UV Screeners and Absorbers

A simple approach to light stabilisation of a substrate is to incorporate therein some substance which will reflect or absorb incident light. It would therefore be thought that incorporation of pigments or dyes 

Some authors have deduced that dyes, especially aroylbenzimidazoles, provided they are suitably thermally stable [7, 8], and flnorescence brighteners such as Leucopor EGM [9] can photostabilise PET. Naphthol AS dye intermediates have been patented as UV screeners applicable in aromatic polyesters [10]. 

A number of classes of additives specifically for use as UV absorbers in polymer formulations are available including, among the better known ones, aromatic salicylates [11], o-hydroxybenzophenones [12], 2-hydroxyphenylbenzotriazoles [12,13], derivatives of cinnamic acid and related materials [14,15], aromatic oxanilides [16], cyclic imino ester derivatives known varionsly as benzoxazines or benzoxazones [17, 18] and hydroxyphenyltriazines [19]. 

Phenyl salicylates are one of the oldest UV stabilisers known. They are not particularly good stabilisers, but they rearrange on exposnre to UV radiation to form 2-hydroxybenzophenones, which may be regarded as the active species. The problem with these materials is that the conversion is not rapid or, in some cases, clean , which can lead to poor performance and some discoloration of the host polymer. These additives have been largely superseded by later developments. 

Cinnamic acid derivatives are said to operate via a mechanism in which the excited state dissipates energy by rotating about the double bond, which is weakened towards such a rotation in the excited state. There are also many vibrational enei y-dissipation possibilities with cinnamates which can allow safe loss of absorbed energy. Once again, the molecule should be capable of imdergoing several of these transformations. 

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The terminology used when referring to UV screeners and absorbers (UVAs) sometimes overlaps. UV screeners physically block, reflect, and scatter UV radiation while also absorbing it to some degree, while nonblocking organic UVAs efficiently absorb UV energy and convert it to harmless forms. 

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