Observations Made During Stretching, Tearing, and Failure of NR and SBR Loaded with Various Amounts of Carbon Black

Using a 35-mm camera with macro attachments, and focused on the tip of a cut, the stretching, tearing, and failure of notched samples of NR and SBR were documented during continuous testing. The propagation of a cut occurs by the stretching, tearing, and collapsing of fine strands, which we propose consists of bundles of long unwinding molecular chains of the rubber. Two main cut-growth regions are identified: a slow-cut-growth region consisting of irregular-shaped nodules or cells of collapsed rubber strands along short tear paths and a fast-cut-growth region consisting of long intersecting tear or cleavage paths leading to catastrophic failure. Addition of 20.6 phr (15 wt.%) carbon black results in a coarsening of the stretching, tearing, and fracture morphologies relative to those seen with the gum rubbers. Addition of 64 phr (35 wt.%) of carbon black results in all these structures being very fine with the phenomenon of knotty, deviated tearing dominating in the slow-cut-growth region. The knotty tearing is frequently revealed by the presence of chevron-like markings which correspond to the intersection of individual disoriented (deviated) tears. The initiation of tearing occurs at relatively low strains for all three carbon black loadings (0, 15, 35 wt.%); however, the strain to initiate tearing relative to the fracture strain was seen to decrease with an increase in carbon black, especially with the SBR series. Both failure strength and elongation increase significantly with carbon-black additions to SBR up to about 39 phr (25 wt.% of carbon black). With further additions, the elongation falls off, although the engineering strength remains unchanged, while the stiffness and tearing energies increase. With reference to the literature, we discuss the role of carbon black and deviated tearing on strengthening rubbers; suggestions are made for further research in this area.