Note: Descriptions are shown in the official language in which they were submitted.
1 321 455
The present invention relates to a method and
apparatus for applying a protectivQ thermoplastic jacket
to a pipe by cross-head extrusion, and in particular to
the jacketing of pipe with high-density polyethylene
(HDPE~.
Steel pipes or tubing which are! intended for
underground installation must be protectively coated
against corrosion. There is a substantial body of a:rt
relating to apparatus and methods for the application of
a thermoplastic covering to a pipe, first covered with an
adhesive CQating.
"Cross-head" extrusion will be understood
throughout the description and claims herein to r~:Eer
yenerally to known processes for c~ating a pipe by
conveying it longitudinally through the annular nozzle or
head of an extruder, operable to extrude tubular coatings
of adhesive film and jacketing material over the pipe as
it passes through the extrusion head. Examples of cross-
head extrusion apparatus and of the use of such in
jacketi~g a steel pipe are referred to in United States
Patents Nos. 4,386,996 (Landgraf et al.); 4,484,877
:(Stucke); 4,451,413 (Stucke et al.); 4,510,007 (Stucke);
and 4,519,863 (Landgraf et al.~
~he process of cross-head extrusion i5 variously
referred to as a l'straight-through" or "endo" process to
distinguish it from the older known technique of
enveloping a steel pipe by spiral wrapping. That method
involves the simult~neous winding of pipe with a double
lay~r of sheet material, the inner being an adhesive strip
and the outer thermoplastic, in screw thread fashion as
disclosed, for example, in United States Patent No.
3,616,006 (Landgra~ et al.~. -
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Where it can be used, the "endo" process for
applying an adhesive-thermoplastic multilayer covering to
pipe generally produces a product having properties
superior to a pipe spiral-wrapped with :Like materials.
Xnown pipe coatings applied using a spiral method are
subject to weak joints at the overlap and poor coverage
of radial or longitudinal welding seams on the pipe. We
hava found that the disadvantages of spiral wrapping are
particularly yreat where HDPE is to be applied as the
outer jacketing material. Pipe which has been spiral
wrapped with that jacketing material often exhibit
r~latively poor low temperature adhesion of the protective
coating.
A number of methods for the jacketing of steel pipe
by the application o~ an adhesive-polyethylene laminate to
a pipe which has first been coated with a curable epoxy
resin/curing agent mixture (which mixtures will be
referred to hereinafter generally as "epoxy primer") have
been described in German printed Patent No~ 19 65 802
(spiral wrapping method) and in United States Patent No.
4,510,007 (Stucke) (straight-thxough extrusion). However,
the "endo" process presents its own practical difficulties
in the effective jacketing of pipe using such materials,
par~icularly when high-density polyethylene is used as the
outer cladding.
Firstly, in the "endo" process the application of
an even layer of epoxy primer, critical to good results,
is not a straightforward matter, unlike in the spiral
process, where liquid primer can be applied to the surface
of a pipe turned about its Iong axis. It has been
proposed, in United States patent No. 4,510,007 referred
to above, to apply primer electrostatically as a powdery
preconden~ated blend of epoxy resin and curing agent.
However, this requires supplemental surface heating o~ the
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1 32 1 455
pipe to melt the powder, and very careful temperature
control to evaporate reaction products from the epoxy cure
and obtain a reasonably uniform layer of primer base
coating. According to the present invention, a novel
annular epoxy primer applicator is employed, which can be
used with a liquid primer such as a 'l100% solids"
composition, for excellent control of the thickness of the
prlmer base coat.
Secondly, the application of sealing pressure to
ths freshly applied jacketiny presents considerable
difficulties when cross-head extrusion is used to coat the
pipe. A pressure roller of the kind useful in applying
thermoplastic strip helically, as in United States Patent
No. 4,178,200, cannot be llsed on the hot tubular ~acket of
thermoplastic or adhesive/thermoplastic multilayer
composite deposited on a pipe coming from a cross-head
extruder.
The desirable result of applying sufficient
pressure to develop adhesion of the coating in a straight-
through process, without dislocating or blemishing thecros~-extruded thermoplastic jacket, has now been achieved
according to the present invention, by the use of an
annular pressure applicator, operable to constrict the
jacketed pipe along a band of sur~ace shortly after
initiation o~ a cooling quench, so that the top layer of
polyethylene or other thermoplastic about the constricted
region is solid and the lower layer still molten. ~he
radial application o~ pressure is then ef~ective to
enhance adhesion, without damag2 to the coating.
Accordingly, in one aspect the present invention
provides a method of applying a protective jacket to a
steel pipe comprising the steps of: (1) applying a
substa~tially even coating o~ epoxy primer in the liquid
state to the outer surface of the pipe whexe the thickness
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1 32 1 455
of the epoxy is between about ~5~ and a~out 100~, (2)
raising the pipe temperature to between about 135C and
about 200C to bring about partial curing of the primer
coating and to pro~ide a fluid surface, (3) coating the
pipe with a jacket of adhesive copolymer before the primer
has gelled and a thermoplastic, such as high-density
polyethylene, using cross-head extrusion apparatus, and,
(4) after the primer has gelled9 applying radial pressure
to the surface of the jacketed pipe at a stage when the
outer surface of the thermoplastic has hardened and while
the inner portion thereof remains above the softening
temperature, thereby achieving good adhesion of the jacket
to the pipe.
The invention also provides apparatus for applying
a uniform coating of liquid epoxy ~rimer to the outer
surface of a steel pipe, comprising a hollow chamber
having front and rear walls with circular openings
ther through to receive and allow passage of the steel
pipe to be coated. The front and rear wall openings are
equipped with flexible annular sealing means operable to
exert steady cons~rictive pressure on the pipe so as to
permit liquid primer from within the chamber to be drawn
smoothly onto the surface of the pipe as it is moved
lonqitudinally through the applicator.
Finally, the present invention provides apparatus
~or sealing the extruded protective jacket onto the pipe
surface by pressure comprising a spray-cooling quench
chamber through which the jacketed steel pip~ is conveyed
longitudinally, a flexible, annular pressure applicator
which receives the pipe as it is conveyed through the
~uench chamber and exerts radial pressure on the pipe
surface at a position within the quench where the outer
surface of the thermoplastic jacket has hardened while the
inner portion thereo~ remains above the softening
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1 321 ~55
temperature of the thermoplastic and where the epoxy
primer has gelled.
Preferred embodiments of the invention will
hereinafter be described with reference made to the
drawings in which:
Figure 1 is a vertical cross-sectional view of an
epoxy primer applicator ~or use in carrying out the method
o~ the invention;
Figure 2 is a side view of a water quench equipped
with a pressure ring ~or use in applying a jacket to a
pipe according to the invention;
Figure 3 is a vertical cross-sectional view of an
alternative epoxy primer applicator;
Figure 4 is a side view of the applicator of Figure
3;
Figure 5 is a vertical cross-sectional view of a
pressure wheel assembly for use in applying the jacket to
the pipe;
Figure 6 is a side view of the pressure wheel
assembly;
Figure 7 is a schematic plan view of the entire
pipe coating process, the pipe direction being from left
to right;
Figure 8 is a vertical cross-sectional view of a
cross-head extruder along lines 8-8 of Figure 9; and
Figure 9 is a horizontal cross-sectional detail of
the two stage cross-head extrusion process where adhesive
and polyethylene coating is applied.
Referring to Figure 7, prior to applying a jacket
of thermoplastic to a pipe 10 according to the method of
the invention, the surface of the pipe should be
abrasively cleaned to a near-white surface condition
preferably by a shot blast cleaner 46 in-line, in order
to remove rust particles and surface contaminants to
create a clean surface to which epoxy primer coat will
1 321 455
adhere well. The pipe may be preheated by preheat station
45, to remove residual moisture and ensure that the clean
pipe is always at least 3C above the dew point and that
the pipe is at the proper temperature for chromate
pretreatment to follow. Optionally, the cleaned pipe may
be chemically pretreated as it is conveyecl longitudinally
through chromate pretreatment station 47, toward the
cross-head extrusion apparatus. An aqueous chromate
solution may be applied and permitted to dry onto the pipe
surface, to provide an additional measure of corrosion
resistance for the pipe.
Passing through the epoxy application station 48,
the pipe is then coated with a substantially uniform
coating of liquid epoxy primer, o~ a thickness between
about 25~ and about 100~. When a thickness of primer less
than about 25~ is applied, the coated pipe which results
has too low a resistance to cathodic disbondment. The
upper limit of epoxy thickness of about 100~ is dictated
by considerations o~ cost and the inflexibility of thicker
epoxy films at low temperatures, presenting the risk of
cracking of the cured primer undercoat when the coated
pipe bends. The epoxy coating may be applied by a number
of conventional methods, such as "rugging" (drawing an
epoxy-wetted piece of rug fabric across the pipe surface).
For very large diameter pipes (greater than about 16
inzhes), liquid epoxy primer may be sprayad unto the pipe
surface.
The epoxy coated pipe 10 is then con~eyed through
heating means such as induction coils 49 to initiate
partial curing of the epoxy primer.
The epoxy coated pipe 10 thereafter proceeds
through a first cross-head extrusion apparatus 50 where
adhesive is applied to a still ~luid epoxy primer and
immediately to a second cross-head extruder 51 where a
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1 321 455
thermoplastic top coat i5 applied. Co-extrusion of thsi
adhesive and top coat through a single die may also be
employed ~lthough not shown in Figure 7.
The top coated pipe 10 thereafter i~ conveyed
through a quench chambeir 2~ comprising initial quench
station 52 and final quench station 54. ]Between the two
quench stations 52 and 54, radially inward pressure is
applied to the outer surface of the pipe at the pressure
applicator station 53 after the epoxy has gelled.
Referring to Figure 1, the pipe i5 coated with
primer in an "endo" process according to one embodiment of
the present invention by conveying the pipe through a
novel applicator such as illustrated. Th~i cleaned,
pretreated and heated pipe 10 is translated longitudinally
by conventional conveying apparatus (not shown) in
direction A toward the cross-head extrusion apparatus (see
Figur~ 7) where adhesive and thermoplastic will be
applied. Pipe 10 passes through the annular reservoir 12
of the primer applicator, by way of circular openings 14a
-~and 16a in front and rear walls 14 and 16, respectively.
Liquid epoxy primer 18, added to reservoir 12 by means o~
~illing hopper 20, is prevented from flowing freely out of
reservoir 12 by flexible front and rear annular sealing
means 14b and 16~.
In the specific apparatus illustrated, 14b and 16b
are natural rubber rings, fractionally smaller in diameter
than pipe lO, and clamped by bolts 15 betwee~ the elements
of doubl~ walls 14 and 16, respectively. ~t will be
appreciated, however, that many equivalent arrangements of
annular sealing means could be employed to achieve the
desired result, viz. a coating and wiping action on the
pipe passing through reservoir 12 so as to apply a uniform
coating of primer theretoO
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"Epoxy primer" is used herein to describe
generally any suitable mixture o~ epoxy resin with curing
agents such as amines. The material favoured in applying
the method of the invention is "100~ solids" epoxy, which
cures from liquid to solid without any app:reciable change
in volume. It has the advantage over solvent-based epoxy
~ormulations that extrusion of topcoat may be carried out
without having to wait until volatile components are
dri~en off. Solvent-based epoxy primer or fusion bond
epoxy coating may be used instead, but such materials may
not lend themselves to application by use of the
applicator described in connection with Figure 1.
An epoxy applicator according to the invention may
advantageously be made with integral carriage means 21
extending from front wall 14 and having centering wheel
pairs 22a and 22b, independently adjustable to assist in
the centering o~ pipe 10 accurately within opening~ 14a
and 16a.
Further, the epoxy applicator of Figure 1 may be
suspended in the path of advancing pipe 10 on a line
supplied with a ccunterbalancing set of weights (not
shown) in order to avoid any net transverse force on the
pipe.
B~cause liquid epoxy primers will eventually
solidify under ambient conditions, it is desirable to
provide means (not show~) for pumping and recirculating
primer through reservoir 12 by way of a cooling system.
From the applicator, pipe 10 is conveyed ~urther to
heating means, e.g.induction coils 49, where the pipe
temperature is raised to between about 135C and about
200C, in order to initiate partial curing of the primer
and to provide a hot fluid surface to receive the extruded
adhesive and thermoplastic coatings. However, it is
essential that the rate of curing of the primer be such
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1321455
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that it is still fluid te.g. not gelled) when the extruded
adhesive is applied. ~his allows a good bond between the
primer and the adhesive.
The heated pipe is then conveyed by a conventional
conveyor to a first cross-head extrusion apparatus 50 where
adhesive is app}ied and immediately thereafter to a second
cross-head extruder 51, where the thermoplastic top coat is
applied.
Referring to ~igure 9l a two stage cross-head
extrusion process is shown. A first cros~-head extrusion die
55 applies a layer of adhesive 58 to the epoxy coated pip~
10. Thereafter a second cross-head extrusion die 55' applies
a layer of thermoplastic top coating 59 of high density
polyethylene upon the adhesive coated pipe 10.
Referring to Figures 8 and 9, in order to avoid the
entrapment of aix bubbles, between the epoxy coated pipe 10
and the adhesive layer 58, and between the adhesive coated
pipe 10 and the top coating 59, vacuum pressure is created
through the use of pressurized air exhausted through air
nozzles 57 and 57'. Air is supplied to nozzles 57 and 57'
through distribution rings 56 and 56' positioned ~etween the
pipe 10 and extruder dies 55 and 55'. The air is exhausted
in the direction opposite the direction of pipe travel. Tha
films of the adhesive layer 58 and top coating 59 are drawn
toward the pipe 10 under the vacuum created by the exhausted
air due to the Venturi efect as air passes through the
restriction caused by the ~istribution rings 56 and 56' in
the annular air passage between the pipe 10 and dies 55 and
55'.
AS adhesive, a copolymer such as LOTADER~ PX 8460
or the like may be used. Such materials are characterized
by the presence of chemically active acrylate and maleic
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1321455
a¢id groups which provide an excellent bond between the
curing epoxy resin and the thermoplastic (HD~E) topcoat.
The cross-head extruder will typically operate at
a temperature of between 1~5 and 200C in the heating
zone, with the extruder adapter set at 200 to 215C, the
extruder die heated to 215 to 225C, and the die lips
significantly cooler than the extruder die, typically at
abou~ 210C. The die lips are pre-set at a thickness of
approximately 2-3 mm and adhesive delivexy rate is such
that the adhesive is drawn down to a thickness of
approximately 150-200~ as it contacts the pipe surface.
The pipe has to be sufficiently hot to keep the adhesive
in the molten ~tate, preferably in the range 135-200C,
otherwise the adhesive may cool down too quickly and
solidify on the pipe surface without forming a proper
bonding link between substrate and topcoat.
When polyethylene is used as the topcoat, the
mechanical properties of the coated pipe generally improve
with increasing density of the polyethylene. A density of
betw~en about 0.930 and about 0.9~0 g/cm3 may usefully be
used, the preferred range being 0.948 to 0.951 g/cm3. The
upper limit of density relates to the practical
difficulties in extruding a film o~ such material. Too,
very high density (highly crystalline) polyethylene is
extremely sti~f at l.ow temperatures and could crack if
there is any flexion of the coated pipe.
The method and apparatus o~ the invention are
equally applicable to the co-extrusion of polyethylene and
the copolymer adhesive through a single die in a cross-
head extruder so as to fvrm a single layer, with the
polyethylene bonded to, hut discrete from the copolymer
adhesive. Co-extrusion assures superior adhesion between
the polyethylene and adhesive layers.
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Whether a two stage extrusion or co extrusion has
been carried out, the still-hot pipe coated with primer,
adhesive and high density polyethylene is then conveyed
into a quench chamber, indicat~d generally at 24 in Figure
2, where it is rapidly cooled with variously directed
streams of water from nozzles 24a~ Radial pressure is
then applied to the surface of the jacketed pipe after the
primer has gelled, over a region which has been quenched
only to the extent that the outer surface of the
lo polyethylene coating has hardened. The inner portion
should still be above the softening temperature, so that
the constriction of the pipe effects the best possible
adhesion of all layers.
A pressure applicator adapted for use of the
lS coating method of the present invention is a flexible
annular pressure applicator indicated generally at 26 in
Figure 2.
The pressure applicator consists o~ a specially
molded rubber ring 26a, of a diameter slightly smaller
than that of pipe 10. Riny 26a is so molded as to allow
for easy entry of pipe 10, but to increase the
constrictive pressure gradually from entry t~ exit of the
pip~ through it. It has been found that a rubber ring
made of a ~urometer 70 rubber, as measured on the hardness
scale, is satisfactory. Ring 26a is mounted by clamping
plates 26b and support brackets 26c at a fixed position
along the path of travel of the pipe through the quench,
selected so that pressure is applied after initial
quanching, but before the underside of the topcoat and the
adhesive layer have cooled and after gelling of the epoxy
layer.
Preferably, provision is made in the mounting of
ring 26a to allow for small tran~verse movement within the
vertical plane, to accommodate any small lateral movement
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1321~5
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of pipe 10 as it travels through the quRnch region and
pressure applicator along the "endo" conveyor.
Tha function of precsure applicator 26 is to apply
radial pressure to the polyethylene coating so as to bring
togeth~r the epoxy, the adhesive and polyethylene in intimate
bonding contact. Ik is essential that th~ position of the
applicator in the quench tank be such that, over the pipe' 5
annular surface region under pressure at any given timel the
polyethylene has cooled down just suf~iciently to develop a
"skinl' at the top, so that it will not tear under the
friction of passage through ring 26a. The lower layer of
polyethylene top coat should still b~ molten to adhere to the
copolymer adhesive.
The following specific experimental example is
illustrative of the invention:
~ four inch pipe was cl~anod a~rasively to a near-
white finish using a mixture o~ steel shots and grit to the
average anchor pattern depth of 50~O
A chromat~ solution, prepared by mixing a 10%
chromate (Pyrene Corporation) with 90% water, was applied by
brushing onto the pipe, previously heated to 60 C . The
thickness of the wet solution was about 25~. The chromate
solution was allowed to evaporate be~ore the pipe was treated
further.
Epoxy primer was prepared by mixing 100 parts o*
Dow~ Epoxy Resin D.E.R. 331 with 50 parts of Versamid~ 125
from Henk~l Corporation and 15 parts of red iron oxide
pigmen~, Bayferox~ from Bayer Chemical Co. Ltd. The primer
was applied to the pipe using the epoxy primer applicator of
of Figure l, equipped with a natural rubber (40 Durometer~
wiping ring, to a thickness of 50 microns as measured on
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1 3~ 1 455
13
top of the anchor pattern. The pipe temperature during
application of epoxy primer was 40C.
Immediately after application of t:he primer, the
pipe proceeded on the "endo" con~eyor to an induction
coil, where the temperature was raised to 150C, followed
immediately by a cross-head extrusion of adhesive, namely
LOTADER PX 8460, obtained from CdF Chimie (France). That
product is a terpolymer containing active groups. The
adhesive was extruded at a thickness of 150~.
The pipe proceeded further to a second cross-head
extruder where high density polyethylene was applied at a
thickness of lmm. The p.ipe proceeded from the second
extruder to a quench bath equipped with a rubber ring
pressure applicator as described above and illustrated in
Fi~ure 2, located approximately 25 seconds of pipe travel
time into the ~uench.
The coated pipe product obtained by the foregoing
procedure exhibited a peel strength adhesion in the range
of 100N/cm-150N/cm. The cathodic disbondment test as
measured at room temperature, at 5V DC, in a 5~ NaCl
solution for thirty days, showed a disbonded radius o~ 4-
8mm. The product is bendable up to 4.76 degreestpipe
diameter at -30C without failure. The impact strength of
the product was found to be in the area of 10J, usin~ a
16mm round tup. In addition, the hot water resistance of
the product was excellent; there was no loss of adhesion
upon exposing the sample to boiling water ~or 24 hours.
In order to achieve the superior peel strength
adhesion which characterizes the jacket applied to the
pipe by the foregoing method, it is most important that
the epoxy primer be applied as a liquid to the pipe
surface as an ~ven film of thickness between about 25~ and
100~. One applicator for this purpose has been described.
7 32 1 455
14
An alternakive applicator or the purpose is shown in
Figures 3 and 4.
This alternativ~ applicator comprises a hopper 27
to which the epoxy primer is supplied, the lower portion
of the hopper defining a reservoir through which the pipe
passe~ as it is conveyed longitudinally, in the
direction of the arrow A, towards the crosr,-head extrusion
apparatus. The hopper 27 has ~latl vertical, front and
rear walls 28, 29 with aligned circular openings 30
through which the pipe 10 passes. Each o~ the openings i5
bounded by an annular peripheral flange 31, 31' secured to
the respective wall 28, 29. The side wall 32 of the
hopper 27 is provided with a cooling jacket 33 supplied
with cooling water via an inlet 34. The water outlet is
shown at 35.
Adjacent to the front opening 30 is an annular
wiper 36 of rubber, which wipes the surface of the pipe
10 and prevents leakage through the opening 30. The wiper
36 is clamped between the flange 31 and a second annular
flanga 37 bolted to it. Adjacent to the rear opening is
a second wiper consisting of an annular rubber bladder 38
which wipes the coated surface o~ the pipe emsrging from
the rear opening. The wipiny pressure of the bladder 38
is adjusted by controlling the air pressure in the bladder
by means of a pressure regulator 39. The bladder i5
supported by an annular support 40 bolted to the rear
~lange 31'.
An alternative pressure applicator for applying
radial pressure to the surface of the jacketed pipe is
shown in Figures 5 and 6. This alternative pressure
applicator consists of a pressing wheel assembly. The
wheels of the assembly are arranged in two sets of four
wheels each, 41, 41', the two sets being axially offset
and staggered in overlapping relation around the
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1321455
circumference of the pipe. The sets of wheels are mounted
on axles 42~ 42' supported from opposite side~ of an
octagonal support frame 43 by adjustable support legs 44,
44'.
The wheels have low pressure tyres which bear
against the pipe jacket at the stage at which its outer
~urfac~ has hardened, thereby to achieve the good adhesion
of the jacket to the pipe.
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