An acid-base indicating film and a method for preparing the same

Abstract

The application provides an acid-base indicating film and a preparation method thereof. The preparation method of the acid-base indicating film provided by the application comprises the following steps: a) mixing polyurethane resin, an organic solvent and acid-base indicator powder to obtain a coating liquid; b) coating the coating liquid onto release paper, and then performing solidification to obtain the acid-base indicating film; wherein the polyurethane resin is a liquid type polyurethane resin. The acid-base indicating film provided by the application can not only accurately test the pH value, but also can be repeatedly used, waste is avoided, and the acid-base indicating film is convenient to carry and can be applied in different occasions, so that the application limitation is broken.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of detecting materials, in particular to an acid-base indicating film and a preparation method thereof. BACKGROUND

[0002] Liquid acidity or alkalinity test or calibration is currently a very common test. There are mainly two common acidity or alkalinity test methods: one is pH test paper, and the other is pH meter. When using the pH test paper, a small piece of test paper is used to drop the detected liquid, different pH values of the liquid will cause different color reactions of the pH test paper to produce different colors, and then the colored test paper is compared with the standard color card, so that the pH value of the detected liquid can be determined. The pH meter is an electronic test equipment with a glass electrode at the end for pH detection. During testing, the calibrated pH meter is only immersed in the detected liquid, and the pH test can be quickly performed. Generally, the pH meter can more accurately display the pH value of the detected liquid than the pH test paper.

[0003] However, the above two test methods each have obvious shortcomings. The pH test paper is usually disposable, which is very wasteful during testing. Although the pH meter has accurate test results, the test process is complex and inconvenient to carry, and it is generally only used in fixed experimental places. SUMMARY

[0004] Therefore, the present application provides an acid-base indicating film and a preparation method thereof. The acid-base indicating film provided by the present application can not only accurately test the pH value, but also can be repeatedly used to avoid waste, and is convenient to carry and can be applied in different occasions, breaking the application limitations.

[0005] The present application provides a preparation method of an acid-base indicating film, comprising the following steps:

[0006] a) mixing polyurethane resin, organic solvent and acid-base indicator powder to obtain a coating liquid;

[0007] b) coating the coating liquid onto release paper, and then curing to obtain an acid-base indicating film;

[0008] Among them:

[0009] The polyurethane resin is a liquid polyurethane resin.

[0010] Preferably, the polyurethane resin is selected from at least one of NW-956, NC-5400 and NC-5008.

[0011] Preferably, the step a) specifically comprises:

[0012] a1) mixing polyurethane resin and organic solvent to obtain a polyurethane solution;

[0013] a2) mixing the polyurethane solution with acid-base indicator powder to obtain a coating liquid.

[0014] Preferably, the mass ratio of the polyurethane resin to the organic solvent is 1:(0.1-3).

[0015] Preferably, the mass ratio of the acid-base indicator powder to the polyurethane solution is 0.05%-20%.

[0016] Preferably, the organic solvent is at least one selected from butanone, dimethylformamide and butyl acetate.

[0017] Preferably, the acid-base indicator powder is at least one selected from methyl violet, cresol red, thymol blue, alizarin yellow R, methyl yellow, bromophenol blue, methyl orange, bromocresol green, methyl red, bromothymol blue, neutral red, phenol red, phenothalin, thymolphthalein and indigo carmine.

[0018] Preferably, the acid-base indicator powder is micro-nano particle powder.

[0019] Preferably, in the step b), the temperature of the solidification is 50-90℃ and the time is 2-10 min.

[0020] The application further provides an acid-base indicator film prepared by the preparation method.

[0021] The acid-base indicating film provided by the application has the advantages that the polyurethane material is insoluble in water, the acid-base indicator is soluble in water, and the acid-base indicator will generate a color reaction after being dissolved in an aqueous solution with different pH values. Specifically, the principle is as follows: the acid-base indicating film has the function of color generation for different pH liquids because the polyurethane material and the acid-base indicator are two incompatible systems, and the acid-base indicator is dispersed in the polyurethane material in the form of micro-nano particles. The polyurethane material is an elastomer, and thus the molecular chain has good activity and large freedom. Due to the surface energy, the acid-base indicator will continuously enrich on the surface of the polyurethane material until the acid-base indicator on the surface of the polyurethane material reaches equilibrium with the acid-base indicator inside the polyurethane. When the measured liquid is dropped on the surface of the film, the acid-base indicator on the surface of the film will be dissolved in the measured liquid to generate a color reaction. The polyurethane film is insoluble in water, and thus the acid-base indicator inside the film cannot contact the measured liquid, and thus cannot generate a color reaction, and the film itself does not change color. When the acid-base indicator on the surface of the film is consumed and the content is reduced after detection, due to the surface energy, the acid-base indicator inside the film will again migrate to the surface of the film, so that the acid-base indicating film can be repeatedly used until the acid-base indicator inside the film is effectively consumed. Therefore, the acid-base indicating film provided by the application can be used for pH testing and has excellent mechanical properties; moreover, the acid-base indicating film can be repeatedly used, greatly improving the service life, and at the same time, the acid-base indicating film is convenient to carry and can be used for testing in different occasions; in addition, the preparation process is simple, and the cost is low.

[0022] The experimental results show that the acid-base indicating film provided by the application can effectively indicate the pH value of the measured liquid. Moreover, the acid-base indicating film can be repeatedly used for more than 30 times, has stable performance, and has a long service life. At the same time, the tensile strength is more than 30 MPa, and the elongation at break is more than 300%, which shows excellent mechanical properties. BRIEF DESCRIPTION OF DRAWINGS

[0023] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description only constitute the embodiments of the application, and for those skilled in the art, other drawings can also be obtained without creative labor on the basis of the provided drawings.

[0024] Figure 1 It is a schematic diagram of the action principle and process of the acid-base indicating film of the application;

[0025] Figure 2 It is an effect diagram of test one in example 4;

[0026] Figure 3 It is an effect diagram of test two in example 4. DETAILED DESCRIPTION

[0027] The application provides a preparation method of an acid-base indicating film, comprising the following steps:

[0028] a) mixing a polyurethane resin, an organic solvent and acid-base indicator powder to obtain a coating liquid;

[0029] b) coating the coating liquid onto release paper and then performing solidification to obtain the acid-base indicating film;

[0030] wherein:

[0031] The polyurethane resin is a one-liquid polyurethane resin.

[0032] [Regarding step a]:

[0033] a) mixing a polyurethane resin, an organic solvent and acid-base indicator powder to obtain a coating liquid.

[0034] In the application, the polyurethane resin is a one-liquid polyurethane resin. In the application, the selection of the polyurethane material is very crucial. First, the polyurethane material is not soluble in water but is soluble in organic solvents such as butanone, dimethylbenzene, DMF and the like; at the same time, a one-liquid polyurethane resin without a curing agent is selected. The crosslinking degree of the polyurethane film will affect the migration of the acid-base indicator in the film. The application adopts a one-liquid polyurethane resin, which can better cooperate with the acid-base indicator and is conducive to the precipitation of the acid-base indicator. If a two-liquid polyurethane containing a curing agent is used, the acid-base indicator cannot be effectively precipitated, and the color change indicating performance is lost. In the application, preferably, the polyurethane resin is selected from at least one of NW-956, NC-5400 and NC-5008, and more preferably NW-956. The use of the above-mentioned polyurethane resins is conducive to fully exerting the color change indicating effect of the indicating film product and better matching the acid-base indicator, thereby ensuring the mechanical properties of the indicating film product.

[0035] In the application, the organic solvent is preferably at least one of butanone, dimethylformamide and butyl acetate, and more preferably butanone. The use of the above-mentioned organic solvents in the application reduces the viscosity of the polyurethane resin, facilitates film coating, and better dissolves the polyurethane resin, thereby ensuring the film coating performance.

[0036] In the application, the type of the acid-base indicator powder is not particularly limited and can be selected according to the pH range of the measured liquid to be detected. Preferably, the acid-base indicator powder is at least one of methyl violet, cresol red, thymol blue, alizarin yellow R, methyl yellow, bromophenol blue, methyl orange, bromocresol green, methyl red, bromothymol blue, neutral red, phenol red, phenolphthalein, thymolphthalein and indigo carmine red. In the application, the acid-base indicator powder is a micro-nano particle, i.e. a micron or nanometer particle.

[0037] In the application, step a) preferably specifically comprises:

[0038] a1) mixing polyurethane resin with organic solvent to obtain polyurethane solution;

[0039] a2) mixing the polyurethane solution with acid-base indicator powder to obtain coating liquid.

[0040] Regarding step a1): the mixing manner is not particularly limited, and the polyurethane resin is fully dissolved in the organic solvent according to the conventional mixing manner in the art, such as stirring mixing. After mixing, the polyurethane solution is obtained.

[0041] Regarding step a2): the mixing manner is not particularly limited, and the acid-base indicator powder is fully dispersed in the polyurethane solution according to the conventional mixing manner in the art, such as stirring mixing. After mixing, the coating liquid is obtained.

[0042] In the present application, the mass ratio of the polyurethane resin to the organic solvent is preferably 1:(0.1-3), and can be 1:0.5, 1:0.8, 1:1, 1:1.5, 1:2, or 1:3.

[0043] In the present application, the mass ratio of the acid-base indicator powder to the polyurethane solution is preferably 0.05%-20%, i.e., the amount of the acid-base indicator powder is 0.05%-20% of the mass of the polyurethane solution, and can be 0.05%, 0.1%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, or 20%. When the amount is controlled within the above range, the color development indicating effect can be effectively achieved, and the specific polyurethane resin described above can be better matched to ensure the mechanical properties of the indicating film. The mass ratio is more preferably 1%-5%. Within the preferred amount range, the color change sensitivity can be further improved, and the mechanical properties can be ensured.

[0044] [Regarding step b]:

[0045] b) after the coating liquid is coated onto the release paper, curing is performed to obtain an acid-base indicating film.

[0046] In the present application, the type of the release paper is not particularly limited, and a conventional release paper can be used. In the present application, the coating manner of the coating liquid coated onto the release paper is not particularly limited, and a conventional coating manner can be used. In the present application, the thickness of the coating is preferably ≥0.05 mm.

[0047] In the present application, after the above coating, curing is performed. In the present application, the curing temperature is preferably 50-90 DEG C, and can be specifically 50 DEG C, 55 DEG C, 60 DEG C, 65 DEG C, 70 DEG C, 75 DEG C, 80 DEG C, 85 DEG C, 90 DEG C. Controlling the curing temperature in the above range can ensure that the coating film is fully cured and the mechanical properties of the film are ensured. If the curing temperature is too high, the mechanical properties of the film can be reduced. The curing time is preferably 2-10 min, and can be specifically 2 min, 3 min, 4 min, 5 min, 6 min, 7 min, 8 min, 9 min, 10 min. In the present application, after the above curing, the release paper is preferably removed, thereby obtaining the acid-base indicator film.

[0048] The present application also provides an acid-base indicator film prepared by the preparation method described in the above technical solution.

[0049] The use method / test method of the acid-base indicator film provided by the present application is as follows: a drop of the liquid to be tested is added to the surface of the acid-base indicator film, color development is waited, and then a standard pH colorimetric card is compared to obtain the pH value of the liquid to be tested. After the above detection is performed, the measured liquid on the surface of the indicator film is removed, the surface of the indicator film is slightly washed and dried, and the color-changing substance in the indicator film will continue to migrate to the surface of the film, so that the indicator film can be reused. The test process of the acid-base indicator film of the present application is simple, the indicator film is convenient to carry, and can be applied in various occasions.

[0050] In the acid-base indicator film provided by the present application, the polyurethane material is insoluble in water, and the acid-base indicator is soluble in water. After the acid-base indicator is dissolved in an aqueous solution with different pH values, a color development reaction occurs. Specifically, the principle is as follows: the acid-base indicator film has the function of developing color for different pH liquids because the polyurethane material and the acid-base indicator are two incompatible systems, and the acid-base indicator is dispersed in the polyurethane material in the form of micro-nano particles. The polyurethane material is an elastomer, and therefore the molecular chain has good activity and large freedom. Due to the surface energy, the acid-base indicator will continuously enrich on the surface of the polyurethane material, until the acid-base indicator on the surface of the polyurethane material and the acid-base indicator inside the polyurethane material reach equilibrium. When the liquid to be tested is dropped onto the surface of the film, the acid-base indicator on the surface of the film will dissolve into the liquid to be tested and develop color. The polyurethane film is insoluble in water, and therefore the acid-base indicator inside the film cannot contact the liquid to be tested, and thus cannot develop color. The film itself does not change color. When the content of the acid-base indicator on the surface of the film is reduced after detection, due to the surface energy, the acid-base indicator inside the film will again migrate to the surface of the film, so that the acid-base indicator film can be repeatedly used until the acid-base indicator inside the film is effectively depleted. Figure 1 , Figure 1 The present application also provides an acid-base indicator film prepared by the preparation method described in the above technical solution.

[0051] The acid-base indicating film provided by the present application contains a color-changing substance. Unlike pH test paper, the color-changing substance in the indicating film can continuously migrate to the surface of the film, so that when the film is in contact with a liquid of different acidity or alkalinity, the color-changing substance can dissolve into the measured liquid to produce a color reaction, while the film itself does not change color. When the measured liquid on the surface of the film is removed and the surface of the indicating film is gently washed and dried, the color-changing substance in the indicating film will continue to migrate to the surface of the film, so that the indicating film can be reused. In addition, since the indicating film itself does not change color and can be reused, the indicating film can be combined with other materials, components or structures to achieve various functions in different forms and in different occasions, overcoming the problems of complex pH meter testing process, inconvenience of carrying and experimental site limitations. The acid-base indicating film provided by the present application can be used alone to test the acidity or alkalinity of a liquid, and can also be used as a film material to form a product that can test the acidity or alkalinity of a liquid by being combined with other related materials, components or structures, such as a waterproof film on the surface of a waterproof zipper, which can not only impart waterproof performance to the zipper, but also display the acidity or alkalinity of rainwater in real time when outdoor sports are encountered in rainy weather. In addition, the preparation process of the present application is very simple, the cost is low, and it is convenient for large-scale production and application.

[0052] In summary, compared with the prior art, the present application has the following beneficial effects:

[0053] 1. The acid-base indicating film of the present application is insoluble in water, the film itself does not change color, and the color-changing substance can migrate to the surface to make the measured liquid change color.

[0054] 2. The acid-base indicator in the film can continuously migrate out of the film, so that the acid-base indicating film can be reused multiple times.

[0055] 3. The acid-base indicating film of the present application is stable in performance and can be combined with other components or structures to prepare various composite structures for occasions where acid-base testing is required.

[0056] 4. The preparation process of the acid-base indicating film of the present application is simple, the cost is low, and it is convenient for large-scale production and application.

[0057] The experimental results show that the acid-base indicating film provided by the present application can effectively indicate the pH value of the measured liquid. Moreover, it can be reused more than 30 times, has stable performance and a long service life. At the same time, the tensile strength is more than 30 MPa, and the elongation at break is more than 300%, showing excellent mechanical properties.

[0058] For a further understanding of the present application, preferred embodiments thereof will be described in conjunction with examples below, it being understood, however, that this description is made only by way of further illustration and not as a limitation on the claimed application.

[0059] Example 1

[0060] NW-956 polyurethane resin was mixed with butanone at a mass ratio of 1:1 and stirred thoroughly to prepare a polyurethane solution. Then, bromothymol blue acid-base indicator powder (used in an amount of 0.1% of the mass of the polyurethane solution) was added and stirred thoroughly to disperse the indicator, obtaining a coating solution. The coating solution was then coated onto a release paper to a thickness of 0.05 mm, cured in an oven at 70°C for 3 min, and the release paper was then peeled off, obtaining an acid-base indicator film.

[0061] Example 2

[0062] Example 1 was implemented, except that the amount of bromothymol blue acid-base indicator powder used was 1% of the mass of the polyurethane solution.

[0063] Example 3

[0064] Example 1 was implemented, except that the amount of bromothymol blue acid-base indicator powder used was 5% of the mass of the polyurethane solution.

[0065] Comparative Example 1

[0066] Example 2 was implemented, except that 1% of a polyisocyanate type curing agent was added during the preparation of the polyurethane solution. The specific process was as follows:

[0067] NW-956 polyurethane resin was mixed with butanone at a mass ratio of 1:1 and stirred thoroughly, and a polyisocyanate type curing agent (used in an amount of 1% of the mass of the mixed solution obtained by stirring) was added and stirred thoroughly to prepare a polyurethane solution. Then, bromothymol blue acid-base indicator powder (used in an amount of 1% of the mass of the polyurethane solution) was added and stirred thoroughly to disperse the indicator, obtaining a coating solution. The coating solution was then coated onto a release paper to a thickness of 0.05 mm, cured in an oven at 70°C for 3 min, and the release paper was then peeled off, obtaining an acid-base indicator film.

[0068] Example 4: Product Testing

[0069] (1) Test One

[0070] The acid-base indicator films obtained in Examples 1-3 and Comparative Example 1 were respectively labeled as No. 1, No. 2, No. 4, and No. 3. One drop of a basic solution with a pH of 8 was added to the surface of each of the four indicator films, and the color development effect was observed, with the results shown in Table 1. Figure 2 Figure 2 The effect diagram of Test One in Example 4. ​

[0071] Bromothymol blue acid-base indicator is orange-yellow before color development, and the color change range is about 6.2-7.6, and it shows yellow in the presence of acid and blue in the presence of base.

[0072] After adding an alkaline solution with a pH of 8 on the surface of the film, it can be seen that: on the film with an indicator content of 0.1% (No. 1), the test solution changes color, but due to the low content of the indicator, the color change is low, and it is light blue; on the film with an indicator content of 1% (No. 2), the test solution is obviously blue; at the same time, as Comparative Example 1, on the film with 1% of the curing agent and 1% of the indicator (No. 3), the test solution cannot develop color and is still transparent because the indicator cannot be effectively precipitated, and the acid-base indication experiment fails; on the film with an indicator content of 5% (No. 4), the test solution becomes dark blue due to the high content of the indicator.

[0073] The above tests prove that the acid-base indicating films of Examples 1-3 can effectively perform pH color development reaction and can be used as pH test films; and among them, the indicator content in Examples 2-3 is 1%-5%, and the test is more sensitive. The film obtained in Comparative Example 1 fails due to the presence of the curing agent, and cannot be used as a pH test film.

[0074] (2) Test two

[0075] The acid-base indicating films obtained in Examples 1-3 were used to test pH, respectively, to test the color development effect after adding an acidic solution with a pH of 6 and an alkaline solution with a pH of 8 to the surface of the film, see Figure 3 , Figure 3 which are the effect pictures of Test two in Example 4.

[0076] It can be seen that when the pH of the test solution is 6, the indicating film is yellow, and when the pH of the test solution is 8, the indicating film is blue, and furthermore, as the content of the acid-base indicator increases, the depth of the test solution after color development gradually deepens, making the test more sensitive.

[0077] In addition, after the above tests, the test solution on the surface of the indicating film in Examples 1-3 was removed and washed and dried, and then the test solution was added again, and the result was still able to develop color. After testing, after 30 cycles of repeated tests, the indicating film still had color development effect.

[0078] (3) Test three

[0079] The acid-base indicating films obtained in Examples 1-3 were subjected to mechanical property tests, and the results are shown in Table 1.

[0080] Table 1: Mechanical properties of the acid-base indicating films of Examples 1-3

[0081] Tensile strength, MPa Elongation at break, % Example 1 45 520 Example 2 43 510 Example 3 39 480

[0082] As shown in the test results in Table 1, the tensile strength of the acid-base indicator film obtained in Examples 1-3 is above 39 MPa, and the breaking elongation strength is above 480%, which shows excellent tensile mechanical properties.

[0083] As shown in Tests 1-3 above, the acid-base indicator film of Examples 1-3 not only can effectively test the pH value, but also has excellent mechanical properties.

[0084] Example 5

[0085] According to Example 1, except that the amount of bromothymol blue acid-base indicator powder is 15% of the mass of the polyurethane solution.

[0086] Example 6

[0087] According to Example 1, except that the amount of bromothymol blue acid-base indicator powder is 20% of the mass of the polyurethane solution.

[0088] Comparative Example 2

[0089] According to Example 1, except that the amount of bromothymol blue acid-base indicator powder is 30% of the mass of the polyurethane solution.

[0090] Example 7: Product Test

[0091] (1) Test 1

[0092] The acid-base indicator films obtained in Examples 5-6 and Comparative Example 2 are used for pH testing, and the color development effect after adding an alkaline solution with a pH of 8 to the surface of the film is tested. The results show that the three indicator films are all blue, which can effectively indicate the pH of the measured liquid.

[0093] (2) Test 2

[0094] The acid-base indicator films obtained in Examples 5-6 and Comparative Example 2 are tested for mechanical properties, and the results are shown in Table 2.

[0095] Table 2: Mechanical properties of the acid-base indicator films of Examples 5-6 and Comparative Example 2

[0096] Tensile strength, MPa Elongation at break, % Example 5 35 430 Example 6 32 410 Comparative Example 2 18 190

[0097] As shown in the test results in Table 2, the tensile strength of the acid-base indicator film obtained in Examples 5-6 is above 30 MPa, and the breaking elongation is above 400%, which shows excellent tensile mechanical properties. However, the mechanical strength of Comparative Example 2 is significantly reduced, which proves that if the amount of acid-base indicator powder is too high, the mechanical properties of the indicator film will be reduced.

[0098] From the above test 1-test 2, it can be seen that the acid-base indicating film of the embodiment 5-6 of the present application can not only effectively test the pH value, but also has excellent mechanical properties. In the comparative example 2, due to the too high content of the indicator, although the pH value can be tested, the mechanical properties of the indicating film are poor, which can easily affect the actual usability and service life.

[0099] Example 8

[0100] According to the implementation of example 2, except that the NW-956 polyurethane resin is replaced by NC-5400 polyurethane resin.

[0101] Example 9

[0102] According to the implementation of example 2, except that the NW-956 polyurethane resin is replaced by NC-5008 polyurethane resin.

[0103] Example 10: product test

[0104] (1) test 1

[0105] The acid-base indicating film obtained from examples 8-9 is used for pH test, and the color development effect after the basic solution with pH 8 is added to the surface of the film is tested. The results show that the two indicating films are blue, which can effectively indicate the pH of the measured liquid.

[0106] (2) test 2

[0107] The mechanical properties of the acid-base indicating film obtained from examples 8-9 are tested, and the results are shown in table 3.

[0108] Table 3: mechanical properties of the acid-base indicating film of examples 8-9

[0109] Tensile strength, MPa Elongation at break, MPa Example 8 38 380 Example 9 34 320

[0110] From the test results in table 3, it can be seen that the tensile strength of the acid-base indicating film obtained from examples 8-9 of the present application is more than 30 MPa, and the elongation at break is more than 300%, which shows excellent mechanical properties.

[0111] From the above test 1-test 2, it can be seen that the acid-base indicating film of the embodiment 8-9 of the present application can not only effectively test the pH value, but also has excellent mechanical properties.

[0112] In addition, from the product test of example 4, example 7 and example 10, it can be seen that the one liquid type polyurethane resin used in the present application can fully develop the color development indicating effect, and can be better matched with the acid-base indicator, so as to ensure the mechanical properties of the indicating film product. In the comparative example 1, the curing agent is added (equivalent to the case of using two liquid type polyurethane resin containing curing agent), which cannot develop color and cannot be used as acid-base indicating film.

[0113] The principles and implementations of the present application are described herein with specific examples, and the above description of the embodiments is only used to help understand the method of the present application and its core idea, including the best mode, and also enable any person skilled in the art to practice the present application, including manufacturing and using any device or system, and implementing any combined method. It should be noted that, for those skilled in the art, some improvements and modifications can be made to the present application without departing from the principles of the present application, and these improvements and modifications also fall within the protection scope of the claims of the present application. The scope of patent protection of the present application is defined by the claims, and can include other embodiments that can be conceived by those skilled in the art. If these other embodiments have structural elements similar to the literal expression of the claims, or if they include equivalent structural elements that are not substantially different from the literal expression of the claims, then these other embodiments should also be included in the scope of the claims.

Claims

1. A method for preparing an acid-base indicator membrane, characterized in that, Includes the following steps: a1) Mix polyurethane resin with an organic solvent to obtain a polyurethane solution; a2) Mix the polyurethane solution with acid-base indicator powder to obtain a coating liquid; The acid-base indicator powder is a micro-nano particle powder; b) After the coating liquid is applied onto the release paper, it is cured to obtain an acid-base indicator film; the coating thickness is ≥0.05mm; in: The polyurethane resin is selected from at least one of NW-956, NC-5400 and NC-5008; The mass ratio of the polyurethane resin to the organic solvent is 1:(0.1~3). The mass ratio of the acid-base indicator powder to the polyurethane solution is 0.05%~20%; The acid-base indicator membrane can be reused; When the acid-base indicator on the surface of the acid-base indicator membrane is consumed and its content decreases after detection, the acid-base indicator inside the acid-base indicator membrane will migrate back to the membrane surface, so that the acid-base indicator membrane can be repeatedly recycled until the acid-base indicator inside the membrane that can effectively migrate to the surface is exhausted.

2. The preparation method according to claim 1, characterized in that, The organic solvent is selected from at least one of butanone, dimethylformamide, and butyl acetate.

3. The preparation method according to claim 1, characterized in that, The acid-base indicator powder is selected from at least one of methyl violet, cresol red, thymol blue, alizarin yellow R, methyl yellow, bromophenol blue, methyl orange, bromocresol green, methyl red, bromothymol blue, neutral red, phenol red, phenolphthalein, thymolphthalein and indigo carmine.

4. The preparation method according to claim 1, characterized in that, In step b), the curing temperature is 50~90℃ and the time is 2~10min.

5. An acid-base indicator membrane prepared by any one of claims 1 to 4.

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