Halogen-free flame retardants are mainly divided into five categories: inorganic hydroxides, phosphorus-based, nitrogen-based, intumescent (phosphorus-nitrogen combination), and silicon-based; there are also a few auxiliary types such as boron-based, metal salts, and bio-based.
1. Inorganic hydroxides (most common, low cost)
– Aluminum hydroxide ATH: decomposition ≈200°C, addition 30%–60%; low smoke, non-toxic, used for PP/PE/EVA, wire and cable, home appliances.
– Magnesium hydroxide MDH: decomposition ≈340°C, addition 25%–50%; higher heat resistance, suitable for PP, ABS, engineering plastics; not suitable for PET/PBT (catalytic hydrolysis).
– Zinc borate: often compounded with ATH/MDH, smoke suppression, anti-drip, used for PVC, nylon, epoxy.
– Antimony oxide: auxiliary flame retardant, compounded with phosphorus/nitrogen, used for engineering plastics, coatings.
2. Phosphorus-based (efficient, low smoke, mainstream for electronics/engineering plastics)
– Ammonium polyphosphate APP: core of intumescent type, addition 15%–30%; charring + gas release, used for PP, PE, coatings, wood.
– Red phosphorus RP: efficient (addition 5%–15%), strong flame retardancy; requires microencapsulation to prevent moisture absorption, used for nylon, PBT, epoxy.
– Organic phosphate esters: such as TPP, RDP, BDP; transparent, low toxicity, used for PC/ABS, PVC, epoxy resin.
– Phosphinates: such as AlPi, ZnPi; good thermal stability (>300°C), efficient with low addition, used for PA, PBT, TPU.
– DOPO and its derivatives: reactive type, phosphorus-containing epoxy/resin, used for PCB, copper-clad laminate, electronic potting.
3. Nitrogen-based (low smoke, high thermal stability, preferred for nylon)
– Melamine cyanurate MCA: Decomposition ≈350°C, low smoke, no dripping; used for PA6/66, PP, EVA.
– Melamine polyphosphate MPP: Phosphorus-nitrogen compound, high thermal stability, used for high-temperature nylon, epoxy.
– Melamine phosphate MP: Intumescent component, compounded with APP.
4. Intumescent flame retardant IFR (phosphorus-nitrogen compound, no dripping, low smoke)
– Typical formula: APP (acid source) + pentaerythritol (carbon source) + melamine (gas source).
– Features: Foaming upon heating to form char layer, heat and oxygen insulation; used for PP, PE, ABS, coatings, wood.
5. Silicon-based (new type, low smoke, anti-dripping, high-end applications)
– Polysiloxane/silicone resin: e.g., PDMS, MQ resin; add 5%–15%, improves processing and toughness; used for PP, PE, epoxy.
– Silicone rubber micro powder: anti-dripping, smoke suppression; used for engineering plastics, wires and cables.
6. Other auxiliary halogen-free flame retardants
– Boron-based: borax, boric acid, metaborate; smoke suppression, char formation; used for wood, coatings, PVC.
– Metal salts: ammonium molybdate, stannates; mainly smoke suppression, used in combination.
– Bio-based: starch, cellulose, lignin; environmentally friendly and low cost; used for PP, PE, composites.
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Selection quick reference (key differences)
– Cost: ATH/MDH < APP < MCA < Phosphinate < Silicon-based
– Flame retardant efficiency: Phosphinate > Red phosphorus > IFR > MCA > ATH/MDH
– Thermal stability: Silicon-based > MDH > MCA > APP > ATH
– Smoke density: Nitrogen-based ≈ Silicon-based < Phosphorus-based < ATH/MDH
