Photodegradation of Cotton Cellulose

Abstract

The relation between the fading and phototendering behaviour of Caledon Yellow GN, 5GK and 4G on cotton cellulose has been examined. A distinction is drawn between the reactions promoted by visible light and those promoted by ultraviolet radiation. Caledon Yellow 5GK is the most efficient phototenderer in visible light and production of the leuco dye accompanies this process. Phototendering by this mechanism can also be identified with Caledon Yellow GN, although to a much smaller extent than with 5GK, and production of the leuco form is accompanied by formation of free radicals. Regeneration of the original dye is best promoted by an atmosphere of moist oxygen. Excitation by short‐ultraviolet radiation leads to irreversible chemical changes in the dyes and degradation of the cellulose. The most efficient phototenderer in visible light (Caledon Yellow 5GK) markedly inhibits the degradation of cotton by ultraviolet radiation, whereas the poor phototenderers in visible light (Caledon Yellow 4G and GN) exert no such marked protection. The mechanisms of these reactions are discussed. Hydrogen abstraction by the excited non‐aggregated dye is the most probable path to the leuco dye and photodegradation in visible light, and the mechanism is thus comparable with the mechanisms elucidated for anthraquinone compounds in solution and on cellulose. Electronic‐energy transfer to the cellulose, in competition with the abstraction process, would be favoured during ultraviolet irradiation, and is the mechanism suggested for the degradation of cellulose under these conditions. The influence of dye aggregation on the reactions and the mechanism of the irreversible chemical changes in the dyes are also discussed.

Photochemical Behaviour of Vat Dyes I—Reaction with the Polymer Substrate

“…The phototropic behaviour of flavanthrone, which has been known for many years, has recently been studied extensively by Feichtmayr and Schlag (22) and by Fiebig (23). Their spectral results show that on irradiation the yellow dye is reduced to the green acid leuco form (AH2) of the dye (A), and not to the blue ionised alkaline leuco form (A2-) as appears to be suggested by Baugh et al (24). In the latter paper the use of the wavenumber scale as abscissa for the reflectance curves of flavanthrone in its reduced and oxidised states makes comparison with the spectrum of the irradiated dyeing inexact, since the interesting spectral changes are in the longer wavelength region, but the work reported by Fiebig (23) illustrates the clear and substantial difference of leuco flavanthrone in the alkaline and acid states.…”

“…Baugh e t al. (24), from a consideration of the behaviour of three vat dyes, found that the dye that promoted the greatest degradation with visible radiation was the most efficient desensitiser to short-wave ultraviolet radiation. On the basis of this finding, they suggested two competing processes for the degradation of the cellulose: (a) direct transfer of electronic energy from the excited dye system to the cellulose, which would lead to efficient degradation, as in direct photolysis in the undyed state, and (b) absorption of 253.7-nm radiation by the vat dye, leading to internal conversion from the X X * state to the n~* state allowing the latter state to participate in hydrogenabstraction reactions.…”