an absolutely dry state was measured. When this cylindrical knitted fabric is stored for 24 hours in a constant temperature and humidity chamber (LH-20-11M manufactured by Nagano Kagaku Kikai Co., Ltd.) adjusted to a standard condition of a temperature of 20 ° C. and a humidity of 65%. The weight (W _s ) of the sample was measured. Thereafter, the moisture absorption rate in a standard state at a temperature of 20 ° C. and a humidity of 65% was calculated using the following formula.
Moisture absorption rate (% by weight) = ((W _s −W ₀ ) / W ₀ ) × 100.

  The raw materials used in the examples are as shown below.

[Sea component A]
Polyethylene copolymerized with ethylene glycol as the glycol component and 5-sodium sulfoisophthalic acid / isophthalic acid / terephthalic acid as the acid component in a ratio of 12/25/63 mol%, respectively, and vacuum-dried at a drying temperature of 150 ° C. for 12 hours. .

[Sea component B]
Poly L-lactic acid having a weight average molecular weight of 168,000 and a melting point of 170 ° C. is vacuum-dried at a drying temperature of 120 ° C. for 8 hours.

[Island component A]
The thermoplastic composition produced by the method shown below was designated as island component A.

  To 100 parts by weight of cotton linter, 240 parts by weight of acetic acid and 67 parts by weight of propionic acid were added and mixed at 50 ° C. for 30 minutes. After the mixture was cooled to room temperature, 172 parts by weight of acetic anhydride cooled in an ice bath and 168 parts by weight of propionic anhydride were added as an esterifying agent, and 4 parts by weight of sulfuric acid was added as an esterification catalyst, followed by stirring for 150 minutes. An esterification reaction was performed. In the esterification reaction, when it exceeded 40 ° C., it was cooled in a water bath. After the reaction, a mixed solution of 100 parts by weight of acetic acid and 33 parts by weight of water was added as a reaction terminator over 20 minutes to hydrolyze excess anhydride. Thereafter, 333 parts by weight of acetic acid and 100 parts by weight of water were added, and the mixture was heated and stirred at 80 ° C. for 1 hour. After completion of the reaction, an aqueous solution containing 6 parts by weight of sodium carbonate was added, and the precipitated cellulose acetate propionate was filtered off, subsequently washed with water, and dried at 60 ° C. for 4 hours using a hot air dryer. In the obtained cellulose acetate propionate (hereinafter referred to as CAP), the degree of ester substitution of the acetyl group and propionyl group was 1.9 and 0.7, respectively, and the weight average molecular weight (Mw) was 177,000. It was.

  22.0 parts by weight of the CAP, 17.9 parts by weight of polyethylene glycol having an average molecular weight of 600, and 0.1 part by weight of bis (2,6-di-t-butyl-4-methylphenyl) pentaerythritol diphosphite After kneading at a resin temperature of 240 ° C. using a shaft extruder and stretching the thermoplastic composition discharged from the extruder into a strand shape, it was put into a pelletizer to obtain 5 mm square pellets. Thereafter, the pellet was vacuum-dried at 95 ° C. for 12 hours. The obtained pellets had a weight average molecular weight (Mw) of 16,000,000.

[Island component B]
The thermoplastic composition produced by the method shown below was designated as island component B.

  In the manufacturing method of the island component A, 93 parts by weight, average molecular weight, cellulose acetate propionate (product name: CAP482-20, acetyl group ester substitution degree 0.1, propionyl group ester substitution degree 2.5) manufactured by Eastman Chemical Using a known biaxial extruder, 6.9 parts by weight of 600 polyethylene glycol and 0.1 part by weight of bis (2,6-di-t-butyl-4-methylphenyl) pentaerythritol diphosphite were used, and the resin temperature was 220. Kneaded at a temperature of 0 ° C. The weight average molecular weight (Mw) of the obtained pellet was 183,000.

[Island component C]
The thermoplastic composition produced by the method shown below was designated as island component C.

  In manufacturing method of island component A, Eastman Chemical cellulose acetate butyrate (product name CAB171, acetyl group ester substitution degree 1.0, butyryl group ester substitution degree 1.7) 87 parts by weight, average molecular weight 600 polyethylene 12.9 parts by weight of glycol and 0.1 part by weight of bis (2,6-di-t-butyl-4-methylphenyl) pentaerythritol diphosphite were kneaded at a resin temperature of 220 ° C. using a known biaxial extruder. did. The weight average molecular weight (Mw) of the obtained pellet was 173,000.

[Example 1]
The sea component A and the island component A are melted separately at a melter temperature of 270 ° C., introduced into a melt spinning pack having a spinning temperature of 270 ° C., and the sea component A and the island component A are distributed using a known die, and the pore diameter D = It was discharged at a discharge rate of 18 g / min from the final die having 0.4 mm, L / D = 1, and the number of holes = 12. The ratio of sea component / island component was 30/70, and the number of islands per single yarn of sea-island composite fiber was 16. The discharged sea-island composite fibers are cooled by cooling air (25 ° C., wind speed 0.3 m / sec) from a distance 65 mm below the final die surface (cooling start point), and are converged by applying an oil agent. It was taken up at a rate of minutes to obtain a sea-island composite fiber of 90 dtex-12 filament.

  Using this sea-island composite fiber, a 27-gauge tubular knitted fabric was produced with a circular knitting machine (NCR-BL type, manufactured by Eikoh Industries, Ltd.) having a pot diameter of 3.5 inches (89 mm). This tubular knitted fabric is immersed in water at room temperature for 20 minutes for deplasticizing, and then this tubular knitted fabric is heated to 60 ° C. in an alkaline aqueous solution (compound: sodium hydroxide, concentration: 1.5% by weight). The core-sheath composite fiber (with a cellulose fatty acid ester as a core component and cellulose with a degree of ester substitution of less than 0.5 as a sheath component) A tubular knitted fabric having a core-sheath ratio = 40: 60) was obtained. The weight ratio (bath ratio) between the sea-island composite fiber and the aqueous solution was 1:20.

  This tubular knitted fabric was immersed in a treatment tank filled with normal temperature acetone for 2 hours and eluted while stirring. Thereafter, washing with water was performed to obtain a tubular knitted fabric made of cellulosic fibers. Further, the tubular knitted fabric is dipped in a soft finish processing solution containing 3% by weight of “Viclon 88T” (manufactured by Yushi Kogyo Co., Ltd.), which is a softener mainly composed of a cationic surfactant. The flexible finishing treatment was performed for 15 minutes while heating to 50 ° C. After the flexible finishing treatment, the tubular knitted fabric was dried with a hot air dryer set at 60 ° C. for 1 hour.

  Table 1 shows the results of evaluation of the tubular knitted fabric obtained in Example 1. From the result of the evaluation, it was judged that a tubular knitted fabric having a light weight, a hygroscopic property, and a good texture made of a hollow cellulose fiber was obtained. The hollow cellulose fiber constituting the tubular knitted fabric had a tensile strength of 1.1 cN / dtex and an elongation of 15.3%.

[Example 2]
In Example 1, sea component B was used, and the alkaline aqueous solution was heated to 90 ° C. to perform sea removal. The results of the evaluation are as shown in Table 1. Since the alkaline aqueous solution was heated to a higher temperature, the hollow cellulosic fiber constituting the tubular knitted fabric had a tensile strength of 1.0 cN / dtex and an elongation of 11.2%. However, there was a slight decline.

[Examples 3 to 4]
In Example 1, island component B and island component C were used. The results of the evaluation are as shown in Table 1.

[Example 5]
In Example 1, the discharge rate was 9 g / min, the sea-island ratio was 65/35, and the obtained sea-island composite fiber of 45 dtex-12 filament was used. The results of the evaluation are as shown in Table 1. The hollow cellulose fiber constituting the tubular knitted fabric had a tensile strength of 0.8 cN / dtex and an elongation of 13.2%.

[Examples 6 to 7]
In Example 1, the treatment time for deesterification was changed to 60 minutes in Example 6 and 15 minutes in Example 7, and the core-sheath ratio was changed to obtain hollow cellulose fibers having different hollow ratios. The results of the evaluation are as shown in Table 1.

[Comparative Example 1]
In Example 1, the core component was not eluted from the core-sheath composite fiber having cellulose fatty acid ester as the core component and cellulose having an ester substitution degree of less than 0.5 as the sheath component (that is, not hollow). The results of the evaluation are as shown in Table 1.

[Comparative Example 2]
The island component A is melted at a melter temperature of 260 ° C., introduced into a melt spinning pack with a spinning temperature of 270 ° C., and a nozzle whose discharge hole is a C-shaped slit (the diameter of the circumscribed circle of the C-shaped slit is 1 mm, the slit width is It was discharged at a discharge rate of 15 g / min from 0.1 mm and the number of holes of 36). The discharged multifilament is cooled by cooling air (25 ° C., wind speed 0.3 m / sec) from a distance of 150 mm below the base surface (cooling start point), taken up at a speed of 1500 m / min, and hollow of 100 dtex-36 filament Of multifilaments were obtained.

  Using this multifilament, a 27-gauge cylindrical knitted fabric was produced with a circular knitting machine (NCR-BL type, manufactured by Eikoh Industry Co., Ltd.) having a hook diameter of 3.5 inches (89 mm). This tubular knitted fabric is immersed in water at room temperature for 20 minutes for deplasticizing, and then this tubular knitted fabric is heated to 60 ° C. in an alkaline aqueous solution (compound: sodium hydroxide, concentration: 1.5% by weight). For 30 minutes so as to obtain a tubular knitted fabric of hollow cellulosic fibers made of cellulose having an ester substitution degree of less than 0.5. This tubular knitted fabric is dipped in a soft finishing liquid containing 3% by weight of “Viclon 88T” (manufactured by Yushi Kogyo Co., Ltd.), which is a softener mainly composed of a cationic surfactant. The flexible finishing process was performed for 15 minutes while heating to 0 degreeC. After the flexible finishing treatment, the tubular knitted fabric was dried with a hot air dryer set at 60 ° C. for 1 hour. The results of the evaluation are as shown in Table 1.

[Comparative Example 3]
In order to obtain hollow cellulosic fibers having a single yarn diameter smaller than that of Comparative Example 2, island component A was melted at a melter temperature of 260 ° C. and introduced into a melt spinning pack having a spinning temperature of 270 ° C., and the discharge hole was a C-shaped slit. From the base (the diameter of the circumscribed circle of the C-shaped slit is 0.5 mm, the slit width is 0.05 mm, the number of holes is 36), and was discharged at a discharge rate of 4 g / min. The discharged multifilament was cooled by cooling air (25 ° C., wind speed 0.3 m / sec) from a distance of 150 mm below the base surface (cooling start point) and tried to be taken up at a speed of 1500 m / min. The strength was low and it could not be wound.

  The hollow and ultrafine cellulosic fiber of the present invention is excellent in light weight, hygroscopicity, particularly soft texture, and therefore suitable for applications where human skin and fibers are in direct contact, such as garments, interiors, and hygiene. Can be used.

1: Spin block 2: Melt spinning pack 3: Spinneret 4: Cooling start point 5: Cooling device 6: Spinning yarn 7: Feeding device 8: First godet roller 9: Second godet roller 10: Package X : Length of long side of the rectangle circumscribing the cross-sectional shape Y: Length of short side of the quadrilateral circumscribing the cross-sectional shape

Claims (5)

A hollow cellulosic fiber having a single yarn diameter of 10 μm or less and an ester substitution degree of less than 0.5. A method for producing a hollow cellulosic fiber, wherein a cellulose fatty acid ester is eluted from a core-sheath composite fiber having a cellulose fatty acid ester as a core component and cellulose having a degree of ester substitution of less than 0.5 as a sheath component. The core-sheath conjugate fiber is obtained by removing a sea component from a sea-island composite fiber containing a thermoplastic composition comprising a cellulose fatty acid ester as an island component and deesterifying the surface layer portion of the island component. The method for producing a hollow cellulose fiber according to claim 2. The hollow according to claim 3, wherein the sea component is a thermoplastic polymer having solubility in an alkaline aqueous solution, and the cellulose fatty acid ester is selected from cellulose acetate propionate and cellulose acetate butyrate. A method for producing cellulosic fibers. A surface layer portion of a fiber of a thermoplastic composition comprising a cellulose fatty acid ester is deesterified to obtain cellulose having an ester substitution degree of less than 0.5, cellulose fatty acid ester as a core component, and cellulose having an ester substitution degree of less than 0.5. A method for producing a hollow cellulose fiber, comprising obtaining a core-sheath composite fiber as a sheath component and eluting a cellulose fatty acid ester from the core-sheath composite fiber.

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