Colour stripping of reactive‐dyed cotton by ozone treatment

Trials have been carried out to investigate the efficiency of ozone treatment in the colour stripping of reactive‐dyed cotton fabrics. The trials were performed on a specially designed apparatus to inject ozone gas into the liquor passing through a perforated beam with fabric rolls on it, just like a beam dyeing machine with ozone venturi injection. Conventional reductive colour stripping was applied as the control treatment, and ozone treatment was applied for three different application times (15, 30, and 45 min). Trials were performed with seven selected reactive dyes having various chromophores and reactive groups. The results indicate that 45 min ozone treatment yielded the best colour stripping results among the three application times. Furthermore, the colour stripping percentages of the 45 min ozone treatment were higher for four of the tested dyes, the same for one of the tested dyes, and lower for two of the tested dyes compared with the control treatment, which consisted of conventional reductive colour stripping. Colour stripping of 90% and above was achieved for all samples of the 45 min ozone treatment. The chemical oxygen demand values of the colour stripping baths were compared: the average value of the 45 min ozone application baths was 105 mg l−1, and it was 1993 mg l−1 for the conventional reductive treatment baths. Consequently, a reduction in chemical oxygen demand of almost 94% was achieved by ozone treatment compared with conventional reductive treatment. The strength values of the fabrics after the respective treatments were similar, with a difference of only 3%.