Magnesium hydroxide is a flame-retardant filler, and research adding it to a calcium-carbonate-filled polypropylene composite shows its fire-performance benefit rises with loading — but its effect on tensile strength doesn’t follow a simple straight line. This page covers that research and where magnesium hydroxide is sourced from.
What is it?
Magnesium hydroxide (Mg(OH)2) is a filler used mainly as a non-halogenated flame retardant in polymer compounds. Per the U.S. Geological Survey’s Mineral Commodity Summaries 2025, magnesium compounds in the U.S. are recovered from seawater (by companies in California and Delaware), from well brines (in Michigan), and from lake brines (in Utah). U.S. imports of magnesium hydroxide specifically were sourced primarily from Mexico (59%), the Netherlands (14%), and Israel (13%) over 2020-23.
Why should I use it?
- Magnesium hydroxide’s flame-retardant action comes from a genuine chemical mechanism (endothermic decomposition releasing water vapor), backed by direct thermal testing — see “How it’s used.”
- Adding it to a filled polypropylene composite measurably raises the limiting oxygen index (LOI), a standard flammability measure, in a dose-dependent way — see Limitations.
How it’s used
In plastics and construction-related polymer compounds, magnesium hydroxide works as a flame retardant by decomposing endothermically at high temperature, releasing water vapor that dilutes combustible gases while leaving a residue that limits heat transfer. A 2024 peer-reviewed study found this dehydration step occurs across a decomposition range of 309°C to 409°C.
Limitations — when it isn’t the right choice
- Adding magnesium hydroxide raises flame resistance with loading, but its effect on mechanical strength doesn’t follow a simple pattern — it needs to be checked at the specific loading level used, not assumed. The same 2024 study tested magnesium hydroxide added to a polypropylene composite already containing 50 wt% calcium carbonate. The limiting oxygen index rose steadily with more magnesium hydroxide: 17.9% for neat PP, 19.7% for PP with calcium carbonate alone, 21.2% with 5 wt% added magnesium hydroxide, and 23.1% with 10 wt%. Tensile strength, however, was not straightforward: it dropped from 31.4 MPa (neat PP) to 23.4 MPa (PP/calcium carbonate) to 21.2 MPa at 5 wt% magnesium hydroxide — its lowest point — before recovering slightly to 22.3 MPa at 10 wt%. This means a higher magnesium hydroxide loading isn’t automatically worse for mechanical strength than a lower one, and these exact figures are specific to this particular calcium-carbonate-filled polypropylene system; they should not be assumed to hold in a different resin or filler combination without separate testing.
