Preparation of lignin-based vinylogous urethane vitrimer materials and their potential use as on-demand removable adhesives

Abstract

Conventional thermosets rely on petrochemical products and cannot be reprocessed, recycled, or reshaped under mild conditions owing to their permanent, highly crosslinked structure. Although some of these materials are down-cycled into lower-value products, most thermosets are incinerated and landfilled, which intensifies the problem of resource shortage and environmental pollution. In this work, wheat straw lignin (Protobind 1000) was used as a raw material to prepare modified lignin by a non-catalytic reaction with t-butyl acetoacetate. The modified lignin was cross-linked with a fatty acid diamine (Priamine™ 1075), and a fully biobased vitrimer material with a high lignin content (40–50 wt%) was successfully prepared. The mechanical properties of these vitrimers could be adjusted by changing the density of the cross-linked network inside the material, which is determined by the ratio of the modified lignin to diamine. In the case of elevated temperatures (>100 °C), dynamic chemical bonds (vinylogous urethane amine exchange) inside the material are activated, which allows the material to be recycled and reused. Rheology was used to study time- and temperature-dependent stress relaxation dynamics. Experimental results show that the relaxation time typically ranges from 104 s at 150 °C to 9 s at 180 °C, exhibiting rapid relaxation throughout the process without the addition of any catalyst. A potential application of this work is in the synthesis of on-demand removable adhesives. The dry shear stress of the adhesive is 12.7 MPa (170 °C, 40 bar, 0.5 h) and the wet shear strength, with a minimum strength of 3.5 MPa, is similar under different conditions. The excellent adhesive strength mainly stems from the formation of abundant hydrogen bonds and the mechanical interlocking between the adhesive and the wood. Combining the special solvent swelling behavior of the vitrimer material and the rapid exchange of dynamic chemical bonds of the material at high temperatures, we propose to use both solvent removal and heat removal methods to remove the adhesive, and the removed adhesive can be recycled. This work provides a case for biobased vitrimers to promote the development and utilization of advanced biobased thermoset materials.