Flame retardant additives

Definition (what it is)

Flame retardant additives are chemical substances incorporated into combustible materials—such as polymers, textiles, foams, coatings, adhesives, and composites—to reduce the likelihood of ignition, slow flame spread, lower heat release, and limit smoke and toxic gas production during a fire. They can be added physically to a formulation (additive flame retardants) or chemically built into the polymer backbone or network (reactive flame retardants), the latter reducing migration and improving permanence.

How they work (mechanisms)

Flame retardants act through one or more mechanisms in the gas and/or condensed phases:

  • Gas-phase radical inhibition: halogenated compounds and some phosphorus species quench flame-propagating radicals (H•, OH•), interrupting combustion chain reactions.
  • Condensed-phase char promotion: phosphorus- and nitrogen-containing systems catalyze dehydration and carbonization, forming a thermally stable, insulating char that reduces heat and mass transfer and can suppress melt dripping.
  • Endothermic cooling and dilution: mineral hydroxides (e.g., aluminum trihydrate, ATH; magnesium hydroxide, MDH) decompose endothermically, absorbing heat and releasing water or other inert gases that dilute flammable volatiles.
  • Intumescence and barrier formation: intumescent packages and expandable graphite expand into a foamed carbonaceous layer that shields the substrate from heat and oxygen.
  • Synergistic effects: certain combinations (e.g., brominated FRs with antimony trioxide; zinc borate with ATH; phosphorus–nitrogen systems) enhance overall efficiency, smoke suppression, and char strength.

Key technical characteristics and selection factors

  • Target performance: selected to meet flammability, smoke, and toxicity criteria for the application (e.g., ignition resistance, flame spread, heat release rate, dripping behavior).
  • Polymer compatibility: tailored to specific polymers (e.g., PP, PE, PA6/66, PBT, PC, ABS, PC/ABS, PPE/PPO, PPS, PEEK, LCP, epoxies, unsaturated polyesters, polyurethanes, silicones).
  • Loading levels (typical ranges): highly efficient phosphorus/halogen systems ~5–20 wt%; intumescent systems ~10–30 wt%; mineral hydroxides often higher, ~30–60 wt% (common at 45–65 wt% in wire/cable compounds). Actual levels depend on polymer, part thickness, and target rating.
  • Property trade-offs: FRs can affect rheology, mechanical strength/impact, electrical properties (CTI), color, thermal stability, and long-term aging. Reactive systems and polymeric/oligomeric FRs often reduce blooming, plasticization, and volatility.
  • Durability and migration: reactive FRs (common in epoxies, polyurethanes, some polyesters) are covalently bound; additive FRs may require encapsulation or stabilizers to mitigate moisture sensitivity, migration, and discoloration (e.g., encapsulated red phosphorus).
  • Sustainability and compliance: growing preference for halogen-free, low-smoke, low-toxicity systems to address environmental regulations (e.g., RoHS, REACH, Stockholm Convention POPs), corrosion concerns from acid gases, and recyclability.

Major classes and examples

  • Halogenated organic FRs: brominated or chlorinated aromatics/aliphatics (e.g., TBBPA for epoxies/PCBs; polymeric brominated styrenics), often with antimony trioxide synergist. Use is increasingly restricted or application-specific due to environmental and smoke/corrosivity concerns; many legacy substances (e.g., decaBDE, HBCD) are regulated.
  • Phosphorus-based FRs: ammonium polyphosphate (APP), red phosphorus (encapsulated), organophosphates and phosphinates/phosphonates (e.g., RDP, BDP, aluminum diethyl phosphinate, AlPi). Widely used in PC, PA, PBT, PC/ABS, epoxies, PUR, and intumescent systems.
  • Nitrogen-based FRs: melamine and derivatives (melamine cyanurate, melamine phosphate/polyphosphate), triazine-based compounds; often synergize with phosphorus in intumescence.
  • Mineral/inorganic systems: ATH, MDH, metal hydroxystannates, zinc borate, molybdate compounds; primarily endothermic coolants, smoke suppressants, and char enhancers; prevalent in polyolefins, elastomers, and certain thermosets.
  • Carbon-based/barrier systems: expandable graphite; nano-additives (e.g., nanoclays, layered double hydroxides) as secondary synergists to reinforce char and reduce permeability.

Applications and relevance (including EV design)

  • Electrical and electronics: housings, connectors, bobbins, relays, enclosures, and PCB-related components requiring UL 94 V-0/5VA classifications, glow-wire resistance, and high CTI.
  • Transportation (automotive and EVs): interior trim and under-hood parts (meeting FMVSS 302/ISO 3795), high-voltage connectors, busbar supports, inverters/OBC/DC-DC housings, potting/encapsulation resins, cable insulation (often LSZH/HFFR), battery pack components (module housings, cell spacers, vent manifolds, composite enclosures) to slow fire propagation and provide additional intervention time during thermal runaway.
  • Construction, appliances, textiles, and foams: insulation, structural composites, upholstery foams, and coatings where flame spread, smoke density, and toxic effluents must be controlled.

Standards and test methods (examples)

  • Small-scale flammability: UL 94 (HB, V-2, V-1, V-0, 5V), limiting oxygen index (LOI, ASTM D2863), vertical/horizontal burn for vehicles (FMVSS 302/ISO 3795).
  • Electrical safety and ignition sources: glow-wire GWFI/GWIT (IEC 60695-2), tracking resistance CTI (IEC 60112), RTI (UL 746B).
  • Fire performance and heat release: cone calorimetry (ISO 5660), single/dual calorimetry per application standards.
  • Smoke and corrosivity (especially for cables): smoke density (e.g., ISO 5659-2), halogen acid gas and acidity/conductivity (IEC 60754 series). OEMs and sectors often specify internal or application-specific protocols, including battery fire/abuse tests for EVs.

Processing and incorporation

  • Melt compounding: twin-screw extrusion to disperse additives and synergists uniformly; use of compatibilizers, coupling agents, and impact modifiers to balance mechanical properties; moisture control is critical for hydrolysis-prone polymers and hygroscopic FRs.
  • Reactive incorporation: co-monomers or reactive oligomers integrated during polymerization or curing (epoxies, polyurethanes, certain polyesters) to achieve inherent flame retardancy and reduced migration.
  • Surface treatments/coatings: intumescent paints, back-coatings for textiles and foams where bulk modification is impractical.
  • Practical considerations: thermal stability of FRs at processing temperature, particle size/shape effects on viscosity and surface finish, colorability (e.g., red phosphorus darkens), and tool corrosion (acidic volatiles).

Design and selection considerations

  • Define the required fire performance (e.g., UL 94 V-0 at a given thickness, heat release targets, smoke/toxicity limits) alongside electrical, mechanical, and thermal requirements.
  • Match FR chemistry to polymer family, processing temperature, and part thickness/geometry.
  • Evaluate synergy packages (e.g., zinc borate with intumescents; antimony with brominated FRs) for efficiency and smoke suppression.
  • Consider end-of-life, regulatory status, and potential impacts on recyclability and secondary use streams.
  • Validate through representative part geometry and full test regimes; lab-scale ratings do not always predict system-level fire behavior.

Synonyms and related terms

  • Synonyms: flame retardants; fire retardants; FR additives.
  • Related: intumescent systems; smoke suppressants; synergists; halogen-free flame retardants (HFFR); low-smoke zero-halogen (LSZH) compounds; additive vs. reactive flame retardants; self-extinguishing compounds.