Subject: Surface Engineering

TERM PAPER
TOPIC: SMART COATINGS

Abstract:
Materials that are capable of adapting their properties dynamically to an external stimulus are called responsive, or smart. The term "smart coating" refers to the concept of coatings being able to sense their environment and make an appropriate response to that stimulus. The standard thinking regarding coatings has been as a passive layer unresponsive to the environment. The current trend in coatings technology is to control the coating composition on a molecular level and the morphology at the nanometer scale. The idea of controlling the assembly of sequential macromolecular layers and the development that materials can form defined structures with unique properties is being explored for both pure scientific research and industrial applications. Several smart coating systems have been developed, examined, and are currently under investigation by numerous laboratories and industries throughout the world. Examples of smart coatings include stimuli responsive, antimicrobial, antifouling, conductive, self-healing, and super hydrophobic systems. In this paper a number of application-wise prominent smart coatings are discussed with their individual merits & demerits.

INTRODUCTION: A new generation of anticorrosion coatings that both possesses passive matrix functionality and actively responds to changes in the local environment has prompted great interest from material scientists. Corrosion is one of the most major destruction processes involved in material loss, and its prevention is paramount in protecting investments. Active corrosion protection aims to restore material properties (functionality) when the passive coating matrix is broken and corrosion of substrate has started. The coating has to release the active and repairing material within a short time after the coating integrity has been breached. This acts as a local trigger for the mechanism that heals the defect.
The oldest types of self-repairing coatings are polymer-based coatings. The concept of these coatings is as follows:

* Micrometer-scale containers, filled with monomers similar in nature to the polymer matrix and an appropriate catalyst or ultraviolet (UV)-sensitive agent to initiate the polymerization of the monomer when released at the damaged spot of the polymer coating, are incorporated inside the coating matrix.

* When these micro containers become mechanically deformed, they release the monomer and catalyst and thus seal the defect.

Repair of macroscopic adhesion defects formed throughout the service life of composites was carried out by filling hollow fibers with a polymer resin; these fibers would then fracture under excessive loading of the structure. The sealing component could be methyl methacrylate, 2-ethylhexyl methacrylate, and tertiary butylaminoethyl methacrylate and lauryl methacrylate for fixing adhesion of the polypropylene matrix.

* Polyelectrolyte-based aqueous poly(L-lysine) - graft-poly (ethylene glycol) was employed as the self-healing agent in an oxide-based tribosystem.

* The barrier properties of a damaged coating can also be recuperated by a simple blocking of the defects with insoluble precipitates.

Fig. 1 The mechanism of self-healing action of a ‘smart’ anticorrosion coating.

* Another approach to hybrid self-healing coatings is based on the use of inhibitors that can be released from the coating system. (1) The direct introduction of inhibitor components into protective coatings very often leads to the deactivation of a corrosion inhibitor and degradation of the polymer matrix. To overcome this, several systems entrapping the inhibitor and preventing its direct interaction with the coating matrix have been developed.

* One, quite simple, approach to inhibitor entrapment is based on the complications of organic molecules by cyclodextrin.

* Another is based on the use of oxide nanoparticles, which can play the role of nanocarriers for corrosion inhibitors adsorbed on their surface.

* Immobilization of Ce3+ ions on the surface of ZrO2 nanoparticles was achieved during the synthesis of a nano -sol by the controlled hydrolyzation of the precursor with a Ce - containing aqueous solution.

* The inhibition of inorganic ions can also be incorporated by exchangeable ions associated with cation- and anion-exchange solids. Ca(II) and Ce(III) cation-exchanged bentonite anticorrosion pigments are prepared by the exhaustive exchange of naturally occurring bentonite. Desorption controlled release

The first example of a coating doped with immobilized inhibitors here presented is the hybrid sol–gel film.

* Hybrid sol–gel films have been extensively studied during the last decade as a possible pre-treatment or primer layer on different metals and alloys and used as a model matrix for different encapsulated inhibitors.

* Silica-based sol–gel films can form a Si O Al conversion layer giving rise to a stable aluminium oxide/sol–gel film interface and consequently decrease of corrosion of aluminium alloys.

* These films are promising pre-treatment systems, since they combine flexibility and compatibility with paint systems (organic component), with good mechanical and adhesive properties (inorganic component).

* From a corrosion perspective, their role is passive, working only as a mechanical barrier. Nevertheless, they are usually applied as thin films and do not provide a perfect barrier; also, the formation of cracks or other type of defects can contribute for a more pronounced disruption of the barrier effect with corrosion processes initiating after some time.

* In order to prevent corrosion activity, complementary approaches have been tried to enhance the protection properties of the sol–gel by addition of ZrO2 nanoparticles to the sol–gel matrix and incorporation of corrosion inhibitors. * A simple strategy for inhibitor immobilization is based on the incorporation of inhibiting cations such as Ce3+ in amorphous oxide nanoparticles. The addition of corrosion inhibitors to coating formulations is not straightforward because the inclusion of active species can have a negative effect on coating barrier properties. In this case, this limitation was overcome by using ZrO2 nanoparticles, both as reinforcement of coating matrix and reservoir for Ce(NO3)3.

* The microscopic observations following immersion tests in NaCl solution showed that sol–gel films with Ce-doped ZrO2 were crack-free, whilst coatings without cerium presented large cracks in the places of intermetallic particles (cathodic sites in pitting corrosion) in 2024 aluminium alloy. The electrochemical characterization of these thin coatings by electrochemical impedance spectroscopy (EIS) clearly demonstrated a well-defined distinction between films without cerium cations, films with Ce(NO3)3 directly added to the siloxane matrix, and films with Ce-doped zirconia nanoparticles. For short immersion times, all the coatings demonstrated similar responses. With the increase of immersion time, the impedance values decreased differently (Fig. 1).

* The smallest changes were observed for films with Ce-doped ZrO2, followed by Ce-doped matrix films and finally undoped films. In the former case, only two time constants could be detected and ascribed to sol–gel film (high frequencies) and aluminium oxide (intermediate frequencies), whereas in the case of Ce-free coatings and Ce-doped matrix a third time constant related to corrosion activity was observed at low frequencies, denoting a higher coating degradation.

* The results indicate a general decrease of the oxide film resistance during test. In the case of hybrid matrix without ZrO2, the oxide resistance decreases more than three orders of magnitude after 1 day of immersion. The inclusion of zirconia nanoparticles results in a smaller decrease of resistance because of extra barrier effect provided by the presence of such nanostructures. * The stability of the intermediate oxide film is the highest for the case when Ce-doped ZrO2 particles are used in sol–gel films, and Ce-doped sol–gel matrix. For immersion times up to 100 h, the film with Ce-doped matrix shows higher oxide resistance values after which a steep decrease follows. * In the case of Ce-doped ZrO2-nannoconainers the rate of oxide resistance decrease is less pronounced. These results can be interpreted in terms of timescale and extent of inhibitor release. For cerium cations “free” in the sol–gel matrix, the release of inhibitor is spontaneous and fast and the stock of inhibitor within the sol–gel ends after a relative short time (100 h), whereas in the case of Ce-doped ZrO2 the release is slower and determined by the interaction between cerium cations and ZrO2 nanoparticles. * XPS analysis revealed that the presence of cerium cations results in a decrease of electron binding energy of Zr in ZrO2, indicating that cerium interacts directly with these nanostructured particles (adsorption, absorption), and is released after immersion of the sol–gel film in NaCl solution. * Additionally, properties, whereas when incorporated into zirconia particles this detrimental effect is minimized. Fig. 2c depicts the evolution of sol–gel (pore) resistance for Ce-doped ZrO2 sol–gel films (Ce3+ 0.5 wt%)-A, Ce-doped ZrO2 sol–gel films (Ce3+ 1 wt%)-C, coating with Ce-doped matrix (0.5 wt%) and ZrO2 nanoparticles-B, coating with ZrO2 nanoparticles only-D. * The observed trends are in agreement with evolution of coating capacitance and associated causes of degradation. The pore resistance decreases with immersion time, with the coating with concentrated Ce-doped (1 wt%) ZrO2 being the worst. * These results show that not only the direct addition of corrosion inhibitor to the matrix reduces coating barrier properties but also the excess of inhibitor in ZrO2 contribute to loss of coating barrier properties [18]. Therefore, a compromise between concentration of inhibitor responsible for active corrosion protection and coating barrier properties directly related to passive protection must be reached in order to achieve overall superior protection. In conclusion, it was demonstrated that the use of oxide nanoparticles as nanocontainers for Ce3+ provides prolonged release of the inhibitor and minimizes the degradation of the sol–gel coating. pH-controlled release : * A different strategy was used to provide coatings with selfhealing ability, based on the release of inhibitor on demand, using local change of pH during corrosion processes as the trigger.

* In the context of corrosion processes, perhaps the most relevant trigger is pH, as the initiation of corrosion activity is accompanied by local changes of pH in anodic and cathodic sites.

* To increase the control over the release of inhibitors, inorganic nanocarriers based on SiO2 were coated with polyelectrolyte shells using the Layer-by-Layer (LbL) assembly technique.

* The corrosion inhibitor benzotriazole was incorporated between oppositely charged polyelectrolyte layers. The aim was to use the permeability dependence of polyelectrolyte shells on pH to provide release of corrosion. inhibitors, only when corrosion processes were about to start or have already started.

* SiO2 nanoparticles were chosen as the inorganic cores of these “smart” nanocontainers due to their compatibility with the sol–gel hybrid matrix. These negatively charged particles were firstly coated with a layer of positively charged poly(ethylene imine) (PEI), followed by adsorption of negatively charged poly(styrene sulfonate) (PSS) and then positively charged benzotriazole (from acidic medium) as corrosion inhibitor.

* The PSS/benzotriazole adsorption was repeated to increase the inhibitor loading in the nanomaterials. Fig. 3a shows the evolution of zeta potential as different layers are adsorbed. The alternation between positive and negative values corresponds to the deposition of positively and negatively charged layers, respectively.

* Besides, the decrease in the zeta potential magnitude following addition of benzotriazole can be due to incomplete recharge of the surface resulting from the low molecular weight of this species when compared to large polyelectrolyte chains.

* Particle size measurements (Fig. 3b) support the above mentioned explanation. The increase of particle size for individual PEI and PSS is 8 nm while for the inhibitor layer is only 4 nm.The optimum number of PSS/benzotriazole layers was found to be 2. In the end of LbL deposition, the SiO2 particles were 100 nm sized as shown by transmission electron microscopy (TEM).

* The suspension of SiO2/PEI/PSS/benzotriazole was then added to ZrO2 and organosiloxane sols as described before [17–19] and the films applied on AA2024 metallic substrates. The obtained films did not show any particle aggregation (atomic force microscopy, AFM) and the estimated thickness was ca. 2 _m. The protection properties were investigated by EIS and by the scanning vibrating electrode techniques (SVET). EIS measurements performed in NaCl solution (0.005 M NaCl, frequency range: 105–10−2 Hz) showed that the highest impedance values were found for the sol–gel films doped with SiO2/PEI/PSS/benzotriazole followed by sol–gel films without additives, and sol–gel films with benzotriazole directly added to the matrix. Although benzotriazole is known as an effective corrosion inhibitor for AA2024 in solution, it detrimentally interacts with the hybrid matrix causing degradation of the coating.

* In the case of hybrid sol–gel films doped with SiO2/PEI/PSS/benzotriazole the barrier properties of sol–gel matrix were retained and the stability of the native intermediate oxide film, present in the aluminium alloy, was found to be the highest amidst the studied systems. Selfhealing of artificial defects was observed both by EIS and SVET measurements for this coating.

* Fig. 4 depicts the local current maps over the AA2024 alloy surface covered with both the unloaded ZrOx/SiOx hybrid film and the film impregnated with high amounts of nanocontainers. The local corrosion zones can be observed on the coated metal surface monitoring the local ionic flow densities

* The artificial defects (200 _m diameter) were formed on the surface of both coatings after 24 h of immersion in 0.05 M NaCl as shown in panels b and f. Well-defined cathodic activity appears in the place of induced defect on the alloy coated with the undoped hybrid film (panel c).

* This activity becomes higher with immersion time (panels d, e). A different behavior was revealed after defect formation on the alloy coated with hybrid film containing nanocontainers.

* In the latter case, corrosion activity could not be detected 4 h after the creation of the defect (panel g). The cathodic activity appears in the zone of the induced defect only after ca. 24 h (panel h).

* Interestingly, the artificial defect becomes passivated again two hours later, remaining “healed” even after 48 h (panel i).

Fig. 4. SVET maps of the ionic currents measured above the surface of AA2024 coated with undoped sol–gel pre-treatment (a, c, d, e) and with pre-treatments impregnated with nanocontainers (g, h, i). The maps were obtained before defect formation (a) and 4 h (c, g), 24 h (d, h) and 48 h (e, i) after defect formation. Scale units: _A cm−2.Scanned area:2mm× 2 mm
.

* These results show that the conditions associated with local corrosion activity cause the release of benzotriazole from the nanocontainers, hindering the corrosion process in the defective area.

The healing of defects can be interpreted in the following way:

1. At cathodic sites of AA2024 (intermetallic Al–Cu–Mg sites) the pH increases by reduction of oxygen or water and formation of hydroxyl anions causing the increase of polyelectrolyte shell permeability and consequent release of benzotriazole inhibitors locally at the defected zones.

2. The effect of SiO2/PEI/PSS/benzotriazole loading concentration on solgel films was also studied .The increase in concentration of these smart nanocontainers leads to increase of the resistance associated with the intermediate aluminium oxide film, without a significant loss of the sol–gel barrier properties, due to the active corrosion protection effect.

* Similar investigation was carried out using hollow natural halloysite nanotubes as inorganic hosts for corrosion inhibitors, coated with polyelectrolyte shells by LbL [27]. Such nanotubes provide significantly higher loading capacity in comparison to the silicananoparticles. The outer surface of halloysites is negatively charged, while the inner surface is positively charged, thereby promoting the loading of negatively charged species within the nanotubes.

* The corrosion inhibitor used in this study was 2-mercaptobenthiazole, while poly(allylamine hydrochloride) (PAH) and PSS polyelectrolytes were used to cover halloysite nanotubes. The maximum loading of 2-mercaptobenzothiazole achieved corresponds to about 5 wt%.

* Halloysite nanotubes loaded with corrosion inhibitors do not have control over the release of the loaded species. Approximately 10% of inhibitor is released from the nanotubes after 6 days in water at neutral pH. Nonetheless, when the nanotubes are coated with polyelectrolyte shells, release of inhibitor could not be detected. The increase of pH up to 10 (pH typically observed during localized corrosion processes) caused the release of inhibitor. This is due to pH-dependent permeability of the polyelectrolyte shells.

* Hybrid sol–gel films doped with 2-mercaptobenzothiazole halloysite/PAH/PSS were also investigated [27]. The resulting films revealed crack- and defect-free surfaces (scanning electron microscopy-SEM, AFM) with the nanotubes included in the sol–gel matrix without any aggregation or accumulation in specific regions. AA2024 substrate coated with halloysite-containing sol–gel film showed higher impedance values, with both smaller corrosion activity and improved barrier properties when compared to blank sol–gel (EIS).

* The application of inorganic host structures containing inhibitors and coated with polyelectrolyte shells provides a simple but effective way of controlling the release of inhibitors, triggered by pH conditions, without affecting the barrier properties of sol–gel coatings.

* When compared to adsorption of corrosion inhibitors on the surface of oxide nanoparticles, the mechanism of release is more effective for protection applications: the release of species being dependent on pH will certainly provide a control not only in time but also spatially because only the nanoreservoirs close to the defects where corrosion activity initiates will sense local pH changes releasing the inhibitor.

Ion-exchange control of release Continuing the increase of complexity in terms of inhibitor release from simple desorption to pH-triggered release of inhibitors, another type of “smart” nanoreservoirs for corrosion inhibitors based on release by ion-exchange was suggested. * Bentonite is a cation-exchanger, consisting of stacks of negatively charged alumosilicate sheets, between which inhibiting cations can be intercalated. Ca2+ and Ce3+-loaded bentonites dispersed in polyester resin layers and applied to galvanized steel substrates display active protection, avoiding coating delamination from the cut edge of samples.

* The performance of different pigments was evaluated by measuring the rate of coating delamination from the cut edge of samples exposed to salt-spray testing during 1000 h. The results show that Ce-bentonites outperform strontium chromate pigments at all times (Fig. 5). In another work, Ce3+-loaded bentonites combined with Ce3+-exchanged Shieldex and dispersed in polyvinyl butyral coatings applied to hot dip galvanised steel were also found to provide corrosion protection by inhibiting of coating delamination

* However, the release of the inhibitor is triggered by metal cations available in the surroundings. The presence of cations may not be directly associated with corrosion processes, which can lead to waste of inhibitor. Another type of ion-exchangers is Layered Double Hydroxides
(LDHs).

* Structurally, LDHs consist of stacks of mixed-metal hydroxides positively charged, stabilized by anions and solvent molecules situated between the positive layers.

* The mechanism of release of species from LDHs is based on anion-exchange. Thus, in the context of corrosion protection the LDHs can be used with a double role:

(i) Release of anionic corrosion inhibitors. (ii) Entrapment of aggressive anionic species. Furthermore, The release of inhibitors can also be indirectly triggered by pH changes:

* At very high pH the inhibitors can be exchanged with hydroxyl anions. * At very low pH the LDHs dissolve releasing the entrapped inhibitors. LDHs can be prepared by different routes including co-precipitation and ion-exchange methods, as well as calcination-rehydration * It constitutes a versatile structure to allocate different inorganic as well as organic anions.

* X-ray diffraction (XRD) results shows that these materials are crystalline, and the peak positions at low 2_ angles allows correlation between the gallery height, size and orientation of different anions in intergallery space.

* Fig. 6 shows SEM and TEM images of LDHs intercalated with vanadates (LDH-VOx). LDHs are polycrystalline particles with a plate-like morphology, and dimensions ranging 200–400 nm diameter and lateral size 20–40 nm. Moreover, both dimensions and morphologies were found to be retained upon anion-exchange.

Early detection of aluminum corrosion via “turn-on” fluorescence in smart coatings :

* Aluminum and its alloys are an extremely useful and abundant group of materials used in various engineering structures .

* Aluminum is a very thermodynamically active metal, which nonetheless advantageously provides its natural corrosion resistivity. A dense inert protective aluminum oxide layer forms rapidl when the metal is exposed to an oxygen containing environment (e.g., air, water) and protects the metal surface from corroding since it is more thermodynamically inactive.

* However, the protective oxide film can be destabilized and as a result corrosion can occur. The oxide is not stable in acidic (pH < 4) or alkaline (pH > 9) solutions nor in the presence of aggressive ions (such as chlorides present in salt water) that might locally attack the oxide and cause pitting . Pits are initiated at weak sites in the oxide and propagate according to the anodic reactions:

Al → Al3+ +3e− (1) Al3+ +3H2O → Al(OH)3 +3H - (2)

The cathodic reaction involves reduction of dissolved oxygen according to: O2 +2H2O + 4e−→ 4OH− (3)

* During pit propagation, pH inside the pit decreases according to reaction (2).When metal is in a salt water environment chloride ions will migrate to the pit to balance the positive charge. As a result hydrogen chloride (HCl) is formed that further dissolves the metal and accelerates pit propagation.

* Different fluorescent probes, such as lumogallion , morin and 8-hydoxyquinoline-5-sulfonic acid , have been used to sense corrosion of aluminum by complexing with Al3+ ions released at the anodic site of corrosion (reaction (1)). Upon this complexation chelation-enhanced fluorescence (CHEF) is produced.

* Another approach in aluminum corrosion sensing involves detection of the localized pH changes at anodic and cathodic sites of corrosion by using acid–base indicators. These indicators change their color or fluorescence depending on pH. They are usually weak organic acids or bases and their dissociated form has a different color or fluorescence than the undissociated form.

* The color or fluorescence of these indicators appeared due to increase in the local pH according to reaction at the cathodic sites of corrosion.

* High-pHresponsive microcapsules that release phenolphthalein in clear or light-colored polyurethane coatings at the alkaline cathodic area of corrosion.

* There were also some attempts to sense low pH observed at the anodic sites of corrosion, where the actual metal dissolution occurs, due to reaction (2).

* The fluorescence of this indicator is quenched in acidic pH below 4. Therefore the sensing coating was initially fluorescent and its fluorescence decreased upon exposure to corrosive environment, making it very difficult to judge the onset of the corrosion.

* An ideal corrosion indicator for aluminum,while sensitive to pH changes, would not possess any functional groups (such as phenol groups) that can be ionized (as is the case for acid–base indictors) or that can react with the epoxy components in such a way that alters their fluorescent properties.Adesired probe should be fluorescence rather than color changing since higher indication sensitivity can be achieved using fluorescent indicators.

* The ideal sensing molecule would also respond to pH changes associated with corrosion resulting in “turn-on” fluorescence. A “turn-on” approach is much more useful and practical than a fluorescence-quenching approach since it is simply easier to detect small areas that fluoresce (against a background that does not) than to observe a slight decrease in overall fluorescence as in the case of quenching reactions.

* The initially nonfluorescent indicator, after being blended into the epoxy coating, would ideally become highly fluorescent at the onset of corrosion before any obvious sign of metal corrosion can be easily observed.

* Spiro [1H-isoindole-1,9_-[9H]xanthen]-3(2H)-one, 3_,6_- bis(diethylamino)-2-[(1-methylethylidene)amino] (“FD1”, Fig. 1) can be used in epoxy coatings to successfully detect early corrosion of steel.

* It was also noted that FD1 fluorescence increases significantly with decreasing pH thus we propose to use this molecule as a “turn-on” sensor for the acidic pH observed at the anodic site of aluminum corrosion where metal dissolution occurs.

Smart coating process of proton-exchange membrane for polymer electrolyte fuel cell :

* For the urgent need of renewable and environmentally friendly energy, the development of fuel cell related techniques have been promoted in recent years

* PEMFCs is one of the most attractive power sources, especially for portable devices due to the relatively lower operating temperatures and smaller system size. The key material of PEMFC is the proton conducting polymer electrolyte in the membrane electrode assembly (MEA).

* The essential conditions for a polymer electrolyte membrane used in PEMFCs are high proton conductivity, high stability against the attack of radical by-products and low cost.

There were many ways which improved the characteristics of PEMFC in recent years:

* The main elements for operational performances of PEMFCs can be attributed the electrolyte|catalyst| electrode interfaces, where the redox reactions occur in fuel cell operation

* The resistance of cell may influence the operation voltage and current of a PEMFC, and there are lots of interface modification processes can overcome this problem.

* Currently, the Nafion_ membrane, which was developed by DuPont Ltd., is still the most adapted electrolytes both in academic researches and commercial applications.

The morphologies of Nafion_117 with different power of treatment were shown by AFM in Fig. 1.

(fig 1. The images of Nafion 117 through (a)N00 (b)BO2-10W (c) BO2-20W (d) BO2-30W

* The morphologies of 20W plasma treated and untreated samples with and without the smart coating catalyst layers are shown in Fig. 2.

* Small clusters dispersed and coated on the surface can be clearly observed. This smart coating method can effectively coat a catalyst layer even on the very rough profiles of the plasma treated membranes (Fig. 2).

* Due to the rough surface caused by plasma treatment, the original 2-dimensional surface area becomes 3-dimensional, which increases the effective area for electrochemical reaction. In addition, the smart coated catalyst layer uniformly coated through the roughened morphologies can act as a charge transfer media, which may reduce the resistance.

*
SOME EXAMPLES OF SMART COATINGS : Antifouling Bio-catalytic Color shifting Conductive Corrosion sensing Defect/degradation sensing Light sensing Molecular electronics Nanotechnology-based Optically active

SOME LIVE EXAMPLES :

Scratch Resistant Self-healing Coating :

New scratches One week later

SUMMARY:
Summarising , these are the commercially available, prominent & promising smart coatings as of today & since they have their own advantages and disadvantages, we have to choose the right one keeping in mind the end application. Conductive polymers and coatings Anti-inflammatory/antimicrobial coatings Nanotechnology-based coatings

Key References :

* Daria V. Andreeva* and Dmitry G. Shchukin, Max Planck Institute of Colloids and Interfaces, Am Mühlenber 1, 14476 Golm, Germany

* Hoang-Jyh Leu , Kuo-Feng Chiu , Chiu-Yue Lin , Department of Materials Science and Engineering, Feng Chia University, Taichung, Taiwan.

* M.L. Zheludkevich∗, J. Tedim, M.G.S. Ferreira
CICECO, DEMaC, University of Aveiro, Aveiro, 3810-193, Portugal

* Anita Augustyniak, Weihua Ming ,Nanostructured Polymers Research Center, Materials Science Program, University of New Hampshire, Durham, NH 03824, United States