Damage tolerance of toughened epoxy at low temperatures

Official URL: https://risc01.sabanciuniv.edu/record=b2767101_(Table of contents)

Epoxies are high performance thermosetting polymers widely used in many engineering applications such as structural adhesives, coatings and matrices of fiber reinforced polymer composites. They possess exceptional mechanical properties, chemical resistance, and high temperature properties. However, the highly cross-linked epoxy networks are inherently brittle, and therefore epoxy resins show a low resistance to crack propagation compared to other polymers. Meanwhile, numerous methods are used to improve their fracture resistance. For example, elastomers, core-shell particles or a second phase via domain precipitation are added to the epoxy to enable larger plastic deformation. Reinforcement with carbon nanoparticles has also been investigated. New applications of epoxy resins and fibre-reinforced plastics can be found in the burgeoning hydrogen economy, which experiences a come-back to the World-stage as a promising protagonist especially with regards to the challenges of climate change to lower the CO2 emission. Since the liquid hydrogen has to be kept in cryogenic conditions and pressurized, the hydrogen storage vessels should be designed based on cryogenic temperature conditions. The composites must be able to withstand very low temperatures and retain their functions even in face of sudden major temperature changes. In addition, they can lead to weight savings for applications where are subjected to uniform pressure that causes equal membrane strains. Like most materials, epoxy resin behaves much more brittle at low temperatures than at room temperature. Common tougheners are not designed for such temperatures and are sometimes based on mechanisms that no longer function when the material is “frozen”. This present work investigates the mechanical properties, fracture performance and toughening mechanisms of a cured epoxy polymer modified by a second phase such as core-shell particles, elastomer particles or carbon-nanotubes (CNTs). To enhance fracture toughness of the epoxy resin, core-shell, elastomer and carbon nanotube (CNTs) tougheners were incorporated using different content ratios. Mechanical tests were carried out at different temperature conditions such as ambient, -40 °C and -80 °C. Different tougheners revealed different properties under different temperature conditions. The single edge notched bending (SENB) test results showed that fracture energy of the modified epoxy at -40 °C increased from 461 kJ/m2 to 610 kJ/m2 with the addition of 30 wt.% elastomer particles. When the temperature lowers, the largest increases in fracture energy observed were for 0.3 wt.% CNTs and 30 wt.% elastomer particles. The toughening mechanisms for such systems were postulated to be rubber-particle cavitation and void growth and debonding and plastic void growth of the silica necklaces.