A study on the improvement of adhesively bonded joint strength at cryogenic temperature with flame and silane treatment

Abstract

Joining of metallic or composite structures with adhesive has been employed for efficient assembling of large and complicated structures, such as LNG (Liquefied natural gas) containment tanks and satellite structures. However, the adhesively bonded joint is vulnerable to decrease the load carrying capability in harsh environments, which decreases durability of the adhesive joint structures. Also as the number of joined structure is increased, an efficient bonding technology should be developed to meet the structural performance. Especially, silane coupling agent treatments have been widely used to improve the bonding durability and fatigue resistance of adhesively bonded joints due to the enhancement of compatibility and interface interaction between inorganic materials and organic materials. Flame treatments using propane gas were conducted to improve surface wettability of epoxy adhesives and to maximize silane treatment effects for large metallic or composite adherends. In this thesis, a proper flame treatment process and coating process for an epoxy-functional silane coupling agent were investigated to improve the adhesion strength and reliability of adhesively bonded joints composed of metallic or composite adherends under harsh environments such as cryogenic temperatures or humid-salty environments. The optimum fabrication condition for the silane solution was investigated considering the relations between silane treatment and surface roughness of the aluminum adherend. The surface free energies of the aluminum adherend were calculated and the lap shear strengths of the adhesively bonded aluminum joint were measured at the cryogenic temperature with respect to the fabrication variables such as the pH and weight percent of the silane solution. Also, the effect of the silane coating for roughened aluminum adherends on the adhesion strength was investigated using XPS (X-ray photoelectron spectroscopy). Since the metal adherends should have high surface fre...