Two routes to fire retardancy
Halogenated fire-retardant resins contain chlorine or bromine chemistry, which interrupts flame reactions in the gas phase. Halogen-free resins avoid those elements and rely on other mechanisms, commonly mineral fillers such as alumina trihydrate that release water when heated.
Both routes can produce laminates that pass demanding fire tests. They differ in what the smoke contains, in how the resin handles and in what specifications allow.
Why specifications choose
Burning halogenated materials can release acidic, corrosive gases. For places people must escape from — trains, ships, some buildings — standards may limit smoke toxicity, and some specifications restrict halogens outright. Where no such restriction applies, a halogenated resin may be entirely acceptable and simpler to process.
Samrat Poly Resins’ three FR grades
SPR 201 FR is the halogenated grade. SPR 201 FR (HF) is the halogen-free orthophthalic grade, listed for halogen-free laminates and marine components. The ISO Fire Retardant Resin is halogen-free, ATH-filled and isophthalic.
Published typical values
The values show a real processing difference: SPR 201 FR (HF) is much more viscous and heavier than the other two, with a longer gel time.
| Property | Fire Retardant Resin (SPR 201 FR) | Halogen-Free Fire-Retardant Resin (SPR 201 FR (HF)) | ISO Fire Retardant Resin (SPR 301 FR) |
|---|---|---|---|
| Chemistry | Not published | Orthophthalic unsaturated polyester, halogen-free fire-retardant | Unsaturated Polyester Resin (Fire Retardant Grade) |
| Appearance | White | White to light-yellow opaque liquid | White |
| Viscosity (typical) | 400 cPs | 1,500–3,500 cPs at 25 °C | 400 cPs |
| Gel time / pot life (typical) | 5.5 Minutes | 15–30 minutes at 25 °C | 5.5 minutes |
| Specific gravity / density (typical) | Not published | 1.25–1.45 | 1.18 g/cm³ |
| Source of values | Fire Retardant Resin TDS | 2026 catalogue; grade TDS | ISO Fire Retardant Resin TDS |
Typical values, reproduced from each product’s own published data. They describe the liquid resin at its reference condition, not a finished part, and are not guaranteed limits.
Proving the choice
Whichever resin is chosen, the fire classification belongs to the finished construction — gelcoat, laminate, thickness and core — tested to the named standard. Choose the resin that the specification allows and that processes well in your shop, then test the build.