Fading of Phenylazo–β–naphthol Dyes on Polypropylene*

The fading of a series of phenylazo–β–naphthol dyes on polypropylene by exposure to either long– or short–wavelength ultraviolet radiation and under nitrogen and oxygen atmospheres, respectively, is reported. The half–lives and relative rates of fading of the dyes were calculated at the wavelength of maximum absorption of the hydrazone band and the hydrazone–azo ratios of the dyes were determined on polypropylene and in ethanol. The relative rates of fading were plotted against the appropriate Hammett constants for the substituents. In general, a linear relation was observed. The graphs had positive slopes, but negative slopes were also obtained with the dyes bearing electropositive groups. Graphs of relative rates of fading against hydrazone˜azo ratio were also drawn. The results were generally similar to those obtained with the plots of Hammett constants, but correlation was better. The results indicate that, on polypropylene, the most important fading mechanism for the hydrazone form of the dye is reductive, although there is evidence that oxidation may occur, especially in the presence of oxygen.