ATO supply is constrained. But qualifying an alternative requires understanding which zinc synergist works best in which substrate. This guide breaks it down by chemistry, mechanism, and application.
FL-ZS vs FL-ZHS: Selection Guide for ATO Reduction
Both zinc stannate and zinc hydroxystannate reduce or replace antimony trioxide (ATO) as flame retardant synergists and smoke suppressants. The difference that usually decides selection is processing temperature. The table compares zinc stannate (FL-ZS) and zinc hydroxystannate (FL-ZHS) on verified properties. You can also compare all three zinc synergists.
| Property | FL-ZS (Zinc Stannate) | FL-ZHS (Zinc Hydroxystannate) |
|---|---|---|
| Form | Anhydrous, ZnSnO3 | Hydrated trihydrate, ZnSnO3·3H2O |
| CAS No. | 12036-37-2 | 12027-96-2 |
| Role vs ATO | Partial replacement or significant loading reduction; synergist in brominated systems | Partial replacement or significant loading reduction; synergist in brominated systems |
| Full ATO elimination | Possible in specific systems; confirm by application-specific fire testing | Possible in specific systems; confirm by application-specific fire testing |
| Flame-retardant mechanism | Gas-phase and condensed-phase | Gas-phase and condensed-phase, plus endothermic cooling from bound-water release |
| Smoke suppression | Yes | Yes |
| Processing temperature | Thermally stable to approximately 570°C; suits sustained processing above 220°C (engineering thermoplastics such as PA, PBT, PET) | Process below approximately 200°C; bound water begins releasing near 220°C |
| Best-fit applications | High-temperature polymers and engineering thermoplastics | PVC and lower-temperature systems, smoke-critical formulations, and thin films or calendered sheet via the micronized FL-ZHS-M grade |
Substitution ratios are not fixed values. They depend on the polymer system and fire standard and should be confirmed by application-specific fire testing (LOI, UL-94). Contact Flamisol for application guidance and available test data.
Three Zinc Synergists: How They Differ
FL-ZS, FL-ZHS, and FL-ZB are all inorganic zinc-based synergists, but they differ meaningfully in chemistry, mechanism, and best-fit application.
| Product | Chemistry | Smoke Suppression | Afterglow Suppression | Halogen-Free Compatible | Best Substrate |
|---|---|---|---|---|---|
| FL-ZS | Zinc stannate | Good | No | No | Rigid/flex PVC, polyolefins |
| FL-ZHS | Zinc hydroxystannate | Excellent | No | No | Flexible PVC, wire & cable |
| FL-ZB | Zinc borate | Good | Yes | Yes | Universal: incl. halogen-free |
How Zinc Synergists Work
ATO works primarily via a gas-phase mechanism: antimony halides form in the flame zone and scavenge radicals that propagate combustion. Zinc synergists work differently and in some cases more broadly:
- FL-ZS / FL-ZHS: Zinc tin compounds decompose at the polymer surface forming char and tin oxide, blocking heat transfer and oxygen access. Also gas-phase activity via ZnO/SnO₂ catalytic action.
- FL-ZB: Multiple mechanisms: boric acid melts to form a glassy char layer, ZnO promotes cross-linking, and water of crystallisation provides endothermic cooling. This multi-mechanism action is why FL-ZB uniquely suppresses afterglow.
Starting Point Loading Guidance
These are starting points for formulation work only. Full fire testing (UL 94, LOI, IEC 60332, etc.) is required to confirm compliance in your specific system.
| System | Typical ATO Baseline | FL-ZS / FL-ZHS Starting Point | FL-ZB Starting Point |
|---|---|---|---|
| Rigid PVC | 3–5 phr | 3–6 phr (1:1 to 1.2:1) | 3–5 phr |
| Flexible PVC (W&C) | 3–6 phr | FL-ZHS 4–8 phr preferred | 3–6 phr |
| Polyolefin (brominated) | 2–4 phr | 3–5 phr | 2–4 phr |
Request Evaluation Samples + TDS
We supply FL-ZS, FL-ZHS, and FL-ZB in evaluation quantities. Include your substrate, target fire standard, and current ATO loading in your RFQ.