The Science Behind Foam Outgassing: What You Need to Know When Selecting The Right Material Solution for Sealed Assemblies
Outgassing Affects More Foam Assemblies Than Most Engineers Expect
Foam outgassing affects more sealed assemblies than most engineers account for during material selection. Silicone foam, polyurethane foam, polyethylene foam, neoprene foam, PVC foam, EPDM foam, vinyl nitrile foam, and silicone sponge each release volatile compounds at different rates when exposed to heat, pressure change, and thermal cycling. The assemblies most vulnerable to those releases include EV battery enclosures, HVAC motor housings, appliance control assemblies, optical modules, and aerospace components.
In many sealed assemblies, failure does not begin with catastrophic material breakdown. Failure often begins with contamination accumulation measured in milligrams rather than pounds. Deposited volatile compounds can reduce optical transmission, increase electrical contact resistance, interfere with sensor calibration, and create intermittent field failures that are difficult to diagnose during validation testing. For OEM manufacturers, this can lead to warranty claims, field returns, reduced product life, and extended root-cause investigations.
Volatile compounds that condense on nearby surfaces, corrode electrical contacts, or degrade optical sensors can originate from foam cushioning, foam gaskets, and foam sealing layers already inside the assembly rather than from an external contamination source. Cell structure, polymer type, density, and post-processing each contribute to the total volatile load introduced into a sealed enclosure, which means outgassing risk is a function of how a foam is built, not just what it is made from.
How Outgassing Is Measured and Why It Is a Design Risk
Outgassing is quantified using ASTM E595-15, which measures two values: total mass loss (TML) and collected volatile condensable materials (CVCM). In the test, a material sample is held at 125°C under vacuum for 24 hours, and the condensed volatile fraction is collected and weighed.
ASTM E595 was originally developed for aerospace contamination screening but has become a practical engineering screening tool for electronics, battery systems, medical devices, and optical assemblies where contamination-sensitive components are enclosed in limited air volume environments.
The standard screening thresholds are TML below 1.00% and CVCM below 0.10%. CVCM is the more critical value in most sealed assemblies because it represents the portion of released volatiles that will condense on a cooler nearby surface. Volatile condensate on optical surfaces, corroded contacts, degraded sensors, and compromised electrical insulation are the failure modes that result when foam materials exceed those thresholds inside a closed enclosure. The applications where that risk is highest include lens mounting gaskets, PCB cushioning, battery cell barriers, motor housing seals, and vibration isolation pads.
Open Cell Foams: Breathable by Design, Volatile by Degree
Open-cell foam is breathable by design. Air, vapor, and liquids move freely through the interconnected cell structure, and in a sealed enclosure that same characteristic means any volatiles released from within the foam bulk can reach the enclosure environment more readily than in a closed-cell material, where volatiles remain trapped within individual cells. Polyurethane is the most widely produced open-cell foam due to its flexibility, broad performance range, and cost, and it is also one of the higher-risk materials in outgassing-sensitive assemblies.
The primary volatile organic compounds (VOCs) generated by polyurethane foam are formaldehyde and acetaldehyde, produced through oxidation of the polyether or polyester soft segment. VOC generation rates can increase significantly during accelerated aging or elevated temperature exposure because diffusion coefficients and oxidation reactions increase with temperature. Engineers evaluating assemblies above 180°F (82°C) should review elevated-temperature aging data rather than relying solely on room-temperature material properties.
Acrolein is a possible byproduct but is more commonly associated with high-heat thermal decomposition than with standard operating conditions. Emission rates rise significantly with temperature, and low-density and high-density grades present different volatile load profiles that must be evaluated independently. Many high-density polyurethane grades, including PORON formulations from Rogers Corporation, pass ISO 6452 fogging requirements and FMVSS 302 flammability requirements, which provides a useful screening baseline but does not substitute for application-specific outgassing data.
Silicone foam presents a different profile. Outgassing does not follow a linear trend across BISCO grades; BF-1000, BF-2000, HT-800, HT-820, and HT-840 each carry different TML and CVCM values that must be confirmed against test data for the grade specified. Post-curing can reduce VOC levels in silicone foam, and the material is rated for continuous use to 200°C (392°F), making it a candidate for high-temperature enclosures where polyurethane would not be suitable.
Closed Cell Foams: Sealed Structure, Lower Migration Risk
Closed-cell foam presents a lower volatile migration risk than open-cell foam in sealed enclosures. The sealed cell structure is impervious to water, vapor, and air, which means gases released within the foam travel a longer path to the surface and enter the enclosure environment more slowly than they would from an open-cell material. That said, closed-cell structure reduces migration rate but does not prevent outgassing. Blowing agents trapped within cells are released gradually over the service life of the foam, and unreacted raw materials from incomplete curing will outgas over an extended period regardless of cell structure.
ASTM D1056 compression testing and compression set measurements should also be evaluated because foam materials experiencing permanent deformation can increase failure modes. For example, it can alter internal air volume and sealing force over time, affecting both contamination risk and long-term enclosure performance. Also, permanent deformation changes enclosure loading and gasket geometry over time. Excessive compression set may create leak paths even when outgassing remains acceptable.
The base polymer is the primary determinant of outgassing profile in any closed-cell foam, but curing process and additives including blowing agents, catalysts, flame retardants, and plasticizers also significantly affect volatile output. Closed-cell elastomeric foams retain the environmental, chemical, and temperature resistance characteristics of their base polymer. EPDM closed-cell foam retains UV and heat resistance; nitrile closed-cell foam retains oil and fuel resistance. These inherited properties are relevant to both application suitability and outgassing behavior, since the same polymer chemistry that governs environmental resistance also governs what volatiles are present and at what release rate.
Each material in the closed-cell category carries its own outgassing profile. Polyethylene foam offers a low outgassing profile relative to urethane and is common in cushioning and protective packaging applications. Closed-cell PVC foam delivers excellent environmental resistance, including weather, fungi, and oxidation, and seals effectively with as little as 30% compression, making it a practical choice for gasketing, sealing, and weatherstripping applications; its outgassing profile should be evaluated by grade, as service temperature and density vary across the product range.
Neoprene and blended neoprene foams combining EPDM, CR, and SBR are widely used in HVAC equipment including gasketing and sealing. EPDM foam provides excellent resistance to UV, ozone, and weathering, making it a common choice for outdoor and HVAC sealing applications. Vinyl nitrile foam, a closed-cell blend of PVC and nitrile rubber (NBR), is common in thermal and acoustic insulation applications and retains the chemical and oil resistance of its nitrile component. Silicone sponge shares the temperature and chemical resistance characteristics of silicone foam but carries a different outgassing profile that must be evaluated independently against the specific grade and construction specified.
Foam Selection Chart
Open-Cell Silicone Foam
| Grade | Density lb/ft³ | TML % | CVCM % | Flame / aerospace compliance |
|---|---|---|---|---|
| BISCO BF-1000 extra soft | 12 | 3.46 | 1.12 | UL 94 V-0/HF-1; FMVSS 302 |
| BISCO BF-2000 ultra soft | 11 | 3.81 | 1.14 | UL 94 V-0 & HF-1; FMVSS 302; ASTM E162; ASTM E662 |
| BISCO HT-870 soft | 15 | 1.19 | 0.34 | UL 94 V-0; FMVSS 302, ASTM E162, ASTM E662 |
| BISCO HT-800 medium | 22 | 0.98 | 0.25 | UL 94 V-0; FMVSS 302; AMS3195 |
| BISCO HT-820 firm | 24 | 2.11 | 0.63 | UL 94 V-0; FMVSS 302; AMS3196 |
| BISCO HT-840 extra firm | 28 | 2.08 | 0.57 | UL 94 V-0; FMVSS 302 |
Open-Cell Polyurethane Foam
| Grade | Density lb/ft³ | Fogging Test | FMVSS 302 | UL flame |
|---|---|---|---|---|
| PORON 4701-40 soft | 15 / 20 / 30 | Pass | Pass | UL 94 HBF |
| PORON 4701-50 firm | 15 / 20 / 30 | Pass | Pass | UL 94 HBF |
| PORON 4701-60 very firm | 15 / 20 / 25 | Pass | Pass | UL 94 HBF |
Closed Cell Foams
| Material | Cell Structure | Temp Range | Primary Application |
|---|---|---|---|
| Vinyl nitrile (NBR/PVC/CR) | Closed | –40°F to 200°F | Automotive and aerospace gaskets; HVAC sealing; excellent oil and fuel resistance |
| Polyethylene foam | Closed | –40°F to 194°F | Cushioning, protective packaging, electronics |
| PVC foam* | Closed | –40°F to 200°F* | Gasketing, sealing, weatherstripping, and insulation; highly conformable; seals with as little as 30% compression; resistant to weather, fungi, and oxidation |
| EPDM foam | Closed | –70°F to 220°F | Outdoor sealing, HVAC, weatherstripping |
| Neoprene / EPDM / CR blended | Closed | –40°F to 200°F | HVAC gasketing, industrial sealing |
| Silicone sponge | Closed | –100°F to 500°F | High-temperature sealing, EV battery construction, electrical enclosures |
* PVC foam service temperature varies by grade: –15°F to 160°F (grades 2713, 2710) up to –40°F to 200°F (grades 111, 108/116).
Application Environment
| Environment | Recommended foam type | Basis |
|---|---|---|
| EV Battery | BISCO HT-800; BISCO HT-820 | HT-800 lowest TML (0.25%); HT-820 AMS3196; both pass all ASTM E595 thresholds |
| HVAC | Vinyl nitrile; EPDM; neoprene blended | Oil and fuel resistance; UV and weather resistance; FMVSS 302 compliant |
| Optical | BISCO HT-840 | Lowest CVCM (0.01%) in lineup — minimizes condensate risk on optical surfaces |
| Aerospace | BISCO HT-820; BISCO HT-800 | HT-820 AMS3196; HT-800 AMS3195 |
| Cleanroom | BISCO HT-800; PORON 4701-40 | HT-800 ASTM E595 confirmed; PORON ISO 6452 fogging confirmed |
Selecting by Application Environment
The application environment determines which foam properties govern the selection decision, and outgassing is only one variable in that evaluation.
In EV battery and energy storage assemblies, compression force deflection (CFD) values drive grade selection alongside outgassing data. Foam used for cell cushioning and module gasketing must compress reliably under assembly loads without permanent set, while also meeting enclosure sealing requirements. BISCO silicone sponge grades serve both functions and carry confirmed ASTM E595 data for the applications where outgassing verification is required.
HVAC enclosures present a different set of variables by location within the system. EPDM foam is the appropriate choice for outdoor seals and battery enclosures where UV, ozone, and moisture resistance are the primary requirements. Neoprene foam is common for indoor ducts and motor-adjacent areas where moderate oil and chemical resistance is needed. In motor compartments where temperatures exceed 180°F (82°C), urethane foam grade verification is required before specifying, as emission rates rise significantly above that threshold, and standard grade data may not reflect in-service conditions.
Optical modules and sealed electronics represent the most contamination-sensitive non-aerospace category. All four BISCO silicone foam grades exceed at least one standard low-outgassing threshold, which means the acceptable tolerance for TML and CVCM must be defined by the assembly's contamination sensitivity before a grade can be specified. CVCM is the value that determines condensate risk on optical surfaces; TML reflects total volatile output regardless of where those compounds migrate.
Aerospace and cleanroom applications carry the most stringent outgassing requirements, and post-cured silicone foam grades with the lowest available CVCM are the primary candidates. Across all markets, the common applications where foam outgassing risk is most concentrated include LCD gaskets, EV battery cushioning, PCB mounting, wire harness protection, motor housing seals, and vibration isolation pads, which are components that are enclosed, compressed, and thermally cycled throughout the service life of the product.
Converting Considerations
Material selection and converting format are decisions that interact with each other, and for assemblies with outgassing requirements, the interaction extends to every layer in the converted part.
Pressure-sensitive adhesive (PSA) systems frequently become overlooked volatile sources within converted constructions. Acrylic, silicone, and rubber-based adhesives each contain different carrier systems, additives, and residual processing compounds that may contribute to overall TML and CVCM values. The complete laminated construction should therefore be evaluated rather than reviewing only the foam substrate.
Silicone foam, urethane foam, and closed-cell elastomeric foams are available as die-cut parts, slit rolls, and sheets. Kiss-cut liner format supports peel-and-apply installation on assembly lines, eliminating handling steps and reducing the risk of contamination at the bonding surface. For thick or dense foam materials where die cutting would cause compression or distortion, waterjet cutting produces clean edges without applying heat or mechanical pressure to the material.
Laser cutting may provide tolerances approaching ±0.005 in (±0.127 mm) for some thin materials but can introduce localized thermal effects. Waterjet cutting avoids thermal input and may therefore be preferred when material heating could influence foam structure or volatile release characteristics.
Multi-layer lamination combines foam with adhesive and film layers into a single precision-converted component, which reduces part count and assembly time. In outgassing-sensitive assemblies, laminated constructions require a complete review because the volatile load from every layer in the construction contributes to the total outgassing profile of the finished part. An adhesive layer or film facing that has not been evaluated for TML and CVCM can introduce volatile compounds into a sealed enclosure even when the foam substrate has been verified. The outgassing review applies to the construction as a whole, not the foam alone.
Wide-web lamination up to 60 inches supports large-format parts and high-volume production runs. Complex lamination using multiple winders and servo-driven stations allows precise registration of dissimilar materials across the web, which is particularly relevant when combining foam carriers with thin film layers or adhesive transfer tapes where alignment tolerances are tight.
FAQs
What is the difference between TML and CVCM, and which matters more in a sealed assembly?
TML measures total mass lost as volatiles during the ASTM E595 test. CVCM measures what fraction of those volatiles condenses on a cooler surface. CVCM has the greater effect on sealed assembly performance because it represents compounds that physically deposit on optical surfaces, contacts, and sensors. Standard screening thresholds are TML below 1.00% and CVCM below 0.10%.
Why does silicone foam density not predict outgassing?
Outgassing is determined by polymer chemistry, cell structure, curing process, and additives not by how much material is present per unit volume. Density alone is not a reliable predictor.
What is the outgassing risk of open-cell polyurethane foam vs. closed-cell foam in a sealed enclosure?
Open-cell foam presents higher migration risk. Its interconnected cell structure allows volatiles to reach the enclosure environment more readily than closed-cell foam, where volatiles release more slowly from within sealed cells. Primary VOCs from polyurethane are formaldehyde and acetaldehyde, and emission rates rise significantly with temperature.
Does outgassing affect HVAC gasket and motor housing applications?
Yes. Volatile compounds in sealed motor housings can corrode contacts and degrade sensors. In motor compartments exceeding 180°F (82°C), urethane foam emission rates rise significantly, and standard grade data may not reflect in-service conditions. EPDM, neoprene, and vinyl nitrile are common HVAC gasket materials, but outgassing data for those grades should be confirmed before specifying in sealed enclosures.
Which foam is best for EV battery cushioning where volatile migration is a concern?
BISCO HT-800 has the lowest confirmed TML in the silicone foam lineup and carries AMS3195 compliance. BISCO HT-820 has the lowest confirmed CVCM and carries AMS3196 compliance. Both pass all ASTM E595 thresholds and are rated from –67°F to 392°F. Confirm CFD values against assembly load requirements before final specification.
Selecting the right foam for a sealed assembly starts with confirmed outgassing data and the right converting partner. Material selection should consider polymer chemistry, outgassing data, compression behavior, environmental exposure, and manufacturing format simultaneously. Engineered Materials supports design teams through material selection, prototyping, precision die-cutting, wide-web lamination, slitting, knife cutting, laser cutting, and waterjet converting to ensure the final converted construction performs as intended in production environments.