Fillers in polymers:

Fillers are an essential part of plastics and rubber products. The word “filler” implies some material which will help in cost reduction of rubber and plastics products.

In rubbers, carbon black is an essential component. Carbon black is a filler, though cannot be called a cheap material. Carbon blacks improve mechanical properties and also processibility (especially during extrusion) of rubbers. The two problems with carbon black are:

i)they increase viscosity of the rubber compound and

ii)they increase hysteresis of the rubber which can reduce the service life of the product especially tyres.

On the other hand, inorganic fillers like ground silica, clay and calcium carbonate do not improve properties to that extent. These fillers are of very low cost.

In plastics, inorganic fillers do not improve strength but help in reducing shrinkage in moulds and thermal expansion coefficients. They are also useful in giving colours to the product to some extent (though they cannot replace colourants totally). Some fillers, either alone or in conjunction with special plasticisers help in increasing flame retardancy of plastics. Only fibrous fillers can improve mechanical properties of plastics and these too only with proper surface treatments.

Inorganic fillers pose problems in mixing with rubbers as they are polar while polymers are much less polar than them. Thus the particles form agglomerates and this is the reason why fillers often reduce tensile and impact strengths of polymers. With proper surface treatments they can be made to improve properties of rubbers and plastics.

Mostly fillers have particle sizes(diameters) in micron range. For the past 20 years, interest in using filler particles in nano scale dimensions is increasing all over the world. Nano fillers are even more difficult to mix in a rubber or a plastic compared with the normal fillers.

Nano composites are made of polymers and fillers(in nano sized particles). Nano fillers may exist in nature or can be made artificially. A few types of nano fillers are: nano clay, nano silica, nano titania, carbon nano tubes. The primary carbon black particles are also nano sized particles-they are pelletized to larger sizes so that they can be easily mixed into the rubbers.

Nano clay – polymer composites:

The important class of nano filler is a type of clay called “nano clay”, available in a few parts of the world. Nano clay consists of particles in the form of sheets of alumina and silica layers(each sheet is about 200-400nm long and wide and less than 10 nm thick) separated by about 7nm distance between the layers. In this gap, sodium ions are present. If these ions can be replaced by cations of larger sizes(example quarternary ammonium cation R4N+ large spaces will be available for polymer molecules to pass through them and get locked up. The R4N+ group will also enable the clay to attract the less polar polymer chains to pass through the gaps(called galleries). Such a surface treated clay is manufactured by a few companies in the world. This filler is very expensive but the improvement in the properties is excellent. We shall see more details about their preparation and uses in the later issues.

Dr B.Kothandaraman
Professor, Rubber and Plastics Technology
MIT Campus, Anna University, Chennai-600044
28.07.2012