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
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
|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|
|Tensile Strength (MPa)||5.8|
|Modulus at 300% (MPa)||3.7|
|Elongation at break (%)||440|
|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
|Minimum torque (lb/in)||9.7|
|Maximum torque (lb/in)||73.4|
|Tensile Strength (Mpa)||10.1|
|Elongation at Break (%)||43.0|
|Modulus at 100%||3.0|
|Hardness (Share A)||64|