A method for synthesizing flame-retardant nylon 6 by in-situ polymerization and flame-retardant nylon 6 prepared thereby

Abstract

The present invention discloses a method for synthesizing flame-retardant nylon 6 by in-situ polymerization, and flame-retardant nylon 6 prepared thereby. The method utilizes low-temperature plasma technology to treat melamine and cyanuric acid, which are then reacted with dibasic acids and diamines in a solid-phase reaction to form salts. The modified melamine dibasic acid salt and cyanuric acid diamine salt are then added to a caprolactam polymerization system, where MCA is formed in situ during the polymerization process to synthesize flame-retardant nylon 6. In the present invention, the low-temperature plasma treatment process is a solid-phase reaction, and the reaction time of the treated melamine and cyanuric acid with the dibasic acid and diamine is shortened, while improving the compatibility of the reactants with nylon 6. This method suppresses deterioration in the mechanical properties of flame-retardant nylon 6 while also achieving a high flame retardancy.

Description

Technical Field

[0001] The invention belongs to the technical field of flame retardant materials, and particularly relates to a method for synthesizing flame retardant nylon 6 by an in-situ polymerization method and the flame retardant nylon 6 prepared by the method. Background Art

[0002] Flame-retardant nylon 6 is a highly effective flame-retardant modified material widely used in both fiber and engineering plastics. In the fiber sector, flame-retardant fabrics are used as protective clothing and everyday workwear in industries such as steel, forging, metallurgy, machinery, chemicals, and firefighting. They can also be used to make carpets, bed sheets, and curtains for public spaces like hospitals, hotels, and airports. In the engineering plastics sector, the development of new energy vehicles is driving growing demand for flame-retardant nylon 6.

[0003] Existing flame-retardant nylon 6 is mostly synthesized via melt blending, where nylon 6 is physically blended with halogen-free flame retardants, such as melamine cyanurate (MCA) and diethyl hypophosphite, in a twin-screw extruder. Due to the poor compatibility of halogen-free flame retardants with nylon 6, a 20% flame retardant level is required to achieve a V0 flame retardancy rating. However, this significantly reduces the mechanical properties of the nylon material, such as elongation at break and notched impact strength.

[0004] During the hydrolysis and polycondensation of caprolactam to produce nylon 6, in-situ MCA generation within the polymerization reactor achieves excellent dispersion of MCA in the nylon melt, improving flame retardant utilization efficiency and reducing the amount of flame retardant added. With a flame retardant addition level of less than 10%, the flame retardancy rating can reach V0. This reduction in flame retardant addition effectively suppresses the degradation of the mechanical properties of flame-retardant nylon, making in-situ polymerization a preferred method for synthesizing flame-retardant nylon 6.

[0005] CN200710192571.X discloses a method for preparing halogen-free flame-retardant nylon 6. The method uses melamine cyanurate (MCA) as a flame retardant, disperses MCA into a caprolactam polymerization system, and produces flame-retardant nylon 6 through in-situ polymerization. To evenly disperse MCA in the polymerization system, the patent utilizes a diamine reacting with cyanuric acid, and a dibasic acid reacting with melamine in water for 0.5 to 3 hours to produce hexamethylenediamine cyanurate and melamine adipate. This improves their compatibility in the polymerization system, allowing their dispersion in the polymerization system to reach the nanometer scale. When the MCA content is 5wt%, the flame-retardant nylon 6 exhibits a tensile strength of 61.1MPa, an elongation at break of 11.2%, and a notched impact strength of 8.7kJ / m 2 , the flame retardant grade reaches V0.

[0006] The literature (Study on the Preparation, Properties and Application of In-situ Polymerized Flame-retardant Nylon, Ph.D. thesis of Hunan University, 2011) proposed a modification method in which hexamethylenediamine and adipic acid react with cyanuric acid and melamine respectively in water. After salt formation, their compatibility with caprolactam is improved. The molar ratios of melamine / H2O and cyanuric acid / H2O are 1 / 4 and 1 / 122.5 respectively. The reaction time is 1 hour. The tensile strength, flexural strength and notched impact strength of the flame-retardant nylon 6 obtained by in-situ polymerization are 64.1MPa, 73.1MPa and 10.4kJ / m respectively. 2 , the flame retardant grade reaches V0. The literature (Preparation and Characterization of a novel Flame retarded MCA-PA6 Resin by In-Situ Polymerization, Advanced Materials Research, 2011, 399-401(9): 444-448) proposed a similar method, in which the molar ratios of melamine / H2O and cyanuric acid / H2O were 1 / 56 and 1 / 28 respectively, and the reaction time was 2 hours. The flame retardant nylon 6 fiber obtained by in-situ polymerization had a tensile strength and elongation at break of 3.11 cN / dtex and 47.25% respectively, a flame retardant grade of V0, and a limiting oxygen index (LOI) of 29.3%.

[0007] Existing technologies utilize in-situ polymerization, whereby dibasic acids and diamines react with melamine and cyanuric acid, respectively, to enhance their compatibility in caprolactam polymerization systems and improve the dispersibility of MCA in nylon 6. Compared to melt blending, flame-retardant nylon 6 synthesized by in-situ polymerization uses a lower amount of MCA while maintaining flame retardancy, thus suppressing the deterioration of the material's mechanical properties. However, the existing technologies require large amounts of water as a reaction medium when reacting dibasic acids and diamines with melamine and cyanuric acid, generating significant amounts of wastewater and resulting in a waste of water resources. Furthermore, the reaction time is longer than one hour, resulting in low efficiency and high energy consumption. Summary of the Invention

[0008] To address the aforementioned problems with the existing technology, the present invention utilizes low-temperature plasma technology to treat melamine and cyanuric acid. These are then reacted with dibasic acids and diamines in a solid-phase reaction to form salts. The modified melamine dibasic acid salt and cyanuric acid diamine salt are then added to a caprolactam polymerization system, where MCA is formed in situ during the polymerization process to synthesize flame-retardant nylon 6. The low-temperature plasma treatment process is a solid-phase reaction, shortening the reaction time of the treated melamine and cyanuric acid with the dibasic acid and diamine, while also improving the compatibility of the reactants with nylon 6. This method not only suppresses the deterioration of the mechanical properties of flame-retardant nylon 6, but also achieves a high flame retardancy.

[0009] In view of this, one of the objects of the present invention is to provide a method for synthesizing flame retardant nylon 6 by in situ polymerization.

[0010] A second object of the present invention is to provide flame-retardant nylon 6 prepared by the above method.

[0011] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0012] In a first aspect, the present invention provides a method for synthesizing flame-retardant nylon 6 by in situ polymerization, comprising the following steps:

[0013] (1) treating cyanuric acid using low-temperature plasma technology in a CO2 atmosphere, and reacting the treated cyanuric acid with a diamine in a solid phase to obtain cyanuric acid diamine salt;

[0014] (2) treating melamine using low-temperature plasma technology in an ammonia atmosphere, and reacting the treated melamine with a dibasic acid in a solid phase reaction to obtain a melamine dibasic acid salt;

[0015] (3) The diamine salt of cyanuric acid and the dibasic acid salt of melamine obtained in the above steps (1) and (2) are subjected to an in-situ polymerization reaction with caprolactam and water to synthesize flame-retardant nylon 6.

[0016] In a specific embodiment, in step (1), the conditions for treating cyanuric acid using low-temperature plasma technology are:

[0017] The processing power is 200-800W, preferably 400-600W, such as 400W, 450W, 500W, 550W, and 600W. When the power is too low, the high-energy electrons generated are small, the activated gas molecules are small, and the processing effect is poor. When the power is too high, the number of high-energy electrons generated will not increase further, but energy consumption will increase.

[0018] The treatment time is 5-50 min, preferably 10-20 min, for example 10 min, 12 min, 15 min, 18 min, 20 min; if the treatment time is too short, fewer activated gas molecules will be produced and the treatment effect will be poor; if the treatment time is too long, the treatment effect will not continue to increase, but energy consumption will increase.

[0019] In a specific embodiment, in step (1), the particle size of cyanuric acid is 10 to 500 microns, preferably 10 to 50 microns.

[0020] In a specific embodiment, in step (1), the diamine is one or more combinations selected from hexamethylenediamine, ethylenediamine, propylenediamine, butylenediamine, pentamethylenediamine, octanediamine, and decanediamine; preferably, 1,6-hexamethylenediamine.

[0021] In a specific embodiment, in step (1), the molar ratio of cyanuric acid to diamine is 1:1 to 1.2, preferably 1:1.1.

[0022] In a specific embodiment, in step (1), the temperature for the reaction of the treated cyanuric acid and the diamine is 25-80°C, preferably 40-50°C, for example, 40°C, 42°C, 45°C, 48°C, 50°C; and the reaction time is 5-30 min, preferably 10-20 min, for example, 10 min, 12 min, 15 min, 18 min, 20 min.

[0023] In a specific embodiment, in step (2), the conditions for treating melamine with low-temperature plasma are:

[0024] The processing power is 100-600W, preferably 300-500W, such as 350W, 400W, 450W, 500W;

[0025] The treatment time is 5-30 min, preferably 5-15 min, for example, 5 min, 8 min, 10 min, 12 min, 15 min.

[0026] In a specific embodiment, in step (2), the particle size of melamine is 10 to 500 microns, preferably 10 to 50 microns.

[0027] In a specific embodiment, in step (2), the dibasic acid is one or more combinations selected from adipic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, suberic acid, and sebacic acid, preferably 1,6-hexanoic acid.

[0028] In a specific embodiment, in step (2), the molar ratio of melamine to dibasic acid is 1:1 to 1.2, preferably 1:1.1.

[0029] In a specific embodiment, in step (2), the temperature of the reaction of the treated melamine and the dibasic acid is 100-200°C, preferably 150-160°C, for example, 150°C, 152°C, 155°C, 158°C, or 160°C; and the reaction time is 5-30 minutes, preferably 5-15 minutes, for example, 5 minutes, 8 minutes, 10 minutes, 12 minutes, or 15 minutes. In a specific embodiment, in step (3), the in-situ polymerization reaction is carried out under inert gas protection, for example, nitrogen protection.

[0030] In a specific embodiment, in step (3), the molar ratio of diamine cyanurate, melamine dibasic acid salt, caprolactam and water is: 1:1:10-40:1-5; preferably 1:1:15-30:2-4.

[0031] In a specific embodiment, in step (3), in the in-situ polymerization reaction, diamine cyanurate and melamine dibasic acid salt react with caprolactam and water at a first temperature for a period of time, and then cool to a second temperature and continue to react for a period of time.

[0032] In a specific embodiment, the first temperature is 220-280°C, preferably 240-260°C, such as 245°C, 250°C, 255°C, 260°C; the reaction time at the first temperature is 1-10 hours, preferably 3-8 hours;

[0033] In a specific embodiment, the second temperature is 210-270°C, preferably 230-250°C, for example 230°C, 235°C, 240°C, 245°C, 250°C; the reaction time for cooling to the second temperature is 1-5 hours, preferably 1-3 hours.

[0034] In a second aspect, the present invention provides flame-retardant nylon 6 prepared by the above method.

[0035] Beneficial effects

[0036] (1) Compared with treating melamine and cyanuric acid by adding adipic acid and hexamethylenediamine to water respectively, the present invention activates ammonia or CO2 by high-energy positive and negative ions generated by low-temperature plasma, and then performs surface modification on melamine and cyanuric acid, thereby increasing the concentration of amino groups or carboxyl groups on their surfaces, thereby significantly improving their reactivity with dibasic acids and diamines, shortening the reaction time (reaction time is less than 30 minutes), and the process is a solid-phase reaction, does not require water consumption, and does not generate wastewater.

[0037] (2) The melamine dibasic acid salt and cyanuric acid diamine salt modified by low-temperature plasma treatment are polymerized with caprolactam, and their compatibility with caprolactam is significantly improved, and the dispersion effect in the melt is better, which inhibits the deterioration of the mechanical properties of flame-retardant nylon 6 and can also achieve a V0 flame retardant grade.

[0038] The endpoints of the ranges and any values disclosed herein are not limited to the precise ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a schematic diagram of a low-temperature plasma processing device.

[0040] Figure 2This is a schematic diagram of the principle of using low-temperature plasma to treat cyanuric acid in a CO2 atmosphere according to the present invention.

[0041] Figure 3 Schematic diagram of the present invention using low-temperature plasma to treat melamine in an ammonia atmosphere.

[0042] Figure 4 It is the infrared chromatogram of melamine, melamine adipate prepared in Comparative Example 1 and melamine adipate prepared in Example 1.

[0043] Figure 5 1 is a SEM image of flame retardant nylon 6 prepared in Comparative Example 1 (A) and Example 1 (B) of the present invention. DETAILED DESCRIPTION

[0044] The present invention uses low-temperature plasma technology to treat melamine and cyanuric acid, thereby increasing their alkalinity and acidity, that is, increasing their reactivity, so that they can better form salts with subsequently added dibasic acids and diamines. The modified melamine dibasic acid salt and cyanuric acid diamine salt are added to the caprolactam polymerization system, and MCA is formed in situ during the polymerization process to synthesize flame-retardant nylon 6. The low-temperature plasma treatment of melamine and cyanuric acid is carried out using a low-temperature plasma device. The present invention uses Figure 1 The low-temperature plasma device shown is provided for illustrative purposes only, but is not limited thereto.

[0045] Figure 1 The low-temperature plasma device shown consists of a reaction chamber, electrodes, a high-frequency power supply, a gas cylinder and a vacuum pump.

[0046] When treating cyanuric acid, first put the cyanuric acid into Figure 1 CO2 is introduced into the reaction chamber of the low-temperature plasma device shown, and then the discharge power of the high-frequency power supply is adjusted to activate the CO2 using the high-energy positive and negative electrons generated. The activated Co2 attacks the surface of cyanuric acid, increasing the carboxyl density on the surface of cyanuric acid and improving the reaction activity with diamine.

[0047] After the surface of cyanuric acid is treated with low-temperature plasma, diamine (such as hexamethylenediamine) is added to react with the surface-treated cyanuric acid. Due to the increase in the density of carboxyl groups, the reactivity of the two is improved. The concentration of diamine bound to the surface of cyanuric acid is increased, which can further improve the compatibility with the caprolactam melt and help improve the dispersibility in the melt. The principle diagram of using low-temperature plasma to treat cyanuric acid in a CO2 atmosphere is shown in Figure 2 .

[0048] When treating melamine, first put the melamine into Figure 1Ammonia is introduced into the reaction chamber of the low-temperature plasma device shown, and then the discharge power of the high-frequency power supply is adjusted to activate the ammonia using the high-energy positive and negative electrons generated. The activated ammonia attacks the surface of melamine, increasing the amino group density on the melamine surface and improving the reaction activity with the dibasic acid.

[0049] After treating the melamine surface with low-temperature plasma, a dibasic acid (such as adipic acid) is added to react with the surface-treated melamine. As the density of amino groups on the melamine surface increases, the reactivity of the two is improved. The increased concentration of dibasic acid bound to the melamine surface can further improve the compatibility with the caprolactam melt and help improve the dispersibility in the melt. See the schematic diagram of using low-temperature plasma to treat melamine in an ammonia atmosphere. Figure 3 .

[0050] The following are detailed descriptions of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are intended only to illustrate and explain the present invention and are not intended to limit the present invention. The present invention will be described in detail below through examples. In the following examples, unless otherwise specified, all materials used can be obtained commercially, and unless otherwise specified, the methods used are conventional methods in the art.

[0051] Reagents and sources:

[0052] Cyanuric acid: purity 98%, purchased from Jiangsu Leien Environmental Protection Technology Co., Ltd.

[0053] 1,6-Hexanediamine: purity 99%, purchased from Shandong Anhe Chemical Co., Ltd.

[0054] Melamine: purity 99%, purchased from Nantong Runfeng Petrochemical Co., Ltd.

[0055] 1,6-Adipic acid: purity 99%, purchased from Yunsheng Chemical (Shandong) Co., Ltd.

[0056] Caprolactam: purity 99.5%, purchased from Shandong Xuchen Chemical Technology Co., Ltd.

[0057] Example 1

[0058] Cyanuric acid and melamine are ground separately, and the particle size is controlled to be 10 to 50 microns.

[0059] 12.9 g (0.1 mol) of ground cyanuric acid was placed in Figure 1 Into the reaction chamber of the low-temperature plasma device shown, CO2 was introduced, the discharge power was adjusted to 500 W, and the treatment was carried out at room temperature for 15 minutes. Then, 12.8 g (0.11 mol) of 1,6-hexanediamine was added and the reaction was stirred at 40°C for 15 minutes to obtain hexamethylenediamine cyanurate.

[0060] 12.6 g of ground melamine was placed in a low-temperature plasma reaction chamber, ammonia was introduced, the discharge power was adjusted to 400 W, and the mixture was treated at room temperature for 10 minutes. Then, 16.1 g (0.11 mol) of 1,6-hexanediol was added, and the mixture was stirred and reacted at 150° C. for 10 minutes to obtain melamine adipate.

[0061] The above two salts were added to a polymerization kettle, 255 g (2.3 mol) of caprolactam was added, 5 g (0.3 mol) of deionized water was added, the air in the reactor was replaced with nitrogen three times, the temperature was raised to 250°C, and the reaction was stirred for 5 hours (magnetic stirrer, speed 100 rpm), and then the pressure was reduced to normal pressure. Subsequently, the pressure in the reactor was reduced to 100 Pa, the temperature was reduced to 240°C, the reaction was continued for 2 hours, the material was discharged, cooled, and pelletized to obtain flame-retardant nylon 6.

[0062] Example 2

[0063] The difference from Example 1 is that the discharge power of the cyanuric acid CO2 low-temperature plasma treatment is 200W, and the discharge power of the melamine ammonia low-temperature plasma treatment is 100W.

[0064] Example 3

[0065] The difference from Example 1 is that the discharge power of the cyanuric acid CO2 low-temperature plasma treatment is 800W, and the discharge power of the melamine ammonia low-temperature plasma treatment is 600W.

[0066] Example 4

[0067] The difference from Example 1 is that the treatment time of the cyanuric acid CO2 low-temperature plasma treatment is 10 minutes, and the treatment time of the melamine ammonia low-temperature plasma treatment is 5 minutes.

[0068] Example 5

[0069] The difference from Example 1 is that the treatment time of the cyanuric acid CO2 low-temperature plasma treatment is 50 minutes, and the treatment time of the melamine ammonia low-temperature plasma treatment is 30 minutes.

[0070] Comparative Example 1

[0071] Cyanuric acid and melamine are ground separately, and the particle size is controlled to be 10 to 50 microns.

[0072] 12.9 g of ground cyanuric acid and 11.4 g of 1,6-hexanediamine were placed in 100 ml of water and stirred at 60° C. for 2 hours. The mixture was then filtered, cooled to room temperature and dried to obtain hexamethylenediamine cyanurate.

[0073] 12.6 g of ground melamine and 14.8 g of 1,6-adipic acid were placed in 100 ml of water and stirred at 120° C. for 2 hours, and then the mixture was filtered, cooled to room temperature and dried to obtain melamine adipate.

[0074] The above two salts were added to a polymerization kettle, 255 g of caprolactam was added, 5 g of deionized water was added, the air in the reactor was replaced with nitrogen three times, the temperature was raised to 250°C, the reaction was carried out for 5 hours, and then the pressure was reduced to normal pressure. Subsequently, the pressure in the reactor was reduced to 100 Pa, the temperature was reduced to 240°C, the reaction was continued for 2 hours, the material was discharged, cooled, and pelletized to obtain flame-retardant nylon 6.

[0075] Comparative Example 2

[0076] Cyanuric acid and melamine are ground separately, and the particle size is controlled to be 10 to 50 microns.

[0077] 12.9 g of ground cyanuric acid and 11.6 g of 1,6-hexamethylenediamine were mixed and stirred at 40° C. for 2 hours to obtain hexamethylenediamine cyanurate.

[0078] 12.6 g of ground melamine and 14.6 g of 1,6-adipic acid were mixed and stirred at 150° C. for 2 hours to obtain melamine adipate.

[0079] The above two salts were added to a polymerization kettle, 255 g of caprolactam was added, 5 g of deionized water was added, the air in the reactor was replaced with nitrogen three times, the temperature was raised to 250°C, and the reaction was stirred for 5 hours (magnetic stirrer, speed 100 rpm), and then the pressure was reduced to normal pressure. Subsequently, the pressure in the reactor was reduced to 100 Pa, the temperature was reduced to 240°C, the reaction was continued for 2 hours, the material was discharged, cooled, and pelletized to obtain flame-retardant nylon 6.

[0080] Test Examples

[0081] Infrared spectroscopy analysis:

[0082] Pure melamine, melamine adipate prepared by reaction in aqueous solution (Comparative Example 1) and melamine adipate prepared by reaction after plasma treatment (Example 1) were analyzed by infrared spectroscopy. Figure 4 In the infrared spectrum, 3200~2800cm -1 In the wavenumber range, the diffuse strong characteristic peak is NH3 + Stretching vibration peak, NH3 + The peak position of the salt formed with -COO- is 1643cm -1 , 1510cm -1 -COO - Asymmetric stretching vibration; 645cm-1 The peaks at -COO- are the swing vibration absorption peaks. The degree of reaction between the two can be judged from the intensity of these peaks.

[0083] from Figure 4 It can be seen that after low-temperature plasma treatment, melamine reacts with adipic acid to form salt at 3300-2800 cm -1 NH3 in the wavenumber range + Stretching vibration peak, 1643cm -1 NH3 + Characteristic peaks of salts formed with -COO-, 1510 cm -1 -COO - Asymmetric stretching vibration, 645cm -1 Department-COO - The rocking vibration absorption peak intensities of the samples were higher than those in aqueous solution, indicating that the reactivity of melamine and adipic acid was indeed enhanced after plasma treatment.

[0084] Scanning electron microscopy (SEM) analysis

[0085] The dispersion of the flame retardant prepared in aqueous solution (Comparative Example 1) and the flame retardant prepared by plasma treatment (Example 1) in the synthesized flame retardant nylon was analyzed by SEM. Figure 5 The results showed that after plasma treatment, the dispersed particle size of the flame retardant in the flame retardant nylon was smaller, below 0.5 microns; while the dispersed particle size of the flame retardant prepared in aqueous solution in the flame retardant nylon was above 2 microns.

[0086] Mechanical properties and flame retardant performance tests:

[0087] The mechanical properties and flame retardancy of the flame retardant nylon 6 prepared in the above examples and comparative examples were tested according to the following test standards. The results are shown in Table 1.

[0088] Tensile strength: ISO 527-2 / 1A / 50;

[0089] Elongation at break: ISO 527-2 / 1A / 50;

[0090] Notched impact strength: ISO 179 / 1eA;

[0091] Limiting oxygen index (LOI): GB / T2406;

[0092] Flame retardant grade: GB / T2408-2008.

[0093] Table 1

[0094]

[0095]

[0096] Comparative Example 1 reacts melamine with adipic acid, and cyanuric acid with hexamethylenediamine in water to synthesize flame-retardant nylon 6. Comparative Example 2 directly mixes melamine and cyanuric acid with a dibasic acid and a diamine to synthesize flame-retardant nylon 6. Examples 1-5 utilize low-temperature plasma to treat melamine and cyanuric acid, thereby increasing their alkalinity and acidity, and improving their reactivity with the dibasic acid and diamine, to synthesize flame-retardant nylon 6. As can be seen from the data in Table 1, comparing Example 1 with Comparative Examples 1 and 2, the use of low-temperature plasma to treat melamine and cyanuric acid not only conserves water resources but also achieves high dispersion of MCA in the nylon matrix. The synthesized flame-retardant nylon 6 achieves good flame retardancy while also effectively suppressing the deterioration of the mechanical properties of the flame-retardant nylon 6.

Claims

1. A method for synthesizing flame-retardant nylon 6 by in-situ polymerization, comprising the following steps: (1) treating cyanuric acid using low-temperature plasma technology in a CO2 atmosphere at a treatment power of 200-800 W for a treatment time of 5-50 min; conducting a solid-phase reaction between the treated cyanuric acid and a diamine to obtain cyanuric acid diamine salt; wherein the molar ratio of cyanuric acid to diamine is 1:1-1.2; (2) treating melamine in an ammonia atmosphere using low-temperature plasma technology at a treatment power of 100-600 W for a treatment time of 5-30 min, wherein the treated melamine reacts with a dibasic acid in a solid phase to obtain a melamine dibasic acid salt; wherein the molar ratio of melamine to the dibasic acid is 1:1 to 1.2; (3) The diamine salt of cyanuric acid and the dibasic acid salt of melamine obtained in the above steps (1) and (2) are subjected to an in-situ polymerization reaction with caprolactam and water to synthesize flame-retardant nylon 6, wherein the molar ratio of the diamine salt of cyanuric acid, the dibasic acid salt of melamine, the caprolactam and the water is 1:1:10-40:1-5.

2. The method according to claim 1, characterized in that In step (1), the conditions for treating cyanuric acid using low-temperature plasma technology are: Processing power of 400-600W; and / or The processing time is 10-20 minutes.

3. The method according to claim 1, characterized in that In step (1), the particle size of cyanuric acid is 10 to 500 microns; and / or In step (1), the diamine is one or more combinations selected from hexamethylenediamine, ethylenediamine, propylenediamine, butylenediamine, pentamethylenediamine, octanediamine, and decanediamine; and / or In step (1), the molar ratio of cyanuric acid to diamine is 1:1.

1.

4. The method according to claim 1, wherein In step (1), the particle size of cyanuric acid is 10 to 50 microns; and / or In step (1), the diamine is 1,6-hexanediamine.

5. The method according to claim 1, wherein In step (1), the temperature for the reaction of the treated cyanuric acid and diamine is 25-80° C., and the reaction time is 5-30 min.

6. The method according to claim 1, characterized in that In step (1), the temperature for reacting the treated cyanuric acid with the diamine is 40-50° C.; and / or the reaction time is 10-20 min.

7. The method according to claim 1, characterized in that In step (2), the conditions for treating melamine with low-temperature plasma are: Processing power of 300-500W; and / or The processing time is 5-15 minutes.

8. The method according to claim 1, characterized in that In step (2), the melamine particle size is 10 to 500 microns; and / or In step (2), the dibasic acid is one or more combinations selected from adipic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, suberic acid, and sebacic acid; and / or In step (2), the molar ratio of melamine to dibasic acid is 1:1.

1.

9. The method according to claim 1, characterized in that In step (2), the melamine particle size is 10 to 50 microns; and / or In step (2), the dibasic acid is 1,6-hexanediol.

10. The method according to claim 1, characterized in that In step (2), the temperature for reacting the treated melamine with the dibasic acid is 100-200° C., and the reaction time is 5-30 min.

11. The method according to claim 1, wherein In step (2), the temperature for reacting the treated melamine with the dibasic acid is 150-160° C.; and / or the reaction time is 5-15 min.

12. The method according to claim 1, characterized in that In step (3), the in-situ polymerization reaction is carried out under the protection of an inert gas; and / or In step (3), the molar ratio of diamine cyanurate, melamine dibasic acid salt, caprolactam and water is 1:1:15-30:2-4.

13. The method according to claim 1, wherein In step (3), in the in-situ polymerization reaction, diamine cyanurate and melamine dibasic acid salt react with caprolactam and water at a first temperature for a period of time, and then cool to a second temperature and continue to react for a period of time.

14. The method according to claim 13, characterized in that The first temperature is 220-280° C.; the reaction time at the first temperature is 1-10 hours; The second temperature is 210-270° C.; the reaction time after cooling to the second temperature is 1-5 hours.

15. The method according to claim 14, characterized in that The first temperature is 240-260° C.; and / or The reaction time at the first temperature is 3-8 hours; and / or The second temperature is 230-250°C; and / or The reaction time of cooling to the second temperature is 1-3 hours.

16. Flame-retardant nylon 6 prepared by the method for synthesizing flame-retardant nylon 6 by in-situ polymerization according to any one of claims 1 to 15.

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