Fully biomass-based biodegradable polymers from lignin and raw castor oil: lignin-graft-castor oil

Abstract

Lignin is the second most abundant renewable biomass polymer. Although lignin has great potential as a sustainable raw material due to its abundance, low price, and high aromatic concentration, lignin's complex and poorly defined chemical structure and excessive reaction sites limit its applications. Castor oil is a natural vegetable oil that can be converted to many useful chemicals, such as ethylene brassylate monomer. Although previous studies considered castor oil-derived ethylene brassylate to be a sustainable raw material, pristine castor oil has not been directly used to synthesize a sustainable polymer. In this work, we report the synthesis and structure–property relationships of a novel 100% biomass-based polymer, lignin-graft-castor oil. To prepare the new polymer, natural lignin was chemically modified by a single step with succinic acid and acetic acid. The acetic acid caps the hydroxyl groups on lignin with acetyl groups to decrease unnecessarily excessive reaction sites (–OH). A carboxylic group on succinic acid provides chemical functionality to link lignin to castor oil. The castor oil was used in its pristine form as a vegetable oil without any modification. These two biomasses were covalently linked through esterification in the presence of carbodiimide to prepare a graft copolymer, lignin-graft-castor oil. The synthesized series of copolymers showed a range of melting temperatures (57–71 °C) and glass transition temperatures (39–60 °C) that enabled thermal processing. The mechanical properties can be controlled by changing the crosslinking density and the mass ratio of lignin and castor oil. The copolymer's highest modulus was 228.43 MPa, which is 40% higher than that of commercial low-density polyethylene (LDPE). Furthermore, its physical properties, including density and water contact angle, were precisely controlled by the mol% of the capping agent and the mass ratio of capped lignin and castor oil. The lowest and highest densities were 0.96 g mL⁻¹ and 1.14 g mL⁻¹, respectively; the lowest and highest contact angles of water on the surface of lignin-graft-castor oil were 70.5° and 92.9°, respectively. The synthesized fully biomass-based polymer, lignin-graft-castor oil, can be a genuine solution to plastic waste because it is a resource of 100% biomass with minimal chemical modification to form an ester linkage that is readily degradable under natural conditions. In addition, precisely and easily controllable properties enable customized applications in various fields.