Paper Coating Optical Brightening Agents (OBA)
The brightness of paper and board has increased dramatically during recent years. The brightness of pulp, fillers and coating pigments is not high enough to reach these brightness targets. Therefore there is a need to use a coating additive called an optical brightener. These products are also known under the names optical whitening agents, optical bleaching agents, or fluorescent whitening agents (FWA).
Fluorescence is a phenomenon where the molecules of a fluorescent substance become electronically excited by absorbing light energy and then emit this energy at a higher wavelength. Fluorescence is usually restricted to compounds with large conjugated systems containing p-electrons. Most of the OBAs on the market are derivatives of bis(triazinylamino)stilbene. Only the trans-isomer exhibits strong fluorescence, the cis-isomer is nonfluorescent. The OBAs used in the paper in¬dustry are natrium salts and are thus water soluble.
There are three types of optical brighteners used in the paper industry, all based on the stilbene molecule. The main difference is the number of solubilizing sulfonic groups. Disulfonated OBAs have two sulfonic groups; the two other substituents could be hydrophilic groups. This OBA has a very good affinity but limited solubility and is mostly used in the wet-end. The most commonly used OBAs are the tetrasulfonated types. Tetra-sulfonated OBAs are versatile products because of their characteristics of medium affinity and good solubility. They can be used in most applications in the paper industry: wet-end, size-press, and coating. The hexasulfonated OBAs are special¬ties used mostly in coatings where high brightness is required.
OBAs absorb ultraviolet radiant energy at 300–360 nm and re-emit the energy in the visible range, mainly in the blue wavelength region. This increases the amount of light emitted, resulting in higher brightness or whiteness. Because the reflected light is bluish, the yellow shade of paper is compensated, contributing to making the paper look still whiter. Whiteness is defined as the measured reflectance of light across the visible spectrum including color components. Brightness again is defined as the reflectance of light at the wavelength 457 nm without color in the measurement. To measure the brightness or whiteness of paper and board con¬taining OBAs requires an instrument having a known amount of UV in the illumi¬nation.
The test method used increasingly is the CIE whiteness (SCAN method P66) instead of the traditional ISO brightness, which does not define the illumi¬nant. An increase in OBAs at lower concentrations results in an increase in white¬ness • As the concentration goes up to 1.5 parts of dry pigment there is no more gain in whiteness when adding more tetrasulfonated OBA. This is called the sat¬uration point or the graying point. The hexasulfonated OBA actually has no gray¬ing point because of its high solubility.