Selecting Pigments for Paints, Coatings and Inks

Are color pigments and dyes different?

Pigment dispersion - The step-by-step process

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

1. Deagglomeration – The breaking down of the agglomerates & aggregates by applying force. A mixture of both crushing and mechanical shearing force is applied here.


2. Wetting – It occurs at the surface of a pigment when a surface-active agent sticks to the pigment's surface. It acts as a connection between the pigment and the binder. Wetting out time depends on the viscosity. Heat produced by the mechanical shearing process increases the temperature of the mixture. This thus reduces the viscosity, helping the wetting out process.


3. 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.


4. Stabilization – It prevents the pigments from re-agglomerating. The pigment dispersion is stabilized by dispersing agents 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.

Which are the common types of pigments?

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

• Carbon black pigments
• Conductive pigments
• Titanium dioxide pigments
• Extenders

Organic vs. inorganic pigments - Property comparison

What are organic pigments?

1. One of vegetable origin, and
2. Other of animal origin

• Organic red pigments
• Organic blue pigments
• Organic green pigments
• Organic violet pigments
• Organic orange pigments
• Organic black pigments
• Organic brown pigments
• Organic yellow pigments

Organic Red Pigments

Organic Blue Pigments

Organic Green Pigments

Organic Violet Pigments

• Water-based emulsions
• Automotive OEM paints
• Coil coatings
• and more

Organic Orange Pigments

• Cadmium reds and molybdate
• Beta-naphthol and BON arylamide pigments

• Organic blacks
• Inorganic blacks: iron oxides, graphite
• Vegetable blacks: peach, charcoal, vine
• Animal blacks: bones, ivory

Organic Brown Pigments

Organic Yellow Pigments

• Brightness of shade Opacity
• Fastness requirements
• Physiological properties, and
• Economic considerations

What are inorganic pigments?

• Red Pigments
• Blue pigments
• Green Pigments
• Black Pigments
• Brown Pigments
• White Pigments
• Yellow Pigments
• Extender Pigments
• Corrosion Inhibiting Pigments

Inorganic Red Pigments

Inorganic Blue Pigments

1. Indanthrone which is used for particular high quality applications
2. Ultramarine and Prussian blue are two inorganic pigments occasionally used

Inorganic Green Pigments

• Organic blacks
• Inorganic blacks: iron oxides, graphite
• Vegetable blacks: peach, charcoal, vine
• Animal blacks: bones, ivory

Brown Pigments

White Pigments for Coatings and Inks

Yellow Pigments for Coatings and Inks

• Brightness of shade
• Opacity
• Fastness requirements
• Physiological properties, and
• Economic considerations

Extender Pigments for Coatings and Inks

• Extender pigments are added in order to reduce the cost of a paint formulation. They are also used to modify the flow (viscosity), sedimentation stability and film strength.
• Most extender pigments appear white and possess a refractive index similar to commonly used binders.
• Most of the extender pigments occur naturally and others can be produced synthetically.
• Aluminum silicate, magnesium silicate (talc), silica, calcium carbonate (synthetic and natural) and barium sulfate are some commonly used extender in paints and coatings.

Corrosion Inhibiting Pigments for Coatings & Inks

• Physically obstructing the passage of water and oxygen
• Protecting the anodic sites that have become pitted
• Providing soluble pacifying ions to protect the metal
• Producing an insoluble film to prevent active corrosion

How to measure the performance of pigments?

Color of pigment

• 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. They have a high reflection in specific areas of the visible spectrum. They also absorb light in areas outside the visible spectrum (i.e., ultra-violets that the human eye cannot detect). They split the energy up, and re-emitting it in the visible spectrum.

Color strength

1. Chemical Structure 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.


2. Particle size also influences the color strength of a pigment. Higher color strength is obtained with smaller particles. Manufacturing conditions are the main factor to consider while formulating. They influence 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. It imparts colloidal stability to the finer particles. Thus avoiding their flocculation and using their full intrinsic color strength.

Heat resistance

Light fastness

• Titanium dioxide promotes the photodegradation of most organic pigments. Therefore, high ratios of titanium dioxide led to poorer levels of light fastness.
• Iron oxide can improve the light fastness of organic pigments. This is due to the fact that it is an effective absorber of UV light.

• pigment concentration,
• crystal modification, and
• particle size distribution

Weather stability

• Outdoor performance required (lifetime, 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

Insolubility

1. Blooming - If the pigment dissolves in the solvent, as the paint dries, the solvent comes to the surface and evaporates. It leaves crystals of pigment on the surface in the form of a fine powder. As solubility increases with temperature, this phenomenon worsens at elevated temperatures.


2. Plate out - The effect of plate out looks similar to blooming but occurs in plastics and powder coatings. This effect is not due to the pigment dissolving. But because of the surface of the pigment that is 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 bead mills. During the milling stage, heat is generated, which dissolves a portion of the pigment. Over some time, the dissolved "pigment" starts to precipitate. It loses its brilliance and color strength. This becomes noticeable in paints containing two different colored pigments with different solubility. 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

Transparency

• their high level of transparency gives an attractive finish, and
• their weatherability resistance improves the weatherability of pigments with which they can be combined.

Chemical stability

• 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. They have an abrasive effect on the pigment. It is essential that if the coating comes in contact with food, the coating is unaffected and the food remains unchanged.


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

Which properties to consider while selecting pigments?

Crystal structure

• Titanium dioxide
• Phthalocyanine blue, and
• Linear Trans quinacridone

Particle shape

• chemical structure,
• crystalline structure, or
• synthesis of a pigment

• Aggregates are connected along crystal boundaries during synthesis or drying. It is difficult to separate them. Hence, pigment manufacturers attempt to avoid their formation during the pigment's production.
• Agglomerates are loose clusters of primary particles. They can break down via an efficient dispersion process. Following this process, it is still possible for particles to re-agglomerate into loosely held groups. These groups are known as flocculates. This commonly occurs when there is a rapid change of state, i.e.,
  
  • Too rapid dilution or
  • The addition of an incompatible substance

Particle size

• 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;

Surface area & oil absorption

Hardness of the pigments

Quantity of Pigment

• the intensity and tinctorial strength
• the required opacity
• the required gloss
• the resistance and durability specified

1. Pigment volume concentration (PVC) -
   
   • It fundamental concern when formulating paints with optimum durability. There is a critical point that represents the densest packing of the pigment particles. This aligns with the degree of dispersion of the system. For systems requiring high gloss, low PVC is required. Primers and undercoats can have much higher PVC - up to 90%.


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

Binders used in formulations

• 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. These 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 on solvents such as xylene, ketones, and esters. They are very powerful and can dissolve pigments with poor or only moderate resistance to solvents.

Paint performance in end application

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