Self-healing material
Self-healing materials are artificial or synthetically created substances that have the built-in ability to automatically repair damages to themselves without any external diagnosis of the problem or human intervention. Generally, materials will degrade over time due to fatigue, environmental conditions, or damage incurred during operation. Cracks and other types of damage on a microscopic level have been shown to change thermal, electrical, and acoustical properties of materials, and the propagation of cracks can lead to eventual failure of the material. In general, cracks are hard to detect at an early stage, and manual intervention is required for periodic inspections and repairs. In contrast, self-healing materials counter degradation through the initiation of a repair mechanism that responds to the micro-damage. Some self-healing materials are classed as smart structures, and can adapt to various environmental conditions according to their sensing and actuation properties.
Although the most common types of self-healing materials are polymers or elastomers, self-healing covers all classes of materials, including metals, ceramics, and cementitious materials. Healing mechanisms vary from an instrinsic repair of the material to the addition of a repair agent contained in a microscopic vessel. For a material to be strictly defined as autonomously self-healing, it is necessary that the healing process occurs without human intervention. Self-healing polymers may, however, activate in response to an external stimulus to initiate the healing processes.
A material that can intrinsically correct damage caused by normal usage could prevent costs incurred by material failure and lower costs of a number of different industrial processes through longer part lifetime, and reduction of inefficiency caused by degradation over time.
History
The ancient Romans used a form of lime mortar that has been found to have self-healing properties. By 2014, geologist Marie Jackson and her colleagues had recreated the type of mortar used in Trajan's Market and other Roman structures such as the Pantheon and the Colosseum and studied its response to cracking. The Romans mixed a particular type of volcanic ash called Pozzolane Rosse, from the Alban Hills volcano, with quicklime and water. They used it to bind together decimeter-sized chunks of tuff, an aggregate of volcanic rock.As a result of pozzolanic activity as the material cured, the lime interacted with other chemicals in the mix and was replaced by crystals of a calcium aluminosilicate mineral called strätlingite. Crystals of platey strätlingite grow in the cementitious matrix of the material including the interfacial zones where cracks would tend to develop. This ongoing crystal formation holds together the mortar and the coarse aggregate, countering crack formation and resulting in a material that has lasted for 1,900 years.
Materials science
Related processes in concrete have been studied microscopically since the 19th century.Self healing materials only emerged as a widely recognized field of study in the 21st century. The first international conference on self-healing materials was held in 2007. The field of self-healing materials is related to biomimetic materials as well as to other novel materials and surfaces with the embedded capacity for self-organization, such as the self-lubricating and self-cleaning materials.
Biomimetics
Plants and animals have the capacity to seal and heal wounds. In all plants and animals examined, firstly a self-sealing phase and secondly a self-healing phase can be identified. In plants, the rapid self-sealing prevents the plants from desiccation and from infection by pathogenic germs. This gives time for the subsequent self-healing of the injury which in addition to wound closure also results in the restoration of mechanical properties of the plant organ. Based on a variety of self-sealing and self-healing processes in plants, different functional principles were transferred into bio-inspired self-repairing materials. The connecting link between the biological model and the technical application is an abstraction describing the underlying functional principle of the biological model which can be for example an analytical model or a numerical model. In cases where mainly physical-chemical processes are involved a transfer is especially promising.There is evidence in the academic literature of these biomimetic design approaches being used in the development of self-healing systems for polymer composites.
The DIW structure from above can be used to essentially mimic the structure of skin. Toohey et al. did this with an epoxy substrate containing a grid of microchannels containing dicyclopentadiene, and incorporated Grubbs' catalyst to the surface. This showed partial recovery of toughness after fracture, and could be repeated several times because of the ability to replenish the channels after use. The process is not repeatable forever, because the polymer in the crack plane from previous healings would build up over time.
Inspired by rapid self-sealing processes in the twining liana Aristolochia macrophylla and related species a biomimetic PU-foam coating for pneumatic structures was developed. With respect to low coating weight and thickness of the foam layer maximum repair efficiencies of 99.9% and more have been obtained. Other role models are latex bearing plants as the weeping fig, the rubber tree and spurges, in which the coagulation of latex is involved in the sealing of lesions. Different self-sealing strategies for elastomeric materials were developed showing significant mechanical restoration after a macroscopic lesion.
Self-healing polymers and elastomers
In the last century, polymers became a base material in everyday life for products like plastics, rubbers, films, fibres or paints. This huge demand has forced to extend their reliability and maximum lifetime, and a new design class of polymeric materials that are able to restore their functionality after damage or fatigue was envisaged. These polymer materials can be divided into two different groups based on the approach to the self-healing mechanism: intrinsic or extrinsic.Autonomous self-healing polymers follow a three-step process very similar to that of a biological response. In the event of damage, the first response is triggering or actuation, which happens almost immediately after damage is sustained. The second response is transport of materials to the affected area, which also happens very quickly. The third response is the chemical repair process. This process differs depending on the type of healing mechanism that is in place. These materials can be classified according to three mechanisms, which can be correlated chronologically through four generations. While similar in some ways, these mechanisms differ in the ways that response is hidden or prevented until actual damage is sustained.
Polymer breakdown
From a molecular perspective, traditional polymers yield to mechanical stress through cleavage of sigma bonds. While newer polymers can yield in other ways, traditional polymers typically yield through homolytic or heterolytic bond cleavage. The factors that determine how a polymer will yield include: type of stress, chemical properties inherent to the polymer, level and type of solvation, and temperature.From a macromolecular perspective, stress induced damage at the molecular level leads to larger scale damage called microcracks. A microcrack is formed where neighboring polymer chains have been damaged in close proximity, ultimately leading to the weakening of the fiber as a whole.
Using polymer scaling laws and the segmental relaxation of a chain, one can relate the kinetics of bond breaking to the frequency, amplitude of the excitation and the friction coefficient of embedded matrix,
Here, is the number of unrelaxed monomer at the fracture interface and is the thermal energy. Using Rouse relaxation time and a single grafted chain at the interface, one can show that.