Method for dissolving cellulose fibers and application thereof

Abstract

The invention belongs to the technical field of cellulose material processing, and particularly relates to a method for dissolving cellulose fibers and application thereof. The method comprises the following steps: S1, mixing cellulosic fibers, paraformaldehyde (PF) and a polar aprotic solvent to obtain a to-be-treated solution; s2, stirring the to-be-treated liquid at room temperature until the paraformaldehyde is dissolved; s3, carrying out three-stage heating treatment on the liquid obtained in S2, and S4, stopping heating, and continuously stirring while cooling the liquid to room temperature until the cellulose fibers are completely dissolved, so as to obtain a cellulose solution. The method is simple in process, low in energy consumption and high in dissolving efficiency, and can be widely applied to natural fibers including cotton, hemp and the like and novel modified cellulose fibers including lyocell and the like.

Description

Technical Field

[0001] The invention belongs to the technical field of cellulose material processing, and particularly relates to a method for dissolving cellulose fibers and an application thereof. Background Art

[0002] As the most abundant natural polymer on Earth, the efficient utilization of cellulose has long been a research priority in fields such as materials science and the textile industry. The dissolution of cellulose is a critical first step in achieving its regeneration and high-value-added applications (such as the manufacture of regenerated fibers, films, and gels). However, due to the extensive and regular hydrogen bond network between cellulose molecular chains, which forms a stable crystalline structure, it is extremely difficult to dissolve in conventional solvents.

[0003] Currently, several methods exist for dissolving cellulose in industry and laboratories. For example, the classic N,N-dimethylacetamide / lithium chloride (DMAc / LiCl) system is a commonly used dissolution system. However, this method has significant drawbacks: First, LiCl is highly hygroscopic, requiring rigorous vacuum drying prior to dissolution. The operating conditions are demanding, as the presence of trace amounts of water can severely affect the dissolution process. Second, this method typically requires prolonged heating and stirring, resulting in high energy consumption. More critically, the molecular structure or presence of additives in recently developed modified cellulose fibers with special properties (such as flame retardancy and antibacterial properties), such as flame-retardant Lyocell, make them difficult to dissolve effectively using the traditional DMAc / LiCl system, significantly limiting the recycling and reuse of these new types of waste textiles.

[0004] Therefore, this field urgently needs a dissolution method with simple process, mild conditions, low energy consumption, and wide applicability, especially one that can effectively dissolve a variety of cellulose fibers including new modified cellulose, so as to promote the development of the circular economy and enhance the application value of waste textiles. Summary of the Invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a method for dissolving cellulose fibers. The method has a simple process, low energy consumption, high dissolution efficiency, and can be widely applied to natural fibers including cotton, linen, and new modified cellulose fibers such as lyocell.

[0006] The technical solution adopted by the present invention to solve its technical problem is:

[0007] A method for dissolving cellulose fibers, the method comprising the following steps: S1: mixing cellulose fibers, paraformaldehyde (PF) and a polar aprotic solvent to obtain a liquid to be treated, wherein the mass ratio of the cellulose fibers to the paraformaldehyde is 1:(1-10), and the mass ratio of the cellulose fibers to the polar aprotic solvent is 1:(50-200); S2: stirring the liquid to be treated at room temperature until the paraformaldehyde is dissolved; S3: performing a three-stage temperature increase treatment on the liquid obtained in S2, the three-stage temperature increase treatment sequentially comprising: a. heating to 40-60°C and keeping warm for 5-20 minutes; b. continuing to heat to 70-100°C and keeping warm for 5-20 minutes; c. heating again to 110-130°C and keeping warm for 5-20 minutes; S4: stopping heating, cooling the liquid to room temperature while continuing to stir until the cellulose fibers are completely dissolved to obtain a cellulose solution.

[0008] In the present invention, in the three stages of S3 heating treatment, the temperature is first raised to 40-60°C to swell the fibers and allow the solvent to enter the fibers; the temperature is then continued to be increased to 70-100°C. The increased temperature increases the kinetic energy of the solvent molecules and further destroys the crystalline regions of the fibers, causing some fibers to dissolve and form a partially homogeneous system; and finally the temperature is raised to 110-130°C. The high temperature provides sufficient energy to dissolve the cellulose fibers.

[0009] Preferably, the cellulose fiber is selected from one or more of cotton fiber, hemp fiber, viscose fiber or lyocell fiber; and the polar aprotic solvent is selected from one or a mixture of at least two of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC) and dimethyl sulfoxide (DMSO).

[0010] Preferably, the polar aprotic solvent is a mixed solvent of N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMAC). The inventors surprisingly discovered that using either DMAC or DMF alone resulted in suboptimal dissolution: the former failed to completely destroy the fiber structure, while the latter failed to dissolve the cellulose at all. However, mixing the two in a specific ratio yielded an excellent synergistic effect, producing a uniform, clear, and completely dissolved cellulose solution, laying the foundation for subsequent high-quality applications.

[0011] Preferably, the volume ratio of DMF to DMAC is 4:6 to 6:4. The best effect occurs when the volume ratio of DMF to DMAC is 5:5.

[0012] Preferably, the cellulose fibers in S1 need to be pretreated, and the pretreatment includes: using methanol to perform solvent exchange on the cellulose fibers, and then drying and crushing. Furthermore, the pretreatment includes: using methanol to exchange 1 to 5 times, filtering and wringing out, drying in an oven at 50 to 80°C, drying for 12 to 24 hours, and then using a crusher to make it fluffy. More preferably, the pretreatment includes: using methanol to exchange 3 to 5 times, filtering and wringing out, drying in an oven at 50 to 60°C, and drying for 12 hours; the purpose of the exchange is to remove stains that may exist on the fiber surface and to activate the fiber for better dissolution; after drying, the fiber enters the crusher to make the fiber fluffy, so that the solution can enter the fiber better and faster during the dissolution process, thereby promoting dissolution.

[0013] Preferably, the mass ratio of the cellulose fiber to the paraformaldehyde is 1:(1-5); the mass ratio of the cellulose fiber to the polar aprotic solvent is 1:(50-100). The ratios used can be adjusted accordingly depending on the type of cellulose fiber. This range is the ratio that allows for uniform dissolution of different types of cellulose fibers when testing, i.e., ensuring the dissolution of all cellulose fibers. Different types have different maximum solubility in equal amounts of solvent.

[0014] Preferably, the temperatures of the three-stage heating treatment in S3 are 50-60°C, 80-100°C, and 110-120°C, respectively. This step-by-step heating strategy is another important aspect of the present invention. It is not a simple heating process, but a targeted, staged process of destroying the cellulose structure. Compared with continuous high-temperature heating, it has lower energy consumption and higher efficiency.

[0015] Preferably, the degree of polymerization of the paraformaldehyde is 50 to 100, the molecular weight is 1500 to 3000 g / mol, and the paraformaldehyde is in the form of blocks, powders, granules or flakes, with a purity of ≥95%.

[0016] Preferably, the cellulose fiber is a modified cellulose fiber, such as flame retardant lyocell fiber.

[0017] A cellulose solution produced by the method of the present invention is used in the preparation of solid electrolyte membranes or functional fiber membranes. The method of the present invention can efficiently dissolve natural, regenerated, and modified cellulose fibers. The cellulose solution produced by this method can be used to prepare high-value-added products (such as functional films).

[0018] The present invention achieves the dissolution of different cellulose fibers through a one-pot process. Compared with the prior art, the present invention has the following beneficial effects:

[0019] 1. The "PF + polar aprotic solvent + three-stage heating" combination proposed in this invention successfully solves the existing technical difficulties. It can completely dissolve new modified cellulose fibers such as flame-retardant Lyocell, which are difficult to treat with traditional methods (such as DMAc / LiCl system). It is also effective for conventional fibers such as cotton and linen, and has a wide range of applications.

[0020] 2. The preferred solution of DMF and DMAC mixed solvent discovered by the present invention is not a simple physical mixture, but rather produces a synergistic effect of 1+1>2. Using either solvent alone is ineffective, but the combination of the two forms a solvent environment with extremely strong dissolving power. The resulting solution is homogeneous and transparent, with no undissolved fiber residue (verified by SEM electron microscopy), providing a guarantee for the preparation of high-quality downstream products.

[0021] 3. The original "three-stage heating" process of the present invention, through the scientific process of "low-temperature swelling-medium-temperature crystal breaking-high-temperature complete dissolution", accurately and efficiently destroys the hydrogen bond network of cellulose, avoiding the high energy consumption caused by long-term continuous high-temperature heating in traditional methods, significantly shortening the dissolution time and improving production efficiency.

[0022] 4. The cellulose solution produced by the method of the present invention is homogeneous, stable, and has excellent processability. For example, it can be directly used to prepare functional fiber membranes through electrospinning, and further to prepare solid electrolyte membranes with lithium ion transference numbers as high as 0.6. This demonstrates its enormous potential for application in high-tech fields such as energy storage, achieving high-value-added conversion of waste textiles. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a diagram of the flame-retardant lyocell fiber after dissolution;

[0024] Figure 2 This is the SEM image of flame-retardant lyocell fiber dissolved in DMAC system;

[0025] Figure 3 This is the SEM image of flame-retardant lyocell fiber dissolved in DMAC / DMF system;

[0026] Figure 4 This is a state diagram of DMF system dissolving flame retardant lyocell fiber;

[0027] Figure 5 This is a state diagram of DMAC / LiCl system dissolving flame retardant lyocell fiber;

[0028] Figure 6 This is a diagram of the static state of the flame retardant lyocell fiber solution after dissolution;

[0029] Figure 7 It is a fiber membrane prepared by electrospinning of DMAC / DMF system;

[0030] Figure 8 The current before and after polarization and the calculated ion transfer number (a) and impedance before and after polarization (b) of the solid electrolyte membrane assembled lithium symmetric battery were measured;

[0031] Figure 9 This is the state diagram of DMSO / DMF dissolving flame retardant lyocell fiber;

[0032] Figure 10 This is the electrospinning state diagram of the DMSO / DMF system. DETAILED DESCRIPTION

[0033] The technical solution of the present invention is further described in detail below through specific examples. It should be understood that the implementation of the present invention is not limited to the following examples, and any form of modification and / or change made to the present invention will fall within the scope of protection of the present invention.

[0034] In the present invention, unless otherwise specified, all parts and percentages are by weight, and the equipment and raw materials used are commercially available or commonly used in the art. The methods in the following examples are all conventional methods in the art unless otherwise specified.

[0035] Unless otherwise specified, the reagents used in the following examples can be purchased from conventional biochemical reagent stores.

[0036] Flame retardant lyocell fiber was purchased from China Textile Science Research Institute Co., Ltd.

[0037] Example 1

[0038] A method for dissolving flame-retardant lyocell fiber, comprising the following steps:

[0039] (1) Prepare flame retardant lyocell fiber solution: exchange the flame retardant lyocell fiber with methanol three times, then put it in a 50℃ oven for 12 hours for drying, and then put the fiber into a crusher to make it fluffy; put the treated flame retardant lyocell fiber, paraformaldehyde (PF) and solvent into a glass sample bottle, wherein the mass ratio of flame retardant lyocell fiber to paraformaldehyde is 1:1, and the solvent is a mixture of DMAC and DMF with a volume ratio of 1:1, and the mass ratio of the solvent to flame retardant lyocell fiber is 100:1 (the mass ratio of DMAC / DMF to flame retardant lyocell fiber is 50:1), and put a magnet into it.

[0040] (2) Stirring to dissolve PF: Cover the flame-retardant lyocell fiber solution in the glass sample bottle with a lid and place it on a stirrer for stirring until the PF is observed to dissolve;

[0041] (3) Heating: After PF is completely dissolved, heat it to 60°C and keep it warm for 15 minutes, then heat it to 90°C and keep it warm for 15 minutes, and finally heat it to 115°C and keep it warm for 15 minutes;

[0042] (4) Cooling and stirring to obtain a flame-retardant lyocell fiber solution: After the last heating and heat preservation step, stop heating, gradually return to room temperature, and continue stirring until it is completely dissolved to obtain a flame-retardant lyocell fiber solution.

[0043] The flame retardant lyocell fiber solution obtained is uniform and clear in macroscopic state. Figure 1 The solution was cast into a film and then SEM test was performed. The results are shown in Figure 3 As shown, no obvious fiber structure was observed in the figure, verifying that the fibers were completely dissolved.

[0044] Comparative Example 1

[0045] The difference between this comparative example and Example 1 is that DMF and DMAC used in the dissolution process are replaced with all DMAC. The rest of the formula and specific dissolution process are the same as those in Example 1.

[0046] After the dissolution process, although a uniform solution is formed macroscopically, when the dissolved solution is poured on a polytetrafluoroethylene plate and dried to form a film, it is placed under an electron microscope for measurement and it is found that there is an obvious fiber structure, indicating that the fiber is not completely dissolved. Figure 2 This result is consistent with Figure 3 In contrast, it was proved that the mixed solvent of DMF and DMAC had a synergistic effect in achieving complete dissolution.

[0047] Comparative Example 2

[0048] The difference between this comparative example and Example 1 is that DMF and DMAC used in the dissolution process are replaced with all DMF. The rest of the formula and specific dissolution process are the same as in Example 1.

[0049] After the dissolution process, it fails to dissolve and the macroscopic state is as follows Figure 4 shown.

[0050] Comparative Example 3

[0051] This comparative example differs from Example 1 in that the specific ratio of DMF to DMAC used in the dissolution process was explored. The remaining formulations and specific dissolution process were the same as in Example 1.

[0052] While the solvent dosage remained constant, only the ratio of DMF to DMAC was changed. Dissolution was performed using volume ratios of DMF to DMAC of 3:7, 4:6, 5:5 (Example 1), 6:4, and 7:3. The results showed that as the amount of DMF increased, the dissolution rate slowed down, and insoluble agglomerated particles appeared, increasing in number with increasing DMF dosage. As the amount of DMAC increased, the dissolution time gradually increased. Macroscopic observation revealed a dissolved state, but under an electron microscope, the particles appeared fibrous. The size of the fibers observed under an electron microscope gradually increased with increasing DMF dosage.

[0053] When the volume ratio of DMF to DMAC is 3:7 or 4:6, DMAC is less volatile than DMF, and this solvent system is more prone to jet adhesion during the electrospinning process, resulting in poor results. Furthermore, while the resulting flame-retardant Lyocell fiber solution appears completely dissolved macroscopically, the tiny fibers observed under an electron microscope may affect the treatment or produce larger, undissolved fiber clumps during subsequent processing. Meanwhile, when the volume ratio of DMF to DMAC is 6:4 or 7:3, while the increased DMF dosage makes the solvent system more volatile, fiber agglomeration occurs, resulting in an uneven solution. Subsequent electrospinning can lead to uneven spinning and clogging of the spinneret, increasing time and costs. If the resulting flame-retardant Lyocell fiber solution is intended for direct electrospinning to produce high-value-added products such as functional fiber membranes, the 3:7 or 7:3 ratios are not recommended. 4:6 and 6:4 ratios can also be used for processes with lower solubility requirements, with the optimal DMF to DMAC ratio of 5:5 yielding the best results.

[0054] Comparative Example 4

[0055] An existing method for dissolving cellulose fibers is to dissolve flame-retardant lyocell fibers. The specific steps are as follows:

[0056] (1) Prepare the original fiber dissolving solution: exchange the flame retardant lyocell fiber with methanol for 3 times, then put it in a 50℃ oven for 12 hours to dry, and then put the fiber into a crusher to make it fluffy; put the treated flame retardant lyocell fiber, LiCl and DMAC into a glass sample bottle, with the mass ratio of flame retardant lyocell fiber to LiCl being 1:1, and the mass ratio of DMAC to flame retardant lyocell fiber being 100:1, and put a magnet into it.

[0057] (2) Dissolving by heating: Cover the sample bottle, place it on a stirrer and stir it, raising the temperature to 100°C.

[0058] The expected effect should be to heat the solution and continue stirring until the fibers dissolve. However, after 12 hours of stirring, the flame-retardant lyocell fiber still did not dissolve, indicating that this method is incapable of dissolving new cellulosic fibers such as flame-retardant lyocell. Furthermore, even when dissolving natural cellulosic fibers such as cotton and linen, the LiCl needs to be dehydrated before dissolution. However, it is highly hygroscopic, and if it contains water, the dissolution effect will be greatly reduced. It is also difficult to control it to be completely dry during the preparation of the original fiber dissolving solution.

[0059] The state of flame retardant lyocell fiber dissolved by existing dissolution methods is as follows Figure 5 shown.

[0060] Application Example 1

[0061] The solution after dissolving in Example 1 was left to stand for 12 hours, and delamination occurred. The delamination state of cotton and flame retardant lyocell fiber after dissolution was shown in the figure. Figure 6 As shown, the upper layer is clear and transparent, and the lower layer is deposited with the corresponding dissolved fibers. After the solution is separated, the main components of the clear liquid in the upper layer are DMAC and DMF, which can be recycled and reused. The dissolved fiber part of the lower layer can be processed accordingly according to needs to obtain derivative products such as cellulose membranes.

[0062] Application Example 2

[0063] A solid electrolyte membrane for lithium-ion batteries made from waste flame-retardant lyocell fibers, the specific preparation method of which is as follows:

[0064] (1) Preparation of electrospinning precursor solution: Add 0.1 g of LiTFSI and 1.5 g of polyvinylidene fluoride (PVDF) to the flame-retardant lyocell fiber solution prepared in Example 1 and stir evenly to prepare an electrospinning precursor solution.

[0065] After investigating various polymers added to the electrospinning precursor solution, the subsequent electrospinning process found that PAN > PVDF, ranked from easiest to most difficult. PEO was insoluble in the solution, while PVDF worked well with cellulose fibers, so PVDF was selected. The amount of PVDF added was 15 wt %, as the electrospinning precursor solution requires a certain viscosity but not excessive thickness. 0.1 g of lithium salt was added to improve the membrane's ionic conductivity.

[0066] (2) Electrospinning: The electrospinning precursor solution is placed in a needle tube for electrospinning to obtain a cellulose membrane.

[0067] The macroscopic image of the cellulose membrane obtained by electrospinning is shown in Figure 2. Figure 7 shown.

[0068] The electrospinning parameters were set as follows: drum speed of 200 rpm, distance between needle and receiver of 15 cm, voltage of 14.5 kV, and pushing speed of 0.8 ml / h.

[0069] (3) Casting: After the electrospinning is completed, the cellulose membrane is formed and the PEO solution is cast. 2 5 ml was evenly poured on the membrane and then dried in an oven at 50°C for 12 h to make it dry.

[0070] The specific composition of the PEO solution is 10ml of acetonitrile, 0.49g of PEO, and 0.32g of LiTFSI. PEO can effectively fill the gaps between the skeletons formed by cellulose fibers and is most compatible with cellulose fibers.

[0071] (4) After drying, it is cut into small discs with a diameter of 16 mm, which can be used for battery applications.

[0072] Assemble lithium symmetric batteries to measure the lithium ion migration number of the battery. The results are as follows Figure 8 Calculations show that the solid electrolyte membrane prepared by electrospinning and casting of dissolved flame-retardant lyocell fibers has a lithium ion transference number of 0.6 after being assembled into a button cell, which is a good effect.

[0073] This application example proves that the solution obtained by the method of the present invention is of excellent quality and can be used to prepare high value-added products.

[0074] Comparative Example 5

[0075] The difference between this comparative example and Example 1 is that the solvent used in the dissolution process is a mixture of DMSO and DMF in a volume ratio of 1:1. The rest of the formula and specific dissolution process are the same as those in Example 1.

[0076] During the dissolution process, it was found that the dissolution system of DMSO+DMF solvent and PF can also successfully dissolve flame retardant lyocell fiber. The state after dissolution is as follows: Figure 9 shown.

[0077] The flame retardant lyocell fiber solution obtained in this comparative example was subjected to electrospinning as in Application Example 2 to prepare a cellulose film. However, during the electrospinning process, it was found that liquid rather than silk was ejected from the needle. The specific state is as follows: Figure 10 As shown in the figure, this is because DMSO is difficult to volatilize, which makes spinning difficult. Therefore, the use of DMSO+DMF solvent system is not conducive to the subsequent high value-added reuse of flame-retardant lyocell fiber solution.

[0078] In summary, the method of the present invention is applicable to nearly all existing types of cellulose fibers, avoiding the insolubility issues of cellulose in traditional dissolution methods due to differences in cellulose structure or the addition of additives. In the method of dissolving cellulose fibers described herein, a solvent system with a DMF:DMAC volume ratio of 4:6 to 6:4 effectively dissolves cellulose fibers, with the optimal effect being achieved when the DMF:DMAC volume ratio is 5:5. The preferred solvent system (DMAC / DMF) described herein not only successfully dissolves cellulose fibers, but also exhibits excellent processing properties, enabling subsequent high-value-added applications.

[0079] The present invention adopts the interaction between the organic solvent and the added trace additive to dissolve the cellulose fiber, and generates less waste pollutants. Since the dissolved solution will cause stratification after precipitation, the upper organic solvent can be recovered and reused after filtration.

Claims

1. A method for dissolving cellulose fibers, characterized in that: The method comprises the following steps: S1: mixing cellulose fibers, paraformaldehyde (PF), and a polar aprotic solvent to obtain a liquid to be treated, wherein the mass ratio of the cellulose fibers to the paraformaldehyde is 1:(1-10), and the mass ratio of the cellulose fibers to the polar aprotic solvent is 1:(50-200); S2 stirring the liquid to be treated at room temperature until the paraformaldehyde is dissolved; S3 performs a three-stage temperature raising process on the liquid obtained in S2, wherein the three-stage temperature raising process sequentially comprises: a. Heat to 40-60°C and keep warm for 5-20 minutes; b. Continue heating to 70-100°C and keep warm for 5-20 minutes; c. Heat again to 110-130°C and keep warm for 5-20 minutes; S4: stopping heating and cooling the liquid to room temperature while continuing to stir until the cellulose fibers are completely dissolved to obtain a cellulose solution.

2. The method according to claim 1, wherein: The cellulose fiber is selected from one or more of cotton fiber, hemp fiber, viscose fiber or lyocell fiber; the polar aprotic solvent is selected from one or a mixture of at least two of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC) and dimethyl sulfoxide (DMSO).

3. The method according to claim 2, wherein: The polar aprotic solvent is a mixed solvent of N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMAC).

4. The method according to claim 1, wherein: S1 The cellulose fibers need to be pretreated, and the pretreatment includes: using methanol to perform solvent exchange on the cellulose fibers, and then drying and crushing.

5. The method according to claim 1, wherein: The mass ratio of the cellulose fiber to the paraformaldehyde is 1:(1-5); the mass ratio of the cellulose fiber to the polar aprotic solvent is 1:(50-100).

6. The method according to claim 1, wherein: The temperatures of the three-stage temperature increase treatment in S3 are 50-60°C, 80-100°C and 110-120°C, respectively.

7. The method according to claim 1, wherein: The polymerization degree of the paraformaldehyde is 50-100, and the molecular weight is 1500-3000 g / mol.

8. The method according to claim 1, wherein: The cellulose fiber is modified cellulose fiber.

9. Use of the cellulose solution prepared by the method according to any one of claims 1 to 8 in the preparation of a solid electrolyte membrane or a functional fiber membrane.

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