TML (Total Mass Loss) testing is a standardized thermal-vacuum outgassing test that quantifies the mass of volatile compounds released by a material when heated under high vacuum, in accordance with ASTM E595 and ECSS-Q-ST-70-02. It is a mandatory qualification requirement for materials used on spacecraft, satellites, optical payloads, and other vacuum-sensitive systems, where molecular contamination can severely degrade performance. This article provides a comprehensive technical overview of TML testing, including sample types, measured parameters, test methods, applications, and frequently asked questions.
Technical Overview
The TML (Total Mass Loss) test is a gravimetric thermal-vacuum outgassing evaluation designed to measure the total quantity of volatile matter released by a material when it is exposed to elevated temperature under high vacuum. During the test, a preconditioned specimen is held at 125 °C for 24 hours in a vacuum environment while escaping molecules condense on a collector plate maintained at 25 °C, allowing simultaneous determination of total mass loss and condensable contamination.
TML testing forms the core of internationally recognized outgassing standards, principally ASTM E595 (developed in cooperation with NASA) and the European standard ECSS-Q-ST-70-02. Space agencies and prime contractors typically require that flight materials meet the following acceptance criteria:
- TML (Total Mass Loss) not exceeding 1.0 % of the initial specimen mass
- CVCM (Collected Volatile Condensable Materials) not exceeding 0.10 % of the initial specimen mass
- Additional project-specific limits on water vapor regained (WVR) where condensation on cold surfaces is a concern
Materials that fail these thresholds are considered unacceptable for spacecraft use because their outgassed molecules can recondense on optical surfaces, solar cells, thermal control coatings, and sensitive detectors, causing measurable performance degradation over the mission lifetime.
Testing Samples
Practically every non-metallic material used in spacecraft, launch vehicles, and high-vacuum equipment is a candidate for TML testing. Specimens are typically prepared in small quantities of approximately 100–300 mg to ensure complete outgassing within the 24-hour exposure window. Common sample categories include:
Polymeric and Composite Materials
Engineering plastics, structural composites, CFRP laminates, printed circuit board substrates, and 3D-printed polymer parts. These materials often contain residual monomers, plasticizers, or absorbed moisture that outgas under vacuum.
Adhesives, Sealants, and Potting Compounds
Epoxies, silicones, polyurethanes, acrylics, and encapsulation gels used for bonding, sealing, and electronic assembly. Incomplete cure or low-molecular-weight additives are frequent sources of excessive mass loss.
Coatings, Paints, and Surface Treatments
Thermal control paints, black anodizing seals, conformal coatings, optical black coatings, and metallized layer systems. Outgassed species from coatings are a leading cause of molecular contamination on adjacent optics.
Films, Tapes, and Insulation Materials
Kapton and other polyimide films, PTFE tapes, multilayer insulation (MLI) components, cable jackets, wire insulation, and pressure-sensitive adhesive tapes used throughout the spacecraft harness and thermal enclosure.
Lubricants, Greases, and Functional Fluids
Testing Items
A complete outgassing evaluation according to ASTM E595 / ECSS-Q-ST-70-02 reports the following measured parameters:
- TML (Total Mass Loss): the percentage of specimen mass lost after 24 hours at 125 °C in vacuum, calculated from pre-test and post-test weighings
- CVCM (Collected Volatile Condensable Materials): the mass of outgassed molecules that recondense on the 25 °C collector plate, expressed as a percentage of the initial specimen mass
- WVR (Water Vapor Regained): the mass of water reabsorbed by the specimen after reconditioning for 24 hours at 23 °C and 50 % relative humidity following the vacuum exposure
- RML (Recovered Mass Loss): the difference between TML and WVR, representing the permanent, non-moisture-related mass loss of the material
Reporting all four parameters allows engineers to distinguish genuine material decomposition from simple moisture desorption, which is essential for accurate contamination risk assessment.
Testing Methods
TML testing follows a tightly controlled gravimetric procedure to ensure reproducibility between laboratories worldwide. The general workflow is summarized below.
Sample Preparation and Conditioning
Specimens are cleaned, cut or portioned to the required mass range, and conditioned for 24 hours in a controlled environment of 23 °C and 50 % relative humidity. Each specimen and its designated collector chip are then pre-weighed on a microbalance with a resolution of one microgram or better.
Thermal-Vacuum Exposure
The weighed specimens are loaded into a TML test chamber and heated to 125 °C ± 1 °C for 24 hours under high vacuum, at a pressure below approximately 10⁻³ Pa (roughly 10⁻⁵ torr). A chromium-plated or equivalent collector plate, held at 25 °C ± 1 °C and positioned a fixed short distance from each specimen, captures condensable outgassed molecules during the exposure.
Post-Test Gravimetric Analysis
After the vacuum exposure, specimens are re-weighed to determine TML, and collector plates are re-weighed to determine CVCM. Specimens are then reconditioned at 23 °C / 50 % RH for a further 24 hours and weighed once more to determine WVR and RML. Duplicate or triplicate specimens are tested for each material to confirm consistency of results.
Applications
TML test data directly support material selection, qualification, and flight acceptance across a wide range of industries, including:
- Spacecraft and satellite material qualification for NASA, ESA, and commercial space programs
- Optical payload verification, including telescopes, imaging sensors, lenses, mirrors, and cryogenic instruments
- Thermal control system design, ensuring radiators, MLI blankets, and coatings remain contamination-free
- Solar array and photovoltaic component screening to prevent power degradation from deposited films
- High-vacuum industrial equipment, semiconductor fabrication tools, and particle accelerator components
- Launch vehicle and missile program material screening, where rapid thermal-vacuum transitions occur
By demonstrating compliance with TML and CVCM limits, manufacturers can trace materials to established outgassing databases such as the NASA Outgassing Database, dramatically reducing qualification cost and schedule risk.
FAQ
What does TML stand for and what does it measure?
TML stands for Total Mass Loss. It measures the percentage of a material's initial mass that is lost when the material is held at 125 °C for 24 hours in high vacuum, and it is the primary indicator of a material's outgassing propensity.
What are the standard acceptance criteria for TML testing?
The widely accepted criteria are TML ≤ 1.0 % and CVCM ≤ 0.10 %, as defined in ASTM E595 and adopted by most space agencies. Some missions with contamination-critical optics impose stricter limits, such as CVCM ≤ 0.05 % or lower.
How is TML different from CVCM?
TML quantifies the total mass of all volatile species leaving the material, including water vapor, whereas CVCM quantifies only the fraction of those species that actually recondense on a surface held at 25 °C. CVCM is often the more contamination-relevant parameter for optics and thermal surfaces.
Why is WVR measured in addition to TML?
A significant portion of TML for many polymers is simply absorbed water. WVR identifies how much of the lost mass was reabsorbable moisture, allowing calculation of RML (Recovered Mass Loss), which reflects the true permanent degradation of the material.
How long does a TML test take?
The vacuum exposure itself lasts 24 hours, but including sample conditioning, pre-weighing, post-test weighing, and reconditioning, a complete test typically requires three to five working days. Additional time may be needed for materials requiring special preparation or full material qualification campaigns.
Can a material with high TML still be used in space?
In some cases yes, if the high TML is dominated by water vapor (high WVR) and the CVCM remains very low. Acceptance decisions also depend on the material's location relative to sensitive surfaces, thermal environment, and the availability of bake-out procedures to reduce outgassing before flight.