RTV Silicone Outgassing: E595 Screen vs E1559 Rates

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.”

If you're buying a custom RTV mold, unmix three jobs: shop degas of mixed rubber before a pour; a mold used to cast a potting or fixture; a fixture or gasket that lives inside the chamber. Tin-cure keeps outgassing. Platinum RTV can be a candidate after air post-cure or vacuum bake. This is not optical LSR lenses (different bake) and never a LIM hot tool.

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 exist in specialty catalogs. They are not craft 1:1 platinum.

E1559 rates: isothermal outgassing with QCMs at more than one collector temperature. Optics and long missions use this when a 24 h mass-loss percentage cannot tell you whether remaining flux will still coat a 25 °C mirror over months. NASA-STD-6016 and ECSS-Q-ST-70-02 may demand E595 plus 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. Fogging of optics tracks condensables.

Common mix-up: mold TDS versus chamber article

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. A platinum RTV glove in the resin room is a different building from a platinum gasket sitting next to a cooled detector.

What to ask when you're buying

• Is the RTV a pour mold, a chamber fixture/gasket, or the potting that flies?
• E595 on a specimen that matches thickness and bake of the article?
• Optics-adjacent: E1559 or a witness-plate protocol?
• Platinum only? Tin excluded?
• Who keeps the master if we recast a baked skin that tore?

Worked example: 2-cavity potting mold for a flight brick

A 2-cavity platinum 20A glove, jacketed, to cast a low-outgassing potting. The mold was standard platinum RTV, vacuum degassed, not E595-certified. The potting compound was a named low-volatile addition grade, air post-cured and vacuum-baked as the program required, E595 on that thickness. Filing the mold TDS as MAPTIS data would have been the mix-up. Tooling charge on the jacket did not include a space-materials listing.

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. 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.

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. Diffusion through 15 mm of rubber is slower than through a 1 mm plaque. Levers after you picked platinum: low-cyclic formulation, oven post-cure in moving air, vacuum bake-out of the article, avoid recent solvent wipes. Do not confuse a helium leak with outgassing.

Lab chambers: platinum RTV only, baked, CVCM mindset even if you never pay for E595. Prefer mechanical clamps over rubber when you can. Aerospace: MAPTIS or ESA lookup, then program-specific TVAC. E595 pass is the door, not the living room. Near optics, CVCM and E1559 dominate.

When this article is the wrong quote

Chocolate and gummy molds don't need E595. Production LSR optics are a parts quote. A tiny shallow tray in air can skip vacuum-bake folklore. Geometry can override: a thick dam next to a detector is an E1559 conversation even at one piece.

Related reading: custom RTV molds, how we make the mold, platinum-cured silicone, what tooling means.

What to send for an RTV mold quote

• Object: pour mold vs chamber fixture vs flight potting
• Platinum-cure RTV, low-volatile grade if the article lives in vacuum; no tin-cure in chamber
• 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) on that article
• Optics-adjacent: ASTM E1559 or witness-plate protocol
• Master, split, jacket, recast

Send whether the rubber stays in the chamber or only casts the potting. We'll refuse “NASA approved silicone” without a method and a specimen.

We would not recommend E595 on a 1 mm plaque standing in for a 25 mm dam

We would not recommend tin-cure inside a vacuum system you care about. We would not bake craft platinum hoping it becomes MAPTIS-listed. We would not raise Shore A first to “outgas less.” We would not file the mold TDS as the potting's flight paper.

RUUIPON Mold Structure Decision: if this is a pour mold, same split rules. Flat back? open-back. Cannot peel? two-part / core. Large wet potting? mother mold. If the rubber lives in vacuum, chemistry and bake override Shore slogans.

Typical shop values: degas vs outgassing

| Item | Typical shop value | | --- | --- | | Shop degas | About −0.095 MPa until foam collapses | | E595 screen | 24 h at 125 °C; TML ≤ 1.0 %, CVCM ≤ 0.10 % historical NASA/ESA door | | Chamber chemistry | Platinum addition RTV, baked; tin keeps outgassing | | Mix / shrink | 1:1 or 10:1; platinum ~0.1–0.3% | | Cavity Shore (molds) | 10A–20A with jacket | | Thickness | Test article should match as-used section |

Reference values only. Confirm material-specific limits against the supplier’s technical data and agree project acceptance criteria before production.

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. Tin copy molds in the resin room are a different building.

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.