Design and Develop Lignin based Recyclable Copolymers for Hydrophobic Coatings

Due to the abundance, renewability, biodegradability, overall hydrophobicity, good compatibility with cellulose, and anti-UV/oxidant abilities, lignin has great application potentials in hydrophobic coatings on cellulose-based substrates. However, lignin's structural heterogeneity and rigidity challenge its value-added utilization. Herein, Kraft lignin (KL), from paper mills, is fractionated into more homogeneous fractions (FL), nanosized into lignin micro-nanospheres (LMNS), chemically modified and copolymerized with other constituents to fabricate hydrophobic coating materials with improved coating performances.

To investigate structure-property relationships of lignin-based copolymers, solvent fractionation is conducted to obtain FLs with different molecular weight (MW) and hydroxyl (OH) contents to prepare copolymers by integrating with ɛ-caprolactone (ɛ-CL), oleylamine (OAm), and bis(3-aminopropyl)-terminated-poly(dimethyl siloxane) (PDMS), respectively. Results show that 1) KL and FLs are successfully copolymerized with ɛ-CL via ring-opening polymerization (ROP) to form lignin-grafted PCL copolymers (lignin-g-PCL), where high-MW FL favors the ROP of ɛ-CL. 2) LA-modified KL (MKL) and FL (MFL) are successfully copolymerized with OAm and PDMS to form lignin-OAm grafted and lignin-PDMS crosslinked polyimines, respectively, via Schiff-base reactions, where low-MW FL favors the copolymerization with amines. 3) The lignin-OAm polyimines can be self-repaired and reprocessed, while lignin-PDMS polyimines can be repaired with assistance. 4) Lignin incorporation improves the hydrophobicity, thermal stability, and UV-blocking abilities of synthesized lignin-based copolymers, benefiting the further applications.

The prepared copolymers are coated on cellulose-based substrates including paper, wood, and fabric for surface hydrophobization. Results show that: 1) Lignin-g-PCL coated filter paper separates hexane-, petroleum ether-, and chloroform-water mixtures with outstanding separation efficiencies up to 99% and excellent reusability. 2) Lignin-PDMS polyimines exhibit good coating hydrophobicity, adhesion, and removability on printing paper. The hydrophobicity is further enhanced by LMNS surface roughening. 3) Robust superhydrophobicity is achieved on the cross-section of the wood block (WB-C) with water contact angle up to 173° and roll-off angle low to 5°, which is mechanically, chemically, and water stable. 4) The polyimines-LMNS coatings improve the stiffness and tensile strength of the paper and improve the stiffness and ductility of the fabric.

These studies are expected to provide new insight into lignin-based polymerization and lignin valorization for hydrophobic coatings.