Tensile Strength Recovery Research Articles

Fabricating epoxy vitrimer with excellent mechanical and dynamic exchange properties via network topology design

Epoxy vitrimers are a novel type of environmentally friendly materials that possess exceptional properties, including self-healing, recyclability, and reprocessability. They offer a solution to the issue of traditional thermosetting epoxy resins, which are not recyclable. However, the incorporation of dynamic bonds typically results in a decrease in mechanical properties. Therefore, achieving a balance between the mechanical properties and dynamic exchange capability is crucial. In this work, a series of epoxy resin systems with various topological network characteristics were prepared by adjusting the proportions of bisphenol A epoxy resin (E51) and polyethylene glycol diglycidyl ether (EGDGE) epoxy resins in combination with a curing agent containing vinylogous urethane (VU) dynamic bonds. Through further study, we found that with the increase of E51 content, the cross-linking density of the network gradually increased, and the flexibility of the macromolecular chains decreased (favorable for enhancing the material's mechanical performance). Simultaneously, the free volume of the network gradually increased, and the tightness of the macromolecular chains decreased (favorable for improving the material's dynamic exchange ability). These two effects combined to make E51-40 exhibit the lowest activation energy for dynamic covalent bond exchange and the lowest topological transition temperature. Compared to E51-0, the tensile strength and glass transition temperature of E51-40 were improved by 22.23 % and 25.30 %, respectively. After remoulding experiment at 140 °C for 1h, the material exhibited a high tensile strength recovery of 91.34 %. Therefore, starting from the design of molecular structure, adjusting the network topology is an effective means to balance the mechanical and dynamic exchange properties of epoxy vitrimers. This provides experimental guidance and theoretical insights for designing high-performance epoxy vitrimer materials.

Shape‐memory thermosets: Tailoring the structure for toughness improvement and scratch healability

AbstractA straightforward but effective approach to fabricate shape‐memory polyurethane thermosets by tailoring the soft phase composition of two‐ and four‐armed polycaprolactone (PCL) polyols is described. As the crystallization of the PCL soft phase has an effect on driving the self‐assembly of H‐bonds, a good balance of tensile performance and thermo‐healability can be achieved by regulating the ratio between PCL‐diol and PCL‐tetrol in the soft phase. The inferior ability of crystallization of the segments near the branching point gives rise to enhancement of the both overall healability and toughness of the network. Healings of punctured holes and cuts are assessed using optical microscopy (OM), field‐emission scanning electron microscopy (FE‐SEM), tensile analysis, and two‐electrode impedence characterization. The obtained network with the best healing behavior has a soft phase composition with the PCL‐diol/PCL‐tetrol molar ratio of 2/1, a tensile toughness of 64 MPa J−1 and a healing efficiency (estimated by recovery of tensile strength) of nearly 80%.Highlights The composition of the soft phase of polyurethane networks was tailored. Two‐ and four‐armed polycaprolactones were used for the network structure. A good balance between mechanical and healing performance was obtained. Healing of complete cuts showed tensile strength recovery of nearly 80%.

Tunable Dual Self-Healing Composite Coatings with Self-Assembled Structures Regulated by Janus Nanoparticles.

The durability of concrete structures can be enhanced in a convenient and permanent manner through the surface protection of cementitious materials with composite polymer coatings. However, polymer coatings are susceptible to various mechanical and physical deterioration in complex and variable environments. In this paper, the theory of polymer microstructure regulation was employed to improve the sustainability of the protective performance of composite coatings. The self-assembled core-shell structure regulated by amphiphilic Janus nanoparticles is employed to modify the tunable polystyrene acrylate-polysiloxane self-healing coatings. The results demonstrate that the adhesion strength of the prepared self-assembled coating reached 3.7 MPa, which is sufficient to resist the damage to the microstructure of cementitious materials caused by physical erosion, seepage, and ionic corrosion. The self-healing coating, regulated by Janus particles, exhibited a residual creep of only 57.03% and a maximum loss angle tangent of 0.381. Furthermore, the material exhibited a superior shape memory function due to the presence of strong hydrogen bonding. The regulated self-healing coating repaired the polymer structural damage under mechanical and thermal deformation by bridging and filling effects. The coating demonstrated a tensile strength recovery of up to 71.23% in a wetted state, accompanied by a rapid restoration of its electrochemical properties and corrosion resistance. Furthermore, the self-healing emulsion penetrates the substrate defects and forms numerous polymer crystalline particles that effectively fill the microcracks.

High-Temperature Stirring Pretreatment of Waste Rubber Particles Enhances the Interfacial Bonding and Mechanical Properties of Rubberized Concrete

This paper innovatively proposes a method of 180 °C high-temperature stirring pretreatment for waste rubber particles and compares this method with untreated, NaOH-treated, and silane coupling agent KH570-treated waste rubber particles. Fourier-transform infrared spectroscopy, X-ray diffraction analysis, water contact angle measurement, scanning electron microscopy, and energy-dispersive X-ray study are used to investigate the effects and mechanisms of different pretreatment methods on waste rubber particles. The results indicate that compared to NaOH-treated and KH570-treated waste rubber particles, the 180 °C high-temperature-stirred pretreated waste rubber particles show significantly improved cleanliness and form a hard oxide film. The study also investigates the effects of different pretreatment methods on the mechanical properties and interface binding performance of rubber concrete made from pretreated waste rubber particles. The results demonstrate that rubber concrete prepared using 180 °C high-temperature-stirred pretreated waste rubber particles substituting 20% fine aggregate exhibits the best mechanical properties and interface bonding performance. The compressive strength recovery rates after 7 and 28 days are 41.6% and 37.3%, respectively; the split tensile strength recovery rates are 47.3% and 60.6%; the axial compressive strength recovery rates are 34.1% and 18.8%; and the static compression moduli of elasticity recovery rates are 46.8% and 26.3%. High-temperature stirring pretreatment of waste rubber particles is simple to operate and suitable for scaled production. Its pretreatment effect is superior to those of the KH570 and NaOH methods, providing a reference value for the scalable application of waste rubber particles as a substitute for fine aggregate in rubber concrete.

Weldable, Reprocessable, and Water-resistant Polybenzoxazine Vitrimer Crosslinked by Dynamic Imine Bonds.

Traditional polybenzoxazine thermosets cannot be reprocessed or recycled due to the permanent crosslinked networks. The dynamic exchangeable characteristics of imine bonds can impart the networks with reprocessabilities and recyclabilities. This study reported a weldable, reprocessable, and water-resistant polybenzoxazine vitrimer (C-ABZ) crosslinked by dynamic imine bonds. It was synthesized through a condensation reaction between an aldehyde-containing benzoxazine oligomer (O-ABZ) and 1,12-dodecanediamine. The resulting C-ABZ was able to be welded and reprocessed due to the dynamic exchange of imine bonds. The tensile strengths of the welded C-ABZ and the reprocessed C-ABZ after three cycles of hot-pressing were 76.7, 81.3, 70.8, and 58.1 Mpa, with corresponding tensile strength recovery ratios of 74.1 %, 78.6 %, 68.4 %, and 56.1 %, respectively. Furthermore, the polybenzoxazine backbone significantly improved the water resistance of the imine bonds. After immersing in water for 30 days at room temperature, the weight gain of C-ABZ was less than 1 % with corresponding tensile strength and tensile strength retention ratio of 59.5 Mpa and 57.5 %, respectively. Although the heat resistance of C-ABZ decreased slightly with increased hot-pressing cycles, a glass transition temperature (Tg, tanδ) of 150 °C was retained after the third hot-pressing. Overall, these findings demonstrate that the C-ABZ possesses excellent comprehensive performances.

Autogenous healing of the interface between hollow natural fiber (HNF) and reactive magnesia cement (RMC) matrix

The fiber-matrix interface healing determines the restoration of tensile strength of fiber reinforced cementitious composites (FRCCs). This work first-timely investigates the autogenous healing of debonded interface between hollow natural fiber (HNF) and reactive magnesia cement (RMC) matrix and subsequent tensile strength recovery. HNF-RMC single-fiber specimens are preloaded to induce interfacial debonding, and then conditioned under water-air cycles for healing. The healing-induced recovery of the interfacial properties in the HNF-RMC group are significantly higher than the control PVA-RMC and PVA-PC groups. Tensile test of notched FRCCs demonstrated that the tensile strength of HNF-RMC can be more than doubled after cracking-healing, and its healing efficiency is around 50% higher than that of PVA-PC and PVA-RMC. Based on microstructural characterization in situ and finite element modeling, the increased healing degree is attributed to two new mechanisms related to the special porous microstructure of HNF. First, the lumens and micropores within cell of HNF can be the pathway for CO2 and water penetration, which promotes the formation of healing products (e.g., hydrated magnesium carbonates) within fiber-matrix interface and hence the recovery of chemical and frictional bonds. Second, the healing products formed within lumens caused lateral fiber expansion, and thus prominently enhanced the frictional bond.

Implementation and precision of CFRP geometric configurations before adhesive bonding repair based on 1064 nm infrared pulsed fiber laser ablation

Before the adhesive bonding repair of the aircraft’s CFRP (Carbon Fiber Reinforced Polymer) structure, the common practice is to remove the damaged area material and form a predetermined repair geometric configuration using the grinding method according to the repair design shape. The geometric accuracy, dimensional accuracy, and surface characteristics of the geometric configuration have a significant impact on the adhesive bonding and repair effectiveness in CFRP structure repair. In the study, a 1064 nm infrared pulse laser was used to selectively and controllably remove the three layers of cured prepreg (fiber and resin) on the surface of the CFRP laminate structure. Based on the analysis of geometric accuracy and surface morphology, the effectiveness of CFRP adhesive bonding repair was verified. The results showed that, under the condition of undamaged CFRP fibers and thorough removal of the surface resin, the laser achieved a dimensional accuracy of ± 50 μm and a thickness accuracy of ± 20 μm for CFRP geometric configuration. The angular accuracy of rectangular configuration was less than ± 0.15°, and the roundness accuracy of circular configuration was greater than 0.995. The mechanical validation of the adhesive bonding repair indicated that the tensile strength recovery rate of CFRP repair structure after laser treatment was 84.7 %, and the failure mode was cohesive failure.

Depth-dependent self-healing capacity and mechanism of cracked fiber-reinforced concrete by bacterial community

The utilization of bacteria to facilitate the healing of engineered cementitious composites (ECC) has been an interesting approach. However, the commonly used microorganism has been the Bacillus strain, effective in forming bacterial calcium carbonate well at crack top surface but is less favorable for deep-crack healing due to its aerobic nature. In this study, a novel method was introduced that incorporated bacterial community to enhance the self-healing behavior along the depth, breaking away from the traditional single bacteria pattern. The depth-dependent healing ratio of for tire polymer (TP) fiber reinforced ECC using bacterial community (i.e., multiple strains mainly including 15 types of bacteria) was investigated. Healing behaviour was comprehensively evaluated in terms of crack-depth healing, water absorption ratio, and axial strength recovery. Micro-quantitative and macro-quantitative analysis of bacterial products at different depths and the entire crack were conducted through mapping and thermogravimetry. The results revealed the self-healing mechanisms and superiority of the TP-ECC with the bacterial community. Along the crack depth, the healing behaviour of TP-ECC incorporating bacterial community was superior to that of the traditional single bacteria approach. Bacterial communities had an advantage in axial tensile strength recovery compared to single bacteria. It was proved to be technically feasible to enhance self-healing capacity of the TP-ECC when incorporating a bacterial community.

Self-healing elastomers from supramolecular random copolymers of 4-vinyl pyridine

For the first time hydrogen bonding complexes between poly(4-vinyl pyridine) (P4VP) and its copolymers were employed as a healing concept for building supramolecular materials with self-healability. P4VP and its random copolymers with stearyl acrylate and stearyl methacrylate were first synthesized via UV light-mediated metal-free polymerization using 4-(10H-phenothiazin-10-yl)-N,N-diphenylaniline as organic photocatalyst. Three complexes between the P4VP homopolymer and the obtained copolymers bearing “hairy” side groups with 3,3′-dithiodipropionic acid (DTDP) as a proton donor crosslinker were studied. Owing to the “solvent”-like mantle formed by lowly ordered stearyl side groups, the complex of poly(4-vinyl pyridine-random-stearyl methacrylate) and DTDP exhibited efficient self-healability of complete cuts at room temperature after 30 min, with tensile strength recovery of above 80%. The results of this work suggested that this dynamic supramolecular concept based on P4VP is promising for developing self-healing materials for various applications.

New Route of Tire Rubber Devulcanization Using Silanes.

The disposal of tires at the end of their lifespan results in societal and environmental issues. To tackle this, recycling and reuse are effective solutions. Among various recycling methods, devulcanization is considered to be a very sustainable option, as it involves the controlled breakdown of crosslinks while maintaining the polymer backbones. The objective of this study is to develop a sustainable devulcanization process for passenger car tire rubber using silanes. In this study, a thermo-mechanical-chemical devulcanization process was conducted to screen six potential devulcanization aids (DAs). Silanes were chosen as they are widely used in tire rubber as coupling agents for silica. The efficiency of the devulcanization was studied by the degree of network breakdown, miscibility of the devulcanized material, and mechanical properties of the de- and revulcanized material. Compared to the parent compound, a 55-60% network breakdown was achieved for the devulcanizate along with 50-55% of tensile strength recovery. In addition to superior devulcanization efficiency, this DA offers a sustainable alternative to the conventional ones, such as di-phenyl-di-sulphide, due to its compliance with safety regulations. The devulcanizate can be utilized in high-performance applications, such as tires and seals, while 100% devulcanizate can be employed in low-strength technical rubber products.

Self–healing capacity assessment of cracked slag modified concrete under different re-curing conditions

The self-healing property of concrete is one of the unique features that can increase reinforced concrete structures’ reliability and service life and significantly reduce repair and maintenance costs. In this regard, self-healing characteristic of concrete specimens containing different quantities of slag was investigated in lime, tap, and 3% salt water. In addition, the cracked concrete specimens were cured in the mentioned solutions to investigate the effect of curing conditions on the corrosion process. Tensile strength recovery, water passing content through the cracked specimens, and chloride ion penetration tests have been performed to investigate the self-healing performance of cracked concrete. Also, the changes in the corrosion potential of embedded steel rebars in healed concrete specimens were measured via an innovative experiment. The outcomes revealed the significant impact of substituting slag with Portland cement on the healing rate. Furthermore, the tensile strength recovery in the specimens containing 15, 30, and 45% slag has improved up to 40, 46, and 59% in tap water re-cured condition. The results from the corrosion potential and the other experiments indicated an equal tendency besides the significant impact of the specimens’ curing circumstances on the self-healing process.

Electrochemical Healing of Fractured Metals.

Repairing fractured metals to extend their useful lifetimes advances sustainability and mitigates greenhouse gas emissions from metal mining and processing. While high-temperature techniques have long been used to repair metals, the increasing ubiquity of digital manufacturing and "unweldable" alloys, as well as the integration of metals with polymers and electronics, call for radically different repair approaches. This paper presents a framework for effective room temperature repair of fractured metals using an area-selective nickel electrodeposition process we refer to as electrochemical healing. Based on a theoretical model that links geometric, mechanical, and electrochemical parameters to the recovery of tensile strength, this framework enables 100% recovery of tensile strength in nickel, low-carbon steel, two "unweldable" aluminum alloys, and a 3D-printed difficult-to-weld shellular structure while using a single common electrolyte chemistry. Through a distinct energy dissipation mechanism, this framework also enables up to 136% recovery of toughness in an aluminum alloy. To facilitate practical adoption, this work reveals scaling laws for the energetic, financial, and time costs of healing, and demonstrates the restoration of a functional level of strength in a fractured standard steel wrench. Empowered with this framework, room-temperature electrochemical healing could open exciting possibilities for the effective, scalable repair of metals in diverse applications. This article is protected by copyright. All rights reserved.

Photo‐RAFT Polymerization for Hydrogel Synthesis through Barriers and Development of Light‐Regulated Healable Hydrogels under NIR Irradiation

AbstractWe report an aqueous and near‐infrared (NIR) light mediated photoinduced reversible addition–fragmentation chain transfer (photo‐RAFT) polymerization system catalyzed by tetrasulfonated zinc phthalocyanine (ZnPcS4−) in the presence of peroxides. Taking advantage of its fast polymerization rates and high oxygen tolerance, this system is successfully applied for the preparation of hydrogels. Exploiting the enhanced penetration of NIR light, photoinduced gelation is effectively performed through non‐transparent biological barriers. Notably, the RAFT agents embedded in these hydrogel networks can be reactivated on‐demand, enabling the hydrogel healing under NIR light irradiation. In contrast to the minimal healing capability (<15 %) of hydrogels prepared by free radical polymerization (FRP), RAFT‐mediated networks display more than 80 % recovery of tensile strength. Although healable polymer networks under UV and blue lights have already been established, this work is the first photochemistry system using NIR light, facilitating photoinduced healing of hydrogels through thick non‐transparent barriers.

Photo-RAFT Polymerization for Hydrogel Synthesis through Barriers and Development of Light-Regulated Healable Hydrogels under NIR Irradiation.

We report an aqueous and near-infrared (NIR) light mediated photoinduced reversible addition-fragmentation chain transfer (photo-RAFT) polymerization system catalyzed by tetrasulfonated zinc phthalocyanine (ZnPcS4-) in the presence of peroxides. Taking advantage of its fast polymerization rates and high oxygen tolerance, this system is successfully applied for the preparation of hydrogels. Exploiting the enhanced penetration of NIR light, photoinduced gelation is effectively performed through non-transparent biological barriers. Notably, the RAFT agents embedded in these hydrogel networks can be reactivated on-demand, enabling the hydrogel healing under NIR light irradiation. In contrast to the minimal healing capability (< 15%) of hydrogels prepared by free radical polymerization (FRP), RAFT-mediated networks display more than 80% recovery of tensile strength. Although healable polymer networks under UV and blue lights have already been established, this work is the first photochemistry system using NIR light, facilitating photoinduced healing of hydrogels through thick non-transparent barriers.

Insights into self‐healing performance of epoxidized deproteinized natural rubber/graphene oxide composite

AbstractThe self‐healing performance of epoxidized deproteinized natural rubber (EDPNR) and EDPNR/graphene oxide (GO) composites was investigated. Composites of EDPNR25 and EDPNR50 with GO contents of 0.5 and 1.0 phr were prepared by adding a GO dispersion into a EDPNR latex and the product was subsequently cast into films. Tensile strengths of the original samples and after self‐healing were used to evaluate self‐healing performance. The tensile strength of EDPNR25 after self‐healing achieved about 65% of tensile strength of original EDPNR25. The recovery of tensile strength for the composite increased to 81% for EDPNR25/GO0.5 and 105% for EDPNR25/GO1.0 for self‐healing at 25°C for 24 h. At 70°C, EDPNR25/GO0.5 and EDPNR25/GO1.0 composite achieved ~100% recovery after 24 h. On the other hand, the tensile strength recoveries of EDPNR50, EDPNR50/GO0.5, and EDPNR50/GO1.0 were relatively low (33%, 28%, and 24%, respectively) at 25°C. At 70°C, recoveries increased to 63%, 67%, and 52%, respectively. This result demonstrated that the interdiffusion of epoxidized natural rubber molecules plays a key role in the self‐healing performance rather than hydrogen bonds between GO and functional groups on EDPNR molecules.

Synthesis and thermal-responsive behavior of a polysiloxane-based material by combined click chemistries

A thermally reversible and high performance polydimethylsiloxane (PDMS)-based network was easily achieved from a novel single molecular silicone-based Diels–Alder crosslinker, which was obtained via the combination of the thiol-acrylate Michael addition and Diels-Alder click chemistries in a straightforward one-pot fashion. The high flexibility of the silicone-derived crosslinker allows for good healing of complete cuts without the need of a pre-healing step to dissociate the dynamic DA bonds. The cut healing process was possible via a single heating step at 70 °C, reaching a relatively good healing efficiency, evaluated by a tensile strength recovery of above 80%.

Polybutadiene derived allophanate as the crosslinker for Diels–Alder type self‐healing polyurethane networks

AbstractIntrinsic self‐healing materials have drawn great interest in recent years. The design of novel building blocks is essential for developing polymer networks with different properties. Allophanate is usually regarded as a side product during the preparation of polyurethanes. During the research of self‐healing polyurethane networks, we have studied the terminal functionalization of hydroxyl‐terminated polybutadiene with furfuryl isocyanate and occasionally obtained the furan‐terminated polybutadiene allophanate. The tetra‐branched molecule is employed as the crosslinker in the construction of a series of Diels–Alder type polyurethane networks. These polyurethane networks exhibit good mechanical performance and excellent self‐healing ability. Damages can be healed via the thermo‐promoted dissociation and recombination cycle, giving good to excellent recovery of tensile strength within three cycles of “damage‐repair” processes.

Metal-coordinated amino acid hydrogels with ultra-stretchability, adhesion, and self-healing properties for wound healing

It is a great challenge to prepare hydrogels with tissue adhesion, ultra-stretchability, and good tensile stress as wound dressings. Here, we reported a poly(acrylamide-histidine methacrylamide) (P(AM-HisMA)) hydrogel polymerized by histidine methacrylamide (HisMA) as a biocompatible amino acid derivative and acrylamide (AM). Since the amino acid HisMA provided physical cross-linking to the P(AM-HisMA) hydrogel, the hydrogel had self-healing properties (tensile strength recovery > 84 %), adhesion (4.1 kPa for porcine skin), and excellent mechanical properties (tensile stress of 77 kPa, tensile elongation of 5800 %, compressive stress of 2.0 MPa) when compared with polyacrylamide (PAM) hydrogels. Moreover, we prepared the P(AM-HisMA)-Fe3+ hydrogel by immersing Fe3+ ions into the P(AM-HisMA) hydrogel. The hydrogel contained histidine-Fe3+ coordination bonds and various non-covalent bonds, which resulted in better physical properties such as reinforced mechanical property (tensile strength of 89 kPa, stretchability of 4900 %), enhanced adhesive strength (11.9 kPa for porcine skin), and better self-healing property (within 5 min). The P(AM-HisMA)-Fe3+ hydrogel showed remarkable biocompatibility and excellent antibacterial property (100 % killing ratio). In vitro wound healing experiments demonstrated that P(AM-HisMA)-Fe3+ hydrogel effectively accelerated the wound healing process. Furthermore, the P(AM-HisMA)-Fe3+ hydrogels have promising application prospects as dressing materials for joint skin wound healing.

STUDY ON THE INFLUENCE OF THE CONFINEMENT EFFECT ON THE BOND STRENGTH RECOVERY IN THE DEFECTIVE GROUTED SLEEVE CONNECTION

Defects emanating from the onsite operation of the grouted sleeve connector have a significant impact on the ultimate tensile capacity of the connector. In this research, an experiment on the capacity of the fully grouted sleeve connector considering different configurations of defects was carried out. The experiment results indicated that the connector is highly sensitive to the location of the defects, which engenders a drop of 15% in the ultimate capacity of the connector. Based on the accurate simulation of the experiment model, a series of parametric analyses were conducted to evaluate the interaction of defects with other mechanical properties of the connector. It was found that the different values of the ratio of the sleeve diameter to that of the bar within the design recommended interval significantly influence the connection's performance. The lowest ratio value engenders approximately 10% to 16% of tensile strength recovery in the weakened configuration, while a bigger ratio value engenders a decline in the capacity. This work proposes the incorporation of a safety constant in the average bond expression.

Carbon nanotube survivability in marine environments and method for biofouling removal

The deep sea survivability and biofouling characteristics of corrosion-resistant bulk carbon nanotubes (CNTs) have been studied after deployment in the Atlantic Ocean over the course of 12 months. Quantification of barnacle count, biofouling density, and non-combustible residue shows cyanoacrylate coatings increase durability and reduce the colonization of biofouling compared to as-received CNTs. Scanning electron microscopy was performed on the biofouled CNTs, and the majority of species were identified as diatoms, consisting of an ordered silica cell wall. Both the as-received and cyanoacrylate-treated CNTs were successfully acid purified to remove biogrowth, leading to complete recovery of tensile strength and electrical transport properties. Thermogravimetric analysis, scanning electron microscopy, contact angle, dynamic mechanical analysis, and current carrying capacity measurements validated the refunctionalization results. Thus, the multifunctional property recovery and enhanced durability confirms that CNTs are electrochemically stable in saltwater environments and are resilient to biofouling conditions in real-world environments after extended exposure.