Pigments Characteristics

1. Crystal Structure
2. Particle Shape
3. Particle Size
4. Surface Area & Oil Absorption
5. Hardness

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 become 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.