Silicone elastomer

Identity and material specification

Dow's SILASTIC 9204-50 gives typical 6.1 MPa tensile strength after a 10 min cure at 120°C. Its 26% compression set after 22 h at 175°C instead uses a 10 min cure at 175°C, ASTM D395 Method B, Type I, 12.5 mm. Carry specimen cure and exposure with each result. Dow SILASTIC 9204-50 data

Silicone elastomer is a crosslinked rubber formulation, not a single grade or a universal seal material. WACKER describes a silicon–oxygen backbone with organic substituents; polymer structure, fillers and additives influence the result. VMQ denotes methyl-vinyl silicone, while FVMQ includes fluorinated substituents and must be identified separately. High-consistency rubber (HCR) is supplied as solid raw rubber; WACKER offers both peroxide- and platinum-addition-curing HCR. Its liquid silicone rubber (LSR) is a two-component, platinum-addition-curing system. WACKER, Solid and Liquid Silicone Rubber, pp. 8–13

Cure family and process choice

WACKER's RTV guide distinguishes these room-temperature-vulcanizing systems:

The guide warns that addition-system mixing ratios are product-specific; changing the ratio can produce incomplete cure or altered properties. Select the ratio and permissible temperature adjustment from the exact product instructions. A two-part silicone is not necessarily an LSR injection-molding compound. WACKER, RTV Silicones, §§3.1–3.2

For production, separately control usable working time, mold filling/de-airing, cure sufficient for demolding, any required post-cure and final inspection conditioning. A tack-free surface or an easy demold is a process observation, not proof that all specified mechanical or extractables requirements have been met.

Named LSR example: preserve cure conditions

Dow's SILASTIC 9204-50 is a self-lubricating, 4 wt% phenyl-oil-filled LSR for injection molding. Selected typical results are:

Dow says these values are not specification-writing values. It supplies lot-matched A/B kits, specifies 1:1 mixing, and recommends metering/mixing without entraining air. Amines, sulfur and organotin complexes can inhibit cure. Dow, SILASTIC 9204-50 TDS, pp. 1–2

A room-temperature casting example

ELASTOSIL M 4641 A/B is an addition-curing RTV-2 molding rubber, not the Dow LSR. WACKER specifies A:B = 10:1 by weight and requires matching batch numbers. Its guide values include 90 minutes pot life at 23°C, defined by reaching 150,000 mPa·s, and cured properties after 24 hours at 23°C: Shore A 43, tensile strength above 4.5 N/mm² under ISO 37 and elongation above 300%. WACKER, M 4641 A/B TDS

Calculated batching illustration: for 550 g total at 10:1 by mass, A is 550 × 10/11 = 500 g and B is 550 × 1/11 = 50 g. This does not authorize volume-based dispensing or establish a pot life at another temperature. Preserve the stated viscosity endpoint when comparing working-time data.

Finished sheet and assembly limits

Saint-Gobain's Norseal 9200 series is finished solid sheet/continuous-length rubber. Its 9235 and 9255 data give Shore A hardnesses of 30 and 50, respectively, and typical compression set of 30% after 70 hours at 302°F (150°C). These exposure conditions differ from the Dow test. The sheet recommends −73 to 204°C for the silicone elastomer but directs users to separate data for pressure-sensitive adhesive options. A backing adhesive therefore cannot inherit the rubber's temperature range. Saint-Gobain, Norseal 9200, Form 1049

For a gasket cut from sheet, specify thickness tolerance, surface condition, attachment and compression geometry. For a molded part, specify flash, parting lines and critical sealing dimensions. Establish compression and leakage acceptance on the actual assembly instead of deriving them from hardness alone.

Service, fluid compatibility and aging

Parker's silicone-rubber screening guidance lists incompatibility with silicone oil/grease, low-molecular chlorinated hydrocarbons, aromatic mineral oil, fuels and aromatic hydrocarbons. It separately lists resistance to aliphatic engine/gearbox oils and animal/vegetable oils and grease; keep these distinctions when screening a fluid. Its handbook also explains that fluid interaction can cause absorption and extraction, producing swelling or shrinkage; temperature, compound composition, thickness and stress state affect the result. Use this as family-level screening, then obtain compound-specific evidence for the actual medium. Special oil-resistant silicone formulations do not make all VMQ compounds interchangeable. Parker, O-Ring Handbook, §§6.3.16–6.4

Qualify the combined service condition: fluid concentration, temperature, duration, pressure, motion and cleaning cycles. Keep air-aging results separate from liquid exposure. For electrical, optical, food-contact or medical use, obtain the exact grade's applicable documentation and finished-part test requirements; a generic silicone label does not establish suitability.

Post-cure, handling and source dates

WACKER explains that post-curing can improve mechanical properties and remove volatiles; requirements depend on formulation and intended use. Its processing guide calls for suitable fresh-air circulation and warns against cross-contamination during post-curing. Follow the grade-specific procedure and current SDS for both components, pigments, curing agents and primers. Do not infer a universal bake cycle from the word silicone. WACKER, Solid and Liquid Silicone Rubber, §6.1

The current WACKER solid/LSR guide prints 6709en/10.25, replacing 6709e/11.22; its RTV guide prints 7461en/10.24, reprint 7461e/02.19. M 4641's TDS explicitly gives most recent change 9 January 2024. Dow's TDS prints ©2017–2021, Form 80-3947-01-0921 S2D. Norseal's PDF prints ©March 2026, despite its product page listing an older publication date. The cited Parker European handbook prints 07/2015, ©2015. These source dates are distinct from this page's review date.

Material selection through production