Cotton, like all natural fibres, has some natural colouring matter, which confers a yellowish brown colour to the fibre. The purpose of bleaching is to remove this colouring material and to confer a white appearance to the fibre. In addition to an increase in whiteness, bleaching results in an increase in absorbency, levelness of pre treatment, and complete removal of seed husks and trash. In the case of the production of full white finished materials, the degree of whiteness is the main requirement of bleaching. The amount of residual soil is also taken into consideration because of the possibility of later yellowing of the material. In the case of pretreatment for dyeing, the degree of whiteness is not as important as, for example, the cleanliness of the material, especially the metal content. Similar demands refer to the production of medical articles. In this case, too, the metal content as well as the ash content are important factors. If whiteness is of primary importance, it requires a relatively large amount of bleaching agent as well as a high operating temperature and a long dwell time.
Accurate regulation of the bleaching bath is a further obligatory requirement. Where the destruction of trash, removal of seed husks and an increase in absorbency is a prime necessity (e.g. for dyed goods), a high degree of alkalinity is all important. It is, however, not the alkali alone that is responsible for these effects. The levelness of pretreatment can only be guaranteed if cotton of the same or equal origin is processed in each bath. If this is not the case, suitable pre-treatment will have to be undertaken to obtain, as closely as possible, the required uniformity. A pre-treatment with acid and/or a chelating agent will even out (better still eliminate) varying quantities of catalytic metallic compounds.
Although there are different bleaching agents that can be used for bleaching cotton, hydrogen peroxide is, by far, the most commonly used bleaching agent today. It is used to bleach at least 90% of all cotton and cotton blends, because of its advantages over other bleaching agents. The nature of the cotton colour, its mechanism of removal with hydrogen peroxide and the basic rules for formulation of bleaching liquors have been presented in detail elsewhere. The mere formulation of the correct initial bath concentration is not sufficient to ensure a controlled bleaching process. Of equal importance are regular checks of the bath composition during the operation. Such checks do not only contribute to an economic bleaching operation but also allow an early tracing of the defects and failures of the system. The important parameters for bleaching with hydrogen peroxide are as follows:
- Concentration of hydrogen peroxide.
- Concentration of alkali.
- Nature and quality of the goods.
- Water hardness and other impurities.
- Types and concentration of auxiliaries.
- Desired bleaching effect.
- Available equipment and stabilizer system employed.
Most of these factors are inter-related, and all have a direct bearing on the production rate, the cost and the bleaching quality. Though they operate collectively, it is better to review them individually for the sake of clarity.
There are two concentrations to be considered: that based on the weight of the goods and that based on the weight of the solution. All other factors being equal, the concentration on the weight of the goods determines the final degree of whiteness. In order to get adequate bleach there must be enough peroxide present from the start. On the other hand, the peroxide concentration based on the weight of the solution will determine the bleaching rate - the greater the solution concentration, the faster the bleaching. No peroxide bleaching system ever uses up its entire peroxide charge for active bleaching, as some is always 'lost' during normal process.
The alkalinity in the system is primary responsible for producing the desired scour properties and maintaining a reasonably constant pH at the desired level throughout the bleaching cycle. The quantity of the alkali to be added depends above all on the character of the goods, the finish required and the kind and quality of the other ingredients in the liquor. The alkalinity is defined as the 'amount' of alkali in the system and should be distinguished from the pH, which is a measure of the hydrogen ion concentration in the solution. The pH value in peroxide bleaching is of extreme importance because it influences bleaching effectiveness, fiber degradation and peroxide stability in bleaching cotton fibers.
With increasing pH, whiteness index increases to a maximum at a pH of 11.0 and then decreases. Fibre degradation is at minimum at a pH of 9.0 but that which occurs at a pH of 10.0 is well within acceptable values. Above a pH of 11.0, fibre degradation is unacceptably severe. A pH range of 10.2-10.7 is considered optimum for bleaching cotton with hydrogen peroxide. Lower pH values can lead to decreasing solubility of sodium silicate stabiliser (see below) as well as lower whiteness due to less activation of the peroxide.
By increasing the temperature, the degree of whiteness as well as its uniformity increases. However, at too high a temperature, there is a possibility of a decrease in the degree of polymerization of the cotton. Moreover, due to good fat removal at high temperatures such as 110C, the handle of the material can become harsh and the sew ability of woven cotton fabrics may also decrease. Time, temperature and concentration of peroxide are all inter-related factors. At lower temperatures, longer times and higher concentrations are required. As the temperature of bleaching increases, shorter times and lower peroxide concentrations can be employed.
The amount of peroxide decomposed is greatly reduced with increasing weight of cotton fibre in the bleach liquor. The raw fibre almost completely suppresses decomposition, while the scoured fibre is somewhat less effective. The demineralised fibre is the least effective stabiliser. While impurities such as magnesium and calcium may have a good stablising effect when present in appropriate amount, other impurities such as iron, copper and manganese can have very harmful effect, resulting in catalytic decomposition of hydrogen peroxide leading to fibre damage.
A good stabilizing system is indispensable in bleaching cotton with hydrogen peroxide. While sodium silicate is one of the most commonly used stabilisers, its use may result in a harsh handle of the fabric as well as resist spots leading to spotty dyeing. The best alternatives to sodium silicate are organic stabilisers or a combination of silicate and organic stabilisers.
In addition to the most important ingredients of the bleaching recipe, namely hydrogen peroxide, caustic soda and the stabilizer, auxiliaries are used sometimes to aid the bleaching process. These may include surfactants and chelating agents. The type and concentration of these auxiliaries also plays an important role in the bleach effect obtained. The desired bleaching effect does not need necessarily be optimal white. For goods-to-be-dyed, the main concern will normally be achieving good and uniform absorbency.
The available equipment plays a role in determining which process criteria must be taken into account such as: cold, hot or HT bleaching; dry-wet or wet-on-wet impregnation; discontinuous or continuous processing; process control. The most common problems in bleaching cotton with hydrogen peroxide are as follows:
- Inadequate mote removal.
- Low degree of whiteness.
- Uneven whiteness (or bleaching).
- Resist marks.
- Formation of ox cellulose
It is not always possible to find the cause of these problems without detailed analyses. The most useful tests that can be carried out to check the effectiveness of the bleaching process are for whiteness, absorbency and tensile strength. Checks and measures are required also to assure level dyeing properties. Control of residual moisture content (e.g. 7% for cotton) is part of the standard pre-treatment, which should be uniform throughout the material.
This article was originally published in the July issue of the magazine, New Cloth Market - the complete textile magazines from textile technologists."