Лако-красочные материалы — производство

Most dyes and pigments owe their colour to the selective absorption of incident light. In some compounds, colour can also be observed as a result of the emission of visible light of specific wavelengths. These compounds are referred to as luminescent. The most commonly encoun­tered luminescent effects are fluorescence and phosphorescence. The transitions which can occur in a molecule exhibiting either fluorescence or phosphorescence are illustrated in Figure 2.6. When the molecule absorbs light it is excited from the lowest vibrational level in its ground state (S0) to a range of vibrational levels in the singlet first excited state (S*). In the case of luminescent organic molecules, this is generally a n-n* electronic transition. During the time the molecule spends in the excited state, energy is dissipated from the higher vibrational levels, and the lowest vibrational level is attained. Fluorescence occurs if the molecule then emits light as it reverts from this level to various vibrational levels in the ground state. Non-radiative processes, the most important of which is generally collisional deactivation, also gives rise to dissipation of energy from the excited state. As a result, there will be a reduction in the intensity of fluorescence and in many cases it will be absent altogether. Another process which may occur is intersystem crossing to a triplet state. Emission of light from the triplet state is termed phosphorescence, a phenomenon which is longer-lived than fluorescence. As a consequence of the loss of vibrational energy in the excited state, fluorescent emission occurs at longer wavelengths than absorption, the difference between the wavelengths of maximum emission and maximum absorption for a fluor­escent compound being referred to as the Stokes’ shift. In the case of a fluorescent dye, the overall visual effect from the dye, whether in solution or when incorporated into a textile fabric, plastic material or surface coating, originates from the colour due to the selective absorption of light supplemented by the colour due to the light emitted. This gives the dye its particularly visual brilliance. Another type of compound which makes use of light-emitting properties is fluorescent brightening agents (FBAs). These are compounds, structurally closely related to fluorescent dyes,

that absorb light in the UV region of the spectrum and re-emit the energy at the lower (blue) end of the visible spectrum. When incorporated into a white substrate, such as a textile fabric or a plastic article, FBAs provide a particularly appealing bluish cast. One of the most important uses of FBAs is in washing powders to impart a bluish whiteness to washed fabrics.