Light Fastness of Dyes

Light Fastness of Dyes

Ronak Raj Gupta
MLV Textile and Engineering College, Bhilwara, India
Email: ronakgupta135@gmail.com

 

Introduction:
Light fastness refers to a dyed textile’s resistance to fading or color change when exposed to light. Fastness to light is one of the most important properties a dyed fiber in order to fulfill its function over a period of time.

The fading involves the change in color’s characteristics like purity, brightness and hue. Sunlight has wave-particle duality, and sunlight that transmits energy in the form of photons has a strong impact on the molecular structure of dyes. 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. For phthalocyanine dyes that have reached 8 grades of light fastness, adding appropriate metal ions in the dyeing and finishing process to form complexes inside the dye molecules can significantly improve the brightness and light fastness of the dyes. In order to improve the light fastness of the dye, dye manufacturers have taken many measures.

Increasing the relative molecular weight of the dye, increasing the chance of internal complexation of the dye, increasing the coplanarity of the dye and the length of the conjugated system can relatively improve the light fastness of the dye.

The effect of solar radiation falling on a dyed textile are manifested in two principle ways: (a) a change in the shade and/or depth of shade of the coloration, and (b) the physical degradation of the substrate fiber as evidenced by changes in tensile strength, abrasion resistance, etc. When cellulose is exposed to light, degradation takes place very gradually, and the change can be followed by the loss of tensile strength and increase in (a) reducing power (copper number), (b) fluidity of cuprammonium solutions, (c) alkali solubility, and (d) methylene blue absorption. After prolonged exposure, the filters become completely brittle and fall to a powder. The formation of oxycellulose and humic substances is accompanied by the evolution of carbon monoxide and dioxide. However the changes in the dye component usually occur in a much shorter time, therefore, the problem of dye fading is of greater significance than fiber degradation.

Mechanism of Fading:
The absorption of one quantum of light of 400nm increases the energy of molecule by about 71 kcal. The bond energies of common bonds are:-

C-C 58.6; C-N 48.6; C-O 70; C=C 100; C=O 147; N=N 80 kcal per mole

By this, it is clear that energy is adequate to rupture bonds of organic molecules. When the basic structure of the chromophore in the dye structure is destroyed by photons, the color of the light emitted by the dye chromophore will change.

By absorption of light, a molecule rises from its ground state of lowest energy to an excited state of higher energy. The life of an excited molecule is exceedingly small: about 10-7 sec. for a permitted transition. The activated molecule may emit radiation in the form of fluorescence or phosphorescence; or lose its energy as heat by collision with other molecules; or dissociate or take part in chemical reaction. The primary photochemical change may be followed by secondary thermal reaction, which again may take place as successive or chain reactions or branch out into several side reactions. The exited dye molecules undergo various processes which result in fading. The processes are:

• Photolysis: 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.
• Photo tendering: The photochemical degradation of cellulose in presence of active vat dyes is a photosensitized process in the sense that energy is transferred from the spot at which light is absorbed to another at which a chemical reaction takes place. The range of photo tendering activity of dyes varies from active to protective on the idea that the dye may accelerate the photochemical decomposition of fiber or it may act as a protective agent. Yellow and orange vat dyes are most active tenderers while blue, green and black are inactive. 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.
• Photo reduction: 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.
• Photo sensitization: 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.
• Photo oxidation: 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.

According to the Einstein law of photochemical equivalence each molecule taking part in a photochemical reaction absorbs one quantum of radiant energy. The quantum yield or efficiency Φ (number of molecules changed/number of quanta absorbed) should be unity if Einstein law is obeyed: it varies, however, from a minute faction (as for dyes) to millions (as for some chain reactions).

Considering the many different chemical types to which dyes belong, it is clear that no single mechanism can explain the fading of dyes as a class.

The fading of dyes might involve three photochemical reactions:- oxidation, reduction and decomposition.

Irradiation of cellulose by ultraviolet light in a nitrogen atmosphere causes considerable degradation. Haller and Ziersch had found thiourea and a mixture of glucose and sodium metaphosphate to protect a number of dyes on cotton from fading. Thiourea and its derivatives, dimethylolurea and other amides can protect dyed rayon form light and gas fading. However, except for coppering of certain direct cotton dyes, no treatment has proved to be of practical value for a substantial increase in the light fastness of fugitive dyes. They noticed that fading is accelerated by oxidizing agents and a variety of organic substances like glycerol, turkey red oil, starch and gums.

Factors Affecting Fading of Dyed Textiles:

1. Selection of dye & its photostability: 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. In general, the chromophore and its substituents is most important in determining the light fastness of 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.
2. The concentration of dye: The light fastness of a dyed fiber usually increase with increasing dye concentration. 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. 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 fibres like polyester and polyamide the unfixed dyes are more susceptible to UV light.
3. The source of light: Not all absorbed wavelengths in the visible and ultraviolet are equally effective in initiating a fading process. Fugitive dyes are faded mainly by visible radiation, while dyes of high light fastness are faded mainly by violet and ultraviolet radiation.
4. The atmosphere: The moisture content of the atmosphere is known to have a pronounced effect on the fading rates of certain dyes, for example wool fades faster in high humidity. 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. Atmospheric contaminants like sulphur dioxide, oxides of nitrogen, hydrogen sulphides, and ozone, are known to react with dyes in absence of light. However, these reactions may accelerate when the dye is in an elctronically excited state
5. The nature of fiber: 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 fibre shows better resistance than disperse dyed polyamide fibers. Each fiber reacts differently under the action of 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. Three kinds of effects appear to be important: (a) the reactivity of a dye in an excited state with certain chemical groups within the fiber, as phototendering of cotton by some vat dyes: (b) the stabilization of a dye by the quenching of the excited state through the agency of some group within the fiber; and (c) the permeability of the fiber to various components of the atmosphere that are capable of reaction with the dye in its excited state.
6. Others: The light fastness of a dyed textile may also depend on the acidity or the alkanity of the fiber after dyeing. The light fastness of cotton fabric is best near neutral. Fastness of mordant depend upon mordant and mordanting methods. 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 light. Formaldehyde containing cationic fixing agents shows poor resistance to light. Also, soft finishing of cotton fabrics with cationic softener will reduce the light fastness of reactive dyes, mainly because the softener will turn yellow after being exposed to the sun, so that the color of the fabric will also change. Generally, the washing fastness of fabrics treated with cationic low- molecular or polyamine fixatives is in the 4-5 grade, but the light fastness of fabrics fixed by such fixatives has decreased.

Standards for Light Fastness Testing of Textiles:

• ISO 105 B01: Color fastness test for textile – color fastness to light: daylight. This standard applies to all types of textiles.
• ISO 105 B02: Color fastness to artificial light: Xenon arc fading lamp test. This standard specifies a method intended for determining the resistance of the color of textiles of all kinds and in all forms to the action of an artificial light source representative of natural daylight (D65). This method is also applicable to white (bleached or optically brightened) textiles.
• ISO 105 B03: Color fastness test for textile – color fastness to weathering: outdoor exposure. This standard specifies a method intended for determining the resistance of the color of textiles of all kinds except loose fibers to the action of weather as determined by outdoor exposure.
• ISO 105 B04: Color fastness test for textile – color fastness to artificial light: Xenon arc fading lamp test. This standard specifies a method intended for determining the resistance of the color of textile of all kinds, except loose fibers, to the action of weather as determined by exposure to simulated weathering conditions in a cabinet equipped with a xenon arc lamp.
• ISO 105 B05: Color fastness test for textile – detection and assessment of photochromism. This standard specifies a method intended for detecting and assessing change in color, after brief exposure to light, of colored textiles which change in color on exposure to light but which virtually return to their original shade when stored in the dark.
• ISO 105 B06: Color fastness test for textile – color fastness and ageing to artificial light at high temperatures: xenon arc fading lamp test.
• ISO 105 B07: Color fastness test for textile – color fastness to light of textiles wetted with artificial perspiration. This standard sepcifies a method for determining the resistance of the color of textiles, of all kinds and in all forms, to the combined effect of wetting with acid or alkaline artificial perspiration solutions and an artificial light source representing natural daylight (D65).

Test Methods:
There are a large number of different light fastness tests available on the market like 1—Enclosed Carbon-Arc Lamp, Continuous Light

2—Enclosed Carbon-Arc Lamp, Alternate Light and Dark 3—Xenon-Arc Lamp, Continuous Light, Black Panel Option 4—Xenon-Arc Lamp, Alternate Light and Dark

5—Xenon-Arc Lamp, Continuous Light, Black Standard Option 6—Daylight Behind Glass

Each has its advantages and disadvantages. The most commonly used are the xenon arc and MBTF lamp; however, carbon arc and natural sunlight are also used.

Earlier fadeometer (a carbon arc lamp) was used to test light fastness. For accelerated ageing tests, exposure in a fadeometer at 63oC + 3oC and relative humidity of 45 + 15% is given.

Fifty hours exposure in a fadeometer under these conditions to six months daylight exposure behind glass.

Nowadays, IS:2454:1967 for artificial light and IS:686:1957 for day light is used. It requires the test sample to be exposed to light alongside a set of selected blue dyes on wool, rated 1 (poor) to 8 (excellent). These dyes are such selected that each dye fades at approx. twice the rate of dye that is a number higher in set. In day light test sample size of 1×6 cm is taken and sample is placed at some angle from horizontal. Exposure time is between 8-24 hours. In xenon light source, color temperature of 5500-6500K is used. Filter should be placed between light source and sample to ensure that its spectrum (wavelength) closely matches the spectrum of natural daylight coming through the glass. The humidity and temperature of the test atmosphere are controlled. Two exposure times are used to determine whether fading progresses steadily or initially at a different rate from the longer-term exposure. Assessment can be done visually or by spectroscopy. . For white textiles also the color fastness is assessed by comparing with a blue wool sample.

The eight dyes according to their ratings are:

8 – C.I. 2nd edition solubilised vat blue 8
7 – C.I. 2nd edition solubilised vat blue 5
6 – C.I. 2nd edition solubilised acid blue 23
5 – C.I. 2nd edition solubilised acid blue 47
4 – C.I. 2nd edition solubilised acid blue 121
3 – C.I. 2nd edition solubilised acid blue 83
2 – C.I. 2nd edition solubilised acid blue 109
1 – C.I. 2nd edition solubilised acid blue 104

ISO 105 B02
The following is the basic procedure for the Xenon arc lamp light fastness test method using ISO 105 B02.

1. Preparation of test materials and light fastness apparatus:
The fading properties of the two sets of blue wool references may be different and therefore the results from the two sets of references are not interchangeable. Opaque material is taken for covering specimens and parts of blue wool references, such as thin aluminium sheets or cardboard covered with aluminium foil. Either a black-panel thermometer (BPT) or black- standard thermometer (BST) is used for temperature sense. A radiometer can be used to control the uniformity of the exposure as the irradiance of the specimen surface is related to the intensity of the light and the distance from the light to the specimen.

2. Adjustment of the humidity:
Expose the humidity control specimens (partially covered) to the light at the same time as the blue wool references until the color difference between the exposed and unexposed parts of the humidity control specimens reached level 4 on the grey scale. During this time, the light fastness of the humidity control specimens is assessed and checked daily and, if necessary, adjust the controller to maintain the specified temperature and humidity.

3. Exposure to xenon arc lamp:
Under predetermined conditions, expose the specimens and the blue wool references to the xenon arc lamp at the same time. There are five methods of exposure, as described in ISO 105 B02:

• Method 1: The exposure time is controlled by inspection of the specimen and therefore a set of blue wool reference is required for each specimen.
• Method 2: The exposure time is controlled by checking the blue wool references, so a large number of specimens can be tested simultaneously with just one set of blue wool references.
• Method 3: Exposing the specimen to the xenon arc lamp with two blue wool references is used to check that the specimen conforms to a performance specification.
• Method 4: The specimen and the reference sample are exposed to the xenon arc lamp together to test whether the specimen is consistent with the reference sample.
• Method 5: The specimen can be exposed to the xenon light alone or together with the blue wool references until the required irradiation is reached, for checking compliance with the approved irradiation energy values.

4. Light fastness rating:
Assess the light fastness rating of the specimens according to the standard ISO 105 B02, writing a test report, and recording the relevant test conditions and test data

Grade	Degree of Fading	Light Fastness Type
8	No fading	Outstanding
7	Very slight fading	Excellent
6	Slight fading	Very good
5	Moderate fading	Good
4	Appreciable fading	Moderate
3	Significant fading	Fair
2	Extensive fading	Poor
1	Very extensive fading	Very poor

Light Fastness of Common Dyes:

• Basic = Poor, excellent on acrylic
• Acid = Generally very good, range poor to excellent
• Acid premetalised = Good to excellent
• Neutral premetalised = Very good to excellent
• Mordant =  Good to excellent
• Substantive = Good to excellent
• Azoic = Good to excellent
• Disperse = Fair to excellent
• Sulphur = Poor to fair for yellows and browns, good to excellent in darker shades
• Vat = Generally excellent
• Reactive = Good to very good, poor to moderate on nylon
• Pigment = Very good to excellent

Improvement Measures:

1. Choice of dye: Firstly, dyes should be selected according to fiber properties and textile applications. For cellulose fiber textiles, dyes with better oxidation resistance should be selected; for protein fibres, dyes with better reduction resistance or containing weak oxidative additives should be selected; for other fibers, dyes should be selected according to the effect on fading. In order to enhance the photo-oxidative stability of the azo group in the dye molecular structure, some strong electron withdrawing groups are usually introduced in the ortho position of the azo group during the dye synthesis process, thereby reducing the electron cloud density of the azo nitrogen atom. In addition, hydroxyl groups can also be introduced into the two vicinal positions of the azo group to use its coordination ability to complex with heavy metals, thereby reducing the electron cloud density of the hydrogen atom of the azo group, and shielding the azo group.

Secondly, dyes with good light resistance stability and compatibility should be selected for color matching. Different dyes have different fading properties and even different photofading mechanisms. Sometimes, the presence of one dye can sensitize the fading of another dye. Reasonable control of the dyeing process, to fully combine the dyes with the fibers, and to avoid the hydrolyzed dyes and unfixed dyes remaining on the fibers is an important way to obtain higher color fastness to light.

2. Improvement of soaping process: In the dyeing process, the amount of hydrolyzed dyes and floating color should be minimized by complete soaping and washing to improve the light fastness of the fabric; at the same time, it should also improve the color fastness to washing, perspiration, rubbing, etc.

3. Selection of fixing agent and softener: Most fixatives like quaternary ammonium salts, sulfonium salts or phosphorus salts form lakes on the fibers. Although the washing fastness is very good, it often reduces the original light fastness of the dyes. Some cationic softeners and amino-modified silicone softeners have disadvantages such as yellowing, discoloration of dyes, and inhibition of fluorescent whitening agents.

4. UV absorbers and light fastness enhancers: These are aromatic organic compounds and can directly absorb the ultraviolet rays irradiated on the fabric and prevent the dyes from being damaged by photo-oxidation. The commonly used UV absorbers are benzophenone, benzotriazole, phenyl salicylate etc.

Working of UV absorbers

Step 1: UV absorbers absorb UV energy and transform it to vibration energy without breaking of bond in its structure.

Step 2: 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.

5. 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.

Brightening Agents:
When sunlight falls on a white fabric with yellowish tint, the colored part absorbs rays of its complementary color, which is blue. The amount of reflected light is therefore, decreased by the amount of light absorbed and the cloth appears yellowish. Optical brightening agents are blueing agent like ultramarine blue which increases the whiteness of white fabric. It absorbs some yellow light from incident light and therefore the reflected light is deficient in yellow, so a white effect is produced.

Fluorescent brightening agents do not absorb light in the visible region, but absorb UV rays from the incident light and re-emit them within visible spectrum. A surface containing fluorescent compound can emit more than the total amount of daylight which falls on it, producing a brilliant white effect. But this effect is only operative in presence of UV, so not in artificial light. In most cases the light and washing fastness of these are not good. Some examples are Tinopal, Blancofhor, Uvitex, Leucophor etc.

Both of these agents are not useful for dyed material because they flatten the shade.

References:

1. The chemistry of synthetic dyes by K. Venkataraman Volume II
2. The chemistry of synthetic dyes by K. Venkataraman Volume VIII
3. Chemical processing of textiles by NCUTE
4. Dyeing and chemical technology of textile fibers by E. R. Trotman
5. https://www.utstesters.com/blog/how-to-improve-the-light-fastness-of-textiles-_b161
6. https://www.testextextile.com/what-is-the-light-fastness-test-for- textiles/
7. https://textilelearner.net/light-fastness-of-textiles-factors- affecting/
8. https://textilelearner.net/ color-fastness-to- light/