Flame-retardant powder coating for battery metal surface and preparation method thereof

Abstract

The present invention relates to a high-leveling, high-thermal-conductivity, flame-retardant powder coating for battery metal surfaces and a preparation method. The powder coating formula comprises, by weight, 550-570 parts of polyester resin, 40-47 parts of TGIC curing agent, 80-110 parts of modified boron nitride powder, 8-12 parts of brightener, and 8-12 parts of leveling agent. The powder coating product utilizes a flame-retardant, high-leveling polyester resin with high bromine and phosphorus contents and excellent compatibility with the modified boron nitride powder. The incorporation of special organosilicon segments imparts excellent high-temperature resistance and melt fluidity to the polyester resin. The resulting powder coating exhibits high leveling, excellent thermal conductivity and flame retardancy, as well as a high insulation rating and excellent high-temperature resistance after curing. It is suitable for coating battery metal surfaces and has the advantages of a simple preparation process and long-lasting excellent properties.

Description

Technical Field

[0001] The invention belongs to the technical field of powder coatings, and in particular relates to a flame-retardant powder coating for battery metal surfaces and a preparation method thereof. Background Art

[0002] Powder coatings are widely used in surface coating of household appliances, automobile industry, metal instruments and equipment due to their advantages of being pollution-free, saving energy and resources, having high mechanical strength of coating films and complete recycling of excess coatings.

[0003] With the rapid development of new energy vehicles, the number of batteries used in these vehicles is increasing. These batteries primarily include lead-acid batteries, nickel-metal hydride batteries, and lithium-ion batteries. The metal surfaces of these power batteries also require corrosion protection. Batteries release heat during charging and discharging, so the coatings used must possess excellent thermal conductivity. Furthermore, as high-energy storage devices, batteries are susceptible to ignition and combustion in the event of a collision. Therefore, the coatings used on the metal surfaces of batteries must exhibit excellent flame retardancy to ensure safety. Due to the high power of batteries used in new energy vehicles, they generate significant heat during operation, requiring high heat resistance from the coating to prevent long-term performance degradation. Currently, the industry primarily uses external flame retardants to achieve these flame retardant properties. However, these external small molecule flame retardants have poor resistance to water, salt spray, and solvents, and are easily lost from the coating, resulting in a gradual loss of flame retardancy over time. This makes them incapable of meeting the long-term coating requirements for new energy vehicle batteries. Developing powder coatings that are simultaneously high-temperature resistant, durable, flame-retardant, and possess excellent thermal conductivity is a major challenge for the industry. Summary of the Invention

[0004] To address these issues, the present invention has developed a flame-retardant powder coating for battery metal surfaces that exhibits high-temperature resistance and high thermal conductivity. This powder coating utilizes a flame-retardant, high-leveling polyester resin with high bromine and phosphorus content and excellent compatibility with modified boron nitride powder. The incorporation of organosilicon segments imparts excellent high-temperature resistance and melt fluidity to the polyester resin. The resulting cured powder coating exhibits high leveling, excellent thermal conductivity and flame retardancy, as well as a high insulation rating and superior high-temperature resistance. Its application in battery metal surface coatings offers the advantages of a simple preparation process and long-lasting excellent properties.

[0005] A flame retardant powder coating comprises the following raw materials in parts by weight: 550-570 parts of polyester resin, 40-47 parts of TGIC curing agent (triglycidyl isocyanurate), 80-110 parts of modified boron nitride powder, 8-12 parts of brightener, and 8-12 parts of leveling agent.

[0006] The flame retardant powder coating as described above, wherein the preparation method comprises the following steps:

[0007] (1) The polyester resin, TGIC curing agent, modified boron nitride powder, brightener, and leveling agent in the formula are uniformly mixed, and then melt-extruded through a twin-screw extruder;

[0008] (2) the material melt-extruded by the twin-screw extruder is passed through a tablet press with a water (e.g., tap water) cooling system to be tableted and coarsely crushed into coarse particles;

[0009] (3) The coarse particles are further ground through a mill, and the particles that meet the requirements are screened out and collected through the mill's induced draft system to obtain a flame retardant powder coating.

[0010] In the method for preparing a flame-retardant powder coating as described above, in step (1), the screw temperature for melt extrusion is 130-140° C., and the screw speed is 400-500 rpm; in step (3), the required particles are 160-180 mesh.

[0011] In the above-mentioned flame-retardant powder coating formula, the polyester resin is prepared by high-temperature polycondensation using the following raw materials in molar proportions: 10-14 parts of octamethylcyclotetrasiloxane (D4), 7-9 parts of phosphorus trichloride, 35-40 parts of toluene, 8-10 parts of ethanol, 10-12 parts of 1,4-dibromo-2,3-butanediol, 15-20 parts of 2,2-bis(bromomethyl)-1,3-propylene glycol, 12-16 parts of 2,5-dibromoterephthalic acid, and 8-12 parts of 2,3-dibromosuccinic acid. The raw materials of the polyester resin also include catalyst 1: tetramethylammonium hydroxide, the amount of which is 0.05-0.08% of the mass of the octamethylcyclotetrasiloxane raw material; catalyst 2: monobutyltin oxide, the amount of which is 0.15-0.25% of the total mass of the raw material; and antioxidant 1010, i.e., pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, the amount of which is 0.3-0.5% of the total mass of the raw material.

[0012] The synthesis steps of the polyester resin include:

[0013] (1) Add the formulated amount of toluene, octamethylcyclotetrasiloxane (D4) and catalyst 1 into a reaction kettle, start stirring, and heat to 110-120° C. to carry out heat-insulating polymerization reaction;

[0014] (2) sampling and testing. When the viscosity of the polymer reaches 200-230 mPa·s (at 25°C), the temperature is lowered to 10-15°C, and the formulated amount of phosphorus trichloride is added dropwise to carry out phosphite esterification reaction. At the same time, nitrogen is introduced for bubbling to discharge hydrogen chloride gas generated in the reaction system. After the addition is completed, the reaction is continued at the heat preservation temperature;

[0015] (3) Sampling and testing. When the hydroxyl value of the polymer is lower than 5 mgKOH / g, it indicates that the terminal hydroxyl groups of the silicone chain segment have been completely reacted. At this time, add the formulated amount of ethanol and continue the phosphite esterification reaction at 10-15°C;

[0016] (4) sampling and testing. When the ethanol content is less than 1%, it indicates that the ethanol reaction is basically complete. At this time, add the formulated amount of 1,4-dibromo-2,3-butanediol and 2,2-bis(bromomethyl)-1,3-propylene glycol, and heat to 80-90°C and keep warm to continue the phosphite esterification reaction;

[0017] (5) Sampling the polymer to detect the acid value. When the acid value of the polymer is lower than 3 mgKOH / g, it indicates that the phosphite esterification reaction has been completed. At this time, the temperature is raised to 125-130°C. During the heating process, volatile substances such as toluene solvent are removed. 2,5-dibromoterephthalic acid is added in the formula amount, and then the temperature is gradually raised to 220-225°C, and the polymerization reaction is carried out by heat preservation.

[0018] (6) Sampling and testing the acid value of the polymer. When the acid value reaches 25-33 mgKOH / g, adding the formulated amount of antioxidant, and simultaneously starting the vacuum system to carry out vacuum polycondensation reaction at 220-225°C;

[0019] (7) When the acid value of the polymer reaches 9-15 mgKOH / g, release the vacuum system, add the formulated amount of 2,3-dibromosuccinic acid, and continue the end-capping reaction at 220-225°C;

[0020] (8) When the acid value of the polymer reaches 29-36 mgKOH / g, stop the reaction, discharge the material while hot, and cool the polyester resin with a steel belt with condensed water, then crush and granulate it to obtain the product.

[0021] In the polyester resin synthesis step, the dropping time in step (2) is controlled to be 1-1.5 hours; in step (5), the temperature is gradually increased to 220-225°C at a heating rate of 15-17°C / h; in step (6), the vacuum degree is maintained between -0.097 MPa and -0.099 MPa.

[0022] The obtained polyester resin has the appearance of colorless and transparent particles, an acid value of 29-36 mgKOH / g, and a softening point of 95-103°C.

[0023] The flame retardant powder coating as described above, wherein the modified boron nitride powder is prepared by a solvent method;

[0024] The formula includes the following raw materials in parts by weight: 50-55 parts of boron nitride powder, 5-7 parts of the polyester resin prepared above, 35-40 parts of toluene, and 1-1.5 parts of polyethylene glycol trimethoxysilyl propyl ether (silane coupling agent 4140);

[0025] The preparation method comprises the following steps: (1) adding a formula amount of toluene, polyethylene glycol trimethoxysilyl propyl ether (silane coupling agent 4140) and the polyester resin prepared above into a reaction kettle, heating the mixture to 80-90° C. to fully dissolve the mixture, then adding boron nitride powder and continuing to maintain the temperature and stir for a modification reaction for 1-1.5 hours, then heating the mixture to 110-115° C. and starting a vacuum system with a vacuum degree controlled at -0.097 MPa to -0.099 MPa to fully remove the toluene solvent, and then drying the mixture in an oven until the volatile matter is less than 0.5 wt % after discharging the mixture, and then adding the mixture to a high-speed grinder for pulverization and sieving to obtain a modified boron nitride powder with a particle size of 90-130 mesh.

[0026] Among the above raw materials, the boron nitride powder has a particle size of 80-120 μm and can be purchased from, for example, Shanghai Zhuzi New Materials Co., Ltd.

[0027] The present invention also relates to the application of the flame retardant powder coating on the metal surface of a battery.

[0028] Beneficial effects:

[0029] The powder coating product described in the present invention uses a flame-retardant high-leveling polyester resin with high bromine and phosphorus content and excellent compatibility with modified boron nitride powder. The introduction of special silicone chain segments results in excellent high-temperature resistance and melt fluidity of the polyester resin. The prepared powder coating product has a high leveling grade after curing, excellent thermal conductivity and flame retardancy, and a high insulation grade and excellent high-temperature resistance. It is used for coating the metal surface of batteries and has the advantages of a simple preparation process and long-lasting excellent properties. DETAILED DESCRIPTION

[0030] The present invention is further described below with reference to the embodiments. It should be noted that, in the absence of conflict, the embodiments in this application and the technical features in the embodiments can be combined with each other. It should be noted that, unless otherwise specified, all technical and scientific terms used in this application have the same meanings as those commonly understood by those of ordinary skill in the art to which this application belongs. The words "including" or "comprising" and the like used in the present invention are intended to mean that the elements or objects appearing before the word cover the elements or objects listed after the word and their equivalents, without excluding other elements or objects.

[0031] The raw materials of the present invention, except for the polyester resin and modified boron nitride powder described below, are all commercially available.

[0032] 1. Polyester resin:

[0033] The polyester resin is prepared by high-temperature polycondensation using the following raw materials in molar proportions: 12 parts octamethylcyclotetrasiloxane (D4), 8 parts phosphorus trichloride, 38 parts toluene, 9 parts ethanol, 11 parts 1,4-dibromo-2,3-butanediol, 18 parts 2,2-bis(bromomethyl)-1,3-propanediol, 14 parts 2,5-dibromoterephthalic acid, and 10 parts 2,3-dibromosuccinic acid. The polyester resin also includes catalyst 1: tetramethylammonium hydroxide, used in an amount of 0.06% by weight of the octamethylcyclotetrasiloxane raw material; catalyst 2: monobutyltin oxide, used in an amount of 0.2% by weight of the total raw material; and antioxidant 1010, used in an amount of 0.4% by weight of the total raw material.

[0034] The synthesis steps of the polyester resin include:

[0035] (1) Add the formulated amount of toluene, octamethylcyclotetrasiloxane (D4) and catalyst 1 into a reaction kettle, start stirring, and heat to 110° C. to carry out heat-insulating polymerization reaction;

[0036] (2) Sampling and testing. When the viscosity of the polymer reaches 200-230 mPa·s at 25°C, the temperature is lowered to 10°C, and phosphorus trichloride in the formula amount is added dropwise to carry out phosphite esterification reaction. At the same time, nitrogen is introduced for bubbling to discharge hydrogen chloride gas generated in the reaction system. The addition time is controlled within 1.5 hours. After the addition is completed, the reaction is continued at the heat preservation temperature.

[0037] (3) Sampling and testing. When the hydroxyl value of the polymer is lower than 5 mgKOH / g, it indicates that the terminal hydroxyl groups of the silicone chain segment have been completely reacted. At this time, the formulated amount of ethanol is added to continue the phosphite esterification reaction at 10°C;

[0038] (4) sampling and testing. When the ethanol content is lower than 1 wt%, it indicates that the ethanol reaction is basically complete. At this time, the formulated amount of 1,4-dibromo-2,3-butanediol and 2,2-bis(bromomethyl)-1,3-propylene glycol are added, and the temperature is raised to 80° C. and kept warm to continue the phosphite esterification reaction;

[0039] (5) Sampling and testing the acid value of the polymer. When the acid value of the polymer is lower than 3 mgKOH / g, it indicates that the phosphite esterification reaction has been completed. At this time, the temperature is raised to 125°C. During the heating process, volatile substances such as toluene solvent are removed. 2,5-dibromoterephthalic acid is added in the formula amount. Then, the temperature is gradually raised to 220-225°C at a heating rate of 16°C / h, and the polymerization reaction is carried out by heat preservation.

[0040] (6) Sampling the polymer to detect the acid value, and when the acid value reaches 25-33 mgKOH / g, adding the formulated amount of antioxidant, while starting the vacuum system, maintaining the vacuum degree at -0.098 MPa, and conducting vacuum polycondensation reaction at 220°C;

[0041] (7) When the acid value of the polymer reaches 9-15 mgKOH / g, release the vacuum system, add the formulated amount of 2,3-dibromosuccinic acid, and continue the end-capping reaction at 220°C;

[0042] (8) When the acid value of the polymer reaches the expected value, stop the reaction, discharge the material while it is still hot, and use a steel belt with condensed water to cool the polyester resin, and then crush and granulate it to obtain the product.

[0043] The obtained polyester resin had the appearance of colorless and transparent particles, an acid value of 32 mgKOH / g, and a softening point of 99°C.

[0044] 2. Modified boron nitride powder:

[0045] The modified boron nitride powder is prepared by modification using a solvent method;

[0046] The formula includes the following raw materials in parts by mass: 52 parts of boron nitride powder, 6 parts of the polyester resin prepared above, 38 parts of toluene, and 1.2 parts of polyethylene glycol trimethoxysilyl propyl ether (silane coupling agent 4140); the boron nitride powder has a particle size of 80-120 μm (purchased from Shanghai Zhuzi New Materials Co., Ltd.).

[0047] The preparation method comprises the following steps: (1) adding a formula amount of toluene, polyethylene glycol trimethoxysilyl propyl ether (silane coupling agent 4140) and the polyester resin prepared above into a reaction kettle, heating the temperature to 80° C. to fully dissolve the mixture, then adding boron nitride powder and continuing to maintain the temperature and stir for a modification reaction for 1.5 hours, then heating the temperature to 110° C. and starting a vacuum system with a vacuum degree controlled at -0.098 MPa to fully remove the toluene solvent, and then drying the material in an oven until the volatile matter is less than 0.5 wt % after discharging the material, and then adding the material into a high-speed grinder for pulverization and sieving to obtain a modified boron nitride powder with a particle size of 90-130 mesh.

[0048] Example 1

[0049] A flame retardant powder coating comprises the following raw materials in parts by mass: 550 parts of polyester resin, 40 parts of TGIC curing agent, 100 parts of modified boron nitride powder, 12 parts of brightener, and 8 parts of leveling agent.

[0050] The flame retardant powder coating as described above, wherein the preparation method comprises the following steps:

[0051] (1) The polyester resin, TGIC curing agent, modified boron nitride powder, brightener, and leveling agent in the formula were mixed uniformly, and then melt-extruded through a twin-screw extruder; wherein the screw temperature of the melt extrusion was 130° C. and the screw speed was 400 rpm;

[0052] (2) the material melt-extruded by the twin-screw extruder is tableted and coarsely crushed into particles by a tablet press equipped with a tap water cooling system;

[0053] (3) The coarse particles are further ground through a mill, and the particles that meet the requirements (160-180 mesh) are screened out and collected through the mill's induced draft system to obtain a flame retardant powder coating.

[0054] Example 2

[0055] A flame retardant powder coating comprises the following raw materials in parts by mass: 570 parts of polyester resin, 47 parts of TGIC curing agent, 110 parts of modified boron nitride powder, 8 parts of brightener, and 12 parts of leveling agent.

[0056] The flame retardant powder coating as described above, wherein the preparation method comprises the following steps:

[0057] (1) The polyester resin, TGIC curing agent, modified boron nitride powder, brightener, and leveling agent in the formula were mixed uniformly, and then melt-extruded through a twin-screw extruder; wherein the screw temperature of the melt extrusion was 140° C. and the screw speed was 500 rpm;

[0058] (2) the material melt-extruded by the twin-screw extruder is tableted and coarsely crushed into particles by a tablet press equipped with a tap water cooling system;

[0059] (3) The coarse particles are further ground through a mill, and the particles that meet the requirements (160-180 mesh) are screened out and collected through the mill's induced draft system to obtain a flame retardant powder coating.

[0060] Example 3

[0061] A flame retardant powder coating comprises the following raw materials in parts by mass: 560 parts of polyester resin, 44 parts of TGIC curing agent, 100 parts of modified boron nitride powder, 10 parts of brightener, and 10 parts of leveling agent.

[0062] The flame retardant powder coating as described above, wherein the preparation method comprises the following steps:

[0063] (1) The polyester resin, TGIC curing agent, modified boron nitride powder, brightener, and leveling agent in the formula were uniformly mixed, and then melt-extruded through a twin-screw extruder; wherein the screw temperature of the melt extrusion was 135° C. and the screw speed was 450 rpm;

[0064] (2) the material melt-extruded by the twin-screw extruder is tableted and coarsely crushed into particles by a tablet press equipped with a tap water cooling system;

[0065] (3) The coarse particles are further ground through a mill, and the particles that meet the requirements (160-180 mesh) are screened out and collected through the mill's induced draft system to obtain a flame retardant powder coating.

[0066] Example 4

[0067] A flame retardant powder coating comprises the following raw materials in parts by mass: 565 parts of polyester resin, 45 parts of TGIC curing agent, 105 parts of modified boron nitride powder, 9 parts of brightener, and 11 parts of leveling agent.

[0068] The flame retardant powder coating as described above, wherein the preparation method comprises the following steps:

[0069] (1) The polyester resin, TGIC curing agent, modified boron nitride powder, brightener, and leveling agent in the formula were mixed uniformly, and then melt-extruded through a twin-screw extruder; wherein the screw temperature of the melt extrusion was 130° C. and the screw speed was 500 rpm;

[0070] (2) the material melt-extruded by the twin-screw extruder is tableted and coarsely crushed into particles by a tablet press equipped with a tap water cooling system;

[0071] (3) The coarse particles are further ground through a mill, and the particles that meet the requirements (160-180 mesh) are screened out and collected through the mill's induced draft system to obtain a flame retardant powder coating.

[0072] Comparative Example 1

[0073] A commercially available ordinary TGIC system powder coating purchased from Anhui Meijia New Materials Co., Ltd. was used, and then 8% by mass of ammonium polyphosphate was added to the powder coating as a flame retardant.

[0074] Paint coating preparation:

[0075] The prepared powder coating was sprayed on the surface-treated tinplate substrate using an electrostatic spray gun to a film thickness of about 150 μm, and was fully cured at 200° C. / 15 min to obtain a coating.

[0076] Coating performance testing was conducted in accordance with GB / T 21776-2008, "Guide to the Testing Standards for Powder Coatings and Their Coatings." Leveling was tested in accordance with JB / T 3998-1999, "Determination of Leveling by Scraping." Adhesion was tested in accordance with GB / T 9286-1998, "Paint and Varnish Film Cross-Cut Test." Flame retardancy was assessed in accordance with GB / T 2408-2008, "Plastics—Determination of Burning Behavior—Horizontal and Vertical Methods." The test results obtained according to these standards are shown in the table below.

[0077] Table 1 Test results of coating performance of examples and comparative examples

[0078]

[0079]

[0080] Table 2 High temperature resistance and insulation test results of the embodiments and comparative examples

[0081]

[0082]

[0083] The coating performance test results in Tables 1 and 2 demonstrate that the powder coating products produced using the present invention's specific raw material combination and process exhibit excellent overall performance. The coatings exhibit a smooth, flat surface, excellent impact resistance, and a high gloss of over 94%. Adhesion is at level 0, and the leveling rating is particularly high, reaching level 7. Furthermore, the coatings exhibit excellent boiling water resistance, with no surface changes after boiling in boiling water for 10 hours. Furthermore, the coatings exhibit excellent flame retardancy and durability, with flame retardancy ratings at level V0 both before and after boiling in boiling water for 10 hours. This is due to the excellent flame retardancy of the coatings cured with a polyester resin containing bromine and phosphorus and a high nitrogen-containing TGIC. At the same time, the polyester resin used in the powder coating product of the present invention contains high-temperature resistant silicone segments, and the final coating film has excellent high-temperature resistance. The coating film surface remains unchanged after a high-temperature test of 320°C / 4h. The insulation performance is also good, meeting the insulation application requirements of battery products. In particular, in terms of thermal conductivity, the product of the present invention uses specially modified high-thermal conductivity material to modify boron nitride powder, so the thermal conductivity coefficient is good, basically above 0.87W / m·K, which is far superior to ordinary TGIC powder coating products.

[0084] It can be seen from the comparative example that the flame retardant performance of ordinary TGIC powder coating products, even if compounded with the flame retardant ammonium polyphosphate, is significantly reduced, especially the flame retardant performance after boiling in boiling water for 10 hours, and can only reach the HB level, indicating that the added flame retardant is severely lost after boiling in boiling water for 10 hours. At the same time, the coating appearance, gloss, leveling grade, high temperature resistance, insulation performance and thermal conductivity are also far inferior to the product of the present invention.

[0085] The above description is merely illustrative of certain exemplary embodiments of the present invention. It is understood that those skilled in the art will be able to modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the above description is illustrative in nature and should not be construed as limiting the scope of the claims.

Claims

1. A flame retardant powder coating, characterized in that: The powder coating comprises the following raw materials in parts by weight: 550-570 parts of polyester resin, 40-47 parts of TGIC curing agent, 80-110 parts of modified boron nitride powder, 8-12 parts of brightener, and 8-12 parts of leveling agent; The polyester resin is prepared by high-temperature polycondensation using the following raw materials in molar proportions: 10-14 parts of octamethylcyclotetrasiloxane, 7-9 parts of phosphorus trichloride, 35-40 parts of toluene, 8-10 parts of ethanol, 10-12 parts of 1,4-dibromo-2,3-butanediol, 15-20 parts of 2,2-bis(bromomethyl)-1,3-propanediol, 12-16 parts of 2,5-dibromoterephthalic acid, and 8-12 parts of 2,3-dibromosuccinic acid; catalyst 1: tetramethylammonium hydroxide, the amount of which is 0.05-0.08% of the mass of the octamethylcyclotetrasiloxane raw material; catalyst 2: monobutyltin oxide, the amount of which is 0.15-0.25% of the total mass of the raw materials; and antioxidant: antioxidant 1010, the amount of which is 0.3-0.5% of the total mass of the raw materials. The synthesis steps of the polyester resin include: (1) Add the formulated amount of toluene, octamethylcyclotetrasiloxane and catalyst 1 into the reactor, start stirring, and heat to 110-120°C for heat preservation polymerization reaction; (2) Sampling and testing: when the viscosity of the polymer reaches 200-230 mPa·s at 25°C, cool to 10-15°C, add the formulated amount of phosphorus trichloride dropwise to carry out phosphite esterification reaction, and simultaneously introduce nitrogen gas for bubbling to discharge the gas generated in the reaction system. After the addition is completed, continue to keep the temperature for reaction; (3) Take samples for testing. When the hydroxyl value of the polymer is lower than 5 mgKOH / g, it means that the terminal hydroxyl groups of the silicone chain have been completely reacted. At this time, add the formulated amount of ethanol and continue the phosphite esterification reaction at 10-15 °C. (4) Sampling and testing. When the ethanol content is lower than 1wt%, it means that the ethanol reaction has been completed. At this time, add the formulated amount of 1,4-dibromo-2,3-butanediol and 2,2-bis(bromomethyl)-1,3-propanediol, raise the temperature to 80-90℃ and keep it warm, and continue the phosphite esterification reaction; (5) Take samples to test the acid value of the polymer. When the acid value of the polymer is lower than 3 mgKOH / g, it means that the phosphite esterification reaction has been completed. At this time, the temperature is raised to 125-130 ° C. During the heating process, volatile substances are removed. Add the formulated amount of 2,5-dibromoterephthalic acid, then gradually raise the temperature to 220-225 ° C and carry out the heat preservation polymerization reaction. (6) Take samples to test the acid value of the polymer. When the acid value reaches 25-33 mgKOH / g, add the formulated amount of antioxidant and start the vacuum system at the same time to carry out vacuum polycondensation reaction at 220-225°C; (7) When the acid value of the polymer reaches 9-15 mgKOH / g, release the vacuum system, add the formulated amount of 2,3-dibromosuccinic acid, and continue the end-capping reaction at 220-225°C; (8) When the acid value of the polymer reaches 29-36 mgKOH / g, stop the reaction, discharge the material while it is still hot, and cool the polyester resin with a steel belt with condensed water, then crush and granulate it to obtain the product; The modified boron nitride powder is prepared by modification using a solvent method; the formula includes the following raw materials in parts by weight: 50-55 parts of boron nitride powder, 5-7 parts of the above-mentioned polyester resin, 35-40 parts of toluene, and 1-1.5 parts of polyethylene glycol trimethoxysilyl propyl ether; the preparation method includes the following steps: (1) adding the formulated amount of toluene, polyethylene glycol trimethoxysilyl propyl ether and the above-mentioned polyester resin into a reactor, heating the reactor to 80-90°C to dissolve them completely, then adding boron nitride powder, continuing to heat and stir the modified reaction for 1-1.5 hours, then heating the reactor to 110-115°C, and starting the vacuum system, controlling the vacuum degree at -0.097Mpa to -0.099Mpa, and fully removing the toluene solvent. After discharging, continue to dry the material until the volatile matter is less than 0.5wt%, then add the material to a high-speed grinder for crushing and sieving to obtain a modified boron nitride powder with a particle size of 90-130 mesh.

2. A flame retardant powder coating according to claim 1, characterized in that: The preparation method of powder coating comprises the following steps: (1) Evenly mix the formulated amount of polyester resin, TGIC curing agent, modified boron nitride powder, brightener, and leveling agent, and then melt extrude them through a twin-screw extruder; (2) The material melt-extruded by the twin-screw extruder is tableted by a tablet press with a water cooling system and coarsely crushed into coarse particles; (3) The coarse particles are further ground through a mill, and the particles that meet the requirements are screened out and collected through the mill's induced draft system to obtain a flame retardant powder coating.

3. A flame retardant powder coating according to claim 2, characterized in that: In step (1) of the preparation method, the screw temperature of the melt extrusion is 130-140°C and the screw speed is 400-500 rpm; in step (3), the required particles are 160-180 mesh.

4. A flame retardant powder coating according to claim 1, characterized in that: In the polyester resin synthesis step, the dropping time in step (2) is controlled to be 1-1.5 hours; in step (5), the temperature is gradually increased to 220-225°C at a heating rate of 15-17°C / h; in step (6), the vacuum degree is maintained between -0.097 MPa and -0.099 MPa.

5. The flame retardant powder coating according to claim 1, characterized in that: In the raw materials for preparing the modified boron nitride powder, the particle size of the boron nitride powder is 80-120 μm.

6. Use of the flame retardant powder coating according to any one of claims 1 to 5 on the metal surface of a battery.

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