Two forces are driving compounders to evaluate antimony trioxide substitution simultaneously: China’s 2024 strategic mineral classification of antimony has pushed ATO spot pricing to multi-year highs with 2.5–3 month lead times, while IARC Group 2B classification for Sb₂O₃ is triggering specification reviews in automotive, wire and cable, and electronics supply chains.
How ATO Works vs How Zinc Synergists Work
ATO operates through a gas-phase mechanism. During combustion, antimony trioxide reacts with halogen (from the polymer or a co-additive) to form antimony halides that scavenge flame-propagating radicals in the gas phase. It has no significant solid-phase activity and provides minimal smoke suppression on its own.
Zinc-based synergists work differently. FL-ZS and FL-ZHS (zinc stannate and hydroxystannate) decompose at the polymer surface, forming a char barrier and releasing tin oxide and zinc oxide that catalyze additional char formation. This solid-phase activity directly reduces smoke output and CO generation, a meaningful difference from ATO in applications where smoke density limits apply. FL-ZB (zinc borate) adds a third mechanism: the boric acid melt forms a glassy surface layer, and the water of crystallization provides endothermic cooling. This is why FL-ZB uniquely suppresses afterglow, a requirement in several transport and construction standards.
Side-by-Side Comparison
| Property | ATO (Sb₂O₃) | FL-ZS | FL-ZHS | FL-ZB |
|---|---|---|---|---|
| FR mechanism | Gas-phase only | Gas + solid phase | Gas + solid phase | Solid phase + endothermic |
| Smoke suppression | Limited | Good | Excellent | Good |
| CO reduction | Minimal | Moderate | Significant | Moderate |
| IARC classification | Group 2B | None | None | None |
| Supply status (Mar 2026) | Constrained | Available | Available | Available |
| Halogen-free compatible | No | No | No | Yes |
| Typical loading (phr) | 3–6 | 3–6 | 4–8 | 3–6 |
| Afterglow suppression | No | No | No | Yes |
Which Zinc Synergist to Test First
The right starting point depends on your substrate:
- Rigid PVC: Start with FL-ZS. Strong synergism with halogenated FR systems, good thermal stability, commercially proven in rigid PVC profiles and sheets.
- Flexible PVC wire & cable: Start with FL-ZHS. Superior smoke suppression and CO reduction make it the recommended choice where IEC 61034 smoke density or NBS smoke chamber limits apply.
- Halogen-free systems: FL-ZB is the only zinc synergist in this group that functions in halogen-free matrices. Works with MPP, APP, and ATH-based systems.
- Universal / multifunctional requirement: FL-ZB where afterglow suppression is also needed (transport, construction); FL-ZHS where primary driver is smoke density reduction.
What to Expect in Formulation Testing
Zinc synergist substitution is not 1:1 by weight with ATO. The effective loading range for zinc synergists is typically 3–8 phr depending on substrate and target fire standard, compared to 3–6 phr for ATO. Recommend parallel testing with the current ATO formulation as a baseline, targeting the same fire test method (UL 94, LOI, IEC 60332, etc.).
Most compounders find that 2–3 formulation iterations are sufficient to match or exceed baseline fire performance at comparable or lower total additive loading. Smoke performance typically improves versus the ATO baseline, which may be commercially useful if customers are approaching smoke limits. Evaluation quantities from 25 kg are available to support direct comparison testing.
Request evaluation quantities of FL-ZS, FL-ZHS, or FL-ZB for direct comparison testing, response within 24 hours.
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