Full definition
Particulate inorganic material added to rubber compounds to reduce cost, modify physical properties, and improve processing characteristics. Major types: (1) Calcium carbonate (CaCO₃, whiting) — the most common non-reinforcing filler; 50-200 phr for cost reduction; ground (GCC) or precipitated (PCC); increases hardness without significantly reinforcing. (2) Kaolin (hydrated aluminum silicate/china clay) — semi-reinforcing; 20-80 phr; improves electrical insulation and acid resistance. (3) Talc (magnesium silicate) — improves tear resistance and surface finish; 10-50 phr. (4) Precipitated silica — reinforcing filler (see separate entry). (5) Barium sulfate (barytes) — high density for sound damping and X-ray shielding. (6) Mica — improves gas barrier properties and dimensional stability. Filler particle size and surface treatment affect reinforcement: smaller particles (<1 μm) and surface-treated (silane, stearic acid) fillers provide better properties. Per ASTM D1765 (carbon black) and D6738 (fillers). Non-reinforcing fillers are sometimes called extenders because they extend (dilute) the more expensive polymer.