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UT-led team uses high-resolution X-ray computed tomography to determine flaws in rubber

A new method to ensure consistency and quality in rubber manufacturing, developed by a research team from the University of Tennessee, Knoxville, and Eastman, is likely to show real-world impact on material sustainability and durability for products such as car tires.

An open-access paper on their approach, “Sulfur Dispersion Quantitative Analysis in Elastomeric Tire Formulations by Using High Resolution X-Ray Computed Tomography”, was published in the journal Rubber Chemistry and Technology.

Good dispersion of compounded ingredients in a rubber formulation is important for mechanical performance. After mixing, certain materials can remain undispersed within the rubber matrix, which could lead to critical flaws, influencing performance according to the Griffith failure criteria. High resolution X-ray computed tomography (XCT) offers a unique opportunity to measure phase domain size and distributions. Fillers such as carbon black or silica can be differentiated from sulfur or zinc oxide, providing an opportunity to determine dispersion characteristics of the various phases.

The XCT technique has become an important characterization tool for three-dimensional and higher dimension material science due to the availability of polychromatic micro-focus X-ray sources and efficient high spatial resolution detectors with superior scintillators. High resolution XCT provides very rich data quantifying mixing efficiency of particulates in a matrix, such as insoluble sulfur or silica particles in rubber. Imaging with X-rays provides attenuation, phase, or scattering contrast and will prove to be a critical method for evaluating the field of rubber crosslinking, considering realistic environments in situ.

This paper highlights methodology development and validation and provides insight on the dispersion of polymeric (insoluble) sulfur in rubber formulations. Dispersion assessment is compared using three techniques: high resolution XCT, population survival analysis in tensile testing, and optical microscopy.

—Penumadu et al.

As consumers in the US and around the globe are increasingly incentivized toward electric vehicles and away from fossil-fuel reliance, current EV users have uncovered an unexpected maintenance issue. Due to the combination of higher weight and higher torque, EVs put more pressure on standard tires, causing them to degrade 30% faster than tires on internal-combustion vehicles.revp

UT’s Fred N. Peebles Professor and IAMM Chair of Excellence Dayakar Penumadu, along with electrical engineering graduate student Jun-Cheng Chin, postdoctoral researcher Stephen Young and three Eastman scientists, recently published research aimed at resolving one of rubber manufacturing’s most common challenges: identifying flaws in the material.

Rubber contains additives such as zinc oxide and sulfur that work to improve strength, elasticity and other favorable traits. When the ingredients are not distributed evenly throughout a rubber product such as a car tire, the material will contain flaws that cause the product to degrade prematurely.

If components such as sulfur do not disperse well, that generates localized hard spots. That hard stuff attracts a lot of mechanical and thermal stresses, making the material degrade prematurely. That leads to safety and economic impacts.

—Dayakar Penumadu

Even a flaw the width of a human hair can decrease the life span of a large rubber component such as a car tire.

Identifying and studying such flaws—a field known as fracture mechanics—is critical to understanding how the material will perform. Yet finding such flaws before they cause problems is an issue that has long plagued the rubber industry.

The current industry approach is to cut out a small sample of rubber, then observe it under an optical microscope. Not only is this tedious and destructive, it’s unreliable. It requires you to guess beforehand where, in an opaque sample, you need to check for inconsistencies.

—Dayakar Penumadu

In addition, optical microscopes cannot differentiate between rubber components—for example, sulfur and zinc oxide both appear as white specks.

Penumadu’s team has overcome this issue by switching from optical analysis to X-ray computed tomography. X-rays that pass through the sample are scattered and absorbed differently depending on the materials they strike. A computer then reconstructs a digital 3D model of the rubber’s interior.

This is a very important point. XCT lets us see the inside of the material noninvasively, and we can actually see the distribution of each component.

—Dayakar Penumadu

The application of this new method increases the rubber industry’s ability to view and predict flaws and will ultimately lead to more consistent quality and longer-lasting rubber products.

Resources

  • Dayakar Penumadu, Jun-Cheng Chin, Stephen Young, Frederick Ignatz-Hoover, Tom Floyd, Peter Chapman (2021) “Sulfur Dispersion Quantitative Analysis In Elastomeric Tire Formulations By Using High Resolution X-Ray Computed Tomography”. Rubber Chemistry and Technology doi: 10.5254/rct.21.79997

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