As the tire industry faces environmental challenges with traditional materials, especially 6PPD, the need for alternatives is becoming obvious. With regulatory bodies like the State of California, the US EPA, and European Chemicals Agency scrutinizing these substances, tire manufacturers and their suppliers face pressure to find viable replacements. In addition, companies are moving towards a more sustainable tire by evaluating a wide variety of new materials. In this Q&A, we spoke to Smithers experts Josh Guilliams and Rory Mumford to explore the challenges and considerations necessary for evaluating 6PPD and other alternative materials in tire production.

What is 6PPD, and why is it a concern in tire manufacturing?

6PPD (N-1,3-dimethylbutyl-N'-phenyl-p-phenylenediamine) is an important antioxidant in tire formulations that helps prevent ozone cracking and oxidative degradation in unsaturated elastomers, such as styrene-butadiene rubber (SBR) and natural rubber. It not only prolongs the lifespan of tires, but also enhances their performance. Its effectiveness stems from its ability to react with ozone and free radicals, forming protective films on the tire surface. The environmental transformation product of 6PPD, 6PPD-quinone (6PPD-Q), has been shown in studies to be toxic to aquatic organisms like coho salmon, raising environmental concerns and prompting scrutiny from tribal nations, environmental groups, States, and trade groups.  The US EPA recently issued an advance notice of proposed rulemaking (ANPRM) to gather information on the potential risks of 6PPD and 6PPD-Q.

What specific performance criteria must alternative antidegradants meet to be considered viable replacements for 6PPD?

While there is a list of potential alternatives to 6PPD, including other antidegradants and novel materials, the performance and environmental impact of some of these substitutes remain somewhat unknown. Each alternative must continue to be evaluated not only for its ability to protect tires from oxidative degradation, but also for its environmental impact, including the potential risk to wildlife.
Viable replacements must maintain or exceed the performance of 6PPD in several key areas:

• Ozone protection: Defense against ozone-induced cracking and deterioration.
• Heat aging resistance: Protecting the tire from thermal degradation.
• Flex fatigue resistance: Ensuring the tire can withstand repeated flexing without failure.
• Compatibility: Must be compatible with existing tire formulations and manufacturing processes.
• Processability: Must be easily incorporated into the rubber compound.
• Retention: Must remain in the tire matrix and not leach out prematurely.
• Environmental Risk: Acceptable environmental risk profile for parent and environmental transformation products, focusing on persistence, bioaccumulation, and toxicity (PBT) criteria.

Are there other chemicals in tire production that are being reviewed for better alternatives from a sustainability or environmental perspective? 

Yes, in addition to 6PPD, other chemicals in tire production, like some sulfur compounds, and accelerators such as zinc oxide also deserve some attention having generated initial inquiries from regulators. Also, tire companies are constantly trying to increase the usage of more sustainable materials such as rice husk silica or recycled carbon black.
Other chemicals of concern include:

• DPG (1,3-diphenylguanidine) is an accelerator used in tire production that has been noted by the European Chemical Agency for reclassification under CLP regulation (classification, labelling, and packaging of products). It was listed in 2024 for public consultation in an information gathering process.  
• Certain sulfur vulcanization accelerators have also been noted as potentially releasing harmful byproducts.

• Zinc Oxide, also used as an activator in vulcanization, can leach into the environment and has been questioned by the State of California as potentially being toxic to aquatic life.

In addition to monitoring scientific studies and regulatory activities for opportunities to develop better chemicals, companies in the tire supply chain continue to try to develop more sustainable options for common tire ingredients. As noted above, rice husks are being investigated as a more sustainable source of silica, which is critical for use in lower rolling resistance tires. Carbon black has also been a target to develop more sustainable sources. Through development efforts, several methods of producing alternative carbon blacks are progressing to increase both performance and capacity. These are just two examples amongst many gaining traction in the tire industry today.

Whether developing more environmentally friendly chemicals or more sustainable options, it is vital to holistically evaluate each option to ensure the optimum alternative is used.

What specific methodologies should the industry employ to comprehensively evaluate new tire materials?

A holistic approach is essential, involving:

• Chemical analysis: Characterizing the composition and properties of materials using techniques like chromatography and spectroscopy.
• Physical property testing: At a compound level, there are indicators of final performance such as DMA, ozone resistance, and others that can provide some initial benchmarks to understand if a new material could meet required performance targets.
• Tire Performance testing: Evaluating tread wear, rolling resistance, wet grip, dynamic fatigue, and other critical properties. This can be conducted both in laboratory settings as well as on-vehicle at proving grounds to truly assess final performance of tires with modified compounds and constructions.
• Environmental risk assessment: During research, initial screening for PBT characteristics can be helpful in the prioritization of 6PPD alternatives. Products identified to be used in production will require an environmental risk assessment evaluating ecotoxicology, environmental fate, and physicochemical properties according to the requirements of the regional regulatory bodies such as the US EPA or ECHA
• Life cycle assessment (LCA): Evaluating the environmental impact of materials across their entire life cycle. This covers not only direct environmental impact, but also indirect effects such as energy or water usage to produce or process.

How does collaboration with a specialized third-party testing and consulting provider like Smithers streamline the evaluation of antidegradant or other material alternatives?

Smithers offers specialized expertise in material science and environmental impact testing, enabling tire manufacturers to:

• Accelerate screening and validation: Utilizing advanced testing capabilities to quickly identify promising alternatives that meet performance and environmental criteria.
• Ensure regulatory compliance: Leveraging in-depth knowledge of global regulations and testing protocols.
• Gain independent and objective data: Relying on unbiased testing and analysis.

Access specialized equipment and expertise: Benefit from Smithers' state-of-the-art facilities and experienced scientists.

As the tire industry embarks on this critical journey toward more sustainable and environmentally friendly materials, thorough evaluation, informed decision-making, and collaboration will be cardinal in replacing harmful chemicals like 6PPD and other legacy materials. Only through diligent research and innovative partnerships with organizations like Smithers, can the industry hope to deliver safer, greener tires for the future at the pace that is required.