Preparation and characterization of high-performance epoxy nanocomposites

A recent trend to address the toughness problems of epoxy resins as most widely used engineering resin is to incorporate nanofillers in epoxy matrix. Considering the practical drawbacks of most of nanofillers such as high cost, availability and difficult dispersion, halloysite nanotubes (HNT) -as cheap, abundant, and easy to process naturally-occurring clay minerals- has provided a great opportunity in the field of polymeric nanocomposites. Although studies showed that HNTs could considerably enhance impact toughness of epoxies, more study is needed yet to determine the best processing parameter to prepare high-performance epoxy nanocomposite. The aim of this research is to provide detailed understanding of the effects of HNTs on the properties of epoxy nanocomposites and to highlight the proper process and material design to get the full benefit of nanofiller (HNTs) for improving the toughness of epoxy resin. In this regards, the effects of various processing parameters (such as mixing methods, curing system, types of HNTs and their surface modification) on the rheological, curing and impact behavior of epoxy nanocomposites was studied. Moreover, new kind of self-healing material with ability to heal the possible cracks in epoxy matrix was developed. For this purpose, the feasibility of using HNTs’ lumen as carrier of healing agent to achieve possible simultaneous self-healing and toughening effects for production of smart self-healing nanocomposite was explored and a new method for preparation of self-healing epoxy composite based on novel electrospun nanofibrous membranes has been developed. This study shows that proper design of process and material has crucial effect on the final properties of epoxy nanocomposite. While the viscosity of the uncured epoxy increased after the incorporation of HNTs, the impact properties of the cured epoxy/HNTs nanocomposites was highly dependent on the characteristics of base epoxy matrix determined by the type of curing agent. Fracture mechanism studies revealed that if shear yielding happened in the epoxy matrix, the incorporation of HNTs deteriorated the impact properties of epoxy nanocomposites. Compared to the commonly used mechanical mixing method, ultrasonication effectively improved the dispersion of HNTs in epoxy matrix, and considerably enhanced the viscosity and impact strength of the epoxy/HNTs nanocomposites, especially at high HNTs loading. Types of HNTs had a significant effect on the impact properties of epoxy/HNTs nanocomposites in a way that those nanocomposites reinforced with longer and more uniform HNTs showed significantly higher impact strength. Furthermore, surface modification of halloysite nanotubes (HNTs) by γ-aminopropyltriethoxysilane (APTES) and Bisphenol-A-diglycidyl-ether (DGEBA) was explored through NaOH pretreatment and found to be effective in improving their toughening effect in epoxy nanocomposites. The quality of modification had a significant effect on the impact properties of epoxy/HNTs nanocomposites. Whilst the crack initiation stage was not affected by incorporation of modified or unmodified HNTs, the crack propagation energy was dramatically increased by the incorporation of HNTs and their modification. The force required for crack propagation was considerably increased in epoxy/modified-HNTs nanocomposites to even higher than those of crack initiation force which is important to maintain the structural integrity of the nanocomposites after crack initiation. In addition, to prepare smart self-healing epoxy composite, feasibility of using HNTs’ lumen space as carrier of the healing agents was explored. It was found that nanosized lumen of HNTs could not provide adequate amount of healing agents required for self-healing structural composites. To have adequate amount of healing agents, new carrier system based on electrospun polyacrylonitrile (PAN) nanofibrous mats was developed. The average healing efficiency of approximately 75% at 50 ºC and 38% at room temperature was achieved in epoxy/electrospun PAN composites. These epoxy composites were capable of repeated self-healing for up to 6 times at room temperature. To have simultaneous benefits of HNTs and self-healing functionality hybrid Epoxy/HNTs/PAN nanocomposites was developed.