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Kaolin in Technical Rubber


Kaolins (clays) find use in a variety of applications, either because of specific technical properties and/or because of their very favourable cost / performance ratio.


Fine kaolins have semi-reinforcing properties and may be used as either the primary filler in rubber or in order to partially replace or extend more highly reinforcing carbon blacks. Less fine (filler) kaolins confer lower levels of reinforcement and may be more readily used at higher volume loadings. In addition, the processing properties of filler clays are often more advantageous, since they are easier to disperse efficiently. If light coloured articles are to be produced, then kaolins such as Speswhite™ & Polwhite™ E or PoleStar™ 200R calcined clay can be used in brightly
coloured compounds.

All kaolin fillers share some common features because of the platelike shape of their particles, which results in some stiffening of the rubber compound. This gives much better extrusion and
calendering properties (exhibiting low die swell and good dimensional stability) than can be obtained from most other mineral fillers.


Washing Machine Seal                                 Flooring Compound


Formulation phr
Vistalon 3666 175
Zinc oxide 5
Stearic acid 1
Titanium dioxide 20
Speswhite™ 100
Polyvest 25 2
Sunpar 2280 25
Struktol WB212 3
TMTD 0.8
Tetrone A 0.8
ZBDC 2.0
Sulphur 2.0
Tensile Strength (MPa) 10.7
Modulus at 300% (MPa) 2.8
Elongation at break (%) 800
Tear Strength (N/mm) 20
Hardness (° Shore A) 45
Compression set at 70°C (%) 24.4
Compression set at 100°C (%) 31.3
Formulation phr
Vistalon 4608 60
Vistalon 2504 40
Zinc oxide 5
Stearic acid 2
Paraffin wax 5
Polwhite™ E 200
Sunpar 150 30
Sulphur 1.5
ZDC 0.5
TMTD 0.5
MBT 1.5
Tensile Strength (MPa) 5.8
Modulus at 300% (MPa) 3.7
Elongation at break (%) 440
Hardness (IRHD) 73
Compression set at 70°C (%) 79.3

































In injection mouldings platy kaolins or talcs can cause anistropy while synthetic silicas or aluminium silicates give a high viscosity causing difficulties in mold filling. Calcined kaolin gives, as a consequence of its shape and size, little anistropy and low viscosity. Properties are also very good (but may be boosted by small amounts of synthetic silica) and a typical EPDM injection moulding for a spark plug cap is given below.


Spark Plug Cap


Formulation phr
Vistalon 7606 100
Polyethylene Wax 3
PoleStar™ 200R 120
Precipitated Silica 30
Plasticiser 30
Polyethylene glycol 3
Sulphur 0.2
Peroxide 8
Curing characteristics  
Minimum torque (lb/in) 9.7
Maximum torque (lb/in) 73.4
T2 (min) 1.0
T90 (min) 7.7
Mechanical Properties  
Tensile Strength (Mpa) 10.1
Elongation at Break (%) 43.0
Modulus at 100% 3.0
Hardness (Share A) 64

For detailed formulation advice please contact us




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