Microencapsulated Healing Agent Research Articles

A 3D micromechanical model to predict the complete stress-strain relation of microencapsulated self-healing concrete

The research on the concrete structure built with self-healing materials brings inspiration to increase the safety and sustainability of underground structures in the whole life cycle. The utilization of microencapsulated healing agents in self-healing concrete has demonstrated efficacy in the repair of microcracks within concrete structures. Nevertheless, there exists a dearth of effective methodologies for assessing the impact of microcapsule parameters on the mechanical properties of self-healing concrete. This study introduces an innovative three-dimensional micromechanical model that can be utilized to analyze the micromechanical response of microencapsulated self-healing concrete under tensile loading conditions. The 3D micromechanical model is accomplished through the utilization of the elastic secant compliance tensor. Subsequently, a comprehensive examination is undertaken to analyze the progression of damage-healing in self-healing concrete incorporating microcapsules. Finally, a parametric investigation is conducted to elucidate the impact of the micro-parameters on the mechanical behavior of self-healing concrete. The present discovery holds significant implications for the development of microencapsulated self-healing concrete for underground structures, particularly in terms of establishing appropriate parameters.

Microencapsulated Diepoxy-Functionalized Ionic Liquids to the Design of Self-Healable Epoxy Networks

An extrinsic self-healing mechanism based on microencapsulated healing agents represents an original way to produce self-healable thermosetting materials without modifying the structural architecture of the co-monomers. In this work, self-healing was achieved through poly(melamine–formaldehyde) (PMF) microcapsules containing a polymerizable diepoxidized ionic liquid monomer denoted as ILEM. First, a synthetic route to design ILEM@PMF microcapsules via in situ polymerization was developed and optimized through the choice of surfactants, core/shell ratios, and stirring speeds. Then, the obtained microcapsules (10 wt %) were incorporated into three different epoxy–amine networks and their effects on the morphology, thermal behavior, i.e., glass transition temperature (Tg) and degradation temperature (Td), as well as on the mechanical properties were investigated. In addition, a pre-crack was generated with a fresh razor blade into the center groove of the epoxy networks and their self-healing performances were observed by scanning electron microscopy before and after the curing process.

Damage-healing analysis of microencapsulated self-healing concrete subjected to tensile loading using a 2D micromechanical model

Self-healing concrete that employs microencapsulated healing agents has been proven to be an effective method for microcrack repairment in the concrete structure. However, there is a lack of efficient tools to evaluate the effect of the parameters of microcapsules on the mechanical behavior of the self-healing concrete. In this paper, the evolution of the damage-healing process of microencapsulated self-healing concrete subjected to tensile loading is numerically analyzed from a microscopic perspective by using a 2D micromechanical model. Based on the deformation and propagation evolution mechanism of microcracks, the contribution of microcracks to the total compliance tensor of microencapsulated self-healing concrete under tensile loading is deduced at various stages. According to the calculated total compliance tensor, the stress-strain and compliance-strain relations of microencapsulated self-healing concrete are discussed with special attention to the stress dropping and strain softening stages. Finally, parametric analysis was conducted using the constructed model to investigate the influence of size and content of microcapsules, the types of healing agent and the initial damage of the concrete on the mechanical behaviors of microencapsulated self-healing concrete. The constructed 2D model is significantly useful for the reasonable selection of the optimal parameters of microencapsulated self-healing concrete.

Self‐Healing of HTPB Based Polyurethane Binder via Ring Opening Metathesis Polymerization

AbstractThe self‐healing capability of Hydroxyl Terminated Polybutadiene (HTPB) based polyurethane binder via Ring Opening Metathesis Polymerization(ROMP) is reported. Microencapsulated healing agent Dicyclopentadiene (DCPD) and wax protected Ru‐based Grubbs catalyst were incorporated in binder matrix (i.e,HTPB) uniformly to heal the cracks. These healing agents were also incorporated into HTPB matrix individually to check healing properties on their own and thereafter cured for five days. The incorporation of these healing agents in binder matrix did not affect the curing of matrix. It was found out that cuts when applied to the matrix were self‐healed at room temperature. Mechanical testing have revealed that around 50 % of tensile strength was regained in the healed specimen.

Nonlinear vibrations and viscoelasticity of a self-healing composite cantilever beam: Theory and experiments

A self-healing beam was fabricated by using a microencapsulated healing agent (5E2N) with dispersed multi-walled carbon nanotubes (MWCNT). Grubbs’ catalyst was dispersed in the epoxy matrix reinforced with MWCNT. The self-healing material was specifically realized for this study modifying a formulation developed for satellites. Experiments were conducted on the self-healing beam in order to identify the viscoelastic material properties by vibration measurements. In addition, large-amplitude vibration experiments were carried out on the self-healing cantilever beam. A refined third-order shear deformation theory was developed to model the nonlinear vibrations of the cantilever beam. The model retains rotary inertia and in-plane and rotational nonlinearities in addition to traditional von Kármán nonlinear terms; it uses the mechanical properties experimentally identified. A satisfactory comparison of experimental and numerical results for nonlinear vibrations of the self-healing cantilever beam is presented for five different excitation levels.

Microencapsulated healing agents for an elevated-temperature cured epoxy: Influence of viscosity on healing efficiency

Among many applications, elevated-temperature cured epoxy resins are widely used for high-performance applications especially for structural adhesive and as a matrix for structural composites. This is due to their superior chemical and mechanical properties. The thermosetting nature of epoxy produces a highly cross-linked polymer network during the curing process where the resulting material exhibited excellent properties. However, due to this cross-linked molecular structure, epoxies are also known to be brittle, and once a crack initiated in the material, it is difficult to arrest the crack propagation. Earlier research found that the inclusion of encapsulated healing agents is able to introduce self-healing ability to the room-temperature cured epoxies. The current study investigated the self-healing behaviour of an elevated-temperature cured epoxy, which incorporated the dual-capsule system loaded with diglycidyl-ether of bisphenol-A (DGEBA) resin and mercaptan. The microcapsules were prepared by the in-situ polymerisation method while the fracture toughness and the self-healing capability of the tapered-double-cantilever-beam (TDCB) epoxy specimens were measured under Mode-I fracture toughness testing. We investigated the effect of temperature on viscosity of the healing agents and how these values influence the formation of uniform healing on the fracture surfaces. It was found that incorporation of the dual-capsule self-healing system onto an elevated-temperature cured epoxy slightly changed the fracture toughness of the epoxy as indicated by the Mode-I testing. In the case of thermal healing at 70°C, the self-healing epoxy exhibited a recovery of up to 111% of its original fracture toughness, where a uniform spreading of the healant was observed. The excellent healing behaviour is attributed to the lower viscosity of the healant at higher temperature and the higher glass transition temperature ( Tg) of the produced healant film. The DSC analysis confirmed that the healing process was not contributed by the post-curing of the host epoxy.

Mechanical responses of microencapsulated self-healing cementitious composites under compressive loading based on a micromechanical damage-healing model

The cementitious composites with microencapsulated healing agents have become a class of hotspots in the field of construction materials, and they have very broad application prospects and research values. The in-depth study on multi-scale mechanical behaviors of microencapsulated self-healing cementitious composites is critical to quantitatively account for the mechanical response during the damage-healing process. This paper proposes a three-dimensional evolutionary micromechanical model to quantitatively explain the self-healing effects of microencapsulated healing agents on the damage induced by microcracks. By virtue of the proposed 3 D micromechanical model, the evolutionary domains of microcrack growth (DMG) and corresponding compliances of the initial, extended and repaired phases are obtained. Moreover, the elaborate studies are conducted to inspect the effects of various system parameters involving the healing efficiency, fracture toughness and preloading-induced damage degrees on the compliances and stress-strain relations. The results indicate that relatively significant healing efficiency, preloading-induced damage degree and the fracture toughness of polymerized healing agent with the matrix will lead to a higher compressive strength and stiffness. However, the specimen will break owing to the nucleated microcracks rather than the repaired kinked microcracks. Further, excessive higher values of healing efficiency, preloading-induced damage degree and the fracture toughness of polymerized healing agent with the matrix will not affect the compressive strength of the cementitious composites. Therefore, a stronger matrix is required. To achieve the desired healing effects, the specific parameters of both the matrix and microcapsules should be selected prudently.

Mechanical response analysis of self-healing cementitious composites with microcapsules subjected to tensile loading based on a micromechanical damage-healing model

For the past several years, research in the field of self-healing construction materials becomes a hotspot with broad application prospects. This paper mainly focuses on the self-healing cementitious composites with microcapsules. To quantitatively interpret the self-healing effect of micro-encapsulated healing agents on microcrack-induced damage, a three-dimensional evolutionary micromechanical model is established to predict the mechanical response of the cementitious composites with the microcapsules subjected to tensile loading during the damage-healing process. The evolutionary domains of microcrack growth (DMG) and the corresponding compliances at the initial, activated and repaired stages are obtained. On the basis of the proposed 3D micromechanical model of the self-healing cementitious composites with microcapsules, elaborate studies of constitutive relations and compliance are conducted to investigate the effects of various system parameters involving the healing efficiency, fracture toughness and preloading-induced damage degrees on the compliances and stress-strain relations. The results indicate that relatively significant healing efficiency, preloading-induced damage degree and the fracture toughness of polymerized healing agent with the matrix will lead to higher tensile strength and stiffness. However, based on the two different failure modes of self-healing concrete, the excessive values of healing efficiency, preloading-induced damage degree and the fracture toughness of polymerized healing agent with the matrix will not affect the tensile strength of the cementitious composites. For the sake of the desired optimal healing effect, the specific parameters of both the matrix and the microcapsules should be selected carefully.

Global Sensitivity Analysis for the Polymeric Microcapsules in Self-Healing Cementitious Composites.

Cementitious composites with microencapsulated healing agents are appealing due to the advantages of self-healing. The polymeric shell and polymeric healing agents in microcapsules have been proven effective in self-healing, while these microcapsules decrease the effective elastic properties of cementitious composites before self-healing happens. The reduction of effective elastic properties can be evaluated by micromechanics. The substantial complicacy included in micromechanical models leads to the need of specifying a large number of parameters and inputs. Meanwhile, there are nonlinearities in input–output relationships. Hence, it is a prerequisite to know the sensitivity of the models. A micromechanical model which can evaluate the effective properties of the microcapsule-contained cementitious material is proposed. Subsequently, a quantitative global sensitivity analysis technique, the Extended Fourier Amplitude Sensitivity Test (EFAST), is applied to identify which parameters are required for knowledge improvement to achieve the desired level of confidence in the results. Sensitivity indices for first-order effects are computed. Results show the volume fraction of microcapsules is the most important factor which influences the effective properties of self-healing cementitious composites before self-healing. The influence of interfacial properties cannot be neglected. The research sheds new light on the influence of parameters on microcapsule-contained self-healing composites.

Microstructure optimizations for improving interlaminar shear strength and self-healing efficiency of spread carbon fiber/epoxy laminates containing microcapsules

The purpose of this study is to improve interlaminar shear strength and self-healing efficiency of spread carbon fiber (SCF)/epoxy (EP) laminates containing microcapsules. Microencapsulated healing agents were embedded within the laminates to impart a self-healing functionality. Self-healing was demonstrated on short beam shear specimens, and the healing efficiency was evaluated by strain energies of virgin and healed specimens. The effects of microcapsule concentration and diameter on apparent interlaminar shear strength and healing efficiency were discussed. Moreover, damaged areas after short beam shear tests were examined by an optical microscope to investigate the relation between the microstructure and the healing efficiency of the laminates. The results showed that the stiffness and the apparent interlaminar shear strength of the laminates increased as the microcapsule concentration and diameter decreased. However, the healing efficiency decreased with decreasing the microcapsule concentration and diameter.

Effect of the incorporation of a microencapsulated healing agent in an epoxy-amine fiber reinforced composite material

The objective of this work was to study the effect of incorporating a microencapsulated healing agent in an epoxy matrix and E-glass fiber reinforced composite. Microcapsules were prepared via oil-in-water emulsion polymerization method with dicyclopentadiene as core material and poly(urea-formaldehyde) (PUF) as shell material. The suitable formulation for the epoxy matrix was selected based on the study of the rheological and mechanical properties of various chemical systems. Different amounts of microcapsules were incorporated and the most appropriate processing method (mixing, curing and post-curing cycle) was evaluated. Furthermore, flexural and fracture tests were carried out and the distribution of the capsules as well as the interfacial adhesion with the epoxy matrix were studied. Finally, the processing of fiber reinforced composites, with and without microcapsules, was carried out by compression molding and the mechanical properties of the composites were studied (modulus and maximum flexural strain) from testing three-point bending. The resulting samples with 32 wt. % of fibers and matrices with no microcapsules were compared. Compression molding technique did not affect the integrity of the microcapsules inside the composites.

Modeling microcapsule-enabled self-healing cementitious composite materials using discrete element method

Concrete with a micro-encapsulated healing agent is appealing due to its self-healing capacity. The discrete element method (DEM) is emerging as an increasingly used approach for investigating the damage phenomenon of materials at the microscale. It provides a promising way to study the microcapsule-enabled self-healing concrete. Based on the experimental observation and DEM, a three-dimensional damage-healing numerical model of microcapsule-enabled self-healing cementitious materials under compressive loading is proposed. The local healing effect can be simulated in our model, as well as the stress concentration effect and the partial healing effect. The healing variable of the DEM model is developed to describe the healing process. We examine the dependence of the mechanical properties of the microcapsule-enabled self-healing material on (a) the stiffness of the solidified healing agent, (b) the strength of the solidified healing agent, (c) the initial damage of specimens, and (d) the partial healing effect. In particular, the proposed numerical damage-healing model demonstrates the potential capability to explain and simulate the physical behavior of microcapsule-enabled self-healing materials on the microscale.

Micromechanical Properties of a New Polymeric Microcapsule for Self-Healing Cementitious Materials.

Self-healing cementitious materials containing a microencapsulated healing agent are appealing due to their great application potential in improving the serviceability and durability of concrete structures. In this study, poly(phenol–formaldehyde) (PF) microcapsules that aim to provide a self-healing function for cementitious materials were prepared by an in situ polymerization reaction. Size gradation of the synthesized microcapsules was achieved through a series of sieving processes. The shell thickness and the diameter of single microcapsules was accurately measured under environmental scanning electron microscopy (ESEM). The relationship between the physical properties of the synthesized microcapsules and their micromechanical properties were investigated using nanoindentation. The results of the mechanical tests show that, with the increase of the mean size of microcapsules and the decrease of shell thickness, the mechanical force required to trigger the self-healing function of microcapsules increased correspondingly from 68.5 ± 41.6 mN to 198.5 ± 31.6 mN, featuring a multi-sensitive trigger function. Finally, the rupture behavior and crack surface of cement paste with embedded microcapsules were observed and analyzed using X-ray computed tomography (XCT). The synthesized PF microcapsules may find potential application in self-healing cementitious materials.

EFFECT OF INHIBITOR AGENTS ADDITION ON CORROSION RESISTANCE PERFORMANCE OF TITANIA SOL–GEL COATINGS APPLIED ON 304 STAINLESS STEEL

Hybrid organic–inorganic coatings are deposited on 304 stainless steel substrates by the sol–gel technique to improve the corrosion resistance. A titania-based nanostructured hybrid sol–gel coating is impregnated with three different microencapsulated healing agents (inhibitors) including cerium, Benzotriazole (BTA), and 8-Hydroxyquinoline (8H). Field-emission scanning electron microscopy (FE-SEM) and electrochemical impedance spectroscopy (EIS) are performed to investigate the barrier performance properties. The optimum conditions to achieve corrosion protective coatings for 304 stainless steel were determined. The Nyquist plots demonstrate that the activation time of the coating containing 8H as an organic healing agent shows improved behavior when compared to other coatings including cerium and BTA. Cerium as an inorganic healing agent is second and BTA is third and minimum. An increase in the impedance parameters such as resistance and capacitance as a function of immersion time is achieved in a 3.5[Formula: see text]wt.% NaCl solution by using healing agents such as BTA. Actually, over the course of immersion, the barrier performance behavior of the coatings changes and reduction of the impedance observed from the coatings containing Ce and 8H discloses deterioration of the protection system after immersion for 96[Formula: see text]h of immersion in the 3.5% NaCl solution. However, after 96[Formula: see text]h of immersion time, the concentration of chloride ions is high and causes increase in defects, micro cracks, hole on the surface of hybrid titania nanostructured coating containing Ce and 8H by destruction of coating, and also hybrid titania nanostructured coating containing BTA; BTA is released from coating to improve the resistance of passive film, which is created on the surface.

Sealing of cracks in cement using microencapsulated sodium silicate

Cement-based materials possess an inherent autogenous self-healing capability allowing them to seal, and potentially heal, microcracks. This can be improved through the addition of microencapsulated healing agents for autonomic self-healing. The fundamental principle of this self-healing mechanism is that when cracks propagate in the cementitious matrix, they rupture the dispersed capsules and their content (cargo material) is released into the crack volume. Various healing agents have been explored in the literature for their efficacy to recover mechanical and durability properties in cementitious materials. In these materials, the healing agents are most commonly encapsulated in macrocontainers (e.g. glass tubes or capsules) and placed into the material. In this work, microencapsulated sodium silicate in both liquid and solid form was added to cement specimens. Sodium silicate reacts with the calcium hydroxide in hydrated cement paste to form calcium-silicate-hydrate gel that fills cracks. The effect of microcapsule addition on rheological and mechanical properties of cement is reported. It is observed that the microcapsule addition inhibits compressive strength development in cement and this is observed through a plateau in strength between 28 and 56 days. The improvement in crack-sealing for microcapsule-containing specimens is quantified through sorptivity measurements over a 28 day healing period. After just seven days, the addition of 4% microcapsules resulted in a reduction in sorptivity of up to 45% when compared to specimens without any microcapsule addition. A qualitative description of the reaction between the cargo material and the cementitious matrix is also provided using x-ray diffraction analysis.

A micromechanical study of the breakage mechanism of microcapsules in concrete using PFC2D

Concrete containing micro-encapsulated healing agents is appealing due to its self-healing capacity. Although much research has been conducted on concrete containing microcapsules, the mechanism for their breakage due to microcracking is still not well understood. By considering microcapsules as circular voids, this paper uses the two-dimensional particle flow code (PFC2D) to study the fracturing behavior of concrete specimens containing one microcapsule and one microcrack. Specifically, the PFC2D model was first developed and validated based on experimental results. Then the validated model was used to systematically study the initiation, propagation and coalescence of cracks and the corresponding stress and strain at each cracking stage. The results indicate that the propagating cracks include both the first (primary) and secondary cracks. The initiation location and stress of the first cracks depend on both the preexisting crack inclination angle and the ligament length. The initiation and propagation of the first and secondary cracks disturb the stress and displacement fields of the specimen. The crack coalescence between the preexisting crack and the hole is also influenced by the preexisting crack inclination angle and the ligament length. The research enhances the understanding of the effect of different factors on the cracking behavior of concrete containing a circular hole and a preexisting crack and paves the way for using discrete element method (DEM) to investigate the breakage mechanism of microcapsules in concrete.

開繊炭素繊維/エポキシ樹脂積層材料の自己修復と強度回復に及ぼす炭素ナノ材料添加の影響

This study examines interlaminar shear strength and self-healing of spread carbon fiber (SCF)/epoxy (EP) laminates. Self-healing is accomplished by incorporating a microencapsulated healing agent and catalyst within the epoxy matrix. Short beam shear specimens of self-healing SCF/EP laminates were prepared and tested. The healing efficiency was evaluated by the strain energies of virgin and healed specimens. The effect of addition of carbon nanomaterials to the matrix on the apparent interlaminar shear strength and the healing efficiency are discussed. The damage area of the healed specimens was also examined by an optical microscope.

グラフェンの超音波分散とグラフェン含有開繊炭素繊維/エポキシ樹脂積層材料の自己修復

Interlaminar damages can cause a reduction in strength and stiffness of laminated composites and is extremely difficult to detect and repair by conventional methods. Thus, self-healing has the potential to mitigate the interlaminar damages. This study examines the interlaminar shear strength and the self-healing of spread carbon fiber/epoxy laminates containing ultrasonicated graphenes. Healing is accomplished by incorporating a microencapsulated healing agent and a catalyst within an epoxy matrix. Healing is triggered by crack propagation through the microcapsules, which then release the healing agent into crack faces. Subsequent exposure of the healing agent to catalyst initiates polymerization and bonding of crack faces. Self-healing is demonstrated on short beam shear specimens and healing efficiency was evaluated by the strain energies of virgin and healed specimens. The damaged area of the specimens was also examined by an optical microscope.

自己修復性を有する開繊炭素繊維／エポキシ樹脂積層材料の強度回復と微視構造最適化

Damage can cause a reduction in the strength and stiffness in FRP and is very difficult to detect and repair. Thus, self-healing has the potential to mitigate damage in FRP. This study examines the effect of microcapsule concentration and diameter on the strength recovery of spread carbon fiber(SCF)/epoxy(EP) laminates encompassing self-healing. Healing is accomplished by incorporating a microencapsulated healing agent and catalyst. Self-healing is demonstrated on short beam shear specimens and the healing efficiency was evaluated by the strain energies of virgin and healed specimens. The damaged area of specimens was also examined by an optical microscope. The results showed that as the microcapsule concentration increased, the apparent interlaminar shear strength decreased and the healing efficiency increased. The trade-off between the strength and the healing efficiency was observed. Furthermore, as the microcapsule diameter increased, the apparent interlaminar shear strength decreased. High healing efficiency was achieved with 50μm microcapsule diameter.

마이크로캡슐을 함유한 폴리머 코팅제의 콘크리트 표면균열 자기치유시스템

자기치유성능을 가지고 있는 자기치유제를 함유하고 있는 마이크로 캡슐을 콘크리트 표면의 고분자 매트릭스층에 적용하였다. 메타크릴레이트를 코어물질로 하고 폴리 우레아포름알데히드를 캡슐막 구성물질로 하는 미세캡슐을 고분자 매트릭스 안에 첨가하여 콘크리트 표면에 코팅하였다. 콘크리트 표면에 미세균열이 형성되고 전파될 때, 균열이 전파되는 위치에 있는 캡슐이 파괴되어 치유체가 균열면 사이로 흘러나오게 되고, 균열에 스며든 단량체가 햇빛에 의해 중합반응을 일으켜 균열이 자기치유 되는 현상을 고찰한다. 자기치유성능평가는 흡수성, 투수성 실험 및 광학현미경을 통한 관찰로 확인하였다. 결과적으로 제시된 자기치유시스템은 콘크리트 균열부의 부분적인 복원을 위해 사용할 수 있음을 확인하였다. In this paper, microencapsulated healing agent was embedded in the polymer matrix to obtain self healing properties. Microencapsulation of methacrylate using polyurea-formaldehyde as a shell material and studied the effect of agitation rate on capsule characteristics such as size, shell thickness, and surface morphology. The formation of microcapsules was confirmed by FTIR and TGA, and capsule characteristics were studied by optical microscopy and SEM. The self-healing effect was evaluated using permeability measurements and further confirmed by surface analytical tools including optical microscope. According to the experimental results, the microencapsulated healing system has the self-heaing ability for artificial cracks.