# Silicone elastomer

Selecting silicone rubber by compound, cure chemistry and supplied form, with bounded molding, sheet, sealing and conditioning examples.

Reviewed: 2026-11-01

Tags: materials, polymers, elastomers

## 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](https://www.dow.com/documents/80/80-3947-01-silastic-9204-50-liquid-silicone.pdf?iframe=true)

**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](https://www.wacker.com/h/medias/6709-EN.pdf)

## Cure family and process choice

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

| System | Cure and design implication |
|---|---|
| RTV-1 | One component; atmospheric moisture initiates condensation cure from the exposed surface inward. Skin formation does not establish through-cure. |
| Condensation RTV-2 | Components are mixed; required moisture is already present in the base. Volatile reaction products and shrinkage require consideration. |
| Addition RTV-2 | Components bring together polymer, platinum catalyst and crosslinker. The addition reaction does not release volatile curing byproducts. |

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](https://www.wacker.com/h/medias/7461-EN.pdf)

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:

| Property | Result | Specimen cure condition |
|---|---:|---|
| Shore A, ASTM D2240 | 50 | 10 min at 120°C |
| Tensile strength, ASTM D412 | 6.1 MPa | 10 min at 120°C |
| Elongation, ASTM D412 | 380% | 10 min at 120°C |
| Tear strength, ASTM D624 B | 41 kN/m | 10 min at 120°C |
| Compression set, ASTM D395 | 26%, after 22 h at 175°C | 10 min at 175°C; method B, Type I, 12.5 mm |

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](https://www.dow.com/documents/80/80-3947-01-silastic-9204-50-liquid-silicone.pdf?iframe=true)

## 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](https://www.wacker.com/h/en-in/medias/ELASTOSIL-M-4641-AB-en-2024.01.09.pdf)

**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](https://www.tapesolutions.saint-gobain.com/en-us/files/2786/download)

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](https://www.parker.com/content/dam/Parker-com/Literature/Praedifa/Catalogs/Catalog_O-Ring-Handbook_PTD5705-EN.pdf)

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](https://www.wacker.com/h/medias/6709-EN.pdf)

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](../grades-tempers-and-conditions.md)