TABLE OF CONTENTS Pigments for PVC must be thermally and light stable, have good dispersability, and be compatible within the formulation. They may also need to satisfy specific regulations covering the end use, e.g., food contact, toys, and so on. Inorganic pigments are the most common type.
Titanium dioxide pigments are used to give bright whiteness and opacity. They play a very significant part in PVC pigmentation.
The opacity of a pigmented material is a function of its surface reflectivity, the refractive index of the pigment relative to the binder system, and its light scattering power. The light scattering capability is a function of particle size and/or absorption of incident light. Rutile TiO 2 has a refractive index of 2.76 in relation to PVC at 1.54, indicating the reason for its use. TiO 2 can have two crystal structures, rutile and anastase. Rutile is used in plastic applications, due to higher refractive index, better light scattering efficiency, and superior UV absorption.
Production for plastics grades is normally via the chloride process where the chloride is generated prior to conversion to the oxide.
Commercial grades are based on multilayer inorganic materials over a TiO 2 core. The layers can be silica (SiO 2) and/or alumina (Al 2O 3) which act as a barrier between the core and the resin to minimise photocatalytic degradation of the polymer. Interaction between UV radiation, moisture, and an uncoated TiO 2 surface produces free...
