**1. Introduction**

Self-healing materials can repair themselves and recover integrity using the resources inherently available or healing mechanisms activated during microfractures. The autorepair process can be autonomic or externally assisted, but it is always triggered by damage to the material. The integration of the self-healing functionality in thermosetting resins is driven by the need to reduce the environmental impact and the speed of resource depletion. This can be achieved by consistently reducing starting materials (primary resources) and, as a consequence, energy consumption and CO2 emissions into the atmosphere. Together with the atmosphere decarbonization contribution, the possibility to auto-repair polymeric materials significantly impacts the speed reduction of producing end-of-life plastic wastes. A strong impact on cost reduction is also expected, deriving from the longer life of materials and reduced demand for resources necessary to produce new ones. Ultimately, all this would translate into a high environmental impact reduction and energy sustainability promotion. Furthermore, together with the reduction of maintenance operations on composite components, the possibility to guarantee the structural integrity of components in spaces inaccessible for maintenance operations is strongly felt in aeronautics and aerospace.

**Citation:** Vertuccio, L.; Calabrese, E.; Raimondo, M.; Catauro, M.; Sorrentino, A.; Naddeo, C.; Longo, R.; Guadagno, L. Effect of Temperature on the Functionalization Process of Structural Self-Healing Epoxy Resin. *Aerospace* **2023**, *10*, 476. https:// doi.org/10.3390/aerospace10050476

Academic Editor: Khamis Essa

Received: 12 March 2023 Revised: 11 May 2023 Accepted: 16 May 2023 Published: 18 May 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

The studies carried out until now on auto-repair polymeric materials have resulted in innumerable healing designs that recently are focused on complex systems capable of supporting multiple cycles [1–10].

Self-healing polymers can be divided as extrinsic and intrinsic [8,11]. Extrinsic polymers are based on auto-repair processes depending on external healing agents in micro/nano vessels (generally in the form of microcapsules o vascular channels). The healing agents are released to seal the damaged regions.

Capsule-based self-healing systems retain the healing agent in microcapsules. When the damage occurs and propagates in the material, the microcapsules are cracked and the self-healing mechanism is triggered, leading to a local healing event. The main systems involve the encapsulation of a liquid healing agent and the dispersion of a catalyst, active in the ring-opening metathesis polymerization (ROMP), inside the polymeric matrix [12–15]. This strategy was also used to impart self-healing features to an epoxy resin modified with CTBN rubber for application as an adhesive by Henghua Jin et al. [16]. In particular, the authors developed a toughened epoxy adhesive where self-healing is achieved via embedded microcapsules containing dicyclopentadiene monomer and Grubbs' catalyst. Vascular self-healing materials sequester the healing agent in one, two, or three-dimensional networks consisting of capillaries or hollow channels [17–20].

Intrinsic self-healing polymers are those in which the reversible bonds (dynamic covalent and noncovalent bonds) active in the material can restore the integrity of the polymer after a damage event [11,21].

Therefore, these self-healing polymers are based on the inherent reversibility of bonding of the matrix polymer. For this typology of self-healing polymers, healing events can be achieved by reversible covalent reactions [22–33], the presence of a dispersed meltable thermoplastic phase [34,35], ionomeric coupling [36–39], via molecular diffusion [40–42] or hydrogen bonding [36–39,43,44]. The intrinsic self-healing materials are less complex than capsule-based and vascular self-healing materials, avoiding the problems related to healing-agent integration, compatibility, catalyst stability, etc. [13,45]. However, most self-healing intrinsic systems have mechanical and chemical properties incompatible with those required by structural applications.

Thermosetting resins, with their combination of thermal stability, performance, and chemical resistance, are extensively used in industry. However, integrating self-healing functionality into these materials is very difficult due to their irreversible network structure and low chain mobility, which impede the chain flow necessary for the common self-healing process [46].

To address this challenge, different approaches have been experimented in literature [46]. In previous works the authors have proposed an intrinsic self-healing system consisting of an epoxy resin covalently modified by a rubber phase containing self-healing fillers [47,48]. The presence of the elastomer has allowed reducing the rigidity of the epoxy chains and promoting the activation of an auto-repair mechanism based on hydrogen bonding interactions. Furthermore, rubber-toughened thermosetting resins manifest several advantages compared with the unmodified resin. Ricciardi et al. [49] used nitrile rubber to toughen glass fiber reinforced EP composites. They found that the modified composites showed smaller delamination, although the absorbed energy was the same and the load was higher. Karger-Kocsis and Friedrich analyzed fatigue crack propagation of carboxylterminated acrylonitrile-butadiene rubber (CTBN) and silicon rubber (SI) modified Epoxy resin [50,51]. The incorporation of CTBN and/or SI dispersion in the EP matrix improved the resistance to fatigue crack propagation.

In order to not significantly alter the mechanical properties of the resin, intended for structural applications (such as aerospace or automotive fields), different fillers or nano-fillers have been solubilized/dispersed into the epoxy matrix [47,48].

This work focuses on the effect of the curing temperature of a rubber-toughened bifunctional epoxy resin filled with self-healing molecules to impart an auto-repair function to the resin. FT/IR analysis and dynamic mechanical analyses (DMA) were used to study the functionalization of the epoxy precursor and to estimate the amount of the elastomeric phase bonded to the epoxy precursor for different temperatures of functionalization. A more significant amount of rubber phase bonded to the matrix was found for the functionalization performed at the higher temperature of 160 ◦C compared to that conducted at 120 ◦C. Thermogravimetric analysis was employed to evaluate the beginning of the degradation temperature for all the formulated systems. Self-healing tests performed through the tapered double cantilever beam (TDCB) evidence a higher healing efficiency for the selected system functionalized at 160 ◦C, containing the healing molecules. A synergistic effect between the interactions determined by the active self-healing fillers and those due the presence of elastomeric domains bonded to the epoxy precursor has been hypothesized.
