Stripping (textiles)

Right-first-time production in textiles entails achieving correctness on the initial dyeing attempt, resulting in a reduced need for stripping, rework, or redyeing.^[1]

Stripping is a method of reprocessing employed in the textile industry, wherein the removal of color from fabric becomes essential during the dyeing process. This approach can be employed to rectify any complications encountered during the dyeing process or to alter the color of surplus fabric for subsequent utilization.^[2][3] The stripping process is as complicated as dyeing and requires the greatest care of multiple factors such as dye type, material, reducing agent, cost, etc.^[4] When it comes to removing dye from fabric, it is important to distinguish between back stripping and destructive stripping.^[5]

Back stripping is a technique employed in the textile industry for the purpose of partial dye removal from fabrics. This method entails the selective extraction or displacement of dye molecules from the textile substrate without causing significant damage to the fabric itself. Back stripping only affects the depth of color.^[5][6] Back stripping primarily relies on the use of appropriate chemicals, solvents, or detergents, often in conjunction with controlled temperature and pH conditions.^[2] The objective is to reverse or weaken the chemical bonds that hold the dye molecules to the textile fibers, allowing them to be lifted or washed away.^[5] For an example by applying salt and alkali treatment at a temperature of 95 degrees Celsius, a reduction in dye of 20–40% can be achieved in reactive dyed materials. This occurs due to the hydrolysis of the dye fiber bond.^[7] This process is generally less aggressive and aims to color correction.^[5][6][2]

Destructive stripping, on the other hand, represents a more aggressive approach to dye removal from fabrics. This method involves the use of harsh chemicals, strong acids. The primary objective of destructive stripping is to eliminate the dye molecules. This technique may be employed when the complete removal of the dye takes precedence. Destructive stripping is commonly used where the fabric is to be completely re-dyed. In case of destructive stripping, for example, dyes with an azo group (N=N-) can be chemically reduced to an amine molecule that is almost colorless by using chemical reducing agents.^[5][2] The approach to comprehensive chemical stripping can differ based on the specific dyes applied.^[5] There are several methods for carrying out destructive stripping, which can involve reduction alone, oxidation alone, or a combination of both. Another option is to carry out reduction followed by oxidation or oxidation followed by reduction treatment.^[7]

Direct dyes are removed by either boiling the fabric in alkaline sodium hydrosulfite, bleaching the fabric with sodium hypochlorite, or boiling the fabric with 1–2% sodium chlorite adjusted to a pH of 3 to 4 using formic or acetic acid.^[2]

Reactive dyes constitute a significant category of colorants employed in the dyeing process of cellulosic substances like cotton and viscose.^[8] Over 80% of the dyes employed in the dyeing of cellulosic materials are reactive dyes.^[8] These dyes, as suggested by their name, possess an inherent capacity to chemically react with the hydroxyl groups present in cellulosic materials, resulting in the formation of covalent bonds. Consequently, this chemical reaction imparts excellent fastness properties to the dyed materials.^[9][8]

Vat dyes exhibit resistance to elimination when subjected to reducing chemicals.^[4]

To remove certain azoic combinations, 1 gram of Hydrosulfite and 6 cubic centimeters of caustic soda 77TW [a 32% solution of Sodium Hydroxide]^[10] is recommended.^[4]

Color removal from hydrophilic fibers, which interact well with water, is usually uncomplicated and relies on dyestuff chemistry. Water-soluble agents that react in water often perform the color removal, making the process easy.^[11] Dealing with hydrophobic fibers that repel water is more difficult for dyers. They encounter challenges in getting the stripping agent and dyestuff to react in the same environment and must navigate chemical intricacies.^[11]