Grantee Research Project Results

Final Report: Recycle & Reuse for Superelastic SMA Tires via Intermediate Thermal Annealing

EPA Contract Number: 68HERC24C0029
Title: Recycle & Reuse for Superelastic SMA Tires via Intermediate Thermal Annealing
Investigators: Weinberg, Charles
Small Business: The SMART Tire Company, Inc.
EPA Contact: Richards, April
Phase: I
Project Period: December 1, 2023 through May 30, 2024
Project Amount: $99,903
RFA: Small Business Innovation Research (SBIR) - Phase I (2024) RFA Text |  Recipients Lists
Research Category: Small Business Innovation Research (SBIR)

Description:

The topic of research for this contract is the use of intermediate thermal annealing as a means to extend the lifespan of used shape memory alloy (SMA) materials. Due to the predominant usage of these materials in medical applications (dental, heart stents etc), post-consumer use is largely unexplored, and practically all research on fatigue life is performed on virgin material and with particular wire diameters. The only known recycling or refurbishing of SMA is by conventional metal alloy melters, but the industry rate of reclamation is unknown and assumed to be relatively small. There is little financial incentive in the medical industry for reuse, even if the very challenging safety concerns were alleviated; however, other industries can greatly benefit from extended lifespans.

A series of experiments were performed to estimate the fatigue life of nitinol wire at various strains with and without intermediate annealing steps. A Dynamic Mechanical Analyzer (DMA) was used to run samples to failure in 3-point-bending tests. Control samples of virgin material were subjected to different levels of maximum strain and then compared to identical samples that had been heat treated after 25, 50 and 75% of their predicted lifetime, to ascertain whether the treatments were beneficial to fatigue life of the material. Three different heat treatments of varied time and temperatures were investigated and compared to the control, and to each other.

Summary/Accomplishments (Outputs/Outcomes):

 

Results of the tests were generally good at different combinations of strain, annealing method and expected time-to-failure. In particular, of the combinations tested, more than 75% performed better on average than the control samples, 67% of those were by at least 1 standard deviation, and only 1 outlier sample was > 1 standard deviation worse than the control.

Notably, analysis was initially performed on the overall expected lifetime of the sample. When comparing the expected remaining lifetime of the samples after treatment, more significant results were obtained. For the highest strain tests, 88% of all combinations survived between 11% and 116% more test cycles than projected, but this also equates to between 51% and 544% longer than expected post-treatment. Depending on the exact evolution of the material and whether results of 50%+ gains can be replicated at larger scales, this would be significant enough to enable at least a 2nd service life for the wire.

Conclusions:

Heat treatment alone is unlikely to restore shape memory alloy materials to their original pristine condition; however, these experiments suggest that specific treatments have a life-extending effect on used nitinol wire. Combined with other potential methods and refinements, it may be possible to reliably refurbish used material for applications that are equivalent or less demanding than the original usage. Further investigation is needed at lower strains and with higher fatigue life material to better understand the magnitude of the effect(s), whether it could be additive with other methods, and how well it scales to millions of cycles and different grades of material. Scanning Electrochemical Microscopy (SECM) would be very informative to further investigate differences in failure modes of the material, as would longer tests, larger sample sizes, and a wider range of alloy formulations.

Commercialization of shape memory alloy tires is just one large-scale structural application, but alone would greatly exceed the current production capacity of nitinol worldwide. In order to be successful at scale, lower cost and sustainability are two of the most important requirements in order to compete with incumbent technologies that are lower cost, but environmentally problematic.

The global tire industry is a $300B market with seemingly insurmountable sustainability issues. Billions of lbs of waste are produced annually, CO2 emissions from manufacturing are globally significant, and tire wear is a leading cause of microplastic pollution and “forever” chemicals. There are, however, few viable alternatives. Creating long-lasting and recyclable tires is one of the most impactful and innovative opportunities in all of transportation. Nitinol is the advanced material most promising as a substitute for pneumatic tires, if it can be delivered at an effective and reliable cost per mile, both of which are greatly enhanced by effective reconditioning methods.

Even a limited, but reliable method of extending the service life of already manufactured forms of nitinol wire will impact the commercial viability of these (and related) products, by:

1. Creating a market for post-consumer material, lowering costs
2. Further reducing CO2 emissions related to manufacturing, melting and/or disposal
3. Supporting new products which do not require virgin material, but might benefit greatly from access to a post-consumer grade at a lower cost.

All in all, significant improvements to this unique advanced material stand to benefit a variety of large industries, starting with but not limited to tires.