TML Flow Disc Testing Method: Total Mass Loss and Outgassing Evaluation for Space Materials

The TML flow disc testing method is a standardized outgassing screening technique, widely performed in accordance with ASTM E595, that quantifies the Total Mass Loss (TML) and Collected Volatile Condensable Materials (CVCM) of spacecraft materials. By heating specimens at 125°C under high vacuum and condensing the effluents onto a temperature-controlled collector disc, the method determines whether a material meets the strict contamination-control acceptance criteria of the aerospace industry. This article explains the principle, applicable samples, testing items, procedures, and applications of the TML flow disc test.

Technical Overview

The TML flow disc testing method is a thermal-vacuum outgassing test used to evaluate the volatility of materials intended for use in spacecraft and other high-vacuum environments. In this method, a pre-weighed specimen is placed inside a small effluent cup and heated to 125°C for 24 hours under a vacuum of less than 5 × 10⁻⁵ torr (approximately 6.7 × 10⁻³ Pa). Volatile substances escaping from the specimen flow outward and condense on a chromium-plated collector disc maintained at 25°C, positioned at a fixed distance above the sample cup.

After the exposure, both the specimen and the collector disc are re-weighed with a microbalance. The percentage of mass lost by the specimen is reported as Total Mass Loss (TML), while the percentage of condensable material deposited on the collector disc is reported as Collected Volatile Condensable Materials (CVCM). A third parameter, Water Vapor Regained (WVR), can be determined by re-conditioning the post-test specimen in a humid environment. The test is primarily governed by ASTM E595, with equivalent or closely related requirements specified in ECSS-Q-ST-70-02 and NASA SP-R-0022A.

The general acceptance criteria adopted by most space agencies require TML ≤ 1.0% and CVCM ≤ 0.10%. Materials exceeding these limits are considered to pose a contamination risk to optical surfaces, thermal control coatings, sensors, and other sensitive spacecraft components.

Testing Samples

Polymeric and Composite Materials

Engineering plastics, potting compounds, fiber-reinforced composites, laminates, and 3D-printed polymer parts are frequently screened because polymers typically contain plasticizers, residual monomers, and curing by-products that outgas under vacuum and elevated temperature.

Adhesives, Coatings, and Encapsulants

Structural adhesives, conformal coatings, thermal control paints, black anodizing sealants, silicone and epoxy encapsulants, and potting resins are tested after full curing to verify that volatile content and condensable fraction remain within specification.

Films, Tapes, and Insulation Materials

Polyimide films, multilayer insulation (MLI) blankets, kapton tapes, cable insulations, and foam materials are common test subjects, since they are used in large surface areas aboard spacecraft and can release significant quantities of volatiles.

Metals, Ceramics, and Inorganic Materials

Although metals and ceramics generally exhibit very low outgassing, surface treatments, lubricants, coatings, and additive-manufactured parts with residual porosity may still require verification by this method.

For each material, specimens are typically prepared with a total mass of 100–300 mg, and multiple specimens (commonly three) are tested to ensure representative results. All specimens must be fully cured or processed through their normal production route before testing.

Testing Items

• TML (Total Mass Loss): the percentage of the initial specimen mass lost during the 24-hour thermal-vacuum exposure, calculated as (m₁ − m₃)/m₁ × 100.
• CVCM (Collected Volatile Condensable Materials): the percentage of condensable effluents collected on the 25°C collector disc relative to the initial specimen mass, calculated as Δm(collector)/m₁ × 100.
• WVR (Water Vapor Regained): the percentage of mass regained by the post-test specimen after re-conditioning at approximately 23–25°C and 50% relative humidity, used to distinguish absorbed water from true material decomposition.
• Acceptance evaluation: comparison of measured TML and CVCM values against the standard limits of TML ≤ 1.0% and CVCM ≤ 0.10%, or against project-specific requirements.

Testing Methods

Specimen Preparation and Pre-Conditioning

Specimens are cleaned, documented, and pre-conditioned at 23 ± 2°C and 50 ± 5% relative humidity for 24 hours. Initial masses (m₁) of both the specimens in their effluent cups and the clean collector discs are measured using a microbalance with a resolution of 1 microgram or better.

Thermal-Vacuum Exposure

The loaded effluent cups and collector discs are assembled in the test chamber, which is evacuated to a pressure below 5 × 10⁻⁵ torr. Specimens are heated to 125°C while the collector discs are held at 25°C, and these conditions are maintained for 24 hours. The temperature gradient drives volatile effluents from the hot specimen toward the cold disc, where condensable species are deposited.

Post-Test Measurement and Calculation

After the chamber returns to ambient conditions, the specimens and collector discs are re-weighed to obtain the final mass (m₃) and the collector mass change. TML and CVCM are then calculated and reported to the nearest 0.01%. If WVR is required, the post-test specimen is re-conditioned at 50% relative humidity for 24 hours and weighed once more.

Quality Control

Reference specimens and blank cups are included in each test run, and the apparatus is periodically verified using reference materials with known outgassing behavior. Key influencing factors include vacuum integrity, temperature stability, balance accuracy, and consistent specimen geometry.

Applications

• Spacecraft material selection and qualification during the design phase of satellites, launch vehicles, and deep-space probes.
• Contamination control for optical payloads, telescopes, mirrors, lenses, and sensors where molecular films can degrade transmittance and reflectance.
• Verification of thermal control materials, including white paints, black coatings, second-surface mirrors, and multilayer insulation.
• Screening of adhesives, potting compounds, cable insulation, printed circuit boards, connectors, and other electronic packaging materials.
• Support for supplier approval, incoming material inspection, change control, and failure analysis in aerospace manufacturing.
• Research and development of low-outgassing formulations for polymers, coatings, and additive-manufactured components.

FAQ

What is the TML flow disc testing method? It is a rapid, standardized screening test based on ASTM E595 in which a material specimen is heated at 125°C under high vacuum for 24 hours, and the escaping volatiles are collected on a 25°C disc, allowing TML and CVCM to be quantified.

What are the pass/fail criteria? The widely accepted criteria are TML ≤ 1.0% and CVCM ≤ 0.10%. Some programs with highly sensitive optics impose stricter limits, while certain applications may accept higher TML if CVCM remains low and WVR confirms the loss is mainly water.

Why is CVCM often more critical than TML? TML measures everything that leaves the specimen, including water vapor, which is usually harmless. CVCM measures only the fraction that re-condenses on cold surfaces, and this condensed film is what actually contaminates optics and thermal surfaces.

How much sample is required? Typically 100–300 mg per specimen, with at least three specimens recommended for each material to ensure statistical reliability.

How long does the test take? The exposure itself lasts 24 hours; including pre-conditioning, weighing, and reporting, a complete test is usually completed within 3 to 5 working days.

Which standards govern the method? The primary standard is ASTM E595, complemented by ECSS-Q-ST-70-02 for European space programs and NASA SP-R-0022A for NASA programs. All three define essentially the same 125°C / 24-hour / high-vacuum test conditions.