Preparation method of ammonium polyphosphate flame retardant based on microencapsulated coating of epoxy resin

Abstract

The application discloses a preparation method of an ammonium polyphosphate flame retardant based on epoxy resin microencapsulation coating, and comprises the following steps: step 1, preparing an epoxy resin emulsion; step 2, preparing fine-particle ammonium polyphosphate, which comprises the following steps: S1, mixing diaphosphorus pentoxide and diammonium hydrogen phosphate; S2, feeding ammonia at a rate of 0.1-0.5 m / h per 100 kg of reaction materials; S3, feeding ammonia at a rate of 2-5 m / h per 100 kg of reaction materials; S4, feeding ammonia at a rate of 0.5-1.9 m / h per 100 kg of reaction materials; S5, discharging; and S6, crushing; step 3, mixing the fine-particle ammonium polyphosphate and hydrogen-containing silicone oil; step 4, adding the epoxy resin emulsion, and performing initial curing and re-curing; step 5, drying; and step 6, screening. 3 3 3 The ammonium polyphosphate flame retardant product prepared by the method has small particle size, little change in particle size and good dispersibility.​​

Description

Technical Field

[0001] This invention specifically relates to a method for preparing an ammonium polyphosphate flame retardant based on epoxy resin microencapsulation. Background Technology

[0002] Ammonium polyphosphate (APP) is an environmentally friendly inorganic flame retardant based on phosphorus and nitrogen. It has wide applications in flame-retardant coatings, resins, rubber, paper, wood, and fibers, characterized by low toxicity and low smoke. Currently, APP particle sizes range from 15 to 20 micrometers. The finer the particle size of an inorganic flame retardant, the larger the surface area, resulting in greater contact with the polymer, better dispersion and compatibility, and a significantly improved flame-retardant effect. However, in plastics, APP with a larger particle size can negatively impact dispersion; therefore, it is generally applied after grinding or pulverizing followed by saturation treatment.

[0003] Patent publication number CN101254908B discloses a method for preparing highly heat-resistant crystalline type II ammonium polyphosphate. This method involves a mixed reaction of diammonium hydrogen phosphate and phosphorus pentoxide, followed by ammonia introduction and spraying with a high-concentration urea solution. The resulting product has a small particle size, with an average particle size of less than 10 micrometers. However, the large amount of water present in the urea solution affects the condensation reaction in the synthesis process, easily leading to uneven polymerization and poor heat resistance, with an initial decomposition temperature of 270–275℃.

[0004] Patent publication CN101597044A discloses a method for preparing crystalline type II ammonium polyphosphate with high polymerization degree and narrow molecular weight distribution. The method involves an equimolar mixture of diammonium hydrogen phosphate and phosphorus pentoxide, with a large amount of inert gas introduced initially. The process involves the introduction, stopping, and re-introduction of ammonia gas, and the reaction is carried out with a surface treatment agent (selected from melamine cyanurate, aminosilane, epoxy resin, reactive melamine, or isocyanate). This method yields a polyphosphate product with a fine particle size (less than 12 micrometers). However, the preparation process requires the introduction of inert gases such as nitrogen, helium, and argon, increasing production costs. Furthermore, the production control process and operating steps are complex, and the equipment requirements are high.

[0005] Patent publication number CN111807343B discloses a method for controlling the fine particle size of ammonium polyphosphate and its application. In the process of ammonium polyphosphate synthesis, 0.5%-3% of ammonia-deficient ammonium polyphosphate powder is added to obtain a crystalline type II ammonium polyphosphate product with controlled fine particle size. This method can control the proportion of fine particles in the final product, with the proportion of fine particles ≤2.19μm being less than or equal to 2%, while the average particle size of the product remains around 17-20 micrometers. Summary of the Invention

[0006] To address the aforementioned shortcomings, this invention provides a method for preparing an ammonium polyphosphate flame retardant based on epoxy resin microencapsulation. The prepared ammonium polyphosphate flame retardant product based on epoxy resin microencapsulation has small particle size, minimal particle size variation, and good dispersibility.

[0007] The technical solution is: a method for preparing ammonium polyphosphate flame retardant based on epoxy resin microencapsulation, comprising the following steps:

[0008] Step 1), prepare epoxy resin emulsion;

[0009] Step 2), preparing fine-particle-size ammonium polyphosphate, includes the following steps:

[0010] The technical solution is: a method for preparing fine-particle-size ammonium polyphosphate, comprising the following steps:

[0011] S1, phosphorus pentoxide and diammonium hydrogen phosphate are stirred in a kneader and heated to 220-250°C;

[0012] S2, start ammonia flow, with an ammonia flow rate of 0.1–0.5 mg / 100 kg of reactants. 3 / h, reaction 20-45min;

[0013] S3, heat to 260-320℃, increase ammonia flow rate to 2-5m³ per 100kg of reactants. 3 / h, reaction time 3-5 hours;

[0014] S4, stop heating and reduce the ammonia flow rate to 0.5–1.9 m³ per 100 kg of reactants. 3 / h, reaction time 1-4 hours;

[0015] S5, stop ammonia feeding, and when the reaction temperature drops to 150-200℃, discharge the material to obtain fine-particle-size ammonium polyphosphate crude product;

[0016] S6, after being crushed, yields fine-particle-size ammonium polyphosphate.

[0017] Step 3): Take the fine-particle-size ammonium polyphosphate prepared in step 2), put it into the reaction vessel, spray in hydrogen-containing silicone oil, and mix.

[0018] Step 4): The epoxy resin emulsion prepared in Step 1) is added to the reaction vessel in Step 3) by spraying, stirred and mixed at room temperature, and then heated for initial curing; then heated for further curing.

[0019] Step 5), dry to remove excess moisture, and obtain the coated initial product;

[0020] Step 6) The water-based epoxy resin-coated ammonium polyphosphate product is obtained by sieving.

[0021] Optionally, the molar ratio of phosphorus pentoxide to diammonium hydrogen phosphate is 1.05:1 to 1:1.

[0022] Optionally, the molar ratio of phosphorus pentoxide to diammonium hydrogen phosphate is 1:1.

[0023] Optionally, in step S1, a process modifier is added to the kneader, which is a mixture of precipitated silica and melamine diamide.

[0024] Optionally, the amount of the process modifier is 0.1 to 0.5% of the sum of the weight of phosphorus pentoxide and diammonium hydrogen phosphate.

[0025] Optionally, the process modifier is made of ammonia, melamine diamide, and silica.

[0026] Optionally, the process modifier is prepared by the following steps:

[0027] (1) Add melamine diamide to ammonia water and stir to form a suspension;

[0028] (2) Spray the suspension of (1) evenly onto the silica in a mixer and dry it to obtain the process improver.

[0029] Optionally, the amount of the process modifier is 0.1 to 0.5% of the sum of the weight of phosphorus pentoxide and diammonium hydrogen phosphate.

[0030] Optionally, the process modifier is made of ammonia, melamine diamide, and silica.

[0031] Optionally, the process modifier is prepared by the following steps:

[0032] (1) Heat the ammonia water to 40-80℃, add melamine diamide, and stir to dissolve for 15-45 minutes;

[0033] (2) Spray the solution of (1) evenly onto the silica in a high-speed mixer and dry it to obtain the process improver.

[0034] Optionally, the ammonia solution is 0.5% to 10% ammonia solution.

[0035] Optionally, the weight ratio of ammonia to melamine diamide is 4:1 to 19:1.

[0036] Optionally, the weight ratio of the solution to silica is 1:20 to 1:5.

[0037] Optionally, the crude product of fine-particle-size ammonium polyphosphate has a particle size of 6-14 micrometers, and the fine-particle-size ammonium polyphosphate has a particle size of 5-12 micrometers.

[0038] Optionally, the ratio of fine-particle-size ammonium polyphosphate: hydrogen-containing silicone oil: self-emulsifying waterborne epoxy resin weight: curing agent is 960-990.5: 2-10: 5-20: 2.5-10.

[0039] Optionally, the initial curing conditions are: temperature 40–60°C, 60–120 minutes; the secondary curing conditions are: curing at 80–120°C for 1–5 hours.

[0040] Invention principle:

[0041] In this invention, the first step of the reaction uses an ultra-low ammonia flow reaction, in which the hydroxyl groups of melamine diamide condense with phosphorus pentoxide to produce melamine diamide phosphate. The ultra-low ammonia flow reaction helps to mitigate the initial violent reaction of phosphorus pentoxide, making the polymerization reaction more uniform, avoiding over-reaction, which causes the degree of polymerization to increase rapidly, and the reaction system to become viscous quickly, making it difficult to disperse the modifier.

[0042] In the second step, when the ammonia flow rate is increased, the temperature rises rapidly. Melamine diamide phosphate participates in the polymerization reaction of ammonium polyphosphate. In the polymerization reaction involving melamine diamide phosphate, the molecular chain grows unidirectionally. During the subsequent cooling process, the melamine diamide end is more likely to form crystal nuclei. At the same time, silica adsorbs melamine diamide, promoting its rapid dispersion in the system and making the reaction more uniform. After heating is stopped, the system begins to cool slowly. The silica adsorption end forms crystal nuclei first, enabling the ammonium polyphosphate molecules in the system to be rapidly and uniformly pulverized, resulting in more uniform and finer particle sizes.

[0043] The fine-particle-size ammonium polyphosphate prepared in this invention is first treated with hydrogen-containing silicone oil. The hydrogen-containing silicone oil contains a large number of active Si-H bonds, which facilitate the formation of hydrogen bonds between the phosphorus-oxygen double bonds in the ammonium polyphosphate, adhering to the surface of the ammonium polyphosphate powder and forming a hydrophobic film. Then, the ammonium polyphosphate is coated with an emulsified waterborne epoxy resin emulsion. Since the ammonium polyphosphate already has a hydrophobic film on its surface, after atomization, the droplets come into contact with the ammonium polyphosphate surface. Upon contact with the powder surface, the lipophilic groups of the droplets dissolve with the hydrogen-containing silicone oil, causing the epoxy resin and curing agent particles to distribute along the surface of the ammonium polyphosphate, making it easier to form a complete film during curing. Because water molecules are located towards the outside, a water layer separates the ammonium polyphosphate particles, making it difficult for the powder particles to adhere during curing. After the product is cured, the hydrogen-containing silicone oil layer is fixed under the epoxy resin coating layer, preventing water from penetrating into the interior and providing protection. With good coating integrity, the water solubility is greatly improved under the double protection of hydrogen-containing silicone oil and epoxy resin coating layers, which can be as low as 0.1g / 100mL water. After coating treatment, the product has small particle size, little particle size change, and good dispersibility. Detailed Implementation

[0044] The present invention will be further described below.

[0045] The specific implementation method is as follows:

[0046] Melamine diamide is commercially available.

[0047] Silica is commercially available.

[0048] Phosphorus pentoxide is commercially available.

[0049] Industrial diammonium hydrogen phosphate is commercially available.

[0050] Hydrogen-containing silicone oil is commercially available.

[0051] The self-emulsifying waterborne epoxy resin is commercially available, containing 50 wt% epoxy resin.

[0052] The curing agent is commercially available.

[0053] Example 1: Preparation of process modifier

[0054] (1) Heat 1000g of ammonia water with a mass percentage of 5wt% to 60℃, add 100g of melamine diamide powder, and stir for 30 minutes to form a solution.

[0055] (2) Spray the solution from (1) evenly onto 10 kg of silica in a high-speed mixer and dry it to obtain a process improver (10.1 kg).

[0056] The process improver prepared in this embodiment is used in Examples 2 to 4 below.

[0057] Example 2

[0058] S1, take 250 kg of phosphorus pentoxide and 500 g of the process improver from Example 1, add them to a kneader, and start stirring for 10 minutes; then add 232 kg of diammonium hydrogen phosphate, stir and heat to 230°C;

[0059] S2, start ammonia flow, with an ammonia flow rate of 0.3 m³ / kg of reactant. 3 / h, reaction time 30 minutes;

[0060] S3, heat to 290℃, increase ammonia flow rate to 4.5m³ per 100kg of reactants. 3 / h, reaction time 4.5 hours;

[0061] S4, stop heating and reduce the ammonia flow rate to 1 m³ per 100 kg of reactants. 3 / h, reaction time 2.5 hours;

[0062] S5, stop ammonia feeding, and wait for the reaction temperature to drop to 160℃ before discharging to obtain fine-particle-size ammonium polyphosphate crude product 1;

[0063] S6, the coarse product is mechanically ground and then air-classified. The feed rate is 25 kg / min, the classifier speed is 1000 r / min, the main unit speed is 1160 r / min, and the induced draft fan volume is 3800 m³ / min. 3 / h, after pulverization, fine-particle ammonium polyphosphate 1 is obtained.

[0064] The results of testing crude polyphosphate product 1 and fine polyphosphate product 1 with fine particle size are shown in Tables 1 and 2 below.

[0065] Example 3

[0066] S1, take 250 kg of phosphorus pentoxide and 1400 g of the process improver from Example 1, add them to a kneader, start stirring for 10 minutes, then add 232 kg of diammonium hydrogen phosphate; stir until the temperature reaches 230°C;

[0067] S2, start ammonia flow, with an ammonia flow rate of 0.3 m³ / kg of reactant. 3 / h, reaction time 30 minutes;

[0068] S3, heat to 290℃, increase ammonia flow rate to 4.5m³ per 100kg of reactants. 3 / h, reaction time 4.5 hours;

[0069] S4, stop heating and reduce the ammonia flow rate to 1 m³ per 100 kg of reactants. 3 / h, reaction time 2.5 hours;

[0070] S5, stop ammonia feeding, and wait for the reaction temperature to drop to 160℃ before discharging to obtain fine-particle-size ammonium polyphosphate crude product 2;

[0071] S6, the coarse product is mechanically ground and then air-classified. The feed rate is 25 kg / min, the classifier speed is 1000 r / min, the main unit speed is 1160 r / min, and the induced draft fan volume is 3800 m³ / min. 3 / h, after pulverization, fine-particle ammonium polyphosphate 2 is obtained.

[0072] The results of testing crude polyphosphate product 2 and fine polyphosphate product 2 with fine particle size are shown in Tables 1 and 2 below.

[0073] Example 4

[0074] S1, take 250 kg of phosphorus pentoxide and 2400 g of the process improver from Example 1, add them to a kneader, start stirring for 10 minutes, then add 232 kg of diammonium hydrogen phosphate; stir and raise the temperature to 230°C;

[0075] S2, start ammonia flow, with an ammonia flow rate of 0.3 m³ / kg of reactant. 3 / h, reaction time 30 minutes;

[0076] S3, heat to 290℃, increase ammonia flow rate to 4.5m³ per 100kg of reactants. 3 / h, reaction time 4.5 hours;

[0077] S4, stop heating and reduce the ammonia flow rate to 1 m³ / 100 kg of reactants. 3 / h, reaction time 2.5 hours;

[0078] S5, stop ammonia feeding, and when the reaction temperature drops to 160℃, discharge the material to obtain fine-particle-size ammonium polyphosphate crude product 3;

[0079] S6, fine-particle-size ammonium polyphosphate crude product 3 is mechanically ground and then air-classified. The feed rate is 25 kg / min, the classifier speed is 1000 r / min, the main unit speed is 1160 r / min, and the induced draft fan volume is 3800 m³ / min. 3 / h, after crushing, fine-particle ammonium polyphosphate 3 is obtained.

[0080] The results of testing crude polyphosphate product 3 and fine polyphosphate product 3 with fine particle size are shown in Tables 1 and 2 below.

[0081] Example 5

[0082] S1, add 250 kg of phosphorus pentoxide and 232 kg of diammonium hydrogen phosphate into a kneader and stir until the temperature reaches 230°C;

[0083] S2, start ammonia flow, with an ammonia flow rate of 0.3 m³ / kg of reactant. 3 / h, reaction time 30 minutes;

[0084] S3, heat to 290℃, increase ammonia flow rate to 4.5m³ per 100kg of reactants. 3 / h, reaction time 4.5 hours;

[0085] S4, stop heating, reduce the ammonia flow rate to 1 m3 / h per 100 kg of reactants, and react for 2.5 hours;

[0086] S5, stop ammonia feeding, and when the reaction temperature drops to 160℃, discharge the material to obtain crude ammonium polyphosphate product 1;

[0087] S6, crude ammonium polyphosphate product 1, is mechanically ground and then air-classified. The feed rate is 25 kg / min, the classifier speed is 1000 r / min, the main unit speed is 1160 r / min, and the induced draft fan volume is 3800 m³ / min. 3 / h, after pulverization, ammonium polyphosphate 1 is obtained.

[0088] The results of testing crude ammonium polyphosphate product 1 and ammonium polyphosphate 1 are shown in Tables 1 and 2 below.

[0089] Example 6

[0090] Take fine-particle-size ammonium polyphosphate 1 from Example 2, fine-particle-size ammonium polyphosphate 2 from Example 3, fine-particle-size ammonium polyphosphate 3 from Example 4, and ammonium polyphosphate 1 from Example 5, and coat them according to the method of this example.

[0091] Step 1: Take self-emulsified waterborne epoxy resin (the weight of self-emulsified waterborne epoxy resin is 12 / 490 of the weight of fine-particle-size ammonium polyphosphate), curing agent (the weight of curing agent is 3 / 490 of the weight of fine-particle-size ammonium polyphosphate) and water (the weight of water is 35 / 490 of the weight of fine-particle-size ammonium polyphosphate), mix them, and shear at high speed for 30 minutes to form an epoxy resin emulsion.

[0092] Step 2: Take ammonium polyphosphate and put it into a mixing reactor. Spray in 1 kg of hydrogen-containing silicone oil (the weight of the hydrogen-containing silicone oil is 1 / 490 of the weight of the ammonium polyphosphate) and mix at high speed for 60 minutes.

[0093] Step 3: Add the epoxy resin emulsion from Step 1 to the mixing reactor from Step 2 by spraying. Stir and mix at high speed at room temperature for 60 minutes, then heat to about 50°C for initial curing for 60-120 minutes; then heat to 80-120°C for curing for 1-5 hours.

[0094] Step 4: Drying to remove excess moisture, resulting in the coated initial product.

[0095] Step 5: Screening yields water-based epoxy resin-coated ammonium polyphosphate product.

[0096] The flame retardant and processing properties of waterborne epoxy resin-coated ammonium polyphosphate products were tested, and the results are shown in Table 3.

[0097] Comparative Example 1

[0098] S1, add 250 kg of phosphorus pentoxide and 232 kg of diammonium hydrogen phosphate into a kneader; stir and raise the temperature to 290°C;

[0099] S2, start ammonia feeding, with an ammonia flow rate of 4.5m³ per 100kg of reactants. 3 / h, reaction time 4.5 hours;

[0100] S3, reduce the ammonia flow rate to 1m³ per 100kg of reactants. 3 / h, reaction time 3 hours;

[0101] S4, stop ammonia feeding, and when the reaction temperature drops to 160℃, discharge the material to obtain crude ammonium polyphosphate product 2;

[0102] S5, crude ammonium polyphosphate product 2, is mechanically ground and then air-classified. The feed rate is 25 kg / min, the classifier speed is 1000 r / min, the main unit speed is 1160 r / min, and the induced draft fan volume is 3800 m³ / min. 3 / h, after pulverization, ammonium polyphosphate 2 is obtained.

[0103] S6, ammonium polyphosphate 2 is taken and pulverized by airflow to obtain ammonium polyphosphate 3. Pulverization conditions: compressed air with an atmospheric dew point of ~25℃ is used, and the pressure is controlled at 0.7MPa, according to 7m 3 The feed rate is 15 kg / min, which is controlled at 1100 r / min. The airflow pulverizer and classifier are fed into the classifier. The feed rate of crude ammonium polyphosphate product 2 is controlled at 15 kg / min.

[0104] The results of testing crude ammonium polyphosphate product 2, ammonium polyphosphate 2, and ammonium polyphosphate 3 are shown in Tables 1 and 2 below.

[0105] Comparative Example 2

[0106] S1, add 250 kg of phosphorus pentoxide and 232 kg of diammonium hydrogen phosphate into a kneader and stir until the temperature reaches 230°C;

[0107] S2, start ammonia feeding, the ammonia rate is 1m³ per 100kg of reactant. 3 / h, reaction time 30 minutes;

[0108] S3, heat to 290℃, increase ammonia flow rate to 4.5m³ per 100kg of reactants. 3 / h, reaction time 4.5 hours;

[0109] S4, stop heating and reduce the ammonia flow rate to 1 m³ per 100 kg of reactants. 3 / h, reaction time 2.5 hours;

[0110] S5, stop ammonia feeding, and when the reaction temperature drops to 160℃, discharge the material to obtain crude ammonium polyphosphate product 3;

[0111] S6, the crude product is mechanically ground and then air-classified. The feed rate is 25 kg / min, the classifier speed is 1000 r / min, the main machine speed is 1160 r / min, and the induced draft fan volume is 3800 m3 / h. After grinding, ammonium polyphosphate 4 is obtained.

[0112] The results of testing crude ammonium polyphosphate product 3 and ammonium polyphosphate product 4 are shown in Tables 1 and 2 below.

[0113] Comparative Example 3

[0114] Take ammonium polyphosphate 2 and ammonium polyphosphate 3 from Comparative Example 1 and ammonium polyphosphate 4 from Comparative Example 2 respectively, and coat them according to the method of this comparative example.

[0115] Step 1: Take self-emulsified waterborne epoxy resin (the weight of self-emulsified waterborne epoxy resin is 12 / 490 of the weight of ammonium polyphosphate), curing agent (the weight of curing agent is 3 / 490 of the weight of ammonium polyphosphate) and water (the weight of water is 35 / 490 of the weight of ammonium polyphosphate), mix them, and shear at high speed for 30 minutes to form an epoxy resin emulsion.

[0116] Step 2: Take ammonium polyphosphate and put it into a mixing reactor. Spray in 1 kg of hydrogen-containing silicone oil (the weight of the hydrogen-containing silicone oil is 1 / 490 of the weight of the ammonium polyphosphate) and mix at high speed for 60 minutes.

[0117] Step 3: Add the epoxy resin emulsion from Step 1 to the mixing reactor from Step 2 via spraying. Mix at high speed for 60 minutes at room temperature, then heat to approximately 50°C for initial curing for 60–120 minutes; then heat to 80–120°C for curing for 1–5 hours.

[0118] Step 4: Drying to remove excess moisture, resulting in the coated initial product.

[0119] Step 5: Screening yields water-based epoxy resin-coated ammonium polyphosphate product.

[0120] The flame retardant and processing properties of waterborne epoxy resin-coated ammonium polyphosphate products were tested, and the results are shown in Table 3.

[0121] Table 1

[0122]

[0123] Table 2

[0124]

[0125]

[0126] As can be clearly seen from Tables 1 and 2, the product of this invention has high heat resistance and finer particle size, making it particularly suitable for polyester processing. It is easy to disperse more evenly and improves flame retardant efficiency.

[0127] Table 3

[0128]

[0129]

[0130] Note: The flame retardant performance, processing test comparison methods, and formulations in Table 3 are as follows:

[0131] The ammonium polyphosphate coated product was applied to PBT resin materials. The experimental samples and polypropylene were mixed evenly in a mixer, and the homogeneous mixture was extruded and granulated using a twin-screw extruder to prepare standard test strips.

[0132] Formula: Polypropylene (F401): 73; Ammonium polyphosphate sample: 22; Synergist: 4.5; Antioxidant: 0.2 (1010∶168=2∶1.2); Lubricant: 0.3. Voltage withstand test: A 20cm*20cm plate with a thickness of 0.4mm was placed in a constant temperature and humidity chamber at 80℃ and 85% humidity for 72 hours. Surface moisture was then absorbed before the voltage withstand test.

[0133] As can be clearly seen from Table 3, the fine-particle-size product of this project significantly improves the flame retardant properties of polypropylene and has excellent processing performance.

[0134] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing an ammonium polyphosphate flame retardant based on epoxy resin microencapsulation, comprising the following steps: Step 1), prepare epoxy resin emulsion; Step 2), preparing fine-particle-size ammonium polyphosphate, includes the following steps: S1, phosphorus pentoxide and diammonium hydrogen phosphate are stirred in a kneader and heated to 220~250℃; S2, start ammonia flow, with an ammonia flow rate of 0.1~0.5 mg / 100 kg of reactants. 3 / h, reaction 20~45min; S3, heat to 260~320℃, increase ammonia flow rate to 2~5m³ per 100kg of reactants. 3 / h, reaction time 3~5 hours; S4, stop heating and reduce the ammonia flow rate to 0.5~1.9m per 100kg of reactants. 3 / h, reaction time 1~4 hours; S5, stop ammonia feeding, and when the reaction temperature drops to 150~200℃, discharge the material to obtain fine-particle-size ammonium polyphosphate crude product; S6, after being crushed, yields fine-particle-size ammonium polyphosphate; Step 3), take the fine-particle-size ammonium polyphosphate prepared in Step 2), put it into the reaction vessel, spray in hydrogen-containing silicone oil, and mix; Step 4): The epoxy resin emulsion prepared in Step 1) is added to the reaction vessel in Step 3) by spraying, stirred and mixed at room temperature, and then heated for initial curing; then heated for further curing. Step 5), dry to remove excess moisture, and obtain the coated initial product; Step 6), the water-based epoxy resin-coated ammonium polyphosphate product is obtained by sieving; In S1, a process improver is also added to the kneader. The process improver is made of ammonia, melamine diamide and silica. The amount of the process improver is 0.1 to 0.5% of the sum of the weight of phosphorus pentoxide and diammonium hydrogen phosphate.

2. The method for preparing ammonium polyphosphate flame retardant based on epoxy resin microencapsulation according to claim 1, characterized in that, The molar ratio of phosphorus pentoxide to diammonium hydrogen phosphate is 1.05:1 to 1:

1.

3. The method for preparing ammonium polyphosphate flame retardant based on epoxy resin microencapsulation according to claim 2, characterized in that, The molar ratio of phosphorus pentoxide to diammonium hydrogen phosphate is 1:

1.

4. The method for preparing ammonium polyphosphate flame retardant based on epoxy resin microencapsulation according to claim 1, characterized in that, The process modifier is prepared through the following steps: (1) Heat the ammonia water to 40~80℃, add melamine diamide, and stir to dissolve for 15~45 minutes; (2) Spray the solution of (1) evenly onto the silica in a high-speed mixer and dry it to obtain the process improver.

5. The method for preparing ammonium polyphosphate flame retardant based on epoxy resin microencapsulation according to claim 4, characterized in that, The ammonia solution is 0.5-10% ammonia solution; and / or The weight ratio of ammonia to melamine diamide is 4:1 to 19:1; and / or The weight ratio of the solution to silica is 1:20 to 1:

5.

6. The method for preparing ammonium polyphosphate flame retardant based on epoxy resin microencapsulation according to any one of claims 1 to 5, characterized in that, The crude product of fine-particle-size ammonium polyphosphate has a particle size of 6-14 micrometers, while the fine-particle-size ammonium polyphosphate has a particle size of 5-12 micrometers.

7. The method for preparing ammonium polyphosphate flame retardant based on epoxy resin microencapsulation according to any one of claims 1 to 5, characterized in that, In step 1), the epoxy resin emulsion is made of self-emulsifying waterborne epoxy resin and curing agent; the ratio of fine-particle-size ammonium polyphosphate: hydrogen-containing silicone oil: weight of self-emulsifying waterborne epoxy resin: curing agent is 960~990.5: 2~10: 5~20: 2.5~10. The initial curing conditions are 40~70℃ for 60~120 minutes; the secondary curing conditions are 80~120℃ for 1~5 hours.

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