The effect of carbon nanotubes loaded with 2-mercaptobenzothiazoIe in epoxy-based coatings.
Keywords Carbon nanotubes, Conductive, Corrosion inhibitor, Nanocontainer, Layer-by-layer technique
Corrosion is a natural process. It is defined as a chemical or electrochemical reaction between a material and its environment, which leads to the deterioration of the material and its properties. (1) It results in the decrease of the strength of the metal, the efficiency of the equipment, loss of fluids, and loss of surface properties of the metal. (2,3) The degradation of materials due to corrosion has become one of the important issues which leads to depreciation of investment goods. (4,5) Hence, the development of an active corrosion protection system for metallic substrates is a subject of major importance. There are five primary methods to avoid corrosion which include a proper material choice, coatings, inhibitors, cathodic protection, and an appropriate design which would avoid probable corrosion. (6,7)
The barrier coatings used for preventing corrosion have proven to be very efficient as they are not only easy to prepare but also produce dense barrier coatings which exhibit low permeability to corrosive agents. (8) A coating is a substance, mostly a liquid, which is applied on the substrate, which appears as either a continuous or discontinuous film after drying. Its basic function is to isolate the metal from the corrosive environment and form a barrier to stifle corrosion. (9) These coatings are applied to improve the performance properties of the substrate such as appearance, adhesion, wettability, and also to provide protection from corrosion. (10,11) Coatings can be organic, inorganic, or even the hybrid of both. Epoxy coatings are used extensively as they offer a number of benefits. They are quick to install, durable, easy to clean, are environmentally friendly, and also provide resistance to chemical substances and improve safety. (12) Similarly, sol-gel coatings, chromium coatings, layered double hydroxides, and silica-based coatings are also used. (13,14) However, the applications of such coatings have some limitations. They lose their protective function once they get damaged, i.e., they provide a passive protection. This drawback can be overcome by providing an active protection using inhibitors. Inhibitors are those substances which may be injected into the system to inhibit the formation of corrosion cells by various mechanisms. (15) In the case of organic inhibitors like amines, the inhibitor is adsorbed on anodic and cathodic sites, which suppresses the corrosion current. A few inhibitors specifically affect either the anodic or cathodic process and others promote the formulation of protective films on the metal surface. (16)
Although the use of inhibitors provides an active corrosion protection, they cannot be directly incorporated into the coating matrix as it might lead to significant shortcomings in the stability and self-repairing activity of the coating. The inhibitor could react with the coating matrix, and in addition, leaching of the inhibitor into the environment might take place. Therefore, active anticorrosive coatings should be prepared in such a way that they could be able to protect the metal from corrosion even when they are not intact. Such functionality can be imparted by the addition of nanocontainers which could encapsulate the inhibitor in them. (17) Recent studies have shown that the use of nanocontainers highly enhances the anticorrosive properties. The main function of a nanocontainer is to distribute itself into the coating matrix and encapsulate the inhibitor by forming a shell possessing controlled permeability which allows the responsive release of the inhibitor. As a result, the inhibitor is kept in a trapped state which disables the undesirable interaction between the active component (inhibitor) and the passive matrix and also avoids leaching of the inhibitor. (18) The selective response of the nanocontainer is shown by allowing certain molecular species to enter or leave the nanoreactor, while others are blocked from entering the cavity. This is achieved by means of intrinsic, semipermeable properties of the boundary layer or by selective pores. (19,20) The containers should be chosen in such a way that they are dispersible with the coating vehicle and remain intact on capsules; loading of the containers should not not affect the other properties of the coating matrix; release kinetics should be fast and sustainable; the added containers' quantity should be minimal so that it does not affect the pigment volume concentration (PVC); and finally, they should be compatible with the coating formulation. (21,22) The best way of synthesizing nanocontainers is to generate polymer nanocapsules by forming a polymer shell around a preformed template particle. The layer-by-layer technique involves a series of deposition steps of oppositely charged polyelectrolytes. The driving force in this method is the electrostatic attraction between the added polymer surface charges. These smart nanocontainers release the corrosion inhibitor on demand. (23) Nanocontainers are nanometer-sized hollow spheres, which are capable of encapsulating a large amount of guest molecules within their core domain. They have the ability to release the inhibitors (guest molecules) in a controlled way. Thus, they provide self-healing properties to the coating. Some commonly used nanocontainers are Ti[O.sub.2] nanospheres, ZnO nanospheres, ceria nanocontainers, carbon nanotubes (CNTs), etc. (24,25) CNTs have attracted the attention of many scientists worldwide. In combination with active materials, they have been used for the corrosion protection. (26,27) The small proportions, strength, and the notable physical properties of these structures make them a very unique material with a whole range of promising applications. CNTs are unique tubular structures of nanometer-scale diameter and large length/diameter ratio. The nanotubes may consist of from one up to tens and hundreds of concentric shells of carbons with adjacent shells' separation of 0.34 nm. (28) The carbon network of the shells is closely related to the honeycomblike arrangement of the carbon atoms in the graphite sheets. The remarkable mechanical and electronic properties of the nanotubes stem from their quasi-one dimensional (ID) structure and the graphite-like arrangement of the carbon atoms in the shells. Thus, the nanotubes have high Young's modulus and tensile strength, which make them preferable candidates for the preparation of composite materials with improved mechanical properties. (29)
The present study deals with the synthesis of 2-mercaptobenzothiazole (MBT)-loaded CNTs and their application in epoxy-based coatings. The report encompasses the three-step procedure for synthesizing loaded nanotubes: (1) loading of polypyrrole layer on the CNT surface; (2) loading of the corrosion inhibitor; and (3) loading of polyacrylic acid layer on inhibitor-loaded CNTs. Mild steel (MS) samples were coated with epoxy coatings incorporated with CNTs loaded with 2-MBT as the inhibitor.
Materials and reagent
All chemicals were of analytic reagent grade. The single-walled carbon nanotubes (SWCNTs) are -COOH functionalized used as received. Sodium lauryl sulphate (SLS), ammonium per sulfate (APS), pyrrole, acetone, sodium chloride (NaCl), polyvinyl pyrrolidone (PVP), ethanol, 2-mercaptobenzothiazole (MBT), polyacrylic acid were obtained from s.d. Fine Chemicals Limited (India) and used without further purification.
Coating of polypyrrole layer on CNT
The method of addition polymerization was used to produce polypyrrole-coated nanotubes. 0.2 g CNT was added into distilled water, and the reaction was carried out in a reactor under continuous stirring. To eliminate the effects of oxygen on the reaction matrix, nitrogen gas was purged inside the reactor continuously. Surfactant was added to develop a negative charge surrounding the nanotubes so that polypyrrole layer could effectively attach through ionic interactions. The polymerization process was initiated with the initiator solution of ammonium per sulfate in 10 mL water. After 20 min of continuous stirring with overhead motor at 800 rpm, 5 mL of pyrrole was added dropwise into the solution for 30 min. To ensure a proper dispersion of CNT, the solution was kept in ultrasonic bath for 1 h at the maintained temperature range of 29-30[degrees]C. The resulting solution was filtered and washed with ethanol followed by distilled water. Thereafter, the resulting polypyrrole-coated CNTs were incubated in 0.5 M NaCl doped with 1 wt% PVP in ethanol for 30 min. The role of PVP is to prevent aggregation of the particles. The resulting dispersions were centrifuged at 2000-3000 rpm for 10 min after washing them with distilled water. The solutions were discarded, and the polypyrrole-coated CNTs were obtained.
[FIGURE 1 OMITTED]
Loading of 2-mercaptobenzothiazole (MBT)
In the next step, 1 g of inhibitor, MBT (solubilized in acetone),was added into acidic medium maintained at pH 3 and the polypyrrole-coated CNTs taken in 0.1 M NaCl solution were mixed and stirred for 20 min. The solution was then filtered after stirring and washed with 0.5 M NaCl. The resulting suspension was centrifuged at 2000-3000 rpm for 5-10 min. The supernatant solution was discarded, and the inhibitor-coated CNTs were obtained.
Coating of polyacrylic acid layer
This layer was the final coating layer in the preparation of the nanocontainers, and it helps in improving the compatibility of the nanocontainer with the epoxy resin used. The products from the above step were taken into 100 mL of 0.5 M NaCl solution, and about 0.2 g of acrylic acid was added. The reaction mixture was stirred for 20 min to result in the formation of the polyacrylic layer on the particles. The formed nanocontainers were then filtered and then thoroughly washed with water. They were dried for 2 days in ambient atmosphere. During the whole process, active material was encapsulated into nanocontainers with a shell possessing controlled permeability properties. As a result, nanocontainers were uniformly distributed in the matrix keeping the inhibitor in a "trapped" state and avoiding the undesirable interaction between the inhibitor and the passive matrix. The flow diagram will help to understand the whole process as shown in Fig. 1.
Application in epoxy-based coatings
The synthesized nanocontainers were used in anticorrosive coatings for MS application. The steel panels were first sanded using grid paper, and then using isopropyl alcohol to remove all the unwanted dirt particles. The coating was prepared by mixing epoxy resin and polyaminoamide (hardener) in the ratio 2:1.064 by considering the epoxy equivalent weight and amine value, respectively, with further addition of 1% of synthesized CNT nanocontainer. A mixture of xylene and butanol was used as a solvent to improve the workability of resin by decreasing the viscosity of the coating. After the hardener is added, the coating should be immediately applied on the steel panels as it starts curing the coating. The settling time for the coating depends on the curing condition only. The coated panels were cured at 120[degrees]C for 60-80 min.
The prepared nanocontainers were characterized by X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FT1R), UV-Vis spectroscopy, Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM). Measurements of XRD were taken on a Rigaku Mini-Flex X-Ray Diffractometer instrument using Cu-K[alpha] radiation source ([lambda] = 0.154 nm). FTIR was performed on a Perkin Elmer Spectrum 100 Spectrophotometer using KBr pellet. UV-Vis spectra were carried out using PerkinElmer UV-Vis Spectrophotometer. Morphological study was done on a JEOL, JSM-6380 LA 15 kV electron microscope. TEM images of nanocontainers were collected using Philips Model CM200 microscope having operating voltages ranging from 20 to 200 kv with the resolution of 2.4 [Angstrom]. The coated MS panels were tested for their corrosion resistance properties according to ASTM B117. The anticorrosion performance of the coated panels was evaluated on Versa STAT-3 instrument (AMETEK, Princeton Applied Research, Oak Ridge, TN). The anticorrosive properties of the coated panel samples were measured using DC polarization technique in 0.05 M NaCl solution. The experiments were performed at room temperature, using a three-electrode electrochemical cell, consisting of working electrode, saturated calomel electrode (SCE) as reference, and platinum as counter electrode. The water absorption test was performed according to ASTM D570.
[FIGURE 2 OMITTED]
Results and discussion
FTIR was used to confirm the presence of every layer on CNT. The formation of the SLS layer on the CNTs was shown by the peaks at 2692, 1750, 1290, and 1100 [cm.sup.-1]. (30) The peak at 1558 [cm.sup.-1] comes from -CC- stretch of pyrrole ring, and the peak at 1033 [cm.sup.-1] is caused by -C-H- in-plane deformation of the pyrrole unit. Also, the peak at 1648 [cm.sup.-1] observed was related to -C-N- stretching vibration in pyrrole. The characteristic peaks of polypyrrole are seen, one at 2935 [cm.sup.-1] which indicates the presence of an aromatic ring, and another at 3624 [cm.sup.-1] which indicates the N-H bond vibration. The peaks at 579, 665, 822, 875, and 1048 [cm.sup.-1] show the formation of 2-MBT layer formation. The slight variations may indicate some variation in the interaction between the intercalated benzoate anion and the cationic layers. The adsorption band at 716 [cm.sup.-1] corresponds to the bending of the five adjacent hydrogen atoms of the ring, which are visible as typical signals of monosubstituted aromatic ring. The absorption band at around 3450 [cm.sup.-1] is assigned to the stretching vibration of the hydroxyl groups and the interlayer water molecules. The bands close to 579 [cm.sup.-1] are typical of the benzene ring vibrations, and the band near to 1518 [cm.sup.-1] is characteristic of the aromatic and the triazole ring stretching vibrations. (31) It is observed that the peaks of the PPy (2800-3624 [cm.sup.-1]), as seen in Fig. 2, are not observed as 2-MBT and PAA interfered with PPy chains. The peak at 1725 [cm.sup.-1] comes from C=0 stretch, and the one at 1410 [cm.sup.-1] comes from COO- stretching of polyacrylic acid.
XRD patterns of the CNT and nanocontainer sample are shown in Fig. 3. The formation of CNTs encapsulated into nanocontainers has been depicted in the figure, and it can be also observed that during the preparation of nanocontainers, CNT phase remains the same. The XRD patterns of the CNT and CNT loaded with PPy-2-MBT-Poly aery lie acid are shown in Fig. 3. (21) In the pattern of the synthesized nanocontainer, there are three main peaks at about 2[theta] = 25[degrees], 44[degrees], and 51[degrees] (Fig. 3). The pure CNTs have characteristic peaks at 25.9[degrees], 44.5[degrees], and 51.8[degrees] which are in good agreement. There was no major peak in view of the fact that no impurity remained in this sample. The peak centered at 2[theta] = 25[degrees] may be ascribed to the periodicity parallel to the polymer chain, while the weak peaks at higher angles may be caused by the periodicity perpendicular to the polymer chain. From Fig. 3, it is clear that the intensity of nanocontainers was suppressed due to three-layer-coated system, since the material became less crystalline by nature. (31)
[FIGURE 3 OMITTED]
The SEM images of functionalized CNTs and CNTs loaded with conductive polymer-inhibitor-polyacrylic acid are shown in Fig. 4a. The crystallite has a uniform characteristic rod-like shape and a relatively narrow particle size distribution (PSD) within the similar range. (32) The TEM images of nanocontainers clearly reveal that the layer-by-layer technique has worked successfully as shown in Fig. 4b. The CNTs are obviously seen in the image as depicted by the continuous black line. This was wrapped after loading of PPy, 2-MBT, and acrylic acid shows rough and uneven surfaces.
The thermogravimetric analysis (TGA) of CNTs and nanocontainers are shown in Fig. 5. From the figure, it is clearly seen that the degradation in CNT nanocontainer is higher compared with CNTs. The TGA analysis of the CNT nanocontainers indicates a weight loss in the range of 30-200[degrees]C, due to degradation of 2-MBT on the surface of nanocontainer. The second weight loss for this nanocontainer ranges between 200 and 500[degrees]C, corresponding to oxidative degradation of 2-MBT. The sharp weight loss in this range was near about 51% and residue 49%. The weight loss was observed because the inhibitor 2-MBT undergoes oxidative degradation which starts to degrade above 200[degrees]C; in addition, polyacrylic acid and poly (pyrrole) start to degrade above 300[degrees]C. (33) This effect shows that inhibitor was encapsulated due to the layer-by-layer assembly and not sustained above 250-300[degrees]C. The TGA diagram of CNTs is included in the Fig. 5 for comparison with loaded CNTs. It shows three main regions of weight loss. In the first temperature region of 30-200[degrees]C, desorption of physically adsorbed water (free and physisorbed water) was observed. In the second region of 200-500[degrees]C attributed to chemisorbed water, the monolayer of [H.sub.2]O molecules directly interact with the solid surface such as CNTs and hydroxyls. In the case of CNTs only 4.37% weight loss was observed in the region between 200 and 500[degrees]C. Above this temperature region, no degradation occurs, so ultimately no weight loss occurrence was observed. (34) The third region of above 500[degrees]C is attributed to the dehydroxylation (release of OH from the structure).
Particle size and zeta potential analysis
As shown in Fig. 6, the graph shows that the average particle size gradually increases. This indicates and confirms the layer-by-layer building of the nanocontainer by following the procedure described in the "Coating of polypyrrole layer on CNT," "Loading of 2-mercaptobenzothiazole (MBT)," "Coating of polyacrylic acid layer" sections. Initially, the particle size of the CNTs was found to be around 180 nm. After the formation of the polypyrrole layer onto the CNT nanocontainer by miniemulsion polymerization, it has been observed that the particle size increased to an average value of 347 nm. Therefore, it can be said that the thicknesses of the layers of polypyrrole show an average thickness of 167 nm (as per analysis taken from dynamic light scattering method). Addition of the 2-MBT molecular layer increases the thickness further by 507 nm. 2-MBT layer thickness is high, which clearly indicates that a large number of molecules were adsorbed on polypyrrole-coated CNTs assembly as the 2-MBT is a smaller molecule than the polypyrrole layer. Finally, the chains of the PAA were embedded into the 2-MBT layers. Increase in the size with the increasing layer thickness indicates the complete formation of the nanocontainer assembly. The average size of the nanocontainer was found to be about 1296 nm as indicated by PSD analysis. (35,36) The zela potential of the initial CNTs nanoparticles is negative as shown in Fig. 7. Electrophoretic measurements indicate the charging of the nanoparticles coated with the adsorbed polyelectrolyte or inhibitor layers upon the addition of each layer. Figure 6 shows a drastic increase of the surface charge after deposition of the first conductive layer due to 2-MBT deposition followed by a similar decrease after acrylic adsorption during the next stage. The increase in the value of zeta potential, and hence the enhanced charging of the surface of layer by 2-MBT, indicates the deep penetration of 2-MBT into the PPy layer. (37)
[FIGURE 4 OMITTED]
Release study of corrosion inhibitor at different pH values
In order to evaluate the responsive release of the inhibitor from nanocontainers, the release performance was studied in aqueous media at pH values of 3, 7, and 9 which was adjusted by adding either dilute acid (HC1) or alkali solution (NaOH). The objective of the investigation was to find out the optimal pH conditions at which the release reaches to maxima. The release concentrations of the corrosion inhibitor at different pH values were measured using UV-Vis spectrophotometer. The release study was carried out over a period of 5 h. Figure 8 shows the release concentration of corrosion inhibitor as per unit mass of nanocontainer at different pH values. It is observed that the maximum release concentration of 2-MBT at the end of 5 h is 0.35 mg [L.sup.-1] [g.sup.-1] of nanocontainer, which is found in the case of solution having pH value of 3. However, in case of solutions having pH values of 5 and 7, the release concentration is found to be 0.23 and 0.16 mg [L.sup.-1] [g.sup.-1] of nanocontainer, respectively. It has been observed that the greater the amount of inhibitor is released in acidic pH compared with neutral and basic pH, which supports the observations reporting that the 2-MBT acts as an inhibitor for ferrous metals under acidic conditions (38) as well as under neutral conditions. (39) Figure 8 shows at any operating pH value, a stable release is observed after 5 h, which indicates that the prepared nanocontainer shows proper encapsulation of the 2-MBT into the polyelectrolyte layer. A greater amount of inhibitor released provides better protection to the ferrous metal by adsorption in its molecular or protonated form leading to the formation of compact passive layer, which results in the enhancement in the inhibition of ferrous metal.
Water absorption test
Figure 9 shows the dependence of water uptake values of the coatings containing pigment, i.e., nanocontainers on immersion time in water. This test was performed according to ASTM D2842. The maximum uptake values of epoxy and nanocontainer/epoxy coatings are shown in Fig. 9. The water uptake of nanocontainer/epoxy coatings increased with the immersion time during the whole immersion period, and it reached the highest value after immersion for 350 h. The water uptake value of epoxy coatings is the lowest among the two species. The water uptake value of nanocontainer coatings is higher than that of epoxy. The presence of nanocontainer in epoxy coating can enhance the density of the coating; reduce the transport paths through which the corrosive electrolyte solution; and make the oxygen penetrate. Therefore, the presence of nanocontainer in epoxy coating is beneficial to enhance its anticorrosion property. (40)
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
Salt spray test
A salt spray test was conducted to check corrosion resistance of coated samples. The neutral salt spray test was used to qualitatively determine the corrosion resistance performance of coated samples. This test was performed according to ASTM B117. Optimized coating systems for steel all performed well over 1000 h in the test, which is usually a typical requirement in the coating industry. Figures 10 and 11 show images of coated steel samples before and after exposure times, respectively, in a salt spray chamber. It is clearly seen that the coating which carries nanocontainers furnishes better result compared with only epoxy-based system. It is understandable that samples show better corrosion resistance than bare metal. It can also be concluded from Fig. 11 that the anticorrosive ability of coating which holds nanocontainers together can be enhanced by combining the effects of CNTs and inhibitor viz 2-MBT. The presence of nanocontainers increases the thickness of the coatings. The trapped inhibitor inside the active matrix is often subjected to spontaneous leakage from the surface during aging. The released inhibitor forms a thin adsorption layer on the damaged metallic surface to sufficiently hinder the anodic and cathodic corrosion processes and passivate the alloy by replacing the damaged area. Thus, the LbL-assembled nanoreservoirs incorporated into the hybrid matrix release the inhibitor on demand to heal the defects in the coating and provide active corrosion protection with direct feedback. (20) Therefore, the nanocontainer plays an important role in the anticorrosive coating and provides adhesion formed throughout the service life of the coating.
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
DC polarization measurement
The corrosion resistance of stainless steel is attributed to the formation of a passive film, which protects the material from nonstop corrosion attack. However, when subjected to a combined corrosion and erosion attack, the passive film can be damaged by solid-particle impingement resulting in the exposure of a bare metal surface to the corrosive medium and thus increasing the corrosion/erosion rate. Approaches for resisting the synergistic attack of erosion and corrosion include alloying elements and surface modification to self-healing characteristics. (18) The corrosion protection properties of the coated samples were estimated using DC polarization technique. Initial DC polarization measurements were performed using 0.05 M NaCl as an electrolyte. The coated panels for each system were immersed in 0.05 M NaCl solution for 125 h and were again evaluated by means of potentiostat. Anodic and cathodic reactions occurring at open circuit may be obtained from the linear regions of the polarization curve, as illustrated in Figs. 12 and 13. From the obtained curves, it can be seen that the epoxy coating has lower electrochemical potential values (corrosion potential ([E.sub.corr])) compared with nanocontainer-based epoxy coatings (Fig. 12). These trends can be explained by the presence of interactions between nanoparticle and inhibitor. These interactions lead to the formation of longer conjugation lengths and thus suppress the rate of electrochemical reaction compared with epoxy coatings resulting in passivation and noble potential thereby providing corrosion protection. (41) The main function of a nanocontainer is that when introduced in a coating matrix, it distributes itself in the passive matrix keeping the inhibitors in a trapped state. This avoids the leaching of the inhibitors into the atmosphere and also avoids the unwanted interaction between the inhibitor and the passive matrix. Nanocontainers release the inhibitor when the passive matrix is exposed to a corrosive agents' attack. The potential values were drastically lowered after 125 h of immersion in salt solution (Fig. 13). At this time also, coatings with inhibitor exhibited more positive potential suggesting better corrosion protection than without any inhibitor.
[FIGURE 10 OMITTED]
[FIGURE 11 OMITTED]
[FIGURE 12 OMITTED]
[FIGURE 13 OMITTED]
The nanocontainers were synthesized successfully with an eco-friendly strategy by using water as a reaction medium. The nanocontainers were confirmed by particle size distribution and morphological study. Experimental substantiation clearly reveals that the nanocontainers/epoxy-based system gives better anticorrosion results than the epoxy-alone system due to the interaction between nanocontainers and the corrosive environment. Combining the nanoparticles (i.e., CNTs and inhibitor) can achieve an enhanced corrosion protection coating for MS as per electrochemical measurements and salt spray test. The enhancement of the barrier properties of the coating are probably due to blocking of the pores from the released inhibitor from the nanocontainer.
K. V. Yeole, S. T. Mhaske ([mail])
Department of Polymer and Surface Engineering, Institute of Chemical Technology, Mumbai 400019, India
K. V. Yeole
I. P. Agarwal
Department of Chemical Technology, National Institute of Technology, Warangal 506004, India
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|Author:||Yeole, K.V.; Agarwal, I.P.; Mhaske, S.T.|
|Publication:||Journal of Coatings Technology and Research|
|Date:||Jan 1, 2016|
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