Why are fabrics bleached? What are the common bleaching methods used in dyeing factories?

After scouring, most natural and man-made impurities on the fabric are removed, and the capillary effect is significantly improved, meeting the processing requirements of some varieties. However, for bleached fabrics and brightly colored light-colored floral and dyed fabrics, it is necessary to further improve whiteness, hence the need for bleaching.

1. Purpose of Bleaching

The purpose of bleaching is to remove natural pigments, giving the fabric the necessary whiteness, and simultaneously removing natural impurities that were not completely removed during scouring.

2. Commonly Used Bleaching Agents: Oxidative Bleaching Agents: Sodium hypochlorite (chlorine bleach), hydrogen peroxide (oxygen bleach), peracetic acid, sodium chlorite (acetic acid bleach), sodium perborate. Reducing Bleaching Agents: Sodium sulfite, sodium dithionite (sodium hydrosulfite).

In actual production, the first two oxidative bleaching agents are most commonly used, with hydrogen peroxide being the most widely used. Oxidative bleaching mainly destroys pigments through oxidation; however, it should be noted that while destroying pigments, the bleaching agent may also damage the fibers.

Reducing bleaching agents, when used to bleach products, cause the reduced pigments to re-oxidize and regain their original color after prolonged exposure to air, leading to a decrease in the whiteness of the fabric. Therefore, they are rarely used nowadays.

3. Bleaching Methods

Bleaching methods are categorized as flat-width and rope-like, single-head and double-headed, loose and tight, continuous and intermittent.

Bleaching methods mainly include immersion bleaching, spray bleaching, and continuous pad bleaching. Immersion bleaching involves soaking the fabric in the bleaching solution. Spray bleaching involves evenly piling the fabric in a tank and continuously spraying the bleaching solution onto the fabric using a pump. Spray bleaching is a non-continuous process and is rarely used nowadays. Continuous pad bleaching involves bleaching the fabric in a large container or other equipment for a period of time after padding with the bleaching solution.

4. Processed Items

Cotton and cotton-type fabrics: Cotton dyeing and printing plants widely use oxidizing bleaching agents such as sodium hypochlorite and hydrogen peroxide. For bleaching cotton and cotton-type fabrics, peracidic compounds such as sodium perborate, peracetic acid, and sodium percarbonate are occasionally used. Sodium chlorite is mostly used for bleaching synthetic fibers and their blends.

Ramella fabrics: Generally, sodium hypochlorite is used in a relatively dilute bleaching solution (available chlorine at 2 g/L) and for a longer time. After chlorine bleaching, deoxygenation with H₂O₂ is used to achieve good bleaching results.

Silk fabrics: Whether silk fabrics need bleaching depends on the type of silk and the requirements of the final product. Bleaching agents that can be used for silk fabrics fall into two main categories: oxidizing bleaching agents and reducing bleaching agents.

Synthetic fibers: Since synthetic fibers do not contain pigments, they are generally not bleached.

5. Bleaching Process

Hydrogen peroxide bleaching: Hydrogen peroxide, also known as hydrogen peroxide solution, has the molecular formula H₂O₂. Hydrogen peroxide bleaching is abbreviated as oxygen bleaching. Under alkaline conditions, hydrogen peroxide solution has very poor stability. Therefore, commercial hydrogen peroxide solution is weakly acidic. Fabrics bleached with hydrogen peroxide exhibit good whiteness and pure color, and are less prone to yellowing during storage. It is widely used for bleaching cotton fabrics. Oxygen bleaching has greater adaptability than chlorine bleaching, but hydrogen peroxide is more expensive than sodium hypochlorite, and oxygen bleaching requires stainless steel equipment, resulting in higher energy consumption and higher costs than chlorine bleaching.

Currently, the most commonly used method in dyeing and printing plants is the flat-width steam bleaching method. This method offers high continuity, automation, and production efficiency, with a simple process flow and no environmental pollution.

The hydrogen peroxide bleaching process is as follows: padding with hydrogen peroxide bleaching solution → steaming → washing. The bleaching solution contains 2–6 g/L of hydrogen peroxide (100%). The pH is adjusted to 10–11 with caustic soda, and appropriate amounts of stabilizer and wetting agent are added. The fabric is padded in the bleaching solution at room temperature, steamed at 95–100℃ for 45–60 minutes, and then washed with water.

Sodium hypochlorite bleaching: Sodium hypochlorite bleaching is mainly used for bleaching cotton fabrics and cotton-vinyl alcohol blends, and sometimes for polyester-cotton blends. However, it cannot be used for bleaching protein fibers such as silk and wool, as sodium hypochlorite has a destructive effect on protein fibers, causing yellowing and damage. During the bleaching process, in addition to the destruction of natural pigments, the cotton fibers themselves may also be damaged. Therefore, it is essential to control the bleaching process conditions to ensure both appearance and internal quality meet standards.

Sodium hypochlorite is easy to manufacture and inexpensive. Sodium hypochlorite bleaching is convenient to operate and requires simple equipment. However, due to its environmentally unfriendly effects, it is gradually being replaced by hydrogen peroxide.

The sodium hypochlorite bleaching process is as follows: Padding with sodium hypochlorite bleaching solution → Stacking → Washing → Dechlorination → Washing. The bleaching solution contains 3-5 g/L of available chlorine. The pH is adjusted to around 10 with caustic soda. The fabric is then soaked in the bleaching solution at room temperature, piled up for 30-60 minutes, and then washed with water to remove the chlorine.

After bleaching cotton fabrics with sodium hypochlorite, a small amount of residual chlorine remains. This is generally removed using reducing agents such as sodium thiosulfate, sodium bisulfite, and hydrogen peroxide.

Sodium chlorite (NaClO2) bleaching: The biggest advantage of using sodium chlorite for bleaching cotton fabrics is that it can destroy pigments and impurities without damaging the fibers. Sodium chlorite is also a good bleaching agent for synthetic fibers, and the whiteness stability of bleached fabrics is better than that of fabrics bleached with chlorine or oxygen. It is not suitable for wool or other protein fibers. However, sodium chlorite is expensive, highly corrosive to metals, requiring the use of titanium or titanium alloys, and produces toxic ClO2 gas during the bleaching process, necessitating well-sealed equipment. Therefore, its use is somewhat limited, and it is currently mostly used for bleaching polyester-cotton blended fabrics.

Unlike chlorine bleaching and oxygen bleaching, semi-acid bleaching is performed under acidic conditions, typically with a pH of 4-4.5. Semi-acid bleaching has low requirements for pretreatment; fabrics can even be bleached without desizing. It has a shorter process route and is particularly suitable for synthetic fabrics. However, fabrics bleached directly without desizing and scouring generally have poor absorbency. Adding an appropriate amount of nonionic surfactant to the bleaching solution can improve the fabric's absorbency. A subsequent oxygen bleaching after semi-acid bleaching improves both the whiteness and absorbency of the fabric. If the fabric is desizing and scouring before semi-acid bleaching, the amount of sodium chlorite can be reduced, but the process is longer.

The commonly used process for semi-acid bleaching is similar to that of oxygen bleaching. It can use a continuous pad-steaming method or a cold bleaching process depending on the equipment. The continuous pad-steaming process for semi-acid bleaching is as follows: Padding solution → Steaming → Washing → Dechlorination → Washing.

Sub-bleaching process for polyester-cotton blended fabrics: The bleaching solution contains 12-25 g/L sodium chlorite, 5-10 g/L ammonium sulfate (activator), and 5-10 g/L of bleaching agent. The bleaching solution is applied at room temperature, steamed at 100-102℃ for 1-1.5 hours, washed with water to remove chlorine, and then thoroughly washed again after piling.

5. Evaluation of Bleaching Effect

The primary purpose of bleaching cotton fabrics is to remove natural pigments. However, the cotton fibers themselves may be damaged during the bleaching process. Therefore, evaluating the bleaching effect requires considering both the achieved appearance quality (whiteness) of the bleached fabric and the potential damage to the fibers during the bleaching process.