Urea formaldehyde (UF) microcapsules loaded with corrosion inhibitor for enhancing the anti-corrosive properties of acrylic-based multi-functional PU coatings

Abstract

The present work focuses on enhancing the anti-corrosive performance of multi-functional polyurethane coatings by encapsulation of the commercially available inhibitors 2-mercaptobenzimidazole and 2-mercapatobenzothiozole, despite their reaction with diisocyanate. Initially, inhibitors were encapsulated by in situ polymerisation of urea and formaldehyde. Formation of microcapsules was confirmed by optical microscopy and FE-SEM. Thermal stability and particle size of microcapsules embedded with inhibitors were estimated by TGA and a particle size analyzer, respectively. Release rates of inhibitors were investigated by UV spectrophotometry. Anti-corrosive coatings were designed by dispersing encapsulated inhibitors in a multi-functional acrylic-based PU coating. The anti-corrosive study of coatings was investigated by the Tafel plot method, weight loss study, immersion study, and FE-SEM of corrosive panels. The impact of microcapsule concentration on the anticorrosive nature of coatings was also evaluated. Our study showed an increase in corrosion inhibition efficiency of PU coatings after incorporation of encapsulated corrosion inhibitors. Comparison of anti-corrosive performance of encapsulated inhibitors demonstrated that 2-MBT is superior to 2-MBI.

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Introduction

Corrosion of metallic substrates is directly responsible for a 3–4% loss in domestic production due to high maintenance, high operating cost, contamination of valuable products, plant shut downs, and affects on safety and reliability.^1,2 At present, different polymeric paints or coatings used as a protective barrier are employed to control corrosion of metal substrates. However, these coatings are inadequate to prevent metal corrosion for prolonged periods or under adverse conditions.³ Therefore, the anti-corrosive properties of polymeric coatings have been improved using inhibiting pigments, hybrid films, corrosion inhibitors, self-healing mechanisms, etc.

Corrosion inhibitors are conventionally employed as active anti-corrosive components. Inorganic corrosion inhibitors like chromates, phosphates, nitrites, etc.⁴ are efficient but toxic. For example, chromate has a tendency to cause diseases like cancer and has adverse environmental impacts.⁵ Consequently, recent research has focused on the development of non-toxic and chromium-free corrosion inhibitors for the protection of metals.^6,7 To this end, many heterocyclic organic compounds have imerged as good inhibitors for the protection of various metals or alloys from corrosion.⁸ Preferred organic corrosion inhibitors as a ecofriendly alternative over toxic inorganic inhibitors are 2-mercaptobenzimidazole (2-MBI) and 2-mercaptobenzothiozole (2-MBT). However, if an inhibitor is directly added in a coating, it may show negative impact on the coating properties.⁹ Moreover, solubility of many inhibitors in coating media is a serious concern and affects inhibitor efficiency.^10,11 Direct addition of an inhibitor in a coating can result in interaction between the two materials that can result into deactivation of the inhibitor.¹² More specifically in polyurethane (PU) coatings, mixing 2-MBI and 2-MBT will yield a thiol-isocyanate coupling reaction.¹³ Hence, it is necessary to impart an anti-corrosive property to PU coatings without affecting the structure of the inhibitor.

Microencapsulation is an efficient way to provide protection for a wide range of applications such as food additives, drug release, pesticide release, etc. Microencapsulation has also been used to enhance the anti-corrosiveness of polymeric coatings in corrosion sensors through encapsulation of phenolphthalein in multi-functional protective coatings¹⁴ and self-healing coatings.¹⁵

Although synthesis and encapsulation of the triazole derivative as an anti-corrosive agent have been achieved,¹⁶ microencapsulation has not been tested with enhancement of the anti-corrosive property of polyurethane or other coatings with commercially available inhibitors, such as 2-MBI and 2-MBT. These inhibitors have proven their efficiency, commercially availability, and offer a well-known mechanism of corrosion inhibition. However, their reaction with isocyanate groups through the thiol-isocyanate coupling reaction is a main obstacle to their use for enhancing anti-corrosive properties of PU coatings.¹³ These limitations directed us to consider an encapsulated form of these inhibitors to enable their use in PU coatings.

For the encapsulation process, selection of core moiety and shell materials depends upon their varied functions such as food additives,¹⁷ self-healing coatings^15,18 dyes,¹⁹ pharmaceuticals,²⁰ pesticide formulations, etc.²¹ Due to their easy formation and robost nature, urea formaldehyde (UF) shells have been found suitable for encapsulating a number of reagents such as dicyclopentadiene, neem seed oils, and multi-maleimide healing agents.^22–24

Hence, we selected UF as a shell material for encapsulation of commercial corrosion inhibitors and used UF for studying the anti-corrosive performance of corrosion inhibitors in preparation of multi-functional acrylic-based PU coatings. The representative reaction scheme for preparation of UF microcapsules is given in ESI Fig. S1.† Varying proportions of UF microcapsules were used in PU coatings based on acrylic polyol. The selection of acrylic-based PU coatings was performed on the basis of previous studies, as it showed formation of strong films under appropriate curing conditions and most extensively used polyols for high-performance coatings.²⁵ The study also compared two organic-based corrosion inhibitors.

Experimental

Materials

Benzyl alcohol, butyl acrylate (BA), methyl methacrylate (MMA), and styrene were purchased from Sigma-Aldrich. Methylene diphenyl diisocyanate (MDI) and hydroxyl ethyl acrylate (HEA) were received from Kishore Polyurethanes Pvt. Ltd., Nasik, India. Acrylic copolymer used in the preparation of PU coatings was synthesized using butyl acrylate, methyl methacrylate, hydroxyl ethyl acrylate, and styrene as per the procedure reported elsewhere.²⁵ Corrosion inhibitors (2-MBI/2-MBT), formalin (37% formaldehyde solution), resorcinol, and sodium lauryl sulfate (SLS) were supplied by S. D. Fine Chem. Ltd., Mumbai. Materials used for microencapsulation such as urea (NH₂CONH₂), ammonium chloride (NH₄Cl), and poly(vinyl alcohol) [PVA] were obtained from Loba Chem. Pvt. Ltd., Mumbai.

Methods

Microencapsulation of corrosion inhibitors

Urea formaldehyde microcapsules containing organic corrosion inhibitors were prepared by in situ polymerization of urea with formaldehyde.²² The typical procedure included dissolution of PVA (2 g), urea (5 g), resorcinol (0.5 g), and ammonium chloride (0.5 g) in deionized water (100 mL) in a three-neck, round-bottom flask. Separate solutions of the inhibitors 2-MBI and 2-MBT were prepared in benzyl alcohol in varying amounts from 0.5 to 2 g. After adjusting the flask solution to pH 7, only one inhibitor solution was added to the round-bottom flask for up to 30 min under stirring to form a stable emulsion. At this stage, formalin (13.51 g, in the form of 37% aqueous formaldehyde solution) was added to the stabilized emulsion. The reaction temperature was slowly increased to 65 °C, and the solution was allowed to stir at 500 rpm for 3 h. The reaction pH was maintained at 3 with hydrochloric acid solution (5 wt%). Formation of microcapsules was verified under an optical microscope. Once stable microcapsules were formed, they were collected with vacuum filtration. After rinsing with water, microcapsules were stored until further use.

Preparation of PU coatings

Preparation of acrylic-based PU coatings was carried out by reacting acrylic polyol with MDI in a NCO/OH ratio of 1.2 : 1 at room temperature as per the reported procedure.²⁶ Excess diisocyanate was treated according to the hydroxyl component in the reaction mixture to ensure reaction of all the hydroxyl groups of polyol, because unreacted hydroxyl groups present in the coating may detoriate performance. Required quantities of polyol, diisocyanate, and microcapsules were added to toluene under constant stirring. After complete mixing, the mixture was applied to (6 × 3 cm) steel panels with a bar applicator to a film thickness of 120 μm.

Characterization

An optical microscope (LABOMED Sigma, 2124001, Texas) was used for primary observations of prepared microcapsules under 40–100× magnification. The dispersion study of microcapsules in PU coatings was performed using a polarising microscope (Leica, Germany). The morphological study of microcapsules was performed using a scanning electron microscope (SEM) (JEOL JSM 6360 and JEOL JSM 5400) in the voltage range from 0.5 to 30 kV. The particle size of microcapsules was determined using a laser particle size analyzer (Mastersizer 2000, M41100167, Malvern, UK). FT-IR spectra were scanned 50 times (Shimadzu 8400, Japan) using a KBr disc method. Thermogravimetric analysis (TGA) was performed on a thermo gravimetric analyzer (Shimadzu TGA 50, Japan) by heating from room temperature to 600 °C under nitrogen at a heating rate of 10 °C min⁻¹.

Determination of core content

The concentration of inhibitor present in microcapsules was determined using a Soxhlet apparatus. First, a fixed amount of dried microcapsules (W_m) sealed in dry filter paper bag (W_s) was placed in a Soxhlet apparatus. Simultaneously, the weight of an empty round-bottom flask was noted (W_O), and an extracting solvent (10 mL) was added into the same. After a 6 h extraction, the sample was removed from the Soxhlet apparatus, dried in an oven for 12 h, and weighed (W_sd). In the intervening time, the core material was separated from the solvent with the help of vacuum distillation and weighed (W_Ov). The following formulae were used to determine percentages of core content (α_c), shell material (α_s), and entrapped solvent in microcapsules (α_sv).

(a) Core content (α_c′ %)

(b) Shell material (α_s′ %)

(c) Entrapped solvent (α_sv′ %)

100 − (α_c + α_s)

Control release study

The control release study of inhibitors 2-MBI and 2-MBT encapsulated within a UF shell was conducted in three different buffer solutions (pH 4, 7, and 9). Known quantities of dry microcapsules placed in a Guch crucible (Grade no. 4) were treated with 10 mL of buffer solution for 1 h. Filtrates were collected for analysis by UV spectrophotometer (Chemito UV-2700 Double Beam). The same procedure was repeated each hour for the next 8 h.

PU coating characterizations

The gloss of PU coatings with and without microcapsules (containing inhibitors) was measured by a gloss meter (BYK Additive & Instruments, Germany). The hardness of prepared PU coatings was measured by a pencil hardness tester (BYK Additive & Instruments, Germany) as per ASTM D-3336. In this test, 9B to 9H grade pencils were used. A cross-cut adhesion tester (Elcometer 107.1542) was used to assess adhesion of each coatings as per ASTM D-3359-02. The adhesion of PU coatings was measured by determining the number of cubes removed compared to the number of cubes remaining on the coating panels.

Corrosion studies

Tafel slope analysis. Electrochemical study was performed using a computerized electrochemical analyser (CH Instruments, HI1140C, USA). A coated panel, platinum electrode, and Ag/AgCl-saturated calomel electrode were used as working, counter, and reference electrodes, respectively. Before applying coatings, panels were polished with emery paper, rinsed with deionised water followed by acetone, and dried. Then, five different panels for each corrosion inhibitor were coated with acrylic-based PU coating containing 1–5% of UF microcapsules containing one of the corrosion inhibitors. At a succeeding step, panels were allowed to cure at room temperature for 48 h. A 1 cm² area on each panel was used as a working electrode for sample testing against 0.5 M HCl test solution. The potential was scanned between −1 V to +1 V at a rate of 0.01 mV s⁻¹ at room temperature. The metal panel composition was Fe-94.98% and C-5.02%.

(a) Corrosion rate (CR)

(b) Degree of surface coverage (θ)

(c) % Inhibition efficiency (% IE)

%IE = (θ × 100)

CR – corrosion rate, (θ) – degree of surface coverage, % IE – percent inhibition efficiency.

Weight loss study

The weight loss study conducted on metal panels (size – 14.8 × 7 × 0.15 cm) included washing panels with water and then with acetone. After drying, the panels were coated with PU containing encapsulated inhibitors. Panels were weighed before immersion into the HCl (1000 mL, 0.5 M) test solution. Then panels were removed from the solution after 24 h and reweighed. The same procedure was repeated for up to 240 h. The weight loss of coated metal panels was obtained from the difference in initial and final weights.

Immersion studies

The coating corrosion study was also performed using an immersion method. The study was conducted at room temperature by dipping samples in 0.5 M HCl and 3.5% NaCl solutions. To record the nature of coatings in these solutions, samples were immersed for up to 240 h, during which images were recorded by a digital camera (Sony 16 megapixel).

Results and discussion

Initially, 2-MBT and 2-MBI without encapsulation were included as inhibitors in PU coatings. After mixing inhibitor, polyol, and diisocyanate in toluene, a precipitation was formed instead of a viscous solution. As a result, coatings were opaque, had surface irregularity, and peeled easily from the substrate. The probable mechanism behind this result is the side reaction between inhibitor and one of the reactants noted in Fig. 1. In this case, the thiol group reacts with an isocyanate group through a thiol-isocyanate coupling reaction.¹³ This reaction deactivates the inhibitor and disturbs the stoichiometric amount of diisocyanate required for preparation of PU. Hence, only PU coatings with encapsulated inhibitors were prepared.

Fig. 1 Structures of 2-MBT, 2-MBI, and the possible thiol-isocyanate coupling reaction.

Surface morphology of microcapsules

Initially, formation of microcapsules was verified under an optical microscope, for details refer to ESI (Fig. S2†). In addition, SEM images were collected to study the nature of the surface, separation, shape, size, and other morphological properties of microcapsules, as illustrated in Fig. 2. In the presence of each of the inhibitors, the morphological study revealed spherical microcapsules with a rough surface. Presence of a rough surface increases area and improves surface adhesion of coated microcapsules.²⁷ Furthermore, the formed microcapsules were distinctly differentiated from each other. These observations are beneficial for improving dispersion of microcapsules in the coating media and hence for enhancing anti-corrosive performance.

Fig. 2 SEM images of microcapsules with inhibitors 2-MBI (a & b) and 2-MBT (c & d) encapsulated at 400 RPM.

FT-IR analysis

FT-IR spectra (Fig. S3†) of encapsulated 2-MBI and 2-MBT clearly shows that inhibitors were encapsulated without any structural changes.

Particle size and thermal analysis of microcapsules

Particle size distribution and average particle sizes of the prepared microcapsules are shown in (Fig. S4†). Thermal analysis (TGA curves) of cores and microcapsules are given in (Fig. S5†).

Release rate of corrosion inhibitors from UF microcapsules

The purpose of this study was to determine the release rate of cores and their stability in encapsulated form against acidic, neutral, or basic conditions. Fig. 3 shows a graphical representation of inhibitor release rate from UF microcapsules. In UF microcapsules containing the 2-MBI core moiety, the release rate of acidic buffer solution (pH −4) was highest at up to 98% after 8 h, whereas those for neutral and basic conditions release rates were 3.2 and 28.5%, respectively, in the same time period.

Fig. 3 Release rate of corrosion inhibitors from UF microcapsules containing (a) 2-MBI and (b) 2-MBT.

The release rate of 2-MBT in acidic medium (pH-4) reached a maximum of 95% of the core moiety. In neutral and basic conditions, release rates were observed up to 3.1 and 34% respectively. These release rates were slow as compared to that in acidic conditions.

Both microcapsule types showed very high release rates in a acidic conditions compared to neutral or basic conditions. However, the neutral condition was more stable than the basic condition. This indicated that the neutral condition did not have an effect on the UF shell wall, but acidic and basic conditions affected the shell during the study period.

Dispersion study

Fig. 4 shows dispersion of UF microcapsules into the PU coatings. The purpose of this study was to verify the dispersion of microcapsules throughout the PU coatings. PU coatings containing fewer microcapsules were located a larger distance from each other, while the closeness of microcapsules increased with increasing percentage of microcapsules. Fig. 4 clearly shows that 1, 2, 3, 4, and 5% microcapsules loaded in PU coatings offered good capsule dispersion.

Fig. 4 Polarising microscopic images of PU coatings containing UF microcapsules in varying percentages.

Electrochemical corrosion study

The electrochemical corrosion study of PU coatings loaded with UF microcapsules containing corrosion inhibitors included: current density (I₀), Tafel slope (β_a or β_c), and corrosion potential (E₀) (Fig. 5). Additional information is furnished in ESI Tables S1 and S2.†

Fig. 5 Tafel plot of PU coatings containing (a) 2-MBI inhibitor-loaded microcapsules tested in a 0.5 M HCl solution, (b) 2-MBT inhibitor-loaded microcapsules tested in a 0.5 M HCl solution.

The values of I₀ and plate corrosion rate revealed a decrease in current density with increased loading of UF microcapsules. These results demonstrated that the corrosion inhibitor molecules adsorbed on the metal surface also played a role as a corrosion inhibitor.²⁸

Upon increasing corrosion inhibitor concentration, current density (I₀) values decreased, which is one of the most important parameters in demonstrating corrosion inhibition efficiency.

As per the trends obtained in the electrochemical data, the inhibition efficiency of benzimidazol derivatives was in the order of 2-MBI < 2-MBT. Shifting of cathodic and anodic sides as per Tafel plots of PU coatings indicated that the inhibitors behaved like mixed types.²⁹

Tafel slopes specified that the cathode area was more polarized (β_c > β_a) than the anodic area, indicating that the present corrosion inhibitors were predominantly cathode-controlling. This is because, in acid medium, organic corrosion inhibitor easily protonates and predominates cathodic control of the inhibition process.^29–31

Upon comparing the two different corrosion inhibitors, it was clearly noted that the 2-MBT-containing UF microcapsules in the acrylic PU coatings showed better inhibition efficiency, corrosion rate (CR), and surface coverage area compared to 2-MBI-containing microcapsules.

The surface coverage area of 2-MBT was higher than that of 2-MBI, suggesting a greater adsorption power of 2-MBTcompared to 2-MBI. Adsorption and formation of a protective barrier of corrosion inhibitor protects a metal surface from aggressive species. Moreover, the lone electron pair (sp²) present on a hetero atom of a corrosion inhibitor molecule interacts with the vacant ‘d’ orbital of the iron metal surface to protect the substrate from corrosion.³²

The graphical form of corrosion rate and inhibition efficiency of PU coatings loaded with microcapsules is shown in Fig. 6. The graph in (a) shows that corrosion rate decreased with increased microcapsule loading. Graph (b) shows an increase in percent inhibition efficiency with increased microcapsule loading. The data demonstrated that PU coatings embedded with the 2-MBT microcapsules showed better performance than the 2-MBI counterpart.

Fig. 6 Graphs of (a) corrosion rate (CR) (myr) of 2-MBI/MBT against 0.5 M HCl and (b) inhibition efficiency (% IE) of 2-MBI/MBT against 0.5 M HCl.

Weight loss study

The weight loss study significantly described the corrosion rate and inhibition efficiency of PU coatings, as shown in Fig. 7.

Fig. 7 Graphs of (a) corrosion rate (CR) of 2-MBI/MBT against 0.5 M HCl, (b) inhibition efficiency (% IE) of 2-MBI/MBT against 0.5 M HCl, (c) weight loss study of MBI against 0.5 M HCl, and (d) weight loss study of MBT against 0.5 M HCl.

Fig. 7(a) illustrates that the corrosion rate decreased with microcapsule loading in PU coatings. Similar results were also observed for inhibition efficiency (% IE), as seen in Fig. 7(b). The maximum inhibition efficiency was found with 5% UF microcapsule-loaded PU coatings, which was higher for 2-MBT than 2-MBI. The superior results for 2-MBT-based coatings may be due to presence of the sulphur atom in the MBT ring structure; this atom has higher interaction energy compared to the nitrogen present in the 2-MBI molecule.³²

The corrosion rate and inhibition efficiency of microcapsule-loaded PU coatings were found to be dependent on the percent loading of microcapsules and corrosion inhibitors.

Fig. 7(c) and (d) are graphical representations of weight loss of UF microcapsule-loaded PU coating panels versus time. The coatings showed initial weight loss up to 192 h, with a subsequent weight gain up to 240 h as Fe was converted into Fe₂O₃, a self-anticorrosive agent. This Fe₂O₃ may have also been responsible for the weight loss reduction of the metal samples after 120 h.

Corrosion inhibitor molecules reflected higher surface coverage area over a metal surface compared with water molecules on a metal surface.

Immersion study

Immersion study is an efficient way to determine corrosion based on visible observations or physical changes occurring on coating panels after immersion in corrosive media. Physical changes were noted on multi-functional acrylic-based PU-coated panels when exposed to 0.5 M HCl and 3.5% NaCl solutions for 240 h. Fig. 8 contains images taken after dipping PU-coated and uncoated panels in different media for 240 h. The panels showed that uncoated steel panels were highly corroded upon exposure to alkali and acidic solutions, while PU-coated panels loaded with encapsulated inhibitors were less corroded. Upon increasing the inhibitor percentage, panel corrosion was decreased in both acidic and alkali media. The observed effect may be due to an increase in inhibitor amount. Among all tested coating panels, the 5% microcapsule-loaded panels were the least corroded.

Fig. 8 Immersion study of PU coatings loaded with encapsulated 2-MBI and 2-MBT corrosion inhibitors against 0.5 M HCl and 3.5% NaCl solutions.

Comparing alkali and acidic conditions, the corrosion rate was higher in the acidic condition than in alkaline condition. Overall, the results demonstrate that the corrosion inhibitor containing microcapsules enhanced the anti-corrosive property of coatings.

SEM images of PU coatings

Uncoated metal panels and panels coated with PU were immersed in 3.5% NaCl or 0.5 M HCl solution for 240 h. After removal from solution, their SEM images were recorded, as shown in Fig. 9.

Fig. 9 SEM images of (a) untreated sample dipped in NaCl solution (3.5%), (b) untreated sample dipped in HCl solution (0.5 M), (c) PU coating containing 2-MBI microcapsules (5%) tested against NaCl solution (3.5%), (d) PU coating containing 2-MBI microcapsules (5%) tested against HCl solution (0.5 M), (e) PU coating containing 2-MBT microcapsules (5%) tested against NaCl solution (3.5%), (f) PU coating containing 2-MBT microcapsules (5%) tested against HCl solution (0.5 M).

Images for uncoated metal showed deposits of salt and corrosion on the panels after immersion into the salt solution, whereas panels dipped in acid solution showed deep cracks with a corroded metal surface. On the contrary, panels coated with PU containing 2-MBI microcapsules tested against salt solution displayed no corrosion and only a small amount of deposited salt. In acid solution, these panels showed corrosion without any salt deposits.

Comparison of panels with different inhibitor microcapsules indicated that 2-MBT-based microcapsules were almost free from corrosion and experienced weak salt deposition. Similar results were reported for panels immersed in acidic solution.

Performance of both inhibitors was better in salt solution compared to acidic solution. Between the tested inhibitors, 2-MBT-based microcapsules showed the best results.

Physico-mechanical properties of PU coatings

Tables 1 and 2 present data for physico-mechanical properties of PU coatings. Time required for drying or curing was slightly lower for both PU coatings containing encapsulated inhibitors. Further, upon increasing the amount of UF microcapsules, both drying and curing times decreased, indicating that the presence of microcapsules increased crosslinking rate. This suggests that unreacted secondary amine groups present on the UF shell increased the crosslinking rate of acrylic-based PU coatings by reacting with isocyanate groups of MDI.³³ Cross-cut adhesion and pencil hardness requirements were met by coatings loaded with or without UF microcapsules. The gloss of acrylated coatings containing inhibitors decreased with increasing percentage of microcapsules, possibly due to the increase in coating opacity. This study demonstrated that addition of encapsulated corrosion inhibitor in acrylic-based coatings has an effect on crosslinking rate and gloss but does not affect cross-cut adhesion or pencil hardness.

Table 1 Coating properties of multi-functional PU coatings loaded with 2-MBI containing UF microcapsules

PU coat (% microcapsules loaded)	Drying time (h)		Cross-cut adhesion	Gloss	Pencil hardness
	Dry to touch	Dry to hard
Uncoated	—	—	—	90.3	—
Coated	6	72	Pass	86.8	4H
1%	5.55	71	Pass	85.6	4H
2%	5.55	69	Pass	61.3	5H
3%	5.45	68	Pass	43.2	5H
4%	5.25	65	Pass	42.1	5H
5%	5.05	60	Pass	40.7	6H

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Table 2 Coating properties of multi-functional PU coatings loaded with 2-MBT containing UF microcapsules

PU coat (% microcapsules loaded)	Drying time (h)		Cross-cut adhesion	Gloss	Pencil hardness
	Dry to touch	Dry to hard
Uncoated	—	—	—	80.5	—
Coated	6.05	71	Pass	76.5	3H
1%	5.45	69	Pass	71.5	3H
2%	5.40	65	Pass	68.1	5H
3%	5.35	63	Pass	65.2	6H
4%	5.30	62	Pass	63.4	6H
5%	5.15	61	Pass	41.8	6H

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Conclusion

In the present study, we encapsulated two organic corrosion inhibitors, 2-MBI and 2-MBT, in urea-formaldehyde microcapsules. The prepared microcapsules retained the structural integrity of both inhibitors under encapsulated conditions. Inhibitor-encapsulated molecules were loaded successfully in varing concentrations within acrylic-based multi-functional PU coatings, which shown better anti-corrosive properties than coatings prepared without microcapsules. Anti-corrosive performance of coatings loaded with microencapsule-containing 2-MBT inhibitor was better than that of 2-MBI-containing microcapsules.

This study also revealed that encapsulated inhibitors offer resistance against corrosion of metallic substrate when incorporated in acrylic-based PU coatings. This study describes an entirely new approach for encapsulation of commercial corrosion inhibitors in UF shells that can be used in the development of new multi-functional coatings for anti-corrosive applications.

Acknowledgements

The authors are thankful to the University Grants Commission (UGC), Government of India, New Delhi, India for providing an RFSMS fellowship to one of the authors. This work was also supported by a National Research Foundation of Korea (NFR: 2012M2A2A6035933) grant funded by the Korean government.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra16327c

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