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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Several names are applied to pigments that show an angle dependent colonr produced by light interference in thin Ahns." " For example nacreous, interference. 

Nacreous and Interference Pigments. The optical principles of nacreous (interference) pigments are shown in Figure 72 for a simplified case of nearly normal incidence without multiple reflection and absorption. At the interface P, between two materials with refractive indices n t and n2, part of the beam of light L, is reflected (L,) and partially transmitted (i.e., refracted) (L2). The intensity ratios depend on n, and n2. In a multilayer arrangement, as found in pearl or pearlescent and iridescent materials (Fig. 71 C), each interface produces partial reflection. After penetration through several layers, depending on the size and difference between n1 and n2,... 

One recent innovation was the introduction of interference colours which are used to add a glow to the product. In sugar confectionery they are used on panned goods, and elsewhere have been very successful in the cosmetic business. Interference pigments have their properties because they cause a selective reflection of light where the wavelength, and hence the colour, of the reflected light depend upon the thickness of... 

In order to understand how three distinct colors from one pigment are possible, it is first necessary to understand the optical properties of pearlescent and interference pigments and also the optical properties of absorption colorants. The manner in which these reflect light differs and this forms the basis of the three color effect. 

Pearlescent and interference pigments reflect light in a specular manner similar to a mirror. The angle at which most of the light is reflected is equal to the angle of the incident light. The... 

Figure 7. Reflection of light by interference pigments coated on diffuse angle of -15/60, the amount of light glass slides. reflected was quite small, and the neutral...

Since many paint layers already exhibit inhomogeneities in such dimensions for the calculation of color data (tristimulus values and chromaticity coordinates), it is necessary to use mean reflectance values derived from three to five individual measurements of different sample areas. For individual measurements, the sample surfaces, especially in the case of paints containing plate-Hke pigments (light interference and/or color variable pigments), should have varying orientations to the adjustable measuring aperture. 

Interference pigments n. Materials used as pigments, which depend on the interference of light for their color. They are generally made from platelets coated with a thin film of higher refractive index. For example, thin films of titanium dioxide coated on mica flakes are one type manufactured and used today. 

Iron oxides may be natural or synthetic. Synthetic oxides are preferred for their high pigmentation capacity, and their chemical and color purities. Natural oxides are cheaper in cost but they contain impurities that may interfere with some of the polymer properties. Iron oxides are used as yellow, red, black, brown shaded, and light-tan pigments. 

The phenomena of light interference can also be produced using difiraction technology. Diffractive pigments are manufactured by vacuum deposition on a specially patterned surface [49, 50]. 

The UV method performs well in the absence of interferences of other additives or pigments but is liable to be in error owing to interference by highly absorbing impurities that may be present in the sample or in the solvent [56,57]. Commercial erucamides with variable light stability have been analysed by means of SEC-UV (254nm)/RI and UV absorption in MeOH [58]. 

Pearlescent pigments give rise to a white pearl effect often accompanied by a coloured iridescence. The most important pearlescent pigments consist of thin platelets of mica coated with titanium dioxide which partly reflect and partly transmit incident light. Simultaneous reflection from many layers of oriented platelets creates the sense of depth which is characteristic of pearlescent lustre and, where the particles are of an appropriate thickness, colours are produced by interference phenomena. Pearlescent pigments are used in automotive finishes, plastics and cosmetics. 

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