# Green-Dyeing Processes of Plant and Animal Fibers Using Folium, an Ancient Natural Dye
## 4 Discussion  
The purpose of this work is to find the best conditions for a dyeing method for plant and animal fibers (i.e., cotton and wool) using an ancient dye called folium without the use of solvents and mordants and via eco-friendly processes.  
The employed textile fibers were first treated with He-plasma, as well as a He-plasma enriched with 20% O\({}_{2}\), to functionalize the surface by increasing its roughness, thus strengthening the binding of dye molecules.  
Infrared characterizations showed that the He-O\({}_{2}\) plasma treatment works best for cotton, whereas pure He-plasma treatment is preferable for wool; in both cases, a 2 min-treatment for both fiber types is best. The processes mentioned above resulted in better wettability of textiles, as well as better absorption of the dye. It is possible to observe, from the FTIR spectra, the increase in the oxygen species (in cotton tissues treated wit He-O\({}_{2}\) plasma) and sulfur-oxygen species (in wool tissues treated with He plasma) that increase the compatibility between the fiber's surface and chitosan molecules.  
Dye tests proved that the method used for samples of raw cotton dyed for 24 h and cotton treated with He-O\({}_{2}\) plasma and dyed for 4 h was not enough to allow the dye to  
Figure 5: (**A**) FORS spectra of raw cotton (a) after plasma treatment with He, (b) dyed with folium, (c) washed; (**B**) FORS spectra of raw cotton (a\({}^{\prime}\)) after plasma treatment with He-O\({}_{2}\), (b\({}^{\prime}\)) dyed with folium, (c\({}^{\prime}\)) washed; (**C**) FORS spectra of raw wool (a\({}^{\star}\)) after plasma treatment with He, (b\({}^{\star}\)) dyed with folium, (c\({}^{\star}\)) washed; (**D**) FORS spectra of raw wool (a\({}^{\star\star}\)) after plasma treatment with He-O\({}_{2}\), (b\({}^{\star\star}\)) dyed with folium, (c\({}^{\star\star}\)) washed.  
stick to the material after washing. Therefore, the dyeing step for the latter samples was carried out at increasingly high temperatures to favor all of the binding steps of the dye onto the textile fibers.  
Samples of cotton (treated with He-O\({}_{2}\) plasma and dyed for 4 and 1.5 h) and wool (treated with He and He-O\({}_{2}\) plasma and dyed for 1.5 h) were previously treated with chitosan, with concentrations of 3.5, 5, and 10 g/L, to verify which concentration could maximize the color fastness. The best results were obtained using a 10 g/L concentration.  
The temperature was kept below 50 \({}^{\circ}\)C, as natural substances can be subject to thermal degradation. Finally, the temperature of the dye bath was increased to 100 \({}^{\circ}\)C for cotton and 80 \({}^{\circ}\)C for wool, for samples of cotton and wool treated with He and He-O\({}_{2}\) plasma and coated with a solution of chitosan 10 g/L and dyed for 1.5 h. These samples showed the best dye absorption and color fastness after the washing procedure (Figure 6).  
The results obtained from the dyeing and washing steps were confirmed by means of fiber optic reflectance spectroscopy (FORS), which displays folium-related peaks in the range of 500-600 nm, specifically with two peaks at 545 nm and 575 nm, also in the spectra acquired for the washed samples.  
Furthermore, a larger amount of dye was observed on the wool samples, compared to the cotton ones; a possible reason behind this is that pigments with a structure like that of folium bind more strongly onto animal fibers rather than onto plant fibers.  
As a matter of fact, animal fibers mainly consist of proteins containing amino- and carboxyl groups, which form ionic bonds with protonated anthocyanins. On the contrary, cellulose does not contain such functional groups; therefore, only weak van der Waals interactions and hydrogen bonds can be formed between the OH- groups of anthocyanins and cellulose.  
This work demonstrates that it is possible to design eco-friendly dyeing methods which do not employ any solvents or mordants, and thus require less water and energy consumption, as well as avoiding the production of potentially hazardous waste.