Antimony Trioxide: Properties, Uses in Plastics and Regulatory Status

Antimony trioxide (ATO, Sb2O3, CAS 1309-64-4) is an inorganic metal oxide used as a synergist for halogenated flame retardants in plastics, which means it adds almost no fire protection on its own and works only when a bromine or chlorine donor is present. That dependence explains both why antimony trioxide appears in almost every brominated flame retardant recipe and why a bromine phase-out removes the reason to use it at all.

Antimony trioxide carries a harmonised classification as a Category 2 carcinogen (H351) under the CLP Regulation (EC) No 1272/2008, the International Agency for Research on Cancer placed trivalent antimony in Group 2A in Monographs Volume 131 (2022), California has listed it for cancer under Proposition 65 since 1 October 1990, it is an authorised EU food-contact additive with a specific migration limit of 0.04 mg/kg expressed as antimony, and it is not on the REACH Candidate List. Antimony trioxide is one of 87 flame-retardant pages in our directory of plastic additives, each with the same identity, dosage and regulatory fields.

This reference sets out the identity of antimony trioxide, the four-step antimony-halogen reaction behind its synergy, its properties, its loading per polymer, its 3 plastics applications, its UL 94 and LOI performance, its behaviour beside other additives, its dated regulatory matrix, its safety profile, the 5 named substitutes and the export-control risk now driving substitution.

Table T1. Identity of antimony trioxide (ATO).

What Is Antimony Trioxide (ATO)?#

Antimony trioxide is the trivalent oxide of antimony, Sb2O3, a white odourless solid that plastics compounders add as a flame retardant synergist rather than as a flame retardant in its own right. Its chemical class is the inorganic metal oxides, its molecular weight is 291.52 g/mol, and its registered identity is CAS 1309-64-4 and EC 215-175-0, the numbers that appear on every safety data sheet and every customs declaration for the material.

Which substances does the label ATO actually cover, and which does it not? The label covers one compound under five common names: diantimony trioxide, antimony(III) oxide, antimony oxide, senarmontite and valentinite, the last two being the mineral forms. It does not cover the antimony(V) compounds, and it does not cover any halogen donor, because the synergist carries no bromine or chlorine of its own. As a synergist, antimony trioxide belongs to the halogenated branch of flame retardants for plastics, the hub that compares every chemical family against the others, and it is the only member of that branch whose entire function depends on a second additive being present in the same compound.

What does ATO stand for, and what is Sb2O3?#

ATO stands for antimony trioxide, and Sb2O3 is its molecular formula: two antimony atoms in the +3 oxidation state bound to three oxygen atoms, giving a molecular weight of 291.52 g/mol. The oxidation state matters, because this is antimony(III), not antimony(V): the +5 oxide Sb2O5 is a separately registered substance under CAS 1314-60-9 with its own CLP entry. ATO on this site is also not antimony-doped tin oxide, the transparent conductive oxide that electronics literature abbreviates the same way, which this reference always writes out in full.

ATO or ATH? Two abbreviations that are not interchangeable#

ATO and ATH are two of the most confused abbreviations in flame retardancy: ATO is antimony trioxide (Sb2O3, CAS 1309-64-4), a synergist that needs a halogen donor, while ATH is aluminum trihydrate (Al(OH)3, CAS 21645-51-2), a mineral flame retardant that works alone by releasing water. ATO is therefore not aluminum trihydrate (ATH), and the working principles are opposites. Antimony trioxide acts in the gas phase and carries halogen into the flame at loadings of a few percent, whereas the endothermic mineral absorbs 1,051 J/g as it decomposes and is dosed at 30 to 65 wt% because its effect scales with mass. One is a multiplier, the other is the flame retardant itself. Aluminum trihydrate (ATH) releases water endothermically and needs no halogen partner, which is exactly the working principle that antimony trioxide lacks.

Table T1b. Antimony trioxide (ATO) versus aluminum trihydrate (ATH).

Is antimony trioxide a flame retardant on its own?#

No: antimony trioxide has no flame-retardant effect on its own, and it only works when the compound already contains a bromine or chlorine donor that can convert it into volatile antimony halides. The finding of no flame-retardant effect alone comes from the mechanism literature rather than from formulating convention: on its own in a polymer the oxide behaves as an inert white filler, because the reaction that produces the active species starts with the hydrogen halide that the donor releases.

The consequence for the formulator is that a flame retardant package is specified as a pair, never as a single ingredient: a halogen source plus a synergist, with the two levels set together. Antimony trioxide is the reference member of the flame retardant synergists, the group that multiplies another additive instead of acting alone, and a halogen-free redesign deletes both lines from the recipe at once.

Is antimony trioxide a natural mineral?#

Antimony trioxide occurs naturally as two minerals, cubic senarmontite and orthorhombic valentinite, and the same two crystal forms exist in the industrial product: the cubic form converts to the orthorhombic form above 606 °C (1,122.8 °F). Both forms have the same formula and the same CAS number, and they differ in density, at 5.2 g/cm3 for the cubic alpha form and 5.67 g/cm3 for the orthorhombic beta form. This reference does not state which crystal form commercial flame retardant grades use, because no such statement is held in our sources.

How Does Antimony Trioxide Work with Halogenated Flame Retardants?#

Antimony trioxide works by converting the hydrogen halide released by a brominated or chlorinated flame retardant into antimony oxyhalides and finally into volatile antimony trihalide, which carries the halogen into the flame and traps the H· and OH· radicals that keep combustion going. The active species is therefore never the oxide itself but the antimony trihalide (SbX3) formed in situ, which is why the two additives have to decompose in the same temperature window to work together.

Why does adding an inert white oxide change how a bromine compound burns? Because a halogenated flame retardant on its own releases hydrogen bromide that scavenges the chain-carrying H· and OH· radicals and then diffuses out of the flame zone, while antimony trihalide is heavier, stays in the reaction zone longer and returns the halogen to it repeatedly. Both the halogen and the antimony act in the gas phase rather than in the condensed phase, so the pair suppresses the flame chemistry instead of building a protective char. The gas-phase and condensed-phase routes are compared on how flame retardants work, where each class is placed on the same scale.

The stepwise antimony-halogen reaction sequence#

The conversion of antimony trioxide into volatile antimony trihalide runs in four steps, and each step releases SbX3 over a different temperature window, which is why the synergist keeps feeding the flame zone instead of being consumed at once. The sequence below follows the mechanism review published in Materials in 2021 (doi 10.3390/ma14247901), where X stands for bromine or chlorine depending on the donor.

1. Oxyhalide formation. Sb2O3 + 2HX → 2SbOX + H2O. The oxide reacts with the hydrogen halide released by the donor and the first antimony oxyhalide appears.
2. First trihalide release. 5SbOX → Sb4O5X2 + SbX3. The oxyhalide disproportionates, volatile antimony trihalide leaves for the gas phase and a more oxygen-rich oxyhalide remains.
3. Second trihalide release. 4Sb4O5X2 → 5Sb3O4X + SbX3. The intermediate condenses further and delivers a second portion of SbX3 at a higher temperature.
4. Third trihalide release and regeneration. 3Sb3O4X → 4Sb2O3 + SbX3. The last oxyhalide gives up its halogen and antimony trioxide is regenerated, which closes the cycle.

The practical reading of that ladder is that the synergist is a halogen shuttle rather than a consumable: each rung delivers antimony trihalide at a different temperature, and the last step puts the oxide back into the condensed phase to start again.

How much halogen does antimony trioxide need?#

The loading is set by the bromine content of the donor, not by a universal ratio: in Albemarle's published HIPS data, 10.7 wt% bromine combined with 5 wt% antimony trioxide reaches UL 94 V-0 at 0.8 mm. That bromine level corresponds to roughly 8 to 12 wt% of decabromodiphenyl ethane in the compound, because the donor carries at least 82 % bromine by weight.

The same 5 wt% of antimony trioxide therefore supports a very different donor loading when the donor's bromine content differs. This reference prints no weight-ratio rule of thumb for bromine to antimony, because no sourced ratio is held in our sources, and compounders confirm the pairing on a rated test bar at the target wall thickness.

What Are the Physical and Chemical Properties of Antimony Trioxide?#

Antimony trioxide is a white odourless solid with a melting point of 656 °C (1,212.8 °F), a boiling point of 1,425 °C (2,597 °F) at which it sublimes, and a density of 5.2 g/cm3 in the cubic form or 5.67 g/cm3 in the orthorhombic form. Table T2 collects the values that a data sheet comparison usually turns on.

Table T2. Physical and chemical properties of antimony trioxide.

The practical point in that table is the thermal stability. A melting point of 656 °C (1,212.8 °F) sits far above the processing window of every thermoplastic that uses the synergist, so antimony trioxide passes through compounding and injection moulding unchanged and only enters the reaction sequence in the flame itself. Values on this page follow PubChem (CID 14794); Google's AI Overview currently prints 655 °C for the melting point, so check the data source when comparing two sheets. Solubility, particle size, refractive index and oil absorption are not stated here, because no sourced value for any of them is held in our sources.

Which Polymers Use Antimony Trioxide, and at What Dosage?#

Antimony trioxide is used in the polymers that carry a halogen source: HIPS, ABS, polypropylene, polyamide and PBT with a brominated flame retardant, and PVC, which supplies its own chlorine. In every one of them the synergist is dosed together with the donor, and the published levels are pair values rather than stand-alone loadings, as Table T3 shows.

Table T3. Antimony trioxide level by polymer.

How do the phr values in a PVC recipe compare with the wt% values in a styrenics recipe? The conversion is arithmetic: wt% equals the phr of the ingredient divided by the total phr of the formulation, multiplied by 100. A flexible PVC compound of 100 phr resin, 55 phr plasticizer, 50 phr aluminum trihydrate and 5 phr antimony trioxide totals 210 phr, so the 5 phr of synergist is 2.4 wt% of the compound. PVC recipes state antimony trioxide in PHR (parts per hundred resin), which converts to weight percent only with the full formulation total in hand, and comparing a phr figure with a wt% figure without that step is the most common dosage error in flame retardant specifications.

Antimony trioxide in HIPS and ABS#

Styrenics are the classic bromine-antimony market: in Albemarle's published HIPS formulation, 10.7 wt% bromine from decabromodiphenyl ethane plus 5 wt% antimony trioxide reaches UL 94 V-0 at a wall thickness of 0.8 mm. The donor in that formulation is DBDPE (decabromodiphenyl ethane), CAS 84852-53-9, which carries at least 82 % bromine and melts above 345 °C (653 °F), a thermal margin that keeps it intact through styrenic compounding.

ABS uses the same pairing with a different donor set. Tetrabromobisphenol A serves as an additive flame retardant in ABS housings, where it is blended rather than reacted into the chain, and the antimony trioxide level again follows the bromine content that the donor delivers. No separate ABS loading figure is stated here, because none is held in this reference. The full package for a flame-retarded housing, including impact modifier and anti-drip agent, is on additives for ABS.

Antimony trioxide in flexible PVC cable compounds#

Flexible PVC needs no separate bromine donor, because the resin itself supplies chlorine, so a cable compound uses antimony trioxide at about 5 phr alongside 50 phr of aluminum trihydrate in Huber's comparison formulations. The chlorine released as hydrogen chloride during PVC decomposition is the halogen that the synergist converts into antimony trichloride, which is why the same fire package that needs 10.7 wt% added bromine in HIPS needs none at all here.

The rest of the recipe matters as much as the synergist. Huber's reference cable formulation of PVC K70 100 phr, DIDP 55 phr, stabilizer 2.7 phr, aluminum trihydrate 45 to 100 phr, zinc borate 5 phr and chalk 10 phr reaches a limiting oxygen index of 26 to 27 % with a UL 94 V-0 rating at 3 mm, and zinc borate at 3 to 6 phr alongside the mineral reduces both dripping and smoke. Smoke suppressants and the rest of the PVC fire package are compared on flame retardants and smoke suppressants for PVC, where each component is separated by function.

Antimony trioxide in polypropylene, polyamide and PBT#

In polypropylene, polyamide and PBT, antimony trioxide is paired with a high-temperature bromine donor such as brominated polystyrene, which survives the 240 to 320 °C (464 to 608 °F) processing window that engineering compounds need. Brominated polystyrene is the thermally stable brominated flame retardant for glass-filled PA66 and PBT, and it is used either with antimony trioxide or with the tin synergists, which makes the synergist choice a formulation decision rather than a fixed pairing.

No antimony trioxide loading figure for polypropylene, polyamide or PBT is held in this reference, so none is printed. Polyesters carry an additional constraint that styrenics and polyolefins do not, because antimony trioxide can attack the polyester chain itself, and the interactions section below sets out why PET and PBT systems commonly use an antimony(V) synergist instead.

What Is Antimony Trioxide Used For? 3 Application Areas in Plastics#

Antimony trioxide has 3 plastics application areas: flame-retarded enclosures and housings in styrenics and engineering plastics, wire and cable compounds, and PVC film and sheet. Its other recorded uses, as a PET polycondensation catalyst, as a glass and ceramic opacifier, in pigments and in textile back-coatings, are not plastics-additive uses and are treated below the contextual border at the end of this page.

• Electrical and electronic enclosures. Flame-retarded HIPS, ABS and engineering compounds for housings, stands and internal parts.
• Wire and cable compounds. Flexible PVC insulation and sheathing, where the resin supplies the chlorine.
• PVC film and sheet. Calendered and extruded products where fire performance and migration behaviour are specified together.

Electrical and electronic enclosures#

Flame-retarded HIPS and ABS enclosures are the largest plastics use of antimony trioxide, because a thin-wall housing has to reach UL 94 V-0 at 0.8 mm without losing impact strength. The synergist earns its place there by letting the compounder hit the rating at a total additive load that a thin wall and a high-gloss surface can still tolerate.

The regulatory counter-force sits in the same application. Since 1 March 2021, Regulation (EU) 2019/2021 has banned halogenated flame retardants in the enclosures and stands of electronic displays, which removes the bromine donor and with it the reason for the synergist, and New York has applied a comparable ban on organohalogen flame retardants in display enclosures since 1 January 2024. The full enclosure formulation, including the anti-drip agent, is on additives for electrical and electronics.

Wire and cable compounds#

PVC cable compounds combine antimony trioxide with aluminum trihydrate and zinc borate, and Huber's reference recipe reaches a limiting oxygen index of 26 to 27 % with a UL 94 V-0 rating at 3 mm. PVC cable insulation is specified to a minimum limiting oxygen index of 26 vol % oxygen, so that recipe clears the requirement with a narrow margin and the mineral filler carries most of the load. The stabilizer, plasticizer and filler side of the same recipe is on additives for wire and cable compounds.

Smoke is the second specification in this application, and it is the one the synergist does not improve. Zinc borate at 3 to 6 phr alongside the mineral reduces dripping and smoke, which is why cable recipes carry it beside the antimony rather than instead of it. Construction Products Regulation classes and the halogen-free alternatives are covered on flame retardants for wire and cable.

PVC film and sheet#

PVC film and sheet are the second chlorine-based use, and here the 0.04 mg/kg specific migration limit for antimony decides whether the compound can also serve a food-contact application. The limit applies to the finished article, not to the additive in the bag, so a film maker has to track antimony migration from every source in the compound at once.

No antimony trioxide loading figure for PVC film is held in this reference, so none is printed here. How the synergist sits beside the stabilizer and plasticizer package is set out on additives for PVC, where the film and sheet recipes are separated from the cable recipes.

How Well Does Antimony Trioxide Perform? UL 94 and LOI Data#

The published performance data for antimony trioxide are always pair data: 5 wt% with 10.7 wt% bromine gives UL 94 V-0 at 0.8 mm in HIPS, and 5 phr with 50 phr aluminum trihydrate sits inside a PVC cable recipe that reaches LOI 26 to 27 % and V-0 at 3 mm. Neither number belongs to the synergist alone, and neither transfers to another polymer, another donor or another wall thickness.

Table T4. Measured performance of systems containing antimony trioxide.

The UL 94 V-0 classification behind the first two rows is a set of four pass criteria, not a single number: no single afterflame longer than 10 s, no more than 50 s of total afterflame over ten flame applications to five specimens, afterglow no longer than 30 s, and no flaming drips that ignite the cotton indicator below the bar. The last criterion is the reason most commercial packages add an anti-drip agent. The V-0, V-1 and V-2 criteria and the thickness rule are explained on UL 94 flammability ratings.

The oxygen index result is the second, independent measure. The 26 to 27 % figure is a limiting oxygen index (LOI) value measured to ISO 4589, and it states the minimum oxygen concentration in a nitrogen-oxygen mixture at which the specimen keeps burning, which makes it a ranking tool rather than a fire-safety classification. No cone-calorimeter, heat-release, smoke-density or mechanical-property value for antimony trioxide is held in this reference, so this page names those test methods without numbers. Pick a starting system for your polymer and target rating with the flame retardant selector.

How Does Antimony Trioxide Interact with Other Additives?#

Antimony trioxide is never the only functional additive in a flame-retardant compound: it needs a halogen donor, it is often partly replaced by zinc borate, it is combined with a PTFE anti-drip agent, and it competes for compatibility with the antioxidant and antistatic package. The four groups below set out what each one does to the synergist.

• Halogen donors. Decabromodiphenyl ethane, TBBPA (tetrabromobisphenol A), brominated polystyrene, decaBDE as a legacy material and chlorinated paraffins, which act as secondary plasticizer and flame retardant at once in PVC and take antimony oxide as their synergist.
• Co-synergists. Zinc borate at 3 to 6 phr in PVC, which promotes char, cuts dripping and smoke, and replaces part of the antimony load.
• Anti-drip agents. PTFE, which fibrillates in the melt and holds the burning polymer together, so the no-igniting-drips criterion of UL 94 V-0 is met.
• Antagonists. Antioxidants, whose effectiveness is reduced by flame retardants, and amine or amide antistats, which react with halogenated flame retardants.

Two of those interactions change the formulation rather than merely the performance. Where the particle size of antimony trioxide causes light scattering in a thin film or costs impact strength in a thin-wall part, formulators move to sub-micron colloidal antimony pentoxide grades. Most commercial packages also add a PTFE anti-drip agent, because UL 94 V-0 forbids flaming drips that ignite the cotton.

Why antimony trioxide is not used in PET and PBT flame retardant systems#

Antimony trioxide is acidic enough to catalyse the depolymerisation of polyesters, so PET and PBT systems use sodium antimonate instead, an antimony(V) synergist that gives the same halogen synergy without attacking the chain. The difference is chemical rather than physical: sodium antimonate, NaSbO3, CAS 15432-85-6, is less acidic and less catalytically active towards polyester depolymerisation than Sb2O3, which is the whole reason the substitution exists.

Glass-filled polyamide and PBT compounds show the same division of labour from the donor side, since brominated polystyrene is paired either with an antimony(V) synergist or with the zinc stannates. Polyester systems use sodium antimonate instead, an antimony(V) synergist that does not attack the chain, and the choice is made at the resin level rather than at the rating level.

What Is the Regulatory Status of Antimony Trioxide?#

Antimony trioxide is registered under REACH, is not a Substance of Very High Concern, carries a harmonised CLP classification as a Category 2 carcinogen (H351), is authorised for EU food-contact plastics with a specific migration limit of 0.04 mg/kg expressed as antimony, and has been on California's Proposition 65 list for cancer since 1 October 1990 (status 23 September 2026). Table T5 sets out every instrument that names the substance or reaches it indirectly.

Table T5. Regulatory matrix for antimony trioxide, status 23 September 2026.

Three cells in that matrix read "status being verified" rather than "no", and the distinction is deliberate: this reference prints a negative only where the underlying list is closed and has been checked, as it is for RoHS and the EU POPs Regulation. What registration does and does not mean is explained on REACH and plastic additives, which separates registration from authorisation and restriction.

Is antimony trioxide REACH registered, and is it an SVHC?#

Yes, antimony trioxide is registered under REACH (Regulation (EC) No 1907/2006) under EC number 215-175-0, and no, it is not a Substance of Very High Concern: it has never appeared on the REACH Candidate List. Registration is a dossier obligation tied to tonnage, so it certifies that data exist and that uses are declared, not that a substance has been assessed as acceptable.

Its partners in the same formulation are on a different footing. DBDPE was added to the SVHC Candidate List on 5 November 2025 for very persistent and very bioaccumulative properties, and is identified rather than restricted, while TBBPA was added on 17 January 2023. In a brominated recipe, the regulatory pressure currently sits on the donor rather than on the synergist.

Is antimony trioxide allowed in food-contact plastics?#

Yes, in the EU: antimony trioxide is listed in Annex I of Regulation (EU) No 10/2011 as FCM substance 398, reference 35760, with a specific migration limit of 0.04 mg/kg expressed as antimony, which is one of the lowest limits in the Union list. That limit is set through the Annex I entry for the substance itself and not through the Annex II table of metal limits, which has a practical consequence: the 0.04 mg/kg ceiling applies to antimony in the food simulant whatever its origin in the article, whether it arrives as a flame retardant synergist or as a polycondensation catalyst residue.

The specific migration limit works alongside the overall migration limit of 10 mg/dm2, which becomes 60 mg/kg for articles intended for infants and young children, so a compliant article satisfies both tests. How Annex I entries and the overall migration limit work is explained on EU 10/2011. The US food-contact position is being verified for this page, so no 21 CFR section or FCN number is stated here.

Is antimony trioxide restricted under RoHS or the EU Ecodesign rules?#

Antimony trioxide is not itself restricted by RoHS or by the EU Ecodesign rules, but both instruments reach it indirectly, because the Ecodesign Regulation (EU) 2019/2021 has banned halogenated flame retardants in the enclosures and stands of electronic displays since 1 March 2021 and a synergist without a halogen donor has no function. The Ecodesign ban is therefore the single most consequential rule on this page even though it never names the substance.

RoHS Directive 2011/65/EU lists ten restricted substances, among them the polybrominated diphenyl ethers and polybrominated biphenyls at 0.1 % by weight in homogeneous material, and antimony trioxide is not among them. The planned addition of TBBPA and medium-chain chlorinated paraffins to that list was dropped in 2024, so the restricted set is unchanged. The ten restricted substances are listed on RoHS and plastic additives, with the exemption structure that governs electronics compounds.

Is antimony trioxide listed under California Proposition 65?#

Yes: antimony trioxide has been on the California Proposition 65 list as a carcinogen since 1 October 1990, where it is named "Antimony oxide (antimony trioxide)". No safe harbour level for the substance is stated here, because no No Significant Risk Level for it is held in this reference. Listing dates for every flame retardant are on California Proposition 65, where TCEP appears from 1 April 1992, TDCPP from 28 October 2011, TBBPA from 27 October 2017 and molybdenum trioxide from 19 March 2021.

Is Antimony Trioxide Safe? Health, Safety and Environmental Profile#

Antimony trioxide carries a harmonised classification as a Category 2 carcinogen (H351, "suspected of causing cancer") under CLP Annex VI, index number 051-005-00-X, and the International Agency for Research on Cancer placed trivalent antimony in Group 2A, probably carcinogenic to humans, in Monographs Volume 131 (2022). Those two statements come from different systems, one a legal classification and the other a hazard evaluation, and they agree on direction while differing in strength. The full evidence position rests on 3 elements.

• Classification. Harmonised CLP entry Carc. 2, H351, under Annex VI index 051-005-00-X; the International Antimony Association self-classification adds STOT RE 2, H373 for repeated-exposure organ toxicity.
• Authoritative reviews. IARC Group 2A for trivalent antimony (Monographs Volume 131, 2022); the US National Toxicology Program listed antimony trioxide in its 15th Report on Carcinogens (December 2021); California listed it under Proposition 65 for cancer on 1 October 1990.
• Occupational handling. The ACGIH threshold limit value is 0.5 mg/m3 expressed as antimony, which is a workplace air concentration and applies to dust exposure during handling, weighing and compounding.

What a harmonised entry outranks, and how a self-classification sits beside it, is explained on CLP classification of plastic additives. This page prints no LD50, no NOAEL and no leaching figure, because no such value is held in this reference.

The exposure route that drives every one of those classifications is inhalation of the dust during handling and compounding, which is why the occupational limit is an airborne concentration and why dust-free delivery forms, closed transfer and local exhaust ventilation are the controls that matter in a compounding plant. Bound synergist in a finished part is governed by the 0.04 mg/kg migration limit instead. Exposure from finished articles, which is a different question from the powder, is covered on flame retardants and human health.

What Are the Alternatives to Antimony Trioxide?#

The 5 substitutes for antimony trioxide named in the flame-retardant literature are zinc stannate, zinc hydroxystannate, zinc borate, sodium antimonate and antimony pentoxide, and a sixth route drops the halogen donor as well and moves the whole compound to a halogen-free system. Table T6 compares them on the four properties that decide a substitution: whether they still need a halogen donor, their classification, their identity and the polymer where they take over.

Table T6. Antimony trioxide compared with 5 synergist alternatives.

Zinc borate's harmonised CLP status is an open verification item in our source library: the Repr. 2, H361d classification is a supplier self-classification, not an ECHA harmonised entry.

One reading of that table matters more than the rest: no alternative removes the halogen donor, so switching synergist changes the classification profile of a compound without changing its combustion chemistry, and only the sixth route below changes both. All six are compared side by side under antimony and other synergists, where the class is treated as a whole.

Antimony trioxide vs zinc stannate and zinc hydroxystannate#

The two tin synergists are the direct functional replacements: zinc stannate (ZnSnO3, CAS 12036-37-2) and zinc hydroxystannate (ZnSn(OH)6, CAS 12027-96-2) give the same halogen synergy while also suppressing smoke, and neither carries a carcinogenicity classification. Zinc stannate, EC 405-290-6, molecular weight 232.1 g/mol, is described in the supplier literature as an antimony trioxide replacement synergist and smoke suppressant, and it is not on the REACH Candidate List. Zinc stannate gives halogen synergy plus smoke suppression without a carcinogenicity classification, which is the single clearest argument for the substitution.

Zinc hydroxystannate, EC 404-410-4, molecular weight 286.1 g/mol, adds an endothermic water release to the tin-catalysed char formation, which is why it behaves as both a synergist and a smoke suppressant. In studies, modified zinc hydroxystannate reduced total smoke production by about 46 to 53 %, and zinc hydroxystannate is used at 3 to 5 phr in PVC research formulations. Neither tin compound is a Substance of Very High Concern.

Antimony trioxide vs zinc borate#

Zinc borate is a partial rather than a full replacement: in PVC it is used at 3 to 6 phr alongside aluminum trihydrate to cut dripping and smoke, and its hydrated grades release water between 290 and 415 °C (554 and 779 °F), but it does not reproduce the gas-phase halogen transport that antimony trioxide provides. Its own mechanism is condensed-phase, through a glassy borate layer and char promotion at the burning surface, so it complements rather than replicates the synergist.

Three hydrate identities appear in the trade literature, CAS 138265-88-0 for the 2ZnO·3B2O3·3.5H2O grade, CAS 12767-90-7 and CAS 149749-62-2, so buyers name the hydrate on the specification. Zinc borate is a partial replacement that also promotes char and cuts afterglow, and it is not a Substance of Very High Concern, although its harmonised CLP status remains an open verification item in this reference.

Antimony trioxide vs sodium antimonate and antimony pentoxide#

The two antimony(V) compounds keep the antimony chemistry but change the physical form: sodium antimonate suits polyesters that antimony trioxide would depolymerise, and colloidal antimony pentoxide suits fibres and films where the particle size of antimony trioxide would scatter light or cost impact strength. Both deliver the same halogen-antimony synergy through the same gas-phase route, so the performance question is one of compatibility rather than of mechanism.

Their classification differs from that of the trioxide in a way that is easy to misread. Sodium antimonate (NaSbO3, 192.75 g/mol) and antimony pentoxide (Sb2O5, CAS 1314-60-9, EC 215-237-7, 323.52 g/mol) are both covered by the harmonised CLP group entry 051-003-00-9 for antimony compounds, which carries H332, H302 and H411, while antimony trioxide has its own entry 051-005-00-X with the Carc. 2 classification. A different entry is not a statement that one substance is safer; it is a statement that the two were assessed separately.

Replacing the whole bromine-antimony package with a halogen-free system#

The sixth route removes the reason for a synergist altogether: a halogen-free system built on aluminum trihydrate, magnesium hydroxide, aluminium diethylphosphinate or an intumescent ammonium polyphosphate package needs no antimony, because none of these acts through gas-phase halogen transport. The US Geological Survey names selected organic compounds and hydrated aluminum oxide as the flame retardant substitutes for antimony.

The trade-off is loading and processing temperature. Aluminum trihydrate releases 1,051 J/g from about 200 °C (392 °F) and is used at 60 to 65 wt% for UL 94 V-1 to V-0 in thermoplastics, magnesium hydroxide is stable to about 320 °C (608 °F) and releases 1,316 J/g, aluminium diethylphosphinate reaches V-0 at 0.8 mm in PA 6T/66 at about 15 wt%, and intumescent polypropylene needs 22 to 30 wt% of an ammonium polyphosphate system for V-0. Loadings, processing ceilings and cost for the whole class are set out on halogen-free flame retardants; the regulatory drivers are the Ecodesign display ban of 1 March 2021 and the New York organohalogen ban of 1 January 2024.

Who Supplies Antimony Trioxide? Grades, Supply and Price Risk#

Antimony trioxide supply is a mining question before it is a chemistry question: the US Geological Survey puts world antimony mine production at 110,000 tonnes of contained antimony in 2025, of which China accounts for 40,000 tonnes, Russia 32,000 and Tajikistan 22,000. No producer, grade name or trade name for antimony trioxide is held in this reference, so no company is named on this page. Grade selection is a particle-size and dispersion question rather than a chemistry question, which is why formulators move to sub-micron colloidal antimony pentoxide where the particle size of the trioxide scatters light or costs impact strength. Company profiles, locations and certifications are in the directory of flame retardant manufacturers and suppliers.

The supply risk is concentrated and recent. China required export licences for antimony ore, metal, oxides and compounds in August 2024 and banned antimony exports to the United States in December 2024, and US antimony oxide imports still rose to an estimated 39,000 tonnes of contained antimony in 2025, against 14,000 tonnes in 2023. As a supply-risk indicator, and not as an oxide price, the US Geological Survey reports an average antimony metal price of USD 25 per pound in 2025 (Argus assessment, 99.65 % Sb, CIF), against USD 10.24 in 2024 and USD 5.49 in 2023. The oxide price and its drivers are tracked on antimony trioxide price.

Table T7. Antimony supply and trade indicators (USGS Mineral Commodity Summaries 2026).

Buyers should ask for the supplier's safety data sheet, a certificate of analysis with the antimony assay, and the country of origin, because the origin now decides both the duty position and the export-licence risk.

How Does Antimony Trioxide Fit into the Synergist Family?#

Antimony trioxide is the reference member of the flame retardant synergist class, the group of substances that carry little fire performance of their own and exist to multiply a primary flame retardant, alongside zinc borate, the tin synergists and the PTFE anti-drip agents. The class also includes antimony pentoxide, sodium antimonate and melamine polyphosphate used with the phosphinates, and its defining property is dependence: every member needs a partner additive to have any effect at all.

Seen from the market rather than from the formulation, the class is small and the parent family is large. Analyst estimates put the flame retardant market at USD 8.1 to 9.3 billion in 2025, and brominated flame retardants accounted for 390,000 tonnes in 2011, about 19.7 % of flame retardant volume, which is the donor volume that the antimony synergists are tied to. Volumes, raw-material exposure and the supply risks of the whole family are on flame retardants market.

Antimony trioxide as a PET polycondensation catalyst#

Antimony trioxide has a second life outside additive chemistry: it is the standard polycondensation catalyst for PET, which is a polymerization catalyst rather than a plastic additive and therefore sits outside the scope of this site, but it is the reason the EU sets a specific migration limit of 0.04 mg/kg for antimony. PET made by that route carries antimony catalyst residue, and the migration limit governs that residue exactly as it governs the synergist in a PVC film.

Storage changes the measured residue. In one survey of 132 brands from 28 countries, antimony in PET-bottled water rose by an average of 90 % over six months of storage in 48 European brands and by 19 % in 14 Canadian brands (Shotyk and Krachler, Environmental Science & Technology, 2007). This reference states the relative change only, because no absolute concentration from that survey is held in it, and the survey is not a finding that any limit was exceeded. Catalyst residues, reheat additives and acetaldehyde scavengers are covered on additives for PET resin.

Non-plastics uses of antimony trioxide (outside this site's scope)#

Outside plastics, antimony trioxide is used as an opacifier in glass and ceramics, as a component of pigments and as a flame retardant in textile back-coatings, none of which is covered on this site. Those categories are named here only so that a reader who arrived from a ceramics, pigment or textile-finishing query can see at once that this page answers a different question.

The scope line is the same one applied across this reference: additives used in plastics are in, and glass, ceramics, coatings, textile finishing, paper chemicals and pharmaceuticals are out. No dosage, process temperature or specification for any of those uses appears on this page.

Is antimony trioxide banned in Europe?#

No: antimony trioxide is registered under REACH, is not on the Candidate List and is authorised for EU food-contact plastics with a 0.04 mg/kg migration limit, although its Annex XVII position is being verified for this page (status 23 September 2026). The restriction entries that do cover plastic additives are listed on REACH Annex XVII restrictions, where each entry names its substances and its scope.

Is antimony trioxide a PFAS?#

No: antimony trioxide is an inorganic metal oxide with no carbon atoms and therefore no perfluorinated carbon chain, so it falls outside every PFAS definition. The question reaches flame retardancy through two other materials, the PTFE anti-drip agents and potassium perfluorobutane sulfonate in polycarbonate, and the fluorinated additives that the question really concerns are covered on PFAS in plastics.

What is the HS code for antimony trioxide?#

Antimony oxide enters the United States under HTS 2825.80.0000, which carries a free duty rate (US Geological Survey, Mineral Commodity Summaries 2026). That is the US tariff line, and only its six-digit root 2825.80 is internationally harmonised, so an EU Combined Nomenclature code or another national code has to be taken from the importing country's own tariff. Tariff lines for every additive family are listed under plastic additive trade.