JP3495391B2 - Polyethylene composition for steel pipe coating - Google Patents

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a polyethylene composition for coating steel pipes. More specifically, the present invention relates to a polyethylene composition for coating a steel pipe suitable for cold regions using a specific polyethylene.

[0002]

2. Description of the Related Art Conventionally, a large-diameter steel pipe forming a pipeline for transporting natural resources such as oil and natural gas, or for passing a cable for communication or power transmission has been For example, it may be buried or installed in the ground, rivers, swamps, seawater, or from cold regions to tropical regions, and is expected to be used for a long period of several decades. For this reason, this kind of steel pipe must be adaptable to various weather conditions or installation conditions on the earth, and durability of the steel pipe, particularly corrosiveness of the steel material of the steel pipe has been a serious problem. Therefore, polyethylene coating has been used in recent years as an anticorrosion coating for steel pipes for pipelines.

[0003]

By the way, oil and gas fields were discovered on the Arctic coast of Alaska and the former Soviet Union.
A large pipeline is being built. Pipelines used in these cold regions are exposed to extremely severe weather conditions, and the anticorrosion coating has low temperature impact resistance at about −45 ° C., flexibility, and a temperature of 80% after pipeline operation.
Since the temperature rises up to ℃, stable quality in a wide range of temperatures such as aging resistance at high temperatures and stress crack resistance is required.

As a polyethylene resin used for coating the steel pipe for cold regions, polyethylene of a high pressure method, particularly ethylene-vinyl acetate copolymer (EVA) is known.
However, EVA has a problem that the cost is high and an acid is generated when it comes into contact with water, so it cannot be said to be industrially advantageous. Also, instead of the above-mentioned high-pressure polyethylene, medium-low-pressure polyethylene, especially medium-high density polyethylene, has poor physical properties in terms of environmental stress crack resistance (stress crack resistance) and low temperature impact resistance. However, it is not suitable for cold regions because it has a large residual internal stress due to its high tensile modulus.

[0005]

Means for Solving the Problems The inventors of the present invention have made extensive studies to solve the problems of the polyethylene resin used for coating the steel pipe for cold regions, and as a result, have found that specific physical properties (melt index, flow ratio) By using a polyethylene composition mainly composed of linear low-density polyethylene having a density and a tensile elongation at break at low temperature), the above-mentioned problems are improved and a polyethylene coating layer having excellent performance is formed. It was found that the present invention has been completed.

That is, the gist of the present invention is that the melt index is 0.1 to 1.5 g / 10 minutes, the flow ratio is 45 or more, and the tensile elongation at break at -45 ° C. is 200%.
The linear low density polyethylene which is the above is at least 70
A polyethylene composition for coating a steel pipe for cold regions, which is characterized in that it is contained in a weight percentage. The present invention will be described in more detail below.

The linear low-density polyethylene used in the present invention is a copolymer of ethylene and another α-olefin, and is different from the conventional low-density polyethylene resin produced by the high pressure method. The linear low-density polyethylene includes, for example, ethylene and other α-olefins such as butene, hexene, octene, decene, and 4-methylpentene-1 in an amount of 4 to 4.
About 17% by weight, preferably about 5 to 15% by weight of copolymerization, which is produced by using a Ziegler type catalyst or Phillips type catalyst used in the production of medium- and low-pressure method high-density polyethylene. Density polyethylene is made into a short branched structure by a copolymerization component, and the density is also appropriately reduced by utilizing this short chain branching, and 0.91 to 0.
It is about 93 g / cm ³ , and is a polyethylene having a structure that is more linear than conventional low-density polyethylene and has more branches than high-density polyethylene.

The linear low-density polyethylene described above is produced by applying a production process of a gas phase polymerization method using a fluidized bed reactor, a stirred bed reactor, a tubular reactor, etc., for example, JP-A-54-14.
It can be produced based on the method (gas phase polymerization method using a fluidized bed reactor) described in No. 8093, No. 54-154488 and the like. The linear low-density polyethylene used in the present invention is manufactured by the method described above, and has a density of 0.91 to 0.93 g /
cm ³ , preferably 0.915 to 0.925 g / cm ³
And the melt index is 0.1-1.5 g / 10 minutes,
It is preferably 0.2 to 0.7 g / 10 minutes, the flow ratio is 45 or more, preferably 50 to 100, and the tensile elongation at break at -45 ° C is 200% or more, preferably 250 to
650%, and stress crack resistance (ESC
R) is 1000 hr or more, preferably 3000 hr or more.

In the present invention, density means ASTMD.
1238, the melt index is a value measured at 190 ° C. according to ASTM D1505, and the flow ratio is a shear force of 10 ⁶ dynes / cm ² (using the melt index measuring device). Load 111
31 g) and the extrusion rate (g / 10 min) of 10 ⁵ dyne / cm ² (load 1113 g),

[0010]

[Equation 1] It is calculated by.

The flow ratio is a measure of the molecular weight distribution of the resin used, and the smaller the flow ratio value is, the narrower the molecular weight distribution is.
The larger the flow ratio value, the wider the molecular weight distribution. The tensile elongation at break is measured according to ASTM D638 with a press piece at −45 ° C., and the stress crack resistance (environmental stress crack resistance (E
(Also referred to as SCR)) is measured in accordance with ASTM D1693.

If the density of the linear low-density polyethylene is less than the lower limit, the hardness at high temperature (60 to 80 ° C.) is insufficient, and if it is higher than the upper limit, the low temperature impact resistance decreases. If the melt index is less than the lower limit, the molding processability will decrease due to a decrease in the extrusion amount, and if it is higher than the upper limit, stress crack resistance and low temperature impact resistance will decrease, and if the flow ratio is less than the lower limit, the extrudability during molding. Is reduced.

When the tensile elongation at break is less than the lower limit, the low temperature impact resistance is insufficient, and when the stress crack resistance is the lower limit, the environmental stress cracking property is insufficient. The composition of the present invention is a linear low density polyethylene 1 having the above-mentioned specific melt index, flow ratio, density and tensile elongation at break.
Although it may be composed of 100% by weight, it may be a composition containing less than 30% by weight of another resin as long as it has at least 70% by weight of the linear low density polyethylene. In this case, the content of the linear low density polyethylene is preferably 80% by weight or more, more preferably 90% by weight or more. If the proportion of the linear low-density polyethylene is less than 70% by weight, the excellent characteristics of the linear low-density polyethylene will be impaired, which is not preferable.

Other resins to be added to the above linear low density polyethylene include linear low density polyethylene outside the range specified in the present invention, high-pressure polyethylene such as ethylene-vinyl acetate copolymer, medium and low pressure. A method polyethylene etc. are mentioned. The blending ratio of these other resins is less than 30% by weight, preferably less than 20% by weight, more preferably less than 10% by weight, based on the entire composition. If the blending amount of the other resin is too large, one of the effects of the present invention is lost, so it is desirable to reduce the blending amount of the other resin as much as possible.

The composition of the present invention contains, in addition to the linear low-density polyethylene described above, additives which have been conventionally compounded in polyethylene compositions for coating steel pipes, such as heat stabilizers, light stabilizers, ultraviolet absorbers and antioxidants. Agents and the like can be added. 1-3% by weight of carbon black as an ultraviolet absorber,
Desirably, it is preferable to add 1.5 to 3% by weight. As antioxidants, phenol-based antioxidants, sulfide-based antioxidants, phosphite-based antioxidants, etc. can be used.
It can be used by blending 01 to 2% by weight. On the other hand, 0.01 to 0.2% by weight of metal soaps may be blended as a lubricant.

The coating of the above polyethylene resin composition on a steel pipe is carried out by extruding the composition into a tubular shape and coating it on the steel pipe, or by extruding a flat sheet of the composition and spirally winding it on the steel pipe. It is possible to adopt a method of coating with the above. In the former method, for example, a crosshead die or an offset die can be used, and in the latter method, for example, a T-die can be used. In order to improve the adhesion upon coating, the steel pipe itself may be subjected to a mechanical treatment or a chemical treatment, or an adhesive or an adhesive may be interposed. Further, in order to impart adhesiveness to the composition layer itself, a part of the composition may be graft-modified with a monomer having a functional group such as maleic anhydride, or a polyolefin graft-modified with the above monomer may be used. A small amount may be blended within a range that does not impair the characteristics of the composition.

Thus, the steel pipe obtained by coating the polyethylene composition of the present invention is suitable for use in cold regions, where cold regions mean regions having an outside air temperature of -20 ° C or lower. Represent

[0018]

EXAMPLES Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples without departing from the gist thereof. Example 1 and Comparative Example 1 100 parts by weight of polyethylene having the physical properties shown in Table 1,
1.5 parts by weight of carbon black and 0.15 parts by weight of phenolic antioxidant, 0.1 of thioether antioxidant
Parts by weight were kneaded using a kneader at 200 ° C. to prepare a polyethylene composition for coating a steel pipe and pelletize it. Using the pellets, a polyethylene sheet material at 250 ° C. is extruded from a T-die of an extruder, and a spiral is made while partially superimposing the polyethylene sheet material on a steel pipe heated to 200 ° C. and coated with an adhesive. It was wrapped in a strip and covered and cooled. Table 1 shows each measurement result of the obtained polyethylene coating layer.

In Table 1, the embrittlement temperature is ASTM D746.
The tensile elastic modulus was measured at a test speed of 1 mm / min in accordance with JIS K7113. The pH at the time of contact with water was measured by immersing 10 g of resin pellets in 100 g of water having pH 7 for 24 hours.

[0020]

[Table 1]

[0021]

The polyethylene composition obtained by the present invention is excellent in stress crack resistance and low temperature impact resistance and is suitable for coating steel pipes in cold regions.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-5-187592 (JP, A) JP-A-54-154488 (JP, A) JP-A-54-148093 (JP, A) JP-B-62- 9138 (JP, B1) (58) Fields surveyed (Int.Cl. ⁷ , DB name) C08L 23/00-23/36 C08F 210/00-210/18 F16L 58/18

Claims (2)

(57) [Claims] 1. A melt index of 0.1 to 1.5 g
/ 10 min, the flow ratio is 45 or more, and the linear low density polyethylene having a tensile elongation at break at 200 ° C. of 200% or more is contained at least 70% by weight. A polyethylene composition for coating a steel pipe for cold regions. 2. The linear low density polyethylene has a density of 0.9.
The polyethylene composition for coating steel pipes for cold regions according to claim 1, which is 1 to 0.93 g / cm ³ .