A coated steel pipe and its preparation method

Abstract

The present invention relates to the technical field of plastic-coated steel pipes, and provides a plastic-coated steel pipe and a preparation method thereof. A plastic-coated steel pipe includes a steel pipe and an anti-corrosion layer disposed on the outer surface of the steel pipe. The anti-corrosion layer includes a fusion-bonded epoxy layer, an adhesive layer, and a polyethylene layer arranged in sequence from the inside to the outside. The raw materials of the polyethylene layer include the following components in parts by mass: 100-110 parts of high-density polyethylene, 20-28 parts of zinc oxide, 6-10 parts of a lubricant, 1-4 parts of an antioxidant, and 6-8 parts of an auxiliary agent. The auxiliary agent includes poly-p-phenylene terephthalamide fiber and polyethylene terephthalate powder. Through the above technical solution, the problem of poor chemical corrosion resistance of the plastic-coated steel pipe in the related art is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of plastic-coated steel pipes, and specifically, to a plastic-coated steel pipe and a preparation method thereof. Background Art

[0002] In the field of industrial fluid transportation, as a typical representative of steel-plastic composite pipes, plastic-coated steel pipes can take into account both the high strength of steel and the corrosion resistance of the plastic anti-corrosion layer. However, they still face multiple technical bottlenecks in extreme chemical environments. The polyethylene layer of existing plastic-coated steel pipes is prone to swelling, interfacial peeling or molecular chain degradation in media such as strong acids, strong alkalis, and organic solvents, resulting in the failure of protection. For example, when the high-density polyethylene in the raw material of the polyethylene layer is in long-term contact with aromatic solvents, the polyethylene layer will soften due to the diffusion of molecular chains; while in a high-temperature alkaline environment, the hydrolysis reaction of ester bonds in the epoxy resin layer will significantly shorten its service life.

[0003] The complexity of soil corrosion further exacerbates the service risk of the polyethylene layer of plastic-coated steel pipes. Buried pipes not only need to cope with the differences in physical and chemical factors such as soil conductivity, water content, and pH value, but also need to resist the erosion of microorganisms and stray currents; in the fields of chemical engineering, petroleum, etc., plastic-coated steel pipes often cause brittle fractures due to the combined action of tensile stress and corrosive media caused by the corrosion of the polyethylene layer. In the field of ocean engineering, the high concentration of chloride ions, microorganisms in seawater, and the wet-dry alternation in the tidal zone significantly accelerate the aging and corrosion of the polyethylene layer.

[0004] In summary, the existing plastic-coated steel pipes have insufficient chemical corrosion resistance, and it is of crucial significance to develop a plastic-coated steel pipe with improved chemical corrosion resistance. Summary of the Invention

[0005] The present invention provides a plastic-coated steel pipe and a preparation method thereof, which solve the problem of poor chemical corrosion resistance of plastic-coated steel pipes in related technologies.

[0006] The technical solution of the present invention is as follows:

[0007] The present invention provides a plastic-coated steel pipe, including a steel pipe and an anti-corrosion layer provided on the outer surface of the steel pipe. The anti-corrosion layer includes a fusion-bonded epoxy layer, an adhesive layer, and a polyethylene layer arranged in sequence from the inside to the outside;

[0008] The raw material of the polyethylene layer includes the following components in parts by mass:

[0009] 100-110 parts of high-density polyethylene, 20-28 parts of zinc oxide, 6-10 parts of lubricant, 1-4 parts of antioxidant, 6-8 parts of additive;

[0010] The additive includes poly-p-phenylene terephthalamide fiber and polyethylene terephthalate powder.

[0011] When a lubricant is added to the polyethylene layer of the plastic - coated steel pipe of the present invention, the lubricant molecules can be inserted between the polyethylene molecular chains. Due to the relatively weak intermolecular force between them, the mutual friction and entanglement between the polyethylene molecular chains are reduced, the internal frictional force between polymer molecules is lowered, making the polyethylene molecular chains move more smoothly during relative movement. As a result, the viscosity of the material is reduced, and the fluidity of the polyethylene layer during the processing is improved. In addition, adding a lubricant can also improve the interfacial lubrication between the polymer and the surface of the processing equipment. When the plastic - coated steel pipe is extruded, the lubricant will form a lubricating film between the polymer and the equipment surface, thereby reducing the adhesion force between the polymer and the equipment surface, making the material flow more smoothly in the equipment, reducing the accumulation and hanging of the material on the equipment surface, and ensuring the continuity and stability of the processing process.

[0012] During the processing of the plastic - coated steel pipe, such as in the extrusion process, polyethylene is affected by factors such as high temperature and mechanical shear force, and is prone to oxidation reactions. Therefore, the present invention adds an antioxidant to the polyethylene layer of the plastic - coated steel pipe. The antioxidant can quickly play a role during the processing to inhibit the oxidation of polyethylene caused by high temperature and mechanical action. During the extrusion process, when the polyethylene material is pushed by the screw through the high - temperature barrel, the antioxidant can timely capture the free radicals generated by high temperature, prevent the breakage and degradation of the polyethylene molecular chains, avoid the deterioration of the material properties caused by oxidation, thereby ensuring the smooth progress of the processing process, improving production efficiency, and reducing product defects caused by oxidation during the processing process.

[0013] As a further technical solution, the particle size of the polyethylene terephthalate powder is 800 mesh.

[0014] In the present invention, high - density polyethylene is used as the base material of the polyethylene layer of the plastic - coated steel pipe. It has a highly regular linear molecular chain, which promotes the formation of a relatively high degree of crystallinity. The high crystallinity endows the polyethylene layer with good strength, toughness and wear resistance. In the plastic - coated steel pipe, high - density polyethylene provides basic physical protection for the steel pipe, enabling it to effectively resist external physical actions such as friction and impact.

[0015] As a further technical solution, the raw materials of the fusion - bonded epoxy layer include the following components in parts by mass: 50 - 60 parts of epoxy resin, 20 - 30 parts of ethylene - vinyl acetate copolymer, 10 - 20 parts of petroleum resin, 20 - 30 parts of titanium hydride, 2 - 5 parts of benzoin, and 1 - 4 parts of adipic dihydrazide.

[0016] As a further technical solution, the raw materials of the adhesive layer include the following components in parts by mass: 15 - 45 parts of ethylene acrylic resin, 8 - 14 parts of polyvinyl butyral, 10 - 20 parts of polyisobutylene, and 1 - 3 parts of dioctyl phthalate.

[0017] As a further technical solution, the mass ratio of the poly(p-phenylene terephthalamide) fiber to the polyethylene terephthalate powder is 7-9:1.

[0018] In the present invention, the mass ratio of the poly(p-phenylene terephthalamide) fiber to the polyethylene terephthalate powder can be 7:1, 7.1:1, 7.2:1, 7.3:1, 7.4:1, 7.5:1, 7.6:1, 7.7:1, 7.8:1, 7.9:1, 8:1, 8.1:1, 8.2:1, 8.3:1, 8.4:1, 8.5:1, 8.6:1, 8.7:1, 8.8:1, 8.9:1, 9:1, preferably 8:1.

[0019] In the polyethylene layer of the plastic-coated steel pipe of the present invention, if the mass ratio of the poly(p-phenylene terephthalamide) fiber to the polyethylene terephthalate powder is higher or lower than 7-9:1, the chemical corrosion resistance of the plastic-coated steel pipe will decrease. When the mass ratio of the poly(p-phenylene terephthalamide) fiber to the polyethylene terephthalate powder is 7-9:1, the chemical corrosion resistance of the plastic-coated steel pipe can be greatly improved. Due to the interaction between the poly(p-phenylene terephthalamide) fiber, the polyethylene terephthalate powder and the high-density polyethylene, a denser microstructure is jointly formed. Among them, the rigid chain segment of the poly(p-phenylene terephthalamide) fiber plays a skeleton support role in the polyethylene matrix, making the molecular arrangement more orderly and reducing the voids between molecules. The polyethylene terephthalate powder fills the voids formed by the polyethylene and the poly(p-phenylene terephthalamide) fiber by virtue of its film-forming property, further densifying the microstructure and effectively blocking the penetration of chemical substances, thereby improving the chemical corrosion resistance of the plastic-coated steel pipe.

[0020] As a further technical solution, the zinc oxide is modified zinc oxide;

[0021] The raw materials of the modified zinc oxide include zinc oxide, a silane coupling agent and 5-carboxybenzotriazole. [[ID=—13]]

[0022] As a further technical solution, the silane coupling agent includes one of KH550, KH560, KH570, preferably KH550.

[0023] In the polyethylene layer of the plastic-coated steel pipe of the present invention, the raw materials of the modified zinc oxide include zinc oxide, silane coupling agent and 5-carboxybenzotriazole. Zinc oxide itself is an inorganic compound with excellent ultraviolet absorption performance, but it has poor compatibility with the organic polymer material polyethylene. The molecular structure of the silane coupling agent contains two different types of groups: one end is an active group that can chemically react with the surface of inorganic substances; the other end is an organic group that can physically or chemically react with organic substances. During the preparation of the modified zinc oxide, one end of the silane coupling agent chemically reacts with the hydroxyl groups on the surface of zinc oxide to form chemical bonds, thereby grafting the silane coupling agent onto the surface of zinc oxide. The other end of the silane coupling agent is connected to 5-carboxybenzotriazole, thus obtaining the modified zinc oxide. The modification process changes the surface properties of zinc oxide, converting it from hydrophilic to organophilic, greatly enhancing the compatibility between zinc oxide and the polyethylene matrix. When the modified zinc oxide is added to the polyethylene layer, it can be better dispersed among polyethylene molecules, forming a more uniform microstructure, improving the ultraviolet resistance of the polyethylene layer. Moreover, after being irradiated by ultraviolet rays, the photogenerated electrons generated inside zinc oxide will be affected by the "withdrawal effect" of the unsaturated groups in 5-carboxybenzotriazole and transfer from zinc oxide to 5-carboxybenzotriazole, increasing the absorption of ultraviolet rays by zinc oxide, and thus improving the ultraviolet resistance of the plastic-coated steel pipe.

[0024] As a further technical solution, the preparation method of the modified zinc oxide includes the following steps:

[0025] A1. Add zinc oxide to ethanol, ultrasonically disperse it evenly, then add the silane coupling agent, stir, and dry to obtain silane-modified zinc oxide;

[0026] A2. Add 5-carboxybenzotriazole to ethanol for the first mixing, then add the silane-modified zinc oxide for the second mixing, concentrate, and dry to obtain the modified zinc oxide.

[0027] As a further technical solution, the mass-volume ratio of the zinc oxide to the ethanol in A1 is 1 g:8 - 10 mL.

[0028] As a further technical solution, the mass-volume ratio of the 5-carboxybenzotriazole to the ethanol in A2 is 1 g:15 - 20 mL.

[0029] As a further technical solution, the mass ratio of the zinc oxide to the silane coupling agent is 1:0.8 - 1;

[0030] The mass ratio of the zinc oxide to the 5-carboxybenzotriazole is 1:0.7 - 0.9.

[0031] In the polyethylene layer of the plastic - coated steel pipe of the present invention, during the preparation of modified zinc oxide, when the mass ratio of zinc oxide to silane coupling agent is 1:0.8 - 1, the silane coupling agent molecules can form a uniform modified layer on the surface of zinc oxide. If the content of the silane coupling agent is too low, it will lead to insufficient modification, poor compatibility between zinc oxide and polyethylene, and easy agglomeration in the polyethylene layer, affecting the performance of the polyethylene layer. If the content is too high, excessive silane coupling agent will undergo side reactions such as self - polymerization in the system, not only wasting raw materials but also damaging the microstructure of the polyethylene layer and reducing the physical properties of the polyethylene layer. When the mass ratio of zinc oxide to 5 - carboxybenzotriazole is 1:0.7 - 0.9, the two can achieve good synergistic effects in properties such as ultraviolet resistance. 5 - carboxybenzotriazole and zinc oxide cooperate with each other, broadening the ultraviolet absorption range and improving the overall ultraviolet resistance efficiency. If the content of 5 - carboxybenzotriazole is too low, it cannot fully compensate for the deficiency of zinc oxide in ultraviolet absorption, and the ultraviolet resistance performance of the polyethylene layer is not significantly improved. If the content is too high, it will affect other properties of the polyethylene layer, such as thermal stability and processing performance, resulting in a decline in the performance of the polyethylene layer.

[0032] As a further technical solution, during the stirring, the temperature is 55 - 65°C, the time is 10 - 12 h, and the rotation speed is 7000 - 8000 rpm.

[0033] In the polyethylene layer of the plastic - coated steel pipe of the present invention, during the preparation of modified zinc oxide, the temperature is in the range of 55 - 65°C. Within this temperature range, the reaction activity of silane coupling agent molecules with the surface hydroxyl groups of zinc oxide is greatly improved, promoting the two to undergo chemical reactions more efficiently, forming stable chemical bonds, and achieving surface modification of zinc oxide. The stirring rotation speed is maintained at 7000 - 8000 rpm, which can ensure that the reactants are fully mixed in the system, promoting the uniform dispersion of the silane coupling agent in the ethanol solution containing zinc oxide, and greatly increasing the collision probability between the silane coupling agent and zinc oxide particles.

[0034] As a further technical solution, during the first mixing, the temperature is 25 - 35°C and the time is 3 - 4 h.

[0035] As a further technical solution, during the second mixing, the temperature is 20 - 30°C and the time is 20 - 24 h.

[0036] As a further technical solution, the lubricant includes one or more of zinc stearate, calcium stearate, and polyethylene wax.

[0037] In the polyethylene layer of the plastic - coated steel pipe of the present invention, the lubricant can be any one or more of conventional lubricants, and can be one or more of magnesium stearate, barium stearate, oxidized polyethylene wax, zinc stearate, calcium stearate, and polyethylene wax. Preferably, it is one or more of zinc stearate, calcium stearate, and polyethylene wax.

[0038] As a further technical solution, the antioxidant includes one or more of antioxidant 1010, antioxidant 1076, and antioxidant 168.

[0039] In the polyethylene layer of the plastic-coated steel pipe of the present invention, the antioxidant can be any one or more of conventional antioxidants, and can be one or more of antioxidant 1098, antioxidant 3114, antioxidant 626, antioxidant 126, antioxidant 1010, antioxidant 1076, and antioxidant 168, preferably one or more of antioxidant 1010, antioxidant 1076, and antioxidant 168.

[0040] The present invention also provides a method for manufacturing a plastic-coated steel pipe for manufacturing the plastic-coated steel pipe described above, including the following steps:

[0041] S1. Blend the raw materials of the fusion-bonded epoxy layer, extrude, and pulverize to obtain epoxy layer powder;

[0042] S2. Fusibly bond the epoxy layer powder to the outer surface of the steel pipe to obtain a fusion-bonded epoxy layer;

[0043] S3. Blend the raw materials of the adhesive layer, extrude, and pulverize to obtain adhesive layer powder; lay the adhesive layer powder on the surface of the fusion-bonded epoxy layer to obtain an adhesive layer;

[0044] S4. Mix and extrude the components of the raw materials of the polyethylene layer to obtain a polyethylene layer, lay the polyethylene layer on the adhesive layer, and apply pressure for lamination to obtain the plastic-coated steel pipe.

[0045] The working principle and beneficial effects of the present invention are as follows:

[0046] The present invention uses poly(p-phenylene terephthalamide) fiber and polyethylene terephthalate powder as additives and applies them to the polyethylene anti-corrosion layer of plastic-coated steel pipes, improving the chemical corrosion resistance of plastic-coated steel pipes. In the prior art, natural plant fibers are generally used to improve the chemical corrosion resistance. However, there is a chemical structure with multiple hydroxyl groups in natural plant fibers, which leads to easy agglomeration of natural plant fibers. In the present invention, this problem is avoided by first using poly(p-phenylene terephthalamide) fiber. Due to its regular and rigid molecular chain structure and strong hydrogen bond interaction between molecular chains, poly(p-phenylene terephthalamide) fiber forms a stable "molecular reinforcement framework" within the polyethylene layer. When the plastic-coated steel pipe encounters chemical substance erosion, this "framework" can effectively block the infiltration of chemical substance molecules. At the same time, the hydrogen bonds promote the polyethylene molecules to be arranged more closely and orderly, greatly reducing the gaps that chemical substances can penetrate. Meanwhile, polyethylene terephthalate powder is added to cooperate with poly(p-phenylene terephthalamide) fiber. Polyethylene terephthalate powder has good compatibility with polyethylene and can be evenly distributed in the polyethylene matrix, filling the tiny gaps between poly(p-phenylene terephthalamide) fibers and polyethylene molecules, further blocking the diffusion path of chemical substances and enhancing the overall structural strength of the polyethylene layer. Moreover, the ester groups in the molecular structure of polyethylene terephthalate powder can resist the attack of some chemical substances, making the polyethylene layer more stable in a chemical environment. Therefore, the joint addition of poly(p-phenylene terephthalamide) fiber and polyethylene terephthalate powder to the polyethylene layer can effectively resist the penetration and erosion of chemical substances, significantly improving the chemical corrosion resistance. The plastic-coated steel pipes prepared by the present invention are more conducive to being applied in scenarios with complex chemical environments such as chemical engineering and sewage treatment, with stronger adaptability and corrosion resistance, reducing problems such as pipeline damage and leakage caused by corrosion, lowering the maintenance cost, and extending the service life of the pipeline. Detailed implementation manners

[0047] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts fall within the scope of protection of the present invention.

[0048] In the following examples and comparative examples, the high-density polyethylene has the model number HE2550, the particle size of zinc oxide is 20 nm, the length of the poly(p-phenylene terephthalamide) fiber is 3 mm, the thickness is 1.5 D, the model number is kevlar 1414, the model number of the polyethylene terephthalate powder is WB-8828, the particle size is 800 mesh, the model number of the epoxy resin is SM601, the model number of the ethylene-vinyl acetate copolymer is EVA910, the model number of the petroleum resin is C9 petroleum resin, the model number of the ethylene acrylic resin is EAA3440, the model number of the polyvinyl butyral is B60H, the model number of the polyisobutene is HRD850, the length of the poplar fiber is 149 μm, and the aspect ratio is 3.22.

[0049] Example 1

[0050] A plastic-coated steel pipe includes a steel pipe and an anti-corrosion layer provided on the outer surface of the steel pipe. The anti-corrosion layer includes a fusion-bonded epoxy layer, an adhesive layer, and a polyethylene layer arranged from the inside to the outside. The raw materials of the polyethylene layer include the following components in parts by mass:

[0051] 110 parts of high-density polyethylene, 28 parts of zinc oxide, 10 parts of zinc stearate, 4 parts of antioxidant 1010, 8 parts of auxiliary agent;

[0052] The auxiliary agent includes poly(p-phenylene terephthalamide) fiber and polyethylene terephthalate powder with a mass ratio of 11:1;

[0053] The raw materials of the fusion-bonded epoxy layer include the following components in parts by mass: 60 parts of epoxy resin, 30 parts of ethylene-vinyl acetate copolymer, 20 parts of petroleum resin, 30 parts of titanium hydride, 5 parts of benzoin, 4 parts of adipic dihydrazide;

[0054] The raw materials of the adhesive layer include the following components in parts by mass: 45 parts of ethylene acrylic resin, 14 parts of polyvinyl butyral, 20 parts of polyisobutene, 3 parts of dioctyl phthalate;

[0055] The preparation method of the plastic-coated steel pipe includes the following steps:

[0056] S1. Blend the raw materials of the fusion-bonded epoxy layer, extrude, and pulverize to obtain epoxy layer powder;

[0057] S2. Fusibly bond the epoxy layer powder to the outer surface of the steel pipe to obtain a fusion-bonded epoxy layer;

[0058] S3. Blend the raw materials of the adhesive layer, extrude, and pulverize to obtain adhesive layer powder; lay the adhesive layer powder on the surface of the fusion-bonded epoxy layer to obtain an adhesive layer;

[0059] S4. Mix and extrude the components of the raw materials of the polyethylene layer to obtain a polyethylene layer, lay the polyethylene layer on the adhesive layer, and press and compound to obtain the plastic-coated steel pipe.

[0060] Example 2

[0061] A coated steel pipe includes a steel pipe and an anti-corrosion layer provided on the outer surface of the steel pipe. The anti-corrosion layer includes a fusion-bonded epoxy layer, an adhesive layer, and a polyethylene layer arranged from inside to outside. The raw materials of the polyethylene layer include the following components in parts by mass:

[0062] 100 parts of high-density polyethylene, 20 parts of zinc oxide, 2 parts of zinc stearate, 2 parts of calcium stearate, 2 parts of polyethylene wax, 0.5 part of antioxidant 1076, 0.5 part of antioxidant 168, 6 parts of auxiliary agent;

[0063] The auxiliary agent includes poly-p-phenylene terephthalamide fiber and polyethylene terephthalate powder with a mass ratio of 5:1;

[0064] The raw materials of the fusion-bonded epoxy layer include the following components in parts by mass: 50 parts of epoxy resin, 20 parts of ethylene-vinyl acetate copolymer, 10 parts of petroleum resin, 20 parts of titanium hydride, 2 parts of benzoin, 1 part of adipic dihydrazide.

[0065] The raw materials of the adhesive layer include the following components in parts by mass: 15 parts of ethylene acrylic resin, 8 parts of polyvinyl butyral, 10 parts of polyisobutylene, 1 part of dioctyl phthalate;

[0066] A preparation method of the coated steel pipe includes the following steps:

[0067] S1. Blend the raw materials of the fusion-bonded epoxy layer, extrude, and pulverize to obtain epoxy layer powder;

[0068] S2. Fusibly bond the epoxy layer powder to the outer surface of the steel pipe to obtain a fusion-bonded epoxy layer;

[0069] S3. Blend the raw materials of the adhesive layer, extrude, and pulverize to obtain adhesive layer powder; Lay the adhesive layer powder on the surface of the fusion-bonded epoxy layer to obtain an adhesive layer;

[0070] S4. Mix and extrude the components of the raw materials of the polyethylene layer to obtain a polyethylene layer, lay the polyethylene layer on the adhesive layer, and apply pressure for compounding to obtain the coated steel pipe.

[0071] Example 3

[0072] A coated steel pipe includes a steel pipe and an anti-corrosion layer provided on the outer surface of the steel pipe. The anti-corrosion layer includes a fusion-bonded epoxy layer, an adhesive layer, and a polyethylene layer arranged from inside to outside. The raw materials of the polyethylene layer include the following components in parts by mass:

[0073] 105 parts of high-density polyethylene, 24 parts of zinc oxide, 8 parts of polyethylene wax, 2 parts of antioxidant 168, 7 parts of auxiliary agent;

[0074] The auxiliary agent includes poly-p-phenylenediamine terephthalate fiber and polyethylene terephthalate powder with a mass ratio of 10:1;

[0075] The raw materials of the fusion-bonded epoxy layer include the following components in parts by mass: 55 parts of epoxy resin, 25 parts of ethylene-vinyl acetate copolymer, 15 parts of petroleum resin, 25 parts of titanium hydride, 4 parts of benzoin, and 2 parts of adipic dihydrazide;

[0076] The raw materials of the adhesive layer include the following components in parts by mass: 35 parts of ethylene acrylic resin, 9 parts of polyvinyl butyral, 15 parts of polyisobutene, and 2 parts of dioctyl phthalate;

[0077] The preparation method of the plastic-coated steel pipe includes the following steps:

[0078] S1. Blend the raw materials of the fusion-bonded epoxy layer, extrude, and pulverize to obtain epoxy layer powder;

[0079] S2. Fusibly bond the epoxy layer powder to the outer surface of the steel pipe to obtain a fusion-bonded epoxy layer;

[0080] S3. Blend the raw materials of the adhesive layer, extrude, and pulverize to obtain adhesive layer powder; lay the adhesive layer powder on the surface of the fusion-bonded epoxy layer to obtain an adhesive layer;

[0081] S4. Mix and extrude the components of the polyethylene layer raw materials to obtain a polyethylene layer, lay the polyethylene layer on the adhesive layer, and press and compound to obtain a plastic-coated steel pipe.

[0082] Example 4

[0083] The difference between this example and Example 3 is only that the mass ratio of poly-p-phenylenediamine terephthalate fiber and polyethylene terephthalate powder in this example is 6:1.

[0084] Example 5

[0085] The difference between this example and Example 3 is only that the mass ratio of poly-p-phenylenediamine terephthalate fiber and polyethylene terephthalate powder in this example is 7:1.

[0086] Example 6

[0087] The difference between this example and Example 3 is only that the mass ratio of poly-p-phenylenediamine terephthalate fiber and polyethylene terephthalate powder in this example is 8:1.

[0088] Example 7

[0089] The difference between this example and Example 3 is only that the mass ratio of poly-p-phenylenediamine terephthalate fiber and polyethylene terephthalate powder in this example is 9:1.

[0090] Example 8

[0091] The difference between this example and Example 6 is only that the zinc oxide in this example is replaced with an equal mass of modified zinc oxide. The preparation method of the modified zinc oxide includes the following steps:

[0092] A1. Add zinc oxide to ethanol (the mass-volume ratio of zinc oxide to ethanol is 1 g:10 mL). After ultrasonic dispersion, add KH550 (the mass ratio of zinc oxide to KH550 is 1:1), and stir at 65 °C at a speed of 8000 rpm for 10 h, then dry to obtain KH550-modified zinc oxide;

[0093] A2. Add 5-carboxybenzotriazole to ethanol (the mass-volume ratio of 5-carboxybenzotriazole to ethanol is 1 g:20 mL), mix at 35 °C for 4 h, then add KH550-modified zinc oxide, mix at 30 °C for 20 h, concentrate and dry to obtain modified zinc oxide, where the mass ratio of zinc oxide to 5-carboxybenzotriazole is 1:0.7.

[0094] Example 9

[0095] The difference between this example and Example 6 is only that the zinc oxide in this example is replaced with an equal mass of modified zinc oxide. The preparation method of the modified zinc oxide includes the following steps:

[0096] A1. Add zinc oxide to ethanol (the mass-volume ratio of zinc oxide to ethanol is 1 g:8 mL). After ultrasonic dispersion, add KH550 (the mass ratio of zinc oxide to KH550 is 1:0.8), and stir at 55 °C at a speed of 7000 rpm for 12 h, then dry to obtain KH550-modified zinc oxide;

[0097] A2. Add 5-carboxybenzotriazole to ethanol (the mass-volume of 5-carboxybenzotriazole to ethanol is 1 g:15 mL), mix at 25 °C for 3 h, then add KH550-modified zinc oxide, mix at 20 °C for 24 h, concentrate and dry to obtain modified zinc oxide, where the mass ratio of zinc oxide to 5-carboxybenzotriazole is 1:0.9.

[0098] Comparative Example 1

[0099] The difference between this comparative example and Example 3 is only that the additive in this comparative example is replaced with an equal mass of poly(p-phenyleneterephthalamide) fiber.

[0100] Comparative Example 2

[0101] The difference between this comparative example and Example 3 is only that the additive in this comparative example is replaced with an equal mass of polyethylene terephthalate powder.

[0102] Comparative Example 3

[0103] The difference between this comparative example and Example 3 is only that there is no auxiliary agent in this comparative example.

[0104] Comparative Example 4

[0105] The difference between this comparative example and Example 3 is only that the auxiliary agent in this comparative example is replaced with poplar fibers of equal mass.

[0106] Experimental Example 1

[0107] The polyethylene layers of the coated steel pipes prepared in Examples 1 to 7 and Comparative Examples 1 to 4 were tested for impact strength according to the method specified in GB / T 1843-2008 "Determination of Plastic Izod Impact Strength". The notch was type A. Then, the samples were immersed in 30 wt% sulfuric acid for 48 h and the impact strength was tested again. The specimen size was 80 mm × 10 mm and the thickness was 4 mm. The test results are shown in Table 1.

[0108] Table 1 Test Results of Impact Strength

[0109]

[0110] As can be seen from Table 1, in the present invention, the decrease in the impact strength of the coated steel pipes prepared in Examples 1 to 7 after acid treatment is smaller than that of Comparative Examples 1 to 4, indicating that the synergistic effect of poly(p-phenylene terephthalamide) fibers and polyethylene terephthalate powder improves the chemical corrosion resistance of the coated steel pipes.

[0111] Experimental Example 2

[0112] The polyethylene layers of the coated steel pipes prepared in Example 6 and Examples 8 to 9 were tested for impact strength according to the method specified in GB / T 1843-2008 "Determination of Plastic Izod Impact Strength". The notch was type A. Then, the samples were subjected to ultraviolet treatment according to the method in GB / T 16585-1996 "Test Method for Artificial Weathering (Fluorescent Ultraviolet Lamp) of Vulcanized Rubber", and the impact strength was tested again. The ultraviolet conditions were 0.89 W / m 2 , the temperature was 50 °C, the time was 480 h, the cycle time was 4 h (4 h of ultraviolet light exposure and 4 h of condensation), the specimen size was 80 mm × 10 mm, and the thickness was 4 mm. The test results are shown in Table 2.

[0113] Table 2 Test Results of Impact Strength

[0114]

[0115] As can be seen from Table 2, in the present invention, the decrease in the impact strength of the coated steel pipes prepared in Examples 8 to 9 after ultraviolet treatment is smaller than that of Example 6. Therefore, in the present invention, the use of a silane coupling agent and 5-carboxybenzotriazole to modify zinc oxide improves the ultraviolet resistance of the coated steel pipes.

[0116] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A coated steel pipe, characterized in that, It includes a steel pipe and an anti-corrosion layer provided on the outer surface of the steel pipe. The anti-corrosion layer includes a fusion-bonded epoxy layer, an adhesive layer, and a polyethylene layer arranged in sequence from the inside to the outside; The raw materials of the polyethylene layer include the following components in parts by mass: 100 - 110 parts of high-density polyethylene, 20 - 28 parts of zinc oxide, 6 - 10 parts of lubricant, 1 - 4 parts of antioxidant, and 6 - 8 parts of auxiliary agent; The auxiliary agent includes poly-p-phenylene terephthalamide fiber and polyethylene terephthalate powder; The mass ratio of the poly-p-phenylene terephthalamide fiber to the polyethylene terephthalate powder is 7 - 9:1; The zinc oxide is modified zinc oxide; The raw materials of the modified zinc oxide include zinc oxide, silane coupling agent, and 5-carboxybenzotriazole.

2. The plastic-coated steel pipe according to claim 1, wherein The preparation method of the modified zinc oxide includes the following steps: A1. Add zinc oxide into ethanol, ultrasonically disperse it evenly, then add the silane coupling agent, stir, and dry to obtain silane-modified zinc oxide; A2. Add 5-carboxybenzotriazole into ethanol for the first mixing, then add the silane-modified zinc oxide for the second mixing, concentrate, and dry to obtain the modified zinc oxide.

3. The plastic-coated steel pipe according to claim 2, characterized in that, The mass ratio of the zinc oxide to the silane coupling agent is 1:0.8 - 1; The mass ratio of the zinc oxide to the 5-carboxybenzotriazole is 1:0.7 - 0.

9.

4. A coated steel pipe according to claim 2, characterized in that, When stirring, the temperature is 55 - 65°C, the time is 10 - 12 h, and the rotation speed is 7000 - 8000 rpm.

5. A plastic-coated steel pipe according to claim 2, characterized in that, When the first mixing is carried out, the temperature is 2 6. A plastic-coated steel pipe according to claim 2, wherein, ​ 7. A plastic-coated steel pipe according to claim 1, wherein, ​ ​ 8. A preparation method of a plastic-coated steel pipe for preparing a plastic-coated steel pipe according to any one of claims 1 to 7, characterized in that, ​ ​ ​ ​ ​

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