Additional information: Bleaching of cotton and bast fibres

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1. DESCRIPTION OF TECHNIQUES, METHODS AND EQUIPMENT


After scouring, cotton becomes more hydrophilic. However, the original colour stays unchanged due to coloured matter that cannot be completely removed by washing and alkaline extraction.

When the material has to be dyed in dark colours it can be directly dyed without need of bleaching. On the contrary, bleaching is an obligatory step when the fibre has to be dyed in pastel colours or when it will need to be subsequently printed. In some cases, even with dark colours a pre-bleaching step may be needed, but this is not a full bleaching treatment.

Bleaching can be performed on all kinds of make-ups (yarn, woven and knitted fabric).

The most frequently used for cellulosic fibres are oxidative bleaches, namely:

  • hydrogen peroxide (H2O2)
  • odium hypochlorite (NaClO)
  • sodium chlorite (NaClO2)

Apart from these, peracetic acid is also applicable. Also optical brightening agents are commonly used to obtain a whitening effect.

Bleaching with hydrogen peroxide

Bleaching can be carried out as a single treatment or in combination with other treatments (e.g. bleaching/scouring or bleaching/scouring/desizing can be carried out as single operations).

The textile is treated in a solution containing hydrogen peroxide, caustic soda and hydrogen peroxide stabilisers at pH 10.5 – 12 (the optimal pH for scavenging of the OH* radical is 11.2). Recent investigations have shown that the bleaching agent of peroxide is not the anion HOO-, but the dioxide radical anion OO*- (also known as superoxide). In competition with the formation of the bleaching agent, the OH* radical is formed, which is responsible for attacking and depolymerising the cellulose fibre. The formation of the OH* radical is catalysed by metals such as iron, manganese and copper. The prevention of catalytic damage as a consequence of uncontrolled formation of OH* is mostly taken care of by using compley formers that inactivate the catalyst (stabilisers). Sodium silicate together with Mg salts (MgCl2 or MgSO4) and sequestering/complexing agents (EDTA, DTPA, NTA, gluconates, phosphates and polyacrylates) are commonly used as stabilisers.

Other auxiliaries used in hydrogen peroxide bleaching are surfactants with emulsifying, dispersing and wetting properties. Employed surfactants are usually mixtures of anionic compounds (alkyl sulphonates and alkyl aryl sulphonates) with non-ionic compounds such as alkylphenol ethoxylates or the biologically degradable fatty alcohol ethoxylates.

Operating temperatures can vary over a wide range from ambient to high temperature. Nonetheless, a good bleaching action occurs when operating at around 60-90 °C.

Bleaching with hydrogen peroxide in neutral conditions (pH range of 6.5 – 8) is also possible in some cases (e.g. when treating cotton in blends with alkali-sensitive fibres such as wool). At these pH conditions activators are required to give bleaching activity. Note that below pH 6.5 H2O2 decomposes into H2O and O2 by HOO*- / O2 disproportionation. Under these conditions hydrogen peroxide is wasted (production of inactive O2 gas).

A wide range of bleaching agent of peroxide is anionic in nature (hydrophilic behaviour), it is not possible with this bleaching method to destroy selectively the coloured hydrophobic material present on natural fibres without attacking the polymer itself.

Bleaching with sodium hypochlorite

The high reactivity of this bleaching agent imposes softer operative conditions than hydrogen peroxide (pH 9-11 and temperatures not above 30°C). Otherwise there is a risk of damage to the cellulose fibre.

The bleaching stage is followed by an anti-chlorine treatment in order to eliminate completely the hypochlorite and decompose the chloroamines generated during bleaching.

Bleaching with sodium hypochlorite can be carried out in batch (e.g. overflow, jet, jigger, winch beck), semi-continuous (pad-batch) or continuous mode. A two-stage process is also in use in which hypochlorite and hydrogen peroxide are used.

The use of hypochlorite as bleaching agent is in decline for ecological reasons. It is still applied for yarn and knitted fabric when a high degree of whiteness is required, for articles that remain white (e.g. linen), or require a white background or in processes where the ground-dye is discharged with a bleach treatment.

Bleaching with sodium chlorite/chlorate

Chlorite/chlorate bleaching, although in decline, is still applied for synthetic fibres, cotton, flax and other cellulosic fibres, often in combination with hydrogen peroxide.

The bleaching agent is the chlorine dioxide gas (ClO2), which follows a completely different working mechanism compared to hydrogen peroxide. Whereas the superoxide radical ion in hydrogen peroxide is hydrophilic and therefore works preferentially in the hydrophilic region of the fibre (attack of the fibre polymer), ClO2 absorbs preferentially on the hydrophobic associated material, such as woody part of bast fibres. For this reason it is an excellent bleaching agent (ensuring a high degree of whiteness and no risk of damage of the fibre) especially for synthetic fibres and for bast fibres such as flax where, compared to cotton, there is a higher percentage of hydrophobic impurities. Because chlorine dioxide is unstable as a gas and can only be stored as a solution of approximately 1% in water, it must be generated on-site as an aqueous solution. There are two ClO2 precursor chemicals in present general industrial use, namely sodium chlorite and sodium chlorate. Although sodium chlorate is considerably less expensive than sodium chlorite, it is more difficult and expensive to convert to ClO2, which explains why it is less commonly used.

Both sodium chlorite and sodium chlorate are used in strong acid conditions (pH 3.5 – 4 by formic or acetic acid). Chlorine dioxide solutions have a great corrosive action on construction materially including stainless steel. Sodium nitrate is used as a corrosion inhibitor to protect the stainless steel parts of equipment. It is also necessary to select detergent/wetting agents because the oxalic acid used for acidification also serves for sequestering metals. The order of introduction of the different auxiliaries has to be controlled to avoid direct contact between the concentrated sodium chlorite/chlorate solution and acids.

The textile material is bleached by padding or in long bath processes. The temperature is normally kept at 95°C, but cold procedures have also been developed to diminish toxicity and corrosion problems, using formaldehydes as an activator for sodium chlorite.

The advantages of chlorine dioxide bleaching are the high degree of whiteness and the fact that there is no risk of damage to the fibre. The main disadvantages are the high stresses to which the equipment is subjected and the chlorine residues that may be left on the fibre, depending on the way chlorite (or chlorate) is produced and activated. Recent technologies using hydrogen peroxide as the reducing agent of sodium chlorate are now available to produce ClO2 without generation of AOX.

Bleaching with peracetic acid

Peracetic acid is produced from acetic acid and hydrogen peroxide. It can be purchased as ready-made product or produced in-situ. Its optimal bleaching action is reached only in a very narrow pH range between 7 and 8. Below pH 7 the degree of whiteness decreases sharply and above pH 9 depolymerisation of the fibre with consequent damage of the fibre occurs.

Peracetic acid is sometimes applied for synthetic fibres (e.g. polyamide) where hydrogen peroxide cannot be used.

Source: BAT in textile industry, 2003


2. NEW TECHNOLOGIES


a) Changes in the process

No information is available.

b) Changes in the heat supply system

No information is available.

c) Changes in the energy distribution system

No information is available.

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