Polyurea micro-/nano-capsule applications in construction industry: A review

2.2.1 Bulk emulsion method

Bulk emulsion technique is one of the common methods for the synthesis of micro-/nano-capsule. Figure 3 demonstrates the step-by-step formation of polyurea micro-/nano-capsules [35]. First, oil droplets of isocyanate have been formed inside water by the agitation of the mechanical stirrer, then diamine is added to the water and dissolved, finally polymerization occurred at the interface of the oil droplets. Besides, there are some studies that used organic phase as continuous phase. Surfactants are employed to provide emulsion through decreasing droplet diameter to micro-/nano-size [54]. The amount of dispersed phase, the ratio of TDI to HDI, PH value, and temperature are important parameters for the synthesis of well-shaped and defect-free micro-/nano-capsules of a desirable size. Also, the diffusion of the solvent into aqueous phase can avoid the appropriate encapsulation [24]. A model also was derived for predicting the crystallinity and molecular weight of polyurea formed in micro-/nano-capsule preparation. Regarding the capability of the current technology, the quantity of polyurea micro-/nano-capsule production via bulk emulsion technique is higher than microfluidic, yet there is a considerable portion of not encapsulated particles observed in the final product. This item plays a key role in the manipulation of microcapsules in construction industry where cargo leakage may lead to decrease in the life of the building materials [21,55]. Although studies showed that parameters, including emulsifier percentage, temperature, and stirring rate determine the particle size of micro-/nano-capsule, the precision of controlling shell thickness is lower in comparison to the microfluidic device. Bulk emulsion with continuous process has been studied for the production of polyurea micro-/nano-capsule, in an appropriate condition, this process led to produce 198 g (capsule)/h [56]. However, the process encountered some limitations; the higher micro-/nano-capsule concentration in the emulsion resulted in more collisions between droplets and the broad range of micro-/nano-capsule size. Therefore, the optimization of flow rate and monodispersed particles are required. This process was then compared with microfluidic method. The flow rate was 0.26 L/h in a recirculation loop reactor and 0.028 L/h in a similar microfluidic device, yet polydispersity is 39.4% in the loop reactor against 2.8% in microfluidic, which indicates the higher size variation in the particles.

Figure 3

Preparation of polyurea micro-/nano-capsule via bulk emulsion technique.

The broad size distribution of droplet is obtained through bulk emulsion technique. The effect of the reactivity of the diisocyanate and diffusivity in different sizes of nano-capsules have been studied. Droplets with 83 nm diameter have been provided with ultrasonication [49]. Results of mathematical model confirmed by experiments showed that reaction of higher reactive diisocyanate underwent diffusion control process, whereas lower reactive isocyanates followed kinetics of reaction. In the bulk emulsion method increasing the agitation rate can lead to production of micro-/nano-capsules with smaller mean size. In a study [31], micro-/nano-spheres with the average diameter of 10.74 ± 0.94 µm and distribution width of 2.27 decreased to 4.20 ± 0.32 µm and 0.93, respectively, by increasing the agitation rate from 6 × 10³ to 12.0 × 10³ rpm. Additionally, rotating packed bed have been used for the preparation of polyurea micro-/nano-capsules [57]. In this method a high gravity is applied by the centrifugal force. The mean diameters of polyurea micro-/nano-capsules provided by this technique were about 11.88, 12.31, and 11.83 μm for 1,200 rpm.

2.2.2 Microfluidic method

Another method for preparing polyurea microcapsules is using microfluid devices, which can be fabricated through different techniques. Similar to the emulsion method, isocyanate is dissolved in oil phase and diamine in water phase. Oil microdroplets are then formed through injecting the oil phase into the continues phase (water) via a capillary tube or channel. There are several techniques for droplet formation in microfluidic devices such as terrace-like, cross-flow, co-flow, and focusing flow which are shown in Figure 4. Among the materials employed for making microfluidic chip, PDMS with planar shape and glass capillary with 3D geometry are the most commonly used. Also, the focusing flow technique is widely used for composite droplets, e.g., core–shell droplets. As shown in Figure 5, the device includes a set of capillary glass tubes and a controllable pumping force to fabricate microcapsules. By the aid of the microfluidic devices, microcapsule with a broad range of thickness and diameter can be provided [58]. As the permeability, stability, and releasing function of microcapsules are highly dependent on their size [59] narrow size distribution is required for their maximum performance [60]. Microfluidic method can offer more control over microcapsules’ preparation and provide uniform droplet size with narrow dispersity [61]. Although controlling over the mean size and wall thickness of polyurea micro-/nano-capsules is higher in this technique, industrial scaling of microfluidic methods is still the subject of many recent research studies [62]. Different flow rates of encapsulated phase were compared in various methods of microfluidic and bulk emulsion. High throughput glass microfluidic device, microfluidics, microchannel emulsification, and micromixer possessed 0.004, 0.0015, 0.025, 0.06 L/h, respectively. In contrast, bulk emulsion methods showed higher yields. Flow rates of encapsulated phase for membrane emulsification with porous glass, static mixer in the recycled loop, and recirculation loop reactor were 0.9, 11.94, and 0.26 L/h, respectively. It seems that in comparison to microfluidic method, emulsion methods have higher production yield in industrial scale. However, microfluidic methods are more desirable for production of monodisperse hybrid shell with two or three layers, due to the possibility of controlling liquid flow and then collecting the emitted microcapsules in the capillary [63].

Figure 4

Different method of creating micro-/nano-capsules by microfluidic devices [64]: left 2D PDMS devices and right 3D glass capillary.

Figure 5

(a) Schematic figure of emulsion drop generation by focusing flow and (b) real micro-/nano-graph of the glass capillary during the micro-/nano-droplet formation [58].

Polyurea monodisperse microcapsules were prepared by controlling the flow rate of water phase and oil phase in microfluidic devices [58]. MDI dissolved in limonene, used as oil phase, TEPA, SDS, and NaCl were dissolved in water phase. Oil droplets were formed in the microfluidic device by the aid of an air pump. In another study, polyethylenimine and TEPA, as well as TDI were used to study the effect of different surfactants in the microfluidic device. Osmotic driven inflation of 0–1,740 kPa resulted in 66–125 µm microcapsule mean size with thickness of 80 to 120 ± 12 nm. Results show that increasing the HLB (hydrophilic to hydrophobic) ratio of the surfactant will enhance the stiffness and permeability of the polyurea shell. It was also found that the more solubility of the amine in the oil phase, the thicker the polyurea shell would be [50].

Bulk emulsion technique possesses poor control over the size of micro/nano-capsules and the distribution size due to the formation of preparing droplet with homogenizer. Conversely, microfluidic devices use adjustable high shear force to fabricate droplets out of the two immiscible flow streams. This provides higher control over size and dispersity [61]. According to the results, at the constant continuous flow rate (Q _c) of 100 µL/min and altering the flow rate of disperse phase (Q _d) from 1, 2, 3, 4, and 5 µL/min, polyurea microcapsules with mean sizes of 27, 30, 32, and 34 µm were obtained. Figure 6 compares the narrow results of polyurea microcapsules size dispersity prepared by microfluidic method at Q _d = 5 µL/min to a typical outcome of bulk emulsion technique. The broad and bimodal distribution of microcapsules’ size in the range of 0–16 µm has been obtained for the bulk emulsion; whereas, the microfluidic method endowed microcapsules with uniformity and narrow dispersity between 32 and 36 µm.

Figure 6

Size distribution of emulsion droplets produced by standard and microfluidic methods. (a and b) Optical microscopic images of o/w emulsions produced by (a) homogenizer and (b) microfluidic chip (Q _c = 100 μL/min, Q _d = 5 μL/min). (c and d) Histograms of droplet size distribution from (a and b) analyzed by ImageJ for (c) homogenized and (d) microfluidic chip produced droplets (Q _c = 100 μL/min, Q _d = 5 μL/min) [61].

3.1.1 PCM

PCMs are nowadays considered as a useful technology for energy saving of buildings. These materials show a higher potential of commercial application in construction industries and several studies investigated the types of materials with the capability of melting and solidifying during the day–night temperature fluctuation. PCMs use the latent heat of reversible crystallization that happened during the liquid-to-solid or solid-to-solid phase transition. Paraffin, n-hexadecane, and tetradecane are some prevalent PCMs. In addition to having an appropriate melting temperature, PCMs have two major characteristics: low expansion during the phase change and large surface area per unit volume, resulting in higher efficiency of heat transfer. Although PCMs have directly been used in passive solar panels [85], micro-/nano-encapsulation provides many benefits for PCMs construction applications such as preventing from reaction with their environment, increasing the compatibility with the media, and enhancing the heat transfer through more surface-to-volume ratio [78]. Considering the requirements, many parameters could be a candidate as material for micro-/nano-capsule shells. The heat absorbing and release rate are related to the type and thickness of the shell wall, ensuring the energy efficiency of the core–shell and the construction material environment. This parameter handles rapid thermal response. At the same time, the mechanical properties of the shell should prohibit any rupture and fracture against production or application processes [80]. Polyurea satisfies these requirements and provides possibilities for designing the required thickness with aforementioned desirable properties [21]. Further improvements on materials of polyurea micro/-nano-capsules have been performed and PCMs were encapsulated through polyurea with GO and hybrid shell. Besides, the method of synthesis has a key role on the properties of the resulting materials where using a rotating packed bed provided well-shaped micro-/nano-capsules with higher mechanical properties and more efficient energy storage. Unique properties of shell produced by polyurea containing paraffin resulted in the fabrication of PCMs and its insulation properties in the presence of solar radiation. In order to increase the thermal conductivity of the micro-/nano-capsules and decrease the strength loss in the resulting building material, flaky graphite-doped polyurea micro-/nano-capsule was also prepared [79]. Results showed that these materials not only lower the temperature of the wall and indoor of the test rooms by a maximum of 13.1 and 6.22°C, but also the flexural and compressive strength of the resulting cement composite was in the satisfying area of standard building materials (14.2 and 4.1 MPa, respectively).

In another study, the application of polyurea microcapsules in gypsum board increased the temperature of the board surface by 2, 4, 5, and 7°C at the radiant flux density of 200, 400, 600, and 800 W/m² [76]. Actual core content, the ratio of fusing enthalpy of polyurea to pure PCM, and the encapsulation efficiency [86] are main parameters of evaluating the amount of energy saving. Besides, mean diameters, distribution sizes, and wall thickness should be adjusted for the maximum energy saving efficiency considering the required mechanical properties [72].

Thermal properties of PCMs with polyurea shell have been studied. The actual core content and the encapsulation efficiency were 42.5 ± 0.5 and 85.0 ± 0.92, respectively, which reached to 79.60 ± 0.17 and 63.67 ± 0.14 by increasing the amount of PCMs in the process of encapsulation. In another study, titania-polyurea microcapsules were used to produce PCM and applied in cement paste [78]. The schematic picture of hybrid polyurea shell is depicted in Figure 9. Scanning electron microscope (SEM) images also confirmed the second layer of titania-polyurea. The melting and freezing cycles indicate the excellent thermal stability and strength of the polyurea/TiO₂ shell. The amount of octane core increases from 56.5 to 72.8 wt% via increasing the microcapsule sizes from 100 to 300 µm. Energy storage efficiency and thermal storage capacity were 77.3 and 99.9%, respectively. In another study, the crystallization of PCM inside polyurea microcapsules was investigated through the addition of nanoparticles of Fe₃O₄ to the n-octadecane which made them magnetic. The use of microfluidic device for the preparation of microcapsules resulted in the uniform sizes of microcapsules and near 100% efficiency of encapsulation. Figure 10a–c shows the uniform polyurea microcapsules, microcapsules with Fe₃O₄ nanoparticles, and a photograph of microcapsules suspension in a vial, respectively. Their response to the magnetic field is also shown in Figure 10d which indicates that nano particles (NPs) inside polyurea microcapsules were stable after washing. The addition of Fe₃O₄ NPs, led to the increase of the heat flow of PCM microcapsules from 159.5 to 169.7 J/g and from 159.1 to 165.7 J/g in cooling and heating process.

Figure 9

Schematic drawing of (a) PCM micro-droplet, (b) microencapsulated PCMs (MEPCMs) with PUA shell, and (c) MEPCMs with TiO₂–PUA shell. Micrographs of (d) individual MEPCM with PUA shell and (e) individual MEPCM with TiO₂–PUA shell. (f) Fabricated free-flowing MEPCMs [78].

Figure 10

Optical images of (a) PCM@polyurea microcapsules without Fe₃O₄ NPs in the discontinuous phase, (b) PCM@polyurea microcapsules generated by encapsulating Fe₃O₄ NPs along with n-octadecane in the discontinuous phase, captured at the reservoir at 35 °C, (c) photograph of PCM@polyurea microcapsules with encapsulated Fe₃O₄ NPs, and (d) magnetically aligned microcapsules in the image (c) under an external magnetic field at room temperature [82].

Nano-capsules of polyurea/Fe₃O₄ nanoparticles with the size of 400–600 nm were prepared via O/W emulsion method through stirring rate of 8,000 rpm [87]. The presence of Fe₃O₄ nanoparticles provided the magnetic property to the polyurea PCM nano-capsules and elevated the thermal conductivity of the shell. However, increasing the amount of Fe₃O₄ nanoparticles from 0 to 6.6% led to the decrease of the melting and freezing latent heat from 101.1 and 105.6 to 83.28 and 87.4 J/g, respectively, due to the higher density of nanoparticles.

3.1.2 Reversible thermal reaction micro-/nano-capsule

The reversible thermal reaction is another form of energy-storing by polyurea micro-/nano-capsules. Crystalline polyols or sugar alcohol, salt hydrate, and anhydrous salts are capable to release their storage energy when their molecular structure is altered in different temperatures. The preparation and heat release of xylitol encapsulated by polyurea/PU have been studied. Water sorption of xylitol releases negative heat solution of −36.5 kcal/kg at 35°C which is higher than several PCMs [83]. Polyurea micro-/nano-capsules with mean pore size of 19.6 ± 1.7 and 15.3 ± 2.0 µm and encapsulated xylitol of 15.1 and 77.1% showed 24.4–124.3 J/g heats of dilution at 35°C, respectively. The yield of shell formation increased from 16.2 to 30.9% by increasing the ratio of MDI from 16.6 to 28.5%. In another study, polyurea micro-/nano-capsules with xylitol core and mean size of 19.3 ± 2.7 μm which had dilution heats of 13.9–179.9 J/g were obtained [84].

Other energy saving materials using the reversible chemical reaction are organic photochromic compounds which can be divided to chemical bond cleavage, cis–trans isomerism, and pericyclic reaction type. These materials can raise the energy saving ability in PCM microcapsules. One of the common chemical bond cleavage types is spirooxazine. The energy storage mechanism of spirooxazine relays on the ring-opening reaction of its C–O bonds where it transformed from a colorless state to its color isomer anthocyanin at the exposure to ultraviolet radiation. In addition, the ring-opening anthocyanin structure had a large conjugated molecular system. Due to the stability of spiral form, the molecule tends to return to this configuration after stopping the ultraviolet radiation, returning energy in a considerable amount [33]. This material was dissolved in butyl stearate as PCM, before encapsulation. Three types of diamines were selected to study their polyurea micro-/nano-capsule size and surface via in situ polymerization. Mean sizes of 0.72–5.64, 1.32–20.11, and 0.86–4.15 µm were obtained as the result of the reaction of TEPA, low NH PAE, and high NH PAE with IPDI, respectively. SEM results also showed that high NH PAE and TEPA had softer surface, whereas micro-/nano-capsules prepared by low NH PAE was thin and brittle because of fast and uncontrollable reaction. As a result of 1:1, 1:1.5, 1:2, and 1:2.5 ratio of IPDI to high NH PAE, micro-/nano-capsules with crystallization enthalpy of −72.94, −72.14, −72.73, and −73.56 J/g were obtained. Results show that the temperature of polyurea microcapsules containing PCM with photochromic materials were 4.9°C higher than the temperature in same microcapsules without photochromic properties.

3.2.1 Sodium silicate

Reaction of sodium silicate with calcium hydroxide not only heals the concrete cracks but also produces calcium silicate hydrate (C–S–H) gel, which is durable and resistant to aggressive environments. Calcium hydroxide is susceptible to acid attack and is water soluble. In conditions that sodium silicate is directly mixed with concrete, processing encounters several problems such as altering the setting time and efflorescence, whereas the encapsulated sodium silicate provides the opportunity for in situ curing after the cracks appear in the concrete. Due to the possibility of using polyurea at higher temperature without losing mechanical performance, their application on oil well cement at a temperature of 80°C has been studied [53]. Stiff and elastic types of polyurea referred as T1 and T2 were employed to prepare microcapsules as self-healing materials for oil well concrete. Microcapsules with mean size of 110 µm contained 90% sodium silicate cargo and 10% polyurea shell decreased the crack depth by 13 and 58% through 2.5–7.5% addition. However, compressive strength was also reduced by 26 and 40% for stiff and elastic polyurea shell. Figure 11 shows the SEM and energy dispersive X-ray spectroscopy (EDX) of the T1 microcapsules. In SEM image, C–S–H gel was found in a form of fibrous materials near the residual shell of the T1 microcapsules on the cracking surface. EDX was used to determine the chemical elements of the fibrous materials. The results of EDX also confirmed the release of sodium silicate cargo of the ruptured microcapsules at the cracking surface and the healing reaction of calcium hydroxide with sodium silicate.

Figure 11

SEM observations of the healed oil well cement pastes containing T1 microcapsules (a) various SEM images from the fractured surfaces and (b) an SEM image of a microcapsule and the corresponding EDX analysis results of ruptured microcapsules [53].

A study compared two commercial types of polyurea and gelatin microcapsules with the core of sodium silicate that were referred as T130 and L500, respectively [93].

Figure 12 compared the sorptivity results of cement paste with two different microcapsules. It was obtained that the addition of polyurea microcapsules had a higher self-healing effect of concrete and reduced the sorptivity of concrete by 45% in 7 days, whereas the sorptivity of concrete enhanced by gelatin microcapsules decreased by only 15% in the same period [94]. The effect of sorptivity reduction in cement paste is observed. Figure 13 shows the improvement in sealing of cracks in cement pastes with polyurea microcapsules.

Figure 12

Sorptivity of cracked specimens containing L500 gelatin microcapsules (blue line) and T130 polyurea microcapsules (red line) at 4% volumetric fraction compared with cracked cement specimens (black line) [94].

Figure 13

Sorptivity measurements are taken over a 28-day healing period. Comparison of later absorbed by cement (left) control cement paste samples and (right) cement paste samples containing 4% T130 microcapsules (polyurea microcapsules). Testing is after a s7-day healing period [94].

Also, the percentage of adding microcapsules above 12% reduced the compressive strength by about 24–27%. In this work, micro-/nano-capsules with the mean sizes of 40–700 µm have been employed to investigate healing of concrete cracks with different width.

The addition of microcapsules with mean size under 5–30 µm (Portland cement sizes) in concrete can fill the concrete voids, whereas higher mean sizes contain more sodium silicate and thus higher healing efficiency. Rheological properties of concrete embedded gelatin microcapsules with mean size of 500 µm also compared with concrete enhanced by polyurea microcapsules with 130 µm mean size. Results showed that concrete viscosity increased by gelatin and polyurea microcapsules by 54 and 47%, respectively, reducing the concrete compressive strength [94].

3.2.2 Calcium nitrate

Calcium nitrate is one of the main materials used as the core of micro-/nano-capsules to provide self-healing abilities in concrete. The self-healing mechanism of calcium nitrate mainly relies on hydration acceleration and crystallization of cement by cations resembling tricalcium silicate and dicalcium silicate. Indeed, the reaction of calcium hydroxide via calcium nitrate results in calcium hydroxy nitrate with higher basic capacity and greater abilities of hydrosilicate formation. The presence of calcium nitrate led to the formation of calcium carbonate in the cracks which intense the healing abilities of calcium carbonate. Calcium nitrite containing micro-/nano-capsule of polyurea/PU has been employed in steel bar-reinforced concrete. The calcium reacted with water anywhere cracks occurred in the concrete, and provided 58% of crack recovery and protection of rebars from corrosion [97]. The addition of nanomagnetic materials to polyurea microcapsules was proposed as an idea in another study [96]. In this method, magnetic polyurea microcapsules can reach to the location of steel embedded inside the reinforced concrete through running an electric current in the wire, which has not yet been studied. The water in oil emulsion method was employed to synthesize magnetic polyurea microcapsules containing calcium nitrate and sodium silicate as core materials. Results showed adding 0.3 wt% hydrophobic nano iron oxides caused phase separation, whereas addition of 0.10 wt% resulted in the stabilized emulsion of polyurea microcapsules with average size of 10 µm. The presence of calcium nitrite provides corrosion resistance abilities at the interface of rebar by penetration through the concrete and repassivating the steel. Magnetized polyurea microcapsules with PCM materials as cargo are shown in Figure 10. Anti-corrosion properties of PUF microcapsules for steel bar in reinforced concrete have been studied [99]. The method mainly relays on the release of calcium hydroxide core at low PH near the rebar to form a passive film.

3.2.3 MgO

The shrinkage of cement causes cracks inside the concrete decreasing the mechanical properties. MgO is one of expanding materials used for the shirking compensation through reaction with water and producing MgOH. In this self-healing technique, MgO increases in volume to 117% which is able to fill the hollow canals. The micro-/nano-encapsulation of MgO for self-healing of the cement has been investigated [100]. As the polyurea shell is serviceable at higher temperatures, the application of MgO and sodium silicate polyurea micro-/nano-capsule has been studied for crack healing of oil well cement [98].

3.2.4 Bacteria encapsulation

Recent technologies are using biological methods for repairing concrete cracks [101,102]. Amongst different methods of using bacteria for healing concrete, micro-/nano-encapsulated bacteria showed higher self-healing properties [103]. In a study, self-healing polyurea micro-/nano-capsules filled with bacteria have been employed to repair concrete cracks. Since the cracks appear in the concrete, the polyurea shell releases the bacteria and nutrition materials. As oxygen and moisture are available, the bacteria began to charge calcium carbonate inside the crack. Polyurea has met the requirements of an appropriate shell because, first, the novel chemical structure and multiple donner acceptor bonds of urea led to intermolecular hydrogen band, substantial mechanical stability as well as chemical and water resistance properties which protect the bacteria from the alkali and wet environment, second, the fast reaction makes the simple and tunable production of the capsule [13]. Figure 14 shows the crack repairment of the concrete through metabolic conversion of organic compounds to calcium carbonate. The mean sizes of polyurea micro-/nano-capsules containing bacteria from 0.2 to 1.3 mm were prepared by stirring polyetheramine and MDI at 400 rpm. Optical density at 600 nm showed that the bacterial spore germination increased from 0.212 nm 0 h to 2.499 nm after 48 h of incubation.

Figure 14

Image taken from crack filled with precipitation of calcium carbonate at the magnifications of (a) 50× and (b) 230× [13].

3.3.1 Active agents

Micro-/nano-capsules of active agents have been used for preparing self-healing polymeric coats. HDI, H-MDI, IPDI, HDA, and epoxy were encapsulated by polyurea [122] and added to epoxy, PU, and polyurea coat. This mechanism was also used for preparing self-healing cement paste [123]. The aim of the isocyanate micro-/nano-capsule preparation is to react with water or epoxy to heal the cracks that appear at the coating surface [119]. Figure 15 shows a schematic image of micro/nano-capsules behavior for self-healing coating. This mechanism enhances the protective abilities and service life of the coat in seawater or other harsh area exposures. However, reactive agent capsulation with polyurea encountered a number of drawbacks. Common solvents for active agents such as isocyanates are of hazardous materials and are dissolved in the matrixes in the long term and require higher mechanical properties. In a study, the effect of functionalization of polyurea micro-/nano-capsules on their mean size distribution and water uptake were investigated [114]. Results showed that the modification increased the amount of encapsulation and decreased the size variation where the core content, mean size, and dispersity of non-functionalized polyurea microcapsule were changed from 49%, 165 µm and 86–83%, 73 µm and 15 by using hexamethyldisilazane. Also, water uptake decreased from 2.4 to 1% for modification of polyurea micro-/nano-capsules via 2-ethylhexylamine. Moreover, fluorinated amine polyurea microcapsule could retain the isocyanate after 2 days of immersion in water by 60%, but the normal polyurea shell loses the encapsulated isocyanate in the same period [114].

Figure 15

Schematic representation of crack healing by micro-encapsulated self-healing agent with a catalyst embedded in a resin matrix [104].

Hybrid polyurea/PUF microcapsules were prepared and dispersed in epoxy. The amount of encapsulation was about 73% and the mean diameter was 158 ± 42 to 60 ± 17 μm with PUF thickness of 422 ± 36 to 309 ± 27 nm. The lower sizes showed lower resistance to water and organic solvent due to lower thickness of PUF. In addition to self-healing properties of H-MDI, its lubrication properties were also studied [117]. In comparison to HDI, H-MDI provided higher protective properties as an active agent for micro/nano-capsules due to its oily nature. The friction coefficient of epoxy-embedded polyurea microcapsules with core of H-MDI decreased by 78.9%. The wear width and wear depth of this epoxy then decreased from 1.3 ± 0.1 mm and 74 ± 36 μm to 0.9 ± 0.1 mm and 20.0 ± 4.1 μm through addition of polyurea microcapsules. HDA encapsulated by polyurea GO was used as a self-healing agent in epoxy. The mean size of 500 nm with shell thickness of 80 nm was obtained through agitation rate of 1,800 rpm and emulsifier dodecyl benzene sulfonate. Results showed 62.5% anti-corrosive self-healing efficiency in epoxy by adding only 3% of GO-PU nano-capsules. In comparison to polyether amine encapsulated by PMMA with 60% healing efficiency by 5% PMMA microcapsules addition [124], polyurea showed more accurate results with lower ratio of microcapsules. Polyurea and lignosulfonate have been used for encapsulation of IPDI and dispersed in polyurea coat. The polyurea microcapsules with the average mean size of 50 μm and the encapsulation efficiency of 75.3% were obtained. The corrosion current as an indicator of self-healing has been analyzed before and after scratching. The steel demonstrated that 2.06 × 10⁻⁰⁴ millimeters per year (mmpy) decreased to 1.54 × 10⁻⁰⁴ mmpy after coating with polyurea while the ruptured polyurea showed 2.49 × 10⁻⁰¹ mmpy, which decreased to 8.95 × 10⁻⁰³ mmpy after autonomous self-healing [7].

3.3.2 Organic oil

Protective coats are often exposed to physical wear or chemical erosion, organic oils are able to reduce the coefficient of friction and wear rate by forming a lubricating film at the surface [125]. Nevertheless, due to their vulnerability against higher temperatures and harsh environments, organic oils require to be protected with an engineered and strong shell. Also, the direct addition of some organic oils such as quinoline resulted in reaction with isocyanate and deactivated the anti-corrosion abilities. Several studies reported the employment of polyurea for encapsulation of quinoline, tung oil, and linseed oil as well as inorganic oils [126]. In order to prepare polyurea microcapsules with anti-corrosion core, quinoline have been dissolved in xylene and ethyl acetate [120]. These two different core materials then encapsulated by polyurea through stirring at 5,000 rpm. Microcapsules with xylene core material possessed mean size of 72 μm, whereas average size of microcapsules was 86 μm for the core of ethyl acetate solvent. Both formed in a broad distribution average size from almost 1 to 200 μm.

The polyurea microcapsules released their quinoline dissolved in xylene with 20.568, 41.332, and 50.703% after 1, 3 and 6 h. Finally using 4% polyurea encapsulated quinoline, a considerable decrease in the corrosion of steel was observed from 2.016 to 0.6506 mm/year led to the increase of average inhibition efficiency to 67.72%. Linseed oil is also used for the corrosion protection of steel. The cross-linkers can be mixed with organic oil and used as core of micro-/nano-capsules. These micro-/nano-capsules are able to provide both self-lubrication and self-curing properties at the same time. Tung oil was encapsulated by polyurea with different types of diamines. Polyurea microcapsules prepared by DETA and TETA showed deformation where 70–80% of them damaged and ruptured after stirring in hot water at 70–80°C. In contrast, composite droplets prepared by PAMAM survived in the same condition with only 5% damage [36]. As can be seen in Figure 16, metallic surface coated with PU contained polyurea microcapsules remained unchanged after 120 h immersed in salt water. In contrast, metallic sheet without polyurea microcapsules encountered severe corrosion on its surface. A few studies used magnetic properties of microcapsules for inhibition of corrosion with targeted delivery which limited to ethyl cellulose microcapsules with tung oil core [127]. Results showed that the average distance of microcapsules from steel decreased by 50% from 160.7 to 80.8 µm under an external magnetic field.

Figure 16

Corrosion test results of steel panels coated with PU coatings with different percentages of polyurea microcapsules containing tung oil before and after testing for 120 h [36].