The photofading of phenylazo‐aniline, ‐pyridone and ‐quinolone disperse dyes on nylon fabric was analysed using the Kubelka–Munk (K/S) spectra of fabrics exposed to a carbon arc in air. The exposure of dyed fabric through ultraviolet and coloured filters, which do not shield the main absorption band, showed a large decrease in the initial rate (KPA) of fading. Compared with the KPA values without filters, the values using filters were half as large for dyes without the nitro groups and a quarter as large for dyes with the nitro groups. The KPA values may be qualitatively explained by the sum of azo scission (decrease of K/S value at λmax) and the conversion of nitro groups to nitroso groups. These phenomena occur, respectively, via thermal disproportionation reactions between hydrazinyl radicals (from the azo group) and the reaction between hydrazinyl and N‐centred nitrosyl hydroxide radicals (from the nitro group). The azo scission is promoted by N‐centred nitrosyl hydroxide radicals via the latter reactions.

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The initial rates of photofading for 30 disperse azo dyes on nylon fabric upon exposure to a carbon arc in air have been analysed by formulating a kinetic equation that describes azo scission via the disproportionation reactions and intramolecular H‐transfer by two kinds of hydrazinyl radicals and the conversion of a nitro group to a nitroso group via the disproportionation reaction of nitrosyl hydroxide radicals. The five reaction rate constants are discussed in terms of the heats of reaction by calculating the heats of formation for the reactants, intermediates and products of each reaction using the PM5 method. Phenylazo‐ and thiazoleazo‐N,N‐substituted anilines and phenylazo‐pyridones exhibited large rate constants of multiple terms, while phenylazo‐phenols had the highest light fastness and very small rate constants for the disproportionation reactions of hydrazinyl radicals. Photofading on nylon fabric was primarily controlled by the thermal reactions of photo‐induced monohydrogenated dye radicals, which occurred via one or two primary multiple routes.

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