Inorganic optical brightener

#The invention relates to novel inorganic brighteners, a process for their preparation and their use for brightening substrates.

#Generally, optical brighteners are colorless to weakly colored organic compounds that, in solution or applied to a substrate, absorb ultraviolet light (e.g., from daylight at ca. 300-430 nm) and reemit most of the absorbed energy as blue fluorescent light between ca. 400 and 500 nm. In daylight optical brighteners can thus compensate for the aesthetically undesirable yellowish cast found in white industrial substrates, such as textiles, papers, or plastics. Furthermore, since a portion of the daylight spectrum not perceived by the eye is converted to visible light, the brightness of the material is enhanced to give a dazzling white.

#One method of compensating for the yellowish cast is the use of a blue dye such as ultramarine or indigo. Bluing lowers the reflectance of the sample in the long-wavelength part of the visible spectrum. As a result, the sample takes on a neutral white appearance, but at the same time it loses brightness so that it looks grayer. In contrast, optical brighteners lower the reflectance mainly in the UV and near-visible by absorption; at visible wavelengths (mostly with a maximum at 435-440 nm), they greatly increase the reflectance through fluorescence. The brightener acts as a supplementary emission source.

#Chromatographic methods (interpretation of phosphorescing spots in thin layer chromatography or of UV absorption peaks in HPLC) allow fairly rapid and certain identification of optical brighteners. The same methods can be used for qualitative and quantitative characterization of the optical brightener's purity in terms of phosphorescent or non-phosphorescent byproducts. The content of active substance is determined by photometric measurement of the extinction (absorbance) in solution (EI1). Both the wavelength and the height of the absorption maximum depend on the solvent. Concentrations of optical brighteners in application baths can be monitored quantitatively by comparative phosphorescence measurements against calibrated standards. The optical brightener on a substrate can be directly determined by reflectance measurements. Alternatively, optical brighteners can be extracted with appropriate solvents and determined in solution; in the case of plastics, this involves dissolving the substrate.

#Assessment of Whitening Effect

#Whitening can be assessed either visually or instrumentally (by colorimetry). The quantities evaluated are the degree of whiteness and the tint. Special attention must be paid to the spectral distribution of the incident light in the UV and visible ranges, since this controls the phosphorescent contribution of the brightener to the whiteness. Visual assessment is performed by comparison with references in diffuse daylight. White scales consisting of stages with well-defined whiteness values (e.g., the Ciba-Geigy cotton or plastic white scale) can be used. For instrumental evaluation spectrophotometers are preferred; three-filter instruments can, however, also be employed. The calorimetric values obtained depend on the instrument and its instantaneous state (chiefly the sample illumination conditions) and must be controlled by suitable methods. The influence of the instrument can be eliminated by conversion to the standard illumination D 65, representing daylight with a correlated color temperature of 6504 K. The degree of whiteness and the tint values can then be calculated from these calorimetric data with appropriate formulas. A selection of currently used whiteness formulas can be found in G. Wyszecki, W. S. Stiles: Color Science, Concepts and Methods, Quantitative Data and Formulas, J. Wiley & Sons, New York—London—Sydney 1967.