Thermoplastic epoxy designed using Vitrimer chemistries

Abstract

Globally, 70 million tons of epoxy thermosets are manufactured at a perplexing rate which constitutes approximately a quarter of polymer generation by weight. A thermosetting polymer contains a permanent crosslinked network and is one that undergoes degradation without going through fluid state on heating at high temperatures. This in turn dramatically reduces its durability and longevity which persuades the requirement of disposing it in landfills or incineration leading to environmental pollution and difficulty in waste management. Unfortunately, this disposal strategy attributes to the rising dangers and concerns for the environment. Epoxy resins are thermosetting polymeric liquids that are viscous and contain epoxides in their chemical structure. Its properties such as light weight, good adhesion, high moisture and chemical resistance, strong durability at high and low temperatures, low shrinkage and great mechanical properties makes it a desirable matrix use in various applications. There is an increasing need for sustainable polymeric materials that not only exceed strict performance requirements but also have the capacity to be recycled and reprocessed in several different industries, including aerospace, automotive, transportation, and medical. In recent times, the emergence of vitrimers with self-healing technology has sparked significant interest in exploring innovative approaches for self-healing on the interface as well as bulk phenomena. This area shows great promise and potential for advancements in various fields. Vitrimers have become a potential class of polymers because of these demands. Unlike other materials, vitrimers have dynamic covalent crosslinks that enable temperature-dependent exchange kinetics. This property enables vitrimers to behave like thermoplastics that can be reprocessed at high temperatures along with retaining and accomplishing the desired resistance to chemical and mechanical stress comparable to thermosets at the application temperatures. These unconventional polymer networks rely on dynamic bonds that can react reversibly, namely covalent adaptable networks (CANs). The CANs could release stress from deformation, allowing the crosslinked polymers to be reprocessed, reshaped, and recycled. The heavy amounts of stress, force, high temperature, and pressure in various applications that the material is subjected to results in cracks and fractures and consequently the life of the material is shortened. In this thesis, we have successfully synthesized single dual and triple dynamics CANs that are epoxy-based vitrimer by using the various types of chemistry like transesterification, disulphide metathesis, silyl ether exchange, imine, and acetal exchange. We developed fast curing Epoxies (which cure within 2 hours) using Vitrimer chemistries. By incorporating this covalent adaptable network, various epoxies were designed which can cure within 2-10 hours. The resulting epoxy vitrimers exhibited high tensile strength, exceptionally high Tg, fast stress relaxation and high an activation energy. Such epoxy-based vitrimer materials could widely be used in various applications in automobile, aerospace technology. Reduction in maintenance costs of structures could be attained through incorporating the vitrimer chemistry. The resulting epoxy vitrimers also exhibited self-healing, reusability and re-processability property triggered by the exchangeable bonds attained by incorporating the vitrimer chemistry presented.