Cellulose carbamate and preparation method thereof, regenerated cellulose membrane and preparation method thereof

Abstract

The invention belongs to the technical field of cellulose carbamate and regenerated cellulose membrane preparation, and provides cellulose carbamate and a preparation method thereof, and a regenerated cellulose membrane and a preparation method thereof. The preparation method comprises the following steps: soaking cellulose pulp in an aqueous solution containing urea and acid, and sequentially dehydrating, crushing and drying to obtain urea / acid-cellulose pulp; and esterifying the urea / acid-cellulose pulp to obtain the urea / acid-cellulose composite material. By adding the acid, the molecular weight of the cellulose carbamate is regulated and controlled, and the dissolving effect of the cellulose carbamate in the low-temperature alkali / urea is greatly improved. An inert organic carrier and cosolvents such as methanol and ethanol are not needed, so that the preparation process is simple, environment-friendly, safe and controllable, and industrial implementation is facilitated. Meanwhile, compared with esterification of a liquid phase system in the prior art, the solid urea / acid-cellulose pulp is esterified, so that the heat consumption is greatly reduced, and the industrial production cost is greatly reduced.

Description

Technical Field

[0001] This invention relates to the field of cellulose carbamate and regenerated cellulose membrane preparation technology, and particularly to a cellulose carbamate and its preparation method, and a regenerated cellulose membrane and its preparation method. Background Technology

[0002] Cellulose is a widely distributed natural polymer on Earth. Due to its wide availability, large annual yield, non-toxicity, renewability, and environmental friendliness, it is a potentially huge alternative to fossil resources. More importantly, cellulose derivatives have been widely used in medicine, food, construction, chemicals, and environmental protection.

[0003] The elemental composition of cellulose is: C=44.44%, H=6.17%, O=49.39%, and its chemical structural formula is (C6H). 10 O5) n Where n is the degree of polymerization. The chemical structure of cellulose consists of D-glucopyranose rings linked together by β-1,4-glycosidic bonds. Its molecules contain a large number of hydroxyl groups, resulting in strong hydrogen bonding between and within molecules. At the same time, the complex aggregate structure of cellulose and its high crystallinity make it difficult to dissolve in common organic and inorganic solvents and it cannot be melt-processed, which seriously limits the practical application of cellulose.

[0004] However, cellulose derivatization can improve its solubility and endow it with new functions and applications. Commonly used cellulose derivatization methods include the viscose process and the cellulose carbamate (CC) process. The traditional viscose process improves the solubility of cellulose by derivatizing it into cellulose sulfonates. However, this process uses and generates CS2 and H2S, both of which are toxic substances, and improper handling can cause harm to the environment and workers' health. Therefore, people have begun to look for greener and more environmentally friendly systems to replace the viscose process, and the cellulose carbamate (CC) process is recognized as one of the most promising technologies to replace the viscose process. As early as 1938, Hill and Jacobsen reported a nitrogen-containing cellulose derivative—cellulose carbamate, abbreviated as CC, which is a product of the reaction between cellulose and urea. Cellulose carbamate can be dissolved in an alkali-urea system, thereby regenerating a regenerated cellulose membrane.

[0005] In the reaction for preparing cellulose carbamate, urea undergoes thermal decomposition to produce isocyanic acid (HNCO) and ammonia. Isocyanic acid, as an intermediate in urea decomposition, reacts with cellulose. Isocyanic acid also reacts with ammonia. The main and side reactions are as follows: Main reaction: Side reactions: Patent CN102898531A discloses a method for preparing cellulose carbamates via a gas-solid phase reaction. It uses cellulose pulp or activated cellulose pulp as raw material, and synthesizes cellulose carbamates by esterification with isocyanate gas under inert gases such as carbon dioxide or nitrogen and under specific temperature and pressure conditions. The synthesized cellulose carbamates can dissolve in NaOH solution to form a stable solution, which can be used for the preparation of regenerated cellulose fibers or membranes. Although this method has advantages in terms of environmental friendliness and process simplification, significant challenges remain in terms of reaction efficiency, equipment requirements, product uniformity, and industrial scale-up.

[0006] Patent CN104497151A discloses a method for modifying cellulose with urea without byproducts. This method prepares a cellulose / urea mixture by impregnating cellulose with a urea solution or by spraying cellulose with molten urea. The cellulose / urea mixture is then heated at 140-200°C to obtain cellulose carbamate. This method uses a small amount of urea, has no side reactions or cross-linking reactions during the reaction, does not require washing of the reaction product, and features simple synthesis steps, low cost, and is economical and environmentally friendly. However, the cellulose carbamate obtained by this method has a single molecular weight.

[0007] Patent CN107325193A discloses a method for synthesizing cellulose carbamate. This method involves pulverizing, ball-milling, pre-reacting, irradiating, and drying the cellulose raw material to obtain the cellulose carbamate. However, this method is complex, some steps are energy-intensive, and the reaction is difficult to control and reproduce.

[0008] How to prepare cellulose carbamate with good solubility in low-temperature alkali / urea through a simple and economical preparation method has been a subject of ongoing research by researchers in this field. Summary of the Invention

[0009] In view of this, the purpose of this invention is to provide a cellulose carbamate and its preparation method, as well as a regenerated cellulose membrane and its preparation method. The preparation method of this invention greatly improves the solubility of cellulose carbamate in low-temperature alkali / urea.

[0010] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for preparing cellulose carbamate, comprising the following steps: Cellulose pulp is soaked in an aqueous solution containing urea and acid, and then dehydrated, pulverized and dried in sequence to obtain urea / acid-cellulose pulp; The urea / acid-cellulose slurry is esterified to obtain the cellulose carbamate.

[0011] Preferably, the cellulose pulp contains cellulose with a mass content of 90% or more; the cellulose pulp includes one or more of cotton pulp, wood pulp, bagasse pulp, straw pulp, reed pulp, hemp pulp, and bamboo pulp.

[0012] Preferably, the aqueous solution containing urea and acid has a urea concentration of 2-5% by mass and an acid concentration of 0.1-1% by mass. The acid is one or more of organic and inorganic acids, the inorganic acid including one or more of sulfuric acid, hydrochloric acid and phosphoric acid; the organic acid includes p-toluenesulfonic acid.

[0013] Preferably, the mass ratio of the cellulose pulp to the aqueous solution containing urea and acid is 1:10~20; The drying temperature is 40~70℃.

[0014] Preferably, the esterification temperature is 135~175℃, the heating rate from room temperature to the esterification temperature is 3~10℃ / min, and the time is 20~80min.

[0015] The present invention also provides cellulose carbamates prepared by the preparation method described in the above technical solution.

[0016] The present invention also provides a regenerated cellulose membrane, which is prepared from the cellulose carbamate described in the above technical solution.

[0017] The present invention also provides a method for preparing the regenerated cellulose membrane described in the above technical solution, comprising the following steps: After the cellulose carbamate and urea solution described in the above technical solution are first mixed, sodium hydroxide solution is then added for a second mixing to obtain a cellulose carbamate solution. The cellulose carbamate solution is used to lay a film, and the resulting wet film is solidified in a coagulation bath to obtain a regenerated cellulose hydrogel; the regenerated cellulose hydrogel is then surface-dried to obtain the regenerated cellulose membrane. The coagulation bath is an aqueous solution containing sulfuric acid and sodium sulfate.

[0018] Preferably, the cellulose carbamate solution contains 6-9% cellulose carbamate by mass.

[0019] Preferably, in the aqueous solution containing sulfuric acid and sodium sulfate, the mass concentration of sulfuric acid is 5-15 wt%, and the mass concentration of sodium sulfate is 5-20%.

[0020] This invention provides a method for preparing cellulose carbamate.

[0021] The preparation method of this invention achieves controllable molecular weight preparation of cellulose carbamates by adding acid, and simultaneously greatly improves the solubility of the prepared cellulose carbamates in low-temperature alkali / urea. The preparation method of this invention does not require an inert organic carrier or co-solvents such as methanol or ethanol, making the preparation process simple, environmentally friendly, safe, and controllable, facilitating industrialization. Furthermore, compared to the esterification of liquid-phase systems in existing technologies, this invention esterifies solid urea / acid-cellulose slurry, significantly reducing heat consumption and greatly lowering industrial production costs.

[0022] Furthermore, in the aqueous solution containing urea and acid, the mass concentration of urea is 2-5%, and the mass concentration of acid is 0.1-1%; the acid is one or more of organic and inorganic acids, the inorganic acid including one or more of sulfuric acid, hydrochloric acid, and phosphoric acid; the organic acid includes p-toluenesulfonic acid. The low concentration of acid added in this invention avoids the carbonization of cellulose pulp by high-concentration acid; simultaneously, the low-concentration acid can fully contact the cellulose pulp and urea, efficiently catalyzing the subsequent esterification of urea and cellulose pulp.

[0023] This invention also provides a regenerated cellulose membrane, prepared from the cellulose carbamate described in the above-described technical solution. The regenerated cellulose membrane provided by this invention exhibits excellent mechanical properties. Attached Figure Description

[0024] Figure 1 The molecular weight diagram of CC prepared by different acid concentrations in Comparative Example 1 and Examples 1-4; Figure 2 The images show the dissolution light microscopy images of CCs prepared with different acid concentrations in Comparative Example 1 and Examples 1-4. Figure 3 FT-IR spectra of cellulose pulp and CC prepared with different acid concentrations in Comparative Example 1 and Examples 1-4; Figure 4 XRD patterns of cellulose pulp and CC prepared by different acid concentrations in Comparative Example 1 and Examples 1-4; Figure 5 These are dissolution micrographs of CCs prepared by esterification at different temperatures and times in Examples 5-24; Figure 6 The mechanical properties of the regenerated cellulose hydrogels obtained in Examples 25 and 27 are shown in the diagram. Figure 7 The mechanical properties of the regenerated cellulose membranes obtained in Examples 25 and 27 are shown in the diagram. Figure 8 This is a photograph of the cellulose product obtained in Comparative Example 2. Detailed Implementation

[0025] This invention provides a method for preparing cellulose carbamate, comprising the following steps: Cellulose pulp is soaked in an aqueous solution containing urea and acid, and then dehydrated, pulverized and dried in sequence to obtain urea / acid-cellulose pulp; The urea / acid-cellulose slurry is esterified to obtain the cellulose carbamate.

[0026] In this invention, unless otherwise specified, all raw materials used are preferably commercially available products.

[0027] The present invention involves impregnating cellulose pulp in an aqueous solution containing urea and acid, followed by dehydration, pulverization and drying to obtain urea / acid-cellulose pulp.

[0028] In this invention, the cellulose pulp preferably contains 90% or more cellulose by mass. In this invention, the cellulose pulp preferably has a molecular weight of less than or equal to 100,000, more preferably 10,000 to 100,000, and even more preferably 70,000 to 90,000. In this invention, the cellulose pulp is preferably a cellulose dissolving pulp. In this invention, the cellulose pulp preferably includes one or more of cotton pulp, wood pulp, bagasse pulp, straw pulp, reed pulp, hemp pulp, and bamboo pulp. In this invention, the wood pulp preferably includes softwood pulp; in one specific embodiment of this invention, the molecular weight of the softwood pulp is preferably 7.67w. In this invention, the cotton pulp is preferably made from cotton linters; the wood pulp is preferably made from unshaped wood; the bagasse pulp is preferably made from sugarcane bagasse; the straw pulp is preferably made from straw; the reed pulp is preferably made from reeds; the hemp pulp is preferably made from hemp stalks; and the bamboo pulp is preferably made from bamboo.

[0029] In this invention, the mass concentration of urea in the aqueous solution containing urea and acid is preferably 2-5%, more preferably 4%; the mass concentration of acid is preferably 0.1-1%, specifically preferably 0.1%, 0.2%, 0.25%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 1%. In this invention, the acid is preferably one or more of organic and inorganic acids, the inorganic acid preferably including one or more of sulfuric acid, hydrochloric acid, and phosphoric acid, more preferably sulfuric acid; the organic acid preferably includes p-toluenesulfonic acid.

[0030] In this invention, the mass ratio of the cellulose pulp to the aqueous solution containing urea and acid is preferably 1:10 to 20, and more preferably 1:15.

[0031] In this invention, the immersion temperature is preferably room temperature, i.e., neither additional heating nor additional cooling is required, and the immersion time is preferably 4 to 12 hours, more preferably 8 hours. In this invention, the immersion is preferably carried out under static conditions.

[0032] In this invention, the dehydration is preferably carried out in a washing machine. In this invention, the pulverization is preferably carried out in a pulverizer. In this invention, the drying temperature is preferably 40-70°C, more preferably 60°C. In this invention, the drying is preferably carried out in an oven.

[0033] In this invention, the water content of the urea / acid-cellulose pulp is preferably less than 5 wt%.

[0034] This invention involves immersing cellulose pulp in an aqueous solution containing urea and acid, followed by dehydration, pulverization, and drying, thereby achieving a uniform distribution of urea and acid in the pulp.

[0035] After obtaining urea / acid-cellulose slurry, the present invention esterifies the urea / acid-cellulose slurry to obtain the cellulose carbamate.

[0036] In this invention, the esterification temperature is preferably 135~175℃, specifically preferably 135℃, 140℃, 145℃, 150℃, 155℃, 160℃, 165℃, 170℃ or 175℃; the heating rate from room temperature to the esterification temperature is preferably 3~10℃ / min, specifically preferably 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min or 10℃ / min; the time is preferably 20~80min, specifically preferably 20min, 30min, 40min, 50min, 60min, 70min or 80min.

[0037] In this invention, the esterification is preferably carried out under sealed conditions. In one specific embodiment of the invention, the urea / acid-cellulose pulp is preferably wrapped in tin foil before esterification.

[0038] After the esterification is completed, the present invention preferably further includes post-treatment, which includes: washing and drying the obtained esterified product sequentially to obtain the cellulose carbamate. In the present invention, the detergent used for washing is preferably water, and the present invention does not specifically limit the amount and number of washing reagents, as long as the esterified product is washed until neutral. In the present invention, the drying temperature is preferably 60°C, and the drying is preferably carried out in an oven. The present invention does not specifically limit the drying time, as long as the water is completely removed.

[0039] The preparation method provided by this invention uses acid as a catalyst, which can catalyze the esterification reaction of cellulose with urea (or its derivatives) to promote the formation of cellulose carbamate; catalyze the decomposition / activation of urea to produce more active intermediates (isocyanate, carbamate); catalyze the hydrolysis of cellulose glycosidic bonds, resulting in molecular weight reduction; the combination of these three effects achieves effective molecular weight reduction, thereby enabling controllable preparation of cellulose carbamate.

[0040] This invention also provides a cellulose carbamate prepared by the method described above. The cellulose carbamate provided by this invention has high solubility, reaching 9% solubility in low-temperature alkali / urea.

[0041] The present invention also provides a regenerated cellulose membrane, which is prepared from the cellulose carbamate described in the above technical solution.

[0042] The present invention also provides a method for preparing the regenerated cellulose membrane described in the above technical solution, comprising the following steps: After the cellulose carbamate and urea solution described in the above technical solution are first mixed, sodium hydroxide solution is then added for a second mixing to obtain a cellulose carbamate solution. The cellulose carbamate solution is used to lay a film, and the resulting wet film is solidified in a coagulation bath to obtain a regenerated cellulose hydrogel; the regenerated cellulose hydrogel is then surface-dried to obtain the regenerated cellulose membrane. The coagulation bath is an aqueous solution containing sulfuric acid and sodium sulfate.

[0043] In this invention, the cellulose carbamate and urea solution described in the above technical solution are first mixed, and then sodium hydroxide solution is added for a second mixing to obtain a cellulose carbamate solution.

[0044] In this invention, the temperature of the first mixing is preferably 0~5°C. In this invention, the temperature of the sodium hydroxide solution is preferably -18°C. In this invention, the second mixing is preferably carried out under stirring conditions.

[0045] In this invention, the cellulose carbamate solution contains 6-9% cellulose carbamate by mass, more preferably 7%. The mass ratio of cellulose carbamate, sodium hydroxide, urea, and water in the cellulose carbamate solution is preferably 6-9:7:8:76-79, more preferably 7:7:8:78. The volume ratio of water in the urea solution to water in the sodium hydroxide solution is preferably 1:1.

[0046] After obtaining the cellulose carbamate solution, the present invention lays the cellulose carbamate solution into a film, and the formed wet film is solidified in a coagulation bath to obtain a regenerated cellulose hydrogel; the regenerated cellulose hydrogel is surface dried to obtain the regenerated cellulose membrane.

[0047] In this invention, the preferred method for film deposition is casting. The preferred substrate for film deposition is plexiglass. In the aqueous solution containing sulfuric acid and sodium sulfate, the mass concentration of sulfuric acid is preferably 5-15%, more preferably 8%, and the mass concentration of sodium sulfate is preferably 5-20%, more preferably 10%. In the aqueous solution containing sulfuric acid and sodium sulfate, the sulfuric acid acts as a neutralizing agent, while the sodium sulfate prevents rapid solidification and shrinkage of the wet film, thus ensuring the performance of the regenerated cellulose membrane.

[0048] Before surface drying the regenerated cellulose hydrogel, the process preferably further includes washing the regenerated cellulose hydrogel, wherein the detergent used for washing is preferably deionized water.

[0049] The following detailed description, in conjunction with embodiments, illustrates the cellulose carbamate and its preparation method, as well as the regenerated cellulose membrane and its preparation method provided by the present invention. However, these descriptions should not be construed as limiting the scope of protection of the present invention.

[0050] Comparative Example 1 Urea was dissolved in water to prepare a 4 wt% urea aqueous solution. The cellulose pulp (here, softwood pulp with a molecular weight of 7.67w) was soaked in the urea solution at a mass ratio of 1:15, stirred and mixed, and then allowed to stand for 8 hours. Subsequently, the pulp was dehydrated by a washing machine, pulverized by a pulverizer, and dried in an oven at 60°C to obtain urea-cellulose pulp (water content of 3.5 wt%).

[0051] Weigh 80g of urea-cellulose pulp, wrap it in aluminum foil and seal it. Place it in an oven and heat it from room temperature to 165℃ at a rate of 5℃ / min. Then keep it at that temperature for 1 hour to complete the esterification. Take it out to obtain cellulose carbamate (CC).

[0052] A certain amount of CC was washed with deionized water until neutral, dried in an oven at 60℃, and then a certain amount of the cleaned CC was used to determine its molecular weight by the viscosity method.

[0053] The raw materials were prepared according to the mass ratio of CC, NaOH, urea, and water of 7:7:8:78. Then, urea was dissolved in 1 / 2 volume of water to obtain a urea solution. CC was immersed in the urea solution and pre-cooled to 0~5℃ to obtain a cellulose carbamate-urea mixture. NaOH was dissolved in the remaining 1 / 2 volume of water and pre-cooled to -18℃ to obtain a NaOH solution. The cellulose carbamate-urea solution and the NaOH solution were mixed and stirred to obtain a 7wt% cellulose carbamate solution. The dissolution was observed under a light microscope. The 7wt% cellulose carbamate solution was centrifuged at 10℃, 8000rpm, and 10min, and its falling ball viscosity was measured.

[0054] The falling ball viscosity analysis revealed that the 7 wt% cellulose carbamate solution obtained in Comparative Example 1 had poor solubility and was prone to gelation. Therefore, the subsequent preparation of regenerated cellulose hydrogel and regenerated cellulose membrane was not carried out.

[0055] Comparative Example 2 Prepare a sulfuric acid aqueous solution with a mass concentration of 0.8 wt%. Soak the cellulose pulp (here, softwood pulp with a molecular weight of 7.67 w) in the sulfuric acid aqueous solution at a mass ratio of 1:15, stir and mix well, and then let it stand for 8 hours. Subsequently, dehydrate it in a washing machine, pulverize it in a pulverizer, and dry the dehydrated and pulverized sample in an oven at 40°C to obtain sulfuric acid-cellulose pulp (water content of 3.6 wt%).

[0056] Weigh 80g of sulfate-cellulose pulp, wrap it in aluminum foil and seal it. Place it in an oven and heat it from room temperature to 165℃ at a rate of 5℃ / min. Then keep it at this temperature for 1 hour to complete the reaction. Take it out to obtain the cellulose product.

[0057] Example 1 The only difference from Comparative Example 1 is that sulfuric acid was added to a 4 wt% urea aqueous solution to obtain a 4 wt% urea / 0.25 wt% H2SO4 aqueous solution. All other steps and parameters were the same as those in Comparative Example 1.

[0058] Example 2 The only difference from Comparative Example 1 is that sulfuric acid was added to a 4 wt% urea aqueous solution to obtain a 4 wt% urea / 0.50 wt% H2SO4 aqueous solution. All other steps and parameters were the same as those in Comparative Example 1.

[0059] Example 3 The only difference from Comparative Example 1 is that sulfuric acid was added to a 4 wt% urea aqueous solution to obtain a 4 wt% urea / 0.80 wt% H2SO4 aqueous solution. All other steps and parameters were the same as those in Comparative Example 1.

[0060] Example 4 The only difference from Comparative Example 1 is that sulfuric acid was added to a 4 wt% urea aqueous solution to obtain a 4 wt% urea / 1.0 wt% H2SO4 aqueous solution. All other steps and parameters were the same as those in Comparative Example 1.

[0061] Example 5 Urea was dissolved in water, and then sulfuric acid was added to prepare an aqueous solution with a mass concentration of 4wt% Urea / 0.80wt% H2SO4. The cellulose pulp (same as Comparative Example 1) was soaked in the 4wt% Urea / 0.80wt% H2SO4 aqueous solution at a mass ratio of 1:15, and stirred until well mixed. Then it was allowed to stand for 8 hours. Subsequently, it was dehydrated by a washing machine, pulverized by a pulverizer, and dried in an oven at 60℃ to obtain urea-cellulose pulp (water content of 3.4wt%).

[0062] Weigh 80g of urea-cellulose pulp, wrap it in aluminum foil and seal it. Place it in an oven and heat it from room temperature to 135℃ at a rate of 5℃ / min. Then keep it at that temperature for 20 minutes to complete the esterification. Take it out to obtain CC.

[0063] A certain amount of CC was washed with deionized water until neutral, dried in an oven at 60℃, and then a certain amount of the cleaned CC was used to determine its molecular weight by the viscosity method.

[0064] The raw materials were prepared according to the mass ratio of CC, NaOH, urea, and water of 7:7:8:78. Then, urea was dissolved in 1 / 2 volume of water to obtain a urea solution. CC was immersed in the urea solution and pre-cooled to 0~5℃ to obtain a cellulose carbamate-urea mixture. NaOH was dissolved in the remaining 1 / 2 volume of water and pre-cooled to -18℃ to obtain a NaOH solution. The cellulose carbamate-urea mixture was mixed with the NaOH solution and stirred to obtain a 7wt% cellulose carbamate solution. The dissolution was observed under a light microscope. The 7wt% cellulose carbamate solution was centrifuged at 10℃, 8000rpm, and 10min, and its falling ball viscosity was measured.

[0065] Example 6 The only difference from Example 5 is that the esterification conditions are: the temperature is increased from room temperature to 135°C at a rate of 5°C / min and then held for 40 min. All other steps and parameters are the same as in Example 5.

[0066] Example 7 The only difference from Example 5 is that the esterification conditions are: the temperature is increased from room temperature to 135°C at a rate of 5°C / min and then held at that temperature for 60 min. All other steps and parameters are the same as in Example 5.

[0067] Example 8 The only difference from Example 5 is that the esterification conditions are: the temperature is increased from room temperature to 135°C at a rate of 5°C / min and then held at that temperature for 80 min. All other steps and parameters are the same as in Example 5.

[0068] Example 9 The only difference from Example 5 is that the esterification conditions are: the temperature is increased from room temperature to 145°C at a rate of 5°C / min and then held for 20 min. All other steps and parameters are the same as in Example 5.

[0069] Example 10 The only difference from Example 5 is that the esterification conditions are: the temperature is increased from room temperature to 145°C at a rate of 5°C / min and then held for 40 min. All other steps and parameters are the same as in Example 5.

[0070] Example 11 The only difference from Example 5 is that the esterification conditions are: the temperature is increased from room temperature to 145°C at a rate of 5°C / min and then held at that temperature for 60 min. All other steps and parameters are the same as in Example 5.

[0071] Example 12 The only difference from Example 5 is that the esterification conditions are the same as in Example 5, except that the temperature is increased from room temperature to 145°C at a rate of 5°C / min and then held at that temperature for 80 min.

[0072] Example 13 The only difference from Example 5 is that the esterification conditions are: the temperature is increased from room temperature to 155°C at a rate of 5°C / min and then held for 20 min. All other steps and parameters are the same as in Example 5.

[0073] Example 14 The only difference from Example 5 is that the esterification conditions are: the temperature is increased from room temperature to 155°C at a rate of 5°C / min and then held for 40 min. All other steps and parameters are the same as in Example 5.

[0074] Example 15 The only difference from Example 5 is that the esterification conditions are: the temperature is increased from room temperature to 155°C at a rate of 5°C / min and then held at that temperature for 60 min. All other steps and parameters are the same as in Example 5.

[0075] Example 16 The only difference from Example 5 is that the esterification conditions are: the temperature is increased from room temperature to 155°C at a rate of 5°C / min and then held at that temperature for 80 min. All other steps and parameters are the same as in Example 5.

[0076] Example 17 The only difference from Example 5 is that the esterification conditions are: the temperature is increased from room temperature to 165°C at a rate of 5°C / min and then held for 20 min. All other steps and parameters are the same as in Example 5.

[0077] Example 18 The only difference from Example 5 is that the esterification conditions are: the temperature is increased from room temperature to 165°C at a rate of 5°C / min and then held for 40 min. All other steps and parameters are the same as in Example 5.

[0078] Example 19 The only difference from Example 5 is that the esterification conditions are: the temperature is increased from room temperature to 165°C at a rate of 5°C / min and then held at that temperature for 60 min. All other steps and parameters are the same as in Example 5.

[0079] Example 20 The only difference from Example 5 is that the esterification conditions are: the temperature is increased from room temperature to 165°C at a rate of 5°C / min and then held at that temperature for 80 min. All other steps and parameters are the same as in Example 5.

[0080] Example 21 The only difference from Example 5 is that the esterification conditions are: the temperature is increased from room temperature to 175°C at a rate of 5°C / min and then held for 20 min. All other steps and parameters are the same as in Example 5.

[0081] Example 22 The only difference from Example 5 is that the esterification conditions are: the temperature is increased from room temperature to 175°C at a rate of 5°C / min and then held at that temperature for 40 min. All other steps and parameters are the same as in Example 5.

[0082] Example 23 The only difference from Example 5 is that the esterification conditions are: the temperature is increased from room temperature to 175°C at a rate of 5°C / min and then held at that temperature for 60 min. All other steps and parameters are the same as in Example 5.

[0083] Example 24 The only difference from Example 5 is that the esterification conditions are: the temperature is increased from room temperature to 175°C at a rate of 5°C / min and then held at that temperature for 80 min. All other steps and parameters are the same as in Example 5.

[0084] Example 25 An 8wt% H₂SO₄ / 10wt% Na₂SO₄ solution was prepared as a coagulation bath. The centrifuged 7wt% cellulose carbamate solution obtained in Example 2 was cast into a membrane using a casting method. The regenerated cellulose hydrogel was regenerated in the coagulation bath, washed with deionized water, and surface-dried on an acrylic plate to obtain a regenerated cellulose membrane. The mechanical properties of the obtained regenerated cellulose hydrogel and regenerated cellulose membrane were tested.

[0085] Example 26 The only difference from Comparative Example 1 is that hydrochloric acid was added to a 4 wt% urea aqueous solution to obtain a 4 wt% urea / 0.8 wt% HCl aqueous solution. All other steps and parameters were the same as those in Comparative Example 1.

[0086] Example 27 An 8 wt% H₂SO₄ / 10 wt% Na₂SO₄ solution was prepared as a coagulation bath. The centrifuged 7 wt% cellulose carbamate solution prepared in Example 26 was used to cast a membrane. The regenerated cellulose hydrogel was regenerated in the coagulation bath, washed with deionized water, and surface-dried on an acrylic plate to obtain a regenerated cellulose membrane. The mechanical properties of the obtained regenerated cellulose hydrogel and regenerated cellulose membrane were tested.

[0087] The molecular weight of the washed cellulose carbamates obtained by esterification in Comparative Example 1 and Examples 1-4 was determined, and the results are as follows: Figure 1 As shown. By Figure 1 It can be seen that the higher the concentration of added sulfuric acid, the stronger the catalytic degradation effect on cellulose carbamate, and the lower the molecular weight of the cellulose carbamate obtained by esterification. Therefore, by varying the amount of sulfuric acid added, the molecular weight of cellulose carbamate can be controlled.

[0088] Light micrographs of the cellulose carbamate solutions obtained in Comparative Example 1 and Examples 1-4 are shown below. Figure 2 As shown, by Figure 2 It can be seen that the higher the concentration of added sulfuric acid, the lower the molecular weight of cellulose carbamate, the better the solubility of cellulose carbamate, and the less undissolved fiber under a light microscope.

[0089] Figure 3 The FT-IR spectra of cellulose pulp and CC prepared with different acid concentrations in Comparative Example 1 and Examples 1-4 are shown. Figure 3 It can be seen that, by comparing cellulose pulp and cellulose carbamate (CC) pulp, CC pulp has a lower viscosity at 1710 cm⁻¹. -1 The presence of a distinct absorption peak at this point is due to the stretching vibration of the carbonyl group (C=O) in cellulose carbamate, indicating the successful synthesis of cellulose carbamate (CC).

[0090] Figure 4 The XRD patterns of cellulose pulp and CC prepared with different acid concentrations in Comparative Example 1 and Examples 1-4 are shown. Figure 4 As shown, in 2 θ The diffraction peaks at 14.8º, 16.5º, and 22.8º all exhibit three typical diffraction peaks, corresponding to cellulose type I (101), (1... 1) and (002) planes. Where (101) and (1) 1) A broad peak is formed. Treatment of cellulose with sulfuric acid of different concentrations to prepare CC does not change the crystalline structure of cellulose, but only plays a catalytic role in depolymerization.

[0091] The falling ball viscosity of CC solutions prepared with different sulfuric acid concentrations is shown in Table 1.

[0092] Table 1. Falling ball viscosity of CC solutions prepared with different sulfuric acid concentrations.

[0093] As can be seen from Table 1, the falling ball viscosity of the cellulose carbamate solution obtained after dissolving the prepared cellulose carbamate decreases with increasing sulfuric acid concentration in the solution.

[0094] The molecular weight determination results of the washed CC obtained by esterification at different temperatures and times in Examples 5-24 are shown in Table 2.

[0095] Table 2. Molecular weight of CC obtained by esterification at different temperatures and times.

[0096] Cellulose pulp soaked in 4wt% Urea / 0.8wt% H2SO4 solution was dehydrated, pulverized, and dried. It was then esterified at different temperatures (135℃, 145℃, 155℃, 165℃, 175℃) and for different times (20min, 40min, 60min, 80min) to obtain samples under different esterification conditions. The effects of different temperatures and times on the esterification process were investigated at 135℃-20min, 135℃-40min, 135℃-60min, 135℃-80min, 145℃-20min, 145℃-40min, 145℃-60min, 145℃-80min, 155℃-20min, 155℃-40min, 155℃-60min, 155℃-80min, 165℃-20min, 165℃-40min, 165℃-60min, 165℃-80min, 175℃-20min, 175℃-40min, 175℃-60min, and 175℃-80min.

[0097] Table 2 shows that there was no significant difference in molecular weight of CC samples prepared at different temperatures and times; CC samples prepared at the same temperature but at different times did not show a decrease in molecular weight with increasing esterification time; CC samples prepared at the same time but at different temperatures also did not show a decrease in molecular weight with increasing esterification time; therefore, the temperature and time of esterification are not the main factors affecting the molecular weight of CC.

[0098] The falling ball viscosities of the CC solutions obtained by esterification at different temperatures and times in Examples 5-24 are shown in Table 3.

[0099] Table 3. Falling ball viscosity of CC solutions prepared by esterification at different temperatures and times.

[0100] As shown in Table 3, under the same esterification temperature, the falling ball viscosity of the CC solution decreases with longer esterification time. Under the same esterification time, the falling ball viscosity of the CC solution decreases with higher esterification temperature.

[0101] Dissolution microscopy images of CCs prepared by esterification at different temperatures and times in Examples 5-24 are shown below. Figure 5 As shown. By Figure 5 As shown, under the same esterification temperature, there was no significant difference in the dissolution effect of CC with longer esterification time (fewer fibers observable under a light microscope indicate better dissolution). Under the same esterification time, there was no significant difference in the dissolution effect of CC with higher esterification temperature.

[0102] Combining Table 2, Table 3 and Figure 5 It is evident that esterification temperature and time are not the primary factors affecting the molecular weight of CC, but higher temperatures and longer esterification times improve sample homogeneity, resulting in lower falling-ball viscosity of the CC solution. CC solutions with suitable falling-ball viscosity are more suitable for preparing regenerated cellulose membranes. Therefore, we can select appropriate esterification conditions based on suitable falling-ball viscosity.

[0103] The cellulose carbamate solution with a concentration of 7 wt% obtained in Example 26 had a falling ball viscosity of 5.96 s (19.3 °C), indicating good dissolution performance.

[0104] The mechanical property test data of the regenerated cellulose hydrogels and regenerated cellulose membranes prepared in Examples 25 and 27 are shown in Tables 4 and 5. Figures 6-7 As shown.

[0105] Table 4. Test data of mechanical properties of regenerated cellulose hydrogel

[0106] Table 5. Test data of mechanical properties of regenerated cellulose membrane

[0107] Combine Table 4 and Figure 6 Table 5 and Figure 7 It can be seen that the regenerated cellulose membrane prepared by CC dissolution under urea / acid conditions has excellent mechanical properties, and a high-strength regenerated cellulose membrane was prepared.

[0108] Figure 8Here is a photograph of the cellulose product obtained in Comparative Example 2. Figure 8 It can be seen that under the conditions of 165℃ and 1 hour, the pulp soaked in sulfuric acid was carbonized and turned black. This indicates that simply adding acid to soak the pulp cannot achieve the effect of esterification; a good esterification effect can only be achieved under the dual action of urea and acid.

[0109] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing cellulose carbamate, characterized in that, Includes the following steps: Cellulose pulp is soaked in an aqueous solution containing urea and acid, and then dehydrated, pulverized and dried in sequence to obtain urea / acid-cellulose pulp; The urea / acid-cellulose slurry is esterified to obtain the cellulose carbamate.

2. The preparation method according to claim 1, characterized in that, The cellulose pulp contains 90% or more cellulose by mass. The cellulose pulp includes one or more of cotton pulp, wood pulp, bagasse pulp, straw pulp, reed pulp, hemp pulp, and bamboo pulp.

3. The preparation method according to claim 1, characterized in that, The aqueous solution containing urea and acid has a urea concentration of 2-5% and an acid concentration of 0.1-1%. The acid is one or more of organic and inorganic acids, the inorganic acid including one or more of sulfuric acid, hydrochloric acid and phosphoric acid; the organic acid includes p-toluenesulfonic acid.

4. The preparation method according to claim 1, 2, or 3, characterized in that, The mass ratio of the cellulose pulp to the aqueous solution containing urea and acid is 1:10~20; The drying temperature is 40~70℃.

5. The preparation method according to claim 1, characterized in that, The esterification temperature is 135~175℃, and the heating rate from room temperature to the esterification temperature is 3~10℃ / min, with a time of 20~80min.

6. The cellulose carbamate prepared by the method according to any one of claims 1 to 5.

7. A regenerated cellulose membrane, characterized in that, It is prepared from the cellulose carbamate described in claim 6.

8. The method for preparing the regenerated cellulose membrane according to claim 7, characterized in that, Includes the following steps: After first mixing the cellulose carbamate and urea solution as described in claim 6, sodium hydroxide solution is added for a second mixing to obtain a cellulose carbamate solution. The cellulose carbamate solution is used to lay a film, and the resulting wet film is solidified in a coagulation bath to obtain a regenerated cellulose hydrogel; the regenerated cellulose hydrogel is then surface-dried to obtain the regenerated cellulose membrane. The coagulation bath is an aqueous solution containing sulfuric acid and sodium sulfate.

9. The preparation method according to claim 8, characterized in that, The cellulose carbamate solution contains 6-9% cellulose carbamate by mass.

10. The preparation method according to claim 8, characterized in that, The aqueous solution containing sulfuric acid and sodium sulfate has a sulfuric acid concentration of 5-15% and a sodium sulfate concentration of 5-20%.

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