Only a small portion of solar radiation consists of invisible ultraviolet (UV) light. But
the light in this spectral range is responsible for photo degradation. Photo degradation
results in discoloration, fading, embrittlement, cracking, chalking and/or loss of
mechanical properties. Chalking gelcoat fiberglass, yellowing plastics, fading and
weakening fabrics and sunburned skin are all familiar problems caused by UV light. Before
UV light can cause harm, it must first be absorbed. If it is not turned into heat or
transferred to a nearby stabilizer molecule called a quencher, it breaks weak chemical
bonds. This is the beginning of UV damage.
Some materials absorb UV light more readily than other materials.
Materials that readily absorb UV light are quickly damaged...rubber, vinyl, gelcoat
fiberglass, and many other plastics. Acrylic is slow to absorb UV light and accordingly
very resistant to photo degradation.
UV stabilizers are a group of chemical agents with the ability to
counteract or neutralize the harmful effects of UV light. Competitive absorbers provide
protection by converting UV light to heat so it can dissipate harmlessly (See Vol. I).
Other UV stabilizers work differently. ALL UV stabilizers are consumed as they do their
job. In a way, they serve as sacrificial molecules, taking the abuse from UV light instead
of the material they are protecting.
This brief overview greatly simplifies this very complex subject.
Discoloring due to absorbers that have absorbability into the visible light range is a
problem. And there are many others.
Two important points: UV stabilizers have to be periodically renewed or
replenished if continuing UV protection is to be achieved. Second, there is no such thing
as a permanent UV stabilizer, a matter of physics, not chemistry.
