Role of Inhibitors in Rubber Molding

Any natural or synthetic rubber material that is used for rubber molding is often mixed with vulcanizing agents, activators, inhibitors, and a variety of other additives. The addition of each of these special compounds offers individual benefits such as solvent resistance, hardness, flexibility, and temperature resistance. While vulcanizing agents initiate the process of curing, inhibitors prolong the curing time to strengthen the elasticity of the part.

Inhibitors are active ingredients, which, when added to a rubber molding mixture, retard a chemical reaction. By doing so, they slow down the vulcanization or curing time or pot life of the rubber mixture. The mixture hence remains in a processing condition for a long time and reacts well with the activators and vulcanizing agents that strengthen the rubber part. Although they behave like catalysts, inhibitors are consumed during the reaction after which the process proceeds naturally. Added at room temperature, inhibitors increase the mechanical property of the rubber mixture.

There are a number of inhibitors that are used by commercial rubber manufacturers. If the resins used are unsaturated polyester resins, then phenolic derivatives and quaternary ammonium salts are used as inhibitors. Listed below are some of the commonly used inhibitors:

  • Ethylene diamine tetra-acetic acid
  • N-cyclohexylthiophthalimide
  • Hydroquinone
  • p-benzoquinone

The presence of even minute traces of copper can degrade natural rubber materials due to the catalytic effect. The sodium salts of ethylene diamine tetra-acetic acid or EDTA are a class of materials that make popular copper inhibitors. They are used as stabilizers to inhibit the effect of copper as well as manganese on polypropylene or natural rubber.

Premature vulcanization of rubber material that occurs before complete flow of rubber in the mold is defined as scorch. Scorch is a common effect found in most silicone rubber molding processes and results in incomplete or distorted profiles. N-cyclohexylthiophthalimide is an inhibitor that prevents pre-vulcanization. Commonly referred to as PVI or pre-vulcanization inhibitor, these compounds do not affect the formation of cross-links and hence are highly safe.

Hydroquinone and p-benzoquinone are inhibitors of free radical polymerization in the rubber molding process. Generally, salts of organic bases are added to these phenolic derivatives for increasing the storage life of rubber. Trimethyl ammonium acetates, bromides, and chlorides are other similar inhibitors.

For increasing UV light resistance, carbon black or UV light absorbers are added to the rubber molding mixture. Typically, waxes are added to prevent light-aging effects.

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