EP0026906B1 - Process for coating metal pipes and application of said pipes - Google Patents

Description

It is known to coat metal pipes with a polyethylene tape coming directly from the extruder. The temperature of the pipes is about 180 ° C. The types of polyethylene used have melt indexes (190 ° C / 2.16 kg) of 0.4 to 0.7 g / 1 min. It takes about 4 minutes to build up layers of about 3.5 mm in thickness with a length of the tubes of 12 to 15 m. However, in order to achieve the above-mentioned layer thickness, the band usually has to be wrapped several times around the tube with regard to its thickness, which can lead to an uneven surface structure. When applying it is also important to ensure that the individual turns overlap or that the individual turns are mutually connected at the same time. Any errors that occur lead to the loss of corrosion protection.

On the other hand, the coating of metal pipes with polyethylene powder is known, the z. B. is applied by Aufriesetn, spin coating or starting on the preheated tube to 300 to 360 ° C. The types of polyethylene used in this process have melt indexes (190 ° C / 2.16 kg) of 1.2 to 1.7 g / 1 min. The melt index must be higher than in the first-mentioned process because easier melting is essential for coating with powder in order to anchor the applied particles on the surface. Depending on the pipe diameter, the coating time for powder is generally about 5 minutes. However, this method has the disadvantage that, although high preheating temperatures are used, adequate corrosion protection due to the application of only one layer is often not achieved.

Another publication describes a composite pipe made of a polyethylene powder layer welded onto a steel pipe and a polyethylene layer welded to it, which, for. B. in the form of a polyethylene film emerging from an extruder onto the hot sintered polyethylene powder layer. When applying the first polyethylene layer, the steel tube is z. B. preheated to 150 ° C. At this temperature, however, it is not possible to produce a smooth, coherent layer in a desired required minimum layer thickness of 1.5 to 4 mm.

According to another known method, a polyethylene layer is applied to a steel pipe, firstly a 0.05 mm thick first layer of an adhesion promoter at a tube temperature about 100 ° C. above the solidification temperature of the adhesion promoter and then the second layer of the adhesion promoter at 20 to Pipe temperature 50 ° C above the solidification temperature, e.g. B. at 140 ° C, is applied. The adhesion promoter of the first layer can be applied in powder form or by wrapping it with a film. The second layer can be applied by pre-extrusion in the form of a double tube or a double wrapping film. Here too, the temperature of 140 ° C. is not sufficient for the purposes desired according to the present invention, that is to say for the required minimum layer thickness.

Another document also discloses coating a steel tube surface with a polyethylene having a higher melt index, e.g. 1 to 1.5 g / 10 min (ASTM-D 1238-53T) and with a lower melt index polyethylene, e.g. B. 0.2 to 0.5 g / 10 min, the second layer being applied to the first coating.

Polyethylene powder is used to form both layers. If two layers are applied in the form of powder on top of one another, the surface sometimes leaves something to be desired in terms of their homogeneity and smoothness.

Another known method (US-A-3 348 995), which was taken into account in the formulation of the preamble of claim 1, describes the application of a layer of polyethylene with a high density and a lower melt index on metal surfaces, which is followed by an application of a thicker one Medium or low density polyethylene layer with high melt index connects, the polyethylene can be used either as a powder or as a film. In this process, it is necessary in any case to first apply the polyethylene with the lower melt index at a temperature of at least 260 ° C. in a thin layer in order to avoid detachment of the coating from the metal surface. A disadvantage of the method is that the metal object to be coated must be kept at such a high temperature - but at least 177 ° C. - during the entire coating period that the polyethylene used is in a molten state until the metal is completely coated.

It was therefore desirable to have a process which avoids the disadvantages of the known processes and provides better products, or at least those with the same good properties as the known processes.

The subject of the invention is the method defined in claim 1.

A further possible embodiment of the invention provides according to claims 3 to 5 that first of all a polyethylene powder with a melt index of 1.2 to 1.7 g / 10 min or a strip of polyethylene emerging directly from an extruder with a melt index of at most 1 , 7 g / 10 min (190 ° C / 2.16 kg) on a metal tube, which in the case of powder coating has a temperature of at least 300 ° C and in the case of the coating from the extruder a maximum of 250 ° C, then the coating is allowed to cool to a temperature of about 110 to 170, preferably 110 to 150 ° C and then in a third stage a self-supporting, light-stabilized, light-colored polyethylene film with a melt index of 0.4 to 1.1 g / 10 min at this temperature.

For example, compounds of the benzotriazole type are suitable as light stabilizers for the light-colored polyethylene film.

A layer thickness of 1.5 to 4 mm is generally required for adequate corrosion protection. In some cases, the layer thickness can also be more than 4 mm. However, the layer thickness must be at least 1.5 mm, because otherwise there is an excessive cathodic protective current requirement.

The process according to the invention has the advantage that, in general, compared to known processes, it surprisingly results in energy savings and nevertheless a considerably higher working speed with at least equally good production properties. Above all, the combination of powder application coating with the tape film coating - in this order mentioned - has succeeded in reducing the energy consumption through targeted temperature control, but at the same time improving the quality of the coating because any pores that occur from the powder coating the film cover is closed by welding. In addition, the light-colored cover layer provides good protection for the pipes against excessive heating when stored outdoors for long periods under the influence of heat, for example due to sunshine or by laying in strongly heated floor layers. The coating of the extruder is also very simple and time-saving without additional equipment. The strip emerging from the extruder advantageously has a melt index of at least 0.4 g / 10 min.

In the first stage, the polyethylene in the powder coating expediently has a particle size of 1 to 600 μm, preferably 100 to 400 μm. The self-supporting polyethylene film is advantageously in the form of a polyethylene tape, for. B. made of polyethylene with a melt index of 0.1 to 1.2 g / 10 min, applied. It can be applied to a rotating tube. This has the advantage that the tape can be wound automatically. The bandwidth can be varied as desired. It is z. B. at least 20 mm, expediently 10 to 1500 mm. A range of up to about 1 m is generally used. In the method according to the invention, it is desirable to ensure when applying that the individual turns overlap or that there is a mutual connection of the individual turns at the same time, however, perfect corrosion protection can generally be achieved even if these conditions are not completely fulfilled because the first coating underneath also contributes to the corrosion protection behavior of the product. The layer thickness of the tapes is usually 100 to 400 microns, preferably 100 to 200 microns. The light colored bands are preferably white. However, another color can also be selected as desired, e.g. B. the warning color yellow, but also light orange, light blue, light green or the like. If necessary, the light-colored tapes can also be used to identify the pipes.

The speed of the coating can vary within wide limits. It depends on the desired layer thickness, the thickness of the tube wall, the length and the diameter of the tube, the outer tube diameter being 50 to 2000 mm, for example. For the coating of a 12 m long tube with a diameter of 1500 mm and a coating of 3.5 mm layer thickness, z. B. about 15 to 45, usually about 30 minutes to cool to 60 ° C.

The coating of a pipe with a diameter of 400 mm and a layer thickness of 1.5 mm (this is the minimum thickness for adequate corrosion protection) generally requires about 8 to 20, usually about 15 minutes for a 12 m long coating.

In order to improve the adhesion of the layer applied in the first stage to the tube base, it is sometimes useful to add an additional resin in the form of a polymer, e.g. B. polyvinyl acetate, ethylene-vinyl acetate copolymer, ethylene-acrylic acid and / or acrylic ester copolymer, optionally with other comonomers, or other polymers, advantageously in a proportion of 5 to 15, preferably 5 to 10 wt .-%, based on the polyethylene powder. According to another embodiment of the invention, it is also possible to apply these polymers to the tube before applying the polyethylene powder. This coating can be carried out by the usual coating methods, by spraying, but preferably by powder coating. The same additional resins can also be contained in the polyethylene tape in a proportion of 2 to 5% by weight, based on polyethylene.

The requirements relating to minimum layer thickness, freedom from pores, peel strength, impact resistance, indentation resistance, elongation at break, specific coating resistance, heat and light aging according to the regulations of DIN 30670 are fully met by the coatings produced according to the invention.

The tubes coated by the method according to the invention are used in a variety of ways. Because of their surface protection, they are especially for laid pipes, e.g. B. in pipelines for the extraction of petroleum, but also of other gaseous, liquid or highly viscous substances, for example natural gas, water, sewage sludge, concrete, waste water, slurries or the like.

It is particularly advantageous to use the pipes produced according to the invention for laying in warm or hot areas, e.g. B. also in the desert.

Examples

• 1) An iron pipe (outer diameter 108 mm, wall thickness 10 mm) is preheated to 220 ° C and then covered with polyethylene powder (melt index 17 to 25) with a layer thickness of 2 mm for 2 minutes. After 4 minutes the coating has melted and the tube temperature has dropped to 160 ° C. Starting at this tube temperature, a 110 11m thick, 50mm wide polyethylene tape with a melt index of 1.2 and a layer thickness of 110 µm is applied at 160 ° C. Then allowed to cool to room temperature either simply by leaving it lying there or by passing a cooling medium through the tube. Immediately after application of the film, a perfect and smooth fusion of both coating layers occurs. After 30 minutes from the start of the coating, the tube has cooled to 60 ° C without any additional cooling.
• 2) An iron tube (outer diameter 90 mm, wall thickness 4.5 mm) is preheated to 250 ° C and then coated for ¹ _1/2 minutes with polyethylene powder (melt index 17 to 25) with a layer thickness of 2 mm. After 3 minutes the coating melted smoothly and cooled to 150 ° C. At this tube temperature, a 40 mm wide polyethylene tape with a melt index of 1.2 and a layer thickness of 110 µm is applied and the tube is then cooled to room temperature. The tape is applied by wrapping the tube rotating around its axis, the spool being moved along the tube. Immediately after the tape has been applied, it fuses perfectly with the first polyethylene layer. After 12 minutes from the start of the pipe coating, the pipe has cooled to 60 ° C without being cooled.

1 V (comparison - state of the art - preheating temperature above 300 ° C -)

The same tube as in Example 1 is used, but with a preheating temperature of 310 ° C. The tube is coated with polyethylene powder with a melt index of 1.2 to 1.7 g / 10 min with a layer thickness of 2 mm. After 10 minutes the coating melted smoothly and reached a temperature of 180 ° C. The cooling time to 60 ° C simply by leaving it lying there is 50 minutes.

2 V (comparison - state of the art - preheating temperature above 300 ° C)

The same tube as in Example 2 is used, but with a preheating temperature of 360 ° C. The tube is covered at this temperature with polyethylene powder having a melt index from 1.2 to 1.7 1 _^1/2 minutes with a layer thickness of 2 mm. The coating melted in the course of 4 minutes and at the same time cooled to 310 ° C. However, reheating is still required for smooth melting. Therefore, after these 4 minutes, the pipe is reheated for one minute. The coating is smooth after this additional minute. The cooling time to 60 ° C from the start of the powder coating and without additional cooling is 42 minutes.

• 3) Es wird gearbeitet wie nach Beispiel 1, jedoch wird anstelle des Polyäthylenpulvers nunmehr ein Pulver in Form eines Gemisches von Polyäthylen mit 10 Gew.-%, bezogen auf Polyäthylen, eines Vinylacetat-Homopolymerisats aufgetragen. Man erhält einen Überzug mit gleich guten Eigenschaften wie nach Beispiel 1 und mit einer einwandfreien glatten Oberfläche.
• 4) Es wird gearbeitet wie nach Beispiel 2, jedoch wird anstelle des Polyäthylenbandes nunmehr ein Band aus einem Gemisch von Polyäthylen und 3,5 Gew.-%, bezogen auf Polyäthylen, eines Vinylacetat-Homopolymerisats verwendet. Man erhält einen glatten und einwandfreien Überzug mit gleich guten Eigenschaften wie nach Beispiel 2.
• 5) Ein Eisenrohr (Außendurchmesser 108 mm, Wanddicke 10 mm), wird auf 310°C vorgewärmt und darauf 2 Minuten mit Polyäthylenpulver (Schmelzindex 1,2 bis 1,7 g/ 10 min) mit einer Schichtdicke von 2 mm überzogen. Nach 20 Minuten ist der Überzug geschmolzen und die Rohrtemperatur auf 160°C abgesunken. Beginnend bei dieser Rohrtemperatur wird bei 160°C ein 200 µm dickes, 50 mm breites Polyäthylenband mit einem Schmelzindex von 0,4 g/10 min in einer Schichtdicke von 200 µm aufgebracht. Dann läßt man das Rohr entweder durch bloßes Liegenlassen oder unter Durchleiten eines Kühlmediums auf Raumtemperatur abkühlen. Unmittelbar nach Aufbringen des Bandes tritt eine einwandfreie und glatte Verschmelzung von beiden Überzugsschichten ein. Nach 40 Minuten ab Beginn der Pulverbeschichtung ist das Rohr beim Liegenlassen ohne zusätzliche Kühlung auf 60°Cabgekühlt.
• 6) Ein Eisenrohr (Außendurchmesser 90 mm, Wanddicke 4,5 mm), wird auf 360°C vorgewärmt und darauf 1¹/₂ Minuten mit Polyäthylenpulver (Schmelzindex 1,2 bis 1,7 g/ 10 min) mit einer Schichtdicke von 2 mm überzogen. Nach höchstens 8 Minuten ist der Überzug glatt geschmolzen und nach 12 Minuten auf 150°C abgekühlt. Bei dieser Rohrtemperatur wird ein 40 mm breites Polyäthylenband mit einem Schmelzindex von 0,4 g/10 min in einer Schichtdicke von 200 µm aufgebracht und darauf das Rohr auf Raumtemperatur abgekühlt. Das Aufbringen des Bandes erfolgt durch Umwickeln des um seine Achse rotierenden Rohrs, wobei die Spule am Rohr entlang bewegt wird. Unmittelbar nach Aufbringen des Bandes tritt eine einwandfreie Verschmelzung mit der ersten Polyäthylenschicht ein. Nach 30 Minuten ab Beginn der Rohrbeschichtung ist das Rohr beim Liegenlassen ohne zusätzliche Kühlung auf 60° C abgekühlt.
• 7) Ein Eisenrohr (Außendurchmesser 500 mm, Wanddicke 6 mm) wird auf 250°C vorgewärmt und mit einem Äthylen-AcrylsäureCopolymer in Pulverform als Haftgrundierung mit einer Schichtdicke von etwa 100 µm beschichtet. Auf diese so überzogenen und noch warmen Rohre wird ein unmittelbar aus einem Extruder austretendes Polyäthylenband mit einem Schmelzindex von 1,2 g/10 min und mit einer Schichtdicke von 250 u.m aufgebracht. Dieser Wickelvorgang wird so lange fortgesetzt, bis die gewünschte Schichtdicke von 4 mm erreicht ist. Das Abkühlen des Rohres auf etwa 140°C erfolgt durch bloßes Liegenlassen. Bei dieser Temperatur wird ein gelbes Polyäthylenband mit einem Schmelzindex von 0,7 g/10 min, einer Breite von 300 mm und einer Schichtdicke von 200 µm aufgebracht. Es tritt eine einwandfreie und glatte Verschmelzung der beiden Polyäthylen- überzugsschichten ein. Nach 30 Minuten ab Beginn der Extruder-Beschichtung ist das Rohr beim Liegenlassen ohne zusätzliche Kühlung auf 60° C abgekühlt.

As the comparison between the examples according to the invention and the comparative examples according to the prior art shows, the energy expenditure, ie the preheating temperature and at the same time the cooling time, is significantly less in the examples according to the invention than in the comparative examples.

• 3) The procedure is as in Example 1, but instead of the polyethylene powder, a powder is now applied in the form of a mixture of polyethylene with 10% by weight, based on polyethylene, of a vinyl acetate homopolymer. A coating is obtained with the same properties as in Example 1 and with a perfect smooth surface.
• 4) The procedure is as in Example 2, but instead of the polyethylene tape, a tape made from a mixture of polyethylene and 3.5% by weight, based on polyethylene, of a vinyl acetate homopolymer is now used. A smooth and flawless coating with the same properties as in Example 2 is obtained.
• 5) An iron tube (outer diameter 108 mm, wall thickness 10 mm) is preheated to 310 ° C and then coated for 2 minutes with polyethylene powder (melt index 1.2 to 1.7 g / 10 min) with a layer thickness of 2 mm. After 20 minutes the coating has melted and the tube temperature has dropped to 160 ° C. Starting at this pipe temperature, a 200 µm thick, 50 mm wide polyethylene tape with a melt index of 0.4 g / 10 min is in at 160 ° C applied a layer thickness of 200 microns. Then the tube is allowed to cool to room temperature either simply by leaving it lying there or by passing a cooling medium through it. Immediately after the tape has been applied, a perfect and smooth fusion of both coating layers occurs. After 40 minutes from the start of the powder coating, the tube is cooled to 60 ° C when left lying without additional cooling.
• 6) An iron pipe (outer diameter 90 mm, wall thickness 4.5 mm), is preheated to 360 ° C and then 1 _^1/2-minute polyethylene powder (melt index 1.2 to 1.7 g / 10 min) with a layer thickness of 2 mm coated. After a maximum of 8 minutes the coating melted smoothly and after 12 minutes it cooled to 150 ° C. At this tube temperature, a 40 mm wide polyethylene tape with a melt index of 0.4 g / 10 min is applied in a layer thickness of 200 µm and the tube is then cooled to room temperature. The tape is applied by wrapping the tube rotating around its axis, the spool being moved along the tube. Immediately after the tape has been applied, it fuses perfectly with the first polyethylene layer. After 30 minutes from the start of the pipe coating, the pipe has cooled to 60 ° C without being cooled.
• 7) An iron pipe (outside diameter 500 mm, wall thickness 6 mm) is preheated to 250 ° C and coated with an ethylene-acrylic acid copolymer in powder form as an adhesive primer with a layer thickness of about 100 µm. A polyethylene tape emerging directly from an extruder and having a melt index of 1.2 g / 10 min and a layer thickness of 250 μm is applied to these tubes, which are still warm. This winding process is continued until the desired layer thickness of 4 mm is reached. The tube is cooled to about 140 ° C by simply leaving it there. At this temperature, a yellow polyethylene tape with a melt index of 0.7 g / 10 min, a width of 300 mm and a layer thickness of 200 µm is applied. A perfect and smooth fusion of the two polyethylene coating layers occurs. After 30 minutes from the start of the extruder coating, the tube has cooled to 60 ° C without any additional cooling.

Claims (10)

1. Process for coating of metal tubes with polyethylene wherein a) polyethylene having a melt index of more than 1 g/10 min. is applied in a first step to a heated metal tube, b) then in a second step the temperature of the coated tube is lowered and c) a self-supporting film of polyethylene is applied in a third stage, characterized in that a) a polyethylene having a melt index of 1.2 to 70 g/10 min. (190°C/2.16 kg) is applied in stage 1 to a metal tube having a temperature of at least 200° C, b) the coated tube is allowed to cool to a temperature of from 110 to 170°C in stage 2, and c) the self-supporting film of polyethylene is applied thereon in stage 3 at this temperature until a minimum layer thickness of the total coating of 1.5 to 4 mm is achieved, the melt index of the film selected from the range of from 0.1 to 7 g/10 min. (190°C/2.16 kg) being lower than that melt index which is selected for the first stage.

2. Process according to claim 1, characterized in that a polyethylene having a melt index of 15 to 70 g/10 min. (190°C/2.16 kg) is applied in the first stage.

3. Process according to claim 1, characterized in that a polyethylene powder having a melt index of from 1.2 to 1.7 g/10 min. (190°C/2.16 kg) or a tape, coming directly from an extruder, having a melt index of at most 1.7 g/10 min. (190°C/2.16 kg) is applied to the metal tube which has a temperature of at least 300°C in the case of powder-coating and of at most 250° C in the case of coating with the extrudated tape and that in the third stage a self-supporting light-stabilized light-colored film of polyethylene having a melt index of 0.4 to 1.1 g/10 min. (190°C/2.16 kg) is applied.

4. A process according to one or more of claims 1 to 3, characterized in that in the second stage the coated tube is cooled to a temperature of 110 to 150° C.

5. Process according to one or more of claims 1 to 4, characterized in that in the third stage the coating of polyethylene with the lower melt index is applied in the form of a polyethylene tape.

6. Process according to claim 5, characterized in that the polyethylene tape is applied to a rotating tube.

7. Process according to one or more of claims 3 to 6, characterized in that the coating of polyethylene is applied in the form of a white polyethylene tape having a thickness of from 100 to 200 µm.

8. Process according to one or more of claims 1 to 7, characterized in that in the first stage the polyethylene powder is applied in admixture with a vinylacetate homo- or copolymer, the proportion of which is preferably 5 to 10% by weight, referred to the polyethylene.

9. Use of the tubes made by the process according to one or more of claims 1 to 8 in pipelines.

10. Use according to claim 9 for the conveyance of petroleum.