Invisible protection system for public transportation: flame retardant mechanism and technological evolution of magnesium hydroxide

Abstract: As an environmentally friendly inorganic flame retardant, magnesium hydroxide significantly improves the fire safety of public transportation materials through the triple mechanisms of thermal decomposition endothermicity, gas phase dilution and solid phase barrier. This paper systematically analyzes its flame retardant mechanism, modification technology breakthroughs and engineering applications in the field of rail transit, providing theoretical support for the design of green flame retardant materials.

I. Challenges and technological transformation of public transportation flame retardant system

1.1 Limitations of traditional flame retardants

Halogen flame retardants (such as bromine and chlorine) release highly toxic gases such as hydrogen cyanide (HCN) and dioxins when burning. HCN can cause coma in people for 5 minutes at a concentration of 120ppm, and the half-life of dioxins exceeds 10 years. Smoke suffocation in closed compartments accounts for the main cause of death from fires (80%), and there is an urgent need to develop low-toxic and efficient alternatives

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1.2 Innovation of flame retardant mechanism of magnesium hydroxide

Magnesium hydroxide (Mg(OH)₂) decomposes at 340–490℃ to achieve triple protection:

Thermal management mechanism: decomposition absorbs 1.3 kJ/g of heat, delaying the thermal decomposition of materials by 40%

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Gas phase dilution: releases 18.6% mass fraction of water vapor, dilutes the oxygen concentration and catalyzes the conversion of CO→CO₂

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Ceramic barrier: generates a dense layer of MgO (specific surface area ≥20 m²/g) to block heat and oxygen transfer

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The whole process only produces H₂O and MgO, which meets the EU EN 45545-2 toxicity release standard (FED<0.1)

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2. Material Engineering Breakthrough and Empirical Research

2.1 Multifunctional Composite Material Design

Subway Smoke Suppression System: Xiongan Smart Subway uses Mg(OH)₂/aerogel composite interior panels, NBS smoke box test shows:

Peak smoke density dropped from 2556 to 375 (a decrease of 85.3%)

Initial smoke generation delay of 210 seconds, HCN concentration stable <5 ppm

High-speed rail lightweight components: 40% nano-Mg(OH)₂ modified polypropylene (PP) luggage rack:

Component weight reduction of 30%, oxygen index of 34.5 (UL94 V-0 grade)

EN 45545-2 certified smoke density is only 54% of traditional materials

2.2 Circular economy technology path

Shenzhen Public Transport Group achieves closed-loop recycling of discarded seats:

Wet separation and recovery of Mg(OH)₂ with a purity of 99.3%

Compounded with recycled PP for new vehicle armrests, carbon footprint reduced by 62% and cost reduced by 40%

III. Key technology innovation and performance optimization

3.1 Surface modification strategy

Silane coupling grafting: KH-570 builds molecular bridges, and the interface bonding force is increased by 300%

Bionic mineralization structure: Simulating the layered growth of shell-like MgO nanosheets, the filling amount of subway cable sheath is reduced to 58% and still meets the UL94 V-0 level

3.2 Nanostructured efficiency enhancement

Hexagonal flake nano Mg(OH)₂ developed by the Qinghai Salt Lake Institute of the Chinese Academy of Sciences:

Particle size 50–80 nm, specific surface area 22 m²/g

Adding 18% to high-speed rail seat leather will achieve a micron-level 40% resistance, and the tensile strength will increase by 30%

3.3 Intelligent response system

Phase change microcapsule technology applied in maglev trains:

Toughening of paraffin/PE matrix at room temperature

Release of Mg(OH)₂ active ingredient in 0.7 seconds when exposed to fire

Passed EN 45545-2 aviation-grade certification

IV. Sustainability certification and ecological benefits

4.1 Green building certification integration

Shanghai Metro Line 17 project:

92% of waste interiors are recycled

Recycled Mg(OH)₂ is used for platform fire retardant coating

Meet the LEED platinum level "construction waste recycling rate ≥ 75%" standard

4.2 Carbon footprint traceability management

Qingdao production base introduces blockchain technology:

Full-process recording of carbon data from magnesium extraction from salt lake to modified process

Certification cycle shortened by 60%, Shenzhen Ping An Financial Center corridor saves 46% energy

4.3 Bio-based modification breakthrough

Genetically engineered strains synthesize plant lipid coating:

Subway handrails pass LEED's highest score for "bio-based materials"

Oxygen index increased by 42%, VOC release decreased by 67%

Conclusion: Paradigm shift from passive protection to active safety

Magnesium hydroxide breaks through the bottleneck of mechanical property degradation caused by high filling content (>60%) through nanostructuring, surface functionalization and intelligent response design, and achieves a synergistic improvement in flame retardant efficiency and material strength. Its application in the field of rail transit shows that:

Poison control: reduce the concentration of toxic gas to the safety threshold (HCN<5 ppm);

Resource recycling: the recycling rate of waste materials exceeds 90%;

Certification value-added: integrated LEED/EN 45545-2 and other multiple standard certifications.

Future research needs to focus on the visualization of in-situ flame retardant mechanism, the full life cycle carbon emission model and the development of bio-inorganic hybrid materials to promote the system upgrade of public transportation from "accident emergency" to "intrinsic safety".