Stability to Light and Ultraviolet Rays

UV absorbers are added to provide protection against harmful UV radiation. The UV absorbers absorb UV light and release the excess energy as heat. Commonly used UV absorbers for PP are derivatives of benzophenone, benzotriazoles and esters. The use of hindered amines has also been reported. 

The efficiency of the UV absorbers to protect the part depends on the part thickness. Stabilisation in thick parts is more effective than the thin parts, films or sheets. 

Carbon black in a suitable concentration can make the article sufficiently protected for 10 years continuous outdoor exposure in temperate climates. The use of carbon black is clearly restricted in fibres and films. In addition, the heat stability of the carbon black modified grades may be poor in comparison to unmodified grades. 

Hindered amine light stabilisers function as free radical scavengers and can double as thermal antioxidants. They are, though costly, effective at very low concentrations. Hindered amines can interact with other additives (e.g., phenolic antioxidants, titanium dioxide) in the PP to produce yellowishness. Halogenated flame retardants can react with hindered amine light stabilisers and render them ineffective. 

The protection of PP against UV radiation and heat is a very complex issue. The idea of this section is not to go in depth into formulation issues which are best left for resin manufacturers/formulator or stabiliser suppliers. However, from the above discussion, it can be appreciated that  

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