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Every single design consideration is important in the costly endeavor to send an object into space. One of these considerations, which at first may not appear to be large but can lead to catastrophic results if poorly considered, is the choice of adhesive to be used in various components of space systems. These adhesives are used for bonding, insulating, damping, conducting, and a host of other reasons. They are often used near or directly adjacent to electronic optical devices where contamination would be of major concern. In general, any material used in space must possess the following characteristics:
• Good resistance against radiation degradation
• Outstanding atomic oxygen resistance
• Excellent micro-cracking resistance against
• Low outgasing characteristics
Like all other polymers, silicone is susceptible to radiation which exists in space due to the absence of any atmospheric gases that would serve as an obstacle in the path of electromagnetic waves and subatomic particles. While radiation can cause changes in the properties of silicone rubber similar to those caused by heat aging, certain specialty silicones possess excellent resistance against radiation (see Table 1). In addition to radiation, atomic oxygen is another factor with which aerospace materials engineers must deal. Atomic oxygen reacts with polymers causing erosion, which is a threat to spacecraft durability. Nearly 90% of the atmosphere from 100 to 350 miles from the Earth's surface is comprised of atomic oxygen. Silicone adhesives would generally not be exposed to this form of oxygen, as they would be encapsulated between two surfaces; however, silicone has been used as a coating in some space applications to protect various polymer substrates.
Outgassing of silicone depends on what grade of silicone is used. In general, the volatiles are low molecular weight polydimethylsiloxanes, which are the remnants and products of the polymerization reaction. Siloxanes with chain lengths of 4 to 10 siloxy groups constitute what are commonly referred to as "total silicone volatiles."1 The number of volatiles that outgas depends not only on the type of silicone used but also on the type of non-polymer filler, additives and curing agents. Processing also plays a role in determining the final properties of any given formulation. Fortunately, the use of special formulation and processing techniques results in a silicone adhesive that meets the requirements of NASA outgassing specifications and retains all of the adhesive benefits that silicone provides.2 Specialty silicones employing optimized cure cycles have been able to achieve as low as 0.06% TML and 0.02% CVCM.
Silicone adhesives are currently being used in a variety of space applications. Electronics for a satellite constructed by the Johns Hopkins University Applied Physics Laboratory for the NASA Goddard Space Flight Center required a silicone adhesive suitable for extremely cold temperatures in order to bond heat sinks to PCBs. This example serves to highlight another unique condition of space - for internal devices, conduction is the only method for heat transfer since there is obviously no convection of air, and radiation would only occur at the surface of the spacecraft. Another example is the use of a silicone adhesive to bond a radiation conduction bar to the mount plate of a charged coupling device (CCD) of a telescope. The proximity of the silicone to the CCD required the development of a very low outgassing formulation since any volatiles could contaminate the device and diminish the telescope's capability. These and other examples show that silicone will increasingly be an adhesive of choice in many aerospace applications.
References1 Toube , Mel. Factors Affecting Silicone Volatile Levels in Fabricated Silicone Elastomers. Rubber World, June 2002, Vol. 226, No. 3
2 See Thermabond in NASA outgassing data at www.outgassing.nasa.gov
For more information:For more information on silicone adhesives, contact Kareem M. Monib, ARLON, Silicone Technologies Div., phone 302-595-1231; e-mail firstname.lastname@example.org;
or visit www.arlon-std.com.