White Paper · Fire Safety
FRP is an organic composite and will burn when exposed to sufficient heat — but not all FRP burns equally. Flame-retardant polyester and vinyl ester grades are formulated to self-extinguish and to limit flame spread and smoke; phenolic FRP is nearly non-flammable, produces very little smoke and is the standard for offshore escape routes and enclosed spaces. This paper explains the ASTM E84 surface-burning test, what flame spread index (FSI) and smoke developed index (SDI) actually mean, and why phenolic resin is the decisive material where fire safety is regulated.
- ASTM E84 (Steiner tunnel) measures how fast a flame spreads across a surface and how much smoke it produces — the two metrics codes care about most.
- Class A requires FSI ≤ 25 and SDI ≤ 450; that is the target for most occupied industrial and commercial walkways.
- Phenolic FRP achieves FSI near 5, low smoke and no dripping, satisfying IMO FTP Code and offshore/escape-route requirements where polyester cannot.
How Surface Burning Is Measured
ASTM E84, Standard Test Method for Surface Burning Characteristics of Building Materials, mounts a 9.7 m (32 ft) sample panel in the ceiling of a gas-fired "Steiner tunnel." A controlled flame (equivalent to a roughly 88 kW fire) is applied at one end for 10 minutes. Two readings are taken:
- Flame Spread Index (FSI):) how far and how fast the flame front advances. Red oak = 100; inorganic cement board = 0. The material is rated against these two references.
- Smoke Developed Index (SDI): optical density of the smoke drawn through the tunnel exhaust, again scaled red oak = 100 / cement = 0.
Building codes then group materials by FSI/SDI into classes. For grating in occupied areas the target is Class A.
What the Code Numbers Mean
| Class | Flame Spread Index (FSI) | Smoke Developed Index (SDI) | Typical Use |
|---|---|---|---|
| Class A | 0 – 25 | 0 – 450 | Corridors, exit access, occupied walkways, offshore decks |
| Class B | 26 – 75 | 0 – 450 | Limited industrial use; rarely for grating |
| Class C | 76 – 200 | 0 – 450 | Exposed exterior only; not for interiors |
Per ASTM E84 / NFPA 255 and building-code adoption. Most ZeAllgrate fire-retardant molded and pultruded grating meets Class A (indicative FSI 5–20, SDI 100–350); verify with the test report for the exact resin batch.
Why Phenolic Is Different
| Resin System | Typical E84 FSI | Smoke | Behaviour in Fire |
|---|---|---|---|
| General-purpose orthophthalic | 40–75 (Class B/C) | Moderate–high | Burns, drips, smokes; not for interiors |
| Fire-retardant isophthalic | 15–25 (Class A) | Moderate | Self-extinguishing; acceptable general industrial |
| Fire-retardant vinyl ester | 10–20 (Class A) | Low–moderate | Self-extinguishing; retains integrity longer |
| Phenolic | ≤ 5 (Class A) | Very low | Chars, does not drip, zero fuel contribution |
Indicative ranges from industry fire-performance data and manufacturer E84 reports. Phenolic is the premium fire/low-smoke grade; FSI and SDI values depend on resin formulation and glass content.
The Offshore & Escape-Route Material
Phenolic resin cross-links into a char on heating. In a fire it does not melt, drip or feed the flame; it forms a stable carbon layer that insulates the glass and limits further decomposition. Smoke is low, and toxic-gas yield (CO, HCN, HCl) is far below halogenated polyester. This is why phenolic grating and handrail are specified for:
- Offshore oil & gas: IMO FTP Code (Resolution A.653(16)) and SOLAS require low flame-spread and low-smoke materials in accommodation, escape routes and control stations.
- Enclosed / below-deck spaces: pump rooms, cable trenches, tunnels where smoke tenability governs evacuation.
- Public and occupied buildings: corridors, stadium access and any space where code demands Class A with low SDI.
The trade-off: phenolic is less chemically resistant than vinyl ester (it is not chosen for strong oxidizers), has a rougher natural surface and costs roughly 2–3× a fire-retardant polyester. It is selected for fire, not for corrosion.
Which Standard Applies Where
Specify the standard family that matches your jurisdiction and vessel/operator; offshore projects often require IMO FTP in addition to E84.
| Standard / Code | What It Governs | Typical Application |
|---|---|---|
| IMO FTP Code (A.653(16)) | Surface flammability, smoke & toxic gas of marine materials | Escape routes, accommodation, control stations |
| SOLAS | Life-saving and fire-safety regulations | Escape and access design |
| ASTM E84 / NFPA 255 | Surface burning (FSI/SDI) | General deck material; Class A target |
| ASTM E662 / NBS smoke | Specific optical density of smoke | Enclosed-space qualification |
| ISO 5659-2 | Smoke density by chamber test | Rail / marine / tunnel supply |
| UL 94 | Vertical burn of plastic specimens (V-0 rating) | Component-level fire classification |
IMO FTP Code, SOLAS, ASTM E84, E662, ISO 5659-2 and UL 94; operator specifications frequently impose stricter internal smoke/toxicity limits than the base code.
Specifying Fire-Safe FRP
Start from the code
Identify the jurisdiction (US building code, EU EN 13501, offshore IMO). For most interior walkways E84 Class A is the practical target; for offshore escape routes require phenolic + IMO FTP approval.
Request the test report
A verbal "Class A" is not enough. Require an ASTM E84 report stating FSI and SDI for the exact resin grade, and confirm the report is current and traceable to a recognized laboratory.
Match resin to fire + chemistry
Fire and corrosion are separate selection axes. Where both are severe (e.g. bleach plant offshore), fire-retardant vinyl ester is usually the compromise; phenolic where fire dominates and chemistry is mild.
Do not assume post-cure fire performance
Fire performance depends on resin formulation and cure. Uncured or under-cured panels have higher FSI. Require post-cure and Barcol readings alongside the fire report.
Why E84 Alone Is Not Enough Offshore
E84 measures flame spread and optical smoke density; it does not measure the toxicity of the gases a burning material produces. Offshore and enclosed-space codes care about tenability during evacuation: how much CO, HCN, HCl and SO₂ a fire releases. This is where phenolic decisively separates from polyester. Halogenated fire-retardant polyesters can pass E84 Class A while producing corrosive HCl smoke; phenolic burns to a stable char and releases far lower toxic-gas yield, which is why IMO FTP Code and North Sea operator specifications call for phenolic in escape routes, accommodation and control stations regardless of E84 alone.
| Material | Flame spread (E84 FSI) | Smoke (SDI) | Toxic gas yield | Dripping |
|---|---|---|---|---|
| FR polyester (general) | 40–75 | High | Moderate (depends on FR) | Yes |
| FR isophthalic / VE | 10–25 | Moderate | Low–moderate | Minimal |
| Phenolic | ≤ 5 | Very low | Very low | None (chars) |
Indicative ranges; toxic-gas yield per ISO 5659-2 / NBS smoke chamber and operator test data. Phenolic is the tenability choice for enclosed and escape-route service.
What to Actually Check
A "Class A" claim on a brochure is not a specification. When you receive an E84 report, verify four things:
- The resin grade matches the panel you are buying. A report on a fire-retardant isophthalic does not cover a vinyl ester or a different glass content.
- The report is current and from a recognized lab (e.g. an independent fire-test laboratory), not an in-house extrapolation.
- Both numbers are stated: FSI and SDI, not just "Class A." Class A requires FSI ≤ 25 AND SDI ≤ 450.
- For offshore, confirm IMO FTP Code / smoke-toxicity data in addition to E84; E84 Class A does not imply IMO approval.
Two Selection Axes, One Panel
Fire and corrosion are separate selection axes, and the "best" resin for one is not always best for the other. Phenolic wins fire and loses to vinyl ester on strong oxidizers; vinyl ester wins chemistry but needs a fire-retardant formulation to reach Class A; isophthalic is the cheap workhorse for both general fire and general corrosion. The correct procedure is to list both requirements and pick the lowest-cost resin that meets the stricter one — then confirm the other on the matrix. Where both are severe (e.g. a bleach plant on an offshore deck), fire-retardant vinyl ester is usually the compromise; phenolic is reserved for fire-dominated, chemically mild service.
Conclusions
Fire-safe FRP is achievable, but it is specified, not assumed. Require ASTM E84 with FSI ≤ 25 and SDI ≤ 450 (Class A) for occupied walkways; upgrade to phenolic resin — FSI near 5, very low smoke, no dripping — for offshore escape routes and enclosed spaces governed by IMO FTP. Always tie the fire claim to a current laboratory report for the exact resin batch, and balance fire selection against chemical resistance, which is a separate design axis.
Sources Cited in This Paper
- Industry molded grating engineering guide (manufacturer PDF) — fire-retardant grades and E84 Class A data. Manufacturer technical data available upon request.
- Industry pultruded grating metric design manual (manufacturer PDF) — phenolic and fire-retardant bar data. Manufacturer technical data available upon request.
- ASTM E84 / NFPA 255 — Surface Burning Characteristics of Building Materials; ASTM E662; ISO 5659-2; UL 94.
- IMO Resolution A.653(16) / FTP Code; SOLAS — Fire safety of marine materials and escape routes.
- ZeAllgrate — Guidelines for the Engineer/Designer (ACMA) — fire-performance specification practice.
Specify a Fire-Rated FRP Package
Tell us the code (E84 / IMO FTP / EN 13501) and your chemicals — ZeAllgrate returns a fire-rated resin recommendation with an E84 report for the batch.