Encapsulated Healing Agent Research Articles

Strengthening and self‐healing of natural fiber composites via PLA core–shell nanofibers as healing agent carrier

AbstractThe resin‐rich region between the fiber laminates of a fiber‐reinforced composite is susceptible to matrix damage impeding the composite's integrity. Hence, this study intends to interlayer core–shell poly(lactic) acid (PLA) nanofibers between the flax fibers as a preventive and reactive mechanism to matrix damages in the resin‐rich area. The nanofibers prevent premature damage by suppressing or mitigating crack propagation and in the event of damage to the nanofibers, it reacts by releasing the encapsulated healing agent. Upon interlayering the flax fiber laminates with self‐healing core–shell nanofibers, the flexural modulus exhibited an increment of 31% whereas the short beam strength improved by 19%. Self‐healing traits of the composites were assessed by subjecting the composites to repeated flexural or short beam strength tests for several load cycles. Upon initial damage to the composites, the composites exhibited a full retention and improvement in mechanical properties. The self‐healing composites were also able to withstand more loading cycles than the conventional composite under repeated short‐beam strength tests. Under both configurations, the damaged self‐healing composites exhibited a residual strength greater than or equivalent to the ultimate strength of a pristine flax fiber composite.Highlights Fabrication of EP/flax self‐healing composites by PLA core–shell nanofibers. Interlayering healing nanofiber mats between flax fabrics in an FRP composite. Nanofiber interlayers exhibited a profound impact on the composite properties. Nano mats facilitate autonomous repair of internal defects within matrices. The self‐healing composites showed full retention and mechanical strength.

Experimental and numerical study on SMA modified with an encapsulated polymeric healing agent

The present study aims at experimentally and numerically investigating the effect an encapsulated healing agent on the mechanical characteristics of a stone mastic asphalt (SMA). As a healing agent a thiol-containing urethane AR-polymer is used in this study. In order to gain a numerical insight into mechanical behavior of the capsules in SMA, a micromechanical finite element modeling is employed. The developed model allows capturing the stresses induced in the capsules at different load cases applied to the SMA on macro-scale. Particular attention is paid presently to the numerical evaluation of the local stress state that arises around capsules during compaction, operation, and also during crack initiation. SMA mixtures with various volumetric contents of healing capsules were manufactured and the capsules survival during mixture production was evaluated based on X-Ray Computed Tomography measurements. The effect of capsules on the self-healing properties of asphalt mixtures has furthermore been examined with repeated compressive strength tests. The obtained experimental results indicate that the absolute majority of capsules survive mixture production, and that their addition increases the SMA strength recovery during the healing period. The experimental and numerical results concerning capsules breakage are found to be in reasonable agreement. The developed micromechanical model may thus potentially provide a useful tool for optimization of capsules mechanical properties in order to improve their survival during mixture production as well as their timely activation.

Assessment of mechanical and healing behaviors of glass fibers‐epoxy composite containing the microcapsules with the nanocomposite shell

AbstractIn the current study, the effect of adding the encapsulated healing agent with neat polymeric and nanocomposite shells on the mechanical‐healing features of glass fibers/epoxy composite under various loads was investigated. Therefore, the microcapsules with urea‐formaldehyde (UF) nanocomposite shell containing the various weight fraction of silica nanoparticles (0, 1, 2, 3) were synthesized. After that, the flexural, tensile, and interlaminar shear strength (ILSS) samples were destructed by using the quasi‐static penetration method with the damaging force of 500, 1050, and 2900 N, respectively. The maximum healing efficiency in the flexural loads belonged to the composite with the 3 wt% nanosilica on the surface of microcapsules, which was 83.9%. Also, it was found the increment in the weight fraction of silica nanoparticles had a negligible effect on the healing ability of composite under tensile loading. Achieving 135.8% healing efficiency in the composite with the 2 wt% nanosilica under the shear loads can be one of the most important findings in this work.

Simulation of the mechanical behavior of self-healing composite materials

PurposeThe purpose of this study is the investigation of self-healing materials containing encapsulated healing agents embedded in a polymer matrix with dispersed catalysts. In recent years, the high performance and design flexibility of composite materials have led to their widespread use in the aeronautics, space, automotive and marine fields. Simultaneously, as the need for advanced material properties has increased, many studies have been conducted on multifunctional materials, focusing on different fields of their desired capabilities.Design/methodology/approachA multiscale model was developed to simulate the effect of microcapsules on the mechanical behavior of the polymer matrix. Furthermore, the effects of microcapsule diameter and microcapsule concentration on the mechanical behavior of the composite were studied. Digimat and Ansys software were used to simulate the self-healing composites.FindingsThere is a trade-off between the efficiency of the microcapsules and the degradation of the properties of the composite material.Originality/valueThe generated model simulated an encapsulated healing agent in a polymeric matrix.

Self-healing capability of asphalt mixture containing polymeric composite fibers under acid and saline-alkali water solutions

Self-healing capability of the asphalt mixtures containing encapsulated healing agent can be affected by different water components as results of their exposure to aggressive environmental conditions such as the acid rain or saline-alkali solutions in coastal areas. In view of this, this paper aims to study the effect of harmful water environments on the self-healing capability of asphalt mixtures containing a kind of novel composite fibers. The Ca-alginate/SiO2 composite fibers encapsulating healing agent was synthesized by using a microfluidic device. Several tests were conducted to characterize the micromorphology, thermal stability and mechanical properties of the fibers. Effectiveness of the composite fibers in asphalt was observed by fluorescence microscopy. Additionally, three-point bending test was performed to measure the effects of different water environments (including acid, alkali and salt-water solutions) on the self-healing properties of asphalt mixtures with, and without fibers. Furthermore, the mechanism of the effect of different water environments on the self-healing performance of the asphalt mixture was studied by Fourier-transform infrared spectroscopy (FT-IR) and scanning electron microscope (SEM) analysis. Main results showed that the composite fiber encapsulated 61.9 wt% of healing agent, and the fibers could meet the temperature and mechanical requirements during the manufacturing process of asphalt mixture. The composite fiber worked and enhanced self-healing capability of asphalt. Harmful water environments did not weaken the effect of the composite fiber. Compared with the drying condition, the self-healing property of asphalt mixtures containing the composite fibers was further improved after being immersed in alkali and salt-water environment. The results indicated that the sodium ions in the alkaline solution and the salt solution could undergo a de-crosslinking chemical reaction with calcium alginate in the fiber wall, as a result of which more healing agent leaked out to soften and rejuvenate asphalt. In short, the composite fibers encapsulating healing agent worked, and these were successfully applicable into asphalt mixture under different water environments.

Crack-Resistance Behavior of an Encapsulated, Healing Agent Embedded Buffer Layer on Self-Healing Thermal Barrier Coatings

In this work, a novel thermal barrier coating (TBC) system is proposed that embeds silicon particles in coating as a crack-healing agent. The healing agent is encapsulated to avoid unintended reactions and premature oxidation. Thermal durability of the developed TBCs is evaluated through cyclic thermal fatigue and jet engine thermal shock tests. Moreover, artificial cracks are introduced into the buffer layer’s cross section using a microhardness indentation method. Then, the indented TBC specimens are subject to heat treatment to investigate their crack-resisting behavior in detail. The TBC specimens with the embedded healing agents exhibit a relatively better thermal fatigue resistance than the conventional TBCs. The encapsulated healing agent protects rapid large crack openings under thermal shock conditions. Different crack-resisting behaviors and mechanisms are proposed depending on the embedding healing agents.

Microcapsule-based self-healing materials: Healing efficiency and toughness reduction vs. capsule size

We report the synthesis of controlled sized Urea-Formaldehyde (UF) microcapsules containing an epoxy healing agent via in situ emulsification polymerization for the study of self-healing epoxy systems. Scanning Electron Microscopy (SEM) confirmed that the capsules possessed rough external surface which enhanced mechanical interlocking. Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA) and Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy were employed so as to determine the capsules thermal stability and decompositions and encapsulated healing agent percentage. To our knowledge it is the first time the solid-state NMR is used for the estimation of encapsulated healing agent. The obtained results clearly indicated that with decreasing capsule size, capsules remained thermally stable at high temperatures (approximetly up to 230 °C). Additionally, capsule size is for the first time directly correlated to both healing efficiency and the reduction of mechanical performance after self-healing system incorporation. Healing efficiency is proportional to capsule size with larger capsules resulting in 68% maximum load recovery. However, smaller capsules result to lower reduction of properties, i.e. 7% as oppose to 18% for larger ones. Although healing efficiency can be enhanced through the use of relatively large capsules, this is in expense of mechanical performance, i.e. there is an optimal capsule size.

Evaluation of sunflower oil’s healing effect in bituminous materials

Recently, sunflower oil has been widely used as encapsulated healing agent for self-healing asphalt. However, few researches have evaluated the healing effect of sunflower oil on aged asphalt binder. As such, this paper investigates the potential healing effect of sunflower oil in bituminous materials. Dynamic shear rheometer (DSR) was employed to evaluate the performance of the asphalt binders. The resultant master curve indicated that by adding 5% sunflower oil, approximately 30% loss of modulus in aged bitumen could be restored. Thus, the test result bear solid testimony that sunflower oil has the potential to fully recover the original rheological properties as well as healing capacity of the asphalt binder.

Rare Earth Triflate Initiators in the Cationic Polymerization of Tung Oil‐Based Thermosetting Polymers for Self‐Healing Applications

ABSTRACTRare earth triflates were successfully used to initiate the cationic polymerization of tung‐oil‐based thermosetting polymers. These bio‐renewable thermosets were developed and evaluated for use as an encapsulated healing agent in self‐healing materials. The tung‐oil‐based polymer is comprised of predominantly tung oil with styrene and divinylbenzene used as a reactive diluent and to improve the initiator solubility and the homogeneity of the resulting thermoset. While traditional cationic polymerization utilizes moisture‐sensitive Lewis acids as initiators, rare earth triflates offer a new approach to polymerization where moisture sensitivity is not a significant concern. Nine different rare earth triflates were investigated: ytterbium, scandium, samarium, cerium, gadolinium, erbium, terbium, holmium, and neodymium. The thermomechanical properties of the thermosets were tested and the analysis included effects of initiator type on the storage modulus, loss modulus, tan δ, glass transition temperature, and thermal stability. Finally, the adhesive properties of the thermosets were studied using compressive lap shear tests, and the fracture surfaces were analyzed using scanning electron microscopy and energy‐dispersive X‐ray spectroscopy.

Room Temperature Self-Healing Thermoset Based on the Diels–Alder Reaction

A self-healing epoxy-amine thermoset based on the compatible functionalization of the thermoset and encapsulated healing agent has been successfully developed. Healing of the thermoset resulted from the reaction of furans in the thermoset and multimaleimides (MMIs) in the healing agent solution. The healing agent, MMI dissolved in phenyl acetate, was encapsulated using a urea-formaldehyde encapsulation method. Autonomic healing of the thermoset was achieved by incorporating microcapsules filled with the healing agent solution within a furan-functionalized epoxy-amine thermoset. The resulting self-healing thermoset recovered 71% of its initial load after fracture.

Detecting the Activation of a Self‐Healing Mechanism in Concrete by Acoustic Emission and Digital Image Correlation

Autonomous crack healing in concrete is obtained when encapsulated healing agent is embedded into the material. Cracking damage in concrete elements ruptures the capsules and activates the healing process by healing agent release. Previously, the strength and stiffness recovery as well as the sealing efficiency after autonomous crack repair was well established. However, the mechanisms that trigger capsule breakage remain unknown. In parallel, the conditions under which the crack interacts with embedded capsules stay black-box. In this research, an experimental approach implementing an advanced optical and acoustic method sets up scopes to monitor and justify the crack formation and capsule breakage of concrete samples tested under three-point bending. Digital Image Correlation was used to visualize the crack opening. The optical information was the basis for an extensive and analytical study of the damage by Acoustic Emission analysis. The influence of embedding capsules on the concrete fracture process, the location of capsule damage, and the differentiation between emissions due to capsule rupture and crack formation are presented in this research. A profound observation of the capsules performance provides a clear view of the healing activation process.

Self-healing efficiency of cementitious materials containing tubular capsules filled with healing agent

Abstract It has been estimated that, in Europe, 50% of the annual construction budget is spent on rehabilitation and repair of the existing structures [1] . Therefore, autonomous crack healing of concrete, a construction material that is highly susceptible to cracking, would be desirable. In this research, an encapsulated healing agent was embedded in the mortar matrix to obtain self-healing properties. Upon crack appearance, the capsules break and the healing agent is released, causing crack repair. By means of Computed Tomography and visual observation of the crack faces, filling of the cracks with healing agent was observed. It was seen that more than 50% of the original strength and stiffness could be regained after self-healing. It was also found that the water permeability could be reduced by a factor 102 to 104 due to autonomous crack healing. As a consequence, the proposed technique may be used for partial restoration of concrete properties after cracking.

Characterization of diene monomers as healing agents for autonomic damage repair

AbstractFor effective autonomic healing of damaged polymers and composites, it is essential to understand how the encapsulated healing agent behaves during and after cure. In this study, two different diene monomers [dicyclopentadiene (DCPD), 5‐ethylidene‐2‐norbornene (ENB)] and their blends were investigated as candidate self‐healing agents, using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). DSC experiments for samples showed that DCPD has a melting transition while the blends and ENB have no melting in the temperature range measured. Samples for DMA were prepared and tested by two different methods in the presence of Grubbs catalyst. In the first case (method I), monomers were mixed with the catalyst directly. In the second case (method II), the catalyst was mixed with an epoxy/amine system and cured into a film that was polished to expose the catalyst. The cure behavior of monomer samples was examined on the epoxy/catalyst film. Method II is considered to be a simulative experiment, which can occur in a real situation for damaged epoxy matrix composite. It was found that acceleration of cure reaction and reduction of catalyst concentration is possible by blending DCPD with ENB from method I. Storage modulus (G′) value after cure in method II showed that a DCPD : ENB blend ratio of 1 : 3 reached the highest G′ value at shorter cure time and lower catalyst levels than other monomer combinations. DCPD and ENB are presumably responsible for increases in rigidity and reactivity, respectively. This may improve the healing efficiency in autonomic damage repairing applications. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1266–1272, 2006