UV absorption-mechanism

Ultraviolet absorbers were among the first organic stabilizers used. They are colorless compounds that strongly, but selectively absorb ultraviolet radiation and harmlessly dissipate it as heat so that it does not lead to photosensitization. They are also characterized by their very good stability to the absorbed radiation. However, based on the UV absorption mechanism alone, they can only provide limited protection to surface layers and thin samples, for example fibers and films. In accordance with the Beer-Lambert absorption law, the amount of radiation reaching any particular layer diminishes exponentially with the distance from the exposed surface. Thus, the effectiveness of protection via screening of the actinic radiation from the polymer by the UV absorber increases with sample thickness. Protection by UV absorbers is most effective when the additive is concentrated on the surface, such as when it is incorporated in a thin film coextruded over the polymer [75]. 

Scheme 18.17 UV absorption mechanism of hydroxybenzophenones, hydroxybenzotriazoles, and hydroxytriazines.

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