Enhancing Mechanical, Impact, and Corrosion Resistance of Self-Healable Polyaspartic Ester Polyurea via Surface Modified Graphene Nanoplatelets

Abstract

Polyaspartic ester polyurea (PEP) elastomers are highly promising for self-healable protective coatings in industrial applications, yet their broader adoption is limited by insufficient mechanical and corrosion resistance. Herein, we develop a multifunctional PEP nanocomposite by incorporating Jeffamine D2000-functionalized graphene nanoplatelets (F-GNPs), prepared through a one-step mechanochemical process. This strategy promotes strong interfacial bonding and uniform dispersion, yielding synergistic property enhancements. At an optimal loading of 0.3 wt%, the PEP/F-GNP nanocomposite exhibited a substantial performance enhancement, with its tensile and tear strengths augmented by 263.0% and 64.2%, respectively. Moreover, the resulting coating delivered an 84.0% boost in impact resistance on aluminum alloy, along with enhanced substrate adhesion. Electrochemical and salt spray tests further confirmed its exceptional anti-corrosion performance. While the reinforcement strategy presented a classic trade-off with self-healing, it is critical to note that the nanocomposite preserved a high healing efficiency of 83.3% after impact damage. Overall, this scalable interfacial engineering strategy simultaneously enhances the material’s mechanical robustness and protective performance, while striking a favorable balance with its intrinsic self-healing capability, paving the way for next-generation coatings.

1. Introduction

Polyurea (PUA) elastomers have emerged as versatile materials for robust protective coatings across diverse applications, owing to their advantageous combination of low cost, light weight, high processability, and mechanical and chemical resistance [1,2,3]. Their exceptional performance is rooted in a unique microphase-separated architecture stabilized by extensive hydrogen bonding [4]. Within this structure, rigid “hard segments”—formed by the rapid reaction of isocyanate and amine groups—aggregate into domains that provide strength and modulus, while flexible, amorphous “soft segments” impart the requisite elasticity and toughness [5]. However, a hallmark of traditional polyurea chemistry is the extremely rapid, catalyst-free reaction that forges this structure. While ideal for industrial spray applications, this near-instantaneous gelation severely restricts the material’s use in manual processes, where it hinders proper leveling and often leads to surface defects [6].

The development of third-generation polyaspartic ester polyurea (PEP) directly addresses these kinetic challenges. At its core is the polyaspartic ester, a sterically hindered secondary diamine whose unique molecular structure—characterized by steric hindrance, electron induction, and hydrogen bonding effects—effectively tempers the isocyanate-amine reaction [7]. This extends the system’s gel time from minutes to hours, overcoming the primary construction difficulties of earlier generations [8]. This breakthrough in processability, combined with a formidable balance of chemical resistance and mechanical durability, has led to PEP’s widespread adoption for protecting metal and concrete in sectors like bridge engineering and marine environments [9]. Nevertheless, PEP systems still harbor significant limitations, notably suboptimal mechanical properties and a propensity for brittle fracture [10], which often prove inadequate for the rigorous demands of extreme protective applications.

To push beyond this performance ceiling, research has increasingly focused on developing mechanically enhanced and multifunctional polyurea materials [5]. A foundational strategy is to tune the polymer’s intrinsic properties by adjusting the ratio of its soft and hard segments. For instance, Tzelepis et al. [11] investigated how varying the hard segment weight fraction directly influences the linear viscoelasticity of the resulting polyurea. A more advanced approach centers on creating functionalized PUA through two primary routes. The chemical modification involves incorporating fluorine or silicon groups to impart capabilities like superhydrophobicity and self-healing [12,13,14,15]. Alternatively, the physical modification involves embedding high-performance fillers—such as aramid fibers or glass fibers—into the polyurea matrix to significantly enhance impact resistance, recyclability, and flame retardancy [16,17,18].

The emergence of nanotechnology offers a promising solution to PEP’s limitations by improving filler dispersion and interfacial interactions. For example, Li et al. [19] enhanced the tensile strength of a PEP matrix by covalently functionalizing MWCNTs with hexamethylene diisocyanate (HDI) trimer, while Meng et al. [10] utilized amino-modified boron carbide (B₄C) nanosheets to elevate mechanical properties and impart environmental resilience. Among these nanofillers, graphene and its derivatives, particularly graphene nanoplatelets (GNPs), have emerged as premier reinforcing agents for polyurea nanocomposites due to their exceptional properties [20,21,22]. However, their practical application is hampered by strong van der Waals forces, which cause them to agglomerate within the polymer matrix [23]. A key strategy to overcome this involves leveraging the residual oxygen-containing groups on the GNP surface to create stronger covalent interfaces with the polymer [24]. For instance, Mei et al. [25] functionalized graphene oxide (GO) with polydopamine (PDA) under mild conditions for reinforcement in a polyurea (PU) matrix. At a strain rate of 7000 s⁻¹, the GO@PDA_1.0%/PU composite showed 33% and 54% increases in flow strength and yield strength. Similarly, Mansourian-Tabaei et al. [26] employed a POSS-functionalization strategy to create PU/GNP-POSS films with desirable thermomechanical properties for force protection. Despite these advances, studies on graphene-based PEP nanocomposites remain limited, particularly in comprehensively enhancing overall protective performance while minimizing any compromise to PEP’s inherent self-healing capabilities.

Accordingly, this study presents a mechanochemical strategy for functionalizing GNPs with Jeffamine D2000, producing F-GNPs that promote enhanced dispersion and robust interfacial bonding in a PEP matrix. We prepared a series of PEP nanocomposites by incorporating F-GNPs at varying weight fractions and conducted comprehensive evaluations of their morphology, mechanical properties, impact resistance, and corrosion protection. Additionally, this comprehensive reinforcement was realized with a slight compromise to the system’s inherent self-healing efficiency. This research thus offers a practical framework for engineering advanced protective coatings that harmoniously balance exceptional durability and self-repair capabilities, opening new avenues for their use in demanding environments.

2. Experiment

2.1. Materials

Expandable graphite (graphite intercalation compound, GIC) was procured from Asbury Carbons Inc. (Asbury, NJ, USA). Matrix components included polyaspartic ester resin (Desmophen NH1420, hereafter NH1420) from Covestro AG (Leverkusen, Germany), isophorone diisocyanate (IPDI) from Bayer (Shanghai, China), and two polyether amines from Huntsman (The Woodlands, TX, USA): Jeffamine D2000 (D2000) and Jeffamine T5000 (T5000). Anhydrous ethanol and dimethylformamide (DMF) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All materials were used as received without further purification.

2.2. Preparation

2.2.1. Preparation of Functionalized Graphene Nanoplatelets

First, GNPs were prepared from expandable graphite, leveraging a synergistic process that coupled high-temperature thermal expansion with subsequent liquid-phase ultrasonic exfoliation [27]. To achieve surface functionalization of the as-prepared GNPs, a mechanochemical approach was employed. In a typical procedure, 0.1 g of the GNPs, 2.0 g of D2000, 20 g of DMF as a dispersing agent, and 20 g of zirconia grinding balls (3 mm diameter) were loaded into a 50 mL zirconia grinding bowl. After being sealed and purged with nitrogen to establish an inert atmosphere, the mixture was subjected to planetary ball milling at 400 rpm. The process was run in intermittent cycles of 20 min of operation followed by a 10 min rest period, accumulating a total milling time of 8 h. Subsequently, the resulting slurry was separated via vacuum filtration, and the collected solid was washed repeatedly with DMF and ethanol to remove any unreacted D2000. Finally, the product was dried in a vacuum oven at 65 °C for 24 h to yield a fine black powder, designated as D2000-functionalized GNPs (F-GNPs).

2.2.2. Preparation of PEP Nanocomposites

A series of PEP/F-GNP nanocomposites with varying F-GNP contents (0.1, 0.2, 0.3, 0.4, and 0.5 wt%) were fabricated using a two-step solution polymerization method, as schematically depicted in Figure 1. First, to prepare the nanofiller suspension for each concentration, a corresponding amount of F-GNPs was wet-ground with the NH1420 in an agate mortar for 30 min. The mixture was then treated with a probe sonicator (10 min, 500 W, 40 kHz) to form a homogeneous F-GNP/NH1420 suspension.

For the synthesis of the matrix prepolymer, all reactants (D2000, T5000, and IPDI) were dehydrated prior to use. In a four-neck flask under a nitrogen atmosphere, calculated amounts of D2000 and T5000 were combined and briefly sonicated to form a uniform mixture. While immersed in an ice-water bath, IPDI was added dropwise with mechanical stirring to achieve a final n(NCO)/n(NH₂) of 1.08. Once the addition was complete, the mixture was stirred for a further 30 min. Subsequently, the system was heated to 80 °C and held for 3 h to yield the NCO-terminated prepolymer.

To integrate the nanofiller and complete the synthesis, the F-GNP/NH1420 suspension was blended with the prepolymer at 60 °C for 30 min to perform the chain extension. The final mixture was vacuum-degassed, cast into a polytetrafluoroethylene (PTFE) mold, and cured at 65 °C for 72 h to obtain the PEP/F-GNP nanocomposites. For comparison, PEP/GNP nanocomposites and the neat PEP polymer were prepared following the same procedure, by substituting F-GNPs with pristine GNPs or by omitting the nanofiller addition entirely, respectively.

2.3. Characterization

The pristine GNPs and functionalized F-GNPs were characterized by Fourier transform infrared (FTIR) spectroscopy and thermal gravimetric analysis (TGA). FTIR spectra were recorded on a Nicolet-6700 spectrophotometer (Nicolet Instrument Co., Madison, WI, USA) from 400 to 4000 cm⁻¹, using samples prepared as KBr pellets. Raman spectra were recorded on a microspectrometer equipped with a 633 nm laser excitation source. TGA was performed on a TAQ-500 thermal analyzer (TA Instruments Inc., New Castle, DE, USA) by heating samples from room temperature to 800 °C at 10 °C/min in a nitrogen atmosphere.

The fracture morphology of the nanocomposites was examined using scanning electron microscopy (SEM, Gemini-SEM 300, Zeiss, Jena, Germany) at an accelerating voltage of 10 kV. Prior to analysis, the samples were cryogenically fractured in liquid nitrogen, and the fresh fracture surfaces were sputter-coated with a thin layer of platinum.

The tensile properties and tear strength of the nanocomposites were measured on an SHK-A104 universal testing machine, following the GB/T 528-2009 [28] and GB/T 529-2008 [29] standards, respectively. The tensile tests utilized Type 1A dumbbell-shaped specimens with a gauge section of 25 mm × 4 mm × 2 mm. For tear strength, right-angle specimens were used, each with a 1.0 ± 0.2 mm precut notch. The final results represent the average of at least five measurements for each sample formulation.

The pull-off adhesion strength and Charpy impact resistance of the coatings on aluminum alloy substrates were evaluated. Adhesion tests were performed on coatings with a thickness of 200 ± 10 μm in accordance with the GB/T 5210-2006 [30] standard. A KT-500Z tester (Shenzhen Ketan Electronic Technology Co., Ltd., Shenzhen, China) was used with a 10 mm diameter dolly, a loading rate of 0.5 MPa/s, and a data acquisition frequency of 100 Hz. The impact resistance was assessed on 2 mm thick coatings as per ISO 179-1:2000 [31], using an XJJY-50 impact tester (Beijing Hangtian Weichuang Equipment Technology Co., Ltd., Beijing, China) equipped with a 25 J pendulum. Each reported value represents the average of five replicate tests.

Electrochemical impedance spectroscopy (EIS) was performed on a CHI600E workstation (Chenhua, Shanghai, China) to assess the corrosion protection of the nanocomposite coatings, following the ISO 16773-1:2016 [32] standard. A three-electrode cell was employed, consisting of a PEP-coated steel plate as the working electrode, a platinum plate as the counter electrode, and a saturated mercuric glycol electrode as the reference. The samples were immersed in either 5 wt% NaCl or 5 wt% H₂SO₄ solution at room temperature. Impedance spectra were swept from 10⁵ Hz down to 10⁻² Hz with a 5 mV sinusoidal perturbation.

The environmental durability of the samples was assessed via a 240 h neutral salt spray test conducted in an XT-Y60 chamber (Dongguan Huitai Machinery Co., Ltd., Dongguan, China). The accelerated aging protocol involved continuous exposure to a 5 ± 1 wt% NaCl fog at 35 ± 2 °C. The resulting material degradation was then quantified by re-evaluating the tensile strength of the samples.

Self-healing degree (η) was quantified via a damage-heal-retest protocol. For tensile, tear, and impact tests, specimens were damaged by complete fracture, pre-loading to 50% peak force, or pre-cracking (≤1.0 mm), respectively. Damaged samples were then healed at 60 °C for 12 h. The efficiency η was calculated using the following equation:

$$\eta ={\displaystyle \frac{P_h}{P_o}}\ast 100\%$$

(1)

where P_h and P_o represent the mechanical properties post-healing and in the pristine state, respectively.

3. Result and Discussion

3.1. Structural and Compositional Characterization of F-GNPs

The successful functionalization of the GNPs was confirmed through comprehensive chemical, structural, and thermal analyses, with the results presented in Figure 2.

As depicted in Figure 2a, the FTIR spectrum of pristine GNPs reveals several characteristic peaks indicative of surface oxygen groups: a broad O–H stretching vibration at 3450 cm⁻¹, C=O stretching from carboxyl groups at 1718 cm⁻¹, and C–O–C epoxy stretching at 1043 cm⁻¹ [33,34]. These abundant functional groups provide ample reactive sites for the subsequent modification. Upon functionalization, the F-GNP spectrum exhibits several critical changes. Most notably, intensified C–H stretching vibrations from the grafted D2000’s aliphatic polyether chains emerge in the 2830–2950 cm⁻¹ region [35]. Concurrently, new absorptions appear at 3670 cm⁻¹ and 1605 cm⁻¹, which are assigned to N–H stretching and bending vibrations, respectively, originating from the amine groups of D2000. Collectively, these spectral transformations indicate that mechanochemical ball milling promotes the anchoring of D2000 molecules onto the GNP surface via van der Waals forces, hydrogen bonding, and potential minor covalent grafting.

As shown in Figure 2b, the Raman spectra of both pristine GNPs and F-GNPs display the characteristic D, G, and 2D bands at approximately 1340, 1580, and 2690 cm⁻¹, respectively. The G band represents the in-plane vibration of the ordered sp² carbon lattice, whereas the D band is associated with structural defects, including the presence of sp³-hybridized carbons [36]. For the pristine GNPs, a low I_D/I_G of 0.05 is observed, indicating a graphitic structure with high crystalline integrity. After the modification, the I_D/I_G ratio for F-GNPs increases from 0.05 to 0.09. This modest increase confirms that the mechanochemical process generates active sites on the GNP surface, facilitating covalent anchoring of D2000 molecules and introducing new sp³-hybridized carbons that heighten structural disorder, without compromising the fundamental sp² framework of the graphene core.

To quantify the extent of this modification, TGA was performed under a nitrogen atmosphere, as shown in Figure 2c. Following a minor initial weight loss below 100 °C from adsorbed water, a more pronounced decomposition stage occurs between 200 and 400 °C. For the pristine GNPs, the modest weight loss in this region is attributed to the removal of residual oxygen-containing functional groups and the initial breakdown of the carbon skeleton. In stark contrast, the F-GNPs demonstrate a significantly higher mass loss in this same range. This is ascribed to the additional thermal decomposition of the grafted D2000 molecules; as a polyetheramine, its ether linkages and amine groups undergo chain scission at these temperatures, releasing small-molecule gases and resulting in a greater mass reduction [35]. Quantitatively, the grafting ratio was determined from the difference in residual weight at 800 °C [37]. At this temperature, the GNPs remain highly thermally stable, while D2000 undergoes near-complete decomposition [38]. Therefore, the additional mass loss in the F-GNP sample can be confidently attributed to the grafted polymer. The final residual weight thus decreased from 98.26% for pristine GNPs to 90.97% for F-GNPs, and this mass difference of 7.29% directly corresponds to the grafting ratio, providing definitive quantitative validation of the functionalization.

In essence, the functionalization mechanism relies on the dual functionality of D2000 during mechanochemical milling. First, as a viscous medium, it mitigates destructive impact forces while enhancing productive shear energy, promoting the efficient exfoliation of graphite into high-quality, ultrathin GNP platelets. Second, D2000 acts as a molecular bridge to ensure robust interfacial integration. One of its terminal amine groups anchors to the GNP surface via both strong non-covalent interactions and covalent grafting with surface epoxide groups. The other terminal amine then bonds with the polyurea’s isocyanate groups, forging a covalent interface crucial for GNP dispersibility and matrix compatibility.

3.2. Mechanical Properties

The mechanical integrity of a protective coating is paramount for its durability, as resistance to tensile stress and tearing is essential for preventing failure under operational loads. To evaluate the reinforcing effect of the fillers, the mechanical properties of PEP/GNP and PEP/F-GNP nanocomposites with varying filler fractions were systematically investigated, with the results summarized in Figure 3.

Across all tested parameters, the PEP/F-GNP nanocomposites consistently outperform their PEP/GNP counterparts, affirming the benefits of the D2000 functionalization. As shown in Figure 3a, the tensile strength of both systems exhibits a non-linear relationship with filler concentration. The PEP/F-GNP nanocomposites achieve a peak tensile strength of 31.6 ± 0.8 MPa at a 0.3 wt% loading, which is a remarkable 39.8% improvement over the peak strength of the PEP/GNP system (22.6 ± 0.9 MPa) and a 263.0% enhancement compared to the pure PEP matrix (8.7 ± 0.8 MPa). This substantial strengthening stems from two synergistic factors: the high intrinsic strength and modulus of the GNPs and the robust interfacial adhesion created by the surface modification. As depicted in Figure 3d, the amino groups on F-GNPs form covalent and hydrogen bonds with the polyurea matrix, creating a strong interface. Under external load, this chemical linkage ensures efficient stress transfer from the polymer matrix to the rigid reinforcing phase and inhibits interfacial slip, thus effectively boosting tensile strength at low concentrations by harnessing the full reinforcing potential of the nanofiller [39]. However, beyond the optimal 0.3 wt% loading, the strength declines due to filler agglomeration, which introduces stress concentration points. Notably, when the F-GNP mass fraction reaches 0.5 wt%, this agglomeration effect becomes so pronounced that the nanocomposite’s tensile strength drops below that of the pure PEP matrix.

Similarly, the ductility quantified by elongation at break, follows an analogous trend, as depicted in Figure 3b. At 0.3 wt% loading, the PEP/F-GNP system’s elongation peaked at 952.6 ± 20.2%, surpassing the PEP/GNP system by 38.6% and the pure PEP by 75.1%. This superior ductility stems from the flexible polyether chains of D2000, which promote energy dissipation via interactions with the PEP matrix. Furthermore, the well-dispersed F-GNP nanosheets orient under shear to form a physical network; this 2D topology restricts the catastrophic movement of surrounding polymer chains, thereby enhancing plastic deformation. In contrast, pristine GNPs create weak interfaces that limit load transfer. At higher loadings, they also act as physical spacers due to agglomeration, directly hindering molecular chain mobility and promoting brittle failure.

The tear strength analysis shown in Figure 3c further corroborates these findings. The PEP/F-GNP nanocomposite achieves a maximum tear strength of 83.6 ± 5.3 N/mm at 0.3 wt%, substantially higher than the 65.8 ± 3.7 N/mm peak for PEP/GNP. The well-dispersed, strongly bonded F-GNP nanosheets serve as effective barriers, forcing the crack front to navigate a more tortuous path. This process dissipates significantly more energy, thereby enhancing the material’s overall toughness and resistance to tearing. In summary, the D2000 functionalization is a highly effective strategy, with 0.3 wt% F-GNPs creating an optimal balance of mechanical properties.

To further elucidate the reinforcing mechanism, the cryo-fractured surfaces of pure PEP and the optimal PEP/F-GNP-0.3% nanocomposite were examined using SEM, as depicted in Figure 4. As shown in Figure 4(a1–a3), the fracture surface of pure PEP appears relatively smooth, featuring randomly extending microcracks that suggest limited energy absorption capacity. In contrast, the PEP/F-GNP fracture surface as shown in Figure 4(b1–b3) exhibits fewer cracks and a reduced degree of rupture, culminating in a smoother overall morphology. This indicates diminished crack generation originating from the crack deflection mechanism, whereby the incorporated F-GNPs redirect propagating cracks and dissipate stress more effectively. Notably, minimal agglomerates are evident, with graphene nanosheets adopting an “embedded” distribution within the matrix, signifying enhanced bonding between the fillers and the polyurea network. This superior dispersion is often accompanied by nanoscale grooves or wrinkled structures, which arise from efficient stress transfer from the graphene layers to the surrounding matrix during deformation.

3.3. Impact Resistance

Polyurea is renowned for its exceptional ability to damp impact loads, effectively dissipating and absorbing energy to protect underlying substrates from damage [40]. This characteristic has led to its widespread use as high-performance, impact-resistant coatings. To fully leverage this intrinsic energy-absorbing capacity, a robust interfacial bond is an absolute prerequisite, preventing delamination upon impact and ensuring the coating and substrate function as a unified system. Therefore, the interfacial bonding strength between the PEP coatings and an aluminum alloy substrate was quantified via pull-off adhesion testing [8]. The results, presented in Figure 5a alongside corresponding test photographs in Figure 5b, demonstrate that the incorporation of both pristine and functionalized GNPs significantly enhances adhesion. A filler loading of 0.3 wt%, which was determined to be optimal for mechanical performance, was used for all nanocomposites hereafter unless otherwise noted.

Specifically, the addition of pristine GNPs increased the adhesion strength by nearly 30%, from 9.1 ± 0.4 MPa for pure PEP to 11.7 ± 0.5 MPa for the nanocomposite. This marked improvement is attributed to a powerful mechanical anchoring effect. The high specific surface area and lamellar structure of the graphene enable the nanosheets to interlock with the micro-topography of the substrate, while interactions between defect sites on the GNPs and the polyurea molecular chains further bolster the interfacial bond [41].

The PEP/F-GNP coating further boosted the adhesion strength to 12.1 ± 0.8 MPa, a 3.4% increase over the PEP/GNP counterpart. This improvement stems from the introduction of a new bonding mechanism at the interface. The amine groups, incorporated via D2000 functionalization, facilitate the formation of hydrogen bonds with the native hydroxyl groups on the substrate’s aluminum oxide surface (Al₂O₃). While the inherent chemical inertness of Al₂O₃ limits the potential for stronger covalent bonding, these hydrogen bonds effectively complement the purely physical anchoring mechanism of the graphene backbone [42].

To evaluate their protective capabilities, the impact resistance of the PEP coatings on an aluminum alloy substrate was systematically investigated. The resulting impact strengths are presented graphically in Figure 5c, with corresponding photographs of the post-impact samples shown in Figure 5d. The bare aluminum alloy substrate recorded an average impact strength of 337.0 kJ/m². Upon applying a layer of neat PEP, this value increased by over 20% to 404.9 ± 2.9 kJ/m², demonstrating the inherent energy-absorbing nature of the polyurea. Notably, the incorporation of nanofillers at the optimal loading of 0.3 wt% led to a substantial further increase in performance. Specifically, the PEP/F-GNP composite reached an impact strength of 620.0 ± 4.1 kJ/m², an impressive 84.0% improvement compared to the bare substrate. This superior protection is visually confirmed in Figure 5d: under a 15 J impact energy, the unprotected Al substrate exhibits pronounced brittle cracks, whereas the PEP/F-GNP coating greatly mitigated the substrate’s brittle cracking, with the coating itself showing no cracks or damage.

The exceptional impact resistance stems from a multi-level energy dissipation mechanism. Primarily, the inherent toughness of the PEP matrix arises from the dynamic behavior of its polymer chains. Upon impact, these long chains rapidly deform, elongate, and reconfigure, while the dense network of hydrogen bonds between the soft and hard segments continuously breaks and reforms [43,44]. This entire process effectively absorbs and dissipates the impact energy. The incorporation of stiff GNP nanosheets enhances this by promoting efficient stress transfer between fillers and the matrix, thereby increasing overall stiffness and strength. F-GNPs provide further advantages through a more stable interface that enables uniform transmission of impact waves for better energy dissipation, coupled with additional hydrogen bonds formed with the matrix, which amplify energy absorption via repeated bond rupture and reformation [45].

3.4. Anti-Corrosion Performance

The long-term durability of protective coatings in harsh environments often hinges on their ability to resist corrosion, particularly in marine or industrial settings where exposure to salts, acids, and moisture is prevalent. To assess the impact of nanofillers on the corrosion resistance of PEP coatings, electrochemical polarization tests and salt spray exposure experiments were conducted, providing insights into both electrochemical behavior and mechanical integrity under corrosive conditions.

The corrosion behavior of the coatings was first assessed in 5 wt% NaCl and 5 wt% H₂SO₄ solutions using potentiodynamic polarization. The resulting Tafel plots are presented in Figure 6a,b, from which the key electrochemical parameters—corrosion potential (E_corr) and corrosion current density (I_corr)—were derived via Tafel extrapolation. In both corrosive media, the addition of GNP and F-GNP nanofillers shifts E_corr to a more positive potential, indicating a reduced thermodynamic tendency for corrosion. Notably, the PEP/F-GNP nanocomposite consistently demonstrated superior corrosion resistance, exhibiting the highest E_corr values (0.225 V in NaCl and 0.301 V in H₂SO₄) and the lowest I_corr values. Since a lower I_corr corresponds directly to a slower corrosion rate, these results provide clear electrochemical evidence that the F-GNP filler is the most effective at inhibiting corrosion on the aluminum substrate [46].

To assess long-term durability, the coatings were subjected to a 240 h neutral salt spray test, after which their mechanical integrity, surface wettability, and visual appearance were evaluated. As shown in Figure 6c, the trend in tensile strength loss after exposure aligns with the electrochemical results. The PEP/F-GNP coating maintained its mechanical integrity remarkably well, exhibiting a minimal tensile strength loss of only 6.7%. This demonstrates its superior resistance to degradation compared to the pristine PEP and PEP/GNP systems. The changes in surface wettability, shown in Figure 6d, offer further insight. While the PEP/GNP coating initially had the highest water contact angle (104.9°), its hydrophobicity degraded significantly after exposure. In contrast, the PEP/F-GNP coating showed the most stable contact angle, indicating superior resistance to surface chemical degradation. The visual evidence in Figure 6e–g corroborates these findings. For the neat PEP coating, moisture and chloride ions (Cl⁻) readily attack the urea linkages, leading to molecular chain scission; this chemical degradation, coupled with salt-induced swelling stresses, results in visible microcracks. The PEP/F-GNP surface, in stark contrast, exhibits superior integrity by remaining largely intact, which visually demonstrates how the reinforced filler mitigates the corrosion progression.

The superior corrosion resistance of the PEP/F-GNP nanocomposite arises from a multi-faceted mechanism. The dense polyurea network forms the foundational physical barrier against corrosive agents. Building upon this, the homogeneously dispersed F-GNP nanosheets create an effective nano-barrier. Their high specific surface area establishes a tortuous path that significantly prolongs the diffusion time for species like H₂O, O₂, and Cl⁻, thus enhancing the coating’s overall impermeability [47]. Moreover, the D2000 functionalization provides a crucial chemical advantage by introducing hydrophobic polypropylene oxide segments alongside the amino groups, which collectively fortify the coating against degradation by limiting water absorption while maintaining matrix compatibility.

3.5. Self-Healing Properties

Intrinsically self-healing materials leverage dynamic internal bonds to repair damage upon external stimuli [22,48]. PEP is a polymer particularly well-known for this capability, which arises from the dense network of reversible hydrogen bonds within its molecular structure [49]. To explore the influence of nanofillers on this behavior, we evaluated the healing efficacy of pure PEP alongside its nanocomposites incorporating 0.3 wt% of GNPs or F-GNPs.

To balance healing effectiveness with energy efficiency and thermal stability, we selected a moderate repair temperature of 60 °C [50]. The optimal healing time was then determined through preliminary trials, as depicted in Figure 7a,b. Following impact-induced damage, the samples initially displayed distinct macroscopic notches. After 6 h, the impact-induced notches showed significant contraction, but near-complete surface reconstruction required 12 h to achieve an apparently healed state. Based on this visual evidence, a 12 h healing duration was identified as ideal and utilized for all subsequent quantitative assessments.

The healing degree for impact strength, tensile strength, elongation, and tear strength were calculated using Equation (1) and are compared in Figure 7c–f. In essence, this metric represents the percentage of a property recovered after a sample is damaged and then healed for 12 h at 60 °C. These results reveal a consistent trend across all four properties: enhancing mechanical properties with nanofillers comes at the cost of healing ability. For instance, as shown in Figure 7c, while the PEP/F-GNP nanocomposites provided superior initial impact protection, its healing degree was the lowest at 83.3%, underperforming both PEP/GNP (84.6%) and neat PEP (91.4%).

The superior self-healing of neat PEP, schematically illustrated in Figure 8, is attributed to its dynamic hydrogen bond network. Upon damage, hydrogen bonds at the fracture interface act sacrificially, breaking to absorb energy and protect the covalent backbone. This breakage liberates flexible chain segments, allowing them to diffuse across the crack interface via Brownian motion. Additionally, the polar aspartate groups may attract ambient moisture, creating a lubricating effect that further enhances chain mobility. As the chains interpenetrate, the polar ureido and aspartate groups readily reform the hydrogen bond network, restoring mechanical integrity. In contrast, the diminished healing in PEP/F-GNP stems from the covalent integration of the nanofillers. These strong covalent links, while providing stability, create points of irreversible failure. Their permanent fracture upon damage restricts the necessary chain reorganization and entanglement for effective healing, thereby inhibiting the dynamic hydrogen bond reformation that drives recovery and leading to markedly lower efficiencies [51].

As shown in Figure 7f, the healing degree of tear strength is significantly lower than that of impact or tensile metrics. This discrepancy can be attributed to the complex, multi-scale fracture mechanics that govern the tearing process [52]. Unlike the break in a tensile test, tearing involves a cascade of failure modes, including matrix-filler debonding, crack deflection, and delamination, which creates a highly tortuous and damaged fracture surface. These processes significantly elevate the energetic barrier for chain diffusion and dynamic bond reconstruction, thereby fundamentally limiting the material’s overall healing potential, especially in the nanocomposites.

To compare the extent to which F-GNPs enhance the mechanical and self-healing properties of PEP in this work with existing research,

Table 1. The improvement comparison of F-GNPs with previous studies.

Additive	Loading	Tensile Strength (MPa)	Matrix Tensile Strength (MPa)	Mechanical Improvement (%)	Repair Rate (%)	Ref.
F-GNPs	0.3 wt%	31.6	8.7	263.0	83.3	This work
IP-GNP	0.05 vol%	15.72	7.55	108.2	72	[22]
DSPU	/	4.18	8.42	101.4	88	[53]
TiO2	3 wt%	29.8	25.7	15.9	/	[54]
MDIBN	/	20.64	17.47	18.1	/	[55]

summarizes the reinforcement efficiency of representative fillers. Meng et al. [22] used IP-GNP to increase the tensile strength of polyurea composites from 7.55 MPa to 15.72 MPa (an increase of 108.2%), achieving a self-healing efficiency of 72%. Dural et al.’s DSPU system increased tensile strength from 4.18 MPa to 8.42 MPa (an increase of 101.4%) while maintaining an 88% healing rate [53]. Similarly, Dural et al. [54] reported that a 3 wt% TiO₂ system only achieved an increase from 25.7 MPa to 29.8 MPa (an increase of 15.9%, with no healing rate reported). Cheng et al.’s MDIBN system saw strength increase from 17.47 MPa to 20.64 MPa (an increase of 18.1%, with no healing rate reported) [55]. In contrast, this work significantly increased the tensile strength of PEP from 8.7 MPa to 31.6 MPa (an increase of 263.0%) with only 0.3 wt% F-GNPs, while still maintaining a self-healing efficiency of 83.3%, demonstrating a significant advantage in achieving both high strength and reparability with low filler content.

4. Conclusions

In this work, a mechanochemical strategy proved instrumental in engineering a high-performance, multifunctional PEP nanocomposite. This approach successfully functionalized GNPs with D2000, a modification validated through comprehensive microstructural analyses. As evidenced by SEM, this functionalization ensured superior dispersion of GNPs and enhanced interfacial compatibility within the PEP matrix. The resultant enhanced synergy yielded remarkable mechanical properties, with the PEP/F-GNP nanocomposite attaining peak tensile strength of 31.6 ± 0.8 MPa and tear strength of 83.6 ± 5.3 N/mm at an optimal 0.3 wt% F-GNP loading. This structural reinforcement directly translated into exceptional protective coating performance, with the nanocomposites exhibiting robust adhesion to aluminum substrates and enhancing their impact resistance by a remarkable 84%. Furthermore, electrochemical and salt spray tests corroborated their superior anti-corrosion capabilities, primarily attributed to the physical barrier effect of the well-dispersed nanoplatelets. Critically, while this reinforcement introduced a fundamental trade-off with autonomous repair, our optimal nanocomposite nonetheless preserved a high self-healing efficiency of 83.3%, as evaluated by impact resistance recovery. This research, therefore, not only delivers a versatile material platform for demanding applications in harsh environments but also offers critical insights into balancing the competing properties of strength and reparability, paving the way for the rational design of next-generation coatings.