Flexible strain sensor material with high monitoring signal stability and preparation method and application thereof

Abstract

The invention discloses a flexible strain sensor material with high stability of monitoring signals and a preparation method and application thereof.The preparation method comprises the steps that S1, polyvinylpyrrolidone, conductive filler and an organic solvent are taken and mixed fully and evenly, then the organic solvent is removed through heating volatilization, and conductive nano filler is obtained; s2, mixing the conductive nano reinforcing filler with methyl vinyl silicone rubber, nano silicon dioxide, hydroxyl silicone oil, vinyl silicone oil and a vulcanizing agent, and mixing to obtain a mixture; and S3, vulcanization: the mixture is subjected to vulcanization in two stages, the vulcanization temperature in the second stage is 20-30 DEG C higher than the vulcanization temperature in the first stage, and the flexible strain sensor material is obtained. The flexible strain sensor material which is high in sensitivity, good in mechanical property and stable in monitoring signal is prepared in an efficient, pollution-free and low-cost mode, and the flexible strain sensor material is suitable for engineering structure health monitoring under large and small deformation and wide in application prospect.

Description

Technical Field

[0001] The invention relates to the field of rubber materials and strain sensing technology, and in particular to a flexible strain sensor material with high stability of monitoring signals, a preparation method and application thereof. Background Art

[0002] During the service process, engineering structures will inevitably be subjected to the coupling effects of loads, environmental corrosion, material aging, fatigue damage and other factors, resulting in damage accumulation, which will lead to structural resistance attenuation and easily cause sudden accidents. With the continuous advancement of science and technology and the increasing safety requirements of society, engineering structural health monitoring has become an important research direction in modern engineering technology as an important means to ensure the safety of various structures such as buildings, bridges, aircraft, and ships.

[0003] Earthquake is a natural disaster that seriously threatens the safety of human life and property. Therefore, countries around the world are committed to doing a good job in engineering earthquake resistance and disaster reduction, and are committed to improving the earthquake resistance level of construction projects and improving the earthquake resistance of construction projects. The use of seismic isolation technology and seismic isolation bearings can isolate or consume the energy of seismic waves, which is currently the most effective earthquake resistance measure. Seismic isolation bearings are a device used for seismic isolation of building structures. Their main function is to reduce the impact of earthquakes on buildings by adding a seismic isolation layer between the foundation and the superstructure of the building. Seismic isolation bearings are often in a high-load working environment of "continuous small earthquakes and occasional strong earthquakes". The bearings are severely damaged and have a short service life. In addition, seismic isolation bearings are mainly made of rubber materials. The aging and failure of rubber will also accelerate the fatigue and damage of the bearings. It can be said that the service performance of seismic isolation bearings in high-intensity earthquake zones directly determines the health of the engineering structure. Therefore, regular inspection of seismic isolation bearings is very important to ensure their normal performance. One of the core issues of structural health monitoring technology of seismic isolation bearings is intelligent sensing technology. Currently, widely used structural health monitoring sensors are mainly made of inorganic non-metallic materials such as fiber gratings and piezoelectric ceramics. The structural and functional characteristics of the rubber isolation bearing itself determine that the sensing material must have the characteristics of large deformation, anti-destruction, long life and high sensitivity. Ordinary inorganic non-metallic smart materials are no longer applicable. Therefore, the research and development of new smart sensing materials is urgent.

[0004] There are many methods for monitoring the health of engineering structures, such as sensor monitoring, acoustic emission monitoring, non-destructive testing, image processing monitoring, data mining and machine learning, ground monitoring, ground photogrammetry, etc. Among the many technical means for structural health monitoring, flexible strain sensors have become one of the cores of research due to their high sensitivity, real-time performance, and ability to effectively reflect the stress state of the structure. Especially in the field of human monitoring and large-scale component monitoring, the research on the application of flexible strain sensors not only provides important support for the maintenance and management of infrastructure, but also provides new ideas for individual health monitoring and industrial safety.

[0005] Common strain sensors include electro-positive strain gauges, fiber Bragg grating strain sensors, piezoelectric strain sensors, etc. Traditional strain sensors are susceptible to environmental interference, have low sensitivity, poor stability after long-term use, and poor anti-interference ability, making it difficult to implement strain monitoring in most structures. In recent years, with the development of materials science and nanotechnology, new strain sensors (such as flexible sensors based on carbon nanotubes and conductive polymers) have better sensitivity, response speed and anti-interference ability, and have gradually been used in the field of high-precision monitoring. Flexible strain sensors output response signals by monitoring the changes in electrical properties caused by the deformation of the object. Generally, flexible strain sensors work based on the principle of resistance change. When strain acts on the flexible strain sensor, the geometric shape of the material changes. For example, stretching causes the length of the sensor to increase and the cross-sectional area to decrease, thereby increasing the resistance; compression is the opposite, and the resistance decreases. The resistance change of the sensor has a certain relationship with the applied strain, and the resistance change is converted into an electrical signal for output. The electrical signal can be processed by amplification, filtering, etc. to provide accurate deformation information of the structure.

[0006] However, flexible strain sensors usually have a shoulder effect in the output resistance / response signal. The shoulder effect refers to the irregular distortion or sudden inflection point of the resistance change curve of the flexible strain sensor when the flexible strain sensor is deformed. It is usually manifested as a slow or stagnant resistance change within a certain strain range, resulting in an unstable sensor output response signal, which affects the judgment of the degree of structural damage. This phenomenon is mainly due to the instability of the contact network between the conductive fillers in the flexible strain sensor. As the strain increases, the conductive network may be reconstructed or disconnected, resulting in discontinuous or nonlinear signal response. The shoulder effect limits the accuracy and stability of the flexible strain sensor within the monitoring range, hindering its application in high-precision strain measurement. In addition, the shoulder effect may also cause the sensor to age during long-term use, further affecting its reliability and repeatability. Therefore, solving the shoulder effect is the key to improving the monitoring stability performance of flexible strain sensors. It is urgent to develop new materials and optimize the design to improve the response characteristics of flexible strain sensors. Summary of the invention

[0007] The purpose of the present invention is to provide a flexible strain sensor material with high stability of monitoring signals and a preparation method and application thereof in order to solve the above problems.

[0008] In order to achieve its purpose, the present invention adopts the following technical solution:

[0009] A first aspect of the present invention provides a method for preparing a flexible strain sensor material with high stability of monitoring signals, comprising the following steps:

[0010] S1. Preparation of Conductive Nano-Reinforced Fillers

[0011] Polyvinyl pyrrolidone, a conductive filler and an organic solvent are mixed, and after being fully mixed, the organic solvent is removed by volatilization by heating to obtain a conductive nanofiller;

[0012] Wherein, the mass ratio of polyvinyl pyrrolidone to conductive filler is 0-1.0:0.1-0.4;

[0013] S2. Preparation of conductive nano rubber composites

[0014] The conductive nano-reinforced filler prepared in step S1 is mixed with methyl vinyl silicone rubber, nano-silicon dioxide, hydroxy silicone oil, vinyl silicone oil, and a vulcanizing agent in a mass ratio of 18-26:500-700:100-140:30-42:0-80:10-14, and kneaded to obtain a mixture;

[0015] S3, vulcanization:

[0016] The mixed material prepared in step S2 is vulcanized in two stages, wherein the vulcanization temperature in the second stage is 20 to 30° C. higher than that in the first stage, to obtain the flexible strain sensor material.

[0017] Preferably, the organic solvent is ethyl acetate or tetrahydrofuran or acetone;

[0018] The conductive filler is selected from multi-walled carbon nanotubes, conductive carbon black or graphene;

[0019] The vinyl silicone oil is a vinyl-terminated silicone oil, and the vinyl-terminated silicone oil is selected from one of vinyl-terminated polydimethylsiloxane (Vi-PDMS) and vinyl-terminated polymethylvinylsiloxane (Vi-PMVS), or a mixture of the two in any proportion;

[0020] The vulcanizing agent is selected from one or more of dicumyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, tert-butyl perbenzoate, and dibenzoyl peroxide.

[0021] In step S1, the mass volume ratio of polyvinyl pyrrolidone, conductive filler and organic solvent is 0.2-1.0 g: 0.1-0.4 g: 100-1200 ml or 0.2-0.8 g: 0.18-0.3 g: 200-1000 ml;

[0022] Polyvinyl pyrrolidone, conductive filler and organic solvent are mixed, ultrasonically stirred and mixed evenly, and the organic solvent is removed by volatilization by heating to obtain conductive nanofiller; the ultrasonic stirring conditions are ultrasonic frequency of 100-150 Hz, stirring rate of 450-600 rpm, and ultrasonic stirring and mixing for 2-4 hours;

[0023] Preferably, magnetic stirring is performed before ultrasonic stirring, and the magnetic stirring condition is stirring at a stirring rate of 450 to 600 rpm for 30 to 60 min;

[0024] The organic solvent is tetrahydrofuran, and the temperature is raised to 50-70° C. to remove the tetrahydrofuran and obtain the conductive nano filler.

[0025] In step S2, the mass ratio of the conductive nano-reinforced filler to the methyl vinyl silicone rubber, nano-silicon dioxide, hydroxy silicone oil, vinyl silicone oil, and vulcanizing agent is 18-26: 500-700: 100-140: 30-42: 40-80: 10-14 or 20-24: 550-650: 110-130: 33-39: 60-80: 11-13;

[0026] The mixing temperature is 40-60°C, the roller distance is 0.5-1mm, the speed is 20-35rpm, and the mixing process is 40-60min.

[0027] In step S3, the first stage of vulcanization has a vulcanization temperature of 150-170°C, a time of 10-20 min, and a pressure of 10-20 MPa, preferably a vulcanization temperature of 160-170°C, a time of 10-15 min, and a pressure of 10-15 MPa;

[0028] Then the second stage vulcanization treatment is carried out, the vulcanization temperature is 180-200°C, and the time is 2-4 hours.

[0029] Preferably, in step S3, the first stage vulcanization is carried out on a flat plate vulcanizer, and the second stage vulcanization is carried out in an oven.

[0030] In the above-mentioned preparation method,

[0031] The vinyl content of the methyl vinyl silicone rubber is 0.16%, and the molecular weight is 7.2×10 5 g / mol;

[0032] The multi-walled carbon nanotubes are 20-30 μm long, 6-8 nm in diameter, and have a specific surface area of ​​500-500 m 2 / g;

[0033] The nano-silicon dioxide has a particle size of 7-40 nm and a specific surface area of ​​200 m 2 / g;

[0034] The vinyl silicone oil has a viscosity of 8000±1000 Pa·s and a vinyl content of 0.1 to 1.2 wt%;

[0035] The hydroxyl content of the hydroxy silicone oil is 7-9% and the viscosity is 25-30 Pa·s.

[0036] A second aspect of the present invention provides a flexible strain sensor material, which is prepared by any of the preparation methods described above.

[0037] A third aspect of the present invention provides use of the above-mentioned flexible strain sensor material in preparing a flexible strain sensor.

[0038] In the application technology solution, the flexible strain sensor material is prepared into a flexible sensor for engineering structure health monitoring or human motion monitoring.

[0039] The beneficial effects of the present invention are:

[0040] (1) Compared with traditional commercial conductive fillers, the conductive nano-reinforced filler self-assembled from multi-walled carbon nanotubes modified with a suitable proportion of non-ionic compound polyvinyl pyrrolidone in the present invention can form a stable adsorption layer with the surface of multi-walled carbon nanotubes through its hydrophilicity and the flexibility of the polymer chain, thereby effectively reducing the aggregation phenomenon between multi-walled carbon nanotubes. At the same time, during the ultrasonic stirring process, the ultrasonic wave propagates in the liquid, generating periodic high-pressure and low-pressure fluctuations to form cavitation bubbles. When the bubbles collapse rapidly in the liquid, local high temperature and high pressure will be generated, generating strong shear force and shock waves, which can effectively break and disperse the multi-walled carbon nanotube agglomerates. This self-assembly strategy not only significantly improves the dispersion uniformity of the conductive nano-reinforced filler in the rubber matrix, but also promotes the reconstruction and stabilization of the conductive network inside the material, thereby giving the composite material excellent deformation adaptability and conductive properties. This method effectively optimizes the microstructure of the composite material, enhances its comprehensive performance, and lays a solid foundation for the development of high-performance functional materials.

[0041] (2) Self-assembled conductive nano-reinforced fillers and end-vinyl silicone oil are introduced into methyl vinyl silicone rubber, and reaction sites are added to the molecular chains of methyl vinyl silicone rubber, resulting in an increase in cross-linking points between molecular chains. The free chains inside the methyl vinyl silicone rubber are converted into cross-linked chains to form highly constrained areas, thereby enhancing the conductive cross-linked network structure inside the methyl vinyl silicone rubber. When the conductive network is reconstructed, the movement of the conductive nano-reinforced fillers attached to the molecular chains of the methyl vinyl silicone rubber is restricted, eliminating the distortion or sudden turning point of the conductive nano-reinforced fillers on the molecular chains of the methyl vinyl silicone rubber when the strain is unloaded, enhancing the interface bonding interaction between the conductive nano-reinforced fillers and the molecular chains of the methyl vinyl silicone rubber, eliminating the shoulder effect, and improving the stability of the output response signal of the flexible strain sensor during the monitoring process.

[0042] The present invention prepares a flexible strain sensor material with high sensitivity, good mechanical properties and stable monitoring signals in an efficient, pollution-free and low-cost manner. The flexible strain sensor material is suitable for health monitoring of engineering structures under large and small deformations, and provides a material with broad application prospects for the development of the field of structural health monitoring. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 It is a process flow chart of the preparation method of the present invention.

[0044] Figure 2 This is a physical picture of the flexible strain sensor material prepared in Example 5.

[0045] Figure 3 These are SEM microscopic morphology images of the flexible strain sensor materials of Comparative Example 1 and Example 5.

[0046] Figure 4 This is a comparison chart of the monitoring signals output by the flexible strain sensor materials of comparative example 1 and embodiment 5.

[0047] Figure 5 This is the output response signal of the flexible strain sensor material of Example 5 monitoring the seismic isolation bearing.

[0048] Figure 6 This is the response signal output by the flexible strain sensor material of Example 5 to human body monitoring. DETAILED DESCRIPTION

[0049] The present invention will be further described below in conjunction with embodiments, but the present invention is not limited thereto.

[0050] The experimental methods in the following examples are conventional methods unless otherwise specified.

[0051] Example

[0052] The raw materials are:

[0053] Non-ionic compounds include polyvinyl pyrrolidone (PVP), multi-walled carbon nanotubes (MWCNT), tetrahydrofuran (THF), methyl vinyl silicone rubber (VMQ), nano-silica, hydroxy silicone oil (PDMS), vinyl-terminated polymethyl vinyl siloxane (Vi-PMVS), and 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (curing agent bis-25).

[0054] Among them, the vinyl content of methyl vinyl silicone rubber is 0.16%, and the molecular weight is 7.2×10 5 g / mol;

[0055] Multi-walled carbon nanotubes are 20-30 μm long, 6-8 nm in diameter, and have a specific surface area of ​​500-500 m 2 / g;

[0056] Nano-silicon dioxide particle size 7-40nm, specific surface area 200m 2 / g;

[0057] The viscosity of Vi-PMVS is 8000 Pa·s and the vinyl content is 0.15 wt%;

[0058] The hydroxyl content of hydroxy silicone oil is 7-9% and the viscosity is 25-30 Pa·s;

[0059] 2,5-Dimethyl-2,5-di(tert-butylperoxy)hexane was of analytical grade.

[0060] The preparation method of the flexible strain sensor material of the present invention (process flow chart as shown in Figure 1 ), follow the steps below:

[0061] S1. Preparation of Conductive Nano-Reinforced Fillers

[0062] Take a non-ionic compound polyvinylpyrrolidone (PVP) and mix it with multi-walled carbon nanotubes (MWCNT) and tetrahydrofuran (THF), stir it for 60 minutes at room temperature with a magnetic stirrer at a stirring rate of 550 rpm to obtain a conductive nanofiller suspension, then put the beaker containing the conductive filler suspension into an ultrasonic cleaner, install a stirrer above the beaker, insert the stirring head into the beaker, set the ultrasonic frequency to 100 Hz, the temperature to 25°C, the stirring rate of the stirrer to 550 rpm, and ultrasonically stir for 3 hours. After the end, raise the temperature to 50°C to remove tetrahydrofuran to obtain a conductive nanofiller (light black powder).

[0063] This process can form a stable adsorption layer on the surface of multi-walled carbon nanotubes through the hydrophilicity of the non-ionic compound polyvinyl pyrrolidone and the flexibility of the polymer chain, thereby effectively reducing the aggregation phenomenon between multi-walled carbon nanotubes. Then, in the next step, it can be evenly dispersed in methyl vinyl silicone rubber. The obtained conductive nano-enhanced filler is a black light powder, which can be evenly mixed with methyl vinyl silicone rubber in a double-roll mill in the subsequent preparation process.

[0064] S2. Preparation of conductive nano rubber composites

[0065] The conductive nano-reinforced filler prepared in step S1 was uniformly mixed with methyl vinyl silicone rubber (VMQ), nano-silica, hydroxy silicone oil (PDMS), Vi-PMVS, and 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (curing agent bis-25) in a double-roll mill, wherein the mixing temperature was 50° C., the roller distance was 1 mm, the speed was 25 rpm, and the mixing process was 50 min. An unvulcanized conductive nano-rubber composite material was obtained.

[0066] This process uses the shear force generated by the double-roll mill to evenly disperse the filler in the methyl vinyl silicone rubber to form a three-dimensional tunneling conductive network. At the same time, the reaction sites are added to the methyl vinyl silicone rubber molecular chain, resulting in an increase in the cross-linking points between the molecular chains. The free chains inside the methyl vinyl silicone rubber are converted into cross-linked chains to form a highly constrained area, which enhances the conductive cross-linking network structure inside the methyl vinyl silicone rubber and enables it to have the signal acquisition capability to monitor under large and small deformations.

[0067] S3, vulcanization

[0068] S3.1, the first stage of vulcanization:

[0069] The conductive nano-rubber composite material of step S2 is placed in a flat vulcanizer and vulcanized at a temperature of 160° C., a pressure of 15 MPa, and a time of 15 min to obtain a first-stage vulcanization product.

[0070] In the first stage of vulcanization, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane is decomposed into free radicals under high temperature conditions. These free radicals stimulate the breakage of the vinyl double bonds in the methyl vinyl silicone rubber and further generate new free radicals. The free radicals promote the cross-linking reaction of the rubber molecular chain, and finally form a network with a highly cross-linked structure. Through this reaction, a block-shaped conductive nano-rubber composite material was successfully prepared.

[0071] S3.2, Second stage vulcanization:

[0072] The conductive nano-rubber composite material that has undergone the first-stage vulcanization is placed in a forced air drying oven and subjected to the second-stage vulcanization at a temperature of 180° C. for 3 hours, thereby obtaining a flexible strain sensor material.

[0073] In the second stage of vulcanization, under high temperature conditions, the cross-linking structure is further optimized through a cross-linking reaction initiated by free radicals, thereby significantly enhancing the mechanical properties, high temperature resistance, elasticity and chemical stability of the flexible strain sensor, and improving its overall performance.

[0074] In the process of the present invention, the vulcanization of the mixture is carried out in two stages. The first stage of vulcanization is mainly to vulcanize the conductive mixture into a shape and form a cross-linked network inside. The second stage of vulcanization is mainly to increase the cross-linking density of the first stage of vulcanization, so that the mechanical properties and aging resistance of the composite material can be improved.

[0075] According to the above method, the conductive nano-rubber composite materials of Examples 1-5 were prepared. The specific raw material proportions of each example are shown in Table 1:

[0076] Table 1

[0077]

[0078] Comparative Example 1 (using the same filler as the present invention, but different process)

[0079] The same raw materials and dosage as those in Example 5 of the present invention are used, but the preparation process adopts the process in the prior art, that is, methyl vinyl silicone rubber, multi-walled carbon nanotubes, polyvinyl pyrrolidone, nano-silicon dioxide, hydroxy silicone oil, Vi-PMVS, and 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane are directly put into an internal mixer for mixing, and the temperature is set to 70°C, the speed is 80rpm, the roller distance is 1mm, and the time is 40min. Then, it is put into a flat vulcanizer at a pressure of 10MPa, a temperature of 170°C, and a time of 10min, and vulcanization molding is performed to obtain the product.

[0080] Comparative Example 2 (using different fillers from the present invention and the same process)

[0081] Different fillers of the present invention are used, specifically, by weight, the fillers include 100 parts of methyl vinyl silicone rubber, 10 parts of commercial graphene, 300 ml of acetone, 10 parts of nano silicon dioxide, and 2 parts of di-tert-butyl peroxide (DTBP). A flexible strain sensor is made by the preparation process of the present invention.

[0082] Specifically, the process includes: stirring graphene and tetrahydrofuran in a magnetic stirrer at 550rpm for 1h at room temperature to obtain a conductive filler suspension. Subsequently, the conductive filler suspension is placed in an ultrasonic cleaner, and ultrasonically stirred for 3h under the conditions of an ultrasonic frequency of 100Hz, a temperature of 25°C, and an ultrasonic stirring rate of 550rpm. Then the temperature is raised to 50°C to remove tetrahydrofuran to obtain a conductive nanofiller. Methyl vinyl silicone rubber is placed in a double-roll mill, and then conductive nanofillers, silica, and di-tert-butyl peroxide are placed and mixed uniformly at a temperature of 50°C, a roller distance of 1mm, a speed of 25rpm, and a time of 50min to obtain an uncured conductive nanomixture. The uncured conductive nanomixture is placed in a flat vulcanizer and subjected to a first-stage vulcanization at a temperature of 160°C, a pressure of 15MPa, and a time of 15min to obtain a conductive nanomixture after vulcanization. Finally, the conductive nanomixture is placed in an oven and subjected to a second-stage vulcanization at 180°C for 3h to obtain a flexible strain sensor.

[0083] Performance Testing

[0084] The performance of the prepared flexible strain sensor material is tested.

[0085] (1) Volume conductivity: The volume conductivity of the conductive nano rubber material is tested according to the GT / T1692-2008 standard. First, the sample is cut into a standard sample of size 40mm×1omm×1mm (the surface is cleaned with anhydrous ethanol), and the resistance value of the sample in the unstrained state is measured using a Keysight 34465A digital multimeter. The average resistance value within 1s of the digital multimeter is collected, and the average value is taken for each group of 3 test strips. The volume conductivity formula is defined as:

[0086]

[0087] In formula (1), σ is the volume conductivity (S / m), ρ is the volume resistivity of the material (Ω·m), R is the volume resistance (Ω), L is the length of the sample (m), and S is the cross-sectional area (m 2 ).

[0088] (2) Sensitivity test: Clamp the two ends of the conductive nano rubber material on an electronic universal testing machine, take a wire, connect one end of the wire to the conductive nano rubber material, and the other end of the wire to a digital multimeter. Set the strain through the universal testing machine. After a single stretching cycle, the maximum resistance value is transmitted to the computer through the digital multimeter, and then the sensitivity is calculated through the resistance and strain according to formula (2).

[0089] Sensitivity calculation method: Formula (2) is used to evaluate the sensitivity of the composite material:

[0090] GF=(ΔR / R 0 ) / ε formula (2)

[0091] In formula (2), ε is the strain. ΔR / R 0 , ΔR=RR 0 , where R 0 is the initial resistance, and R is the test resistance.

[0092] (3) Test method of resistance / strain response signal: The conductive nano rubber material is cut into strips of 40 mm × 40 mm × 1 mm, the sample is fixed on an electronic universal testing machine for a cyclic loading-unloading test, and the change of the resistance / strain response signal is recorded with a digital multimeter.

[0093] (4) Mechanical properties test method: According to GB / T528-2009, the mechanical properties of dumbbell-shaped standard conductive nano rubber material specimens were measured using an electronic universal testing machine (DDL10) at a tensile rate of 200 mm / min- 1 , and take the average value of 3 groups of experimental data.

[0094] The performance test results are shown in Table 2.

[0095] Table 2: Flexible strain sensor material performance results

[0096]

[0097] Figure 2 This is a physical picture of the flexible strain sensor material prepared in Example 5.

[0098] It can be seen from the various embodiments and comparative examples that the volume conductivity of the various embodiments of the present invention is significantly reduced compared with comparative example 1. In particular, compared with comparative example 1, the volume conductivity of embodiment 5 using the same filler is reduced more significantly, indicating that the process of the present invention can reduce the volume conductivity to the greatest extent.

[0099] All the examples can achieve good sensitivity. Compared with Comparative Example 1 or Comparative Example 2 using fillers different from those of the present invention and using different preparation methods, the sensitivity of each example is improved to varying degrees.

[0100] By comparing the SEM microscopic morphology of Example 1 with that of Example 5 ( Figure 3 ) It can be seen that the conductive filler of Comparative Example 1 has serious agglomeration in the rubber matrix, which hinders the continuity of the conductive filler in the rubber matrix. The self-assembled conductive filler of Example 5 has good dispersion in the rubber matrix, which is also an important factor in obtaining low volume conductivity, high sensitivity, good mechanical properties, and stable resistance / strain response signals.

[0101] Tensile strength and elongation at break are mainly related to the dispersibility of the conductive nanofiller in the matrix. Comparative Example 1 uses the same raw materials as the example but a different process. Since all the raw materials are mixed at one time, the conductive filler has poor dispersibility during the mixing process with the rubber matrix, so no conductive rubber nanocomposite material with good dispersibility is formed. Comparative Example 2 does not perform self-assembly modification on commercial graphene, and its dispersibility in the rubber matrix is ​​also poor. Comparative Example 1 or Comparative Example 2 does not have self-assembled conductive nanofillers, and the tensile strength and elongation at break of the composite material finally obtained are low. The tensile strength and elongation at break of Example 5 using the filler ratio and process of the present invention are significantly improved.

[0102] Comparison of the output monitoring signals of the flexible strain sensor of Comparative Example 1 and Example 5: The flexible strain sensor materials prepared in Comparative Example 1 and Example 5 were cut into strips of 40 mm × 40 mm × 1 mm, respectively, and the samples were fixed on an electronic universal testing machine for a cyclic loading-unloading test, and the changes in the resistance / strain response signals were recorded with a digital multimeter. Figure 4 It can be seen that the monitoring signal graph of the material in comparative example 1 shows a sudden increase in resistance (called the shoulder effect); while the monitoring signal graph of the material in Example 5 shows a continuous signal, reflecting a stable monitoring signal, and no shoulder effect. Example 5 combines the self-assembled conductive filler with Vi-PMVS to increase the reaction sites on the rubber matrix molecular chain, resulting in an increase in the cross-linking points between the molecular chains, and the free chains inside the rubber are converted into cross-linked chains to form a highly constrained area, thereby enhancing the conductive cross-linked network structure inside the rubber matrix. When the conductive network is reconstructed, the movement of the self-assembled conductive filler attached to the rubber molecular chain is restricted, reducing the sudden increase in resistance generated by the self-assembled conductive filler on the molecular chain when the rubber is strain unloaded, enhancing the interface binding interaction between the self-assembled conductive filler and the rubber molecular chain, and eliminating the shoulder effect.

[0103] By the method of the present invention, a conductive nano-reinforced filler with a specific non-ionic compound polyvinyl pyrrolidone and a multi-walled carbon nanotube mass ratio and a methyl vinyl silicone rubber are combined to produce a flexible strain sensor with low volume conductivity, high sensitivity and good mechanical properties. Moreover, the innovative ratio of the raw materials of the present invention enables the flexible strain sensor to eliminate the shoulder effect in the monitoring signal (the smaller the resistance / strain response hysteresis rate (%) result value in Table 2, the weaker the shoulder effect affects the stability of the signal), so that it has a stable monitoring signal, which is more conducive to real-time structural health monitoring of the size deformation of the structure. At the same time, the flexible strain sensor can overcome the defects of traditional strain sensors such as brittleness, low sensitivity and narrow monitoring range. The monitoring signal output during the monitoring process cooperates with the perfect deformation of the measured structure to obtain the health status of the measured engineering structure in real time.

[0104] Application Example 1

[0105] The flexible strain sensor material prepared in Example 5 was cut into blocks of 40mm×10mm×1mm, and the cut flexible strain sensor was attached to the high damping seismic isolation rubber support in the laboratory of this research team using special silicone rubber glue. One end of the wire was attached to the sensor material with strong tape, and the other end of the wire was connected to a digital multimeter. When the high damping rubber support deformed, the sensor was deformed, and the sensor deformation caused the resistance to change. The real-time resistance / strain response signal was output on the computer to evaluate the deformation degree of the high damping rubber support, so as to monitor the high damping seismic isolation rubber support. When the rubber support deformed greatly under certain experimental conditions, the sensor was deformed greatly, resulting in a significant change in the resistance / response signal, indicating that the rubber support was damaged or aged.

[0106] The entire test process was carried out on a shearing machine, with the working conditions of frequency: 0.03-0.12Hz, deformation displacement: 30-80mm, pressure: 15MPa-30MPa. Deformation direction: bidirectional shear deformation. The test results are as follows Figure 5 As shown, as the high damping rubber bearing deforms, the sensor can capture it with high sensitivity and stably output resistance / strain response signals, and the detection results are accurate and reliable.

[0107] Most of the isolation bearings currently on the market are rubber bearings. The flexible strain sensor material prepared by the present invention has a rubber matrix, which can solve the compatibility problem with rubber isolation bearings. At the same time, the flexible strain sensor has high sensitivity, stable resistance / strain response signal output of the deformation of the isolation bearing, and excellent deformation ability, which can achieve perfect coordinated deformation with the isolation bearing. In addition, since the isolation bearings are often in harsh environments, compared with the fragility, low sensitivity, and poor aging resistance of traditional strain sensors, which cannot perform real-time monitoring of the isolation bearings under large deformation, the flexible strain sensor material of the present invention is particularly suitable for real-time monitoring of the structural health of the isolation bearings. The sensor of the present invention is used to monitor the isolation bearings in real time, and there is no need to disassemble each bearing and take it back to the laboratory for testing, which greatly saves manpower, material resources and testing costs.

[0108] Application Example 2

[0109] The flexible strain sensor material prepared in Example 5 was cut into a block of 40 mm × 10 mm × 1 mm and taped to the elbow joint of a human arm. One end of the wire was attached to the sensor material and the other end was connected to a digital multimeter. The elbow joint was subjected to reciprocating flexion and extension motions. The sensor was able to capture the resistance / strain response signal output during the elbow joint motion with high sensitivity. During the whole process, the resistance / strain response signal did not show a shoulder effect, and the resistance / strain response signal remained stable ( Figure 6 ), the test results are accurate and reliable.

Claims

1. A method for preparing a flexible strain sensor material with high stability of monitoring signals, characterized in that: The steps include: S1. Preparation of Conductive Nano-Reinforced Fillers Polyvinyl pyrrolidone, a conductive filler and an organic solvent are mixed, and after being fully mixed, the organic solvent is removed by volatilization by heating to obtain a conductive nanofiller; Wherein, the mass ratio of polyvinyl pyrrolidone to conductive filler is 0-1.0:0.1-0.4; S2. Preparation of conductive nano rubber composites The conductive nano-reinforced filler prepared in step S1 is mixed with methyl vinyl silicone rubber, nano-silicon dioxide, hydroxy silicone oil, vinyl silicone oil, and a vulcanizing agent in a mass ratio of 18-26:500-700:100-140:30-42:0-80:10-14, and kneaded to obtain a mixture; S3, vulcanization: The mixed material prepared in step S2 is vulcanized in two stages, wherein the vulcanization temperature in the second stage is 20 to 30° C. higher than that in the first stage, to obtain the flexible strain sensor material.

2. The preparation method according to claim 1, characterized in that: The organic solvent is ethyl acetate or tetrahydrofuran or acetone; The conductive filler is selected from multi-walled carbon nanotubes, conductive carbon black or graphene; The vinyl silicone oil is a vinyl-terminated silicone oil, and the vinyl-terminated silicone oil is selected from one of vinyl-terminated polydimethylsiloxane (Vi-PDMS) and vinyl-terminated polymethylvinylsiloxane (Vi-PMVS), or a mixture of the two in any proportion; The vulcanizing agent is selected from one or more of dicumyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, tert-butyl perbenzoate, and dibenzoyl peroxide.

3. The preparation method according to claim 1, characterized in that: In step S1, The mass volume ratio of polyvinyl pyrrolidone, conductive filler and organic solvent is 0.2-1.0g:0.1-0.4g:100-1200ml or 0.2-0.8g:0.18-0.3g:200-1000ml; Polyvinyl pyrrolidone, conductive filler and organic solvent are mixed, ultrasonically stirred and mixed evenly, and the organic solvent is removed by volatilization by heating to obtain conductive nanofiller; the ultrasonic stirring conditions are ultrasonic frequency of 100-150 Hz, stirring rate of 450-600 rpm, and ultrasonic stirring and mixing for 2-4 hours; Preferably, magnetic stirring is performed before ultrasonic stirring, and the magnetic stirring condition is stirring at a stirring rate of 450 to 600 rpm for 30 to 60 min; The organic solvent is tetrahydrofuran, and the temperature is raised to 50-70° C. to remove the tetrahydrofuran and obtain the conductive nano filler.

4. The preparation method according to claim 1, characterized in that: In step S2, the mass ratio of the conductive nano-reinforced filler to the methyl vinyl silicone rubber, nano-silicon dioxide, hydroxy silicone oil, vinyl silicone oil, and vulcanizing agent is 18-26: 500-700: 100-140: 30-42: 40-80: 10-14 or 20-24: 550-650: 110-130: 33-39: 60-80: 11-13; The mixing temperature is 40-60°C, the roller distance is 0.5-1mm, the speed is 20-35rpm, and the mixing process is 40-60min.

5. The preparation method according to claim 1, characterized in that: In step S3, The first stage of vulcanization has a vulcanization temperature of 150-170°C, a time of 10-20 min, and a pressure of 10-20 MPa. The preferred vulcanization temperature is 160-170°C, a time of 10-15 min, and a pressure of 10-15 MPa. Then the second stage vulcanization treatment is carried out, the vulcanization temperature is 180-200°C, and the time is 2-4 hours.

6. The preparation method according to claim 5, characterized in that: In step S3, the first stage vulcanization is carried out on a flat plate vulcanizer, and the second stage vulcanization is carried out in an oven.

7. The preparation method according to claim 2, characterized in that: The vinyl content of the methyl vinyl silicone rubber is 0.16% and the molecular weight is 7.2×10 5 g / mol; The multi-walled carbon nanotubes are 20-30 μm long, 6-8 nm in diameter, and have a specific surface area of ​​500-500 m 2 / g; The nano-silicon dioxide has a particle size of 7-40 nm and a specific surface area of ​​200 m 2 / g; The vinyl silicone oil has a viscosity of 8000±1000 Pa·s and a vinyl content of 0.1 to 1.2 wt%; The hydroxyl content of the hydroxy silicone oil is 7-9% and the viscosity is 25-30 Pa·s.

8. A flexible strain sensor material, characterized in that: The invention is prepared by the preparation method according to any one of claims 1 to 7.

9. Use of the flexible strain sensor material according to claim 8 in preparing a flexible strain sensor.

10. The use according to claim 8, characterized in that: The flexible strain sensor material is prepared into a flexible sensor for use in engineering structure health monitoring or human body motion monitoring.

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