Tyre Asia – May-June 2018

Tyre Asia April/May 2018 95

the current, power, ram movement and
rotor speed process parameters of a
reactive base compound produced in
an internal mixer. First, the fillers are
incorporated until the dispersion is
complete and there is no free filler. At
the same time, the temperature increases
until the reaction temperature is
reached. The rotor speed is then reduced
and regulated to keep the compound
temperature constant. An overshoot
would be harmful, as the compound
could start to cure. At this point, it
becomes clear again that the cooling
behaviour of the rotors is important
because an efficient process must be able
to approach the maximum permissible
reaction temperature as closely as
possible. According to Arrhenius [2],
the reaction speed is directly influenced
by the temperature - the higher the
temperature, the faster the reaction
will be.

Assuming that the dispersion phase
lasts as long as the reaction phase, the
process could be divided exactly in the
middle with a tandem mixing system
compared to a single internal mixer. As
already described, the dispersion phase
would have to run in the upper mixer and
the reactive phase in the tandem lower
mixer.

Images 7 and 8 show the individual
processes in the upper and lower mixers.
It can be seen that the process sequence
in the lower mixer looks different from
the corresponding process in the internal
mixer as shown in Image 6. First, a warm
and almost homogeneous mixture is
conveyed into the lower mixer, and is
drawn in by the latter. Intake problems
cannot be present as performance is
required immediately. Because the
mixture is transferred warm, no power
peaks are recognisable, as can be seen
in the internal mixer at the start of the
process. This means that the drive can
be “relatively small” and does not require
high overload factors.

If, as shown in this example, it is possible
to divide the process precisely in the
middle, then the tandem process would
halve the cycle time or double the
throughput of the line compared to the
conventional internal mixer.

Degassing and adhesion

issues

A major problem with reactive mixtures

today is their porosity and stickiness.

Porosity is a sign of incomplete degassing

of reaction products and water from the

raw materials. In this respect, the open

system of the tandem lower mixer gives

it a great advantage (missing ram). It

is very easy to connect an extraction

system between the upper and lower

mixer from the transfer shaft, which

can effectively remove the resulting

reaction products and evaporating water.

It is readily conceivable that extrusion

processes can be improved with lower

porosity of the mixture.

The problem of stickiness of reactive

mixtures can be explained, among other

things, by an incomplete reaction. The

tandem principle ensures that just as

much time is available for the reaction

sequence, as is for the incorporation and

dispersion phase (upper mixer process).

As explained in Section 2.3, relatively

high speeds can be applied in the tandem

sub-machine a) higher rotational speeds

can be driven, b) larger surfaces are

offered to the material to be mixed, and

thus thinner layers are present. This

should reduce the tendency of adhesive

bonding on discharge rollers, e.g. on

conVeX discharge extruders

(see Image 9).

Economic aspects

The question about possible application

possibilities of the tandem process will

be examined below and related economic

aspects discussed. For this purpose,

simple examples of possible process

divisions are defined, which certainly

need to be understood as theoretical

borderline cases.

In principle, these considerations

should be addressed based on the

possibility of the process division. “Black”

mixtures and reactive mixtures have

to be distinguished in the preparation

of base compounds. In the case of

“black” base compounds at least the

so-called “black incorporation time”

(BIT) must be overrun so that there is

no open filler present. Discharge cannot

take place beforehand. Base mixes

are often mixed for longer so that the

dispersion is advanced further. Image

10 illustrates this schematically, where

a clear overrun of the BIT is assumed. It

can be estimated that up to 25 per cent

of the mixing time could be saved by

transferring the process from a single

internal mixer to a tandem line. The BIT

also needs to be safely overrun in the

tandem upper mixer before transfer to

the lower mixer can take place.

In addition to the 25 per cent saving

on mixing time or throughput increase,

the approach would have the added

advantage that the mixture stays just as

long in the lower mixer as in the upper

mixer, which actually would lead to a real

extension of the mixing time of 50% in

the sample calculation. As many mixtures

in the tyre sector are run in several

stages, the possibilities for reducing the

mixing step are opened up.

As previously described, for reactive

batches such as silica compounds the

process can be divided down the middle,

as shown in picture 11. This results in

half the mixing time, or rather a 100%

increase in throughput.

Image 6: Example for mixing of reactive mixtures
with a mixer with ram

Image 7: Example for mixing of reactive mixtures

with Tandem mixer – Process in the upper mixer

Image 8: Example for mixing of reactive master

batch with Tandem mixer – Process in the

lower mixer

Image 9: Adhesive bonding on discharge rollers

Image 10: Schematically process illustration

“black” masterbatch