Carbon black is regarded as the best reinforcement agent in rubber.  Filler materials such as precipitated calcium carbonate (PCC), zinc oxide, calcium silicates, silicas, clays and magnesium carbonate are used in non-black products.  Kaolin is the main non-black filler mineral used in rubber, accounting for over 50% of all the non-black filler used.  

The majority of the kaolin used in rubber is airfloated hard kaolin with calcined, delaminated and water washed varieties making up the balance.  Water washed grades are also used in some applications such as extrusions where freedom from grit is especially important.  Calcined kaolin is used in heavy-duty insulation rubber and there is also a small but increasing demand for surface treated kaolin in rubber.

 1.1 Specifications of kaolin used in rubber

The specific gravity of a filler mineral needs to be accurately determined to allow accurate formulation of a rubber compound.  Compounds are mixed in parts per hundred of elastomer by weight so the costs of compounds are calculated on the basis of their volume.

High grade clays used in rubber, such as ball clay and kaolin, have a low content of coarse particle defined as greater than 44m.  The typical particle size analysis of kaolin used in rubber is 99.5% less than 44m and between 50% and 97% minus 10m.  A higher proportion of coarse particles will reduce the tensile strength and result in premature flex-cracking, the cause of dielectric failure in insulating rubbers.  Coarse particles also cause premature wear and tear on mixing machines and extruder dies.

If the particle size of the filler mineral used exceeds the polymer interchain distance, it introduces an area of localised stress.  This can weaken the rubber when it is subjected to flexing or stretching.  Fillers with particle sizes larger than 10µ are not generally usually used in rubber.  Fillers with a particle size of 1µ to 10 µ are used as diluents and have no significant elect on the rubber product.  Semi-reinforcing fillers have a particle size of 0.1µ to 1µ.  Reinforcing fillers have particle sizes ranging from 0.01µ to 0.1µ

Metal impurities in fillers adversely affect the wear and ageing properties of filled rubber.  Standard ageing tests are used to evaluate the extent to which the presence of iron, copper and manganese reduce the durability of rubber.  The level of impurities present, and the specific impurity, leads to variations in the durability of different types of rubber.  For example, iron has a more serious effect on the durability of styrene butadiene rubber (SBR) than on natural rubber.  Producers of different rubber products often have quite different requirements for the levels of metallic impurities in minerals they use.

The pH of a kaolin grade for use in rubber is generally specified at between 4.5 and 5, although more alkaline clays are used in epoxy and vinyl systems.  Fillers with an excessively acidic pH accelerate the curing of a rubber formulation.  This causes premature curing in the mould, a process known as scorching.  Fillers that are too alkaline have the opposite effect and require the addition of accelerators to maintain the production process, increasing costs.

Clays used in rubber are classified as either hard or soft, depending mainly on particle sizes and the resultant effects on rubber formulations.  Hard clays generally have a slightly smaller particle size than soft clays and their use produces a stiffer uncured compound.  The use of hard clays results in a higher tensile strength and abrasion resistance in the cured compound compared to soft clays.

A water-settling test is used to determine if a clay is hard or soft.  A dry sample of the clay is suspended in distilled water by means of vigorous stirring and the suspended material is then allowed to settle.  Since the coarse particles separate from the fines, the height of the settled clay in the column after twenty-four or forty-eight hours gives an indication of the particle distribution of the clay and therefore its probable reinforcing properties.  

Kaolins classified as hard are mainly used in non-black rubber goods where high abrasion resistance is important.  These applications include as shoe heels and soles, floor tiles and mats, wire and cable insulation, conveyor belts and bicycle tyres.  The addition of hard clays to uncured rubber increases the stiffness of the product, which is important to prevent sagging or collapse of products.  Soft kaolin grades are principally used in household products, toys, rubber clothing and other moulded goods.  Soft kaolin is also used as an additive in rubber to prevent problems in the mould, which might be caused by the softness of the uncured rubber, which are not severe enough to require the use of hard kaolin.

The leading use for both synthetic and natural rubber is in the manufacture of tyres.  The main filler used in rubber for tyres was formerly carbon black, today a mixture of carbon black and silica is also used.  Untreated kaolin cannot provide the reinforcement required but silane treated kaolins, which have small and irregular particles, can be used in rubber destined for tyres.  However, these kaolins are more expensive than carbon black or silica and produce a tyre with a lower equivalent strength so are not widely used at present.