difficulties, particularly at the d

difficulties, particularly at the dosing stage, but its popularity is growing and it is
becoming clear that it is complementary to masterbatch, rather than in
competition with it. It is widely considered best suited to long runs, where dosing
can be more controlled, but the latest equipment (such as Colortronic's new lowrate
gravimetric additive feeder, Graviblend S) is claimed to bring colour changes
down to seconds.
Recent development has concentrated on liquid colours for PET, to meet the
huge demand from the packaging sector. Ferro's SpectraFlo Type 99 is opaque,
complementing its range of transparent colours, meeting FDA rules and offering
a rapid colour change at a let-down ratio of 1000:1. A clear-tint PET green from
Milliken is said to have advantages over other liquid pigment dispersions for PET,
with cost savings over pre-coloured PET.
6.2 Addition of Colorants
Originally, pigments were dispersed in plastics simply by dry blending in a
tumble-mixer - or in an oil drum. The plasticizing function of an extruder or
injection moulding machine also offered a reasonably good method of dispersing
a pigment but the penetration of plastics into more critical markets brought
demands for greater homogeneity and consistency. To achieve best results,
however, pigmented compounds need to be prepared before moulding, using
dedicated equipment such as a compounding extruder, or pigments must be
formulated as concentrates, in a form giving trouble-free mixing with virgin
material alongside the moulding machine.
Concentrates or masterbatches are consistent, simple, and safe. They usually
come in a granular form, in which the concentrated pigment is dispersed in a
polymer carrier (such as polyethylene) that is compatible with the matrix resin.
The let-down ratio is usually 0.5-2%, depending on colour, host material, and
part thickness.
The advantages claimed include cost savings due to less handling, quick
colour changes, savings with lower inventory and reduced storage
requirements, no pollution from colour particles in the air, and easy and clean to
use. Colour dispersion is so good that precise measuring and mixing are not
essential (but good screw back-pressure is recommended), while there is no
adverse effect on the physical properties of the host material.
A typical range will cover around 400 colours, with a minimum order
quantity of 25 kg, but some suppliers can supply down to 1 kg at a surcharge.
Colour matching can also be done, usually on a minimum quantity of 50 kg.
There are around 200 producers of colour masterbatch, the leaders being Cabot,
Schulmann, Ampacet, andFerro.
Whether the colour is added by a specialist compounder or by a good technical
processor, the quality depends on dispersion and match. The pigment is dispersed
by 'wetting' the particles with the resin, and the size and shape of the particles
are therefore of great importance, as also are the rheological properties of the
resin matrix. Apart from colour fidelity, good dispersion also plays a role in