Colouring of plastics: how to improve the yield of a pigment

We can distinguish two types of colorants, pigments or dyes, for plastics colouring process. The first ones can be totally insoluble in the polymeric matrix if they are inorganic pigments or partially soluble if they are organic. However, dyes are entirely soluble in plastics.

The choice between dyes and pigments depends on the type of the polymer and the type of colouring that we are looking for. High opacity colouring usually requires inorganic pigments and low opacity or fully transparent ones will need organic pigments or dyes.

There are other important aspects when choosing a pigment, but that will not be discussed in this post.

The yield of a pigment could be defined as the best balance between a stronger colouring and the percentage of this to achieve it, so a pigment will get higher yield than other if it achieves the same or higher tinting capacity with a lower percentage in weight in the final formula.

The yield of a colour is closely linked to the dissemination quality of a pigment in the polymeric matrix. When talking about plastics colouring, one of the key terms to take into account is colorant dispersion.

Dispersion is the capacity of disaggregate a colorant reducing its particle or drop size to the minimum in a plastic and depends on several factors: particle pigment size, processing conditions, shear, applied equipment and method, polymer viscosity, chemical compatibility between pigments and polymers and, finally, additives or compatibilizing agents.

The better the pigment dispersion is, the higher the volume will occupy in plastic and the higher yield. The most important consequence of an improvement of the dispersion is that we will need a lower pigment quantity to achieve the same capacity of staining.

This aspect is very important in SPC or monocolour concentrates, where optimum dispersion can bring down prices and make our masterbatches more competitive in quality and price.

In order to improve the dispersion, it is necessary to validate it by the intern methods of the company, but if they do not have, by defining them (DIN EN 13900-5 or ISO 18553: 2002) and establish a quality parameter or “mixing index”. Then, the modification of the compounding process is mandatory to reduce agglomerate levels detected in the measurements. Aspects such as re-design of screw configuration, change in the production, modification of machine parameters (speed and temperatures), as well as the use of dispersing agents can help to improve significantly the pigment dispersion in the matrix and, consequently, its colouring strength and its yield.