Characterization of antioxidant concentration using terahertz time-domain spectroscopy (THz-TDS) in Ethylene-Vinyl-Acetate-Copolymer (EVA)

Ethylene-vinyl acetate copolymer (EVA) is a significant material for sealing and insulation with crucial applications in the nuclear power industry. As the service life increases, aerobic conditions induce an oxidation reaction that accelerates the aging process of material properties. This leads to a gradual deterioration of physical and mechanical properties, which can result in safety hazards.
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Adding antioxidants to the material can effectively slow down the aging process. Terahertz spectroscopy is a noncontact, fast, and non-destructive testing method with great potential for simultaneous material ageing testing.
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Thus, EVA samples with varying concentrations of 1010 antioxidant were analyzed using terahertz time-domain and dielectric constant spectra of the samples. The results indicated that the refractive indices and dielectric constants of the EVA samples decreased within the tested frequency range as the antioxidant content increased. Furthermore, the absorption coefficient, dispersion coefficient, and dielectric loss were used as criteria for evaluation. Various methods were used to construct a multi-input model for antioxidant content, including Random Forest (RF), Multilayer Perceptron (MLP), Partial Least Squares (PLSR), and eXtreme Gradient Boosting (XGboost). Classification and discrimination results were obtained through visual processing. The RF and xgboost models achieved the highest accuracy rate. Also, molecular dynamics simulations were used to model the effect of functional group content in 1010 antioxidants, ranging from $0.5-2.5 \mathrm{THz}$, on the dielectric constant. The purpose was to explain the changes in physical parameters caused by the antioxidant content. Our work contributes to the fusion of terahertz time-domain spectroscopy and artificial intelligence techniques, this paves a way to fast and nondestructive measurements of insulating materials.