Pigments (Latin pigmentum for " color ", " make-up ") are colorants , so coloring substances. In contrast to dyes , they consist of particles and are practically insoluble in the application medium. The application medium is the substance into which the pigment is incorporated, for example in binders such as oils or plastics . In biology , the term pigment describes all substances that give color in a living organism.
Pigments can be differentiated according to their chemical structure (inorganic or organic), according to their optical properties (white, colored, black, effects) and according to their technical properties (corrosion protection, magnetism). The color stimulus arises from absorption and remission ( scattering or reflection ) of certain frequency components of visible light . Solid body properties such as crystal structure , crystal modification , particle size and particle size distribution are decisive for the properties of the pigments , the latter due to the specific surface area .
Pigments are z. B. used in oil paints , varnishes , emulsion paints , printing inks and colored pencils , as a coating pigment ( white pigments ) and filler in paper production and in the coloring of textiles , plastics, cosmetics and candles .
Evidence for the use of earth colors goes back well into prehistory . In rock and cave paintings , naturally achievable earth colors have been proven, mainly based on iron oxide, such as ocher . It can be assumed that these have been used for body painting since the early days of mankind . Often white lime color to ocher yellow and rust-red to brown color gradations are present in natural pigments, such as red chalk, a clay colored with hematite (a red iron oxide). Important inorganic colorants of the early days were red chalk (a yellow iron oxide) and brownstone . A colorant that has been used since ancient times is white, see lime paint . Greenish earths and black minerals are less common. With the mastery of fire, prehistoric painters had black charcoal or charcoal and brick-red burnt ocher at their disposal. Products that emerge from the campfire can be classified as artificial pigments, although ocher and charcoal can also be created from natural fires. Under such circumstances, the schematic grouping of natural and artificial pigments in the tradition of painting has little use in historical terms. The history of the manufacture and use of pigments must be considered in detail, and the determination of the pigments based on traditional names is a problem. Pigments for ceramic decoration have been proven , and one of the oldest white pigments is calcium oxide , which is obtained from limestone by burning .
In ancient times , the pigment processing technique was widely used. Black pigments ( iron oxide black, manganese black), which were formed from clays and ocher in ceramic firing at temperatures around 1000 ° C, were found in particular in the case of pigments processed with ceramics . Carbon black, which was deposited on the ceramic vessel bodies by smoking processes, was of great importance . Red pigments are based primarily on the iron oxide reds , obtained by burning ocher or iron-containing clays. Colloidally disperse copper , which was formed from basic copper carbonates ( malachite , azurite ) during reducing firing , was suitable for red tones. For white pigments in addition to standing calcium carbonate , the kaolinite available.
Cold processed pigments for wall, stone and wood painting are also based on ocher and clays (black and red pigments). Green pigments were based on copper compounds, such as basic copper carbonate and copper hydroxychlorides , which were formed by immersing copper sheets in salt solutions. The “ Egyptian blue ”, a copper calcium silicate, which was probably produced by melting basic copper carbonate , calcium carbonate and quartz sand , was in great demand because of its clear color . Cobalt aluminate blue, which was only rediscovered as Thénards blue in 1804, had also long been used as a blue pigment .
In later painting, white lead [Pb (OH) 2 · 2 PbCO 3 ] was the only available white pigment for a long time . From the beginning of the 19th century this was replaced by zinc white (ZnO) because of its toxicity . Nowadays titanium white (TiO 2 ), which was only developed in the 20th century, is used almost exclusively .
Up to the 18th century, one of the most expensive colors was blue, for which there was previously only the rare gemstone lapis lazuli as raw material in addition to the synthetically produced smalt pigments . The latter produces ultramarine blue after a labor-intensive process .
The first industrially produced inorganic pigment was Berlin Blue in 1704. The first organic pigment, Pararot , CI Pigment Red 1, an azo pigment of the β-naphthol group followed in 1885. Copper phthalocyanine followed in 1935 and quinacridone in 1955. Which was discovered in 1986 chemical group diketo-pyrrolo-pyrrole was the last pigment type to gain significant market share.
Pigments are used in many industries: paints, varnishes, printing inks and plastics, as well as cosmetics, paper, building materials, ceramics and glass. In the textile industry, soluble dyes are increasingly being replaced by pigments, referred to there as disperse dyes . In 2006 the global market for inorganic, organic and specialty pigments reached a volume of around 7.4 million tons. In terms of volume, Asia has the largest share, followed by Europe and North America. Sales of around 17.6 billion US dollars (around 13 billion euros) were achieved. In 2009 pigments were sold worldwide for around 20.5 billion US dollars, approx. 1.5% - 2% more than in the previous year. In 2010 sales will reach around 24.5 billion US dollars and in 2018 around 27.5 billion US dollars.
Pigments typically arise in the form of the primary particles . The primary particles can grow together to form aggregates over their surfaces . From agglomerates is when primary particles and / or aggregates edges are connected via their corners /. The pigment agglomerates are comminuted by the dispersing process ( dispersion ) when the pigments are incorporated into an application medium. Smaller agglomerates, aggregates and primary particles are formed. If present, these are wetted by a dispersing medium. Ideally, they are distributed statistically across the application medium.
In solid form, the pigment can be used pure ( primary pigment ), as a solid mixture of two or more pigments or as a mixture with one or more fillers . Mixing with fillers reduces the color strength , which means that small quantities can be better dosed. This option is used with powder coatings . Due to the spatial proximity, primary pigments have a more intense effect ( simultaneous contrast ).
In the case of liquid paints, (prepared) pigment preparations that either contain binders or are binder-free are often used . These pigment preparations are formulated like the paint itself; pre-dispersed, they contain high pigment concentrations in additives, solvents, water or binders, depending on the area of application. The advantage of pigment preparations is that they can be incorporated easily and precisely because the pigment is already dispersed and standardized. Additives can have a disadvantageous effect, as the pigment preparation may no longer be compatible with all paint systems.
As Tönsystem (English Tinting system ), the combination of multiple (usually 12-20) pigment preparations, an automatic dosing and a Rezeptiersoftware designated. This method is used for architectural paints . Pigment preparations can be in the form of a mixture with other pigments or fillers. In addition to the liquid pigment preparations that are frequently used, granulated preparations made with readily soluble binders are available if additional solvents are undesirable in the coating formulation.
A third option, which is particularly widespread in the plastics industry, is the use of solid or liquid pigment preparations, masterbatches or liquid colors . In masterbatch production, the pigments are extruded or kneaded into a binder matrix at a higher processing temperature . After cooling, the resolidified masterbatches are usually granulated so that they produce more precise and reproducible color tones when they are incorporated into the plastic . Masterbatches can contain several pigments or fillers depending on the desired effect. Liquid pigment preparations are produced in batches at room temperature. For this purpose, the formulation components are distributed in a binding agent previously selected for the respective application and then dispersed. The most optimal possible break-up of agglomerates is crucial in order to ensure that the color concentrates and / or the functional process additives are highly effective. Dissolvers , bead mills and roller mills are mostly used here.
Pigments are usually named with trivial names , trade names or names from the Color Index ( CI Generic Name ), as systematic nomenclatures according to IUPAC ( International Union of Pure and Applied Chemistry ) or CAS ( Chemical Abstracts Service ) lead to unwieldy and complicated names.
- Common name: brilliant yellow
- Trade names: Aureolin , Benzimidazolone Yellow
- Registered trade name: Hostaperm (TM) Yellow H4G
- C. I. Generic Name: C. I. Pigment Yellow 151
- IUPAC name: 2 - [[1 - [[(2,3-Dihydro-2-oxo-1 H -benzimidazol-5-yl) amino] carbonyl] -2-oxopropyl] azo] -benzoic acid
- CAS index name: Benzoic acid, 2 - [[1 - [[(2,3-dihydro-2-oxo-1 H -benzimidazol-5-yl) amino] carbonyl] -2-oxopropyl] azo] -
Classification according to properties
Pigments with common properties are grouped together which, depending on the purpose, lead to different structures. DIN 55943 initially divides colorants into organic and inorganic colorants . Each of the two groups is divided into dyes and pigments. The next level is divided according to the visual effect. A distinction is made between white pigments, colored pigments and dyes, black pigments and dyes, effect pigments and luminous pigments and dyes. The groups of white dyes and effect dyes are physically not possible, since the effect as a pigment is based exclusively on scattering (white pigments) or reflection (effect pigments). This requires an interface which the dissolved dyes do not have.
The inorganic colorants are not subdivided further, since it is a standard from the paint sector and no inorganic colorants are used there.
A list of individual pigments sorted by color is given under List of Pigments .
Division into natural and synthetic pigments
In the case of inorganic pigments, a distinction is made between natural and synthetic pigments. The first group includes earths and minerals ( earth colors , mineral white ) that do not require any or only mechanical processing (mostly drying and grinding) for their use. The second group includes inorganic pigments such as metallic effect pigments , carbon black , white pigments , iron oxide pigments or zirconium silicates , i.e. synthetic products from different manufacturing processes. Synthetic inorganic pigments are produced industrially due to their more stable quality and higher purity.
It is not always necessary to choose the classification or can be determined from the material whether it is of natural or artificial origin. Such a subdivision is difficult with the iron oxide-containing paint layers of prehistoric painting. The indication cinnabar , the red modification of mercury sulphide , does not provide any information about a natural origin. Was also "vermilion" in ancient times a synonym for any red and synonymous with the red lead , the minium. The division of inorganic pigments into natural and artificial did not emerge until the 1940s and says nothing about their chemical structure.
Classification according to chemical classes
Chemically, the most important industrial pigments can be divided into eight substance classes. Specifically, these are titanium dioxide , carbon black , Bismutpigmente , oxides and hydroxides, Eisencyanblau , ultramarine , cadmium pigments and chromate pigments .
The group of oxides and hydroxides is further divided into iron oxide pigments , chromium oxide and mixed phase as cobalt green , (the latter with the subgroups Spinel , hematite pigments , Inverse Spinel and Rutilderivate ). The group of chromate pigments is further divided into chrome yellow , chrome green and molybdates .
Soot has a special position here. Soot is inorganic by definition. Due to the small particle size and the resulting application properties, it is often classified as an organic pigment.
Most inorganic pigments are characterized by the fact that they do not react chemically with the oxygen in the air and are therefore extremely resistant to aging and retain their hue for practically any length of time, although this is due to the aging of an organic painting medium such as oil with the Time can be affected.
Its high heat resistance makes it possible to use it in porcelain painting . Only inorganic pigments can be used here, since organic pigments are not temperature-stable and are destroyed when fired. In industrial applications, a high level of heat resistance is important for coloring plastics , powder coatings or coil coatings , although heat-resistant organic pigments can be used because of the lower temperatures.
The hue of inorganic colored pigments is often described as cloudy compared to organic pigments. This applies without restriction to pigments such as iron oxide pigments or chrome oxide green, but there are still some inorganic pigments with a pure hue. Of the pigments used industrially, this is essentially bismuth vanadate , and the pigments, which have fallen into disrepute, cadmium sulfide, lead chromate or molybdate red show a brilliant color tone with good hiding power at the same time. In addition, there are comparatively rarely used pigments such as cobalt blue or ultramarine .
Because of their industrial importance and distribution, white pigments occupy a special position. In the paper industry alone, well over 10 million tons per year are used in Europe, with the white minerals with calcium carbonate making up by far the largest share.
In the paint sector, white is of paramount importance. In emulsion paints , it is the base color for tinting systems and also the main color. In terms of value and production volume, with around 60% of all pigments, titanium dioxide has by far the largest share. In 2006 almost 4.5 million tons of titanium dioxide were consumed worldwide. The white pigment reached this position during the 1960s. Titanium white replaced white lead due to its fastness properties, and there has been a sharp increase in the total amount used in industrialized countries. The easily accessible iron oxide pigments follow in terms of value with 8% and in terms of production volume with 22% in second place in world pigment production, followed by carbon black with 9% and 4% of the volume. The other inorganic and organic pigments divide into the remaining amount. Due to the much higher price level, however, these reach almost 30% in terms of value.
Among the other inorganic pigments, chromium (III) oxide , ultramarine , bismuth vanadate , zirconium silicates and the group of mixed-phase oxide pigments are particularly important. Due to its refractive index, calcium carbonate is preferably used in the paint industry not as a pigment, but as a filler .
Natural organic pigments
Organic pigments occur in nature as "animal" or "plant colors" . Some of these pigments are easy to make. Vine black is an incompletely burned wine wood. Some historically important pigments, such as the brightly colored Indian yellow from the urine of cows, lost their exclusivity due to the wide range of synthetic pigments. The soluble, almost colorless leuco form of indigo, the leuco indigo or indigo white, becomes the colored, insoluble pigment indigo through oxidation with atmospheric oxygen .
Synthetic organic pigments
Synthetic organic pigments are classified according to their chemical structure. The most diverse and at the same time the largest group are the azo pigments . These pigments make up over 50% of the amount of organic pigments sold. The other group is summarized as the polycyclic pigments or, colloquially, non-azo pigments .
Azo pigments are pigments whose chromophore properties are essentially based on the delocalization of electrons starting from an azo group (-N = N-). Azo pigments are pigments that contain at least one azo group. The azo pigments are further subdivided into classes, the chemistry of which allows a rough statement about the authenticity of the pigments. The actual authenticity depends essentially on the substituents used and the particle size. A distinction is made between monoazo and disazo pigments based on the number of azo bonds it contains. A further distinction is made according to the respective substituents.
The monoazo pigments include the simple monoazo pigments, such as the β-naphthol pigments as well as the naphthol AS pigments and the laked azo dyes. Some of the most important organic pigments used belong to this group, at the same time it is the oldest industrially available group. Examples are the arylide yellow pigments CI Pigment Yellow 1, 3 and 74, CI Pigment Orange 5 or CI Pigment Red 112.
A special case are the benzimidazolone pigments , which are in turn monoazo pigments and have polycyclic substituents . These lead to very good weather resistance, so that these pigments achieve the highest fastness properties among the azo pigments. Examples are CI Pigment Yellow 154 or CI Pigment Orange 36. The disazo pigments include the diaryl yellow pigments (CI Pigment Yellow 83 ), the disazo condensation pigments (CI Pigment Yellow 128) or the acetoacetic anilide pigments (CI Pigment Yellow 155). Azo metal complex pigments are a special case because, strictly speaking, they do not contain a real azo group.
Laked pigments, i.e. originally soluble dyes converted into salts with metals , are used in textile dyeing. Color lacquers mean that soluble dyes are fixed as (coloring agents) on the fiber by reaction with metal salts or tannin.
Polycyclic pigments are compounds whose chromophore property is created by the delocalization of electrons over a more or less extensive ring system.
The majority of the polycyclic pigments are made up of copper phthalocyanine pigments , which make up about half of the polycyclic pigments. The most important representatives of this group are the various types of phthalocyanine blue and the halogenated types ( phthalocyanine green ). Other important polycyclic pigment classes are quinacridones , diketopyrrolopyrrole pigments , dioxazines , perylenes , isoindolines and inthanthrones .
In addition to these two substance groups, there are also a number of organic pigments of different composition. They often have a specific area of application. Often only a chemical compound of this structure is suitable as a pigment for economic considerations or requirements for authenticity . Representatives of this group are laked dyes which, as salts of heavy metals, have lost their solubility and are therefore pigments.
Organic pigments differ from inorganic pigments mostly in their higher color strength , lower hiding power , higher color (chroma) and lower weather resistance. In addition, organic pigments are often more expensive. Organic pigments are aftertreated in order to improve certain application properties such as dispersibility or hiding power. The aftertreatment also sets the particle size, which is responsible for the level of fastness, color strength and the fine-tuning of the coloring .
With regard to the toxicology of organic pigments, the general rule is that these pigments, taken by themselves, are physiologically practically inert due to their low solubility . Health concerns arise as fine dust . Organic pigments are considered to be practically non-biodegradable. Since pigments are used in the intermediate or end product using dispersants, binders, solvents or the like, the toxicological effect of these substances may have to be checked.
Degradation products of pigments that occur when irradiating with laser light, for example when removing pigments from tattoos, can be toxicologically questionable. When the tattoo pigment CI Pigment Red 22 (CAS No. 6448-95-9) was split by laser light, the toxic and carcinogenic 2-methyl-5-nitroaniline was detected; when irradiated with copper phthalocyanine , hydrogen cyanide was formed .
- Acid-base indicators do not count among the pigments: they are dyes whose color changes with the pH of the solution
- Substrate colors consist of a coloring component and a more or less colorless pigment, the substrate . In a conversion process, both components are bonded to one another, making them insoluble in water and binding agents. In antiquity and in the Middle Ages, plant dyes ( dye plants ) were mostly grown on a white substrate such as chalk or white lead ; In this mordants such as alum and sodium carbonate added, the improved connection between the dye and substrate.
Metallic effect pigments
Brass and aluminum are the most important pigments for creating a metal effect . Brass particles give colors a golden appearance, while aluminum in a matching platelet shape creates a silvery appearance. Previously used descriptions are Silver Bronze for aluminum pigments and depending on color and alloy gold bronze , pale gold , rich pale gold and rich gold brass pigments.
The visual impression depends on the angle. In the top view (almost perpendicular), the lighter- looking metallic effect pigment can be seen, while the mostly dark basic shade emerges at a flat angle. This effect due to the platelet shape of the particles is known as a flop . Aluminum platelets in a particle size suitable for pigments produce a silver sheen, almost spherical particles of the same particle size form a uniform gray surface. Since untreated aluminum pigments are only moderately stable, especially in aqueous systems and under the influence of weathering, surface-treated brands have been developed that compensate for this disadvantage.
The color depth is related to the grain size. The exact appearance of the pigment is essentially determined by the particle size and the regularity of the particle shape. Coarse particles create a glittering impression called a sparkle . Fine particles create a smoother flop, i.e. a smoother transition when changing the viewing angle. Both types are often used at the same time to achieve the desired effect.
These pigments are called interference pigments . They consist of platelet-shaped carrier substrates with a low refractive index, mostly natural mica , silicon dioxide or very thin glass platelets, which are coated with one or more extremely thin and very uniform oxide layers with a high refractive index. Titanium dioxide , iron (III) oxide or zirconium dioxide are preferred , and mixed oxides thereof are also used. Sol-gel , CVD or PVD processes are mainly used as coating processes . The layer thicknesses produced are in the range of 100 nm. During production, precise control of the coating thickness (to ± 3 nm) and its homogeneity is crucial.
By choosing the coating parameters, primarily the refractive index , layer thickness and layer sequence, almost any color and shade can be achieved through the effect of interference . Under certain conditions, viewing angle-dependent color flop colors can be generated in which the hue changes depending on the viewing angle of the observer.
Luminous pigments are, on the one hand, brightly colored fluorescent pigments for fluorescent colors ("neon colors") and, on the other hand, phosphorescent substances that are luminescent . They are used in luminous inks . Usually made from fluorescent pigments incorporated in a matrix of fluorescent dyes , thereby obtained pigment properties. As afterglow are doped inorganic substances used with phosphorescence. The green luminous pigments based on zinc sulfide are widespread and are used to mark escape routes.
The radioactive illuminants are not counted among the pigments, although they are insoluble. They are self-luminous objects that are not emitted by UV or daylight, but by radioactive excitation.
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