Study on photofading of two natural dyes sodium copper chlorophyllin and gardenia yellow on cotton

The light fastness of fabrics dyed with natural dyes is not satisfactory, which limits their applications in the textile industry. So far, the detailed photofading mechanism of natural dyes remains unclear. To improve the light fastness of cotton fabrics dyed with two natural dyes (sodium copper chlorophyllin and gardenia yellow), two additives ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) and sodium citrate were used to treat the dyed cotton fabrics via the dipping-padding method. The performance results showed that both the two additives had good effects on inhibiting the photofading of the dyed fabrics. The photofading mechanism of the two natural dyes was also investigated. The results from 1,1-Diphenyl-2-picrylhydrazyl free radical (DPPH) scavenging assay and cyclic voltammetry tests demonstrated very limited antioxidant properties of the two dyes. Singlet oxygen and hydroxyl radical were detected in the photosensitizing process of the two dyes by electron paramagnetic resonance method. The photo-induced chemiluminescence (PICL) tests confirmed that the color changes of the dyed cotton fabrics were reduced because the reactivities of the two dyes to react with reactive oxygen species were dramatically reduced in the presence of EDTA-2Na and sodium citrate. End products generated in the photofading of the two dyes were detected by high-performance liquid chromatography-mass spectrometry (HPLC-MS). Based on the comprehensive analysis of above results, possible degradation pathways of the two dyes were proposed. The mechanism study in this research may help shed light on proposing universal solutions to the poor light fastness of most natural dyes on cotton.