Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02045749 1999-08-10
Title: NOVEL TAPE COATING
Baclsaround of the Invention
The present invention relates in general to protective
coatings of tubular objects and specifically to protective coatings
of pipes, and even more particularly to inground pipes.
The art is replete with protective pipeline coatings which
provide varying degrees of resistance to impact, mechanical
penetration, storage, indentation, abrasion, soil stresses and
cathodic disbondment. Four major categories of protective pipeline
coating are presently employed by the pipeline industry. They are:
(1) Hot Applied coal-tar enamel and asphalt mastics in
relatively thick layers (100 mils) and commonly reinforced on the
outside with glass or asbestos sheets. While such coatings are
reported to represent over half of the plant-applied coatings in
the United States, the hazards presented by their use foretell a
decreasing popularity of this category. Further, the products show
poor impact resistance, poor resistance to mechanical penetration,
poor abrasion resistance, poor stability to soil stress conditions,
and are only deemed fair in regard to indentation resistance, pipe
bending, cathodic disbonding and resistance to hydrocarbon
solvents.
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(2) Extrusion coatings of a thermoplastic resin: (typically
40 mils). In practice, polyethylene has virtually a monopoly in
this area. The technique may involve a seamless tubular extrusion
over the pipe or a flat die sheet extrusion wrapped over the pipe.
In most cases, the polymer is appli~d to a first-applied mastic
layer (e. g. bituminous). These coatings show improved properties
in regard to those mentioned above for the Hot-Applied enamel and
coal tar coatings except insofar as resistance to hydrocarbon
solvents.
(3) Fusion-bonded coatings - A thermoplastic powder is
electrostatically applied to hot pipe where it "melts", adheres to
the metal and fuses to itself. Only three basic materials have
been widely used--polyethylene, vinyl and epoxy powders with only
the latter enjoying commercial success in the United States.
Chemically,~these are, generally, bis-phenol A polymers with epoxy
end groups.
The epoxies require a thermal curing to the thermoset form and
usually a catalyst is used in the system (e.g. amines, acids, boron
halides, etc.). Often times a liquid epoxy primer is used prior
to the powder coating. Typically epoxy coatings have been 12-14
mils in thickness to provide at least acceptable resistance to
cathodic disbonding, although in M.D. Simpson's paper "External
Protection of Steel Pipes Using Epoxy Powder Coatings"
(contribution SI) presented at the Second International Conference
on the Internal and External Protection of Pipes (in England Sept.
1977) he states (page X2)
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~'Bitumen coal tar and polyethylene are required to be fed
relatively thick, but epoxy powder coatings g i v a a x c a 11 a n t
protection with only 3 mm of coatings."
Apparently and obviously "3 mm" should be -0.3 MM-(12 mils)
nevertheless, this value (i.e. 0.3 MM) still represents a
relatively thick coating and its attendant disadvantages, e.g.
brittleness and lack of flexibility and stresses at the pipe-epoxy
interface.
In order to effect a satisfactory epoxy coating which would
have satisfactory resistance to impact, mechanical penetration,
indentation, abrasion, soil stresses, and cathodic disbondment it
has been accepted that about a 12-14 mil thickness coating is
required, which is very costly to produce.
(4) Tape Coating Systems (typically 20-80 mils thick)
This method entails spirally wrapping a corrosion protective tape
around a rubber based primer coating, referred to in the art as the
innerwrap, followed by applying a second plastic outerwrap tape in
a similar fashion as the innerwrap.
Many improvements on this tape coating system involving an
inner and an outerwrap, have been advanced all of which have at
their essence the primary task of promoting a tight bond thereby
creating a coating which insulates the pipe from degradative
external forces.
Accordingly, some tapes comprise polyethylene backings with
a pressure-sensitive adhesives, or primer-activated adhesive
coating thereon. The properties exhibited by these pipe coatings
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are similar to those of extrusion coated pipe coatings.
U. S. Patent No. 4, 213, 486 issued to Samour et al. and assigned
to the present assignee, discloses a polyethylene outerwrap
carrying a means for effecting bonding to the innerwrap epoxy layer
wherein the means may be a hot melt adhesive or a
pressure-sensitive adhesive.
U.S. Patent No. 3,874,418 issued to Hielema and assigned to
the present assignee discloses:
"A method of coating a pipe and a pipe coated thereby, said
method comprising the steps of progressively spirally winding
a corrosion protective adhesive coated plastic tape onto the
outer surface of the pipe with a spiral overlap, covering the
coated pipe by progressively winding a film thereon with a
predetermined overlap, and, as the film is wound onto the
coated pipe, introducing and distributing under pressure a
hot melted adhesive into intimate contact with the surface of
the marginal portion of the trailing edge of the film and the
surfaces of the overlapped portion along the leading edge
thereof and of the portion of the tape immediately adjacent
the leading edge of the film."
Still a further advancement in the art of tape coat systems
is disclosed in U.S. Patent No. 4,806,400 issued to Sancaktar and
assigned to the present assignee, wherein the improvement consists
of tapering the opposed edges of the tape to enhance a tighter
closure by being less subject to soil stress.
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64407-129
While all the aforementioned tape coat systems provide
for successful protective coatings, they still comprise separate
layers. Accordingly, the task of the present invention can be
described as being directed to improving the present tape pipewrap
systems against degradative external forces by providing for a
continuous and seamless protective tape coating system.
Brief Description of the Invention
In accordance with the present invention, the
aforementioned task is solved in a simple and elegant manner by
heat fusing a polymeric outerwrap to a polymeric innerwrap tape
coating, by incorporating a heat fusible material into said inner
and outer wrap, which when heated and cooled forms a completely
closed protective tape coating.
According to one aspect of the present invention there
is provided a tape wrap system adapted for protecting tubular
articles comprising an innerwrap covering the surface of the
article to be protected and an outerwrap placed over said
innerwrap, said innerwrap comprising a blend of an impact
resistant moiety and a heat fusible moiety carrying on adhesive
layer on its inner surface, said outerwrap comprising blend of an
impact resistant moiety and a heat fusible moiety, said tape wrap
system fusing said innerwrap and outerwrap together when heated
and thereafter cooled.
According to a further aspect of the present invention
there is provided a process for externally protecting a pipe which
comprises the steps of:
(ay cleaning said pipe;
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CA 02045749 1999-08-10
(b) applying primer to said pipe;
(c) wrapping said pipe with a protective coating of a tape wrap
system as defined above;
(d) heating said pipe from about 225°F to about 325°F to fuse
adjacent surfaces of the heat-fusible moieties; thereafter
(e) cooling said pipe.
Detailed Description of the Invention
As mentioned previously, the present invention relates in general to
protective coatings of tubular objects and specifically to protective tape
coatings
for inground pipes.
The novel coating system of this invention comprises a polymeric
innerwrap and a polymeric outerwrap. The polymeric innerwrap yields an A/B/C
or AB/C layered construction wherein the A layer consists essentially of a
heat
fusible material from about 0.5 to about 10.0 mils thick; preferred heat
fusible
materials are
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ethylene vinyl acetate having a melting point below the application
temperature, ethyl methyl acrylate, and low density polyethylene,
ethylene vinyl acetate being particularly preferred. The 8 layer
consists essentially of an impact resistant polyolefin material,
preferably polyethylene, and most preferably a mixture of low and
high density polyethylene from about 5.0 to about 30.0 mils thfckt
and the C layer consists essentially of an adhesive, preferably a
thermosetting adhesive from about 2.0 to about 20.0 mils thick..
Exemplary thermosetting adhesives are thermosetting rubber-based
adhesives such as butyl rubber, natural rubber and styrene
butadiene, butyl rubber and styrene butadiene, as wall as Kratons.
In still a further embodiment the innerwrap comprises a single
layer ranging from about 5 to about 30 mils in thickness and having
an A B blend corresponding to the aforementioned description.
The novel outerwrap comprises an A/B/A or an A/B layer
construction corresponding to the aforementioned description
wherein the A layer is from about 0.5 to about 10.0 mils thick,
the B layer is from about 5.0 to about 30.0 mils thick, and the A
layer is from about 0.5 to about 10.0 mils thick. In still a
further embodiment the outerwrap comprises a single layer ranging
from about 15 to about 30 mils in thickness and having an A B blend
corresponding to the aforementioned description.
The manufacture of such coatings is well known in the art
comprising such well known processes as calendering, extrusion and
coextrusion, and as such comprises no part of the present
invention. Notably, both the inner and the outerwrap may contain
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additional ingredients performing specific desired functions. As
illustrations of such additives mention may be made of fillers such
as carbon black, zinc oxide, clays, chalk, whitings, calcium
silicate, barium sulfate and the like in order to reduce the cost,
increase the specific gravity, and/or to raise~the viscosity;
plasticizers and softeners such as mineral oil, lanolin, etc.;
antioxidants, e.g. aromatic amine antioxidants, substituted
phenols, hydroquinone (p-dihydroxybenzenej, etc.: curing agents
such as sulfur, organic peroxides and the like; accelerators:
sequesting agents; biocides such as bactericides, etc.
The general process of producing a tape coating system
comprising the novel invention includes well known plant tape
coating methods. The pipe may be first preheated to approximately
200°F and then prepared by any of the conventional ways known in
the art such as by shot or grit blasting. Primer is then applied
to the pipe by way of well known processes such as spraying or
brushing. Next, the pipe is coated with the A/B/C or AB/C
innerwrap by progressively spirally winding said tape onto the
outer surface of the pipe thereby maintaining a marginal overlap.
Notably, the innerwrap is applied in such a way that layer C is
placed on the pipe while layer A or the AB blend layer is on the
outer surface. Outerwrap A/B/A, A/B or single layer blend A B is
then applied to cover the coated pipe by progressively winding said
tape onto the innerwrap coated pipe with a predetermined overlap.
Next, flame or oven heat is applied to the coated pipe to attain
a tape temperature ranging from about 225°F to about 325°F.
Lastly,
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the pipe is cooled by any of the known methods, exemplary of which
is water cooling.
The present invention has particular application to small pipe
segment patching. However, this application requires minor
modifications in the aforementioned general process. First, the
outerwrap must be crosslinked prior to pipe application.
Crosslinking may be accomplished by any of the well known methods
such as chemical, radiation, etc. Second, the inner and/or
outerwrap is preferably applied by enveloping the pipe segment in
the wraps) as opposed to spirally winding although spirally
winding is not excluded; and overlapping the longitudinal edges of
said wrap(s).
Third, the area to be fused may be protected with a heat
resistant sheet prior to applying heat to prevent tape coating
dimensional distortion. Said sheet comprises a composition able
to resist the heat and flame of a torch for a time sufficient to
allow the underlying outerwrap and innerwrap to malt and fuse to
each other and to the pipe. Materials which have been found
suitable are polyamide film (i.e. "Kapton", trademark of Dupont)
and aluminum foil. Other materials which could be used are Kevlar
fabric, fiberglass fabric, laminate of fiberglass fabric with
aluminum foil, flame resistant cotton, etc. Fiberglass fabric is
particularly preferred. Last, flame or oven heat is applied to the
coated pipe to attain a tape temperature ranging from about 225°F
to about 3 2 5°F .
In accordance with this novel process, the outerwrap is
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completely heat fused to the innerwrap thereby forming a uniform,
. continuous, and completely closed protective coating which fully
protects against external degradative forces. More specifically,
the ethylene vinyl acetate components namely layers A or the A
moiety in blend AB are heat fused, thereby creating a tight bond
between the inner and outerwrap so ws to achieve essentially a one
layer seamless coating. Said seamless coating is highly
advantageous in that no openings are present which when subjected
to externai forces create potential portals of entry for pips
corrosion factors. In sum, the present invention provides for a
completely closed internal pipewrap environment which
advantageously maintains pipe integrity.
Additionally, the present invention is particularly useful for
wrapping pipe segments, such as in j oining the exposed ends of pipe
segments before ground implantation. Notably, during pipe
production pipes are cut into particular length to make
transportation possible. Pipe coatings are either applied in the
plant or in the field. If they are applied in plant the ends are
left exposed. Thus before ground implantation the adjoining pipe
ends need to be joined and coated. The present invention offers
a particularly advantageous joint coating.
The following examples show by way of illustration and not
limitation the novel characteristics of the present invention.
xamplg (Innerwrap)
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Layer Ingredients) Thickness (mils)
A Ethylene Vinyl Acetate 4
(softening point* 59°C)
B 64% Low Density Polyethylene 11
32% High Density Polyethylene
2.5% Black Concentrate
1.0% Antioxidant/Low Density
*as determined by the American Society for Testing and Material
D-1525.
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~amnle ~ (Outerwrap)
Layer Ingredient($) Thickness (mils)
A 96% Ethylene Vinyl Acetate 3
(softening point 67°C)
4% White Concentrate
B 66-96% High Density Polyethylene 19
0-30% Low Density Polyethylene
4% White Concentrate
A 96% Ethylene Vinyl Acetate 2.5 mils
4% White Concentrate
Coatings prepared by the previously mentioned process and in
accordance with example I and II were subjected to Cathodic
Disbondment, Impact, and Soil Stress Testing. Prior art tapes
comprising an outerwrap with a polyethylene backing and a
pressure-sensitive adhesive coated thereon and an innerwrap
consisting of polyethylene, prepared in the aforementioned process
but omitting both the heating and cooling steps, were used as
controls.
The following data illustrates the advantageous
characteristics of the novel invention and consequently the
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longevity and integrity of a pipe coated with the novel invention.
T~~; a I
Cathodic Disbondment (measured after 30 days)
System Temperature Cathodic Disbondment
(°F) (inches squared)
Test Sample 140 2.44
Control 140 6.00
Test 70 0.75
Control 70 0.6-0.75
Impact
(administered and measured in accordance with ASTM G-14
guidelines)
Test Sample 60 in/lb
Control 45 in/lb
Soil Stress at 90°F
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CA 02045749 1999-08-10
(Disclosed in U.S. Patent No. 4,483,197 issued to Jordan
Kellner)
Test Sample . No peel back of outerwrap
Control Outerwrap peeled back
These test results demonstrate the advantageous
characteristics and hence resistance to external degradative
forces. Specifically, cathodic disbondment is more than 50%
decreased at 140°F, which is particularly advantageous given the
implantation of pipes in hot temperature regions. The results of
impact testing will show an advantage over the control yet the most
important soil stress testing shows a highly significant
improvement, no peeling of the outerwrap at 90°F. Resistance to
soil stress is highly significant since pipeline tape
deterioration, corrosion, is mostly caused by soil stress imparted
on the inground pipe.
Example III is illustrative of a single layer blend outerwrap.
Example III
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Ingredient
A - Ethylene vinyl acetate copolymer 35
(softening point 59°C)
B - Low density polyethylene blend 59
White concentrate 5
Antioxidant 1
By way of recapitulation, heat fusing polymeric outerwrap tape
to a polymeric innerwrap tape by incorporating an heat fusible
moiety in both wraps, advantageously prolongs the integrity and
hence longevity of inground pipes.
Since certain changes may be made without departing from the
scope of the invention herein described, it is intended that all
matter contained in the foregoing description, including the
examples, shall be taken as illustrative and not in a limiting
sense.
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