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Announcing the Results of Groundbreaking Research on Natural Rubber Failure
-Elucidating the Details of Crystallization Behavior at Crack Tips-

 

March 12, 2019

 

Sumitomo Rubber Industries, Ltd. is pleased to announce that, as a result of our joint research with the Leibniz Institute for Polymer Research*1 (Dresden, Germany), we have succeeded in shedding further light on the behavior of strain induced crystallization*2 occurring at the tips of cracks in natural rubber. The results of this groundbreaking research, which represent a first in the history of polymer science, were first unveiled at the Tire Technology Expo 2019, which was held this past March 5th through 7th in Hanover, Germany. Moving forward, the Sumitomo Rubber Group will put these results to use in the ongoing development of rubber materials that feature superior wear resistance compared to conventional compounds, which in turn will lead to the development of tires that can maintain high performance for longer.

*1Founded in 1948 as a textile research laboratory attached to a spinning mill, the Leibniz Institute for Polymer Research is now one of the leading polymer research facilities in Germany and is actively engaged in joint research with top research organizations from around the world.
*2Here, crystallization refers to the phenomenon in which natural rubber molecules become aligned in the direction of elongation when the rubber is stretched.
 
 

Previously, our company had elucidated the mechanisms that cause voids (the origin of rubber failure) within synthetic rubber to develop into cracks. Then, with an eye toward future environmental issues and the growing need for long-lasting tire performance, we next turned our attention to failure phenomena in natural rubber (one of the main raw materials that make up a tire) as a new focal point for our ongoing research into the causes of rubber wear and failure.


■Experimental Overview
It has long been known that crystallization occurs in natural rubber when it is stretched and that the rigidity of the rubber increases in areas where crystallization is occurring. As crystallization does not occur in SBR (the most common type of synthetic rubber used in tires), it is believed that crystallization must be a major factor behind the formation of cracks in natural rubber (which ultimately lead to rubber failure). As tire rubber rotates while in contact with the ground, it deforms repeatedly and cyclically while subjected to strain constraint.*3 Thus, we saw a need to observe the state of deformation occurring in the tips of cracks in natural rubber when it is subjected to strain constraint.

*3Strain constraint refers to a state in which rubber is unable to deform freely. For example, imagine a thin, disc-shaped specimen of synthetic rubber that has been affixed to a disc-shaped metal plate and then stretched in a direction that is perpendicular to (i.e. away from) the face of the plate. When stretched in this way, the natural tendency of the rubber is to contract in the direction of width (i.e. radially). However, because the rubber is affixed to a metal plate, the area of the rubber in contact with the plate is unable to contract in the direction of width unless and until it peels off. As a result, the volume of the specimen appears (externally) to expand.

■Experimental Methodology
We first selected a flat sheet-shaped specimen of natural rubber featuring sufficient width in the direction perpendicular to the direction of elongation in order to accurately reflect the real-world relationship between the forces acting on the tips of cracks in rubber and strain and also to allow for X-ray analysis of the crystalline structures forming within the specimen. We next repeatedly stretched and relaxed the specimen while using wide-angle X-ray scattering (WAXS) to observe the crystallization behavior occurring at the tips of cracks*4 within the rubber.

*4The use of a flat sheet-shaped natural rubber specimen featuring sufficient width in the direction perpendicular to the direction of elongation made it possible to sufficiently suppress contraction in the direction of width when the specimen was stretched. As a result, we were able to recreate the behavior of crack tips when rubber is subjected to strain constraint.

■Results
When a thin strip-shaped specimen of natural rubber is stretched, most of the rubber molecules will align in the direction of elongation (forming crystallites). When a flat sheet-shaped specimen of natural rubber with a sufficient width is stretched, strain constraint occurs. As a result, fewer of the rubber molecules orient in the direction of elongation than in a thin strip-shaped specimen. We found that, when this is the case, crystallization occurs in various directions, seemingly at random.

 

 
 
Changes in X-Ray Diffraction Image When Specimen Rotated in Axial Direction (Azimuth Angles)
If crystallites were solely oriented in the direction of elongation, the diffraction image would be identical regardless of the rotation angle. Here, the yellow spots (indicating the orientation of crystallites) increasingly fade and disappear as the rotation angle increases, indicating that crystallization is occurring in various directions. (Note: The axis of rotation is the tensile axis, which is perpendicular to the X-ray beam.)


In addition, we also found that when a specimen of natural rubber contains the filler material carbon black, the crystallites that form are smaller in comparison with those in a specimen that does not contain carbon black.

Further, as the result of our observations of crystallization during repeated stretching and relaxation of the specimen, we found that the crystallites that form when the specimen is stretched will dissipate when the specimen returns to its original shape. However, the crystallites are larger when the specimen is returning than when it is stretching.

With the automotive industry currently facing many sweeping changes, the Sumitomo Rubber Group has responded by formulating our SMART TYRE CONCEPT, a new tire technology development concept that aims to achieve "Even Greater Safety Performance" and "Even Greater Environmental Performance." We believe that the results of this research in crystallization occurring at the tips of cracks in natural rubber will enable us to better control the alignment of crystals that form in rubber, thus contributing to the development of rubber that is more resistant to wear and less prone to failure. The Sumitomo Rubber Group will continue working to accelerate our efforts to establish our "Performance Sustaining Technology" for tire rubber as a step toward fully realizing our SMART TYRE CONCEPT in the near future.

<Reference>
News Release Issued January 9, 2019
"Announcing the Results of Research into Rubber Failure with Potential to Improve Rubber Wear Resistance Performance" (http://www.srigroup.co.jp/english/news/2019/sri/2019_001.html)



 

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