Sumitomo claims breakthrough in rubber failure study

TOKYO—Sumitomo Rubber Industries says it has cracked the mystery surrounding rubber failure, in a joint study with the Dresden, Germany-based Leibniz Institute for Polymer Research.

The Japanese tire and rubber company said the "ground-breaking" research sheds light on the unknown mechanism that causes microscopic voids within the rubber which lead to the formation and propagation of cracks.

According to SRI, previous research had failed to "fully explicate" the theory that the fracturing of rubber molecules and the formation of voids within rubber at the microscopic scale led to the formation and growth of cracks in the rubber and eventually rubber failure.

In 2015, using its "advanced 4D Nano design," Sumitomo advanced rubber development technology by managing to suppress void formation. With this latest joint research, the company says it has succeeded in "directly observing internal structural changes in combination with the mechanical behavior of molecules within actual synthetic rubber specimens."

SRI claims that the findings of the research will give "greater control over viscoelastic properties of the rubber" and could help develop rubber materials with "extremely high durability."

The company will continue to work on the results to translate them into "high-performance tires that resist wear to maintain their performance for longer lifetime."

The company said the breakthrough was achieved through two different types of experiments.

One involved using computed tomography to analyze changes in force, strain and volume in a thin rubber disk.

Here, a thin rubber disk specimen was placed between and affixed to two metal disk plates and stretched in a direction perpendicular to the contact surface (i.e. so that the plates were moving in opposing directions) in order to observe the relationship between applied stress and changes in volume as a function of apparent strain. At the same time, CT was utilized to directly observe the formation of voids within the rubber.

Due to the inherent properties of rubber, when a thin disk-shaped specimen is stretched in this way, its natural inclination is to contract radially (i.e. perpendicular to the direction of tensile stress). However, because the specimen was affixed to the metal plates, the rubber directly attached to the plates could not contract radially as long as it remained solidly affixed to the plates. As a result, the rubber was forced to expand, allowing us to directly observe the formation of voids through CT observation of the rubber interior.

According to SRI, with this experiment, the company also learned that the circumstances of void formation can vary.

"In particular, we found that rubber failure in synthetic rubber containing filler material such as silica or carbon black occurred due to the formation of voids between agglomerates of filler material," the company noted. This is while rubber failure in synthetic rubber containing no filler material occurred due to the formation of voids resulting from the sliding of rubber molecules.

A second test involved the elucidation of synthetic rubber failure behavior through analysis using small-angle X-ray scattering (SAXS)

In this experiment, a notched sheet specimen of synthetic rubber was stretched laterally (i.e. in the planar direction) while small-angle x-ray scattering was utilized to observe the formation and growth of voids within the rubber at the notch point.

Using small-angle x-ray scattering to measure internal rubber density at the point of the notch in the sheet specimen of synthetic rubber, SRI found that the density of the rubber nearer to the notch was lower than the rubber in areas farther away from the notch. This suggested that many voids were forming within the rubber near the notch point. These results indicate that, when sheet rubber is stretched laterally (i.e. in the planar direction), voids exist at the point where the rubber tears.

"Thus, we have discovered that voids are directly involved in rubber failure, as had long been suspected," the company said.

Sumitomo said it anticipated that the results of this research would lead to the development of rubber that is "more resistant to failure and wear than any type of rubber that has come before."