Preparation method and application of layer-by-layer self-assembly flame retardant material

Abstract

The present invention belongs to the technical field of new materials, and specifically relates to a method for preparing a coating with flame retardant and fire warning functions and its application, wherein a polyurethane foam (PU) is treated alternately with anionic solution and cationic solution to obtain a double-coated PU. The present invention utilizes the interaction between anions and cations to self-assemble layer by layer into FPUF, and the anions and cations synergistically flame retardant during the combustion process, showing excellent flame retardancy, and the drastic change in its resistance can realize a short fire warning signal, thus realizing the function of fire warning.

Description

Technical Field

[0001] The invention relates to the technical field of flame retardant coatings, and in particular to a method for preparing a layer-by-layer self-assembled flame retardant material and its dual applications of fire warning and flame retardancy. Background Art

[0002] In recent years, polyurethane composites have been widely used due to their various excellent properties, but the fire hazards caused by their flammability cannot be ignored. The main methods for flame-retardant polyurethane are additive and coating flame retardants. Additive flame retardants are easy to destroy the structure of composite materials and deteriorate mechanical properties, while coating flame retardants are a better choice. Their coating not only enhances the flame retardancy of composite materials, but also ensures the structural stability of composite materials. Studies have found that the application of multiple flame retardants can play a synergistic role and better suppress the release of smoke and heat in fires.

[0003] The study found that ammonium polyphosphate and sodium alginate are often used to configure anionic solutions in layer-by-layer self-assembly. Ammonium polyphosphate is rich in phosphorus and nitrogen flame retardant elements, has excellent flame retardant effect and is inexpensive; sodium alginate is widely used due to its own structural characteristics and good fire resistance during combustion. In addition, MXene is a two-dimensional sheet structure composed of transition metal carbides, nitrides or carbonitrides. It was first reported in 2011 and has received widespread attention from researchers and has been developed and applied in many fields such as energy, sensing, catalysis, and flame retardancy. Since the surface of MXene materials has abundant terminal groups, such as hydroxyl or terminal oxygen groups, it can be configured as anionic solution. In contrast, porous MOFs with amino groups can be configured as cationic solutions. Both the transition metal part and the organic ligand part of MOF can play a flame retardant and smoke suppression effect. In addition, the application of NH2-MOFs in flame retardant and thermal insulation coatings has rarely been reported, and it has not been reported that it has been prepared as a cationic solution.

[0004] Chinese invention patent publication CN104711856A discloses a method for preparing a self-healing super-hydrophobic flame-retardant coating and a technical solution for using anions and cations to form a flame-retardant composite coating. However, the arrangement of the anions and cations and the combination with the substrate need to be improved. Summary of the invention

[0005] In view of the problems existing in the prior art, the present invention provides a coating with flame retardant and fire warning functions, a preparation method thereof and an application thereof.

[0006] The present invention is achieved through the technical solution:

[0007] The present invention provides a layer-by-layer self-assembled flame retardant material, which is a coating with flame retardant and fire warning functions, and is a flame retardant composited by NH2-MOFs and anionic solution. The layer-by-layer self-assembled flame retardant material is a layer-by-layer self-assembled flame retardant material formed by arranging anionic solution and cationic solution alternately coated on the outside of a substrate of polyurethane foam (PU) to form a coating, and utilizes the interaction between anions in the anionic solution and cations in the cationic solution to self-assemble layer by layer to form a coating with flame retardant and fire warning functions; the weight of each layer of the anionic solution coating accounts for 0.1-1.9%; the weight of each layer of the cationic solution coating accounts for 0.15-2%, the number of layers of the anionic solution coating is 3-15 layers, the number of layers of the cationic solution coating is 3-15 layers, and the coating of the anionic solution and the coating of the cationic solution are arranged alternately.

[0008] The cationic substance in the cationic solution is a metal organic framework material with an amino group, and the metal organic framework material is one or more of NH2-MIL-53(Fe), NH2-MIL-53(Al), NH2-UiO-66(Hf), NH2-UiO-66(Zr), NH2-MIL-101(Cr), NH2-MIL-101(Fe) or NH2-MIL-101(Al).

[0009] The anion substance in the anion solution is one or more of MXene, ammonium polyphosphate (APP) or sodium alginate (SA).

[0010] A method for preparing a layer-by-layer self-assembled flame retardant material, wherein the layer-by-layer self-assembled flame retardant material is the above-mentioned layer-by-layer self-assembled flame retardant material; the preparation method comprises the following steps:

[0011] (1) Preparation of self-assembled cationic solution: dissolving the metal organic framework material in deionized water, and then slowly adding an acidic solution to adjust the pH value of the mixed solution to 5.0-6.8; the ratio of the metal organic framework material to deionized water is 0.01-5 g: 10-500 mL; preparing a self-assembled cationic solution;

[0012] (2) Preparation of self-assembled anion solution: placing an anion substance in deionized water to prepare an anion solution, and preparing a self-assembled anion solution with a mass percentage concentration of 1 wt % to 5 wt %;

[0013] (3) Polyurethane foam pretreatment: immerse the entire polyurethane foam in a solution containing anionic surfactant for 5 to 10 minutes to form a negatively charged surface on the surface of the polyurethane foam, and then wash the immersed polyurethane foam with deionized water until it is neutral;

[0014] (4) immersing the polyurethane foam obtained in step (3) into the self-assembled cationic solution obtained in step (1) for a soaking time of 1 to 10 minutes, then taking it out and immersing it into the self-assembled anionic solution obtained in step (2) for a soaking time of 0.5 to 8 minutes, and before each immersion in the self-assembled cationic solution and in the self-assembled anionic solution, washing the polyurethane foam with deionized water;

[0015] (5) Repeat step (4) 3 to 15 times, then place the obtained sample in an oven and dry it at a temperature of 65 to 75° C. for 12 to 48 hours to obtain a layer-by-layer self-assembled flame retardant material with flame retardant and fire warning functions.

[0016] Preferably, the acidic solution in step (1) is an acetic acid solution.

[0017] Preferably, the anionic surfactant in step (3) is one or more of polyacrylic acid (PAA), sodium dodecylbenzene sulfonate (SDBS) or sodium dodecyl sulfate (SDS).

[0018] Preferably, in step (1), acetic acid solution is added dropwise to adjust the pH value of the mixed solution to 6.0-6.5; and in step (3), the mass percentage concentration of the anionic surfactant is 0.01wt%-0.6wt%.

[0019] Preferably, in step (4), the immersion time in the self-assembly cation solution is 1 to 5 minutes, and the immersion time in the self-assembly anion solution is 1 to 5 minutes; and the duration of washing with deionized water is 1 to 3 minutes.

[0020] Preferably, the polyurethane is hard polyurethane or soft polyurethane.

[0021] Preferably, in step (5), drying is carried out at a temperature of 70°C.

[0022] An application of a layer-by-layer self-assembled flame retardant material, wherein the application is a dual application of the layer-by-layer self-assembled flame retardant material in fire warning and flame retardancy.

[0023] Preferably, in the dual application of fire warning and flame retardancy, the coating of the anionic solution and the coating of the cationic solution in the coating synergistically exert a flame retardant effect. At the same time, during the flame retardant process, the fire warning signal is transmitted through the drastic change of resistance, thereby achieving the role of fire warning.

[0024] Preferably, the preparation method of the metal organic framework material is:

[0025] (1) Dissolve the metal compound in a container containing N,N-dimethylformamide (DMF) in a ratio of 2-3 g: 50-150 mL.

[0026] (2) Dissolve 2-aminoterephthalic acid in DMF solution in a ratio of 1.6-1.9 g: 60-80 mL.

[0027] (3) The solution obtained in step (1) and the solution obtained in step (2) are mixed and stirred at room temperature to obtain a homogeneous suspension.

[0028] (4) The homogenized suspension obtained in step (3) is placed in a high pressure reactor and transferred to an oven, where it is maintained at 105-115° C. for 20-28 hours.

[0029] (5) After the reaction of step (4) is completed, the product obtained in step (4) is cooled, centrifuged, washed with DMF, and then vacuum dried at 65-90° C. to obtain the product metal organic framework material.

[0030] Preferably, the polyurethane is hard polyurethane or soft polyurethane.

[0031] Preferably, the polyurethane is flexible polyurethane (FPU).

[0032] The beneficial effects of the present invention are:

[0033] The present invention obtains an alternating double-material coating PU by alternately treating a base polyurethane foam (PU) with a specific anion solution and a cationic solution, and utilizing the interaction between anions and cations to self-assemble layer by layer into FPUF. The obtained PU composite material not only maintains good mechanical properties, but also has excellent thermal stability, flame retardancy and smoke suppression properties. During the combustion process, the anions and cations have a synergistic flame retardant effect, showing excellent flame retardancy, and the drastic change in its resistance can realize a short fire warning signal, thus achieving the role of fire early warning. In particular, it is highly innovative to improve the flame retardancy of PU composite materials by using the layer-by-layer self-assembly technology of NH2-MOFs and anionic MXene solutions, and the present invention provides a new idea for its flame retardant application.

[0034] Since the surface of MXene materials has abundant terminal groups, such as hydroxyl or terminal oxygen groups, it can be configured as an anion solution. Common anion solutions also include ammonium polyphosphate and sodium alginate; porous MOF with amino groups can be configured as a cationic solution. The anion-cation synergistically coordinated coating obtained by the specific configuration of the present invention not only plays a synergistic flame retardant role, but also the drastic change of its resistance during the combustion process can realize a short fire warning signal, thus realizing the role of fire early warning.

[0035] The present invention combines the structural and performance advantages of MXene or APP or sodium alginate and porous MOF through the coordination of specific parameters of each step, synergistic flame retardancy, and gives full play to the gas phase and condensed phase flame retardant mechanism. First, the lamellar structure of MXene effectively blocks the internal and external interaction of heat and gas, and the porous structure of MOF can complicate its propagation path, that is, the specific combination of the two has a very strong coordination effect; secondly, the phosphorus and nitrogen flame retardant elements of APP and the carbon-based MOF can form an expansion flame retardant system, and the two also have a strong coordination effect; finally, the transition metal of MOF can catalyze the polymer to quickly form a protective carbon layer and inhibit the smoke release of the composite material, so that the coordination and cooperation between the various components constitute the technical solution of the material of the present invention with good technical effects as a whole. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a schematic diagram of the structure of the layer-by-layer self-assembled flame retardant material of the present invention. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention is clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0038] Example 1

[0039] This embodiment provides a coating with flame retardant and fire warning functions, which is a MXene and NH2-MIL-101 (Fe) coated FPU composite material.

[0040] This embodiment also provides a method for preparing the above-mentioned flame retardant and fire warning coating, and the specific steps are as follows:

[0041] (1) Dissolve 2.7 g of FeCl3·6H2O in a round-bottom flask containing 75 mL of N,N-dimethylformamide (DMF).

[0042] (2) 1.81 g of 2-aminoterephthalic acid was dissolved in 75 mL of DMF solution.

[0043] (3) Mix the solution of step (1) and the solution of step (2), and stir them thoroughly at room temperature to obtain a homogeneous suspension.

[0044] (4) The homogenized suspension from step (3) was placed in a high pressure reactor and transferred to an oven at 110° C. for 24 h.

[0045] (5) After the reaction is completed, the product of step (4) is cooled, centrifuged, washed with DMF and dried in vacuo at 80°C to obtain the product NH2-MIL-101(Fe).

[0046] (6) Dissolve NH2-MIL-101(Fe) in deionized water, slowly add acetic acid solution to adjust the pH value of the mixed solution to about 6.0-6.5, and obtain a cationic solution.

[0047] (7) Prepare 9 M hydrochloric acid solution with concentrated hydrochloric acid and deionized water, and slowly add 1 g LiF and 1 g Ti3AlC2 several times in an oil bath at 35 °C for 48 h.

[0048] (8) After the reaction is completed, the product of step (7) is washed with deionized water several times until the supernatant is neutral.

[0049] (9) The precipitate obtained in step (8) was ultrasonically dispersed in 200 mL of deionized water for 2 h to obtain a chromatographic MXene colloidal solution.

[0050] (10) The suspension of step (9) was centrifuged at 3500 r / min for 25 min to remove the unexfoliated Ti3C2 powder and obtain a MXene anion solution.

[0051] (11) FPU was pretreated by immersing the PU in a solution containing SDS to form a negatively charged surface, and then washed with deionized water until it was neutral.

[0052] (12) The pretreated FPU in step (11) was alternately immersed in NH2-MOFs and MXene solutions for 30 min to complete a cycle of growth. Before each immersion in the self-assembly solution, the foam was washed in deionized water for 5 min to remove excess material and squeeze out water under a specified pressure.

[0053] (13) When the number of layers reached 4, the sample was dried in an oven at 70 °C overnight to obtain the final product, which was named: FPU / 4-NH2-MIL-101(Fe) / MXene.

[0054] Example 2

[0055] This embodiment provides a coating with flame retardant and fire warning functions, which is an APP and NH2-MIL-101 (Fe) coated FPU composite material.

[0056] This embodiment also provides a method for preparing the above-mentioned flame retardant and fire warning coating, and the specific steps are as follows:

[0057] (1) Dissolve 2.7 g of FeCl3·6H2O in a round-bottom flask containing 75 mL of N,N-dimethylformamide (DMF).

[0058] (2) 1.81 g of 2-aminoterephthalic acid was dissolved in 75 mL of DMF solution.

[0059] (3) Mix the solution of step (1) and the solution of step (2), and stir them thoroughly at room temperature to obtain a homogeneous suspension.

[0060] (4) The homogenized suspension from step (3) was placed in a high pressure reactor and transferred to an oven at 110° C. for 24 h.

[0061] (5) After the reaction is completed, the product of step (4) is cooled, centrifuged, washed with DMF and dried in vacuo at 80°C to obtain the product NH2-MIL-101(Fe).

[0062] (6) dissolving NH2-MIL-101(Fe) in deionized water, slowly adding acetic acid solution to adjust the pH value of the mixed solution to about 6.0-6.5, and obtaining a cationic solution;

[0063] (7) APP is dissolved in deionized water, and the mixture is stirred thoroughly at room temperature to disperse the mixture into a homogeneous suspension to obtain an APP anion solution.

[0064] (8) FPU was pretreated by immersing the PU in a solution containing SDS to form a negatively charged surface, and then washed with deionized water until it was neutral.

[0065] (9) The pretreated FPU in step (11) was alternately immersed in NH2-MIL-101(Fe) and APP solutions for 30 min to complete a cycle of growth. Before each immersion in the self-assembly solution, the foam was washed in deionized water for 5 min to remove excess material and squeeze out water under a specified pressure.

[0066] (10) When the number of layers reached 4, the sample was dried in an oven at 70 °C overnight to obtain the final product, which was named: FPU / 4-NH2-MIL-101(Fe) / APP.

[0067] Example 3

[0068] This embodiment provides a coating with flame retardant and fire warning functions, which is a sodium alginate and NH2-MIL-101 (Fe) coated FPU composite material.

[0069] This embodiment also provides a method for preparing the above-mentioned flame retardant and fire warning coating, and the specific steps are as follows:

[0070] (1) Dissolve 2.7 g of FeCl3·6H2O in a round-bottom flask containing 75 mL of N,N-dimethylformamide (DMF).

[0071] (2) 1.81 g of 2-aminoterephthalic acid was dissolved in 75 mL of DMF solution.

[0072] (3) Mix the solution of step (1) and the solution of step (2), and stir them thoroughly at room temperature to obtain a homogeneous suspension.

[0073] (4) The homogenized suspension from step (3) was placed in a high pressure reactor and transferred to an oven at 110° C. for 24 h.

[0074] (5) After the reaction is completed, the product of step (4) is cooled, centrifuged, washed with DMF and dried in vacuo at 80°C to obtain the product NH2-MIL-101(Fe).

[0075] (6) dissolving NH2-MIL-101(Fe) in deionized water, slowly adding acetic acid solution to adjust the pH value of the mixed solution to about 6.0-6.5, and obtaining a cationic solution;

[0076] (7) Sodium alginate (SA) is dissolved in deionized water, and its pH value is adjusted to about 5. The mixture is fully stirred at room temperature to disperse into a homogeneous suspension to obtain a SA anion solution.

[0077] (8) FPU was pretreated by immersing the PU in a solution containing SDS to form a negatively charged surface, and then washed with deionized water until it was neutral.

[0078] (9) The pretreated FPU in step (11) was alternately immersed in NH2-MIL-101(Fe) and SA solutions for 30 min to complete a cycle of growth. Before each immersion in the self-assembly solution, the foam was washed in deionized water for 5 min to remove excess material and squeeze out water under a specified pressure.

[0079] (10) When the number of layers reached 4, the sample was dried in an oven at 70 °C overnight to obtain the final product, which was named: FPU / 4-NH2-MIL-101(Fe) / SA.

[0080] Comparative Example 1

[0081] This comparative example provides a kind of FPU, which is a pure FPU sample without any flame retardant additive.

[0082] Comparative Example 2

[0083] This comparative example provides a FPU / 8-NH2-MIL-101(Fe) / MXene composite material. The preparation method of the FPU / 8-NH2-MIL-101(Fe) / MXene is substantially the same as that of Example 1, except that the number of coating layers is 8.

[0084] Comparative Example 3

[0085] This comparative example provides a FPU / 12-NH2-MIL-101(Fe) / MXene composite material. The preparation method of the FPU / 12-NH2-MIL-101(Fe) / MXene is substantially the same as that of Example 1, except that the number of coating layers is 12.

[0086] Comparative Example 4

[0087] This comparative example provides a FPU / 8-NH2-MIL-101(Fe) / APP composite material. The preparation method of the FPU / 8-NH2-MIL-101(Fe) / APP is substantially the same as that of Example 2, except that the number of impregnation layers is 8.

[0088] Comparative Example 5

[0089] This comparative example provides a FPU / 12-NH2-MIL-101(Fe) / APP composite material. The preparation method of the FPU / 12-NH2-MIL-101(Fe) / APP is substantially the same as that of Example 2, except that the number of impregnation layers is 12.

[0090] Comparative Example 6

[0091] This comparative example provides a FPU / 8-NH2-MIL-101(Fe) / SA composite material. The preparation method of the FPU / 8-NH2-MIL-101(Fe) / SA is substantially the same as that of Example 3, except that the number of impregnation layers is 8.

[0092] Comparative Example 7

[0093] This comparative example provides a FPU / 12-NH2-MIL-101(Fe) / SA composite material. The preparation method of the FPU / 12-NH2-MIL-101(Fe) / SA is substantially the same as that of Example 3, except that the number of impregnation layers is 12.

[0094] The FPU composite materials obtained in Examples 1 to 3 and Comparative Examples 1 to 7 were respectively subjected to oxygen index tests, and the results are shown in Table 1.

[0095] Table 1

[0096]

[0097]

[0098] It can be seen from Table 1 that the LOI index of the multilayer coating provided in the present invention is greatly improved compared with the pure FPU sample of Comparative Example 1, and within the scope of the present invention, the LOI index also increases with the increase in the number of layers.

[0099] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A layer-by-layer self-assembled flame retardant material, characterized in that: The layer-by-layer self-assembled flame retardant material is a layer-by-layer self-assembled flame retardant material formed by alternately coating anion solution and cation solution on the outside of a polyurethane foam substrate, and utilizing the interaction between anions in the anion solution and cations in the cation solution to self-assemble layer by layer to form a coating with flame retardant and fire warning functions; The weight of each layer of anionic solution coating accounts for 0.1-1.9%; the weight of each layer of cationic solution coating accounts for 0.15-2%, the number of layers of anionic solution coating is 3-15 layers, the number of layers of cationic solution coating is 3-15 layers, and the coatings of anionic solution and cationic solution are arranged alternately; The cationic substance in the cationic solution is a metal organic framework material with an amino group, and the metal organic framework material is one or more of NH2-MIL-53(Fe), NH2-MIL-53(Al), NH2-UiO-66(Hf), NH2-UiO-66(Zr), NH2-MIL-101(Cr), NH2-MIL-101(Fe) or NH2-MIL-101(Al); The anionic substance in the anionic solution is one or more of MXene or ammonium polyphosphate; The preparation method of the metal organic framework material is as follows: (1) Dissolve the metal compound in a container containing N, N-dimethylformamide (DMF) in a ratio of 2-3 g: 50-150 mL; (2) Dissolve 2-aminoterephthalic acid in DMF solution in a ratio of 1.6-1.9 g: 60-80 mL; (3) mixing the solution obtained in step (1) and the solution obtained in step (2), and stirring them thoroughly at room temperature to obtain a homogeneous suspension; (4) placing the homogenized suspension from step (3) in a high pressure reactor, transferring it to an oven, and maintaining it at 105-115° C. for 20-28 hours; (5) After the reaction of step (4) is completed, the product obtained in step (4) is cooled, centrifuged, washed with DMF, and then vacuum dried at 65-90° C. to obtain the product metal organic framework material.

2. A method for preparing a layer-by-layer self-assembled flame retardant material, characterized in that: The layer-by-layer self-assembled flame retardant material is the layer-by-layer self-assembled flame retardant material according to claim 1; the preparation method comprises the following steps: (1) Preparation of self-assembled cationic solution: dissolve the metal organic framework material in deionized water, and then slowly add an acidic solution to adjust the pH value of the mixed solution to 5.0-6.8; the ratio of the metal organic framework material to deionized water is 0.01-5 g: 10-500 mL; prepare a self-assembled cationic solution; (2) Preparation of self-assembled anion solution: anion substances are placed in deionized water to prepare anion solution, and a self-assembled anion solution with a mass percentage concentration of 1 wt% to 5 wt% is prepared; (3) Polyurethane foam pretreatment: immerse the entire polyurethane foam in a solution containing anionic surfactant for 5 to 10 minutes to form a negatively charged surface on the surface of the polyurethane foam, and then wash the immersed polyurethane foam with deionized water until it is neutral; (4) immersing the polyurethane foam obtained in step (3) into the self-assembled cationic solution obtained in step (1) for a soaking time of 1 to 10 minutes, then taking it out and immersing it into the self-assembled anionic solution obtained in step (2) for a soaking time of 0.5 to 8 minutes, and washing the polyurethane foam with deionized water before each immersion in the self-assembled cationic solution and before immersing it in the self-assembled anionic solution; (5) Repeat step (4) 3 to 15 times, then place the obtained sample in an oven and dry it at a temperature of 65 to 75° C. for 12 to 48 hours to obtain a layer-by-layer self-assembled flame retardant material with flame retardant and fire warning functions.

3. The preparation method according to claim 2, characterized in that: The acidic solution in step (1) is an acetic acid solution; in step (1), the acetic acid solution is added dropwise to adjust the pH value of the mixed solution to 6.0-6.

5.

4. The preparation method according to claim 2, characterized in that: The anionic surfactant in step (3) is one or more of polyacrylic acid (PAA), sodium dodecylbenzene sulfonate (SDBS) or sodium dodecyl sulfate (SDS).

5. The preparation method according to claim 2, characterized in that: The mass percentage concentration of the anionic surfactant in step (3) is 0.01wt%~0.6wt%.

6. The preparation method according to claim 2, characterized in that: In step (4), the immersion time in the self-assembled cationic solution is 1 to 5 minutes, and the immersion time in the self-assembled anionic solution is 1 to 5 minutes; the deionized water washing time is 1 to 3 minutes.

7. The preparation method according to claim 2, characterized in that: The polyurethane is hard polyurethane or soft polyurethane.

8. The preparation method according to claim 2, characterized in that: In step (5), drying is performed at a temperature of 70°C.

9. An application of a layer-by-layer self-assembled flame retardant material, characterized in that: The application is the dual application of the layer-by-layer self-assembled flame retardant material prepared by the preparation method described in claims 2 to 8 in fire warning and flame retardancy.

10. The use according to claim 9, characterized in that: In the dual application of fire warning and flame retardancy, the coating of the anionic solution and the coating of the cationic solution in the coating synergistically exert a flame retardant effect. At the same time, during the flame retardant process, the fire warning signal is transmitted through the drastic change of resistance, thereby achieving the role of fire warning.

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