Sustainable chemically modified benzoxazine-based shape memory thermosets: Design strategies,recyclability,and prospects

Abstract

Advances in stimuli-responsive materials have led to increasing popularity due to their ability to adapt intelligently and be capable of “remembering” their original shape after adopting temporary deformed shapes in various applications. At the same time, the environmental and sustainability challenges of end-of-life (EOL) disposal for these materials are particularly concerning. This review synthesizes current knowledge on how sustainable chemistry and functional material design can be bridged by integrating waste, as an effort to reach a closed-loop circular economy, into high-performance shape memory benzoxazine-based thermosets. Both agricultural and industrial waste streams, including lignin, vanillin, eugenol, diphenolic acid, and cardanol, were systematically discussed, exploiting each unique functional characteristic, such as phenol, aldehyde, allylic, carboxyl, and alkyl groups, to be utilized as valuable target sites for chemical modifications from phenolation, esterification, amination, imination, hydrothiolation and thiol functionalization, inverse vulcanization, and direct condensation to enable the shape memory effect with a controllable end-of-life (EOL) scenario. Thermoset materials initially designed with degradable linkages can be reshaped, healed, or degraded under specific triggers and conditions, retaining their properties for several reprocessing cycles until they reach their performance limits. In contrast, networks lacking dynamic functionality usually display long-term stability during service, but they require more advanced technologies for EOL management. In such cases, methods such as catalytic oxidation can be used to recover valuable fiber and resin fragments, which typically necessitates more complex processing procedures. The choice between these recycling methods should be based on the intended use, the service environment, and the desired EOL option. Implementing the waste-to-value concept with these advanced strategies undoubtedly offers environmental benefits, but it also entails “hidden costs,” such as increased processing complexity, catalyst requirements, or purification procedures, which must be weighed carefully to avoid outweighing the advantages. We conclude by outlining key areas of future advancement, with a particular emphasis on the need for thorough life-cycle and techno-economic evaluation of these waste-derived benzoxazine-based systems to transition them from laboratory proof-of-concept to operational, predictive, and industrial-scale circular systems.

Graphical Abstract