BISCO® RS-840 Very Firm
BISCO® RS-840 is a very firm cellular silicone sponge with a density of 30 lb/ft³ and a compression force deflection of 22.5 psi per ASTM D1056, designed to retain the lightness of a foam while providing the enhanced sealing capability of a traditional sponge rubber. The material has a temperature range of -67°F to 392°F, passes UL 94 V-0 and UL 94 HF-1 at a third-party laboratory, and is UV and ozone resistant. RS-840 is an alternative to HT-840.
Key Properties
- Compression Force Deflection: 22.5 psi (ASTM D1056)
- Compression Set, 22 Hours @ 212°F @ 50%: 4.0% (ASTM D1056)
- Water Absorption: 1.0% (ASTM D1056)
- Density: 30 lb/ft³
- Temperature Range: -67°F to 392°F
- UL 94 V-0 / UL 94 HF-1: Pass (Meets at third party lab)
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- Specifications
- Applications
- General Info
- FAQs
Physical Properties
| Property | Value | Test Method |
| Thickness | 1/8″, 3/16″ | — |
| Color | Gray | — |
| Material/Substrate | Silicone Sponge | — |
| Density | 30 lb/ft³ | — |
Mechanical Properties
| Property | Value | Test Method |
| Compression Force Deflection | 22.5 psi | ASTM D1056 |
| Compression Set, 22 Hours @ 212°F @ 50% | 4.0% | ASTM D1056 |
| Water Absorption | 1.0% | ASTM D1056 |
| Low Temperature Brittleness | -67°F | ASTM D746 |
Thermal Properties
| Property | Value | Test Method |
| Temperature Range | -67°F to 392°F | — |
Flame & Fire Performance
| Property | Value | Test Method |
| UL 94 V-0 | Pass | Meets at third party lab |
| UL 94 HF-1 | Pass | Meets at third party lab |
Environmental Performance
| Property | Value | Test Method |
| UV Resistance | Yes | — |
| Ozone Resistance | Yes | — |
Compliance
- UL 94 V-0 — Compliant (Meets at third party lab)
- UL 94 HF-1 — Compliant (Meets at third party lab)
- Environmental seals against dust, moisture, air, and light penetration in outdoor enclosures including lighting fixtures, HVAC units, and electronic cabinets requiring high closure force resistance
- Durable, high-closure-force gaskets for enclosures where maximum firmness within the RS series is required
- Press pads requiring greater conformability and even pressure distribution under elevated temperature conditions
- Environmental sealing in assemblies subject to high mating flange loads or thermally demanding press applications
- Replacement or alternative sealing for HT-840 applications
Common industries: Electronics, HVAC, industrial manufacturing, press lamination, outdoor enclosures.
BISCO® RS-840 is a very firm-grade cellular silicone sponge in the BISCO® RS series, positioned as a direct alternative to HT-840. Like RS-820, it is formulated to 30 lb/ft³ density and 22.5 psi CFD, but RS-840 is offered in 1/8″ and 3/16″ thicknesses only and is specifically described for applications requiring high-closure-force gaskets, press pads demanding greater conformability, and even pressure distribution at high temperatures.
The press pad application is unique to RS-840 within the RS series and reflects the material’s capacity to distribute load evenly under elevated thermal conditions.
The material achieves a compression set of 4.0% at 22 hours, 212°F, 50% deflection per ASTM D1056. Low temperature brittleness is rated at -67°F per ASTM D746, consistent with the full temperature range of -67°F to 392°F. Water absorption is 1.0% per ASTM D1056. UV resistance and ozone resistance are both confirmed Yes. The material exhibits temperature independency across its service range as stated on the TDS.
Compliant with UL 94 V-0 and UL 94 HF-1 (meets at third party lab).
What differentiates RS-840 from the other grades in the BISCO® RS series?
RS-840 shares the same density (30 lb/ft³) and CFD (22.5 psi) as RS-820 but is distinguished by two characteristics unique to this grade on its TDS: it is the only RS series grade specifically described for press pad applications requiring greater conformability and even pressure distribution at high temperatures, and it is the only very firm grade limited to two thicknesses (1/8" and 3/16"), whereas RS-820 is available in an additional 1/4" thickness. RS-840 is positioned as an alternative to HT-840.
Why is compression set critical for a very firm cellular silicone sponge used in press pad and high-closure-force sealing applications?
In press pad applications, the sponge must distribute load evenly across the pressing surface repeatedly over the service life of the tool. A high compression set means the material permanently flattens under the press load, creating uneven pressure distribution across the pad and reducing its effective service life. For high-closure-force gaskets, permanent set means the sponge no longer rebounds sufficiently to maintain the required contact force against the mating flange. RS-840's compression set of 4.0% at 22 hours, 212°F, 50% deflection per ASTM D1056 confirms the material recovers to near-original geometry through repeated compression at elevated temperature.
What is the temperature range for RS-840?
The temperature range is -67°F to 392°F as listed on the TDS. Low temperature brittleness is rated at -67°F per ASTM D746.
Is RS-840 suitable for flame-rated environments?
Yes. RS-840 passes UL 94 V-0 and UL 94 HF-1 at a third-party laboratory as listed on the TDS.
How should RS-840 be specified?
Specify grade (RS-840), thickness (1/8" or 3/16"), color (Gray), applicable compliance requirements (UL 94 V-0, UL 94 HF-1), dimensional tolerances, and finished format (sheet, slit roll, or die-cut gasket/press pad).
