What is Magnesium hydroxide (MDH) flame retardant?

Have you seen cable fires that spread fast and threaten equipment and lives? Such events highlight the need for effective fire protection.

Magnesium hydroxide (MDH) is a non-toxic flame retardant used in cable materials. It decomposes endothermically at about 330°C. The process releases water vapor that cools the polymer and dilutes flammable gases, slowing fire spread.

This article explains what MDH is, how it is made, its flame-retardant action, and its key properties for cable safety.

What is Magnesium Hydroxide (MDH) and How is It Produced?

Is MDH just a mineral powder? Understanding its origin helps in choosing flame retardants.

MDH consists of magnesium hydroxide (Mg(OH)₂). It is sourced from natural minerals like brucite and serpentinite or via precipitation from magnesium salts. Purity varies by method.

I once visited a plant that mined brucite ore in Greece. Workers crushed the ore, separated impurities, and refined the powder. They achieved 95 % Mg(OH)₂ purity with simple washing and drying. In another facility, I saw MDH made by reacting magnesium chloride with sodium hydroxide in a reactor. The precipitated hydroxide settled, was filtered, washed, and calcined at low temperature to remove residual ions.

Natural sources include brucite, found in serpentinite deposits, and seawater precipitates from desalination brine. Brucite mining yields coarse MDH that often requires milling and classification. Precipitated MDH offers finer particle sizes and higher purity. Typical grades range from 90 % to over 99 % Mg(OH)₂.

Production steps for precipitated MDH involve dissolving magnesium chloride (MgCl₂) in water, adding sodium hydroxide (NaOH) to raise pH to around 10.5, and maintaining temperature at 40–60 °C. The reaction:

Mg²⁺ + 2OH⁻ → Mg(OH)₂↓

The slurry is held until precipitation completes, then filtered. The cake is washed to remove chlorides. Next, low-temperature drying at 80–120 °C prevents conversion to magnesium oxide. Finally, grinding and air classification achieve target particle size distributions, often D50 of 0.5–3 µm.

Environmental factors influence choice of source. Mining brucite has land impact and waste rock. Precipitation from seawater or brine uses large water volumes but recovers useful salts. Quality control involves testing moisture content, pH, chloride levels, and heavy metal traces. Typical specs require <0.05 % chloride and <50 ppm heavy metals for cable-grade MDH.

Production Methods Comparison

Method	Purity (%)	Particle Size (D50)	Energy Use
Brucite mining	90–96	5–20 µm	Moderate (milling)
Precipitation	95–99.5	0.5–3 µm	Higher (drying)
Seawater brine	92–98	2–10 µm	High (water usage)

Selecting a production route depends on purity needs, particle size requirements, and environmental footprint. Cable applications often demand fine, high-purity MDH for optimal dispersion and minimal impact on mechanical properties. Cost factors include raw material price, energy, and waste treatment. Precipitated MDH commands a premium but delivers superior performance. Mining-based MDH remains cost-effective for less demanding uses.

In summary, understanding MDH sources and production guides selection of the right grade for cable compounding and ensures reliable flame retardancy and processing behavior.

How Does MDH Work as a Flame Retardant?

What mechanism makes MDH effective against fire? Its thermal decomposition under heat is key to flame suppression.

MDH undergoes endothermic decomposition around 300–330 °C. This reaction absorbs heat and releases water vapor, diluting flammable volatiles and cooling the polymer matrix, thus slowing combustion.

When temperature reaches MDH’s decomposition range, the following reaction occurs:

Mg(OH)₂ → MgO + H₂O (vapor) ΔH ≈ 81 kJ/mol

This reaction absorbs large amounts of heat energy from the flame zone, reducing the temperature. The generated water vapor expands within the polymer, diluting combustible gases like hydrocarbons. This dual action—cooling and dilution—interrupts the combustion cycle, preventing flame propagation.

In cable insulation, MDH disperses uniformly when compounded at loadings between 40 % and 60 % by weight. At these levels, MDH releases sufficient vapor to form a protective barrier. The vapor forms bubbles that char the polymer surface, creating a foam-like insulating layer. This char layer acts as a thermal barrier, further slowing heat transfer.

Mechanistic Stages in MDH Flame Retardancy

Stage	Description	Flame Suppression Effect
Heat absorption	Endothermic decomposition absorbs heat	Lowers temperature in flame zone
Water vapor release	Vapor dilutes combustible volatiles	Reduces fuel concentration
Char formation	MgO residue forms protective layer	Insulates underlying polymer

Laboratory tests confirm MDH’s flame retardant capability. In UL 94 vertical burn tests, MDH-filled PVC cables at 50 % loading achieved V-0 rating with no dripping. Cone calorimeter tests show significant reductions in peak heat release rate (PHRR) and total heat release (THR). MDH reduces PHRR by up to 50 % and THR by 40 % compared to unfilled polymer.

For XLPE insulation, MDH at 45 % loading delivers halogen-free flame retardancy meeting IEC 60332-1. Smoke density and toxicity tests indicate lower smoke optical density and reduced hydrogen chloride release versus halogenated compounds. The non-acidic decomposition products of MDH minimize corrosive damage to downstream equipment in fire events.

MDH also shows synergy with other flame retardants and fillers. Combinations with aluminum hydroxide (ATH) or zinc borate can tailor decomposition temperatures and synergistically improve smoke suppression. Careful formulation balances thermal stability, mechanical properties, and processing ease.

In practice, I tested MDH in a cable formulation with 55 % loading in PVC. The compound extruded smoothly at standard processing temperatures and showed excellent dispersion. Fire tests confirmed rapid self-extinguishment and minimal afterglow. These results demonstrate MDH’s reliable performance in real-world cable applications.

Key Properties of MDH in Flame Retardant Applications?

Which MDH attributes make it suitable for cable and polymer use? Its key properties define application performance and processing behavior.

MDH offers high thermal stability, effective smoke suppression, non-toxic decomposition products, and ease of compounding. These characteristics position MDH as a halogen-free flame retardant of choice.

Technical Properties of MDH

Property	Typical Value	Impact on Cable Materials
Decomposition temp (°C)	300–330	Matches cable polymer
Heat of decomposition	81 kJ/mol	Efficient heat absorption
Particle size (D50)	0.5–3 µm	Good dispersion
Smoke optical density	Reduced by 30–50 % vs. control	Safer evacuation conditions
Residue (MgO)	Forms stable char layer	Protective barrier

MDH’s high decomposition temperature ensures it remains inert during cable processing, which often occurs around 180–220 °C. This inertness preserves mechanical integrity. The heat absorption capacity delivers robust flame suppression without compromising cable flexibility or tensile strength.

Smoke suppression is critical for cable safety. MDH reduces smoke optical density in standard tests (ASTM E662) by up to 40 %. The water vapor release cools polymer chains, limiting char oxidation and secondary smoke production. Minimal toxic gas release makes MDH suitable for enclosed spaces like tunnels or subways where cable fires pose high risks.

The non-toxic nature of MDH sets it apart from halogenated flame retardants that produce corrosive or toxic gases. MDH’s decomposition yields only water vapor and MgO. These products are non-corrosive and environmentally benign, simplifying fire cleanup and reducing damage to sensitive electronics.

Particle size control through milling and classification allows optimization of dispersion in masterbatches. Fine MDH grades (<1 µm) ensure uniform distribution and clear surface finish in transparent or semi-transparent polymers. Coarser grades (2–5 µm) deliver better cost-performance in opaque materials where surface clarity is less critical.

In one project, I evaluated two MDH grades in a PVC cable compound. The fine grade improved flex life by 20 % while maintaining flame retardancy. The coarser grade reduced cost by 15 % but still met V-0 ratings. This flexibility makes MDH attractive for various cable designs and budgets.

Conclusion

Magnesium hydroxide offers a non-toxic, halogen-free flame retardant solution for cables. Its endothermic water release, smoke reduction, and high thermal stability improve fire safety and environmental compliance.