California's Zinc-in-Tires Proposal: What It Means for Global Tire Formulation and How to Prepare

Quick Summary

California's tire regulation does not stay in California. Because global tire makers rarely maintain separate formulations for one market, a zinc rule adopted in Sacramento tends to reset the formulation baseline for the entire industry, the same way the state's 6PPD action did.

The California Department of Toxic Substances Control (DTSC) has proposed to list motor vehicle tires containing zinc as a Priority Product under its Safer Consumer Products (SCP) program. If finalized, manufacturers selling zinc-containing tires in California would have to notify the state and evaluate whether safer, commercially viable alternatives exist. The driver is Tire and Road Wear Particles (TRWP), which release zinc into waterways through stormwater runoff. Regulators treat zinc as a controllable source of aquatic toxicity; the tire industry maintains that zinc oxide is currently irreplaceable for tire safety and performance.

No fully zinc-free tire technology has been commercially established. The realistic near-term direction is zinc reduction, not zinc elimination, and that shift is already creating demand for low-zinc activator technologies across the global tire supply chain.

Why This Matters Beyond California: The "California Effect"

The most important point for anyone in the tire supply chain is not the regulation itself but its reach. DTSC's Safer Consumer Products program requires manufacturers of listed products to conduct an alternatives analysis to determine whether safer product chemistries or designs are feasible. That obligation applies to any manufacturer selling into California, which means it applies, in practice, to nearly every major global brand.

Tire manufacturing runs on economies of scale and formulation consistency. Maintaining a separate "California-compliant, low-zinc" product line alongside a conventional "high-zinc" line is logistically and financially difficult for global brands such as Bridgestone, Michelin, and Goodyear. The more practical response is to move the entire production platform toward lower zinc loading, which is exactly how a single state regulation becomes a worldwide formulation standard.

International regulators are watching. The European Chemicals Agency (ECHA), operating under REACH, has historically tracked California's Green Chemistry initiatives closely. If DTSC succeeds in compelling the industry to evaluate and reduce zinc, matching regulatory activity in Europe and Asia becomes more likely, not less. The recent 6PPD episode showed how quickly a regional tire-chemistry rule can reshape global formulation priorities.

What the DTSC Proposal Actually Requires

Under the SCP framework, listing a product as a Priority Product does not ban it outright. Instead, manufacturers must notify DTSC and conduct a formal Alternatives Analysis (AA) to determine whether a substitute can reduce or eliminate zinc without compromising performance or shifting the environmental burden elsewhere.

If no viable alternative is identified, or if the available alternatives carry equal or greater hazards, DTSC holds authority to impose a regulatory response. These responses range from mandatory disclosure and use restrictions to, in some cases, prohibiting the sale of the product in California.

One development that changes the timeline calculus deserves attention. Under Senate Bill 502, DTSC gained enhanced authority over the SCP program. SB 502 gave DTSC the authority to move directly to a regulatory response instead of requiring manufacturers to conduct an alternatives analysis, in which case DTSC would use publicly available studies or existing evaluations of alternatives. For manufacturers, this means the traditional assumption that an AA provides a multi-year runway before any binding requirement may no longer hold in every scenario.

Why Zinc Oxide Is Still Central to Tire Chemistry

Understanding why this regulation is difficult requires understanding what zinc oxide does in a tire compound. It is not a minor additive; it is structural to the cure.

Zinc oxide is a core component of sulfur vulcanization, the reaction that converts raw rubber into a durable, elastic, crosslinked network. In a typical formulation, zinc oxide works as an activator alongside stearic acid and organic accelerators such as sulfenamides and thiazoles. During the cure, zinc oxide reacts with stearic acid to form soluble zinc complexes; these interact with sulfur and the accelerator system to generate the reactive intermediates that drive efficient crosslinking and optimize crosslink distribution across the rubber matrix.

Beyond cure activation, zinc oxide carries several functions at once:

• Processing safety: reduces the risk of premature vulcanization (scorching) during mixing and extrusion.
• Heat resistance: helps form short, stable crosslinks that improve resistance to heat build-up during high-speed operation.
• Fatigue and tear resistance: supports the dynamic durability a tire needs under continuous mechanical stress.
• Aging resistance: contributes secondary protection against oxidative and UV degradation over the tire's service life.
• Structural stability: strengthens the overall integrity of the cured rubber network.

Because zinc oxide performs these roles simultaneously, replacing it is far harder than swapping a single-function additive. This is the basis for the U.S. Tire Manufacturers Association (USTMA) position that tires meeting current safety and performance standards cannot presently be manufactured without zinc oxide.

The Environmental Concern: Tire and Road Wear Particles

The regulatory pressure traces back to a documented pathway. As vehicles travel, friction between tread and road surface continuously sheds microscopic particles. These Tire and Road Wear Particles contain rubber polymers, fillers such as carbon black and silica, and the compounding ingredients embedded in the matrix, including zinc.

Rainfall washes TRWP off roadways into stormwater systems and onward into rivers, streams, and coastal waters. Over time, zinc leaches from these particles into aquatic ecosystems. Environmental studies have linked elevated zinc concentrations to acute and chronic toxicity in aquatic organisms, with salmonids and other freshwater fish among the more sensitive species; effects include disruption of enzymatic processes and impairment of gill function.

From the regulator's standpoint, zinc from tire wear is a controllable, non-point source of contamination that is best addressed upstream through product design rather than downstream through stormwater treatment.

Where the Stakeholders Stand

The debate is not a simple environment-versus-industry split. Several groups bring distinct and partially valid positions.

The real question is whether alternative technologies can match zinc oxide's combination of safety, durability, manufacturability, and now environmental performance.

Regulatory Timeline and Current Status

The current proposal is the result of more than a decade of evaluation.

The 2023 Final Rationale Document has locked in the scientific basis for DTSC's position that zinc added during vulcanization contributes meaningfully to aquatic toxicity via runoff. The scientific foundation has been established; what manufacturers are now tracking is the transition from a pending proposal to a binding rule.

If Zinc-Containing Tires Become a Priority Product: The Alternatives Analysis

Once a product is listed, manufacturers enter the SCP Alternatives Analysis, a structured two-stage evaluation.

Stage 1: Identification and Screening.

Manufacturers must map every function zinc oxide performs in the formulation (cure activation, scorch prevention, and the rest) and screen candidate substitutes. This stage filters out any alternative that introduces immediate safety, performance, or toxicological hazards.

Stage 2: Lifecycle and Economic Evaluation.

Surviving candidates undergo full physical validation against federal safety mandates, including high-speed endurance and wet braking traction. Manufacturers then perform a cradle-to-grave Life Cycle Assessment to confirm the alternative does not create new environmental trade-offs, alongside analysis of supply availability and retooling cost. Candidates must be evaluated across high-speed performance, endurance, wet traction, manufacturing feasibility, lifecycle impact, supply chain availability, and cost.

The purpose of the framework is to ensure that solving one environmental problem does not quietly create another.

What Tire Manufacturers and Suppliers Can Do Now

The settled science and the SB 502 authority together argue against waiting for a final rule. Organizations across the supply chain can take concrete steps now:

• Audit current zinc loading. Establish a documented baseline of zinc oxide levels across existing formulations before any reporting obligation arrives.
• Evaluate low-zinc activator technologies. Begin lab-scale assessment of active zinc oxide and other high-efficiency activators that can hold cure performance at reduced zinc levels.
• Run validation studies early. High-speed, endurance, and wet-traction testing takes time; starting now shortens the eventual compliance path.
• Assess lifecycle impacts of candidates. A substitute that fails the LCA stage is not a substitute; screen for this early rather than late.
• Engage chemical suppliers on roadmap and capacity. Confirm which suppliers already have qualified low-zinc products and the production capacity to support a formulation shift at scale.

Manufacturers that begin evaluating alternatives now will be in a stronger position whenever the rule is finalized, rather than scrambling against a deadline.

Low-Zinc Activator Technology: The Practical Near-Term Path

A fully zinc-free system has not emerged as an industry-wide solution. The achievable near-term goal is to improve zinc efficiency: achieving the same vulcanization performance with a lower total zinc load. One such example of this technology is the active zinc oxide CS AZO®, produced by Pan-Continental Chemical for the rubber and tire industry.

CS AZO® is a patented controllable-structure active zinc oxide. Its core-shell technology leads to performance and sustainability improvements that are highly relevant to a low-zinc transition:

• Lower zinc content. Compared to conventional zinc oxide in the same formulation and composition, CS AZO® contains only half the amount of zinc. This speaks directly to the TRWP runoff concern at the center of the DTSC proposal.
• Improved processing efficiency. Its unique structure allows for better dispersion in rubber, for improved vulcanization performance.
• Reduced carbon footprint. The carbon emissions of producing CS AZO® are only 20% of those of conventional zinc oxide.
• Regulatory readiness. CS AZO® is REACH registered, REACH SVHC, and RoHS compliant.

For tire and rubber manufacturers building a low-zinc roadmap, CS AZO® is a commercially established option that has already been adopted across multiple segments of the rubber industry.

To request a sample or technical data sheet for evaluation in your formulation, contact Pan-Continental Chemical.

Frequently Asked Questions

Q: Is California banning zinc oxide in tires?

A: No. The current proposal would require manufacturers to conduct an Alternatives Analysis, not an immediate ban. However, under SB 502, DTSC has authority in some circumstances to move directly to a regulatory response without first requiring an AA.

Q: Why is zinc oxide used in tires?

A: It acts as a vulcanization activator and simultaneously supports processing safety, heat resistance, fatigue and tear performance, and long-term aging resistance.

Q: What are Tire and Road Wear Particles (TRWP)?

A: Microscopic particles generated by normal tire-road friction during driving. They carry rubber polymers, fillers, and compounding ingredients, including zinc, into the environment through stormwater runoff.

Q: Can zinc oxide be completely replaced?

A: Not at present. No commercially established zinc-free technology matches zinc oxide's performance across all tire applications. The realistic near-term path is zinc reduction through higher-efficiency activators such as CS AZO®, which can lower total zinc loading while maintaining cure performance. Manufacturers evaluating this route should begin lab validation early and confirm supplier capacity for qualified low-zinc products.

Q: Why is California's proposal important globally?

A: Because global manufacturers generally avoid maintaining separate regional formulations, a California rule tends to influence worldwide product development. International regulators under frameworks such as REACH also monitor California's approach closely.

Q: What is an Alternatives Analysis?

A: A structured DTSC evaluation that assesses whether safer, commercially viable substitutes exist for a chemical of concern, covering performance, safety, lifecycle impact, and economic feasibility.

Conclusion

The DTSC zinc proposal marks a shift in how tire environmental performance is judged, expanding the lens from rolling resistance and fuel efficiency to include heavy-metal shedding during everyday use. The scientific basis has been established, the regulatory process is in motion, and SB 502 has compressed the timeline assumptions manufacturers once relied on.

The decisive question is no longer whether zinc can be reduced, but whether the industry can adopt activator technologies that hold the line on safety, durability, and manufacturability while cutting zinc release into waterways. Manufacturers and suppliers that evaluate lower-zinc solutions now, rather than after a rule is final, will be the ones positioned to respond on their own timeline instead of a regulator's.