Solvent-resistant polyurethane composite material and preparation method thereof

Abstract

The invention relates to the technical field of high polymer materials, and discloses a solvent-resistant polyurethane composite material and a preparation method thereof, and the material is prepared by reacting the following components: an isocyanate component, a polyol component and a functional chain extender component; the functional chain extender is a derivative of ortho-amino aromatic carboxylic acid protected by a thermally unstable ester group, and the protecting group is decomposed under a thermal treatment condition and releases a functional group in situ, wherein the functional group can be subjected to cyclization reaction with ureido on a polymer chain to form a heterocyclic cross-linked structure; the preparation method comprises the following steps: (a) reacting an isocyanate component, a polyol component and a functional chain extender to obtain a soluble and processable linear polyurethane precursor; and (b) carrying out high-temperature heat treatment on the molded precursor. Through the design of the latent functional group and the two-step process, the contradiction between the processability and the final performance of the cross-linked polyurethane is solved, and the prepared material has excellent solvent resistance, thermal stability and good mechanical properties.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer materials, in particular to a solvent-resistant polyurethane composite material and a preparation method thereof. Background Art

[0002] Polyurethanes are widely used in coatings, adhesives, elastomers, fibers, and other fields due to their excellent mechanical properties and adjustable structure. However, conventional linear thermoplastic polyurethanes, whose molecular chains are primarily connected by secondary bonds such as hydrogen bonds, easily dissociate in highly polar solvents, causing the material to swell severely or even dissolve. Furthermore, their relatively limited thermal stability significantly restricts their application in harsh environments such as chemical corrosion and high temperatures.

[0003] To solve this problem, the method of introducing a chemical cross-linking network is usually used to improve the solvent resistance and thermal stability of polyurethane. However, the existing chemical cross-linking method has an inherent technical contradiction. If a conventional multifunctional cross-linking agent is directly introduced during the polymerization stage, the viscosity of the reaction system will increase sharply as the reaction proceeds, and irreversible gelation will easily occur before the polymerization is completed, causing the material to lose fluidity and processability, making it impossible to carry out subsequent forming operations such as coating, spinning or molding. On the other hand, if a post-cross-linking method is adopted to ensure processability, the conventional post-cross-linking reaction is often harsh and inefficient, making it difficult to form a uniform and dense cross-linking network in the solid state, and the performance improvement effect of the final material is therefore limited. Therefore, developing a method that can maintain the excellent processing properties of the polyurethane precursor and form a high-density, high-stability chemical cross-linking network through mild and controllable subsequent treatment, thereby preparing a solvent-resistant polyurethane material with excellent comprehensive performance, is a technical problem that needs to be solved urgently in this field. Summary of the Invention

[0004] The invention of this application aims to solve the technical problem in the prior art that it is difficult to strike a balance between the chemical solvent resistance, processability and mechanical toughness of conventional polyurethane materials.

[0005] In a first aspect, the present application provides a solvent-resistant polyurethane composite material, which adopts the following technical solution: A solvent-resistant polyurethane composite material is prepared by reacting the following raw material components in parts by weight: Isocyanate component: 100 parts; Polyol component: 80-200 parts; Functional chain extender component: 10-50 parts; Among them, in order to achieve the technical effect of the present invention, the key is that the functional chain extender is a derivative of an ortho-amino aromatic diamine with a thermally unstable protective group, wherein the protective group decomposes under specific heat treatment conditions, thereby in situ generating a functional group that can undergo an efficient cyclization reaction with the urea group formed by the amino group on the polymer chain to form a heterocyclic cross-linked structure.

[0006] By adopting the above technical solution, this functional chain extender with a specific structure and a thermally unstable protective group is introduced into the polyurethane synthesis system, so that while the polyurethane chain segments are formed, pre-designed latent reaction sites for subsequent chemical conversion are introduced into the polymer main chain.

[0007] This protecting group remains chemically inert during the polyurethane polymerization stage and does not participate in the reaction, ensuring the stable preparation of a linear, soluble, and processable precursor. During the subsequent heat treatment stage, the protecting group decomposes upon thermal decomposition, generating highly reactive functional groups (such as carboxyl groups) in situ. This functional group, along with the urea groups already present in the polyurethane segments, is precisely positioned at the ortho position of the aromatic ring. This functional group then undergoes intramolecular or intermolecular dehydration condensation reactions with the spatially adjacent urea groups, forming a highly stable heterocyclic ring. Those skilled in the art generally believe that similar reactions between ester and urea groups require a catalyst, such as a strong base. However, one of the unique features of this invention lies in the "intramolecular catalysis" effect brought about by the ortho position design, which significantly reduces the reaction energy barrier, enabling the cyclization reaction to occur efficiently under high-temperature solid-phase conditions without the need for any external catalyst. This process covalently links the initial linear polymer chains through these newly generated heterocyclic structures, constructing a three-dimensional chemically cross-linked network.

[0008] Because the heterocyclic structure has better chemical and thermal stability than the carbamate bond and urea bond, the final composite material obtains the ability to resist swelling and dissolution against chemicals such as strong polar solvents and chlorinated hydrocarbons without significantly losing the inherent mechanical toughness of polyurethane.

[0009] The functional chain extender of the present invention is a derivative of ortho-amino aromatic carboxylic acid protected by a thermally unstable ester group.

[0010] By adopting this technical solution, ester groups, particularly tert-butyl ester groups, are used as thermally unstable protecting groups. Their thermal decomposition temperature window (typically above 200°C) is clearly distinct from the conventional polyurethane processing and polymerization temperatures (typically below 120°C). This ensures that the protecting groups do not decompose prematurely during the first stage of polymerization and molding, ensuring process stability. Furthermore, the thermal decomposition products of the ester group (such as olefins and carboxylic acids) react cleanly, making by-products easily removable.

[0011] Preferably, the heterocyclic cross-linked structure is a quinazolinedione heterocyclic structure.

[0012] By adopting this technical solution, quinazolinedione forms a stable structure with a conjugated aromatic heterocyclic system, and its formation process is thermodynamically favorable. Using the quinazolinedione ring as a crosslinking node can provide excellent chemical inertness to the final composite network, further improving the material's solvent and heat resistance.

[0013] Preferably, the functional chain extender is 3,3'-di(tert-butyloxycarbonyl)-4,4'-diaminodiphenylmethane. Its chemical structure is as follows:

[0014] By adopting this technical solution, the specific molecular structure, in which the tert-butyloxycarbonyl group serves as a protecting group and the amino group serves as a group reactive with isocyanate, embodies the core design concept of the present invention: the carboxyl group generated after deprotection and the urea group formed by the amino group reacting are positioned ortho to the aromatic ring. This spatial arrangement promotes intramolecular and intermolecular cyclization reactions, facilitating the efficient formation of a quinazolinedione cross-linked network.

[0015] Preferably, the 3,3'-di(tert-butyloxycarbonyl)-4,4'-diaminodiphenylmethane is prepared by the following steps: (a) Using 2-nitro-5-methylbenzoic acid and formaldehyde as raw materials, reacting in concentrated sulfuric acid at 95-105°C to obtain 3,3'-dicarboxy-4,4'-dinitrodiphenylmethane; (b) subjecting the product of step (a) to hydrogenation reduction in the presence of a Pd / C catalyst at a hydrogen pressure of 3.0 to 4.0 MPa and a temperature of 60 to 70° C. to obtain 3,3′-dicarboxyl-4,4′-diaminodiphenylmethane; (c) subjecting the product of step (b) to a reflux esterification reaction with tert-butyl alcohol under acid catalysis and azeotropic dehydration conditions to obtain the 3,3'-di(tert-butyloxycarbonyl)-4,4'-diaminodiphenylmethane.

[0016] Preferably, the raw material components further include 0 to 20 parts by weight of a conventional chain extender component, wherein the conventional chain extender is selected from one or more of small molecule diols or small molecule diamines.

[0017] By adopting the above technical solution, the introduction of conventional chain extenders can be used to adjust the content, structure, and regularity of the final material's hard segment. By adjusting its type and dosage, mechanical and thermal properties such as the modulus, hardness, and glass transition temperature of the composite material can be fine-tuned, thereby achieving customization of material properties within a certain range.

[0018] Preferably, the isocyanate component is selected from one or more of methylene diphenyl diisocyanate, toluene diisocyanate, isophorone diisocyanate or hexamethylene diisocyanate; and the polyol component is selected from one or more of polytetramethylene ether glycol, polypropylene glycol or polyester glycol.

[0019] By employing this technical approach, the selection of isocyanates and polyols with different chemical structures directly determines the basic structure of the hard and soft segments in the polyurethane precursor. For example, using aromatic isocyanates can create a more rigid hard segment, while using aliphatic isocyanates can improve the material's weather resistance. The choice of different polyol soft segments directly affects the material's flexibility, low-temperature performance, and affinity for specific chemicals.

[0020] In a second aspect, the present application provides a method for preparing a solvent-resistant polyurethane composite material, which adopts the following technical solution: A method for preparing a solvent-resistant polyurethane composite material comprises the following steps: (a) Precursor preparation stage: reacting an isocyanate component with a polyol component to form an isocyanate-terminated prepolymer; then carrying out a chain extension reaction between the prepolymer and the functional chain extender and an optional conventional chain extender at a temperature below the decomposition temperature of the protecting group to obtain a soluble and meltable linear polyurethane precursor; (b) Heat treatment and curing stage: After the linear polyurethane precursor obtained in step (a) is formed, it is heat treated under conditions higher than the decomposition temperature of the protective group to decompose the protective group, and the synergistic effect of the ortho-functional groups designed by the present invention is utilized to induce an in situ cyclization and cross-linking reaction to form a three-dimensional network structure.

[0021] By adopting the above technical solution, the method divides the material preparation process into two independent and orthogonal stages.

[0022] In the first stage, all reactions are carried out under mild conditions. At this time, the system is a linear polymer with the fluidity required for solution or melt processing and can be easily formed into films, fibers or molded products.

[0023] In the second stage, external energy (heat) is applied to trigger a pre-designed, programmed chemical transformation. This transformation occurs in tandem: first, thermal decomposition of the protecting groups, followed by efficient cyclocrosslinking between the newly generated functional groups and the urea groups, driven by the ortho-effect. This design, which decouples processability from the development of final properties in terms of time and process conditions, overcomes the technical issues of traditional cross-linked polyurethanes, such as the rapid increase in viscosity and narrow processing window during polymerization.

[0024] Preferably, in step (a), the temperature of the chain extension reaction is controlled at 0-30°C; and in step (b), the temperature of the heat treatment is 200-300°C.

[0025] By adopting the above technical solution, precise temperature control is the key to achieving the separation of the two-stage reaction. The chain extension temperature of 0-30°C ensures the efficient reaction of amino groups with isocyanate groups, while completely suppressing the decomposition of thermally unstable protective groups such as tert-butyl ester. The heat treatment temperature of 200-300°C can effectively provide the activation energy required for the decomposition of the protective group and the subsequent cyclodehydration reaction, and is sufficient to activate the subsequent cyclodehydration reaction catalyzed by the ortho-position structure of the present invention, ensuring that the network construction is completed within a reasonable time. These two non-overlapping temperature ranges form the basis of the operability of this method.

[0026] Preferably, the heat treatment curing in step (b) specifically includes performing solvent volatilization treatment at 80-120° C., then heating to 220-280° C. and maintaining the temperature to perform deprotection and cyclization cross-linking reactions.

[0027] By adopting this technical solution and a programmed heat treatment process with step-by-step temperature increases, residual solvent in the precursor is first removed at a relatively low temperature (80-120°C). This prevents product defects (such as bubbles and pinholes) caused by rapid solvent volatilization during the subsequent high-temperature curing stage. Subsequently, the temperature is raised to the reaction temperature range (220-280°C), allowing the deprotection and cyclization reactions to proceed smoothly within the uniform solid-phase matrix, facilitating the formation of a final material with uniform structure and consistent properties.

[0028] In summary, the present invention includes at least one of the following beneficial technical effects: 1. The present invention exhibits excellent resistance to chemical solvents. By pre-arranging the reactive groups in the functional chain extender at the molecular level in the "ortho" position, efficient in-situ deprotection and cyclization reactions can be triggered during the subsequent heat treatment phase without the need for any external catalyst, forming a highly stable quinazolinedione heterocyclic cross-linked structure. This results in a dense and well-organized three-dimensional chemical network. This network effectively constrains the polyurethane molecular chains, resisting the penetration and swelling of highly polar solvents (such as DMAc and NMP), thereby endowing the material with structural stability and solvent resistance in chemical environments that traditional polyurethanes struggle to withstand.

[0029] 2. The present invention solves the contradiction between material processability and ultimate high performance, and has high process controllability. The latent chemical design and two-stage preparation method adopted by the present invention, during the first stage of low-temperature polymerization, obtain a soluble and fusible linear polyurethane precursor with good fluidity and a wide processing window, which is convenient for forming by conventional methods such as solution casting, molding or spinning. In the second stage, the latent cyclization cross-linking reaction mediated by the ortho-structure is triggered by programmed heating. This design that clearly separates processing and molding from performance curing in terms of process conditions overcomes the technical difficulties of traditional cross-linking systems in which the viscosity rises sharply during the polymerization process and is difficult to process.

[0030] 3. The present invention improves solvent resistance while effectively maintaining the inherent excellent mechanical toughness of polyurethane materials. The cross-linked network of the present invention is constructed based on pre-formed linear polyurethane polymer chains composed of flexible soft segments and rigid hard segments. The formation of cross-linking points does not destroy the basic segment structure that gives polyurethane its elasticity and toughness, but rather reinforces it through moderate cross-linking. Therefore, the final material avoids the increased brittleness often seen in traditional high-crosslink density materials, achieving a good balance between the two key properties of solvent resistance and mechanical toughness. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a Fourier transform infrared spectrum of the functional chain extender 3,3'-di(tert-butyloxycarbonyl)-4,4'-diaminodiphenylmethane prepared in Preparation Example 1 of the present invention; Figure 2 This is the hydrogen nuclear magnetic resonance spectrum of the functional chain extender 3,3'-di(tert-butyloxycarbonyl)-4,4'-diaminodiphenylmethane prepared in Preparation Example 1 of the present invention; Figure 3 This is a Fourier transform infrared spectrum of the structural isomer 5,5'-di(tert-butyloxycarbonyl)-3,3'-diaminodiphenylmethane prepared in Preparation Example 4 of the present invention; Figure 4 This is the hydrogen nuclear magnetic resonance spectrum of the structural isomer 5,5'-di(tert-butyloxycarbonyl)-3,3'-diaminodiphenylmethane prepared in Preparation Example 4 of the present invention; Figure 5 This is a thermogravimetric analysis curve of comparative example CS2 of the present invention; Figure 6 4. The Fourier transform infrared spectra of comparative example CS2 and example S1 of the present invention are compared. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention will be described in further detail below with reference to the embodiments. However, it should be understood that these embodiments of the present invention are only intended to illustrate the present invention and are not intended to limit the scope of the present invention. Various modifications or variations made to the present invention without departing from the spirit and substance of the present invention all fall within the scope of protection of the present invention.

[0033] The sources and specifications of the main raw materials and reagents used in the following examples and comparative examples are as follows. Reagents not specifically specified are all commercially available products of analytical grade or higher.

[0034] 4,4'-Methylene diphenyl diisocyanate (MDI): CAS No.: 101-68-8; Specification: 4,4'-isomer content ≥99.5%; NCO group content: 33.4%~33.6%.

[0035] Polytetramethylene ether glycol (PTMEG): CAS number: 25190-06-1; linear polyether glycol with a repeating unit of (-O-CH2CH2CH2CH2-); specifications: number average molecular weight (Mn) 2000 g / mol; hydroxyl value: 54.0-58.0 mg KOH / g; moisture content ≤0.03 wt%.

[0036] Polypropylene glycol (PPG): CAS number: 25322-69-4; linear polyether diol with a repeating unit of (-O-CH(CH3)-CH2-); specifications: number average molecular weight (Mn) 2000 g / mol; hydroxyl value: 54.5-57.5 mg KOH / g; moisture content ≤0.03wt%.

[0037] p-Toluenesulfonic acid (PTSA): CAS number: 104-15-4; analytical grade.

[0038] 4,4'-Methylenebis(2-chloroaniline) (MOCA): CAS No.: 101-14-4; Specification: Purity ≥ 99.0%; Melting point: 108-110°C.

[0039] Functional chain extender: The full name is 3,3'-di(tert-butyloxycarbonyl)-4,4'-diaminodiphenylmethane, abbreviated as DC-DDM-tBu, which is prepared by ourselves. The preparation method is shown in the following preparation example.

[0040] Its molecular formula is: C 23 H 30 N2O4; Its structural formula is as follows: ; The core structure of this molecule is a 4,4'-diaminodiphenylmethane backbone. On each benzene ring, the tert-butyloxycarbonyl group (-C(=O)OC(CH3)3) is located ortho to the amino group (-NH2). This ortho-positioning arrangement is the technical foundation for the subsequent thermally induced cyclization and crosslinking reaction of the present invention.

[0041] Preparation Examples 1-3: Preparation of Functional Chain Extender DC-DDM-tBu Preparation Example 1: This preparation example provides a method for preparing a functional chain extender DC-DDM-tBu, and the steps are as follows: ; (a) Synthesis of 3,3'-dicarboxy-4,4'-dinitrodiphenylmethane: To a 500 mL three-necked flask equipped with a mechanical stirrer, reflux condenser, and thermometer, add 250 mL of concentrated sulfuric acid. While stirring, add 36.2 g (0.2 mol) of 2-nitro-5-methylbenzoic acid in portions. Once completely dissolved, slowly add 8.9 g (0.11 mol) of 37% formaldehyde solution dropwise. After the addition is complete, heat the reaction system to 100°C and maintain this temperature for 6 hours. After the reaction is complete, cool the reaction mixture to room temperature and slowly pour it into a beaker containing 1000 g of crushed ice, whereupon a large amount of yellow solid precipitates. Filter the mixture, wash the filter cake thoroughly with deionized water until neutral, and then dry it in a vacuum oven at 80°C for 12 hours to yield 35.5 g of the product as a yellow powder.

[0042] (b) Synthesis of 3,3'-dicarboxy-4,4'-diaminodiphenylmethane: 30.0 g (approximately 0.083 mol) of 3,3'-dicarboxy-4,4'-dinitrodiphenylmethane obtained in step (a) and 1.5 g of a 10% Pd / C catalyst were added to a 500 mL autoclave, followed by 250 mL of methanol as the solvent. The air in the autoclave was replaced three times with nitrogen and then three times with hydrogen. The hydrogen pressure was raised to 3.5 MPa, stirring was initiated, and the autoclave was heated to 65°C. Under these conditions, a hydrogenation reduction reaction was carried out until the pressure in the autoclave no longer decreased. Heating was stopped, the reaction mixture was cooled to room temperature, and the pressure in the autoclave was slowly released. The reaction mixture was filtered to remove the Pd / C catalyst, and the filtrate was concentrated to dryness under reduced pressure to obtain 23.1 g of an off-white solid product.

[0043] (c) Synthesis of 3,3'-di(tert-butyloxycarbonyl)-4,4'-diaminodiphenylmethane: 20.0 g (approximately 0.07 mol) of 3,3'-dicarboxy-4,4'-diaminodiphenylmethane obtained in step (b), 0.7 g of p-toluenesulfonic acid, and 200 mL of tert-butanol were added to a 500 mL round-bottom flask equipped with a Dean-Stark trap. The mixture was heated to reflux, and the water produced by the reaction was removed azeotropically through the trap. After 24 hours of reaction, the mixture was cooled to room temperature. The reaction solution was concentrated under reduced pressure, and the residue was dissolved in 200 mL of ethyl acetate and washed sequentially with saturated sodium bicarbonate solution and saturated brine. The organic phase was dried over anhydrous magnesium sulfate, filtered, and the solvent was evaporated under reduced pressure. The crude product was purified by silica gel column chromatography (eluent: petroleum ether / ethyl acetate = 3 / 1, v / v) to obtain 21.5 g of the product as a white solid.

[0044] The final product is a white crystalline powder with a melting point of 148-150°C. Figure 1 and Figure 2 Its chemical structure was determined by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy ( 1 H NMR) analysis confirmed.

[0045] like Figure 1 As shown, the infrared spectrum (KBr pellet method) shows: at 3472 cm -1 and 3365 cm -1 There are clear double peaks of asymmetric and symmetric stretching vibration of NH bond at 1718 cm -1 A strong and sharp absorption peak appears at , which is attributed to the stretching vibration of the ester group C=O in the tert-butyloxycarbonyl group.

[0046] like Figure 2 As shown, the nuclear magnetic resonance hydrogen spectrum (400 MHz, solvent CDCl3) shows: a single peak at the chemical shift of δ1.54 ppm, the integrated area corresponds to 18 protons, attributed to two equivalent tert-butyl groups (-C(CH3)3); a single peak at δ3.88 ppm, the integrated area corresponds to 2 protons, attributed to the methylene group (-CH2-) connecting the two benzene rings; a broad single peak at δ4.25 ppm, the integrated area corresponds to 4 protons, attributed to two primary amino groups (-NH2); a group of multiple peaks appear in the aromatic region of δ6.80~7.55 ppm, the integrated area corresponds to 6 protons, attributed to the hydrogen on the benzene ring.

[0047] Preparation Example 2: (a) Synthesis of 3,3'-dicarboxy-4,4'-dinitrodiphenylmethane: Except for controlling the reaction temperature at 95° C., the rest of the feeding and operation steps are exactly the same as step (a) of Preparation Example 1.

[0048] (b) Synthesis of 3,3'-dicarboxy-4,4'-diaminodiphenylmethane: Except that the hydrogen pressure was controlled at 3.0 MPa and the reaction temperature was controlled at 60° C., the rest of the feeding and operation steps were exactly the same as step (b) of Preparation Example 1.

[0049] (c) Synthesis of 3,3'-di(tert-butyloxycarbonyl)-4,4'-diaminodiphenylmethane: The product obtained in step (b) was subjected to the same operation as in step (c) of Preparation Example 1 to obtain a white solid product.

[0050] The final product is a white crystalline powder with a melting point of 147-149° C. Its infrared spectrum and hydrogen nuclear magnetic resonance spectrum data are consistent with those of the product obtained in Preparation Example 1.

[0051] Preparation Example 3: (a) Synthesis of 3,3'-dicarboxy-4,4'-dinitrodiphenylmethane: Except for controlling the reaction temperature at 105° C., the rest of the feeding and operation steps are exactly the same as step (a) of Preparation Example 1.

[0052] (b) Synthesis of 3,3'-dicarboxy-4,4'-diaminodiphenylmethane: Except that the hydrogen pressure was controlled at 4.0 MPa and the reaction temperature was controlled at 70° C., the rest of the feeding and operation steps were exactly the same as step (b) of Preparation Example 1.

[0053] (c) Synthesis of 3,3'-di(tert-butyloxycarbonyl)-4,4'-diaminodiphenylmethane: The product obtained in step (b) was subjected to the same operation as in step (c) of Preparation Example 1 to obtain a white solid product.

[0054] The final product is a white crystalline powder with a melting point of 148-151° C. Its infrared spectrum and hydrogen nuclear magnetic resonance spectrum data are consistent with those of the product obtained in Preparation Example 1.

[0055] Preparation Example 4: Preparation of the functional chain extender structural isomer 5,5'-di(tert-butyloxycarbonyl)-3,3'-diaminodiphenylmethane In the early stages of this invention's development, the following structural isomer was synthesized to investigate the influence of functional group position on subsequent cyclocross-linking reactions. In this isomer, the amino group and the ester group are in a meta position on the benzene ring. Experimental results indicate that this structure is unable to achieve the technical problem addressed by this invention. Its preparation method and performance data are documented below for subsequent comparative experiments.

[0056] Chemical name: 5,5'-di(tert-butyloxycarbonyl)-3,3'-diaminodiphenylmethane; Molecular formula: C 23 H 30 N2O4; The structural formula is as follows: ; The synthesis steps are as follows: ; (a) Synthesis of 5,5'-dicarboxy-3,3'-dinitrodiphenylmethane: To a 500 mL three-necked flask equipped with a mechanical stirrer and a thermometer, add 200 mL of concentrated sulfuric acid and cool to 10°C. While stirring, slowly add 33.4 g (0.2 mol) of 3-nitrobenzoic acid (CAS: 121-92-6). Once completely dissolved, slowly add 8.9 g (0.11 mol) of 37% formaldehyde solution dropwise, maintaining the temperature above 20°C. After the addition is complete, return the mixture to room temperature and continue the reaction for 12 hours. The reaction mixture is slowly poured into 1000 g of crushed ice, resulting in the precipitation of an off-white solid. Filter the mixture, wash with deionized water until neutral, and dry in a vacuum oven at 80°C to yield 33.2 g of the product.

[0057] (b) Synthesis of 5,5'-dicarboxy-3,3'-diaminodiphenylmethane: 30.0 g (about 0.087 mol) of the intermediate obtained in step (a) and 1.5 g of 10% Pd / C catalyst were added to a 500 mL autoclave, and 250 mL of ethyl acetate was added as a solvent. The air in the autoclave was replaced with nitrogen three times and then replaced with hydrogen three times. The hydrogen pressure was increased to 3.5 MPa, stirring was started, and the autoclave was heated to 65°C for hydrogenation reduction reaction until the pressure in the autoclave no longer decreased. After cooling to room temperature, the Pd / C catalyst was filtered out, and the filtrate was concentrated to dryness under reduced pressure to obtain 23.8 g of an off-white solid product.

[0058] (c) Synthesis of 5,5'-di(tert-butyloxycarbonyl)-3,3'-diaminodiphenylmethane: 20.0 g (about 0.07 mol) of the product obtained in step (b), 0.7 g of p-toluenesulfonic acid, and 200 mL of tert-butanol were added to a 500 mL round-bottom flask equipped with a Dean-Stark water trap. The mixture was heated to reflux, azeotropically dehydrated, and reacted for 24 hours. After cooling, the reaction solution was concentrated under reduced pressure, dissolved in ethyl acetate, and washed sequentially with saturated sodium bicarbonate aqueous solution and saturated brine. The organic phase was dried over anhydrous magnesium sulfate, filtered, and the solvent was evaporated under reduced pressure. The crude product was purified by silica gel column chromatography to obtain 20.5 g of a white solid product.

[0059] The final product was a white powder with a melting point of 162-164°C. Figure 3 and Figure 4 Its chemical structure was confirmed by Fourier transform infrared spectroscopy (FT-IR) and proton nuclear magnetic resonance (¹H NMR) analyses.

[0060] like Figure 3 As shown, the infrared spectrum (KBr pellet method) shows: at 3480 cm -1 and 3370 cm -1 There are clear double peaks of asymmetric and symmetric stretching vibration of NH bond at 1715 cm -1 A strong and sharp absorption peak appears at , which is attributed to the stretching vibration of the ester group C=O in the tert-butyloxycarbonyl group.

[0061] like Figure 4 As shown, the H NMR spectrum (400 MHz, solvent DMSO-d6) shows: a singlet at chemical shift δ1.52 ppm, with an integrated area corresponding to 18 protons, attributed to two equivalent tert-butyl groups (-C(CH3)3); a singlet at δ3.85 ppm, with an integrated area corresponding to 2 protons, attributed to the methylene group (-CH2-) connecting the two benzene rings; a broad singlet at δ5.30 ppm, with an integrated area corresponding to 4 protons, attributed to two primary amino groups (-NH2); a group of multiplets appear in the aromatic region at δ6.60-7.30 ppm, with an integrated area corresponding to 6 protons, attributed to the hydrogen on the benzene ring.

[0062] Examples 1-4: Preparation of solvent-resistant polyurethane composite materials Example 1: An embodiment of the present invention provides a method for preparing a solvent-resistant polyurethane composite material, comprising the following steps: (a) Preparation of linear polyurethane precursor: In a 500 mL three-necked flask equipped with a mechanical stirrer, a thermometer, and a nitrogen atmosphere, add 100 g of 4,4'-diphenylmethane diisocyanate (MDI) and 150 g of polytetramethylene glycol (PTMEG, Mn = 2000). Under nitrogen, heat the system to 70°C and react for 2 hours to obtain an NCO-terminated prepolymer. The prepolymer is then cooled to room temperature and diluted with 250 g of N,N-dimethylacetamide (DMAc).

[0063] In another beaker, 30 g of 3,3'-di(tert-butyloxycarbonyl)-4,4'-diaminodiphenylmethane (DC-DDM-tBu) prepared by the method of Preparation Example 1 was dissolved in 100 g of DMAc to form a chain extender solution.

[0064] The chain extender solution was slowly added dropwise to the prepolymer solution under vigorous stirring and cooling in an ice-water bath. The reaction temperature was kept below 10°C throughout the addition. After the addition was complete, the ice-water bath was removed and the reaction was continued at 25°C for 4 hours to obtain a light yellow, transparent, viscous linear polyurethane precursor solution with a solids content of approximately 40%.

[0065] (b) Heat treatment curing: The precursor solution obtained in step (a) was evenly cast onto a clean horizontal glass plate, and the wet film thickness was controlled using a film scraper. The glass plate was placed in a forced air drying oven and heat treated according to the following procedure: first, it was dried at a constant temperature of 80°C for 12 hours to remove most of the DMAc solvent; then, the temperature was raised to 250°C and kept at a constant temperature of 250°C for 2 hours to carry out deprotection and cyclization cross-linking reactions. After the reaction was completed, the heating was turned off and the sample was allowed to cool naturally to room temperature in the oven. Carefully peeled off from the glass plate to obtain a light yellow, tough and transparent polyurethane composite film, recorded as S1.

[0066] Example 2: This embodiment of the present invention provides a method for preparing a solvent-resistant polyurethane composite material. Except for adjusting the amount of the functional chain extender DC-DDM-tBu to 45 g, the types and amounts of all other raw materials, the equipment used, and all the steps are identical to those in Example 1. A polyurethane composite film is obtained, designated S2.

[0067] Example 3: This embodiment of the present invention provides a method for preparing a solvent-resistant polyurethane composite material, wherein the amount of the isocyanate component (MDI) is 100 g, and the amount of the functional chain extender (DC-DDM-tBu) is 30 g. As the polyol component, 90 g of polypropylene glycol (PPG, Mn = 2000) replaces PTMEG. Aside from the type and amount of the polyol, the types and amounts of all other raw materials, the equipment used, and all the steps are identical to those in Example 1. A polyurethane composite film is obtained, designated S3.

[0068] Example 4: This embodiment of the present invention provides a method for preparing a solvent-resistant polyurethane composite material. Except that, when preparing the chain extender solution, 30 g of DC-DDM-tBu was replaced with a mixture of 20 g of DC-DDM-tBu and 5 g of 1,4-butanediol (BDO), the types and amounts of all other raw materials, the equipment used, and all the steps were identical to those in Example 1. A polyurethane composite film was ultimately obtained, designated S4.

[0069] Comparative Examples 1 to 7: Comparative Example 1: Compared to Example 1, this method differs from the previous one in that, instead of using 30g of the functional chain extender DC-DDM-tBu, an equivalent molar amount (18.8g) of the conventional aromatic amine chain extender 4,4'-methylenebis(2-chloroaniline) (MOCA) was used. The types and amounts of all other raw materials, equipment, and procedures were identical. The resulting film is designated CS1.

[0070] Comparative Example 2: Compared with Example 1, the difference is that in the heat treatment curing step (b), after the precursor solution is cast into a film, it is only vacuum dried at 80°C to constant weight, and the subsequent high-temperature heat treatment step at 250°C is omitted. The resulting film is designated CS2.

[0071] Comparative Example 3: Compared to Example 1, this comparative example attempted to prepare a cross-linked polyurethane using a conventional one-step method. The difference was that all raw materials (100g MDI, 150g PTMEG, 30g DC-DDM-tBu, and 350g DMAc) were added to the reaction flask at once, and the polymerization reaction was carried out directly at 70°C. It was observed that the reaction gelled within a short period of time, and a processable solution could not be obtained. This product is designated as CS3.

[0072] Comparative Example 4: Compared to Example 1, this comparative example employed a conventional urethane crosslinking system. The difference was that, instead of using DC-DDM-tBu as the chain extender, 10 g of 1,4-butanediol (BDO) was used for chain extension, and 15 g of trimethylolpropane (TMP) was added as a crosslinker. The remaining steps were identical. The resulting film was designated CS4.

[0073] Comparative Example 5: Compared to Example 1, the difference was that instead of using 30 g of the functional chain extender DC-DDM-tBu, an equivalent molar amount (30 g) of its structural isomer 5,5'-di(tert-butyloxycarbonyl)-3,3'-diaminodiphenylmethane, prepared by the method of Preparation Example 4, was used. All other raw materials, equipment, and operating procedures were identical to those in Example 1. The resulting film was designated CS5.

[0074] Comparative Example 6: Compared to Example 1, the difference was that the amount of functional chain extender DC-DDM-tBu was reduced to 5 g (i.e., 5 parts per 100 g of MDI). All other raw materials, equipment, and operating procedures were identical. The resulting film was designated CS6.

[0075] Comparative Example 7: Compared to Example 1, the difference was that the amount of functional chain extender DC-DDM-tBu was increased to 70 g (i.e., 70 parts per 100 g of MDI). All other raw materials, equipment, and operating procedures were identical. The resulting film was designated CS7.

[0076] In order to verify the comprehensive performance of the polyurethane material prepared in the present invention, the film samples prepared in Examples 1 to 4 and Comparative Examples 1 to 7 (respectively denoted as S1 to S4 and CS1 to CS7) were subjected to the following performance tests.

[0077] 1) Mechanical properties test: Test standard: Refer to the national standard GB / T 528-2009 "Determination of tensile stress-strain properties of vulcanized rubber or thermoplastic rubber".

[0078] Sample Preparation: After 24 hours of conditioning all prepared films (S1-S4, CS1, CS2, CS4-CS7) at 25°C and 50% relative humidity, cut them into standard specimens using a dumbbell cutter (blade width 4 mm). At least five valid specimens should be cut from each sample.

[0079] The test steps are as follows: 1. Use electronic universal material testing machine for testing.

[0080] 2. Clamp the two ends of the dumbbell-shaped specimen in the upper and lower clamps of the testing machine, ensuring that the specimen is straight and has appropriate tightness.

[0081] 3. Set the tensile rate of the testing machine to 200 mm / min.

[0082] 4. Start the testing machine and stretch the specimen until it breaks. The instrument automatically records and calculates the tensile strength and elongation at break of the specimen.

[0083] 5. Repeat the above process for all valid specimens of each sample and take the arithmetic mean of the final result.

[0084] 2) Solvent resistance (swelling rate) and gel content test: Test solvent: N,N-dimethylacetamide (DMAc).

[0085] The test steps are as follows: 1. Accurately cut a square specimen of approximately 15 mm × 15 mm from each film sample.

[0086] 2. Place the sample in a vacuum oven at 80°C and dry it to constant weight. After cooling to room temperature in the desiccator, weigh its initial dry weight using a 1 / 10,000 analytical balance and record it as m0.

[0087] 3. Completely immerse the weighed sample in a sealed glass bottle containing sufficient DMAc solvent and let it soak at 25°C for 72 hours to allow it to fully swell.

[0088] 4. Remove the swollen sample and quickly absorb any residual solvent on its surface with filter paper. Immediately weigh its wet weight after swelling, recording it as m1. For samples that dissolve or disintegrate in the solvent, record their state.

[0089] 5. Place the wet weight of the sample in a vacuum oven at 80°C and dry it to constant weight. After cooling, weigh its final dry weight and record it as m2.

[0090] 6. Calculate the swelling ratio and gel content according to the following formula: Swelling rate (%) = [(m1-m0) / m0] × 100%; Gel content (%) = (m2 / m0) × 100%; 3) Thermal stability test: Testing instrument: Thermogravimetric analyzer.

[0091] The test steps are as follows: 1. Take about 5 to 8 mg of each film sample and place it in an alumina crucible.

[0092] 2. Place the crucible on the sample holder of the thermogravimetric analyzer.

[0093] 3. Set the test program: Under high-purity nitrogen protection (gas flow rate: 50 mL / min), heat from room temperature (about 30°C) to 700°C at a heating rate of 10°C / min.

[0094] 4. The instrument automatically records the curve of sample mass change with temperature. The temperature at which the sample loses 5% of its weight (T_d5) is read from the curve as an indicator for evaluating the thermal stability of the material.

[0095] The test results are summarized in Table 1: Table 1 Performance test results of examples and comparative examples:

[0096] Based on the performance test data shown in Table 1, the following conclusions can be drawn: The materials (S1-S4) prepared in Examples 1 to 4 of the present invention all exhibit low swelling ratio, high gel content and high thermal decomposition temperature. Specifically, the swelling ratio of sample S1 in Example 1 in DMAc solvent is as low as 4.8%, the gel content reaches 98.6%, and its 5% thermal weight loss temperature is 363.1°C. This performance is attributed to the two-step process of the present invention. First, the terminal amino group is reacted with the isocyanate group at low temperature to obtain a processable linear polyurethane precursor containing a tert-butyloxycarbonyl protective group; then, in a high-temperature heat treatment stage at 250°C, the tert-butyloxycarbonyl group is thermally decomposed to generate a highly reactive carboxyl group in situ. Since in the molecular structure of the functional chain extender, the amino group and the tert-butyloxycarbonyl group are in an ortho position on the benzene ring, the newly generated carboxyl group can undergo an efficient intramolecular cyclodehydration reaction with the reacted amino group on the adjacent chain segment to form a quinazolinedione heterocyclic structure with stable chemical and thermal properties. These heterocyclic structures form effective chemical cross-linking points between polymer chains, thereby constructing a dense cross-linked network.

[0097] The results of the comparative examples demonstrate the essentiality of the various technical features of the present invention from various perspectives. Sample CS2 from Comparative Example 2, after omitting the 250°C high-temperature heat treatment step, completely dissolved in DMAc, had a gel content of zero, and exhibited a thermal weight loss onset temperature of only 208.9°C, demonstrating that high-temperature treatment is essential for forming a solvent-resistant crosslinked network. Sample CS5 from Comparative Example 5, which utilizes a structural isomer with amino and ester groups in a meta position as a chain extender, exhibited a gel content of only 64.7% and a swelling ratio of 86.4%. This demonstrates that the ortho-positioned arrangement of functional groups on the benzene ring is a key structural foundation for efficient cyclocrosslinking and excellent solvent resistance. The results of Comparative Examples 1 and 4 further demonstrate that the solvent resistance achieved by forming a quinazolinedione crosslinked network in the present invention is significantly improved compared to a conventional linear polyurethane (CS1, completely dissolved) and a conventional trimethylolpropane-crosslinked polyurethane (CS4, swelling ratio 28.5%). Furthermore, the instant gelation phenomenon in Comparative Example 3 demonstrates the necessity of the two-step process employed in the present invention for obtaining a processable precursor. The results of Comparative Examples 6 and 7 demonstrate that there is an optimal range for the amount of functional chain extender used. A too low amount (CS6) results in insufficient crosslinking density and poor solvent resistance, while a too high amount (CS7) leads to excessive crosslinking density, manifested as a low elongation at break (33%) and brittle material.

[0098] In summary, the detailed data of the above embodiments and comparative examples jointly confirm that the present invention, by adopting a functional chain extender with an amino group arranged in an ortho position and a latent functional group that is removed by heat, combined with a specific two-step preparation process, can form a highly stable quinazolinedione chemical cross-linking network in situ in the polymer matrix through high temperature treatment while ensuring good processability of the precursor solution, thereby giving the final composite material excellent solvent resistance, thermal stability and comprehensive mechanical properties.

[0099] Test Example 1: In order to directly observe and confirm the chemical transformation process of the present invention at the molecular level, FT-IR and TGA analyses were performed on key samples.

[0100] Step 1: Quantitative confirmation of thermal deprotection reaction (TGA analysis) To quantitatively verify the thermal decomposition of thermally labile protecting groups in the two-stage reaction of the present invention, thermogravimetric analysis (TGA) was performed on the linear polyurethane precursor CS2 (whose solid components were identical to those in Example 1) before heat treatment. The theoretical weight loss of the linear polyurethane precursor (CS2) was calculated as follows: 1) According to the formulation of Example 1, the total mass of the solid raw materials of the polymer is: 100g (MDI) + 150g (PTMEG) + 30g (functional chain extender) = 280g; 2) The weight fraction (W_f) of the functional chain extender 3,3'-di(tert-butyloxycarbonyl)-4,4'-diaminodiphenylmethane in the final polymer solid is: W_f=30g / 280g≈10.71%; 3) It is known that the theoretical weight loss rate (W_loss_monomer) of the functional chain extender monomer after removing the two tert-butyloxycarbonyl protecting groups (in the form of isobutylene) is 24.69%.

[0101] 4) Therefore, the total theoretical weight loss rate (W_loss_polymer) of the entire linear polyurethane precursor (CS2) during thermal decomposition should be: W_loss_polymer=W_f×W_loss_monomer=10.71%×24.69%≈2.64%; like Figure 5 The TGA curve shows a weight loss step between 200°C and 300°C, indicating that the expected chemical decomposition reaction occurs within this temperature range. The experimentally measured weight loss rate for this step is 2.59%.

[0102] The experimentally measured weight loss (2.59%) was highly consistent with the theoretical weight loss (2.64%) accurately calculated based on the formulation of Example 1. This confirmed that the decomposition of the thermally labile protecting groups and the in situ release of the active functional groups in the two-stage reaction mechanism of the present invention occurred according to the expected stoichiometric relationship.

[0103] Step 2: Confirmation of the cyclization structure of quinazolinedione (FT-IR analysis) like Figure 6 As shown, by comparing the infrared spectra before heat treatment (CS2, solid line) and after heat treatment (S1, dotted line), the occurrence of the reaction can be observed: Complete disappearance of the characteristic peak of the reactant: located at ∼1720 cm before heat treatment -1 The ester C=O peak and ∼1650 cm -1 The urea C=O peak disappears completely after heat treatment.

[0104] The clear generation of the product characteristic peak: At the same time, after heat treatment, the peak at ~1770 cm -1 and ~1710 cm -1 A new pair of strong and sharp absorption doublets appeared at 400 nm. This pair of characteristic doublets is the signature signal of the two imide carbonyl groups in the specific heterocyclic structure of quinazolinedione.

[0105] The disappearance of the characteristic peak of the reactant and the generation of the characteristic peak of the product confirm that at high temperature, the ester and urea groups pre-arranged on the polymer chain by the "ortho-position" design of the present invention are indeed converted into the expected quinazolinedione cross-linked structure through a cyclization reaction.

[0106] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A solvent-resistant polyurethane composite material, characterized in that: It is prepared by reacting the following raw material components in parts by weight: Isocyanate component: 100 parts; Polyol component: 80-200 parts; Functional chain extender component: 10-50 parts; Wherein, the functional chain extender is a derivative of an ortho-amino aromatic carboxylic acid protected by a thermally unstable ester group.

2. The solvent-resistant polyurethane composite material according to claim 1, characterized in that: The heat-labile ester protective group of the functional chain extender decomposes under heat treatment conditions, thereby in situ generating a carboxyl group that can undergo a cyclization reaction with the urea group on the polymer chain to form a quinazolinedione heterocyclic cross-linked structure.

3. The solvent-resistant polyurethane composite material according to claim 1, characterized in that: The functional chain extender is 3,3'-di(tert-butyloxycarbonyl)-4,4'-diaminodiphenylmethane.

4. The solvent-resistant polyurethane composite material according to claim 3, characterized in that: The 3,3'-di(tert-butyloxycarbonyl)-4,4'-diaminodiphenylmethane is prepared by the following steps: (a) Using 2-nitro-5-methylbenzoic acid and formaldehyde as raw materials, reacting in concentrated sulfuric acid at 95-105°C to obtain 3,3'-dicarboxy-4,4'-dinitrodiphenylmethane; (b) subjecting the product of step (a) to hydrogenation reduction in the presence of a Pd / C catalyst at a hydrogen pressure of 3.0 to 4.0 MPa and a temperature of 60 to 70° C. to obtain 3,3′-dicarboxyl-4,4′-diaminodiphenylmethane; (c) subjecting the product of step (b) to a reflux esterification reaction with tert-butyl alcohol under acid catalysis and azeotropic dehydration conditions to obtain the 3,3'-di(tert-butyloxycarbonyl)-4,4'-diaminodiphenylmethane.

5. The solvent-resistant polyurethane composite material according to claim 1, characterized in that: The raw material components further include 0 to 20 parts by weight of a conventional chain extender component, wherein the conventional chain extender is selected from one or more of small molecule diols or small molecule diamines.

6. The solvent-resistant polyurethane composite material according to claim 1, characterized in that: The isocyanate component is selected from one or more of methylene diphenyl diisocyanate, toluene diisocyanate, isophorone diisocyanate or hexamethylene diisocyanate; the polyol component is selected from one or more of polytetramethylene ether glycol, polypropylene glycol or polyester glycol.

7. A method for preparing the solvent-resistant polyurethane composite material according to any one of claims 1 to 6, characterized in that: The following steps are involved: 1) Precursor preparation stage: reacting an isocyanate component with a polyol component to form an isocyanate-terminated prepolymer; then, carrying out a chain extension reaction between the prepolymer and the functional chain extender and a conventional chain extender at a temperature below the decomposition temperature of the protecting group to obtain a soluble and meltable linear polyurethane precursor; 2) Heat treatment and curing stage: After the linear polyurethane precursor obtained in step 1) is formed, heat treatment is performed under conditions higher than the decomposition temperature of the protective group to decompose the protective group and initiate an in-situ cyclization and cross-linking reaction to form a three-dimensional network structure.

8. The preparation method according to claim 7, characterized in that In step 1), the temperature of the chain extension reaction is controlled at 0-30°C.

9. The preparation method according to claim 7, characterized in that In step 2), the heat treatment temperature is 200-300°C.

10. The preparation method according to claim 7, characterized in that The heat treatment curing in step 2) specifically includes: (i) performing a solvent volatilization treatment at 80-120° C.; (ii) then raising the temperature to 220-280° C. and maintaining the temperature to carry out deprotection and cyclization cross-linking reactions.

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