Self-Protecting Epoxy Coatings with Anticorrosion Microcapsules.

The corrosion of steel substrates causes damage that is costly to repair or replace. Current protective coatings predominately rely on environmentally harmful anticorrosive agents and toxic solvents to protect the underlying substrate. The use of lawsone (2-hydroxy-1,4-napthoquinone) together with a water-based epoxy coating provides an environmentally friendly alternative for common protective coatings. Microencapsulated lawsone embedded in an epoxy coating allows the anticorrosive agent to remain dormant until released by damage and delivered directly onto the steel substrate. UV-vis analysis confirms successful encapsulation of lawsone in a polyurethane shell wall and reveals up to 8 wt % lawsone in the capsule cores. Uniform dry film thickness and inflicted damaged are verified with ultrasound and optical microscopy. Visual and electrochemical analysis demonstrates that this self-protective scheme leads to a 70% corrosion inhibition efficiency in a neutral salt water solution.

Introduction

The steel used in bridges, automobiles, or pipelines is vulnerable to corrosion damage unless protected from environmental exposure. Protective coatings and corrosion inhibitors are used to prevent or slow the damage that occurs because of corrosion and reduce the costs associated with repair and replacement of damaged coatings and substrates. Protective coatings rely on solvents and other compounds that contain high volatile organic compounds (VOCs). Defined by the United States Environmental Protection Agency (EPA) in 1970, VOCs are chemicals released in the atmosphere that undergo photochemical reactions releasing environmentally harmful peroxides and ozone, including those from paints and coating applications.[1] These chemicals are binders, coalescing agents, plasticizers, freeze/thaw stabilizers, defoamers, surfactants, and viscosity modifiers.[2] During the drying and curing process, the release of VOCs causes unpleasant odors, possible skin or eye irritation, and allergic reactions.[3] The coating material used in this work contains water as the dispersing solvent with no VOCs.

Corrosion inhibitors, used in combination with protective coatings, slow the corrosion process by inhibition of specific corrosion mechanisms. Some common corrosion inhibition additives include zinc, phosphates, and chromates.[4−6] However, zinc and phosphates are harmful for humans and the environment and chromates have been mostly banned because of their carcinogenicity.[7−9] The industry standard for incorporating a corrosion inhibitor into a coating is by direct addition. Although this approach is straightforward, the inhibitor may not be directly exposed upon damage to the coating, potentially limiting its effectiveness. Similarly, coating materials have been fabricated to have an intrinsic anticorrosion or self-healing mechanism such as a sol–gel or a water-activated polyelectrolyte coating.[10,11] However, these coatings lack the structural integrity of other coating materials (e.g., epoxy coatings).

Another approach to corrosion inhibition is to encapsulate the desired inhibitor and embed the container in the coating.[12−16] Encapsulation provides protection of the inhibitor by preventing uncontrollable release or undesired chemical reactions with the coating material and has been shown to increase the corrosion protection of the substrate when compared to directly adding the inhibitor into the coating material.[17,18] The inhibitor chemical agent (e.g., anticorrosive, antibacterial, and self-healing) remains sequestered and protected within the capsule until released by damage to the coating. Modified nanoparticles are commonly employed to contain the anticorrosive agent.[17,19] Shchukin et al.[18] applied polyelectrolyte layers to silica nanoparticles entrapping the anticorrosive agent within the multiple layers. Upon corrosion damage, the anticorrosive agent is released to prevent further corrosion. Self-healing coatings can autonomously prevent corrosion by repairing coating damage whenever and wherever it occurs.[20,21] Some self-healing coatings use microencapsulated vegetable oils and their derivatives as the healing agent.[22−25] However, the timescale for repair may be slow and the volume of the released healing agent may be insufficient to fully prevent corrosion from proceeding. The vegetable oils are drying oils and can require 24 h or more to repair coating damage. Microcapsules with color and fluorescent indicators have also been added to coatings for autonomous damage indication.[26,27] However, the release of damage indicators alone does not repair coating damage or prevent the corrosion process.

Here, we introduce a self-protecting polymer coating with a microencapsulated anticorrosion agent distributed throughout the coating. Lawsone (2-hydroxy-1,4-napthoquinone), an extract of the henna plant, has known anticorrosive properties based on the chelation of metal cations.[28−33] The metal complexes formed (Figure ) adsorb onto the underlying metal surface and form a protective barrier. Lawsone is safe for humans and the environment and is used for recreational tattoos and skin markings in medical procedures.[34,35] Lawsone has been previously incorporated into a corrosive solution to provide corrosion protection of a substrate but not encapsulated. Encapsulated lawsone in combination with a no VOC water-based coating provides an environmentally friendly protective coating for steel substrates. Lawsone must be encapsulated in this self-protecting system because it is insoluble in the water-based epoxy and forms large agglomerations. Corrosion analysis of this system shows corrosion protection immediately after damage without the need for a healing/drying time or external human intervention.

Figure 1

Lawsone structure interacts with a metal ion (M) during the corrosion process and forms a 1:1 or 2:1 metal complex where Z = 2 or 3.

The coating system is schematically illustrated in Figure . Microcapsules containing the green anticorrosive agent (lawsone and core solvent) are initially embedded in a coating on a steel substrate. Upon mechanical damage to the coating, the microcapsules are ruptured and the lawsone is released into the damaged area where it forms a protective barrier on top of the underlying steel substrate. An environmentally friendly carrier solvent (hexyl acetate) is coencapsulated with lawsone to provide a low viscosity, highly wetting solution that easily coats the steel surface. The carrier solvent then evaporates or diffuses away leaving behind a solid protective barrier of lawsone–metal complexes. Other carrier solvents initially investigated are shown in the Supporting Information. In this paper, we demonstrate the anticorrosive effect of microcapsules containing the anticorrosive agent lawsone in a water-based epoxy coating with no VOCs.

Figure 2

Schematic of a self-protecting coating. (a) Undamaged coating with embedded microcapsules containing an anticorrosive agent (e.g., lawsone). (b) Mechanical damage ruptures embedded capsules releasing their content into the damage area. (c) Evaporation and diffusion of the carrier solvent produces a solid protective barrier that passivates and protects the underlying steel substrate.

Results and Discussion

Microcapsule Characterization

Optical (Figure a) and scanning electron microscopy (SEM) cross section (Figure b) images of microcapsules were used to measure the average capsule diameter (Figure c) and shell wall thickness (Figure d), respectively. SEM cross section images were taken after the microcapsules were embedded in an epoxy (Epofix) and freeze-fractured to view the capsule cross section. The average capsule diameter was 30 ± 9.5 μm, and the measured shell wall thickness was 510 ± 110 nm.

Figure 3

Characterization of microcapsule diameter and shell wall thickness. (a) Optical image of microcapsules as prepared. (b) SEM image of a capsule cross section embedded in epoxy. (c) Histogram of capsule diameter as measured from optical images. (d) Capsule shell wall thickness measured from SEM cross-sectional imaging. False color added to highlight the shell interior (green), the shell wall (yellow), and the surrounding epoxy (blue).

UV–vis was used to measure the lawsone content of the extracted capsule core (Figure ) using the procedure outlined in the Supporting Information. As shown in Figure , the concentration of lawsone in the capsules plateaus at a maximum value of 8%. This plateau is likely due to a concentration-dependent nucleation of the solute (lawsone) as described by La Mer.[36]

Figure 4

Lawsone content of microcapsules as measured by UV–vis analysis. The dashed blue line represents the amount of lawsone initially added into the encapsulation beaker and the amount of lawsone that would be present inside the microcapsules if all of the lawsone was successfully encapsulated.

Representative dynamic thermogravimetric analysis (TGA) results are shown in Figure . The microcapsules exhibit no appreciable weight loss before 200 °C, indicating good thermal stability. Degradation of the shell wall material occurs thereafter followed by rapid weight loss. Notably, the carrier solvent (hexyl acetate) evaporates well before 100 °C, whereas type 0 capsules are stable until nearly 230 °C, indicating successful encapsulation of the core material. The trace for neat lawsone shows that a thermally stable residual mass exists above 400 °C. Comparing type 0 with the other microcapsule types, the residual mass above 400 °C in types 2, 5, and 10 capsules confirms the presence of lawsone in the core material.

Figure 5

TGA results of type 0, 2, 5, and 10 microcapsules along with pure core materials.

Isothermal TGA traces are shown in Figure a. The hexyl acetate-filled capsules lose almost 50% of its mass in an isothermal analysis at 29 °C. This environment is slightly above room temperature and has a constant nitrogen flow allowing for quick evaporation of the hexyl acetate core. Isothermal TGA of lawsone and the shell wall polyurethane (PU) shows no mass loss in the TGA environment. From this we conclude that the mass loss of the capsules near room temperature is due to the loss of the inactive carrier solvent, hexyl acetate, and not the active anticorrosive agent, lawsone. The TGA environment is not representative of the actual storage conditions. The capsules are stored at room temperature (22 °C) and in stagnant air or in a closed container. The capsules stored in open containers at room temperature showed a mass loss of 14% after 4 h and only 25% after 100 h (Figure b). Elevating the temperature slightly (35 °C, coating cure temperature) shows a significantly different response. The capsules lose 50% mass in the first 24 h and almost 80% of their original mass after 100 h. However, the most representative storage condition is in a closed storage container kept at room temperature. In this case, after 100 h, the capsules lose less than 3% mass.

Figure 6

Thermal stability of microcapsule components. (a) Isothermal TGA near room temperature (29 °C) for 24 h of hexyl acetate-filled microcapsules, lawsone, and the shell wall material (PU). (b) Mass of microcapsules kept under various storage conditions over time at room temperature (22 °C) and at an elevated temperature (35 °C).

Anticorrosion Coating Performance

Confirmation of Lawsone Release

The release of lawsone from the capsules was confirmed optically using an opaque phenolic coating containing 20 wt % of type 10 capsules. Control coatings with 20 wt % hexyl acetate capsules were also prepared. Figure contains optical images of the two coatings after damage. In Figure a, a distinctive orange color is observed in the scribed region. In contrast, no color change is observed for the control coatings (Figure b).

Figure 7

Optical confirmation of lawsone release following scribe damage in an opaque phenolic coating. Optical images of (a) self-protecting coating with lawsone-filled microcapsules. (b) Control coating with hexyl acetate-filled microcapsules.

Observation of the Corrosion Product in the Damage Zone

Anticorrosion performance of water-based epoxy coatings containing lawsone capsules was evaluated through optical imaging, SEM imaging, and electrochemical measurements. For these studies, the damage induced by a corrocutter was kept as consistent as possible across all coatings. A reconstructed image of the damage created from a series of digital microscope images is shown in the Supporting Information. On average, the scribe damage of water-based epoxy coating samples measured 82 ± 10 μm wide at the opening, 193 ± 17 μm deep from the flat undamaged coating, and 3.8 cm long. Optical and SEM images were taken of damaged samples after they were submerged in a 5 wt % NaCl solution for 5 days (Figure ). Figure a,c are images of a control coating with 20 wt % type 0 capsules, and Figure b,d are images of a self-protecting coating with 20 wt % type 10 microcapsules. The optical images show that the corrosion product has not spread as far in the self-protecting coating when compared to the control coating. Similarly, there are significantly less corrosion product crystals (Figure c inset) seen in the SEM image of the self-protected coating. Because of the similar color of the lawsone microcapsules and coating, the optical images in Figure have been enhanced by digitally subtracting out the background color and adding a false color to the corrosion product image. Both the original and color change images are shown in the Supporting Information.

Figure 8

Reduction in the corrosion product in coatings with lawsone capsules. Optical (a,b) with a false colored corrosion product and SEM (c,d) images of the corroded samples after submersion in the NaCl solution for 5 days. (a,c) Coatings containing 20 wt % type 0 capsules. (b,d) Coatings containing 20 wt % type 10 capsules.

Electrochemical Characterization of Corrosion

Electrochemical (EC) analysis of damaged epoxy coatings was carried out using the test cell in Figure . The average and one standard deviation of the EC parameters, open-circuit potential (Eoc), corrosion current (Icorr), and inhibition efficiency (IE %) are shown in Table for samples containing 20 wt % of the five types of microcapsules. The less negative open-circuit potential with an increasing amount of lawsone in the microcapsules indicates an anodic-type inhibition. Coating specimens with the lowest corrosion current (type 10 and 20 capsules) provide the best corrosion protection. An additional set of experiments were carried out for coatings with 10 wt % of type 0 and type 10 capsules only. The corrosion current of coatings with 10 wt % type 10 capsules is summarized in Table . As expected, control coatings with type 0 capsules had similar properties regardless of capsule loading. Coatings with 20 wt % type 10 capsules had a smaller corrosion current than coatings with 10 wt % type 10 capsules, indicating that the corrosion current depends on the amount of lawsone delivered.

Figure 11

Electrochemical test cell with three electrodes and a 5 wt % NaCl electrolyte (75 mL). The steel samples measured 30.5 by 10.0 by 0.5 cm. The silicone seal was 0.5 cm thick. The electrolyte chamber measured 7.6 by 10 cm at the base and had a 4 cm diameter opening at the top. The coated steel substrate acts as the working electrode, a platinum wire (7.5 cm long, 0.5 mm diameter) serves as the counter electrode, and a silver/silver chloride electrode (7.5 cm long, 6 mm diameter) serves as the reference electrode.

Table 1

Corrosion Potential and Corrosion Current of Coatings Containing 20 wt % of Various Types of Microcapsules

capsule type	Eoc (mV)	Icorr (μA)	IE % (%)
type 0	–604 ± 15.1	0.83 ± 0.22	0 ± 21.9
type 2	–585 ± 34.2	0.66 ± 0.06	20.4 ± 7.1
type 5	–546 ± 8.60	0.44 ± 0.04	47.2 ± 5.0
type 10	–545 ± 13.5	0.36 ± 0.03	55.9 ± 4.0
type 20	–550 ± 3.80	0.33 ± 0.03	59.9 ± 3.4

Table 2

Corrosion Potential and Corrosion Current of Coatings Containing 10 wt % of Various Types of Microcapsules

capsule type	Eoc (mV)	Icorr (μA)	IE % (%)
type 0	–600 ± 4.44	0.76 ± 0.17	0 ± 22.3
type 10	–554 ± 13.0	0.52 ± 0.05	31.3 ± 6.9

The corrosion current for all samples is plotted as a function of lawsone loading in Figure . For all coatings, the corrosion current decreases with increasing lawsone content in the microcapsules, resulting in less corrosion of the substrate. Figure summarized the inhibition efficiency (IE %) as calculated by eq of all of the coatings. Maximum and minimum Io values were used to calculate the IE % for the different sample types giving the maximum and minimum possible range of IE % values. Coatings with 20 wt % loading of type 10 and 20 microcapsules had a similar lawsone content and a similar anticorrosion performance. Both capsule types had a maximum inhibition efficiency of almost 70%. The IE % of coatings with 10% loading of type 10 capsules had almost half the IE % of coatings with 20% loading of type 10 capsules. The amount of corrosion decreases almost linearly with an increasing lawsone concentration within the microcapsules. The amount of lawsone that can be encapsulated is a limiting factor that prevents a better anticorrosion performance with this protection scheme.

Figure 9

Corrosion current as a function of lawsone loading for coatings containing 20 wt % microcapsules and 10 wt % microcapsules. Each data point represents one sample.

Figure 10

Corrosion inhibition efficiency as a function of lawsone loading for coatings containing 20 wt % microcapsules and 10 wt % microcapsules. Maximum and minimum IE % values were calculated for each data point shown in Figure .

Conclusions

Self-protective water-based epoxy coatings were fabricated with 10 and 20 wt % microcapsules containing the henna plant extract lawsone dissolved in a carrier solvent hexyl acetate. This coating system is a safer and more environmentally benign alternative to current anticorrosion coating systems used commonly in industry. The lawsone-filled microcapsules also can be combined with a variety of other types of materials where extra corrosion prevention is desired. Coatings with lawsone capsules developed a significantly less corrosion product in the damage region after exposure to 5 wt % NaCl solution. Increasing the lawsone content in microcapsules led to a nearly linear reduction in the corrosion current and increase in the inhibition efficiency as measured by EC analysis. Coatings fabricated with 20 wt % capsules containing the highest lawsone content of 8 wt % exhibited up to 70% inhibition efficiency.

Experimental Methods

Materials

The microencapsulation materials gum arabic, hexyl acetate, and lawsone were purchased from Sigma-Aldrich (St. Louis, MO, USA) and were used as received. The microcapsule shell wall material was prepared from a PU prepolymer (Desmodur L 75) provided by Bayer MaterialScience. Hot rolled steel substrates following ASTM A569 were purchased from Metals Depot (Winchester, KY, USA). The epoxy coating consisted of Epirez resin 6520-WH-53 and Epikure curing agent 6870-W-53 and were obtained from Chemical Marketing Concepts Inc. Phenoline 187 phenolic resin was obtained from Carboline (St. Louis, MO, USA).

Microcapsule Fabrication

The microencapsulation procedure was modified from Mangun et al.[37] The surfactant gum arabic (9 g) was slowly dissolved in 80 mL of deionized (DI) water in a 400 mL beaker. The core solution consisting of 40 g of hexyl acetate together with a range of concentrations of lawsone was mixed with 5.0 g of the PU prepolymer (shell wall) in a separate beaker. The core solution was then slowly added to the water and surfactant solution while undergoing mechanical agitation with a 51 mm diameter low shear mixing blade at 1200 rpm. Mixing was allowed to continue for 2 h at 85 °C in a temperature-controlled water bath. The resulting capsules were allowed to float in a separatory funnel and rinsed in DI water until the solution was clear and free of excess lawsone. The microcapsules were then freeze-dried to remove all excess water. The naming convention and lawsone content of the core solution for microcapsules that were fabricated for this study are summarized in Table .

Table 3

Variation of Lawsone Content in Different Microcapsule Types

designation	Lawsone concentration of core solution (wt % wrt capsule core)
type 0	0
type 2	2
type 5	5
type 10	10
type 20	20

Capsule Core Analysis

The final lawsone content of the encapsulated core material was determined by extracting the core material and performing UV–vis spectroscopy (Shimadzu 2401-PC) of the sample as detailed in the Supporting Information. Using a calibration curve (Supporting Information Figure S2), the lawsone content can be calculated from the measured peak absorbance of the extracted capsule core material. TGA was performed on both microcapsules and isolated core materials to determine thermal stability by heating to 650 °C at a rate of 10 °C/min under nitrogen purge. Three isothermal analyses were completed on capsules and components to determine capsule stability under storage conditions. First, 24 h isothermal TGA was completed near room temperature (29 °C) on capsules and capsule components. Second, capsules were stored in an open container at room temperature (22 °C) and at an elevated temperature (35 °C) for 100 h and were weighed every 24 h. Last, capsules stored at room temperature in a closed container were also weighed every 24 h to more closely replicate storage conditions.

Coating Preparation and Damage Protocol

Steel substrates measuring 30.5 by 10.0 by 0.5 cm were prepared prior to the coating application. The substrates were sandblasted to a “white metal blast” according to ASTM D7055-09 with 180 grit aluminum oxide blast media. They were cleaned with compressed air to remove all excess grit, rinsed in acetone to remove dirt and oils, and then let air-dry at room temperature. The water-based epoxy coating was then applied with a micrometer-controlled doctor blade. The epoxy coating mixture consisted of four components. The first two were a stoichiometric mixture of Epirez resin 6520-WH-53 and Epikure curing agent 6870-W-53 (weight ratio of 10:4). Both the resin and curing agent are 53 wt % solids and 47 wt % water. To this mixture, 2 g of DI water was added for every 10 g of Epirez resin for viscosity modification. The final component was 10 or 20 wt % microcapsules with respect to the epoxy solid mass (resin and curing agent). The coatings containing microcapsules with no lawsone (type 0) were prepared as a control, and the self-protecting coatings with lawsone-filled microcapsules contained up to 20 wt % lawsone. The applied coating material was cured for 24 h at 35 °C. Fully cured and undamaged coatings were optically imaged and analyzed with an EC analysis to show the change in the barrier properties due to the addition of the microcapsules. These results are detailed in the Supporting Information. The coatings were also damaged with a corrocutter (Erichsen 639) affixed with a razor blade to inflict uniform scribe damage to the coating. The scribe damage size was measured with a digital microscope. After damage, the coatings were subjected to ultrasonic, visual, and EC measurements to show the anticorrosion performance of the lawsone-filled microcapsules.

Additional specimens were prepared with a phenolic resin (Phenoline 187, made by Carboline) for enhanced optical imaging. The manufacturer-suggested mix ratio and cure time were used to prepare the phenolic coatings with 20 wt % type 0 and type 10 microcapsules which were then applied to steel substrates similar to the epoxy coating. The coatings were damaged with a corrocutter and optically imaged to view the release of lawsone. The release of lawsone was confirmed optically by its orange color.

Ultrasonic Analysis of Coating Thickness

An ultrasonic transmission analysis was completed in pulse-echo mode to measure the coating thickness. A custom transducer with a frequency of 80 MHz and focal length of 1 cm was used. The sampling frequency was 1 GHz, and the spot size of the transducer at the focal point was 60 μm. The speed of sound within the coating material was first determined (see Supporting Information). The speeds of sound of the epoxy coating and the microcapsule-embedded epoxy coating were 2800 and 2600 m/s, respectively.

The samples were placed in a water tank (degassed and DI) and the time difference of the signal between the water/coating interface and the coating/steel interface was measured. The thickness of the coating was then calculated using the measured speed of sound of the coating material. A full field thickness plot of the sample (see Supporting Information Figure S5) was acquired by x–y scanning of the transducer, and from these data, the average thickness and standard deviation were calculated.

As a comparison, coating samples were also cross-sectioned and polished. The coating thickness was measured with a scanning electron microscope in this case. The ultrasonic and SEM thickness measurements were in good agreement (see Supporting Information). The thickness of the coatings used in this work had a cured thickness of 120 ± 11.8 μm.

EC and Visual Characterization

The EC analysis was completed in a specially designed test cell shown schematically in Figure . The coated steel substrate was damaged and then placed beneath the electrolyte chamber. The entire cell was clamped to provide a sealed cell for analysis. A 5 wt % NaCl solution was used as the electrolyte. The cell consists of three electrodes with the steel substrate as the working electrode, a platinum wire as the counter electrode (BASi MW-1032), and a silver/silver chloride reference electrode (BASi MF-2052).

Linear polarization measurements at room temperature (23 °C) were performed with a Bio-Logic VSP potentiostat to measure the corrosion current immediately after damage was introduced to the coating. The open-circuit potential was first measured for 20 min; then, a stepwise voltage from −100 to 100 mV at a rate of 0.2 mV/s was applied with respect to the open-circuit voltage and the current measured throughout. The EC analysis was completed on at least three samples for each of the different types of microcapsules.

Tafel extrapolation was used to determine the corrosion current associated with steady-state corrosion. The corrosion current for the control (Io) and self-protecting samples (Ii) was measured and used to calculate the inhibition efficiency (IE %)[32,38]

The inhibition efficiency provides a quantitative metric for cultivating the performance of self-protecting coatings through the reduction in corrosion current. The Tafel analysis was used as a tool to determine the self-protecting capabilities of the coating and not to determine the underlying corrosion mechanisms of lawsone as they are already understood.[28−30]

A similar test cell was used for visual analysis, except the top of the electrolyte chamber was closed to prevent evaporation of the electrolyte solution. The electrolyte solution (5 wt % NaCl in DI water) was loaded into the cell immediately after damage, and the samples were left submerged in the solution for 5 days. Submersion visual tests were chosen to show the self-protection properties days after damage was introduced to the coating as opposed to salt-fog analysis that can simulate months of corrosion damage. Afterward, both optical and SEM imaging was performed on the test samples. Because of the similar color of the lawsone microcapsules and coating, some of the optical images were enhanced by adding false color to the corrosion product after digitally subtracting out the background color.