Cellulose solution and preparation method thereof

Abstract

The invention discloses a cellulose solution and a preparation method thereof. The preparation method comprises the following steps: a solvent preparation step: mixing set ionic liquid with N-methylmorpholine-N-oxide hydrate according to a set mass ratio; a dissolving step: under a normal pressure condition, taking a first temperature as an initial reaction temperature, adding absolutely dry pulp into the solvent prepared in the solvent preparation step, and stirring to disperse the pulp; after uniform mixing, raising the temperature to a second temperature, and swelling cellulose in the pulp in the temperature raising process; reacting for a set time after the temperature reaches a second temperature, so that the cellulose is thoroughly dissolved; the second temperature is higher than the first temperature. According to the cellulose solution and the preparation method thereof, the problems that in the prior art, a viscose technology is high in toxicity, NMMO has the explosion risk, ionic liquid is high in price, and energy consumption is increased due to the fact that vacuum dehydration is needed for dissolution can be solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of solution preparation, and relates to a solution preparation method, in particular to a cellulose solution and a preparation method thereof. Background Art

[0002] Cellulose is the most abundant natural polymer material in nature. Due to its characteristics such as renewability, biocompatibility, and environmental friendliness, it is considered to be the main chemical raw material for realizing sustainable development in the future. Regenerated cellulose materials are made into different forms, such as filaments, films / membranes, microspheres, hydrogels / aerogels, etc. They are widely used in textile, packaging, biomedicine, water treatment, agriculture, food and other fields. However, cellulose molecular chains have a large number of hydroxyl groups. Due to the intermolecular and intramolecular hydrogen bond interactions, it can neither be melted by heating nor dissolved in common solvents, resulting in difficulties in its dissolution and processing.

[0003] The traditional viscose process uses the reaction of cellulose with NaOH and CS2 to generate cellulose xanthate that can be dissolved in alkali solution, so that spinning or film-making processing can be carried out. However, this process will produce toxic gases such as H2S and CS2, causing serious environmental pollution. The industrial Lyocell process directly dissolves cellulose in N-methylmorpholine-N-oxide (abbreviated as NMMO) to prepare various regenerated cellulose materials. For example, Patent CN106222771B mixes cellulose pulp with an NMMO aqueous solution with a mass concentration of 70% - 76%, and after dehydration swelling, dissolution and defoaming, a cellulose spinning dope is obtained, and then cellulose fibers are obtained after subsequent spinning process treatment. This process has a non-toxic solvent and can be recycled, but the solvent NMMO has poor stability during the dissolution process and has potential explosive safety problems.

[0004] Ionic liquids (ILs), as a green solvent for preparing regenerated cellulose materials, have the advantages of low vapor pressure, non-flammability, good thermal stability, and recyclability. Cellulose ionic liquid solutions are directly obtained by dissolving cellulose in ionic liquids, and then anti-solvents are used to solidify and form them into regenerated cellulose materials, making the process more environmentally friendly. For example, Patent CN105670026B pre-mixes an aqueous solution of 1-allyl-3-methylimidazolium chloride (AmimCl) and cellulose raw materials, then dehydrates them, dissolves the cellulose, and finally regenerates it into a cellulose membrane. However, the viscosity of ionic liquid solutions is relatively high, making it difficult to transport between different devices. Moreover, a small amount of water will seriously affect the solubility, and the raw material cost is high, with a price of about 1.6 to 4 million yuan per ton.

[0005] The existing methods for preparing cellulose solutions mainly include the viscose method and the NMMO method. The viscose method not only causes great harm to human health during the production process but also causes serious environmental pollution. As a non-derivatized solvent that can physically dissolve cellulose, NMMO causes little environmental pollution and can be recycled. However, the stability of NMMO is not good, and additives need to be added to reduce side reactions. Moreover, there is an explosion risk during the solvent use and recovery process. The pulping methods for preparing cellulose solutions with NMMO are mainly divided into two types: dry method and wet method. Both methods need to increase the NMMO concentration by vacuum pumping during the dissolution process, resulting in a large amount of energy consumption. For example, Patent CN111892718B provides a method for dry-dissolving cellulose. The cellulose pulp board is mechanically crushed and mixed with an aqueous NMMO solution with a concentration of 70% - 87% at a temperature of 70 - 110°C and left standing for a period of time, and then dehydrated and dissolved under vacuum conditions to obtain a cellulose solution. For example, Patent CN116854996B provides a method for preparing a cellulose mixed pulp. The cellulose pulp without crushing is directly mixed with the NMMO solution for pulping. The mixed pulp consists of cellulose pulp, NMMO solution, hydroxylamine, propyl gallate, sodium hydroxide, and water. Vacuum pumping for dehydration is still required during the dissolution process.

[0006] Ionic liquids can also be used to physically dissolve cellulose. The solvent can be recycled, and it has low vapor pressure, is not flammable, and has good thermal stability. Ionic liquids have strong dissolving ability and fast dissolution rate, which easily leads to the pulp wrapped inside not being dissolved, resulting in undissolved white cores, thereby affecting the dissolution quality of the cellulose solution. To avoid the above situation, Patent CN105670026B adds a certain amount of water to the ionic liquid, controls the mass concentration of the ionic liquid between 55.0% and 98.5%, adjusts the viscosity of the ionic liquid and its swelling ability for cellulose, and controls the mixing temperature and time to make the pulp reach the effect of swelling but not dissolving, and then performs vacuum dehydration and dissolution. However, vacuum pumping will generate a large amount of energy consumption and increase production costs. And the dissolution of cellulose in ionic liquids is often accompanied by degradation. In the NMMO system, degradation can be inhibited by adding propyl gallate, but this method has no effect on the ionic liquid system. For example, Patent CN109384940B proposes to dilute the ionic liquid with pure water and then treat it with an alkaline anion exchange resin to make its pH value 8-12 in order to remove the acidic impurities in the ionic liquid that cause cellulose degradation, and then remove the pure water to obtain the ionic liquid. This method is not only cumbersome but also has high energy consumption during the dehydration process.

[0007] To prepare a cellulose solution, in addition to the industrialized viscose method and NMMO method, some other solvent systems have been developed, including lithium chloride / N,N-dimethylacetamide (DMAc / LiCl), ionic liquids, NaOH / urea aqueous solution solvent systems, etc. For example, Patent CN116218013B uses corn straw as raw material, and after alkali treatment, bleaching, pH adjustment, and ultrasonic treatment, obtains corn straw fiber raw material, and uses the DMAc / LiCl system to treat the raw material to obtain a cellulose solution for film preparation. For example, Patent CN116120609B mixes the bagasse pulp cellulose powder obtained after ball milling treatment with the ionic liquid AmimCl and then heats and dissolves it to obtain a cellulose solution. For example, Patent CN113788966B mixes urea, alkali, and deionized water in proportion to prepare an alkali / urea aqueous solution, adds inorganic fillers to disperse to obtain a dispersion, cools it to -14 to -10 °C, and then adds cellulose and stirs to obtain a cellulose solution. However, these solvent systems respectively have problems such as high cost, high solution viscosity, the need for a relatively low temperature and a highly alkaline environment during the dissolution process, and non-recyclability.

[0008] In view of this, there is an urgent need to design a new method for preparing a cellulose solution to overcome at least some of the above defects existing in the existing preparation methods. Summary of the Invention

[0009] The present invention provides a method for preparing a cellulose solution, which can solve the problems in the prior art that the viscose process is highly toxic, NMMO has an explosion risk, ionic liquids are expensive, and vacuum dehydration is required for dissolution, increasing energy consumption.

[0010] To solve the above technical problems, according to one aspect of the present invention, the following technical solutions are adopted:

[0011] A method for preparing a cellulose solution, the preparation method comprising:

[0012] Solvent preparation step: Take a set ionic liquid and N-methylmorpholine-N-oxide hydrate and mix them according to a set mass ratio to form a composite solvent; the viscosity of the composite solvent is lower than that of the same pure ionic liquid.

[0013] Dissolution step: Under atmospheric pressure conditions, taking the first temperature as the initial reaction temperature, adding absolute dry pulp into the solvent prepared in the solvent preparation step, and dispersing the pulp by stirring; after mixing evenly, heating up to the second temperature, during the process of increasing the temperature, the cellulose in the pulp swells; after the temperature reaches the second temperature, reacting for a set time to make the cellulose dissolve thoroughly; the second temperature is greater than the first temperature.

[0014] As an embodiment of the present invention, in the dissolution step, the first temperature is 50-70 °C, the second temperature is 70-100 °C; the proportion of pulp is 5-14 wt%; during the dispersion process of the pulp, the stirring speed is 10-30 r / min.

[0015] As an embodiment of the present invention, in the solvent preparation step, the ionic liquid includes at least one of 1-allyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium chloride, 1-ethyl-3-methylimidazolium chloride, and 1-butyl-3-methylimidazolium acetate.

[0016] As an embodiment of the present invention, before the dissolution step, the preparation method further includes a solute pretreatment step: taking a set amount of absolute dry pulp sheet and mixing it with a set ethanol solution, the mass of the absolute dry pulp sheet accounts for 0.8-1.5% of the mass of the ethanol solution; stirring it at a speed of 1000-2000 r / min for 20000-40000 r to break it up, the loose pulp is filtered by suction and then replaced with absolute ethanol to remove moisture, and it is dried at a low temperature of less than or equal to 60 °C as the dissolution raw material.

[0017] As an embodiment of the present invention, after the dissolution step, the preparation method further includes a film-making step: after the prepared cellulose solution is defoamed, an artificial film is scraped using a single-sided preparation device, the obtained regenerated cellulose film is soaked in deionized water for more than 2 h, and the water is changed frequently to wash the solvent in the film, and it is dried at a low temperature of less than or equal to 60 °C to obtain a dry regenerated cellulose film.

[0018] As an embodiment of the present invention, in the step of preparing the solvent, the mass ratio of the ionic liquid to N-methylmorpholine-N-oxide hydrate is 1-9:1.

[0019] As an embodiment of the present invention, in the step of preparing the solvent, a set amount of water is also added (a small amount of water can be added, and the specific content is related to the type of composite solvent).

[0020] According to another aspect of the present invention, the following technical solution is adopted: a cellulose solution prepared according to the above preparation method.

[0021] The beneficial effects of the present invention are as follows: The cellulose solution and its preparation method proposed by the present invention can solve the problems in the prior art, such as the high toxicity of the viscose process, the explosion risk of NMMO, the high price of ionic liquids, and the increased energy consumption due to the need for vacuum dehydration during dissolution.

[0022] The cellulose solution preparation process mentioned in the present invention includes three main processes: dispersion, swelling, and dissolution of pulp. After mixing the ionic liquid and NMMO·H2O, the viscosity of the composite solvent is lower than that of the same pure ionic liquid, which is beneficial to the dispersion of cellulose pulp in the solvent, avoiding phenomena such as white cores caused by direct and rapid dissolution, and thus improving the dissolution quality. By adjusting the temperature to control the swelling and dissolution of cellulose pulp in the composite solvent, compared with the traditional method of increasing the proportion of water in the system for adjustment, the dehydration process is reduced, and the energy consumption in the dissolution stage can be significantly reduced. And the ionic liquid / NMMO composite solvent used has lower toxicity than the traditional viscose process, lower explosion risk than the Lyocell process, and at the same time reduces the dosage of ionic liquid, making the price cost lower. Also, due to the relatively low viscosity of the composite solvent itself, when other conditions (such as cellulose degree of polymerization, dissolution amount, dissolution temperature, etc.) remain unchanged, the movement ability of cellulose molecular segments in the solution system is enhanced, which can effectively improve the uniformity of the solution, and the viscosity of the prepared cellulose solution is also relatively low, reducing the difficulty of solution transportation and processing. In addition, no other additives are added to the composite solvent to reduce the degradation of cellulose during dissolution. For example, NMMO needs to add propyl gallate as a stabilizer, and the ionic liquid needs to be adjusted to an appropriate pH value. This not only reduces the use of chemicals but also simplifies the solvent preparation steps. Therefore, the present invention can be used for the preparation of cellulose solutions. Description of the Drawings

[0023] Figure 1 It is a flowchart of the preparation method of the cellulose solution in an embodiment of the present invention.

[0024] Figure 2 It is the 1H NMR spectrum of the ionic liquid, NMMO·H2O, and the composite solvent in an embodiment of the present invention.

[0025] Figure 3 Comparison chart of zero-shear viscosity of ionic liquid, NMMO·H20 and composite solvent with a mass ratio of 1:1 for each component in an embodiment of the present invention (under the condition of 90 °C).

[0026] Figure 4 Physical pictures of cellulose solutions in the examples and comparative examples of the present invention in an embodiment of the present invention.

[0027] Figure 5 Microscope pictures of cellulose solutions in the examples and comparative examples of the present invention in an embodiment of the present invention.

[0028] Figure 6 Comparison chart of weighted relaxation time spectra of cellulose solutions in Examples 1 and 2 and Comparative Examples 4 and 5 of the present invention (under the condition of 95 °C).

[0029] Figure 7 Physical pictures of regenerated cellulose membranes of Examples 1 and 2 and comparative samples of the present invention. Detailed Description of the Invention

[0030] The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0031] In order to further understand the present invention, the preferred embodiments of the present invention will be described below in conjunction with examples. However, it should be understood that these descriptions are only for further explaining the features and advantages of the present invention, rather than limiting the claims of the present invention.

[0032] The description of this part only focuses on several typical embodiments, and the present invention is not limited to the scope described in the embodiments. The mutual replacement of the same or similar prior art means and some technical features in the embodiments are also within the scope of the description and protection of the present invention.

[0033] Unless otherwise specified, implied from the context or in accordance with the convention of the prior art, all parts and percentages in this application are based on weight, and the test and characterization methods used are synchronized with the filing date of this application. Where applicable, any patents, patent applications or disclosures involved in this application are incorporated herein by reference in their entirety, and their equivalent family patents are also incorporated by reference. In particular, the definitions of synthetic techniques, products and processing designs, polymers, comonomers, initiators or catalysts, etc. in the art disclosed in these documents. If the definition of a specific term disclosed in the prior art is inconsistent with any definition provided in this application, the definition provided in this application shall prevail.

[0034] The numerical ranges in this application are approximate values, so unless otherwise stated, they may include values outside the ranges. The numerical ranges include all values from the lower limit value to the upper limit value incremented by 1 unit, provided that there is an interval of at least 2 units between any lower value and any higher value. For example, if it is stated that a component, physical or other property (such as molecular weight, melt index, etc.) is from 100 to 1000, it means that all individual values are clearly listed, such as 100, 101, 102, etc., as well as all sub-ranges, such as 100 to 166, 155 to 170, 198 to 200, etc. For ranges containing values less than 1 or fractions greater than 1 (such as 1.1, 1.5, etc.), 1 unit is appropriately regarded as 0.0001, 0.001, 0.01 or 0.1. For ranges containing single-digit numbers less than 10 (such as 1 to 5), 1 unit is usually regarded as 0.1. These are merely specific examples of what is intended to be expressed, and all possible combinations of the values between the lowest and highest values listed are considered to be clearly recited in this application. It should also be noted that the terms "first", "second", etc. in this article do not limit the order of precedence, but are only used to distinguish substances with different structures.

[0035] When using chemical compounds, unless explicitly stated, the singular includes all isomeric forms, and vice versa (for example, "hexane" individually or collectively includes all isomers of hexane). Additionally, unless explicitly stated, a noun described by "a", "an" or "the" also includes its plural forms.

[0036] The terms "comprising", "including", "having" and their derivatives do not exclude the existence of any other components, steps or processes, and are independent of whether these other components, steps or processes are disclosed in this application. To eliminate any doubts, unless explicitly stated, all compositions using the terms "comprising", "including", or "having" in this application may contain any additional additives, excipients or compounds. In contrast, except for those necessary for operational performance, the term "consisting essentially of" excludes any other components, steps or processes from the scope described below any such term. The term "consisting of" does not include any components, steps or processes not specifically described or listed. Unless explicitly stated, the term "or" refers to the individual members listed or any combination thereof.

[0037] The expressions of the steps in each embodiment in the specification are only for convenience of description, and the implementation manner of this application is not limited by the order of step implementation.

[0038] The present invention discloses a method for preparing a cellulose solution, and the preparation method includes the following steps:

[0039] Solvent preparation step: Take a set ionic liquid and N-methylmorpholine-N-oxide hydrate and mix them in a set mass ratio to form a composite solvent; the viscosity of the composite solvent is lower than that of the same pure ionic liquid. In an embodiment of the present invention, in the solvent preparation step, the ionic liquid includes at least one of 1-allyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium chloride, 1-ethyl-3-methylimidazolium chloride, and 1-butyl-3-methylimidazole acetate; the mass ratio of the ionic liquid to N-methylmorpholine-N-oxide hydrate is 1-9:1.

[0040] Dissolution step: Under atmospheric pressure conditions, using the first temperature as the initial reaction temperature, add absolute dry pulp to the solvent prepared in the solvent preparation step, and disperse the pulp by stirring; after mixing evenly, raise the temperature to the second temperature. During the temperature increase process, the cellulose in the pulp swells; after the temperature reaches the second temperature, react for a set time to make the cellulose dissolve thoroughly; the second temperature is greater than the first temperature. In an embodiment of the present invention, in the dissolution step, the first temperature is 50-70°C, the second temperature is 70-100°C; the pulp proportion is 5-14 wt%; during the pulp dispersion process, the stirring speed is 10-30 r / min.

[0041] Before the dissolution step, the preparation method may further include a solute pretreatment step: Take a set amount of absolute dry pulp board and mix it with a set ethanol solution, stir it at a speed of 1000-2000 r / min for 20000-40000 r to break it up, and after the loose pulp is filtered by suction, replace the water with absolute ethanol to remove moisture, and dry it at a low temperature of less than or equal to 60°C as the dissolution raw material.

[0042] The concentration range of the ethanol solution can be 0-100%. Using an ethanol aqueous solution is to reduce the keratinization of the pulp fibers during the drying process, which is beneficial to achieving a swelling effect with the subsequent use of the composite solvent. The mass of the absolute dry pulp board accounts for 0.8-1.5% (about 1%) of the mass of the ethanol solution. Too high a pulp concentration is not conducive to dispersing the pulp board into loose pulp.

[0043] After the dissolution step, the preparation method may further include a film-forming step: After the prepared cellulose solution is defoamed, use a single-sided coater for manual film scraping. The obtained regenerated cellulose film is soaked in deionized water for more than 2 h, and the water is changed frequently to wash the solvent in the film, and then dried at a low temperature of less than or equal to 60°C to obtain a dry regenerated cellulose film.

[0044] Figure 1 It is a flowchart of the preparation method of the cellulose solution in an embodiment of the present invention; please refer to Figure 1 , in an embodiment of the present invention, the preparation method of the cellulose solution of the present invention includes the following steps:

[0045]

Step S1

[0046] Take 5 - 15 g of absolute dry pulp board and mix it with 1 - 2 L of ethanol solution with a volume concentration of 50%. Use a fiber disintegrator to stir it at a speed of 1000 - 2000 r / min for 20000 - 40000 r to disperse it. The loose pulp is filtered by suction and then replaced with absolute ethanol to remove moisture, and it is dried overnight at a low temperature (≤60°C) as the dissolving raw material.

[0047]

Step S2

[0048] Take one of 1 - allyl - 3 - methylimidazolium chloride (AmimCl), 1 - butyl - 3 - methylimidazolium chloride (abbreviated as BmimCl), 1 - ethyl - 3 - methylimidazolium chloride (abbreviated as EmimCl), 1 - butyl - 3 - methylimidazolium acetate (abbreviated as BmimAc) and N - methylmorpholine - N - oxide monohydrate (abbreviated as NMMO·H20) and mix them in a mass ratio of 9:1 - 1:1 (a small amount of water can be added, and the specific content is related to the type of composite solvent). Stir and melt it into a clear liquid at the lowest melting temperature, and keep the two components in the composite solvent mixed evenly and there are no un - melted particles.

[0049]

Step S3

[0050] Under normal pressure conditions, take the lowest temperature (50 - 70°C) at which the composite solvent in Step S2 does not crystallize as the initial reaction temperature, add the absolute dry pulp board treated in Step S1, and the proportion of the pulp board is 5 - 14 wt%. The stirring paddle mixes the pulp board and the composite solvent at a stirring speed of 10 - 30 r / min. This part is the dispersion process of the pulp board. After the two are mixed evenly, raise the temperature to 70 - 100°C. During the process of raising the temperature, the cellulose in the pulp board swells. This part is the swelling process of the pulp board. After the temperature reaches 70 - 100°C, extend the reaction time until the cellulose solution is completely dissolved and then end the reaction. This part is the dissolving process of the pulp board.

[0051]

Step S4

[0052] After the prepared cellulose solution is defoamed, it is manually cast using a single-sided applicator. The obtained regenerated cellulose membrane is soaked in deionized water for more than 2 h, and the water is changed frequently to wash the solvent in the membrane, and then dried at a low temperature (≤60 °C) to obtain a dry regenerated cellulose membrane.

[0053] Tests related to the composite solvent used in the present invention:

[0054] The lowest melting temperature of the composite solvent in the present invention is related to the mass ratio between the two components constituting the composite solvent. After mixing BmimAc and NMMO·H2O in different mass ratios to explore the melting point of the composite solvent (Table 1), it can be seen that increasing the proportion of NMMO·H2O will cause the melting point of the BmimAc + NMMO·H2O composite solvent to rise.

[0055] To confirm whether a chemical reaction occurs during the preparation process of the composite solvent in the present invention: Three ionic liquids (AmimCl, BmimAc, BmimCl) and NMMO·H2O are respectively mixed in a mass ratio of 1:1, and then heated to 90 °C and stirred for 30 min to obtain an ionic liquid + NMMO·H2O composite solvent. During the heating and mixing process, no obvious experimental phenomena (such as white smoke, gas, heat release, etc.) are found. The 1H NMR spectra of the pure solvent and the composite solvent are respectively tested. Through the obtained spectra ( Figure 1 ) it can be seen that: compared with the 1H NMR spectra of the ionic liquid and NMMO·H2O, the peak shapes at each position of the composite solvent remain unchanged, and no new peaks appear; the peak positions do not shift (due to the presence of crystal water in NMMO·H2O, there will be a slight shift in the peaks at adjacent positions); after integrating the peaks of the ionic liquid and NMMO·H2O in the composite solvent respectively, it is found that the ratio is not significantly different from that of the pure product.

[0056] When preparing the composite solvent, it is mixed according to the mass ratio of ionic liquid to NMMO·H2O of 1:1, and the corresponding molar ratios are calculated respectively. Then, according to the 1H NMR spectrum, the molar ratio of the two components in the composite solvent is calculated to confirm that the two components of the ionic liquid and NMMO·H2O are only mixed.

[0057] However, since both components in the composite solvent are prone to absorb water, and the ionic liquid has a higher water absorption degree. Therefore, there is a certain gap between the molar ratio calculated according to the spectrum and the molar ratio calculated according to the mass. The content of NMMO·H2O is higher than the theoretical value, which is in line with the material characteristics. Therefore, combining Figure 1 and the test data in Table 2, it can be seen that no chemical reaction occurs after mixing these three ionic liquids with NMMO·H2O.

[0058] Table 1 Melting temperature table of BmimAc and NMMO·H2O composite solvents with different mass ratios in the present invention ("×" means not completely melted, "√" means completely melted)

[0059]

[0060] Table 2 Calculation data of the molar ratio of the two components in the composite solvent of three ionic liquids and NMMO·H2O

[0061]

[0062] Table 3 Comparison of rheological data of cellulose solutions in Examples 1 and 2 of the present invention and Comparative Examples 4 and 5 (at 95°C)

[0063]

[0064] Table 4: Tensile test data of regenerated cellulose films of Examples 1 and 2 of the present invention and comparative samples

[0065]

[0066] Example 1 Composite solvent normal pressure dissolution experiment

[0067] Take 6g of absolute dry pulp (with a characteristic viscosity of about 450) and mix it with 1L of ethanol solution with a volume concentration of about 50%, use a fiber disintegrator to stir it at a speed of 1500r / min for 30000r to break it up, filter the loose pulp and then replace it with anhydrous ethanol to remove moisture, and dry it at low temperature overnight as a dissolved raw material.

[0068] Take 47g of BmimCl powder (without pH adjustment) and 47g of NMMO·H20 powder and mix them at 90℃. After stirring and melting, the reaction temperature is lowered to 70℃ to ensure that the two components in the composite solvent are evenly mixed and there are no unmelted particles.

[0069] At a reaction temperature of 70°C, 6g of the treated absolute dry pulp was added to a total weight of 94g of the composite solvent, and the pulp and composite solvent were mixed with a stirring blade at a stirring speed of 25r / min. After stirring for 60min, the reaction temperature was increased to 90°C, and the stirring blade speed was reduced to 15r / min. After the pulp was dissolved for 90min, a cellulose solution (such as Figure 4 shown).

[0070] Microscopic observation showed that most of the fibers in the solution were dissolved, with only a small amount of intact fibers remaining (e.g. Figure 5 The degradation amount was tested and the intrinsic viscosity value decreased by 90 units. The rheological test data of the cellulose solution showed that the viscosity was lower than that of the cellulose solution obtained by dissolving in Comparative Example 4 (as shown in Table 3). The weighted relaxation time spectrum showed that the viscosity average molecular weight was smaller than that of the cellulose solution obtained by dissolving in Comparative Example 4 (as shown in Table 3). Figure 6 shown).

[0071] Prepare a regenerated cellulose membrane using the cellulose solution obtained in Example 1 (as Figure 7 shown). The tensile test results are shown in Table 4: the elastic modulus increased by about 2 times compared with the comparative sample. This cellulose membrane has high rigidity and shape stability and is suitable for static or low-dynamic load environments and fields resistant to deformation. It lost some breaking strength but still reached the medium strength of the regenerated cellulose membrane material. The elongation at break decreased significantly, and the ductility was low. Repeated bending should be avoided.

[0072] Example 2 Composite Solvent Atmospheric Pressure Dissolution Experiment

[0073] Take 5 g of absolutely dry pulp sheet (intrinsic viscosity about 450) and mix it with 1 L of ethanol solution with a volume concentration of about 50%. Use a fiber defibrator to stir it at a speed of 1500 r / min for 30000 r to break it up. After the loose pulp is filtered by suction, it is then replaced with absolute ethanol to remove moisture, and it is dried at low temperature overnight as the dissolution raw material.

[0074] Take 39 g of BmimAc, 39 g of NMMO·H20 powder and 3.9 g of deionized water and mix them. After stirring and melting at 90 °C, lower the reaction temperature to 50 °C to keep the two components in the composite solvent mixed evenly and there are no unmelted particles.

[0075] At a reaction temperature of 50 °C, add 5 g of the treated absolutely dry pulp sheet to the composite solvent with a total weight of 81.9 g, and mix the pulp sheet and the composite solvent with a stirring speed of 25 r / min by a stirring paddle. After stirring for 90 min, raise the reaction temperature to 70 °C, and at the same time lower the stirring paddle speed to 15 r / min. After the pulp sheet is dissolved for 60 min, a cellulose solution is obtained (as Figure 4 shown).

[0076] After microscopic observation, most of the fibers in the solution are dissolved, and only a small amount of intact fibers are not dissolved (as Figure 5 shown). Test the degradation amount, and the intrinsic viscosity value drops by 44 units. From the rheological data of the cellulose solution, it can be seen that the viscosity is greater than that of the cellulose solution obtained by dissolution in Example 1 (as shown in Table 3). From the weighted relaxation time spectrum, it can be seen that the viscosity-average molecular weight is smaller than that of the cellulose solution obtained by dissolution in Example 1 (as Figure 6 shown).

[0077] The tensile test data of the regenerated cellulose membrane prepared from the cellulose solution in Example 2 (as Figure 7 shown) is similar to that in Example 1. This cellulose membrane has the characteristics of high rigidity, medium strength and extremely low ductility, and is suitable for applications in scenarios with high requirements for shape stability and anti-deformation ability but small dynamic loads.

[0078] Comparative Example 1 Composite Solvent Vacuum Dissolution Experiment

[0079] Take 6 g of absolutely dry pulp board (intrinsic viscosity is about 450) and mix it with 1 L of ethanol solution with a volume concentration of about 50%. Use a fiber defibrator to stir it at a speed of 1500 r / min for 30000 revolutions to disperse it. The loose pulp is filtered by suction and then replaced with absolute ethanol to remove moisture, and it is dried at low temperature overnight as the dissolution raw material.

[0080] Take 47 g of BmimCl powder (pH not adjusted) and 47 g of NMMO·H2O powder and mix them. Stir and melt them at 90 °C to keep the two components in the composite solvent evenly mixed and there are no unmelted particles.

[0081] At a reaction temperature of 90 °C, add 6 g of the treated absolutely dry pulp board to a composite solvent with a total weight of 94 g. The stirring paddle mixes the pulp board and the composite solvent at a stirring speed of 25 r / min. There are many large pieces of pulp board wrapped in the solution that are visible to the naked eye. After stirring for 150 min, start to evacuate (the final vacuum degree is kept below 300 Pa), and at the same time reduce the rotation speed of the stirring paddle to 15 r / min. After stirring for 60 min, there are still many large pieces of pulp board wrapped in the solution. After raising the reaction temperature to 100 °C and stirring for 30 min, the wrapped pulp board in the solution does not decrease.

[0082] Take a sample of the unwrapped part in the cellulose solution (as Figure 4 shown). After microscopic observation, there are still undissolved complete fibers in the solution (as Figure 5 shown).

[0083] It can be seen from Comparative Example 1 that when the composite solvent and the pulp board are mixed at a relatively high reaction temperature (90 °C), the outer pulp board will dissolve first, which will affect the swelling and dissolution of the inner pulp board, resulting in insoluble wrapped pulp in the solution. Even if the vacuum is evacuated, it is difficult to dissolve the wrapped pulp, which affects the dissolution quality.

[0084] Comparative Example 2 Composite Solvent Vacuum Dissolution Experiment

[0085] Take 6 g of absolutely dry pulp board (intrinsic viscosity is about 450) and mix it with 1 L of ethanol solution with a volume concentration of about 50%. Use a fiber defibrator to stir it at a speed of 1500 r / min for 30000 revolutions to disperse it. The loose pulp is filtered by suction and then replaced with absolute ethanol to remove moisture, and it is dried at low temperature overnight as the dissolution raw material.

[0086] Take 47 g of BmimCl powder (pH not adjusted), 47 g of NMMO·H2O powder and 9.4 g of deionized water and mix them. Stir and melt them at 90 °C to keep the two components in the composite solvent evenly mixed and there are no unmelted particles.

[0087] At a reaction temperature of 90 °C, 6 g of treated dry pulp was added to a composite solvent with 10% water and a total weight of 103.4 g. The stirring paddle mixed the pulp and the composite solvent at a stirring speed of 25 r / min. Samples were taken under these conditions for microscopic observation, and all were found to be intact and unswollen fibers. After stirring for 60 min, vacuum pumping was started (the final vacuum was maintained below 300 Pa), and at the same time, the rotation speed of the stirring paddle was reduced to 15 r / min. After the pulp was dissolved for 60 min, a cellulose solution was obtained (as Figure 4 shown).

[0088] After microscopic observation, there were no obvious undissolved fibers in the solution (as Figure 5 shown). The degradation amount was tested, and the intrinsic viscosity value decreased by 73 units.

[0089] It can be seen from Comparative Example 2 that at a relatively high reaction temperature (90 °C), adding a certain amount of water to the composite solvent is beneficial to the uniform dispersion of the pulp. However, if the amount of added water is too large, it is necessary to dissolve thoroughly under vacuum pumping conditions later, increasing the energy consumption for preparing the solution.

[0090] Vacuum pumping dissolution experiment of the composite solvent in Comparative Example 3

[0091] 6 g of dry pulp board (with an intrinsic viscosity of about 450) was mixed with 1 L of ethanol solution with a volume concentration of about 50%. It was dispersed by using a fiber defibrator at a rotation speed of 1500 r / min for 30000 r. The loose pulp was filtered and then replaced with anhydrous ethanol to remove moisture, and it was dried at low temperature overnight as the dissolution raw material.

[0092] 47 g of BmimAc, 47 g of NMMO·H20 powder and 9.4 g of deionized water were mixed and stirred and melted at 90 °C to keep the two components in the composite solvent mixed evenly and without undissolved particles.

[0093] At a reaction temperature of 90 °C, 6 g of treated dry pulp was added to a composite solvent with 10% water and a total weight of 103.4 g. The stirring paddle mixed the pulp and the composite solvent at a stirring speed of 15 r / min. Many large lumps of wrapped pulp were visible to the naked eye in the solution. After stirring for 60 min, the reaction temperature was raised to 95 °C. After stirring for 120 min at this temperature, there were still many large lumps of wrapped pulp in the solution. Vacuum pumping was started (the final vacuum was maintained below 300 Pa), and after stirring for 90 min, the cellulose solution gradually changed from smooth, transparent and brown to rough and light yellow (as Figure 4 shown).

[0094] After microscopic observation, there were no obvious undissolved fibers in the solution (as Figure 5 shown).

[0095] It can be seen from Comparative Example 3 that after replacing the BmimCl component in the composite solvent with BmimAc, due to the stronger dissolution ability of BmimAc for cellulose, at a relatively high reaction temperature (90 °C), even if a certain amount of water (10% water content) is added to the composite solvent, the outer pulp will dissolve first, resulting in insoluble wrapped pulp, and it is difficult to dissolve the wrapped pulp even when the reaction temperature is increased by 5 °C subsequently.

[0096] Comparative Example 4 Pure Solvent Dissolution Experiment (BmimCl)

[0097] Take 6 g of absolutely dry pulp sheet (intrinsic viscosity of about 450) and mix it with 1 L of ethanol solution with a volume concentration of about 50%. Use a fiber defibrator to stir it at a speed of 1500 r / min for 30000 r to disperse it. After the loose pulp is filtered by suction, it is replaced with absolute ethanol to remove moisture, and then dried at low temperature overnight as the dissolution raw material.

[0098] At a reaction temperature of 100 °C, add 6 g of the treated absolutely dry pulp sheet to 123.5 g of pure BmimCl solvent with a total water content of 23.9% (adjusted pH = 10), and mix the pulp sheet and the solvent with a stirring paddle at a speed of 40 r / min. Start vacuum pumping after 15 min (the final vacuum degree is maintained below 100 Pa), and at the same time reduce the stirring paddle speed to 25 r / min. After 30 min, reduce the stirring paddle speed to 15 r / min. After the vacuum pumping time reaches 3 h, end the dissolution to obtain a cellulose solution (as Figure 4 shown).

[0099] After microscopic observation, there are no obvious undissolved fibers in the solution (as Figure 5 shown). Test the degradation amount, and the intrinsic viscosity value drops by 39 units.

[0100] Since the viscosity of the cellulose solution with a pulp concentration of 6% is relatively high and it is difficult to scrape the film manually, the pulp concentration is reduced to 3.5% (the other conditions remain unchanged), and then the film is scraped. After washing and drying, a comparison sample is obtained for tensile testing (as shown in Table 4).

[0101] Comparative Example 5 Pure Solvent Dissolution Experiment (BmimCl)

[0102] Take 6 g of absolutely dry pulp sheet (intrinsic viscosity of about 450) and mix it with 1 L of ethanol solution with a volume concentration of about 50%. Use a fiber defibrator to stir it at a speed of 1500 r / min for 30000 r to disperse it. After the loose pulp is filtered by suction, it is replaced with absolute ethanol to remove moisture, and then dried at low temperature overnight as the dissolution raw material.

[0103] At a reaction temperature of 100 °C, 6 g of treated dry pulp was added to 117.1 g of pure BmimCl solvent containing 19.73% water by weight (pH not adjusted, pH = 6.14), and the pulp and solvent were mixed with a stirring paddle at a speed of 40 r / min. After 15 min, vacuum pumping was started (the final vacuum was maintained below 100 Pa), and at the same time, the speed of the stirring paddle was reduced to 25 r / min. After 30 min, the speed of the stirring paddle was reduced to 15 r / min. After the vacuum pumping time reached 3 h, the dissolution was ended to obtain a cellulose solution (as Figure 4 shown).

[0104] After microscopic observation, there were no obvious undissolved fibers in the solution (as Figure 5 shown). The degradation amount was tested, and the intrinsic viscosity value decreased by 244 units. From the rheological test data of the cellulose solution, it can be seen that the viscosity was much lower than that of the cellulose solutions obtained by dissolution in Examples 1 and 2 (Table 3). From the weighted relaxation time spectrum, it can be seen that the viscosity-average molecular weight was also smaller than that of the cellulose solutions obtained by dissolution in Examples 1 and 2 (as Figure 6 shown). This is related to the large degradation caused by the low pH value of the solvent.

[0105] Dissolution experiment of pure solvent (NMMO) for Comparative Example 6

[0106] 6 g of dry pulp board (intrinsic viscosity about 450) was directly mixed with 159.53 g of NMMO aqueous solution containing 50% water, and 0.5 wt% of propyl gallate was added to the pulp board.

[0107] At a reaction temperature of 95 °C, the pulp and solvent were mixed with a stirring paddle at a speed of 40 r / min. After 15 min, vacuum pumping was started (pressure 6000 Pa). After 2 h, the pressure dropped to 2500 Pa. After the vacuum pumping time reached 3.5 h, the dissolution was ended to obtain a cellulose solution (as Figure 4 shown).

[0108] After microscopic observation, there were no obvious undissolved fibers in the solution (as Figure 5 shown). The degradation amount was tested, and the intrinsic viscosity value decreased by 50 units.

[0109] The ionic liquid and NMMO·H2O were mixed at a mass ratio of 1-9:1 (a small amount of water can be added, and the specific content is related to the type of composite solvent).

[0110] Table 5 Calculation data table of water content of composite solvent

[0111]

[0112] As can be seen from Table 5, the calculated water content of the ionic liquid in the composite solvent used in the present invention is 1.46% to 11.74%, and the calculated overall water content of the composite solvent is 1.33% to 6.65%.

[0113] ILs directly dissolving cellulose (intrinsic viscosity value above 400) requires a water content below 1.0% and a temperature above 80 °C, while the NMMO solution directly dissolving cellulose has a water content of 5 - 13.3%, and as the water content decreases, the dissolution temperature increases, with a minimum requirement of 80 °C. The water in the composite solvent is mainly introduced by NMMO·H20. According to the change in the ratio of ILS and NMMO·H20 in the composite solvent, the proportion of water also changes. If only considering the ratio of ILs and water, the water content is 1.46 - 11.74%, and at this time, ILs do not have the ability to directly dissolve cellulose; if only considering NMMO·H20, when ILS:NMMO·H20 is 9:1, the concentration of cellulose in NMMO·H20 is 40%, exceeding the maximum theoretical concentration of NMMO·H20 for dissolving cellulose. Therefore, the dissolution mechanism of the composite solvent for cellulose is different from that of NMMO·H20 or ILS alone for cellulose dissolution, and all three components of ILS, NMMO, and water play a key role in the dissolution of cellulose.

[0114] The present invention also discloses a cellulose solution prepared according to the above preparation method.

[0115] In summary, the cellulose solution and its preparation method proposed by the present invention can solve the problems of high toxicity in the viscose process, explosion risk of NMMO, high price of ionic liquids, and increased energy consumption due to vacuum dehydration required for dissolution in the prior art.

[0116] In order to prepare a cellulose solution, in addition to the industrialized viscose method and NMMO method, some other solvent systems have been developed, including lithium chloride / N,N-dimethylacetamide (DMAc / LiCl), ionic liquids, NaOH / urea aqueous solution solvent systems, etc. For example, Patent CN116218013B uses corn straw as a raw material, and after alkali treatment, bleaching, pH adjustment, and ultrasonic treatment, a corn straw fiber raw material is obtained, and the DMAc / LiCl system is used to treat the raw material to obtain a cellulose solution for film preparation. For example, Patent CN116120609B mixes the bagasse pulp cellulose powder obtained after ball milling treatment with the ionic liquid AmimCl and then heats and dissolves it to obtain a cellulose solution. For example, Patent CN113788966B mixes urea, alkali, and deionized water in proportion to prepare an alkali / urea aqueous solution, adds inorganic fillers to disperse to obtain a dispersion, cools it to -14 to -10 °C, and then adds cellulose and stirs to obtain a cellulose solution. However, these solvent systems respectively have problems such as high cost, high solution viscosity, the need for a low temperature and a highly alkaline environment during the dissolution process, and non-recyclability.

[0117] The cellulose solution preparation process mentioned in the present invention includes three main processes: dispersion, swelling, and dissolution of pulp. After mixing ionic liquid with NMMO·H2O, the viscosity of the composite solvent is lower than that of the same pure ionic liquid, which is beneficial to the dispersion of cellulose pulp in the solvent, avoiding phenomena such as white core caused by direct and rapid dissolution, and thus improving the dissolution quality. By adjusting the temperature to control the swelling and dissolution of cellulose pulp in the composite solvent, compared with the traditional method of increasing the proportion of water in the system for adjustment, the dehydration process is reduced, and the energy consumption in the dissolution stage can be significantly reduced. Moreover, the ionic liquid / NMMO composite solvent used has lower toxicity than the traditional viscose process, smaller explosion risk compared with the Lyocell process, and at the same time reduces the dosage of ionic liquid, making the price cost lower. Also, due to the relatively low viscosity of the composite solvent itself, when the other conditions (such as cellulose degree of polymerization, dissolution amount, dissolution temperature, etc.) remain unchanged, the movement ability of cellulose molecular chain segments in the solution system is enhanced, which can effectively improve the uniformity of the solution, and the viscosity of the prepared cellulose solution is also relatively low, reducing the difficulty of solution transportation and processing. In addition, no other additives are added to the composite solvent to reduce the degradation of cellulose during dissolution. For example, NMMO needs to add propyl gallate as a stabilizer, and the ionic liquid needs to be adjusted to an appropriate pH value. This not only reduces the use of chemicals but also simplifies the solvent preparation steps. Therefore, the present invention can be used for the preparation of cellulose solutions.

[0118] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope described in this specification.

[0119] The description and application of the present invention here are illustrative and do not intend to limit the scope of the present invention to the above embodiments. The effects or advantages involved in the embodiments may not be reflected in the embodiments due to various factors, and the description of the effects or advantages is not used to limit the embodiments. The deformations and changes of the embodiments disclosed here are possible, and the substitutions and equivalents of various components in the embodiments are well-known to those of ordinary skill in the art. Those skilled in the art should clearly understand that the present invention can be implemented in other forms, structures, arrangements, proportions, and with other components, materials, and parts without departing from the spirit or essential characteristics of the present invention. Other deformations and changes can be made to the embodiments disclosed here without departing from the scope and spirit of the present invention.

Claims

1. A method for preparing a cellulose solution, characterized in that, The preparation method includes the following steps: Solvent preparation step: Take a set ionic liquid and N-methyl morpholine-N-oxide hydrate and mix them in a set mass ratio to form a composite solvent; the viscosity of the composite solvent is lower than that of the same pure ionic liquid. Dissolution step: Under normal pressure conditions, with the first temperature as the initial reaction temperature, add absolute dry pulp to the solvent prepared in the solvent preparation step, and disperse the pulp by stirring; after mixing evenly, raise the temperature to the second temperature. During the temperature increase, the cellulose in the pulp swells; after the temperature reaches the second temperature, react for a set time to make the cellulose dissolve thoroughly; the second temperature is greater than the first temperature.

2. The preparation method of the cellulose solution according to claim 1, wherein: In the dissolution step, the first temperature is 50-70 °C, the second temperature is 70-100 °C; the proportion of pulp is 5-14 wt%.

3. The preparation method of the cellulose solution according to claim 2, wherein: In the dissolution step, during the dispersion of the pulp, the stirring speed is 10-30 r / min.

4. The preparation method of the cellulose solution according to claim 1, wherein: The ionic liquid includes at least one of 1-allyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium chloride, 1-ethyl-3-methylimidazolium chloride, and 1-butyl-3-methylimidazole acetate.

5. The preparation method of the cellulose solution according to claim 1, wherein: Before the dissolution step, the preparation method further includes a solute pretreatment step: Take a set amount of absolute dry pulp board and mix it with a set ethanol solution; the mass of the absolute dry pulp board accounts for 0.8-1.5% of the mass of the ethanol solution; stir it at a speed of 1000-2000 r / min for 20000-40000 r to break it up. The loose pulp is filtered by suction and then replaced with anhydrous ethanol to remove moisture, and it is dried at a low temperature of less than or equal to 60 °C as the dissolution raw material.

6. The preparation method of the cellulose solution according to claim 1, wherein: After the dissolution step, the preparation method further includes a film preparation step: After the prepared cellulose solution is defoamed, use a single-sided preparation device to perform manual film scraping. The obtained regenerated cellulose film is soaked in deionized water for more than 2 h, and the water is changed frequently to wash the solvent in the film, and it is dried at a low temperature of less than or equal to 60 °C to obtain a dry regenerated cellulose film.

7. The preparation method of the cellulose solution according to claim 1, wherein: In the solvent preparation step, the mass ratio of the ionic liquid to N-methyl morpholine-N-oxide hydrate is 1-9:

1.

8. The preparation method of the cellulose solution according to claim 1, wherein: In the solvent preparation step, a set amount of water is also added.

9. The preparation method of the cellulose solution according to claim 8, wherein: The mass of the added water is related to the type of the composite solvent.

10. A cellulose solution prepared by the preparation method according to any one of claims 1 to 9.

Images