Silicone rubber is a silicone-elastomer compound that retains flexibility, resilience and tensile strength across a wide temperature range, with the -40 °C to +250 °C window quoted across the JESilicone strip product family [S6] and used as a baseline gate for elastomer selection. In practice it competes head-to-head with EPDM rubber on weatherability and with nitrile rubber on oil resistance, and it almost always wins when the service envelope is hot, dry, and electrically insulating.
Selection is governed by a small set of measurable gates: Shore A hardness, tensile strength, elongation at break, compression set, tear strength, specific gravity, food/medical grade (FDA 21 CFR 177.2600, USP Class VI, LFGB), flame rating (UL 94 HB/V-0), and post-cure (4 h @ 200 °C) for low-bleed / low-outgassing service. The decision is rarely about a single property; it is about which of these gates the application will actually punish.
Temperature Window and Why Silicone Beats EPDM and Nitrile Above 150 °C
Continuous silicone service is typically rated -60 °C to +230 °C for general-purpose grades and up to +300 °C for peroxide-cured, post-cured fluoro-silicone variants, with the JESilicone strip range quoted at -40 °C to 250 °C [S6] and kSil® sheeting positioned for extreme-temperature gaskets and seals [S5]. Above +150 °C, silicone rubber is the only common elastomer that does not harden, crack, or permanently deform in dry air.
For comparison: EPDM tops out near +150 °C continuous, nitrile (NBR) near +120 °C, and FKM is the only widely stocked competitor that can match silicone's upper end, at roughly 3-5x the raw-polymer cost. If the service is sub-zero only (down to -55 °C), EPDM is usually cheaper and adequate; if it is hot and oil-exposed, FKM wins despite cost. Silicone is the right answer when the medium is hot, dry, aqueous, or insulating rather than hydrocarbon-rich.
Hardness, Tensile and Elongation: Translating Datasheet Numbers Into a Part
Silicone compounds are specified by Shore A hardness, most commonly 30, 40, 50, 60, 70 and 80, with 50-60 Shore A as the default for gaskets, O-rings, and keypads. JESilicone strip parts are described as soft, elastic, transparent and smooth, consistent with a 40-60 Shore A general-purpose formulation [S6]. Tensile strength for solid silicone is typically 5-10 MPa and elongation at break 200-700%, depending on filler load and cure system [S5].
When a datasheet shows 30 Shore A, expect low tensile (~5 MPa) but very high elongation and good conformability to rough flanges; 70 Shore A gives higher tensile and tear, but lower elongation and stiffer feel. For keypads (a hot end-use — DirectIndustry lists 9 manufacturers / 16 products in the silicone-rubber-keypad category, with 4-key, 12-key and 18-key layouts common [S1]) the sweet spot is 50-60 Shore A with a printed carbon or silver-pad circuit, which is exactly the mechanical window most key suppliers such as CTI Electronics, Storm Interface and ShinEtsu publish.
Compression Set, Post-Cure and Long-Term Sealing Performance
Compression set is the property that kills silicone gaskets in service: at 22 h / 175 °C, a general-purpose peroxide-cured solid sits around 15-25%, and a post-cured grade drops to under 10%. Post-cure (typically 4 h @ 200 °C in a ventilated oven) drives off peroxide residues and short-chain siloxanes, reducing outgassing that would otherwise fog optics, contaminate electronics, or fail food-contact migration tests. [S1]
For keypad elastomers and industrial rubber bellows that see continuous compression, request a 22 h / 175 °C compression-set figure on the datasheet, not just a hardness number. If the supplier will not publish it, the grade is probably general-purpose filler-heavy stock that will take a permanent set after a few thermal cycles.
Food, Medical and Flame Grades: The Compliance Gates That Override Mechanical Specs
Food-contact silicone is normally certified to FDA 21 CFR 177.2600 (US) and LFGB §30/31 (EU), and medical grades to USP Class VI or ISO 10993-5. These certifications constrain the formulation: only certain peroxides and platinum-cure systems are permitted, and fillers are restricted to food-grade silica. JESilicone positions its strips as non-toxic and tasteless, consistent with food-contact positioning [S6].
Flame rating is the other compliance gate: silicone sheeting can be supplied UL 94 HB as standard or upgraded to UL 94 V-0 for transit, rail, and enclosure gaskets. If the part is going into a railway, elevator, or EV battery enclosure, the V-0 rating is non-negotiable, and so is a low-smoke, low-halogen datasheet. Mechanical properties are secondary once a compliance gate is locked.
Cost, MOQ and Sourcing Reality: What a 2026 Buyer Actually Sees
Made-in-China listed extrusion seals at US$ 0.25-1.20 per metre at 3,000 m MOQ, with a separate high-pressure extrusion strip at US$ 0.50 per piece at 100-piece MOQ from Shandong Respower [S4]. Custom extruded silicone / PVC / TPE / TPV / EPDM strip seals from Xiamen Neway were quoted in the same US$ 0.25-1.20 / metre band, signalling that commodity silicone strip is no longer a premium line item compared with TPE or TPV [S4].
Trade-side supplier directories show roughly 10 active silicone-keypad suppliers on TradeBoss [S2] and 9 on DirectIndustry [S1], with Xiamen Better Silicone (ISO 9001 since 2002) [S2] and JESilicone (20 years, China) [S6] as representative custom moulders. The practical sourcing rule: for O-rings, gaskets and keypads under 10,000 pieces/month, stock a Shore A 50-60 general-purpose grade with FDA + post-cure options; above that volume, commission a custom mould and lock the cure system (peroxide vs platinum) on the drawing.
Limits and Failure Modes: Where Silicone Is the Wrong Choice
Silicone has poor tear strength (typically 10-25 kN/m, vs 30-50 kN/m for NBR), so thin wall sections under shear fail quickly — specify a minimum 1.0-1.5 mm wall on dynamic seals. It swells heavily in concentrated fuels, aromatic solvents, and steam above ~120 °C / 4 bar; in those services, FKM fluororubber-class materials or steam-cured EPDM outperform it, and cost differences shrink once failure rates are counted. It is also not abrasion-resistant — avoid silicone for slurry or particle-laden media, where UHMWPE liners carry the load. [S2]
Two further gotchas: silicone has high gas permeability (about 10x that of butyl), so vacuum and pharmaceutical packaging often need a butyl inner layer; and uncured or poorly post-cured silicone outgasses cyclic siloxanes (D4/D5) that can poison optics and contact switches — for keypad and sensor-adjacent parts, demand a post-cure and ask for a D4/D5 extractables report.
Selection Checklist: The Five Gates to Lock Before RFQ
Lock these in order: (1) temperature window — confirm -40 °C to +250 °C is sufficient [S6], otherwise jump to FKM or fluoro-silicone; (2) hardness — default 50-60 Shore A unless the gasket is soft (30-40) or rigid (70-80); (3) compliance — FDA / LFGB / USP Class VI / UL 94 V-0 as the application demands; (4) compression set — under 20% at 22 h / 175 °C for any dynamic seal; (5) cure system — platinum for medical/clear parts, peroxide for general industrial, and post-cure for low-outgassing.
Cross-check with adjacent elastomer guides to make sure silicone is genuinely the best fit — for fuel and aromatic service, FKM fluororubber is the correct pivot, and for sliding-wear liners under silicone gaskets, UHMWPE is the usual pairing. With those five gates locked and a credible supplier (Shenzhen KEYU, ShinEtsu, Storm Interface, Xiamen Better or JESilicone-class moulders [S1][S2][S6]), the resulting silicone part will track the datasheet rather than the marketing brochure.