Lignin-Based Covalent Adaptable Network Polymers─When Bio-Based Thermosets Meet Recyclable by Design

Abstract

Covalent adaptable networks (CANs) play an important role in polymer chemistry, as they provide an innovative link between thermoplastics and thermosets. The breakthrough idea behind CANs is to at least partially replace irreversible crosslinks in classic thermoset polymers with dynamic covalent bonds that allow for reversible polymer character and recyclability. Besides, CANs also offer other popular features such as self-healing, weldability, configurability, and shape memory. Most CANs are still petroleum-based, yet shifting toward more sustainable approaches is of the utmost interest. Considering this and the high abundance of lignocellulosic biomass, this perspective focuses on all the research found on lignin-based CANs, including both those starting from fractionated lignin and from lignin-based monomers. This is clearly a new branch within bio-based CANs that holds great potential in various industries. Additionally, some examples of thermoset polymers derived from the same lignin building blocks are given to showcase important chemical transformations that can be used for CANs design in the future. Although CAN design has been extended to many different types of bonds, imines and disulfides largely prevail in the current literature. Lastly, a SWOT (strengths, weaknesses, opportunities, and threats) analysis is presented, considering the performance, competition, opportunities, and drawbacks of lignin-derived CANs.

Organosolv biorefinery: resource-based process optimisation, pilot technology scale-up and economics

“…For example, lignins having a high content of carboxylic acids can be used for the preparation of polymers such as polyesters, and lignin with a high content of hydroxyl groups can be used for the preparation of polymers having covalent adaptable networks to improve the recycling of thermoset materials such as resins. 147,148 It is important to underline that the preparation of such materials is possible thanks to the purity and limited dispersity of the organosolv lignin. Otherwise, the lignin must be purified and fractionated.…”

Recycling of Epoxy Resins with Degradable Structures or Dynamic Cross-Linking Networks: A Review

“…In the 140 years since the discovery of the Schiff base structures, a large number of related research works have been reported, and in recent years it has become one of the hot research topics due to researchers’ attention to degradable materials. In addition to the aforementioned properties, compounds used for synthesizing Schiff base are gradually being replaced by biobased materials, among which lignin vanillin derivatives are widely used. , …”

“…In addition to the aforementioned properties, compounds used for synthesizing Schiff base are gradually being replaced by biobased materials, among which lignin vanillin derivatives are widely used. 117,118 2.4. Disulfide Bonds.…”

Covalent Adaptable Networks with Tailorable Material Properties Based on Divanillin Polyimines

“…Also, due to the fast kinetics of the exchange reactions taking place in the network, self-healing behavior and fast stress relaxation has been observed in this class of CANs . In addition, the biobased pool of aldehydes is rather extensive, and includes vanillin, − syringaldehyde, , furfural, − and functionalized lignin. − Imine formation is a well-known reaction in which a primary amine reacts with an aldehyde, yielding a CN moiety, with water as the sole byproduct . Taken together, imine-based CANs represent a promising platform as a substituent for nonrenewable materials since dynamic covalent chemistries allow closed-loop recyclability .…”