# Pigment Dyeing of Atmospheric Pressure Plasma-Treated Cotton Fabric
## 3 Results and Discussion
### Colour Levelness  
Pigment is insoluble in water, but it disperses well in the dyeing solution. However, pigment particles in the dyeing solution may get aggregated in water medium with binder. Therefore, an unlevelness of colour may occur in pigment-dyed cotton fabric due to pigment aggregation. In normal practice, a dispersing agent is added to the dyeing solution in order to avoid the aggregation of pigment particles. However, in this study, a dispersing agent was not used, because we wanted to not only study the effect of atmospheric pressure plasma treatment on the colour levelness of pigment application to cotton fabric; we also wanted to study the effect of pigment aggregation on the colour levelness of pigment-dyed cotton fabric. The colour levelness of pigment-dyed cotton fabric is expressed as an RUI value; a high RUI value indicates a poor colour levelness of the pigment-dyed cotton fabric.  
After atmospheric pressure plasma treatment, the colour levelness of some pigment-dyed samples is improved, but some pigment-dyed samples have serious colour levelness problems. The RUI values of all of the pigment-dyed cotton fabric samples are less than 0.2, which means that all of the pigment-dyed (1 g/L and 5 g/L) cotton fabrics achieved excellent colour levelness in visual appearance. If the RUI values are compared based on the results shown in Figures 2 and 3, the colour levelness problem is more serious in 5 g/L pigment-dyed cotton fabric. This may have been due to the high amount of pigment particles in the dyeing solution, which might have possibly caused pigment aggregation and poor migration during the drying process. Even if we treated the pigment-dyeing solution with ultrasound for particle dispersion and distribution, the reduction in effective diameter from 206.0 nm to 199.9 nm (with about 3% reduction) is not significant. The pigment particle aggregation and poor migration during the drying process can lead to a low colour levelness quantitatively.  
Figure 2: Relative unlevelness index (RUI) of 1 g/L pigment-dyed cotton samples (Front: atmospheric pressure plasma-treated fabric side; Back: non-atmospheric pressure plasma-treated fabric side).  
Figure 4 illustrates that pigment particles are aggregated on a fibre surface, and the effective diameters of some pigment particles, for example, are 416.7 nm (circle mark) and 213.0 nm (square mark). Figure 5 is the SEM image showing that abundant pigment particles are encased by a binder layer on the cotton fibre surface. The uneven distribution of pigment particles on the cotton fibre surface may cause issues regarding an unlevelness of colour. The fluctuation of colour levelness is mainly due to pigment particle migration or aggregation, but not atmospheric pressure plasma treatment.  
Figure 4: SEM image of atmospheric pressure plasma-treated cotton fibre after 5 g/L pigment dyeing (atmospheric pressure plasma treatment condition: discharge power = 170 W; oxygen flow rate = 0.6 L/min; head moving speed = 1 mm/s; and head-to-substrate distance = 3 mm).  
Figure 3: RUI of 5 g/L pigment-dyed cotton samples (Front: atmospheric pressure plasma-treated fabric side; Back: non-atmospheric pressure plasma-treated fabric side).