Date | 2026-05-11 09:52:27
In rail vehicle material selection, “flame retardant” is the requirement everyone mentions. For interior and equipment non-metallic materials, however, the real safety threshold is not whether the material ignites — it is how much smoke and how much toxic gas it releases while burning. Those two decide how far people can see and how long they can survive in a fire.
The short version: the real gate is smoke density (R22) and gas toxicity (R23), not flammability alone. A high oxygen index does not mean passing smoke and toxicity — brominated flame retardants raise LOI but can generate corrosive gas, while inorganic systems based on aluminium hydroxide or magnesium hydroxide absorb heat, release water vapour, dilute combustible gas and suppress both smoke and toxicity. Three things matter in practice: itemised R22/R23 reports rather than a blanket “meets HL2”, data at the actual wall thickness because smoke density is thickness-sensitive, and typical value ranges rather than best-case numbers.
EN 45545-2 divides vehicle fire risk into 26 requirement sets, R1 through R26. For BMC and SMC insulation parts, R22 (smoke density) and R23 (toxicity) are the two that decide overall safety.
Under EN 45545-2:2020 Annex C and Annex D, smoke density follows ISO 5659-2 at a radiant heat flux of 25 kW/m², in both flaming and non-flaming modes, and reports Ds max. R22 sets two hazard levels: HL2 generally requires Ds max ≤300, HL3 requires Ds max ≤150.
Toxicity follows EN 17084 or NF X 70-100, assessing eight gases including CO, HCN, HCl and SO₂ and combining them into a conventional toxicity index, CIT. HL2 requires CIT ≤0.9; HL3 requires CIT ≤0.75.
The level a part must reach depends on where it sits in the vehicle — interior or exterior, horizontal or vertical surface — and on what the component does. BMC parts for electrical equipment such as insulators and supports, when installed in a passenger compartment, are commonly required to meet HL2 or HL3 on both R22 and R23.
BMC consists of unsaturated polyester resin, glass fibre, mineral filler and trace additives. The resin system and the flame-retardant synergist make a large difference to smoke and toxicity. Conventional brominated formulations can lift the oxygen index, yet they tend to generate hydrogen bromide and other corrosive gases during combustion, pushing CIT up and making HL2 toxicity hard to hold reliably.
That raises a practical question for material selection: a material already certified to GB 8624 B1 or the withdrawn BS 6853 — does it still satisfy the smoke and toxicity requirements once placed inside the EN 45545-2 framework?
The halogen-free flame-retardant BMC grades developed for rail applications show coverage of both HL2 and HL3 on R22 and R23. Typical measured ranges at 3 mm thickness, tested to ISO 5659-2 and EN 17084:
Smoke density (R22), Ds max in flaming mode: typically 120 to 180, with some batches below 150 — inside the HL3 ceiling.
Toxicity (R23), CIT: generally 0.4 to 0.7, leaving margin against both HL2 and HL3.
The mechanism is a combined aluminium hydroxide and magnesium hydroxide system paired with low-styrene-emission control. The inorganic fillers release water vapour as they decompose, absorbing heat and diluting combustible gas at the surface; optimised interfacial bonding between filler and resin reduces how much organic content takes part in pyrolysis, which limits smoke and toxic gas at the source.
Oxygen index alone is not enough. LOI describes how easily a material sustains combustion at a given oxygen concentration; it says nothing about smoke or toxicity. A material with LOI ≥35 can still perform poorly on R22 and R23.
Ask for itemised reports. A complete EN 45545-2 report should show separate measured results for R22 and R23, not a general statement that the material “meets HL2”.
Check thickness sensitivity. Smoke density correlates with specimen thickness, so request data at the wall thickness your part actually uses — 2 mm or 3 mm, for example.
Confirm production consistency. Laboratory samples and production batches can differ; ask for a typical range rather than only the best result.
Wenzhou Jintong maintains dedicated BMC formulations for rail applications, with test capability against R22 and R23 under the EN 45545-2 framework, and can propose a compliant solution based on the installation location and hazard level your project requires.
Test specimen: BMC 1616, white, 4 mm thickness. Testing body: SGS-CSTC, report AJFS2005003731RS (15 June 2020). Conclusion: the material meets the HL3 hazard level for R22 and R23 in Table 5 of EN 45545-2:2013+A1:2015. The 3 mm ranges quoted above are typical production figures and are not the same measurement basis as this single-grade third-party report — grade, thickness and batch all differ, so selection should rest on a report for the actual grade at the actual wall thickness.
R22 assesses smoke density: Ds max measured to ISO 5659-2 at 25 kW/m² radiant heat flux. R23 assesses gas toxicity: the weighted CIT of CO, HCN, HCl, SO₂ and others to EN 17084 or NF X 70-100. Both feed directly into escape conditions in a fire and are mandatory for vehicle material approval.
No. BS 6853 has been withdrawn, and GB 8624 uses different test items and judgement methods from EN 45545-2. Vehicles entering the European market, or designed to the EN framework, need itemised R22/R23 testing and a corresponding report; older certificates cannot be substituted.
LOI only describes how easily the material sustains combustion at a given oxygen concentration; it does not characterise smoke or toxicity. Brominated formulations can raise LOI while producing hydrogen bromide and other corrosive, irritant gases, which makes CIT worse. Selection has to look at LOI, Ds max and CIT together.
For technical data sheets, grade recommendations, or a discussion about your specific requirements:
📧 wendy.qiu@smcbmc.com 📞 +86-13868305300
