Kinetic study on the sunlight-induced degradation of acid azo dyes on silk

The effects of solar irradiation on the two azo dyes CI Acid Red 1 and CI Acid Orange 8 were investigated both on dyed silk and in aqueous solution at pH 2¯4.5. When absorbed on silk, the dyes were more photoreactive, both undergoing photodegradation, though at different rates, and displaying a protective action towards the fibre substrate, as evidenced by viscosimetric analysis. In aqueous solution CI Acid Orange 8 underwent photodegradation, with a quantum yield around 10-3 mol einstein-1, while the predominant reaction occurring from CI Acid Red 1 was photocyclization, especially at low pH.