Amid the global advancement of "halogen-free and halogen-limited" environmental policies and stricter fire safety standards, intumescent flame retardant (IFR) systems have become mainstream, replacing traditional halogen-based counterparts due to low smoke, low toxicity, high efficiency and environmental friendliness. As the core carbon source of IFR systems, the catalytic char-forming mechanism of char-forming agents directly dictates flame retardant and thermal insulation performance, forming a "flame-retardant and thermal-insulating barrier" for substrates via precise regulation of high-temperature chemical transformations and structural construction. This article analyzes their working principles from three core aspects: catalytic char-forming reactions, char layer construction, and synergistic flame retardant mechanisms.

I. Core Catalytic Char-Forming Reactions: From Molecules to Char

Catalytic char formation is a complex synergistic process involving acid sources, gas sources and substrates at high temperatures, dominated by two key reactions: catalytic dehydration and cross-linking polymerization, accompanied by endothermic cooling and gas regulation to lay the foundation for char layer formation.

Catalytic dehydration, coordinated with acid sources like ammonium polyphosphate (APP), initiates char formation. At 200-300℃ (typical initial fire temperature), APP decomposes to produce phosphoric acid, which catalyzes char-forming agents (e.g., pentaerythritol, carboxylated polysaccharides) to dehydrate, forming unsaturated carbon skeleton precursors. This reduces flammable gas emission, absorbs heat, and releases water vapor to cool the system and dilute gases. Research by Professor Zhang Shuidong’s team (South China University of Technology) shows carboxylated regenerated cellulose (ORC) forms stable P-OH bonds with phosphoric acid, boosting char residue by 9.7 times versus traditional pentaerythritol.

Cross-linking polymerization determines char layer strength and compactness. Precursors undergo cross-linking to form a 3D network, accelerated by synergists (e.g., molybdates, nickel phosphate) that enhance graphitization. Professor Cao Kun’s team (Zhejiang University) developed an "interface autocatalysis" system, where in-situ generated boron phosphate (BPO₄) nanocrystals form a composite char layer (outer ceramic layer + inner intumescent layer). Matching decomposition temperatures is critical—e.g., pentaerythritol (decomposition temp. >250℃) pairs with APP to avoid premature degradation.

II. Char Layer Construction: From Foaming to Optimization

Efficient flame retardancy relies on a continuous, compact char layer with good mechanical properties. Char-forming agents cooperate with gas sources (e.g., melamine) through three stages: intumescent foaming, curing, and structural optimization.

At ~340℃, gas sources release non-flammable gases (N₂, CO₂) that form bubbles in softened precursors, creating a porous expanded char layer. Char-forming agent hydroxyl content controls foaming rate, while carbon content affects carbonization. Addition amount (5%-20%, lower than acid source) is critical—excess limits expansion, insufficient causes collapse. The EP/MFAPP/ORC27 system achieves a 41.5x expansion ratio, cutting peak heat release rate by 55.6%.

Expanded char layers further carbonize and compact at high temperatures. Synergists (nano-clay, zinc borate) fill pores, and in-situ BPO₄ forms a 5.5μm dense ceramic layer. FTIR/XPS characterizations confirm P-O-C/P-C bond recombination enhances graphitization (I_D/I_G: 1.92→1.65), maintaining structural integrity at 550℃.

III. Synergistic Mechanism: Multi-Dimensional Combustion Interruption

Char layers achieve efficient flame retardancy via "physical barrier + chemical inhibition + thermal management", cutting the combustion chain at the source.

Physical barrier is core: it blocks O₂/flammable gas contact and slows heat transfer with low thermal conductivity. In steel structure coatings, expanded char layers keep backplane temperatures below 300℃ for 50 minutes, ensuring structural stability.

Chemical inhibition and thermal management enhance performance: gas-phase non-flammable gases dilute fuels and quench free radicals; condensed-phase reactions absorb heat to prevent re-ignition. Char-forming agents also reduce dripping and smoke—41.8% char residue cuts total smoke production by 62.2%.

IV. Key Influencing Factors & Optimization Directions

Char-forming efficiency depends on material properties, system ratio and application scenarios. Select agents matching substrate processing temperatures (e.g., dipentaerythritol for engineering plastics, 250-300℃) and optimize acid/gas/char ratios with synergists to improve char layer performance.

Frontier research focuses on green, efficient agents (e.g., recycled cellulose-based) and strategies like "interface autocatalysis", balancing expansion and compactness while resolving high addition and reduced substrate mechanical properties issues.

Conclusion

The catalytic char-forming mechanism realizes a "chemical transformation-structural formation-efficiency synergy" loop, offering an eco-friendly flame-retardant solution. Jiangxi Dstone Mineral Fiber Technology Co., Ltd. exemplifies commercialization. As a Jiangxi-based high-tech and specialized SME, it focuses on R&D, production and service of flame-retardant materials. Its smoke-suppressing char-forming agents integrate eco-friendliness and efficiency, forming optimized synergistic systems with acid/gas sources, excelling in dehydration/cross-linking efficiency, char compactness and smoke suppression—suitable for new energy batteries, electronics, aviation/rail transit and green building materials.

Founded in 2021 with 30 million RMB registered capital, Jiangxi Dstone holds 17 patents and operates a professional R&D center with over 100 advanced devices, covering bench-scale to pilot production. Adhering to "Upholding Integrity and Perseverance", it provides customized masterbatches and solutions, contributing to material safety and new quality productive forces. Future technological iteration will drive high-quality development in fire safety materials.