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Environmental Friendly Flame Retardants—Melamine and Its Salts

With increasingly strict global environmental regulations and rising public awareness of safety, traditional halogen-antimony flame retardants are facing restrictions due to high smoke output, toxic gas release and strong corrosion during combustion. Melamine and its salts have emerged as premium nitrogen-based and nitrogen-phosphorus synergistic flame retardants, widely recognized as top choices for environmental friendly flame retardants solutions.

As core components of intumescent flame retardants (IFR), melamine and its salts deliver dual flame retardant effects in both condensed phase and gas phase. Featuring low toxicity, low smoke, high thermal stability and cost efficiency, they are extensively applied in plastics, textiles, coatings, rubber and wood products. This article comprehensively introduces their core advantages, working mechanisms, common product types, practical applications and production features.

Outstanding Advantages of Melamine and Its Salt Flame Retardants

Melamine and its salts stand out among mainstream flame retardants for their excellent comprehensive performance and environmental compatibility, compared with halogen-antimony systems, organophosphorus compounds and inorganic hydrates.

Low Toxicity and Low Corrosion

Melamine is low-toxic and non-irritating. Its oral LD50 for rats exceeds 3000 mg/kg. Long-term inhalation of melamine dust at 200 mg/m³ causes no poisoning symptoms. Most melamine salts also have low toxicity:
  • Melamine Cyanurate (MCA): Oral LD50 > 4000 mg/kg for rats
  • Melamine Polyphosphate (MPP): Oral LD50 > 2000 mg/kg for rats
These materials exhibit far weaker corrosion than halogen-antimony flame-retardant materials. After 100 hours of contact with stainless steel at 300°C, the corrosion of MPP and MCA systems is only 1/7 and 1/50 of halogen-antimony systems, respectively, effectively protecting processing molds and equipment.

Low Smoke and Few Harmful Combustion Products

During thermal decomposition and combustion, melamine and its salts release non-toxic, inert gases such as N₂, CO₂, and NH₃, with no halogenated acids (HX) generated.
  • For PA6 flame-retarded by MCA, the maximum smoke density in flaming and smoldering states is only 1/4 of that using halogen-antimony additives.
  • MPP flame-retarded PA66 produces nearly no HBr and minimal HCN and CO. Its VOF index (evaluating smoke and toxic substances) is just 1/5 of halogen-antimony systems.
This characteristic fully complies with fire safety standards for public places and EU environmental requirements for building materials.

Excellent Thermal Stability

Most melamine and its salts start to decompose above 250°C, which is suitable for the processing temperature of general plastics and engineering plastics:
  • Melamine, melamine phosphate and melamine pyrophosphate: Initial decomposition at around 250°C
  • MCA and MPP: Resist temperatures over 320°C; MPP only loses 2% mass at 370°C
They will not decompose or fail during high-temperature plastic processing.

Nitrogen-Phosphorus Synergistic Effect

Phosphate-based melamine salts contain high contents of nitrogen (14%–15%) and phosphorus (35%–40%). The combination of nitrogen and phosphorus produces a strong synergistic flame-retardant effect, greatly improving overall fire resistance.

Other Practical Merits

  • Dual flame retardant performance in condensed phase and gas phase;
  • Minor impact on plastic processability: Materials remain workable even with 30%–40% addition;
  • Low density (1.55 ~ 1.80 g/cm³), lighter than halogen-antimony flame retardants;
  • Mostly white powder, easy to blend with various materials;
  • Abundant raw materials, simple production process and competitive cost.
Main Limitations
The primary drawback is relatively moderate flame-retardant efficiency. A dosage of 25%–30% is generally required for ideal results. Some melamine salts may slightly weaken the mechanical properties of base materials, and a few varieties have slight water solubility or sublimation issues.

Comprehensive Flame Retardant Mechanisms

Unlike single-function flame retardants, melamine and its salts act through six synergistic pathways to suppress combustion, operating in both the condensed and gas phases.

Endothermic Cooling

Sublimation, volatilization and thermal decomposition of melamine and its salts absorb large amounts of heat. The heat absorbed during melamine decomposition at 250–450°C reaches 2000 kJ/mol, significantly reducing the combustion zone temperature and inhibiting flame spread.

Release Inert Gases

Decomposition generates N₂, CO₂, NH₃ and water vapor. These gases dilute oxygen and flammable gas concentrations around the flame, forming a protective gas layer that isolates air.

Promote Char Formation

Melamine salts form cross-linked polymers at high temperatures, with high char yields: MPP reaches 40%, and melamine pyrophosphate reaches 50% at 700°C. The dense char layer prevents heat and mass transfer and protects internal materials from burning. Higher char yield means better flame resistance.

Form Intumescent Barrier Layer

As key components of intumescent flame retardants (IFR), they work with acid sources and carbon sources to form a thick intumescent barrier layer under high heat, blocking heat conduction and fuel volatilization.

Heat Removal by Molten Drips

Some melamine salts, such as oxalate and phthalate, cause molten drips during combustion. The drips carry away heat quickly and extinguish the flame. For PA6, adding only 3%–5% melamine oxalate can raise the limiting oxygen index (LOI) above 35%.

Capture Active Free Radicals

Decomposition products trap active free radicals in the gas phase and terminate combustion chain reactions. Free radicals can also be neutralized by collision with the intumescent char layer.

Common Types of Melamine-Based Flame Retardant Salts

Melamine is a weak base and can react with inorganic and organic acids to form stable salts. The mainstream commercial varieties and basic properties are listed below:
Product NameNitrogen ContentPhosphorus ContentAppearanceTypical Commercial Brands
Melamine (MA)66.64%0Colorless crystal–
Melamine Cyanurate (MCA)49.40%0White crystalBudit315, Melapur MC
Melamine Phosphate (MP)37.52%13.82%White powderFyrol MP, Amgard NH
Melamine Pyrophosphate (DMPY)39.07%14.40%White powderBudit311, FRCROS-491
Melamine Polyphosphate (MPP)40.78%15.02%White crystalMelapur 200, Budit 3141
Melamine Borate (MB)44.71%–White powder–
Melamine Oxalate (MO)38.89%–White powder–
Among them, MCA and MPP are the most widely used industrial products, featuring extremely low water solubility (MPP: 0.05 g/100mL water at 25°C) and stable performance.

Wide Application Scenarios

Since the 1980s, melamine and its salts have been applied in flame-retardant industries. They can be used alone or compounded with other flame retardants and additives, covering multiple industries.

Engineering Plastics & Thermoplastics

They are ideal for PP, PE, PS, PVC, PC, PA, PET, PBT and epoxy resin.
  • MCA: Dominates non-reinforced flame-retardant PA. It accounts for nearly 40% of flame retardants used in Western European non-reinforced PA for electronics and electrical appliances.
  • MPP: Suitable for glass-fiber reinforced PA and PBT. Formulas like PA66/Glass fiber/MPP can achieve a UL94 V-0 rating at 1.6 mm thickness, with excellent LOI values.

Rubber & Elastomers

Used for flame-retardant EPDM and other elastomers. Formulas compounded with PPO and kaolin can achieve UL94 V-0 performance.

Textiles, Coatings, Paper & Wood

Applied as flame retardant additives for textiles, anti-flaming coatings, fire-resistant paper and treated wood, meeting civil and construction fire protection requirements.

Polyurethane Foam

Combined with halogenated phosphate esters for flexible polyurethane foam, delivering outstanding moisture aging resistance and meeting British BS flame retardant standards.

Products using melamine-series flame-retardants have high Comparative Tracking Index (CTI), low density and low smoke, making them especially suitable for electrical, electronic and automotive plastic parts.

Conclusion-Environmental Friendly Flame Retardants

Melamine and its salts are mature, eco-friendly, halogen-free flame retardants that integrate low toxicity, low smoke, high thermal stability and cost-effectiveness. Their multi-path flame-retardant mechanisms make them reliable alternatives to traditional halogen-antimony flame retardants.

Driven by global environmental protection policies and fire safety upgrading, the market demand for melamine-based flame retardants keeps growing. For the industry, future development directions include optimizing product quality, developing composite flame retardant masterbatches, conducting surface modification and expanding application fields.

For material manufacturers, choosing melamine and its salts can not only meet strict environmental and fire standards but also control production costs. They will remain a mainstream development direction for the flame-retardant industry in the long run.

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