TML properties testing measures the Total Mass Loss and Collected Volatile Condensable Materials of a material exposed to a thermal-vacuum environment, providing essential qualification data for spacecraft and high-vacuum applications. Performed in accordance with ASTM E595 and ECSS-Q-ST-70-02, the test determines whether non-metallic materials meet the strict outgassing limits demanded by the aerospace industry. This article explains the principle, testing items, methodology, acceptance criteria, and typical applications of TML properties testing.
Technical Overview
TML properties testing is a thermal-vacuum outgassing evaluation widely used to qualify materials for satellites, spacecraft, and other high-vacuum systems. TML stands for Total Mass Loss, defined as the percentage of a specimen's mass that is lost through evaporation, sublimation, or outgassing when the material is heated in a vacuum. The test is most commonly performed per ASTM E595, which simultaneously measures CVCM (Collected Volatile Condensable Materials), the fraction of outgassed species that recondense on a cooler surface positioned near the specimen.
Why Outgassing Control Is Critical
In the vacuum environment of space, polymeric and non-metallic materials release low-molecular-weight volatiles such as plasticizers, residual solvents, curing by-products, oligomers, and absorbed moisture. These molecules can migrate through the spacecraft and redeposit onto sensitive surfaces, including optical lenses, mirrors, sensors, solar cells, and thermal radiators, causing contamination, transmission loss, and degraded performance. TML testing therefore serves as a standardized screening method to identify and exclude materials with excessive outgassing potential before flight hardware is built.
Governing Standards
- ASTM E595 – Standard Test Method for Total Mass Loss (TML) and Collected Volatile Condensable Materials (CVCM) of Outgassing in a Vacuum Environment
- ECSS-Q-ST-70-02 – Thermal vacuum outgassing test method for spacecraft materials (European Cooperation for Space Standardization)
- NASA SP-R-0022A – Vacuum stability requirements of polymeric materials for spacecraft applications
Testing Samples
Typical Material Categories
- Engineering plastics, elastomers, and polymer films
- Adhesives, sealants, potting compounds, and encapsulants
- Paints, conformal coatings, and thermal control coatings
- Pressure-sensitive tapes, film adhesives, and label stocks
- Lubricants, greases, and release agents
- Cable insulation, wire jacketing, and connector materials
- Composite laminates, printed circuit board substrates, and prepregs
- 3D-printed parts and additive manufacturing feedstock materials
Sample Requirements
ASTM E595 typically requires a specimen mass between 100 mg and 300 mg, presented in small copper sample boats with a defined exposed surface area. Specimens should be prepared in the same cured, post-processed, or as-delivered condition as the intended flight application, since incomplete curing dramatically increases outgassing. Duplicate or triplicate specimens are recommended to verify result repeatability.
Testing Items
- Total Mass Loss (TML): the total percentage of specimen mass lost during the 24-hour thermal-vacuum exposure, calculated from the initial and final microbalance readings.
- Collected Volatile Condensable Materials (CVCM): the percentage of outgassed matter that recondenses on a collector plate maintained at 25 °C, representing the contamination risk to nearby critical surfaces.
- Water Vapor Regained (WVR, optional): the percentage of mass regained after the post-test specimen is reconditioned in a humid atmosphere, used to determine how much of the TML was only water.
The universally applied acceptance limits are TML no greater than 1.0 percent and CVCM no greater than 0.10 percent. Materials exceeding these thresholds are generally rejected for unrestricted spacecraft use unless it can be demonstrated that the mass loss is attributable to water vapor or that the material is located far from contamination-sensitive surfaces.
Testing Methods
Sample Conditioning
Specimens are first conditioned in a controlled climate, typically 24 hours at 25 °C and 50 percent relative humidity as specified in ASTM E595, to stabilize their initial moisture content. Each specimen is then weighed on a high-precision microbalance with microgram-level resolution, and the collector plates are weighed and assembled into the test apparatus.
Thermal-Vacuum Exposure
The weighed specimens are loaded into small copper holders inside the outgassing chamber. The chamber is evacuated to a pressure of 5 × 10^-5 torr (approximately 6.7 × 10^-3 Pa) or lower, and the specimens are heated to 125 °C for 24 hours. Throughout the exposure, a temperature-controlled collector plate maintained at 25 °C faces the specimens, so that condensable volatiles leaving the samples are trapped and quantified.
Post-Test Measurement and Calculation
After the exposure cycle, the specimens and collector plates are re-weighed under identical conditions. The results are calculated as follows: TML (percent) equals the initial mass minus the final mass, divided by the initial mass, multiplied by 100. CVCM (percent) equals the mass gained by the collector, divided by the initial specimen mass, multiplied by 100. For WVR, the tested specimen is reconditioned at the original temperature and humidity for 24 hours and weighed once more, and the regained mass is expressed as a percentage of the initial mass.
Quality Assurance
Reliable TML testing requires calibrated microbalances, leak-tight vacuum systems with accurate temperature control, blank reference boats, and periodic verification with reference materials whose outgassing behavior is well documented. Test reports should clearly state the applied standard, conditioning history, individual and mean TML, CVCM, and WVR values, and a pass or fail conclusion against the 1.0 percent and 0.10 percent limits.
Applications
- Spacecraft and satellite material qualification and screening during design and procurement
- Contamination control for optical payloads, telescopes, star trackers, and scientific detectors
- Thermal control subsystems, including coatings, multilayer insulation, and radiator surfaces
- Avionics and electrical assemblies using adhesives, coatings, potting compounds, and cables
- Materials for semiconductor manufacturing equipment and ultra-high-vacuum systems
- Verification of supplier data against published NASA and ESA outgassing databases
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 mass that is lost when heated to 125 °C for 24 hours in a vacuum, reflecting the total quantity of volatile substances the material can release.
What are the pass or fail limits for TML testing?
According to ASTM E595-based acceptance criteria used by NASA, ESA, and most space agencies, a material passes when TML is 1.0 percent or less and CVCM is 0.10 percent or less. Both criteria must be satisfied for unrestricted use near sensitive surfaces.
Can a material with a TML above 1.0 percent still be used?
Yes, in certain cases. If the Water Vapor Regained measurement shows that most of the mass loss was moisture rather than organic volatiles, the material may still be acceptable. Alternatively, an extended vacuum bake-out before assembly can reduce outgassing and bring the material into compliance.
How much sample is needed and how long does the test take?
A specimen of roughly 100 mg to 300 mg is required, ideally tested in duplicate. The complete workflow, including conditioning, 24-hour thermal-vacuum exposure, reconditioning, weighing, and reporting, typically takes three to five working days.
Which industries require TML data?
Primarily the aerospace and satellite industries, and additionally semiconductor equipment manufacturers, optical instrument builders, and any developer of products that must operate in high vacuum without contaminating adjacent components.