Interactions
of Lignin with Optical Brightening Agents and Their Effect on Paper
Optical Properties
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Abstract
Optical
brightening agents (OBAs) are widely used in the production
of uncoated and coated paper grades to improve their optical properties.
The presence of lignin in the pulp furnishes is well-known to have
a significant effect on the OBA brightening efficiency, but how OBA
interacts with lignin is still not well understood. In this study
we used wood lignin to investigate the lignin/OBA interactions and
its effect on OBA brightening. Three lignin samples isolated from
spruce, pine, and aspen were used. Both di- and tetra-sulfonated OBAs
were studied. It was found that the OBA addition can effectively improve
the optical properties of paper, such as ISO brightness, CIE whiteness,
and <i>b</i>*, but disulfonated OBA was found to be more
effective at a lower dosage (less than 0.6%) than the tetra-sulfonated
OBA. The addition of a small amount of lignin (0.4%) onto filter paper
had negative effects on the optical properties, but the impact depends
strongly on lignin structures (lignin samples from spruce, pine, and
aspen), which explain the early results that mechanical pulps from
different wood species respond very differently to OBA brightening.
A modified Kubelka–Munk equation was used to predict and model
the brightness and whiteness response of different lignin types and
OBA, which can be used to provide guidance in determining the amount
of OBA needed to reach specified optical property target