PVC is often described as a self-extinguishing polymer. This is due to the high chlorine content in the polyvinyl chloride structure: once the ignition source is removed, rigid unplasticized PVC will often not sustain combustion.
However, this does not mean that every PVC compound automatically meets fire-safety requirements. In a practical formulation, PVC resin is combined with plasticizers, fillers, stabilizers, lubricants, pigments, modifiers and other components. These materials affect the behaviour of the compound when heated and exposed to fire.
The fire properties of a PVC product are therefore determined by the formulation as a whole. Additional flame-retardant components are often used in cable compounds, PVC roofing membranes, protective coatings and other materials with specific fire-performance requirements. Antimony trioxide is one of the most widely used synergists in such systems.
When Synergists Are Needed
A synergist enhances the action of other components in a flame-retardant system, although it may not provide the required performance on its own. In halogen-containing compounds, antimony trioxide commonly serves this purpose.
System performance depends on the complete formulation. The type and level of plasticizer, fillers, stabilizers, ATO dosage and dispersion quality can strengthen or weaken its effect.
ATO: Antimony Trioxide
ATO, or antimony trioxide (Sb2O3), is widely used as a synergist in halogen-containing systems. PVC itself contains chlorine, while some formulations may also include chlorinated paraffins or other components that affect fire performance.
ATO increases the effectiveness of a flame-retardant system and helps achieve the required level of fire resistance without excessively increasing the dosage of other components. It is used in cable compounds, membranes, coatings and other PVC materials required to meet fire-test criteria.
At the same time, ATO is not a universal solution for every formulation. Its effectiveness depends on the plasticizer type, filler content, stabilizer system, the presence of other flame-retardant components and the requirements of the specific test method.
When selecting an ATO grade, it is important to consider the content of the main substance and impurities, particle size and particle-size distribution, whiteness, the type of surface treatment and the product’s ability to disperse uniformly in the PVC compound.
ATO Mechanism of Action
Flame retardancy is achieved primarily in the gas phase above the material surface. For antimony oxide to work effectively, it and the halogen source must participate in reactions that occur at temperatures above 315 °C.
When heated, antimony halides and oxyhalides are formed and subsequently converted into volatile antimony compounds. In the gas phase, these compounds inhibit the chain radical reactions that sustain combustion. ATO therefore does not act in isolation; it functions as a synergist in a halogen-containing flame-retardant system.
Limitations of ATO Use
The high effectiveness of ATO does not eliminate its processing and economic limitations.
Standard antimony trioxide is a fine powder. It can generate dust during handling and dosing, while consistent performance requires uniform particle distribution throughout the compound. Dispersion quality depends on product properties, equipment, mixing conditions and formulation composition.
Increasing the ATO dosage can affect density, colour, surface appearance, mechanical properties and the cost of the finished material. The high price of antimony trioxide is itself a significant constraint in formulations that require substantial loadings.
Another factor is product availability. The global antimony market depends on a limited number of countries and producers. In 2024, China introduced export controls on antimony and certain related materials. Supply licensing, trade disputes and possible tighter restrictions create risks of delays, shortages and sharp price fluctuations. Antimony prices reached historic highs in 2025 and remained elevated even after a subsequent correction.
ATO is also not a complete answer to every fire-safety requirement. It primarily improves the effectiveness of a halogen-containing flame-retardant system, but does not guarantee reduced smoke generation. Fire resistance, smoke density, acidity and toxicity of combustion products must be assessed separately.
What to Check During Testing
When developing a flame-retardant PVC compound, it is important to assess not only whether it passes one test, but also the overall balance of properties. Key parameters to check include:
- fire performance according to the target test method;
- oxygen index, if included in the test programme;
- smoke generation;
- acidity and toxicity of combustion products, where these indicators are specified;
- ATO distribution quality in the compound;
- the effect of the additive on processing;
- surface quality;
- colour and colour stability;
- mechanical properties;
- material density;
- formulation cost;
- result consistency across repeat tests.
This approach helps identify whether a limitation is caused by insufficient fire resistance, smoke generation, dispersion quality, processability or formulation cost.
An Alternative Approach: Revaguard RG69P
Despite the limitations described above, ATO remains an effective and well-understood component of flame-retardant PVC formulations. The objective is therefore not necessarily to eliminate antimony trioxide entirely, but to use its properties more efficiently.
Revaguard RG69P is a flame-retardant masterbatch for PVC. It contains no more than 70% antimony trioxide, together with additional components that help reduce smoke generation.
RG69P makes it possible to retain the benefits of an ATO-based flame-retardant system while reducing the amount of pure antimony trioxide. This reduces the formulation’s dependence on its price and availability, which is particularly relevant in the context of export restrictions and global market volatility.
The masterbatch format simplifies dosing and incorporation into the compound, reduces dust formation and supports more uniform distribution of the components during mixing.
In selected PVC formulations, Revaguard RG69P can be evaluated as a replacement for standard ATO at the same initial dosage. Replacement feasibility and the final dosage must be confirmed by testing the specific material for fire performance, smoke generation, processability and mechanical properties.
Revada Plastic specialists can help determine an initial RG69P dosage, prepare a comparative test programme and assess its suitability for an existing formulation. A technical consultation and a Revaguard RG69P sample can be requested for testing.
