Materials that appear stable under normal conditions may release volatile compounds when exposed to elevated temperatures under high-vacuum. This phenomenon, known as outgassing, can result in molecular contamination that affects the performance of optical systems, detectors, sensors, electronic assemblies, and other contamination-sensitive equipment.
To evaluate outgassing behaviour in a consistent and reproducible way, laboratories and manufacturers can rely on ASTM E595-15 (2021) standard. Originally developed for space applications, the standard is now widely adopted across aerospace, defence, semiconductor manufacturing, optics, and other industries where material cleanliness is critical.
Outgassing testing evaluates the amount and nature of volatile compounds released by a material when exposed to controller vacuum and high temperatures.
The objective is to determine whether a material is suitable for contamination-sensitive applications by measuring both the amount of material released and its potential to contaminate nearby surfaces.
Unlike atmospheric conditions, molecules released under vacuum do not readily disperse. Instead, they may migrate through the chamber and condense onto colder surfaces.
This contamination can affect optical components, infrared windows, detectors, mirrors, sensors and precision electronic assemblies, potentially reducing performance or long-term reliability.
For this reason, outgassing testing plays a critical role during material qualification for high-reliability systems.
The number of specimens depends on the design of the test system.
Modern instruments can typically accommodate 6 to 12 samples within a single test cycle. The available positions may be fully occupied by test specimens to maximize throughput, allowing multiple materials or production batches to be evaluated under identical conditions. Alternatively, part of the chamber can be reserved for system functionality checks or reference samples, while the remaining positions are used for testing the specimens.
ASTM E595 specifies specimens measuring 8 mm max diameter x 10 mm height. These dimensions provide a standardized specimen geometry, ensuring consistent thermal exposure and reproducible outgassing measurements.
When testing sample which are bigger, the specimen should be prepared in accordance with the standard while remaining representative of the final application.
The standard exposure lasts 24 hours under controlled vacuum and temperature conditions.
Typical conditions include the following parameters:
| Specimen temperature | 125 °C ± 1 °C |
| Exposure time | 24 hours |
| Vacuum level | Below 7 × 10⁻³ Pa (5 × 10⁻⁵ Torr) |
| Collector temperature | 25 °C ± 1 °C |
Maintaining these conditions is essential to obtain comparable results between different laboratories.
The overall laboratory time may be longer, as it also includes sample preparation, weighing before and after testing, chamber stabilisation, and any additional analyses performed on the collected condensates.
If Water Vapor Regained (WVR) is measured, further conditioning under controlled ambient conditions is required before the final weighing.
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The purpose of heating the material is to accelerate the release of volatile compounds that could be emitted during service.
The test is not intended to reproduce the exact operating temperature of every application. Instead, it provides standardized conditions that enable meaningful comparison between different materials.
The collector plate is intentionally maintained at 25 °C to simulate surfaces that are cooler than the heated specimen. Volatile molecules released from the specimen condense on this controlled-temperature surface, allowing the amount of Collected Volatile Condensable Material (CVCM) to be accurately determined.
The collector therefore represents a simplified model of how contamination may accumulate on sensitive components such as optical windows, mirrors, detectors or sensors.
ASTM E595 provides three primary parameters:
The percentage of mass lost by the specimen during thermal vacuum exposure. It indicates the overall amount of volatile material released.
TML acceptance limit: ≤ 1.00 %
Measures the fraction of released compounds that condense onto a cooled collector surface. This parameter is particularly important because it directly reflects the contamination risk for nearby sensitive components.
After testing, some materials reabsorb moisture from the atmosphere. WVR quantifies this effect, helping distinguish water uptake from the actual loss of volatile compounds and improving interpretation of the results.
Together, these three parameters provide a standardized basis for material qualification and comparison.
The acceptance limits were originally established by NASA and are now commonly referenced in international space qualification programs. Nevertheless, acceptance criteria may vary depending on the application, customer specifications, or industry requirements. This is an important distinction: ASTM E595 defines how to perform the test, while the acceptance criteria are usually defined by the end user or by application-specific standards.
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Not necessarily. A material meeting the typical TML and CVCM limits demonstrates low outgassing under the standardized test conditions, but real operating environments may differ significantly. Factors such as:
can all influence outgassing behaviour.
For critical applications, ASTM E595 should therefore be considered a qualification baseline rather than a complete material assessment.
ASTM E595 quantifies the amount of condensable material but does not identify its chemical composition.
By replacing the standard collector plate with an infrared-transparent window, the deposited material can be analysed directly by FT-IR spectroscopy. This approach provides additional information on the chemistry of the condensed species, helping distinguish between silicones, hydrocarbons, plasticizers, lubricants and other volatile compounds.
The combination of ASTM E595 with FT-IR transforms the test from a purely quantitative evaluation into a comprehensive contamination analysis, supporting material selection, process optimization and failure investigations.
ASTM E595-15 (2021) has become one of the most widely recognised methods for evaluating outgassing behaviour in contamination-sensitive applications.
While TML, CVCM and WVR provide the quantitative basis for material qualification, combining the standard test with complementary analytical techniques enables a more comprehensive understanding of the nature of released compounds and their potential impact on system performance.
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