BISCO® HT-840 Extra Firm
Short Description
BISCO® HT-840 is an extra firm closed cell silicone foam engineered for applications demanding the highest closure force, superior sealing capability, and maximum mechanical durability in the HT-800 series. Rated from −67°F to 392°F, HT-840 delivers 20.6 psi compression force deflection at 25% and 1.8% compression set (22 hrs @ 212°F, 50%), providing sponge-rubber-comparable performance in foam form.
Key Properties
- Compression Force Deflection @ 25%: 20.6 psi (142 kPa) typical; spec 16–26 psi (110–179 kPa)
- Compression Set: 1.8% typical; spec <5% (22 hrs @ 212°F, 50%)
- Density: 28 lb/ft³ (448 kg/m³) typical
- Water Absorption: 0.5%; spec <5%
- Thermal Conductivity: 0.09 W/m°K
- Recommended Service Temperature: −67°F to 392°F
We want to help!
Enter your information and we'll get you in touch with our knowledgable staff. We know how important to the right product for your project.
Request a Consultation
- Specifications
- Applications
- General Info
- FAQs
Physical Properties
| Property | Value | Test Method |
| Material | Closed Cell Silicone Foam | — |
| Density | 28 lb/ft³ (448 kg/m³) typical | — |
| Thermal Conductivity | 0.09 W/m°K | ASTM C518 |
| Water Absorption | 0.5%; spec <5% | Internal (24 hrs, 2″ below surface) |
| Thicknesses Available | 0.063″, 0.094″, 0.125″, 0.188″, 0.250″ | — |
| Colors | Gray | — |
Mechanical Properties
| Property | Value | Test Method |
| Compression Force Deflection @ 25% | 20.6 psi (142 kPa) typical; spec 16–26 psi (110–179 kPa) | ASTM D1056 |
| Compression Set (22 hrs @ 212°F, 50%) | 1.8% typical; spec <5% | ASTM D1056 |
Flame & Fire Performance
| Property | Value | Test Method |
| Flame Resistance | V-0 | UL 94 (File E83967) |
| Flame Spread Index (Is) | <35, Flaming Mode | ASTM E162 |
| Smoke Density (Ds) @ 1.5 Minutes | <100, Flaming Mode | ASTM E662 |
| Smoke Density (Ds) @ 4 Minutes | <200, Flaming Mode | ASTM E662 |
| Burn Length | <100 mm/min | FMVSS 302 |
Thermal & Environmental Performance
| Property | Value | Test Method |
| Recommended Use Temperature | −67°F to 392°F (−55°C to +200°C) | Internal |
| Low Temperature Flexibility | Pass at −67°F (−55°C) / 3 min | ASTM D746 |
| Low Temperature Brittleness | Pass at −67°F (−55°C) / 5 hrs | ASTM D1056 |
| UV Resistance | Yes | — |
| Ozone Resistance | Yes | — |
Electrical Properties
| Property | Value | Test Method |
| Dielectric Strength | 57 V/mil | ASTM D149 |
| Dielectric Constant @ 1 kHz | 1.8 | ASTM D150 |
| Dissipation Factor @ 1 kHz | 0.006 | ASTM D150 |
| Dry Arc Resistance | 149 sec | ASTM D495 |
| Volume Resistivity | 10¹⁴ Ω·cm | ASTM D257 |
Outgassing (ASTM E595)
| Property | Value | Test Method |
| Total Mass Loss (TML) | 2.08% | ASTM E595 |
| Collected Volatile Condensable Materials (CVCM) | 0.57% | ASTM E595 |
| Water Vapor Regain (WVR) | 0.01% | ASTM E595 |
Compliance
- UL 94 V-0 — Compliant (File E83967)
- FMVSS-302 — Compliant
- High closure force enclosure and communication cabinet gasketing
- Press pad applications requiring uniform pressure distribution at elevated temperatures
- Fine dust and wind-driven rain sealing in outdoor electronics and lighting enclosures
- Structural sealing interfaces requiring maximum foam density and durability
- High-load vibration isolation and cushioning in industrial equipment
Common industries: telecommunications, electronics, lighting, industrial equipment, and transportation.
HT-840 is the extra firm, highest-density grade in the HT-800 series, designed for enclosures and applications requiring a durable, high-closure-force gasket with sealing performance comparable to traditional sponge rubber. Its patented chemistry and compact cell structure provide superior sealing against fine particles, wind-driven rain, and fire. HT-840 offers the highest tear and tensile strength in the HT-800 series, supporting use in the most mechanically demanding sealing and gasketing environments.
In addition to enclosure gasketing, HT-840 is well-suited for press pad applications requiring greater conformability and uniform pressure distribution at elevated temperatures. Resistance to UV, ozone, and extreme temperature cycling ensures consistent long-term performance across harsh outdoor and industrial environments.
What differentiates HT-840 from other HT-800 series grades?
HT-840 is the extra firm, highest-density grade in the HT-800 series at 20.6 psi CFD typical. It provides the highest closure force, greatest mechanical strength, and maximum sealing durability — specified when the application demands sponge-rubber-level performance in a cellular foam form.
Why is compression set critical?
Compression set measures long-term recovery after sustained compression. Lower values indicate better sealing force retention over the life of the application, reducing maintenance costs from gasket failure.
What is the operating temperature range?
−67°F to 392°F (−55°C to +200°C)
Is HT-840 suitable for regulated environments?
Yes. HT-840 complies with UL 94 V-0 and FMVSS-302, supporting use in electronics and transportation applications with flame performance requirements.
How should this material be specified?
Specify grade (HT-840), thickness, compliance requirements, dimensional tolerances, and finished format. Note: available in Gray only.
