The Effect of Complex Emulsifier on the Structure of Tung Oil and Phenolic Amides Containing Microcapsules and Its Anti-Fouling and Anti-Corrosion Performances

Abstract

In this study, the urea-formaldehyde (UF)-tung oil solution of phenolic amide (PA) microcapsules to realize anti-fouling and anti-corrosion integration was synthesized by the in situ polymerization method. The compounds and structures were optimized by investigating six kinds of different emulsifiers. The results showed that high-core-content and narrow-particle-size-distribution microcapsules could be synthesized with sodium dodecyl benzene sulfonate (SDBS)/polyvinyl alcohol (PVA), and the core content of the microcapsules was 75 wt% at microcapsule sizes from 24.07 to 71.33 µm. The results of self-healing coatings showed that when the content of microcapsules in the coating exceeded 10 wt%, the healing agent released from the scratched surface could cover the naked metal effectively, which could pass a 7 day neutral salt spray test without rust at the scratched area. A sufficient dose anti-fouling agent can be provided to prevent diatoms and mussels from adhering. The present work shows that the complex emulsifier can better control the particle size distribution and microstructure of the microcapsules, and the admixture of the microcapsules into the resin epoxy coating can realize excellent anti-corrosion and anti-fouling functions.

1. Introduction

With the rapid development of the marine industry, metal equipment has been used in the marine service [1]. The marine environment causes harsh corrosion and fouling, which have always been the main problems faced by marine ships. Corrosion will reduce the strength of ship materials, destroy the integrity of the ship, and shorten the service life of equipment. The fouling caused by microbial attachment not only increases the driving resistance but also reduces the vessel speed. The common way to protect a metallic material against corrosion is to paint anti-corrosion coatings on its surface. However, the coatings will be damaged in the process of manufacturing and service, causing micro-cracks, which will destroy the protective performance of the coatings. If not repaired in time, the contact of the metallic substrates with corrosive media could lead to the production of metal corrosion, accelerating the failure of the coatings [2,3]. Microbes could also attach to the micro-cracks more easily than into the completed coating, which will destroy the anti-corrosion layer, thereby accelerating the corrosion and affecting the speed and maneuverability of the ship [4].

A microcapsule self-healing concept was first proposed by White to repair the coating. Microcapsules are hollow spheres with a core–shell structure, which fracture upon mechanical damage, releasing the healing agents to the damage zones and covering the small cracks by their forming films [5]. When the microcapsules-embedded organic coating produces microcracks under external stress, repairing is achieved by filling the cracks with microcapsules flowing out of repairing agents at the cracks, and, as a consequence, the coating service life is dramatically prolonged. Among the self-healing methods, the external self-healing coating with microcapsules has attracted attention because of its self-healing without any external assistance [6]. At present, the types of healing agents include dicyclopentadiene (DCPD), epoxy resins [7], isocyanates, and drying oils. Dry oils have gained widespread attention because of their low cost and environmental friendliness [8,9]. The corrosion resistance and repair efficiency of the coating are directly related to the particle size of the microcapsules. However, the current research on particle size control is not perfect, and the prepared particle size distribution range is wide and the particle size is too large. The uneven particle size distribution of the microcapsules will greatly reduce the self-repair efficiency of the microcapsules in the coating. Considering the influence of microcapsules on the strength of polymers, it is extremely important to optimize their size and spacing [10]. In the process of preparing microcapsules, emulsifiers are of great importance for the dispersion and stabilization of the emulsion, and they directly affect the particle size and surface morphology of microcapsules [11]. In addition, marine microorganisms will adhere to the cracks and affect the self-repairing efficiency of the coating. In order to realize anti-corrosion and prevent microorganisms from adhering, the addition of an anti-fouling agent is necessary. The anti-fouling agent is dissolved in dry oil as the core, which has a certain influence on the dispersion performance of the emulsion. Therefore, it is necessary to carry out research work on the optimization of the particle size distribution of microcapsules and the content of microcapsules in the self-healing coating.

The main objective was to synthesize an integration of anti-corrosion and anti-fouling microcapsules with an ideal size distribution and a sufficient healing and anti-fouling effect. To achieve this purpose, complexing emulsifiers were adopted to fabricate the integration of anti-corrosion and anti-fouling microcapsules. The characteristics of microcapsules synthesized under different emulsifiers were tested by Scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), and Thermogravimetric analysis (TGA) to verify the feasibility of diameter control. The anti-fouling and anti-corrosion performances were also examined with Electrochemical impedance spectroscopy (EIS), salt spray, and mussel and diatom adhesion test to evaluate the anti-corrosion and anti-fouling performances in an epoxy coating. The study has far-reaching implications for addressing corrosion and anti-fouling at microcracks and also provides a basis for the application of real sea engineering.

2. Materials and Methods

2.1. Raw Materials

The tung oil used as the self-healing agent was obtained from Aladdin Co., Ltd. (Shanghai, China). Urea, formaldehyde solution (37 wt%), hydrochloric acid (HCl), ammonium chloride (NH₄Cl), sodium chloride (NaCl), 1-octanol, ethanol, and resorcinol were purchased from Sino-pharm Beijing Reagent Co., Ltd. (Beijing, China). SDBS, PVA, and OP-10 used as the emulsifiers were purchased from Macleans. Q₂₃₅ steel was purchased from Shanghai BaoSteel, China. The 725-H06-51 curing agent and epoxy resin were purchased from Shanghai Shuangrui Paint Co., Ltd. (Shanghai, China). Phenolic amides were obtained from Luoyang Ship Material Research Institute, China.

2.2. Preparation of MC(T) and MC(P and T)

Microcapsules containing tung oil (MC(T)) and microcapsules containing tung oil and phenolic amides (MC(P and T)) were synthesized by in situ polymerization [12]. The phenolic amide was dispersed in tung oil by ultrasonic vibration, which is insoluble in water. Figure 1 shows a schematic diagram of the synthesis of MC(P and T). Briefly, 5.00 g of urea, 0.50 g of resorcinol, 0.50 g of ammonium chloride, and 6.5 g of sodium chloride were loaded with 260 mL of deionized water and dissolved with stirring at a speed of 1000 rpm at ambient temperature. The 2% HCl was used to adjust the PH at 3.0. Then, a 0.25 wt% emulsifier aqueous solution and 20 mL of tung oil were loaded into a three-necked flask and emulsified for 30 min at room temperature. Then, 12.67 mL of formaldehyde was added into the three-necked flask through an acid burette, and the temperature was slowly raised to 60 °C. Stirring was continued at 800 rpm until the reaction was complete. The MC(T) was filtered from the suspension by filter paper, washed repeatedly with denatured alcohol and ultrapure water, and dried in an oven at 45 °C. In this study, six different emulsifier types including SDBS, PVA, OP-10, SDBS/PVA, SDBS/OP-10, and PVA/OP-10 were investigated, and the content ratio of the compound emulsifier was 1:1 in weight.

2.3. Characterization of MC(T) and MC(P and T)

The shape and surface morphology of the microcapsules were observed using an optical microscope (OM) (KH-8700, Haoshi Instrument Technology Co., Ltd. Shanghai China) and SEM (Hitachi SU8020, Tokyo, Japan). Image-J was used to measure and count at least 200 microcapsule sizes in the SEM images to determine the average diameter. FT-IR (Thermo Scientific Nicolet 8700, Waltham, MA, USA) was used to detect the chemical structure of the raw materials and corresponding capsules in a wavenumber range of 4000–500 cm⁻¹ [13,14]. TGA (STA-449F5, Netzsch manufacturing Co., Ltd., Selb, Germany) was used for thermal stability of the material. The shell and core content of the microcapsules was roughly calculated by the weight loss curve [15].

2.4. Preparation of the Coating Samples

Various weight contents of MC(P and T) were added into the epoxy resin (0, 5, 10, and 15 wt%) under stirring for 15 min. The samples were applied on the carbon steel panels using an applicator. The applied coatings were cured for 48 h at 40 °C. The layers were measured to be about 350 μm after curing.

2.5. Characterization of Coating Samples

The KH-8700 three-dimensional video image in situ acquisition system (OM) was used to observe the distribution of microcapsules in epoxy coatings and the morphology of microcapsules. The cross-section of the epoxy coating samples after curing was observed to analyze whether the microcapsules were uniformly distributed. The DeFelsko PosiTest AT pull-out adhesion tester (DeFelsko, New York, USA) was used to investigate the effects of microcapsules loading on coating bonding strength. After the surfaces of 150 mm × 100 mm × 4 mm carbon steel plates were mechanically ground and sandblasted, epoxy coatings loaded with microcapsules of 0 wt%, 5 wt%, 10 wt%, and 15 wt% were applied by the coating method. The coating samples were cured at 40 °C for 48 h, and the thickness of the coating was measured to be about 350 μm. Five sets of data were obtained for each sample with 3M glue to obtain the average bonding. The adhesion of the coatings was tested according to the ISO4624 Pull-Off Adhesion Test Standard.

The anti-corrosion performance was characterized by the salt spray test and EIS. Damaged samples were prepared by creating a 1 cm × 50 μm scratch on the 20 mm × 20 mm × 2 mm-coated samples through a No.11 scalpel blade in air, with the scratch reaching the metallic substrate. For the neutral salt spray test, samples were exposed to 5% NaCl solution at 35 °C, and the salt spray deposition volume was 1 to 2 mL/cm²·h. The samples under different salt spray times were photographed and recorded. The PARSAT2273 electrochemical workstation of American AMEEK company was used for EIS. A three-electrode arrangement was used for the EIS test, including the exposed sample that served as the working electrode, saturated calomel as the reference electrode, and primary niobium wire as the auxiliary electrode. The test frequency was 100 kHz–10 mHz, and a sine wave with an amplitude of 20 mV was used as the disturbance signal.

The anti-fouling performance was characterized by the number of diatoms and mussels (collected from the water near Qingdao, China) on the coating after the abrasion treatment. The samples were placed in a test solution containing single-celled marine diatoms, and the amount of diatom adhesion was observed and recorded with a fluorescence microscope after 24 h. For the mussel fouling test, an average of five mussels were placed on the sample and immersed in 20 °C seawater, and the number of foot silks attached after different times were recorded.

3. Results and Discussion

3.1. Emulsification System

Typical SEM images of the microcapsules with different emulsifier types are shown in Figure 2. However, Figure 2a,b,f show that agglomeration occurred at the surface of microcapsules synthesized with SDBS, PVA, and PVA/OP-10 emulsifiers, respectively. In addition, as evidenced by the SEM images shown in Figure 2c,e, the surface of microcapsules was depressed and deformed in OP-10 and SDBS/OP-10 emulsions, respectively, and the formation of the deformed shell resulted from the high surface activity and strong emulsification ability of OP-10. It can be seen from Figure 2d that synthesized microcapsules with PVA/SDBS were spherical with no inter-capsule bonding from micrographs, which is because the stability of the emulsion improved by PVA and the anionic emulsifier SDBS is beneficial to the deposition of positively charged urea-formaldehyde resin [16].

Figure 3a shows the TGA curves of MC(T), tung oil, and UF under N₂ atmosphere from room temperature to 850 °C. It can be observed that the decomposition temperature of the tung oil was between 350 and 490 °C and the weight loss completed quickly. The UF decomposition temperature was 210–340 °C. The mass loss of MC(T) was 220–350 °C and 350–490 °C, corresponding to PUF and tung oil. It can be determined from Figure 3a that the core material content reached 75%. FT-IR spectra of MC(T) from Figure 3b showed that the peak at 3410cm⁻¹ is assigned to the superposition of –OH and –NH stretching vibration peaks, the peak at 1640 cm⁻¹ is assigned to the –C=O stretching vibration, and 1564 cm⁻¹ corresponds to the –NH bending vibration peak. The peaks of tung oil at 2930 cm⁻¹, 2854, 1745, 1456, 1380, and 992 cm⁻¹ correspond to the antisymmetric and symmetric stretching vibrations of –CH₂, the characteristic of –C = O in the –COOH absorption peak, and the –CH₃ asymmetric and symmetrical deformation vibration peak. MC(T) contains the characteristic peaks of tung oil and UF. It was determined that the synthesis of self-healing microcapsules was successful and had good thermal stability.

Figure 4 shows the particle size distribution of MC(T) synthesized under different emulsification systems. The particle size and particle size distribution determined the self-healing effect in the coating to a certain extent. Figure 4 shows that the average particle size of microcapsules synthesized with SDBS, PVA, OP-10, SDBS/PVA, SDBS/OP-10, and PVA/OP-10 were 58.34, 35.02, 64.06, 47.70, 70.36, and 50.02 µm, respectively. The synthesized microcapsules had the smallest particle size distribution in the SDBS/PVA system, and 95% of the microcapsules’ size was in the particle size range from 24.07 to 71.33 µm. This is because the synergistic effect of the composite emulsifier makes the emulsion have a lower surface activity and maintains the stability of the oil-in-water emulsion, which can make the oil droplet particle size more evenly distributed, and the particle size of the synthesized microcapsules is relatively small. From the TGA data of microcapsules with different emulsifiers, it is known that when the emulsifier was SDBS/PVA, the content of core material was highest, and the content of tung oil and PUF was 75% and 10%, respectively. Therefore, the SDBS/PVA was identified as the optimum compound emulsifier for synthesizing microcapsules in this work.

3.2. Characterizations of Synthesized MC(P and T)

In order to verify the practicability of microcapsules and realize the idea of integrating anti-corrosion and anti-fouling by introducing an anti-fouling agent into microcapsules, the microcapsules with an anti-fouling agent were studied.

Figure 5 shows the effect of phenolic amides on the particle size of microcapsules. It can be seen that the average particle size of the microcapsules synthesized with tung oil as the core material after adding the anti-fouling agent was reduced. After the viscosity test, it was found that the phenolic amides dissolved in the tung oil, and the viscosity of the core material dropped from 394.3 to 350.8 cP, which affected the emulsification behavior of the tung oil in water. The decrease in the viscosity of the core material is equivalent to an increase in the shear force of the emulsion at the same emulsification stirring speed, which reduces the particle size of the synthesized MC(P and T).

The mass loss curves of tung oil, UF, PA, and MC(P and T) are shown in Figure 6. It can be seen that the decomposition temperature of the phenolic amides was between 190 and 310 °C. The mass loss curve of MC(P and T) consisted of two parts: the degradation at 220–350 °C corresponds to the loss of UF and phenolic amides of about 28%, and 350–490 °C corresponds to the loss of tung oil about 55%. The phenolic amides dissolved in tung oil in the early stage was 10% of the core material content, which shows that the core material content of the total MC(P and T) was about 60%.

The FT-IR spectra of the chemical structure of tung oil, UF, PA, and MC(P and T) are presented in Figure 7. It can be seen from the FT-IR spectrum of MC(P and T) that the absorption peak of phenolic hydroxyl at 3410 cm⁻¹ was swallowed by the broad peak of urea-formaldehyde resin; the phenolic hydroxyl -CO absorption peaks at 1238, 1498, and 1585 cm⁻¹ were the corresponding characteristic peaks of benzene ring stretching; and 740 cm⁻¹ was the characteristic peak of the ortho-substituted benzene ring, which are all characteristic peaks of phenolic amides. MC(P and T) contained the characteristic peaks of tung oil, UF, and anti-fouling agent, indicating the formation of urea-formaldehyde resin self-healing microcapsules containing tung oil and phenolic amides.

3.3. Characterization of Self-Healing Coating

3.3.1. Microstructure of Coating with MC(P and T)

It can be seen from Figure 8 the cross-sectional diagram of the distribution of different contents of microcapsules in the coatings. Figure 8a–d show that as the content of microcapsules increased, the number of microcapsules at the cross-section gradually increased. When the content of microcapsules reached 10%, the cross-section was obviously covered with a large amount of tung oil, and the microcapsules were uniformly distributed in the coating without obvious adhesion and agglomeration. It can also be found that the microcapsules were in complete form and no crashed microcapsules could be found at the cross-sectional surface of the coating, which suggests that no breaking occurred to most microcapsules. The stirring strength of the coating preparation process could be endured, tolerated by the synthesized microcapsules. However, the microcapsules were broken as the coating was fractured, allowing the release of tung oil and phenolic amides.

3.3.2. Bonding Strength of Coating with MC(P and T)

Bonding strength is an important factor for the service life of the coating. The coating surface was subjected to a tension perpendicular to the coating surface in the bonding strength test, and the tension increased until the coating fell from the substrate. Figure 9 shows that with the content increase of the MC(P and T), the bonding strength decreased. When the content of microcapsules reached 15 wt%, the bonding strength of the coating decreased significantly to 4.5 MPa. The results obtained revealed that adding MC(P and T) decreased the bonding strength of the coating samples. The decrease in the bonding strength of the coating having more MC(P and T) can be attributed to the insufficient strength of the microcapsules, their weak interface adhesion, and defects caused by UF resin particles. Apparently, the yielding strength of UF loaded with tung oil was smaller than that of the resin epoxy. Corrosion resistance was partly dependent on the bonding strength of the coating. The mechanical performance could be exacerbated as the content was too high. Therefore, a suitable amount of addition of microcapsules is very important to maintain the coating physical performances.

3.3.3. EIS of Coating with MC(P and T)

EIS was used to study the effect of MC(P and T) core release on the anti-corrosion property of the coating [17]. Figure 10 shows EIS Nyquist plots and Bode plots of coating samples after scratching and immersing in seawater, and Figure 11 shows the change in impedance modulus (|Z|_0.01Hz) of the different-microcapsules-content scratched coatings with the time of immersion in seawater.

The Nyquist plot shows the change in the corrosion resistance of organic coatings with immersion time and the comparison of the corrosion resistance of different coatings. The coating resistance at low frequency (|Z|_{0.01 Hz}) is often used to evaluate the corrosion resistance of the samples. Figure 11 shows that the corrosion resistance at |Z|_0.01Hz was higher compared with those of the rest of the immersion times at 2 h of immersion, due to the smaller scratches, the slower penetration of seawater, and the air hindering the transfer of ions and temporarily increasing the resistance. However, as there was no self-healing layer to protect the Q₂₃₅ carbon steel substrate from corrosion, the Nyquist plot semicircle diameter of the pure epoxy sample became smaller over time, and it can be seen that the corrosion resistance continued to decline. This is because there was no self-healing agent, so seawater quickly penetrated into the substrate, leading to corrosion. For the self-healing coating containing microcapsules, the value of |Z|_0.01Hz tended to be stable after a slight increase in 48–72 h, and the overall value was larger than those of 0% coating samples |Z|_0.01Hz. The increase in resistance at the scratched region indicates that the ruptured microcapsules released tung oil, which filled the cracks and prevented the infiltration of corrosion products, showing a certain anti-corrosion effect. However, the decrease in the |Z|_0.01Hz value indicates that the corrosion resistance of the tung oil layer was lower than that of the epoxy coating. The |Z|_0.01Hz value of the epoxy resin coatings with 5 wt% polyurea-ethylenediamine (EDA) microcapsules decreased from 2.31 × 10⁵ to 9.10 × 10⁴ Ω·cm² [14]. Compared with polyurea-ethylenediamine (EDA) microcapsules, it can be seen from Figure 11 that UF-tung oil had better corrosion resistance.

The corresponding equivalent circuit models was obtained by fitting the immersion after 24 h of samples data by using the nonlinear least squares method with the chi square value as the indicator. All EIS data and underlying physicochemical processes must be considered when determining the optimum model in the fitting process [18]. Based on the EIS and fitting results, the healing process and equivalent circuit models can be summarized as Figure 12.

Table 1. Impedance values of coating samples with UF-tung oil microcapsules, Ω·cm².

Time (h) Samples	24	48	72	96	168
0%	1.47 × 105	1.11 × 105	1.75 × 105	1.30 × 105	1.17 × 105
5%	3.39 × 105	3.12 × 105	3.63 × 105	2.86 × 105	2.54 × 105
10%	3.69 × 105	3.62 × 105	3.32 × 105	3.23 × 105	2.93 × 105
15%	4.26 × 105	3.67 × 105	3.63 × 105	2.97 × 105	2.75 × 105

shows the scratched coating samples resistance values after fitting, which was calculated as the sum of R_oxide or R_healing and R_ct. From Figure 12a and the equivalent circuit model R(Q(RW(QR))), it can be seen that the corrosion products oxide layer built up at the scratches of the neat coating, but corrosion products were not dense enough to inhibit the corrosion. However, there was an oxygen diffusion process between the substrate and the solution, and the oxygen transport can be restricted to a certain extent. R_s is the solution resistance, W is the diffusion component, Q_oxide and R_oxide are the capacitance and resistance of the oxide layer, respectively, and Q_dl and R_ct are the capacitance and the resistance of the double layer, respectively. The equivalent circuit model R(Q(R(QR))) was more suitable for the self-healing coating. Q_healing and R_healing are the capacitance and resistance of the tung oil film, respectively. The microcapsules were broken and tung oil was released to have better anti-corrosion performance, which can form an oil film to heal cracks by reacting with the surrounding oxygen and provide protection for the carbon steel substrate.

3.3.4. Neutral Salt Test of Coating with MC(P and T)

It can be seen from Figure 13 that rusting appeared on the scratches after 72 h of pure epoxy coatings, and became more serious with time. There was basically no obvious corrosion of the coating with microcapsules within 72 h. It can be seen from Figure 13 that the coating containing 5 wt% microcapsules was corroded after 7 days, and the coating containing 10 wt% and 15 wt% microcapsules had slight degrees of corrosion at the scratches after 14 days of exposure. It can be concluded that the corrosion resistance increased with the content of microcapsules. Therefore, it is necessary to increase the content of microcapsules in the coating as much as possible while maintaining the physical properties. Compared to the results of EIS, it can be found that 5 wt% microcapsules addition was not enough to protect the scratch sufficiently.

3.3.5. Anti-Fouling of Coatings with MC(P and T)

Diatoms are single-celled marine social algae that easily adhere to organic coatings [19], which are usually used for the anti-fouling performance test for organic coatings. Figure 14 shows the effect of different microcapsule contents on the coating with the adhesion of diatoms. It can be seen from Figure 14a,b that there were a lot of diatoms attached to the neat epoxy coating surface and a small amount of diatoms attached to the coating surface with 5 wt% microcapsules. In Figure 14c,d, it can be seen that there were almost no diatoms attached to the coating with microcapsules additions, indicating that the coating of the anti-diatom attachment was better as the microcapsules content increased, and the microcapsules content reaching 10 wt% had a good anti-diatom attachment effect.

Mussels release foot silks through the foot silk glands, which are a kind of protein substance. Mussel foot thread is very sensitive to the attached surface, so its sensitivity was used to study the anti-fouling performance of the coating section. The effect of different microcapsule contents on the coating adhesion of mussel foot silks can be seen from Figure 15. The amount of foot silks attachment dropped significantly after adding microcapsules, and the amount of foot silks attachment gradually decreased with the increase in the microcapsule content. When the content of microcapsules was 10 wt%, the effect of preventing foot silks adhesion was not much different from that when the content of microcapsules was 15 wt%, and it already had a good anti-fouling effect.

In the process of biological fouling, first, a lipids and protein molecules thin film is formed on the material surface, micro-organisms such as bacteria and diatoms adhere to the thin film to form a biofilm, and then large fouling organisms begin to adhere [20]. Zhang et al. [21] discovered that the double bond groups and ketone carbonyl were necessary elements for the anti-fouling effect, which can achieve the purpose of anti-fouling by inhibiting fouling biological activity. The phenolic amides anti-fouling agent used in the test is an anti-fouling active compound with a special molecular structure that is screened out using biological extraction and technology combined with chemical synthesis technology. In this study, the layer of tung oil film containing phenolic amides was formed on the surface of the specially treated coating. The phenol amides in the oil film enters the fouling organism and destroys the cell function and basic metabolism of organisms through a specific functional group structure, and inhibits and kills marine biological fouling.

4. Conclusions

Based on the analysis of microcapsules and coating characterization of the anti-corrosive and anti-fouling performance, the following conclusions can be drawn:

• The complex emulsifier of SDBS/PVA had the best emulsifying effect among the six emulsifiers, which could achieve the narrowest particle size distribution of the microcapsules (from 24.07 to 71.33 µm) with 75% core content.
• The addition of 10–15 wt% of microcapsules in the epoxy coating could provide enough healing agent to repair the damage, which made coatings have better anti-corrosion and anti-fouling effects, and the amide included in the healing area was sufficient for preventing 90% of diatoms and 80% of mussel foot silk from adhering to the scratching.
• The anti-corrosion and anti-fouling integration of the self-healing coating was realized, which provides a practical direction for future engineering applications, but the addition of microcapsules will reduce the bonding strength of the coating. The future research direction is to solve the problem of how to ensure the bonding strength of the coating while adding microcapsules.