Published: November 5, 2026 | Last Updated: November 5, 2026
SEO Title Tag: Magnesium Hydroxide vs ATH: The 110 °C Gap | Hezhen Meta Description: Both hydroxides cool a fire by releasing water. Only one survives a polypropylene melt. Decomposition window, loading, smoke and the regional details that decide the choice.
Aluminium hydroxide and magnesium hydroxide work by the same mechanism: heated, they decompose endothermically, release water vapour and leave an oxide layer over the burning surface. They are sold at very different prices, which is why the cheaper one is so often specified into a compound it cannot survive. What decides most projects is not cost or purity but roughly 110 °C of decomposition temperature — whether that window sits above or below the temperature at which your polymer is processed.
Aluminium hydroxide begins to decompose at roughly 180–220 °C, releasing about 34.6% of its mass as water and absorbing around 1.0 kJ per gram. Magnesium hydroxide begins at roughly 330–340 °C, releases about 30.9% of its mass as water and absorbs around 1.3–1.4 kJ per gram.
Read those numbers in the order a compounder reads them, not the order a chemist does. The water fraction favours ATH slightly; the heat absorption favours MDH; but neither matters if the filler has already decomposed in the extruder barrel. A polypropylene melt runs at 200–230 °C, polyamide at 240–280 °C and PBT at 240–260 °C. ATH is above its decomposition temperature across that entire range. It foams, the compound loses elongation and the surface turns chalky before the pellet is even cut.
For PVC at 170–200 °C and EVA foam at 160–190 °C, ATH is comfortably inside its window and remains the sensible and cheaper choice. That is the honest boundary: ATH is not obsolete, it is simply restricted to polymers that process below about 200 °C.
Choosing MDH is not free. It costs more per kilogram and, at the same loading, gives up a little of the water release that does the cooling. What it buys is the ability to flame-retard the polymers that ATH cannot touch, at loadings a compounder can actually run.
Both are used at 50–65% by weight in polyolefin systems to reach a limiting oxygen index of 28–38. An unfilled polyolefin sits around 17–18. Below roughly 50% loading, neither hydroxide usually reaches a V-0 result, and above 65% the melt viscosity and the impact strength both become the problem rather than the fire test. The window is narrow, which is why the coupling agent matters as much as the filler.
One further difference is the oxide left behind. ATH leaves aluminium oxide, which is hard and ceramic-like; MDH leaves magnesium oxide, which forms a coherent char with the polymer residue. In smoke-density testing, that char is usually why MDH-filled compounds beat ATH-filled compounds at comparable loading — the layer is more continuous, so less fuel reaches the flame and less smoke leaves it.
Two hidden costs decide the real comparison. The first is dispersion: both hydroxides are hydrophilic, both agglomerate in a non-polar melt, and both need surface treatment before high loading is possible. A buyer who compares an untreated ATH price with a silane-treated MDH price is comparing two different products.
The second is moisture. Magnesium hydroxide picks up water on the surface, and every tenth of a percent that reaches the extruder becomes a bubble in a cable sheath and a dielectric failure in service. Cable grades are therefore specified with moisture limits typically at or below 0.5%, and often with a conductivity limit on the aqueous extract. Neither figure appears on a price list, and both decide whether the compound passes.
Screw wear is often raised as a reason to prefer one. Both minerals are soft — 2.5–3 on the Mohs scale, far below barrel steel. Wear comes from the loading, the particle size and above all from silica and iron in the ore, not from the hydroxide: a 60% loading of a clean fine grade wears less than 45% of a coarse grade containing quartz.
In Vietnam, Thailand, Malaysia and Indonesia, the compounding plants are often running older twin-screw lines in ambient humidity above 70% for much of the year. Two consequences follow. First, the filler must be stored and fed dry: sealed bags, a dryer on the feed throat, and a moisture spec written into the purchase order rather than assumed. Second, a low-moisture surface-treated grade is worth paying for in this climate in a way it may not be in a European plant with a climate-controlled resin dryer.
The same humidity argues for MDH over ATH in one specific case that is easy to miss: recycled and mixed polyolefin streams. ATH begins to decompose close enough to typical reprocessing temperatures that a poorly controlled barrel profile can strip water from the filler, and reprocessed material rarely tolerates that.
Gulf cable plants run with ambient temperatures that make barrel cooling harder and temperature overshoot more likely. That reinforces the same rule: the further the decomposition onset sits above the processing temperature, the more forgiving the process. On the product side, outdoor cable in Gulf conditions faces UV and sustained heat, which is a compound design question — the hydroxide does not provide UV stabilisation and should never be presented as doing so.
| Property | Aluminium hydroxide (ATH) | Magnesium hydroxide (MDH) |
|---|---|---|
| Decomposition onset | approx. 180–220 °C | approx. 330–340 °C |
| Water released | approx. 34.6% by mass | approx. 30.9% by mass |
| Endothermic heat | approx. 1.0 kJ/g | approx. 1.3–1.4 kJ/g |
| Residue | aluminium oxide, ceramic-like | magnesium oxide char |
| Typical loading | 50–65 wt% | 50–65 wt% |
| LOI at that loading | 28–38 | 28–38 |
| Compatible polymers | PVC, EVA, flexible PE | PP, PE, EVA, PA, PBT, LSZH cable |
| Mohs hardness | approx. 2.5–3 | approx. 2.5–3 |
| Moisture sensitivity | moderate | higher; low-moisture grades needed |
| Relative cost | lower | higher |
Three cases justify ATH. Flexible PVC, where processing runs far below 180 °C. EVA foam and carpet backing, where a lower decomposition temperature is an advantage. And cost-driven parts that never see a hot melt, such as a pressed panel. Choosing MDH where ATH works is paying for a temperature margin the process does not need.
1. Can ATH and MDH be blended? Yes, and it is done deliberately: the two decompose at different temperatures, so a blend can release water over a wider range and improve performance in a multi-stage fire. The blend ratio has to be tested against the actual fire scenario, not assumed.
2. Does MDH give a higher LOI than ATH at the same loading? Marginally, because it absorbs more heat per gram, but the difference is smaller than the difference made by the coupling agent and the dispersion. Treat LOI as a system result, not a filler property.
3. Why does my MDH-filled compound foam during extrusion? Almost always moisture — either surface water on the filler or insufficient venting. Dry the filler, check the feed zone temperature, and confirm the moisture specification with the supplier before changing the formulation.
4. Is MDH suitable for outdoor cable in hot climates? The hydroxide is thermally stable in service; the limits come from the polymer, the UV package and the jacket design. Magnesium hydroxide does not provide UV protection and should not be specified as if it did.
Send the polymer, the processing temperature profile, the target loading and the test you have to pass — UL94, LOI, smoke density or a customer specification. The Hezhen technical team will confirm whether magnesium hydroxide or a blend with ATH is the right route, recommend the particle size and surface treatment, and supply samples with moisture, conductivity and particle size distribution data so the grade can be qualified on your own line.
