Note: Descriptions are shown in the official language in which they were submitted.
CA 02233599 1998-03-31
WO 98/06511 PCT/EP97/04230
"PROCESS FOR REPAIRING PLASTIC COATINGS OF METAL PIPES"
The present invention concerns a process for repairing
' plastic coatings of metal pipes, that allows the repairs to be
made in a very short time.
' Metal pipes, usually made of steel, coated with plastic
material, are widely used for building pipelines conveying
fluid materials, such as oil, natural gas, or water, even for a
long distance.
Said pipelines must be able to withstand the most diverse
environmental conditions, and, in certain cases, the high
temperatures that are maintained in the proximity of pumping
stations, for example, or that may be typical of certain fluids
extracted from the subsoil.
In order to avoid weak points along the line it is necessary,
therefore, that the damaged or removed segments of the plastic
coating (where the pipes are welded together, for example) be
repaired in such a way that the mechanical properties are as
homogeneous as possible with the ones of the original coating.
As a response to said needs there are materials and
methods known in the art which allow one to reach said
objective with satisfactory results.
However, when the damaged portion of the coating is quite
extensive (for example when the coating has been removed in
order to weld the pipes), one needs complicated repairing
techniques requiring long times, and sophisticated and costly
apparatus, such as induction ovens.
Now has been perfected a particularly fast and efficient
repair method which allows one to limit the use of the
induction oven to the heating phase preceding the application
of the primer, and to carry out the other heating phases by
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using a flame shielded by inert gas (such as nitrogen, for
example).
Said process is particularly suited for repairing
polyolefin coatings, preferably polypropylene and/or
polyethylene coatings. '
Therefore, the object of the present invention is a
process for repairing coatings of metal pipes made of plastic
materials (in particular polyolefin, preferably polypropylene
and/or polyethylene coatings) comprising the following
operating phases:
1) sanding (optionally preceded by removal of greasy and
encrusting substances) of the exposed portion of the metal
pipe (i.e., the one without coating),
2) application of shells made of a rigid material (such as
steel, for example} on the edges of the existing coating
surrounding the exposed portion,
3) heating the exposed portion by way of induction (in
particular with induction ovens) to a temperature ranging
from 140 to 270 °C, preferably from 170 to 240°C,
4) application on the exposed portion of an epoxy primer in
powder form, and a polyolefin adhesive in powder form,
5) removal of the rigid shells and application of the
coating, in the form ofa polyolefin tape, by way of
wrapping and heating with an inert gas-shielded flame.
Preferably, the size of the shells used in phase 2) is
such that they overlap the existing coating by about 10
centimeters from the edges.
Said shells serve the purpose of preventing the coating
from splitting and lifting during the heating of phase 3).
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In phase 4), the epoxy primer in powder form and the
polyolefin adhesive inpowder form are preferably applied by
using guns that are fed from a fluid bed by way of a pump which
aspirates the powders using the Venturi effect.
Said guns can be equipped with a system for the
electrostatic charge of the powders.
Various alternative solutions are possible for phase 5).
In particular phase 5) may comprise the following steps:
5i) removal of the rigid shells,
5ii) heating the exposed portion and the surrounding part
of the existing coating by way of an inert gas-
shielded flame,
5iii) application of a polyolefin adhesive in powder form
on the existing coating by way of flame spraying,
5iv) application, by wrapping, of a polyolefin tape (in
particular a polypropylene and/or polyethylene tape)
having preferably a thickness of 800 microns, on the
exposed portion and on the surrounding area of the
existing coating, preferably overlapping the edges
of the tape being wrapped by 50-70~ of its width,
5v) heating the entire wrapping with the above mentioned
flame, and optionally,
5vi) cooling said wrapping, for example with atomized
water.
In step 5iv), the area of the existing coating,
surrounding the exposed portion, which is covered with the tape
consists preferably of at least 1.5 - 2 turns of said tape at
both sides of the pipe surrounding said exposed portion.
As an alternative, phase 5) may comprise the following
steps:
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5.1) optional application, by flame spraying, of an
additional layer of-polyolefin adhesive in powder
form (in the case that one would want to increase the
thickness of the coating),
5.2) application, by wrapping, of a polyolefin tape (in
particular a polypropylene and/or polyethylene tape)
on the exposed portion and on the shells made of
rigid material, preferably with an overlapping of
about 505 of the width of the tape, both at the edges
of the tape being wrapped as well as where said tape
is wrapped around the shells,
5.3) removing said shells as well as the portions of the
tape overlapping the shells,
5.4) heating, by way of an inert gas-shielded .flame, the
entire wrapping as well as the area around the
existing coating and the coating just applied, and
applying a polyolefin adhesive in powder form on the
latter area by way of flame spraying.
The purpose of step 5.4) is to form a bond between the
existing coating and the one just applied.
As a further alternative phase 5) may comprise the
following steps:
5a) removal of the rigid shells,
5b) application by way of flame spraying of an additional
layer of polyolefin adhesive in powder form,
preferably having a thickness of about 500-600
microns,
5c) application of a polyolefin tape (in particular a
polypropylene and/or polyethylene tape), without
overlapping it on the existing coating, said tape
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having been preheated (preferably to 80-90°C), being
as wide as the entire exposed portion, and preferably
having a thickness of 1.2-1.3 mm,
5d) heating the area where the existing coating meets the
one just applied, by way of an inert gas-shielded
flame, and applying a polyolefin adhesive in powder
form to said area by way of flame spraying.
Step 5c) requires a certain degree of accuracy in applying
the tape on the area to be coated without incorporating air
bubbles.
Step 5d) serves the purpose of forming a bond between the
existing coating and the one just applied.
In all the steps above that require flame heating,
excluding the particular case of flame spraying, the
temperature of said heating ranges preferably from 80 to 200°C.
As mentioned above, the flame used is shielded by an inert
gas, such as nitrogen for example.
The purpose of said shielding is to avoid or reduce the
contact between the air and the polymer that is melted by the
flame, thus avoiding the oxidation and degradation of the
polymer, which causes a marked reduction of the life of the
coating, especially when the operating temperature of the
coated pipe exceeds 80°C.
Said shielded flame can be obtained with guns or sprayers
that have a central nozzle for spraying powders, and some
concentric circles of nozzles.
A combustible gas (such as methane, propane, butane for
example) is fed through one or more circles of nozzles, while
the inert gas is fed through one or more concentric circles of
nozzles to provide the shielding. Thus one obtains a shielded
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flame (i.e_, capable of preventing the degradation of the
polymer with which it comes in c-ontact) by ignition of the
combustible gas.
Said guns or sprayers are also used for the application of
the polyolefin adhesive in powder form by way of-the flame
spraying method. In this case the surface-of the powder
particles of the polyolefin adhesive is heated to a temperature
ranging from 50 to 240°C by way of the flame.
Preferably, the adhesive in powder form is made of a
polyolefin composition comprising by weight:
A) from 30~ to 940, preferably from 50~ to 85~, of
polypropylene or polyethylene (HDPE, LDPE, or LLDPE), or a
propylene/ethylene or propylene/ethylene/C4-C1a oc-olefin
crystalline random copolymer;
B) from Oo to 70~, preferably from 5~ to 40~, of an
ethylene/propylene or ethylene/1-butene elastomeric
polymer;
C) from 0.5~ to 10~, preferably from 2o to 4~, of
polypropylene or polyethylene (HDPE, LDPE, or LLDPE)
modified with polar groups (in particular by way of
partial or total grafting ofmalefic anhydride or
isophoronebismaleamic acid) in quantities ranging from 0.5
to 10~;
D) from 0~ to 0.5~, preferably from 0.1 to 0.5~, of a
nucleating agent, such as dibenzylidenesorbitol or talc,
for example;
E) from 0~ to 10~, preferably from 6~ to lDo, of -titanium
dioxide;
said composition having a melt index L {ASTM D 1238 L) ranging
from 3 to 150, preferably from 5 to 90 g/10 minutes if the
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composition is predominantly polypropylene based, or a melt
index E (ASTM D 1238 E) ranging from 0.1 to 20, preferably from
0.5 to 15 g/10 minutes if the composition is predominantly
polyethylene based, and being in powder form with a diameter of
' the particles not exceeding 600 micrometers, with the following
particle size distribution (percentages by weight):
- not more than 25~, preferably not more than 4~, of the
powder having a diameter of the particles ranging from 300
to 450 micrometers;
- not more than 100, preferably not more than 0.6~, of the
powder having a diameter of the particles greater than 450
micrometers.
Preferably, a "predominantly polypropylene based"
composition is a composition where component (A) is made of
polypropylene or a propylene/ethylene or propylene/ethylene/C4-
C1o cc-olefin crystalline random copolymer, while a
"predominantly polyethylene based" composition is a composition
where component (A) is made of polyethylene.
Examples of C4-C1p oc-olefins optionally present in
component {A) are: 1-butene; 1-hexene; 1-octene; 4-methyl-1-
pentene.
When present, the C4-C1o Cc-olefins range in quantity from
2~ to 10~ by weight.
Examples of the above mentioned polyolefin composition are
described in published European patent application n. 603 906.
The polyolefin tape mentioned above can be essentially
made of a propylene or ethylene homopolymer, or a copolymer of
propylene with ethylene and/or higher oc-olefins (in particular
C4-Cg}, or a copolymer of ethylene with the above mentioned
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higher ~c-olefins, or a blend of the above mentioned
( co ) polymers .
Examples of C4-Cg oc-olefins are the ones given above for
the polyolefin adhesive in powder form.
Other components commonly used in the art, such as fillers '
or pigments, may be present in the composition of the
polyolefin tape.
In particular, the polyolefin tape can be made of a
polyolefin composition comprising:
a) 10-60, preferably 20-50 parts by weight of polypropylene
homopolymer with an isotactic index greater than 90,
particularly ranging from 95 to 98, or-a crystalline
copolymer of propylene with ethylene and/or a CH2=CHR oc-
olefin where R is an alkyl radical with 2-6 carbon atoms,
containing more than 85~ by weight of propylene, and
having an isotactic index greater than 85~;
b) 10-40 parts by weight of a polymer fraction containing
ethylene, insoluble in xylene at ambient temperature;
c) 30-60, preferably 30-50 parts by weight of a amorphous
ethylene-propylene copolymer fraction optionally
containing minor proportions of a dime, soluble in xylene
at ambient temperature, and containing 40-70~ by weight of
ethylene,
said polyolefin composition being cross-linked by way of
grafting an alkenyl derivative of alkoxysilane and reaction
with water, until one obtains hot set values, according to .
regulation CEI 20-31, lower than 175, preferably lower than
120, at 200°C, and lower than 10~ at 23°C.
Polymer compositions comprising (a)+(b)-+-(c) are described
in published European patent application n. 400333.
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Normally the total content of polymerized ethylene in the
above mentioned compositions ranges from 20 to 60~ by weight,
Zn fraction (a) , the CH2=CHR oc-olefin where R is an alkyl
radical with 2-6 carbon atoms is preferably selected from 1-
butene, 1-pentene, 4-methyl-1-pentene.
The dime in fraction (c) is preferably selected from
butadiene, 1,4-hexadiene, 1,5-hexadiene, ethylidene-1-
norbornene.
The molecular weight of the various fractions (determined
by measuring the intrinsic viscosity in tetrahydronaphthaline
at 135°C) varies in function of the nature of the components
and the total melt index of the composition.
Preferably it is comprised within the following limits:
- 0_5 - 3 dl/g for fraction (a);
- 2 - 8 dl/g for fraction (b) + (c).
The electron microscopic exam of the compositions
indicates that the dispersed phase is made of the amorphous
ethylene-propylene copolymer, and its average particle size is
less than 2 microns.
Preferred examples of alkenyl derivatives of alkoxysilanes
used to obtain the cross-linking of the above mentioned
compositions are vinyltriethoxysilane, allyltriethoxysilane,
and divinyldimethoxysilane.
Generally the alkenyl derivatives of alkoxysilanes are
grafted to the composition by way of extrusion in the presence
of peroxides.
Examples of the above mentioned polyolefin composition are
described in published European patent application n. 619 343.
As an alternative to the above mentioned cross-linked
polyolefin composition one can use, for the polyolefin tape,
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appropriately cross-linked polyethylene (HDPE, LDPE, or LLDPE),
using, for example, the above mentioned grafting technique of
an alkenyl derivative of alkoxysilane and reaction with water. ,
According to a preferred variation, the polyolefin tape
comprises a layer essentially made of the above mentioned
polyolefin materials, and an additional layer, coextruded for
example, of a polyolefin adhesive.
Naturally, during the application of the tape the adhesive
polyolefin layer is made to come in contact with the surface to
be coated.
Preferred examples of the polyolefin adhesive composition
that can be used for the above mentioned additional layer of
the polyolefin tape are the ones previously described for the
polyolefin adhesive in powder ~orm.
As a way of example, the repair of the coating of a welded
joint between two steel pipes, having a diameter of 508 mm and
a thickness of 8 mm, from which the preexisting polypropylene
coating around the joint has been removed, may be carried out
using the process of the present invention comprising the steps
from 5i) to 5vi), using the following operating times:
phase 1): 60 seconds
phases 2 and 3): 60 seconds
phase 4): 60 seconds
steps 5i), 5ii), and 5iii) 40 seconds
step 5iv): 60 seconds
step 5v): 40 seconds ~
step 5vi): 40 seconds
thus obtaining a coating, in the area being repaired, with a
thickness of 2.5 mm ca.
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This shows the operational speed that one can obtain with
the process of the present invention.
r
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