As you can see, there are advantages and disadvantages with using Hypochlorite bleaching; however, if this bleach is used properly, the desired end result will be good. This makes the process conditions very important in the use of sodium or calcium hypochlorite. One of the most important conditions is the pH of the bleach bath, both in the beginning and at the end of the process.
The pH of the bleach bath determines the percent of hypochlorous acid (HOCL) liberated during the bleach process. This percentage should be kept at the lowest possible level for optimum bleaching to ensure evenly bleached shades of the cotton garments and minimal damage to the garments or fabric.
At a pH of 8 or less chlorine gas begins to develop. This gas is very dangerous to people in the work area, especially when using open paddle tubs, becks, or washer-extractors which are not vented outside the plant and other unvented open equipment. This condition also causes severe shading and extreme loss of tensile strength in the garments or fabric being bleached.
The pH of the bleach bath should be kept above 9.0, preferably between a pH of 10.0 - 11.0, for optimum bleaching results. To achieve the optimum pH of 10.0 - 11.0 there are several procedural steps to follow.
1. If bleaching after desizing, make sure to use free-rinsing chemicals in the desize process. We recommend our buffered and sequestered alkali desize for most desizing operations, (VIRCOSCOUR NEW-LF). This product will rinse freely and the pH will revert to a pH of 7.0 - 7.5 depending on the pH of the water supply. By using a free rinsing desize system, it will not alter the pH of the bleach bath.
2. If bleaching after a neutral stable cellulase enzyme process, make sure to incorporate at least one rinse prior to your bleach bath and two rinses when an acid stable enzyme process is used. All cellulase enzymes have some type of buffered system to obtain maximum abrasion at a desired pH. These buffers will affect the pH of the bleach bath if a rinse or rinses are not incorporated in the procedure. This will produce inconsistent bleaching results and possibly shaded garments.
3. A buffered alkali works well in sodium or calcium hypochlorite bleaching. Soda ash, caustic and other unbuffered alkali can be neutralized during the bleaching process. As the hypochlorous acid is formed, the pH will drop and produce inconsistent bleaching from load to load or uneven shades within the bleach load.
We recommend several different buffered alkali depending on the desired standard. They are as follows:
A. VIRCOSCOUR T-101 or VIRCOSCOUR TX-101 - These products produce high contrasting white on abraded denim garments and high whites on white woven or knitted garments or fabrics.
B. VIRCOSCOUR NEW-LF,VIRCOLYE 200, VIRCOLYE 2PF, and VIRCOLYE 2LP These products produce a neutral to flat white contrast on abraded denim garments and neutral to flat white on knitted or woven garments or fabric.
All of the above alkali can be used with sodium perborate or hydrogen peroxide to aid in cleaning up denim and maximizing the white contrast without affecting the hand of the end product.
Most hypochlorite bleaches are processed at a temperature of 130°F to 150°F for 10 to 25 minutes. With the proper pH, bleaches have been run at the boil with little or no fiber damage. Having a proper pH can provide for a more efficient use of hypochlorite and consistent shades from load to load.
Attention should be paid to the use of reliable instrumentation for checking and recording time and temperature. Variations in time or temperature will cause inconsistent results from load to load.
