from pigments based, notably, on ca

from pigments based, notably, on cadmium and the search is on to replace these
with others of equal effectiveness.
Table 6.1 At a glance: pigments, dyes, special effects
Function Colorants act either by absorbing parts of the spectrum and reflecting
other parts (solid pigments, dyes), or by transmitting only certain
wavelengths (transparent colours); these effects can be combined
with multi-layer structures, also using interference patterns to
achieve an effect
Properties affected Colour, appearance: some pigments can also give shielding against
UV light
Materials/characteristics Organic; inorganic; pearlescent; metallic; special effects
Disadvantages Can contaminate other materials/equipment unless kept separate or
used in dust-free/non-polluting form; migration if not correctly
compounded; thermal stability in processing at high temperatures:
replacement of heavy metals with pigments of comparable
performance
New developments Easier forms for use/incorporation in compounds; colour
concentrates, liquid colours; improved dispersibilty, better thermal
stability; replacement of heavy metals; introduction of new
chemistry
6.1 Main Types of Pigment and Colorant
Pigments are usually supplied as dry powders of various specific gravity and
bulking value. Care is needed in handling, not simply because of the cost of the
pigment, but also because it may present a dust hazard. Safe, dust-free
formulations are now the general rule, but with so many other priorities many
processors have clearly decided that colour is not their problem, and prefer to
buy material ready-coloured.
6.7.7 Mixed metal oxides
These pigments are synthetic minerals. The desired colour is achieved by
selecting specific metal oxides, based on metals such as chromium, nickel,
antimony, titanium, manganese, cobalt, aluminium, zinc, iron, and copper.
They are characterized by good heat stability, insolubility, excellent chemical
resistance, non-bleeding/non-migrating, excellent weathering and lightfastness,
good dispersability, good hiding power, strong, bright colours, and nonwarping.

6.7.2 Dyes
These are transparent and give bright colours in light. Most have relatively poor
light fastness and limited heat stability, but will tend to retain their colour better