compounds
In addition to one or more base polymers, typical rubber compounds contain a number of additional ingredients to modify their performance and cost. These formulations usually evolve over time and may contain between 10 and 50 separate ingredients. The formulations are usually kept secret within an organisation and are often the key differentiator between competing products. Large organisations # may have many thousands of formulations to choose from for a particular application.
The typical ingredients in a formulation include:
| raw polymer | synthetic or natural rubber | 20% to 70% of compound |
| fillers | carbon black (a wide variety of carbon blacks are available, each imparting specific properties to the compound), silicas, clays, etc | could be up to 80% of compound |
| pigments | wide range of pigments available for non carbon black applications | up to 20% |
| anti-degradants (to protect from UV, ozone, etc) | e.g. amines, phenolics, peroxides | up to 10% |
| process aids | e.g. oils, resins, soaps, blowing agents | up to 10% |
| cure system | curing agents (e.g. sulphur, peroxide) accelerators and retardants (e.g. dithiocarbamates, thiurams, thiazoles, sulphenamides, CBS, MBS, MBT, zinc oxide) |
typically 1% to 5% |
Note: the percentages of materials used are only provided as an indication of relative ranges. Real formulations may vary from these ranges.
As the choice of polymer is key to the performance of the compound, a rough comparison of base polymer performances is given below. Any material selection should be based on the supplier's material data sheets and should also include compound and process performance characteristics such as filler reinforcing, crosslink density and cure time.
Key to chart
All the ingredients are mixed together (compounded), usually either on an open mill (small volumes, critical materials) or using an internal mixer such as tangential or interlocking rotor machines. These mixers discharge to a dump mill or dump extruder which works the materials further before the forming and processes.
(pictures courtesy of www.chinamixing.com)
