Abstract
The purposes of this study were to use fluorescent optical brightening agents (OBAs) to study
the effect of the bleached chemi-thermomechanical pulp (BCTMP) content in papermaking stock
on the optical properties of the resulting paper, and find means to improve problems associated
with BCTMP use, such as paper yellowing and the tendency for such paper to have lower brightness
and apparent whiteness. The OBAs used were commercial di- (2S) and tetra-sulfonic (4S)
types which were added to pulps with different BCTMP contents. The optical properties, including
brightness, apparent whiteness, CIE L*a*b* values, and light-fastness, of the resulting handsheets
were then tested. The results indicated that the 2S-OBA group exhibited greater efficacies with
regard to brightness and apparent whiteness than the 4S-OBA group. At an OBA dosage of < 1.0%
and a BCTMP content of 10%, the whiteness-enhancing effect was maximized. At OBA dosages
of > 1.0%, however, 15% of BCTMP in the furnish yielded the best effect. The 2S-OBA tended to
shift the paper color more toward the green-blue axis than did the 4S-OBA. The saturation point
occurred when the 2S-OBA dosage reached 1.0~1.5%; and for 4-S OBA it was 1.5%. Yellowing of
paper tended to increase with an increasing BCTMP content. In BCTMP-containing paper, adding
OBAs decreased the yellowing trend which was proportional to the OBA dosage. Yellowing was
more moderate with the 2S-OBA than the 4S-OBA with regard to apparent whiteness; however; for
brightness measurements, both OBA groups had similar yellowing trends. Summarizing the above
results, the 2S-OBA was more suited to BCTMP-containing furnishes.