A color-changing silica gel grading indicator and its preparation method

Abstract

This invention discloses a color-changing silica gel grading indicator and its preparation method, belonging to the technical field of functional porous humidity indicator materials. The indicator consists of three color-changing silica gel components with different peak positions in pore size distribution. The pores of the three carriers are respectively loaded with cobalt chloride hexahydrate or bromophenol blue, methyl orange, and bromothymol blue. Citric acid and disodium hydrogen phosphate can be introduced as pH buffer components. The outer surface of the particles can be hydrophobically passivated using an anhydrous toluene solution of dimethyldichlorosilane. The preparation method includes dividing the same batch of fine-porous silica gel into three portions, expanding the pores under different hydrothermal conditions to obtain three carriers, then loading the indicator separately using an equal-volume impregnation method and drying them, and finally passivating the outer surface after mixing. This invention achieves graded color reading of humidity, with the three indicators operating independently to avoid interference. The passivation treatment simultaneously improves threshold sharpness and cycle durability. The process has strong repeatability and scalability, making it suitable for applications such as moisture protection for electronic equipment, artifact storage, and indicator packaging.

Description

Technical Field

[0001] This invention belongs to the technical field of functional porous humidity indicator materials, specifically relating to a color-changing silica gel grading indicator and its preparation method. Background Technology

[0002] Color-changing silica gel, a porous desiccant with visual humidity indication, is widely used in applications such as moisture protection for electronic equipment, sealing of optical instruments, storage of cultural relics, and indicator packaging for food and pharmaceuticals. It provides users with an intuitive humidity reading by changing color in response to ambient humidity. Traditional color-changing silica gel is typically represented by fine-porous silica gel loaded with cobalt chloride hexahydrate, relying on the reversible hydration reaction of anhydrous cobalt chloride transforming into pink cobalt chloride hexahydrate upon absorbing water to achieve the color change. However, this traditional color-changing silica gel has revealed several problems in practical applications.

[0003] First, traditional color-changing silica gel particles have a uniform pore size distribution, and the overall color change is concentrated around a certain relative humidity threshold. Users find it difficult to determine the specific relative humidity level (low, medium, or high) from the color alone, failing to meet the need for categorized humidity identification. Second, to achieve a deeper initial blue color and higher moisture absorption contrast, commercial color-changing silica gel typically contains a high proportion of cobalt. This contradicts the recent trend of restricting cobalt and its compounds under EU REACH and RoHS regulations, limiting its application in export packaging and food / pharmaceutical contact applications. Third, some studies have attempted to introduce multiple indicators into the same silica gel particle to expand the corresponding range of hue and humidity. However, acid-base indicators and cobalt-based indicators undergo electrostatic, hydrogen bonding, and coordination interactions within the same pore, causing interference between the color rendering ranges of various indicators. Ultimately, this results in a significant decrease in hue contrast and poor categorized color rendering. Furthermore, the silanol groups on the outer surface of traditional silica gel particles have a strong adsorption affinity for water molecules. Before reaching the capillary condensation threshold, non-pore-controlled moisture absorption occurs on the particle surface, raising the color change threshold front and reducing its sharpness. Simultaneously, the thin water film on the particle surface, during repeated moisture absorption and desorption cycles, drives indicator molecules to migrate and accumulate on the particle surface, reducing the product's cycle durability. Finally, traditional impregnation processes are mostly qualitative over-impregnation, lacking precise control over the actual indicator loading in each particle, resulting in poor color consistency between batches.

[0004] Therefore, it is of great significance to design color-changing silica gel products that can solve the above problems, achieve continuous graded humidity indication, avoid mutual interference of indicators, reduce cobalt content, and have high threshold sharpness and cycle durability, as well as their corresponding preparation methods. Summary of the Invention

[0005] To address the problems existing in the background art, the present invention provides a color-changing silica gel grading indicator, comprising a first color-changing silica gel component, a second color-changing silica gel component, and a third color-changing silica gel component in a mass ratio of (28-42):(28-42):(22-38); wherein: The first color-changing silica gel component includes a carrier A and a first indicator loaded within the pores of the carrier A. The raw material for the first indicator is selected from cobalt chloride hexahydrate or bromophenol blue. Based on 100 parts by weight of the carrier A, the amount of cobalt chloride hexahydrate is 3-6 parts or the amount of bromophenol blue is 0.8-1.5 parts. The second color-changing silica gel component includes a carrier B and methyl orange loaded within the pores of the carrier B. Based on 100 parts by weight of the carrier B, the amount of methyl orange is 0.5-1.5 parts. The third color-changing silica gel component includes a carrier C and bromothymol blue loaded within the pores of the carrier C. Based on 100 parts by weight of the carrier C, the amount of bromothymol blue is 0.8-2.0 parts.

[0006] In a preferred embodiment, the peak positions of the pore size distributions of the carriers A, B, and C, calculated using the nitrogen adsorption-desorption isotherm desorption branch BJH method, are respectively located in the ranges of 2.8-3.2 nm, 4.5-5.5 nm, and 10-12 nm.

[0007] In a preferred embodiment, the second color-changing silica gel component further includes citric acid. Based on 100 parts by mass of carrier B, the amount of citric acid is 0.05-0.20 parts. The citric acid and methyl orange are co-loaded within the pores of carrier B. The third color-changing silica gel component further includes disodium hydrogen phosphate. Based on 100 parts by mass of carrier C, the amount of disodium hydrogen phosphate is 0.10-0.30 parts. The disodium hydrogen phosphate and bromothymol blue are co-loaded within the pores of carrier C. The outer surface of the particles of the color-changing silica gel grading indicator undergoes a surface hydrophobic passivation treatment. The surface hydrophobic passivation treatment agent is an anhydrous toluene solution of dimethyldichlorosilane, and the mass ratio of dimethyldichlorosilane to anhydrous toluene is 1:90 to 1:110.

[0008] This invention also provides a method for preparing a color-changing silica gel grading indicator, comprising the following steps: S1: Divide the initial fine-pore silica gel particles of the same batch into three parts, and perform pore-expansion treatment under three different hydrothermal conditions with different temperatures, different sodium carbonate concentrations and different times. Then, calcine and stabilize them in a unified manner so that the peak positions of the pore size distribution of the three carriers fall within the range of 2.8-3.2nm, 4.5-5.5nm and 10-12nm, respectively. These are denoted as carrier A, carrier B and carrier C. S2: Using an aqueous solution of cobalt chloride hexahydrate as impregnation solution A, the carrier A is impregnated by an equal-volume impregnation method, and after drying, the first color-changing silica gel component is obtained. S3: Using an ethanol-water solution containing methyl orange as impregnation solution B, the carrier B is impregnated by an equal-volume impregnation method, and after drying, the second color-changing silica gel component is obtained; S4: Using an ethanol-water solution containing bromothymol blue as impregnation solution C, the carrier C is impregnated by an equal-volume impregnation method, and after drying, the third color-changing silica gel component is obtained. S5: The first color-changing silica gel component, the second color-changing silica gel component, and the third color-changing silica gel component are mechanically mixed evenly in a dry environment to obtain the color-changing silica gel grading indicator.

[0009] Furthermore, step S1 specifically includes the following sub-steps: S11: Using type A fine-pore spherical silica particles with a pore size of 2-4 mm and a peak position of pore size distribution in the range of 2.4-2.7 nm as determined by the BJH method according to claim 1 as the initial fine-pore silica particles, they are divided into three parts by mass and denoted as precursor A, precursor B and precursor C respectively. S12: The A precursor is placed in a sodium carbonate aqueous solution with a concentration of 0.02-0.04 mol / L, and the mass ratio of the A precursor to the sodium carbonate aqueous solution is 1:5 to 1:8. After sealing, it is hydrothermally treated at 85-92℃ for 1.0-2.0 hours. After cooling, the particles are taken out and washed with deionized water until the pH of the washing solution is 7±0.5 and the conductivity is not higher than 10μS / cm. Then it is dried at 105℃ for 4 hours to obtain the carrier A precursor. S13: The B precursor is placed in a sodium carbonate aqueous solution with a concentration of 0.05-0.08 mol / L, and the mass ratio of the B precursor to the sodium carbonate aqueous solution is 1:5 to 1:8. After sealing, it is hydrothermally treated at 105-115℃ for 2.0-3.0 hours. After cooling, the particles are taken out and washed with deionized water until the pH of the washing solution is 7±0.5 and the conductivity is not higher than 10μS / cm. Then it is dried at 105℃ for 4 hours to obtain the carrier B precursor. S14: The C precursor is placed in a sodium carbonate aqueous solution with a concentration of 0.10-0.15 mol / L, and the mass ratio of the C precursor to the sodium carbonate aqueous solution is 1:5 to 1:8. After sealing, it is hydrothermally treated at 125-135℃ for 3.0-4.0 hours. After cooling, the particles are taken out and washed with deionized water until the pH of the washing solution is 7±0.5 and the conductivity is not higher than 10μS / cm. Then it is dried at 105℃ for 4 hours to obtain the carrier C precursor. S15: The precursors of carrier A, carrier B, and carrier C are calcined at 350°C for 2 hours under nitrogen protection, and then naturally cooled to room temperature to obtain carrier A, carrier B, and carrier C respectively. Step S15 is used to stabilize the porous skeleton obtained by hydrothermal pore expansion and remove residual moisture.

[0010] Furthermore, step S2 specifically includes the following sub-steps: S21: Mix cobalt chloride hexahydrate with deionized water at a mass ratio of 8:100 to 18:100, and stir at room temperature until dissolved to obtain the clear impregnation solution A; S22: Determine the amount of impregnation solution A to be weighed according to the following formula: ; In the formula, The volume of the impregnation solution A is measured in mL. This is the impregnation allowance coefficient, with a value ranging from 1.00 to 1.05; The total pore volume of the carrier A is expressed in mL / g. The mass of carrier A is expressed in grams. After adding the carrier A to the weighed impregnation solution A, let it stand and impregnate for 1 hour at room temperature and pressure, stirring gently every 15 minutes until there is no obvious free liquid on the surface of the carrier A particles. S23: The carrier A, after being impregnated in step S22, is placed in a vacuum drying oven and slowly dried for 4 hours under vacuum conditions of 40°C and gauge pressure of -0.085MPa to -0.075MPa. Then, the temperature is raised to 105°C and switched to atmospheric pressure to continue drying for 2 hours to obtain the first color-changing silica gel component. The mass ratio of cobalt chloride hexahydrate to deionized water in step S21 is adjusted so that the mass ratio of cobalt chloride hexahydrate in the first color-changing silica gel component to the mass ratio of carrier A is (3-6):100.

[0011] Furthermore, step S3 specifically includes the following sub-steps: S31: Mix methyl orange, anhydrous ethanol and deionized water in a mass ratio of (0.6-3.0):(15-25):(75-85), stir in a 50°C water bath until dissolved, and filter through a 0.45μm filter membrane after natural cooling to room temperature to obtain the clear impregnation solution B. S32: Determine the amount of impregnating solution B to be weighed according to the following formula: ; In the formula, The volume of the impregnation solution B is measured in mL. This is the impregnation allowance coefficient, with a value ranging from 1.00 to 1.05; The total pore volume of the carrier B is expressed in mL / g. The mass of carrier B is expressed in grams. After adding the carrier B to the weighed impregnation solution B, let it stand and impregnate for 1 hour at room temperature and pressure, stirring gently every 15 minutes until there is no obvious free liquid on the surface of the carrier B particles. S33: The carrier B, after being impregnated in step S32, is placed in a forced-air drying oven and dried at 60°C for 3 hours, then heated to 105°C and dried for another 2 hours to obtain the second color-changing silica gel component; the mass ratio of methyl orange, anhydrous ethanol and deionized water in step S31 is adjusted so that the mass ratio of methyl orange in the second color-changing silica gel component to the mass ratio of carrier B is (0.5-1.5):100.

[0012] Furthermore, step S4 specifically includes the following sub-steps: S41: Mix bromothymol blue, anhydrous ethanol and deionized water in a mass ratio of (0.8-3.0):(60-70):(30-40), stir in a 30°C water bath until dissolved, and filter through a 0.45μm filter membrane to obtain the clear impregnation solution C; S42: Determine the amount of the impregnation solution C to be weighed according to the following formula: ; In the formula, The volume of the impregnation solution C is measured in mL. This is the impregnation allowance coefficient, with a value ranging from 1.00 to 1.05; The total pore volume of the carrier C is expressed in mL / g. The mass of the carrier C is expressed in grams. After adding the carrier C to the weighed impregnation solution C, let it stand and impregnate for 1 hour at room temperature and pressure, stirring gently every 15 minutes until there is no obvious free liquid on the surface of the carrier C particles. S43: The carrier C, after being impregnated in step S42, is placed in a vacuum drying oven and dried in three stages: first, it is slowly dried for 3 hours under vacuum conditions of 50°C and a gauge pressure of -0.075MPa to -0.065MPa; then, it is dried for 2 hours at 80°C under normal pressure; and finally, it is dried for 1 hour at 105°C under normal pressure to obtain the third color-changing silica gel component. The purpose of the three-stage drying is to prevent the bromothymol blue from migrating to the outer surface of the carrier C particles by evaporating in stages. The mass ratio of the bromothymol blue, the anhydrous ethanol, and the deionized water in step S41 is adjusted so that the mass ratio of the bromothymol blue in the third color-changing silica gel component to the carrier C is (0.8-2.0):100.

[0013] Furthermore, step S5 specifically includes the following sub-steps: S51: The first color-changing silica gel component, the second color-changing silica gel component, and the third color-changing silica gel component are respectively placed in a vacuum dryer equipped with 3A molecular sieve desiccant and left to stand at 25-30°C for 48 hours to reduce the water content of each component, as determined by the 105°C loss method, to below 2% by mass, thereby obtaining the pre-dried first color-changing silica gel component, the pre-dried second color-changing silica gel component, and the pre-dried third color-changing silica gel component. S52: Weigh the pre-dried first color-changing silica gel component, the pre-dried second color-changing silica gel component, and the pre-dried third color-changing silica gel component according to the mass ratio described in claim 1, and place them in a closed mixer with a relative humidity of not more than 10%. Turn on the mixer and mix at a speed of 10-30 r / min for 10-20 minutes to obtain the color-changing silica gel grading indicator.

[0014] Furthermore, following step S5, a step S6 is included to perform a surface hydrophobic passivation treatment on the outer surface of the particles of the color-changing silica gel grading indicator. Step S6 specifically includes the following sub-steps: S61: Dimethyldichlorosilane and anhydrous toluene are mixed at a mass ratio of 1:90 to 1:110 to prepare a hydrophobic treatment solution. Step S61 and subsequent sub-steps of step S6 are carried out in a dry environment with a water content of less than 50 ppm. S62: Add the color-changing silica gel grading indicator to the hydrophobic treatment solution, wherein the mass ratio of the color-changing silica gel grading indicator to the hydrophobic treatment solution is 1:3 to 1:4, and let it stand and soak at 25-30°C for 30 minutes without stirring. S63: Take out the particles processed in step S62, wash them twice with anhydrous toluene to remove unreacted dimethyldichlorosilane and hydrogen chloride released by the reaction, then place them in a vacuum drying oven at 105°C and purge them with a dry nitrogen stream for 4 hours to remove residual dimethyldichlorosilane, toluene and hydrogen chloride. After naturally cooling to room temperature, the color-changing silica gel grading indicator with hydrophobic passivation is obtained.

[0015] The beneficial effects achieved by this invention are as follows: The color-changing silica gel grading indicator of this invention is based on pore-selective capillary condensation as the physical basis for grading response. Three carriers with different pore size distribution peak positions are each loaded with a corresponding color-developing indicator to form three color-changing silica gel components. The pore sizes of the carriers in the three components increase sequentially. According to the physical correspondence revealed by the Kelvin equation, capillary condensation color development is triggered sequentially at three relative humidity thresholds: low, medium, and high. This allows the overall product to exhibit a continuous, monotonous hue transition between blue, purple, orange, and green as the ambient relative humidity rises from low to high. Users can directly read the humidity level based on the appearance hue, obtaining graded humidity readings without the need for electronic sensing modules or power supplies. This invention also provides two implementation schemes where the first indicator is selected from cobalt chloride hexahydrate or bromophenol blue. The bromophenol blue alternative version can achieve color development at low humidity levels under cobalt-free conditions, expanding its application range.

[0016] This invention involves independently impregnating three indicators in three carriers with different pore sizes, and then mechanically mixing the resulting three color-changing silica gel components. This ensures that the three indicators are located within independent carrier particles in the final product, with no liquid phase channels between the particles. This structure avoids the mutual interference caused by electrostatic interactions, hydrogen bonding, and coordination interactions that may occur between acid-base indicators and cobalt-based indicators within the same pore. This allows for full utilization of the color-changing contrast at each level, resulting in a higher cumulative color difference between the three levels compared to a single-batch impregnation process with three different pore sizes. Furthermore, this invention introduces citric acid as a pH buffer in the hydrated state into the second color-changing silica gel component and disodium hydrogen phosphate as a pH buffer in the hydrated state into the third color-changing silica gel component. This results in better batch-to-batch repeatability and long-term hue stability of the colors exhibited by methyl orange and bromothymol blue after humidity triggering.

[0017] This invention further provides a step of short-term, mild hydrophobic passivation treatment on the outer surface of the obtained color-changing silica gel grading indicator particles. By limiting the treatment time and temperature and using a static, non-stirring method, the hydrophobic reaction is controlled at the exposed silanol sites on the outer surface of the particles. This not only shields the non-pore-controlled moisture absorption on the outer surface of the particles but also retains the hydrophilicity inside the pores to ensure the humidity selectivity of pore-selective capillary condensation. After hydrophobic passivation, non-pore-controlled moisture absorption before reaching the pore capillary condensation threshold on the outer surface of the particles is suppressed. The leading edge of each color-changing threshold is no longer raised due to premature moisture absorption by the thin water film on the outer surface of the particles, thus improving the threshold sharpness of the product. At the same time, the phenomenon of indicator molecules migrating and accumulating on the outer surface of the particles due to repeated moisture absorption and desorption cycles is also suppressed, improving the cycle durability of the product. Thus, within a suitable process window, both threshold sharpness and cycle durability are improved simultaneously.

[0018] The preparation method of this invention uses the same batch of initial fine-pored silica gel particles as starting material. Through three sets of systematically gradient hydrothermal pore-expansion conditions, carriers with three different pore size distributions are obtained, avoiding initial compositional differences introduced by different batches of raw materials. This ensures that the peak positions of the pore size distributions of the three types of carriers can repeatedly fall within the preset three target ranges. The equal-volume impregnation method of this invention is based on the process relationship that the volume of the impregnation liquid equals the total pore volume that the carrier can absorb, and introduces an impregnation margin coefficient slightly greater than one as a process allowance. This provides a precise control over the actual indicator loading in each carrier particle, ensuring the colorimetric consistency of the product across different batches. The raw materials used in the preparation process of this invention are all bulk chemical reagents. The process mainly involves mature unit operations such as hydrothermal treatment, equal-volume impregnation, hot air and vacuum drying, and surface silanization, exhibiting strong repeatability and scalability, and is suitable for industrial mass production. Attached Figure Description

[0019] Figure 1 This is a comparison chart of the carrier property characterization and pore size-humidity threshold mapping between Examples 1 to 3 and Comparative Example 1, wherein... Figure 1 (a) is a comparison diagram of the pore size distribution of the BJH desorption branch. Figure 1 (b) is the aperture-relative humidity threshold mapping predicted by the Kelvin equation.

[0020] Figure 2 This is a comparison chart of the ΔE-RH response curves of Examples 1 to 4 with Comparative Examples 1, 2 and 6.

[0021] Figure 3 This is a comparison chart of the hue-humidity correspondence between Examples 1 and 4 and Comparative Example 1, in which... Figure 3 (a) is the chromaticity locus diagram of the ab plane. Figure 3 (b) is a graph showing the change of hue angle with relative humidity.

[0022] Figure 4 These are comparison diagrams of the surface passivation effects of Examples 1 to 3 and Comparative Examples 1, 4, and 5, in which... Figure 4 In figure (a), the threshold sharpness dE / dRH varies with relative humidity. Figure 4 (b) is a line graph showing the ΔE retention rate after 50 moisture absorption and desorption cycles.

[0023] Figure 5 This is a flowchart of a method for preparing a color-changing silica gel grading indicator according to the present invention. Detailed Implementation

[0024] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. In addition, the forms of the various structures described in the following embodiments are merely illustrative. The present invention is not limited to the structures described in the following embodiments. All other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] The color-changing silica gel graded indicator provided by this invention is a graded humidity indicator material based on pore size selective capillary condensation. The main body is composed of three color-changing silica gel components mechanically mixed in a certain mass ratio. These three components are designated as the first, second, and third color-changing silica gel components. Each component has a carrier A, carrier B, and carrier C with progressively increasing pore size distribution peak positions, corresponding to low, medium, and high relative humidity thresholds, respectively. Each component's carrier pores are loaded with a corresponding color-developing indicator. Based on the different relative humidity thresholds corresponding to the carrier pore sizes, the three components are triggered sequentially at different relative humidity levels, providing a continuous graded indication of ambient humidity.

[0026] The physical basis of the aforementioned aperture-selective capillary condensation phenomenon is the Kelvin equation. A common form of the Kelvin equation is... In the formula Represents the natural logarithm operation; It is the relative vapor pressure at which capillary condensation occurs, with a dimension of 1, and is numerically equivalent to the decimal expression of relative humidity. The surface tension of water at the test temperature (N / m); The molar volume of water (m 3 / mol); Let be the cosine of the contact angle of water on the hydroxylated silica wall; the contact angle on hydrophilic silica approaches zero. The value is around 1; The radius of the pores where capillary condensation occurs (m); The ideal gas constant (J / (mol·K)) is 8.314. Let K be the absolute temperature (K). The negative sign on the right side of the formula indicates that the saturated vapor pressure of a concave liquid surface is lower than that of a flat liquid surface. From this equation, it can be seen that when the orifice radius... When the relative vapor pressure decreases, the relative vapor pressure at which capillary condensation occurs also decreases, and the relative humidity of the environment that triggers condensation also decreases accordingly. Guided by this equation, this invention designs the peak positions of the pore size distribution of carriers A, B, and C to be three progressively larger levels, so that the moisture absorption trigger humidity corresponding to the three components increases sequentially, with the three levels covering the low, medium, and high relative humidity ranges, respectively.

[0027] In this invention, the peak position of the pore size distribution is determined using the desorption branch BJH method. The reason is that for the mesoporous range of 2.8 nm to 12 nm, the measurement of the desorption branch is constrained by the meniscus stability implicit in the Laplace-Kelvin equation. The obtained peak position is more consistent with the hygroscopic condensation mechanism of interest in this invention and has good comparability with data in most literature on similar mesoporous materials.

[0028] The first color-changing silica gel component is responsible for indicating the low humidity level. The peak position of the pore size distribution of carrier A is set within the aforementioned relatively small range, corresponding to a lower level of capillary condensation relative humidity. When the ambient humidity rises from a low level, carrier A is the first to absorb moisture through its pores. The first indicator is loaded within the pores of carrier A. The raw material for the first indicator is selected from cobalt chloride hexahydrate or bromophenol blue. When the raw material is cobalt chloride hexahydrate, its molecular formula is CoCl2·6H2O, and it is a pink crystal at room temperature. During the high-temperature drying stage of the preparation process, cobalt chloride hexahydrate loses its water of crystallization to form anhydrous cobalt chloride, with the molecular formula CoCl2, which is dark blue, causing the first color-changing silica gel component to appear blue overall in the dry state. There is a reversible hydration equilibrium between anhydrous cobalt chloride and water molecules, with the reaction formula CoCl2 + 6H2O ⇌ CoCl2·6H2O. The forward reaction changes the product from a blue anhydrous salt to a pink hexahydrate salt, and the reverse reaction can be achieved by further drying. When capillary condensation occurs in the pores of carrier A, the liquid water appearing within the pores comes into contact with anhydrous cobalt chloride and gradually hydrates, causing the first color-changing silica gel component to change from blue to pink. When the raw material is bromophenol blue, bromophenol blue is a sulfonyl acid-base indicator with the molecular formula C0. 19 H 10 Br4O5S, when dry, is yellowish-brown. After hydration or dissolution, it turns yellow or purple depending on the pH of the medium. This provides a low-humidity color development scheme that does not contain cobalt, which can be used as an alternative in applications where the user has restrictions on the amount of cobalt.

[0029] The second color-changing silica gel component is responsible for indicating the medium humidity level. The peak position of the pore size distribution of carrier B is set in the medium range, corresponding to a median capillary condensation relative humidity. Methyl orange is loaded within the pores of carrier B. Methyl orange is an azo acid-base indicator; its sodium salt form has the molecular formula C2. 14 H 14 N3NaO3S, pK a The value is approximately 3.47; the dry powder of methyl orange is reddish-orange, and when dissolved in water, it turns red when the pH is below 3.1, yellow when the pH is above 4.4, and orange in the transition zone. In the second color-changing silica gel component, citric acid can be simultaneously loaded into the B channel of the carrier as a pH buffer component. Citric acid has the molecular formula C6H8O7 and is a weak organic acid with tertiary dissociation pK aThe values ​​are approximately 3.13, 4.76, and 6.40, respectively, which can provide a relatively stable weakly acidic local environment in the hydrated state. In the dry state, methyl orange exists as a red-orange solid in the pores of carrier B; when capillary condensation occurs in the pores of carrier B at the medium humidity threshold, liquid water appears in the pores, methyl orange dissolves and coexists with the citric acid buffer, and the color changes from the solid phase red-orange to the color exhibited by the dissolved state at the buffer pH, and the entire component undergoes a significant hue change.

[0030] The third color-changing silica gel component is responsible for indicating the high humidity level. The peak position of the pore size distribution of carrier C is set within a relatively large range, corresponding to a higher level of capillary condensation relative humidity. Bromothymol blue is loaded within the pores of carrier C. Bromothymol blue is a sulfonylurea acid-base indicator with the molecular formula C0. 27 H 28 Br2O5S, pK a The pH value is approximately 7.0; the dry powder of bromothymol blue is yellowish-brown to yellow, and when dissolved in water, it is yellow when the pH is below 6.0, blue when the pH is above 7.6, and green in the transition zone. In the third color-changing silica gel component, disodium hydrogen phosphate can be simultaneously loaded into the pores of carrier C as a pH buffer component. Disodium hydrogen phosphate has the molecular formula Na2HPO4 and forms a buffer pair with sodium dihydrogen phosphate, providing a local environment in the neutral to slightly alkaline range in the hydrated state. When capillary condensation occurs in the pores of carrier C at a high humidity threshold, liquid water appears in the pores, bromothymol blue dissolves and coexists with the disodium hydrogen phosphate buffer, turning blue or blue-green at a slightly alkaline pH.

[0031] The resulting color-changing silica gel grading indicator, obtained by mechanically mixing the three color-changing silica gel components at a set mass ratio, exhibits a continuous hue transition between blue, purple, orange, and green as the relative humidity gradually increases from a low level. Specifically, the blue color is dominated by the dry state of the cobalt-based indicator in carrier A; when the humidity rises above the threshold of carrier A, the cobalt-based component changes from blue to pink, and the overall color shifts from blue to purplish-red; when the humidity continues to rise above the threshold of carrier B, the methyl orange in carrier B changes from a solid-phase red-orange to a dissolved state, resulting in a transition from red to orange and then to a yellowish hue; when the humidity continues to rise above the threshold of carrier C, the bromothymol blue in carrier C changes from yellow to blue, superimposing with the already color-changing methyl orange-yellow, resulting in an overall greenish hue. A stable and monotonous correspondence is formed between the hue and the relative humidity, allowing users to directly read the humidity level based on the hue without relying on electronic sensors. The hue-humidity correspondence can be quantitatively modeled in the CIELAB color space. The resulting hue-humidity curve maintains good repeatability after multiple moisture absorption and desorption cycles, supporting the use of the indicator in indicative packaging, cultural relic storage, moisture protection for electronic devices, and other applications.

[0032] To further reduce crosstalk between the three components caused by condensation on the outer surface of the particles and to improve overall storage and transportation stability, the outer surface of the obtained color-changing silica gel grading indicator particles can be subjected to hydrophobic passivation treatment. The treatment agent is an anhydrous toluene solution of dimethyldichlorosilane. Dimethyldichlorosilane has the molecular formula (CH3)2SiCl2, which undergoes a silanization reaction with the silanol groups on the outer surface of the silica gel particles to generate a hydrophobic linkage structure of the type ≡Si-O-Si(CH3)2-O-Si≡ and release hydrogen chloride. The reaction formula is (CH3)2SiCl2 + 2≡Si-OH → ≡Si-O-Si(CH3)2-O-Si≡ + 2HCl. After short-term mild passivation, a hydrophobic protective layer is formed on the outer surface of the particles, which can reduce non-pore-controlled moisture absorption on the outer surface of the particles. The passivation treatment time and temperature need to be controlled within an appropriate range to avoid the reaction penetrating into the pores and blocking the pore openings, so as to retain the hydrophilicity and capillary coagulation characteristics inside the pores.

[0033] The preparation method of the color-changing silica gel grading indicator is described in detail below. (Refer to...) Figure 5 The process includes steps S1, S2, S3, S4, and S5, and may further include step S6 after step S5. First, in step S1, initial fine-pore silica particles from the same batch are subjected to differentiated pore expansion under three different hydrothermal conditions to obtain carriers A, B, and C whose pore size distribution peak positions fall within three target ranges, respectively. Then, in steps S2, S3, and S4, the three carriers are impregnated and dried in equal volumes, introducing three indicators into the pores of the corresponding carriers to obtain a first color-changing silica component, a second color-changing silica component, and a third color-changing silica component. Next, in step S5, the three components are mechanically mixed uniformly under a dry environment to obtain the color-changing silica graded indicator. Finally, in step S6, the outer surface of the obtained indicator particles undergoes a surface hydrophobic passivation treatment.

[0034] Step S1 involves differentiated hydrothermal pore-expansion treatment of the initial fine-pored silica particles, consisting of sub-steps S11, S12, S13, S14, and S15 sequentially. In sub-step S11, type A fine-pored spherical silica particles with a pore size of 2 mm to 4 mm and a peak position in the pore size distribution determined by the aforementioned BJH method are selected as the initial fine-pored silica particles. Type A silica is a fine-pored silica with a small initial pore size, facilitating the subsequent hydrothermal pore-expansion steps to obtain three target pore sizes. This initial raw material is divided into three equal parts by mass, designated as precursor A, precursor B, and precursor C, respectively, for use in the subsequent preparation of carrier A, carrier B, and carrier C. The three raw materials are from the same source and batch, effectively reducing the impact of initial composition differences on the resulting pore size distribution. In sub-steps S12, S13, and S14, precursors A, B, and C are subjected to hydrothermal pore-expansion treatment, respectively. A systematic gradient exists among the three hydrothermal conditions, progressively increasing in sodium carbonate concentration, hydrothermal temperature, and hydrothermal time. The physicochemical basis of hydrothermal pore expansion is the Oswald ripening mechanism. The silica skeleton partially dissolves in a weakly alkaline sodium carbonate aqueous solution to form soluble silicate ions. Dissolution preferentially occurs at small particle sites with greater curvature and weak pore wall sites. Driven by the concentration gradient, the dissolved silicate ions migrate and redeposit at sites with less curvature, resulting in increased pore size, thicker pore walls, and improved skeleton stability. The more stringent the hydrothermal conditions, the deeper the ripening process, leading to larger pore size peaks. After hydrothermal treatment, the particles are cooled and removed, and repeatedly washed with deionized water until the pH of the washing solution falls into the neutral range and the conductivity is no higher than the lower threshold of 10 μS / cm, indicating that sodium carbonate and sodium ion residues have been effectively removed. Excessive residual carbonate may continue to interfere with the color of acid-base indicators in subsequent applications. The washed particles are dried at a relatively high temperature of around 105°C to obtain precursors A, B, and C. In sub-step S15, the three precursors are respectively kept at a moderate temperature of around 350°C for a suitable time under an inert atmosphere, and then naturally cooled to room temperature before being removed. The purpose of calcination is to stabilize the porous framework obtained through hydrothermal pore expansion and remove residual moisture. The calcination temperature needs to be higher than the dehydration temperature of the hydrated sodium silicate mesophase generated during hydrothermal treatment, but lower than the temperature at which silica gel undergoes significant sintering and pore size collapse; therefore, a moderate temperature range is preferred. The solids obtained after calcination are carriers A, B, and C, and their pore size distribution peak positions should fall within the three target ranges, respectively.

[0035] Step S2 involves impregnating carrier A with an equal volume of an aqueous solution of cobalt chloride hexahydrate as impregnation solution A and then drying it. This step is comprised of sub-steps S21, S22, and S23 sequentially. In sub-step S21, cobalt chloride hexahydrate and deionized water are mixed at a specific mass ratio and stirred at room temperature until dissolved, yielding a clear impregnation solution A. Cobalt chloride hexahydrate has high solubility in water at room temperature, and the selected mass ratio is far below its saturation value, ensuring that impregnation solution A does not crystallize at room temperature. In sub-step S22, according to the formula... Determine the amount of impregnation solution A to be weighed in the formula. The volume (mL) of impregnation solution A is taken. This is the impregnation allowance coefficient, with a dimension of 1; The total pore volume of carrier A is (mL / g). Let be the mass (g) of carrier A. This formula reflects the basic principle of the equal-volume impregnation method, namely, that the volume of the impregnation liquid is approximately equal to the total pore volume that the carrier can absorb, while introducing an impregnation allowance coefficient slightly greater than 1. As a process allowance; when When the value is too large, excess liquid will remain on the outer surface of the particles, forming a free liquid film. When the value is less than 1, some channels cannot be fully wetted by the impregnating liquid, therefore Taking values ​​within a small range slightly greater than 1 ensures that the impregnation liquid is fully immersed in the pores while there is no obvious free liquid on the outer surface of the particles. The equal-volume impregnation method is a recognized method for preparing supported materials and has a long and widespread application in the field of heterogeneous catalysis. Support A is added to the weighed impregnation liquid A and allowed to stand at room temperature and pressure for a suitable impregnation time, gently stirring at regular intervals until there is no obvious free liquid on the surface of the support A particles. The entire impregnation process relies on capillary force to ensure that the impregnation liquid is mainly distributed within the pores. In sub-step S23, support A, after impregnation in sub-step S22, is placed in a vacuum drying oven and first slowly dried for a suitable time at a lower temperature and negative pressure (around 40°C). Then, the temperature is raised to a higher temperature (around 105°C) and switched to atmospheric pressure to continue drying for a suitable time, yielding the first color-changing silica gel component. The purpose of low-temperature, slow-speed vacuum drying is to allow water in the impregnation solution to migrate to the particle surface at a lower evaporation rate, avoiding surface enrichment caused by rapid evaporation carrying solutes to the outer surface of the particles. A second-stage drying process, involving raising the temperature to a higher level and switching to atmospheric pressure, further removes residual water from the pores. During the drying process, the hydration number of cobalt chloride hexahydrate decreases, resulting in anhydrous or low-hydration cobalt chloride distributed within the pores of carrier A. In sub-step S21, the mass ratio of cobalt chloride hexahydrate to deionized water should be within a suitable range and appropriately adjusted so that the mass ratio of cobalt chloride hexahydrate to carrier A in the resulting first color-changing silica gel component falls within the range defined in the main claim.

[0036] Step S3 involves impregnating carrier B with an equal volume of an ethanol-water solution containing methyl orange as impregnation solution B and then drying it. This step consists of sub-steps S31, S32, and S33 sequentially. In sub-step S31, methyl orange, anhydrous ethanol, and deionized water are mixed in a certain mass ratio and stirred in a gently heated water bath until dissolved. After naturally cooling to room temperature, the mixture is filtered through a 0.45 μm filter membrane to remove any insoluble impurities, resulting in a clear impregnation solution B. Introducing anhydrous ethanol as a solubilizing agent into the preparation of impregnation solution B improves the solubility of methyl orange at room temperature, reduces the saturation concentration limit, and adjusts the wettability of the impregnation solution on carrier B. Simultaneously, the surface tension of the ethanol-water mixture is lower than that of pure water, which helps the impregnation solution to spread fully within the pores of the medium-pore carrier B. If a citric acid buffer component is included, citric acid can be dissolved together with methyl orange in the ethanol-water solution in this sub-step and introduced into the pores of carrier B in a predetermined proportion as a pH buffer component in the hydrated state. In sub-step S32, according to the formula Determine the amount of impregnation solution B to be weighed, in the formula. The volume (mL) of impregnation solution B is taken. This is the impregnation allowance coefficient, dimensionless, and is used in sub-step S22. Same meaning; The total pore volume of carrier B (mL / g); The weighed mass (g) of carrier B is given. Carrier B is added to the weighed impregnation solution B and allowed to stand at room temperature and pressure for a suitable impregnation time, gently stirring at regular intervals until there is no obvious free liquid on the surface of the carrier B particles. In sub-step S33, the carrier B impregnated in sub-step S32 is placed in a forced-air drying oven. It is first dried at a lower temperature of around 60°C to allow the ethanol molecules in the solvent to evaporate predominantly, and then the temperature is raised to a higher temperature of around 105°C to continue drying to remove residual moisture, obtaining the second color-changing silica gel component. The forced-air drying oven maintains strong airflow exchange, which helps to promptly remove ethanol vapor and avoids ethanol vapor accumulation and reflux in the system. In sub-step S31, the mass ratio of the methyl orange, the anhydrous ethanol, and the deionized water should be within a suitable range and appropriately adjusted so that the mass ratio of methyl orange to carrier B in the obtained second color-changing silica gel component falls within the range defined in the main claim.

[0037] Step S4 involves impregnating the carrier C with an equal volume of an ethanol-water solution containing bromothymol blue as the impregnation solution C and then drying it. This step is comprised of sub-steps S41, S42, and S43 sequentially. In sub-step S41, bromothymol blue, anhydrous ethanol, and deionized water are mixed in a specific mass ratio and stirred in a gentle water bath until dissolved. The mixture is then filtered through a 0.45 μm filter membrane to obtain a clear impregnation solution C. Bromothymol blue has low solubility in pure water, with a saturation concentration at room temperature on the order of 0.1% by mass. Therefore, the ethanol content in impregnation solution C is significantly higher than that in impregnation solution B, allowing the bromothymol blue to reach the concentration required for the target loading. If a disodium hydrogen phosphate buffer component is included, disodium hydrogen phosphate can be dissolved together with the bromothymol blue in this sub-step and introduced into the pores of the carrier C along with the impregnation solution in a predetermined proportion, serving as a pH buffer component in the hydrated state. In sub-step S42, according to the formula... Determine the amount of impregnation solution C to be weighed in the formula. The volume (mL) of impregnation solution C is taken. This is the impregnation allowance coefficient, dimensionless, used in sub-steps S22 and S32. Same meaning; The total pore volume of carrier C (mL / g); The mass (g) of carrier C is measured. Carrier C is added to the weighed impregnation solution C and allowed to stand at room temperature and pressure for a suitable impregnation time, gently stirring at regular intervals until there is no obvious free liquid on the surface of the carrier C particles. In sub-step S43, the carrier C impregnated in sub-step S42 is placed in a vacuum drying oven and dried in three stages. The first stage involves slow drying at a lower temperature (around 50°C) and negative pressure for a suitable time to allow the ethanol to evaporate slowly. The second stage involves switching to atmospheric pressure and raising the temperature to a moderate temperature (around 80°C) for a suitable time to further remove moisture. The third stage involves raising the temperature to a higher temperature (around 105°C) for a suitable time to remove residual organic solvents and moisture, yielding the third color-changing silica gel component. The purpose of the three-stage drying process is that bromothymol blue has a large molecular weight and tends to crystallize and precipitate under low solvent conditions. Reducing the total solvent volume in stages and controlling the drying rate at each stage can prevent rapid crystallization and migration of the solute in the later stages of drying. If a single high-temperature rapid drying method is used, bromothymol blue molecules may flow with the solvent vapor to the outer surface of the particles, causing bromothymol blue to accumulate on the outer surface of the particles rather than be uniformly distributed inside the pores, reducing the color contrast of the indicator after humidity triggering. In sub-step S41, the mass ratio of the bromothymol blue, the anhydrous ethanol, and the deionized water should be within a suitable range and appropriately adjusted so that the mass ratio of the bromothymol blue in the resulting third color-changing silica gel component to the carrier C falls within the range defined in the main claim.

[0038] Step S5 involves pre-drying and mechanically mixing the three obtained color-changing silica gel components, and consists of sub-steps S51 and S52 sequentially. In sub-step S51, the obtained first, second, and third color-changing silica gel components are placed in a vacuum desiccator equipped with 3A molecular sieve desiccant and allowed to stand for an appropriate time at a temperature slightly above room temperature, so that the water content of each component, as determined by the 105°C loss-in-weight method, drops to a low level of less than 2% by mass, resulting in pre-dried first, second, and third color-changing silica gel components. 3A molecular sieve has high adsorption selectivity for water molecules, which maintains a low water vapor partial pressure in the air inside the desiccator, promoting the slow release of residual water from each component. The specific operation of the loss-in-weight method involves weighing a quantitative sample into a flat weighing bottle, drying it to constant weight in an oven at 105°C, and directly calculating the total water content of the sample from the weight loss ratio. Pre-drying each component to a low moisture content avoids color fluctuations caused by residual moisture migration between the three components during subsequent mixing. Especially when temperature changes occur, moisture migration may reach the local capillary condensation threshold in a particular component, triggering color changes and affecting the initial appearance consistency of the finished product. In sub-step S52, the three components pre-dried in sub-step S51 are weighed according to a set mass ratio and placed in a closed mixer with controlled relative humidity. The mixer is turned on and mixed at an appropriate speed for a suitable time to obtain the color-changing silica gel grading indicator. Controlling the relative humidity of the mixing environment to a low level below 10% prevents localized condensation of any component due to ambient humidity during mixing. The mixer's flipping operation allows for sufficient staggered doping of the three components in three dimensions, avoiding particle wear and pore structure damage that may occur with high-speed stirring.

[0039] The present invention further includes step S6 after step S5, which involves surface hydrophobication and passivation treatment of the outer surface of the obtained color-changing silica gel grading indicator particles, consisting of sub-steps S61, S62, and S63 in sequence. In sub-step S61, dimethyldichlorosilane and anhydrous toluene are mixed at a certain mass ratio to prepare a hydrophobic treatment solution. Dimethyldichlorosilane undergoes hydrolysis upon contact with water, releasing hydrogen chloride and generating silanol and its oligomers. Therefore, sub-step S61 and the remaining sub-steps of sub-step S6 should be carried out in a dry environment with a water content of less than 50 ppm. Common practice is to operate in a drying glove box or in a closed reactor with a dry inert gas, using anhydrous toluene that has been dehydrated by molecular sieves. In sub-step S62, the obtained color-changing silica gel grading indicator is added to the prepared hydrophobic treatment solution, the relative amount of particles to the treatment solution is controlled according to a set mass ratio, and the mixture is left to stand and impregnate for a suitable time at a temperature slightly above room temperature without stirring. The arrangement of standing without stirring is because stirring may accelerate the penetration of the liquid phase into the interior of the particles and extend the hydrophobic reaction into the pore area, blocking the pores and impairing the humidity selectivity of capillary condensation. By limiting the treatment time and temperature and adopting a standing method, the hydrophobic reaction mainly occurs at the relatively exposed silanol sites on the outer surface of the particles, while the silanol sites inside the pores largely retain their original hydrophilic state. In sub-step S63, the particles treated in sub-step S62 are taken out, washed several times with anhydrous toluene to remove unreacted dimethyldichlorosilane and hydrogen chloride released by the reaction, and then placed in a vacuum drying oven at a relatively high temperature of around 105°C with a dry nitrogen flow for a suitable time to remove residual dimethyldichlorosilane, toluene, and hydrogen chloride. After naturally cooling to room temperature, the color-changing silica gel grading indicator with surface hydrophobic passivation treatment is obtained. The process of washing with anhydrous toluene followed by vacuum drying with a nitrogen stream is more effective than vacuum drying alone in removing highly polar hydrogen chloride molecules from the system, thus preventing residual hydrogen chloride on the particles from interfering with the color of acid-base indicators such as methyl orange and bromothymol blue.

[0040] The color-changing silica gel grading indicator prepared through steps S1 to S5 and the optional step S6, compared to traditional silica gel indicators obtained by impregnating a single commercial silica gel with multiple indicators, features three indicators loaded into the pores of three carriers with different peak positions of pore size distribution. The three carriers correspond to three relative humidity thresholds, allowing the indicator's hue to exhibit a continuous and monotonous transition as the ambient relative humidity increases, enabling more detailed grading of humidity levels. Since the three indicators are located in independent carrier particles and do not directly contact each other, the potential interference between acid-base indicators and cobalt-based indicators within the same particle is avoided. Especially after further surface hydrophobic passivation treatment of the particle's outer surface, non-pore-controlled moisture absorption is significantly reduced, further lowering the risk of crosstalk. The raw materials used in the entire preparation process are all bulk chemical reagents, and the relevant testing methods are all supported by current national or industry standards. The process mainly involves mature unit operations such as hydrothermal treatment, equal-volume impregnation, hot air and vacuum drying, and surface silanization. The method has strong repeatability and scalability. The resulting color-changing silica gel grading indicator can be placed together with the items to be protected from moisture, and the relative humidity level can be read directly based on the appearance color. No additional power supply or electronic sensing module is required, making it suitable for applications that are sensitive to transportation and storage costs.

[0041] Example 1: Steps S1 to S6 of this example are performed as follows. Step S1 includes steps S11, S12, S13, S14, and S15, where the initial fine-pored silica particles are subjected to differentiated hydrothermal pore expansion to obtain three types of carriers. In sub-step S11, 300g of type A fine-pored spherical silica particles with a peak position of 2.50nm in the desorption branch BJH pore size distribution and a particle size of 3.0mm are taken as the initial fine-pored silica particles and divided into three equal parts of 100g each, labeled as precursor A, precursor B, and precursor C, respectively. In sub-step S12, precursor A is placed in 600g of a 0.030mol / L sodium carbonate aqueous solution and hydrothermally treated at 90℃ for 1.5h. After cooling, it is repeatedly washed with prepared deionized water until the pH of the washing solution is 7.0±0.3 and the conductivity is not higher than 8μS / cm. It is then dried at 105℃ for 4h to obtain carrier precursor A. In sub-step S13, precursor B is placed in 600g of a 0.060mol / L sodium carbonate aqueous solution and hydrothermally treated at 110℃ for 2.5h. Washing and drying are performed as in sub-step S12 to obtain precursor B. In sub-step S14, precursor C is placed in 600g of a 0.120mol / L sodium carbonate aqueous solution and hydrothermally treated at 130℃ for 3.5h. Washing and drying are performed as in sub-step S12 to obtain precursor C. In sub-step S15, the three precursors are calcined at 350℃ for 2h under nitrogen protection, and then naturally cooled to room temperature to obtain supports A, B, and C, respectively. Step S2 consists of sub-steps S21, S22, and S23 sequentially, and is used to prepare the first color-changing silica gel component. In sub-step S21, 12g of cobalt chloride hexahydrate is dissolved in 100g of deionized water and stirred at room temperature until dissolved to obtain a clear impregnation solution A. In sub-step S22, 100g of carrier A is mixed with the following formula: The calculated 42 mL impregnation solution A was mixed and allowed to stand for 1 hour at room temperature and pressure, with gentle stirring every 15 minutes until there was no obvious free liquid on the surface of the carrier A particles. In sub-step S23, the impregnated carrier A was placed in a vacuum drying oven and first slowly dried at 40 °C and gauge pressure of -0.080 MPa for 4 hours, then the temperature was raised to 105 °C and switched to atmospheric pressure to continue drying for 2 hours, to obtain the first color-changing silica gel component.

[0042] Step S3 consists of sub-steps S31, S32, and S33, and is used to prepare the second color-changing silica gel component. In sub-step S31, 1.5 g of methyl orange, 0.10 g of citric acid, 20 g of anhydrous ethanol, and 78.4 g of deionized water are mixed and stirred in a 50°C water bath until dissolved. After natural cooling, the mixture is filtered through a 0.45 μm filter membrane to obtain a clear impregnation solution B. In sub-step S32, 100 g of carrier B is mixed with... The calculated 65 mL impregnation solution B was mixed, and the impregnation method was the same as in sub-step S22. In sub-step S33, the impregnated carrier B was placed in a forced-air drying oven and dried at 60°C for 3 h, and then the temperature was raised to 105°C and dried for another 2 h to obtain the second color-changing silica gel component.

[0043] Step S4 consists of sub-steps S41, S42, and S43, and is used to prepare the third color-changing silica gel component. In sub-step S41, 1.5 g of bromothymol blue, 0.20 g of disodium hydrogen phosphate, 65 g of anhydrous ethanol, and 33.3 g of deionized water are mixed and stirred in a 30°C water bath until dissolved. The solution is then filtered through a 0.45 μm filter membrane to obtain a clear impregnation solution C. In sub-step S42, 100 g of carrier C is mixed with... The calculated 95 mL impregnation solution C was mixed, and the impregnation method was the same as in sub-step S22. In sub-step S43, the impregnated carrier C was placed in a vacuum drying oven and dried in three stages: first, it was slowly dried at 50 °C and gauge pressure of -0.070 MPa for 3 h; then, it was dried at atmospheric pressure and 80 °C for 2 h; and finally, it was dried at atmospheric pressure and 105 °C for 1 h to obtain the third color-changing silica gel component.

[0044] Step S5, consisting of sub-steps S51 and S52, is used to pre-dry and mechanically mix the three color-changing silica gel components. In sub-step S51, the three color-changing silica gel components are placed in a vacuum dryer equipped with a 3A molecular sieve and allowed to stand at 28°C for 48 hours. The water content of each component is found to be less than 1.5% by mass, as determined by the loss-in-weight method at 105°C. In sub-step S52, the three pre-dried components are weighed in a mass ratio of 35:35:30 and placed in a closed mixer with a relative humidity of less than 8%. The mixture is then turned and mixed at 20 r / min for 15 min to obtain 100 g of the color-changing silica gel grading indicator of Example 1. Step S6, consisting of sub-steps S61, S62, and S63, is used to hydrophobically passivate the outer surface of the obtained color-changing silica gel grading indicator particles. In sub-step S61, 0.30 g of dimethyldichlorosilane and 30 g of anhydrous toluene are mixed in a drying glove box to prepare a hydrophobic treatment solution. In sub-step S62, the above-mentioned color-changing silica gel grading indicator is added to the treatment solution and allowed to stand and soak at 28°C for 30 minutes without stirring. In sub-step S63, the particles are removed, washed twice with anhydrous toluene, placed in a vacuum drying oven at 105°C and dried with a stream of dry nitrogen for 4 hours, and then allowed to cool naturally to room temperature to obtain the final product of Example 1.

[0045] Example 2: In this example, the target BJH peak values ​​of the desorption branches of carriers A, B, and C are 2.85 nm, 4.65 nm, and 10.20 nm, respectively. The hydrothermal conditions in sub-step S12 are 86 °C, 0.025 mol / L, and 1.2 h; in sub-step S13, they are 106 °C, 0.055 mol / L, and 2.2 h; and in sub-step S14, they are 126 °C, 0.105 mol / L, and 3.2 h. In sub-step S21, the impregnation solution A is prepared in a ratio of 8:100; in sub-step S31, the mass ratio of methyl orange, anhydrous ethanol, and deionized water is 0.7:18:80, and citric acid is 0.05 parts; in sub-step S41, the mass ratio of bromothymol blue, anhydrous ethanol, and deionized water is 0.9:62:32, and disodium hydrogen phosphate is 0.10 parts. The mass ratio of the three components is 28:30:24. The remaining steps are the same as in Example 1.

[0046] Example 3: In this example, the target BJH peak values ​​of the desorption branches of carriers A, B, and C are 3.15 nm, 5.40 nm, and 11.80 nm, respectively. The hydrothermal conditions in sub-step S12 are 91 °C, 0.038 mol / L, and 1.8 h; in sub-step S13, they are 114 °C, 0.075 mol / L, and 2.8 h; and in sub-step S14, they are 134 °C, 0.135 mol / L, and 3.8 h. In sub-step S21, the ratio of impregnation solution A is 17:100; in sub-step S31, the mass ratio of methyl orange, anhydrous ethanol, and deionized water is 2.5:22:78, and citric acid is 0.18 parts; in sub-step S41, the mass ratio of bromothymol blue, anhydrous ethanol, and deionized water is 2.6:68:36, and disodium hydrogen phosphate is 0.28 parts. The mass ratio of the three components is 40:38:35. The remaining steps are the same as in Example 1.

[0047] Example 4: In sub-step S21 of this example, the impregnation solution A was changed to a mixture of 1.2g bromophenol blue, 20g anhydrous ethanol, and 79g deionized water, dissolved and filtered through a 0.45μm filter membrane. The parameters of the remaining steps are the same as in Example 1.

[0048] Comparative Example 1: This comparative example uses a single-peak commercial silica gel impregnation method, directly employing 300g of type A fine-pore spherical silica gel without hydrothermal pore-expansion treatment. 4.5g of cobalt chloride hexahydrate, 1.5g of methyl orange, 1.5g of bromothymol blue, 0.30g of citric acid, and 0.30g of disodium hydrogen phosphate were dissolved in a mixed solvent of 60g anhydrous ethanol and 230g deionized water to obtain a mixed impregnation solution. This mixed impregnation solution was then immersed in the silica gel in a single pass and dried in a vacuum drying oven at 105℃ for 4 hours to obtain the product of Comparative Example 1; the remaining surface hydrophobicity and passivation treatments were performed in step S6 of Example 1.

[0049] Comparative Example 2: This comparative example uses a three-mode pore size but single-batch mixed impregnation method. The preparation of carriers A, B, and C is the same as sub-steps S11 to S15 of Example 1. The three carriers are physically mixed in a mass ratio of 35:35:30, then impregnated in the same mixed impregnation solution as described in Comparative Example 1 in one step and dried; the surface hydrophobicity passivation treatment is the same as step S6 of Example 1.

[0050] In Comparative Example 3, the amount of citric acid in step S31 was changed to 0; the amount of disodium hydrogen phosphate in sub-step S41 was changed to 0; the parameters of the remaining steps were the same as in Example 1.

[0051] Comparative Example 4: This comparative example is a version without surface passivation and does not use all steps S6 sub-steps of Example 1; the parameters of the remaining steps are the same as those of Example 1.

[0052] In Comparative Example 5, the immersion time in sub-step S62 of this comparative example was extended from 30 min to 90 min; the parameters of the remaining steps were the same as in Example 1.

[0053] Comparative Example 6: This comparative example uses commercially available type A blue color-changing silica gel with a cobalt chloride hexahydrate content of approximately 8.0% by mass and a particle size of 2mm to 4mm, without any further treatment.

[0054] In Experiment 1, nitrogen adsorption-desorption isotherms were measured using the multi-point BET method and the desorption branch BJH method on the original type A silica gel, carriers A, B, and C of Examples 1 to 3, and the silica gel of Comparative Example 1. The instrument used was a nitrogen adsorption-desorption type specific surface area and pore size distribution analyzer, with liquid nitrogen temperature at 77 K and relative pressures ranging from 0.01 to 0.99. Samples were degassed under vacuum at 200 °C for 6 h before testing. Each sample was measured three times, and the arithmetic mean was taken. The obtained BET specific surface area, total pore volume measured by the single-point method at a relative pressure of 0.99, peak position of the desorption branch BJH pore size distribution, and half-peak width are listed in Table 1.

[0055] Table 1. Characterization results of BJH pore size distribution for different samples:

[0056] The physical property characterization corresponding to the data in Table 1 is as follows: Figure 1 As shown, Figure 1 In the middle (a), the superimposed comparison of the BJH desorption branch pore size distribution curves dV / dlog(d)-d for each sample is shown, and the three target ranges of the present invention are displayed in superimposed three light-colored backgrounds. Figure 1 (b) shows the mapping curve between pore size and relative humidity threshold calculated according to the Kelvin equation, and overlays the operating points and measured color change thresholds of the three carriers in Example 1. Figure 1As can be seen in (a), the original type A silica gel and the silica gel of Comparative Example 1 obtained by impregnation with it only exhibit a sharp pore size distribution peak around 2.50 nm; after differential hydrothermal pore expansion, the BJH peak positions of carriers A, B, and C in Examples 1 to 3 deviated significantly from the initial peak positions of the raw materials, falling into three target ranges: 2.8 nm to 3.2 nm, 4.5 nm to 5.5 nm, and 10 nm to 12 nm, respectively. The interval between the three peak positions showed a systematic slight shift from the lower limit to the upper limit as the parameter values ​​of the examples were taken. Figure 1 As can be seen in (b), the relative humidity threshold predicted by the Kelvin equation decreases monotonically as the effective pore radius of the Kelvin decreases; the theoretical values ​​and measured color change thresholds of the three carriers in Example 1 are consistent in a physical sense, and the three pore sizes correspond to the low, medium and high relative humidity thresholds, respectively.

[0057] From the above Figure 1 It can be seen that sub-steps S12 to S14 of this invention can repeatedly prepare carriers with three target pore sizes through differentiated hydrothermal conditions; the three pore sizes and the relative humidity threshold that triggers capillary condensation conform to the physical correspondence given by the Kelvin equation. This conclusion directly proves that the limitation of the pore size distribution peak positions of carriers A, B, and C described in this invention is feasible. The physicochemical basis of hydrothermal pore enlargement is the Oswald ripening mechanism. The silica skeleton partially dissolves in a weakly alkaline sodium carbonate aqueous solution to form soluble silicate ions. Dissolution preferentially occurs at sites with larger curvature. The dissolved silicate ions migrate under the drive of the concentration gradient and redeposit at sites with smaller curvature, resulting in an overall increase in pore size, thicker pore walls, and improved skeleton stability. The simultaneous increase in hydrothermal temperature, sodium carbonate concentration, and hydrothermal time makes the ripening process more in-depth, resulting in larger pore size peak positions, which forms a systematic process gradient between sub-steps S12, S13, and S14.

[0058] Experimental Example 2: In this experiment, the colors of the products from Examples 1 to 4 and Comparative Examples 1, 2, and 6 were measured in a controlled humidity chamber using a spectrophotometer with a D65 light source, a 10° field of view, and reflection mode, according to the CIELAB color space and the CIE76 ΔE color difference formula. Humidity was increased from 5% RH in 5% increments up to 95% RH, with each level stabilized for 6 hours before data collection. Five particles were measured for each sample at each level, and the arithmetic mean was taken. The ΔE value for each level was calculated using the color at 5% RH as the baseline. Table 2 shows the key statistical indicators.

[0059] Table 2. Comparison of ΔE-RH response characteristics between the examples and the comparative examples:

[0060] The ΔE-RH response curves corresponding to the data in Table 2 are summarized as follows: Figure 2As shown, the horizontal axis represents relative humidity, and the vertical axis represents ΔE. From Figure 2 It can be seen that the curves of Examples 1 to 4 all exhibit a three-stage stepped upward shape, with the three threshold values ​​consistent with Table 2. Furthermore, the magnitude of each step's rise is similar, resulting in a regular, stepped gradation of the overall curve. Comparative Examples 1 and 6 both show a single, sharp rise only between RH 30% and 40%, after which they essentially remain flat in the mid-to-high RH region. Comparative Example 2, while exhibiting three segments near RH 30%, 55%, and 80%, shows a significantly reduced step difference in each segment, making the entire curve tend to be flattened.

[0061] The cumulative ΔE of Examples 1 to 4 is approximately twice that of Comparative Examples 1 and 6, indicating that the three-stage pore size selective capillary condensation response scheme of the present invention exhibits high color change contrast at the product level, corresponding to capillary condensation color development occurring sequentially at three different relative humidity thresholds for carriers A, B, and C. The ΔE step difference of Examples 1 to 4 is approximately twice that of Comparative Example 2, indicating that the process of independently impregnating the three indicators in steps S2 to S4 of the present invention and then mixing them effectively avoids the mutual interference between acid-base type indicators and cobalt-based indicators. The mixed impregnation method of Comparative Example 2 allows the three indicators to simultaneously enter the pores of all carriers. Cobalt ions, as Lewis acids, perturb the azo or sulfophthalein electron distribution of methyl orange and bromothymol blue, and change the ionization balance of the indicators through hydrogen bonding and electrostatic interactions, resulting in a significant decrease in the color change contrast of each stage. The present invention isolates the three indicators in independent carrier particles, with no liquid phase channels between the particles, thus avoiding the above-mentioned interference. The curve of Example 4 starts at a different point in the low RH region than that of Example 1, but the three thresholds and the cumulative ΔE are on the same order of magnitude. This indicates that the first indicator of the present invention, selected from either cobalt chloride hexahydrate or bromophenol blue, can achieve the color development function in the low humidity range, providing a cobalt-free alternative for application scenarios with restrictions on cobalt.

[0062] Experimental Example 3: Under the same humidity chamber conditions described in Experimental Example 2, the values ​​of L, a, and b for Examples 1, 2, 3, 4, and Comparative Example 1 at various RH levels from 5%RH to 95%RH were recorded. * Data, by The hue angle was calculated, and the monotonic correlation between the hue angle and relative humidity was calculated using the Spearman rank correlation coefficient. The hue continuity and hue angle coverage range for each sample are listed in Table 3.

[0063] Table 3. Hue-humidity correspondence characteristics of the examples and comparative examples:

[0064] Figure 3In (a), the chromaticity trajectories of Example 1, Example 4 and Comparative Example 1 are drawn on the two-dimensional plane ab, and the values ​​of several RH levels are marked on the trajectory. Figure 3 Figure (b) shows the curves of hue angle versus relative humidity for Examples 1, 2, 3, and Comparative Example 1. Figure 3 It can be seen that, Figure 3 In (a), the trajectory of Example 1 is determined by a. * Negative and b * Starting in the negative blue quadrant, it transitions sequentially through the purple-red and orange-yellow quadrants to a. * Negative and b * The trajectory, biased towards the blue-green quadrant, forms a nearly closed loop around the origin, covering most quadrants of the ab plane; the trajectory in Example 4 is formed by a... * Partial and b * Starting in the tan quadrant, the trajectory follows a similar shape but from a different starting quadrant; the trajectory in Comparative Example 1 shows displacement only in the low to medium RH region, slows down upon entering the medium RH region, and even shows a slight regression upon entering the high RH region, failing to form a monotonically extending path in the ab plane. Figure 3 In (b), the hue angle curves of Examples 1 to 3 all transitioned monotonically with relative humidity, exhibiting high Spearman correlation coefficients; in Comparative Example 1, the curve changed from rising to falling after RH exceeded 60%, showing a significantly lower Spearman correlation coefficient. Figure 3 As can be seen, the overall product obtained by this invention exhibits a continuous transition of hue along a monotonous path as the relative humidity increases from low to high. Users can read the humidity level based on the appearance hue. Mechanistically, the color changes of the three indicators are concentrated at three different relative humidity thresholds. The color changes of the three levels overlap, causing the overall hue to slide sequentially through multiple hue ranges on the CIELAB color wheel, forming a monotonous correspondence between the hue angle and relative humidity. In contrast, Comparative Example 1, lacking a pore size gradient, has the three indicators located in particles with the same pore size. The mixed color development does not show a monotonous correspondence of relative humidity levels. Furthermore, in the high humidity area, the hydration reaction of the cobalt-based indicator is saturated, and the local pH of the acid-base indicator shifts due to the lack of a buffer microenvironment for leveling. As a result, the overall hue recedes in the high humidity area, further reducing the hue monotony.

[0065] In Experiment 4, the threshold sharpness was obtained by calculating the first-order numerical derivative dE / dRH with respect to relative humidity from the ΔE-RH curve measured in Experiment 2. A cyclic durability test procedure was performed under constant temperature and humidity conditions. The environmental chamber was switched between 40℃ and 85%RH and 25℃ and 25%RH conditions, with one complete cycle every 4 hours, for a total of 50 cycles. The final-state ΔE was measured every 10 cycles, and the ΔE retention rate was obtained by normalizing the ΔE from the first cycle. Response time. Defined as the time required for the product's ΔE to reach 90% of its steady-state final value after a step change in humidity from 25%RH to 85%RH, it is obtained from in-situ video recording and colorimeter sampling. Residual Cl - The content was determined by ion chromatography.

[0066] Table 4. Impact of surface passivation on key performance indicators:

[0067] Figure 4 Figure (a) shows the curves of dE / dRH as a function of relative humidity for Example 1, Comparative Example 4 and Comparative Example 5. The three curves correspond to the three process states of standard passivation, no passivation and over-passivation, respectively. Figure 4 Figure (b) presents line graphs of ΔE retention rates for Examples 1, 2, and 3, and Comparative Examples 1, 4, and 5 over 50 cycles. From... Figure 4 It can be seen that, Figure 4 In (a), the curve of Example 1 exhibits sharp peaks with a peak height of approximately 4.5ΔE per 1%RH near the three threshold levels, and the peak positions are consistent with the three threshold levels of Example 1 in Table 2; the three peak heights of Comparative Example 4 synchronously decrease to approximately 2.6ΔE ​​per 1%RH, and the three peak positions generally shift slightly forward towards lower RH, with a broadened peak shape; the three peak heights of Comparative Example 5 synchronously decrease to approximately 3.4ΔE per 1%RH, and the three peak positions generally shift slightly backward towards higher RH. Figure 4 In (b), the ΔE retention rates of Examples 1 to 3 after 50 cycles were all above 90%; Comparative Example 4 decreased to 80%; Comparative Example 5 decreased to 75%; and Comparative Example 1 decreased to 84%. From the above... Figure 4 It can be seen that the hydrophobic passivation treatment of the particle outer surface described in step S6 of the present invention can simultaneously improve threshold sharpness and cycle durability within a suitable process window. Mechanistically, in Comparative Example 4, without passivation, the silanol groups on the particle outer surface directly adsorb water molecules from the environment. When the relative humidity has not yet reached the capillary condensation threshold, non-pore-controlled moisture absorption has already occurred on the particle outer surface, causing the threshold leading edge to rise and the threshold sharpness to decrease. Simultaneously, during repeated moisture absorption and desorption cycles, the particle outer surface drives indicator molecules to migrate and accumulate on the particle outer surface, leading to a decrease in cycle durability. In Comparative Example 5, with excessive passivation, the hydrophobic reaction penetrates into the pore opening region, causing some pore openings to be blocked by dimethylsilyl groups. Water molecules need to overcome higher diffusion resistance to enter the pore interior, resulting in an upward shift in the trigger humidity for pore-selective capillary condensation, a prolonged response time, and a simultaneous decrease in the threshold amplitude. Step S6 of this invention controls the hydrophobication reaction at the exposed silanol sites on the outer surface of the particles by limiting the time and temperature of the passivation treatment and by using a static, non-stirring method. This not only shields the non-porous controlled hygroscopicity of the outer surface of the particles but also retains the hydrophilicity inside the pores, thereby improving the threshold sharpness and cycle durability.

[0068] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A color-changing silica gel grading indicator, characterized in that, It includes a first color-changing silica gel component, a second color-changing silica gel component, and a third color-changing silica gel component in a mass ratio of (28-42):(28-42):(22-38); wherein: The first color-changing silica gel component includes a carrier A and a first indicator loaded within the pores of the carrier A. The raw material for the first indicator is selected from cobalt chloride hexahydrate or bromophenol blue. Based on 100 parts by weight of the carrier A, the amount of cobalt chloride hexahydrate is 3-6 parts or the amount of bromophenol blue is 0.8-1.5 parts. The second color-changing silica gel component includes a carrier B and methyl orange loaded within the pores of the carrier B. Based on 100 parts by weight of the carrier B, the amount of methyl orange is 0.5-1.5 parts. The third color-changing silica gel component includes a carrier C and bromothymol blue loaded within the pores of the carrier C. Based on 100 parts by weight of the carrier C, the amount of bromothymol blue is 0.8-2.0 parts.

2. The color-changing silica gel grading indicator according to claim 1, characterized in that, The peak positions of the pore size distributions of carriers A, B, and C, calculated using the nitrogen adsorption-desorption isotherm desorption branch BJH method, are located in the ranges of 2.8-3.2 nm, 4.5-5.5 nm, and 10-12 nm, respectively.

3. The color-changing silica gel grading indicator according to claim 1, characterized in that, The second color-changing silica gel component further includes citric acid. Based on 100 parts by mass of carrier B, the amount of citric acid is 0.05-0.20 parts. The citric acid and methyl orange are co-loaded within the pores of carrier B. The third color-changing silica gel component further includes disodium hydrogen phosphate. Based on 100 parts by mass of carrier C, the amount of disodium hydrogen phosphate is 0.10-0.30 parts. The disodium hydrogen phosphate and bromothymol blue are co-loaded within the pores of carrier C. The outer surface of the particles of the color-changing silica gel grading indicator undergoes a surface hydrophobic passivation treatment. The surface hydrophobic passivation treatment agent is an anhydrous toluene solution of dimethyldichlorosilane, and the mass ratio of dimethyldichlorosilane to anhydrous toluene is 1:90 to 1:

110.

4. A method for preparing the color-changing silica gel grading indicator according to any one of claims 1-3, characterized in that, Includes the following steps: S1: Divide the initial fine-pore silica gel particles of the same batch into three parts, and perform pore-expansion treatment under three different hydrothermal conditions with different temperatures, different sodium carbonate concentrations and different times. Then, calcine and stabilize them in a unified manner so that the peak positions of the pore size distribution of the three carriers fall within the range of 2.8-3.2nm, 4.5-5.5nm and 10-12nm, respectively. These are denoted as carrier A, carrier B and carrier C. S2: Using an aqueous solution of cobalt chloride hexahydrate as impregnation solution A, the carrier A is impregnated by an equal-volume impregnation method, and after drying, the first color-changing silica gel component is obtained. S3: Using an ethanol-water solution containing methyl orange as impregnation solution B, the carrier B is impregnated by an equal-volume impregnation method, and after drying, the second color-changing silica gel component is obtained; S4: Using an ethanol-water solution containing bromothymol blue as impregnation solution C, the carrier C is impregnated by an equal-volume impregnation method, and after drying, the third color-changing silica gel component is obtained. S5: The first color-changing silica gel component, the second color-changing silica gel component, and the third color-changing silica gel component are mechanically mixed evenly in a dry environment to obtain the color-changing silica gel grading indicator.

5. The method according to claim 4, characterized in that, Step S1 specifically includes the following sub-steps: S11: Using type A fine-pore spherical silica particles with a pore size of 2-4 mm and a peak position of pore size distribution in the range of 2.4-2.7 nm as determined by the BJH method according to claim 1 as the initial fine-pore silica particles, they are divided into three parts by mass and denoted as precursor A, precursor B and precursor C respectively. S12: The A precursor is placed in a sodium carbonate aqueous solution with a concentration of 0.02-0.04 mol / L, and the mass ratio of the A precursor to the sodium carbonate aqueous solution is 1:5 to 1:

8. After sealing, it is hydrothermally treated at 85-92℃ for 1.0-2.0 hours. After cooling, the particles are taken out and washed with deionized water until the pH of the washing solution is 7±0.5 and the conductivity is not higher than 10μS / cm. Then it is dried at 105℃ for 4 hours to obtain the carrier A precursor. S13: The B precursor is placed in a sodium carbonate aqueous solution with a concentration of 0.05-0.08 mol / L, and the mass ratio of the B precursor to the sodium carbonate aqueous solution is 1:5 to 1:

8. After sealing, it is hydrothermally treated at 105-115℃ for 2.0-3.0 hours. After cooling, the particles are taken out and washed with deionized water until the pH of the washing solution is 7±0.5 and the conductivity is not higher than 10μS / cm. Then it is dried at 105℃ for 4 hours to obtain the carrier B precursor. S14: The C precursor is placed in a sodium carbonate aqueous solution with a concentration of 0.10-0.15 mol / L, and the mass ratio of the C precursor to the sodium carbonate aqueous solution is 1:5 to 1:

8. After sealing, it is hydrothermally treated at 125-135℃ for 3.0-4.0 hours. After cooling, the particles are taken out and washed with deionized water until the pH of the washing solution is 7±0.5 and the conductivity is not higher than 10μS / cm. Then it is dried at 105℃ for 4 hours to obtain the carrier C precursor. S15: The precursors of carrier A, carrier B, and carrier C are calcined at 350°C for 2 hours under nitrogen protection, and then naturally cooled to room temperature to obtain carrier A, carrier B, and carrier C respectively. Step S15 is used to stabilize the porous skeleton obtained by hydrothermal pore expansion and remove residual moisture.

6. The method according to claim 4, characterized in that, Step S2 specifically includes the following sub-steps: S21: Mix cobalt chloride hexahydrate with deionized water at a mass ratio of 8:100 to 18:100, and stir at room temperature until dissolved to obtain the clear impregnation solution A; S22: Determine the amount of impregnation solution A to be weighed according to the following formula: ； In the formula, The volume of the impregnation solution A is measured in mL. This is the impregnation allowance coefficient, with a value ranging from 1.00 to 1.05; The total pore volume of the carrier A is expressed in mL / g. The mass of carrier A is expressed in grams. After adding the carrier A to the weighed impregnation solution A, let it stand and impregnate for 1 hour at room temperature and pressure, stirring gently every 15 minutes until there is no obvious free liquid on the surface of the carrier A particles. S23: The carrier A, after being impregnated in step S22, is placed in a vacuum drying oven and slowly dried for 4 hours under vacuum conditions of 40°C and gauge pressure of -0.085MPa to -0.075MPa. Then, the temperature is raised to 105°C and switched to atmospheric pressure to continue drying for 2 hours to obtain the first color-changing silica gel component. The mass ratio of cobalt chloride hexahydrate to deionized water in step S21 is adjusted so that the mass ratio of cobalt chloride hexahydrate in the first color-changing silica gel component to the mass ratio of carrier A is (3-6):

100.

7. The method according to claim 4, characterized in that, Step S3 specifically includes the following sub-steps: S31: Mix methyl orange, anhydrous ethanol and deionized water in a mass ratio of (0.6-3.0):(15-25):(75-85), stir in a 50°C water bath until dissolved, and filter through a 0.45μm filter membrane after natural cooling to room temperature to obtain the clear impregnation solution B. S32: Determine the amount of impregnation solution B to be weighed according to the following formula: ； In the formula, The volume of the impregnation solution B is measured in mL. This is the impregnation allowance coefficient, with a value ranging from 1.00 to 1.05; The total pore volume of the carrier B is expressed in mL / g. The mass of carrier B is expressed in grams. After adding the carrier B to the weighed impregnation solution B, let it stand and impregnate for 1 hour at room temperature and pressure, stirring gently every 15 minutes until there is no obvious free liquid on the surface of the carrier B particles. S33: The carrier B, after being impregnated in step S32, is placed in a forced-air drying oven and dried at 60°C for 3 hours, then heated to 105°C and dried for another 2 hours to obtain the second color-changing silica gel component; the mass ratio of methyl orange, anhydrous ethanol and deionized water in step S31 is adjusted so that the mass ratio of methyl orange in the second color-changing silica gel component to the mass ratio of carrier B is (0.5-1.5):

100.

8. The method according to claim 4, characterized in that, Step S4 specifically includes the following sub-steps: S41: Mix bromothymol blue, anhydrous ethanol and deionized water in a mass ratio of (0.8-3.0):(60-70):(30-40), stir in a 30°C water bath until dissolved, and filter through a 0.45μm filter membrane to obtain the clear impregnation solution C; S42: Determine the amount of the impregnation solution C to be weighed according to the following formula: ； In the formula, The volume of the impregnation solution C is measured in mL. This is the impregnation allowance coefficient, with a value ranging from 1.00 to 1.05; The total pore volume of the carrier C is expressed in mL / g. The mass of the carrier C is expressed in grams. After adding the carrier C to the weighed impregnation solution C, let it stand and impregnate for 1 hour at room temperature and pressure, stirring gently every 15 minutes until there is no obvious free liquid on the surface of the carrier C particles. S43: The carrier C, after being impregnated in step S42, is placed in a vacuum drying oven and dried in three stages: first, it is slowly dried for 3 hours under vacuum conditions of 50°C and a gauge pressure of -0.075MPa to -0.065MPa; then, it is dried for 2 hours at 80°C under normal pressure; and finally, it is dried for 1 hour at 105°C under normal pressure to obtain the third color-changing silica gel component. The purpose of the three-stage drying is to prevent the bromothymol blue from migrating to the outer surface of the carrier C particles through staged evaporation. Adjust the mass ratio of bromothymol blue, anhydrous ethanol, and deionized water in step S41 so that the mass ratio of bromothymol blue in the third color-changing silica gel component to the carrier C is (0.8-2.0):

100.

9. The method according to claim 4, characterized in that, Step S5 specifically includes the following sub-steps: S51: The first color-changing silica gel component, the second color-changing silica gel component, and the third color-changing silica gel component are respectively placed in a vacuum dryer equipped with 3A molecular sieve desiccant and left to stand at 25-30°C for 48 hours to reduce the water content of each component, as determined by the 105°C loss method, to below 2% by mass, thereby obtaining the pre-dried first color-changing silica gel component, the pre-dried second color-changing silica gel component, and the pre-dried third color-changing silica gel component. S52: Weigh the pre-dried first color-changing silica gel component, the pre-dried second color-changing silica gel component, and the pre-dried third color-changing silica gel component according to the mass ratio described in claim 1, and place them in a closed mixer with a relative humidity of not more than 10%. Turn on the mixer and mix at a speed of 10-30 r / min for 10-20 minutes to obtain the color-changing silica gel grading indicator.

10. The method according to claim 4, characterized in that, Following step S5, a step S6 is further included to perform a surface hydrophobicity passivation treatment on the outer surface of the particles of the color-changing silica gel grading indicator. Step S6 specifically includes the following sub-steps: S61: Dimethyldichlorosilane and anhydrous toluene are mixed at a mass ratio of 1:90 to 1:110 to prepare a hydrophobic treatment solution. Step S61 and subsequent sub-steps of step S6 are carried out in a dry environment with a water content of less than 50 ppm. S62: Add the color-changing silica gel grading indicator to the hydrophobic treatment solution, wherein the mass ratio of the color-changing silica gel grading indicator to the hydrophobic treatment solution is 1:3 to 1:4, and let it stand and soak at 25-30°C for 30 minutes without stirring. S63: Take out the particles processed in step S62, wash them twice with anhydrous toluene to remove unreacted dimethyldichlorosilane and hydrogen chloride released by the reaction, then place them in a vacuum drying oven at 105°C and purge them with a dry nitrogen stream for 4 hours to remove residual dimethyldichlorosilane, toluene and hydrogen chloride. After naturally cooling to room temperature, the color-changing silica gel grading indicator with hydrophobic passivation is obtained.

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