Academic literature on the topic 'MICROENCAPSULATED HEALING AGENTS'

The concept of introducing self-healing capabilities in polymer materials and systems has been based on mimicking biological self-healing materials and systems, for example, materials like proteins have phenomenal capabilities in self-healing damaged biological structures. This work has been extended to investigate self-healing capabilities of fibre reinforced epoxy composites. Microencapsulated epoxy and mercaptan healing agents were incorporated into a glass fibre reinforced epoxy matrix to produce a polymer composite capable of self-healing. The specimens containing the microencapsulated epoxy and mercaptan healing agents did gain excellent strength and achieved a healing efficiency up to 140%.

Identification of non-structural damage in concrete infrastructure and actuation of preventive repair solutions is an established approach to avoid further structural damages and more expensive repair regimes. However the repair of concrete itself is not infallible with 55% of reported repairs in the EU failing within 5 years of service. Thus the already once repaired concrete structure is then subject to a constant cycle of repeated repair and a cumulative associated life cycle cost. The development of external repair material with self-healing capabilities, can affect a real step-change on the life-cycle costs and maintenance of existing and new infrastructure. Developed polymeric microcapsules containing liquid sodium silicate were used to impart autonomic self-healing to readily available commercial repair mortars for the first time. These materials cover a range of potential real time repair applications. Initially the compatibility between the developed self-healing agents and commercial products was established and the self-healing performance of the novel composite system was then evaluated. The results underlined the huge potential for the proposed composite systems as a stepping stone toward commercial uptake of self-healing microcapsule-based cementititious materials.

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.

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.

Asphalt self-healing by encapsulated rejuvenating agents is considered a revolutionary technology for the autonomic crack-healing of aged asphalt pavements. This paper aims to explore the use of Bio-Oil (BO) obtained from liquefied agricultural biomass waste as a bio-based encapsulated rejuvenating agent for self-healing of bituminous materials. Novel BO capsules were synthesized using two simple dripping methods through dropping funnel and syringe pump devices, where the BO agent was microencapsulated by external ionic gelation in a biopolymer matrix of sodium alginate. Size, surface aspect, and elemental composition of the BO capsules were characterized by optical and scanning electron microscopy and energy-dispersive X-ray spectroscopy. Thermal stability and chemical properties of BO capsules and their components were assessed through thermogravimetric analysis (TGA-DTG) and Fourier-Transform Infrared spectroscopy (FTIR-ATR). The mechanical behavior of the capsules was evaluated by compressive and low-load micro-indentation tests. The self-healing efficiency over time of BO as a rejuvenating agent in cracked bitumen samples was quantified by fluorescence microscopy. Main results showed that the BO capsules presented an adequate morphology for the asphalt self-healing application, with good thermal stability and physical-chemical properties. It was also proven that the BO can diffuse in the bitumen reducing the viscosity and consequently self-healing the open microcracks.

Self-healing of concrete is the process in which the material regenerates itself repairing inner cracks. This process can be produced by autogenous or autonomous healing. Autogenous healing is a natural process, produced by carbonation and/or continuing hydration. Autonomous healing is based on the use of specific agents to produce self-healing, which can be added directly to the concrete matrix, embedded in capsules or introduced through vascular networks. Some examples are superabsorbent polymers, crystalline admixtures, microencapsulated sodium silicate, and bacteria. This review is structured into two parts. The first part is an overview of self-healing concrete that summarises the basic concepts and the main advances produced in the last years. The second part is a critical discussion on the feasibility of self-healing concrete, its possibilities, current weaknesses, and challenges that need to be addressed in the coming years.

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.

The primary motivation behind the present research work is to study the effect of inclusion of healant loaded microcapsules on specific properties (thermal, structural and mechanical) of a representative epoxy thermoset. In addition, we also explore alternate chemistry for introducing self healing functionality in epoxy composites. Two distinct healing systems have been investigated, namely cycloaliphatic epoxy and unsaturated polyester. The healants were encapsulated in urea-formaldehyde microcapsules by adopting an in-situ dispersion polymerization route and in polystyrene shell through solvent evaporation process. The effect of operating parameters particularly stirring speed on the particle size distribution has been studied. Under optimal conditions, the core content of the epoxy loaded microcapsuleswas found to be 65 ± 4%for microcapsules prepared by dispersion polymerization route and 38 ± 2%for microcapsules prepared using solvent evaporation route. It is to be noted that the healing efficiency is strongly influenced by the internal microstructure of the microcapsule and we also developed an analytical model for predicting the amount of healant released in the event of microcapsule rupture. In microcapsules possessing “reservoir” type microstructure, the healant exists as a single droplet, where the entire content is expected to be released upon rupture. On the other hand, in monolithic microcapsules, the healant is dispersed in the form of discrete micro-droplets, and only the healant available within the cracked microcapsule is expected to be released and cause healing. Our model predicted that significantly lower amounts of healant is released in monolithic microspheres in comparison to reservoir microcapsules, especially when the micro-droplet dimensions and core content, both are low. V Triethylenetetramine (TETA) hardener was encapsulated by adopting two methods, namely interfacial polymerization and physical entrapment technique. The effects of experimental parameters, namely reaction temperature (50-75°C), stirring speed (400-700 rpm) and epoxy: amine concentration ratio (10:1.2-10:4.3) on the microcapsule properties was investigated. A polymeric surfactant was used to stabilize the suspension in order to modulate the particle size distribution of the resultant microcapsules. Highest encapsulation efficiencies resulted when the reaction medium was maintained at 70°C under a stirring speed of 600 rpm, while maintaining an epoxy: amine ratio of 10:3.2. The microcapsule dimensions and core content could be tailored, following the interfacial polymerisation route. Under optimal conditions, spherical microcapsules with 100 % yield and 12% core content were obtained. Physical entrapment approach was also explored for the immobilisation of amine hardener in mesoporous silica. For this purpose, mesoporous silica (SBA-15) was synthesised using polymer-templated technique and employed as a substrate for immobilization. Vacuum infiltration of TETA led to its entrapment within the porous structure of SBA-15 with loadings as high as 5g/g, which could be attributed to hydrogen bonding and acid–base interactions. The curing kinetics of self-healing epoxy compositions was investigated by non-isothermal differential scanning calorimetric (DSC) studies. Epoxy loaded microcapsules and immobilised amine was dispersed into epoxy resin, and cured using TETA. DSC studies revealed the autocatalytic nature of epoxy curing, which remained unaltered due to addition of the fillers responsible for introducing self healing functionality. The kinetic parameters of the curing process were determined using both Friedman and Kissinger–Akahira–Sunose (KAS) method. The activation VI energy at different degree of conversion (E  ) was found to decrease with increasing degree of cure (  ). Although urea-formaldehyde possess secondary amine functionalities, which have the potential to react with the epoxy groups, no significant differences in the curing kinetics of the base resin were observed.Kinetic parameters were used to predict the curing behaviour of compositions at higher heating rates using KAS method. As expected, the onset curing temperature (Tonset) and peak exotherm temperature (Tpeak) of epoxy shifted towards higher temperatures with increased heating rate; however introduction of fillers do not affect these characteristic temperatures significantly. Also, the overall order of reaction does not vary significantly. The results suggest that although 2° amino groups are available with the urea-formaldehyde (UF) resin, these do not directly participate in the curing reaction, as the primary amino groups in TETA are more easily accessible. To evaluate the effect of self healing additives on the mechanical properties and healing efficiency, epoxy composites containing UF and PS microcapsules (5 30%, w/w) were prepared by room temperature curing and their mechanical behaviour and healing efficiency was studied under both quasi-static and dynamic loadings. The tensile strength, modulus and impact resistance of the matrix was found to decrease with increasing amount of microcapsule in the formulation, the detrimental effect being less pronounced for polystyrene microcapsules due to its monolithic internal microstructure. Morphological investigations on the cracked surface revealed features like crack pinning, crack bowing, microcracking and crack path deflection, which were used to explain the toughened nature of microcapsule containing epoxy composites. VII Healing efficiency was quantified in terms of the ratio of impact strength before and after healing. For the purpose of validation of the developed analytical model, composites were prepared using epoxy encapsulated microcapsules with varied internal structures. Ni and Cu-imidazole complexes were prepared for use as latent hardeners for epoxy. Both the imidazole-metal complexes could effectively cure the epoxy released from within the microcapsules in the event of damage followed by thermal treatment. In line with our predictions, the extent of healing was much lower in the case of samples containing monolithic microcapsules. At 20% w/w microcapsule loadings, healing efficiencies close to 60% was observed upon introduction of reservoir type microcapsules, while under similar loadings, only 10% healing could be evidenced in the presence of monolithic microcapsules. For reservoir type microcapsules, complete healing (efficiency ~ 100±2%) could be effected at 30% microcapsule loading, in the presence of metal imidazole complexes. In comparison, the complete healing was evidenced at relatively lower microcapsule loading (20%, w/w) when amine immobilised SBA-15 was used. The potential of encapsulated unsaturated polyester resin (USP) towards introduction of healing functionality was also explored. USP was encapsulated in urea-formaldehyde shell and polystyrene shell by dispersion polymerization and solvent evaporation technique respectively both resulting in the formation of free flowing microcapsules. Calorimetric studies confirmed the chemical activity of the encapsulated USP, which spontaneously polymerised in the presence of a free radical initiator, 2,2‟-Azobis(2-methylpropionitrile) (AIBN), at temperature as low as 80°C. Temperature triggered healing of epoxy-microcapsule composites was performed at 110° C and healing efficiency was quantified as the ratio of impact strength of healed and virgin specimens. The same was found to increase with increasing amount of VIII microcapsule in the formulation and reached a maximum (100 ± 2%) at 20% (w/w) loading. Fractographic analysis of the surface revealed the flow pattern of chemically active polyester resin from the ruptured microcapsules, which subsequently cured in the presence of AIBN available within the matrix.

For the last decade, many researchers have been working to develop self-healing materials, and have obtained good results in the field of polymers, these components with microencapsulated healing agent have exhibited noticeable mechanical performance and regenerative property The research described in this paper applies the concept of self healing to simulate self healing polymer matrix composites, with the aid of models developed by the authors for the manufacturing processes and self-healing behavior. The development of self-healing is a novel idea that has not been totally explored in great detail yet. The concept of self-healing described in this paper consists of simulation of a healing agent dicyclopentadiene (DCPD) inside of a microvascular network within a polymer matrix coating with catalyst forming a self-healing composite (SHC). When this SHC is damaged or cracked, the healing agent by capillary action will flow inside of the microvascular network; when the liquid enter in contact with the catalyst will form a polymer structure and sealing the crack. The study consists of theoretical analysis and Computational Fluid Dynamics of a self-healing polymer. The objective of the study reported here was to find the influence and efficiency of the microvascular network in healing a polymer matrix. To check this effect a computational model was created to simulate the healing treatment, thus a crack was created on the matrix surface piercing the microvascular network filled with healing agent and the method to simulate healing behavior of the composite allows assessment of the effects of the autonomously repairing repeated damage events.