JP2988302B2 - Polyolefin-coated steel pipe and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polyolefin which can be used in a wide temperature range from -60.degree. C. to 60.degree. The present invention relates to a coated steel pipe and a method for manufacturing the same.

[0002]

2. Description of the Related Art A polyolefin-coated steel pipe is a steel pipe in which the outer peripheral surface of a steel pipe is coated with a polyolefin resin such as polyethylene or polypropylene for the purpose of preventing corrosion of steel, and because of its excellent corrosion resistance, transportation of crude oil and natural gas. Used as a line pipe for

[0003] As the development of oil fields has progressed in polar regions and deserts due to an increase in energy demand in recent years, the use environment of line pipes has been becoming severer. In particular, the operating temperature of line pipes in the Russian region has been
Although the temperature was up to 45 ° C, recently there has been a demand for an environment up to -60 ° C.

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.). This requirement could not be met in terms of mechanical properties and corrosion resistance.

[0005] In general, 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 are reduced. For this reason, a method of coating a steel material with a polyolefin resin layer via 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, 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 such as summer / winter or day / night, the adhesion between the steel pipe surface and the polyolefin resin layer is reduced. As a countermeasure, a primer layer such as an epoxy resin is provided below the modified polyolefin adhesive layer.

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

[0008] After preheating the steel pipe treated as described above, if necessary, apply a primer to the outer peripheral surface of the steel pipe, immediately heat the steel pipe by gas heating, high-frequency induction heating, etc., and pass it through the next transport roller. Within a short period of time (usually within about 1 minute), the primer coating is cured until a coating strength is obtained that does not damage the coating by passing through a transport roller. After passing through the next conveyor 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 powder coating or melt extrusion. Upon coating, an olefin-coated steel pipe is obtained.

A typical primer is an epoxy primer containing a bisphenol A type epoxy resin and an amine type curing agent as described in JP-A-57-113871. Polyolefin-coated steel pipes manufactured using primers are excellent in cathodic electrolytic peeling resistance at room temperature, but have low adhesive strength at room temperature and low-temperature impact resistance in a temperature range below -45 ° C. Is also inferior.

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 pipe coated with a crystalline ethylene-propylene block copolymer via an adhesive layer is described. This coated steel tube has high mechanical properties and corrosion resistance at high temperatures.
(Cathode electrolytic peeling resistance, hot water secondary adhesion) is excellent, but in the low temperature range of -30 ° C or less, the coating layer becomes brittle,
Becomes unusable.

Japanese Patent Application Laid-Open 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 type curing agent) A polyolefin-coated steel tube having an epoxy primer layer, also of high Tg type, formed from a composition containing, and inorganic pigments is described. 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, there is a problem that cracks occur on the coating surface, and further, peeling and destruction occur. Have.

The heating of the steel pipe after the application of the primer serves not only to cure the primer coating but also to preheat the steel pipe for laminating the following adhesive layer made of modified polyolefin and a 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 around 180 ° C for polyethylene resin coating, so that the coating layer melts and adheres well during lamination. ,
In the case of coating with a polypropylene resin, the temperature was usually adjusted to a steel pipe temperature of about 200 ° C.

For example, Japanese Patent Publication No. 12516/1986 discloses that after a metal surface is heated to a temperature of 150 to 250 ° C., an epoxy resin composition is applied thereon, and the temperature of an object to be coated is 140 ° C. or higher. It is described to coat a polyolefin resin in between,
Japanese Patent Publication No. 3-29588 discloses that a steel pipe is coated by preheating a steel pipe to 180 to 210 ° C. and then coating a polypropylene via a modified polypropylene adhesive layer.

However, when a polyolefin-coated steel pipe is manufactured by a conventional method, there is a problem that a phenomenon in which the thickness of the coating resin layer becomes thinner at the weld bead portion of the steel pipe (that is, uneven thickness) occurs. For this reason, it is necessary to cut the bead of the steel pipe in advance or to increase the thickness of the coating layer in anticipation of the thickness deviation, which is disadvantageous in terms of manufacturing cost.

[0015]

SUMMARY OF THE INVENTION The object of the present invention is to solve the problem of -60.
Polyolefin-coated steel pipe that can be used in a wide temperature range from 60 ° C to 60 ° C, that is, the resin coating shows excellent adhesion and corrosion resistance throughout this temperature range, withstands long-term cooling and heating cycles, and has a temperature of -60 ° C. However, it is an object of the present invention to provide a polyolefin-coated steel pipe and a method for producing the same, which do not cause embrittlement of the coating layer and exhibit good impact resistance. Another object of the present invention is to provide a polyolefin-coated steel pipe in which the thickness of a coating layer in a weld bead portion of the steel pipe is suppressed, and a method for producing the same.

[0016]

The cause of the above-mentioned uneven thickness of the coating layer at the weld bead portion may be that the temperature of the steel pipe at the time of coating layer lamination is too high. The lamination of the polyolefin resin coating layer is performed immediately after the steel pipe is preheated to 150 ° C or higher, which also serves to cure the previously applied primer coating.Therefore, the decrease in the temperature of the steel pipe from preheating to the lamination of the coating layer is very small. (At most about 10 ° C). Therefore, especially when the coating layer is a polyethylene resin layer, the temperature of the steel pipe at the time of lamination of the coating layer considerably exceeds the melting point of the polyethylene resin to be coated (generally, about 110 to 135 ° C), and the laminated polyethylene resin melts. It is presumed that the uneven thickness at the bead portion of the coating layer becomes remarkable due to the fluidization.

Therefore, it is considered that the uneven wall thickness can be prevented by lowering the temperature of the steel pipe at the time of lamination. However, conventionally, in order to harden the primer coating film in a short time of 1 minute or less, the temperature of the steel pipe is set to 150 ° C. or more. It has been considered that heating is required, and that the coating layer must be laminated at a temperature higher than the melting temperature of the coating resin in order to ensure the adhesion of the coating layer.

The present inventors have found that an epoxy 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 a conventional epoxy type primer. And the adhesion between the steel pipe and the modified polyolefin adhesive layer can be enhanced in a wide temperature range, whereby the primer coating film can be shortened by heating so that the steel pipe temperature during lamination is lower than the melting point of the coated polyolefin resin. It has been found that the composition can be cured in time, the adhesion of the coating layer is secured, and the above object is achieved.

Here, the present invention relates to 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 order from the bottom, wherein the primer layer is a bisphenol A type epoxy resin 40 to It was formed from a primer composition containing 60 parts by weight, 0 to 30 parts by weight of a novolak 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, which is an epoxy-based primer layer.

The primer composition has a coating film of 90 to 150
It can be cured within one minute in a temperature range lower than the conventional temperature of ° C. As a result, lamination of the polyolefin resin layer,
This can be performed at a steel pipe temperature lower than the melting point of the resin, and the uneven thickness of the coating layer at the bead portion can be suppressed.

Accordingly, from another aspect, the present invention provides a method for manufacturing a steel pipe comprising a primer layer, a modified polyolefin adhesive layer,
And The method of manufacturing a polyolefin-coated steel pipe of laminating a polyolefin resin layer in this order, the steel pipe temperature at the time of stacking of the polyolefin resin layer at the 90 ° C. or higher, Ri temperatures below der melting point of the polyolefin resin, the primer layer
Is a bisphenol A type epoxy resin 40 to 60 parts by weight,
Volak type epoxy resin 0 to 30 parts by weight, phenyl glyci
15 to 35 parts by weight of a gill ether, 10 to 40 parts by weight of a curing agent, and
Composition containing 1.5 to 4 parts by weight of reaction accelerator
This is a method for producing a polyolefin-coated steel pipe , which is an epoxy-based primer layer formed from a product . Such lamination of the coating layer at a lower temperature than before becomes possible 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 modified temperature of -50 ° C. or less at a low temperature embrittlement temperature and a modification rate of 0.2% or more. Polyethylene resin layer, (3)
The curing agent for the primer layer is an amine-based curing agent,
(4) The primer layer has a glass transition temperature of 60 to 80 ° C., and (5) a primer treatment layer, particularly preferably a chromate treatment layer, is provided below the primer layer. Thereby,
Polyolefin coating with resin coating layer showing adhesion and corrosion resistance in a wide temperature range from -60 ° C to 60 ° C, withstanding long-term cooling and heating cycles, no embrittlement at -60 ° C, and showing good impact resistance 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 on the outer peripheral surface of the steel pipe in order from the bottom, like the conventional product, and preferably further has a layer below the primer layer. It has an undercoat layer, particularly preferably a chromate film layer.

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

Before coating, as a pretreatment, it is preferable that the outer peripheral surface of the steel pipe is degreased, and that the mill scale or the like is removed by grit or shot blasting and cleaned. The primer composition may be immediately applied to the surface of the cleaned steel pipe.However, in order to increase 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 the 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 (such as zinc phosphate). A preferred undercoating layer is a chromate treatment layer. The chromate treatment solution to be used is not particularly limited, but a coating type chromate treatment solution containing silica fine powder and hexavalent and trivalent chromium ions is preferable from the viewpoint of corrosion resistance. Alternatively, if desired, phosphoric acid can be further used.

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

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

Examples of the bisphenol A epoxy resin include bisphenol A diglycidyl ether and bisphenol A having an epoxy equivalent of about 170 to 3000.
Mold epoxy resin can be used. Small amounts of other bisphenol-type epoxy resins (eg, hydrogenated bisphenol A diglycidyl ether, bisphenol AD diglycidyl ether, bisphenol F diglycidyl ether)
(Amount of 30% by weight or less of the bisphenol A type epoxy resin) can be used in combination.

The novolak type epoxy resin may be either a cresol novolak type or a phenol novolak type. Bifunctional bisphenol A as epoxy resin
By using a 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 promoted. Phenyl glycidyl ether is a reactive diluent added for adjusting the viscosity of the primer composition and improving the curing speed. The phenyl group of the phenyl glycidyl ether may have an alkyl group or another substituent.

As the curing agent, any epoxy curing agent can be used, but amine curing agents such as a heterocyclic diamine modified product, a fatty acid polyamine modified product, an aromatic polyamine modified product and a polyamideamine modified product are preferred. From the viewpoint 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-based reaction accelerator can be used. Examples of commercially available products include those marketed by Shikoku Chemicals under the trade name Curesol.

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

If the proportion of the bisphenol A type epoxy resin is too small, the anticorrosion property decreases. On the other hand, if the proportion of the bisphenol A type epoxy resin or phenylglycidyl ether is too large, or if the proportion of the novolak type epoxy resin, curing agent or reaction accelerator is too small, the curing rate of the primer coating film at a low temperature of 150 ° C. or lower can be obtained. And the coating film is likely to be damaged at the time of contact with the next transport roller.
Conversely, if the proportion of the phenylglycidyl ether is too small, or if the proportion of the novolak type epoxy resin, the curing agent or the reaction accelerator is too large, the viscosity of the obtained primer composition increases, resulting in a decrease in pot life. In addition, gelling of the primer composition proceeds during the continuous coating operation, and the coating workability deteriorates.

The epoxy primer composition can be prepared as a one-pack or two-pack composition. In the case of the one-pack type, the reaction accelerator and the curing agent are microencapsulated, and are dispersed in a resin solution containing the remaining components 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 applied. In order to make the primer composition have a viscosity suitable for application, if necessary,
It may be diluted with an organic solvent such as toluene or xylene.

The primer composition can be applied by a conventionally known technique such as spray coating, roller coating, brush coating, ironing, and flow coating. Thereafter, the surface of the steel pipe is heated to cure the primer coating, thereby forming an epoxy-based primer layer. The Tg of the epoxy-based primer layer formed after curing is preferably in the range of 60 to 80C. The Tg of the primer layer can be adjusted by the ratio of the above two types of epoxy resin and phenyl glycidyl ether. If Tg is lower than 60 ° C, high-temperature corrosion resistance is reduced, and if Tg is higher than 80 ° C, low-temperature impact resistance at -60 ° C is deteriorated. A more preferred Tg is in the range of 65-75C. The thickness of the epoxy-based primer layer is 5 to 100 μm,
Particularly, the thickness is preferably in the range of 10 to 50 μm.

The heating of the steel pipe for curing the primer coating is performed so that the temperature of the steel pipe when laminating the polyolefin resin layer in a continuous line is 90 ° C. or higher and the melting point of the polyolefin resin to be coated. To do. Since the lamination of the polyolefin resin layer is performed immediately after the curing of the primer coating film, the temperature of the steel pipe from heating to 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 hardening the primer coating to such an extent 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 layer is laminated.

The specific temperature of the steel pipe after applying the primer is
About 95-130 when the coating resin is polyethylene resin
C., and preferably about 95-145.degree. 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 one minute. Thereby, damage to the primer layer at the time of passing through the next transport roller can be avoided.

When a chromate film is provided as a base treatment layer under the primer layer, the steel tube is already heated for baking the chromate film, and the steel tube 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 the application of the primer composition. Thereby, the time required for heating for curing the primer coating film can be reduced. 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 film.

On the epoxy-based primer layer, an adhesive layer made of a modified polyolefin resin is coated with a polyolefin resin layer as a target coating layer. The coating of both layers is performed by extruding a film of each resin simultaneously (coextrusion method) or sequentially by an extrusion method such as a T-die or a round die or the like, so that the preheated steel pipe is coated on the steel pipe. It can be performed by stacking. In particular, extrusion coating by the T-die method is preferable. The coating of the adhesive layer can also be performed by electrostatically applying a modified polyolefin resin powder. Thereafter, when the steel pipe is cooled by water cooling or the like, the polyolefin-coated steel pipe of the present invention is obtained.

The thickness of each layer is such that the adhesive layer has a thickness of 0.1 to 0.6 mm, especially 0.2 to 0.5 mm, and the coating layer has a thickness of 2 to 5 mm, especially 2.5
It is preferably in the range of ４4 mm. As the modified polyolefin resin of the adhesive layer, polyethylene, a polyolefin resin such as polypropylene, maleic acid, acrylic acid, unsaturated carboxylic acid such as methacrylic acid, or those modified by copolymerization with an acid anhydride thereof, or those of these Mixtures of modified and polyolefin resins can be used.

As the polyolefin resin of the coating layer, low density polyethylene, medium density polyethylene, high density polyethylene, polypropylene and the like can be used. Polyethylene can also be used in copolymers with small amounts of other α-olefins (eg acrylic acid). Further, it is preferable to use polypropylene 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 type as the polyolefin resin of the coating layer for the adhesive layer. That is, when the coating layer is a polyethylene resin, it is preferable to use a modified polyethylene resin as the adhesive layer, and when the coating layer is a polypropylene resin, it is preferable to use a modified polypropylene resin 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 is a low-temperature embrittlement temperature of -50 ° C. or less, and a modification ratio of 0.2% or more. Of the modified polyethylene resin. More preferably, from the viewpoint of low-temperature resistance, the coating layer is a low-density or medium-density polyethylene,
The adhesive layer is composed of a linear low-density polyethylene modified with maleic acid or maleic anhydride, especially one having a low-temperature embrittlement temperature of -60 ° C or less and a modification rate of 0.20% or more. Here, the modification ratio means the content (copolymerization ratio, weight%) of the modified material (eg, maleic acid or maleic anhydride) in the entire resin.
It is. The modification ratio of the modified polypropylene resin is preferably 0.12% or more.

According to the present invention, the temperature of the steel pipe at the time of laminating the polyolefin resin of the coating layer is set to a temperature not lower than 90 ° C. and not higher than the melting point of the polyolefin resin of the coating layer. Conventionally, the coating layer is generally laminated at a temperature higher than the melting point of the polyolefin resin of the coating layer, so in the present invention, the coating layer is laminated, and thus, the curing of the previous primer coating is performed at a lower temperature than before. Become. Thus, even when 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 can be formed in a wide temperature range. It is possible to obtain a polyolefin-coated steel pipe which can maintain high adhesion and can be used in a wide temperature range from -60 ° C to 60 ° C, which was impossible 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 hardly occurs, 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 in a short time of less than 1 minute, and the coating film is damaged by 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 thickness of the bead portion of the coating layer becomes uneven, but also the heat energy required for heating the steel pipe becomes large, which is disadvantageous in cost. The preferable steel pipe temperature at this time is 95 to 125 ° C. when the coating layer is made of polyethylene resin, and when the coating layer is made of polypropylene resin.
It is in the range of 120-145 ° C.

The heating temperature of the steel pipe for hardening the primer layer is too high or too low, so that the temperature of the steel pipe at the time of lamination is low.
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 temperature of the steel pipe at the time of lamination of the coating layer may be adjusted to this range by cooling (forced or natural cooling) or reheating. This is undesirable because it complicates the manufacturing process or increases the line length. Therefore, it is preferable that the temperature of the steel pipe at the time of lamination of the coating layer be within the above range by adjusting the heating temperature for curing the primer coating film.

[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 shot-blasted carbon steel pipe (outside diameter 24 inches x wall thickness 15 mm x length 10 m) was cut at 1 m / min.
After passing through the first conveying roller while moving between the conveying rollers at a moving speed of, a silica-containing coating type chromate treatment liquid (Cr ⁶⁺ / total Cr ratio = 0.59 to 0.65) is rubbed on the outer peripheral surface thereof, and ℃ baking, total Cr adhesion 250mg / m
^{A second} chromate treated layer was formed.

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

After passing through the third conveying roller, a maleic anhydride-modified polyethylene resin to be an adhesive layer (modification ratio: 0.22%,
The low-temperature embrittlement temperature is −60 ° C.) and the low-density polyethylene resin (density 0.924 g / cm ³ , melting point 114 ° C.) serving as the coating layer is formed by the T-die method so that the film thickness becomes 0.4 mm and 3.0 mm, respectively. Extrusion coating was performed, followed by water cooling to obtain a polyethylene-coated steel pipe. The temperature of the steel pipe surface at the time of laminating the coating layer was 130 ° C.

[0052]

Example 2 A medium density polyethylene (density 0.935 g / cm ³ , melting point 126 ° C.) was used as the polyethylene of the coating layer, and the temperature of the steel pipe at the time of laminating the coating layer was 130 ° C.
A polyethylene-coated steel pipe was obtained in the same manner as described above.

[0053]

Example 3 As an epoxy primer, 40 parts of bisphenol A type epoxy resin, 0 parts of phenol novolak 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 modified heterocyclic diamine as a curing agent and 3 parts of an imidazole-based reaction accelerator was used.

[0054]

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

[0055]

Comparative Example 2 As an epoxy primer, 30 parts of bisphenol A type epoxy resin, 0 parts of phenol novolak 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 modified heterocyclic diamine as a curing agent and 3 parts of an imidazole-based reaction accelerator was used.

[0056]

Comparative Example 3 As an epoxy primer, 50 parts of bisphenol A epoxy resin, 10 parts of phenol novolak epoxy resin, 25 parts of phenylglycidyl 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 examined for adhesion 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 number of linear scratches having a width of 10 mm were made in the polyethylene coating layer of the polyethylene coated steel tube in the lateral direction (circumferential direction) at a depth reaching the steel surface, and after a part of the coating layer was peeled off. , 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 out a cut sample of 300 × 300 mm, it was immersed in a mixed bath of liquid nitrogen and Freon, and cooled to −60 ° C. The cooled sample was subjected to a drop impact test (drop impact 30J) in accordance with the ASTM G-14 method. 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 × 200 mm cut sample, drill a 9 mm diameter drill hole until the steel surface substrate is exposed.
The sample was immersed in a 3% saline solution at a constant temperature of 60 ° C., and a voltage was applied to the sample to set the voltage for the SCE standard electrode to −1.5 V.
After leaving for 30 days under these conditions, the polyethylene layer in the drill hole was cut off, and the peel diameter was measured.

(Cooling and Heat Cycle Resistance) A cut sample of 150 × 70 mm was taken from a polyethylene-coated steel pipe, and was taken out at −60 ° C. × 1.
A series of cooling / heating cycles of time → temperature rising 3 hours → 70 ° C. × 1 hour → temperature falling 7 hours was 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 a temperature range from -60 ° C to 60 ° C, and also has a good thermal 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, all of the low-temperature impact resistance, the cathodic electrolytic peeling resistance, and the thermal cycling resistance were reduced. Therefore, it can be seen that it is advantageous to perform a base treatment, particularly a chromate treatment, in order to be able to use in the above wide temperature range.

[0065]

[Example 5] Shot-blasted steel pipe (outside diameter 24 inches x wall thickness 15 mm x length 10 m, weld bead height 0.7 mm)
Was continuously processed 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 conveying roller, the surface of the steel pipe is
Immediately after passing through the second conveyor roller, the primer composition having the composition shown in Table 2 was preheated to 0 ° C.
Apply by spraying so as to have a thickness of 20 μm, immediately heat the steel pipe to a predetermined temperature by high-frequency induction heating for 1 minute,
Before passing through the third transport roller, the primer coating film was cured to form a primer layer.

After passing through the third transport roller, the presence or absence of coating damage due to the contact of the primer layer with the roller immediately after the passage was visually determined. If no damage to the primer coating was observed, immediately after passing through the third transport roller, the modified polyolefin resin of the adhesive layer and the polyolefin resin of the coating layer were placed on the resulting primer layer by the T-die method. The polyolefin-coated steel pipe was obtained by extrusion-coating to a thickness of 0.3 mm and 3.0 mm, respectively, and then water-cooling. The temperature of the steel pipe at the time of lamination of the polyolefin resin of the coating layer was determined by measuring the surface temperature of the steel pipe immediately before the lamination using a contact surface thermometer. The temperature of the steel pipe was achieved by heating after the application of the primer composition, and the temperature drop from this heating to the lamination of the coating layer was approximately 5 ° C.

As the polyolefin resin of the coating layer, low density, medium density and high density polyethylene (PE) resin and polypropylene (PP) resin were used. The melting point of each resin was as shown in the margin of Table 3. As the modified polyolefin resin of the adhesive layer, a maleic anhydride-modified polyethylene resin having a modification ratio of 0.22% when the coating layer is polyethylene, and a modification ratio of 0.0 when the coating layer is polypropylene.
A 15% maleic anhydride-modified polypropylene resin was used.

Workability at the time of applying the primer composition (whether or not gelled), whether or not the coating film was damaged when passing through the third transport roller, the temperature of the steel pipe at the time of laminating the coating layer, and the obtained polyolefin coating Table 3 shows the measurement results of the thickness deviation value of the weld bead portion of the steel pipe. In addition, as shown in FIG. 1, the thickness deviation value is the difference between the thickness A of the polyolefin resin layer (coating layer + adhesive layer) at the non-bead portion and the thickness B of the polyolefin resin layer at the bead portion. The thicknesses A and B were determined 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 lamination of the coating layer was changed to the melting point of the polyolefin resin of the coating layer. The primer coating film can be cured in a shorter time at a lower heating temperature than before, and coating film damage when passing through the transport roller was avoided. By laminating the coating layer at a relatively low temperature, the phenomenon of thickness reduction (uneven thickness) of the coating layer at the weld bead portion of the steel pipe was significantly reduced.

On the other hand, in the case of a primer composition outside the scope of the present invention, the composition gels and cannot be applied, or the curing speed of the coating film at a low temperature is low, and the coating is sufficiently cured by heating for a short time. Without contact, damage to the coating occurred on contact with the transport rollers. In addition, even when the primer composition was within the range of the present invention, if the temperature of the steel pipe at the time of lamination of the coating layer exceeded the melting point of the resin of the coating layer, the uneven thickness increased.

[0074]

According to the present invention, the following effects can be obtained. The epoxy-based primer layer is formed from the primer composition within the scope of the present invention, and preferably by using a chromate treatment in combination with the base, so that the initial adhesion can be maintained even when a cooling / heating cycle is applied for a long period of time. At low temperature.

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, the coating layer can be further strongly adhered, Strength is increased, and high-temperature corrosion resistance is improved. As described above, the polyolefin-coated steel pipe of the present invention can be used in a wide temperature range from -60 ° C to 60 ° C.

Since the primer coating film can be cured at a lower temperature and in a shorter time than before, the coating layer can be laminated at a steel pipe temperature equal to or lower than the melting point of the polyolefin resin to be coated. Meat is reduced. As a result, there is no need to cut the bead of the steel pipe or increase the thickness of the coating layer in anticipation of the thickness variation, and the energy cost required for heating the steel pipe can be reduced, which is economically advantageous.

[Brief description of the drawings]

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

Continuation of the front page (56) References JP-B-56-43031 (JP, B2) JP-B-63-4490 (JP, B2) (58) Fields investigated (Int. Cl. ⁶ , DB name) B32B 1 / 00-35/00

Claims (4)

(57) [Claims] 1. 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 a steel pipe in order from the bottom, wherein the primer layer is a bisphenol A type epoxy resin 40 to
60 parts by weight, 0-30 parts by weight of novolak type epoxy resin, 15-35 parts by weight of phenyl glycidyl ether, 10-40 parts of curing agent
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 method according to claim 1, wherein 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 ratio of 0.2% or more, and curing of the primer layer. 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 80C. 3. The polyolefin-coated steel pipe according to claim 1, further comprising a base treatment layer below the primer layer. 4. 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, wherein the temperature of the steel pipe at the time of laminating the polyolefin resin layer is 90.
C. or higher and a temperature lower than the melting point of the polyolefin resin, and the primer layer is 40 to 60 parts by weight of a bisphenol A type epoxy resin, 0 to 30 parts by weight of a novolak type epoxy resin, 15 to 35 parts by weight of phenyl glycidyl ether, and is cured. A method for producing a polyolefin-coated steel pipe, which is an epoxy-based primer layer formed from a primer composition containing 10 to 40 parts by weight of an agent and 1.5 to 4 parts by weight of a reaction accelerator.