Bleaching is the process of removing or weakening unwanted coloration, especially eliminating yellowing . When bleach is known according to substances that unwanted color from raw materials of paper and textile industries or discoloration due to aging or light yellowing remove. In addition to the process of bleaching, the bleaching agent is sometimes referred to as "bleach". The bleaching of pulp and paper as well as textiles is of particular industrial importance .
The word “bleach” is related to “blanc”, which means white or colorless in Romance languages . In German it is changed to “blank” in the meaning of “clear”, “pure”. The word “pale” also describes a weak color: the pale moon, pale with shock.
Linen , hemp and cotton fibers were spun and woven in their natural color, so that the finished fabrics were yellowish to gray-brown and had to be bleached before being dyed or sold. Freshly woven textiles as well as textiles soiled through use were subjected to lawn bleaching: on a meadow near a river, the bleaching area or the bleaching area, the fabrics that were still wet from the laundry, but also yarns, were laid flat or stretched and kept continuously moist (hence the proximity to the river). Some bleachers also sprinkled the textiles with lye.
The peroxides formed intermediately under the influence of light and atmospheric oxygen as well as the reactive oxygen species formed during photosynthesis of the lawn caused the bleaching effect. The treatment to the desired degree of whiteness could take weeks or even a whole summer.
In Germany, household linen was dried and bleached on the "lawn" or "bleaching" under the laundry stakes until the 1970s. This method is still used in other countries today. The chemical processes involved in bleaching lawns are the basis of modern, oxygen-based washing and bleaching agents.
In addition to the dyers and washers , there was the bleacher's own profession. These had their own guild in many cities . In the valley of the Wupper the yarn was nourished , through an official privilege a regional monopoly for bleaching arose . The bleaching place was also called bleaching for short . In many cities there are streets whose name suggests former bleaching places, e.g. B. Bleiche, Tuchbleiche, Bleichstrasse, Bleichwiese, Bleichwiesenstrasse, Bleichplatz, On the Bleiche, On the Bleiche.
Bleaching agents are all oxidizing and reducing agents or substances with an adsorptive effect, only selectively acting compounds are of practical importance. Just as unwanted accompanying substances in natural fibers are destroyed during bleaching with bleaching agents, there is the possibility that all organic molecules are destroyed and discolored in the process. The point of attack for the bleaching agents should preferably be the dyes or, in the case of polymers, their chromophoric groups.
What all bleaches have in common is their attack on colored conjugated π-electron systems . When most natural fibers are bleached, the fibers are also damaged and the degree of polymerization decreases. The bleaching agent is unable to differentiate between the undesirable brown aging products and the desired fiber molecules. Bleaching agents attack the coloring substances by destroying the chromophores . Different colored compounds can be more easily attacked by oxidation or reduction , so that oxidative or reductive bleaching methods have very different effectiveness depending on the dye to be decolorized.
Oxygen and peroxides
Peroxides are compounds that contain an -O – O- group, they are two oxygen atoms that are directly connected to each other. The oxygen-oxygen bond of peroxides is not stable and breaks down with the formation of reactive radicals . Due to the O = O double bond, oxygen itself is not a peroxide and is less reactive than this.
- Gaseous oxygen
- is used extensively in pulp bleaching to complete the delignification of sulphate and sulphite pulp. The reaction takes place under pressure , at up to 8 MPa in continuous reactors at temperatures up to over 90 ° C. In the yellowing of wood-containing papers, atmospheric oxygen plays an important role together with traces of metal such as iron.
- Hydrogen peroxide
- with the formula H 2 O 2 is used for bleaching textile fibers such as cotton fabrics and flax fibers (linen), which today are mainly bleached with it. It can also be used to bleach wood pulp and wood pulp , but is too expensive compared to more economical bleaches.
- The bleaching reaction takes place under alkaline conditions, and the perhydroxyl anion (HOO - ) attacks the dyes in a nucleophilic manner . It works against quinones and conjugated aldehydes . The oxidation products are water-soluble under the bleaching conditions and are removed by washing. The polymer structure of the cellulose fibers is not attacked under the alkaline conditions if heavy metals such as iron have been removed beforehand by acid washing or the use of complexing agents such as diethylenetriaminepentaacetic acid (DTPA).
- were eponymous for Persil . More recently, percarbonates have also been used. They have the same effect as hydrogen peroxide in aqueous solution, but are easier to store as a solid . This is more advantageous for use in detergents.
- Peroxyacetic acid
- The reactive CH 3 CO 3 H is mainly used in laundries for disinfection. The end product, acetic acid, is readily biodegradable and therefore environmentally friendly. In the bleaching of pulp peroxyacetic acid is used to activate the alkaline peroxide bleaching under TCF ( T otal- C hlor- F used ree-bleach) conditions.
- itself is hardly ever used industrially for bleaching, since halogenation also takes place in addition to oxidation . When bleaching with chlorine, chlorinated hydrocarbons are formed, and toxic dioxins are particularly problematic .
- Commercially available as bleach or " Eau de Javel " are aqueous solutions that are used as household bleaches and as stain removers. The active substance is usually sodium hypochlorite (NaClO), less often potassium hypochlorite (KClO). The basic hypochlorite solutions are sometimes referred to in English as "liquid chlorine" (liquid chlorine). The following equilibrium is established in the watery state:
- Under alkaline conditions, the hypochlorite anion reacts nucleophilically and discolours impurities or the bleaching effect is based on the formation of reactive singlet oxygen .
- Diluted solutions contain hypochlorous acid , HClO, which is much more reactive, but can also damage the bleached material through non-selective oxidation.
- Hypochlorous acid as an oxidizing agent .
- When acidifying, there is a risk of toxic chlorine gas being produced because the solubility of chlorine in water is low. To reduce this risk, the commercially available solutions usually contain an alkaline buffer.
- Chlorine dioxide (ClO 2 )
- is the world's leading bleaching agent for pulp . Under acidic conditions, chlorine dioxide reacts selectively with the phenolic structures of the residual lignin, the polymer cellulose is not attacked. When bleaching linen, chlorine dioxide is used alternately with hydrogen peroxide. The oxidation potential of chlorine dioxide is relatively low at +0.93 V (at pH = 0), the reactivity is due to the electron gap of the chlorine dioxide radical.
By reduction , quinones (yellow to red in color) can be converted into colorless phenol derivatives. The most important reaction in reducing bleaching is the destruction of colored metal complexes or the degradation of dyes. Natural phenol derivatives form intensely colored inks with iron or copper salts. If the reaction products are not washed out, the chromophores can easily be regressed by oxidation with atmospheric oxygen.
- The sodium salt of dithionic acid (Na 2 S 2 O 4 ) is the main reducing bleaching agent. It is used for wood pulp bleaching, wool bleaching and vat dyeing .
- Sulfur dioxide
- is used as bisulfite (NaHSO 3 ) or in gaseous form and can bleach unwanted discolouration, especially when dyeing with synthetic textile dyes. In special cases, bleaching with gaseous SO 2 is more suitable because this easily escapes.
Removal of the dye
Strong complexing agents such as DTPA and EDTA or weak complexing agents such as citric acid and oxalic acid are particularly effective in the case of discoloration caused by organometallic complexes. The resulting new complexes are colorless and water-soluble and can be removed by washing.
Binding substances such as activated charcoal or fuller's earth (mostly kaolin ) are used for bleaching through adsorption. Natural oils, such as palm oil , contain large amounts of dark impurities, including carotenes . These can be removed by adsorption. Oxidizing bleaching of such oils is also technically possible, but not common because of the undesirable side reactions with the double bonds of the fatty acid chains.
Prepared enzymes are used as additives in detergents . These substances are predominantly proteases, which break down proteins biologically, which can then be washed out.
Applications of bleach
Bleach is primarily the textile-chemical process to remove the undesired coloration of natural fibers with the help of an oxidative but also reductive bleaching agent . In the case of cotton and rayon , bleaching is generally used as a pretreatment during dyeing in order to prepare the fiber structure. For modern synthetic fibers, bleaching of the raw material is not necessary because of the manufacturing conditions. However, colored by-products can arise from auxiliary materials during the manufacturing process.
Bleaching agents are added to detergents to remove unwanted aging products and to destroy staining soiling. These then have a chemical effect, in addition to wetting agents and the mechanical washing movement. The content of bleach in modern detergents is a complex mixture that should be effective at higher washing temperatures, but must not damage the colors of the fabrics.
The bleaching process is mostly carried out with hydrogen peroxide, caustic soda and surfactants (and other auxiliary chemicals: bleach stabilizers). Depending on whether plant fibers (cellulose such as cotton ) or animal fibers (proteins such as sheep's wool) are to be bleached, the pH value is different. Cotton is bleached alkaline and wool, on the other hand, in acid.
The chemical components react with each other and the influence of the bleach stabilizers controls and slowly splits off the oxygen from the hydrogen peroxide. In the event of spontaneous, uncontrolled decomposition, the molecular chain length of the cellulose would be reduced, which is noticeable through reduced tear strengths.
There are several goals with bleaching:
- Discoloration through the oxidation of the basic dye
- Removal of vegetable contamination such as leaf residues, fiber impurities.
- Removal of staining fat impurities
- In addition, the absorbency for subsequent processes is increased and more even.
A distinction is made between continuous and discontinuous bleaching processes. In the continuous process, the bleaching liquor (mixture of hydrogen peroxide, sodium hydroxide solution, stabilizer and water) is applied and the decomposition of the hydrogen peroxide is activated in a flow steamer close to the 100 ° C mark. The advantage is a fast process with high energy costs.
In the discontinuous process, the bleaching liquor is applied to the fabric and stored in a rotating manner on large rollers (pegs or docks) for up to 24 hours. The advantage here is the high energy savings, as no additional process heat is required.
The discontinuous process also includes the bleaching of yarns on bobbins or warp beams. Here the bleaching process is carried out in large vessels under pressure and high temperatures in the bleaching liquor.
Depending on the type of process, the individual components of the liquor are added to the bleaching liquor in different mixing ratios.
After each bleaching process, the bleaching chemicals that have remained on the fabric must be removed again by washing so that subsequent processes in the printing or dyeing works are not disrupted, as any remaining hydrogen peroxide destroys the dyes.
New developments in this sector can greatly reduce the risk of over- bleaching .
The jeans dyes are applied as a vat . Küpe is a water-soluble form of such dyes that absorbs perfectly on cotton. When air is subsequently admitted, the vat shape breaks down oxidatively and the insoluble pigment remains trapped in the molecular ball of the fibers. One sign of this type of fiber binding is the poor rub fastness of “real” denim fabrics. This process is reversed when bleaching with reducing substances . The pigment becomes water-soluble and can be rinsed off, the fabric is bleached.
Oxygen-based bleaching agents can be found in modern laundry detergents and in stain remover . Oxygen-based bleach means that this bleach releases oxygen from hydrogen peroxide (or ozone). Such bleaching agents are adjusted to the special bleaching conditions by various magnesium-containing stabilizers for commercial products.
A widely used bleach in heavy duty detergents was perborate. It was pushed back by other means, because its boron compounds are difficult to break down and, in high concentrations in the water, are harmful to plants. Percarbonate is used in stain tabs, stain removers and modular detergents. Its complex composition makes it difficult to integrate it into heavy-duty detergents. Since perborate and percarbonate only become active from a temperature of 60 degrees, so-called bleach activators are used as additives.
With the help of bleaching agents, unwanted color stains and yellowing can be removed from the laundry. Bleaching agents also have a disinfectant effect on fungi and bacteria , but can attack the laundry. In heavy-duty detergents , bleach only becomes effective at higher temperatures. In fine - and stained laundry detergents , however, no bleaches are included. Oxygen-based bleach is almost always used for textile cleaning in the household , especially hydrogen peroxide in liquid and sodium perborate in powdered bleach. Chlorine-based bleach is still used in industrial textile cleaning and textile processing .
In the paper industry, pulp is bleached with chlorine dioxide or hypochlorite . The ecologically questionable bleach with elemental chlorine is disappearing. Environmentally friendly, completely chlorine-free processes with hydrogen peroxide or ozone are more cost-intensive and are mainly used in the Scandinavian countries. Pulp bleached with chlorine dioxide is also known as ECF (elementary chlorine free) pulp; bleached with hydrogen peroxide or ozone than TCF (totally chlorine free) pulp.
The naturally stored pigments are destroyed by bleaching or bleaching the hair. If you want to achieve lighter shades than natural hair when coloring your hair, this process is necessary beforehand. Annoying body hair is lightened by bleaching instead of removing the hair.
In some cases, unwanted pigment spots on human skin, such as freckles (zones with increased melanin density) are treated with special bleaching creams. Hydrogen peroxide and peroxo compounds such as sodium perborate , magnesium perborate or urea peroxide are used as oxidizing agents . Substances such as mercury and hydroquinone are also used to lighten the skin .
Bleaching of surfaces (antique wood, teeth or bones, more recently also human skin) is often used to do justice to the fashionable white. The preparation of further renovation steps or the discoloration for light colors can also be the cause here. A dilute or very dilute solution of hydrogen peroxide is widely used for this purpose. Other remedies are citric and oxalic acid, fruit acids for skin whitening and carbamide for teeth whitening. Organic iron compounds are often yellowish, or they are simply yellow iron oxyhydroxide (up to rust), which lose their color when converted into iron oxalo or citronate complexes.
In the preparation technique , bones are bleached in order to obtain cleaned pieces suitable for collections and exhibitions. Hunters bleach their hunting trophies in order to have presentable memories.
Although they are also referred to as “optical bleaching agents”, optical brighteners , laundry blue or a horse chestnut extract trigger a different process. The addition of a little blue color compensates for the yellow tint, although it leads to a darkening, it shifts the visual impression towards achromatic . This is a purely optical effect with no chemical reaction. Optical brighteners absorb UV light and convert it into visible light, so their effect on yellowish artificial light is very limited.
The eye detects “white” as an impression when the color stimulus (the light in its spectral composition) excites all three sensors for color perception evenly. Due to the molecular vibrations, organic substances absorb in that spectral range of the ultraviolet that connects to the short-wave end of visible light. The absorption can reach into the blue and green areas, rarely into the red. The absorption of "blue photons " leads to the reflection of wavelengths, which then give a yellow, orange or brown tone on white surfaces. The brown-yellow yellow, i.e. the yellowing or, more generally, the undesired discoloration, can be optically neutralized by applying the complementary color blue. In the case of laundry blue, actually associated with a certain loss of “whiteness”, this light neutral gray still appears more acceptable than the yellow color cast. Optical brighteners that compensate for the yellow tone with additional blue have a better effect . Due to the fluorescence effect, these substances absorb photon energy in the invisible UV and emit light in the blue spectral range, thus increasing the total amount of visible light emitted. Optically brightened textiles then shimmer typically bluish in intense sunlight or, for example, in black light in the discotheque.
- Sohn, Joh. Paul: Exact and detailed description of the new invention and improvement made in natural and lawn bleaching of linen and hemp yarns and the canvases made from them ... Olmütz 1853. S. 6 ff
- Gülich, Jeremias Elias: Complete coloring and blue book . Ulm 1781. Volume 3, p. 146.
- H. Sixta: Handbook of Pulp . Wiley-VCH 2006, ISBN 3-527-30999-3 .
- Monika Pohl and Jean Pütz : Hobby Tip der Hobbythek: Washing clothes with a white vest , number 305, no year (link checked on January 4, 2014).
- Günter Vollmer and Manfred Franz: Chemical products in everyday life , Georg Thieme Verlag Stuttgart, 1985, pp. 119-120, ISBN 3-13-670201-8 .