Process for the preparation of dicyandiamide pyrophosphate and its use

Abstract

This invention provides a method for preparing melamine pyrophosphate and its application, belonging to the field of flame retardant technology. The invention uses pyrophosphoryl chloride and melamine as raw materials, reacting them in an alcohol-water mixed solvent. After the reaction, the mixture is neutralized with alkali, and post-processed to obtain melamine pyrophosphate. This invention uses pyrophosphoryl chloride as the phosphorus source, solving the problems of insufficient reaction due to the high viscosity of raw materials in the traditional pyrophosphate method and the hygroscopic and difficult-to-store raw materials in the phosphorus pentoxide method. The reaction process does not require strong acid catalysis; the byproduct hydrogen chloride is converted to ammonium chloride after neutralization with ammonia water, avoiding the generation of high-salt wastewater, making the process environmentally friendly. The method of this invention features mild reaction conditions, simple operation, high product purity, and good thermal stability. The obtained melamine pyrophosphate has a purity ≥95% and a 5% thermal decomposition temperature ≥300℃, making it suitable as a halogen-free flame retardant for widespread application in epoxy resins, polyolefins, nylon, coatings, rubber, and other fields.

Description

Technical Field

[0001] This invention relates to the field of flame retardant technology, and in particular to a method for preparing melamine pyrophosphate and its application. Background Technology

[0002] Dimelamine pyrophosphate (DMPY) is a nitrogen- and phosphorus-containing intumescent flame retardant. Its molecular structure contains a triazine ring and pyrophosphate ions, which promote charring of the substrate at high temperatures, forming an expanded char layer that provides thermal and oxygen-barrier flame retardancy. This type of flame retardant has advantages such as good thermal stability, high flame retardant efficiency, good compatibility with polymer materials, low smoke density during combustion, and halogen-free environmental friendliness. Therefore, it is widely used in fields requiring improved fire safety performance, such as polyolefins, nylon, epoxy resins, coatings, and rubber products.

[0003] Currently, common methods for industrially preparing melamine pyrophosphate mainly revolve around the selection of phosphorus sources. One method uses pyrophosphate as the phosphorus source, which can react directly with melamine. However, pyrophosphate itself is a viscous liquid, easily encapsulating solid raw materials during the reaction, leading to uneven mass transfer and limited reaction conversion. Furthermore, pyrophosphate requires special storage and transportation conditions due to its hygroscopic nature, affecting the stability of the raw material. Another method uses phosphorus pentoxide as the phosphorus source. Phosphorus pentoxide has high reactivity, but its strong hygroscopicity poses significant difficulties for raw material storage and handling. The reaction process is also complex, prone to side reactions, and increases the difficulty of subsequent purification processes. A third traditional method uses phosphates such as sodium pyrophosphate as the phosphorus source, reacting in a strongly acidic environment. While the raw materials are readily available, this method consumes large amounts of acid, generates large amounts of saline wastewater during the reaction, resulting in high subsequent treatment costs and environmental pollution.

[0004] In conclusion, developing a process for preparing di- and tri-melamine pyrophosphate that is easy to process, has high reaction efficiency, produces high-purity products, and is environmentally friendly is of great significance for promoting technological progress and green production in the flame retardant industry. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing di- and tri-melamine pyrophosphate and its application, which solves the problems in the prior art, such as the high viscosity of raw materials leading to insufficient reaction, the easy absorption of moisture by raw materials making them difficult to store, and the generation of high-salt wastewater during the reaction process leading to a heavy environmental burden.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for preparing di- and tri-melamine pyrophosphate, comprising the following steps: using pyrophosphoryl chloride and melamine as raw materials, reacting them in an alcohol-water mixed solvent, neutralizing with alkali solution after the reaction, and obtaining di- and tri-melamine pyrophosphate after post-treatment.

[0007] Preferably, the molar ratio of pyrophosphoryl chloride to melamine is 1:1.8 to 1:2.2; The volume ratio of alcohol to water in the alcohol-water mixed solvent is 3:1 to 5:1; The alcohol is anhydrous ethanol.

[0008] Preferably, the reaction temperature is 35°C to 45°C; The reaction takes 3 to 6 hours.

[0009] Preferably, the alkaline solution is dilute ammonia solution with a mass fraction of 4% to 8%; The neutralization step involves adjusting the pH of the reaction system to between 6.5 and 7.5. The post-processing steps include solid-liquid separation, washing, and drying of the neutralized reaction products; The pyrophosphoryl chloride is pretreated by grinding and sieving before use, and the sieving is through a 200-mesh sieve; The grinding and sieving pretreatment is carried out in an inert atmosphere with an oxygen content of less than 5 ppm and a water content of less than 1 ppm, and the grinding time is 15 to 20 minutes.

[0010] Preferably, the melamine is dried before use to reduce its moisture content to less than 0.2 wt%. The drying process is carried out at a temperature of 100°C to 110°C, a vacuum degree of -0.095MPa to -0.085MPa, and a drying time of 5 to 7 hours. The amount of the alcohol-water mixed solvent used is 5 to 7 times the total mass of the raw materials; The reaction is carried out under the protection of an inert gas; Before the reaction, an inert gas is introduced into the reaction system for 20 to 40 minutes to replace the air, with a gas flow rate of 40 mL / min to 90 mL / min.

[0011] Preferably, the reaction steps include: first dissolving pyrophosphoryl chloride in an alcohol-water mixed solvent, and then adding melamine in batches; The pyrophosphoryl chloride is dissolved at a temperature of 30°C to 40°C, and the stirring time is 10 to 25 minutes. The melamine is added in 5 to 8 parts, with an interval of 8 to 12 minutes between the addition of two adjacent parts; During the addition of melamine, the temperature of the reaction system is controlled to be between 38°C and 42°C. The stirring speed for the reaction is 350 rpm to 650 rpm; In the step of neutralizing with alkali solution, the alkali solution is added dropwise at a rate of 1 mL / min to 6 mL / min; After the step of neutralizing with alkali is completed, the reaction system is heated to 60°C to 75°C and kept at that temperature for 20 to 50 minutes. The solid-liquid separation is performed by centrifugation, with a centrifugation speed of 5000 rpm to 7000 rpm and a centrifugation time of 8 to 12 minutes.

[0012] Preferably, the washing involves washing with water 2 to 4 times until no chloride ions are detected in the washing solution; The statement that chloride ions are undetectable means that the washing solution shows no white turbidity when tested with 0.1 mol / L silver nitrate solution; The drying process is vacuum drying, with a drying temperature of 70°C to 90°C and a drying time of 10 to 14 hours. The post-processing step also includes pulverizing the dried product to control its particle size D50 to between 4 μm and 11 μm.

[0013] The present invention also provides melamine pyrophosphate prepared by the above preparation method.

[0014] This invention also provides the application of the above-mentioned di- and tri-melamine pyrophosphate in the preparation of flame retardant materials.

[0015] Preferably, the flame-retardant material is epoxy resin, polyolefin, glass fiber reinforced nylon, coating, or rubber product.

[0016] Compared with the prior art, the method for preparing melamine pyrophosphate provided by the present invention has the following beneficial effects: First, it boasts excellent environmental friendliness. This invention uses pyrophosphate chloride as the phosphorus source, reacting it with melamine in an alcohol-water mixed solvent. It eliminates the need for strong acids such as sulfuric acid or hydrochloric acid as catalysts, thus avoiding the use of strong acids at the source. The byproduct hydrogen chloride generated during the reaction is neutralized with dilute ammonia water and converted into ammonium chloride. Ammonium chloride is readily soluble in water and can be easily removed by washing. The resulting washing liquid, mainly composed of ammonium chloride, can be recycled as a byproduct. Compared to traditional processes that use salts such as sodium pyrophosphate as the phosphorus source, require strong acid conditions, and generate large amounts of high-salt sodium chloride wastewater, this invention produces almost no difficult-to-treat high-salt wastewater and solid waste, significantly reducing wastewater treatment costs. It is an environmentally friendly green synthesis process.

[0017] Secondly, the product quality is high. This invention uses pyrophosphoryl chloride as the phosphorus source. It is a solid powder, non-sticky, and highly soluble in an alcohol-water mixed solvent, allowing for thorough contact and reaction with melamine. This avoids the problems of unreacted material encapsulation and incomplete reaction caused by the high viscosity of the raw material in the traditional pyrophosphoric acid method. The ethanol-water mixed solvent system has excellent dissolving and dispersing capabilities for the raw material, with mild reaction conditions and few side reactions. Testing shows that the melamine pyrophosphate product obtained by this invention has a purity of over 95%, a 5% thermal decomposition temperature above 300℃, nitrogen and phosphorus content meeting theoretical values, low impurity content, and excellent overall product quality.

[0018] Third, the process exhibits good stability and is suitable for industrial production. Pyrophosphoryl chloride is in solid form and is relatively stable, making it easier to store and transport compared to liquid pyrophosphoric acid, requiring no special insulation or anti-condensation facilities. Compared to phosphorus pentoxide, its hygroscopicity is significantly reduced, requiring less stringent storage conditions and offering higher operational safety. The reaction conditions of this invention are mild, with a reaction temperature of only 35-45℃, eliminating the need for high-temperature and high-pressure equipment, thus requiring less sophisticated production equipment and consuming less energy. The post-processing is simple, requiring only centrifugation, washing, drying, and pulverization to obtain the finished product. The process flow is short, the operation is simple, and it is easy to achieve continuous and large-scale industrial production.

[0019] Fourth, the product exhibits excellent application performance. When the di- and tri-melamine pyrophosphate prepared according to this invention is added to epoxy resin at a dosage of only 9%, the limiting oxygen index of the material increases from 19.5% to 28.7%, and the flame retardant rating reaches UL-94V-0, demonstrating excellent flame retardant efficiency. Simultaneously, the retention rate of mechanical properties such as tensile strength, flexural strength, and impact strength is all above 90%, and the flexural modulus is even improved, indicating that this flame retardant has good compatibility with the matrix resin, minimal impact on the material's mechanical properties, and possesses significant practical value.

[0020] The di- and tri-melamine pyrophosphate prepared by this invention is a high-performance nitrogen-phosphorus intumescent halogen-free flame retardant with advantages such as high thermal stability, good flame retardant efficiency, good compatibility with polymer materials, and low smoke and low toxicity during combustion. It can be widely used in many industrial fields.

[0021] In the electronics and electrical fields, it can be used to prepare flame-retardant epoxy resin encapsulation materials, copper-clad laminates, printed circuit boards, connectors, relay housings, etc., meeting the requirements of electronic components for halogen-free flame retardancy and high reliability. In the new energy vehicle field, it can be used for flame-retardant modification of components such as charging pile housings, battery boxes, high-voltage connectors, and wire harness sheaths, improving the fire safety of new energy vehicles. In the rail transit and aerospace fields, it can be used for flame-retardant treatment of interior materials, seats, partitions, wire ducts, and other components, meeting the stringent standards of relevant industries for material combustion performance.

[0022] In the building materials sector, it can be used in flame-retardant coatings, insulation materials, sealing materials, wall panels, pipes, etc., to improve the fire resistance rating of buildings and reduce fire risk. In the textile sector, it can impart flame-retardant properties to synthetic fiber fabrics such as polyester and nylon through coating or impregnation processes, and is used in the production of flame-retardant textiles such as fire suits, tents, curtains, and carpets. Furthermore, it can be applied to multiple fields such as rubber products, polyolefin films, wire and cable sheaths, foaming materials, and adhesives, providing efficient and environmentally friendly flame-retardant solutions for various polymer materials, and has broad market application prospects. Attached Figure Description

[0023] Figure 1 The infrared spectrum of melamine pyrophosphate; Figure 2 This is the liquid chromatogram of the sample. Detailed Implementation

[0024] This invention provides a method for preparing di- and tri-melamine pyrophosphate, comprising the following steps: using pyrophosphoryl chloride and melamine as raw materials, reacting them in an alcohol-water mixed solvent, neutralizing with alkali solution after the reaction, and obtaining di- and tri-melamine pyrophosphate after post-treatment.

[0025] In this invention, pyrophosphoryl chloride is a phosphorus source compound with the chemical formula P2O3Cl4, also known as pyrophosphoryl chloride, diphosphoryl chloride, or tetrachloropyrophosphoric acid, and is an inorganic compound. Pyrophosphoryl chloride is a white crystalline solid at room temperature with a melting point of approximately 56°C and a relatively high boiling point. It is readily soluble in organic solvents such as ethanol, ether, and acetone, and undergoes hydrolysis in water to produce pyrophosphoric acid and hydrogen chloride. As a phosphorus source, pyrophosphoryl chloride has advantages over traditional pyrophosphoric acid in that it is a solid, non-sticky, easy to store and transport, does not easily absorb moisture and decompose, and has good solubility in organic solvents, allowing for sufficient contact with reactants. This invention does not impose any special restrictions on the source of the pyrophosphoryl chloride; commercially available analytical grade or industrial grade products can be used, such as those purchased from Aladdin Reagents, Maclean Reagents, Sigma-Aldrich, and other domestic and international reagent companies.

[0026] In this invention, melamine is a nitrogen-containing heterocyclic organic compound with the chemical formula C3H6N6, also known as melamine, cyanamide, or cyanuramide triamide. Melamine's molecular structure contains a triazine ring, and its nitrogen content is as high as approximately 66%, making it an important raw material for preparing nitrogen-based and nitrogen-phosphorus-based flame retardants. Melamine is a white monoclinic crystal, slightly soluble in water, insoluble in cold ethanol, but soluble in hot ethanol. This invention does not impose any special restrictions on the source of the melamine; commercially available industrial-grade products can be used, typically requiring a purity of ≥99%. For example, products purchased from domestic and international manufacturers such as Sichuan Chemical Industry, Luxi Chemical Industry, and Jinhe Industrial Co., Ltd. are acceptable. Before use, it is preferable to dry the melamine to remove adsorbed moisture and avoid adverse effects on subsequent reactions.

[0027] In this invention, the alcohol-water mixed solvent refers to a solvent system composed of an alcohol solvent and water mixed in a certain proportion. The alcohol-water mixed solvent combines the organic solubility of alcohol with the polarity of water, enabling it to simultaneously dissolve pyrophosphoryl chloride and melamine, providing a good reaction medium for both. This invention does not impose any special limitations on the alcohol solvent; commonly used lower alcohols in the art, such as methanol, ethanol, propanol, isopropanol, and butanol, are acceptable, with ethanol being preferred, and anhydrous ethanol being more preferred. Ethanol is a common organic solvent with low toxicity, moderate volatility, low price, easy recovery, and good miscibility with water. The water is preferably deionized water or distilled water to reduce interference from impurity ions on the reaction. The volume ratio of alcohol to water is preferably 3:1 to 5:1, for example, 3:1, 3.5:1, 4:1, 4.5:1, or 5:1. Within this ratio range, the solvent system can provide suitable solubility and a suitable reaction environment, ensuring both sufficient dissolution of pyrophosphoryl chloride and maintaining a moderate hydrolysis rate. As a further preferred embodiment, the volume ratio of alcohol to water can be from 3.8:1 to 4.2:1, with 4:1 being the most preferred. This ratio exhibits the best reaction effect in the examples.

[0028] In this invention, the reaction is a process in which pyrophosphoryl chloride and melamine react chemically in an alcohol-water mixed solvent to generate the target product, dimelamine pyrophosphate. The reaction mechanism may involve the partial hydrolysis of pyrophosphoryl chloride in the alcohol-water mixed solvent to generate a pyrophosphate intermediate, which then undergoes a neutralization reaction with melamine to generate dimelamine pyrophosphate, with hydrogen chloride as a byproduct. Appropriate temperature and time control are necessary during the reaction to ensure complete reaction and avoid side reactions. The preferred reaction temperature is 35°C to 45°C, for example, 35°C, 38°C, 40°C, 42°C, or 45°C. Within this temperature range, the reaction rate is moderate, ensuring the reaction completes within a reasonable time while avoiding solvent evaporation or exacerbated side reactions due to high temperatures. As a further preferred embodiment, the reaction temperature can also be 38°C to 42°C, with 40°C being the most preferred, as this temperature exhibited the best product yield and purity in the examples. The reaction time is preferably 3 to 6 hours, for example, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours, or 6 hours. Within this time range, the reaction can proceed to completion, and the product yield reaches a high level. As a further preferred embodiment, the reaction time can also be 4 to 5 hours, with 4 hours being the most preferred, as this time is sufficient to ensure complete reaction in the examples.

[0029] In this invention, after the reaction is complete, the acidic substances in the system need to be neutralized with an alkaline solution. The purpose of this neutralization step is to convert the byproduct hydrogen chloride generated in the reaction into harmless salts, while adjusting the pH of the system to near neutral for easier subsequent processing. The alkaline solution is preferably dilute ammonia water, i.e., an alkaline solution formed by ammonia gas dissolved in water. Ammonia water is weakly alkaline and can gently neutralize hydrogen chloride to generate ammonium chloride without damaging the product. Ammonia water is also volatile, and excess ammonia easily escapes during the subsequent drying process and will not remain in the product. This invention does not have any special restrictions on the source of the ammonia water; commercially available analytical grade or industrial grade ammonia water can be used, and it needs to be diluted to a suitable concentration before use. The mass fraction of the ammonia water is preferably 4% to 8%, for example, it can be 4%, 5%, 6%, 7%, or 8%. Within this concentration range, the ammonia water can effectively neutralize the acid in the system without causing local over-alkalinity that could damage the product due to excessive concentration. As a further preferred embodiment, the mass fraction of the ammonia water can also be 5.5% to 6.5%, and most preferably 6%, as this concentration showed the best neutralization effect in the examples. The neutralization step preferably adjusts the pH of the reaction system to 6.5 to 7.5, for example, 6.5, 6.8, 7.0, 7.2, or 7.5. Within this pH range, the system is near neutral, which is beneficial for subsequent solid-liquid separation and washing operations, while ensuring the stability of the product. As a further preferred embodiment, the pH can also be 6.8 to 7.2, with 7.0 being the most preferred, as this pH value yielded the best product quality in the examples.

[0030] In this invention, the post-processing steps include solid-liquid separation, washing, and drying of the neutralized reaction product. These operations are conventional product separation and purification methods in chemical synthesis. Solid-liquid separation is used to separate the solid product from the mother liquor; washing is used to remove impurities and salts adsorbed on the product surface; and drying is used to remove water from the product to obtain a dried final product.

[0031] Preferably, the pyrophosphoryl chloride undergoes a grinding and sieving pretreatment before use, specifically passing through a 200-mesh sieve. The purpose of grinding and sieving is to pulverize the pyrophosphoryl chloride raw material into fine particles, increasing its specific surface area, which facilitates rapid dissolution and uniform dispersion in the solvent. Grinding can be performed using equipment such as an agate mortar, ball mill, or pulverizer. Passing through a 200-mesh sieve means that the particle size is less than 75 μm. This invention does not strictly limit the grinding time, as long as all particles pass through the 200-mesh sieve. As a further preferred embodiment, the grinding and sieving pretreatment is carried out in an inert atmosphere with an oxygen content of less than 5 ppm and a water content of less than 1 ppm, for a grinding time of 15 to 20 minutes. An inert atmosphere, such as high-purity nitrogen or argon, can prevent the pyrophosphoryl chloride from absorbing moisture or oxidizing due to frictional heating during grinding. The grinding time can be, for example, 15, 16, 17, 18, 19, or 20 minutes, within which sufficient pulverization effect can be ensured.

[0032] Preferably, the melamine is dried before use to reduce its moisture content to below 0.2 wt%. The purpose of drying is to remove adsorbed moisture from the melamine raw material, preventing moisture from entering the reaction system and interfering with the reaction process. Melamine has a certain degree of hygroscopicity and may absorb moisture from the air during prolonged storage. This invention does not impose any particular limitation on the drying method; methods such as vacuum drying, forced-air drying, and infrared drying can be used. Vacuum drying is preferred because vacuum conditions facilitate rapid moisture evaporation. As a further preferred embodiment, the drying temperature is 100°C to 110°C, for example, 100°C, 102°C, 105°C, 108°C, or 110°C; the vacuum degree is -0.095MPa to -0.085MPa, for example, -0.095MPa, -0.092MPa, -0.09MPa, -0.088MPa, or -0.085MPa; and the drying time is 5 hours to 7 hours, for example, 5 hours, 5.5 hours, 6 hours, 6.5 hours, or 7 hours. Under these conditions, moisture in melamine can be effectively removed while avoiding excessively high temperatures that could cause melamine to sublimate or decompose. The most preferred drying conditions are 105°C, -0.09MPa, and 6 hours of drying, which achieved satisfactory drying results in the examples.

[0033] Preferably, the amount of the alcohol-water mixed solvent is 5 to 7 times the total mass of the raw materials. The total mass of the raw materials refers to the sum of the masses of pyrophosphoryl chloride and melamine. The amount of solvent used affects the concentration of reactants and the reaction rate. If the amount of solvent is too small, the reaction system becomes viscous, making mass transfer difficult; if the amount of solvent is too large, the reactant concentration becomes too low, the reaction rate becomes slow, and the equipment utilization rate becomes low. A volume of 5 to 7 times is a preferred range, for example, it can be 5 times, 5.5 times, 6 times, 6.5 times, or 7 times. As a further preferred embodiment, the amount of solvent can also be 5.8 to 6.2 times, and most preferably 6 times, which achieved the best reaction effect in the examples.

[0034] Preferably, the reaction is carried out under an inert gas atmosphere. The purpose of the inert gas atmosphere is to isolate oxygen and moisture from the air, prevent the hydrolysis or oxidation of pyrophosphoryl chloride, and also prevent side reactions. This invention does not have a particular limitation on the type of inert gas; nitrogen, argon, helium, etc., can be used, with nitrogen or argon being preferred due to their wide availability and low cost. As a further preferred embodiment, before the reaction, an inert gas is introduced into the reaction system for 20 to 40 minutes to replace the air, at a rate of 40 mL / min to 90 mL / min. The purpose of replacing the air is to completely remove oxygen and moisture from the reaction system, creating a stable inert environment. The replacement time can be, for example, 20, 25, 30, 35, or 40 minutes; the aeration rate can be, for example, 40 mL / min, 50 mL / min, 60 mL / min, 70 mL / min, 80 mL / min, or 90 mL / min. As a further preferred option, the replacement time can also be 28 to 32 minutes, with the most preferred option being 30 minutes; the ventilation rate can also be 50 mL / min to 80 mL / min, with the most preferred option being 60 mL / min. These conditions ensured good protective effects in the examples.

[0035] Preferably, the reaction steps include: first dissolving pyrophosphoryl chloride in an alcohol-water mixed solvent, and then adding melamine in batches. This order and method of addition is to control the reaction rate and avoid side reactions caused by excessively high local concentrations. Dissolving the pyrophosphoryl chloride first allows it to be uniformly dispersed in the system, and then slowly adding melamine ensures a stable reaction. As a further preferred embodiment, the pyrophosphoryl chloride is dissolved at a temperature of 30°C to 40°C, and the stirring and dissolution time is 10 minutes to 25 minutes. Within this temperature range, pyrophosphoryl chloride can dissolve rapidly while avoiding premature hydrolysis due to excessively high temperatures. The dissolution temperature can be, for example, 30°C, 32°C, 35°C, 38°C, or 40°C; the dissolution time can be, for example, 10 minutes, 15 minutes, 20 minutes, or 25 minutes. As a further preferred embodiment, the dissolution temperature can also be 33°C to 37°C, most preferably 35°C; the dissolution time can also be 15 minutes to 20 minutes, most preferably 15 minutes. These conditions, in the examples, achieve complete dissolution of the pyrophosphoryl chloride.

[0036] As a further preferred embodiment, the melamine is added in batches of 5 to 8 parts, with an interval of 8 to 12 minutes between each batch. Batch addition effectively controls the exothermic reaction and prevents a sudden rise in system temperature. The number of parts can be, for example, 5, 6, 7, or 8 parts; the time interval can be, for example, 8, 9, 10, 11, or 12 minutes. As a further preferred embodiment, the number of parts can also be 5 to 6 parts, with 5 parts being the most preferred; the time interval can also be 9 to 11 minutes, with 10 minutes being the most preferred. This condition, in the examples, achieves stable feeding.

[0037] As a further preferred embodiment, during the addition of melamine, the reaction system temperature is controlled between 38°C and 42°C. The temperature needs to be closely monitored during the addition process, and if necessary, the temperature should be maintained stable by adjusting the water bath or cooling medium. The temperature control range can be, for example, 38°C, 39°C, 40°C, 41°C, or 42°C. As a further preferred embodiment, the temperature during the addition process can also be controlled between 39.5°C and 40.5°C, with 40°C being the most preferred. This temperature, in the examples, ensures the stable progress of the reaction.

[0038] As a further preferred embodiment, the stirring speed of the reaction is 350 rpm to 650 rpm. The purpose of stirring is to ensure uniform mixing of the reactants and enhance mass and heat transfer. The stirring speed needs to be adjusted according to the viscosity and scale of the reaction system. The speed range can be, for example, 350 rpm, 400 rpm, 450 rpm, 500 rpm, 550 rpm, 600 rpm, or 650 rpm. As a further preferred embodiment, the stirring speed can also be 400 rpm to 600 rpm, preferably around 400 rpm during the feeding stage and preferably 500 rpm to 600 rpm during the reaction stage. This condition ensures good mixing in the examples.

[0039] As a further preferred embodiment, in the neutralization step with alkali, the alkali solution is added dropwise at a rate of 1 mL / min to 6 mL / min. Controlling the dropping rate aims to avoid localized over-alkaliness leading to product decomposition or side reactions. The dropping rate can be, for example, 1 mL / min, 2 mL / min, 3 mL / min, 4 mL / min, 5 mL / min, or 6 mL / min. As a further preferred embodiment, the dropping rate can also be 2 mL / min to 5 mL / min, most preferably 3 mL / min, which, in the examples, achieves a mild neutralization process.

[0040] As a further preferred embodiment, after the neutralization step with alkali solution is completed, the reaction system is heated to 60°C to 75°C and held at this temperature for 20 to 50 minutes. The purpose of heating and holding at this temperature is to promote the complete dissolution of residual impurities and inorganic salt byproducts, facilitating subsequent washing and removal. The holding temperature can be, for example, 60°C, 65°C, 70°C, or 75°C; the holding time can be, for example, 20 minutes, 30 minutes, 40 minutes, or 50 minutes. As a further preferred embodiment, the holding temperature can also be 65°C to 70°C, with 68°C being the most preferred; the holding time can also be 30 minutes to 45 minutes, with 30 minutes being the most preferred. This condition achieved good impurity removal effect in the examples.

[0041] As a further preferred embodiment, the solid-liquid separation is centrifugal separation, with a centrifugation speed of 5000 rpm to 7000 rpm and a centrifugation time of 8 to 12 minutes. Centrifugal separation is a method of rapidly separating solid products from liquid mother liquor using centrifugal force. The centrifugation speed and time need to be adjusted according to the size and density of the solid particles. The centrifugation speed can be, for example, 5000 rpm, 5500 rpm, 6000 rpm, 6500 rpm, or 7000 rpm; the centrifugation time can be, for example, 8 minutes, 9 minutes, 10 minutes, 11 minutes, or 12 minutes. As a further preferred embodiment, the centrifugation speed can also be 5500 rpm to 6500 rpm, most preferably 6000 rpm; the centrifugation time can also be 9 minutes to 11 minutes, most preferably 10 minutes. These conditions achieved good solid-liquid separation results in the examples.

[0042] Preferably, the washing involves washing with water 2 to 4 times until chloride ions are undetectable in the washing solution. The purpose of washing is to remove soluble salt impurities such as ammonium chloride carried in the product. The washing water is preferably deionized water or distilled water. The number of washing cycles can be, for example, 2, 3, or 4. As a further preferred embodiment, the number of washing cycles is preferably 3, which is sufficient in the examples to remove chloride ions. Undetectable chloride ions means that the washing solution shows no white turbidity when tested with 0.1 mol / L silver nitrate solution. Chloride ions react with silver ions to form a white precipitate of silver chloride, which is a sensitive method for detecting the presence of chloride ions. After each washing, a small amount of washing solution can be taken, and a few drops of silver nitrate solution can be added to observe whether white turbidity is produced. If no turbidity is observed, it indicates that chloride ions have been substantially removed, and the washing endpoint has been reached.

[0043] Preferably, the drying is vacuum drying, with a drying temperature of 70°C to 90°C and a drying time of 10 to 14 hours. Vacuum drying is a method of heating and drying under negative pressure, which can reduce the drying temperature, shorten the drying time, and avoid damage to the product caused by high temperatures. The drying temperature can be, for example, 70°C, 75°C, 80°C, 85°C, or 90°C; the drying time can be, for example, 10 hours, 11 hours, 12 hours, 13 hours, or 14 hours. As a further preferred embodiment, the drying temperature can also be 75°C to 85°C, most preferably 80°C; the drying time can also be 11 hours to 13 hours, most preferably 12 hours. This condition, in the examples, achieves sufficient drying of the product.

[0044] Preferably, the post-processing step further includes pulverizing the dried product to control its particle size D50 within the range of 4 μm to 11 μm. The purpose of pulverization is to obtain a powder product with uniform fineness for easy subsequent application. Pulverization can be performed using equipment such as an ultrafine pulverizer, an air jet mill, or a ball mill. Particle size D50 refers to the particle size corresponding to a cumulative particle size distribution percentage of 50%, and is a commonly used indicator of the average particle size of powder. D50 is controlled within the range of 4 μm to 11 μm, for example, it can be 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, or 11 μm. As a further preferred embodiment, D50 can also be controlled within the range of 5 μm to 10 μm, and most preferably around 8 μm. This range achieved satisfactory product fineness in the examples, which is beneficial for uniform dispersion in the matrix resin.

[0045] This invention also provides melamine pyrophosphate prepared by the above-described method. The melamine pyrophosphate, prepared by the method of this invention, possesses specific physicochemical properties and application performance. As a preferred embodiment, the purity of the di- and tri-melamine pyrophosphate is ≥95%, for example, 95%, 96%, 97%, 98%, or 99%; the 5% thermal decomposition temperature is ≥300℃, for example, 300℃, 305℃, 310℃, 315℃, or 320℃; the particle size D50 is 5μm to 10μm, for example, 5μm, 6μm, 7μm, 8μm, 9μm, or 10μm; the nitrogen content is 37% to 40%, for example, 37%, 38%, 39%, or 40%; the phosphorus content is 13% to 15%, for example, 13%, 13.5%, 14%, 14.5%, or 15%; and the conductivity of its 25% aqueous solution at 25℃ is less than 500μs / cm, for example, 400μs / cm, 420μs / cm, 450μs / cm, 480μs / cm, or 490μs / cm. These indicators all demonstrate that the product of this invention has high purity, high thermal stability, suitable particle size, and good control of ion residues.

[0046] This invention also provides the application of the aforementioned melamine pyrophosphate in the preparation of flame-retardant materials. The flame-retardant material refers to a polymeric material or composite material that exhibits flame-retardant properties after the addition of a flame retardant. Melamine pyrophosphate, as a highly efficient nitrogen-phosphorus intumescent flame retardant, can promote char formation during material combustion, forming an expanded char layer that provides heat and oxygen insulation, thereby inhibiting flame propagation. As a preferred embodiment, the flame-retardant material is an epoxy resin, polyolefin, glass fiber reinforced nylon, coating, or rubber product.

[0047] The epoxy resin mentioned above is a type of thermosetting resin containing epoxy groups in its molecular structure. It possesses excellent adhesion, mechanical properties, and chemical resistance, and is widely used in electronic packaging, coatings, and composite materials. Commonly used epoxy resins include bisphenol A type epoxy resin, bisphenol F type epoxy resin, phenolic epoxy resin, and alicyclic epoxy resin, such as grades E-51, E-44, and E-20. The product of this invention can be used to prepare flame-retardant epoxy resin casting materials, potting compounds, copper-clad laminates, and laminates.

[0048] The polyolefins referred to are thermoplastic resins polymerized from olefin monomers, mainly including polyethylene, polypropylene, polybutene, polymethylpentene, and their copolymers and modified products. Polyolefins have advantages such as low density, good mechanical properties, chemical resistance, excellent electrical insulation, and easy processing and molding, and are widely used in packaging, agriculture, construction, wire and cable, automotive parts, and other fields. The products of this invention can be used to prepare flame-retardant polyolefin films, sheets, pipes, injection molded parts, wire and cable sheaths, etc.

[0049] The glass fiber reinforced nylon refers to a composite material made of nylon (polyamide) as the matrix resin and glass fiber as the reinforcing material. Nylon itself has excellent mechanical properties, wear resistance, and oil resistance. After being reinforced with glass fiber, its strength, stiffness, and heat resistance are further improved, and it is widely used in automotive engine peripheral parts, mechanical parts, power tool housings, etc. Commonly used nylons include nylon 6, nylon 66, nylon 46, and nylon 12. The product of this invention can be used to prepare flame-retardant glass fiber reinforced nylon products, meeting the flame-retardant performance requirements of the electronics, electrical, and automotive industries.

[0050] The coating refers to a material that, when applied to the surface of an object, forms a firmly adhering, continuous thin film, providing protection, decoration, or other special functions. Coatings can be categorized by base material type, such as epoxy coatings, polyurethane coatings, acrylic coatings, and alkyd coatings. The product of this invention can be used to prepare flame-retardant coatings, applicable to fields such as fireproofing of steel structures, fireproofing of cables, and flame retardancy of wood.

[0051] The rubber products mentioned refer to products made from natural or synthetic rubber, possessing properties such as high elasticity, wear resistance, sound insulation, and cushioning. Commonly used synthetic rubbers include styrene-butadiene rubber (SBR), butadiene rubber (BR), nitrile rubber (NBR), ethylene propylene rubber (EPR), and chloroprene rubber (CBR). Rubber products are widely used in tires, seals, hoses, belts, shock absorbers, and other fields. The product of this invention can be used to prepare flame-retardant rubber products, meeting the needs of flame-retardant rubber in the coal mining, electronics, and rail transportation industries.

[0052] In summary, the method for preparing melamine pyrophosphate provided by this invention uses readily available raw materials, is environmentally friendly, simple to operate, and produces high-quality products. The resulting products show promising flame-retardant application prospects in various polymer materials. The technical solutions provided by this invention are described in detail below with reference to embodiments, but these should not be construed as limiting the scope of protection of this invention.

[0053] Example 1 This embodiment provides a method for preparing melamine pyrophosphate: (1) Raw material pretreatment Pyrophosphoryl chloride pretreatment: Place commercially available pyrophosphoryl chloride solid (analytical grade) in a glove box filled with high-purity nitrogen or argon (oxygen content <5ppm, water content <1ppm), and manually grind it for 15-20 minutes using an agate mortar and pestle until it passes through a 200-mesh standard sieve (pore size 75μm). The resulting fine powder is then placed in a dry, sealed bottle for later use. Melamine pretreatment: Place commercially available melamine (industrial grade, purity ≥99%) in a vacuum drying oven and dry it for 6 hours at 105℃ and a vacuum of -0.09MPa. Take a sample and test it to ensure that the moisture content is below 0.2% (wt). After taking it out, immediately place it in a desiccator containing color-changing silica gel to cool to room temperature for later use.

[0054] (2) Construction of the reaction system Solvent preparation: Measure anhydrous ethanol and deionized water in a volume ratio of 4:1, and mix them evenly in a beaker to prepare a mixed solvent; Apparatus preparation: In a 500mL four-necked flask equipped with a mechanical stirrer, a constant temperature water bath, a reflux condenser, and a thermometer, add 300mL of the above mixed solvent (the total amount of solvent should be controlled to be 6 times the total mass of the raw materials to be added). Atmosphere replacement: Turn on the stirrer (200-300 rpm), introduce high-purity nitrogen into the liquid surface at a rate of 50-80 mL / min, and continue to replace for 30 minutes to remove dissolved oxygen and air from the system. Then maintain a slight positive pressure nitrogen protection.

[0055] (3) Synthesis reaction Dissolution by adding feed: Turn on the constant temperature water bath and control the solvent temperature at 35℃. Under nitrogen protection, slowly add 21.5g (about 0.1mol) of pretreated pyrophosphoryl chloride powder into the flask through a powder feeder, maintain a speed of 400rpm, and stir for 15-20 minutes until the solid is completely dissolved and a transparent solution is formed; Batch addition: Divide 25.2g (approximately 0.2mol) of pretreated melamine into 5-8 equal portions, adding one portion every 10 minutes. During the addition process, maintain the reaction system temperature at 40±2℃ by adjusting the water bath. Isothermal reaction: After the feeding is complete, increase the stirring speed to 500-600 rpm and maintain the system at 40℃ for 4-5 hours. During the reaction, the system can be observed to gradually change from transparent to milky white suspension.

[0056] (4) Neutralization and impurity removal pH adjustment: After the reaction is complete, reduce the rotation speed to 300 rpm. Add 6% (w / min) dilute ammonia solution dropwise using a peristaltic pump at a rate of 2-5 mL / min, while monitoring the pH in real time with a precision pH meter. Stop adding the solution when the pH of the system rises to 6.5-7.5 (this process converts the byproduct HCl into easily soluble ammonium chloride). Heating to aid dissolution: After stopping the dropwise addition, raise the temperature to 65-70℃ and keep stirring for 30-45 minutes to use the high temperature to promote the full dissolution of residual impurities and inorganic salt byproducts.

[0057] (5) Post-processing Separation: Cool the reaction solution to room temperature (about 25°C) naturally, transfer it to a centrifuge, centrifuge at 6000 rpm for 10 minutes, discard the supernatant and collect the white solid filter cake; Washing: Resuspend the filter cake in 80-100 mL of deionized water and stir to wash. Repeat the centrifugation washing 3 times until the supernatant shows no obvious white turbidity when tested with 0.1 mol / L silver nitrate solution (indicating that chloride ions have been basically removed). Finished product: The washed solid was placed in an 80℃ vacuum drying oven and dried for 12 hours until constant weight. After removal, it was pulverized using an ultrafine pulverizer to control the particle size D50 to 5-10μm, finally obtaining a white powdery melamine pyrophosphate product.

[0058] This embodiment uses optimal parameters, as follows: Molar ratio: Pyrophosphoryl chloride : melamine = 1 : 2 Mixed solvent: anhydrous ethanol: deionized water = 4:1 (volume ratio) Solvent ratio: The solvent mass is 6 times the total mass of the raw materials. Reaction temperature: 40℃ ± 2℃ Neutralizing alkaline solution: 6% (wt) dilute ammonia water Endpoint pH: 7.0 ± 0.5.

[0059] Reaction principle and equation: Pyrophosphate chloride undergoes partial hydrolysis in an ethanol-water mixture to generate a pyrophosphate intermediate. This intermediate then reacts with melamine to form dimelamine pyrophosphate. Simultaneously, the generated hydrogen chloride byproduct is neutralized by ammonia water to convert it into ammonium chloride. The reaction equation is as follows: P2O3Cl4 + 2C3H6N6 + 2H2O → (C3H6N6)2 · H4P2O7 + 4HCl FT-IR spectroscopy analysis The structure of the synthesized product in this embodiment was characterized by Fourier transform infrared spectroscopy, with a wavenumber range of 4000-400 cm⁻¹ and a resolution of 4 cm⁻¹. Figure 1 The image shows the infrared spectrum of melamine pyrophosphate.

[0060] The strong and broad absorption peak at 3380.73 cm⁻¹ is attributed to the stretching vibration of the amino group (NH) in melamine, and the broadening of the peak indicates the presence of intermolecular hydrogen bonding. The strong absorption peaks at 1663.21 cm⁻¹ and 1570.26 cm⁻¹ correspond to the C=N stretching vibration and ring skeletal vibration of the triazine ring, respectively, which are characteristic peaks of melamine. The absorption peaks at 1507.74 cm⁻¹ and 1406.61 cm⁻¹ further confirm the existence of the triazine ring structure.

[0061] In the fingerprint region, the strong absorption peak at 1059.42 cm⁻¹ is attributed to the antisymmetric stretching vibration of pyrophosphate (PO), and the absorption peak at 983.58 cm⁻¹ corresponds to the stretching vibration of the POP bridging bond; these are characteristic absorptions of pyrophosphate. The absorption peaks at 779.21 cm⁻¹, 691.55 cm⁻¹, and 603.59 cm⁻¹ are attributed to the bending and skeletal vibrations of PO.

[0062] Infrared spectroscopy simultaneously revealed characteristic absorption peaks for both melamine and pyrophosphate, confirming the successful synthesis of dimelamine pyrophosphate. Slight shifts in certain peak positions compared to the pure component indicate electrostatic and hydrogen bonding interactions between the cations and anions.

[0063] Liquid Chromatography Analysis Figure 2 The HPLC chromatogram of the sample is shown in Table 1. The statistical table of HPLC peak data is also shown in Table 1.

[0064] Table 1. Statistical Table of HPLC Peak Data for Samples Peak Retention time (min) Peak area (mAU*s) Peak height (mAU) Area percentage (%) 1 3.077 17,987 2,092 0.193 2 13.803 13,578 2,162 0.146 3 (Main Peak) 14.181 8,965,836 630,553 96.161 7 16.490 100,001 14,827 1.073 11 25.356 103,016 9,390 1.105 As shown in Table 1, the sample exhibited a strong characteristic absorption peak at a retention time RT = 14.181 min. Calculations using the peak area normalization method showed that the area percentage of principal component 3 was 96.161%. Under these analytical conditions, a total of 11 chromatographic peaks were detected. Besides the main peak, the impurity peaks were mainly distributed at 16.490 min (1.073%) and 25.356 min (1.105%). The presence of impurities with high retention times may be attributed to condensation byproducts generated during synthesis or trace amounts of unreacted starting material derivatives.

[0065] Analysis of the data shows that the main peak height reaches 630,553 mAU, baseline noise is well controlled, and the signal-to-noise ratio (S / N) far exceeds the limit of quantitation. The resolution (RT) between the main peak 3 and the adjacent impurity peaks 2 (RT = 13.803 min) and 4 (RT = 14.754 min) is greater than 1.5, which meets the technical specifications for chromatographic purity analysis of core journals.

[0066] Based on the above HPLC data, the chemical purity of the sample reached over 96%.

[0067] The relevant product indicators are shown in Table 2 below: Table 2 Product-related indicators Indicator Name Test results Nitrogen (N) content, % 38.4 Phosphorus (P) content, % 14.3 Moisture 0.06 pH value (10g / L) 4.0 5% thermal weight loss (°C) (heating rate 20°C / min) 325 Electrical conductivity (25% aqueous solution, room temperature) (µs / cm) 430 Particle size (D50), μm 9.2 Appearance White powder Example 2

[0068] Application of melamine pyrophosphate in epoxy resins Formula: Epoxy resin (bisphenol A type E-51): 100 parts (parts by weight); Curing agent (methyltetrahydrophthalic anhydride, MeTHPA): 85 parts; Di- and tri-melamine pyrophosphate (DMPY): 9 parts; Accelerator (2-methylimidazole): 0.5 parts.

[0069] The flame retardant properties are shown in Table 3 below: Table 3 Flame retardant properties of samples Test Project Pure epoxy resin EP / 9%DMPY Improvement range Limiting Oxygen Index (LOI) (%) 19.5 28.7 +47% UL-94 rating Not approved V-0 - The addition of melamine pyrophosphate in this invention significantly improves the flame retardant properties of epoxy resin. Experimental results show that after adding 9% DMPY, the limiting oxygen index (LOI) of the epoxy resin increased from 19.5% to 28.7%, an increase of 47%. This improvement indicates that melamine pyrophosphate can effectively increase the oxygen demand of epoxy resin and reduce its flammability. Furthermore, the UL-94 flame retardant rating improved from failing to V-0, further demonstrating the flame retardant effect of this flame retardant on epoxy resin.

[0070] The mechanical properties are shown in Table 4 below: Table 4 Mechanical properties of the samples Test Project Pure epoxy resin EP / 9%DMPY retention rate Tensile strength (MPa) 78.5 75.2 95.8% Bending strength (MPa) 125 118 94.4% Impact strength (kJ / m²) 17.2 15.8 91.9% Flexural modulus (GPa) 2.95 3.08 104.4% The addition of melamine pyrophosphate in this invention has a certain impact on the mechanical properties of epoxy resin. Although tensile strength, flexural strength, and impact strength decrease slightly, their retention rates are high, indicating that the material enhances flame retardancy while maintaining good performance. Specifically, tensile strength decreased from 78.5 MPa to 75.2 MPa, with a retention rate of 95.8%; flexural strength decreased from 125 MPa to 118 MPa, with a retention rate of 94.4%; and impact strength decreased from 17.2 kJ / m² to 15.8 kJ / m², with a retention rate of 91.9%. However, the flexural modulus increased from 2.95 GPa to 3.08 GPa, an improvement of 104.4%, indicating that this additive has a certain enhancing effect on the rigidity and structural stability of epoxy resin.

[0071] As shown in the above embodiments, this invention provides a method for preparing di-melamine pyrophosphate by reacting pyrophosphoryl chloride and melamine in an alcohol-water mixed solvent. Infrared spectroscopy analysis confirmed that the synthesized product simultaneously possesses the characteristic structures of melamine and pyrophosphate, indicating the successful synthesis of the target product. Liquid chromatography analysis showed that the product obtained by this method has high content of the main component, few impurities, and ideal product purity. Product performance tests showed that the di-melamine pyrophosphate has a high thermal decomposition temperature, a good nitrogen-phosphorus content ratio, and suitable electrical conductivity and particle size distribution. Its application in epoxy resin systems can significantly improve the limiting oxygen index and flame retardant rating of the material, while maintaining good performance of the main mechanical properties, demonstrating excellent comprehensive application performance.

[0072] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing melamine pyrophosphate, characterized in that, Includes the following steps: Using pyrophosphoryl chloride and melamine as raw materials, the reaction was carried out in an alcohol-water mixed solvent. After the reaction was completed, the mixture was neutralized with alkali solution and post-treated to obtain dimelamine pyrophosphate.

2. The preparation method according to claim 1, characterized in that, The molar ratio of pyrophosphoryl chloride to melamine is 1:1.8 to 1:2.2; The volume ratio of alcohol to water in the alcohol-water mixed solvent is 3:1 to 5:1; The alcohol is anhydrous ethanol.

3. The preparation method according to claim 1, characterized in that, The reaction temperature is 35°C to 45°C; The reaction takes 3 to 6 hours.

4. The preparation method according to claim 1, characterized in that, The alkaline solution is dilute ammonia water with a mass fraction of 4% to 8%; The neutralization step involves adjusting the pH of the reaction system to between 6.5 and 7.

5. The post-processing steps include solid-liquid separation, washing, and drying of the neutralized reaction products; The pyrophosphoryl chloride is pretreated by grinding and sieving before use, and the sieving is through a 200-mesh sieve; The grinding and sieving pretreatment is carried out in an inert atmosphere with an oxygen content of less than 5 ppm and a water content of less than 1 ppm, and the grinding time is 15 to 20 minutes.

5. The preparation method according to claim 1, characterized in that, The melamine is dried before use to reduce its moisture content to below 0.2 wt%. The drying process is carried out at a temperature of 100°C to 110°C, a vacuum degree of -0.095MPa to -0.085MPa, and a drying time of 5 to 7 hours. The amount of the alcohol-water mixed solvent used is 5 to 7 times the total mass of the raw materials; The reaction is carried out under the protection of an inert gas; Before the reaction, an inert gas is introduced into the reaction system for 20 to 40 minutes to replace the air, with a gas flow rate of 40 mL / min to 90 mL / min.

6. The preparation method according to claim 1, characterized in that, The reaction steps include: first dissolving pyrophosphoryl chloride in an alcohol-water mixed solvent, and then adding melamine in batches; The pyrophosphoryl chloride is dissolved at a temperature of 30°C to 40°C, and the stirring time is 10 to 25 minutes. The melamine is added in 5 to 8 parts, with an interval of 8 to 12 minutes between the addition of two adjacent parts; During the addition of melamine, the temperature of the reaction system is controlled to be between 38°C and 42°C. The stirring speed for the reaction is 350 rpm to 650 rpm; In the step of neutralizing with alkali solution, the alkali solution is added dropwise at a rate of 1 mL / min to 6 mL / min; After the step of neutralizing with alkali is completed, the reaction system is heated to 60°C to 75°C and kept at that temperature for 20 to 50 minutes. The solid-liquid separation is performed by centrifugation, with a centrifugation speed of 5000 rpm to 7000 rpm and a centrifugation time of 8 to 12 minutes.

7. The preparation method according to claim 1, characterized in that, The washing process involves washing with water 2 to 4 times until no chloride ions can be detected in the washing solution. The statement that chloride ions are undetectable means that the washing solution shows no white turbidity when tested with 0.1 mol / L silver nitrate solution; The drying process is vacuum drying, with a drying temperature of 70°C to 90°C and a drying time of 10 to 14 hours. The post-processing step also includes pulverizing the dried product to control its particle size D50 to between 4 μm and 11 μm.

8. The melamine pyrophosphate prepared by the preparation method according to any one of claims 1 to 7.

9. The application of the melamine pyrophosphate according to claim 8 in the preparation of flame retardant materials.

10. The application according to claim 9, characterized in that, The flame-retardant material is epoxy resin, polyolefin, glass fiber reinforced nylon, coating or rubber product.

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