Pearlescent coatings
Encyclopedia
Pearlescent or nacre
ous coatings or pigments possess optical effects that not only serve decorative purposes (such as cosmetics, printed products, industrial coatings, or car paints), but also provide important functional roles, such as security printing or optical filters.
ous pigments have become popular in the creation of luster effects in coatings. This has enabled the generation of new and unique color effects for automotive, industrial, cosmetic and pharmaceutical applications. The pearlescent effect is produced by the specular reflection
of light
from the many surfaces of the platelets with parallel orientation at various depths within the coating. Light striking the platelets is partially reflected and partially transmitted through the platelets. A pearly luster effect
is produced by the dependence of reflection on viewing angle, and the sense of depth is created by reflection from many layers. The platelets must be extremely smooth to maximize reflected light, and any surface roughness diminishes the lustrous effect. Non-fine particles or pigments with rough edges can also negatively affect the lustrous appearance.
, and angle-dependent optical effects are now commonplace and are found in many products. These optical effects not only serve decorative purposes (such as cosmetics, printed products, industrial coatings, or car paints), but also provide important functional roles, such as security printing
or optical filters. In the security field, for example, angle-dependent optical effects obtained from pearlescent pigments cannot be easily copied with photocopier
machines or photographic techniques, making forgery much more complicated and expensive.
In decorative applications, several advantages result from the use of special effect pigments. The first is the illusion of optical depth, which is created by the arrangement of a multiplicity of platelet-like semi-transparent particles of a pearlescent pigment. The achieved impression is the result of reflection of light at the different interfaces between pigment and binder and at the boundary layers of the effect pigment itself. The effect is especially strong when extended areas are profiled as in automotive fenders
. Currently, pearlescent pigments can be found in the paints of more than 40% of the cars in the United States and 30% in Europe. The second advantage is the subtle to startling eye-catching effect of an angle-dependent color. This greatly enhances aesthetics and promotional eye-catching appeal of articles.
Optical effects such as directed reflection
, multiple reflection, interference, and color travel (strong angle-dependent optical effects) cannot only be achieved with special pigments in parallel alignment in an application system, but also by utilizing extended multilayer films. Such films consist of continuous, non-pigmented inorganic or organic polymeric materials, which are manufactured in a thickness of a few to several hundred micrometers. These films are structured typically as multilayer systems with alternating layers of different refractive index
. Effect pigments, on the other hand, generate their optical attractiveness in the application system because of the ability of easy parallel orientation of a multitude of platelet-like particles. Also in powder coating
applications, where an optimal orientation of the pigment platelets is difficult in some cases, strong lustrous color effects are achievable. Reflection, multiple reflection, and interference is here possible, also under circumstances where the particles are partly disoriented, only the achieved effects can be different.
Pigments without a layer structure are the well-known metal effect pigments, such as aluminium
or copper
–zinc
(brass
) platelets. Flaked transparent effect pigments such as single-crystalline BiOCl
and polycrystalline TiO2
also belong to this group. These non-metallic flakes are mostly very thin to achieve a certain interference color. This can lead to a lower mechanical stability compared with flakes based on substrate platelets.
Only a few materials are known to crystallize in thin flakes with suitable size and thickness for use as effect pigments. Substances which do not crystallize in this manner can be used only with thin supporting platelets, the substrates or templates onto which the pigment materials can be deposited. The best-known examples are pearlescent pigments based on platelets of natural or synthetic mica
, alumina flakes (Xirallic®)
coated with thin layers of TiO2, and Fe2O3
. Pigments based on transparent substrates, such as mica-based products, can be easily combined with absorption pigments or with metal effect pigments even in thicker coatings. The advantage of metal effect pigments is their strong hiding power in the application system. Mixtures of transparent pearlescent pigments and more hiding metallic pigments are often used in automotive coatings.
alloys ("gold bronzes"), zinc, or other metals. The metallic effect is caused by the reflection of light at the surfaces of the pigment particles. The luster effect decreases as the proportion of light scattered at edges and corners of the particles increases. Larger particles are better reflectors, leading to higher brilliance and brightness. The metallic appearance depends also on the orientation of the metal flakes in the application system, the particle shape, the transparency of the binder
matrix, and the presence of other colorants. The required particle size of the pigments depends on the intended use and can vary from a few micrometers (offset printing
) to medium grades (10–45 μm, automotive coatings, gravure and flexographic printing
) and coarser grades (corrosion-inhibiting
systems, plastics). The thickness of the flakes can vary from less than 0.1 to 1 μm.
There are leafing and non-leafing metal effect pigments on the market. Leafing pigments float on the surface of paint
or printing ink
films because of their high interfacial tension, achieved by the use of stearic acid
during the pigment manufacture. On the other hand, non-leafing pigments are completely wetted by the application medium and are dispersed homogeneously throughout the coating. Non-leafing properties are created by using lubricant
s that consist of branched-chain or unsaturated
fatty acid
s (e.g., oleic acid
) or polar
substances (e.g., fatty amine
s).
Metal effect pigments are produced by treating metal granules with stamping machines. Ball mill
s using dry milling (Hametag process) or wet milling (Hall process) are mostly used to produce metal flakes. During the ball milling process, a lubricant is added to prevent cold fusion and to achieve the desired leafing or non-leafing properties.
Standard aluminium pigments are produced in "cornflake" and "silverdollar" types depending on the quality and shape of the starting granules and on the milling conditions. A special type is PVD aluminium, also known as VMP (Vacuum Metallized Pigment), produced by a vacuum process where the aluminium is deposited on a web. After releasing the deposited aluminium from the web, very thin flakes are obtained, with improved mirror-like effects when incorporated into coating systems.
The current world market for metal effect pigments can be estimated at more than 20,000 tonnes per year. Depending on the production process and the application, the pigments are supplied in powder form or as solvent
-containing preparations (pastes, granulates). Stabilized aluminium pastes with water or water-miscible solvents are available for waterborne coating or printing systems. Pigments coated with special organic (e.g., acrylics
) or inorganic materials (e.g., silica) are achievable for powder coatings.
derived from fish scale
s, skin, or bladder. The pigment particles in the suspension are platelet-shaped with a high aspect ratio. They consist of 75–97% guanine
and 3–25% hypoxanthine
. The ratio of these two purines depends on the fish species (e.g., herring
s, sardine
s). One ton of fish yields less than 250 g of guanine. An industrial synthetic process for producing purine
s with this crystal shape does not exist. An aqueous suspension of fish scales is, therefore, extracted with organic solvents to dissolve and remove the protein
s. The remaining dispersion contains purine crystals and scale, which are separated from one another by a complicated washing and phase-transfer process.
Pigment platelets tend to agglomerate
and are therefore only handled as dispersion
s. These dispersions are used almost exclusively in cosmetic
applications (nail enamels, lotion
s, shampoo
s) because of the very high price. The pigment particles of natural pearl essence show a high but soft luster (nD = 1.79 (parallel) to 1.91 (perpendicular)) and a relatively low density of 1.6 g/cm³, which reduces settling in liquid formulations. The world production of natural pearl essence in 2004 is estimated to be less than 50 tonnes.
from aqueous solutions. Carbon dioxide
is reacted with these solutions under carefully controlled conditions. The resulting platelet-shaped particles are less than 0.05 μm thick and have diameters of about 20 μm, yielding an aspect ratio of about 200. Due to their high refractive index of 2.0 and their even surfaces, the platelets exhibit a very strong luster.
(BiOCl) effect pigments are produced by hydrolysis
of very acidic bismuth
salt solutions in the presence of chloride
. The crystal quality can be adjusted by the chosen reaction parameters, such as bismuth salt concentration, temperature, pH-value, reactor geometry, and addition of surfactant
s. The usually tetragonal bipyramidal crystal geometry can be flattened to platelets with high aspect ratio. Pigments with an aspect ratio of 10–15 show low luster and very good skin feeling and are used as filler
s in cosmetics. Crystals with higher aspect ratio show strong luster and are mainly used for nail polish
. The low light stability, the fast settling behavior caused by a density of 7.73 g/cm³, and the lack of mechanical stability limit the use of bismuth oxychloride in technical applications. Therefore, it is mainly used in cosmetics, but also in buttons and jewelry. The current world market is about 400 t per year.
consists of pure or doped α-iron oxide (α-Fe2O3, hematite
). It is found already in nature in form of platelets. This natural product with a density of 4.6–4.8 g/cm³ and a dark gray color with low luster is nearly exclusively used in corrosion protection coatings. Micaceous iron oxide can also be obtained as platelets by hydrothermal reaction in alkaline media. If substantial amounts of doping materials are incorporated, the aspect ratio can be increased up to 100, resulting in much better luster. The color can be also shifted from dull dark to a more attractive reddish brown allowing the use of the platelets for decorative applications. Al2O3
, SiO2
, and Mn2O3
are the most important doping constituents. Fe(OH)3 or better FeOOH
as starting material is heated in an alkaline suspension together with the doping constituents to temperatures above 170 °C, typically 250–300 °C. Platelets of doped micaceous iron oxide are formed after several minutes to hours. In a second reaction phase, the pH-value is further increased leading to the growth of flat basal faces.
flakes can be produced breaking down continuous films of TiO2. Such films can be obtained using a web-coating process where TiOCl2 is thermally hydrolyzed on the surface of the web. Substrate-free TiO2 flakes can also be achieved from TiO2-mica pigments by dissolving the substrate in strong acids or hydroxides. The so-obtained titanium dioxide flakes are not single crystals but polycrystalline and slightly porous. They show only limited mechanical stability in most cases and can, therefore, not be used in technical applications where stress is exerted.
s. However, the difference of their refractive index and that of the typical application media is too small to generate strong interference colors and luster effects. In most cases, the aspect ratio of these crystals is also much smaller than that of the substrate-free and the substrate-based inorganic effect pigments.
s (LCP) can also be used in form of flakes or large films to achieve interference and especially angle-dependent interference color effects. The structure of these materials consist of parallel oriented chiral cholesteric (nematic) liquid crystal
line layers having their director rotated by a certain angle with respect to an adjacent layer, building up a helical array
. The helical structure is responsible for interference effects because the refractive index is changing from layer to layer. It is also possible to get such optical effects by helical superstructures. A number of LCP pigments and films are based on polysiloxanes. The first step of the manufacture is the formation of a thin cross-linked film of a liquid crystalline polymer. After a UV-curing step for polymerization
, the so formed solid film is ground to small platelets. These particles can be used to achieve angle-dependent effects when having a thickness of more than 4 μm in most cases. Up to now the application of these pigments is still limited because of the thickness and some stability problems.
-forming methods). Importantly, this approach could be used to protect pharmaceuticals from counterfeiting and adulteration. Coatings to which pearlescent pigments have been incorporated can further enhance the color appeal of coated tablet and capsule pharmaceutical products. Possible systems for this application utilise polymers like polyvinyl alcohol
, hypromellose
, Carrageenan
, or microcrystalline cellulose
, as the carrier materials in levels up to 50%, a pearlescent pigment or combinations, e.g., micacious titanium dioxide in levels between 1–10%, and dispersants like tween
, lecithin
and polyethylene glycol
in levels between 0–15%, to act as stabilisers
.
Nacre
Nacre , also known as mother of pearl, is an organic-inorganic composite material produced by some mollusks as an inner shell layer; it is also what makes up pearls. It is very strong, resilient, and iridescent....
ous coatings or pigments possess optical effects that not only serve decorative purposes (such as cosmetics, printed products, industrial coatings, or car paints), but also provide important functional roles, such as security printing or optical filters.
Description
Pearlescent or nacreNacre
Nacre , also known as mother of pearl, is an organic-inorganic composite material produced by some mollusks as an inner shell layer; it is also what makes up pearls. It is very strong, resilient, and iridescent....
ous pigments have become popular in the creation of luster effects in coatings. This has enabled the generation of new and unique color effects for automotive, industrial, cosmetic and pharmaceutical applications. The pearlescent effect is produced by the specular reflection
Specular reflection
Specular reflection is the mirror-like reflection of light from a surface, in which light from a single incoming direction is reflected into a single outgoing direction...
of light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
from the many surfaces of the platelets with parallel orientation at various depths within the coating. Light striking the platelets is partially reflected and partially transmitted through the platelets. A pearly luster effect
Lustre (mineralogy)
Lustre is a description of the way light interacts with the surface of a crystal, rock, or mineral. The word lustre traces its origins back to the Latin word lux, meaning "light", and generally implies radiance, gloss, or brilliance....
is produced by the dependence of reflection on viewing angle, and the sense of depth is created by reflection from many layers. The platelets must be extremely smooth to maximize reflected light, and any surface roughness diminishes the lustrous effect. Non-fine particles or pigments with rough edges can also negatively affect the lustrous appearance.
Advantages of pearlescent pigments
Lustrous, iridescentIridescence
Iridescence is generally known as the property of certain surfaces which appear to change color as the angle of view or the angle of illumination changes...
, and angle-dependent optical effects are now commonplace and are found in many products. These optical effects not only serve decorative purposes (such as cosmetics, printed products, industrial coatings, or car paints), but also provide important functional roles, such as security printing
Security printing
Security printing is the field of the printing industry that deals with the printing of items such as banknotes, passports, tamper-evident labels, product authentication, stock certificates, postage stamps and identity cards...
or optical filters. In the security field, for example, angle-dependent optical effects obtained from pearlescent pigments cannot be easily copied with photocopier
Photocopier
A photocopier is a machine that makes paper copies of documents and other visual images quickly and cheaply. Most current photocopiers use a technology called xerography, a dry process using heat...
machines or photographic techniques, making forgery much more complicated and expensive.
In decorative applications, several advantages result from the use of special effect pigments. The first is the illusion of optical depth, which is created by the arrangement of a multiplicity of platelet-like semi-transparent particles of a pearlescent pigment. The achieved impression is the result of reflection of light at the different interfaces between pigment and binder and at the boundary layers of the effect pigment itself. The effect is especially strong when extended areas are profiled as in automotive fenders
Fender (automobile)
Fender is the US English term for the part of an automobile, motorcycle or other vehicle body that frames a wheel well . Its primary purpose is to prevent sand, mud, rocks, liquids, and other road spray from being thrown into the air by the rotating tire. Fenders are typically rigid and can be...
. Currently, pearlescent pigments can be found in the paints of more than 40% of the cars in the United States and 30% in Europe. The second advantage is the subtle to startling eye-catching effect of an angle-dependent color. This greatly enhances aesthetics and promotional eye-catching appeal of articles.
Optical effects such as directed reflection
Reflection (physics)
Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...
, multiple reflection, interference, and color travel (strong angle-dependent optical effects) cannot only be achieved with special pigments in parallel alignment in an application system, but also by utilizing extended multilayer films. Such films consist of continuous, non-pigmented inorganic or organic polymeric materials, which are manufactured in a thickness of a few to several hundred micrometers. These films are structured typically as multilayer systems with alternating layers of different refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
. Effect pigments, on the other hand, generate their optical attractiveness in the application system because of the ability of easy parallel orientation of a multitude of platelet-like particles. Also in powder coating
Powder coating
thumb|right|Aluminium extrusions being powder coatedPowder coating is a type of coating that is applied as a free-flowing, dry powder. The main difference between a conventional liquid paint and a powder coating is that the powder coating does not require a solvent to keep the binder and filler...
applications, where an optimal orientation of the pigment platelets is difficult in some cases, strong lustrous color effects are achievable. Reflection, multiple reflection, and interference is here possible, also under circumstances where the particles are partly disoriented, only the achieved effects can be different.
Types of pearlescent pigments
Pearlescent pigments can basically be classified into two groups:- platelets that consist of only one optically homogeneous material (substrate-free pigments);
- platelets that have a layered structure and consist of at least two optically different layer materials (pigments with layer-substrate structure or multilayer pigments without a substrate).
Pigments without a layer structure are the well-known metal effect pigments, such as aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
or copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
–zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
(brass
Brass
Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...
) platelets. Flaked transparent effect pigments such as single-crystalline BiOCl
Bismuth oxychloride
Bismuth oxychloride is a chemical compound of bismuth, oxygen, and chlorine, with the formula BiOCl. It exists in nature as the mineral bismoclite which is part of the matlockite mineral group.-Structure:...
and polycrystalline TiO2
Titanium dioxide
Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
also belong to this group. These non-metallic flakes are mostly very thin to achieve a certain interference color. This can lead to a lower mechanical stability compared with flakes based on substrate platelets.
Only a few materials are known to crystallize in thin flakes with suitable size and thickness for use as effect pigments. Substances which do not crystallize in this manner can be used only with thin supporting platelets, the substrates or templates onto which the pigment materials can be deposited. The best-known examples are pearlescent pigments based on platelets of natural or synthetic mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
, alumina flakes (Xirallic®)
Alumina effect pigment
Alumina effect pigment are pearlescent pigments based on alumina .The most important class of special effect pigments today exists of natural mica platelets coated with thin films of transparent metal oxides like titanium oxide and iron oxide....
coated with thin layers of TiO2, and Fe2O3
Iron(III) oxide
Iron oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron oxide , which is rare, and iron oxide , which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe2O3 is the main...
. Pigments based on transparent substrates, such as mica-based products, can be easily combined with absorption pigments or with metal effect pigments even in thicker coatings. The advantage of metal effect pigments is their strong hiding power in the application system. Mixtures of transparent pearlescent pigments and more hiding metallic pigments are often used in automotive coatings.
Metal effect pigments
By far the most important effect pigments without layer structure are the metal effect pigments. They consist of flakes, or lamellae, of aluminium ("aluminium bronzes"), copper and copper-zincBrass
Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...
alloys ("gold bronzes"), zinc, or other metals. The metallic effect is caused by the reflection of light at the surfaces of the pigment particles. The luster effect decreases as the proportion of light scattered at edges and corners of the particles increases. Larger particles are better reflectors, leading to higher brilliance and brightness. The metallic appearance depends also on the orientation of the metal flakes in the application system, the particle shape, the transparency of the binder
Binder (material)
-See also:*Adhesive or Glue*Cement*Paint...
matrix, and the presence of other colorants. The required particle size of the pigments depends on the intended use and can vary from a few micrometers (offset printing
Offset printing
Offset printing is a commonly used printing technique in which the inked image is transferred from a plate to a rubber blanket, then to the printing surface...
) to medium grades (10–45 μm, automotive coatings, gravure and flexographic printing
Flexography
Flexography is a form of printing process which utilizes a flexible relief plate. It is basically an updated version of letterpress that can be used for printing on almost any type of substrate including plastic, metallic films, cellophane, and paper...
) and coarser grades (corrosion-inhibiting
Corrosion inhibitor
A corrosion inhibitor is a chemical compound that, when added to a liquid or gas, decreases the corrosion rate of a material, typically a metal or an alloy. The effectiveness of a corrosion inhibitor depends on fluid composition, quantity of water, and flow regime...
systems, plastics). The thickness of the flakes can vary from less than 0.1 to 1 μm.
There are leafing and non-leafing metal effect pigments on the market. Leafing pigments float on the surface of paint
Paint
Paint is any liquid, liquefiable, or mastic composition which after application to a substrate in a thin layer is converted to an opaque solid film. One may also consider the digital mimicry thereof...
or printing ink
Ink
Ink is a liquid or paste that contains pigments and/or dyes and is used to color a surface to produce an image, text, or design. Ink is used for drawing and/or writing with a pen, brush, or quill...
films because of their high interfacial tension, achieved by the use of stearic acid
Stearic acid
Stearic acid is the saturated fatty acid with an 18 carbon chain and has the IUPAC name octadecanoic acid. It is a waxy solid, and its chemical formula is CH316CO2H. Its name comes from the Greek word στέαρ "stéatos", which means tallow. The salts and esters of stearic acid are called stearates...
during the pigment manufacture. On the other hand, non-leafing pigments are completely wetted by the application medium and are dispersed homogeneously throughout the coating. Non-leafing properties are created by using lubricant
Lubricant
A lubricant is a substance introduced to reduce friction between moving surfaces. It may also have the function of transporting foreign particles and of distributing heat...
s that consist of branched-chain or unsaturated
Unsaturated compound
In organic chemistry, a saturated compound is a chemical compound that has of a chain of carbon atoms linked together by single bonds and has hydrogen atoms filling all of the other bonding orbitals of the carbon atoms. Alkanes are an example of saturated compounds...
fatty acid
Fatty acid
In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...
s (e.g., oleic acid
Oleic acid
Oleic acid is a monounsaturated omega-9 fatty acid found in various animal and vegetable fats. It has the formula CH37CH=CH7COOH. It is an odorless, colourless oil, although commercial samples may be yellowish. The trans isomer of oleic acid is called elaidic acid...
) or polar
Chemical polarity
In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...
substances (e.g., fatty amine
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
s).
Metal effect pigments are produced by treating metal granules with stamping machines. Ball mill
Ball mill
A ball mill is a type of grinder used to grind materials into extremely fine powder for use in mineral dressing processes, paints, pyrotechnics, and ceramics.-Description:...
s using dry milling (Hametag process) or wet milling (Hall process) are mostly used to produce metal flakes. During the ball milling process, a lubricant is added to prevent cold fusion and to achieve the desired leafing or non-leafing properties.
Standard aluminium pigments are produced in "cornflake" and "silverdollar" types depending on the quality and shape of the starting granules and on the milling conditions. A special type is PVD aluminium, also known as VMP (Vacuum Metallized Pigment), produced by a vacuum process where the aluminium is deposited on a web. After releasing the deposited aluminium from the web, very thin flakes are obtained, with improved mirror-like effects when incorporated into coating systems.
The current world market for metal effect pigments can be estimated at more than 20,000 tonnes per year. Depending on the production process and the application, the pigments are supplied in powder form or as solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
-containing preparations (pastes, granulates). Stabilized aluminium pastes with water or water-miscible solvents are available for waterborne coating or printing systems. Pigments coated with special organic (e.g., acrylics
Acryl group
In organic chemistry, the acryloyl group is the functional group with structure H2C=CH–C–; it is the acyl group derived from acrylic acid. The preferred IUPAC name for the group is prop-2-enoyl, and it is also known as acrylyl or simply acryl...
) or inorganic materials (e.g., silica) are achievable for powder coatings.
Natural pearl essence
Natural pearl essence, also called natural fish silver, is a pigment suspensionSuspension (chemistry)
In chemistry, a suspension is a heterogeneous fluid containing solid particles that are sufficiently large for sedimentation. Usually they must be larger than 1 micrometer. The internal phase is dispersed throughout the external phase through mechanical agitation, with the use of certain...
derived from fish scale
Fish scale
Fish scale may refer to:* The rigid plates on the skin of a fish* Fishscale, an album by Ghostface Killah* Fishscale cocaine* Fish Scales, a rapper...
s, skin, or bladder. The pigment particles in the suspension are platelet-shaped with a high aspect ratio. They consist of 75–97% guanine
Guanine
Guanine is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine . In DNA, guanine is paired with cytosine. With the formula C5H5N5O, guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with...
and 3–25% hypoxanthine
Hypoxanthine
Hypoxanthine is a naturally occurring purine derivative. It is occasionally found as a constituent of nucleic acids where it is present in the anticodon of tRNA in the form of its nucleoside inosine. It has a tautomer known as 6-Hydroxypurine. Hypoxanthine is a necessary additive in certain cell,...
. The ratio of these two purines depends on the fish species (e.g., herring
Herring
Herring is an oily fish of the genus Clupea, found in the shallow, temperate waters of the North Pacific and the North Atlantic oceans, including the Baltic Sea. Three species of Clupea are recognized. The main taxa, the Atlantic herring and the Pacific herring may each be divided into subspecies...
s, sardine
Sardine
Sardines, or pilchards, are several types of small, oily fish related to herrings, family Clupeidae. Sardines are named after the Mediterranean island of Sardinia, around which they were once abundant....
s). One ton of fish yields less than 250 g of guanine. An industrial synthetic process for producing purine
Purine
A purine is a heterocyclic aromatic organic compound, consisting of a pyrimidine ring fused to an imidazole ring. Purines, including substituted purines and their tautomers, are the most widely distributed kind of nitrogen-containing heterocycle in nature....
s with this crystal shape does not exist. An aqueous suspension of fish scales is, therefore, extracted with organic solvents to dissolve and remove the protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s. The remaining dispersion contains purine crystals and scale, which are separated from one another by a complicated washing and phase-transfer process.
Pigment platelets tend to agglomerate
Agglomerate
Agglomerates are coarse accumulations of large blocks of volcanic material that contain at least 75% bombs...
and are therefore only handled as dispersion
Dispersion (chemistry)
A dispersion is a system in which particles are dispersed in a continuous phase of a different composition . See also emulsion. A dispersion is classified in a number of different ways, including how large the particles are in relation to the particles of the continuous phase, whether or not...
s. These dispersions are used almost exclusively in cosmetic
Cosmetics
Cosmetics are substances used to enhance the appearance or odor of the human body. Cosmetics include skin-care creams, lotions, powders, perfumes, lipsticks, fingernail and toe nail polish, eye and facial makeup, towelettes, permanent waves, colored contact lenses, hair colors, hair sprays and...
applications (nail enamels, lotion
Lotion
A lotion is a low- to medium-viscosity, topical preparation intended for application to unbroken skin. By contrast, creams and gels have higher viscosity.Lotions are usually applied to external skin with bare hands, a clean cloth, cotton wool or gauze...
s, shampoo
Shampoo
Shampoo is a hair care product used for the removal of oils, dirt, skin particles, dandruff, environmental pollutants and other contaminant particles that gradually build up in hair...
s) because of the very high price. The pigment particles of natural pearl essence show a high but soft luster (nD = 1.79 (parallel) to 1.91 (perpendicular)) and a relatively low density of 1.6 g/cm³, which reduces settling in liquid formulations. The world production of natural pearl essence in 2004 is estimated to be less than 50 tonnes.
Basic lead carbonate
Basic lead carbonate (Pb(OH)2•2PbCO3) can be synthesized in the form of thin hexagonal platelets by precipitationPrecipitation (chemistry)
Precipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...
from aqueous solutions. Carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
is reacted with these solutions under carefully controlled conditions. The resulting platelet-shaped particles are less than 0.05 μm thick and have diameters of about 20 μm, yielding an aspect ratio of about 200. Due to their high refractive index of 2.0 and their even surfaces, the platelets exhibit a very strong luster.
Bismuth oxychloride
Bismuth oxychlorideBismuth oxychloride
Bismuth oxychloride is a chemical compound of bismuth, oxygen, and chlorine, with the formula BiOCl. It exists in nature as the mineral bismoclite which is part of the matlockite mineral group.-Structure:...
(BiOCl) effect pigments are produced by hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
of very acidic bismuth
Bismuth
Bismuth is a chemical element with symbol Bi and atomic number 83. Bismuth, a trivalent poor metal, chemically resembles arsenic and antimony. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores. The free element is 86% as dense as lead...
salt solutions in the presence of chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
. The crystal quality can be adjusted by the chosen reaction parameters, such as bismuth salt concentration, temperature, pH-value, reactor geometry, and addition of surfactant
Surfactant
Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or that between a liquid and a solid...
s. The usually tetragonal bipyramidal crystal geometry can be flattened to platelets with high aspect ratio. Pigments with an aspect ratio of 10–15 show low luster and very good skin feeling and are used as filler
Filler (materials)
Fillers are particles added to material to lower the consumption of more expensive binder material or to better some properties of the mixtured material...
s in cosmetics. Crystals with higher aspect ratio show strong luster and are mainly used for nail polish
Nail polish
Nail polish, or nail varnish, is a lacquer applied to human fingernails or toenails to decorate and/or protect the nail plate.-History:...
. The low light stability, the fast settling behavior caused by a density of 7.73 g/cm³, and the lack of mechanical stability limit the use of bismuth oxychloride in technical applications. Therefore, it is mainly used in cosmetics, but also in buttons and jewelry. The current world market is about 400 t per year.
Micaceous iron oxide
Micaceous iron oxideIron(III) oxide
Iron oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron oxide , which is rare, and iron oxide , which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe2O3 is the main...
consists of pure or doped α-iron oxide (α-Fe2O3, hematite
Hematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...
). It is found already in nature in form of platelets. This natural product with a density of 4.6–4.8 g/cm³ and a dark gray color with low luster is nearly exclusively used in corrosion protection coatings. Micaceous iron oxide can also be obtained as platelets by hydrothermal reaction in alkaline media. If substantial amounts of doping materials are incorporated, the aspect ratio can be increased up to 100, resulting in much better luster. The color can be also shifted from dull dark to a more attractive reddish brown allowing the use of the platelets for decorative applications. Al2O3
Aluminium oxide
Aluminium oxide is an amphoteric oxide with the chemical formula 23. It is commonly referred to as alumina, or corundum in its crystalline form, as well as many other names, reflecting its widespread occurrence in nature and industry...
, SiO2
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...
, and Mn2O3
Manganese(III) oxide
Manganese oxide is the chemical compound of formula Mn2O3.-Preparation and chemistry:Heating MnO2 in air at below 800°C α-Mn2O3 is produced . γ-Mn2O3 can be produced by oxidation followed by dehydration of manganese hydroxide...
are the most important doping constituents. Fe(OH)3 or better FeOOH
Iron(III) oxide-hydroxide
A number of species are dubbed iron oxide-hydroxide. These chemicals are oxide-hydroxides of iron, and may occur in anhydrous or hydrated forms...
as starting material is heated in an alkaline suspension together with the doping constituents to temperatures above 170 °C, typically 250–300 °C. Platelets of doped micaceous iron oxide are formed after several minutes to hours. In a second reaction phase, the pH-value is further increased leading to the growth of flat basal faces.
Titanium dioxide flakes
Titanium dioxideTitanium dioxide
Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
flakes can be produced breaking down continuous films of TiO2. Such films can be obtained using a web-coating process where TiOCl2 is thermally hydrolyzed on the surface of the web. Substrate-free TiO2 flakes can also be achieved from TiO2-mica pigments by dissolving the substrate in strong acids or hydroxides. The so-obtained titanium dioxide flakes are not single crystals but polycrystalline and slightly porous. They show only limited mechanical stability in most cases and can, therefore, not be used in technical applications where stress is exerted.
Flaky organic pigments
Some organic pigments can also be forced to crystallize in form of flakes. Typical examples are 1,4-diketo-3,6-diarylpyrrolo(3,4-c)-pyrrole (DPP), 2,9-dichlorochinacridone, and metal phthalocyaninePhthalocyanine
Phthalocyanine is an intensely blue-green coloured macrocyclic compound that is widely used in dyeing. Phthalocyanines form coordination complexes with most elements of the periodic table...
s. However, the difference of their refractive index and that of the typical application media is too small to generate strong interference colors and luster effects. In most cases, the aspect ratio of these crystals is also much smaller than that of the substrate-free and the substrate-based inorganic effect pigments.
Pigments based on liquid crystal polymers
Liquid crystal polymerLiquid crystal polymer
Liquid-crystal polymers are a class of aromatic polyester polymers. They are extremely unreactive and inert, and highly resistant to fire.-Background:...
s (LCP) can also be used in form of flakes or large films to achieve interference and especially angle-dependent interference color effects. The structure of these materials consist of parallel oriented chiral cholesteric (nematic) liquid crystal
Liquid crystal
Liquid crystals are a state of matter that have properties between those of a conventional liquid and those of a solid crystal. For instance, an LC may flow like a liquid, but its molecules may be oriented in a crystal-like way. There are many different types of LC phases, which can be...
line layers having their director rotated by a certain angle with respect to an adjacent layer, building up a helical array
Helix
A helix is a type of smooth space curve, i.e. a curve in three-dimensional space. It has the property that the tangent line at any point makes a constant angle with a fixed line called the axis. Examples of helixes are coil springs and the handrails of spiral staircases. A "filled-in" helix – for...
. The helical structure is responsible for interference effects because the refractive index is changing from layer to layer. It is also possible to get such optical effects by helical superstructures. A number of LCP pigments and films are based on polysiloxanes. The first step of the manufacture is the formation of a thin cross-linked film of a liquid crystalline polymer. After a UV-curing step for polymerization
Polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
, the so formed solid film is ground to small platelets. These particles can be used to achieve angle-dependent effects when having a thickness of more than 4 μm in most cases. Up to now the application of these pigments is still limited because of the thickness and some stability problems.
Use of pearlescent pigments in pharmaceutical products
Pharmaceutical manufacturers attempt to increase brand awareness and satisfaction through product presentation (for instance, colors incorporated in coatings, encapsulation and other tabletTablet
A tablet is a pharmaceutical dosage form. It comprises a mixture of active substances and excipients, usually in powder form, pressed or compacted from a powder into a solid dose...
-forming methods). Importantly, this approach could be used to protect pharmaceuticals from counterfeiting and adulteration. Coatings to which pearlescent pigments have been incorporated can further enhance the color appeal of coated tablet and capsule pharmaceutical products. Possible systems for this application utilise polymers like polyvinyl alcohol
Polyvinyl alcohol
Polyvinyl alcohol is a water-soluble synthetic polymer .-Properties:...
, hypromellose
Hypromellose
Hypromellose , short for hydroxypropyl methylcellulose , is a semisynthetic, inert, viscoelastic polymer used as an ophthalmic lubricant, as well as an excipient and controlled-delivery component in oral medicaments, found in a variety of commercial products.As a food additive, hypromellose is an...
, Carrageenan
Carrageenan
Carrageenans or carrageenins are a family of linear sulfated polysaccharides that are extracted from red seaweeds. There are several varieties of carrageen used in cooking and baking. Kappa-carrageenan is used mostly in breading and batter due to its gelling nature...
, or microcrystalline cellulose
Microcrystalline cellulose
Microcrystalline cellulose is a term for refined wood pulp and is used as a texturizer, an anti-caking agent, a fat substitute, an emulsifier, an extender, and a bulking agent in food production. The most common form is used in vitamin supplements or tablets...
, as the carrier materials in levels up to 50%, a pearlescent pigment or combinations, e.g., micacious titanium dioxide in levels between 1–10%, and dispersants like tween
Polysorbate
Polysorbates are a class of emulsifiers used in some pharmaceuticals and food preparation. They are often used in cosmetics to solubilize essential oils into water-based products. Polysorbates are oily liquids derived from PEG-ylated sorbitan esterified with fatty acids...
, lecithin
Lecithin
Lecithin is a generic term to designate any group of yellow-brownish fatty substances occurring in animal and plant tissues, and in egg yolk, composed of phosphoric acid, choline, fatty acids, glycerol, glycolipids, triglycerides, and phospholipids .The word lecithin was originally coined in 1847 by...
and polyethylene glycol
Polyethylene glycol
Polyethylene glycol is a polyether compound with many applications from industrial manufacturing to medicine. It has also been known as polyethylene oxide or polyoxyethylene , depending on its molecular weight, and under the tradename Carbowax.-Available forms:PEG, PEO, or POE refers to an...
in levels between 0–15%, to act as stabilisers
Stabilizer (chemistry)
In chemistry a stabilizer is a chemical which tends to inhibit the reaction between two or more other chemicals. It can be thought of as the antonym to a catalyst...
.