Low-temperature curing nano-silver-graphene composite conductive ink for paper-based humidity sensor and preparation method of low-temperature curing nano-silver-graphene composite conductive ink

Abstract

The invention is applicable to the field of technical improvement of printed electronic materials, and provides low-temperature cured nano-silver-graphene composite conductive ink for a paper-based humidity sensor, and the low-temperature cured nano-silver-graphene composite conductive ink comprises 45-60% of nano-silver particles; 0.5 to 3% of hydroxylated graphene; 0.5-2% of a dispersant; 5-8% of a low-temperature cross-linking agent; 10-20% of a rheology modifier; the solvent is a mixed system of ethylene glycol and water, the mass ratio of ethylene glycol to water is 7: 3, and the solvent accounts for the balance of the conductive ink. The curing temperature of the conductive ink is less than or equal to 80 DEG C and is far lower than the thermal deformation temperature of a paper base material, the problems of carbonization and deformation of the base material caused by high temperature are thoroughly solved, and the conductive ink is compatible with various paper base materials.

Description

Technical Field

[0001] This invention belongs to the field of printed electronic materials technology improvement, and particularly relates to a low-temperature curing nano-silver-graphene composite conductive ink for paper-based humidity sensors and its preparation method. Background Technology

[0002] The manufacturing of paper-based electronic devices currently faces several key technical challenges: traditional conductive inks, such as silver paste and carbon paste, require high-temperature curing processes above 150°C, which can lead to deformation, carbonization, or even combustion of the paper substrate; some low-temperature conductive inks, although curing at lower temperatures (around 120°C), suffer from problems such as excessively high sheet resistance (>1Ω / sq) and poor adhesion; furthermore, existing conductive inks have poor printability on paper substrates, easily exhibiting phenomena such as ink bleeding and diffusion, making it difficult to form high-precision conductive patterns. Although recent research has attempted to use novel conductive materials such as silver nanowires and graphene, silver nanowires are prone to oxidation and are expensive, while the conductivity of pure graphene inks is insufficient to meet the requirements of sensor electrodes. These problems severely restrict the development and application of paper-based humidity sensors.

[0003] With the rapid development of smart packaging, disposable medical testing, and environmental monitoring, the demand for low-cost, biodegradable paper-based sensors is increasing. In the food packaging industry, real-time monitoring of humidity changes inside packaging is necessary to prevent food spoilage. In the medical field, disposable humidity test strips require good biocompatibility and biodegradability. In warehousing and logistics, a large number of low-cost environmental monitoring labels are needed. These applications place new demands on paper-based humidity sensors: First, the manufacturing process must be compatible with the paper substrate, and the curing temperature must not exceed 100°C; second, the conductive material needs to have good environmental stability, maintaining stable conductivity even under humid conditions; third, the entire sensor system should meet environmental protection requirements, preferably using biodegradable materials. However, existing technologies struggle to meet these requirements: high-temperature processes damage the paper substrate; conventional conductive materials experience significant performance degradation in humid environments; and the cold chain transportation of vaccines creates a huge demand for low-cost, disposable temperature and humidity monitoring labels, for which traditional technologies cannot provide suitable solutions. Summary of the Invention

[0004] The purpose of this invention is to provide a low-temperature curing nano-silver-graphene composite conductive ink for paper-based humidity sensors and its preparation method, in order to solve the above-mentioned technical problems.

[0005] This invention is achieved by providing a low-temperature curing nano-silver-graphene composite conductive ink for paper-based humidity sensors, wherein the conductive ink comprises the following components by weight percentage: Nano silver particles: 45-60%; Hydroxylated graphene: 0.5-3%; Dispersant: 0.5-2%; Low-temperature crosslinking agent: 5-8%; Rheology modifier: 10-20%; Solvent: The solvent is a mixture of ethylene glycol and water, with a mass ratio of ethylene glycol to water of 7:3, and the solvent accounts for the remainder of the conductive ink.

[0006] A further technical solution of the present invention is that, by weight percentage, the conductive ink comprises the following components: Nano silver particles: 50-55%; Hydroxylated graphene: 1-2%; Dispersant: 1-2%; Low-temperature crosslinking agent: 6-8%; Rheology modifier: 15-20%; Solvent: The solvent is a mixture of ethylene glycol and water, with a mass ratio of ethylene glycol to water of 7:3, and the solvent accounts for the remainder of the conductive ink.

[0007] A further technical solution of the present invention is that, by weight percentage, the conductive ink comprises the following components: Nano silver particles: 52%; Hydroxylated graphene: 1.5%; Dispersant: 1.5%; Low-temperature crosslinking agent: 7%; Rheology modifier: 18%; Solvent: The solvent is a mixture of ethylene glycol and water, with a mass ratio of ethylene glycol to water of 7:3, and the solvent accounts for the remainder of the conductive ink.

[0008] A further technical solution of the present invention is: the particle size of the nano-silver particles is 40-60nm, the carboxylated graphene has a 2-4 layer structure; the dispersant is PVP K30; the low-temperature crosslinking agent is modified polyurethane; and the rheology modifier is hydroxypropyl cellulose.

[0009] A further technical solution of the present invention is: the curing temperature of the conductive ink is ≤80℃, the sheet resistance on the paper substrate after curing is <0.1Ω / sq, and the resistance change rate after 5000 bending cycles at a bending radius of 3mm is <3%; at a temperature of 25℃, the viscosity of the ink is 50-100cP, and the surface tension is 30-35mN / m.

[0010] Another object of the present invention is to provide a method for preparing a low-temperature curing nano-silver-graphene composite conductive ink for paper-based humidity sensors, the preparation method comprising the following steps: S1. Weigh out the following ingredients by weight percentage: silver nanoparticles, hydroxylated graphene, dispersant, low-temperature crosslinking agent, rheology modifier, and a mixed solvent prepared at a mass ratio of ethylene glycol to water of 7:3. S2. Add the weighed silver nanoparticles to 1 / 2 volume of mixed solvent. After adding the dispersant, stir at 300-500 r / min for 10-15 minutes at 25-30℃. Then, disperse the mixture with ultrasonic power of 300-500W for 20-30 minutes to obtain a silver nanoparticle dispersion. S3. Add the weighed hydroxylated graphene to the remaining 1 / 2 volume of the mixed solvent, stir at 300-500 r / min for 5-10 minutes at 25-30℃, and then disperse with ultrasonic power of 300-500W for 30-40 minutes to obtain a graphene dispersion. S4. Slowly add the graphene dispersion to the nano silver dispersion, stirring at 600-800 r / min during the addition process. After all the graphene dispersion has been added, stir for 20-30 minutes. Then add the low-temperature crosslinking agent and rheology modifier in sequence, and adjust the stirring speed to 1000-1200 r / min for high-speed shear mixing for 30-45 minutes to obtain the initial mixed system. S5. Transfer the initial mixed system to a ball mill and grind it with zirconia balls at a speed of 200-300 r / min for 1-2 hours until the particles in the system are of uniform size and there is no obvious agglomeration. S6. Detect the viscosity and surface tension of the system after grinding. Adjust the viscosity to 50-100 cP and the surface tension to 30-35 mN / m by adding a small amount of mixed solvent at 25°C to obtain the low-temperature curing nano-silver-graphene composite conductive ink. S7. After printing the obtained composite conductive ink onto the paper-based substrate, cure it at 70°C for 15-20 minutes, and then cure it at 80°C for 25-30 minutes to complete the curing.

[0011] A further technical solution of the present invention is: the nano-silver particles are prepared by chemical reduction method, and the sheet resistance after solidification is maintained at <0.1Ω / sq when the silver content is ≤50wt%; The hydroxylated graphene is prepared by in-situ reduction via redox method, and its interfacial bonding strength with paper-based substrates reaches level 4B or above under the ASTM D3359 standard. The low-temperature crosslinking agent is a modified polyurethane, which can be fully crosslinked at 80°C to form a three-dimensional network structure that can tightly fix the silver nanoparticles and carboxylated graphene. After curing, the sheet resistance change rate of the conductive layer after aging at 85°C / 85% RH for 1000 hours is <5%. The composite conductive ink has a contact angle of 40-60° with the paper substrate, and there is no ink bleeding or diffusion after printing. When using inkjet printing, the minimum line width can reach 15-20μm, and the edge roughness is <0.3μm. When using a 250-350 mesh screen and a squeegee pressure of 0.2-0.3MPa for screen printing, the conductive pattern has a uniform thickness of 5-8μm. The composite conductive ink has a shear thinning index n=0.3-0.5, is compatible with roll-to-roll (R2R) continuous production processes, and can achieve a production speed of 5-10m / min.

[0012] A further technical solution of the present invention is: the frequency of ultrasonic dispersion in step S2 is 20-40kHz, and the frequency of ultrasonic dispersion in step S3 is 25-45kHz, to ensure that the nano-silver particles and carboxylated graphene are not agglomerated and are uniformly dispersed. In step S7, both pre-curing and final curing are carried out in a temperature and humidity controlled system. The relative humidity of the curing environment is controlled at 40-60% to avoid cracking of the conductive layer during the curing process.

[0013] Another object of the present invention is to provide a paper-based humidity sensor, wherein the electrodes of the paper-based humidity sensor are formed by printing and curing the composite conductive ink.

[0014] A further technical solution of the present invention is that the electrode linewidth is 15-20μm and the sheet resistance is less than 0.1Ω / sq; The paper-based humidity sensor exhibits a sheet resistance change rate of less than 5% after aging for 1000 hours at 85℃ / 85%RH.

[0015] The beneficial effects of this invention are as follows: This composite conductive ink has the following advantages over existing conductive inks: Low-temperature curing, compatible with paper substrates: curing temperature ≤80℃, far below the heat distortion temperature of paper substrates, completely solving the problems of substrate carbonization and deformation caused by high temperature, and compatible with various paper substrates; Low sheet resistance and high adhesion: The nano-silver-graphene composite conductive network significantly reduces contact resistance, with a sheet resistance of <0.1Ω / sq after curing; the three-dimensional network formed by the cross-linking of modified polyurethane is tightly bonded to the paper substrate fibers, with adhesion reaching ASTM D3359 standard level 4B or above, and no peeling after repeated rubbing. High-precision printing, suitable for miniaturization: The ink viscosity (50-100Cp), surface tension (30-35mN / m) and contact angle (40-60°) of the paper substrate are precisely matched. Inkjet printing can achieve a minimum linewidth of 15-20μm and an edge roughness of <0.3μm; screen printing can obtain conductive patterns with a uniform thickness of 5-8μm, meeting the electrode precision requirements of miniaturized paper-based sensors. Superior environmental stability: The barrier effect of graphene inhibits the oxidation and migration of silver particles, and the cross-linked polymer network provides encapsulation protection. After aging at 85℃ / 85% RH for 1000 hours, the sheet resistance change rate is <5%, and after 100 thermal cycles at -40~85℃, the resistance change rate is <2.5%. It maintains stable performance in humid environments over long periods. Low-cost scalability: Nano-silver is prepared using a chemical reduction method (60% lower cost than nano-silver wire), and graphene is reduced in situ using a redox method (80% lower cost than mechanical exfoliation). Low sheet resistance is maintained even with a silver content ≤50wt%. The ink shear thinning index n=0.3-0.5, compatible with roll-to-roll (R2R) continuous production, with a production speed of 5-10m / min, significantly reducing industrialization costs. High compatibility and customizable functionality: The surface energy of the conductive layer can be tuned to 40-45 mN / m, and the interfacial bonding strength with moisture-sensitive materials such as PEDOT:PSS reaches level 4B or higher, with a sensor response time of <3 seconds; the mechanical strength can be optimized by adjusting the graphene content (1-3 wt%), or antibacterial function can be imparted by adding ZnO nanoparticles, adapting to the needs of different application scenarios; Environmental compliance: The solvent system is an ethylene glycol / water mixture, which does not contain heavy metals (such as lead and cadmium) and fully complies with RoHS (2011 / 65 / EU) and REACH (EC No. 1907 / 2006) environmental standards, avoiding environmental pollution during the printing process and safety risks to the end product. Attached Figure Description

[0016] Figure 1 This is a flowchart of the preparation method of composite conductive ink provided in the embodiments of the present invention.

[0017] Figure 2 This is a technical roadmap provided by an embodiment of the present invention. Detailed Implementation

[0018] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0019] In the description of this invention, it should be understood that the terms "length," "width," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientation or positional relationships, are based on the orientation or positional relationships shown in the accompanying drawings and are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. Furthermore, in the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0020] Market demand and technological bottlenecks have driven us to develop a novel low-temperature curing conductive ink. This ink needs to simultaneously meet several key indicators: curing temperature below 100℃, sheet resistance less than 0.1Ω / sq, good printability and adhesion on paper substrates, and stable performance in humid environments. This invention successfully developed a conductive ink that meets these requirements by compositing graphene with nano-silver, providing a crucial material foundation for the industrial application of paper-based humidity sensors.

[0021] like Figure 1 The flowchart shown below illustrates the preparation method of the low-temperature curing nano-silver-graphene composite conductive ink for paper-based humidity sensors provided by this invention, and its details are as follows: Step S1: Prepare raw materials. Weigh the following by weight percentage: silver nanoparticles, hydroxylated graphene, PVPK30, modified polyurethane, hydroxypropyl cellulose, and a mixed solvent prepared at a 7:30 mass ratio of ethylene glycol to water. The silver nanoparticles are prepared by chemical substitution method. When the silver content is ≤50wt%, the sheet resistance after curing is maintained at <0.1Ω / sq. In this formulation, the silver nanoparticles are the main conductive phase with a particle size of 40-60nm. The hydroxylated graphene is prepared by in-situ reduction using a redox method. The interfacial bonding strength with the paper substrate reaches level 4B or higher under the ASTM D3359 standard. The hydroxylated graphene is used to improve the stability of the conductive network and inhibit silver migration. PVP K30 is used as a dispersant to prevent particle agglomeration. The modified polyurethane is used for low-temperature crosslinking and cured at 80℃. Hydroxypropyl cellulose is used to adjust the rheology and improve printability.

[0022] Step S2: Prepare a nano-silver dispersion. Add the nano-silver particles weighed according to the weight percentage to half the volume of the mixed solvent, then add PVP K30 and stir at 300-500 r / min for 10-15 minutes at 25-30℃. Then, use ultrasound with a power of 300-500W to disperse the mixture for 20-30 minutes to obtain the nano-silver dispersion. In this process, PVP K30 is used as a dispersant to prevent the silver particles from agglomerating. The frequency of ultrasonic dispersion is 20-40kHz to ensure that the nano-silver particles are not agglomerated and are uniformly dispersed.

[0023] Step S3: Prepare graphene dispersant. Add the graphene weighed according to the weight percentage to the remaining 1 / 2 volume of the mixed solvent. Stir at 200-300 r / min for 5-10 minutes at 25-30℃. Then, disperse the mixture with ultrasound at a power of 300-500W for 30-40 minutes to obtain a graphene dispersion. The frequency of the ultrasound dispersion is 25-45kHz to ensure that the carboxylated graphene does not agglomerate and is evenly dispersed.

[0024] Step S4: Prepare the mixing system. Slowly add the graphene dispersion prepared in step S3 to the nano-silver dispersion obtained in step S2. During the addition, stir the liquid at 600-800 r / min. After the graphene dispersion is added, continue stirring for 20-30 minutes. Then, add the modified polyurethane and hydroxypropyl cellulose sequentially. After adding, adjust the stirring speed to 1000-1200 r / min and mix using high-speed shear for 30-45 minutes to obtain the initial mixed system. The modified polyurethane can be completely cross-linked at 80℃ to form a three-dimensional network structure that can tightly fix the nano-silver particles and carboxylated graphene. After curing, the sheet resistance change rate of the conductive layer after aging at 85℃ / 85% RH for 1000h is <5%.

[0025] Step S5, refine the grinding: transfer the initial mixed system obtained in step S4 to a ball mill, use zirconia balls with a ball-to-material ratio of 3:1, and grind at a speed of 200-300 r / min for 1-2 hours until the particle size in the system is uniform and there is no obvious agglomeration.

[0026] Step S6: Adjust performance. Detect the viscosity and surface tension of the system after grinding. By adding a small amount of mixed solvent at 25°C, adjust the viscosity to 50-100 cP and the surface tension to 30-35 mN / m to obtain the low-temperature curing nano-silver-graphene composite conductive ink. The contact angle between the composite conductive ink and the paper substrate is 40-60°. There is no ink bleeding or diffusion after printing. When using inkjet printing, the minimum linewidth can reach 15-20 μm, and the edge roughness is <0.3 μm. When using a 250-350 mesh screen and a squeegee pressure of 0.2-0.3 MPa for screen printing, the conductive pattern thickness is uniform and 5-8 μm. The shear thinning index of the composite conductive ink is n=0.3-0.5, compatible with roll-to-roll (R2R) continuous production processes, and the production speed can reach 5-10 m / min.

[0027] Step S7, curing treatment: After printing the obtained composite conductive ink onto the paper-based substrate, it is first cured at 70℃ for 15-20 minutes, and then finally cured at 80℃ for 25-30 minutes to complete the curing process. Both pre-curing and final curing are carried out in a temperature and humidity controlled system, and the relative humidity of the curing environment is controlled at 40-60% to avoid cracking of the conductive layer during the curing process.

[0028] The apparatus for preparing composite conductive ink mainly consists of horizontal and vertical conveyor tracks on a control console, a fixed mounting platform, a mobile mounting platform, a 3D scanner, a circular mold positioning and corrector, a mobile rust-preventive sprayer, a trigger-type hydraulic device, an automatic lifting door, and a temperature and humidity control system. Material formulation: This invention uses a composite system of nano-silver particles (40-60nm) and few-layer graphene (2-4 layers) as conductive filler. Conductivity is improved by precisely controlling the mass ratio of the two (30:1 to 50:1). Nano-silver provides the main conductive pathways, while graphene not only acts as a bridge to fill conductive gaps but also effectively inhibits the migration and oxidation of nano-silver. The ink formulation also includes a specially designed dispersant (PVP K30) and a low-temperature crosslinking agent (modified polyurethane), enabling the ink to achieve complete curing below 80℃ while maintaining good dispersion stability.

[0029] Table 1. Ink Formulation Composition and Functions Low-temperature curing mechanism: Traditional conductive inks rely on high-temperature sintering to fuse silver particles, while this invention achieves low-temperature conductivity through the following mechanism: First, the PVP modified on the surface of nano-silver partially decomposes during heating, exposing fresh silver surfaces; second, the modified polyurethane undergoes a cross-linking reaction at 80°C, forming a three-dimensional network structure that tightly fixes the conductive particles; finally, the graphene sheets form close contact with the silver particles under pressure, significantly reducing contact resistance. This curing mechanism allows the ink to achieve excellent conductivity at temperatures far below the heat distortion temperature of the paper substrate (approximately 150°C).

[0030] Printability: Considering the porous and hydrophilic nature of paper substrates, the ink of this invention achieves excellent printability through the following design: viscosity controlled at 50-100 Cp (25℃), surface tension adjusted to 30-35 mN / m, and contact angle optimized with various types of paper within the range of 40-60°. These parameters ensure that the ink can moderately penetrate the paper substrate to enhance adhesion without excessive diffusion affecting pattern accuracy. By adding a specific ratio of ethylene glycol / water mixed solvent (7:3), a moderate drying speed is ensured while avoiding cracking problems caused by rapid drying.

[0031] High-precision pattern optimization capability: Employing inkjet printing and precision screen printing, the ink exhibits excellent jetting stability in inkjet printing, with droplet volume controllable to 3-5 pL and a minimum linewidth of 15-20 μm. In screen printing, by optimizing screen parameters (250-350 mesh) and squeegee pressure (0.2-0.3 MPa), conductive patterns with sharp edges and uniform thickness (5-8 μm) can be obtained. This high-precision characteristic is crucial for the fabrication of miniaturized humidity sensors.

[0032] Excellent mechanical flexibility: Thanks to the reinforcing effect of graphene and the introduction of elastic polymers, the cured conductive film exhibits excellent mechanical flexibility. Tests show that after 5000 bending cycles at a bending radius of 3mm, the resistance change rate is less than 3%. This characteristic makes this product particularly suitable for applications requiring frequent bending, such as foldable packaging and wearable devices. SEM observation of the cross-section of the bent sample reveals that the conductive network remains intact, with no obvious cracks.

[0033] Environmental Resistance: The conductive layer formed after ink curing exhibits excellent environmental stability. After aging for 1000 hours under high temperature and high humidity conditions of 85℃ / 85%RH, the sheet resistance change rate is less than 5%. This is mainly attributed to: the barrier effect of graphene effectively slowing down the oxidation of silver; the cross-linked polymer network providing good encapsulation protection; and the strong interaction between carboxyl-based graphene and the substrate. Furthermore, the ink formulation does not contain harmful heavy metals and fully complies with RoHS and REACH environmental requirements.

[0034] Table 2. Environmental stability test data Excellent compatibility with humidity-sensitive materials: Through surface energy modulation (40-45 mN / m) and functional group design (-COOH), a strong interaction is formed between the conductive layer and the humidity-sensitive layer, achieving an interfacial bonding strength of 4B or higher (ASTM D3359). This excellent compatibility ensures that the sensor will not delaminate during long-term operation in humid environments.

[0035] The manufacturing process is simple: In actual production, the ink is fully compatible with roll-to-roll (R2R) continuous production processes. With appropriate drying equipment (such as infrared or hot air), a production speed of 5-10 m / min can be achieved. The rheological properties of the ink (shear thinning index n=0.3-0.5) make it suitable for high-speed printing while maintaining good pattern resolution. This characteristic makes large-scale industrial production possible.

[0036] Wide range of applications: Performance can be customized by adjusting the formula to meet different application needs. For example, increasing the graphene content (up to 3%) can improve mechanical strength but slightly reduce conductivity; adjusting the resin ratio can change the flexibility of the cured film; adding a small amount of other nanomaterials (such as ZnO) can endow the ink with special functions such as antibacterial properties. This adjustability greatly expands the application range of the product.

[0037] Low material cost: Nano-silver is prepared using a chemical reduction method, and the silver content is reduced to below 50% through process optimization while maintaining good conductivity. Graphene is prepared using a lower-cost redox method, and the cost is further reduced through in-situ reduction technology, creating conditions for the large-scale application of paper-based electronic devices.

[0038] Low-temperature curing nanocomposite system: It uses nano-silver-graphene composite conductive filler and special low-temperature crosslinking agent to cure below 80℃.

[0039] Environmentally friendly solvent formulation: An ethylene glycol / water (7:3) mixed solvent system was developed, which is completely free of heavy metals, complies with RoHS standards, and achieves the best penetration-diffusion balance for paper substrates.

[0040] High-precision printing compatibility: Optimized viscosity (50-100 cP) and surface tension (30-35 mN / m), supports inkjet printing with a linewidth of 15-20 μm and an edge roughness of <0.3 μm.

[0041] A phased curing process: a 70℃ pre-curing + 80℃ final curing strategy, solves the delamination problem at the interface of multi-layer structures, and achieves a production speed of 5-10m / min (R2R compatible).

[0042] Breakthrough in cost control: The chemical reduction of silver + graphene oxide process reduces material costs, and the silver content is less than 50% while still maintaining excellent conductivity.

[0043] Intelligent packaging adaptation design: Develop an energy level matching interface with moisture-sensitive materials such as PEDOT:PSS, with a response time of <3 seconds to meet the real-time monitoring needs of cold chain logistics.

[0044] This invention provides a low-temperature curing nano-silver-graphene composite conductive ink for paper-based humidity sensors, wherein the conductive ink comprises the following components by weight percentage: Nano silver particles: 45-60%; Hydroxylated graphene: 0.5-3%; Dispersant: 0.5-2%; Low-temperature crosslinking agent: 5-8%; Rheology modifier: 10-20%; Solvent: The solvent is a mixture of ethylene glycol and water, with a mass ratio of ethylene glycol to water of 7:3, and the solvent accounts for the remainder of the conductive ink.

[0045] Example 1: Preparation and Properties of Basic Formulation Conductive Ink 1.1 Raw material ratio (total mass 100g) Nano-silver particles (particle size 40-50nm, prepared by chemical reduction method): 50g (50wt%), carboxylated graphene (2-4 layers, in-situ reduction by redox method): 1g (1wt%), PVP K30: 1g (1wt%), modified polyurethane (80℃ cross-linked type): 6g (6wt%), hydroxypropyl cellulose: 15g (15wt%), mixed solvent (ethylene glycol: water = 7:3): 27g (balance).

[0046] 1.2 Obtained according to the above-described method for preparing composite conductive ink 1.3 Performance Test Results Example 2: Basic Formula 2.1 Raw material ratio (total mass 100g) Nano silver particles: 52g, carboxylated graphene: 1.5g (1wt%), PVP K30: 1.5g, modified polyurethane: 7g, hydroxypropyl cellulose: 18g, mixed solvent (ethylene glycol: water = 7:3): 20g (balance).

[0047] 2.2 Performance Test Results Sheet resistance after curing: 0.08Ω / sq; Bending performance (3mm radius, 5000 cycles): Resistance change rate +1.8%; Aging at 85℃ / 85% RH (1000h): Shear resistance change rate +4.1%.

[0048] Example 3: High graphene content formulation (enhanced mechanical properties) 3.1 Raw material ratio (total mass 100g) Nano silver particles: 48g, carboxylated graphene: 2.5g, PVP K30: 1.5g, modified polyurethane: 7g, hydroxypropyl cellulose: 18g, mixed solvent: 23g (balance).

[0049] 3.2 Performance Test Results Sheet resistance after curing: 0.08Ω / sq; Bending performance (3mm radius, 5000 cycles): Resistance change rate +1.2%; Aging at 85℃ / 85% RH (1000h): Shear resistance change rate +2.8%.

[0050] Example 4: ZnO-containing antibacterial formula (suitable for medical scenarios) 4.1 Raw material ratio (total mass 100g) Nano-silver particles: 55g, carboxylated graphene: 1.2g, PVP K30: 0.8g, modified polyurethane: 5.5g, hydroxypropyl cellulose: 12g, ZnO nanoparticles (15nm): 0.3g, mixed solvent: 25.2g (balance).

[0051] 4.2 Performance Test Results Sheet resistance after curing: 0.05Ω / sq; Antibacterial properties (GB / T 21866-2008): Inhibition rate against Escherichia coli and Staphylococcus aureus >99%; Adhesion: 4B grade (ZnO addition does not affect the interface bonding strength).

[0052] Another object of the present invention is to provide a paper-based humidity sensor, wherein the electrodes of the paper-based humidity sensor are formed by printing and curing the composite conductive ink.

[0053] The electrode linewidth is 15-20 μm, and the sheet resistance is less than 0.1 Ω / sq; The paper-based humidity sensor exhibits a sheet resistance change rate of less than 5% after aging for 1000 hours at 85℃ / 85%RH.

[0054] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A low-temperature curing nano-silver-graphene composite conductive ink for paper-based humidity sensors, characterized in that, The conductive ink comprises the following components by weight percentage: Nano silver particles: 45-60%; Hydroxylated graphene: 0.5-3%; Dispersant: 0.5-2%; Low-temperature crosslinking agent: 5-8%; Rheology modifier: 10-20%; Solvent: The solvent is a mixture of ethylene glycol and water, with a mass ratio of ethylene glycol to water of 7:3, and the solvent accounts for the remainder of the conductive ink.

2. The low-temperature curing nano-silver-graphene composite conductive ink according to claim 2, characterized in that, The conductive ink comprises the following components by weight percentage: Nano silver particles: 50-55%; Hydroxylated graphene: 1-2%; Dispersant: 1-2%; Low-temperature crosslinking agent: 6-8%; Rheology modifier: 15-20%; Solvent: The solvent is a mixture of ethylene glycol and water, with a mass ratio of ethylene glycol to water of 7:3, and the solvent accounts for the remainder of the conductive ink.

3. The low-temperature curing nano-silver-graphene composite conductive ink according to claim 1 or 2, characterized in that, The conductive ink comprises the following components by weight percentage: Nano silver particles: 52%; Hydroxylated graphene: 1.5%; Dispersant: 1.5%; Low-temperature crosslinking agent: 7%; Rheology modifier: 18%; Solvent: The solvent is a mixture of ethylene glycol and water, with a mass ratio of ethylene glycol to water of 7:3, and the solvent accounts for the remainder of the conductive ink.

4. The low-temperature curing nano-silver-graphene composite conductive ink according to claim 3, characterized in that, The nano-silver particles have a particle size of 40-60 nm, and the carboxylated graphene has a 2-4 layer structure; the dispersant is PVP K30; the low-temperature crosslinking agent is modified polyurethane; and the rheology modifier is hydroxypropyl cellulose.

5. The low-temperature curing nano-silver-graphene composite conductive ink according to claim 1, characterized in that, The conductive ink has a curing temperature of ≤80℃, a sheet resistance of <0.1Ω / sq on a paper substrate after curing, and a resistance change rate of <3% after 5000 bending cycles at a bending radius of 3mm; at a temperature of 25℃, the ink has a viscosity of 50-100cP and a surface tension of 30-35mN / m.

6. A method for preparing a low-temperature curing nano-silver-graphene composite conductive ink according to any one of claims 1-5, characterized in that, The preparation method includes the following steps: S1. Weigh out the following components according to the weight percentages described in any one of claims 1-5: silver nanoparticles, hydroxylated graphene, dispersant, low-temperature crosslinking agent, rheology modifier, and a mixed solvent prepared at a mass ratio of ethylene glycol to water of 7:

3. S2. Add the weighed silver nanoparticles to 1 / 2 volume of mixed solvent. After adding the dispersant, stir at 300-500 r / min for 10-15 minutes at 25-30℃. Then, disperse the mixture with ultrasonic power of 300-500W for 20-30 minutes to obtain a silver nanoparticle dispersion. S3. Add the weighed hydroxylated graphene to the remaining 1 / 2 volume of the mixed solvent, stir at 300-500 r / min for 5-10 minutes at 25-30℃, and then disperse with ultrasonic power of 300-500W for 30-40 minutes to obtain a graphene dispersion. S4. Slowly add the graphene dispersion to the nano silver dispersion, stirring at 600-800 r / min during the addition process. After all the graphene dispersion has been added, stir for 20-30 minutes. Then add the low-temperature crosslinking agent and rheology modifier in sequence, and adjust the stirring speed to 1000-1200 r / min for high-speed shear mixing for 30-45 minutes to obtain the initial mixed system. S5. Transfer the initial mixed system to a ball mill and grind it with zirconia balls at a speed of 200-300 r / min for 1-2 hours until the particles in the system are of uniform size and there is no obvious agglomeration. S6. Detect the viscosity and surface tension of the system after grinding. Adjust the viscosity to 50-100 cP and the surface tension to 30-35 mN / m by adding a small amount of mixed solvent at 25°C to obtain the low-temperature curing nano-silver-graphene composite conductive ink. S7. After printing the obtained composite conductive ink onto the paper-based substrate, cure it at 70°C for 15-20 minutes, and then cure it at 80°C for 25-30 minutes to complete the curing.

7. The preparation method according to claim 6, characterized in that, The nano-silver particles are prepared by chemical reduction, and the sheet resistance after solidification remains <0.1Ω / sq when the silver content is ≤50wt%. The hydroxylated graphene is prepared by in-situ reduction via redox method, and its interfacial bonding strength with paper-based substrates reaches level 4B or above under the ASTM D3359 standard. The low-temperature crosslinking agent is a modified polyurethane, which can be fully crosslinked at 80°C to form a three-dimensional network structure that can tightly fix the silver nanoparticles and carboxylated graphene. After curing, the sheet resistance change rate of the conductive layer after aging at 85°C / 85% RH for 1000 hours is <5%. The composite conductive ink has a contact angle of 40-60° with the paper substrate, and there is no ink bleeding or diffusion after printing. When using inkjet printing, the minimum line width can reach 15-20μm, and the edge roughness is < 0.3μm. When using a 250-350 mesh screen and a squeegee pressure of 0.2-0.3MPa for screen printing, the conductive pattern has a uniform thickness of 5-8μm. The composite conductive ink has a shear thinning index n=0.3-0.5, is compatible with roll-to-roll (R2R) continuous production processes, and can achieve a production speed of 5-10m / min.

8. As described in claim 7, characterized in that, In step S2, the ultrasonic dispersion frequency is 20-40kHz, and in step S3, the ultrasonic dispersion frequency is 25-45kHz, to ensure that the nano-silver particles and carboxylated graphene are not agglomerated and are uniformly dispersed. In step S7, both pre-curing and final curing are carried out in a temperature and humidity controlled system. The relative humidity of the curing environment is controlled at 40-60% to avoid cracking of the conductive layer during the curing process.

9. A paper-based humidity sensor, characterized in that, The electrodes of the paper-based humidity sensor are formed by printing and curing the composite conductive ink as described in any one of claims 1-5.

10. The paper-based humidity sensor according to claim 9, characterized in that, The electrode linewidth is 15-20 μm, and the sheet resistance is less than 0.1 Ω / sq; The paper-based humidity sensor exhibits a sheet resistance change rate of less than 5% after aging for 1000 hours at 85℃ / 85%RH.

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