After scouring, most of the natural and man-made impurities on the fabric have been effectively removed, significantly enhancing the capillary effect and making it suitable for processing certain types of fabrics. However, for bleached fabrics, light-colored, or brightly colored cotton textiles, further whitening is often required. This means that any remaining pigments on the fabric need to be removed to achieve a whiter finish. Even after scouring—especially when using atmospheric pressure steaming—some stubborn impurities like cottonseed hulls may still remain. These can be completely eliminated by the action of a bleaching agent.
Oxidative bleaching agents such as sodium hypochlorite and hydrogen peroxide are commonly used in cotton printing and dyeing operations. For cotton and cotton blends, peracid compounds like sodium perborate, peracetic acid, and sodium percarbonate are occasionally used, while sodium chlorite is mainly applied to synthetic fibers and their blended fabrics. Sodium hypochlorite bleaching is typically referred to as chlorine bleaching, hydrogen peroxide as oxygen bleaching, and sodium chlorite as sub-bleaching.
**1. Sodium Hypochlorite Bleaching**
Common forms of sodium hypochlorite include bleaching powder and sodium hypochlorite solution. Bleaching powder is produced by reacting chlorine gas with slaked lime, while sodium hypochlorite is obtained by passing chlorine through a caustic soda solution. Although both contain calcium hypochlorite, sodium hypochlorite offers better performance. Large-scale textile mills often produce or purchase sodium hypochlorite locally, while smaller factories in rural areas may still use bleaching powder due to its availability. However, bleaching powder is more expensive and less commonly used today.
Sodium hypochlorite is widely used for bleaching cotton and cotton-blended fabrics, and sometimes for polyester-cotton blends. It should not be used on protein fibers like silk or wool, as it can damage them and cause yellowing. During the bleaching process, natural pigments in the fibers are broken down, resulting in a whiter appearance. However, this process can also harm the fiber structure if not carefully controlled.
There are two main methods for sodium hypochlorite bleaching: drenching and continuous rolling. Drenching involves immersing the fabric in a solution and allowing it to soak at room temperature before washing and acid treatment. Continuous rolling, on the other hand, uses a machine to apply the bleaching solution and then roll the fabric. The process typically includes steps like bleaching, washing, acid treatment, and drying.
The amount of effective chlorine in the bleaching solution is crucial. For scoured fabrics, 1.5–2 g/L is standard, while higher concentrations may be needed for lower-quality cotton. Acid treatment with sulfuric acid is essential to neutralize the bleach and prevent residual chlorine from damaging the fabric during storage.
Several factors affect the effectiveness of sodium hypochlorite bleaching:
- **pH Level:** The best results occur at pH 9.5–10.5. At lower or higher pH levels, fiber damage increases.
- **Temperature:** Higher temperatures speed up the reaction but can also weaken the fibers. A typical range is 20–30°C.
- **Concentration:** Too high a concentration can reduce fabric strength, so it's often balanced with longer bleaching times.
- **Dechlorination:** After bleaching, residual chlorine must be removed to avoid yellowing and damage during storage. Hydrogen peroxide is often used for this purpose.
**2. Hydrogen Peroxide Bleaching**
Hydrogen peroxide, also known as peroxide, is another popular bleaching agent. It provides excellent whiteness and color purity, and the fabric is less likely to yellow over time. Compared to chlorine bleaching, hydrogen peroxide is more versatile but also more expensive. It requires stainless steel equipment and consumes more energy, which increases costs.
The hydrogen peroxide bleaching process typically involves padding the fabric with a solution containing 2–5 g/L of hydrogen peroxide, adjusting the pH to 10.5–10.8 with caustic soda, and then steaming at 95–100°C for 45–60 minutes. Afterward, the fabric is washed thoroughly.
Key factors affecting hydrogen peroxide bleaching include:
- **Concentration:** Higher concentrations do not necessarily lead to better results and may damage the fabric.
- **Temperature:** Increasing the temperature speeds up the bleaching process but must be balanced to avoid fiber degradation.
- **pH Level:** The optimal pH range is 9–10, where the bleaching efficiency is highest.
- **Metal Ions and Stabilizers:** Metal ions like iron and copper can catalyze hydrogen peroxide decomposition. Stabilizers such as water glass or organic phosphonates help control this, though they may leave deposits on equipment.
Other hydrogen peroxide bleaching methods include the chlorine-oxygen double bleaching method, which combines chlorine and oxygen bleaching to reduce the need for high hydrogen peroxide concentrations. Another method is the cold-rolled pile technique, which is useful for small-batch production and does not require specialized equipment.
In all cases, safety is a priority. High-concentration hydrogen peroxide can cause severe skin burns, so proper protective gear must be worn during handling.
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