Influence of fluorescent dyes for dyeing of regenerated cellulose fabric

Abstract

This study attempted to dye regenerated cellulose fiber fabric with fluorescent dyes in different conditions (dyeing time, temperature, pH, concentration, etc.) and investigated their effects on color strength, fluorescence intensity, and other properties of dyed fabrics. The exhaust dyeing method was applied in this experiment for cellulose based knitted fabrics. The physical properties of dyed knitted fabrics before and after treatment with fluorescent dyes were determined and evaluated. After fluorescent staining, the results showed that a large number of granular fluorescent particles were coated on the fiber surface. Higher K/S values were obtained under conditions of a dyeing temperature of 80℃, dyeing time of 30 min, and NaCl concentration of 1 wt% than other conditions. Moreover, the fluorescence intensity of the dyed fabric was also highest at 80℃. The color strength and bursting strength of dyed knitted fabrics were increased with the increase of the concentration of fluorescein sodium. Thus, cellulose based knitted fabric dyed with fluorescent dyes exhibited excellent fluorescent properties and could be used in fluorescent clothing.

Keywords

fluorescent cellulose knitted fabric dyeing

Fluorescence compounds are special substances that can absorb incident light of a certain wavelength, such as ultraviolet and x-ray, and then can emit visible light of a larger wavelength.^1,2 Once the incident light stops, the emitted light also disappears. Fluorescent dyes have many varieties which owe their commercial exploitation in a broad range of applications to their bright appearance and easy identification.³ At present, fluorescent dyes are widely used in textiles,³ food and drugs,^4,5 fluorescent markers,^6,7 safety protection,⁸ and other fields.⁹ In the textile application field, fluorescent dyes are used according to fashion trends in leisurewear clothes, especially sportswear and outdoor workwear.¹⁰ Published papers show the fluorescent dyes suitable for fabric dyeing mainly include disperse and acidic dyes. And previous research on dyeing properties and conditions have mainly focused on polyester¹¹ and nylon.^8,12

Fluorescent dyes have a long history. Dating back to the 1930s, the combination of certain dyes and resins was explored and produced brighter color. In 1971, Bayer designated the fluorescent dye discovered as “fluo-rescine.”¹⁰ To this day, fluorescent dyes are becoming more and more versatile. Knitted fabrics such as cotton, polyester, acrylic, nylon, etc. have been studied and are dyed by fluorescent dyes.⁸ Jacob Winkler et al. used fluorescent dye DTAF to stain cotton fabrics;¹³ Lucjan Szuster et al. analyzed and studied polyester fabrics with fluorescent properties;¹⁴ Pik-ling Lam et al.¹⁵ and Manjaree Satam et al.¹⁶ synthesized fluorescent disperse dyes with different structures for acrylic fabrics; Moreover, Satam et al. also synthesized acid fluorescent dyes with high dye fastness and suitable for wool, silk, and nylon fabrics.¹⁶ However, few studies have been done on dyes and related technological conditions suitable for ultrafine regenerated cellulose fibers.

Fluorescein sodium (FS) salt is called uranine and is a yellow–green laser dye. It has a higher efficiency of laser energy conversion.¹⁷ In biology, it is used as a tracer in retinal angiography and as a diagnostic aid for revealing corneal trauma and fitting contact lenses.^18,19 The molecular structure of FS is small and it’s easily dispersed in water.²⁰ So it can fully contact with ultrafine regenerated cellulose fibers and has excellent affinity. In previous studies, the fluorescence intensity of FS solution was affected by many factors, such as temperature, pH, and so on.^21,22 At the same time, fluorescence quenching theory proves that the fluorescence intensity increases with the increase of the fluorescence concentration on the fabric. When the concentration reaches a critical value, the fluorescence intensity is the highest, and when the concentration exceeds this critical value, the fluorescence intensity decreases.²³ Therefore, exploring the fluorescent dyeing properties and conditions of ultrafine regenerated cellulose fabrics is of great significance for saving energy and improving the fluorescent effect.

Knit fabrics are formed by the intermeshing of loops, which have a high degree of stretch and elasticity better than woven fabric. In our work, regenerated cellulose knitted fabrics are used as substrate for fluorescence treatment. On the basis of its excellent comfort, the fabric can obtain bright color and a strong fluorescence effect. In the fluorescence dyeing process, the effect of physical parameters such as the types of fluorescent dyes, dye concentration, pH, dyeing temperature, dyeing time, and concentration of electrolyte salts in dye solution were discussed.

Materials and methods

Materials

Knitted fabrics (130.2 ± 0.1 g m⁻², single jersey) were made by our lab using two kinds of regenerated microfine cellulose fiber blend yarns with different fineness. The specification of the blended yarns was: yarn count 80 s, blended ratio 20/80. Fluorescein sodium (FS, C.I.45350) and rhodamine B (RB, C.I.45170) were obtained from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). Acridine orange (AO, C.I.46005) was purchased from Shanghai Macklin Biochemical Co. Ltd. Other reagents (sodium chloride etc.) were also bought from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China).

Preparation of fluorescent fabric

The fluorescent fabrics were prepared as follows: Firstly, knitted fabric was treated in a dye bath at a temperature of 40℃ for 10 min. Then, the dye bath was prepared for FS, RB, and AO separately with a concentration range from 0.25–3 wt%, and the liquor ratio was 1:50. Samples were added to each dye bath, and dyeing was started for 10 min at 40℃ then the temperature was raised to 90℃ for 30 min (each 10 min, 0.5 g NaCl was added). Then, the temperature was gradually lowered to 50℃. The dyed samples were washed with deionized water and dried in an air oven. In the same dyeing process, the experimental parameters were changed as follows: temperature (70℃ to 95℃), dyeing time (20 min to 60 min), pH (5.5 to 10.5), NaCl concentration (0 to 10 wt%) and fluorescent dye (0.25 wt% to 3 wt%). The process flow chart is shown in Figure 1.

Figure 1.

Flow chart of fluorescent dyeing process.

Characterization

Scanning electron microscopy (SEM)

The morphology of knitted fabrics before and after fluorescent dyeing was examined by scanning electron microscopy (SEM, S4800, Hitachi, Japan). Gold plating was required before samples were observed. The scanning voltage of SEM was 10 KV.

Fourier transform infrared spectroscopy (FTIR)

The structure of knitted fabrics before and after fluorescent dyeing was analyzed by FTIR on a Nicolet5700 (Thermo Nicolet Company, USA) in absorbance mode. For each measurement, each spectrum was obtained by the performance of 32 scans with the wavenumber ranging from 400 to 4000 cm⁻¹ with a resolution of 4 cm⁻¹. The FTIR tableting technique was adopted. First, the sample was mixed with KBr evenly at a ratio of 1:200. Then, the sample was dried at high temperature and ground in an agate mortar until the particle size was less than 2 µm. Finally, the sample was pressed into a transparent sheet on a tablet press for testing.

Color strength

The color depth of the fabric surface was proportional to the K/S value, and the bigger the K/S value, the darker the color. The color intensity was usually calculated using the Kubelka–Munk equation.^24–28 At the same time, the relationship between dye concentration and reflectivity was expressed by the relative absorption and scattering of incident light. The equation (1) is as follows:

\frac{K}{S} = \frac{(1 - R) 2}{2 R}

(1)

where K is the absorption coefficient, S is the scattering coefficient of the colorant, and R is the percentage of reflectivity.²⁹

The percentage of reflectivity (R value) was measured under illuminant D65 using the 10° standard observer by an X-rite SP60 series spectrophotometer (Xrite company, USA). The color strength (K/S value) of samples was calculated using the above formula. Each dyed fabric was folded three times during the test and measured six times in different positions to obtain an average value.

Fluorescence emission spectrum

The fluorescence emission spectra of fabric were carried out using a Hitachi F-4600 spectrofluorometer (Hitachi High-Technologies Corporation, Japan). During the test, the sample was folded into two layers and laid on a solid support of a fluorescence spectrophotometer. The source was a xenon lamp. The operating voltage was set to 250 V. The slit width of the excitation light was 2.5 nm and the emitted light was 5 nm. The excitation wavelength of FS was chosen to be 470 nm, and the emission wavelength ranged between 400 and 700 nm. The excitation wavelength of RB was 264 nm. The excitation wavelength of AO was 274 nm.

Fluorescence fastness

The rubbing fastness was analyzed using the YG(B)571-III Rubbing Colour Fastness Tester (Wenzhou Darong Textile Instrument Co., Ltd, China) at room temperature. The experiment referred to national standard GB/T 3920-2008. The washing fastness test was carried out according to AATCC61:2010, which was examined by the SW-20B washing fastness tester (Quanzhou Meibang Instruments Co., Ltd., China). The experimental temperature was 40℃, the time was 45 min, and the soap concentration was 0.37 wt%. The number of steel balls was 10. Finally, the fluorescence intensity was used as the evaluation index of firmness, rather than the color grade evaluation method.

Results and discussion

Morphology analysis

In our experiment, samples were knitted fabrics made of plied yarns containing two kinds of different regenerated microfine cellulose fibers. The knitted fabrics had excellent properties of hygroscopicity and breathability and their appearance was smooth and soft (Table 1). Compared with traditional single jersey, the edge-roll of samples had been improved.^30,31 Figure 2(A),(B) depicts SEM images of the front of the knitted fabrics. It can be clearly seen that each coil had two yarns. At the same time, yarns were slightly fluffy after high-temperature fluorescence treatment for a certain period of time (Figure 2(B)). A large number of granular fluorescent particles were coated on the fiber surface (Figure 2(b)). Moreover, samples were treated with three different fluorescent dyes and their fluorescence images were obtained (Figure 3). Samples were brightly colored and had excellent fluorescence effects.

Figure 2.

SEM of knitted fabric before (A, a) and after (B, b) fluorescent treatment.

Figure 3.

Fluorescence images of knitted fabrics treated by different fluorescent dyes: (a) fluorescein sodium, (b) rhodamine B, (c) acridine orange, (d) blank sample.

Table 1.

Basic properties of the samples

Samples	Property
Microfine/modal	20/80
Stitch structure	Single jersey
Thickness (mm)	0.54 ± 0.01
Weight (g/m²)	130.2 ± 0.1
Moisture regain (%)	13.0
Wicking height (mm)	111
Water vapor transmission (g/m²·d)	8120
Air permeability (mm/s)	3428.9
Softness (mN)	87.3

Structural analysis

The structural changes of knitted fabrics before and after fluorescence treatment were analyzed by FTIR (Figure 4). Table 2 shows the main characteristic peaks of cellulose. From Figure 4 and Table 2, the absorption peak of samples at 3450–3414 cm⁻¹ was the stretching vibration absorption of the OH group, which was the characteristic band of cellulose.^32,33 The absorption peak at 3000–2800 cm⁻¹ was attributed to the stretching vibration peak of C–H. In this range, RB and AO have obvious characteristic peaks, and the percentage change of the peak value of fluorescent fabrics after treatment is slightly higher than that of the characteristic peaks on both sides.

Figure 4.

Infrared spectra of the fluorescence dyes and knitted fabrics with fluorescence treatment: (A) FS, (B) RB, (C) AO, (a) fluorescence dyes, (b) untreated samples, (c) treated samples.

Table 2.

Band assignment of cellulose by infrared spectroscopy

Wavenumber/cm⁻¹	Assignment
3450–3420	OH stretching hydrogen bonded (cellulose II)
3375–3350	OH stretching (intramolecular hydrogen bonding)
2910–2870	CH₂ stretching
1635	Adsorbed water
1430–1374	CH₂ bending
1336	OH in-plane bending
1317	CH₂ wagging
1162–1110	Asymmetrical in-phase ring stretching
1000	C–O stretching

For this phenomenon, we can initially speculate that dyes are successfully dyed. During dyeing, hydrogen bonds, electrostatic, and other physical effects also play a significant role in addition to chemical crosslinking. Color fastness testing can verify the hypothesis.^34–36 In Figure 5, the chemical structure of the three dyes is very similar, and the infrared spectral peak of the treated samples was not significantly different. However, due to the small proportion of dyes in dyeing, the characteristic peaks of samples in the infrared spectrum did not change significantly. Thus, the characteristic peaks of all samples were similar, which were not affected by fluorescence dyes.

Figure 5.

The general structure and reaction mechanism of fabrics and dyes: (a) fabrics; (b) FS; (c) RB; (d) AO.

Comparison of fluorescein types

Three different fluorescent agents (RB, AO, and FS) were used to dye knitted fabric. In the dyeing process, the experimental parameters were: fluorescent agents concentration 0.75 wt%, dyeing temperature 80℃, NaCl concentration 1 wt%. Figure 6 shows that the fluorescence intensity of FS was similar to that of AO but stronger than RB. The three dyes have different excitation wavelengths, but their emission wavelengths are between 500 and 600 nm.

Figure 6.

Effect of different fluorescent dyes on fluorescence intensity. (A)–(C) are the emission spectra of the fluorescent fabrics. (a)–(c) are the excitation spectra. (A),(a) fluorescein sodium; (B),(b) rhodamine B; (C),(c) acridine orange.

The sensitivity of the human eye to light is related to the wavelength of light. When the light is bright, the human eye is most sensitive to the yellow–green light at 550 nm, while when the light is weak, it is most sensitive to the blue–green light at 510 nm. The excitation wavelength of FS samples was 470 nm and the emission wavelength ranged between 400 and 700 nm. Therefore, FS fabric has a bright fluorescence in both day and night. The wavelength spectrum of common white LED ranges from 450 to 460 nm, which belongs to the optimum excitation range of FS. By contrast, the excitation wavelength of RB samples was 264 nm. The excitation wavelength of AO samples was 274 nm. So FS has a significant advantage over RB and AO.

Fluorescence fastness of dyed fabrics

The results of fastness testing for the fluorescent fabrics are shown in Figure 7. The results show that the color strength of the fabrics decreased obviously and the fluorescence intensity decreased slightly after 5000 cyclic friction tests with standard friction cloth. The fabrics had good fluorescence fastness. As shown in Figure 7(B)–(D), the affinity and fastness of both FS and AO to the fabric were significantly stronger than RB.

Figure 7.

Fluorescence fastness of dyed fabrics after friction treatment. (A) The loss of color strength of fabric after friction. (B)–(D) The loss of fluorescence intensity after friction treatment of three kinds of fluorescent fabrics: (B) fluorescein sodium; (C) rhodamine B; (D) acridine orange; (a) treated samples; (b) original samples.

As shown in Figure 8, the fluorescence intensity of the three kinds of fluorescent fabrics showed a decreasing trend after soaping. It can be determined that physical adsorption is the main factor between the dye and the fabric and a small amount is chemical bond binding. From the analysis of the washing fastness and rubbing fastness of fluorescent fabrics, it can be seen that the fluorescence intensity of the FS fabric remains good despite some losses. Therefore, FS fluorescent fabric has value for further research and a potential market. In the following parts, we selected FS as dye and analyzed the influence of external factors on dyeing properties.

Figure 8.

Fluorescence fastness of dyed fabrics after soaping. (A)–(C) The loss of fluorescence intensity after soaping of three kinds of fluorescent fabrics: (A) fluorescein sodium; (B) rhodamine B; (C) acridine orange; (a) treated samples; (b) original samples.

Fluorescence effect and color strength analysis

Effect of NaCl concentration

FS was a kind of laser dye with a yellow–green area that had a high efficiency of laser energy conversion. The excitation wavelength interval was 530–570 nm. During the dyeing process, some inorganic electrolyte salt (such as NaCl) was added that could effectively promote the coagulation of fluorescent dyes and reduce the solubility of the dyes in the dye bath for better coloring of fabrics.^37,38

The color strength (K/S value) and fluorescence emission spectrum of dyed fabrics were measured by changing the concentration of NaCl (Figure 9). FS is a water-soluble anionic organic salt, and regenerated cellulose fibers also have slight negative charges in water. A slight repulsive force will be produced between the dye and the fabric. When sodium chloride is added to the dyeing bath, the cation ionized by NaCl in water will adsorb and wrap on the surface of the fiber, which will greatly reduce the energy resistance caused by the Coulomb force and improve the dyeing rate. In this study, the concentration of FS was 0.75 wt%. In Figure 9(A), K/S values gradually increased with the increase of NaCl concentration and then tended to steady-state values. The dyeing effect was very obvious. In Figure 9(B), the fluorescence intensity of dyed fabric was strongest when NaCl concentration was 1 wt%. When the concentration of NaCl was more than 1 wt%, the fluorescence intensity decreased with the increase of salt concentration. The reason for fluorescence intensity fluctuation was the FS deposited on the surface of the knitted fabric, resulting in fluorescence quenching. There is actually a wide variety of quenching processes, including excited state reactions, molecular rearrangements, ground state complex formation, energy transfer processes, and so forth. With the increase of NaCl concentration, the collision between a singlet excited dye molecule and salt ions in solution becomes more frequent, which causes fluorescent molecules to release heat back to the environment and to transit back to the ground state in the form of non-radiation. At the same time, a small amount of salt ions attract and aggregate with dyes, resulting in a quenching effect.^39,40 In testing the fluorescent solution, we also found that fluorescence quenching occurred when the concentration of NaCl in dyeing solution was too high.³⁹

Figure 9.

Influence of NaCl concentration in dye solution on fabric color strength (A) and fluorescence intensity (B); The concentration of NaCl in the dye solution was (a) 0, (b) 0.4, (c) 1, (d) 4, (e) 5, and (f) 10 wt%, respectively.

Effect of pH on fluorescence staining of knitted fabric

Figure 10 depicts the effect of pH on the dyeing strength of the knitted fabrics. Fabrics were knitted using regenerated cellulose blend yarn, which had excellent alkali resistance but not acid. Under conditions of strong acid, FS would lose its fluorescence effect and ,at the same time, neutralization or alkalization would occur again. So, fluorescence staining was considered suitable under weak acid and alkaline conditions. Acetic acid and sodium hydroxide solution (concentration 1 mol L⁻¹) were used to adjust the pH of the dye bath in the range from 5.5 to 10.5. In Figure 10, the K/S values of samples decrease from 5.69 to 0.92 with an increase of pH values. When the pH was 8, the K/S value was only 1.38. Under alkaline condition, FS existed in the form of quinone molecules, amphoteric ions, and ester molecules in solution. With the increase of pH, neutral molecules decreased while the concentrations of monovalent anions and bivalent anions increased gradually. Cellulose molecules had negative electrons in water and the electrostatic repulsion between dye molecules and fibers enhanced, causing a decrease in dyeing rate, color strength, and K/S value. While pH declined, the concentration of cations increased gradually in the dye bath, leading to enhancement of electrostatic forces and dyeing rate increased.

Figure 10.

Effect of pH value on dyeing strength of knitted fabrics.

Effect of dyeing temperature

Dyeing temperature was also one of the important factors affecting the color strength and fluorescence intensity of the dyed fabrics. In this experiment, dyeing temperature ranged from 70–95℃. Figure 11(A) shows the relationship between K/S value and dyeing temperature. At 80℃, the K/S value reached 1.56 higher than at other temperature conditions. At the same time, the fluorescence intensity of the dyed fabric was also highest at 80℃. The reason may be that the movement of the molecular chain of regenerated cellulose fiber increases with the rise in temperature, which expands the intermolecular gap of the fiber in the fabric. This will facilitate the diffusion of dye molecules into the fiber. In addition, the movement of dye molecules becomes intense with the increase of temperature, which improves the accessible region and contact opportunity of the dye in the fiber.

Figure 11.

Effect of dyeing temperature on color strength (A) and fluorescence intensity (B); dyeing temperature was (a) 80℃, (b) 70℃, and (c) 90℃, respectively.

When the temperature was lower than 80℃, the fluorescence intensity gradually increased with the increase of temperature. Fluorescence quenching refers to some processes causing a decrease of fluorescence intensity.⁴⁰ There are many reasons for fluorescence quenching, including excited state reactions, molecular rearrangements, ground state complex formation, energy transfer processes, and so forth.¹ When the temperature exceeds 80℃, the dye uptake rate gradually approaches the maximum with the increase of temperature. And with the intermolecular π–π interactions of the chromophore continuing to strengthen, intermolecular interactions of a singlet excited dye molecule with a ground state dye molecule give rise to the energy-transfer to quench the fluorescence¹. The fluorescence intensity decreases gradually due to a quenching effect. Therefore, the best fixation temperature is 80℃.

Effect of fluorescein sodium concentration

Figure 12(A) shows the color strength of dyed fabrics affected by the concentration of FS. The concentration of NaCl in solution was 2 wt%. From Figure 12(A), the color strength of dyed fabrics gradually increased from 0.05 to 1.32 with the increase of FS concentration. On the whole, shown in Figure 12(B), the fluorescence intensity first increased and then decreased with the concentration of FS, and the intensity was the highest around 1 wt%. While the content of fluorescein on the fabric exceeds a certain amount, the fluorescence intensity of the fabric decreases. Fluorescence quenching in solution is observed along with an increase of dye concentration and/or the formation of an excimer; the result conforms to the fluorescence quenching theory.^39,40

Figure 12.

Effect of concentration of fluorescein sodium on color strength (A) and fluorescence intensity (B) of dyed fabrics. (a) 0.25 wt%; (b) 0.5 wt%; (c) 1 wt%; (d) 2 wt%; (e) 3 wt%.

Bursting strength of dyed fabrics

When the concentration of FS was changed and other conditions remained unchanged, the bursting properties of the dyed fabrics were tested. Figure 13 depicts the bursting properties of the dyed fabrics affected by different concentrations of FS solution. For the control sample, the bursting property was 184 N. When the concentration of FS solution was 0.5–2 wt %, the bursting property of the dyed fabric was about 160 N. So we can determine that the concentration of sodium fluorescein had little effect on the bursting strength. But, the bursting strength of the dyed fabric was lower than the control sample. There are probably two reasons. On the one hand, cellulose fiber was easy to expand after moisture absorption during the dyeing process and then dried quickly in the oven, which caused certain damage to the fibers, weakened the cohesive force between fibers in the yarn, and made the yarn slightly fluffy (Figure 2(B)). On the other hand, the wet strength of the regenerated cellulose fiber is reduced by 50% under wet state conditions and the elongation is also increased significantly. Under conditions of dampness and heat, the sample will undergo slight plastic deformation after continuous stirring by a mechanical external force. As a result, the strength of the sample also decreased to a certain extent after drying, but is in an acceptable range.

Figure 13.

Bursting strength of dyed fabrics after fluorescent treatment.

Conclusion

In this present study, knitted fabrics were dyed with fluorescent dyes and showed bright colors and excellent fluorescence effects. SEM results depicted that a large number of granular fluorescent particles were coated on the fiber surface. At the same time, influencing factors (dyeing time, temperature, pH, etc.) were examined to characterize the color strength and fluorescence intensity of the dyed fabric. The results showed that at conditions of dyeing temperature 80℃, dyeing time 30 min, NaCl concentration 1 wt%, K/S values were higher than at other conditions. The color strength of dyed fabrics gradually increased from 0.05 to 1.32 with an increase of fluorescein sodium concentration. Moreover, the fluorescence intensity of dyed fabric was also highest at 80℃. In addition, the bursting strength of dyed knit fabrics was increased from 154 to 167 N with an increase in the concentration of fluorescein sodium. Thus, the results showed cellulose based knit fabric dyed with fluorescent dyes exhibited excellent fluorescent properties and could be used in fluorescent clothing.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following support for the research, authorship, and/or publication of this article: This work was financially supported by Qingdao Innovation Project (17-1-1-68-jch), China Postdoctoral Science Foundation (2016M592141), Shandong Postdoctoral Innovation Project (201603067), National Natural Science Foundation of Shandong Province (BS2015CL017), and Industrial Research Institute of Nonwovens & Technical Textiles, Qingdao University.

ORCID iD

Jinfa Ming

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