Flame Retardancy

Silicone-Based Flame Retardants for Wire & Cable, Electronics, and Construction

Silicones occupy a unique position in the flame-retardant market: instead of releasing halogenated radicals (the dominant chemistry for traditional FRs) or generating phosphorus-rich char (the leading halogen-free alternative), silicones form a glassy silica char layer when burned. This char acts as a thermal barrier that protects the underlying polymer from continued combustion, while releasing minimal smoke and zero toxic halogens.

Three silicone-FR chemistries dominate commercial use:

Silicone-rubber jacketed cable: HTV silicone rubber compounds with ATH (alumina trihydrate) co-filler, used in low-smoke zero-halogen (LSZH) cable jackets for railway, marine, and tunnel applications
Silicone-modified plastics: silicone-acrylate copolymers and silicone-phosphazene blends as additive flame retardants for engineering plastics (PC, PC/ABS, PBT)
Silicone resins as binders: methyl-phenyl silicone resins as char-forming binders for fire-protective coatings on steel and structural members

Silicone Cable Jacket Compounds

The transition from PVC to LSZH cable jackets has been a multi-decade trend driven by fire safety regulations in public infrastructure (rail tunnels, ships, hospitals, schools). Silicone-rubber LSZH compounds replace the halogenated FRs in PVC with a synergistic mix:

Silicone rubber gum (HTV, Shore A 60–70 base): the polymer matrix
ATH (Aluminum trihydrate, 40–60 phr): releases water at 220 °C, cooling the flame
MDH (Magnesium hydroxide, 30–50 phr): releases water at 340 °C for higher-temperature protection
Fumed silica (5–10 phr): dimensional reinforcement
Vulcanization system: peroxide cure typical for HTV

Performance targets:

IEC 60332-1 vertical flame test: pass
IEC 60332-3 bunched cable: Category C pass
IEC 60754 acid gas: less than 0.5% HCl
IEC 61034 smoke density: light transmittance >60% (vs 5–15% for PVC)
BS 6387 fire-survival: F2 (3 hours flame at 950 °C)

Silicone-jacketed LSZH cable carries 30–60% price premium over PVC but is mandatory in many public-infrastructure projects worldwide.

Silicone-Modified Engineering Plastics

For polycarbonate (PC) and PC/ABS used in IT/electronics enclosures, silicone-acrylate copolymers (e.g., Mitsubishi Metablen S-2001) at 1–3 wt% loading provide UL 94 V-0 rating without bromine or phosphorus. The mechanism: during burning, the silicone migrates to the polymer surface and forms a silica char that quenches combustion.

Silicone-phosphazene blends provide an even stronger flame retardant effect for high-stiffness applications (laptop chassis, server housings) but at higher cost.

These chemistries are differentiated from "silicone-impact-modifier" usage (where silicone-acrylate is used for low-temperature toughness rather than flame retardancy) but the same chemical structure provides both benefits.

Silicone-Resin Char-Forming Coatings

Methyl-silicone resin and methyl-phenyl-silicone resin coatings for steel structural members function differently: rather than slowing combustion of an underlying polymer, they form a char layer at high temperature that insulates the steel from heat. Key applications:

Intumescent fireproofing for steel beams (when combined with ammonium polyphosphate, melamine, and pentaerythritol)
High-temperature exhaust-stack coatings
Engine-bay heat-shield paints

Silicone resins are typically applied at dry film thickness 50–500 μm, depending on the steel cross-section and required fire rating (e.g., 60-minute rating for office buildings, 120-minute for tunnel infrastructure).

Test Standards

The dominant flame-retardancy standards are:

UL 94 (plastics flammability): V-0 rating is the practical minimum for electronics, achieved by halogenated, phosphorus-based, or silicone-based chemistries
EN 13501-1 (Euroclasses for construction): A2 (non-combustible) or B-s1, d0 (limited combustibility, low smoke, no flaming droplets) are typical targets for public buildings
NFPA 130 (rail-vehicle fire safety): Class B-s2, d0 with smoke and toxicity limits
IEC 60695-11-10: needle-flame and glow-wire tests for electrical equipment
IMO FTP Code Part 5: marine applications require non-combustible materials per international shipping regulation

Sourcing and Cost

Silicone flame retardants are higher-cost than halogenated alternatives but increasingly required by regulation:

LSZH cable compounds: $4–7 USD/kg (vs $2–3 for FR PVC)
Silicone-acrylate FR additive: $15–25 USD/kg, used at 2–3 wt% (incremental cost ~$0.30–0.75/kg of finished plastic)
Silicone-resin fireproofing paints: $8–20 USD/kg, varies with formulation complexity