Method for preparing melamine polyphosphate at low temperature

Abstract

The invention provides a method for preparing melamine polyphosphate at low temperature, and relates to the field of melamine polyphosphate. The method for preparing melamine polyphosphate at low temperature comprises the following steps: dispersing polyphosphoric acid, reacting at low temperature and post-treating. The method for preparing melamine polyphosphate at low temperature can solve the problems of high intermediate dehydration temperature, complex process, high production cost and energy consumption, poor product chromaticity and stability, difficult solvent recovery and prominent safety and environmental protection risks in the existing production process; the melamine polyphosphate with good chromaticity and stability can be prepared in one step on the premise that high-temperature dehydration and solvent recovery are not needed, the process is simple and controllable, the production cost is low and the equipment requirement is low.

Description

A method for preparing melamine polyphosphate at low temperature Technical Field

[0001] This invention relates to the field of melamine polyphosphate, and in particular to a method for preparing melamine polyphosphate at low temperature. Background Technology

[0002] Melamine polyphosphate (MPP) is a high-performance halogen-free intumescent flame retardant with advantages such as high flame retardant efficiency, low smoke density, low toxicity, and good thermal stability. As a key acid and gas source in intumescent flame retardant systems, MPP can be used alone or in combination with char sources such as pentaerythritol. It is widely used in glass fiber reinforced nylon, polybutylene terephthalate (PBT), polyolefins, wires and cables, fire-retardant coatings, and other fields. It consistently meets the UL94 V-0 flame retardant requirements and complies with international environmental regulations such as RoHS and REACH, demonstrating irreplaceable application value in the field of flame retardant materials.

[0003] Currently, the main method for preparing MPP is a two-step process using phosphoric acid and melamine as raw materials, as exemplified by Chinese patent CN105924651A. The first step involves reacting phosphoric acid with melamine to generate melamine phosphate intermediate; the second step involves dehydrating and polymerizing the melamine phosphate intermediate at a high temperature of 300-320℃ to form melamine polyphosphate. Although the method has been industrialized, there are a series of technical defects that are difficult to overcome, namely: (1) High energy consumption cost: The high temperature dehydration stage requires a large amount of heat energy to be continuously input, which directly leads to high production energy consumption and significantly increases product cost; (2) Difficulty in process control: The high temperature environment is prone to material oxidation and degradation, which leads to the product color darkening or even turning black. Therefore, strict nitrogen protection measures must be adopted, which puts extremely high requirements on the sealing performance of the production equipment; at the same time, the high temperature reaction conditions have strict requirements on the high temperature resistance and corrosion resistance of the equipment, which further increases the cost of equipment purchase and maintenance; (3) Poor product quality stability: The high temperature system has high reactivity and poor stability. The key process parameters such as the degree of polymerization of the product are difficult to control accurately, which easily leads to uneven molecular weight distribution of the product, which ultimately causes fluctuations in flame retardant performance and affects the downstream application effect; (4) Safety and environmental protection hazards: A small amount of harmful gas may be generated during the high temperature dehydration process, and dust is easily generated during the discharge and equipment cleaning process, which not only pollutes the environment but may also pose a threat to the health of on-site operators.

[0004] To overcome the above shortcomings, researchers have conducted improvements to the two-step method. For example, Yu Zhiyuan et al., Synthesis of High Thermal Stability Melamine Polyphosphate, Synthetic Chemistry, 2017-25(1):76-79, disclosed a method for synthesizing melamine polyphosphate using heteropolyacid catalysis and multi-temperature condensation reaction. Specifically, phosphoric acid and melamine are used as raw materials. After preparing melamine phosphate under heteropolyacid catalysis, it is dehydrated and condensed at a high temperature of 300-350℃ to obtain melamine polyphosphate. The color and thermal stability of the melamine polyphosphate prepared by this method have been significantly improved, but the reaction temperature in the later stage still needs to be increased to more than 300℃, which cannot solve the problems of high energy consumption, safety and environmental protection.

[0005] To circumvent the drawbacks of high temperatures and safety / environmental concerns, Chinese patent CN108084101B discloses a method for preparing melamine polyphosphate. Using melamine and polyphosphoric acid as raw materials and an organic solvent as the reaction solvent, the method first promotes the full reaction of melamine and polyphosphoric acid under reflux to form an intermediate. This intermediate is then mixed with treatment agents such as urea and ammonium bicarbonate. The addition of urea and ammonium bicarbonate lowers the subsequent heat treatment temperature. Finally, melamine polyphosphate is obtained through a dehydration condensation reaction at 180-230℃. While this method can lower the reaction temperature required for heat treatment, the reduction in temperature depends entirely on the addition of treatment agents. Furthermore, the addition of these agents leaves residues, affecting product purity, quality, and downstream application performance. Further purification and other processing steps are required, making the post-processing cumbersome. Moreover, the decomposition products of the treatment agents (such as ammonia and carbon dioxide) can affect product color, corrode production equipment, and increase the burden on the condensation system, hindering stable industrial production. Furthermore, this method requires the use of large quantities of organic solvents such as glacial acetic acid, methanol, and ethanol, resulting in high solvent consumption, high costs for solvent recovery and treatment, and the flammability and explosiveness of these organic solvents, posing safety hazards. Finally, this method remains a two-step process and does not achieve one-step preparation of melamine polyphosphate.

[0006] In summary, existing industrial preparation methods for melamine polyphosphate all involve a two-step process: fully reacting polyphosphoric acid with melamine followed by high-temperature dehydration and condensation. This approach suffers from problems such as high intermediate dehydration temperatures, complex processes, high production costs and energy consumption, poor product color and stability, difficulties in solvent recovery, and significant safety and environmental risks. It is impossible to prepare melamine polyphosphate in a single step while overcoming these issues. Therefore, providing an industrial preparation method for melamine polyphosphate that eliminates the need for high-temperature dehydration and solvent recovery, features a simple and controllable process, low production costs, less stringent equipment requirements, and improves product color and stability, would effectively address the technical pain points in this field and has significant technical importance and research value. Summary of the Invention

[0007] To address the technical problems existing in the prior art, this invention provides a method for preparing melamine polyphosphate at low temperature. This method solves the problems of high intermediate dehydration temperature, complex process, high production cost and energy consumption, poor product color and stability, difficult solvent recovery, and prominent safety and environmental risks in existing production processes. It can produce melamine polyphosphate with good color and stability in one step without high-temperature dehydration, solvent recovery, simple and controllable process, low production cost, and relaxed equipment requirements.

[0008] To solve the above technical problems, the present invention adopts the following technical solution: a method for preparing melamine polyphosphate at low temperature, comprising the following steps: polyphosphate dispersion, low-temperature reaction, and post-treatment; the method for dispersing polyphosphate is to add polyphosphate with a degree of polymerization of 4-6 to deionized water at 0-5℃ and disperse it evenly to obtain an aqueous polyphosphate system; the method for low-temperature reaction is to add powdered melamine to the aqueous polyphosphate system at 0-5℃ under stirring conditions, and stir the reaction after the melamine is added to obtain the reaction material; the method for post-treatment is to separate and collect the solids in the reaction material, and wash and dry the solids to obtain melamine polyphosphate.

[0009] Preferably, in the polyphosphoric acid dispersion, the mass fraction of polyphosphoric acid, calculated as 100% H3PO4, is 116-118%.

[0010] Preferably, in the polyphosphoric acid dispersion, the mass ratio of deionized water to polyphosphoric acid is 2-7:1.

[0011] Preferably, in the low-temperature reaction, the molar ratio of melamine to phosphorus in polyphosphoric acid is 1:1.0-1.3.

[0012] Preferably, in the low-temperature reaction, the particle size D of the powdered melamine is... 98 <10-50μm.

[0013] Preferably, in the low-temperature reaction, the stirring rate is 200-300 r / min.

[0014] Preferably, in the low-temperature reaction, melamine is added within 30-300 minutes.

[0015] Preferably, in the low-temperature reaction, the stirring reaction time after the melamine is added is 2-4 hours.

[0016] Furthermore, the post-processing method involves filtering and collecting the solids from the reaction materials, washing the solids with deionized water 2-3 times, and then drying them to obtain melamine polyphosphate; the mass ratio of deionized water used in each washing to polyphosphate dispersion is 0.1-0.5:1.

[0017] Preferably, in the post-processing, the drying temperature is 100-120℃.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: The method for preparing melamine polyphosphate at low temperature of the present invention completely abandons the two-step process route of "first synthesizing melamine phosphate and then high-temperature dehydration" in the traditional process. The inventors have found that the existing two-step methods all involve first promoting the full reaction of polyphosphoric acid and melamine, and then performing high-temperature dehydration condensation until the degree of polymerization of the MPP product reaches about 5. The present invention starts from the initial reaction raw materials, specifically selecting polyphosphoric acid and melamine powder with a degree of polymerization of 4-6 (mass fraction of 116-118%) as raw materials. Using deionized water to provide a solvent environment, under the temperature conditions of 0-5℃, while effectively inhibiting the decomposition of polyphosphoric acid, polyphosphoric acid and melamine are directly synthesized into melamine polyphosphate with a degree of polymerization of 4-6 through a one-step reaction of acid-base neutralization to form salt. There is no need to adjust the degree of polymerization of the product through the traditional high-temperature dehydration condensation process, and thus no need for the heat treatment process of the traditional two-step method. The method for preparing melamine polyphosphate at low temperature solves the problems of high intermediate dehydration temperature, complex process, high production cost and energy consumption, poor product color and stability, difficult solvent recovery, and prominent safety and environmental risks in the existing "two-step" production process. It can produce melamine polyphosphate with good color and stability in one step without high-temperature dehydration, solvent recovery, simple and controllable process, low production cost, and relaxed equipment requirements. Specifically: 1) In the preparation process of melamine polyphosphate of the present invention, polyphosphoric acid with a degree of polymerization of 4-6 (mass fraction of 116-118%) is directly reacted with melamine to obtain melamine polyphosphate with a degree of polymerization of 4-6. This eliminates the need to adjust the product's degree of polymerization through the traditional high-temperature dehydration condensation process, thus omitting the traditional high-temperature dehydration condensation process. Compared with the traditional "two-step" synthesis process, energy consumption is significantly reduced. The present invention reacts at a low temperature of 0-5℃, eliminating the need for the 300-320℃ high-temperature dehydration process in the traditional "two-step" process, thus greatly reducing energy consumption and production costs.

[0019] 2) The method for preparing melamine polyphosphate of the present invention has high stability of the low-temperature reaction system, the preparation process is simple and easy to control, and the process parameters such as reaction temperature and stirring rate are easy to precisely control, which can avoid the problem of product performance fluctuation under high-temperature reaction conditions.

[0020] 3) The preparation method of melamine polyphosphate of the present invention has relaxed equipment requirements. The reaction process does not require high-temperature resistant equipment. Ordinary glass-lined reactors can meet the production requirements, which greatly reduces the equipment purchase and maintenance costs and facilitates industrial promotion.

[0021] 4) The method for preparing melamine polyphosphate of the present invention is environmentally friendly and highly safe; no harmful gases or dust are generated during the low-temperature reaction process, which reduces environmental pollution and avoids the safety hazards of high-temperature reactions.

[0022] 5) The method for preparing melamine polyphosphate of the present invention has a high yield, and the obtained melamine polyphosphate has high whiteness and good stability. Its performance is better than that of the product prepared by the traditional "two-step method", and it can be widely used in various flame retardant materials. Attached Figure Description

[0023] Figure 1 shows the carbon NMR spectrum of the polyphosphate melamine prepared in Example 1 of this invention. 13 Figure 2 shows the phosphorus NMR spectrum of the polyphosphate melamine prepared in Example 1 of this invention. 31 P NMR). Detailed Implementation

[0024] To provide a clearer understanding of the technical features, objectives, and effects of this invention, specific embodiments are now described. It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0025] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments of the present invention. As used herein, "first," "second," etc., are used to distinguish similar objects and are not used to describe a particular order or sequence. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0026] This invention provides a method for preparing melamine polyphosphate at low temperature. Using polyphosphoric acid and melamine as raw materials, and deionized water as the solvent environment, melamine polyphosphate is directly synthesized in a one-step reaction under low temperature conditions through a reaction between polyphosphoric acid with a degree of polymerization of 4-6 and melamine. The reaction equation for this method is as follows: nC3H6N6 + H n+2 P n O 3n+1 = (C3H5N6) n ⋅H n P n O 3n+1The method for preparing melamine polyphosphate at low temperature includes the following steps: S01, dispersing polyphosphate: Deionized water is added to a reactor equipped with a stirrer, cooling device, and thermometer, and the temperature of the deionized water is lowered to 0-5℃ and kept at this temperature; at this temperature, polyphosphate with a degree of polymerization of 4-6 is added to the reactor, stirring is started, and stirring is carried out for 15-120 minutes to uniformly disperse the polyphosphate in the water; S02, low-temperature reaction: Subsequently, under the condition that the material temperature is 0-5℃ and under stirring, powdered melamine is added, and the melamine addition time is controlled to be 30-300 minutes (preferably 120-160 minutes). After the addition is completed, the reaction is continued at 0-5℃ for 2-4 hours to obtain the reaction material; S03, post-treatment: After the reaction is completed, the reaction material is filtered, and the obtained filter cake is washed with deionized water 2-3 times. The washed filter cake is dried to constant weight to obtain melamine polyphosphate.

[0027] Preferably, in step S01, the polyphosphoric acid, based on 100% H3PO4, has a mass fraction of 116-118% (corresponding to a degree of polymerization of 4-6 for the polyphosphoric acid).

[0028] Preferably, in step S02, the particle size D of the melamine is... 98 <10-50 μm; more preferably, the particle size D of the melamine is... 98 <30μm.

[0029] Preferably, in step S02, the molar ratio of melamine to phosphorus in polyphosphoric acid is 1:1.0-1.3, more preferably 1:1.0-1.1.

[0030] Preferably, in step S01, the mass ratio of deionized water to polyphosphoric acid is 2-7:1.

[0031] Preferably, in step S02, the stirring speed is 200-300 r / min.

[0032] Preferably, in step S03, the mass ratio of deionized water to polyphosphoric acid used in each wash during the filter cake washing process is 0.1-0.5:1, more preferably 0.2-0.3:1.

[0033] Preferably, in step S03, the drying temperature of the washed filter cake is 100-120℃.

[0034] The low-temperature preparation method of melamine polyphosphate in this invention abandons the traditional two-step route of "first synthesizing melamine phosphate and then dehydrating at high temperature". Starting from the initial reaction raw materials, polyphosphoric acid and melamine powder with a degree of polymerization of 4-6 (mass fraction of 116-118%) are selected as raw materials. Deionized water is used to provide the solvent environment. Under the temperature conditions of 0-5℃, while effectively inhibiting the decomposition of polyphosphoric acid, polyphosphoric acid and melamine are directly synthesized into melamine polyphosphate with a degree of polymerization of 4-6 through a one-step reaction of acid-base neutralization to form salt. There is no need to adjust the degree of polymerization of the product through the traditional high-temperature dehydration condensation process, and thus no need for the heat treatment process of the traditional two-step method. The method for preparing melamine polyphosphate at low temperature can solve the problems of high intermediate dehydration temperature, complex process, high production cost and energy consumption, poor product color and stability, difficult solvent recovery and prominent safety and environmental risks in the existing "two-step" production process. It can produce melamine polyphosphate with good color and stability in one step without high temperature dehydration, solvent recovery, simple and controllable process, low production cost and relaxed equipment requirements.

[0035] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described below in conjunction with some specific embodiments.

[0036] In all embodiments, the NMR equipment used was an AVANCE Ⅲ HD400MHz NMR spectrometer manufactured by Bruker AG, Switzerland. For solid-state carbon NMR spectroscopy, the test resonance frequency was 100.625MHz, using a cp pulse, an H / X dual-resonance solid-state probe with a spectral width of 50kHz, a 4mm ZrO2 rotor with a rotation speed of 12kHz, a cycle delay of 2s, a spin rate of 5kHz, a time domain size of 2108, 1600 scans / hour, a contact pulse of 2ms, and a capture time of 0.034s. For solid-state phosphorus NMR spectroscopy, the test resonance frequency was 161.98MHz, using a cp pulse, an H / X dual-resonance solid-state probe with a spectral width of 80kHz, a 4mm ZrO2 rotor with a rotation speed of 12kHz, a cycle delay of 2s, a spin rate of 10kHz, a time domain size of 2108, 1600 scans / hour, a contact pulse of 2ms, and a capture time of 0.034s.

[0037] Thermogravimetric analysis (TGA) was performed using a Mettler TGA2 instrument in a nitrogen atmosphere at a flow rate of 60 mL / min and a heating rate of 10 °C / min. The temperature range was 50–600 °C.

[0038] Whiteness was measured using a ZB-B whiteness meter manufactured by Hangzhou Zhibang Instrument Co., Ltd.

[0039] In each embodiment, the polyphosphoric acid used is an industrial product provided by Yangzhou Ruiyang Chemical Co., Ltd.; the melamine is an industrial product provided by Shandong Shuntian Chemical Group Co., Ltd., and is crushed to a predetermined particle size before use.

[0040] Example 1 This example provides a method for preparing melamine polyphosphoric acid at low temperature. The specific steps are as follows: 1) Dispersing polyphosphoric acid: Add 150g of deionized water to a 500mL four-necked flask. Place the reaction flask in a low-temperature constant temperature water bath and control the system temperature at 1℃. At this temperature, add 50g of polyphosphoric acid with a mass fraction of 116.4% (degree of polymerization n=4.3). Turn on the stirring device and control the stirring speed at 200r / min. Stir and disperse for 30min to make the polyphosphoric acid uniformly dispersed in the water; 2) Low-temperature reaction: Then, under the condition that the material temperature is 1℃, slowly add 74.9g of melamine (the molar ratio of melamine to phosphorus is 1:1.0) at a rate of 0.5g / min. The addition process lasts for 149.8min. After the addition is completed, continue stirring and reacting for 2h to obtain the reaction material; wherein, the particle size D of the melamine used is... 98 It is 20μm.

[0041] 3) After the post-treatment reaction is completed, the reaction material is vacuum filtered, and the resulting filter cake is washed 3 times with 15g of water each time. Then it is placed in a drying oven and dried at 120℃ for 3h to obtain 123.03g of melamine polyphosphate (theoretical yield 124.9g), with a yield of 98.49%. The water solubility (25℃) is 0.03g / L, the whiteness is 97.5, the temperature of 1wt% thermal weight loss is 371℃, the temperature of 2wt% is 378℃, the temperature of 5wt% is 390℃, and the temperature at the maximum rate of thermal weight loss is 420℃.

[0042] The structure of the product obtained in this embodiment was characterized by solid-state carbon NMR spectroscopy and phosphorus NMR spectroscopy. The polyphosphate melamine obtained in Example 1... 13 The C NMR spectrum is shown in Figure 1; the melamine polyphosphate obtained in Example 1 is shown in Figure 1. 31 The P NMR spectrum is shown in Figure 2. In Figure 1, 164.49 ppm and 157.79 ppm are carbon peaks in the triazine ring. Due to the influence of the amino group and phosphate salt formation, the carbon peak in the triazine ring splits into two peaks, with 157.79 ppm being the carbon peak connected to the amino group that forms a salt with the phosphate group. In Figure 2, -20.75 ppm and -23.16 ppm are P peaks on the main chain of melamine polyphosphate (MPP), and the two symmetrical multiplets appearing on both sides of this peak are the rotational sideband peaks of this P peak; 0.75 ppm and 10.25 ppm are P peaks on the end groups of melamine polyphosphate (MPP). The above characteristic peaks match those of melamine polyphosphate (MPP), indicating that the product prepared in this example is melamine polyphosphate (MPP).

[0043] Example 2 This example provides a method for preparing low-temperature polyphosphoric acid melamine. The specific steps are as follows: 1) Polyphosphoric acid dispersion: Add 300g of deionized water to a 500mL four-necked flask, place the reactor in a low-temperature constant temperature water bath, and control the system temperature at 2℃; add 50g of polyphosphoric acid with a mass fraction of 116.4% (degree of polymerization n=4.3) to the above low-temperature aqueous phase, turn on the stirring device, control the stirring speed at 200r / min, and stir and disperse for 30min to make the polyphosphoric acid uniformly dispersed in the aqueous phase.

[0044] 2) Low-temperature reaction: Subsequently, under the condition that the material temperature is 1℃, 74.9g of melamine (the molar ratio of melamine to phosphorus is 1:1.0) was slowly added to the uniformly dispersed polyphosphoric acid aqueous phase system at a rate of 0.5g / min. The addition process lasted for 149.8min. After the addition was completed, the reaction was stirred for another 2h to obtain the reaction material. Among them, the particle size D of the melamine used was... 98 It is 20μm.

[0045] 3) After the post-treatment reaction is completed, the reaction material is vacuum filtered, and the resulting filter cake is washed 3 times with 15g of water each time. Then it is placed in a drying oven and dried at 120℃ for 3h to obtain 122.4g of melamine polyphosphate (theoretical yield 124.9g), with a yield of 98.0%. The water solubility (25℃) is 0.027g / L, the whiteness is 97.8, the temperature of 1wt% thermal weight loss is 373℃, the temperature of 2wt% is 379℃, the temperature of 5wt% is 391℃, and the temperature at the maximum rate of thermal weight loss is 422℃.

[0046] The product obtained in this embodiment was characterized according to the method in Example 1, indicating that the product prepared in this embodiment is the target product, melamine polyphosphate (MPP).

[0047] Example 3 This example provides a method for preparing low-temperature polyphosphoric acid melamine. The specific steps are as follows: 1) Add 300g of deionized water to a 500mL four-necked flask and place the reaction flask in a low-temperature constant temperature water bath. Control the system temperature at 1℃. At this temperature, add 50g of polyphosphoric acid with a mass fraction of 116.4% (degree of polymerization n=4.3) to the reaction flask. Turn on the stirring device and control the stirring speed at 200r / min. Stir and disperse for 30min to make the polyphosphoric acid uniformly dispersed in the water.

[0048] 2) Low-temperature reaction: Subsequently, under the condition that the material temperature is 1℃, 68.1g of melamine (the molar ratio of melamine to phosphorus is 1:1.1) was slowly added at a rate of 0.5g / min. The addition process lasted for 136.2min. After the addition was completed, the reaction was stirred for another 2h to obtain the reaction material. Among them, the particle size D of the melamine used was... 98It is 20μm.

[0049] 3) After the post-processing reaction is completed, the reaction material is vacuum filtered, and the resulting filter cake is washed 3 times with 15g of water each time. Then it is placed in a drying oven and dried at 120℃ for 3h to obtain 115.86g of melamine polyphosphate (theoretical yield 118.10g), with a yield of 98.1%. The water solubility (25℃) is 0.028g / L, the whiteness is 97.8, the temperature of 1wt% thermal weight loss is 375℃, the temperature of 2wt% is 380℃, the temperature of 5wt% is 392℃, and the temperature at the maximum rate of thermal weight loss is 423℃.

[0050] The product obtained in this embodiment was characterized according to the method in Example 1, indicating that the product prepared in this embodiment is the target product, melamine polyphosphate (MPP).

[0051] Example 4 This example provides a method for preparing low-temperature polyphosphoric acid melamine. The specific steps are as follows: 1) Add 150g of deionized water to a 500mL four-necked flask and place the reaction flask in a low-temperature constant temperature water bath. Control the system temperature at 1.5℃. At this temperature, add 50g of polyphosphoric acid with a mass fraction of 118.07% (degree of polymerization n=6) to the reaction flask. Turn on the stirring device and control the stirring speed at 200r / min. Stir and disperse for 30min to make the polyphosphoric acid uniformly dispersed in the water.

[0052] 2) Low-temperature reaction: Subsequently, under the condition that the material temperature is 1.5℃, 75.98g of melamine (the molar ratio of melamine to phosphorus is 1:1.0) was slowly added at a rate of 0.5g / min. The addition process lasted for 152min. After the addition was completed, the reaction was stirred for another 2h to obtain the reaction material. Among them, the particle size D of the melamine used was... 98 It is 20μm.

[0053] 3) After the post-treatment reaction is completed, the reaction material is vacuum filtered, and the resulting filter cake is washed 3 times with 15g of water each time. Then it is placed in a drying oven and dried at 120℃ for 3h to obtain 124.56g of melamine polyphosphate (theoretical yield 125.98g), with a yield of 98.87%. The water solubility (25℃) is 0.02g / L, the whiteness is 97.4, the temperature of 1wt% thermal weight loss is 375℃, the temperature of 2wt% is 380℃, the temperature of 5wt% is 395℃, and the temperature at the maximum thermal weight loss rate is 430℃.

[0054] The product obtained in this embodiment was characterized according to the method in Example 1, indicating that the product prepared in this embodiment is the target product, melamine polyphosphate (MPP).

[0055] Example 5 This example provides a method for preparing low-temperature polyphosphoric acid melamine. The specific steps are as follows: 1) Add 150g of deionized water to a 500mL four-necked flask and place the reaction flask in a low-temperature constant temperature water bath. Control the system temperature at 1℃. At this temperature, add 50g of polyphosphoric acid with a mass fraction of 118.07% (degree of polymerization n=6) to the reaction flask. Turn on the stirring device and control the stirring speed at 200r / min. Stir and disperse for 30min to make the polyphosphoric acid uniformly dispersed in the water.

[0056] 2) Low-temperature reaction: Subsequently, under the condition that the material temperature is 1℃, 69.07g of melamine (the molar ratio of melamine to phosphorus is 1:1.1) was slowly added at a rate of 0.5g / min. The addition process lasted for 138.1min. After the addition was completed, the reaction was stirred for another 2h to obtain the reaction material. Among them, the particle size D of the melamine used was... 98 It is 20μm.

[0057] 3) After the post-processing reaction, the reaction material was vacuum filtered, and the resulting filter cake was washed three times with 15g of water each time. Then it was placed in a drying oven and dried at 120℃ for 3h to obtain 117.4g of melamine polyphosphate (theoretical yield 119.07g), with a yield of 98.60%. The water solubility (25℃) was 0.018g / L, the whiteness was 97.2, the temperature at which the thermal weight loss was 1wt% was 378℃, the temperature at which the thermal weight loss was 2wt% was 383℃, the temperature at which the thermal weight loss was 5wt% was 397℃, and the temperature at which the maximum thermal weight loss rate was 432℃.

[0058] The product obtained in this embodiment was characterized according to the method in Example 1, indicating that the product prepared in this embodiment is the target product, melamine polyphosphate (MPP).

[0059] Comparative Example 1: Melamine polyphosphate was prepared using a traditional two-step method: 150g of deionized water and 50g of 85% phosphoric acid were added to a 500mL reactor and stirred until homogeneous. Then, 54.7g of melamine was added, and the mixture was heated to 80℃ and kept at that temperature for 2 hours to generate melamine phosphate intermediate. The intermediate was filtered, washed, and then poured into a kneader. Nitrogen gas was introduced for protection, and the mixture was heated to 320℃ and kept at that temperature for 3 hours for high-temperature dehydration condensation. After cooling, 94.3g of melamine polyphosphate product was obtained, with a whiteness of 92 and a temperature of 1wt% thermal weight loss at 365℃.

[0060] Compared with Example 1 of the present invention, this comparative example is a two-step reaction, which is complicated and requires a high reaction temperature for high-temperature dehydration in the later stage, resulting in a significant increase in equipment energy consumption, high requirements for the manufacture of high-temperature equipment, and a significant increase in equipment cost. At the same time, the product obtained by this comparative example has poor color and low whiteness.

[0061] Unless otherwise stated, all percentages used in this invention are mass percentages.

[0062] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing melamine polyphosphate at low temperature, characterized in that, The process includes the following steps: polyphosphoric acid dispersion, low-temperature reaction, and post-treatment. The polyphosphoric acid dispersion method involves adding polyphosphoric acid with a degree of polymerization of 4-6 to deionized water at 0-5℃ and dispersing it evenly to obtain an aqueous polyphosphoric acid system. The low-temperature reaction method involves adding powdered melamine to the aqueous polyphosphoric acid system at 0-5℃ under stirring conditions. After the melamine is added, the mixture is stirred to obtain the reaction material. The post-treatment method involves separating and collecting the solids from the reaction material, washing and drying the solids to obtain melamine polyphosphate.

2. The method for preparing melamine polyphosphate at low temperature according to claim 1, characterized in that, In the polyphosphoric acid dispersion, the mass fraction of polyphosphoric acid, calculated as 100% H3PO4, is 116-118%.

3. The method for preparing melamine polyphosphate at low temperature according to claim 1, characterized in that, In the polyphosphoric acid dispersion, the mass ratio of deionized water to polyphosphoric acid is 2-7:

1.

4. The method for preparing melamine polyphosphate at low temperature according to claim 1, characterized in that, In the low-temperature reaction, the molar ratio of melamine to phosphorus in polyphosphoric acid is 1:1.0-1.

3.

5. The method for preparing melamine polyphosphate at low temperature according to claim 1, characterized in that, In the low-temperature reaction, the particle size D of the powdered melamine is... 98 <10-50μm.

6. The method for preparing melamine polyphosphate at low temperature according to claim 1, characterized in that, In the low-temperature reaction, the stirring rate is 200-300 r / min.

7. The method for preparing melamine polyphosphate at low temperature according to claim 1, characterized in that, In the low-temperature reaction, melamine is added completely within 30-300 minutes.

8. The method for preparing melamine polyphosphate at low temperature according to claim 1, characterized in that, In the low-temperature reaction, the stirring reaction time after the melamine is added is 2-4 hours.

9. The method for preparing melamine polyphosphate at low temperature according to claim 1, characterized in that, The post-processing method is as follows: the solid matter of the reaction material is collected by filtration, the solid matter is washed with deionized water 2-3 times and then dried to obtain melamine polyphosphate; the mass ratio of deionized water used in each washing to polyphosphate dispersion is 0.1-0.5:

1.

10. The method for preparing melamine polyphosphate at low temperature according to claim 1, characterized in that, In the post-processing, the drying temperature is 100-120℃.

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