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Application of Dyes and Pigments
Colorants for Plastic Industry
Colour selection is one of the vital things to be considered in the plastics industry. Colour is an integral part of the plastic material and it should not be considered as an after thought. The whole system of colouring has to be deal with using what is called a total systems approach. No doubt the colour enhances the part's functionality, but it has also the potential to degrade certain material properties like impact strength, if not properly applied.

Colourants for Plastic Industry

The colourants that are used in the Plastic Industries are the dyes and pigments. Both dyes and pigments exhibit colour through the process of visible light absorption and scattering. Typically colour formulations generally have four colourants. Black and white are used basically to control the value and Chroma, while two coloured pigments or dyes are applied to establish the Hue. The resulting color that is achieved is governed by the laws of subtractive mixing, the same law that is used in printing and painting.

Dyes that are used in the plastics industry must be very strong, show good heat stability and be transparent. Typically the dyes are used to tint or shade a resin. In the plastics industry dyes are limited in use, as such they can be used for only a selected number of resins. Pigments used in this industry can be organic or inorganic. While pigments offer more choices to the manufacturers due to its opacifying ability, broader chroma range, and better heat stability for most of the resins. Pigments are generally dispersed within a resin matrix, while dyes solubilize in the resin. Titanium Dioxide is a pigment that has a wide use in Plastics industry. It is used to make the plastics more whiter, brighter and durable.

Dyes or Pigments?

It is actually the base polymer that dictates the choice of the colorant selected. Pigments are typically used in polyolefins as most of dyes show a tendency to migrate in polyolefins. While Dyes are often successfully used with polycarbonate, polystyrene, acrylic, and other types polymers because of their color strength and transparency. The final word is whatever be the colorant selected it must show compatibility with the base resin. This is in order to prevent a color shift occurring over time.

Let us look at the properties of few of the colourants that constitutes both dyes and pigments:

Type of Effect Pearlescent Metallic Fluorescent Phosphorescent Thermochromic Photochromic
Cost Low Low Low-High Medium High High
Polymer compatibility All All Dyes: PMMS, PET, ABS, Pigments:PE, PP PMMS, PE, PS All All
Durability Excellent Excellent Fair Good Fair Fair

Notes:
PMMA: Poly Methyl Methacrylate
PET: Polyethylene Terephthalate
SAN: Styrene-Acrylonitrile Copolymer
ABS: Acrylonitrile Butadiene Styrene

Factors in selecting the right colourant

  • Processing Temperature
  • Dispersion method
  • Resin type
  • Dimensional stability
  • Heat stability: A careful choice of inorganic and organic pigments from the view point of temperature is vital for maximizing color and performance
  • Gloss
  • The exact application (whether general purpose, food contact item, toy, package subject to CONEG regulation etc.)
  • Type of polymer(s) being used
  • How is the polymer(s) being processed
  • What kinds of functional additives are required like plasticizers, stabilizers, flame retardants, slip agents, etc
  • End-use application requirements
  • Weather fastness
  • Colourant loading

Use of Solvent dyes in coloring Plastics

Solvent dyes are an excellent medium to dye plastics. Basically a solvent dye is a dye that gets soluble in organic solvents. Selecting a solvent dye must be based on the following points:
  • On dissolving in plastic, it must be bright, clear and transparent.
  • Color intensity should be high, PS Resin: 0.01-0.1%, for ABS: 0.03-0.5%).
  • It should have high thermo-stability.
  • It should mix properly with other organic and inorganic pigments.
  • It should show high light resistant property.
  • Should show stability in other kinds of chemicals.


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