MASCHINEN UND ANLAGEN
MACHINERY AND EQUIPMENTS
Continuous mixing Constistand process Twin screw extruder Rubber compounds Quality
In recent years a lot of research work has
been done in the field of continuous mixing
of rubber compounds and the trend is that
research on this topic will increase. Additionally powdery and free flowing raw
materials become available, at least in small
quantities. Despite these almost all rubber
compounds are produced in internal mixers
or on open mills.
The present study compares the benefits and
disadvantages of continuous mixing with
respect to batch mixing. Both technological
and economical aspects are viewed in the
present study. It will be shown that only in
case of a drastically change in mind setting or
in case that leading global customers require
a far better consistent production continuous mixing will break through. Even in this
case it is questionable whether the present
rubber industry including machinery suppliers and raw material companies is able to
develop continuous mixing systems on a
commercial scale. A global cooperation
between customer, tyre and technical rubber
articles manufacturers, raw material suppliers and the machine industry is an absolute
necessity to introduce this challenging
technology on a commercial scale successfully.
Kontinuierliche Mischungsherstellung, eine herausfordernde
Möglichkeit
kontinuierliches Mischen robuste
Verfahren Zweischneckenextruder Kautschukmischungen Qualität
Die kontinuierliche Herstellung von
Kautschukmischungen erfreut sich in den
letzten Jahren eines starken Aufschwungs. Darüberhinaus stehen rieselfähige Rofstoffe für die kontinuierliche
Mischungsherstellung in bestimmten
Mengen zur Verfügung. Dennoch werden
für die Mischungsherstellung überwiegend Kneter und Walzwerke eingesetzt.
Diese Arbeit befasst sich mit den Vor- und
Nachteilen der kontinuierlichen Mischungsherstellung in Bezug auf die
klassischen Chargenverfahren. Sowohl
die verfahrenstechnischen als auch wirtschaftlichen Aspekte werden in diese
Studie einbezogen. Gezeigt wird, dass nur
ein drastisches Umdenken oder die
Nachfrage weltweit führender Kunden
eine weit bessere Mischgüte fordern, da
kontinuierliche Mischungsherstellung
zum Durchbruch verhelfen würde. Eine
weltweite Zusammenarbeit zwischen
Kautschukverarbeitern, Rohstoffherstellern und Maschinenherstellern ist notwendig für den Erfolg kontinuierlicher
Mischverfahren.
430
Continuous Mixing,
a Challenging Opportunity1
For almost a century rubber compounds
are produced by internal mixers. These mixers are batch mixers. Especially in the tyre
manufacturing the mixing process has
been optimised with respect to productivity. High rotor speeds combined with high
fill factors enable to increase the productivity and are applied widely. Various text
books deal with the mixing process in
such mixers [1]. Many papers have been
published which deal with the mixing quality obtained by means of batch mixers [2 –
4]. It is well known that the compound
quality is limited due to the energy balance, the flow behaviour in the mixer, and
batch to batch variations. By a adequate
compound design one could correct for
the influence of the mixing process on
the final properties of such a compound.
However the viscosity of the rubber compound and therefore its processability is
depending on the mixing process as
well. This yields the maintaining of rather
big geometry tolerances on extruded profiles [5] and other products made of rubber.
It is well known from thermoplastics processing that continuous mixed compounds
are very consistent and very narrow tolerated. However the feeding of such devices
requires free flowing material. Obviously
such raw materials used to be rarely available for rubber compounds. Recently
powdery rubber became available [6, 7].
Research work on continuous mixing of
rubber compounds have shown that the
degree of mixing could be very high and
its variation is remarkably low [8]. Continuous mixing of rubber compounds has
gained attention recently. Especially since
Pirelli announced the continuous compounding mixer [9].
It is the purpose of this paper to give an
overview on available technologies for
continuous mixing and to compare its benefits with traditional batch mixing. Further work to be done before continuous
mixing is widely accepted and an outlook
in the next future will be discussed as well.
1
Presented at a meeting of the Rubber Division,
American Chemical Society
Requirements to rubber products
of the future
Among a variety of rubber goods the passenger car radial tyre is one of the most
known high technology product. Fig. 1
shows the design of such a PCR tyre
[10]. It is a composition of various extruded
and calendared products which are built
on a tyre building machine before vulcanisation in a tyre curing press. Obviously
wide tolerances have to be maintained
with respect to tread profiles, side wall profiles, inner liners etc. due to varying processing properties of the applied compounds.
In order to ensure a minimum thickness on
each part in the tyre the set point of dimensions has to be increased to become capable. It means that each tyre contains a surplus of rubber material, causing higher
costs. Also the tyre performance is influenced by the surplus of rubber. Both environmental and performance preconditions
will require a much more constant and
consistent production and therefore a
much better and more constant mixing degree.
There is no reason to assume that it would
not yield for technical rubber parts. Generally speaking there will be a drive towards
a much better and more constant mixing
process.
Mixing by means of internal
mixers
Fig. 2 shows the concept of an internal mixer. It is characterised by a mixer entry
chute through which the material can be
fed. The ram or floating weight pushes
the raw material towards the mixing chamber. The mixing chamber has got the shape
of a horizontal “8” in which 2 rotors coun-
G. Nijman, Enschede (The Netherlands)
Corresponding author:
Dr. Gerard Nijman
Vredestein Banden BV
P.O. Box 27, 7500 AA Enschede,
Netherlands
E-mail: nijmang@vredestein.com
KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 9/2004
Fig. 3. Typical fingerprint for masterbatch mixing in a 400 litre internal
mixer
Fig. 1. Concept of a passenger car radial tyre
Fig. 2. Basic concept of an internal mixer
ter rotate. At the bottom a drop door can
be opened to discharge the stock into a
downstream device. If both the ram and
drop door are closed the mixing process
is taking place in a closed mixing chamber.
All the material takes part at the mixing
process.
Basically two rotor types are applied in
the internal mixer, namely the tangential
rotor type and the intermeshing rotor
type. The latter one interferes with the adjacent rotor and turns at the same rotor
speed compulsorily. In the drop door or
through the side plates a thermo couple
is mounted to record the temperature of
the rubber.
The material flow in an internal mixer is
oriented vertically. On top of the internal
KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 9/2004
mixer a more or less sophisticated feeding
system can be found. Underneath the mixer a downstream device, such as an sheeting extruder with one or two tangential
screws, or two roll mills.
The shape of the mixer entry chute allows
the feeding of raw materials in almost any
trade form. Rubber bales, the most widely
trade form for rubbers and polymers, can
be fed as one piece in an industrial sized
mixer. Feeding of material is very convenient, especially for tangential type of mixers.
By recording the motor power, the temperature and the position of the ram the progress of the mixing process can be followed. Such a diagram of a complete mixing
process is called a fingerprint of the batch.
The integrated power, which is the total
energy input, can be added in the fingerprint. Fig. 3 shows a typical fingerprint for
a masterbatch produced on an industrial
sized mixer. It is characterised by a high
temperature increase and a highly fluctuating power input. It can be clearly seen that
the mixing process is highly non isothermal. The maximum temperature determines the total mixing time.
It is well known that the rheo curve, a diagram in which the curing characteristics
are given, is a fairly good measure to determine the consistency of batch mixing.
Fig. 4 is a recognised example for a sufficient batch to batch consistency of a compound with the same recipe. Variations
in curing characteristics are caused partly
by variations in the mixing process. Viscosity variations and therefore variations in
the compound’s processability can be expected as well. This yields variations in extruded profile cross section geometry.
Fig. 5 shows the concept of the internal
mixer proposed by Banbury [11]. It can
431
Fig. 4. Rheocurves of various batches of a compound with the same recipe
Trade forms for polymers and
carbon black
Most polymers do have their final physical
properties at the end of the polymerisation
process. However, a lot of down stream
processes are necessary to get the polymers in their final trade form, see Tab. 1
and 2. Rubber bales are wrapped with a
plastic foil in order to avoid stickiness
with the adjacent bale. Rubber shows a
high tackiness in its uncured stage. This
is the main reason that polymer granulates
are seldom used. A lot of anti tacking coating should be used in order to keep the
Tab. 1. Processing steps during the final finishing of the production of rubber after
polymerisation
Polymerisation in water
Precipitation
Fig. 5. The concept of the internal mixer as
suggested by Banbury (1917)
Isolation from water
Drying
Production of bales
be seen easily that the basic concept of the
internal mixer did not change since almost
90 years. Obviously the advantage of the
easily feeding is preferred with respect to
the rather poor between batch consistencies, which can be achieved with internal
mixers.
Packaging of bales
Tab. 2. Processing steps in the production of
carbon black after the carbon black has got
its final properties
Production of fluffy black
Pelletisation
Drying
432
granulate free flowing, which increases
the raw material cost price.
Carbon black is normally supplied in pellets. It is a cluster of carbon agglomerates,
which can be broken down quite easily.
The size of a pellet is of order 2 mm. A
well pelletised carbon black can be conveyed almost free from dust. However if
the pellet breaks down into carbon black
dust it will not only cause dirt but also longer dosing time at the mixer. After production of fluffy black the pelletisation has to
be done for transportation reasons but the
final properties are not affected.
After dosing the materials into the mixing
chamber of the internal mixer the polymer
bales are ground and the carbon black is
dispersed. Obviously an important task
of the internal mixer is to destroy the trade
form of raw materials before the incorporation and mixing process can start.
Powder rubbers are masterbatches of carbon black and rubber which are put together immediately after ceasing the polymerisation process [8]. The product is a free
flowing granulate, which can be fed automatically into the internal mixer. Another
new development is the gas phase polymerised EPDM together with carbon black [9].
The availability of free flowing materials allows the application of continuous compounding machines, which major disadvantage is the necessity to apply very dedicated particle geometry for a fair feeding.
KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 9/2004
Fig. 6. Typical screw configurations for co-rotating twin screw extruders
Recent developments in
continuous mixing of rubber
compounds
In the last twenty years several attempts
have been made to apply continuous mixing devices for the production of rubber
compounds [14]. The following devices
have been applied:
– Co-rotating twin screw extruders
– Counter rotating twin screw extruders
– Multiscrew extruders
– Planetary gear extruders
– Others.
Co-rotating twin screw extruders are widely applied in the compounding of thermoplastics. The extruder consists of 2 screws
which intermesh each other. The screw
concept consists of modules with different
screw elements. Fig. 6 shows typical examples of screws which are applied in
twin screw extrusion. Fig. 7 shows typical
screw elements which have been used in
twin screw extruder applications in rubber
compounding. A few attempts to apply corotating twin screw extruders into rubber
mixing were reported [8, 15]. The first report deals with the mixing of powder rub-
Fig. 7. Typical screw elements as applied in co-rotating twin screw extruders
KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 9/2004
ber and the main results obtained were a
very high degree of dispersion and a remarkable consistency. The second report deals
with the mixing of rubber out of raw materials which were supplied in their normal
trade form. Polymers were granulated just
prior to their feeding into the extruder. The
basic outcome of this paper is that from a
technological point of view it is possible to
mix rubber compounds by means of counter rotating twin screw extruders. Limper
and Keuter proposed a model for the pressure built up along the screw axes of a counter rotating twin screw extruder [16].
They assumed that such a device can be
presented as a series of reaction vessels.
A typical result of a model calculation is
shown in Fig. 8. It can be clearly seen
that the co-rotating twin screw extruder
is a poor pressurizer. This is well known
from practise.
Counter rotating twin screw extruders are
applied in PVC compounding and processing. This concept has been used by Farrel
for rubber mixing and they call this aggregate the Farrel Continuous Mixer [17]. The
concept, shown in Fig. 9, consists of a
short feeding zone, a mixing zone which
looks similar to typical rotor design as applied in tangential internal mixers and an
adjustable orifice gate. The feeding of
the mixer is starved. The residence time
in the mixer and hence the degree of mixing is determined by the discharge orifice
gate. Although high degree of mixing can
be achieved, due to its low throughput
rate, this concept did not find a wide application.
Multiscrew extruders like the Bühler Ring
extruders are proposed to be applicable
for continuous rubber mixing as well
[18]. This extruder type is in fact an extension of the co-rotating twin screw extruder,
refer to Fig. 10. Several screws are arranged in a ring and 2 adjacent screws intermesh each other, creating relatively more
shear zones than twin screw extruders at
similar throughput ranges. In order to
achieve the same mixing degree a shorter
extruder could be applied with respect to
twin screw extruder, but a homogeneous
feeding seems to be less beneficial.
Planetary gear extruders are widely applied
in PVC processing, since its said favourable
temperature control [19]. The concept is
shown in Fig. 11 and exists of a main
screw, partially responsible for the feeding
and partially acting as a drive for various
gears which rotate between the drive
and the barrel. The pressure built up is
very poor; a gear extruder or a single screw
433
venting extruder and in its upstream a
sort of tangential mixing unit. (Refer to
Fig. 12) The screw speed together with
the rotor speed determines the mixing degree. The feeding units and the reciprocating ram determine the throughput.
Today’s status quo of continuous
mixing
Fig. 8. Typical pressure built up along a screw axis of a co-rotating TSE
extruder should be added in order to pressurise the rubber. As far as known no experimental data are available on mixing of
rubber with the planetary gear extruder.
Among other devices the Farrel MVX extruder seems being applied successfully at
least in some niches of rubber mixing
[20]. It basically consists of a single screw
Tab. 3 contains all benefits and disadvantages of batch mixing with respect to batch
mixing. Tab. 4 contains basically the same
for continuous mixing.
The major advantages of internal mixers are
1. it accepts almost any trade form
2. it is widely applied for several decades. It
is based on proven technology.
The disadvantages of the internal mixers
are benefits of the continuous mixer, namely
1. the absolute degree of mixing is not very
high and
2. the between batch consistency is poor,
especially at high throughput requirements
So if raw materials should be available in
free flowing form, continuous mixing
could be applied in order to ensure high
quality compounds.
Another benefit of continuous mixing is
the better temperature control. This enables the control of a chemical reaction, if
any, and the application of thermal sensitive raw materials, which cannot be used in
internal mixers.
Today it can be stated that continuous mixing can be applied for rubber compounding successfully. The very recent announcement of the CCM (Continuous Compounding Machine) by Pirelli shows that
the benefits of continuous mixing have
found evidence [21]. The basic precondition of applying continuous mixing devices
is the availability of all raw materials in free
flowing form in order to achieve a controlled feeding of the aggregate. Either the
polymers or rubbers have to be supplied
as powders or granulates or advanced granulator techniques (including anti tacking
facilities) have to be applied.
Future scenarios and outlook
Fig. 9. Farrel Continous Mixer
434
The technological benefits of continuous
mixing are indisputable. At present maximum throughput rates of 250 kg/hr have
been reported [22]. For typical tyre applications throughput requirements are at least
ten times higher. Up-scaling of continuous
mixers to fulfil these requirements is necessary but not yet practised.
KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 9/2004
complete program of TSE for rubber mixing [23]. Based on the total turn over of
rubber machinery suppliers the total estimated R&D budget for all companies
might be of order 7 million US Dollar. It
feels that it is too small to finance all activities to be done.
Conclusions: A challenging
opportunity?
Fig. 10. The Bühler Ring extruder
Furthermore obviously no research work is
done in the development of alternative trade forms of traditional raw materials. Advanced and dedicated upstream equipment for a proper feeding of continuous
mixing devices with traditional trade formed raw materials is not yet available.
In special cases continuous mixing is already applied, however:
Continuous mixing might be considered
only in case of required increase of mill
room productivity, or product performance
requires a higher and more constant quality,
and free flowing rubber is sufficiently available, or continuous mixing can be followed
in line by profile extrusion or calendaring.
A dead lock might be raised between general availability of free flowing materials
and the development of continuous mixing devices for rubber. A vicious circle
of this problem clearly shows that initiatives towards a break through in the industrial application of continuous mixing cannot be expected soon.
Furthermore mechanical engineering attention seems to be needed in case of Maximum torque of long screws at high compound viscosity and wear resistance of dispersion screw elements.
Following a recent list of machinery business in rubber the present equipment suppliers seem to be too small to develop a
Continuous Mixing of rubber compounds
is a very promising method in order to obtain a more consistent quality at very high
levels. (E.g. better dispersion and far better
consistency)
However, neither are present trade forms
of traditional raw materials suitable for
continuous mixing processes nor R&D activities on this subject have been reported
so far.
Powdery rubber is an excellent material for
continuous mixing but replaces as a specialty commodity materials.
A break through of application of Twin
Screw Extruders in Rubber Mixing cannot
be expected until the performance of
the final product desires a substantially
more consistent mixing quality, or global
consortia, which include research institutes, raw material suppliers, machine suppliers and rubber processors, are going
to cooperate in the field of continuous mixing.
It would be a great mistake if the rubber
industry would miss this challenging opportunity.
Fig. 11. The Planetary Gear Extruder
KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 9/2004
435
Literature
Fig. 12. The Farrel MVX extruder
Tab. 3. Benefits of Internal Mixers
Benefits
Disadvantages
*
Accepts almost any trade form
*
High power peaks
*
Proven technology
*
Batch to batch variation
*
No need for special dosing equipment
*
heat history
*
Both automatic and manual control
*
material weight variation
*
Efficient dispersion effect
*
Dust forming by air movement
*
Flexibility in run length
*
Labour intensive
*
“easy” to maintain and robust machine
*
Many adjustable parameters
*
High installation costs
*
Complicated downstream
*
Wide application field
Tab. 4. Benefits of continuous mixing devices
Benefits
*
*
*
*
*
*
*
*
Steady power supply, i.e. Steady process and
constant heat history
fine-tuning possible
Labour extensive
Constant high degree of mixing, no between
batch var.
lower erection costs
Higher degree of dispersion because of lack of
dead spots
Direct Extrusion possible
Direct calendaring possible
436
Disadvantages
*
*
*
*
*
*
*
Free flowing raw materials only
Developing technology
complicated dosing machines necessary
Automatic control only
beneficial for long runs only
High mass temperatures (to some extend)
Screw optimisation for one compound
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Kautschukverarbeitung, Carl Hanser Verlag,
München (1989).
[2] W. M. Wiedmann, H. M. Schmid, Rubber Chem.
Technol. 55 (1982) 363.
[3] James L. White, Rubber Processing Technology Materials - Principles; Hanser Publishers, Munich,
Vienna, New York (1995).
[4] G. Nijman, DIK Symposium on Mixing, Hannover
(1993).
[5] G. Nijman, R. Luscalu, Rubberworld, 221 (1999)
27.
[6] U. Görl, K. H. Nordsiek, Kautsch. Gummi Kunstst.
51 (1998) 250.
[7] E. T. Italiaander, Rubber Technology International,
26 (1997) 177.
[8] U. Görl, M. Schmitt, A. Amash, M. Bogun.
Kautsch. Gummi Kunstst. 55 (2002) 22.
[9] Pirelli World, 32 (2002) 4.
[10] www.vredestein.com
[11] F. H. Banbury, U.S. Patent (filed Nov 18, 1916)
1,200,070 (1916).
[12] K. H. Nordsiek, G. Berg, Kautsch. Gummi Kunstst.
28 (1975) 397.
[13] E. T. Italiaander, Gummi Fasern Kunstst. 50 (1997)
456.
[14] DIK Dymposium on Continuous Mixing (german)
held on October 25 and 26, 1999.
[15] G. Capelle, Gummi Fasern Kunstst. 49 (1996)
470.
[16] A. Limper, H. Keuter, Symposium on Mixing DIK
German Rubber Institute; Sept. 11, 2000 (Hannover, Germany).
[17] F. J. Borzenski, Symposium on Mixing DIK German
Rubber Institute; Sept. 11, 2000 (Hannover, Germany).
[18] F. Innerebner, Symposium on Mixing DIK German
Rubber Institute; Sept. 11, 2000 (Hannover, Germany).
[19] M. Roth, Symposium on Continuous Mixing DIK
German Rubber Institute; Oct. 25 and 26, 1999
(Hannover, Germany).
[20] F. J. Borzenski, Symposium on Continuous Mixing
DIK German Rubber Institiute; Oct. 25 and 26,
1999 (Hannover, Germany).
[21] ERJ 184 (2002) 30.
[22] A. Amash, M. Bogun, R. H. Schuster, ACS, Pittsburgh, Oct 2002.
[23] ERJ 183 (2001) 29.
KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 9/2004