Study on the Flame Retardant Mechanism and Thermal Degradation Behavior of Fireproof Glue – Analysis of Action Mechanisms in Multiple Temperature Ranges

As a key functional material in construction, aerospace, and electronics industries, the flame retardant performance of fireproof glue is directly related to fire safety and structural stability. This paper systematically analyzes the core flame retardant mechanisms of fireproof glue, combined with thermal degradation processes in typical temperature ranges (300°C, 600°C, 1000°C), to reveal the dynamic action mechanisms of flame retardant components (such as inorganic fillers, organic flame retardants, and intumescent composite systems). This provides a theoretical basis for the formulation design and engineering application of high-performance fireproof glue.

Core Flame Retardant Mechanisms of Fireproof Glue

The flame retardant performance of fireproof glue relies on the synergistic effect of multiple physical and chemical mechanisms, mainly including the following four categories:

1. Endothermic Cooling Mechanism

Flame retardants absorb heat through endothermic decomposition or phase transition, reducing the surface temperature of the substrate and inhibiting thermal decomposition. Typical representatives include:

• Metal hydroxides(such as aluminum hydroxide ATH and magnesium hydroxide MDH): Dehydrate to release crystalline water in the temperature range of 180-350°C. Each gram of ATH absorbs approximately 1.96 kJ of heat, reducing the temperature rise rate of the polymer matrix.
• Borates/phosphates: Melt to form a glassy coating at high temperatures, delaying heat transfer through physical heat absorption.

2. Covering and Isolation Mechanism

Flame retardants form dense char layers or ceramic layers upon heating to block oxygen and heat exchange:

• Intumescent flame retardants (IFR): Composed of acid sources (such as phosphate esters), carbon sources (such as pentaerythritol), and gas sources (such as melamine), they generate expanded char layers at high temperatures. The thickness of the char layer can reach 50-100 times the initial thickness, with a thermal conductivity as low as 0.1-0.3 W/(m·K).
• Silicone-based flame retardants: Organosiloxanes form SiO₂ ceramic layers at high temperatures, which can withstand temperatures above 1200°C, suitable for ultra-high temperature scenarios.

3. Gas-Phase Flame Retardant Mechanism

Flame retardants decompose to produce non-combustible gases or free radical scavengers, inhibiting the combustion chain reaction:

• Halogen-based flame retardants: Decompose to produce HX (X=Cl, Br) gases, which capture H· and OH· free radicals in combustion, reducing the reaction rate. For example, the free radical capture efficiency of bromine-based flame retardants is 2-3 orders of magnitude higher than that of hydroxides.
• Nitrogen-based flame retardants: Release inert gases such as NH₃ and N₂ to dilute oxygen concentration. When the NH₃ concentration exceeds 15%, combustion can be suffocated.

4. Condensed-Phase Flame Retardant Mechanism

Reduce the generation of combustible gases by promoting char formation or altering the pyrolysis pathway:

• Phosphorus-based flame retardants: Form phosphate/pyrophosphate layers in the condensed phase, catalyzing the dehydration and carbonization of polymers, with a char yield increased by 10%-30%. For example, phosphate ester flame retardants can increase the residual char rate of epoxy resin from 5% to 25%.
• Nanofillers: Montmorillonite (MMT), graphene, etc., delay the diffusion of small molecules through physical barrier effects and promote the graphitization of char layers, enhancing char layer strength.

 

Thermal Degradation Processes and Flame Retardant Mechanisms in Typical Temperature Ranges

1. Low-Temperature Stage (≤300°C): Endothermic Dominance and Initial Char Formation

Thermal Degradation Characteristics:

• The polymer matrix (such as acrylate and epoxy resin) begins mild pyrolysis, with main chain scission generating low-molecular-weight olefins and alkanes. The heat release rate (HRR) starts to increase, and the oxygen index (OI) remains at 18%-21%.
• Behavior of flame retardants:
  • Metal hydroxides: ATH begins to dehydrate at 220-250°C, and MDH at 320-350°C. The released water vapor dilutes combustible gas concentration, while physical filling reduces the thermal conductivity of the matrix (e.g., adding 30% ATH can reduce the thermal conductivity from 0.2 W/(m·K) to 0.12 W/(m·K)).
  • Phosphorus-nitrogen synergistic systems: Ammonium polyphosphate (APP) begins to decompose to produce NH₃ and phosphoric acid. Phosphoric acid reacts with carbon sources to form phosphate ester intermediates, laying the foundation for subsequent char formation.

 

2. Medium-Temperature Stage (300-600°C): Char Strengthening and Free Radical Suppression

Thermal Degradation Characteristics:

• The polymer undergoes vigorous decomposition, with massive C-C bond scission. HRR reaches its peak (e.g., HRR of epoxy resin at 450°C can reach 500 kW/m²), producing combustible gases such as CO and CH₄, and OI decreases to 15%-18%.
• Behavior of flame retardants:
• Intumescent flame retardant systems:
  • Acid sources (APP) decompose to form phosphoric acid/pyrophosphoric acid, carbon sources (pentaerythritol) dehydrate to form char, and gas sources (melamine) release N₂ and NH₃ to promote char layer expansion.
  • The char layer structure transitions from initially loose and porous (porosity >70%) to dense (porosity <40%), with char layer thickness increasing to 50-80 μm and thermal resistance increasing by 3-5 times.
• Halogen-antimony synergistic systems:
  • Bromine-based flame retardants (such as decabromodiphenyl ether) decompose to produce Br· free radicals at 350-450°C, which react with Sb₂O₃ to form SbBr₃. Its gas-phase density (5.7 g/L) is higher than air, forming a covering layer to inhibit oxygen diffusion.
  • SbBr₃ condenses into droplets to capture H· and OH· free radicals, reducing the reaction rate constant by 40%-60%.

3. High-Temperature Stage (>600°C): Ceramization and Limiting Oxygen Maintenance

Thermal Degradation Characteristics:

• The polymer is completely carbonized, and the char layer oxidizes to form CO₂. HRR gradually decreases but continues to release heat, with high-temperature oxidation reactions (C+O₂→CO₂) becoming the main heat source. OI rebounds to 21%-25% (depending on residual flame retardant activity).
• Behavior of flame retardants:

1. Silicone/aluminum composite systems:
   • Organosiloxanes crosslink to form Si-O-Si networks, which further oxidize to form SiO₂-Al₂O₃ ceramic layers, resistant to temperatures above 1000°C, with a thermal conductivity as low as 0.05 W/(m·K) (close to air).
   • Crystalline phases such as mullite (3Al₂O₃·2SiO₂) are generated, enhancing the mechanical strength of the char layer, with erosion resistance increased by 5-8 times.
2. Phosphate-borate systems:
   • Form glass phases (such as Na₂O-B₂O₃-SiO₂) to fill char layer pores, reducing oxygen permeability (from 10⁻¹⁰ m²/s to 10⁻¹² m²/s), while releasing BO· free radicals to inhibit CO combustion (CO+BO·→CO₂+B·).

 

Temperature Response Characteristics and Synergistic Design of Flame Retardant Components

1. Temperature Limitations of Single Flame Retardants

• Metal hydroxides: Highly effective at low temperatures (≤300°C), but lose endothermic capacity after dehydration at high temperatures, with insufficient char layer strength prone to cracking.
• Halogen-based flame retardants: High free radical suppression efficiency in the medium-temperature stage (300-600°C), but poor long-term effectiveness due to halide volatilization at high temperatures, and generate toxic smoke (e.g., lethal concentration of HBr is 500 ppm).
• Silicone-based flame retardants: Excellent performance at high temperatures (>600°C), but poor compatibility with the matrix at low temperatures, resulting in a 20%-30% decrease in adhesive strength.

2. Synergistic Effects of Composite Flame Retardant Systems

• Phosphorus-nitrogen-silicon ternary synergy:
• Low temperature: APP dehydrates to form phosphoric acid, promoting carbonization; melamine releases NH₃ to dilute oxygen.
• Medium temperature: Siloxane grafts to the char layer to improve char layer toughness; silicon phosphate esters form glass phases to enhance barrier properties.
• High temperature: Generate SiO₂-C ceramic layers to inhibit char layer oxidation. For example, adding 5% silane coupling agent can increase char layer oxidation resistance by 50%.
• Reinforcement mechanisms of nanofillers:
• The lamellar structure of montmorillonite (MMT) (layer spacing 1.5-3 nm) hinders small molecule diffusion, reducing pyrolysis gas release rate by 30%-40%.
• Graphene (with <5 layers) enhances char layer graphitization through π-π interactions, increasing char layer electrical conductivity from 10⁻⁴ S/m to 10² S/m and accelerating heat dissipation.

3. Design Principles for Temperature-Adaptive Formulations

• Gradient functional design: Rely on endothermic flame retardants (such as ATH/MDH) in low-temperature regions, strengthen char formation (such as IFR) in medium-temperature regions, and introduce ceramization components (such as silicates) in high-temperature regions.
• Interface compatibility regulation: Modify the surface of inorganic fillers with silane coupling agents (such as KH-560) to increase filler-matrix interface binding energy from 20 mJ/m² to 50 mJ/m², preventing filler detachment at high temperatures.
• Environmental tolerance optimization: For hot and humid environments, use hydrophobic flame retardants (such as coated ATH), with water absorption reduced from 3% to 0.5% to avoid flame retardant component loss.

 

Characterization Techniques and Application Challenges

1. Characterization Methods for Thermal Degradation Behavior

• Thermogravimetry-differential scanning calorimetry (TG-DSC): Precisely measure mass changes and enthalpy values in each temperature range. For example, the weight loss rate at 300°C reflects the dehydration degree of hydroxides, and the residual char rate at 600°C characterizes char formation capacity.
• Fourier transform infrared spectroscopy (FTIR): Track changes in characteristic functional groups. For example, the weakening of the hydroxyl peak at 3400 cm⁻¹ indicates ATH dehydration, and the enhancement of the Si-O bond at 1020 cm⁻¹ indicates siloxane crosslinking.
• Scanning electron microscopy (SEM): Observe the microstructure of char layers. Intumescent systems exhibit honeycomb-like porous structures (pore size 5-10 μm), while silicone-based systems form dense glassy coatings.

2. Key Challenges in Engineering Applications

• Adhesive stability at high temperatures: Traditional organic glues carbonize above 600°C, with adhesive strength decreasing to less than 10% of the initial value. It is necessary to develop ceramic-based or metal-based adhesive systems (such as aluminosilicate-phosphate adhesives), with high-temperature strength retention >60%.
• Balance between environmental protection and flame retardancy: The EU REACH regulation restricts the use of halogen-based flame retardants. Under the trend of halogen-free, it is necessary to improve the efficiency of phosphorus-nitrogen-based and silicone-based flame retardants, such as enhancing the thermal stability of APP (decomposition temperature increased from 250°C to 320°C) through microencapsulation technology (capsule particle size 1-5 μm).
• Multifunctional integration in complex scenarios: Aerospace applications require fireproof glue to simultaneously have radiation resistance (≥10⁴ Gy) and oil corrosion resistance (strength retention >90% after fuel immersion for 72 hours), necessitating the introduction of multifunctional fillers such as nanosilicon carbide (SiC).

Conclusions and Prospects

The flame retardant mechanism of fireproof glue is a dynamic process of multi-temperature range and multi-mechanism synergy: endothermic cooling dominates at low temperatures, char formation and gas-phase inhibition are relied upon at medium temperatures, and physical-chemical barriers of ceramic layers are used at high temperatures. Future research should focus on the following directions:

1. Intelligent responsive flame retardant systems: Develop temperature-sensitive flame retardants (such as phase change material-loaded flame retardants) to achieve adaptive switching of flame retardant mechanisms in different temperature zones.
2. Green preparation technologies: Explore bio-based flame retardants (such as lignin phosphate esters and chitosan derivatives) to reduce VOC emissions (target ≤50 g/L) and halide content.
3. Cross-scale simulation technologies: Combine molecular dynamics (MD) and finite element analysis (FEA) to establish multi-scale prediction models from molecular structures to macroscopic properties, shortening the formulation development cycle.

By deeply analyzing the temperature response laws of flame retardant components and optimizing the synergistic mechanisms of composite systems, fireproof glue will exhibit broader application prospects in high-end equipment manufacturing, new building materials, and other fields.

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