Optical brighteners

from Wikipedia, the free encyclopedia

Optical brighteners (colloquially whiteners , also whiteners ) are fluorescent substances whose function is to increase the whiteness of materials, in particular by compensating for the yellow tint.

Liquid detergent. Right: glowing under ultraviolet light

commitment

They are most frequently used in the detergent, textile, fiber, paper and plastics industries in order to compensate for the yellowness of the substances to be lightened due to residual substances and not completely eliminated by bleaching .

history

Horse chestnuts were used as detergents at an early stage (because of the saponins they contain ), as early as the end of the 18th century it was noticed that horse chestnut extracts (containing the fluorescent esculin ) improved the effect of bleaching liquors , the fluorescence of the extracts was investigated by Paul Krais in 1929 , which led to this for the development of optical brighteners for detergents.

Mode of action

Optical brighteners require ultraviolet radiation to work . The white then appears whiter in strong sunshine and clear, blue skies outdoors or under suitable artificial lighting (with a high UV component) . Fluorescence is the cause of the brightening effect of optical brighteners. These are substances that absorb from the invisible ultraviolet (at 290–400 nm) and, after an intramolecular interplay, re- emit most of the absorbed energy in visible light . Fluorescent substances which emit at 400-480 nm, if possible at 430-440 nm, are most suitable. Suitable groupings for the molecules of modern optical brighteners are heterocyclic five-membered rings such as oxazole , triazole and imidazole radicals. They have a high quantum yield of fluorescence.

By emitting additional blue light, the remission ( color saturation ) increases to over 100%, which makes the base material appear brighter ( whiter than white ). In addition, the additive admixture of blue light results in a covering of yellow tones. Incidentally, a blue ( cool ) white has a more intense effect , it is more easily associated with high white . The "gray veil" is covered better than with slightly red or greenish white.

White papers and white textiles are now usually optically brightened. Textiles and paper appear brighter as a result. Under ultraviolet light (" black light "), for example in the discotheque , the emitted fluorescent light is perceived particularly intensely due to the high light intensity in the surrounding darkness, and with a color stimulus around 440 nm it is also clearly blue. Similar conditions can be found in the tanning studio. In contrast to the normal daytime environment with bound body colors and structures, "radiant" light colors are seen through the emissions .

Substance groups

Chemical structure CI Fluorescent Brightener 260 (Blankophor BBH)

Optical brighteners used today can be divided into six groups,

  • Stilbene compounds, in particular triazinyl derivatives of 4,4'-diamino-2,2'-stilbene sulfonic acid (flavonic acid). This group of optical brighteners was launched in 1940 by Bayer AG under the brand name Blankophor . With a share of around 80% of all optical brighteners produced, it is the largest group and is suitable for cellulose and polyamides.
  • Ethylene , phenylethylene and thiophene derivatives that contain two hetero-aromatic radicals (e.g. the benzoxazole radical) (hydrophobic, as additives in melt spinning processes)
  • Coumarin - and carbostyril derivatives (not used today more)
  • 1,3-diphenyl-2-pyrazoline (for the optical brightening of protein fibers, cellulose acetate and polyamides )
  • Naphthalimide (most important representative of N -methyl-4-methoxy-naphthalimide , very stable)
  • Compounds in which a condensed aromatic is directly connected to a heteroaromatic , such as thiazoles , pyrazoles , oxadiazoles or triazines (for example 2,4-dimethoxy-6- (1'-pyrenyl) -1,3,5-triazine)

Today around 400 different substances are produced as optical brighteners with a total amount of over 33,000 t / a. This number includes different forms of application of the same body. Of the optical brighteners whose absorption maximum wavelength shorter than visible light, the colorful must fluorescent colors to distinguish, in the green and yellow ( Brillantflavin ) and Red ( rhodamine fluorescence) and find their application for effect purposes and security here.

use

laundry detergent

In the detergent sector, optical brighteners are primarily intended for washing light-colored textiles with heavy-duty detergents. The function of detergent brighteners is to replace brighteners that are washed out or photochemically degraded during wear. In this way, increasing yellowing and fading of the textiles is prevented and the service life is increased. The content of optical brighteners in detergents ranges from 0.03% to 0.3% (dry matter) with an average value of 0.15%.

Today's detergents contain almost exclusively the two stilbene derivatives DAS1 (disodium 4,4-bis [(4-anilino-6-morpholino-1,3,5-triazin-2-yl) amino] stilbene-2,2-disulfonate ) and DSBP (disodium-4,4-bis (2-sulfostyryl) -biphenyl). The market ratio of DAS1 to DSBP in Europe is approx. 85% to 15%. DAS1 and DSBP are sodium salts of aromatic (with DAS1 also heterocyclic) sulfonic acids. Due to the high acidity of sulfonic acids, they are present in dissolved form as anions at pH values ​​of 6 to 8. The sulfonate groups are responsible for the moderate water solubility of the otherwise hydrophobic brighteners.

The laundry blue used earlier made the laundry appear gray because the absorption of blue light by the laundry ( yellowing ) was compensated for by the absorption of all other light components.

textiles

As early as the 19th century, there were reports about the fluorescence of esculin or watery extracts from horse chestnut bark irradiated by sunlight. Around 1929, the German chemist Paul Krais (1866–1939) observed that textile goods (wool and flax fibers) appeared whiter than usual after treatment with aesculin (from an extract of the horse chestnut). The 4-methylumbelliferone derived from aesculin was the first to be industrially produced Brightener. These optical brighteners, which were initially used, were, however, not very lightfast, waterfast and washfast. The first optical brighteners that adhered well to the fiber were the derivatives of 4,4'-diamino-2,2'-stilbene disulfonic acid.

paper

Optical brighteners are formulated in the manufacture of some types of paper . Papers are so "bright" white. They are used for magazine papers and poster papers in order to achieve better brightness. The white pigment for paper is titanium dioxide , which increases the degree of whiteness; fluorescent organic substances are added for optical brightening. In addition, finely ground and extremely white or precipitated calcium carbonate is often used as a replacement for titanium dioxide .

In the event of an overdose of optical brighteners, the absorption can reach into the visible, the opposite effect occurs, the paper does not become lighter, but the color becomes greenish.

Fluorescent color

Colorants that are not themselves fluorescent can be turned into bright colors with a fluorescent color character by adding certain optical brighteners. Due to the special absorption and emission maxima, however, this composition is only suitable for blue to blue-green tones. Since optical brighteners are manufactured for the absorption of ultraviolet and emission in the blue range, this limitation is immanent. This use plays a certain role separately for signal colors .

Environmental relevance

  • During the washing process, the optical brighteners couple to the fabric fiber. Since it is an equilibrium process, some of it remains in the washing solution and thus ends up in the wastewater and thus further into the sewage treatment plant. Up to 50% of the brighteners go into the sewage sludge, the rest is carried into the waters (receiving waters). Because of the resistance of the brighteners to microbial degradation in the soil and in the water, a critical accumulation can occur. In water bodies and on the surface of the soil, photolytic degradation takes place slowly in sunlight.
  • A hormonal effect of certain classes of substances is suspected. Optically brightened textiles are ascribed a damaging effect due to direct skin contact and take over into the body. Ecologically working textile, detergent and paper manufacturers therefore do without optical brighteners.

Figurative meaning

In a figurative sense, euphemistic, distorting PR methods are also referred to as whitening methods in public relations . Mostly the conceptually more established greenwashing is meant, which stands for the self-portrayal of companies that specifically want to launch an environmentally conscious image.

Individual evidence

  1. ↑ Johann Georg Krünitz, Friedrich Jakob Floerke, Heinrich Gustav Floerke, Johann Wilhelm David Korth, Ludwig Kossarski, Carl Otto Hoffmann: Oekonomische Encyclopaedie or general system of state urban household and agriculture in alphabetical order by Johann Georg Krünitz [continued from - Vol 73-77: Friedrich Jakob Floerke, Vol 78-123: Heinrich Gustav Flörke, Vol 124-225: Johann Wilhelm David Korth, partly Ludwig Kossarski a. Carl Otto Hoffmann, 226-242: Carl Otto Hoffmann]. ; Berlin, 1785; Page 646 ( limited preview in Google Book search).
  2. ↑ Burkhard Fugmann: RÖMPP Lexikon Naturstoffe, 1st edition, 1997. ISBN 978-3-131-79541-0 , p. 184 ( limited preview in the Google book search).
  3. ↑ a b Uwe Claussen: Applied fluorescence: whitener . In: Chemistry in Our Time . tape 7 , no. 3 , 1973, p. 141-147 , doi : 10.1002 / ciuz.19730070503 .
  4. ↑ External identifiers of or database links to Disodium 4,4'-bis [(4-anilino-6-morpholino-1,3,5-triazin-2-yl) amino] stilbene-2,2'-disulphonate : CAS number: 16090-02-1, EC number: 240-245-2, ECHA InfoCard: 100.036.571 , GESTIS substance database : 495129 , PubChem : 6433657 , ChemSpider : 4938753 , Wikidata : Q27286897 .
  5. ↑ a b groups of substances
  6. ↑ a b c Entry on optical brighteners. In: Römpp Online . Georg Thieme Verlag, accessed on May 1, 2014.
  7. ↑ Blankophor history. Blankophor GmbH, accessed on July 10, 2019 .
  8. ^ Bertram Philipp, Peter Stevens: Grundzüge der Industrielle Chemie , VCH Verlagsgesellschaft mbH, 1987, p. 299, ISBN 3-527-25991-0 .
  9. ↑ JC Poggendorf (Ed.): Annalen der Physik , Vol. 4, Verlag JA Barth, Leipzig 1854. P. 313
  10. ^ HJ Meyer (Ed.): New Konversations-Lexikon - A dictionary of general knowledge , Vol. 6, Verlag Bibliographisches Institut, Hildburghausen 1863. S. 936
  11. ↑ History
  12. ↑ Stefan Hächler: Mass Conservation of Paper: An Overview of the Common Methods (PDF; 268 kB), page 9.

Web links