Optimization of Sodium Lignosulfonate Treatment on Nylon Fabric Using Box–Behnken Response Surface Design for UV Protection

Nylon filament having a linear density of 149 deniers was procured from M/s JCT Ltd. Hoshiarpur, Punjab, India. Sodium lignosulphonate power was procured from M/s Lignotech, Umkomass, South Africa.

Nylon filament was woven into the 2/2 t will fabric of 90.8 g × m⁻² areal density with 26 threads per cm in the warp and 31 threads per cm in a weft direction using a loom. The fabric was woven with the suitable thread density for the preparation of the fabric of the above areal density with the available reed on the loom.

The fabric was scoured with 5 g × L⁻¹ non-ionic detergent for 30 min at 60°C before LS treatment. Various concentrations of LS solution (10%, 15%, and 20%) were prepared in an aqueous medium and the nylon fabric was treated in an infrared heating chamber for 30 min by varying the temperature and pH.

The LS concentrations, treatment temperature, and pH were taken as independent variables. The levels of these parameters are shown in Table 1. The levels are selected based on a preliminary experiment where satisfactory results were obtained. The coating of LS over the textile surface depends on the temperature and pH of the solution. Depending on the pH of the treatment solution, the color of LS coating varies, which was evidenced by Ultraviolet-visible (UV-Vis) spectroscopy analysis. The color of the sample may ultimately affect the UPF of the treated fabrics. The difference in the color of LS at different pH is shown in Figure 1(a). It is observed from the figure that the color of LS is sensitive to change in pH. The color of the LS solution is pale brown in a slightly acidic and neutral pH. However, it turned dark brown in an alkaline pH. The main cause of the dark brown color of lignin is the groups of quinonoid structures. As mentioned earlier, the catechol structure in LS is easy to oxidize to quinonoid structures, which render the lignin strongly colored [14]. Figure 1(b) shows the UV-Vis spectra of the LS solution. The solution shows the absorbency at 280 nm. The peak is due to the content of guaiacyl compound. Similar observations have been reported by Yang et al. (2015) [14].

Figure 1

The process parameters were optimized during the application of LS by a 3³ Box–Behnken design response surface design to obtain the best results. The Box–Behnken design of the experiment is widely used for the optimization of chemical processes as a rotatable design. In the design, the experimental runs are formulated in such a way that no run has all parameters in their extreme levels. As a result, the harsh conditions of the chemical treatment can be avoided and optimum levels of the treatment conditions can be identified using response surface methodology. Experimental design software (Design-Expert version 6.0.8) was used for the design of test runs and statistical evaluation of responses.

The FTIR spectra of untreated and LS-treated nylon samples were recorded using a Bruker Alfa FTIR with a scan rate of 4 cm⁻¹ in the wave range of 4,000–500 cm⁻¹. The surface topography of control and LS-treated fabric samples was scanned by Sigma field emission scanning electron microscope from Carl Zeiss.300. Gold coating of samples was done by using a sputter coating set-up. The EDX study of control and LS-treated fabric samples was performed on the same FESEM instrument with EDX attachment. Thermal stability of the LS coated nylon fabrics was investigated by a Perkin Elmer 4000 TGA instrument in a temperature range of 40°–400°C with a heating arte of 10°C per min.

The UPF of the fabric samples was estimated with Gester GT-C30 UPF tester, according to AS/NZS4399 standards against the radiation range of 290–400 nm. Ten samples of 2.5 cm × 2.5 cm were prepared and ten observations were taken from each sample. The color values of the LS-treated samples were evaluated with the aid of the Computer Color Matching system (Konica Minolta, model No. 4808) enabled with JAY pack software. The Gloss Meter (CHN Spec technology, Hangzhou model No. CS300S) was used to analyze the lustrous appearance of fabric samples by estimating the amount of reflected light at an angle of 60°. Tensile strength and elongation of untreated and LS-treated fabric samples were measured by an Instron 4200 tester following ASTM standard.

Q-Sun weather-o-meter was used to evaluate the light fastness property of fabric samples as per ISO 105-B02. The fabric samples were exposed for 10 h by keeping irradiance −1.10 W × m⁻², wavelength −420 nm, and the temperature of the chamber −50°C. The washing fastness and color staining were as per ISO 2.

Response surface methodology was used to establish an empirical relationship between the response variables, UPF, and a set of experimental factors. The process parameters such as concentration, temperature, and pH of LS solutions were optimized using 3³ Box–Behnken response surface designs. The concentration of LS solution was kept at 10% (−1), 15% (0), and 20% (+1) level, temperature of LS solution was kept at 60 °C (−1), 75 °C (0), and 90 °C (+1), and pH of LS solution was kept at 5.5 (−1), 7.0 (0), and 8.5 (+1) respectively. According to this design, a total of 17 samples were prepared. UPF-values of all these 17 fabric samples are measured and reported in Table 2.

3.1.1

Effect of concentration of LS solution and temperature on UPF

Figure 2 shows the contour and response surface plot between concentration and temperature of LS solution on fabric UPF. The contour plot shows that UPF increases with an increase in the concentration of the LS solution. UPF initially increases and then decreases with an increase in treatment temperature. The maximum value of UPF is observed at the highest concentration of LS solution (20%) around the mid-value of the treatment temperature (75 °C). The increase in UPF with an increase in the concentration of LS solution is attributed to an increase in the add-on of lignin on fabric. Earlier studies have also verified that lignin is a good absorber of UVR [15].

Figure 2

3.1.2

Effect of temperature and pH on UPF

The contour and response surface plot of UPF factor against temperature and pH of LS solution is shown in Figure 3. It can be seen from the contour diagram that the UPF factor initially marginally increases and then decreases with an increase in temperature and pH of LS solution. It is observed from Table 2 that the highest UPF is observed with Treatment 4. (at 75 °C temperature and 5.5 pH). From the table, it is ascertained that the pH of the LS solution is an important factor in determining the UPF. The UPF-values were found to be higher in acidic pH, in comparison with alkaline pH.

Figure 3

3.1.3

Effect of LS concentration and pH on UPF

Figure 4 shows the contour and response surface plot of the influence of concentration and pH of LS solution on UPF. The contour plot clearly indicates that UPF increases with an increase in the concentration of LS solution. It initially increases then decreases with an increase in the pH of the solution. The increase in UPF with an increase in the concentration of LS solution is attributed to increasing lignin add-on on fabric, leading to an increase in the ability to absorb the UVR. It is also indicated that an acidic medium leads to an increase in fabric UPF rather than an alkali medium.

Figure 4

The proposed mechanism of reaction between nylon molecule with sodium lignosulphonate is shown in Figure 5. Nylon 6 and sodium lignosulphonate molecules possess –NH– and –free radical group –OH– respectively. Free radical of OH group of LS attaches with –NH– group of nylon 6, consequently releasing H₂O molecule during reaction.

Figure 5

FTIR spectra of uncoated nylon and LS coated nylon are shown in Figure 6. FTIR spectra of pure nylon fiber shows absorption band at 3,297 cm⁻¹ that may be attributed to the vibration of –OH groups, band at 2,922 cm⁻¹ that may be attributed to –CH asymmetric stretching, band at 1,635 cm⁻¹ that may be attributed to amide I band, band at 1,537 cm⁻¹ that may be attributed to –CH₂ asymmetric deformation of amide II, band at 1,462 cm⁻¹ that may be attributed to –CH₂ scissoring and –NH deformation, and band at 1,369 cm⁻¹ that may be attributed to – CH₂ wagging of amide III. The FTIR spectra of LS-treated nylon shows all vital peaks of nylon fiber as well as characteristic peaks of lignin. Shifting of the peak of 1,635 cm⁻¹ of nylon to a new position at 1,590 cm⁻¹ after LS treatment indicates the intermolecular interaction between –NH– group of nylon with –OH group of lignin [16]. The prominent peak at 1,024 cm⁻¹ indicates C–H Plane deformation of lignin [17, 18].

Figure 6

The surface topographic FESEM images of control and LS-treated nylon 6 fabric samples are shown in Figure 7. It is evident from the figure that there is no prominent deposition which is visible on the fiber surface. This may be because of the chemical reaction between the nylon and LS, rather than a physical coating on the surface.

Figure 7

For evidencing the presence of LS on the surface of nylon, EDX analysis was carried out for all samples. The EDX spectra of untreated and 20% LS-treated nylon fabric are shown in Figure 8. It is visible from EDX analysis that the control nylon fabric showed three peaks for carbon, nitrogen, and oxygen elements. In addition to the said peaks, the LS-treated fabric sample showed extra peaks for sulfur and sodium, due to LS treatment. From Table 4, it is observed that the pure nylon fabric sample has 64.78% carbon, 17.56% nitrogen, and 17.65% oxygen. The element-composition is changed after LS treatment on nylon fabric in terms of sodium and sulfur. The LS-treated nylon fabric sample has 63.87% carbon, 16.77% nitrogen, 19.04% oxygen, 0.24% sodium, and 0.07% sulfur.

Figure 8

The fabric color strength of control and LS-treated fabric at different concentrations are measured and the results are shown in Table 5. Lignin is colored brown due to the presence of chromophoric groups such as catechols, aromatic ketones, coniferaldehyde, stilbenes, and conjugated phenolics, [19, 20]. These chromophores can interact with nylon to impart a yellow color to the fabric (Figure 9). Table 5 demonstrates that the increase in the concentration of sodium LS drives the increase in the value of color strength linearly. These increases in color-strength values of LS-treated fabric samples were found to be statistically significant, as proved by the ANOVA test. It is reported that the UPF of the fabric harbors a positive relationship with the color values of the fabric, because a dark color absorbs higher UV radiation than light shades [21]. In our study also, we have observed a direct positive correlation between the UPF and K/S.

Figure 9

The light fastness properties of LS-treated fabric samples tested under the xenon arc lamp are shown in Table 5. All LS-treated fabric samples reveal a very good lightfastness rating. It is evident from light fastness data that sodium LS has an excellent affinity with nylon fiber surface under the xenon arc light. The washing fastness of LS-treated nylon samples is excellent and exhibits staining neither on cotton nor on scoured nylon fabric after washing. This may be attributed to the fact that sodium LS has an effective affinity with nylon fabrics. It is also noted that the soaping process given after the LS treatment is one of the reasons for getting good fastness properties.

Fabric luster is one of the attributes which affects the visual appearance of a fabric. It is the amount of specular light the fabric reflects. The luster behavior of control and LS-treated fabrics at different concentrations are shown in Table 5. The untreated nylon fabric was found to be highly lustrous (4.55 Gloss 60°). The presence of LS on treated nylon fabric samples suppressed the luster intensity and the concentration of LS is inversely proportional to the luster intensity. The reduction in luster intensity in the presence of sodium LS is attributed to its coloring potential which exhibits higher absorption of light in the visible region.

Tensile strength, modulus, elongation of untreated nylon fabrics, and all LS-treated nylon fabrics are shown in Table 6. It can be seen that after LS treatment there is a slight improvement in the tensile properties of nylon fabric. Therefore, it can be concluded that the LS treatment is enhancing the strength of the nylon fabrics and is therefore very safe for the modification for bringing about the UV protection properties in it. The enhancement of the tensile strength of nylon fabric after LS treatment is due to the significant intermolecular interaction as shown in Figure 5.

TGA analysis of untreated nylon and 20%-LS-solution-treated–nylon-sample was conducted to understand the thermal degradation behavior. The TGA thermogram is shown in Figure 10. It can be seen that LS treatment does not cause any significant deterioration of the thermal stability of nylon. The insignificant weight loss at 400°C may be due to the loss of LS molecules from the nylon surface without affecting the nylon 6 polymer.

Figure 10