Self-healing materials represent a class of smart systems designed to repair damage autonomously or non-autonomously, restoring mechanical integrity and functionality while improving durability [1-3]. These materials can be classified as follows: (i) intrinsic systems, which facilitate multiple healing events via reversible chemical bonds or the incorporation of a thermoplastic phase; and (ii) extrinsic systems, which enable a single healing event by releasing an encapsulated healing agent into cracks [1-4]. Self-healing activation can be autonomous or non-autonomous. Autonomous activation is triggered by the damage itself (e.g. capsule rupture), while non-autonomous activation is initiated by external stimuli such as temperature, light, magnetic or electrical fields [2,3]. Self-healing materials offer a novel way of extending the lifespan of components that are usually the weakest part of large structures, such as the structural adhesives, mostly epoxy based, used in wind turbine blades [4]. Although the incorporation of thermoplastic healing agents into thermosetting polymers has been studied, developing structural adhesives that can deliver both high mechanical performance and effective self-healing in a single material remains challenging [3,5].

In this context, the present study investigated a new type of thermally mendable structural adhesive consisting of an epoxy resin (EP) as the structural matrix, a cyclic olefin copolymer (COC) as the thermoplastic healing agent, and carbon nanotubes (CNTs) as the conductive nanofiller. Different EP and COC formulations (i.e. from 15 wt% to 30 wt%) were investigated, as were different CNT contents (i.e. from 0.1 phr to 1.0 phr), which were dispersed in the epoxy matrix. The adhesives were characterized in terms of their rheological, morphological, thermal and mechanical properties. Finally, thermal healing was assessed at different temperatures, i.e. 145 °C and 175 °C, as the recovery of the adhesives fracture toughness. Furthermore, single-lap bonded joints were prepared by applying an optimized formulation of the obtained adhesives between two composite adherends. These were then tested using lap shear tests, with the optimized mending temperature being used to activate the healing mechanism.

References

1- Z. P. Zhang, M. Z. Rong, M. Q. Zhang Prog. Polym. Sci. 2023, 144, 101724.

2- A. Kontiza, I.A. Kartsonakis Polymers 2024, 16, 2115.

3- E.K. Kumar, S.S. Patel, V. Kumar, S.K. Panda, S.R. Mahmoud, M. Balubaid Arch. Comput. Methods Eng. 2023, 30, 1041.

4- L. Mishnaevsky Jr. Materials 2022, 15, 2959

5- F. Zhang, L. Zhang, M. Yaseen, K. Huang J. Appl. Polym. Sci. 2021, 138, 50260.

Acknowledgments

The authors acknowledge the financial support of the European Union - Next Generation EU - PNRR, Mission 4 Component 2, Investment 1.3 - PE MICS Spoke 5 - LOLIMAR Project (PE00000004, CUP D43C22003120001).