JP3168871B2 - Polyolefin coated steel pipe - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an outer surface polyolefin-coated steel pipe which can be used in a wide temperature range from -60.degree.

[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 to prevent corrosion of steel materials. Used for line pipes. However, in recent years, due to the progress of oil field development accompanying the increase in energy demand, the use environment has become ever more severe, such as in polar regions and deserts. In particular, the guaranteed temperature of the use environment of line pipes in extremely cold regions such as Russia has been up to -45 ° C conventionally,
Demand in environments up to -60 ° C has also been observed. For this reason, there is a growing need for a coated steel pipe that can be used in a wide range from low to high temperatures, but the conventional manufacturing method cannot meet this need.

[0003] 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. are doing. For this reason, a method in which a polyolefin resin layer is coated on a steel pipe through an adhesive layer made of a modified polyolefin resin obtained by copolymerizing an olefin with an unsaturated carboxylic acid or an anhydride thereof has conventionally been used.

However, although this modified polyolefin resin layer is excellent in initial adhesion at room temperature, it is exposed to a high temperature and a humid environment for a long time, or is subjected to a long-term cold heat due to a difference in temperature between summer and winter or day and night. When subjected to the cycle, the adhesion between the steel pipe surface and the polyolefin resin layer is reduced.

Therefore, as a countermeasure, a method of subjecting the surface of the steel pipe to a chemical conversion treatment of phosphoric acid or chromic acid, a method of applying a primer such as an epoxy urethane resin, an epoxy resin, or polyvinyl alcohol to the surface of the steel pipe, or A method using both of them is used.

Accordingly, polyolefin-coated steel pipes are usually
It is manufactured by the following procedure. First, after the surface of the steel pipe is degreased, the oxide film is removed by grid blast or shot blast, and a chemical conversion treatment with phosphoric acid or chromic acid is performed. Thereafter, a primer composed of, for example, an epoxy resin and an amine-based curing agent is applied, and the steel tube surface is cured by heating to a predetermined temperature. Next, on the cured primer layer, a polyolefin-coated steel pipe is obtained by sequentially or simultaneously laminating and coating a modified polyolefin resin to be an adhesive layer and a polyolefin resin to be a coating layer by a powder coating method or a melt extrusion method. .

As a primer, there is an epoxy primer comprising a bisphenol A type epoxy resin and an amine type curing agent as described in JP-A-57-113871. Although the polyolefin-coated steel pipe thus obtained is excellent in cathodic electrolytic peelability at room temperature, it has a low adhesive strength at room temperature and is inferior in low-temperature impact resistance in a temperature range lower than -45 ° C.

Japanese Patent Application Laid-Open No. 4-19041 proposes a polyethylene coated steel pipe in which a crosslinked polyethylene is laminated on the outer surface of a steel pipe via an epoxy primer layer and a modified polyethylene coating layer. However, although this polyethylene-coated steel pipe is excellent in durability at high temperatures, the coating layer becomes brittle at extremely low temperatures of -60 ° C, and when subjected to an impact test,
There is a problem that cracks occur on the film surface.

Also, a polypropylene coated steel pipe using a specific crystalline ethylene-propylene block copolymer as a coating layer, proposed in Japanese Patent Publication No. 3-29588,
Excellent mechanical properties and corrosion resistance at high temperatures, but -30 ° C
In the following low-temperature region, there is a problem that the coating layer becomes low-temperature embrittlement and cannot be used.

[0010]

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 ℃ to 60 ℃, that is, the coating shows excellent adhesion and corrosion resistance throughout this temperature range, withstands long-term cooling and heating cycles, even at -60 ℃ An object of the present invention is to provide a polyolefin-coated steel pipe which does not have embrittlement of a coating layer and exhibits good impact resistance.

[0011]

Means for Solving the Problems In order to solve the above problems, the present inventors have conducted intensive studies on the improvement of the low-temperature toughness of the coating layer polyolefin material of the polyolefin-coated steel pipe, its film thickness, and the underlying treatment. The present inventors have found a coating system having excellent mechanical properties and corrosion resistance in a wide temperature range from 60 ° C. to 60 ° C., and also having good cooling and heat cycle properties, and completed the present invention.

Here, the gist of the present invention is to provide a polyolefin-coated steel pipe in which an outer surface of a steel pipe is subjected to a base treatment and then coated with a polyethylene resin via a primer layer and a modified polyolefin adhesive resin layer. A composition in which the total thickness of the adhesive resin layer and the polyolefin-coated resin layer is 2 to 6 mm, and the polyolefin-coated resin layer contains 100 parts by weight of linear low-density polyethylene and 10 to 50 parts by weight of high-pressure low-density polyethylene. And a polyolefin-coated steel pipe.

In the above polyolefin-coated steel pipe, the polyolefin-coated resin layer contains 100 parts by weight of a linear low-density polyethylene, 10 to 50 parts by weight of a high-pressure low-density polyethylene, and 7 parts by weight or less of an ethylene-vinyl acetate copolymer. May be formed from the composition.

The present inventors previously used a primer composition comprising a bisphenol-type epoxy resin, a novolak-type epoxy resin, phenylglycidyl ether, a curing agent and a reaction accelerator as an epoxy primer, and a low-temperature embrittlement temperature of -50 ° C. By using a modified polyolefin resin having a modification ratio of 0.2% or more as an adhesive layer, a polyolefin-coated steel pipe which can be used in a wide temperature range from -60 ° C to 60 ° C has been proposed (Japanese Patent Application No. 7-17125). No.) This polyolefin-coated steel pipe can be used in a low-temperature environment at -60 ° C.
According to the results of the drop weight impact test based on the STM method, the coating does not crack even at an impact value of up to 30 J. However, when handling and transporting polyolefin-coated steel pipes in cold regions, further severe low-temperature impact evaluation was required.
However, a polyolefin-coated steel pipe that does not crack on the surface of the film even when the film is cast at -60 ° C. is desired.

The polyolefin-coated steel pipe of the present invention satisfies such a severe requirement of low-temperature impact resistance. For example, the steel pipe is excellent in that cracking does not occur on the coating surface even when the coating is shot at −60 ° C. Has low-temperature impact characteristics.

[0016]

Hereinafter, the present invention will be described in detail. The polyolefin-coated steel pipe of the present invention has, on the outer surface of the steel pipe, four layers of a chromate treatment layer, a primer layer, a modified polyolefin adhesive resin layer, and a polyolefin resin layer in this order from the bottom.

The type and dimensions of the steel pipe (original pipe) to be coated are not particularly limited, and may be any type (for example, alloy steel such as carbon steel and stainless steel). Is usually a carbon steel tube. Also, a plated steel pipe can be used as a raw pipe. It is desirable that the outer surface of the steel pipe is preliminarily degreased with an alkali and then polished with a grid or a shot to remove mill scale and the like before coating.

Further, the chromate treatment as another base treatment is performed by applying a chromate treatment solution to the cleaned outer surface of the steel pipe by a conventionally known method such as air spray, airless spray, and ironing. Is done.

The chromate treatment solution to be used is not particularly limited, but a coating type chromate treatment solution containing fine silica powder and hexavalent and trivalent chromium ions is preferred from the viewpoint of corrosion resistance. Alternatively, phosphoric acid can be further added for use.

After the application of the chromate treatment liquid, the steel pipe is heated to bake a 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 the chromate film attached is preferably in the range of 50 to 1000 g / m ² as the total amount of Cr attached.

Next, a primer is applied to the surface. For primer application, spray coating, roller coating,
Conventional techniques such as brush coating, ironing, and flow coating can be used.

The primer layer is composed of a bisphenol type epoxy resin, phenyl glycidyl ether, a curing agent and a reaction accelerator from the viewpoint of anticorrosion properties and low-temperature impact properties, and, if necessary, an epoxy primer containing a novolak type epoxy. The glass transition temperature (Tg) formed from the composition is preferably 60 to 80 ° C., but is not limited thereto, and a primer layer is formed using a primer such as an epoxy urethane resin and polyvinyl alcohol. Is also good.

The bisphenol type epoxy resin, novolak type epoxy, phenyl glycidyl ether, curing agent and reaction accelerator used in the preferred epoxy primer composition are described below.

As the bisphenol type epoxy resin,
Epoxy resins having an epoxy equivalent of about 170 to 3000, such as diglycidyl ether of bisphenol A, can be used. A small amount of glycidyl ether of another bisphenol type epoxy resin, for example, hydrogenated bisphenol A or bisphenol F (30% by weight or less of bisphenol A diglycidyl ether) may be used in combination.

The novolak type epoxy resin may be either a cresol type or a phenol novolak type. By using a bifunctional bisphenol-type epoxy resin and a polyfunctional novolak-type epoxy resin in combination, it is possible to improve the primer curing rate at a relatively low temperature. Phenyl glycidyl ether is added to adjust the viscosity of the primer composition.

Examples of the curing agent include modified heterocyclic diamines, modified aliphatic polyamines, modified aromatic polyamines, modified polyamidoamines, and the like. For this reason, modified heterocyclic diamines are preferred. As the reaction accelerator, for example, an imidazole-based reaction accelerator can be used.

The epoxy-based primer composition is preferably prepared as a two-part composition, similarly to the conventional one. After applying the primer, the surface of the steel pipe is heated to cure the primer layer. As a method of heating the steel pipe for curing the epoxy primer layer, a conventionally known method such as high-frequency induction heating, far-infrared heating, and gas heating is applied.

An adhesive resin layer made of a modified polyolefin is coated on the epoxy primer layer applied and cured on the steel pipe surface, and a polyolefin resin layer serving as a coating layer is coated thereon. The coating of these two layers can be performed by preheating the steel pipe and then simultaneously extruding (co-extrusion method) or laminating each resin so as to cover the steel pipe by an extrusion method such as a T-die or a round die. it can. The coating of the adhesive resin 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.

According to the present invention, the total thickness of the primer layer, the modified polyolefin adhesive resin layer, and the polyolefin resin coating layer is in the range of 2 to 6 mm. When the total film thickness is less than 2 mm, the high-temperature impact resistance at 60 ° C. decreases, and when the total thickness exceeds 6 mm, the low-temperature impact resistance at -60 ° C. and the thermal cyclability decrease due to an increase in the residual stress of the coating layer. In addition, since the polyolefin unit consumption is deteriorated, the cost is disadvantageous.

The thickness of each of the primer layer, the modified polyolefin adhesive resin layer and the polyolefin-coated resin layer is 10 to 50 μm for the primer layer and 10 μm for the modified polyolefin adhesive resin layer.
0 to 800 μm, and the thickness of the polyolefin-coated resin layer is preferably in the range of 2 to 5 mm.

The modified polyolefin resin forming the adhesive resin layer is obtained by modifying a polyolefin resin such as polyethylene by copolymerization with an unsaturated carboxylic acid such as maleic acid, acrylic acid, methacrylic acid or an acid anhydride thereof, or A mixture of these modified products and a polyolefin resin can be used, but a low-temperature embrittlement temperature of −50 ° C. or less,
It is preferable to use a modified polyolefin resin having a modification ratio of 0.2% or more. A particularly preferred adhesive resin is a linear low-density polyethylene modified with maleic anhydride. Among them, a resin having a low-temperature embrittlement temperature of −60 ° C. or less and a modification rate of 0.25% or more is preferred. Here, the modification ratio is a content (copolymerization ratio, weight%) of a modified material (for example, maleic anhydride) in the whole resin.

The polyolefin resin forming the coating layer is
The composition comprises 100 parts by weight of a linear low-density polyethylene and 10 to 50 parts by weight of a high-pressure low-density polyethylene, and further contains, if necessary, 7 parts by weight or less of an ethylene-vinyl acetate copolymer.

If the blending ratio of the high-pressure polyethylene is less than 10 parts by weight based on 100 parts by weight of the linear low-density polyethylene, the throughput at the time of extrusion coating is reduced, and the productivity in the line is deteriorated. When the amount exceeds the weight part, the low temperature impact resistance at -60 ° C is inferior. When the ethylene-vinyl acetate copolymer is blended, if the blending amount exceeds 7 parts by weight, the high-temperature impact resistance of the polyolefin resin-coated steel pipe is inferior due to a decrease in the Vicat softening point of the polyolefin resin composition.

The polyolefin-coated steel pipe of the present invention obtained by the above-described method has the following advantages. By using a polyolefin resin with excellent low-temperature toughness and using both chromate treatment and primer coating,
The low-temperature impact resistance at 60 ° C is significantly improved. Good properties are also obtained with respect to thermal cyclability, high-temperature impact resistance and high-temperature corrosion resistance.

As a result of the above, the polyolefin-coated steel pipe of the present invention can be used in a wide temperature range from -60 ° C to 60 ° C.

[0036]

The present invention will be described below in detail with reference to examples according to the present invention.

[0037]

[Examples 1 to 6] Steel pipe (24 inches x 15 mmt x 10 m length)
Was subjected to shot blasting, and a silica-based chromate solution was rubbed on the surface and baked by heating to 80 ° C. to form a chromate-treated layer having a total Cr adhesion of 250 g / m ² .

Next, the two-part epoxy primer composition is applied to the surface of the chromate treatment layer by air spray so as to have a thickness of 30 μm, and is cured by heating at 150 to 190 ° C. to obtain a Tg.
An epoxy primer layer at 74 ° C. was formed. The two-part epoxy primer composition used contains 60 parts by weight of a bisphenol A-type epoxy resin, 20 parts by weight of phenylglycidyl ether, 35 parts by weight of a curing agent, and 3 parts by weight of a reaction accelerator after mixing two parts. is there.

Then, while maintaining the steel pipe at a pipe temperature of 150 to 190 ° C., the maleic anhydride-modified polyethylene resin as the adhesive resin and the polyethylene-coated resin composition of the present invention having the composition shown in Table 1 were immediately subjected to the T-die method. And extrusion coated. The total thickness of the primer layer, the modified polyethylene resin adhesive layer and the polyethylene-coated resin layer, and the T-die extrusion amount are as shown in Table 1. Next, a polyethylene-coated steel pipe was obtained by water cooling.

[0040]

Comparative Examples 1 to 4 A polyethylene-coated steel pipe was obtained in the same manner as in the Example except that the total thickness of the primer layer, the adhesive layer and the coating resin layer, and the composition of the polyolefin resin coating layer were as shown in Table 1.

Tests of the low-temperature impact resistance, high-temperature impact resistance, high-temperature corrosion resistance, and thermal cyclability of the polyolefin-coated steel tubes obtained in the above Examples and Comparative Examples were carried out by the following methods. The results are summarized in Table 1.

(Low-Temperature Impact Test) A cut sample of 300 × 300 mm was cut out from a polyolefin-coated steel pipe, immersed in a mixed bath of liquid nitrogen and Freon, and cooled to −60 ° C. Cool the sample surface manually after cooling (commercially available, blade length 22m
m) was injected, and the presence or absence of occurrence of cracks on the sample surface was visually determined.

(High-temperature impact test) From polyethylene-coated steel pipe
After cutting out a cut sample of 300 × 300 mm, the sample was left in a constant temperature room at 60 ° C. to control the temperature of the sample. The sample after temperature control at 60 ° C is subjected to a drop weight impact test in accordance with the ASTM G-14 method.
The impact of the sample was determined by inspecting the impact portion with a pinhole tester at a voltage of 10 KV.

(High temperature corrosion resistance) from polyolefin-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 standing for 30 days, the polyolefin layer at the drill hole was cut off, and the peel radius was measured.

(Cooling / Heat Cycle Properties) A 150 × 70 mm cut sample was taken from a polyethylene-coated steel pipe, and a series of heat cycles of −60 ° C. × 1 hour → heating 3 hours → 70 ° C. × 1 hour → temperature lowering 7 hours was performed. Granted times. The sample after the test was visually inspected for the presence or absence of peeling of the polyethylene film on the end face.

As can be seen from the results shown in Table 1, the polyethylene-coated steel pipe of the present invention exhibits excellent impact resistance and corrosion resistance in a wide temperature range from -60 ° C to 60 ° C, and exhibits a thermal cycling resistance test. Was also good.

On the other hand, when the composition of the resin coating layer is out of the range of the present invention, the extruded amount decreases or cracks occur in a low-temperature impact test. Further, even when the composition of the resin coating layer is within the range of the present invention, when the film thickness is out of the range of the present invention, low-temperature impact resistance and cold-heat resistance are poor, or high-temperature impact resistance is poor.

[0048]

[Table 1]

[0049]

According to the present invention, there is provided a polyolefin-coated steel pipe which exhibits excellent mechanical properties and corrosion resistance in a wide temperature range from -60 ° C to 60 ° C and can withstand a long-term cooling / heating cycle. In particular, even in a severe impact test at −60 ° C., the coating layer shows extremely excellent low-temperature impact resistance without causing embrittlement, and can be used in a wide temperature range from −60 ° C. to 60 ° C.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ identification code FI F16L 58/10 F16L 58/10 (58) Investigated field (Int.Cl. ⁷ , DB name) B32B 15/08 B05D 7/14 F16L 58/10

Claims (2)

(57) [Claims] 1. A polyolefin-coated steel pipe coated with a polyolefin resin via a primer layer and a modified polyolefin adhesive resin layer after subjecting an outer surface of the steel pipe to a base treatment, wherein the primer layer, the modified polyolefin adhesive resin layer, and the polyolefin-coated resin The total thickness of the layers is 2
A polyolefin-coated steel pipe having a thickness of 6 mm and a polyolefin-coated resin layer comprising a composition containing 100 parts by weight of linear low-density polyethylene and 10 to 50 parts by weight of high-pressure low-density polyethylene. 2. The polyolefin-coated resin layer is composed of 100 parts by weight of linear low-density polyethylene,
The polyolefin-coated steel pipe according to claim 1, comprising a composition containing 0 to 50 parts by weight and 7 parts by weight or less of an ethylene-vinyl acetate copolymer.