Rubber Compounding Additives for Cord Adhesion

The HRH systems that are used in rubber compounding today are very proprietary however, they generally consist of several rubber compounding additives, which will now be discussed. 

One of the original uses for precipitated hydrated silica was its use in the original HRH adhesion compounds. Even though silica is also used today as a very important inorganic filler to improve rolling resistance in tire tread compounds, it is still used extensively in the wire coat or breaker compounds to impart steel tire cord adhesion. 

Originally the tire industry used straight resorcinol as the R component in the HRH compound for good steel tire cord adhesion. However, with environmental and safety concerns in handling free resorcinol in the rubber mixing operations, resorcinol is now used in the tire and rubber industry as a solid resorcinol formaldehyde resin (or RF resin), which is somewhat safer to work with. 

In addition, a third class of chemical additives is used in the HRH rubber formulations. This third additive is known as a methylene donor because it actually reacts with the resorcinol formaldehyde resin during the mixing in the rubber compound. The most common methylene donor used by the rubber industry today is hexamethoxymethylmelamine (HMMM). However, sometimes another chemical, hexamethylene tetramine (HMT), may be used. 

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