Varnish
The laking is a method of colorant preparation , with from water-soluble dyes by adding precipitants insoluble pigments are generated. From a chemical point of view , these pigments are salts , but are known industrially as “laked pigments” or “colored lacquer”, not to be confused with the coating material paint .
Working method
Chemically, laking is the formation of salts through precipitation with the aim of obtaining reaction products that are insoluble in water, i.e. insoluble salts.
Many simple and economical dyes are water soluble, especially anionic and cationic dyes. This property can interfere with certain applications, for example in offset printing . To color fastness reach are pigments required. In order to obtain inexpensive pigments from the water-soluble dyes, the functional groups responsible for the water-solubility are changed by reaction with suitable counterions.
- Acid dyes (anionic dyes)
Anionic dyes contain sulfonates or carboxylates . Barium ions and calcium ions are suitable as laking agents for sulphone groups . Carboxylates are usually coated with lead ions, which is becoming less important from the point of view of environmental protection. Aluminum , iron and other heavy metal ions are also used.
- Basic (cationic) dyes
Cationic dyes contain amines as a reaction group. They are brilliant and strong in color and are well suited for painting. Laking the basic groups with heteropoly acids leads to pigments with sufficient color fastness.
- Basic substances
Are of interest to the varnish
- Anthraquinone dyes ,
- Thiazine dyes and
- Triphenylmethane , but above all
- Azo dyes , which are very variable with their chromophore.
For quality control purposes, heavy metal salt is added to the dye solution until there is no more leakage (“color halo”) around the sample on a filter paper.
Printing pigments
Printing pigments must
- have suitable color stimuli (e.g. for four-color printing),
- have high opacity even with thin layers,
- be sufficiently colourfast ,
- be dispersible in the binder (which depends on particle shapes and sizes).
Lacquer red C
The barium varnished version of the azo pigment C.I. Pigment Red 53 is one of the most widely used red pigments in printing. It is produced by coupling diazotized 4-chlorotoluidic acid with 2-naphthol . The solubility of the sodium salt (bronze orange, lithosol red C) is also low, but still too high for use as a pigment for offset printing inks. Sufficiently insoluble pigments can be produced by lacquering as a calcium salt, better as a barium salt. They have both the desired color properties of the azo dye and the technically required properties.
The calcium varnish Pigment Red 53: 2, which has been produced since 1900, is still used today as a color pigment, hence the name “Lacquer Red C”. The barium salt Pigment Red 53: 1 has better technical properties and is of greater economic importance.
The use of lacquer red C, barium salt in cosmetic articles is prohibited by EC regulation No. 1223/2009, Annex 2 .
Check dye AS
Because of its characteristic with a clear color change, the dye is suitable as a base-acid indicator for printing securities . Due to the water solubility of the basic dye, there are significant restrictions on the printing processes that can be used. The main body is a carboxylic acid that can be coated with lead ions. The indicator property is retained and the lead content is acceptable for the purpose.
Rhodamine varnish
The cationic (basic) dye rhodamine B corresponds to a good approximation to the magenta shade required for scale printing . It can be used directly as a dye in flexographic printing . In general, however, it is not suitable as a printing pigment if it is not treated. Especially in offset printing, the dye would get into the water and spread evenly over the printing surface. Complex metal acids are suitable for practical use; the rhodamines are usually coated with phosphor-tungsten-molybdenum acid (PTM). PTM lacquers based on rhodamine are suitable as magenta in four-color printing and approved in letterpress and offset printing in accordance with DIN 16508/9. However, the paint fastness and the alkali fastness are often too low.
Mordant dyes
This group of dyes usually gets its good color fastness through complex formation with metal ions. The technique is often used in textile dyeing. To do this, the soluble dyes are first applied to the fiber, and then it is “ pickled ” with metal salts . Fixation on the fiber by complex or salt formation is achieved mainly with chromium , which greatly improves the wash fastness. However, the heavy metal pollution of the fibers and the dyeing waste water is a disadvantage and ecologically critical.
Color lakes of biological origin
Obtained from plants
Hematein varnish
Extractively, hematoxylin can be obtained from blue wood in the form of colorless crystals, which oxidize to brown hematein in the air . Hematein forms brown to purple chelate complexes with Zn 2+ , Fe 3+ , Cr 3+ , Al 3+ . These colored lacquers are not very lightfast, but are used as staining agents in histology .
Madder paint
Extraction can from the plant " True madder " an Alizarin - glycoside are obtained in the form of yellowish crystals. Alizarin is formed during drying. Alizarin forms with Ca 2+ ; Fe 3+ ; Cr 3+ ; Al 3+ ; Ti 3+ red to red-violet chelate complexes (for example the alizarin madder (Al) ). These colored lakes are used to detect the corresponding cations in analytical and clinical chemistry . In the past, wool and silk were dyed red with alizarin, today synthetic dyes have displaced naturally obtained alizarin from the market.
Flavonoid varnishes
Basic substances for the varnishing can be obtained from different plants by extraction.
| plant | Latin name | dye |
|---|---|---|
| Reseda dye | Reseda luteola | Luteolin |
| Yellowwood or dyer's mulberry tree | More tinctoria | Morin |
| Dyeing areas | Quercus tinctoria | Quercetin |
| Crossberry | Rhamnus | Rhamnetine , flavones |
With various metal ions, flavones form colored lacquers in all colors. Because of their low lightfastness, they are of no importance in technical use. But they occur in flower petals, so that the flower colors vary depending on the location, depending on the geologically existing metal traces, through the formation of the colored varnish.
Obtained from animals
Carmine lacquer
Carmine lacquer is a dark red powder. Carminic acid can be extracted from cochineal , a type of lice . The dried bodies of female Coccus cacti (L.) contain approximately 10% carminic acid. This color acid is extracted in acidic solution and preferably lacquered with aluminum-calcium, aluminum or aluminum-tin salts in order to obtain a color lacquer with a brilliant shade. In the color index it has the designation CI 75470 or Natural Red 4. Other designations are cochineal lacquer and monk lacquer. The light and weather fastness is only low, the water fastness depends on the coating agent. The colored lacquer is insoluble in organic solvents. It is more heat-resistant than other food colors and hardly susceptible to oxidation or reduction.
Carminic acid forms dark red chelate complexes that are not very lightfast with Ca 2+ and Al 3+ , and scarlet chelate complexes with Sn 2+ . The formation of the colored lacquer is used in analytical chemistry as evidence for Al 3+ . Carminic acid varnishes have been used in lipsticks .
| Lamination metal | hue |
|---|---|
| aluminum | crimson |
| Aluminum / tin | Scarlet fever |
| barium | dull purple |
| chrome | purple |
| copper | burgundy |
| iron | purple |
| lead | red-brown |
| magnesium | pink |
| mercury | scarlet |
| tin | scarlet |
| uranium | green |
| zinc | purple |
Historically, carmine lacquer was an important natural coloring agent because it was pure in color. It was widely used for dyeing textiles and as a pigment until well over the middle of the 19th century. The "red coats" in the British Army were colored in this way until Azoscharlach was introduced for them in 1878. Today carmine varnish is important as a good colorant for food, medicines and cosmetics. It is mainly used for water coloring, baked goods, cosmetics and also for printing inks. It fulfills the condition of being a natural dye - i.e. not a tar dye - and is permanently listed in the relevant lists without any known incidents. Its fastness properties are better than those of other food coloring, and it is the only natural pigment that is suitable for eye make-up.
Indian yellow
Indian yellow is a historical magnesium-calcium color varnish of euxanthic acid . Euxanthinic acid is a pathological metabolic product of cattle that are fed mango tree leaves and suffer from dehydration. These form an intensely yellow-colored urine, from which the euxanthic acid is deposited when concentrated. Indian yellow is a xanthene dye. It used to be used as paint.
literature
- W. Fühler (Ed.): Composition, production and use of printing inks. Gebr. Schmidt, Stuttgart 1975.
- Paul Rys, Heinrich Zollinger: Guide to dye chemistry . Verlag Chemie, Weinheim 1976
Web links
Individual evidence
- ↑ a b DIN 55943 . In: German Institute for Standardization e. V. (Ed.): Colorants 1 . 7th edition. DIN-Taschenbuch 49.Berlin, Vienna, Zurich 2012, ISBN 978-3-410-23202-5 , pp. 508, 518 .
- ↑ Willy Herbst, Klaus Hunger: Industrial Organic Pigments . Wiley-VCH, Weinheim 2009, ISBN 978-3-527-62496-6 , pp. 330 ( limited preview in Google Book search).
- ↑ Patent EP0965617 : New crystal modification of the pigment CI Pigment Red 53: 2 (gamma phase). Registered on June 5, 1999 , published on December 22, 1999 , applicant: Clariant GmbH, inventor: Martin U. Schmidt, Hans Joachim Metz.
- ↑ External identifiers or database links for Pigment Red 53: 1, barium salt : CAS number: 5160-02-1, EC number: 225-935-3, ECHA InfoCard: 100.023.578 , PubChem : 21238 , ChemSpider : 17215143 , Wikidata : Q27155937 .
- ↑ Regulation (EC) No. 1223/2009 (PDF)