JPH08187820A - Polyolefin-covered steel pipe and manufacture thereof - Google Patents

Abstract

PURPOSE: To obtain covered steel pipe, which is excellent in resin covering properties in wide range temperature region and has adhesion and corrosion resistance, by a method wherein primer layer contains the specified amount of bisphenol A type epoxy resin, novolak type epoxy resin, phenyl glycidyl ether, hardener and reaction accelerator. CONSTITUTION: The polyolefin-covered steel pipe has primer layer, modified polyolefin adhesive bond and polyolefin resin layer in the order named on the outer peripheral surface of the steel pipe. In this case, the primer layer contains 40-60 pts.wt. of bisphenol A type epoxy resin, 0-30 pts.wt. of novolak type epoxy resin, 15-35 pts.wt. of phenyl glycidyl ether, 10-40 pts.wt. of hardener and 1.5-4 pts.wt. of reaction accelerator. The film of the primer composition can be hardened within the temperature range of 90-150 deg.C or that lower than the conventional one within one minute. As a result, the lamination of the polyolefin resin layer can be performed at the temperature of the steel pipe, which is lower than the melting point of the resin.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is a polyolefin which can be used in a wide temperature range from -60 ° C to 60 ° C and in which the phenomenon of reducing the thickness (uneven thickness) of the coating layer in the weld bead portion of a steel pipe is suppressed. The present invention relates to a coated steel pipe and a manufacturing method thereof.

[0002]

2. Description of the Related Art Polyolefin-coated steel pipe is a steel pipe whose outer peripheral surface is coated with a polyolefin resin such as polyethylene or polypropylene for the purpose of corrosion protection of steel materials. Since it has excellent corrosion resistance, it transports crude oil and natural gas. It is used as a line pipe for.

As the oil field development in recent years has progressed in polar regions, deserts and the like due to the increase in energy demand, the environment in which line pipes are used is becoming severer. Especially, in the environment where the line pipe is used in the Russian polar region, the guaranteed temperature has been
The temperature was up to 45 ℃, but recently demand for the environment up to -60 ℃ has come to be seen.

Under these circumstances, there is a demand for a polyolefin-coated steel pipe that can be used in a wide temperature range from a low temperature (-60 ° C) to a high temperature (60 ° C). It was not possible to meet this requirement in terms of mechanical properties and corrosion resistance.

Generally, a polyolefin resin is a non-polar polymer material and therefore has excellent chemical stability and mechanical properties, but has a problem of poor adhesion to a polar surface such as a steel pipe. is there. Due to this lack of adhesion,
Corrosion resistance and mechanical properties of the polyolefin-coated steel pipe deteriorate. Therefore, a method of coating a steel material with a polyolefin resin layer through an adhesive layer made of a modified polyolefin resin obtained by copolymerizing an olefin with an unsaturated carboxylic acid or an anhydride thereof is generally adopted.

However, although this modified polyolefin adhesive layer has excellent initial adhesion at room temperature,
When exposed to a humid environment for a long time or subjected to a long-term cooling / heating cycle due to cold / hot conditions such as summer / winter or day / night, the adhesion between the steel pipe surface and the polyolefin resin layer deteriorates. As a countermeasure against this, a primer layer such as an epoxy resin is provided under the modified polyolefin adhesive layer.

Therefore, a general polyolefin-coated steel pipe is manufactured continuously by moving the steel pipe by a conveying roller and sequentially passing through the following steps.
First, the outer peripheral surface of the steel pipe is cleaned in a pretreatment step (eg, by degreasing and / or shot blasting, etc.). Thereafter, if necessary, a base treatment for improving the adhesion may be performed. This base treatment is generally performed by chemical conversion treatment such as phosphate treatment or chromate treatment.

If necessary, after preheating the steel pipe treated as described above, a primer is applied to the outer peripheral surface of the steel pipe, and the steel pipe is immediately heated by gas heating, high frequency induction heating, etc., and passed through the next conveying roller. Within a short period of time (usually within about 1 minute), the primer coating is cured until the coating strength is obtained so that the coating is not damaged by passing through the transport roller. After passing through the next conveying roller, the modified polyolefin resin to be the adhesive layer and the polyolefin resin to be the outer coating layer are sequentially or simultaneously laminated on the cured primer layer by a powder coating method or a melt extrusion method. When coated, an olefin coated steel pipe is obtained.

A typical example of the primer is an epoxy type primer containing a bisphenol A type epoxy resin and an amine type curing agent as described in JP-A-57-113871. The polyolefin-coated steel pipe manufactured using the primer has excellent resistance to cathodic electrolytic debonding at room temperature, but it has low adhesive strength at room temperature and low-temperature impact resistance in the temperature range below -45 ° C. Is inferior to

JP-A-60-4054 discloses that a modified polyolefin resin having a low temperature embrittlement temperature of -20 ° C. or lower is formed on a high Tg type epoxy primer layer having a glass transition temperature (Tg) of 80 ° C. or higher. A coated steel tube coated with a crystalline ethylene-propylene block copolymer through an adhesive layer is described. This coated steel pipe has high temperature mechanical properties and corrosion resistance.
Although it is excellent in (cathodic electrolytic peeling resistance and hot water secondary adhesion), the coating layer becomes brittle in the low temperature range of −30 ° C. or lower,
It becomes unusable.

Japanese Unexamined Patent Publication (Kokai) No. 4-175159 discloses a bisphenol diglycidyl ether type bifunctional epoxy resin, an aminophenol type trifunctional epoxy resin, a condensate of m-xylene diamine and epichlorohydrin (amine curing agent). , And a polyolefin-coated steel pipe formed from a composition containing an inorganic pigment, also having a high Tg type epoxy primer layer. However, although this coated steel pipe is excellent in high temperature corrosion resistance, when subjected to an impact test at an extremely low temperature of -60 ° C, cracking on the coating surface, further peeling, and breakage occur. Have.

The heating of the steel pipe after the primer is applied not only cures the primer coating film, but also preheats the steel pipe for laminating the following adhesive layer made of modified polyolefin and coating layer made of polyolefin resin. Conventionally, the heating of the steel pipe at this time is 150 ° C or higher, which is higher than the melting point of the coating resin, typically about 180 ° C in the case of polyethylene resin coating, so that the coating layer melts and adheres well during lamination. ,
In the case of polypropylene resin coating, it was usual to carry out the steel pipe temperature around 200 ° C.

For example, in Japanese Examined Patent Publication No. 61-12516, a metal surface is heated to a temperature of 150 to 250 ° C., an epoxy resin composition is applied thereto, and the temperature of an object to be coated is 140 ° C. or more. It is described that a polyolefin resin is coated between
Japanese Examined Patent Publication (Kokoku) No. 3-29588 discloses that a steel pipe is preheated to 180 to 210 ° C. and then coated with polypropylene through a modified polypropylene adhesive layer to coat the steel pipe.

However, when a polyolefin-coated steel pipe is manufactured by the conventional method, there is a problem that a film thickness of the coating resin layer becomes thin at the weld bead portion of the steel pipe (that is, uneven thickness). Therefore, the bead of the steel pipe must be cut in advance or the thickness of the coating layer must be increased in consideration of the uneven thickness margin, which is also disadvantageous in terms of manufacturing cost.

[0015]

DISCLOSURE OF INVENTION Problems to be Solved by the Invention
Polyolefin coated steel pipe that can be used in a wide temperature range from ℃ to 60 ℃, that is, the resin coating shows excellent adhesion and corrosion resistance throughout this temperature range, withstands long-term cooling and heating cycles, -60 ℃ However, it is an object of the present invention to provide a polyolefin-coated steel pipe and a method for producing the same, which does not cause brittleness of the coating layer and exhibits good impact resistance. Another object of the present invention is to provide a polyolefin-coated steel pipe in which uneven thickness of the coating layer in the weld bead portion of the steel pipe is suppressed, and a method for producing the same.

[0016]

As a cause of the uneven thickness of the coating layer at the weld bead portion, it is considered that the temperature of the steel pipe when the coating layer is laminated is too high. The lamination of the polyolefin resin coating layer is performed immediately after preheating the steel pipe to 150 ° C or higher in order to harden the primer coating applied earlier, so the decrease in the temperature of the steel pipe from the preheating to the lamination of the coating layer is very slight. (At most about 10 ℃). Therefore, especially when the coating layer is a polyethylene resin layer, the temperature of the steel pipe when the coating layer is laminated will exceed the melting point of the polyethylene resin to be coated (generally about 110 to 135 ° C), and the laminated polyethylene resin will melt. It is presumed that the uneven thickness at the bead portion of the coating layer becomes significant due to the flow of the metal.

Therefore, it is considered that uneven thickness can be prevented by lowering the steel pipe temperature during lamination, but conventionally, in order to cure the primer coating film in a short time within 1 minute, the steel pipe temperature of 150 ° C. or higher is required. It has been considered that heating is required and that the coating layer is laminated at a temperature higher than the melting temperature of the coating resin in order to secure the adhesion of the coating layer.

The present inventors have found that an epoxy-based primer obtained by combining a bisphenol A type epoxy resin or a novolak type epoxy resin with phenylglycidyl ether as a reactive diluent can be cured at a lower temperature than conventional epoxy-based primers. In addition, the adhesiveness between the steel pipe and the modified polyolefin adhesive layer can be enhanced over a wide temperature range, and as a result, the primer coating can be shortened by heating such that the steel pipe temperature during lamination is lower than the melting point of the coated polyolefin resin. It has been found that the above object can be achieved by allowing the coating to be cured in a time and ensuring the adhesiveness of the coating layer.

The present invention provides a polyolefin-coated steel pipe having a primer layer, a modified polyolefin adhesive layer, and a polyolefin resin layer on the outer peripheral surface of the steel pipe in this order from the bottom, wherein the primer layer is a bisphenol A type epoxy resin 40- Formed from a primer composition containing 60 parts by weight, 0 to 30 parts by weight of a novolac type epoxy resin, 15 to 35 parts by weight of phenylglycidyl ether, 10 to 40 parts by weight of a curing agent, and 1.5 to 4 parts by weight of a reaction accelerator. A polyolefin-coated steel pipe characterized by being an epoxy-based primer layer.

The coating film of this primer composition is 90-150.
It can be cured within 1 minute in a lower temperature range of ℃ than before. As a result, the lamination of the polyolefin resin layer,
It is possible to carry out at a steel pipe temperature lower than the melting point of this resin, and it is possible to suppress uneven thickness of the coating layer at the bead portion.

Accordingly, from another aspect, the present invention provides a primer layer, a modified polyolefin adhesive layer, and an outer peripheral surface of a steel pipe.
And a method for producing a polyolefin-coated steel pipe in which polyolefin resin layers are laminated in order, wherein the temperature of the steel pipe during lamination of the polyolefin resin layer is 90 ° C. or higher and the melting point of the polyolefin resin or lower, the polyolefin coating It is a method of manufacturing a steel pipe. Such lamination of the coating layer at a lower temperature than before can be achieved by forming the primer layer from the above primer composition.

In a preferred embodiment, (1) the polyolefin resin layer is a polyethylene resin layer, and (2) the modified polyolefin adhesive layer has a low temperature embrittlement temperature of -50 ° C or less and a modification rate of 0.2% or more. Polyethylene resin layer, (3)
The curing agent of the primer layer is an amine curing agent,
(4) The glass transition temperature of the primer layer is 60 to 80 ° C., and (5) a primer treatment layer, particularly preferably a chromate treatment layer, is provided below the primer layer. Thereby,
A polyolefin coating that has a resin coating layer that exhibits adhesion and corrosion resistance in a wide temperature range from -60 ° C to 60 ° C, withstands long-term cooling / heating cycles, does not become brittle even at -60 ° C, and has good impact resistance. A steel pipe can be obtained.

[0023]

Hereinafter, the present invention will be described in detail. The polyolefin-coated steel pipe of the present invention has three layers of a primer layer, a modified polyolefin adhesive layer, and a polyolefin resin coating layer in order from the bottom on the outer peripheral surface of the steel pipe, similarly to the conventional product, and preferably further under the primer layer. It has an undercoat layer, particularly preferably a chromate film layer.

The type and size of the steel pipe (original pipe) to be coated are not particularly limited, and a steel pipe made of any steel type (eg, carbon steel, alloy steel, stainless steel, etc.) may be used. In the case of a line pipe for transporting crude oil or natural gas, it is usually a carbon steel pipe. Also, a plated steel pipe can be used as the raw pipe.

Prior to coating, it is preferable that the outer peripheral surface of the steel pipe is degreased, and mill scale is removed by polishing with grit or shot blast to clean the steel pipe. The primer composition may be applied immediately to the surface of the cleaned steel pipe, but in order to enhance the corrosion resistance of the polyolefin-coated steel pipe and the adhesion of the resin coating layer, the primer composition should be applied after forming a base treatment layer. Is preferred.

The undercoat layer can be formed by a chemical conversion treatment such as a chromate treatment or a phosphate treatment (zinc phosphate, etc.). A preferred base treatment layer is a chromate treatment layer. The chromate treatment liquid to be used is not particularly limited, but from the viewpoint of anticorrosion property, a coating type chromate treatment liquid containing fine silica powder and hexavalent and trivalent chromium ions is preferable. Alternatively, if desired, phosphoric acid may be further added for use.

The chromate treatment liquid may be applied to the outer peripheral surface of the steel pipe by a known method such as airless spraying, spray coating, and ironing. After applying the chromate treatment liquid, the steel pipe is heated to bake the chromate film. As a method for heating the steel pipe, conventionally known methods such as high frequency induction heating, far infrared heating, gas heating and the like can be applied. Chromate coating amount is 50-1000 mg /
It is preferably m ² , particularly in the range of 100 to 500 mg / m ² .

According to the present invention, the epoxy-based primer layer comprises a bisphenol A type epoxy resin or a mixture of a bisphenol A type epoxy resin and a novolac type epoxy resin mixed with phenyl glycidyl ether, a curing agent, and a reaction accelerator. Formed from a primer composition. As a result, it is possible to cure the primer coating at a low temperature of 150 ° C or less in a short time, and from -60 ° C.
It is possible to obtain a polyolefin-coated steel pipe that exhibits excellent adhesion and corrosion resistance in a wide temperature range up to 60 ° C.

Examples of the bisphenol A type epoxy resin include bisphenol A diglycidyl ether and bisphenol A having an epoxy equivalent of about 170 to 3000.
Type epoxy resin can be used. Other bisphenol type epoxy resins (eg hydrogenated bisphenol A diglycidyl ether, bisphenol AD diglycidyl ether, bisphenol F diglycidyl ether) are also available in small amounts.
(Amount of 30% by weight or less of the bisphenol A type epoxy resin) can be used together.

The novolac type epoxy resin may be either cresol novolac type or phenol novolac type. Bifunctional bisphenol A as an epoxy resin
By using the polyfunctional novolak type epoxy resin in combination with the type epoxy resin, the curing speed of the primer coating film at a relatively low temperature is further accelerated. Phenylglycidyl ether is a reactive diluent added for adjusting the viscosity of the primer composition and improving the curing rate. The phenyl group of phenylglycidyl ether may have an alkyl group or other substituents.

Although any epoxy curing agent can be used as the curing agent, an amine curing agent such as a heterocyclic diamine modified product, a fatty acid polyamine modified product, an aromatic polyamine modified product, or a polyamidoamine modified product is preferable. From the viewpoints of low temperature impact resistance and cathodic electrolytic peeling resistance, a heterocyclic diamine modified product is particularly preferable.

As the reaction accelerator, for example, a known imidazole type reaction accelerator can be used. Examples of commercially available products include those commercially available under the trade name Cureazole from Shikoku Kasei.

The mixing ratio of each component in the primer composition is as follows. Bisphenol A type epoxy resin: 40 to 60 parts by weight, novolac type epoxy resin: 0 to 30 parts by weight, phenylglycidyl ether: 15 to 35 parts by weight, curing agent: 10 to 40 parts by weight, reaction accelerator: 1.5 to 4 parts by weight Department.

If the proportion of the bisphenol A type epoxy resin is too small, the anticorrosion property is deteriorated. On the other hand, if the proportion of bisphenol A type epoxy resin or phenylglycidyl ether is too high, or if the proportion of novolac type epoxy resin, curing agent or reaction accelerator is too low, the curing rate of the primer coating at a low temperature of 150 ° C or less And the coating film is likely to be damaged at the next contact with the transport roller.
On the other hand, if the proportion of phenylglycidyl ether is too small, or if the proportion of the novolac type epoxy resin, the curing agent or the reaction accelerator is too large, the viscosity of the obtained primer composition increases, leading to a decrease in pot life. The gelation of the primer composition during the continuous coating work progresses to deteriorate the coating workability.

The epoxy primer composition can be prepared as a one-pack type or a two-pack type composition. In the case of the one-pack type, the reaction accelerator and the curing agent are microencapsulated and are dispersed in a resin liquid consisting of the remaining components in advance before use to be applied. In the case of a two-pack type, a solution containing a curing agent and a resin solution containing the remaining components can be mixed before use and provided for coating. In order to make the viscosity of the primer composition suitable for coating, if necessary,
You may dilute with organic solvents, such as toluene and xylene.

The primer composition can be applied by a conventionally known technique such as spray coating, roller coating, brush coating, ironing coating and flow coating. Then, the surface of the steel pipe is heated to cure the primer coating film and form an epoxy-based primer layer. The Tg of the epoxy primer layer formed after curing is preferably in the range of 60 to 80 ° C. The Tg of the primer layer can be adjusted by the ratio of the above two types of epoxy resins and phenylglycidyl ether. When Tg is lower than 60 ° C, high temperature corrosion resistance is lowered, and when Tg is higher than 80 ° C, low temperature impact resistance at -60 ° C is deteriorated. A more preferable Tg is in the range of 65 to 75 ° C. The thickness of the epoxy primer layer is 5-100 μm,
In particular, the range of 10 to 50 μm is preferable.

The heating of the steel pipe for curing the primer coating film is carried out in a continuous line so that the temperature of the steel pipe at the time of laminating the polyolefin resin layer is 90 ° C. or higher and not higher than the melting point of the polyolefin resin to be coated. To do. Since the lamination of the polyolefin resin layer is carried out immediately after the curing of the primer coating film, the temperature drop of the steel pipe from the heating to the lamination of the coating layer is at most 10 ° C or less. Therefore, it is necessary to limit the heating temperature of the steel pipe for curing the primer coating film to such a temperature that the temperature of the steel pipe is not higher than about 10 ° C. higher than the melting point of the polyolefin resin to be coated. Further, the lower limit of the heating temperature is about 95 ° C in consideration of the temperature decrease until the coating layers are laminated.

The temperature of the steel pipe after the specific application of the primer is
About 95-130 when the coating resin is polyethylene resin
It is preferably in the range of about 95 to 145 ° C when the coating resin is a polypropylene resin. Within this range,
The heating temperature is selected so that the primer coating cures within about 1 minute. As a result, it is possible to avoid damage to the primer layer during the passage of the next conveying roller.

When the chromate film is provided as the undercoat layer under the primer layer, the steel pipe is already heated for baking the chromate film, and the steel pipe is heated to a predetermined temperature required for hardening in a relatively short time. Can be heated. When the primer composition is directly applied to the steel pipe without providing the chromate film, the steel pipe may be preheated to an appropriate temperature before applying the primer composition. Thereby, the time required for heating for curing the primer coating film can be shortened. However, it goes without saying that such preheating must be lower than the heating temperature for curing the primer coating film. Similarly, the baking temperature of the chromate film is preferably lower than the heating temperature for curing the primer coating film.

An adhesive layer made of a modified polyolefin resin is coated on the epoxy-based primer layer, and a polyolefin resin layer, which is a target coating layer, is coated thereon. The coating of both layers is carried out by a conventional method, for example, by extrusion of a resin such as a T-die or a round die at the same time (coextrusion method) or in order to extrude the resin film onto the steel pipe so as to coat the preheated steel pipe. It can be performed by stacking. In particular, extrusion coating by the T-die method is suitable. The coating of the adhesive layer can also be performed by electrostatically coating the modified polyolefin resin powder. Then, the steel pipe is cooled by water cooling or the like to obtain the polyolefin-coated steel pipe of the present invention.

The thickness of each layer is within the range of 0.1 to 0.6 mm for the adhesive layer, particularly 0.2 to 0.5 mm, and 2 to 5 mm for the coating layer, especially 2.5.
It is preferably within the range of up to 4 mm. As the modified polyolefin resin for the adhesive layer, a polyolefin resin such as polyethylene or polypropylene which is modified by copolymerization with an unsaturated carboxylic acid such as maleic acid, acrylic acid or methacrylic acid, or an acid anhydride thereof, or these A mixture of the modified product and the polyolefin resin can be used.

As the polyolefin resin for the coating layer, low density polyethylene, medium density polyethylene, high density polyethylene, polypropylene or the like can be used. Polyethylene can also be used as a copolymer with small amounts of other α-olefins (eg acrylic acid). Further, polypropylene is preferably used as a copolymer with a small amount of ethylene from the viewpoint of preventing low temperature embrittlement.

From the viewpoint of the adhesion of the coating layer, it is preferable to use a modified polyolefin resin of the same kind as the polyolefin resin of the coating layer for the adhesive layer. That is, when the coating layer is a polyethylene resin, a modified polyethylene resin is preferably used as the adhesive layer, and when the coating layer is a polypropylene resin, a modified polypropylene resin is preferably used as the adhesive layer.

In order to obtain a polyolefin-coated steel pipe which can be used in a wide temperature range from -60 ° C to 60 ° C, the coating layer is made of polyethylene resin, the adhesive layer has a low temperature embrittlement temperature of -50 ° C or less, and a modification rate of 0.2% or more. It is preferable to form the modified polyethylene resin. More preferably, from the viewpoint of low temperature resistance, the coating layer is low density or medium density polyethylene,
It is composed of a linear low-density polyethylene whose adhesive layer is modified with maleic acid or maleic anhydride, with a low-temperature embrittlement temperature of -60 ° C or less and a modification rate of 0.20% or more. Here, the modification rate means the content rate of the modified material (eg, maleic acid or maleic anhydride) in the whole resin (copolymerization rate, weight%).
Is. The modification rate of the modified polypropylene resin is preferably 0.12% or more.

According to the present invention, the temperature of the steel pipe when laminating the polyolefin resin of the coating layer is set to 90 ° C. or higher and not higher than the melting point of the polyolefin resin of the coating layer. Conventionally, since the coating layer was generally laminated at a temperature higher than the melting point of the polyolefin resin of the coating layer, in the present invention, the lamination of the coating layer, and therefore the curing of the primer coating before that, is performed at a lower temperature than before. Become. Thus, even if the heating temperature of the steel pipe is low, by forming the primer layer from the above primer composition, the primer coating film can be cured in a short time, and the polyolefin resin of the coating layer also has a wide temperature range. It is possible to obtain a polyolefin-coated steel pipe that can maintain high adhesion and can be used in a wide temperature range of -60 ° C to 60 ° C, which was not possible with conventional products. Further, since the temperature of the steel pipe at the time of laminating the coating layer is equal to or lower than the melting point of the coating resin, unnecessary flow of the coated polyolefin resin is unlikely to occur, and uneven thickness of the coating layer at the bead portion is significantly reduced.

If the temperature of the steel pipe at the time of laminating the coating layer is lower than 90 ° C., the curing reaction of the primer coating film does not proceed sufficiently within a short time of less than 1 minute, resulting in coating film damage due to contact with the transport roller. . On the other hand, when the temperature of the steel pipe exceeds the melting point of the polyolefin resin, not only the uneven thickness of the bead portion of the coating layer becomes large, but also the thermal energy required for heating the steel pipe becomes large, which is disadvantageous in terms of cost. The preferable steel pipe temperature at this time is 95 to 125 ° C when the coating layer is polyethylene resin, and when the coating layer is polypropylene resin.
Within the range of 120 to 145 ° C.

The heating temperature of the steel pipe for hardening the primer layer is too high or too low, and the temperature of the steel pipe during lamination is
If the temperature does not fall within the range of 90 ° C or higher and the melting point of the coating layer resin or lower, the steel pipe temperature during lamination of the coating layer may be adjusted to this range by cooling (forced or natural cooling) or reheating. This is not desirable because it complicates the manufacturing process or increases the line length. Therefore, it is preferable to adjust the heating temperature for curing the primer coating film so that the temperature of the steel pipe during the lamination of the coating layer falls within the above range.

[0048]

EXAMPLES The present invention will be specifically described below with reference to Examples. In the examples, all parts are parts by weight.

[0049]

Example 1 A carbon steel tube (outer diameter: 24 inch × wall thickness: 15 mm × length: 10 m), which was shot-blasted, was 1 m / min.
While passing between the transport rollers at a moving speed of, after passing through the first transport roller, apply silica-containing coating type chromate treatment liquid (Cr ⁶⁺ / total Cr ratio = 0.59 to 0.65) to the outer peripheral surface of Heated to ℃ and baked, total Cr adhesion amount is 250 mg / m
A chromate treated layer of ² was formed.

After passing through the second conveying roller, the two-pack type epoxy primer composition was applied on the chromate-treated layer by air spraying so that the film thickness after drying was about 20 μm, and immediately at 130 ° C. The coating film was cured by heating for 1 minute to form an epoxy primer layer having a Tg of 74 ° C. The composition of the epoxy-based primer composition used after mixing the two liquids was as follows: 50 parts of bisphenol A type epoxy resin (bisphenol A diglycidyl ether), 10 parts of phenol type novolac type epoxy resin, 20 parts of phenylglycidyl ether.
Parts, 30 parts of a heterocyclic diamine-modified product of a curing agent, and an imidazole-based reaction accelerator (CUREZOL manufactured by Shikoku Kasei) 2
The solid content was 75 wt% (solvent: toluene).

After passing through the third conveying roller, a maleic anhydride modified polyethylene resin (modification rate 0.22%,
Low temperature embrittlement temperature -60 ℃) and low density polyethylene resin (density 0.924 g / cm ³ , melting point 114 ℃) to form the coating layer by T-die method so that the film thickness becomes 0.4 mm and 3.0 mm respectively in this order. A polyethylene-coated steel pipe was obtained by extrusion coating and then water cooling. The temperature of the steel pipe surface when the coating layer was laminated was 130 ° C.

[0052]

Example 2 Example 1 was repeated except that medium density polyethylene (density 0.935 g / cm ³ , melting point 126 ° C.) was used as the polyethylene for the coating layer, and the temperature of the steel pipe when the coating layer was laminated was 130 ° C.
A polyethylene-coated steel pipe was obtained in the same manner as in.

[0053]

Example 3 As an epoxy primer, 40 parts of bisphenol A type epoxy resin, 0 part of phenol type novolac type epoxy resin, 25 parts of phenyl glycidyl ether,
A polyethylene-coated steel pipe was obtained in the same manner as in Example 1 except that a composition containing 25 parts of a heterocyclic diamine modified product of a curing agent and 3 parts of an imidazole reaction accelerator was used.

[0054]

Comparative Example 1 The composition described in JP-A-4-175159,
That is, 100 parts of bisphenol A type epoxy resin (bisphenol A diglycidyl ether), aminophenol type trifunctional epoxy resin (YX-
4) 100 parts, and a condensation product of m-xylediamine and epichlorohydrin (Gascamin G328 manufactured by Mitsubishi Gas Chemical Co., Inc.) 5
A polyethylene-coated steel pipe was obtained in the same manner as in Example 1 except that an epoxy-based primer layer was formed using the composition of 8.6 parts and the temperature of the steel pipe at the time of laminating the coating layer was 190 ° C.

[0055]

Comparative Example 2 As an epoxy type primer, 30 parts of bisphenol A type epoxy resin, 0 part of phenol type novolac type epoxy resin, 20 parts of phenyl glycidyl ether,
A polyethylene-coated steel pipe was obtained in the same manner as in Example 1 except that a composition containing 30 parts of a heterocyclic diamine modified product of a curing agent and 3 parts of an imidazole-based reaction accelerator was used.

[0056]

[Comparative Example 3] As an epoxy type primer, 50 parts of bisphenol A type epoxy resin, 10 parts of phenol type novolac type epoxy resin, 25 parts of phenyl glycidyl ether,
A polyethylene-coated steel pipe was obtained in the same manner as in Example 1 except that a composition containing 7 parts of a modified heterocyclic diamine as a curing agent and 3 parts of an imidazole-based reaction accelerator was used.

The polyethylene-coated steel pipes obtained in the above Examples and Comparative Examples were tested for adhesive strength, low temperature impact resistance, high temperature corrosion resistance, and cold / heat cycle resistance by the following methods. The results are summarized in Table 1.

(Adhesive Strength) A large number of linear scratches having a width of 10 mm in the transverse direction (circumferential direction) were inserted into the polyethylene coating layer of the polyethylene-coated steel pipe at a depth reaching the steel surface, and after peeling off a part of the coating layer. , The adhesion between the steel surface and the polyethylene film
It was measured by a spring balance method at a constant temperature of ° C. At this time, the peeling angle was 180 °, and the peeling speed was 10 mm / min.

(Low temperature impact resistance) From polyethylene-coated steel pipe
After cutting a cut sample of 300 × 300 mm, it was immersed in a mixed bath of liquid nitrogen and chlorofluorocarbon and cooled to −60 ° C. The cooled sample was subjected to a drop impact test (drop impact 30J) according to the ASTM G-14 method, and after the test was completed, the presence or absence of cracks on the sample surface was visually determined.

(High Temperature Corrosion Resistance) The high temperature corrosion resistance was evaluated by the cathodic electrolytic peeling resistance at 60 ° C. From polyethylene coated steel pipe
After cutting out a 200 x 200 mm cut sample, drill a drill hole with a diameter of 9 mm until the steel surface substrate is exposed.
The sample was immersed in a 3% saline solution at a constant temperature of 60 ° C, a voltage was applied to the sample, and the voltage for the SCE standard electrode was set to -1.5V.
After standing for 30 days under these conditions, the polyethylene layer in the drill hole portion was cut off, and the peeled diameter was measured.

(Cold / heat cycle resistance) A 150 × 70 mm cut sample was taken from a polyethylene-coated steel pipe, and the temperature was −60 ° C. × 1.
A series of cooling / heating cycles of time → heating 3 hours → 70 ° C. × 1 hour → cooling 7 hours were applied 20 times. With respect to the sample after the test, the presence or absence of peeling of the polyethylene film on the end face was visually observed.

[0062]

[Table 1]

As can be seen from the test results in Table 1, the polyethylene-coated steel pipe of the present invention has excellent mechanical properties and corrosion resistance in the temperature range from -60 ° C to 60 ° C, and also has good cold-heat cycle resistance. there were. Therefore, it can be seen that it can be used in a wide temperature range from -60 ° C to 60 ° C.

When the chromate treatment was omitted in Example 1, the low temperature impact resistance, the cathodic electrolytic peeling resistance, and the cold-heat cycle resistance were all deteriorated. Therefore, it can be seen that it is advantageous to perform the undercoat treatment, particularly the chromate treatment, in order to make it usable in the above wide temperature range.

[0065]

[Example 5] Shot-blasted steel pipe (outer diameter 24 inches x wall thickness 15 mm x length 10 m, weld bead height 0.7 mm)
Was continuously treated as follows while moving at a moving speed of 1 m / min between transport rollers provided at intervals of 5 m.

After passing through the first transport roller, the surface of the steel pipe was
Immediately after being preheated to 0 ° C. and passing through the second transport roller, the primer composition having the composition shown in Table 2 had a film thickness after drying of about
It is applied by spray method so that it becomes 20 μm, and immediately the high temperature induction heating heats the steel pipe to a predetermined temperature for 1 minute,
The primer coating was cured before passing through the third transport roller to form a primer layer.

After passing through the third conveying roller, the presence or absence of coating film damage due to contact of the primer layer immediately after passing with the roller was visually judged. If no damage was found on the primer coating, immediately after passing through the third transport roller, the modified polyolefin resin for the adhesive layer and the polyolefin resin for the coating layer were formed on the produced primer layer by the T-die method. The polyolefin-coated steel pipe was obtained by extrusion-coating in order to obtain film thicknesses of 0.3 mm and 3.0 mm, and then water cooling. The temperature of the steel pipe at the time of laminating the polyolefin resin of the coating layer was determined by measuring the surface temperature of the steel pipe immediately before the laminating with a contact type surface thermometer. The temperature of the steel pipe was achieved by heating after applying the primer composition, and the temperature decrease between heating and laminating the coating layer was about 5 ° C.

Low density, medium density and high density polyethylene (PE) resins and polypropylene (PP) resins were used as the polyolefin resin for the coating layer. The melting point of each resin was as shown in the margin of Table 3. As the modified polyolefin resin for the adhesive layer, a maleic anhydride modified polyethylene resin having a modification rate of 0.22% when the coating layer is polyethylene, and a modification rate of 0 when the coating layer is polypropylene.
A 15% maleic anhydride modified polypropylene resin was used.

Workability (presence or absence of gelation) at the time of applying the primer composition, presence or absence of coating film damage when passing through the third conveying roller, steel pipe temperature at the time of coating layer lamination, and the obtained polyolefin coating Table 3 shows the measurement results of the uneven thickness value of the weld bead portion of the steel pipe. The thickness deviation value is the difference between the thickness A of the polyolefin resin layer (coating layer + adhesive layer) in the non-bead portion and the thickness B of the polyolefin resin layer in the bead portion, as shown in FIG. The thicknesses A and B were obtained from a micrograph of a cross section of the coated steel pipe.

[0070]

[Table 2]

[0071]

[Table 3]

As can be seen from the results shown in Table 3, when the epoxy primer layer was formed from the primer composition within the scope of the present invention, the temperature of the steel pipe at the time of laminating the coating layer was the melting point of the polyolefin resin of the coating layer. The primer coating film can be cured in a short time at a lower heating temperature, which is lower than before, and damage to the coating film when passing through the transport roller was avoided. By laminating the coating layer at this relatively low temperature, the phenomenon of uneven thickness (uneven thickness) of the coating layer in the weld bead portion of the steel pipe was significantly reduced.

On the other hand, in the case of the primer composition outside the scope of the present invention, the composition is gelled and cannot be applied, or the coating film cures slowly at a low temperature and is sufficiently cured by heating for a short time. Without it, the coating film was damaged by the contact with the transport roller. Even if the primer composition was within the scope of the present invention, when the temperature of the steel pipe during the lamination of the coating layer exceeded the melting point of the coating layer resin, the uneven thickness increased.

[0074]

The following effects can be obtained by the present invention. Forming an epoxy-based primer layer from a primer composition within the scope of the present invention, preferably by using in combination with a chromate treatment on the base, it is possible to maintain the initial adhesion even after long-term cooling cycle, at -60 ℃ The low temperature impact resistance of is significantly improved.

When the coating layer is a polyethylene resin and the adhesive layer is a modified polyethylene resin having a low temperature embrittlement temperature of −50 ° C. or less and a modification rate of 0.2% or more, further strong adhesion of the coating layer is possible and the adhesion is improved. Strength is increased and high temperature corrosion resistance is improved. From the above, the polyolefin-coated steel pipe of the present invention can be used in a wide temperature range of -60 ° C to 60 ° C.

Since the primer coating film can be cured at a temperature lower than that conventionally used in a short time, the coating layer can be laminated at a steel pipe temperature lower than the melting point of the polyolefin resin to be coated, and the weld bead portion of the steel pipe can be unevenly distributed. Meat is reduced. As a result, there is no need to cut the bead of the steel pipe or to thicken the coating layer in consideration of the uneven thickness allowance, and the energy cost required for heating the steel pipe can be reduced, which is economically advantageous.

[Brief description of drawings]

FIG. 1 is a diagram showing an uneven thickness value in a bead portion of a polyolefin resin pipe.

Claims (5)

[Claims] 1. In a polyolefin-coated steel pipe having a primer layer, a modified polyolefin adhesive layer, and a polyolefin resin layer on the outer peripheral surface of the steel pipe in this order from the bottom, the primer layer comprises a bisphenol A type epoxy resin 40-
60 parts by weight, novolac type epoxy resin 0 to 30 parts by weight, phenyl glycidyl ether 15 to 35 parts by weight, curing agent 10 to 40
A polyolefin-coated steel pipe, which is an epoxy-based primer layer formed from a primer composition containing 1 part by weight and 1.5 to 4 parts by weight of a reaction accelerator. 2. The polyolefin resin layer is a polyethylene resin layer, the modified polyolefin adhesive layer is a modified polyethylene resin layer having a low temperature embrittlement temperature of −50 ° C. or less and a modification rate of 0.2% or more, and the primer layer is cured. The polyethylene-coated steel pipe according to claim 1, wherein the agent is an amine-based curing agent, and the glass transition temperature of the primer layer is 60 to 80 ° C. 3. The polyolefin-coated steel pipe according to claim 1, further comprising an undercoat layer under the primer layer. 4. In a method for producing a polyolefin-coated steel pipe, in which a primer layer, a modified polyolefin adhesive layer, and a polyolefin resin layer are sequentially laminated on the outer peripheral surface of the steel pipe, the steel pipe temperature during lamination of the polyolefin resin layer is 90.
A method for producing a polyolefin-coated steel pipe, characterized in that the temperature is not lower than 0 ° C and not higher than the melting point of the polyolefin resin. 5. The primer layer is bisphenol A.
Type epoxy resin 40 to 60 parts by weight, novolac type epoxy resin 0 to 30 parts by weight, phenyl glycidyl ether 15 to 35 parts by weight, curing agent 10 to 40 parts by weight, and reaction accelerator 1.5 to 4
The method for producing a polyolefin-coated steel pipe according to claim 4, which is an epoxy-based primer layer formed from a primer composition containing parts by weight.