A dynamically covalently crosslinked polyurethane-based composite material, and a preparation method and application thereof

Abstract

The application belongs to the technical field of heat-conducting composite materials, and discloses a kind of dynamic covalent crosslinking polyurethane-based composite material and its preparation method and application, specifically including dynamic covalent crosslinking polyurethane matrix particles and modified filler, modified filler is combined on the surface of dynamic covalent crosslinking polyurethane matrix particles and dispersed in the gap between dynamic covalent crosslinking polyurethane matrix particles.The dynamic covalent crosslinking polyurethane-based composite material prepared by the heat-conducting filler has excellent heat conductivity, mechanical properties and insulation characteristics, and has wide practical application value in the field of electronic device thermal interface materials, etc.

Description

Technical Field

[0001] This invention belongs to the field of thermally conductive composite materials technology, specifically relating to a dynamically covalently crosslinked polyurethane-based composite material, its preparation method, and its application. Background Technology

[0002] With the rapid development of information and communication technologies, the performance and integration of electronic products are constantly increasing, making thermal management and heat dissipation of electronic devices increasingly important. Polymers possess advantages such as processability, low density, electrical insulation, corrosion resistance, and low cost. Therefore, polymer-based thermally conductive materials have attracted widespread attention from researchers. However, the thermal conductivity of common polymers is very low (0.1-0.5 W / (m·K)), far from meeting the application requirements of thermally conductive materials. Introducing thermally conductive fillers (such as boron nitride, graphite, metal particles, and carbon nanotubes) into polymers to prepare thermally conductive composite materials is the mainstream method for improving the thermal conductivity of materials.

[0003] Since the thermal conductivity of composite materials depends on the thermal conductivity path formed by the thermally conductive filler, traditional methods for preparing thermally conductive composite materials typically require relatively high filler contents, leading to difficulties in processing, low ductility, high cost, and poor filler distribution. Furthermore, with increasing demands on thermally conductive composite materials, composites prepared by direct filling with a single filler have limited thermal conductivity. Poor compatibility between inorganic fillers and polymers also results in high interfacial thermal resistance and decreased mechanical properties, all of which severely hinder the improvement of the overall performance of thermally conductive composite materials. Therefore, there is an urgent need to develop a simple and effective method to prepare crosslinked dynamically covalently crosslinked polyurethane thermally conductive composite materials with low filler contents and excellent mechanical and thermal conductivity. Summary of the Invention

[0004] In view of the problems and shortcomings of the existing technology, the purpose of this invention is to provide a dynamic covalent crosslinked polyurethane-based composite material, its preparation method and application.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] The first aspect of the present invention provides a dynamically covalently crosslinked polyurethane-based composite material, comprising dynamically covalently crosslinked polyurethane matrix particles and modified fillers, wherein the modified fillers are covalently bonded to the surface of the dynamically covalently crosslinked polyurethane matrix particles and dispersed in the voids between the dynamically covalently crosslinked polyurethane matrix particles;

[0007] The dynamically covalently crosslinked polyurethane matrix particles are mainly crosslinked by suspension polymerization of reactive monomer A containing two functional groups I, reactive monomer B containing two functional groups II, and crosslinking agent C containing at least three functional groups III; wherein, the functional group I is an isocyanate group; the functional group II is a hydroxyl group; the functional group III is a thiol group; the dynamic covalent bond in the dynamically covalently crosslinked polyurethane matrix is ​​a thiourethane bond; and the functional group of the modified filler is at least one of amino, thiol, epoxy group, and double bond.

[0008] Preferably, the reactive monomer A is a diisocyanate. More preferably, the reactive monomer A is at least one selected from hexamethylene diisocyanate, 4,4'-diphenylmethane diisocyanate, toluene diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, and isophorone diisocyanate.

[0009] Preferably, the reactive monomer B is an oligomeric diol, more preferably a linear oligomeric diol; the number average molecular weight of the oligomeric diol is 2000-4000 g / mol. More preferably, the reactive monomer B is at least one selected from polycaprolactone diol (PCL diol), polytetrahydrofuran ether diol, polycarbonate diol, polyethylene glycol, and polypropylene glycol.

[0010] Preferably, the crosslinking agent C is a polythiol compound. More preferably, the crosslinking agent C is at least one selected from pentaerythritol tetra-3-mercaptoacrylate (PTME), pentaerythritol tetra(mercaptoacetic acid) ester, trimethylolpropane tris(3-mercaptopropionate), and trimethylolpropane tris(mercaptoacetic acid).

[0011] More preferably, the reactive monomer A is hexamethylene diisocyanate (HDI); the reactive monomer B is polycaprolactone diol (PCL diol) with a number average molecular weight of 2000 g / mol; and the crosslinking agent C is pentaerythritol tetra-3-mercaptoacrylate (PTME).

[0012] Preferably, functional group III in the crosslinking agent C reacts with functional group I and / or functional group II in step (1). Preferably, the molar amounts of the reactive monomer A, reactive monomer B, and crosslinking agent C satisfy the relationship n. A ∶(n B +n C The mass ratio of the three components is (0.98-1.05):1, and the mass of each component satisfies the relationship (m). A +m C ) / (m A +m B +m C ) = (0.2 - 0.3); In the aforementioned relation, n A n B n CThese represent the molar amounts of reactant monomer A, reactant monomer B, and crosslinking agent C, respectively. A m B m C These represent the masses of reactant monomer A, reactant monomer B, and crosslinking agent C, respectively.

[0013] It should be noted that n A ∶(n B +n C The ratio of ) is the isocyanate index R. In this invention, the isocyanate index R is controlled between 0.98 and 1.05, and its calculation formula is as follows:

[0014]

[0015] In the above formula, m1 is the mass of diisocyanate; m d The mass of the oligomeric diol; m p M1 is the mass of PTME; M1 is the relative molecular mass of the diisocyanate; M d M represents the relative molecular mass of the oligomeric diol; p denoted as PTME, where is the relative molecular mass.

[0016] (m) A +m C ) / (m A +m B +m C The ratio of the hard segment content Ch in the crosslinked polyurethane is used to calculate the hard segment content Ch. In this invention, the hard segment content Ch is controlled between 20% and 30%, and the calculation formula is as follows:

[0017]

[0018] In the above formula, m1 is the mass of diisocyanate; m d The mass of the oligomeric diol; m p For the quality of PTME.

[0019] Preferably, by mass percentage, the content of reactive monomer A in the dynamically covalently crosslinked polyurethane-based composite material is 6wt%-15wt%, the content of reactive monomer B is 55wt%-80wt%, the content of crosslinking agent C is 8wt%-18wt%, and the content of modified filler is 3wt%-20wt%.

[0020] Preferably, the modified filler is a modified thermally conductive filler; the modified filler includes one-dimensional modified fillers and / or two-dimensional modified fillers. Further, the one-dimensional modified filler is a modified carbon nanotube; the two-dimensional modified filler is a modified boron nitride; wherein the modified boron nitride is prepared by modifying hexagonal boron nitride with a silane coupling agent; the modified carbon nanotube is prepared by modifying hydroxylated carbon nanotubes with a silane coupling agent; the functional group of the silane coupling agent is at least one selected from amino, mercapto, epoxy, and double bonds. More preferably, the silane coupling agent is at least one selected from KH-550, KH-560, KH570, KH-590, and KH-792.

[0021] More preferably, the preparation method of the modified boron nitride specifically involves: dispersing hexagonal boron nitride evenly in a hydrolyzed silane coupling agent solution, heating to 85-95℃ and stirring for 6-8 hours, followed by filtration, washing, and drying to obtain the modified boron nitride; wherein the size of the hexagonal boron nitride is 3-35 μm; and the amount of silane coupling agent added for the modified boron nitride is 1%-15% of the mass of the hexagonal boron nitride. Further, the specific preparation steps of the hydrolyzed modified boron nitride silane coupling agent solution are as follows: adding the silane coupling agent to a 95% ethanol aqueous solution, then adding formic acid to adjust the pH to 3-5, and stirring for 15 minutes to obtain the modified boron nitride.

[0022] More preferably, the modified carbon nanotubes are prepared by: dispersing hydroxylated carbon nanotubes evenly in a hydrolyzed silane coupling agent solution, heating to 65-75℃ and stirring for 6-8 hours, followed by filtration, washing, and drying; wherein the hydroxylated carbon nanotubes are carbon nanotubes with hydroxyl groups on their surface, with a diameter of 10-50 nm and a length of 1-50 μm; the amount of silane coupling agent added to the modified carbon nanotubes is 3-10 times the mass of the hydroxylated carbon nanotubes. Further, the hydrolyzed modified carbon nanotube silane coupling agent solution is obtained by adding the silane coupling agent to ethyl acetate and stirring for 15 minutes.

[0023] Preferably, the modified filler is a mixture of modified boron nitride and modified carbon nanotubes; the mass ratio of the modified boron nitride to the modified carbon nanotubes is (8-10):1.

[0024] A second aspect of the present invention provides a method for preparing the dynamically covalently crosslinked polyurethane-based composite material described in the first aspect, comprising the following steps:

[0025] (1) Dissolve reactive monomer A containing two functional groups I and reactive monomer B containing two functional groups I in a solvent and stir to obtain a prepolymer; wherein, the functional group I is an isocyanate group and the functional group II is a hydroxyl group;

[0026] (2) The catalyst and a crosslinking agent C containing at least three functional groups III are added to the prepolymer and the reaction is continued to obtain a crosslinked polyurethane intermediate solution; wherein the functional group III is a mercapto group;

[0027] (3) The cross-linked polyurethane intermediate solution is added to the suspending agent solution to form a suspension. The suspension is stirred and reacted until solid particles are obtained. The solid particles are washed and dried to obtain dynamic covalently cross-linked polyurethane matrix particles. The dynamic covalent bond in the dynamic covalently cross-linked polyurethane matrix is ​​a thiourethane bond.

[0028] (4) The modified filler is mixed evenly with the dynamic covalently crosslinked polyurethane matrix particles and then subjected to hot pressing to obtain the dynamic covalently crosslinked polyurethane matrix composite material; wherein the functional group of the modified filler is at least one of amino, mercapto, epoxy group and double bond.

[0029] Preferably, the reactive monomer A is a diisocyanate. More preferably, the reactive monomer A is at least one selected from hexamethylene diisocyanate (HDI), 4,4'-diphenylmethane diisocyanate, toluene diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, and isophorone diisocyanate.

[0030] Preferably, the reactive monomer B is an oligomeric diol, more preferably a linear oligomeric diol; the number average molecular weight of the oligomeric diol is 2000-4000 g / mol. More preferably, the reactive monomer B is at least one selected from polycaprolactone diol (PCL diol), polytetrahydrofuran ether diol, polycarbonate diol, polyethylene glycol, and polypropylene glycol.

[0031] Preferably, the crosslinking agent C is a polythiol compound. More preferably, the crosslinking agent C is at least one selected from pentaerythritol tetra-3-mercaptoacrylate (PTME), pentaerythritol tetra(mercaptoacetic acid) ester, trimethylolpropane tris(3-mercaptopropionate), and trimethylolpropane tris(mercaptoacetic acid).

[0032] More preferably, the reactive monomer A is hexamethylene diisocyanate (HDI); the reactive monomer B is polycaprolactone diol (PCL diol) with a number average molecular weight of 2000 g / mol; and the crosslinking agent C is pentaerythritol tetra-3-mercaptoacrylate (PTME).

[0033] Preferably, functional group III in the crosslinking agent C reacts with functional group I and / or functional group II in step (1). Preferably, the molar amounts of the reactive monomer A, reactive monomer B, and crosslinking agent C satisfy the relationship n. A ∶(n B +n CThe mass ratio of the three components is (0.98-1.05):1, and the mass of each component satisfies the relationship (m). A +m C ) / (m A +m B +m C ) = (0.2 - 0.3); In the aforementioned relation, n A n B n C These represent the molar amounts of reactant monomer A, reactant monomer B, and crosslinking agent C, respectively. A m B m C These represent the masses of reactant monomer A, reactant monomer B, and crosslinking agent C, respectively.

[0034] Preferably, by mass percentage, the content of reactive monomer A in the dynamically covalently crosslinked polyurethane-based composite material is 6wt%-15wt%, the content of reactive monomer B is 55wt%-80wt%, the content of crosslinking agent C is 8wt%-18wt%, and the content of modified filler is 3wt%-20wt%.

[0035] Preferably, the solvent is dimethyl carbonate (DMC); the amount of solvent used is 3-4 times the sum of the masses of reactant monomer A, reactant monomer B and crosslinking agent C.

[0036] Preferably, the catalyst is dibutyltin dilaurate (DBTDL); the amount of the catalyst is 1.5%-2% of the total mass of reactant monomer A, reactant monomer B and crosslinking agent C.

[0037] It should be noted that the crosslinked polyurethane intermediate solution is a liquid crosslinked polyurethane in which monomers A, B and C undergo partial crosslinking; the crosslinked polyurethane particles are solid crosslinked polyurethane after complete crosslinking.

[0038] More preferably, in step (2), the catalyst and crosslinking agent C are first dissolved in a solvent and then added to the prepolymer to continue the reaction. The solvent used is the same as the solvent in step (1).

[0039] Preferably, the suspending agent solution in step (3) is an aqueous solution of the suspending agent; the mass fraction of the suspending agent solution is 0.9%-1.2%, more preferably 1%. More preferably, the suspending agent is polyvinyl alcohol (PVA); the degree of alcoholysis of the polyvinyl alcohol is 98.0%-99.8%, and the viscosity is 54.0-66.0 mPa•s.

[0040] Preferably, the amount of water in the suspension solution is 6-10 times the mass of the solvent. More preferably, the amount of water in the aqueous solution of polyvinyl alcohol is 8-10 times the mass of dimethyl carbonate.

[0041] Preferably, the solid particles obtained in step (3) are cooled before being washed and dried. More preferably, the washing step specifically involves washing with ultrapure water three times to remove the PVA from the surface of the solid particles; the drying step specifically involves freeze drying to remove the moisture from the surface of the solid particles.

[0042] Preferably, the reaction conditions for obtaining the prepolymer in step (1) are: first react at 60-80℃ for 1.5-2.5h, then cool down to 55-65℃ and then add solvent and stir for 0.5-1h (to reduce the concentration of the prepolymer solution, prevent excessively fast reaction leading to explosive polymerization or premature crosslinking, and facilitate the completion of subsequent suspension polymerization crosslinking polymerization reaction); the reaction conditions for obtaining the crosslinked polyurethane intermediate solution in step (2) are: react at 55-65℃ for 3-5h; the reaction conditions for obtaining the solid particles in step (3) are: stir for 1.5-2.5h at 55-65℃.

[0043] Preferably, the modified filler is a thermally conductive modified filler; the modified filler includes one-dimensional modified fillers and / or two-dimensional modified fillers. Further, the one-dimensional modified filler is a modified carbon nanotube; the two-dimensional modified filler is a modified boron nitride; wherein the modified boron nitride is prepared by modifying hexagonal boron nitride with a silane coupling agent; the modified carbon nanotube is prepared by modifying hydroxylated carbon nanotubes with a silane coupling agent; the functional group of the silane coupling agent is at least one selected from amino, mercapto, epoxy, and double bonds. More preferably, the silane coupling agent is at least one selected from KH-550, KH-560, KH570, KH-590, and KH-792.

[0044] More preferably, the preparation method of the modified boron nitride specifically involves: dispersing hexagonal boron nitride evenly in a hydrolyzed silane coupling agent solution, heating to 85-95℃ and stirring for 6-8 hours, followed by filtration, washing, and drying to obtain the modified boron nitride; wherein the size of the hexagonal boron nitride is 3-35 μm; and the amount of silane coupling agent added for the modified boron nitride is 1%-15% of the mass of the hexagonal boron nitride. Further, the specific preparation steps of the hydrolyzed modified boron nitride silane coupling agent solution are as follows: adding the silane coupling agent to a 95% ethanol aqueous solution, then adding formic acid to adjust the pH to 3-5, and stirring for 15 minutes to obtain the modified boron nitride.

[0045] More preferably, the modified carbon nanotubes are prepared by: dispersing hydroxylated carbon nanotubes evenly in a hydrolyzed silane coupling agent solution, heating to 65-75℃ and stirring for 6-8 hours, followed by filtration, washing, and drying; wherein the hydroxylated carbon nanotubes are carbon nanotubes with hydroxyl groups on their surface, with a diameter of 10-50 nm and a length of 1-50 μm; the amount of silane coupling agent added to the modified carbon nanotubes is 3-10 times the mass of the hydroxylated carbon nanotubes. Further, the hydrolyzed modified carbon nanotube silane coupling agent solution is obtained by adding the silane coupling agent to ethyl acetate and stirring for 15 minutes.

[0046] Preferably, the modified filler is a mixture of modified boron nitride and modified carbon nanotubes; the mass ratio of the modified boron nitride to the modified carbon nanotubes is (8-10):1.

[0047] Preferably, the mixing method in step (4) is mechanical mixing. This mechanical mixing process allows the mixed modified filler (such as the modified boron nitride and modified carbon nanotubes of the present invention) to be uniformly coated on the surface of the dynamically covalently crosslinked polyurethane matrix particles. More preferably, the mechanical mixing method is at least one of mechanical stirring, ball milling, and mechanical vibration. When the mechanical mixing method is mechanical stirring, the mechanical stirring conditions are preferably stirring at a temperature of 15-30°C for 1 hour.

[0048] Preferably, the hot-pressing temperature in step (4) is the heat exchange temperature of the dynamic covalent bonds, which facilitates the softening of the dynamic covalently crosslinked polyurethane matrix particles due to the exchange of dynamic covalent bonds, making them suitable for processing and molding. When the dynamic covalent bonds are thiourethane bonds, the hot-pressing conditions are: temperature 165-180℃, pressure 2-3MPa, heating time 3-8min, and pressurization time 25-40min.

[0049] A third aspect of this invention provides the application of the dynamically covalently crosslinked polyurethane-based composite material described in the first aspect above in thermally conductive materials. When the modified filler is a thermally conductive modified filler, the dynamically covalently crosslinked polyurethane-based composite material described in any of the first aspects above is used in the preparation of thermally conductive materials, especially in the preparation of thermal interface materials, such as the preparation of thermal interface materials for LED and computer heat dissipation systems.

[0050] Furthermore, when the modified filler is modified boron nitride and modified carbon nanotubes in a mass ratio of (8-10):1, the dynamically covalently cross-linked polyurethane-based composite material is a thermally conductive and insulating material.

[0051] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0052] (1) In this invention, dynamic covalently cross-linked polyurethane matrix particles are first prepared by suspension polymerization, and then modified filler is mixed with the dynamic covalently cross-linked polyurethane matrix particles and hot-pressed to obtain a dynamic covalently cross-linked polyurethane matrix composite material. The modified filler is a mixture of two-dimensional modified boron nitride and one-dimensional modified carbon nanotubes. The composite material prepared by this invention utilizes the hot-pressing method of wrapping the thermally conductive modified filler on the surface of the cross-linked polyurethane particles to make the modified filler aggregate at the interface of the cross-linked polyurethane particles, thus constructing an efficient three-dimensional thermal conductivity pathway on a macroscopic scale. Therefore, high thermal conductivity can be obtained with low filler content. In addition, the "cage" structure formed by the modified filler prepared by this invention can effectively hinder crack propagation, so that the composite material maintains high mechanical properties; at the same time, it may also undergo dynamic bond exchange with the cross-linked polyurethane matrix during hot pressing, enhancing the interfacial bonding force and thus strengthening the mechanical properties of the composite material. In one embodiment, the modified boron nitride / modified carbon nanotube / polyurethane composite material prepared by this invention exhibits a thermal conductivity as high as 0.96 W / (m·K), a tensile strength of 7.3 MPa, and an elongation at break as high as 696%, which are 19% higher in thermal conductivity, 70% higher in tensile strength, and 445% higher in elongation at break than the boron nitride / carbon nanotube / polyurethane composite material with unmodified filler. Furthermore, the composite material prepared by this invention also possesses insulating properties, thus having broad practical application value in fields such as thermal interface materials for electronic devices.

[0053] (2) The present invention utilizes linear reactive monomers and multifunctional crosslinking agents to prepare solid particles containing dynamic covalent bonds in a suspension polymerization solution using a suspension polymerization method. This facilitates the dynamic bond exchange reaction between the particles and fillers containing modified groups under hot pressing, thereby enhancing the interaction between the crosslinked polyurethane matrix and the fillers, and between the fillers themselves. Consequently, the mechanical properties and solvent resistance of the composite material prepared by the present invention are superior to those of thermoplastic polymer-based composite materials.

[0054] (3) This invention uses two-dimensional modified boron nitride as the primary thermal filler and one-dimensional modified carbon nanotubes as the secondary thermal filler to prepare a hybrid filler. On the one hand, the relationship between the two-dimensional boron nitride and the one-dimensional carbon nanotubes forms a more effective heat conduction path, improving the thermal conductivity of the composite material. On the other hand, this invention also functionalizes the boron nitride and carbon nanotubes, which not only increases the compatibility between the filler and the cross-linked polyurethane matrix and reduces the contact thermal resistance between the filler and the cross-linked polyurethane matrix, but also facilitates the exchange of dynamic bonds between the filler and the cross-linked polyurethane matrix and between the fillers during hot pressing, thereby enhancing the interaction between the two and strengthening the mechanical properties of the composite material.

[0055] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0056] Figure 1 An optical microscope image of the composite material prepared in Example 1 of this invention;

[0057] Figure 2 This is a schematic diagram of the preparation process of the modified boron nitride / modified carbon nanotube / polyurethane composite material of the present invention;

[0058] Figure 3 The infrared spectra are before and after boron nitride modification;

[0059] Figure 4 The images show the infrared spectra of hydroxylated carbon nanotubes before and after modification. Detailed Implementation

[0060] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0061] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0062] This invention employs a universal mechanical tensile testing machine (SANS) to assess the tensile properties of composite film samples prepared in the examples and comparative examples. The test parameters were as follows: the sample film (thickness d = 0.55 ± 0.03 mm) was cut into dumbbell-shaped strips (gauge length 13 mm, width 2 mm) using a standard cutter; the strain rate was 100 mm / min; the test conditions were room temperature (approximately 25 ± 5 °C) and relative humidity 45 ± 5%. Each sample was tested five times under the same conditions, and the average value was taken as the tensile property result for that sample.

[0063] This invention uses a DRL-Ⅲ thermal conductivity meter from Xiangtan Xiang Instrument Co., Ltd. to test the thermal conductivity of composite material samples prepared in the examples and comparative examples. The temperatures of the hot plate and cold plate of the thermal conductivity meter are 20℃ and 80℃, respectively, and the pressure is 12N. The sample size is a circular disc with a diameter of 25mm and a thickness of approximately 1.5mm.

[0064] (I) The effect of filler modification on the properties of composite materials

[0065] To investigate the effect of filler modification on the properties of composite materials, the inventors conducted the following experiments: Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 4. The types and amounts of fillers used and the properties of the composite materials prepared with them are shown in Table 1.

[0066] Example 1

[0067] This embodiment provides a modified boron nitride / modified carbon nanotube / polyurethane composite material, such as... Figure 1 As shown, the composite material consists of cross-linked polyurethane particles (matrix) with a hard segment content of 28% and modified fillers. Based on the total mass of the composite material, the modified filler content is 9.9 wt%. The modified filler consists of modified boron nitride and modified carbon nanotubes in a mass ratio of 10:1.

[0068] The preparation method of the above-mentioned modified boron nitride / modified carbon nanotube / polyurethane composite material is as follows: Figure 2 As shown, the specific steps include the following:

[0069] (1) Preparation of modified boron nitride (FBN): Weigh 95g of ethanol and 5g of deionized water to prepare a 95wt% ethanol solution, and adjust the pH to 3-5 using formic acid. Add 50 mL of the above ethanol solution to a three-necked flask equipped with a condenser, add 0.15g of silane coupling agent KH560, and stir continuously for 15 min. Then, add 1g of BN to the above solution and stir at 90 ℃ for 6 h. After the solution cools, filter and wash three times with deionized water and ethanol. Dry the product in a vacuum oven at 80 ℃ for 12 h to obtain pure modified boron nitride, denoted as FBN.

[0070] Infrared spectra of boron nitride before and after modification were obtained, such as... Figure 3 As shown, the modified boron nitride exhibits a concentration at 2923 cm⁻¹. -1 The peak observed is the absorption peak of the CH stretching vibration at 2852 cm⁻¹. -1 An asymmetric stretching vibration peak of -CH2- appears at 1110 cm⁻¹. -1 The presence of a distinct Si-O bond vibration peak indicates that the silane coupling agent KH-560 has been successfully grafted onto the surface of boron nitride.

[0071] (2) Preparation of modified carbon nanotubes FCNT: 0.5 g of hydroxylated carbon nanotubes were added to a 250 mL three-necked flask equipped with a condenser. 100 mL of ethyl acetate and 5 g of KH-550 were added to the three-necked flask, and the mixture was stirred at 70 °C for 6 h. The mixture was filtered and washed several times with ethyl acetate. The product was dried in a vacuum oven at 80 °C for 12 h to obtain modified carbon nanotubes, denoted as FCNT.

[0072] Infrared spectra of hydroxylated carbon nanotubes before and after modification, such as... Figure 4 As shown, the infrared spectrum of the modified carbon nanotubes can be observed at 3435 cm⁻¹. -1 The enhancement of the absorption peak at 1554 cm⁻¹ -1 A deformation vibration peak of -NH2 appears at 2924 cm⁻¹. -1 A stretching vibration peak of -CH2 appears at 801 cm⁻¹. -1 The presence of a characteristic Si-OH absorption peak indicates that the silane coupling agent KH-550 has been successfully grafted onto the surface of the hydroxylated carbon nanotubes.

[0073] (3) Preparation of crosslinked polyurethane particles PTUM: 9.60 g PCL diol (0.0048 mol) was placed in a dry three-necked flask equipped with a magnetic stirrer and heated under vacuum in an oil bath at 120 °C for 2 h to remove moisture. The mixture was then cooled to 80 °C. 15 mL of DMC was added to dissolve the polyurethane particles, and 2.04 g of HDI (0.0121 mol) was added and reacted for 2 h. After cooling to 60 °C, 10 mL of DMC was added and stirred for 1 h to obtain a polyurethane prepolymer solution. Then, 0.27 g DBTDL dissolved in 10 mL of DMC and 1.69 g PTME (0.0035 mol) were added to the flask and the reaction was continued for 4 h to obtain a polyurethane intermediate solution. 3.00 g of PVA (purchased from Zhengzhou Penny Chemical Reagent Factory, item number 20221125) and 300 g of deionized water were added to a 500 mL single-necked flask and stirred at 70 °C. After the polyvinyl alcohol was completely dissolved, the temperature was lowered to 60 °C. The polyurethane intermediate solution was added dropwise to the PVA solution to form a suspension, and stirred at 60 °C for 2 h. The suspension was allowed to settle naturally at room temperature and washed three times with ultrapure water. After freeze-drying, pure cross-linked polyurethane particles (solid) were obtained, denoted as PTUM.

[0074] (4) Preparation of FCNT / FBN / PTUM composite material: 10.00g of cross-linked polyurethane particles, 1.00g of modified boron nitride and 0.10g of modified carbon nanotubes were mixed evenly by mechanical stirring to ensure that the filler was uniformly coated on the surface of the cross-linked polyurethane particles, thus obtaining a mixed powder. The mechanical stirring was preferably carried out at 15-30℃, 200 rpm, and for 1 hour. The mixed powder obtained after stirring was hot-pressed into a modified boron nitride / modified carbon nanotube / polyurethane composite material, denoted as FCNT / FBN / PTUM. The hot-pressing conditions were as follows: heating temperature 170℃, pressure 2.5MPa, heating time 5min, and holding pressure for 30min.

[0075] The volume resistivity of the above-mentioned modified boron nitride / modified carbon nanotube / polyurethane composite sample is greater than 10. 9 It has excellent insulation properties with a temperature of Ω·cm and a thermal conductivity as high as 0.96 W / (m·K).

[0076] Comparative Example 1

[0077] This comparative example provides a boron nitride / carbon nanotube / polyurethane composite material, which consists of cross-linked polyurethane particles with a hard segment content of 28% and fillers. Based on the total mass of the composite material, the filler content is 9.9 wt%. The filler consists of boron nitride (unmodified) and carbon nanotubes (unmodified) in a mass ratio of 10:1.

[0078] The preparation method of the above boron nitride / carbon nanotube / polyurethane composite material is basically the same as that of Example 1. The difference is that: the preparation of step (1) modified boron nitride and step (2) modified carbon nanotube is not carried out; the unmodified boron nitride is directly replaced by the modified boron nitride, and the unmodified carbon nanotube is replaced by the modified carbon nanotube for the preparation of step (4) boron nitride / carbon nanotube / polyurethane composite material, which is denoted as CNT / BN / PTUM.

[0079] Comparative Example 2

[0080] This comparative example provides a modified boron nitride / polyurethane composite material, which consists of cross-linked polyurethane particles with a hard segment content of 28% and modified fillers. Based on the total mass of the composite material, the modified filler content is 9.9 wt%; wherein, the modified filler is modified boron nitride.

[0081] The preparation method of the modified boron nitride / polyurethane composite material is basically the same as that of Example 1. The difference is that the preparation of modified carbon nanotubes in step (2) is not carried out; no modified carbon nanotubes are added in step (4), the amount of modified boron carbide is 1.10g, and finally the modified boron nitride / polyurethane composite material is obtained, which is denoted as FBN / PTUM.

[0082] Comparative Example 3

[0083] This comparative example provides a boron nitride / polyurethane composite material, which consists of cross-linked polyurethane particles with a hard segment content of 28% and fillers. Based on the total mass of the composite material, the filler content is 9.9 wt%; wherein the filler is boron nitride (unmodified).

[0084] The preparation method of the above boron nitride / polyurethane composite material is basically the same as that of Example 1. The difference is that: the preparation of modified boron nitride in step (1) and modified carbon nanotubes in step (2) is not carried out; no modified carbon nanotubes are added in step (4), the amount of boron carbide is 1.10g, and the boron nitride / polyurethane composite material is finally obtained, which is denoted as BN / PTUM.

[0085] Comparative Example 4

[0086] This comparative example provides a modified boron nitride / modified carbon nanotube / polyurethane composite material, which is composed of cross-linked polyurethane particles with a hard segment content of 28% and modified fillers. Based on the total mass of the composite material, the modified filler content is 9.9 wt%. The modified filler is composed of modified boron nitride and modified carbon nanotubes in a mass ratio of 10:1.

[0087] The preparation method of the modified boron nitride / modified carbon nanotube / polyurethane composite material is basically the same as that of Example 1. The difference is that: in step (3), the suspension polymerization method is not used to prepare cross-linked polyurethane particles. Instead, the obtained polyurethane intermediate solution is cooled to room temperature, 1.34g of modified boron nitride and 0.13g of modified carbon nanotube are added and stirred rapidly for 15min. After sonication for 5min, it is poured into a polytetrafluoroethylene mold and dried at 80℃ for 48h. The resulting modified boron nitride / modified carbon nanotube / polyurethane composite material is denoted as FCNT / FBN / PTU. At the same time, the preparation of step (4) is not performed.

[0088] Table 1. Performance test data of the composite materials prepared in Example 1 and Comparative Examples 1-4

[0089]

[0090] Comparing Example 1 and Comparative Example 4 in the table above, it can be seen that even with the exact same raw materials, different preparation methods will achieve completely different technical effects: Example 1 (suspension polymerization) has higher thermal conductivity and elongation at break than Comparative Example 4 (conventional polymerization liquid mixed with filler before polymerization). The thermal conductivity of Comparative Example 4 is only 51% of that of Example 1. Regarding thermal conductivity, although filler modification improves the compatibility between the filler and the crosslinked polyurethane matrix, the filler is still difficult to disperse well in the crosslinked polyurethane during solution polymerization. Furthermore, with a small amount of filler, even adding two types of filler makes it difficult to form a three-dimensional filler network in the crosslinked polyurethane matrix to create an efficient heat conduction path. Regarding elongation at break, on the one hand, the "cage" formed by the filler in the three-dimensional filler network structure hinders crack propagation, maintaining a high elongation at break; on the other hand, modified BN and modified CNT effectively reduce the contact thermal resistance between the filler and the matrix, while enhancing the interaction between the filler and the matrix, and between fillers themselves, thereby enhancing the mechanical properties of the material.

[0091] Comparing Example 1 and Comparative Example 1 in the table above, it can be seen that, under the same filler content, the modified filler (Example 1) exhibits superior thermal conductivity, tensile strength, and elongation at break compared to the unmodified filler (Comparative Example 1). While the thermal conductivity of the unmodified filler is lower than that of the modified filler, the difference is not significant; however, the tensile strength and elongation at break of the unmodified filler show a sharp decline compared to the modified filler. This is mainly because the unmodified filler has poor compatibility with the polymer, and the hybrid filler is distributed at the interface of the crosslinked polyurethane particles, hindering polymer fusion and causing numerous structural defects.

[0092] Since carbon nanotubes also possess a certain degree of electrical conductivity, the content of carbon nanotubes needs to be controlled to prepare insulating and thermally conductive materials, with boron nitride as the main thermally conductive material. Based on this, the inventors also prepared Comparative Examples 2 and 3, using only boron nitride as filler. Comparing Comparative Examples 2 and 3 in the table above, it can be seen that the thermal conductivity of the unmodified filler is still slightly lower than that of the modified filler, but the tensile strength and elongation at break are significantly lower. This is because BN distribution at the PTUM interface forms structural defects; the lamellar structure of BN coats the PTUM surface, hindering dynamic bond exchange reactions. Specifically, the lack of interaction between unmodified BN and the crosslinked polyurethane matrix significantly reduces the mechanical properties of the composite material; however, the modified functional groups on the modified BN can increase the interaction with the matrix. For example, the thiol groups in the crosslinked polyurethane matrix PTUM can react with the epoxy groups in KH560, increasing the interaction between BN and the matrix, thus significantly increasing the elongation at break.

[0093] However, under the premise of the same modified / unmodified filler content, comparing Example 1 and Comparative Example 2, it can still be found that the thermal conductivity, tensile strength, and elongation at break of the composite material with modified carbon nanotubes are significantly higher than those of the composite material without modified carbon nanotubes. Comparing Comparative Example 1 and Comparative Example 3, it can be found that the thermal conductivity, tensile strength, and elongation at break of the composite material with unmodified carbon nanotubes are not significantly different from those of the composite material without unmodified carbon nanotubes. In terms of thermal conductivity, the introduction of FCNT can build "bridges" to connect FBN sheets on the basis of the three-dimensional filler network structure, forming a second layer of filler three-dimensional network, further improving the thermal conductivity of the composite material. In terms of mechanical properties, the amino groups on the surface of FCNT may synergistically enhance the interaction between fillers and between fillers and crosslinked polyurethane with the epoxy groups on the surface of FBN, further enhancing the mechanical properties of the FCNT / FBN / PTUM composite material.

[0094] (II) Effect of modified filler content on the properties of composite materials

[0095] To investigate the effect of modified filler content on the properties of composite materials, the inventors conducted the following experiments, namely Examples 1-4. The corresponding modified filler content and the properties of the composite materials prepared therefrom are shown in Table 2.

[0096] Example 2

[0097] This embodiment provides a modified boron nitride / modified carbon nanotube / polyurethane composite material, which is composed of cross-linked polyurethane particles with a hard segment content of 28% and modified fillers. Based on the total mass of the composite material, the modified filler content is 3.2 wt%. The modified filler is composed of modified boron nitride and modified carbon nanotubes in a mass ratio of 10:1.

[0098] The preparation method of the modified boron nitride / modified carbon nanotube / polyurethane composite material is basically the same as that of Example 1. The difference is that the amount of modified boron nitride added in step (4) is 0.30g and the amount of modified carbon nanotube added is 0.03g.

[0099] Example 3

[0100] This embodiment provides a modified boron nitride / modified carbon nanotube / polyurethane composite material, which is composed of cross-linked polyurethane particles with a hard segment content of 28% and modified fillers. Based on the total mass of the composite material, the modified filler content is 5.2 wt%. The modified filler is composed of modified boron nitride and modified carbon nanotubes in a mass ratio of 10:1.

[0101] The preparation method of the modified boron nitride / modified carbon nanotube / polyurethane composite material is basically the same as that of Example 1. The difference is that the amount of modified boron nitride added in step (4) is 0.50g and the amount of modified carbon nanotube added is 0.05g.

[0102] Example 4

[0103] This embodiment provides a modified boron nitride / modified carbon nanotube / polyurethane composite material, which is composed of cross-linked polyurethane particles with a hard segment content of 28% and modified fillers. Based on the total mass of the composite material, the modified filler content is 8.1 wt%. The modified filler is composed of modified boron nitride and modified carbon nanotubes in a mass ratio of 10:1.

[0104] The preparation method of the modified boron nitride / modified carbon nanotube / polyurethane composite material is basically the same as that of Example 1. The difference is that the amount of modified boron nitride added in step (4) is 0.80g and the amount of modified carbon nanotube added is 0.08g.

[0105] Table 2 Performance test data of the composite materials prepared in Examples 1-4

[0106]

[0107] As shown in the table above, the thermal conductivity increases significantly with increasing modified filler content, while the mechanical properties of the composite material gradually decrease. Regarding thermal conductivity, on the one hand, a higher modified filler content results in a wider thermal conduction path formed by the three-dimensional filler network structure, leading to better thermal conductivity. On the other hand, higher contents of two-dimensional modified boron nitride and one-dimensional modified carbon nanotubes in the hybrid filler result in more branches of the microscopic thermal conduction path, further improving thermal conductivity. Regarding mechanical properties, although the table shows a decrease in tensile strength and elongation at break of the composite material prepared in the embodiments of this invention, the decrease is much slower compared to the unmodified composite material with different filler contents prepared by the inventors. This is because with increasing filler content, the number of amino groups on the FCNT surface and epoxy groups on the FBN surface increases, enhancing the interaction between fillers and between the filler and the crosslinked polyurethane, thus maintaining good mechanical properties for FCNT / FBN / PTUM.

[0108] Example 5

[0109] This embodiment provides a modified boron nitride / modified carbon nanotube / polyurethane composite material, which is composed of cross-linked polyurethane particles with a hard segment content of 28% and modified fillers. Based on the total mass of the composite material, the modified filler content is 9.9 wt%. The modified filler is composed of modified boron nitride and modified carbon nanotubes in a mass ratio of 8:1.

[0110] The preparation method of the modified boron nitride / modified carbon nanotube / polyurethane composite material is basically the same as that of Example 1. The difference is that the amount of modified boron nitride added in step (4) is 0.98g and the amount of modified carbon nanotube added is 0.12g.

[0111] The above-mentioned modified boron nitride / modified carbon nanotube / polyurethane composite material sample has a thermal conductivity of 1.04 W / (m·K), a tensile strength of 6.8 MPa, and an elongation at break of 688%.

[0112] Example 6

[0113] This embodiment provides a modified boron nitride / modified carbon nanotube / polyurethane composite material, which is composed of cross-linked polyurethane particles with a hard segment content of 28% and modified fillers. Based on the total mass of the composite material, the modified filler content is 20.2 wt%. The modified filler is composed of modified boron nitride and modified carbon nanotubes in a mass ratio of 10:1.

[0114] The preparation method of the modified boron nitride / modified carbon nanotube / polyurethane composite material is basically the same as that of Example 1. The difference is that the amount of modified boron nitride added in step (4) is 2.3g and the amount of modified carbon nanotube added is 0.23g.

[0115] The above-mentioned modified boron nitride / modified carbon nanotube / polyurethane composite material sample has a thermal conductivity of 1.51 W / (m·K), a tensile strength of 5.5 MPa, and an elongation at break of 486%.

[0116] Application performance testing:

[0117] The composite materials prepared in Examples 1-4 and Comparative Examples 2 and 4 of this invention were used as thermal interface materials (TIMs) and placed in a computer CPU cooling system to compare their heat transfer capabilities within the system. Specifically, the CPU cooling system consisted of TIMs placed on the surface of the computer CPU and covered with a heatsink. A fan was connected to the heatsink, and the heat generated in the CPU was directed to the heatsink through the TIMs. The fan then drew in cool air, which carried away the heat from the heatsink through airflow. The cooling system was tested using professional CPU performance testing software (AIDA64). The CPU was kept at full load for 15 minutes, and the average core temperature was recorded. The fact that the CPU core temperature remained below 80°C indicates that the CPU cooling system had good heat transfer performance.

[0118] The results showed that the core temperature of the composite material sample prepared in Example 2 with a modified filler content of 3% was close to 80°C; the core temperature of the composite material sample prepared in Example 2 with a modified filler content of 10% even reached 66°C; while the core temperature of the composite material sample prepared in Comparative Example 4 slightly exceeded 80°C. In summary, the composite material prepared by this invention has great application potential in the field of thermal interface materials for computer CPU cooling systems.

[0119] In summary, this invention effectively overcomes the shortcomings of the prior art and has high industrial applicability. The above embodiments are intended to illustrate the substantive content of this invention, but are not intended to limit the scope of protection of this invention. Those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this invention without departing from the essence and scope of protection of this invention.

Claims

1. A dynamically covalently crosslinked polyurethane-based composite material, characterized in that, It includes dynamically covalently crosslinked polyurethane matrix particles and modified fillers, wherein the modified fillers are bonded to the surface of the dynamically covalently crosslinked polyurethane matrix particles and dispersed in the voids between the dynamically covalently crosslinked polyurethane matrix particles; The dynamically covalently crosslinked polyurethane matrix particles are mainly formed by crosslinking and polymerization of reactive monomer A containing two functional groups I, reactive monomer B containing two functional groups II, and crosslinking agent C containing at least three functional groups III; wherein, the functional group I is an isocyanate group; the functional group II is a hydroxyl group; the functional group III is a mercapto group; and the dynamic covalent bond in the dynamically covalently crosslinked polyurethane matrix is ​​a thiourethane bond; The reactive monomer A is a diisocyanate, the reactive monomer B is an oligomeric diol, the crosslinking agent C is a polythiol compound, and the modified filler is a mixture of modified boron nitride and modified carbon nanotubes; the mass ratio of modified boron nitride to modified carbon nanotubes is (8-10):1; the modified boron nitride is prepared by modifying hexagonal boron nitride with KH-560 silane coupling agent; the modified carbon nanotubes are prepared by modifying hydroxylated carbon nanotubes with KH-550 silane coupling agent. The preparation method of the dynamically covalently crosslinked polyurethane-based composite material includes the following steps: (1) Dissolve reactive monomer A containing two functional groups I and reactive monomer B containing two functional groups I in a solvent and stir the reaction to obtain a prepolymer; (2) The catalyst and a crosslinking agent C containing at least three functional groups III are added to the prepolymer and the reaction is continued to obtain a crosslinked polyurethane intermediate solution; (3) The cross-linked polyurethane intermediate solution is added to the suspending agent solution to form a suspension. The suspension is stirred until solid particles are obtained. The solid particles are washed and dried to obtain dynamic covalently cross-linked polyurethane matrix particles. (4) After the modified filler is mixed evenly with the dynamic covalently crosslinked polyurethane matrix particles, hot pressing is performed to obtain the dynamic covalently crosslinked polyurethane matrix composite material.

2. The dynamically covalently crosslinked polyurethane-based composite material according to claim 1, characterized in that, The number-average molecular weight of the oligomeric diol is 2000-4000 g / mol.

3. The dynamically covalently crosslinked polyurethane-based composite material according to claim 2, characterized in that, The reactive monomer A is at least one of hexamethylene diisocyanate, 4,4'-diphenylmethane diisocyanate, toluene diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, and isophorone diisocyanate; the reactive monomer B is at least one of polycaprolactone diol, polytetrahydrofuran ether diol, polycarbonate diol, polyethylene glycol, and polypropylene glycol; and the crosslinking agent C is at least one of pentaerythritol tetra-3-mercaptoacrylate, pentaerythritol tetra(mercaptoacetic acid) ester, trimethylolpropane tri(3-mercaptopropionate), and trimethylolpropane tri(mercaptoacetic acid).

4. The dynamically covalently crosslinked polyurethane-based composite material according to claim 3, characterized in that, The molar amounts of the three reactant monomers A, B, and C satisfy the relationship nA∶(nB+nC)=(0.98-1.05)∶1; the mass of the three satisfies the relationship (mA+mC) / (mA+mB+mC)=(0.2-0.3).

5. The dynamically co-crosslinked polyurethane-based composite material according to any one of claims 1-4, characterized in that, By mass percentage, the content of reactive monomer A in the dynamically covalently crosslinked polyurethane-based composite material is 6wt%-15wt%, the content of reactive monomer B is 55wt%-80wt%, the content of crosslinking agent C is 8wt%-18wt%, and the content of modified filler is 3wt%-20wt%.

6. The method for preparing the dynamically covalently crosslinked polyurethane-based composite material according to any one of claims 1-4, characterized in that, Includes the following steps: (1) Dissolve reactive monomer A containing two functional groups I and reactive monomer B containing two functional groups I in a solvent and stir the reaction to obtain a prepolymer; (2) The catalyst and a crosslinking agent C containing at least three functional groups III are added to the prepolymer and the reaction is continued to obtain a crosslinked polyurethane intermediate solution; (3) The cross-linked polyurethane intermediate solution is added to the suspending agent solution to form a suspension. The suspension is stirred until solid particles are obtained. The solid particles are washed and dried to obtain dynamic covalently cross-linked polyurethane matrix particles. (4) After the modified filler is mixed evenly with the dynamic covalently crosslinked polyurethane matrix particles, hot pressing is performed to obtain the dynamic covalently crosslinked polyurethane matrix composite material.

7. The preparation method according to claim 6, characterized in that, The reaction conditions for obtaining the prepolymer in step (1) are: first react at 60-80℃ for 1.5-2.5h, then cool down to 55-65℃ and then add solvent and stir for 0.5-1h; the suspending agent solution in step (3) is an aqueous solution of suspending agent; the amount of water in the suspending agent solution is 6-10 times the mass of the solvent.

8. The application of the dynamically covalently crosslinked polyurethane-based composite material according to any one of claims 1-5 in thermally conductive materials.

9. The application according to claim 8, characterized in that, When the modified filler is modified boron nitride and modified carbon nanotubes in a mass ratio of (8-10):1, the dynamically covalently cross-linked polyurethane-based composite material is a thermally conductive and insulating material.

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