Cellulose fibers, resin composition, molded article, and method for producing cellulose fibers

Abstract

Provided are a resin composition, a molded article, and a method for producing cellulose fibers. When dyed by a dye, the cellulose fibers have a ratio (L' / L) of the skin layer brightness (L') and the core layer brightness (L) of 0.70 or less. The skin layer brightness (L') and the core layer brightness (L) of the cellulose fiber cross section are obtained by measuring the R, G, and B values of each of the layers, and calculating using the formula of [(maximum value of the R, G, and B values) / 255] * 100 [%].

Description

Cellulose fiber, resin composition, molded article, and method for producing cellulose fiber

[0001] The present invention relates to a cellulose fiber, a resin composition, a molded article, and a method for producing a cellulose fiber.

[0002] Fiber composite materials, which contain high-strength, high-elasticity fibers such as glass fibers to increase the strength and rigidity of plastics, are used in a variety of fields, including automotive parts, sporting goods, building materials, and general merchandise.

[0003] Glass fiber reinforced plastics, which have been used as lightweight, high-strength materials, exhibit excellent properties during use. However, the use of glass fiber as a reinforcing fiber creates residues upon disposal, which poses a significant environmental burden.

[0004] Glass fiber is also used as a base material for printed wiring boards to improve their insulation and rigidity, but the use of glass fiber also creates a problem of environmental impact due to the residue generated when the board is disposed of.

[0005] Therefore, the use of cellulose fibers, which have excellent properties such as high mechanical properties, dimensional stability, low thermal expansion, electrical insulation, and low specific gravity, is being considered as reinforcing fibers for fiber-reinforced resin materials and as base materials for printed wiring boards.

[0006] For example, Patent Document 1 discloses cellulose fibers containing cellulose type II and containing 1 mass % or less of an imidazolium salt.

[0007] International Publication No. 2021 / 241539

[0008] The cellulose fibers described in Patent Document 1 are excellent, but with the recent increase in demand for cellulose fibers, there is a need for further development of cellulose fibers. In particular, there is a demand for cellulose fibers that have both a high elastic modulus and excellent elongation. However, elastic modulus and elongation are contradictory properties, and it is difficult to achieve both. The present invention aims to solve this problem and to provide cellulose fibers that have a good balance between elastic modulus and elongation, as well as resin compositions, molded articles, and methods for producing cellulose fibers that use the cellulose fibers.

[0009] In light of the above-mentioned problems, the present inventors conducted research and found that the above-mentioned problems can be solved by adjusting the cellulose density in the skin layer and core layer of a cellulose fiber. Specifically, the above-mentioned problems were solved by the following means. <1> A cellulose fiber having a ratio (L' / L) of the lightness of the skin layer (L') to the lightness of the core layer (L) when dyed with a dye of 0.70 or less, wherein the lightness of the skin layer (L') and the lightness of the core layer (L) in a cross section of the cellulose fiber are values ​​calculated by measuring the R, G, and B values ​​of each layer and calculating according to [(maximum value among the R, G, and B values) / 255]*100[%]. <2> The cellulose fiber according to <1>, having a crystallinity of 40% or more as measured by X-ray diffraction (XRD). <3> The cellulose fiber according to <1> or <2>, which has been stretched. <4> The cellulose fiber according to <1>, having a crystallinity of 40% or more as measured by X-ray diffraction (XRD) and which has been stretched. <5> The cellulose fiber according to any one of <1> to <4>, having a number average fiber length of 1 mm or more and less than 10 mm. <6> The cellulose fiber according to any one of <1> to <5>, which is a chopped strand. <7> The cellulose fiber according to any one of <1> to <4>, which is a continuous fiber. <8> A resin composition comprising the cellulose fiber according to any one of <1> to <7> and a resin. <9> A molded article formed from the resin composition according to <8>. <10> A method for producing a cellulose fiber, which comprises immersing a cellulose solution containing raw cellulose in a coagulation liquid, spinning the resulting fiber, and then washing the resulting fiber with a washing liquid containing water. <11> The method for producing a cellulose fiber according to <10>, wherein the cellulose fiber has a ratio (L' / L) of the lightness of the skin layer (L') to the lightness of the core layer (L) when dyed with a dye of 0.70 or less, the lightness of the skin layer (L') and the lightness of the core layer (L) in the cross section of the cellulose fiber are values ​​calculated by measuring the R, G, and B values ​​of each layer and calculating according to [(maximum value among the R, G, and B values) / 255]*100[%], the degree of crystallinity measured according to X-ray diffraction (XRD) is 40% or more, and the cellulose fiber is stretched.

[0010] The present invention makes it possible to provide cellulose fibers having a good balance between elastic modulus and elongation, as well as resin compositions, molded articles, and methods for producing cellulose fibers, all of which use the cellulose fibers.

[0011] Figure 1 is a schematic diagram showing an example of an apparatus and process for producing cellulose fibers. Figure 2 is a schematic diagram showing a cross section of cellulose fibers after dyeing and washing. Figure 3 is a photograph of a cross section of the cellulose fibers of Example 6 after dyeing and washing.

[0012] Hereinafter, an embodiment of the present invention (hereinafter simply referred to as "the present embodiment") will be described in detail. Note that the following present embodiment is an example for explaining the present invention, and the present invention is not limited to only this embodiment. Note that in this specification, the word "to" is used to mean that the numerical values ​​written before and after it are included as lower and upper limits. In this specification, various physical property values ​​and characteristic values ​​are those at 23°C unless otherwise specified. When the measurement methods etc. described in the standards shown in this specification vary depending on the fiscal year, they are based on the standards as of January 1, 2022, unless otherwise specified.

[0013] The cellulose fiber of this embodiment is a cellulose fiber in which the ratio (L' / L) of the lightness of the skin layer (L') to the lightness of the core layer (L) when dyed with a dye is 0.70 or less. The lightness of the skin layer (L') and the lightness of the core layer (L) in the cross section of the cellulose fiber are values ​​calculated by measuring the R, G, and B values ​​of each layer and calculating [(maximum value of R, G, and B values) / 255]*100[%]. By making the skin layer denser than the core layer in this way, a cellulose fiber with a well-balanced and excellent elastic modulus and elongation can be obtained. This is advantageous in that it achieves both the elastic modulus and elongation, which are in a trade-off relationship. That is, in this embodiment, the cellulose fiber was dyed, washed, and then observed with a digital microscope. Here, low lightness indicates that the dye is difficult to remove and that the cellulose is densely present. In contrast, high lightness indicates that the dye is easily removed and that the cellulose is relatively not dense. It is presumed that by making the skin layer relatively denser than the core layer, phase separation within the fiber is less likely to occur and the degree of crystallinity of the fiber can be increased, thereby achieving high rigidity.

[0014] The brightness ratio (L' / L) is preferably 0.70 or less, and more preferably 0.68 or less, and may further be 0.66 or less, 0.64 or less, 0.62 or less, or 0.60 or less. The lower limit of the brightness ratio (L' / L) is preferably 0.30 or more, more preferably 0.35 or more, even more preferably 0.40 or more, even more preferably 0.45 or more, and even more preferably 0.49 or more, and may be 0.51 or more, 0.53 or more, 0.54 or more, 0.55 or more, 0.56 or more, 0.57 or more, 0.58 or more, 0.59 or more, or 0.60 or more. By setting the brightness ratio at or above the lower limit, the effects of the present invention tend to be more effectively exhibited. The brightness ratio is measured by the method described in the examples. Note that if the fiber surface has been surface-treated, the surface treatment agent is removed by a known method before measurement. As a method for adjusting the lightness ratio (L' / L), for example, when cellulose fibers are produced by immersing a cellulose solution containing raw cellulose in a coagulation liquid and spinning the resulting fibers, the fibers may be washed for a long period of time (for example, 6 hours or more and 24 hours or less) with a washing liquid containing water after spinning. By washing, the proportion of cellulose in the skin layer can be made dense.

[0015] In the cellulose fiber of this embodiment, the brightness of the skin layer (L') is preferably 20% or more, more preferably 25% or more, even more preferably 30% or more, even more preferably 35% or more, even more preferably 40% or more, and may even be 41% or more, 43% or more. By setting the brightness at or above the lower limit, the breaking elongation tends to be higher. Furthermore, the brightness of the skin layer (L') is preferably 65% ​​or less, more preferably 60% or less, even more preferably 55% or less, even more preferably 50% or less, even more preferably 48% or less, and even more preferably 45% or less. By setting the brightness at or below the upper limit, the rigidity tends to be higher.

[0016] In the cellulose fiber of this embodiment, the brightness of the core layer (L) is preferably more than 60%, more preferably 65% ​​or more, even more preferably 68% or more, even more preferably 70% or more, even more preferably 73% or more, and even more preferably 75% or more. By making the brightness equal to or greater than the lower limit, rigidity tends to be increased. Furthermore, the brightness of the core layer (L) is preferably 95% or less, more preferably 90% or less, even more preferably 85% or less, even more preferably 83% or less, even more preferably 81% or less, and may be 79% or less. By making the brightness equal to or less than the upper limit, breaking elongation tends to be increased.

[0017] In the cellulose fiber of this embodiment, the difference in lightness (L-L') between the core layer (L) and the skin layer (L') is preferably 15% or more, more preferably 20% or more, even more preferably 25% or more, and may even be 30% or more, and is preferably 45% or less, more preferably 40% or less, and even more preferably 35% or less.

[0018] The cellulose fibers of this embodiment preferably have a high degree of crystallinity. A high degree of crystallinity results in cellulose fibers with a higher modulus of elasticity. The crystallinity of the cellulose fibers of this embodiment, as measured by X-ray diffraction (XRD), is preferably 40% or more, more preferably 50% or more, and even more preferably 52% or more, and may be 55% or more, 60% or more, 65% or more, 70% or more, 75% or more, or 80% or more. Furthermore, the upper limit of the crystallinity of the cellulose fibers is practically 90% or less, and may further be 88% or less, 85% or less, 84% or less, 80% or less, 75% or less, 70% or less, 65% or less, 60% or less, or 55% or less.

[0019] The crystalline orientation degree of the cellulose fibers of this embodiment is preferably 85% or more, more preferably 87% or more, even more preferably 90% or more, even more preferably 91% or more, and even more preferably 92% or more. The upper limit of the crystalline orientation degree may be 100%, but practical values ​​are 99% or less, 98% or less, 97% or less, 96% or less, and 95% or less.

[0020] The cellulose fibers of this embodiment are preferably stretched. Stretching aligns the orientation of cellulose in the cellulose fibers, increasing the proportion of hydrogen bonds and making the skin layer denser. The stretching ratio (draft ratio) is preferably 10 times or more, and may be 20 times or more, 30 times or more, 35 times or more, or 50 times or more depending on the application. Setting the stretching ratio at or above the lower limit tends to increase the orientation of cellulose chains, thereby increasing the elastic modulus of the fiber. Furthermore, the stretching ratio is preferably 100 times or less, more preferably 80 times or less, even more preferably 70 times or less, even more preferably 60 times or less, and even more preferably 50 times or less. Depending on the application, the stretching ratio may be 40 times or less, 30 times or less, 25 times or less, 20 times or less, or 15 times or less. Setting the stretching ratio at or below the upper limit tends to effectively achieve both stable continuous production and maintenance of physical properties.

[0021] The cellulose fiber of this embodiment is a cellulose fiber in which the ratio (L' / L) of the lightness of the skin layer (L') to the lightness of the core layer (L) when dyed with a dye is 0.70 or less, and the lightness of the skin layer (L') and the lightness of the core layer (L) in the cross section of the cellulose fiber are values ​​calculated by measuring the R, G, and B values ​​of each layer and calculating according to [(maximum value of R, G, and B values) / 255]*100[%], and it is particularly preferable that the crystallinity measured according to X-ray diffraction (XRD) is 40% or more and that the fiber is stretched.

[0022] The cellulose fiber of this embodiment contains cellulose. The cellulose is not particularly limited, and may be natural cellulose materials such as wood pulp, cotton, cotton linter, hemp, bamboo, and abaca, regenerated cellulose fibers such as rayon, cupra, and lyocell, and regenerated cellulose such as paper and clothing made from these. In one example of this embodiment, the cellulose is regenerated cellulose.

[0023] The cellulose used in this embodiment preferably has a degree of polymerization of 200 or more, more preferably 400 or more, even more preferably 600 or more, still more preferably 800 or more, and even more preferably 1000 or more. By making the degree of polymerization equal to or greater than the lower limit, the effect of increasing the strength of the fiber tends to be further improved. Furthermore, the cellulose used in this embodiment preferably has a degree of polymerization of 5000 or less, more preferably 3000 or less, even more preferably 2000 or less, still more preferably 1500 or less, and even more preferably 1200 or less. By making the degree of polymerization equal to or less than the upper limit, the effect of improving physical properties due to the high concentration of cellulose tends to be further improved. The degree of polymerization is measured by size exclusion chromatography.

[0024] The cellulose content of the cellulose fiber is usually 90% by mass or more, preferably 95% by mass or more, and more preferably 97% by mass or more. Only one type of cellulose may be used, or two or more types may be used. When two or more types are used, it is preferable that the total amount is within the above range.

[0025] The cellulose fiber of the present embodiment may contain components other than cellulose. Specific examples include additives such as antioxidants, and, as will be described in detail later, the ionic liquid used in spinning cellulose. An example of the ionic liquid is an imidazolium salt.

[0026] In the cellulose fibers of this embodiment, the content of the imidazolium salt is preferably 1% by mass or less, more preferably 0.8% by mass or less, even more preferably 0.7% by mass or less, and even more preferably 0.65% by mass or less. By setting the content below the upper limit, the heat resistance of the cellulose fibers can be improved. In the cellulose fibers of this embodiment, the content of the imidazolium salt is also preferably 0.01% by mass or more, more preferably 0.02% by mass or more, even more preferably 0.05% by mass or more, even more preferably 0.08% by mass or more, and even more preferably 0.1% by mass or more. In this way, by including a trace amount of imidazolium salt in the cellulose fibers, the smoothness of the surface is effectively maintained, and handleability tends to be improved. The cellulose fibers of this embodiment may include only one type of imidazolium salt, or may include two or more types. When two or more types are included, the total amount is preferably within the above range.

[0027] The fiber diameter of the cellulose fiber of this embodiment is preferably 5 μm or more, more preferably 7 μm or more, even more preferably 8 μm or more, even more preferably 9 μm or more, and even more preferably 10 μm or more. By making the fiber diameter equal to or greater than the lower limit, the effect of maintaining spinnability tends to be further improved. Furthermore, the fineness of the cellulose fiber of this embodiment is preferably 30 μm or less, more preferably 17 μm or less, even more preferably 16 μm or less, even more preferably 15 μm or less, and even more preferably 14 μm or less. By making the fineness equal to or less than the upper limit, the reinforcing effect when made into a fiber-reinforced resin tends to be further improved. The fineness of the cellulose fiber is measured according to the description of the examples described below.

[0028] The form of the cellulose fiber of this embodiment is not particularly limited. A first embodiment of the cellulose fiber of this embodiment is a continuous fiber. Continuous fiber refers to a fiber having a number average fiber length of more than 10 mm, preferably 10 cm or more, and more preferably 1 m or more. There is no particular upper limit, but it is 100,000 m or less. Continuous fiber is preferably stored and transported as a wound body wound around a core material (including a bobbin, etc.). Continuous fiber may be a spun yarn obtained by spinning short fibers or long fibers, but is preferably not a spun yarn. Continuous fiber is preferably a fibrous fiber obtained by spinning.

[0029] A second embodiment of the cellulose fiber of this embodiment is a long fiber. Long fibers are, for example, chopped strands obtained by cutting continuous fibers to a certain length. The fiber length of the long fiber is preferably 1 mm or more, more preferably 2 mm or more, and preferably less than 10 mm, more preferably 9 mm or less, and even more preferably 8 mm or less. The number average fiber length is the average value of the lengths of the individual fibers, and in the case of chopped strands, this usually corresponds to the cut length.

[0030] The tensile modulus of the cellulose fiber of this embodiment is preferably 20 GPa or more, more preferably 25 GPa or more, even more preferably 30 GPa or more, even more preferably 32 GPa or more, and even more preferably 35 GPa or more. There is no particular upper limit to the tensile modulus of the cellulose fiber of this embodiment, but 50 GPa or less is practical. The breaking elongation of the cellulose fiber of this embodiment is preferably 3% or more, more preferably 4% or more, even more preferably 5% or more, even more preferably 6% or more, even more preferably 6.5% or more, and even more preferably 10% or more. There is no particular upper limit to the breaking elongation of the cellulose fiber of this embodiment, but 20% or less is practical, and even 15% or less will fully satisfy the required performance. The tensile modulus and breaking elongation are measured as described in the examples below.

[0031] <Method for producing cellulose fibers> Next, a method for producing the cellulose fibers of this embodiment will be described. The cellulose fibers of this embodiment can be produced by known methods, but are preferably produced by a method including immersing a cellulose solution containing raw cellulose and a solvent in a coagulation liquid, spinning the fibers, and then washing with a washing liquid containing water. Washing the cellulose fibers with a washing liquid containing water after spinning densifies the cellulose in the skin layer, making it easier to achieve the desired brightness ratio. The brightness ratio can also be adjusted by adjusting the production scale. In this embodiment, the cellulose fibers are more preferably produced by a method including immersing a cellulose solution containing raw cellulose and an imidazolium salt in a coagulation liquid, spinning the fibers, and then washing with a washing liquid containing water.

[0032] The cellulose content (cellulose concentration) in the cellulose solution is preferably 1% by mass or more, more preferably 3% by mass or more, even more preferably 5% by mass or more, and even more preferably 7% by mass or more. By setting the cellulose content at or above the lower limit, the cellulose solution has a viscosity that facilitates molecular orientation during spinning, and fibers with a higher elastic modulus tend to be obtained. Furthermore, the cellulose content in the cellulose solution is preferably 20% by mass or less, more preferably 17% by mass or less, even more preferably 14% by mass or less, and may be 11% by mass or less depending on the application, etc. By setting the cellulose content at or below the upper limit, the viscosity of the cellulose solution is not too high, making it possible to further reduce the burden on the equipment, etc., during spinning. Other details of the raw cellulose are the same as those described in the section on cellulose fibers above.

[0033] In this embodiment, the solvent for the cellulose solution can be an ionic liquid, a tertiary amine oxide, dimethyl sulfoxide, or dimethylacetamide; an ionic liquid or a tertiary amine oxide is more preferred, and an ionic liquid is even more preferred. The ionic liquid is preferably an imidazolium salt, and is preferably a salt composed of a cation having an imidazole ring and an anion. Examples of the anion include chloride anion, bromide anion, acetate anion, phosphate anion, propionate anion, and formate anion; chloride anion and bromide anion are preferred, and chloride anion is more preferred. The tertiary amine oxide is preferably N-methylmorpholine-N-oxide. The imidazolium salt in this embodiment preferably has a molecular weight of 100 to 500. Specific examples of the imidazolium salt in this embodiment include 1-alkyl-3-methylimidazolium chloride, 1-ethyl-3-methylimidazolium acetate, 1-ethyl-3-methylimidazolium diethyl phosphate, 1-butyl-3-methylimidazolium acetate, 1-butyl-3-methylimidazolium propionate, 1-butyl-3-methylimidazolium formate, and 1-butyl-3-methylimidazolium diphosphate. Examples of the alkyl group include methyl phosphate, 1,3-dimethylimidazolium acetate, 1-ethyl-3-methylimidazolium propionate, 1-ethyl-3-methylimidazolium formate, 1-ethyl-3-methylimidazolium bromide, 1-ethyl-3-methylimidazolium dimethyl phosphate, and 1-allyl-3-methylimidazolium chloride, and preferably includes 1-alkyl-3-methylimidazolium chloride. The alkyl group in the 1-alkyl-3-methylimidazolium chloride preferably has 2 to 6 carbon atoms. The alkyl group in the 1-alkyl-3-methylimidazolium chloride is preferably a butyl group. The butyl group may be a t-butyl group, an n-butyl group, a sec-butyl group, or an isobutyl group, but an n-butyl group is preferred. More preferably, the 1-alkyl-3-methylimidazolium chloride is 1-butyl-3-methylimidazolium chloride.

[0034] In this embodiment, the method for obtaining an imidazolium salt such as 1-alkyl-3-methylimidazolium chloride is not particularly limited, and known techniques can be used. For example, 1-alkyl-3-methylimidazolium chloride can be obtained from the reaction product of 1-methylimidazole and RCl (R is an alkyl group having 2 to 6 carbon atoms). In this embodiment, when using an imidazolium salt such as 1-alkyl-3-methylimidazolium chloride, it is preferable to remove the acid component with a base.

[0035] The cellulose solution may contain an aprotic polar solvent, such as dimethyl sulfoxide, pyridine, N,N-dimethylacetamide, N,N-dimethylformamide, or N-methyl-2-pyrrolidone.

[0036] In the cellulose fiber production method of this embodiment, in a cellulose solution (hereinafter sometimes simply referred to as the "cellulose solution") containing raw cellulose and a solvent (preferably an imidazolium salt), the content of the solvent (preferably an imidazolium salt and / or a tertiary amine oxide, more preferably an imidazolium salt) is preferably 80% by mass or more, more preferably 83% by mass or more, and even more preferably 86% by mass or more. Furthermore, the content of the solvent (preferably an imidazolium salt and / or a tertiary amine oxide, more preferably an imidazolium salt) in the cellulose solution is preferably 99% by mass or less, more preferably 97% by mass or less, even more preferably 95% by mass or less, and even more preferably 93% by mass or less. Furthermore, in the cellulose solution, the imidazolium salt and / or tertiary amine oxide (preferably an imidazolium salt) preferably accounts for 90% by mass or more of the solvent contained in the cellulose solution, more preferably 95% by mass or more, even more preferably 98% by mass or more, and even more preferably 99% by mass or more. The imidazolium salt may be used alone or in combination of two or more. When two or more types are used, the total amount is preferably within the above range.

[0037] The cellulose solution may contain a stabilizer in addition to the above, examples of which include pyrocatechin, pyrogallol, gallic acid, methyl gallate, ethyl gallate, propyl gallate, isopropyl gallate, ellagic acid, oxalic acid, phosphoric acid, sodium hexametaphosphate, tannin, and tannic acid.

[0038] The method for producing cellulose fibers according to this embodiment will be described below with reference to Fig. 1. It goes without saying that the production method according to this embodiment is not limited to this.

[0039] FIG. 1 is a schematic diagram illustrating an example of an apparatus for producing cellulose fibers according to this embodiment, with 1 representing a cellulose solution, 2 a nozzle, 3 cellulose fibers, 4 a coagulation liquid, and 5 a winder. Furthermore, in this embodiment, the apparatus preferably includes, or a separate apparatus from the apparatus, a washing bath containing a washing liquid for washing the produced cellulose fibers. In this embodiment, cellulose solution 1 is discharged from nozzle 2. When discharged from the nozzle, cellulose solution 1 tends to have high viscosity and poor fluidity. Therefore, to improve fluidity, it is preferable to heat and discharge the solution. The temperature of the cellulose solution during discharge is preferably 70°C or higher, more preferably 80°C or higher. By maintaining the temperature at 70°C or higher, the fluidity of the cellulose solution tends to be further improved. Furthermore, the upper limit of the temperature of the cellulose solution during discharge is preferably 130°C or lower, more preferably 120°C or lower. By maintaining the temperature at 130°C or lower, cellulose decomposition can be more effectively suppressed. Furthermore, the nozzle diameter during discharge can be, for example, 0.1 to 0.5 mm. The fibrous cellulose solution discharged from the nozzle 2 is immersed in the coagulation liquid 4. By immersing the fibrous cellulose solution in the coagulation liquid, it is spun. Spinning may be a continuous method in which the concentration of the solvent is maintained at a certain level or less while adding the coagulation liquid, or a batch method in which the coagulation liquid is replaced when the concentration of the solvent in the coagulation liquid reaches a certain level or more. The solvent dissolved in the coagulation liquid may be recovered and recycled again for the production of cellulose fibers. The air gap between the fibrous cellulose discharged from the nozzle 2 and the coagulation liquid 4 is preferably 3 to 30 cm.

[0040] The discharge rate from the nozzle during spinning is preferably 0.015 cc / min or more per hole, more preferably 0.025 cc / min or more, even more preferably 0.03 cc / min or more, even more preferably 0.05 cc / min or more, and even more preferably 0.08 cc / min or more. By setting the discharge rate at or above the lower limit, productivity tends to be further improved. Furthermore, the discharge rate is preferably 1 cc / min or less per nozzle, more preferably 0.8 cc / min or less, even more preferably 0.5 cc / min or less, even more preferably 0.2 cc / min or less, and even more preferably 0.12 cc / min or less. By setting the discharge rate at or below the upper limit, solvent removal in the coagulation bath can be effectively performed. The number of nozzles may be 5,000 or more, or 10,000 or more, depending on the production scale. The spinning speed (winding speed) is preferably 50 m / min or more, more preferably 60 m / min or more, even more preferably 70 m / min or more, even more preferably 80 m / min or more, and even more preferably 90 m / min or more. By setting it to the lower limit or higher, productivity tends to be further improved. Furthermore, the spinning speed is preferably 1000 m / min or less, more preferably 500 m / min or less, even more preferably 300 m / min or less, even more preferably 200 m / min or less, and even more preferably 100 m / min or less. By setting it to the upper limit or less, solvent removal in the coagulation bath can be effectively carried out.

[0041] The coagulation liquid can be water at a temperature ranging from 0°C to 100°C, or a lower alcohol, polar solvent, nonpolar solvent, or the like at a temperature ranging from -40°C to 100°C. Considering economic efficiency and the working environment, a solvent containing water is preferred. The solvent is released from the spun cellulose solution during immersion in the coagulation liquid or by subsequent washing, resulting in cellulose fibers. In this embodiment, the total immersion time in the coagulation liquid depends on conditions such as the volume and temperature of the liquid, but is preferably 0.1 seconds or more, and more preferably 6 seconds or more. By setting the immersion time at or above the lower limit, the amount of solvent (e.g., imidazolium salt) in the resulting cellulose fibers can be effectively reduced. The upper limit of the immersion time in the coagulation liquid is preferably 48 hours or less, more preferably 24 hours or less, and may even be 10 hours or less, 3 hours or less, or 2 hours or less. By setting the content to the upper limit or less, a small amount of solvent (especially the imidazolium salt) remains in the cellulose fibers, making it possible to maintain the fiber surface in a smooth state, which tends to further improve handleability.

[0042] In this embodiment, the spun cellulose fibers are preferably washed with a cleaning solution containing water. Washing with a cleaning solution can make the skin layer of the cellulose fibers denser. Furthermore, the crystallinity of the cellulose fibers can be increased. Examples of water-containing cleaning solutions include a solution consisting of water alone and a cleaning solution containing water and at least one polar solvent. In this embodiment, the water content of the cleaning solution is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 98% by mass or more. In this embodiment, for example, after the spun cellulose fibers are wound, the wound body is immersed in the cleaning solution. The time for washing with the cleaning solution is preferably 3 hours or more, more preferably 5 hours or more, even more preferably 10 hours or more, even more preferably 15 hours or more, and even more preferably 20 hours or more. By setting the time to be equal to or greater than the lower limit, the remaining solvent can be more appropriately removed and the skin layer can be made denser. Furthermore, the time for washing with the cleaning solution is preferably 10 days or less, more preferably 5 days or less, and even more preferably 3 days or less. By setting the temperature to the upper limit or less, productivity tends to be further improved. The temperature of the washing liquid is preferably 5°C or higher, more preferably 9°C or higher, even more preferably 11°C or higher, still more preferably 13°C or higher, and even more preferably 15°C or higher. By setting the temperature to the lower limit or higher, the solvent removal rate tends to be further improved. The temperature of the washing liquid is also preferably 100°C or lower, more preferably 90°C or lower, even more preferably 80°C or lower, still more preferably 70°C or lower, and even more preferably 60°C or lower. By setting the temperature to the upper limit or less, destruction of the cellulose structure tends to be effectively suppressed.

[0043] Furthermore, the temperature difference between the coagulation liquid and the washing liquid (absolute value, preferably the temperature of the washing liquid minus the temperature of the coagulation liquid) is preferably 0°C or higher, more preferably 1°C or higher, even more preferably 2°C or higher, even more preferably 3°C or higher, and even more preferably 4°C or higher. By making the temperature equal to or higher than the lower limit, impurities and the like in the cellulose fibers tend to be washed more effectively. Furthermore, the temperature difference between the coagulation liquid and the washing liquid (absolute value, preferably the temperature of the washing liquid minus the temperature of the coagulation liquid) is preferably 90°C or lower, more preferably 80°C or lower, even more preferably 70°C or lower, even more preferably 60°C or lower, and even more preferably 50°C or lower. By making the temperature equal to or lower than the upper limit, destruction of the cellulose structure tends to be effectively suppressed.

[0044] The cellulose fibers after immersion in the coagulation liquid are wound up by a winder 5. The resulting cellulose fibers can be stretched by adjusting the winding speed of the winder, thereby adjusting the fiber diameter of the cellulose fibers. Alternatively, a stretching roll or the like may be provided in addition to the winder. When the cellulose fibers are stretched by adjusting the winding speed relative to the discharge speed from the nozzle, the ratio (winding speed / discharge speed) is preferably 10 to 100 times, more preferably 30 to 70 times.

[0045] <Applications of Cellulose Fibers> The cellulose fibers of this embodiment can be used in a wide variety of known applications, but are preferably used as a resin composition containing a resin and cellulose fibers. Examples of the resin composition include a resin impregnated with the cellulose fibers of this embodiment (e.g., prepreg) and a resin blended with the cellulose fibers of this embodiment (e.g., resin pellets). When the resin composition is in the form of pellets, they are preferably long-fiber pellets. Long-fiber pellets refer to pellets whose pellet length is equivalent to the average fiber length of the cellulose fibers contained therein. "Equal" means that the average fiber length of the cellulose fibers contained therein is 95 to 105% of the pellet length, preferably 99 to 101%. Such long-fiber pellets can be produced, for example, by impregnating the cellulose fibers with a molten resin component while opening a roving of cellulose fiber bundles, then pulling them into strands and cutting them to the desired pellet length (e.g., 1 mm or more, 30 mm or less, or even less than 10 mm). On the other hand, when the resin composition contains a thermosetting resin, the resin composition includes not only a fiber-reinforced resin material in which the thermosetting resin is fully cured, but also a prepreg in which the thermosetting resin is semi-cured. The resin contained in the resin composition may be a thermoplastic resin or a thermosetting resin. Examples of thermoplastic resins include polyamide resin (nylon), polyacetal resin, polycarbonate resin, polyvinyl chloride resin, ABS resin, polysulfone resin, polyethylene resin, polyolefin resin, polystyrene resin, (meth)acrylic resin, and fluororesin. Examples of thermosetting resins include unsaturated polyester resin, epoxy resin, melamine resin, and phenolic resin. Examples of polyolefin resins include polypropylene resin. Furthermore, when a polyolefin resin is used, it is preferable that a portion of the polyolefin resin is an acid-modified polypropylene resin. As the acid-modified polypropylene resin, an acid-modified polyolefin resin modified with maleic anhydride and / or maleic acid (preferably maleic anhydride-modified polypropylene resin) is preferable.Furthermore, the resin composition may contain additives such as low shrinkage agents, flame retardants, flame retardant assistants, plasticizers, antioxidants, UV absorbers, colorants, pigments, and fillers, as necessary. The molded article of this embodiment is formed from the resin composition of this embodiment. For details of the resin composition using cellulose fibers of this embodiment, please refer to the descriptions in WO 2019 / 066069 and WO 2019 / 066070, the contents of which are incorporated herein by reference.

[0046] The molded article of this embodiment is formed from the resin composition of this embodiment. The resin composition of this embodiment can be used, for example, as a transparent resin material, a three-dimensional modeling material, a cushioning material, a repair material, an adhesive, a pressure-sensitive adhesive, a sealing material, a heat insulating material, a sound-absorbing material, an artificial leather material, a paint, an electronic material, a packaging material, an automotive part, or a fiber composite material. The shape of the molded article may be a sheet or a film such as a coating film. The molded article can be produced by a known method, for example, the method described in paragraphs 0092 to 0114 of JP 2019-119983 A, the contents of which are incorporated herein by reference.

[0047] The present invention will be explained in more detail below with reference to examples. The materials, amounts used, ratios, treatment contents, treatment procedures, etc. shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention is not limited to the specific examples shown below. If the measuring instruments used in the examples are difficult to obtain due to discontinuation or the like, measurements can be made using other instruments with equivalent performance.

[0048] Example 1 <Spinning> Kyoward 500SN (Kyowa Chemical Industry Co., Ltd.) was added at 1% by mass to a 60% by mass aqueous solution of 1-butyl-3-methylimidazolium chloride (BmimCl, manufactured by Jinkai Processing Products, product number: 20042301 (ionic liquid)), and the mixture was stirred at room temperature for 5 hours. After filtration through No. 4A hard filter paper, the filtrate was dehydrated to obtain BmimCl for spinning. To this BmimCl, raw material pulp (manufactured by Georgia-Pacific Cellulose, product number: V-81) was added and the mixture was stirred at 100°C to prepare a cellulose solution with a concentration of 9% by mass. Cellulose fibers were produced from the cellulose solution prepared above using an apparatus such as that shown in FIG. 1. That is, cellulose solution 1 was loaded into a syringe and discharged from nozzle 2, which had a nozzle diameter of 0.26 mm, at a rate of 0.1 cc / min. The cellulose solution was then wound at a winding speed to achieve the draft ratio shown in Table 1, and the cellulose fiber was passed through (immersed in) water (coagulation liquid 4) at 15°C for 0.7 seconds to coagulate the cellulose fiber, which was then wound onto a bobbin using winder 5. The bobbin (wound body) was then immersed in water (cleaning liquid) at 20°C for 1 day to wash it. The cellulose fiber was stretched by adjusting the winding speed to achieve the above draft ratio. The obtained cellulose fiber was measured and evaluated as follows.

[0049] <Dyeing> The cellulose fiber wound on a bobbin was unwound and embedded in Refine Tech epoxy resin (Epomount base: Epomount hardener II = 10:2 (mass ratio)). After complete hardening, an ultrathin section of 1 μm thickness was prepared using an ultramicrotome (Leica, EM UC7) and a shim knife XAC type (SYMX3055) and bonded to a glass preparation with a silicone adhesive. 0.15 mL of a dyeing solution containing 1% by mass of BrilliantBlue R (Tokyo Chemical Industry, product number: C0700) and 10% by mass of sodium chloride (Fujifilm Wako Pure Chemical, product number: 191-01665) was dropped onto this ultrathin section and heated on a hot plate at 70 ° C for 3 minutes. After heating, the section was rinsed with pure water, ethanol, glycerin, and ethanol, in the above order, for 1 minute each. Figure 2 is a schematic diagram of a cross section of a dyed fiber, with 10 representing the fiber cross section, 11 representing the skin layer, and 12 representing the core layer. The skin layer 11 retained its color after dyeing and washing, whereas the core layer 12 lost its color after washing.

[0050] <Lightness Measurement> The cross-sections of ultrathin sections of the dyed and washed cellulose fibers were observed using a digital microscope. Observation was performed using transmitted light of 225, mixed illumination light of 100 (coaxial 100%), and a magnification of 1500x. Images were obtained using manual depth composition. Sampling of the cellulose fibers was performed on five cross-sections, excluding 5% of the lengthwise area from the start and end of the fiber wound around the bobbin, and the average value was calculated. Fibers whose cross-sections could be clearly observed using the digital microscope were used as the measurement targets. During microscopic observation, the RGB values ​​of the core layer were measured by selecting the central portion of the fiber cross-section, and the RGB values ​​of the skin layer were measured by selecting the approximate center of the skin layer portion (i.e., the dyed portion) of the fiber. From the measured values, the lightness of the skin layer (L') and core layer (L) was calculated using the following formula: Lightness = [(maximum value of R, G, B values) / 255] * 100 [%] The lightness ratio (L' / L) of the skin layer (L') to the core layer (L) was calculated from the lightness of the skin layer and the core layer. The digital microscope used was a VHX-7000 manufactured by Keyence. Figure 3 shows a cross-sectional photograph of Example 6. Figure 3(a) is a partially enlarged view of the image observed with the digital microscope, and the object to be measured was one in which a clean shape, as shown in Figure 3(b), could be observed.

[0051] <Crystallization degree of cellulose fiber> Cellulose fiber (undyed) wound on a bobbin was unwound and finely cut with scissors to a fiber length of approximately 1 mm. The fiber was dispersed in water and then subjected to suction filtration to prepare a fiber plate. The crystallinity of the prepared fiber plate was measured using X-ray diffraction (XRD). The crystallinity was calculated using the Segal method from the (002) plane peak intensity value and the amorphous peak intensity value. A MiniFlex 600 manufactured by Rigaku was used for the measurement. The crystallinity of the cellulose fiber was expressed in %.

[0052] <Fiber diameter of cellulose fiber> Cellulose fiber was unwound, a 0.1 g weight was attached, and the fineness was measured at a measurement length of 2.5 cm using an auto-blow type fineness measuring instrument (DENICON DC-21 manufactured by Search). The fineness (dtex) value was converted to fiber diameter (μm) using a fiber specific gravity of 1.52, and this value was used in the tensile test.

[0053] <Tensile Test> The cellulose fibers whose fineness had been measured were attached to a tensile tester (Shimadzu EZTest, a small tabletop tester) via clips with the weight still attached, and a tensile test was carried out in accordance with JIS L 1013 to measure the tensile modulus (unit: GPa) and elongation at break (unit: %). A total of 15 measurements were carried out for each sample, and the average value excluding the maximum and minimum values ​​was used. The fiber samples used were vacuum dried at 60°C for 24 hours, and the tensile test was carried out in a constant temperature and humidity chamber at 22°C and 50% RH. Evaluation was carried out according to the following classification. <<Tensile Modulus>> S: 30 GPa or more A: 20 GPa or more but less than 30 GPa B: Less than 20 GPa (outside of practical use level) <<Elongation at break>> S: 10% or more A: 5% or more but less than 10 B: Less than 5% (outside of practical use level)

[0054] Example 2 The same procedure as in Example 1 was carried out except that the ionic liquid was changed to 1-ethyl-3-methylimidazolium diethyl phosphate (EmimDEP). Example 3 A fiber was produced in the same manner as in Example 1, except that the draft ratio was changed to 13 and winding was carried out.

[0055] Example 4 Fibers were produced in the same manner as in Example 2, except that the draft ratio was changed to 13 and winding was performed.

[0056] Example 5 Fibers were produced in the same manner as in Example 1, except that the concentration of the cellulose solution was changed to 10% by mass and the draft ratio was changed to 51 before winding.

[0057] Example 6 Fibers were produced in the same manner as in Example 1, except that the concentration of the cellulose solution was changed to 10% by mass and the draft ratio was changed to 40. The degree of crystalline orientation was measured according to the following method. <<Degree of crystalline orientation>> The fiber was fixed to a sample holder with the fiber axis direction (fiber length direction) as the reference axis, and an XRD profile was measured. The azimuth angle distribution intensity of the diffraction angles of the strong diffraction intensities obtained in the XRD profile was measured by transmission measurement. In the azimuth angle distribution curve, orientation peaks were observed near 0° and 180°, and the average half-width of these peaks, Φ １／２ The degree of crystal orientation was calculated using the following formula: Crystal orientation (%) = [1 - (Φ １／２ / 180°)]×100 Measurement was performed using EMPYREAN manufactured by Spectris.

[0058] Example 7 Fibers were produced in the same manner as in Example 1, except that the concentration of the cellulose solution was changed to 8.7% by mass and the draft ratio was changed to 51.

[0059] Example 8 Fibers were produced in the same manner as in Example 1, except that the concentration of the cellulose solution was changed to 8.7% by mass and the draft ratio was changed to 40. The degree of crystal orientation was measured in the same manner as in Example 6.

[0060] Example 9 Fibers were produced in the same manner as in Example 1, except that the ionic liquid was changed to (NMMO monohydrate, N-methylmorpholine-N-oxide hydrate, manufactured by Angene, product number: ANG-05911), the concentration of the cellulose solution was changed to 12% by mass, the draft ratio was changed to 40, and 0.5% by mass of propyl gallate (manufactured by Fujifilm Wako Pure Chemical Industries, Wako First Grade) was added to the raw pulp when the raw pulp was added to the solvent. The degree of crystal orientation was measured in the same manner as in Example 6. The step of obtaining BmimCl for spinning in Example 1 was omitted.

[0061] Comparative Example 1 The same procedure as in Example 1 was carried out except that the amount of Kyoward 500SN (manufactured by Kyowa Chemical Industry Co., Ltd.) added was changed to 3 mass % and the bobbin after winding was not immersed in water (cleaning liquid).

[0062] Comparative Example 2 Several fibers were taken from a commercially available rayon fiber bundle (manufactured by Cordenka, product number: RT700, number average fiber diameter: 12 μm). After immersion in acetone for 6 hours to remove the surface treatment agent, the fibers were dyed in the same manner as in Example 1, and the brightness was calculated. The tensile modulus and elongation at break were also measured in the same manner as in Example 1. The degree of crystal orientation was also measured in the same manner as in Example 6.

[0063]

[0064] In Tables 1 and 2, "polymerization of cellulose" refers to the degree of polymerization of the cellulose raw material. As is clear from the results in Tables 1 and 2, the cellulose fibers of the present invention were able to improve the physical properties of modulus and elongation, which are in a trade-off relationship, in a well-balanced manner. In contrast, the fibers of the comparative examples were rated "B" for modulus, and were unable to improve the modulus and elongation in a well-balanced manner.

[0065] 1 Cellulose solution 2 Nozzle 3 Cellulose fiber 4 Coagulation liquid 5 Winder

Claims

1. A cellulose fiber having a ratio (L' / L) of the lightness of the skin layer (L') to the lightness of the core layer (L) when dyed with a dye of 0.70 or less, wherein the lightness of the skin layer (L') and the lightness of the core layer (L) in the cross-section of the cellulose fiber are values ​​calculated according to the formula [(maximum value among R, G, and B values) / 255] * 100 [%] by measuring the R, G, and B values ​​of each layer.

2. The cellulose fiber according to claim 1, wherein the degree of crystallinity measured by X-ray diffraction (XRD) is 40% or more.

3. A cellulose fiber according to claim 1 or 2, which is stretched.

4. The cellulose fiber according to claim 1, wherein the degree of crystallinity measured according to X-ray diffraction (XRD) is 40% or more, and the fiber is stretched.

5. A cellulose fiber according to claim 1 or 2, wherein the number average fiber length is 1 mm or more and less than 10 mm.

6. A cellulose fiber according to claim 1 or 2, which is a chopped strand.

7. A cellulose fiber according to claim 1 or 2, which is a continuous fiber.

8. The crystallinity measured according to X-ray diffraction (XRD) is 40% or more, and the material is stretched. The average fiber length is 1 mm or more and less than 10 mm. The cellulose fiber according to claim 1, which is a chopped strand.

9. A resin composition comprising the cellulose fibers and resin according to claim 1, 2, or 8.

10. A molded article formed from the resin composition described in claim 9.

11. A method for producing cellulose fibers according to claim 1, 2, or 8, comprising immersing a cellulose solution containing raw cellulose in a coagulation solution, spinning the fibers, and then washing them with a washing solution containing water.

12. A method for producing cellulose fibers, comprising immersing a cellulose solution containing raw cellulose in a coagulation solution, spinning the fibers, and then washing them with a washing solution containing water, A method for producing cellulose fibers, wherein the cellulose fiber has a ratio (L' / L) of the lightness of the skin layer (L') to the lightness of the core layer (L) when dyed with a dye of 0.70 or less, and the lightness of the skin layer (L') and the lightness of the core layer (L) in the cross-section of the cellulose fiber are values ​​calculated according to the formula [(maximum value among R, G, and B values) / 255] * 100 [%] by measuring the R, G, and B values ​​of each layer, and the crystallinity measured according to the X-ray diffraction method (XRD) is 40% or more, and the fiber is stretched.