MASCHINEN UND ANLAGEN
MACHINERY AND EQUIPMENTS
Planetary roller extruder Continuous
mixing Rubber/Filler composites
This paper describes the experimental
work carried out during investigations on
the continuous production of a silica filled
rubber mixture with a planetary roller
extruder. For the investigation, free
flowing Rubber/Filler Composites (RFC)
were used. These Composites are based on
an emulsion styrene butadiene rubber
(E-SBR) with silica and silane. The reaction
between the filler and the silane is performed during the production process of
the composites. Therefore the composites
contain a completely silanized silica. Under utilization of the high cooling performance of a planetary roller extruder, all
mixture components, including the crosslinking chemicals were dosed into one
feeding section. No scorch of the produced
mixture is determined. The mixture characteristics, like filler dispersion and crosslinking behaviour, and the properties of
the vulcanised rubber, like tensile strength
and hardness correspond to the characteristics of the reference mixture obtained
from the internal mixer. With the continuous process the specific energy input is
three times less than that in the internal
mixer.
Der Planetwalzenextruder als
kontinuierliches Mischaggregat für
kieselsäuregefüllte Kautschukmischungen
Planetwalzenextruder Kontinuierliches Mischen Rubber/Filler Composites
Die experimentelle Arbeit stellt Untersuchungen zur kontinuierlichen Herstellung einer kieselsäuregefüllten
Kautschukmischung dar. Es werden
rieselfähige Rubber/Filler-Composites
(RFC) mit abgeschlossener Silanisierungsreaktion eingesetzt. Unter Ausnutzung der hohen Kühlleistung des
Planetwalzenextruder werden alle
Mischungsbestandteile, inklusive der
Vernetzungschemikalien in das Füllteil
des Extruders zugegeben. Es wird keine
Anvulkanisation der so erzeugten Fertigmischung im Planetwalzenextruder
festgestellt. Die Mischungseigenschaften, wie Füllstoffdispersion und Vernetzungsverhalten, und die Vulkanisateigenschaften, wie Reißfestigkeit
und Härte entsprechen den Eigenschaften der Referenzmischung aus
dem Innenmischer. Charakteristisch ist
ein um den Faktor 3 geringerer spezifischer Energieeintrag im kontinuierlichen Verfahren.
Continuous Mixing of Silica Filled
Rubber Mixtures with a Planetary
Roller Extruder
During the production process of a rubber
product, the mixing of the compound represents the first and most important
step. Traditionally the mixing takes place
in large volume internal mixers, which consist of mixing chambers with 2 revolving
rotors. This technology has remained essentially unchanged over the last 90 years.
Since the patenting of the internal mixer
for rubber in the year 1914 [1], advancements within this technology were concerned mainly with specific solutions as for
example different rotor geometries or the
variation of the distance between the axes
[2, 3].
During the discontinuous process the
quantities of different components are
fed into the internal mixer according to
fixed recipes and orders. The advantage
of this procedure is the high flexibility
regarding different recipes and mixing orders. A disadvantage of the discontinuous,
batch production of rubber mixtures in the
internal mixer is however that this process
can lead to a difference in the mixture quality from one batch to another [4]. For
example, well known is the so-called “First
Batch Effect”. This effect arises with the
first batch of one recipe due to a too
low chamber temperature while starting
the internal mixer and this leads to a mixture of lower quality. Therefore, frequently
the first up to the fourth mixture are rejected. A further disadvantage of the discontinuous process is that, due to the rising
temperature of the mixture in the internal
mixer the cross-linking chemicals have to
be added in cost intensive separate stages.
With the help of continuous mixing processes, batch to batch differences in the
quality can be prevented due to the constant dosage of the mixture components
and constant process conditions. The continuous mixing can be done with multiscrew extruders like planetary roller extruders, twin screw or ring extruders. Due to
the high cooling capacity of multi-screw
extruders, the temperature development
along the process unit can be controlled.
Further advantages of this process are reduction of energy consumption, simplified
process control and smaller environmental
KGK Kautschuk Gummi Kunststoffe 58. Jahrgang, Nr. 7-8/2005
impacts because of the decrease in dust,
gases and reaction products. These advantages are obtained with comparable or
better mixture and product properties
like the filler dispersion or the tensile
strength [5 – 12].
Since the introduction of the silica filled
rubber mixtures [13], these mixtures
form the basis of passenger car tires
with small rolling friction, good wet slide
and winter characteristics. These mixtures
are based on solution styrene butadiene
(L-SBR), butadiene rubber (BR), high-activity silica and a bifunctional organosilane as
coupling agent (e.g. SI 69). Due to silanol
groups on the silica surface and the resulting high interaction forces between the
particles as well as the incompatibility of
silica and the rubber, silanes are added
into the mixture. These silanes react with
the silica and improve the dispersion behaviour, so that the incompatible phases silica
and rubber are linked with one another by
covalent bonds. The production of silica
filled rubber mixtures is usually done in
the internal mixer [2, 3] within at least
three process step. This is highly energy
and cost-intensive. The internal mixer serves as a chemical reactor. The reaction runs
in a temperature range of 140 – 150 C
and takes place in about 3 – 4 minutes.
In order to maintain the temperature range, it is necessary that the production of
the base mix takes place in 2 steps. During
the reaction, ethanol is produced. The addition of cross linking chemicals takes place in an additional third production step
with a maximum temperature of 120 C
[14, 15].
The continuous production of silica filled
rubber mixtures is described by Eswaran
S. Luther, M. Bogun, Hannover,
H. Rust, Bochum
Corresponding author:
Dr. Sabine Luther
DIK e.V.
Eupener Str. 33
30519 Hannover
Tel.: 05 11/8 42 01-0
Fax: 05 11/8 38 68 26
371
Fig. 1. Sectional drawing of a cylinder assembly
et al. [16] in a patent from the company
Goodyear. Herein, several concepts are
introduced using single and double screw
extruders and combinations of it. The
Continuous Compound system (CCM)
invented by Pirelli [7] is also a continuous
production process of silica filled rubber
mixtures. Due to long reaction time of
the reaction and different process temperatures, the mixing line consists of two
double screw extruders coupled with a
buffer silo. The first extruder works within
a temperature range of 150 C to 200 C.
Within this extruder, the reaction takes
place. The manufactured base mix is
then granulated and stored temporarily
in a buffer silo. The addition of the
cross-linking chemicals takes place within
the second extruder with a maximum
mass throughput temperature of 120 C.
In the described continuous processes by
Goodyear and Pirelli, granulated rubber
and free flowing silica are used as raw materials.
Fig. 2. Roller model with “old” and “new” thermodynamics
toothed cylinder. Due to the helical gearing
of the planetary system a continuous rolling out of the material is built up at simultaneous discharge in direction outlet of the
roller part. This guarantees the best selfcleaning effect of all compounding extruders. When using an open stop ring,
practically all the material – except by small
material residues – will be rolled out of the
roller part so that e. g. very economic changes in colour resp. formulations can be
achieved. Due to extremely thin wall thickness (Fig. 2) of the cylinder assembly and
the central spindle, an excellent controlling
of temperature in the contact surface is obtained. With the introduction of pressure
water heating and an improved cooling
channel construction which allows a product-near temperature control and thus influencing decisively the heat transmission.
Hence this system has found an increasing
acception.
The modular design means that several cylinders can be flanged together. Whereas
the central spindle is covering the whole
processing length, every module can be
equipped with different number of planetary spindles. The single cylinder sections
are connected to each other via intermediate stop rings. The planetary spindles
are running against these stop rings.
Due to the planetary spindles and the variation of the stop ring diameter, the dwell
time of the melt and pressure build-up can
be varied. As presented in Fig. 3 planet
spindles with different tooth geometry
are available. The free cross-sectional
area can be adjusted over grooves in the
central spindle or the diameter of the dispersion rings (Fig. 4).
Motivation
The motivation of this work is the development and optimization of a continuous
production process for silica filled rubber
mixtures by a planetary roller extruder
with use of free flowing Rubber/Filler
Planetary roller extruder
Planetary roller extruders are used today
for mixing of powder coatings, thermoplastics, products of the chemical and
pharmaceutical industry as well as in the
food industry. The process part of the planetary roller extruder (Fig. 1) essentially
consists of a central spindle, a cylinder
and circulating planet spindles. The central
spindle and the cylinder are temperature
controlled. The dosage of the material
into the process part takes place in a temperature controlled feeding zone. The drive is carried out via the central spindle
which, in turn, distributes the torque to
the planets. These roll off in the 45 helical
372
Fig. 3. Variations of planetary spindles, special design
Fig. 4. Grooves in the
central spindle and
dispersion Rings
KGK Kautschuk Gummi Kunststoffe 58. Jahrgang, Nr. 7-8/2005
Tab. 1. Composition of the RFC Types
type
latex
silica
silane
zinc soap
RFC 1215
E-SBR 1712
100 phr
U 7000 FC
76 phr
Si 75
6,2 phr
EF 44
4 phr
RFC 1212
E-SBR 1721
100 phr
U 7000 FC
76 phr
Si 75
6,2 phr
EF 44
4 phr
Tab. 2. Recipe
Fig. 5. Free flowing Rubber/Filler Composites
based on E-SBR/silica/silane
Component
phr
RFC 1215
132,9
oil
35
Premix 1
Composites (RFC). The Rubber/Filler Composites (RFC) are available as development
products. The RFC contains completed
silanized silica [19, 20]. The investigations
show that with the utilization of the high
cooling efficiency of planetary roller extruders the cross-linking chemicals can be added directly into the extruder and therefore
the mixing can take place in a single-step
continuous process. The maximum mass
temperature of the rubber mixture of
120 C is not exceeded. The comparison
of the properties obtained for the rubber
mixture and the vulcanised product as
well as the energy inputs show a high potential for the continuous process.
Materials and process conditions
Two different RFC types based on E-SBR/
silica/silane are used. The RFC additionally
contain 4 phr zinc soap as a processing aid
during mixing. The zinc soap shall lower
the Mooney viscosity [21, 22]. The RFC
granulates have a size range of 0.5 –
1.5 mm (Fig. 5). The composition of the
RFC types and the used recipes of the
rubber mixture are represented in Tab. 1
and 2.
Premix 2
RFC 1212
51,1
Tab. 4. Characteristic Data of the planetary
roller extruder Entex TP-WE 70/1200 M3
Parameter
Length of cylinders
400 mm
Number of cylinders
3
26 kW
ZnO
3
Power
stearic acid
2
Max. torque
170 Nm
6PPD
1,5
Max. screw speed
140 1/min
wax
1
DPG
2
CBS
1,5
S
2,3
Sum
232,3
Tab. 3. Mixing order
Tab. 5. Configuration of planetary spindles
type
cyl. 1
cyl. 2
cyl. 3
planetary spindle
1
2
2
nap spindle 1
3
3
1
nap spindle 2
3
2
3
Step time
1
0 – 2 min
2 – 4 min
RFC 1215 und RFC 1212,Úl,
ZnO, Stearic acid, Wax, 6PPD
mixing (final temp. = 150 C)
24 h storage
2
0 – 2 min
2 – 5 min
batch step 1
mixing (final temp. = 150 C)
24 h storage
3
0 – 2 min
batch step 2+DPG, CBS, S
(final temp. = 100 C)
The production of RFC reference mixtures
are made in an 1.5 l laboratory internal
mixer with intermeshing rotors (Werner
& Pfleiderer) according to the mixing order
shown in Tab. 3.
The characteristic data of the planetary
roller extruder and the planet spindle configuration used in this work from the com-
pany Entex Rust & Mitschke are listed in
Tab. 4 and Tab. 5. The structure of procedure is illustrated in Fig. 6.
The addition of RFC and further components like activators, antiaging- and ozone
agents as well as cross-linking chemicals
into the planetary roller extruder is done
via gravimetric dosing systems. For the reduction of dosing equipment, premixes are
made from the mixture components. Premix 1 consists of RFC 1212, ZnO, stearic
acid, 6PPD and wax. The cross-linking chemicals DPG, CBS and S are mixed as premix
2. The softener oil is heated to a moderate
temperature of 60 C and injected into the
roller cylinder between the feeding section
and first cylinder.
Fig. 6. Process configuration
for the continuous mixing process
with a planetary roller extruder
KGK Kautschuk Gummi Kunststoffe 58. Jahrgang, Nr. 7-8/2005
373
Tab. 6. Processing parameter and mixture
characteristics for the reference mixture
mixed in the internal mixer
Spec. energy input [kWh/kg]
1,680
Mooney viscosity
61
Dispersion index [%]
98
0
0
Smax
ÿ Smin
[dNm]
14,5
Scorch time [min]
3,8
Characterization
The manufactured mixtures are milled to
4 mm thick sheets on a roller mill and vulcanised in an electrical press as 2 mm and
6 mm thick plates at a temperature of
165 C. The properties of the raw rubber
mixture, the Mooney viscosity as well as
the properties of the vulcanised plates
(e.g. tensile strength) are measured according to the standardized procedures. The
determination of the filler dispersion are
carried out with the top illumination-microscopic dispersion index analysis system
(DIAS) [23].
Results and discussion
The characteristics of the rubber mixture
mixed in the internal mixer are specified
in Tab. 6. Here a high specific energy input
can be recognized.
Usually during the continuous production
of rubber mixtures the cross-linking chemicals are added shortly before the extrusion
exit [5 – 10] similar to the procedure during
the discontinuous mixing with an internal
mixer. In this work the cross-linking chemicals as well as all further mixture compo-
Tab. 7. Comparison of process parameters and properties of the vulcanizates for the
continuous and discontinuous mixing process
internal mixer
throughput
spec.energy [kWh/kg]
0
SMax
ÿ
0
SMin
–
15 kg/h
40 kg/h
3 steps
11 min
70 1/min
120 1/min
1,680
0,6
0,425
14,5
14,4
14,9
tensile strength[MPa]
[dNm]
23,3
22,1
22,0
elongation at break [%]
497
639
616
hardness(Shore A)
65
63
65
nents are added together at the feeding
section.
In Fig. 7, the specific energy input and the
Mooney viscosity is represented as a function of mass throughput for constant filling
degrees of the extruder. It is recognizable
that with increasing mass throughput
the specific energy input decreases, and
the Mooney viscosity increases. These effects are due to the fact that with increasing mass throughput the rotation speed is
increased for adjustment of the constant
filling degrees from 70 1/min for a mass
throughput of 15 kg/h upto 140 1/min
for a mass throughput of 50 kg/h. The increase of the rotation speed leads to a rise
of the mass temperature in the process
volume and thus to a reduction of the mixture viscosity and the shear stress. In Fig. 8
the adjusting mass temperature between
the roller cylinders as well as the exit temperature is represented as a function of
mass throughput. For a throughput of
15 kg/h in the roller cylinders and at the
extrusion exit a constant mass temperature
Fig. 7. Mooney Viscosity and specific energy input as function of mass
throughput for constant filling degree of the extruder
374
planetary roller extruder
of 75 C is measured. If the throughput is
increased, the mass temperatures increases linearly. In roller cylinders 1 and 2 equal
temperatures are measured. At the exit of
cylinder 3 lower temperatures result. A maximum permissible mass temperature of
120 C is reached with a mass throughput
of 40 kg/h.
Tab. 7 shows the energy inputs, the crosslinking density (S’Max-S’Min) as well as the
properties of the vulcanised product like
ultimate tensile strength, elongation at
break and hardness of the silica filled rubber mixtures from the discontinuous and
the continuous mixing process. The continuous mixing process with a planetary
roller extruder is characterised by a clearly
decreased specific energy input compared
with the discontinuous process of around
35%. The specific energy input of the continuously manufactured rubber mixture results with approximately identical properties of the vulcanised product as ultimate
tensile strength and hardness. The filler
dispersion and the scorch time are only
Fig. 8. Mass temperature as a function of throughput at different
positions along the extruder axis
KGK Kautschuk Gummi Kunststoffe 58. Jahrgang, Nr. 7-8/2005
Literature
extruder, it is possible to add the crosslinking chemicals together with all further
mixture components in the feeding section
of the extruder. No scorch is evident in the
manufactured mixture. The mixture characteristics, like filler dispersion and crosslinking density (S’Max-S’Min) and the
properties of the vulcanised product, like
tensile strength and hardness correspond
to the characteristics of the mixture mixed
on the internal mixer. The specific energy
input is three times smaller with the continuous process. The higher values of the
Mooney viscosity in relation to the reference mixtures are to be reduced in further investigations by optimisation of the process
characteristics as well as by the employment of new types of silica filled RFC containing oil.
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[2] W. Hoffmann, H. Gupta: Handbuch der Kautschuktechnologie, Dr. Gupta Verlag, Rattingen,
2001.
[3] F. Röthemeyer, F. Sommer: Kautschuktechnologie,
Werkstoffe-Verarbeitung-Produkte, Carl Hanser
Verlag, München; Wien, 2001.
[4] E. Haberstroh, Chr. Linhart, GAK 55 (2002) 722.
[5] G. Capelle: GAK 49 (1996) 470.
[6] E. T. Italiaander, GAK 50 (1997) 456.
[7] D. Shaw, European Rubber Journal 184 (2002)
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[9] A. Amash, M. Bogun, R. H. Schuster, U. Görl, KGK
55 (2002) 367.
[10] M. Bogun, R. H. Schuster, U. Görl, H.-J. Radusch,
Tire Technology International, June 2003, 22.
[11] M. Kewitz, T. Malzahn, Plastverarbeiter 52 (2001)
138.
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März 2003.
[13] Michelin, European Patent 0 051 227 (1995).
[14] U. Görl, GAK 51 (1998), 416.
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[16] V. Eswaran, C. Kiehl, F. Magnus, P. Handa, USPatent 5,711,904 (1998).
[17] D. Shaw, European Rubber Journal 184 (2002)
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[20] U. Görl, M. Schmitt, KGK 55 (2002) 502.
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Acknowledgement
The authors
We thank the Degussa AG for the supply
of the Rubber/Filler Composites.
S. Luther is head of the processing department
of the German Institute of Rubber Technology,
(Deutsches Institut für Kautschuktechnologie e.V.,
DIK). M. Bogun is a PhD student also at the DIK
and H. Rust is company owner and managing director of Entex Rust & Mitschke GmbH.
Fig. 9. Scorch time
and Dispersion index
as function of mass
throughput
slightly affected by the output (Fig. 9). The
values of these mixture characteristics are
similar to the values achieved during the
discontinuous mixing process. The constant scorch time and cross-linking density
(S’Max-S’Min) for the entire throughput
range confirm that it is possible to add
the cross-linking chemicals at the same
time with all further mixture components
in the feeding section. This is due to the
high cooling capacity in the planetary roller
extruder.
Summary
Based on free flowing rubber/filler Composites (RFC), investigations were accomplished for the continuous production of silica filled rubber mixtures with a planetary
roller extruder. With this single step process, silica filled rubber mixtures were
manufactured with the use of RFC based
on E-SBR/silica/silane. Due to the high
cooling performance of the planetary roller
KGK Kautschuk Gummi Kunststoffe 58. Jahrgang, Nr. 7-8/2005
375