After scouring, most of the natural and man-made impurities on the fabric have been removed, and the capillary effect has significantly improved. This makes the fabric suitable for processing certain types of textiles. However, for bleached fabrics, light-colored, and brightly colored cottons, further whitening is often required. This means that any remaining pigments on the fabric must be removed to achieve a higher level of whiteness. Even after scouring, especially when using atmospheric pressure steaming, some impurities like cottonseed hulls may still remain. These can be effectively eliminated by the action of a bleaching agent.
Oxidative bleaching agents such as sodium hypochlorite and hydrogen peroxide are widely used in cotton printing and dyeing processes. For cotton and its blends, peracid compounds like sodium perborate, peracetic acid, and sodium percarbonate are occasionally used. Sodium chlorite, on the other hand, is mainly used for synthetic fibers and their blended fabrics. Hypochlorite bleaching is commonly referred to as chlorine bleaching, hydrogen peroxide as oxygen bleaching, and sodium chlorite as sub-bleaching.
(1) Sodium Hypochlorite Bleaching
Common forms of hypochlorites include bleaching powder and sodium hypochlorite. Bleaching powder is formed when chlorine gas reacts with slaked lime. It can also be produced by passing chlorine through a caustic soda solution, resulting in sodium hypochlorite. While calcium hypochlorite is the active ingredient in bleaching powder, its overall effectiveness is not as good as that of sodium hypochlorite. Many large-scale textile mills produce sodium hypochlorite themselves or purchase it from nearby chemical plants. Bleaching powder is more commonly used in small-scale operations where access to sodium hypochlorite is limited due to cost or availability.
Sodium hypochlorite bleaching is known for its simple process and equipment, making it ideal for treating cotton and cotton-blended fabrics. It is sometimes used for polyester-cotton blends but cannot be applied to protein-based fibers like silk or wool, as it can damage these materials and cause yellowing.
During the bleaching process, natural pigments in the fibers are broken down, leading to a loss of color. However, this process can also cause some damage to the cotton fibers. Therefore, careful control of the bleaching conditions is essential to maintain both the quality and strength of the fabric.
The two-time sodium hypochlorite bleaching method includes drenching and continuous rolling. Drenching involves stacking the fabric in a box and continuously spraying the solution over it at room temperature for 1–1.5 hours before washing and acid leaching. This method is discontinuous and rarely used today. Continuous rolling, however, is more common, involving padding the fabric with bleaching solution, then steaming and washing.
The process for rope continuous bleaching with sodium hypochlorite typically follows this sequence: bleaching solution → stacking → rolling → washing → acid rolling → washing. The effective chlorine content in the solution is usually between 1.5–2 g/l for scoured fabric, and 3 g/l for steamed fabric. Low-quality cotton may require an additional 0.5 g/l of effective chlorine.
Acid pickling is done using sulfuric acid, with concentrations ranging from 1–3 g/l for rope fabric and 2–3 g/l for flat fabric. After acid treatment, the fabric is stacked at 30–40°C for 10–15 minutes. Small factories may use cobblestone floors for this step, but care should be taken to ensure worker safety.
Several factors influence the effectiveness of sodium hypochlorite bleaching:
(a) pH: The optimal pH range for bleaching is 9.5–10.5. At lower pH levels, chlorine gas is released, which can be harmful to workers and equipment. Higher pH values (9–11) result in better whiteness with minimal fiber damage.
(b) Temperature: Higher temperatures increase bleaching speed but can also accelerate cellulose oxidation. A typical operating temperature is 20–30°C, with cooling measures needed if temperatures exceed 35°C.
(c) Concentration: The concentration of available chlorine in the solution determines the bleaching effect. Too high a concentration can damage the fabric, so many mills opt for lower concentrations with longer bleaching times to preserve fiber strength.
(d) Dechlorination: Residual chlorine can cause yellowing and reduce fabric strength during storage. To address this, dechlorination agents like hydrogen peroxide or sodium bisulfite are often used to remove excess chlorine.
(2) Hydrogen Peroxide Bleaching
Hydrogen peroxide, also known as peroxide, is widely used for bleaching cotton fabrics. It provides excellent whiteness, maintains color purity, and resists yellowing during storage. Compared to chlorine bleaching, hydrogen peroxide offers greater adaptability, though it is more expensive and requires stainless steel equipment, which increases energy consumption and costs.
1. Hydrogen Peroxide Bleaching Process
Hydrogen peroxide bleaching can be carried out continuously or in batches, using either steaming or cold-rolling methods. Flat steaming is currently the most popular technique in textile mills due to its efficiency, automation, and environmental benefits.
The standard process involves: padding with hydrogen peroxide solution → steaming at 95–100°C for 45–60 minutes → washing. The bleaching solution typically contains 2–5 g/l of hydrogen peroxide, with pH adjusted to 10.5–10.8 using caustic soda. Stabilizers and wetting agents are added to enhance performance.
2. Factors Affecting Hydrogen Peroxide Bleaching
(a) Concentration: A concentration of 5 g/l is usually sufficient for effective bleaching. Increasing the concentration beyond this point does not improve whiteness and may cause fiber brittleness. Thin fabrics may require lower concentrations.
(b) Temperature: Higher temperatures speed up hydrogen peroxide decomposition, allowing shorter bleaching times. At 90–100°C, the decomposition rate reaches 90%, and the fabric achieves maximum whiteness.
(c) pH: Hydrogen peroxide is stable in acidic conditions but decomposes rapidly in alkaline environments. The optimal pH for bleaching is around 10, where whiteness is maximized without excessive fiber damage.
(d) Metal Ions and Stabilizers: Iron, copper, and other metal ions can catalyze hydrogen peroxide decomposition, reducing its effectiveness. Stabilizers like water glass or organic phosphonates are added to prevent this. Water glass is cost-effective but can lead to silicon scale buildup, while non-silicate stabilizers offer better long-term performance at a higher cost.
3. Other Hydrogen Peroxide Bleaching Methods
In addition to steam bleaching, alternative methods include:
(a) Chlorine-Oxygen Double Bleaching: This combines chlorine and oxygen bleaching, reducing the need for high hydrogen peroxide concentrations. The process involves first applying sodium hypochlorite, followed by hydrogen peroxide bleaching.
(b) Cold-Roll Pile Method: Used in small-scale operations without oxygen bleaching equipment, this method involves high hydrogen peroxide concentrations and extended soaking times. Although less efficient, it offers greater flexibility for small batches.
Hydrogen peroxide is a strong oxidizing agent and can cause severe skin burns. Proper protective measures must be taken when handling concentrated solutions.
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