Introduction to Different Mechanisms of Fabric Flame Retardant Finishing

Introduction to Different Mechanisms of Fabric Flame Retardant Finishing

In modern society, the flame retardant properties of fabrics are of critical importance, with extensive applications across various fields including home furnishings, transportation, industrial protection, and fire safety. Fabric flame retardant finishing serves as a key technique for enhancing these properties, supported by complex and sophisticated principles. From a chemical standpoint, the mechanisms of flame retardant finishing primarily rely on interactions between chemical flame retardants and fibers. Chemical flame retardants, the core components enabling fabric flame retardancy, can be categorized based on their mode of action into gas-phase, condensed-phase, and interrupt-heat-exchange flame retardants. Current research allows for a broad classification of flame retardant mechanisms into categories such as endothermic reactions, radical formation, melting theory, generation of non-combustible gases, and condensed-phase flame retardancy.

I. Chemical Flame Retardancy

Gas-Phase Flame Retardants function by decomposing at high temperatures to produce substances capable of scavenging free radicals. During combustion, free radicals play a pivotal role in sustaining the chain reaction. For instance, halogen-based flame retardants, upon exposure to heat, decompose to produce hydrogen halide gases. These hydrogen halides react with highly reactive free radicals in the flame, such as hydrogen radicals (H·) and hydroxyl radicals (OH·), forming less reactive halogen radicals (X·) and water. This reaction effectively interrupts the combustion chain reaction, preventing sustained burning. Research by numerous scholars demonstrates that in combustion tests of fabrics containing halogen-based flame retardants, the flame spread rate is significantly reduced, and under certain conditions, self-extinguishment can be achieved.

Condensed-Phase Flame Retardants primarily operate within the condensed phase. Phosphorus-based flame retardants are typical examples. When subjected to heat, these retardants decompose to form compounds like phosphoric acid and metaphosphoric acid. These acidic substances promote the dehydration and carbonization of the fiber, leading to the formation of a dense char layer. This char layer exhibits excellent thermal insulation and oxygen barrier properties, effectively hindering oxygen diffusion into the fiber and suppressing the release of flammable gases. Research data indicate that cotton fabrics treated with phosphorus-based flame retardants show a significantly increased char formation rate and a markedly reduced heat release rate.

Interrupt-Heat-Exchange Flame Retardants work by undergoing physical changes that absorb substantial heat, thereby lowering the fabric's surface temperature and making sustained combustion difficult. Hydrated metal oxides, such as aluminum hydroxide and magnesium hydroxide, are common examples. When heated, these compounds decompose endothermically. For instance, the decomposition reaction of aluminum hydroxide, 2Al(OH)₃ → Al₂O₃ + 3H₂O, absorbs approximately 1967 kJ/kg of heat. This heat absorption effectively cools the fabric surface below the temperature required for sustained combustion. In practical applications, fabrics incorporating hydrated metal oxides can maintain a lower surface temperature for a period when exposed to a flame, thereby delaying the onset of combustion.

II. Physical Flame Retardancy

Beyond chemical mechanisms, physical barrier formation is another significant approach in flame retardant finishing. Inorganic fillers like mica and talc can be incorporated into fabrics to create physical barriers. These fillers possess good high-temperature resistance and, during combustion, can form a continuous protective layer on the fabric surface, impeding the transfer of flame and heat. Industrial protective fabrics often utilize inorganic fillers like mica to enhance flame retardancy. For example, adding an appropriate amount of mica powder to protective garments used in high-temperature environments can effectively reduce the fabric's thermal conductivity, safeguarding the wearer from heat injury.

From a fiber structure perspective, modifying the fiber's microstructure can also impart flame retardancy. Chemical modification or physical treatments can enhance a fiber's thermal stability and flame resistance. For instance, copolymerization modification of polyester fibers to introduce flame-retardant functional groups can improve their inherent flame retardancy. Such modified polyester fibers exhibit a higher molecular chain scission temperature and a slower thermal decomposition rate when burned, thereby reducing the generation of flammable gases.

III. Principles of Phosphorus-Based Flame Retardants

When phosphorus-based flame retardants are exposed to high temperatures, they decompose, generating phosphoric acid. This phosphoric acid acts like a remarkable "catalyst," promoting the dehydration and carbonization of oxygen-rich polymers. This process induces significant changes in the polymer's molecular structure, substantially reducing the production of combustible gases. Concurrently, a compact, robust, and hard-to-burn char layer forms, akin to equipping the material with a resilient suit of "flame-retardant armor."

Furthermore, the generated phosphoric acid does not remain inactive, it undergoes further dehydration polymerization, forming polyphosphoric acid. This polyphosphoric acid creates a glassy melt layer that acts like a transparent yet sturdy "protective shield," uniformly covering the material's surface. It functions as a loyal "guardian," effectively blocking oxygen access and suppressing the release of volatile pyrolysis products, thereby providing an excellent protective barrier.

When polyester is heated, phosphorus-based flame retardants decompose, releasing phosphoric acid. This phosphoric acid has a unique chemical effect: it can promote the scission of ester bonds within the polyester molecules, initiating a degradation process. As these ester bonds break, the polyester forms molten droplets. Under forces like gravity, these droplets can drip away from the flame zone, helping to prevent further flame spread and providing an effective mechanism for suppressing combustion.

In summary, from the mechanisms described, it is clear that phosphorus-based flame retardants primarily achieve their effect by promoting fiber carbonization. Since the carbonization process is generally closely related to the oxygen content within the fiber, phosphorus-based retardants are typically more effective on oxygen-containing fibers compared to non-oxygen-containing ones.

IV. Conclusion

The principles underlying fabric flame retardant finishing constitute a multifaceted and complex system. Chemical flame retardants act through gas-phase, condensed-phase, and interrupt-heat-exchange mechanisms, while physical barriers and fiber structure modifications also provide crucial support for flame retardant properties. With continuous scientific and technological progress, our understanding of these principles is steadily deepening. Future developments will likely yield even more efficient and environmentally friendly flame retardant technologies, offering more reliable protection for life and property safety.

Yinsu Flame Retardant, dedicated to the deep cultivation and innovation of flame retardant technology, is committed to providing efficient, eco-friendly, and customized flame retardant solutions for diverse sectors including textiles, building materials, and electronic appliances. We safeguard safety and empower sustainable industrial development.