Treating the cause: Molecular Rebar Design studies replacement for 6ppd

AUSTIN, Texas—The U.S. EPA has awarded Molecular Rebar Design L.L.C. a grant of just less than $100,000 to study the "reduction or replacement of 6ppd" in commercial tires through improved tire cracking resistance.

The $99,989 grant, awarded Nov. 28 to the 14-year-old company that works in carbon nanotube technology, will allow MRD to study whether CNTs can be a drop-in replacement for 6ppd; complement the chemical additive to allow lesser loadings of 6ppd per tire; or be used in conjunction with another, less effective but perhaps more environmentally friendly chemical additive, like 77PD.

"We are looking at whether you can take a combination of technologies and get something commercial in a couple years," said August Krupp, director of rubber development for MRD. "Improvements for treadwear, within the rubber itself, is our focus."

The chemical 6ppd is an antidegradant used in tire manufacturing that prevents breakdown and cracking due to oxygen and ozone exposure, as well as from wide temperature swings.

However, at some point during tire abrasion, 6ppd can morph into 6ppd-quinone, a previously unknown transformational chemical that is killing coho salmon—and possibly other species—in the Pacific Northwest.

By design, 6ppd migrates to the tire surface during use to create a protective film against degradation.

But herein lies the conundrum.

Since it needs to migrate to the tire surface to work, 6ppd-quinone then has easy access into the waterways of the Pacific Northwest.

"Our focus is on the sidewall, where cracking is visible to consumers," Krupp said. "The question is, can you reduce the migratory effects of 6ppd?"

6ppd exists everywhere within a tire—in the tread, carcass and sidewall—because it is a small-molecule chemical intended to be mobile.

As stated, this mobility also allows the chemical to get into tire roadway runoff very easily.

"And that's the conundrum—6ppd has to come to the surface to be effective," Krupp said.

The Phase I grant from the U.S. EPA is a Small Business Innovation Research award, according to MRD.

Research began Dec. 1 and will be performed over six months at MRD's facilities in Austin, Krupp said, with the contract for the grant expiring May 31, 2024.

The timeline for finding an alternative to 6ppd has reached a fever pitch in the last year, as Native American tribes and fishing groups in the Pacific Northwest have filed legal notices and a lawsuit against the world's largest tire manufacturers, demanding an end to using 6ppd as an additive in tire manufacturing.

CNTs may provide a replacement for the embattled 6ppd—but not as a small-molecule chemical drop-in replacement, as other tire manufacturers and some private companies are analyzing.

"Treating the effects of polymer ozonation, rather than preventing the initial ozone degradation, is a unique approach that can be implemented commercially more quickly than the evaluation of a new small-molecule antiozonant—which may require exhaustive environmental testing, in addition to the rubber compound tests," Krupp said.

Essentially, MRD is looking at preventing or minimizing the microcracking that can lead to larger crack growth—"treating the cold itself, rather than the symptoms of a cold."

"We will treat the underlying cause rather than the symptoms, preventing the crack growth that is caused by ozonation," Krupp said. "Finding a new molecule is focusing on the treatment of the problem ... and it still may be a small-molecule chemical that can easily move around."

Krupp explained that carbon nanotubes are produced using a petrochemical process, with propane or ethylene and some type of catalyst.

CNTs can have different geometries, the determination of which is part of the CNT-6ppd research, Krupp said.

"They can be single- or multi-walled, and at MRD we can convert that carbonaceous group into something else," he said.

While some in the nanotechnology field believe carbon nanotubes—essentially graphene, nano-scale tubes of single-atom layer thickness or multiple layers wrapped to form a hollow core, comprised of carbon—may someday stand alone as the optimal performance additive in both tire and non-tire elastomers, others see a hybrid of CNT and 6ppd, and CNT and carbon black or silica, as the fillers of the future.

CNTs are more reinforcing than carbon black, "have fantastic electrical and thermal conductivity" and can be used as a reinforcing filler in many applications, Krupp said.

They decrease rolling resistance and increase abrasion resistance—molding the performance triangle into parameters not seen before.

"With the use of MR, ozonation-related microcrack propagation will be lessened, preventing coalescence of microcracks, and thus reducing larger crack occurrence," Krupp said. "With this additional reinforcement in a tire compound, the quantity of 6ppd used can be reduced, or 6ppd can be replaced with a less effective, but environmentally safer antiozonant like 77PD."

In Phase I, MRD will determine what type of carbon nanotube surface functionality is most effective at preventing crack propagation, and then formulate tire sidewall elastomer compounds with those nanotubes "while varying the type and quantity of antiozonant present in the sidewall compounds."

"These rubber compounds will undergo ozone aging to simulate real-world conditions, whereafter the aged samples will be tested for differences in crack resistance, lifetime, tear strength and intrinsic strength," Krupp said.

The analysis will culminate in a sidewall compound formulation "that is less environmentally damaging than incumbent compounds that utilize a full loading of 6ppd," Krupp said.

The resulting experimental compound hopefully will demonstrate the same, or better, expected lifetime as an incumbent sidewall compound.

The ever-stubborn performance triangle never has been asked to be more flexible, especially in the tire wear corner, as electric vehicles are expected to increase abrasion by more than 30 percent compared to internal combustion engine vehicles.

While this makes for an enormously large rubber compounding problem, Molecular Rebar and Goodyear are going small to find a remedy.

Molecular Rebar, the awardee of a $1.15 million Department of Energy grant in November 2022 to develop, test and produce an EV tire tread, is banking on its carbon nanotube technology as a filler (with a carrier agent, like silane) in the tread to provide a solution.

Goodyear, considered a Phase II contractor as part of the DOE pact, ultimately will test the EV tires with the prototype tire tread. The project will look to commercialize the Molecular Rebar for passenger vehicles first.

Thus far, results look promising.

Implications of the project are far-reaching, as success with the prototype development and demonstration represent key steps toward commercialized tire production. The project, which began in August 2022, will look to an August 2024 completion date when the grant period expires.

"The tire industry is risk-averse, for good reason, and we see challenges in the timing of development for EV tires," Krupp said. "The EV industry is moving at a rapid pace, and we believe an inherent challenge is the pace at which the tire industry and its suppliers innovate versus the pace at which the EV industry is innovating."

To be clear, Molecular Rebar is being used in the tire tread for this research and development project, not the rubber in the carcass.