Cold bleaching for cotton

[ Introduction ]

Calls for environmental concerns directed at the processing industry are continuously on the increase, coming mainly from consumers. The textile industry is no exception. Textile processing plants, with their limited number of experienced staff, are required to respond to demanding requests and sudden calls. Cold bleaching is one of the processes that contribute to the homogenization of cotton finishing, be it in small or big lots.

With regard to dyeing, the cold pad batch method is now widely used in line with the spread of reactive dyestuffs. As for bleaching, cold bleaching using hydrogen peroxide is in use.

The following are points touching on cold bleaching.

Kimura Shoji Group promotes textile chemicals production and their effects

[ Features ]

As shown in Fig.1, cold bleaching using hydrogen peroxide has many advantages, though it involves some time constraints. Reduced use of thermal energy, effective use of chemicals with corresponding reduction in the load on wastewater and in the operational load on a limited number of experienced workers in particular are of great advantage. In addition, it gives fabric a soft hand without causing damage, and having no machinery corrosion or problems in the operational environment it imparts good water absorbency, resulting from superior, uniform bleaching and scouring, which guarantee level dyeing in the following stage.

Fig.1 Advantages of cold bleaching method

[ Process ]

The process is shown in Fig.2.

Fig.2 Process of cold bleaching method

Time required for bleaching using hydrogen peroxide alters considerably according to the temperature used.

Room temperature: 40-60: 90-100: 120:

16-24 hours 5-8 hours 60-90 minutes 15-25 minutes

Boiling by the batch method (1 hour) or cold bleaching is generally carried out in the finishing of towels. Despite strict time controls and vast steam consumption, the high-temperature, high-pressure method is widely used for cheese yarns as it allows quick bleaching.

Pick-up for the padding stage is usually 80-110%, but it is 100-110% for towels. To prevent uneven bleaching along the edges of fabric, batch-up rolls should be left airtight under a plastic sheet at room temperature for 20-24 hours.

Next comes the washing stage. Here, there is a variety of processing methods, from which an appropriate one is selected according to the kind of facilities and the size of lots used. A process shown in Fig.3 is used for towels.

Fig.3 Traditional washing process (towels)

( Click the image for enlargement )

Treatment using caustic soda and an activator in the initial washing stage improves the water absorbency of the finished product. This is an indispensable process for products such as towels, in which water absorbency holds importance.

Neutralization by acetic acid is necessary for the prevention of unevenness in reactive-dyeing that follows next. Special attention is required on this point with towels, which are often dyed in pale shades.

The batch method, though with less operational efficiency, is often used instead in small-lot operation, before proceeding to dyeing or drying.

[ Recipes ]

As old bleaching is combined with the scouring process, chemicals required for hydrogen peroxide bleaching as well as an alkali-resistant scouring agent, effective at low temperatures, are added.

Raw cotton is dark brown in color due to the natural pigment in the plant. Many impurities adhere, especially to the surface. They consist mainly of protein, pectin, ash and wax. With the agglutination of these impurities, the fiber surface displays water repellency, which contributes to the survival of the species.

Occasionally, remains of pesticides and fertilizers, though in small quantities, are contained in fiber. Cold bleaching is a method that swells, breaks down and removes these impurities, as well as making fiber white through pigment decomposition.

Airi Chemical's standard recipe is shown in Fig. 4.

Fig.4 Airichemical's cold bleaching standard recipe

Hydrogen peroxide (35%) 75 g/L Caustic soda (flake) 15 g/L ACT-2 STA-1 KT-DA MULTITOL TK OX-50 (HAKKOL BY-SP) 28 g/L 4 g/L 1 g/L 5 g/L 5 g/L (X) ag/L ACT-2 is added along with STA-1 as a stabilizer in the decomposition of hydrogen peroxide and as an adjuster to achieve uniform bleaching. The recipe includes an activator that helps the dispersion of stripped impurities, the sequestering of metal ions, especially iron ions, the penetration of the scouring agent, the swelling of impurities and the decomposition of thickeners. Plain cotton weave Pick-up 20 x 20 hours The amount of hydrogen peroxide and other chemicals and pick-up will alter according to the fabric and process conditions used.

Caustic soda(50%)/Batch Hydrogen peroxide(35%)/Batch Cold bleaching recipe Cold bleaching recipe 2 g/L 85 x 45minutes 8 % owf 90 x 45minutes Airi's standard recipe A competitor's recipe-ON

A good degree of whiteness was produced from bleaching using sodium chlorite as well as in cold bleaching with hydrogen peroxide, as opposed to poor results in the batch method using hydrogen peroxide.

Airi Chemical and a competitor's recipes both resulted in whiteness superior to other methods. Each produced good water absorbency with little difference the two, using the dropping method.

Although bleaching with the use of sodium chlorite has advantages in the production of superior whiteness based on a simple recipe and in the oxidation of impurities such as pectin and protein, there are considerable disadvantages, such as the generation of harmful gas (chlorine dioxide), metal corrosion and maintenance costs required for sound operational environment and facilities.

Fig.5 shows results of whiteness testing on towels, using the above-mentioned cold bleaching method with the amount of sodium peroxide altered: 24g, 50g, 80g/L, and 50g/L x 2 times, at 110% pick-up.

Fig.5 Cold bleaching method: Difference in whiteness by the amount of hydragen peroxide used

As shown in Fig.5, the best whiteness was produced when bleaching was carried out twice, using 50g/L of hydrogen peroxide each time. However, it may be difficult, except for some particular circumstances, to accept this little difference made in whiteness, given the total costs with regard to efficiency, time and the amount of chemicals used. In the case with 24g/L of hydrogen peroxide used, a yellowish tint was recognizable even to the naked eye. In comparing cases of 50g/L and 80g/L, the difference in whiteness was small for the amount of sodium peroxide added. The appropriate amount of sodium peroxide, therefore, seems to be up to around 80g/L.

The addition of Multitol-TK (1g/L) and caustic soda (2g/L) to the washing bath in the following stage will result in a good degree of water absorbency given to the substrate. Test results with instant absorption have been produced using the dropping method.

[ Conclusion ]

The following is a reference to thermal energy, with regard to cotton bleaching using hydrogen peroxide.

The amount of steam required for raising 10 at a typical finishing plant is 22-23kg per 1 ton of water.

• Batch bleaching at 95
  The temperature difference will be 80 when water used is 15.
  

• The amount of water required for a single treatment
  1,000kg, for 300 bath towels, 100kg in weight, at the liquor ratio of 1:10
  

• The amount of steam required for raising temperature
  22 x 8 x 1=176kg
  

• The cost of steam (4/kg)
  176 x 4=704
  

The amount of heat release required for maintaining this temperature for 45 minutes, 15kg in steam, is not so much. The cost of steam is 60, therefore, it makes 764 for 300 bath towels. This figure should be something that cannot be overlooked for a manufacturer who deals with several hundred lots a month. The use of the cold bleaching method will eliminate these costs (excluding the washing stage).

Moreover, the softer hand produced by cold bleaching makes the method best suited for treating towels.

This method is worth consideration, as mentioned above, with advantages in terms of cost performance and efficiency over the batch method.