Flame Retardant Mechanisms of Flame-Retardant PP Materials

The flame-retardant mechanism of flame-retardant PP (polypropylene) materials centers on interrupting or delaying the “heat-oxygen-combustible gas” cycle (the three essential elements of combustion: combustibles, oxidizers, and ignition sources) through physical or chemical actions. Based on the types of flame retardants used and their modes of action, these mechanisms can be primarily categorized into five types, which often work synergistically to enhance flame-retardant performance.

1. Gas-Phase Flame Retardation Mechanism

This mechanism functions by interfering with the gas-phase reaction process of combustion. Its core lies in either “inhibiting free radical reactions in the flame” or “diluting the concentration of combustible gases,” and it is commonly found in systems such as halogen-containing flame retardants and some phosphorus-based flame retardants.

• Free Radical Scavenging: When heated, flame retardants decompose to release active substances (e.g., hydrogen halides, PO· free radicals). These substances can combine with high-energy free radicals (e.g., H·, OH·) generated during combustion reactions to form stable small-molecule compounds (e.g., H₂O, HCl), interrupting the “free radical chain reaction” (the key to sustaining combustion) and thereby extinguishing or suppressing the flame.
  For example, bromine-containing flame retardants (such as decabromodiphenyl ether) decompose when heated to produce Br·, which combines with H· to form HBr. HBr then reacts with OH· to generate H₂O and Br· (Br· can act cyclically), continuously blocking the combustion chain.
• Gas Dilution: Flame retardants decompose to produce large amounts of inert gases (e.g., N₂, CO₂, H₂O vapor). These gases do not participate in combustion; instead, they dilute the concentrations of O₂ and combustible gases (e.g., alkanes, alkenes produced by PP pyrolysis) in the combustion zone, lower the flame temperature, and ultimately inhibit combustion.

2. Condensed-Phase Flame Retardation Mechanism

This mechanism acts on the solid or molten bulk of PP materials. It modifies the polymer’s pyrolysis behavior and forms a “protective barrier” to prevent the transfer of heat and oxygen, serving as the core mechanism for halogen-free flame retardancy (e.g., phosphorus-nitrogen intumescent systems).

• Promoting Char Formation / Creating a Flame-Retardant Char Layer: When heated, flame retardants (e.g., phosphorus-based, phosphorus-nitrogen synergistic systems) catalyze the PP molecular chain to undergo “cross-linking, cyclization, and aromatization” reactions, forming a dense, continuous char layer. This char layer performs the following functions:
  1. Physical Barrier: Prevents external O₂ from entering the interior of the material while blocking the escape of internal combustible gases into the flame zone.
  2. Heat Insulation: The char layer has poor thermal conductivity, which reduces the transfer of external heat to the interior of the material and slows down the further pyrolysis of PP.
     For example, in intumescent flame-retardant PP (IFR-PP), the acid source (such as ammonium polyphosphate, APP) releases phosphoric acid when heated, catalyzing the dehydration and carbonization of the carbon source (such as pentaerythritol). Meanwhile, the gas source (such as melamine) releases gas to expand the char layer, forming a “honeycomb-like porous char layer” with stronger barrier effects.
• Inhibiting Pyrolysis Reactions: Some flame retardants (e.g., metal hydroxides) can lower the surface temperature of the material through endothermic decomposition or react with active groups generated by PP pyrolysis, inhibiting the decomposition of PP into combustible small molecules and reducing the production of combustibles.

3. Heat Exchange Interruption Mechanism

This mechanism works by absorbing heat generated from combustion to lower the temperature of the material surface and the combustion zone, keeping the temperature below PP’s ignition temperature (approximately 420°C) and thus terminating combustion. It is commonly used in metal hydroxide-based flame retardants (e.g., aluminum hydroxide, magnesium hydroxide).

• Specific Process: Metal hydroxides (such as Al(OH)₃) decompose when heated at 200–300°C, releasing crystal water and absorbing a large amount of heat (the decomposition heat of Al(OH)₃ is approximately 1967 J/g). The water vapor produced by decomposition not only absorbs heat but also dilutes combustible gases. The generated metal oxides (e.g., Al₂O₃) adhere to the material surface, further enhancing the heat insulation and barrier effects.
• Characteristics: This mechanism relies on a high addition amount of flame retardants (usually 30%–60%). Although it is environmentally friendly and halogen-free, it may affect the mechanical properties of PP (e.g., toughness, processing fluidity).

4. Synergistic Flame Retardation Mechanism

A single flame retardant often struggles to balance “high flame-retardant efficiency, low addition amount, and excellent mechanical properties.” Therefore, in practical applications, compounding multiple flame retardants is common, leveraging the synergy of different mechanisms to improve performance. Common compound systems include:

• Phosphorus-Nitrogen (P-N) Synergy: Phosphorus-based flame retardants (e.g., APP) promote char formation, while nitrogen-based flame retardants (e.g., melamine) release inert gases to expand the char layer. Additionally, nitrides can react with phosphides to form more stable phosphorus-nitrogen compounds, enhancing the density of the char layer. This synergy improves efficiency by 2–3 times compared to using phosphorus or nitrogen flame retardants alone.
• Halogen-Antimony (Halogen-Sb) Synergy: Halogen-containing flame retardants (e.g., bromine-based ones) decompose to produce hydrogen halides, which react with antimony trioxide (Sb₂O₃) to form antimony halides (e.g., SbBr₃). These halides not only scavenge free radicals but also form a “glassy coating” on the material surface, combining both gas-phase and condensed-phase flame-retardant effects and significantly reducing the addition amount of halogen-based flame retardants.
• Inorganic-Organic Synergy: For instance, compounding nano-montmorillonite (inorganic) with intumescent flame retardants (organic). Nano-montmorillonite can disperse uniformly in the PP matrix, promoting char layer formation and improving its mechanical strength while reducing the addition amount of organic flame retardants.

5. Nanocomposite Flame Retardation Mechanism

With the development of nanotechnology, nanoscale fillers (e.g., nano-clay, nano-silica, carbon nanotubes) are introduced into the PP matrix. Through “nanoscale dispersion,” flame retardancy is achieved with a low addition amount (usually 1%–5%). The core mechanisms are “optimizing the char layer structure” and “inhibiting pyrolysis volatiles.”

• When heated, nano-fillers migrate to the material surface and combine with PP pyrolysis products to form a “dense nanocomposite char layer.” This char layer is thinner and more uniform than traditional char layers, with higher efficiency in blocking O₂ and heat.
• The layered or tubular structure of nano-fillers can “physically constrain” the PP molecular chain, slowing down its pyrolysis rate and reducing the production of combustible gases.
• For example, in nano-montmorillonite/PP composites, the layered structure of montmorillonite forms a “brick-and-mortar” char layer during combustion, significantly improving the material’s flame-retardant rating (e.g., from UL 94 HB to V-1) while barely affecting the toughness of PP.

Summary: Mechanism Focus of Different Flame-Retardant Systems

Halogen-containing flame-retardant (halogen-antimony) systems primarily rely on gas-phase free radical scavenging and condensed-phase coating, with representatives such as decabromodiphenyl ether + Sb₂O₃, offering high flame-retardant efficiency and low addition amounts. Halogen-free phosphorus-nitrogen intumescent systems focus on condensed-phase char formation (intumescent char layer), typified by APP + pentaerythritol + melamine, which are environmentally friendly, low-smoke, and low-toxic. Metal hydroxide systems work through heat exchange interruption (endothermic reaction + water vapor dilution), with Al(OH)₃ and Mg(OH)₂ as typical examples, featuring halogen-free properties, low smoke, and low cost. Nanocomposite flame-retardant systems depend on condensed-phase nanocomposite char layers, represented by nano-montmorillonite and nano-silica, enabling low addition amounts and excellent mechanical properties.

In practical applications, the final flame-retardant performance of flame-retardant PP is usually the result of the combined action of multiple mechanisms. For example, phosphorus-nitrogen intumescent systems involve both “condensed-phase char formation” and “gas-phase gas dilution,” while halogen-antimony systems combine “gas-phase free radical inhibition” and “condensed-phase coating.”