Nanocontainer-Based Active Systems: From Self-Healing Coatings to Thermal Energy Storage.

We highlight the development of nanocontainer-based active materials started in 2006 at the Max Planck Institute of Colloids and Interfaces under the supervision of Prof. Helmuth Möhwald. The active materials encapsulated in the nanocontainers with controlled shell permeability have been first applied for self-healing coatings with controlled release of the corrosion inhibitor. The nanocontainers have been added to the paint formulation matrix at 5-10 wt % concentration, which resulted in attaining a coating-autonomous self-healing ability. This research idea has attracted the attention of many scientists around the world (>1500 publications during the last 10 years) and has already been transferred to the commercialization level. The current trend in nanocontainer-based active systems is devoted to the multifunctionality of the capsules which can combine self-healing, antibacterial, thermal, and other functionalities into one host matrix. This article summarizes the previous research done in the area of nanocontainer-based active materials together with future perspectives of capsule-based materials with antifouling or thermoregulating activity.

Introduction

Development of the functional micro- and nanocontainers (or capsules) has attracted interest for various research areas such as biotechnology, medicine, cosmetics, catalysis and bulk multifunctional autonomic materials. The main advantage of the nanocontainers is the possibility to isolate the encapsulated active agent from the surrounding environment combined with its targeted release where and when it is needed. In general, research on nanocontainer formation and loading requires the ability to form a nanocontainer shell, which should be stable and permeable to release/upload materials and should also possess other desired functionalities (magnetic, catalytic, conductive, targeting, etc.). One has to combine several properties in the shell structure and composition to achieve this goal. Depending on the nature of the “smart” materials (e.g., polymers, nanoparticles, and nanocarbons) introduced into the container shell, various stimuli can induce reversible and irreversible shell modifications: variation of the pH, ionic strength, temperature ramp, ultrasonic treatment, alternating magnetic field, and electromagnetic field. Different responses can be observed to vary from fine effects such as tunable permeability for molecular-sized corrosion inhibitors to the total rupture of the container shell.

Several approaches have been developed so far to fabricate microcontainers and nanocontainers.[1] One approach is based on the self-assembly of lipid molecules or amphiphilic block copolymers into spherically closed bilayer structures (vesicles).[2,3] The second approach is to use dendrimers or hyperbranched polymers as nanocontainers.[4,5] The third procedure involves suspension and emulsion polymerization around oil or water nanodroplets forming a cross-linked polymer shell, also involving ultrasound. This method allows one to obtain hollow nanoshells with a size starting from 20 nm in a facile, one-step procedure. The method is well studied, and several reviews exist on the emulsion polymerization technique.[6,7] Contrary to organic nanocontainers, inorganic nanocontainer scaffolds (such as mesoporous silica or titania and halloysite nanotubes) with pH-controllable pore nanovalves are more mechanically robust and cheaper (e.g., 3–6 USD per 1 kg of halloysites) to use in large-scale production.[8,9] This article highlights the current achievements in the area of self-healing coatings, which was started in 2006 at the Max-Planck Institute of Colloids and Interfaces (Golm, Germany) together with new trends in the development of nanocontainer-based multifunctional materials. The first part will contain a description of the autonomous self-healing coatings and their commercialization potential, and the other parts will be focused on added multifunctionalities such as antifouling activity and heat storage. The use of green energy as new energy sources (heat generation from biomass, generation of electricity from photovoltaic cells, and the photochemical production of hydrogen) has becomes more popular now and requires new approaches to the synthesis of nanomaterials for energy applications.

Concept of Nanocontainer-Based Self-Healing Coatings

The general approach to impart a feedback functionality to a coating by the incorporation of encapsulated active material is represented in Figure .[10−12] The possibility of interplay among shell properties, encapsulated active agent (inhibitor), coating formulation, and metal substrates makes this approach almost universal for multifunctional coating solutions. The general understanding of the self-healing coating is the autonomic or stimulated ability of the coating to restore its main functionality: protection of the metal substrate against corrosion. However, the general concept faces difficulties in interaction between coating components and the nanocontainer shell. There has been no universal solution until now for nanocontainers loaded with corrosion inhibitors for applications in different types of coatings. Therefore, many types of nanocontainers are developed in the scientific literature (we estimated around 200), but only a few are a direct route to commercialization. Here, we highlight the mostly developed self-healing systems.

Figure 1

Schematic representation of nanocontainer-based self-healing coatings.[10]

Nanocontainer-based self-healing coatings can be classified according to the external stimulus to which they respond. Various external stimuli of a physical or chemical nature can cause a change in the coating:

(1) Mechanical impact is the first external stimulus demonstrated for microcapsule-based coatings by White et al.[13] and later by Blaizick et al.[14] and Zhao et al.[15] White et al. encapsulated a healing agent (dicyclopentadiene) into polymer microcapsules embedded in an epoxy coating matrix containing the Grubbs catalyst. The embedded microcapsules are ruptured upon crack formation, and the healing agent is released into the crack by capillary forces and comes into contact with the catalyst. The result is in crack healing and recovery of the barrier properties of the coating. The next step in the mechanically triggered self-healing coating was the development of a microvascular network loaded with the reagents necessary to cure a healing polymer inside the crack. Coatings based on a microvascular network are out of the scope of our article, and we refer the reader to the main papers in this area.[16,17] Another method is to use a curing agent from the local environment employing highly air-sensitive or moisture-reactive materials, such as metal oxide precursors[18] and organosiloxanes,[19] as encapsulated cargo. In such coatings, the active material is first released as a result of crack propagation and forms an impermeable layer only after oxidation and hydrolysis by atmospheric oxygen or water. Successful self-healing by such a triggering mechanism requires a capsule size large enough (50–200 μm) to enable easy rupture and delivery of a sufficient amount of active agent. However, the integration of large microcapsules into thin coatings is limited by the coating thickness. Microcapsule shells should be rigid to preserve the capsule’s integrity during embedding in the coating matrix and brittle enough to facilitate capsule rupture upon crack formation, and at the same time, the active agent should have low viscosity and vapor pressure.

(2) Electromagnetic irradiation belongs to the physical stimulus, which affects the nanocontainer shell because of either local heating or photochemical reactions. The direct incorporation of photocatalytic TiO2 particles into polymer coatings leads to the oxidation of the organic components of the coating matrix and the loss of its barrier properties. However, the application of TiO2 and Ag nanoparticles as triggering components of the nanocontainer shell makes it possible to release the encapsulated agent without damaging the coating matrix. A polyelectrolyte shell, which modifies the outer surface of TiO2-based containers, prevents the spontaneous leakage of the loaded material and revealed the UV-stimulated release of the benzotriazole corrosion inhibitor under UV irradiation.[20] For the coatings where very fast release of the inhibitor is important, the use of UV irradiation as an external trigger is strongly preferable. A similar effect, but with a slower inhibitor release, was found by Skorb et al. for nanocontainers coated with a polyelectrolyte shell containing IR-sensitive Ag nanoparticles.[21] One could terminate the corrosion process, in this case, by intensive local IR laser irradiation. The healing ability under laser irradiation was proven by the scanning vibration electrode technique (SVET). It was also noted that, by applying a polyelectrolyte shell with noble metal particles over the mesoporous titania via layer-by-layer assembly, it is possible to fabricate micro- and nanoscaled containers sensitive to both UV and IR irradiation.

(3) The internal chemical impact from the coating/metal substrate interface to trigger inhibitor release is the change in the local pH in the corroded area. It is currently most popular for nanocontainer-based self-healing coatings, and the nanocontainers based on it were commercialized in Europe and China (both multinational ones like BASF and SMEs like Smallmatec). Utilizing the pH shift as a stimulus for corrosion inhibitor release is the natural way to design anticorrosive coatings with a high self-healing response because corrosion leads to a local pH decrease in anodic areas and a local pH increase in cathodic ones. There are several ways to make pH-sensitive nanocontainers for self-healing coatings. The first developed one is the layer-by-layer (LbL) assembly of oppositely charged species (e.g., polyelectrolytes) on templating colloidal nanoparticles. The template can be either removed to form a hollow structure or retained to provide better mechanical stability.[22,23] The advantages of this method are the variety of charged species suitable for shell construction and the adjustable layer thickness and flexibility. For example, silica nanocontainers with a benzotriazole (BTA)-loaded LbL shell were dispersed in sol–gel coatings and demonstrated good corrosion resistance.[24] Another methodology used to increase the amount of inhibitor in LbL nanocontainers is to incorporate the inhibitor in a porous core protected by the LbL shell. This was demonstrated by inserting organic inhibitors into porous metal oxide nanoparticles (TiO2 and SiO2), followed by coating them with polyelectrolyte multilayers. Such pH-sensitive nanocontainers were dispersed in SiO/ZrO sol–gel coatings and improved the coatings’ corrosion inhibition properties reducing the corrosion to zero.[25] However, the complexity of such nanocontainers restricts their scaling up and industrial application.

Commercially available porous inorganic materials can be applied as inorganic nanocontainers for self-healing coatings. Only inhibitor loading and formation of the trigger-sensitive blockers (valves) are fabrication steps required to produce ready-to-use inhibitor-loaded nanocontainers. Inorganic nanocontainers can be mesoporous silica[26] or titania,[27] ion-exchange nanoclays,[28] and halloysite nanotubes.[29] Mesoporous silica and titania are commercial products, and despite the fact that they are produced in thousand ton volumes, they are still more expensive than halloysites. The industrially mined, viable, and inexpensive halloysite nanotubes have high potential as inhibitor nanocontainers for commercial applications. Halloysites are two-layered aluminosilicates with hollow tubular structure. Their size varies within 1–15 μm for the length and within 10–150 nm for the lumen inner diameter. Halloysite nanotubes were loaded with the inhibitor, 2-mercaptobenzothiazole and covered by a polyelectrolyte stopper layer to improve control over the inhibitor release.[30,31] Anticorrosion performance tests using the industrial neutral salt-spray test (ISO 9227 standard, 5 wt % NaCl, 35 °C) were conducted to check the halloysite efficiency in industrial coatings.[32,33]

As one can see in Figure , the direct addition of Korantin SMK corrosion inhibitor, which is an alkylphosphoric ester produced by BASF with a chain length of alkyls in the ester group ranging from C6 to C10, to the coating in free form (1 wt %) drastically reduces the corrosion protection performance even after 500 h of the neutral salt-spray test. On the contrary, inhibitor encapsulated inside halloysite nanotubes increased the corrosion protection 5-fold. These results are due to the favorable halloysite structure, which provides good inhibitor storage in the lumen and limits spontaneous inhibitor leakage due to the small-diameter (20–50 nm) ends covered by the pH-sensitive stoppers.[34] Salt-spray tests demonstrate clear evidence on an industrial level that halloysite nanotubes can develop a new, revolutionary generation of self-healing anticorrosion coatings on the mass-production level.

Figure 2

Neutral salt-spray test results for pure polyepoxy coating (A, 1000 h), polyepoxy coating directly loaded with Korantin SMK corrosion inhibitor (B, 500 h), and polyepoxy coating in the presence of Korantin SMK-loaded halloysite nanotubes (C, 1000 h).[33]

(4) More rare external triggers used for capsules and nanocontainers in self-healing coatings are ultrasonic treatment,[35,36] temperature,[37] ionic strength,[38] and different electrochemical potentials on the surface of the corroded metal substrate under the coating matrix.[39−41] However, only a few papers have been published in this field without continuous significant interest from academia and industry.

The current level of the nanocontainer development for autonomous self-healing coatings with single functionality or responsive only to one triggering mechanism had already reached a sufficient number of scientific publications on the laboratory scale and now requires an increase in the possible functionalities. However, the multiple functionality of the coatings may be limited by the number of nanocontainers which can be incorporated into the coatings without changing their main properties, such as the barrier, color, and so forth. Usually, if the number of nanocontainers exceeds 10 wt % in the cured coating, then its main passive properties become considerably reduced.[42] Therefore, the second generation of the multifunctional nanocontainer-based coatings should involve nanocontainers with multiple functionality. This direction is rapidly developing now with an increasing number of publications. In the next two parts, we would like to focus on two additional functionalities: (i) antifouling, which can be used for maritime applications, and (ii) thermal energy storage for domestic applications, for example, in zero-energy houses.

Nanocontainers with Antifouling Ability

Biofouling is a natural process that refers to the accumulation and growth of microorganisms, algae, or plants on any natural or artificial wetted surface.[43] The first establishment by microorganisms and unicellular algae occurs within the first minute of immersion of the surface in seawater.[44] This initial stage allows the formation of a conditioning film consisting of physically adsorbed organic molecules followed by the settlement and growth of bacteria, protozoa, and diatoms, creating a complex biofilm matrix within the first 24 h of immersion (microfouling). The existence of this complex biofilm provides sufficient food for the formation of microscopically visible algae, spores, seaweeds, and invertebrates after 2 to 3 weeks of immersion (soft macrofouling). Finally, the increased capture of microscopically visible organisms stimulates the settlement of larvae of marine organisms such as mussels, barnacles, and sponges as well as spores of macroalgae after several weeks of immersion.[45] Biofouling in the maritime milieu leads to severe economic disturbances in the marine industries as well as a negative environmental impact. Biofouling is estimated globally to cost USD 150 billion per year, a cost that is a result of the maintenance and cleaning of submerged man-made surfaces on buoys, membrane bioreactors, desalination units, cooling water systems, and oil pipelines.[46] Therefore, it is very important to stop biofouling in the first stage of bacterial growth.

Historically, the most successful antifouling paints were biocide-based self-polishing coatings based on the well-known organotin compounds, especially tributyltin (TBT). However, its high toxicity in humans and the negative environmental impact led to the worldwide direct ban of TBT usage by the International Maritime Organization in 2008.[47] Therefore, the antifouling paint manufacturers were forced to study and develop TBT-free environmentally friendly antifouling paints. An effective methodology for addressing this problem is the controlled release of the encapsulated biocide, which controls the release rate of the biocides and, at the same time, protects them from the surrounding environment.[48,49] If the surface of the nanocontainer is modified by the stable, nonreleasing antifouling agent, then the additional dual antifouling activity can be achieved–the first from the encapsulated biocide and the second from the antifouling surface of nanocontainers.

Quaternary ammonium salts (QASs) have been indexed as antimicrobial compounds for more than 70 years.[50,51] They were used against the growth of a broad range of microorganisms in several applications, including food and pharmaceutical products, antiseptics, disinfectants, biocides, fungicides, cosmetics, and water treatment.[52,53] The advantage of the QAS-modified materials is their attachment to the fillers of the coatings, which allows a permanent antifouling effect of the coating without the release of the biocide material. Usually in the literature nonporous silica nanocontainers are used only for the grafting of biocide agents onto the surface and mesoporous silica nanoparticles (MSNs) are used only for biocide encapsulation. However, it is possible to demonstrate dual-function antibacterial/antifouling MSNs based on the combination of two strategies (surface modification and encapsulation) in one system (Figure ).[54] For this purpose, spherical mesoporous MCM-48 silica nanoparticles with a dual synergetic antimicrobial effect were synthesized and tested against Gram positive and Gram negative bacteria as well as in antifouling coatings. For the preparation of MSNs, the modified Stöber’s method was used according to Schumacher et al. because it is an easy and fast way to obtain MCM-48 at room temperature.[55] MCM-48 spherical nanoparticles were chosen for two main reasons. First, their interwoven, branched 3D mesostructure makes them excellent candidates for the loading and release of biocides. Second, this is the first study that reports the surface modification of MCM-48 with QAS for secondary biocide activity and the controlled release of an encapsulated antifouling agent.

Figure 3

Encapsulation of biocide in the QAS-modified MCM-48 and solid materials before and after the encapsulation.

The specific surface area of the mesoporous MCM-48 nanocontainers exhibited a high value of 1300 m2/g. The average pore diameter was 3.2 nm. The encapsulation of ecofriendly Parmetol S15 biocide (active component DCOIT) reached 30 wt % yield. Glass slides (22 mm × 22 mm) were coated with a spin coater by using 1 mL of antifouling nanocontainers dispersed in ethanol solution (2 wt %) and tested against E. coli bacteria. As can been seen in Figure , the QAS-modified MSNs reduced the number of viable E. coli by around 90% as compared to pristine MCM-48. The addition of the second functionality via Parmetol S15 encapsulation resulted in enhanced antibacterial performance by killing all of the exposed bacteria during the whole test duration (at 37 °C for 3 h).

Figure 4

Relative number of viable bacteria (E. coli) after testing on glass slides spin-coated with pristine MCM-48, Parmetol S15 (Par) loaded/unloaded dimethyloctadecyl [3-(trimethoxysilyl) propyl] ammonium chloride (QC18), and dimethyltetradecyl[3-(triethoxysilyl) propyl] ammonium chloride (QC14) surface-modified MCM-48.[54]

A panel field test took place in the northern Red Sea (Eilat, Israel) for 5 months in order to evaluate the antifouling efficacy of the modified MCM-48 nanoparticles incorporated into ship coatings. PVC panels coated with pristine paint (biocide-free paint from Jotun, Norway) were used as control samples. Nanocontainers were suspended mechanically via a homogenizer in a polymeric paint matrix. The concentration of the nanoparticles in the paints was adjusted to 5 wt %, and four different paint formulations were synthesized. Paint 1: pristine paint + 5 wt % QC18-modified MCM-48 loaded with Parmetol S15. Paint 2: pristine paint + 5 wt % QC14-modified MCM-48 loaded with Parmetol S15. Paint 3: pristine paint + 5 wt % QC18-modified MCM-48. Paint 4: pristine paint + 5 wt % QC14-modified MCM-48. The coated PVC panels were placed on a floating structure made of a stainless steel frame submerged to a depth of 8 to 9 m. Figure shows photographs of the exposed coated panels in the first day of deployment and after 5 months of exposure.

Figure 5

Photographs of PVC panels coated with pristine paint and paints 1–4 during the field test trial in the Red Sea (a) on the first day of deployment and (b) after 5 months of exposure.[54]

At the end of the field test, the control sample had 39% biofouling coverage while paints 1–4 presented significantly lower biofouling coverage below 10%. Paints 1 and 2 containing the modified nanoparticles with the dual antifouling effect illustrated superior performance with 6.7 and 7.8% biofouling coverage due to the biocide encapsulated in the QAS-modified nanoparticles. The demonstrated results show a simple and facile approach toward multifunctional coatings combining self-healing and antifouling abilities.

Nanocontainers for Thermal Energy Storage

There are three main types of materials for thermal energy storage: materials for sensible heat storage (such as water), materials for thermochemical heat storage, where heat is converted into chemical energy, and phase-change materials where heat is stored as the phase-transition enthalpy. Solid–liquid PCMs show a good balance between energy capacity and volume changes upon phase transition. The ideal properties for a PCM are (i) a suitable melting temperature, (ii) high latent heat, (iii) high thermal conductivity, (iv) congruent melting and no supercooling, (v) nonflammability, (vi) noncorrosiveness, and (vii) low cost.[56] However, there are no PCMs found so far which fit all of these criteria.

Mostly developed PCMs are organic paraffins and crystallohydrates.[57] Paraffin waxes are linear alkanes containing 8–40 carbon atoms. Their disadvantages include low thermal conductivity, flammability, and high costs. Commercial paraffin contains formaldehyde and vinyl chloride as well as benzene, toluene, naphthalene, and methyl ethyl ketone which are volatile and carcinogenic in nature, so care must be taken while using these materials in building applications.[58] Salt hydrates (also known as crystallohydrates) are the major class of inorganic PCMs and are most promising for applications due to their high latent heat, high energy storage density, low cost, and wide range of melting temperatures available. Disadvantages of salt hydrates include incongruent melting, phase separation, supercooling, and corrosiveness toward container materials (heat carriers), especially metals. Incongruent melting is incomplete melting of the salt hydrate, leading to the irreversible formation of a salt of lower hydration number. This leads to zero latent heat and also rendering salt hydrates chemically unstable often after very few melting/freezing cycles.

The practical use of PCMs is hindered by their limitations. The encapsulation of PCMs into microcontainers and nanocontainers can considerably improve their energy storage properties (Figure ). However, the nanocontainer shell must possess “smart” multifunctional properties: controlled thermal energy release, protection against corrosion and degradation during heat uptake/release cycles, increased environment/PCM surface area and heat conductivity, and the possibility to use such energy capsules in powder or paste form as additives to the bulk materials (concrete, foam, paint, etc.) to attain thermal energy storage properties.[59] All of these properties are crucial to PCM use in practical applications, so encapsulation can almost be a “one size fits all” solution.

Figure 6

Cartoon showing ideal capsule behavior for the salt hydrate core.[57]

Inverse Pickering emulsions, interfacial polymerization, and solvent evaporation–precipitation methods are the most common chemical methods described in the literature for the encapsulation of inorganic PCMs.[60] The first successful core–shell capsules containing salt hydrates specifically for energy storage were developed by Sarier et al. for thermally regulating fibers.[61] They have used a mix of PEG1000, hexadecane, and sodium carbonate decahydrate (Na2CO3·10H2O) as core material encapsulated in a urea-formaldehyde shell. Salaün et al. encapsulated Na2PO4·12H2O in a polyurea/polyurethane shell.[62] They used the solvent evaporation technique, dissolving cellulose acetate butyrate in a volatile solvent (chloroform) which polymerizes as the solvent evaporates. The crystallohydrate load was up to 79 wt %, and Graham et al. demonstrated a simple method to nanoencapsulate magnesium nitrate hexahydrate, employing in situ miniemulsion polymerization with ethyl-2-cyanoacrylate as the monomer.[63] Using sonication to prepare miniemulsions improved the synthesis by reducing the amount of surfactant required as a stabilizer, and then the inverse miniemulsion polymerization method was used for the encapsulation of magnesium nitrate hexahydrate nanodroplets, resulting in 100–200 nm PCM nanocontainers. Figure visually demonstrates stabilized PCM after encapsulation.

Figure 7

Bulk Mg(NO3)2·6H2O (a and c) and nanoencapsulated salt hydrate (b and d) before heating to 100 °C (top) and after cooling back to room temperature (bottom).[63]

Before melting, pure Mg(NO3)2·6H2O is a crystalline solid (Figure a). After melting, it recrystallizes to the solid surrounded by water, showing that a volume change occurs during phase transition and the recrystallized solid forms a compact block (Figure c). Nanoencapsulated salt hydrate (Figure b,d) shows no volume increase or change in appearance before and after heating to 100 °C. This indicates chemical and structural stability of the encapsulated PCM at the transition temperature. The absence of leakage means that the salt hydrate is fully protected by encapsulation, which is consistent with the thermal stability demonstrated by DSC.

Formation of the crystallohydrate eutectics provides an effective instrument to regulate the melting temperature for desired applications.[64] The following paper describes the encapsulation of crystallohydrate eutectic mixture (Mg(NO3)2·6H2O and Na2SO4·10H2O).[65] SEM images of the nanocontainers are shown in Figure . Initial crystallohydrates are large, >1 μm crystals combined into agglomerates of up to 100 μm in size.

Figure 8

SEM images of (A) bulk Mg(NO3)2·6H2O, (B) Na2SO4·10H2O, and (C) a 1:1 wt % Mg(NO3)2·6H2O:Na2SO4·10H2O mixture. (D–F) Demonstration of energy nanocontainers with Mg(NO3)2·6H2O, Na2SO4·10H2O, and a 1:1 wt % Mg(NO3)2·6H2O:Na2SO4·10H2O core, respectively. Each scale bar for images A–C is 1 μm, and each scale bar for images D–F is 100 nm.[65]

After encapsulation, the size of the crystallohydrate core is reduced to 100–200 nm, with the smooth capsule shell providing complete coverage of the core, with no pores present. This is important for ensuring that the core material is protected from the environment and for preventing water loss during heating. The images show how the nanocontainers tend to aggregate; however, single nanocontainers can be clearly seen. The nonspherical shape of the nanocontainers is caused by the solid form of the crystallohydrate core appearing after cooling of the initially liquid crystallohydrate core droplets during the preparation of energy nanocontainers.

DSC results demonstrated a high thermal stability of nanoencapsulated single and mixed crystallohydrates, which remained unchanged after 100 thermal cycles (Figure ). Encapsulation of the crystallohydrate mixtures also reduced supercooling (Figure c,d). The encapsulated 1:2 wt % Mg(NO3)2·6H2O:Na2SO4·10H2O eutectic mixture has one well-defined phase-transition peak with TM = 15.4 °C and TF = −1.1 °C. The transition is stable for >100 heat uptake/release cycles and has a latent heat capacity of 126.8 J·g–1, which showed 67% encapsulation efficiency. Additive mixtures of crystallohydrate-loaded nanocontainers have a high potential to design multitemperature heat storage systems containing energy nanocontainers with different PCM cores sensitive to different transition temperatures.

Figure 9

DSC data for (A) the 1:1 wt % Mg(NO3)2·6H2O:Na2SO4·10H2O bulk mixture, (B) the 1:2 wt % Mg(NO3)2·6H2O:Na2SO4·10H2O bulk mixture, (C) encapsulated 1:1 wt % Mg(NO3)2·6H2O:Na2SO4·10H2O, and (D) encapsulated 1:2 wt % Mg(NO3)2·6H2O:Na2SO4·10H2O.[65]

Huang et al. used Na2PO4·12H2O as a core material and methyl methacrylate as a monomer along with ethyl acrylate as a cross-linker.[66] The shell was made by a suspension polymerization combined with solvent evaporation. They found that upon encapsulation the PCM was partially dehydrated to form Na2PO4·7H2O, which resulted in an increase in the melting temperature from 36 to 51 °C. Importantly, upon encapsulation the thermal conductivity increased from 1.01 W·m–1·K–1 for pure Na2PO4·12H2O to 1.426 W·m–1·K–1 for the encapsulated Na2PO4·7H2O. Platte et al. encapsulated different mixtures of sodium sulfate, sodium phosphate, and sodium carbonate that were hydrated by dissolution in water, and the shell was formed by surface-thiol Michael addition polymerization.[67] Schoth et al. developed a surfactant-free method to encapsulate sodium sulfate decahydrate.[68] They utilized the Pickering emulsion technique to create the initial emulsion. It was shown that Na2SO4·10H2O could be encapsulated up to 20 wt %, which is its solubility limit in water. Strong interface interactions between core and shell materials can influence the heat uptake/release of the capsules, similar to that observed for the encapsulation of organic PCMs.[69] A relatively large specific surface area of the encapsulated crystallohydrates accelerates the nucleation of hydrated salts, leading to a reduction in subcooling.[70]

Conclusions and Outlook

Research on the nanocontainer-based self-healing coatings was started in 2005 in the Department of Interfaces, Max Planck Institute of Colloids and Interfaces under the guidance of Prof. Helmuth Möhwald. Since that time, this topic has attracted considerable attention around the scientific world with >1500 publications during the past decade. This approach is very versatile and can be applied not only to autonomous self-healing coatings but also to other multifunctional responsive materials.

The main advantage of self-healing coatings is their autonomic response to corrosion or any other defects in the coating. After the damage is terminated, the release of the encapsulated active agents stops and the coating restores its characteristics. The current main challenge is the transfer of the research achievements in the self-healing coatings to the commercialization level. This research work requires close collaboration with paint producers to adapt developed nanocontainers to the commercial paint formulations and pass all industrial test requirements.

Another great opportunity is to extend a self-healing approach to the multifunctional nanocontainers able to encapsulate several active materials and respond to different triggering impacts. This can impart to bulk systems additional functionality (such as demonstrated antifouling or thermoregulation) responding to various environmental changes. Much efforts has been expended to focus on the mechanisms of bulk matrix–shell and nanocontainer–nanocontainer interactions.