Epoxy resins are used as high-performance thermosetting linings to protect substrates under corrosive environments.
However, in a severe corrosive chemical solution, such protective layers may degrade with long time due to penetrations
of solvent and solute molecules into resin network. In this regard, the terahertz time-domain spectroscopy (THz-TDS)
is a promising tool for non-destructive evaluation of the penetrant amounts due to high transparency of such plastic materials
and high sensitivity to the molecular vibrations in terahertz spectral range. In this work, the complex refractive indexes n
and κ of epoxy specimens were measured after immersion into sulfuric acid solutions and compared with penetrated mass
fractions of water and acid ions. It was found that n and κ depended linearly with water and sulfuric acid mass fraction in
specimens, and κ of sulfuric acid immersed specimens was lager at higher frequency. While the calculated Δκ agreed well
with THz-TDS measurement by THz-TDS, the calculated Δn was higher than the measurement. The difference may be attributed
to the water and sulfuric states in the specimen.
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