Polyethylene-based composite coating and preparation method and application thereof

Abstract

The present application discloses a polyethylene-based composite coating and a preparation method and application thereof, and relates to the technical field of polymer materials. The polyethylene-based composite coating comprises polyethylene and cross-linked polyurethane. The polyethylene-based composite coating provided by the present application has excellent corrosion resistance and flame retardant properties.

Description

Technical Field

[0001] The present application relates to the technical field of polymer materials, and in particular to a polyethylene-based composite coating and a preparation method and application thereof. Background Art

[0002] Polyethylene composite steel pipe combines the high strength and high modulus of metal with the excellent chemical corrosion resistance of plastic, and is widely used as buried or underwater steel oil, gas, water and heating pipelines. However, the linear structure and low surface energy of polyethylene coating lead to limited anti-corrosion performance, which restricts its long-term use in certain highly corrosive environments (such as strong acid, strong alkali, high humidity, etc.). Summary of the invention

[0003] In view of this, the present application provides a polyethylene-based composite coating and a preparation method and application thereof.

[0004] The embodiment of the present application is implemented as follows: the embodiment of the present application provides a polyethylene-based composite coating, including polyethylene and cross-linked polyurethane; wherein the structural formula of the cross-linked polyurethane is as shown below:

[0005] ;

[0006] in, for , for .

[0007] Optionally, in some embodiments of the present application, the polyethylene is high-density polyethylene; and / or

[0008] In the polyethylene-based composite coating, the mass ratio of the polyethylene to the cross-linked polyurethane is (10-50): (15-30); and / or

[0009] In the polyethylene-based composite coating, the polyethylene and the cross-linked polyurethane form a semi-interpenetrating network structure.

[0010] Accordingly, the present invention also provides a method for preparing a polyethylene-based composite coating, comprising the following steps:

[0011] Providing 2,2,3,3-tetrafluoro-1,4-butanediol and 1,6-hexamethylene diisocyanate, mixing, reacting, and forming a polyurethane prepolymer;

[0012] Polyethylene and trihydroxymethyl phosphorus oxide are provided, mixed with the polyurethane prepolymer, and reacted. The trihydroxymethyl phosphorus oxide and the polyurethane prepolymer react to form a cross-linked polyurethane, and the polyethylene and the cross-linked polyurethane react to form a polyethylene-based composite coating.

[0013] Optionally, in some embodiments of the present application, the mixing of the 2,2,3,3-tetrafluoro-1,4-butanediol and the 1,6-hexamethylene diisocyanate further comprises: adding a first solvent to form a first mixed liquid, reacting to form a polyurethane prepolymer.

[0014] Optionally, in some embodiments of the present application, the first solvent is selected from one or more of ethyl acetate, tetrahydrofuran, chlorobenzene, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, diethylene glycol dimethyl ether, dimethyl sulfoxide, acetone, acetophenone, N,N-dimethylformamide, and pyrrole; and / or

[0015] In the first mixed solution, the mass concentration of the 2,2,3,3-tetrafluoro-1,4-butanediol is 20 g / L to 50 g / L; and / or

[0016] In the first mixed solution, the mass concentration of 1,6-hexamethylene diisocyanate is 100 g / L to 150 g / L.

[0017] Optionally, in some embodiments of the present application, the mass ratio of the 2,2,3,3-tetrafluoro-1,4-butanediol to the 1,6-hexamethylene diisocyanate is (4-10): (10-15); and / or

[0018] The reaction temperature of the 2,2,3,3-tetrafluoro-1,4-butanediol and the 1,6-hexamethylene diisocyanate is 50° C. to 80° C.; the reaction time is 2 h to 5 h; and / or

[0019] The structural formula of the polyurethane prepolymer is shown below:

[0020] ;

[0021] in, for .

[0022] Optionally, in some embodiments of the present application, providing polyethylene includes: providing a polyethylene dispersion, wherein the polyethylene dispersion includes the polyethylene and a second solvent.

[0023] Optionally, in some embodiments of the present application, the polyethylene is high-density polyethylene; and / or

[0024] The second solvent is selected from one or more of benzene, toluene, xylene, dimethyl ether, tetraethylene glycol dimethyl ether, n-octane, isooctane, n-hexane, and cyclohexane; and / or

[0025] In the polyethylene dispersion, the mass concentration of the polyethylene is 100 g / L to 500 g / L.

[0026] Optionally, in some embodiments of the present application, the preparation method further comprises providing a catalyst, and mixing it with the polyethylene, the trimethylolphosphine oxide, and the polyurethane prepolymer; the catalyst comprises a tin catalyst; the tin catalyst comprises one or more of dibutyltin dilaurate, dibutyltin dilaurate, stannous octoate, dibutyltin didodecylsulfide, and dibutyltin diacetate; and / or

[0027] The mass ratio of the polyethylene, the trihydroxymethylphosphine oxide and the polyurethane prepolymer is (10-50): (1-5): (14-25); and / or

[0028] The reaction of the polyethylene, the trihydroxymethylphosphine oxide and the polyurethane prepolymer includes a first heat treatment and a second heat treatment performed sequentially; the temperature of the first heat treatment is 50°C to 100°C, and the reaction time is 5min to 30min; the temperature of the second heat treatment is 100°C to 150°C, and the reaction time is 1h to 5h; and / or

[0029] The structural formula of the cross-linked polyurethane is shown below:

[0030] ;

[0031] in, for , for .

[0032] Correspondingly, an embodiment of the present application further provides a steel pipe, which includes a coating, and the material of the coating includes the above-mentioned composite coating, or includes the composite coating prepared by the above-mentioned preparation method.

[0033] The polyethylene-based composite coating provided in the present application includes polyethylene and cross-linked polyurethane. The cross-linked structure and high fluorine content in the cross-linked polyurethane give it a higher surface energy, which can significantly improve the corrosion resistance of the composite coating. The cross-linked polyurethane can also significantly improve the flame retardant properties of the composite coating. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For those skilled in the art, other drawings can be obtained based on these drawings without paying any creative work.

[0035] Figure 1 This is a flow chart of a method for preparing a polyethylene-based composite coating provided in an embodiment of the present application. DETAILED DESCRIPTION

[0036] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without making creative work belong to the scope of protection of the present application. In addition, it should be understood that the specific implementation methods described herein are only used to illustrate and explain the present application, and are not used to limit the present application.

[0037] In this application, unless otherwise stated, directional words such as "upper" and "lower" generally refer to the upper and lower parts of the device in actual use or working state, specifically the drawing direction in the accompanying drawings; while "inner" and "outer" refer to the outline of the device. In addition, in the description of this application, the term "including" means "including but not limited to". The terms first, second, third, etc. are used only as labels and do not impose numerical requirements or establish an order.

[0038] In this application, "and / or" describes the association relationship of associated objects, indicating that there may be three relationships. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone. A and B can be singular or plural.

[0039] In this application, "at least one" means one or more, and "plurality" means two or more. "One or several", "at least one of the following" or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c", or "at least one of a, b, and c" can all mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0040] Various embodiments of the present application may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity, and should not be understood as a rigid limitation on the scope of the present application; therefore, the range description should be considered to have specifically disclosed all possible sub-ranges and single numerical values ​​within the range. For example, the range description from 1 to 6 should be considered to have specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, which apply regardless of the range. In addition, whenever a numerical range is indicated herein, it is meant to include any cited number (fractional or integer) within the indicated range.

[0041] The structural formula and molecular weight of some chemical reagents used in this application are described as follows:

[0042] 2,2,3,3-Tetrafluoro-1,4-butanediol: , molecular weight = 162.08;

[0043] 1,6-hexamethylene diisocyanate (HDI): , molecular weight = 168.19;

[0044] Tris(hydroxymethyl)phosphine oxide: , molecular weight = 140.08.

[0045] The technical solution of this application is as follows:

[0046] In a first aspect, an embodiment of the present application provides a polyethylene-based composite coating, comprising polyethylene and cross-linked polyurethane; wherein the structural formula of the cross-linked polyurethane is as shown below:

[0047] ;

[0048] in, for , for .

[0049] The polyethylene-based composite coating provided in the present application includes polyethylene and cross-linked polyurethane. The cross-linked structure and high fluorine content in the cross-linked polyurethane give it a higher surface energy, which can significantly improve the corrosion resistance of the composite coating. The cross-linked polyurethane can also significantly improve the flame retardant properties of the composite coating.

[0050] In some embodiments, the polyethylene is high-density polyethylene (HDPE). High-density polyethylene has good hardness, tensile strength, creep, wear resistance, electrical insulation, toughness and cold resistance, as well as high chemical stability.

[0051] In some embodiments, in the polyethylene-based composite coating, the mass ratio of the polyethylene to the cross-linked polyurethane is (10-50): (15-30), for example, it can be 12:28, 15:25, 20:22, 30:15, 28:20, 40:18, 45:20, 48:16 or a range between any two ratios. Within the mass ratio range, the cross-linked polyurethane and the polyethylene are conducive to synergistic action, thereby improving the corrosion resistance and flame retardancy of the composite coating.

[0052] In some embodiments, in the polyethylene-based composite coating, the polyethylene and the cross-linked polyurethane form a semi-interpenetrating network structure. The semi-interpenetrating network structure can enhance the toughness of the composite coating and improve the corrosion resistance and flame retardancy of the composite coating.

[0053] Second, see Figure 1 The present application also provides a method for preparing a polyethylene-based composite coating, comprising the following steps:

[0054] S11, providing 2,2,3,3-tetrafluoro-1,4-butanediol and 1,6-hexamethylene diisocyanate, mixing and reacting to form a polyurethane prepolymer;

[0055] S12, providing polyethylene and trihydroxymethyl phosphorus oxide, mixing with the polyurethane prepolymer, and reacting, wherein the trihydroxymethyl phosphorus oxide and the polyurethane prepolymer react to form a cross-linked polyurethane, and the polyethylene and the cross-linked polyurethane react to form a polyethylene-based composite coating.

[0056] The preparation method of the polyethylene-based composite coating provided in the present application has a high content of fluorine in 2,2,3,3-tetrafluoro-1,4-butanediol. The high fluorine content can effectively improve the surface energy of the composite coating, improve the corrosion resistance and flame retardancy of the composite coating, the raw materials are easily available, the cost is low, the operation is convenient, and a high-performance composite coating can be prepared.

[0057] In the S11:

[0058] In some embodiments, the mixing of the 2,2,3,3-tetrafluoro-1,4-butanediol and the 1,6-hexamethylene diisocyanate further comprises: adding a first solvent to form a first mixed liquid, and reacting to form a polyurethane prepolymer.

[0059] In some embodiments, the first solvent is selected from one or more of ethyl acetate, tetrahydrofuran, chlorobenzene, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, diethylene glycol dimethyl ether, dimethyl sulfoxide, acetone, acetophenone, N,N-dimethylformamide, and pyrrole.

[0060] In some embodiments, in the first mixed solution, the mass concentration of the 2,2,3,3-tetrafluoro-1,4-butanediol is 20 g / L to 50 g / L, for example, 25 g / L, 30 g / L, 35 g / L, 40 g / L, 45 g / L, or a range between any two values, etc. Within the mass concentration range, the 2,2,3,3-tetrafluoro-1,4-butanediol is facilitated to be uniformly dissolved and dispersed.

[0061] In some embodiments, in the first mixed solution, the mass concentration of the 1,6-hexamethylene diisocyanate is 100 g / L to 150 g / L, for example, 110 g / L, 120 g / L, 130 g / L, 140 g / L, or a range between any two values, etc. Within the mass concentration range, the 1,6-hexamethylene diisocyanate is conducive to uniform dissolution and dispersion.

[0062] In some embodiments, the mass ratio of the 2,2,3,3-tetrafluoro-1,4-butanediol to the 1,6-hexamethylene diisocyanate is (4-10): (10-15), for example, 5:15, 6:10, 7:12, 8:13, or a range between any two ratios. Within the mass ratio range, the 2,2,3,3-tetrafluoro-1,4-butanediol and the 1,6-hexamethylene diisocyanate are conducive to the efficient reaction to generate a polyurethane prepolymer, thereby improving the product yield.

[0063] In some embodiments, the reaction temperature of the 2,2,3,3-tetrafluoro-1,4-butanediol and the 1,6-hexamethylene diisocyanate is 50°C to 80°C, for example, 60°C, 70°C, 80°C, or any range between two values, etc.; the reaction time is 2h to 5h, for example, 2.5h, 3h, 3.5h, 4h, 4.5h, or any range between two values, etc. In this way, under the reaction conditions, it is beneficial to promote the reaction of the 2,2,3,3-tetrafluoro-1,4-butanediol and the 1,6-hexamethylene diisocyanate and improve the reaction efficiency.

[0064] In some embodiments, the polyurethane prepolymer has a structural formula as shown below:

[0065] ;

[0066] in, for .

[0067] In some embodiments, the reaction equation of the 2,2,3,3-tetrafluoro-1,4-butanediol and the 1,6-hexamethylene diisocyanate is as shown below:

[0068] ;

[0069] in, for .

[0070] In S12:

[0071] In some embodiments, the polyethylene is high-density polyethylene (HDPE). High-density polyethylene has good hardness, tensile strength, creep, wear resistance, electrical insulation, toughness and cold resistance, as well as high chemical stability.

[0072] In some embodiments, providing polyethylene includes: providing a polyethylene dispersion, wherein the polyethylene dispersion includes the polyethylene and a second solvent.

[0073] In some embodiments, the second solvent is selected from one or more of benzene, toluene, xylene, dimethyl ether, tetraethylene glycol dimethyl ether, n-octane, isooctane, n-hexane, and cyclohexane.

[0074] In some embodiments, in the polyethylene dispersion, the mass concentration of the polyethylene is 100 g / L to 500 g / L, for example, 150 g / L, 200 g / L, 250 g / L, 300 g / L, 350 g / L, or a range between any two values, etc. Within the molar concentration range, the polyethylene is conducive to uniform dissolution and dispersion.

[0075] It should be noted that the polyethylene dispersion can be obtained by adding polyethylene to the second solvent. In order to promote the dissolution of polyethylene, it can be stirred under heating. The heating temperature is 80°C to 120°C, for example, it can be 85°C, 90°C, 105°C, 110°C, 115°C or a range between any two values.

[0076] In some embodiments, the preparation method further comprises providing a catalyst and mixing it with the polyethylene, the tris(hydroxymethyl)phosphine oxide, and the polyurethane prepolymer.

[0077] Further, the catalyst includes a tin catalyst. The tin catalyst includes one or more of dibutyltin dilaurate, dibutyltin dilaurate, stannous octoate, di(dodecylsulfide) dibutyltin, and dibutyltin diacetate. The catalyst can accelerate the reaction efficiency.

[0078] In some embodiments, the mass ratio of the polyethylene, the trihydroxymethylphosphine oxide and the polyurethane prepolymer is (10-50): (1-5): (14-25), for example, 15:2:15, 20:3:18, 25:4:18, 30:5:20, 40:2:20 or a range between any two ratios. Within the mass ratio range, the trihydroxymethylphosphine oxide and the polyurethane prepolymer are conducive to fully reacting to form a cross-linked polyurethane, and the cross-linked polyurethane and the polyethylene are conducive to reacting to form a composite coating, thereby improving the product yield.

[0079] In some embodiments, the reaction of the polyethylene, the trishydroxymethylphosphine oxide, and the polyurethane prepolymer includes a first heat treatment and a second heat treatment performed sequentially.

[0080] Furthermore, the temperature of the first heat treatment is 50°C~100°C, for example, it can be 60°C, 70°C, 80°C, 90°C or a range between any two values, etc.; the reaction time is 5min~30min, for example, it can be 10min, 15min, 20min, 25min or a range between any two values, etc.

[0081] The temperature of the second heat treatment is 100°C~150°C, for example, it can be 110°C, 120°C, 130°C, 140°C or a range between any two values; the reaction time is 1h~5h, for example, it can be 2h, 3h, 4h or a range between any two values.

[0082] Thus, the first heat treatment can promote the full and uniform mixing of the components, and the second heat treatment can promote the cross-linking reaction of the components to generate a composite coating with a semi-interpenetrating network structure.

[0083] In some embodiments, the cross-linked polyurethane has the following structural formula:

[0084] ;

[0085] in, for , for .

[0086] In a third aspect, the present application also provides an application of the polyethylene-based composite coating prepared by the above-mentioned preparation method.

[0087] Specifically, a steel pipe includes a coating, and the material of the coating includes the above-mentioned composite coating, or includes the composite coating prepared by the above-mentioned preparation method.

[0088] In some embodiments, the thickness of the coating is 0.2 mm to 0.5 mm, for example, it can be 0.25 mm, 0.3 mm, 0.35 mm, 0.4 mm, 0.45 mm or a range between any two values.

[0089] Preferably, the thickness of the coating is 0.25 mm to 0.3 mm.

[0090] It should be noted that, in the process of preparing the composite coating, after the first heat treatment, it can be spin-coated into a wet film layer of a desired thickness, and then subjected to a second heat treatment to obtain a coating.

[0091] The present application is described in detail below through specific embodiments. The following embodiments are only partial embodiments of the present application and are not limitations of the present application.

[0092] Embodiment 1: This embodiment provides a polyethylene-based composite coating, and its preparation method is as follows:

[0093] Step 1: Weigh 6.5 g of 2,2,3,3-tetrafluoro-1,4-butanediol and 50 g of anhydrous tetrahydrofuran into a 100 mL three-necked flask, heat to 60° C., stir evenly, then add 10.1 g of 1,6-hexamethylene diisocyanate, pass nitrogen gas for protection, react at 60° C. for 4 h, and form an isocyanate-terminated polyurethane prepolymer with an average molecular weight of 823;

[0094] Step 2: Weigh 43.2 g of high-density polyethylene (HDPE HMA016, purchased from ExxonMobil Chemical Co., Ltd.) and add it to 100 mL of xylene, heat it to 100 °C, and stir until the HDPE is completely dissolved;

[0095] Step 3: Pour the polyurethane prepolymer solution obtained in step 1 into the HDPE / xylene solution obtained in step 2, stir and mix evenly, add 1.9g of trihydroxymethylphosphine oxide and 0.2g of dibutyltin dilaurate at 60°C, stir evenly, react at 60°C for 10 minutes, pour the reaction solution on a stainless steel sheet, and apply it evenly with a scraper. The coating thickness is 0.3mm. Heat treat at 120°C for 2h to remove xylene to obtain a coating with a semi-interpenetrating network. The coating material includes a composite coating.

[0096] Example 2: This example is basically the same as Example 1, except that the mass of the high-density polyethylene in this example is 27.8 g.

[0097] Example 3: This example is basically the same as Example 1, except that the mass of the high-density polyethylene in this example is 18.5 g.

[0098] Example 4: This example is basically the same as Example 1, except that the mass of the high-density polyethylene in this example is 12.3 g.

[0099] Comparative Example: This comparative example provides a coating, and the preparation method is as follows:

[0100] Weigh 43.2 g of high-density polyethylene (HDPE HMA016, purchased from ExxonMobil Chemical Co., Ltd.) and add it to 100 mL of xylene, heat it to 100 °C, and stir until the HDPE is completely dissolved;

[0101] Pour the HDPE / xylene solution on a stainless steel sheet and apply it evenly with a scraper. The coating thickness is 0.3 mm. Heat treat at 120° C. for 2 h to remove the xylene to obtain a coating. The coating material includes a polyethylene coating.

[0102] The water contact angle, corrosion resistance and flame retardancy of the coatings of Examples 1 to 4 and the comparative example were tested. The test results are shown in Table 1.

[0103] Among them, the water contact angle test is carried out in accordance with the standard GB / T 30693-2014;

[0104] According to the standard GB / T 10125-2012 "Artificial atmosphere corrosion experiment salt spray test", the corrosion resistance test uses acetic acid salt spray (AASS), the pH of the salt solution is controlled at 3.0~3.1, the pH value of the collected liquid in the salt spray box is 3.1~3.3, the test cycle is 240h, and the corrosion resistance is evaluated by the mass loss before and after the test;

[0105] The flame retardant performance test is carried out in accordance with standard UL94;

[0106] .

[0107] It can be seen from Table 1 that the water contact angles of the coatings provided in Examples 1 to 4 are all greater than those in the comparative example, wherein the water contact angle of Example 4 is the largest, which is 169°. This is because a high content of fluorine is introduced to increase the surface energy. The corrosion resistance and flame retardancy of the coatings provided in Examples 1 to 4 are far superior to those in the comparative example, wherein the mass loss before and after the salt spray test in Example 4 is the smallest, which is only 1.5%. The flame retardancy grades of both Examples 3 and 4 are V-1, which have good flame retardancy.

[0108] The technical solutions provided by the embodiments of the present application are introduced in detail above. Specific examples are used herein to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the method of the present application and its core idea. At the same time, for technical personnel in this field, according to the idea of ​​the present application, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as a limitation on the present application.

Claims

1. A method for preparing a polyethylene-based composite coating, characterized in that: The steps include: Providing 2,2,3,3-tetrafluoro-1,4-butanediol and 1,6-hexamethylene diisocyanate, mixing and reacting them to form a polyurethane prepolymer, wherein the mass ratio of the 2,2,3,3-tetrafluoro-1,4-butanediol to the 1,6-hexamethylene diisocyanate is (4-10): (10-15), the reaction temperature of the 2,2,3,3-tetrafluoro-1,4-butanediol and the 1,6-hexamethylene diisocyanate is 50° C. to 80° C., and the reaction time is 2 h to 5 h; Providing polyethylene and tris(hydroxymethyl)phosphine oxide, mixing with the polyurethane prepolymer, and reacting, wherein the tris(hydroxymethyl)phosphine oxide and the polyurethane prepolymer react to form a cross-linked polyurethane, and the mass ratio of the polyethylene, the tris(hydroxymethyl)phosphine oxide and the polyurethane prepolymer is (10-50):(1-5):(14-25); The reaction of the polyethylene, the trimethylolphosphine oxide and the polyurethane prepolymer includes a first heat treatment and a second heat treatment performed in sequence; the temperature of the first heat treatment is 50°C to 100°C, and the reaction time is 5min to 30min; the temperature of the second heat treatment is 100°C to 150°C, and the reaction time is 1h to 5h; Wherein, the structural formula of the cross-linked polyurethane is shown as follows: ； Among them, R2 is , R1 is .

2. The preparation method according to claim 1, characterized in that: The mixing of the 2,2,3,3-tetrafluoro-1,4-butanediol and the 1,6-hexamethylene diisocyanate further comprises: adding a first solvent to form a first mixed liquid, reacting to form a polyurethane prepolymer.

3. The preparation method according to claim 2, characterized in that: The first solvent is selected from one or more of ethyl acetate, tetrahydrofuran, chlorobenzene, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, diethylene glycol dimethyl ether, dimethyl sulfoxide, acetone, acetophenone, N,N-dimethylformamide, and pyrrole; and / or In the first mixed solution, the mass concentration of the 2,2,3,3-tetrafluoro-1,4-butanediol is 20 g / L to 50 g / L; and / or In the first mixed solution, the mass concentration of 1,6-hexamethylene diisocyanate is 100 g / L to 150 g / L.

4. The preparation method according to claim 2, characterized in that: The structural formula of the polyurethane prepolymer is shown below: ； Among them, R1 is .

5. The preparation method according to claim 2, characterized in that: The providing of polyethylene includes: providing a polyethylene dispersion, wherein the polyethylene dispersion includes the polyethylene and a second solvent.

6. The preparation method according to claim 5, characterized in that: The polyethylene is high-density polyethylene; and / or The second solvent is selected from one or more of benzene, toluene, xylene, dimethyl ether, tetraethylene glycol dimethyl ether, n-octane, isooctane, n-hexane, and cyclohexane; and / or In the polyethylene dispersion, the mass concentration of the polyethylene is 100 g / L to 500 g / L.

7. The preparation method according to claim 2, characterized in that: The preparation method also includes providing a catalyst and mixing it with the polyethylene, the trimethylolphosphine oxide and the polyurethane prepolymer; the catalyst includes a tin catalyst; the tin catalyst includes one or more of dibutyltin dilaurate, stannous octoate, dibutyltin didodecylsulfide and dibutyltin diacetate.

8. A steel pipe, characterized in that: The steel pipe includes a coating, and the coating includes a composite coating prepared by the preparation method according to any one of claims 1 to 7.

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