Design of smart resins for structural applications

The growing interest in the development of composite materials arises from the need to satisfy fundamental requirements in the field of structural materials (aircrafts, ships, wind turbine blades, automotive components or electronic devices, etc.). These requirements are: a) weight reduction - to maximize the performance; b) control of pollution during the manufacturing process of the materials and their use in service; c) low consumption of fuel and resources; d) reduction of the manufacturing and operating costs (life cycle costs) etc. The use of aeronautical thermosetting resins is still limited because of several drawbacks, such as the absence of electrical and thermal conductivity and the poor impact damage resistance (vulnerability of non-metallic materials to environmental hazards such as rain, storms, turbulence, icing, lightning, wind speed, etc.). These limitations can lead to the damage accumulation process, thus significantly compromising the integrity of the structure. An important contribution to increase the composite structural application can be achieved by designing smart materials through the implementation of the strategy of autonomous damage-repair and other specific functions integrated in the material structure. Different innovative approaches have been considered to impart self-healing function to multifunctional resins. As an instance, for microencapsulated self-healing systems, a new ruthenium initiator for Ring-Opening Metathesis Polymerization (ROMP), which is able to tolerate reactive components, high temperatures during the curing cycles, and preserve its activity at very low concentration in combination with Diaminodiphenylsulfone (DDS) hardener, has been employed in aeronautical self-healing systems. Very recently, new relevant achievements have been obtained with different supramolecular chemistry approaches. The supramolecular systems are usually characterized by repeatable and autonomous self-healing capability, but they have been developed for applications where high mechanical performance of thermosetting resins is not required. Chemical strategies utilizing supramolecular chemistries for the design of self-healing functional materials, include dynamic covalent bonds and non-covalent interactions. Several attempts have also been made to extend these mechanisms to structural systems requiring high mechanical performance and integrated functionalities. In this context, hybrid materials or nanomaterials have been functionalized with hydrogen bonding moieties to activate self-healing mechanisms and introduce additional functionalities. This work proposes a further successful strategy in the field of supramolecular chemistry, aimed at developing self-healing, load-bearing structures with all functionalities integrated in a single material able to meet many important industrial requirements. In particular, the authors propose to use, as self-healing fillers, molecules having functional groups able to act as hydrogen bonding donors and acceptors at the same time, to add into a toughened epoxy resin in combination with pristine Multiwall Carbon Nanotubes (MWCNTs), with the aim to activate self-healing mechanisms and to impart electrical conductivity to the material, simultaneously.