Remark: Jinwei car paint employ reliable pigments for the high hidding power and high chroma.
Pigments are particulate solids that are dispersed in paints to provide certain characteristics to them, including color, opacity, durability, mechanical strength, and corrosion protection for metallic substrates. In order to achieve the desired results, pigments have to possess certain properties. Pigments may be inorganic and organic materials. Organic pigments are used primarily for decorative purposes, while inorganic materials have traditionally been added for protective properties.
Extenders are used in conjunction with pigments to modify the properties of the paint. The size and shape of the pigment particles are important considerations, since they affect the agglomeration or packing within the paint binder or matrix. Mostpigments are crystalline and the crystal form often affects the characteristics of the
pigment. Particle size also affects the finished gloss, settling of the pigment during storage, and wetting by the binder. Other factors such as color, tint strength, colorfastness, and opacity are inherent characteristics of the pigment. Density is another important factor affecting not only settling, but also the volume of pigment for a given weight addition.
Pigments have to be uniformly dispersed in the binder to provide a consistent appearance. They have to remain in suspension or be easy to disperse if settling occurs. When the paint is mixed, the pigments are incorporated mechanically in a mill. The pigment has to be able to withstand the milling process.
A wide variety of coloring agents can be used to provide a considerable range of colors and shades. The number of pigments used and the amount of each is related to color, hiding, and many other film properties. Some common pigments are discussed in the following section.
Almost all paint formulas include some white pigments to adjust the lightness– darkness of the final color. Some of common white pigments are listed as follows.
1. Titanium Dioxide
Titanium dioxide is a widely used, nontoxic, stable synthetic pigment used for both protective and decorative paints. It comes in two different crystalline forms, rutile and anatase. The rutile crystalline form is more compact and has a higher specific gravity, greater refractive index, enhanced stability, and enhanced durability.
The high refractive index of titanium dioxide (2.7 for rutile and 2.5 for anatase) enables it to provide a higher degree of opacity in paint films than any other white pigment. Rutile titanium dioxide is photochemically inert and the pigment protects paint films from degradation by scattering absorbed light. In contrast, the anatase form is photochemically active and paints formulated with this pigment are subject to heavy chalking. The anatase grade is used mainly for interior applications due to its cleaner white color.
2. Zinc Oxide
A synthetic based pigment that will inhibit mold growth, if present in high levels (about 30% by weight). The basic nature of zinc oxide leads to interaction with paints of high acid value with the formulation of zinc soaps. These can mechanically reinforce the film but can cause embrittlement on exterior exposure.
A synthetic inert pigment often used with chlorine-containing binders in the formulation of fire-retardant paints. When exposed to flames, the antimony chloride vapor that is released is heavier than air and acts as a blanket to the fire. This pigment also has good opacifying characteristics.
4. White Lead
Asynthetic basic pigment used for centuries, now limited in use because of its toxicity. Being basic, white lead will react with acidic paints to form lead soaps, which will enhance film elasticity, useful for timber primers. However, the pigment has a high chalking rate and a tendency to darken in polluted atmospheres due to the formation of lead sulfide.
Yellow pigments are used to adjust the yellow–blue axis of the color spectrum.
Azo dyes, ranging in color from orange to green-shade yellows. They have good light stability in full shade unless the dye is reduced with white pigments. They provide good opacity and are nontoxic, so they are often used instead of lead chromates.
Yellow dyes are soluble in ketones, esters, and aromatic solvents, but they have poor solubility in aliphatic solvents. This makes them suitable for use in air-dry coatings that contain aliphatic hydrocarbons or in emulsion systems.
Insoluble azo dyes prepared in a range of shades from yellow to red. They have good opacity and are insoluble in most solvents used in paint formulations. They are nontoxic and have good resistance to acids and alkalis, and they can withstand temperatures up to 300◦F. The benzidine yellows are not lightfast, even when used at full shade, so their use is generally restricted to interior applications.
3. Lead Chromates
Chromate pigments are rarely used today because of the environmental problems associated with chrome.Awide range of synthetic pigments, ranging from pale yellow to deep orange in color, have been used in paint formulation. Lead chromes have high tint strengths, good lightfastness, and opacity, but they also darken in polluted air and they will bleach in the presence of sulfur dioxide. Lead chromates interactwith alkaline substrates resulting in color loss. Nevertheless, the lead chromates areuseful coloring pigments and are used in undercoats and finishing systems. The color
of the pigment is determined by the process conditions during manufacture.
Three forms of zinc chromate are used for pigmentation of paints. The pigment grades have good lightfastness and are stable in polluted air, but they are of low opacity. The zinc chromates are slightly basic and will react with acid paints, leading to a thickening of the paint during storage. The form of zinc chromate used for corrosion-protection must be free from residual chloride, which is usually present in pigment grades.
A third form of zinc chromate, the tetroxichromate, is used in metal pretreatment primers.
5.Yellow Iron Oxides
Naturally occurring iron oxides, ranging in color from dull yellow to dark yellowbrown, but more restricted in the range of color shades than synthetically prepared oxides. Natural oxides have cleaner and brighter colors combined with greater color strength because of their greater purity. Both natural and synthetic oxides are resistant to alkalis and organic acids, but they can be discolored by mineral acids.
Shade variations can occur at elevated temperatures. Iron oxides will absorb UV radiation and have a protective effect in exterior paints.
Synthetic pigments varying in color from primrose to orange, the shade being determined by manufacturing process conditions. They are stable at high temperatures. They are unaffected by alkalies, but they can be decomposed by acids. They may be usedon a variety of substrates, especially where alkali resistance or temperature resistance is required.
Cadmium, like chrome and lead, was a staple in the formulation of paint, especially performance coatings, for many years. These heavy metals have long been recognized as undesirable components in paint due to their negative impact on the environment and have been all but phased out of paint formulation.
Green pigments are used to adjust the blue–yellow axis of the color.
A toner, which, unlike the azo dyes, is not purely organic in nature but is a salt-like compound. Toners are formed by the reaction of a basic dyestuff with a complex acid or by the reaction of an acidic dyestuff with a metal or metal salt; they are more resistant to solvents than the organic dyes.
Phthalocyanine green pigment has a blue–green color with good opacity in paints.
It has good resistance to solvents, heat, and alkalis, but it can be decomposed by acids.
It is widely used in decorative finishes.
Chromium oxide is a dull green, synthetic pigment with good stability to light, heat, acids, and alkalinity, but it provides poor opacity. It is useful as a pigment for paints that requires high chemical and photochemical resistance. Chrome oxide is also used as a pigment for concrete and cement. Once again, chrome is no longer a common pigment used in paint formulation in North America.
3.Lead Chrome Greens
Synthetic pigments ranging from grass green to deep green in color, the shade being controlled by the Prussian blue admixed with the lead chrome yellow. They have good opacity, but they tend to discolor in polluted air. They also tend to float in paint formulations. Owing to toxicity, these products are not often used today.
Blue pigments are used to adjust the blue–yellow axis of the color.
These vary in color from reddish-blue to yellowish-green; they tend to have high tint strength and high opacity along with very good lightfastness. They are nontoxic,temperature stable, and resistant to most solvents and chemicals except strong acids.
These pigments are used in a great many paints and finishes.
This is a bright blue synthetic pigment having high staining power but low opacity. It is lightfast and acid resistant, but it can decompose to iron oxide by exposure to alkalinity and elevated temperature.
Acomplex synthetic aluminosilicate pigment, it occurs naturally as the semiprecious stone lapislazuli. The color of the pigment is related to the presence of sodium sulfide. The shade can be altered by the amount of silica used in its manufacture. The pigment has a coarse texture and is subject to hard settling, but it has good lightfastness, heat resistance, and alkali resistance. It will decompose in an acid.
Red pigments are used to adjust the red–green axis of the color.
Nontoxic azo dyes characterized by bright, clean colors, good lightfastness, and high opacities although the color stability is reduced when they are mixed with white pigments. Toluidine reds have good color stability, are resistant to acids and alkalis, and heat up to 350◦F for short periods. They are soluble in aromatic solvents and slightly soluble in aliphatic solvents and alcohols, so they are susceptible to bleeding in many systems.
Nontoxic azo dyes ranging in color from orange to red or deep crimson. They have good lightfastness even if reduced with white pigments and they are temperature stable up to 300◦F. Arylamide reds are slightly soluble in solvents. They are resistant to acids and alkalis and are used in many decorative exterior applications.
3.Red Iron Oxide
Natural red oxides, mined as hematite ore, and synthetic red iron oxides are used as pigments. The presence of impurities together with the variation in the iron oxide content provides pigments with colors ranging from orange-red to deep brown. The natural oxides are used in timber stains and low-cost metal primers.
Synthetic oxides have softer texture and greater purity, giving brighter, cleaner colors, and greater tinting strength. Colors are determined by manufacturing condition. Natural and synthetic oxides are resistant to alkalis and organic acids but may react to mineral acids and high temperatures. They absorb UV radiation and increase the resistance of paint films to fading.
Two other inorganic red pigments, red lead and basic lead silicochromate, were used in the past, primarily for their corrosion-protecting properties. They are seldom used today due to the environmental risks associated with such materials.
Black pigments are present in many paint formulas to adjust the lightness–darkness of the coating.
1.BlackIr on Oxide
A synthetic pigment of low tinting strength, mainly used as a colorant for fillers, primers, and undercoats. The pigment is reddened by heat, but otherwise the oxide has good chemical resistance.
66 Paint Technology Handbook
Pigments that are produced by the carbonization of hydrocarbon minerals, plants, and animals. Properties vary according to the source of the carbon and the impurities present. Purer carbons are of a finer texture, have better color and opacity than carbon of lower purity. Carbon blacks are lightfast, insoluble in acids and alkalis, and are resistant to solvents. They provide good opacity, even at low addition levels. Carbon blacks are very common pigments used in paint formulations.
Metallic pigments are very popular for paints that will be used on products such as automobiles, motorcycles, metal office furniture, and other products that require exceptional appearance characteristics. While aluminum is the most commonly used metallic pigment, stainless steel, lead, and zinc have also been used in paint formulation.
Apure metal that is rendered resistant to many aggressive agents by the presence of a surface film of alumina. It is available in two powdered forms, leafing and nonleafing grades. Particles of the leafing grades, which are coated with stearic acid or a similar surface-active agent, tend to float toward the surface of a paint film and become oriented parallel to the plane of the film.
The close packed layer of aluminum platelets provides the film with a reasonably efficient moisture barrier and functions as a protective pigment. Normally, low levels of aluminum are used, often with no other pigments. The aluminum may also prevent corrosion by being a sacrificial anode when the surface of the coating has been damaged.
The nonleafing grade does not migrate to the surface of the coating but is more randomly distributed throughout the film. Again, low levels of the nonleafing aluminum are used and the pigment’s main function is to provide a sparkle effect to the film.
A naturally occurring potassium alumino-silicate used in the form of fine platelets. The lamellar structure of mica results in a leafing effect like that in leafing-grade aluminum that reduces water permeability in the paint film. The incorporation of mica improves weathering resistance in paint by reducing the tendency to fail by crackingand checking. These properties permit mica to be used in exterior decorative coatings.
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