A method for producing high-strength lyocell regenerated cellulose fiber

Abstract

The application discloses a preparation method of high-strength Lyocell regenerated cellulose fiber, and aims to solve the problem of insufficient mechanical strength of existing regenerated cellulose fiber caused by too wide molecular weight distribution of pulp. The method is characterized in that, through the synergistic effect of a mild pre-hydrolysis kraft (PHK) cooking process and an optimized multi-stage elemental chlorine-free (ECF) bleaching sequence (D0-E-D1), the lignin and hemicellulose are efficiently removed, and the long molecular chain of cellulose is maximally protected; through multi-stage gradient cold alkali extraction and purification treatment, the alkali concentration and low-temperature conditions are accurately controlled, and short-chain cellulose is selectively dissolved, so that the active regulation and narrowing of the molecular weight distribution of cellulose are realized, and a dissolving pulp with high polymerization degree and concentrated molecular weight distribution is obtained. Finally, the high-quality dissolving pulp is used to prepare Lyocell fiber with significantly enhanced mechanical properties through NMMO solvent system dry-jet wet spinning.

Description

Technical Field

[0001] This invention relates to a method for preparing high-strength Lyocell regenerated cellulose fiber, belonging to the field of bio-based fiber material technology. Background Technology

[0002] Lyocell fiber, a green regenerated cellulose fiber using N-methylmorpholine-N-oxide (NMMO) as a solvent, is highly favored for its environmentally friendly production process and excellent fabric properties. Its overall performance, especially its mechanical strength, largely depends on the quality of the solvent pulp used. However, the strength of existing Lyocell fibers still falls short of fully meeting the growing demands of the high-end textile and industrial textile sectors.

[0003] Currently, the industry generally believes that increasing the average degree of polymerization of dissolving pulp is one of the key pathways to enhance the strength of Lyocell fibers. Researchers have conducted numerous explorations to this end. For example, the low-eutectic solvent pretreatment technology proposed by Shen Kuizhong's team at the Chinese Academy of Forestry in 2020 (patent CN112482069B) can effectively remove hemicellulose and lignin, but it does not focus on controlling the degree of polymerization of cellulose, resulting in pulp with a degree of polymerization mostly at the conventional level (approximately 500-700). In 2022, Wang Qiang's team at Qilu University of Technology in Shandong developed a method for refining dissolving pulp using γ-valerol (patent CN112144308B), which improved the purity of α-cellulose, but because the treated material was already degraded chemical pulp, the degree of polymerization of the finished dissolving pulp product was difficult to exceed 800.

[0004] However, the average degree of polymerization is not the only key factor determining fiber strength; the molecular weight distribution of cellulose is equally crucial. An overly broad molecular weight distribution means that the pulp contains both long molecular chains and too many short molecular chains (which may affect dissolution and spinning). These short-chain cellulose molecules cannot effectively bear stress during fiber formation and instead become weak points in the structure, thus limiting the improvement of the fiber's final strength.

[0005] Therefore, existing technologies face the following prominent problems and limitations:

[0006] (1) Single technical objective: Most dissolving pulp preparation processes still focus on removing lignin and hemicellulose to improve purity and whiteness, lacking awareness and means to effectively control the molecular weight distribution of cellulose.

[0007] (2) The strength bottleneck remains unresolved: The pulp prepared by existing methods often has a wide molecular weight distribution, resulting in insufficient mechanical strength of Lyocell fibers produced from it.

[0008] (3) Insufficient process synergy: Although cold alkali extraction is used for purification, its potential functions in selectively removing short-chain cellulose and narrowing molecular weight distribution have not been fully recognized and utilized.

[0009] To address the aforementioned technical challenges, this invention aims to provide a method for preparing dissolving slurries that combine high polymerization degree and concentrated molecular weight distribution, thereby achieving stable preparation of high-quality raw materials for high-performance Lyocell fibers. Summary of the Invention

[0010] The purpose of this invention is to provide a method for preparing high-strength Lyocell regenerated cellulose fiber, which solves the problem of insufficient mechanical strength of existing regenerated cellulose fibers due to the excessively wide molecular weight distribution of pulp.

[0011] To achieve the above objectives, the present invention adopts the following technical solution:

[0012] A method for preparing high-strength Lyocell regenerated cellulose fiber, comprising the following steps:

[0013] (1) Pretreatment: Screen the raw materials and perform pre-hydrolysis treatment to obtain pretreated raw materials with low hemicellulose content;

[0014] (2) Cooking: The pre-hydrolyzed raw materials are cooked using the sulfate method, and then washed and screened to obtain pre-hydrolyzed sulfate slurry;

[0015] (3) Multi-stage elementless chlorine bleaching ECF and chelation treatment: The pre-hydrolyzed sulfate pulp is subjected to multi-stage elementless chlorine bleaching ECF, which includes the first stage of chlorine dioxide bleaching D0, alkali treatment E and the second stage of chlorine dioxide bleaching D1. The bleached pulp is then chelated with a chelating agent for chelation treatment.

[0016] (4) Multi-stage gradient cold alkali extraction and purification: The chelated pulp is subjected to multi-stage gradient cold alkali extraction and purification treatment, including the first stage cold alkali extraction treatment and the second stage cold alkali extraction treatment, to obtain a dissolving pulp with high polymerization degree and concentrated molecular weight distribution.

[0017] (5) Wet spinning: The obtained high degree of polymerization dissolution slurry is placed in a reaction vessel and mixed with NMMO solution and antioxidant to obtain spinning solution. High-strength Lyocell fiber is obtained by dry-spray wet spinning.

[0018] Preferably, the raw material is one or more of coniferous wood, broadleaf wood, bamboo, wheat straw, and hemp.

[0019] Preferably, the specific conditions for the pre-hydrolysis treatment in step (1) are: temperature 150~165℃, heating time 30~60min, holding time 30~60min, liquid ratio 1:5, and P factor 100~250.

[0020] Preferably, the specific process conditions for sulfate cooking in step (2) are: temperature 150~165℃, heating time 30~60min, holding time 30~120min, liquid ratio 1:4, cooking H factor 300~800, alkali amount 10%~20% relative to the oven-dry raw material, and sulfidation degree 20%~30% relative to the oven-dry raw material.

[0021] Preferably, the specific process conditions for the first stage of chlorine dioxide bleaching D0 in step (3) are as follows: ClO2 dosage is 3%~5% of the dry pulp dosage, temperature is 60~75℃, time is 15~45min, and pulp concentration is 5~15%; the conditions for alkali treatment E are as follows: NaOH dosage is 0.5%~2% of the dry pulp dosage, temperature is 60~75℃, time is 30~90min, and pulp concentration is 5~15%; the conditions for the second stage of chlorine dioxide bleaching D1 are as follows: ClO2 dosage is 1%~3% of the dry pulp dosage, temperature is 60~75℃, time is 30~90min, and pulp concentration is 5~15%; the chelating agent used in the chelation treatment is disodium ethylenediaminetetraacetate, dosage is 0.5%~2% of the dry pulp dosage, temperature is 60~75℃, time is 30~90min, and pulp concentration is 5~15%.

[0022] Preferably, the specific conditions for the first stage of cold alkali extraction in step (4) are: NaOH dosage is 3-6% of the dry pulp dosage, temperature is 15-20℃, time is 30-60min, and pulp concentration is 5-15%.

[0023] Preferably, the specific conditions for the second stage of cold alkali extraction in step (4) are: NaOH dosage is 10-15% of the dry pulp dosage, temperature is 20-25℃, time is 60-90min, and pulp concentration is 5-15%.

[0024] Preferably, in step (5), the antioxidant is propyl gallate, and the amount of antioxidant is 0.05-0.1% of the total mass of the high degree of polymerization dissolving slurry and NMMO solution; the mass fraction of NMMO solution is 70-75%; and the mass ratio of high degree of polymerization dissolving slurry to NMMO solution is 1:(15-40).

[0025] Furthermore, the mass ratio of the high degree of polymerization dissolving slurry to the NMMO solution is 1:(20-35).

[0026] Preferably, the specific conditions for wet spinning are as follows: the spinning solution is metered by a metering pump under a pressure of 0.2 to 0.5 MPa and then extruded through the micro-holes of the spinneret. It enters a coagulation bath with a concentration of 5 to 20% through an air section of 10 to 40 mm and coagulates in the coagulation bath at a temperature of 10 to 30°C. The draw ratio is 3 to 10. After washing and drying, high-strength Lyocell regenerated cellulose fibers are obtained.

[0027] The core working principle of this invention lies in the synergistic effect of a mild pre-hydrolyzed sulfate (PHK) cooking process and an optimized multi-segment elemental chlorine-free (ECF) bleaching sequence (DO-E-D1). This process efficiently removes lignin and hemicellulose while maximally protecting long cellulose molecular chains. Simultaneously, by utilizing a multi-segment gradient cold alkali extraction step and precisely controlling conditions such as alkali concentration and low temperature, short-chain cellulose is selectively dissolved while achieving deep purification of the pulp, thus actively regulating and narrowing the molecular weight distribution of cellulose. Finally, the resulting high-quality dissolving pulp, characterized by both high polymerization degree and concentrated molecular weight distribution, is spun using a dry-jet wet spinning method with an NMMO solvent system to produce Lyocell fibers with significantly enhanced mechanical properties. This represents a leap from "purity" to "structure," fundamentally improving fiber strength. This invention is the first to use the "cellulose molecular weight distribution" of pulp as a core quality control indicator. By selectively removing short-chain cellulose that cannot effectively bear the strength load through multi-stage gradient cold alkali extraction, the cellulose molecular chain length of the final pulp becomes more uniform. This uniform chain structure can distribute stress more evenly during fiber forming, thereby significantly improving the dry and wet breaking strength of Lyocell fiber and solving the strength bottleneck of traditional fibers due to their wide molecular weight distribution.

[0028] Compared with the prior art, the beneficial effects of the present invention are:

[0029] (1) A two-stage gradient extraction process is adopted, first mild and then enhanced. The first stage (low alkali and low temperature) mainly removes residual hemicellulose and very short chain fragments, while the second stage (higher alkali concentration) selectively dissolves medium-length short-chain cellulose with a degree of polymerization in the range of 200-800. This graded treatment method achieves precise trimming of cellulose components, significantly reducing the polydispersity index (PDI) of the final dissolving pulp to below 2.3, and successfully obtaining high-quality pulp with a highly concentrated molecular weight distribution;

[0030] (2) The mild pre-hydrolyzed sulfate method (PHK) and multi-stage elemental chlorine-free (ECF) bleaching process adopted in this invention retains the long cellulose chain to the maximum extent, keeping the degree of polymerization (DP) of pulp at a high level of 1300-1450. The synergistic effect of high degree of polymerization and narrow distribution, PHK cooking and ECF bleaching lay the foundation for retaining high degree of polymerization. The subsequent molecular weight distribution control not only improves the average degree of polymerization, but also removes the low molecular weight components that affect the stability of dissolution and spinning processing. This makes the final spinning solution have good rheological properties and spinnability, which is conducive to the stable preparation of high-quality fibers through dry-jet wet spinning process.

[0031] (3) The Lyocell fibers prepared by this method have significantly improved key mechanical properties such as dry strength, wet strength and modulus, meeting the requirements of high-end textiles and industrial textiles. At the same time, the entire preparation process abandons elemental chlorine bleaching and adopts ECF technology, resulting in low wastewater pollution load. It is a clean and environmentally friendly high-performance fiber production route. Detailed Implementation

[0032] The technical solution of the present invention will be further described below with reference to specific embodiments, but it is not limited thereto.

[0033] Example 1

[0034] A method for preparing high-strength Lyocell regenerated cellulose fiber, the specific steps of which are as follows:

[0035] (1) Pretreatment: Take 150g of raw material (broadleaf wood) and determine its initial moisture content. Weigh wood chips equivalent to 0.14kg of oven-dry weight, place them in a rotary cooker, add deionized water at a liquid ratio of 1:5, set the maximum temperature to 160℃, the heating time to 60min, the holding time to 20min, and calculate the P factor to be approximately 170. After the holding time is completed, exhaust the gas, wash and dry to obtain pre-hydrolyzed wood chip raw material.

[0036] (2) Cooking: Take 0.1 kg of pre-hydrolyzed wood chips (octane-dry weight) and place them in a rotary cooker. Add 20% NaOH (calculated as NaOH) as the alkali dosage and 30% Na2S (calculated as Na2S) as the sulfidation degree, based on the octane weight of the raw material. Add 0.5% (octane-dry weight) of disodium ethylenediaminetetraacetate (EDTA). Add deionized water at a liquid ratio of 1:4. Set the maximum temperature to 160℃, the heating time to 60 min, and the holding time to 60 min. Calculate the H factor to be approximately 402. After cooking, wash until neutral, sieve the pulp (sieve gap 0.25 mm) to obtain fine pulp. After balancing the moisture content, measure the yield to obtain pre-hydrolyzed sulfate hardwood pulp. Under these cooking conditions, the coarse pulp yield is 53.25%, the fine pulp yield is 43.73%, the kappa number is 23.13, the degree of polymerization (DP) is 1684.3, the α-cellulose content is 82.14%, and the S content is 402%. 18 It has a content of 7.82% and a lignin content of 3.75%.

[0037] (3) Multi-stage elemental chlorine-free bleaching of ECF and chelation treatment:

[0038] The elemental chlorine-free bleaching process consists of three steps: D0, E, and D1, followed by a chelation treatment. Step D0: Take oven-dried pulp, add 5% ClO2 (by weight of oven-dried pulp), pH 3.0-4.0, react at 70℃ and 10% pulp concentration for 30 minutes. During the reaction, manually knead the mixed pulp every 10 minutes to ensure uniform reaction. After the reaction, wash the pulp with deionized water until neutral and dry in a 50℃ oven for 12 hours. Step E: Alkali treatment: Add 1% NaOH (by weight of oven-dried pulp), treat at 70℃ and 10% pulp concentration for 60 minutes, manually kneading the mixed pulp every 10 minutes. After the treatment, wash until neutral and dry at 50℃ for 12 hours. Step D1: Bleaching: Add 3% ClO2 (by weight of oven-dried pulp), react at 70℃ and 10% pulp concentration for 60 minutes, wash and dry. Chelation treatment: Add 0.5% EDTA (by weight of oven-dried pulp), treat at 70℃ and 10% pulp concentration for 60 minutes, wash and dry.

[0039] (4) Multi-stage gradient cold alkali extraction and purification:

[0040] The first stage of cold alkali extraction was performed sequentially: 3% NaOH (octane-dry pulp weight) was added, and the reaction was carried out at 15°C and 10% pulp concentration for 60 minutes, with the pulp being kneaded every 10 minutes to ensure thorough mixing between the pulp and the reagent. After the reaction, the pulp was thoroughly washed until neutral and then dehydrated. The washed and dehydrated pulp was then treated with 10% NaOH (octane-dry pulp weight) at 20°C and 10% pulp concentration for 90 minutes. After the reaction, the pulp was washed until neutral and then dried at 50°C for 12 hours, ultimately yielding a dissolving pulp with high polymerization degree and concentrated molecular weight distribution.

[0041] (5) Wet spinning: Weigh 20g of completely dry dissolving paste, add 584g of 72wt% NMMO aqueous solution and 0.5g of propyl gallate, mix and swell for 0.5h, then vacuum at 110℃ for 2h to obtain spinning solution.

[0042] Spinning conditions: spinning pressure 0.4MPa, temperature 110℃, spinneret orifice diameter 0.2mm, 30 holes, air section 10-15mm, coagulation bath concentration 10%, temperature 15℃, draw ratio 5, followed by washing and drying to obtain Lyocell fiber.

[0043] Example 2

[0044] A method for preparing high-strength Lyocell regenerated cellulose fiber, the specific steps of which are as follows:

[0045] (1) Pretreatment: Take 180g of raw material (coniferous wood + bamboo, mass ratio 1:1) and determine the initial moisture content. Weigh 0.16kg of raw material (ocean dry weight) and place it in a rotary cooker. Add deionized water at a liquid ratio of 1:5. Set the maximum temperature to 155℃, the heating time to 45min, and the holding time to 40min. The calculated P factor is approximately 130. After the holding time is completed, exhaust the gas, wash and dry the raw material to obtain the pre-hydrolyzed raw material.

[0046] (2) Cooking: Take 0.12 kg of pre-hydrolyzed raw material (octane-dry weight) and place it in a rotary cooker. Add 15% NaOH (octane-dry weight of raw material), 25% Na2S (calculated as Na2S) as the degree of sulfidation, and 0.8% (octane-dry weight of raw material) of disodium ethylenediaminetetraacetate (EDTA). Add deionized water at a liquid ratio of 1:4. Set the maximum temperature to 158℃, the heating time to 40 min, and the holding time to 90 min. Calculate the H factor to be approximately 550. After cooking, wash until neutral, sieve the pulp (sieve gap 0.25 mm) to obtain fine pulp, and measure the moisture content to obtain pre-hydrolyzed sulfate pulp. Under these cooking conditions, the coarse pulp yield is 51.87%, the fine pulp yield is 42.35%, the kappa number is 21.46, the degree of polymerization (DP) is 1726.5, the α-cellulose content is 83.27%, and the S... 18 It has a content of 7.21% and a lignin content of 3.23%.

[0047] (3) Multi-stage elemental chlorine-free bleaching of ECF and chelation treatment:

[0048] D0 stage bleaching: Take oven-dried pulp, add 4% ClO2 (by weight of oven-dried pulp), pH 3.0-4.0, react at 65℃ and pulp concentration 8% for 40 minutes, manually kneading the mixed pulp every 10 minutes during the reaction. After the reaction, wash until neutral and dry at 50℃ for 12 hours.

[0049] Section E alkali treatment: Add 1.2% NaOH (ocean-dry pulp weight), treat at 68℃ and pulp consistency 8% for 75 minutes, manually kneading and mixing the pulp every 10 minutes during treatment. After treatment, wash until neutral and dry at 50℃ for 12 hours.

[0050] D1 bleaching: Add 2% ClO2 (ocean-dry pulp weight), react at 65℃ and pulp concentration of 8% for 75 minutes, then wash and dry.

[0051] Chelation treatment: Add 1.0% EDTA (ocean-dry pulp weight), treat at 68℃ and pulp concentration of 8% for 75 minutes, then wash and dry.

[0052] (4) Multi-stage gradient cold alkali extraction and purification:

[0053] First stage cold alkali extraction: Add 4.5% NaOH (ocean-dry pulp weight), react at 18℃ and 8% pulp concentration for 45 minutes, kneading the pulp every 10 minutes during the reaction. After the reaction, wash thoroughly until neutral and dehydrate.

[0054] The second stage of cold alkali extraction: 12% NaOH (octane dry weight) was added to the above dehydrated slurry, and the reaction was carried out at 22°C and 8% slurry concentration for 75 minutes. After the reaction was completed, the slurry was washed until neutral and dried at 50°C for 12 hours to obtain a dissolving slurry with high polymerization degree and concentrated molecular weight distribution.

[0055] (5) Wet spinning: Weigh 24g of oven-dry dissolving paste, add 688g of 73wt% NMMO aqueous solution, add 0.6g of propyl gallate, mix and swell for 1h, then vacuum at 115℃ for 1.5h to obtain spinning solution.

[0056] Spinning conditions: spinning pressure 0.3MPa, temperature 115℃, spinneret orifice diameter 0.18mm, 75 holes, air section 20-25mm, coagulation bath concentration 12%, temperature 20℃, draw ratio 6, followed by washing and drying to obtain Lyocell fiber.

[0057] Example 3

[0058] A method for preparing high-strength Lyocell regenerated cellulose fiber, the specific steps of which are as follows:

[0059] (1) Pretreatment: Take 200g of raw material (wheat straw + hemp, mass ratio 2:1) and determine the initial moisture content. Weigh 0.17kg of raw material (octane dry weight) and place it in a rotary cooker. Add deionized water at a liquid ratio of 1:5. Set the maximum temperature to 163℃, the heating time to 35min, and the holding time to 50min. Calculate the P factor to be approximately 220. After the holding time is completed, exhaust the gas, wash and dry the raw material to obtain the pre-hydrolyzed raw material.

[0060] (2) Cooking: Take 0.13 kg of pre-hydrolyzed raw material (dry weight), add 18% NaOH (based on alkali) and 28% Na2S (based on Na2S) as the degree of sulfidation, and add 1.0% (dry weight of raw material) of disodium ethylenediaminetetraacetate (EDTA). Add deionized water at a liquid ratio of 1:4, set the maximum temperature to 162℃, the heating time to 50 min, and the holding time to 45 min. The calculated H factor is approximately 380. After cooking, wash until neutral, sieve the pulp (sieve gap 0.25 mm) to obtain fine pulp, and measure the moisture content to obtain pre-hydrolyzed sulfate pulp. Under these cooking conditions, the yield of coarse pulp is 45.34%, the yield of fine pulp is 36.68%, the kappa number is 22.79, the degree of polymerization (DP) is 1653.8, the α-cellulose content is 81.89%, and the S content is 1653.8%. 18 It has a content of 7.69% and a lignin content of 4.05%.

[0061] (3) Multi-stage elemental chlorine-free bleaching of ECF and chelation treatment:

[0062] D0 stage bleaching: Take oven-dried pulp, add 3.5% ClO2 (by weight of oven-dried pulp), pH 3.0-4.0, react at 72℃ and 12% pulp concentration for 25 minutes, manually kneading the mixed pulp every 10 minutes during the reaction. After the reaction, wash until neutral and dry at 50℃ for 12 hours.

[0063] Section E alkali treatment: Add 0.8% NaOH (ocean-dry pulp weight), treat at 72℃ and pulp consistency 12% for 50 minutes, manually kneading and mixing the pulp every 10 minutes during treatment. After treatment, wash until neutral and dry at 50℃ for 12 hours.

[0064] D1 bleaching: Add 1.5% ClO2 (ocean-dry pulp weight), react at 72℃ and pulp concentration of 12% for 80 minutes, then wash and dry.

[0065] Chelation treatment: Add 1.5% EDTA (ocean-dry pulp weight), treat at 72℃ and pulp concentration of 12% for 50 minutes, then wash and dry.

[0066] (4) Multi-stage gradient cold alkali extraction and purification:

[0067] First stage cold alkali extraction: Add 5.5% NaOH (ocean-dry pulp weight), react at 19℃ and 12% pulp concentration for 35 minutes, kneading the pulp every 10 minutes during the reaction. After the reaction, wash thoroughly until neutral and dehydrate.

[0068] The second stage of cold alkali extraction: 14% NaOH (octane dry weight) was added to the above dehydrated slurry, and the reaction was carried out at 24°C and 12% slurry concentration for 85 minutes. After the reaction was completed, the slurry was washed until neutral and dried at 50°C for 12 hours to obtain a dissolving slurry with high polymerization degree and concentrated molecular weight distribution.

[0069] (5) Wet spinning: Weigh 28g of dissolving paste, add 787g of 74wt% NMMO aqueous solution, add 0.7g of propyl gallate, mix and swell for 1.2h, then vacuum at 120℃ for 1.8h to obtain spinning solution.

[0070] Spinning conditions: spinning pressure 0.45MPa, temperature 120℃, spinneret orifice diameter 0.12mm, 100 holes, air section 30-35mm, coagulation bath concentration 18%, temperature 25℃, draw ratio 7, followed by washing and drying to obtain Lyocell fiber.

[0071] Comparative Example 1

[0072] A method for preparing high-strength Lyocell regenerated cellulose fiber is the same as the preparation steps in Example 1, except that there is no multi-stage gradient cold alkali extraction purification step after bleaching and chelation.

[0073] Comparative Example 2

[0074] A method for preparing high-strength Lyocell regenerated cellulose fiber is identical to that in Example 1, except that only a single cold alkali extraction purification step is used after bleaching and chelation. Specifically, 3% NaOH (octane-dry pulp mass) is added, and the reaction is carried out at 15°C with a pulp concentration of 10% for 60 minutes. During this period, the pulp is kneaded every 10 minutes to ensure thorough mixing between the pulp and the chemical solution. After the reaction, the pulp is thoroughly washed until neutral and dried at 50°C for 12 hours to obtain a dissolving pulp with high polymerization degree and concentrated molecular weight distribution.

[0075] Comparative Example 3

[0076] A method for preparing high-strength Lyocell regenerated cellulose fiber is identical to that in Example 1, except that only a single cold alkali extraction purification step is used after bleaching and chelation. Specifically, 10% NaOH (octane-dry pulp mass) is added, and the mixture is reacted at 20°C and 10% pulp concentration for 90 minutes for the second stage of treatment. After the reaction, the mixture is washed until neutral and dried at 50°C for 12 hours to finally obtain a dissolving pulp with high polymerization degree and concentrated molecular weight distribution.

[0077] Comparative Example 4

[0078] A method for preparing high-strength Lyocell regenerated cellulose fibers is the same as that in Example 1, except that the temperature differs in the multi-stage gradient cold alkali extraction purification. Specifically:

[0079] The first stage of cold alkali extraction was performed sequentially: 3% NaOH (octane-dry pulp weight) was added, and the reaction was carried out at 10°C and 10% pulp concentration for 60 minutes. During this process, the pulp was kneaded every 10 minutes to ensure thorough mixing between the pulp and the reagent. After the reaction, the pulp was thoroughly washed until neutral and then dehydrated. The washed and dehydrated pulp was then treated with 10% NaOH (octane-dry pulp weight) at 20°C and 10% pulp concentration for 90 minutes. After the reaction, the pulp was washed until neutral and then dried at 50°C for 12 hours to obtain a dissolving pulp with high polymerization degree and concentrated molecular weight distribution.

[0080] Comparative Example 5

[0081] A method for preparing high-strength Lyocell regenerated cellulose fibers is the same as that in Example 1, except that the temperature differs in the multi-stage gradient cold alkali extraction purification. Specifically:

[0082] The first stage of cold alkali extraction was performed sequentially: 3% NaOH (octane-dry pulp weight) was added, and the reaction was carried out at 15°C and 10% pulp concentration for 60 minutes, with the pulp being kneaded every 10 minutes to ensure thorough mixing between the pulp and the reagent. After the reaction, the pulp was thoroughly washed until neutral and then dehydrated. The washed and dehydrated pulp was then treated with 10% NaOH (octane-dry pulp weight) at 30°C and 10% pulp concentration for 90 minutes. After the reaction, the pulp was washed until neutral and then dried at 50°C for 12 hours, ultimately yielding a dissolving pulp with high polymerization degree and concentrated molecular weight distribution.

[0083] Performance testing

[0084] The products obtained in Examples 1-3 and Comparative Examples 1-5 were subjected to the following tests and comparative analyses.

[0085] 1. Dissolving slurry performance test

[0086] Test indicators: α-cellulose content (%), hemicellulose content (as S) 18 Values ​​are expressed as %, ash (%), degree of polymerization (DP), molecular weight distribution (expressed as polydispersity index PDI), and whiteness (%) ISO.

[0087] Test method: Conducted in accordance with relevant national standards or industry-standard methods. Specific results are shown in Table 1.

[0088] 2. Lyocell fiber mechanical property testing:

[0089] Test parameters: dry fracture strength (cN / dtex), wet fracture strength (cN / dtex), dry fracture elongation (%), and wet fracture elongation (%). Specific results are shown in Table 2.

[0090] Table 1 Results of dissolving slurry performance tests

[0091]

[0092] Table 2 Fiber Mechanical Property Test Results

[0093]

[0094] As can be seen from the results in Tables 1 and 2 above, the α-cellulose content of the dissolving pulp obtained by the preparation method of the present invention is as high as 93% or more, the degree of polymerization (DP) is maintained at a high level of 1387-1458, and the polydispersity index (PDI) is significantly reduced to below 2.3, indicating a highly concentrated molecular weight distribution. In Examples 1-3, multi-stage gradient cold alkali extraction significantly reduced the PDI (narrower molecular weight distribution) while maintaining a high degree of polymerization, demonstrating the ability to actively regulate the cellulose chain length. The dry and wet breaking strengths of the prepared Lyocell fibers are significantly higher than those of the fibers in Comparative Examples 1-5. Compared with Comparative Example 1, the dry breaking strength of the fibers in Example 1 increased by 20.6%, and the wet breaking strength increased by 24.2%. Compared with Comparative Examples 2 and 3 (which only used a one-step cold alkali extraction purification step), the fiber strength of Example 1 also increased significantly, indicating the importance of multi-stage gradient cold alkali extraction. Comparative Examples 4 and 5 show that temperature control has a significant impact on PDI and strength, indicating that the precision of process conditions is crucial. This invention utilizes the synergistic effect of a mild pre-hydrolyzed sulfate (PHK) cooking process and an optimized multi-segment elemental chlorine-free (ECF) bleaching sequence, along with multi-segment gradient cold alkali extraction purification treatment, to selectively remove short-chain cellulose while protecting the long molecular chains of cellulose, thus significantly improving the final strength of the fiber.

[0095] It should be noted that the above embodiments are merely some preferred embodiments of the present invention, and not all embodiments. Obviously, based on the above embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

Claims

1. A method for preparing high-strength Lyocell regenerated cellulose fiber, characterized in that, It includes the following steps: (1) Pretreatment: Screen the raw materials and perform pre-hydrolysis treatment to obtain pretreated raw materials with low hemicellulose content; (2) Cooking: The pre-hydrolyzed raw materials are cooked using the sulfate method, and then washed and screened to obtain pre-hydrolyzed sulfate slurry; (3) Multi-stage elementless chlorine bleaching ECF and chelation treatment: The pre-hydrolyzed sulfate pulp is subjected to multi-stage elementless chlorine bleaching ECF, which includes the first stage of chlorine dioxide bleaching D0, alkali treatment E and the second stage of chlorine dioxide bleaching D1. The bleached pulp is then chelated with a chelating agent for chelation treatment. (4) Multi-stage gradient cold alkali extraction and purification: The chelated pulp is subjected to multi-stage gradient cold alkali extraction and purification treatment, including the first stage cold alkali extraction treatment and the second stage cold alkali extraction treatment, to obtain a dissolving pulp with high polymerization degree and concentrated molecular weight distribution. In the first stage of cold alkali extraction treatment, the amount of NaOH used is 3-6% of the amount of oven-dry pulp, and the temperature is 15-20℃. In the second stage of cold alkali extraction treatment, the amount of NaOH used is 10-15% of the amount of oven-dry pulp, and the temperature is 20-25℃. (5) Wet spinning: The obtained high degree of polymerization dissolution slurry is placed in a reaction vessel and mixed with NMMO solution and antioxidant to obtain spinning solution. High-strength Lyocell fiber is obtained by dry-spray wet spinning.

2. The method for preparing high-strength Lyocell regenerated cellulose fiber according to claim 1, characterized in that, The raw materials are one or more of the following: coniferous wood, broadleaf wood, bamboo, wheat straw, and hemp.

3. The method for preparing high-strength Lyocell regenerated cellulose fiber according to claim 1, characterized in that, The specific conditions for the pre-hydrolysis treatment in step (1) are: temperature 150~165℃, heating time 30~60min, holding time 30~60min, liquid ratio 1:5, and P factor 100~250.

4. The method for preparing high-strength Lyocell regenerated cellulose fiber according to claim 1, characterized in that, The specific process conditions for sulfate cooking in step (2) are as follows: temperature 150~165℃, heating time 30~60min, holding time 30~120min, liquid ratio 1:4, cooking H factor 300~800, alkali amount 10%~20% relative to the oven-dry raw material, and sulfidation degree 20%~30% relative to the oven-dry raw material.

5. The method for preparing high-strength Lyocell regenerated cellulose fiber according to claim 1, characterized in that, The specific process conditions for the first stage of chlorine dioxide bleaching D0 in step (3) are as follows: ClO2 dosage is 3%~5% of the dry pulp dosage, temperature is 60~75℃, time is 15~45min, and pulp concentration is 5~15%; the conditions for alkali treatment E are as follows: NaOH dosage is 0.5%~2% of the dry pulp dosage, temperature is 60~75℃, time is 30~90min, and pulp concentration is 5~15%; the conditions for the second stage of chlorine dioxide bleaching D1 are as follows: ClO2 dosage is 1%~3% of the dry pulp dosage, temperature is 60~75℃, time is 30~90min, and pulp concentration is 5~15%; the chelating agent used in the chelation treatment is disodium ethylenediaminetetraacetate, dosage is 0.5%~2% of the dry pulp dosage, temperature is 60~75℃, time is 30~90min, and pulp concentration is 5~15%.

6. The method for preparing high-strength Lyocell regenerated cellulose fiber according to claim 1, characterized in that, The specific conditions for the first stage of cold alkali extraction in step (4) are: time 30-60 min, pulp concentration 5-15%.

7. The method for preparing high-strength Lyocell regenerated cellulose fiber according to claim 1, characterized in that, The specific conditions for the second stage of cold alkali extraction in step (4) are: time 60-90 min, pulp concentration 5-15%.

8. The method for preparing high-strength Lyocell regenerated cellulose fiber according to claim 1, characterized in that, In step (5), the antioxidant is propyl gallate, and the amount of antioxidant is 0.05-0.1% of the total mass of the high degree of polymerization dissolving slurry and NMMO solution; the mass fraction of NMMO solution is 70-75%; and the mass ratio of high degree of polymerization dissolving slurry to NMMO solution is 1:(15-40).

9. The method for preparing high-strength Lyocell regenerated cellulose fiber according to claim 1, characterized in that, The specific conditions for wet spinning are as follows: the spinning solution is metered by a metering pump under a pressure of 0.2 to 0.5 MPa and then extruded through the micro-holes of the spinneret. It enters a coagulation bath with a concentration of 5 to 20% through an air section of 10 to 40 mm and coagulates in the coagulation bath at a temperature of 10 to 30°C. The draw ratio is 3 to 10. After washing and drying, high-strength Lyocell regenerated cellulose fibers are obtained.

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