WOOL FABRIC DYEING WITH ACID DYE AFTER UV IRRADIATION

The present work focused on the possibility of improving wool dyeing in terms of kinetics and final dye bath exhaustion, with good fastness of dyed materials, at lower temperatures than those used in conventional processes, by a simple pre-irradiation of wool with ultraviolet rays. Ecofriendly aspects of radiation processes are well known: energy savings (low-temperature process), low environmental impact with no VOC emissions, simple, cheap and small equipment, high treatment speed. The effect of different radiation times (30 s, 2, 5, 20 min) and lamp powers (providing about 50mW/cm2 or 900mW/cm2 radiation on the substrate) were investigated in order to optimize the process. First, the treated samples were characterized by SEM and FTIR-ATR analysis revealing a surface modification of wool fibers without affecting their morphology. UV light, coupled with the ozone formed in contact with air, oxidized mainly the cuticular region of wool fibre, increasing the OH external groups and forming cystine oxides and cysteic acid. As consequence, lower values of water contact angle were measured on treated fabrics with respect to untreated ones, denouncing an increased hydrophilicity. No effect was revealed by bursting test on mechanical properties of treated fabrics, confirming that the fibers modification is confined to the surface, not involving the bulk, while Datacolor measurements showed some yellowing, more pronounced for high power and longer times treated samples. For what concerns the dyeability, it was tested differently treated and untreated samples with an acid dye at 70°C, 80°C and 90°C. At each temperature, the radiated samples showed better results in terms of kinetics and final bath exhaustion reached. Of relevance was the comparison between the dyeing of the radiated sample at 80°C with the untreated sample at 90°C: in the first case higher final bath exhaustion and faster kinetics were found. It means the real possibility to reduce dyeing temperature and times. Datacolor measures on dyed samples confirmed a coloration more intense on radiated samples, in good agreement with bath exhaustion evaluations. Moreover fastness tests to dry and wet rubbing and to domestic laundering showed best results on radiated samples, confirming also an improved adhesion of dye molecules to wool fibers. Finally, also a pilling test was carried out with good results. UV exposure makes surface fibers weaker, so no anchor fibers are available to secure pills to the fabric surface. Hence it was found that the UV treatment can also prevent pilling in knitwear, without severe loss in weight and strength of the fabric. In conclusion, UV radiation can be indicated as a valid ecofriendly pretreatment to confer peculiar properties to wool fabrics. In particular treating wool for just 5 minutes with the low power or 30 s with higher power lamp, it can be dyed at 80°C, at least a 20°C lower temperature than conventional dyeing, with a fast kinetic, reaching the total bath exhaustion and good fastness.