Buyer's Guide to Pigments

Pigments find enormous types of application in broad range of industries and encompasses a huge variety of Colorants.

High performance inorganic pigments offer comparatively weak color strength, but can give excellent opacity and durability. To give another example, high cost organic pigments though offer excellent color strength but comes with the limitation of poor opacity. Thus Formulators are left with very difficult choices to make.

One thing to remember is that, pigment selection in most cases is application-specific. Some of the important factors that plays a vital role can be of choices like:

• Color and Tinctorial strength.
• Long-term performance vis-a-vis coloring cost.
• High melt temperature of polymers: For example take the case of fluoropolymers. Typical melt temperatures are high and can vary from 165°C- 370°C. Here the pigment selection is restricted and the majority of pigments applied are inorganic in nature.

Here are few tips that are valuable in pigment selection:

• Selection of Pigment generally is balancing of many types of variables, therefore changes made in the existing commercial formulations should be done judiciously considered and only after color rematching. For example a manufacturer who randomly switches colorant suppliers or reduces expenditure by adopting less costly colorants is under the risk of problems during processing and end-use.
• Consideration should be given towards particle size of the Pigment as it affects light scattering, which in turn can affect shading. To cite an example, finer particles has a tendency to shift a red pigment's shade towards blue, while coarser particles tends to move it towards yellower shades.
• In the case of application in plastics, adding pigments and dyes asks for matching the right mix of colorants to the shade and intensity of the required colors.
• Organic pigments generally can withstand temperatures 400°F and 475°F, while inorganic pigments has been seen to withstand 550°F or even more. In case of some complex inorganic pigments, there is a greater heat stability than some polymers. In case of inorganic pigments like yellow iron oxide, a surface treatment is required to increase their heat stability above 400°F. So, it is imperative that a careful selection of inorganic and organic pigments must be done from the standpoint of temperature so as to maximize color and performance.
• It has been seen that in some cases, organic pigments has a tendency to cause part warpage. This generally occurs when pigments interfere with the polymer crystallization. Now the point here is that warpage issues arises more with some kind of pigments than with others. Examples are that of green or blue phthalocyanine pigments. Resins like high-density polyethylene, are more prone to warpage due to their rapid crystallization.
• The pigment that is chosen must show compatibility with the base resin so as to prevent a color shift over time.