A Practical Approach to Rheology and Rheometry

Rheology
10.6.5 Testing the oil absorption of carbon blacks
Carbon blacks are of utmost importance for the quality of rubber products and
especially of car tires. Carbon blacks are graded in several aspects out of
which the following two are very important: the free surface area of the primary
carbon black particles given in units of square meters per gram – a value of
150 m 2 /g being not unusual – and the so-called “structure” of the carbon black
defining the ability of this filler to adhere onto rubber molecules. Both
characteristics of carbon black are difficult to determine. Mixing different
batches of carbon blacks with rubbers which may be of unknown molecular
structure will leave an uncertainty with respect to the carbon black
performance. This leads to a revised test method: instead of testing the absorption
of carbon blacks onto rubbers one determines the absorption of a low viscosity
DBP-plasticizer – dibutylphthalat – in a batch of carbon black. DBP is an
oily chemical which can be easily produced within tight specifications and it
therefore provides the standard for the ability of the carbon black to absorb it.
Again a mixer test provides an automated test of carbon black batches. Special
mixer sensors – e.g. of the Cabot design – are filled with a specified volume
of the carbon black powder. While the rotors rotate tumbling this dry powder
a burette is feeding the DBP into the mixing chamber at a rate of 4 ml/min. The
normal time scale on the abscissa of the rheogram can be converted into a
scale of the added DBP in units of ml/min. Depending on the type of carbon
black and its structure, the mixed powder and oil suddenly will ball-up: they
agglomerate to some high viscous dough resulting in a very sudden torque
increase. – Fig. 154 a. A computerized evaluation of this rheogram will define
the amount of DBP added before the torque curve reaches e.g. 70% of the
peak torque and it is taken as a characteristic value that can be linked to the
absorption of the filler into rubber compounds. Of course, one can also
incorporate carbon black samples directly in polymers and determine what the
relative effect the addition of a particular carbon black has on the torque/
viscosity of the rubber compound – Fig. 154 b.
These tests are typical relative tests for the quality control of substances in
which the change of composition is rheologically determined resulting in data
only meaningful on the background of similar tests with standard carbon
blacks. Especially the the DPB-absorption test which initially was probably
quite empirical has proven its success in perhaps more than 100 000 tests
worldwide for the differentiation of carbon black batches.
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