# Pigment Dyeing of Atmospheric Pressure Plasma-Treated Cotton Fabric
## 3 Results and Discussion
### Colour Yield  
Table 4 shows the colour yield (expressed as \(K/S\) value) of cotton fabrics dyed with pigment concentrations of 1 g/L and 5 g/L, respectively. Samples 1 to 9 represent cotton fabric treated with atmospheric pressure plasma with different combinations of operational parameters (Table 1). It is noted that the K/S values of the atmospheric pressure plasma-treated cotton fabric are higher than untreated cotton fabric (i.e., control), which indicates more pigment particles in the atmospheric pressure plasma-treated cotton fabrics than in the untreated cotton fabric. Since atmospheric pressure plasma treatment can improve the water absorption ability of cotton fabric by inserting oxygen-containing functional groups and generating cracks/grooves in the fibre surface [9; 10], both chemical and physical surface modifications facilitate pigment absorption in the atmospheric pressure plasma-treated cotton fabric. As a result, a better colour yield was obtained in the atmospheric pressure plasma-treated cotton fabric than in the untreated control cotton fabric. In Table 4, the \(K/S\) values of 1 g/L of pigment-dyed cotton fabrics were not significantly affected in relation to the discharge power (130 W to 170 W). Conversely, in the case of 5 g/L, the \(K/S\) values of 5 g/L pigment-dyed cotton fabrics had a positive relationship with the discharge power (Table 4). Under the influence of atmospheric pressure plasma treatment, the cotton fibre surface can be modified by generating oxygen-containing functional groups (which are hydrophilic in nature) and cracks/grooves along the fibre axis [9; 10]. Generally speaking, the discharge power at a high level can produce deep cracks/grooves along the fibre axis, because a high discharge power implies a high active species formation rate [10; 23]. With the increasing of level of discharge power from 130 W to 170 W, deep cracks/grooves are generated in the cotton fibre surface, and hence, more space is provided for accommodating the pigment particles. This makes the pigment-dyeing solution diffuse at a faster rate along the fibre axis for enhancing pigment absorption, resulting in an increased colour yield. In the case of the oxygen flow rate, it is noted that the oxygen gas used can contribute an active plasma species for enhancing surface etching and the oxidizing effect [2; 3; 4]. After atmospheric pressure plasma treatment, the newly inserted oxygen-containing groups (e.g., -OH and -C-O) in the cotton fibre surface are capable of forming more hydrogen bonds with water molecules [10]; then, the pigment-dyeing solution absorption is enhanced as a result. However, based on the \(K/S\) values of samples 3 to 6, there is no obvious relationship between colour yield and the oxygen flow rate, although samples 3 to 6 did achieve a better colour yield than the untreated cotton fabric. To conclude, the increase in the \(K/S\) values of the atmospheric pressure plasma-treated cotton fabrics is influenced by the combined effect of both discharge power and the oxygen flow rate. The increase in the \(K/S\) values of the atmospheric pressure plasma-treated cotton fabric is mainly due to the formation of cracks/grooves in the fibre surface. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Concentration** & **Control** & **1** & **2** & **3** & **4** & **5** & **6** & **7** & **8** & **9** \\ \hline
1 g/L & 0.92 & 0.97 & 0.96 & 0.95 & 0.98 & 0.98 & 0.97 & 1.00 & 0.98 & 0.97 \\
5 g/L & 4.54 & 4.68 & 4.71 & 4.90 & 4.86 & 4.84 & 4.93 & 4.94 & 4.84 & 4.75 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: \(K/S\) values of 1 g/L and 5 g/L pigment-dyed cotton samples. As shown in Table 4, an increase in the head moving speed (Samples 7 to 9) shows a negative effect on the colour yield of the yellow pigment-dyeing application (1 g/L and 5 g/L). When the head moving speed is kept at 1 mm/s, the \(K\)/\(S\) value is the highest (Sample 7).