Investigation of effects of ultrasonic vibration on wetting, heating, and curing of structural adhesives

The application of structural adhesives to join materials is limited by their slow bond strength development behaviors. Conventional heating methods are unable to cure the adhesives quickly. Therefore, the development of rapid heating and curing methods is important. The main objective of this research is to investigate the effects of ultrasonic vibration on wetting, heating, and curing of structural adhesives.;Feasibility study of using ultrasound, radio frequency, microwave, and infrared to rapidly heat and cure structural adhesive was performed. The effect of operating parameters on lap shear strength of adhesive joints was examined. The cure time of the adhesive to achieve a prescribed handling strength was significantly reduced from 25 minutes to less than 3 minutes. The advantages and limitations of each method are discussed. A pulsed ultrasonic heating and curing (PUHC) technique was developed to enhance adhesive curing. The effect of process parameters on heating rate of the adhesive was investigated. Rapid curing of structural adhesive was successfully demonstrated by this technique. Thermal analysis of the reaction kinetics of structural adhesive cured by PUHC was then carried out by Differential Scanning Calorimeter. A chemical model based on a four-parametric semi-empirical equation was developed to distinguish between ultrasonic thermal effects and ultrasonic vibration effects. It was found that the produced non-thermal effects were significant at the beginning of the curing process. The conversion of the adhesive produced by PUHC after 50 seconds was almost three times higher than that obtained by pure thermal curing.;Extensive study of the effect of ultrasonic vibration on enhancing the wettability of a liquid/solid system was investigated. Contact angles of liquid droplets on epoxy-coated steel with and without the presence of ultrasonic vibration were measured. Results showed that ultrasonic vibration does increase the kinetics of wetting and reduce the static contact angle drastically. A dimensionless equation which correlates the dynamic contact angle to material properties, thermal effect, and ultrasonic parameters, was developed to predict the motion of droplets under ultrasonic forced wetting. Good agreement between the predicted values and the experimental results was obtained. The dimensionless equation is suitable for determining the dynamic contact angle in both spontaneous and ultrasonic forced wetting conditions.