Bleaching is the most important part of wet processing. Bleaching is done for removing natural color of the textile materials. During pre treatment of the textile materials different parameter is maintained strictly. Bleaching is done by different types of bleaching agents.
Bleaching agents are applied on different types of fiber. Textile fiber may be natural or synthetic. Depending on fiber types bleaching agents also differ. Bleaching agents mainly classified as the follows:
- Ozone (O3)
- Bleaching Powder
- Hydrogen Per Oxide (H2O2)
- Sodium Hypochloride (NaOcl)
- Sodium Chlorite (NaclO2)
- Potassium Dichromate (K2Cr2O7)
- Potassium Permanganate (KmnO4)
- Per Acetic Acid (CH3COOH)
- Sodium Per Oxide (Na2O2)
It is mentioned that;
Above no. 1 and 2 agent uses for home purpose.
Above no.3, 4 and 5 agent uses for industrial purpose.
Above no. 6, 7, 8 and 9 agent uses for washing purpose.
- Ferrose Sulphate (FeSO4)
- Zinc Dust (ZnO)
- Stannous Chloride (SnCl2)
- Sulpher Dioxide (SO2)
- Sodium Hypo Sulphite (Na2S2O2)
- Hydrogen Sulphide (H2S)
Bleaching agents have different applicability depending on fiber types.
Different types of bleaching agents use on different fibers. Among all of the bleaching agents hydrogen per oxide is called universal bleaching agent. There have some reasons behind this specialty. They are as follows:
- Hydrogen per oxide is a very gentle bleaching agent.
- It is very easy for application.
- Permanent treatment is possible with hydrogen per oxide.
- It is easy to apply with any types of fiber.
Bleaching agents are applying on fiber by two processes. They are:
- Discontinuous or Semi-continuous process: In this method bleaching agent apply on fiber discontinuously or semi continuously. Bleaching tank, Jigger, Padder, Kier and many others use for treatment with bleaching agent.
- Continuous process: In this process bleaching treatment is performed as per sequence of treatment. J-Box use for continuous process.
During bleaching; following points must be considered for get better performance:
- Bleaching Agent
- Wetting Agent
- Sequestering Agent
- Materials and Liquor ratio
- Application time
Different types of machine manufacturing company manufacture this type of machine. Machine should have enough properties which is required for performing the operation.
Anyhow, when we will select the chemicals we have to ensure the chemical properties, which is required. Well known chemical manufacturing companies also supply the guide line of chemical use. We will use right bleaching agent for right fiber.
Whiteness of textile materials is liked to everybody. White fabric colored with dyes. Sometimes we want to get a white fabric; which is obtained by bleaching process. After scouring process bleaching is done for get white fabric. Bleaching is done for removing natural color which is inherent in the fiber.
Bleaching is the second steps of pretreatment of textile materials as well as wet processing technology. Bleaching process can be defined as; the destruction of natural coloring matter from the textile materials in order to achieve a clean white end product. Different types of bleaching agent use during bleaching.
Objectives of Bleaching: Different types of objectives are gained by the bleaching process. Followings are the objectives of bleaching:
- The main objectives of bleaching are to get a sufficiently high and uniform degree of whiteness in the textile materials.
- To get a high and uniform absorptivity in the textile materials.
- Bleaching agent occur some damage to the textile materials. So bleaching must be accompanied with minimum fiber damage.
- To preserve a good user and technological properties of the textile materials.
- The process must be ecologically and financially sensible.
- To accelerate the next dyeing process.
Bleaching agent is alkaline in properties. Before using it one should have deep knowledge about different types of bleaching agents. Different types of bleaching agent are available in the market. Some are strong and some are weak in alkaline nature. So, we have to idea about which bleaching agent is suitable for which type of fabric. Following process parameters for bleaching must be considered. Points are as follows:
- Bleaching agents have a chemical construction. During bleaching; the chemistry of bleaching.
- The concentration of bleaching agents and other chemicals which use in the bleaching process.
- Bleaching is generally done in alkaline medium of the bath. Bath pH decides the performance of bleaching. So pH of the bath should be considered.
- Temperature of the bath should be considered during process of bleaching.
- Bleaching performance also affected by the duration of bleaching. So, time is an important factor.
- Materials and liquor ratio also considered during bleaching process.
- Scouring is done for removing impurities of fabric or fiber. If scouring process is not successfully done then presence impurities hamper the bleaching performance. So, presence of impurities in the materials is important point for bleaching operation.
- Bleaching agents are sensitive to atmospheric condition. So, storage condition of bleaching agent is important. The place of storage should be neat, clean and dry. Otherwise the working activities could be damaged.
Bleaching is done in different types of machine. Bleaching agents are manufactured by different types of chemical manufacturing companies. All of the bleaching agents have not same characteristics. In bleaching process H202 is used widely for bleaching.
So, bleaching should be perfect for get a better dyeing performance. After bleaching, materials need to neutralize for next process. After neutralization process materials get ready for dyeing. It also called that scouring and bleaching is the half of dyeing.