Improved Lignin Polyurethane
Properties with Lewis
Acid Treatment
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Abstract
Chemical modification strategies to improve the mechanical
properties
of lignin-based polyurethanes are presented. We hypothesized that
treatment of lignin with Lewis acids would increase the concentration
of hydroxyl groups available to react with diisocyanate monomers.
Under the conditions used, hydrogen bromide-catalyzed modification
resulted in a 28% increase in hydroxyl group content. Associated increases
in hydrophilicity of solvent-cast thin films were also recorded as
evidenced by decreases in water contact angle. Polyurethanes were
then prepared by first preparing a prepolymer based on mixtures of
toluene-2,4-diisocyanate (TDI) and unmodified or modified lignin,
then polymerization was completed through addition of polyethylene
glycol (PEG), resulting in mass ratios of TDI:lignin:PEG of 43:17:40
in the compositions investigated here. The mixture of TDI and unmodified
lignin resulted in a lignin powder at the bottom of the liquid, suggesting
it did not react directly with TDI. However, a homogeneous solution
resulted when TDI and the hydrogen bromide-treated lignin were mixed,
suggesting demethylation indeed increased reactivity and resulted
in better integration of lignin into the urethane network. Significant
improvements in mechanical properties of modified lignin polyurethanes
were observed, with a 6.5-fold increase in modulus, which were attributed
to better integration of the modified lignin into the covalent polymer
network due to the higher concentration of hydroxyl groups. This research
indicates that chemical modification strategies can lead to significant
improvements in the properties of lignin-based polymeric materials
using a higher fraction of an inexpensive lignin monomer from renewable
resources and a lower fraction an expensive, petroleum-derived isocyanate
monomer to achieve the required material properties