## 3 Results and Discussion
### Tensile Strength and Weight  
The mean value of tensile strength and tensile strength loss for selected samples are depicted in Figures 2 and 3. The results reveal that a desized denim fabric has a higher tensile strength [23]. For two kinds of denim fabrics, the lowest tensile strength is related to the bleached sample with potassium permanganate, and the tensile strength of treated samples noticeably decreases compared with desized samples. The principle of chemical bleaching (potassium permanganate, sodium hypochlorite and hydrogen peroxide) is that the chemical reagents are used to decompose the dye. The oxidbillity of potassium permanganate is stronger than sodium hypochlorite and hydrogen peroxide, the effect of bleaching is better, the cotton fibers are also damaged, so the tensile strength loss is highest. Figure 2: Effects of bleaching types on tensile strength: (**a**) Fabric 1 (warp), (**b**) Fabric 1 (weft), (**c**) Fabric 2 (warp), (**d**) Fabric 2 (weft). The CO\({}_{2}\) laser treatment uses the heat to etch the fibers onto the surface of the fabrics in a short time, whereupon the dye disappears, but it does not hurt the fibers inside. The usage of cellulose enzyme is to decompose the cellulose fibers, and after treatment, the tensile strength of denim fabrics declines. Because the enzymatic bleaching needs a long time to set off a chemical reaction, and this experimental length was just 15 min, the chemical reaction was not completed. In general, laser bleaching has been similar to cellulose enzyme, and causes a decrease in tensile strength. From the Table 1, the warp density of kapok/cotton denim fabrics (338 threads/10 cm) is higher than that of cotton denim fabrics (302 threads/10 cm), which leads to the higher tensile strength of kapok/cotton denim fabrics. In contrast, the tensile strength in the weft direction of kapok/cotton denim fabrics is much lower than that of the cotton denim fabrics, and this may be caused by the fact that the weft of two denim fabrics is made up of different materials [24]. Fabric 1 is made of 100% cotton, but Fabric 2 is a composite of 70% cotton and 30% kapok. Because of different materials, the fabrics have different properties. The kapok fiber is a hollow fiber of which its hollow degree is more than 90% and the volume ratio is as four times that of cotton. Kapok fiber has a short length, low intensity and poor cohesion, so the fabric made of kapok fiber had a poor mechanical property compared with the other materials. Compared to the warp, the tensile strength loss in the weft of Fabric 2 is smaller, and the kapok's special structure is the main cause of it. Though the mechanical property of kapok/cotton denim is poorer than cotton denim fabric, it arrives as the standard of the garment. So, the kapok/cotton denim material is suited for industrial production. The change of weight of different denim fabrics is reported in Table 5. It is a similar situation with the tensile strength change. The results show that the weights of all the samples were declined, but the weight loss was acceptable (less than 8.74%). The fibers of denim fabrics etched by laser or decomposed by chemicals had all been damaged, and the dye had degraded, so the weights of the samples did decrease. Because of the aforementioned reasons, the denim with potassium permanganate bleaching has the highest weight loss, and the mechanical properties are mostly destroyed. Both the enzymatic bleach and laser bleach act upon the fibers, and this takes a short experimental time, so it shares a  
Figure 3: Effects of bleaching types on tensile strength loss: (**a**) Fabric 1 (warp), (**b**) Fabric 1 (weft), (**c**) Fabric 2 (warp), (**d**) Fabric 2 (weft). similar trend of weight loss. Except for bleaching with cellulose enzyme, the weight loss of Fabric 2 is all less than Fabric 1. The main cause is that the kapok fiber is a kind of hollow fiber, whose hollow degree is more than 90%.