Effect of cellulase enzyme concentrations on denim garment washing
The physical and mechanical properties of denim garments treated at different cellulase concentrations were measured and the results are summarized in Table 1. It can be seen from Table 1 that, treatment of denim garments with cellulase at 0.5% concentration caused significantly decrease in tensile strength and this decrease was higher at higher cellulase concentrations up to 3.5%. During washing, cellulase hydrolyzed cotton. First, it attacked on projecting fibers (micro-fibrils) on surface, then attacked on yarn portion, hydrolyzed them slowly and upon time penetrated inside the fabric. The result of this reaction is that the primary wall (outer layer) of the cotton fiber is loosened and broken down quicker with the frictional action (mechanical forces) of rotating cylinder of the washing machine. This effect also depends on the washing conditions. Hydrolysis of cellulose would certainly affect fabric properties, namely, tensile strength, stiffness, elongation at break, water absorption, moisture regain, moisture content and fabric surface characteristics. Similar results for tensile strength have previously been obtained with undyed cotton cellulose (Heikinheimo et al. 1998; Kleman-Leyer et al. 1996). From, Table 1 it can be seen that at 3% cellulase concentration the strength loss is higher. By considering all the properties of treated denim it can be seen from Table 1 that 2% cellulase concentration is the optimum result for denim garment washing.
Denim hydrolysis was measured by monitoring the color fading from treated garments (Table 1). It can be seen from the Table 1 that the color shade of denim garments decreased significantly after they were exposed to cellulase treatment particularly at higher concentrations of 2.0% to 3.0%. During washing, the part of the primary wall of cotton is always in contact with cellulase, so at the contact point, fiber surfaces are hydrolyzed by the catalysis of the cellulase and then treated garments become duller and color is faded. The hair-like cotton fibrils are degraded first and partly detached from the main fiber chain and indigo dye bonds are broken from the yarn surface. Rotating garments inside washing machine hydrolyzes more bonds due to mechanical friction, restores their original white color. The results disclose that increasing the cellulase concentration from 3.0 to 3.5% has no effect on color shade change, because with 3.0% cellulase concentration most of the indigo dyes are loosened from the fabric surface and no dyes are remain on fabric surface. Therefore no effect found on color shade change when increased cellulase concentration 3.0 to 3.5%. From Table 1 it can also be seen that 2% cellulase concentration is the optimum result for color fading.
Denim hydrolysis by measuring the stiffness of denim garments was also monitored. The stiffness of the denim garments decreased at 0.5% cellulase concentration, and the decrease was more pronounced at higher cellulase concentrations up to 3.0%. After treatment with cellulase, the starch of warp yarns was removed. As a result, bending length was less and stiffness was decreased in comparison to untreated. The decrease bending length was more pronounced at higher cellulase concentrations up to 3%, and stiffness was decreased to 44.6% of the original garment. At 2% cellulase concentration the stiffness decreased to 41.8% which is the closest to desired level. Water absorption rate of denim garment was 155%, whereas unwashed denim was 126% absorption (Table 1).
Moisture content is a measure of the level of water in the denim samples. It can also be seen from Table 1 that the moisture content of the treated samples are similar and in the range of 8.81-8.93%, whereas untreated sample was 7.4%. The same holds also true for elongation at break. Elongation at break of the treated denim garments are very close to similar and lengthwise denim elongation was 34-37% and widthwise denim elongation was 20-22%, whereas lengthwise elongation of untreated denim was 21% and widthwise elongation was 16% respectively.
It can also be seen from Table 1 that cellulase treatment by washing of the denim garments with cellulase at 0.5% concentration caused increase in GSM (fabric weight) of the garments, whereas weight loss (GSM decreased) results have previously been obtained with direct and reactive dyed cotton fabric substrate. During weaving, cotton fabrics are subjected to considerable tensions, particularly in the warp direction. In subsequent finishing processes, such as calendaring this stretch is increased and temporarily set in the fabric. The fabric is then in a state of dimensional instability (Cookson 1992). Subsequently when the denim garment was thoroughly wetted in enzyme washing, it tended to revert its more stable dimensions which results in the contraction of the yarns. This effect is usually greater in the warp direction than in the weft direction. This is known as relaxation shrinkage. Due to relaxation shrinkage, PPI (picks per inch) is higher in treated, compared to untreated denim garments. As a result, fabric GSM is increased at different cellulase concentrations and 2% cellulase concentration is optimum result (Table 1). Although hydrolysis occurred in cellulase washing, at the same time relaxation shrinkage occurred, and the GSM (fabric weight) of denim garments increased slightly. But the use of higher concentrations of cellulase (3.5%) brought losses in fabric weight due to more hydrolysis of cellulose. It was observed that 2% concentration of cellulase results in the maximum increase in fabric weight, compared to the other concentrations.
Thus, 2% cellulase is optimal because this concentration of cellulase exhibited the maximum decrease in fabric stiffness and shrinkage; and maximum increase in water absorption, elongation at break and GSM as compared with the other concentrations.
Effect of temperature in cellulase enzyme washing on denim garment
The effects of 2% cellulase in denim washing under the influence of 40°C, 45°C, 50°C, 55°C, 60°C and 70°C for 40 min was investigated. The effects of temperature on tensile strength, stiffness, color fading, fabric weight, elongation at break, water absorption, moisture regain, moisture content and shrinkage of denim garments are shown in Table 2. The results show that raising the temperature from 40 to 65°C has an effect on tensile strength, stiffness and color fading. At 60 and 65°C, the color shade decreased, and a higher temperature (70°C) does not cause any further decrease in color shade, because the action of cellulase is lowest at higher temperature. The effect of temperature on stiffness and water absorption is clear particularly when washing was performed at 50°C and 60°C, because of the loosening of surface fibers caused by washing temperature. GSM was lower at 65°C and 70°C but substantially higher GSM values were obtained at 55°C. The effect of temperature on color fading and surface roughness is clear particularly when cellulase was performed at 55°C.
Effect of time in cellulase enzyme washing on denim garment
The effects of 2% cellulase in denim washing at 55°C for 20, 30, 40, 45, 50, 55 and 60 min were investigated. The effect of time on tensile strength, stiffness, color fading, fabric weight, elongation at break, water absorption, moisture regain, moisture content and shrinkage is shown in Table 3. The results indicate that there are marginal differences in moisture content, moisture regain, EPI and PPI. But effects on fabric strength loss, color fading, stiffness, shrinkage and GSM are observed with increases of time up to 60 min. It can also be seen from Table 3 that water absorption increased after washing and the rate of water absorption varies between washed and unwashed denim and it increased rapidly up to certain time (40 min), then increased slowly (55 min) but a higher duration (60 min) does not cause any further increase in water absorption. As mentioned previously (Kwon & Sarmadi 1995), the water absorption increased after laundering and the rate of water absorption varies upon fabric status (finished/unfinished) and it increased up to certain level of washing then water absorption decreased, which is co-related to our obtained results of water absorption in denim garment washing. So considering fabric properties 40 min washing time is optimum for denim washing with cellulase.
Scanning electron micrograph
The morphological of the denim garments was examined by scanning electron microscopy (SEM) on the untreated and treated samples. The washing treatment is affected by 2% cellulase for 40 min at 55°C. Figure 1(a) shows SEM images (15.0 kV × 35, magnification 525) of untreated cotton denim garment and Figure 1(b) shows SEM images (15.0 kV × 500, magnification 7500) of untreated cotton denim garment (magnified). The Figure 1 shows parallel ridges and no fibrils (projecting fibers) and ruptures visible in the images, because yarns are coated with size materials and projecting fibers are not visible on surface.
Figure 2(a) shows SEM images (15.0 kV × 35, magnification 525) of enzyme treated cotton denim garment and Figure 2(b) shows SEM images (15.0 kV × 500, magnification 7500) of enzyme treated cotton denim garment (magnified). For Figure 2, the washing condition is followed by 2% cellulase for 40 min at pH 5.5 and 55°C in the fiber-liquor ratio of 1:530. Figure 2 shows loosened, disoriented and wrinkled surfaces due to fiber degradation by hydrolysis and abrasion were due to mechanical friction by the washing machine during processing. As mentioned previously (Betrabet et al. 1980; Li & Hardin 1998), the enzyme attacks the cellulose of cotton progressively, the primary wall being the first target. As observed in Figure 2, there are more cracks on the surface of fibers. This is caused by cellulase washing of cotton denim garments.