A review of the recent development in self-healing rubbers and their quantification methods

Abstract

The emerging self-healing concept in rubber materials appears to be a good option for reducing rubber waste since it can enhance the longevity of the product's structure in conjunction with standard development goals to ensure sustainable consumption and efficient use of resources. Self-healing elastomers are fabricated to heal cracks and damage both at the micro and macroscopic levels to restrict failure by incorporating healing agents extrinsically or intrinsically. The major successful techniques to fabricate self-healing rubber were reviewed. The review also focuses on testing and analysis methods to understand the healing phenomena and quantification methods to measure the healing performance. The review also includes an overview of the self-healing elastomers for tyre applications.

Keywords

Self-healing rubber quantification of self-healing analysis of self-healing

Introduction

Rubber has been enormously employed in various engineering applications such as tyre and automotive parts, hoses, cables and flexible electronic devices due to its excellent dynamic performance, resistance to wear, creep and cyclic deformation. Traditionally, the vulcanisation system used in crosslinked rubbers is thermoset by nature; it holds the properties of mechanical robustness with the outstanding capacity of resistance to deforming upon receiving a large force and is able to withstand thermal and chemical. However, notwithstanding their excellent initial properties, elastomers still eventually encounter failure through fatigue or fracture processes, and the structures require manual labour attention to identify and rectify the problem. Typically, regular inspection and maintenance are scheduled before the damage goes undetected until failure occurs. The irreversible nature of the rubber crosslinked network results in the discarding of humongous rubber products yearly, all of which are alarming and jeopardising our environment if no immediate action is taken by any parties to solve the waste related to conventional rubber products. In that case, the overall universal waste will increase globally by 70% by 2050, as reported by Kaza et al.¹ Irreversible sulfur and peroxide vulcanisation commercially employed in the rubber compounding system must be replaced with reversible networks to reduce pollution related to rubbers. The development of healable rubber materials that can detect and repair the damage themselves appears to be a possible way to enhance structural safety, increasing the material's lifespan and improving environmental sustainability, thus providing a new technique for reshaping and recycling rubber products.

There is a growing concern about self-healing rubbers in industrial or engineering applications that can self-mend and regain functionality when damaged without requiring any detection or repair by manual labour. Recently in reviews, Idumah et al.² discuss the state-of-the-art self-healing mechanisms of polymeric hydrogels, shape memory, and stretchable materials, while Barrios et al.³ overview the evolution of self-healing elastomers. To date, the encapsulation approach is one of the most successful concepts that depend on the healing agents sequester in discrete capsules. Upon rupture, the healing agent is released from the microcapsule in the damaged region and local healing is depleted. However, crack initiation caused deterioration in mechanical properties due to the typically weak adherent of the capsule's shell to the rubber matrix. It can also influence rubber's rheological behaviour and cure characteristics due to the presence of micrometer-sized structures.⁴ Due to these limitations, the intrinsic healing mechanism offers more benefits in elastomers than the extrinsic mechanism and will be the main focus of this review.

Intrinsic self-healing materials can undergo a healing process multiple times at the same site. Such an approach possesses a self-healing functionality through built-in material properties through reversible cleavage and reformation of supramolecular networks (hydrogen bonds,^5,6 polar interactions,⁷ ionic bonding^8–10 metal-lig and,^11,12 disulfide bonds^13–15 Diels- Alder,^16,17 transesterification reactions,^18,19 shape memory,²⁰ and combinations between these mechanisms).^21–23 However, a significant challenge arises when there is a contradiction between outstanding mechanical properties that relies on the rigid and stable covalently bonded three-dimensional molecular network while self-healing behaviour depends on flexible and chain mobility between the polymer strand.²⁴

To understand the healing capabilities of the mechanism utilised in the elastomers, an analysis of healing efficiency is done by comparing the change in the properties between the original and healed material. The test to be conducted must be suitable to the desired applications. For example, to obtain the healing information of self-healing of elastomer or soft material, a mechanical fracture test is more practical rather than a tensile test alone. For coating applications, scratch healing tests must be counted in the testing method range. Tensile test and tapered double cantilever beam (TDCB) are more suitable for investigating the restoration of mechanical properties, especially for brittle materials. To optimise the self-healing performance, a guide in designing the material and mechanism to be implemented, the understanding of underlying mechanism up to the molecular level, is a must.²⁵ Different visualisation tools, spectroscopy techniques and thermal characterisation will also be reviewed in this article to evaluate their mechanisms.

Many reviews are describing the recent advances of self-healing elastomers.^4,26–32 However, to the best of the author’s knowledge, a limited review focuses exclusively on its quantification method. This review initially expresses insight into the recent development of self-healing elastomers, both tailor-made and commercially available elastomers. In particular, concentrate on assessment methods used to quantify self-healing efficiency associated with self-healing elastomers.

Approaches of self-healing mechanism in elastomer

In general, there are four generations of self-healing mechanisms; (i) encapsulation of external healing agents, (ii) intrinsic mechanism, (iii) mechanism inspired on living organisms such as plants, animal and human skin, where the healing agents are contained in vascular network and (iv) overlapping of different healing mechanisms as illustrated in Figure 1. Yang and Urban³³ categorized the self-healing mechanisms based on physical, chemical and physico-chemical processes. The general category used by many researchers, intrinsic and extrinsic mechanisms, is still applicable and broadly applied.^34–36 Even if there are different classifications of healing mechanisms highlighted by the material scientists, the purpose is still the same, to make the elastomer recover itself physically, chemically, thermally, and functionality when the damage is present in the material’s body. It is worth mentioning that capsule-based extrinsic mechanism is not widely used in elastomers because the outer shell of capsules is poorly adhered to the rubber matrix and serves as a spot for crack initiation. It could affect the rheological, cure behaviour and deteriorate the mechanical properties.⁴ The intrinsic mechanism is the foremost approach employed in the rubber system to functionalize the self-healing behaviour in commercially available and tailormade rubbers.

Figure 1.

The generations of self-healing materials.

Extrinsic approach

Extrinsic self-healing materials provide the repairability of the destroyed bonds in elastomer by allowing liquid healent to flow to the damaged area via ruptured microcapsules or vascular network. These two types of extrinsic mechanisms are also identified as physico-chemical approaches since the physical and chemical processes can cooperate in providing self-healing to the materials.³⁷

Capsular-based healing

Healing by the encapsulated approach can be classified into three main categories, which are; i) encapsulated monomer together with the catalyst inside one microcapsule, ii) two types of monomers encapsulated in different capsules and iii) liquid oligomers. Zhu et al.³⁸ in their overview, further classified microencapsulated healing systems that have proven efficient into five categories: single capsule, capsule/dispersed catalyst, phase-separated droplet/capsules, double capsule and all-in-one microcapsule. The images of the system are represented in Figure 2. Bollinger et al.³⁹ are considered the pioneer group that explored the application of microcapsules as a healing mechanism in polymer networks. In their work, dicyclopentadiene (DCPD) and ruthenium-based catalysts contained in different microcapsules were dispersed in the epoxy polymer matrix. Then the two content of ruptured capsules polymerised through the ring-opening metathesis polymerisation (ROMP). Hager et al.⁴⁰ mentioned that incorporating microcapsules into the elastomeric materials to functionalised the self-healing property is not widely applied because the capsule shells are hard to survive the typical processing steps of elastomers.

Figure 2.

Types of capsular based healing system have proven efficient.³⁸

Other than the stability of the shell layer, the capsule's size is another crucial parameter to be considered. The big capsule allows for healing a larger crack because it can hold a greater volume of healing agents. However, it affects the material's properties in terms of the roughness of its surface and leads to the propagation of cracks. Smaller size capsules cannot afford to heal the larger damaged area; thus, more capsules are needed in the polymer matrix and cause changes in rheological properties. Keller et al.⁴¹ reported on the two components healing microcapsule filled with high molecular weight PDMS resin with vinyl functionalised with platinum catalyst and another microcapsule filled with PDMS copolymer with active sites. Both microcapsules are then dispersed in the PDMS matrix, sustaining up to 50% bulk stretch of the matrix without apparent damage. Via the tear testing, it reveals the self-healable PDMS recovered at least 70% from the original property and is also capable of achieving 100%. In other studies, graphene oxide microcapsules (GOMCs) containing linseed oil as the healing agent inside were assembled by liquid crystalline GO sheets. The microcapsules were incorporated in a waterborne PU matrix to be applied as coating applications on the galvanised steel plates.⁴² Yin et al.⁴³ fabricated a self-healing rubber based on a PDMS matrix embedded with urea-formaldehyde microcapsule encased with Sylgard 184A gum in capsule I and hydrogen silicone oil in capsule II. Both shell microcapsules were developed with poly (urea formaldehyde) (PUF). Upon the release of the dimethyl vinyl-terminated dimethylsiloxane that acts as resin monomer and methylhydrogen dimethylsiloxane as a crosslinker, polymerisation would occur by sealing the damaged area as depicted in Figure 3. This strategy enabled a maximum healing efficiency of 72%, revealed by the tensile test of a virgin and healed sample. Yu et al.³⁸ quoted that materials equipped with extrinsic self-healing mechanisms can be easily integrated into commercially available polymers due to no structural amendment on the molecular level needed. However, this method can only be applied for a single healing process because of the exhaustion of the healing agent once the capsule releases its content.

Figure 3.

SEM images of the crack-healed behaviour of the damaged PDMS composites filled with Sylgard 184A and hydrogen oil microcapsules with the healing time of (a) 0h, (b) 6h, (c)12h and (d) 24h.

Vascular-based healing

Three-dimensional micro-vascular network containing a healing agent was designed by the researcher to overcome the limitation of the microcapsule concept. Considering animal tissues, plants and other living organisms’ self-healing systems, researchers have several attempted to introduce bioinspired healing activity into polymeric materials. The vascular based is similar to the capsular healing system based on its principle but differs in terms of the fabrication method, shape and matrix integration. Factors that could lead to the effectiveness of vascular healing systems are network mechanical characteristics, triggering mechanism, and performance of healing agent.⁴⁴ Encapsulated healents give way to flow towards the damaged parts through a vein-like vascular network fabricated through the electrospinning or solution-blowing method.⁴⁵ These concepts have been investigated in thermosets,⁴⁶ but their application is very limited in elastomeric materials. A significant amount of vascular network might be required to rectify unpredictable damage, and this will cause the fabrication process very difficult.⁴⁷ Lee et al.⁴⁸ utilized dual emulsion electrospinning to develop core-shell fibre coatings in nano-sized with the dimethyl siloxane (DMS) resin as a healing agent and dimethyl-methyl hydrogen-siloxane act as crosslinker separately in the cores. In their research, cured poly (dimethyl siloxane) PDMS is intercalated in the coating pores. DMS resin and cured PDMS will be released separately and mixed to heal the scratch or microcracks when the damage occurs. Figure 4(a)) shows the SEM images of transparent nanofibers containing the healing agent and crosslinker (core materials). The squeezed-out of core materials image was presented in Figure 4(b)) to confirm the contents inside the nanofibers since it is invisible.

Figure 4.

SEM micrographs of (a) zoomed-in nanofibers in the mat (b) squeezed-out core materials.⁴⁸

Intrinsic non-covalent interaction

Intrinsic self-healing mechanisms provide multiple healing reactions using reversible non-covalent and covalent crosslinks in the elastomer networks activated via an external stimulant such as heat or light.^49,50 Non-covalent systems of intrinsic self-healing mechanisms are comprised of weak bonding interactions such as Van der Waals, hydrogen bonding, ionic interactions, metal-ligand coordination, π-π stacking, host-guest interaction and dipole-dipole interactions. In self-healing elastomers, hydrogen bonding is a non-covalent system that mostly chooses and usually combines with other types of interaction to balance mechanical properties and healing capabilities.⁵¹ Chen et al.⁵² have reported a self-healing polyurethane (PU) elastomer based on intrinsic nnon-covalenthydrogen bond and van der Walls forces mechanism. It was mentioned in their published work that an intrinsic covalent system is complicated to synthesize, costly, toxic materials, and challenging to apply on an industrial scale.

Ionic association

Ionic bonds are said to be the second most popular choice of non-covalent mechanisms utilized in self-healing rubber.⁵¹ It involves the electrostatic attraction between the different charges of ions which associate into pairs and form clusters. Cao et al.⁵³ designed a self-healable natural rubber based on the supramolecular network without undergoing any chemical modification. The restoration of local damages (up to 100%) after 5 min of the healing process was evident at ambient temperature. This could be attributed to the rearrangement and reconnection of the reversible ionic network, showing that the developed material has excellent self-healing performance. However, such recovery was only achieved for the samples undergoing one-minute heat treatment at 140°C. Dahlke et al.⁵⁰ prepared zwitterionic polymers attached to two different functional side groups; with shorter spacer 3-((2-(methacryloyloxy) ethyl) dimethylammonium) propane sulfonate (MAPS) and another type with longer spacer 3-((2-(methacryloyloxy) ethyl) dimethylammonium) butane sulfonate (MABS). Polymers with MAPS as side groups are able to heal up to 98% after being exposed to heat at 120°C for 2 h. However, for material with more extended spacer side groups, the healing performance is 89% after 6 h at 120°C.

Another zwitterionic self-healable elastomer was developed by blending zwitterionic polyol and conventional polyol.⁵⁴ The material was reported to heal by water-triggered and demonstrated excellent self-healing capabilities with significantly enhanced surface hardness, surface elastic modulus, and Young's modulus, which attributed to the strong interactions between zwitterions clusters. Yang et al.⁵⁵ investigated the application of ionic crosslinking in self-healing strain sensors based on brominated NR. Their strategy involved NR's bromination and mixing with histidine (His) to form His- Br – coordination bonds. The healing ability is 93.98%, with a recorded healing time is 10 s for mechanical healing. Figure 5(a) and (b), captured by a three-dimensional microscope, the cut at the damaged area almost disappeared due to the treatment process. After about 10 s, the self-healed sample, as shown in Figure 5(c), could be twisted and stretched into an extended elongation without fracture. The prepared sample was coated with a conductive layer to check its ability to recover its electrical functionality. Visual observation was conducted by connecting the sensor to a conductive light-emitting diode (LED) bulb circuit, as in Figure 6. The blue light is turned off due to the sample splitting and lighted back after being brought into contact in only 2 seconds.

Figure 5.

The images of (a) cut and (b) healed samples in super depth of field three-dimensional microscope. Images of the healed sample given the stretching and twisting process.⁵⁵

Figure 6.

Images of sensor’s electrical functionality revovery when connected to the (LED) bulb conducting circuit.⁵⁵

ENR is one of the commercially available rubbers that are primarily used in the self-healing field.^56,57 This is because ENR contain epoxide group (polar oxirane) that ease the integration of self-amendment properties to the material. One of the successful self-healable ENR has been conducted by Mandal et al.⁵⁸ by utilizing controlled release mixed metals ions (Fe-Zn) and diamine as crosslinking agents. The reformation of the network is aided by the slow release of metal ions from the metal amine complex to the ultra-active oxirane groups. Diamine is the compound that responsible to release active ferric ions in a control manner to oxirane ring to activate the formation of new networks at the damaged site. Figure 7

Figure 7.

Current signals and pictures of PAA-rGO elastomer in the monitoring of external movement of (a) finger bending, (b) wrist bending, (c) slight bend and (d) large bend.⁵⁹

Hydrogen bonding

Hydrogen bonding has a relatively weak strength of bond (∼10 to 65 kJ/mol), but it is still employed widely in self-healing materials because it is considered the simplest and most effective bond that has autonomous healing properties. However, it is still a significant challenge to prepare autonomous hydrogen bond-derived self-healing material that combines high modulus and toughness. Yang et al.⁵⁹ constructed a room temperature conductive self-healing elastomer based on hydrogen intermolecular networks for strain sensor application, which was applied on human skin to detect external movement such as finger bending, wrist bending, slight bend and large bend as display in Figutre 7. The material synthesized from poly (acrylic acid) (PAA) embedded with reduced graphene oxide (rGO) is identified as PAA-rGO elastomer. The prepared material increased in mechanical properties and decreased in electrical resistance with the rGO content. Formation of overlapping between rGO sheets enabling the conductive channel to be built in the material. Maximum recovery of mechanical and electrical properties after cutting achieved at room temperature is 95% only in 30 s, and it was reported accurately detecting the current signal even after undergoing the healing process.

Robust self-healing TPU elastomers that are able to recover at 100°C fully were developed by Hu et al.⁶⁰ by integrating dangling 2-ureido-4 [1H]-pyrimidione (UPy) side group consisting of four hydrogen bonds into the main disulfide mechanism. The result in Figure 8 revealed that as the amount of UPy ratio increased, the tensile strength decreased, and the highest strength recorded was up to 25 MPa. To impart self-healing behaviour into this material, UPy-diol is embedded together with SS-diol as chain extenders and acts as thermo reversible crosslinkers. Figure 9 illustrates the production routes of self-healing TPU elastomers. The synthesis involves the reaction between hydrogenated 4,40 -methylenediphenyl diisocyanate (HDMI) as diisocyanate with polytetramethylene ether glycol (PTMEG) as polyol with the presence of dibutyltin dilaurate (DBTL) as a catalyst. The feeding ratio of SS-diol and UPy-diol chain extenders were varied. The result produces TPU with a dynamic main chain and consists of thermally reversible quadruple hydrogen bonds between dangling UPy side chains.

Figure 8.

The original strength and the post healing treatment strength at different temperatures. Reproduced from.⁶⁰

Figure 9.

The synthetic routes of self-healing TPU elastomers with dynamic main chain and self-complementary quadrapule hydrogen bonds between dangling UPy side chain.⁶⁰

Intrinsic dynamic covalent bonds

Dynamic covalent systems are said to have better performance in self-healing performance than non-covalent systems. Therefore, dynamic covalent bonds play a key role in formulating self-healing material. In the bond reforming process, stability and reactivity are the important factors to be considered. For instance, free radicals formed after cleavage or the other reactive groups must remain reactive and have sufficient time to re-bond before they undergo oxidative processes. Strategies such as Diels- Alder reaction and disulfide exchange reaction are exploited under the dynamic covalent systems.

Diels-Alder reaction

Incorporating Diels- Alder (DA) reaction as the intrinsic self-healing mechanism is considered the most successful strategy, which can closely mimic the biological models. A number of researchers investigated thermally reversible DA routes to achieve dynamic reversible crosslinking due to its characteristics of low coupling and high decoupling temperatures.^33,61–65 One of the most common examples of DA is the reaction between furan and maleimide as shown in Figure 10^61,66,67

Figure 10.

The reversible DA coupling of a furan heterocycle (diene) with a maleimide (dienophile) counterpart applied to a polymerization system.⁶⁸

Figure 9 shows the formation of cyclohexene adduct through [4+2] cycloaddition via the interaction between an electron-rich diene and an electron-deficient dienophile.⁶⁸ After the construction, cyclohexene can be broken back into diene and dienophile by retro-DA (rDA) reaction at high temperatures. In a study by Tanasi et al.,⁶⁹ furan moieties were grafted to maleated rubber where maleic anhydride was first attached to NR chains and the pending furans were formed crosslinked with bismaleimide. The mechanical properties and crosslinking density of NR bonds with the DA process are comparable to those of vulcanized NR with low sulfur content. Jia et al.⁷⁰ applied a thermoreversible crosslinked DA reaction in commercial ethylene propylene diene monomer (EPDM) rubber embedded with silane-modified silica. EPDM grafted with maleic anhydride (EPDM-g-MA) was reacted with furfuryl amine to yield furan functionalized EPDM (EPDM-ga-FA), then it was further crosslinked with 3-methacryloxypropyltrymethoxysilane (as poor electron agent). The resulting material can be recycled and yield comparable mechanical properties to the original samples. Crosslinking and de-crosslinking via DA mechanisms were proven by DSC analysis and solubility tests. The fabricated samples show an endothermic peak at 72°C, associated with DA, and an endothermic peak at about 137°C belongs to rDA at 160°C. From the solubility test, the samples are not soluble in toluene at room temperature. However, the sample dissolve when heated in the same type of solvent at 160°C in 30 min and turn into a gel-like solid when the temperature drops during a cooling process. Self-healing ability of EPDM/silica composites was monitored by observing the recovery of the cut sample. Figure 10(a) shows a cut on the sample sheet and healed sample in Figure 11(b) after heating to 160°C for 10 min, then cooled to 80°C. As shown in the figure, the recovered sample cannot be easily broken when applied with external force and proof that the material was able to self-heal under a heating-cooling cycle. However, there are no monitoring results of self-healing properties based on tensile tests reported in the work.

Figure 11.

(a) EPDM/silica composite sheet with a break; (b) Heating-cooling cycle healed EPDM/silica composite sheet under external force crosslinked.⁷⁰

Disulfide exchange reaction

In preparation for dynamic self-healing polymers on sulfur-based chemistry, the two most common mechanisms used are thiol-disulfide exchange and disulfide exchange through radical-mediated mechanism.⁷¹ The exchange mechanism of thiol-disulfide is via nucleophilic displacement of a thiolate anion attached to disulfide by another existing thiolate anion. By raising the pH above neutral, the exchange of thiolate anion can be catalytically controlled, and typically amine base accelerators are employed. However, aerial oxidation of non-attach thiolates at neutral or basic conditions will be the significant problem in this type of reaction which might reduce the system’s dynamic behaviour.

Disulfide exchange can be classified into two categories: i) aliphatic disulfide exchange and ii) aromatic disulfide exchange mechanism. Two competitive mechanisms were considered in disulfide exchange: metathesis and radical-mediated mechanisms. In the concept of disulfide bond for self-healing elastomer, radicals will be formed upon the mechanical breakage and recombine or undergo the rapid exchange with S-S bonds. This process can prevent the complete collapse of the structure by releasing the energy accumulated by initial elastic deformation.⁷² An et al.⁷³ reported dual-sulfide-disulfide networks self-healing polymer by combining methacrylate copolymers bearing pendant vinyl groups (P2-ene) with a polythiol by photo-induced thiol-ene click-type radical addition. It was found that dynamic disulfide crosslinkages were generated with excess iodine-modified thiol (S-H) groups, and the materials exhibited rapid self-healing properties at room temperature with the absence of external healing agents without external stimuli applied. However, the healed cuts were all under 0.1 mm in width. Therefore it is not sufficient to consider that the fabricated materials led to the complete restoration of the mechanical properties.²⁴ Recently, Gao et al.⁷⁴ synthesised acrylate-based elastomers via thiol-terminated polysulfide crosslinkers containing rich disulfide bonds that allowed the elastomer to be healable and reprocessable. To investigate and compare the thiol-ene click reaction via different mechanisms, they synthesise the materials through the radical pathway, such as redox-initiator and photo-initiator systems, and catalytic processes mediated by bases (Michael addition). From the investigation, the acrylate-based elastomers could be cured by an alkaline system, redox system and photo-initiator. However, the free radical initiation with no heating system failed due to lower activity. Alkali in acrylate-based elastomer (alkaline- AE) helps catalyse and promote disulfide bond exchange by showing better properties than those prepared by photo-initiator and radical pathway. An optical microscope and tensile test were utilised to monitor the healing process progress for self-healing capability. From the kinetic curve in Figure 12(a), which resembles the reading from the measurement of width changes of scratches, 5 h are taken for alkaline- AE to restore 90% of its mechanical properties. The highest tensile strength value for the pristine alkaline- AE was only 0.36 MPa, and the recovery retest value is 83%, calculated from the healed ratio to that of original after being treated at 100°C.

Figure 12.

The self-healing efficiency of acrylate-based elastomers (a) based on changes of scratch widths after treated at different healing times at 100°C and (b)the tensile stress-strain curves of alkaline-AE, redox-AE and photo-AE before and after self-healing at 100°C.⁷⁵

Matxain et al.⁷⁵ synthesised epoxidised vegetable oil (EVO) based recyclable thermoset from epoxidised linseed oil (ELO) crosslinks with the combination of aliphatic disulfide (3,3-dithiodipropionic acid (DA)) and aromatic disulfides (2,2-dithiodibenzoic acid (BA)). Utilising the combination of disulfide-containing crosslinkers can help the material to achieve desirable combinations in material properties and processability because an aliphatic crosslinker can provide good flexibility and weather ability properties to the polymer, while aromatic crosslinker not only increase the mechanical properties by its rigid structure but will expedite the disulfide metathesis reactions which enhancing reprocessing ability due to low bond dissociation energy. Combining the two diacids has proven to increase the reactivity of the reaction by lowering the activation energy (Ea). The value of Ea for material containing only aliphatic disulfide is 109 kJ/mol, and the value reduces to 75 kJ/mol when both aliphatic and aromatic disulfide is used as crosslinkers. The value of activation energy obtained by the Kissinger and Ozawa methods is in correlation with the DSC result, which shows that material containing BA led to higher reactivity as revealed by the higher reaction enthalpy. Aliphatic and aromatic disulfide exchange reaction was able to occur only at a higher temperature above 150°C since there were no signals detected in the BA/DA disulfide mixture heated at 130°C for 5 h, conducted by H NMR spectra.

Vitrimers

Vitrimers are polymer network materials designed with dynamic reversible covalent crosslinks, where the constructed bonds generated via associative covalent bonds undergo very fast dissociation and association reactions simultaneously with the ability to secure the network integrity firmly. Implementing the vitrimer concept in elastomer allows the covalent network to be self-healed, reprocessable and recyclable.⁷⁶ Thus it appears as a unique method to fabricate self-healable rubber products without sacrificing the mechanical properties.³ There are two mechanisms of vitrimer; dissociative dynamic covalent crosslink and associative covalent crosslinked and by taking the Diels-Alder reaction as an example for the first mechanism (Figure 13(a)), Hayashi et al.⁷⁷ sketched a diagram displaying the dissociation of furan and maleimide groups when heated. When the heat source is removed, the two groups are re-associated again via the retro Diels-Alder reaction. This bond exchange mechanism allows the material to undergo thermoplastic-like processing, such as injection moulding and extrusion. Weak physical crosslinks dissociate gradually, causing the material’s viscosity to decrease slowly. In contrast, the latter mechanism has been mentioned to have simultaneous and rapid bond exchange reaction that allow the network integrity to maintain and sustaining the density of crosslink. Figure 13(b) demonstrates the transesterification reaction between the ester and alcohol group in base condition.

Figure 13.

Example of (a) dissociative dynamic covalent bonds and (b) associative dynamic covalent bonds, and the schematic for the difference in the bond exchange mechanism in the network.⁷⁷

Wang et al.⁷⁶ reported a recycle-reinforcing elastomer vitrimer synthesised by mixing epoxidised natural rubber (ENR) and carboxylated nitrile rubber (XNBR) with the absence of a catalyst. The sample can be recycled, and interestingly, the mechanical performance of the sample improves after the recycling process, from 2.8 Mpa to more than 6 Mpa after the third recycles. Figure 14 revealed the stress-strain property of the pristine and recycled samples. Figure 15 shows the images of the sample that was cut into smaller sizes, masticated and underwent the moulding process to re-development into the same shape to verify its recycling properties. Zhu and co-workers⁷⁸ also reported on a catalyst-free vitrimer synthesised from ENR with 50% of epoxidation degree. The strategy involves the segregation of sodium alginate (SA) in an elastomer matrix, then embedded with exchangeable hydroxyl ester bonds as crosslinkers at the epoxidised natural rubber–sodium alginate (ENR-SA) interfaces. The results show that SA act as both a crosslinking and reinforcing agent since the tensile strength and modulus increased with SA loading. In this study, vitrimers with segregated structure identified as defect-tolerant, solid and tough elastomeric vitrimer was developed as a target material. Unstructured dispersion of SA particles in the ENR matrix was produced as a control sample to illustrate the importance of microstructural design. From the reported results, the sample’s mechanical properties with a segregated phase are slightly higher than the latter, revealing an enhancement of load-bearing and sacrificial ability. In comparison, with SA loading of 20 phr, the tensile strength value for the sample with segregated and randomly dispersed particles are 26.5 MPa and 22.4 MPa, respectively. Various associative exchange reactions can be employed to fabricate vitrimers, such as sulphur-sulphur metathesis, boronic ester exchange, imine exchange, transcarbomylation, transamination and transalkylation.

Figure 14.

Stress-strain curves of the original and recycled XNBR.⁷⁸

Figure 15.

Images of XNBR. (a) Original sample, (b) first, (c) second and (d) third time recycled sample.⁷⁸

Combination of different intrinsic mechanisms

Instead of a single reversible network, combining two or more self-healing mechanisms in one material’s system is designed by the researchers with the purpose of achieving good qualities of both self-healing efficiency and mechanical properties. A work reported by Qu et al.⁵⁷ utilized a dynamic dual network of self-healing elastomer based on boron/oxygen dative bonds and hydrogen bonds to fabricate highly flexible polyborosiloxane/polysiloxane (PBS/PDMS) with rate sensing capability. The tensile strength of a single dynamic bond of boron/oxygen is 0.04 Mpa, and the strength increases up to 0.42 Mpa after being combined with a hydrogen bond from PDMS, with maximum elongation at break of 1171%. The mechanical properties of the damaged material are restored almost entirely after a 2 to 4 h healing process at 80°C. To realize the PBS/PMS self-healing elastomer as a flexible sensor, MWCNT was embedded into the material. Figure 16 shows that the light-emitting diode (LED) can light up after the recovery process, showing that the prepared sample has good electrical conductivity after the healing treatment.

Figure 16.

The electrifying situation of LED in the circuit with PBS/PDMS- MWCNT sensor, (a) original sensor, (b) cutting sensor, and (c) healed sensor.¹²⁹

Very recently, Chen et al.⁷⁹ developed rapid underwater self-healing elastomer by combining π – π interaction and water-stable debye forces onto poly (benzyl methacrylate) -poly (ethyl acrylate) (PBMA-PEA) networks. From the mechanical tests, the self-healing elastomer has the maximum high Young’s modulus at 48 Mpa. The material is subjected to be applied for underwater stuff such as the diver’s oxygen tube. Figure 17 demonstrates the leakage oxygen tube able to be repaired after pressing for a few seconds underwaters. 90% of mechanical recovery tested under harsh aqueous conditions is recorded in just 10 s at room temperature.

Figure 17.

Simulation of tube transport 0.5 L/min oxygen (a) cut by a blade, (b) leakage, (c) gently pressed underwater, (d) recovered from leakage.⁷⁹

A self-healing bromo butyl rubber (BIIR) was designed by hydrogen and ionic bonding hybrid.⁸⁰ Triple hydrogen networks were created by the interaction between uracil groups and diamidopyridyl bi-linker to further analyse the potential of ionised BIIR from prior work.¹⁰ The added hydrogen bonding as a secondary reversible network softened the rubber compound even more, thus improving the material’s flexibility owing to the spacer length increase, leading to withstand additional force during deformation. However, the result from repeated tensile tests after damage revealed that the elastomer’s ability to recover the damage itself with a dual healing mechanism is 39% only, lower than the fabricated sample with the solely ionic network, which achieved 74%.

Biobased self-healing elastomer

The biobased self-healing elastomer can be defined as materials that can self-mend fabricated from living or once-living substances. The research to replace petroleum-based elastomers with biobased is a growing concern due to the shortage of non-renewable resources.⁸¹ In a study conducted by Lai et al.⁸² natural rubber (NR) was melt-blended with polycaprolactone (PCL) to develop a self-healing elastomer. The resultant elastomer holds shape memory properties, assisting the healing process. Acrylic acid was grafted onto the PCL molecular chains to enhance healing efficiency through supramolecular hydrogen bonds. The highest tensile strength is about 25 Mpa, and the healing efficiency demonstrated by the modified elastomer is 62.8% at 80°C.

In another study conducted by Cao et al.⁵³ high aspect ratio cellulose nanocrystals (t-CNs) originated from marine biomass were embedded in self-healing epoxidized natural rubber (ENR), which was prepared from simple latex mixing. Both desired properties, self-mending and mechanical strength, were significantly enhanced due to reversible hydrogen bonding between the oxygen-containing groups in ENR and hydroxyl groups of t-CNs. Self-healable, stretchable with the photoluminescence properties of carboxymethylcellulose-based (CMC-based) hydrogel was developed by Chen et al.⁸³ Such properties are achieved through ionic bonding interaction between Al3+ ions with COO- of citric acid derivatives and CMC.

Utilizing different content of cellulose nanocrystals, Li et al.⁸⁴ designed a self-healing gel by integrating disulfide bonds of thiuram through the polymerization process. The restoration mechanism was via the reconnection between dithiocarbamate radical intermediates and the delocalization of unpaired electrons on the dithiocarbamate structure generated when the gel bonding is cleaved. In a study by Liu et al.⁸⁵ self-healing polysaccharide hydrogel based on enamine bonds was prepared by mixing cellulose acetate aqueous solution with chitosan. Their strategy is to react cellulose with tert-butyl acetate in ionic liquid 1-allyl-3-methylimidazolium chloride (AMIMCl). The enamine bonds were created between the amino groups of carbonyl group from cellulose acetate and chitosan to develop multi-component hydrogel network. Shao et al.⁸⁶ applied the reversible DA reaction in the development of self-healing nanocomposite hydrogel from polyethylene glycol (PEG) with a maleimide terminal group embedded with cellulose nanocrystal (CNC) modified by furanyl. Other than reinforcing filler, CNC-modified furanyl also acted as a multifunctional crosslinker. The results show that the mechanical properties and self-healing efficiency can be adjusted by controlling the per cent of furyl functional groups substitution and the mole ratio of furyl to maleimide.

Self-healing rubbers for tyre application

Over the decades, research on self-healing elastomers has been extensively explored and is still being digging^87–89 because of the humongous potential to be implemented into various applications such as in the automotive industry^90,91 electric and electronic devices^21,92 sensors^55,93–95 coating,⁹⁶ aerospace industry,⁹⁷ etc^98–100 due to their ability to increase service lifetime. In the automotive industry, investigations on self-healing elastomers have become more and more interesting because it consumes more than 75% of world rubber production and is even considered to have the most prolific relationship.¹⁰¹ Particularly, tyre applications since elastomers are a fundamental part of a tire’s composition. The current tyre industry is not just concentrating on the development of new technologies to increase consumers’ safety and overall performance of the tyre only, but also with the purpose to focus on the environmental aspects by employing sustainable materials which may become a new standard for many industries.

Tyres is often subjected to tough and intense service environments, such as expose to harsh weather conditions and numerous abrasions with the road surface. Tyres is constantly damaged by punctures and have expiration date, necessitating their replacement or repair and producing an excessive amount of waste tyres. xxxx denoted that the advanced materials for future tyre applications should be equipped with self-healing behaviour, self-adapting and self-controlled over itself. This can be accomplished by using intelligent materials. When considering the environmental impacts from the tyre applications, Naddeo et al. (2021) also emphasised the same facts, the possibility of recyclable sustainable tyres made of sustainable materials with low rolling resistance indices, as well as low rolling noise indices.

NR is the main material used to make tyres because it has the highest failure strain, the best damping properties, and the best resistance to wear, creep, and cyclic deformation. Due to this fact, very recently, Masraff and Shuib (2023)⁹⁰ reported a self-healing sealant rubber tape intended for puncture-proof pneumatic tubeless tyres based on natural rubber. Self-healing ability was activated by integrating reversible ionic crosslinks derived from zinc thiolate that grafted on widely available NR chains. The fabricated tape was then consolidated onto the inner liner of the tire. The punctured test was conducted using varied diameters of nails (1.69, 2.48, 3.37 and 4.03 mm), after the nail was punched onto the tire, it was removed, and the air pressure was measured and analysed. The results of the test after the tyres were left for 24 h were revealed in Figure 18 below. It shows that the material was able to heal the damage caused by smaller diameter nails (1.69 and 2.48 mm) almost 100%, then reduced its efficiency when the larger diameter of nails was used. A repeated puncture test was also done and evident that the material is able for repeated punctures at the same spot and maintain its structural integrity at different test conditions such as pressure, temperature, and time.

Figure 18.

The healing percentage of self-healing rubber tape punctured with different nail diameter.⁹⁰

Previously, a team of researchers from Harvard University⁶⁸ created a synthetic elastomer for use in flat tyres by embedding imidazolium into commercially available BIIR with covalent and reversible ionic non-covalent networks, which they believed would simultaneously enhance the mechanical strength and self-healing performance of the rubber. In a separate study, Chumnum et al.⁶⁹ designed a self-healing BIIR modified with ionic liquid (IL) and butyl imidazole (IM) for use as inner liners and tire treads. Carbon black and CNT hybrid fillers were incorporated into 70:30 and 80:20 phr (BIIR: NR) mixtures. The sample with the IM modifier, denoted as BIIR/NRCNTCB-IM, displayed the least reduction in conductivity after three recycling cycles and conductivity tests when compared to the initial test piece (Figure 19). It indicates that the healing process of the composites not only rejoins the fractured molecular chains, but also allows the fillers to reconnect. The mechanical properties of undamaged and repaired samples are tabulated in Tables 1 and 2, respectively, and as can be seen, a low self-healing percentage was observed when measuring self-healing performance based on tensile strength reading.

Figure 19.

Electrical conductivity of pure BIIR, pure NR and their composites with and without IL and IM modifiers. BIIR:NR ratios of 70:30 and 80:20 after recycling from 1,2, and 3 rounds using the compression molding of 10,000 Pa.¹³⁰

Table 1.

Mechanical properties in terms of 100% moduli, tensile strength, and elongation at break of the pure rubbers and their composites before self-healing propagation.¹³⁰

Samples	100% Modulus (MPa)	Tensile strength (MPa)	Elongation at break (%)
Pure NR	1.31 ± 0.02	14.50 ± 0.15	1412.45 ± 5.34
Pure BIIR	0.36 ± 0.05	0.12 ± 0.10	3124.45 ± 6.55
BIIR:NRratio = 70:30
BIIR/NR	0.35 ± 0.04	2.32 ± 0.12	1504.17 ± 14.56
BIIR/NR-CNT_CB	0.48 ± 0.11	4.47 ± 0.25	1164.04 ± 23.16
BIIR/NR-CNT_CB-IL₅	0.96 ± 0.10	3.75 ± 0.10	551.82 ± 16.65
BIIR/NR-CNT_CB-IM₅	0.50 ± 0.06	3.31 ± 0.22	1743.16 ± 34.55
BIIR:NRratio = 80:20
BIIR/NR	0.35 ± 0.05	1.57 ± 0.15	1159.44 ± 21.76
BIIR/NR-CNT_CB	0.44 ± 0.01	2.45 ± 0.51	1064.14 ± 62.98
BIIR/NR-CNT_CB-IL₅	0.70 ± 0.02	2.82 ± 0.32	564.75 ± 44.23
BIIR/NR-CNT_CB-IM₅	0.40 ± 0.04	1.79 ± 0.57	2476.55 ± 24.45

Table 2.

Mechanical properties of the pure rubbers and their composites after self-healing propagation.¹³⁰

Samples	100% Modulus (MPa)	Tensile strength (MPa)	Elongation at break (%)
Pure NR	—	—	—
Pure BIIR	0.33 ± 0.12	0.27 ± 0.15	674.65 ± 23.34
BIIR:NRratio = 70:30
BIIR/NR	0.34 ± 0.05	0.48 ± 0.12	396.59 ± 11.33
BIIR/NR-CNT_CB	0.42 ± 0.06	0.51 ± 0.11	257.80 ± 32.23
BIIR/NR-CNT_CB-IL₅	0.50 ± 0.01	0.50 ± 0.08	166.10 ± 14.43
BIIR/NR-CNT_CB-IM₅	0.48 ± 0.14	0.78 ± 0.19	497.80 ± 15.18
BIIR:NRratio = 80:20
BIIR/NR	0.33 ± 0.04	0.46 ± 0.02	336.73 ± 32.33
BIIR/NR-CNT_CB	0.41 ± 0.16	0.47 ± 0.05	283.77 ± 16.43
BIIR/NR-CNT_CB-IL₅	0.53 ± 0.07	0.54 ± 0.23	191.13 ± 18.55
BIIR/NR-CNT_CB-IM₅	0.50 ± 0.12	0.68 ± 0.45	502.25 ± 25.78

Several well-known tire manufacturers, such as Continental, had adopted self-healing technology. ContiSealTM is an innovative tire installed on the Volkswagen Passat CC that can seal 80% of punctures promptly. It was created by coating the inner tire with a viscous and adhesive self-healing agent. Goodyear's company announced a patented technology called DuraSeal Technology® for self-healing truck tyres that integrate a yellow gel-like rubber sealant to heal damaged areas. The punctured site triggered the gel to flow and heal itself. Based on the findings, the healing procedure at the same location can be repeated multiple times, extending the tire's service life by up to six times compared to conventional truck tires. Figure 20 depicts the procedure for repairing a Finnix self-sealing tire made from thermoplastic elastomer that has developed a fissure. This technology was developed by Finixx New Technology Co. Ltd using mix thermoplastic elastomer Styrene Isoprene Styrene (Mix SIS) block polymer.

Figure 20.

Mix SIS fills a puncture void of Finnix seal leakage tyre.

Assessment and monitoring of healing performance

The growing interest and successful development of self-healing polymer required a methodology to measure the material's performance to recover its structural and functional properties following damage. Different characterization strategies must be applied for different types of self-healing polymeric materials. Scratching, cracking, puncture, and delamination are examples of damages that self-healing polymers can heal, and some researchers also include conductivity and fluorescence recovery properties as the study object of functional polymers. Damage type, the trigger factor, the external stimulation, and the healing treatment itself are the factors to be considered in defining the healing ability.⁵⁰ Each test method is designed for specific applications. For example, scratch repair testing is suitable for studying self-healing of coating layers and only for early characterization of the repair behaviour for the lab scale specimens which small in quantity. However, such test cannot be considered for determining the recovery of structured material’s mechanical properties. In this case, tensile test or double cantilever beam tests are more suitable methods, but such techniques are limited to relatively brittle polymers and the latter is frequently used for extrinsic system utilization.²⁴ Fracture mechanics testing is more suitable for characterizing the healing of ductile polymers or deformable materials. Therefore, a functional relationship exists between polymer type, required application and test method. In this section, the testing method and characterization techniques employed to evaluate the healing performance of self-healing rubbers occurring at both microscopic and macroscopic levels, either quantitatively or qualitatively, were overviewed. The methods involve physical testing to determine the mechanical properties and characterization to reveal the underlying healing mechanism. However, healing assessment of other functions such as electrical, electrochemical, fluorescence, and thermal properties will not be included in this review. The focus is only on the mechanical damage and the related testing.

Bulk mechanical analysis

The healing evaluation of self-healing rubbers involves quantification of healing performance, η and it is defined as the recovery percentage of virgin rubber properties after healing as shown in equation (1)^102,103

η = \frac{P_{h e a l e d}}{P_{o r i g i n a l}} x 100 %

(1)

Where η is the healing efficiency, P_healed is the properties after healing, and P_original is the properties before healing. It requires repeated physical testing methods of self-healing rubber, which is measurable to be conducted before and after healing. Different tests are employed using different damage modes, depending on the applications of self-healing rubbers. During testing, the material is exposed to external mechanical stress until failure, and the healing reaction takes place to repair the damage. The material is then re-tested to compare the properties of virgin and healed samples to evaluate healing efficiency. Few physical characterisations are commonly used to evaluate the healing performance of self-healing rubbers. Most are under quasi-static fracture methods such as tensile test and tapered double cantilever beam (TDCB). The crack formation and propagation that occurred following the tests are controlled and predictable, associated with small fracture volume and quantifiable fracture parameters. Fatigue and impact are the other primary fracture loading conditions often evaluated in self-healing rubbers.¹⁰² All these methods were discussed in the following sections.

Tensile test

Tensile testing is one of the basic tests for the measurement of strength and deformability for elastomers. However, in some cases of self-healing elastomers, tensile testing is not suitable for studying the healing behaviour. Many researchers utilized the tensile in the testing range of self-healing polymer to gain information on the healing efficiency via changes in the tensile strength of healed and pristine material. It can also be used to get rough details on the time dependence of the healing process.²⁵ During the process, the researcher must concentrate on the presence of any surface roughness and ensure a proper alignment of fracture surfaces because these two factors often lead to weaker tensile properties due to the porosity of rubber samples. The test conditions, such as the test sample shape and dimension, crosshead speed of the machine and testing temperature, follow either ASTM or ISO standards to minimize the occurrence of variations, while ASTM D412 is mostly employed to conduct tensile tests for an elastomer involving a dumbbell-shaped specimen. In order to evaluate the healing performance of a self-healable rubber, the rubber sample is subjected to stretching until the break, and the fracture surfaces are joined together and sent for healing treatment and re-tested under the same testing conditions^104–106

Chen et al.¹⁰⁷ studied the tensile properties of self-healing epoxidized natural rubber (ENR) after different healing conditions and the healing efficiencies of the rubber compound with different dithiol-bearing boronic ester (BDB) content, as shown in Figure 21. In this work, BDB was used as a crosslinker to create a rearrangeable crosslink network based on the reaction between thiols and epoxy groups, contributing to the self-healing ability of ENR. Figure 21(a) demonstrated that the healed sample could undergo stretching due to the re-establishment of the crosslinked network. Based on Figure 21(b) and 21(c), it can be summarized that the rubber sample healed at 80°C for 24 h and showed the most comparable tensile properties with that of the virgin rubber sample, indicating that this healing condition yields the highest healing efficiency. ENR with 3% and 5% BDB, labelled BE3 and BE5, respectively, showed the highest healing efficiency among other rubber compounds. This means that the 3% and 5% BDB content provided the optimum crosslink density, in which higher crosslink density may restrict the chain mobility to diffuse across the interface for the self-healing process to occur, while relatively low crosslink density was not sufficient to restore the tensile properties of the healed sample.

Figure 21.

(a) Healing of self-healable ENR; stress-strain curves of ENR compounds healed (b) at different temperatures for 24 h and (c) at 80°C for different periods and (d) healing efficiencies of ENR with different BDB content healed at 80°C for 24 h.¹⁰⁷

Fracture mechanics

The tensile strength of a broken sample is not a suitable test to quantify interfacial healing because it cannot provide information on chemical activities at the healing interfaces nor the effective restoration of mechanical integrity.²⁵ The recovery process between two fracture surfaces should be treated as an interfacial phenomenon. Test protocol based on fracture mechanics such as TDCB should be utilized to acquire quantitative information on the healing behaviour, especially on brittle polymer.¹⁰⁸ Applying the J-integral parameter in the fracture mechanics testing procedure on soft materials such as elastomer to determine the process of interfacial healing was reported more accurately than tensile testing. The accuracy in assessing the interfacial healing is attributed to properly measuring mechanical integrity restoration as a function of healing time and temperature by considering the interface structural discontinuity in its various stages.

Tapered double-cantilever beam

A tapered double-cantilever beam (TDCB) offers a unique testing method where the fracture toughness does not depend on the crack length with the geometry, as shown in Figure 22. This method can be used for multiple healing cycles when the crack length varies with each healing cycle and provides a more accurate self-healing performance than other methods, in which the results may be affected by the variation of crack length. Another advantage of this method is that the realignment of the geometry is relatively easy when a partial fracture occurs.¹⁰⁹ TDCB has been widely used to test extrinsic self-healing polymers such as the self-healing epoxy nanocomposites designed by Guadagno et al.¹¹⁰ In this study, they evaluated the healing efficiency of epoxy composites loaded with different types of carbon nanotubes. The composite samples were introduced with a pre-crack to sharpen the crack tip and subjected to displacement to propagate the crack along the centreline of the sample. The crack then underwent healing at room temperature and was re-tested again until failure. The healing efficiency was determined as the ratio of fracture toughness of healed and virgin samples. Zheng et al.¹¹¹ developed self-healing polyurethane (PU) coating loaded with improved stability and strength of microcapsules designed by a multi-layered shell structure filled with dicyclopentadiene (DCPD). Importantly, they focus on the convenience of synthesizing multi-layered shells and assessing the chemical reactivity of core material, which previous researchers may overlook. In this study, the TDCB fracture test was applied to measure the self-healing efficiency of multilayer microcapsules. It reveals the healing efficiency of PU coating was about 118%, and multilayer microcapsules provide the excellent self-healing capability.

Figure 22.

Tapered double-cantilever beam (TDCB) geometry.¹¹⁰

Fatigue

Fatigue is the most common dynamic damage mode that can be found in elastomer products. Also, the major failure that can shortens the service life of rubbers because they are often subjected to cyclic deformation when utilized in engineering applications. It permanently damages elastomeric materials due to fluctuating stresses and strains, producing a crack after sufficient fluctuation.⁴⁴ The stress needed to fail a material during repeated loading cycles is much lower than in a single loading event. Thus, it is crucial to evaluate the fatigue lifespan and fatigue resistance of self-healing rubbers, which can potentially be exposed to high cyclic loading when in service.^102,109 The healing process occurs simultaneously when the energy accumulates within the material system during fatigue loading. Therefore, it is suggested that a faster healing rate is desired to enhance fatigue crack resistance.¹¹² The healing performance of the material under fatigue loading can be assessed by determining the fatigue life extension using the calculation shown in equation (2).

λ = \frac{N_{h e a l e d} - N_{c o n t r o l}}{N_{C o n t r o l}}

(2)

Where

λ

is the fatigue life extension ratio, N_healed is the number of cycles to failure of a self-healing sample, and Ncontrol is the number of cycles to failure of a control sample without healing ability. Cheng et al.¹¹³ included the fatigue stretching test in their design of self-healing and recyclable ENR based on dual self-healing systems: disulfide metathesis and hydrogen bonding. The material was subjected to cyclic stretching up to 60 000 times at constant strain, which was about 250% and then sent for healing at 120°C for 6 h. A tensile test was carried out for both pristine and self-mended specimens to determine the mechanical properties of self-healing ENR after being subjected to fatigue deformation, as shown in Figure 23. The material showed superior healing performance in which the original strength was recovered up to 98% after healing the fatigue-damaged sample. Even though the material was further subjected to three times fatigue stretching tests, it was still able to achieve 87% healing efficiency, indicating that the material was capable of healing the fatigue damage zone and could undergo multiple healing cycles.

Figure 23.

(a) Fatigue stretching test and (b) Stress-strain curves before and after healing.¹¹³

Ballistic impact

The quantification of impact damage is relatively more complex than static and fatigue damage. It is because the damage caused by impact involves the dynamic response of both the material and impacting object. However, it provides an attractive condition for the materials to exhibit self-healing ability due to the short formation of damage and the relatively long period of time provided to heal the impact damage. In 2007, Kalista et al.¹¹⁴ discovered the recovery of ionomers with self-healing properties after ballistic impact. A sharp projectile was used to puncture the thick ionomer films. Ballistic impact forms a bullet impact zone on the material surface, associated with high temperature caused by friction and strain energy release. Ballistic healing occurs within the ionomer system, in which the healing reaction generates heat through a frictional process. The heat is used to convert the polymeric material to a viscoelastic melt state and heal the damage by reconstructing ionic interactions within the polymer system.^4,¹⁰⁹

The local heat is said to approach the melting temperature; however, the extent of heat is not easy to measure. Rahman et al.⁵⁶ reported the self-healing properties of ENR with two different epoxidation levels (25 and 50 mol % epoxidation, respectively) based on the ballistic puncture test. The optical images of the impact zone of ENR25 (25 mol % epoxidation) and ENR50 (50 mol % epoxidation) are shown in Figure 24. The impact zone of ENR50 in Figure 24(a) showed complete closure of the hole, while an apparent hole was observed in the impact zone of ENR25 in Figure 24(b). The presence of a hole in ENR25 indicated that the material could not repair the damage by itself, showing poor self-healing ability. The complete hole enclosure observed in ENR50 revealed a high chain diffusion level at the damaged area, healing the hole formed by ballistic impact. The damaged ENR50 sample was then sent for morphological characterization using SEM, as shown in Figure 25. The SEM image further supported the discussion in which the surface of the impact zone was smooth with no crack or hole observed, indicating that the integrity of the material has been fully recovered.

Figure 24.

Optical images of impact zone created during ballistic test for (a) ENR50 and (b) ENR25.⁵⁶

Figure 25.

SEM image of the surface of impact zone in ENR5.

Characterization of underlying mechanism of self-healing rubbers

The previous sections discussed various methods used to qualitatively assess the healing efficiency of self-healing rubbers. Other testing techniques are also involved in assessing the healing performance, which serve as a purpose to understand the mechanism of recovery at the elastomer network or molecular level. This is important because it can correlate to the healing efficiency obtained from the physical testing methods. It also provides an opportunity to design novel self-healing materials with better functional properties. These methods can be further classified into imaging techniques, spectroscopic techniques, thermal characterization, etc. This section overviewed a few techniques extensively used to investigate the underlying mechanism of self-healing rubbers.

Scanning electron microscopy

Microscopy is often used as an imaging technique for morphological characterization to visualise the self-healing ability of material. It is generally used before and after healing treatment to confirm the success of self-healing material. Different size of morphology requires different types of microscopy, including optical microscopy (OM), scanning electron microscopy (SEM) or transmittance electron microscopy (TEM).¹¹⁵ The most widely used microscopy in the studies of self-healing elastomers is scanning electron microscopy (SEM) to identify the surface fracture due to its ability to examine fracture patterns at higher resolutions and provide a three-dimensional image of both the outer and inner surface of an elastomer.¹¹⁶ This helps to study the relationship between the healing efficiency of a self-healing system and its morphological characteristics and gives a qualitative assessment of the healing performance of the material. An example of a study involving SEM characterization is the design of self-healing elastomer via Diels-Alder chemistry and ionic interactions by Peng et al.¹¹⁷ The specimens were cut and underwent healing at 25°C for 17 h and another 7 h at 60°C. The morphology of samples was determined using the SEM model Nova NanoSEM450 before and after the healing treatment, as illustrated in Figure 26. The DRN-0-6, which only had Diels-Alder bonds without ionic interactions, showed a clear cut line after healing at room temperature. However, it started to heal when healed at 60°C, indicating that the Diels-Alder reversible reaction only occurred at high temperature. However, the cut line did not wholly disappear, showing that the thermodynamic reaction of Diels-Alder chemistry was relatively slow. As for the DRN-24-6, which had ionic interactions within the system, the cut line was partly healed after 17 h at 25°C, meaning that the ionic interactions are dynamic at room temperature. Moreover, the cut line completely faded after heating at 60°C, showing a high healing efficiency contributed by the synergistic re-bonding of both ionic interactions and Diels-Alder chemistry.

Figure 26.

SEM micrographs of (i) DRN-0-6 and (ii) DRN-24-6 before and after healing process.¹¹⁷

Atomic force microscopy

Atomic force microscopy (AFM) is considered an excellent device to assess topographic changes after preparing nanometers to micrometre scale scratches on the surfaces of self-healing materials. Combining AFM with microthermal analysis is an interesting approach to investigating the recovery behaviour of the material in coating application at the microscopic level.¹¹⁸ However, AFM is unsuitable for a deep cuts inducer, contact mode scanning under large tip deflection conditions must be applied for the case of such cuts, and it cannot detect a rapid healing process.⁷² An et al.¹¹⁹ investigated a room-temperature self-repairability polyurethane (PU) elastomer developed from polytetramethylene ether glycol (PTMEG) blended with isophorone diisocyanate (IPDI). The reaction was carried out at 80°C and was catalyzed by dibutyltin dilaurate (DBTDL). AFM was used in this study to obtain information on the phase morphology. The result reveals that the elastomers have nanosized and loosely packed hard domains due to the partial hard segments of IPDI and weak hydrogen bonding. In another study of strong and tough self-healing elastomers by combining ionic networks and DA networks, AFM demonstrated the material with only ionic bonds in the structure without DA bonds separated into different sizes of aggregates, as in Figure 27. The broadness of the glass transition region was contributed by the sizes of aggregates formed. Bigger aggregates tend to be stronger and dissociate at a higher temperature, while small aggregates are weaker and dissociate at a lower temperature.¹¹⁷

Figure 27.

AFM phase image of self-healing polymer with ionic bonds only.¹¹⁷

Fourier transform infrared spectroscopy

FTIR is a spectroscopic technique commonly used to study the self-healing system at the molecular level by identifying the chemical functional groups in a material. It also helps to monitor the self-healing reaction by comparing the presence of certain chemical groups before and after incorporating a self-healing system into the material.¹¹⁵ It can be used for the characterization of various self-healing systems in elastomers, including both covalent adaptable network (CAN) and supramolecular networks, to ensure the self-healing system is successfully introduced into the material. Xu et al.¹²⁰ involved FTIR characterization in monitoring the presence of an ionic supramolecular network to fabricate self-healing carboxylated styrene butadiene rubber (XSBR) via the addition of zinc oxide. They suggested that the reaction between carboxylic groups in XSBR and zinc oxide generates Zn2+ ions which tend to aggregate into ionic clusters. The ionic clusters eventually form an ionic crosslink network due to electrostatic interaction. According to the FTIR result, as depicted in Figure 28, the neat XSBR peaked at 1694 cm-1, representing the stretching vibration of C=O in the carboxylic group. The XSBR/ZnO that was prepared at ambient temperature showed a similar spectrum with neat XSBR, indicating that only minimal reaction occurred to form ionic crosslinking. The absorption peak at 1694 cm-1 disappeared for the XSBR/ZnO composite cured at 100°C and 150°C. Instead, the spectrum showed a new peak at 1551 cm-1,, corresponding to the asymmetrical stretching of C=O and C-O bonds attaching to the same carbon.¹²¹ This indicated that the reaction between carboxylic groups of XSBR and ZnO mainly occurred at elevated temperatures, forming Zn2+ salt bonding that gave rise to the ionic supramolecular network.

Figure 28.

FTIR spectrum of (a) Neat XSBR, (b) XSBR/ZnO compounded at room temperature, (c) XSBR/ZnO composite cured at 100°C and (d) XSBR/ZnO composite cured at 150°C.¹²⁰

Differential scanning calorimetry

Differential scanning calorimetry (DSC) is a thermal characterisation of self-healing rubbers by understanding molecular behaviour. DSC analysis can be used to determine the difference in glass transition temperature (Tg) of the prepared sample before and after incorporating the self-healing mechanism so that the molecular changes can be detected and related to the mechanical properties of self-healing rubber. Besides, it can also detect the presence of reversible bonds in rubber systems because the breakage of these dynamic bonds requires energy which shows an endothermic peak in the DSC curves.¹²² Many researchers have involved DSC analysis in their studies to investigate the thermal behaviour of self-healing rubbers, especially those with intrinsic self-healing systems^56,70,123 This is because the reversibility of this system allows the material to heal under repeated cycles, allowing the material to be heated and cooled repeatedly during DSC analysis to analyse the thermal behaviour of the material.¹²⁴ For instance, Jia et al.⁷⁰ used DSC to monitor the Diels-Alder (DA) chemistry in the self-healable EPDM/silica composites. Figure 29 shows the DSC curves of EPDM composites with various silica loading. The results showed the emergence of an exothermic peak at about 72°C and an endothermic peak at about 140°C observed in the DSC curves for all composites. The exothermic peak corresponded to the DA reaction, while the endothermic peak corresponded to the rDA reaction since it involved the breakage of covalent bonds, which required energy. The DSC results provided evidence of reversible DA reactions in the EPDM/silica composites. Besides, there was also an exothermic peak observed at about 165°C which could be due to the thermal influences of polymerisation and decomposition of maleimides at elevated temperatures.¹²⁵

Figure 29.

DSC curves of EPDM/silica composite.⁷⁰

Raman spectroscopy

Raman spectroscopy is an excellent tool to follow chemical reactions and detect the chemical presence.²⁵ Two articles published by the same group of authors²⁴ reported self-healing natural rubber based on a modification of the crosslinking process in conventional NR vulcanized with sulfur formulations without the presence of a catalyst. They use a conventional sulfur system in NR and partially cure the rubber. In this study, Electron Spin Resonance (ESR) is used to evaluate the role of free sulfur radicals and Raman spectroscopy is employed to examine the disulfide/polysulfide ratio. A straight cut along the width of rectangular samples was manually generated by a new scalpel blade, and macroscopic damage was introduced for self-healing study purposes. The broken samples were then appropriately placed inside a home-built healing cell within 5 min of being cut at 70°C, pressure 1 bar, and 7 h to activate the healing process before being retested with a tensile test machine to acquire the self-healing efficiency. From the results obtained by Raman spectroscopy, all the samples produce more polysulfidic than disulfidic crosslinks, which correlates with the theory that conventional sulfur systems produce more polysulfidic. Higher sulfur content leads to a higher degree of crosslinking increase in stiffness and tensile strength.

Scratch analysis for coating applications

Optical-tactile measurement

For this type of quantification, optical and mechanical techniques are combined, allowing screening elements below 50 μm.¹²⁶ Dahlke et al.⁵⁰ quoted that analysis and quantification of scratch healing by optical microscopy only are not too accurate due to the low intensity of light reflection from the inner wall of scratches. Recently, they measured the healing efficiency of the novel zwitterionic through tactile profile scans which enable the volume of a whole scratch to be obtained rather than the surface area that should be counted in. By utilizing this method, complex and real forms of damage, such as fish-scale patterns, can be studied. Indenters made from diamond and MST scratch tester were used to form scratches over a length of 2500 μm. The device used to form scratches allow the material’s scratch to be reproducible with the same pattern. Abend et al.¹²⁷ have reported a technique specifically for the measurement of scratch-healing polymers, which assesses the mechanical properties using the combination of scratch tester MST3 and laser scanning microscopy (LSM) for optical analysis. This approach provided three-dimensional images of the scratches and enabled the quantification of scratch volume during the healing treatment. The comparison of scratch healing among different materials is also allowable under this technique. After LSM produced the raw data sets, they were processed automatically by a new algorithm (MATLAB@ script). 2D crack analysis was performed by the previous researcher¹²⁸ to measure the damaged area from the top view using an optical microscope. However, this method has limitations since the depth and volume of the scratch cannot be quantified.

Conclusions

In general, the repairability property is possible to be embedded in rubbers and elastomeric materials via external interaction such as capsular and vascular-based, intrinsic mechanisms, including dynamic covalent and non-covalent or any combinations of the concept. Self-healing materials can prolong the product’s service life and help reduce rubber waste. However, designing self-healing material with balance properties between mechanical and good healing performance is still a significant challenge that needs to be addressed by the researcher. In this paper, different strategies employed in recently reported self-healing rubbers have been overviewed. Understanding the different healing mechanisms, different classes of materials, expected type of damage, and targeted applications guide the selection of testing protocols for self-healing rubbers and elastomers. Appropriate quantification methods are essential to assess the healing efficiency, thus helping optimise the building of self-healing materials.

Footnotes

Acknowledgements

The first author gratefully acknowledges Universiti Teknologi MARA (UiTM) and Ministry of Higher Education (MoHE) Malaysia for awarding the PhD scholarship.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Universiti Sains Malaysia under GERAN PENYELIDIKAN PEMADANAN USM-INDUSTRI (1001.PBAHAN.8070021).

ORCID iDs

Noor Faezah Mohd Sani

Noor Aishatun Majid

Abdul Rehman

Raa Khimi

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