Thermosensitive double-shell microcapsule flame retardant and preparation method and application thereof

Abstract

This invention relates to the field of flame retardant material preparation technology, specifically to a temperature-sensitive double-shell microcapsule flame retardant, its preparation method, and its application. The preparation method includes the following steps: (1) preparing powdered APP@SiO2 composite particles; (2) preparing the APP@SiO2@EC temperature-sensitive double-shell microcapsule flame retardant. The temperature-sensitive double-shell microcapsule flame retardant prepared by this invention has the characteristics of high initial decomposition temperature, strong weather resistance, and precise temperature-sensitive release, and has broad application prospects in the field of logistics fire prevention and control.

Description

Technical Field

[0001] This invention relates to the field of flame retardant material preparation technology, specifically to a temperature-sensitive double-shell microcapsule flame retardant, its preparation method, and its application. Background Technology

[0002] To meet the growing demands for energy security, logistics, and fire prevention, research on flame-retardant protection for electrochemical energy storage devices such as supercapacitors and high-performance paper-based packaging materials is rapidly emerging. Ammonium polyphosphate (APP), as an intumescent flame retardant, is considered an ideal flame-retardant additive due to its high phosphorus content, environmental friendliness, low toxicity, and excellent char-promoting ability in fires. However, in practical applications, APP faces serious performance bottlenecks: First, the large number of polar hydroxyl groups on the APP surface leads to its extremely high hygroscopicity, causing a sharp drop in flame-retardant efficiency in humid environments, and it is prone to migration to the substrate surface; second, raw APP particles have extremely poor compatibility with non-polar cellulose substrates (such as kraft paper), and direct addition can severely reduce the mechanical strength of the substrate.

[0003] To improve the environmental stability, thermal stability, and interface performance of an application surface (APP), an effective strategy is to introduce a functional shell layer onto the APP surface to construct a core-shell structure. Two-dimensional (2D) materials or inorganic nanolayers (such as SiO2) have excellent thermal shielding performance and physical strength, but when used alone for coating, they suffer from high brittleness and insufficient hydrophobicity. While organic polymers (such as EC) have good film-forming properties and are hydrophobic, they tend to melt too quickly in the early stages of a fire and cannot provide long-term protection.

[0004] If the physicochemical morphological characteristics of inorganic inner shells and organic shells can be combined to design inorganic / organic double-shell composite flame retardant materials, it is expected that the advantages of both can be combined to achieve a deep integration of "long-term moisture protection" and "temperature-sensitive release," meeting the urgent needs of industrial fire prevention. Therefore, this invention provides a temperature-sensitive double-shell microcapsule flame retardant, its preparation method, and its application. Summary of the Invention

[0005] The technical problem to be solved by this invention is to provide a temperature-sensitive double-shell microcapsule flame retardant, its preparation method, and its application. The aim is to provide a temperature-sensitive double-shell microcapsule flame retardant of an organic-inorganic hybrid composite material.

[0006] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: In a first aspect, a method for preparing a thermosensitive double-shell microcapsule flame retardant includes the following steps: (1) An inorganic inner shell of silica (SiO2) was coated on the surface of ammonium polyphosphate (APP) using the sol-gel method to obtain inorganic modified APP@SiO2 composite particles; the inorganic modified APP@SiO2 composite particles were then peeled, dispersed and dried in sequence to obtain powdered APP@SiO2 composite particles. (2) Ethyl cellulose (EC) and the powdered APP@SiO2 composite particles are dispersed in an organic solvent and ultrasonically treated to anchor the molecular chains of ethyl cellulose to the functional groups or active sites on the surface of the powdered APP@SiO2 composite particles. Then, by solvent evaporation, ethyl cellulose completes self-assembly and coating on the surface of the powdered APP@SiO2 composite particles to form an organic outer shell layer, thus obtaining APP@SiO2@EC thermosensitive double-shell microcapsule flame retardant.

[0007] Based on the above technical solution, the present invention can be further improved as follows.

[0008] Furthermore, the preparation of inorganically modified APP@SiO2 composite particles in step (1) includes the following specific steps: Ammonium polyphosphate was added to a mixed solvent of alcohol and deionized water, followed by ammonia catalyst. Then, silicon compound was added dropwise at a rate of 0.1-0.3 mL / min under conditions of 40-55℃ and 500-1000 rpm for 3.5-6 h. After the reaction was completed, the product was washed until the pH was 7-8.5 to obtain inorganically modified APP@SiO2 composite particles.

[0009] The alcohols include ethanol and methanol; the ammonia solution has a mass concentration of 25% to 28%. The cleaning product can be washed by centrifugation with ethanol and deionized water at 4500 to 6500 rpm.

[0010] Furthermore, the ratio of the amount of ammonium polyphosphate to the amount of the mixed solvent is (10~25) g : (40~70) mL; The ratio of ammonium polyphosphate to ammonia water is (10~25) g : (2~5) mL; The ratio of ammonium polyphosphate to silicon compound is (10~25) g: (8~16) mL; The volume ratio of alcohol to deionized water in the mixed solvent is 1:1 to 1:1.8; The silicon compound includes at least one of tetraethyl orthosilicate (TEOS), methyl silicate, methyltriethoxysilane, 3-aminopropyltriethoxysilane, and water glass (sodium silicate).

[0011] Further, the specific method of peeling, dispersing and drying in step (1) is as follows: the inorganic modified APP@SiO2 composite particles are sonicated under a protective gas for 1~2 h, and then centrifuged at 5500~8500 rpm for 0.5~1.5 h to obtain a suspension; the suspension is freeze-dried at -55~-65℃ for 24~48 h to obtain layered or granular inorganic modified APP@SiO2 composite particles.

[0012] The protective gas ultrasound is performed in an ice bath environment under an Ar or N2 protective atmosphere to prevent the APP from undergoing surface oxidation or hydrolysis in a strong sound field.

[0013] Furthermore, in step (2), the mass ratio of the ethyl cellulose to the powdered APP@SiO2 composite particles is 3~6:15~22; In step (2), the ratio of ethyl cellulose to organic solvent is (3~6) g : (40~90) mL; The organic solvent mentioned in step (2) includes at least one of ethyl acetate, butyl acetate, dichloromethane, ethanol, isopropanol, and n-butanol.

[0014] Furthermore, the conditions for ultrasonic treatment in step (2) are: ice water bath, power of 200W~400W, and time of 0.5~2h; The specific process of solvent evaporation in step (2) is as follows: heating at 55~75℃ and stirring at 400~800 rpm until the organic solvent is completely evaporated. It should be noted that after the organic solvent is completely evaporated, it is rinsed 3~5 times with deionized water and ethanol respectively, and then the product is dried in a vacuum oven at 65~80℃ for about 24 h to obtain APP@SiO2@EC thermosensitive double-shell microcapsule flame retardant.

[0015] Secondly, a temperature-sensitive double-shell microcapsule flame retardant is prepared by the aforementioned preparation method.

[0016] Thirdly, a flame-retardant kraft paper packaging material, wherein the flame-retardant kraft paper packaging material comprises the aforementioned temperature-sensitive double-shell microcapsule flame retardant.

[0017] Fourthly, a method for preparing a flame-retardant kraft paper packaging material includes the following steps: preparing a slurry from the aforementioned thermosensitive double-shell microcapsule flame retardant, dispersant, and binder; coating the slurry onto the surface of the kraft paper to be treated, followed by drying and sheeting to obtain the flame-retardant kraft paper packaging material. The drying is vacuum drying for 24-48 hours; the sheeting process uses a pressure of 10-15 MPa.

[0018] Furthermore, the mass ratio of the thermosensitive double-shell microcapsule flame retardant, the dispersant, and the binder is 6~10:1:1; The dispersant includes at least one of sodium lignosulfonate, sodium hexametaphosphate, and sodium polyacrylate; the binder includes at least one of sodium carboxymethyl cellulose (CMC-Na) and polyvinyl alcohol.

[0019] The thermosensitive double-shell microcapsule flame retardant of this invention is an organic-inorganic hybrid composite material. It first constructs a rigid inorganic framework (SiO2) and then anchors a flexible organic coating layer (ethyl cellulose). The preparation process is simple and reproducible. When applied to flame-retardant packaging materials such as kraft paper, it has broad application prospects in the fields of energy storage safety and logistics fire prevention.

[0020] The beneficial effects of this invention are: (1) Significantly improved thermal stability: The presence of the SiO2 inner shell forms an effective thermal shielding effect, which prevents the overflow of thermal degradation products of internal APP, and the initial decomposition temperature can be increased by more than 40℃.

[0021] (2) Excellent hydrophobic and water-resistant properties: The outermost EC organic film provides a tight hydrophobic barrier. In actual tests, after 48 hours of exposure in an 85% RH environment, its moisture absorption rate was reduced by more than 80% compared with the original APP, solving the problem of APP failure.

[0022] (3) Precise temperature-sensitive release: The organic shell cracks or melts at a specific fire temperature (260~310℃), thus acting as a "switch" to release the internal high-efficiency flame retardant, achieving precise flame retardant intervention.

[0023] (4) Improve substrate strength: EC has good chemical compatibility with kraft paper fiber. Compared with the original APP, this product can effectively maintain or even enhance the tensile strength of kraft paper. Attached Figure Description

[0024] Figure 1 This is a scanning electron microscope (SEM) image of the original APP particles of this invention; Figure 2 This is a scanning electron microscope (SEM) image of the inorganic inner shell modified APP@SiO2 of the present invention; Figure 3 This is a scanning electron microscope (SEM) image of the APP@SiO2@EC double-shell microcapsule flame retardant (MAPP) synthesized in Example 1 of the present invention; Figure 4 Fourier transform infrared (FTIR) spectra of the original APP particles, inorganic inner shell modified APP@SiO2, and APP@SiO2@EC prepared in Example 1 of this invention; Figure 5 Thermogravimetric analysis (TGA) comparison curves of the original APP, APP@SiO2 and APP@SiO2@EC prepared in Example 1 of this invention; Figure 6 Comparison of contact angle tests for the original APP, APP@SiO2, and APP@SiO2@EC prepared in Example 1; Figure 7 This is a comparison of the morphology of the original APP, Comparative Example 1, and Example 1 flame retardant materials under vertical burning test (VBT). Detailed Implementation

[0025] The principles and features of this invention are described below. The examples given are for illustrative purposes only and are not intended to limit the scope of the invention. Where specific techniques or conditions are not specified in the embodiments, they should be performed according to the techniques or conditions described in the literature in this field, or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased through legitimate channels.

[0026] This embodiment relates to a method for preparing a temperature-sensitive double-shell microcapsule flame retardant, comprising the following steps: (1) An inorganic inner shell of silica (SiO2) was coated on the surface of ammonium polyphosphate (APP) using the sol-gel method to obtain inorganic modified APP@SiO2 composite particles; the inorganic modified APP@SiO2 composite particles were then peeled, dispersed and dried in sequence to obtain powdered APP@SiO2 composite particles. (2) Ethyl cellulose (EC) and the powdered APP@SiO2 composite particles are dispersed in an organic solvent and ultrasonically treated to anchor the molecular chains of ethyl cellulose to the functional groups or active sites on the surface of the powdered APP@SiO2 composite particles. Then, by solvent evaporation, ethyl cellulose completes self-assembly and coating on the surface of the powdered APP@SiO2 composite particles to form an organic outer shell layer, thus obtaining APP@SiO2@EC thermosensitive double-shell microcapsule flame retardant.

[0027] Preferably, in step (1) of this embodiment, the preparation of inorganically modified APP@SiO2 composite particles includes the following specific steps: Ammonium polyphosphate was added to a mixed solvent of alcohol and deionized water, followed by ammonia catalyst. Then, under conditions of 40-55℃ and 500-1000 rpm (e.g., 40℃, 50℃, 55℃, 500 rpm, 800 rpm, 1000 rpm), silicon compound was added dropwise at a rate of 0.1-0.3 mL / min (e.g., 0.1 mL / min, 0.2 mL / min, 0.3 mL / min) for 3.5-6 h (e.g., 3.5 h, 4 h, 6 h). After the reaction was completed, the product was washed until the pH was 7-8.5 to obtain inorganically modified APP@SiO2 composite particles.

[0028] The alcohols include ethanol and methanol; the ammonia solution has a mass concentration of 25% to 28%. The cleaning product can be washed by centrifugation with ethanol and deionized water at 4500 to 6500 rpm.

[0029] Preferably, in this embodiment, the ratio of ammonium polyphosphate to the mixed solvent is (10~25) g: (40~70) mL, for example, 10 g: 40 mL, 15 g: 55 mL, 25 g: 70 mL, etc. The ratio of ammonium polyphosphate to ammonia water is (10~25) g: (2~5) mL, for example, 10 g: 2 mL, 15 g: 3 mL, 25 g: 5 mL, etc. The ratio of the ammonium polyphosphate to the silicon compound is (10~25) g: (8~16) mL, for example, 10 g: 8 mL, 15 g: 12 mL, 25 g: 16 mL, etc. The volume ratio of alcohol to deionized water in the mixed solvent is 1:1 to 1:1.8, for example, 1:1, 1:1.4, 1:1.8, etc. The silicon compound includes at least one of tetraethyl orthosilicate (TEOS), methyl silicate, methyltriethoxysilane, 3-aminopropyltriethoxysilane, and water glass (sodium silicate).

[0030] Preferably, the specific method of peeling, dispersing and drying in step (1) of this embodiment is as follows: the inorganic modified APP@SiO2 composite particles are sonicated under a protective gas for 1-2 h, for example, 1 h, 2 h, etc., and then centrifuged at 5500-8500 rpm for 0.5-1.5 h, for example, centrifuged at 5500 rpm, 7000 rpm, 8500 rpm for 0.5 h, 1 h, 1.5 h, etc., to obtain a suspension; the suspension is freeze-dried at -55~-65℃ for 24-48 h to obtain layered or granular inorganic modified APP@SiO2 composite particles.

[0031] The protective gas ultrasound is performed in an ice bath environment under an Ar or N2 protective atmosphere to prevent the APP from undergoing surface oxidation or hydrolysis in a strong sound field.

[0032] Preferably, in step (2) of this embodiment, the mass ratio of ethyl cellulose to the powdered APP@SiO2 composite particles is 3~6:15~22, for example 3:15, 4.5:18, 6:22, etc.; The ratio of ethyl cellulose to organic solvent in step (2) is (3~6) g: (40~90) mL, for example 3 g: 40 mL, 4 g: 60 mL, 6 g: 90 mL, etc.; The organic solvent mentioned in step (2) includes at least one of ethyl acetate, butyl acetate, dichloromethane, ethanol, isopropanol, and n-butanol.

[0033] Preferably, the conditions for ultrasonic treatment in step (2) of this embodiment are: ice water bath, power of 200W~400W, preferably 300W, and time of 0.5~2 h; The specific process of solvent evaporation in step (2) is as follows: heating at 55~75℃ and stirring at 400~800 rpm, for example, heating at 65℃ and stirring at 600 rpm, until the organic solvent is completely evaporated. It should be noted that after the organic solvent is completely evaporated, it is rinsed 3~5 times with deionized water and ethanol respectively, and then the product is dried in a vacuum oven at 65~80℃ for about 24 h to obtain APP@SiO2@EC thermosensitive double-shell microcapsule flame retardant.

[0034] This embodiment also relates to a temperature-sensitive double-shell microcapsule flame retardant, which is prepared by the aforementioned method.

[0035] This embodiment also relates to a flame-retardant kraft paper packaging material, which includes the aforementioned temperature-sensitive double-shell microcapsule flame retardant.

[0036] This embodiment also relates to a method for preparing a flame-retardant kraft paper packaging material, comprising the following steps: preparing a slurry from the aforementioned thermosensitive double-shell microcapsule flame retardant, dispersant, and binder; coating the slurry onto the surface of the kraft paper to be treated, followed by drying and sheeting processes to obtain the flame-retardant kraft paper packaging material. The drying is vacuum drying for 24-48 hours; the sheeting process uses a pressure of 10-15 MPa.

[0037] Preferably, the mass ratio of the thermosensitive double-shell microcapsule flame retardant, the dispersant, and the binder in this embodiment is 6~10:1:1, for example, 8:1:1, etc. The dispersant includes at least one of sodium lignosulfonate, sodium hexametaphosphate, and sodium polyacrylate; the binder includes at least one of sodium carboxymethyl cellulose (CMC-Na) and polyvinyl alcohol.

[0038] As can be seen, the thermosensitive double-shell microcapsule flame retardant of the present invention is an organic-inorganic hybrid composite material. First, a rigid inorganic framework (SiO2) is constructed, and then a flexible organic coating layer (ethyl cellulose) is anchored. The preparation process is simple and has good repeatability. Specific examples are provided below for further explanation.

[0039] Example 1 The preparation of the thermosensitive double-shell microcapsule flame retardant in this embodiment includes the following steps: (1) Preparation of inorganic inner shell modified APP (APP@SiO2 particles): 20 g of original ammonium polyphosphate (APP) particles were prepared, and their scanning electron microscope (SEM) images are shown below. Figure 1 As shown, the mixture was added at a rate of 0.3 g / time to a solution consisting of 40 mL ethanol and 50 mL deionized water. 3 mL of ammonia was added to adjust the pH. Under constant temperature water bath conditions of 800 rpm and 45℃, 12 mL of TEOS (tetraethyl orthosilicate, Si(OC2H5)4) was continuously added dropwise at a rate of 0.15 mL / min. The reaction was carried out for 5 h. The product was repeatedly centrifuged and washed at 5500 rpm until pH = 7.2, yielding inorganically modified APP@SiO2 composite particles. The scanning electron microscope (SEM) image is shown below. Figure 2 As shown.

[0040] (2) Peeling, dispersion and freeze drying: The inorganic modified APP@SiO2 composite particles were subjected to ice bath sonication for 1.5 h under Ar gas protection, followed by centrifugation at 6500 rpm for 1 h. The precipitate was collected and placed in a freeze dryer and vacuum dried at -60℃ for 36 h to obtain loose powdered APP@SiO2 composite particles.

[0041] (3) Organic shell self-assembly coating: Take 18 g of powdered APP@SiO2 composite particles obtained in step (2) and 3 g of ethyl cellulose (EC) and disperse them in 50 mL of ethyl acetate solution. Disperse them by ultrasonication for 1.5 h. Transfer the mixture to a constant temperature stirrer, control the speed to 600 rpm, and heat it to 60℃.

[0042] (4) Product Collection and Post-processing: After the ethyl acetate has completely evaporated, the solid product is taken out, rinsed with a small amount of ethanol, and then dried in a vacuum oven at 80℃ for 24 h to obtain the final product APP@SiO2@EC thermosensitive double-shell microcapsule flame retardant (MAPP). Its scanning electron microscope (SEM) image is shown below. Figure 3As shown.

[0043] Example 2 The difference between Example 2 and Example 1 is that in step (3): 5 g EC is selected and the solvent is replaced with dichloromethane; in step (4), the evaporation temperature is reduced to 50°C; the coating thickness of the obtained product APP@SiO2@EC thermosensitive double-shell microcapsule flame retardant is increased by about 25% compared with Example 1, and the measured hydrophobic properties are further optimized, making it suitable for extremely humid industrial environments.

[0044] Example 3 The preparation of the thermosensitive double-shell microcapsule flame retardant in this embodiment includes the following steps: (1) Take 10 g of raw APP particles and add them to a solvent consisting of 40 mL of mixed solution (ethanol to deionized water volume ratio: 1:1.8); add 2 mL of ammonia catalyst, and add 8 mL of tetraethyl orthosilicate dropwise at a rate of 0.1 mL / min under the conditions of 500 rpm and 40℃, and react for 6 h. Wash until the pH is 7.0.

[0045] (2) Sonicate under protective gas for 2 h, centrifuge at 5500 rpm for 1.5 h, and then freeze dry at -65℃ for 48 h.

[0046] (3) Take 15 g of the obtained powder and 3 g of ethyl cellulose (EC) and disperse them in 40 mL of ethanol. After sonication for 2 h, stir at 55 °C until the solvent is completely evaporated.

[0047] (4) The prepared MAPP particles have a complete double shell structure, which demonstrates that they still have a good coating effect even at low dosage.

[0048] Example 4 The preparation of the thermosensitive double-shell microcapsule flame retardant in this embodiment includes the following steps: (1) Take 25 g of raw APP particles and add them to a solvent consisting of 70 mL of mixed solution (ethanol and deionized water volume ratio: 1:1); add 5 mL of ammonia catalyst, and add 16 mL of tetraethyl orthosilicate dropwise at a rate of 0.3 mL / min under the conditions of 1000 rpm and 55℃, and react for 3.5 h. Wash until the pH is 8.5.

[0049] (2) Sonicate under protective gas for 1 h, centrifuge at 8500 rpm for 0.5 h, and then freeze dry at -55℃ for 24 h.

[0050] (3) Take 22 g of the obtained powder and 6 g of ethyl cellulose (EC) and disperse them in 90 mL of ethyl acetate. After sonication for 0.5 h, stir at 75 °C until the solvent is completely evaporated.

[0051] (4) The product has excellent hydrophobic properties, extremely low moisture absorption rate after exposure to 85%RH environment for 48 h, and stronger film adhesion on kraft paper surface.

[0052] Comparative Example 1 This comparative example provides a method for preparing a single inorganic inner shell flame retardant: compared with Example 1, only steps (1) and (2) in Example 1 are performed. After obtaining APP@SiO2, no further organic coating process is performed. Instead, it is directly made into flame retardant paper according to the method in Example 2.

[0053] Comparative Example 2 This comparative example provides a method for preparing a single organic shell flame retardant: compared with Example 1, steps (1) and (2) are skipped, and 20 g of raw APP and 3 g of EC are directly mixed in ethyl acetate and coated by evaporation and crystallization according to step (4).

[0054] Test case (1) Performance testing: ① Fourier transform infrared spectroscopy: The prepared APP@SiO2@EC double-shell microcapsule flame retardant, APP@SiO2 and the original APP were tested by TGA ( Figure 4 The results showed that, through comparative analysis of FTIR spectra, the material retained the core characteristic peak of ammonium polyphosphate (APP) (1435 cm⁻¹). -1 NH4 + Bending vibration and 1250 cm -1 Based on the P=O stretching vibration, APP@SiO2 at 1080 cm⁻¹ -1 The peak shape at 800 cm⁻¹ is significantly broadened. -1 The signal changes at this location confirmed the successful deposition of the first layer of SiO2; while the 2850-2980 cm⁻¹ region in APP@SiO2@EC... -1 Area and 1375 cm -1 The newly added CH characteristic peak confirms the outermost ethyl cellulose (EC) coating; combined with the physical shielding weakening trend of the core material characteristic signals at each stage as the number of shell layers increases, it strongly proves the successful preparation of the APP@SiO2@EC double-shell microcapsule structure.

[0055] ② Thermogravimetric analysis: The prepared APP@SiO2@EC double-shell microcapsule flame retardant, APP@SiO2, and the original APP were subjected to TGA testing ( Figure 5 The results showed that the initial decomposition temperature of APP@SiO2@EC was increased by more than 40℃ compared with the original APP, indicating that the double-shell structure has excellent thermal stabilization properties.

[0056] ③ Hydrophobicity test: Contact angle tests were performed on the prepared APP@SiO2@EC double-shell microcapsule flame retardant, APP@SiO2, and the original APP. Figure 6 The results showed that the contact angle of APP@SiO2@EC was about twice that of the original APP, indicating that the double-shell structure has excellent waterproof performance.

[0057] (2) Application example: The preparation of a flame-retardant kraft paper includes the following steps: MAPP material prepared according to the examples or samples prepared according to the comparative examples are mixed with dispersant (sodium lignosulfonate) and binder (CMC-Na) at a mass ratio of 8:1:1, and an appropriate amount of water is added to grind into a uniform slurry; the slurry is then uniformly coated onto a 120 g / m² sheet using an automated coating machine. 2 The surface of the kraft paper was dried under vacuum for 24 hours, and then flattened under a pressure of 12 MPa to obtain a flame-retardant kraft paper sample for the following experimental verification.

[0058] ① Flame retardant performance test: Vertical burning test (VBT) was conducted on the flame retardant kraft paper prepared in Example 1, Comparative Example 1, and Comparative Example 2. Figure 7 The vertical combustion procedure is as follows: First, the sample is cut to standard size and conditioned under constant temperature and humidity conditions, then vertically fixed on a support; a Bunsen burner is used to apply a first flame (10 s) at the center of the bottom of the sample, and the flaming time t1 is recorded after the flame is removed; immediately after the flame is extinguished, a second 10 s flame application is performed, and the second flaming time t2 and the smoldering time t3 are recorded, while monitoring for ignition of molten drips through the degreased cotton laid under the sample throughout the process. Finally, the flame retardancy rating of the material is comprehensively evaluated based on the values ​​of t1, t2, and t3, as well as whether drips ignite the cotton. The results show that Example 1, using APP@SiO2@EC double-shell microcapsule flame retardant, has the best flame retardant effect, and the paper base structure remains intact, proving that the double-shell structure achieves a highly efficient synergistic flame retardant effect.

[0059] In summary, (1) thermal stability is significantly improved: the presence of the SiO2 inner shell forms an effective thermal shielding effect, which prevents the overflow of thermal degradation products of the internal APP, and the initial decomposition temperature can be increased by more than 40℃. (2) excellent hydrophobic and water-resistant properties: the outermost EC organic film provides a tight hydrophobic barrier. In actual tests, after exposure to 85% RH environment for 48 h, its moisture absorption rate was reduced by more than 80% compared with the original APP, solving the problem of easy failure of APP. (3) precise temperature-sensitive release: the organic shell cracks or melts at a specific fire temperature (260~310℃), thus acting as a "switch" to release the internal high-efficiency flame retardant, realizing precise flame retardant intervention. (4) improved substrate strength: EC has good chemical compatibility with kraft paper fiber. Compared with the original APP, this product can effectively maintain or even enhance the tensile strength of kraft paper.

[0060] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for preparing a temperature-sensitive double-shell microcapsule flame retardant, characterized in that, Includes the following steps: (1) An inorganic silica inner shell layer was coated on the surface of ammonium polyphosphate using the sol-gel method to obtain inorganic modified APP@SiO2 composite particles; the inorganic modified APP@SiO2 composite particles were then peeled, dispersed and dried in sequence to obtain powdered APP@SiO2 composite particles. (2) Ethyl cellulose and the powdered APP@SiO2 composite particles are dispersed in an organic solvent and ultrasonically treated to anchor the molecular chains of ethyl cellulose to the functional groups or active sites on the surface of the powdered APP@SiO2 composite particles. Then, by solvent evaporation, ethyl cellulose completes self-assembly and coating on the surface of the powdered APP@SiO2 composite particles to form an organic outer shell layer, thus obtaining APP@SiO2@EC thermosensitive double-shell microcapsule flame retardant.

2. The method for preparing a thermosensitive double-shell microcapsule flame retardant according to claim 1, characterized in that, The preparation of inorganically modified APP@SiO2 composite particles in step (1) includes the following specific steps: Ammonium polyphosphate was added to a mixed solvent of alcohol and deionized water, followed by ammonia catalyst. Then, silicon compound was added dropwise at a rate of 0.1–0.3 mL / min under conditions of 40–55 °C and 500–1000 rpm for 3.5–6 h. After the reaction was completed, the product was washed until the pH was 7–8.5 to obtain inorganically modified APP@SiO2 composite particles.

3. The method for preparing a thermosensitive double-shell microcapsule flame retardant according to claim 2, characterized in that, The ratio of ammonium polyphosphate to the mixed solvent is (10~25) g : (40~70) mL; The ratio of ammonium polyphosphate to ammonia water is (10~25) g : (2~5) mL; The ratio of ammonium polyphosphate to silicon compound is (10~25) g: (8~16) mL; The volume ratio of alcohol to deionized water in the mixed solvent is 1:1 to 1:1.8; The silicon compound includes at least one of tetraethyl orthosilicate, methyl silicate, methyltriethoxysilane, 3-aminopropyltriethoxysilane, and water glass.

4. The method for preparing a thermosensitive double-shell microcapsule flame retardant according to claim 1, characterized in that, The specific method for peeling, dispersing and drying in step (1) is as follows: the inorganic modified APP@SiO2 composite particles are sonicated under a protective gas for 1-2 hours, and then centrifuged at 5500-8500 rpm for 0.5-1.5 hours to obtain a suspension; the suspension is freeze-dried at -55 to -65℃ for 24-48 hours to obtain the inorganic modified APP@SiO2 composite particles.

5. The method for preparing a thermosensitive double-shell microcapsule flame retardant according to claim 1, characterized in that, The mass ratio of ethyl cellulose to the powdered APP@SiO2 composite particles in step (2) is 3~6:15~22; In step (2), the ratio of ethyl cellulose to organic solvent is (3~6) g : (40~90) mL; The organic solvent mentioned in step (2) includes at least one of ethyl acetate, butyl acetate, dichloromethane, ethanol, isopropanol, and n-butanol.

6. The method for preparing a thermosensitive double-shell microcapsule flame retardant according to claim 1, characterized in that, The conditions for ultrasonic treatment in step (2) are: ice water bath, power of 200W~400W, and time of 0.5~2 h; The specific process of solvent evaporation in step (2) is as follows: heating at 55~75℃ and stirring at 400~800 rpm until the organic solvent is completely evaporated.

7. A temperature-sensitive double-shell microcapsule flame retardant, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 6.

8. A flame-retardant kraft paper packaging material, characterized in that, The flame-retardant kraft paper packaging material includes the thermosensitive double-shell microcapsule flame retardant as described in claim 7.

9. A method for preparing a flame-retardant kraft paper packaging material according to claim 8, characterized in that, The process includes the following steps: preparing a slurry from the thermosensitive double-shell microcapsule flame retardant, dispersant, and binder as described in claim 7; coating the slurry onto the surface of kraft paper to be treated, and then sequentially drying and pressing the slurry to obtain flame-retardant kraft paper packaging material.

10. The method for preparing a flame-retardant kraft paper packaging material according to claim 9, characterized in that, The mass ratio of the thermosensitive double-shell microcapsule flame retardant, the dispersant, and the binder is 6~10:1:1; The dispersant includes at least one of sodium lignosulfonate, sodium hexametaphosphate, and sodium polyacrylate; the binder includes at least one of sodium carboxymethyl cellulose and polyvinyl alcohol.

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