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 Selection

Pigment Selection
These significant characteristics have been explained below for better selection.

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 amorphorous 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. a 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 results in an uneven increase in tinctorial strength, depending on how much shear has been developed 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 & Oil 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

Hardness is usually based on Mohs Hardness Scale (a non-linear scale, used as a comparison chart). 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 ustensil 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. It is a complex calculation but essential for paints that have to meet the highest performance standards with respect to durability.

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

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

Paint type-Binder

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 insoluble, or almost insoluble in white spirit, so it rarely narrows the choice of pigments.

Industrial finishes can be based on broad variety of solvents. To take an example, solvents such as xylene, ketones, and esters are very powerful and can dissolve pigments with poor or only moderate resistance to solvents. If such paints are dried by stoving, the high temperature poses even greater demands on the pigment used.

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.

End use

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. Common classifications set apart paints used for building, architectural or decorative, automotive finishes, OEM (original equipment manufacturers) or VR (vehicle refinishes) and those used for industrial finishes. In the same way, pigments used in paint do not require same properties than pigment used for inks applications.

The price difference between a cheap organic pigment and a high performance pigment can be a factor of around 20. High performance specialty pigments are often complex in composition and have limited application, which accounts for their high price.

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