Process for the preparation of heat and water resistant polyimine and its application in natural plant fiber composites

Abstract

The application discloses a preparation method of heat-resistant and water-resistant polyimine and application of the polyimine in natural plant fiber composite materials, and belongs to the technical field of high polymer materials. The preparation method of the heat-resistant and water-resistant polyimine is as follows: polyvinyl imine, aromatic polyaldehyde and aromatic diamine are gradient heated at <= 120 DEG C to synthesize the polyimine which is heat-resistant and has low water absorption, the glass transition temperature of the polyimine is > 145 DEG C, and the water absorption of the polyimine is <= 0.4%. The polyimine is applied to the natural plant fiber composite materials to prepare the composite materials which have the properties of high strength, heat resistance, low water absorption and the like, the tensile strength of the composite materials is > 180 MPa, the bending strength is > 150 MPa, the water absorption is < 0.5%, and the composite materials can be degraded and recycled. Compared with existing plastics and composite materials, the polyimine and the composite materials have better heat resistance, strength and hydrolysis resistance, and can be used as light-weight and recyclable materials required in the fields of automobiles, high-speed rails, airplanes and the like, and as environment-friendly materials required in the fields of furniture, buildings and sports.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high molecular materials, in particular to a preparation method of heat-resistant and water-resistant polyimine and its application in natural plant fiber composites. BACKGROUND

[0002] Lightweight and environmentally friendly natural fiber composites can improve the power and fuel efficiency of vehicles, high-speed trains, and aircraft, and reduce emissions. The market is rapidly developing, and the market value is expected to reach 2.3 billion US dollars by 2032 (Global Markets Insights, Nature fibers composites market size). However, the current natural fiber composites mainly use non-degradable thermoplastic and thermosetting polymer matrices, making it difficult to recycle and reuse the composites, which are usually directly landfilled or incinerated, causing a large amount of resource waste and secondary pollution (Zhao X, et al. Resour Conserv Recy., 2022, 177: 105962).

[0003] Polyimine Vitrimer has dynamic reversible covalent bonds and can be degraded or converted into recyclable materials within a few hours under specific conditions. Based on such matrices, recyclable natural fiber composites can be prepared (Zhou Lisheng et al., Materials Review, 2020, 34(S1): 585~591). Moreover, the imine groups in polyimine Vitrimer can form hydrogen bonds with the hydroxyl groups of the fibers. The application of polyimine to fiber composites can improve the performance of the composites without modifying the fibers, making it an ideal matrix for preparing recyclable natural fiber composites (Su Z, et al. J Mater Chem A., 2020, 8(28): 14082~14090).

[0004] However, the current polyimine curing temperature is higher than the tolerance temperature of natural fibers, resulting in damage to the mechanical properties of natural fibers. Moreover, polyimine has high water absorption and poor hydrolysis resistance, which limits its use in producing exterior parts of vehicles and other transportation tools. Therefore, there is an urgent need to develop a method for synthesizing polyimine with high glass transition temperature and low water absorption at a temperature that does not damage the mechanical properties of natural fibers, thereby preparing natural fiber composites with excellent mechanical, thermal, and hydrolysis resistance. SUMMARY

[0005] In view of the above prior art deficiencies, one of the purposes of the present application is to provide a low-temperature preparation method of heat-resistant and water-resistant polyimine, which solves the problems of high curing temperature and high water absorption of polyimine, and further causes the natural plant fiber composite material used for the production of vehicle exterior parts to be difficult to have mechanical and thermal properties and hydrolysis resistance, and difficult to be recycled.

[0006] To achieve the above-mentioned purposes, the specific technical solutions of the present application are as follows:

[0007] The preparation method of heat-resistant and water-resistant polyimine comprises the following steps: uniformly mixing aromatic polyaldehyde, polyethylene imine and aromatic diamine in a solvent, and then heating to obtain heat-resistant and water-resistant polyimine.

[0008] Preferably, the molar ratio of the aromatic polyaldehyde, the aromatic diamine and the polyethylene imine is 1:(0.2~0.7):(0.2~0.5); and the mass ratio of the aromatic polyaldehyde to the solvent is 1:(10~30).

[0009] Preferably, the heating mode is gradient heating.

[0010] Preferably, the gradient heating temperature is: the first gradient heating temperature is 15~30℃, the second gradient heating temperature is 60~90℃, and the third gradient heating temperature is 100~120℃.

[0011] Preferably, the gradient heating time is: the first gradient heating time is 4~6h, the second gradient heating time is 3~6h, and the third gradient heating time is 2~4h.

[0012] Preferably, the aromatic polyaldehyde is one of p-xylylene glycol, o-xylylene glycol, 1,3-benzene glycol and 4,4-biphenyl glycol.

[0013] Preferably, the weight average molecular weight of the polyethylene imine is one of 300~18000.

[0014] More preferably, the weight average molecular weight of the polyethylene imine is one of 300, 600, 800, 1800, 10000 and 18000.

[0015] Preferably, the aromatic diamine is one of tetramethyl-p-phenylenediamine, 4,4'-(1,3-phenylenebis(oxy))diphenylamine, 4,4'-(((perfluoropropane-2,2-diyl)bis(4,1-phenylene))bis(oxy))diphenylamine, 4,4'-methylenebis(2,6-diethylaniline), 4,4'-methylenebis(2-ethyl) aniline and 4-(4-amino-3,5-dimethylbenzyl)-2,6-dimethylaniline.

[0016] Preferably, the solvent is one of tetrahydrofuran and dichloromethane.

[0017] The method provided by the application selects polyethyleneimine, aromatic polyaldehyde and aromatic diamine as raw materials, controls the ratio among polyethyleneimine, aromatic polyaldehyde and aromatic diamine, controls the heating mode and temperature, and prepares the heat-resistant and low-water-absorption polyimine under the condition of gradient heating at ≤120℃. The polyethyleneimine can play a crosslinking role in the reaction, and has a macromolecular long-chain structure. The special network structure formed by the polyethyleneimine, the aromatic polyaldehyde and the aromatic diamine can make the glass transition temperature of the polyimine higher than the highest synthesis temperature, so that the polyimine with a higher glass transition temperature can be synthesized at a lower temperature. In addition, compared with aliphatic diamine, the aromatic diamine can better prevent the invasion of water molecules due to the hydrophobic effect of the benzene ring, and the polyimine material prepared by using the aromatic diamine has a lower water absorption. However, the existing reaction system is prone to phase separation, which leads to a large decrease in the water resistance and mechanical properties of the material. In the method, the polyethyleneimine, the aromatic polyaldehyde and the aromatic diamine produce a synergistic effect, so that the phase separation in the reaction process is avoided, and the problem of a large decrease in the mechanical properties and water resistance of the prepared material caused by the phase separation is solved. The method provided by the application can synthesize the polyimine with heat resistance and low water absorption at ≤120℃, the glass transition temperature of the polyimine is >145℃, and the water absorption is ≤0.4%. The water absorption is reduced by 50-60 times compared with the polyimine prepared by using tris(2-aminoethyl)amine as a crosslinking agent.

[0018] Another purpose of the application is to provide the application of the heat-resistant and water-resistant polyimine in natural plant fiber composites.

[0019] Preferably, the method for applying the heat-resistant and water-resistant polyimine in natural plant fiber composites comprises the following steps: uniformly mixing the aromatic polyaldehyde, the polyethyleneimine and the aromatic diamine in a solvent, then pouring the mixture into a mold containing natural plant fibers, and gradient heating to obtain a composite material.

[0020] Preferably, the molar ratio of the aromatic polyaldehyde, the aromatic diamine and the polyethyleneimine is 1:(0.2-0.7):(0.2-0.5), and the mass ratio of the aromatic polyaldehyde, the solvent and the natural plant fibers is 1:(10-30):(2-6).

[0021] Preferably, the natural plant fibers are one of ramie fibers, flax fibers, hemp fibers, bamboo fibers and straw fibers.

[0022] Compared with the prior art, the application has the following advantages:

[0023] (1) The polyimine prepared by the method has heat resistance and low water absorption, the glass transition temperature is greater than 145 DEG C, and the water absorption is less than or equal to 0.4%, which is superior to most plastics such as epoxy resin; when applied to natural plant fiber composites, the prepared composite has excellent mechanical properties, heat resistance and low water absorption, the glass transition temperature is greater than 140 DEG C, the tensile strength and bending strength are higher than 180 MPa and 150 MPa respectively, the tensile modulus and bending modulus are higher than 2.5 GPa and 9.2 GPa respectively, and the water absorption is less than 0.5%, and the composite can be degraded and recycled, and the performance of the re-prepared composite has no significant difference.

[0024] (2) The raw material polyethylene imine in the method has the function of a crosslinking agent, and the price of 100 g of polyethylene imine is only 90 yuan, which is lower than that of the crosslinking agent tri(2-aminoethyl)amine (100 mL, about 2500 yuan) used in the prior art, and is more suitable for industrialization.

[0025] (3) The heat resistance, mechanical strength, hydrolysis resistance and other properties of the composite prepared by the method are superior to most plastics and natural plant fiber composites for automobiles and other vehicles on the current market, and the composite can be degraded and recycled, and has a wide application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 Tan delta of polyimine prepared for example 1 changes with temperature curve;

[0027] Figure 2 Tan delta of polyimine prepared for example 2 changes with temperature curve;

[0028] Figure 3 Tan delta of polyimine prepared for example 3 changes with temperature curve;

[0029] Figure 4 Tan delta of polyimine prepared for comparative example 1 changes with temperature curve;

[0030] Figure 5 Tan delta of polyimine prepared for comparative example 2 changes with temperature curve;

[0031] Figure 6 Tan delta of polyimine prepared for comparative example 3 changes with temperature curve;

[0032] Figure 7 Tan delta of polyimine prepared for comparative example 4 changes with temperature curve;

[0033] Figure 8 Tan delta of polyimine prepared for comparative example 5 changes with temperature curve;

[0034] Figure 9 Tan delta versus temperature curve of the composite material prepared for application example 1;

[0035] Figure 10 Tensile stress-strain curve of the composite material prepared for application example 1;

[0036] Figure 11 Flexural stress-strain curve of the composite material prepared for application example 1;

[0037] Figure 12 Tan delta versus temperature curve of the composite material prepared for application example 2;

[0038] Figure 13 Tensile stress-strain curve of the composite material prepared for application example 2;

[0039] Figure 14 Flexural stress-strain curve of the composite material prepared for application example 2;

[0040] Figure 15 Tan delta versus temperature curve of the composite material prepared for application example 3;

[0041] Figure 16 Tensile stress-strain curve of the composite material prepared for application example 3;

[0042] Figure 17 Flexural stress-strain curve of the composite material prepared for application example 3. DETAILED DESCRIPTION

[0043] 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.

[0044] The present application provides a preparation method of heat-resistant and water-resistant polyimine, comprising the following steps: uniformly mixing aromatic polyaldehyde, polyethylene imine and aromatic diamine in a solvent, and then heating to obtain heat-resistant and water-resistant polyimine.

[0045] In some examples, the molar ratio of the aromatic polyaldehyde, the aromatic diamine and the polyethylene imine can be 1:(0.2-0.7):(0.2-0.5); and the mass ratio of the aromatic polyaldehyde to the solvent can be 1:(10-30).

[0046] In some examples, the heating mode is gradient heating; the temperature of the gradient heating can be: a first gradient heating temperature of 15-30 DEG C, a second gradient heating temperature of 60-90 DEG C, and a third gradient heating temperature of 100-120 DEG C.

[0047] In some examples, the time of the gradient heating can be: a first gradient heating time of 4-6 h, a second gradient heating time of 3-6 h, and a third gradient heating time of 2-4 h.

[0048] In some examples, the aromatic polyaldehyde can be one of terephthaldehyde, o-phthaldehyde, 1,3-benzenedialdehyde, and 4,4-biphenyldialdehyde.

[0049] In some examples, the weight average molecular weight of the polyethylene imine can be one of 300-18000.

[0050] In some examples, the weight average molecular weight of the polyethylene imine can be one of 300, 600, 800, 1800, 10000, and 18000.

[0051] In some examples, the aromatic diamine can be one of tetramethyl-p-phenylenediamine, 4,4'-(1,3-phenylenebis(oxy))dianiline, 4,4'-(((perfluoropropane-2,2-diyl)bis(4,1-phenylene))bis(oxy))dianiline, 4,4'-methylenebis(2,6-diethylaniline), 4,4'-methylenebis(2-ethyl)aniline, and 4-(4-amino-3,5-dimethylbenzyl)-2,6-dimethylaniline.

[0052] In some examples, the solvent can be one of tetrahydrofuran and dichloromethane.

[0053] The application also provides application of the heat-resistant and water-resistant polyimine in natural plant fiber composites.

[0054] The application of the heat-resistant and water-resistant polyimine in natural plant fiber composites includes the following steps: uniformly mixing aromatic polyaldehyde, polyethylene imine, and aromatic diamine in a solvent, pouring the mixture into a mold containing natural plant fibers, and gradient heating to obtain a composite material.

[0055] In some examples, the molar ratio of the terephthaldehyde, the aromatic diamine, and the polyethylene imine can be 1:(0.2-0.7):(0.2-0.5); and the mass ratio of the aromatic polyaldehyde, the solvent, and the natural fiber can be 1:(10-30):(2-6).

[0056] In some examples, the natural plant fiber can be one of ramie fiber, flax fiber, hemp fiber, bamboo fiber, and straw fiber.

[0057] In the following examples, the ramie fibers, flax fibers, and hemp fibers used are commercially available products, which are purchased from Hubei Jinghua Textile Group.

[0058] Example 1

[0059] A heat-resistant and water-resistant polyimine, the molar ratio of the raw materials p-xylylene glycol, tetramethyl-p-phenylenediamine, and polyethyleneimine is 1:0.2:0.5, and the mass ratio of p-xylylene glycol to tetrahydrofuran is 1:10, and the preparation method is as follows:

[0060] Dissolve p-xylylene glycol in tetrahydrofuran, then add polyethyleneimine with a weight average molecular weight of 300 and tetramethyl-p-phenylenediamine, stir uniformly, and then pour into a mold, first heat at 15℃ for 6h, then heat at 90℃ for 3h, and finally heat at 120℃ for 2h to obtain a heat-resistant and water-resistant polyimine. The glass transition temperature of the polyimine prepared in this example reaches 185℃ (Tg). Figure 1 After the polyimine is soaked in water for 24h, the water absorption rate is 0.40%.

[0061] Example 2

[0062] A heat-resistant and water-resistant polyimine, the molar ratio of the raw materials o-xylylene glycol, 4,4'-(1,3-phenylenebis(oxy))diphenylamine, and polyethyleneimine is 1:0.4:0.4, and the mass ratio of o-xylylene glycol to dichloromethane is 1:20, and the preparation method is as follows:

[0063] Dissolve o-xylylene glycol in dichloromethane, then add polyethyleneimine with a weight average molecular weight of 600 and 4,4'-(1,3-phenylenebis(oxy))diphenylamine, stir uniformly, and then pour into a mold, first heat at 20℃ for 5h, then heat at 75℃ for 5h, and finally heat at 110℃ for 3h to obtain a heat-resistant and water-resistant polyimine. The glass transition temperature of the polyimine prepared in this example reaches 155℃ (Tg). Figure 2 After the polyimine is soaked in water for 24h, the water absorption rate is 0.36%.

[0064] Example 3

[0065] A heat-resistant and water-resistant polyimine, the molar ratio of the raw materials 4,4-biphenyldimethylol, 4,4'-(((perfluoropropane-2,2-diyl)bis(4,1-phenylene))bis(oxy))diphenylamine, and polyethyleneimine is 1:0.7:0.2, and the mass ratio of 4,4-biphenyldimethylol to tetrahydrofuran is 1:30, and the preparation method is as follows:

[0066] The terephthalaldehyde was dissolved in tetrahydrofuran, then polyethyleneimine with a weight average molecular weight of 1800 and 4,4'-(((perfluoropropane-2,2-diyl)bis(4,1-phenylene))bis(oxy))diphenylamine were added, stirred uniformly, poured into a mold, heated at 15℃ for 6h, then heated at 60℃ for 6h, and finally heated at 100℃ for 4h to obtain the heat-resistant and water-resistant polyimine. The glass transition temperature of the polyimine prepared in this example reached 145℃ Figure 3 , and the water absorption rate of the polyimine after being soaked in water for 24h was 0.39%.

[0067] Comparative Example 1

[0068] The polyimine of this comparative example was basically the same as the preparation method of Example 1, except that equimolar amount of tris(2-aminoethyl)amine was used to replace polyethyleneimine. The glass transition temperature of the synthesized polyimine was 160℃ Figure 4 , and the water absorption rate of the polyimine after being soaked in water for 24h was 25.6%.

[0069] Comparative Example 2

[0070] The polyimine of this comparative example was basically the same as the preparation method of Example 2, except that equimolar amount of tris(2-aminoethyl)amine was used to replace polyethyleneimine. The glass transition temperature of the synthesized polyimine was 135℃ Figure 5 , and the water absorption rate of the polyimine after being soaked in water for 24h was 20.1%.

[0071] Comparative Example 3

[0072] The polyimine of this comparative example was basically the same as the preparation method of Example 3, except that equimolar amount of tris(2-aminoethyl)amine was used to replace polyethyleneimine. The glass transition temperature of the synthesized polyimine was 125℃ Figure 6 , and the water absorption rate of the polyimine after being soaked in water for 24h was 24.2%.

[0073] Comparative Example 4

[0074] The polyimine of this comparative example was basically the same as the preparation method of Example 1, except that equimolar amount of polyethyleneimine was used to replace tetramethyl-p-phenylenediamine. The glass transition temperature of the synthesized polyimine was 150℃ Figure 7 , and the water absorption rate of the polyimine after being soaked in water for 24h was 34%.

[0075] Comparative Example 5

[0076] The preparation method of the polyimide in this comparative example is basically the same as that in Example 1, except that an equimolar amount of ethylenediamine is used instead of tetramethyl-p-phenylenediamine. The glass transition temperature of the synthesized polyimide is 160°C. Figure 8 The water absorption rate of polyimide after soaking in water for 24 hours was 30.1%.

[0077] Application Example 1

[0078] The application of a heat- and water-resistant polyimide in a natural plant fiber composite material, wherein the molar ratio of the raw materials terephthalaldehyde, tetramethyl-p-phenylenediamine, and polyethyleneimine is 1:0.2:0.5, and the mass ratio of terephthalaldehyde, tetrahydrofuran, and ramie fiber is 1:10:6, and the preparation method is as follows:

[0079] Terephthalaldehyde was dissolved in tetrahydrofuran, then polyethyleneimine with a weight-average molecular weight of 300 and tetramethyl-p-phenylenediamine were added. After thorough mixing, the mixture was poured into a mold containing ramie fibers. The mixture was heated first at 15°C for 6 hours, then at 90°C for 3 hours, and finally at 120°C for 2 hours to obtain the composite material. The glass transition temperature of the composite material prepared in this application example reached 195°C. Figure 9 The tensile strength and modulus reached 203 MPa and 3.01 GPa, respectively. Figure 10 The flexural strength and modulus reached 213 MPa and 12.37 GPa, respectively. Figure 11 The water absorption rate of the composite material after soaking in water for 24 hours was 0.50%.

[0080] Application Example 2

[0081] The application of a heat- and water-resistant polyimide in natural plant fiber composite materials, wherein the molar ratio of raw materials phthalaldehyde, 4,4'-(1,3-phenylenebis(oxy))diphenylamine and polyethyleneimine is 1:0.4:0.4, and the mass ratio of phthalaldehyde, dichloromethane and flax fiber is 1:20:4, is as follows:

[0082] Phthalate was dissolved in dichloromethane, followed by the addition of polyethyleneimine (weight average molecular weight 600) and 4,4'-(1,3-phenylenebis(oxy))diphenylamine. The mixture was stirred thoroughly and poured into a mold containing flax fibers. The mixture was heated first at 20°C for 5 hours, then at 75°C for 5 hours, and finally at 110°C for 3 hours to obtain the composite material. The glass transition temperature of the composite material prepared in this application example reached 180°C. Figure 12 The tensile strength and modulus reached 185 MPa and 2.53 GPa, respectively. Figure 13 The flexural strength and modulus reached 154 MPa and 12.33 GPa, respectively. Figure 14 The water absorption rate of the composite material after soaking in water for 24 hours was 0.46%.

[0083] Application Example 3

[0084] Application of heat-resistant and water-resistant polyimine in natural plant fiber composite material, the molar ratio of raw materials 4,4-biphenyldicarboxaldehyde, 4,4'-(((perfluoropropane-2,2-diyl)bis(4,1-phenylene))bis(oxy))diphenylamine, polyethyleneimine is 1:0.7:0.2, the mass ratio of 4,4-biphenyldicarboxaldehyde, tetrahydrofuran, hemp fiber is 1:30:6, the preparation method is as follows:

[0085] Dissolve p-xylylene glycol using tetrahydrofuran, then add polyethyleneimine with a weight average molecular weight of 1800 and 4,4'-(((perfluoropropane-2,2-diyl)bis(4,1-phenylene))bis(oxy))diphenylamine, pour into the mold containing hemp fiber after stirring uniformly, first heat at 15℃ for 6h, then heat at 60℃ for 6h, and finally heat at 100℃ for 4h to obtain the composite material. The glass transition temperature of the composite material prepared in this application example reaches 155℃ ( Figure 15 ), the tensile strength and modulus reach 187MPa and 3.21GPa ( Figure 16 ), the bending strength and modulus reach 177MPa and 9.21GPa ( Figure 17 ). The water absorption rate of the composite material after soaking in water for 24h is 0.43%.

[0086] Application Example 4

[0087] The composite material prepared in application example 1 is immersed in tetrahydrofuran containing tetramethyl-p-phenylenediamine, the amount of tetramethyl-p-phenylenediamine and tetrahydrofuran is consistent with the amount used in the preparation of the composite material in application example 1, and the fiber composite material in application example 1 is stirred at 50rpm for 1h, and the polyimine will depolymerize, so that the ramie fiber will separate from the polyimine matrix, and a degraded polyimine mixture is obtained. The ramie fiber is placed in the mold again, and polyethyleneimine and aromatic polyaldehyde are added to the degraded polyimine mixture according to the reaction system of application example 1, and then poured into the mold after mixing. According to the method described in application example 1, the composite material is prepared again. The tensile strength and modulus, bending strength and modulus of the re-prepared composite material respectively reach 200MPa and 3.11GPa, 210MPa and 12.13GPa, and the water absorption rate of the re-prepared composite material after soaking in water for 24h is 0.49%, which is basically the same as that of the composite material in application example 1.

[0088] Application Example 5

[0089] The composite material prepared in Application Example 2 was immersed in dichloromethane containing 4,4'-(1,3-phenylenebis(oxy))diphenylamine, wherein the amounts of 4,4'-(1,3-phenylenebis(oxy))diphenylamine and dichloromethane were consistent with those used in the preparation of the composite material in Application Example 2, and stirring was performed at 50 rpm for 1 h. The polyimine in the fiber composite material of Application Example 2 was depolymerized, so that the flax fibers were separated from the polyimine matrix, and a degraded polyimine mixture was obtained. The flax fibers were placed in a mold again, and polyethyleneimine and aromatic polyaldehyde were added to the degraded polyimine mixture according to the reaction system of Application Example 2, and then the mixture was poured into the mold. The composite material was prepared again according to the method described in Application Example 2. The tensile strength and modulus, the bending strength and modulus of the re-prepared composite material reached 180 MPa and 2.32 GPa, 150 MPa and 12.11 GPa, respectively. The water absorption rate of the re-prepared composite material after immersion in water for 24 h was 0.46%, and the performance was basically the same as that of the composite material in Application Example 2.

[0090] Application Example 6

[0091] The composite material prepared in Application Example 3 was immersed in tetrahydrofuran containing 4,4'-(((perfluoropropan-2,2-diyl)bis(4,1-phenylene))bis(oxy))diphenylamine, wherein the amounts of 4,4'-(((perfluoropropan-2,2-diyl)bis(4,1-phenylene))bis(oxy))diphenylamine and tetrahydrofuran were consistent with those used in the preparation of the composite material in Application Example 3, and stirring was performed at 50 rpm for 1 h. The polyimine in the fiber composite material of Application Example 3 was depolymerized, so that the hemp fibers were separated from the polyimine matrix, and a degraded polyimine mixture was obtained. The hemp fibers were placed in a mold again, and polyethyleneimine and aromatic polyaldehyde were added to the degraded polyimine mixture according to the reaction system of Application Example 3, and then the mixture was poured into the mold. The composite material was prepared again according to the method described in Application Example 3. The tensile strength and modulus, the bending strength and modulus of the re-prepared composite material reached 182 MPa and 3.02 GPa, 173 MPa and 9.23 GPa, respectively. The water absorption rate of the re-prepared composite material after immersion in water for 24 h was 0.43%, and the performance was basically the same as that of the composite material in Application Example 3.

[0092] As can be seen from Examples 1-3 and Comparative Examples 1-3, the method provided by the present application can be used to synthesize polyimide with high heat resistance and low water absorption at low temperature. In addition, the inventors found that during the establishment of the method of the present application, one-time heating preparation can cause some components to react faster, and the molecular chains in some areas to solidify first, causing the remaining components to be unable to participate in the polymerization reaction, and thus the glass transition temperature and mechanical properties of the prepared polyimide to be low. As can be seen from Application Examples 1-6, the polyimide prepared by the method of the present application is applied to natural plant fiber composites, and the prepared composites have the properties of high strength, heat resistance, and low water absorption, and can be recycled and reused. The polyimide preparation method of the present application and the natural plant fiber composites synthesized therefrom have wide application prospects in the fields of automobiles, high-speed rails, airplanes, and the like.

[0093] The above detailed description of the embodiments of the present application, but the present application is not limited to the specific details of the above-described embodiments. Within the scope of the claims and technical concepts of the present application, the technical solutions of the present application can be subjected to various simple modifications and changes, and these simple modifications all belong to the protection scope of the present application.

Claims

1. A process for the preparation of heat- and water-resistant polyimine, characterized in that, The method comprises the following steps: The aromatic polyaldehyde, polyethylene imine and aromatic diamine are mixed uniformly in a solvent, and then heated to obtain a heat-resistant and water-resistant polyimine; The molar ratio of the aromatic polyaldehyde, the aromatic diamine and the polyethylene imine is 1:(0.2-0.7):(0.2-0.5); and the mass ratio of the aromatic polyaldehyde to the solvent is 1:(10-30); The aromatic polyaldehyde is one of p-phthaldehyde, o-phthaldehyde, 1,3-benzene dicarboxaldehyde and 4,4-biphenyl dicarboxaldehyde; The heating mode is gradient heating, and the gradient heating temperature is 15-30 DEG C for the first gradient heating, 60-90 DEG C for the second gradient heating and 100-120 DEG C for the third gradient heating; The gradient heating time is 4-6 h for the first gradient heating, 3-6 h for the second gradient heating and 2-4 h for the third gradient heating; The solvent is one of tetrahydrofuran and dichloromethane.

2. The method for preparing heat-resistant and water-resistant polyimide according to claim 1, characterized in that, The weight average molecular weight of the polyethylene imine is one of 300-18000.

3. The method for preparing heat-resistant and water-resistant polyimide according to claim 1, characterized in that, The aromatic diamine is one of tetramethyl-p-phenylenediamine, 4,4'-((1,3-phenylenebis(oxy))dianiline, 4,4'-(((perfluoropropane-2,2-diyl)bis(4,1-phenylene))bis(oxy))dianiline, 4,4'-methylenebis(2,6-diethylaniline), 4,4'-methylenebis(2-ethyl)aniline and 4-(4-amino-3,5-dimethylbenzyl)-2,6-dimethylaniline.

4. The heat-resistant and water-resistant polyimine prepared by the preparation method of any one of claims 1-3 is applied to natural plant fiber composite materials.

5. Use of the heat- and water-resistant polyimine according to claim 4 in natural plant fiber composites, characterized in that, The natural plant fiber is one of ramie fiber, flax fiber, hemp fiber, bamboo fiber and straw fiber.

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