TML Testing in Accordance with Chinese Domestic Standards: Total Mass Loss Evaluation for Spacecraft

TML domestic standard testing measures the Total Mass Loss of materials exposed to a simulated space vacuum environment in accordance with Chinese national and aerospace industry standards. This test verifies that polymeric and non-metallic materials used in spacecraft do not release excessive volatile contaminants that could degrade optical surfaces, sensors, and thermal control systems. This article explains the technical principles, sample requirements, test items, testing methods, and typical applications of TML testing under domestic Chinese standards.

Technical Overview

TML (Total Mass Loss) testing is a thermal-vacuum outgassing test that quantifies the amount of volatile mass released by a material when it is heated in a high-vacuum environment. In China, the test is performed in accordance with domestic standards, including national standards (GB/T series) and aerospace industry standards (QJ series), which are technically aligned with internationally recognized methods such as ASTM E595 and ECSS-Q-ST-70-02. The purpose of the test is to screen materials before they are used on spacecraft, because volatile compounds released in orbit can recondense on cold surfaces such as lenses, mirrors, detectors, and thermal control coatings, causing performance degradation.

During a standard test, preconditioned specimens are held at 125°C for 24 hours in a vacuum of approximately 1.3 × 10^-4 Pa or lower, while volatile effluents condense on a collector plate maintained at 25°C. The mass change of the specimens determines the TML value, and the mass change of the collector determines the CVCM value. Space agencies and satellite manufacturers generally require materials to meet a TML of 1.0 percent or less and a CVCM of 0.10 percent or less before flight qualification is granted.

Testing Samples

Sample Categories

TML domestic standard testing is applicable to a wide range of engineering materials, including:

• Polymers and engineering plastics, such as PEEK, PTFE, polyimide, and polycarbonate
• Elastomers and rubber compounds, including silicone and fluororubber seals and gaskets
• Adhesives, sealants, potting compounds, and encapsulation resins
• Coatings, paints, films, tapes, and label materials
• Thermal control materials, such as multilayer insulation films and radiator coatings
• Printed circuit board laminates, cables, and connectors
• Composite materials, ceramics, lubricants, and additive-manufactured parts

Sample Preparation Requirements

Specimens are typically prepared with a mass in the range of approximately 100 to 300 milligrams and a thickness representative of the actual application, generally not exceeding a few millimeters to ensure uniform heating. Before testing, specimens must be fully cured or processed according to the manufacturer specification, since incomplete curing significantly increases outgassing. Specimens are then conditioned at 23 ± 2°C and 50 ± 5 percent relative humidity for at least 24 hours to stabilize their moisture content. All handling must be performed with clean gloves, tweezers, or fixtures to prevent contamination by fingerprints, oils, or airborne particles. In most cases, a minimum of three specimens per material condition is tested to ensure statistical reliability.

Testing Items

Total Mass Loss (TML)

TML is the percentage of mass lost by a specimen after the 24-hour thermal-vacuum exposure. It represents the total quantity of volatile substances, including water and other outgassing products, released by the material under simulated space conditions.

Collected Volatile Condensable Materials (CVCM)

CVCM is the percentage of volatile effluents that recondense on a chromium-plated collector plate maintained at 25°C during the test. Because this fraction is what can actually contaminate sensitive surfaces in flight, CVCM is often the most critical acceptance parameter.

Water Vapor Regained (WVR)

WVR measures the amount of water vapor reabsorbed by the specimen after exposure when it is returned to standard laboratory humidity conditions. This item helps distinguish absorbed moisture from genuine material decomposition or weight loss.

Recovered Mass Loss (RML)

RML is calculated as TML minus WVR and represents the permanent, non-recoverable mass loss of the material. It is particularly useful when evaluating materials such as adhesives and polymers whose TML is dominated by absorbed water.

Testing Methods

Test Conditions

The standard test condition specified by domestic standards, consistent with ASTM E595, requires a vacuum pressure of 1.3 × 10^-4 Pa or lower, a specimen temperature of 125 ± 1°C, an exposure duration of 24 hours, and a collector plate temperature of 25 ± 1°C. A microbalance with a resolution of 1 microgram or better is used for all mass measurements, and the weighing room is maintained under controlled temperature and humidity conditions.

Test Procedure

The typical testing sequence is as follows:

• Condition the specimens and collector plates at 23 ± 2°C and 50 ± 5 percent relative humidity for at least 24 hours
• Weigh the conditioned specimens and collector plates using a calibrated microbalance to obtain initial masses
• Load the specimens into the sample chamber and position the collector plate at the specified distance above the specimens
• Pump the system down to the required vacuum level and heat the specimen chamber to 125°C
• Maintain the exposure condition for 24 hours while volatiles migrate to and condense on the 25°C collector
• Return the system to ambient pressure using dry nitrogen or clean air, then recondition the specimens under standard laboratory conditions before re-weighing
• Weigh the specimens and collector plates again, perform an additional 24-hour humidity reconditioning step when WVR is required, and record all final masses

Calculation and Acceptance Criteria

TML is calculated as the difference between the initial and final specimen masses divided by the initial specimen mass, expressed as a percentage. CVCM is calculated as the mass gained by the collector divided by the initial specimen mass, also expressed as a percentage. WVR is calculated as the mass regained by the specimen after humidity reconditioning divided by the initial specimen mass. The commonly accepted pass criteria are TML less than or equal to 1.0 percent and CVCM less than or equal to 0.10 percent. Materials exceeding these limits may still be acceptable in specific applications if RML meets the requirement or if a contamination-control waiver is approved by the spacecraft system engineering authority.

Applications

TML domestic standard testing is widely applied across the aerospace and high-reliability electronics industries. Typical applications include:

• Material selection and screening during spacecraft and satellite design
• Flight qualification and periodic requalification of approved materials
• Contamination control for optical payloads, sensors, cameras, and detectors
• Verification of multilayer insulation, thermal blankets, and radiator coatings
• Qualification of adhesives, potting compounds, and encapsulants for space electronics
• Incoming material inspection and supplier quality verification for aerospace supply chains
• Research and development of low-outgassing polymer formulations and coatings
• Generation of test reports demonstrating compliance with customer and standards requirements

FAQ

What is the difference between TML and CVCM?

TML measures the total mass a material loses during the thermal-vacuum exposure, including water and all volatiles. CVCM measures only the fraction of those volatiles that recondense on a surface at 25°C, which indicates the actual contamination risk to nearby sensitive surfaces. A material can have a relatively high TML but still pass if its CVCM is low.

Which standards govern TML testing in China?

Domestic testing is conducted according to Chinese national standards and aerospace industry standards of the QJ series, which define test apparatus, conditions, and procedures consistent with ASTM E595 and ECSS-Q-ST-70-02. This alignment allows test results to be accepted in both domestic and international space programs.

What are the pass criteria for TML testing?

The widely used acceptance limits are TML less than or equal to 1.0 percent and CVCM less than or equal to 0.10 percent. For materials whose mass loss is dominated by absorbed moisture, the recovered mass loss (RML) may be evaluated in place of TML by agreement between the customer and the test laboratory.

How many specimens are required for a test?

A minimum of three specimens per material is typically required, along with reference or blank collector plates. Additional specimens may be needed when a material is tested in multiple conditions, such as different cure states, thicknesses, or surface treatments.

How long does a TML test take?

The vacuum exposure itself lasts 24 hours. Including preconditioning, microbalance weighing, post-test reconditioning, and reporting, a complete test cycle usually requires three to five working days, assuming the equipment schedule and balance calibration are available.

Can a material that fails the TML limits still be used?

Yes, in certain cases. If the excess mass loss is attributable to water vapor, the material may pass based on RML. Alternatively, a vacuum bake-out process can be applied to reduce outgassing, or a contamination-control waiver can be granted when the material is located far from sensitive optics or thermal surfaces and the mission risk assessment supports its use.