Preparation Method and Application of Crystalline Ammonium Polyphosphate Type II with a Lamellar Structure

Abstract

The present invention provides a method for preparing crystalline ammonium polyphosphate type II with a layered structure and its application. The method includes: S1: Add phosphorus pentoxide and diammonium hydrogen phosphate, stir evenly, then introduce ammonia. The ammonia introduction amount is in a ratio with the reactants, and the ammonia introduction amount per 100 kg of reactants is 0.5 - 2 m³ / h. Heat and react for 1 h - 3 h, then stop introducing ammonia; S2: Open the vacuum, stir the product obtained in step S1, and spray and add an aqueous urea solution preheated to 65°C - 70°C within 0.5 h - 2 h. After adding, continue to stir for 10 minutes - 20 minutes, and then close the vacuum; S3: Stir, introduce preheated ammonia gas at 120°C - 150°C, and the ammonia introduction amount is in a ratio with the reactants, and the ammonia introduction amount per 100 kg of reactants is 1 m³ / h - 6 m³ / h. Heat and react; S4: Stop heating and stir, introduce un-preheated ammonia gas, and the ammonia introduction amount is in a ratio with the reactants, and the ammonia introduction amount per 100 kg of reactants is 0.5 m³ / h - 2 m³ / h. When the material temperature drops below 150°C, discharge, cool, and pulverize to obtain crystalline ammonium polyphosphate type II with a layered structure.

Description

Technical Field

[0001] The present invention relates to the technical field of ammonium polyphosphate modification, and particularly to a preparation method and application of crystalline type II ammonium polyphosphate with a layered structure. Background Art

[0002] Ammonium polyphosphate is a phosphorus-nitrogen type halogen-free flame retardant, which mainly serves as an acid source in the halogen-free intumescent flame retardant system and also has a certain gas source function. It is known that ammonium polyphosphate has six crystal structures, and the most commonly used ones are crystalline type I and crystalline type II. The crystal type can be judged by the X-ray diffraction pattern, and the strongest peak of type II appears at 15.52°. Crystalline type II ammonium polyphosphate has lower water solubility and more complete crystals than crystalline type I. In the application of ultra-thin steel structure fire retardant coatings, crystalline type II ammonium polyphosphate has better storage stability, higher fire resistance grade and anti-migration performance. As is well known, after the temperature of steel reaches 550°C, its strength is not as good as that of wood. Therefore, the fire retardant coating for steel needs to have characteristics such as high magnification expansion in a high-temperature environment to reduce fire losses and casualties.

[0003] In recent years, the flame retardancy of ammonium polyphosphate has been improved by intercalating ammonium polyphosphate into inorganic materials such as zirconium phosphate and hydrotalcite. However, the addition of inorganic materials in the above methods inevitably reduces the phosphorus content of ammonium polyphosphate, and interferes with the crystal morphology of ammonium polyphosphate. In addition, some methods need to be carried out in a medium, the process is complex, and there is wastewater containing sodium chloride. Summary of the Invention

[0004] On the premise of not reducing the phosphorus content of ammonium polyphosphate and not interfering with the crystal morphology of ammonium polyphosphate, the present invention proposes a method for rapid crystallization through a process and urea to prepare crystalline type II ammonium polyphosphate with a layered microstructure. The prepared crystalline type II ammonium polyphosphate with a layered structure has good storage stability and a high flame retardant and fire protection grade in waterborne ultra-thin intumescent fire retardant coatings.

[0005] In order to solve at least one of the above technical problems, a preparation method of crystalline type II ammonium polyphosphate with a layered structure provided by the present invention includes:

[0006] S1: Add phosphorus pentoxide and diammonium hydrogen phosphate and stir evenly, then introduce ammonia. The amount of ammonia introduced is in a ratio of 0.5 m 3 / h - 2 m 3 / h per 100 kg of reactants, heat and react for 1 h - 3 h and then stop introducing ammonia;

[0007] S2: Evacuate to a vacuum degree of -0.02 MPa to -0.04 MPa, stir the product obtained in step S1 at a stirring speed of 120 r / min - 180 r / min, slowly spray and add an aqueous urea solution preheated to 65°C - 70°C within 0.5 h - 2 h, continue stirring for 10 min - 20 min after addition, and then close the vacuum;

[0008] S3: Stir at a reduced stirring speed of 100 r / min - 160 r / min, introduce ammonia preheated to 120°C - 150°C, and the ratio of ammonia amount to reactants is that the ammonia introduction amount per 100 kg of reactants is 1 m 3 / h - 6 m 3 / h, and heat and react for 2 h - 6 h;

[0009] S4: Stop heating and stir at a stirring speed of 50 r / min - 80 r / min, introduce non-preheated ammonia, and the ratio of ammonia introduction amount to reactants is that the ammonia introduction amount per 100 kg of reactants is 0.5 m 3 / h - 2 m 3 / h. When the material temperature drops below 150°C, discharge, cool, and pulverize to obtain crystalline ammonium polyphosphate type II with a layered structure.

[0010] Optionally, the molar ratio of phosphorus pentoxide to diammonium hydrogen phosphate is 100:(95 - 105), and the molar ratio of phosphorus pentoxide, diammonium hydrogen phosphate, and urea is 100:(95 - 105):(0.01 - 10). Preferably, the molar ratio of phosphorus pentoxide, diammonium hydrogen phosphate, and urea is preferably 100:(95 - 105):(0.5 - 5).

[0011] Optionally, the urea concentration of the aqueous urea solution is 68 wt.% - 72 wt.%, for example, it can be 70 wt.%.

[0012] Optionally, the molar ratio of urea to water in the preparation of the aqueous urea solution is 1:1.

[0013] Optionally, the content of phosphorus pentoxide ≥ 99.5%, the active R value ≥ 1.7, and the proportion of reducing substances ≤ 0.005%

[0014] Optionally, in step S1, the stirring speed is 50 r / min - 80 r / min, and stir for 20 min - 30 min.

[0015] Optionally, in step S3, the ratio of ammonia introduction amount to reactants is that the ammonia introduction amount per 100 kg of reactants is 1 m 3 / h - 4 m 3 / h.

[0016] Optionally, step S1 is carried out in a horizontal reactor. When heating the reaction, the heat transfer oil temperature is set at 150°C - 180°C. In step S3, when heating the reaction, the heat transfer heating oil temperature is set at 200 - 240°C.

[0017] Optionally, the excess ammonia gas is absorbed by phosphoric acid to prepare diammonium hydrogen phosphate for use as a raw material.

[0018] The present application also provides the use of the crystalline ammonium polyphosphate of type II with a layered structure prepared by any of the above preparation methods. It is characterized in that the crystalline ammonium polyphosphate of type II is used in a waterborne ultra-thin intumescent fireproof coating after being combined with an auxiliary agent, solvent water, vinyl acetate emulsion, pentaerythritol, and melamine. Here, the ultra-thin is a consensus in the industry, relative to cement mortar.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: a layered ammonium polyphosphate salt is formed, which has a flaky structure and can accelerate the expansion of APP. At the same time, the phosphorus content of the obtained layered ammonium polyphosphate salt remains above 31% (by mass), and the flame retardant efficiency is not affected.

[0020] Moreover, the ultra-thin steel structure fireproof coating using ammonium polyphosphate can achieve high-magnification expansion in a fire, forming a carbon layer with a dense surface and porous interior, having heat insulation and fire resistance properties, and having characteristics such as resistance to external physical impact, explosion shock, earthquake resistance, and anti-cracking. It can delay and prevent the temperature rise of the steel structure, reducing fire losses and casualties. Description of the Drawings

[0021] Figure 1 It is the SEM image of the crystalline ammonium polyphosphate of type II with a layered structure provided in Example 1 of the present invention;

[0022] Figure 2 It is the infrared spectrum of the crystalline ammonium polyphosphate of type II with a layered structure provided in Example 1 of the present invention;

[0023] Figure 3 It is the infrared spectrum of commercially available crystalline ammonium polyphosphate of type II;

[0024] Figure 4 It is the X-ray diffraction pattern of the crystalline ammonium polyphosphate of type II with a layered structure provided in Example 1 of the present invention;

[0025] Figure 5 It is the TGA curve of the crystalline ammonium polyphosphate of type II with a layered structure provided in Example 1 of the present invention;

[0026] Figure 6 It is the SEM image of the crystalline ammonium polyphosphate of type II provided in Comparative Example 2 of the present invention. Detailed Embodiments

[0027] The following will further describe in detail the specific implementation manners of the present invention in conjunction with the accompanying drawings and embodiments. The following examples are used to illustrate the present invention, but are not used to limit the scope of the present invention.

[0028] Example 1

[0029] S1. Add 100 kg (704.5 moles) of phosphorus pentoxide and 92.1 kg (697.5 moles) of diammonium hydrogen phosphate into a horizontal reactor, with a stirring speed of 70 r / min. After stirring evenly for 20 - 30 minutes, introduce ammonia. The ratio of the ammonia introduction amount to the reactants is 1.0 m 3 / h, set the heat conduction oil temperature to 170 °C, and stop introducing ammonia after reacting for 2 h;

[0030] S2. Open the vacuum, with a vacuum degree of -0.03 MPa, adjust the stirring speed to 150 r / min, and slowly spray and add 1.81 kg (containing 21.13 moles of urea) of 70% wt urea aqueous solution preheated to 65 - 70 °C within 1.5 h. After adding, continue to stir for 10 - 20 minutes, and then close the vacuum;

[0031] S3. Adjust the stirring speed to 120 r / min, introduce ammonia, preheat the ammonia to 120 - 150 °C, and the ratio of the ammonia introduction amount to the reactants is 3 m 3 / h, set the heat conduction heating oil temperature to 220 °C, and react for 3 hours;

[0032] S4. Stop heating, adjust the stirring to 70 r / min, continue to introduce ammonia, without preheating the ammonia, and the ratio of the ammonia introduction amount to the reactants is 1 m 3 / h. When the material temperature drops below 150 °C, discharge, cool, and pulverize to obtain crystalline type II ammonium polyphosphate with a layered structure.

[0033] SEM shows Figure 1 and infrared shows Figure 2 and X - diffraction shows Figure 4 and TGA shows Figure 5 .

[0034] Example 2

[0035] S1. Add 100 kg (704.5 moles) of phosphorus pentoxide and 93 kg (704.5 moles) of diammonium hydrogen phosphate into a horizontal reactor, with a stirring speed of 70 r / min. After stirring evenly for 20 - 30 minutes, introduce ammonia. The ratio of the ammonia introduction amount to the reactants is 1 m 3 / h, set the heat conduction oil temperature to 150 °C, and stop introducing ammonia after reacting for 2 h;

[0036] S2. Open the vacuum to a vacuum degree of -0.03 MPa, adjust the stirring speed to 160 r / min, slowly spray and add 1.21 kg (containing 14.09 moles of urea) of 70% wt urea aqueous solution preheated to 65 - 70 °C within 1 hour, continue stirring for 10 - 20 minutes after addition, and then close the vacuum;

[0037] S3. Adjust the stirring speed to decrease by 100 r / min, introduce ammonia, preheat the ammonia gas to 120 - 150 °C, the ammonia introduction amount is in a ratio of 4 m per 100 kg of reactants, set the heat conduction heating oil temperature to 220 °C, and react for 2 hours; 3 / h, set the heat conduction heating oil temperature to 220 °C, and react for 2 hours;

[0038] S4. Stop heating, adjust the stirring to 70 r / min, continue to introduce ammonia, without preheating the ammonia gas, the ammonia introduction amount is in a ratio of 1 m per 100 kg of reactants, discharge the material when the material temperature drops below 150 °C, cool and pulverize to obtain crystalline type II ammonium polyphosphate with a layered structure. 3 / h, discharge the material when the material temperature drops below 150 °C, cool and pulverize to obtain crystalline type II ammonium polyphosphate with a layered structure.

[0039] Example 3

[0040] S1. Add 100 kg (704.5 moles) of phosphorus pentoxide and 97.65 kg (739.7 moles) of diammonium hydrogen phosphate into a horizontal reactor, set the stirring speed to 70 r / min, stir for 20 - 30 minutes until uniform, then introduce ammonia, the ammonia introduction amount is in a ratio of 2 m per 100 kg of reactants, set the heat conduction oil temperature to 170 °C, and stop introducing ammonia after reacting for 1 h; 3 / h, set the heat conduction oil temperature to 170 °C, and stop introducing ammonia after reacting for 1 h;

[0041] S2. Open the vacuum to a vacuum degree of -0.02 MPa, adjust the stirring speed to 180 r / min, slowly spray and add 0.31 kg (containing 3.52 moles of urea) of urea aqueous solution preheated to 65 - 70 °C within 0.5 hour, continue stirring for 10 - 20 minutes after addition, and then close the vacuum;

[0042] S3. Adjust the stirring speed to decrease by 140 r / min, introduce ammonia, preheat the ammonia gas to 120 - 150 °C, the ammonia introduction amount is in a ratio of 2 m per 100 kg of reactants, set the heat conduction heating oil temperature to 240 °C, and react for 4 hours; 3 / h, set the heat conduction heating oil temperature to 240 °C, and react for 4 hours;

[0043] S4. Stop heating, adjust the stirring to 80 r / min, continue to introduce ammonia, without preheating the ammonia gas, the ammonia introduction amount is in a ratio of 2 m per 100 kg of reactants, discharge the material when the material temperature drops below 150 °C, cool and pulverize to obtain crystalline type II ammonium polyphosphate with a layered structure. 3 / h, discharge the material when the material temperature drops below 150 °C, cool and pulverize to obtain crystalline type II ammonium polyphosphate with a layered structure.

[0044] Example 4

[0045] S1. Add 100 kg (704.5 moles) of phosphorus pentoxide and 88.40 kg (669.3 moles) of diammonium hydrogen phosphate into a horizontal reactor. Set the stirring speed at 70 r / min. After stirring for 20 - 30 minutes until homogeneous, introduce ammonia. The ammonia introduction rate is 0.5 m 3 / h per 100 kg of reactants. Set the heat transfer oil temperature at 180°C. Stop introducing ammonia after reacting for 3 h;

[0046] S2. Turn on the vacuum with a vacuum degree of -0.04 MPa. Adjust the stirring speed to 120 r / min. Slowly spray and add 3.0 kg (containing 35.22 moles of urea) of urea aqueous solution preheated to 65 - 70°C within 2 hours. After adding, continue stirring for 10 - 20 minutes, and then turn off the vacuum;

[0047] S3. Adjust the stirring speed to 160 r / min. Introduce ammonia with the ammonia preheated to 120 - 150°C. The ammonia introduction rate is 1 m 3 / h per 100 kg of reactants. Set the heat transfer heating oil temperature at 200°C and react for 6 hours;

[0048] S4. Stop heating. Adjust the stirring speed to 50 r / min. Continue to introduce ammonia without preheating the ammonia. The ammonia introduction rate is 0.5 m 3 / h per 100 kg of reactants. Discharge the material when the material temperature drops below 150°C, cool it, and pulverize it to obtain crystalline type II ammonium polyphosphate with a layered structure.

[0049] Comparative Example 1

[0050] S2. Turn on the vacuum with a vacuum degree of -0.04 MPa. Adjust the stirring speed to 150 r / min. Slowly spray and add 6.65 kg (containing 77.50 moles of urea) of 70% wt urea water mixture preheated to 65 - 70°C within 3.0 hours. After adding, continue stirring for 10 - 20 minutes, and then turn off the vacuum;

[0051] The rest is the same as in Example 1.

[0052] Comparative Example 2

[0053] S1. Add 100 kg (704.5 moles) of phosphorus pentoxide and 93 kg (704.5 moles) of diammonium hydrogen phosphate into a horizontal reactor. Set the stirring speed at 70 r / min. After stirring for 20 - 30 minutes until homogeneous, raise the temperature. Set the heat transfer oil temperature at 155 - 165°C. After half an hour, raise the temperature again and set the heat transfer oil temperature at 275 - 285°C. Continue stirring until the material melts, and then introduce ammonia. The ammonia introduction rate is 5.0 m 3per hour, while spraying 21.16 kg of urea aqueous solution at 80 °C and 80% concentration (containing 281.8 moles of urea), the spraying time is 15 minutes. After the spraying is completed, continue to heat up and stir the reaction, control the internal temperature within 300 °C, and adjust the ammonia gas volume to 2.0 m of ammonia gas per 100 kg of reactants 3 per hour, and react for 3 h;

[0054] S2. Add the above materials to another high-shear reactor preheated to 275 - 285 °C, and introduce ammonia. The ammonia gas volume per 100 kg of reactants is 1.0 m 3 per hour, continue to react for 2 hours, cool and crush to obtain crystalline type II ammonium polyphosphate. The particle morphology is shown in Figure 6 .

[0055] Comparative Example 3

[0056] S1. Add 14.2 kg (100 moles) of phosphorus pentoxide, 13.2 kg (100 moles) of diammonium hydrogen phosphate, and 1.80 kg (30 moles) of urea to a 50 L horizontal reactor, stir at a speed of 70 r / min, stir for 20 - 30 minutes until uniform, then heat up, set the heat transfer oil temperature to 245 - 255 °C, introduce ammonia, and the ratio of ammonia gas volume to reactants is 2.0 m of ammonia gas per 100 kg of reactants 3 per hour, react for 1 hour, and the pressure in the reactor is 0.1 MPa;

[0057] S2. Set the heat transfer oil temperature to 290 - 310 °C, add 3.15 kg of zinc-aluminum layered double hydroxide, continue to stir and introduce ammonia to react for 1.5 hours, and the ratio of ammonia gas volume to reactants is 1.0 m of ammonia gas per 100 kg of reactants 3 per hour, cool and crush to obtain crystalline type II ammonium polyphosphate.

[0058] The properties of the ammonium polyphosphate prepared in Examples 1 - 4 and Comparative Example 1 are shown in Table 1.

[0059] Table 1: Properties of Ammonium Polyphosphate in Examples 1 - 4 and Comparative Examples

[0060]

[0061] For the above examples, the detection methods are as follows:

[0062] 1) pH: Prepare a 10% aqueous solution, soak it in a water bath at room temperature of 25 °C for 30 min, and test it with a pH meter;

[0063] 2) Viscosity: Prepare a 10% aqueous solution, soak it in a water bath at room temperature of 25 °C for half an hour, and test it with an NDJ-79;

[0064] 3) Solubility: Conduct according to the provisions of 6.8 in HG / T2770 - 2020.

[0065] Remark: 1. The measured phosphorus content of Comparative Example 3 is 28.8%.

[0066] Application Examples 1 - 7

[0067] Apply Examples 1 - 4, Comparative Example 1, commercially available crystalline ammonium polyphosphate type II, and commercially available crystalline ammonium polyphosphate type I to the applications in waterborne ultra-thin intumescent fire retardant coatings corresponding to Application Examples 1 - 7 respectively. The basic formula is as follows (weight percentage):

[0068]

[0069] According to the ISO 834 heating curve, load: 115 kg, conduct the performance test of the ultra-thin intumescent fire retardant coating in a medium-scale furnace for refractory limit. Sample size: 1.56 m I-beam, coating thickness is about 2 mm. The fire resistance time is the time when the temperature on the back of the steel beam reaches 538 °C or the steel beam deforms and bends beyond the specified degree (as long as one of them reaches, it is judged to terminate, and this time is the fire resistance limit time).

[0070] Coating storage stability, the time when the ultra-thin intumescent fire retardant coating gels during storage at 5 °C.

[0071] Table 2: Performance of the ultra-thin intumescent fire retardant coating in Application Examples

[0072]

[0073]

[0074] Remark:

[0075] 1. Calculated according to the phosphorus content, 21.9% is added to the formula of Comparative Example 3 to keep the phosphorus content similar to that of other application examples. In this application example, the initial expansion is slower, the final expansion ratio is low, and the carbon layer does not fall off.

[0076] 2. During the test of commercially available crystalline ammonium polyphosphate type I, the carbon layer expands rapidly at the beginning, with a faster initial expansion speed than that of commercially available crystalline ammonium polyphosphate type II and the examples of this patent. However, as the test time prolongs, the strength of the carbon layer is insufficient and it continuously powders and falls off, and finally there is a phenomenon of local and overall peeling on the surface of the tested steel beam.

[0077] It was found from infrared spectra and X-ray diffraction analysis that the ammonium polyphosphate with a layered structure prepared by the method of this patent is crystalline ammonium polyphosphate type II. It was found by SEM electron microscopy comparison that the ammonium polyphosphate prepared by the method of this patent has a layered structure, while the ammonium polyphosphate in Comparative Example 2 is a non-layered structure, indicating that adding a urea-aqueous solution rapidly at the initial stage of the material melting reaction plays the role of providing a large amount of ammonia gas in a short time. It can also be found in the performance and application examples that its performance is similar to that of commercially available crystalline ammonium polyphosphate type II. At the same time, it was found in Comparative Example 2 that with the rapid addition of the urea-aqueous solution, a large amount of carbon dioxide gas is generated and the torque of the reactor changes violently. In the present invention, it is added slowly at a certain stage of the reaction, and ammonia gas is generated by the decomposition of urea-water, so that rapid crystallization and solidification continuously occur on the surface of the material, and finally a layered structure is formed. In Comparative Example 3, layered double hydroxide was used for nano-composite modification of ammonium polyphosphate, and its water solubility was significantly reduced compared with commercially available crystalline ammonium polyphosphate type II, but the phosphorus content decreased from 31.5%-32% of ammonium polyphosphate to 28.8%, and the 2% TGA also decreased significantly. It was found in the application example that it has an obvious effect on the expansion of the carbon layer.

[0078] From the comparison of thermogravimetric analysis of Examples 1-4 and Comparative Example 1, the addition of urea still has a certain effect on the thermodynamic properties of ammonium polyphosphate. Especially in Comparative Example 1, when more urea was added, the 2% thermogravimetric temperature decreased significantly, and the storage stability of the waterborne ultra-thin intumescent fireproof coating decreased significantly.

[0079] From the application examples, the fire resistance of the ultra-thin intumescent fireproof coating depends on the expansion height and quality of the carbon layer, and the two complement each other. The preparation method of adding a urea-aqueous solution in the middle adopted in this patent significantly improves the fire resistance time of the prepared waterborne ultra-thin intumescent fireproof coating, indicating that the layered structure is effective in improving the fire resistance time.

[0080] During the test, the initial carbon layer expansion rate of commercially available crystalline ammonium polyphosphate type I was better than that of commercially available crystalline ammonium polyphosphate type II and the examples of this patent. However, as the test time extended, the strength of its carbon layer was insufficient, the carbon layer continuously powdered and escaped, and the expansion ratio decreased instead. Finally, local overall shedding occurred on the surface of the tested steel beam. It shows that crystalline ammonium polyphosphate type II is superior to crystalline ammonium polyphosphate type I in terms of fire resistance time and storage stability of waterborne ultra-thin intumescent fireproof coatings.

[0081] Although the present invention has been disclosed above with the preferred embodiments, it is not intended to limit the present invention. Any person skilled in this art can make some modifications and refinements without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the scope defined by the claims.

Claims

1. A preparation method of crystalline ammonium polyphosphate type II with a layered structure, characterized in that, Including: S1: Add phosphorus pentoxide and diammonium hydrogen phosphate and stir evenly, then introduce ammonia. The ammonia introduction amount is in a ratio with the reactants, and the ammonia introduction amount per 100 kg of reactants is 0.5 m³ / hour - 2 m³ / hour. Stop introducing ammonia after heating and reacting for 1 h - 3 h; S2: Open the vacuum, with the vacuum degree being -0.02 MPa to -0.04 MPa. Stir the product obtained in step S1, with the stirring speed being 120 r / minute - 180 r / minute. Spray and add the urea aqueous solution preheated to 65°C - 70°C within 0.5 hour - 2 hours. After adding, continue to stir for 10 minutes - 20 minutes, and then close the vacuum; S3: Stir, with the stirring speed reduced to 100 r / minute - 160 r / minute. Introduce ammonia preheated to 120°C - 150°C. The ammonia amount is in a ratio with the reactants, and the ammonia introduction amount per 100 kg of reactants is 1 m³ / hour - 6 m³ / hour. Heat and react for 2 hours - 6 hours; S4: Stop heating and stir, with the stirring speed being 50 r / minute - 80 r / minute. Introduce non-preheated ammonia. The ammonia introduction amount is in a ratio with the reactants, and the ammonia introduction amount per 100 kg of reactants is 0.5 m³ / hour - 2 m³ / hour. When the material temperature drops below 150°C, discharge, cool, and pulverize to obtain crystalline type II ammonium polyphosphate with a layered structure; Among them, the molar ratio of phosphorus pentoxide to diammonium hydrogen phosphate is 100:(95 - 105), and the molar ratio of phosphorus pentoxide, diammonium hydrogen phosphate, and urea is 100:(95 - 105):(0.5 - 5).

2. The preparation method of crystalline ammonium polyphosphate type II with a layered structure according to claim 1, characterized in that, The urea concentration of the urea aqueous solution is 68 wt.% - 72 wt.%.

3. The preparation method of crystalline ammonium polyphosphate type II with a layered structure according to claim 1, characterized in that, When preparing the urea aqueous solution, the molar ratio of urea to water is 1:

1.

4. The preparation method of crystalline ammonium polyphosphate type II with a layered structure according to claim 2, characterized in that, The content of phosphorus pentoxide ≥ 99.5%, the active R value ≥ 1.7, and the proportion of reducing substances ≤ 0.005%.

5. The preparation method of crystalline ammonium polyphosphate type II with a layered structure according to claim 1, characterized in that, In step S1, the stirring speed is 50 r / minute - 80 r / minute, and stir for 20 minutes - 30 minutes.

6. The preparation method of crystalline ammonium polyphosphate type II with a layered structure according to claim 1, characterized in that, In step S3, the ammonia introduction amount is in a ratio with the reactants, and the ammonia introduction amount per 100 kg of reactants is 1 m³ / hour - 4 m³ / hour.

7. The preparation method of crystalline ammonium polyphosphate type II with a layered structure according to claim 1, characterized in that, Step S1 is carried out in a horizontal reactor. When heating and reacting, set the heat transfer oil temperature to 150°C - 180°C. In step S3, when heating and reacting, set the heat transfer heating oil temperature to 200 - 240°C.

8. Use of the crystalline ammonium polyphosphate of type II in a layered structure prepared by the preparation method according to claim 1, characterized in that, The crystalline type II ammonium polyphosphate is used in a waterborne ultra-thin intumescent fireproof coating after being combined with an auxiliary agent, solvent water, vinyl acetate emulsion, pentaerythritol, and melamine.

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