|Material||12: Paints and inks|
|General Information||Most commercial paints are unsuitable for use in space. Painted materials or structures present a large surface area to environmental conditions. Only with careful selection, application and control of processes can paints fulfil the specified function|
|Use in Spacecraft||The most critical use of paints is in the thermal-control subsystem. They can also be employed for corrosion protection. Inks are used for marking and for identification purposes.|
|Main Categories||Common organic binders are epoxies, acrylics, silicones, and polyurethanes. Inorganic bases are mainly silicates. Pigments are chosen to produce the specified optical properties:
- white pigments for low solar absorptance and high emittance (zinc oxide (ZnO), titanium dioxide (TiO2 ), zinc orthotitanate (Zn2 TiO4 ) and zirconium dioxide (ZrO2 ) are the most common);
- aluminium flakes for medium absorptance and emittance;
- carbon black for high absorptance and emittance.
Electrically conductive thermal-control paints were developed to avoid charging and discharging in geostationary orbit. Some, which are based on metal or carbon pigments, can be used when a a /ratio close to one is acceptable. Electrically conductive white paints are based on semi-conductive pigments; their stability in the space environment has now been assessed in some cases. Paints normally contain several proprietary components intended to give them good application properties. They come in the form of one-part or two-part, rather viscous liquids. They are brought to the right viscosity by mixtures of solvents. Solvent-free paints also exist.
|Processing and Assembly||Hardening of the paint layer is due first to the evaporation of the solvents followed by some chemical reaction producing an insoluble film; the reaction can be catalysed by the atmosphere (oxygen or moisture) in one-part paints or by an added catalyst in two-part systems.
Processing generally needs mixing: application of one or more coats by brush or by spray with intermediate partial drying; final drying and curing at room temperature or in an oven. Inorganic paints are more difficult to apply and good adhesion is difficult to obtain.
- Paints with limited shelf-lives shall be stored under controlled conditions and meet the requirements of ECSS-Q-ST-70-22. Pot life shall also be controlled. Special pretreatment shall be carried out of the item to be painted (e.g. cleaning, abrading and priming, as for adhesives, coatings or varnishes) to achieve good adhesion of the paint coating. Paints tend to remain sticky for a long time and should not be applied in dusty atmospheres. Paint layers are fragile and can be damaged by abrasion and shocks.
- Contamination by oils and chemicals shall be avoided since the cleaning of a painted item is a difficult operation: this is particularly acute for inorganic base paints.
|Hazardous and Precluded||It is very difficult to find a good “space” paint, particularly a white one, in view of the different requirements of mechanical resistance, space environment stability and outgassing. This normally results in the recommendation of rather lengthy and difficult cure schedules and extreme cleanliness precautions during and after application.
Most of the solvents used in paints are toxic or flammable.
|Effects of Space environment|
- Vacuum exposure of paints results in high outgassing due mainly to solvent residues (which are reduced by a baking process) and also condensable products. The only way to reduce this inconvenience is to prescribe extremely long and difficult cure processes, sometimes under vacuum. Even in this case, only very few of the commercial paints can qualify. The method is in any case frequently impracticable since the painted items cannot resist the cure temperature if they contain electronic or other sensitive devices. During the outgassing period, paint layers harden and become more brittle, but the main risk is contamination of optics and electronics in the vicinity Inorganic paints are generally less contaminating, since they evolve mainly water.
- Radiation is the most damaging environmental factor for paints used on the exterior of spacecraft. Particles and UV tend to embrittle paint layers. Their main effect, however, is the degradation of optical properties: emittance of paints is in general stable under radiation. Some black paints bleach slightly under the combined effects of vacuum, particles and UV. These factors are very dangerous for white paints, which undergo a drastic increase in absorptance. This effect can be studied only by measurements under vacuum, since atmospheric gases can bleach the defects created in the paint. The increase in absorptance is due to changes in both pigment and binder. In the former, colour centres are created which absorb at specific wavelengths; in the latter the absorptance edge of the UV side is moved towards longer wavelengths and sometimes new bands appear.
- Inorganic-based white paints (silicate binder) are more stable than those with an organic base, and some of them are quite good from the optical-properties point of view. The stability of white paint under radiation depends to a large extent on the physico-chemical purity of the pigment used.
- High temperature degrades paints (“smoking” under ascent conditions). In this respect, silicones and silicates are best. Heat can be beneficial in accelerating the bleaching of certain colour centres in pigments, but normally increases the yellowing of binders. Thermal cycling can cause deterioration in paints that are not flexible enough to cope with the substrate’s dimensional changes: flaking, blistering, cracking can occur. Paints with inorganic binders are rather sensitive in this respect.
- Atomic oxygen in low Earth orbit attacks paints. Those with a silicone and perfluorinated base seem better. Silicate bases are resistant.
|Some Representative Products||No commercial white paint is perfectly satisfactory for space use, since most of them outgas too much or are unstable under radiation. The situation is a bit less critical for black paints. Many commercial paints were flown but with moderate success. Many space paints, and particularly white, are not commercial items and are prepared in-house or under contract by spacecraft builders. Materials which can be considered are:
- Aeroglaze H332 (former Chemglaze),
- Aeroglaze L300 (former Chemglaze),
- Aeroglaze Z306 (former Chemglaze),
- Acheson Electrodag 501,
Silver loaded paint:
- MAP-SG 121 FD,