Amidst the escalating influence of global climate change, nations have implemented and progressively reinforced regulations and policies to constrain high-polluting and high-carbon industries, urging a shift towards low-carbon, eco-friendly, and sustainable practices (Sovacool et al., 2022). Among these, the coatings industry, recognized as a significant polluter, confronts pressures from both regulatory mandates and market competition, compelling the development of products aligned with low-carbon and environmental standards to meet market demands (Pathak et al., 2022; Pleshivtseva et al., 2023).

Waterborne coatings, prized for their eco-friendly attributes, stand as a pivotal strategy for the coatings industry’s pursuit of low-carbon transitions. Among these, waterborne epoxy resin emulsions, serving as pivotal constituents for various functional coatings, have garnered considerable scholarly and industrial focus in both production and application realms (Steinbrück et al., 2022). However, prevailing waterborne epoxy resin formulations often fall short of meeting demands for high-temperature conditions. Consequently, the development of waterborne epoxy resin emulsions capable of harmonizing low-carbon environmental performance with high-temperature resilience assumes critical importance, not only for advancing the low-carbon evolution of the coatings sector but also for presenting substantial industrial and market application opportunities in the realm of high-temperature-resistant materials utilizing waterborne epoxy resins. The significance of attaining a low-carbon footprint in the preparation process is twofold: it aligns with the global imperative to mitigate climate change and bolster environmental integrity, while also addressing the inherent need for sustainable growth within the coatings industry. Embracing low-carbon technologies and materials enables the industry to mitigate its environmental footprint, complying with stricter regulations and meeting evolving market expectations. Additionally, these technologies can bolster material performance and efficiency, fostering innovation in both materials and processes, thereby catalyzing the green transition and fostering high-quality development across the coatings industry worldwide.

This study begins by reviewing the current research landscape both domestically and internationally. It then outlines the main research methodologies employed, encompassing the preparation of waterborne phenol-formaldehyde epoxy resins, surface modification of nano silica (SiO₂), investigation into SiO₂-modified waterborne phenol-formaldehyde epoxy resin composites, and the in situ synthesis of nano SiO₂-modified waterborne phenol-formaldehyde epoxy resins. Subsequently, the prepared samples undergo testing, and the resultant findings are discussed. Ultimately, the research concludes by presenting its key findings and implications.

The primary research objectives of this study are outlined as follows:

(1) Synthesis of waterborne phenol-formaldehyde epoxy resin employing a phase transfer catalyst and a two-step alkaline method.

The experiments involve various reagents. Table 1 presents the information for all reagents used in this experiment.

The instruments utilized in the experiments are outlined in Table 2.

The reagents and equipment utilized in this study are outlined previously. Next, the synthesis process of waterborne phenol-formaldehyde epoxy resin is elucidated. In a four-necked flask equipped with a condenser, 20.00 g of phenol, 3.00 g of deionized water, and 0.24 g of oxalic acid were added under an N2 atmosphere. The mixture is stirred magnetically at 60°C for 15 min, followed by the dropwise addition of 14.00 g of formaldehyde solution. After the completion of the addition, another 0.24 g of oxalic acid is added, and the temperature is raised to 75°C for a 2-h reaction. Subsequently, the temperature is further increased to 90°C, and the reaction continued for 2 h (Liu et al., 2022; Ma et al., 2022; Oyebanji et al., 2022). Following the reaction, the product is washed with water to neutrality and subjected to vacuum distillation to remove water, yielding the phenol-formaldehyde resin (I). The principle of this reaction is illustrated in Figure 1.

Under an N2 atmosphere, an excess of ECH and 0.48 g of tetra-n-butylammonium bromide (TBAB) is added to the previously obtained compound (I). The reaction temperature is set at 90°C for 3 h. Subsequently, the temperature is reduced to 70°C, and a sodium hydroxide (NaOH) solution is added dropwise, followed by an additional 2-h reaction period. Upon completion of the reaction, the product is washed with water to neutrality and subjected to vacuum distillation to remove the solvent. Finally, epoxy phenol novolac (EPN) (II) is obtained (Cohen et al., 2022; Wangari et al., 2022; Abdurahmonov et al., 2023). The principle of this reaction is illustrated in Figure 2.

A specified quantity of EPN (II) is dissolved in tetrahydrofuran (THF) at 65°C with stirring. DEA is then added according to the prescribed amount, and the reaction proceeded for a specific duration. Upon completion of the reaction, oxalic acid is introduced to neutralize the system for 0.5 h, followed by a temperature reduction to 55°C. Deionized water is subsequently slowly added while stirring for 1 h to yield the waterborne phenol-formaldehyde epoxy resin emulsion. This emulsion is labeled Diethanolamine Water Epoxy Phenol Novolac (D-WEPN) (Sun et al., 2022). The preparation principle of this emulsion is depicted in Figure 3.

Nano SiO₂ undergoes surface modification using γ-aminopropyltriethoxysilane (APTES) to decrease surface hydroxyl (-OH) content and graft organic groups, thereby improving its hydrophobicity for enhanced dispersion in the resin (Gonçalves et al., 2022a; Kyei et al., 2022; Sonnendecker et al., 2022). More efficient surface modification is achieved by finely controlling the dosage of APTES and adjusting reaction conditions such as time and temperature. This results in a reduction of the hydroxyl content on the surface of nano SiO₂, thereby enhancing its dispersibility and hydrophobicity in phenolic epoxy resin, consequently improving the resin’s high-temperature resistance. Subsequently, employing phenol-formaldehyde prepolymer, ECH, DEA, and acetic acid as primary constituents, the nanoscale SiO₂-modified waterborne phenol-formaldehyde epoxy resin is synthesized via the prepolymer method (Sharmila et al., 2022; Romão et al., 2022).

To achieve uniform dispersion of nano SiO₂ in the phenolic epoxy resin, this study employs an in situ polymerization method (Wen et al., 2022). The surface hydroxyl groups of nano SiO₂ are copolymerized with monomeric phenolic compounds and formaldehyde to obtain SiO₂-modified phenolic resin. Subsequently, it is further reacted with ECH, DEA, and acetic acid to produce a SiO₂-modified phenolic epoxy resin water-based emulsion (Steinbrück et al., 2022).

The prepared samples undergoes testing, including infrared analysis, emulsion stability assessment, and film performance evaluation. Following the removal of solvent from the samples, a FTIR test is conducted with a scanning range of 4000 to 500 cm⁻¹, 32 scans per sample, and a resolution of 4.0 cm⁻¹. Figure 4 depicts the analysis results of the waterborne novolak epoxy resin.

In Figure 4, the phenolic resin reveals numerous phenolic hydroxyl groups, evident through a broad absorption peak near 3300 cm⁻¹ and characteristic benzene ring peaks at 1500–1600 cm⁻¹. Upon addition of epichlorohydrin, a distinctive epoxy peak emerges at 912 cm⁻¹, and most hydroxyl peaks near 3300 cm⁻¹ diminish, indicating the completion of the epoxy reaction and the substitution of most hydroxyl groups by epoxy groups. Subsequent addition of DEA leads to some epoxy groups undergoing addition reactions to form hydroxyl groups, resulting in a weakened epoxy peak at 912 cm⁻¹ and a strengthened hydroxyl peak near 3300 cm⁻¹. Neutralization with acetic acid yields a carboxyl carbonyl peak at 1734 cm⁻¹. Infrared spectroscopic analysis confirms the successful synthesis of phenolic epoxy resin, with the obtained product exhibiting the expected structure. Figure 5 presents the infrared test analysis results of nano SiO₂ modification before and after.

In Figure 5, the spectra reveal characteristic peaks of nano SiO₂ at 3435 cm⁻¹, 1105 cm⁻¹, 808 cm⁻¹, and 471 cm⁻¹. The peak at 3435 cm⁻¹ corresponds to the absorption of hydroxyl groups (-OH) on the surface of SiO₂. The peak at 1105 cm⁻¹ signifies the symmetric stretching vibration of Si-O-Si, while the peak at 808 cm⁻¹ represents the antisymmetric stretching vibration of Si-O-Si. Additionally, the peak at 471 cm⁻¹ corresponds to the bending vibration of Si-O-Si. Comparatively, the other two curves exhibit absorption peaks attributed to -CH₂ and -CH₃ groups near 2900 cm⁻¹, with the third peak being more prominent, indicating the introduction of -CH₂ and -CH₃ groups from 3-APTES. The enhancement of peaks near 1100 cm⁻¹, 800 cm⁻¹, and 470 cm⁻¹ suggests the formation of Si-O-Si bonds. The broadening of the peak around 3400 cm⁻¹ is attributed to the overlap between the -NH2 groups introduced by 3-APTES and the -OH groups.

Figure 6 depicts a comprehensive analysis of the infrared test results for the nano SiO₂-modified water-based phenolic epoxy resin.

In Figure 6, the Simple A curve reveals the presence of abundant phenolic hydroxyl groups in the phenolic resin, evidenced by the prominent and broad hydroxyl peak at 3400 cm⁻¹. Following the reaction with ECH, depicted in the Simple B curve, the hydroxyl groups undergo epoxidation, leading to a notable decrease in the hydroxyl peak at 3400 cm⁻¹ and the emergence of an epoxy peak at 910 cm⁻¹. These data affirm the successful reaction between ECH and hydroxyl groups, evidenced by the attenuation of the hydroxyl peak. Upon the addition of DEA, depicted in the Simple C curve, some epoxy groups react with DEA to form hydroxyl groups, resulting in a reduction of the epoxy peak at 910 cm⁻¹ and an augmentation of the hydroxyl peak at 3400 cm⁻¹. After neutralization with acetic acid, the Simple D curve reveals a carboxyl carbonyl peak at 1732 cm⁻¹. Throughout the process, a faint Si-O-Si bending vibration peak at 471 cm⁻¹ persists, indicating the formation of chemical bonds between SiO₂ and the EPN molecular chain.

Emulsion stability testing encompasses the assessment of various aspects, including the influence of DEA on the stability of the water-based phenolic epoxy resin emulsion, the impact of nano SiO₂ on the stability of SiO₂/WEPN composite material emulsion, and the effect of nano SiO₂ on the stability of modified water-based phenolic epoxy resin emulsion. The specific test results are outlined below, with detailed data on the impact of DEA dosage on the average particle size and zeta potential of the emulsion provided in Table 4.

When the ratio of n₁/n₂ exceeds 1/3, the absolute value of the system’s zeta potential surpasses 32 mV, indicating high emulsion stability. With increasing DEA dosage, the emulsion particle size gradually decreases. However, when the n₁/n₂ ratio falls below 1/3.5, the particle size exceeds 1265 nm, signifying instability, possibly leading to gel formation. Within the range of 1/3.25 to 1/3, the emulsion particle size ranges from 324 to 859 nm, indicating moderate particle size conducive to stable storage. A n₁/n₂ ratio exceeding 1/3 results in particle size below 324 nm, signifying stable emulsion. Table 5 provides detailed insights into the impact of DEA dosage on emulsion stability.

As DEA dosage increases, the emulsion transitions from a pale yellow hue to milky white, and precipitate particles diminish. When the n₁/n₂ is ratio is ≤1/3.5, precipitation occurs, rendering the emulsion unstable. Conversely, with a n₁/n₂ ratio ≥1/3.25, minimal precipitation is observed, ensuring stable storage.

In Table 6, when the nano SiO₂ content exceeds 2%, the centrifugation time of the emulsion decreases rapidly due to nano SiO₂ aggregation. Consequently, the emulsion color gradually shifts to pale yellow, and stability decreases, resulting in shortened storage life, rendering it impractical. Based on dispersion and emulsion stability analyses, a nano SiO₂ content below 3% is optimal, with a recommendation of 2%.

In Table 7, when the nano SiO₂ content is below 2%, the absolute value of the Zeta potential exceeds 30 mV, indicating good system stability. However, as the nano SiO₂ content increases, the average particle size of the emulsion gradually increases. Once the nano SiO₂ content surpasses 2%, the average particle size of the emulsion exceeds 1000 nm, resulting in instability and the formation of precipitate particles. Therefore, it is advisable to maintain the nano SiO₂ content below 2%, ideally around 1.5%.

Table 8 displays the impact of nano SiO₂ content on various application properties of the coating films.

Table 8 displays the variation in coating properties with increasing nano SiO₂ content. Initially, the acid and alkali resistance of the coatings rises before declining. Similarly, the impact resistance increases from 40 cm to 50 cm, then declines. Pencil hardness gradually elevates to 3H, while adhesion grade decreases from 0 to 1. Additionally, both surface and bulk drying times gradually decrease.

In Figure 6, the phenolic resin exhibits numerous phenolic hydroxyl groups, evident from the prominent and broad peak at 3400 cm⁻¹ in the Simple A curve. Upon reaction with ECH, the hydroxyl groups undergo epoxidation, leading to a notable decrease in the hydroxyl peak at 3400 cm⁻¹ and the emergence of an epoxy peak at 910 cm⁻¹ in the Simple B curve. These data signify the effective reaction of ECH with hydroxyl groups and the attenuation of the hydroxyl peak. With the addition of DEA, some epoxy groups undergo an addition reaction with DEA, forming hydroxyl groups. This results in a reduction of the epoxy peak at 910 cm⁻¹ and an amplification of the hydroxyl peak at 3400 cm⁻¹ in the Simple C curve. Following neutralization with acetic acid, the Simple D curve displays a carboxyl carbonyl peak at 1732 cm⁻¹. Notably, a weak Si-O-Si bending vibration peak at 471 cm⁻¹ persists throughout the process, indicating chemical bonding between SiO₂ and the EPN molecular chain.

The enhanced water-based phenolic epoxy resin emulsion, distinguished by its remarkable high-temperature resistance, chemical stability, environmental compatibility, and superior mechanical attributes, holds immense promise across various applications. It finds suitability in surface coatings within sectors such as automotive, aerospace, and industrial machinery, offering enduring corrosion protection. Moreover, serving as a matrix resin, it augments material high-temperature performance, making it ideal for high-performance composite materials fabrication. Additionally, it meets the requirements for high-strength bonding in elevated-temperature environments, applicable in industrial equipment and aerospace component bonding. In contrast to the research endeavors of others, exemplified by Pan and Yan (2023), focusing predominantly on enhancing high-temperature resistance through alterations in resin cross-linking density, this study fortifies high-temperature resistance via optimized preparation processes, nano SiO₂ surface modification, and composite material techniques. This approach not only elevates high-temperature resistance but also ensures environmental compatibility and widens application horizons (Pan and Yan, 2023). Similarly, juxtaposed with the research conducted by Zhao et al. (2022), which concentrates on leveraging diverse chemical modification methods to bolster resin physical properties, this study, through comprehensive methodological optimization, not only achieves enhancements in physical attributes but also broadens material application realms by incorporating nano-fillers, particularly in demanding high-temperature and high-performance sectors (Zhao et al., 2022).