**4. Conclusions**

In this study, a bifunctional epoxy precursor was functionalized with a rubber phase. Then, self-healing molecules were added to the rubber-toughened bifunctional epoxy resin to confer it auto-repair function. The functionalization process allows obtaining elastomeric domains not exceeding 500–600 nanometers. After the curing cycle of the formulated selfhealing samples, the effect of the temperature of epoxy precursor functionalization on the properties of the samples was evaluated in terms of self-healing efficiency, DMA, and TGA. It was found that the higher functionalization temperature of 160 ◦C can better promote the reaction between the rubber phase and the epoxy precursor during the functionalization process. The more efficient interactions affect the resin structure and its thermal and mechanical properties. Self-healing efficiency analysis highlights the crucial role of functionalization temperature in increasing self-healing ability. Samples functionalized at 160 ◦C manifest healing efficiency higher than 69%. The highest value (88%) was detected for the sample with DBA filler. The healing efficiency of the same sample functionalized at 120 ◦C decreases to 52%. Results from DMA evidence that a lower value of Tg (observed for the sample functionalized at the higher temperature of 160 ◦C) allows for obtaining the highest healing efficiency. Thermal stability in air higher than 250 ◦C was observed for all formulated samples.

**Supplementary Materials:** The following supporting information can be downloaded at https:// www.mdpi.com/article/10.3390/aerospace10050476/s1: Figure S1: Dimensions of the TDCB geometry specimen (on the left); EP-R-160 sample located in the INSTRON instrument (on the right). Numerical values of the lengths are expressed in mm; Figure S2: DSC curves of the samples EP-R-160, EP-R-160- DBA, EP-R-160-T, EP-R-160-M, before the curing process (after the functionalization reaction) and after the curing process; Figure S3-1: Photos of the self-healing fillers after incorporation into the Gurit precursor ECC at room temperature; Figure S3-2: Photos of the self-healing fillers before incorporation into the Gurit hardener MHHPA; Figure S3-3: Photos of the self-healing fillers after incorporation into the Gurit hardener MHHPA; Figure S3-4: Visualization of the complete solubilization of the self-healing fillers in the Gurit hardener MHHPA; Figure S4: (a) Load-Displacement curves for the sample Ep-R-120-DBA; and (b) histogram illustrating the healing efficiency values for the samples Ep-R-120-DBA and EP-R-160-DBA.

**Author Contributions:** Conceptualization, methodology, supervision, project administration, writing—original draft, writing–review and editing L.G.; Formal analysis; investigation M.R., C.N., R.L. and A.S.; Resources M.C.; Conceptualization, formal analysis, data curation, writing–original draft, investigation, writing–review and editing E.C. and L.V. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research leading to these results was funded by the European Union's Seventh Framework Programme for Research, Technological Development and Demonstration, under Grant Agreement N 313978.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
