Abstract
Carbon nanotubes (CNTs) could add in traditional thermal curing epoxy adhesive as effective strengtheners. The addition of CNTs not only strengthens the epoxy but also make it able to absorb microwave energy and conduct electrical current. In this work, processes of adhesive bonding by curing of carbon nanotube (CNT)/epoxy composite adhesives with microwave and direct application of electrical current were developed for fast and energy save bonding of materials.Using microwave to cure epoxy adhesive containing multi-wall carbon nanotubes not only leads to large time and energy savings, but also results in much higher strength. The curing time by microwave heating was shortened to just 1/3 - 1/4 of that required by conventional heating means. The well-dispersed CNTs in the epoxy resin absorb microwave energy, acting as heat sources to cure the epoxy resin uniformly and effectively. Microwave curing can diminish the deterioration effect of lowering cross-link reaction by CNTs as observed in conventional heating. Thus, as much as three times addition of CNTs in the epoxy is allowed for optimum strength. As a result, 56% increase of the shear strength was observed when the epoxy adhesive doped with 3wt% CNTs was cured by microwave heating.However, microwave curing of adhesive bonding cannot easily apply to materials that absorb or reflect microwave radiations strongly. The high equipment cost, size of microwave chamber and complexity in uniform power control put additional limitations to the application of microwave curing process. Therefore, there is definitely a need to develop a simple, time saving and cost effective process for curing CNT/epoxy adhesives. So, we developed the processes of thermal curing CNT/epoxy composite adhesives with direct application of electrical current.Since CNTs are largely semi-conductive in nature, when apply electrical current directly through a CNT/epoxy composite adhesive film, the adhesive will burn out along a narrow path while leave other parts of the adhesive totally uncured due to the positive feedback effect. Therefore, two methods were developed to eliminate the detrimental positive feedback effect during Joule heating.For joining conductive materials, the base materials to be bonded are used as current leads that make the current pass from one plate through the conducting CNTs/epoxy composites film to the other plate. For joining non-conductive materials, buckypaper was used as current carrier in a buckypaper-epoxy adhesive plaster patch. The electrical current passes through the buckypaper to raise the temperature of the epoxy resin for curing. With this process, not only the positive feedback effect can be avoided but also more uniform curing of the CNT/epoxy adhesive can be obtained with a beneficial negative feedback effect.This process requires only simple equipment and is theoretically the most time and energy saving process for curing CNT/epoxy adhesive. The addition of CNTs in the epoxy can provide better heat transfer and strengthening of the epoxy adhesive. With the use of buckypaper/0.5% CNT/epoxy adhesive cured by Joule heating, the bonding strength increased significantly to be 67% higher compared to conventionally cured specimen with pure epoxy.