maintaining the physical properties of the compound, while inadequately mixed
pigment may even damage the processing equipment. Dispersion of a critical
pigment in a ^difficult' resin matrix may be promoted by special dispersant
additives.
Pigments are generally robust, but over-mixing can be as serious a problem as
under-mixing. Stability of the pigment is an important factor. Many pigments
can deteriorate when exposed to excessive heat and, where the matrix is an
engineering thermoplastic, a pigment with higher thermal stability is needed, to
resist the higher processing temperature.
Generally, large-particle pigments, such as titanium dioxides, are easier to
handle - but carbon blacks, by their very nature, demand specialist handling
(and are usually compounded in dedicated sealed units). Organic pigments also
have fairly large particles, but they tend to be light and 'fluffy', and may also
carry electrostatic charges, all of which makes them difficult to disperse. Smallerparticle
pigments provide a denser colour in the plastics matrix but their specific
gravity can provide problems in metering/weighing. The sequence of blending,
as well as the use of dispersing aids, is critical.
6.3 Replacement of Cadmium
Probably the main challenge to pigment development during the past 10 years
has been the ecological drive to replace heavy metals which, it was feared, could
leach out from landfills into the environment. Lead had already been phased out
on the grounds of toxicity but, in Europe, there were national and supranational
moves to ban the use of cadmium pigments also. The latest study (by the EU itself)
concludes that cadmium pigments do not present any significant threat to
human health or to the environment. A more far-reaching report on cadmium
and cadmium oxide is expected.
Technically it is not easy to produce an exact 'drop-in' replacement (especially
at the same cost level). A key problem has been to produce a yellow that is as
effective as cadmium. Economically there are also problems. Respondents to a
survey by the Cadmium Association said that costs increased by 2-5 times: over
50% found that productivity fell by 10-25% when moving to non-cadmium
pigments, particularly with polyolefins.
Organic pigments are bringing their own strengths and weaknesses as they
move into markets formerly held by cadmium and lead. In general they have
excellent colour characteristics, but they can have lower stability, while
increasing formulation costs. New pigments are being designed as blends of
organic and inorganic compounds, using features of both to tailor to a specific
application - but blends can retain weak properties from their individual
constituents.
The need to replace heavy metals has also stimulated the introduction of new
chemistry. An important development has been that by Rhone-Poulenc of
inorganic pigments based on sulphur, with a crystalline structure that can be
doped with various metal elements (rare earths). The colour strength is reported