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This review will present a comprehensive view of the use of microcapsules in coatings. Beginning with the general self healing strategy, this paper will discuss the traditional repair methods used in polymers. It will then continue to summarize the recent advances in the field of smart materials pertaining to applications in coatings. The review summarizes the work of many research groups across the globe and presents the recent advances in the coatings industry with the incorporation of these smart microcapsules capable of performing various protective functions to increase the life time of the coatings chosen for the selected area of application.
Fracture mechanics in polymers
In situ curing of the resin
Man's dependence on materials has been everlasting, which is evident from the representation of these materials in the names of the ages such as the Bronze Age, Iron Age etc., thus making material science one of the oldest and the most dynamic disciplines of science. Such dependence on materials has not only led to inventions that satisfy human needs but also has raised expectations for materials that do much more. One such expectation is the ability to autonomously respond to changing conditions. The advent nanotechnology seems to be one of the ways to meet this rising expectation. 'Smart' or responsive materials are materials that respond to external stimuli. The stimulus can be electrical, magnetic, light, pH etc. The inspiration for responsive materials has been derived from complex biological processes in humans and animals[2-4] and mimicking them . One such example is the micro-vascular style delivery of the healing agent which continuously monitors the 'health' of the system.
Smart materials are classified into various types depending on the type of stimuli they respond to and the action that they perform in response. One such application of responsive materials is in self healing of organic coatings. "Self healing can be defined as the ability of a material to heal (recover) damages automatically and autonomously without external intervention". Microencapsulation, a process by which a micron sized solid particles, liquids or gases are incorporated an inert shell enables the isolation and protection of the material from the environment[8-9]. Encapsulation techniques have long been used for a vast array of applications. In 1930s Barrett Green studied colloidal chemistry and created the first gelatin microcapsules by coacervation technique which was later used to create a colorless dye based paper which upon application of mechanical pressure using a pen would release visible color. The use of polymers as encapsulating materials came to existence in the early 1960s, when the need for materials to act as a rate controlling device or a container began to gain importance. Release technology is classified into various types depending on the kinetics and mode of release. The various types are (a) Erodible devices that disappear; (b) Membrane encapsulated reservoir devices; (c) Matrix devices; (d) Reservoir devices without a membrane. Pharmaceutical industry has been the pioneer in the use of this technique to protect drugs from the acidic environment of the stomach and target the drug to a specific area in the body, or provide sustained release of the same [13-15]. The food industry uses microcapsules to encapsulate flavors, retain aroma, increase product shelf life and protect ingredients from oxidation [16-17].
There are very few materials that are distinctly visible and yet so unnoticed and paints (coatings) is one of them. Coating is generally defined as material which when applied to a surface appears as a continuous film. However, the application of such a protective layer on a material of choice is also considered coating. Paints, a type of coating, are dispersions of various types of binders, volatile components, pigments and additives, formulated for a specific application. Coatings are multilayered systems which generally contain a top coat and a primer. Coatings are mainly applied to surfaces for decorative and protective purposes. Functional coatings are coatings with the same properties as that of regular coatings with additional capabilities. These additional capabilities include antifouling, self cleaning[19, 21-22], scratch resisting[23-24], antibacterial, antigrafitti, and anticorrosive[27-28] functions. Incorporation of various components into coatings makes such coatings functional. By definition, metal rich primers are functional coatings too. However, functional coatings also have the additional feature of sensing damage. Functional coatings also fall under the category polymer composites.
2. Fracture mechanics in polymers
Polymer composites have found use in many structural materials and damage due to chemicals, heat, environment etc is a common occurrence. Crack propagation, is the most common method of failure and mechanics of failure has been modeled and researched [29-31]. For a crack to propagate, the energy supplied to the system must be greater or equal to the energy required to create new surfaces on the material. The crack growth models that have been developed are mostly based on the parameter called KI [33-34]. In case of the model which involves the crack opening failure growth, KI is related to the crack depth, material geometry and the applied stresses. The crack growth takes place when the maximum stress intensity factor is reached (KIQ). During fatigue-type damage, crack propagation is related to both the change in KI Âand the maximum stress intensity factor (KImax) during cycling. Thus, for a surface to self-heal the fractured surfaces need to be resealed or crack growth be impaired.
Figure 2.1. Mode I failure mechanism 
Extrinsic crack growth retardation used in self healing generally involves the dissipation of energy away from the propagating crack tip via a mechanical change behind the crack tip. Traditional methods used for healing or repairing include, patching, welding, and in situ curing of new resins.
Welding is the rejoining of fractured surfaces or fusing new materials to the damaged regions of a polymer composite. The physical property of the material is expected to be restored to the virgin state and chain entanglements helps to achieve the same. The process of welding involves various transitions and rearrangements. Once the rearrangements are complete the two surfaces are fused together and are said to be welded. The various factors that need to be taken into account to ensure proper welding are the welding temperature[36-37], surface roughness, chemical bonding between the surfaces and the presence of solvents[40-41]. The use of welding not only has applications for thermoplastic resins but also has been explored for themosets [42-45].
Patching is another technique in which the damaged area is replaced using a new material. The new material is attached via mechanical fastening or adhesive bonding. The approaches used in case of patching include superficial patches or superficial patches after the removal of the damaged material. The extent of recovery is dependent on the adhesion at the interface of the patch and the original material and the thickness of the patch.
2.3 In situ curing of a resin
This technique is very similar to patching however it can be understood as a variant of patching in which a new resin is injected into the damaged area. The uncured resin diffuses into the matrix and holds the patch in place.
Thermoplastics are repaired using fusion bonding and adhesive bonding. Both the methods work very similar to welding and patching. The traditional repairs that have been used are used for external and more accessible damages instead of damages caused due to internal micro-cracks. The self healing polymers thus find applications in this area.
3. Material design
Degradation, be it natural or artificial, in materials is inevitable. Commonly used design principles include damage prevention and damage management. On a plot of damage level vs. time, for current man made materials, the rate of damage formation is either zero or positive at all times.
The approach where the materials are designed in such a way that they have a higher yield point and the rate of damage is less are the goals of the damage prevention approach. The alternative approach to that of damage prevention is the damage management approach. It is based on the concept that the formation of the damage is not problematic as long as the damage is counteracted by a so called "healing" step which restores the material back to its pristine state.
(a) (b) (c)
Figure 3.1: Schematic of three types of self healing materials (borrowed from reference ).
Figure 3.1(a) shows a single stage healing which repairs the damage created and introduces a negative d(damage)/dt and the Figures 3.1(b) and (c) show two and multiple recoveries respectively. However, even though the damage level decreases after each recovery step, the recovered state is not where the pristine material would be. It can also be said that the material is self healing but only for a finite time. The negative damage formation could be mathematically described as,
The final material properties and material lifetime hence depends on the rate of damage formation and rate of recovery. Self healing is essentially a process in which new matter would have to be created in place of the matter that has been removed due to various failure mechanisms. Thus the mobility of the called healing agents must be a parameter that needs to be taken into consideration. Moreover, not only should the healing agents be able to reach the site of damage but also be able to heal the site through various processes such as chemical reaction, absorption etc. Thus to have such a system, the presence of a trigger or a stimulus becomes essential. Another major area to focus is the range or the area around which the self healing is pronounced. The damage recovery at different size scales provides insights into the area and the recovery efficiency. There are several types of stresses in a system, which eventually lead to crack formation.
Figure 3.2. Damage escalation and Damage recovery in a self healing composite 
Damage recovery in coatings follows a micro-meso-macro hierarchy. Since damage recovery in organic coatings involves material transport, the material needed for recovery purposes can be transferred from the bulk of the coatings or from the bulk of the three interfaces (air, substrate or filler). Polymeric materials in general permit limited material transport and hence the material transport across a layer of organic coating is minimal. Also the timing of recovery becomes important, as the increase in the damage scale requires more material to restore it to the initial state. Thus, the idea is to be able to minimize the scale and hence minimize the amount of material required. This concept is also referred to as preemptive healing. Figure 3.2 summarizes the various length scales and the ease of recovery at various stages.
The quantitative methods to calculate the healing efficiency proposed by Wool and O' Connor has been summarized by the following equations.
where R, Ïƒ, Îµ, E and I are the recovery ratios relating to fracture stress, elongation at break, fracture energy and molecular parameters respectively. A comparison between an ideal and a more practically achievable self healing material shows that we have a long way to go before such materials could be available to use in day to day life.
Table 1: Properties of ideal self healing materials.
Ideal self healing materials
More practical self healing material
Heals damage multiple times
Atleast a single recovery
Heals defects of any size
Applicable to small defects
Complete recovery of the material
Partial damage recovery
External assistance required for healing
Equal or superior properties as compared to the current materials
More or less equal to the current materials
Cheaper than current materials
Cost is more yet acceptable
4. pH stimulus
Encapsulation technique also finds an application in the area of corrosion inhibition. The use of microcapsules for the corrosion inhibition is gaining a lot of importance. When a metal corrodes there are two reactions that occur simultaneously - oxidation and reduction. The oxidation site is where the electron is released and the reduction takes places where the released electron is consumed. The site where the oxidation occurs is also referred to as an anode and the reduction site is called cathode. Referring to these sites in terms of pH we can say that the anode is acidic and the cathode is basic. This principle is used in designing of the pH sensitive microcapsules. The use of materials that undergo bond cleavage in the presence of hydroxyl ions makes the microcapsules sensitive to pH. The incorporation of a corrosion inhibitor in such microcapsules causes the cathodic reactions to stop and hence inhibits corrosion.
Self assembly of opposite charged polyelectrolytes is often times used for the fabrication of thin films by Layer-by-Layer technique[54-55]. Recently, polyelectrolyte assemblies have been employed as containers to store the corrosion inhibitor in protective coatings. The distinct advantages of using this technique is the isolation of the inhibitor and providing on demand release of the inhibitor in response to changing environmental conditions. There also other more obvious disadvantages to this too, compatibility with the coating is one of them and size to ensure uniform distribution within the coating. Change in local pH as a trigger is again preferred because corrosion reactions generally involve change in pH in the anodic and cathodic areas. Zheludkevich et al. [57-59] demonstrated the use of SiO2 particles coated with poly(ethylene imine)/poly(styrene sulfonate) (PEI/PSS) as nanocontainers embedded in a epoxy functionalized ZrO2/SiO2 sol-gel coating. They also demonstrated the used of halloysite nanotubes with corrosion inhibitors. The corrosion performance of the synthesized PEI/PSS nanocontainers was evaluated visually and using Electrochemical Impedance Spectroscopy. To begin with aluminum 2024 samples were coated with the sol-gel matrix with benzotriazole (corrosion inhibitor) dissolved directly in the sol-gel matrix and the nanocontainers. It was observed that the coatings containing the nanocontainers showed better corrosion resistance against sodium chloride solutions of various concentrations when immersed for 14 days.
Figure 4.1 Aluminum 2024 alloy coated with sol-gel coatings containing (a) benzotriaole 14 days immersion in 0.005 M NaCl (b) nanocontainers 14 day immersion in 0.5 M NaCl.
EIS results of the Aluminum 2024 samples after 190 h of immersion in 0.005 M NaCl show that the undoped hybrid coatings with different concentrations of the nanocontainers show similar behavior beyond 0.1 Hz and the measured capacitance was found to be similar as well. The sample containing the highest concentration of the nanocontainers showed almost pure capacitive behavior at low frequencies and hence seemed to demonstrate good corrosion protection even after long immersion times. The lower nanocontainer concentration coatings showed a good corrosion performance however, formation of resistances at low frequencies show active defect formation. The undoped sol-gel coatings show the lowest corrosion resistance. It was also seen that two other samples prepared with direct impregnation of benzotriazole showed lower resistances.
Figure 4.2 Impedance spectra of different sol-gel films after 190 h of immersion in 0.005 M NaCl.
SVET method was also used to measure the fluxes of cations and anions on the surface. Figure 4.3 shows the SVET map of undoped ZrOX/SiOX film and the ZrOX/SiOX film with maximum nanocontainers. After 24 h of immersion artificial defects (200 Âµm dia.) were introduced on the coatings. A well defined cathodic activity was observed in the area of the defect which became more intense with immersion time in case of the undoped ZrOX/SiOX film. However in case of the ZrOX/SiOX film doped with nanocontainers some cathodic activity was observed after 24 h of the defect formation. 2h later the defect was found to have been passivated and the defect remained "healed" even after 48 h.
Figure 4.3 SVET maps of ionic currents measured above AA2024 panels with undoped sol- gel ( a,c,d,e) and with pretreatments impregnated by nanocontainers (g-i). The maps were obtained before defect formation (a) and for 4 h (c,g), 24 h (d,h) and 48 h ( e,i) after defect formation. Scanned area: 2 mm X 2 mm
Multi-walled carbon nanotube (MWCNT) embedded microcapsules have also been fabricated by LbL self assembly technique and their electrochemical behavior evaluated. The use of MWCNT is favorable due to its low cost compared to single walled CNT. Figure 4.4 illustrates the fabrication of the MWCNT embedded microcapsules.
Figure 4.4 Representation of LbL assembly of MWCNT embedded microcapsule. PAH - poly(allylamine hydrochloride), PSS - Poly(sodium 4-styrenesulfonate).
Figure 4.5 TEM images of (PSS/PAH)/MWCNT microcapsules before (a,b) and after (c,d) dissolving the core, (e) hollow (PSS/PAH) microcapsules. SEM images of (PSS?PAH)/MWCNT microcapsules before (f,g) and after (h) dissolving the core at different magnifications.
Cyclic voltammetry was conducted on the (PSS/PAH) MWCNT on a GC electrode using a phosphate buffer at pH 6.86 at different scan rates and a well defined reversible voltammogram was obtained at -0.05 V. Increase in scan rate lead to shifting of the peaks a little and redox current linearly scaled with the increase in the scan rate. Since the (PSS/PAH) in the GC electrode could not exhibit redox peaks, the observed peaks were attributed to the MWCNT. The change in the behavior of the MWCNT with changing pH was also studied and was understood that by increasing the pH the peak potential decreased due to the decrease in the concentration of protons at higher pH.
Figure 4.6 Cyclic Voltammogram of (PSS/PAH)/MWCNT microcapsules at different scan rates. Reductions and oxidation peaks as function of scan rates and (a,b,c) as function of pH at 0.1 V/s.
Thiourea has also been used as an inhibitor and has been encapsulated in glutin and polyvinyl alcohol (PVA). Polarization and EIS was performed on the samples after the addition of the microcapsules every 6 h. The release rate was measured using UV vis spectrophotometer and it was seen that the release time for microcapsules that were sealed once with gluten (#1) was 12 h, the time for the microcapsules sealed once with PVA was 18 h and the time for the ones that were sealed twice with PVA was found to be 48 h. Thus the use of PVA delayed the release rate over glutin. With the EIS study it was observed that the diameter of the curve in the Nyquist plot increased with time and reached a stable value once all the inhibitor was released. The corrosion rate decreased with release of the microcapsules and remained constant after the complete release of the inhibitor.
Figure 4.7 Concentration of thiourea and time in 0.12 L 0.1 M sulfuric acid solution as measured using UV spectrophotometric method.
Other functional capsule membranes have be also been designed that have capabilities to release large dyes in response to the changing pH of the outer medium. Porous nylon capsule membrane with surface grafted-polyelectrolytes have been designed which respond to external pH. The grafts act as permeation valves change in their conformation in response to the pH. The permeation of NaCl and water soluble naphthalene molecules was studied. Large nylon 2, 12 capsules were synthesized using ethylenediamine and 1,10-bis(chlorocarbonyl)decane by interfacial polymerization and the vinyl groups were grafted on to the surface. The permeation measurement was made by detecting the electrical conductance of the surrounding solution. The plot of specific conductance of different types of capsules shows the effect of grafting on the permeation rate. It was shown that the poly(vinly pyridine) (PVP) grafted capsules showed faster NaCl permeation in a pH 2 over a basic environment of pH 12. When poly(methacrylic acid) was grafted a reverse of what was observed for PVP grafted capsules was seen. The PVP grafted capsule membrane formed a higher barrier to NaCl permeation in the neutral pyridine form (pH >7) of the grafted polymer and not in the cationic pyridinium form (pH<6). The permeability of PMA grafter capsule was decreased when the polymer was in the neutral carboxylic acid form (pH <5) and anionic carboxylate form (pH>6). The ungrafted original capsule did not show any signs of permeation at the whole pH 2-12 range. When the graft polymer is in the ionized form, the polymer chains are repelled by charge repulsion between ionic side chains or by hydrophilic properties which allow easy permeation of NaCl.
Figure 4.8 Schematic of the structure and functional nylon capsules.
However, when the graft polymer is in the neutral form, the porous membrane was found to be covered by the entangled polymer, hence reducing the permeation significantly. It was concluded that the permeability controlled pH can be selected by the graft homo- or copolymers having dissociative side chains. The amount of NaCl permeation was also found to be dependent on the hydrophobicity of polymers. The valve of the graft polymers was also found to be dependent on the temperature or redox reaction  in addition to the pH changes. One very similar study was done on a biomimetic membrane capsule used for drug delivery.
Figure 4.8. (a) pH sensitive permeation of NaCl responding to ambient pH changes at 25 Â°C (b) pH- rate profiles of NaCl permeation at 25 Â°C. In both the plots (a) ungrafted capsules (b) PVP capsules (c) OMA capsules.
Akamatsu et al.  developed a novel method to obtain pH responsive core shell microcapsule reactors using plasma graft polymerization. The pH responsive gating function was achieved using a copolymer of N isopropylacrylamide and acrylic acid. The microcapsules that were formulated were filled with glucose oxidase using the "bottle in" method. The responsiveness desired was achieved using the gating functionality of the copolymer formed in the pores of the shells using the plasma grafting technique. Figure 4.3 illustrates the mechanism of operation of the microcapsule reactors. The polymer in the pores, act as pH gates. The polymer hydrates at pH 5 and dehydrates at pH 4. Thus the microcapsules are designed such that they have a high rate of reaction at a high pH due to the increase in the diffusivity across the membrane. The FTIR spectra of the samples that were made showed characteristic peaks at 1650 and 1550 cm-1 which are characteristic of N isopropylacrylamide (NIPAM) and at 1720 cm-1 which is characteristic of acrylic acid (AA) confirming the formation of the copolymer of NIPAM and AA.
Figure 4.9 Schematic representation of the pH responsive microcapsules containing glucose oxidase (GOD).
Figure 4.10 FE- SEM micrographs of core shell microcapsules showing (a) the general morphology (b) cross section (c) outer surface and (d) inner surface.
The performance of these microcapsule reactors was done by monitoring the amount of oxygen as it is the substrate of the reaction and it was seen that at a fixed temperature (40Â°C) the reaction rate at pH 5 was 2.7 times higher than at pH 4. It can be explained by the fact that at pH 5 the pH responsive NIPAM - co - AA was in its hydrophilic state allowing the glucose to pass through the shell walls and the GOD reactions to take place and at pH 4 due to the hydrophobic state of the copolymer, the penetration of glucose through the pores was not assisted and hence the GOD reaction did not take place.
Figure 4.11 Amount of oxygen consumed by the pH responsive microcapsules.
Another chemistry that has been developed for pH sensitivity is the thiol chemistry. Shirley et al. developed variable release microcapsules to encapsulate insecticides such as chloropyrifos or lambda- cyhalothrin or an herbicide butylate. The disulfide linkages undergo hydrolysis in the presence of hydroxyl groups. This is utilized in the variable release microcapsules.
Figure 4.12 Schematic diagram illustrating disulfide linkages used in pH sensitive microcapsules.
5. Mechanical Stimulus
Structural polymers are bound to be damaged due to formation of cracks and the location of such cracks is generally where the detection is difficult and repair almost impossible. The size and the length scales of such cracks also vary depending on the nature of stresses. The use of rubbery particles or rigid inorganic fillers to enhance the fracture toughness was one the ways to enhance the toughness of the composites without losing the mechanical properties [68-69]. The addition of such components to an existing coating adds more complexity to the coating. Brown et al. showed that there was a 126% increase in the fracture toughness by the addition of glass microspheres into a matrix. Shukla et al.  showed a 200% rise in the fracture toughness using aluminum silicate microspheres. The use of microcapsules in a polymer matrix not only strengthens the matrix but also provide space for some kind of a healing agent. Polymeric microcapsules are mostly prepared using miniemulsion polymerization [72-73] where submicron sized oil droplets are dispersed into a water phase. Majority of the self healing composites use urea formaldehyde walls and dicyclopentadiene as the liquid healing agent. The surface roughness added to the microcapsules synthesized using miniemulsion polymerization allows the dispersion of these into polymer matrices. The particle sizes of the synthesized microcapsules vary from 10-1000 Âµm with inner membranes that are 160-220 nm. The trigger for the release of the healing agent is through mechanical rupture. It is also important that the microcapsules are fabricated with the right wall thickness and with optimum mechanical properties. Keller and Sottos determined the effect of the stiffness of the microcapsules and the stiffness of the surrounding medium and found that the stiffer the microcapsule walls more is the tendency of the crack to deflect away from the capsule. It was also determined that more complaint shell wall tends to attract the crack towards the microcapsules.
Figure 5.1. Stress state in the vicinity of a planar crack as it approaches a spherical filler particle embedded in a linearly elastic matrix. The image on the left corresponds to an inclusion three times stiffer than the surrounding matrix, and the image on the right corresponds to an inclusion three times more compliant than the surrounding matrix.
The size of the microcapsules also plays an important role in the performance of the system. It determines the amount of healing agent available and also the toughness of the system. The microcapsule size is controlled by the rate of agitation during the encapsulation process. It has been reported that smaller the size of the microcapsules greater is the toughness of the system at lower concentrations. In terms of the amount of healing agent delivered and the recovery efficiency it found that larger the microcapsules, more was the recovery due to a larger volume of the healing agent delivered to the system. Since there is a tradeoff between the size and the amount of healing agent available, Brown et al determined that the amount of best healing agent was delivered at 10 wt % of 386 Âµm sized microcapsules which translates to 4.5 mg of healing agent delivered per unit crack area. Blaiszik et al.  reported nanosized capsules as small as 220 nm made using ultrasonication and miniemulsion techniques.
Figure 5.2 (a) SEM image of nanocapsules produced using hexadecane costabilizer (b) TEM image showing the core - shell morphology.
The rate of healing and the kinetics of healing are very important as well because if the rate of crack growth is faster than the healing process itself then no healing would occur. Also since the healing is based on a catalyst aiding the reaction, if the rate of healing polymerization was faster than catalyst dissolution then recovery at isolated regions was observed. However, if the healing agent cured slowly and sufficient time was given, then maximum recovery of mechanical strength was seen. The adhesion effects and the long term recovery effect was seen in case of carefully designed microcapsules.
One of the most thoroughly studied systems is the Dicyclopentadiene(DCPD)/Grubbs' catalyst system. Ring opening metathesis polymerization is initiated by the ruthenium (IV) catalyst which is responsible for the recovery of the damaged areas. White et al. designed such a system in 2001 which has been studied extensively. According to the system, the DCPD is incorporated in urea formaldehyde based microcapsules which are incorporated in a composite. When damage occurs, the crack propagates through the specimen and ruptures the microcapsules. Liquid healing agent would then flow through the crack via capillary action and upon contact with the catalyst would polymerize and fill up the crack restoring the mechanical strength of the material. In this study White et al. observed an increase in the load bearing capacity of neat epoxy. Initially, they also observed that there was a recovery of 75 % of the virgin fracture load. The testing that was used by this group was a tapered double cantilever beam, (TDCB) illustrated in the figure 5.3. Three samples were made - reference samples, self activated samples and self healing samples. The reference sample was the one in which manual injection of the polymer was made. The reference sample showed about 51% recovery which was then optimized to 99 %. The self activated samples initially showed 20% recovery which was later optimized to 73% and the self healing autonomous samples showed 38% recovery and later were optimized to 66% after heating at 80 Â°C. All the percent recoveries observed were average percent recoveries.
Figure 5.3 (a) Autonomous healing concept proposed by White et al. (b) Optical micrograph image of the healing concept in action
Figure 5.4 Schematic representation of the test parameters and the self healing process with in composite materials.
Figure 5.5 (a) Reference specimen where the premixed healing agent and the catalyst are injected into the damaged area (b) Self activated specimen where the catalyst is embedded into the epoxy and the DCPD is injected into the damaged area (c) self healing specimen where both the catalyst and the healing agent are embedded into the epoxy .
The fracture toughness of the epoxy coating is also something to be considered, the addition of the DCPD and the Grubbs' catalyst was found to increase the fracture toughness up to 15%. The use of other monomers in the microcapsules walls was also explored by Sun and Zhang. They used melamine formaldehyde resin, urea formaldehyde resin and gelatin-gum arabic coacervate. They found that the gelatin microcapsules did not show breakage use to compression whereas the melamine formaldehyde and the urea formaldehyde microcapsule showed clear signs of rupture when compressed. Lie et al. explored the other monomers inside the capsule shell. They tested a blend of two monomers as the healing agent while maintaining the desired mechanical properties as achieved by White et al. The polymerization was found to be faster with the addition of ethylene norbornene and was found to be completed using lesser amounts of the catalyst. Different catalysts have also been explored as far as the DCPD polymerization is considered by Wilson et al. An ideal catalyst is required to have rapid dissolution in the healing agent, fast initiation of polymerization, thermal stability, high processing and working temperature and chemical inertness to the surrounding matrix. The rate constants of the first generation Grubbs' catalyst was 1.45E-4 s-1 and 4.3E-3 s-1 for the second order Grubbs' catalyst and the rate was too fast to be measured for Hoveyda - Grubbs' catalyst. The robustness of the catalyst when exposed to the epoxy polymer matrix was also tested. The first generation Grubbs' catalyst was found to turn from purple to brown showing catalyst deactivation. The second generation Grubbs' catalyst showed color change from brown to green whereas the Hoveyda- Grubbs' catalyst was found to show no color change. The second order Grubbs' catalyst showed better thermal stability and hence better healing at 125 Â°C. Alternative to ruthenium catalysts have also been investigated but have found to be expensive and have limited availability. Kamphaus et al investigated tungsten (VI) catalyst as an alternative to ruthenium catalysts however the tungsten based catalysts showed lower fracture toughness and also the healing efficiency was found to be 20 % for an autonomous healing sample with 12 wt% WCl6.
Other routes have also been explored due to the cost associated with the Grubbs' catalyst and the thermal stability. Cho et al. have explored the possibility of poly(dimethylsiloxane) (PDMS) based self healing materials. The siloxane based healing agent was phase separated in the matrix while the catalyst was encapsulated throughout an epoxy matrix. This ensures an even distribution of the healing agent throughout the matrix and the catalyst has a better stability in the system. This system also uses the same idea proposed by White et al. By manual injection technique the upper limit of the healing was determined to be 24% for 3.6 wt % microcapsules. Following up with this study, Keller et al.[88-89] proposed a new matrix that consisted of the PDMS instead of epoxy. The system that he proposed consisted of two separate types of microcapsules (1) high molecular weight vinyl functionalized PDMS and platinum catalyst complex (2) PDMS copolymer with active sites that would link to the vinyl functionalized PDMS. With this system 75% healing efficiency was obtained with 10 wt% resin and 5 wt% initiator capsules. Interestingly, some samples showed an efficiency of 100% or greater because of the fact that the matrix was similar to the healing agent and hence the crack could propagate via different pathways.
Following the same train of thought, scientist has also tried incorporating epoxy/hardener microcapsules into an epoxy matrix. Yin et al. were the first to try the epoxy hardener system which contained an uncured epoxy resin and imidazole-metal hardener. 30 - 70 micron size particles were obtained and urea formaldehyde was used as the shell wall material. Even though the incorporation of the two in an epoxy matrix increased the fracture toughness, the system in use was not fully autonomous. The imidazole metal hardener requires a temperature range of 130Â°C. However, the healing efficiency for this system was found to be 111 % using 10 wt % epoxy microcapsules and 2 wt% latent hardener. Yaun et al. designed a fully autonomous system with epoxy resin as the healing agent and mercaptan as the hardener.
Figure 5.6. Size distributions and SEM images of (a) epoxy loaded capsules (b) mercaptan loaded capsules.
The maximum healing was found to occur when the ratio of the microcapsules was kept at 1:1 and was found to level off with the addition of 5 wt% microcapsules. Efficiencies of over 100 % were achieved and reason for this was found to be similar to that of the PDMS based microcapsules. It was also seen that the healing could be achieved at a lower temperatures of the order of 0 - 10 Â°C with an efficiency of 86% thus broadening the operating temperature range of these capsules. The diameter of the particles that have been synthesized could also be controlled by adjusting the pH, concentration of the surfactants, the time and the heating rates during the microencapsulation process. More chemically resistant and stable polythiol capsules have also been encapsulated in melamine formaldehyde.
Isocyanates are the latest addition to the chemicals that have been encapsulated which can react when exposed to humid or wet environments to mimic healing. Isophorone diisocyanate has been encapsulated and has shown stability over time. Healing by many solvents [40-41, 95-96] has also been one of the not so explored areas. Lin et al. showed that solvent could be used to promote healing during the wetting and diffusion stages. Wetting here means the swelling of the bulk polymer and the interlocking of the polymer chains across the defect. It was determined that by dissolving the polymer in various solvents the glass transition temperature could be lowered allowing the material to heal at a lower temperature. Caruso et al incorporated solvent capsules in a polymer matrix and determined that the healing efficiency was directly proportional to solvent polarity in the dielectric constant range on 32-47. It was also determined that the solvents were difficult to incorporate in the urea formaldehyde capsules however chlorobenzene was successfully incorporated and showed efficiencies of the order of 86% for 20 wt% of the capsules.
Microcapsules with oil core have been synthesized fairly on a large scale and researched extensively. However, another challenge in the field of incorporation is formation of water core microcapsules. Loxley et al. synthesized poly (methylmethacrylate) microcapsules with water to be core liquid. The water core microcapsules are fairly difficult to synthesize as the spreading coefficients are to be taken into account while formulating the emulsion. The various particle morphologies can be obtained with different spreading coefficients. Torza and Mason  have investigated the equilibrium morphology adopted by droplets of immiscible liquids (phases 1 and 3) when brought in contact with a mutually immiscible liquid (phase 2). The three possible combinations as stated by them are as follows,
where S is the spreading coefficient of the given phase. If the conditions in eq. 1 are satisfied then core shell morphology were obtained, the eq. 2 gave rise to "acorn" shaped particles and the eq. 3 gave two separate droplets.
Figure 5.7 Four possible two phase morphologies (a) core shell (b) occluded (c) acorn (d) hetroaggregate.
The technique used for the encapsulation of water in the cores of the polymer capsules is achieved using a technique known as internal phase separation. An o/w emulsion was prepared which consists of the oil phase, the mixture of the polymers, high boiling poor solvent and the low boiling poor solvent. A suitable emulsifier was used to disperse the mixture and form an emulsion. The mixture is then subjected to reduced pressure and elevated temperature to remove the good solvent. Thus polymerization takes place around the non-solvent and hence encapsulates it. Tiarks et al.  used the similar technique and reported the formation of nano sized particles. Since water can be encapsulated a large array of water soluble compounds could also be potentially incorporated using this technique.
Figure 5.8 Schematic steps involved in the internal phase separation process. N.v.n.s - nonvolatile nonsolvent v.s- volatile solvent
Figure 5.9. Electron micrographs of microcapsules produced from pre emulsions containing no acetone in the oil phase.
6. Photo stimulus
Photoinitiated cycloaddition reactions which undergo cyclization upon irradiation of a certain wavelengths of light and cleavage upon irradiation with certain other smaller wavelength of light are used in the photo initiated damage recovery. Chung et al.  incorporated cinnamoyl groups into a polymer system leading to photochemically induced healing systems. The polymer networks can also be crosslinked via photochemical cycloaddition of the cinnamoyl groups to form cyclobutane dimers. Due to the high ring stain in the cyclobutane groups, it was assumed to break when the crack propagated within the specimen. When wavelengths greater than 310 nm was shined on the specimen the cyclobutane crosslinks were formed hence restoring the mechanical strength of the specimen. IR spectroscopy was used to confirm the same. The highest efficiency that was around 14% and it was found to increase to 26% when both photochemical and thermal healing was used together.
Another interesting development recently reported by Pastine et al.  was the phototriggerable microcapsules. The phototriggerable microcapsules contained optothermally active species, such as carbon nanotubes, which could rapidly heat up the liquid content and cause rupture due to increase in the internal pressure. The microcapsules in the range of 100-100 microns were synthesized which could be released optothermally by using a laser irradiation to release the incorporated toluene. The duration of exposure was a low as 0.02 s.
Figure 6.1 Toluene filled microcapsules containing 1 wt% CNT. (a) Optical image of the scintillation vail (b) Optical image of the microcapsules in oil (c) SEM image of the microcapsules (d) SEM images of crushed microcapsules with visible interiors.
Self healing using microcapsules has become one of the most interesting and ever growing areas of research with new and more interesting publications every year. Microencapsulation has proved to be a very successful and useful technique in the fields of pharmaceuticals sciences and has found many applications in the field of organic coatings as well. Even though the technology has been explored a lot in many fields of science the area of organic coatings still remains untouched. Thus, the use of this technique for many different applications such as anticorrosives, mechanical strength healants etc. are still to be explored. While some methods in this field are very well studied and understood there are new ones that need to be understood. Some of the well known methods are now looking at optimization and commercialization whereas the ones that have been discovered now will someday be commercialized. The use of various stimuli to trigger and use of various materials to make cheaper and more versatile microcapsules are the driving forces of this field of science. Some of the challenges such as thermal stability and ease in application are areas that require attention. As researchers deepen their understanding of various techniques and broaden the number of attainable materials the field of encapsulation grows.