Opacity & Tinting Strength
Opacity
The opacity of a pigmented object is a function of its surface reflectivity, the refractive index of the pigment relative to the binder system, its light scattering power – which is also a function of its particle size, and/or its absorption of the incident light.
Thus whilst a perfect mirror wholly reflects the incident light (in one direction), a pure white surface may directly reflect some incident light but may also scatter the light in many directions. A pure matt black surface absorbs the incident light and colored materials absorb some parts of the spectrum and return the remainder.
The opacity or covering power of a white paint, ink, plastic or paper application is a function of the refractive index of its pigmentation relative to the surrounding binder, or in the case of a paper sheet, to air. The greater the difference in refractive index, the higher the amount of light scattering. Before titanium dioxide, white paints and paper had been pigmented with relatively low refractive index pigments and fillers with significantly lower refractive indices than either form of TiO2. Figure 6.1 compares the refractive index values for TiO2 , fillers and binders.
Within a pigmented article, light scattering occurs via reflection, refraction and diffraction. Thus when light is incident on a pigment particle:
- some is directly reflected from the pigment particle's surface
- some is refracted through the crystal, with the path of the light being bent and scattered more with high refractive index pigments
- And some is diffracted around the particle.
With a TiO2 particle size of approximately half the wavelength of light, the particle may bend several times as much light as that incident on it, with a commensurate increase in scattering power.
Providing the coating or film is sufficiently thick, the common factor is that the viewer cannot see what is under the surface because all of the incident light is either returned to the surface by scattering within the film or absorbed without reaching the underlying material. To the viewer. the substrate is therefore completely obliterated. See Figure 6.2 for a graphic of opacity factors.
Opacity, or hiding power, may be measured in several ways. The simplest is to measure the 'contrast ratio' which is the ratio of a film's reflectance over a black substrate divided by its reflectance over a white substrate. Whilst simple, this method does not separate opacity due to scattering from that due to absorption.
To assess the true light scattering potential of a pigment requires the calculation of its 'scattering coefficient' which takes into account light absorption and also film thickness.
Tinting Strength
The tinting strength of a pigment is closely related to its opacity. For a white pigment such as TiO2, tinting strength is sometimes called its 'lightening power' – in effect how much it can 'lighten' the color of a colored pigment mixed with it. The term therefore describes the light scattering of the TiO2 relative to the light absorbing properties of colored pigments. In a well dispersed system, opacity and tinting strength are directly proportional but this relationship may break down if either component is flocculated.
The particle size of a TiO2 pigment also affects the color of systems, especially those containing both scattering (e.g. white) and absorbing (e.g. black) pigments due to its 'undertone' Tinting strength and undertone are both closely controlled by TiO2 manufacturers to ensure consistent color matches in tinted systems such as automotive refinish and in-store tinted paints.
|
