Recyclable water-soluble cellulose composite material and preparation method thereof

Abstract

The invention relates to a recyclable water-soluble cellulose composite material and a preparation method thereof, and the preparation method comprises the following steps: (1) placing cellulose in an organic alkali mixed solution, introducing gas, stirring and dissolving to obtain a cellulose organic alkali solution; polylactic acid is placed in an organic solvent and stirred to be dissolved, and a polylactic acid organic solution is obtained; (2) putting isocyanate into the polylactic acid organic solution, adding a catalyst, introducing gas for protection, and performing grafting synthesis to obtain an isocyanate grafted polylactic acid solution; (3) mixing the cellulose organic alkali solution with the isocyanate grafted polylactic acid solution, introducing gas for protection, and carrying out grafting synthesis; (4) pouring the solution grafted in the step (3) into a coagulating bath for regeneration; and (5) collecting the regenerant in the step (4), air-drying, blade-coating to form a film by using a film maker or shaping by using a mold under a hydrothermal condition, and air-drying again. According to the invention, the processability breakthrough of cellulose is realized, and meanwhile, the problem of natural degradation caused by'pseudo degradation 'of a polylactic acid material is solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of cellulose composite materials, and particularly relates to a recyclable water-soluble cellulose composite material and a preparation method thereof. Background Art

[0002] Plastic pollution has become a global environmental crisis, with over 10 million tons of plastic waste entering marine ecosystems annually, threatening biodiversity and exacerbating the spread of microplastics. Although biodegradable materials are considered an ideal alternative to petroleum-based plastics, the current mainstream materials, polylactic acid (PLA) and cellulose, face difficult technical challenges. While PLA can be processed through conventional processes such as melt extrusion and injection molding, its degradation relies heavily on high-temperature industrial composting conditions (55-70°C), requiring years to decompose in the natural environment, making it essentially a "pseudo-degradable" material. Cellulose, the most abundant biopolymer in nature, can achieve a soil degradation rate of >95% within 28 days. However, its dense hydrogen bond network and rigid β-1,4-glycosidic bonds between its molecular chains result in a melting temperature (theoretical value >260°C) far above its thermal decomposition temperature, making it impractical for direct thermoforming. This forces conventional processes to rely on costly and highly toxic ionic liquids or N-methylmorpholine-N-oxide (NMMO) solvent systems, severely restricting its industrial application.

[0003] To overcome the bottleneck in cellulose processing, existing technologies primarily focus on two modification strategies: polyester blending and small molecule grafting. The polyester blending method involves physically blending cellulose with polyesters such as polylactic acid, polyethylene terephthalate, and polybutylene terephthalate-adipate. This method uses the melting point of the polyester as the melting point of the cellulose-polyester mixture, which does not change the inherent "infusibility" of cellulose. Furthermore, this method of blending the cellulose and polyester phases can easily cause macroscopic phase separation, leading to a decrease in properties such as transparency and mechanical properties. The small molecule grafting method, which introduces a dynamic hydrogen-bonding network into aminocellulose to produce heat-processable cellulose plastics, achieves a breakthrough in cellulose "melting" properties. However, the product is a powder that requires hot pressing to form a film, making it impossible to process cellulose materials into 3D profiles. Therefore, both polyester blending and small molecule grafting methods suffer from varying degrees of deficiencies in imparting processability to cellulose. Furthermore, existing technologies have consistently failed to resolve the processing-degradation paradox of cellulose-based materials: the high mechanical properties imparted by the strong hydrogen-bonding network are inherently at odds with ease of processing and rapid degradation. Therefore, developing a new cellulose-based material that can achieve hydrothermal dual-response processing and easy degradation and recycling has become a core challenge to solve the dilemma of bioplastic industrialization.

[0004] Patent document with application number 202210337304.1 discloses a method for achieving thermal processing of cellulose through solvent restriction. Under the condition of adding plasticizer, a certain amount of solvent and moisture are retained through precise control of the residual solvent to prepare a heat-sealable cellulose film. The prepared heat-sealable cellulose film has high transparency, but the mechanical properties of the cellulose film are poor and still need further improvement. Summary of the Invention

[0005] Based on the above-mentioned shortcomings and deficiencies in the prior art, one of the objects of the present invention is to at least solve one or more of the above-mentioned problems in the prior art. In other words, one of the objects of the present invention is to provide a recyclable water-soluble cellulose composite material and a preparation method thereof that meets one or more of the above-mentioned needs.

[0006] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical solutions:

[0007] A method for preparing a recyclable water-soluble cellulose composite material comprises the following steps:

[0008] (1) placing cellulose in an organic alkali mixed solution, introducing gas, stirring and dissolving, and obtaining a cellulose organic alkali solution;

[0009] placing polylactic acid in an organic solvent and stirring to dissolve the polylactic acid to obtain a polylactic acid organic solution;

[0010] (2) placing isocyanate in a polylactic acid organic solution, adding a catalyst, introducing gas protection, and grafting synthesis to obtain an isocyanate-grafted polylactic acid solution;

[0011] (3) mixing the cellulose organic alkali solution with the isocyanate grafted polylactic acid solution, passing through a gas protection, and grafting synthesis;

[0012] (4) pouring the grafted solution from step (3) into a coagulation bath for regeneration;

[0013] (5) The regenerated product in step (4) is collected and air-dried, and then coated into a film or formed into a mold under hydrothermal conditions using a film-making device, and air-dried again.

[0014] As a preferred embodiment, the mass ratio of the cellulose to the polylactic acid is 1:(2-3).

[0015] As a preferred embodiment, in step (1), the organic base mixed solution is a mixed solution of dimethyl sulfoxide and an organic base, the organic base includes one of 1,8-diazabicyclo[5.4.0]undec-7-ene and tetramethylguanidine, and the gas is carbon dioxide; wherein the molar ratio of cellulose to the organic base is 1:(2-4).

[0016] As a preferred embodiment, the cellulose includes plant fiber or microcrystalline cellulose, and the plant fiber includes one or more of cotton, flax, wood pulp, and sugarcane bagasse.

[0017] As a preferred embodiment, in step (1), the organic solvent includes one of tetrahydrofuran and chloroform, and the solid-liquid ratio of polylactic acid to the organic solvent is 1 g: (10-15) mL.

[0018] As a preferred embodiment, in step (2), the isocyanate is one of toluene diisocyanate, diphenylmethane diisocyanate, isophorone diisocyanate, and dicyclohexylmethane-4,4'-diisocyanate, and the gas is nitrogen;

[0019] The usage ratio of polylactic acid to isocyanate is 1g: (0.8-1.2)mmol.

[0020] As a preferred embodiment, in step (2), the catalyst is one of dibutyltin dilaurate, dibutyltin diacetate, and di-n-butyltin dilaurate, and the amount of the catalyst is 0.2-0.35 wt% of the mass of the polylactic acid.

[0021] As a preferred embodiment, in step (4), the coagulation bath is any one of ethanol, methanol, and acetone.

[0022] As a preferred embodiment, in step (5), the hydrothermal conditions are: ultrapure water at 30-45°C;

[0023] The air-drying conditions are: ambient temperature of 15-20° C. and humidity of 40-60%.

[0024] The present invention also provides a water-soluble cellulose composite material prepared by the preparation method described in any of the above schemes.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] (1) The water-soluble cellulose composite material prepared by the present invention is degradable and easy to separate and recycle;

[0027] (2) The raw materials used are bio-polyester and cellulose, which are bio-friendly, non-toxic and harmless;

[0028] (3) Compared with the mechanical property defects of polyester blending and the decreased degradation and recyclability of small molecule grafting, the water-soluble cellulose composite material of the present invention has the characteristics of high strength and high transparency. It can be water-processed at room temperature and heat-processed at about 100°C. It is easy to separate and recycle in mixed waste plastics, achieving a breakthrough in the processability of cellulose materials. At the same time, it solves the natural degradation problem caused by the "pseudo-degradation" of polylactic acid materials, which is conducive to expanding the application market of cellulose and polyester materials in agriculture, packaging and biomedicine. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 Schematic diagram comparing the synthetic routes of each embodiment of the present invention and comparative examples 1-4;

[0030] Figure 2 This is a photo of the product obtained by water processing of the asymmetric plastic of Example 1 of the present invention;

[0031] Figure 3 This is a photo of a product obtained by thermal processing of the asymmetric plastic according to Example 1 of the present invention;

[0032] Figure 4 These are photos of the samples of Example 1 of the present invention and Comparative Examples 1-4 in water. DETAILED DESCRIPTION

[0033] In order to more clearly illustrate the embodiments of the present invention, some embodiments of the present invention are described below. Obviously, for those skilled in the art, other implementation methods can be obtained by replacing these embodiments without creative work.

[0034] The recyclable water-soluble cellulose composite material prepared by the present invention is analyzed by the Gibbs free energy change in the modification of cellulose processing properties under the background of thermodynamics and kinetics: polyester blending increases the disorder of the mixed system by introducing flexible segments, thereby increasing the mixing entropy change to achieve a free energy change ΔG < 0, while small molecule grafting increases the internal energy of cellulose by destroying the cellulose hydrogen bond network, thereby reducing the enthalpy change to achieve a Gibbs free energy change ΔG < 0. Based on the fact that current modification technologies are all limited by the dilemma of "single parameter optimization": polyester blending only increases the mixing entropy change but ignores the cost of enthalpy increase, and small molecule grafting focuses on enthalpy reduction but sacrifices the potential for entropy increase, both of which cannot meet the full parameter synergy condition of the Gibbs free energy equation (ΔG = ΔH-TΔS ≤ 0). Therefore, the present invention realizes the two-way regulation of "entropy and enthalpy" by grafting polylactic acid and cellulose, and controls the content of flexible structure polylactic acid to achieve the introduction of flexible structure while destroying the cellulose hydrogen bond network and avoid macroscopic phase separation. The prepared water-soluble cellulose composite material exhibits shape-processing capabilities that are sensitive to both water and heat, which is also the source of its "asymmetry." During water processing, the hydrophilic dynamic hydrogen bond network of cellulose and the hydrophobic barrier of polylactic acid jointly participate in the shape reshaping of the water-soluble cellulose composite material (also known as "asymmetric plastic"). During thermal processing, the rigid segments of cellulose and the heat-sensitive segments of polylactic acid jointly participate in the shape reshaping of the asymmetric plastic.

[0035] Specifically, the method for preparing the recyclable water-soluble cellulose composite material of the present invention comprises the following steps:

[0036] (1) placing cellulose in an organic alkali mixed solution, introducing gas, and stirring to dissolve the cellulose until it becomes a transparent light yellow viscous state, thereby obtaining a cellulose organic alkali solution;

[0037] Placing polylactic acid in an organic solvent, stirring and dissolving the solution until it becomes uniform and transparent, thereby obtaining a polylactic acid organic solution;

[0038] (2) placing isocyanate in a polylactic acid organic solution, adding a catalyst, introducing gas protection, and grafting synthesis to obtain an isocyanate-grafted polylactic acid solution;

[0039] (3) mixing the cellulose organic alkali solution with the isocyanate grafted polylactic acid solution, passing through a gas protection, and grafting synthesis;

[0040] (4) pouring the grafted solution from step (3) into a coagulation bath for regeneration;

[0041] (5) The regenerated product in step (4) is collected and air-dried, and then coated into a film or formed into a mold under hydrothermal conditions using a film-making device, and air-dried again.

[0042] The mass ratio of the cellulose to the polylactic acid is 1:(2-3), which can be determined within the mass ratio range according to actual application requirements.

[0043] In the above step (1), the organic base mixed solution is a mixed solution of dimethyl sulfoxide (DMSO) and an organic base, the organic base includes one of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and tetramethylguanidine (TMG), and the gas is carbon dioxide; wherein the molar ratio of cellulose to the organic base is 1:(2-4), which can be determined within the molar ratio range according to actual application requirements.

[0044] The cellulose mentioned above includes plant fiber or microcrystalline cellulose, and the plant fiber includes one or more of cotton, flax, wood pulp, and sugarcane bagasse.

[0045] In the above step (1), the organic solvent includes one of tetrahydrofuran and chloroform, and the solid-liquid ratio of polylactic acid to the organic solvent is 1g:(10-15)mL, which can be determined within the range of solid-liquid ratio according to actual application requirements.

[0046] In the above step (2), the isocyanate is one of toluene diisocyanate TDI, diphenylmethane diisocyanate MDI, isophorone diisocyanate IPDI, and dicyclohexylmethane-4,4'-diisocyanate HMDI, and the gas is nitrogen;

[0047] The usage ratio of polylactic acid to isocyanate is 1 g: (0.8-1.2) mmol, and the specific usage ratio can be determined within the range of the usage ratio according to actual application requirements.

[0048] In the above step (2), the catalyst is one of dibutyltin dilaurate, dibutyltin diacetate, and di-n-butyltin dilaurate, and the amount of the catalyst is 0.2-0.35wt% of the mass of the polylactic acid, which can be determined within the mass percentage range according to actual application requirements.

[0049] In the above step (4), the coagulation bath is any one of ethanol, methanol, and acetone.

[0050] In the above step (5), the hydrothermal conditions are: ultrapure water at 30-45°C;

[0051] The film thickness of the film former is 250-750μm; the mold is a customized 3D mold, including spoons, cups, bowls, etc.

[0052] The air-drying conditions are: ambient temperature of 15-20° C. and humidity of 40-60%.

[0053] The present invention also provides a water-soluble cellulose composite material prepared by the above preparation method.

[0054] The recyclable water-soluble cellulose composite material and its preparation method of the present invention are further explained below through specific examples.

[0055] Example 1:

[0056] like Figure 1 As shown, the preparation method of the water-soluble cellulose composite material of this embodiment includes the following steps:

[0057] (1) 1 g of microcrystalline cellulose with a molecular weight of 162.14 was dissolved in a mixed solution of 18.2 mL of dimethyl sulfoxide and 2.74 g of DBU, and a high-flux CO2 solution was introduced. The solution was stirred at a constant speed at 50°C for dissolution until a transparent light yellow viscous state was obtained.

[0058] (2) Dissolve 2 g of polylactic acid in 20 mL of tetrahydrofuran solution and stir at a constant speed at 60°C until the solution becomes transparent and uniform;

[0059] (3) 1.6 mmol of isophorone diisocyanate was added to the polylactic acid solution in step (2), 0.004 g of di-n-butyltin dilaurate as a catalyst was added, high-flux gas N2 was introduced, and the mixture was stirred at a constant speed at 60°C for dissolution, and the reaction conditions were maintained for 20 hours;

[0060] (4) mixing the cellulose solution obtained in step (1) with the polylactic acid solution obtained in step (3), introducing high-flux gas N2, stirring at a constant speed at 60°C for dissolution, and maintaining the reaction conditions for 36 hours;

[0061] (5) pouring the grafted mixed solution in step (4) into a 500 mL ethanol coagulation bath for regeneration, and then air-drying at an ambient temperature of 18° C. and a humidity of 40% to obtain an asymmetric plastic;

[0062] (6) Hydro / thermal processing of asymmetric plastics;

[0063] Water processing: 200 wt% of the relatively asymmetric plastic was added to 30°C ultrapure water to dissolve the plastic. After dissolution, the plastic was coated with a film forming apparatus and air-dried at an ambient temperature of 18°C ​​and a humidity of 40% to obtain a strip film for subsequent mechanical property testing of tensile strength.

[0064] In addition, the plastic can be injected into the mold after it is dissolved, such as Figure 2 As shown, the shape structure of the spoon is obtained;

[0065] Thermal processing: Thermal bonding of asymmetric plastics at 100°C, such as Figure 3 As shown, the strip film is heat-bonded end to end to obtain a loop material.

[0066] Example 2:

[0067] like Figure 1 As shown, the preparation method of the water-soluble cellulose composite material of this embodiment includes the following steps:

[0068] (1) Dissolve 1 g of microcrystalline cellulose in a mixed solution of 18.2 mL of dimethyl sulfoxide and 2.74 g of dibutyl sulfoxide (DBU), introduce high-flux CO2, and stir at a constant speed at 50°C to dissolve until a transparent, light yellow, viscous state is obtained.

[0069] (2) Dissolve 3 g of polylactic acid in 30 mL of tetrahydrofuran solution and stir at a constant speed at 60°C until the solution becomes transparent and uniform;

[0070] (3) adding 3 mmol of isophorone diisocyanate to the polylactic acid solution in step (2), adding 0.009 g of di-n-butyltin dilaurate as a catalyst, passing high-flux gas N2, stirring at a constant speed at 60°C for dissolution, and maintaining the reaction conditions for 24 hours;

[0071] (4) mixing the cellulose solution obtained in step (1) with the polylactic acid solution obtained in step (3), introducing high-flux gas N2, stirring at a constant speed at 60°C for dissolution, and maintaining the reaction conditions for 48 hours;

[0072] (5) pouring the mixed solution grafted in step (4) into a 500 mL methanol coagulation bath for regeneration and air-drying to obtain an asymmetric plastic;

[0073] (6) Hydro / thermal processing of asymmetric plastics;

[0074] Water processing: 150 wt% of the relatively asymmetric plastic was added to 35°C ultrapure water to dissolve the plastic. After dissolution, the plastic was coated with a film forming apparatus and air-dried at an ambient temperature of 20°C and a humidity of 40% to obtain a strip film for subsequent mechanical property testing of tensile strength at break.

[0075] In addition, the plastic can be injected into the mold after being dissolved and air-dried at an ambient temperature of 20°C and a humidity of 40%.

[0076] Thermal processing: Thermal bonding of asymmetric plastics at 100°C.

[0077] Example 3:

[0078] like Figure 1 As shown, the preparation method of the water-soluble cellulose composite material of this embodiment includes the following steps:

[0079] (1) Dissolve 1 g of microcrystalline cellulose in a mixed solution of 18.2 mL of dimethyl sulfoxide and 2.74 g of dibutyl sulfoxide (DBU), introduce high-flux CO2, and stir at a constant speed at 50°C to dissolve until a transparent, light yellow, viscous state is obtained.

[0080] (2) Dissolve 2.5 g of polylactic acid in 37.5 mL of chloroform (CH2Cl2) solution and stir at a constant speed at 60°C until the solution becomes transparent and uniform;

[0081] (3) 2 mmol of toluene diisocyanate was added to the polylactic acid solution in step (2), 0.0075 g of di-n-butyltin dilaurate as a catalyst was added, high-flux gas N2 was introduced, and the mixture was stirred at a constant speed at 60°C for dissolution, and the reaction conditions were maintained for 12 hours;

[0082] (4) mixing the cellulose solution obtained in step (1) with the polylactic acid solution obtained in step (3), introducing high-flux gas N2, stirring at a constant speed at 60°C for dissolution, and maintaining the reaction conditions for 24 hours;

[0083] (5) pouring the mixed solution grafted in step (4) into a 500 mL ethanol coagulation bath for regeneration and air-drying to obtain an asymmetric plastic;

[0084] (6) Hydro / thermal processing of asymmetric plastics;

[0085] Water processing: 200 wt% of the relatively asymmetric plastic was added to 30°C ultrapure water to dissolve the plastic. After dissolution, the plastic was coated with a film forming apparatus and air-dried at an ambient temperature of 20°C and a humidity of 50% to obtain a strip film for subsequent mechanical property testing of tensile strength.

[0086] In addition, the plastic can be injected into the mold after being dissolved and air-dried at an ambient temperature of 20°C and a humidity of 50%.

[0087] Thermal processing: Thermal bonding of asymmetric plastics at 100°C.

[0088] Example 4:

[0089] like Figure 1 As shown, the preparation method of the water-soluble cellulose composite material of this embodiment includes the following steps:

[0090] (1) Dissolve 1 g of microcrystalline cellulose in a mixed solution of 18.2 mL of dimethyl sulfoxide and 2.76 g of tetramethylguanidine (TMG), introduce high-flux CO2, and stir at a constant speed at 50°C to dissolve until a transparent light yellow viscous state is obtained;

[0091] (2) Dissolve 2 g of polylactic acid in 20 mL of tetrahydrofuran solution and stir at a constant speed at 60°C until the solution becomes transparent and uniform;

[0092] (3) adding 2.4 mmol of diphenylmethane diisocyanate (MDI) to the polylactic acid solution in step (2), adding 0.004 g of di-n-butyltin dilaurate as a catalyst, introducing high-flux gas N2, stirring at a constant speed at 60°C for dissolution, and maintaining the reaction conditions for 12-24 hours;

[0093] (4) mixing the cellulose solution obtained in step (1) with the polylactic acid solution obtained in step (3), introducing high-flux gas N2, stirring at a constant speed at 60°C to dissolve, and maintaining the reaction conditions for 24-48 hours;

[0094] (5) pouring the mixed solution grafted in step (4) into a 500 mL ethanol coagulation bath for regeneration and air-drying to obtain an asymmetric plastic;

[0095] (6) Hydro / thermal processing of asymmetric plastics;

[0096] Water processing: 200 wt% of the relatively asymmetric plastic was added to 30°C ultrapure water to dissolve the plastic. After dissolution, the plastic was coated with a film forming apparatus and air-dried at an ambient temperature of 20°C and a humidity of 60% to obtain a strip film for subsequent mechanical property testing of tensile strength.

[0097] In addition, the plastic can be injected into the mold after being dissolved and air-dried at an ambient temperature of 20°C and a humidity of 60%.

[0098] Thermal processing: Thermal bonding of asymmetric plastics at 100°C.

[0099] Example 5:

[0100] like Figure 1 As shown, the preparation method of the water-soluble cellulose composite material of this embodiment includes the following steps:

[0101] (1) Dissolve 1 g of microcrystalline fiber in a mixed solution of 18.2 mL of dimethyl sulfoxide and 1.73 g of tetramethylguanidine (TMG), introduce high-flux CO2, and stir at a constant speed at 50°C to dissolve until it becomes a transparent light yellow viscous state;

[0102] (2) Dissolve 3 g of polylactic acid in 45 mL of tetrahydrofuran solution and stir at a constant speed at 60°C until the solution becomes transparent and uniform;

[0103] (3) adding 2.4 mmol of dicyclohexylmethane-4,4'-diisocyanate (HMDI) to the polylactic acid solution in step (2), adding 0.009 g of di-n-butyltin dilaurate as a catalyst, introducing high-flux gas N2, stirring at a constant speed at 60°C for dissolution, and maintaining the reaction conditions for 12-24 hours;

[0104] (4) mixing the cellulose solution in step (1) with the polylactic acid solution in step (3), introducing high-flux gas N2, stirring at a constant speed at 60°C to dissolve, and maintaining the reaction conditions for 24-48 hours;

[0105] (5) pouring the mixed solution grafted in step (4) into a 500 mL acetone coagulation bath for regeneration and air-drying to obtain an asymmetric plastic;

[0106] (6) collecting the air-dried product in step (5) and performing water / heat processing;

[0107] Water processing: 200 wt% of the relatively asymmetric plastic was added to 45°C ultrapure water to dissolve the plastic. After dissolution, the plastic was coated with a film forming apparatus and air-dried at an ambient temperature of 15°C and a humidity of 40% to obtain a strip film for subsequent mechanical property testing of tensile strength.

[0108] In addition, the plastic can be injected into the mold after being dissolved and air-dried at an ambient temperature of 20°C and a humidity of 50%.

[0109] Thermal processing: Thermal bonding of asymmetric plastics at 100°C.

[0110] Comparative Example 1:

[0111] like Figure 1 As shown, the preparation method of the water-soluble cellulose composite material of this comparative example is different from that of Example 1 in that:

[0112] In step (2), 4 g of polylactic acid was dissolved in 40 mL of tetrahydrofuran solution, and the other steps and process parameters were the same as those in Example 1;

[0113] That is, the mass ratio of microcrystalline fiber to polylactic acid is 1:4.

[0114] Comparative Example 2:

[0115] like Figure 1 As shown, the preparation method of the water-soluble cellulose composite material of this comparative example is different from that of Example 1 in that:

[0116] In step (2), 1 g of polylactic acid was dissolved in 10 mL of tetrahydrofuran solution, and the other steps and process parameters were the same as those in Example 1;

[0117] That is, the mass ratio of microcrystalline fiber to polylactic acid is 1:1.

[0118] Comparative Example 3:

[0119] like Figure 1 As shown, the preparation method of the water-soluble cellulose composite material of this comparative example is different from that of Example 1 in that:

[0120] The PLA solution from steps (1) and (2) is directly mixed with the cellulose solution, the mixed solution is directly cast into a film, and then placed in ethanol for regeneration, and then air-dried to form a film. That is, the PLA and cellulose are regenerated by physical mixing, and no covalent bond is formed between the two.

[0121] Comparative Example 4:

[0122] like Figure 1 As shown, the difference between Comparative Example 4 and Example 1 is that:

[0123] Cellulose nanocrystals (CNCs) were prepared using a conventional sulfuric acid method (64 wt% H2SO4). Deionized water was used to disperse the CNCs, and acetone was used as an intermediate solvent to exchange the water-dispersed CNCs for toluene. L-lactide, a precursor to PLA, was pre-dissolved in toluene. The CNC / toluene suspension was mixed with the L-lactide / toluene solution, and a catalyst (stannous octoate) was added. The polymerization reaction continued for 24 hours and was terminated by adding a few drops of dilute hydrochloric acid solution. The resulting product was precipitated in an excess of cold methanol and dried under vacuum at 60°C for 72 hours. The resulting product was a nanocomposite. This means that cellulose was grafted onto PLA in the form of CNCs. This means that cellulose was grafted onto PLA in the form of nanocrystals, resulting in a powder.

[0124] The following performance tests and analyses were performed on the samples obtained from Example 1 and Comparative Examples 1-4:

[0125] The products of Comparative Examples 1 and 2 do not have thermal processing properties, and their water processing properties are as follows: Figure 4 As shown. In ultrapure water, due to the high content of hydrophobic polylactic acid in Comparative Example 1, the water-soluble cellulose composite material (i.e., asymmetric plastic) cannot be dissolved, and then it cannot be processed anymore; because the content of hydrophobic polylactic acid in Comparative Example 2 is too low, the hydrogen bond network of the hydrophilic cellulose is completely broken up in water, and the hydrogen bonds cannot be reorganized during the dehydration process and cannot be reshaped. In the embodiments of the present invention, by controlling the content of polylactic acid in the asymmetric plastic to 67%-75%, the covalent bonding of the hydrophilic cellulose phase and the hydrophobic polylactic acid phase is achieved without macroscopic phase separation, so that the cellulose and polylactic acid components can synergistically participate in regeneration and molding during water processing and heat processing, and finally achieve water / heat synergistic processing.

[0126] Table 1 Tensile strength at break of samples of Example 1 and Comparative Examples 1-4

[0127] sample Tensile breaking strength / MPa Example 1 120±2.6 Comparative Example 1 25±1.2 Comparative Example 2 10±0.9 Comparative Example 3 28±1.3 Comparative Example 4 powder

[0128] The product of Comparative Example 3 is a blend of cellulose and PLA, which has thermal processing properties and water processing properties. Figure 1 The mechanical properties are shown in Table 1. Comparative Example 3 exhibited neither swelling nor dissolution in ultrapure water and therefore could not be further processed in water. The cellulose and polylactic acid phases in Comparative Example 3 were physically combined, but differences in hydrophilicity, hydrophobicity, molecular chain rigidity, and flexibility between the two phases easily led to macroscopic phase separation, resulting in a decrease in mechanical properties.

[0129] The product of Comparative Example 4 is a grafted product of CNC and PLA, and its water processing performance is as follows: Figure 1 The mechanical properties are shown in Table 1. The product of Comparative Example 4 is a powdery nano-mixture, and its dispersion state in water mainly depends on the dispersibility of CNC in water, which is difficult to shape.

[0130] Table 2 Tensile strength at break of samples from Example 1 after cyclic processing

[0131] Number of cycles Tensile breaking strength / MPa 0 120±2.6 1 119±2.1 2 120±1.9 3 116±0.8 4 119±1.1 5 117±1.0 6 118±1.3

[0132] In summary, the water / heat collaboratively processed asymmetric plastic prepared in the embodiment of the present invention is high in strength and transparency, can be water-processed at room temperature, and can be heat-processed at around 100°C. It can still maintain its original mechanical properties after multiple cycles, as shown in Table 2; and its water solubility makes it easy to separate and recycle in mixed waste plastics, achieving a breakthrough in the processability of cellulose materials, while solving the natural degradation problem caused by the "pseudo-degradation" of polylactic acid materials, which is conducive to expanding the application market of cellulose and polyester materials in agriculture, packaging, and biomedicine.

[0133] Since there are many embodiments of the present invention and the experimental data of each embodiment is huge and voluminous, it is not suitable to list them one by one here. However, the content required for verification and the final conclusions obtained in each embodiment are similar. Therefore, the verification content of each embodiment will not be described one by one here, and only Examples 1-5 are used as representatives to illustrate the excellence of the present invention.

[0134] The above description is only a detailed description of the preferred embodiments and principles of the present invention. For ordinary technicians in this field, there may be changes in the specific implementation methods based on the technical concepts provided by the present invention, and these changes should also be regarded as the scope of protection of the present invention.

Claims

1. A method for preparing a recyclable water-soluble cellulose composite material, characterized in that: The following steps are involved: (1) placing cellulose in an organic alkali mixed solution, introducing gas, stirring and dissolving, and obtaining a cellulose organic alkali solution; placing polylactic acid in an organic solvent and stirring to dissolve the polylactic acid to obtain a polylactic acid organic solution; (2) placing isocyanate in a polylactic acid organic solution, adding a catalyst, introducing gas protection, and grafting synthesis to obtain an isocyanate-grafted polylactic acid solution; (3) mixing the cellulose organic alkali solution with the isocyanate grafted polylactic acid solution, passing through a gas protection, and grafting synthesis; (4) pouring the grafted solution from step (3) into a coagulation bath for regeneration; (5) The regenerated product in step (4) is collected and air-dried, and then coated into a film or formed into a mold under hydrothermal conditions using a film-making device, and air-dried again.

2. The preparation method according to claim 1, characterized in that The mass ratio of the cellulose to the polylactic acid is 1:(2-3).

3. The preparation method according to claim 1, characterized in that In the step (1), the organic base mixed solution is a mixed solution of dimethyl sulfoxide and an organic base, the organic base includes one of 1,8-diazabicyclo[5.4.0]undec-7-ene and tetramethylguanidine, and the gas is carbon dioxide; wherein the molar ratio of cellulose to the organic base is 1:(2-4).

4. The preparation method according to claim 1, characterized in that The cellulose includes plant fiber or microcrystalline cellulose, and the plant fiber includes one or more of cotton, flax, wood pulp, and sugarcane bagasse.

5. The preparation method according to claim 1, characterized in that In the step (1), the organic solvent includes one of tetrahydrofuran and chloroform, and the solid-liquid ratio of polylactic acid to the organic solvent is 1 g: (10-15) mL.

6. The preparation method according to claim 1, characterized in that In the step (2), the isocyanate is one of toluene diisocyanate, diphenylmethane diisocyanate, isophorone diisocyanate, and dicyclohexylmethane-4,4'-diisocyanate, and the gas is nitrogen; The usage ratio of polylactic acid to isocyanate is 1g: (0.8-1.2)mmol.

7. The preparation method according to claim 1, characterized in that In the step (2), the catalyst is one of dibutyltin dilaurate, dibutyltin diacetate, and di-n-butyltin dilaurate, and the amount of the catalyst is 0.2-0.35 wt% of the mass of the polylactic acid.

8. The preparation method according to claim 1, characterized in that In the step (4), the coagulation bath is any one of ethanol, methanol, and acetone.

9. The preparation method according to claim 1, characterized in that In the step (5), the hydrothermal conditions are: ultrapure water at 30-45°C; The air-drying conditions are: ambient temperature of 15-20° C. and humidity of 40-60%.

10. The water-soluble cellulose composite material obtained by the preparation method according to any one of claims 1 to 9.

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