The hiding power of a pigment can be determined using the phenomena of light absorption and reflection. Chromatic strongly colored pigments, black pigments absorb light and light colored and white pigments, on the contrary, reflect.
The refractive index of film-forming substances is about 1.4 to 1.8. If the refractive index of a pigment is greater than 1.65, it is called hiding. If the refractive index of the pigment is close to that of the film-forming agent, it is called tinting (non-coating). White-leafing pigments are often used as a filler. The highest hiding power of all white pigments is titanium dioxide. The value of hiding power depends very much on how fragmented the pigment (from its dispersion).
Only a small part of the light flux is reflected from the surface layer of the paint coating. The rest of the sun's rays are scattered throughout the entire volume of the paintwork and reflected from almost every constituent particle of the coating. This is why hiding power is directly dependent on the pigment content of the paint material. The pigment content can be determined using the OKP (Volume Pigment Concentration) value. OKP is the ratio of film-forming agent to pigment. With the increase of pigment content the hiding power of the paintwork material increases linearly (approximately by 10 - 15%). Then the increase in hiding power slows down a little, passes through the maximum value and gradually begins to fall. The point is that with a high pigment content in the coating material, the pigment particles get closer and the light fluxes that are reflected from them overlap. Therefore, it is also important that the pigment particles are evenly distributed throughout the entire volume of the paint material.
There are three main methods by which the hiding power can be determined: instrumental-mathematical, visual and contrast ratio method. The instrumental-mathematical method is based on the Gurevich-Kubelka-Munk theory. This method is carried out with the use of a computer (calculating machine). Reflection coefficients (brightness) of coatings of different thickness on black and white substrate are determined. Then the thickness of the protective layer and the hiding power are calculated. The visual method requires a black and white checkerboard. The paint material is applied to the glass plate until white and black squares are visually distinguishable (not visible) when applied to the checkerboard. Knowing the exact composition of the paint material, its mass and the area of the glass plate, the hiding power is calculated quickly and easily. The contrast ratio method is based on calculating the contrast ratio.
The contrast ratio is defined as the ratio of the brightness ratio of the paint coating on a black substrate to the brightness ratio of the same coating, only on a white substrate. If the contrast ratio is 0.98, the surface is considered covered.
Lightfastness is the ability of a pigment to retain a constant composition and optical properties when exposed to light. Virtually all pigments change some of their characteristics under prolonged exposure to light, e.g. they may fade or change their shade.
Pigments of organic origin, which are prone to photochemical redox reactions, are more susceptible to fading (or discoloration). Bleaching of a pigment is a reduction in the saturation of its color.
Changing hue and darkening are changes characteristic of pigments of inorganic origin. Pigments containing metal cations, such as Hg, Pb, CrO42- anions, etc., most often darken and change their shade. Color change can be reversible and irreversible. The phenomenon of reversible color change is called phototropy and is characteristic, for example, of the white lithopone pigment. When exposed to light, the pigment darkens, because ZnS is photochemically decomposed and Zn2+ is reduced to metallic zinc. The equilibrium of this reaction is reversed in the dark, and the pigment becomes lighter.
The lightfastness of the pigment is determined in artificial light or in sunlight (natural light).