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Basic Rubber Compounding

In any molded rubber product, the three major influences of part quality that we control are the mold, the process and the rubber formulation. By far the most influential is the latter. In plastics, designers can use material suppliers’ specification data to determine what material will work best in their application, but in rubber, that information is not readily available. The reason is the plastic molder purchases their materials in a ready to process form and the resulting physical and environmental resistance properties are controlled by that supplier. In contrast, the rubber molder purchases ingredients from many suppliers and mixes them to form the material that is processed in the mold, therefore, each molder controls the material's and resulting end product's properties.

Often designers have little experience with rubber and have no idea how to select the rubber that is best for their application. Searching through some general reference literature they may discover (for example) that nitrile is rated excellent at resisting petroleum hydrocarbons and believe that specifying that and a specific durometer (hardness) will control the end products material properties.

What the designer doesn't realize is that rubber compounders have over 180 different commercial grades of nitrile from which to select! These vary by acrylonitrile (ACN) content, viscosity, chemistry, and supplier ("equivalent" grades usually are not) among other parameters. On top of that, we will combine the base polymer (often two or more grades) with approximately 10 - 20 other ingredients (See example formula below.) These other ingredients enable us to achieve the desired physical, environmental resistance and processing properties required to mold the end product. In addition, we have literally thousands of chemicals to choose from when selecting those other ingredients. As you can see the possible combinations and resulting material variations are staggering.

In the majority of materials, just adding vulcanizing chemicals will produce a product so weak, in its mechanical and environmental resistance characteristics that it is almost unusable. Even natural rubber can have its very good physical traits enhanced by putting in other ingredients in the mixture.

Not only can we improve an elastomer’s physical strengths by compounding other ingredients into the mixture of elastomer and sulfur, but also we can improve other characteristics and even create some that do not naturally occur.

Compounding can be broken into five main systems as follows:

1. Elastomer System

  • One or more elastomers for characteristics
  • Elastomer for improved processing

2. Filler System

  • Reinforcing filler
  • Semi and non-reinforcing fillers
  • Plasticizers

3. Protectants

  • Antioxidents
  • Antiozonants
  • Inhibitors/promoters

4. Process Aids

  • Mixing aids
  • Modling aids

5. Cure System

  • Vulcanizing Agents
  • Activators
  • Accelerators
  • Scorch retarders

One important detail to note is that nearly all compounders use the unit of measure of "parts per hundred" (PHR) for their formula. This is a unit of weight for the relationship between the elastomer system and the other systems. If we always utilize 100 parts of elastomer for all formulas then it is much simpler to change the other systems to create changes and different formulas.

The reason this is so important is that the cure system reacts only with the elastomer system. Thus, as we change all the other systems, the relationship between the elastomer system and the cure system remain constant with a few exceptions that we won’t address here.

When creating a new compound there are three main criteria compounders use to guide them. Listed in order of importance they are as follows:

  • 1. Customer requirements
  • 2. "Processability"
  • 3. Cost

Nearly all new compounds are modifications of some existing formulations. Nowadays, development of a completely new compound is seldom attempted. Moreover, such an attempt is usually unnecessary. In order to be efficient and effective in rubber compounding, chemist should take full advantage of technical information readily available inside as well as outside of his organization. He must be analytical, resourceful, and innovative.

The following is a useful procedure to guide compound development:

  • 1. Set specific objectives (properties, price, etc.)
  • 2. Select base elastomer(s).
  • 3. Study test data of existing compounds.
  • 4. Survey compound formulations and properties data presented by material suppliers in their literature.
  • 5. Choose a starting formulation
  • 6. Develop compounds in laboratory to meet objectives.
  • 7. Estimate cost of compound selected for further evaluation.
  • 8. Evaluate processability of compound in the factory.
  • 9. Use compound to make a product sample.
  • 10. Test product sample against performance specification.

Rubber compounding is one of, if not the most difficult and complex subjects to master in the field of rubber technology. Compounding is not really a science. It is part art, part science. In compounding, one must cope with literally hundreds of variables in material and equipment. There is no infallible mathematical formulation to help the compounder. That is why compounding is so difficult a task.

Example Formula:

Click the button below to open a separate window which illustrates a sample formula.