Quick answer: Antimony trioxide (Sb₂O₃, “ATO”) is not a flame retardant on its own — it is a synergist. Paired with a brominated flame retardant it forms volatile antimony halides that scavenge the H• and OH• radicals driving combustion, so a bromine + antimony system reaches UL94 V-0 at a far lower total loading than bromine alone. The usual bromine-to-antimony ratio is around 3:1. The catch in 2026: after China’s 2024 export controls, antimony prices spiked roughly 4x before easing — so how much ATO sits in your formula now moves your cost sheet.
Ask a formulator what antimony trioxide does and you get a one-liner: “it makes bromine work harder.” True, but it hides the part that matters to your budget and your compliance file. Let us unpack how the synergy actually works, what ratio to run, and why the antimony line on your BOM got so volatile.
Is antimony trioxide a flame retardant?
Strictly, no. Put Sb₂O₃ into a polymer with no halogen and you get almost no flame retardancy — just an expensive white filler. Its whole value is the synergistic effect it has with halogenated systems. Combined with a brominated (or chlorinated) flame retardant, the two together suppress combustion far more than the sum of their separate contributions. That is why you never see antimony sold as a finished flame retardant, only as the partner in a bromine + antimony package.
How antimony trioxide works — the synergy mechanism
Why does adding a non-retardant make bromine so much better? Because it changes where and when the bromine acts. The sequence, in the flame:
- Antimony halide generation. Above ~250 °C, Sb₂O₃ reacts with the decomposing bromine source to form volatile antimony tribromide (SbBr₃) and oxybromides. This pulls the bromine into the gas phase earlier and at lower temperature.
- Radical scavenging. In the gas phase those antimony halides are potent radical traps. They mop up the reactive H• and OH• radicals that keep the combustion chain reaction alive — effectively poisoning the flame.
- Physical shielding. A secondary effect: molten antimony species and the released gases (HBr, CO₂, water vapour) blanket the surface, dilute the fuel and shut out oxygen.
The net result is a system that self-extinguishes at a much lower additive load. That efficiency is the entire reason brominated chemistry stays competitive — and it is the backbone of our brominated flame retardant additives. For the full menu of bromine sources that pair with antimony, see our breakdown of brominated flame retardants examples.
| System | Acts where | Flame retardant result |
|---|---|---|
| Antimony trioxide alone | — | Negligible (not a flame retardant) |
| Brominated alone | Gas phase | Moderate; needs high loading |
| Bromine + antimony (synergy) | Gas phase + surface | Strong; UL94 V-0 at low loading |
The bromine-to-antimony ratio
How much antimony? The workhorse figure is a 3:1 bromine-to-antimony ratio by weight — enough antimony to convert the available bromine, without wasting it. Push the ratio too lean on antimony and the synergy drops off; overload it and you are just paying for filler that dulls the plastic and raises cost. The exact split shifts with the resin, the bromine source and the wall thickness you are rating.
| Resin | Typical bromine : antimony | Target |
|---|---|---|
| ABS / HIPS | ~3 : 1 | UL94 V-0 @ 1.6 mm |
| PP | ~3 : 1 to 4 : 1 | UL94 V-0 / V-2 |
| PE / crosslinked PE | ~2.5 : 1 to 3 : 1 | UL94 V-0, cable jacket |
These are starting points, not gospel — the only ratio that counts is the one that passes on your part, which is why we bracket a few loadings in the lab and burn them before quoting. Getting the loading right also feeds the LOI: most brominated compounds land in the LOI 26–32 range once the antimony ratio is dialled in.
2026: antimony’s price rollercoaster and your formula
Here is the part that turned a chemistry footnote into a purchasing headache. In September 2024 China — which controls roughly 48% of mined antimony and 74% of antimony trioxide refining — imposed export controls. Prices that had sat around $15,000–$18,000 per tonne through early 2024 ran up to nearly $60,000 per tonne by mid-2025, close to a 4x jump. In November 2025 China suspended the export ban for US-bound shipments and prices eased, but the episode showed how exposed any bromine + antimony formula is to one country’s policy.
What does that mean when you spec a flame retardant? Two things. First, the antimony fraction is now the swing factor on your compound cost — the same V-0 result costs very differently depending on how lean you can run the antimony. Second, buyers are re-examining whether every part truly needs a halogenated system at all. There is a separate regulatory nudge too: ECHA has classified certain antimony compounds as Substances of Very High Concern (SVHC), which does not ban them in plastics but does raise the documentation and scrutiny bar.
Cutting antimony — substitutes and the halogen-free exit
Can you use less antimony, or none? There are two directions, and they are not equal:
- Partial replacement. Co-synergists like zinc borate can replace a portion of the antimony while holding the rating, and add smoke suppression and anti-drip as a bonus. Iron oxides and some tin compounds play similar supporting roles. This trims antimony cost without redesigning the system.
- Full exit to halogen-free. The only way to remove antimony entirely is to drop the halogen it serves and switch to a phosphorus or mineral (ATH/MDH) system. That is a bigger reformulation, but it also deletes the smoke, the acid gas and the RoHS/REACH conversation in one move.
Note the honest limit: there is no drop-in, one-to-one substitute for antimony in a brominated system, and when antimony prices fell in late 2025 a lot of substitution projects quietly slowed down. So the real decision is strategic, not just a swap. If your market is halogen-sensitive or you want off the antimony supply risk for good, the cleaner answer is a halogen-free flame retardant system. Weighing that trade-off is exactly what our halogen-free vs halogenated flame retardants comparison walks through.
How Rectivas formulates bromine + antimony systems
We supply pre-synergized brominated systems — the bromine source and antimony ratio already balanced for your resin — as powder blends and as brominated flame retardant masterbatch. Every formula is compounded on our partner factory’s SHJ-75 twin-screw line and burn-tested at our partner factory’s UL94 chamber before it leaves, and each lot ships with an accredited third-party laboratory test report so your incoming QC has data, not adjectives. Where antimony cost is biting, we will quote both a lean-antimony (zinc borate co-synergist) version and a halogen-free alternative so you can compare on real numbers.
Send us the resin, the UL94 target and wall thickness, and your view on halogen and antimony. We will bracket the loadings, burn samples, and come back with the ratio that passes at the lowest cost. Request a quote with your spec to get started. Antimony trioxide also carries an occupational classification worth knowing about — we cover it honestly in are flame retardants toxic.
FAQ
Is antimony trioxide a flame retardant on its own?
No. On its own it does almost nothing. It is a synergist that only becomes effective when combined with a halogenated (usually brominated) flame retardant, where it sharply boosts the flame-poisoning effect.
What is the ratio of antimony trioxide to brominated flame retardant?
A bromine-to-antimony weight ratio of about 3:1 is the common starting point. It varies with resin and bromine source — PP often runs leaner on antimony (3:1 to 4:1) than ABS. The correct ratio is the one that passes UL94 at your wall thickness, confirmed by burn test.
Is antimony trioxide banned or RoHS restricted?
It is not one of the six RoHS-restricted substances, so it is not banned in electronics the way PBB/PBDE are. However, ECHA lists certain antimony compounds as SVHC, so expect tighter documentation and keep an eye on future REACH action — it is a “watch and declare” material, not a “banned” one.
What can replace antimony trioxide?
For partial replacement, zinc borate is the most common co-synergist and adds smoke suppression. To remove antimony completely you have to leave halogenated chemistry and move to a phosphorus or mineral halogen-free system — there is no exact drop-in substitute within a brominated formula.
Those LOI figures come with a caveat worth knowing before you optimise against them: raising oxygen index and passing UL94 are not the same goal, and dripping pushes them in opposite directions. Explained in limiting oxygen index.
If You Are Trying to Cut Antimony: Where to Start
This is the question we get asked most on this subject, and most sources answer it with methodology rather than numbers. Here are the numbers we would start a trial from — with the caveat that follows them, which matters as much as the figures do.
| Your Situation | Starting Point | What It Costs You |
|---|---|---|
| Antimony price is the driver No policy requirement | Zinc borate, partial replacement of up to roughly 75% of the Sb₂O₃, at 2–10 phr | Usually the cheapest route. Smoke improves. Expect to re-run burn testing |
| Antimony must go entirely Customer or market policy | Zinc stannate or hydroxystannate at 1–5 phr — note this is lower than the antimony it replaces | Higher price per kilo, partly offset by lower loading and better smoke |
| Concrete anchor Flexible PVC, published case | A system running 7.5% ATO reached comparable fire performance on about 2.5% zinc hydroxystannate — roughly 1:3 | Use as a first trial point, not as a specification |
| Demanding brominated polyolefin | Partial replacement first, laddered in steps with burn testing at each step | Slower, but full swap in one move fails here more often than it works |
Now the caveat, and it is not a hedge. There is no fixed substitution ratio, because the answer moves with the polymer system, the halogen source, the filler package and the fire standard you have to meet. Anyone who gives you a single universal number without asking what resin you run is guessing. Those figures above are trial starting points, and they have to be confirmed with LOI and UL94 on your own formulation before they go anywhere near a specification.
What we would actually advise, in order: first confirm whether your driver is price or policy, because that alone decides which of the two routes above you should be testing. If it is price, do not remove antimony entirely — partial replacement with zinc borate captures most of the saving at a fraction of the reformulation risk. If it is policy, budget for a re-certification cycle rather than assuming a drop-in swap. And in either case, ladder the substitution across three or four trial points instead of jumping to the target in one step; the failure mode is almost always a formulation that passes at the bench and drifts at production loading.
Final Thoughts
Antimony is a synergist, not a flame retardant, which is why cutting it changes the whole package rather than one line item. If cost or policy is pushing you to reduce it, zinc borate replaces a substantial share in most PVC systems, and zinc stannate takes it out entirely where antimony must go. Both change smoke behaviour too. Send your current loading and target rating and we will work out the substitution ratio for your formulation.
If reducing antimony is the actual project rather than understanding how it works, the routes and starting ratios are worked through separately in cutting or replacing antimony trioxide — including the case where the honest answer is to reduce rather than remove. The antimony line also varies sharply by region — the UK index sits near double China’s — with the figures set out in what moves the decabromodiphenyl ethane price.
—— Rectivas Materials 团队 技术工程师老陈