Degradation :

Polypropylene is liable to chain degradation from exposure to heat and UV radiation such as that present in sunlight.
Oxidation usually appears at the tertiary carbon atom existing in every repeat unit. A free radical is formed here, and then reacts further with oxygen, followed by chain scission to yield aldehydes and carboxylic acids.
In external applications, it shows up as a network of fine cracks and crazes that become deeper and more severe with time of exposure.

For external applications, UV-absorbing additives must be used in application which using in external. Carbon black also offers some defense from UV attack.
The polymer could oxidize at high temperatures, a common problem during molding operations. Anti-oxidants are normally added to avoid polymer degradation.