TML (Total Mass Loss) testing is the industry-standard outgassing screening method used to qualify non-metallic materials for spacecraft, satellites, and other high-vacuum applications. Following recognized procedures such as ASTM E595 and ECSS-Q-ST-70-02, TML measurement methods quantify the percentage of mass a specimen loses when heated to 125 °C under high vacuum for 24 hours. This guide explains the technical principle, applicable sample types, test items, step-by-step procedures, calculation formulas, and typical applications of TML testing.
Technical Overview
TML (Total Mass Loss) is defined as the percentage of its initial mass that a material loses when it is exposed to a specified thermal-vacuum environment. In the widely adopted test protocol based on ASTM E595 and ECSS-Q-ST-70-02, specimens are conditioned at room temperature, weighed, heated at 125 °C for 24 hours in a high-vacuum chamber, and then weighed again. The relative mass difference is reported as TML.
The purpose of TML testing is contamination control. In the vacuum of space, polymeric and organic materials release low-molecular-weight volatiles through outgassing. These volatiles can recondense on cold or sensitive surfaces such as optical lenses, mirrors, sensors, solar cells, and thermal control coatings, degrading their performance. TML testing therefore serves as a rapid and reproducible screening method to identify materials that are safe for spacecraft use before flight qualification.
TML testing is normally performed together with CVCM (Collected Volatile Condensable Materials) and WVR (Water Vapor Regained) measurements, which together describe the complete outgassing behavior of a material. Typical acceptance limits used by most space agencies are:
- TML ≤ 1.0 % for general spacecraft materials
- CVCM ≤ 0.10 % to protect optical and thermal surfaces from condensed contamination
- WVR is reported for information and is used to correct TML when moisture loss dominates
Testing Samples
TML testing applies to virtually any non-metallic material intended for use in a space or high-vacuum environment. The most commonly tested sample categories are listed below.
Polymeric and Thermoplastic Materials
Engineering plastics such as PEEK, PTFE, polyimide, polycarbonate, POM, and nylon are tested because processing residues, plasticizers, and absorbed moisture can outgas under vacuum thermal cycling.
Adhesives, Sealants and Elastomers
Epoxy adhesives, silicone elastomers, acrylic adhesives, potting compounds, and O-ring materials frequently contain volatile siloxanes, solvents, and cure by-products, making them a primary target for TML and CVCM screening.
Coatings, Paints and Inks
Thermal control coatings, black anodizing sealers, structural paints, conformal coatings, marking inks, and metallized finishes are tested to ensure that residual solvents and organic binders do not contaminate adjacent hardware.
Films, Tapes and Insulation Materials
Polyimide films, multilayer insulation (MLI) blanks, adhesive transfer tapes, cable wraps, and electrical insulation tapes are tested as-received, since film products often carry residual low-molecular-weight species from manufacturing.
Electronic and Encapsulation Materials
Conformal coatings for printed circuit boards, encapsulants, cable insulations, connector housings, potting resins, and increasingly 3D-printed polymer parts are submitted for TML testing as part of spacecraft electronic assembly qualification.
Testing Items
A complete TML test campaign normally reports the following parameters, all expressed as a percentage of the initial conditioned specimen mass:
- TML (Total Mass Loss): the total percentage of mass lost by the specimen during the 125 °C vacuum exposure, calculated as TML = (m1 − m2) / m1 × 100 %, where m1 is the conditioned initial mass and m2 is the mass after vacuum exposure.
- CVCM (Collected Volatile Condensable Materials): the percentage of volatile material that recondenses on a collector plate held at 25 °C, calculated as CVCM = (m4 − m3) / m1 × 100 %, where m3 and m4 are the collector masses before and after the test.
- WVR (Water Vapor Regained): the percentage of mass regained by the specimen after storage for 24 hours at 23 °C and 50 % relative humidity following the vacuum exposure, calculated as WVR = (m5 − m2) / m1 × 100 %.
- RML (Recovered Mass Loss): the difference between TML and WVR, representing the permanent, non-moisture-related mass loss, which is the parameter of greatest concern for long-duration missions.
Testing Methods
The TML measurement method consists of four stages: specimen preparation, pre-conditioning and initial weighing, thermal-vacuum exposure, and post-exposure measurement. The complete sequence is described below.
Specimen Preparation
For each material, multiple specimens (typically three) are prepared so that results can be averaged and outliers identified. Specimens are cut, poured, or deposited into standardized sample cups. The specimen mass and geometry are controlled within the limits defined by the applied standard, since excessive thickness can artificially suppress measured outgassing by limiting diffusion.
Pre-Conditioning and Initial Weighing
Specimens and collector plates are conditioned in a controlled environment, typically 23 °C and 50 % relative humidity for 24 hours, to stabilize their moisture content. All mass measurements are then performed on a high-precision microbalance with a resolution on the order of 10 micrograms, operated inside a temperature- and humidity-controlled enclosure. The conditioned specimen mass m1 and the initial collector mass m3 are recorded.
Thermal-Vacuum Exposure
The specimens, in their cups, are loaded into the outgassing test chamber, with a chromium- or gold-plated collector plate maintained at 25 °C positioned at a fixed distance directly above each specimen. The chamber is evacuated to a high vacuum level on the order of 10⁻³ Pa or better, and the specimens are heated to 125 °C for 24 hours. Volatiles leaving the specimens either are pumped away or condense on the cooled collector surfaces, exactly as they would on nearby cold hardware in orbit.
Post-Exposure Measurement and TML Calculation
After 24 hours, heating is stopped, the chamber is backfilled with dry nitrogen, and specimens are removed and reweighed as quickly as possible to minimize moisture uptake during handling. The post-exposure mass m2 is recorded, and TML is calculated as (m1 − m2) / m1 × 100 %. The result is compared against the applicable acceptance limit, normally 1.0 %.
CVCM and WVR Determination
The collector plates are reweighed to obtain m4, from which CVCM is calculated and compared against the 0.10 % limit. For WVR, the tested specimens are stored for 24 hours at 23 °C and 50 % relative humidity and weighed again to obtain m5. Reporting TML, CVCM, and WVR together allows engineers to distinguish true material decomposition from simple moisture desorption.
Applications
TML testing methods are applied wherever outgassing contamination could compromise the performance or lifetime of hardware operating in vacuum. Key application areas include:
- Spacecraft and satellite material selection and qualification before flight hardware integration
- Optical payloads, telescopes, lenses, mirrors, and sensors where molecular contamination directly reduces throughput and image quality
- Thermal control surfaces, radiators, optical solar reflectors, and MLI blankets whose emissivity and absorptivity are degraded by condensable films
- Solar array substrates, cable harnesses, connectors, and potting compounds for spacecraft electronics
- Manned spacecraft and space station interior materials, where outgassed compounds can affect crew health and cabin optics
- High-vacuum laboratory and semiconductor process equipment, vacuum furnaces, and particle accelerator components
- Verification testing for material suppliers, research institutions, and quality acceptance in launch vehicle and CubeSat programs
FAQ
What is the difference between TML and CVCM?
TML measures the total mass a specimen loses during vacuum heating, while CVCM measures only the fraction of that outgassed material that recondenses on a surface held at 25 °C. CVCM is often the more critical parameter for optical and thermal hardware, because condensed contaminants cause the actual performance degradation.
Why is the test performed at 125 °C for 24 hours?
The 125 °C / 24-hour condition is an accelerated screening point defined by ASTM E595 and adopted by ECSS and NASA standards. It is severe enough to drive off volatiles from materials used near moderately warm spacecraft equipment, while remaining short enough for efficient material qualification screening.
Can a material with high TML still be acceptable?
Yes. If most of the mass loss is water, the material may be acceptable. The WVR measurement identifies how much of the lost mass is reabsorbed moisture, and the corrected value RML (TML minus WVR) is then evaluated against the acceptance criterion.
How many specimens are required for a TML test?
Most standards require testing multiple specimens per material, typically three, so that average values can be reported and anomalous results caused by specimen preparation defects can be detected and investigated.
What balance accuracy is needed?
Because CVCM acceptance limits are as low as 0.10 % of specimen mass, mass measurements must be made with a microbalance offering resolution on the order of 10 micrograms, used within a controlled temperature and humidity environment to guarantee reproducibility.
Which standards govern TML testing methods?
The principal standards are ASTM E595, the European ECSS-Q-ST-70-02, and NASA SP-R-0022A, together with equivalent national aerospace standards. These documents specify the test temperature, vacuum level, exposure duration, collector temperature, and reporting format.