## 3 Results and Discussion  
### K/S Values  
The K/S value indicates the degree of dyeing of the fabric. The Kubelka-Munk theory is used to calculate the absorption coefficient K and the scattering coefficient S of the measured object, and establishes a certain functional relationship with the concentration C of the colored substance in the solid sample. The result is the K/S value. The smaller the K/S value obtained by calculation, the lighter the surface color of the fabric; that is, the lower the concentration of the colored substance, the better the bleaching effect [25]. Table 4 shows the K/S values of denim fabrics with different bleaching methods. It is a fact that after laser or potassium permanganate treatment, the K/S values decrease dramatically. This shares a similar trend with the cellulose enzyme treatment, sodium hypochlorite treatment and hydrogen peroxide treatment on K/S values loss. The different mechanism of color-fading causes this huge difference. The changes of kapok/cotton and cotton denim fabrics are similar. The principle of sodium hypochlorite bleaching is that sodium hypochlorite generated ClO' ions which oxidize organic pigment on the fabric in the alkaline condition. During the bleaching, a partial double-bond of the pigment would get saturated, and the original conjugated double-bond would be broken. The chromophore of dyes was damaged, and the color was faded. The theory of hydrogen peroxide bleaching is similar with sodium hypochlorite bleaching. Hydrogen peroxide produces HO\({}_{2}\)' ions with strong oxidizing in alkaline condition, then the organic pigment oxidizes, and the conjugated structure would be destroyed. Finally, the denim fabrics would be discolored. With potassium permanganate under the condition of the acid decomposition of active oxygen with special strong oxidizing [O], the fibers could be damaged and the dye disappeared, so as to achieve the purpose of color-fading. Bleaching mechanisms, such as oxygen bleaching, chlorine bleaching and manganese drift lotion, involve the use of the mechanism of water oxidation which acts directly on the pigment. But for potassium permanganate bleaching, both the dye and the fibers of these fabrics \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **K/S (Fabric1)** & **K/S (Fabric1) Loss (\%)** & **K/S (Fabric2)** & **K/S (Fabric2) Loss (\%)** \\ \hline Desized & \(15.833\pm 1.102\) & 0 & \(14.252\pm 1.011\) & 0 \\ Laser & \(3.899\pm 0.072\) & \(75.374\pm 1.392\) & \(4.219\pm 0.080\) & \(70.397\pm 1.335\) \\ Enzyme & \(13.310\pm 0.091\) & \(15.935\pm 0.109\) & \(11.773\pm 0.075\) & \(17.394\pm 0.111\) \\ NaClO & \(13.675\pm 0.098\) & \(13.630\pm 0.098\) & \(11.214\pm 0.073\) & \(21.316\pm 0.139\) \\ KMnO\({}_{4}\) & \(7.703\pm 0.111\) & \(51.348\pm 0.740\) & \(6.570\pm 0.103\) & \(53.901\pm 0.845\) \\ H\({}_{2}\)O\({}_{2}\) & \(12.789\pm 0.871\) & \(19.226\pm 1.309\) & \(10.546\pm 0.064\) & \(26.003\pm 0.158\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: K/S Values of samples with different bleaching methods. are decomposed, so the K/S value of the denim fabric with potassium permanganate bleaching is the smallest. The bleaching mechanism of the enzyme is that the cellulose enzyme decomposes cellulose fiber (cotton fiber), resulting in the pigment on the cellulose fiber bleaching out. Laser bleaching uses the heat produced by the laser machine to etch the fiber directly onto the fabric surface in a short time. The fibers are damaged, and the dye which adheres to the fibers would not be in existence. Both enzyme bleaching and laser bleaching decomposes the fiber, but the fiber degradation by this enzyme is a slow process, whereas the fiber decomposition by laser etching is a fast process. Compared with enzyme bleaching, the laser bleaching is a more efficient method to wash denim fabrics, and the K/S value decreases greatly. For the five kinds of bleaching above, the effect of laser bleaching was the best, and the K/S value loss is greater than others.