"Invisible firefighters" in public transportation: Application and innovation of magnesium hydroxide in flame retardant materials

In public transportation systems, fire safety has always been a key issue to ensure the safety of passengers. In recent years, with the rapid development of new halogen-free and environmentally friendly flame retardant materials, magnesium hydroxide (Mg(OH)₂) has become an ideal flame retardant choice in rail transportation, aviation and urban public transportation due to its excellent thermal stability, low toxicity and environmentally friendly properties. This article will systematically explain its physical and chemical mechanisms, engineering applications, technological breakthroughs and green sustainable development.

1. Flame retardant mechanism and performance advantages of magnesium hydroxide

Magnesium hydroxide is a typical endothermic halogen-free flame retardant with a decomposition temperature of about 340°C. The following reaction occurs at this temperature:

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This reaction has a triple flame retardant effect:

Heat absorption effect: Each mole of magnesium hydroxide decomposes and absorbs about 80 kJ of heat, effectively delaying the temperature rise on the surface of the material and delaying the start of combustion;

Dilution isolation effect: The released water vapor can dilute the oxygen concentration and inhibit the spread of flames; At the same time, the generated magnesium oxide (MgO) covers the surface of the material to form a dense ceramic layer, further blocking the diffusion of heat and combustible gases;

Toxic gas conversion effect: Water vapor can catalyze the conversion of carbon monoxide (CO) into carbon dioxide (CO₂), reducing the toxicity of smoke.

Compared with traditional halogen-containing flame retardant systems (such as bromine or chlorine compounds), magnesium hydroxide does not produce toxic gases (such as hydrogen cyanide, dioxins, etc.), meets the requirements of international environmental regulations such as RoHS and REACH, and the combustion residue is harmless and has good ecological compatibility.

II. Typical applications of magnesium hydroxide in public transportation materials

(I) Improved smoke suppression performance in subway car interior materials

Taking the Xiong'an New Area Smart Metro project as an example, the interior panels prepared with magnesium hydroxide and silica aerogel composite masterbatch showed significantly improved smoke suppression performance in the standard NBS smoke box test:

The peak smoke density dropped from 2556 to 375;

The initial smoke generation time was extended to 210 seconds;

The hydrogen cyanide concentration was controlled below 5 ppm, which is far below the occupational exposure limit (OSHA standard is 10 ppm);

The peak heat release rate decreased by 58%.

These data show that magnesium hydroxide can not only effectively suppress the spread of fire, but also significantly increase the probability of passengers surviving in a fire.

(II) Flame retardant optimization of lightweight structural parts of high-speed rail

A high-speed EMU luggage rack uses a polypropylene (PP) matrix filled with 40% magnesium hydroxide:

The material oxygen index (LOI) reaches 34.5%, meeting the highest fire protection level of EN45545-2 R1 HL3;

The combustion smoke density is only 54% of traditional flame retardant materials;

While the amount of flame retardant added is reduced, the tensile strength of the material is increased by 15%, achieving a win-win situation of lightweight and safety.

(III) Recycling of recycled materials for bus seats

Shenzhen Bus Group recovers magnesium hydroxide from discarded seats through wet separation, and compounds it with recycled polypropylene to make a new handrail material:

The purity of recycled magnesium hydroxide reaches 99.3%;

The oxygen index of the composite material is maintained above 32%;

The overall carbon footprint is reduced by 62%, and the cost is reduced by 40%;

It is in line with the concept of circular economy and promotes the transformation of green manufacturing.

3. Technical challenges and innovative solutions of magnesium hydroxide

Although magnesium hydroxide has many advantages, its high filling amount (usually >60%) can easily lead to the deterioration of polymer mechanical properties, limiting its wide application. In recent years, researchers have conducted in-depth explorations around interface compatibility, nano-modification and responsive functional design.

(I) Interface enhancement technology

Silane coupling agent treatment: KH-570 is used to modify the surface of magnesium hydroxide particles, significantly improving the interfacial bonding between them and the polymer matrix, and increasing the tensile strength of the material by 300%.

Bionic mineralization technology: Simulating the shell structure to grow magnesium oxide nanosheets, constructing a multi-level pore structure, improving flame retardant efficiency and reducing the filling amount.

(II) Application of nano-magnesium hydroxide

Hexagonal nano-magnesium hydroxide (particle size 50–80 nm, specific surface area 22 m²/g) developed by the Qinghai Salt Lake Research Institute of the Chinese Academy of Sciences:

Adding 18% to the leather of high-speed rail seats can achieve a flame retardant effect of 40% at the micron level;

The tensile strength of the material increased by 30%, breaking through the technical bottleneck of "high filling = high brittleness".

(III) Intelligent responsive flame retardant system

Phase change microcapsule technology is introduced into the cable sheath of maglev trains:

Paraffin coating enhances the toughness of the material at room temperature;

After the temperature rises to the critical point, the active ingredient of magnesium hydroxide is released within 0.7 seconds;

Successfully passed the aviation-grade EN 45545-2 certification, realizing the integration of rapid response and efficient fire extinguishing.

IV. Green certification and sustainable development trends

Magnesium hydroxide not only performs well in functionality, but also shows unique advantages in the context of green manufacturing and carbon neutrality.

(I) Material recycling and closed-loop utilization

The Shanghai Metro Line 17 project achieves efficient recycling of magnesium hydroxide from waste interior materials and uses it for platform fire retardant coatings:

The recycling rate is as high as 92%;

Meets the LEED Platinum certification standard of "construction waste recycling rate ≥ 75%";

Achieve closed-loop resource management and reduce dependence on raw materials.

(II) Full life cycle carbon footprint tracking

A hydrogen magnesium factory in Qingdao introduces a blockchain traceability system:

Realizes carbon emission records from salt lake magnesium extraction, calcination, modification to terminal application;

The certification cycle is shortened by 60%, which helps green procurement and carbon trading;

Applied to the corridor system of Shenzhen Ping An Financial Center, with an energy saving rate of 46%.

(III) Bio-based synergistic modification strategy

A new modification process based on gene-edited strains secreting plant lipid coatings:

Significantly improves the dispersibility of magnesium hydroxide in bio-based polymers;

The VOC release of subway handrail materials is reduced by 67%, and the oxygen index is increased by 42%;

By passing the highest score of LEED "bio-based materials", the application scenarios of green buildings are expanded.

As a new generation of green flame retardant materials, magnesium hydroxide is reshaping the safety boundaries and environmental protection standards of public transportation systems. From subway tunnels to high-speed rail carriages, from bus seats to aviation cables, its multifunctional integration, intelligent response and ecological cycle make it not only a passive safety protector, but also a technology engine that actively promotes green development.