Лако-красочные материалы — производство

Gerhard Pfaff

7.1

Introduction

Special effect pigments with the most important types, pearl luster and interfer­ence pigments, are based on natural or synthetic materials. They show outstand­ing qualities of luster, brilliance, and iridescent color effects based upon optically thin layers. This visual impression develops by reflection and scattering of light on thin multiple layers. In nature this is not limited to pearls and mussel shells alone; there are a multitude of birds, fish, precious stones and minerals, even insects which demonstrate a pearl luster effect. Experiments to understand the optical principles of natural pearl luster demonstrate that the brilliant colors are based upon structured biopolymers and upon layered structures which are devel­oped by biomineralization.

Figure 7.1 illustrates the various optical principles of conventional pigments (absorption pigments), metal effect pigments, and pearls and pearl luster pig­ments. In the case of absorption pigments, the interaction with light is based upon absorption and/or diffuse scattering. A completely different optical behavior can be observed with the group of effect pigments including pearl luster and met­al effect pigments. Metal effect pigments consist of small metal platelets (for example aluminum, titanium, copper) which operate like little mirrors and almost completely reflect the incident light.

Pearl luster pigments simulate the luster of natural pearls. These consist of alternating transparent layers with differing refractive indices. The layers consist of CaCO3 (high refractive index) and proteins (low refractive index).

This difference in refractive indices, arising equally on the interface between air/oil film or oil film/water, is a prerequisite for the well-known iridescent color images in these media. Small highly refractive platelets of pearlescent pigments align themselves parallel in optically thin systems such as paints, printing inks, or plastics. Interference effects develop when the distances of the various layers or the thicknesses of the platelets have the right values.

Synthetic pearlescent pigments are either transparent or light-absorbing platelet shaped crystals. They can be monocrystalline as in Pb(OH)2 x 2 PbCO3 and BiOCl

High Performance Pigments. Edited by Edwin B. Faulkner andRussell J. Schwartz Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31405-8

Natural Pearl Pearl Luster Pigment

Figure 7.1 Optical properties of absorption pigments, effect pigments, and natural pearls.

or possess a multilayered structure in which the layers have differing refractive indices and light absorption properties.

7.2