The conventional bleaching of textiles is carried out at high process temperatures and involves considerable amounts of water, wastewater, and energy. A responsible use of energy and resources for the protection of the environment is possible by using the ecological and economical low-temperature bleaching technology COLD WHITE.
Comparison of various bleaching processes
State of the art
Discontinuous bleaching of cotton in jet or yarn machines is usually performed with hydrogen peroxide. The purpose of bleaching is to give a high degree of whiteness and good absorbency to the textile material, thus optimally preparing the substrate to the subsequent dying, printing or production of white goods. Furthermore, only little damage to the cotton and consistent bleaching results (reproducibility) are desired. A conventional bleaching process is performed at 95 to 98°C or under HT conditions up to 110°C. The high temperatures, as well as the addition of alkali, are necessary to activate the required hydrogen peroxide. Also, fiber fats are emulsified in a better way and impurities of cotton (contaminant load) are easily washed off in an alkaline medium. However, high process temperatures require more energy and consequently lead to a larger carbon footprint.
COLD WHITE process
The ecological and economic advantages of a low-temperature bleaching such as lower energy and water consumptions, higher productivity and lower unit cost are obvious. But does this also ensure sufficient process safety for the textile finisher? The team of Zschimmer & Schwarz took up the challenge to develop an innovative process for bleaching that provides a guarantee for ecological and economic efficiency. This COLD WHITE bleaching process includes two key components, which enable a powerful bleaching at temperatures below 80°C:
Component 1) The environmentally friendly OPTAVON LTB is a special hydrogen peroxide activator for the low-temperature bleaching of cotton and its blends. The product accelerates the decomposition of hydrogen peroxide enabling a significant reduction of the bleaching temperature and bleaching time. Using OPTAVON LTB, an excellent bleaching effect is obtained at 70°C already. This allows a high degree of whiteness whereas the damage to the cotton is low. OPTAVON LTB is GOTS 4.0 certified and bluesign® approved.
Component 2) The low-foaming detergent and wetting agent TISSOCYL POW ensures a rapid and uniform wetting of the fabric. Being especially developed for the COLD WHITE system the detergent is able to reliably solve natural fiber impurities as well as fats and waxes, to disperse and to emulsify them and to increase the rewettability of the fabric after bleaching. The bleaching at significantly reduced temperatures requires a strong emulsifying power of the surfactant because emulsification takes place mainly below the melting point of native waxes and paraffins.
The application of the COLD WHITE system yields the following interesting benefits for the textile finisher:
The reduction of the bleaching temperature results in savings of energy and time, a lower carbon dioxide emission and decreased wastewater pollution. In total, the COLD WHITE process allows a noticeable reduction of production cost, an increase in production and preservation of the environment. Additionally, the mild bleaching conditions result in a lower degreasing of the cotton. So the weight loss of the fabric is significantly reduced which increases the yield. Further effects are a smooth fabric passage in the jet machine and a much lower tendency to knot formation. The quantity of a crease preventing agent required for goods that tend to creasing can be reduced. The pilling propensity of the fabrics is diminished as well. Bleaching at low temperatures also causes less damage to the cotton fiber (higher degree of polymerization). Final terry fabrics, in particular, appear softer and more voluminous (fig. 1).
The fact that knitting oils can be removed easier at temperatures below 80°C is often disregarded. At higher temperatures, there is the risk that the oil emulsion is destabilized and stains are formed.
Nevertheless, it is important to take into account that in principle a compromise has to be found between the ecological and economic benefits of a low-temperature bleaching and the demands on the absorbency of the bleached goods to be obtained. In general, the absorbency can be neglected if the fabric pretreated by the exhaustion process is not dried before dyeing. The key for a level dyeing is rather the fact that the fabric is completely wetted and residues of natural waxes are evenly distributed on the fabric (fig. 2).
Standard bleaching vs. low-temperature bleaching
Several application tests were carried out with fabrics consisting of 100 % CO, CO/EL and blends of CO and synthetic fibers in different make-ups (tricot knitted fabric, terry fabric, cross-wound package) that were pretreated in a jet machine and in a package dyeing machine, respectively, using both the standard process and the COLD WHITE low temperature bleaching. Subsequently, the bleached fabrics were dyed.
Recipes and process parameter
1) Standard bleaching
|TISSOCYL RLB||0.7 ml/l||(highly concentrated, non ionic detergent)|
|REDUSTAB OS||1 ml/l||(organic stabilizer for the hydrogen peroxide bleaching)|
|OPTAVON SV||1 ml/l||(complexing and dispersing agent)|
|hydrogen peroxide 35%||4 – 6 ml/l*|
|caustic soda 50%||2 – 2.5 ml/l*|
Liquor ratio 1:8, bleaching 45 min at 98°C, rinsing 10 min at 80°C, rinsing and neutralizing with acetic acid (60%) 10 min at 60°C, rinsing and residual peroxide decomposition 10 min at 40°C, ready for dyeing (see fig. 3).
2) COLD WHITE low-temperature bleaching
|TISSOCYL POW||1 ml/l||(nonionic special detergent for COLD WHITE)|
|OPTAVON LTB||1.5 ml/l||(hydrogen peroxide activator)|
|hydrogen peroxide 35%||6 – 8 ml/l*|
|caustic soda 50%||2 – 2.5 ml/l*|
* Concentration of chemicals depends on the degree of whiteness of the greige fabric and on the whiteness level which has to be reached.
Liquor ratio 1:8, bleaching 40 min at 70°C, rinsing and neutralizing with 0.5 g/l SETACID AB conc 10 min at 40°C, residual peroxide decomposition in the same bath 10 min at 40°C, ready for dyeing (see fig. 3).
Table: Results of the whiteness measurement.
|Whiteness 1) Standard bleaching
45 min 98°C
|Whiteness 2) COLD WHITE
40 min 70°C
|100 % CO single jersey ready-for-dye||72.0 CIE||70.6 CIE|
|100% CO single jersey full-white||154.1 CIE||150.6 CIE|
|100% CO yarn ready-for-dye||71,6 CIE||70.8 CIE|
|100% CO terry fabric ready-for-dye||72.7 CIE||71.2 CIE|
|95/5 CO/EL jersey ready-for-dye||70.3 CIE||68.1 CIE|
Discussion of the results
Degree of whiteness
The results of the whiteness quality achieved for both ready-for-dye and full white articles by the application of the COLD WHITE low-temperature bleaching process were nearly equivalent to those achieved by to the conventional bleaching. All COLD WHITE bleached goods provided for dyeing were optimally prepared for the subsequent dyeing process. The dyeing results were level final color appearances and shades of uniform depth. Our practical experience shows that a higher amount of hydrogen peroxide is often required in the COLD WHITE bleaching in order to get equivalent results.
All fabrics show a softer and more voluminous appearance because of the lower degreasing effect at lower bleaching temperatures. The handle of the terry fabric was rated as particularly soft and fleecy, hence the amounts of soft handle finishing agents could be reduced (fig. 1).
Further investigations of cotton fabrics of different presentations confirm that a high-quality pretreatment is achieved by using the COLD WHITE process which ensures an excellent reproducibility of the final fabric appearance after dyeing.
Water consumption and effluent
The neutralizing agent SETACID AB conc shows very good compatibility with conventional catalases. This enables a reduced number of rinsing baths. One rinsing cycle can be saved after the COLD WHITE bleaching allowing to carry out the neutralization as well as the decomposition of the residual peroxide in one bath at 40°C. The result is a water and effluent saving of at least 20% compared with the conventional process.
A conventional hot bleaching usually leads to a strong detachment of organic components of the cotton which requires more intensive rinsing after bleaching. The COLD WHITE process enables a reduced extraction of cotton due to milder conditions resulting in a higher yield of the bleached fabrics. This contributes to a significant reduction of the chemical oxygen demand (COD) of the wastewater load.
Lower temperatures of the bleaching and rinsing baths and shorter process times reduce the energy demand by about 15% in comparison with conventional bleaching processes.
Process time/production capacity
Shorter heating and cooling times, the reduction of the number of baths and a reduced bleaching time at 70°C result in a reduction of the total process time by about 20%. The plant capacity can be increased without additional technical efforts. Thus, the COLD WHITE process offers an opportunity to increase the production capacity and the efficiency of a textile factory and to reduce the fixed cost considerably.
The pretreatment process COLD WHITE developed by Zschimmer & Schwarz is an alternative method for the discontinuous hydrogen peroxide bleaching of knitted fabrics and yarns of cotton or its blends with synthetic fibers. It represents a perfect basis for customized pretreatment formulations providing flexibility by adapting them to the respective production conditions and grey fabric qualities with simple adjustments. The low-temperature bleaching COLD WHITE not only involves a responsible dealing with our environment but also considering economic aspects like high production safety, consistant quality and improved competitiveness of our customers.
OPTAVON, TISSOCYL, SETACID = product names of the company ZSCHIMMER & SCHWARZ MOHSDORF GmbH & Co KG.