Investigation of Anti-Corrosion Behavior of Epoxy-Based Tannic Acid/Benzoxazine and Embedded ZnO Nanocomposites

Abstract

Corrosion is a major issue in many industries, leading to material degradation, increased maintenance costs, and safety hazards. Conventional protective coatings frequently rely on hazardous chemicals, which has driven demand for environmentally friendly materials that can enhance the durability of infrastructure. The present study investigates the structural, mechanical, anticorrosive, and tensile properties of a novel polymer composite based on tannic acid-benzoxazine monomer (TA-BZ), reinforced with epoxy resin and zinc oxide (ZnO) nanoparticles. The composite formulations are designated as Epoxy-TA-BZ1-ZnO (A), Epoxy-TA-BZ2-ZnO (B), and Epoxy-TA-BZ4-ZnO (C). The objective of this research is to develop a sustainable material system with improved anticorrosive and mechanical performance. The composites were synthesized through the crosslinking of TA-BZ with epoxy resin and the incorporation of ZnO nanoparticles, known for their corrosion-inhibiting properties and contributions to tensile strength. The materials were evaluated using Fourier Transform Infrared (FT-IR) spectroscopy, Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), potentiodynamic polarization techniques, and tensile testing. Among the tested formulations, Epoxy-TA-BZ4-ZnO exhibited outstanding anticorrosive performance, achieving a minimal corrosion rate of 0.06 mm/year. This performance is attributed to the favorable dispersion of ZnO nanoparticles at 5 wt%, which serve as effective barriers to corrosive agents under the conditions studied. These findings highlight the potential of TA-BZ-based composites as environmentally sustainable alternatives to conventional coatings in corrosion-sensitive applications.

1. Introduction

Corrosion continues to be a major concern across several sectors, leading to material deterioration and presenting substantial economic and safety hazards. Traditional anticorrosion methods often depend on hazardous coatings and metals, which pose environmental and health risks. As a result, there is an increasing demand for sustainable, biocompatible materials exhibiting exceptional anticorrosion capabilities. Advancing these materials is essential for prolonging infrastructure lifespan, reducing maintenance costs, and minimizing environmental impacts [1,2,3,4].

Benzoxazine resins are recognized for their high thermal stability, mechanical robustness, and flame resistance, positioning them as strong candidates for advanced coatings [5]. These resins, known for their low water uptake and stability under extreme conditions, effectively minimize moisture penetration—a primary contributor to metal corrosion [6]. Benzoxazine polymers also offer environmental benefits, as they can be synthesized without toxic catalysts, and display excellent adhesion and self-healing properties, making them highly protective for metal substrates [7,8,9]. Studies indicate that adding various components further enhances their barrier capabilities, surpassing conventional coatings like epoxy. Benzoxazines are typically produced from phenols and primary amines using a Mannich-type reaction with formaldehyde, and they undergo ring-opening polymerization to form high-performance thermosets [5,6]. Nevertheless, researchers continue to seek alternative raw materials and synthesis techniques to expand benzoxazine applications in more challenging environments [7,8,9,10]. In the past decade, environmental concerns have significantly influenced corrosion prevention strategies. Plant-based extracts, particularly polyphenols, are gaining attention for their cost-effectiveness, biodegradability, and environmental friendliness [11,12]. These polyphenols, studied for their anti-corrosive efficiency, have shown inhibition rates exceeding 85%, which have propelled further electrochemical and theoretical studies. Recent advances have highlighted the potential of plant-derived polyphenols as sustainable and safe alternatives to conventional petroleum-based raw materials in thermosetting polymers. Liu et al. reviewed the utilization of aromatic biobased feedstocks, such as tannic acid, emphasizing their low toxicity, renewability, and multifunctional properties, including antioxidant and antimicrobial activities—which make them particularly suitable for green polymer design [13]. Complementing this, Klfout et al. discussed the development of bio-based polybenzoxazine systems, demonstrating their promising performance as adhesive coatings with improved thermal stability and environmental compatibility. These studies support the rationale behind our selection of tannic acid in the current work, aligning with broader trends in sustainable polymer research [14]. Tannic acid, a natural polyphenol, has emerged as an effective alternative to synthetic phenols for creating benzoxazine monomers. It contains numerous reactive phenolic hydroxyl groups, which not only impart antioxidant properties but also provide several sites for benzoxazine synthesis. Some research also explores alternative corrosion inhibitors, such as Schiff base-based cationic Gemini surfactants, examining their effectiveness in protecting carbon steel in hydrochloric acid environments [11,12,13,14,15,16,17,18]. Incorporating tannic acid into benzoxazine synthesis aligns with the shift toward bio-based, sustainable materials, addressing the demand for eco-friendly polymers. The addition of tannic acid enhances the hydrogen bonding network in the polymer matrix, thereby improving both mechanical strength and thermal stability. Its antioxidant nature also resists oxidative degradation, making tannic acid-modified benzoxazines particularly suitable for use in corrosive settings [19,20,21]. The synthesis process often involves the reaction of tannic acid with aniline and paraformaldehyde, which enhances the polymer’s rigidity and thermal resistance due to aniline’s structural properties. Ethanol and toluene are commonly used to maintain solubility and reaction efficiency, resulting in high purity benzoxazine monomers [6,7,22,23,24,25,26,27,28,29,30,31,32,33]. Recent studies demonstrate the effectiveness of tannic acid-based benzoxazine coatings for corrosion protection, as evidenced by X. Cao et al., who found that these coatings significantly improve corrosion resistance on steel substrates by inhibiting the ingress of water and oxygen [34,35]. Similarly, another study observed that tannic acid-enriched benzoxazine coatings enhance saltwater corrosion resistance in marine environments, outperforming traditional epoxy coatings due to their improved adhesion and barrier properties [34,36]. The incorporation of zinc oxide (ZnO) nanoparticles significantly enhances the anticorrosive and mechanical properties of polymer coatings [37]. ZnO nanoparticles are recognized for their exceptional barrier characteristics, contributing to the formation of a denser and more compact polymer network that effectively impedes the diffusion of corrosive agents such as water, oxygen, and ions. This improved impermeability is critical for prolonging the lifespan of coatings, particularly in aggressive and corrosive environments [15,38,39].

Recent investigations underscore the efficacy of adding ZnO nanoparticles to polymer coatings, as this incorporation markedly enhances both corrosion resistance and mechanical performance. Specifically, ZnO nanoparticles fortify the coating’s impermeability by establishing a robust barrier that minimizes the penetration of moisture, oxygen, and ionic species. Furthermore, the addition of ZnO is known to improve vital mechanical properties, including abrasion resistance and adhesion, which are essential for maintaining durability in demanding conditions. Additionally, ZnO contributes photocatalytic properties and UV stability, rendering it ideal for protective applications in corrosive settings [8,9]. Moreover, ZnO reinforces the mechanical integrity of the composite, enhancing tensile strength and abrasion resistance, which is crucial for industrial applications subjected to mechanical stress. The photocatalytic and antimicrobial characteristics of ZnO further augment the coating’s protective abilities by inhibiting microbial growth that could otherwise accelerate material degradation. ZnO also serves as a UV stabilizer, effectively preventing the degradation of the polymer matrix under sunlight exposure, thus preserving the coating’s structural and aesthetic qualities over time [15,39,40,41,42,43,44]. Studies indicate that incorporating epoxy into benzoxazine composites further amplifies their corrosion resistance and mechanical properties, particularly when combined with ZnO nanoparticles. Epoxy-benzoxazine coatings, especially those containing ZnO nanofillers, exhibit a dual crosslinked network structure that significantly reduces corrosion rates, thus proving highly effective for applications on metal substrates within aggressive environments. The presence of ZnO nanoparticles not only contributes to a denser, impermeable barrier but also enhances the coating’s resistance to moisture, oxygen, and ionic penetration, thereby offering substantial protection against corrosive agents [8,16,19,20,45,46,47].

The integration of ZnO into these composites not only strengthens their physical structure but also improves UV resistance and mitigates photodegradation, which is essential for prolonging the service life of coatings exposed to harsh environmental conditions. ZnO’s photocatalytic properties additionally provide self-cleaning benefits by minimizing the accumulation of organic deposits that could facilitate microbial growth, thereby further reducing corrosion risks. The mechanical enhancements offered by ZnO-epoxy-benzoxazine composites—such as improved adhesion, hardness, and abrasion resistance—are particularly advantageous for industrial applications where high durability and corrosion resistance are paramount [15,16,48]. Research conducted by Zhou et al. illustrates that benzoxazine-epoxy-ZnO composite coatings applied to mild steel substrates exhibit significantly lower corrosion currents compared to traditional coatings [19]. This finding highlights the synergistic benefits of epoxy’s dual-network structure in conjunction with ZnO’s protective attributes, emphasizing their potential as robust, long-term corrosion protection solutions in industrial settings. This performance underscores the applicability of these advanced composites in replacing conventional materials in critical applications where corrosion resistance is of utmost importance [10,12]. Conventional epoxy coatings are widely used in protective applications but often suffer from limited durability in aggressive environments due to their hydrophilic nature, susceptibility to thermal degradation, and limited resistance to long-term oxidative and hydrolytic stress. These challenges highlight the need for advanced hybrid coatings with improved structural and environmental stability [3,4]. In response to these challenges, this study explores the synthesis of a novel composite material comprising tannic acid-benzoxazine (TA-BZ), crosslinked with epoxy resin, and reinforced with zinc oxide (ZnO) nanoparticles. Tannic acid, a natural polyphenolic compound, provides mechanical strength, thermal stability, and inherent bioactivity, making it a suitable candidate for applications in fields such as drug delivery and tissue engineering. Benzoxazine chemistry further enhances the composite by facilitating the formation of robust crosslinked networks, which improves the material’s overall performance [5,6,7,8,33]. The addition of ZnO, known for its antibacterial properties and UV-filtering capabilities, further augments the composite’s functionality, making it an attractive candidate for use in biocompatible materials and environmentally sustainable applications [9,10,11,12,19,49,50,51,52,53,54,55,56,57]. The primary objective of this research is to investigate the structural, mechanical, and anticorrosion properties of the epoxy-TA-BZ-ZnO composite. The study evaluates the uncured samples, systematically comparing their performance. Additionally, the inclusion of ZnO in the composite is expected to further improve its anticorrosion properties, along with enhancing its resistance to environmental degradation. The findings of this study are expected to contribute to the development of multifunctional, sustainable materials with significant potential for applications in anticorrosion coatings and environmental remediation. By integrating natural and synthetic components, this research offers a promising solution to corrosion-related challenges across various industrial and technical sectors. It highlights the importance of balancing corrosion resistance with mechanical strength and promoting sustainable materials for industrial use.

2. Results and Discussion

2.1. Chemistry

The molecular structure presented in Figure 1 represents a hybrid polymer composite consisting of a benzoxazine-epoxy matrix embedded with ZnO nanoparticles. The system is characterized by the combination of benzoxazine units and epoxy groups, which contribute distinct and complementary properties to the overall material, particularly for anticorrosion applications. The epoxy groups are known for their high reactivity enabling crosslinking with the TA-BZ matrix. This crosslinked network enhances the mechanical integrity and thermal stability of the polymer. In anticorrosion coatings, this network serves as a barrier to corrosive agents such as water, oxygen, and chloride ions, preventing their penetration and thereby reducing the risk of corrosion to the underlying substrate. The polymer backbone predominantly consists of benzoxazine rings, denoted by the aromatic structures highlighted in red. Benzoxazines are well-regarded for their excellent thermal and chemical stability, low water absorption, and superior char yield upon decomposition—all critical features for protective coatings. Low water absorption, in particular, plays a crucial role in anticorrosion applications by limiting moisture ingress, a key contributor to corrosion, thereby extending the lifetime of the protective layer. Additionally, the presence of hydroxyl groups (-OH) within the polymer backbone further enhances intermolecular hydrogen bonding, which strengthens the polymer network and provides additional sites for interaction with ZnO nanoparticles. This improves adhesion to the substrate, reducing the likelihood of delamination—a common issue in corrosion-prone environments. ZnO nanoparticles, shown as orange circles, serve as functional fillers and active agents for corrosion resistance. ZnO is recognized for its reinforcing properties, especially in enhancing the material’s tensile strength, modulus, and thermal conductivity. Furthermore, ZnO enhances UV resistance, hence improving the material’s endurance against ultraviolet radiation, which is crucial for outdoor applications where coatings are subjected to degradation. ZnO demonstrates significant anticorrosion properties via a passivation mechanism, creating a protective oxide layer on metallic surfaces. This layer serves as a physical and chemical barrier, inhibiting direct contact between the metal and corrosive surroundings, including salty or acidic situations. The proper dispersion of ZnO in the polymer matrix is essential; effective incorporation reduces agglomeration and maximizes the reinforcing and anticorrosive properties of the nanoparticles. The red-highlighted benzoxazine units confer rigidity and structural stability to the polymer due to their inherent aromatic nature. These rigid aromatic structures enhance the overall thermal stability of the composite, making it resistant to high-temperature environments where corrosion is accelerated. Additionally, the combination of flexible epoxy groups with rigid benzoxazine units creates a well-balanced polymer architecture that offers both toughness and resilience while maintaining high thermal performance and excellent barrier properties against corrosive agents. The presence of ZnO within the polymer matrix also facilitates molecular-level interactions, particularly with hydroxyl groups along the benzoxazine backbone. These interactions likely contribute to enhanced nanoparticle dispersion, improved polymer-filler adhesion, and overall matrix stability, resulting in superior mechanical performance, thermal properties, and long-term corrosion resistance.

The FTIR spectrum analysis of the Epoxy-TA-BZ-ZnO composite as shown in Figure 2 and (

Table 1. Enhanced FTIR Peak Assignments for TA-BZ Synthesis Confirmation.

Wavenumber (cm−1)	Assignment	Molecular Origin	Significance for TA-BZ Formation
3200–3500	O-H stretching (phenolic)	Residual tannic acid hydroxyl groups	Confirms retention of polyphenolic character
2850–2950	C-H stretching (aliphatic)	Methylene bridges in benzoxazine rings	Indicates successful Mannich condensation
1580–1600	Aromatic C=C stretching	Benzene rings from TA and aniline	Confirms aromatic structure preservation
1500–1550	Aromatic C=C stretching	Substituted benzene rings	Secondary aromatic vibrations
1220–1260	C-O-C asymmetric stretch	Oxazine ring formation	Primary confirmation of benzoxazine synthesis
1150–1180	C-N stretching	Tertiary amine in oxazine ring	Supporting evidence for ring closure
950–970	C-H out-of-plane deformation	Trisubstituted benzene adjacent to oxazine	Characteristic benzoxazine fingerprint
800–850	C-H out-of-plane bending	Aromatic substitution patterns	Confirms aromatic substitution
<600	Zn-O stretching	ZnO nanoparticle incorporation	Validates ZnO presence in composite

Bold entries indicate diagnostic peaks for benzoxazine formation.

) verifies the effective integration of the principal components: epoxy, tannic acid benzoxazine (TA-BZ), and zinc oxide (ZnO). The peaks detected between 1500 and 1600 cm⁻¹ are attributed to the aromatic C=C stretching of both bisphenol A from the epoxy group and TA-BZ, therefore confirming the existence of the aromatic structure in both components. The benzoxazine ring, derived from TA-BZ, is indicated by absorption bands in the 1220–1260 cm⁻¹ range (C-O-C stretching), together with typical vibrations of the benzoxazine ring observed at about 950–970 cm⁻¹, as seen in the samples in Figure 2 (Epoxy-TA-BZ-ZnO). Furthermore, the broad band in the 3200–3500 cm⁻¹ range is ascribed to O-H stretching, indicative of hydroxyl groups from tannic acid, as the epoxy interacts with TA-BZ. The existence of ZnO is further validated by the pronounced absorption band beneath 600 cm⁻¹, indicative of the Zn-O stretching vibrations. This corroborates the assumption that ZnO was effectively incorporated into the network. The FTIR spectra collectively indicates that the Epoxy-TA-BZ-ZnO system has achieved effective integration, as evidenced by the anticipated chemical interactions among the epoxy resin, TA-BZ, and ZnO, which appear in the typical absorption bands. This thorough investigation verifies the structural integrity and chemical composition of the synthesized material, emphasizing the effective integration of ZnO and the interaction between epoxy and TA-BZ.

Nuclear Magnetic Resonance (NMR) spectroscopy was utilized to determine the chemical structure and composition of the uncured TA-BZ samples. ¹H-NMR analyses were performed to gain detailed insights into their molecular architecture. The ¹H-NMR spectrum of the uncured TA-BZ samples (the starting monomer), as shown in (Figures S1 and S2), revealed distinct characteristic peaks providing crucial information about the initial structural features. A sharp singlet observed at δ 5.2 ppm corresponds to methylene protons (-CH₂-) bridging aromatic rings in the benzoxazine structure, serving as a key indicator of an intact oxazine ring. Peaks in the aromatic region (δ 6.5–7.5 ppm) were attributed to protons on the benzene rings associated with both the benzoxazine and tannin moieties, underscoring the intricate aromatic framework of the hybrid system. A broad signal at δ 4.8 ppm, assigned to oxazine ring protons, further validated the presence of the benzoxazine structure. Furthermore, the aliphatic region (δ 0.8–2.5 ppm) exhibited multiple signals, likely originating from the complex tannin component, highlighting the structural diversity within the tannin-benzoxazine hybrid.

The scanning electron microscopy (SEM) images as shown in Figure 3 provide critical insights into the morphological characteristics of the samples, highlighting the presence and distribution of ZnO filler particles within the polymer matrix. In Figure 3a,b, (Epoxy-TA-BZ-4-ZnO), a significant agglomeration of ZnO particles is observed as a bright, irregular cluster at the center, approximately 10 μm in diameter. This agglomerate exhibits a complex structure with surface irregularities, such as fractures or fissures, likely indicative of interactions between the filler particles and the polymer matrix. The surface morphology shows considerable complexity, where the large agglomerate appears to have induced visible cracks within the polymer matrix, likely due to differential thermal expansion or contraction during synthesis. These findings suggest that optimizing filler content and dispersion techniques may be necessary to achieve a more uniform distribution of ZnO particles and enhance polymer-filler interaction. Addressing the challenges associated with large agglomerates could improve the overall performance of the thin films. Regulating filler size and distribution is essential for reducing stress and flaws in the polymer matrix, ultimately improving the material’s mechanical and functional properties.

The TEM images (Figure 4) provide valuable insights into the morphology and distribution of ZnO nanoparticles within the Epoxy-TA-BZ matrix for samples (A–C). In the samples, a heterogeneous distribution of ZnO nanoparticles with varying sizes and shapes is observed. Sample A (Epoxy-TA-BZ1-ZnO) contains relatively large, irregularly shaped ZnO particles, ranging from 50 to 100 nm. These particles appear loosely aggregated, indicating limited integration with the epoxy matrix. Sample B (Epoxy-TA-BZ2-ZnO) demonstrates a higher density of ZnO particles, primarily in the 30–80 nm range. These particles have a propensity to agglomerate, indicating possible agglomeration challenges with varying TA-BZ ratios. Sample C (Epoxy-TA-BZ4-ZnO) exhibits the highest particle density among the samples, with ZnO particle sizes ranging from 20 to 100 nm. The optimized TA-BZ ratio in this sample led to more pronounced agglomeration, forming larger clusters of connected particles.

It is important to note that the pristine ZnO nanoparticles used in this study have a manufacturer-reported size range of 30–50 nm. The larger particle sizes observed within the composites are therefore likely a result of nanoparticle agglomeration during mixing and interactions with the polymer matrix. These observations underscore the significant influence of composite formulation and structure on the final nanoparticle dispersion. As such, these morphological characteristics are expected to have a direct impact on the mechanical reinforcement, thermal performance, and anticorrosion behavior of the resulting nanocomposites, as elaborated in subsequent sections.

The X-ray diffraction (XRD) analysis as shown in Figure 5 was conducted on uncured (Epoxy-TA-BZ-1-ZnO (A), Epoxy-TA-BZ-2-ZnO (B), Epoxy-TA-BZ-4-ZnO (C)) thin film polymer samples containing zinc oxide (ZnO) filler to examine the presence of ZnO and crystallinity. The XRD patterns showed distinct peaks characteristic of ZnO, notably at 2θ values of 31.7°, 34.4°, and 36.3°, corresponding to the (100), (002), and (101) crystallographic planes of hexagonal wurtzite ZnO. Additional peaks were identified at 47.5°, 56.6°, and 62.9°, corresponding to the (102), (110), and (103) planes, confirming the successful incorporation of crystalline ZnO filler in the samples. The epoxy matrix exhibited a broad, diffuse peak centered around 17–25°, characteristic of its amorphous nature. This peak persisted in the samples, indicating that the epoxy network retained its amorphous structure [58,59]. The semi-crystalline nature of the polymer matrix, as indicated by broad peaks in the low 2θ region, reflects the nature of the polymer structure. Additionally, the XRD patterns revealed a correlation between the different TA-BZ samples and crystallinity, with samples B and C displaying progressively stronger ZnO peaks relative to the polymer background. This suggests that the varying mole ratios of TA-BZ in these samples influence the degree of crystallinity within the polymer matrix. These structural characteristics, including ZnO crystallization and polymer-filler integration, are expected to impact the mechanical and thermal properties of the TA-BZ-ZnO thin film composites. Further investigation into these performance characteristics is warranted to understand the material’s full potential for advanced applications.

The thermogravimetric analysis (TGA) was conducted on uncured (Epoxy-TA-BZ-1-ZnO (A), Epoxy-TA-BZ-2-ZnO (B), Epoxy-TA-BZ-4-ZnO (C)) thin film polymer samples to comprehensively evaluate their thermal stability characteristics and degradation mechanisms. The thermal behavior of these novel composites provides critical insights into their potential performance in high-temperature applications and their underlying structure–property relationships.

The TGA result exhibited characteristic multi-stage degradation profiles, starting with an initial plateau region extending to approximately 300 °C, followed by a series of well-defined weight loss stages, and culminating in a final residual mass at 800 °C. This multi-stage degradation pattern is indicative of complex thermal decomposition mechanisms involving various chemical moieties within the polymer structure.

The initial thermal stability, quantified by T₁₀ (temperature at 10% weight loss), increased systematically with higher TA-BZ content, ranging from 398 °C for sample A to 408 °C for sample C (

Table 2. Thermal degradation parameters of uncured Epoxy-TA-BZ-ZnO samples derived from TGA analysis.

Sample	T10 (°C)	T20 (°C)	T30 (°C)	T50 (°C)	Residue at 800 °C (%)
Epoxy-TA-BZ1-ZnO (A)	398	425	442	465	1.45
Epoxy-TA-BZ2-ZnO (B)	403	429	448	467	1.67
Epoxy-TA-BZ4-ZnO (C)	408	435	452	469	1.89

). This enhancement in thermal stability can be attributed to the molecular architecture of the TA-BZ component, where the rigid benzoxazine rings provide thermal resistance through their aromatic structure and potential for forming dense crosslinked networks. The presence of ZnO nanoparticles, while maintained at constant concentration across all samples, likely contributes to this thermal stability through reinforcement effects and possible catalytic influence on the degradation pathway. The thermal stability can be further characterized by examining the temperature ranges for specific degradation stages, as detailed in

Table 3. Temperature ranges for different degradation stages of the uncured samples.

Sample	Stage I (°C)	Stage II (°C)	Stage III (°C)	Residual Mass Transition (°C)
Epoxy-TA-BZ1-ZnO (A)	300–385	385–465	465–550	550–650
Epoxy-TA-BZ2-ZnO (B)	305–390	390–470	470–555	555–660
Epoxy-TA-BZ4-ZnO (C)	310–395	395–475	475–560	560–670

.

The first degradation stage (Stage I) can be attributed to the decomposition of less thermally stable components, such as unreacted epoxy groups or side chains. The second stage (Stage II) likely corresponds to the main chain scission of the polymer backbone, while the third stage (Stage III) may involve the breakdown of more thermally stable aromatic structures and crosslinked networks. The final residual mass transition represents the carbonization process and the formation of thermally stable char. The char-yield characteristics at different temperatures provide insights into the material’s thermal resistance and potential flame-retardant properties, as shown in

Table 4. Char yield characteristics of the uncured Epoxy-TA-BZ-ZnO samples at various temperatures.

Sample	Char at 400 °C (%)	Char at 500 °C (%)	Char at 600 °C (%)	Char at 800 °C (%)
Epoxy-TA-BZ1-ZnO (A)	85.2	28.7	1.98	1.45
Epoxy-TA-BZ2-ZnO (B)	87.5	30.4	2.05	1.67
Epoxy-TA-BZ4-ZnO (C)	89.8	32.1	2.13	1.89

and Figure S3.

The char yield progressively increases with higher TA-BZ content across all temperature points, suggesting that the benzoxazine component enhances the formation of thermally stable carbonaceous structures during pyrolysis. This improved char formation has significant implications for flame retardancy, as the char layer acts as a physical barrier, reducing heat transfer and mass transport during combustion. The IPDT values, representing the overall thermal stability of the material, increase with higher TA-BZ content, confirming the enhancement of thermal resistance. Similarly, the CTI and Thermal Stability Index values show a consistent upward trend, indicating that the incorporation of higher TA-BZ content systematically improves the thermal durability of these composites. The TGA weight loss derivative curves (DTG) provide further insights into the degradation mechanisms, with peak temperatures and areas indicating the relative contributions of different degradation processes, as summarized in

Table 5. DTG peak characteristics for the uncured Epoxy-TA-BZ-ZnO samples.

Sample	Peak 1 Temp (°C)	Peak 1 Area (%)	Peak 2 Temp (°C)	Peak 2 Area (%)	Peak 3 Temp (°C)	Peak 3 Area (%)
Epoxy-TA-BZ1-ZnO (A)	345	18.5	432	65.7	510	15.8
Epoxy-TA-BZ2-ZnO (B)	348	17.2	438	67.9	515	14.9
Epoxy-TA-BZ4-ZnO (C)	352	16.1	445	70.2	520	13.7

.

The shift in peak temperatures toward higher values with increasing TA-BZ content confirms the enhanced thermal stability of these composites. The relative areas of the peaks also change, with Peak 2 (corresponding to main chain degradation) becoming more dominant in samples with higher TA-BZ content. This suggests that the polymer network structure becomes more uniform and homogeneous with increased TA-BZ, leading to a more concentrated degradation process at higher temperatures.

In summary, the comprehensive TGA analysis demonstrates that the thermal stability of Epoxy-TA-BZ-ZnO composites is significantly influenced by the TA-BZ ratio, with higher TA-BZ content resulting in enhanced thermal resistance, increased char formation, higher activation energy for degradation, and more controlled degradation kinetics. These improvements in thermal properties make the Epoxy-TA-BZ4-ZnO composition particularly promising for applications requiring exceptional thermal durability, such as high-temperature coatings, aerospace components, and electronic materials subject to thermal cycling.

The differential scanning calorimetry (DSC) analysis of the uncured Epoxy-TA-BZ-ZnO samples (A, B, and C) provides crucial information about their thermal transitions, curing behavior, and thermodynamic properties. This thermal characterization is essential for understanding the processing-structure-property relationships in these novel composite materials and optimizing their performance for specific applications.

The DSC thermograms revealed complex thermal behavior with multiple transitions, reflecting the multi-component nature of these composites.

Table 6. Primary thermal events in the DSC analysis of uncured Epoxy-TA-BZ-ZnO samples.

Sample	Glass Transition Temperature (°C)	Exothermic Peak Temperature (°C)	Exothermic Peak Height (mW)	Enthalpy of Curing (J/g)
Epoxy-TA-BZ1-ZnO (A)	152	348	3.0	310
Epoxy-TA-BZ2-ZnO (B)	155	349	3.2	290
Epoxy-TA-BZ4-ZnO (C)	158	352	3.4	270

summarizes the primary thermal events observed in the uncured samples.

A notable endothermic transition was observed around 152–158 °C, corresponding to the glass transition temperature (Tg) of the uncured polymer matrix. The systematic increase in Tg with higher TA-BZ content indicates that the rigid benzoxazine structures restrict molecular mobility within the polymer matrix, resulting in enhanced thermal stability at the molecular level. This relationship between composition and glass transition behavior has significant implications for the mechanical properties and dimensional stability of these materials at elevated temperatures. The most prominent feature in the DSC thermograms was the large exothermic peak between 300 and 375 °C, with maximum intensity at approximately 348–352 °C. This exothermic event corresponds to the ring-opening polymerization of the benzoxazine rings, a key curing mechanism that leads to the formation of a crosslinked network structure. The detailed characteristics of this curing exotherm are presented in

Table 7. Detailed characteristics of the curing exotherm for uncured Epoxy-TA-BZ-ZnO samples.

Sample	Onset Temperature (°C)	Peak Temperature (°C)	Endset Temperature (°C)	Peak Width (°C)	Cure Index
Epoxy-TA-BZ1-ZnO (A)	325	348	365	40	0.78
Epoxy-TA-BZ2-ZnO (B)	328	349	368	40	0.81
Epoxy-TA-BZ4-ZnO (C)	332	352	372	40	0.85

and Figure S4.

The thermal barrier capability and heat transfer performance of the TA-BZ-epoxy-ZnO composites can be rationalized based on their molecular architecture and the thermal properties observed. The inherently low thermal conductivity of the system is primarily attributed to the dense, crosslinked polymeric network formed via benzoxazine polymerization, which is further reinforced by the presence of aromatic moieties from both tannic acid and aniline. These structural features are characteristic of organic thermosets known for their poor phonon transport and, consequently, low thermal conductivity [60,61].

The inclusion of ZnO nanoparticles introduces an additional factor due to the inherently high thermal conductivity of bulk ZnO (~60 W·m⁻¹·K⁻¹) [61]. Nonetheless, at the relatively low loading level employed in this study (5 wt%), and considering the tendency of nanoparticles to agglomerate, their contribution to the overall heat transfer is expected to be limited. As such, the composite’s effective thermal conductivity is projected to remain within the low range of 0.20–0.30 W·m⁻¹·K⁻¹ [60].

Moreover, the thermal barrier performance of the materials is supported by the high glass transition temperatures (152–158 °C), indicating the polymer matrix retains its structural integrity and insulating capabilities under elevated or cyclic thermal conditions. This is particularly advantageous for applications that demand long-term thermal stability.

For context, typical thermal conductivity values for epoxy resins range from 0.15 to 0.25 W·m⁻¹·K⁻¹, while benzoxazine polymers usually fall within the range of 0.12 to 0.20 W·m⁻¹·K⁻¹ due to their tightly crosslinked networks [60]. In comparison, conventional commercial thermal barrier coatings often exhibit thermal conductivities in the range of 0.8–2.0 W·m⁻¹·K⁻¹ [60]. These comparisons suggest that the developed bio-based composites offer competitive thermal insulation performance, with the added benefit of enhanced corrosion resistance. The synergistic combination of low thermal conductivity and high thermal stability positions these materials as promising candidates for multifunctional protective coatings in demanding service environments.

2.2. Long-Term Stability Prediction Based on Polymer Structure

While accelerated aging experiments were not conducted within the scope of this study, the structural attributes of the TA-BZ-epoxy-ZnO composites offer strong indicators of long-term durability. Differential Scanning Calorimetry (DSC) analysis revealed high glass transition temperatures (152–158 °C), indicative of excellent thermal stability and restricted polymer chain mobility under typical service conditions.

Additionally, the antioxidant properties of tannic acid—a polyphenolic component of the system—are expected to contribute significantly to long-term stability. Tannic acid is a well-documented radical scavenger, capable of inhibiting oxidative degradation processes without the need for additional stabilizing additives. This intrinsic antioxidant behavior may extend the service life of the composites, particularly in outdoor and high-temperature environments [61].

The cross-linked network formed through benzoxazine polymerization further enhances durability by providing superior hydrolytic stability compared to polymer systems incorporating ester or urethane linkages. The ring-opening polymerization of benzoxazines generates chemically robust C–N and C–C bonds, which are inherently resistant to hydrolytic cleavage. Previous kinetic studies have demonstrated the minimal susceptibility of such networks to chemical degradation, even under acidic or alkaline conditions [62]. Moreover, bio-based benzoxazine coatings have demonstrated strong resistance to environmental degradation in several accelerated aging studies. Polybenzoxazine composites incorporating natural fibers or nanoparticle reinforcements have exhibited excellent retention of thermal and mechanical properties following prolonged exposure to UV radiation, high humidity, and elevated temperatures. These findings underscore the resilience of benzoxazine-based systems and support the anticipated long-term durability of TA-BZ composites in demanding outdoor and marine environments, where thermal, oxidative, and hydrolytic stability are critical for sustained protective performance [63].

The AFM images demonstrate significant variations in surface morphology between the different uncured polymer thin films, highlighting the impact of differing TA-BZ ratios on these surfaces (Figure 6). Distinct surface characteristics are detected in the uncured samples (A, B, and C). Sample A (Epoxy-TA-BZ1-ZnO) has a reasonably smooth surface punctuated by larger, rounded protrusions, perhaps representing ZnO particles or tiny agglomerates. Conversely, Sample B (Epoxy-TA-BZ2-ZnO) has a more textured surface characterized by elongated, interconnecting structures, indicating improved dispersion of filler particles. Sample C (Epoxy-TA-BZ4-ZnO) exhibits bigger, more pronounced protrusions, potentially indicative of elevated ZnO agglomerates or filler particles. The AFM investigation substantiates the significant influence of ZnO filler content and the varying TA-BZ ratios on the surface morphology of thin film polymers. The interplay of these elements is crucial in determining the final surface characteristics of the material, underscoring the importance of filler content in optimizing polymer surface qualities.

The analysis of stress-strain curves in Figure 7 for the uncured (Epoxy-TA-BZ-ZnO-1, 2, 4, designated as A, B, and C) thin film polymer samples, all containing a constant ZnO filler content, reveals a significant influence of TA-BZ ratios on mechanical properties. For instance, Epoxy-TA-BZ-ZnO-1 (sample A) exhibits a moderate initial slope, achieving a stress of 30 MPa at 1% strain, indicating the stiffness of the material. The ultimate tensile strength (UTS) of this sample reaches approximately 100 MPa at 10% strain. Although the ZnO content remains constant, the varying TA-BZ ratios result in distinct mechanical responses in the uncured samples. In the uncured state, Epoxy-TA-BZ4-ZnO (sample C) exhibits the highest initial modulus and UTS, reaching 110 MPa at 10% strain, indicating that the composition of the polymer matrix can significantly impact its mechanical properties. This suggests that the higher TA content in sample C leads to improved polymer network interactions, ultimately enhancing mechanical performance.

These observations underscore the importance of maintaining constant ZnO filler content and highlight how variations in TA-BZ ratios critically influence the polymer network’s structure and its interaction with the ZnO filler. The findings emphasize the necessity of optimizing the TA-BZ ratio to improve filler dispersion, stress distribution, and overall mechanical performance in the polymer matrix [64,65,66,67].

2.3. Anticorrosion Properties of Uncured Epoxy-TA-BZ-ZnO Samples

The anticorrosion performance of the uncured (A, B, and C) thin-film polymer composites, with a constant concentration of ZnO filler, was systematically evaluated using electrochemical potentiostat techniques. The investigation focused on elucidating the impact of variations in the mole ratios of TA-BZ on the corrosion resistance of the composites. The results demonstrate a distinct shift in the corrosion potential (Ecorr), with uncured samples exhibiting values between −412 mV and −405 mV vs. SCE, which shifted positively with the mole ratio of TA-BZ as presented in

Table 8. Electrochemical Corrosion Parameters of Epoxy-TA-BZ-ZnO Composites in 3.5% NaCl Solution.

Sample	Ecorr (mV vs. SCE)	Icorr (µA/cm2)	βa (mV/Decade)	βc (mV/Decade)	Corrosion Rate (mm/Year)
Epoxy-TA-BZ1-ZnO (A)	−412 ± 8	5.8 ± 0.3	95 ± 4	88 ± 3	0.067 ± 0.004
Epoxy-TA-BZ2-ZnO (B)	−408 ± 6	5.5 ± 0.2	97 ± 3	90 ± 4	0.064 ± 0.003
Epoxy-TA-BZ4-ZnO (C)	−405 ± 5	5.2 ± 0.3	98 ± 5	91 ± 3	0.060 ± 0.004

Values represent mean ± standard deviation (n = 3). Table’s keys: Ecorr (mV vs. SCE): The corrosion potential, which reflects the material’s susceptibility to corrosion. More negative values of E_corr indicate a higher tendency of the material to corrode; Icorr (µA/cm²): The corrosion current density, which quantifies the rate of electron transfer during the corrosion process. Higher I_corr values correspond to a faster rate of corrosion; βa (mV/decade): The anodic Tafel slope, representing the kinetics of metal dissolution during the corrosion process; βc (mV/decade): The cathodic Tafel slope, which describes the rate of the cathodic reaction, commonly oxygen reduction, during corrosion; Corrosion rate (mm/year): A value derived from the corrosion current density, indicating the material loss due to corrosion over time, expressed in millimeters per year.

and Figure S5. This trend suggests that variations in the polymer matrix composition may enhance the electrochemical nobility of the material. In terms of corrosion current density (Icorr), uncured samples exhibited values between 5.2 and 5.8 µA/cm², implying that the TA-BZ composition affects the corrosion rate. Interestingly, the Icorr values decreased as the mole ratios of TA-BZ varied, suggesting that ZnO, despite its constant content, contributes to lowering the corrosion rate by impeding electrolyte penetration through the composite. Furthermore, the Tafel slopes (βa and βc) displayed a gradual increase with variations in the mole ratios of TA-BZ for the uncured samples, indicating that the electrochemical kinetics of both anodic and cathodic reactions are influenced by the polymer composition. The calculated corrosion rates for the uncured samples demonstrate moderate values (0.060–0.067 mm/year). Notably, the corrosion rates decreased as the mole ratios of TA-BZ varied across the samples, underscoring the protective role of ZnO in enhancing the composite’s resistance to corrosive environments. Cyclic voltammetry analysis revealed narrow hysteresis loops for the samples, indicative of lower charge storage capacity and superior corrosion resistance. The anticorrosion rate of the Epoxy-TA-BZ-ZnO samples in this study was measured at a level significantly lower than the rate observed in previous studies of TA-BZ without epoxy and ZnO additions, where TA-BZ A4 exhibited an anticorrosion rate of 0.131 mm/year. This reduction in the anticorrosion rate suggests that incorporating epoxy and ZnO favorably enhances the corrosion resistance of the TA-BZ matrix. Although epoxy and ZnO are often added to improve other material properties, such as mechanical strength and UV resistance, our findings indicate a potential synergistic effect that enhances anticorrosion performance.

Overall, these findings highlight the complex interplay between polymer composition and ZnO filler in governing the anticorrosion properties of epoxy-based composites. Although the constant ZnO content provides some level of protection, optimizing the mole ratios of TA-BZ is crucial for balancing the mechanical and anticorrosion performance of these materials in high-temperature and corrosive environments.

3. Experimental

3.1. Chemicals and Materials

All chemicals and reagents, including Tannic Acid (99.5%), paraformaldehyde (95%), Aniline, Ethanol, Methanol, chloroform (99.5%), toluene (99.5), ZnO, the epoxy resin and its hardener used in this research were used as supplied by Sigma-Aldrich (St. Louis, MO, USA) and Fisher Scientific (Waltham, MA, USA) without further purification. Zinc oxide nanoparticles (99.9% purity, average particle size 30–50 nm, specific surface area 15–25 m²/g) were obtained from Sigma-Aldrich (Product No. 721077). The epoxy resin system consisted of diglycidyl ether of bisphenol A (DGEBA, epoxy equivalent weight 182–192 g/eq) with a poly(oxypropylene)diamine hardener (amine value 380–420 mg KOH/g). The resin viscosity at 25 °C was 11,000–14,000 mPa·s as specified by the manufacturer. All materials were stored under nitrogen atmosphere prior to use to prevent moisture absorption.

3.2. Instrumentation

Fourier transform infrared (FT-IR) spectra were obtained using a PerkinElmer Spectrum 100 FT-IR spectrometer (PerkinElmer, Waltham, MA, USA) within the 4000–4500 cm⁻¹ range. Nuclear magnetic resonance (NMR)) spectra were investigated using an AVANCE III 400 MHz NMR Spectrometer (Bruker Corporation, Berlin, Germany) for 1H-NMR analyses. The nanoparticles were analyzed using an X-ray diffractometer (D8-Focus, Bruker, Madison, WI, USA) with CuKα radiation (1.5418 Å). The instrument operated at a current of 40 mA, voltage of 40 kV, and a step filter of 0.01 ° to determine the design creation and transparent stage. Scanning electron microscopy (SEM) was used to visualize the composite nanoparticles and investigate their morphology. The surface morphology was investigated using a scanning electron microscope (SEM) from JEOL Ltd. (Tokyo, Japan). The SEM images were captured with a Zeiss LEO Supra 55VP Field Emission and SEM Zeiss 1530 (Carl Zeiss AG, Oberkochen, Germany). Composite nanoparticle suspensions were diluted tenfold with their dispersion media for sample preparation. A drop of the diluted nanoparticle suspension was then placed directly on a polished metal (aluminum) sample holder. The specimens were dehydrated in a vacuum. The samples were subsequently coated with gold using an EMITECH K450X sputter coater (Quorum Technologies, East Sussex, UK). Thermal stability was investigated using a Shimadzu D60 thermogravimetric analyzer (Shimadzu Corporation, Kyoto, Japan). Mechanical properties were evaluated using an Instron 5967 Universal Testing Machine (Instron, Norwood, MA, USA). Stress-strain measurements were conducted at a crosshead speed of 1 mm/min, providing information on material strength, ductility, and potential susceptibility to stress-corrosion cracking.

The anticorrosion properties were directly assessed using a PARSTAT 4000A Potentiostat/Galvanostat (Princeton Applied Research, Oak Ridge, TN, USA). Electrochemical measurements were performed in a standard three-electrode cell configuration, with the sample as the working electrode, a platinum wire counter electrode, and a saturated calomel reference electrode. Tests were conducted in a 3.5 wt% NaCl aqueous solution at 25 ± 2 °C. A 3.5 wt% NaCl solution was selected to simulate seawater conditions, representing one of the most prevalent and aggressive environments faced by protective coatings in marine applications. This concentration reflects the average salinity of ocean water and is widely recognized as a standard testing medium in corrosion studies, as outlined in ASTM G31 [68] and ISO 17475 [32,69]. Although alternative corrosive media (e.g., acidic or alkaline solutions) could yield additional insights, the NaCl environment was prioritized due to its direct relevance to real-world marine corrosion scenarios in which such coatings are commonly employed.

Potentiodynamic polarization curves were recorded from −250 mV to +250 mV versus the open circuit potential at a scan rate of 0.5 mV/s. Working electrodes were prepared by mounting steel substrates (1 cm²) in epoxy resin, leaving only the test surface exposed. Prior to coating application, the substrates were sequentially polished with 400, 800, and 1200 grit SiC papers, followed by polishing with 0.3 μm alumina paste to achieve a mirror finish. The electrodes were then degreased with acetone and ethanol, dried under nitrogen, and immediately coated with the test samples. The coating thickness was maintained at 50 ± 5 μm using a wet film thickness gauge.

Before electrochemical measurements, the coated electrodes were immersed in a 3.5 wt% NaCl solution for 30 min to establish a stable open circuit potential (OCP). The composition used for composite preparation was TA-BZ monomer (10 wt%), epoxy resin (85 wt%), and ZnO nanoparticles (5 wt%). All potentiodynamic polarization tests were initiated from −250 mV vs. OCP and terminated at +250 mV vs. OCP to prevent excessive coating degradation.

The purpose of this comprehensive analytical approach was to elucidate the underlying mechanisms of corrosion resistance and evaluate the effectiveness of protective treatments applied to the samples. By combining surface, structural, thermal, and electrochemical analyses, this study aimed to provide a holistic understanding of the factors influencing anticorrosion properties. The results are expected to contribute to the development of more effective corrosion mitigation strategies for the studied materials in aggressive environments.

3.3. Methods

3.3.1. Synthesis of Tannic Acid Benzoxazine (TA-Bz) Monomer

The synthesis was conducted using a one-pot Mannich condensation method. TA-BZ monomers were prepared in three distinct molar ratios, designated as TA-BZ1, TA-BZ2, and TA-BZ4, as shown in Scheme 1. Specifically, 0.588 mmol of tannic acid (TA) was reacted with varying amounts of aniline (29.3 mmol, 4.73 mmol, and 1.83 mmol) and paraformaldehyde (58.7 mmol, 14.65 mmol, and 5.87 mmol, respectively) in a round-bottom flask containing a toluene–ethanol mixture (1:2 v/v) at 70 °C for 24 h under reflux. After completion of the reaction, the solvent was removed under reduced pressure. These ratios (1:6:12, 1:4:8, and 1:2:4, respectively) were selected based on the classical Mannich reaction stoichiometry (phenol: amine: paraformaldehyde of 1:1:2), while accounting for the multifunctional nature of tannic acid, which contains approximately 8–10 phenolic hydroxyl groups. Adjusted ratios were thus employed to accommodate the statistical likelihood of benzoxazine ring formation at multiple sites and to manage steric hindrance, enabling control over the degree of substitution and the potential crosslink density in the final structure [70]. The crude products were recrystallized from methanol to eliminate unreacted monomers and residual by-products. To further purify the monomers, the recrystallized solids were dried at 50 °C under vacuum overnight. While this temperature is sufficient to remove residual toluene under reduced pressure, paraformaldehyde is expected to be fully consumed during the Mannich condensation reaction. Therefore, no significant amount of free paraformaldehyde is anticipated in the final product. This purification protocol was implemented to enhance the chemical safety of the monomers and to align with the study’s sustainability objectives.

3.3.2. Preparation of Pure Epoxy Resin

According to the methodology indicated by Hussein et. al., the epoxy resin, which serves as the matrix, was produced in this study by a simple dissolving process followed by ultrasonication [2]. Subsequently, a small amount of chloroform was introduced to achieve a 1:1 weight ratio of ‘A’ and ‘B’ components. These combined materials were then exposed to ultrasonication for 15 min. After receiving a further 10 min sonication to ensure a homogeneous blending, the resulting mixture was placed onto a petri dish. The dish was left at room temperature overnight until the solvent evaporated. Acquired were precise, thin sheets of epoxy resin, which were subsequently dried in a desiccator before their initial usage.

3.3.3. The Preparation of TA-BZ Blend Epoxy-ZnO

After the preparation of the TA-BZ samples, they were combined with the previously prepared epoxy resin and 5 mg of ZnO nanoparticles to form the composite. The resulting mixture was subjected to sonication for 30 min to ensure uniform dispersion of the nanoparticles. After sonication, the mixture was transferred into a Petri dish and left to cure at room temperature overnight, yielding smooth films. These films were designated as Epoxy-TA-BZ1-ZnO (A), Epoxy-TA-BZ2-ZnO (B), and Epoxy-TA-BZ4-ZnO (C).

4. Conclusions

In conclusion, the polymer structure described represents a highly engineered benzoxazine-epoxy-ZnO composite, wherein the benzoxazine backbone ensures excellent thermal stability, the epoxy groups provide crosslinking flexibility, and ZnO nanoparticles act as both reinforcing agents and active anticorrosive components. This combination is tailored to yield advanced materials with improved mechanical strength, thermal resistance, and superior anticorrosion performance, making it suitable for high-performance applications in industries such as marine, automotive, and infrastructure, where protection against corrosion is critical for long-term durability and reliability.

This study successfully demonstrates the development of a novel tannic acid-benzoxazine (TA-BZ)-based polymer composite reinforced with epoxy and zinc oxide (ZnO) nanoparticles. The investigation highlights the significant performance of uncured samples in terms of anticorrosion and mechanical properties. The uncured Epoxy-TA-BZ4-ZnO sample exhibited superior anticorrosion properties due to optimal ZnO dispersion, with enhanced mechanical strength and tensile properties. Despite variations in performance with different TA-BZ ratios, the improved structural integrity and anticorrosion properties make these materials strong candidates for applications demanding high performance in corrosive environments. This research contributes to the ongoing development of high-performance materials for industrial applications, offering a valuable balance between corrosion resistance and mechanical strength.