**4. Dyeing properties**

**3.4. Color properties**

88 Eco-Friendly Textile Dyeing and Finishing

UMP particles which are dispersed finely in dispersions enhance the color depth of UMP dispersions (e.g. transmittance, chroma, and lightness) and improve color display perform‐ ance. The transmittances were enhanced with decreasing the mean size of dispersion (Figure 4). A color analysis program estimated the lightness (L\*) and chroma (C\*) from a spectrum of dispersion samples at a wavelength of 380 to 780 nm based on the L\*a\*b\* Color System and D65 light source. The finer sizes of UMP particles in dispersion resulted in the higher lightness (L\*) and chroma (C\*). Serious aggregation of UMP particles without a dispersant in the dispersion would be performed. Holding the supercritical fluid-assisted dispersion

process at the supercritical region conduced to better dispersion [43].

**Figure 4.** The color analyses from the four representative samples of dispersions. (ο), Lightness; (•), chroma

Color properties are presented the color of the UMP dispersion. The absorbance of UMP hy‐ drous dispersion is slightly larger than that of containing organic solvents. The change is mainly attributed to the disparate polarities of the solutions. In these two solutions, while the polarity of solvent UMP dispersion, which contains more ethanol, is weaker. The color of UMP is aroused by the π→π\* energy transition when the chromophore groups are irradi‐ ated. The weak ionization of the UMP increases in a solution with high polarity (such as H2O), and the electric charge in the conjugated system can transport more easily, which leads to an increases in the absorbance. However, the maximum absorption wavelengths for both systems with or without solvent nearly remain the same. This indicates that the chro‐ mophore groups of the UMP are not damaged in the UMP dispersion containing organic solvent and the conjugated systems are essentially not altered, therefore, the color hue of the solvent UMP dispersion remains identical to that of the UMP hydrous dispersion [35].
