Cryogenic Mechanical Properties Of Nanofiller Modified DGEBF Epoxy Resins

With the rapid developments in spacecraft and superconducting cable technologies, and large cryogenic engineering projects such as the International Thermonuclear Experimental Reactor (ITER) etc., epoxy resins have been increasingly employed in cryogenic engineering applications as impregnating materials, adhesives and matrices for advanced composites. However, pure epoxy resins normally have poor crack resistance at room temperature and could be more brittle at cryogenic temperatures, which makes them unsuitable for some cryogenic engineering applications that demand epoxy resins to have high cryogenic mechanical properties. Therefore, nitrile -butadiene nano-rubber (VP-501), multiwall carbon nanotubes (MWCNTs)/n-butyl glycidyl ether (BGE) and MWCNTs/poly(ethersulfone) (PES) were employed in this dissertation to modify the DGEBF epoxy resin in improving the cryogenic mechanical properties especially cryogenic fracture toughness. The effects of modifier content, type and microstructure on the tensile and fracture toughness properties at room temperature (RT) and liquid nitrogen temperature (77 K) have been studied. The toughening mechanisms at cryogenic temperature through various modifiers were discussed. Moreover, since good thermal properties of modified systems are required in practical applications, studies on the effects of modifier type and content on the glass transition temperature (Tg) were also conducted. Finaly, the residual MWCNTs length plays a critical role in determining the mechanical properties of polymer nanocomposites and is thus regarded as the main reason responsible for the unexpected low mechanical properties. A simple but appropriate methodology based on the modified rule of mixtures is proposed to simultaneously determine the MWCNTs strength and the MWCNTs-polymer interfacial strength. The results have shown that the addition of 5 phr VP-501 can strengthen DGEBF/DETD system at both RT and 77 K. With VP-501 content increasing, the fracture toughness was increased, while the tensile modulus was decreased at both RT and 77 K. The addition of 0.5 phr MWCNTs to diglycidyl ether of bisphenol-F epoxy with BGE via the ultrasonic technique can enhance the cryogenic tensile strength, Young's modulus, failure strain and impact strength at 77 K. Moreover, synthetic sequence leads to selective dispersion of MWCNTs in the brittle primary phase but not in the soft second phase in the two-phase epoxy matrix. The addition of 0.5 phr MWCNTs to diglycidyl ether of bisphenol-F epoxy with thermoplastic PES using the three-roll calandering method can enhance the cryogenic tensile strength, Young's modulus, failure strain and fracture toughness at both RT and 77 K. Moreover, the tensile strength and Young's modulus at 77 K were higher than those at RT at the same composition while failure strain and fracture toughness showed the opposite results.Differential scanning calorimetry (DSC) analysis shows that Tg of the selected systems does not obviously change with increasing the VP-501 content. On the other hand, Tg of the epoxy resins is increased by the addition of MWCNTs.The research work on modification of cryogenic epoxy resins in this dissertation provides the basic knowledge and data for application of modified DGEBF epoxy resins in cryogenic temperature environment.