Rubber compounds are far more complex than plastic and metal alloys. They are not standardised, but are subject to individual formulation. This makes it impossible for users to specify by ingredients and therefore focus should be directed towards properties and of course approvals required for the actual application.
Basically, any rubber formulation consists of polymer, filler, plasticiser and a curing system. In addition, anti-degradants, processing aids and other functional components may be applied. The polymer provides the main rubber characteristics, depending on its chemical design.
In this example, we have chosen EPDM. Three monomers can be combined in chains with varying length, side branches, ratio and sequence. All of these parameters are of decisive importance to the characteristics such as strength, permanent deformation, etc.
After having chosen the polymer, it is time to look into the fillers. The most important group is carbon black, which provides additional strength, regulates friction, flexibility, thermal and conductive properties. Depending on the structure, carbon black can alter the properties significantly.
In addition to carbon black there is a number of “white fillers”, e.g. silica, chalk and kaolin. In general, these fillers provide less reinforcement, and most of them have an adverse effect on chemical resistance.
Now it is time to select the plasticisers. They regulate the hardness and act as homogenizers. For EPDM there is a range of plasticisers of which mineral oil is the most common.
Finally, it is time to look into the curing system. Basically, two systems are relevant for EPDM: Sulphur and peroxide. When using a sulphur system, the compound will generally be fast curing, while a peroxide system will provide low compression set, low taste and smell and much better thermal stability. On the down side, one should be aware that an additional process step is required: Post curing. This is in order to eliminate decomposition products that would otherwise cause bad smell and taste.