Specification of Fillers for Elastomers

The specification for fillers for rubber varies considerably according to the natnre of the product. Users of fillers must also be quite clear of the fact that no two fillers are exactly the same. Conventions that categorise fillers such as the American Society for Testing and Materials (ASTM) system for blacks provide only a system for grouping products with similar characteristics. If two fillers with identical specification values are compared in critical products, significant differences in performance will still often be found. Filler interchangeability must be approached with a certain degree of caution. 

Fillers vary in particle size by many orders of magnitude, their production and processing methods vary enormously, and the chemical nature of the types available varies considerably. Probably the most important single requirement of any product is batch to batch uniformity. Properties most usually described are  

Moisture content Mean particle size Colour (for non-blacks) Surface area Oil absorption pH 

Sieve residue Particle size distribution Impurity levels Iodine number (blacks) Chemical composition Density 

Moisture content should be controlled to be as low as possible, bearing in mind the surface area and chemistry of the filler. Low surface area fillers with hydrophobic surfaces will naturally maintain a low moisture content once dried. High surface area fillers with hydrophilic surfaces, such as precipitated silica, will regain moisture from the atmosphere rapidly and relatively high levels of moisture (5-7%) are unavoidable. Other minerals such as kaolin are also sensitive to moisture regain, but to more acceptable levels of moisture (0.5-2.0%). Total absence of moisture, however, should be avoided as water plays a subtle role in many crosslinking reactions. 

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