Structure of Filled Natural Rubber Near the Tip of a Fatigue Crack

Natural rubber is a well-known elastomer commonly used in the tyre industry. Because of its remarkable toughness it is often used in the most mechanically demanding parts of the tyre and in particular near the steel cord where shear stresses are high and the need to bond to the metal imposes an excess of sulfur and a higher crosslink density.

The toughness of natural rubber is generally attributed to its aptitude to crystallize under strain. While the existence of crystallization in front of the crack tip is now well-documented, existing studies on filled systems have been carried out on natural rubbers with a low crosslink density and at room temperature^1-3, the experiment has never been carried out at high temperature, with realistic materials or in aged samples. Yet natural rubber in truck tyres can function for hours at 100°C and thermal (oxygen free) aging can lead to changes in the crosslinking structure.

While it is qualitatively clear that the presence of SIC at the crack tip slows down the propagation of fatigue cracks the detailed mechanism linking the extent of strain induced crystallization to the toughness G is still unclear.

In order to shed some light on this question, we carried out an extensive investigation of the extent of crystallization in well crosslinked and filled NR in uniaxial extension and at the crack tip by using wide-angle X-ray diffraction with synchrotron radiation. Using a scanning X-ray microbeam (20 microns diameter), we also mapped the strain–induced crystallization near the crack tip in static loading conditions at different values of applied energy release rates G. We specifically investigated the effect of the crosslinking density, the effect of thermal (oxygen-free) aging and the effect of temperature (between 23 and 100 °C). Several novel findings are reported : a significant amount of crystallization was still present at the crack tip at 100°C, thermal aging (in the absence of oxygen) greatly reduces the amount of crystallization at the crack tip without much effect on the room temperature resistance to fatigue crack propagation of the material, and an increase in crosslinking density reduces the extent of crystallinity at the crack tip for the same applied G. These findings question some aspects of the role played by SIC in NR toughening.

1. Beurrot-Borgarino, S.; Huneau, B.; Verron, E.; Rublon, P., Strain-induced crystallization of carbon black-filled natural rubber during fatigue measured by in situ synchrotron X-ray diffraction. Int J Fatigue 2013, 47(0), 1-7.

2. Trabelsi, S.; Albouy, P.-A.; Rault, J., Stress-Induced Crystallization around a Crack Tip in Natural Rubber. Macromolecules 2002, 35(27), 10054-10061.

3. Brüning, K.; Schneider, K.; Roth, S. V.; Heinrich, G., Strain-induced crystallization around a crack tip in natural rubber under dynamic load. Polymer 2013, 54 (22), 6200-6205.