Anti-aging directly-buried thermal insulation pipe and preparation method thereof

Abstract

The invention relates to the technical field of heat preservation pipes, and provides an anti-aging directly-buried heat preservation pipe and a preparation method thereof.The anti-aging directly-buried heat preservation pipe sequentially comprises a steel pipe, a polyurethane heat preservation layer and a polyethylene outer protection pipe layer from inside to outside, and the polyethylene outer protection pipe layer comprises a polyethylene outer protection pipe inner layer and a polyethylene outer protection pipe outer layer; the polyethylene outer protective pipe inner layer is prepared from the following raw material components in parts by weight: 60 parts of first polyethylene and 6 to 16 parts of first carbon black; the outer layer of the polyethylene outer protective tube comprises the following raw materials in parts by weight: 60 parts of second polyethylene and 8-20 parts of second carbon black; the weight of the second carbon black is greater than that of the first carbon black; the first polyethylene and the second polyethylene respectively and independently comprise high-pressure polyethylene and low-pressure polyethylene in a weight ratio of (1: 9)-(9: 1). By means of the technical scheme, the problems that in the prior art, a polyethylene outer protection pipe layer outside a direct burial heat preservation pipe is low in mechanical strength and poor in impact resistance are solved.

Description

Technical Field

[0001] This invention relates to the field of thermal insulation pipe technology, specifically to an anti-aging direct-buried thermal insulation pipe and its preparation method. Background Technology

[0002] Direct-buried insulated pipes, as a type of high-efficiency insulated transportation pipeline, are widely used in various fields such as urban centralized heating and cooling, petrochemicals, and municipal water supply and drainage. The structure of a direct-buried insulated pipe typically consists of a working steel pipe, an insulation layer, and a polyethylene outer protective layer. The outer protective layer, as a key structure directly in contact with underground soil, moisture, microorganisms, and the external environment, plays a crucial role in protecting the insulation layer, isolating it from external corrosion and aging, and resisting mechanical impact. In particular, the anti-aging properties of the polyethylene outer protective layer are of great significance for ensuring the service life and operational safety of direct-buried insulated pipes in harsh environments.

[0003] Polyethylene (PE) is the main material for the outer protective layer of direct-buried insulated pipes, providing better matching with the internal insulation layer. However, PE itself has poor aging resistance; prolonged exposure to light or oxygen can cause the molecular chains of PE to break, reducing the aging resistance of the outer protective layer. To address the aging problem of the PE outer protective layer, carbon black is usually added. Carbon black's absorption of ultraviolet light and its shielding effect against oxygen penetration improve the stability of the PE material, thus enhancing the aging resistance of the outer protective layer to some extent. However, while the addition of carbon black improves the resistance of polyolefins to photo-oxidative aging, it reduces the mechanical strength and impact resistance of the PE outer protective layer. This is because the interfacial cohesion between carbon black and the PE matrix is ​​relatively poor, and the carbon black exhibits significant local aggregation in the entire mixture, failing to effectively disperse external stress, resulting in relatively poor mechanical strength and impact resistance of the PE outer protective layer.

[0004] Therefore, developing a direct-buried insulated pipe that not only has good anti-aging properties but also improves the mechanical strength and impact resistance of its polyethylene outer protective layer is of great significance for extending the service life of direct-buried insulated pipes in harsh environments. Summary of the Invention

[0005] This invention proposes an anti-aging direct-buried insulated pipe and its preparation method, which solves the problems of low mechanical strength and poor impact resistance of the polyethylene outer protective layer of direct-buried insulated pipes in related technologies.

[0006] The technical solution of the present invention is as follows: This invention proposes an anti-aging direct-buried insulated pipe, which includes, from the inside out, a steel pipe, a polyurethane insulation layer, and a polyethylene outer protective pipe layer. The polyethylene outer protective pipe layer includes an inner polyethylene outer protective pipe layer and an outer polyethylene outer protective pipe layer. The raw material for the inner layer of the polyethylene outer sheath comprises the following components in parts by weight: 60 parts of first polyethylene and 6-16 parts of first carbon black; The raw material for the outer layer of the polyethylene outer sheath comprises the following components in parts by weight: 60 parts of second polyethylene and 8-20 parts of second carbon black; The weight of the second carbon black is greater than the weight of the first carbon black; The first polyethylene and the second polyethylene each independently comprise high-pressure polyethylene and low-pressure polyethylene in a weight ratio of 1:9 to 9:1.

[0007] As a further technical solution, the weight ratio of high-pressure polyethylene to low-pressure polyethylene in the first polyethylene is 0.5~1:1.5.

[0008] As a further technical solution, the weight ratio of high-pressure polyethylene to low-pressure polyethylene in the second polyethylene is 3~5:1.

[0009] In the polyethylene outer sheath layer of the anti-aging direct-buried thermal insulation pipe of this invention, by adjusting the content ratio of high-pressure polyethylene and low-pressure polyethylene in the inner and outer layers of the polyethylene outer sheath, when the weight ratio of high-pressure polyethylene to low-pressure polyethylene in the first polyethylene layer is 0.5~1:1.5 and the weight ratio of high-pressure polyethylene to low-pressure polyethylene in the second polyethylene layer is 3~5:1, the mechanical strength of the polyethylene outer sheath layer can be further improved, and its tensile strength can be increased to over 25.2 MPa. The speculated reason may be that the relatively high carbon black content in the outer layer of the polyethylene outer sheath, with a high-pressure polyethylene to low-pressure polyethylene weight ratio of 3~5:1, allows the polyethylene matrix material to better encapsulate the relatively high carbon black content, thus better regulating the structural stability of the outer layer. Conversely, the relatively low carbon black content in the inner layer of the polyethylene outer sheath, with a high-pressure polyethylene to low-pressure polyethylene weight ratio of 0.5~1:1.5, is more conducive to reducing the migration of carbon black in the inner layer, thereby better dispersing external stress and ultimately further improving the overall mechanical strength of the polyethylene outer sheath layer.

[0010] As a further technical solution, the raw materials of both the inner layer of the polyethylene outer sheath and the outer layer of the polyethylene outer sheath include antioxidants, ultraviolet absorbers, and compatibilizers. The antioxidant includes one or both of antioxidant 1010 and antioxidant 1076; The ultraviolet absorber includes one or two of benzotriazole and 2,4-dihydroxybenzophenone; The compatibilizer includes maleic anhydride-grafted polyethylene.

[0011] As a further technical solution, the amount of antioxidant added to the inner layer of the polyethylene outer sheath and the outer layer of the polyethylene outer sheath is independently 2 to 4 parts each; The amount of ultraviolet absorber added to the inner layer of the polyethylene outer sheath and the outer layer of the polyethylene outer sheath is independently 0.5 to 1 part each; The amount of compatibilizer added to the inner layer of the polyethylene outer sheath and the outer layer of the polyethylene outer sheath is 2 to 4 parts each.

[0012] As a further technical solution, the raw materials for the outer layer of the polyethylene outer sheath also include chitosan and aromatic aminocarboxylic acid compounds in a weight ratio of 3 to 9:1.

[0013] In the polyethylene outer protective layer of the anti-aging direct-buried thermal insulation pipe of this invention, when the raw materials of the polyethylene outer protective layer also include chitosan and aromatic aminocarboxylic acid compounds, the addition of aromatic aminocarboxylic acid compounds and chitosan can make the carbon black more uniformly dispersed in the polyethylene matrix, thereby improving the bonding effect with the polyethylene matrix, thus better shielding ultraviolet rays and resisting oxygen penetration. At the same time, the amino groups in the aromatic aminocarboxylic acid compounds can also consume peroxides generated during the aging process of polyethylene to a certain extent, thereby inhibiting the aging of polyethylene and effectively improving the anti-aging performance of the outer layer of the polyethylene outer protective layer, thus effectively improving the anti-aging performance of the direct-buried thermal insulation pipe.

[0014] As a further technical solution, the aromatic aminocarboxylic acid compound includes one or more of 3-aminophthalic acid, 3,5-diaminobenzoic acid, and m-aminobenzoic acid, preferably 3,5-diaminobenzoic acid.

[0015] In this invention, the aromatic aminocarboxylic acid compound can be 3-aminophthalic acid, 3,5-diaminobenzoic acid, or m-aminobenzoic acid. When the aromatic aminocarboxylic acid compound is 3,5-diaminobenzoic acid, it best improves the anti-aging performance of the outer layer of the polyethylene outer sheath. The reason is speculated to be that compared with 3-aminophthalic acid and m-aminobenzoic acid, 3,5-diaminobenzoic acid contains more amino active groups, which can better disperse carbon black and inhibit the aging effect of polyethylene.

[0016] As a further technical solution, the weight ratio of the chitosan and the aromatic aminocarboxylic acid compound to the second carbon black is 4~8:20.

[0017] As a further technical solution, the steel pipe includes one of seamless steel pipe and straight seam steel pipe, preferably seamless steel pipe.

[0018] This invention proposes a method for preparing an anti-aging direct-buried thermal insulation pipe, which includes the following steps: A1. The raw materials for the inner layer of the polyethylene outer sheath are blended and extruded to obtain the inner layer of the polyethylene outer sheath. A2. The raw materials of the outer layer of the polyethylene outer sheath are blended and extruded onto the outer surface of the inner layer of the polyethylene outer sheath to obtain a polyethylene outer sheath layer including the inner layer of the polyethylene outer sheath and the outer layer of the polyethylene outer sheath. A3. After fixing the polyethylene outer protective layer, the steel pipe is sent into the polyethylene outer protective layer and fixed. The polyurethane insulation material is poured between the polyethylene outer protective layer and the steel pipe to obtain the anti-aging direct buried insulation pipe.

[0019] This invention also proposes a method for preparing an anti-aging direct-buried insulation pipe, which includes the following steps: S0. The aromatic aminocarboxylic acid compound is dispersed in ethanol, the second carbon black is added, and the mixture is mixed for the first time, concentrated, and dried to obtain the second carbon black treated product I. The chitosan is dispersed in an aqueous ethanol solution, the second carbon black treated product I is added, and the mixture is mixed for the second time, concentrated, and dried to obtain the second carbon black treated product II. S1. The raw materials for the inner layer of the polyethylene outer protective tube are blended and extruded to obtain the inner layer of the polyethylene outer protective tube. S2. The second polyethylene, the second carbon black treated material II, and the remaining components of the outer layer of the polyethylene outer sheath except for the second polyethylene, the second carbon black, the aromatic amino carboxylic acid compound, and chitosan are blended and extruded onto the outer surface of the inner layer of the polyethylene outer sheath to obtain a polyethylene outer sheath layer including the inner layer of the polyethylene outer sheath and the outer layer of the polyethylene outer sheath. S3. After fixing the polyethylene outer protective layer, the steel pipe is sent into the polyethylene outer protective layer and fixed. The polyurethane insulation material is poured between the polyethylene outer protective layer and the steel pipe to obtain the anti-aging direct buried insulation pipe.

[0020] In this invention, aromatic aminocarboxylic acid compounds and chitosan are first surface-composite with carbon black, and then the resulting carbon black-treated material is blended with other components in the outer layer of the polyethylene outer sheath to prepare the outer layer of the polyethylene outer sheath, which is more effective in improving the anti-aging performance of the outer layer of the polyethylene outer sheath.

[0021] As a further technical solution, in step S0, the stirring speed of the first mixing is 400~600 rpm and the time is 30~60 min; The second mixing temperature is 30~40℃, the stirring speed is 500~800rpm, and the time is 1.5~2.5h.

[0022] The working principle and beneficial effects of this invention are as follows: In this invention, the anti-aging direct-buried insulation pipe comprises a two-layer structure: an inner polyethylene outer sheath and an outer polyethylene outer sheath. Both layers utilize both high-pressure and low-pressure polyethylene, and the carbon black content is adjusted so that the carbon black content in the inner polyethylene outer sheath is less than that in the outer polyethylene outer sheath. This effectively improves the mechanical strength and impact resistance of the polyethylene outer sheath layer in the direct-buried insulation pipe. Specifically: The outer layer of the polyethylene outer sheath has a high carbon black content. When external stress is applied to the outer sheath, the relatively large number of carbon black particles effectively disperses the stress within the surrounding polyethylene matrix and the inner layer of the polyethylene outer sheath. The inner layer of the polyethylene outer sheath has a relatively low carbon black content, which disperses the stress transmitted to the outer layer without affecting the bonding between the inner layer and the polyurethane insulation layer due to excessive carbon black, thus enhancing the overall stability of the direct-buried insulation pipe structure. Furthermore, both the inner and outer layers of the polyethylene outer sheath utilize high-pressure and low-pressure polyethylene. The interfacial cohesion between the high-pressure polyethylene (flexible) and low-pressure polyethylene (rigid) is adjusted to improve the overall stability of the polyethylene outer sheath layer, thus enhancing its overall stability. Therefore, the polyethylene outer sheath layer consists of a two-layer structure: an inner polyethylene outer sheath layer and an outer polyethylene outer sheath layer. Both layers contain high-pressure polyethylene and low-pressure polyethylene, which improves the interfacial bonding between polyethylene and carbon black. The carbon black content in the inner polyethylene outer sheath layer is adjusted to be less than that in the outer polyethylene outer sheath layer, forming a carbon black gradient distribution from the inside to the outside. This effectively disperses external stress and ultimately improves the mechanical strength and impact resistance of the polyethylene outer sheath layer, thereby improving the mechanical stability of the direct-buried insulation pipe. Detailed Implementation

[0023] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0024] In the following examples and comparative examples, the high-pressure polyethylene is LDPE FB5005; the low-pressure polyethylene is HDPE PX2413; the carbon black is N330; the chitosan has a degree of deacetylation of 90% and a weight-average molecular weight of 150,000; and the maleic anhydride-grafted polyethylene is E100.

[0025] Example 1 A method for preparing an anti-aging direct-buried thermal insulation pipe includes the following steps: A1. 6 parts high-pressure polyethylene, 54 parts low-pressure polyethylene, 6 parts carbon black, 2 parts antioxidant 1010, 0.5 parts 2,4-dihydroxybenzophenone and 2 parts maleic anhydride grafted polyethylene are blended and then extruded to obtain the inner layer of the polyethylene outer sheath. A2. 6 parts high-pressure polyethylene, 54 parts low-pressure polyethylene, 8 parts carbon black, 2 parts antioxidant 1010, 0.5 parts 2,4-dihydroxybenzophenone and 2 parts maleic anhydride grafted polyethylene are blended and then extruded onto the outer surface of the inner layer of the polyethylene outer sheath to obtain a polyethylene outer sheath layer containing a polyethylene outer sheath inner layer and a polyethylene outer sheath outer layer. A3. After fixing the polyethylene outer protective layer with a pipe clamp, the seamless steel pipe is bundled with the support and sent into the polyethylene outer protective layer and fixed. The polyurethane insulation material is poured between the polyethylene outer protective layer and the seamless steel pipe to obtain an anti-aging direct buried insulation pipe.

[0026] Example 2 A method for preparing an anti-aging direct-buried thermal insulation pipe includes the following steps: A1. 30 parts high-pressure polyethylene, 30 parts low-pressure polyethylene, 10 parts carbon black, 3 parts antioxidant 1010, 1 part 2,4-dihydroxybenzophenone and 3 parts maleic anhydride grafted polyethylene are blended and extruded to obtain the inner layer of the polyethylene outer sheath. A2. 30 parts high-pressure polyethylene, 30 parts low-pressure polyethylene, 20 parts carbon black, 3 parts antioxidant 1010, 1 part 2,4-dihydroxybenzophenone and 3 parts maleic anhydride grafted polyethylene are blended and then extruded onto the outer surface of the inner layer of the polyethylene outer sheath to obtain a polyethylene outer sheath layer containing a polyethylene outer sheath inner layer and a polyethylene outer sheath outer layer. A3. After fixing the polyethylene outer protective layer with a pipe clamp, the seamless steel pipe is bundled with the support and sent into the polyethylene outer protective layer and fixed. The polyurethane insulation material is poured between the polyethylene outer protective layer and the seamless steel pipe to obtain an anti-aging direct buried insulation pipe.

[0027] Example 3 A method for preparing an anti-aging direct-buried thermal insulation pipe includes the following steps: A1. 54 parts high-pressure polyethylene, 6 parts low-pressure polyethylene, 16 parts carbon black, 4 parts antioxidant 1010, 1 part 2,4-dihydroxybenzophenone and 4 parts maleic anhydride grafted polyethylene are blended and extruded to obtain the inner layer of the polyethylene outer sheath. A2. 54 parts high-pressure polyethylene, 6 parts low-pressure polyethylene, 20 parts carbon black, 4 parts antioxidant 1010, 1 part 2,4-dihydroxybenzophenone and 4 parts maleic anhydride grafted polyethylene are blended and then extruded onto the outer surface of the inner layer of the polyethylene outer sheath to obtain a polyethylene outer sheath layer containing the inner layer of the polyethylene outer sheath and the outer layer of the polyethylene outer sheath. A3. After fixing the polyethylene outer protective layer with a pipe clamp, the seamless steel pipe is bundled with the support and sent into the polyethylene outer protective layer and fixed. The polyurethane insulation material is poured between the polyethylene outer protective layer and the seamless steel pipe to obtain an anti-aging direct buried insulation pipe.

[0028] Example 4 The only difference between this embodiment and Embodiment 2 is that in the preparation of the inner layer of the polyethylene outer sheath in this embodiment, 15 parts of high-pressure polyethylene and 45 parts of low-pressure polyethylene are added; in the preparation of the outer layer of the polyethylene outer sheath in this embodiment, 45 parts of high-pressure polyethylene and 15 parts of low-pressure polyethylene are added.

[0029] Example 5 The only difference between this embodiment and Embodiment 2 is that in the preparation of the inner layer of the polyethylene outer sheath in this embodiment, 24 parts of high-pressure polyethylene and 36 parts of low-pressure polyethylene are added; in the preparation of the outer layer of the polyethylene outer sheath in this embodiment, 50 parts of high-pressure polyethylene and 10 parts of low-pressure polyethylene are added.

[0030] Example 6 The only difference between this embodiment and Embodiment 5 is that the preparation method of the anti-aging direct-buried insulation pipe in this embodiment is different, specifically: S0. Disperse 8 parts of 3,5-diaminobenzoic acid in 50 parts of ethanol, add 20 parts of carbon black, stir at 400 rpm for 60 min, concentrate, and dry to obtain the second carbon black treated product. S1. 24 parts high-pressure polyethylene, 36 parts low-pressure polyethylene, 10 parts carbon black, 3 parts antioxidant 1010, 1 part 2,4-dihydroxybenzophenone and 3 parts maleic anhydride grafted polyethylene are blended and extruded to obtain the inner layer of the polyethylene outer sheath. S2. 50 parts of high-pressure polyethylene, 10 parts of low-pressure polyethylene, 3 parts of antioxidant 1010, 1 part of 2,4-dihydroxybenzophenone, 3 parts of maleic anhydride grafted polyethylene and the above-mentioned second carbon black treatment material are blended and extruded onto the outer surface of the inner layer of the polyethylene outer sheath to obtain a polyethylene outer sheath layer containing a polyethylene outer sheath inner layer and a polyethylene outer sheath outer layer. S3. After fixing the polyethylene outer protective layer with a pipe clamp, the seamless steel pipe is bundled with the support and sent into the polyethylene outer protective layer and fixed. The polyurethane insulation material is poured between the polyethylene outer protective layer and the seamless steel pipe to obtain an anti-aging direct buried insulation pipe.

[0031] Example 7 The only difference between this embodiment and Embodiment 6 is that step S0 in the preparation method of the anti-aging direct-buried insulation pipe in this embodiment is different, specifically: S0. Disperse 8 parts of chitosan in 50 parts of ethanol aqueous solution (ethanol weight fraction is 20%), add 20 parts of carbon black, stir at 400 rpm for 60 min, concentrate, and dry to obtain the second carbon black treated product.

[0032] Example 8 The only difference between this embodiment and Embodiment 6 is that step S0 in the preparation method of the anti-aging direct-buried insulation pipe in this embodiment is different, specifically: S0. Disperse 2 parts of 3,5-diaminobenzoic acid in 50 parts of ethanol, add 20 parts of carbon black, stir at 400 rpm for 60 min, concentrate, and dry to obtain second carbon black treatment product I; disperse 6 parts of chitosan in 50 parts of ethanol aqueous solution (ethanol weight fraction is 20%), add the above second carbon black treatment product I, stir at 30℃ and 500 rpm for 2.5 h, concentrate, and dry to obtain second carbon black treatment product II.

[0033] Example 9 The only difference between this embodiment and Embodiment 6 is that step S0 in the preparation method of the anti-aging direct-buried insulation pipe in this embodiment is different, specifically: S0. Disperse 1 part of 3,5-diaminobenzoic acid in 50 parts of ethanol, add 20 parts of carbon black, stir at 400 rpm for 60 min, concentrate, and dry to obtain the second carbon black treatment product I; disperse 7 parts of chitosan in 50 parts of ethanol aqueous solution (ethanol weight fraction is 20%), add the above second carbon black treatment product I, stir at 30℃ and 500 rpm for 2.5 h, concentrate, and dry to obtain the second carbon black treatment product II.

[0034] Example 10 The only difference between this embodiment and Embodiment 6 is that step S0 in the preparation method of the anti-aging direct-buried insulation pipe in this embodiment is different, specifically: S0. Disperse 0.8 parts of 3,5-diaminobenzoic acid in 50 parts of ethanol, add 20 parts of carbon black, stir at 400 rpm for 60 min, concentrate, and dry to obtain second carbon black treated product I; disperse 7.2 parts of chitosan in 50 parts of ethanol aqueous solution (ethanol weight fraction is 20%), add the above second carbon black treated product I, stir at 30℃ and 500 rpm for 2.5 h, concentrate, and dry to obtain second carbon black treated product II.

[0035] Example 11 The only difference between this embodiment and Embodiment 10 is that step S0 in the preparation method of the anti-aging direct-buried insulation pipe in this embodiment is different, specifically: S0. Disperse 0.4 parts of 3,5-diaminobenzoic acid in 50 parts of ethanol, add 20 parts of carbon black, stir at 600 rpm for 30 min, concentrate, and dry to obtain second carbon black treated product I; disperse 3.6 parts of chitosan in 50 parts of ethanol aqueous solution (ethanol weight fraction is 20%), add the above second carbon black treated product I, stir at 40℃ and 800 rpm for 1.5 h, concentrate, and dry to obtain second carbon black treated product II.

[0036] Example 12 A method for preparing an anti-aging direct-buried thermal insulation pipe includes the following steps: S1. 24 parts high-pressure polyethylene, 36 parts low-pressure polyethylene, 10 parts carbon black, 3 parts antioxidant 1010, 1 part 2,4-dihydroxybenzophenone and 3 parts maleic anhydride grafted polyethylene are blended and extruded to obtain the inner layer of the polyethylene outer sheath. S2. 50 parts high-pressure polyethylene, 10 parts low-pressure polyethylene, 3 parts antioxidant 1010, 1 part 2,4-dihydroxybenzophenone, 3 parts maleic anhydride-grafted polyethylene, 20 parts carbon black, 2 parts 3,5-diaminobenzoic acid and 6 parts chitosan are blended and then extruded onto the outer surface of the inner layer of the polyethylene outer sheath to obtain a polyethylene outer sheath layer containing an inner layer of polyethylene outer sheath and an outer layer of polyethylene outer sheath. S3. After fixing the polyethylene outer protective layer with a pipe clamp, the seamless steel pipe is bundled with the support and sent into the polyethylene outer protective layer and fixed. The polyurethane insulation material is poured between the polyethylene outer protective layer and the seamless steel pipe to obtain an anti-aging direct buried insulation pipe.

[0037] Comparative Example 1 The only difference between this comparative example and Example 2 is that in this comparative example, the low-pressure polyethylene in both the inner and outer layers of the polyethylene outer sheath is replaced with an equal amount of high-pressure polyethylene.

[0038] Comparative Example 2 The only difference between this comparative example and Example 2 is that in this comparative example, the high-pressure polyethylene in both the inner and outer layers of the polyethylene outer sheath is replaced with an equal amount of low-pressure polyethylene.

[0039] Comparative Example 3 The only difference between this comparative example and Example 2 is that in this comparative example, 10 parts of carbon black in the inner layer of the polyethylene outer sheath are replaced with 15 parts of carbon black; and 20 parts of carbon black in the outer layer of the polyethylene outer sheath are replaced with 15 parts of carbon black.

[0040] Comparative Example 4 The only difference between this comparative example and Example 2 is that in this comparative example, 10 parts of carbon black in the inner layer of the polyethylene outer sheath are replaced with 20 parts of carbon black; and 20 parts of carbon black in the outer layer of the polyethylene outer sheath are replaced with 10 parts of carbon black.

[0041] Experimental Example 1 The direct-buried insulated pipes prepared in Examples 1-5 and Comparative Examples 1-4 were subjected to the following performance tests: (1) Tensile strength: The polyethylene outer protective layer of the direct-buried insulated pipes prepared in Examples 1-5 and Comparative Examples 1-4 was tested for tensile strength according to the method in GB / T 1040.1-2018 "Determination of tensile properties of plastics - Part 1: General Rules". During the test, the specimen was a type 1A specimen with a size of 170mm×10mm×4mm and the test speed was 100mm / min. The test results are shown in Table 1. (2) The impact resistance test was carried out according to the method in GB / T 29047-2021 "High-density polyethylene outer protective pipe rigid polyurethane foam prefabricated direct buried insulated pipe and fittings". During the test, a 3.0 kg drop hammer was dropped from a height of 2 m to impact the polyethylene outer protective pipe layer under the condition of -20℃. The polyethylene outer protective pipe layer was observed to see if there were visible cracks. The test results are shown in Table 1.

[0042] Table 1 Performance test results of Examples 1-5 and Comparative Examples 1-4

[0043] As can be seen from Table 1, compared with Comparative Examples 1-4, the tensile strength of the polyethylene outer sheath layer of the direct-buried insulated pipe prepared in Examples 1-5 is improved, reaching over 22.8 MPa. Furthermore, after impact testing, no cracks were found on the surface of the polyethylene outer sheath layer. This indicates that the polyethylene outer sheath layer comprises a two-layer structure: an inner polyethylene outer sheath layer and an outer polyethylene outer sheath layer. The use of both high-pressure and low-pressure polyethylene in both layers, along with a lower carbon black content in the inner polyethylene outer sheath layer compared to the outer polyethylene outer sheath layer, effectively solves the problems of low mechanical strength and poor impact resistance of the polyethylene outer sheath layer in direct-buried insulated pipes.

[0044] Experiment Example 2 The direct-buried insulated pipes prepared in Examples 4-12 were subjected to axial shear strength tests before and after aging according to the method in GB / T 29047-2021 "High-density polyethylene outer protective pipe, rigid polyurethane foam prefabricated direct-buried insulated pipe and fittings". The aging test conditions were: steel pipe temperature 170℃, aging time 1450h. The test results are shown in Table 2 below.

[0045] Table 2 Performance test results of Examples 4-12

[0046] The axial shear strength retention rate was calculated as (axial shear strength after aging / axial shear strength before aging) × 100%. Compared with Examples 4-7 and 12, the axial shear strength retention rate of the direct-buried insulation pipes prepared in Examples 8-11 was improved after aging tests, reaching over 94.8%. This indicates that when the raw materials of the outer layer of the polyethylene outer sheath also include chitosan and aromatic aminocarboxylic acid compounds, firstly performing surface composite treatment of the carbon black in the outer layer of the polyethylene outer sheath with chitosan and aromatic aminocarboxylic acid compounds, and then blending the resulting carbon black treatment material with other components in the outer layer of the polyethylene outer sheath to prepare the outer layer of the polyethylene outer sheath, the aging resistance of the outer layer of the polyethylene outer sheath can be effectively improved, thereby effectively improving the aging resistance of the direct-buried insulation pipe.

[0047] The above are merely 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 principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An anti-aging direct-buried insulated pipe, comprising, from the inside out, a steel pipe, a polyurethane insulation layer, and a polyethylene outer protective layer, characterized in that, The polyethylene outer sheath layer includes a polyethylene inner sheath layer and a polyethylene outer sheath layer; The raw material for the inner layer of the polyethylene outer sheath comprises the following components in parts by weight: 60 parts of first polyethylene and 6-16 parts of first carbon black; The raw material for the outer layer of the polyethylene outer sheath comprises the following components in parts by weight: 60 parts of second polyethylene and 8-20 parts of second carbon black; The weight of the second carbon black is greater than the weight of the first carbon black; The first polyethylene and the second polyethylene each independently comprise high-pressure polyethylene and low-pressure polyethylene in a weight ratio of 1:9 to 9:

1.

2. The anti-aging direct-buried heat-insulating pipe according to claim 1, characterized in that, The weight ratio of high-pressure polyethylene to low-pressure polyethylene in the first polyethylene is 0.5~1:1.

5.

3. The anti-aging direct-buried heat-insulating pipe according to claim 1, characterized in that, The weight ratio of high-pressure polyethylene to low-pressure polyethylene in the second polyethylene is 3~5:

1.

4. The anti-aging direct-buried heat-insulating pipe according to claim 1, characterized in that, The raw materials for both the inner layer and the outer layer of the polyethylene outer sheath include antioxidants, ultraviolet absorbers, and compatibilizers. The antioxidant includes one or both of antioxidant 1010 and antioxidant 1076; The ultraviolet absorber includes one or two of benzotriazole and 2,4-dihydroxybenzophenone; The compatibilizer includes maleic anhydride-grafted polyethylene.

5. An anti-aging direct-buried heat-insulating pipe according to any one of claims 1 to 4, characterized in that, The outer layer of the polyethylene outer sheath also includes chitosan and aromatic aminocarboxylic acid compounds in a weight ratio of 3 to 9:

1.

6. The anti-aging direct-buried heat-insulating pipe according to claim 5, characterized in that, The aromatic aminocarboxylic acid compounds include one or more of 3-aminophthalic acid, 3,5-diaminobenzoic acid, and m-aminobenzoic acid.

7. The anti-aging direct-buried heat-insulating pipe according to claim 5, characterized in that, The weight ratio of the chitosan and the aromatic aminocarboxylic acid compound to the second carbon black is 4~8:

20.

8. The anti-aging direct-buried heat-insulating pipe according to claim 1, characterized in that, The steel pipe includes either seamless steel pipe or straight seam steel pipe.

9. A method for preparing an anti-aging direct-buried thermal insulation pipe, used to prepare the anti-aging direct-buried thermal insulation pipe according to any one of claims 5 to 7, characterized in that, Includes the following steps: S0. The aromatic aminocarboxylic acid compound is dispersed in ethanol, the second carbon black is added, and the mixture is mixed for the first time, concentrated, and dried to obtain the second carbon black treated product I. The chitosan is dispersed in an aqueous ethanol solution, the second carbon black treated product I is added, and the mixture is mixed for the second time, concentrated, and dried to obtain the second carbon black treated product II. S1. The raw materials for the inner layer of the polyethylene outer protective tube are blended and extruded to obtain the inner layer of the polyethylene outer protective tube. S2. The second polyethylene, the second carbon black treated material II, and the remaining components of the outer layer of the polyethylene outer sheath except for the second polyethylene, the second carbon black, the aromatic amino carboxylic acid compound, and chitosan are blended and extruded onto the outer surface of the inner layer of the polyethylene outer sheath to obtain a polyethylene outer sheath layer including the inner layer of the polyethylene outer sheath and the outer layer of the polyethylene outer sheath. S3. After fixing the polyethylene outer protective layer, the steel pipe is sent into the polyethylene outer protective layer and fixed. The polyurethane insulation material is poured between the polyethylene outer protective layer and the steel pipe to obtain the anti-aging direct buried insulation pipe.

10. The method for preparing an anti-aging direct-buried heat-insulating pipe according to claim 9, characterized in that, In step S0, the stirring speed for the first mixing is 400-600 rpm, and the time is 30-60 min; The second mixing temperature is 30~40℃, the stirring speed is 500~800rpm, and the time is 1.5~2.5h.

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