|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.
- Coatings and varnishes: Current polymer bases are alkyd, epoxy, polyester, polyimide, polyurethane, silicone, polyesterimide and polybenzimide. Coatings appear as one- or two-component systems, frequently containing solvents (thinners) to give the necessary low viscosity. Some are crystal clear, while some contain organic dye (mainly for quality control in the application). There are also products containing fillers. From the mechanical point of view, all grades are found from quite rigid to elastic products. As in the case of adhesives, coatings are frequently proprietary mixtures, the composition of which is difficult to trace.
- Some adhesive coatings have quite different properties above or below their Tg. This shall be carefully considered during selection for a particular application.
|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:
- Low temperature (50 ºC) and pressure (2 kg/cm2) for epoxy/amines.
- High temperature (150 ºC) and low pressure for epoxy/anhydrides.
- High temperature and high pressure (5 kg/cm2 to 20 kg/cm2) for phenolics.
- Very high temperature (250 ºC) and high pressure for polyimides.
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:
Application is by brush, dipping, flow or spray processes. Curing is very similar to that of adhesives but no pressure is applied. Since coatings and varnishes frequently contain solvents, these shall be dried out before curing commences (air drying or forced air drying). Solvent retention frequently occurs and tends to increase as the square of the film thickness, it is reduced by a high-temperature bake.Highviscosity sometimes creates flow problems which can be corrected by the use of thixotropic
|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.
- The adhesive shall be physically and chemically compatible with the component parts to be bonded:
Many adhesives or curing agents are harmful to human beings and care in their handling is necessary.
Adhesive bonding is in general quite sensitive to small changes in the process. This can lead to considerable variations in performance if strict control is not exercised. Moisture, either contained in the constituents, condensed on the adherends or in the atmosphere, can impair the cure of some adhesives (epoxies for example): humidity shall therefore be controlled. Atmospheric carbon dioxide can react with some curing agents and affect the properties of the cured product.
The storage, shelf life and pot life of coatings and varnishes shall be controlled in the sameway as for adhesives.When the purpose of varnishes and coatings is to protect or insulate the underlying item, care shall be taken to produce a continuous and adherent layer. Adhesion can be promoted by the use of suitable surface treatments (e.g. when a conformal coating shall adhere to Teflon insulation) and priming. Debubbling under low pressure with careful control of the process aids the formation of an intact protective surface.
The proportion of catalyst, the temperature of the applied coating or varnish product and of the substrate, along with the topography and orientation of the substrate shall all be controlled. Thick coatings can generate mechanical stresses and fairly high temperatures during their cure, and any damaging effects of these on the item to be protected shall be assessed by testing. Coatings cured at high temperature contain residual stresses at lower temperatures.Somecatalysts also give rise to corrosion problems with certain metals (copper, silver). Corrosion appears also where impurities (solder flux, moisture) are trapped in voids or cracks in the coating. Finally, thinners as well as base compounds can be toxic or flammable.
Material selection factors for conformal coatings are detailed in ESA SP1173 and are dependent on electrical requirements and anticipated service environment: dielectric constant, insulation resistance and corona suppression; and processing-related factors such as cure temperature and exothermic heat of reaction.
- Physically, the adhesive shall attach itself to the two surfaces to be bonded and in general this needs special pretreatment (cleaning, etching, priming). The adhesive shall also be capable of accommodating dimensional changes in the bonded surfaces (expansion-coefficient matching).
- Chemically, the adhesive shall not be corrosive to the adherents used (corrosion action is frequently due to hardeners).
|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”.
- Structural adhesives for which themaximumtemperature of use is low(60 ºC to 70 ºC) are likely to evolve contaminants at quite low temperature under vacuum (epoxy/amine). Adhesives which need atmospheric moisture to cure shall not be used in confined areas (large bonds between nonporous surfaces).
- Most coatings and varnishes which rely only on solvent evaporation to harden (solvent types, e.g. cellulose varnishes or dispersions, e.g. acrylics) are not suitable for space applications. These products are most likely to be profuse outgassers even after long drying periods. Solvent elimination is an exponential function of absolute temperature (Arrhenius equation) and is inversely proportional to the square of the coating’s thickness. Other coatings, containing solvent as a thinner but relying on a subsequent curing reaction to harden, should be avoided.
NOTE It is sometimes difficult to attain the specified viscosity without using a solvent.
- Solvents sometimes attack insulation in the device to be coated. Alkyd and polyester are in general not good enough for space use. Polysulphides, which are unstable in a thermal-vacuum environment, should also be avoided.
- Coatings and varnishes usually present very large surfaces to the space environment, this makes them particularly dangerous when not well chosen.
|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.
- Particle radiation at the level encountered in space is not harmful for adhesives, which are in any case protected by the items (adherends) they are bonding. Only coatings on satellite surfaces experience exposure to radiation, often combined with UV; see comments for UV. Insulating varnishes used inside “black boxes” are well protected against particle fluxes.
- UV radiation can darken optical adhesives. In this regard silicones are superior to epoxies. UV and particle radiation can both increase the outgassing rate of adhesives.UVradiation and proton fluxes are themain factors and can cause darkening and hardening of coatings and increase the outgassing rate. Insulating varnishes used in “black boxes” are not subjected to UV.
- High temperature degrades adhesives.For long-term exposure polyimide can be used up to more than 300 ºC; the best epoxies are normally limited to 170 ºC. Phenolics and silicones lie between. High temperature accelerates outgassing. Silicone-type coatings and varnishes are recommended for high temperatures. For very high temperatures, “ladder-polymers”, such as polyimide or polybenzimide, are the only possible candidates. When flammability is a property to be considered, silicone materials should be chosen in preference to polyurethane coatings.
- Low temperature stiffens adhesives and causes brittle bonds. Some polyurethane adhesives are still useful at very low temperatures (cryogenic). A similar effect is seen with coatings which tend to harden, shrink and crack.
- Thermal cycling leads to failure of the adhesive bond when the expansion coefficients of the adherents and adhesives are not matched and when the adhesive is not flexible enough to cope with the strain. Thick layers of rigid adhesives are prone to high stresses. Coatings and varnishes experience thermal-cycling due to shadow-sunlight passage or to variable internal heat sources caused by switching equipment on and off. Mismatch of expansion coefficients between coating and coated items gives rise to high stresses and eventually to cracks. Thermal insulation by the coating can lead to overheating of high-power components, particularly in vacuum.
- Atomic oxygen (inLEO)is only applicable to adhesives exposed to ATOX (such as those on solar-cell and panel assemblies) which can be attacked. Exposed coatings are susceptible: silicones are resistant.
|Some Representative Products||Adhesives that can be considered are:
Coatings and varnishes that can be considered are:
- Araldite AV 138,
- DC 6--1104,
- Eccobond Solder 56C,
- Redux 312,
- RTV 566,
- RTV S 691,
- RTV S 695,
- Scotch Weld EC 2216,
- Solithane 113.
- DC 93500,
- MAPSIL 213,
- RTV S 695,
- Uralene 5750.