Water-resistant flame-retardant polyethylene composite material as well as preparation method and application thereof

Abstract

The invention relates to the technical field of flame-retardant polyethylene composite materials, in particular to a water-resistant flame-retardant polyethylene composite material as well as a preparation method and application thereof. The water-resistant flame-retardant polyethylene composite material is prepared from the following raw materials in parts by weight: 60 to 72 parts of polyethylene, 20 to 26 parts of water-resistant coke pipe, 8 to 14 parts of flame retardant, 0.1 to 1 part of dispersing agent, 0.1 to 1 part of antioxidant and 0.1 to 1 part of ethylene bis stearamide. The water-resistant coke piperidine is prepared by the following method: synthesizing diphosphate of a piperazine derivative by taking the piperazine derivative and phosphoric acid as raw materials; and carrying out polymerization reaction on the diphosphate of the piperazine derivative to obtain the water-resistant coke piperidine. According to the technical scheme, the technical problem that the water resistance of a flame-retardant polyethylene composite material in the prior art is not ideal can be solved, and the novel water-resistant coke pipe is provided and has ideal popularization and application prospects.

Description

Technical Field

[0001] The invention relates to the technical field of flame-retardant polyethylene composite materials, and in particular to a water-resistant flame-retardant polyethylene composite material and a preparation method and application thereof. Background Art

[0002] Polyethylene material is non-toxic and odorless, and has the advantages of high acid and alkali resistance, electrical insulation, excellent mechanical properties, etc. It is widely used in the manufacture of packaging materials, containers, pipes, wires and cables, daily necessities, etc. However, it is flammable and requires the use of flame retardants to reduce its fire hazard.

[0003] As the world's environmental awareness continues to improve, phosphorus-containing flame retardants including: piperazine pyrophosphate, ammonium polyphosphate (APP), melamine polyphosphate (MPP), red phosphorus, etc., occupy an important position in the domestic flame retardant market, but they generally have the disadvantage of poor water resistance, which will cause the performance of flame retardants to deteriorate in high humidity or underwater environments, thereby affecting the overall performance of composite materials. Specifically, when the water resistance of the flame retardant is poor, it may hydrolyze or deteriorate after contact with water or water vapor, or even the flame retardant may migrate and precipitate to the surface of the composite material, thereby reducing its protective effect on the substrate. In addition, this decomposition or deterioration process may directly affect the mechanical properties of the composite material, including its strength, toughness and durability, and ultimately lead to performance degradation or failure of the material during use.

[0004] Therefore, in order to improve the performance stability of composite materials in water or humid environments, it is necessary to research and develop flame retardants with better water resistance. This includes improving the chemical structure of flame retardants, using surface modification technology or developing new flame retardant systems to overcome the problem of poor water resistance and ensure that composite materials can maintain good performance under various environmental conditions. However, the existing technology still lacks environmentally friendly flame retardants with good water resistance that can be used for polyethylene composite materials, which needs further research and development. Summary of the invention

[0005] The present invention is intended to provide a water-resistant flame-retardant polyethylene composite material to solve the technical problem that the water resistance of the flame-retardant polyethylene composite material in the prior art is not ideal.

[0006] In order to achieve the above object, the present invention adopts the following technical scheme:

[0007] A water-resistant flame-retardant polyethylene composite material, comprising the following raw materials, by weight: 60-72 parts of polyethylene, 20-26 parts of water-resistant flame retardant, 8-14 parts of flame retardant, 0.1-1 parts of dispersant, 0.1-1 parts of antioxidant, and 0.1-1 parts of vinyl bisstearamide;

[0008] The water-resistant pyrochlore is any one of the compounds represented by formula (1-2), the compounds represented by formula (3-2), the compounds represented by formula (4-2), the compounds represented by formula (5-2), the compounds represented by formula (7-2), the compounds represented by formula (8-2), and the compounds represented by formula (9-2);

[0009]

[0010] Among them, n=500-2000.

[0011] Furthermore, the polyethylene includes high melt index polyethylene and low melt index polyethylene in a mass ratio of 1:0.8-1.6.

[0012] Furthermore, the flame retardant includes at least one of melamine polyphosphate, melamine cyanurate and ammonium polyphosphate.

[0013] Further, the water-resistant coke is prepared by the following method:

[0014] S1: Using piperazine derivatives and phosphoric acid as raw materials, synthesizing diphosphates of piperazine derivatives;

[0015] S2: After the diphosphate ester of piperazine derivative is polymerized, water-resistant pyropiperazine is obtained.

[0016] Furthermore, in S1, the aqueous solution of the piperazine derivative is added dropwise to the aqueous solution of phosphoric acid to obtain the diphosphate ester of the piperazine derivative.

[0017] Furthermore, in S1, the molar ratio of the piperazine derivative to the phosphoric acid is 1:0.8-1.5.

[0018] Further, the piperazine derivative includes any one of aminoethylpiperazine, 2,2-dimethylpiperazine, 2,5-dimethylpiperazine, 2,6-dimethylpiperazine, 2-ethylpiperazine, 2-isopropylpiperazine and N,N'-di(aminoethyl)-piperazine.

[0019] Further, in S2, in a nitrogen environment, the diphosphate of the piperazine derivative is reacted at a temperature of 200-300°C and a pressure of 0.05-0.3MPa for 3-8 hours to obtain water-resistant coke; during the reaction, the flow rate of nitrogen is maintained at 0.1-5L / min.

[0020] The technical scheme also provides a method for preparing a water-resistant flame-retardant polyethylene composite material, wherein the raw materials are dried and mixed evenly, and then a twin-screw extruder is used to melt-extrude and granulate the mixed raw materials to obtain pellets of the water-resistant flame-retardant polyethylene composite material; the pellets are injection-molded by an injection molding machine to obtain the final product.

[0021] Furthermore, the temperature of each section of the twin-screw extruder is 180±10℃, 200±10℃, 200±10℃, 200±10℃, and the main screw speed is 30-40HZ; the temperature of each section of the injection molding machine is 180±10℃, 200±10℃, 200±10℃, and 200±10℃.

[0022] In summary, the technical solution synthesizes water-resistant piperazine pyrophosphate that can solve the water resistance problem of flame-retardant materials by improving the chemical structure of flame retardants, and applies it to the preparation of flame-retardant polyethylene to obtain a water-resistant flame-retardant polypropylene composite material. When water-resistant piperazine pyrophosphate is applied to the production of polyethylene materials, the raw material piperazine derivative of water-resistant piperazine pyrophosphate adopts any one of aminoethyl piperazine, 2,2-dimethyl piperazine, 2,5-dimethyl piperazine, 2,6-dimethyl piperazine, 2-ethyl piperazine, 2-isopropyl piperazine and N,N'-di(aminoethyl)-piperazine, and the obtained polyethylene composite material has the most ideal water resistance and flame retardant properties, and the water-resistant piperazine pyrophosphate has good compatibility with other components in the polyethylene composite material, and the composite material can maintain relatively ideal mechanical properties. DETAILED DESCRIPTION

[0023] The present invention is further described in detail below in conjunction with examples, but the embodiments of the present invention are not limited thereto. Unless otherwise specified, the technical means used in the following examples and experimental examples are conventional means well known to those skilled in the art, and the materials, reagents, etc. used can be obtained from commercial sources.

[0024] Example 1: Preparation of water-resistant coke

[0025] The piperazine derivative is first reacted with phosphoric acid to obtain an intermediate product (diphosphate of the piperazine derivative), and then the intermediate product is thermally polymerized to obtain water-resistant pyropiperazine. Piperazine derivatives include aminoethylpiperazine (see formula (1-1), CAS NO.: 140-31-8), 2-methylpiperazine (see formula (2-1), CAS NO.: 109-07-9), 2,2-dimethylpiperazine (see formula (3-1), CAS NO.: 84477-72-5), 2,5-dimethylpiperazine (see formula (4-1), CAS NO.: 106-55-8), 2,6-dimethylpiperazine (see formula (5-1), CAS NO.: 108-49-6), trans-2,5-diethylpiperazine (see formula (6-1), CAS NO.: 6189-24-8), 2-ethylpiperazine (see formula (7-1), CAS NO.: 13961-37-0), 2-isopropylpiperazine (see formula (8-1), CAS NO.: 84468-53-1), N,N'-di(aminoethyl)-piperazine (see formula (9-1), CAS NO.: 6531-38-0), 2-piperazinone (see formula (10-1), CAS NO.: 5625-67-2).

[0026] The molecular formula of the corresponding synthesized water-resistant pyropiperazine (or non-water-resistant pyropiperazine) can be found in formula (1-2), formula (2-2), formula (3-2), formula (4-2), formula (5-2), formula (6-2), formula (7-2), formula (8-2), formula (9-2), and formula (10-2), and n=500-2000. The piperazine derivatives used to synthesize water-resistant pyropiperazine include: derivatives formed by replacing one or two hydrogen atoms on one or more carbon atoms of piperazine with an alkyl group, or derivatives formed by replacing one or two hydrogen atoms on one or two nitrogen atoms of piperazine with an aminoalkyl group. Among them, the alkyl group is preferably a methyl group, an ethyl group, or a propyl group, and the aminoalkyl group is preferably an aminoethyl group.

[0027]

[0028]

[0029]

[0030]

[0031] Taking aminoethyl piperazine formula (1-1) as an example, the synthesis process is shown in formula (11) and formula (12). Aminoethyl piperazine is first reacted with phosphoric acid to obtain aminoethyl piperazine diphosphate. Aminoethyl piperazine diphosphate is polymerized to obtain water-resistant pyropiperazine.

[0032] In contrast, in the prior art, the structural formula of common piperazine pyrophosphate is shown in formula (13).

[0033]

[0034] The synthesis process is roughly as follows: dissolve the piperazine derivative in deionized water at a ratio of 0.4-0.6 mol: 50-150 mL (preferably 0.5 mol: 100 mL) to obtain a piperazine derivative solution. Dissolve phosphoric acid in deionized water at a ratio of 1-1.5 mol: 50-200 mL (preferably 1 mol: 100 mL) at 25-90° C. to obtain a phosphoric acid solution. Add the phosphoric acid solution to the piperazine derivative solution at a rate of 1-10 drops / second. After the pH value is constant, the reaction is completed, washed and dried at 60-120° C. to obtain the diphosphate ester of the piperazine derivative (i.e., the intermediate product). Next, a polymerization reaction is carried out, and 0.1-1 mol of the diphosphate ester of the piperazine derivative is added to a tubular atmosphere furnace, and heated and mixed at a temperature of 200-300° C. for 3-8 hours under a nitrogen atmosphere (0.05-0.3 MPa). Nitrogen was introduced from the beginning of heating to the end of cooling (maintaining a nitrogen flow rate of 0.1-5 L / min). Afterwards, the product was rinsed with deionized water to remove the raw materials (unreacted monomers were rinsed with deionized water), and then dried to obtain water-resistant pyrolysis piperidine.

[0035] More specifically, the preparation process of water-resistant piperazine is described by taking aminoethyl piperazine as an example (named as: Preparation Example 1). 0.5 mol of aminoethyl piperazine (AEP, 64.68 g) is dissolved in 100 ml of deionized water to obtain an aminoethyl piperazine solution for standby use. 1 mol of phosphoric acid (using 85% phosphoric acid mother solution, 98 g / 0.85 = 115.3 g of phosphoric acid mother solution needs to be added) is dissolved in 100 ml of water to obtain a phosphoric acid solution for standby use. At room temperature (about 25° C.), the phosphoric acid solution is transferred to a three-necked flask, and the aminoethyl piperazine solution is slowly added dropwise (the specific dropping speed is 5 drops / second) to the phosphoric acid solution. The pH value in the measurement system is determined, and the reaction is terminated after the pH remains constant. The product is washed and dried at 100° C. to obtain aminoethyl piperazine diphosphate. Next, a polymerization reaction was carried out by adding 0.5 mol of aminoethylpiperazine diphosphate into a tubular atmosphere furnace and heating and mixing at 200°C for 3 hours under a nitrogen atmosphere (0.1 MPa). Nitrogen (1 L / min) was introduced from the start of heating to the end of cooling. After that, the product was rinsed with deionized water to remove the raw materials (unreacted monomers) and dried to obtain water-resistant pyropiperazine.

[0036] The other preparation examples are basically the same as Preparation Example 1, except for the selection of specific raw materials and parameters, as shown in Tables 1 and 2. The yield of aminoethylpiperazine diphosphate, the water contact angle of water-resistant pyrocarbonate, and the number of days (caking status) when the newly prepared dried water-resistant pyrocarbonate begins to agglomerate under normal temperature and humidity conditions were tested and counted, as shown in Tables 1 and 2.

[0037] Table 1: Preparation examples (wherein, for the deionized water item, the amount of solvent used to dissolve the piperazine derivative is indicated before the “ / ”, and the amount of solvent used to dissolve the phosphoric acid is indicated after the “ / ”; the domestically produced coke is produced by Chongqing Kejufu New Materials Co., Ltd., model QZ10-2; piperazine is used as a raw material for the production of coke. Due to the solubility of the piperazine raw material, the temperature for obtaining the intermediate product is 50°C, which is the best condition for producing piperazine diphosphate. For details, see Preparation Examples 4-6)

[0038]

[0039] Table 2: Preparation Examples (Continued from Table 1, for the deionized water item, the amount of solvent used to dissolve the piperazine derivative is indicated before the “ / ”, and the amount of solvent used to dissolve phosphoric acid (85wt%) is indicated after the “ / ”; in the preparation process of the diphosphate of the intermediate product piperazine derivative, different preparation examples used different reaction temperatures, all of which were the best conditions to ensure the synthesis efficiency of the intermediate product)

[0040]

[0041] It can be seen from the experimental results that the use of piperazine derivatives as raw materials to make piperazine pyrophosphate analogs is more excellent than the water resistance of traditional piperazine pyrophosphate, greatly improving the water resistance of flame retardant materials (increased water contact angle and improved anti-caking ability). The inventor analyzed that the reason is that the piperazine derivatives have added specific groups relative to piperazine, forming a certain steric hindrance, thereby improving the water resistance of the overall material, and its flame retardant properties are not affected. Introducing a certain alkyl chain into the molecular structure of piperazine pyrophosphate forms a certain steric hindrance effect, thereby giving piperazine pyrophosphate water resistance characteristics.

[0042] Among them, the water-resistant pyropiperazine synthesized by using trans-2,5-diethylpiperazine, 2-ethylpiperazine, 2-isopropylpiperazine and N,N'-di(aminoethyl)-piperazine as piperazine derivatives has a more ideal hydrophobic property and can ensure that no caking occurs within 90 days. The second is the use of 2,2-dimethylpiperazine, 2,5-dimethylpiperazine, 2,6-dimethylpiperazine, aminoethylpiperazine and 2-methylpiperazine, which can ensure that no caking occurs within 60-70 days.

[0043] The water resistance of the pyrocarbon synthesized by piperazine is very unsatisfactory, as can be seen in Preparation Examples 4-6 and domestic piperazine. Moreover, not all types of piperazine derivatives prepared at random can achieve the effect of increasing water resistance. For example, Preparation Example 13 uses 2-piperazinone as a derivative of piperazine to synthesize pyrocarbon, and the water resistance of the finished product is not only not improved, but seriously decreased. Therefore, in order to ensure the water resistance of the finished pyrocarbon, it is necessary to ensure that the following forms of piperazine derivatives are used: derivatives formed by replacing one or two hydrogen atoms on one or more carbon atoms of piperazine with an alkyl group, or derivatives formed by replacing the hydrogen atoms on one or two nitrogen atoms of piperazine with an aminoalkyl group; wherein the alkyl group is preferably a methyl group, an ethyl group, or a propyl group, and the aminoalkyl group is preferably an aminoethyl group.

[0044] Example 2: Preparation of polyethylene composite material

[0045] In parts by weight, the components of the composite material include: 60-72 parts of polyethylene, 20-26 parts of water-resistant pyrochlore flame retardant (water-resistant pyrochlore), 8-14 parts of common flame retardant (flame retardant), 0.1-1 part of dispersant, 0.1-1 part of antioxidant (168), and 0.1-1 part of vinyl bisstearamide (EBS).

[0046] Wherein, polyethylene includes two polyethylene homopolymers, namely, high melt index polyethylene (for example: HDPE 5000S) and low melt index polyethylene (for example: LLDPE-7042). The mass ratio between high melt index polyethylene and low melt index polyethylene is maintained at 1:0.8-1.6. Common flame retardants include any one of melamine polyphosphate (MPP, produced by Chongqing Kejufu New Materials Co., Ltd.), melamine cyanurate (MCA) and ammonium polyphosphate (APP). The dispersant can adopt conventional PE wax in the prior art.

[0047] After drying the above raw materials, they were mixed evenly in a high mixer (1000rpm), and then melt-extruded and granulated by a twin-screw extruder. The obtained pellets were dried at 80°C for 12h and then injection-molded by an injection molding machine to obtain test samples. The temperatures of each section of the twin-screw extruder were 180±10°C, 200±10°C, 200±10°C, 200±10°C (180°C, 200°C, 200°C, 200°C in the specific implementation process), and the main screw speed was 30-40HZ (35HZ in the specific implementation process); the temperatures of each section of the injection molding machine were 180±10°C, 200±10°C, 200±10°C, 200±10°C (180°C, 200°C, 200°C, 200°C in the specific implementation process). The specific amounts of raw materials for the polyethylene composite material are shown in Table 3. A water-resistant flame-retardant polyethylene composite material was obtained, and the test results are shown in Table 4. Among them, the UL-94 (1.6mm) test method refers to the standard GB / T2408-2008 / IEC; the double "85" test (1000h) test method refers to the standard GB / T 2423.50 Part 2, and the test method is test Cy; the mechanical property retention rate (%) test method refers to the standard GB / T 1040.2, which specifically refers to the ratio of tensile properties before and after the double "85" test.

[0048] Table 3: Raw material dosage of polyethylene composite materials (parts)

[0049]

[0050] Table 4: Test results (A: UL-94 (1.6 mm); B: double "85" test (1000 h); C: mechanical property retention rate (%); D: UL-94 (1.6 mm, after double "85 test"))

[0051]

[0052] It can be seen from the above experimental data that the flame retardant properties of water-resistant coke are consistent with those of domestic coke (or self-prepared coke, tests 4-6), but the water resistance of the composite material is significantly improved. Test 7 uses 2-methylpiperazine to prepare water-resistant coke. Although the water resistance of water-resistant coke itself is relatively ideal (for example, it is similar to the water resistance parameters of Q1 and Q2), it is applied to the preparation process of a specific polyethylene composite material, but it is found that it is difficult to effectively improve the water resistance of the composite material, and there is a slight precipitation phenomenon during the double "85" test. Test 9 uses trans-2,5-diethylpiperazine to prepare water-resistant coke. Although the water resistance of water-resistant coke itself is relatively ideal (see Table 2 Preparation Example 9), it is applied to the preparation process of a specific polyethylene composite material, but it is found that it is difficult to effectively improve the water resistance of the composite material, and there is a surface precipitation phenomenon during the double "85" test, and the flame retardant level is difficult to maintain at the V-0 level, and the mechanical properties of the overall composite material are deteriorated to a certain extent. It can be seen that the water resistance of water-resistant pyrolysis pipe (for example: the agglomeration of water-resistant pyrolysis pipe and the contact angle) is not the determining factor for judging the water resistance and flame retardancy of the final polyethylene composite material. When water-resistant pyrolysis pipe is applied to composite materials, other factors such as the compatibility of water-resistant pyrolysis pipe in the overall composite environment need to be considered. Although the water resistance of water-resistant pyrolysis pipe involved in Test 7 and Test 9 is relatively excellent (reflected in the contact angle and agglomeration), when it is actually applied to polyethylene composite materials, whether the water resistance of water-resistant pyrolysis pipe can be exerted is not simply judged based on the water resistance of water-resistant pyrolysis pipe itself. Therefore, the water-resistant pyrolysis pipe flame retardant suitable for application to polyethylene composite materials should be prepared using the following piperazine derivatives: aminoethyl piperazine, 2,2-dimethyl piperazine, 2,5-dimethyl piperazine, 2,6-dimethyl piperazine, 2-ethyl piperazine, 2-isopropyl piperazine, N,N'-di(aminoethyl)-piperazine.

[0053] In addition, the inventors also tried to introduce longer alkyl chains into piperazine to form piperazine derivatives in order to find the most suitable water-resistant flame retardant for polyethylene composite materials. For example, the inventors used 1,4-bis(3-aminopropyl)piperazine (CAS NO.: 7209-38-3) to prepare water-resistant flame retardant. The specific technical parameters and preparation method can be found in Preparation Example 12 (only the piperazine derivative is replaced). The water-resistant flame retardant prepared by 1,4-bis(3-aminopropyl)piperazine shows good water resistance and does not agglomerate within 90 days. Then, a polyethylene composite material was prepared according to the method of test 12, and it was found that the UL-94 (1.6 mm) test results, double "85" test results, and UL-94 (1.6 mm, after double "85 test") test results of the polyethylene composite material (hereinafter referred to as comparative composite material A) showed slight precipitation on the surface compared with test 12 (using N, N'-di(aminoethyl)-piperazine as the raw material of the piperazine derivative), and the mechanical property retention rate dropped to below 80%; and the initial tensile properties of the comparative composite material A (before the double "85" test) decreased by more than 10% compared with test 6 (using piperazine as the raw material). This shows that although increasing the carbon chain length of the piperazine derivative will increase the hydrophobicity of the water-resistant polyurethane, the introduction of too many carbon chains will also affect the compatibility of the water-resistant polyurethane and the polyethylene composite material, resulting in a significant decrease in the mechanical properties of the composite material.

[0054] In addition to 1,4-bis(3-aminopropyl)piperazine as a raw material for piperazine derivatives, the inventors have also tried to use 2-isobutylpiperazine (CAS NO.: 84477-68-9). For specific technical parameters and preparation methods, please refer to Preparation Example 11. The water-resistant coke prepared by 2-isobutylpiperazine was prepared according to the method of Test 11 to obtain a polyethylene composite material (hereinafter referred to as: Comparative Composite Material B). It was found that the UL-94 (1.6mm) test results, double "85" test results, and UL-94 (1.6mm, double "85 test") test results of the polyethylene composite material were more precipitated than those of Test 11 (using 2-isopropylpiperazine as a raw material for piperazine derivatives), and the mechanical property retention rate dropped to less than 75%. In addition, the initial tensile properties of the comparative composite material B (before the double "85" test) decreased by more than 15% compared with the tensile properties of Test 6 (using piperazine as a raw material). This shows that although increasing the carbon chain length of the piperazine derivative will increase the hydrophobicity of the water-resistant polypropylene, the introduction of too many carbon chains will also affect the compatibility of the water-resistant polypropylene and polyethylene composites, resulting in a significant decrease in the mechanical properties of the composites.

[0055] Therefore, the type of piperazine derivatives for water-resistant piperazine is very important. In order to ensure both water resistance and mechanical properties, piperazine derivatives such as aminoethylpiperazine, 2,2-dimethylpiperazine, 2,5-dimethylpiperazine, 2,6-dimethylpiperazine, 2-ethylpiperazine, 2-isopropylpiperazine, and N,N'-di(aminoethyl)-piperazine are preferably used.

[0056] The above is only an embodiment of the present invention, and the common knowledge such as the known specific technical solutions and / or characteristics in the solution is not described in detail here. It should be pointed out that for those skilled in the art, without departing from the technical solution of the present invention, several modifications and improvements can be made, which should also be regarded as the protection scope of the present invention, and these will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A water-resistant flame-retardant polyethylene composite material, characterized in that: The raw materials include, by weight: 60-72 parts of polyethylene, 20-26 parts of water-resistant pyrochlore, 8-14 parts of flame retardant, 0.1-1 parts of dispersant, 0.1-1 parts of antioxidant, and 0.1-1 parts of vinyl bisstearamide; The water-resistant pyrochlore is any one of the compounds represented by formula (1-2), the compounds represented by formula (3-2), the compounds represented by formula (4-2), the compounds represented by formula (5-2), the compounds represented by formula (7-2), the compounds represented by formula (8-2), and the compounds represented by formula (9-2); Among them, n=500-2000.

2. The water-resistant flame-retardant polyethylene composite material according to claim 1, characterized in that: The polyethylene includes two types: high melt index polyethylene and low melt index polyethylene with a mass ratio of 1:0.8-1.

6.

3. The water-resistant flame-retardant polyethylene composite material according to claim 1, characterized in that: The flame retardant includes at least one of melamine polyphosphate, melamine cyanurate and ammonium polyphosphate.

4. The water-resistant flame-retardant polyethylene composite material according to claim 1, characterized in that: The water-resistant coke is prepared by the following method: S1: Using piperazine derivatives and phosphoric acid as raw materials, synthesizing diphosphates of piperazine derivatives; S2: After the diphosphate ester of piperazine derivative is polymerized, water-resistant pyropiperazine is obtained.

5. The water-resistant flame-retardant polyethylene composite material according to claim 4, characterized in that: In S1, an aqueous solution of phosphoric acid is added dropwise to an aqueous solution of a piperazine derivative to obtain a diphosphate ester of the piperazine derivative.

6. The water-resistant flame-retardant polyethylene composite material according to claim 5, characterized in that: In S1, the molar ratio of the piperazine derivative to phosphoric acid is 1:0.8-1.

5.

7. The water-resistant flame-retardant polyethylene composite material according to claim 6, characterized in that: The piperazine derivative includes any one of aminoethylpiperazine, 2,2-dimethylpiperazine, 2,5-dimethylpiperazine, 2,6-dimethylpiperazine, 2-ethylpiperazine, 2-isopropylpiperazine and N,N'-di(aminoethyl)-piperazine.

8. The water-resistant flame-retardant polyethylene composite material according to claim 4, characterized in that: In S2, in a nitrogen environment, the diphosphate of the piperazine derivative is reacted at a temperature of 200-300°C and a pressure of 0.05-0.3MPa for 3-8 hours to obtain water-resistant coke. During the reaction, the flow rate of nitrogen is maintained at 0.1-5L / min.

9. A method for preparing a water-resistant flame-retardant polyethylene composite material according to any one of claims 1 to 6, characterized in that: The raw materials are dried and mixed evenly, and then a twin-screw extruder is used to melt and extrude the mixed raw materials into granules to obtain granules of a water-resistant flame-retardant polyethylene composite material; the granules are injection-molded by an injection molding machine to obtain a final product.

10. The method for preparing a water-resistant flame-retardant polyethylene composite material according to claim 9, characterized in that: The temperature of each section of the twin-screw extruder is 180±10℃, 200±10℃, 200±10℃, 200±10℃, and the speed of the main screw is 30-40HZ; the temperature of each section of the injection molding machine is 180±10℃, 200±10℃, 200±10℃, and 200±10℃.

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