Colouring of Polymers and Composites - Introduction to Polymeric Composites with Rice Hulls - page 106

comparatively large wood shavings or rice hulls shells, and acquire strong, bright
and lightfast colours. A more efficient colouration of cellulose polymer composites is
possible in this way because in cellulose particles the main constituent of a composite
is coloured and not just the quantitatively subordinate polymer matrix. The liquid-
colourant preparation can comprise 0.5-10% by weight of dye based on the pigment.
This pigment may consist of an organic or inorganic pigment plus at least one dye, a
dispersant and water, or a mixture of water and a water-retaining agent.
Preferably, the pigments should be in finely divided form, with typical average
particle sizes of 0.1-0.5 µm. Depending on the effect desired, the pigments may be
used in transparent or other forms. Organic pigments are typically organic chromatic
and black pigments. Inorganic pigments can be colour pigment-chromatic, black and
white pigments, and also lustre pigments. Typical colourants are carbon black (black
colour), titanium dioxide (white colour) and iron oxide (red colour).
8.4 Methods of Colouring Polymers
Colour selections and processing methods are decisions made by the producers of
polymer composites supported by the manufacturers or suppliers of the required
colours or systems, who provide guidance on how to obtain the best results. Several
methods are available but probably the general standard ones used in processing
plastics are masterbatches, cube blends and pre-colours.
8.4.1 Masterbatch Concentrates
The most common method used by plastic processors are masterbatches consisting
of concentrated pigments dispersed into a polymer carrier resin. During processing,
the masterbatch is mixed with natural resin and can be compounded with other
ingredients in a composite mixture. The percentage use of the colourant will depend
on the final colour desired but, in general, small amounts as a percentage of the
mixture are sufficient. Masterbatches are the most economical method to colour
plastics and can be available in small volumes. Depending on the carrier selected,
the same masterbatch may be used to colour several resins or chosen for maximum
compatibility and processing ease. If a special colour(s) is needed, a supplier could
oblige in a few days because most of them are geared with modern technology and
equipment.