Colouring of plastics: how to choose a colorant
The colouring of a plastic product is not a simple task, despite of being completely familiar with plastic products coloured in countless tonalities, colours or effects, this process requires several important steps. One of them is the colorant selection process.
The colouring selection will depend on several factors, but they could be mainly divided in two groups: the plastic to be coloured and the final application.
Regarding plastics, some aspects have to be taken into consideration, such as the melt flow rate, if it is transparent or not, if it is semi-crystalline or amorphous, if it is a glossy or matt plastic, its melt or processing temperature or if the plastic adds some kind of filler additives or fibre in its formulation, among other aspects.
Colorants can be dyes or pigments (organic or inorganic). Dyes are soluble in plastic, so they are easily mixable and they are usually used to colour transparent plastics, normally for amorphous polyesters. Pigments are partially soluble (organic) and insoluble (inorganic). The first can be used to colour transparent plastics, too. Both, dyes and organic pigments have much more capacity to migrate than inorganic pigments. Inorganic pigments are used to colour any type of plastic that requires a high level of opacity; this is its main characteristic.
From the point of view of tinting capacity or colouring strength, organic pigments are more effective than the inorganic ones, creating brighter and glossier colours at lower percentages, 0.2-0.5% in contrast with 2% approx.
When we take into consideration the final application, the selection of the colorant has to bear in mind the durability of the coloured product, its final price, environmental factors (UV radiation, use temperature, aggressive chemically environments, etc.) or possible migrations.
In the colouring terminology, different resistances to different phenomenon that have an impact on its performance and that influence in its selection are usually mentioned. These resistances are known as fastness. We can find:
- Lightfastness, that refers to the persistence of a colour after receiving a determined quantity of UV radiation.
- Chemical fastness, that refers to a colour resistance in aggressive chemically environments, whether acids or basics.
- Thermal fastness, colour resistance to temperatures, particularly during the processing.
- Migration fastness. Some organic pigments or dyes can migrate or being extracted from plastic. Its fastness would indicate the difficulty of pigment migration.
Other important aspects are the pigment dispersability, different special functions that the pigment and its format contain, chemical plastic-pigment compatibility, if it is available in a masterbatch, powder, liquid, etc. shape.