Light Fastness of Textiles – Factors Affecting and Control Measures
K. Senthil Kumar
Modern Testing Services India Private Limited
Light fastness of textiles is the resistance of dyes and pigments to the effect of sun heat and light energy. Sun electromagnetic spectrum consists of Gamma to Radio waves. Out of that the radiation ranging from UV to IR reaches the earth surface. Particularly UV rays are having high energy which accelerates the fading of dye. UV rays can be divided into UV-A, UV-B and UV-C. UV-A have long wave length of 320-400nm which is Light fastness is the resistance of dyes & pigments to the effect of sun heat & light energy. Sun electromagnetic spectrum consists of Gamma to Radio waves. Out of that the radiation ranging from UV to IR reaches the earth surface. Particularly UV rays are having high energy which accelerates the fading of dye. UV rays can be divided into UV-A, UV-B and UV-C. UV-A have long wave length of 320-400nm which is not absorbed by the atmospheric ozone, UV-B have medium wavelength of 280-320nm which is partly absorbed & UV-C have short wavelength of 100-280nm which is completely absorbed. Since UV-A & UV-B is not completely absorbed by ozone in the atmosphere, it reaches the earth surface with photon energy of 315-400 (kj/mol) which exceeds the carbon to carbon single bond energy of 335 (kj/mol). Thus results in the fading of dyes & pigments not absorbed by the atmospheric ozone, UV-B have medium wavelength of 280-320nm which is partly absorbed & UV-C have short wavelength of 100-280nm which is completely absorbed. Since UV-A & UV-B is not completely absorbed by ozone in the atmosphere, it reaches the earth surface with photon energy of 315-400 (kj/mol) which exceeds the carbon to carbon single bond energy of 335 (kj/mol). Thus results in the fading of dyes & pigments.
2. MECHANISM OF FADING
When UV light falls on dyes & pigments the UV photons with high energy excites the electrons of dye from ground state to exited state. The dye at the exited state is highly reactive and unstable so it comes back to the original ground state. During the quenching of dyes from exited state to ground state, atmospheric triplet oxygen reacts with dyes to form singlet oxygen. Under the presence of visible light the exited dye molecules react with the atmospheric oxygen to form super oxide radical. The singlet oxygen & super oxide radical formed is highly reactive and capable of destroying the dyes. The exited dye molecules undergo various processes which result in fading. The processes are:
When high energy photons of UV light falls on the dyes with suitable bond dissociation energy, homolytic cleavage occurs in the chromophoric system of dye which results in fading.
The exited dye molecule undergoes oxidation process in the presence of oxygen. During the process, the chromophoric system of the dye reacts with the oxygen to form nonchromophoric system which results in fading. The dye which has the carbonyl group as chromophore can easily undergoes oxidation to form non carbonyl chromophore.
The dye molecule with unsaturated double bond as a chromophore undergoes reduction in the presence of hydrogen to form saturated chromophoric system. Due to saturation the length of the chromophoric system decreases which results in fading.
When cellulosic dyed material is exposed to sunlight the dyes will abstract hydrogen from the cellulose in which photo reduction of cellulosic substrate occurs. At the same time dyes will undergo oxidation in the presence of atmospheric oxygen by which photo oxidation of dye occurs. During these processes, both the fading of dyes and the strength loss of substrate occurs. In this case both the dye and the material are responsible for the fading.
Under the presence of UV light the material supplies hydrogen to the dyes which causes reduction. As hydrogen is abstracted from the material it undergoes oxidation. In this case, the material is the only responsible for the fading of dyes.
3. FACTORS AFFECTING THE LIGHT FASTNESS OF TEXTILES
The factors affecting the light fastness of textiles are as follows:
Selection of dyes
The selection of dyes plays a vital role in determining the fastness to light. The Natural dyes are least resistant to light; reactive dyes show moderate resistance, whereas the Vat dyes show good resistance to light. Dye with larger particle size shows good resistance to fading eg: Indigo dyes. Dyes with good aggregation property show good resistance to UV light. eg: Vat Dyes.
The below shows the dyes with increasing order of light fastness:
Natural Dyes > Reactive Dyes > Vat Dyes > Indigo Dyes
Metal complex dyes show better resistance to fading than acid dyes since the metal can absorb UV energy and transforms into heat energy.
Brighter shades of dye show poor resistance to light. The electron mobility of brighter shade dyes is high and so the electrons can easily move to the exited state causing breakage in the chromophoric system of the dye which results in fading.
Depth of shade
Darker shades of dye show good resistance to light, whereas the lighter shades show poor resistance to light. Due to the presence of large number of dye molecules in dark shades, only very few dye molecules involve in fading and rest of the dye helps in maintaining the depth of shade. But due to the presence of less dye molecules in lighter shades, the dye molecules affected by light is more so the shade becomes duller.
Amount of unfixed dyes
In cotton materials the reactive dye form covalent bond with the cellulose of the fiber. The dye particles which are properly fixed to the fiber show good resistance to UV light whereas the dyes which are unfixed to the fiber show poor resistance. In the synthetic fibers like polyester and polyamide the unfixed dyes are more susceptive to UV light.
Light fastness of the same dye differs when it is dyed with two different fibers. So it is necessary to know the importance of fiber characteristics. Disperse dyed polyester fiber shows better resistance than disperse dyed polyamide fibers. Each fiber reacts differently under the action of UV light. Polyester and Polyamide shows good resistance to UV light since the de lustering agent (Tio2) present in the fiber absorbs UV light. Wool is also a good absorber of UV light. But cotton and silk shows poor resistance to UV light as it pass through the fiber and undergoes photo degradation process like photo tendering, photo oxidation etc.
After treatment with chemical auxiliary
The residual substances like size and other chemical auxiliary like thickeners affects the light fastness. OBA used as a brightening agent shows poor resistance to UV as it is easily damaged by UV light. Formaldehyde containing cationic fixing agents shows poor resistance to light.
Moisture content and heat are the two main environmental factors that affect the light fastness. Under the presence of UV light the moisture reacts with atmospheric oxygen to form hydrogen peroxide and other active radical. These active agents formed will degrade the fiber. Fibers show poor resistance to light under high moisture and heat.
4. CONTROL MEASURES
The light fastness of dyes can be improved by either reducing the effect of UV light on dyes or making the dye more resistant to UV light.
Interaction with Light
UV absorbers are aromatic organic compounds that are used to reduce the effect of UV light on dyes. The commonly used UV absorbers are benzophenone, benzotriazole, Phenyl salicylate etc…
Working of UV Absorbers
UV absorbers absorb UV energy and transform it to vibration energy without breaking of bond in its structure.
The next step is the transformation of vibration energy to the surrounding material in the form of heat.
Benzotriazole on absorbing UV light gets converted into structure II and it reverts to the original structure with the loss of thermal energy.
Interaction with dye Stuff
Anti-oxidants are organic substances that are added to minimize the photo oxidation of textile materials. The commonly used anti-oxidants are gallic acid, cafeic acid, ascorbic acid, hindered phenols and amines.
These agents work either by trapping the radicals or by decomposing the peroxides. It absorbs free singlet oxygen formed during the quenching of exited dyes.
Fastness to light plays a vital role in maintaining the aesthetic value of garments during its usage. Even the use of UV absorbers and antioxidants can improve the light fastness only up to 1 grade. Hence care should be taken care while
- Selection of dye like Natural or direct or Vat or reactive or indigo etc.
- Selection of Depth of the shade either dark shade or light shade.
- Maintaining proper process parameters during dyeing.
- Removal of unfixed dyes during washing
- Use of suitable chemical & auxiliary during finishing.
- Drying and storage during the actual use.
- Thiagarajan P and Nalankilli G, “Improving light fastness of reactive dyed cotton fabric with antioxidant and UV absorbers”, Indian Journal of Fibre & Textile Research, Vol.38, pp.161-164
- Schindler W D, Hauser P J, “Chemical Finishing of Textiles”, pp.149-153
- Schindler W D, Hauser P J, “Chemical Finishing of Textiles”, pp. 157-163
- Agnes Timar-Balazsy and Dinah Eastop, “Chemical principles of textile conservation”, pp.88-92
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It’s me, a Textile Consultant, Blogger & Entrepreneur. I’m working as a textile consultant in several local and international companies. I’m also a contributor of Wikipedia.