Under the action of sunlight, the color and light of the pigment remain unchanged, which is called the lightfastness of the pigment. There is no absolutely light-resistant pigment: under the action of sunlight, the shade of all inorganic pigments and organic pigments has varying degrees of change. Differences in the lightfastness of the individual pigments can only be seen in the timing of the onset of these changes. Some pigments lighten, fade, lose their original color to gray or mottled colors under the action of sunlight; this phenomenon is called fading (color fading). Most organic pigments can be classified as fast fading pigments. (Related instrument: Colorimeter)
Other pigments, on the contrary, darken under the action of sunlight and become dark and blurred shades, especially if they are used in oil-based film formers. Pigments that fall into this category include chrome yellow, natural cinnabar, zinc silver, etc.
The action of light can cause the pigment to fade or darken; insufficient sunlight can cause the pigment to turn yellow (especially white pigments), or cause other changes in color (such as colored pigments).
Yellowing of white coatings is mostly found in poorly lit rooms. It is often seen that the color that has changed in the indoor light and shade returns to its original color in the sun, or on the contrary, it changes color in the sun and recovers in the shade. This phenomenon is called phototropism, which is caused by the absorption of ultraviolet light or ordinary short-wave light by the object. For example, titanium dioxide loses its yellow color after it is placed in the sun, and the zinc-silver white that has become clear in the sun returns to its original white color when it is bright and clear.
In testing, the following items have a significant influence on the precision of the results obtained:
1) The change of the test temperature, because when the intensity of the ultraviolet light machine is constant, the higher the temperature, the greater the change in the color of the pigment. (Related instruments: thermometer paper)
2) Changes in humidity, because we know that the speed of photochemical reactions increases due to the presence of moisture (moisture); (related instruments: temperature and humidity meter)
3) There are wide differences in film-forming substances, for example, the light resistance of a pigment in oil is significantly different from the light resistance of the pigment in colloidal solution. Therefore, when conducting comparative tests, the same film former should always be used.
It should also be considered that the physical and chemical properties of the film-forming substance itself, such as its purity, purity and other properties, also have a major impact on the light resistance of the pigment.
It is necessary to distinguish the three kinds of light fastness of pigments, film formers and finished color paints. The light fastness of the pigment is affected by the film former, and the order in which one layer of paint is applied to another layer and the primer is also affected. See here . All these factors must be taken into account when determining the lightfastness of a pigment, and the test must always be carried out under completely defined conditions. When carrying out the test of the light fastness of colored pigments, pulp is generally not used, but wood boards, cardboards or paper sheets colored by the pigments to be tested are used. In this case glue, casein, gelatin etc. solutions or oils are used as binders.
