For outdoor applications, pigments used for coloring should be selected for their
weather resistance characteristics. Closely related to
light fastness, weatherability adds the extra dimension of atmospheric conditions
(including salt from the sea, waste gases from industrial areas, or very low humidity
from desert conditions). Weather resistant pigments are usually lightfast but the
reverse is not always the case.
The selection of pigments for outdoor use depends on:
- outdoor performance required (life time, climatic region/ Kilo Langley)
- binder type
- concentration of the pigment
- presence of titanium dioxide (which typically accelerates fading)
- concentration and type of light stabilizers used.
Performance can also be influenced by the surface of the painted object and by the
processing heat history.
Once the above variables have been defined, the best way to assess weathering resistance
in service is by using outdoor exposure tests in the climatic region(s) concerned.
This is clearly not always feasible. The widely used alternative is accelerated
testing. Machines are available which in addition to a xenon lamp, include wet cycles
interspersed between longer dry cycles. Weatherability is designated in terms of
the 1-5 Grey Scale. 5 represents no change and 1 a severe change.