Scientists put stability of natural rubber to the test
PETRA III reveals strain response of rubber on short time scales
Researchers from the Leibniz-Institut für Polymerforschung Dresden, Technische Universität Dresden, and DESY have used the PETRA III light source to study structural changes in natural rubber upon stretching. The scientists have followed these changes with an unprecedented time resolution of seven milliseconds, corresponding to over 140 static images per second. Considering that a large portion of the world’s natural rubber production goes into the manufacturing of automobile tires, which have to withstand large amounts of strain, the new research findings promise to be of interest for the global tire industry. The study has recently been published in the journal “Macromolecules” (DOI: 10.1021/ma3011476).
Natural rubber, which is extracted from the bark of rubber trees, is widely used in technical applications and products. It is flexible, elastic, and waterproof. Its main components are long chains of the organic compound isoprene. Cross-linking these chains through “vulcanization” and filling the material with silica or carbon black makes the rubber stronger and more suitable for heavy duty applications. “Since World War II, researchers have been trying to produce synthetic alternatives to natural rubber,” says scientist Karsten Brüning, who is affiliated with both Dresden institutes. “Until today, however, filled natural rubber is the first choice for demanding applications such as the use in truck tires.”
One reason for the strength of natural rubber is “strain-induced crystallisation”. Without any strain, the rubber’s polyisoprene chains have no long-range periodic order. When the material is stretched, however, the chains start orienting into a periodic, crystalline structure. This transformation enhances the stability of the material. “Let’s say you have a nail or a pebble stone stuck in your car’s tire. The damage, a crack in the tire, exerts a local strain on the rubber material,” Brüning says. “Strain-induced crystallisation in that location leads to a self-reinforcement where it is most needed, and it can stop the damage from spreading.”
Brüning and his colleagues have followed strain-induced crystallisation in natural rubber using the wide-angle X-ray diffraction (WAXD) technique. At the Micro- and Nanofocus Beamline (MiNaXS) at PETRA III, the scientists placed their rubber samples into the intense X-ray beam and recorded the intensity distribution of the scattered X-rays behind the sample. As they stretched the rubber, the scattering pattern changed and intensity peaks appeared, indicating the formation of crystallites in the sample.
Strain responses of natural rubber have been investigated previously, but none of the earlier studies has provided a “real-time” look at the events. “To obtain good signals at a millisecond time resolution, it is crucial to have very intense x-rays bundled into a small and parallel beam,” DESY scientist Stephan Roth explains. “The MiNaXS beamline is one of only a few beamlines in the world that is capable of performing this type of experiment.”
The research team performed two types of stress tests. In the first experiment, they monitored the degree of crystallisation in rubber samples, while repeatedly cycling them between a minimum and a maximum strain. One cycle was completed within approximately one second. “Similar testing procedures are used to examine fatigue in tires,” Brüning says. The maximum crystallinity in the rubber turned out to be lower for periodic stretching than for a static strain of the same magnitude. Since crystallinity has a reinforcing effect, material tests under static conditions may overestimate the material’s strength.
The researchers have also demonstrated that the stabilizing effect of crystallisation in natural rubber increases with the minimum strain in the cycling experiment. “This result is somewhat counterintuitive,” Brüning points out. “The material’s resistance to crack propagation, for instance, is weaker when the material is under a smaller strain.” Although the phenomenon was empirically known before, only the recent study proves that it originates from strain-induced crystallisation.
In the second stress test, the researchers quickly stretched rubber samples only once in less than ten milliseconds and then analysed the build-up of crystallinity in the sample over a period of one minute after impact. The data suggests that crystallisation is a two-step process: a rapid initial formation of crystallisation nuclei, followed by slower crystal growth.
“At present there is no generally accepted model for strain-induced crystallisation,” Brüning says. “Our studies aim at throwing light on the mechanisms of this phase transition and at understanding in detail how the molecular properties of natural rubber affect its mechanical characteristics.” The researchers also plan further studies on natural and synthetic materials as well as studies of fast processes such as crack propagation. “To watch a crack propagate through a material in our experiments should be within reach in the next few years,” Roth says.
Karsten Brüning, Konrad Schneider, Stephan V. Roth, and Gert Heinrich, Kinetics of Strain-Induced Crystallization in Natural Rubber Studied by WAXD: Dynamic and Impact Tensile Experiments, Macromolecules (DOI: 10.1021/ma3011476).