The dyeing and finishing of read-made jeans garments has gained increasing prominence in recent years. It has gained a position of ever-increasing importance in the clothing market, due to wearing comfort and fashion. Designers learned to make jeans attractive to an ever increasing number of customers through innovative modifications and styling.
Bleaching denim, of the garment washed, are stone-washed and bleached with different chemicals and treatments. Bleaching is a treatment of textiles in order to lighten the fabric for the final shades.
Let us now discuss the different bleaching processes, their benefits and features, and also the problems of the various treatments. Most bleaching processes are made on open-pocket washing machines or barrel machines (sometimes in Asia), in which many denim products can be treated together. The difference between these machine types is the diameter of the drum and the speed of rotation. The recipes and running times in this report are based on recipes for open-pocket machines. But there are also some mechanical and technical treatments, which are carried out for each single garment.
Bleaching with sodium hypochlorite is by far the most common bleaching process. It is used when the indigo's color needs to be lightened as required by fashion, as well as clean up the pocket linen and the inner face of the jeans (weft). This particular bleaching process is preferably used whenever a strong contrast between blue and white fibers is desired, since it gives the bluest cast of all bleaching processes. Moreover, this process is very cost-effective, however, a major disadvantage is that the AOX content, a measure of organic halogen compounds, often exceeds' the permitted effluent pollution.
Sodium hypochlorite is a powerful oxidizing agent with a corresponding high redox potential. Hypochlorite consequently reacts relatively unselective; it reacts even with the fibers. The danger of fiber damage with this bleaching agent is much greater compared to other bleaching processes. Due to its high oxidation agent potential, hypochlorite bleaching is carried out at low temperatures (approx. 40C).
actual bleaching agent in bleaching liquors is not sodium hypochlorite (NaOCI)
itself, but the hypochlorous acid (HOCI) which is formed from it in a pH
sensitive equilibrium. In order to ensure mild bleaching conditions, a pH range
between 9.0-11.5 has been found as optimum range for effective bleaching.
Since, under practical conditions, pH levels below 10 are difficult to keep
constant, an initial pH range in
11.0-12.5 is selected, as a rule to take into account the consumption of soda ash or caustic soda.
The control of the pH value is very important. The strongest bleaching activity is at pH 9.5 - 12.5; at pH 5 - 7 the activity is the lowest. After reaching the required bleaching effect, the bleaching process has to be stopped in the same bath by using hydrogen peroxide or sodium bisulfite.