Flame breakthrough resistance

Definition (what it is)

The ability of a material, component, or assembly to prevent a flame from penetrating through its thickness and igniting or sustaining flaming on the unexposed side for a specified time under defined thermal and mechanical conditions. It is typically expressed as time to flame penetration (breakthrough time) or as a pass/fail outcome based on integrity and non-exposure-side criteria set by a test.

Function and purpose (key technical characteristics)

  • Maintains a fire barrier: limits flame propagation, hot gas and particle egress, and heat transfer to protected areas.
  • Thermal performance: low through-thickness heat flux and delayed temperature rise on the unexposed side; effective char/intumescent or ceramic layer formation.
  • Integrity under fire: resists melting, perforation, cracking, delamination, erosion by high-velocity flames, and joint or seam failure.
  • Mechanical stability at elevated temperature: retains enough strength/cohesion to avoid rupture under thermal expansion, vibration, or pressure differentials.
  • Controlled surface behavior: limited afterflame/afterglow, minimal melt-drip or ember formation, and low flame spread on the exposed face.

How it is measured and tested

  • Typical exposures: direct flame impingement (including jet-like flames), radiant heat, or combined thermal–mechanical loads with defined flame temperature, heat flux, airflow, and duration.
  • Common metrics: time to burnthrough; presence/absence of holes or cracks; ignition or sustained flaming on the unexposed side; backside temperature rise thresholds; mass loss; afterflame time and dripping.
  • Representative standards and practices (chosen per sector and hazard):
    • Burnthrough and jet-fire focused: ISO 22899-1 (jet fire), ISO 2685/aviation powerplant fire tests, ASTM E2816 (aircraft fuselage burnthrough), UL 1709 (rapid-rise hydrocarbon fire).
    • Assembly fire resistance: ASTM E119, ISO 834/EN 1363 (evaluate integrity and insulation of walls/doors/floors where flame breakthrough is one failure mode).
    • Materials and screening: UL 94 and IEC 60695 flammability tests, limiting oxygen index (LOI), ASTM D6413 (textiles). These address ignitability and flame spread and are not direct measures of flame breakthrough.
    • Protective clothing and flexible barriers: ISO 17492, ASTM F1939 (heat transmission and burnthrough resistance of fabrics).
    • Vehicle and battery systems: OEM- and regulator-specific tests (e.g., UL 2580, SAE, UN/ECE and national EV standards) that simulate thermal runaway, directed flame jets, and external fire exposures to assess enclosure burnthrough and occupant protection.
  • Important distinction: many common flammability tests (e.g., UL 94, LOI) do not replicate the heat fluxes or erosive action of real flame jets; burnthrough resistance must be verified with exposure-appropriate methods.

Relevance and applications

  • Electric vehicles (EVs): battery pack enclosures and covers, module-to-module partitions, underbody shields, and cabin/battery bulkheads require burnthrough resistance to contain thermal runaway flames and hot gases, delay flame egress, and provide evacuation time. High-voltage cable jackets, busbar insulation, potting, and power-electronics housings benefit similarly to prevent cascading failures and compartment-to-compartment fire spread.
  • Transportation and aerospace: thermal/acoustic insulation, firewalls, nacelle components, and fuselage burnthrough protection.
  • Buildings and infrastructure: fire doors, walls, penetration seals, cable transit systems, and fire stops.
  • Marine/offshore and energy: jet-fire resistant passive fire protection on structures and equipment.
  • Protective equipment: multilayer textiles in garments and flexible barriers where preventing flame penetration and limiting heat transfer are critical.

Design strategies and typical materials

  • Multilayer barriers: combine a structural layer (metal or composite) with insulating and sacrificial layers (intumescent or ceramic-forming coatings, ceramic papers/felts) to reduce heat flux and resist perforation.
  • Metals: steel, stainless steel, and aluminum sheets or honeycombs for structural integrity and high breakthrough resistance; often paired with internal insulation.
  • Inorganic insulations: mica sheets, glass fiber mats, mineral wool, and alumina–silica ceramic papers/felts for high-temperature resistance.
  • Polymer composites: phenolic, high-Tg epoxy, cyanate ester, or BMI matrices with glass/carbon/aramid fibers; often enhanced with intumescent or ceramic-forming surface layers.
  • High-temperature polymers and elastomers: silicones and certain fluoropolymers for gaskets, seals, cable jackets, and coatings that form protective char or ceramic-like residues.
  • Textiles and foils: aramid and oxidized PAN fabrics, basalt fabrics, and aluminized or metallic foils for wraps, shields, and radiant heat reflection.
  • Flame-retardant systems and fillers: expandable graphite, phosphorus–nitrogen chemistries, mineral hydrates (ATH, MDH), melamine derivatives, nano-clays, and glass/ceramic microspheres to promote char, reduce thermal conductivity, and suppress melt-drip.
  • Joints and penetrations: design overlaps, folded seams, fire-resistant gaskets/sealants, and thermal-barrier adhesives to eliminate weak points where breakthrough typically initiates.
  • System-level measures: vent design to redirect jets away from barriers, pressure relief to limit mechanical loading on hot barriers, and mechanical support for intumescent/char layers.

Factors affecting performance

  • Material properties: thermal stability, char yield/strength, softening/melting behavior, emissivity, thermal conductivity, heat capacity, and thickness.
  • Exposure conditions: flame velocity/temperature, heat flux, duration, orientation, oxygen availability, and combined mechanical loads.
  • Real-world degraders: vibration, pre-damage, cyclic thermal aging, contamination (oils/electrolyte), and assembly tolerances at seams and fasteners.

Synonyms and related terms

  • Synonyms/near-related: burnthrough resistance, flame penetration resistance, jet-fire resistance (context-specific).
  • Related but distinct: flame resistance/retardancy (ignitability and flame spread behavior), fire-resistance rating of assemblies (overall time-to-failure including integrity and insulation), thermal propagation resistance (battery-specific), smoke/toxicity performance.

Notes and cautions

  • A high flammability rating (e.g., UL 94 V-0) or high LOI does not guarantee flame breakthrough resistance; targeted burnthrough or fire-resistance tests are required.
  • Select tests that realistically represent the hazard (e.g., localized jet flames from venting cells versus large pool fires), and validate full assemblies, including joints and penetrations, not just flat coupons.