Most fabric that is dyed, printed, or finished must first be prepared, with the exception of denim and certain knit styles. Preparation, also known as pretreatment, consists of a series of various treatment and rinsing steps critical to obtaining good results in subsequent textile finishing processes. . In preparation, the mill removes natural impurities or processing chemicals that interfere with dyeing, printing, and finishing. Typical preparation treatments include desizing, scouring, and bleaching. Preparation steps can also include processes, such as singeing and mercerizing, designed to chemically or physically alter the fabric. For instance, the mercerizing stage chemically treats the fabric to increase fiber strength and dye affinity, or ability to pick up dyes. This, in turn, increases the longevity of fabric finishes applied during finishing. Many of the pollutants from preparation result from the removal of previously applied processing chemicals and agricultural residues. These chemical residues can be passed on to subsequent stages with improper preparation.
Most mills can use the same preparation equipment for the entire range of products they produce. In most cases, facilities favor continuous rather than batch preparation processes for economic and pollution control reasons. A number of mills, however, prepare goods, particularly knits, batchwise on dyeing machines to simplify scheduling and handling. Sometimes, facilities operate batchwise to reduce high capital costs required for high productivity and the complexity of storing and tracking goods through continuous wet processing operations.
Because preparation is relatively uniform across most of a mill's production, preparation is usually the highest-volume process in a mill and hence an important area for pollution prevention. If fabrics contained no contamination upon arrival for wet processing, preparation processes would be unnecessary, eliminating about half the pollution outputs from wet processing and a significant amount of wastewater. The primary pollutants from preparation is wastewater containing alkalinity, BOD, COD, and relatively small amounts of other contaminants such as metals and surfactants.
Following are major fabric preparation techniques