Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Continuous chromate-free fluidized-bed pipe coating
Coated pipes for the automotive industry have
hitherto been produced by using chromium VI compounds
(chromates). Chromates are used in order to achieve very
good adhesion values in the extrusion process employed
hitherto. To this end, use is made of chromate-treated
steel pipes, chromate-treated aluminium pipes, and steel
pipes which are first aluminium-treated and then chromate-
treated. However, the automotive industry is demanding
chromium-free pipes from the year 2003.
It was desired therefore to provide a process
which permits continuous chromium-free pipe coating. There
are known processes for the continuous coating of pipes.
For example, the journal "Kunststoffe", Volume 57, No. 1,
pages 21-24, describes a process which uses fluidized-bed
coating to coat pipes with PVC, but the information given
there does not concern good adhesion values and homogeneous
layer thickness distributions. This does not therefore
provide a means of complying with the substantial
requirements of the automotive industry.
The disadvantages of the prior art are, in
particular, unsatisfied adhesion values and non-uniform
layer thickness of thin layers (120-150 Vim).
Thus, the present invention provides a process for
chromate-free coating an external surface of a metal pipe by
fluidized-bed coating using a pulverulent fusible polymer as
a coating material, which comprises:
(1) cleaning the pipe in a pretreatment system to remove a
grease;
(2) applying a primer to the pipe;
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(3) with a first medium-frequency induction coil, baking
the primer, and, when a solvent-containing primer is used,
evaporating the solvent;
(4) when the solvent-containing primer is used, rapidly
dissipating the evaporated solvent, by using a radial fan;
(5) preheating the pipe;
applying the pulverulent fusible polymer to the pipe,
by using a fluidized-bed coating pan;
(7) eliminating powder accumulations and eliminating powder
deficits and resultant pores on an underside of the pipe, by
using internals in the fluidized-bed coating pan composed of
an air-flush system above the pipe and metal flow-guide
panels below the pipe;
(8) smoothing an incompletely molten pulverulent fusible
polymer;
(9) thoroughly melting the adherent pulverulent fusible
polymer and producing a smooth melt, in a melting section;
(10) preliminarily cooling the pipe surface, by using an
air-flush system; and
(11) further cooling and hardening the coating, by using
water,
wherein, depending on a thickness of the coating to be
formed:
(a) the preheating of the pipe of step (5) is
conducted by using a second medium-frequency induction coil
and the application of the pulverulent fusible polymer to
the pipe of step (6) is conducted by using the fluidized-bed
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pan having an integrated third medium-frequency induction
coil;
(b) the preheating of the pipe of step (5) is
conducted by using a second medium-frequency induction coil
and the smoothing of the incompletely molten pulverulent
fusible polymer of step (8) is conducted by using a third
medium-frequency induction coil;
(c) the preheating of the pipe of step (5) is
conducted by using a second medium-frequency induction coil,
the application of the pulverulent fusible polymer to the
pipe of step (6) is conducted by using the fluidized-bed pan
having an integrated third medium-frequency induction coil,
the smoothing of the incompletely molten pulverulent fusible
polymer of step (8) is conducted by using a third medium-
frequency induction coil;
(d) the application of the pulverulent fusible
polymer to the pipe of step (6) is conducted by using the
fluidized-bed pan having an integrated third medium-
frequency induction coil; or
(e) the application of the pulverulent fusible
polymer to the pipe of step (6) is conducted by using the
fluidized-bed pan having an integrated third medium-
frequency induction coil and the smoothing of the
incompletely molten pulverulent fusible polymer of step (8)
is conducted by using a third medium-frequency induction
coil.
Fig. 1 is a schematic view of a system employed in
a preferred embodiment according to the process of the
present invention.
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The process is preferably operated automatically
and continuously and serves for the external coating of
pipes by fluidized-bed coating. The process may be carried
out by a system composed of the following parts:
1) a pretreatment system for cleaning the pipes, which are
mostly supplied in a greased form;
2) a primer-adhesion promoter pan for applying an adhesion
promoter between steel surface and polymer coating layer
(spray system or immersion system);
3) a first medium-frequency induction coil for baking the
primer and, if a solvent-containing primer is used, for
evaporating the solvent;
4) a radial fan for faster dissipation of the evaporated
solvent;
Z5 5) a second medium-frequency induction coil for ;preheating
the pipe;
6) a fluidized-bed coating pan with an integrated third
medium-frequency induction coil for applying a coating
material. The dissipation factor of the coating material is
too low for it to become heated, whereas the preheated steel
pipe passing through the system is heated very rapidly to
the desired temperature. Often decisive factors controlling
the coating layer thickness during the flui.dized-bed coating
are a preheat temperature and an immersion time. In the
case of a pipe passing through the system, this means that
the coating layer thickness can be changed via a generator
power and an advance rate of the pipe. The two factors can
be controlled independently of one another from a control
desk;
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7) internals in the fluidized-bed coating pan, composed of
an air-flush system above the pipe for eliminating powder
accumulations and metal flow-guide panels below the pipe for
eliminating powder deficits and resultant pores on an
5 underside of the pipe. Uniform coating layer thickness,
both radially and axially, can be ensured by using the
specific internals;
8) a fourth medium-frequency induction coil for smoothing
the incompletely molten coating;
9) a melting section, needed for thorough melting of the
adherent coating deposit after the pipe emerges from the
fourth medium-frequency induction coil and for producing a
smooth melt. During passage through the system, the layer
is still hot and soft and is therefore easily damaged.
Passing of the pipe over rollers in this phase is therefore
not permissible;
10) an air-flush system for preliminary cooling the pipe
surface. The pipe surface temperature is thus controlled to
below the melting point of the coating material; and
11) a water-based cooling system. The pipe runs into a
water trough in which the coating layer undergoes further
cooling and hardening, and guiding over rollers therefore
becomes possible again here.
Depending on the desired layer thickness, the
second, third and fourth induction coils under 5), 6) and 8)
may be operated in various combinations and with varying
power. The possibilities of use of the induction coils are:
the second and fourth induction coils under 5) and 8);
the second and third induction coils under 5) and 6);
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the second, third and fourth induction coils under 5), 6)
and 8 ) ;
the third induction coil 6); and
the third and fourth induction coils under 6) and 8).
In all cases, the pipes are heated by medium-
frequency induction coils. There are stated approximation
formulae for the electrical energy required, the coating
rate of this type of system, and also the powder usage.
Using the process it is possible to couple together pipe
sections with any desired length to give a continuous line,
and to coat these pipes externally with polymer powder as
they pass through the system horizontally. Suitable coating
materials are fusible polymers capable of fluidization, and
mixtures of these polymers. Polyamide powders are
particularly suitable, especially those based on nylon.-11 or
nylon-12. Powders which give particularly good processing
here are those prepared as in DE 29 06 64'7 (Huls AG),
marketed under the trade-mark VESTOSINT (Degussa AG), since
these powder particles have a particularly round granular
shape due to preparation by the precipitation process. A
commercially available adhesion promoter is first applied to
the pipe surface. Suitable primers here are any of the
familiar grades for polymers, in particular those for
polyamides. They may be applied in solution, suspension, or
in powder form. Particularly suitable adhesion promoters
for VESTOSINT* are those specifically adapted to VESTOSINT*.
If use is made of a solvent-containing adhesion promoter
whose solids content is about 80, the layer thickness of the
primer after air-drying is from 5 to 8 ~,m. The process of
the invention can achieve uniform coating layer thicknesses
*Trade-mark
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of from 50 to 1000 ~,m. Preferred coating layer thicknesses
are from 50 to 300 ~Cm. The process of the invention can
achieve coating layer thickness tolerances of ~ 30%,
preferably a commercially available adhesion promoter (e. g.
VESTOSINT* adhesion promoter WS 5) is first applied to the
pipe surface. The layer thickness of the primer after air-
drying is typically from 5 to 8 ~um. If a solvent-containing
adhesion promoter is used its solids content is generally
about 8%.
The pipes produced by 'the process of the invention
are particularly suitable as hydraulic piping and brake
piping, e.g. for the automotive industry.
The process of the invention will be described in
more detail below.
Preheating by medium-frequency induction
Medium-frequency induction heating was selected
because when applied to the continuous process it is a
heating method which is readily controllable but
nevertheless very fast, and gives the further advantage that
the induction coils which heat the pipe as it passes through
the system can be arranged directly within the fluidized
powder, with the result that there are no heat losses. At
10,000 Hz and with a pipe wall thickness of 2 mm, heating at
300°C takes 1 second. At lower frequencies the greater
penetration depth makes the heating process even faster, and
at 2000 Hz the time would be only 0.'~3 second under the same
conditions. The induction coil takes the form of a coiled
tubing and is cooled by passage of water. It remains cold,
as does the powder. A generator system is composed of a
*Trade-mark
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machine generator which generates the high frequency, a
control cabinet with a control desk, a capacitor battery,
and an induction coil. The system may simply be regarded as
a transformer with high-frequency electrical energy fed into
its primary side and with the workpiece forming its
secondary side composed of just one winc~.ing. The resultant
very high current density in a second circuit results in
fast heating. Workpieces passing through the system have to
have uniform cross section and uniform wall thickness,
examples being articles with rotational symmetry, such as
wires, pipes, rods, and the like.
Energy consumption and coating rate
A throughput rate (advance rate) of the pipes
depends on a pipe diameter and the wall thickness, i.e. on
I5 the weight of the pipe per unit length, and also on the
generator power. Naturally, a substantial part is also
played by the generator efficiency and the required degree
of heating of the pipe. However, since these latter
variable can initially be regarded as constant or at least
not subject to great Variation, appropriate average
numerical values can be assumed. The generator power
required is:
Bt
N-G.c . _ (1)
860r~
where N is the generator power in kW, G is the pipe weight
in kg/h passing through the system, cp is the' specific heat
of steel pipe (about 0.12 kcal/kg degree), et- is the
required pipe temperature increase, and ~7 is the overall
efficiency of the generator system (approximately from 0.6
to 0.75). The coefficient 860 derives from t=he conversion
1 kW = 860 kcal/h. If the equation (1) is solved for G 'when
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r~ is 0.7 and at is about 240°C, the result is a very useful
rule-of-thumb formula for the maximum amount of steel in
kg/h which can be heated with a particu:Lar generator power
(valid for the present conditions and similar conditions):
G = 20 N (2) .
Using a 36 kW medium-frequency generator,
therefore, about 720 kg/h of steel pipe can be heated to
240°C. This calculated value was confirmed in the example.
For quick approximations, an approximate guideline
value for power required (under the present conditions or
similar conditions) is:
N~50k~h (3) .
g
These rule-of-thumb formulae are naturally not
dimensionally correct since the numerical value has been
used for quantities which have dimensions (e. g. specific
heat cp ). However, these dimensionally incorrect equations
have proven to be very useful for operating requirements.
If the formulae for the weight of pipe passing through the
system are combined with the formula for the generator power
required the result is a simple relationship for the maximum
throughput rate (advance rate) of a steel pipe of density
y = 7.85 kg/dm3 for a given generator power. If, for example
there is a frequent need for pipes of various diameters and
wall thicknesses to be coated in. a pipe-coating system, the
following formula rapidly gives a guideline value for the
maximum throughput rate. The numerical factor has to be
altered somewhat for other conditions:
N
Amax ~ I $ lda - S'~-S ( 4 ) ,
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The throughput rate vmaX has to be inserted here in
m/min, the generator power N in kW, the external pipe diameter
da in mm, and the pipe wall thickness s likewise in mm.
Example:
5 The pipe-coating system (Figaare 1) is composed of:
1) a pretreatment system 1 for cleaning the pipes, which
are mostly supplied in a greased form;
2) a primer-adhesion promoter pan 3 for applying an
adhesion promoter between steel surface and polymer coating
10 layer (spray system or immersion system);
3) a first medium-frequency induction coil 4,5 for baking
the primer and for evaporating the solvent;
4) a radial fan 6,7 for faster dissipation of the
evaporated solvent;
5) a second medium-frequency induction coil 9 for
preheating the pipe;
6) a fluidized-bed coating pan 11 with an integrated third
medium-frequency induction coil for applying polyamide (PA)
powder, e.g., nylon-1~?. The dissipation factor of the PA
powder is too low for it to become heated, whereas the
preheated steel pipe passing through the system is heated very
rapidly to the desired temperature. The decisive factors
controlling layer thickness during fluidized-bed coating are
preheat temperature and immersion time. In the case of a pipe
passing through the system this means that the coating layer
thickness can be changed via the generator power and the
advance rate of the pipe. The two factors ca.n be controlled
independently of one another from the control desk;
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7) internals in the fluidized-bed coating pan, composed of
an air-flush system above the pipe for eliminating powder
accumulations and of metal flow-guide panels below the pipe
for eliminating powder deficits and resultant pores on the
underside of the pipe. Uniform coating layer thickness,
both radially and axially, can be ensured by using the
specific internals;
8) a fourth medium-frequency induction coil for smoothing
the incompletely molten coating;
9) a melting section, needed for thorough melting of the
adherent coating deposit after the pipe emerges from the
fourth medium-frequency induction coil, and for producing a
smooth melt. During passage through the system, the coating
layer is still hot and soft and is therefore easily damaged.
Passing of the pipe over rollers in this phase is therefore
not permissible;
10) an air-flush system for preliminary cooling the pipe
surface. The pipe surface temperature is thus controlled to
below the melting point of the coating material; and
11) a water-based cooling system. The pipe runs into a
water trough 16 in which the coating layer undergoes further
cooling and hardening, and guiding over rollers therefore
becomes possible again here.
The results of a series of experiments on the
system described are given in Table 1. All of Examples 1 to
7 used VESTOSINT* 2157 precipitated nylon-12 powder from
Degussa AG. In all the examples given there was no
chromate-pretreatment.
* Trade-mark
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Tests on primed pipes
a) TL 222 corrosion-protection coatings on .brake pipes
(surface-protection requirements) D ~n/PA
Corrosion resistance: test time 500 h with scribe mark
to DIN 53 167;
Scribe mark creep: < 2mm;
Corrosion resistance: test time 500 h following rock
impact test to PV 1213; no underlying metal corrosion;
Corrosion resistance: test time 1000 h; no zinc
corrosion, no underlying metal corrosion, and no
breakaway of PA layer
Chemicals resistance: to TL222, item 5; no blistering or
softening of the polymer coating layer occurred, after 24
hours of air-drying and then winding around a mandrel
(360°) of dimension 16 mm, no visible cracks or peeling
of the PA coating occurred.
b) Adhesion tests on primed pipes after storage in water,
tests using knife tip:
1) Pipes without scribe mark
Dry test, one day after coating: very good adhesion
Dry test, one day after coating, on a wound pipe (around
16 mm mandrel): very good adhesion
3 days' storage in water, directly after removal, very
good adhesion
2) Pipes with scribe mark
Dry test, one day after coating: very good adhesion
3 days' storage in water, directly after removal: very
good adhesion.
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Description
of Figure
1
1 Cleaning system
2 First drive
3 Primer unit
4 First induction coil (I primer drying)
-
5 Second induction coil (V - primer drying)
6 First radial fan
7 Second radial fan
8 Second drive
9 Third induction coil (preheating)
10 First lay-on roller
1
11 Fluidized-bed sinter pan including fourth induction coil
12 Third drive
13 PIPE
14 Blow-off nozzle
15 Second lay-on roller
16 Water bath
17 Third lay-on roller
18 Fourth lay-on roller
19 Fourth drive
20 Fifth drive (caterpil lar draw-off)
21 Take-off
