California sets 6ppd rule, USTMA to mobilize research

WASHINGTON—The marching orders have been issued by the California Department of Toxic Substances Control for tires containing 6ppd, and the mobilization to find an alternative for the antidegradant continues.

The DTSC on July 26 officially adopted a rule that requires manufacturers of tires (domestic or foreign) for sale in California to evaluate safer alternatives to 6ppd, an additive that helps reduce tire cracking and wear.

The antioxidant and antiozonant also morphs into 6ppd-quinone at some point during tire abrasion, a transformational chemical that has a mortality rate of about 50 percent in coho salmon.

The new rule applies to any tire in "the stream of commerce" in the state and takes effect Oct. 1.

Domestic and foreign manufacturers have until Nov. 30 to notify DTSC if they manufacture 6ppd-containing tires that are used in California, DTSC spokesman Russ Edmondson said.

"We are working with the U.S. EPA, other states, researchers and the (U.S.) Tire Manufacturers Association to find a path to make tires safer for our environment without compromising on-road safety," DTSC Director Meredith Williams told Rubber News.

The Washington-based USTMA immediately responded to the new rule from the California DTSC, announcing an official consortium of 16 of the world's largest tire producers to tackle the critical alternative research.

The companies—which include Michelin, Bridgestone, Goodyear and Continental, among many others—will be guided in their research by California's Safer Consumer Product Regulations, and the listing of 6ppd as a "priority product" within the program.

The SCPR framework helps products and chemicals comply with Federal Motor Vehicle Safety Standards and other consumer, vehicle and tire manufacturer requirements.

"This consortium will power a collaborative approach to the alternatives analysis process, which will yield the most effective and exhaustive review possible of whether a safer alternative to 6ppd in tires currently exists," Anne Forristall Luke, USTMA president and CEO, said July 26. "With the DTSC taking this action, we will continue our active and transparent work to complete the required alternatives analysis and collaborate with other stakeholders."

According to DTSC Director Karl Palmer, the SCPR provides a "rigorous and transparent" framework for the study of possible toxic substances and alternative analysis work.

Currently there are no known, viable alternatives to replace 6ppd, and the search for one has drawn the attention of chemical companies like Flexsys America L.P.

"6ppd plays a crucial role in the safety of tires on California's roads and, currently, there are no widely available safer alternatives," Palmer said. "For this reason, our framework is ideally suited for identifying alternatives to 6ppd that ensure the continued safety of the tires on California's roads while protecting California's fish populations and the communities that rely on them."

According to the USTMA, any potential alternative to 6ppd "must provide equivalent tire safety and performance to 6ppd, while improving environmental metrics."

The USTMA believes a full mitigation strategy also must be undertaken, one that includes infrastructure efforts to filter or eliminate tire road wear particles, which contain roughly 50 percent tire abrasion particles and 50 percent road-related particles.

"USTMA's primary focus will be on maintaining these critical tire and vehicle safety benefits in any potential replacement product to ensure tire safety and performance," the organization said July 26.

USTMA members include the aforementioned tire production leaders, as well as Giti Tire (USA) Ltd.; Hankook Tire America Corp.; Kumho Tire U.S.A. Inc.; Nokian Tyres Inc.; Pirelli Tire North America; Sumitomo Rubber Industries; Toyo Tire Holdings of Americas Inc.; and Yokohama Tire Corp. USTMA's statistical affiliate members, including Double Coin North America L.L.C.; Maxxis International USA; Nexen Tire America Inc. and Sailun Tire Americas, also will be involved.

According to a study completed in 2021 at the University of Washington (Tian et al.), 6ppd reacts with ozone (O3) in the ambient air to form 6ppd-quinone, which kills coho salmon as they migrate upstream to spawn in the waterways of the Pacific Northwest.

This toxic chemical—6ppd-quinone—has been fatal to about half of the coho salmon studied in the laboratory. The juvenile salmon were offered for the studies by the Puyallup Indian tribe, who have lived along the shores of Puget Sound for thousands of years.

In California, in particular, the loss of coho salmon has "significantly affected" Native American tribes, which rely on the fish, according to the DTSC.

"Mounting evidence indicates that 6ppd and 6ppd-quinone also pose threats to other aquatic organisms," Palmer said.

The two-year-old Tian et al. study is the root motivation for the process today that has 6ppd in the crosshairs of the California EPA, which oversees the DTCS.

"While heavy metals and polycyclic aromatic hydrocarbons have long been regulated pollutants in stormwater, recent studies have reported numerous emerging organic contaminants such as various pesticides, pharmaceuticals, plasticizers, and vehicle and tire rubber-related contaminants," the Tian study states.

Calling it "stormwater-linked urban runoff mortality syndrome," death rates for coho salmon across Pacific Northwest sub-basins correlated with road density and traffic intensity, according to the study.

Five outdoor sites were used in the study, with two roadway runoff sites measured in Seattle and one in Los Angeles; and one site "receiving water" in Seattle (Puget Sound) and another in San Francisco.

Coho salmon are integral to Native American diets and cultures and have declined to a fraction of what they were at their peak in the 1960s, at around 55,000 "new individuals" per year counted through fish ladders in the Pacific Northwest.

Cryo-milled tire tread (CMTT), developed jointly by the USTMA and the World Business Council for Sustainable Development's Tire Industry Project, now is available free of charge for researchers, governments and academic institutions to study the 6ppd/6ppd-quinone conundrum.

CMTT is not a direct replacement for the study of TRWP, rather it allows researchers to isolate and focus on the tire tread component. The CMTT methodology developed by the Tire Industry Project essentially eliminates the infinite variables presented by the natural environment.

The CMTT uses treads from passenger, light truck and bus tires, all from North American manufacturers.

USTMA members are funding production of the material.

More information is available at ustires.org/cmtt.