Magnesium Hydroxide Flame Retardant: Complete Guide

2026-09-19 18:29:40

Regulatory pressure and end-market expectations keep pushing formulators away from halogenated systems. RoHS and WEEE restrict halogenated additives in electronics, railway and marine standards such as EN 45545 demand low smoke and low toxicity, and consumers increasingly expect "halogen-free" on the label. Magnesium hydroxide (MDH, Mg(OH)2) is the workhorse mineral flame retardant that answers all three demands - if you choose the right grade and use it correctly.

This guide covers the working mechanism, the grades that matter, realistic loading levels, and the specification points that separate a good MDH from a cheap one.

What Is Magnesium Hydroxide Flame Retardant?

Magnesium hydroxide is a white, non-toxic mineral powder produced either by grinding natural brucite (mineral-derived grades) or by chemical synthesis from brine or magnesium salts (synthetic grades with higher purity and controlled morphology). Unlike halogenated flame retardants, MDH does its job through simple physical chemistry - no corrosive or toxic combustion products, no dioxin concerns, and excellent colorability in the final product.

How MDH Flame Retardancy Works: Four Mechanisms

1. Endothermic decomposition. At approximately 330-350 degrees C, Mg(OH)2 decomposes to MgO and water, absorbing roughly 1.3-1.4 kJ/g of heat. This cools the polymer below its sustained-burning temperature.

2. Fuel dilution. The released water (about 31% of the compound mass) dilutes combustible gases and oxygen concentration at the flame front.

3. Protective barrier. The MgO residue is a refractory oxide that accumulates on the burning surface, promoting a ceramic-like char layer that insulates the underlying polymer.

4. Smoke suppression. By promoting solid-phase char formation instead of gas-phase combustion, MDH significantly reduces soot and toxic fumes - a decisive advantage in enclosed spaces such as trains, ships and basements.

Why the Decomposition Temperature Matters

This is the single most important number when comparing mineral hydroxide retardants:

·         MDH releases water from about 330-350 degrees C.

·         Aluminum Hydroxide (ATH) already starts releasing water at about 180-220 degrees C.

If your compound is processed at 200-230 degrees C (typical for PP, EVA and PE cable compounds), ATH begins decomposing inside the extruder, causing porosity, voids and property loss. MDH tolerates the same processing window with a comfortable safety margin. This is why MDH dominates polypropylene and EVA applications, while ATH dominates lower-temperature resins such as epoxy, unsaturated polyester and PU. (See our dedicated comparison: ATH vs MDH.)

Typical Loading and Formulation Guidance

Mineral hydroxide retardants work by sheer mass action, so loading is high:

·         40-60 wt% is the common working range to reach UL94 V-0 at practical thicknesses in polyolefins.

·         At these levels, surface modification (stearic acid, silanes) and compatibilizers such as PP-g-MA are essential to keep tensile strength and elongation acceptable.

·         Synergists stretch performance: char-forming agents, zinc borate, and silicone powders each add IEC/UL margin or reduce total loading.

A well-formulated MDH compound balances flame rating, mechanical properties, cost, and - increasingly - smoke data.

MDH Grades and Where Each One Fits

Wanfeng produces the full spectrum, from mineral-derived magnesium hydroxide base powder to cable-grade chemically modified MDH and synthesized hexagonal plate grades.

Where MDH Is Used

·         LSZH / HFFR cable compounds (EVA/PE base) - metro, tunnel, marine and building wire

·         Polypropylene compounds - battery housings, appliance internals, construction profiles

·         Appliance housings and heating-element insulation (see our appliances solution page)

·         Construction boards and insulation panels (see construction solutions)

·         Rubber goods such as conveyor belts and mats, often paired with rubber-specific retardants

Selecting a Supplier: The Five Questions

1. Purity: is MgO content and CaO/Fe2O3 controlled per batch with a COA?

2. Morphology: is particle size (D50) and crystal habit consistent lot to lot?

3. Surface treatment: can they supply modified grades, not just raw powder?

4. Testing: do they operate in-house LOI, oxygen index and smoke density testing? (Wanfeng's Rubber & Plastic Flame Retardant Technology Center runs oxygen index, smoke density and rheology testing on every development.)

5. Scale: can they hold quality at volume - Wanfeng runs six automated production lines and exports to 30+ countries?

FAQ

What is the maximum processing temperature for MDH compounds?

Keep compounding below about 230 degrees C. MDH is stable to roughly 330 degrees C, so normal polyolefin and EVA processing windows are safe; prolonged overheating will start early decomposition.

How much magnesium hydroxide do I need for UL94 V-0?

Most polyolefin compounds land in the 40-60 wt% range, depending on polymer grade, part thickness and synergists. There is no universal number - a small formulation study with your resin is the only reliable answer.

Should I use surface-treated or untreated MDH?

Untreated powder is cheaper but makes high-loading compounds brittle. Treated (cable-grade) MDH disperses better and preserves elongation - the standard choice for cables and demanding profiles.

MDH or ATH - which should I choose?

Match the decomposition temperature to your processing temperature: above ~200 degrees C processing, choose MDH; below, ATH is usually more economical. Read our full ATH vs MDH comparison for the detailed table.

Request MDH Samples and Technical Support

Wanfeng supplies the complete MDH portfolio - raw powders, high-purity grades, platelet morphology and pre-modified cable grades - with application testing support. Contact us for a grade recommendation, or browse the magnesium hydroxide product family.