Photochemical stability of cellulose textile surfaces painted with some reactive azo-triazine dyes
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- Rosu, L., Rosu, D., Gavat, C. et al. J Mater Sci (2014) 49: 4469. doi:10.1007/s10853-014-8144-0
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The influence of ultraviolet irradiation of different doses (λ > 300 nm) on the structural and color modifications of cotton fabrics painted with four different azo-triazine dyes (Reactive Yellow 143, Reactive Orange 13, Reactive Red 183, and Reactive Red 2) was studied. High irradiation doses up to 3500 J cm−2 led to changes in the dyes structures. Structural changes before and after the complete irradiation were compared by applying FTIR, UV–Vis, and near infrared chemical imaging techniques. Color modifications were also investigated. Color differences significantly increased with the irradiation dose for all the studied samples.
Although the skin protection against the ultraviolet (UV) radiation provided by textile fabrics has been a topic for some years, there still exists a lack of knowledge on the subject. It is wrongly believed that any textile article would be appropriate for UV protection. Natural fabrics such as cotton, silk, and wool provide a low skin protection due to the low absorption degree of UV radiation . In fact, the protection provided by a garmet depends on fiber type, structural characteristics, color and dyeing intensity, and the presence of optical brightening agents or UV absorbers [2–8]. Most of the textiles remain colored throughout inside and outside and are exposed to varying doses of sunlight. Solar lights intensity is an important parameter of photochemical processes . The color perception of dyed textiles is explained by the selective absorption at a certain wavelength of the incident visible light by the dyestuff molecules of the material surface. The unabsorbed wavelengths are reflected by the material leading to its coloration. Besides the aesthetical function, the color of the fabric is one of the most important variables on the protection against UV radiation . There are several characteristics, such as chemical structures of the dyestuff and the shade and intensity of the color that may influence the improvement of UV protection. Dark colors have been reported as providing better protection than light colors [4, 6, 11, 12]. UV radiation with 300 ≤ λ ≤ 400 nm, representing up to 5 % of the total incident sunlight on the earth surface, has the highest quantum energy [1, 13]. This energy may affect the structure of fabric substrate, the structure of dye molecules, and the chemical bonding between the dye and the fabric support. The reactive azo-triazine dyes are among the most common classes of substances used for coloring textiles. During the dyeing process, the reactive dyes are covalently bonded to the textile . The chemical structures of these dyes contain one or more azo bonds (R-N = N-R) that provide visible light absorption . A wide range of chromophores with excellent fastness can be selected including various triazine derivatives . Unlike direct dyes, which create weak hydrogen bonding with the cellulose substrate, the reactive dyes can also be attached to the fiber by covalent bonding between the reactive dye group, such is a halo-heterocycle (i.e. halo-triazine) or an activated double bond, and nucleophilic fiber group, thus exhibiting a greater light stability and fast washing characteristics [15, 16]. Halo-triazine structures are among the most common reactive groups able to react with –OH or –NH2 groups belonging to cellulose or proteic fibers. Therefore, dyeing with reactive dyes has become the most common method for coloring of some cellulosic textile materials such as cotton . Because of its simplicity, the method is successfully used both in industry and at home or in the art studio. Usually, the colored fabrics remain in contact with the human skin for several hours every day . It is known that the azo dyes can undergo some biochemical changes leading to colorless compounds with aromatic amine structure, which are easily absorbed by the skin . An important role in the chemical transformations of dyes molecules which come in contact with the skin surface is played by bacteria. Although some of the metabolites show low toxicity compared with the original compounds , some papers report that the aryl amines structures and the free radicals are potentially carcinogenic [19–21]. It is also known that the dye base triazines are the major sources of pollution to the environment. The preliminary pathological studies on rats have shown that some reactive dyes with azo-triazine structures have a high toxic potential (ReactiveYellow-143 and Reactive Orange-13) while others presented moderately toxic characteristics (Reactive Red-2 and Reactive Red-183) . Although studies on the photostability of reactive dyes in solution were reported in the literature [14, 23, 24], the influence of UV radiation on textiles painted with azo-triazine dyes is less known. Since the stability of the fabric-dye complex may affect human health, the aim of this paper consists in the assessment of the photodecomposition behavior of the dyed textile under the action of UV radiation with λ > 300 nm.
Materials and methods
The characteristics of the reactive azo-triazine dyes
Reactive Yellow 143 (RY-143)/Hutsman Textile effects
217, 265, 422
Reactive Orange 13 (RO-13)/Hutsman Textile effects
226, 284, 488
Reactive Red 183 (RR-183)/ Hutsman Textile effects
227, 280, 503
Reactive Red 2 (RR-2)/ PRO Chemical & Dye
217, 289, 544
According to the literature data, the peaks with the maximum absorbtion signals values of ~280 and ~230 nm correspond to triazine and to benzene entities . Also, the signals ranging between 400 and 600 nm are specific to the conjugated electronic system which characterizes the chromophores. The cleavage of the cromophoric entities under UV irradiation leads to the decoloration of the dyes structures  and of the dyed fabrics, as a consequence. The accumulation of chromophores resulted from photo-oxidation processes of dye and/or textile support leads to darkening of irradiated samples. A fabric, manufactured mainly of alkaline cleaned and bleached cotton fibers, was obtained from a commercial source (IASITEX S.A. Iasi, Romania). Fabric dyeing was achieved by fleet depletion technique in alkaline media (pH 9–10) with computerized plant Mathias Polycolor Uniprogramer 2002 type (Werner Mathias AG, Switzerland). Aqueos solutions containing 5 % dye were used to paint the fabric. The temperature in the dyeing bath increased from 21 to 80 °C with a heating rate of 5 °C min−1. Rinsing with hot water, cold water, and finally, with distilled water was used to remove the traces of unreacted dye.
49-cm2-dyed fabric was irradiated with a medium pressure OSRAM HQE-40 lamp, as an artificial light source, in the field range of 240–400 nm. The light source power was 100 W. Irradiation of fabric samples was performed in air by using a rotating hexagonal prism shaped device, as sample carrier, with the light source positioned on the central ax of the prism. Samples were protected from thermal degradation with a distilled water filter and a fan. The more energetic radiations with λ < 300 nm, not present in the natural light spectrum, were eliminated with a quartz/borosilicate filter having a maximum transparency at 365 nm. The irradiance value, measured at a distance of 60 mm from the source, was 9.7 mW cm−2. This value was about 4.4 times higher than the average UV irradiance measured outdoors on a clear summer day (2.2 mW cm−2). A PMA 2100 radiometer provided with a UVA detector, type PMA 2110 (Solar Light Co., USA) was used for measurements of irradiance and of radiance exposure which is dependent on the irradiation time. The temperature measured inside of the irradiation device was 20–22 °C and the relative air humidity (RH) was 53 %. A thermo-hygrometer model JK-HTM-3 (Shanghai Jingke Scientific Instrument Co., China) was used to monitor the temperature and RH values during irradiation. Samples were extracted from the irradiation device every 20 h and analyzed.
Samples were analyzed for color changes according to DIN 6174 (Farbmetrische Bestimmung von Farbabständen bei Körperfarben nach der CIELAB-Formel, 1979).
The UV–Vis spectra and quantitative determination of the dyes detached from the cellulosic fibers, as a result of exposure of fabrics to UV radiation, were made with UV–Vis Cole Palmer 1100 RS apparatus provided with Unico 1100 SS-1.11 version software. Measurements were carried at the visible wavelengths corresponding to the absorption peaks that characterize each dye. Concentrations of extracted dyes were calculated using the standard previously plotted curves. The dyes extraction solutions were prepared according to the standard SR EN ISO 105-E04, January 1998, classification index L22, Textiles—Tests for color fastness to perspiration, using the prescriptions given below .
For the alkaline extraction, a freshly prepared solution containing 0.1 g α-l alanine, 1 g NaCl, and 0.5 g Na2HPO4·2H2O was used. The solid substances were dissolved in 200 mL distilled water and the pH values were adjusted to 8 using a 0.1 mol L−1 NaOH solution. For the acid extraction, a freshly prepared solution containing 0.1 g α-l alanine, 1 g NaCl, and 0.5 g NaH2PO4 2H2O was used. The solid substances were dissolved in 200 mL distilled water and the pH values were adjusted to 5.5 using a 0.1 mol L−1 NaOH solution. The pH value was monitored with an AB 15 Plus Cole Parmer device provided with a glass electrode Accumet BASIC.
Near infrared chemical imaging (NIR-CI) spectra were recorded on a Specim’s Ltd. SisuCHEMA device controlled with Evince software package for processing the original image data. The system includes a chemical imaging workstation for 1000–2500 nm, NIR 130 domains. The original image for each sample was taken with a NIR model spectral camera, respectively an imaging spectrograph type ImSpector N17E with 320 and 640 pixel spatial resolution at a rate of 60–350 Hz.
Results and discussion
Color changes during irradiation and photodecomposition mechanism
Color differences significantly increased with the irradiation dose for all the studied samples. The samples painted with light colored dyes, such as RY-143 and RO-13, presented the highest damage caused by irradiation. It is possible in these cases that the dyes layers may be easily penetrated by the UV radiations thereby causing a more advanced photochemical degradation of the cellulosic fabric. The fabrics painted with dyes RR-183 and RR-2 were less sensitive to irradiation. In this case, the color differences recorded during irradiation were comparable and significantly lower than those recorded for the samples painted with RY-143 and RO-13.
Variation of chromatic coefficients with radiation dose
(J cm−2 × 102)
As an observation, the significant decrease in L* values may be correlated with the substitution degree and number of amine moieties, which is higher for the fabrics painted with RY-143 and RO-13 compared to RR-2 and RR-183, thus leading to bathochrome displacement effect and blue and green chromophore accumulation. The hypochrome effect may be also due to photochemical cleavage of azo bond .
The modifications of UV–Vis spectra may explain the changes in color parameters of the dyed textile fabric during irradiation. Even the cellulosic fabric could undergo some photo-oxidative degradation during exposure to UV radiation, the dye acting as a photo-senzitizer. It is known that the UV radiation may cause the generation of free radicals that are able to initiate photo-degradative reactions in the cellulosic materials such as depolymerization, dehydrogenation, dehydroxylation, dehydroxymethylation, and the release of hydrogen, carbon monoxide, and carbon dioxide . Photo-oxidative reactions are involved mainly in cellulose depolymerization . The appearance of new chromofore groups with conjugated unsaturated structures as a result of photo-oxidation processes in the cellulose fibers can also explain the color changes of colored textiles .
The broad band between 3600 and 3000 cm−1 is attributed to the cellulosic OH groups stretching vibration. The peak at 2901 cm−1 corresponds to C–H stretching vibrations. The peak at 1428 cm−1 is assigned to a symmetric CH2 bending vibration. According to the literature , this band may give information on the crystallinity of the cellulosic substrate. The peak at 1645 cm−1 is assigned to absorbed OH or conjugated C=O entities. The peak at 1365 cm−1 is attributed to symmetric C–H deformation, the peak at 1316 cm−1 to CH2 wagging, the one at 1203 cm−1 to OH deformation and the peak at 1158 cm−1 to C–O–C asymmetric vibration. The region from 1107 to 900 cm−1 corresponds to asymmetric glucose ring and C–O stretching vibrations . The peak at 900 cm−1 is related to β-(1 → 4)-glycosidic bonds stretching vibration. This band may provide information on the amorphous state of the cellulosic substrate .
Two observations can be made regarding Fig. 7. Firstly, an increase in the extracted dye quantity with irradiation dose can be observed. This aspect can be related to a more intense photo-degradation of the cellulose substrate. Secondly, higher values of the pH lead to the extraction of higher dyes quantities from the cellulose textiles.
Structural modifications studies by NIR-CI spectroscopy
The experimental signal values for each functional group were assigned by using IR reference spectra within the NIST spectral library database.
NIR absorption bands assigned to the dyed and UV irradiated textile fabrics for 100 h: Y-143: 985 nm (N–H second overtone); 2135 nm (O–H stretch/C=O stretch combination/symmetric C–H deformation); 2280 nm (C–H stretch/CH2 deformation); RO-13: 1540 nm (O–H stretch first overtone); 2110 nm (O–H bend/C–O stretch combination and asymmetric C–O–O stretch third overtone from the new peroxide structures probably formed during irradiation); 2295 nm (C–H bend second overtone) 2450 nm (C–H combination and symmetric C–N–C stretch overtone); RR-183: 1790 nm (O–H combination); 1980 nm (asymmetric N–H stretch/N–H in-plane bend C–N stretch combination); 2100 nm (asym C–O–O stretch third overtone from new peroxide structures formed during irradiation); RR-2: 1505 nm (N–H stretch first overtone); 1705 nm (C–H stretch first overtone).
Structural and color modifications of textile materials painted with four different azo-triazine dyes were studied during 100 h UV irradiation time and an irradiation dose up to 3500 J cm−2. Structural changes during irradiation were compared by applying FTIR, UV–Vis, and NIR-CI techniques. A darkening tendency of fabrics painted with RY-143 and RO-13 during irradiation was found, while an insignificant modification of the L* values of the fabrics painted with RR-2 and RR-183 was observed. This aspect was explained by the accumulation of blue and green chromophores during irradiation, due to photo-oxidation reactions on the cellulose substrate surface. Changes in visible absorption spectra as a result of irradiation supported these observations. A photodecomposition mechanism of dyes was proposed. It was concluded that the UV exposure led to the partial detachment of the dyes from the textile surfaces, together with glucose units and dye photo-degradation by C–N bond cleavage and destruction of azo chromophore groups and aromatic rings. NIR spectra showed possible peroxidation processes in the structures of the fabrics dyed with RO-13 and RR-183.
Authors acknowledge the financial support of a grant of the Romanian National Authority for Scientific Research, CNCS—UEFISCDI, Project Number PN-II-ID-PCE-2011-3-0187. Authors are grateful to Ph.D. student Manuela-Tatiana Nistor of the “Petru Poni” Institute of Macromolecular Chemistry, Iasi, Romania, for the recording of the NIR spectra. Paper dedicated to the 65th anniversary of “Petru Poni” Institute of Macromolecular Chemistry of Romanian Academy, Iasi, Romania.