Evaluation of the Compatibility and Exudation of Plasticizers in PVC Plastics by the Proton Spin–Spin Relaxation Times T ₂ with TD-NMR

In order to evaluate the compatibility of PVCs and plasticizers as well as predict the degree of plasticizer exudation, various samples were prepared with plasticizers of varying contents, and their proton spin–spin relaxation time (T2) measurements were performed by TD-NMR using three pulse sequences, i.e., the solid echo, Hahn echo and Carr–Purcell–Meiboom–Gill (CPMG) methods. The free induction decay (FID) curves obtained by the solid echo method and CPMG methods were converted into the normalized derivative signals. The normalized derivative signals indicated the differences in the FID data of these prepared samples which allowed visualization of the degree of their plasticization. The relatively long relaxation T2L and relatively short relaxation T2S times, and their respective fractions F2L and F2S for each motion component of the PVC plastic samples were calculated by fitting the FID curves obtained with the Hahn echo method to the Weibull function. Contact migration tests were then conducted to investigate the extent of plasticizer exudations from the prepared samples to polystyrene foam (PSP) sheets. As a result, a correlation could be observed between the change in the loss of plasticizer mass from the samples (Δmass) and the change in the difference of the T2 values for each molecular motion component (ΔT2 = T2L – T2S). It was found that the more similar the molecular mobility of the components within the PVC plastic, the more minimal the plasticizer exudation, thus presenting a new analytical indicator, the validity of which was verified in this work.