Preparation of Environmentally Friendly Anticorrosive Coatings with Aniline Trimer-Modified Waterborne Polyurethane

Abstract

Eco-friendly waterborne coatings frequently exhibit poor corrosion resistance, high solvent content, and extended curing times, attributed to the excessive employment of hydrophilic groups and petroleum-derived polyols. In this work, aniline trimer (ACAT) and polyethylene glycol (PEG) were used as chain extenders. E-44 epoxy resin was subsequently utilized to modify the system and an aniline trimer-modified waterborne polyurethane (AT-WPU) dispersion was prepared and characterized. The chemical structure of the synthesized ACAT was characterized employing 1H NMR, ESI-MS, and FTIR spectroscopy. The structure and coating performance of the AT-WPU dispersion were investigated utilizing FTIR, particle size analysis, thermogravimetric analysis, DSC, TEM, SEM, and electrochemical corrosion testing. The results demonstrate that the aniline trimer-modified waterborne polyurethane dispersion was successfully synthesized. Additionally, the DSC analysis results and thermogravimetric graphs indicate that the glass transition temperature and thermal stability of the coatings increased with the addition of aniline trimer. As the aniline trimer content increased, the hardness and adhesion of the coatings were significantly enhanced. In the electrochemical corrosion assessment, the corrosion current density of AT-WPU-3 attained 7.245 × 10⁻⁹ A·cm⁻², and the corrosion rate was as low as 0.08 μm·Y⁻¹, indicating excellent corrosion resistance. The present study provides promising practical applications in the domain of metal material protection.

1. Introduction

Metallic corrosion is a pervasive issue with wide-ranging effects on human life and national economic development. According to relevant data, the global annual economic cost of metallic corrosion surpasses USD 700 billion. In our country, economic losses from metallic corrosion have consistently accounted for approximately 5% of the gross domestic product [1]. Clearly, the economic losses resulting from corrosion have a substantial impact [2]. Consequently, the development of simple and effective corrosion protection technologies is essential for mitigating economic losses and supporting national economic growth. Common metal anticorrosion methods include corrosion-resistant alloys, anticorrosive organic coatings, anodic passivation, and cathodic protection. Among these, organic anticorrosive coatings have been widely used due to their straightforward manufacturing process, low cost, and substantial protective effect [3,4,5]. However, in practical engineering applications, traditional solvent-borne anticorrosive coatings release volatile organic compounds (VOCs) during manufacturing, production, and construction, which significantly harm the atmosphere, environment, and human health. Consequently, waterborne coatings have become the industry’s primary development focus due to their safety, non-toxicity, and broad applicability [6,7].

Furthermore, epoxy resin demonstrates exceptional adhesion, high hardness, corrosion resistance, and thermal stability, in addition to superior solvent resistance and excellent color retention properties. These characteristics make epoxy resins particularly advantageous for improving the physical performance of waterborne anticorrosive coatings [8,9]. By modifying waterborne epoxy coatings, their corrosion resistance can be further improved. This includes modifications using oxidized graphene [10], nano-TiO₂ [11], siloxane [12], polydopamine [13], as well as conductive polymers [14]. Among these, conductive polymer-modified anticorrosive coatings are considered a novel type of metal anticorrosive coating, primarily composed of polyaniline (PANI), polythiophene (PTH), polypyrrole (PPY) and their derivatives, all of which possess significant application potential [15]. Polyaniline-based conductive polymers have garnered profound interest owing to their easy preparation, low cost, favorable conductivity, and environmental stability [16,17,18]. The anticorrosion mechanism of polyaniline is complex and multifaceted, involving anodic protection, cathodic separation, inhibitor release, reduced ion penetration, and electric field effects. Additionally, it can provide an anticorrosion effect by forming a passivated metal oxide layer on the metal surface through redox reactions [19,20].

Although polyaniline demonstrates excellent anticorrosion performance, its poor solubility in common organic solvents has impeded its processing and practical application across various material fields. In contrast, polyaniline oligomeric derivatives, such as aniline trimer and tetramer, exhibit higher solubility in a variety of organic solvents. Consequently, incorporating high-solubility polyaniline oligomeric derivatives into the resin molecular structure can effectively enhance the anticorrosion performance of the coating [21,22]. In a study by Huang et al. [23], aniline trimer was used to modify waterborne polyurethane, leading to the successful preparation of electroactive waterborne polyurethane (EWPU). Electrochemical testing revealed that the EWPU coating exhibited strong anticorrosion performance on cold-rolled steel (CRS) electrodes. In a study by Zeng et al. [24], bio-based air-drying waterborne polyurea dispersion products were successfully prepared by synthesizing an alcohol-based intermediate (AK) as a polyol and then using an aniline trimer (AT) as a chain extender. By evaluating the coating performance and anticorrosion properties, the researchers found that the incorporation of an alkyd intermediate (AK) and aniline trimer (AT) resulted in a coating with low viscosity, balanced performance, and excellent anticorrosion capabilities. Oxidized graphene was modified with aniline trimer (ATGO) and then incorporated into an epoxy coating to produce an aniline trimer-modified oxidized graphene composite coating (ATGO/EP) in a study by Guo et al. [25]. The resulting composite coating was spray-applied onto Q235 steel plates for subsequent characterization and corrosion testing. The results demonstrated that the incorporation of ATGO significantly enhanced the corrosion resistance of the coating.

Although aniline trimer has been used to modify single-component polyurethane or polyurea coatings, these coatings still predominantly relied on petroleum-based polyols, which suffer from drawbacks such as long drying times and high solvent content. In this study, the researchers successfully synthesized amine-terminated aniline trimer. Subsequently, aniline trimer-modified waterborne polyurethane emulsions were prepared by replacing the petroleum-based polyols with polyethylene glycol (PEG) as a hydrophilic chain extender and introducing E-44 epoxy resin. By measuring the solid content of the emulsion, we determined the appropriate addition amount of the allnex CYMEL^®325 amine-type curing agent as the crosslinking agent (Component B). After completing the curing process, the researchers prepared the final aniline trimer-modified waterborne polyurethane anticorrosion coating, and then spray-applied the composite coating onto 90 mm × 120 mm × 0.2 mm tin-plated steel panels. Using characterization techniques including mass spectrometry (MS), nuclear magnetic resonance (NMR) spectroscopy, Fourier-transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), electrochemical impedance spectroscopy (EIS), and adhesion tests, we analyzed the optimal addition amount of aniline trimer and determined the best modification process of aniline trimer for the waterborne polyurethane.

2. Materials and Methods

2.1. Materials

Aniline (purified by reduced pressure distillation), p-phenylenediamine sulfate (PPD), ammonium persulfate (APS), and isophorone diisocyanate (IPDI) were purchased from Aladdin Reagent Co., Ltd., a chemical supplier located in Shanghai, China. Polyethylene glycol (PEG) with an average molecular weight of 200 g/mol and PEG with an average molecular weight of 2000 g/mol were also obtained from Chengdu Kelong Chemical Co., Ltd., a chemical supplier located in Chengdu, China. The higher molecular weight PEG (2000 g/mol) was dried at 80 °C under vacuum (75 mmHg) for 2 h before use. Additionally, dibutyltin dilaurate (DBTDL), acetone, and N, N-dimethylformamide (DMF) were provided by the same Chengdu Kelong Chemical Co., Ltd. (Chengdu, China). The epoxy resin (E-44) was supplied by Shandong Youso Chemical Technology Co., Ltd. (Heze, China). Allnex CYMEL^® 325 is offered by Zhanxin Resins (Shanghai) Co., Ltd. (Shanghai, China).

2.2. Methodologies

2.2.1. Synthesis of Amine-Capped Aniline Trimer (ACAT)

The aniline trimer was synthesized following the method originally proposed by Liu et al. [26]. Initially, 200 mL of 1 M HCl solution and 80 mL of anhydrous ethanol were added to a round-bottom flask equipped with a magnetic stir bar. Subsequently, 2.956 g of p-phenylenediamine sulfate and 1.853 g of aniline (purified by reduced pressure distillation) were added, and the mixture was cooled to −5 °C in an ice-salt bath. Lastly, 4.541 g of ammonium persulfate was dissolved in 50 mL of 1 M HCl solution and slowly added dropwise to the reaction mixture using a dropping funnel. Once the addition was completed, the reaction mixture was stirred for an additional 30 min. The resulting solution was then poured into a Büchner funnel and filtered. The product was initially washed with 1 M HCl solution for 1 h, followed by rinsing with deionized water. Afterward, the product was washed with 10% (by mass) ammonia water for 1 h, followed by rinsing with deionized water. Finally, the product was dried in a 60 °C oven for 24 h, yielding a purple-black solid product. The synthetic principle and a photograph of the aniline trimer are shown in Figure 1.

2.2.2. Preparation of Waterborne Polyurethane Modified with Aniline Trimer

The composition of the water-based polyurethane (WPU) emulsion modified by the aniline trimer is presented in

Table 1. Compositions of waterborne polyurethane dispersions modified with aniline trimer.

Types	AT Content (1)	Component/g
		IPDI	E-44	DBTDL	PEG200	PEG2000	Acetone	DMF	AT
AT-WPU-0	0%	6.67	22.7	0.3	2.3	22	20	0	0
AT-WPU-1	3%	7.33	22.7	0.3	2.3	22	30	20	0.8
AT-WPU-2	9%	8.23	22.7	0.3	2.3	22	50	30	2.21
AT-WPU-3	15%	9.34	22.7	0.3	2.3	22	70	40	3.72

⁽¹⁾ Aniline trimer content refers to the proportion of aniline trimer in the chain extender.

. The detailed experimental procedure can be divided into four main steps as follows: In a four-neck round-bottom flask equipped with a stirrer, condenser, and nitrogen inlet, 40 g of DMF was added as the solvent, and 0.1 g of DBTDL was added as the catalyst. Subsequently, IPDI and PEG-200 were added, the mixture was placed in a water bath at 60 °C under nitrogen protection, with heating and stirring (300 rpm) for one hour. Subsequently, the aniline trimer was dissolved in DMF (20 g) and added dropwise using a constant pressure funnel to ensure complete reaction, with continued stirring for 1 h. The isocyanate content was monitored and adjusted to the theoretical value using a butylamine back-titration method [27]. The temperature of the water bath was raised to 70 °C, and the preheated E-44 epoxy resin and dehydrated PEG-2000 were added for a continuous reaction lasting four hours. The viscosity of the system was monitored and controlled by adding acetone during the third stage of the reaction to prevent gelation. After the reaction was completed, the temperature was cooled to 40 °C, and 70 mL of deionized water was slowly added dropwise under high-speed stirring (800 rpm) for 30 min to form the emulsion. Four groups of emulsions with different amounts of ACAT addition were prepared, designated as AT-WPU-0, AT-WPU-1, AT-WPU-2, and AT-WPU-3. The synthetic route is depicted in Figure 2.

2.2.3. Preparation of Waterborne Polyurethane Modified with Aniline Trimer Coatings

The coating substrate consisted of 90 mm × 120 mm × 0.2 mm tinplate sheets. Before applying the coating, the tinplate surface was mechanically ground to remove any oil stains, rust, and other contaminants. The prepared coating emulsion was then blended with 30 wt.% of the allnex CYMEL^®325 amine-based curing agent as the solid constituent. The resulting blend was evenly applied to the pre-treated surface of the tinplate by using a high-pressure spray gun. Subsequently, the coated samples were subjected to thermal curing at 120 °C for a period of three hours in a drying oven, resulting in the production of the final test specimens.

3. Characterization

3.1. 1H NMR

1H NMR characterization was performed using a Bruker 500M nuclear magnetic resonance (NMR) spectrometer, which was manufactured by Bruker in Karlsruhe, Germany. The measurement was conducted at a frequency of 500 MHz, with tetramethylsilane (TMS) serving as the internal standard. The acquired data were subsequently processed using MestReNova 14.0 software.

3.2. ESI-MS

Mass spectrometry characterization was performed using a Waters ZQ2000 mass spectrometer (Milford, MA, USA). Prior to analysis, the samples were first dissolved in methanol solvent, then filtered through a needle filter, and subsequently loaded into the sample vial. The measurements were conducted in the positive ion acquisition mode.

3.3. FTIR Spectroscopy

FTIR spectra of the samples were acquired using a Bruker Instruments FTIR spectrometer (Model: Vector-22, Billerica, MA, USA), which was manufactured in Karlsruhe, Germany. The spectral range was recorded from 500 to 4000 cm⁻¹, Before the measurements, the samples were uniformly coated on KBr discs and then heated at 70 °C in a vacuum drying oven for 3 h.

3.4. Particle Size Testing

The water-borne polyurethane emulsion modified with aniline trimer was diluted with deionized water to approximately 0.02 wt.%. The average particle size was measured using a nanoparticle size analyzer (Nano-ZS, Malvern Ltd., Malvern, UK). The experimental values represent the average of three repeated measurements.

3.5. TEM

Transmission electron microscopy (TEM) was employed to observe the morphology of the samples using a JEM-2100 transmission electron microscope, which was manufactured by JEOL Ltd. in Tokyo, Japan. The diluted emulsion particles were stained with a 2 wt.% phosphotungstic acid solution prior to TEM imaging.

3.6. Fundamental Property Testing of the Coatings

The coating thickness was measured using a coating thickness gauge in accordance with the GB/T 13452.2-2008 standard method [28]. The hardness, adhesion, and flexibility of the different AT-WPU coatings were evaluated in accordance with the Chinese national standards GB/T 6739-2022 [29], GB/T 9286-2021 [30], and GB/T 1731-2020 [31].

3.7. SEM

Initially, all the samples were mounted on the sample holder using a conductive adhesive. Subsequently, the samples were sprayed with a thin layer of gold under vacuum. Finally, the samples were observed using a scanning electron microscope (VEGA3, TESCAN Ltd., Brno, Czech Republic) at magnifications of 1000×, 2000×, 5000× and an accelerating voltage of 10.0 kV.

3.8. Thermogravimetric Analysis Test

The thermal performance of the aniline trimer-modified water-borne polyurethane thin films was evaluated using thermogravimetric analysis (TGA). The TGA experiments were conducted using a thermogravimetric analyzer (Q500, TA Instruments, New Castle, DE, USA) with a heating rate of 10 °C/min. The temperature range of the TGA experim0ents was from room temperature to 550 °C under an inert nitrogen atmosphere.

3.9. Differential Scanning Calorimetry

Differential scanning calorimetry was performed on a DSC25 Differential Scanning Calorimeter from TA Instruments, USA. Amounts of 4–8 mg of the films were selected and ramped from room temperature to 250 °C at a ramp rate of 10 °C/min.

3.10. Electrochemical Corrosion Studies

The aniline trimer-modified water-based polyurethane anti-corrosion coatings were fabricated by high-pressure spraying and then cured at 130–150 °C for 3 h. Subsequently, the coated samples were immersed in a 3.5 wt.% sodium chloride (NaCl) solution for 48 h, followed by electrochemical corrosion tests in the same 3.5 wt.% NaCl solution using a CHI660E electrochemical workstation. A typical three-electrode system was used, with the aniline trimer-modified water-based polyurethane (AT-WPU) coated tin-plated steel as the working electrode, a platinum electrode as the counter electrode, and a saturated KCl electrode as the reference electrode. All electrochemical corrosion tests were carried out at least three times to ensure the reliability and reproducibility of the experimental results. The voltage scanning rate was set at 0.5 mV/s, and the scanning range was from −800 mV to 1500 mV relative to the open-circuit potential. From the obtained Tafel polarization curves, the corrosion potential (E_corr) and corrosion current density (I_corr) of the samples were calculated, which were then used to analyze the corrosion rates (R_corr) of the different coatings. The corrosion rate (R_corr) was calculated using the following equation: [32]

$$R_{corr}\left(mm/year\right)=3270\times {\displaystyle \frac{[I_{corr}·M]}{[V·d]}}$$

(1)

where 3270 = 0.01 × [1 year (in seconds)/96,497.8] and 96,497.8 represents 1 faraday. Additionally, M denotes the atomic mass, V represents the valence, and d signifies the density in g/cm³, in coulombs.

4. Results and Discussion

4.1. Structural Characterisation of Amine-Capped Aniline Trimer (ACAT)

The synthesized amine-terminated aniline trimer was initially characterized. In the positive-ion mode of electrospray ionization mass spectrometry (ESI-MS), the molecular ion peaks typically appear as “M+” or “MH+”. As shown in the mass spectrum in Figure 3, a prominent molecular ion peak was observed at 289.4, consistent with the molecular weight of ACAT. As shown in the FTIR (KBr, cm⁻¹) spectrum in Figure 4, the aniline trimer displayed characteristic absorption peaks: the terminal amino group –NH₂ at 3201 and 3311 cm⁻¹, the C=C stretching vibrations of the quinone ring and benzene ring at 1596 and 1502 cm⁻¹, respectively, the C–N stretching vibration at 1271 cm⁻¹, and the out-of-plane bending vibration of the C–H bond on the para-substituted benzene ring at 830 cm⁻¹. The 1H NMR (500 MHz, d6-DMSO) spectrum in Figure 5 revealed four prominent proton absorption peaks, indicating the highly symmetric molecular structure of ACAT. The singlet peak at 5.43 ppm was attributed to the active hydrogen a of the amino group at the ends of the ACAT molecule. The resonance peaks around 6.55 ppm were assigned to the hydrogen atoms b and c on the benzene rings adjacent to ACAT, while the multiple around 6.95 ppm was attributed to the hydrogen d on the quinone ring in the center. The relative integral values of the observed peaks were close to the theoretical values, confirming the successful synthesis of ACAT.

4.2. FTIR Analysis of AT-WPU

The synthesized AT-WPU was analyzed by Fourier transform infrared (FTIR) spectroscopy. Figure 6a presents the FTIR spectrum of the WPU without ACAT addition, which exhibits the characteristic polyurethane -NH and -OH absorption peaks at 3502 and 3330 cm⁻¹, C-H stretching vibration bands at 2953 and 2882 cm⁻¹, C=O stretching vibration of the amide group at 1712 cm⁻¹, C-N stretching vibration at 1509 cm⁻¹, and strong peaks at 1240 and 1110 cm⁻¹ attributed to the -O-C=O group in the polyurethane soft segment. Figure 6b displays the FTIR spectrum of the synthesized AT-WPU-1, which features an absorption peak around 3300 cm⁻¹ attributed to the N-H stretching vibration of the urea group, typically slightly lower in wavenumber than the N-H stretching of the amide group. The ureido group was synthesized by the reaction of the terminal amino group of the aniline trimer with the isocyanate group, indicating that ACAT was successfully grafted into the long chain of the waterborne polyurethane. The C-H stretching vibration bands of the chain segments are observed at 2953 and 2882 cm⁻¹, while the C=O absorption peak of the urea group appears at 1712 cm⁻¹, similar to the amide group, and the C-N stretching vibration is detected at 1509 cm⁻¹. The FTIR absorption peaks at 1240 and 1105 cm⁻¹ are characteristic of the aromatic N-H group [23]. The absorption peak at 835 cm⁻¹ is attributed to the para-substitution of the benzene ring in AT-WPU-1, which serves as confirmation of the successful grafting of the aniline trimer. Figure 6c presents the FTIR spectrum of the isocyanate compound IPDI, which exhibits the characteristic absorption peak of the -NCO group at 2262 cm⁻¹. Analysis of the spectra indicates that neither AT-WPU-0 nor AT-WPU-1 displays a significant signal around 2260 cm⁻¹; this suggests that all isocyanate groups have reacted with the quantitative amino and hydroxyl groups and the reaction has reached completion [33].

4.3. Particle Size Analysis

The particle size distribution of the synthesized AT-WPU latexes was analyzed using a nanoparticle analyzer. The figure shows that the particle sizes of all the latexes are within the same order of magnitude, suggesting that the addition amount of the aniline trimer chain extender has negligible impact on the polymer particle size [34], and it will not destroy the stability of the emulsion. Generally, a small range of particle sizes results in the formation of well-dispersed waterborne polyurethane emulsions. As shown in Figure 7, the size distributions of the four different latexes all display a single peak, with an average particle size ranging from 200 to 600 nm. For instance, the AT-WPU-2 latex has an average particle size of 420.2 nm and a polydispersity index (PI) of 0.34; this evidence indicates a narrow size distribution and uniform dispersion of the AT-WPU-2 latex in water [35].

4.4. TEM Images

Typical transmission electron microscopy (TEM) images of the different water-borne polyurethane (WPU) latex particles are shown in Figure 8. Figure 8a depicts the unmodified WPU latex AT-WPU-0, which consists of relatively uniform spherical particles approximately 200 nm in size. This observation aligns with previous particle size analysis, indicating a stable dispersion of the particles within the latex. Figure 8b,c present the magnified images of the two aniline trimer-modified WPU latex particles (AT-WPU-1 and AT-WPU-2). These particles primarily exhibit an elliptical or near-spherical morphology, with sizes ranging from approximately 400 to 500 nm and a distinct external boundary. This morphological change is attributed to the introduction of hydrophilic moieties, such as hydroxyl and ether groups, in the modified resin. During the emulsification process in deionized water, the hydrophobic segments of the macromolecular chains aggregate to form micelle-like particles, with the hydrophobic segments forming the core and the hydrophilic segments constituting the outer shell. Under the influence of surface tension and electrostatic forces, these particles adopt a spherical shape and exhibit a certain degree of repulsion, leading to a stable dispersion in the aqueous medium. Figure 8d presents a transmission electron microscopy (TEM) image of AT-WPU-3 at a lower magnification, which reveals a more pronounced three-dimensional mesh structure of the macromolecular chains and the spherical particles present in the emulsion. This phenomenon may be attributed to the fact that as the aniline trimer content increases, its terminal amino group reacts with the isocyanate group, forming more urea groups and creating a multi-block copolymer. The urea groups are capable of forming both intramolecular and intermolecular hydrogen bonds, which results in the strengthening of the interchain interactions. Consequently, the polymer chains demonstrate enhanced resistance to sliding under shear forces and are dispersed in larger aggregates.

4.5. Fundamental Properties of Coatings

Table 2. Fundamental properties of AT-WPU coatings.

Sample	Thickness (μm)	Adhesion	Pencil Hardness	Flexibility (mm)	Impact Resistance (cm)
AT-WPU-0	41.2–43.7	1	2H	2	120
AT-WPU-1	52.6–58.4	1	3H	2	80
AT-WPU-2	54.2–61.6	0	4H	2	60
AT-WPU-3	55.6–63.3	0	4H	2	60

summarizes the fundamental properties of the various water-borne polyurethane (WPU) coatings, including film thickness, adhesion, pencil hardness, flexibility, and impact resistance. Test results for adhesion, impact resistance, and flexibility are shown in Figure 9. As indicated in the table and figures, the film thicknesses of the aniline trimer-modified WPU coatings are similar to those of the unmodified coating, remaining around 55 μm. This can be attributed to the consistency maintained in the latex content, the curing agent ratio, and the spray application parameters throughout the high-pressure spray coating process. Compared to the unmodified coating, the modified coatings demonstrate higher pencil hardness, and it was observed that the hardness of the coating increased in proportion to the addition of aniline trimer. Two factors contribute to this: (1) the amine-type curing agent can react with the terminal epoxy groups of the resin during the high-temperature curing process, resulting in increased crosslinking of the coating film, as illustrated in Figure 10; and (2) the three benzene rings in the aniline trimer structure enhance the rigidity of the polymer backbone. Consequently, the impact resistance is somewhat diminished, but the increased overall cross-linking of the coating mitigates the discrepancy in impact resistance to a considerable extent. Furthermore, the modified coatings also retain comparable flexibility compared to the unmodified coating.

4.6. Investigation of Film Surface

The surface morphology of waterborne polyurethane coatings modified by the trimerization product of aniline is presented in Figure 11. Figure 11a corresponds to the polyurethane coating AT-WPU-1 with 3% ACAT content, which exhibits some uneven areas on the coating surface, but the overall appearance is relatively flat due to the low ACAT content. Figure 11b,c correspond to the polyurethane coatings AT-WPU-2 and AT-WPU-3 with 9% and 15% ACAT content, respectively. It can be clearly observed that their surfaces display many stripe-like white textures, and the white textures expand with the increase in ACAT content. The possible reason is that during the ACAT synthesis, a small amount of other aniline oligomers (such as aniline tetramer, aniline hexamer, etc.) are also generated; this is unavoidable, and the mass spectra of ACAT exhibit the presence of minute signals with mass-to-charge ratios of approximately 380 and 575, corresponding to the molecular weights of aniline tetramer and aniline hexamer, respectively. These small amounts of aniline oligomers will also react with the isocyanate groups, resulting in the formation of stripe-like copolymers that are deposited on the coating surface. This increases the coating thickness, and due to the presence of these larger aniline oligomers, the number of benzene rings on some macromolecular chains increases, leading to increased rigidity and reduced impact resistance of the coatings, which is also verified by the fundamental property tests of the coatings. With increasing ACAT content, more terminal amino groups react with isocyanate groups, promoting the growth of resin molecular chains, resulting in a more complex structure and higher crosslinking density of the coatings. It is evident that, aside from the white texture, the surface of the coating film exhibits improved regularity, surpassing that of AT-WPU-1 (Figure 11a). The fundamental properties of the coating, including adhesion, hardness, and flexibility, continue to show optimal performance.

Figure 11d–f show the scanning electron microscopy (SEM) images of the coated films of AT-WPU-1, AT-WPU-2, and AT-WPU-3 after immersion in a 3.5 wt.% NaCl solution for 168 h. In order to improve the clarity of the images, a flatter interface was selected as a reference. Comparison of the three images reveals that after 168 h of immersion, the surface of the coating with 3% aniline trimer displays a considerable number of holes and corrosion traces. And a substantial crater is presented in the upper right of the coating with 9% addition, although corrosion elsewhere is minimal. However, after 168 h of immersion, the coating with 15% aniline trimer exhibited only a minor defect in the upper left region, and the surface remains largely intact with no significant signs of corrosion. Consequently, the coatings exhibited enhanced corrosion resistance when the addition amount of aniline trimer was increased from 3% to 15%.

4.7. Thermogravimetric Analysis

Thermogravimetric analysis (TGA) is a widely employed technique that evaluates the thermal stability of materials by measuring the weight change of a sample under a constant heating rate [36]. Figure 12 illustrates the thermogravimetric analysis (TGA) curves of various waterborne coating materials, which exhibit three distinct stages of thermal degradation.

The weight loss below 100 °C can be ignored, which is primarily attributed to the evaporation of residual water on the coating surface and the loss of solvents. The first stage of thermal degradation occurs in the temperature range of 200–300 °C, characterized by modest weight loss primarily due to the dissociation of polyurethane bonds, resulting in the formation of isocyanates, alcohols, primary amines, secondary amines, alkenes, and carbon dioxide [37]. The second stage occurs in the range of 300–400 °C, representing the primary phase of weight loss. During this period, the major contributors are the cleavage and degradation of the polyethylene glycol chains, the thermal degradation of the alkyl groups and the conjugated aromatic groups, as well as the overall degradation of the resin molecular chains at elevated temperatures. The range of 400–550 °C primarily corresponds to the thermal oxidative decomposition of the polyurethane films.

Table 3. Thermogravimetric data of different AT-WPU films.

Sample	T5%/°C	T10%/°C	T50%/°C	Tg
AT-WPU-0	274	310	402	79.53
AT-WPU-1	276	323	418	88.27
AT-WPU-2	309	337	422	93.53
AT-WPU-3	318	341	425	101.71

presents the temperature data for 5%, 10%, and 50% weight loss of the samples. It can be observed that in the process of thermal degradation, the modified waterborne polyurethane coatings (AT-WPU-1, AT-WPU-2, and AT-WPU-3) exhibit higher thermal degradation temperatures compared to the unmodified polyurethane coating (AT-WPU-0), indicating that the addition of the aniline trimer can effectively enhance the thermal stability of the waterborne polyurethane films.

The DSC curves are shown in Figure 13. As the aniline trimer content increases, the T_g of the modified waterborne polyurethane coatings rises significantly compared to that of AT-WPU-0. In the hard segment region of the polyurethane macromolecule chain, the urea group formed by the reaction of the terminal amino group of the aniline trimer with the isocyanate group binds to the aryl functional group through hydrogen bonding. This intermolecular hydrogen bonding reduces the pliability of the macromolecule chain, restricting its free movement and rotation, which significantly increases T_g. Additionally, the rigid benzene ring structure of the aniline trimer further slows down chain migration. The analysis indicated that the T_g values of AT-WPU-0, AT-WPU-1, AT-WPU-2, and AT-WPU-3 were 79.53 °C, 88.27 °C, 93.53 °C, and 101.71 °C, respectively. Another primary reason is that the amine curing agent CYMEL^®325 undergoes a ring-opening reaction with E-44 epoxy resin during high-temperature curing, which significantly increases the cross-linking density of the waterborne polyurethane coatings. This cross-linking density directly influences the T_g [38].

4.8. Potentiodynamic Measurements

Electrochemical corrosion parameters, including E_corr, I_corr, R_p, and R_corr, were used to assess the anti-corrosion performance of various waterborne polyurethane coatings applied to SPTE electrodes in a 3.5 wt.% sodium chloride solution. The Tafel polarization curves of SPTE electrodes coated with different waterborne polyurethane coatings are presented in Figure 14. Generally, a lower corrosion current density (I_corr) and a higher corrosion potential (E_corr) indicate enhanced corrosion resistance of the coatings, as reported in the literature [39]. The results indicate that the corrosion potential of the waterborne polyurethane coatings containing aniline trimer is significantly higher than that of the coating without aniline trimer. The electrochemical corrosion parameters of SPTE electrodes coated with different waterborne polyurethane coatings are summarized in

Table 4. Electrochemical corrosion measurements of SPTE electrodes coated with different AT-WPU coatings.

Sample	Ecorr (v)	Icorr (A·cm−2)	ba (mv)	bc (mv)	Rp (MΩ·cm−2)	Rcorr (μm·Y−1)
AT-WPU-0	−0.045	1.731 × 10−6	350	150	26.32	20.12
AT-WPU-1	0.18	7.524 × 10−7	250	150	56.15	8.75
AT-WPU-2	0.52	1.947 × 10−8	74.5	125	1036.9	0.23
AT-WPU-3	0.67	7.245 × 10−9	137	187	4735.5	0.08

. As presented, the E_corr value of AT-WPU-0 is −0.045 V, while the E_corr values of AT-WPU-1, AT-WPU-2, and AT-WPU-3 are all positive, reaching a maximum of 0.67 V. In terms of corrosion current density, the I_corr of AT-WPU-3 reaches 7.245 × 10⁻⁹ A·cm⁻², which is three orders of magnitude lower than that of AT-WPU-0, suggesting a substantial enhancement in the anti-corrosion performance. Furthermore, the trend reveals that as the aniline trimer content increases, the corrosion potential gradually increases, and the corrosion current density decreases by one order of magnitude, implying that within a specific range, a higher aniline trimer content correlates with improved anti-corrosion performance of the corresponding coating. The polarization resistance R_p was determined using the Stern–Geary equation and Tafel plot, with the calculation equation provided as follows:

$$R_P={\displaystyle \frac{b_a{b}_c}{2.303(b_a+b_c)i_{corr}}}$$

(2)

Here, I_corr represents the corrosion current, calculated from the intersection of the linear regions of the anodic and cathodic curves, while b_a and b_c denote the anodic and cathodic Tafel slopes (ΔE/Log I), respectively.

The polarization resistance, R_p, was calculated using the Stern–Geary equation and Tafel plot analysis. The table shows that the R_p values for the aniline trimer-modified waterborne polyurethane coatings AT-WPU-1, AT-WPU-2, and AT-WPU-3 in a 5% sodium chloride solution were 56.15 kΩ·cm⁻², 1036.9 kΩ·cm⁻², and 4735.5 kΩ·cm⁻², respectively. These values are significantly higher than the R_p of the unmodified waterborne polyurethane coating AT-WPU-0, which was 26.32 kΩ·cm⁻². As shown in the last column of the table, the corrosion rate of the aniline trimer-modified waterborne polyurethane coating AT-WPU-3 is as low as 0.08 μm·y⁻¹, which is significantly lower than the corrosion rate of the unmodified waterborne polyurethane coating AT-WPU-0 (R_corr = 20.12 μm·y⁻¹). These findings indicate that the aniline trimer-modified waterborne polyurethane coatings exhibit superior anti-corrosion performance compared to the unmodified coatings.

4.9. Corrosion Protection Mechanism of Coating

To investigate the anti-corrosion mechanism of the AT-WPU coatings and ensure experimental rigor, the coatings were immersed in a 3.5 wt.% sodium chloride solution for 48 h. The AT-WPU coatings covering the SPTE electrodes were removed using a blade, and SEM observations were subsequently conducted. The results revealed that a dense metal oxide layer formed between the AT-WPU coating and the SPTE electrode, like a scale-like structure embedded in the electrode surface, as depicted in Figure 15c. Simultaneously, as shown in Figure 15b, the surface morphology of the SPTE pure electrode was also observed. It can be seen that its surface is relatively smooth and flat, in contrast to the metal oxide layer in Figure 15c. These findings are consistent with those reported by Wessling et al. [40,41,42], which primarily attributed the enhanced anti-corrosion performance to the formation of a conductive polymer-metal composite. Wei et al. [43] proposed that the conductive polymer participated in the interfacial reactions of the metal, thereby promoting the formation of metal oxides. Furthermore, some researchers have developed new conductive polymer coatings and investigated their anti-corrosion mechanisms for iron and other oxidizable metals [44].

Generally, the conductive polymer can capture the electrons generated from the reactions of the metal substrate, thereby facilitating the conversion of the conductive polymer’s state from the oxidized state (PB) to the reduced state (LEB). In the presence of oxygen, iron ions can readily transform into dense Fe₂O₃ and Fe₃O₄, forming a passivation film that adheres to the metal substrate, thus suppressing metal corrosion through anodic protection. The conversion between PB and LEB is reversible, and LEB can be oxidized back to PB, releasing electrons to accelerate the formation of the passivation film [45], as illustrated in Figure 15a. The corrosion process in neutral chloride solutions can be described as follows [46,47]:

Anodic reaction:

$$\mathrm{F}\mathrm{e}\to {\mathrm{F}\mathrm{e}}^{2+}+2{\mathrm{e}}^{-}$$

(3)

Cathodic reaction:

$${\displaystyle \frac{1}{2}}{\mathrm{O}}_2+2{\mathrm{e}}^{-}+{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{O}\mathrm{H}}^{-}$$

(4)

and a chemical process:

$$2{\mathrm{O}\mathrm{H}}^{-}+{\mathrm{F}\mathrm{e}}^{2+}\to {\mathrm{F}\mathrm{e}\left(\mathrm{O}\mathrm{H}\right)}_2\to {\mathrm{F}\mathrm{e}\left(\mathrm{O}\mathrm{H}\right)}_3\to {\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_3$$

(5)

5. Conclusions

This study successfully fabricated waterborne polyurethane coatings modified with aniline trimer. Through the structural characterization of the aniline trimer and the detailed investigation of the structure and properties of the modified waterborne polyurethane coatings, the following conclusions can be drawn:

(1)

The DSC analysis results and thermogravimetric graphs indicate that the introduction of E-44 epoxy resin and the use of the amine-type curing agent for high-temperature curing facilitated a ring-opening reaction, which effectively increased thermal stability and the crosslinking density of the waterborne polyurethane coating. Additionally, the coating exhibits superior fundamental properties compared to the conventional waterborne polyurethane coating AT-WPU-0, such as improved hardness and adhesion.

(2)

Electrochemical evaluation of the aniline trimer-modified waterborne polyurethane coatings reveals that when the addition amount of aniline trimer was increased from 3% to 15%, the coatings exhibited better corrosion resistance. The corrosion potential of AT-WPU-3 reaches 0.67 V, and the corrosion current density is 7.245 × 10⁻⁹ A·cm⁻², which is three orders of magnitude lower than that of the unmodified conventional waterborne polyurethane coating. This improvement is primarily ascribed to the ability of the aniline trimer to induce the formation of a passivation layer on the substrate metal through its redox properties.