The influence of the healing agent characteristics on the healing performance of epoxy coatings: Assessment of the repair process by EIS technique
Graphical abstract
Introduction
Polymeric materials are of broad interest as thin films when acting as a protecting layer to hamper the corrosion process, an exergonic reaction, that can detrimentally affect the mechanical properties of metallic materials resulting in failures [1,2]. However, polymers are prone to failure caused by external factors that decrease the efficacy of using them in practical applications [[3], [4], [5]]. Among all the proposed solutions for this problem, the concept of self-healing polymeric materials has gained considerable attention [[6], [7], [8], [9]]. The key mechanism in the self-healing process is stopping the crack growth (induced from an external factor) by reducing the crack-tip stress intensity by producing a wedge of polymerized healing agent. These mechanisms require the healing agent to be sufficiently polymerized after release [[10], [11], [12]]. Different strategies have been used to assure the recovery in the self-healing polymers according to their applications and use [[13], [14], [15], [16]]. The ability to retard and ultimately arrest fatigue cracks hinges on the ability of the healing chemistry to produce the polymer in the crack plane at a rate that is comparable to the rate of crack propagation [17]. Therefore, several types of research have been conducted to study the effects of healing agents' characteristics on healing kinetics and efficiency [18].
Jones and coworkers reported different healing performances for the several crystal morphologies of Grubbs' catalyst. They found that smaller catalyst particles can dissolve easier in dicyclopentadiene (DCPD) monomers and perform faster healing reactions. However, they are more susceptible to deactivation by the amine molecules present in the matrix. Their conclusions imply the necessity of having a balance between different crystal morphologies/dimensions to assure the best healing performance [19]. In another research, they investigated the effects of catalyst recrystallization and wax protection to take the advantage of faster healing kinetics without being suffered by catalyst deactivation. They achieved greater fatigue life by accelerating the healing kinetics [17]. Mauldin and coworkers studied the effects of different DCPD stereoisomers on self-healing kinetics. They found that exo-DCPD isomer is capable of healing approximately 20 times faster than endo-isomer, but with a lower healing efficiency [20]. Cromwell et al. studied the effects of healing kinetics in an intrinsic system. They used different telechelic di‑boronic ester molecules to heal 1,2-diol-containing polymer chains by crosslinking them with variable kinetics. Their research showed that the diboronic ester molecule with faster reaction kinetics can perform enhanced and accelerated healing compared to the slower one [21].
Regarding the necessity of using fast and stable healing agents to offer better performance and higher efficiency, polyurea-based extrinsic healing systems have gained attention due to their great potential. Guo et al. introduced the amine/isocyanate-reactive system as a promising healing agent for nonconventional environmental conditions [22]. Ma et al. also studied the performance of a polyurea-based self-healing system in epoxy coating via EIS tests to assess the effect of seawater immersion on the healing efficiency [23]. In one of our previous research, Koochaki et al., separately encapsulated methylene diphenyl diisocyanate (MDI) and Jeffamine D230 polyetheramine, as the two portions of the polyurea-based dual capsule healing system. The performance of the bi-component system embedded in an epoxy coating was investigated by recording impedance spectra of the scratched coated steel plate immersed in 3.5 wt% NaCl [24]. Subsequently, they studied the influence of polyetheramine modification on the healing performance of a polyurea-based dual capsule to improve the healing efficiency in wet conditions. By employing EIS as an in situ method, they revealed that grafting catechol side groups on the backbone of polyetheramine significantly enhanced the underwater healing performance of the resulting polyurea [25].
To deep understand the effect of healing agents' molecular characteristics on the performance of polyurea-based dual capsule healing systems, the focus of this research was devoted to i) the molecular weight and ii) functionality of amine-based agents. Therefore, three different kinds of commercially available polyetheramines were selected, while the nature of the isocyanate counterpart was kept constant. In this regard, bi-functional Jeffamine D400 (molecular weight = 400 g/mol) and tri-functional Jeffamine T403 (molecular weight = 440 g/mol) are used, respectively, to study the effects of molecular weight and number of active functionalities for comparison with Jeffamine D230 (230 g/mol, two functionalities) taken as reference amine agent. After encapsulation of healing agents within the SAN shell, the obtained MCs were embedded into an epoxy coating applied on carbon steel plates. The influence of the healing agent characteristics on the kinetics and performance of the healing process was evaluated by the EIS technique as well as the salt spray corrosion tests.
Section snippets
Materials
For the preparation of the self-healing coatings containing dual-capsule, the following chemicals were purchased. Poly styrene-co-acrylonitrile (SAN, used as the capsule shell material) and dimethylformamide (DMF, used as the solvent in preparing the polymer solutions) were purchased from Sigma-Aldrich. Methylene diphenyl diisocyanate-based prepolymer (CORONATE™1391), used as core material for assembling related MCs, was provided by TOSOH Corporation. Different kinds of polyetheramines (used as
Morphology of the capsules
Fig. 1 shows the SEM micrographs of the mc-D400 (mc-X: MCs containing X core) and mc-T403 and their corresponding size distribution diagrams. Micrographs (collected in secondary electron mode) confirm that the prepared MCs have a spherical shape. The mean diameters of the two types of polyetheramine-SAN MCs are measured to be 1.01 ± 0.29 μm and 0.81 ± 0.34 μm for mc-D400 and mc-T403, respectively. The mean diameters are in good agreement with the MC size of the mc-D230 (Fig. S1) [14]. The
Conclusions
As part of a broader systematic study carried out by the group, the effects of molecular characteristics of polyetheramines on i) the kinetics of the healing process and ii) the related self-healing performance were studied to obtain more efficient corrosion protection coatings that limit the detrimental effects of damages affecting the physical integrity of the polymeric barrier. A dual capsule extrinsic system was embedded into an epoxy matrix to prepare self-healing coatings that exploit the
CRediT authorship contribution statement
Mohammad Sadegh Koochaki: Conceptualization, Methodology, Investigation, Data curation, Writing – original draft, Writing – review & editing, Visualization. Rasoul Esmaeely Neisiany: Conceptualization, Methodology, Validation, Data curation, Writing – original draft, Writing – review & editing, Visualization. Saied Nouri Khorasani: Conceptualization, Validation, Resources, Writing – original draft, Writing – review & editing, Supervision, Project administration, Funding acquisition. Ali Ashrafi:
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors express their sincere gratitude to the Iranian Ministry of Science, Research, and Technology (MSRT) for the monetary support of a research visit to Università degli Studi di Milano for the accomplishment of this work. This work was also supported through a grant from the Iran National Science Foundation (INSF, Grant No. 97008660). The authors thank the UNITECH NoLimits and Dr. Nadia Santo for the SEM investigation. SmartMatLab Centre and Dr. Serena Cappelli (Department of Chemistry,
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