Comparison of the Strength of Normal and Edge-Cut Tensile Specimens of Filled Natural Rubber with Different Carbon Black Types and Similar Crosslink Density – Part II

The strength of unfilled natural rubber and SBR is compared in the previous paper¹. With no intentional cut, gum natural rubber is an order of magnitude stronger than SBR with the similar crosslink density. With an edge-cut, the strength change of natural rubber highly depends on the cut size and its behavior is very different from SBR. The authors believe this is due to the strain-induced crystallization in natural rubber which is lack in amorphous SBR.

In this paper, normal and edge-cut tensile specimens of natural rubber vulcanizates with N115 or N660 were tested. Although NR vulcanizates with 50 phr of N115 or N660 black have similar normal (no cut) strengths, strengths with an edge cut reveal that N115 provides greater reinforcement. However, surprisingly, with addition of a small amount of either black (12 phr), the filled NR tears more easily than the gum NR, with the N115 causing a larger reduction in strength. A small amount of black impedes strain-induced crystallization. Upon increasing the N115 concentration to 15 phr, some specimens fail with a knotty crack pattern; at similar cut depth, these show higher strength than the gum NR. Swelling tests confirm that incipient bound rubber is formed at this N115 black concentration. With the coarse black (N660), similar results are found, but at a higher black concentration, because of its lower surface area.