Non-Contact Ultrasonic Monitoring of Vinyl Ester Resin Pre-Cure Maturation for Sheet Moulding Compound Manufacturing

This study presents a non-contact method for monitoring the 17-day pre-cure maturation of vinyl ester resin, a widely used matrix for Sheet Moulding Compound manufacturing. The proposed approach is based on air-coupled ultrasonic thickness resonance spectroscopy, which allows estimation of key parameters like longitudinal velocity, attenuation, and Q-factor. To validate the sensitivity of the proposed method, we compared resin samples with and without 3% MgO thickener under controlled conditions. Samples with thickener exhibited a consistent decrease in Q-factor, with significant slope changes around day 3 and day 7. This reduction, coupled with increasing attenuation, reflects early molecular interactions and correlates with expected viscosity increase. This marks the beginning and end of the key maturation stage temporal window. In contrast, samples without thickener showed negligible changes. Notably, longitudinal velocity remained stable in both cases, indicating minimal variation of the elastic modulus along the sample height during this phase. These findings confirm non-contact resonant ultrasound spectroscopy sensitivity to thickener-induced viscoelastic changes, offering a real-time, non-destructive tool for monitoring pre-cure resin states.