Abstract
In this study, we investigated the effects of the number of white water recirculation rounds on
the whitening performance of added optical brightening agents (OBAs). The results could be useful
in practical mill application to determine the variability estimates when using OBAs in printing
and writing paper production. The variables of this study included OBA types [disulfo-OBA (2SOBA)
and tetrasulfo-OBA (4S-OBA)], dosage (0, 0.3, 0.6, 1.2, and 2.4% of the dry pulp), and the
number of white water recirculation (0~7) rounds. The particle charge (PCD), pH, OBA retention,
etc. of the white water samples were tested. The brightness, apparent whiteness, and ash content of
the handsheets prepared from each round were tested. Results of this study indicated that during the
stock preparation process, the bleached hardwood kraft pulp (BHKP) and calcium carbonate filler
did not notably affect the electrical charge of white water; while the addition of OBA and cationic
starch modified the white water charge balance. The addition of 4S-OBA significantly influenced
the PCD; while the effect of adding 2S-OBA was less significant. Adding either OBAs had no appreciable
effect on the white water pH value. In the 2S-OBA group, the OBA dosage and number
of white water recirculation rounds had little effect on the PCD of the white water; whereas in the
4S-OBA experimental group, with an increasing dosage and number of white water recirculation
rounds, the PCD moved toward more-negative charges. The recirculation retention rates of the 2SOBA
and ash (filler) increased with its dosage and the number of white water recirculation rounds;
those of the 4S-OBA, however, decreased with increasing dosage and number of white water recirculation
rounds. Under the same dosage, the 4S-OBA tended to have a better whitening effect than
the 2S-OBA. Also, the tonal stability of the 4S-OBA was superior to that of the 2S-OBA.