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A poor fastness of natural dyes in comparison with synthetic ones is established beyond question. The cause of natural dyes, that, they are not being used for textile goods at commercial scale is due to their poor fastness properties. To face such limitations, the photochemical behavior of natural dye i.e. Turmeric (Curcuma longa L.) and synthetic dyes were investigated. It has been demonstrated that uv-protective effect was strongly dependent on the absorption characteristics of dyes for ultra violet radiations (UVR).Therefore, the improvement of light fastness is sought at the present time. In this respect, various kinds of photo stabilizer (UV-absorbers) were synthesized and applied on to the substrate along with some naturally occurring to prolong the life of coloured species in the natural as well as synthetic dye stuff. At the end, the reactivity of uv-absorbers with the substrate and their properties as auxiliaries increasing UV-protection and dyeing properties were assessed by spectrophotometeric techniques. Significances of uv-absorbers with mordanting agent have been evaluated in different aspects to make the eco-friendly dyes into economical leading commercial dyes.
Dyeing was practiced in ancient times and has undergone many changes in reaching today's development. In the early age, textiles were dyed using plants and animal sources, but with the advent of synthetic dyes, the use of natural dyes in textiles was eliminated since synthetic dyes give variety of shade and colour (Deo and Desai 1999).
Recent studies show that synthetic dyes are non-biodegradable, allergic and some are carcinogenic. Many countries are reluctant to use, import and export the products that are not eco-friendly. Again the world trend is inclined towards natural dyes in textiles as well as in food processing (Sachan and Kapoor 2004).
Good quality dyes must satisfy many requirements in order to be used for colouring of textile materials, one of them is light fastness-an important property determining the fitness of the articles and the durability of their usage (Violeta et al. 1996). Most natural dyes have poor to moderate light fastness, while synthetic dyes represent the full range of light fastness properties from poor to excellent (Daniela and Vilarem 2006). The photochemical behavior of dyes and of the materials which are coloured by them is influenced not only by environmental factors such as light sources and their spectral distribution, temperature, humidity and atmospheric composition, but to a large extent by their chemical structure and the state of the dye in the polymer (Giles et al. 1975).
In last two decades one of the often realized research programmes in the field of textile materials was work on their protective properties against sunlight UV radiation. As it is known radiation of this type emitted by sun in the range between 100 and 400 nm is subdivided into UV-C (100-280 nm), UV-B (280-315 nm) and UV-A (315-400 nm). Higher layers of the atmosphere absorb the radiation of the UV-C range which is most dangerous to living species. Human skin has to be protected against excessive radiation of the UV-B type. It was proved that the excessive radiation of this range is the origin of premature skin ageing, sunburns, allergies and even skin cancer. The UV-A radiation is less dangerous but also its overdose can result in similar effects as described above (Wojciech et al. 2006).
Light fastness of dyed textiles is related to the chemical structure and physical structure of the fiber itself. It is attributed the fading on cellulose to an oxidative process, whereas on protein it have a reductive nature (Cumming et al. 1956). It is stated that indigo is much more light resistant on wool than on cotton (Padfield and Landi et al. 1996). An oxidative pathway is involved in the fading of indigo dyed cotton. As the fading on non-protein substrate is reductive, the indigoid chromophore which is resistant to photoreduction shows high fastness on wool (Roshan et al.1996).
During the photo degradation of azo dyes, an essential role is played also by two reactions (oxidation and reduction) leading to irreversible conversion of azo dyes into the azo group and destruction of the chromophore system. Which is the predominating reaction is determined by the condition of the irradiation and by the medium in which dye is distributed (Violeta et al.1998).
There are normally two ways to improve light fastness of the colour on fabric, to select the dyes with better light fastness and/or to use UV-absorbers to improve light fastness of the dyed samples (Yiqi and Naarani 2007)
UV absorbers are additives used to prevent the photo degradation of polymeric materials by UV-rich sunlight and artificial light. These additives absorb UV radiation and reemit it as fluorescent or infrared radiation. The energy of the excited molecule which causes photo degradation is released as thermal energy (Lappin, 1971). The UV absorbers must have the following characteristics:-
Recent studies show that synthetic dyes are non-biodegradable, allergic and some are carcinogenic. Many countries are reluctant to use, import and export the products that are not eco-friendly. On the other hand natural dyes don't have excellent fastness properties at all; especially turmeric a natural yellow colour produces poor washing and light fastness rating. Such problems resist using natural dyes as economical source at industrial level. Not even natural dyes but most of the reactive dyes have moderate to good fastness properties. To overcome such limitations of industrial dyes some additives were synthesized and applied along with some naturally occurring. These additives applied on the white fabric and also along with natural and synthetic dyes especially to protect the textile articles from solar ultraviolet radiations. In this regard, the induced UV absorbers and their reducing effect of photo degradation of color was subject of interest. Along with improvement in light fastness, some other physico-chemical properties like crock and washing fastness were also studied. The whole study project concluded that the applied UV absorbers significantly increased the light and wash fastness properties of all the natural and synthetic dyes but these are unable to enhance the crock (rubbing) fastness at appreciable extent. During study it was the keen result that synthetic as well as natural absorbers improve the exhaustion and fixation of all the dyes synergistically during dyeing. The significances of these UV absorbers are helpful to increase the fastness properties of industrially used dyes including reactive, direct and natural curcumin dyes.
So the result showed that the application of UV absorbers is very helpful to increase exhaustion, fixation and fastness properties of treated fabric with UV absorbers. Protection against the UVR is very helpful to avoid the skin and dyed article from damaging ultraviolet radiations, which is economically and hygienically handy in textiles.