The short answer is:
The long answer is:
While plastic is usually considered an "artificial" material,
it is typically a polymer based on organic petrochemical compounds.
Polymers can be described as large molecules made up of simple repeating units.
The word polymer is derived from the Greek words poly meaning "many" and mer meaning "part".
Many types of polymers can be created by varying the molecular composition of the repeating unit.
polymers are therefore subject to degradation and breakdown through natural oxidisation.
Degradation may be initiated or accelerated by numerous factors including ultraviolet light,
oxidisation can result in the breaking and rearranging of chemical bonds along the polymer chain.
This can begin even during the manufacturing process,
due to the conditions of extreme heat or pressure that may be used to form the polymer.
To delay the onset of degradation as long as possible,
both process stabilisers and long-term stabilisers are added to the polymer.
The most important primary anti-oxidants used for long-term stabilisation are the sterically hindered phenols.
these exhibit an undesirable side effect when exposed to ultraviolet radiation.
They develop deeply coloured molecular structures resulting in irreversible yellowing of the plastic.
To minimise this effect,
polymers are also provided with an ultra-violet stabilisation package.
This package is often made up of ultraviolet absorbers (UVA)
and hindered amine light stabilisers (HALS).
UVAs protect physically through molecular absorption of the high-energy fractions of sunlight.
The absorption transforms the molecule into an excited state,
and when it collapses back to its original state,
the absorbed energy is released as a thermal emission (heat).
HALS protect chemically by combining with oxygen when exposed to sunlight to form stable nitroxide radicals.
These trap the oxidising free radicals released when the polymer is exposed to ultraviolet radiation.
the stabilisers are either consumed or migrate to the surface of the polymer where they are abraded away.
The polymer has then lost its protection and deterioration proceeds rapidly when exposed to the factors mentioned above.
after-market protectorant products provide a coating to replace or supplement the original UVAs or HALS in the polymer.
reverse any yellowing which is the result of degradation of the sterically hindered phenols within the polymer itself.