Despite containing planar aromatic ring systems, highly fluorescent dye molecules often contain also structural characteristics that confer a considerable degree of non-planarity. Planar molecules have a tendency to aggregate, in extreme cases becoming pigmentary, and dissipate excited state energy through relaxation processes which involve intermolecular interactions. The classical example of a fluorescent dye, fluorescein, provides an example of a rigid yet non-planar molecular structure which leads to high fluorescence intensity [35].
This fluorescent dye is acidic in nature due to the presence of COOH acid groups in its conjugated structure. At an acid-neutral pH, fluorescein has no charged group, which limits its solubility in water. In order to improve this solubility and to obtain a good dispersion into the bath, a dispersing agent is added. Therefore its application on polyester will take place thanks to its dispersed character. Dispersed dyes were applied to polyester without and with the inclusion of a carrier in the dye bath. The use of these carriers has caused many problems for the dyer, including the residual odor of the carrier on fabrics and the reduction in lightfastness of dyed fabrics due to the residual carrier [29]. Therefore, the process was carried out without the addition of catalyst.
The effect of temperature on the reflectance of fluorescent sample is studied at first. The curve allows a minimum at point 460 nm and a maximum at point 560 nm corresponds respectively to the absorption and emission wavelength of the dye molecule.
3.1 Effect of temperature on the reflectance
The dyeing temperature is an important factor, it plays a crucial role in the coloration of thermoplastic fibers, especially for hydrophobic polyester according to traditional dyeing theory.
According to the curves above (Figure 4), it is clear that the reflectance increases proportionally with temperature. In fact, heating the dye liquor accelerates the strike rate and the diffusion of the dyes inside the fiber structure. Within the fiber polymer, the dye molecules are held by van der Waals forces. The use of maximum process temperature of 140°C leads to a rather small reflectance value which is very far from the reflectance of standard polyester (white).
Figure 3 describes the variation of the reflectance as a function of wavelength by varying the process temperature 80°C, 100°C, 130°C and 140°C for each concentration of the fluorescent marker while the process duration is set to 5 min.
According to this figure, the four curves obtained have the same tendency. For a concentration of 0.1%, 0.2% and 0.3% the reflectance increases until reaching a threshold at 100°C whereas for the concentration of 0.4% the maximum reflectance is present at a temperature of 130°C. By applying the fluorescent marker at a temperature lower than the temperature of the glass transition of the polyester, the polyester chains are quite rigid, which makes diffusion difficult. Therefore, by increasing the temperature up to 100°C, the development of amorphous zones in the fibrous structure is favored which facilitates the penetration of dye inside the textile and thus increases fluorescence [36]. A temperature of 130 °C and 140 °C can decrease the fluorescence intensity, the quenching phenomenon is observed (figure 5).
The phenomenon of quenching can be explained by the significant collision of fluorescent molecules that will create the π-staking phenomenon. In fact, this phenomenon corresponds to the non-radiative relaxation of excited electrons towards the fundamental state. This relaxation can be explained by the excimer effect : formation of excimers by the collision of the fluorophore in the excited state and a molecule of the same type not excited [37].
According to the obtained results, two temperature levels 100°C and 130°C are selected in order to maximize the fluorescence intensity under the UV lamp.
3.2 Effect of irradiation time on the reflectance
Figure 6 illustrates the effect of irradiation time on the reflectance of prepared PET fabrics. Three times 5 min, 10 min and 15 min for two concentrations of the fluorescent product 0.3% and 0.4% at 100°C and 130°C are chosen. The reflectance varies inversely proportional to the irradiation time. This variation is important during the duration of 5 min to 10 min and decreases with 15 min and a stabilization of the reflectance in the time interval from 10 min to 15 min is given. In fact, the increase in irradiation time promotes the rise and diffusion of the dye inside the polyester fiber and thus reduces the reflectivity of the textile until it has a maximum dispersion.
According to the figure 7, it is clear that the effect of time irradiation on the reflectance with a fluorophore concentration of 0.1% and 0.2% is less important than that obtained with a concentration of 0.3% and 0.4%. The intensity of the fluorescence increases by increasing the duration of the irradiation from 5 min to 15 min. This can result in the accessibility of excited molecules to fluorescent radiative return, which depends on the initial number of molecules in the fundamental state. Therefore, by increasing the application time, the increase of the number of fluorescent molecules in the fiber is promoted. So and referring to the previous part, for further investigation the process durations 5 and 15 min are used and evaluated. In fact, 5 min leads to the maximum reflectance and the minimum fluorescence and vice versa for 15 min.
3.3 Effect of fluorescein concentration on the reflectance
Figure 8 shows the variation of the reflectance as a function of the fluorescein concentration from 0.1% to 0.4% with a step of 0.1% at two temperatures 100°C and 130°C.
First of all, the increase of the fluorescein concentration in the bath favors its rise on the textile fiber in a significant way and reduces the reflectance. Then, for an application time of 5 min, the reflectance varies from 44.6%, for a marker concentration of 0.1% at 100°C, to a value of 40% for a marker concentration of 0.4% at 130°C. While setting the process duration to 15 min, the reflectance decreases by increasing the concentration of the fluorescent product from 0.1% to 0.4% respectively from 45% to 38% with a temperature of 100°C and varies from 44% to 35% for the temperature 130°C. Finally, in order to minimize the coloration of the samples and to get closer to the reflectance of white polyester, a reduction of the fluorescent tracer concentration interval takes place.
Figure 9 shows the effect of dye concentration on the reflectance of fluorescent polyester according to its emission wavelength. Using a process duration of 5 min at 100°C and 15 min at 130°C, the fluorescence increases while amplifying the fluorescein concentration from 0.1% to 0.3%, then having its maximum at 0.3% before decreasing at a fluorescein concentration of 0.4%. This can be explained by the increase in the number of excited molecules in the fundamental state by increasing the concentration up to a threshold at which the fluorescence decrease is noticed. This decrease is the result of quenching phenomenon of fluorescence whatever by static inhibition, i.e. the fluorophore establishes a stable and non-fluorescent complex with the dispersed agent and the formation of the complex takes place in the fundamental state and/or either by dynamic inhibition, i.e. the fluorescein is desactivated by contact with the dispersed agent [38].
For the combination of 15 min and 100°C, fluorescence follows the increase in concentration from 0.1% to 0.4% with a slight variation from 0.3% to 0.4%. For the last couple (temperature, time) an increase in fluorescence by accentuating the quantity of the fluorescein is noticed, which reaches its maximum at 0.4%.
In order to obtain a good fluorescence under UV lamp and to keep the white color of the fluorescent polyester under daylight, the optimal conditions are a fluorescent concentration of 0.3%, and an irradiation time of 5 min and 15 min at 100°C and 130°C.
3.4 Fluorescence Imaging of treated Polyester Fabrics
Table 2 illustrates pictures of non-treated polyester and fluorescent samples under visible light and under UV lamp excitation at 465 nm using a light cabin.
It is obvious that by increasing the process duration and process temperature of application the textile becomes a little colored under daylight and gives a yellow-greenish coloration when it is excited under a UV lamp.
Table 2: Photographs of polyester fabrics before and after dyeing. The photographs are taken under different illumination conditions.
![](https://myfiles.space/user_files/69515_16346c490bab499e/69515_custom_files/img1628262947.png)
The commercial fluorescent dyes recommended for textiles are often criticized for light fastness properties which are unable to meet the standards required by more demanding applications. The chemical skeleton of the dye used in actual study is rich of double and single bonds which makes it sensitive to light [40]. However, the use of UV absorber overlayers can produce a fluorescent colour with reasonable light stability [31].
In order to improve the durability of the treatment against light, a UV absorber is introduced in the bath of the fluorescent solution.
3.5 The addition of UV absorber
UV absorbers are colorless organic/inorganic compounds that exhibit strong absorption in the UV wavelength range between 290 nm and 360 nm [41-46]. Once incorporated into fibers, they convert electronic excitation energy into thermal energy. When a UV absorber is excited by high-energy UV radiation and the short-wavelength UV absorber, the absorbed energy can be dissipated as longer-wavelength radiation [42]. Noting that an effective UV absorber must be able to absorb across the spectrum to remain stable against ultraviolet radiation and dissipate the absorbed energy to avoid degradation or color loss [39]. Indeed, many UV absorbers reduce the bleaching effect of common fluorescent brighteners (FAs) because their efficiency in the UV range reaches the efficiency range of these brighteners. For these reasons, it is necessary to use broadband UV absorbers that do not extend into the absorption band of the fluorescent brightener and are covalently bound to the treated tissue [35].
From a chemical point of view, several classes of UV absorbers are available. Important groups of UV absorbers are: 2-hydroxybenzophenones, 2-hydroxyphenylbenzotriazoles, 2 hydroxyphenyl-s-triazines [41,42]. For the evaluation of a UV-absorber finishing realized by a microwave-assisted finishing process, polyester woven materials are treated with the UV absorber Tanuval UVL from Tanatex, Netherlands. The finishing process is performed using a focused microwave synthesis system. The UV-light contains wavelength below 400 nm. For this, the efficiency of the UV-protective finishing is characterized with transmission spectra presenting the diffuse transmission of UV-light. When radiation strikes a fibre surface, it can be reflected, absorbed, transmitted through the fibre or pass between fibres (Figure 10) [47]. The minimum transmission defines the UV protection efficiency. The lower transmission value, the more effective UV absorber is. The application of a UV absorber decreases the transmission value. The addition of UV absorber to the bath has a lesser influence on the color of the fluorescent samples and leads to a yellowish color [48].
According to its technical data sheet four concentrations of UV absorber from 0.5% to 2% with a step of 0.5% are used. The study of this treatment is done by measuring the reflectance, fluorescence, and transmission of the textile support.
3.5.1 Effect of UV absorber on the reflectance value
The curve above represents the evolution of the reflectance as a function of UV absorbers concentration at 100 °C and 130 °C for 5 and 15 min (Figure 11).
It is clear that the addition of UV absorber influences the final coloration of the textile support. This treatment decreases the reflectance once the UV absorber concentration is increased. The evolution of the curves is identical, in fact with a temperature of 100 °C pendant a radiation time of 5 min, the reflectance decreases from 42% to 36% by adding to the fluorescent bath a concentration of 0.5% UV absorber. This decrease is explained by the yellow coloration caused by this absorber. In fact, some UV absorbers produce an undesired yellowish color on treated fabrics, adversely affecting the dyeings obtained. By passing from a concentration of UV absorber from 0.5% to 2% the reflectance decreases slightly and then stabilizes.
From this figure 9, it can also be noted that with the maximum of the temperature and during the minimum of process duration, the curve obtained is superimposed on that obtained with the minimum of the temperature and the maximum of duration.
With a temperature of 130 °C and process duration of 15 minutes, the coloration of the textile fabric is outstanding and the reflectance varies from 38% to 31% by adding 2% of UV absorber to the bath.
3.5.2 Effect of UV absorber on the fluorescence properties
Figure 12 shows the variation of the reflectance as a function of the UV absorber concentration from 0.5% to 2% with a temperature of 100 ° C for 5 and 15 min.
The intensity of fluorescence varies inversely with the increase of the UV absorber concentration. Indeed, for a process duration of 5 min, the reflectance varies from 100% without the addition of UV absorber to 92% for a UV absorber concentration of 2%. This variation is more accentuated in the concentration range of 1.5% to 2% due to the increase of the number of UV absorber molecules in the fluorescent bath. This increase facilitates and favours the collision between fluorescein and UV absorber either by establishing a non-fluorescent complex stable in its fundamental state and in this case it can be spoken of static quenching, or by deactivating the fluorophore by subbing contact with the UV absorber molecule and in this case this phenomenon of dynamic quenching will be in place.
It is clear that with the addition of a concentration of 0.5% UV absorber the reflectance curve gives us the minimum variation of fluorescence compared to the treated sample without UV absorber. In fact, for an irradiation time of 5 min and 15 min the difference in reflectance compared to the fluorescent fabric without UV absorber is 0.51% and 0.91% respectively.
3.5.3 Effect of the concentration of UV absorber on the transmission
The effect of adding UV absorber to the dye bath is well confirmed by observing the figure 13 illustrating the evolution of transmission as a function of wavelength.
When analyzing the different transmission spectra, it is obvious, that the treatment with an UV-absorber generally decreases the transmission of the UV-light in comparison to the untreated reference. The untreated reference sample has no UV-protective function for a range of wavelength from 400 nm to nearly 300 nm. However, the transmission decreases to nearly zero around 300 nm wavelength. A reason might be the aromatic structure of polyester that offers certain UV-protection [36]. Further, all samples finished with the UV-absorber Tanuval, no matter which finishing process is applied, show a decreasing transmission in the range of wavelength from 400 nm to 350 nm, to nearly zero at 350 nm.
A recap of the results reached with different parameters shows that for a process duration of 5 min, the transmission decreases by increasing the UV absorber concentration from 0.5% to 1% and then stabilizes and tends to zero for the concentration range from 1% to 2%. The only difference compared to the irradiation time of 5 min is that the UV absorber activity range widens and tends towards the visible region up to 380 nm while passing from a concentration of 0.5% to 2%.
For the application of UV absorber on the textile support, with a concentration of 0.5% and a time of 5 min and 15 min, the maximum reflectance, the maximum fluorescence, and a more acceptable transmission value are obtained. Therefore, the evaluation of the washing fastness and light fastness of the fluorescent textile support in the presence of UV absorber is made by selecting the sample prepared with 0.3% fluorescent tracer concentration and 0.5% UV absorber concentration for 5 min and 15 min at 100°C.
3.6 Fluorescence Imaging of treated Polyester Fabrics with UV absorber
Table 3 illustrates pictures of non-treated polyester and fluorescent samples under daylight and under UV lamp excitation using a light cabin. The irradiation time of the treated samples is 5 min.
Table 3: Photographs of polyester fabrics dyed without and with UV absorber at different concentrations
![](https://myfiles.space/user_files/69515_16346c490bab499e/69515_custom_files/img1628263231.png)
It is clear that under the daylight, the addition of UV absorber causes an additional yellowish coloration on the surface of the treated textile. This coloration slightly decreases the intensity of fluorescence seen under UV lamp.
3.7 Surface Morphologies of fluorescent Polyester Fabrics in presence of UV absorber
To verify the existence of the anti-counterfeit product at the surface of polyester fabric, treated samples are examined by scanning electron microscope (SEM) (Figure 14).
The SEM analysis shows the presence of the fluorescent agent on the surface and between the fibers of the treated polyester.
3.8 Effect of the addition of UV absorber on light and wash fastness
Generally, to study the effectiveness of such a treatment, it is obviously necessary to study its durability over time. Therefore, the washing and light fastness of prepared fluorescent samples is evaluated.
3.8.1 Washing fastness
The evaluation of the washing fastness is given in the Table 4. The samples to be evaluated are treated with a fluorescent marker concentration of 0.3% in the presence of 0.5% UV absorber at a heating temperature of 100°C for 5 and 15 min.
The results indicate excellent fastness properties of the dyed samples with fluorescent tracer in the presence of UV absorber. By comparing them with the results obtained without UV absorber, it can be said that the washing fastness is slightly influenced by the addition of UV absorber. This can be explained by the presence of dispersing agent in the UV absorber composition responsible for the best dispersion of fluorescent tracer in the dyeing solution and thus a better affinity towards the textile fiber.
Table 4: Evaluation of washing fastness
Fluorescein concentration
|
Irradiation time
|
Temperature
|
Greyscale
|
Without UV absorber
|
With UV absorber
|
0.3%
|
5 min
|
100°C
|
4/5
|
4/5
|
15 min
|
100°C
|
5
|
5
|
3.8.2 Daylight fastness
The evaluation of the light fastness is given in Table 5. The samples to be evaluated are treated with a fluorescent marker concentration of 0.3% in the presence of 0.5% UV absorber at a heating temperature of 100°C for 5 and 15 min.
Table 5: Evaluation of light fastness
Fluorescein concentration
|
Irradiation time
|
Temperature
|
Fluorescence life time
|
Blue scale
|
Without UV absorber
|
With UV absorber
|
Without UV absorber
|
With UV absorber
|
0.3%
|
5 min
|
100°C
|
6h
|
8h
|
4
|
4/5
|
15 min
|
100°C
|
6h
|
8h
|
4
|
4/5
|
The addition of a UV absorber to the fluorescent bath improves the light fastness of the dyed samples. The durability of such a fluorescent treatment after exposure to light reaches 8 hours and 6 hours without UV absorber.
The blue scale with the values found, informs us about the sensitivity of fluororesin to light. Despite the presence of a UV absorber, this value is slightly improved. This can be explained by the photobleaching phenomenon of most organic fluorophores.