JPS5940093B2 - Thermoplastic resin coated metal tube - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a metal tube whose outer surface is coated with a thermoplastic resin, and particularly to a coated metal tube whose coating has little residual strain.

The conventional Atsumachi plastic resin-coated metal tube is made by forming a metal tube (hereinafter referred to as a steel tube) 1 as shown in the process explanatory diagram in Figure 1 and the partial cross-sectional view of the Atsumachi plastic resin-coated metal tube in Figure 2. The steel pipes 1 are preheated by a preheating device 2, and connected to each other using a connecting jig.
An adhesive 10 made of asphalt, synthetic rubber, resin, etc. is applied in an adhesive coating device 3, and an extruder 4 is used to apply a adhesive 10 made of asphalt, synthetic rubber, resin, etc. to polypropylene, low-density polyethylene, high-density ethylene, etc. A first anti-corrosion layer 11 is formed by covering the resin with an extrusion die, and is air-cooled or water-cooled with a cooling device 5.

Next, the steel pipe 1 is coated with an anti-corrosion layer 11 by an anti-adhesion agent coating device 6.
An anti-adhesion agent 12 such as polybutene or silicone is applied to prevent adhesion with the next protective layer, and an extruder 7 is used to heat a material with excellent mechanical strength such as polypropylene, low-density polyethylene, or high-density polyethylene. The plastic resin is extruded from an extrusion die to cover the protective layer 13, cooled to room temperature by air cooling or water cooling in the cooling device 8, and cut into the original length of the steel pipe by the running cutter 9 to manufacture the pipe. However, thermoplastic resin-coated steel pipes manufactured using this conventional process are exposed to Netsucho plastic resin (Natsumachi plastic resin) at high temperatures (usually 150 to 300'C)
(hereinafter referred to as plastic), a residual strain of 3 to 5% occurs in the coating due to volume contraction due to temperature drop to room temperature or molecular orientation during coating, and due to this residual strain, the coating after commercialization Exposure of the steel pipe material at the end due to shrinkage,
Or ESC (Environmental, St.
Problems such as deterioration of Clock and Clock occur. Plastic-coated steel pipes with reduced residual strain are produced by coating with a protective layer, cooling to the minimum necessary temperature, cutting to specified dimensions, and cooling to room temperature to shrink the coating and reduce residual strain in the coating. Coated steel pipes manufactured by this method have been proposed, but for coated steel pipes that require coating up to the pipe ends, a steel pipe connection jig that takes into account the shrinkage length of the coating is required, which causes an increase in manufacturing costs. . In view of the above-mentioned current situation, the present invention aims to find a film that can be manufactured by a conventional manufacturing process and has as little residual strain as possible, and as a result of numerous experimental studies, By using an anti-adhesion agent applied to the outer surface of the anti-corrosion layer, which has a special ratio of low shear adhesion at high temperatures and high shear adhesion at room temperature, heat shrinkage is prevented. Significantly reduces the residual strain of the film caused by

I found out that it can be done. In other words, the plastic-coated steel pipe of the present invention has an anti-adhesion layer on the surface of the anti-corrosion layer applied to the surface of the steel pipe by a conventional method, which exhibits low shear adhesion at high temperatures and high shear adhesion at room temperature. It is characterized in that a protective layer is formed by coating the plastic with an agent. The present invention will be described in further detail below with reference to the accompanying drawings. In the coated steel pipe of the present invention, in the conventional manufacturing process shown in FIG. For example, high melting point waxes, mixtures of aliphatic hydrocarbons, or mixtures of these with copolymers of polyethylene and polypropylene have the special property of exhibiting low shear adhesion at high temperatures and high shear adhesion at room temperature. Use one with

This anti-adhesion agent 12 has a high melting point wax of 33 to 50%,
Aliphatic hydrocarbons 33-500! ), a mixture of 0 to 20% copolymer of polyethylene and polypropylene is preferred from the viewpoint of achieving the intended purpose. The above-mentioned anti-adhesion agent 12 is applied using a conventional applicator, for example, as shown in FIG. A ring-shaped box-like body 6a equipped with flexible sealing plates 6d on both sides to seal the gap between the
An anti-adhesion agent 12'' is supplied from a supply pipe 6b provided on the upper surface into an anti-adhesion agent coating device 6 formed in the above, and the steel pipe 1 passing through the through hole is stored in the device 6. The anti-adhesion agent 12' inside the device 6 is refluxed through the discharge pipe 6c.

The metal tube 1 coated with the anti-adhesion agent 12 in this way is coated with plastic in the extruder 7 in the next step to form the second protective layer 13, and then the plastic-coated steel tube is formed through the steps shown in FIG. is manufactured. In the above manufacturing process, the effect of the adhesion preventing agent 12 used in the present invention will be explained based on FIGS. 4 and 5.

4 and 5 schematically show the dynamics of the coating when the protective layer is applied, FIG. 4 being the conventional one and FIG. 5 being the present invention. In the conventional method shown in FIG. 4, a protective layer is coated on the outer surface of the steel pipe coated with an anti-adhesion agent using molten cylindrical polyethylene 13' extruded from an extrusion die 14. In this case, polyethylene 13'
The running speed Vp of the steel pipe 1 is made larger than the extrusion speed v of
By pulling the coating on the molten cylindrical body, the inner diameter and wall thickness are reduced, that is, the coating is performed with the dynamics of t1, T3, and T2 as shown in the figure. However, in the conventional method using an anti-adhesion agent that does not have the characteristics of the density-inhibiting agent, which is coated with the behavior shown in FIG. The coating coated with length 13 is stretched as the steel pipe 1 runs and contracts during the cooling process, but since the coating itself does not move in the pipe running direction, the contraction is only in the thickness direction. Therefore, at the cooling end point, the length near the cooling start position is 1
11, which has the same length as 3, is cooled and shrinks in volume. Therefore, in the axial direction of the steel pipe, if α is the linear expansion coefficient of polyethylene, T is the extrusion temperature, and t is the elasticity after cooling, a film with residual thermal strain of (T-t) × α is coated. That's what happened. On the other hand, in FIG. 5, the extrusion die 1
The molten cylindrical polyethylene 13' extruded from 4 is coated on the outer surface of the steel pipe 1 as in the case of FIG. Since it has a shear adhesion force, it is stretched as the steel pipe 1 runs, and the film thickness v at the beginning of the coating becomes T2, and the film has a length of 12 and a cross-sectional area of S2 near the cooling start position. It moves and contracts in the axial direction (in the direction of the thick arrow in the figure), and at the cooling end point its length is reduced to 11 and at the same time its thickness is increased to t1, resulting in a cross-sectional area S1 that is almost the same as the cross-sectional area S2 near the cooling start position. ,
Strain caused by thermal contraction is converted into so-called molecular orientation strain due to stretching. Therefore, the axial direction of the steel pipe is coated with a film with little residual strain.It is generally known that molecular orientation strain increases as the drawdown ratio or stretching ratio increases, and conversely decreases as it decreases. The value of this molecular orientation strain is extremely smaller than the value of strain due to thermal contraction, so it can be ignored.

Furthermore, in the case of the present invention, the strain that occurs when the coating contracts in the axial direction of the steel pipe is calculated as (T- t)

The protective layer 13 of a 50A, 80A dual-layer polyethylene-coated steel pipe having the structure shown in FIG. 2 was coated with the anti-adhesion agent mixed in Table 1.

In this case, the shear adhesion strength of the anti-adhesion agent 12 in each temperature range was as shown in Table 2. Table 3 shows the relationship between the shear adhesion force of each anti-adhesion agent shown in Table 1 and the length of movement of the coating in the tube axis direction when the protective layer is applied. Table 4 shows the relationship between the residual strain of the product coating and the shrinkage length of the product coating. 3rd above
As is clear from Tables 4 and 4, the coated steel pipe of the present invention shows that the protective layer coating at high temperatures moves significantly in the pipe axial direction even if the work process is carried out using the usual method. During the cooling process from the start of cooling to room temperature, with any of the anti-adhesion agents shown in Table 1, the length of movement of the protective layer film in the tube axis direction was 70 TILTL or more, and the shrinkage length of the film was 10 m.
m or less, and the residual strain due to thermal contraction of the protective layer can be reduced to 2.5% or less.

As explained in detail above, the present invention uses an anti-adhesion agent with a special agent that exhibits a low shear adhesion force at high temperatures and a high adhesion shear force at room temperature in the manufacturing process of polyethylene-coated steel pipes by a conventional method. Through this simple method, coated steel pipes with significantly reduced residual strain in the protective layer coating can be obtained, minimizing coating shrinkage in products, and improving the ESC resistance of the coating, making it a highly reliable polyethylene product. Coated steel pipes can be provided easily and at low cost.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram showing the manufacturing process of polyethylene-coated steel pipe using a conventional method, Fig. 2 is a partial cross-sectional view of polyethylene-coated steel pipe, Fig. 3 is an explanatory diagram showing an example of an anti-adhesion agent application device, and Fig. 4 FIG. 5 is an explanatory diagram showing the dynamics of a film during coating with a conventional protective layer, and FIG. 5 is the same diagram according to the present invention. In the figure, 1: steel pipe, 2: preheating device, 3: adhesive coating device,
4, 7: Extruder, 5, 8: Cooling device, 6: Anti-adhesion agent coating device, 9: Intercutting machine, 10: Adhesive layer, 11: Anti-corrosion layer, 12: Anti-adhesion return, 13: Protection Layer, 13″: polyethylene, 14: die.

Claims (1)

[Claims] 1 A thermoplastic resin-coated metal tube, which has a low shear adhesion force at high temperatures on the surface of the anticorrosion layer applied to the metal tube surface,
A thermoplastic resin-coated metal tube characterized by forming a protective layer by coating a thermoplastic resin with an anti-adhesion agent that exhibits high shear adhesion at room temperature.