The Universal Selection Source: Coatings Ingredients

Techno Brief: Pigment Technologies

Pigments are important constituents in paints and coatings. They are added to paints and coatings formulations to impart color, bulk or a desired physical and chemical property to the wet or dry film. This selection guide will familiarize you with pigments in paints, their types, differences between pigments and dyes, pigment properties, different families of pigments, pigment dispersions, pigment performances, applications in the paints & coatings industry and much more.

Pigment Performances

Organic pigments are mainly used for applications needing high tinting strength and brilliant shades.

On the other hand inorganic pigments are mainly useful where high opacity is needed.

Pigment performances and properties mainly depend on:
  • Chemical structures
  • Surface properties
  • Crystallinity
  • Particle size and
  • Size distribution

We have prepared a comprehensive section for you to check out the required performance for your coating.

Color

The color of a pigment is mainly dependent on its chemical structure, which is determined by the selective absorption and reflection of various wavelengths of light at the surface of the pigment.

Colored pigments

These absorb part of all the wavelengths of light. For example, a blue pigment reflects the blue wavelengths of the incident white light and absorbs all of the other wavelengths. Hence, a blue car in orange sodium light looks black, because sodium light contains virtually no blue component.

Black pigments absorb almost all the light, whereas white pigments reflect virtually all the visible light falling on their surfaces.

Fluorescent pigments have an interesting characteristic. As well as having high reflection in specific areas of the visible spectrum, they also absorb light in areas outside the visible spectrum (ultra-violets that human eye can not detect), splitting the energy up, and re-emitting it in the visible spectrum. Hence, they appear to emit more light than actually falls upon them, producing their brilliant color.

Pigment Color
 TiO2 Excellent
 Iron Oxide Fair
 Prussian blue
Excellent
 Lead chromate
Excellent
 Carbon black
Excellent
 Monoazo
Excellent
 Disazo
Excellent
 Phthalocyanine
Excellent

Color Strength

As well as color, color strength (or tinctorial strength) must be considered when choosing a pigment. Color strength is the facility with which a colored pigment maintains its characteristic color when mixed with another pigment. The higher the color strength, the less pigment is required to achieve a standard depth of shade.

Chemical structure

It is one of the factors that influence the color strength of a pigment.
  • In organic pigments, color strength depends on the ability to absorb certain wavelengths of light. Highly conjugated molecules and highly aromatic ones show increased color strength.
  • Inorganic pigments that are colored due to having metals in two valency states, show high color strength. In contrast, those that have a cation trapped in a crystal lattice are weakly colored.

Particle size also influences the color strength of a pigment. Higher color strength is obtained with smaller particles. Manufacturing conditions are the main factor that influences the particle size of pigment crystals. Pigment manufacturers play a crucial role:
  • They can reduce the size of the particles by preventing the growth of crystals during synthesis,
  • and they can increase color strength by efficient dispersion.

Pigment dispersion will also take a major role in the color strength of the paint. Indeed, it will impart colloidal stability to the finer particles, avoiding their flocculation and using their full intrinsic color strength.

Pigment

Color Strength

Titanium Dioxide

Excellent

Iron Oxide

Poor-Fair

Prussian blue

Good

Lead chromate

Fair

Carbon black

Excellent

Monoazo

Good-Excellent

Disazo

Excellent

Phthalocyanine

Excellent


Heat Resistance

Few pigments degrade at temperatures normally associated with coatings. However, at higher temperatures, pigments become more soluble and shading can occur. Thus, for organic pigments, heat stability is closely related to solvent resistance. Pigments that prove to be satisfactory at a certain stoving temperature may be totally inadequate in an application requiring 10°C more.

Chemical stability is also likely to be critical at elevated temperatures. This is typically the case in powder coating systems. Another key area is coil coatings, as metal complex pigments may react with stabilizers at elevated temperatures, causing major shifts in shade. Modifications can also occur in the crystal structure of pigments when subjected to elevated temperatures. Pigments with a highly crystalline structure are usually more heat resistant than polymorphic pigments, where the different crystal modifications may respond differently to heat. Typically, inorganic pigments have enhanced heat stability, though an exception is yellow iron oxide, which loses water from the crystal at high temperatures. Heat stability is system dependent and this must be reflected in any test. All tests assess color at various temperature intervals and evaluate the color difference between the sample in question and a standard that has been processed at the minimum temperature.

Pigment

Color


Excellent

Iron Oxide

Good-Excellent

Prussian blue

Good

Lead chromate

Good

Carbon black

Good

Monoazo

Poor-Fair

Disazo

Good

Phthalocyanine

Excellent


Light Fastness

Light fastness is evaluated in relation to the whole pigmented system, not just the pigment. The binder imparts a varying degree of protection to the pigment, so the same pigment will tend to have better light fastness in a polymer than it will in paint. Pigments will nearly always have a much poorer light fastness in a printing ink system, where there is less resin to protect the pigment, and where there is a double effect of light passing through the pigmented layer, being reflected by the substrate and back through the pigmented layer.

Other pigments that may influence light fastness in a pigmented system are mentioned below:
  • Titanium dioxide promotes the photodegradation of most organic pigments. Therefore, high ratios of titanium dioxide lead to poorer levels of light fastness.
  • Iron oxide can improve the light fastness of organic pigments, due to the fact that it is an effective absorber of UV light.
When the association of two pigments gives a better light fastness, it is called a synergistic effect and when the light fastness obtained is lower, it is called an antagonistic effect. Some inorganic pigments are unchanged by exposure to light, but most pigments, and all organic pigments, are changed in some way: darkening or complete fading can occur.

A pigment's ability to resist light is influenced considerably by chemical constitution. Other less significant influences are pigment concentration, the crystal modification, and particle size distribution. Additionally, factors in the environment can dramatically affect results, such as the presence of water and chemicals in the atmosphere or in the paint system.

The light fastness of a pigmented system can only truly be tested in the final formulation and application. Light fastness tests must be carried out only under carefully controlled test conditions.

Weather Stability

For outdoor applications, pigments used for coloring should be selected for their weather resistance characteristics. Closely related to light fastness, weatherability adds the extra dimension of atmospheric conditions (including salt from the sea, waste gases from industrial areas, or very low humidity from desert conditions). Weather resistant pigments are usually lightfast but the reverse is not always the case.

The selection of pigments for outdoor use depends on:
  • outdoor performance required (life time, climatic region/ Kilo Langley)
  • binder type
  • concentration of the pigment
  • presence of titanium dioxide (which typically accelerates fading)
  • concentration and type of light stabilizers used

Performance can also be influenced by the surface of the painted object and by the processing heat history.

Once the above variables have been defined, the best way to assess weathering resistance in service is by using outdoor exposure tests in the climatic region(s) concerned. This is clearly not always feasible. The widely used alternative is accelerated testing. Machines are available which in addition to a xenon lamp; include wet cycles interspersed between longer dry cycles. Weatherability is designated in terms of the 1-5 Grey Scale. 5 represents no change and 1 a severe change.

Insolubility

A pigment must be insoluble in the vehicle (the medium in which it is dispersed), and it must not react with any of the components of the paint, such as crosslinking agents. Pigments are required to retain these properties even when the paint is being dried, which is frequently carried out at elevated temperatures. Once in the dried film, the pigment must also remain unaffected by the substrate and to agents with which it comes into contact, including water, which may simply be in the form of condensation, or acidic industrial atmospheres.

Under certain conditions, pigments may dissolve, leading to application problems. Organic pigments may dissolve to a limited extent in organic solvents, and inorganic pigments may be affected by other components. Solubility of a pigment generates the following problems:
1) Blooming

If the pigment dissolves in the solvent, as the paint dries, the solvent comes to the surface and evaporates, leaving crystals of the pigment on the surface in the form of a fine powder. As solubility increases with temperature, this phenomenon is made worse at elevated temperatures.


2) Plate out

The effect of plate out looks similar to blooming, but occurs in plastics and powder coatings. However, it is not due to the pigment dissolving, but rather to the surface of the pigment not being properly wetted out. It usually occurs mainly with complex pigments and once wiped from the surface does not reappear.


3) Bleeding

Pigments in a dried paint film may dissolve in the solvent contained in a new coat of paint applied on top of the original film. If the topcoat is a different color, particularly a white or pale color, the result can be disastrous. Again elevated temperatures exacerbate the problem.


4) Recrystallization

This phenomenon was almost unknown until the introduction of beadmills. During the milling stage, heat is generated, which dissolves a portion of the pigment. Over a period of time, the dissolved "pigment" starts to precipitate out, loses brilliance and color strength. This becomes especially noticeable in the case of paints containing two differently colored pigments that have different solubility characteristics. The more soluble pigment dissolves and then as it comes out of solution and precipitates, the paint will take the shade of the second pigment. Recrystallization can even take place in aqueous systems. It can be avoided by using less soluble pigments and/or by controlling the temperature during the dispersion process.

Opacity

Hiding power is the ability of a pigmented coating to obliterate the surface. It is dependent on the ability of the film to absorb and scatter light. Naturally, the thickness of the film and the concentration of the pigment play a fundamental role. The color is also important.

Hiding power of pigments
Figure. Hiding power

Dark, saturated colors, such as blacks and deep blues, absorb most light falling upon them, whereas yellows do not. However, carbon black and most organic blue pigments are fairly transparent because they do not scatter the light that falls on them. In contrast, titanium dioxide absorbs almost no light, yet its capacity to scatter light ensures that at a sufficiently high concentration it will cover the substrate being coated. It is common practice to use a combination of pigments to achieve the best results.

A key factor in the opacity of a pigment is its refractive index (RI), which measures the ability of a substance to bend light. The opacifying effect is proportional to the difference between the refractive index of the pigment and that of the medium in which it is dispersed. This is one of the main reasons why titanium dioxide is now almost universally used as the white pigment in paint.
Medium
RI
Air
1.0
Water
1.33
Film Formers
1.4-1.6
Pigment / Filler
RI
Calcium carbonate
1.58
China clay (aluminium silicate)
1.56
Talc (magnesium silicate)
1.55
Barytes (barium sulphate)
1.64
Lithopone 30% (zinc sulphide/barium sulphate)
1.84
Zinc oxide
2.01
Zinc sulphide
2.37
Titanium dioxide: 
Anatase
Rutile
 
2.55
2.76

Inorganic pigments have a high refractive index and organic pigments have much lower values. Consequently, most inorganic pigments are opaque, whereas organic pigments are transparent. The particle size distribution of the pigment is another factor that also plays an important role in opacity. As the particle size increases, the ability of the particle to scatter light increases, up to a maximum. It then starts to decrease. This ability to scatter light increases the hiding power of the pigment, and therefore the hiding power also reaches a maximum and then decreases as the particle size increases.

Effect of particle size on scattering
Figure. Effect of particle size on scattering

Whereas the refractive index of a compound cannot be altered, the pigment manufacturer can influence the particle size of pigments; consequently particle size selection has become one of the principal developments in pigment technology in recent years.

Measurement of opacity

The coating is applied in a wedge shape over a contrast chart. The film thickness is built up over the length of the chart, which is attached to a metal panel. The point at which complete obliteration is observed is noted and the film thickness at that point measured.

Transparency

Usually, transparency is obtained by reducing pigment particle size as possible. This is achieved by surrounding the particles as soon as they are formed with a coating, which prevents the growth of crystals. The most common products used for this coating are rosin or rosin derivatives. This is particularly useful for printing ink pigments that are required to have high transparency and it has the added advantage that such pigments are more easily dispersed.

Iron oxide pigments can be opaque or transparent. The transparent variety are an important group of inorganic pigments as they are widely used for metallic finishes, where their high level of transparency gives an attractive finish, and their weatherability resistance improves the weatherability of pigments with which they can be combined. This is known as a synergistic effect. Transparent iron oxides depend on the particles being unusually small, and also having a crystal shape. The dispersion process can influence transparency, as it involves breaking up agglomerates of particles to individual primary particles. However, primary particles are not split up by the dispersion process. All one can do is to make full use of the pigments original particle size. Good dispersion will maximize the transparency of a small particle.

Measurement of transparency

Transparency is simply assessed by applying the coating over a black and white contrast chart and measuring the color difference. The greater the color difference, the higher the transparency.

Chemical Stability

Resin, crosslinking agents, UV-initiators, and any other additive may react with the pigment and alter its performance. At the time when UV-cured coatings were new to the market, additives significantly reduced storage stability, causing the coating to gel in the can. A great deal of care must be taken when selecting pigments for powder coatings, as the initiator can change the pigment shade and reduce fastness properties. Reputable pigment manufacturers publish data on such systems and can often offer assistance in the case of difficulties. Another adverse effect can come from:

Chemicals that the coating gets in contact with. Water, in the form of condensation, can seriously affect a paint film, particularly in bathrooms and kitchens. Many of the detergents used for cleaning paintwork are harsh and have an abrasive affect upon the pigment. Should the coating come into contact with food, it is essential firstly, that the coating is unaffected and secondly, that the food remains unchanged.

Many testing processes concerning chemical stability consist of applying the chemical to the surface of the coating, keeping them in contact for a given time, then measuring the discoloration of the coating and/or the staining of the chemical concerned.

Pigment
Color
Titanium Dioxide
Excellent
Iron Oxide
Excellent
Prussian blue
Poor
Lead chromate
Good
Carbon black
Excellent
Monoazo
Excellent
Disazo
Excellent
Phthalocyanine
Excellent

Covestro
Channel Alerts

Receive weekly digests on hot topics

Receive your alerts

Back to Top