Ionic liquid polyurethane composite material, and preparation method and application thereof

Abstract

The application belongs to the technical field of wave-absorbing materials, and particularly relates to an ionic liquid polyurethane composite wave-absorbing material and a preparation method and application thereof. The preparation method comprises the following steps: mixing hexamethylene diisocyanate and polyethylene glycol in a reaction kettle in a DMF solvent to generate a prepolymer containing long-chain end isocyanate groups; adding ionic liquid into the reaction kettle during the generation of the prepolymer; adding glycerol as a crosslinking agent to connect the long-chain end isocyanate groups to generate a transparent chain network with good fluidity, so that the ionic liquid is bound in the chain network, that is, the chain network realizes physical loading of the ionic liquid; pouring the reaction product into a mold, and then volatilizing the solvent to form a ionic liquid polyurethane composite material with wave-absorbing performance. The maximum ionic liquid loading of the composite wave-absorbing material prepared by the application is 30 wt%, and the composite wave-absorbing material has a high electromagnetic wave absorption rate (up to 90%), and can be widely used for preparing electromagnetic wave-absorbing devices.

Description

Technical Field

[0001] This invention belongs to the field of microwave absorbing materials technology, specifically relating to an ionic liquid polyurethane composite microwave absorbing material, its preparation method, and its application. Background Technology

[0002] Electromagnetic wave losses in absorbing devices are mainly divided into two parts: dielectric loss and ohmic loss. Ionic liquids, due to their high conductivity and high dielectric loss factor in the microwave band, can be used in the design of electromagnetic absorbing devices with high dielectric loss.

[0003] Polyurethane is a high-molecular-weight elastomer with excellent flexibility and mechanical properties. The transparency of polyurethane materials can be achieved by controlling the crystallinity of the soft and hard segments within the material. Ionic liquids, due to their unique dispersive properties, are promising green, broadband microwave absorbing materials. Loading ionic liquids into polyurethane films can compensate for the leakage issues inherent in microwave absorbing devices and also facilitate the casting and molding of these devices, allowing for the design of more complex superunit structures. Lower crystallinity of the soft and hard segments in the polyurethane material results in higher transparency, and vice versa.

[0004] Electromagnetic absorbing devices based on ionic liquids can be realized by designing a super-unit structure, while conventional electromagnetic absorbing devices require a shell to contain the ionic liquid.

[0005] The preparation mechanism of the composite material involves physically loading ionic liquids into the polyurethane matrix during the process of polyurethane crosslinking to form a chain-like network structure.

[0006] In the preparation of polyurethane-loaded ionic liquid films, it is important to consider that isocyanates readily react with water and oxygen to form biuret, which can cause the solution to become turbid, and that the aqueous system can affect polymer formation. Therefore, water molecules in the reaction reagents must be removed before material synthesis, and the reaction vessel must be kept sealed during the reaction to prevent oxygen from reacting with isocyanates and affecting the reaction results. Summary of the Invention

[0007] The purpose of this invention is to provide a polyurethane composite microwave absorbing material with a large amount of ionic liquid and high electromagnetic wave absorption rate, as well as its preparation method and application.

[0008] The present invention provides a method for preparing ionic liquid polyurethane composite microwave absorbing materials, using DMF as a solvent and hexamethylene diisocyanate (HDI), polyethylene glycol (PEG600), and glycerol as raw materials; the specific preparation steps are as follows:

[0009] (1) First, in DMF solvent, HDI and PEG are mixed in a reactor to generate a prepolymer containing long-chain terminal isocyanate groups, while ionic liquid is added to the reactor during the process of generating the prepolymer.

[0010] (2) Then, glycerol is added as a crosslinking agent to connect the end isocyanate groups of the long chain in step (1) to generate a transparent chain network with good fluidity, so that the ionic liquid is bound in the chain network, that is, the chain network can realize the physical loading of the ionic liquid.

[0011] (3) Finally, the reaction product is poured into a mold and allowed to evaporate and solidify. If dried in a blower drying oven or a vacuum drying oven, an ionic liquid polyurethane composite material with microwave absorption properties is obtained. The composite material is flexible, transparent and has high attenuation performance.

[0012] In step (1), the molar ratio of HDI to PEG is 2.5 to 3.5; the reaction temperature in the reactor is 45 to 55°C, and the reaction time is 2.5 to 6 hours.

[0013] In step (1), the ionic liquid is an imidazole ionic liquid. This is because the cations of the imidazole ionic liquid can form hydrogen bonds with the imino groups in the polyurethane, thereby enabling the polyurethane to support the ionic liquid. Preferably, the three ionic liquids with the largest loss tangent are [EMIm][BF4], [EMIm][N(CN)2], or [EMIm][NTf2].

[0014] In step (1), the maximum loading of the ionic liquid is 30 wt% of the polyurethane. Typically, the amount of the ionic liquid added is 10 wt% to 30 wt% of the polyurethane.

[0015] In step (2), the number of moles of glycerol added satisfies that the total number of hydroxyl groups is equal to the total number of cyanate groups; the reaction temperature is 75-85℃ and the reaction time is 3-5h.

[0016] In step (3), the solvent evaporation molding involves pouring the reactants into a mold, first placing it in a forced-air drying oven for drying at 75–85°C for 3–10 hours; after most of the solution has evaporated, it is placed in a vacuum drying oven for drying at 10°C. -3 -10 -2 The material is dried and molded under a pressure of mbar, with a temperature of 75–85°C and a drying time of 1–3 hours.

[0017] The ionic liquid polyurethane composite microwave absorbing material prepared by this invention can achieve a maximum ionic liquid loading of 30wt%. With the dielectric constant of the 30wt% loading, the designed spherical cap-shaped supercell structure can achieve more than 90% absorption of electromagnetic waves in the 4-50GHz range. It has application prospects in the field of transparent electromagnetic microwave absorbing materials and can be used to prepare wide-bandwidth transparent electromagnetic microwave absorbing devices.

[0018] The absorption principle of the supercell structure comes from the superposition of multiple resonant modes. To achieve wide-bandwidth absorption of the supercell structure, two conditions must be met: the bandwidth of the resonant modes must be wide enough; and the resonant modes must be distributed throughout the entire absorption bandwidth.

[0019] Because the propagation coefficient of electromagnetic waves in lossy media satisfies It can be known that when While the magnitude of the electromagnetic wave decreases twice with increasing frequency, the magnitude of its propagation coefficient remains constant. Furthermore, the change in the dielectric constant of the polyurethane film carrying the ionic liquid ensures the stability of the propagation coefficient of the electromagnetic wave within the material at different frequencies. This means that the optical path length of the electromagnetic wave within the absorbing device does not change significantly with increasing frequency, providing sufficient conditions for a wide bandwidth of the resonant mode.

[0020] On the other hand, due to the geometric continuity and symmetry of the spherical cap-shaped supercell structure, it exhibits multiple resonant modes in the 4–50 GHz frequency range. These include magnetic resonance of electromagnetic waves within the supercell structure and periodic grating effects between dielectrics. Furthermore, higher-order magnetic resonances occur within the supercell structure at higher frequencies. This ensures that the corresponding absorption modes are within the 4–50 GHz range, thus enabling the design of wide-bandwidth electromagnetic absorbing devices.

[0021] Meanwhile, because the curvature change of the spherical cap-shaped superunit structure is more uniform compared to that of cubes and cylinders, the film is easier to remove from the mold. Attached Figure Description

[0022] Figure 1 This is a comparison diagram of the real parts of the relative permittivity of Examples 1-3 and Example 6.

[0023] Figure 2 This is a comparison diagram of the imaginary part of the relative permittivity in Examples 1-3 and Example 6.

[0024] Figure 3 This is a comparison diagram of the real parts of the relative permittivity for Examples 1 and 4-6.

[0025] Figure 4 This is a comparison diagram of the imaginary part of the relative permittivity for Examples 1 and 4-6.

[0026] Figure 5 This is a physical image of Example 1.

[0027] Figure 6 The image shown is the infrared spectrum of Example 1.

[0028] Figure 7 The X-ray diffraction results are for Examples 1 and 4-6.

[0029] Figure 8 The images are SEM images from Examples 1 and 4-6.

[0030] Figure 9 This is a schematic diagram of a supercell structure designed based on the dielectric constant of Example 2.

[0031] Figure 10 This is a schematic diagram of the overall structure of the absorbing device.

[0032] Figure 11 The figure shows the absorption performance curve of this superunit structure. Detailed implementation method:

[0033] The present invention will be further described below with reference to specific embodiments and accompanying drawings.

[0034] Example 1

[0035] The reactants were pretreated by vacuum distillation at 50–70°C to remove water from HDI and PEG600, and by vacuum distillation at 80–90°C for 0.5–3 hours. 4A molecular sieves were heated in a muffle furnace at 400°C for 4 hours, and then immersed in DMF to adsorb water molecules contained in the DMF.

[0036] The dehydration system for the experiment consisted of a heat-collecting, thermostatically heated magnetic stirrer, a round-bottom flask, a vacuum evacuation connector, and a vacuum pump. The round-bottom flask was completely submerged in the heat-collecting, thermostatically heated magnetic stirrer to prevent backflow of condensed water vapor. One end of the vacuum evacuation connector was connected to the top of the round-bottom flask, and the other end was connected to the vacuum pump via a rubber tube. The magnetic stirrer was kept rotating during the dehydration process, and the vacuum pump was kept running continuously.

[0037] For HDI and PEG600, the temperature needs to be maintained between 50 and 70°C. This is because HDI and PEG600 have good fluidity, making it easier to remove excess water during vacuum distillation. Secondly, at excessively high temperatures, HDI and PEG600 react with oxygen, and the isocyanate in HDI undergoes self-condensation at high temperatures, introducing new impurities that affect the final reaction result.

[0038] The reason why the temperature needs to be maintained at 80-90℃ for glycerol and ionic liquids is that glycerol and ionic liquids themselves have good stability at high temperatures and are not easily reacted with oxygen. Increasing the temperature helps to increase the evaporation rate of water in the reagents, thereby improving the dehydration efficiency. On the other hand, since glycerol has poor fluidity at lower temperatures, it is necessary to increase the temperature to improve its fluidity, making it easier for water to be removed during vacuum distillation.

[0039] The dehydration time is generally 0.5–3 hours. The standard for complete removal of moisture is to stop stirring and wait 1–2 minutes. If no bubbles are generated, it indicates that the moisture in the reagent has been largely removed. Since the reagent is prone to reaction under light, the dehydrated reagent should be placed in a brown reagent bottle and dried with activated 4A molecular sieve to improve its shelf life. The activation process of the 4A molecular sieve requires maintaining a temperature of 400℃ for 4 hours in a muffle furnace.

[0040] Specific preparation process:

[0041] Dissolve 1 mol of PEG600 in 30 ml of DMF organic solution and stir thoroughly at 50°C. Since PEG may be weakly alkaline, add one drop of phosphoric acid to the solution before the reaction.

[0042] b mol HDI was slowly added dropwise to the solution over a period of 30 minutes. The ratio of a:b was 2.5–3.5, where the specific value of a depended on the size and thickness of the thin film being prepared.

[0043] The reaction equation for PEG600 and HDI is as follows:

[0044]

[0045] The molar ratio of HDI to PEG600 should be maintained between 2.5 and 3.5. If the ratio is less than 2.5, the resulting polymer network will have small gaps and will not be able to effectively carry ionic liquids. If the ratio is greater than 3.5, the average molecular weight of the prepolymer long chain will be too small, resulting in film formation failure. In the example, the molar ratio of HDI to PEG600 is 3.

[0046] Without the addition of phosphoric acid, PEG600 will react with HDI at a relatively fast rate to form solid polyurethane long chains, with a reaction time of 20 to 30 minutes.

[0047] 30 wt% of [EMIm][BF4] ionic liquid was added dropwise to the solution, and the mixture was stirred uniformly at 50°C for 2.5–6 h until a prepolymer was formed. In the example, the reaction time was 4 h.

[0048] If the time for prepolymer formation is too short, the reaction between isocyanate and hydroxyl groups will be incomplete, resulting in a low average molecular weight and film formation failure. If the time is too long, side reactions will occur. Since isocyanate is in excess during prepolymer formation, some isocyanate will remain when the reaction is almost complete. The remaining isocyanate will react with urethane, causing the prepolymer to deteriorate.

[0049] Add c mol of glycerol dropwise to the solution obtained in the first step of the reaction, raise the temperature to 80°C, and stir the mixture uniformly for 3–5 hours until a chain network is formed. In the example, the reaction time was 4 hours. To achieve the conservation of isocyanate and hydroxyl groups, the amount of glycerol added, c, should satisfy 3c = 2(ba).

[0050] The reaction between glycerol and the prepolymer is as follows:

[0051]

[0052] If the crosslinking time is too short (less than 3 hours), the raw materials will not react completely. Subsequent placement of the solution in a forced-air drying oven will result in white bubbles forming on the film surface. This is because, firstly, continued reaction of the raw materials in the drying oven leads to uneven heating and bubble formation. Secondly, HDI reacts with oxygen at high temperatures to produce white biuret particles. If the crosslinking time is too long, the viscosity of the reaction solution will continuously increase, eventually causing the solution to stick to the reaction vessel and become impossible to remove.

[0053] The experimental reaction system consisted of a heat-collecting, constant-temperature magnetic stirrer, a round-bottom flask, and a glass stopper. The glass stopper was tightened to prevent moisture and oxygen from entering the round-bottom flask.

[0054] The solution obtained from the reaction is poured into a mold and placed in a forced-air drying oven for drying at 80°C for 3 to 10 hours.

[0055] After most of the solution has evaporated, place the solution in a vacuum drying oven and dry at 10°C. -3 -10 -2 The material is dried and shaped under a pressure of mbar, with a reaction temperature of 80℃ and a drying time of 1 to 3 hours.

[0056] If the solution is placed directly into a vacuum drying oven, the DMF solvent in the solution will evaporate too quickly, and the resulting film is prone to bubble formation. If the solution is only placed in a forced-air drying oven for drying, the solution will not evaporate completely, resulting in a film with poor tensile properties and difficulty in peeling it off the mold.

[0057] Example 2

[0058] Replace the [EMIm][BF4] ionic liquid added in Example 1 with the [EMIm][N(CN)2] ionic liquid, while keeping other conditions unchanged.

[0059] Example 3

[0060] The [EMIm][BF4] ionic liquid added in Example 1 was replaced with [EMIm][NTf2] ionic liquid, while other conditions remained unchanged.

[0061] Example 4

[0062] The mass fraction of the [EMIm][BF4] ionic liquid in Example 1 was changed to 20 wt%, while other conditions remained unchanged.

[0063] Example 5

[0064] The mass fraction of the [EMIm][BF4] ionic liquid in Example 1 was changed to 10 wt%, while other conditions remained unchanged.

[0065] Example 6

[0066] In Example 1, no ionic liquid was added, and a pure polyurethane film without ionic liquid was synthesized.

[0067] Figures 1 to 4 The dielectric constant data in the data are all obtained by inversion of the S-parameters determined by the rectangular waveguide method.

[0068] For polyurethane films supporting different ionic liquids, both the real and imaginary parts of their relative permittivity are improved to some extent, such as... Figure 1 , Figure 2 As shown, the increase in the real part of the dielectric constant is due to two main reasons: firstly, the polarization of the ionic liquid itself is stronger than that of the polyurethane film; using polyurethane to support the ionic liquid increases the overall relative dielectric constant. Secondly, it is due to the presence of [EMIm] in the ionic liquid. + The hydrogen bonds formed by the imino groups in polyurethane also have a polarizing effect on electromagnetic waves, increasing the relative permittivity of the film. Regarding the imaginary part of the relative permittivity, on the one hand, the ionic liquid itself exhibits orientation polarization, and the polarization loss increases the composite material's attenuation ability for electromagnetic waves, thus increasing the imaginary part of the electromagnetic wave permittivity. On the other hand, the presence of microchannels within the polyurethane film that allow the ionic liquid to flow creates conductivity losses, further increasing the imaginary part of the film's relative permittivity.

[0069] For polyurethane films loaded with the same ionic liquid at different mass fractions, the real and imaginary parts of their dielectric constant are as follows: Figure 3 , Figure 4As shown, the real part of the dielectric constant increases uniformly with increasing mass fraction, indicating that the real part of the dielectric constant mainly depends on the polarization effects of the polyurethane and the ionic liquid, while the hydrogen bonds between the ionic liquid and the polyurethane have a relatively small impact on the real part of the dielectric constant. With increasing frequency, the imaginary parts of the dielectric constant of polyurethane films with different ionic liquid mass fractions gradually converge. Since the magnitude of conductivity loss is inversely proportional to frequency, while the polarization loss of hydrogen bonds is directly proportional to frequency in the range of 2–18 GHz, it can be concluded that the imaginary part of the dielectric constant mainly originates from the microchannels that allow the ionic liquid to flow.

[0070] The physical object diagram in Example 1 is as follows Figure 5 As shown, the prepared polyurethane film has excellent light transmittance.

[0071] The infrared spectrum of Example 1 is as follows: Figure 6 As shown, in all spectra, 2265cm -1 No characteristic absorption peaks of isocyanate (-NCO) in HDI were observed in the vicinity. However, peaks were observed at 3325 and 1642 cm⁻¹. -1 Absorption peaks belonging to the tensile and bending vibrations of NH bonds were observed at 1716 cm⁻¹. -1 Absorption peaks belonging to the stretching vibrations of the C=O bond were observed; these peaks are characteristic vibrational peaks of urethane esters. This indicates that the reactants reacted completely and polyurethane was successfully synthesized. (Cyclops 3150 and 2942 cm⁻¹) -1 The stretching vibrations of the CH bonds, attributed to asymmetric and symmetric imidazole rings respectively, reached 1169 cm⁻¹. -1 The absorption peaks are attributed to the stretching motion of the CN bonds on the imidazole ring. These results also indicate that ionic liquids have been introduced into the polyurethane film system.

[0072] X-ray diffraction results at different concentrations are as follows Figure 7 As shown, when 2θ is 21.2 ° At the left and right positions, both pure polyurethane and polyurethane films loaded with different mass fractions of ionic liquid exhibited broad diffraction peaks, indicating that the material has low crystallinity. The low crystallinity also corresponds to the high transparency exhibited by the material.

[0073] Figure 8 Images (a), (b), (c), and (d) are SEM images of Examples 6, 5, 4, and 1, respectively. It can be seen that as the mass fraction of the loaded ionic liquid increases, the size of the precipitate particles increases, and the aggregation phenomenon on the surface of the composite material sample intensifies. The main reason is that as the ionic liquid content in the polyurethane increases, the asymmetry between the anions and cations increases, ultimately leading to a decrease in the compatibility between the ionic liquid and the polyurethane, thus enhancing the aggregation phenomenon on the surface of the composite material sample.

[0074] Figure 9 This is a schematic diagram of the microwave absorbing device designed based on the dielectric constant of Example 2. The period length of the supercell structure is p. The bottom layer of the absorbing device is an ITO backplate with a thickness of 0.125 mm. The middle layer is a flat plate structure with the composite material obtained in Example 2 as the dielectric, and a thickness of h. The upper layer is a spherical cap structure with the ITO backplate as the center, where the radius of the sphere is r.

[0075] In the process of designing the super unit structure, simulations were performed on spherical cap structures with period length p of 15–30 mm, dielectric plate thickness h of 0–5 mm, and radius r of 3 mm–15 mm. The radius of the spherical structure in the spherical cap was always less than 0.5 times the period length. The results showed that the absorption of electromagnetic waves in the full bandwidth of 4–50 GHz could be achieved when the size of the absorber satisfies the following relationship.

[0076] Where p = 28 mm, h = 3 mm, r = 11.5 mm, the height of the spherical cap relative to the dielectric plate is 8.5 mm, and the radius of the spherical cap as seen from the dielectric plate is 11.1 mm. The overall thickness of the absorbing device is 11.625 mm.

[0077] Figure 11 The diagram shows the reflection loss of the absorbing device. The mechanism involves one portion of the electromagnetic wave directly entering the structure, while another portion passes through gaps between structures and then undergoes total reflection through the backplate before entering the structure itself. Electromagnetic waves with opposite directions and a phase difference of π interfere to generate standing waves, which convert the energy of the electromagnetic waves into internal energy within the supercell structure, ultimately achieving absorption. From the perspective of the phase of the reflected electromagnetic waves, it can be said that the electromagnetic waves undergo destructive interference, but its essence is still caused by the loss of electromagnetic waves within the supercell structure.

Claims

1. A method for preparing an ionic liquid polyurethane composite microwave absorbing material, characterized in that, Using DMF as solvent and hexamethylene diisocyanate (HDI), polyethylene glycol (PEG), and glycerol as raw materials, the specific preparation steps are as follows: (1) First, in DMF solvent, HDI and PEG are mixed in a reactor to generate a prepolymer containing long-chain terminal isocyanate groups, while an ionic liquid is added to the reactor during the prepolymer generation process; wherein: The ionic liquid is an imidazole-based ionic liquid, specifically [EMIm][BF4]. 、 [EMIm][N(CN)2] or [EMIm][NTf2]; the maximum loading of the ionic liquid is 30 wt% of the polyurethane. (2) Then, glycerol is added as a crosslinking agent to connect the long-chain end isocyanate groups in step (1) to generate a transparent chain network with good fluidity, so that the ionic liquid is bound in the chain network, that is, the chain network realizes the physical loading of the ionic liquid. (3) Finally, the reaction product is poured into a mold and the solvent is allowed to evaporate and solidify to obtain an ionic liquid polyurethane composite material with microwave absorption properties. The composite material is flexible, transparent and has high attenuation performance.

2. The preparation method according to claim 1, characterized in that, The molar ratio of HDI and PEG in step (1) is 2.5 to 3.

5.

3. The preparation method according to claim 2, characterized in that, In step (1), the reaction temperature in the reactor is 45-55℃ and the reaction time is 2.5-6 h.

4. The preparation method according to claim 3, characterized in that, In step (2), the number of moles of glycerol added satisfies that the total number of hydroxyl groups and their cyanate groups are equal; the reaction temperature is 75-85℃ and the reaction time is 3-5 h.

5. The preparation method according to claim 4, characterized in that, The solvent evaporation molding described in step (3) involves pouring the reactants into a mold, first placing it in a forced-air drying oven for drying at 75–85°C for 3–10 hours; after most of the solution has evaporated, it is placed in a vacuum drying oven at 10°C. -3 -10 -2 The material is dried and molded under a pressure of mbar, with a temperature of 75–85°C and a drying time of 1–3 hours.

6. The ionic liquid polyurethane composite microwave absorbing material obtained by the preparation method according to any one of claims 1-5.

7. The application of the ionic liquid polyurethane composite absorbing material as described in claim 6 in the preparation of electromagnetic absorbing devices.

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