Method for preparing high glass transition temperature polyimide by low temperature synthesis and applications thereof

Abstract

The application discloses a preparation method of low-temperature synthesis of high glass transition temperature polyimine and application thereof, and belongs to the technical field of polymer materials. The preparation method comprises the following steps: dissolving aromatic polyaldehyde in a solvent, adding polyethylene imine and aliphatic diamine, and gradient heating to obtain polyimine; and the molar ratio of the aromatic polyaldehyde, the aliphatic diamine and the polyethylene imine is 1:(0.1-0.8):(0.1-0.6). The preparation method can be used for synthesizing polyimine with a glass transition temperature higher than 135 DEG C at a temperature lower than or equal to 120 DEG C. The preparation method can be used for preparing plant fiber composite materials with high strength and heat resistance, the glass transition temperature of the plant fiber composite materials is higher than 140 DEG C, the tensile strength and modulus are respectively higher than or equal to 200 MPa and 2 GPa, and the bending strength and modulus are respectively higher than or equal to 150 MPa and 13 GPa; and the plant fiber composite materials can be degraded and recycled; compared with traditional plastics, the plant fiber composite materials are more heat-resistant, higher in strength, easier to recycle and degradable, and can be used in the fields of light-weighted decoration parts of vehicles such as automobiles, airplanes and high-speed trains, indoor furniture and sports equipment and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high polymer materials, in particular to a preparation method of low-temperature synthesized high glass transition temperature polyimine and application thereof. BACKGROUND

[0002] In the past decade, the market of natural fiber composites has been rapidly developing. In 2022, the global market of natural fiber composites reached about 320 million US dollars per year (Global Markets Insights, Nature fibers composites market size). However, at present, natural fiber composites mainly use traditional non-degradable thermoplastic and thermosetting polymer matrices, and the fibers and matrices cannot be separated, so the damaged matrices or fibers cannot be replaced. Although the waste thermoplastic polymer matrix natural fiber composites can be crushed and treated at high temperature to melt and reshape, the performance is greatly reduced, so they are often directly incinerated or landfilled, causing a large amount of resource waste and secondary pollution (Zhao X, et al. Resour Conserv Recy., 2022, 177: 105962).

[0003] Based on dynamic covalent chemistry, Vitrimer can depolymerize within a few hours under specific conditions and can be recycled in a closed loop. Using such matrices, recyclable natural fiber composites can be prepared (Zhou L, et al. Mater. Rev., 2020, 34(S1): 585-591). Among them, polyimine Vitrimer can improve the performance of composites because of the hydrogen bonds formed between the imine groups and the hydroxyl groups of the fibers, without the need for modified fibers, making it an ideal matrix for preparing recyclable natural fiber composites (Li P, et al. Chem Eng J., 2023, 457: 141341). However, the curing temperature of polyimine is higher than the tolerance temperature of natural fibers, which limits the promotion and application of recyclable natural fiber composites. Therefore, it is urgent to develop a preparation method of low-temperature synthesized high glass transition temperature polyimine and application thereof. SUMMARY

[0004] In view of the problems existing in the prior art, the first aspect of the present application provides a preparation method of low-temperature synthesized high glass transition temperature polyimine. The second aspect of the present application provides a low-temperature synthesized high glass transition temperature polyimine. The preparation method of the present application can synthesize polyimine with a higher glass transition temperature (>135℃) at a lower temperature (≤120℃). The third aspect of the present application provides a preparation method of a fiber composite material. The fiber composite material prepared by the present application has high strength and excellent heat resistance. The above technical effects are achieved by the following technical solutions.

[0005] The first aspect of the present application provides a preparation method of a high glass transition temperature polyimine synthesized at low temperature, comprising the following steps:

[0006] dissolving the aromatic polyaldehyde in an organic solvent, then adding and mixing the polyethylene imine and the aliphatic diamine, and gradient heating to obtain the polyimine;

[0007] The molar ratio of the aromatic polyaldehyde, the aliphatic diamine and the polyethylene imine is 1:(0.1-0.8):(0.1-0.6).

[0008] Compared with the prior art, the preparation method of the present application uses short-chain aliphatic diamine; compared with long-chain aliphatic diamine, the short-chain aliphatic diamine has a shorter chain after reacting with the aromatic polyaldehyde, so that the crosslinking network of the prepared polyimine is more compact; in addition, the polyethylene imine has more primary amine side chains, which can further improve the density of the crosslinking network and also accelerate the reaction rate. In addition, the compact crosslinking network can promote the formation of hydrogen bonds between the hydroxyl groups on the secondary amine side chains of the polyethylene imine and the imine bonds, thereby improving the intermolecular forces of the crosslinking network. Thus, a polyimine with a glass transition temperature > 135℃ can be prepared at a temperature ≤ 120℃.

[0009] Preferably, the weight average molecular weight of the polyethylene imine is 300-18000.

[0010] Preferably, the aromatic polyaldehyde includes at least one of p-phthaldehyde, o-phthaldehyde, 1,3-benzenedialdehyde and 4,4-biphenyldialdehyde.

[0011] Preferably, the aliphatic diamine is at least one of ethylenediamine, 1,3-propanediamine and 1,4-butanediamine.

[0012] Preferably, the organic solvent is at least one of dichloromethane and tetrahydrofuran.

[0013] Preferably, the mass ratio of the aromatic polyaldehyde to the solvent is 1:(10-100).

[0014] Preferably, the gradient heating conditions are as follows: the first heating temperature is 20-40℃, and the time is 3-5h; the second heating temperature is 70-90℃, and the time is 3-5h; and the third heating temperature is 100-120℃, and the time is 1-3h.

[0015] The second aspect of the present application provides a high glass transition temperature polyimine synthesized at low temperature, which is prepared by the above preparation method.

[0016] The third aspect of the present application provides a preparation method of a fiber composite material, wherein natural plant fibers are added when the low-temperature synthetic high glass transition temperature polyimide is prepared by using the above preparation method, so as to prepare the fiber composite material.

[0017] Alternatively, the natural plant fibers are added after the low-temperature synthetic high glass transition temperature polyimide is prepared by using the above preparation method.

[0018] Preferably, the preparation method of the fiber composite material comprises the following steps:

[0019] The aromatic polyaldehyde is dissolved in an organic solvent, then the polyethylene imine and the aliphatic diamine are mixed and poured into a container containing the natural plant fibers, and gradient heating is performed to obtain the fiber composite material.

[0020] Preferably, the molar ratio of the aromatic polyaldehyde, the aliphatic diamine and the polyethylene imine is 1:(0.1-0.8):(0.1-0.6).

[0021] Preferably, the mass ratio of the aromatic polyaldehyde and the natural plant fibers is 1:(1-10).

[0022] Preferably, the mass ratio of the aromatic polyaldehyde and the solvent is 1:(10-100).

[0023] Preferably, the natural plant fibers comprise at least one of ramie fibers, flax fibers, hemp fibers, bamboo fibers and straw fibers.

[0024] Compared with the prior art, the present application has the following advantages:

[0025] 1. The preparation method of the low-temperature synthetic high glass transition temperature polyimide can synthesize polyimide with a high glass transition temperature (>135℃) at a relatively low temperature (≤120℃);

[0026] 2. Compared with the existing fiber composite materials, the fiber composite material of the present application has high strength and excellent heat resistance, with a glass transition temperature >140℃, a tensile strength ≥200MPa, a tensile modulus ≥2GPa, a bending strength ≥150MPa and a bending modulus ≥13GPa; and the fiber composite material of the present application can be degraded after reaching the service life, and each component can be reused to prepare a new composite material, realizing closed-loop recycling, and the performance of the newly prepared fiber composite material has no significant difference.

[0027] 3、The existing polyimine composite preparation method mainly uses tri(2-aminoethyl)amine as a crosslinking agent to synthesize polyimine, and the selling price of 100ml is as high as 2500 yuan or more; and the polyethylene imine used in the application can play a crosslinking role, and the selling price of 100g is only 90 yuan, so the production cost of the polyimine of the application is greatly reduced, and it is more suitable for industrial application;

[0028] 4、The fiber composite material of the application is heat-resistant, high-strength, light, and easy to recycle, which can solve the problems of traditional plastics, such as not heat-resistant, low strength, not degradable and difficult to recycle, and can replace traditional plastics, and has a wide application prospect in the fields of automotive, aircraft, high-speed rail interior trim parts, indoor furniture and sports equipment. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 Tan δ of the polyimine of Example 1 changes with temperature;

[0030] Figure 2 Tan δ of the fiber composite material of Example 1 changes with temperature;

[0031] Figure 3 Tan δ of the polyimine of Example 2 changes with temperature;

[0032] Figure 4 Tan δ of the fiber composite material of Example 2 changes with temperature;

[0033] Figure 5 Tan δ of the polyimine of Example 3 changes with temperature;

[0034] Figure 6 Tan δ of the fiber composite material of Example 3 changes with temperature;

[0035] Figure 7 Tan δ of the polyimine of Comparative Example 1 changes with temperature;

[0036] Figure 8 Tan δ of the polyimine of Comparative Example 2 changes with temperature;

[0037] Figure 9 Tensile stress-strain curve of the fiber composite material of Example 1;

[0038] Figure 10 Bending stress-strain curve of the fiber composite material of Example 1;

[0039] Figure 11 Tensile stress-strain curve of the fiber composite material of Example 2;

[0040] Figure 12A bending stress-strain curve graph for the fiber composite material of Example 2;

[0041] Figure 13 A tensile stress-strain curve graph for the fiber composite material of Example 3;

[0042] Figure 14 A bending stress-strain curve graph for the fiber composite material of Example 3. DETAILED DESCRIPTION

[0043] In order to further illustrate the present application, the technical solutions provided by the present application are described in detail below in conjunction with examples, but they should not be understood as limiting the protection scope of the present application.

[0044] The present application provides an exemplary method for preparing a low-temperature synthetic high glass transition temperature polyimide, comprising the following steps:

[0045] The aromatic polyaldehyde is dissolved in an organic solvent, then polyethyleneimine and aliphatic diamine are added and mixed, and gradient heating is performed to obtain the polyimide.

[0046] The molar ratio of the aromatic polyaldehyde, aliphatic diamine and polyethyleneimine can be 1:(0.1-0.8):(0.1-0.6).

[0047] In some examples of the present application, the weight average molecular weight of the polyethyleneimine can be 300-18000.

[0048] In some examples of the present application, the aromatic polyaldehyde includes at least one of p-phthaldehyde, o-phthaldehyde, 1,3-benzenedialdehyde and 4,4-biphenyl dialdehyde.

[0049] In some examples of the present application, the aliphatic diamine can be at least one of ethylenediamine, 1,3-propanediamine and 1,4-butanediamine.

[0050] In some examples of the present application, the organic solvent can be at least one of dichloromethane and tetrahydrofuran.

[0051] In some examples of the present application, the conditions for gradient heating are as follows: the first heating temperature is 20-40℃, and the time is 3-5h; the second heating temperature is 70-90℃, and the time is 3-5h; the third heating temperature is 100-120℃, and the time is 1-3h.

[0052] In some examples of the present application, the mass ratio of the aromatic polyaldehyde to the solvent can be 1:(10-100).

[0053] The present application also exemplarily provides a method for preparing a fiber composite material, comprising the following steps:

[0054] The aromatic polyaldehyde is dissolved in an organic solvent, then polyethyleneimine and aliphatic diamine are mixed and poured into a container containing natural plant fibers, and then gradient heating is performed to obtain a fiber composite material.

[0055] In some examples of the present application, the molar ratio of the aromatic polyaldehyde, the aliphatic diamine and the polyethyleneimine can be 1:(0.1-0.8):(0.1-0.6).

[0056] In some examples of the present application, the mass ratio of the aromatic polyaldehyde and the natural plant fibers is 1:(1-10).

[0057] In some examples of the present application, the mass ratio of the aromatic polyaldehyde and the solvent is 1:(10-100).

[0058] In some examples of the present application, the natural plant fibers include at least one of ramie fibers, flax fibers, hemp fibers, bamboo fibers and straw fibers.

[0059] The technical solutions of the present application will be described clearly and completely below. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0060] Embodiment 1

[0061] The present embodiment provides a preparation method of low-temperature synthesis of high glass transition temperature polyimine, comprising the following steps:

[0062] The terephthaldehyde is dissolved in tetrahydrofuran, then polyethyleneimine with a weight average molecular weight of 300 and ethylenediamine are mixed and poured into a mold, heated at 20℃ for 5h, then heated at 90℃ for 3h, and finally heated at 120℃ for 1h to obtain polyimine;

[0063] The mass ratio of the terephthaldehyde, the ethylenediamine and the polyethyleneimine is 1:0.8:0.1, and the mass ratio of the terephthaldehyde and the tetrahydrofuran is 1:10.

[0064] The present embodiment also provides a preparation method of a fiber composite material, comprising the following steps:

[0065] The terephthaldehyde is dissolved in tetrahydrofuran, then polyethyleneimine with a weight average molecular weight of 300 and ethylenediamine are mixed and poured into a mold containing ramie fibers, heated at 20℃ for 5h, then heated at 90℃ for 3h, and finally heated at 120℃ for 1h to obtain a fiber composite material;

[0066] The molar ratio of the p-phthaldehyde, ethylenediamine and polyethyleneimine is 1:0.8:0.1; and the mass ratio of the p-phthaldehyde, tetrahydrofuran and ramie fiber is 1:10:1.

[0067] Embodiment 2

[0068] The embodiment provides a preparation method of a high glass transition temperature polyimine synthesized at a low temperature, and the method comprises the following steps:

[0069] The p-phthaldehyde is dissolved in dichloromethane, then the polyethyleneimine with a weight average molecular weight of 600 and 1,3-propanediamine are uniformly mixed, and then the mixture is poured into a mold, and heated at 30 DEG C for 4h, then heated at 80 DEG C for 4h, and finally heated at 110 DEG C for 2h to obtain the polyimine.

[0070] The molar ratio of the p-phthaldehyde, 1,3-propanediamine and polyethyleneimine is 1:0.4:0.47; and the mass ratio of the p-phthaldehyde and dichloromethane is 1:50.

[0071] The embodiment further provides a preparation method of a fiber composite material, and the method comprises the following steps:

[0072] The p-phthaldehyde is dissolved in dichloromethane, then the polyethyleneimine with a weight average molecular weight of 600 and 1,3-propanediamine are uniformly mixed, and then the mixture is poured into a mold containing flax fibers, and heated at 30 DEG C for 4h, then heated at 80 DEG C for 4h, and finally heated at 110 DEG C for 2h to obtain the fiber composite material.

[0073] The molar ratio of the p-phthaldehyde, 1,3-propanediamine and polyethyleneimine is 1:0.4:0.47; and the mass ratio of the p-phthaldehyde, dichloromethane and flax fibers is 1:50:5.

[0074] Embodiment 3

[0075] The embodiment provides a preparation method of a high glass transition temperature polyimine synthesized at a low temperature, and the method comprises the following steps:

[0076] The 4,4-biphenyl dicarboxaldehyde is dissolved in tetrahydrofuran, then the polyethyleneimine with a weight average molecular weight of 1800 and 1,4-butanediamine are uniformly mixed, and then the mixture is poured into a mold, and heated at 40 DEG C for 3h, then heated at 70 DEG C for 5h, and finally heated at 100 DEG C for 3h to obtain the polyimine.

[0077] The molar ratio of the 4,4-biphenyl dicarboxaldehyde, 1,4-butanediamine and polyethyleneimine is 1:0.1:0.6; and the mass ratio of the 4,4-biphenyl dicarboxaldehyde and tetrahydrofuran is 1:100.

[0078] The embodiment further provides a preparation method of a fiber composite material, and the method comprises the following steps:

[0079] 4,4-Biphenyldicarboxaldehyde was dissolved in tetrahydrofuran, and then polyethyleneimine and 1,4-butanediamine with a weight average molecular weight of 1800 were added and mixed. The mixture was poured into a mold containing hemp fibers and heated at 40°C for 3 hours, then at 70°C for 5 hours, and finally at 100°C for 3 hours to obtain a fiber composite material.

[0080] The molar ratio of 4,4-biphenyldicarboxaldehyde, 1,4-butanediamine, and polyethyleneimine is 1:0.1:0.6; the mass ratio of 4,4-biphenyldicarboxaldehyde, tetrahydrofuran, and hemp fiber is 1:100:10.

[0081] Comparative Example 1

[0082] This comparative example is basically the same as Example 1, except that in the preparation method of low-temperature synthesis of high glass transition temperature polyimide, tris(2-aminoethyl)amine is used to replace polyethyleneimine in an equal amount.

[0083] Comparative Example 2

[0084] This comparative example is basically the same as Example 1, except that in the preparation method of low-temperature synthesis of high glass transition temperature polyimide, ethylenediamine is replaced by an equal amount of decanediamine.

[0085] Application Example 1

[0086] The polyimide-fiber composites of Examples 1-3 and the polyimide of Comparative Examples 1-3 were all heated at a rate of 3℃ / min using the tensile mode of a DMAQ800 dynamic thermomechanical analyzer. The sample size was 25mm × 5mm, and the thickness was measured using digital calipers. The glass transition temperature was determined. The results are shown in Table 1 below.

[0087] Example 1: The Tan δ of polyimide as a function of temperature is shown in the figure. Figure 1 As shown. By Figure 1 It can be seen that the glass transition temperature of polyimide reaches 162℃. The Tan δ curve of the fiber composite material in Example 1 as a function of temperature is shown below. Figure 2 As shown. By Figure 2 It can be seen that the glass transition temperature of fiber composite materials reaches 175℃.

[0088] Example 2: The Tan δ of polyimide as a function of temperature is shown in the figure. Figure 3 As shown. By Figure 3 It can be seen that the glass transition temperature of polyimide reaches 154℃. The Tan δ curve of the fiber composite material in Example 2 as a function of temperature is shown below. Figure 4 As shown. By Figure 4 It can be seen that the glass transition temperature of fiber composite materials reaches 160℃.

[0089] Example 3: The Tan δ of polyimide as a function of temperature is shown in the figure. Figure 5 As shown. By Figure 5 It can be seen that the glass transition temperature of polyimide reaches 136℃. The Tan δ curve of the fiber composite material in Example 3 as a function of temperature is shown below. Figure 6 As shown. By Figure 6 It can be seen that the glass transition temperature of fiber composite materials reaches 145℃.

[0090] The Tan δ curve of polyimide in Comparative Example 1 as a function of temperature is shown below. Figure 7 As shown. By Figure 7 It is known that the glass transition temperature of polyimide is only 93℃.

[0091] The Tan δ curves of Comparative Example 2 (polyimide) as a function of temperature are shown below. Figure 8 As shown. By Figure 8 It is known that the glass transition temperature of polyimide is only 65℃.

[0092] Table 1. Measurement data of Examples 1-3 and Comparative Examples 1-3

[0093]

[0094] As shown in Table 1, the glass transition temperature of the polyimide in Examples 1-3 is significantly higher than that of Comparative Examples 1-3, and the glass transition temperature of the fiber composite materials in Examples 1-3 is all above 145°C.

[0095] Application Example 2

[0096] Recycling Example 1: The fiber composite material from Example 1 was immersed in a tetrahydrofuran solution containing ethylenediamine and stirred at 50 rpm for 1 hour. The polyimide in the fiber composite material of Example 1 depolymerized, resulting in a degraded polyimide mixture, thereby separating the hemp fibers from the polyimide matrix. The hemp fibers were placed back into the mold, and polyethyleneimine and aromatic polyaldehyde were added to the degraded polyimide mixture according to the reaction system of Example 1. After mixing, the mixture was poured back into the mold, and the fiber composite material was prepared again according to the preparation method of the fiber composite material in Example 1.

[0097] Recovery Example 2: The fiber composite material of Example 2 was immersed in a tetrahydrofuran solution containing 1,3-propanediamine and stirred at 50 rpm for 1 hour. The hemp fibers in the fiber composite material of Example 2 separated from the polyimide matrix. After separation, the fiber composite material was prepared again according to the preparation method of the fiber composite material in Example 2.

[0098] Recovery Example 3: The fiber composite material of Example 3 was immersed in a dichloromethane solution containing 1,4-butanediamine and stirred at 50 rpm for 1 hour. The hemp fibers in the fiber composite material of Example 3 separated from the polyimide matrix. After separation, the fiber composite material was prepared again according to the preparation method of the fiber composite material in Example 3.

[0099] The fiber composite materials of Examples 1-3 and the fiber composite materials reprocessed from recycled Examples 1-3 were subjected to tensile strength, tensile modulus, flexural strength, and flexural modulus measurements according to ASTM D638 and ASTM D790-17 standards, respectively. The results are shown in Table 2 below.

[0100] Example 1: Tensile stress-strain curve of fiber composite material as shown in Figure 1 Figure 9 As shown. By Figure 9 It can be seen that the tensile strength of the fiber composite material is 220 MPa. The bending stress-strain curve of the fiber composite material in Example 1 is shown below. Figure 10 As shown. By Figure 10 It can be seen that the flexural strength of the fiber composite material is 155 MPa.

[0101] Example 2: Tensile stress-strain curve of fiber composite material as shown in Figure 2 Figure 11 As shown. By Figure 11 It can be seen that the tensile strength of the fiber composite material is 210 MPa. The bending stress-strain curve of the fiber composite material in Example 2 is shown below. Figure 12 As shown. By Figure 12 It can be seen that the flexural strength of the fiber composite material is 152 MPa.

[0102] Example 3: Tensile stress-strain curve of fiber composite material as shown in Figure 3. Figure 13 As shown. By Figure 13 It can be seen that the tensile strength of the fiber composite material is 215 MPa. The bending stress-strain curve of the fiber composite material in Example 3 is shown below. Figure 14 As shown. By Figure 14 It can be seen that the flexural strength of the fiber composite material is 150 MPa.

[0103] Table 2. Measurement results of Examples 1-3 and Recovery Examples 1-3

[0104]

[0105] As shown in Table 2, the tensile strength and flexural strength of the fiber composites in Examples 1-3 can reach 220 MPa and 155 MPa, respectively. Furthermore, when the fiber composites in Examples 1-3 are recycled and then reprocessed into fiber composites, their tensile strength, modulus, flexural strength, and modulus do not show a significant decrease and remain similar to the mechanical properties of the fiber composites before recycling.

[0106] As shown in Table 1 and Table 2, the polyimine and the fiber composite material prepared by the application have excellent heat resistance. The fiber composite material of the application also has high strength, can be recycled in a closed loop, and the performance of the composite material prepared after recycling has no significant difference.

[0107] The above detailed description of the embodiments of the application, but the application is not limited to the specific details of the above embodiments. Within the scope of the claims and technical concepts of the application, the technical solutions of the application can be modified and changed in many simple ways, and these simple changes all belong to the protection scope of the application.

Claims

1. A method for the preparation of a low temperature synthetic high glass transition temperature polyimide, characterized in that, The method comprises the following steps: dissolving aromatic polyaldehyde in an organic solvent, then adding polyethylene imine and aliphatic diamine, and mixing, and gradient heating to obtain polyimine; The molar ratio of the aromatic polyaldehyde, the aliphatic diamine, and the polyethylene imine is 1:(0.1-0.8):(0.1-0.6). The aliphatic diamine is at least one of ethylenediamine, 1,3-propanediamine, and 1,4-butanediamine. The aromatic polyaldehyde comprises at least one of p-phthaldehyde, o-phthaldehyde, 1,3-benzene dicarboxaldehyde, and 4,4-biphenyl dicarboxaldehyde. The gradient heating is performed at a first heating temperature of 20-40 DEG C for 3-5 hours, a second heating temperature of 70-90 DEG C for 3-5 hours, and a third heating temperature of 100-120 DEG C for 1-3 hours. The organic solvent is at least one of dichloromethane and tetrahydrofuran.

2. The production method according to claim 1, characterized by, The weight average molecular weight of the polyethylene imine is 300-18000.

3. A low temperature synthesis of high glass transition temperature polyimides, characterized in that, The polyimine is prepared by the preparation method in any one of claims 1-2.

4. A method of producing a fiber composite material, characterized by In the preparation of the low-temperature synthetic high glass transition temperature polyimine by the preparation method in any one of claims 1-2, natural plant fibers are further added to prepare the fiber composite material. Alternatively, the low-temperature synthetic high glass transition temperature polyimine is prepared by the preparation method in any one of claims 1-2, and then natural plant fibers are added to prepare the fiber composite material.

5. The preparation method according to claim 4, characterized in that, The method comprises the following steps: dissolving aromatic polyaldehyde in an organic solvent, then adding polyethylene imine and aliphatic diamine, and mixing, and gradient heating to obtain polyimine; 6. The preparation method according to claim 4, characterized in that, The molar ratio of the aromatic polyaldehyde, the aliphatic diamine, and the polyethylene imine is 1:(0.1-0.8):(0.1-0.6).

7. The method of any one of claims 4 to 6, wherein the method further comprises the step of: The natural plant fibers comprise at least one of ramie fibers, flax fibers, hemp fibers, bamboo fibers, and straw fibers.

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