Silicone elastomer (VMQ/PMQ family) and fluororubber (FKM) anchor two non-overlapping corners of the industrial seal spec sheet — the first for cold flexibility, FDA contact and dry-heat endurance, the second for hydrocarbon, acid and amine exposure [S3].
FKM is a copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP), often with tetrafluoroethylene (TFE) or perfluoromethylvinyl ether (PMVE) for peroxide-curable perfluoroelastomer (FFKM) grades; silicone rubber is a polydimethylsiloxane (PDMS) backbone with vinyl, phenyl or trifluoropropyl side groups, the last giving fluorosilicone (FVMQ) as a hybrid candidate [S2][S5].
Temperature Service Window: Cold Flexibility vs Hot Oil Endurance
General-purpose silicone rubber (VMQ) keeps elastomeric behaviour down to roughly -60°C and survives continuous dry heat around 200–230°C, with peak excursions to 250°C; FKM is normally rated from about -26°C (standard copolymer) to +205°C continuous and +230°C peak, with low-temperature FKM and FVMQ grades pushing the floor to -40°C [S2][S3]. In an oven seal, food-grade gasket or steam-line static joint, silicone wins on the low end; in a transmission seal, fuel injector O-ring or amine-rich process fluid, FKM wins on the high end.
Below the glass transition silicone does not embrittle as sharply as FKM, so VMQ retains sealing force at freezer or aerospace altitude; FKM below its grade limit stiffens and loses contact stress, which is why HNBR or low-temperature FKM is sometimes layered in [S1].
Chemical Resistance: Where the Two Families Diverge
FKM resists aliphatic and aromatic hydrocarbons, mineral oil, diesel, gasoline, methanol, jet fuel, many acids and amines, and ozone; the three-dimensional solubility parameter (HSP) work in [S1] quantifies this through the Flory-Huggins χ parameter, showing low χ (good compatibility) for FKM in non-polar fuels and higher χ in polar ketones and esters where volume swell still occurs. Standard VMQ swells in non-polar fuel and aromatic solvents but holds in dry heat, hot air, water, dilute acids and food media [S3][S4].
FVMQ (fluorosilicone) was developed as a hybrid: its trifluoropropyl side group gives FKM-like fuel resistance, while the siloxane backbone preserves low-temperature flexibility, with the trade-off of poorer hot-air ageing than FKM and a useful ceiling around 200°C [S5].
Mechanical, Wear and Compression-Set Behaviour
Tensile strength of unfilled silicone is low (typically 3–7 MPa), so reinforcing fumed silica (20–40 phr) is the norm; FKM unfilled also sits low, with carbon black or mineral fillers taking it to 8–15 MPa, and nano-Al2O3 filler studies report further improvements in wear resistance and hardness for dynamic FKM parts such as valve-stem seals [S4]. Compression set is where FKM separates from VMQ in long-duration static seals: peroxide-cured FKM typically returns 15–25% set after 70 h / 200°C, while general VMQ lands closer to 25–40%, which is why silicone dominates short-cycle and easy-replace applications and FKM dominates ten-year service static seals.
Hardness range: VMQ 30–80 Shore A, FKM 50–90 Shore A, FVMQ 40–80 Shore A — relevant when the mating gland tolerates only narrow compression limits, for example an O-ring in a medical or aerospace housing.
Cure System, Process Window and Post-Cure
Silicone is typically peroxide-cured (peroxide type 2,4-dichlorobenzoyl peroxide, dicumyl peroxide) and demands a post-cure of 4 h / 200°C to drive off peroxide by-products and stabilise compression set; FKM uses bisphenol AF (curative I) for diamine-resistant compounds, peroxide (curative V) for amine-, acid- and steam-resistant parts, with the peroxide system increasingly specified for sour service [S3]. The post-cure step costs line time but is the reason FKM retains <30% compression set after years of hot-oil immersion, while improperly post-cured VMQ can reversion-fail in confined steam.
Processing temperature: silicone moulds at 150–180°C, FKM at 160–200°C, and FKM mould fouling is the practical reason many moulders keep separate tooling and operators between the two families.
Selection Decision Matrix: When to Pick Silicone, When to Pick FKM
Use silicone rubber (VMQ/PMQ) when the service envelope is dry heat, FDA/USP contact, electrical insulation, outdoor weathering or sub-zero flexibility — typical roles: oven door gaskets, food-grade static seals, high-voltage insulators, cryogenic flanges; consult silicone rubber grade and cure data before finalising. Use FKM when the media is hydrocarbon, sour crude, amine, methanol, hot oil or aggressive chemical — typical roles: fuel-system O-rings, AEM/AFM seals, chemical-pump stators, automotive crankshaft seals; the FKM selection logic is detailed in FKM type, temperature and chemical gates. Use FVMQ only when the design genuinely needs both fuel resistance and -55°C flexibility and can accept the 200°C ceiling. [S1]
For nitrile (NBR) or EPDM applications adjacent to the same service point, see the nitrile rubber service envelope and EPDM heat, steam and weather behaviour reference pages. For a broader cross-section of industrial elastomer grades, the industrial rubber material index maps compound families against media and temperature.
Failure Modes and Limits Engineers Still Hit
Silicone fails by reversion (siloxane backbone rearrangement) in confined steam above ~150°C, by explosive decompression in high-pressure CO2/N2, and by tear propagation in dynamic glands; FKM fails by rapid gas decompression (RGD) in high-pressure gas, by amine-induced degradation in unpostcured parts, and by cold stiffening below its grade limit, where seals vent rather than seal [S1][S4]. The single most common call-back on FKM is missing post-cure (compression set doubles); the most common call-back on silicone is wrong hardness (too hard, no conformability; too soft, extrusion).
Spec sheet must therefore fix: grade, hardness Shore A ±5, tensile and elongation minimum, compression set at stated time/temperature, post-cure cycle, and a media-immersion volume swell target — without all five, the seal is a guess.
Cost, Lead Time and Sourcing Reality (June 2026)
Raw FKM gum runs roughly 4–6× the price of general-purpose VMQ gum at mid-2026 spot, with perfluoroelastomer (FFKM) grades 15–25× the silicone baseline; lead times on Chinese extruder-supplied silicone tube and hose lines from suppliers such as Hangzhou Bili Rubber & Plastic CO.,LTD stay inside 2–3 weeks for catalogue profile, while FKM O-ring compounds from major compounders (Daikin, Solvay, 3M Dyneon, DuPont Viton) hold at 3–6 weeks for peroxide-cured and 6–10 weeks for FFKM [S6].
For buyers comparing FKM options against the same media, the FKM fluororubber selection criteria page lays out type, temperature and chemical gates; for the silicone side, silicone rubber selection criteria covers hardness, temperature, cure and grade gates in matching depth. Two trackable signals to watch: FKM price easing if HFP supply normalises through Q3 2026, and the gradual pull-through of FVMQ in aerospace fuel bladders, which is where the silicone–FKM boundary is moving fastest.