Study on hydraulic spray atomizing system as a new resource-efficient dyeing-finishing method for wool fabric

Gerard Ortiz

The results from this study are presented in two parts; Part 1 presents the results related to the dyeing performance of resultant wool fabrics prepared through two-steps or one-step HS method in relation to the conventional exhaust dyeing method. Part 2 presents the results related to the finishing performance of resultant wool fabrics prepared through HS methods (two-steps or one-step) in relation to the conventional pad-dry-cure method.

Part 1: Analysis of dyeing performance

A comparative study to assess the dyeing performance of the two-steps and one-step HS method in relation to the conventional exhaust dyeing method were carried out through color strength (K/S) measurements, color difference (ΔECMC) measurements, and fastness to washing and abrasion based on color measurements of the dyes wool fabrics.

Color strength (K/S) measurement

All dyed wool fabrics with both acid and reactive dyes were evaluated through color strength measurements to identify the difference between the conventional and HS dyeing method. In principle, the color strength value provides evidence related to the depth of the color on the dyed fabric surface22. Results presented in Fig. 2b and c show the plots of K/S values of the dyed wool fabric samples prepared with acid dye and reactive dye. Results from both acid and reactive dyed samples showed that there is a significant difference in color strength depending on the method of dyeing used, which is also visible by the naked eye (Fig. 2a). Conventionally dyed samples showed higher color strength values than HS dyed samples. This can be due to the possible diffusion limitation of dyes in the HS dyeing method compared to the conventional dyeing method, which restricted the dyes to be evenly distributed on the pores of the wool fabric23.

Figure 2
figure 2

(a) Digital photographs of the samples; (b) Color strength (K/S) measurements of Acid dyed wool fabric; (c) Color strength (K/S) measurements of reactive dyed wool fabric ( = Conventional;   = Two-steps HS method;  = One-step HS method).

Higher diffusion in conventional methods may occur due to the use of electrolytes in the conventional method (which was not used in HS method) that influences the solubility and adsorption of dyes into the fibers24. The wool fiber swells in liquid, and the acidic conditions charge the amino acids on the surface, making it possible for the dye to enter the fiber and make strong bonds with the fibers25. As the dye fixation in the HS method is a rather dry process, where the fabric is subjected to only dry heat while being moist from the spray liquid, the wool fiber swells less, and affects the dye fixation process. The fabric surface keeps less moisture (the characteristic of wool) during the fixation, and the dyes migrate into a damper environment with lower pH. The level of unfixed dyes is not higher, so the fixation seems to take place deeper into the fabric. Nevertheless, the K/S value of samples dyed using new HS methods showed significant color strength as high as 14.0 which is suitable for commercial application.

A close look at the results shows that, there is a noticeable difference in color strength between the one-step and two-steps HS dyed-finished samples. One-step HS dyed samples showed better color strength than that of two-steps spray dyed samples. The poor color strength in the two-steps HS dyeing method can be due to meddling applied to the color during the finishing step. Although the dyeing method for both cases was the same, in the one-step method, dyes and finishes were mixed and sprayed over the wool fabric together, whereas in the two-steps method, dyes and finishes were sprayed separately over the samples to form a layer-by-layer assembly of dyes and finishes. The extend of the difference in color strength between the one-step and two-steps dyed samples were found to be influenced by the type of finishes used and the weight of the fabric (see Fig. 2). Finish 2 (Ruco-Dry DHE) was found to result in more color difference than Finish 1 (Ruco-Dry ECO DCF). Lighter weight wool fabric (W2/ 264 GSM) provided better color strength in HS dyed samples compared to heavier weight wool fabric (W1/469 GSM).

Color difference (ΔECMC) measurement

To further understand the dyeing performance of the conventional and HS method, the color difference of both acid and reactive dyed wool fabric samples were evaluated. At first, the comparative color difference analysis of the HS dyed samples (one-step) was carried out in respect to the samples prepared through conventional method (see Fig. 3a). After that, the color difference between one-step and two-steps HS dyed samples was studied, as well (Fig. 3b). Results from the color difference between HS dyed and conventional exhaust dyed wool fabric samples shows a significant color difference that can be detected by the naked eye as all samples showed a ΔECMC value of over 1.0. This indicates the possible color difference of the samples due to the dyeing condition in different combination of HS method (one-step or two-steps), wool fabric (W1 or W2), and finishing agent (F1 and F2) which can be subjected to optimization before bulk processes in industrial scale. Nevertheless, the color difference for W1 fabric (469 GSM) was found strongest with 6.6 for acid dyes and 6.7 for reactive dyes. For W2 fabric (264 GSM), the color differences are 4.5 for acid dyes and 4.1 for reactive dyes. This emphasizes the characteristics of two different textile processes to achieve altered product performances. A close look at the results reveals that acid dyes account for a higher color difference than reactive dyes. Reactive dyes are a better fit for a continuous dyeing process, as the dyeing mechanism is less dependent on the swelling of the wool fiber under high temperatures and presence of water26,27. On the other hand, the color difference in a one-step HS method is lower than two-steps method (see Fig. 3a). Further analysis on the color difference measurement between the two-steps and one-step dyed samples (see Fig. 3b) shows that W1 fabric dyed with either acid dyes or reactive dyes and finished with F1 has shown ΔECMC values, which are high enough to be detected by the naked human eye28. On the contrary, the color difference of W2 fabric dyed with acid dyes and finished with F1 has shown ΔECMC value less than 1, which indicates the existence of color difference beyond the detection limit of the human eye.

Figure 3
figure 3

Color difference (ΔECMC) between (a) HS dyed (two-steps) and conventional exhaust dyed samples; (b) One-step and two-steps HS dyed samples (shaded square = color difference points).

Fastness to washing based on color strength (K/S) measurement

Color fastness is an essential analysis to determine the performance of dyeing. Resultant wool fabric samples prepared through either conventional or HS dyeing method were subjected to fastness to washing analysis. The dyeing performance over fastness to wash has been evaluated based on color strength measurements, which is plotted in Fig. 4. Results show that, regardless of the method used, K/S values of most samples have decreased after washing. The decrease in K/S after washing can be explained as the loss of loosely fixed dyes from the fabric during washing29,30. Some samples showed surprising increase in color strength after four washing cycles compared to one cycle, which can be due to the attainment of the evenness of dyes on the fabric surface after possible patches of dyes were removed. This novel study has opened several new discussions through its findings, where investigation of reported phenomenon of color strength related to washing is one of them. Although this is out of the scope of this work, but it certainly can be explored for better understanding of the HS technology for dyeing and finishing.

Figure 4
figure 4

Fastness to washing based on color strength (K/S) measurement; (a) Acid dyed wool fabric; (b) Reactive dyed wool fabric ( = Standard K/S (left);   = K/S after 1 wash (middle);  = K/S after 4 washes (right)).

Fastness to washing based on color difference (ΔECMC) measurement

Color fastness of resultant wool fabric in terms of washing has been further evaluated based on color difference of samples before and after washing. Results are presented in Table 2, which showed that the samples dyed with acid dyes are significantly different in color after washing (observed for both conventional and HS methods). Result shows that, W2 fabric holds its color better after washing than W1 fabric, as the ΔECMC values have a lower significant difference. A comparison between acid and reactive dyes shows that reactive dyes have a better washing fastness than acid dyes, as most ΔECMC values stay close to or < 1, which can be found for both types of fabric dyed with reactive dyes. In general, the color difference increased with the number of washes for samples prepared with all three methods (one-step HS method, two-steps HS method, and conventional method). A close look on the results provides evidence of comparatively higher color difference on the HS dyed samples then the conventionally dyed samples. The ΔECMC in HS1-W1@AF1 after one wash was 3.73 that rose to 5.34 after four washes, whereas C-W1@AF2 has an initial ΔECMC of 0.71 after one wash that rose to 1.50 after four washes. This can be due to the successive impact of the washing cycles on the interaction of the loosely attached/bonded dyes with the fabric surface that causes the abstraction of dyes from the fabric31. Nevertheless, despite the loss of dyes, the strength of color is high enough to retain the characteristics of the dyed fabric as a colored material as supported by the K/S analysis.

Table 2 Fastness to washing based on color difference (ΔECMC) measurement.

Fastness to abrasion based on color strength (K/S) measurement

Color fastness of selected dyed wool fabrics in respect to abrasion was studied based on color strength K/S (before and after abrasion) according to the method described earlier (material characterizations section). Results shows that samples prepared through hydraulic spray atomizing system exhibited no significant difference in color strength after abrasion test regardless of one-step and two-steps process as well as dyes used. This phenomenon is particularly important as the hydraulic spray atomizing system is a continuous coloration process that excludes several after treatment process compared to conventional method. A detailed study can be carried out as a further study to understand the mechanism of superior color fastness of selected dyed wool fabric in respect to abrasion.

Part 2. Analysis of finishing performance

To understand the effect of each preparation method and the performance of the hydrophobic finishes, all finished samples were comparatively studied through water contact angle measurement and their fastness in respect to washing and abrassion. A one-way ANOVA analysis was performed on the data to determine the significant difference between the samples.

Water contact angle ((theta _mathrmH_2mathrmO)) measurement

To assess the hydrophobicity of the samples, contact angle measurements were performed as described earlier in material characterizations section. Figure 5 shows the (theta _mathrmH_2mathrmO) of the wool fabric prepared with either Ruco-Dry ECO DCF (F1) or Ruco-Dry DHE (F2) finishes. Results show a significant difference in (theta _mathrmH_2mathrmO) of the finished wool fabric depending on the preparation method used. In general, all samples finished with any of the two finishes showed higher water contact angle on the fabric when it is prepared with the HS method compared to samples prepared through conventional padding method. This can be explained with the hydrophobic nature of wool which repels liquid to enter the core of the fiber or fabric26,32. As the finishing liquid most likely does not fully penetrate into the fabric, the water-repellent chemicals will primarily react with the fibers on the fabric surface, which results in higher contact angles for the HS finished samples compared to the conventional padded samples.

Figure 5
figure 5

Water contact angle analysis of hydrophobic finished wool fabrics; (a) Ruco-Dry ECO DCF (F1); (b) Ruco-Dry DHE (F2) and (c) digital picture of hydrophobicity of wool fabric.

As for Ruco-Dry ECO DCF (F1), samples prepared with the conventional padding method for W1 fabric, a (theta _mathrmH_2mathrmO) of 125° was recorded, which can be increased by 9° if prepared through a two-steps HS method and by 14° if prepared through one-step HS method. This indicates a better finishing performance of the HS method over conventional padding method once a surface effect, in this case a water-repellency, is desired. Comparing the one-step and two-steps HS method shows that the one-step HS method is more efficient than the two-steps method. The high finishing performance is related to the even and uniform distribution of the finishes on the surface of the wool fabric. On the other hand, a comparison between W1 fabric and W2 fabric shows that there is no significant difference in finishing performance contrary to the difference in dyeing performance. For samples prepared with Ruco-Dry DHE (F2), samples prepared with the HS method show better finishing performance than conventionally padded samples. However, no significant difference was found between the one-step and the two-steps HS method. In general, the finishing performance of the samples prepared with Ruco-Dry DHE was found to be higher than that of Ruco-Dry ECO DCF. Samples finished with Ruco-Dry DHE approach superhydrophobic properties with contact angles between 140 and 150° for one-step HS samples33,34. Notable is also that there is no significant difference in average contact angles between the two different fabrics W1 and W2.

Fastness of hydrophobic finishes of wool fabrics to washing

The fastness properties of the applied finishes on the two wool fabrics have been studied in respect to washing as presented in Table 3. Results show that, in general, almost all samples decrease in the finishing performance after washing, which can be related to the removal of loosely attached or bonded finishes on the fabric surface. Comparing conventionally padded, one-step and two-steps HS finished samples, the loss of performance is more prominent in samples prepared with the one-step HS method followed by the two-steps HS method and lastly the conventional padding method. The differences in (theta _mathrmH_2mathrmO) for the samples prepared with HS methods can be related to the fact that the hydrophobic agents seem to make weaker bonds in a direct spraying process. As it is likely that there are more negatively than positively charged amino acids on the surface, the water repellent agents form weaker bonds with the fiber surface. During washing, these bonds are easily broken causing the fabric to lose some of its hydrophobicity35. Besides, there is no significant difference in average contact angles for all samples after washing, the samples of all three processes show similar contact angles. For W1 fabrics finished with Ruco-Dry Eco DCF these contact angles are 129° prepared by the conventional method and 132° and 130° in the two-steps and one-step HS method, respectively. This is similar for W2 fabrics with the same finish, where the contact angles respectively vary from 131° (conventional padding) to 133° (two-steps HS) and 132° (one-step HS). After a washing cycle, the samples were tumble dried to restore the full effect of the water-repellent finish. The realignment of the hydrophobic agent on the fiber surface can cause the contact angle to increase after washing, as seen with the conventional samples. In general, Ruco-Dry DHE performed worse in the washing test than Ruco-Dry Eco DCF, whereas the initial contact angles of DHE were higher than those of DCF as presented in Table 3.

Table 3 Fastness of hydrophobic finishes to washing based on water contact angle of the finished wool fabrics.

To further understand the differences among the samples prepared through all three methods, results were analyzed through a paired t-test. Table S1 of the supplementary information discusses the P-values of the performed paired t-test. If the P-value is below 0.05, the null hypothesis, i.e., the mean of the differences is 0, should be rejected. This means that the differences in means before and after washing are significantly different, when the P-value is lower than 0.05. A few samples show an insignificant difference in their contact angle before and after washing, although a consistency in the values is missing. Generally, the samples dyed and finished conventionally show a lower significance of difference.

Fastness of hydrophobic finishes of wool fabrics in respect to abrasion

Another factor that can affect the performance of finishes is abrasion. Therefore, the fastness of hydrophobic finishes on wool in respect to abrasion has also been studied based on the water contact angle measurement. Similar to fastness to washing, the performance of hydrophobic finishes was also affected by abrasion. In general, the loss of performance is more prominent in samples prepared with one-step HS followed by the two-steps HS and lastly by conventional padding as presented in Table 4. As was mentioned earlier, direct spraying in the HS method causes the hydrophobic finish to form less strong bonds because of the lack of positively charged amino acids on the fiber surface, which are thus more easily rubbed off.

Table 4 The t-values and P-values of the paired t-test performed on the contact angle measurements before and after abrasion.

To determine whether the difference between the means before and after abrasion is significantly different, a paired t-test was carried out. Table 4 presents the t- and P-values that were gathered from the experiment as well as mean water contact angles before and after abrasion. If the P-value is below 0.05, the null hypothesis, i.e., the mean of the differences is 0, should be rejected. In most cases, this means that the contact angle of the samples dyed and finished conventionally are not significantly different before or after abrasion. All wool fabric samples dyed and finished in the HS methods, except for HS2-W1@F1, C-W1@F2, HS2-W2@F1 and C-W2@F2, however, show that there is no significant difference in the contact angle measurements before and after abrasion, as the P-values are below 0.05.

Sustainable aspects of the HS atomizing process

The sustainable aspect of HS atomizing process has been investigated in terms of use of water, energy, and chemicals. Proposed HS methods are continuous dyeing-finishing process; thus, the processing time was not in the scope of the study. Nevertheless, the speed of the process is subjected to real-time optimization during bulk productions. Table 5 shows an overview of the resource consumption in different HS method compared to conventional methods. Results shows that, the HS methods attributes promising resource-efficient properties with saving of up to 88% water, 100% chemicals depending on the fabric and process it replaced. However, over 200% more dyestuff is used in this process, as the HS dyebath is much more concentrated which need to be optimized before large scale industrial applications. Along with records from Imago, it can be seen that, the new HS method reduces the consumption of energy, water and produces less waste that is identical to the sustainable trifecta -net zero energy, water and waste. Calculations on a large scale are very dependent on many variables, but a comparison of the lab scale methods can at least be made, which may allow predictions for industrial production. The contents of these baths can be translated to their contents per gram of fabric. When comparing the results between the one-step and two-steps HS method, it can be seen that the one-step method uses 50% less water because it combines two baths into one. Because of the shift in pick-up percentage from 60 to 80% of the water-repellent chemicals, 25% less of these chemicals were added to the one-step bath. Because of the higher content of the more alkaline wetting agent in the bath, more acetic acid needed to be added to adjust the pH value.

Table 5 Sustainable aspect of HS methods compared to conventional method.

The reactive dyebath is less acidic and thus requires less acidic acid to balance the pH. The reduction in energy used comes from the dye fixation process. In an exhaust dyeing process, the bath liquid has to stay at a certain temperature throughout the process, which is energy consuming. The different method for dye fixation, in an autoclave as opposed to a heated and moving bath, cause the reduction in energy use. The wastewater generated in the HS method is also less than in a conventional process. The dye or finish liquid is used almost entirely, reducing the wastewater from the dyeing, and finishing process.

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