Tearing of Black-Filled Synthetic Polyisoprene Rubber Vulcanizates

Natural rubber (NR) has outstanding tack and strength in the unvulcanized state and high tensile and tear strength when vulcanized¹. These features are attributed to its ability to crystallize rapidly on straining. Synthetic isoprene rubber (IR), synthesized employing a stereospecific catalyst, although close to natural rubber in chemical composition, has reported to be inferior in strength² compared to NR. This occurs because IR crystallizes more slowly than NR.

Edge-cut tensile specimens of gum natural rubber vulcanizates show an abrupt drop in strength at a critical cut size and normal (no cut) tensile specimens show a sharp decrease in strength at a critical temperature³. These two effects are due to bulk strain crystallization at small cut sizes or low temperatures, but the lack thereof at large cut depth or high temperature. The addition of carbon black affects tearing; this depends on the type and concentration of black.

In this paper, the tearing behavior of IR with various amount of N660 carbon black was investigated, and the results are compared to those obtained with natural rubber. The addition of low concentrations of carbon black decrease cut resistance. When carbon black content increases, anisotropy in strength increases, while critical cut size and exclusion zone decrease. There is a critical level of black needed for reinforcement of IR; this level is higher than that needed for natural rubber. When containing the same amount of black, NR compounds have at least the tear strength of the corresponding IR compounds, and in some cases the NR compounds are much stronger.

References:

1. A. N. Gent, S. Kawahara and J. Zhao, Rubber Chem. Technol., 1998, 71, 668

2. R. Clamroth and Th. Kempermann, Polym. Test. 1986, 6, 3

3. A.G. Thomas and J.M. Whittle, Rubber Chem. Technol., 1970, 43, 222