Chemistry of optical whitening agents and their activation for better performance in paper coating

Besides physical properties of paper, its appearance in terms of brightness, whiteness and shade is of primary concern to papermakers as well as to end users. The common methods to increase the brightness and whiteness of paper include bleaching of pulp to higher level, addition of high bright fillers/pigments and optical whitening agent (OWA) to the paper. It is well known that OWA plays a significant role in improving the optical properties of the paper. The result of the study revealed that sulphonic groups present in the OWA also affect the optical properties of coated paper. The optical properties of coated paper are directly proportional to the number of sulphonic groups present in the OWA. For manufacturing coated paper through multilayer coating and to have better effect of OWAit is better to add it in top-coating colour formulation rather than in under-coating formulation. The result also revealed that increased dosage of OWAs improved the optical properties upto greening point but it is not possible to further improve the same. Addition of OWA carrier increases the brightness and whiteness of coated paper beyond the greening point and the efficacy of OWAcould be increased several times if used along with carrier/activator.