Preparation method of high-strength regenerated cellulose material based on DMAc / LiCl system

Abstract

The invention discloses a preparation method of a high-strength regenerated cellulose material based on a DMAc / LiCl system, and belongs to the technical field of biodegradable high polymer materials. The preparation method comprises the following steps: activating a cellulose raw material with water / DMAc double solvents, and dissolving the activated cellulose raw material in a LiCl / DMAc system to form a solution with the concentration of 5-15wt%; the mechanical property of the material is improved through high-temperature gel treatment, and finally the regenerated cellulose material is obtained through water-phase regeneration treatment. The high-modulus fiber is successfully prepared, the tensile strength of the cellulose fiber regenerated through high-temperature gel treatment reaches 2.2 cN / dtex, and the tensile modulus reaches 139 cN / dtex. And compared with the fiber which is not subjected to gel treatment, the tensile strength is improved by 43%, and the tensile modulus is improved by 37%. The method disclosed by the invention is simple and controllable in process and environment-friendly, conforms to the development trend of green chemistry, and has a wide industrial application prospect.

Description

Technical Field

[0001] The present invention belongs to the technical field of biodegradable polymer materials, and particularly relates to a method for preparing a high-strength regenerated cellulose material based on a DMAc / LiCl system. Background Art

[0002] With the excessive consumption of fossil resources (coal, oil, and natural gas account for more than 80% of the global energy material supply), the sharp decline in the reserves of non-renewable resources and the problem of non-degradable polymer pollution have become the core contradictions restricting sustainable development. As a renewable carbon source with an annual output of over a hundred billion tons (accounting for 40-60% of the total biomass), cellulose's biodegradability, functional group modifiability, and structural diversity provide a strategic solution for replacing petroleum-based materials. Since Anselme Payen determined the molecular formula of cellulose (C6H 10 O5) in 1838, its applications have gradually expanded from traditional textiles and building materials to the field of chemical modification: the industrial production of nitrocellulose (celluloid) in 1870 first verified the potential of cellulose-derived materials. However, the multi-level structure of cellulose (hydrogen bond network in the crystalline region - amorphous region) makes it difficult to melt process and has extremely poor solubility in conventional solvents (the solubility is generally <5 wt%), severely limiting the development of high-value-added products.

[0003] Currently, there is a very challenging problem with cellulose: many important applications of cellulose involve its dissolution. This biological macromolecule has a complex structure, and the partially crystalline structure and intermolecular non-crystalline structure interact through hydrogen bonds, making the chemical processing of cellulose quite difficult. Cellulose neither melts nor dissolves in common aqueous solutions and organic solvents, which severely restricts and hinders the industrial application and development of cellulose.

[0004] The forming process of cellulose materials not only involves the dissolution of cellulose but also the gelation and regeneration of cellulose solutions. Therefore, studying the mechanisms of cellulose dissolution, gelation, and regeneration is also of great significance for the structure and property regulation of cellulose products. Summary of the Invention

[0005] Aiming at the problems in the forming process of current cellulose materials, through process optimization, the present invention provides a method for preparing a high-performance regenerated cellulose material based on a LiCl / DMAc solvent system.

[0006] To solve the above technical problems, the present invention adopts the following technical solutions:

[0007] A method for preparing a high-strength regenerated cellulose material based on a DMAc / LiCl system, comprising the following steps:

[0008] Step 1: After activating the cellulose raw material with a water / DMAc double solvent, dissolve it in the LiCl / DMAc system to form a solution with a concentration of 5-15 wt%.

[0009] Step 2: High-temperature gel treatment of the cellulose solution: Subject the dissolved cellulose solution to high-temperature gel treatment at 50-70 °C to induce an irreversible sol-gel transition and form a three-dimensional network structure.

[0010] Step 3: Preparation of the regenerated material: Immerse the gelled solution in an aqueous coagulation bath, and obtain the regenerated cellulose material through stretching and drying.

[0011] Furthermore, the temperature for high-temperature gel treatment in Step 2 is 60 °C.

[0012] Furthermore, the time for high-temperature gel treatment in Step 2 is 3-7 days.

[0013] A high-strength regenerated cellulose material based on the DMAc / LiCl system, the tensile strength of the regenerated cellulose material is 2.2 cN / dtex, and the tensile modulus reaches 139 cN / dtex.

[0014] Compared with the prior art, the present invention has the following advantages:

[0015] 1) The present invention reveals the influence of the activation of MCC on the cellulose structure. The activation of MCC will significantly change the physical structure of cellulose. During the activation process of MCC, the hydrogen bond strength between cellulose molecules in the amorphous part of cellulose will be weakened, but it cannot act on the crystalline part of cellulose. After being activated by DMAc, cellulose will form nanopores. Especially with the prolongation of the activation time, the fibers on the surface of cellulose will peel off.

[0016] 2) The present invention reveals the relationship between the activation of MCC and cellulose in LiCl / DMAc. Cellulose dissolves in LiCl / DMAc, and the last step requires activation by DMAc. The presence of DMAc is beneficial to the dissolution of cellulose. Moreover, the higher the activation temperature, the faster the cellulose dissolution rate. When cellulose activated by DMAc forms nanopores, this is conducive to the infiltration of the LiCl / DMAc solvent. During the dissolution process, Cl - ions combine with the hydroxyl groups of cellulose to form hydrogen bonds. The cellulose molecules carry negative charges, and at the same time, the entire cellulose solution is ionized, which is conducive to the dissolution of cellulose. For cellulose without being activated by DMAc, during the dissolution process, LiCl / DMAc can only stay on the surface of cellulose and cannot penetrate into the interior of cellulose, only causing the dissolution of the cellulose surface. Without activated MCC, only 19% can be dissolved, and even after cellulose is activated by water for ten days, only about 35% can be dissolved.

[0017] 3) The present invention reveals the role of temperature in the dissolution and gelation of cellulose / LiCl / DMAc. The research results show that temperature plays an important role in the dissolution and gelation processes of fiber / LiCl / DMAc. Cellulose dissolves more thoroughly and has a higher solubility when dissolved in LiCl / DMAc at low temperatures. At 25 °C, only 8 wt% of cellulose can be completely dissolved. At 5 °C, 11 wt% of cellulose can be completely dissolved, and at -15 °C, 12 wt% of cellulose can be completely dissolved. If cellulose is dissolved at -5 °C, it needs to be pre-treated at 110 °C for 1 h. The cellulose undergoes a gel transition at 60 °C. The results of WAXD show that when the cellulose / LiCl / DMAc solution is placed at 60 °C for a long time, an ordered structure will form in the solution. When the placement time is extended to 7 days, a more regular ordered structure will form. For the cellulose solution treated by gelation, it is regenerated, freeze-dried, and then the morphology is observed by electron microscopy. The results show that the cellulose solution forms a three-dimensional network structure after high-temperature gelation, undergoes a sharp contraction during the regeneration and freeze-drying process, and has a denser structure. While for the solution without gelation treatment, only a slight contraction occurs during the regeneration and freeze-drying process, and it has a loose structure.

[0018] 4) The present invention successfully prepared fibers with high modulus: The tensile strength of the regenerated cellulose fibers treated by high-temperature gelation reaches 2.2 cN / dtex, and the tensile modulus reaches 139 cN / dtex. Compared with the fibers without gelation treatment, the tensile strength is increased by 43%, and the tensile modulus is increased by 37%. The compressive strength of the hydrogel regenerated by high-temperature gelation treatment reaches 2.1 MPa, and the compressive modulus reaches 2.9 MPa. Compared with the non-gelled hydrogel, the compressive modulus is increased by 4.5 times, and the compressive strength is increased by nearly 2.3 times. The present invention also verifies the theoretical possibility of improving the fiber modulus by the high-temperature gelation method. Brief Description of the Drawings

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0020] Figure 1 Graph of the elastic modulus and viscous modulus of cellulose / LiCl / DMAc solution varying with temperature;

[0021] Figure 2 Graph of the elastic modulus and viscous modulus of cellulose / LiCl / DMAc solution treated with different gelation times varying with frequency;

[0022] Figure 3WAXD intensity curves of cellulose / LiCl / DMAc treated with different gel times;

[0023] Figure 4 SEM images of aerogels prepared by regeneration of cellulose / LiCl / DMAc without gelation (A) and with high-temperature gelation (B); (1) surface, (2) cross-section, (3) fracture;

[0024] Figure 5 Tensile stress-strain curves of regenerated cellulose fibers with different gel times;

[0025] Figure 6 Schematic diagrams of tensile strength and tensile modulus of mechanical properties of regenerated cellulose fibers regenerated with different solvents;

[0026] Figure 7 Tensile cross-sections (top) and surface morphologies (bottom) of regenerated cellulose fibers with different high-temperature gel times;

[0027] Figure 8 Cross-sectional view of regenerated cellulose fiber;

[0028] Figure 9 WAXD diffraction patterns of regenerated cellulose fibers treated with high-temperature gelation for (A) 0 d, (B) 3 d, and (C) 7 d, (D) azimuthal scan data, and (E) radial scan data of the (110) peak. Detailed implementation manners

[0029] To deeply understand the present invention, we will describe it comprehensively and meticulously. However, the present invention has various implementation manners and is not limited to the specific examples listed herein. The presentation of these examples aims to deepen the comprehensive understanding of the disclosed content of the present invention.

[0030] A method for preparing a high-strength regenerated cellulose material based on the DMAc / LiCl system, comprising the following steps:

[0031] Step 1: Activate the cellulose raw material with a water / DMAc double solvent and dissolve it in the LiCl / DMAc system to form a solution with a concentration of 5-15 wt%.

[0032] Step 2: High-temperature gel treatment of the cellulose solution: Subject the dissolved cellulose solution to high-temperature gel treatment at 50-70 °C to induce an irreversible sol-gel transition and form a three-dimensional network structure;

[0033] Step 3: Preparation of the regenerated material: Immerse the gelled solution in an aqueous coagulation bath, and obtain the regenerated cellulose material through stretching and drying.

[0034] Furthermore, the dissolution concentration of the cellulose raw material in Step 1 is 8-12 wt%.

[0035] Furthermore, the temperature for the high-temperature gel treatment in Step 3 is 60°C.

[0036] Furthermore, the time for the high-temperature gel treatment in Step 3 is 3 to 7 days.

[0037] A high-strength regenerated cellulose material based on the DMAc / LiCl system, wherein the tensile strength of the regenerated cellulose material is 2.2 cN / dtex and the tensile modulus reaches 139 cN / dtex.

[0038] Example 1

[0039] A preparation method of a high-strength regenerated cellulose material based on the DMAc / LiCl system, comprising the following steps:

[0040] Step 1, 40 g of microcrystalline cellulose MCC is activated by water / DMAc, dissolved in 8% LiCl / DMAc, magnetically stirred at 110°C for 50 minutes, and stirred at 5°C for 24 hours to form an optically homogeneous solution

[0041] Step 2, high-temperature gel: The dissolved cellulose solution is treated at 60°C for 7 days to induce gel formation.

[0042] Step 3, spinning and testing: Using the dry-jet wet spinning technique, the gelled cellulose solution is spun into fibers. Through tensile testing, the tensile modulus of the fibers is measured to be 139 cN / dtex and the tensile strength is 2.2 cN / dtex.

[0043] Experimental tests

[0044] I. Influence of temperature on cellulose solution gelation

[0045] A 10 wt% cellulose / LiCl / DMAc solution is placed in an oven at 60°C for 3 d and 7 d. Then rheological tests are performed on the solution. When the viscous modulus of the cellulose solution is greater than the elastic modulus, it exhibits viscous behavior and is called the sol state; when the elastic modulus of the cellulose is greater than the viscous modulus, it exhibits elastic behavior and is called the gel state. Figure 1 This is the temperature scan of the cellulose solution. As can be seen from the figure, when the solution is slowly heated from 25°C to 80°C, the rheological behavior of the cellulose solution changes from viscous behavior to elastic behavior. Then, when the solution is cooled from 80°C to 25°C, the cellulose solution still has elastic characteristics and the viscosity coefficient further increases, indicating that irreversible gelation behavior occurs after the cellulose is treated at high temperature.

[0046] Frequency scans are performed on solutions treated at high temperature for different times, as Figure 2As shown, before the high-temperature treatment, the loss coefficient of the solution in the low-frequency region was always greater than 1, indicating that the solution was always in a sol state. After treatment for 3 days and 7 days, the loss coefficient of the solution in the low-frequency region was greater than 1, indicating that the solution was always in a gel state. This further confirmed the irreversible sol-gel transition of the fiber after high temperature. At the same time, we also observed that after the cellulose solution was treated for 3 days, the elastic modulus fluctuated to some extent at low temperature, and when the treatment time increased to 7 days, the fluctuation became more obvious, which was due to the decrease in the uniformity of the solution after the cellulose solution gelled.

[0047] The solutions with different high-temperature gel times were characterized by WAXD. As Figure 3 shown, no diffraction peaks were observed in the cellulose solution in the q value range of 1.5 to 2.4 nm before treatment, while a diffraction peak was found at 1.6 nm after the cellulose solution was treated for 3 days. This indicated that after 3 days of high-temperature gelation, an ordered structure appeared in the solution. When the high-temperature treatment of the cellulose solution reached 7 days, we observed diffraction peaks at 1.6 nm -1 and 2.3 nm -1 in the WAXD. This showed that after longer gel treatment, there were more ordered structures in the cellulose solution. The gelation of the cellulose solution after high-temperature treatment was because the cellulose molecular chains stacked to form physical cross-linking points. -1 and 2.3 nm -1 in the WAXD. This showed that after longer gel treatment, there were more ordered structures in the cellulose solution. The gelation of the cellulose solution after high-temperature treatment was because the cellulose molecular chains stacked to form physical cross-linking points.

[0048] To further observe the changes after the gelation of the cellulose solution, the cellulose solution was regenerated with water. To preserve its structural integrity, water was then replaced with tert-butanol. Then it was freeze-dried into a sample, and its morphological structure was observed by field emission scanning electron microscopy. The structural morphology is as Figure 4As shown, the morphologies of the samples regenerated from the cellulose solution before and after high-temperature gelation are very different. The cellulose sample without gel regeneration contains a large amount of three-dimensional network structure. Moreover, various textures can be observed on the surface of the sample. In the cross-section of the cellulose sample, a layered structure formed by stacked cellulose can be observed. For the cellulose sample regenerated by high-temperature gelation, both the surface and the cross-section are dense structures without a three-dimensional network structure. That is to say, the cellulose sample after gelation is a dense solid structure. Moreover, by comparing the sizes of the aerocellulose sample before and after drying, it can be found that the cellulose sample treated by high-temperature gelation undergoes severe shrinkage, while the cellulose sample without high-temperature gelation only undergoes slight shrinkage. Crystallization occurs inside the gelled cellulose solution, and overall shrinkage occurs during regeneration and drying. Even freeze-drying cannot prevent its shrinkage behavior. For the cellulose sample without high-temperature gelation, since there are no physical cross-linking points between cellulose molecules, during regeneration and drying, adjacent cellulose molecules freely aggregate into cellulose fibers, and then during freeze-drying, the internal structural characteristics of the cellulose are preserved. Moreover, the structure of the cellulose sample without gel treatment is more like that formed by stacked cellulose, rather than a three-dimensional network structure in the traditional sense.

[0049] II. Mechanical Properties of Regenerated Cellulose Fibers

[0050] Figure 5 Figure shows the tensile stress-strain curves of regenerated cellulose fibers treated by high-temperature gelation for different times. As shown, after the cellulose is treated by high-temperature gelation, the tensile strength and tensile modulus of the cellulose are greatly improved, and the elongation at break decreases. The specific numerical values of the mechanical properties are summarized in Table 1. It can be seen from the table that the tensile strength of the regenerated cellulose without high-temperature gelation treatment is 1.52 ± 0.08 cN / dtex, the tensile modulus is 101.0 ± 5.4 cN / dtex, and the elongation at break is 7.0 ± 0.9%. After three days of high-temperature gelation treatment, the tensile strength of the regenerated cellulose increases to 1.88 ± 0.09 cN / dtex, the tensile modulus increases to 128.1 ± 6.3 cN / dtex, while the elongation at break decreases to 6.1 ± 0.9%. As the time of high-temperature gelation increases to 7 days, the tensile strength of the regenerated cellulose reaches 2.20 ± 0.08 cN / dtex, which is 42% higher than that of the fiber without high-temperature gelation treatment, and the tensile modulus reaches 139.9 ± 4.2, an increase of 37%. The elongation at break decreases to 5.6 ± 1.1%. The current position of the mechanical properties of our fibers is as Figure 6 shown. The tensile modulus of the fiber prepared by high-temperature gelation regeneration has approached that of lyocell fiber. Since low-molecular-weight cellulose is used, the fiber strength is only 2.2 cN / dtex, but it also reaches the strength of viscose fiber.

[0051] Table 1

[0052]

[0053] III. Surface and tensile fracture morphology of regenerated cellulose

[0054] Figure 7 The tensile fracture (upper) and surface (lower) morphology diagrams of cellulose fibers regenerated with different high-temperature gel days are shown. As can be seen from the figure, both the surface and tensile fracture of the regenerated cellulose fibers are independent of the high-temperature gel time. All the regenerated fibers have a smooth surface without a groove structure similar to that of viscose fibers. Moreover, no obvious necking phenomenon can be seen in the cross-sections of all the regenerated cellulose fibers, nor a serrated cross-section similar to that of viscose cellulose. The cross-section of the regenerated cellulose fibers was observed by optical microscope, as Figure 8 shown, which is basically a circular structure.

[0055] IV. Crystalline structure of regenerated cellulose fibers

[0056] Figure 9 The two-dimensional diffraction pattern of the regenerated cellulose fibers is shown. The black arrows point to the positions of each diffraction peak. Figure 9 (E) is the azimuthal scan data of the regenerated fiber cellulose. It can be seen that the regenerated cellulose is a typical cellulose II structure. 20.6° and 21.6° correspond to the (110) and (020) crystal planes of cellulose respectively. 12.7° is the (-110) crystal plane of the cellulose II structure. After calculation, the crystallinities obtained are 55%, 60%, and 60% respectively. This data is slightly lower than that of Sameer (65% - 67%). That is to say, the crystallinity of the cellulose fibers regenerated after the high-temperature gel treatment of the cellulose solution has increased slightly, which is within the error range, that is, the high-temperature gel does not significantly improve the crystallinity of the fibers.

[0057] The orientation degree of the fibers was characterized, and the results are shown in Table 1. The orientation degree of the cellulose fibers regenerated after the high-temperature gel treatment of the cellulose solution increased from 0.70 at 0 days to 0.7 (gel 3d) and 0.73 (7d). Since the crystallinity of the fibers did not change significantly, it indicates that the increase in the orientation degree of the fibers led to a significant increase in the modulus of the fibers.

[0058] It can be inferred from the SEM images and solution WAXD data of the previously prepared cellulose samples that a three-dimensional network structure is formed after the gel treatment of the cellulose solution. During the extrusion and stretching process, the spinning solution is arranged regularly, and finally fibers with a higher orientation degree are formed.

[0059] V. Influence of high-temperature gel on the regeneration of cellulose solution

[0060] The cellulose solution is regenerated to prepare a hydrogel, and then the mechanical properties are measured. It is found that the cellulose hydrogel treated by high-temperature gelation has higher compressive modulus and compressive strength. However, the mechanical properties of the hydrogels after 3 days and 7 days do not change much. The cellulose prepared by high-temperature gelation treatment of the cellulose solution not only has a more regular shape but also excellent mechanical properties. After the hydrogel regenerated by high-temperature gelation treatment is compared with that without high-temperature gelation treatment, the compressive strength is increased from 0.38 Mpa to 2.1 MPa, an increase of 450%. The compressive modulus is increased from about 0.89 MPa to about 2.9 MPa, an increase of 230%. This is because during the replacement process of the cellulose solution with a three-dimensional network structure, the internal structure is more uniform, and at the same time, the formed hydrogels are cross-linked with each other. While for the hydrogel without a three-dimensional network structure, a concentration difference will be formed during the replacement process, then defects will be formed, and at the same time, the three-dimensional network cross-linking points will be greatly reduced, which will greatly reduce the compressive modulus and compressive strength of the hydrogel.

[0061] In summary, 1. High temperature will cause irreversible gelation of the cellulose / LiCl / DMAc solution. Long-term high-temperature treatment will cause crystallization inside the cellulose solution, form a three-dimensional network structure, and at the same time cause the inhomogeneity of the cellulose solution. 2. After the cellulose / LiCl / DMAc solution is regenerated and dried after gelation treatment, it will shrink sharply and obtain a denser structure. The solution without gelation treatment will shrink slightly during regeneration and drying and obtain a structure with certain voids. 3. The regenerated cellulose fiber prepared by the high-temperature gel method has a very high modulus. Compared with the fiber without high-temperature gelation, its tensile modulus is increased by 37% and its tensile strength is increased by 43%. The hydrogel prepared by the high-temperature gel method has very good mechanical properties. The strength reaches 2.1 MPa and the modulus is also close to 2.9 MPa. Compared with the non-gelled hydrogel, the compressive strength is increased by 4.5 times and the compressive modulus is increased by 2.3 times. The regenerated cellulose fiber prepared by high-temperature gelation has a higher degree of orientation, which is increased by 0.03 compared with that without gelation. The regenerated cellulose fiber has a smooth surface and a cross-section close to a circle.

[0062] The content not described in detail in the specification of the present invention belongs to the prior art well-known to those skilled in the art. Although the illustrative specific embodiments of the present invention are described above for the understanding of those skilled in the art of the present technology, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those ordinary skilled in the art of the present technology, as long as various changes are within the spirit and scope of the present invention defined and determined by the appended claims, these changes are obvious, and all inventions and creations using the concept of the present invention are within the scope of protection.

Claims

1. A method for preparing a high-strength regenerated cellulose material based on the DMAc / LiCl system, characterized in that: It includes the following steps: Step 1: Activate the cellulose raw material with a water / DMAc double solvent and then dissolve it in a LiCl / DMAc system to form a solution with a concentration of 5-15 wt%. Step 2: High-temperature gel treatment of the cellulose solution: Subject the dissolved cellulose solution to high-temperature gel treatment at 50-70 °C to induce an irreversible sol-gel transition and form a three-dimensional network structure. Step 3: Preparation of the regenerated material: Immerse the gelled solution in an aqueous coagulation bath and obtain the regenerated cellulose material through stretching and drying.

2. The preparation method of a high-strength regenerated cellulose material based on the DMAc / LiCl system according to claim 1, characterized in that: The temperature for the high-temperature gel treatment in Step 2 is 60 °C.

3. The preparation method of a high-strength regenerated cellulose material based on the DMAc / LiCl system according to claim 1, characterized in that: The time for the high-temperature gel treatment in Step 2 is 3-7 days.

4. A high-strength regenerated cellulose material based on the DMAc / LiCl system prepared by the preparation method according to any one of claims 1 to 3, characterized in that: The tensile strength of the regenerated cellulose material is 2.2 cN / dtex, and the tensile modulus reaches 139 cN / dtex.

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