WO2010057292A1

WO2010057292A1 - Polymeric fusible powder coating on elongate tubular article

Info

Publication number: WO2010057292A1
Application number: PCT/CA2009/001523
Authority: WO
Inventors: Nicolas Cunningham; Stephen J . Edmondson; Dennis T. H. Wong
Original assignee: Shawcor Ltd.
Priority date: 2008-11-24
Filing date: 2009-10-29
Publication date: 2010-05-27
Also published as: CA2644625A1

Abstract

A method for applying a polymeric heat-bondable powder coating on an elongate tubular article having a surface extending circumf erentially around the article, for example a pipe. The article is heated to a temperature at which the powder bonds to the surface. The powder is spray applied to the surface of the article while hot, the powder is permitted to bond to the surface to form a coating on the surface. Heat or pressure or heat and pressure are applied. The pressure is applied uniformly on the whole of the coating on the surface in order to densify the coating. The coated article is then allowed to cool.

Description

POLYMERIC FUSIBLE POWDER COATING ON ELONGATE TUBULAR ARTICLE

This application claims the benefit of the filing date of Canadian Patent Application No. 2,644,625 filed November 24, 2008 under the title POLYMERIC FUSIBLE POWDER COATING ON ELONGATE TUBULAR ARTICLE.

The content of the above patent application is hereby expressly incorporated by reference into the detailed description hereof.

Thick anti-corrosion coatings such as three layer polyethylene are typically applied by extruding a thick film of polyethylene (PE) onto a pipe that has been coated with fusion bonded epoxy (FBE) and adhesive. This technique has a number of drawbacks, the most notable of which is the tendency of the polyethylene to form a tent over a raised weld. In our patent application number WO 2007/095741 published August 30, 2007, we described how spray applying

FBE, adhesive and some PE prior to applying the polyethylene sheet can overcome this problem, however this entails adding an additional layer of PE (to give a total of four layers) which can add cost. A preferred solution may be to apply a two layer system comprising FBE followed by only one layer of a modified polyolefin to the FBE, as described in our Canadian patent application number 2,632,802 filed May 30, 2008. However, applying a two layer system may give a limitation with respect to the thickness that can be achieved, and hence a limitation as to the resistance to impact damage that can be achieved. If a thin coat of the modified polyolefin outer layer is applied for example 0.25 mm thick, it will not give the same protection to damage as a conventional three layer coating but may be easier to apply. If a thicker coating, for example greater than 0.7 mm thick, of the modified polyolefin is applied it may form a textured appearance, which will contain voids, and may have poor anti-corrosion properties and poor resistance to moisture .

The present invention provides a method for applying a polymeric heat bondable powder coating on an elongate tubular article having a surface extending circumferentially around the article, comprising heating the article to a temperature at which the powder bonds to the surface, spray applying the powder to the surface of the article while hot and permitting the powder to bond to the surface to form a coating on said surface, and applying heat or pressure or heat and pressure, the pressure applied uniformly on the whole of the coating on the surface, thereby densifying the coating, and permitting the coated article to cool.

With the method of the invention, the heat or uniform pressure or heat and pressure can cause the coating to flow and form a practically void free film.

In the preferred form the above-mentioned surface extends around the entire circumference of the article.

In conventional in-line powder coating of pipe, some pressure may be applied by support members such as tires that support the coated pipe as it is rotated about its axis before cooling. However such pressure is applied along discrete helical tracks along the pipe and is not applied uniformly to the whole of the coating.

In the preferred form, a fusion bonded epoxy coating is applied to an exterior surface of the article before said polymeric heat bondable powder is applied.

A final product obtainable with the method of the present invention has a thick two layer coating with minimal voids present, good weld coverage and good moisture resistance, providing a viable alternative to a conventional three layer polyethylene coating.

In the case in which heating is applied to densify the coating, any conventional heating technique may be employed. Preferably the heat is applied by directing radiant heat at the surface of the coating and, preferably, the heat is applied uniformly on the whole of the coating.

Preferably the radiant heat is infrared radiation.

In one form of the invention, heat and pressure are applied by playing a jet of hot gas on the coating, and, preferably, the heat and pressure are applied uniformly on the whole of the coating. The jet of hot gas may be, for example a jet of hot air, from a hot air blower or hot air knife. One advantage of using a hot air knife is that by directing the flow of hot air, it is possible to force the coating to flow in a desired direction, because of the pressure applied by the hot gas impacting on the molten coating. More preferably, pressure is applied uniformly to the coating using solid rollers that are biased toward the surface of the article. For example, solid rollers made of material such as silicon rubber, fluoroelastomer, such as Viton (trade-mark for fluoroelastomer available from DuPont Canada, Mississauga, Ontario) , and of fluorinated ethylene rubber are used to apply pressure uniformly to a powder coating that has been sprayed on an article, usually a metal article, such as a pipe.

Techniques for applying powder coatings of polymers to elongate tubular articles such as pipe are well known to those skilled in the art and need not be described here in detail. Such techniques are described, for example in our above-mentioned patent application No. WO 2007/095741, the disclosure of which is incorporated herein by reference.

A procedure for applying pressure uniformly to a polymeric coating on a pipe or other elongate tubular article is described in our U.S. Patent No. 5,026,451

(Trzecieski et al), particularly with reference to Figures 5 and 6 of Trzecieski et al ' s patent, and in U.S. Patent No. 3,868,265, Sakai, et al, granted February 25, 1975, and the disclosures of both of these patents is incorporated herein by reference.

Neither of these patents discloses applying pressure rollers to a freshly sprayed powder coating, however, as in the preferred form of the present invention.

Desirably, in carrying out the present method, the use of polymeric powder coatings that have excessively good flow properties when applied to a hot surface is avoided. Such powder materials may give rise to a problem of the coating tending to sag under the influence of gravity down the side of the tubular article if applied as a thick, for example greater than 0.7 mm thick, coating. One of ordinary skill in the art will readily determine by simple trial and experiment whether a given polymeric material has such excessively good flow properties that it may give rise to a sagging problem.

Advantageously a polymeric powder coating material as described in our above-mentioned Canadian patent application 2,632,802 is employed in carrying out the present invention. It will be appreciated, however, that the invention is by no means limited to such material and other polymeric fusible powder coating materials adapted to be applied to hot tubular articles as coatings in powder form can be used. Such materials are well known to those skilled in the art.

Preferably the article coated in the present invention comprises metal pipe, usually steel pipe. It will be appreciated, however, that the invention can be employed to advantage for anti-corrosion coating of other elongated tubular articles, for example metal lamp posts, utility poles and pilings.

In preferred forms of the present invention, the densified coating has a thickness of 0.5 to 4 mm, more preferably 0.7 to 4 mm, and still more preferably about 1 to 2 mm. The invention will be more fully described by way of example only with reference to the accompanying drawings in which

Figure 1 shows somewhat schematically a pipe coating process;

Figure 2 is a cross-section through a portion of a pipe having a densified coating formed in accordance with the method of the invention. The photograph shows an interface with a region to which no pressure has been applied; and

Figure 3 is a top view of the portion of Fig. 2.

Figures 4 to 6 are top views of products obtained as described in Examples 5 and 6 below.

In Figure 1, arrow 11 indicates the progress of a metal, for example steel, pipe 12 from a zone where the pipe is subjected to surface preparation and heating in conventional manner. The pipe is heated to a temperature making it receptive to polymeric powder coatings.

In the example shown, fusion bonded epoxy powder 13 is applied to the hot pipe from an applicator 14.

Subsequently, a coating of polymeric powder 16 is applied from an applicator 17. While the coatings are applied, the pipe is spun about its axis, so that the coatings are applied to the entire surface of the length of pipe that is to be coated. Rollers 18, such as those described in

Trzecieski U.S. Patent 5,026,451 mentioned above are then applied to the coating provided by the polymeric powder 16 in order to densify the coating. The assembly of rollers may preferably be spun so that it rotates along with the pipe. In the case in which the coating of powder 16 is curable, the densified coating may be subjected to a curing treatment at station 19, for example as described in our above-mentioned patent application No. 2,632,802. The pipe is then passed along the path indicated by the arrow 21 in Figure 1 to a conventional cooling zone, where the hot coating is cooled, for example by application of a water quench.

Preferably, the rollers 18 have a Shore A hardness of 18, more preferably between 5 and 35, and still more preferably between 15 and 20 measured in accordance with ASTM D 2240-05.

The rollers 18 may be cooled to limit their surface temperature by using an internal or external cooling system.

The rollers 18 may be made of a single piece, or may be made of distinct pieces joined together with a flexible or articulated joint.

Although the above description provides ample information to one or ordinary skill in the art to carry out the method of the invention, for the avoidance of doubt, some detailed Examples will be given.

Example 1

In this example, panels were employed, simulating a receptive surface of a tubular article. Grit blasted steel panels were etched with a weak solution of phosphoric acid (5%vol/vol) and thoroughly rinsed using deionized water. The panels were preheated to a temperature of about 240⁰C. Using an electrostatic spray gun apparatus, a 0.2 to 0.3 mm thick layer of Scotchkote (trade-mark) 6233 HG fusion bond epoxy (FBE) from 3M was applied to the outside of the thus heated panels. At which point the surface temperature of the panels had dropped to about 230⁰C. A topcoat layer of a coating as described in above-mentioned Canadian patent application 2,632,802 was then sprayed unto the gelling layer of FBE. The topcoat was sprayed until the desired thickness was achieved and immediately thereafter, a roller made of silicone rubber was manually rolled across the surface of the topcoat.

Following rolling, the coating can be reheated to ensure complete curing to simulate post-cure in a plant environment .

Example 2

A 24 inch diameter steel pipe was first sandblasted and cleaned of remaining blasting media. It was mounted on a pipe rotator able to maintain 15 revolutions per minutes (rpm) . A propane heating torch was inserted inside the pipe and the pipe heated to a temperature of about 240⁰C at which point the torch was removed. Using an electrostatic spray gun apparatus, a 0.2 to 0.3 mm thick layer of Scotchkote 6233 HG fusion bond epoxy (FBE) was applied to the outside of the thus heated pipe. At which point the surface temperature of the pipe had dropped to about 230⁰C. A topcoat layer of the same coating material used in Example 1 was then sprayed unto the gelling layer of FBE. The topcoat was sprayed until the desired thickness was achieved and immediately thereafter, a roller made of silicone rubber was lowered unto the surface of the topcoat. During the whole spraying process, the pipe kept rotating at 15 rpm. The roller was left in contact with the topcoat for no more than 1 minute and was then lifted out of contact with the pipe.

The procedure was carried out with thin topcoat layers 0.2 to 0.4 mm thick, and with thick topcoat layers 1 to 2 mm thick.

Example 3

A mixture with the following composition was compounded as described in above-mentioned Canadian patent application 2,632, 802.

^Hindered amine light stabilizer.

Grinding

Particles passing a 425 μm sieve (-425 microns) were retained for use in spraying.

Spraying:

Comparative Example 3a

Steel panels were grit blasted and thermally pickled in conventional manner. The panels were preheated in an oven to 240⁰C and a 0.2 to 0.3 mm thick layer of the compounded mixture particles of Example 3 was sprayed using a modified spray gun fitted with a small funnel. The panels were then immediately placed back in the oven maintained at 240⁰C for a period of no less than 3 minutes and then dipped in a bucket of water at room temperature.

Example 3b

Roller method:

Steel panels were grit blasted and thermally pickled in conventional manner. The panels were preheated in an oven to 240⁰C and a 0.2 to 0.3 mm thick layer of the compounded mixture particles of Example 3 was sprayed using a modified spray gun fitted with a small funnel. Pressure was applied using a silicone roller of 18 Shore A hardness that was rolled once over the freshly sprayed powder. The plate was then placed back in the oven maintained at 240⁰C for a period of no less than 3 minutes and then dipped in a bucket of water at room temperature.

Results :

Holiday testing was performed according to CSA Z-245- 20.06 and the regions under the roller were found to be mostly free of holidays and those not under the roller had multiple holidays when the particle size was -425 microns. Furthermore, a cathodic disbondment test was performed at 65°C for 24 hours using an impressed voltage of 3.5V according to CSA Z-245-20.06. It was found that the region under the roller passed the CSA requirements of less than 6.5 mm but those not rolled failed. Comparatively, without using the roller, a particle size of less than 180 microns is required to achieve acceptable performance in the same cathodic disbondment test.

Example 4

Example 2 was repeated except a topcoat layer of coating material of Composition 1 ground in such a way as to ensure that all the powder passed through a 300 micron sieve was employed.

Figure 2 shows a cross-section of the product having a steel substrate 22, an FBE layer 23 and a topcoat layer 24. In Fig. 2 there is visible on the left a region 26 that was untouched by the roller and on the right a region 27 that was compacted under the roller. As is further seen in the top view in Fig. 3, the region 27, 27a that was under the roller is obviously smoother and denser and possessed less surface defects than the region 26, 26a not compressed by the roller .

Example 5

A sample was prepared and sprayed according to the method described in Example 3a, however, a jet of hot air at about 500⁰C was applied on the freshly sprayed surface. In Figure 4, arrow 28 represents the approximate direction of the hot air jet (from the bottom left corner to the top right corner) .

Results :

The region under the hot air jet was much smoother and performed better in all tests required for meeting pipe coating standard CSA Z245-20-06.

Example 6

A sample was prepared and sprayed according to the method described in Example 3a. A top view of the product is seen in Figure 5. A second sample was prepared according to the method described in Example 3a but immediately after spraying the plate was exposed to IR for a period of 30 seconds. A top view of the product is seen in Figure 6. Re su lt s :

The coating on the plate post heated with IR (Figure 6) was much smoother and performed better in all tests required for meeting pipe coating standard CSA Z245-20-06.

The method of the invention provides a number of distinct advantages including:

1. Increasing the particle size that can be sprayed unto the pipe or other article thus lowering the grinding cost while maintaining the same level of performances. As a general rule, larger particles are less costly to grind than smaller particles.

2. Improving the quality of the sprayed coating by lowering the porosity and improving its smoothness.

3. Allowing the application of a thick, for example a greater than 1.3 mm thick layer of sprayed material to be used as a pipe coating.

Claims

CLAIMS :

1. Method for applying a polymeric heat-bondable powder coating on an elongate tubular article having a surface extending circumferentially around the article, comprising heating the article to a temperature at which the powder bonds to the surface, spray applying the powder to the surface of the article while hot and permitting the powder to bond to the surface to form a coating on said surface, and applying heat or pressure or heat and pressure, the pressure applied uniformly on the whole of the coating on the surface thereby densifying the coating, and permitting the coated article to cool .

2. Method according to claim 1 wherein the heat and pressure are applied by playing a jet of hot gas on the coating.

3. Method according to claim 1 or 2 wherein the heat and pressure are applied uniformly on the whole of the coating.

4. Method according to claim 1 wherein the heat is applied by directing radiant heat at the coating.

5. Method according to claim 1 or 4 wherein the heat is applied uniformly on the whole of the coating.

6. Method according to claim 4 or 5 wherein said radiant heat is infrared radiation.

7. Method according to claim 1 wherein the pressure is applied by applying rollers to said coating.

8. Method according to claim 7 wherein said rollers have a Shore A hardness of 5 to 35.

9. Method according to claim 7 wherein said rollers have a Shore A hardness of 10 to 20.

10 Method according to claim 7 wherein said rollers have a Shore A hardness of 16 to 18.

11. Method according to any one of claims 1 to 10 wherein the thickness of the densified coating is 0.5 to 4 mm.

12. Method according to any one of claims 1 to 10 wherein the thickness of the densified coating is 0.7 to 4 mm.

13. Method according to any one of claims 1 to 10 wherein the thickness of the densified coating is 1 to 2 mm.