Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The coating of metal tubes with a strip of
polyethylene coming directly ~rom an extruder is
known. In such a process the metal tube is first
preheated to a temperature of about 180~. The types
of polyethylene used in the process generally have
melt indices (190C/2.16 kg) of 0.4 to 0.7 g/10 minutes.
A time of about 4 minutes is required to form a layer
of polyethylene on the metal of about 3.5 mm thick.
The coating of metal tubes with polyethylene
powder is also known and the powder is applied e.g.
by sprinkling, flinging or throwing onto the tube
which is preheated to 300 to 360C. The polyethylene
types which may be used in this process generally
have melt indices (190C/2.16 kg) of 1.2 to 1.7 9/10
minutes. In this process, the melt index of the
polyethylene must be higher than that mentioned for
the ab~ve-mentioned process, since easier melting
is essential. Depending on the tube diameter, the
coating time in such powder coating processes is
generally about 5 minutes. However, this type of
process has the disadvantage that a relatively high
preheating temperature, and consequently a high consumption
of energy is required to melt the polyethylene, as
the melt index should not be too low to ensure sufficient
corrosion protection.
A composite metal tube coating has been described
which consists of a polyethylene powder layer welded
onto a steel tube together with a polyethylene layer
which is welded onto the powder coating and which
has been wound, for example in the form of a polyethylene
film from an extruder, onto the hot sintered-on poly-
ethylene powder layer. When the first polyethylenelayer is applied the steel tube should have been
preheated e.g. to 150C. ~owever, at this temperature
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it is not possible to produce a smooth coherent layer
in a desired requisite minimum layer thickness of
1.5 to 4 mm.
In another known process, a polyethylene layer
is applied to a steel tube, after first coating the
tube with a layer, about 0.05 mm thick, of an adhesion
promoter which is applied at a tube temperature iying
approximately 100C above the setting point of the
adhesion promotor. The layer of polyethylene onto
the adhesion promotor is applied at a tube temperature,
e.g. at 140C, lying 20 to 50C above the setting
point. The adhesion promotor of the first ~ayer
can be applied in powder form or by winding a film
round the tube. The application of the second layer
may be effected by pre-extrusion in the form of a
double tubing or of a double winding film. In this
process, also, the temperature of 140C is not sufficient
for the purposes desired according to the present
invention, namely for a requisite minimum layer thickness.
The covering of a steel tube surface with a
polyethylene having a high melt index, for example
1 to 1.5 9/10 minutes (ASTM-D 12 38-53T) has also
been disclosed. A second layer of a polyethylene
with a low melt index, for example 0.2 to 0.5 9/10
minutes, is applied to the covering. Both layers
are applied in the form of a polyethylene powder.
If two layers are applied on top of one another in
the form of powder, the surface sometimes leaves
much to be desired in respect of its homogeneity
and its smoothness.
A process with a smaller consumption of energy
and time is therefore desired which would give products
- with properties as good as those from the known processes.
The present invention provides a process for
coating metal tubes by the application of polyethylene
to the preheated tubes, a polyethylene with a melt
index of over 1 g/10 minutes being applied ~irst
to the preheated metal tube and a polyethylene with
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a low melt index being applied thereupon, said process being characterised in
that in a first step a polyethylene with a melt index of 1.2 to 70, advantage-
ously 15 to 70, and preferably 17 to 25 g/10 minutes (190C/2.16 kg) is first
applied to the metal tubes which have been preheated to a temperature of at
least 200C, whereafter in a second step the coating is cooled to a temperature
of about 110 to 170C, advantageously 110 to 150, and preferably to 120C, and
in a third step a self-supporting film of polyethylene having a melt index of
0.1 to 7 g/10 minutes is thereupon applied at this temperature, so that the
total coating has minimum thickness of 1.5 and to 4 mm.
This invention also provides a coated article which comprises:
a) a metal tube and
b) a plurality of coatings based on polyethylene material applied
in at least two steps,
the first coating having a melt index of 1.2 to 70 g/10 minutes (190C/2.16kg)
and the second coating, in the form of a self-supporting film of 0.1 to 7 g/10
minutes, the total coating has a minimum thickness of 1.5 to 4 mm.
One embodiment of the invention provides that a polyethylene powder
with a melt index of about 1.2 to 1.7 g/10 minutes or a tape of polyethylene
coming directly from an extruder and having a melt index of not more than 1.7
g/10 minutes ~190C/2.16 kg) is first applied to the metal tube which has been
preheated to a temperature of at least 300C in the case of powder coating and
at most 250C in the case of coating with extruded tape, the coating is there-
after cooled in the second step to a temperature of about 110 to 170, preferably
110 to 150C, and in a third step a self-supporting photo-stabilised light-
coloured film of polyethylene with a melt index of 0.4 to 1.1 g/10 minutes is
thereupon applied at this temperature.
Examples of photo-stabilisers for the light-coloured polyethylene
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film are e.g. compounds of the benzotriazole type.
The minimum layer thickness of 1.5 to 4 mm is necessary to ensure
sufficient corrosion protection for the metal tube. In many cases the layer
thickness may also be more than 4 mm.
The process according to the invention has the advantage that it
surprisingly entails a high saving of energy compared with known processes
and, nevertheless, a considerably higher working speed
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with at least equally good product properties. Moreover,
a light-coloured coating layer provides good protection
of the tubes against ~trong heating during any lengthy
storage in the open air under strong thermal action,
for example from solar radiation, or when laid in
strongly heated soil strata. The coating from the
extruder may be effected very simply and in a time-
saving manner without additional expenditure on apparatus.
The tape coming from the extruder advantageously
has a melt index of at least 0.4 9/10 minutes.
Generally, in powder coating, the polyethylene
for the first step has a particle size of 1 to 600 jum,
preferably 100 to 400 ~m. The self-supporting polyethylene
film is applied advantageously in the form of a poly-
ethylene tape for example of polyethylene witha melt index of 0.1 to 1.2 9/lO minutes. The application
may be effected by rotating the tube. This affords
the advantage that the tape can be wound automatically.
The tape width can be varied as desired. It may
be conveniently 10 to 1500 mm e.g. at least 20 mm.
In general, a tape width up to about 1 m is used.
During application it is necessary to ensure that
the individual coil layers overlap or that the individual
coil layers are simultaneously mutually joined together,
in order to achieve satisfactory corrosion protection.
The layer thickness of the tapes is usually 100
to 400 ~m, preferably 100 to 200 ~um. Light-coloured
tapes are preferably white. Depending on the required
end use, however, another colour may also be chosen,
for example the warning colour yellow, and also light
orange, light blue, light green or the like. In
this way, the light-colour~d tapes may also serve
to identify the tubes.
The speed of coating may vary over wide limits.
It depends on the desired layer thickness and on
the tube diameter; the outside tube diameter may
be, for example, from 50 to 2000 mm. For coating
a length of 12 m of a tube with an outside diameter
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of 1500 mm and a coating layer thickness of 3.5 mm
e.g. about lS to 45, generally about 30 minutes are
required in the process of the invention. For coating
a tube 400 mm in diameter with a layer thickness
of 1.5 mm (this is the minimum thickness for sufficient
corrosion protection), generally about 8 to 20, mostly
about 15 minutes are required for a coating 12 m
long.
In order to further improve the adhesion of
the layer applied in the first step to the tube substrate,
it is sometimes appropriate to admix with the polyethylene
powder an additional resin in the form of a polymer,
e.g. polyvinyl acetate, ethylene-vinyl acetate copolymer,
ethylene-acrylic acid and/or acrylate copolymer,
optionally with further comonomers, or other polymers,
advantageously in a proportion of 5 to 15, preferably
5 to 10% by weight relative to the polyethylene powder.
According to another embodiment of the invention
it is also possible to apply such polymers to the
tube before the polyethylene powder is applied.
This coating may be effected according to the conventional
coating processes, e.g. by spraying, but preferably
by powder coating. The same additional resins may
also be present in the polyethylene tape in a proportion
of 2 to 5% by weight relative to the polyethylene.
The requirements regarding minimum layer thickness,
freedom from pores, resistance to paring, i~pact
strength, indentation resistance, elongation at break,
specific sheath resistance and ageing due to heat
and light in accordance with the provisions of DIN
30670 are fully satisfied by the coatings made according
to th~ invention.
The tubes coated by the process according to
the invention have a variety of uses. Owing to the
surface protection which they provide th~y are suitable,
above all, for laid pipes, e.g. in pipelines for
conveying petroleum, and also gaseous or other liquid
substances or substances of higher viscosity, e.g.
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natural gas, water, settling sludge, concrete, waste
waters, suspensions or the like.
Especially advantageous is the use of the tubes
coated according to the invention for laying in warm
or hot areas, e.g. in desert regions.
Examples
1) An iron tube (outside diameter 108 mm, wall
thickness 10 mm) is preheated to 220C and then coated
over 2 minutes with polyethylene powder tmelt index
17 to 25) in a layer thickness of 2 mm. After 4
minutes the covering has melted and the tube temperature
has dropped to 160C. Starting at this tube temperature,
a polyethylene tape 110 Jum thick and 50 mm wide
with a melt index of 1.2 is applied at 160C in a
layer thickness of 110 Jum. The tube is then cooled
to room temperature either by merely allowing it
to stand or by passing a cooling medium through the
tube. Immediately after the film has been applied
a perfect and smooth fusing of both covering layers
takes place. After 30 minutes from the start of
the powder coating the tube has cooled to 60C by
being allowed to stand without additional cooling.
2) An iron tube (outside diameter 90 mm, wall
thickness 4.5 mm) is preheated to 250C and is then
coated over 1~ minutes with polyethylene powder (melt
index 17 to 25) in a layer thickness of 2 mm. After
3 minutes the coating has melted smooth and cooled
to 150C. At this tube temperature a polyethylene
tape ~4D mm wide with a melt index of 1.2 and a layer
thickness of 110 ~um is applied and the tube is then
cooled to room temperature. The tape is applied
by being wound round the tube which is rotated about
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its axis, the coil being moved along the tui~e. Immed-
iately after the tape has been applied a perfect
fusing with the first polyethylene layer takes place.
After 12 minutes from the start of the tube coating
the tube has cooled to 60C by being allowed to stand
without additional cooling.
lC) (Comparison - state of the art - preheating
temperature above 300C)
The same tube as in Example 1 is used r but
with a preheating temperature of 310C. The tube
is coated with a polyethylene powder having a melt
index of 1.2 to 1.7 g/10 minutes in a layer thickness
of 2 mm. After 10 minutes the coating has melted
smooth and has reached a temperature of 180C. The
time of cooling to 60C by merely allowing the tube
to stand is 50 minutes.
2C (Comparison - state of the art - preheating
temperature above 300C)
The same tube as in Example 2 is used, but
with a preheating temperature of 360C. The tube
is coated at this temperature with a polyethylene
powder having a melt index of 1.2 to 1.7 over 1~
minutes in a layer thickness of 2 mm. After 4 minutes
the covering has melted and simultaneously cooled
to 310C. However, further heating is necessary
for melting smooth. The tube is therefore further
heated for 1 minute after the expiry of these 4 minutes.
The covering is thus smooth after this further minute.
The time of cooling to 60C from the start of the
powder coating and without additional cooling is
42 minutes.
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As a comparison between the Examples according
to the in~ention and the comparative Examples according
to the state of the art shows, the consumption of
energy, that is the preheating temperature ~Ind simul-
taneously also the cooling time, is substantiallysmaller in the Examples according to the invention
than in the comparative Examples.
~) Work is carried out as in Example 1, but, instead
of the polyethylene powder, a powder in the form
of a mixture of polyethylene with 10% by weight,
relative to the polyethylene, of a vinyl acetate
homopolymer is applied. A coating is obtained with
properties as good as those according to Example
1 and with a perfect smooth surface.
4) Work is carried out as in Example 2, but, instead
of the polyethylene tape , a tape consisting of
a mixture of polyethylene and 3.5% by weight, relative
to the polyethylene, of a vinyl acetate homopolymer
is used. A smcoth and perfect coating is obtained
with properties as good as those according to Example
2.
; 5) An iron tube (outside diameter 108 mm, wall
thickness 10 mm~ is preheated to 310C and then coated
over 2 minutes with polyethylene powder (melt index
1.2 to 1.7 g/10 minutes) in a layer thickness of
2 mm. After 20 minutes the coating had melted and
the tube temperature had dropped to 160C. Starting
at this tube temperature a polyethylene tape 200 ~m
thick and 50 mm wide with a melt index of 0.4 g/10
30 minutes is applied at 160C in a layer thickness
of 200 ~m. The tube is then cooled to room temperature
either by merely allowing it to stand or by passing
it through a cooling medium. Immediately after the
tape has been applied a perfect and smooth fusing
35 of both coating layers takes place. After 40 minutes
from the start of the powder coating the tube has
cooled to 6~C by being allowed to stand without
additional cooling.
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6~ An iron tube (outside diameter 90 mm, wall
thickness 4.5 mm~ is preheated to 360C and then
coated over 11 minut~s with polyethylene powder (melt
index 1.2 to 1.7 ~/10 minutes) in a layer thickness
of 2 mm. After at most 8 minutes, the coating has
melted smooth and after 12 minutes cooled to 150C.
At this tube temperature a polyethylene tape 40 mm
wide with a melt index of 0.4 g/10 minutes is applied
in a layer thickness of 200 ~um and the tube is then
cooled to room temperature. The tape is applied
by being wound round the tube which is rotated about
its axis, the coil being moved along the tube. Immediately
after the tape has been applied a perfect fusing
with the first polyethylene layer takes place. After
30 minutes from the start of the tube coating the
tube has cooled to 60C by being allowed to stand
without additional cooling.
7) An iron tube (outside diameter 500 mm, wall
thickness 6 mm) is preheated to 250C and coated
with an ethylene-acrylic acid copolymer in powder
form as an adhesive primer in a layer thickness of
100 pm. A polyethylene tape coming directly from
an extruder and having a melt index of 1.2 g/10 mm
and a layer thickness of 250 ~m is applied to this
still hot tube covered in this way. The winding
operation is continued until the desired layer thickness
of 4 mm is obtained. The tube is cooled to about
140C by merely being allowed to stand. At this
temperature a yellow polyethylene tape with a melt
index of 0.7 g/10 minutes, a width of 300 mm and
a layer thickness of 200 jum is applied. A perfect
and smooth, amalgamation of the two polyethylene
covering layers takes place. After 30 min~tes from
the start of the extruder coating the tube has cooled
to 60C by being allowed to stand without additional
cooling.
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It is not intended that the examples given herein should
be construed to limit the invention thereto, but rather
they are submitted to illustrate some of the specific em-
bodiments of the invention. Resort may be had to various
modifications and variations of the present inventicn
withcut departing from the spirit of the discovery or the
scope of the appended claims.
