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Paint pigments

Pigments are colorful compounds that give color to other materials. Pigments that are used in the paint industry, are natural or synthetic and organic or inorganic. The functions of pigments in paint are,

  1. To provide color
  2. To wipe out the previous color
  3. To improve the strength of the paint film
  4. To improve the adhesive properties of the paint film
  5. To improve the durability and weathering properties
  6. To increase the protection against corrosion
  7. To reduce gloss
  8. To modify flow and application properties

Properties of paint pigments

Tinting strength

Tinting strength is a measurement of its opacity. It gives an idea of how much pigment is required to get a particular color. If a lot of pigment is required to get the desired color, tinting strength is poor. Pigments with higher tinting strength do not change their color significantly when it is added to white color pigments. Sometimes the tinting strength can be increased by blending the pigment into a fine powder.

Lightfastness

Lightfastness is the resistance to changing color (fading) when exposed to light.

Hiding power

Hiding power is the ability to hide the substrate. It depends on the wavelength of the light, the ability of the film to absorb and scatter light, the refractive index of the pigment, the particle size of the pigment, etc. Pigments with high light absorption coefficients show higher hiding power.

White paint pigments like Titanium dioxide get their hiding power from their ability to scatter light. Pigments with a higher refractive index have higher scattering.

hiding power
Figure 01: Hiding power

Particle size

The scattering of light depends on the size of the pigment particles. Particle size should be higher than the wavelength of light to scatter the light.

PigmentParticle size (μm)
Iron blue0.01 – 0.2
Titanium dioxide0.02 – 0.3
Red iron oxide0.3 – 0.4
Natural crystalline silica.
(White color)
1.5 – 9
Strontium chromate
(SrCrO4) – Yellow color
0.3 - 20
Table 01: Particle sizes of paint pigments

Particle shape

The shape of the paint pigments determines the properties of the paints like flow properties, setting properties, barrier properties, the durability of the paint film, etc. based on the crystallographic structure, pigments can be divided into three categories.

  1. Spherical particles
    • Titanium dioxide
  2. Needle-like or rod-shaped particles
    • Zinc oxide
  3. Platelets or leaf-like particles
    • Mica

Rod shape pigments will poke through the paint film. This will reduce the gloss of the paint. Needle shape pigments are crack initiators. They will cause cracks in the paint film. Platelets or leaf-like pigments can arrange parallel to the substrate. So, it will prevent the substrate from moisture, oils, radiation, etc. normally pigments with rough surfaces are used in undercoats.

Specific gravity

The ratio of the weight of the pigments to its net volume.

Paint pigments eq 01

Classification of paint pigments according to their functions

Colored pigments

Colored pigments improve the appearance and provide color to the paint. It depends on the particle size, shape, and distribution of the pigment.

Hiding pigments

Hiding pigments provide opacity to the paint film. Most of the time, hiding pigments are carbon black or Titanium dioxide (TiO2)

Barrier pigments

Barrier pigments are sheet-like micro-size fillers. Their flat surfaces are settled parallel to the air–polymer (binder) surface. Therefore, water and Oxygen molecules cannot penetrate directly into the substrate. Those molecules have to flow through tortuous pathways.

Barrier pigments
Figure 02: Barrier pigments

Reinforcing pigments

Reinforcing pigments are commonly used in the rubber industry. They give additional strength. The most common reinforcing pigment is carbon black.

Extender pigments

Extender pigments are added to the pigments to increase the pigment volume. So, it causes to reduce the cost of the pigment.

e.g., silica, Calcium carbonate (CaCO3), Barium sulfate

Sacrificial pigments

Sacrificial pigments are used in anticorrosive paints. Pigments sacrifice themselves to protect iron substrate (Cathodic protection). Sacrificial pigments contain metals that are higher in the electromotive series than the metal of the substrate to be protected.

Without anticorrosive paint, the iron surface will corrode as follows,

  • Anodic reaction (Oxidation)
Paint pigments eq 02

  • The cathodic reaction (Reduction)
Paint pigments eq 03

Under corrosion conditions, the sacrificial pigments are more reactive than the substrate. Therefore, the substrate will be the cathode and the pigments will be the anode. If Zinc based pigments are used for the iron substrate, the following reactions occur.

  • Anodic reaction (Oxidation)
Paint pigments eq 04

  • The cathodic reaction (Reduction)
Paint pigments eq 05

Inhibitive pigments

Inhibitive pigments reduce the rate of corrosion by forming a protective layer (a barrier layer) over the substrate. These inhibitive pigments release soluble species from the pigments to the solvent. They will penetrate the metal surface and form a protective layer. This is a passive way of corrosion inhibition.

e.g.,

  • Red lead pigments (Pb3O4)
  • Chromate pigments
  • Phosphates
  • Silicates

Pigments vs Dyes

Both pigments and dyes are aggregated in dry form. But when they dissolve in a solvent, dyes make a transparent solution where pigments give their color and opacity to the solvent.

Pigments vs Dyes
Figure 03: Pigments vs Dyes

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References and Attributes

Figures:

The cover image was created using an image by André Zivic from Pixabay


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