Photofading and light stability of dyed and pigmented polymers

• Published in: Polymer degradation and stability Vol. 44; no. 3; pp. 357 - 374
• Main Author: Allen, N.S.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 1994
• ISSN: 0141-3910; 1873-2321
• DOI: 10.1016/0141-3910(94)90095-7

The photofading and photostability of dyed and pigmented polymers is a commercial problem involving a complex interplay of phenomena and mechanisms many of which remain unresolved. Dye fading, for example, involves reactions not only associated with the dye itself but also those involving the polymer as well as dye-polymer interactions and these, in turn, are further complicated by the nature of the environmental conditions. All these processes are described for a series of azo, anthraquinoid and triphenylmethane based chromophoric systems. Thus, in the case of dye structure the nature of the substituent, i.e. electron withdrawing or electron accepting group, will influence its photophysical and photochemical behaviour as will the position of substitution on the chromophore. In the latter case intramolecular hydrogen bonding is significant for anthraquinone chromophores. The interaction of the photoexcited dye chromophore with the polymer matrix is an additional process which will not only control the stability of the dye but that of the polymer as well. In a number of cases the dye may sensitise the photochemical breakdown of the polymer, a process often termed ‘phototendering’. This is more dominant in reductive polymer environments such as the nylons. Here, the complex role of reduction and oxidation reactions involving hydrogen (electron) abstraction and singlet oxygen reactions remain unresolved. In many cases these mechanisms are relevant to the photofading of dyes. The interactions of the dye and polymer matrix may also involve energy transfer and quenching with photoactive units or impurities giving rise to phototendering and/or stabilisation effects. The state of aggregation and concentration of the dye are also important parameters which are interrelated in controlling dye stability with the former being controlled by the nature of the polymer matrix. Pigments also play a major role in controlling the stability of a polymer matrix as well as that of a dye. In this regard titanium dioxide is important as a delustrant in textile fibres as well as its use in thermoplastic articles. In addition to these processes the stability of dyed and pigmented polymers is controlled by the nature of the atmosphere, e.g. presence of oxygen, the humidity which can often accentuate fading and degradation reactions, the light source and the temperature of the irradiation conditions. Many industrial developments in the dye and pigment worlds are hindered by such problems although some solutions are feasible in terms of effective stabilisation.