Halogen-free flame retardants can be classified into the following main categories based on their chemical composition, flame retardant mechanism and application characteristics. The classification method is carried out in combination with technical principles and actual application scenarios:
1. Classification by chemical element composition
Phosphorus-based flame retardants
Mechanism of action: Dual flame retardancy through the condensed phase (promoting the formation of the carbonized layer) and the gas phase (releasing free radical scavengers).
Representative type:
Organophosphorus compounds: such as phosphate esters and phosphonate esters, suitable for polyurethane, epoxy resin, etc.
Inorganic phosphorus compounds: such as red phosphorus (which requires surface modification to enhance stability), are often used in polyolefins.
Features: High flame retardant efficiency, but some organophosphorus compounds may affect the material’s hydrolysis resistance.
Nitrogen-based flame retardants
Mechanism of action: High-temperature decomposition generates inert gases such as nitrogen and ammonia, diluting the concentration of flammable gases and inhibiting the combustion chain reaction.
Representative type: Melamine and its salts (such as melamine cyanurate, MCA).
Features: Low toxicity and low smoke, but it needs to be used in combination with phosphorus-based or intumescent flame retardants to enhance the effect.
Silicon-based flame retardants
Mechanism of action: Forms a silica protective layer to isolate oxygen and improve the thermal stability of the material.
Representative types: silicone oil, silicone resin, silicate (such as montmorillonite).
Features: Excellent flame retardancy and durability, but relatively high cost. It is mostly used in high-end engineering plastics.
Inorganic flame retardant
Representative type:
Aluminum hydroxide (ATH) : Decomposes to absorb heat and release water vapor, suitable for thermoplastics.
Magnesium hydroxide (MDH) : It has better thermal stability than ATH and is suitable for high-temperature processing materials.
Features: Large filling amount (usually 30%-60%), which may affect the mechanical properties of the material, but the cost is low.
2. Classification by flame retardant mechanism
Intumescent flame retardant
Composition: It usually includes an acid source (such as phosphoric acid), a carbon source (such as pentaerythritol), and a gas source (such as melamine).
Mechanism of action: A porous carbonized layer is formed during combustion, which isolates the transfer of oxygen and heat.
Application: Widely used in polyolefin, polyurethane foam, etc.
Condensed phase flame retardant
Mechanism of action: By promoting the carbonization of materials or forming a glassy protective layer, it reduces the release of flammable gases.
Representative types: Phosphorus-based and silicon-based flame retardants and some inorganic fillers (such as zinc borate).
Gas-phase flame retardant
Mechanism of action: Releases inert gases (such as nitrogen, carbon dioxide) or free radical scavengers (such as hydrogen halide substitutes) to inhibit the combustion chain reaction.
Representative types: Nitrogen-based flame retardants, partial phosphorus-nitrogen synergistic systems.
3. Classification by the type of applied materials
Flame retardants for thermoplastics
Demand characteristics: It needs to adapt to high-temperature processing (such as injection molding and extrusion), and has high requirements for the dispersion and thermal stability of flame retardants.
Representative types: Phosphorus-based, intumescent flame retardants and ATH/MDH composite systems.
Flame retardants for thermosetting plastics
Demand characteristics: It is necessary to form a stable cross-linked structure with the resin matrix, and the flame retardant needs to be resistant to high-temperature curing processes.
Representative types: red phosphorus, organophosphorus compounds and silicon-based flame retardants.
Flame retardants for elastomers and rubber
Demand characteristics: Flexibility, aging resistance and flame retardancy need to be taken into account.
Representative types: Phosphorus-nitrogen synergistic system, ATH/MDH modified products.
4. Classification by functional complexity
Single-function flame retardant
Features: It only has flame-retardant properties and needs to be used in combination with other additives (such as plasticizers and stabilizers).
Representative types: Inorganic flame retardants such as ATH and MDH.
Multifunctional flame retardant
Features: Integrates multiple functions such as flame retardancy, toughening, and anti-dripping.
Representative types: Phosphorus-nitrogen synergistic intumescent flame retardants, silicon-phosphorus composite systems.
5. Classification by stage of technological development
Traditional flame retardants
Representative types: ATH, MDH, red phosphorus, etc. The technology is mature, but there may be problems such as large addition amount and performance impact.
New type of flame retardant
Representative type:
Nano flame retardants: such as nano montmorillonite and layered double hydroxide (LDH), enhance flame retardant efficiency through the nano effect.
Bio-based flame retardants: such as lignin and chitosan, derived from renewable resources and environmentally friendly.
Reactive flame retardants: They are chemically bonded and embedded in polymer chains to achieve permanent flame retardancy without migration.
Conclusion
The classification of halogen-free flame retardants needs to comprehensively consider chemical composition, flame retardant mechanism, application scenarios and technological development trends. In practical applications, the selection of flame retardants needs to balance flame retardant efficiency, material performance, cost and environmental friendliness, and achieve a synergistic effect through compounding or modification technologies. With the advancement of green chemistry and the demand for sustainable development, new halogen-free flame retardants (such as nano flame retardants and bio-based flame retardants) will become the focus of future research and development.
