ATH or Magnesium Hydroxide — Choosing a Halogen-Free Mineral Flame Retardant
Direct answer
Processing temperature decides. Aluminium trihydrate releases its water at around 200–220 °C, so it works in PVC, polyolefin cable compounds, EVA and thermoset composites processed below that. Magnesium hydroxide decomposes at around 300–330 °C, so it is the choice for polypropylene, polyamide and any compound that sees higher melt temperatures. Use ATH above its decomposition onset and it foams the melt; that is not a formulation problem you can dose your way out of.
How mineral flame retardants work
Both minerals are endothermic hydrates. In a fire they absorb heat and release water vapour, cooling the polymer below its decomposition temperature and diluting the flammable gases at the flame front. The residual oxide forms a ceramic-like char layer that slows heat and mass transfer. There is no halogen, no antimony synergist and no toxic gas contribution — which is precisely why they are specified for low-smoke zero-halogen cable, tunnels, rolling stock and public buildings.
The mechanism is physical, not catalytic, so it only works at high loading. Meaningful fire performance in a polyolefin generally starts around 50 % by weight and often runs to 60–65 %. Anything described as a mineral flame retardant at 5 % is doing something else.
The consequences of loading that high
At 60 % mineral, the compound is more mineral than polymer, and every property follows:
- Elongation at break and impact fall steeply; tensile strength depends almost entirely on the filler-polymer interface.
- Melt viscosity rises; output falls and screw wear rises.
- Water uptake and electrical properties change, which matters directly in cable insulation and sheathing.
- Compound density rises, so a quoted price per kilogram translates into a larger cost per metre of cable than a naive comparison suggests.
This is why surface treatment and a coupling chemistry are not optional at these loadings. Silane-treated grades and maleic anhydride grafted compatibilizers are what keep elongation and tensile strength inside specification.
Grade variables that change the outcome
- Particle size (D50) and surface area. Finer grades disperse better and give better mechanical retention, but drive viscosity up faster.
- Surface treatment. Untreated, stearate coated, or silane treated — see the treatment article; for FR compounds this is the main mechanical lever.
- Purity and soda content. In ATH, residual Na2O affects electrical properties and water extractables in cable compounds.
- Smoke suppression. Both minerals suppress smoke; MDH generally gives additional char stability at high temperature.
Where each is normally specified
ATH: LSZH cable compounds based on EVA or polyolefin, unsaturated polyester and epoxy composites, carpet backing, cast solid surface, PVC compounds.
MDH: polypropylene and polyamide compounds, higher-temperature cable formulations, and any application where the processing window makes ATH unusable.
What to send with the enquiry
The base polymer and processing temperature, the fire standard you must meet (and whether the requirement is ignition, smoke, or both), the minimum tensile and elongation the part must retain, and the current loading if you have one. Fire performance and mechanical performance are one problem at these loadings — they cannot be specified separately.