OK
The Universal Selection Source:
Coatings Ingredients

Pigments for Paints, Coatings and Inks

Pigments are important constituents in paints, coatings and inks. 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.

Are you searching for the right pigment for your formulation?
Explore this guide and understand:


What are Pigments?


Pigments for Coatings Pigments are finely ground natural or synthetic, insoluble particles used to impart color when added to paints and coatings formulations. They are also used to impart bulk or a desired physical and chemical property to the wet or dry film. Some of the main pigment classes include:

  • Organic pigments
  • Inorganic pigments
  • Functional pigments
  • Special effect pigments 

While organic pigments do not disperse easily and form agglomerates (clumps of pigment particles), inorganic pigments get more easily dispersed in the resin. Functional fillers impart a desired property to the coating like corrosion inhibition and special effect pigments create optical effects like metallic, hammer finish and diverse color perceptions depending on the angle.


Difference between Pigments and Dyes


Pigments are organic or inorganic, colored, white or black materials that are practically insoluble in the medium in which they are dispersed. They are distinct particles, which gives the medium their color and opacity.


Pigment Dispersion

Pigments Dispersion


The smallest units are called primary particles. The structure and shape of these particles depends on the crystallinity of the pigment. During the pigment production process, primary particles generally aggregate and generate agglomerates. During the dispersion of the pigment into the polymer, high shear is generally needed to break up these agglomerates (improved tinting strength). Pigments are thus required to resist dissolving in solvents that they may contact during application, otherwise problems such as "bleeding" and migration may occur. In addition, depending on the demands of the particular application, pigments are required to be resistant to light, weathering, heat and chemicals such as acids and alkalis.


Polymer Soluble Dyes


Dye is a substance that is applied in order to impart color with some degree of permanence. Polymer Soluble Dyes are soluble in the medium in which they are dispersed. This means that there are no visible particles and the transparency of the medium is unchanged.

Dye Dispersion

Dyes Dispersion



Types of Pigments & Their Properties


There are two types of pigments:

  1. Organic Pigments
  2. Inorganic Pigments

Each pigment types has distinct properties which, in the past, were used to distinguish one from the other.

Pigment Properties Inorganic Pigments Organic Pigments
Classical High-performance
Color, Purity Often dull Usually bright
Opacity High More or less transparent
Color strength Medium to Low Normally High
Light Fastness (Blue scale) Good to High (7-8) Low to Middle (< 7) Good to High (7-8)
Weather Resistance Varies (depending on chemistry) Insufficient Middle to High
Heat resistance In general > 500°C
Rarely < 200°C
150 to 220°C 200 to 300°C
Fastness to solvents - Bleed resistance High Middle to Good Good to High
Resistance to chemicals Varies (depending on chemistry) High (except for salts) High
Price Low to Medium Medium High


Pigment Dispersion


High quality coatings of high brilliance and color strength are characterized by:

  • A perfect pigment dispersion
  • Optimal pigment particle size
  • Long-term stabilization of the dispersed particle in the formulation.

Most organic pigments show better transparency as dispersion improves, while in the case of the larger particle size inorganic pigments, opacity is improved by good dispersion.

Pigment Dispersion


The dispersion process consists of the permanent breaking down of agglomerates into, as far as possible, primary particles. There are four aspects to the dispersion process:

  • Deagglomeration – The Breaking down of the agglomerates and aggregates by the mixture of crushing action and mechanical shearing force.

  • Wetting – It occurs at the surface of a pigment when a surface active agent sticks to the pigment's surface and acts as a connection between the pigment and the binder.

    Wetting out time depends on the viscosity. Heat produced by the mechanical shearing process causes the temperature of the mixture, hence reduces the viscosity, thus helping the wetting out process.

  • Distribution – It demands the pigment to be equally dispersed throughout the binder system. A lower viscosity tends to lead to a more even pigment distribution.

  • Stabilization – It prevents the pigments from re-agglomerating. The pigment dispersion is stabilized by dispersing agents in order to prevent the formation of uncontrolled flocculates. The resultant suspension is stabilized due to the adsorption of binder species or molecules at the pigment surface.

Now let’s discuss about various factors Influencing Pigment Performances…

Pigment performances and properties mainly depend on several factors such as chemical structure, particle size, surface properties etc. Hence to achieve required performance for your coatings, let’s learn more about how different characteristics of pigments are taken into consideration.


Pigment Performance


Performance of a pigment can be measured by the folloeing properties:


Let's discuss them in detail:

Color of Pigment


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 absorb part of all the wavelengths of light. For example:

  • 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.
  • 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 cannot 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.

Color Strength


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

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
  • Increase color strength by efficient dispersion

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

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.

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. These include:

  • 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


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
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
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 Selection



Pigment Selection


Understand all of these in detail or click on the specific selection criteria based on:


Crystal Structure


Pigments can be crystalline or non-crystalline (amorphous). In crystalline pigments the atoms within each molecule are arranged in a well-structured pattern, however, in amorphous pigments the atoms are randomly arranged. It is also possible for materials to have several different crystalline forms - known as polymorphism.

Color is dependent on these different structures. There exists pigments which have chemically identical entities in different crystal forms, yet these polymorphic pigments are not suitable for use as a pigment. Titanium dioxide, phthalocyanine blue and linear Trans quinacridone are examples of such polymorphic pigments.

Techniques for influencing the formation of a desired crystal form and particle distribution, for the purpose of optimizing the commercial product for end applications, are currently being developed by pigment manufacturers.

Particle Shape


The chemical structure, the crystalline structure or the synthesis of a pigment determine the shape of particles. The primary particles of a pigment may be nodular, spherical, prismatic, acicular or lamellar. Primary particles are composed of single particles. The smaller these particles, the greater their surface energy and therefore the more likely it is that they will clump together during manufacturing. It is not practical to supply pigments in the form of primary particles as they would be more like smoke than a powder. In practice, they only exist as the pigment is synthesized. When the particles clump together during the manufacturing process they form either aggregates or agglomerates.

Aggregates are connected along crystal boundaries during synthesis or drying. Due to the difficulty of separating them, pigment manufacturers attempt to avoid their formation during the pigment's production. Agglomerates are loose clusters of primary particles which can be broken down via an efficient dispersion process.

Following the dispersion process, it is still possible for particles to re-agglomerate into loosely held groups, known as flocculates. This commonly occurs when there is a rapid change of state, ie.

  • Too rapid dilution or
  • The addition of an incompatible substance

Flocculation results in a loss of tinctorial strength. However, flocculates are usually easier to separate than true agglomerates, and even normal shear such as brushing out is sufficient. This result in an uneven increase in tinctorial strength, depending on how much shear has been developing.

Another important point is during brushing out. Small particles are more susceptible to flocculation than larger ones, so pigments most at risk are grades of carbon black and organic pigments, such as phthalocyanine and dioxazine violet pigments. There are an increasing number of flocculation-stable grades being released on the market.

Particle shape can influence the shade of a pigment and properties of the paint.

Particle Size


Pigment particles are not usually spherical. They can have different dimensions depending on whether one measures the length, width or height. Particle size is an average diameter of primary particles. Typical ranges are:

  • Carbon black - 0.01 to 0.08 µm;
  • Titanium dioxide - 0.22 to 0.24 µm;
  • Organics - 0.01 to 1.00 µm;
  • Inorganics - 0.10 to 5.00 µm;

Extender pigments can be among the coarsest pigment particles, up to 50 µm, but other types can be exceptionally fine (e.g. the precipitated silicas).

The pigment's particle size can affect its color, hide and settling characteristics. Large particles usually settle faster than smaller ones, and smaller ones are harder to disperse. Light scattering is also often influenced by pigment size. And the distribution will also affect the colloidal stability and color.

Surface Area & Oïl Absorption


The surface area is the total area of the solid surface. It is measured in squared units (m2) and is usually defined for 1 gram of pigment (typical values for organic pigments are between 10 and 130m2). This surface area is determined by an accepted measurement technique such as the BET (Brunauer, Emmett, and Teller) method using nitrogen adsorption. This technique consists in calculating the adsorption properties of the pigment.

The surface area is closely linked to the pigment's demand for binder. Larger particles have a smaller surface area and therefore a lower demand for binder. As the size of particle of pigment is small, the area of surface becomes large. As a result, the paint need large amount of binder to wet each of pigment particles during the dispersion process.

The amount of oil that is required to "wet out" 100 grams of pigment and to make paint with a pigment is called oil absorption. Oil Absorption is expressed in number of grams of oil per 100 grams of pigment (or volume relationship from weight). This value varies depending upon the pigments physical nature and particle size. The amount of oil affects the time of dryness. In general, large amount of oil causes yellowing and delay of dryness.

Hardness of the Pigments


Hardness is usually based on Mohs Hardness Scale. The hardness of the pigment is measured by comparison with the ten classes of the Mohs scale.

In the absolute scale of the hardness (of Rosiwal), the abrasion resistance is measured with proofs from laboratory, and by attributing to the corindone the value 1000.

Also for the Knoop scale, the values of hardness are absolute. They depend on the depth of the signs engraved on the minerals due to a special utensil with a diamond point, with which a standard of force is applied.

Mineral Mohs Scale Rosiwal Scale Knoop Scale
Gold 0 - -
Talc 1 0.03 1
Gypsum 2 1.25 32
Calcite 3 4.5 135
Fluorite 4 5 163
Apatite 5 6.5 430
Orthoclase 6 37 560
Quartz 7 120 820
Topaz 8 175 1340
Corundum 9 1000 1800
Diamond 10 140000 7000


These scales help define how hard a pigment is and if it will be easily abraded. The hardness of the pigment can affect the durability and abrasion resistance of the film.

The hardness scales also allow the formulator to better define milling equipment needs and end use. Some pigments are soft and can be damaged by milling, especially when placed in a ball mill for extended periods of time.

Another important point to consider is the pigment's solubility and what effect the solvent will have on the pigment's hardness and structure.

Quantity of Pigment


The amount of pigment used in paint is determined by:

  • Its intensity and tinctorial strength
  • The required opacity
  • The gloss required
  • The resistance and durability specified

The paint technologist works on one of the two main concepts, either:

  • Pigment volume concentration (PVC) or
  • Pigment to binder ratio (P:B)

The PVC is of fundamental concern when formulating paints that are required to have optimum performance with respect to durability. It is known that there is a critical point that represents the densest packing of the pigment particles commensurate with the degree of dispersion of the system.

For systems requiring high gloss, low PVC is required, whereas primers and undercoats can have much higher PVC - up to 90%.

The P: B ratio, by weight or occasionally by volume, is a much simpler calculation, often used to assist in formulating a good mill-base and for balancing a formulation for gloss and opacity.

Binders Used in Coatings and Paints Formulations


The binder in the paint system plays a key role in terms of determining the pigment and the type of solvent in which it is dissolved.

  • A common choice for a solvent is water as it is compatible with most polymers, except some toners.
  • White spirit is a commonly used solvent for long oil alkyd paints, which are widely used in decorative gloss paints. A large majority of pigments are almost insoluble in white spirit, so it rarely narrows the choice of pigments.
  • Industrial finishes can be based solvents such as xylene, ketones, and esters. They are very powerful and can dissolve pigments with poor or only moderate resistance to solvents.

It is also necessary to consider whether the coating will be overcoated. For example, in the case of a car getting repaired, the pigment used on the original finish will have to be fast to overcoating.

In powder coatings crosslinking agents can affect the pigment. For this reason, the pigments must be compatible with these agents at temperatures employed during application. It is therefore evident that the type of resin and solvent used remain key factors in the choice of pigment.

Paint Performance in End Application


An awareness of the end use of the paint is essential, as durability and chemical resistances requirements. The maximum price that can be tolerated for improved performance depends on this knowledge.

For example, a low-quality pigment would be insufficient in an automotive finish, just as a high-quality pigment would be unnecessary for use in a gardening tool.

Paints can be classified according to the market in which they are used, such as:

  • Building, architectural or decorative
  • Automotive finishes, OEM (original equipment manufacturers) or VR (vehicle refinishes), and
  • Industrial finishes

In the same way, pigments used in paint do not require same properties than pigment used for inks applications.


Commercially Available Pigments


Explore all types of pigment grades (inorganic, organic, special-effect...) available in market today!




Polymer Application Get Detailed Information on Inorganic Pigments Polymer Application Get Detailed Information on Organic Pigments

ENG's TECHSCOUT 2018: Strategic Venturing & Innovation Excellence
Channel Alerts

Receive weekly digests on hot topics

1 Comments on "Pigments for Paints, Coatings and Inks"
David W Jun 15, 2018
Fantastic Overview of pigments, should be required reading for young (and old) chemists and technicians.

Leave a comment





Your email address and name will not be published submitting a comment or rating implies your acceptance to SpecialChem Terms & Conditions
Back to Top