Since I love exposing marketing hype that is "over the edge," one of the brilliant ideas that marketing experts had was to try to differentiate how yellow their epoxy looks compared to others, hence new marketing terms like “low yellowing epoxies” or “non yellowing hardeners” have crept into the marketing hype. Most of this is garbage for several reasons, and it cannot be ignored that there are more distributors that actual manufacturers, each adding their two cents to the hype... :
Short version:
1) Subjected to UV radiation, all resins will degrade over time. Epoxy, polyurethane, acrylic, whatever…they will all break down eventually with UV exposure.
2) The amount of "damage" depends on the intensity of UV radiation. This can vary country to country, state to state, blah blah...
3) The epoxy resin itself (Part A) is an aromatic chemical structure, which in simple terms means the chemical bonds are susceptible to UV. Companies, to mitigate this, add absorbers or uv stabilizers to slow the process, but the truth is these additives are also UV reactive componds that have a finite life span.
The affects can be offset using selective absorption by modifying the light stable top-coats with pigments or other absorbers/reflectors, or by properly pigmenting the under-coats. But there are problems that require that you be "smarter" than the marketing hype.
Certain pigments also absorb UV radiation, which may be made available to attack UV light sensitive materials. Some pigments themselves change radiant energy into chemical energy, which will disrupt the covalent bonds of the organic molelcules in the polymer.
It is said that the best solution next to not exposing organic compounds/polymers to UV radiation e.g. keep it indoors, is to use families of aliphatic polymers that do not absorb UV radiation, rather are transparent to UV radiation such as acrylic, vinyl acetate, aliphatic urethanes, etc. Using such things as bisphenol-A epoxies, and phenolics "soak up the rays" and degrade. (BPA over-reaction irrelevant in this context.)
Zinc Oxide, zinc sulfide, red iron oxide, carbon black, and rutile titanium dioxide are all excellent absorbers, and provide good protection for UV sensitive polymers. Zinc Oxide is rated very high in that it absorbs all UV radiation at dangerous levels, and offers its binder exellent protection as well.
