Aerospace & Defense Technology - December 2023 - 22

Test & Measurement
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0.45
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0.1
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0 81624324048566472
Time (h)
CV-1142
SCV-2585
SCV1-2596
SCV1-2590
Figure 3: Comparison of the TML percentage over time for silicones tested by ASTM E1559.
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CV-1152
CV4-1161-5
CV10-2568
CV-1144-0
CV-2680-12
SCV-2586
CV-2646
CV-2289-1
CV16-2500
applied. For example, a liquid adhesive
may be a better choice than a tape
for an application with very tight and
complex geometries. If the application
requires silicone to be applied
to a vertical surface, a material that
is thixotropic/non-slump would be
preferred over a low viscosity system.
Process time is also a key consideration.
Cure chemistry can influence
the pot life and cure time, with additional
cure formulations being able to
cure more quickly than condensation
cure formulations. Other factors that
play a role in determining process time
include temperature, humidity and the
need for alternate cure schedules. When
all these factors - function, purity and
process - are considered holistically,
the proper space-grade silicone solution
can be identified and implemented.
Comparing Controlled Volatility
Silicones
NASA developed the ASTM E595 test
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0.02
0.01
method to evaluate the volatile content
of materials used in space applications.
ASTM E595 measures outgassing after a
sample is heated under vacuum (< 10-5
torr) at 125 °C for 24 hours and provides
data on total mass loss (TML), collected
volatile condensable material (CVCM)
and water vapor regained (WVR). The
ASTM E595 standard establishes acceptance
limits at ≤ 1.0 percent TML of the
material and ≤ 0.1 percent CVCM.
Materials used on a spacecraft's more
0 81624324048566472
Time (h)
CV-1142
SCV-2585
SCV1-2596
SCV1-2590
Figure 4: Comparison of the VCM percentage (298 K QCM) over time for products tested by ASTM E1559.
adjacent materials, requiring the use
of a room temperature vulcanizing
(RTV) silicone instead. Purity: Based
on the application and where the
silicone will be used, it is important
to understand how the risk of outgassing
relates to mission success.
For the most sensitive applications,
even minimal amounts of outgassed
22
species can create the potential for
mission failure. These situations
might dictate more stringent volatility
requirements where higher purity
ultra-low outgassing silicones would
be the best option. Process: Once
the material function and purity levels
have been determined, consider
where and how the material will be
mobilityengineeringtech.com
CV-1152
CV4-1161-5
CV10-2568
CV-1144-0
CV-2680-12
SCV-2586CV16-2500
CV-2646
CV-2289-1
sensitive areas may require more stringent
and detailed testing, with acceptance
limits at ≤ 0.01 percent CVCM. In
these cases, NASA recommends ASTM
E1559 testing, which is a more exhaustive
testing process that records outgassing
levels as a function of time and
temperature, thus providing outgassing
kinetic data.
A silicone manufacturer used both
ASTM E595 and E1559 to analyze a
suite of its products, including condensation
and addition cure materials and
sheeting products. The purpose was to
give an overview of the company's low
outgassing (CV product designation)
and ultra-low outgassing (SCV product
designation) products and how
the outgassing kinetics differ between
them, to aid design engineers in mateAerospace
& Defense Technology, December 2023
% TML
% VCM

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Aerospace & Defense Technology - December 2023

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