Technical Notebook: Processing additives for silica—a historical perspective

Precipitated silicas have been used as a reinforcing filler in rubber formulations for more than 80 years. In the early years it was the filler of choice in non-marking applications such as shoe soles and heels. The use of silica in tire compounds such as tread, plies and belt compounds was at that time limited by its processing difficulty and high cost relative to carbon black.

Following the famous Michelin patent of 1993, silica use increased greatly in tire treads, owing to its unusually good wet-traction performance. Loadings of 80-100 phr soon became commonplace.

Also, the combination of silica together with a silane coupling agent became the norm as the means to improve rubber processing and enhance the rubber’s resistance to wear. Today, it is not unusual to find rubber tread recipes which include as much as 150 phr of silica.

This paper focuses on the historical use of precipitated silica, the reasons behind its initial use, and will bring to light some of the unknowns which continue to limit its applications. The paper also reveals how the proper selection of a processing additive can lead to profound differences in rubber performance.