Article pubs.acs.org/IECR
Halloysite Nanotube as Multifunctional Component in Epoxy Protective Coating Poornima Vijayan P,*,† Yara Mohamed Hany El-Gawady,† and Mariam Ali S. A. Al-Maadeed†,‡ †
Center for Advanced Materials, Qatar University, P.O. Box 2713, Doha, Qatar Materials Science and Technology Program, Qatar University, P.O. Box 2713, Doha, Qatar
‡
S Supporting Information *
ABSTRACT: The current research explores the use of halloysite nanotube as a multifunctional filler in epoxy coating for carbon steel. Epoxy monomer loaded halloysite was incorporated into epoxy coating along with amine hardener immobilized mesoporous silica. The waterproofing, self-healing, anticorrosive abilities, and stability under weathering of the coating were evaluated. The halloysite nanotubes are able to impart better waterproofing property to the coating. The released epoxy monomer encapsulated inside the halloysite cavity upon reaction with amine curing agent immobilized in mesoporous silica recovers the damage and thereby facilitates self-healing in epoxy coating. Apart from offering healing ability to the coating, the halloysite nanotubes are able to protect the coatings for a longer period from severe weathering conditions.
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INTRODUCTION The use of halloysite nanotube clay mineral as reservoir for active materials in the development of drug delivery systems and self-healing materials is getting attraction due to their loading and control releasing ability.1,2 Halloysite nanoclay is chemically similar to kaoline clay having multilayer tubular structure with outer diameter of 50−70 nm and inner diameter of 10−30 nm.2,3 Self-healing coating is a facile and economic way for the protection of metals from cracks and damages even at a microscopic level to avoid further corrosion4−6 and thereby reduce the large economic losses to the automotive, aerospace, and oil−gas industries. There are various strategies adopted by researchers for the safe and efficient encapsulation of healing agents for anticorrosive self-healing coatings and self-healing composites structures.6,7 Micropolymer/nanopolymer capsules,8−11 hollow glass fibers/bubbles,12,13 inorganic nanoporous/ nanotube materials, 14,15 core−shell microfibers/nanofibers,16−19 etc. are reported in the literature as containers to load either the polymer healing agents or corrosion inhibitors. These encapsulation methods impose tedious and costly preparation of containers. In this aspect, the halloysite nanotube, an easily available, low cost, naturally occurring clay mineral, offers potential commercialization of self-healing anticorrosive coatings for metal substrate. The efficiency of halloysite nanotubes as containers for healing agents/corrosion inhibitors is recently studied by different researchers to develop self-healing protective coatings. The sol−gel coating containing corrosion inhibitor loaded halloysite nanotube on aluminum substrate was developed so as © XXXX American Chemical Society
to release the inhibitor from the inner tube cavity within 1 h at corrosion sites.20 Further, there is an opportunity to cover the outer surface of the halloysite nanotube with polyelectrolyte multilayers to respond and release the corrosion inhibitor from inner tube upon pH changes associated with the corrosion.21 Later, various corrosion inhibitors loaded halloysite nanotubes embedded inside the polyurethane and acrylic paints on copper were used to release the corrosion inhibitor upon crack formation to further protect the metal from corrosion.22 Different from these efforts, the current study aims to encapsulate the polymer self-healing agent (epoxy monomer) in the halloysite nanotubes for the first time. The width of epoxy polymer is around 0.5 nm,23 which is much lower than the halloysite inner tube diameter, making it possible for the easy penetration of epoxy into the tube cavity. The corresponding cross-linking agent (amine curing agent) for the epoxy monomer is immobilized in mesoporous silica. The loaded halloysite is incorporated into epoxy coating along with the immobilized mesoporous silica on the carbon steel substrate. The wettability of the coatings is evaluated using water contact angle measurements. The self-healing and anticorrosion ability of these two components coating system are evaluated using EIS and accelerated salt immersion test. The investigation of the stability of such self-healing coating and their individual components under weathering conditions Received: July 18, 2016 Revised: September 29, 2016 Accepted: October 10, 2016
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DOI: 10.1021/acs.iecr.6b02736 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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solution was subjected to repeated vacuum using vacuum jar for a time period of 6 h to complete the insertion of epoxy inside the clay nanotube. Preparation of Protective Coating on Carbon Steel. The epoxy encapsulated Epon 815C/15 phr HNT solution was diluted with Epon 826 to make epoxy/7.5 phr HNT solution. This homogeneous solution was then mixed with 0.8 phr curing agent immobilized mesoporous silica. 11 phr curing agent (11 g for 100 g of epoxy 826) was added to the mixture and was degassed by sonicating for 10 min. The mixtures were then coated on one side of the carbon steel to obtain a coating of average thickness of 300 μm. The coating was kept aside for curing for 24 h. The coating is named as EP/7.5 HNT coating. Similarly, a higher concentration of loaded HNT incorporated epoxy coating also prepared (named as EP/15 HNT). For making control epoxy coatings, epoxy 815C and epoxy 826 were mixed separately with hardener in the ratio 100:10.9 using magnetic stirrer for a duration of 10 min. The mixtures were coated on one side of the carbon steel to obtain a coating of average thickness of 300 μm. The coating was kept aside for curing for 24 h. Characterization Techniques. The halloysite clay and the synthesized mesoporous silica were characterized with Nova NanoSEM field emission scanning electron microscopy and FEI TECNAI GF20 S-TWIN transmission electron microscopy. Benchtop scanning electron microscopy (JCM 6000, Jeol) was used to observe the weathering exposed coating surface. Wettability of coatings were evaluated using water contact angle measurements. The contact angle measurements were carried out in a Kruss G40 contact angle goniometer at room temperature following the sessile drop principle. Surface roughness of the coating samples was evaluated using profilometer (Leica DCM8 system). Electrochemical impedance spectroscopic (EIS) studies of the coatings were carried out using Gamry reference 600 potentiostat/galvanostat/ZRA. The reference electrode used was Ag/AgCl electrode and a stainless steel counter electrode arranged parallel to the exposed sample to complete the cell. A 3.5 wt % NaCl aqueous solution was used as the electrolyte solution inside the cell. The sample was in touch with the electrolyte solution. The measurements were carried out at ambient temperature. EIS measurements were carried out at in the frequency range 0.01−100 000 Hz. Corrosion resistance of the coatings was also tested by salt immersion test. The coatings were immersed in 10 wt % salt solutions, and the corrosion was observed at different time intervals. Accelerated weathering test was done by using QUV chamber (QUV-Spray model) by following ASTM G154. The cycle consists of 8 h of UV (λ = 340 nm) radiation at 60 °C and 4 h of condensation at 50 °C. The irradiation intensity was 0.89 W m−2. The coating panels were exposed for different time interval up to 45 days.
helps to predict the service life of the prepared coating. In this work, the long-term stability of the prepared coatings under accelerated weathering exposure is also evaluated.
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EXPERIMENTAL SECTION Materials. The halloysite (HNT) clay mineral was supplied by Sigma-Aldrich. The SEM and TEM images of the halloysite nanotubes are shown in Figure 1. The measured average outer
Figure 1. (a) SEM and (b) TEM images of halloysite nanotube.
tube diameter is ∼75 nm, and average inner tube diameter is ∼22 nm. Poly(ethylene glycol)-block-poly(propylene glycol)block-poly(ethylene glycol) (EO20PO70EO20) (PEG−PPG− PEG, PluronicR P-123) with an average Mn of 5800 supplied by Sigma-Aldrich was used as template for the synthesis of mesoporous silica. Tetraethyl orthosilicate (TEOS) (reagent grade 98%) from Sigma-Aldrich was used as silica source. Epon 826, Epon 815C, and amine curing agent (Epikure 3223) were purchased from Miller-Stephenson Chemical Co., USA. Epon 815C is a diluted (with butyl glycidyl ether) form of Epon 826. Polished carbon steels were used as substrate for coating. Methods. Synthesis of Mesoporous Silica (SBA-15) and Immobilization of Curing Agent in Mesoporous Silica. SBA15 mesoporous silica was synthesized using the amphiphilic triblock copolymer poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (EO20PO70EO20) as described in ref 24. The TEM images of the synthesized mesoporous silica is given are Figure 2. The calculated average
Figure 2. TEM image of (a) mesoporous SBA-15 single particle and (b) details of (a).
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RESULTS AND DISCUSSION A qualitative estimation of the encapsulation of epoxy prepolymer in hallyosite nanotubes was studied by SEM. The presence of epoxy prepolymer inside the nanotube was observed by SEM analysis of the fractured surface of the coating. The SEM image (Figure 3) indicates the accumulation of mass at the halloysite tube ends and shows the release of epoxy prepolymer from the nanotube core upon fracture.
pore diameter of the cylindrical pore is 4 nm (Figure 2b). Thus, synthesized mesoporous silica was incubated with excess of Epikure 3223 for 24 h at 25 °C using medium shaking rate in order to immobilize the amine molecules in pores of SBA-15. Encapsulation of Epoxy Prepolymer into HNT. Epon 815C/15 phr HNT solution was prepared by mixing in a magnetic stirrer for 30 min at 1000 rpm. The homogeneous B
DOI: 10.1021/acs.iecr.6b02736 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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a hydrophilic additive into a matrix can increase the water contact angle due to modified surface composition.26−28 In order to understand the surface and bulk morphology of the coating, the SEM images of the surface and fracture surface of the coating were investigated. The SEM images are given in Figure 5. SEM image of the surface of the coating is not
Figure 5. SEM images of the (a) surface and (b) cross section of epoxy/7.5 HNT coating.
smooth, which indicates the presence of HNT on the surface. This further indicates the high surface roughness of epoxy/7.5 HNT composite coating when compared to control epoxy coating. At the same time the SEM images of fractured surface showed that HNT is also buried inside the composite coating. The presence of mesoporous silica can also increase the water contact angle value. Since the amount of mesoporous silica (0.8 phr) is comparatively small in EP/7.5 HNT coating the effect is mainly believed to be from the HNT. Electrochemical impedance spectroscopy (EIS) is commonly used to evaluate the corrosion performance of organic coatings.29 As an extension, this technique is used to follow active self-healing anticorrosive process in coatings.30,31 The increase of low frequency impedance of scratched coating during immersion in NaCl solution is related to the suppression of active corrosion processes and healing of the failed areas.32,33 Hence, EIS technique can effectively be employed to study the role of epoxy monomer loaded HNT on self-healing performance of the coating. The electrochemical impedance spectroscopic evaluation of both unscratched and scratched coatings was done. The impedance value at low frequency can be used to compare corrosion protection performance of coatings at different immersion times for different coatings. The impedance values at 0.01 Hz for both unscratched and scratched coatings are
Figure 3. SEM images of fracture surface of epoxy/7.5 HNT coating. The dotted red circle shows the accumulated mass at the halloysite nanotube ends.
Moreover, most of the halloysite nanotube ends were closed due to the presence of epoxy prepolymer inside. It is important to evaluate the waterproofing property of a protective coating. Hence, the wettability of the coatings was investigated using contact angle measurements. The water contact angles for control epoxy coating and EP/7.5 HNT coatings are 32.47° and 65.69°, respectively (Figure 4). Halloysite nanotubes are highly hydrophilic, with a water contact angle of 10°.25 However, the presence of halloysite nanotubes in epoxy reduces the hydrophilicity of the EP/7.5 HNT nanocomposite coating. Apart from the nature of the components in the coating, the contact angle highly depends on the surface morphology. As shown in surface profile of the coatings (Figure 4), EP/7.5 HNT coating has higher surface roughness than the control epoxy coating. The calculated surface roughness of control epoxy coating is 11.40 μm, and for EP/7.5 HNT coating it is 20.19 μm. The increased surface roughness upon the incorporation of HNT (epoxy loaded HNT) is responsible for the reduction of hydrophilicity and hence improves the waterproofing properties. Previously, it was reported that for biopolymer/HNT composites, the presence of
Figure 4. Surface profile of (a) control epoxy and (b) EP/7.5 HNT coatings. Inset shows water droplet on respective coatings. C
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reaction of monomer released from HNT with the amine immobilized in mesoporous silica. The release of epoxy monomer from the halloysite lumen into the scratch and its further cross-linking reaction with the immobilized amine curing agent filled the scratch with chemically unique epoxy thermoset material. Even though the recovery is not up to mark, an increase in amount of loaded HNT in the coating would generate a self-healing epoxy coating with more efficiency. Due to this reason, EIS analysis of protective coating with a higher amount of loaded HNT (15 phr HNT) was carried out (Figure 6c and Figure 6f). Here, different from EP/ 7.5 HNT coating, the retrieval of impedance value of EP/15 HNT occurs after 48 h of immersion and it successively increased up to 336 h and further on it remains same. The increment in impedance value at 0.01 Hz is calculated as 128.07%. Higher amount of loaded HNT facilitates more recovery of the scratch, and the recovery process lasts for longer immersion time period in epoxy/15HNT coating when compared with epoxy/7.5 HNT coating. This is because in epoxy/15 HNT coating, more epoxy monomer leaked into the scratch to react with the amine curing agent, resulting in better filling of the scratch with epoxy thermoset. In addition to EIS analysis, salt immersion test was carried out in order to estimate the corrosion protection efficiency and self-healing activity of the coatings. Figure 7 shows the photographs of scratched control epoxy coating and EP/7.5 HNT coating after immersing the coating in 10 wt % NaCl solution at different immersion times. From the second day itself, the rusting of coating especially on the edges of the scratch was evident in the control epoxy coating. On the fourth day of immersion, the coating is observed to be subject to a severe corrosion. However, the EP/7.5 HNT coatings are observed to be intact upon salt immersion. On the fourth day of immersion, even though the texture of the coating becomes faded, the coating is still not attacked by rust. Here, the epoxy monomer loaded HNT proved to enhance the corrosion resistance and impart self-healing activity to the coating. Depending on the application area, coatings are exposed to different environments, which affects negatively their physical or mechanical properties and chemical composition by the formation of new functional moieties or fragmentation of the cross-linked macromolecules. UV radiation, water, and O2 are the three most critical factors for coating’s degradation under weathering exposure.36 On the basis of coating degradation theory, various accelerated weathering tests were developed. Among them, QUV chamber is a commonly used accelerated weathering test method in industry and academia to test the weathering durability of coatings.37 Figure 8 shows the photographs of the coating samples with and without exposure to accelerated weathering conditions. After 45 days of exposure, the control epoxy coating samples completely failed to protect the metal. Compared to the control coating, the EP/7.5 HNT coatings gave better protection to the metal substrate. Table S1 shows the SEM images of the coating surface before and after weathering exposure. The surface of exposed control epoxy coating shows severe void formation, and the voids were enlarged upon increasing exposure time. Also the degradation on the control epoxy coating surface is not uniform. In addition to the voids, visible cracks are formed on the surface of control epoxy coating (especially after 30 days of exposure). As the exposure day increases, more crack are developed on the surface. Thus, formed crack allows the penetration of water through the polymer coating into the metal substrate. Thus, the
given in Table 1. The impedance at 0.01 Hz of unscratched coatings is found to increase with the increase in amount of Table 1. Low Frequency Impedance Values and Coating Resistance for Scratched and Unscratched Control Epoxy Coatings coating EP without scratch EP with scratch
EP/7.5 HNT without scratch EP/7.5 HNT with scratch
EP/15 HNT without scratch EP/15 HNT with scratch
time (h) 0 24 48 72 96 0 24 48 72 96 0 24 48 96 168 192 240 264 336 432
Z (Ω cm2) at 0.01 Hz 1.101 × 108 61180 18400 18990 18300 18610 1.494 × 108 380600 18700 28360 39990 41160 3.411 × 109 270000 86400 10400 28900 96600 105000 158000 223000 335000 345800
halloysite loading in the epoxy coating. In general, the enhancement in corrosion protection of epoxy coating upon the addition of nanofillers is due to the improved barrier performance and better adherence of the coating to the underlying substrate.34 In the case of halloysite filled epoxy coating, the halloysite acts as good barrier by reducing the porosity of the epoxy coating matrix and zigzag the diffusion path available by deleterious species.35 As the amount of halloysite increases in the coating, the barrier effect increases to further enhance the corrosion protection ability of the coating. Figure 6a and Figure 6d shows EIS bode plot and Nyquist plots for control epoxy coating with scratch at regular intervals of immersion time for 96 h. The impedance value at 0 h dropped to its minimum value in successive immersion periods (24−96 h). Figure 6b and Figure 6e show EIS bode plot and Nyquist plots for EP/7.5 HNT coating with scratch at regular intervals of immersion time. The fully declined impedance curve for scratched EP/7.5 HNT coating after 24 h of immersion starts to increase upon further immersion intervals (48−96 h). Since the impedance curves does not show significant change on further immersion time after 96 h for control epoxy coating and EP/7.5 phr HNT coating, to avoid the overlapping of curves, the results after 96 h are not shown here. For EP/7.5 HNT coating with scratch, it is found that the dropped low frequency impedance value starts to increase from 48 h of immersion time onward. The increment in impedance value at 0.01 Hz is calculated as 10.81%. This behavior gives confirmation of the recovery of the anticorrosive property of the scratched coating during immersion time. Here, the healing phenomenon arises from the recovery of the scratch by the D
DOI: 10.1021/acs.iecr.6b02736 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 6. EIS Bode diagrams for (a) control epoxy coating, (b) EP/7.5 phr HNT coatings, and (c) EP/15 phr HNT in 3.5 wt % NaCl solution at different immersion times. Nyquist plots for (d) scratched control epoxy coating, (e) scratched EP/7.5 phr HNT coating, and (f) scratched EP/15 phr HNT coating at different immersion times.
Figure 8. Photographs of coatings before and after exposure of accelerated weathering test (sample size, length × width = 7 cm × 4 cm). Figure 7. Immersed coatings in 10 wt % NaCl solution: control epoxy coating on (a) first day, (b) second day, and (c) fourth day; EP/7.5 HNT coating on (d) first day, (e) second day, and (f) fourth day (sample size, length × width = 7 cm × 4 cm).
protective properties of the control coating are completely degraded after exposure to UV and moisture. In the nonexposed EP/7.5 HNT sample, either HNT or mesoporous silica is deeply embedded in resin coating. The exposed surface E
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(5) Fayyad, E. M.; AlMaadeed, M. A.; Jones, A.; Abdullah, A. M. Evaluation techniques for the corrosion resistance of self-healing coatings. Int. J. Electrochem. Sci. 2014, 9, 4989. (6) Vijayan, P.; AlMaadeed, M. A. "Containers" for self-healing epoxy composites and coating: Trends and advances. eXPRESS Polym. Lett. 2016, 10, 506. (7) Zheludkevich, M. L.; Hughes, A. E. Delivery systems for selfhealing protective coatings. In Active Protective Coatings: NewGeneration Coatings for Metals; Hughes, A. E., Mol, J. M.C, Zheludkevich, M. L., Buchheit, R. G., Eds.; Springer: Dordrecht, The Netherlands, 2016; pp 157−199. (8) Blaiszik, B. J.; Caruso, M. M.; McIlroy, D. A.; Moore, J. S.; White, S. R.; Sottos, N. R. Microcapsules filled with reactive solutions for selfhealing materials. Polymer 2009, 50, 990. (9) Zhao, Y.; Zhang, W.; Liao, L.; Wang, S.; Li, W. Self-healing coatings containing microcapsule. Appl. Surf. Sci. 2012, 258, 1915. (10) Blaiszik, B. J.; Sottos, N. R.; White, S. R. Nanocapsules for selfhealing materials. Compos. Sci. Technol. 2008, 68, 978. (11) Zhao, Y.; Fickert, J.; Landfester, K.; Crespy, D. Encapsulation of self-healing agents in polymer nanocapsules. Small 2012, 8, 2954. (12) Trask, R. S.; Bond, I. P. Biomimetic self-healing of advanced composite structures using hollow glass fibres. Smart Mater. Struct. 2006, 15, 704. (13) Zhang, H.; Yang, J. Etched glass bubbles as robust microcontainers for self-healing materials. J. Mater. Chem. A 2013, 1, 12715. (14) Skorb, E. V.; Fix, D.; Andreeva, D. V.; Möhwald, H.; Shchukin, D. G. Surface-modified mesoporous SiO2 containers for corrosion protection. Adv. Funct. Mater. 2009, 19, 2373. (15) Arunchandran, C.; Ramya, S.; George, R. P.; Kamachi Mudali, U. Self-healing corrosion resistive coatings based on inhibitor loaded TiO2 nanocontainers. J. Electrochem. Soc. 2012, 159, C552. (16) Sinha-Ray, S.; Pelot, D. D.; Zhou, Z. P.; Rahman, A.; Wu, X.-F.; Yarin, A. L. Encapsulation of self-healing materials by coelectrospinning, emulsion electrospinning, solution blowing and intercalation. J. Mater. Chem. 2012, 22, 9138. (17) Wu, X.-F.; Rahman, A.; Zhou, Z.; Pelot, D. D.; Sinha-Ray, S.; Chen, B.; Payne, S.; Yarin, A. L. Electrospinning core-shell nanofibers for interfacial toughening and self-healing of carbon-fiber/epoxy composites. J. Appl. Polym. Sci. 2013, 129, 1383. (18) Lee, M. W.; Yoon, S. S.; Yarin, A. L. Solution-blown core−shell self-healing nano- and microfibers. ACS Appl. Mater. Interfaces 2016, 8, 4955. (19) Vahedi, V.; Pasbakhsh, P.; Piao, C. S.; Seng, C. E. A facile method for preparation of self-healing epoxy composites: using electrospun nanofibers as microchannels. J. Mater. Chem. A 2015, 3, 16005. (20) Fix, D.; Andreeva, D. V.; Lvov, Y. M.; Shchukin, D. G.; Möhwald, H. Application of inhibitor-loaded halloysite nanotubes in active anti-corrosive coatings. Adv. Funct. Mater. 2009, 19, 1720. (21) Shchukin, D. G.; Lamaka, S. V.; Yasakau, K. A.; Zheludkevich, M. L.; Ferreira, M. G. S.; Möhwald, H. Active anticorrosion coatings with halloysite nanocontainers. J. Phys. Chem. C 2008, 112, 958. (22) Abdullayev, E.; Abbasov, V.; Tursunbayeva, A.; Portnov, V.; Ibrahimov, H.; Mukhtarova, G.; Lvov, Y. Self-healing coatings based on halloysite clay polymer composites for protection of copper alloys. ACS Appl. Mater. Interfaces 2013, 5, 4464. (23) Suzuki, N.; Kiba, S.; Yamauchi, Y. Fabrication of mesoporous silica/polymer composites through solvent evaporation process and investigation of their excellent low thermal expansion property. Phys. Chem. Chem. Phys. 2011, 13, 4957. (24) Thielemann, J. P.; Girgsdies, F.; Schlogl, R.; Hess, C. Pore structure and surface area of silica SBA-15: influence of washing and scale-up. Beilstein J. Nanotechnol. 2011, 2, 110. (25) Abdullayev, E.; Price, R.; Shchukin, D.; Lvov, Y. Halloysite tubes as nanocontainers for anticorrosion coating with benzotriazole. ACS Appl. Mater. Interfaces 2009, 1, 1437. (26) Cavallaro, G.; Donato, D. I.; Lazzara, G.; Milioto, S. Films of halloysite nanotubes Sandwiched between two layers of biopolymer:
of EP/7.5 HNT is less prone to void formation. Epoxy monomer loaded halloysite nanotubes help to keep the bulk coating intact for longer time. The embedded HNT protects the epoxy coating from crack formation. Also, the degradation on EP/HNT coating surface is uniform. Degraded products deposited on the surface were evident in SEM images of EP/7.5 HNT coating after 30 and 45 days of exposure. In the case of EP/7.5 HNT coating, the UV irradiation may facilitate the curing reaction of epoxy monomer (UV curing) released during degradation of coating to further protect the coating. Thus, the ability of HNT to give a long-term sustainability for the coating was also confirmed from the weathering results.
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CONCLUSIONS Halloysite nanotubes were loaded with epoxy monomer and were incorporated along with amine immobilized mesoporous silica into epoxy coating for carbon steel. The epoxy protective coating thus prepared was evaluated for their performance. It was found that the presence of loaded HNT made the coating more waterproof when compared to the control coating. The EIS analysis gave evidence for the self-healing ability of the coating preferably at higher loading (15 phr) of epoxy monomer loaded HNT. The salt immersion test further confirmed the self-healing and anticorrosive ability of the coating owing to the presence of epoxy monomer loaded HNT. The QUV weathering studies confirmed the long-term stability offered by epoxy/HNT coating for outdoor applications. The current work successfully sketched the use of HNT as a multifunctional filler in epoxy coating.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b02736. SEM images of surfaces of control epoxy coating and EP/7.5 HNT coating at different weathering exposure periods (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +97477510838. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This manuscript was made possible by PDRA Grant PDRA11216-13014 from the Qatar National Research Fund (a member of Qatar Foundation). The findings achieved herein are solely the responsibility of the authors.
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REFERENCES
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