(44a) Kraft Lignin Color Reduction for Potential UV Absorbent Applications in Transparent and UV Protection Films

The dark brown color of kraft lignin (KL) restricts its potential utilization as
structural material or functional nanofiller in engineering plastics and
composites. Lignin in its native state in wood is nearly colorless. However, it
is transformed into the dark brown colored material during kraft
pulping process. Several efforts are discussed in literature related to lignin
color reduction; however, most of them degrade the lignin aromatic structure
reducing its UV absorption properties. The goal of this research is to improve
the scope of kraft lignin as an UV absorbent
nanomaterial in transparent and UV protecting polymer composite films. This
will be carried out through its modification for reducing color chromophores
without significantly degrading the lignin aromatic structure. The current
study will investigate the effect of peroxide and chlorite
bleaching of kraft lignin with and without blocking
of phenolic hydroxyl groups on its optical properties. Phenolic hydroxyl groups
can be blocked using acetylation or carboxymethylation of KL, where latter
eliminates the use of organic solvent during bleaching step due to the
solubility carboxymethylated KL in aquoues medium.The preliminary results of the acetylated kraft lignin (Ac-KL) and peroxide bleached kraft lignin (PB-KL) showed substantial increase in their
visible light transmission without significantly affecting UV blocking
properties (Figure.1). 0.1 wt.% solution of Ac-KL and PB-KL in dioxane/water
(0.5/0.5 v/v) showed ~160% and ~130% increase in their visible light (550 nm)
transmission compared to unmodified kraft lignin.
Moreover, the UV absorptivity of the Ac-KL and PB-KL did not change
significantly as seen from their UV-Vis spectra and extinction coefficient (Ɛ) measured
at 255 nm. Acetylation of KL replaced its phenolic hydroxyl groups with the
acetyl groups which reduced the lignin color by preventing the formation of
quinone and quinone methide chromophores. Whereas, color reduction in peroxide
bleaching of KL is attributed to the oxidation of existing chromophores to
colorless bodies. In order to obtain maximum color reduction, two-stage
modification will be employed wherein acetylated/carboxymethylated lignin will
be oxidized using peroxide bleaching. Similarly, these
modified lignin will also be treated using chlorite
bleaching. These approaches can synergistically reduce the lignin color by
reducing the chromophore forming side reactions of unmodified lignin during
bleaching and through the oxidation of the existing chromophores to colorless
bodies. Finally, transparent, UV protection films will be processed through the
casting of two-stage modified KL and polyvinyl alcohol aqueous mixtures. The
resultant films will be characterized for their optical, thermal and mechanical
properties.