Vacuum outgassing materials may produce optical surface contamination, sensor contamination, and so forth. To ascertain the compatibility of materials used in space, ASTM has developed a series of test methods to measure outgassing. The ASTM E595 test is popular because it is easy to conduct and yields consistent results for screening materials on total mass loss (TML) and collected volatile condensable materials (CVCM), which are highly significant in determining the potential for product contamination. Contrastingly, ASTM E1559 conditions are more realistic, and they determine dynamic outgassing behavior and condensation on cold surfaces. This technique is suitable for high-precision work that requires detailed information on contamination.
Methodology
In ASTM E595, a specimen sample material is subjected to a vacuum chamber and heated at 125°C for 24 h. Currently, the loss of mass as volatile material is noted, and condensable materials are captured on a cooled plate. The data obtained are then used in computing TML, CVCM, and water vapor regain (WVR). In ASTM E1559, a more complex arrangement is used where the materials are exposed to a high-vacuum system with real-time observation of the outgassed species. The weight of the condensable materials deposited over time can also be measured using a cooled quartz crystal microbalance (QCM), whereas the chemical species can be identified using mass spectrometry.
Instrumentation
The apparatus of the E595 is comparatively simple and consists of a vacuum oven, a cooling collection plate (typically maintained at 25°C), and a highly sensitive balance for weighing the mass. The ASTM E1559 test involves the use of sophisticated instruments, which require high-vacuum, cryogenic cooling stages, QCMs, and possibly mass spectrometers. It enables the continuous tracking of outgassing and allows for the precise measurement of numbers on simulated cold surfaces, a feature particularly applicable in the context of critical space hardware testing.
Strengths
ASTM E595 is a straightforward, cost-effective, and widely accepted method by agencies such as NASA and ESA for screening materials. To select adhesives, coatings, elastomers, and polymers for spacecraft construction, it is most suitable to choose them at an early stage. Instead, ASTM E1559 gives a thorough understanding of the outgassing rate, chemical composition of volatiles, and deposition rates over time. This level of detail is of significant value in the design of contamination-sensitive systems, such as optical payloads and scientific instruments.
Limitations
Although ASTM E595 can be helpful in simple qualifications, it fails to disclose data about the individual identity and real-time behavior of the outgassed species. Neither does it consider fluctuating thermal conditions since it also assumes constant heating. Although it is strong, ASTM E1559 is more complicated and resource-intensive. It focuses on highly specialized instrumentation and personnel, making it accessible only to specialized aerospace laboratories.
