Graphene heating material and application thereof

Abstract

The application relates to the technical field of water-based heating materials, and discloses a graphene heating material and application thereof, which comprises deionized water, propylene glycol methyl ether, tannic acid, sodium lignosulfonate, hexamethylenetetramine, graphene microsheets, conductive carbon black, a defoaming agent, a wetting agent, an anionic water-based polyurethane dispersion and epsilon-caprolactam blocked aliphatic polyisocyanate. In the heat curing stage, the tannic acid reacts with the hexamethylenetetramine to reduce the thickness of the insulating polymer coating layer on the surface of the graphene microsheets; meanwhile, the blocked polyisocyanate is unblocked at high temperature and crosslinked with the water-based polyurethane to form a three-dimensional polymer network to wrap the internal conductive path. The application reduces the contact resistance of adjacent particles by in-situ polycondensation to thin the insulating layer, and slows down the power attenuation phenomenon of the heating element in the long-term use process by using the physical wrapping and chemical anchoring effect of the crosslinked network on the conductive framework.

Description

Technical Field

[0001] This invention relates to the field of water-based heating materials technology, specifically to a graphene heating material and its applications. Background Technology

[0002] With the development of flexible wearable devices and smart cars, carbon-based heating elements with planar heating characteristics are increasingly used in electric heating protective gear, heated clothing, and car seat heating. For environmental protection and production safety considerations, water-based carbon-based heating coatings are gradually becoming the main development direction in this field. Graphene, with its excellent electrical and thermal conductivity, is often used as the core conductive filler in this type of material.

[0003] Existing aqueous graphene heating materials still have some technical shortcomings in practical applications. To improve the problem of graphene microsheets easily agglomerating in aqueous systems, it is necessary to add a large amount of macromolecular dispersants and polymer film-forming matrix to the formulation. While maintaining the dispersion of the slurry, these organic substances form a thick insulating polymer coating layer on the surface of the graphene microsheets. This barrier phenomenon increases the distance between conductive particles, resulting in higher contact resistance inside the heating coating, thus affecting the overall electrothermal conversion efficiency of the material.

[0004] Heating elements undergo frequent temperature cycles during operation. Traditional single-component water-based heating coatings, after curing, typically lack a dense three-dimensional cross-linked network structure in the polymer matrix to provide strong anchoring for the conductive filler. Under the thermal stress of long-term thermal cycling, the conductive framework within the coating is prone to microscopic deformation, relative slippage, or even network breakage. This leads to a gradual increase in local resistance over time, resulting in power attenuation during use.

[0005] The film-forming quality and mechanical flexibility of water-based heating coatings directly affect the environmental adaptability of end products. During conventional heating, baking, and curing processes, if moisture and solvents evaporate too quickly, they can easily become trapped inside the coating and cause localized boiling, leaving microscopic defects such as pinholes or bubbles in the film. When such defective and less flexible coatings are applied to complex stress scenarios such as human wearables or seats, conventional carbon paste films are prone to brittleness and detachment under repeated bending, rubbing, or heavy pressure. Traditional metal heating wires also have the risk of breakage, making it difficult to balance stable electrothermal performance with long-term structural reliability. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a graphene heating material and its application, solving the problems of high contact resistance due to insulating polymer coating, power attenuation caused by damage to the conductive network structure under long-term thermal cycling, and easy brittleness under stress caused by insufficient film quality and mechanical flexibility in existing water-based carbon-based heating materials.

[0007] To achieve the above objectives, the present invention provides the following technical solution: In a first aspect, the present invention provides a graphene heating material, which adopts the following technical solution: A graphene heating material comprising the following raw materials in parts by weight: Deionized water 24.85-25.0 parts, propylene glycol methyl ether 2.0-5.0 parts, tannic acid 1.0-3.0 parts, sodium lignosulfonate 0.5-1.5 parts, hexamethylenetetramine 0.5-1.5 parts, graphene microplates 5.0-10.0 parts, conductive carbon black 1.0-3.0 parts, defoamer 0.2-0.6 parts, wetting agent 0.1-0.4 parts, anionic waterborne polyurethane dispersion 40.0-50.0 parts, ε-caprolactam blocked aliphatic polyisocyanate 5.0-10.0 parts; By adopting the above technical solutions, the electrothermal conversion efficiency and service life of heating materials can be improved. Graphene microsheets are prone to agglomeration in aqueous systems. To achieve uniform dispersion, macromolecular dispersants or polymer film-forming base materials are usually introduced. However, these substances form a thick insulating polymer coating layer on the surface of the graphene microsheets, increasing the quantum tunneling distance between conductive particles and causing an increase in contact resistance. This invention introduces a system of tannic acid and hexamethylenetetramine, combined with blocked polyisocyanate and waterborne polyurethane to form a synergistic reaction system. The specific reaction process is mainly as follows: Under normal temperature fluid conditions, tannic acid utilizes its own polyphenolic hydroxyl structure and combines with the amphiphilic properties of sodium lignosulfonate to adsorb onto the surface of graphene microsheets through π-π non-covalent conjugation, forming steric hindrance, thereby maintaining the homogeneous dispersion of carbon-based materials in aqueous polyurethane dispersions. During the post-coating curing stage, hexamethylenetetramine undergoes thermal decomposition, releasing formaldehyde and ammonia molecules. The polyphenolic groups in the tannic acid structure react with formaldehyde molecules in a phenolic condensation reaction catalyzed by ammonia. This in-situ polymerization reaction occurs directly on the surface of the graphene microsheets, consuming some of the small organic molecules that originally served as a dispersed insulating layer. This causes the originally loose insulating polymer coating to densify, shrink, and thin, exposing carbon-based conductive contact sites, which helps reduce the contact resistance between adjacent graphene microsheets and conductive carbon black. As the curing temperature further increases, the ε-caprolactam-blocked aliphatic polyisocyanate reaches its deblocking temperature, and the ε-caprolactam blocking agent is removed, exposing free isocyanate groups. These free isocyanates undergo urethane crosslinking with the carboxyl groups, terminal hydroxyl groups, and unreacted tannic acid residual hydroxyl groups on the side chains of the anionic aqueous polyurethane dispersion, constructing a three-dimensional polymer crosslinked network. This network encapsulates and anchors the thinned conductive filler skeleton, locking the conductive pathways and reducing the risk of conductive network breakage under frequent thermal stress from temperature fluctuations, thus helping to maintain long-term resistance stability.

[0008] Preferably, the raw materials comprise the following parts by weight: 24.85 parts deionized water, 3.5 parts propylene glycol methyl ether, 2.0 parts tannic acid, 1.0 part sodium lignosulfonate, 1.0 part hexamethylenetetramine, 7.5 parts graphene microsheets, 2.0 parts conductive carbon black, 0.4 parts defoamer, 0.25 parts wetting agent, 50.0 parts anionic waterborne polyurethane dispersion, and 7.5 parts ε-caprolactam blocked aliphatic polyisocyanate.

[0009] By adopting the above technical solution and fixing the specific mass ratio of each component, the molar equivalent matching degree of tannic acid and hexamethylenetetramine is relatively good under this ratio condition. This can not only enable the surface insulating layer to fully condense and thin, but also minimize the impact of excessive residue on film density. At the same time, the ratio of isocyanate to active hydrogen in waterborne polyurethane is moderate, which is conducive to obtaining better mechanical flexibility and consistent heating power.

[0010] Preferably, the preparation steps of the anionic aqueous polyurethane dispersion include: Polytetrahydrofuran ether glycol was heated to 110°C under vacuum for 2 hours to dehydrate it, then cooled to 60°C, the vacuum was released and nitrogen gas was introduced for protection to obtain the dehydrated material. Isophorone diisocyanate was added to the dehydrated material, the temperature was raised to 80°C, and the mixture was stirred at a constant temperature for 2 hours to obtain isocyanate-terminated polyurethane prepolymer. 2,2-Dimethylolpropionic acid was added to the isocyanate-terminated polyurethane prepolymer, and the mixture was stirred at 80°C for 3 hours. The temperature was then lowered to 40°C to obtain the pretreated system. Triethylamine was added dropwise to the pretreatment system to carry out a neutralization and salt formation reaction. After the addition was complete, the mixture was stirred at a constant temperature for 30 minutes to obtain the salt formation system. Deionized water was added dropwise to the salt-forming system at a shear stirring speed of 2000 rpm for emulsification and dispersion. The mixture was stirred continuously for 1 hour to obtain an anionic waterborne polyurethane dispersion with a solid mass fraction of 35.0% and a pH value of 8.0.

[0011] By employing the above technical solution, using polytetrahydrofuran ether diol as the soft segment and isophorone diisocyanate as the hard segment, a specific anionic aqueous polyurethane dispersion was synthesized. The polytetrahydrofuran ether soft segment imparts good low-temperature flexural properties to the final heating film; the carboxyl groups introduced by 2,2-dimethylolpropionic acid, after being neutralized by triethylamine to form a salt, not only provide aqueous phase dispersibility but also participate in the reaction as active crosslinking sites in the subsequent crosslinking stage, which is beneficial to improving the adhesion of the coating to the substrate surface.

[0012] Preferably, the preparation steps of ε-caprolactam-blocked aliphatic polyisocyanate include: Hexamethylene diisocyanate trimer and propylene glycol methyl ether acetate were stirred under nitrogen protection and heated to 65°C to obtain a base mixture. Dibutyltin dilaurate was added to the base mixture, followed by ε-caprolactam in batches, with the temperature controlled not to exceed 75°C during the feeding process, to obtain a closed reaction system; The molar ratio of isocyanate to ε-caprolactam in the hexamethylene diisocyanate trimer is 1:1.05; After the addition of materials is completed, the temperature of the closed reaction system is adjusted to 70℃ and stirred at a constant temperature for 4 hours to obtain a constant temperature reaction system. Monitor the isothermal reaction system up to 2270 cm⁻¹ in the infrared spectrum. -1 The characteristic absorption peak of isocyanate at the ion completely disappeared. Heating was stopped, and the mixture was allowed to cool naturally to room temperature to obtain ε-caprolactam-blocked aliphatic polyisocyanate.

[0013] By employing the above technical solution, a curing agent with an unsealing temperature matching the aforementioned polycondensation reaction was prepared. Hexamethylene diisocyanate trimer possesses high crosslinking degree and is resistant to yellowing. After being sealed with ε-caprolactam, it does not undergo pre-reaction when mixed with aqueous components at room temperature, thus ensuring a certain pot life for the heating slurry. Setting the sealing molar ratio to 1:1.05 helps to fully seal the isocyanate ions, preventing water molecules from consuming isocyanate ions and generating bubbles, thereby reducing the impact of pinhole defects inside the heating coating on local resistivity.

[0014] Preferably, the graphene heating material contains conductive color paste with a fineness of less than 15 μm, and the preparation process of the graphene heating material includes: Deionized water, propylene glycol methyl ether, tannic acid, sodium lignosulfonate and hexamethylenetetramine were added to a reaction vessel and stirred at 800-1200 rpm for 20-40 min to obtain a premixed solution. Add graphene microsheets and conductive carbon black to the premixed liquid, mix evenly, and control the material temperature at 30-40℃ for 2-4 hours of cyclic grinding to obtain conductive color paste; Add defoamer, wetting agent, anionic waterborne polyurethane dispersion and ε-caprolactam blocked aliphatic polyisocyanate to conductive color paste, stir continuously at 300-500 rpm for 40-60 min and filter to obtain graphene heating material. The defoamer is one or more of the following: silicone defoamers, polyether defoamers, and mineral oil defoamers; The wetting agent is one or more of the following: polyether-modified siloxane wetting agent, fluorocarbon wetting agent, and acetylenic diol wetting agent.

[0015] By adopting the above technical solution, and by standardizing the feeding sequence and physical grinding process, carbon materials are added after mixing the additive solution for sand milling. The mechanical shearing force, combined with the surface activity of sodium lignosulfonate and tannic acid, is used to peel off and refine graphene micro-sheets to below 15μm, which is beneficial for constructing a long-range conductive network. Conductive carbon black, as point-like conductive nodes, is interspersed between the sheet graphene to form a multi-dimensional conductive pathway that combines points, lines, and surfaces. By limiting the specific types of defoamers and wetting agents, the surface tension of the slurry can be reduced, the possibility of broken lines or bubbles in the screen printing process can be reduced, and the uniformity of coating resistance distribution can be improved.

[0016] Preferably, after the graphene heating material is coated onto the substrate, it is cured into a film using a three-stage gradient heating process. The three-stage gradient heating process specifically includes: The first stage involves maintaining the temperature at 70-80℃ for 20-30 minutes. The second stage involves maintaining the temperature at 125-135℃ for 25-35 minutes. The third stage involves maintaining the temperature at 150-160℃ for 25-40 minutes.

[0017] By adopting the above technical solution, thermodynamic driving conditions that are consistent with the chemical reaction mechanism of the material system are provided. The first stage of low-temperature insulation allows the deionized water and propylene glycol methyl ether in the system to evaporate smoothly, reducing the phenomenon of coating cracking caused by solvent boiling. The second stage of medium-temperature step-up mainly triggers the phenolic condensation reaction of tannic acid and the decomposition products of hexamethylenetetramine, completing the thinning process of the insulating layer on the graphene surface. The third stage of high-temperature step-up reaches the desealing threshold temperature of ε-caprolactam, initiating the chemical cross-linking of isocyanate, hydroxyl, and carboxyl groups, and finally curing into a conductive network with structural memory effect.

[0018] Secondly, the present invention provides an application of graphene heating material in the preparation of flexible heating elements, employing the following technical solution: An application of graphene heating material in the preparation of flexible heating elements involves uniformly printing the graphene heating material onto a polyimide film substrate and forming a heating film through a three-stage gradient heating process. Electrodes are formed at both ends of the heating film by printing conductive paste and leading out wires. Subsequently, the flexible heating element is produced by hot-pressing and encapsulating with an insulating polymer film.

[0019] By employing the above technical solution, utilizing the high-temperature resistance and insulation properties of polyimide film as a carrier, and combining the adhesion provided by the cross-linked network after the graphene heating material is film-formed, a planar heating substrate is obtained. Through electrode fabrication and hot-press lamination encapsulation, the carbon-based heating film is isolated from the external environment, mitigating the oxidation of the carbon material and poor contact caused by air and moisture intrusion. This results in a flexible heating element with a complete structure and waterproof insulation function, realizing the conversion of electrical energy into far-infrared heat energy.

[0020] Preferably, the wet film thickness is controlled to be 20-40 μm when the graphene heating material is uniformly printed on the polyimide film substrate.

[0021] By adopting the above technical solution, the wet film printing thickness is controlled. Within this thickness range, moisture and solvent can be discharged from bottom to top in a laminar flow manner when the temperature is increased by gradient, reducing the probability of skin forming on the surface first and the internal solvent being trapped and bubbling. Moreover, the thickness of the cured dry film is sufficient to form a complete closed two-dimensional permeation conductive network, achieving the sheet resistance range required under safe working voltage.

[0022] Preferably, the flexible heating element is integrated into any one of the electric heating protective gear, heated clothing, and automotive seat heating systems.

[0023] By adopting the above technical solutions, flexible heating elements are transformed into specific heating terminal products. Thanks to the reduced contact resistance and improved mechanical flexibility provided by the aforementioned material system, the resulting heated clothing, electric heating protective gear, and car seat heating systems can slow down the attenuation of heating power and provide uniform infrared radiation heat after being subjected to stress conditions such as bending, mechanical rubbing, or heavy pressure from passengers. This improves upon the shortcomings of traditional metal heating wires, such as being uncomfortable to wear and easily broken, and conventional carbon film heating elements, which suffer from rapid attenuation.

[0024] This invention provides a graphene heating material and its applications. It has the following beneficial effects: 1. This invention introduces tannic acid and hexamethylenetetramine into the formulation, and utilizes the phenolic condensation reaction that occurs during the heating and curing stage to consume some of the macromolecular dispersants and other organic matter originally attached to the surface of graphene microsheets. This reaction causes the insulating polymer coating layer on the graphene surface to densify, shrink and thin, exposing carbon-based conductive contact sites, reducing the contact resistance between adjacent conductive particles, thereby improving the problem of high contact resistance of carbon-based heating coatings and improving the overall electrothermal conversion efficiency of the material.

[0025] 2. This invention utilizes ε-caprolactam-blocked aliphatic polyisocyanate in combination with anionic waterborne polyurethane dispersion. During the high-temperature stage of gradient curing, isocyanate deblocking is triggered, and crosslinking occurs with the waterborne polyurethane. The resulting urethane three-dimensional polymer network encapsulates and anchors the conductive filler skeleton, reducing the risk of conductive network breakage or relative slippage under the thermal stress impact of frequent temperature rises and falls. This improves the power stability of the heating material under long-term thermal cycling and slows down the power decay of the heating element during use.

[0026] 3. This invention employs a three-stage gradient heating process to cure the coating, allowing moisture and solvents to evaporate steadily at different temperature zones. This reduces the occurrence of pinholes or bubbles caused by solvent trapping and boiling within the coating. The coating cured by this process exhibits good adhesion to the substrate and mechanical flexibility. When applied to electric heating protective gear, heated clothing, or automotive seat heating systems, it can withstand physical stress conditions such as bending or heavy pressure, thereby optimizing the film-forming quality of the water-based heating coating and its adaptability in flexible end products, and reducing the incidence of easy breakage of traditional metal wires and brittle cracking of conventional carbon paste films under stress. Attached Figure Description

[0027] Figure 1 These are in-situ infrared monitoring curves of thermal reaction kinetics for embodiments and comparative examples of the present invention. Figure 2 This is a graph showing the loss factor of the cured films in the embodiments and comparative examples of the present invention as a function of temperature. Figure 3 The graphs show the resistance change rate test curves of the flexible heating films in the embodiments and comparative examples of the present invention under electrothermal cycling. Detailed Implementation

[0028] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] Preparation Examples 1-2: Preparation Example 1: This preparation example provides a method for preparing an anionic aqueous polyurethane dispersion, including the following steps: (1) Polytetrahydrofuran ether diol was added to a four-necked reactor equipped with a stirrer and a condenser. It was heated to 110°C under vacuum for 2 hours to dehydrate, and then cooled to 60°C. The vacuum was released and nitrogen gas was introduced for protection to obtain the dehydrated material. (2) Add isophorone diisocyanate to the dehydrated material in the reactor, heat to 80°C, and stir at a constant temperature for 2 hours to obtain isocyanate-terminated polyurethane prepolymer. (3) Add 2,2-dimethylolpropionic acid to the isocyanate-terminated polyurethane prepolymer, continue to stir at 80°C for 3 hours, and then cool down to 40°C to obtain the pretreated system. (4) Triethylamine was slowly added dropwise to the pretreatment system to carry out the neutralization and salt formation reaction. After the addition was completed, the mixture was stirred at a constant temperature for 30 minutes to obtain the salt formation system. (5) Deionized water was slowly added dropwise to the salt formation system at a shear stirring speed of 2000 rpm for emulsification and dispersion. The mixture was stirred continuously for 1 h to obtain an anionic waterborne polyurethane dispersion with a solid mass fraction of 35.0% and a pH value of 8.0.

[0030] Preparation Example 2: This preparation example provides a method for preparing ε-caprolactam-blocked aliphatic polyisocyanate, including the following steps: (1) Add hexamethylene diisocyanate trimer and propylene glycol methyl ether acetate to a reaction vessel, start stirring under nitrogen protection, and heat to 65°C to obtain a base mixture; (2) Add dibutyltin dilaurate to the base mixture, and then add ε-caprolactam in batches. Control the temperature of the reaction system during the feeding process to not exceed 75°C to obtain a closed reaction system; wherein the molar ratio of isocyanate to ε-caprolactam in the hexamethylene diisocyanate trimer is 1:1.05. (3) After the addition of materials is completed, the temperature of the closed reaction system is adjusted to 70°C and the reaction is stirred at a constant temperature for 4 hours to obtain a constant temperature reaction system; (4) Monitor the isothermal reaction system using a Fourier transform infrared spectrometer until the infrared spectrum reaches 2270 cm⁻¹. -1 The characteristic absorption peak of isocyanate at the ion completely disappeared. Heating was stopped, and the mixture was allowed to cool naturally to room temperature to obtain ε-caprolactam-blocked aliphatic polyisocyanate.

[0031] Examples 1-3: Example 1: This embodiment provides a graphene heating material and its application, including the following steps: (1) Preparation of novel graphene heating ink: 25.0 parts by weight of deionized water, 2.0 parts by weight of propylene glycol methyl ether, 1.0 part by weight of tannic acid, 0.5 parts by weight of sodium lignosulfonate and 0.5 parts by weight of hexamethylenetetramine were added to a reaction vessel and stirred at 800 rpm for 20 min to obtain a premixed solution. Under stirring, 5.0 parts by weight of graphene micro-sheets and 1.0 part by weight of conductive carbon black were added to the premixed solution, mixed evenly, and then pumped into a horizontal sand mill. The material temperature was controlled at 30°C. The mixture was circulated and ground at ℃ for 2 hours to obtain a conductive color paste with a fineness of less than 15 μm. The color paste was then transferred to a mixing tank, and 0.2 parts of an organosilicon defoamer and 0.1 parts of polyether-modified siloxane wetting agent were added. At 300 rpm, 40.0 parts of the anionic aqueous polyurethane dispersion prepared in Preparation Example 1 and 5.0 parts of the ε-caprolactam-blocked aliphatic polyisocyanate prepared in Preparation Example 2 were slowly added. The mixture was stirred continuously for 40 minutes, and after filtration, a novel graphene heating ink was obtained. (2) Preparation of flexible heating film: Using a 150-mesh screen printing plate, the ink obtained in step (1) is uniformly printed on the polyimide film substrate, and the wet film thickness is controlled to be 40 μm; then the substrate is sent into a tunnel oven and cured according to the following three-stage gradient heating process: the first stage is held at 70°C for 20 min; the second stage is held at 125°C for 25 min; the third stage is held at 150°C for 25 min, and then naturally cooled to obtain the graphene flexible heating film of this embodiment; (3) Application: The graphene flexible heating film obtained in step (2) is cut into a preset size, and electrodes are formed at both ends by screen printing conductive silver paste. After the wires are led out, the flexible heating element is made by hot pressing and encapsulating with an insulating polymer film. This element can be further integrated into electric heating protective products.

[0032] Example 2: This embodiment provides a graphene heating material and its application, including the following steps: (1) Preparation of novel graphene heating ink: According to the weight, 24.85 parts of deionized water, 3.5 parts of propylene glycol methyl ether, 2.0 parts of tannic acid, 1.0 parts of sodium lignosulfonate and 1.0 parts of hexamethylenetetramine were added to the reaction vessel and stirred at 1000 rpm for 30 min to obtain a premixed solution; under stirring, 7.5 parts of graphene micro flakes and 2.0 parts of conductive carbon black were added to the premixed solution, mixed evenly and then pumped into a horizontal sand mill, and the material temperature was controlled. The mixture was circulated and ground at 35°C for 3 hours to obtain a conductive color paste with a fineness of less than 15 μm. The color paste was then transferred to a mixing tank, and 0.4 parts of polyether defoamer and 0.25 parts of fluorocarbon wetting agent were added. At a speed of 400 rpm, 50.0 parts of the anionic aqueous polyurethane dispersion prepared in Preparation Example 1 and 7.5 parts of the ε-caprolactam blocked aliphatic polyisocyanate prepared in Preparation Example 2 were slowly added. The mixture was stirred continuously for 50 minutes, and after filtration, a novel graphene heating ink was obtained. (2) Preparation of flexible heating film: Using a 200-mesh screen printing plate, the ink obtained in step (1) is uniformly printed on the polyimide film substrate, and the wet film thickness is controlled to be 30 μm; then the substrate is sent into a tunnel oven and cured according to the following three-stage gradient heating process: the first stage is held at 75°C for 25 min; the second stage is held at 130°C for 30 min; the third stage is held at 155°C for 30 min, and then naturally cooled to obtain the graphene flexible heating film of this embodiment; (3) Application: The graphene flexible heating film obtained in step (2) is cut into a preset size, and electrodes are formed at both ends by screen printing conductive silver paste. After the wires are led out, the flexible heating element is made by hot pressing and encapsulating with an insulating polymer film. This element can be further integrated into heating clothing products.

[0033] Example 3: This embodiment provides a graphene heating material and its application, including the following steps: (1) Preparation of novel graphene-based heating ink: 25.0 parts by weight of deionized water, 5.0 parts by weight of propylene glycol methyl ether, 3.0 parts by weight of tannic acid, 1.5 parts by weight of sodium lignosulfonate, and 1.5 parts by weight of hexamethylenetetramine were added to a reaction vessel and stirred at 1200 rpm for 40 min to obtain a premixed solution. While stirring, 10.0 parts by weight of graphene microsheets and 3.0 parts by weight of conductive carbon black were added to the premixed solution. After mixing evenly, the mixture was pumped into a horizontal sand mill, and the material temperature was controlled at [temperature value missing]. The conductive color paste with a fineness of less than 15 μm was obtained by circulating grinding at 40℃ for 4 hours. The color paste was transferred to a mixing tank, and 0.6 parts of mineral oil defoamer and 0.4 parts of acetylenol wetting agent were added. At 500 rpm, 40.0 parts of anionic aqueous polyurethane dispersion prepared in Preparation Example 1 and 10.0 parts of ε-caprolactam blocked aliphatic polyisocyanate prepared in Preparation Example 2 were slowly added. The mixture was stirred continuously for 60 min, and after filtration, the novel graphene heating ink was obtained. (2) Preparation of flexible heating film: Using a 200-mesh screen printing plate, the ink obtained in step (1) is uniformly printed on the polyimide film substrate, and the wet film thickness is controlled to be 20 μm; then the substrate is sent into a tunnel oven and cured according to the following three-stage gradient heating process: the first stage is kept at 80°C for 30 min; the second stage is kept at 135°C for 35 min; the third stage is kept at 160°C for 40 min, and then naturally cooled to obtain the graphene flexible heating film of this embodiment; (3) Application: The graphene flexible heating film obtained in step (2) is cut into a preset size, and electrodes are formed at both ends by screen printing conductive silver paste. After the wires are led out, the flexible heating element is made by hot pressing and encapsulating with an insulating polymer film. This element can be further integrated into the automotive seat heating system.

[0034] Comparative Examples 1-5: Comparative Example 1: The difference from Example 2 is that tannic acid was not added in step (1), but the rest are the same.

[0035] Comparative Example 2: The difference from Example 2 is that hexamethylenetetramine was not added in step (1), but all other steps are the same.

[0036] Comparative Example 3: Compared with Example 2, the difference is that in step (1), the blocked isocyanate obtained in Preparation Example 2 is replaced by an equal amount of conventional non-blocked waterborne polyurethane crosslinking agent, and the rest are the same.

[0037] Comparative Example 4: Compared with Example 2, the difference is that in step (2), the three-stage gradient process was not used for curing. Instead, the printed substrate was directly sent into a tunnel oven at 155°C for constant temperature curing for 85 minutes. All other aspects are the same.

[0038] Comparative Example 5: Compared with Example 2, the difference is that tannic acid, hexamethylenetetramine and the blocked isocyanate prepared in Example 2 were not added in step (1), and the three-step gradient process was not used for curing in step (2). Instead, the printed substrate was directly sent into a tunnel oven at 155°C for constant temperature curing for 85 minutes. All other aspects are the same.

[0039] Test Examples 1-4: Test Example 1: This test case aims to verify the reaction kinetics and thermodynamic timing of the orthogonal latent curing system during the three-step temperature gradient process using in-situ infrared spectroscopy and variable temperature testing techniques, and to prove that each component can undergo independent cross-linking reactions according to the preset temperature steps.

[0040] Experimental steps: The novel graphene heating ink prepared in Example 2 and the ink prepared in Comparative Example 3 were used as test objects. They were coated onto potassium bromide salt sheets, and the wet film thickness was controlled to be 15 μm. They were placed in a vacuum drying oven and pre-dried at 40 °C for 30 min to remove free solvents on the surface.

[0041] A potassium bromide salt plate with ink samples adhering to its surface was placed in the variable-temperature test cell of an in-situ Fourier transform infrared spectrometer, and the instrument's scanning range was set to 4000 cm⁻¹. -1 Up to 400cm -1 The resolution is 4cm. -1 .

[0042] The heating program of the variable temperature test chamber was set to simulate the actual three-stage gradient curing process: the initial temperature was 70℃, and the temperature was gradually increased to 170℃ at a heating rate of 2℃ / min. During the heating process, infrared spectral data was automatically collected every 5℃.

[0043] Spectral data of Example 2 and Comparative Example 3 at different temperature nodes were extracted, and the data were analyzed at 1010 cm⁻¹. -1 The characteristic peak of CN stretching vibration at 2270 cm⁻¹ is attributed to hexamethylenetetramine. -1 The peak area of ​​the characteristic peak attributable to the isocyanate-NCO asymmetric stretching vibration is calculated by integrating the peak area.

[0044] Using the maximum peak area of ​​each characteristic peak in the initial state or immediately after release as a benchmark, the relative peak intensity at different test temperatures is calculated to quantitatively characterize the degree of reaction consumption of the relevant components.

[0045] Table 1. Relative absorption peak intensities of characteristic groups in Example 2 and Comparative Example 3 at different test temperatures. in conclusion: Based on the data in Table 1 and the appendix Figure 1 In Example 2, the characteristic absorption peak intensity of HMTA showed little change within the temperature range of 70°C to 120°C, indicating that no significant chemical reaction occurred in the initial physical film formation stage. When the temperature rose to the range of 125°C to 135°C, the relative peak intensity of HMTA decreased significantly, and during this period, the relative intensity of the isocyanate characteristic peak in Example 2 remained below 0.05. This result indicates that the first component decomposed at this stage and triggered interfacial crosslinking shrinkage. Simultaneously, since the crosslinking agent of the polyurethane resin had not yet been unsealed, the matrix had not undergone crosslinking, thus not spatially restricting the relative movement of the graphene microsheets. When the test temperature reached above 150°C, the characteristic absorption peak intensity of isocyanate in Example 2 first increased and then rapidly decreased, indicating that the second component unsealed at this stage and underwent a crosslinking reaction with the matrix.

[0046] In contrast, Comparative Example 3, which used a non-blocked crosslinking agent, showed a decrease in the intensity of its isocyanate characteristic peak in the 70°C to 100°C range, dropping from 0.863 to 0.421. This indicates that the unblocked isocyanate groups reacted with the polyurethane resin at low temperatures, leading to premature crosslinking of the matrix. This premature curing restricts the activity of molecular chain segments, thereby hindering the crosslinking shrinkage effect of tannic acid in the subsequent heating stage. The comparison of the two sets of data shows that the orthogonal latent curing system used in the examples can react separately in different temperature ranges, avoiding the adverse effects of premature matrix curing on the optimization of the interfacial conductive network.

[0047] Test Example 2: This test case mainly uses gel rate and dynamic thermomechanical analysis to test the degree of crosslinking and macroscopic mechanical state of materials under different curing processes and formulations, and verifies the fixing effect of polymer crosslinking network on conductive network.

[0048] Experimental steps: The inks prepared in Examples 2, 3, 4, and 5 were extracted and uniformly coated onto a polytetrafluoroethylene glass fiber cloth substrate with a release layer. The wet film thickness was controlled to be 30 μm.

[0049] The coated substrates were processed according to their respective curing process parameters. Examples 2 and 3 were heat-treated according to a preset three-stage gradient process, while Comparative Examples 4 and 5 were placed in a tunnel oven at 155°C for constant temperature curing for 85 minutes.

[0050] After the sample has cooled naturally, the cured coating is peeled off from the release substrate to obtain the corresponding free cured film test sample.

[0051] Weigh a certain mass of the cured film sample (record the initial mass W1), place it in a Soxhlet extractor, and reflux extract with acetone as solvent for 24 h. After extraction, place the remaining gel in an 80℃ vacuum drying oven and dry to constant weight. Weigh the mass of the dried gel (W2) and calculate the gelation rate (W2 / W1×100%).

[0052] The remaining free-cured film was cut into rectangular strips of standard size and clamped in the tensile fixture of the Dynamic Thermomechanical Analyzer (DMA). The test frequency was set to 1 Hz, the heating rate to 3 °C / min, and the test temperature range to 20 °C to 180 °C. The storage modulus and loss factor (tanδ) of the material were recorded as a function of temperature, and the peak temperature of the loss factor curve was recorded as the glass transition temperature (Tg) of the sample.

[0053] Table 2. Crosslinking degree and macroscopic mechanical test data of the cured films of the examples and comparative examples in conclusion: Based on the data in Table 2 and the appendix Figure 2 The gel rates of Examples 2 and 3 were 88.6% and 91.2%, respectively, and the storage modulus at 25°C was greater than 2400 MPa. Comparative Example 5 had a gel rate of 42.3%, a storage modulus of 874.3 MPa, and a peak loss factor at 56.2°C, which was relatively high. These data indicate that the crosslinking agent in the examples reacted with the polyurethane matrix after high-temperature desealing to form a crosslinked structure. The values ​​of gel rate and storage modulus reflect the restricted slippage of the material's molecular chains, macroscopically manifested as system solidification, which is beneficial for fixing the graphene conductive network formed in the upstream process.

[0054] Comparative Example 4 had the same formulation as Example 2, employing a one-step high-temperature isothermal curing process. Its gelation rate was 86.4%, and its storage modulus was 2135.4 MPa. Figure 2 The peak temperature of the loss factor in Comparative Example 4 was 111.5℃, and all related values ​​were lower than those in Example 2. When directly introduced into a high-temperature environment without staged temperature control, solvent evaporation, decomposition of the first component, and decomposition and crosslinking of the second component occurred simultaneously, resulting in a faster reaction rate and affecting the uniformity of the crosslinked network structure. The results of Examples 1 and 2 (Comparative Example 4) demonstrate that the three-stage gradient curing process helps to form a more complete matrix crosslinked network.

[0055] Test Example 3: This test case mainly tests the surface sheet resistance of the cured coating to verify the influence of material formulation and curing process on the connectivity of the internal conductive network of the coating.

[0056] Experimental steps: Extract the cured flexible heating film samples prepared in Examples 1 to 3 and Comparative Examples 1 to 5.

[0057] The above samples were cut into square specimens with a size of 50mm × 50mm.

[0058] Under ambient conditions of 25℃ and 50% relative humidity, the resistance of each sample was tested using a four-probe sheet resistance meter. Ten test points were randomly selected on the surface of each sample, the displayed values ​​were read, and the arithmetic mean was calculated and recorded as the sheet resistance of that sample.

[0059] Table 3. Sheet resistance test results of the flexible heating film samples in the examples and comparative examples. in conclusion: According to the data in Table 3, the sheet resistance of Examples 1 to 3 ranged from 15.76 to 45.38 Ω / sq. The sheet resistance of Comparative Example 1 without tannic acid was 128.73 Ω / sq, and the sheet resistance of Comparative Example 2 without hexamethylenetetramine was 115.42 Ω / sq. The comparison shows that the sheet resistance of the coating decreased when both tannic acid and hexamethylenetetramine were present. Their reaction helps to reduce the polymer insulating layer on the surface of the graphene microsheets, thereby reducing the contact resistance between the microsheets.

[0060] Comparative Example 3, using a non-blocking crosslinking agent, showed a sheet resistance of 86.25 Ω / sq, higher than the example. The non-blocking crosslinking agent initiates crosslinking of the polyurethane matrix at low temperatures; the early-formed matrix network restricts the relative movement of the filler, affecting the dense arrangement of the conductive network. Comparative Example 4, using a one-step isothermal curing process, showed a sheet resistance of 68.91 Ω / sq. Without segmented temperature control, the crosslinking reaction sequence within the system overlapped, hindering the establishment of conductive channels. The comprehensive data comparison shows that the specific component combination with the three-stage gradient curing process reduces the sheet resistance of the heating film.

[0061] Test Example 4: This test case mainly tests the resistance stability and surface temperature distribution of the cured heating film under long-term electrothermal cycling conditions, and verifies the fixing effect of the cross-linked network on the internal conductive structure.

[0062] Experimental steps: The cured flexible heating film samples prepared in Examples 1 to 3, as well as Comparative Examples 3, 4 and 5, were used as test subjects.

[0063] Each sample was cut into a square specimen with a size of 100mm×100mm. Silver conductive paste was uniformly coated on the two opposite edges of the specimen and dried to prepare a test electrode.

[0064] Connect the sample with electrodes to the automatic electrothermal cycling test system. Set the single electrothermal cycle program: apply a constant DC voltage to heat the sample surface to 90°C, maintain this state for 10 minutes, then turn off the power to allow it to cool naturally to room temperature (25°C) and stand for 5 minutes.

[0065] The total number of cycles was set to 2000. The initial resistance (R0) of the sample was measured and recorded before the test. After the test began, the instantaneous resistance (R0) of the sample was measured every 200 cycles during the resting period at room temperature. t ), calculate the rate of change of resistance ((R) t -R0) / R0×100%).

[0066] During the 1000th cycle of power-on maintenance, 16 temperature measurement points were arranged at equal intervals on the membrane surface using a multi-channel temperature monitoring instrument. The temperature values ​​at each point during steady-state operation were recorded, and the range (ΔT) between the highest and lowest temperatures was calculated.

[0067] Table 4. Electrothermal cycling test and temperature uniformity data of the flexible heating films in the examples and comparative examples. in conclusion: Based on the data in Table 4 and the appendix Figure 3 After 2000 electrothermal cycles, the resistance change rate of Examples 1 to 3 was 1.8% to 3.4%, and the film surface temperature difference was 2.1℃ to 2.7℃. Comparative Example 5, under the same conditions, had a resistance change rate of 36.5% and a film surface temperature difference of 14.8℃. The resistance change rate and temperature difference of the Examples were lower than those of Comparative Example 5. Under the stress of thermal expansion and contraction, the conductive filler lacking a cross-linked network experienced relative displacement, causing some conductive contacts to break, resulting in increased resistance and uneven current distribution. The polymer cross-linked network in the Examples fixed the conductive pathways of the graphene, reducing the displacement of the microsheets under thermal stress.

[0068] Comparative Example 3, using a non-blocking crosslinking agent, showed a resistivity change rate of 18.3% and a temperature range of 8.5℃ after 2000 cycles. Comparative Example 4, employing a one-step isothermal curing process, showed a resistivity change rate of 14.2% and a temperature range of 6.3℃. The uniformity of the polymer crosslinked network formed in Comparative Examples 3 and 4 was affected due to early crosslinking of the matrix or overlapping reaction sequences. Under cyclic thermal stress, stress concentration occurred within the crosslinked network, causing localized deterioration of the conductive structure. Based on the above physical mechanisms and test data, the combination of specific component formulation and a three-stage gradient curing process improves the electrothermal stability of the heating film during long-term operation.

Claims

1. A graphene heating material, characterized in that, The ingredients comprise the following parts by weight: 24.85-25.0 parts deionized water, 2.0-5.0 parts propylene glycol methyl ether, 1.0-3.0 parts tannic acid, 0.5-1.5 parts sodium lignosulfonate, 0.5-1.5 parts hexamethylenetetramine, 5.0-10.0 parts graphene microplates, 1.0-3.0 parts conductive carbon black, 0.2-0.6 parts defoamer, 0.1-0.4 parts wetting agent, 40.0-50.0 parts anionic waterborne polyurethane dispersion, and 5.0-10.0 parts ε-caprolactam blocked aliphatic polyisocyanate.

2. The graphene heating material according to claim 1, characterized in that, The ingredients comprise the following parts by weight: 24.85 parts deionized water, 3.5 parts propylene glycol methyl ether, 2.0 parts tannic acid, 1.0 part sodium lignosulfonate, 1.0 part hexamethylenetetramine, 7.5 parts graphene microsheets, 2.0 parts conductive carbon black, 0.4 parts defoamer, 0.25 parts wetting agent, 50.0 parts anionic waterborne polyurethane dispersion, and 7.5 parts ε-caprolactam blocked aliphatic polyisocyanate.

3. The graphene heating material according to claim 1, characterized in that, The preparation steps of the anionic aqueous polyurethane dispersion include: Polytetrahydrofuran ether glycol was heated to 110°C under vacuum for 2 hours to dehydrate it, then cooled to 60°C, the vacuum was released and nitrogen gas was introduced for protection to obtain the dehydrated material. Isophorone diisocyanate was added to the dehydrated material, the temperature was raised to 80°C, and the mixture was stirred at a constant temperature for 2 hours to obtain an isocyanate-terminated polyurethane prepolymer. 2,2-Dimethylolpropionic acid was added to the isocyanate-terminated polyurethane prepolymer, and the mixture was stirred at 80°C for 3 hours. The temperature was then lowered to 40°C to obtain the pretreated system. Triethylamine was added dropwise to the pretreatment system to carry out a neutralization and salt formation reaction. After the addition was complete, the mixture was stirred at a constant temperature for 30 minutes to obtain the salt formation system. Deionized water was added dropwise to the salt-forming system at a shear stirring speed of 2000 rpm for emulsification and dispersion, and stirring was continued for 1 hour to obtain the anionic waterborne polyurethane dispersion with a solid mass fraction of 35.0% and a pH value of 8.

0.

4. The graphene heating material according to claim 1, characterized in that, The preparation steps of the ε-caprolactam-blocked aliphatic polyisocyanate include: Hexamethylene diisocyanate trimer and propylene glycol methyl ether acetate were stirred under nitrogen protection and heated to 65°C to obtain a base mixture. Dibutyltin dilaurate was added to the base mixture, followed by the addition of ε-caprolactam in batches, with the temperature controlled not to exceed 75°C during the addition process, to obtain a closed reaction system; The molar ratio of isocyanate to ε-caprolactam in the hexamethylene diisocyanate trimer is 1:1.05; After the feeding is completed, the temperature of the closed reaction system is adjusted to 70°C and stirred at a constant temperature for 4 hours to obtain a constant temperature reaction system; Monitor the isothermal reaction system up to 2270 cm⁻¹ in the infrared spectrum. -1 The characteristic absorption peak of isocyanate at the point completely disappeared. Heating was stopped, and the mixture was allowed to cool naturally to room temperature to obtain the ε-caprolactam-blocked aliphatic polyisocyanate.

5. The graphene heating material according to claim 1, characterized in that, The graphene heating material contains conductive color paste with a fineness of less than 15 μm, and the preparation process of the graphene heating material includes: The deionized water, propylene glycol methyl ether, tannic acid, sodium lignosulfonate and hexamethylenetetramine were added to a reaction vessel and stirred at 800-1200 rpm for 20-40 min to obtain a premixed solution. The graphene microsheets and the conductive carbon black are added to the premixed liquid, mixed evenly, and the material temperature is controlled at 30-40℃ for 2-4 hours of cyclic grinding to obtain the conductive color paste. The defoamer, the wetting agent, the anionic aqueous polyurethane dispersion, and the ε-caprolactam-blocked aliphatic polyisocyanate are added to the conductive color paste, and the mixture is stirred continuously at 300-500 rpm for 40-60 minutes and then filtered to obtain the graphene heating material.

6. The graphene heating material according to claim 1, characterized in that, The defoamer is one or more of the following: silicone defoamer, polyether defoamer, and mineral oil defoamer; The wetting agent is one or more of the following: polyether-modified siloxane wetting agent, fluorocarbon wetting agent, and acetylenic diol wetting agent.

7. The graphene heating material according to claim 1, characterized in that, The graphene heating material is coated onto a substrate and then cured into a film using a three-stage gradient heating process. The three-stage gradient heating process specifically includes: The first stage involves maintaining the temperature at 70-80℃ for 20-30 minutes. The second stage involves maintaining the temperature at 125-135℃ for 25-35 minutes. The third stage involves maintaining the temperature at 150-160℃ for 25-40 minutes.

8. The application of the graphene heating material according to any one of claims 1-7 in the preparation of flexible heating elements, characterized in that, The graphene heating material is uniformly printed on a polyimide film substrate and formed into a heating film through a three-stage gradient heating process. Electrodes are formed at both ends of the heating film by printing conductive silver paste and wires are led out. Then, the flexible heating element is formed by hot pressing and encapsulation with an insulating polymer film.

9. The application according to claim 8, characterized in that, The thickness of the wet film when the graphene heating material is uniformly printed on the polyimide film substrate is controlled to be 20-40 μm.

10. The application according to claim 8, characterized in that, The flexible heating element can be integrated into any one of electric heating protective gear, heated clothing, and car seat heating systems.

Images