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COLORANTS FOR PLASTICS

Theory of Colour
When white light shines on a plastic material, it may be absorbed, reflected and or
transmitted. Based on the wavelength of light absorbed and transmitted by the
plastic, will determine its colour as it appears to us.
Classification: Colourants are generally classified as inorganic or organic.
Inorganic colours generally are coarser particles of lower colouring efficiency,
stable to heat, light and chemical environment and less expensive. Titanium dioxide
used as opaque pigment and whitner, Iron oxides for reds, yellows, browns and
blacks but rather dull in appearance, Cadmiums range of yellow to red colours are
bright, fairly expensive, Chromium and molybdenum offer a range of yellow, orange
and green colours. Iron blues are attractive and inexpensive; Cobalt blues are
expensive but attractable.
Organic colours are finer particles of higher colouring efficiency, less stable and
more expensive, generally non-toxic, but they must be monitored for possible
impurities that could cause problems. Nigresine dye is a much more effective black
colourant than carbon Black. Carbozoles, perylenes and quinacridones produce
reds, anilines produce orange, monoazes produce orange to yellows
anthraquinones, dyarylides, and nickel azes produce yellows ; and
phthalocyanines produce greens to blues.
Colourants may also be classified as dyes and pigments. Both give transparent
colours. They are mainly organic compounds and very efficient colorants, but they
tend to be unstable and extractable. Pigments are insoluble particles or less that of
dyes properties, producing opaque colours, they may be inorganic or organic. In the
plastics industry, pigments are commonly used to produce opaque colours; dyes
are used only as specialties where a transparent colour is particularly desired.
Criteria for choosing colorants : A typical checklist includes dispensability,
rheology, plate-out, thermal stability, appearance, light fastness, weathering,
migration and toxicity in both processing and use, particularly in leaching from solid
waste.
Market Analysis : In worldwide tonnage quantities, titanium dioxide leads the
figures. In plastics, inorganic white pigments approximately constitute about 72
percent of the total market, inorganic coloured pigments 8 per cent, carbon black 13
percent, organic pigments 5 percent, and dyes 2 percent. But recent trends indicate
the replacement of inorganic by organic colourants.
Specific Critical Requirements for plastics
1) Must be stable at 250°C for 15 minutes preferably at even 300°C.
2) Must have uniform particle size for uniform dispersion.
3) Should not agglomerate or form lumps. 4) Must not bleed.
5) Must not impart any odour.
6) Must be resistant to U.V. Degradation, when used for outdoor applications.
7) Must have a FDA approval and conform to BS Standards for Food Contact
Applications.

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