A method for esterifying cellulose, and a method for producing cellulose formate yarn

Abstract

The present application belongs to the technical field of cellulose regenerated material, and particularly relates to a cellulose esterification method and a preparation method of cellulose formate filament material. The present application provides a cellulose esterification method, which comprises the following steps: mixing cellulose and an esterification system to perform esterification reaction; the esterification system comprises formic acid, zinc chloride and water, and the amount-of-substance ratio of the formic acid, the zinc chloride and the water is 2-4:1:1-2. The present application utilizes the esterification system with a specific amount-of-substance ratio to homogeneously esterify cellulose macromolecules at room temperature. In the examples, the degree of substitution of cellulose formate is all above 0.6, and thus high-strength cellulose formate filament material can be prepared.

Description

Technical Field

[0001] This invention belongs to the field of cellulose regeneration materials technology, specifically relating to a method for esterification of cellulose and a method for preparing cellulose formate filament materials. Background Technology

[0002] Cellulose is the most abundant natural organic polymer, possessing a highly ordered hierarchical structure, excellent physical and mechanical properties, unique chemical properties, and advantages such as low cost, availability, renewability, complete biodegradability, and excellent biocompatibility. These outstanding advantages make it suitable for preparing sustainable high-performance materials to replace non-degradable petroleum-based products and alleviate the pressure of environmental pollution and excessive energy consumption. However, the abundance of hydroxyl groups in the cellulose molecular chain leads to the formation of dense intramolecular and intermolecular hydrogen bonds, as well as complex aggregate and crystalline structures. This hinders the chemical modification of cellulose molecules to improve their solubility and processing properties, thereby endowing them with new functions, expanding their application areas, and promoting the high-value utilization of cellulose resources.

[0003] Viscose fiber is the mainstream product of regenerated cellulose fiber. It has a low production cost and good softness, breathability, and moisture absorption. However, it has low strength, is easy to break, and has poor heat resistance and antibacterial properties. Moreover, it generates a large amount of toxic and harmful gases and waste liquids during the production process, and has been gradually phased out. Lyocell fiber has high strength and toughness and good durability, but its production cost is high. The NMMO (4-methylmorpholine-N-oxide) solvent used is easily oxidized and poses an explosion hazard during use, which restricts its industrial production.

[0004] The literature “Tong Z, Zeng S, Tang H, et al. A room temperature dissolution solvent and mechanism for natural biopolymers: hydrogen bonding interaction investigation[J]. Green Chemistry, 2023.DOI:10.1039 / d3gc01098h” describes the use of a mixed solvent of zinc chloride, formic acid, and water to construct various recycled materials at room temperature. However, due to the strong acidity of the solvent, the cellulose macromolecular structure was degraded, resulting in low strength of the recycled materials, less than 200 MPa.

[0005] The literature “Zhao Z, Gao H, Zhou L, et al. Preparation of regenerated cellulose fibers by microfluidic spinning technology using ionic liquids as the solvents[J]. Cellulose, 2023, 30(12): 7535-7549. DOI: 10.1007 / s10570-023-05301-w” describes the preparation of regenerated cellulose fibers using microfluidic technology with 1-ethyl-3-methylimidazolium acetate / dimethyl sulfoxide as the solvent. However, the resulting material has low strength, with a maximum tensile strength of only 218.14 MPa.

[0006] Cellulose can undergo esterification when mixed with concentrated formic acid solution (99 wt%). Cellulose can dissolve when the degree of formyl substitution exceeds 1.3, but this process takes a considerable amount of time (over 24 hours) and cannot dissolve high molecular weight cellulose. Increasing the temperature or adding a catalyst (such as sulfuric acid, hydrochloric acid, or phosphoric acid) can accelerate the esterification reaction, but strong acid solutions will cause cellulose degradation, making it difficult to homogeneously esterify high molecular weight cellulose (degree of polymerization exceeding 1000), thus preventing the production of high-strength recycled cellulose materials. Summary of the Invention

[0007] In view of this, the purpose of this invention is to provide a method for esterifying cellulose and a method for preparing cellulose formate filament materials. The cellulose esterification method provided by this invention can homogeneously esterify cellulose, thereby obtaining high-strength cellulose formate filament materials.

[0008] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0009] This invention provides a method for esterifying cellulose, comprising the following steps:

[0010] The cellulose and esterification system are mixed and esterified for esterification reaction;

[0011] The esterification system includes formic acid, zinc chloride, and water, wherein the molar ratio of formic acid, zinc chloride, and water is 2–4:1:1–2.

[0012] Preferably, the molar ratio of formic acid, zinc chloride and water is 3:1:2.

[0013] Preferably, the mass ratio of the cellulose to the esterification system is 7-10:100.

[0014] Preferably, the mass ratio of cellulose to esterification system is 7.6 to 8.7:100.

[0015] Preferably, the esterification reaction is carried out at room temperature for 1 to 12 hours.

[0016] Preferably, the esterification reaction takes 2 to 4 hours.

[0017] This invention also provides a method for preparing cellulose formate filament material, comprising the following steps:

[0018] According to the esterification method of cellulose described in the above technical solution, a reaction solution containing cellulose formate is obtained;

[0019] After the reaction solution is filamentized, it is sequentially coagulated, washed, and dried to obtain the cellulose formate filament material.

[0020] Preferably, the coagulation includes passing the filaments after they have been spun into fibers through a calcium chloride aqueous solution coagulation bath and an ethanol coagulation bath in sequence.

[0021] Preferably, the concentration of the calcium chloride aqueous solution is 20–45 wt%.

[0022] The present invention also provides a cellulose formate filament material obtained by the preparation method described above, wherein the diameter of the cellulose formate filament material is 20-100 micrometers.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] This invention provides a method for esterifying cellulose, comprising the following steps: mixing cellulose and an esterification system for an esterification reaction; the esterification system includes formic acid, zinc chloride, and water, wherein the molar ratio of formic acid, zinc chloride, and water is 2–4:1:1–2. This invention utilizes an esterification system with a specific molar ratio to homogeneously esterify cellulose macromolecules at room temperature. Zinc chloride provides zinc ions, chloride ions, and formic acid provides hydrogen ions to form hydrogen bonds with oxygen or hydrogen atoms of the cellulose hydroxyl groups, thereby disrupting the cellulose hydrogen bond network and releasing free hydroxyl groups. Unionized formic acid molecules and free hydroxyl groups then form cellulose formate through the esterification reaction. Water molecules promote the ionization of zinc chloride and formic acid, regulating their hydrolytic effect on cellulose and preserving the macromolecular structure of cellulose. In the examples, the degree of substitution of cellulose formate is above 0.6, thus enabling the production of high-strength cellulose formate filament materials.

[0025] This invention also provides a method for preparing cellulose formate filament materials. The method involves using a reaction solution containing cellulose formate obtained from cellulose esterification, followed by "dry-jet-wet spinning" and air drawing to promote the orientation of the cellulose formate molecular chains. Calcium ions are used to crosslink and anchor the oriented molecular chains, while ethanol molecules induce a tight bond between the oriented chains, resulting in ultra-high strength cellulose formate filament materials. Example data shows that the tensile strength of the cellulose formate filament material prepared by this invention can reach over 1 GPa. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is a schematic diagram of cellulose esterification using formic acid / zinc chloride / aqueous solution in the examples;

[0028] Figure 2 The XRD patterns of cellulose raw material and regenerated cellulose in Example 4 are shown.

[0029] Figure 3 Infrared images of cellulose raw material and regenerated cellulose in Example 4;

[0030] Figure 4 Example 4: Cellulose raw material and regenerated cellulose 13 C solid-state NMR spectrum;

[0031] Figure 5 This is a schematic diagram of the "dry-jet-wet spinning" process in the embodiment;

[0032] Figure 6 This is a photograph of the actual syringe pump used in the embodiment.

[0033] Figure 7 This is a photograph of an air-drawn wire at room temperature, as shown in the example.

[0034] Figure 8 This is a photograph of the cellulose formate filament material from Example 4.

[0035] Figure 9 The tensile mechanical curve of the cellulose formate filament material in Example 4 is shown. Detailed Implementation

[0036] This invention provides a method for esterifying cellulose, comprising the following steps:

[0037] The cellulose and esterification system are mixed and esterified for esterification reaction;

[0038] The esterification system includes formic acid, zinc chloride, and water, wherein the molar ratio of formic acid, zinc chloride, and water is 2–4:1:1–2.

[0039] Unless otherwise specified, all materials and equipment used in this invention are commercially available products in the field.

[0040] In this invention, the molar ratio of formic acid, zinc chloride, and water is preferably 4:1:1, 2:1:2, or 3:1:2. Zinc chloride provides zinc ions, chloride ions, and formic acid provides hydrogen ions to form hydrogen bonds with the oxygen or hydrogen atoms of the cellulose hydroxyl groups, thereby disrupting the cellulose hydrogen bond network and releasing free hydroxyl groups. The unionized formic acid molecules and the free hydroxyl groups then undergo esterification to form cellulose formate. Water molecules promote the ionization of zinc chloride and formic acid, regulating their hydrolytic effect on cellulose and preserving the macromolecular structure of cellulose.

[0041] In this invention, the esterification system is preferably obtained by mixing formic acid, zinc chloride and water. This invention does not have special requirements for the method of mixing formic acid, zinc chloride and water, as long as they are mixed evenly. In specific embodiments of this invention, such as mixing by continuous stirring at room temperature.

[0042] In this invention, the cellulose is preferably pulp cellulose, the degree of polymerization of the pulp cellulose is preferably greater than 1500, more preferably 1539, and the crystallinity is preferably 30-80%, more preferably 40-60%, and even more preferably 53.58%.

[0043] In this invention, the mass ratio of cellulose to esterification system is preferably 7-10:100, more preferably 7.6-9.9:100, and even more preferably 8.7:100.

[0044] In this invention, the esterification reaction is preferably carried out at room temperature, and the reaction time is preferably 1–12 h, more preferably 2–4 h, and even more preferably 2.5–3 h. The esterification reaction is preferably carried out under stirring conditions. This invention does not have special requirements for the stirring speed; speeds commonly used by those skilled in the art are acceptable.

[0045] In this invention, the reaction equation for the esterification reaction is shown in Formula I. During the esterification reaction, the hydroxyl groups of cellulose react with the carboxyl groups of formic acid to generate cellulose formate.

[0046]

[0047] This invention also provides a method for preparing cellulose formate filament material, comprising the following steps:

[0048] According to the esterification method of cellulose described in the above technical solution, a reaction solution containing cellulose formate is obtained;

[0049] After the reaction solution is filamentized, it is sequentially coagulated, washed, and dried to obtain the cellulose formate filament material.

[0050] The present invention describes a cellulose esterification method according to the above technical solution, in which cellulose and an esterification system are mixed and subjected to an esterification reaction to obtain a reaction solution containing cellulose formate.

[0051] In this invention, after mixing cellulose and the esterification system for esterification reaction, it is preferable to further remove air bubbles from the solution, and the method for removing air bubbles from the solution is preferably centrifugation.

[0052] After obtaining the reaction solution containing cellulose formate, the present invention spins the reaction solution into fibers, and then sequentially coagulates, washes and dries them to obtain the cellulose formate fiber material.

[0053] In this invention, the method for forming filaments from the reaction solution preferably includes "dry-jet-wet spinning" and air drawing. In a specific embodiment of this invention, the reaction solution is loaded into a syringe, and then a solution containing cellulose formate is extruded from the syringe using a syringe pump. The syringe outlet is a certain distance from the surface of the coagulation bath, and the extruded filaments are oriented and stretched in the air due to their own gravity. The diameter of the syringe outlet is preferably 900 micrometers; the height of the syringe outlet from the surface of the coagulation bath is preferably 25 cm. The height specified in this invention will not cause filament breakage, and the resulting filament material has a suitable diameter and good tensile mechanical properties.

[0054] In this invention, the coagulation includes passing the filaments after filamentation through a calcium chloride aqueous solution coagulation bath and an ethanol coagulation bath in sequence to obtain cellulose formate gel fibers. The purpose of the coagulation is to anchor and tightly assemble the oriented cellulose formate molecular chains.

[0055] In this invention, after the filaments are formed, in the calcium chloride aqueous solution coagulation bath, due to the cross-linking effect of calcium ions, the ability of the stretched and oriented filaments to return to a free coiled state is weakened, that is, the disorientation of the long cellulose chains is eliminated, thereby anchoring the oriented cellulose molecular chains; in the ethanol coagulation bath, ethanol molecules cause the oriented cellulose molecular chains to come closer to each other and assemble into tightly packed cellulose formate gel fibers.

[0056] In this invention, the concentration of the calcium chloride aqueous solution is preferably 20-45 wt%, more preferably 40 wt%.

[0057] In this invention, the washing is preferably done with water, and the purpose of the water washing is to remove solvent and impurities.

[0058] In this invention, the drying is preferably carried out at room temperature, and the drying time is preferably 4 hours. The purpose of the drying is to evaporate water molecules.

[0059] The present invention also provides a cellulose formate filament material obtained by the preparation method described above, wherein the diameter of the cellulose formate filament material is 20-100 micrometers.

[0060] In this invention, the diameter of the cellulose formate filament material is preferably 20 to 40 micrometers.

[0061] This invention utilizes "dry-jet-wet spinning" and air drawing technology to achieve highly oriented cellulose molecular chains. Then, calcium ions are used to crosslink and anchor the oriented cellulose molecular chains, while ethanol molecules induce the oriented cellulose molecular chains to bind tightly, ultimately yielding ultra-high strength cellulose formate filament materials (reaching over 1 GPa).

[0062] To further illustrate the present invention, the esterification method of cellulose and the preparation method of cellulose formate filament materials provided by the present invention will be described in detail below with reference to the accompanying drawings and embodiments, but these should not be construed as limiting the scope of protection of the present invention.

[0063] In an embodiment of the present invention, the cellulose is industrial-grade pulp cellulose with a degree of polymerization of 1539 and a crystallinity of 53.58%.

[0064] Example 1

[0065] 7.6 g of pulp cellulose was added to 100 g of formic acid / zinc chloride / water solution, with a molar ratio of formic acid, zinc chloride, and water of 4:1:1. The mixture was stirred at room temperature for 2 hours to obtain a solution containing cellulose formate. After centrifugation to remove air bubbles, the solution was loaded into a syringe and extruded using a syringe pump. The syringe nozzle was 25 cm above the coagulation bath surface, and the extruded filaments were oriented and stretched in the air. The filaments were then passed through two coagulation baths: a 40 wt% calcium chloride aqueous solution and an ethanol bath, forming cellulose formate gel fibers. Finally, the solvent and impurities were washed away with water, and the sample was placed in the air to evaporate the water molecules, yielding cellulose formate filament material (regenerated cellulose) with a diameter of 20–40 micrometers.

[0066] Example 2

[0067] 7.6 g of pulp cellulose was added to 100 g of formic acid / zinc chloride / water solution, with a molar ratio of formic acid, zinc chloride, and water of 2:1:2. The mixture was stirred at room temperature for 2.5 h to obtain a solution containing cellulose formate. After centrifugation to remove air bubbles, the solution was loaded into a syringe and extruded using a syringe pump. The syringe nozzle was 25 cm above the coagulation bath surface, and the extruded filaments were oriented and stretched in the air. The filaments were then passed through two coagulation baths: a 40 wt% calcium chloride aqueous solution and an ethanol bath, forming cellulose formate gel fibers. Finally, the solvent and impurities were washed away with water, and the sample was placed in the air to evaporate the water molecules, yielding cellulose formate filament material (regenerated cellulose) with a diameter of 20–40 micrometers.

[0068] Example 3

[0069] 7.6 g of pulp cellulose was added to 100 g of formic acid / zinc chloride / water solution, with a molar ratio of formic acid, zinc chloride, and water of 3:1:2. The mixture was stirred at room temperature for 2.5 h to obtain a solution containing cellulose formate. After centrifugation to remove air bubbles, the solution was loaded into a syringe and extruded using a syringe pump. The syringe nozzle was 25 cm above the coagulation bath surface, and the extruded filaments were oriented and stretched in the air. The filaments were then passed through two coagulation baths: a 40 wt% calcium chloride aqueous solution and an ethanol bath, forming cellulose formate gel fibers. Finally, the solvent and impurities were washed away with water, and the sample was placed in the air to evaporate the water molecules, yielding cellulose formate filament material (regenerated cellulose) with a diameter of 20–40 micrometers.

[0070] Example 4

[0071] 8.7g of pulp cellulose was added to 100g of formic acid / zinc chloride / water solution, with a molar ratio of formic acid, zinc chloride, and water of 3:1:2. The mixture was stirred at room temperature for 3 hours to obtain a solution containing cellulose formate. After centrifugation to remove air bubbles, the solution was loaded into a syringe. The cellulose formate solution was then extruded from the syringe using a syringe pump, with the syringe nozzle 25cm above the coagulation bath surface. The extruded filaments were oriented and stretched in the air. The filaments were then passed through two coagulation baths: a 40wt% calcium chloride aqueous solution and an ethanol solution, forming cellulose formate gel fibers. Finally, the solvent and impurities were washed away with water, and the sample was placed in the air to evaporate the water molecules, yielding cellulose formate filament material (regenerated cellulose) with a diameter of 20–40 micrometers.

[0072] Example 5

[0073] 9.9g of pulp cellulose was added to 100g of formic acid / zinc chloride / water solution, with a molar ratio of formic acid, zinc chloride, and water of 3:1:2. The mixture was stirred at room temperature for 4 hours to obtain a solution containing cellulose formate. After centrifugation to remove air bubbles, the solution was loaded into a syringe and extruded using a syringe pump. The syringe nozzle was 25cm above the coagulation bath surface, and the extruded filaments were oriented and stretched in the air. The filaments were then passed through two coagulation baths: a 40wt% calcium chloride aqueous solution and an ethanol bath, forming cellulose formate gel fibers. Finally, the solvent and impurities were washed away with water, and the sample was placed in the air to evaporate the water molecules, yielding cellulose formate filament material (regenerated cellulose) with a diameter of 20–40 micrometers.

[0074] Table 1 shows the test results of the cellulose formate filament materials in Examples 1-5:

[0075] Table 1. Test results of cellulose formate filament materials in Examples 1-5

[0076]

[0077] Note: Solubility = cellulose mass / (cellulose mass + solvent mass) × 100%; the degree of polymerization of cellulose was determined by the copper ethylenediamine method, and the degree of substitution (DS) of cellulose formate was determined by the back-titration method. Tensile strength was measured using a micrometer to measure the diameter of the filament. The specimen was subjected to a constant tensile load of 10 mm / min using an A-700S universal testing machine until fracture, and the stress-strain curve was obtained.

[0078] Figure 1 The diagram illustrates the esterification of cellulose with formic acid / zinc chloride / aqueous solution in the example. Cellulose undergoes an esterification reaction in the esterification system. Zinc ions, chloride ions, and hydrogen ions in the esterification system first break the hydrogen bonds of cellulose, causing cellulose to dissolve into a solution and release free hydroxyl groups. Then, the free hydroxyl groups and unionized formic acid molecules undergo an esterification reaction to obtain a reaction solution containing cellulose formate, which is a viscous, flowable liquid. Subsequently, it is fed into a syringe via a casting method for spinning.

[0079] Figure 2 The XRD patterns of cellulose raw material and regenerated cellulose in Example 4 are shown. The diffraction peaks of the cellulose raw material are at 2θ = 15.2° and 22.5°, which are both natural cellulose type I. The diffraction peak of the chemically modified cellulose is at 2θ = 20.3°, which is cellulose type II. The change in the cellulose crystal structure indicates that the modification of cellulose is a homogeneous dissolution and esterification process.

[0080] Figure 3 Infrared images of cellulose raw material and regenerated cellulose in Example 4; Figure 4Example 4: Cellulose raw material and regenerated cellulose 13 C10 solid-state NMR spectrum; Infrared image shows regenerated cellulose at 1710 cm⁻¹ -1 The appearance of a new peak at 163 ppm in the solid-state NMR spectrum and the appearance of a new peak at 163 ppm in the regenerated cellulose both prove that cellulose has undergone esterification to obtain cellulose formate.

[0081] Figure 5 This is a schematic diagram of the "dry-jet-wet spinning" process in the embodiment; Figure 6 This is a photograph of the actual syringe pump used in the embodiment. Figure 7 The image shown is a photograph of an air-drawn filament at room temperature in the example. At room temperature, the orientation of cellulose formate molecular chains is promoted by "dry-jet-wet spinning" and air drawing. The orientation of the molecular chains is anchored by calcium ions and induced by ethanol molecules. The molecular chains are tightly bound together. After washing and drying, ultra-high strength cellulose formate filament material is finally prepared.

[0082] Figure 8 Here is a photograph of the cellulose formate filament material from Example 4; Figure 9 The tensile mechanical properties of the cellulose formate filament material in Example 4 are shown. The tensile strength of the cellulose formate filament material reached 1024 MPa, which is superior to that of regenerated cellulose filament materials prepared by existing methods.

[0083] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, not all embodiments. People can obtain other embodiments based on the present invention without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A process for the preparation of a cellulose formate filament material, characterized in that, Includes the following steps: The esterification reaction is carried out by mixing cellulose and the esterification system to obtain a reaction solution containing cellulose formate. After the reaction solution is filamentized, it is sequentially coagulated, washed and dried to obtain the cellulose formate filament material; The esterification system consists of formic acid, zinc chloride, and water, with a molar ratio of formic acid, zinc chloride, and water of 3:1:

2. The mass ratio of cellulose to esterification system is 8.7:100; the cellulose is pulp cellulose, and the degree of polymerization of the pulp cellulose is greater than 1500. The esterification reaction was carried out at room temperature for 3 hours. The coagulation process includes passing the filaments after they have been spun into fibers through a calcium chloride aqueous solution coagulation bath and an ethanol coagulation bath in sequence. The concentration of the calcium chloride aqueous solution is 40 wt%.

2. A cellulose nitrate filament material obtained by the production method according to claim 1, characterized in that, The diameter of the cellulose formate filament material is 20-100 micrometers.

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