Araldite F, solvent-free and unmodified bisphenol- A epoxy resin was purchased from Huntsman group. Its epoxy contents are 5.20–5.35 Equiv/Kg (ISO 3001), having a density range from 1.15–1.20 g/cm³. Acrylic acid (AA, >99% purity) was supplied by power chemical industries (PVT) LTD. Pakistan. Benzimidazole (>99% purity) was purchased from Alfa Aesar (A Johnson Matthey Company) Karlsruhe Germany. Halloysite nanotubes (HNTs) were purchased from natural nano, United States of America. Potassium persulfate (KPS) and 2-hydroxy-2-methylpropiohenone (analytical grade) were used as a thermal curing agent and a photo-initiator. Deionized water (DDW) was used in all experiments. Potassium hydroxide (KOH, >99% purity) was used as a standard for titration (0.1 N). FTIR spectra were recorded on Perkin-Elmer Spectrum two spectrophotometer.

Table 1 summarizes the compositions used to prepare different coatings.

The reaction between diglycidyl ether of bisphenol A and acrylic acid was monitored by measuring acid values at regular time intervals. The acid value of the reaction samples decreased as the reaction between epoxy and acrylic acid proceeded. A decrease in acid value provides a sufficient clue of the reaction between acrylic acid and epoxy.

The initial rate of the reaction between acrylic acid and epoxy appears to be greater than the subsequent rate (Figure 1). The mixture became more viscous as the reaction progressed. An increase in viscosity may slow the reaction by limiting the effective collisions between the acrylic acid and epoxy species. The reaction between the epoxy group and acrylic acid causes the epoxy ring to open. The reaction is depicted schematically in Figure 2.

Figure 3 shows the FTIR spectra of prepared samples. An FTIR analysis of the product showed the disappearance of the absorption peak for the oxirane group, which indicates the epoxy ring opening. The band at 1720 cm^-1 is due to the carbonyl group of the ester formed. The acryloyl double bond attributed to a band at 1637 cm^-1, indicated the formation of a vinyl ester. The band at 3300–3600 cm^-1 in the vinyl ester spectrum is associated with the hydroxyl group with increased absorption intensity compared to the epoxy resin spectrum.

The epoxy groups have completely reacted, as indicated by the absence of an oxirane ring absorption band in the vinyl ester resin spectrum.

The presence of terminal vinyl groups on epoxy acrylate oligomer is an important feature used to further structural modifications. Emulsion polymerization of the epoxy-acrylate oligomer and butyl methacrylate produced an aqueous core-shell latex.

The latex preparation is a consecutive two-step polymerization reaction. In the first step, butyl methacrylate (or vinyl acetate) is polymerized to form the core particles. Later, epoxy acrylate is introduced into the polymerization mixture to form the shell out of the core structure (Figure 4).

HNTs have hydroxy groups on their lumen structure and tend to agglomerate. Sonication was used to disperse these in the latex for 30 min before proceeding with the rest of the reaction.

Table 1 presents the sample codes and the corresponding compositions.

Epoxy resins exhibit superb adhesion to many substrates, good chemical and corrosion resistance, excellent electrical insulation, water resistance, and thermal stability. Coating applications require a high degree of adhesion, chemical, and corrosion resistance. The superb adhesion performance of epoxy resin is due to polar hydroxyl and ether groups lying at the backbone of epoxy resins (Lee et al., 1967; Caşcaval et al., 1997; Oprea et al., 2000). Most coating applications utilize acrylates due to better weather stability, gloss, and surface properties (Bajpai et al., 2002; Sharmin et al., 2004). Literature reports indicate that the strong interaction between epoxy coatings and the metal substrate can be attributed to both chemisorption and physisorption interactions. (El-Aouni et al., 2021).

Typically, acrylates cure via a mechanism involving free radicals and result in three-dimensional networks. Epoxy acrylates and epoxy novolac are widely used due to their superior adhesion, hardness, chemical resistance, and non-yellowing properties. Mechanical and thermal properties of cured epoxy resins are determined by the chemical and structural nature of the epoxy resins and crosslinker, the type, the concentration of cure initiators, cross-linking reaction conditions, epoxy conversion, and crosslinker to epoxy acrylate ratio.

The prepared HGEAs coatings showed excellent adhesion to steel plates. Improved cross-linking of different polymer chains with each other enhances the desirable characteristics. But if the cross-link density of polymeric chains is too high, then the coatings become too hard, and fragile leading to poor adhesion to steel substrates. Generally, conventionally recognized inorganic materials contain some hydroxyl groups on their surfaces due to the impact of the moisture present in the atmosphere (Tang et al., 2006). Possibly strong hydrogen bonding can occur between hydroxyl groups on the metal substrates and oxygen present in the polymer chains. Consequently, the hydrogen bonding between the oxygen of polymer chains and hydroxyl groups present on metal substrates significantly improved the adhesion of coatings on steel substrates.

UV and thermal curing characteristics are described in Table 2. It is observed that below a 5% concentration of photo-initiator (2-hydroxy-2-methylpropiohenone) resulted in incomplete curing showing wet or tacky coatings even after exposure to UV-radiation of the intensity of 2 × 80 W/cm for more than 2 h. However, a 5% photo-initiator concentration resulted in the non-tackiness of coating films. After 90 min of UV curing, all the film mixtures were still sticky, and after 120 min, all the mixtures were no longer tacky.

In the thermal curing method (potassium persulphate as thermal curing agent), all samples were non-tacky at a temperature of 140°C for 170 min of exposure to heat.

The non-tacky drying of the films was observed for all compositions containing 5% photo-initiator and for thermal curing at 140°C for 170 min. All cured films showed an overall good performance.

The gloss of the films was excellent, and all films showed good water, acid, and alkali resistance. These tests were performed in 7 days immersion of sample in water, acidic and alkaline solution. 10% hydrochloric acid and 10% sodium hydroxide solutions were used to check the acid and alkali resistance of the coatings. The coatings were intact from any degradation after 7 days.

The water resistance behavior of coatings was studied by using contact angle measurements. The water contact angles for GEA coatings and HNTs-modified HGEAs coatings were 34.51° and 66.71°, respectively. The contact angle depends on the surface morphology; apart from the nature of the components, the hydrophilicity of coatings is reduced by incorporating HNTs in epoxy coatings. The surface roughness of HNTs-based coatings is greater than that of epoxy coatings. The increased surface roughness of the HNT-loaded coating could lower hydrophilicity and improve its resistance to water, acids, and alkalis. It has been observed that with an increasing proportion of halloysite, there is an enhancement in resistance to water, acids, and alkalis.

Another study has also reported halloysite nanotubes as capable of forming a protective barrier that effectively hinders the ingress of corrosive agents, thereby affording active corrosion protection to carbon steel. Furthermore, this barrier serves as an impediment to water infiltration within the coating (Liu and Li, 2023).

The thermal stability of the coating material plays a pivotal role, with its characteristics significantly shaped by factors such as network structure, chemical composition, type and concentration of residual polar groups, cohesive energy among molecular chains, molecular chain rigidity, various interaction parameters, and other chemical structural aspects, including steric strain and conformational arrangements of functional groups, among others. Thermogravimetric analysis of the various coating materials was performed to evaluate the thermal stability of the coating material.

Figure 5 shows the TGA plots for selective samples. The nanocomposite with HNTs shows an increase in thermal stability with the increase in filler content.

The initial phase of degradation occurs up to 150°C, primarily ascribed to the evaporation of residual solvents and the release of entrapped moisture within the coatings. This stage is marked by a gradual weight loss of approximately 4% up to 150°C, followed by a more pronounced weight reduction of 50% within the temperature interval of 151°C–400°C.

The principal decomposition of the samples transpires during the second stage of decomposition, occurring above 150°C. This rapid decomposition process involves thermal cracking, gasification, disproportionation processes, and secondary dehydrogenation reactions.

The corrosion resistance of GEA and HNTs modified GEA coatings were evaluated by the electrochemical impedance spectroscopy. The corrosion resistance behavior of the different coating samplse immersed in 3.5% NaCl solution was studied. Initially, the open circuit voltage of GEA coating and HNTs modified GEA coating (HGEA-3) are approximately −0.65 and −0.47 V, respectively. As expected, the values of both the coatings started to decrease overtime gradually indicating the penetration of the corrosive solution through the coating. After 30 days, there is significant difference in the OCP values of both coatings where the HNTs modified GEA coating showed the better resilience (Figure 6). We do not see a clear correlation between HNTs loading and the corrosion resistance ability, but a 1.5% loading provided the optimum results.

We have used the lowest frequency impedance modulus (Z_0.01 Hz) as a quantitative indicator of corrosion resistance properties (Figure 7). Again GEA coating loaded with 1.5% HNTs provided the optimum results. This analysis additionally demonstrates that there is not a substantial disparity in the corrosion protection efficacy between GEA and HGEAs. Nonetheless, it is evident that HNTs have enhanced the corrosion protection capabilities of the coating to some extent.

To assess the visual impact of the coatings, samples coated with GEA and GEA modified with HNTs were subjected to immersion in a 5% NaCl solution for a duration of 400 h. Rusting of the coating, particularly on the scratch edges, was visible in the GEA epoxy coatings (Figure 8). However, after salt immersion, the HNTs modified coatings remained intact. After 400 h (16 days) of immersion, the texture of the layers has slightly faded, but the coating has not been attacked by corrosion.

Samples with varying quantities of HNTs were tested. It was found that a 1.5% HNTs loading provided the optimum loading to achieve the best anti-corrosion results. The HNTs-modified epoxy coating has a lower void formation rate. Its film aids in keeping the bulk coating intact for a longer period. Furthermore, the degradation on the HGEA coating surface is uniform. Thus, the ability of HNTs to provide long-term coating sustainability is speculated. These results are in line with the EIS findings.

HNTs are characterized by a diameter of less than 100 nm. Figure 9 shows the scanning electron microscopic image of a nanocomposite. The micrographs show fine dispersion of HNTs distributed in a regular pattern all around the matrix. The fine dispersion of HNTs in nanocomposites is due to mechanical and ultrasonic dispersion treatments. Furthermore, the epoxy acrylate matrix leaches into the lumen of nanotubes, improving interfacial interactions and bonding.