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|PRODUCT DATA of 10: Adhesives, Coatings|
|Material||10: Adhesives, Coatings|
|General Information||Information and specific requirements are given for polymer-based adhesives (structural and non-structural), coatings and varnishes. For ceramic-type adhesives and applied coatings, see 20: miscellaneous|
|Use in Spacecraft||Structural adhesives appear where high load-bearing capability is needed, e.g. in the face-to-core bond of honeycombs. They are characterized by having high strength and modulus; good toughness and peel strength are important factors for structural adhesive bonds.
Non-structural adhesives (glues, bonding agents) are found particularly in, for example, solar-cell assembly, optical-component bonding and screw locking. Adhesives are most useful in the bonding of dissimilar materials which are difficult (or impossible) to assemble by other means: e.g. glass and ceramics. Some ensure good thermal contact and low stress concentration at the joint, but such assemblies are difficult to take apart after fabrication. Electrically conductive adhesives find a use as grounding points for conductive surfaces.
Coatings and varnishes appear as electrical insulating layers, corrosion protection and mechanical protection mainly in electronic circuitry. Finished layers can be thin (e.g. varnishes) or rather thick (e.g. conformal coatings).
|Main Categories||Adhesives: in current use are epoxies, phenolics, "modified" epoxies, acrylates, polyurethanes, silicones, polyimides and cyano-acrylates. Their consistency is quite variable: liquid, paste, powder, supported or unsupported films. Some have to be kept cold until used; others are prepared by mixing two or more components just before application. They can contain fillers or be clear and transparent. Adhesives are in general quite complex (and proprietary) formulations, and appear on the market under many trade names; it is frequently difficult to determine their basic chemical nature from the manufacturer's data. Anything can be bonded with adhesives, but no adhesive exists that can effectively bond everything.
|Processing and Assembly||Adhesives: Processing varies from simple room temperature curing under contact pressure to intricate pressure or temperature exposures depending upon the category and type of adhesive. Typical examples in the cases of structural adhesives are:
Many non-structural adhesives cure under contact pressure at moderate temperatures, e.g. RTV silicone rubbers (some of which cure with atmospheric moisture), cyano-acrylates (moisture cure) and anaerobics (which cure by air exclusion) and polyurethanes. Some of these adhesives are quite sensitive to contaminants, the presence of which sometimes prevents correct curing.
Coatings and varnishes:
|Precautions||It is very easy to misuse adhesives, particularly in critical applications. They normally have a limited shelf life (marked on the packaging and suppliers’ data sheets) which shall be respected, and the conditions under which they are stored shall be adequately controlled (see ECSS-Q--70-22). They frequently have a short “pot-life” or “working life” after their component parts are mixed or brought to activation temperature.
|Hazardous and Precluded||Many adhesives on the market are solutions or emulsions. Although these products can be excellent for their intended terrestrial use, they shall not be applied to space vehicles since they are potential outgassers. It shall be ensured that adhesives are quoted “100 % solid”.
NOTE It is sometimes difficult to attain the specified viscosity without using a solvent.
|Effects of Space environment||Exposure of adhesives to vacuum provokes outgassing. The major components which outgass are unreacted compounds, low-molecular-weight constituents and the bi-products from chemical reactions. As the exposed surface is small (only the bondline), outgassing rates can be quite low. Effects of vacuumalone onthe bond integrity are normally not observed, but someof the evolved constituents can be condensable and can create a contamination danger in a spacecraft (“coating” of electrical or optical components). Many epoxies are acceptable from an outgassing point of view, but are rather sensitive to humidity conditions at the time of curing. “Modified” epoxies, particularly the flexible ones, can have outgassing rate.Nearly all RTV silicones are known to be contaminant, but some manufacturers have developed special compounds for space use. All coatings and varnishes outgas. This is particularly noticeable for types containing solvent. This phenomenon can sometimes be reduced by extended curing at high temperature and under vacuum, but such a method is not very practical and is not always successful. Atmospheric gases trappedwithin cracks and voids in the coating can leak out under vacuum and produce pressures in the “corona range”. Cracks formed under vacuum can fill with outgassing products up to the same pressures. These two phenomena lead to troubles when high electric field strengths are present during spacecraft equipment operation.
|Some Representative Products||Adhesives that can be considered are: