6.4.7 Aluminium pigments
These offer effects such as bright metallic lustre, bright sparkle, pearlescent
effects, and glitter. The pigments are entirely encapsulated by the polymer
carrier, allowing easy and gentle dispersion. Dosage uses standard volumetric
and gravimetric units, with little tendency to contaminate. A high metal content
permits low dosage: 0.1-3% for moulded articles and up to 5% for film will
provide good opacity. In engineering plastics a pigment content of 1-5% may
produce a 5-15% reduction in strength, which may be offset by using a higher
grade. Plastics coloured with aluminium can be recycled, and it is also possible to
improve the appearance of recycled thermoplastics by the addition of aluminium
pigments.
Metallic pigments, especially for automobile applications such as bumpers,
door panels, grips, buttons, hub covers, and scooter panels and wings, are a focal
point of current development by Clariant Masterbatches Division. The company
also claims it has the first colour masterbatch series for HDPE packaging with an
anodized aluminium appearance, giving high reflectivity, chrome, and lustre.
6.4.2 Pearlescents
These are pigments based on thin platelets of transparent mica, coated with
titanium dioxide or iron oxide, producing interference patterns. Titanium
dioxide (Ti02)-coated mica pigments have a 2-10 )am particle size, for increased
lustre, whiteness, and coverage; there are also silver sparkle effects. A two-tone
effect can be obtained with absorption colours deposited directly on interference
pigments of TiOi-coated mica (such as Mearlin Dynacolor).
A 'frosty' effect can be obtained by combining pearlescent pigments with
ultrafine titanium dioxide of 20 |im particle size, compared with the normal
200 |im, as a coating on the mica platelets The pigments (from Kemira Pigments)
reflect different colours according to the angle of viewing. The coating
strongly absorbs radiation in the UV part of the spectrum and shows most of its
scattering power in the blue. TekPearLite colour concentrates give pearlescent
effects: they include FDA-approved grades for cosmetics, toys, and food
packaging.
Colour and effect masterbatches have been developed by Hanna's subsidiary
Victor International. They include a highly concentrated colour masterbatch
(Hi-Strength) that can reduce inventory costs and is seen as particularly suitable
for packaging where high opacity is required. There is also a new translucent
pearl effect masterbatch range (Trans-Pearl) that includes thermochromic,
marble, pearlescent, edge-glow, photochromic, laser marking, frosted, metallic,
and speckled effects.
6.4.3 Light interference pigments
These have recently been taken a stage further with the development of so-called
'flip-flop' pigments that present a different colour according to the angle from
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