Stretchable conductive silver paste, silver film and preparation method thereof

Abstract

This invention belongs to the field of materials technology and discloses a stretchable conductive silver paste, a silver film, and a method for preparing the same. The raw materials for the stretchable conductive silver paste include polyurethane elastomer, silver sheets, silver nanowires, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, aliphatic polyisocyanate trimer, ethyl acetate, butanone, and fumed silica. The mass ratio of silver sheets to silver nanowires is 5:1 to 8:1. This invention employs a multi-scale conductive network composed of silver sheets and silver nanowires, combined with ionic liquid plasticization, low-temperature crosslinking, and silver sheet surface modification, enabling the silver paste to possess high conductivity, excellent tensile stability, and low-temperature curing properties. It is compatible with TPU and PDMS dual substrates and is suitable for flexible electronics fields such as flexible electrodes, strain sensors, and electronic skin.

Description

Technical Field

[0001] This invention belongs to the field of materials technology, specifically relating to a stretchable conductive silver paste, a silver film, and a method for preparing the same. Background Technology

[0002] Stretchable conductive silver paste is a key functional material in the field of flexible electronics, widely used in flexible electrodes, strain sensors, electronic skin, wearable flexible circuits, and flexible display touchscreens. As flexible electronic devices develop towards higher stretchability, higher stability, and lower cost, higher requirements are placed on the stretch conductivity stability, curing temperature, and process adaptability of conductive silver paste. A typical stretchable conductive silver paste consists of an elastomer binder, silver-based conductive filler, and organic solvent. It is formed into a conductive layer after curing through methods such as screen printing, blade coating, and spraying.

[0003] However, existing stretchable conductive silver pastes still have many shortcomings. Regarding conductive fillers, most products use a single type of silver filler. Pure silver sheets are prone to slippage and loss of contact under stretching, while pure silver nanowires suffer from high contact resistance and agglomeration, resulting in poor conductivity stability under stretching and significant resistance drift after cyclic stretching. In terms of curing processes, most existing silver pastes require high-temperature curing above 100℃, making them unsuitable for thermosensitive flexible substrates such as TPU and PDMS, limiting their application in flexible electronics. Regarding rheological properties, the rheological characteristics of most silver pastes are not optimized for the blade coating process, leading to problems such as silver sheet sedimentation and sagging after blade coating, affecting film uniformity.

[0004] Regarding substrate compatibility, existing silver pastes are typically only compatible with a single type of elastic substrate, making it difficult to simultaneously meet the adhesion requirements of two commonly used flexible substrates: TPU and PDMS, thus limiting the flexibility of device design. In terms of interfacial bonding, the interfacial bond between the silver filler and the elastomer matrix is ​​weak, and the silver filler is prone to detaching from the substrate during stretching, leading to damage to the conductive network and an irreversible increase in resistance. These issues collectively restrict the further application of stretchable conductive silver paste in high-performance flexible electronic devices, necessitating the development of a stretchable conductive silver paste and its preparation method that combines high stretchable conductivity stability, low-temperature curing properties, adaptability to coating processes, and dual-substrate compatibility. Summary of the Invention

[0005] To address the above problems, this invention provides a stretchable conductive silver paste, a silver film, and a method for preparing the same.

[0006] In a first aspect, the present invention provides a stretchable conductive silver paste, the raw materials of which include the following components: polyurethane elastomer, silver sheet, silver nanowire, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, aliphatic polyisocyanate trimer, ethyl acetate, butanone and fumed silica; wherein the mass ratio of the silver sheet to the silver nanowire is 5:1 to 8:1.

[0007] In some specific embodiments, the raw materials, by mass parts, include the following components: 10-20 parts of polyurethane elastomer, 30-40 parts of silver flakes, 4-7 parts of silver nanowires, 1-3 parts of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, 1-2 parts of aliphatic polyisocyanate trimer, 20-32 parts of ethyl acetate, 10-15 parts of butanone, and 0.5-2 parts of fumed silica; wherein the mass ratio of silver flakes to silver nanowires is 5:1 to 8:1.

[0008] Preferably, in some specific embodiments, the raw materials include the following components by mass: 14-18 parts of polyurethane elastomer, 31-36 parts of silver flakes, 4.5-6.5 parts of silver nanowires, 1.1-2.7 parts of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, 1.1-1.8 parts of aliphatic polyisocyanate trimer, 24-32 parts of ethyl acetate, 10-14 parts of butanone, and 0.5-2 parts of fumed silica; wherein the mass ratio of silver flakes to silver nanowires is 5:1-8:1.

[0009] Furthermore, the polyurethane elastomer is a hydroxyl-terminated polyurethane elastomer, synthesized using polytetrahydrofuran ether diol as the soft segment, isophorone diisocyanate as the hard segment, and 1,4-butanediol as the chain extender.

[0010] Furthermore, the average molecular weight of the polytetrahydrofuran ether diol is 1000-3000; preferably, the average molecular weight of the polytetrahydrofuran ether diol is 1500-2500.

[0011] Furthermore, the silver sheet has a diameter of 8-15 μm and a thickness of 200-500 nm; the silver nanowire has a diameter of 40-60 nm, a length of 16-40 μm, and an aspect ratio greater than 400; the total mass of the silver sheet and the silver nanowire accounts for 38%-42% of the mass of the stretchable conductive silver paste.

[0012] Specifically, the diameter of the silver sheet is 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, or any combination of these values; preferably, the diameter of the silver sheet is 8~12 μm. Specifically, the thickness of the silver sheet is 200 nm, 220 nm, 240 nm, 260 nm, 280 nm, 300 nm, 320 nm, 340 nm, 360 nm, 380 nm, 400 nm, 420 nm, 440 nm, 460 nm, 480 nm, 500 nm, or any combination of these values; preferably, the thickness of the silver sheet is 200~400 nm.

[0013] Specifically, the diameter of the silver nanowire is 40 nm, 42 nm, 44 nm, 46 nm, 48 nm, 50 nm, 52 nm, 54 nm, 56 nm, 58 nm, 60 nm, or any combination of these values; preferably, the diameter of the silver nanowire is 45~55 nm.

[0014] Specifically, the length of the silver nanowires is 16 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, or any combination of these values; preferably, the length of the silver nanowires is 20~30 μm. Specifically, the mass ratio of silver flakes to silver nanowires is 5:1, 5.5:1, 6:1, 6.5:1, 7:1, 7.5:1, 8:1, or any combination of these values; preferably, the mass ratio of silver flakes to silver nanowires is 5:1~7:1. The mass ratio of silver flakes to silver nanowires is a key parameter affecting the stability of the multi-scale conductive network. When the mass ratio is too high, the silver nanowire content is relatively insufficient, resulting in a decrease in the bridging effect at the gaps between silver flakes, stretching gaps, and microcracks. During stretching, the conductive network is prone to breakage, and the resistance drift increases. Simultaneously, an excessively high proportion of silver flakes increases the cost of the slurry and increases viscosity, which is detrimental to coating. When the mass ratio is too low, the silver nanowire content is relatively too high, and the one-dimensional nanofiller is prone to agglomeration, forming local conductive defects. In addition, excessive silver nanowires will increase the viscosity of the slurry and reduce the surface-to-surface contact efficiency of the silver sheet, which will reduce the initial conductivity.

[0015] Specifically, the total mass of the silver sheet and silver nanowires accounts for 38%, 38.5%, 39%, 39.5%, 40%, 40.5%, 41%, 41.5%, 42% of the mass of the stretchable conductive silver paste, or any combination of these values; preferably, the total mass of the silver sheet and silver nanowires accounts for 38% to 40% of the mass of the stretchable conductive silver paste.

[0016] Further, the amount of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt is 8% to 15% of the solid mass of the polyurethane elastomer. Specifically, the amount of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt is 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15% of the solid mass of the polyurethane elastomer, or any combination of these values; preferably, the amount of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt is 8% to 12% of the solid mass of the polyurethane elastomer. The 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, as an ionic liquid plasticizer, is inserted between polyurethane segments, lowering the glass transition temperature, improving low-temperature flexibility, and improving the interfacial wetting of the silver filler and the matrix. When the dosage is too low, the plasticizing effect is not obvious, the reduction in glass transition temperature is limited, the improvement in low-temperature flexibility is insufficient, and the interfacial wetting between the silver filler and the matrix is ​​inadequate. When the dosage is too high, excessive ionic liquid may undergo micro-phase separation in the polyurethane matrix, resulting in a decrease in the mechanical strength of the silver film and an increase in resistivity. Furthermore, excessive ionic liquid may migrate to the surface during film formation, affecting the appearance and adhesion of the film.

[0017] Further, the fumed silica is hydrophobic fumed silica, and the amount of fumed silica used is 0.5% to 2% of the mass of the stretchable conductive silver paste. Specifically, the amount of fumed silica used is 0.5%, 0.6%, 0.8%, 1.0%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0% of the mass of the stretchable conductive silver paste, or any combination of these values; preferably, the amount of fumed silica used is 0.6% to 1.4% of the mass of the stretchable conductive silver paste. Fumed silica acts as a thixotropic agent in the paste, imparting thixotropy to the paste through the formation of a reversible hydrogen bond network between particles. When standing, the network recovers and the viscosity is high, which can prevent the silver filler from settling; during scraping, the network is destroyed by shearing action, and the viscosity decreases, facilitating leveling. When the amount of fumed silica is too low, the thixotropic network strength is insufficient, and the silver filler is prone to sedimentation during the storage and coating process of the slurry, and the wet film is prone to sagging, affecting the uniformity of film formation. When the amount of fumed silica is too high, as a non-conductive inorganic filler, excessive addition of fumed silica will physically block the contact between silver fillers, increase the contact resistance of the conductive path, and lead to an increase in the resistivity of the silver film.

[0018] Further, the silver sheet is surface-treated with 3-mercaptopropyltrimethoxysilane, and the amount of 3-mercaptopropyltrimethoxysilane used is 0.3% to 1% of the mass of the silver sheet. Specifically, the amount of 3-mercaptopropyltrimethoxysilane used is 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0% of the mass of the silver sheet, or any combination of these values; preferably, the amount of 3-mercaptopropyltrimethoxysilane used is 0.3% to 0.8% of the mass of the silver sheet. 3-mercaptopropyltrimethoxysilane chemically bonds to the silver surface through its mercapto groups, and the silane ends form an interfacial bond with the polyurethane matrix, thereby reducing the debonding of the silver sheet from the matrix during stretching and enhancing the interfacial bonding force.

[0019] Further, the molar ratio (NCO / OH molar ratio) of the isocyanate group (-NCO) in the aliphatic polyisocyanate trimer to the hydroxyl group (-OH) in the polyurethane elastomer is 1.00~1.20. Specifically, the NCO / OH molar ratio is 1.00, 1.02, 1.05, 1.08, 1.10, 1.12, 1.15, 1.18, 1.20, or any combination of these values; preferably, the NCO / OH molar ratio is 1.03~1.10. The NCO / OH molar ratio controls the degree of crosslinking and curing. When the NCO / OH molar ratio is too low, the amount of crosslinking agent is insufficient, the crosslinking density of the silver film is not high enough, the elastic recovery ability after stretching is poor, the resistance drift is serious, and the conductive network is difficult to recover. When the NCO / OH molar ratio is too high, the excess -NCO remains after curing and can react with moisture in the air to generate carbon dioxide gas, forming bubbles inside the film layer, affecting the film layer's density and conductivity. At the same time, excessively high crosslinking density will lead to increased brittleness of the silver film, decreased elongation at break, and deterioration of tensile properties.

[0020] Furthermore, the aliphatic polyisocyanate trimer is a hexamethylene diisocyanate (HDI) trimer.

[0021] Secondly, the present invention provides a method for preparing stretchable conductive silver paste, comprising the following steps: Synthesizing polyurethane elastomer solution: Under dry nitrogen protection, polytetrahydrofuran ether diol and isophorone diisocyanate are added to a solvent to react and obtain a prepolymer. Then, 1,4-butanediol chain extender is added to react and obtain a hydroxyl-terminated polyurethane solution. Silver sheet surface modification: Silver sheets are dispersed in anhydrous ethanol, 3-mercaptopropyltrimethoxysilane is added to react, and the mixture is filtered, washed and dried to obtain modified silver sheets; Preparation of silver paste: Add 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt to the polyurethane elastomer solution and stir evenly. Add the modified silver sheet and grind and disperse. After dispersing the silver nanowires, add them to the system and stir. Add fumed silica and disperse. Add the aliphatic polyisocyanate trimer and stir. Degas under vacuum to obtain conductive silver paste.

[0022] Furthermore, the molar ratio of polytetrahydrofuran ether diol to isophorone diisocyanate, calculated as NCO / OH, is 1.03~1.2:1.

[0023] Further, the reaction temperature of polytetrahydrofuran ether diol with isophorone diisocyanate is 65~75℃, and the reaction time is 1~3 h. Specifically, the reaction temperature of polytetrahydrofuran ether diol with isophorone diisocyanate is 65℃, 66℃, 68℃, 70℃, 72℃, 74℃, 75℃, or any combination of these values; preferably, the reaction temperature of polytetrahydrofuran ether diol with isophorone diisocyanate is 68~72℃. When the reaction temperature is too low, the reaction rate between the isocyanate groups and hydroxyl groups is too slow, the prepolymer reaction is incomplete, and the reaction time is significantly prolonged, which is detrimental to production efficiency; when the reaction temperature is too high, side reactions of the isocyanate groups may be triggered, resulting in a wider molecular weight distribution and increased branching degree of the prepolymer, affecting the subsequent chain extension reaction and the mechanical properties and elastic recovery ability of the polyurethane elastomer.

[0024] Furthermore, the reaction temperature for adding 1,4-butanediol chain extender is 45~55℃, and the reaction time is 0.5~2 h.

[0025] Furthermore, the mixed solvent of ethyl acetate and butanone has a mass ratio of 7:3.

[0026] Furthermore, the solid content of the hydroxyl-terminated polyurethane solution is 30-40 wt%.

[0027] Further, after adding 3-mercaptopropyltrimethoxysilane, the reaction was stirred at 50-70°C for 1-3 hours; the reaction product was filtered, washed with ethanol, and dried at 50-70°C for 3-5 hours.

[0028] Further, after adding 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, stir at 400-600 rpm for 5-15 min.

[0029] Furthermore, after adding the modified silver flakes, the mixture is ground and dispersed 2-4 times using a three-roll mill, with roller gaps of 40-60 μm, 20-40 μm, and 10-20 μm respectively. Gradual dispersion is achieved by progressively decreasing the roller gap: first, a larger roller gap is used for initial deagglomeration of the silver flake agglomerates and wetting of the matrix; then, a medium roller gap is used to further disperse secondary agglomerates; finally, a smaller roller gap is used for fine dispersion, ensuring the silver flakes are uniformly dispersed in the polyurethane matrix while avoiding excessive breakage of the silver flakes. If too small a roller gap is used all at once, the silver flake agglomerates may experience localized overheating due to friction under high shear force, leading to abnormal changes in the viscosity of the polyurethane binder or even localized gelation; simultaneously, high shear force may cause deformation and breakage of the silver flakes, damaging their two-dimensional sheet-like structure, reducing surface-to-surface contact conductivity, and thus affecting the conductivity of the silver film.

[0030] Further, the silver nanowires were pre-dispersed in ethyl acetate and ultrasonically dispersed for 5-15 min; the silver nanowire dispersion was added to the silver sheet / polyurethane mixture under stirring at 300-500 rpm, and stirring was continued for 10-20 min.

[0031] Further, fumed silica was added and dispersed at high speed at 800~1200 rpm for 5~15 min.

[0032] Further, aliphatic polyisocyanate trimer was added, and the mixture was stirred at 200-400 rpm for 3-8 min; vacuum degassing was performed for 10-20 min at a vacuum degree of -0.09 MPa.

[0033] Thirdly, the present invention provides a stretchable conductive silver film, which is obtained by coating the above-mentioned stretchable conductive silver paste into a film and then curing it.

[0034] Further, the dry film thickness of the stretchable conductive silver film is 25~40 μm. Specifically, the dry film thickness of the stretchable conductive silver film is 25 μm, 26 μm, 27 μm, 28 μm, 29 μm, 30 μm, 32 μm, 34 μm, 36 μm, 38 μm, 40 μm, or any combination of these values; preferably, the dry film thickness of the stretchable conductive silver film is 30~35 μm.

[0035] Fourthly, the present invention provides a method for preparing a stretchable conductive silver film, comprising the following steps: S1. Film formation by scraping: The above stretchable conductive silver paste is scraped onto the substrate and allowed to dry at room temperature. S2. Curing: After the surface-dried stretchable conductive silver paste is cured, a stretchable conductive silver film is obtained.

[0036] Further, in step S1, the coating parameters are: blade gap 80~120 μm; coating speed 10~30 mm / s; wet film thickness 80~120 μm; and room temperature surface drying time 10~20 min.

[0037] Furthermore, the curing temperature is 60~80℃, and the curing time is 30~60 min.

[0038] Furthermore, the substrate includes a TPU substrate or a PDMS substrate.

[0039] Furthermore, when the substrate is TPU, the TPU substrate surface is wiped with methyl ethyl ketone (MEK) before coating; when the substrate is PDMS, a polyurethane primer containing siloxane segments is first coated on the PDMS substrate surface, and after drying, the silver paste is applied. In the polyurethane primer containing siloxane segments, the siloxane segments are compatible with the PDMS substrate, and the polyurethane segments are compatible with the silver paste binder, forming a transition layer between the PDMS substrate and the silver paste, thus improving interfacial adhesion.

[0040] Furthermore, the polyurethane primer containing siloxane segments is coated with a polyurethane primer liquid containing siloxane segments; the polyurethane primer liquid containing siloxane segments comprises, by weight, 5-10 parts of polyurethane containing siloxane segments and 90-95 parts of ethyl acetate.

[0041] Furthermore, the preparation method of the polyurethane primer containing siloxane segments is as follows: under nitrogen protection, aminopropyl-terminated polydimethylsiloxane and isophorone diisocyanate trimer are added at an NCO / NH2 molar ratio of 1.0~1.2:1, ethyl acetate is used as solvent, the solid content is 25~35 wt%, and dibutyltin dilaurate catalyst is added at an amount of 0.01%~0.03% of the solid mass. The reaction is carried out at 50~70℃ for 2~4 h to obtain the polyurethane primer containing siloxane segments.

[0042] Furthermore, the polyurethane primer containing siloxane segments has a coating solids content of 5-10 wt% and a coating thickness of 2-5 μm. After coating, it is dried at 50-70°C for 5-15 min, and then silver paste is applied by scraping.

[0043] Compared with the prior art, the beneficial effects of the present invention are: The stretchable conductive silver paste of this invention employs a multi-scale conductive filler composed of silver flakes and silver nanowires, combined with a polyurethane elastomer binder, ionic liquid plasticizer, low-temperature crosslinking agent, and thixotropic agent. The synergistic effect of these components endows the silver paste with excellent conductivity, tensile stability, and low-temperature curing properties. The polyurethane elastomer uses polytetrahydrofuran ether diol as the soft segment, isophorone diisocyanate as the hard segment, and 1,4-butanediol as the chain extender, exhibiting low-temperature flexibility, high elasticity, and mechanical strength. The silver flakes act as the main conductive framework, providing surface-to-surface contact, while the silver nanowires act as bridging fillers, maintaining conductive pathways at tensile gaps and microcracks, synergistically constructing a stable multi-scale conductive network. 1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt is inserted between polyurethane segments to lower the glass transition temperature, improve low-temperature flexibility, and enhance interfacial wetting. The silver flakes are surface-treated with 3-mercaptopropyltrimethoxysilane to enhance the interfacial bonding between the silver filler and the matrix, reducing tensile debonding. Aliphatic polyisocyanate trimers react with polyurethane hydroxyl groups at 60~80℃ to form a cross-linked network, achieving low-temperature curing and making them suitable for heat-sensitive flexible substrates.

[0044] Fumed silica, acting as a thixotropic agent, imparts suitable thixotropic properties to the paste, preventing silver flake sedimentation and sagging after coating, thus adapting to the coating process. The silver paste preparation method is carried out in steps, with clearly defined parameters for each step, ensuring process control and good repeatability, making it suitable for large-scale production. The silver film preparation method employs low-temperature curing after coating, resulting in a simple process. The curing temperature of 60~80℃ is suitable for TPU and PDMS substrates.

[0045] The PDMS substrate employs a polyurethane undercoat containing siloxane segments. The siloxane segments are compatible with PDMS, and the polyurethane segments are compatible with the silver paste, forming a transition layer to improve interfacial adhesion. Combined with the direct-coating curing characteristic of the TPU substrate, this achieves compatibility between TPU and PDMS dual substrates. The stretchable conductive silver paste of this invention possesses high conductivity, excellent tensile stability, low-temperature curing properties, and adaptability to coating processes, making it suitable for applications such as flexible electrodes, strain sensors, electronic skin, wearable flexible circuits, and flexible display touchscreens. Detailed Implementation

[0046] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. 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 protection scope of the present invention.

[0047] Example 1 A stretchable conductive silver paste, based on 100 parts by weight, comprises the following components: 16 parts of polyurethane elastomer (solid), 33.6 parts of modified silver sheets, 5.6 parts of silver nanowires, 1.6 parts of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, 1.4 parts of HDI trimer, 28.56 parts of ethyl acetate, 12.24 parts of butanone, and 1.0 part of fumed silica; wherein the mass ratio of silver sheets to silver nanowires is 6:1, the amount of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt is 10% of the mass of the polyurethane elastomer solid, and the fumed silica accounts for 1% of the total mass of the silver paste.

[0048] A method for preparing a stretchable conductive silver paste includes the following steps: (1) Synthesis of polyurethane elastomer solution: Under dry nitrogen protection, polytetrahydrofuran ether diol (average molecular weight 2000, based on 100 mmol -OH) and isophorone diisocyanate were fed at an NCO / PTMG-OH molar ratio of 1.05. A mixed solvent of ethyl acetate and butanone (mass ratio of ethyl acetate to butanone 7:3) was added, and the mixture was reacted at 70℃ for 2 h to obtain an NCO-terminated prepolymer. The temperature was lowered to 50℃, and 2.63 mmol of 1,4-butanediol (based on 1.05 times the molar equivalent of hydroxyl groups of residual NCO) was added. The reaction was continued for 1 h, and the above mixed solvent was added to adjust the solid content to 35 wt% to obtain a hydroxyl-terminated polyurethane solution.

[0049] (2) Surface modification of silver sheet: take silver sheet (10 μm in diameter and 300 nm in thickness) and disperse it in anhydrous ethanol. Add 3-mercaptopropyltrimethoxysilane at a mass ratio of silver sheet, 3-mercaptopropyltrimethoxysilane and anhydrous ethanol of 100:0.5:400 (the amount of 3-mercaptopropyltrimethoxysilane is 0.5% of the mass of silver sheet). Stir the reaction at 60°C for 2 h. After filtration and washing with ethanol, the reaction product is dried at 60°C for 4 h to obtain modified silver sheet.

[0050] (3) Preparation of silver paste: Take 16 parts of the above polyurethane solution containing 16 parts of solid polyurethane elastomer, add 1.6 parts of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, stir at 500 rpm for 10 min, then add 33.6 parts of the above surface-modified silver sheet, and grind and disperse three times using a three-roll mill with roller gaps of 50 μm, 30 μm, and 15 μm respectively; take 5.6 parts of silver nanowires (50 nm in diameter and 25 μm in length), pre-wet with 2 parts of ethyl acetate, ultrasonically disperse for 10 min, and add to the silver sheet / polyurethane mixture under stirring at 400 rpm, and continue stirring for 15 min; add 1.0 part of fumed silica (hydrophobic Aerosil R972), and disperse at high speed at 1000 rpm for 10 min; add 1.4 parts of HDI trimer (NCO / OH molar ratio with polyurethane elastomer is 1.05), and stir at 300 rpm for 5 min. min; adjust with the remaining ethyl acetate and butanone to make the total amount of ethyl acetate 28.56 parts and the total amount of butanone 12.24 parts; finally, degas under vacuum at -0.09 MPa for 15 min to obtain a stretchable conductive silver paste that can be coated.

[0051] The silver film is prepared using the above-mentioned stretchable conductive silver paste, and the preparation method includes the following steps: S1. The substrate is a TPU film (100 μm thick). The surface is wiped with methyl ethyl ketone (MEK) before coating. The coating parameters are: 100 μm blade gap, 20 mm / s coating speed, and 95 μm wet film thickness. The film is allowed to dry at room temperature for 15 min.

[0052] S2. Then, it is placed in a 60℃ oven for 60 min to cure and then naturally cooled to room temperature to obtain a stretchable conductive silver film with a dry film thickness of 30 μm.

[0053] Example 2 A stretchable conductive silver paste, based on 100 parts by weight, comprises the following components: 18 parts of polyurethane elastomer (solid), 35.68 parts of modified silver sheets, 4.46 parts of silver nanowires, 2.7 parts of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, 1.8 parts of HDI trimer, 24.752 parts of ethyl acetate, 10.608 parts of butanone, and 2.0 parts of fumed silica; wherein the mass ratio of silver sheets to silver nanowires is 8:1, the amount of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt is 15% of the mass of the polyurethane elastomer solid, and the fumed silica accounts for 2% of the total mass of the silver paste.

[0054] A method for preparing a stretchable conductive silver paste includes the following steps: (1) Synthesis of polyurethane elastomer solution: Under dry nitrogen protection, polytetrahydrofuran ether diol (average molecular weight 3000, based on 100 mmol -OH) and isophorone diisocyanate were fed at an NCO / PTMG-OH molar ratio of 1.2. A mixed solvent of ethyl acetate and butanone (mass ratio of ethyl acetate to butanone 7:3) was added, and the mixture was reacted at 75℃ for 1 h to obtain NCO-terminated prepolymer. The temperature was lowered to 55℃, and 10.5 mmol of 1,4-butanediol (based on 1.05 times the molar equivalent of hydroxyl groups of residual NCO) was added. The reaction was continued for 0.5 h, and the above mixed solvent was added to adjust the solid content to 40 wt% to obtain hydroxyl-terminated polyurethane solution.

[0055] (2) Surface modification of silver sheet: take silver sheet (diameter 15 μm, thickness 500 nm) and disperse it in anhydrous ethanol. Add 3-mercaptopropyltrimethoxysilane and feed it at a mass ratio of silver sheet, 3-mercaptopropyltrimethoxysilane and anhydrous ethanol of 100:1:400 (the amount of 3-mercaptopropyltrimethoxysilane is 1% of the mass of silver sheet). Stir the reaction at 70℃ for 1 h. After filtering and washing with ethanol, the reaction product is dried at 70℃ for 3 h to obtain modified silver sheet.

[0056] (3) Preparation of silver paste: Take 18 parts of the above polyurethane solution containing solid polyurethane elastomer, add 2.7 parts of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, stir at 600 rpm for 5 min, then add 35.68 parts of the above surface-modified silver sheet, and grind and disperse twice using a three-roll mill with roller gaps of 60 μm, 40 μm, and 20 μm respectively; take 4.46 parts of silver nanowires (diameter 60 nm, length 40 μm), pre-wet with 2 parts of ethyl acetate, ultrasonically disperse for 10 min, and add to the silver sheet / polyurethane mixture under stirring at 500 rpm, and continue stirring for 10 min; add 2.0 parts of fumed silica (hydrophobic Aerosil R972), and disperse at high speed at 1200 rpm for 5 min; add 1.8 parts of HDI trimer (NCO / OH molar ratio with polyurethane elastomer is 1.2), and stir at 400 rpm for 3 min. min; adjust with the remaining ethyl acetate and butanone to make the total amount of ethyl acetate 24.752 parts and the total amount of butanone 10.608 parts; finally, degas under vacuum at -0.09 MPa for 10 min to obtain a stretchable conductive silver paste that can be coated.

[0057] The silver film is prepared using the above-mentioned stretchable conductive silver paste, and the preparation method includes the following steps: S1. The substrate is a TPU film (100 μm thick). The surface is wiped with methyl ethyl ketone (MEK) before coating. The coating parameters are: blade gap 120 μm, coating speed 30 mm / s, wet film thickness 110 μm; surface dry at room temperature for 20 min.

[0058] S2. Then, it is placed in an 80℃ oven for 30 min to cure and then naturally cooled to room temperature to obtain a stretchable conductive silver film with a dry film thickness of 40 μm.

[0059] Example 3 A stretchable conductive silver paste, based on 100 parts by weight, comprises the following raw materials: 14 parts of polyurethane elastomer (solid), 31.750 parts of modified silver sheets, 6.35 parts of silver nanowires, 1.120 parts of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, 1.167 parts of HDI trimer, 31.580 parts of ethyl acetate, 13.533 parts of butanone, and 0.5 parts of fumed silica; wherein the mass ratio of silver sheets to silver nanowires is 5:1, the amount of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt is 8% of the mass of the polyurethane elastomer solid, and the fumed silica accounts for 0.5% of the total mass of the silver paste.

[0060] A method for preparing a stretchable conductive silver paste includes the following steps: (1) Synthesis of polyurethane elastomer solution: Under dry nitrogen protection, polytetrahydrofuran ether diol (average molecular weight 1000, based on 100 mmol -OH) and isophorone diisocyanate were fed at an NCO / PTMG-OH molar ratio of 1.03. A mixed solvent of ethyl acetate and butanone (mass ratio of ethyl acetate to butanone 7:3) was added, and the reaction was carried out at 65℃ for 3 h to obtain NCO-terminated prepolymer. The temperature was lowered to 45℃, and 1.58 mmol of 1,4-butanediol (based on 1.05 times the molar equivalent of hydroxyl groups of residual NCO) was added. The reaction was continued for 2 h, and the above mixed solvent was added to adjust the solid content to 30 wt% to obtain hydroxyl-terminated polyurethane solution.

[0061] (2) Surface modification of silver sheet: take silver sheet (8 μm in diameter and 200 nm in thickness) and disperse it in anhydrous ethanol. Add 3-mercaptopropyltrimethoxysilane and feed it in a mass ratio of silver sheet, 3-mercaptopropyltrimethoxysilane and anhydrous ethanol of 100:0.3:400 (the amount of 3-mercaptopropyltrimethoxysilane is 0.3% of the mass of silver sheet). Stir the reaction at 50°C for 3 h. After filtering and washing with ethanol, the reaction product is dried at 50°C for 5 h to obtain modified silver sheet.

[0062] (3) Preparation of silver paste: Take 14 parts of the above polyurethane solution containing solid polyurethane elastomer, add 1.120 parts of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, stir at 400 rpm for 15 min, then add 31.75 parts of the above surface-modified silver sheet, and grind and disperse it 4 times using a three-roll mill with roller gaps of 40 μm, 20 μm and 10 μm respectively; take 6.35 parts of silver nanowires (diameter 40 nm, length 16 μm), pre-wet with 2 parts of ethyl acetate, ultrasonically disperse for 10 min, and add to the silver sheet / polyurethane mixture under stirring at 300 rpm, and continue stirring for 20 min; add 0.5 parts of fumed silica (hydrophobic Aerosil R972), and disperse at high speed at 800 rpm for 15 min; add 1.167 parts of HDI trimer (NCO / OH molar ratio with polyurethane elastomer is 1.00), and stir at 200 rpm for 8 min. min; adjust with the remaining ethyl acetate and butanone to make the total amount of ethyl acetate 31.580 parts and the total amount of butanone 13.533 parts; finally, degas under vacuum at -0.09 MPa for 20 min to obtain a stretchable conductive silver paste that can be coated.

[0063] (4) Preparation of PDMS primer and coating of primer layer: Aminopropyl-terminated polydimethylsiloxane and isophorone diisocyanate trimer were added at an NCO / NH2 molar ratio of 1.1:1, with ethyl acetate as solvent and a solid content of about 30 wt%. Dibutyltin dilaurate was added as a catalyst at an amount of 0.02% of the solid mass. The reaction was carried out at 60℃ for 3 h to obtain a polyurethane solution containing siloxane segments. The solution was diluted with ethyl acetate to prepare a primer, which consisted of 8 parts polyurethane containing siloxane segments and 92 parts ethyl acetate per 100 parts by mass. The primer was coated on the surface of the PDMS substrate to a thickness of about 3 μm and dried at 60℃ for 10 min to obtain a polyurethane primer layer containing siloxane segments.

[0064] The silver film is prepared using the above-mentioned stretchable conductive silver paste, and the preparation method includes the following steps: S1. Apply silver paste to the above base layer with a blade gap of 100 μm, a blade speed of 10 mm / s, a wet film thickness of 95 μm, and allow it to dry at room temperature for 10 min.

[0065] S2. Then, it is placed in a 60℃ oven for 60 min to cure and then naturally cooled to room temperature to obtain a stretchable conductive silver film with a dry film thickness of about 25 μm.

[0066] Example 4 A stretchable conductive silver paste, silver film and its preparation method are based on Example 1, with the following differences: the amount of HDI trimer is 1.733 parts, the NCO / OH molar ratio of HDI trimer to polyurethane elastomer added in silver paste preparation step (3) is 1.30, the solvent is adjusted according to the mass ratio of ethyl acetate to butanone 7:3 to make up 100 parts by mass, and the remaining steps and parameters are the same as in Example 1.

[0067] Example 5 A stretchable conductive silver paste, silver film and its preparation method are based on Example 1, with the following differences: the amount of HDI trimer is 1.067 parts, the NCO / OH molar ratio of HDI trimer to polyurethane elastomer added in silver paste preparation step (3) is 0.80, the solvent is adjusted according to the mass ratio of ethyl acetate to butanone 7:3 to make up 100 parts by mass, and the remaining steps and parameters are the same as in Example 1.

[0068] Example 6 A stretchable conductive silver paste, silver film and its preparation method are based on Example 1, the difference being that: in the silver paste preparation step (3), the amount of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt added is 2.56 parts (accounting for 16% of the solid mass of the polyurethane elastomer), the solvent is adjusted according to the mass ratio of ethyl acetate to butanone 7:3 to make up 100 parts by mass, and the remaining steps and parameters are the same as in Example 1.

[0069] Example 7 A stretchable conductive silver paste, silver film and its preparation method are based on Example 1, the difference being that: in the silver paste preparation step (3), the amount of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt added is 1.12 parts, accounting for 7% of the solid mass of the polyurethane elastomer, and the solvent is adjusted accordingly according to the mass ratio of ethyl acetate to butanone of 7:3 to make up 100 parts by mass. The remaining steps and parameters are the same as in Example 1.

[0070] Example 8 A stretchable conductive silver paste, silver film and its preparation method are based on Example 1, the difference being that: the amount of fumed silica added in silver paste preparation step (3) is 2.5 parts, accounting for 2.5% of the mass of stretchable conductive silver paste, and the solvent is adjusted accordingly according to the mass ratio of ethyl acetate to butanone 7:3 to make up 100 parts by mass. The remaining steps and parameters are the same as in Example 1.

[0071] Example 9 A stretchable conductive silver paste, silver film and its preparation method are based on Example 1, the difference being that: the amount of fumed silica added in silver paste preparation step (3) is 0.1 parts, accounting for 0.1% of the mass of the stretchable conductive silver paste, and the solvent is adjusted according to the mass ratio of ethyl acetate to butanone 7:3 to make up 100 parts by mass. The remaining steps and parameters are the same as in Example 1.

[0072] Example 10 A stretchable conductive silver paste, silver film and its preparation method are based on Example 1, the difference being that: the silver sheet is not surface treated with 3-mercaptopropyltrimethoxysilane, and unmodified silver sheet is used directly, while the remaining steps and parameters are the same as in Example 1.

[0073] Example 11 A stretchable conductive silver paste, silver film and its preparation method are based on Example 1, the difference being that: in the silver paste preparation step (1), the reaction temperature of polytetrahydrofuran ether diol and isophorone diisocyanate is 80°C, and the other steps and parameters are the same as in Example 1.

[0074] Example 12 A stretchable conductive silver paste, silver film and its preparation method are based on Example 1, but the difference is that the reaction temperature of polytetrahydrofuran ether diol and isophorone diisocyanate in silver paste preparation step (1) is 60°C, and the other steps and parameters are the same as in Example 1.

[0075] Example 13 A stretchable conductive silver paste, silver film and its preparation method are disclosed. Based on Example 1, the difference is that the curing temperature in step S2 of silver film preparation is 90°C, while the remaining steps and parameters are the same as in Example 1.

[0076] Example 14 A stretchable conductive silver paste, silver film and its preparation method are disclosed. Based on Example 1, the difference is that the curing temperature in step S2 of silver film preparation is 50°C, while the remaining steps and parameters are the same as in Example 1.

[0077] Comparative Example 1 This comparative example is based on Example 1, except that: no silver nanowires are added, the amount of silver sheet used is 39.2 parts, and the remaining steps and parameters are the same as in Example 1.

[0078] Comparative Example 2 This comparative example is based on Example 1, except that the mass ratio of silver sheet to silver nanowire is 4:1, the amount of silver sheet is 31.36 parts, and the amount of silver nanowire is 7.84 parts. All other steps and parameters are the same as in Example 1.

[0079] Comparative Example 3 This comparative example is based on Example 1, except that the mass ratio of silver sheet to silver nanowire is 9:1, the amount of silver sheet is 35.28 parts, and the amount of silver nanowire is 3.92 parts. All other steps and parameters are the same as in Example 1.

[0080] Comparative Example 4 This comparative example is based on Example 1, except that: 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt is not added, the solvent is adjusted according to the mass ratio of ethyl acetate to butanone of 7:3 to make up 100 parts by mass, and the remaining steps and parameters are the same as in Example 1.

[0081] Comparative Example 5 This comparative example is based on Example 1, except that: no fumed silica is added, the solvent is adjusted according to the mass ratio of ethyl acetate to butanone of 7:3 to make up 100 parts by mass, and the remaining steps and parameters are the same as in Example 1.

[0082] The performance of the stretchable conductive silver films prepared in the above embodiments and comparative examples was tested, and the specific test methods and results are as follows: 1. Silver paste viscosity test After the silver paste was allowed to stand at 25±1℃ for 30 min to degas, the apparent viscosity was tested using a Brookfield rotational viscometer at 25±1℃ and 10 rpm. Each sample was tested in parallel three times, and the average value was taken.

[0083] 2. Initial Shear Resistance Test The initial sheet resistance of the stretchable conductive silver film was tested using a four-probe sheet resistance tester at room temperature (25℃). Five points were randomly selected for testing each sample, and the average value was taken.

[0084] 3. Tensile resistance change test Using a universal testing machine with a digital source meter, the silver film sample was clamped on a tensile fixture and stretched at a constant rate to 30% strain. The change in resistance during the stretching process was recorded in real time. The resistance change rate ΔR / R0 (ΔR=R-R0) at 30% stretching was calculated based on the initial resistance R0 before stretching.

[0085] 4. Cyclic tensile stability test A universal testing machine with a digital source meter was used to perform cyclic tensile tests on silver film samples with strains ranging from 0% to 30%. The cycle rate was 0.5 Hz, and the number of cycles was 500. The resistance values ​​of the silver film before and after the cycles were measured, and the rate of change of resistance after 500 cycles was calculated. 500 -R0) / R0×100%].

[0086] The test results are shown in Table 1. As can be seen from the comparison between Example 1 and Comparative Examples 1-3, both excessively high and excessively low silver sheet / silver nanowire mass ratios lead to a significant decrease in tensile and cyclic stability.

[0087] As can be seen from the comparison between Example 1 and Comparative Examples 4-5, ionic liquids and fumed silica have an irreplaceable synergistic effect. Ionic liquid plasticization can reduce the Tg of polyurethane, improve interfacial wetting, and enhance tensile recovery; the thixotropic network of fumed silica can prevent filler sedimentation and ensure film uniformity and electrical conductivity stability.

[0088] A comparison of Examples 1 and 4-5 shows that the NCO / OH molar ratio needs to be 1.00-1.20, preferably 1.03-1.10. Insufficient ratio results in incomplete cross-linking, while excessive ratio produces bubbles and brittleness.

[0089] Based on the comparison between Example 1 and Examples 6-7, it can be seen that in Example 6, the mechanical strength decreased due to micro-phase separation caused by excessive ionic liquid; in Example 7, the plasticizing effect was insufficient, and the improvement in low-temperature flexibility was limited.

[0090] Based on the comparison between Example 1 and Examples 8-9, it can be seen that Example 8 has excessive non-conductive filler that hinders conductive contact; Example 9 has a dosage of 0.1% and a viscosity that is too low (9200 mPa·s), causing silver flakes to settle, and its performance is significantly worse than that of Example 1.

[0091] A comparison of Example 1 and Example 10 shows that, due to the weak bonding between the untreated silver sheet and the substrate interface, it is easy to detach during stretching, resulting in severe damage to the conductive network.

[0092] According to the comparison between Example 1 and Examples 11-12, the reaction temperature of Example 11 was 80°C, and the mechanical properties of the elastomer decreased due to the side reaction caused by the high temperature; the reaction temperature of Example 12 was 60°C, the reaction was incomplete, the prepolymer conversion rate was low, and there were many defects in the crosslinking network.

[0093] A comparison of Examples 1 and 13-14 shows that Example 13, with a curing temperature of 90°C, caused high-temperature damage to the TPU substrate and embrittlement of the film layer; Example 14, with a curing temperature of 50°C, resulted in insufficient crosslinking reaction and poor stability of the conductive network. A curing temperature of 60-80°C ensures both low-temperature curing and good overall performance, making it suitable for both TPU and PDMS thermosensitive substrates.

[0094] The present invention has been further described above with reference to specific embodiments. However, it should be understood that the specific description herein should not be construed as limiting the nature and scope of the present invention. Various modifications made to the above embodiments by those skilled in the art after reading this specification are all within the scope of protection of the present invention.

Claims

1. A stretchable conductive silver paste, characterized in that, The raw materials include the following components: polyurethane elastomer, silver sheet, silver nanowire, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, aliphatic polyisocyanate trimer, ethyl acetate, butanone, and fumed silica. The mass ratio of the silver sheet to the silver nanowire is 5:1 to 8:

1.

2. The stretchable conductive silver paste according to claim 1, characterized in that, The polyurethane elastomer is a hydroxyl-terminated polyurethane elastomer, synthesized using polytetrahydrofuran ether diol as the soft segment, isophorone diisocyanate as the hard segment, and 1,4-butanediol as the chain extender.

3. The stretchable conductive silver paste according to claim 1, characterized in that, The silver sheet has a diameter of 8-15 μm and a thickness of 200-500 nm; the silver nanowire has a diameter of 40-60 nm, a length of 16-40 μm, and an aspect ratio greater than 400; the total mass of the silver sheet and the silver nanowire accounts for 38%-42% of the mass of the stretchable conductive silver paste.

4. The stretchable conductive silver paste according to claim 1, characterized in that, The amount of the 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt is 8% to 15% of the solid mass of the polyurethane elastomer.

5. The stretchable conductive silver paste according to claim 1, characterized in that, The molar ratio of isocyanate groups in the aliphatic polyisocyanate trimer to hydroxyl groups in the polyurethane elastomer is 1.00 to 1.

20.

6. The stretchable conductive silver paste according to claim 1, characterized in that, The silver sheet is surface-treated with 3-mercaptopropyltrimethoxysilane, wherein the amount of 3-mercaptopropyltrimethoxysilane used is 0.3% to 1% of the mass of the silver sheet.

7. A method for preparing the stretchable conductive silver paste according to any one of claims 1 to 6, characterized in that, Includes the following steps: Synthesizing polyurethane elastomer solution: Under dry nitrogen protection, polytetrahydrofuran ether diol and isophorone diisocyanate were reacted in a mixed solvent of ethyl acetate and butanone to obtain a prepolymer. Then, 1,4-butanediol chain extender was added to react and a hydroxyl-terminated polyurethane solution was obtained. Silver sheet surface modification: Silver sheets are dispersed in anhydrous ethanol, 3-mercaptopropyltrimethoxysilane is added to react, and the mixture is filtered, washed and dried to obtain modified silver sheets; Preparation of silver paste: Add 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt to the polyurethane elastomer solution and stir evenly. Add the modified silver sheet and grind and disperse. After dispersing the silver nanowires, add them to the system and stir. Add fumed silica and disperse. Add the aliphatic polyisocyanate trimer and stir. Degas under vacuum to obtain conductive silver paste.

8. A stretchable conductive silver film, characterized in that, The silver paste is prepared by coating and curing the stretchable conductive silver paste as described in any one of claims 1 to 6.

9. A method for preparing a stretchable conductive silver film, characterized in that, Using the stretchable conductive silver paste as described in any one of claims 1 to 6, the method includes the following steps: S1. Film formation by scraping: The stretchable conductive silver paste is scraped onto the substrate and allowed to dry at room temperature. S2. Curing: The surface-dried stretchable conductive silver paste is cured at 60-80℃ to obtain a stretchable conductive silver film.

10. The method for preparing a stretchable conductive silver film according to claim 9, characterized in that, The substrate includes a TPU substrate or a PDMS substrate.

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