**2. The basic principle and theory of damage healing**

Mimicking biosystems, in synthetic materials, damage repair is performed by three steps including actuation of healing, later transport of healing precursors to the damage site, and finally chemical repair process occurring with contact of catalyst or curing species at an angstrom level in which matrix is restructured by bonding of cleaved bonds at the damage site. Macroscopically, healing is proceeding by two consecutive mechanics; these included physical and chemical process. In physical process, the flow of healing precursors or segmental movement of chains is

**21**

materials as shown in equation no. 1.

sample, respectively.

*<sup>η</sup>* <sup>=</sup> *fhealed* <sup>−</sup> *fdamaged* \_\_\_\_\_\_\_\_\_\_\_

*fvirgin* − *fdamaged*

where f is the property of interest of material, and *fhealed*, *fdamaged*, and *fvirgin* are the property of interest of healed sample, the damaged sample, and the virgin

Damage volume is a deciding factor of maximum healing efficiency of various repair systems. Each technique demonstrated diverse healing efficiency for different damage volume. Intrinsic systems are preferred to heal small damage volume and heal at molecular level due to the close proximity of damaged site which is mandatory for re-bonding of the cleaved site. Microvascular network heals large

(1)

*Self-Healing Polymer Composites for Structural Application*

mandatory for damage repair. These events proceed continuously and are controlled by kinetics and thermodynamics. The repair event is determined by the kinetic energy of chains and entropy changes meanwhile by chain diffusions. These intrinsic properties have a great impact on entropy contribution to Gibbs free energy during repair event [31]. Another considerable factor is a free volume in a matrix which is desirable to the mobility of polymeric chains [32]. To obtain free volume, some stimuli-responsive units are also incorporated into nonreversible systems by copolymerization, and during mechanical stress, the entropy ∆S increased due to segmental mobility, and finally rebonding of cleaved sites is possible [33]. During self-healing process, voids/free volume facilitated segmental mobility of chains and matrix macromolecular chains. Void-less system are rigid and subsequently damage sensitive. Heterogeneities are critical parameters which offer the design of self-repair concept. Different types of polymers like block, branched, and/or star polymers showed self-repairs in range of nm to μm. Microphase separation and microcapsules and inorganic particles are embedded into polymers and responsible for macrodamages. In polymeric systems, heterogeneities are developed by phase separation utilized by copolymers or composite materials [27, 28, 34] and shape memory polymers [35]. In chemical process, different polymerization reactions of healing precursor or entanglement of polymer chains or reversible covalent bonding according to base matrix materials is dominant. All these stages are balanced by the damage rates to healing rate. The rate of damage is defined by various factors such as loading frequencies, strain rate, and the amplitude of stress. However, healing rate can be monitored by concentrations of precursor species and/or intrinsic temperature using varying reaction kinetics. In thermoset, encapsulation is an effective healing strategy, but in thermoplastics induced-healing is reported in which healing is possible on heating of polymers above its glass transition temperature (Tg) or using solvents by depression of the effective Tg as compared to below room temperature [36]. In autonomic healing, the healing agent is incorporated or phase separated by matrix so that the healing of crack/failure takes place without external intervention at ambient temperature. It is fully self-contained and responds to external stimuli. Healing is achieved by one-capsule system, dual-capsule system, and hollow fiber and vascular network-based system. In non-autonomic healing, human intervention is mandatory. It is inherent and intrinsically similar to biological structures. It is a partially self-contained healing system. Healing functionality is an intrinsic part of base matrix, but additional heat or radiation is required to proceed. Generally, for high healing efficiency, the healing agent forms a homogeneous mixture although it is difficult to process in terms of large-scale production in industries. The healing efficiency represents the recovery of mechanical integrity of components. To quantify healing efficiency, many definitions have been proposed. Basically, healing efficiency (*η*) is a ratio of change in a property of interest of

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

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

*Functional Materials*

used to recover mechanical integrity.

form of micro-cracking. Micro-cracks degrade polymeric properties inevitably and result into the failure. Single or multiple damage modes result into damage sites and are required to repair the components to continue to the service. If microcracks are untreated, they lead to larger macroscopic cracks, and finally catastrophic loss of the structure is an adverse outcome. To resist these failures, the new-generation materials having autonomic healing capability to damage repair are needed to develop. Conventionally, different lay-up repair techniques are adopted in thermoset. Similarly, in thermoplastics which are known for their mechanical performances, service temperature, and solvent resistance, fusion bonding techniques are

However, these repair strategies are very costly, time-consuming, followed by complicated procedure, and assisted only by expert technicians. So, we required such unique functionality which intimates the damages at nascent stage and sometimes repaired the damages. Self-healing of cracks is an eminent and efficient possible solution of these issues. In healing systems, a damage incident triggers the internal processes that generate the healing response which cured the damages. The bio-systems have damage detection and subsequent prevention techniques; those are source of innovation to design such functionality by introducing the self-healing functionality into artificial materials. These techniques are demanding for manned materials and structures and assisted by the biomimetic approaches. The major concern is focused on recovery of mechanical properties of polymer composites against quasi-static fracture. Initially, Mercier developed a self-healing rubber that can reseal on puncture damage [1]. The concept of healing is progressed with developing efficient vehicles and other systems such as space suits to protect from micrometeoroid impacts [2]. Currently, damage healing of polymer structure is being demonstrated via different approaches, which include extrinsic and intrinsic methodologies. Extrinsic methodology is being performed via encapsulation [3–8], hollow fibers [9–11], microvascular systems [12–16] and intrinsic damage healing is offered by reversible covalent bonding [17–21] and supramolecular interactions [22–30]. In extrinsic methodology, healing is restricted only once, and the delivery of healing precursor's amount is limited. To overcome the above concern, different intrinsic healing systems are developed that offer multiple healing at macroscopic damages. Multiple healing of the same crack is achieved by microvascular 3D system, thermoreversible networks, and supramolecular interaction. These strategies help to design various self-healing systems. In literature, various high-performance systems such as self-healing coating, self-healing ceramics, and self-healing metals are reported. Self-healing nanocomposites are also reported. Self-healing technology provides public safety and reduced maintenance cost of the structure. Healing approaches offer longer lasting with faulttolerant components across various fields including coatings, electronics, robotics, transportations, energy, etc. In the following section, the author is mainly focused to elaborate current trends and the leading research field of remendable polymers for

**20**

structural composites.

**2. The basic principle and theory of damage healing**

Mimicking biosystems, in synthetic materials, damage repair is performed by three steps including actuation of healing, later transport of healing precursors to the damage site, and finally chemical repair process occurring with contact of catalyst or curing species at an angstrom level in which matrix is restructured by bonding of cleaved bonds at the damage site. Macroscopically, healing is proceeding by two consecutive mechanics; these included physical and chemical process. In physical process, the flow of healing precursors or segmental movement of chains is

mandatory for damage repair. These events proceed continuously and are controlled by kinetics and thermodynamics. The repair event is determined by the kinetic energy of chains and entropy changes meanwhile by chain diffusions. These intrinsic properties have a great impact on entropy contribution to Gibbs free energy during repair event [31]. Another considerable factor is a free volume in a matrix which is desirable to the mobility of polymeric chains [32]. To obtain free volume, some stimuli-responsive units are also incorporated into nonreversible systems by copolymerization, and during mechanical stress, the entropy ∆S increased due to segmental mobility, and finally rebonding of cleaved sites is possible [33]. During self-healing process, voids/free volume facilitated segmental mobility of chains and matrix macromolecular chains. Void-less system are rigid and subsequently damage sensitive. Heterogeneities are critical parameters which offer the design of self-repair concept. Different types of polymers like block, branched, and/or star polymers showed self-repairs in range of nm to μm. Microphase separation and microcapsules and inorganic particles are embedded into polymers and responsible for macrodamages. In polymeric systems, heterogeneities are developed by phase separation utilized by copolymers or composite materials [27, 28, 34] and shape memory polymers [35]. In chemical process, different polymerization reactions of healing precursor or entanglement of polymer chains or reversible covalent bonding according to base matrix materials is dominant. All these stages are balanced by the damage rates to healing rate. The rate of damage is defined by various factors such as loading frequencies, strain rate, and the amplitude of stress. However, healing rate can be monitored by concentrations of precursor species and/or intrinsic temperature using varying reaction kinetics. In thermoset, encapsulation is an effective healing strategy, but in thermoplastics induced-healing is reported in which healing is possible on heating of polymers above its glass transition temperature (Tg) or using solvents by depression of the effective Tg as compared to below room temperature [36]. In autonomic healing, the healing agent is incorporated or phase separated by matrix so that the healing of crack/failure takes place without external intervention at ambient temperature. It is fully self-contained and responds to external stimuli. Healing is achieved by one-capsule system, dual-capsule system, and hollow fiber and vascular network-based system. In non-autonomic healing, human intervention is mandatory. It is inherent and intrinsically similar to biological structures. It is a partially self-contained healing system. Healing functionality is an intrinsic part of base matrix, but additional heat or radiation is required to proceed. Generally, for high healing efficiency, the healing agent forms a homogeneous mixture although it is difficult to process in terms of large-scale production in industries. The healing efficiency represents the recovery of mechanical integrity of components. To quantify healing efficiency, many definitions have been proposed. Basically, healing efficiency (*η*) is a ratio of change in a property of interest of materials as shown in equation no. 1.

$$\eta = \frac{f\_{\text{handed}} - f\_{\text{damping}}}{f\_{\text{voyin}} - f\_{\text{damping}}} \tag{1}$$

where f is the property of interest of material, and *fhealed*, *fdamaged*, and *fvirgin* are the property of interest of healed sample, the damaged sample, and the virgin sample, respectively.

Damage volume is a deciding factor of maximum healing efficiency of various repair systems. Each technique demonstrated diverse healing efficiency for different damage volume. Intrinsic systems are preferred to heal small damage volume and heal at molecular level due to the close proximity of damaged site which is mandatory for re-bonding of the cleaved site. Microvascular network heals large

damage volume and potentially attempts highest healing efficiency. Encapsulation strategy covered the regime between intrinsic and vascular systems. Most of the repair systems established high damage volume to low healing rate. To achieve high healing efficiency, the damage rate should be equal to damage healing. Only some systems based on capsule and intrinsic system are matched to healing rate to damage rate. The exact nature of the self-healing method to be deployed depends upon (i) the nature and location of the damage, (ii) the choice of repair resin, (iii) the influence of the operational environment, and (iv) proximity of damage site and healing precursor container. The stability and durability of the final material can be increased by repairing the damage in an autonomic way. Currently, a more dynamic strategy based on damage acceptance and management has been explored and growing exponentially shown in **Figure 1**.

Multiple healing is possible through the intrinsic approaches which have intrinsic functionality. This approach can be practical to thermoplastic, thermoset polymers and elastomers. Intrinsic self-healing is achieved by the recovery of the former interactions, with or without an external trigger. A certain magnitude of stress (i.e., chemical, mechanical, or thermal) enhanced the mobility of polymer network. On impact, the sudden drop of viscosity in matrix occurs due to transfer of impact energy in the form of heat to localized zone, which allows the local deformation and mobility of polymer chains or network. Upon cooling, network restores the initial values of viscosity, and materials achieved virgin mechanical and thermal stability. Moreover, to increase mechanical properties of intrinsic system, more than one chemical healing principle may be required to combine. The damage interfaces disappear when chain entanglements and chemical or physical cross-links formed a network as strong as the bulk material. This process can be obtained by physical and chemical interactions and a combination thereof. The most accepted theory leading to interfacial physical healing is proposed by Wool and O'Connor [37] which is based on molecular interdiffusion leading to chain entanglements. This process can occur at higher temperatures as compared to the bulk polymer glass transition temperature or through local external trigger such as a solvent and temperature beyond the melting point in thermoplastics (welding) [38]. In the case of reversible chemistries, the enhanced mobility leads to a viscous flow of the material in the vicinity of the damage site. It is remarkable that chain interdiffusion has been observed also at temperatures theoretically below the bulk Tg which highlights the potential difference in Tg between the bulk and the surface in freshly damaged materials influencing the healing process. From a mechanical and theoretical point of view, up to 100% healing of an

**23**

**Figure 2.**

*Self-Healing Polymer Composites for Structural Application*

system with some new encapsulation techniques.

**3.1 Microcapsule embedment**

**3. Passive self-healing based on extrinsic techniques**

interface can only be obtained if the new interface has exactly the same properties

In passive mode, healing is generated by incorporation of foreign functionalities. The extrinsic healing process is based on the use of a healing agent contained in the matrix as a separate phase. The healing agent is usually in the liquid state, placed into reservoirs which may be microcapsules or hollow fibers or microvascular network. In most approaches, the healing agent is used with a catalyst, which can also be encapsulated or dissolved in the matrix. Different extrinsic healing approaches are explored. In some cases, the catalyst is not required to initiate the healing process; the healing agent can also react to itself. The extrinsic healing concept is based on the response after or at the onset of damage. Current research is concerned with the improvement of healing agents in terms of compatibility and catalyst-free

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**.

*Demonstration of healing phenomenon. (a) Capsule-based healing [3] and (b) hollow fiber embedment [11].*

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

as the bulk material.

**Figure 1.** *The concept of damage healing using intrinsic methodology.*

*Functional Materials*

and growing exponentially shown in **Figure 1**.

damage volume and potentially attempts highest healing efficiency. Encapsulation strategy covered the regime between intrinsic and vascular systems. Most of the repair systems established high damage volume to low healing rate. To achieve high healing efficiency, the damage rate should be equal to damage healing. Only some systems based on capsule and intrinsic system are matched to healing rate to damage rate. The exact nature of the self-healing method to be deployed depends upon (i) the nature and location of the damage, (ii) the choice of repair resin, (iii) the influence of the operational environment, and (iv) proximity of damage site and healing precursor container. The stability and durability of the final material can be increased by repairing the damage in an autonomic way. Currently, a more dynamic strategy based on damage acceptance and management has been explored

Multiple healing is possible through the intrinsic approaches which have intrinsic functionality. This approach can be practical to thermoplastic, thermoset polymers and elastomers. Intrinsic self-healing is achieved by the recovery of the former interactions, with or without an external trigger. A certain magnitude of stress (i.e., chemical, mechanical, or thermal) enhanced the mobility of polymer network. On impact, the sudden drop of viscosity in matrix occurs due to transfer of impact energy in the form of heat to localized zone, which allows the local deformation and mobility of polymer chains or network. Upon cooling, network restores the initial values of viscosity, and materials achieved virgin mechanical and thermal stability. Moreover, to increase mechanical properties of intrinsic system, more than one chemical healing principle may be required to combine. The damage interfaces disappear when chain entanglements and chemical or physical cross-links formed a network as strong as the bulk material. This process can be obtained by physical and chemical interactions and a combination thereof. The most accepted theory leading to interfacial physical healing is proposed by Wool and O'Connor [37] which is based on molecular interdiffusion leading to chain entanglements. This process can occur at higher temperatures as compared to the bulk polymer glass transition temperature or through local external trigger such as a solvent and temperature beyond the melting point in thermoplastics (welding) [38]. In the case of reversible chemistries, the enhanced mobility leads to a viscous flow of the material in the vicinity of the damage site. It is remarkable that chain interdiffusion has been observed also at temperatures theoretically below the bulk Tg which highlights the potential difference in Tg between the bulk and the surface in freshly damaged materials influencing the healing process. From a mechanical and theoretical point of view, up to 100% healing of an

**22**

**Figure 1.**

*The concept of damage healing using intrinsic methodology.*

interface can only be obtained if the new interface has exactly the same properties as the bulk material.
