Outgassing Rates of RTV Silicones in Vacuum Chambers and Aerospace Use

A vacuum chamber does not care that your RTV “passed NASA low outgassing” on a brochure. It cares what still leaves the rubber this week, at this temperature, onto that cold optic. ASTM E595 is a 24 h, 125 °C, high-vacuum screen of total mass loss (TML) and collected volatile condensable material (CVCM). ASTM E1559 is a rate method with quartz crystal microbalances. They answer different questions. Mixing them is how a fixture elastomer fogs a witness plate after it “passed E595.”

This article is poured RTV used as gaskets, fixtures, dams, and mold rubber inside chambers and on spacecraft-adjacent hardware. It is not LSR optical lenses (different bake) and not the shop step of degassing mixed RTV before a pour. Unmix tin-cure and platinum-cure. Unmix a 2 mm plaque from a 20 mm fixture.

Two vacuums, three numbers

Shop degas (about −0.095 MPa, minutes) pulls air from mixed RTV so the pour is not foamy. It does not strip cyclics to space-grade.

E595 screen: 125 °C, vacuum on the order of 5 × 10⁻⁵ torr (the method’s < 7 × 10⁻³ Pa language), 24 h. Historical NASA/ESA screening levels: TML ≤ 1.0 %, CVCM ≤ 0.10 %. Water vapor regained (WVR) can be reported so you do not punish a wet specimen. Passing means the tested specimen, with its cure and bake, is a low-outgassing *candidate*. Ultra-low lines (TML and CVCM an order of magnitude tighter) exist in specialty catalogs. They are not craft 1:1 platinum.

E1559 rates: isothermal outgassing with QCMs at more than one collector temperature, often paired with mass spectrometry. Optics and long missions use this when a 24 h mass-loss percentage cannot tell you whether the remaining flux will still coat a 25 °C mirror over months. NASA and ESA programs (NASA-STD-6016 thermal-vacuum stability language; ECSS-Q-ST-70-02 on the ESA side) may demand E595 plus engineering judgment, or E1559, or both.

TML is mostly water plus non-condensables plus condensables. CVCM is the fraction that stuck to a 25 °C collector in E595. A material can pass TML and fail CVCM if the volatiles like to condense. That is the fog.

Tin RTV is a poor vacuum citizen

Condensation-cure RTV keeps making small molecules after demold. In a chamber those molecules are a source term with no off switch. The tool also shrinks (dimensional clock). Do not use tin-cure as a fixture, gasket, or mold inside a vacuum system you care about. A tin copy mold in the resin room is a different building.

Platinum RTV can be formulated and post-cured / vacuum-baked down to an E595 pass. Industrial filled platinum with extending fluids can still fail CVCM. Optical and space-grade addition RTVs are stripped of cyclics on purpose. Buy that grade; do not bake a hardware-store translucent hoping it becomes MAPTIS-listed.

Peroxide HCR leftovers are another condensable family. If the chamber article is millable rubber, post-cure it as if food-contact volatiles mattered, then test. LSR fixtures are heat-cured; they still need a volatile spec if they live in vacuum.

Inhibition tack is wet chemistry. A sticky platinum face outgasses and contaminates. Scrap it.

Rates depend on thickness, bake, and temperature

E595 does not predict years at 40 °C. The method itself says 24 h at 125 °C is a comparison, not a life model. Diffusion through 15 mm of rubber is slower than through a 1 mm plaque; a thick fixture can pass a thin-specimen screen in the lab and still bleed in the chamber for weeks. Conversely, a thin gasket bakes out faster.

Levers that change the rate after you already picked platinum:

• Formulation: low-cyclic, no extending fluid, addition-cure
• Oven post-cure in moving air (drives off cyclics that can leave)
• Vacuum bake-out of the article at a temperature the hardware allows, for hours to days, until a witness QCM or a residual-gas scan is quiet enough
• Avoid recent solvent wipes; you will measure alcohol, not silicone

If service temperature exceeds ~65-70 °C, E595’s 125 °C assumption may be too close; the method discusses raising test T. Hot stages next to a mold rubber are an E1559 conversation.

Do not confuse a helium leak with outgassing. Leak detectors stay quiet while a cold plate fogs. That is CVCM behavior.

Chamber practice versus flight hardware

Lab vacuum chamber (optics, deposition, SEM fixtures): platinum RTV only, baked, with a CVCM mindset even if you never pay for E595. Prefer mechanical clamps over rubber when you can. If you need a gasket, buy a listed low-outgassing grade and bake the lot.

Aerospace / spacecraft: materials often go through MAPTIS or an ESA database lookup, then a program-specific thermal-vacuum test. E595 pass is the door, not the living room. Near optics, solar arrays, or thermal control surfaces, CVCM and E1559 dominate. Ultra-low grades exist because “less than 0.1 % CVCM” is still too much next to a cooled detector.

Mold rubber used to cast a spacecraft potting is a process aid. The potting’s own E595 file is the flight paper. Do not file the mold TDS.

RFQ that names the method

“Platinum-cure RTV (addition), low-volatile grade, no tin-cure. Specimen: production cure plus [air post-cure / vacuum bake] at [T / t], thickness [as-used]. Screen: ASTM E595 TML ≤ 1.0 % and CVCM ≤ 0.10 % (or program-tighter). If optics-adjacent: ASTM E1559 rate data or a chamber witness-plate protocol at [T]. MAPTIS / ECSS data if claimed, with specimen ID. Thickness and bake of the flown or chamber article shall match the test article as far as practical.”

Refuse E595 on a different hardness, a different bake, or a 1 mm plaque standing in for a 25 mm dam. Refuse “NASA approved silicone” without a method and a specimen.

FAQ

If my platinum RTV passed ASTM E595, will it stop fogging the chamber window?

Not necessarily. E595 is a 24 h mass screen at 125 °C on a specimen. Thick fixtures, cooler service, and remaining cyclic flux can still condense on a cold optic. Optics jobs often add a vacuum bake and sometimes ASTM E1559 rates.

Why is tin-cure RTV discouraged in vacuum and aerospace fixtures?

Condensation chemistry continues to release small molecules and the rubber keeps shrinking. That is a long-lived source term. Platinum addition RTV, baked, is the candidate chemistry.

What is the difference between TML and CVCM?

TML is total mass lost in the E595 exposure. CVCM is the portion that condensed on a 25 °C collector. Fogging of optics tracks condensables. A pass on TML with a high CVCM is still a contamination material.

Is NASA low outgassing the same as ECSS-Q-ST-70-02?

They are related screening cultures (TML/CVCM-class limits; ESA also discusses recovered mass loss). They are not the same document. Cite the program’s standard and the specimen bake.

Does degassing mixed RTV in a shop chamber meet aerospace outgassing?

No. Shop degas removes air from a pour. Aerospace outgassing is leftover siloxanes and other volatiles under high vacuum and heat. That takes formulation plus post-cure or vacuum bake, then E595/E1559 on that article.