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Keywords:

  • polymeric nanocontainers;
  • pH-responsive;
  • self-healing;
  • organic coatings;
  • corrosion protection

Self-healing, a process of automatic recovery, attracts tremendous attention because of its applications in biological system, microelectronics, and aerospace engineering.[1] Nature provides many self-healing patterns.[2] Bio-inspired materials have been designed to mimic structure–function relationship in nature to endow self-healing ability for biomedical applications.[3] The concept of self-healing has also been applied to new material networks based on reversible bond reconstruction[4] or supramolecular interaction.[5] When external energy such as light, electricity or electromagnetic wave is applied, the network can be self-adaptive to reform materials on the cracks or broken areas. Microencapsulated and microvascular systems have been pioneered by the White group for self-healing coatings based on the auto-release of healing materials from microcapsules broken near the crack surfaces.[6] Polyelectrolyte multilayer (PEM) films have been fabricated with the capability of restoring superhydrophobicity and electrical conductivity.[7]

Metal corrosion destroys materials causing a great economic loss (>3% the world's GDP).[8] Industrial coatings rely mainly on passive materials for protection, but passive coatings have a limited lifetime. Therefore, introduction of an active process like the self-healing mechanism is highly demanded.[9] Chromate conversion coatings were once widely utilized to form a self-adaptive barrier to inhibit corrosive agents accessing metal surfaces.[10] However, the high toxicity and carcinogenic properties of Cr (VI) have great impact on the environment and healthcare. Thus many efforts have been made towards the development of new coatings which are environmentally friendly and protect metal materials efficiently.[11] Silica nanocontainers in inorganic coatings have been fabricated for long-term corrosion protection.[12] The release of corrosion inhibitor encapsulated in nanocontainers could be realized by external stimuli to suppress corrosion expansion. Different stimuli-responsive polymeric nanoparticles have been reported for drug delivery systems, tissue engineering, biosensors, as well as electromechanical systems in the past ten years.[13] However, there are only few reports on polymeric nanoparticles for organic coatings in corrosion protection.

The design of nanoparticles with well-defined morphology and surface properties is of great interest and importance for composite coatings.[14] For instance, compatibility of nanocontainers with coating matrix will directly affect the stability and practical performance of coatings. Herein, we report facile design and synthesis of pH-responsive polystyrene core–shell nanocontainers loaded with corrosion inhibitor for organic self-healing coatings. The as-prepared polymeric core–shell nanocontainers possess a variety of advantages such as low cost and narrow size distribution for the regulation of corrosion inhibitor release. In contrast to silica nanocontainers, polymeric macrocapsules and halloysite nanotubes, the polystyrene core–shell nanoparticles in the present work show a high dispersibility in aqueous solution for water-borne processing and excellent compatibility with organic coating matrix. Due to a good compatibility of polymeric nanocontainers with organic coatings, epoxy composite coatings can not only serve as a passive barrier, but are also capable of healing as an active barrier on metal surfaces. The self-healing mechanism for corrosion protection is a totally automatic healing process without any requirement of external energy input.

The synthesis of polymeric core–shell nanocontainers is sketched in Scheme 1. Corrosion inhibitor (benzotriazole, BTA) is added into reaction systems prior to initiation of polymerization, and then BTA molecules are incorporated into the polystyrene (PS) matrix during the polymerization process. The highly branched polyethylenimine (PEI) chains are adsorbed onto the PS surfaces because of the electrostatic interaction between PSSNa on particle surfaces and PEI chains. The PEI surface coating on the core–shell container surfaces helps to minimize the spontaneous leakage of BTA from the containers and also provides a pH-responsive polymeric shell for controlled release of corrosion inhibitor. Figure 1a,b shows the TEM images of the PS-BTA/PEI core–shell nanocontainers with a BTA feed content of 15 wt% (Table S1). A distinctive core–shell structure has been observed, which consists of a BTA loaded hydrophobic PS domain as a core and a PEI/PSSNa hydrophilic layer as a shell. The SEM image (Supporting Information, Figure S1) also suggests that the PS-BTA/PEI nanocontainers have a well-defined morphology. A photo of the as-prepared PS-BTA/PEI core–shell nanocontainers in 200 mL aqueous solution (0.06 g/mL) is shown in Figure 1c. The weight yield of polymeric containers is in the range of 70–80%, which indicates that the synthesis of PS-BTA/PEI core–shell nanocontainers is highly efficient.

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Scheme 1. Illustration of the synthesis of corrosion inhibitor-loaded PS-BTA/PEI core–shell polymeric nanocontainers. Free inhibitor benzotriazole (BTA) is encapsulated in the nanocontainers during the polymerization process, followed by a polyethylenimine (PEI) layer adsorption to avoid spontaneous leakage of BTA and to respond to external pH stimuli leading to a controlled release as well.

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image

Figure 1. A,B) TEM image of the PS-BTA/PEI core–shell nanocontainers; C) A picture of as-prepared PS-BTA/PEI core–shell nanocontainers in 200 mL of aqueous solution (0.06 g/mL); D) Hydrodynamic diameter and PDI measurements of as-obtained polymeric nanocontainers versus inhibitor content; E) Curve of the benzotriazole content in the polymeric nanocontainer versus benzotriazole feed content. The scale bar in (A,B) is 100 and 50 nm, respectively.

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Dynamic light scattering (DLS) and gel permeation chromatography (GPC) were further carried out to characterize the as-prepared polymeric nanocontainers (Supporting Information, Tables S1 and S2). The effect of BTA feed contents on the hydrodynamic diameter (Dh) and size distribution is shown in Figure 1d. The hydrodynamic diameter of the polymeric particles increases from 74 to 101 nm as the feed content of BTA molecules is increased from 0 to 20 wt%. The polystyrene core–shell containers have polydispersity index in the range of 1.004 and 1.019. The narrow size distribution is very important for uniform particle systems to regulate controlled release dynamics of active molecules from nanocontainers.[15] The molecular weight (Mw) and PDI of PS chains are lowered as the BTA feed content increases from 0 to 20 wt%. It is probably due to an earlier phase separation of polymers to form particles in the presence of higher amount of BTA molecules. The relationship between the BTA content encapsulated in the polymeric nanocontainers and initial feed content is shown in Figure 1E. The amount of BTA molecules encapsulated in the polymeric nanocontainers is lower than the initial feed content. The zeta potential (ζ) of the PS-BTA and PS-BTA/PEI core–shell nanocontainers with −41 and +32 mV, respectively, indicates that the polymeric nanocontainers are highly stable in aqueous solution.

We further studied the controlled release of the BTA molecules from the polymeric nanocontainers. The release dynamics of BTA from the PS-BTA/PEI core–shell nanocontainers at different pH is shown in Figure 2A. There are three stages (stage I: induction period; stage II: sustained release; stage III: sustained release to level off). For the nanocontainers at pH 10, the release rate is faster than that at pH 7 and pH 4. From the equation of microscopic theory (D = <X>2/2t), diffusion coefficient (D) is inversely proportional to time (t) if we assume that the diffusion path (shell thickness, X) is the same. From Figure 2A, the times t1 (pH 4) = 50 h, t2 (pH 7) = 53 h and t3 (pH 10) = 18 h are obtained after which half of the BTA molecules are released out of the polymeric nanocontainers. The thickness of the polymeric shell (Figure 1B) was measured to be around 6 nm. Accordingly, the calculated diffusion coefficients D1 at pH 4, D2 at pH 7 and D3 at pH 10 are 9.9 × 10−19 cm2/s, 9.5 × 10−19 cm2/s and 2.8 × 10−18 cm2/s, respectively. The diffusion coefficients are comparable to literature values for small molecules in a polymeric matrix.[16] To further ensure the pH-dependent release, the PS-BTA/PEI nanocontainers were initially dispersed in aqueous medium of pH 7 for a short time and then the pH was tuned to 10 by addition of sodium hydroxide solution under vigorous stirring. The in situ fluorescence signals in Figure 2B were utilized to monitor the release dynamics of BTA. It is observed that the release rate of BTA has been accelerated upon environmental pH increases from 7 to 10. As the pH increases, the charge density of PEI decreases which leads to the PEI branched polymeric chains to shrink. The shrinkage of PEI chains and their further desorption from the container surfaces result in the fast release of BTA from the container interior (Figure 2C). The well-defined morphology, function and pH-dependent sustained release of inhibitor suggest that the as-prepared PS-BTA/PEI core–shell nanocontainers serve as a good candidate for self-healing coatings on metal surfaces for long-term corrosion protection.

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Figure 2. A) Release dynamics of benzotriazole from the PS-BTA/PEI core–shell nanocontainers at pH 4, 7, and 10, respectively; B) The effect of pH change on release dynamics of benzotriazole from nanocontainers; C) Schematic illustration of pH-dependent release of BTA from the PS-BTA/PEI core–shell nanocontainers.

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It is well known that compatibility between additives and host materials plays an important role in the performance of composite materials.[17] We first doped SiOx/ZrOy sol-gel inorganic coatings with the PS-BTA/PEI nanocontainers. Anodic currents versus immersion time in 0.1 M NaCl obtained by SVET measurement over the scratched area are shown in Figure 3A. The addition of polymeric nanocontainer into inorganic coatings could suppress the corrosion occurrence. However, it is far away from an ideal system because the gradual increase of current on the crack areas with released inhibitor has been observed. This is due to a lack of compatibility between polymeric nanocontainers and sol-gel inorganic coatings, as shown in Figure 3C. For the epoxy organic coatings, the as-prepared PS-BTA/PEI nanocontainers in aqueous solution exhibit good compatibility with aqueous epoxy primer and hardener for processing. The epoxy organic composite coatings doped with polymeric nanocontainers exhibit a good appearance. Figure 3E shows the epoxy coatings doped without and with nanocontainers on metal surfaces. The addition of polymeric nanocontainers into epoxy coatings did not cause any appearance change, which suggests a good integrity/compatibility between polymeric nanocontainers and epoxy organic matrix. A scratch corrosion test of pure epoxy coating and nanocontainer-doped coatings was further performed (Figure 3F, Samples I and II). In contrast to pure epoxy coatings, the epoxy coatings with PS-BTA/PEI nanocontainers displayed an improved anti-corrosion protection on the crack areas. No corrosion propagation was observed for epoxy coating with nanocontainers. The anodic currents characterization monitored by SVET (Figure 3B) show an excellent autonomic healing process with an immersion time in 0.1 M NaCl. The currents on the crack areas were lowered in 2 h and then remained at a negligible state, suggesting that the electrochemical reactions of corrosion have been inhibited.

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Figure 3. Anodic current versus immersion time in 0.1 M NaCl by SVET measurement over the scratched area on different steel coatings: A) SiOx/ZrOy sol-gel coating, B) epoxy organic coatings; schematic distributions of the PS-BTA/PEI polymeric nanocontainers in C) SiOx/ZrOy sol-gel coatings and D) epoxy organic coatings; E) Optical micrographs of epoxy organic coatings on steel in the absence (Sample I) and presence (Sample II) of the PS-BTA/PEI-2 polymeric nanocontainers (3 wt%); F) corrosion test of defined scratched steel samples which are coated with a pure epoxy coating (sample I) and an epoxy coating doped with the PS-BTA/PEI-2 nanocontainers (sample II) after immersion in 0.1 M NaCl for 48 h. The PS-BTA/PEI-2 and PS-BTA/PEI-3 core–shell nanocontainers with a BTA feed content of 10 wt% and 15 wt%, respectively. The size of steel substrates is 1 cm in width × 2 cm in length.

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The self-healing mechanism for long-term anticorrosion protection is illustrated in Figure 4A. If the coating on the metal surface is scratched, the coatings on metal surfaces cannot serve any more as a passive barrier to block corrosive species from the metal. Thus the exposure of metal surfaces causes a fast corrosion expansion. The self-healing coating doped with corrosion inhibitor-loaded nanocontainers allows a fast self-healing process and has remarkable corrosion resistance. The inhibitor-loaded nanocontainers respond to the environmental pH changes caused by initiation of the electrochemical corrosion process,[18] releasing the inhibitor quickly to corrosion areas thus suppressing corrosion and restoring coating functionality in the scratched area. In addition to fracture of nanocontainers, pH-response of the nanocontainers is the main factor to release the inhibitor onto the metal surface to form a dense nanobarrier.

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Figure 4. A) The self-healing mechanism of polymeric nanocontainer-doped epoxy organic coatings on metal surfaces for corrosion protection. The electrochemical reactions during the corrosion induce inhibitor release from pH-responsive nanocontainers leading to a new protective nanobarrier on metal surfaces; Corrosion tests of epoxy coatings on steel in 0.1 M NaCl for 1 month: B) epoxy coating doped with 3 wt% PS-BTA/PEI-3 nanocontainers (the three substrates are with the same coatings after the same corrosion time); C) commercial epoxy coating with zinc phosphate as inhibiting pigment (the three substrates are with the same coatings after the same corrosion time). Red circles indicate pitting corrosions (blistering). The size of steel substrates is 1 cm in width × 2 cm in length.

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We further compare in Figure 4B,C nanocontainer-doped coatings with free inhibitor zinc phosphate-doped coatings for anticorrosion activity. It is obvious that the epoxy coatings doped with nanocontainers exhibit a remarkable resistance to corrosion after the steel substrates were immersed in 0.1 M NaCl for 1 month, although the content of nanocontainers in the coating is only 1–3% in weight (the BTA content in the coating is only 0.15–0.3%). In contrast to the pure coatings without any nanocontainers and even commercial coatings with zinc phosphate inhibiting pigment of around 5 wt% show much lower resistance to corrosion under the same conditions. The deterioration of coatings on steel appears and blistering (pitting corrosion) has been observed (Figure 4C and Figure S3). The long-term anticorrosion property of the PS-BTA/PEI nanocontainer-doped coatings is due to the fast adsorption of BTA onto steel surfaces to heal the initial corrosion spot. The donor-acceptor interaction between the heteroatoms N, O or Π-electrons of BTA and the vacant d-orbitals of iron surface atoms leads to adsorption of corrosion inhibitor on steel surfaces. The inhibitor on the steel surfaces forms a dense barrier which blocks the corrosion species diffusion and access to the metal surfaces. As shown in Figure 4A, the corrosive species such as H2O, Cl and O2 will be repelled away by the newly formed inhibitor complex barrier. Thus the primary corrosion site has been healed and the initial electrochemical reactions which would lead to corrosion extension could be terminated.

In summary, design and fabrication of polymeric nanocontainers for organic self-healing coatings on metal surfaces have been demonstrated. The synthesis process is highly efficient and can be easily upscaled for industrial application. The as-prepared corrosion inhibitor-loaded polymeric nanocontainers are inexpensive, of well-defined morphology and stimuli-responsive function. In addition, the polymeric nanocontainers exhibited good compatibility with epoxy organic coatings which could serve as self-healing coatings for prolonged anticorrosion protection without any external energy input. The resistance of nanoreservoir-containing coatings is higher than that of the reference coatings in a corrosive salt environment when a defined scratch is made on the steel surfaces. The inhibitor-loaded nanocontainers can quickly respond to environmental changes caused by the electrochemical corrosion process, release the inhibitor quickly to corrosion areas, forming an organic barrier to block molecular diffusion and terminating further corrosion expansion. Remarkably, the nanocontainer-doped epoxy coatings can restore integrity and retain good corrosion resistance in 0.1 M NaCl corrosive solution for the measurement period of one month, which even outperforms commercial anticorrosion coatings with inhibiting pigment of zinc phosphate. The polymeric nanocontainers for organic protective coatings based on self-healing mechanisms are also catalyst-free and environmental benign, which promises a broad range of perspectives.

Experimental Section

  1. Top of page
  2. Experimental Section
  3. Acknowledgements
  4. Supporting Information

Synthesis of Corrosion Inhibitor-Loaded Polymeric Core–Shell Nanocontainers: In a typical synthesis, 4-styrenesulfonic acid, sodium salt hydrate (0.4 g, 1.94 mmol) and sodium hydrogen carbonate (0.1 g, 1.19 mmol) were dissolved in 180 mL of de-ionized water and heated to 75 °C under vigorous stirring. Styrene (20 mL, 0.175 mol) was then added into the aqueous solution and followed by stirring for 1 h. To initiate polymerization, 0.1 g of potassium persulfate (0.37 mmol) and benzotriazole (2.7 g, 15 wt% of monomer) were added into the mixture. The feed ratio of benzotriazole relative to styrene was controlled from 0, 5%, 10%, 15% and 20%, respectively. The polymerization process lasts for 24 h under vigorous magnetic stirring. As the temperature cooled down, 8 mL of aqueous solution of highly branched poly(ethylenimine) (0.05 g/mL, pH 5) was added into the reaction mixture to wrap one PEI layer on particle surface leading to PS-BTA/PEI core–shell nanocontainers.

Release Dynamics of the PS-BTA/PEI Core–Shell Nanocontainers: The inhibitor BTA content in the polymeric nanocontainers was determined by UV-vis spectroscopy using analytical curves obtained at pure BTA standard concentration ranging from 10−3 to 10−1 mg/mL in DI water. About 10 mg of the BTA-loaded nanocontainers was dispersed in 10 mL of DI water and monitored at an absorption wavelength of 275 nm for the UV–vis measurement in the following two weeks.

The release dynamics was investigated by dispersing nanocontainers in different pH aqueous solutions. The BTA-loaded nanocontainer suspension was constantly stirred with a magnetic stir bar during the release process. The initial concentration of the BTA-loaded polymeric nanocontainers was 1 mg/mL, and then the release dynamics of BTA from the polymeric nanocontainers was monitored via the UV–vis spectra and fluorescence spectra. The diffusion coefficient was calculated according to the Einstein relation below:

  • display math

where D (cm2/s): diffusion coefficient, <X>2: variation of the position, t: time.

Self-Healing Coatings on Steel Surfaces: The steel plates were pretreated in a mixture of xylene and ethyl acetate in ultrasonic bath and were cut into different sizes for coatings and corrosion characterization. The self-healing coatings on steel surfaces were prepared by dispersing the nanocontainers into a SiOx/ZrOy sol-gel precursor[19] and epoxy primer, respectively. For sol-gel coating, the zirconium oxide sol was prepared by hydrolyzing tetra-propoxyzirconium (TPOZ, 70 wt% 2-propanol) in ethylacetoacetate (1:1 v:v) at room temperature for 20 min. The organosiloxane sol was prepared by hydrolyzing 3-glycidoxypropyltrimethoxysilane (CPTMS) in a mixture of 2-propanol/acidified water (HNO3, pH 0.5) for 20 min. Then, the zirconia sol and organosiloxane sol were mixed (1/2 v/v), stirred for 1 h and aged overnight at room temperature. The corrosion inhibitor-loaded nanocontainers were mixed with the above SiOx/ZrOy precursor with a controlled ratio by sonication. The films on steel surfaces were fabricated by a dip-coating procedure. The carbon steel was immersed into the sol-gel SiOx/ZrOy precursors for 120 s and then was withdrawn at a speed of 18 cm/min. Finally, the steel plates decorated with self-healing coatings were cured at 130 °C for 1 h. The thickness of sol-gel coatings was around 2 μm, which was measured by AFM.[20]

For the epoxy coating, the PS-BTA/PEI nanocontaineres were dispersed into commercial epoxy primer, followed by mixing hardener in water at room temperature (water/hardener/primer ratio: 1/3/7.5). Spray coating technology has been used to produce epoxy organic coatings on steel. The epoxy coatings on steel are further cured at 80 °C for 1 h. After drying, the thickness of the epoxy coating on steel was measured in the range of around 40–70 μm by a coating thickness gauge of Surfix@Pro S (PHYNIX, Germany).

Characterization: The morphology of the polymeric core–shell nanocontainers was characterized by transmission electron microscopy (TEM, Zeiss EM912 Omega) and scanning electron microscopy (SEM, Zeiss Gemini LEO 1550). TEM samples were dispersed in a solvent and a drop of the dispersion was spread onto the surface of a copper grid coated with a carbon membrane and then dried in vacuum at room temperature for TEM characterization. Hydrodynamic diameter and Zeta potential of the polymeric nanocontainers were measured in aqueous solution (0.5 mg/mL) by a ZetaSizer Nano ZS (Malvern Instruments). Gel permeation chromatography (GPC) with simultaneous UV and RI detection was performed in tetrahydrofuran at a flow rate of 1 mL/min. Calibration was done with polystyrene standards. The release kinetics of BTA inhibitor was determined via optical absorption (8453 UV–Visible Spectrophotometer, Agilent Technologies). The scanning vibrating electrode technique (SVET, Applicable Electronics) was employed for monitoring of the current densities with time during the self-healing process. A straight scratch was made in the coating with a scalpel (Bayha blade NO. 24).[21] The pitting corrosion in 0.1 M NaCl solution was investigated by SVET to record vertical current density at a height of 300 μm over an area of 3 mm × 3 mm.

Acknowledgements

  1. Top of page
  2. Experimental Section
  3. Acknowledgements
  4. Supporting Information

G. L. Li acknowledges support by a research fellowship from the Alexander von Humboldt Foundation and support from the EUFP7 “Nano-Barrier” project. We thank Dr. Dimitriya Borisova for the SEM images and Rona Pitschke for the TEM measurements.

Supporting Information

  1. Top of page
  2. Experimental Section
  3. Acknowledgements
  4. Supporting Information

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