Note: Descriptions are shown in the official language in which they were submitted.
CA 02345922 2001-03-15
WO 00/16931 PCT/EP99/06850 1
Process for coating a surface arith a parting agent
The invention relates to processes for coating
a surface with a parting agent.
The problems which arise when shaping
nonferrous metals are explained in the following text
on the basis of the example of shaping aluminum. The
processing of aluminum blocks to give the profiles
takes place at typical processing temperatures of
approximately 450°C, by means of extrusion. In this
process, a preheated aluminum block (A1 block), which
is preferably round in cross section, via a container,
is pressed through the shaping extrusion dye by means
of an extrusion ram, with the result that it is shaped
into a profile, and is then cooled after i_t leaves the
dye. The front part of the extrusion ram, which is also
known as the extrusion plate, must fulfill the
following tasks:
1. It must seal the container in t:he opposite
direction to the direction of extrusion;
2. When the extrusion ram is withdrawn, it must be
easy to part from the aluminum.
The remaining A1 extrusion discard is removed with a
shearing blade (extrusion discard cutter) before the
next Al block moves in for processing.
Owing to the extremely high level of corrosion
exhibited by conventional metallic materials with
respect to aluminum and other nonferrous metals at
typical processing temperatures, contact points between
nonferrous metal and the mold must be treated with
so-called parting agents, so that uninterrupted
operation is possible.
Soot is used as a conventional parting agent
for separating extrusion plate and A1 block; the soot
is deposited on an endface of the A1 block by burring
an acetylene flame. This operation has to be repeated
for each new Al block.
To coat the extrusion discard cutter, it is
customary to use oils which evaporate on coming into
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contact with the hot A1, thus producing a parting
effect.
Parting agents are used not only for extrusion
but also for all processes used for shapin g nonferrous
metals, such as for example gravity dye casting,
low-pressure dye casting, pressure dye casting,
continuous casting.
Generally, pigment-free formations are used,
based on water or mineral oils with additives . In some
cases, pigment-containing formations of parting agents
are also used (cf. for example, EP (310046) . The
pigments used comprise a whole series of solids, such
as for example graphite, MoS2, BN or talc.
If the abovementioned in-situ formation of soot
,_5 is disregarded as a special case, a common factor of
all parting agents known to date are that they have to
be used in the form of liquid suspensions or emulsions.
They are usually applied by being sprayed onto the warm
to very hot surfaces. At high temperatures, there is a
risk of readily volatile components or any solvents
igniting or of the active compound being lost when it
suddenly evaporates or. coming into contact with the hot
surface. Furthermore, the evaporation products
represent a considerable burden on the .ambient air,
which is increased further by the formation of
decomposition products. Generally, a large proportion
of the components pass into the water circuit which is
installed in the area of the plants, and have to be
eliminated at considerable cost at this point.
Therefore, the object of the invention is to
provide a process for coating a surface with a parting
agent which is not subject to the abovementioned
problems of the prior art. The coating is to be carried
ou~ uniformly, rapidly and at low cost.
The object is achieved by means c>f a process
wherein a BN powder is applied to the surface by means
of electrostatic coating.
The surface to be coated may, for example, be
the surface of the shaping extrusion dye, e.g. of the
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extrusion plate or the extrusion discard cutter. In
principle, however, it is also possible for the surface
of the nonferrous metal which comes into contact with
the extrusion ram to be treated in this way. Since the
parting coating applied by means of the process
according to the invention has good surface adhesion,
it is preferable for those surfaces of the shaping
extrusion dye which come into contact with the
nonferrous metal to be treated, since this treatment
i0 has to be repeated after three extrusion cycles at the
earliest.
It has been found that not all BN powders are
equally suitable for coating purposes. Some powders
lead, inter alia, to a high level of consumption,
irregular coating of the surface, and blockage of the
nozzle of the application device.
Irregular coating may lead to uncontrolled
bonding or adhesion of the nonferrous metal to the
contact point between the nonferrous metal and the
metallic material.
It is preferable to use BN powders with a
fineness which corresponds to a specific surface area
of 10-50 m'/g. Particularly preferably, the BN powders
have a specific surface area of 10--30 m2/g, in
particular a specific surface area of 10-20 m2/g.
The powder furthermore preferably contains less
than 50, particularly preferably less than 0.50, of
boric acid and boron oxides, indicated as B203. The
level of the various boric acids, such as for example
metaboric acid or orthoboric acid and of boron oxides
is, as is customary in the prior art., indicated
according to their analysis as B203 content.
Preferably, the moisture content of the powder,
indicated as H20 content, is less than 1%, particularly
preferably less than 0.20.
Particular preference is given to powders with
a bulk density of less than 0.5 g/cm3, in particular
less than 0.3 g/cm3.
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Particular preference is given to BN powders
which fulfill all the abovementioned parameters.
To carry out the process according to the
invention, the BN powder is fluidized in a storage
container by means of a fluidizing gas. The fluidizing
gas is usually compressed air. Other inert gases, such
as for example N2, noble gases or gas mixtures may also
be used.
The fluidized BN powder is electrostatically
charged. This is carried out, for example, as is
customary for powder coatings, in a commercially
available spray-coating device for powder coatings. A
voltage of 75-80 kV is preferably used to charge the BN
particles.
The fluidized BN powder is sprayed onto the
surface in the charged state and coats the desired
parts uniformly. This is carried out in a similar way
to a conventional powder coating.
Preferably, the BN powder throughput is
adjusted to levels which are suitable for the
particular coating operation by means of the
delivery-gas pressure and the nozzle geometry
(primarily the nozzle diameter). The delivery air is
therefore preferably used at 0.5-1.5 bar, the
fluidizing air at 0.3-0.8 bar. The total volume of air
is approximately 5-8 m3/h.
The operation may either be carried out
manually, or else it is entirely possible for the
process to be carried out fully automatically. The
amount applied and the adhesion can be regulated by
varying the voltage and the pressure.
Surprisingly, it has emerged that the BN
powders which are applied by means of the process
according to the invention exhibit sufficiently high
adhesion to the medium to be coated (e. g. an extrusion
plate) without there being any need to add binders.
The invention therefore also relates to the use
of BN powders as a parting agent when shaping
nonferrous metals and to a parting agent consisting of
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BN powder. Preferably, these powders are the BN powders
referred to as being preferred.
The coating carried out by means of the process
according to the invention lasts longer than the
conventional parting-agent coating. It allows the
extrusion cycle to be repeated several times before a
further application is required.
It has been found that the coating also
succeeds at corners and edges and, furthermore, runs
,~0 around these areas.
The coating process according to the invention
has proven to be extremely simple, quick and easy to
carry out.
Powders which are suitable for use in the
process according to the invention are commercially
available with a wide range of properties.
A BN powder which is preferably suitable for
use in the process according to the invention. may also
be produced from commercially available hexagonal BN
powder.
If the level of boric acids and B203 in the
conventi onal powders is too high, the B203 level in the
powder is set to the desired value by washing, for
example with water or alcohols.
Excessive moisture contents are reduced by
means of drying, for example in conventional drying
cabinets, under certain circumstances with protective
gas or vacuum drying, in order to achieve the levels
required for the powder according to the invention.
If necessary, the bulk density which is
required according to the invention for the BN powder
is set by grinding the BN powders, which may have been
pretreated. In doing so, it must preferably be ensured
that there is no excessive moisture uptake during
grinding.
The grinding units used are a series of
conventional "dry" methods, such as for example by
means of pinned disc mills, rotary mills, ball mills,
~Il Var10ll5 fOrmS.
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Tests have confirmed the ease of application of
the BN powder and the excellent parting action of BN
with respect to A1. The use of the powders is not
limited to A7_; the application according to the
invention of BN powder by means of f~lectrostatic
coating without any binders and free of other
substances such as solvents is also suitable for
processing a whole series of other nonferrous metals,
such as for example Cu, Zn, Mg, Pb, Sn or alloys
thereof.
The invention therefore also relates to the use
of the process according to the invention for coating
surfaces when extruding nonferrous metals.
The nonferrous metals are preferably Cu, Zn,
Mg, Pb, Sn and alloys thereof.
The following examples serve to describe the
invention further:
Example 1:
A BN powder which is com~-nercially available
from Elektroschmelzwerk Kempten GmbH, Munich under the
designation Sl, with a fineness which corresponds to a
BET specific surface area of 19.5 m2/g, a B203 content
of 0.120, a moisture of 0.250 H20 and a bulk density of
0.55 g/cm3 (powder I) was ground in a pinned disc mill
with a throughput of 20 kg/h. The resultant powder
(powder II) had a BET specific surface area of
14.5 mz/g, a B203 content of 0.2o, a moisture of 0.3%
and a bulk density of 0.2 g/cm3.
Powder II was applied to an extrusion plate
with a diameter of 20 cm by means of electrostatic
coating (Gema Company), (80 kV; 0.8 bar delivery air on
inlet side, 0.4 bar fluidizing air, 5 m3/h air volume
for delivery). The powder was instantaneously and
uniformly deposited on the extrusion plate. Approx.
0.5 g of BN powder is consumed. The coating only had to
be renewed after a further three extrusion cyc7_es.
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Example 2:
A hexagonal BN powder with a fineness which
corresponds to a speci fic surface area of 6 m2/g and a
8203 content of 90 (powder III) with a bu:Lk density at
0.6 g/cm3 was ground in ball mill untia specific
surface area of 15 m2/g was reached. The resultant
powder IV was dispersed and washed a number of times
with water, until the B203 content had been reduced to
0.1%. Then, the powder was dried in vacuo at 60°C, to
give a residual moisture of 0.2s, and was ground in a
rotary mill in order to break up the agglomerates . The
resultant powder V (bulk density 0.19 g/cm3, other
parameters as mentioned) was used to electrostatically
coat an A1 block, as described in Example 1. The
consumption was the same as that in Example 1. The
coating was uniform and allowed extrusion plates which
came into contact with the layer to be used three times
for shaping A1 before the coating had to be renewed.
