JPS60595B2 - plastic extrusion coated steel pipe - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in the coating of polyethylene extrusion coated steel pipes.

Polyethylene extrusion coated steel pipes are widely used as steel pipes for transportation such as water supply, and the coating of such steel pipes generally consists of a two-layer coating consisting of an anti-corrosion layer and a protective layer for protecting this anti-corrosion layer.

In the case of this two-layer coating, it is necessary to prevent welding between the anticorrosive layer and the protective layer, and various methods for this purpose have been developed in the past.

For example, as disclosed in Japanese Utility Model Publication No. 50-2083, there is a method of preventing arrival at a port by applying an anti-welding agent made of a low-molecular polyolefin resin or a mixture thereof between an anti-corrosion layer and a protective layer. However, according to this method, a step of applying an anti-welding agent is required in the manufacturing process, and when such steel pipes are connected by welding etc. at the piping construction site, after removing the protective layer at the pipe end, It is an unavoidable fact that it is necessary to remove the anti-settling agent remaining on the anti-corrosion layer, and that this process requires a lot of effort.
There is also a method of scattering inorganic powders such as calcium carbonate, talcum, etc. or starch powders on the anti-corrosion layer as a welding preventive agent, as disclosed in Tokukan Sho 50-139422.

However, when using this method, it is necessary to uniformly spread the powder to an appropriate thickness, and it is not only difficult to spread the powder evenly in this way, but also cause welding to occur in areas where the amount of adhesion is small. However, it is difficult to completely achieve the purpose of preventing twilling. In addition, JP-A-49-13
0956, forced cooling of the extrusion-coated corrosion protection layer,
Another method is to apply a protective layer by extrusion immediately after cooling. Although this method is superior in that it does not use any anti-fusing agent, it is not a preferred method because the equipment cost for forced cooling is enormous and the production efficiency is significantly reduced. The present invention solves the above-mentioned problems, and the lower anti-corrosion layer and the upper protective layer do not weld together even if double extrusion coating is performed continuously without using any anti-scattering agent. The aim is to provide plastic-coated steel pipes that are free of plastic-coated steel pipes.

Therefore, the present invention provides a plastic-coated steel pipe in which two layers, an anti-corrosion layer and a protective layer, are extruded onto a steel pipe using a cross-head die extrusion method, in which either the anti-corrosion layer or the protective layer is made of polyethylene alone, and the other layer is made of polyethylene. 2
Its basic feature is that it is coated with a copolymer or blend resin comprising 0 to 4% by weight of polypropylene and 60 to 8% by weight of polypropylene. The reason why polyethylene and polypropylene are blended in the above blending ratio in the present invention is as follows.

That is, as a result of various studies on resins with various blending ratios, we found that it is possible to prevent welding with a 100% polyethylene layer in copolymerized or blended resins containing 20% or less polyethylene and 80% or more polypropylene, but crosshead die extrusion When molding is performed, the resin ejected from the extrusion die does not adhere well to the joints, easily cracks, and has insufficient mechanical strength, making it impractical. Also, 40% or more polyethylene
If the polypropylene content is 60% or less, welding will occur if it is stacked directly on a 100% polyethylene layer. Next, the present invention will be described with reference to specific examples.

Example 1 The blend of polyethylene and polypropylene was changed to polyethylene 0
Resin and polyethylene 1 with various blends between ~100%
Each press sheet (thickness 2 yen) of 0.00% resin was heated to 1902C, overlapped and pressed together with a press, and the presence or absence of settling was confirmed.

The results are shown in Table 1, and polyethylene was 50%
If the content exceeds that amount, settling will occur. Table 1 ○: No welding ×: Welding Example 2 Similar to Example 1, a polyethylene polypropylene blend resin with a polyethylene blending ratio of 0 to 100% was extruded onto a steel pipe using a crosshead die under the following conditions. A tensile test was performed on the resin seam and the base material of the coating.

The results are shown in Table 2. The lining conditions were as follows: Eight steel pipes were used as the original pipes, and high-density polyethylene (density 0.947) was extruded from a crosshead die to a skin thickness of 0.6 as the lower anticorrosive layer. A blended resin coating of 1. Rib was extruded and coated as a protective layer from the next crosshead die.

Table 2 It can be seen from Table 2 that when the polyethylene content ratio is lower than 20%, the strength at the seam of the extrusion coating is extremely low compared to the strength of the base material, which poses a practical problem. .

Example 3 High-density polyethylene (density -0.947M) was used as the anti-corrosion layer.
IO. 25), ethylene-propylene copolymer (ethylene content 30%) (density 0.910M) as a protective layer
IO. Double coating was carried out in the same manner as in Example 2 for steel pipes of different diameters using 50).

The results are shown in Table 3. Table 3 As is clear from this table, the present invention showed good results in forming all of the steel pipes well and with no arrival at the port.

As can be understood from the above examples, according to the present invention, a layer of 100% polyethylene and a layer of 20 to 40% polyethylene are used.
%, and a layer composed of 60 to 80% polypropylene is not completely welded even when directly superimposed, and therefore the outer layer can be easily peeled off at any point on the pipe.

Claims (1)

[Claims] 1. In a plastic-coated steel pipe in which two layers, an anticorrosive layer and a protective layer, are extruded and coated using a cross-head die method, one of the anticorrosive layer and the protective layer is made of polyethylene alone, and the other layer is made of polyethylene 20 to 40% by weight and polypropylene 60%. 1. A plastic extrusion coated steel pipe characterized by being made of a copolymer or blend resin containing up to 80% by weight.