JP2005088299A - Method for producing polyolefin resin-coated steel pipe excellent in surface appearance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a polyolefin resin-coated steel pipe excellent in surface appearance which appropriately prevents the occurrence of a sink mark in a coat overlapping end part during cooling. <P>SOLUTION: In the method, the surface of the steel pipe is coated with the molten polyolefin resin extruded in the form of a sheet while the resin is wounded spirally. While the resin is melted, the overlapping end part of the resin applied spirally is pressed by a draft roll whose surface temperature is at least the melting point of the resin. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for producing a polyolefin resin-coated steel pipe, and more particularly, to a method for producing a polyolefin resin-coated steel pipe having an excellent surface appearance with no appearance defects such as irregularities on the outer surface.

  As a method for producing a polyolefin resin-coated steel pipe, a molten polyolefin is formed on the outer surface of the steel pipe while the heated steel pipe is rotated forward by a conveying roll (skew roll) disposed obliquely with respect to the rotating direction of the steel pipe. A method is known in which a resin sheet is continuously wound in a spiral shape and coated. In this method, it is necessary to rapidly cool and solidify the coating resin after the resin coating before contacting the next conveyance roll for the purpose of preventing the coating resin from being crushed by the conveyance roll. Due to the shrinkage of the polyolefin resin due to the water cooling, a spiral dent, that is, a sink, is formed at the overlapping end portion of the resin sheet, resulting in poor appearance.

  In particular, this phenomenon tends to occur when the coating line speed is increased in order to improve productivity. Due to this sinking problem, the line speed cannot be increased sufficiently and the productivity is hindered. Was a factor.

  When the above polyolefin resin sheet is wound around a steel pipe, a support roll is usually used to press the sheet of the winding portion against the steel pipe. FIGS. 6 and 7 show the state when the belt-shaped molten resin sheet coming out of the T die is wound around the steel pipe. As shown in FIG. 6, this apparatus is equipped with a polyolefin resin extruder and a modified polyolefin resin extruder that forms an adhesive layer, and a strip-shaped molten resin comprising a polyolefin resin layer co-extruded from a T-die and a modified polyolefin resin layer. The sheet is wound around the surface of the steel pipe that is rotating in the right direction of the drawing while rotating. A support roll is provided in the winding portion. FIG. 7 is a cross-sectional view showing a state when the sheet is wound. This support roll is used to prevent the sheet from being loosened or causing air bubbles or the like to be caught between the sheet layers when the belt-shaped molten resin sheet is attached to the steel pipe.

  As a means to prevent the above-mentioned appearance defects, after coating with polyolefin resin, a nozzle is arranged along the overlapping edge of the sheet, and compressed air is sprayed while spraying water particles, and the overlapping edge of the sheet is cooled. A method of solidifying and then further water-cooling the entire outer periphery is disclosed (see Patent Document 1).

JP 11-291398 A

  However, in the above prior art method, during continuous operation, it is difficult to intensively cool the overlapping edges due to the deviation of the spraying position of coolant such as water fine particles, and industrially stable surface appearance. It was difficult to keep good. On the other hand, when the coolant is sprayed on the entire coated resin sheet to cool it, there is a problem that the occurrence of sink marks cannot be prevented as in the case of normal cooling.

  Accordingly, an object of the present invention is to provide a method for producing a polyolefin resin-coated steel pipe that appropriately prevents the occurrence of sink marks at the overlapping end portions of the coating during cooling and has an excellent surface appearance.

  As a result of studying the cause of sink marks occurring at the sheet overlap end, the present inventors have rapidly increased the temperature by water cooling when the compatibility between the overlapped resin layers is insufficient as shown in the schematic diagrams of FIGS. It was found that due to the force that the resin shrinks when cooled, the stress concentrates on the insufficiently melted part between the layers, causing a shift between the layers, resulting in a sink.

  That is, in order to prevent the occurrence of sink marks, the present inventors are the first to demonstrate that it is effective to promote melting / integration between layers of overlapping resins so as not to cause a shift between layers due to shrinkage. I found it.

  In order to promote the fusion / integration between the layers of the overlapping portion of the resin, pressurization with a roll is effective. At this time, the melting is performed by maintaining the temperature of the surface of the roll above the melting point of the resin. It has been found that melting and integration between layers can be promoted while the molten state is maintained without cooling the resin at the power overlap portion, and the occurrence of sink marks can be effectively suppressed.

  This cannot be solved by the conventional support roll alone. The reason is as follows.

  In order to improve the compatibility between the resin layers, if the support roll is pressed strongly, the film thickness of the belt-like molten resin sheet at the time of pipe fitting becomes thin and easily cut, and in particular, a steel pipe having a weld bead portion as shown in FIG. In this case, it is because the pipe is cut when the tube is piped due to the step of the bead portion.

  The present invention has been made on the basis of the above findings, and in a method for producing a polyolefin resin-coated steel pipe formed by coating a molten polyolefin resin extruded into a sheet shape while spirally winding the steel pipe surface, the resin is Production of a polyolefin resin-coated steel pipe with excellent surface appearance, characterized in that, while in the molten state, the overlapping end of the resin coated in a spiral shape is reduced with a reduction roll whose surface temperature is equal to or higher than the melting point of the resin. It is about the method.

  According to the present invention, in a polyolefin resin coating method in which a molten polyolefin resin sheet is continuously spirally wound around the outer surface of a steel pipe, a polyolefin resin-coated steel pipe having an excellent surface appearance with no sink marks in the overlapping portion of the sheets is obtained. be able to.

  Although the kind and dimension of the steel pipe used by this invention are not restrict | limited in particular, For example, an electric sewing pipe, a spiral steel pipe, a UOE steel pipe, and a plated pipe are mentioned.

  First, if necessary, the outer surface of the steel pipe is subjected to rust removal treatment such as alkali degreasing / acid treatment or blast treatment to clean the surface. In order to impart further excellent anticorrosion properties, the surface of the steel pipe may be subjected to a ground treatment such as chromate treatment or phosphate treatment. Further, a primer layer such as an epoxy resin may be provided on the upper layer in order to enhance the adhesion with the polyolefin resin. Examples of the form of the primer before forming the primer layer include a powder primer and a liquid primer.

  When forming the primer layer, the steel pipe is usually heated to about 100 ° C to 250 ° C. Examples of the heating method for the steel pipe include high-frequency induction heating and far-infrared heating gas direct flame heating. As a method for forming the primer layer, methods such as spray coating, roll coating, brush coating, and ironing are used.

  Although the steel pipe of the present invention covers a polyolefin resin, it is preferable to provide an adhesive layer because the polyolefin resin generally has poor adhesion. Preferred examples of the adhesive layer include modified polyolefin resins and ionomer resins. As the modified polyolefin resin, a polyolefin resin used for the polyolefin resin layer may be modified with an unsaturated carboxylic acid such as maleic acid, acrylic acid or methacrylic acid or an acid anhydride thereof. The thickness of the adhesive layer is about 0.1 to 2 mm, preferably about 0.5 to 1.0 mm.

  Known polyolefin resins such as low density polyethylene, medium density polyethylene, high density polyethylene, linear low density polyethylene, homopolypropylene, random polypropylene, block polypropylene, polybutene, polymethylpentene, etc. Can be used. The melting point of the polyolefin resin varies depending on the structure, but in the case of polyethylene, it is about 102 ° C to 137 ° C. The range used for normal extrusion coating is about 110 ° C to 125 ° C, and in the case of polypropylene, about 130 ° C to 170 ° C, usually used for extrusion coating. The range to be used is about 140 to 165 ° C, in the case of polybutene, about 115 ° C to 135 ° C, the range usually used for extrusion coating is about 120 to 130 ° C, and in the case of polymethylpentene, about 220 ° C to 240 ° C. The polyolefin resin coating has a thickness of about 1 to 5 mm, usually about 2 to 4 mm.

  In the method of the present invention, a polyolefin resin is heated and melted and extruded into a sheet shape. The melting temperature of the polyolefin resin is about 10 to 160 ° C. higher than the melting point, preferably about 60 to 140 ° C. for polyethylene and polybutene, and about 50 to 110 ° C. for polypropylene. In the case of polymethylpentene, the temperature is increased by about 40 to 70 ° C. The die to be extruded into a sheet shape may be a known one, and a T die or the like can be used. The width of the sheet is not particularly limited, but is about 350 to 550 mm.

  When the adhesive layer is provided, the steel pipe may be coated prior to the polyolefin resin, but it is easy to co-extrude the polyolefin resin.

  A polyolefin resin sheet in a molten state extruded from a die is spirally wound around the surface of a steel pipe that has been subjected to a base treatment if necessary. In this case, the temperature of the steel pipe is about 0 to 130 ° C. higher than the melting point of the resin when the polyolefin resin is polyethylene, and about 0 to 100 ° C. when the polyolefin resin is polypropylene resin. The overlap width of the sheet side edges wound spirally, that is, the overlap width of the portion having the largest overlap thickness is about 10 to 50 mm, usually about 15 to 30 mm. The number of stacked end portions of the side edges of the sheet varies depending on the pitch at which the sheet is spirally wound, and there are two layers or three layers as shown in FIGS.

  The present invention is characterized in that while the spirally wound polyolefin resin is in a molten state, the overlapping end portion is reduced with a roll having a surface temperature equal to or higher than the melting point of the polyolefin resin. According to the present invention, the overlapping portion of the polyolefin resin sheet pressure-bonded to the surface of the steel pipe with the support roll is squeezed with the squeezing roll while the part is still in a molten state, and the polyolefin resin layers in contact with each other are fused. Can be made.

  This roll reduction promotes the compatibilization of the overlapping end portion and fuses it, so that only the overlapping end portion may be reduced, or the entire overlapping portion may be reduced. For this reason, the roll width is 1.0 to 30 times, preferably 12 to 20 times the width of the portion having the largest number of layers, including the overlapping end portion of the outermost surface. Here, the term “reducing” means to pressurize the resin plane, and the reducing roll keeps the distance from the steel pipe constant and rotates along the outer surface. The surface temperature of the rolling roll is a temperature equal to or higher than the melting point of the polyolefin resin, preferably about 1 to 10 ° C. higher than the melting point.

  The material of the surface of the reduction roll is preferably a flexible material that has heat resistance equal to or higher than the melting point of the resin and does not excessively crush the molten resin during reduction. Examples thereof include silicon-based, fluorine-based, and NBR (acrylonitrile / butadiene rubber) -based resins. The surface portion of the rolling roll that comes into contact with the molten resin is preferably made of any one resin selected from silicon resins, fluorine resins, and NBR resins. A silicon-based resin refers to a resin containing silicon. Further, the fluororesin refers to a resin based on fluororubber. NBR resin refers to a resin based on NBR resin or hydrogenated NBR resin.

  In addition, as a method of maintaining the roll surface temperature above the melting point, a known method such as flowing a temperature-controlled heating medium in a hollow portion inside the roll or increasing the temperature by contact with a molten resin in advance can be used. .

  The reduction roll is located downstream of the support roll, and is preferably disposed about 0.2 to 10 seconds, preferably about 0.5 to 8 seconds after the polyolefin resin surface passes through the support roll.

  The polyolefin resin sheet that has passed through the reduction roll of the present invention is cooled, for example, by spraying water or the like, but after this roll reduction, the coating resin surface temperature until the start of cooling is set to a temperature equal to or higher than the melting point of the resin. It has also been found that the compatibilization is further promoted by keeping. The time for maintaining the surface temperature of the coating resin at a temperature equal to or higher than the melting point is preferably 10 seconds or longer.

[Examples 1-8, Comparative Examples 1-3]
JIS
A UOE steel pipe having an STPY 40 of G-3457, a pipe length of 12 m, an outer diameter of 914.4 mm, and a pipe thickness of 9 mm was used, and the outer surface of the steel pipe was grit blasted. It was conveyed while rotating on the skew roll, and first, the chromate treatment liquid was applied to the surface of the steel pipe using a transfer roll. After coating, the steel pipe temperature was raised to 100 ° C. by the first induction heater and the chromate layer was formed by the first induction heater until the coated surface was in contact with the next skew roll. Subsequently, the temperature of the steel pipe was raised to 210 ° C. with a second induction heater, and an epoxy powder coating (density 1.49, particle size 250 μm or less) was coated on the outer surface with a film thickness of 350 to 400 μm with an electrostatic powder coating machine. . Next, a 0.2 to 0.5 mm thick modified polyethylene resin adhesive layer (maleic anhydride modified polyethylene, melting point 121 ° C., MFR 1.0) melted at 220 ° C. and a 3.0 mm thick polyethylene resin layer (high density polyethylene) , A double layer melt extruded resin sheet having a density of 0.943, a melting point of 124 ° C., and an MFR of 0.24) in a spiral shape so that the width of the portion of the side edge that is the heaviest or most numerous is 20 mm. Wound around.

The support roll attached to this device is made of hollow silicon and has a diameter of 60 mm and a width of 700 mm, and the reduction roll is provided at a downstream side with a distance of 450 mm (distance between shaft centers), a diameter of 180 mm and a width of The thing of 350 mm was used. The pressing pressure of the reduction roll was 2 kg / cm ² . The coating resin was pressure-bonded with this rolling roll, and the roll temperature at that time and the time until cooling thereafter were performed under the conditions shown in Table 1. And the coated steel pipe excellent in the surface external appearance was obtained. Table 1 shows the rolling roll temperature, the time to roll-down cooling, and the appearance evaluation results at that time. The appearance was evaluated according to the following criteria.

Appearance evaluation After coating with polyolefin resin, rolling down with a rolling roll, and cooling, the state of the coated resin overlap end is visually confirmed, the sink width is examined, no sink marks are found: ◎, sink width is 3 mm or less, and sink marks are generated. The number of places was 5 or less: ◯, the sink width was greater than 3 mm, or the number of sinks was 5 or more: x. From the result of Table 1, it turns out that Examples 1-8 of this invention are excellent in surface appearance compared with Comparative Examples 1-4.

  The polyolefin resin-coated steel pipe obtained by the production method of the present invention is used for anticorrosion steel pipes such as gas pipes, water pipes, line pipes, steel pipe piles.

It is a side view which shows the example which manufactures the polyolefin resin coating steel pipe by the method of this invention. It is the figure which showed typically the structure of the overlap part of the resin coat | covered spirally in the cross section. It is a figure which shows the side and longitudinal cross-section of the steel pipe by which the polyolefin resin sheet was spirally wound and covered. It is the figure which showed typically the structure of the overlap part of the resin coat | covered spirally in the cross section. It is the figure which showed typically the state which generate | occur | produced the shift | offset | difference by the overlapping part of FIG. 4 by cooling contraction. It is a figure which shows the state which is winding-coating the conventional polyolefin resin helically. It is the figure which showed typically the longitudinal cross-section of the steel pipe part of FIG.

Claims (3)

  In a method for producing a polyolefin resin-coated steel pipe, in which a molten polyolefin resin extruded in a sheet shape is coated while being spirally wound around the surface of a steel pipe, the surface temperature is the melting point of the resin while the resin is in a molten state. A method for producing a polyolefin resin-coated steel pipe excellent in surface appearance, characterized in that the overlapping end portion of the resin coated in a spiral shape is squeezed with the squeezing roll as described above   2. The method for producing a polyolefin resin-coated steel pipe having excellent surface appearance according to claim 1, wherein after the roll pressure, the surface temperature of the polyolefin resin is maintained at a temperature equal to or higher than the melting point of the resin for 10 seconds or more.   3. The surface appearance according to claim 1 or 2, wherein the material of the surface of the reduction roll is any one selected from silicon resin, fluorine resin, and NBR resin Manufacturing method of polyolefin resin-coated steel pipe

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