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Outgassing and Contamination: Reading ASTM E595 TML and CVCM on a Spacecraft Material TDS
A sensor loses sensitivity in orbit, an optic hazes, and no one ever opened the enclosure. The culprit is often outgassing: ordinary materials, stable at the bench, slowly release volatiles in vacuum that travel until they condense on a cold surface like a lens or detector. The contamination is invisible at assembly and irreversible in flight. This article explains what ASTM E595 measures, why CVCM is the number that bites, how outgassing contaminates a spacecraft, and how to read the screening on a TDS.
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The lens fogs and nobody touched it: outgassing contamination explained
A sensor loses sensitivity in orbit, an optic develops a haze, a solar array dims faster than the models predicted, and no one ever opened the enclosure. The culprit is often outgassing: ordinary materials, perfectly stable at the bench, slowly release volatile compounds in vacuum, and those vapors travel until they find a cold surface to condense on. A lens, a detector, or a radiator is exactly that cold surface.
The contamination is invisible at assembly and irreversible in flight, which is why outgassing screening happens long before a material is allowed near a payload.
What ASTM E595 actually measures
ASTM E595 is the standard screening test for spacecraft materials. A sample is held at elevated temperature in vacuum, and the method reports how much of it leaves and how much of what leaves will redeposit. The two headline metrics are total mass loss (TML), the fraction of the sample's mass that volatilizes, and collected volatile condensable material (CVCM), the fraction that condenses onto a cooled collector plate.
Some data sets add water vapor regained (WVR), the portion of the mass loss that was simply absorbed water returning. The point of the screen is to flag materials whose volatiles would contaminate sensitive hardware before they are ever specified.
Why CVCM is the number that bites
TML and CVCM tell you different things, and confusing them is a common mistake. TML measures how much material left, which matters for dimensional stability and mass budgets. CVCM measures how much of what left will land and stay on a cold surface, which is the contamination that fogs an optic or coats a detector. A material can have a modest TML and still deposit enough condensable material to matter near optics, so for anything sharing a view or a cold sink with sensitive hardware, CVCM is the metric that governs.
Read both, but watch CVCM near anything optical or cold.
How outgassing contaminates a spacecraft
Cold surfaces are the trap
Volatiles migrate from warmer sources to colder sinks and condense where it is coldest. Radiators, cryogenic surfaces, and shaded optics are the natural collectors, so the contamination concentrates exactly where the spacecraft can least afford a film. A material far from the optic can still contaminate it if a line of sight and a thermal gradient connect them.
Optics and sensors are the victims
A condensed film a fraction of a wavelength thick is enough to scatter or absorb light, degrade a detector, or shift a thermal-control coating's properties. The hardware most sensitive to contamination is usually the hardware the mission depends on, which is why contamination control is a discipline of its own, and why material screening is its first line of defense.
Vacuum and heat make it worse
Vacuum removes the atmospheric pressure that would otherwise hold volatiles in, and heat accelerates their release, so the launch and on-orbit environment is precisely the condition that drives outgassing. A material that seems inert in air can be a steady vapor source once it is hot and in vacuum, which is the environment E595 deliberately recreates.
What Each Outgassing Number Tells You
| Metric | What it tells you | Why it matters |
|---|---|---|
| TML (total mass loss) | How much of the sample volatilized | Dimensional stability and mass budget |
| CVCM (condensable material) | How much will condense on a cold surface | Contamination of optics, sensors, radiators |
| WVR (water vapor regained) | How much of the loss was absorbed water | Distinguishes water from true volatiles |
| Bake-out and TDS lot data | How processing and lot affect the numbers | The actual values live on the maker's TDS |
Qualitative descriptions only; this article states no acceptance limits and quotes no test results. Confirm TML, CVCM, and any WVR figures on each grade's current technical data sheet.
The pattern: screen for CVCM near anything cold or optical
The selection rule follows the mechanism: wherever a material shares a vacuum, a line of sight, or a thermal path with optics, sensors, or cold surfaces, choose from grades whose TDS reports ASTM E595 screening and weigh CVCM heavily. Grades we convert that carry the screening include SSP2390 platinum-cured solid silicone, Low Outgassing Silicone Sheeting, PORON 4701-60 and PORON 4701-50, SOLIMIDE AC-550H polyimide foam, and the polyimide films used in MLI. The table in the next section lists the numbers as each maker publishes them. Where bake-out is part of your process, the screening still sets the starting point.
Name the proximity to sensitive hardware first, then let the CVCM line, not the thermal number, gate the shortlist.
Which grades we convert carry E595 data, and which do not
Two things decide where a rubber or foam lands on the E595 screen: what is in the compound, and how it was cured. Peroxide-cured silicones leave cure by-products behind that come off in vacuum unless the sheet is post-baked; platinum (addition) cure leaves none, which is why a platinum-cured solid silicone can measure an order of magnitude below the limits. Cellular materials carry the second penalty: more surface area and more residual blowing agent, so a silicone foam from the same family as a passing solid can fail outright. The numbers below are the maker's published values for the grades we convert. A blank means the maker has not published a result for that grade; it does not mean the grade passes.
| Grade | Form and hardness | TML | CVCM | Source |
|---|---|---|---|---|
| SSP2390-60D (also 50D) | Solid silicone, platinum cure, 59 Shore A (50D: 48) | 0.05% | 0.004% | Third-party ASTM E595-15 report on the 60D grade (Exova-OCM); WVR 0.04% |
| Low Outgassing Silicone Sheeting | Solid silicone, post-cured, 40 / 50 / 60 / 70 Shore A | ≤1.00% | ≤0.10% | Maker's TDS states the NASA-STD-6016 limits as compliance thresholds, not measured values; ships with a COA |
| SSP2575 (available on request) | Solid silicone, peroxide cure, 65 Shore A, low-temperature grade with satellite and launch-vehicle heritage | 0.17% | 0.04% | Maker's ASTM E595 report; WVR 0.10% |
| PORON 4701-60 Very Firm | Microcellular urethane, 240 / 320 / 400 kg/m³ (Shore A 30 / 42 / 53) | 0.6 / 0.7 / 0.7% | 0.05 / 0.02 / 0.03% | Rogers data sheet 17-018, ASTM E595-93; 90°C continuous, 121°C intermittent |
| PORON 4701-50 Firm | Microcellular urethane, 240 / 320 / 480 kg/m³ (Shore A 13 / 18 / 42) | 0.6 / 0.8 / not published | 0.04 / 0.05 / 0.06% | Rogers TDS, ASTM E595-93 |
| PORON AquaPro 37 | Microcellular urethane, 304 kg/m³, 6–16 psi CFD (soft; sealing, not structural) | 0.58% | 0.02% | Rogers TDS, ASTM E595; WVR 0.09% |
| SOLIMIDE AC-550H and PMD polyimide foam | Open-cell polyimide, 0.4–0.5 lb/ft³ (insulation, no compression rating) | <1.0% | <0.1% | Maker's TDS, ASTM E595 |
| BISCO HT-800 | Cellular silicone, 22 lb/ft³ | 0.98% | 0.25% | Rogers TDS; passes TML, exceeds the 0.10% CVCM limit |
| BISCO HT-820 / HT-840 | Cellular silicone, 24 / 28 lb/ft³ | 2.11 / 2.08% | 0.63 / 0.57% | Rogers TDS; fails both limits |
Everything else in our foam and rubber families, including neoprene, EPDM (solid and sponge), vinyl nitrile, PVC, polyethylene, EVA, fluorosilicone sponge, the general-purpose and FDA solid silicones, and the FKM sheet grades, has no published E595 result from the maker. Those grades are not candidates for a contamination-controlled assembly until someone screens them, and we will say so rather than send a sample. For a part that has to hold cylindrical cells, a battery module, or an instrument in place, the working shortlist is a 50 to 65 Shore A solid silicone from the top of the table where the part only needs to locate, or PORON 4701-60 at 320 or 400 kg/m³ where the part also needs to cushion; solid rubber at that hardness bulges rather than compresses, so a cushioning design needs somewhere for the material to go.
How H-O Products helps
H-O Products die-cuts, waterjet-cuts, laminates, and kits low-outgassing materials to your drawing in Winsted, Connecticut; we do not manufacture the films or foams in-house. We convert E595-screened solid silicone, PORON, polyimide-foam, and polyimide-film parts to pattern, kit MLI layers, and supply the maker's TDS with the outgassing data attached, all under our ISO 9001:2015 certified quality management system with lot-coded traceability. Tell us what sits near the part and we will steer toward grades that carry the screening.
Frequently asked questions
What is ASTM E595 outgassing screening?
It is the standard screening test for spacecraft materials. A sample is held at elevated temperature in vacuum, and the method reports total mass loss (TML), the fraction that volatilizes, and collected volatile condensable material (CVCM), the fraction that condenses on a cooled collector. Some data adds water vapor regained (WVR). The screen flags materials whose volatiles could contaminate sensitive hardware before they are specified; the values live on each grade's TDS.
What is the difference between TML and CVCM?
TML is how much material left the sample, which matters for dimensional stability and mass budgets. CVCM is how much of what left will condense and stay on a cold surface, which is the contamination that fogs optics and coats sensors. A material can have a modest TML and still deposit enough condensable material to matter near optics, so CVCM is the metric that governs near cold or optical hardware.
How does outgassing actually contaminate a spacecraft?
Volatiles migrate from warm sources to cold sinks and condense where it is coldest, so radiators, cryogenic surfaces, and shaded optics become collectors. A condensed film a fraction of a wavelength thick can scatter light, degrade a detector, or shift a coating's thermal properties. Vacuum and heat accelerate the release, which is exactly the on-orbit environment E595 recreates.
Which materials carry E595 data?
Among the grades we convert, published ASTM E595 results exist for SSP2390 platinum-cured solid silicone (TML 0.05%, CVCM 0.004% on the 60 Shore A grade), Low Outgassing Silicone Sheeting (TDS states TML ≤1.00%, CVCM ≤0.10%), PORON 4701-60 and 4701-50 microcellular urethane (TML 0.6–0.8%, CVCM 0.02–0.06%), PORON AquaPro 37, SOLIMIDE AC-550H and PMD polyimide foams, and the polyimide films used in MLI. BISCO HT-800 passes TML but exceeds the CVCM limit, and HT-820 and HT-840 fail both. Neoprene, EPDM, PVC, polyethylene, and the general-purpose solid silicones carry no published result. The screening data belongs to the maker's TDS for a specific grade, not to the converted part.
Does H-O run the outgassing test or certify contamination control?
No. ASTM E595 results live on the maker's TDS; H-O converts screened materials to your drawing and carries that documentation on the order. We perform no outgassing testing and make no contamination-control or flight-certification determination. The contamination-control plan and any required testing belong to your program; what we provide is the accurate converted part and its screening documentation.
Have an outgassing-sensitive part reviewed
Tell us what the part sits near (optics, sensors, cold surfaces), the temperature and vacuum exposure, and the geometry, and we will steer toward E595-screened grades and quote converted parts to your drawing.
Samples typically ship in 3 to 5 business days for common die-cut configurations on materials we keep on hand; standard production runs about two weeks after drawing approval; expedited service is available. MOQ varies by material and part.
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Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References
- ASTM E595 (total mass loss and collected volatile condensable material from outgassing in a vacuum environment), cited qualitatively; no acceptance limits or results are stated here.
- ASTM D5470 (thermal impedance); ASTM D149 (dielectric strength). Cited by designation; H-O performs no outgassing or flight-qualification testing.
- Manufacturer technical data sheets: PORON grades reporting E595 screening, ePTFE sheet and membrane vent, Kapton and Apical polyimide film, Cryogel Z and Pyrogel XTE aerogel, SOLIMIDE polyimide foam.
This article summarizes material-selection principles for spacecraft and launch-vehicle hardware using the families and method designations cited on the parent application page. Comparisons are qualitative; thermal, dielectric, and outgassing values are per each grade's current technical data sheet, and no test results are quoted.
ASTM E595 and other designations are named for orientation only; H-O performs no outgassing or qualification testing and makes no flight-certification determinations. All parts are custom manufactured to order; minimum production run quantities vary by material and part. H-O is a converter, not a raw-material producer.