|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.