Retention of optical brightening agents (OBA) and their brightening efficiency on HYP-containing paper sheets

Optical brightening agents (OBA) or fluorescent whitening agents (FWA) are commonly used in the paper industry to improve the optical performance of paper products. The increased use of bleached chemi-thermo-mechanical pulp (BCTMP) or high-yield pulp (HYP) in printing and writing paper grades has initiated research topics on the brightening efficiency of OBA on mechanical pulp-containing furnishes. In this study, process parameters that may affect the retention of OBA were investigated, such as furnish composition, OBA charge, contacting time, water hardness, anionic trash, and fines in HYP. It was found that OBA had lower retention on HYP fibers than on bleached kraft pulp (BKP) fibers. Efforts were also made to understand the underlying mechanism.