Ultrasonic Cure Monitoring of Thermosetting Composite Matrices

In the recent years, the application of thermosetting resins in civil engineering field has shown an impressive increase, mainly due to their use in restoration of degraded structures. A prominent benefit of these materials is their barrier properties against corrosion for both the concrete and its steel reinforcement. The performances of thermosetting resins are greatly influenced by the curing conditions, among which the temperature is the most important. While in industrial processes of cure of thermosetting resins the temperature is a controlled variable that can be exactly set, this is not possible when the cure takes place in external environment without any control like in civil engineering applications. This lack of control on the processing parameters leads to an uncertainty on the polymer performances in terms of glass transition temperature, stiffness and hardening times. A possible solution to this problem would be the development and implementation of “in-situ” sensors which could monitor non-destructively the cure of thermosetting resins. These sensors would provide information correlated to the hardening of the reactive adhesives or thermoset resin based mortars enabling to predict the structural properties of the resulting composites. This would result in time save and improved product quality and, consequently, in minimizing the overall costs. The ultrasonic wave propagation, being related to the material viscoelastic behaviour, has a big potential for an “in-situ” cure monitoring. Ultrasonic waves, acting as a high-frequency dynamic-mechanical stress applied to the material, can follow non-destructively the evolution of the complex modulus during the cure. The non-destructive character of this technique is provided by the very small amplitudes of vibrations applied to the material. In this work, the ultrasonic technique has been used to monitor the progress of the cure of epoxy and unsaturated polyester resins, commonly used as matrices for fiber-reinforced composites and structural and non-structural adhesives. A correspondence between the changes in the velocity and attenuation and the phase transformations during the cure (gelation and vitrification) have been observed. A DSC analysis has been carried out to compare the evolution of the degree of reaction with that of the ultrasonic modulus.