Air-Coupled Ultrasonic Cure Monitoring of Unsaturated Polyester Resins

Air-coupled ultrasound in one-side transmission mode has been applied to monitor the curing process of an unsaturated polyester resin, commonly used as a matrix in glass reinforced composites. The evolution of the resin mechanical properties during cure has been measured from the variation of longitudinal velocity. The technique has demonstrated to be a very powerful tool for detecting the structural changes occurring at gelation and vitrification. The no-contact ultrasonic results have been compared with those obtained using conventional contact ultrasonic measurements, characterized by direct coupling between the transducer and the resin, and with the rheological measurements. The good agreement among the results of the different techniques demonstrates the reliability of air-coupled ultrasound in monitoring the changes of viscoelastic properties occurring during the cure of thermosetting polymers. A further advantage of the proposed one-side ultrasonic technique is the absence of physical contact between the transducers and the sample, which is relevant during composite manufacturing, where probe contact could adversely affect the part quality or access from both sides is not practicable. No-contact ultrasonic cure monitoring is suitable for both stationary and moving liquid or solid samples in several process conditions such as moulding, filament winding, etc., opening the way to new applications of ultrasound in the composite industry.