TML Test Procedure: Complete Experimental Steps for Total Mass Loss Testing

TML (Total Mass Loss) testing is a critical outgassing evaluation used to screen materials for spacecraft, satellites, and high-vacuum applications before flight qualification. This article provides a detailed, step-by-step guide to the TML experimental procedure in accordance with ASTM E595, covering sample preparation, conditioning, vacuum exposure, weighing, and result calculation. By following these standardized steps, laboratories can generate reliable, repeatable outgassing data for material acceptance decisions.

Technical Overview

Total Mass Loss (TML) testing is a thermal-vacuum outgassing test designed to quantify the amount of mass a material loses when exposed to a simulated space environment. The test is performed in accordance with ASTM E595, the internationally recognized standard developed through NASA and ESA cooperation for screening non-metallic materials intended for spacecraft use.

During the test, a pre-weighed specimen is heated to 125°C for 24 hours inside a vacuum chamber maintained at a pressure of 5×10⁻⁵ torr or lower. Volatile substances evaporate from the sample, and a portion of these effluents condense on a collector plate held at 25°C, which is evaluated separately as Collected Volatile Condensable Materials (CVCM). The most widely applied acceptance criteria require TML ≤ 1.0% and CVCM ≤ 0.10%.

• Test standard: ASTM E595 (equivalent to ECSS-Q-ST-70-02 / ECSS Q-ST-70-02)
• Exposure temperature: 125°C ± 1°C for the test specimen
• Collector temperature: 25°C ± 1°C
• Exposure duration: 24 hours under high vacuum
• Chamber pressure: 5×10⁻⁵ torr or better
• Balances: microbalance with 10 microgram (or better) resolution

Testing Samples

Non-metallic materials used in spacecraft and vacuum systems are the primary candidates for TML evaluation. Specimens must be representative of the cured or fully processed state of the material as it will be used in the final application.

Eligible Material Types

• Thermoplastics, thermosets, and engineering polymers
• Elastomers, O-rings, and rubber compounds
• Adhesives, sealants, and potting or encapsulation compounds
• Paints, coatings, conformal coatings, and thermal-control finishes
• Films, tapes, laminates, and dielectric insulating materials
• Composite materials, circuit boards, and cable insulation

Specimen Preparation Requirements

• Nominal specimen mass: typically 100–300 mg per sample
• Specimens should be cut into small pieces to maximize outgassing surface area
• Contamination must be avoided; handle specimens with clean gloves and clean tools
• Materials must be fully cured or processed before testing, as incomplete cure artificially inflates TML values

Testing Items

A complete ASTM E595 outgassing evaluation typically reports three measured parameters:

• TML (Total Mass Loss): the percentage of specimen mass lost during the 24-hour thermal-vacuum exposure
• CVCM (Collected Volatile Condensable Materials): the mass of outgassed products that condense on the 25°C collector plate, expressed as a percentage of the original specimen mass
• WVR (Water Vapor Regained): an optional follow-on measurement of the mass of water reabsorbed by the specimen after exposure to a controlled humidity environment, used to distinguish moisture loss from true material degradation

Testing Methods

The TML experimental procedure follows a strict sequence of preparation, conditioning, exposure, and measurement steps. Each stage is essential to achieving accurate and reproducible results.

Step 1: Pre-Test Conditioning

All specimens and collector plates are stored in a controlled environment of 23°C ± 2°C and 50% ± 5% relative humidity for a minimum of 24 hours before initial weighing. This conditioning normalizes the moisture content of all test articles and establishes a consistent baseline.

Step 2: Initial Weighing

• Weigh each conditioned specimen on a calibrated microbalance and record the initial mass (W₁)
• Weigh each chromium-plated collector disc and record its initial mass (C₁)
• Handle all components with clean tweezers or gloves to prevent contamination

Step 3: Chamber Loading

Each specimen is placed into an individual copper sample cup within the test apparatus, and a pre-weighed collector plate is positioned directly above each cup. The geometry ensures that outgassed molecules must pass the collector, allowing condensables to deposit on the chromium-plated surface.

Step 4: Thermal-Vacuum Exposure

• Evacuate the test chamber to a pressure of 5×10⁻⁵ torr or lower
• Raise the sample compartment temperature to 125°C ± 1°C
• Maintain the collector plates at 25°C ± 1°C
• Hold these conditions continuously for 24 hours

Step 5: Cool-Down and Reconditioning

After the 24-hour exposure, the chamber is allowed to cool, vacuum is broken with dry nitrogen or clean air, and the specimens and collectors are returned to the controlled humidity environment (23°C, 50% RH) for reconditioning before final weighing.

Step 6: Final Weighing

• Weigh each specimen and record the final mass (W₂)
• Weigh each collector plate and record the final mass (C₂)
• For WVR determination, weigh the specimen after additional humidity conditioning and record the regained mass

Step 7: Calculation of Results

• TML (%) = (W₁ − W₂) / W₁ × 100
• CVCM (%) = (C₂ − C₁) / W₁ × 100
• WVR (%) = (mass after humidity exposure − W₂) / W₁ × 100

Results are compared against the acceptance thresholds of TML ≤ 1.0% and CVCM ≤ 0.10%. Materials exceeding these limits are generally rejected for unrestricted spacecraft use unless a documented waiver or requalification is approved.

Applications

TML testing supports contamination control and material qualification across multiple industries:

• Spacecraft and satellite material selection for NASA, ESA, and commercial space programs
• Optical payload qualification, where molecular contamination can degrade lenses, mirrors, and sensors
• Thermal control coatings and multi-layer insulation verification
• Adhesives and encapsulants used in avionics and space electronics
• High-vacuum laboratory equipment and semiconductor process tooling
• Supplier material datasheet verification and incoming inspection

FAQ

What are the pass/fail criteria for TML testing? The universally applied criteria from ASTM E595 and ECSS standards are TML ≤ 1.0% and CVCM ≤ 0.10%. Some programs impose stricter limits for contamination-sensitive applications such as optical instruments.

Why is the sample heated to 125°C? The elevated temperature accelerates outgassing so that a 24-hour test can approximate the total mass loss a material would experience over a much longer exposure in the space environment.

How much sample material is required? A nominal specimen mass of 100–300 mg is standard. Smaller samples reduce sensitivity of the mass-loss measurement, while the balance resolution must remain adequate relative to specimen size.

What is the difference between TML and CVCM? TML measures all mass lost by the specimen, including water and other species, while CVCM measures only the fraction of outgassed products that recondense on a surface at 25°C, which indicates the potential for molecular contamination of nearby hardware.

Why is WVR measured? Water Vapor Regained identifies how much of the measured TML was simply absorbed moisture. A material may exceed the TML limit due to water content yet still be acceptable if WVR demonstrates the excess loss was water rather than volatile organics.