JPH0136784B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a steel pipe coated with plastic on the outside and used in a wide temperature range from -30°C to 120°C. (Prior Art) Currently, polyethylene-coated steel pipes are widely used as external plastic-coated steel pipes. Polyethylene is an excellent material for coating steel pipes because it is inexpensive, chemically stable, and has good mechanical properties (impact resistance, flexibility, etc.) at low temperatures, but it has a low softening point and generally exceeds 80°C. It is considered unsuitable for use at high temperatures. In recent years, high-pressure operation of pipelines and the transportation of heavy crude oil have come to be carried out, and there has been a demand for plastic-coated steel pipes that can withstand higher temperatures to replace conventional polyethylene-coated steel pipes. In addition, in cold regions, burial work is carried out even at temperatures as low as -30°C, so it is also essential to have excellent impact resistance and flexibility even at such low temperatures. Polypropylene has a higher softening temperature than polyethylene, is inexpensive, and is chemically stable, so it is promising as a coating material for high-temperature coated steel pipes. However, conventional polypropylene has poorer mechanical properties at low temperatures than polyethylene. Application to steel pipe coatings was very limited. The present inventors have already discovered that polypropylene prepared by adjusting the ethylene content to an appropriate range using a block copolymer of propylene and ethylene has improved mechanical properties at low temperatures while maintaining a high softening temperature (Tsuchiya et al. : Proceedings of the 4th International Conference on Coating and Coating of Steel Pipe, 307 pages, 1981). The present inventors further investigated the application of such polypropylene to coated steel pipes, and found that the only adhesive that can provide good adhesive strength even at high temperatures is a modified polyolefin adhesive. For adhesives, ASTMG14 is used for the coating layer of coated steel pipes.
It has become clear that when impact is applied using the method shown in , the coating layer cracks at temperatures below -20°C, and does not meet the impact resistance required at -30°C in cold regions. In addition, even if a conventional epoxy primer layer is formed as a pretreatment, the peeling of the coating layer (hereinafter referred to as cathodic peeling) when cathodic potential is applied to the steel pipe for cathodic protection increases significantly as the temperature increases. It became clear. On the other hand, even if the pipe is a polypropylene-coated steel pipe, if it is buried underground and used at high temperatures, penetrating scratches may occur due to the encroachment of surrounding pebbles. Therefore, it is desirable that the indentation depth at high temperatures be as small as possible. However, none of the conventionally known products has been found that satisfies all of the above conditions. (Objective of the Invention) In order to solve the above-mentioned difficulties, the present invention has investigated various pretreatment methods for coating materials, adhesives, and brimer paints, and has developed an outer surface that can be used in a wide temperature range from -30°C to about 120°C. The present invention aims to provide plastic-coated steel pipes. (Structure of the Invention) That is, the present invention provides the following features: (1) As shown in FIG. A resin whose main component is an ethylene-propylene block copolymer and whose low-temperature embrittlement temperature is higher than 0°C and 15°C or lower is used, and maleic anhydride-modified crystalline polypropylene with a low-temperature embrittlement temperature of -30°C or lower is used as an adhesive. A polypropylene-coated steel pipe characterized by using a maleic anhydride-modified polypropylene adhesive consisting of a mixture of unmodified crystalline polypropylene and polyolefin rubber. (2) After cleaning the outer surface of the steel pipe by blasting or the like as a pretreatment, an epoxy primer layer 4 with a glass transition temperature of 80°C or higher is formed between the steel pipe 1 and the adhesive layer 2, as shown in Figure 2. The polypropylene-coated steel pipe according to claim 1, characterized in that: (3) As a pretreatment, the outer surface of the steel pipe is cleaned by blasting or the like, and then a chromic acid-based chemical conversion treatment 5 is applied between the steel pipe 1 and the adhesive layer 2, as shown in Fig. 3. A polypropylene-coated steel pipe according to claim 1. (4) As a pretreatment, after cleaning the outer surface of the steel pipe by blasting, etc., as shown in Figure 4, the steel pipe 1
Chromic acid-based chemical conversion treatment 5 between the adhesive layer 2 and the adhesive layer 2
The polypropylene-coated steel pipe according to claim 1, wherein an epoxy primer layer 4 having a glass transition temperature of 80° C. or higher is formed. It is. The low-temperature embrittlement temperature is a value measured in accordance with ASTM standard D746 using a 2 mm thick test piece with a 0.1 mm deep notch in the width direction on the impact surface. The material of the polypropylene coating layer 3 in the present invention is mainly composed of a crystalline ethylene-propylene block copolymer, and its low temperature embrittlement temperature is 0.
℃ higher than 15℃ or less, and the penetration depth at 120℃ is preferably 0.25mm, particularly preferably
Polypropylene of 0.2 mm or less can be used. In this case, the polypropylene is not only a crystalline block copolymer of propylene and ethylene, but also
The copolymer may contain polyethylene having a density of 0.93 g/cm ³ or more (ethylene homopolymers and ethylene copolymers containing α-propylene, butene-1, pentene, hexene, heptene, octene, etc., whose content in the copolymers is about 15% by weight or less). (including ethylene-α-olefin copolymers with olefins) and polyolefin rubbers (ethylene-propylene copolymer rubber, ethylene-propylene-nonconjugated diene ternary copolymer rubber, ethylene-propylene.
A mixture of (butene-1 ternary copolymer rubber, polyisobutylene, 1,2-polybutadiene) or ethylene-vinyl acetate copolymer rubber can be used. Among these, the following ( Consisting of each component A) to (C), the proportion of each component is (A) 95 to 55% by weight of a polypropylene component that is insoluble in paraxylene at room temperature, (B) 2 to 40% by weight of a polyethylene component that is soluble in paraxylene at room temperature, (C) Polypropylene having a normal temperature paraxylene soluble content of 3 to 20% by weight is preferred. In particular, the isotacticity of the component (A) is preferably 90% or more, preferably 95% or more. These polypropylenes preferably have a melt flow rate (MFR) of 0.05 to 5 g/10 minutes. In addition, it is preferable to add additives such as antioxidants, ultraviolet absorbers, and pigments to these polypropylenes as necessary. In particular, in the present invention, it is preferable that the material for the polypropylene coating layer 3 contains 0.01 to 2 parts by weight of a nucleating agent per 100 parts by weight of polypropylene. Here, if the low temperature embrittlement temperature of the polypropylene coating layer 3 is higher than 15°C, the impact resistance at -30°C is not satisfied. Moreover, if the low temperature embrittlement temperature is 0° C. or lower, the indentation depth at high temperatures becomes large, which is not preferable. Next, as the material for the adhesive layer 2 in the present invention, a modified polyolefin adhesive having a low-temperature embrittlement temperature of -30°C or lower can be used. Specifically, the anhydrous resin is composed of maleic anhydride-modified crystalline polypropylene, unmodified crystalline polypropylene, and polyolefin-in rubber, and the proportion of the polyolefin-in rubber in the total of 100% by weight of each of these components is 5 to 40% by weight. Maleic acid-modified polypropylene adhesives are preferred. The maleic anhydride-modified crystalline polypropylene can be produced by a heating reaction known per se in the presence of a radical reaction initiator between crystalline polypropylene and a maleic anhydride compound, such as initiation of a radical reaction between crystalline polypropylene and a maleic anhydride compound. It can be obtained by a melt reaction in the presence of an agent. The crystalline polypropylene mentioned above includes a crystalline polypropylene homopolymer, a crystalline block copolymer of ethylene and propylene, a combination of ethylene and propylene, or a combination of ethylene and propylene and other α-olefins (butene, hexene, octene, etc.). Examples include crystalline random copolymers. The crystalline polypropylene preferably has a melt flow rate (MFR) of 0.1 to 20 g/10 min. In addition, the grafting rate of the maleic anhydride compound of crystalline polypropylene is 1 g of crystalline polypropylene.
It is preferably in the range of 1×10 ^-7 to 1×10 ^-3 mol, more preferably 1×10 ^-6 to 5×10 ^{-4 mol} .
Preferably it is in the molar range. As the unmodified crystalline polypropylene,
Crystalline polypropylene homopolymers, crystalline block copolymers of ethylene and propylene, ethylene and propylene or ethylene and propylene with other α-olefins (butene, hexene,
crystalline random copolymers with octene, etc.),
or 30% by weight of these crystalline polypropylenes
Examples include those in which the following are replaced with polyethylene (high-density polyethylene, medium-density polyethylene, low-density polyethylene, or linear low-density polyethylene). The unmodified crystalline polypropylene preferably has a methane fluoride (MFR) of 0.1 to 50 g/10 min. Examples of the polyolefin rubber include ethylene-propylene copolymer rubber, ethylene-propylene-nonconjugated diene ternary copolymer rubber, ethylene-propylene-butene-1 ternary copolymer rubber, polyisobutylene, and 1,2-polybutadiene. Examples include. Among these, ethylene-propylene copolymer rubber or ethylene-propylene-nonconjugated diene ternary copolymer rubber is preferred. A portion of the polyolefin rubber is made of rubber such as styrene-butadiene copolymer rubber, butadiene-styrene-acrylonitrile copolymer rubber, cis-1,4-polybutadiene, natural rubber, polyurethane rubber, ethylene-vinyl acetate copolymer rubber, etc. Substituted rubbers can also be used as polyolefin rubber. The maleic anhydride-modified polypropylene adhesive used in the present invention can be obtained by mixing the above-mentioned components by a method known per se. The materials for the epoxy primer layer 4 in the present invention include an epoxy resin whose main component is a bisphenol A type epoxy resin having an epoxy equivalent of 170 to 3000, an amine curing agent that cures by reacting with the epoxy resin at room temperature, and Paints containing pigments, mainly inorganic pigments, can be used. Here, examples of other types of epoxy resins to be added to the bisphenol A type epoxy resin include hydrogenated bisphenol A diglycidyl ether resin, bisphenol A side chain type diglycidyl ether resin, and brominated bisphenol A type epoxy. resins, novolac type epoxy resins, urethane-modified epoxy resins, resorcing glycidyl ether epoxy resins, glycidyl ester type epoxy resins, alicyclic type epoxy resins, glycidylamine epoxy resins, and the like. The amount of epoxy resin added is preferably 0 to 50 parts by weight per 100 parts by weight of bisphenol A type epoxy resin.
It is particularly preferable to add 30 parts by weight from the viewpoint of hot water resistance and high temperature cathode peeling resistance. The epoxy equivalent of this epoxy resin is 170 to 3000, preferably 190 to 1925, from the viewpoint of hot water resistance, high temperature cathode peelability, and painting workability. Examples of amine-based curing agents include heterocyclic modified diamine curing agents, modified fatty acid polyamine curing agents, modified aromatic polyamine curing agents, and modified polyamide amine curing agents. Among the curing agents, from the viewpoint of hot water resistance and high temperature cathode peeling resistance,
Particularly preferred are heterocyclic modified diamine curing agents and modified aromatic polyamine curing agents. There is no particular limitation on the usage ratio of the epoxy resin and the curing agent, and it can be determined in the conventional manner according to the epoxy equivalent of the epoxy resin used. Pigments added to this paint include titanium oxide, mica, barium sulfate, talc, calcium carbonate, clay, strontium chromate,
Zirconium silicate, mapico, carbon black, silica, etc. are preferred, and these pigments may be used alone or as a mixture of two or more in an amount of 5 to 50 parts by weight, preferably 5 to 50 parts by weight, based on 100 parts by weight of the total amount of the epoxy resin and amine curing agent. 5 to 25 parts by weight is selected from the viewpoint of painting workability. Incidentally, if the amount is less than 5 parts by weight, no improvement effect on high temperature cathode peelability is observed. The material for the chromic acid-based chemical conversion treatment layer 5 in the present invention is an aqueous solution of a composite oxide of hexavalent chromium oxide and trivalent chromium obtained by partially reducing hexavalent chromium oxide, or an aqueous solution of a composite oxide of hexavalent chromium oxide and trivalent chromium, or an accelerated reduction thereof. substances such as glycerin, polyvinyl alcohol,
Alternatively, an aqueous solution containing silica sol to improve adhesion to the coating layer can be used. In the present invention, the polypropylene is limited to polypropylene whose main component is a crystalline ethylene-propylene block copolymer, even though ordinary crystalline ethylene-propylene random copolymers do not have much improved low-temperature mechanical properties compared to homopolypropylene. However, in the case of a crystalline ethylene-propylene block copolymer, as shown in FIG.
do not dissolve and form independent regions, and the ethylene-propylene rubber phase 7 is formed between them, resulting in good impact resistance and low-temperature mechanical properties without almost losing mechanical strength at high temperatures. Because you can get it. That is, due to the presence of a continuous phase of polypropylene, the melting point and high temperature mechanical properties of crystalline ethylene-propylene block copolymers differ little from those of homopolypropylene. On the other hand, there are islands of polyethylene and ethylene-propylene rubber within the polypropylene phase, which absorb impact energy, so crystalline ethylene-propylene block copolymers as a whole have much better impact resistance than homopolypropylene. and low-temperature mechanical properties. Incidentally, FIG. 5 is a schematic diagram showing the structures of a polypropylene phase, a polyethylene phase, and an ethylene-propylene rubber phase of a crystalline ethylene-propylene block copolymer. Next, the reason why the low-temperature embrittlement temperature of the polypropylene coating layer mainly composed of crystalline ethylene-propylene block copolymer was set higher than 0℃ and below 15℃, and the low-temperature embrittlement temperature of the adhesive was limited to below -30℃. As a result of various studies conducted by the present inventors, the impact resistance of polypropylene-coated steel pipes is greatly affected not only by the low-temperature embrittlement temperature of the polypropylene coating layer but also by the low-temperature embrittlement temperature of the adhesive. In the impact test conducted, in order to keep the lowest temperature at which cracks do not occur in the coating layer to below -30℃, the low-temperature embrittlement temperature of the polypropylene coating layer and adhesive layer must be
This is because it has been found that it is necessary to keep the invention within the scope of the claims. That is,
If the low-temperature embrittlement temperature of the polypropylene coating layer and adhesive is in the shaded area 9 in Figure 6, the polypropylene-coated steel pipe meets ASTM standards.
Even if impact is applied to the coating layer according to G14, no cracks will occur even at -30℃. In addition, Figure 6 shows the region 9 in which no cracks occur in the coating layer in an impact test at -30°C, with the horizontal axis representing the low-temperature embrittlement temperature of the polypropylene coating layer and the vertical axis representing the low-temperature embrittlement temperature of the adhesive. It is a diagram showing diagonal lines. The reason why a chromic acid-based chemical conversion layer and/or an epoxy primer layer is formed between the steel pipe and the adhesive layer is that these chemical conversion treatment layers and primer layers can prevent cathodic peeling, sub-paint corrosion, and the coating layer. This is because it has a remarkable effect on preventing water from entering from the end face to the adhesive interface. Then, if both the chemical conversion treatment phase and the primer layer are formed,
A more reliable prevention effect can be obtained. The glass transition temperature of the epoxy primer layer was specified to be 80°C or higher because if the glass transition temperature of the epoxy primer layer is lower than 80°C, the epoxy primer layer will soften at high temperatures. This is because sufficient effects cannot be expected to prevent cathode peeling and corrosion under the paint film. Furthermore, the polypropylene-coated steel pipe according to the present invention has a low-temperature embrittlement temperature of polypropylene ranging from 0°C to 15°C.
℃, so the penetration depth at high temperatures is small, and even if it is used at high temperatures while buried underground, there is a risk that penetrating scratches will occur in the coating layer over many years due to the encroachment of surrounding pebbles. Less is. and the low-temperature embrittlement temperature of the crystalline ethylene-propylene block copolymer of the coating layer is 15°C or less,
In addition, since the low-temperature embrittlement temperature of the adhesive is -30°C or lower, no cracks will occur even at -30°C even if the coating layer receives a large impact. In addition, if necessary, a chromic acid-based chemical conversion treatment layer, an epoxy primer layer with a glass transition temperature of 80°C or higher, or both are formed between the steel pipe and the adhesive layer, so in this case, the coating layer is It has excellent durability against cathodic peeling, under-coating corrosion, etc., especially at high temperatures. Therefore, the polypropylene coated steel pipe according to the present invention has excellent coating layer durability and anti-corrosion function in a wide temperature range from a high temperature environment of 120°C to a low temperature environment of -30°C.
This technology is of great industrial significance, as it can be applied to areas where conventional technology has no effective corrosion protection measures, such as pipelines for high-pressure operation of natural gas in cold regions and pipelines for high-temperature transportation of heavy oil. The effects of the present invention will be explained in more detail with reference to Examples below. In addition, in the example, the indentation depth is
Values measured according to DIN standard 30670. Example 1 Shot blasted steel pipe (SGP, 400A)
was preheated to 180°C, and the following composition and blending ratio Maleic anhydride modified crystalline polypropylene homopolymer melt flow index 45g/10 min Maleic anhydride content 3.5×10 ^-5 mol/g Adhesive Compounding ratio 20% by weight Unmodified crystalline ethylene-propylene block copolymer melt flow index 1.0g/10 min Compounding ratio in adhesive 60% by weight Ethylene-propylene copolymer rubber Mooney viscosity 70 Compounding ratio in adhesive 20% by weight %, the low temperature embrittlement temperature is -56℃, the melt flow index is 2.1g/10min, and the density is
0.887g/cm ³ , maleic anhydride content 7.0×
A 10 ^-6 mol/g maleic anhydride-modified polypropylene adhesive was melt-extruded into a circular die to form an adhesive layer with a thickness of 200 μm. This is immediately followed by a cold embrittlement temperature of 10°C and 120°C.
Crystalline ethylene-propylene block copolymer (antioxidant) with an indentation depth of 0.18 mm, a melt flow index of 1.0 g/10 min, a density of 0.922 g/cm ³ , a carbon black concentration of 0.5%, and a nucleating agent content of 0.2%. and an ultraviolet absorber) were melt-extruded using a round die to form a polypropylene coating layer with a thickness of 3 mm on the surface of the steel pipe to obtain a polypropylene-coated steel pipe. For this polypropylene coated steel pipe, measurement of indentation depth at a specified temperature (according to DIN standard 30670), falling weight impact test (ASTM standard G14), cathodic peel test (ASTM standard G8, voltage: -1.5V vsSC)
E., period: 30 days) and hot water immersion experiments were conducted. In the hot water immersion experiment, the coating layer with a width of 10 mm was immersed in hot water at 90°C for 1000 hours, and then peeled at 90 degrees at a peeling rate of 50 mm/min at 23°C using an Instron tensile tester to determine the peel adhesive strength. The results of these tests are summarized in Table 1. Example 2 Shot blasted steel pipe (SGP, 400A)
was preheated to 210℃, and the following composition and blending ratio Maleic anhydride modified crystalline polypropylene homopolymer melt flow index 45g/10 min Maleic anhydride content 3.5×10 ^-5 mol/g Adhesive Compounding ratio 20% by weight Unmodified crystalline ethylene-propylene block copolymer melt flow index 1.0g/10 min Compounding ratio in adhesive 60% by weight Ethylene-propylene copolymer rubber Mooney viscosity 70 Compounding ratio in adhesive 20% by weight %, the low temperature embrittlement temperature is -58℃, the melt flow index is 2.0g/10min, and the density is
0.885g/cm ³ , maleic anhydride content 7.0×
A powdered maleic anhydride-modified polypropylene adhesive of 10 ^-6 mol/g was applied and melted on the surface of the steel pipe to form an adhesive layer with a thickness of 130 μm. This was immediately followed by a low temperature embrittlement temperature of 3°C, an indentation depth of 0.19 mm at 120°C, a melt flow index of 1.0 g/10 min, a density of 0.920 g/cm ³ and a carbon black concentration of 0.5%. A polypropylene coating with a thickness of 3 mm is made by melt-extruding a crystalline ethylene-propylene block copolymer (added with an antioxidant and a UV absorber) with an additive amount of 0.2% using a T-die and spirally wrapping it around the surface of the steel pipe. Obtained steel pipe. Example 1 about this polypropylene coated steel pipe
Performance tests were conducted in the same manner. The results are summarized in Table 1. Example 3 Shot blasted steel pipe (SGP, 400A)
After preheating to 210°C, apply thermosetting epoxy paint (two-component curing type, 100 parts of bisphenol A type epoxy resin with an epoxy equivalent of 190, 40 parts of aromatic amine as a hardening agent, and titanium oxide as a pigment) to the surface. A mixture of 25 parts (25 parts) was applied by spraying and cured to form an epoxy primer layer with a thickness of 30 μm. At this time, the glass transition temperature of the cured coating film was 94°C. Immediately thereafter, Example 2
In exactly the same way as above, the following composition and blending ratio: Maleic anhydride-modified crystalline polypropylene homopolymer melt flow index 45g/10 minMaleic anhydride content: 3.5×10 ^-5 mol/g Mixing ratio in adhesive: 20% by weight Unmodified crystalline ethylene-propylene block copolymer melt flow index 1.0g/10 min Proportion in adhesive 60% by weight Ethylene-propylene copolymer rubber Mooney viscosity 70 Proportion in adhesive 20% by weight A performance test was conducted on the polypropylene-coated steel pipe obtained by applying a melt-kneaded adhesive and coating with polypropylene. The test results are shown in Table 1. Example 4 Shot blasted steel pipe (SGP, 400A)
A paint-on chromate treatment agent was applied to the surface to form a chromate film with a total chromium deposition amount of 600 mg/m ² . Thereafter, it was preheated in exactly the same manner as in Example 2, and the following composition and blending ratio: Maleic anhydride-modified crystalline polypropylene homopolymer melt flow index 45 g/10 minutes Maleic anhydride content: 3.5 x 10 ^-5 mol/g Adhesion Compounding ratio in adhesive: 20% by weight Unmodified crystalline ethylene-propylene block copolymer melt flow index 1.0 g/10 min Compounding ratio in adhesive: 60% by weight Ethylene-propylene copolymer rubber Mooney viscosity 70 Compounding in adhesive A performance test was conducted on a polypropylene-coated steel pipe obtained by applying an adhesive prepared by melt-kneading a proportion of 20% by weight and coating with polypropylene. The results are shown in Table 1. Example 5 Shot blasted steel pipe (SGP, 400A)
A paint-on chromate treatment agent was applied to the surface to form a chromate film with a total chromium deposition amount of 550 mg/m ² . Thereafter, the steel pipe was preheated to 210°C, and the same epoxy paint as in Example 3 was applied to its surface by a spray method, and cured to form an epoxy primer layer with a thickness of 30 μm. Immediately thereafter, Example 2
In exactly the same way as above, the following composition and blending ratio: Maleic anhydride-modified crystalline polypropylene homopolymer melt flow index 45g/10 minMaleic anhydride content: 3.5×10 ^-5 mol/g Mixing ratio in adhesive: 20% by weight Unmodified crystalline ethylene-propylene block copolymer melt flow index 1.0g/10 min Proportion in adhesive 60% by weight Ethylene-propylene copolymer rubber Mooney viscosity 70 Proportion in adhesive 20% by weight A performance test was conducted on a polypropylene-coated steel pipe obtained by applying an adhesive formed by melt-kneading and coating with polypropylene. The results are shown in Table 1. Next, a comparative example using a method other than the present invention will be shown. Comparative example 1 Shot blasted steel pipe (SGP, 400A)
was preheated to 210°C, and the following composition and blending ratio Maleic anhydride-modified crystalline polypropylene block copolymer melt flow index 15.0 g/10 min Maleic anhydride content 1.0 × 10 ^-5 mol/g in adhesive Compounding ratio of 90% by weight Ethylene-propylene copolymer rubber Mooney viscosity 70 Compounding ratio in adhesive 10% by weight Low temperature embrittlement temperature is -5℃, melt flow index is 4.5g/10 min and density is 0.896 g/
cm ³ of powdered maleic anhydride-modified polypropylene adhesive is applied and melted on the steel pipe surface to reduce the thickness.
A 130μ adhesive layer was formed. Immediately following this, a polypropylene coating layer was applied in exactly the same manner as in Example 2, and a performance test was conducted on the resulting polypropylene coated steel pipe. The result is the first
Shown in the table. Comparative example 2 Shot blasted steel pipe (SGP, 400A)
Preheat to 210℃, and apply the following composition and blending ratio maleic anhydride-modified crystalline ethylene-propylene random copolymer melt flow index ^15g /10 minutes on the surface of the adhesive. Blend ratio in adhesive 67% by weight Unmodified crystalline ethylene-propylene block copolymer melt flow index 3.0g/10 min Blend ratio in adhesive 30% by weight Ethylene-propylene copolymer rubber Mooney viscosity 70 Blend ratio in adhesive 3% by weight with a low temperature embrittlement temperature of +14°C, a melt flow index of 6.2g/10min, and a density of 0.888.
A powdered maleic anhydride-modified polypropylene adhesive of g/cm ³ was applied and melted on the surface of the steel pipe to form an adhesive layer with a thickness of 130 μm. This is immediately followed by a cold embrittlement temperature of 30°C or higher and 120°C.
The indentation depth is 0.10 mm, the melt flow index is 0.89 g/10 minutes, the density is 0.918 g/cm ³ ,
A polypropylene-coated steel pipe with a thickness of 3 mm was obtained by melt-extruding homopolypropylene (added with an antioxidant and an ultraviolet absorber) having a carbon black concentration of 0.5% using a T-die and spirally wrapping it around the surface of the steel pipe. Example 1 about this polypropylene coated steel pipe
Performance tests were conducted in the same manner. The results are summarized in Table 1.

[Table] As is clear from Table 1, the indentation depth of the polypropylene-coated steel pipe according to the present invention is 0.2 mm or less at 120°C, and cracks do not occur even when a strong impact is applied to the coating layer at a low temperature of -30°C. Therefore, the coating layer has excellent mechanical properties over a wide temperature range from -30°C to 120°C. Furthermore, as is clear from the test values of comparative examples other than the present invention in Table 1, even if coated steel pipes using homopolypropylene for the coating layer or crystalline ethylene-propylene block copolymer for the coating layer are used, If the low-temperature embrittlement temperature of the coating layer and adhesive layer is outside the scope of the claims of the present invention, the coating layer will crack due to strong impact at temperatures above -30℃, so use at -30℃ is prohibited. I can't stand it. In addition, when cathodic protection is applied to coated steel pipes or when coated steel pipes are used in humid environments, the values of cathodic peel tests and hot water immersion tests become important; in such cases, cleaning treatment is not necessary. As shown in Examples 1 and 2 and the comparative example, the polypropylene-coated steel pipes coated with adhesive directly showed significant peeling in the cathodic peel test, and the adhesion of the coating layer was low after the hot water immersion test. Although only the values are shown, the peel distance in the cathodic peel test was the same for coated steel pipes that were coated with an epoxy primer as in Example 3 before adhesive application and coated steel pipes that were treated with chromic acid-based chemical conversion as in Example 4. The adhesive strength of the coating layer after the hot water immersion test is also significantly improved. This effect is even greater for coated steel pipes that have been subjected to both chromic acid-based chemical conversion treatment and epoxy primer application before applying the adhesive. As detailed above, the present invention has been developed by specifying the molecular structure and physical properties of the materials used for the polypropylene coating layer and adhesive layer, thereby improving the indentation depth of the coating layer at high temperatures while maintaining excellent impact resistance at low temperatures. In addition, if necessary, by forming a chromate-based chemical conversion treatment layer and/or an epoxy primer layer with a high glass transition temperature between the steel pipe surface and the adhesive layer, excellent high-temperature resistance can be achieved. This is an unprecedented invention in that it provides cathode peeling resistance and hot water resistance, and is of great industrial significance.

[Brief explanation of drawings]

Fig. 1 is a cross-sectional view of a polypropylene-coated steel pipe according to the present invention, and Fig. 2 is a part of a cross-section of a polypropylene-coated steel pipe in which an epoxy primer layer with a glass transition temperature of 80°C or higher is formed between the steel pipe and an adhesive layer. , Figure 3 shows a part of a cross section of a polypropylene-coated steel pipe that has been treated with a chromic acid-based chemical conversion treatment between the steel pipe and the adhesive layer, and Figure 4 shows a part of a cross-section of a polypropylene-coated steel pipe that has been treated with a chromic acid-based chemical conversion treatment between the steel pipe and the adhesive layer. This shows a part of a cross section of a polypropylene-coated steel pipe that was subjected to chemical conversion treatment and then formed with an epoxy primer layer with a glass transition temperature of 80°C or higher. FIG. 5 shows the composition of the polypropylene phase and polyethylene phase of a crystalline ethylene-propylene block copolymer. Further, in FIG. 6, appropriate combinations of the low-temperature embrittlement temperature of the polypropylene coating layer and the low-temperature embrittlement temperature of the adhesive are indicated by diagonal lines. DESCRIPTION OF SYMBOLS 1... Steel pipe, 2... Adhesive layer, 3... Polypropylene coating layer, 4... Epoxy primer layer, 5
...Chromic acid-based chemical conversion treatment layer, 6... Polypropylene phase, 7... Ethylene-propylene rubber phase,
8...Polyethylene phase, 9...Appropriate range of low temperature embrittlement temperature of polypropylene coating layer and adhesive layer.

Claims (1)

[Scope of Claims] 1. A steel pipe whose outer surface is pretreated and then coated with polypropylene via an adhesive, the polypropylene being mainly composed of a crystalline ethylene-propylene block copolymer and having a low-temperature embrittlement temperature of 0°C. Maleic anhydride-modified polypropylene made of a mixture of maleic anhydride-modified crystalline polypropylene with a low-temperature embrittlement temperature of -30°C or lower, unmodified crystalline polypropylene, and polyolefin-in rubber, using a resin with a temperature higher than 15℃ or lower as an adhesive. A polypropylene-coated steel pipe characterized by using a polypropylene adhesive. 2. The polypropylene-coated steel pipe according to claim 1, wherein the surface of the steel pipe is cleaned by blasting or the like as a pretreatment, and then an epoxy primer layer having a glass transition temperature of 80° C. or higher is formed. 3. The polypropylene-coated steel pipe according to claim 1, wherein the surface of the steel pipe is cleaned by blasting or the like as a pretreatment and then subjected to a chromic acid-based chemical conversion treatment. 4. Claims characterized in that the surface of the steel pipe is cleaned by blasting or the like as a pretreatment, then subjected to a chromic acid-based chemical conversion treatment, and then an epoxy primer layer with a glass transition temperature of 80°C or higher is formed. The polypropylene coated steel pipe according to item 1.