Polyurethane (PU)
coatings with high lignin content and tunable
properties were made using a combination of fractionation and partial
catalytic depolymerization as a novel strategy to tailor lignin molar
mass and hydroxyl group reactivity, the key parameters for use in
PU coatings. Acetone organosolv lignin obtained from pilot-scale fractionation
of beech wood chips was processed at the kilogram scale to produce
lignin fractions with specific molar mass ranges (Mw1000–6000 g/mol) and reduced polydispersity.
Aliphatic hydroxyl groups were distributed relatively evenly over
the lignin fractions, allowing detailed study of the correlation between
lignin molar mass and hydroxyl group reactivity using an aliphatic
polyisocyanate linker. As expected, the high molar mass fractions
exhibited low cross-linking reactivity, yielding rigid coatings with
a high glass transition temperature (Tg). The lowerMwfractions showed increased
lignin reactivity, extent of cross-linking, and gave coatings with
enhanced flexibility and lowerTg. Lignin
properties could be further tailored by lignin partial depolymerization
by reduction (PDR) of the beech wood lignin and its high molar mass
fractions; excellent translation of the PDR process was observed from
laboratory to the pilot scale necessary for coating applications in
prospective industrial scenarios. Lignin depolymerization significantly
improved lignin reactivity, and coatings produced from PDR lignin
showed the lowestTgvalues and highest
coating flexibility. Overall, this study provides a powerful strategy
for the production of PU coatings with tailored properties and high
(>90%) biomass content, paving the path to the development of fully
green and circular PU materials.