During the past decades, research in self-healing materials has focused on the improvement in the mechanical properties, making stronger materials, able to bear increasing solicitations. This strategy proved to be costly and in some cases inefficient, since materials continue to fail, and maintenance costs remained high. Instead of preparing stronger materials, it is more efficient to prepare them to heal themselves, reducing repairing costs and prolonging their lifetime. Several different self-healing strategies, applied to different material classes, have been comprehensively studied. When new materials are subject of research, the attention is directed into the formulations, product processing and scale-up possibilities. Efforts to measure self-healing properties have been conducted considering the specific characteristics of each material class. The development of comprehensive service conditions allowing a unified discussion across different materials classes and the standardization of the underlying quantification methods has not been a priority so far. Until recently, the quantification of self-healing ability or efficiency was focused mostly on the macroscale evaluation, while micro and nanoscale events, responsible for the first stage in material failure, received minor attention. This work reviews the main evaluation methods developed to assess self-healing and intends to establish a route for fundamental understanding of the healing phenomena
Among latex-producing plants, mainly the latex of Hevea brasiliensis has been studied in detail so far, while comprehensive comparative studies of latex coagulation mechanisms among the more than 20,000 latex-bearing plant species are lacking. In order to give new insights into the potential variety of coagulation mechanisms, the untreated natural latices of five latex-bearing plants from the families Euphorbiaceae, Moraceae and Campanulaceae were visualised using Cryo-SEM and their particle size compared using the laser diffraction method. Additionally, the laticifers of these plants species were examined in planta via Cryo-SEM. Similar latex particle sizes and shape were found in Ficus benjamina and Hevea brasiliensis. Hence, and due to other similarities, we hypothesize comparable, mainly chemical, coagulation mechanisms in these two species, whereas a physical coagulation mechanism is proposed for the latex of Euphorbia spp. The latter mechanism is based on the huge amount of densely packed particles that after evaporation of water build a large surface area, which accelerates the coagulation procedure.
Superabsorbent polymers (SAPs) are well known for their ability to absorb and hold high water amounts accompanied by a high volume expansion. In this work we show the benefits of this underlying property of SAPs to induce underwater crack closure with subsequent barrier restoration in damaged protective coatings. For the proof of concept, three layer epoxypolyester (EP) powder coating systems were developed and applied on carbon steel. In these systems the middle EP layer (also called functional layer) contained crosslinked acrylamide/acrylic acid copolymer SAPs in different amounts ranging from 0 to 40 wt%. The capability of the SAPs to close damages and extend barrier and corrosion protection was evaluated by electrochemical impedance spectroscopy (EIS), NaCl aqueous solution immersion test and optical microscopy. It was found that coatings loaded with a 20 wt% SAP led to the best overall corrosion protection for the studied systems. In order to proof the potential use of this extrinsic healing concept for multiple healing events wet-dry cycles on scratched systems
Polymer based elements are frequently subject to high mechanical load. It is well known, that such components can spontaneously break although the mechanical stress has not reached the average maximum load. These fatigue fractures are caused by micro-cracks. A smart approach would be to implement a self-healing function that is able to heal a crack in an early stage and thus avoid crack propagation. Fraunhofer UMSICHT and the Plant Biomechanics Group Freiburg together with cooperation partners develop biomimetic self-healing elastomers having the capability to repair micro-cracks automatically without any intervention from outside.
Elastomeric polymers are nowadays used in a broad variety of highly demanding applications. Due to alternating loads, microsized cracks may occur in the material, even before its loading-and lifetime-limit. The consequences can be drastic -failure of components often leads to the loss of production, delays, raising costs or facilities and -in rarely cases -personal injuries. Our endeavour is the equipment of such technically relevant elastomers with a self-healing agent. If microcracks occur in the material, this system should be able to prevent further growing and seal parts of the crack or even the complete crack to restore the mechanical properties.The idea to equip an elastomeric matrix with a self-healing agent is bioinspired: In case of breaks, a variety of plants segregate latex particles and proteins that crosslink in an addition reaction and close the fissure.The matrix elastomers investigated within the presented project are EPDM (ethylene propylene diene-terpolymer type M), NBR (nitrile butadiene rubber) and SEBS (styrene ethylene butadiene styrene), a thermoplastic elastomer. After a centrical splitting of died-cut elastomer strips, SEBS exhibits minor autonomous, intrinsic self-healing effects which are probably caused by molecular inter-diffusion processes as postulated by Wool and O'Connor. EPDM and NBR show no such intrinsic self-healing which can be ascribed to their rather stiff und cross-linked structure. Injured specimens from EPDM and NBR do not exhibit subsequent vulcanisation that might initiate intrinsic selfrepairing. It was also found that blending the elastomeric matrix with middle or high molecular polymers till a limit of 30% leads to distinctive self-healing results for EPDM and SEBS. Another presented strategy is the partial microencapsulation of two-component adhesives. In case of a crack, the encapsulated component is released and initiates a polymeric reaction with the second component which is directly embedded into the elastomeric matrix.
Polymer based elements are frequently subject to high mechanical load. It is well known, that such components can spontaneously break although the mechanical stress has not reached the average maximum load. These fatigue fractures are caused by micro-cracks. A smart approach would be to implement a self-healing function that is able to heal a crack in an early stage and thus avoid crack propagation. Fraunhofer UMSICHT and the Plant Biomechanics Group Freiburg together with co-operation partners develop biomimetic self-healing elastomers having the capability to repair micro-cracks automatically without any intervention from outside.
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