Zinc Borate vs Antimony Trioxide vs ATH vs MDH: Four Flame Retardants Compared

Four products dominate halogen-free and low-halogen flame retardancy: aluminium trihydroxide, magnesium hydroxide, zinc borate and antimony trioxide. They are not competitors – they do different jobs at different dose levels – but purchasing departments routinely put them in one comparison table, and that is where bad decisions start. This article puts them side by side on the parameters that actually decide the choice.

1. The four side by side

Property	ATH (aluminium trihydroxide)	MDH (magnesium hydroxide)	Zinc borate	Antimony trioxide
CAS	21645-51-2	1309-42-8	138265-88-0	1309-64-4
Formula	Al(OH)₃	Mg(OH)₂	2ZnO·3B₂O₃·3.5H₂O	Sb₂O₃
Decomposition / dehydration onset	About 180-220 °C	About 330-340 °C	Above 320 °C	Does not decompose in this sense – it volatilises as antimony halides in the flame
Water released	About 34% by weight	About 31% by weight	About 14.5% by weight	None
Typical loading	50-65%	50-60%	A few phr	A few phr
Role	Primary flame retardant	Primary flame retardant	Synergist, smoke suppressant, char promoter	Synergist for halogenated systems
Needs a co-additive?	No	No	Yes – halogen, phosphorus or intumescent system	Yes – halogenated flame retardant is mandatory
Smoke	Suppresses	Suppresses	Suppresses, especially afterglow	Increases smoke in some systems
Toxicology / regulatory pressure	Low	Low	Low; halogen-free	Higher – antimony trioxide carries a carcinogenicity classification in the EU, and supply is concentrated

2. What each one is actually for

Product	Use it when	Do not use it when
ATH	You need a halogen-free system, the processing temperature stays below about 200 °C, and you can tolerate a high filler load (cables, SMC/BMC, carpet backing)	Processing exceeds about 200 °C – the ATH decomposes in the extruder and foams the melt
MDH	Same as ATH but the polymer processes hotter: polyamide, PBT, cross-linked cable compounds	Cost is the dominant constraint – MDH is significantly dearer than ATH
Zinc borate	You already have a primary FR system and want less smoke, less afterglow, more char, or a way to cut antimony	You need the primary flame retardant – it cannot do that job at any sensible loading
Antimony trioxide	A halogenated FR is present and the specification is written around antimony; cost and toxicity are acceptable	You are moving to halogen-free, or the customer’s specification restricts antimony

3. Why zinc borate is not a substitute for ATH

The most common mistake is treating these as interchangeable on a weight basis. Compare the water yield: ATH releases about 34% of its mass as water; zinc borate about 14.5%. To release the same amount of water you would need more than twice the mass of zinc borate – and at that loading you would destroy the mechanical properties without reaching the flame-retardant class, because zinc borate’s contribution is not primarily the endotherm.

The correct mental model: ATH and MDH are the extinguisher; zinc borate is the catalyst for the char layer. One removes heat in bulk, the other changes what the burning surface does. Both can be in the same compound, and often are.

4. Replacing antimony: how far you can actually go

Approach	What changes	Practical limit
Direct partial substitution	Part of the Sb₂O₃ is replaced by zinc borate at similar or slightly higher loading	The flame-retardant class can usually be held while removing a substantial share of the antimony – the exact proportion has to be established per compound
Smoke-driven substitution	Zinc borate is added on top rather than instead	Smoke density and afterglow improve without touching the FR class
Full antimony removal	Requires a different primary system, usually ATH/MDH or a phosphorus chemistry	Zinc borate alone cannot replace antimony; the halogen/antimony pair is a gas-phase mechanism and zinc borate is largely condensed-phase

That last row is the one that decides projects. Moving from a halogen/antimony system to halogen-free is a reformulation, not a substitution – and zinc borate is a component of the new formulation rather than a drop-in for the old one.

5. Selection summary

Your situation	Primary choice	Add
PVC cable, halogenated plasticiser, want less smoke and less antimony	Keep the halogen system	Zinc borate at a few phr
Halogen-free cable compound, processing below 200 °C	ATH at 50-65%	Zinc borate optionally, for char and smoke
Halogen-free engineering thermoplastic (PA, PBT)	MDH or a phosphorus system	Zinc borate for char and afterglow
Intumescent coating	Acid source + char former + blowing agent	Zinc borate in the char-forming package
Regulated on antimony content	Reformulate to halogen-free	Zinc borate + ATH/MDH + phosphorus, as the new system

6. What to compare when you compare prices

Cost per unit of active function, not per kilogram: ATH delivers 34% water, zinc borate 14.5%, and antimony delivers no water at all but works only with halogen present.
Loading penalty: 60% ATH changes tensile strength and elongation dramatically; 3 phr zinc borate does not. That difference is worth more than the price spread.
Processing window: choosing MDH over ATH costs money and buys 100+ °C of headroom. If your process needs it, there is nothing to negotiate.
Regulatory cost of antimony: classification, customer restrictions and supply risk all belong in the comparison, even though none of them appear on a quotation.

7. Packaging and supply

All four are mineral-type powders supplied in 25 kg bags or big bags; zinc borate is normally a lighter powder (0.4-0.5 g/cm³ bulk) than ATH or MDH, which matters for feeder sizing. Each lot ships with a COA and SDS, FOB or CIF against destination, and a halogen-free declaration where the finished article requires one.