Note: Descriptions are shown in the official language in which they were submitted.
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POLYMERIC FUSIBLE POWDER COATING ON ELONGATE TUBULAR ARTICLE
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
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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
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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.
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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.
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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
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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
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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 240 C. Using an electrostatic spray gun
apparatus, a 0.2 to 0.3 mm thick layer of Scotchkote (trade-
mark) 6233 11G fusion bond epoxy (FBE) from 3M was applied to
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the outside of the thus heated panels. At which point the
surface temperature of the panels had dropped to about 230 C.
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 240 C 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
11G 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 230 C. 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.
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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.
Composition 1
Type Supplier Grade g/kg
PE Nova RMS 539 588.2
Black Masterbatch Ampacet MB 19717 5.000
Fusabond E-MB
PEGMA DuPont 265D 50.00
EPOXY DOW DER 6155 150.0
HALS* Ciba Tinuvin 144 2.500
FILLER Nyco Nygloss 8 50.00
Bulk Filler Nyco NYAD 400 150.0
CURE CVC DDA10 4.292
ACCELERATOR CVC U24M 0.000
TOTAL 1000
*Hindered amine light stabilizer.
Grinding
Particles passing a 425 m sieve (-425 microns) were
retained for use in spraying.
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Spraying:
Comparative Example 3a
Steel panels were grit blasted and thermally pickled in
conventional manner. The panels were preheated in an oven to
240 C 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 240 C 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
240 C 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 240 C for a period
of no less than 3 minutes and then dipped in a bucket of
water at room temperature.
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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.
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Example 5
A sample was prepared and sprayed according to the
method described in Example 3a, however, a jet of hot air at
about 500 C 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.
Results:
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.
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The method of the invention provides a number of
distinct advantages including:
l. 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.
