**3.1 Microcapsule embedment**

Encapsulation strategy is mainly studied for polymers and coating. The basic principle of strategy is healing by incorporated healing functionality or reactive constituents into capsules followed by chemical reactions. These reactions take place by various mechanisms including ring opening metathesis polymerization (ROMP) [3], cycloreversion [39], cycloaddition [40], cross-linking reactions [41], or a mechanochemical catalytic activation [42]. Damage acts as a stimulus to initiate the healing process. Damages rupture the microcapsule, and subsequent release of the core material (healing agent) is possible. The healing precursor reached at the damage site by capillary action and spreads itself over the two fracture surfaces due to the surface tension. Further, precursors interact with embedded adjacent catalysts (**Figure 3**) leading to a network formation by following the above chemistries, which terminate the further growth of crack or damage and restore mechanical integrity. White et al. [3] designed a "dicyclopentadiene (DCPD) Grubbs' system" based on capsule healing which achieved 75% recovery of virgin fracture toughness of TDCB specimens. Capsule- and hollow fiber-based healing systems are shown in **Figure 2**.

Mainly, the adhesive and cohesive mechanisms are responsible for the failure of interfaces. Better healing efficiency can be achieved by improving the adhesive tendency of the poly-(DCPD) with fracture surface of the matrix without compromise with the cohesive strength of the poly-(DCPD) (shown in **Figure 4**). The average diameter of the microcapsule is ranging from ∼300 to ∼700 μm and shell walls with thicknesses from 5 to 20 μm. Generally, the core materials of microcapsules are made of poly(urea-formaldehyde) (PUF) [43]. The controlled release of the precursor is the typical job of encapsulation strategy. Some of the eco-friendly catalyst-free healing methods are also introduced because the catalyst-based approach is costineffective and related to some compromise of mechanical properties of matrix. Various critical factors define the performance of encapsulation strategy [44, 45]. There are various different ways to proceed capsule healing; that is, (i) the encapsulated liquid agent can be combined with a dispersed catalyst, (ii) both the healing agent and the catalyst can be embedded in different capsules, (iii) the healing agent can also directly react with a functionality of the matrix under an external stimulus, and (iv) the healing agent and the catalyst can be placed in the matrix as a separate phase. Different capsule-based healing systems are summarized in **Table 1**.

The encapsulation strategy is mainly focused on meltable dispersion and in situ and interfacial encapsulation techniques for capsules. Meltable dispersion is the method of dispersing the healing agent in a melted polymer to form the capsules after solidification of the polymer [47]. In situ and interfacial techniques have been used for PUF or TETA microcapsules. In this technique, the shell is developed by polymerization at the interface of healing agent droplets and the oil-in-water emulsion.

The triggering mechanism of encapsulation strategy is validated by optical microscopy of a fracture plane showing ruptured capsules, by infrared spectroscopy (IR), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) of the fracture plane. Different microencapsulation methods are optimized such as physical methods, chemical methods, and physicochemical methods. Robust, in terms of mechanically and thermally stable, microcapsules having healing precursor have been synthesized for self-healing polyurethane matrix [48]. Triethylenetetramine (TETA) microcapsules for wear-resistant polymer composites [49] and poly(methyl methacrylate) microcapsules with high storage and thermal stability [50] have been manufactured and implemented. The switching behavior of microcapsule geometry between dry and wet condition is a critical healing phenomenon. Polydimethylsiloxane-based self-healing elastomers


**25**

**Figure 4.**

**Figure 3.**

*(d) Hoveyda-Grubbs second generation.*

*Self-Healing Polymer Composites for Structural Application*

also are reported [7]. Dual component self-healing epoxy system containing epoxy (DGEBA) and different variants of hardener microcapsules are investigated [51].

*(a) Grubbs first generation, (b) Grubbs second generation, (c) Hoveyda-Grubbs first generation, and* 

Encapsulation techniques offered single healing due to unavailability of healing precursor into capsules which is earlier invested in damage repair. However, these are limited by processing difficulties and inhomogeneous distributions of two components. Deteriorates of some mechanical integrity is common due to the addition of external chemical constituent's limiting the strategy. To deliver a larger amount of healing agent, hollow glass fibers were used. This fiber reinforcement is based on the bleeding ability of bio-system. For polymer composite systems, the hollow fiber

Hollow fibers are used to deliver a larger amount of liquid healing agent. These are embedded within either glass fiber-reinforced plastic (GFRP) or carbon fiber-reinforced plastic (CFRP) composites. Healing-agent-filled hollow fibers

Various catalysts for ROMP of DCPD are shown in **Figure 3**.

*The ROMP of encapsulated DCPD by Grubbs catalyst [3].*

embedment approach has been more appropriate (**Figure 4**).

**3.2 Hollow fiber embedment**

*DOI: http://dx.doi.org/10.5772/intechopen.82420*

**Table 1.**

*Various self-healing systems based upon encapsulation strategy.*

*Self-Healing Polymer Composites for Structural Application DOI: http://dx.doi.org/10.5772/intechopen.82420*

#### **Figure 3.**

*Functional Materials*

Mainly, the adhesive and cohesive mechanisms are responsible for the failure of interfaces. Better healing efficiency can be achieved by improving the adhesive tendency of the poly-(DCPD) with fracture surface of the matrix without compromise with the cohesive strength of the poly-(DCPD) (shown in **Figure 4**). The average diameter of the microcapsule is ranging from ∼300 to ∼700 μm and shell walls with thicknesses from 5 to 20 μm. Generally, the core materials of microcapsules are made of poly(urea-formaldehyde) (PUF) [43]. The controlled release of the precursor is the typical job of encapsulation strategy. Some of the eco-friendly catalyst-free healing methods are also introduced because the catalyst-based approach is costineffective and related to some compromise of mechanical properties of matrix. Various critical factors define the performance of encapsulation strategy [44, 45]. There are various different ways to proceed capsule healing; that is, (i) the encapsulated liquid agent can be combined with a dispersed catalyst, (ii) both the healing agent and the catalyst can be embedded in different capsules, (iii) the healing agent can also directly react with a functionality of the matrix under an external stimulus, and (iv) the healing agent and the catalyst can be placed in the matrix as a separate

phase. Different capsule-based healing systems are summarized in **Table 1**.

The encapsulation strategy is mainly focused on meltable dispersion and in situ and interfacial encapsulation techniques for capsules. Meltable dispersion is the method of dispersing the healing agent in a melted polymer to form the capsules after solidification of the polymer [47]. In situ and interfacial techniques have been used for PUF or TETA microcapsules. In this technique, the shell is developed by polymerization at the interface of healing agent droplets and the oil-in-water emulsion. The triggering mechanism of encapsulation strategy is validated by optical microscopy of a fracture plane showing ruptured capsules, by infrared spectroscopy (IR), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) of the fracture plane. Different microencapsulation methods are optimized such as physical methods, chemical methods, and physicochemical methods. Robust, in terms of mechanically and thermally stable, microcapsules having healing precursor have been synthesized for self-healing polyurethane matrix [48]. Triethylenetetramine (TETA) microcapsules for wear-resistant polymer composites [49] and poly(methyl methacrylate) microcapsules with high storage and thermal stability [50] have been manufactured and implemented. The switching behavior of microcapsule geometry between dry and wet condition is a critical healing phenomenon. Polydimethylsiloxane-based self-healing elastomers

**S.N. Healable systems Healing mechanism Ref. (s) Healing** 

2 Epoxides/amine Curing mechanism [4] 91 3 Epoxy/DCPD-WCl6 catalyst ROMP [5] 20 4 Epoxy/mercaptan Curing mechanism [6] >100

6 Thiol/maleimide Michal addition reaction [46] >100

CuBr2(2-methylimidazole)4

ROMP [3, 43] 99

Polycondensation reaction [7] >100

1 DCPD/PUF-microcapsules/Grubbs

(polydimethylsiloxane)/Pt catalyst

7 Woven glass fabric/epoxy laminates Epoxy/

*Various self-healing systems based upon encapsulation strategy.*

catalyst

5 Polysiloxane

**efficiency (%)**

[8] 68–79

**24**

**Table 1.**

*(a) Grubbs first generation, (b) Grubbs second generation, (c) Hoveyda-Grubbs first generation, and (d) Hoveyda-Grubbs second generation.*

**Figure 4.** *The ROMP of encapsulated DCPD by Grubbs catalyst [3].*

also are reported [7]. Dual component self-healing epoxy system containing epoxy (DGEBA) and different variants of hardener microcapsules are investigated [51]. Various catalysts for ROMP of DCPD are shown in **Figure 3**.

Encapsulation techniques offered single healing due to unavailability of healing precursor into capsules which is earlier invested in damage repair. However, these are limited by processing difficulties and inhomogeneous distributions of two components. Deteriorates of some mechanical integrity is common due to the addition of external chemical constituent's limiting the strategy. To deliver a larger amount of healing agent, hollow glass fibers were used. This fiber reinforcement is based on the bleeding ability of bio-system. For polymer composite systems, the hollow fiber embedment approach has been more appropriate (**Figure 4**).

#### **3.2 Hollow fiber embedment**

Hollow fibers are used to deliver a larger amount of liquid healing agent. These are embedded within either glass fiber-reinforced plastic (GFRP) or carbon fiber-reinforced plastic (CFRP) composites. Healing-agent-filled hollow fibers

are introduced into the matrix by the vacuum-assisted resin transfer molding (VARTM) process. Vascular self-healing materials have appropriate healing agent in a network in the form of capillaries or hollow channels, which may be interconnected one dimensionally (1D), two dimensionally (2D), or three dimensionally (3D), upon damage. One-dimensional system is designed by glass pipettes that are embedded in epoxy resins [52]. Resin-filled hollow glass fibers impart healing capability on low-velocity impact damage in CFRP [9]. Large-diameter capillaries are not feasible to demonstrate damage healing. Smaller hollow glass fibers filled with resin have been also used, but they were unable to deliver the resin into the crack due to the high-viscous epoxy resins. Later, borosilicate hollow glass fibers (with diameters from 30 to 100 μm with 55% of hollowness) were produced to store the healing precursor resin. This approach offered certain advantages, such as the higher volume of healing agent to deliver, performed by different activation methods; visual inspection of the damaged site is possible, and embedment of hollow fibers to conventional reinforcing fibers is easier. The fracture of hollow fibers is mandatory to release healing precursors which limited the approach. The low viscosity of healing agent is favorable to facilitate fiber infiltration, which is necessary. The reinforcement of hollow glass fibers into CFRP also affects the coefficient of thermal expansion so that multistep fabrication stages of the hollow fiber are another challenge. A novel hybrid multi-scale carbon fiber/epoxy composite reinforced with self-healing core-shell nanofibers at interfaces has been demonstrated [10]. The ultrathin self-healing fibers were fabricated by means of co-electrospinning, in which liquid DCPD as the healing agent was enwrapped into polyacrylonitrile (PAN) to form core-shell DCPD/PAN nanofibers. To enhance the healing efficiency, vascular method is adopted in which a 3D microvascular network is developed into the matrix to store the healing agents for transport in longer distance.

## **3.3 The microvascular embedment**

A microvascular technique is inspired from the respiratory system of livings. The incorporation of micro-channels with a diameter ranging from 1 μm to 1 mm within a polymer composite offers multiple healing. Self-healing materials that use hollow fibers or a mesoporous network are called vascular materials. Microvascular fabrication is possible by various techniques including laser micromachining, soft lithography, electrostatic discharge, fugitive inks, and hollow glass fibers. Healing precursors have been introduced into these channels either by pumping or through capillary forces. However, hollow glass fibers are restricted to the 1D network, but in order to obtain 2D and 3D interconnected networks, steel wires of ca. 0.5 mm could be used [53]. For the interface between plies in laminated composites, two-dimensional networks are suitable. Microvascular channels into polymer composites offer the benefits of added functionality and increased autonomy (i.e., the ability to distribute active healing material for crack healing which is difficult by conventional methods into monolithic materials). The introduction of sacrificial fibers into woven preforms enables the continuous fabrication of 3D microvascular composites that are both strong and multifunctional [12, 13]. To employ damage healing in microvascular systems, functional fluids that act as a healing agent are released upon fracture of vascular network. Further, the healing agent polymerized with adjacent catalyst formed a network and restricts the growth of damage. Active cooling microvascular systems continuously circulate a fluid into, through, and out of the matrix in order to absorb and remove excess heat. Replacement of some reinforcement fibers of FRPs by individual hollow fibers is a well-known method to achieve microvascular composites [54]. Schematic diagram of self-healing materials

**27**

**Figure 6.**

*Self-Healing Polymer Composites for Structural Application*

with 3D microvascular networks is shown in **Figure 5**. The effect of optimum size and orientation of hollow fibers in microvascular architecture in the epoxy matrix

*Schematic diagram of self-healing materials with 3D microvascular networks. (a) Capillary network in skin having a cut in the epidermis layer. (b) Vascular network into epoxy coating having catalyst in a four-point bending configuration monitored with an acoustic emission sensor. (c) Crack propagation toward micro-*

A novel type of hollow fiber called "compartmented fibers" has been developed [55]. However, hollow fibers have a greater influence on the mechanical properties of composites than microcapsules. Indeed, using this type of fibers, a localized healing response can be activated. Vasculature-based healing allowed the efficient delivery of the healing agent and additionally healed a large area. The large-scale production of self-healing fiber-reinforced composites is not feasible due to complex vasculatures and lack of scalability. After the first healing, the network may be refilled for the next healing. Initially, a brittle polymer coating is applied to a more ductile polymer substrate which contains the interconnected network of micro-channels. In contrast, synthetic self-healing composites have high strength-

Ultralow-temperature damage healing is achieved by incorporating a 3D microvascular network (**Figure 6**). Hollow vessels are used to deliver healing agents, and

*DOI: http://dx.doi.org/10.5772/intechopen.82420*

has been studied [14].

**Figure 5.**

to-weight ratios with less dynamic functionality.

*(a) Internal structure of the composites and (b) damage-bleeding healing process [16].*

*channel. (d) Optical image of released healing agent in the coating [15].*

*Self-Healing Polymer Composites for Structural Application DOI: http://dx.doi.org/10.5772/intechopen.82420*

#### **Figure 5.**

*Functional Materials*

longer distance.

**3.3 The microvascular embedment**

are introduced into the matrix by the vacuum-assisted resin transfer molding (VARTM) process. Vascular self-healing materials have appropriate healing agent in a network in the form of capillaries or hollow channels, which may be interconnected one dimensionally (1D), two dimensionally (2D), or three dimensionally (3D), upon damage. One-dimensional system is designed by glass pipettes that are embedded in epoxy resins [52]. Resin-filled hollow glass fibers impart healing capability on low-velocity impact damage in CFRP [9]. Large-diameter capillaries are not feasible to demonstrate damage healing. Smaller hollow glass fibers filled with resin have been also used, but they were unable to deliver the resin into the crack due to the high-viscous epoxy resins. Later, borosilicate hollow glass fibers (with diameters from 30 to 100 μm with 55% of hollowness) were produced to store the healing precursor resin. This approach offered certain advantages, such as the higher volume of healing agent to deliver, performed by different activation methods; visual inspection of the damaged site is possible, and embedment of hollow fibers to conventional reinforcing fibers is easier. The fracture of hollow fibers is mandatory to release healing precursors which limited the approach. The low viscosity of healing agent is favorable to facilitate fiber infiltration, which is necessary. The reinforcement of hollow glass fibers into CFRP also affects the coefficient of thermal expansion so that multistep fabrication stages of the hollow fiber are another challenge. A novel hybrid multi-scale carbon fiber/epoxy composite reinforced with self-healing core-shell nanofibers at interfaces has been demonstrated [10]. The ultrathin self-healing fibers were fabricated by means of co-electrospinning, in which liquid DCPD as the healing agent was enwrapped into polyacrylonitrile (PAN) to form core-shell DCPD/PAN nanofibers. To enhance the healing efficiency, vascular method is adopted in which a 3D microvascular network is developed into the matrix to store the healing agents for transport in

A microvascular technique is inspired from the respiratory system of livings. The incorporation of micro-channels with a diameter ranging from 1 μm to 1 mm within a polymer composite offers multiple healing. Self-healing materials that use hollow fibers or a mesoporous network are called vascular materials. Microvascular fabrication is possible by various techniques including laser micromachining, soft lithography, electrostatic discharge, fugitive inks, and hollow glass fibers. Healing precursors have been introduced into these channels either by pumping or through capillary forces. However, hollow glass fibers are restricted to the 1D network, but in order to obtain 2D and 3D interconnected networks, steel wires of ca. 0.5 mm could be used [53]. For the interface between plies in laminated composites, two-dimensional networks are suitable. Microvascular channels into polymer composites offer the benefits of added functionality and increased autonomy (i.e., the ability to distribute active healing material for crack healing which is difficult by conventional methods into monolithic materials). The introduction of sacrificial fibers into woven preforms enables the continuous fabrication of 3D microvascular composites that are both strong and multifunctional [12, 13]. To employ damage healing in microvascular systems, functional fluids that act as a healing agent are released upon fracture of vascular network. Further, the healing agent polymerized with adjacent catalyst formed a network and restricts the growth of damage. Active cooling microvascular systems continuously circulate a fluid into, through, and out of the matrix in order to absorb and remove excess heat. Replacement of some reinforcement fibers of FRPs by individual hollow fibers is a well-known method to achieve microvascular composites [54]. Schematic diagram of self-healing materials

**26**

*Schematic diagram of self-healing materials with 3D microvascular networks. (a) Capillary network in skin having a cut in the epidermis layer. (b) Vascular network into epoxy coating having catalyst in a four-point bending configuration monitored with an acoustic emission sensor. (c) Crack propagation toward microchannel. (d) Optical image of released healing agent in the coating [15].*

with 3D microvascular networks is shown in **Figure 5**. The effect of optimum size and orientation of hollow fibers in microvascular architecture in the epoxy matrix has been studied [14].

A novel type of hollow fiber called "compartmented fibers" has been developed [55]. However, hollow fibers have a greater influence on the mechanical properties of composites than microcapsules. Indeed, using this type of fibers, a localized healing response can be activated. Vasculature-based healing allowed the efficient delivery of the healing agent and additionally healed a large area. The large-scale production of self-healing fiber-reinforced composites is not feasible due to complex vasculatures and lack of scalability. After the first healing, the network may be refilled for the next healing. Initially, a brittle polymer coating is applied to a more ductile polymer substrate which contains the interconnected network of micro-channels. In contrast, synthetic self-healing composites have high strengthto-weight ratios with less dynamic functionality.

Ultralow-temperature damage healing is achieved by incorporating a 3D microvascular network (**Figure 6**). Hollow vessels are used to deliver healing agents, and

#### **Figure 6.**

*(a) Internal structure of the composites and (b) damage-bleeding healing process [16].*

a porous conductive wire defrosts the system by internal heating, and further healing reactions are proceeded [16]. The concept may be used to develop self-healing in aerostructure at high altitude having low temperature.
