Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to protective coating of metal pipe particular-
ly steel pipe for use in pipe lines for conveying gas or oil.
It has previously been proposed to apply corrosion protective coatings
to metal pipe by first applying a layer of self-sealing mastic or thermoplastic
adhesive or heat insulation to the pipe while the pipe is either heated or
,.,
. unheated, followed by extruding a band of thermoplastic material and helically
`- wrapping it with an overlap in one or more layers over the first coating, as
~' described in United States Patents Landgraf 3,616,006, MacLean et al. 3,687,765,
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Emmons 3,802,908 and Hielema 3,823,045. It has also been commercial practice
~, lO- to apply to such metal pipe a primer coating, followed by a helical wrap of a
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high grade, pinhole-free corrosion-protectlve band of thermoplastlc together
with an a&esive and a second helical wrap of thermoplastic sheet over the
first to provide protection of the first wrap against mechanical abrasion or
tearing, either during shipment of the pipe or during the laying or covering
of it in a trench. The second helical wrap in such case has been held in
place either by an overall coating of pressure-sensitive adhesive over the first
helical wrap or by heat-sealing or adhesively bonding the margin of the outer
helical wrap to itself and/or to the underlying layer. It has also been com-
mer~j:al practice to apply to such pipe protective coating by heating the steel
pipe and bringing the heated pipe into contact with a thermosetting resin pow-
der such as an epoxy resin powder which a&erss to the pipe surface, and there-
after further heating the pipe to sin~er or fuse and cure the adhered epoxy re-
sin particles on the pipe surface. Such coatings tend to exhibit thin spots
or pinholes where sintering or fusing of the particles is incomplete. Extrus-
ion coating of pipe has also been taught by Colombo United States Patent
2,820,249.
It has been considered essential for effective corrosion protection
to ensure that the first coating is substantially completely free from pinholes
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and thin spots. This has been true even in those cases in which a D.C. elec-
trical potential has been maintained on the pipe after laying in order to provide
cathodic protection against corrosion because it has been found that the pres-
ence of a pinhole usually results in generation of hydrogen gas and hydroxyl
ions by electrolytic action, both of which tend to cause destruction of the
adhesive bond and progressive separation of the coating from the pipe. The
necessity for avoiding thin spots and pinholes in the coating has made it very
difficult to employ epoxy resin powder coatings for coating pipe with fully
satisfactory results even thQugh such coatings display remarkably good adhes-
ion to the steel pipe surface. Furthermore, in the case of a first coating
made from a helical wrap of preformed sheet material, the necessity for avoid-
ing pinholes in the corrosion protecting coating makes it important to employ
only expensive prime quality thermopalstic raw material in the manufacture
of the sheet material for the first wrap and greatly reduces the possibility
of using less expensive scrap or second grade raw material because of the tend-
ency of the latter to form pinholes during sheet formation.
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According to the present invention there is provided a method of coat-
ing pipe which comprises the steps of adhering a layer of corrosion protective
material to the surface of the pipe to form a coating thereon, and extruding
directly onto said coating a layer comprising polyolefin to form a protective
outer sheath over said coating.
The outer layer effectively protects against mechanical abrasion and
tearing and at the same time reduces the number of pinholes and thin spots.
The polyolefin layer may comprise polyethylene or polypropylene in
heated, softened condition. In a preferred embodiment the extruded polyolefin
` layer forms a part of a coextruded composite sheet, the inner portion or layer
of which comprises low density polyethylene while the outer portion or layer
comprises high density polyethylene. In another preferred embodiment, the
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underlying layer on the pipe is formed by fusing and curing at least partially
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a layer of powdered epoxy resin to form a thin film corrosion~p *ectivu layer
~ and thereafter coextruding directly onto the surface of the corrosion protect-
ive layer a composite sheet, the inner portion or layer of which comprises
. a copolymer of ethylene and acrylic acid and the outer portion or layer of which
comprises polyethylene, preferably high density polyethylene. The epoxy resin
forms a strongly adherent bond to the surface of the steel pipe while the copo-
lymer reacts chemically with the i~completely cured epoxy resin to form a
strongly adherent bond thereto; the coextrusion of polyethylene with the copo-
lymer produces a strong bond between the two coextruded layers; as a consequence
all parts of the finished coating are bonded very strongly to the surface of the
steel pipe. Moreover, in each case any pinholes or thin spots in the under-
lying corrosion protective coating are covered by the adherent extruded or co-
extruded polyolefin layer; while the polyolefin layer may itself have occasional
pinholes, there is little likelihood that these pinholes will match up with
those in the underlying corrosion protective layer. The polyolefin layer can
be extruded either in tubular form directly onto the coated pipe as the latter
advances through the annular extruder die or it can be extruded in the form of
a band or ribbon which is immediately and directly deposited on and helically
wrapped about the coated pipe as the latter is simultaneously rotated and ad-
vanced along its axis with respect to the extruder die. The method of the
present invention provides adhesion of the outer extruded (or coextruded) poly-
olefin to the underlying corrosion resistant coating over the complete inner
surface of the outer polyolefin and decreases the cost of protecting the pipe
because it makes it possible to tolerate a substantial number of pinholes in
the underlying corrosion protective coating as well as in the protective thermo-
plastic material of the outer helical wrap itself without loss of protection
or decrease in ~he durability of the protective coating.
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The first step of adhering a layer of corrosion protective material
to the surface of the pipe may be carried out by applying a powdered epoxy coat-
` ing, a heat activated adhesive coating, or a butyl based pressure-sensitive
adhesive coating to the pipe surface. In the case of the powdered epoxy coat-
ing the procedure involves heating the pipe, bringing the heated pipe into con-
- tact with powdered epoxy resin to adhere particles of the resin to the pipe
surface, then further heating the resin particles to fuse and at least partially
cure them in place on the pipe surface. In the case of heat-activated adhe-
sive the pipe must also be heated sufficiently so that the adhesive coating is
`~ 10 maintained in activated condition during the subsequent extrusion application
of the polyolefin layer. In the case of the butyl-based pressure-sensitive
; adhesive, the coating may be applied to the pipe which is at room temperature
or slightly higher and maintained at that temperature.
` The outer polyolefin layer is extruded or coextruded directly onto the
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surface of the corrosion protective material in heated and softened form
either in the form of a tube enveloping the coated pipe while advancing the
latter with respect to the die~ or in the form of a band uhile simultaneously
rotating the coated pipe about its axis and advancing it along its axis to
wrap the band helically about the pipe. ~hen the outer polyolefin layer is
coextruded in the form of a composite band or sheet having at one face an
outer layer or layers of tough tear-resistant and abrasion-resistant protective
non-adhesive thermoplastic material and at the inner face at least one layer of
- thermoplastic adhesive material, the coextruded composite sheet is wrapped hel-
ically upon the pipe previously coated with a corrosion protective material,
with the lateral margins of the composite sheet preferably overlapping, while
the inner adhesive layer is in heat-softened condition. By coextrusion is
meant the well-known process of simultaneous extrusion of two or more different
thermoplastics into a multi-layered composite sheet as described, for example,
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in TAPPI MONOGRAPH REPORT NO. CA~43, Chapter 6 (1973~. The composite sheet
is formed either immediately before or immediately after ~within a fraction
of an inchA) coextrusion from the die orifice. Because of the intimate con-
tact of the two molten coextruded layers of the composite sheet with each other
throughout the extent of their interfacial contact surface, the bond between
the two is very strong. Furthermore, because the continuous layer of thermo-
plastic adhesive after application of the composite sheet to the underlying
corrosion protective coating effectively isolates any pinholes in the latter,
occasional pinholes can be tolerated both in the underlying corrosion resis- -
tant coating and in the second or outer protective helical wrap as well be-
cause the likelihood of aligning pinholes in the two separately applied helical
wraps is very remote. The method of the present invention therefore provides
not only improved protection of the corrosion resistant coating against abras-
ion or tearing, but it makes it possible to reduce the cost of the corrosion
resistant coating as well by eliminating or restricting the bad effects hitherto
resulting from the presence of pinholes in the underlying corrosion protective
resistant coating. It also makes it possible to obtain a well bonded outer
wrap without re`gard to the nature of the inner corrosion protective layer.
The temperature of the pipe during application of the initial corros-
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ion protective coating depends largely upon the nature of the coating as
pointed out above. When the corrosion protective coating is powdered epoxy
resin, the pipe must be preheated to a temperature sufficiently high, of the
order of 300 to 600F., to cause the epoxy resin to wet and coat the pipe
throughout its surface> by fusion of the powder particles of the coating, and
then to cure the coating. The corrosion protective coating preferably has a
thickness of 2 to 10 mils and is cured within 1 to 4 minutes. Suitable epoxy
compositions are well known in the art being described for example in Uhited
States Patent No. 4~060J655. ~hen the corrosion protective coating does not
require heating to bond it to the pipe or to cause it to cure, the pipe may
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be at the temperature of the ambient atmosphere provided it is free from con-
densed moisture and is clean.
n the appended drawings,
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~` Figure 1 is a schematic isometric representation of one embodiment
j
of the present invention~ and
Figure 2 is a view in section taken along line 2-2 of Figure 1.
As shown in Figure 1 of the drawing, pipe 10 which has first been coated
with a corrosion protective layer 11 and dried. Lower quality coatings hav-
ing occasional pinholes can be used for layer 11 than has heretofore been the
case without loss of the quality of protection of the finished pipe. In one
embodiment, the corrosion protective coating 11 consists of epoxy resin powder
and is electrostatically applied to the pipe after preheating the latter to
a temperature at least 25F. above the melting point of the epoxy resin pow-
der to cause it to fuse and wet the pipe within 5 to 35 seconds, followed by
curing within a total time of 1 to 4 minutes. ~hen the subsequently applied
~` coating includes a layer of ethylene-acrylic acid copclvmer as described below
.. ~
; the epoxy resin coating 11 is preferably only partially cured to enhance
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subsequent chemical reaction with and bonding of the epoxy resin to the copoly^
- mer.
~ 20 Pollowing application of the corrosion protective coating, the coated
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pipe is advanced axially past extruder die 16 and simultaneously rotated while
there is coextruded through die 16 a band or composite sheet 18 having at its
lower ~or inner surface at least one layer comprising adhesive 20 and at its
upper or outer surface at least one layer comprising heat softened polyolefin
. f
; 22 to form a protective outer sheath. The coextruded band 18 is wrapped helic-
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ally directly on the outer surface of layer ll of the coated pipe with a small
marginal overlà~ of band 18 as shown. Because the adhesive layer 20 remains
; hot and relatively fluid as it passes directly from extrusion die 16 to its
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contact with the coated pipe, and because the area of the adhesive layer is
coextensive with the area of the outer polyolefin layer 22, uniform and com-
plete adhesion throughout the area of interfacial contact between band î6 and
layer 11 on coated pipe 10 is achieved, thus at the same time sealing and iso-
lating any pinholes in coating 11 when the latter is formed from powdered
epoxy resin, for example. After cooling of the pipe, the coating includes
corrosion protective coating 11 adhered to the pipe and outer composite sheet
18 adhered to the entire outer surface of the corrosion protective coating.
~t is essential for best results that the rate of extrusion and the rate of
rotation and advance of the pipe be adjusted to each other so that dimensions
of the composite sheet 18 and the tension under which it is applied to the
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pipe carrying corrosion protective coating be maintained substantially uniform.
The tension should preferably be maintained as high as possible to ensure com-
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: plete contact of the heat softened thermoplastic adhesive layer 20 with the
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outer surface of corrosion protective `layer 11 despite any irregularities
in the latter.
The adhesive may be any of those compositions conventionally em-
ployed for adhesion to metal or plastic surfaces and may be a single material
or a mixture of two or more different materials; among suitable materials are
butyl based adhesives, propylene-acrylic acid copolymer, an ethylene-acrylic
; acid copolymer, ethylene acrylate or vinyl acetate copolymer. The outer pro-
tective polyolefin layer may be any conventional tough and tear resistant poly-
olefin which is heat softenable and extrudable in the same range o~ temperaturesas the thermoplastic adhesive; polyolefins such as medium and high density
polyethylene and high impact polypropylene (containing rubbery ethylenepropylenecopolymer) are preferred. The thickness or gauge of the composite sheet 18
can be varied at will from about 7 mils to about 120 mils of which the thick-
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ness of the thermoplastic adhesive layer may vary from 2 to 25 mils and the
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48711
thickness of the protective thermoplastic layer from 5 to 118 mils. Prefer-
ably the thickness of the adhesive layer is no more than half the total
thickness of the composite sheet, usually from 5 to 30% of the total thickness,
depending on the total thickness of the composite sheet which in turn varies
depending on pipe diameter and the extent of impact resistance desired.
In the case in which the corro~ion protective layer 11 consists of
heat-activated adhesive or of butyl-bascd adhesive which is pressure sensitive,
the outer protective polyolefin layer may be extruded alone, without coextrus-
ion of an adhesive layer, so that composite sheet 18 is replaced by a homo-
geneous sheet of polyolefin. In the case in which the corrosion protectivelayer 11 consi~sts of fused and cured epoxy resin powder, the coextruded ad-
hesive layer preferably comprises a copolymer of ethylene or propylene with
, acrylic acid, which copolymer interacts with the epoxy resin, particularly when
. the latter is incompletely cured at the time of coextrusion of the adhesive.
n the case of a butt seam, in order to ensure sealing of the seam,
it IS also possible as an optional feature to apply over the helical seam
formed by composite sheet 18, either lapped seam or butt seam, as an additional
step a ribbon or tape 24 comprising a heat shrinkable thermoplastic material
~such as oriented polyethylene or polypropylene containing rubber such as ethy-
lene-propylene rubber or the like. A heat source 26 such as an infrared heater
or a flame is provided to heat the tape after it has been applied to cause
it to shrink about the coated pipe along the seam. The coated pipe may be
cooled to room temperature before or after applying and shrinking tape 24 in
place. Ribbon or tape 24 may also be a molten extrusion of the same com-
~4r position as the outer coating layer 22, in which case heat source 26 can be
;~ omitted.
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