Bleaching is the process to remove or weaken, in particular to eliminate yellowing unwanted colorations. When bleach is called according to substances that remove unwanted color from raw materials of paper and textile industries or discoloration due to aging or light yellowing. In addition to the process of bleaching and the bleaching agent is referred to as " bleaching " sometimes. Industrial importance especially the bleaching of pulp and paper, and textiles.
- 3.3.1 enzymes
- 4.1 textiles 4.1.1 denims
- 4.1.2 detergent
The word "pale " is related to " blanc", the white in the Romance languages or referred colorless. In German it is " blank" to the meaning of " clear ", " pure" converted. The word " pale " also describes a weak staining: the pale moon, pale with fright.
The root word " alba " as "white" is found in "Bleach " (Latin Albator ) again.
Even before about 7000 years bleaches were used for textiles, based on sulfur. The actual process material is the reductive effect of sulfur dioxide, as tetravalent sulfur compounds are eager to fill their octet.
Eau de Javel, Germanized Javel water, is considered the first chemical bleach. Its effectiveness was demonstrated in 1785 by Claude -Louis Berthollet.
For textiles were used linen and wool in Central Europe until the 20th century. These fibers contained as raw colored residual substances from the fiber production, also form through the use brown degradation products. To achieve the "pure" impression, Laundry pieces were designed to the river meadows near lavatories outside the cities. The " fading " was done by the sun. The tissues were irrigated alternately with sour milk and the extract of wood ash (potash ). The peroxides formed as intermediates under the influence of light and air and the resulting oxygen during photosynthesis of the lawn reactive oxygen species caused the bleaching effect. The treatment until the desired whiteness could take weeks.
The photocatalytic bleaching (lawn bleach) is not only caused by sunlight or UV radiation. In addition to the direct attack of the photons in a suitable molecular bonds of the unwanted yellow aging products in the natural fibers ozone or singlet oxygen formation on the discoloration acts. For cotton fabrics classified the grass bleaching is also suitable. The household was dried in Germany until well into the 1970s on the " lawn " or "Lead Changer" under the laundry piles and bleached. This method is still used in other countries today. In the grass bleaching running chemical processes are the basis of modern detergents and bleach.
For linen this post was important to remove the lignin from the tissues. This colored " Faserleim " the flax gave the linen the coarse " peasant " reputation. " Fine " linen was consuming processed, bleached intensively and definitely more expensive.
In addition to the dyers and launderers, there was the own profession of bleachers. These had their own guild in many cities, and went on suitable places like river meadows their trade after. In the valley of the Wupper was the Garnnahrung, by a magisterial Royal privilege was a regional monopoly for bleaching.
Bleaching agents are all oxidizing and reducing agents or adsorptive active substances, practically important are selectively active compounds. As undesirable impurities in the bleaching of natural fibers are destroyed with bleaching agents, there is a possibility that all of the organic molecules are destroyed and decolorized it. Target for the bleach should preferably be the dyes or polymers in their chromophoric groups.
Allen bleaching agents have in common is the attack on colored conjugated π -electron systems. In the bleaching of most natural fibers, the fiber is damaged, the degree of polymerization decreases. The bleaching agent can not be distinguished in a position between the undesirable brown aging products and the desired fiber molecules. Bleach attack the coloring substances, by destroying the chromophores. Different colored compounds can be vulnerable, so that oxidative or reductive bleaching methods are very effective in different ways depending on decolorizing dye either easier by oxidation or by reduction.
Oxygen and peroxides
Peroxides are compounds that contain a -OO group, there are two oxygen atoms are directly connected. The oxygen -oxygen bond of peroxides is not stable and decomposes with the formation of reactive radicals. Oxygen itself is due to the double bond itself no peroxide and less reactive than this.
By reduction, (yellow to red colored ) quinones can be converted into colorless phenol derivatives. The most important reaction in the reductive bleaching is the destruction of colored metal complexes or the degradation of dyes. Natural phenolic derivatives form intensely colored inks with iron or copper salts. If the reaction products are not washed out, a regression of the chromophores by oxidation with atmospheric oxygen is easily possible.
Removal of the dye
Strong complexing agents such as EDTA and DTPA or weak complexing agents such as citric acid and oxalic acid are particularly effective in discoloration, whose cause are organometallic complexes. The resulting new complexes are colorless and water-soluble and can be removed by washing.
Sequestrants, such as activated carbon or bleaching earth (usually kaolin ) can be used for bleaching by absorption. Natural oils, for example palm oil, contain large amounts of the dark impurities, including carotenoids. These can be removed by adsorption. An oxidising bleach such oils is also technically possible, due to the undesirable side reactions with the double bonds of the fatty acid chains but not common.
In detergents treated as additional enzymes. These substances are mainly proteases that cleave proteins biologically, which can be washed in a row.
Applications of the bleaching
Bleaching is first the textile chemical processes to eliminate by means of an oxidation, as well as reductive bleaching agent acting the undesired staining of natural fibers. On cotton and viscose staple, the bleaching pre-treatment as in dyeing common practice to prepare the fiber structure. For modern synthetic fibers, bleaching of the raw material is not necessary for their preparation conditions. However, from aids in the production process colored by-products arise.
Added bleaches in detergents used to eliminate unwanted aging products and the destruction of coloring contaminants, these then act chemically, in addition to network resources and the mechanical washing action. The content of bleaching agents in modern detergent is a complex mixture that is to be effective at high wash temperatures, but may not affect the color of the tissue.
The bleaching process is usually with hydrogen peroxide, caustic soda and surfactants ( and other auxiliary chemicals: bleach stabilizers ) performed. Depending on whether the plant ( such as cotton cellulose ) or animal fiber ( wool like proteins ) are to be bleached, the pH is different. Cotton is bleached alkaline and wool, however, under acidic conditions.
The chemical components reacting with each other and the influence of the bleach stabilizers of the oxygen is controlled from the hydrogen peroxide and removed slowly. In a spontaneous and uncontrolled decomposition of the molecular chain length of the cellulose would be reduced, which is reflected by reduced tensile strength. With the bleach has several goals:
A distinction is made between continuous and discontinuous bleaching process. In the continuous process the bleaching liquor is (a mixture of hydrogen peroxide, sodium hydroxide, stabilizer, and water ) and eluted at a flow damper near the 100 - ° C, enabled mark the decomposition of hydrogen peroxide. Advantage is a rapid procedure albeit with high energy costs.
In batch process the bleaching liquor is applied to the fabric, and most rotatably supported on large rolls ( feed rolls or skeins ) for up to 24 hours. Advantage is the high energy saving, because no additional process heat is required.
The batch process also includes the bleaching yarns on spools or warp beams. Here, the bleaching process is carried out in large tanks under pressure and high temperatures in the bleaching liquor.
Depending on the nature of the process, the individual components of the liquor may be added at different mixture ratios of the bleaching liquor.
After each bleaching process remaining on the fabric bleaching chemicals must be removed by washing, so that subsequent processes in the printing or dyeing will not be disturbed, since any leftover hydrogen peroxide destroys the dyes.
New developments in this sector, the risk of over- bleaching can be greatly reduced.
A reducing bleach is effective for the dyed with indigo clothing such as blue jeans. So colored substances can discolor efficiently with dithionite.
The Jean dyes are applied as vat. Vat is a water-soluble form of such dyes, who raises great on cotton. In subsequent admission of air the Küpenform decays by oxidation and it remains the insoluble pigment molecule trapped in the tangle of fibers. A sign of this type of fiber bonding is the poor rub fastness "real" denim. The bleaching with reducing substances, this process is reversed. The pigment is water soluble and can wash out, the fabric is bleached.
Bleach based on oxygen can be found in modern detergents and stain remover. Bleach based on oxygen means that this bleach oxygen from hydrogen peroxide ( or ozone) releases. Such bleaching agents are set by various magnesium-containing stabilizers for commercial products to the specific bleaching conditions.
A widely used bleaching agent in heavy-duty detergents was perborate. It was pushed back by other means, for his boron compounds can be mined and hard work in high concentration in the water phytotoxic. In Fleckentabs, stain salts and detergents modular percarbonate. It can be difficult to integrate due to their complicated composition in heavy-duty detergent. Since perborate and percarbonate be active until a temperature of 60 degrees, is given to so-called bleach activators as additives.
With the help of bleaches unwanted color spots and yellowing can be removed from the laundry. This bleach disinfectant effect on fungi and bacteria but can attack the laundry. In heavy-duty detergents bleaches are effective at higher temperatures. In delicate and colored detergents, however, no bleaching agents are included. For dry cleaning in the household today almost always oxygen- based bleach is used, particularly hydrogen peroxide in liquid and sodium perborate in powdered bleaching. In industrial dry cleaning and textile processing chlorine- based bleach is still used.
In the paper industry, wood pulp is bleached with chlorine dioxide or hypochlorite. The ecologically harmful bleaching with elemental chlorine is disappearing. Environmentally friendly, completely chlorine-free process with hydrogen peroxide or ozone are more expensive and are used mainly in the Scandinavian countries. With chlorine dioxide bleached pulp as ECF ( elementary chlorine free) pulp is referred to; with hydrogen peroxide or ozone bleached as TCF ( totally chlorine free) pulp.
By bleaching or bleaching the hair naturally embedded pigments are destroyed. If lighter tones are to be achieved as the natural hair in hair dyeing, this operation is previously necessary. Disturbing Body is lightened by bleaching, rather than to remove hair.
In dentistry is in translation from English: bleaching teeth whitening as "pale " named. Is used, for example, Carbamide peroxide
Partial unwanted pigmentation on the human skin, such as freckles (zones with increased melanin density ) are treated with special bleaching creams. Suitable oxidizing agents are here hydrogen peroxide and peroxy compounds, such as sodium perborate, Magnesiumperborat or urea used. For skin whitening, substances such as mercury and hydroquinone are used.
Bleaching of surfaces ( antique wood, tooth or bone, more recently, human skin) is often used to be the fashionable white justice. Also the preparation of further remedial steps or discoloration of light colors can be the cause here. Used is a dilute or very dilute solution of hydrogen peroxide in the application. Additional funds are citric acid and oxalic acid for skin lightening and fruit acids, for teeth whitening carbamide also. Frequently iron-organic compounds are yellowish, or it just is yellow iron oxyhydroxide ( to rusting ), which by conversion into Eisenoxalo or lemon complexes lose their color.
In the preparation technique bones are bleached to obtain appropriate purified pieces for collections and exhibitions. Hunter bleach their hunting trophies to possess tative memories.
Although they are referred to as " optical brightener " solve optical brighteners, laundry blue, or even a horse chestnut tree extract from another operation. The addition of a little blue color compensates for the yellow tinge, although it leads to a darkening, the visual impression, however, shifts towards " achromatic ". This is a purely optical effect without chemical reaction. Optical brighteners absorb UV light and convert it into visible light, so their effect is very limited with yellowish artificial light.
The eye represents "white" fixed impression as if the color stimulus ( the light in spectral composition ) excites all three sensors for color perception evenly. Organic substances absorb due to the molecular vibrations in that spectral range of ultraviolet, which adjoins to the short wavelength end of the visible light. The absorption can certainly rarely extend into the blue and green areas to the red. The absorption of " blue photons" leads then give a yellow, orange or brown tone to white areas to the rear radiation of wavelengths. The brown-yellow Gilb, so the yellowing or general undesirable discoloration can be optically neutralized by applying the complementary color blue. In the case of laundry blue actually associated with some loss of "White ", this bright neutral gray appears still more acceptable than the yellow color cast. A better effect cause optical brighteners, which compensate with additional blue to yellow. Fluorescence effect these substances absorb in the invisible UV photon energy and emit light in the blue spectral range, thus the total radiated amount is increased to visible light. Optically brightened fabrics shimmer then typically bluish in intense sunlight or about black light disco.