Aluminum Hydroxide vs Magnesium Hydroxide Flame Retardants: How to Choose (ATH vs MDH)

2026-09-19 18:30:17

Aluminum Hydroxide (ATH, also called aluminum trihydrate) and magnesium hydroxide (MDH) are the two pillars of halogen-free mineral flame retardancy. Both are non-toxic, low-smoke, inexpensive powders that quench fire by releasing water. Yet they are not interchangeable: the difference in their decomposition temperatures effectively decides which polymers each one can serve. This guide gives formulators and buyers a direct, side-by-side comparison - and a selection matrix you can act on today.

The Quick Comparison

How Each One Works

Both retardants share the same three-step mechanism:

1. Endothermic decomposition absorbs combustion heat and cools the polymer.

2. Released water vapor dilutes flammable gases and oxygen at the flame front.

3. Oxide residue (Al2O3 or MgO) builds a protective, char-promoting barrier on the burning surface.

The difference is entirely in *when* the water arrives. ATH sheds its water early and gently; MDH holds it until a much higher temperature.

The Processing-Temperature Rule (The One That Matters Most)

·         If your compound is processed below ~190 degrees C, ATH is usually the economical choice: epoxy, unsaturated polyester (SMC/BMC), polyurethane, latex, EVA at low temperatures, and most rubber goods.

·         If your compound is processed above ~200 degrees C - polypropylene, HDPE, and high-temperature EVA cable compounds - ATH begins to decompose inside the extruder, releasing water, causing porosity, voids, surface defects and degraded properties. Here MDH is not optional; it is the only mineral hydroxide that survives the process.

This single rule answers 80% of real-world "ATH or MDH?" questions.

Flame-Retardant Efficiency and Loading

Both work by mass action and both demand high loadings:

·         UL94 V-0 in polyolefins typically requires 40-60 wt% of either mineral, adjusted for part thickness and synergists.

·         At these volumes, particle size, morphology and surface treatment dominate mechanical outcomes. High-purity, controlled-morphology grades (such as synthesized hexagonal plate MDH) preserve elongation far better than coarse ground fillers.

·         Surface-treated grades are the pragmatic default at high loading; untreated powder can cost more in scrap than it saves per kilo.

Smoke and Toxicity: Where MDH Edges Ahead

Both minerals are dramatically cleaner than halogenated systems - no corrosive HBr/HCl evolution, no dioxin concerns, low toxic-gas load. Between the two, MDH generally delivers lower smoke density and better char promotion, which is why it is the standard in LSZH cable compounds for tunnels, metros, ships and high-rise buildings, where smoke obscuration and toxicity - not just flame spread - define compliance.

Cost and Color

·         ATH is cheaper per kilo and wins wherever the processing window allows it - a major reason it remains the world's highest-volume mineral flame retardant.

·         Whiteness is excellent in both families; high-purity grades support bright whites and pastel colors in appliance housings and construction panels.

·         Total formulation cost should include loading level, compatibilizer demand and scrap rate - not just powder price.

Which Polymers Suit Which Retardant

Using Both Together

The two are not enemies. Hybrid MDH/ATH packages appear in practice to balance cost and performance, and both benefit from synergists: char-forming agents, zinc borate, silicone powders and nano-fillers. A char-forming agent in particular pairs naturally with MDH in cable compounds, building a stronger protective residue at lower total mineral loading.

FAQ

Which is better, ATH or MDH?

Neither universally - the processing temperature of your compound decides. Above ~200 degrees C, MDH; below, ATH is usually the better economic choice.

Can ATH be used in polypropylene?

Only with very careful, low-temperature processing and often reduced loading; otherwise pre-decomposition causes porosity. MDH is the standard for PP.

Which one gives lower smoke?

MDH generally achieves lower smoke density and better char formation, which is why LSZH cable specifications lean on it.

Do I need surface-treated grades?

At loadings above ~40 wt%, treated grades protect mechanical properties and processability enough to pay for themselves in most cable and appliance applications.

Get Both Minerals from One Technical Source

Wanfeng manufactures the full ATH and MDH families - including high-purity, platelet-morphology and pre-modified cable grades - with in-house oxygen index, smoke density and rheology testing. Explore aluminum hydroxide and magnesium hydroxide grades, or contact our engineers for a formulation review.