Quick answer: Before you replace antimony trioxide, work out what is actually driving it — price, a customer policy, or a regulatory read. The answer changes the route completely. If it is cost, you do not need to remove antimony at all: zinc borate replaces a large share of it cheaply. If antimony must go entirely, zinc stannate does it at lower loading than the antimony it replaces. Leaving halogenated chemistry altogether is the most expensive of the three and often unnecessary.
Antimony trioxide is a synergist, not a flame retardant. It does almost nothing on its own — it works by making halogenated chemistry more efficient in the flame. That single fact is why replacing it is a package decision rather than a one-for-one swap, and why the substitution ratio question does not have one universal answer.
First: Which of the Three Drivers Is Yours?
We get asked “what replaces antimony” far more often than “why are you replacing it”, and the second question is the one that saves money.
| Your Driver | What You Should Actually Do |
|---|---|
| Antimony price or supply volatility | Do not remove it — reduce it. Partial replacement with zinc borate captures most of the saving at a fraction of the reformulation risk. Full removal to solve a price problem is the expensive answer. |
| A customer or market policy says antimony-free | Then it has to go entirely, and zinc stannate or hydroxystannate is the practical route. Budget for re-testing, not just a material swap. |
| A regulatory read on halogen chemistry generally | Removing antimony does not make a brominated system halogen-free. If the real requirement is halogen-free, you are changing the whole package — see what leaving halogenated chemistry costs. |
That third row is the most common expensive mistake we see: a buyer removes antimony believing it addresses a halogen-free clause. It does not. The bromine is still there.
Where Antimony Actually Stands, Stated Plainly
We supply brominated packages with antimony, so take this as a supplier being straight rather than a warning designed to sell you something else.
- It is legal and in wide use. Nothing here stops you buying or shipping it today.
- IARC classified it Group 2B — possibly carcinogenic to humans — back in 1989.
- The US NTP lists it as reasonably anticipated to be a human carcinogen, based on inhalation studies that produced lung tumours in rats and mice.
- It sits on ECHA’s CoRAP, the rolling list of substances flagged for evaluation.
- It appears on California’s Proposition 65 list.
What we will not claim: we have not verified that it currently sits on the REACH SVHC Candidate List or in Annex XIV, so we are not going to assert either. If that distinction matters to your compliance file, check the ECHA substance page directly rather than taking any supplier’s word for it — including ours.
The practical read: the exposure concern is concentrated in powder handling during compounding, not in the finished part. That is the same distinction that runs through the whole flame retardant toxicity question.
The Substitution Numbers
This is the question that brings most people to this page, so here are the figures we would start a trial from — with the caveat after them, which matters as much as the numbers.
| Route | Starting Point | What It Costs |
|---|---|---|
| Partial — zinc borate | Replaces up to roughly 75% of the Sb₂O₃ in many PVC systems, at 2–10 phr | Cheapest route. Smoke usually improves. Expect to re-run burn testing |
| Full — zinc stannate / hydroxystannate | Typically 1–5 phr — note this is lower than the antimony it replaces | Higher price per kilo, partly offset by lower loading and better smoke performance |
| Published anchor point | A flexible PVC system running 7.5% ATO reached comparable fire performance on about 2.5% zinc hydroxystannate — roughly 1:3 | Use as a first trial point, never as a specification |
| Demanding brominated polyolefin | Ladder the substitution in three or four steps, burn testing at each | Slower — but a full swap in one move fails here more often than it works |
The caveat, and it is not a hedge. There is no fixed substitution ratio, because it moves with the polymer system, the halogen source, the filler package and the standard you must meet. Any supplier who gives you a single universal number without asking what resin you run is guessing. Confirm with LOI and UL94 on your own formulation before anything goes near a specification.
What Changes Besides the Rating
Three side effects that catch people, all of them worth knowing before the first trial.
Smoke usually gets better, not worse. Both zinc borate and zinc stannate suppress smoke, which antimony does not. If your specification has a smoke clause as well as a flame clause, cutting antimony can help you on both — this is the one place where the change works in your favour.
Tracking performance may shift. Antimony residues are part of why brominated systems track poorly. If you are chasing a CTI requirement as well, removing antimony is a step in the right direction, though rarely enough on its own to reach PLC 0.
Colour and dosing change. Zinc stannate runs at much lower loading than the antimony it replaces, which alters your let-down arithmetic and can shift the colour baseline your colour matching was built on. Re-check both.
What We Would Actually Advise
Settle the driver first. Price and policy lead to different routes, and choosing the wrong one costs either money or a failed audit. Ten minutes of asking where the requirement came from is the highest-return thing you can do here.
If it is price, do not remove antimony. Partial replacement with zinc borate captures most of the saving with far less reformulation risk. We would rather sell you a synergist than a whole new package you did not need.
Ladder it, do not jump. Three or four substitution steps with burn testing at each will find the failure point before production does. Compounds that pass at the bench and drift at production loading are the normal failure mode here.
And the case where you should not change anything: if nobody is asking you for antimony-free, your smoke clause is being met, and antimony pricing is not hurting you, then reformulating is spending money to solve a problem you do not have. That happens more often than the industry likes to admit.
FAQ
What is the substitution ratio from antimony trioxide to zinc stannate?
There is no universal ratio. A published flexible PVC case replaced 7.5% ATO with about 2.5% zinc hydroxystannate at comparable fire performance — roughly 1:3 — and zinc stannate generally runs at 1–5 phr, below the antimony it replaces. Treat that as a trial starting point and confirm with LOI and UL94 on your own system, because the answer moves with polymer, halogen source and filler package.
Is antimony trioxide banned?
No. It is legal and widely used. It carries an IARC Group 2B classification from 1989, the US NTP lists it as reasonably anticipated to be a human carcinogen, and it sits on ECHA’s CoRAP evaluation list — but none of that prevents you buying or shipping it today.
Does removing antimony make my compound halogen-free?
No, and this misunderstanding is expensive. Antimony is a synergist for halogenated chemistry; taking it out leaves the bromine or chlorine exactly where it was. If your requirement is genuinely halogen-free, you are replacing the flame retardant itself, not the synergist.
Will an antimony-free system cost more?
Per kilo, usually yes. Per finished part, not always — zinc stannate runs at lower loading, and if your specification has a smoke clause you may recover some of it through better smoke performance. Run the comparison on cost per acceptable part, not on price per kilo.
Can I just reduce antimony instead of removing it?
Yes, and for a cost-driven project that is usually the right answer. Zinc borate can replace a substantial share of the antimony in many PVC systems while improving smoke. You keep the brominated chemistry and most of its efficiency, and you avoid a full re-qualification.
What testing do I need after switching?
UL94 at your actual wall thickness plus LOI as a development metric, and the smoke clause if your specification has one. If the part carries a listing, budget for re-certification — a synergist change is a formulation change, whatever the rating says afterwards.
Final Thoughts
Most antimony reduction projects fail for one of two reasons: nobody checked whether price or policy was driving it, or somebody swapped the whole package in a single step. Settle the driver, then ladder the substitution. Send us your current loading, base polymer and target rating and we will work out a starting ratio for your formulation rather than quoting a universal number — zinc borate for lean-antimony packages, zinc stannate where it has to go entirely, and the additive range if the answer turns out to be a different system altogether.
If it turns out the requirement is genuinely halogen-free rather than antimony-free, the shortlist is a different one entirely — see the full list of halogen-free flame retardants.
—— Rectivas Materials 团队 技术工程师老陈