Colour your thinking on the optical properties of thermoplastics

This month, polymer expert and technical blog author, Dr Charlie Geddes, looks at what determines colour in thermoplastics

Very few commercial polymers have chemical groups (chromophores), which absorb light in the visible region of the spectrum. Consequently, the base polymers are colourless. If you need colour, you have to add dyes (soluble in the polymer phase) or pigments (insoluble). Dyes fell out of favour because of problems of migration out of the polymer into other materials (‘bleeding’) and because of the development of a wide range of efficient organic pigments, with appropriate heat and weathering resistance.  Mixing of pigments can match any specific combination of hue (shade), chroma (intensity) and lightness.

To generate colour in a natural thermoplastic, the preferred method is to add granules of the same polymer (or a compatible polymer) loaded up with a high concentration of pigment (colour concentrate or masterbatch).  The secret of consistent, efficient colour is to ensure the pigment is broken down into very small particles (dispersion) and evenly mixed (distribution), both requiring careful application of shear in the melt state.

Colourants are added in such small amounts that they do not affect mechanical properties significantly. The exception is impact resistance. Different shaped pigment particles can affect crack initiation. It was a standing joke with a colleague that, after impact test results from one particular consultancy project, we politely declined green hard hats on factory visits.

Although the base polymer is generally colourless, a slight yellowish tinge can appear from polymerisation residue, polymer decomposition during processing and service and, more commonly, from other additives. This yellowness can be cancelled out by optical brighteners, a special class of colourant, which absorb UV light from the invisible part of sunlight or artificial light and emit it in the blue region, giving a cleaner transparency or more intense white and sharper colours. To check for optical brightener, just observe the thermoplastic under a UV lamp.