Fig. 5. (a) FTIR spectra of epoxy in TBD–PG at different thickness after treating at 170 °C for 40 min. The inserted figure show the distance-dependent intensity of ester band. (b) Normalized ester content at different thickness after treating in TBD–PG at 170 °C for 40 min using ester band (v_{C O} = 1730 cm⁻¹) with phenol band (v_{C O} = 1608 cm⁻¹) as a reference by FTIR spectra.