Basic Rubber Compounding
In general, the ingredients of a rubber compound may be divided into five groups. This statement is, by necessity, an oversimplification and is intended as a guide only. Some ingredients in any given formulation might serve two, or even three purposes. Also, any given ingredient might function differently in various elastomers. With these caveats in mind, the following represent the standard ingredients of a rubber compound.
1. Base Elastomer: There are approximately twenty-two commercially available polymers in the U.S.A. A base polymer, or sometimes a blend of polymers, is chosen based on its cured characteristics: abrasion resistance, oil resistance, high and low temperature characteristics, etc. Most always, the over-riding consideration is cost.
2. Fillers and Plasticizers: Most rubber formulations would be useless without some type of filler incorporated. Fillers can be grouped into three categories: reinforcing, semi-reinforcing, and extenders.
A. Reinforcing fillers include the small particle-sized carbon blacks such as N110-0, N220, N330 and light-colored reinforcing agents such as Hisil and Cabosil.
B. Semi-reinforcing fillers include black carbon with larger particle sizes such as N762 and N990. Examples of semi-reinforcing fillers include treated clays and zeolex.
C. Extending fillers are often necessary to meet high-quality specifications such as soft printing rolls where a low durometer and resistance to the inks solvents are required.
Plasticizers: Generally plasticizers are liquids or very soft semi-solids. They must be carefully chosen on the basis of their compatibility with the elastomer. Those that are incompatible can interfere with the curing systems, leach out after a period of time, or both. In products that must be bonded, plasticizer choice often depends upon individual accelerated, long-term testing, or both.
3. Antidegradents or Protective System: Anti-oxidants and anti-ozonants are generally incorporated from 1.0 to 2.0 parts per 100 parts base elastomer. The chemistry of reactions, whether intended or unintended, is complex. Choosing a compatible antidegradant that will not "bloom" often requires individual compound testing.
4. Special Ingredients: Often ingredients are required for a specific purpose. Examples include flame retardants, coloring agents, and process aides. Ingredients such as PEG 3350 are used to overcome the effects of light-colored fillers. The types and amounts of ingredients used vary considerably.
5. Curing Systems: First, we must assume that all of the above ingredients have been chosen so as not to interfere with cross-linking. Second, we will assume a sulfur-based (not dicumyl peroxide nor radiation) curing.
A. The activators: Generally the combination of zinc oxide and stearic acid work together to provide suitable conditions for curing to take place.
B. Acceleration Systems: Any combination of a host of generally organic compounds that dramatically reduce sulfur cross-linking time.
C. Sulfur: The cross-linking agent.