*Electrically Conductive Self-Healing Epoxy Composites for Flexible Applications: A Review DOI: http://dx.doi.org/10.5772/intechopen.1003037*

dual functionality: enabling electric current conductivity while mechanically bonding diverse substrates. Typically, conductive fillers like Ag, Au, Ni, carbon materials, or MXene are integrated into acrylic, silicone, or epoxy resins to create ECAs. Notably, epoxy resin-based ECAs have gained significant traction in electronics, serving purposes such as die attachment and solderless interconnection. This popularity stems from their exceptional attributes: a blend of chemical and thermal resistance, robust mechanical qualities, strong adhesion, and compatibility with various substrates and additives.

Of remarkable significance are epoxy-based ECAs with self-healing capabilities, which can autonomously repair minor damage in the conductive pathways of the adhesive matrix. These properties hold great promise for industries like flexible electronics, wearables, and automotive electronics, offering advantages through self-healing conductive adhesives. Their pliable nature makes them suitable for scenarios where traditional rigid soldering techniques are impractical. An example by Zhang et al. [63] involves a novel epoxy resin synthesized using the curing agent 1,4,5-oxadithiepane-2,7-dione (DSAA), containing reversible disulfide bonds. This epoxy resin demonstrated selfhealing and malleability, showing potential as a reusable adhesive. However, effective self-healing required a relatively high level of DSAA (22.7 wt%).

Advancements in "green chemistry" aim to develop non-petroleum-based matrices for self-healing fiber-reinforced composites. Hu's group [64] obtained fully biobased vitrimers from camphoric acid (CPA) and epoxidized soybean oil (ESO), using them as matrices for a carbon fiber-reinforced composite (**Figure 4**). Network rearrangements via transesterification reactions (TERs) provided CF/CPA/ ESO composites with reprocessing, self-adhesion, and repair capabilities at elevated

#### **Figure 4.**

*Depictions showcasing the dynamic crosslinking rearrangements within CPA/ESO networks, and the creation of a composite laminate involving carbon fiber-reinforced composite. Reproduced from Ref. [64] with permission from the Royal Society of Chemistry.*

temperatures (200°C). Carbon fibers could be fully recycled by degrading CF/CPA/ ESO composites with ethylene glycol. In another study, Xu and colleagues [65] developed carbon fiber reinforced composites utilizing a degradable bio-based epoxidized menthane diamine-adipic acid (EMDA-AA) matrix. Notably, the EMDA-AA vitrimer exhibited topological rearrangement through dynamic transesterification reactions, facilitated by autocatalysis of tertiary amines, even in the absence of a catalyst. These developed vitrimers displayed self-healing behavior at 180°C for 30 mins (healing efficiency = 92.9%) and shape memory at 100°C, applicable to both EMDA-AA and EMDA-AA-CF systems. The reported epoxy vitrimer-based composite showcased commendable adhering properties (lap shear strength = 3.8 MPa) when two EMDA-AA-CF composites were bonded under a hot press at 180°C and 15 MPa.

Zhang et al. [66] introduced a fully bio-based epoxy vitrimer for recoverable adhesives, utilizing ozonated lignin and sebacic acid-derived epoxy cured with a zinc catalyst (Zn(acac)2). This adhesive achieved a lap shear strength of 6.5 MPa when applied to coarse aluminum sheets. Moreover, separated halves of aluminum plates could be rebonded using catalyst-accelerated transesterification at 190°C for 1 h, a feat not achievable with traditional adhesives. This indicates potential for vitrimers as self-healing and recoverable adhesives from bio-based materials. Beyond practical advantages, these adhesives align with eco-friendly principles and cost-saving potential, positioning them as sustainable choices for future electronics. As research and development progress, self-healing conductive adhesives hold exciting promise for revolutionizing electronics manufacturing and maintenance paradigms.

#### **3.2 Coatings**

Coating represents one of the most prevalent applications for epoxy resins. Epoxy materials are extensively employed in coatings to safeguard surfaces and provide decorative attributes, owing to their exceptional mechanical properties and chemical resistance. However, when a coating becomes damaged, its intended functions are compromised. The removal of such coatings from substrates is challenging due to their robust interfacial bonding. To mitigate waste and extend the service life, the optimal approach likely involves repairing the coating and reinstating it for use. Yet, repairing epoxy coatings is intricate due to their permanent cross-linked structure, differing from conventional thermoplastics. A recent advancement by Han and colleagues involved the development of a catalyst-free epoxy vitrimer coating, exhibiting impressive self-healing characteristics [67]. This innovative coating was formulated by curing hyperbranched epoxy (HBE) prepolymers with succinic anhydride (SA). The abundance of hydroxyl groups in HBE facilitated rapid stress relaxation and efficient self-healing at elevated temperatures (>120°C). When applied as a vitrimer layer onto metal plates (with a thickness of approximately 80 μm), this coating not only demonstrated excellent adhesion and hardness but also displayed efficient selfhealing at 150°C for 1 h. Furthermore, the healed coating effectively safeguarded the metal plates from electrolyte erosion, confirming its potent self-healing capability. Incorporating DGEBA (DER 331) into this system resulted in a repairable vitrimer coating that provided robust protection against salt and electrochemical corrosion. This type of epoxy coating holds potential for prolonging the lifespan of thermosets and reducing waste. Raspberry-like polydopamine@ polypyrrole (PDA@PPy) nanoparticles were synthesized to serve as a photothermal agent and subsequently incorporated into an epoxy resin, creating a self-healing coating [68]. Following 808 nm near-infrared (NIR) irradiation for 10 mins, the impact of varying amounts

#### *Electrically Conductive Self-Healing Epoxy Composites for Flexible Applications: A Review DOI: http://dx.doi.org/10.5772/intechopen.1003037*

of PDA@PPy on scratch closure efficiency was explored. Elevated PDA@PPy content correlated with improved healing results, although excessive quantities hindered the repair process due to restrictions on the movement of epoxy resin molecular chains. Hence, precise incorporation of photothermal fillers is pivotal for optimizing the self-healing capacity of coatings. When evaluating corrosion resistance, the linear sweep voltammetry curve illustrated that the corrosion current of a healed coating diminished comparably to an intact coating, underscoring the effective safeguarding of X70 steel.

In a research endeavor undertaken by Hao et al. [69], they carried out a study where they modified Kraft lignin using esterification alongside 4-methylcylohexane-1,2-dicarboxylic anhydride. This chemical process resulted in a novel form of Kraft lignin referred to as L-COOH. The innovative aspect of this modification was its increased solubility in ethanol. By subsequently subjecting it to a reaction with poly(ethylene glycol) diglycidyl ether, utilizing a zinc catalyst, they successfully produced a vitrimer with a notable lignin content exceeding 47 wt%. The vitrimer's dynamic exchange mechanism, primarily involving ester bonds, was triggered at elevated temperatures beyond 140°C. This temperature-induced activation facilitated the efficient repair of coatings within a brief duration of 15 mins at 190°C, with the assistance of glycol. Notably, this coating also exhibited a distinctive swelling capability when exposed to alkaline aqueous solutions. This behavior was attributed to the conversion of phenol and carboxyl groups into sodium phenolate or sodium carboxylate, which caused a transformation in the chemical nature of the coating. As a result, the bonding between the coating and the substrate was weakened, leading to easier coating removal. Liu et al. [70] introduced an example of free radical polymerization based on a dynamic transesterification network. They synthesized a UV-curable oligomer (TMG) from tung oil using microwave technology, which was subsequently photopolymerized alongside a biobased-reactive diluent derived from malic acid (MA). This strategy yielded UV-curable coatings or materials featuring multiple hydroxyl and ester groups. These materials exhibited the capacity for repair and reshaping through dynamic transesterification reactions, activated at higher temperatures with the aid of a zinc catalyst. Importantly, the tensile strengths of welded (12.2 MPa) or reshaped materials (28.7 MPa) exhibited significant enhancement compared to the original material (7.1 MPa). This enhancement was attributed to an elevated crosslinking density resulting from C〓C polymerization and esterification during high-temperature heating.

Feng and collaborators designed tung oil-loaded polyurethane (PU) microcapsules [71], with conductive polyaniline (PANI) impregnated into the microcapsule wall structure. Corrosion tests highlighted the exceptional anti-corrosion properties of the self-healing epoxy coating incorporating 10 wt% tung oil-loaded PU/ PANI microcapsules. These microcapsules, with a tung oil core and PANI wall, acted as corrosion protectors, forming a self-healing film and a passivation layer. In a recent study by Alias et al. [72], linseed oil was encapsulated for corrosion protection of magnesium (Mg). Coatings containing microcapsules were applied to a bare Mg substrate and subjected to scratching to evaluate their self-healing capacity. Electrochemical measurements in a 3.5 wt% NaCl solution demonstrated that the epoxy coating combining linseed oil and urea-formaldehyde substantially reduced the corrosion current density (icorr) on the Mg sheet (1.552 A cm−2) compared to the bare Mg sheet (109.8 μA cm−2). Consequently, these self-healing coatings exhibited notable recovery in both self-healing and corrosion resistance on Mg alloys.

In another investigation, Haddadi et al. [73] reported the creation of aminofunctionalized MXene (Ti3C2) nanosheets through an etching technique and a modification process involving 3-aminopropyltriethoxysilane. MXene's favorable interface interaction and stability with organic/polymeric materials position it as a promising anticorrosion nanofiller for enhancing the passive barrier properties of organic coatings [74]. It also serves as a suitable host for loading corrosion inhibitors [75]. Utilizing cerium (Ce3+) cations as corrosion inhibitors, MXene nanosheets encapsulating these cations were employed to produce self-healing epoxy composite coatings, exhibiting robust corrosion protection performance (**Figure 5**). In a study by Niu et al. [76], a robust omniphobic slippery coating was employed to create self-healing coatings on ultrathin MXene multilayers. Upon exposure to just 10 mins of solar irradiation, the surface temperature of the coating rapidly rose to approximately 50°C. The successful fabrication of MXene multilayers on diverse metal substrates suggests the potential for developing a range of self-healing coatings. Consequently, the utilization of photothermal activation for self-healing emerges as a pivotal element within light-responsive coating technology. Wang and his colleagues [77] introduced a novel Ti3C2Tx/layered double hydroxide (LDH) pigmented epoxy composite hybrid coating that demonstrated remarkable corrosion resistance and self-healing capabilities on AZ31 magnesium alloy. The incorporation of environmentally friendly organic acid anions from cysteine acid (Cys) as corrosion inhibitors and the growth of Ti3C2Tx/MgAl-LDH (TML) heterostructure nanosheets via in-situ assembly led to the self-healing function. The TML heterostructure displayed excellent dispersion and compatibility with epoxy resin. The composite coating exhibited exceptional corrosion resistance, with a corrosion current density of 1.4 × 10−9 A cm−2

#### **Figure 5.**

*Diagram illustrating the inhibition mechanism within the coating phase involving Ti3C2 MXene-Ce3+. In the MXene-Ce3+@EP coating containing a defect, Ce3+ ions have the potential to migrate out from Ti3C2 MXene-Ce3+ nanosheets. These ions subsequently get deposited onto the surface of the mild steel plate within the scribed zone, effectively obstructing cathodic sites. Reprinted with permission from [73]. Copyright 2021 American Chemical Society.*

### *Electrically Conductive Self-Healing Epoxy Composites for Flexible Applications: A Review DOI: http://dx.doi.org/10.5772/intechopen.1003037*

and an impedance value of 1.66 × 107 Ω cm−2 at low frequency. Furthermore, the selfhealing efficiency of the epoxy-TML composite coating reached 56.17% after 6 days of immersion in a corrosive environment.

Lorwanishpaisarn et al. [78] conducted a study with the goal of developing a self-healing epoxy vitrimer/CNT nanocomposite for use as a coating material. They employed two bio-based curing agents, namely cashew nut shell liquid (CNSL) and citric acid (CA), to create adaptable networks held together by covalent bonds. Within the epoxy/CNSL/CA matrix, they incorporated CNTs using probe sonication and agitation, spanning concentrations from 0 to 0.5 wt% (referred to as V-CNT0- 0.5). The research outcomes indicated that the thermomechanical properties of the V-CNT nanocomposites improved as the CNT content increased. The study also found that a bond exchange reaction involving esterification could be triggered by near-infrared (NIR) light. Among the tested compositions, V-CNT0.5 exhibited the highest self-healing efficiency, as evidenced by a Shore D hardness of 97.34%. To evaluate corrosion resistance, coated steel samples containing V-CNT0 and V-CNT0.5 were immersed in a 3.5 wt% NaCl solution for a duration of 7 days. In this context, the corrosion rate of steel coated with V-CNT0.5 significantly decreased from 9.53 × 102 MPY to 3.12 × 10−5 MPY. Additionally, the protection efficiency surged by 99.99%. Capitalizing on these remarkable self-healing and anti-corrosion attributes, it is evident that V-CNT0.5 holds promise as an appealing material for organic anti-corrosion coatings. In a novel achievement, a thermally robust and high-performance pH-responsive anti-corrosive nanoreservoir system was developed [79]. This system is built upon a double-ligand zinc phosphate framework (ZPF) that decorates a graphene oxide skeletal template (G-ZPF) using the ligand exchange theory. The outcomes revealed that the G-ZPF nanoreservoirs loaded with structural inhibitors offer a combination of active (self-healing) and passive (ion-water barrier) attributes within the organic coating. As a result, the epoxypolyamide coating embedded with G-ZPF nanoreservoirs demonstrated remarkable corrosion prevention capabilities and exhibited exceptional thermal and mechanical performance.
