Hydrated sodium silicate and intercalated graphite are the two pressure-generating intumescent chemistries most commonly built into fire door edge seals, glazing gaskets, and air transfer grilles, each with a distinct activation temperature, expansion ratio, and char morphology [S1][S2][S5].
Specifiers running an FD30 cold smoke or FD60 high-temperature cell on the same door set routinely need both chemistries in stock: sodium silicate for the perimeter strip that reacts first, graphite for ironmongery protection kits, grilles, and wider clearances that benefit from volumetric fill [S1][S4][S5]. The fire door assembly as a whole is only as good as the seal at its weakest gap, and the seal chemistry defines that envelope.
Activation Temperature and Expansion: Side-by-Side Numbers
Hydrated sodium silicate begins expanding at roughly 100–120°C (212–248°F) and reaches about 3–5× its original volume, with the char described as a hard, predominantly unidirectional rigid foam that also generates significant closing pressure [S1][S2]. Intercalated graphite starts at 150–220°C (302–428°F) depending on formulation and swells to roughly 15–30× volume, producing a softer, "fluffy" multi- or unidirectional char whose pressure can be tuned across a wide range [S1][S2][S7].
Mono ammonium phosphate (MAP) is the third common chemistry, activating around 180°C and expanding up to 40×, but it forms a low-pressure flexible foam, which is why it shows up in hinge and lock protection kits rather than perimeter strips [S1][S5]. For most FD30 applications the reference figure is a 10 mm or 15 mm strip routed into the door edge or stop, sized to the tested door-leaf and frame combination [S4].
Char Behaviour, Pressure Class, and Smoke Output
Pressure-generating intumescent chemistries, specifically sodium silicate and graphite, are the categories specified where the seal must close a gap against fire-side pressure, and the rigid char is the mechanism that resists distortion of the surrounding construction [S5][S6]. Soft-char materials, in contrast, insulate rather than squeeze, which is why MAP is fitted behind ironmongery where insulation, not closure force, is the requirement [S5].
Sodium silicate expands without producing visible smoke during activation, a property that makes it the default for clean environments and document-rated enclosures, while graphite typically generates smoke as the intercalated layers blow apart [S2]. A practical spec note: if the fire safety brief calls for both early smoke control and a high-volume fill later in the fire curve, sodium silicate goes on the perimeter strip and graphite goes behind larger cavities, grilles, and the ironmongery kit [S2][S5].
Typical Use Sites on a Fire Door Assembly
On a typical FD30 timber leaf, sodium silicate strips sit in the head and jamb rebates as the primary perimeter seal, with sodium silicate or graphite also used for glazing beads where the aperture is small and integrity matters more than fill volume [S1][S4]. Graphite dominates the larger void-filling applications, including fire-rated door air transfer grilles, letterplates, and pipe- or cable-penetration gaskets, because its 15–30× expansion reliably closes openings up to 25–30 mm without a hard-pressure spike on the frame [S1][S3][S7].
Ironmongery protection kits (hinges, locks, closers, letterplates) almost universally use MAP because the soft foam insulates the metal without distorting the rebate or transmitting force into the leaf; using a pressure-generating chemistry in a hinge kit is a common mis-spec that binds the hardware after activation [S1][S5]. For thresholds and drop-down seals, both chemistries appear in combination with smoke brush or fin elements, and the same dual-material logic applies: low-temperature smoke seal first, high-temperature fill second [S4].
Decision Matrix: Which Chemistry for Which Gap
Across the four criteria that drive seal selection (activation temperature, expansion ratio, pressure class, smoke output), the comparison resolves cleanly: sodium silicate wins on low activation, smoke-free expansion, and rigid high-pressure char; graphite wins on volumetric fill, multi-directional expansion, and the ability to be tuned for low- or high-pressure service [S1][S2][S5]. Expansion start temperature is 100–120°C for sodium silicate versus 180–220°C for graphite; expansion ratio is 3–5× versus 20–30×; pressure is hard and unidirectional versus tunable; smoke output is none versus yes [S2].
The practical mapping: specify sodium silicate for perimeter edge seals, glazing seals up to about 6 mm gap, document and electrical-cabinet doors, and any smoke-sensitive zone [S1][S2]. Specify graphite for air transfer grilles, letterplates, service penetrations 10–30 mm, and any industrial enclosure where the fire curve is expected to run hot for the full rating [S1][S2][S7]. Specify MAP behind ironmongery and for any application needing a soft insulating foam rather than a force-generating seal [S1][S5].
What Goes Wrong: Mismatches and Field Failure Modes
Using a pressure-generating chemistry in a low-clearance rebate is one of the more common over-spec failures: a sodium silicate or hard-graphite strip in too tight a gap will bow the door edge or jam the latch before the rated temperature is reached, and the door fails the integrity test not because the seal is bad but because the geometry was wrong [S1][S5]. The reverse, putting a low-pressure MAP strip in a 25 mm service penetration, gives an insulating foam that cannot crush a softening plastic pipe, so the penetration opens within minutes of fire exposure and fire tracks the void [S5].
On air transfer grilles, choosing graphite for a smoke-control door is a second common mis-spec, because the 180–220°C activation temperature means smoke has been moving through the grille for several minutes before the seal reacts; sodium silicate at 100–120°C is the correct material when the door schedule is cold-smoke-rated [S2]. Specifiers should also treat manufacturer expansion claims as formulation-specific, not category-wide: a 3–5× sodium silicate and a 20–30× graphite bracket a wide range, and a particular supplier's grade can sit at either end of that band depending on the additive package [S1][S2][S7]. For doors that integrate detection and annunciation, the electrical fire monitor side of the system should be coordinated with seal selection, since the seal defines the integrity rating the detection system is designed against.
Sourcing, Standards, and What to Ask the Supplier
Both chemistries sit inside the same third-party-tested fire door assembly: the certificate covers the leaf, frame, ironmongery, and seal combination, not the seal in isolation, so any substitution of strip material, width (commonly 10 mm or 15 mm for FD30), or supplier must be cross-checked against the test evidence [S4]. Request the test report's seal schedule and confirm both the chemistry (sodium silicate, graphite, or MAP) and the activation temperature band, not just the generic "intumescent strip" label [S1][S4].
Two further checks pay off on every order: confirm the strip is non-hygroscopic if the door serves a humid plant room (graphite is non-hygroscopic, sodium silicate is moisture-sensitive and can be unsuitable for unheated external risers) [S1]; and confirm the smoke-seal element is a separate silicone or brush fin, not an intumescent-only strip, where cold-smoke rating is on the door schedule [S4]. For warehouse and industrial enclosure work where the door is a fire extinguisher cabinet or riser access rather than a corridor leaf, the same chemistry logic applies but the gap sizes and rating durations usually push the spec toward graphite-filled or hybrid kits [S1][S7].
Trackable signals for the next quarter: (1) any FD30-to-FD60 re-rating projects will surface strip width changes from 10 mm to 15 mm or 20 mm as the dominant move, and sodium silicate strips will need to be cross-checked for pressure at the wider size [S4]; (2) the trend toward hybrid strips (graphite core with sodium silicate face) is appearing in catalogue data, which gives early smoke sealing plus late high-volume fill in a single rebate, and is worth benchmarking on the next door schedule review [S1][S2].
For related coverage, see Duplex vs Triplex Plunger Pump: Flow Smoothness Decision Guide.